Post on 08-Feb-2017
UNIVERSIDAD DE CASTILLA-LA MANCHA
FACULTAD DE CIENCIAS Y TECNOLOGÍAS QUÍMICAS
DEPARTAMENTO DE INGENIERÍA QUÍMICA
NUEVAS APLICACIONES ELECTROCATALÍTICAS PARA
PROCESOS ENERGÉTICOS Y DE REMEDIACIÓN
MEDIOAMBIENTAL
Memoria que para optar al grado de Doctor en Ingeniería Química
presenta:
NURIA GUTIÉRREZ GUERRA
Directores: Dr. José Luis Valverde Palomino
Dr. Antonio de Lucas Consuegra
Composición del tribunal: Dr. Jesús Arauzo Pérez
Dra. Paula Sánchez Paredes
Dr. Juan Carlos Serrano Ruíz
Profesores que han emitido informes favorable de la tesis: Dra. Amaya Romero Izquierdo
Dr. Ángel Caravaca Huertas
Ciudad Real, Noviembre de 2015
D. José Luis Valverde Palomino, Catedrático de Ingeniería
Química de la Universidad de Castilla-La Mancha, y D. Antonio de
Lucas Consuegra, Profesor Contratado Doctor de Ingeniería
Química de la Universidad de Castilla-La Mancha,
CERTIFICAN: Que el presente trabajo de investigación titulado:
“Nuevas aplicaciones electrocatalíticas para procesos energéticos y de
remediación medioambiental” constituye la memoria que Dª. Nuria
Gutiérrez Guerra para aspirar al grado de Doctor en Ingeniería
Química y que ha sido realizada en los laboratorios del
Departamento de Ingeniería Química de la Universidad de Castilla-
La Mancha bajo su supervisión.
Y para que conste a los efectos oportunos, firman el presente
certificado
En Ciudad Real, a 15 de Octubre de 2015
D. José Luis Valverde Palomino D. Antonio de Lucas Consuegra
i
TABLE OF CONTENTS
DESCRIPCIÓN DEL TRABAJO REALIZADO 1
A. Introducción 2
A.1. El hidrógeno como vector energético 2
A.2. El gas de síntesis como fuente de combustibles sintéticos 11
A.3. El dióxido de carbono como gas de efecto invernadero 16
A.4. Reactores de membrana de electrolito sólido (SEMRs) 19
A.5. Reactores electroquímicos de membrana polimérica (PEM) 28
A.6. Objetivo del presente trabajo 32
B. Instalaciones experimentales 33
B.1. Reactor SEMR de cámara sencilla 33
B.2. Reactor SEMR de doble cámara 34
B.3. Reactor PEM 35
C. Resultados y discusión 36
D. Conclusiones y recomendaciones 44
E. Bibliografía 46
CHAPTER 1: Direct Production of Flexible H2/CO Synthesis Gas
via Steam Electrolysis and Ethanol Partial Oxidation 55
Abstract 55
1.1. Introduction 56
1.2. Experimental 58
1.2.1. Catalytic activity measurements 58
1.2.2. Preparation of the solid electrolyte cell 60
1.2.3. Characterization measurements 61
1.3. Results and discussion 61
1.4. Conclusions 74
1.5. References 74
ii
CHAPTER 2: Simultaneous Production and Separation of H2 and
C2 Hydrocarbons via Steam Electrolysis and Methane Partial
Oxidation.
81
Abstract 81
2.1. Introduction 82
2.2. Experimental 84
2.2.1. Catalytic activity measurements 84
2.2.2. Preparation of the solid electrolyte cell 85
2.2.3. Characterization measurements 87
2.3. Results and discussion 88
2.4. Conclusions 99
2.5. References 100
CHAPTER 3. Electrochemical Reforming vs. Catalytic
Reforming of Ethanol: A Process Energy Analysis for Hydrogen
Production
107
Abstract 107
3.1. Introduction 108
3.2. Methodology 111
3.3. Process description 112
3.3.1. Catalytic steam reforming of ethanol-water process 112
3.3.2. Electrochemical reforming of ethanol-water process 117
3.4. Process simulation and energetic evaluation 119
3.5. Conclusions 129
3.6. References 130
CHAPTER 4: Electrochemical Promotion of Ni with Alkali ions
in the CO2 Hydrogenation Toward CO and CH4 137
Abstract 137
4.1. Introduction 138
4.2. Experimental 143
4.2.1. Catalytic activity measurements and EPOC parameters 143
4.2.2. Preparation of the solid electrolyte cell 145
4.2.3. Characterization measurements 147
iii
4.3. Results and discussion 148
4.3.1. Influence of the preparation method of the catalyst film 148
4.3.2. Kinetic study and electrochemical promotion experiments 158
4.4. Conclusions 169
4.5. References 170
CHAPTER 5: Gas Phase Electrocatalytic Conversion of CO2 on
Cu Carbon-based Catalyst-electrodes Toward Fuels 177
Abstract 177
5.1. Introduction 178
5.2. Experimental 180
5.2.1. Catalytic activity measurements 180
5.2.2. Preparation of the electrochemical catalyst 183
5.2.3. Characterization measurements 185
5.3. Results and discussion 186
5.3.1. Characterization of the Cu cathodic-catalyst and deposited
electrodes 186
5.3.2. Electrocatalytic experiments for CO2 conversion 197
5.4. Conclusions 205
5.5. References 206
CHAPTER 6: General Conclusions and Recommendations 213
6.1. Conclusions 213
6.2. Recommendations 214
List of Publications and Conferences 219
Anexo. Patente 221
A. Introducción
A.1. El hidrógeno como vector
energético
A.2. El gas de síntesis como fuente de
combustibles sintéticos
A.3. El dióxido de carbono como gas de
efecto invernadero
A.4. Reactores de membrana de
electrolito sólido (SEMRs)
A.5. Reactores electroquímicos de
membrana polimérica (PEM)
A.6. Objetivo del presente trabajo
B. Instalaciones experimentales
B.1. Reactor SEMR de cámara sencilla
B.2. Reactor SEMR de doble cámara
B.3. Reactor PEM
C. Resultados y discusión
D. Conclusiones y recomendaciones
F. Bibliografía
Descripción del Trabajo
Realizado
1
ste trabajo forma parte de un amplio programa de investigación
sobre la aplicación de sistemas electrocatalíticos en procesos de
interés energético y medioambiental que se viene desarrollando,
durante los últimos años en el Departamento de Ingeniería Química de la
Universidad de Castilla-La Mancha.
En particular, esta Tesis Doctoral tiene como objetivo el uso de nuevos
sistemas electrocatalíticos para la producción de hidrógeno e hidrocarburos
y la valorización de CO2. Este trabajo ha sido financiado por el centro de
investigación ABENGOA Research, a través del proyecto Aplicación de la
electrocatálisis en la producción de H2 acoplada a otros procesos de interés
industrial y medioambiental (proyecto Electrocatal).
Esta tesis doctoral se ha realizado en colaboración con el instituto de
Ciencia de Materiales de Madrid (CSIC) y con el centro de investigación
ABENGOA Research.
E
Descripción del trabajo realizado
2
A. Introducción
A.1. El hidrógeno como vector energético
El modelo energético actual, basado en combustibles fósiles, presenta
serios problemas de sostenibilidad. En primer lugar, por el agotamiento
progresivo del petróleo y las sucesivas crisis que afectan periódicamente a
su producción, y que repercuten en la estabilidad internacional y afectan al
precio del crudo, alterando, de este modo, el equilibrio económico mundial.
En segundo lugar, por los impactos ambientales que se derivan del uso
intensivo de los recursos energéticos fósiles. Por todo ello, resulta evidente
la necesidad de buscar nuevas alternativas energéticas.
Una de ellas se basa en el uso del hidrógeno como vector energético
(“economía del hidrógeno”). Grandes potencias como Estados Unidos,
Japón y la Unión Europea apuestan por el cambio del modelo energético
actual, basado en el uso de combustibles fósiles, por otro basado en el
hidrógeno. Tres son las causas que motivan este escenario [1]:
- Elevada eficiencia energética: la energía química del hidrógeno puede
convertirse de forma directa en electricidad, sin necesidad de emplear un
ciclo termodinámico intermedio, alcanzándose de este modo elevados
rendimientos energéticos. Esta transformación directa se lleva a cabo en
las llamadas celdas de combustible, capaces de convertir, de manera
electroquímica, la energía química del hidrógeno en energía eléctrica.
- Reducción de la dependencia energética: En la actualidad hay una
fuerte dependencia a escala mundial de los combustibles fósiles.
Numéricamente, los hidrocarburos aportan más de la mitad de la energía
primaria consumida a escala planetaria. El petróleo, en particular, aporta
el 32 % del consumo energético primario global, lo que la convierte en la
fuente energética más utilizada [2]. Las reservas de petróleo podrían
Descripción del trabajo realizado
3
agotarse en el escenario actual de producción y de reservas conocidas en
40 años, las del gas natural en 60 y las de carbón en 200 [2]. Esta
limitación de reservas va unida en muchas ocasiones a una elevada
concentración de los yacimientos, lo que facilita presiones políticas por
parte de los países productores.
- Razones medioambientales: La combustión de hidrógeno con oxígeno
puro solo libera vapor de agua, libre de CO2 [3].
A.1.1. Procesos de producción de hidrógeno
Debido a que el hidrógeno no se encuentra aislado en la naturaleza, es
preciso obtenerlo a partir de otras materias primas. En la actualidad
existen procesos muy diferentes que permiten la obtención de hidrógeno a
partir de una gran variedad de materias primas. A continuación se
exponen las formas de producción más relevantes.
i) Procesos biológicos
Estos métodos están menos desarrollados que los que se derivan del
uso de hidrocarburos, aunque su importancia aumenta debido a las
bajas emisiones de CO2 que generan.
- Conversión de biomasa
La valoración de la biomasa puede realizarse a través de cuatro
procesos básicos (combustión, digestión anaerobia, gasificación y
pirólisis) mediante los cuales se puede transformar en calor y
electricidad.
- Producción biológica de hidrógeno
La producción biológica de hidrógeno se lleva a cabo en biorreactores
usando cianobacterias y algas verdes que aprovechan la luz solar. La
presencia de la enzima hidrogenasa en condiciones anaeróbicas permite
a estos organismos realizar, con la ayuda de la fotosíntesis, la reducción
catalítica de protones a H2.
Descripción del trabajo realizado
4
ii) Procesos de conversión química
Los procesos de reformado son los más habituales para la obtención de
hidrógeno. La producción de hidrógeno mundial está alrededor de 500
billones Nm3/año, de la que cerca del 96 % corresponde con procesos de
reformado con vapor de agua de combustibles fósiles, como gas natural
(cuyo componente principal es el metano) que es en la actualidad la
principal fuente para la producción de hidrógeno (49 %) [4].
- Reformado con vapor de agua (SR)
Como se ha comentado anteriormente, la principal vía industrial de
producción de hidrógeno es el reformado de metano con vapor de agua.
Sin embargo, el hidrógeno también se puede obtener por reformado de
otras materias primas como el carbón, el metanol y el etanol. La
reacción, altamente endotérmica, que tiene lugar en estos procesos de
reformado con vapor de agua es la siguiente:
CnHm + nH2O → nCO + (n +m
2)H2 (A.1)
En los procesos convencionales de reformado con vapor de agua, el
combustible reacciona a elevada temperatura y presión (entre 500-950
ºC y 1-25 atmósferas de presión) con la finalidad de producir una mezcla
de H2 y de CO (gas de síntesis) [5]. Los catalizadores más utilizados son
los basados en Ni soportados sobre sílice, alúmina, ceria o zirconia [6].
El uso de este tipo de catalizadores conlleva sin embargo una serie de
inconvenientes como son: las elevadas temperaturas de operación
necesarias y los problemas de desactivación del catalizador originados
por deposición de filamentos de carbono [7].
En las últimas décadas se han incrementado los esfuerzos por
desarrollar catalizadores que sean eficientes a temperaturas de
operación inferiores y reduzcan los problemas asociados a la
Descripción del trabajo realizado
5
desactivación. En este sentido, el uso de catalizadores nobles como Pt [8]
o Rh [9] ha demostrado que puede satisfacer ambos requerimientos.
Asociados a los procesos industriales de reformado de hidrocarburos
con vapor de agua, suelen coexistir otros procesos de conversión como el
de desplazamiento de CO con vapor de agua, convencionalmente
conocido como “Water Gas Shift” (WGS). Este proceso permite
incrementar la producción de hidrógeno mediante la reacción del
monóxido de carbono presente en el medio de reacción con vapor de
agua: CO + H2O ↔ CO2 + H2. Debido a motivos termodinámicos
(reacción exotérmica) y cinéticos, la reacción WGS se realiza con ayuda
de catalizadores en dos pasos: el primero a alta temperatura (HTS) y el
segundo a baja temperatura (LTS). El reactor HTS opera a elevadas
temperaturas (473-523 K) [10] mientras que el reactor LTS lo hace a
temperaturas inferiores (473-523 K) [10]. En el proceso a elevada
temperatura suelen utilizarse catalizadores de Fe3O4/Cr2O3, mientras
que en el de baja temperatura suelen emplearse catalizadores de Cu-
ZnO-Al2O3. En conjunto, se obtiene una conversión final de CO superior
al 99,5 % [7].
- Oxidación parcial (PO)
En los procesos de oxidación parcial de hidrocarburos, el combustible
se hace reaccionar con oxígeno en una proporción inferior a la requerida
para la combustión completa del mismo. La reacción que tiene lugar en
la oxidación parcial es la siguiente:
CnHm + n
2O2 → nCO +
m
2H2 (A.2)
Este proceso se puede llevar a cabo en ausencia de catalizador a
temperaturas comprendidas entre 1150 y 1350ºC [11-13], obteniéndose
un gas constituido por H2, CO, CO2 y CH4 [13]. Sin embargo, el uso de
catalizadores presenta ciertas ventajas operativas: disminución de la
temperatura de trabajo y de los problemas asociados a la deposición del
Descripción del trabajo realizado
6
carbono (bloqueo del reactor, incremento de la presión, etc.). Los
catalizadores más utilizados en este proceso son los basados en Ni, Rh o
Pt, aunque también se utilizan aleaciones metálicas (Al2O3, La2O3 y
MgO) que aseguran condiciones apropiadas de actividad y selectividad
durante 100 horas de operación [11]. La reacción de oxidación parcial es
una reacción exotérmica que no requiere de sistemas auxiliares para
mantener la temperatura de reacción. Por otro lado, el control de la
temperatura es complicado como consecuencia de la exotermicidad de la
reacción ya que se pueden formar puntos calientes [14], que pueden
originar desactivación de los catalizadores por fenómenos de
sinterización.
Igual que en el proceso de reformado descrito anteriormente, este
proceso se puede acoplar a una unidad de desplazamiento de CO con
vapor de agua (WGS) con la que incrementar la producción de
hidrógeno.
Atendiendo a la economía de hidrógeno, los procesos de reformado
resultan más eficientes, ya que la cantidad de hidrógeno producida en
estos es el doble que la producida mediante su oxidación parcial [1].
- Reformado autotérmico (ATR)
El reformado autotérmico es un proceso muy estudiado y se ha usado
por la industria durante más de 50 años [15]. Se trata de un proceso que
combina los de reformado y oxidación parcial, anteriormente descritos.
En este caso se utiliza la exotermicidad de la reacción de oxidación
parcial para aportar el calor necesario a la reacción de reformado con
vapor de agua, buscando un balance de energía neto nulo. Al igual que
en los procesos anteriores, el CO producido puede ser desplazado con
vapor de agua lo que permite incrementar la producción de hidrógeno.
La reacción que se produce se muestra a continuación:
CnHm +n
2H2O +
n
4O2 → nCO + (
n
2+
m
2) H2 (A.3)
Descripción del trabajo realizado
7
El valor de la temperatura de operación oscila entre los máximos de
las temperatura de los otros dos procesos mientras que los catalizadores
empleados son muy similares a los que se utilizan en el reformado con
vapor de agua y en el proceso de oxidación parcial [16]. Al igual que en
este último proceso, se requiere de complejas unidades de separación
con objeto de que el oxígeno alimentado al reactor sea puro.
iii) Procesos electroquímicos. Electrólisis
El proceso de electrólisis consiste en la ruptura de moléculas por la
acción de una corriente eléctrica. Este es el caso, por ejemplo, de la
molécula de agua con la que se obtendrían corrientes puras de H2 y O2. La
electrólisis del agua presenta como principales ventajas la ausencia de
emisiones nocivas para el medio ambiente siempre que se use la fuente de
energía adecuada (eólica, solar, etc.) y la enorme disponibilidad del agua
(constituye aproximadamente 3/4 de la superficie terrestre). La electrólisis
del agua se puede llevar a cabo en tres tipos de electrolizadores:
electrolizadores alcalinos, que operan a bajas temperaturas,
electrolizadores basados en membranas poliméricas (electrolizadores de
baja temperatura, tipo PEM) y electrolizadores de electrolito sólido
(electrolizadores de alta temperatura, tipo SOEC). La electrólisis alcalina
de agua es, debido a su sencillez, uno de las tecnologías más desarrolladas
y aplicadas por la industria para la producción de H2. Sin embargo,
presentan un alto consumo energético, elevados inmovilizados y costes
mantenimiento, siendo limitadas la durabilidad y la seguridad de uso. Esto
está provocando que se estén desarrollando tecnologías más eficientes para
la producción de hidrógeno [17]. Una de estas tecnologías es la basada en
el uso de electrolizadores tipo PEM. A diferencia de los electrolizadores
alcalinos, los PEM requieren un electrolito no líquido, lo que simplifica el
diseño significativamente. Además, los electrolizadores PEM presentan
Descripción del trabajo realizado
8
más ventajas frente a los electrolizadores alcalinos como son: un diseño
más compacto, elevada pureza del H2 obtenido, menor consumo energético,
alta conductividad protónica, mayor nivel de seguridad, un fácil manejo y
mantenimiento [18]. Los electrolizadores de alta temperatura, por su
parte, permiten producir hidrógeno con un menor rendimiento energético
debido a las mayores temperaturas de operación [4].
Existen otros métodos de producción de hidrógeno que se basan en la
ruptura de la molécula de agua mediante procesos de termólisis (por acción
del calor) y fotoelectrólisis (por acción de la luz solar). Sin embargo, ambos
procesos presentan eficiencias inferiores a las obtenidas con el proceso de
electrólisis.
A.1.2. Aplicaciones del hidrógeno
i) Materia prima de procesos químicos
En la actualidad, el hidrógeno se utiliza a gran escala como materia
prima en procesos de hidrogenación de la industria química y
petroquímica [19]. Se utiliza también en otras industrias como la
electrónica, la metalúrgica y la farmacéutica [20]. En este sentido, el 95 %
del hidrógeno producido se consume in-situ, utilizándose
fundamentalmente como materia prima de procesos químicos. En la Figura
A.1 se indican las fuentes de producción y las aplicaciones del hidrógeno.
Descripción del trabajo realizado
9
FUENTE
DEMANDA
Figura A.1. Fuentes de producción y aplicaciones del hidrógeno.
Dentro de los usos del H2 en procesos químicos, destaca la síntesis de
amoniaco por el proceso Haber-Bosh (N2 + 3H2 → 2NH3) que opera a
elevadas presiones (alrededor de 250 atm) y temperaturas de operación de
450 ºC [21].
Como se ha comentado anteriormente, la aplicación más importante del
hidrógeno es en procesos de hidrogenación de la industria del refino. El
objetivo principal es la obtención de fracciones ligeras de crudo a partir de
fracciones pesadas, aumentando su contenido en hidrógeno y
disminuyendo su peso molecular. Estos procesos permiten la eliminación
de forma simultánea de elementos indeseados como azufre, nitrógeno y
metales [20].
Como se ha mencionado anteriormente, la mayor parte del hidrógeno
consumido a escala mundial se produce a partir de combustibles fósiles,
principalmente a partir del reformado con vapor de gas natural o metano.
La producción de hidrógeno a partir de hidrocarburos conduce a una
mezcla de gases formada principalmente por hidrógeno y monóxido de
carbono (con trazas de CO2), lo que comúnmente se denomina gas de
síntesis. El gas de síntesis se utiliza fundamentalmente en procesos
Descripción del trabajo realizado
10
químicos, como la síntesis de metanol, síntesis de Fischer-Tropsch,
hidroformilación de olefinas (síntesis oxo) y síntesis de metanol y etileno,
entre otros compuestos [22].
ii) Generación de energía
El hidrógeno se puede transformar en energía de manera indirecta a
través de su combustión en turbinas de gas y ciclos combinados o
directamente como combustible de motores. Las principales ventajas del
uso del hidrógeno en estos procesos se relacionan con el elevado
rendimiento que puede alcanzarse en su combustión, el hecho de obtener
vapor de agua como único producto de reacción, y la ausencia de NOx en los
gases efluentes al poder controlar la temperatura a la que reaccionan el
nitrógeno y el oxígeno atmosféricos, y de CO2, menor impacto sobre el
calentamiento global [20].
La ruta más directa para convertir el hidrógeno en energía es a través
de pilas de combustible [23]. El descubrimiento de la pila de combustible
por el jurista inglés sir William Robert Grove en 1839 supuso un punto de
inflexión en la tecnología del hidrógeno, siendo numerosos los expertos que
coinciden en su gran importancia futura como sistema de generación de
energía eléctrica. La pila de combustible es un sistema electroquímico que
produce energía por la combinación de hidrógeno y oxígeno en una reacción
química. Esta conversión de energía química en forma de energía eléctrica
se realiza separando el combustible y el comburente (oxígeno o aire) con
una membrana conductora iónica (por ejemplo, de iones oxígeno o
protones) que no permita el transporte de electrones. Esta membrana es
un electrolito. Los electrones necesarios para completar la reacción se
transportarán a través del circuito externo, donde realizarán el trabajo
útil. Por ejemplo, en una pila de combustible de baja temperatura
(PEMFC), el hidrógeno se alimenta al ánodo, donde un electrodo de Pt
Descripción del trabajo realizado
11
facilita su electro-oxidación, formándose electrones y protones. Estos
últimos atraviesan una membrana conductora de protones, permitiendo su
combinación con oxígeno y electrones. Esta reacción está catalizada por Pt
[24]. Los electrones fluyen del ánodo al cátodo a través del circuito externo
pudiendo alimentar a algún dispositivo eléctrico [24, 25]. Este hidrógeno
debe ser de elevada pureza, conteniendo menos de 20 ppm de CO, lo que
evita la desactivación del catalizador [26], tal y como ha sido descrito en el
Capítulo 3 de esta memoria. Las principales ventajas que presenta el uso
industrial de pilas de combustible frente a otros sistemas tradicionales son:
admisión de diversos combustibles, flexibilidad de emplazamiento,
capacidad de cogeneración (pilas de alta temperatura), carácter modular,
seguridad energética e independencia de la red de suministro eléctrico.
A.2. El gas de síntesis como fuente de combustibles sintéticos
El uso del gas de síntesis (mezclas gaseosas de CO + H2) como materia
prima para la producción de combustibles sintéticos y de otros productos
importantes de síntesis supuso a lo largo del siglo XX el comienzo de una
nueva era de la industria química mundial [27]. La obtención de
combustibles líquidos sintéticos ofrecen alternativas interesantes a los
combustibles tradicionales. Estos combustibles suelen ser producidos a
partir de gas de síntesis, mediante las tecnologías X-To-Liquids (XTL).
Según la procedencia del gas de síntesis se distingue entre el proceso Gas-
To-Liquids (GTL), cuando el gas de síntesis proviene del gas natural; el
proceso Coal-To-Liquids (CTL), cuando el gas de síntesis proviene del
carbón; y el proceso Biomass-To-Liquids (BTL), cuando el gas de síntesis
procede de la biomasa. El interés en estas tecnologías está aumentando en
los últimos años debido a la disponibilidad de gas natural y carbón, las
ventajas medioambientales derivadas del uso de la biomasa, y los
inconvenientes e incertidumbres que existen alrededor del petróleo.
Descripción del trabajo realizado
12
A.2.1. Procesos de producción de gas de síntesis
El gas de síntesis empezó a producirse a partir de la gasificación del
coque de hulla o de la destilación de lignitos con aire y vapor de agua.
Después de la segunda Guerra Mundial se introdujeron combustibles
fósiles líquidos y gaseosos, petróleo y gas natural. La principal ventaja de
estos combustibles reside en su contenido en hidrógeno. La proporción
aproximada de H/C del carbón es de 1:1, del petróleo 2:1 y de los gases
naturales ricos en metano 4:1 [27]. Como se verá posteriormente, un
parámetro importante es la razón H2/CO del gas de síntesis producido ya
que determina el tipo de aplicación para la que se va a utilizar éste. A
continuación, se describirán con más detalle los principales métodos
químicos para la producción de gas de síntesis:
i) Reformado del carbón
Este proceso consiste en hacer reaccionar vapor de agua con cualquier
material carbonoso de origen natural, según el siguiente esquema global
endotérmico:
H2O + C → CO + H2 (A.4)
La proporción de H2 y CO puede modificarse, como se ha comentado en
apartados anteriores, por medio de la reacción de desplazamiento con
vapor de agua, water gas shift (WGS) (H2O + CO → CO2 + H2), que
consume CO y genera H2. Los procesos de gasificación de carbón se
caracterizan por la elevada necesidad de energía del proceso que, por otra
parte, requiere de elevadas temperaturas (entre 900 y 1000 ºC) con las que
alcanzar una velocidad de reacción satisfactoria [27].
ii) Reformado de metano con vapor de agua
En la actualidad, el proceso más extendido para la producción de gas de
síntesis es el de reformado de metano (gas natural) con vapor de agua
Descripción del trabajo realizado
13
sobre un catalizador metálico, debido a la elevadas disponibilidades y
reservas existentes de este gas [6]:
CH4 + H2O → CO + 3H2 (A.5)
La conversión completa del metano permite alcanzar razones H2/CO en
la corriente efluente del reactor de reformado de 3. Esta relación es mayor
que la necesaria para la síntesis de productos como metanol o los obtenidos
por medio del proceso de Fischer-Tropsch. Es por ello necesario ajustar la
relación H2/CO hasta un valor adecuado a través de la reacción WGS, lo
que encarece el proceso global.
Debido a la naturaleza endotérmica de la reacción (reacción A.5) se
necesitan temperaturas elevadas para maximizar la conversión de metano,
lo que implica el aporte de calor al sistema mediante la combustión de gas
natural usado como alimentación.
iii) Oxidación parcial de metano
Consiste en la oxidación parcial del CH4 con oxígeno a alta temperatura
mediante la siguiente reacción [28]:
2CH4 + O2 → 2CO + 4H2 (A.6)
Sin embargo, este método no ha sido usado industrialmente debido a
problemas derivados de la desactivación del catalizador. Sin embargo, se
ha desarrollado una variante electroquímica de este proceso que mejora
algunos aspectos del mismo [11]. En esta configuración, se utiliza un
reactor electroquímico de membrana con electrolito sólido (SEMR) al que
se alimenta CH4 (ánodo) y O2 (cátodo). Este sistema será estudiado en
detalle en el Capítulo 2.
Descripción del trabajo realizado
14
A.2.2. Aplicaciones del gas de síntesis
El nombre de “gas de síntesis” proviene de su uso para la obtención de
gas natural sintético en la producción de amoniaco o metanol. El hidrógeno
presente en dicho gas, una vez purificado, puede ser utilizado directamente
en pilas de combustible tanto para la generación de electricidad como
combustible de vehículos eléctricos [27].
Durante muchos años, los procesos de gasificación del carbón se
utilizaron en la producción de gas de alumbrado (gas de hulla) usado en las
ciudades antes de que la iluminación eléctrica fuera una realidad.
El gas de síntesis también puede ser utilizado como combustible. Sin
embargo, posee menos de la mitad de densidad energética que el gas
natural lo que limita su utilización en este tipo de aplicaciones. Por este
motivo, se utiliza principalmente en la producción de combustibles para el
transporte y como producto intermedio para la síntesis de otros
compuestos químicos. Además, el gas de síntesis producido en las grandes
instalaciones de gasificación de residuos puede ser utilizado para generar
electricidad in-situ, disminuyendo los costes operativos de estas plantas.
El gas de síntesis también se utiliza como producto intermedio en la
producción de petróleo sintético, para su uso como lubricante o
combustible, a través del proceso de Fischer-Tropsch. Esta etapa puede
representar más del 50 % de los costes totales de inversión y una gran
parte de los costes operativos [29]. Del gas de síntesis también se puede
obtener gasolina a partir de metanol mediante el proceso Mobil. En la
Figura A.2 se resumen las principales aplicaciones del gas de síntesis.
Descripción del trabajo realizado
15
Gas de síntesis
Metanol
Nafta
IntermediosAmoniaco
Metano- Olefinas
- Aromáticos
- Tolueno
- Etilenglicol
- Isobutano
Haber-Bosch
- Urea
- Hidrazina
- Metilaminas
- Ácido Nítrico
- Acrilonitrilo
- Fertilizantes
H2
CO
Metanación
Proceso
MOBIL- Olefinas aromáticas
(gasolinas)
- Metanol
- Ácido acético
- Formaldehido
- Metilamina
Figura A.2. Principales aplicaciones del gas de síntesis
La relación molar H2/CO del gas de síntesis es un parámetro
fundamental a la hora de determinar su posterior uso, ya que en función
de esta relación se podrán sintetizar unos compuestos determinados u
otros, tal y como se muestra en la Tabla A.1
Tabla A.1. Composición química del gas de síntesis y presencia de co-
reactantes en la producción de varios compuestos químicos y combustibles a partir
del mismo.
Proceso/producto Composición óptima Co-reactantes
Metanol H2/CO = 2 -
Fischer-Tropsch H2/CO = 2 -
Ácido acético CO Metanol
Etanol H2/CO ≈ 2 Metanol
Alcoholes de cadena larga H2/CO ≈ 1 Olefinas
H2 industrial 99,99% H2 -
H2 para celdas de combustible < 20 ppm de CO -
Descripción del trabajo realizado
16
A.3. El dióxido de carbono como gas de efecto invernadero
Según la Convención Marco de las Naciones Unidas existen fuertes
evidencias de que las actividades antropogénicas son la causa del aumento
de la temperatura media en la Tierra debido al incremento de emisiones de
gases de efecto invernadero (GEIs) [30]. En la actualidad, estas emisiones
de gases de efecto invernadero son las mayores de la historia. Se cree que
como consecuencia del aumento de la temperatura media global, el clima
del planeta está cambiando, efecto que se conoce como cambio climático.
Los principales gases de efecto invernadero responsables del aumento de
temperaturas son: dióxido de carbono, metano, óxidos de nitrógeno y un
grupo de compuestos gaseosos que contienen cloro y flúor, como
perfluorocarburos, halogenuros de carbono y hexafluoruro de azufre. Entre
estos, el CO2 es el GEIs más importante debido principalmente a su mayor
emisión.
A.3.1. Principales fuentes y evolución de las emisiones
El consumo global de energía y las emisiones de CO2 asociadas a este
consumo continúan creciendo en los primeros años del siglo XXI. Los
combustibles fósiles son la fuente de energía más utilizada en el mundo; el
86 % de la energía mundial procede de fuentes fósiles y su combustión es
responsable del 75 % de las actuales emisiones antropogénicas. En la
Figura A.3 se representan los porcentajes de emisiones totales
antropogénicas de GEIs por sectores económicos relativas a 2010.
Descripción del trabajo realizado
17
Energía Industria Transporte Construccion Agricultura, silvicultura y otros usos resto
Electricidad y
producción de calor
25 %
Otras energías
9.6 %
EMISIONES DE GEIS POR SECTORES ECONÓMICOS
Electricidad y produccion de calor Agricultura, silvicultura y otros usos Construcción
Transporte Industria Otras energias
Industria
21 %
Transporte
14 %
Construcción
6.4 %
Agricultura,
silvicultura y otros usos
24%
Energía
1,4 %Industria
11 %
Transporte
0,3 %
Construcción
12 %
Agricultura,
silvicultura y otros usos
0,87 %
Total: 49 Gt CO2-eq
(2010)
Emisiones directas de GEIs Emisiones indirectas de GEIs
Figura A.3. Porcentaje de emisiones antropogénicas de GEIs por sectores
económicos relativas a 2010 [30].
En el año 2010, el 35 % de las emisiones de GEIs provenían del sector
de la energía, el 24 % (emisiones netas) de la silvicultura y de otros usos
agrícolas, el 21 % de la industria, el 14 % del transporte y el 6.4 % del
sector de la construcción.
A.3.2. Mecanismos para la eliminación y reducción de las emisiones
En la actualidad, se han propuesto diversas opciones de adaptación y
mitigación que pueden ayudar a abordar el problema del cambio climático.
Sin embargo, una única opción no es suficiente por sí misma desde un
punto de vista tecnológico o económico. Según la Convención Marco de las
Naciones Unidas para el Cambio Climático (UNFCCC, United Nations
Framework Convention on Climate Change) la estabilización de las
Descripción del trabajo realizado
18
emisiones de GEIs se alcanzará cuando las emisiones de GEIs
antropogénicas sean equiparadas a las que pueden ser absorbidas de forma
natural por el planeta.
Existe una gran variedad de tecnologías que pueden reducir la
concentración de CO2 en la atmósfera. Los objetivos de reducción, los
costes, el potencial de cada tecnología, el impacto ambiental, y factores
sociales, como la aceptación pública, serán determinantes para la elección
de la tecnología más adecuada para cada situación concreta. Las diferentes
opciones para la reducción de emisiones de efecto invernadero puede ser,
de acuerdo al Intergovernmental Panel on Climate Change (IPCC) [30], las
siguientes:
- Mejora de la eficiencia energética en la conversión, transporte y uso
final de la energía.
- Aumento de las fuentes de energía de baja emisión: renovables y
nuclear. Aunque el desarrollo de ambas tecnologías ha sido muy
significativo en los últimos años, especialmente en el campo de las
energías renovables, éstas presentan todavía grandes limitaciones.
Por un lado, las fuentes de energía renovables no van a ser capaces
de aportar toda la demanda energética que se requiere para los
próximos años, especialmente en los países con economías
emergentes, a lo que se suma los problemas asociados a su
intermitencia. Por otro lado, la energía nuclear presenta dificultades
de aceptación política y pública debido a problemas relacionados con
la seguridad y el posterior almacenamiento de residuos.
- Captura y secuestro del CO2 (CCS, CO2 Capture and Sequestration)
y almacenamiento de CO2 en sumideros naturales o biológicos. La
primera tecnología consiste en capturar el CO2 emitido por una
central de generación de energía convencional y su posterior
Descripción del trabajo realizado
19
almacenamiento en el subsuelo. Por otro lado, el secuestro del CO2
en sumideros naturales o biológicos como son las plantas y los
océanos, permite la fijación del CO2 en las primeras gracias a
procesos fotosintéticos y a la absorción de este gas por parte de los
segundos. La fijación fotosintética puede potenciarse con prácticas
correctas de agricultura y reforestación.
- Conversión química del CO2 y almacenamiento de la energía en
moléculas orgánicas. El almacenamiento de energía en moléculas
orgánicas reduce los problemas de transporte y almacenamiento.
Además la conversión de CO2 se podría utilizar como un
intermediario para la incorporación de energías renovables en el
proceso industrial. Los procesos catalíticos de CO2 se postulan como
una de las soluciones más prometedoras para la reducción de las
emisiones del CO2 y, por tanto, de la mitigación del cambio climático.
Existen numerosos procesos de conversión catalítica de CO2:
reformado de metano con CO2, síntesis de dimetil carbonato a partir
de metanol y CO2, síntesis de carbonatos cíclicos a partir de CO2 y
epóxidos y sales de amonio, y procesos de hidrogenación para
obtener metanol [31]. En los últimos años este último proceso ha
adquirido gran relevancia ya que permite reemplazar el uso de CO
por CO2 en la producción de metanol.
A.4. Reactores de membrana de electrolito sólido (SEMRs)
Los reactores de membrana de electrolito sólido (Solid Electrolyte
Membrane reactor, SEMRs) son un tipo de reactores compuestos por un
electrolito sólido cerámico y por dos electrodos porosos depositados a ambos
lados del mismo. Los electrolitos sólidos son materiales que deben ser
químicamente estables, tener una elevada conductividad iónica y ser
impermeables a cualquier otra especie no cargada eléctricamente [32]. Las
Descripción del trabajo realizado
20
membranas de electrolito sólido se pueden clasificar en: membranas de
conductores mixtos de iones-electrones (Mixed Ion-Electron Conductors,
MIEC) y membranas de electrolitos sólidos puros. Mientras que las
primeras se caracterizan por tener valores comparables de conductividad
iónica-electrónica, en las segundas la conductividad electrónica es al menos
dos órdenes de magnitud inferior a la conductividad iónica. Es por ello, que
los electrolitos sólidos puros requieren un circuito externo para el
transporte de los electrones. Con este tipo de configuraciones se puede
controlar de un modo eficiente el transporte de iones hacia el electrodo que
se desee mediante la aplicación de potenciales eléctricos o intensidades en
un sentido o en otro [32-34].
A.4.1. Tipos de reactores de membrana de electrolito sólido y modos de
operación
En función de cómo estén expuestos los electrodos a la atmósfera de
reacción, los SEMRs se pueden clasificar en reactores de cámara sencilla y
reactores de doble cámara. Los reactores de cámara sencilla se
caracterizan por tener ambos electrodos (ánodo y cátodo) expuestos a la
misma atmósfera de reacción. Por el contrario, los reactores de doble
cámara se caracterizan por tener los electrodos expuestos a diferentes
mezclas de reacción.
Una de las principales ventajas de los reactores de cámara sencilla
respecto a los de doble cámara es su mayor sencillez, así como la mayor
facilidad para ser implementados en sistemas en los que el soporte
catalítico se sustituye por el electrolito sólido. Por el contrario, los
reactores de doble cámara ofrecen la posibilidad de separar los productos
obtenidos al mismo tiempo que son formados, incrementando de este modo
la economía del proceso.
Descripción del trabajo realizado
21
Los SEMRs pueden operar de acuerdo a los siguientes modos de
operación [33]:
Condiciones de circuito abierto (O.C.C, Open Circuit Conditions).
En este modo de operación no se aplica corriente eléctrica a
través del electrolito sólido, siendo la diferencia de potencial
químico la verdadera fuerza impulsora de la celda.
Condiciones de circuito cerrado. En estas condiciones se aplica
una corriente eléctrica que permite transferir los iones a través
del electrolito sólido, de un electrodo a otro, y reaccionar así con
los reactivos presentes en la fase gas. De este modo, el reactor
podría trabajar como una celda de combustible si el objetivo es
obtener energía. Por otro lado, si el objetivo fuera obtener un
determinado producto de reacción, se podría aplicar cierta
corriente eléctrica (equivalente al flujo de iones transferidos a
través del electrolito sólido) en la dirección deseada, con el
objetivo de favorecer la reacción de dichos iones y de los reactivos
presentes en la atmósfera de reacción.
A.4.2. Electrolitos sólidos
Se conocen numerosos electrolitos sólidos que se suelen clasificar de
acuerdo al ion móvil que se desplaza a su través. A día de hoy se han
descubierto conductores de O2-, F-, H+, K+, Na+, Cu+, Ag+ and Li+ [35].
Debido a la importancia industrial de los procesos catalíticos de oxidación e
hidrogenación, son precisamente los electrolitos sólidos conductores de
iones de oxígeno (O2-) y protones (H+) los más ampliamente utilizados en
los SEMRs.
Los conductores de O2- son disoluciones en estado sólido de cationes
divalentes o trivalentes (Y2O3, CaO, Yb2O3) en óxidos de metales
Descripción del trabajo realizado
22
tetravalentes (ZrO2, ThO2, CeO2) [33]. La conductividad de los iones
oxígeno está basada en las vacantes de O2- creadas en la matriz del óxido
metálico tetravalente cuando se dopa con el óxido metálico divalente o
trivalente. Debido a su estabilidad química y mecánica, el conductor de O2-
más popular es el conocido como YSZ (Yttria Stabilized Zirconia, 6-10 %
Y2O3 e ZrO2).
Por otro lado, los conductores protónicos de alta temperatura consisten
en óxidos de la familia de las perovsquitas, normalmente basados en
SrCeO3 o BaCeO3. A principios de la década de 1980, el grupo de trabajo de
Iwahara y col, [36] descubrieron que estos materiales presentaban
conductividad protónica a elevadas temperaturas (T > 600 ºC). La principal
aplicación de este tipo de conductores protónicos es la obtención de
hidrógeno. Este tipo de perovsquitas presentan unas conductividades del
orden de 10-2-10-3 S cm-1 entre 600 - 1000 ºC cuando se exponen a
atmósferas de reacción que contiene hidrógeno [36].
Otra familia importante de electrolitos solidos son los basados en β”-
alúmina. Este tipo de materiales presentan conductividad de iones Na+ y
K+ a baja temperatura (> 150 ºC). Cabe destacar que los materiales de esta
familia permiten la promoción electroquímica sobre superficies metálicas
[37], fenómeno que será explicado en detalle en apartados posteriores.
A.4.3. Aplicaciones de los SEMRs
i) Conducción selectiva de iones
Las primeras aplicaciones de los SEMRs estaban relacionadas con su
capacidad de conducción selectiva de iones [33]. Existen celdas basadas en
conductores de iones O2- que se han utilizado en la separación del oxígeno
del aire o de cualquier otra mezcla gaseosa. Otra ventaja que ofrece la
conductividad selectiva de iones es que favorecen el suministro de especies
Descripción del trabajo realizado
23
libres de impurezas que puedan afectar al proceso. Un ejemplo de ello es la
síntesis industrial de amoniaco. En este proceso, una parte significativa
del coste proviene de la preparación y purificación de la corriente gaseosa;
por ejemplo, el proceso de síntesis de amoniaco mediante el uso de
conductores protónicos permite el suministro de H+ libres de O2 [38], que es
el causante del envenenamiento irreversible del catalizador.
ii) Estudio del mecanismo de reacción en procesos catalíticos
Se puede extraer información potenciométrica cuando los sistemas
SEMR operan en condiciones de circuito abierto, es decir con una
intensidad eléctrica nula, que combinada con medidas catalíticas, permite
estudiar los mecanismos de la reacción que se está produciendo, De este
modo, se ha demostrado que si el contraelectrodo de una celda de doble
cámara está expuesto al aire (con una presión parcial de oxígeno PO2 = 0,21
bar), la actividad termodinámica del oxígeno atómico adsorbido viene dada
por la siguiente ecuación [33, 39, 40] :
ɑ0 = (0,21)0,5exp (2FE
RT) (A.7)
donde F es la constante de Faraday, R es la constante de los gases ideales,
T es la temperatura absoluta, E es la fuerza electromotriz de la celda (emf)
y α0 es la actividad del oxígeno adsorbido. Esta técnica, denominada como
Potenciometría de Electrolito Sólido (Solid Electrolyte Potenciometry,
SEP), se ha utilizado en el estudio de importantes sistemas catalíticos:
oxidación de óxido de azufre [41], CO [42], etileno [43] y propano [44] sobre
catalizadores de Pt y Ag. Esta técnica también ha sido utilizada para
estudiar cambios de fase.
Descripción del trabajo realizado
24
iii) Promoción electroquímica de la catálisis
El fenómeno de promoción electroquímica de la catálisis
(Electrochemical Promotion Of Catalysis, EPOC) o efecto NEMCA (Non-
Faradaic Electrochemical Modification of Catalytic Activity) ha sido
definido como “una herramienta importante de la electroquímica que
permite alterar de manera pronunciada, reversible y predecible la actividad
y selectividad catalítica de catalizadores conductores soportados sobre
electrolitos sólidos” [37]. En este sentido, se puede afirmar que es un tipo
especial de catálisis en el que las propiedades catalíticas se modifican
mediante la aplicación de un campo eléctrico.
Este fenómeno fue descubierto en el año 1981, por el grupo del profesor
Vayenas, quienes observaron que la actividad y la selectividad de un
catalizador depositado sobre un electrolito sólido podía ser
electroquímicamente modificada in-situ durante el propio proceso de
reacción [45].
El origen del efecto NEMCA tiene lugar cuando se aplica un voltaje o
una intensidad eléctrica entre un electrodo metálico (que actúa como
catalizador y electrodo de trabajo) depositado a un lado de un electrolito
sólido, y un segundo electrodo (contra-electrodo) depositado en el lado
opuesto de dicho electrolito. De este modo, si sobre el electrodo de trabajo
está teniendo lugar una reacción catalítica heterogénea, la aplicación de
corriente eléctrica puede provocar un incremento pronunciado de la
velocidad de reacción, pudiéndose llegar a ser de 10 a 105 veces el valor que
predice la ley de Faraday. Este hecho motivó que este fenómeno también
sea conocido como modificación electroquímica no faradaica de la actividad
catalítica (efecto NEMCA en sus siglas en inglés).
Descripción del trabajo realizado
25
- Origen y mecanismo del fenómeno de promoción electroquímica
En las últimas décadas, la comunidad científica ha emprendido
investigaciones con las que determinar el origen del fenómeno de
promoción electroquímica de la catálisis. Así, el origen ha sido atribuido al
movimiento de especies promotoras controlado electroquímicamente [37].
Estas especies son generadas en la región conocida como tbp (three-phase
bounderie; la interfase entre el electrolito sólido, catalizador-electrodo de
trabajo y la fase gas). En determinadas condiciones, estas especies migran
hacia la superficie del metal distribuyéndose a lo largo de él y modificando
así la capacidad de quimisorción de las moléculas de reactivo sobre la
superficie del catalizador. Esta teoría, que ha sido demostrada mediante
una gran variedad de técnicas de caracterización tanto catalíticas,
electrocatalíticas y de análisis de superficies (XPS, TPD, PEEM, STM,
Voltamperometrías cíclicas) [37], ha puesto de manifiesto que este
fenómeno es análogo al de la promoción química convencional de la
catálisis heterogénea. Sin embargo, el fenómeno EPOC cuenta con la
ventaja adicional de poder controlar, de un modo preciso y reversible, la
cantidad de promotor en el catalizador durante el propio proceso de
reacción. En el caso de la promoción electroquímica, las especies
promotoras son iones que migran de forma controlada desde el electrolito
sólido al metal, y viceversa, y cuyo sentido depende del signo de la
intensidad aplicada. Así por ejemplo, la aplicación de un potencial o
intensidad negativa genera la migración de iones positivos (como por
ejemplo Na+ o K+) desde el electrolito sólido hasta el metal en un conductor
catiónico. De la misma forma, se producirá la migración de iones negativos
(como por ejemplo O2-) desde el metal hasta el electrolito sólido si se trata
de un conductor aniónico.
Descripción del trabajo realizado
26
Por otro lado, durante el proceso de migración, el movimiento de estos
iones está acompañado por el correspondiente ión de compensación de
carga, lo que da lugar a dipolos neutros superficiales. Estos dipolos se
distribuyen a lo largo de toda la superficie metálica, constituyendo lo que
se conoce como doble capa efectiva. La formación de la doble capa efectiva
produce una modificación de la función de trabajo del metal, es decir, una
variación de la densidad electrónica, modificando de este modo su
capacidad de enlace con cada una de las moléculas de reactivo tal y como
se muestra en la siguiente ecuación:
∆Φ=e∆UWR (A.8)
donde ∆Φ es la variación de la función de trabajo, e es la carga del electrón y ∆UWR
es la modificación de la diferencia de potencial. Así pues, cambiando la
función de trabajo de la película metálica se puede alterar la capacidad de
enlace con cada una de las moléculas de reactivo, lo que se traduce en una
modificación del comportamiento catalítico del metal, que dependerá de la
naturaleza del promotor [46]. De este modo, si la especie promotora es
electronegativa (O2-) se produce un incremento en la función de trabajo lo
que favorece la quimisorción de adsorbatos donadores de electrones y se
desfavorece la de aceptores de electrones. Por el contrario, una
disminución de la función de trabajo mediante la adición de promotores
electropositivos (Na+) favorece la quimisorción de especies aceptoras de
electrones.
- Tipos de reacción basadas en el fenómeno de promoción
electroquímica
Se pueden distinguir cuatro tipos de reacciones basadas en el fenómeno
de promoción electroquímica teniendo en cuenta las interacciones
atractivas o repulsivas entre los promotores y adsorbatos, [37]:
Descripción del trabajo realizado
27
- Reacciones electrofóbicas: son aquellas reacciones que muestran un
incremento de la velocidad de reacción para valores positivos de
potencial. Este tipo de comportamiento tiene lugar cuando la cinética
es de orden positivo en el donador de electrones y de orden cero o
negativo en el aceptor de electrones, es decir, el donador de electrones
es el que se encuentra más débilmente adsorbido sobre el catalizador.
- Reacciones electrofílicas: son aquellas reacciones que muestran un
incremento de la velocidad de reacción para valores negativos del
potencial. Este tipo de reacciones se dan cuando la cinética es de
orden positivo en el aceptor de electrones y de orden cero o negativo
en el donador de electrones, es decir, el aceptor de electrones se
muestra más débilmente adsorbido sobre el catalizador.
- Reacciones tipo volcán: estas reacciones presentan un máximo local
de la velocidad de reacción respecto al potencial aplicado. Este
comportamiento tiene lugar cuando tanto el donador como el aceptor
de electrones se encuentran fuertemente adsorbidos sobre el
catalizador.
- Reacciones tipo volcán invertido: son aquellas que presentan un
mínimo en la velocidad de reacción respecto al potencial aplicado. En
este caso, este tipo de comportamiento tiene lugar cuando el donador
y el aceptor de electrones se encuentran débilmente adsorbidos sobre
el catalizador.
- Aplicaciones del fenómeno de promoción electroquímica
Desde su descubrimiento, el fenómeno de promoción electroquímica ha
sido demostrado en más de 80 sistemas catalíticos diferentes, lo que parece
indicar que no está limitado a ningún tipo de catalizador, electrolito sólido
o reacción catalítica en particular [37, 39]. De este modo, el fenómeno
EPOC ha sido estudiado en diversos procesos de interés industrial y
Descripción del trabajo realizado
28
medioambiental, sobre una amplia variedad de catalizadores (Pt, Pd, Rh,
Ag, Au, Ni, Cu, Fe, IrO2, RuO2) depositados sobre distintos electrolitos
sólidos (Na-βAl2O3, K-βAl2O3, YSZ, Nasicon, V2O5-K2S2O7, CaZr0,9In0,1O3-a).
En particular, la promoción electroquímica ha sido investigada en
reacciones de hidrogenación de CO2 [47-58]. La mayoría de los estudios de
hidrogenación se han desarrollado utilizando YSZ como electrolito sólido y
metales nobles como Pt, Pd, Ru y Rh [47-50, 52, 53, 55, 58]. En menor
medida, esta reacción también se ha llevado a cabo con electrolitos sólidos
catiónicos [48, 51, 53, 55-57] y utilizando metales no nobles como Ni o Cu
[47, 50, 51, 54, 56].
A.5. Reactores electroquímicos de membrana polimérica (PEM)
A.5.1. Características generales de los reactores de membrana
polimérica
En los últimos años, como consecuencia del auge en el estudio de los
procesos de obtención de hidrógeno se han desarrollado configuraciones con
membranas de intercambio protónico (Proton Exchange Membrane/
Polymer Electrolyte Membrane, PEM). Estas membranas son poliméricas
y consisten en una estructura semipermeable diseñada para conducir
protones (H+) a temperaturas relativamente bajas (20-200 ºC). Si bien es
cierto que el principal uso de estos reactores reside en la obtención de
energía, en modo celda de combustible (tal y como se comentó en apartados
anteriores), estos también pueden ser utilizados para la producción de
hidrógeno a través de un proceso de electrólisis.
En los años 60, General Electric desarrolló el primer electrolizador de
agua basado en electrolitos poliméricos [59]. El modo de operación de los
electrolizadores tipo PEM es el inverso al llevado a cabo en el modo celda
de combustible. De este modo, se aplican potenciales eléctricos superiores
al potencial de equilibrio dando lugar a la ruptura de la molécula de agua
Descripción del trabajo realizado
29
en el ánodo, produciendo O2, H+ y electrones de acuerdo con la siguiente
ecuación:
2H2O → O2 + 4H+ + 4e- (A.9)
A continuación, los protones migran hacia el cátodo de la celda a través
de la membrana polimérica, donde reaccionan con los electrones que se
mueven desde el ánodo al cátodo a través de un circuito eléctrico externo,
dando lugar a la producción de hidrógeno puro [4]:
2H+ + 2e- → H2 (A.10)
Un electrolizador tipo PEM presenta una configuración muy similar a la
de una celda de combustible tipo PEM. Dichos electrolizadores están
formados por un membrana polimérica y dos electrodos porosos situados a
ambos lados de dicha membrana formando el conjunto denominado MEA
(Membrane Electrode Assembly). También se compone de colectores de
corriente y placas bipolares, que permiten la polarización del sistema y el
flujo de gas o líquido a través de los electrodos.
A.5.2. Membranas de intercambio protónico
La función de las membranas de intercambio iónico (normalmente
protónicas) es permitir el paso de cargas iónicas desde un electrodo a otro
de la celda, cerrando el circuito iónico de la celda electroquímica y
actuando como barrera para el paso de los gases a través de la misma.
Estas membranas deben poseer elevadas estabilidades mecánicas,
químicas (debido a que están en contacto con medios de reacción extremos
como gases oxidantes y reductores en un entorno ácido [17]), y
conductividades iónicas elevadas para evitar pérdidas óhmicas y cierta
durabilidad y fiabilidad.
Los materiales utilizados en este tipo de membranas son polímeros
fluorocarbonados parecidos al teflón, que dotan a la membrana de una
Descripción del trabajo realizado
30
gran resistencia química, mecánica y térmica, y la hacen insoluble al agua.
A estos polímeros se añaden grupos sulfónicos (SO3) que permiten el
tránsito de cargas iónicas a través de la membrana y le proporcionan cierto
carácter hidrófilo [60]. El tipo de membrana más usada es conocida como
Nafion® desarrollada por Dupont [4] y cuya estructura se muestra en la
Figura A.4.
Figura A.4. Estructura química del Nafion®
Además de las membranas Nafion®, existen otras membranas
comerciales perfluorosulfonadas como son Dow® (Dow Chemicals),
Flemion® (Asashi Glass), Aciplex® (Asashi Chemicals) [61] y Sterion®
(Hydrogen Works) [62].
A.5.3. Aplicación de los reactores PEM en procesos de producción de
hidrógeno y valorización de CO2
i) Electrólisis de agua.
Los procesos de electrólisis de agua en configuraciones tipo PEM han
sido muy estudiados en los últimos años. Los electrolizadores tipo PEM se
suelen construir con membranas de intercambio protónico
perfluorosulfonadas. En cuanto a los electrodos, se suele utilizar Pt
soportado sobre materiales carbonosos como catalizador catódico. Sin
embargo, gran parte de los trabajos realizados hasta la fecha han estado
Descripción del trabajo realizado
31
enfocados hacia el estudio del catalizador anódico. En este sentido, el óxido
de Ru es conocido por ser el electrolizador más activo para la evolución de
O2 en el ánodo. Sin embargo, suele ser bastante inestable, por lo que se ha
de estabilizar con óxidos como el de iridio [63].
ii) Electrólisis o reformado electroquímico de alcoholes
Los procesos de reformado electroquímico de alcoholes (electrólisis de
alcoholes) han sido propuestos como una alternativa prometedora para la
producción de hidrógeno puro a partir de energía eléctrica, solventando los
problemas relacionados con los procesos de electrólisis de agua.
Se han reportado costes energéticos asociado a la electrólisis de
alcoholes inferiores a los de la electrólisis de agua (17-30 kWh·kg-1 H2 vs.
53-70 kWh·kg-1 H2, respectivamente [5, 64]) debido a que los primeros
suelen llevarse a cabo a potenciales muy inferiores. También se ha
estudiado la electrólisis de mezclas metanol-agua [65-68], y glicerol agua
[69, 70].
iii) Reducción electrocatalítica de CO2
Los procesos de reducción electrocatalítica de CO2 han sido propuestos
en los últimos años como una alternativa para disminuir las emisiones de
gases de efecto invernadero. Por otra parte, el uso reactores tipo PEM en
reacciones de valorización de CO2 pretende solventar los problemas
relacionados con los procesos convencionales de hidrogenación de CO2. Los
primeros operarían a temperaturas menores de reacción, no requerirían de
alimentación alguna de hidrógeno y podrían utilizar energía eléctrica de
origen renovable.
Hasta el momento, se han llevado a cabo pocos estudios de reducción
electrocatalítica en fase gas utilizando membranas de intercambio
protónico. Genovese y col. [71] usaron electrocatalizadores basados en
Descripción del trabajo realizado
32
nanoestructuras soportadas sobre nanotubos carbono que difieren de los
convencionales utilizados en operaciones en fase acuosa [72]. Modificando
las propiedades de los electrodos nanoestructurados durante su síntesis es
posible mejorar la eficiencia del proceso de reducción y de su productividad
y ajustar la selectividad hacia la formación de mayores cadenas de
hidrocarburos y de otros productos químicos [73].
A.6. Objetivo del presente trabajo
El objetivo global del presente trabajo era estudiar y explorar nuevas
configuraciones electrocatalíticas, basadas tanto en configuraciones SEMR
como configuraciones PEM para la producción de hidrógeno y gas de
síntesis así como para la valorización de CO2 para obtener combustibles
líquidos. De este modo, se desarrolló un programa de trabajo con las
siguientes etapas:
- Revisión bibliográfica y puesta a punto de las distintas instalaciones
experimentales (equipos de análisis y polarización, configuraciones de los
reactores, calibración de equipos de análisis, etc).
- Desarrollo de celdas de electrolito sólido para la producción de gas de
síntesis con una razón H2/CO variable en un reactor SEMR de cámara
sencilla. Este proceso combinaba la electrólisis de vapor de agua (para la
obtención de H2) y la oxidación parcial de etanol (para la obtención de gas
de síntesis).
- Desarrollo de celdas de electrolito sólido en configuraciones SEMR de
doble cámara para la producción y separación simultánea de H2 e
hidrocarburos C2S. Este proceso combinaba la electrólisis de vapor de agua
(para la obtención de H2) y la reacción de acoplamiento selectivo de metano
(para la obtención de C2S).
Descripción del trabajo realizado
33
- Comparativa energética de la producción de H2 a partir de reformado
electroquímico y del reformado catalítico de etanol, mediante su simulación
con Aspen HYSYS.
- Investigación del fenómeno de la promoción electroquímica de la
catálisis con catalizadores no nobles (Ni) sobre conductores de K+ en el
proceso de hidrogenación de CO2 hacia la formación de CO y CH4.
- Desarrollo de celdas tipo PEM para la conversión en fase gas de CO2 a
combustibles líquidos sobre catalizadores de Cu.
B. Instalaciones experimentales
B.1. Reactor SEMR de cámara sencilla
Los experimentos relacionados con la producción de gas de síntesis de
razón variable, vía reformado de vapor de agua y oxidación parcial de
etanol (Capítulo 1) y el estudio de la promoción electroquímica en el
proceso de hidrogenación de CO2 para la producción de CO y CH4 (Capítulo
4) fueron llevados a cabo en una instalación como la que se describe a
continuación. Dicha instalación consta de cuatro partes bien diferenciadas:
sistema de alimentación, sistema de reacción, sistema de polarización y
sistema de análisis. El esquema de la instalación está descrito en detalle
en el Capítulo 1 de esta memoria.
El sistema de alimentación estaba constituido por cuatro líneas de flujo
continuo, análogas e independientes para la alimentación de los diferentes
gases de reacción. CO2, H2 y N2 como gas portador para los experimentos
de hidrogenación de CO2 y N2, H2 y N2 como gas portador para los
experimentos de gas de síntesis. Además, estos últimos experimentos se
realizaron en presencia de vapor de agua y de etanol. El contenido de
ambos compuestos en la corriente alimento fue regulado mediante
saturación a temperatura controlada. Todas las líneas a la salida del
Descripción del trabajo realizado
34
saturador fueron precalentadas con la finalidad de evitar la condensación
del vapor de agua.
El sistema de reacción estaba constituido por un reactor de membrana
de electrolito sólido de cámara sencilla (como los descritos en el apartado
A.4) construido en cuarzo, de tal forma que todos los electrodos se
encontraban bajo la misma atmósfera de reacción. A la salida del reactor,
los gases atravesaban una unidad de frío en la que se condensaban los
posibles productos líquidos. Los métodos de preparación de las celdas de
electrolito sólido, dependiendo de los estudios a realizar, son muy variados
por lo que serán explicados en detalle en los diferentes capítulos de esta
memoria.
El sistema de polarización consistía en un potenciostato-galvanostato
que permitía la aplicación controlada de corrientes eléctricas o potenciales
sobre la celda electroquímica.
Por último, el sistema de análisis estaba constituido por un micro-
cromatógrafo de gases con dos canales que permitía la separación y
cuantificación de los distintos reactivos y productos obtenidos a la salida
del sistema de reacción.
B.2. Reactor SEMR de doble cámara
Los experimentos para la producción/separación simultánea de H2 e
hidrocarburos C2s (Capítulo 2) se llevaron a cabo en una instalación muy
similar a la anterior. La principal diferencia radica en el sistema de
reacción que en este caso estaba constituido por un reactor de doble
cámara dotado de dos entradas y dos salidas, cada una de ellas a un
compartimento. De este modo, mientras que el vapor de agua fue
alimentado mediante saturación de una corriente de nitrógeno a
temperatura controlada a la cámara interior, la cámara exterior fue
Descripción del trabajo realizado
35
alimentada con una corriente de CH4/N2. El hidrógeno puro en la cámara
interior, obtenido mediante el proceso de electrólisis fue calculado de
acuerdo con la ley de Faraday, mientras que los productos obtenidos en la
cámara exterior fueron medidos con el micro-cromatógrafo de gases
anteriormente descrito.
B.3. Reactor PEM
Los experimentos de reducción electrocatalítica en celda PEM (Capítulo
5) fueron llevados a cabo en una instalación formada por cuatro secciones:
sistema de alimentación, sistema de reacción, sistema de polarización y
sistema de análisis y que se describirá en detalle en el Capítulo 5 de esta
memoria.
El sistema de alimentación está constituido por dos líneas de flujo
continuo, análogas e independientes para la alimentación de los diferentes
gases de reacción (CO2 y N2). Todos los experimentos de reacción se
realizaron en presencia de vapor de agua. Como en el apartado anterior, el
contenido de vapor de agua de la corriente alimento fue regulado mediante
saturación en una corriente de N2 a temperatura controlada. Igualmente,
la línea a la salida del saturador fue precalentada con la finalidad de evitar
la condensación de vapor de agua.
El sistema de reacción estaba constituido por una celda PEM. Tanto el
sistema como los métodos de preparación de los electrodos y de ensamblaje
de la membrana-electrodo se describen en detalle en el Capítulo 5 de esta
memoria.
El sistema de polarización estaba formado por un potenciostato-
galvanostato que permitía la aplicación de forma controlada de potenciales
o corrientes eléctricas a la celda PEM.
Descripción del trabajo realizado
36
Finalmente, el sistema de análisis estaba constituido por un
cromatógrafo de gases formado por dos canales de análisis independientes
en los que se analizaba los diferentes productos de reacción mediante
inyección manual con una jeringa de gases.
C. Resultados y discusión
En el Capítulo 1 se ha estudiado la posibilidad de obtener gas de
síntesis de razón variable a partir de la combinación de los procesos de
electrólisis de vapor de agua y oxidación parcial de etanol. Para ello, se
desarrolló un catalizador electroquímico con la configuración Pt/YSZ/Pt.
Cada una de las películas catalíticas de Pt, depositadas a ambos lados del
pellet circular de YSZ, fueron preparadas mediante la técnica de
deposición de arco catódico (CAD), en colaboración con el Instituto de
Ciencia de Materiales de Madrid (CSIC), dando lugar a un contenido final
de 0,8 mg Pt cm-2.
La caracterización del sistema mediante difracción de rayos X (DRX) y
microscopía electrónica de barrido (SEM) demostró la formación de una
película porosa de Pt con un tamaño de cristal de 15 nm con una adecuada
conductividad eléctrica y adhesión al electrolito sólido.
En primer lugar se realizó un experimento galvanostático mediante la
aplicación de intensidades eléctricas (-80 y +80 mA) en la atmósfera de
reacción: C2H5OH/H2O=0,7 %/2 %. En condiciones de circuito abierto (sin
aplicar intensidad) se produjo la reacción de reformado de etanol en ambos
electrodos de Pt dando lugar a H2, CO y CO2 y, en menor medida, a trazas
de CH4, C2H6 y C2H4. Adicionalmente, en el sistema de reacción, compuesto
por un reactor de cámara sencilla, se pudieron producir reacciones como la
reacción de water gas shift o las reacciones de hidrogenación de CO y CO2
dando lugar a una razón molar H2/CO comprendida entre 2,5-3. Por otro
lado, se pudo observar un incremento de la velocidad de producción de
Descripción del trabajo realizado
37
todos los productos, principalmente de H2, durante la imposición de
corrientes eléctricas (-80 y +80 mA). Este incremento de la producción de
hidrógeno fue consecuencia del proceso de electrólisis de vapor de agua que
ocurrió en el cátodo de Pt y a la acción de los iones O2- suministrados
electroquímicamente a través del electrolito sólido hacia el ánodo de Pt que
reaccionaron con el etanol vía oxidación parcial y total. De esto modo, se
obtuvo una razón H2/CO de 8,2 y 8,7 para una intensidad de +80 y -80 mA,
respectivamente, demostrando la posibilidad de modificar la razón H2/CO
a partir de la intensidad aplicada.
A continuación se estudió el comportamiento del sistema a diferentes
temperaturas (T = 600, 650 y 700 ºC) en la atmósfera de reacción
anteriormente mencionada, tras la aplicación de distintos potenciales. Se
pudo observar un incremento de la velocidad de producción de H2, CO y
CO2 al incrementar el potencial aplicado debido a un incremento de la
actividad electrocatalítica. Adicionalmente, se pudo apreciar un
incremento de la velocidad de producción de los compuestos anteriormente
mencionados al incrementar la temperatura para un potencial fijo, debido
a efectos cinéticos. Se calcularon las energías de activación aparentes tanto
en condiciones de circuito abierto como en condiciones de polarización,
obteniéndose una disminución prácticamente lineal de dichas energías al
incrementarse el potencial aplicado.
A continuación se calculó la eficiencia faradaica del proceso de
producción de hidrógeno, observándose que esta fue para todos los casos
menor que 1 indicando que no hay efecto de promoción electroquímica en la
reacción de reformado catalítico de etanol. Además, parte del hidrógeno
producido en el reactor de cámara sencilla reaccionó con otros productos
como CO, CO2, C2H4 y O2 disminuyendo la eficiencia faradaica respecto al
hidrógeno.
Descripción del trabajo realizado
38
Finalmente, se estudió la variación de la razón H2/CO con el potencial
aplicado a diferentes temperaturas observándose que aquella puede ser
fácilmente modificada y, por tanto, ajustada para la producción de
diferentes compuestos de interés industrial.
En el Capítulo 2 se utilizó un SEMR de doble cámara (Pt-
YSZporosa/YSZ/Pt) para llevar a cabo la producción y separación simultánea
de H2 e hidrocarburos C2S. En la cámara interna formada por una película
metálica de Pt depositada sobre YSZ densa se introdujo vapor de agua
mediante saturación de una corriente de nitrógeno; mientras que por la
cámara exterior, formada por Pt impregnado es una capa porosa de YSZ, se
alimentó una corriente de metano. La aplicación de corriente eléctrica en
la cámara interior provoca la electrólisis de vapor de agua, produciéndose
H2 e iones O2-. Estos iones migran hacia la cámara exterior, a través de la
membrana, que está alimentada por una corriente de CH4 y que conduce a
la formación de H2, CO, CO2 y C2S.
Este sistema catalítico fue caracterizado mediante SEM y DRX. Con las
micrografías SEM se pudo comprobar la buena distribución de las
partículas de Pt impregnadas sobre la matriz de la YSZ porosa. Por otro
lado, la técnica de difracción de rayos X demostró la presencia de
partículas de Pt metálico con un tamaño de cristal de 32 nm.
En primer lugar se realizó un experimento galvanostático mediante la
aplicación de intensidades eléctricas de +50 mA. En condiciones de circuito
abierto, en la cámara externa se produjo la reacción de descomposición
térmica de metano TCD (CH4 (g) → C(s) + 2H2 (g)) para producir hidrógeno
y carbón. El carbón se depositó sobre los centros activos del Pt provocando
una disminución progresiva de la actividad catalítica con el tiempo.
Durante esta etapa también se produjo gas de síntesis con una razón
H2/CO ≈ 2 por la reacción del carbón depositado con el oxígeno que
Descripción del trabajo realizado
39
provenía de la YSZ (migración térmica) y el hidrógeno obtenido a partir de
la reacción TCD. Esta razón H2/CO ≈ 2 es la típicamente usada en la
producción de combustibles sintéticos a partir de la síntesis de Fischer-
Tropsch. La aplicación de intensidades positivas permitió obtener
hidrógeno en la cámara interna (por electrólisis de agua) y CO2, CO y C2 en
la cámara externa, atribuidos a la reacción de los iones O2- con el metano y
con el carbón previamente depositado sobre la superficie del catalizador,
regenerando el sistema in-situ.
A continuación, el sistema se caracterizó electroquímicamente mediante
voltamperometrías lineales para diferentes condiciones de reacción. De
esta forma se demostró el efecto depolarizante del carbón y del metano.
Por tanto, la presencia de estos compuestos permite la disminución del
potencial de la celda (para la misma intensidad), reduciendo la cantidad de
energía que hay que suministrar para realizar la electrólisis del vapor de
agua.
Finalmente, el sistema fue chequeado durante largos periodos de
operación con objeto de analizar su durabilidad y reproducibilidad
mostrando unos resultados prometedores con vistas a su posible aplicación
práctica.
Resumiendo, el sistema desarrollado en este capítulo fue capaz, por un
lado, de producir y separar H2 y C2S con elevados rendimientos mediante la
introducción separada de metano y vapor de agua. Además, esta
configuración y modo de operación permitió la valorización in-situ del
carbón producido como agente depolarizante del proceso de electrólisis de
vapor de agua, disminuyendo así el consumo eléctrico asociado al mismo.
Continuando con los diversos métodos de producción de hirógeno. En el
Capítulo 3 se realizó un análisis energético de la producción de hidrógeno
mediante dos métodos muy diferenciados: reformado catalítico de etanol y
Descripción del trabajo realizado
40
reformado electroquímico de etanol. Se propuso un diagrama de flujo
completo para ambos sistemas que se simularon mediante Aspen HYSYS
empleando condiciones de operación y resultados reportados por la
literatura.
Durante el proceso de reformado catalítico, además de la reacción
catalítica de reformado de etanol para obtener hidrógeno y CO2 (C2H5OH +
3H2O → 2CO2 + 6H2), se produce una segunda reacción de reformado que
produce CO (C2H5OH + H2O → 2CO + 4H2). Esta situación implica el que
haya que introducirse dos etapas de reacción adicionales al proceso con el
objetivo de reducir la cantidad de CO presente en la corriente efluente del
proceso principal: reacción de desplazamiento de vapor de agua (Water Gas
Shift, WGS) y oxidación preferencial de CO (COPROX) con la que se
obtiene como subproductos etileno y metano. Por otro lado, en el reformado
electroquímico de etanol, la principal reacción que tiene lugar es la de
electro-oxidación de etanol para producir protones y acetaldehído (C2H5OH
→ C2H4O + 2H+ + 2e-). Estos protones han de ser selectivamente
transportados hacia el cátodo de la membrana de intercambio protónico
para formar hidrógeno (2H+ + 2e- → H2).
En el reformado catalítico se obtuvo el mayor consumo energético en el
heater el cual proporciona la energía necesaria para calentar la corriente
alimento a la temperatura requerida en el reactor de reformado catalítico
(T = 800 ºC). Por otro lado, en el reformado electroquímico de etanol, el
mayor consumo energético correspondió a la energía consumida en la
celda PEM (173,6 kJ mol-1 H2).
Del análisis de los balances de materia se pudo comprobar que el mayor
rendimiento hacia la formación de hidrógeno se obtuvo con el proceso de
reformado electroquímico (0,0436 kg H2/kg C2H5OH) frente al obtenido con
el proceso de reformado catalítico (0,0304 kg H2/kg C2H5OH). Asimismo, se
Descripción del trabajo realizado
41
pudo determinar que el menor consumo energético se obtuvo con el proceso
de reformado electroquímico (29,2 vs. 32,70 kWh / kg H2).
De este modo, los resultados obtenidos en este capítulo demostraron el
interés del proceso de reformado electroquímico de etanol para obtener
hidrógeno de alta pureza en una simple etapa de reacción/separación,
presentando una interesante alternativa al clásico proceso de reformado
catalítico.
En capítulos anteriores se ha puesto de manifiesto la importancia de la
producción de hidrógeno libre de emisiones de CO2. Por ello, en el
Capítulo 4 se estudió la aplicación del fenómeno de la promoción
electroquímica (efecto EPOC, anteriormente descrito en el apartado A.4.3)
en procesos de hidrogenación de CO2. Se desarrollaron tres nuevos
catalizadores electroquímicos con la siguiente configuración: Ni/K-
βAl2O3/Au, Ni-αAl2O3/K-βAl2O3/Au y Au-Ni(30%)-αAl2O3/K-βAl2O3/Au).
Estos catalizadores fueron preparados mediante deposición, sobre un
electrolito sólido de K-βAl2O3, de una pasta de Ni (catalizador denominado
“N”) y de una pasta sintetizada a partir de la mezcla de pasta comercial de
Ni y un polvo de αAl2O3 (catalizador “NA”). Finalmente, se preparó un
tercer electrodo mediante la deposición de una pasta sintetizada a partir
de la mezcla de pasta comercial de Au y un catalizador en polvo de Ni
(30%) impregnado sobre αAl2O3 (catalizador “GNA”).
En primer lugar, se comprobó la influencia de la temperatura de
reducción del catalizador-electrodo de trabajo de Ni con un experimento de
reducción a temperatura programada (TPR) in-situ. El consumo de
hidrógeno fue medido mediante un micro-cromatógrafo de gases situado a
la salida del sistema de reacción. Simultáneamente, se midió la resistencia
de la película metálica de Ni. De esta forma, se demostró que la reducción
de la película metálica tenía lugar a temperaturas superiores a 350 ºC,
Descripción del trabajo realizado
42
aumentando notablemente el consumo de hidrógeno a partir de la misma y
disminuyendo simultáneamente la resistencia eléctrica hasta hacerse cero
lo que indicaba la presencia de una película totalmente conductora
(resistencia < 0,3 Ω). Esta presencia de Ni metálico fue verificada mediante
la técnica difracción de rayos X.
A continuación, se llevaron a cabo experimentos de reacción en
condiciones de circuito cerrado, pudiéndose observar que la aplicación de
potenciales eléctricos negativos dio lugar a la activación del catalizador en
el proceso de reverse water gas shift, para la producción de CO. Este
fenómeno puede ser explicado de acuerdo al fenómeno EPOC. La aplicación
de polarizaciones negativas indujo la migración de especies de K+ desde el
electrolito sólido hacia el catalizador-electrodo de trabajo, favoreciendo la
quimisorción del CO2 con respecto al H2 y originando un incremento
notable de la producción de CO, especialmente con el catalizador NA, y una
disminución de la producción de CH4.
Se realizó un estudio cinético confirmándose que la velocidad de
producción de CO presentó un orden positivo con respecto al reactante
aceptor de electrones (CO2) y un orden negativo u orden cero con respecto
al donador de electrones (H2).
Finalmente, se estudió el efecto del potencial aplicado y de la
concentración de hidrógeno sobre la selectividad de los productos (CO y
CH4). De esta forma se pudo comprobar, como la selectividad y la actividad
podían ser controladas y modificadas mediante el efecto EPOC.
El estudio llevado a cabo en este capítulo supone un avance importante
en lo que se refiere a la aplicación del fenómeno de promoción
electroquímica para la activación de metales no nobles (como el Ni) en
Descripción del trabajo realizado
43
procesos de hidrogenación de CO2, debido a la posibilidad de controlar in-
situ la formación de gas de síntesis o de metano,
Continuando con los métodos de valorización del CO2 en productos de
interés industrial y medioambiental, en el Capítulo 5 se desarrolló un
sistema para llevar a cabo la reducción electrocatalítica a baja
temperatura del CO2 en fase gas. Productos como gas de síntesis, metano,
monóxido de carbono, metanol, acetaldehído, acetona, formato de metilo,
etanol, 2-propanol y n-propanol pueden actuar como vector energético,
permitiendo, en cierto modo, el almacenamiento de energía eléctrica en
forma de energía química. Con este objetivo, se prepararon tres
catalizadores de cobre soportados sobre: grafito (G), carbón activo (AC) y
nanofibras de carbono (CNF) y caracterizados mediante difracción de rayos
X, reducción a temperatura programada, microscopía de transmisión
electrónica, adsorción de N2 y voltamperometrías cíclicas. Se demostró que
el catalizador de cobre soportado sobre carbón activo presentó una mayor
área superficial y tamaño de poro, así como una mayor dispersión de la
fase activa y un menor tamaño de cristal de Cu. La presencia de Cu
metálico fue verificada mediante difracción de rayos X.
Una vez caracterizados los catalizadores se procedió a su ensayo en
reacción. Se observó un incremento progresivo de la velocidad de reacción
al aplicar una intensidad constante de I = -20 mA y que la mayor velocidad
de reacción de CO2 se obtuvo con el catalizador de cobre soportado sobre
carbón activo que era, precisamente, el que presentaba mayores valores de
de área superficial, porosidad y dispersión. Los principales productos
obtenidos fueron: metanol a partir del catalizador de cobre soportado sobre
grafito (Cu-G), y acetaldehído a partir de los catalizadores de cobre
soportados sobre nanofibras de carbono (Cu-CNF) y carbón activo (Cu-AC).
Descripción del trabajo realizado
44
Se pudo demostrar que los productos de reacción procedían de procesos
esencialmente electrocatalíticos y no catalíticos.
A continuación, se estudió la influencia de la intensidad eléctrica
aplicada en estado estacionario y de la temperatura de reacción sobre el
comportamiento de los diferentes sistemas catalíticos ensayados. Se pudo
verificar que el consumo de CO2 aumentaba al hacerlo las intensidades
eléctricas y la temperatura de reacción.
Estos experimentos ponen de manifiesto la posibilidad de modificar y
controlar la actividad catalítica y la selectividad hacia ciertos productos de
interés variando las condiciones de operación.
D. Conclusiones y recomendaciones
De los resultados obtenidos en esta investigación se pueden obtener las
siguientes conclusiones finales:
o El catalizador electroquímico desarrollado en el Capítulo 1
(Pt/YSZ/Pt) permitía producir gas de síntesis y controlar in-situ,
mediante la intensidad aplicada, la razón H2/CO.
o El sistema de doble cámara Pt-YSZporosa/YSZ/Pt desarrollado
permitía producir y separar, simultáneamente, H2 y C2S. Esta
configuración y modo de operación permitía la valorización in-situ
del carbón producido como agente depolarizante del proceso de
electrólisis de vapor de agua, disminuyendo el consumo energético
asociado a éste.
o El estudio desarrollado en el Capítulo 3 permitió la comparación del
consumo energético de dos métodos para la producción de H2:
reformado catalítico de etanol y reformado electroquímico de etanol.
El mayor rendimiento en la producción de hidrógeno y el menor
Descripción del trabajo realizado
45
consumo energético se obtuvo con el reformado electroquímico de
etanol.
o La celda electroquímica Ni-αAl2O3/K-βAl2O3/Au desarrollada
permitía la activación del proceso reverse water gas shift de acuerdo
al fenómeno EPOC, incrementando la producción de CO.
Adicionalmente, este sistema permitía la posibilidad de controlar la
actividad y selectividad catalítica del Ni hacia la producción de CO
y CH4 mediante la migración controlada de iones K+ desde el
electrolito sólido hacia la película de catalizador.
o Los diferentes catalizadores sintetizados Cu-G, Cu-AC y Cu-CNF
permitieron, sin alimentar directamente hidrógeno, la obtención de
compuestos como metanol, acetaldehído, metano, a partir de la
reducción electrocatalítica del CO2 en fase gas a baja temperatura.
Con el objetivo de ampliar y completar los resultados obtenidos en esta
investigación se recomienda:
o Investigar el uso de electrodos basados en metales no nobles en el
proceso de electrólisis de vapor de agua y oxidación parcial de
etanol, así como para la producción/separación simultánea de H2 y
C2S, incrementando, así, la economía del proceso.
o Realizar un estudio exergético que permita seleccionar las
condiciones óptimas de operación para el proceso de reformado
catalítico de etanol y reformado electroquímico de etanol así como
localizar los sumideros de energía y su magnitud.
o Incrementar la producción de CH4 frente a la de CO en el proceso de
hidrogenación de CO2 mediante la activación de la reacción de
metanación de CO2 a través de procesos EPOC utilizando YSZ como
electrolito sólido.
Descripción del trabajo realizado
46
o Incrementar la eficiencia del proceso de valorización de CO2 en la
celda PEM utilizando diferentes catalizadores preparados a partir
de distintas técnicas y aumentando la temperatura del proceso
utilizando membranas Sterion® dopadas con H3PO4 con mayor
resistencia térmica que las usadas en la presente investigación.
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1.1. Introduction
1.2. Experimental
1.2.1 Catalytic activity measurements
1.2.2. Preparation of the solid
electrolyte cell
1.2.3. Characterization measurements
1.3. Results and discussion
1.4. Conclusions
1.5. References
CHAPTER 1:
Direct Production of Flexible H2/CO
Synthesis Gas via Steam Electrolysis
and Ethanol Partial Oxidation
INTRODUCTION
EXPERIMENTAL CHARACTERIZATION
SEM analysis
RESULTS
C2H5OH/H2O = 0.7%/ 2 %, N2 balance, T = 600 ºC
Mixtures of hydrogen and carbon monoxide (syn-gas) are used in a wide range
of industrial applications such as petrochemicals, ammonia, petroleum
refining, methanol synthesis etc. Both kinds of products are the primary
feedstock streams in the refinery and biorefinery industry for the different
equipment and conversion process from petroleum and biomass.
The development of novel configurations for the production of synthesis gas (syn-gas) of flexible H2/CO ratio is of great importance to reduce the cost for the
synthesis of synfuels and high-value chemicals.
Single chamber solid electrolyte cell reactor
- The cross section revealed a good
contact between the porous layer
and the dense electrolyte.
XRD analysis
-Pt particle size = 15 nm
-Cathodic arc deposition method allows to better
control the deposition conditions and hence a better
reproducibility of the catalyst/electrode films.
Influence of the applied current Variation of the H2 Faradaic Efficiency vs. current
C2H5OH/H2O = 0.7%/ 2 %, N2 balance
Influence of the applied potential and temperature on the H2/CO ratio
- The ratio H2/CO can be largely modified and
controlled via the applied polarization
In this work, we propose a radically novel approach to the direct production of syn-gas with flexible H2/CO ratio by means of a solid electrolyte membrane
reactor (SEMRs). For that purpose, a single chamber solid electrolyte membrane reactor based on yttria-stabilized zirconia (YSZ), has been developed
(Pt/YSZ/Pt).
Outlet Inlet
Counter Electrode (CE)
Catalyst-working
Electrode (WE)
Reactor cap Cooling
Quartz
tube
Au wires
Alumina tube
with 4 bores 20 30 40 50 60 70 80 90
YS
Z (
4 2
0)
YS
Z (
4 0
0)
YS
Z (
2 2
2)
Pt
(2 2
2)
Pt
(3 1
1)
Pt
(2 2
0)
Pt
(2 0
0)
Pt
(1 1
1)
YS
Z (
3 3
1)
YS
Z (
2 2
0)
YS
Z (
2 0
0)
Inte
nsi
ty,
a.u
.2 º
YS
Z (
1 1
1)
Pt
YSZ
0
5
10
15
20
25
30
35
0 15 30 45 60 75 90 105 120 135 150
0.00
0.04
0.08
0.12
0.6
0.9
1.2
1.5
1.8
rH2
rCO
rCO2
H2/CO = 2.5H2/CO = 8.7H2/CO = 2.5H2/CO = 8.2
I= -80 mAI = 80 mA O.C.CO.C.C
(r /
mol·
s-1)
x 1
08
O.C.C
H2/CO = 2.8
rCH4
rC2H4
rC2H6
(r /
mol·
s-1)
x 1
08
Time / min
- H2/CO ratio between 2.5 in open circuit
transitions regimes to 8.7 in closed circuit
transitions regimes
0
30
60
90
120
150
180
0
1
2
3
4
3
6
9
12
15
0.0 0.5 1.0 1.5 2.0
0
2
4
6
I /
mA
(rH
2
/ m
ol·s
-1)·
10
7
(rC
O /
mol
·s-1)·
10
8
T = 600 ºC
T = 650 ºC
T = 700 ºC
(rC
O2
/ m
ol·s
-1)·
10
8
UWC
/ V
Influence of the applied potential and temperature
- An increase in the applied potential
led to an increase in the production rate
of the main products: H2, CO and CO2
- An increase in the production rate of
different products at fixed potential with
the explored temperatures.
0 30 60 90 120 150 180 210
0.3
0.4
0.5
0.6
0.7
0.8
0.9
I / mA
T = 600 ºC
T = 650 ºC
T = 700 ºC
Faraday Law: Λ= (r -r0) /(I/nF)
0.0 0.5 1.0 1.5 2.0
0
2
4
6
8
10
12
H2/C
O
Potential / V
T = 600 ºC
T = 650 ºC
T = 700 ºC
CHAPTER 1. DIRECT PRODUCTION OF FLEXIBLE
H2/CO SYNTHESIS GAS VIA STEAM ELECTROLYSIS
AND ETHANOL OXIDATION
55
Abstract
n this chapter, a novel approach for the direct production of syngas
with flexible H2/CO ratio by means of a solid electrolyte membrane
reactor (SEMRs) is proposed. This way, a single chamber solid
electrolyte membrane reactor based on yttria-stabilized zirconia (YSZ) was
developed (Pt/YSZ/Pt), where both active Pt catalysts-electrodes were
exposed to the same reaction atmosphere (C2H5OH/H2O = 0.7 %/2 %). The
application of different polarizations at a range of temperatures (600-700
ºC) allowed to control the H2/CO ratio of the obtained syngas, i.e., the ratio
was varied between 1.5-12 under polarization conditions. Unlike
conventional catalytic partial oxidation processes, the H2/CO adjustment
was tuned without the requirement of external O2 feeding to the reactor. An
increase in the applied current or potential caused the H2/CO ratio to
increase if compared to that of the open circuit conditions. Ethanol
reforming occurred on the Pt catalyst/electrodes which is due to an increase
in the rate of the electrocatalytic processes. On the other hand, a decrease in
the H2/CO ratio with increasing temperatures at fixed potentials was
achieved as a consequence of the occurrence of further reaction of the
produced H2 with the rest of species present in the gas phase, leading to a
decrease in the faradaic efficiency. The proposed configuration may be of
great interest especially for biorefinery applications where H2, syngas and
electricity may be produced from bioethanol.
I
Chapter 1
56
1.1. Introduction
Hydrogen is considered a promising candidate to be the energy carrier
of the future as it can be used to produce electricity via fuel cells. In
addition, precise mixtures of hydrogen and carbon monoxide (syngas) are
also used in a wide range of industrial applications, such as
petrochemicals, ammonia, petroleum refining, methanol synthesis, etc [1-
3]. Both kinds of products (hydrogen and syngas) are the primary feedstock
streams in refinery and biorefinery industries. In the latter case,
bioethanol (a renewable resource) could be used for the production of fuels,
power, heat, and value-added chemicals [4]. As mentioned in a previous
chapter (Descripción del trabajo realizado), autothermal reforming (ATR),
steam reforming (SR), catalytic partial oxidation (CPOX), and a
combination of two latter are the most common paths to convert such fuels
into syngas [5]. Each process has advantages and drawbacks. Briefly,
steam reforming is a strong endothermic process so a significant amount of
energy is required to drive the process. The main advantage of the partial
oxidation and the autothermal reforming, is that they can be carried out at
lower reaction temperatures without any external heat supply due to that
they are exothermic. However, the addition of an O2 stream free of N2 is
required for the reaction, which implies a previous O2/N2 air separation
unit. On the contrary, depending on the use of the stream exiting from the
ethanol reformer, several additional reaction and separation steps, such as
water-gas-shift, preferential oxidation (PROX), methanation and pressure
swing adsorption, are required for the purification and further adjustment
of the H2/CO ratio. If the final application is the electricity production in a
fuel cell, a high value of H2/CO ratio is required to decrease the further CO
separation/purification additional steps. If the syngas is required for
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
57
synthesis applications, a H2/CO ratio below 3 is usually required. The
direct production of flexible H2/CO synthesis gas could avoid or limit the
H2/CO adjustments units and thus energy conservation and cost reduction
should be achieved. Consequently, there is a growing interest to find
solutions for providing flexible H2/CO ratio [6, 7].
In this chapter, a radically novel approach for the direct production of
synthesis gas with flexible H2/CO ratio based on the use of a solid
electrolyte membrane reactor (SEMRs) is proposed [8]. As described in
previous chapter, SEMR mainly consists of a ceramic solid electrolyte
(ionic conductor material e.g., O2- or H+ conductor in most of cases) in
which two metal or metal oxide catalysts-electrodes are deposited on both
sides of the solid electrolyte [9]. These kinds of configurations allow to
supply one of the reactives electrochemically (e.g., O2− for an oxidation
reaction) by a Faradaic operation [9-14]. The option of using this kind of
reactors presents several advantages such as it is possible to enhance the
catalytic activity and selectivity, to better integrate the different processes
and to control the reaction rate in a fast and easy way [15]. In addition, for
the case of the ethanol partial oxidation reaction, pure O2 free of N2 can be
in-situ supplied through an integrate steam electrolysis process, which
would allow a further production of H2, avoiding an energetic intensive air
separation unit. These reactor configurations have already been used for
other catalytic reactions such as the oxidative coupling of methane, the
oxidation and reduction of NOx and the ammonia synthesis [10, 16-19].
Chapter 1
58
Vent
N2
H2
N2
FIC
FIC
FIC
FIC
N2
PC Potenciostat-
galvanostat
Flowmeter
300
300
300 32
300
BROOKS INSTRUMENT
Micro gas
cromatograph
Temperature controller
Cooler
Flow controller
Furnace
Reactor
PGZ 301
VoltaLab
cell
FIC
FIC
FIC
FIC
Nitrógeno
Hidrógeno
Oxígeno/
Aire
Nitrógeno
TIC
TIC
Salida de gasesRegistro
temperatura
TIC
Saturador
Agua
Saturador
Metanol
REACTOR
AUTOLAB
PGSTAT320-N
BRUKER 450-GC
Flujómetro
de burbuja
BRONKHORST EL-FLOW
S-1
S-1
S-1
S-1
S-1
S-1
S-1
Water saturator
Heating wire
FIC
FIC
FIC
FIC
Nitrógeno
Hidrógeno
Oxígeno/
Aire
Nitrógeno
TIC
TIC
Salida de gasesRegistro
temperatura
TIC
Saturador
Agua
Saturador
Metanol
REACTOR
AUTOLAB
PGSTAT320-N
BRUKER 450-GC
Flujómetro
de burbuja
BRONKHORST EL-FLOW
S-1
S-1
S-1
S-1
Ethanol saturator
-2 ºC
S-1
S-1
S-1
S-1
S-1S-1S-1S-1S-1S-1 S-1
S-1
1.2. Experimental
1.2.1. Catalytic activity measurements
The catalytic activity measurements were carried out in an
experimental set-up as shown in Figure 1.1.
Figure 1.1. Scheme of the experimental set-up for the steam reforming and
ethanol partial oxidation experiments
The catalytic experiments were carried out at atmospheric pressure
with an overall gas flow rate of 100 Nml·min-1, in the temperature range
between 600 and 700 ºC. The feed stream composition was: C2H5OH/H2O =
0.7 %/2 %, N2 balanced. The gas flow and composition was controlled by
two N2 calibrated mass flow-meters (Brooks 5850 E) flowing through two
independent ethanol and water temperature controlled saturators in order
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
59
Outlet Inlet
Counter Electrode (CE)
Catalyst-working
Electrode (WE)
Reactor cap Cooling
Quartz
tube
Au wires
Alumina tube
with 4 bores
to achieve liquid–vapour equilibrium. Thus, the content of C2H5OH and
H2O in the final reaction mixture was controlled by using their vapour
pressure at the temperature of the saturators (38 ºC and 45 ºC,
respectively). All lines placed downstream from the saturator were heated
above 100 ºC to prevent condensation.
The reactor was a single chamber type since all the two electrodes
(catalyst-working (WE) and counter (CE) electrodes) are in the same
chamber and are simultaneously exposed to the reactants and products
[20]. Figure 1.2 shows a schematic drawing of the single chamber reactor
and the corresponding electrode configuration.
Figure 1.2. Schematic drawing of the single chamber reactor
The cell reactor with a volume of 30 cm3 was made of quartz. The tube
was closed at one end and exhibited CSTR (continuous stirred tank
Chapter 1
60
reactor) behaviour. The open-end of the tube was mounted on a stainless
steel cap, which had provisions for the introduction of reactants and
removal of products as well as for the insertion of a thermocouple and the
electrical connection with the electrodes of the cell. The electrical contact of
the two Pt electrodes was carried out by gold wires, which were in turn
connected to a potentiostat-galvanostat Voltalab 21 (Radiometer
Analytical).
Reactant and product gases were analysed with an online microgas-
chromatograph (Varian CP-4900) equipped with two columns (Molsieve
and Poraplot Q column) and two thermal conductivity detectors (TCD).
The molsieve column used Ar as the carrier gas and operated at T=80 ºC
and 20 psi. On the other hand, Poraplot Q column operated at T = 70 ºC
and 20 psi, using He as the carrier gas. Before the analysis, the water and
ethanol were trapped by a condenser at -2 ºC. The main detectable
products were H2, CO, CO2, CH4, C2H6 and C2H4. All analysis outputs, as
well as the potentiostat-galvanostat ones (current and potential), were
continuously monitored and recorded.
1.2.2. Preparation of the solid electrolyte cell
The solid electrolyte cell, also called electrochemical catalyst, consisted
of a continuous Pt thin film (geometric area of 2.01 cm2) deposited on both
sides of a 19-mm-diameter, 1-mm-thick YSZ disc (Tosoh-Zirconia), as
shown in Figure 1.3. Both Pt electrodes, which were identical, were
symmetrically deposited on both sides of the YSZ solid electrolyte using
the pulsed cathodic arc technique [21]. The individual pulses were 1 µs
long and had a current of approximately 300 A. A total of 2800 pulses were
used for the deposition of each Pt layer. The substrate holder was at
ground during the deposition and was rotated at 2 r.p.m.
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
61
Catalyst-working
electrode (Pt)Counter
electrode (Pt)
Solid electrolyte
(YSZ)
20 mm
1 mm
Figure 1.3. Scheme drawing of the Pt/YSZ/Pt electrochemical catalyst.
This electrode preparation technique [22] allowed the preparation of a
thin reduced state Pt film with high adhesion to the substrate that did not
require of any further calcination procedure. The final Pt loading was
around 0.8 mg Pt/cm2.
1.2.3. Characterization measurements
The cathodic Pt catalyst-electrode film was characterized by X-ray
diffraction (XRD) with a Philips PW 1710 instrument using Ni-filtered Cu
K radiation. The diffractograms were compared with the JCPDS-ICDD
references. The Pt catalyst film was also characterized via scanning
electron microscopy (SEM) using a JEOL 6490 LV microscope.
1.3. Results and discussion
In this chapter a single chamber solid electrolyte membrane reactor
based on yttria-stabilized zirconia (YSZ), which is an ionic O2- conductor
material, has been developed (Pt/YSZ/Pt), where both active Pt catalysts-
Chapter 1
62
electrodes were exposed to the same reaction atmosphere (a mixture of
C2H5OH and H2O). In this case, no O2 was directly fed from the gas phase
for the partial oxidation of ethanol, and hence, the active O2- ions and O2
molecules for the oxidation reactions were in-situ electrochemically
produced from the supplied H2O, through a steam electrolysis process.
Hence, steam was electrolyzed in the Pt cathode of the cell to form
hydrogen gas and oxygen ions (O2-), which were transported through the
YSZ solid electrolyte (O2- ionic conductor material) to the Pt anodic
electrode, leading to the ethanol electro-catalytic partial oxidation reaction
(Figure 1.4). As discussed below, an overall modification of the H2/CO ratio
of the produced syngas, which could be in-situ controlled via the applied
potential or current (H2 and O2- ions production rate), could occur.
Figure 1.4. Scheme of the solid electrolyte single chamber reactor.
XRD patterns of the Pt cathodic catalyst-electrode film are given in
Figure 1.5. The peak assignments based on JCPDS standards were
consistent with a Pt face-centered cubic (FCC) structure and a YSZ phase
corresponding to the solid electrolyte. This analysis indicated the purity of
both phases with no contamination of the catalyst-electrode film during
the preparation stage. According to previous studies [23], the Pt mean
particle size can be estimated by using the Scherrer’s equation
corresponding to the (1 1 1) peak, resulting in a Pt crystal size of 15 nm.
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
63
20 30 40 50 60 70 80 90
YS
Z (
4 2
0)
YS
Z (
4 0
0)
YS
Z (
2 2
2)
Pt
(2 2
2)
Pt
(3 1
1)
Pt
(2 2
0)
Pt
(2 0
0)
Pt
(1 1
1)
YS
Z (
3 3
1)
YS
Z (
2 2
0)
YS
Z (
2 0
0)
Inte
nsi
ty,
a.u
.
2 º
YS
Z (
1 1
1)
Figure 1.5. XRD patterns of the Pt catalyst electrode prepared by cathodic
arc deposition
Remarkably, this value is considerably lower than that typically
obtained (i.e., around 60 nm) by direct impregnation of Pt precursor
solution salts [24] or the application of metal pastes (values up to 500 nm)
[25], which are the most common techniques for the preparation of such
kind of catalytic/electrocatalytic films. It is worth noting that the
preparation of Pt catalyst/electrodes films with small grain sizes is of great
research interest due the higher kinetic activity [25]. In addition, the
cathodic arc deposition method allowed to better control the deposition
conditions and hence to obtain a better reproducibility in the manufacture
of the catalyst/electrode films. This reactor configuration based on a 19-
mm-diameter, 1-mm-thick YSZ pellet of low geometric area (2 cm2) is
typically employed for lab-scale studies. However, the reproducibility of
the used preparation technique may simplify, in view of practical
applications, by scaling it up to other configurations.
Chapter 1
64
a)
b)
c) Pt
YSZ
Pt
Figure 1.6 shows the top (a) and cross section SEM analysis (b and c) of
the Pt catalyst-electrode film.
Figure 1.6. Top (a) and cross section SEM micrographs (b and c) of the Pt
catalyst-electrode film
The micrograph shows a porous Pt layer with a foam structure where
the Pt particles were networked with each other, leading to a suitable
value of electrical conductivity (in-plane measured electrical resistance
value was below 2 ohms). No cracks or delimitations of the porous layer
were observed after the preparation. The cross section analysis also
revealed a good contact between the porous layer and the dense electrolyte.
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
65
Figure 1.7 represents the rate of the different obtained products (e.g.,
H2 CO2, CO, CH4, C2H4 and C2H6) with the time on stream under both open
circuit conditions (O.C.C) and current imposition of 80 mA and -80 mA at
C2H5OH/H2O = 0.7 %/2 %, N2 balanced, T=600 ºC. The Figure 1.7 also
indicates the H2/CO ratio obtained in each polarization regime (30 min of
duration). Firstly, it can be observed that the reforming products (e.g., H2,
CO and CO2) were the main products in the outlet reaction. Hence, under
O.C.C conditions (no current application) the catalytic ethanol reforming
reaction occurred on both Pt catalyst-electrodes, which symmetrically
contacted with the reaction mixture [26] via:
C2H5OH + H2O 4H2 + 2CO (1.1)
C2H5OH + 3H2O 6 H2 + 2 CO2 (1.2)
It can also be observed the presence of certain amount of CH4, C2H4 and
traces of C2H6 that can be attributed to the direct ethanol hydrogenation
reaction under the explored conditions:
C2H5OHH2+CO+CH4 (1.3)
C2H5OH C2H4 + H2O (1.4)
C2H4 + H2 C2H6 (1.5)
Other related catalytic processes may also occurred in the reaction
mixture such as the water gas shift and the CO and CO2 hydrogenation
reactions, leading to the experimentally H2/CO ratio between 2.5-3
observed under different open circuit transition regimes. The occurrence of
the water gas shift reaction is not very significant due to its exothermic
Chapter 1
66
0
5
10
15
20
25
30
35
0 15 30 45 60 75 90 105 120 135 150
0.00
0.04
0.08
0.12
0.6
0.9
1.2
1.5
1.8
rH2
rCO
rCO2
H2/CO = 2.5H2/CO = 8.7H2/CO = 2.5H2/CO = 8.2
I= -80 mAI = 80 mA O.C.CO.C.C
(r /
mol·
s-1)
x 1
08
O.C.C
H2/CO = 2.8
rCH4
rC2H4
rC2H6
(r /
mol·
s-1)
x 1
08
Time / min
character. In addition, the use of high temperatures (> 600 ºC) led to a
shift of the equilibrium to the left, limiting the conversion of CO.
Figure 1.7. Influence of the applied current (where O.C.C. denotes Open Circuit
Conditions) on the dynamic value of the reaction rates of the obtained products.
Conditions: C2H5OH/H2O = 0.7 %/2 %, N2 balanced, T=600 ºC.
On the other hand, it can be observed an increase in the reaction rates
of all the obtained products during the current impositions (80 and -80 mA.
The effect of the polarizations could be explained attending to the steam
electrolysis process on the Pt cathode and to the electrochemical supplying
of O2- ions to the Pt anode during the polarizations [13]. It is well known
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
67
[27] that YSZ solid electrolyte behaves as a pure oxygen anion conductor
material (O2-). Then, under polarization conditions, a considerable amount
of H2 is released into the gas phase at the cathode via reaction (1.6)
H2O + 2e- H2 + O2- (1.6)
while O2- ions were transported through the YSZ solid electrolyte to the Pt
anodic electrode leading to the electrocatalytic partial oxidation reactions
of ethanol (see Figure 1.4), increasing the production rate of the different
ethanol oxidation products:
C2H5OH+O2-3H2+2CO+2e- (1.7)
C2H5OH+3O2-3H2O+2CO2+6e- (1.8)
In addition, other reaction such as the oxygen formation, the ethanol
and CO catalytic oxidation and H2 electro and catalytic oxidation may also
occur at the explored conditions:
O2-O2+2e- (1.9)
C2H5OH+O2H2+CO+CO2 (1.10)
CO+1/2O2 CO2 (1.11)
H2 + O2- H2O + 2e- (1.12)
H2 + 1/2O2 H2O (1.13)
Hence, under the steady state conditions, a H2/CO ratios of 8.2 and 8.7
were achieved at 80 and -80 mA, respectively, showing the possibility of
increasing the overall H2/CO ratio . On the other hand, it can be observed
that similar production rate transitions behaviors were obtained under the
Chapter 1
68
positive and negative polarization. It indicates not only a suitable
geometry of the electrochemical cell and a similar morphology of both Pt
catalyst electrodes which can, therefore, act as cathode or anode depending
on the applied potential but also the reproducibility of the catalyst-
electrode preparation method on the two faces of the YSZ solid electrolyte
by the technique of cathodic arc deposition. It can also be observed that for
all the transitions, the system returned to the initial OCC production rate
values, demonstrating the reversibility of the polarization effect and the
stability of the electrodes under the explored reaction conditions.
According to these remarks, henceforth positive polarizations will be
applied for the subsequent reaction experiments, the rate of the primary
reaction products (H2, CO and CO2) being uniquely displayed.
Figure 1.8 shows the steady state variation of the main reaction
products: H2, CO, and CO2, and the obtained steady state current during
different potentiostatic impositions carried out at different reaction
temperatures (600-700 ºC) by using the same reaction mixture:
C2H5OH/H2O = 0.7 %/2 %, N2 balanced. Firstly, an increase in the applied
potential led to an increase in the production rate of the main products: H2,
CO and CO2. This effect can be easily explained considering that an
increase in the applied potential and hence in the current led to an
increase in the rate of the different electrocatalytic processes [13]: steam
electrolysis at the cathode (reaction 1.6) and ethanol electro-oxidation at
the anode, which increased the amount of partial and complete ethanol
oxidation products (reactions 1.7-1.11). Consequently, at higher current
values more amount of H2 was produced since higher amount of oxygen
ions (O2-) were supplied to the anodic electrode, which in turn increased
the rate of the electrocatalytic and catalytic partial oxidation of ethanol.
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
69
0
30
60
90
120
150
180
0
1
2
3
4
3
6
9
12
15
0.0 0.5 1.0 1.5 2.0
0
2
4
6
I /
mA
(rH
2
/ m
ol·
s-1)·
10
7
(rC
O /
mol·
s-1)·
10
8
T = 600 ºC
T = 650 ºC
T = 700 ºC
(rC
O2
/ m
ol·
s-1)·
10
8
UWC
/ V
Figure 1.8. Influence of the applied potential and the reaction temperature on the
steady state current and reaction rates of the main products. Conditions:
C2H5OH/H2O = 0.7 %/2 %, N2 balanced.
On the other hand, it can also be observed an increase in the production
rate of different products with the temperature at fixed potential. This
increase could be attributed to the increase of both the kinetics of the
catalytic and electrocatalytic activity, when increasing the temperatures,
and the ionic conductivity of the YSZ solid electrolyte. Hence, the obtained
current at fixed potential increased with the reaction temperature which
would again explain the increase in the electrocatalytic kinetics [12, 15].
Chapter 1
70
The apparent activation energy calculated via Arrhenius plot under
open circuit conditions and closed circuit conditions was obtained. The
apparent activation energy calculated under O.C.C was 106.83 kJ·mol-1.
This value is in the same range as that reported in previous works of
catalytic steam reforming of ethanol. For instance, an activation energy of
187.7 kJ·mol-1 was reported for the steam reforming of ethanol over Ni
catalyst [28]. On the other hand, the apparent activation energies under
closed circuit conditions (0.5 V, 1 V, 1.5 V and 2 V) were 98.50 kJ·mol-1,
87.30 kJ·mol-1, 55.96 kJ·mol-1 and 49.00 kJ·mol-1, respectively. As it can be
observed, calculated activation energy linearly decreased with increasing
applied potential. The activation energy under open circuit conditions was
106.83 kJ·mol-1 whereas that obtained at closed circuit conditions (2 V) was
49.00 kJ·mol-1. This value is in the same range that the activation energy
obtained under closed circuit conditions for the electro-oxidation of ethanol
over Pt in a phosphoric acid media (36 kJ·mol-1) [29].
Figure 1.9 shows the variation of the obtained H2 faradaic efficiency ().
This factor was calculated via Faraday´s law from the experimentally
measured H2 production rate under current imposition. This way, the
overall hydrogen consumption and production reactions were taken into
account.
= (r -r0) /(I/nF) (eq. 1.1)
where r is the H2 production rate under the applied current, ro is the open-
circuit catalytic rate due to the catalytic steam reforming of ethanol, I is
the applied current, n is the charge of the ionic species (2) and F is the
Faraday’s constant.
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
71
0 30 60 90 120 150 180 210
0.3
0.4
0.5
0.6
0.7
0.8
0.9
I / mA
T = 600 ºC
T = 650 ºC
T = 700 ºC
Figure 1.9. Variation of the H2 Faradaic Efficiency () vs. current at diferent
temperatures.
This parameter related the H2 produced at each current (I/nF) with the
overall hydrogen production reactions (r-r0). It can be observed that all the
displayed faradic efficiencies values were lower than 1, which indicated
that no electrochemical promotion effect (NEMCA) occurred on the Pt
catalyst/electrode for the ethanol reforming reaction [30] or at least was
negligible vs. the previous explained electrocatalytic processes. These
results are in agreement with EPOC studies which clearly demonstrated
that the desorption of O2- promoting ionic species on Pt is quite fast at
temperatures above 600 ºC. Vernoux et al [31] reported an O2-TPD study,
in which the promoting ionic oxygen species desorbed from the Pt surface
at 600 ºC. That the O2 evolution reaction is a fast reaction at temperature
above 600 ºC is evident from the high value of the applied current (high
electrocatalytic activity) in comparison with some EPOC studies where low
current values are used [20]. Furthermore, the high operating
temperatures used in this chapter did not favor the formation of the
Chapter 1
72
effective double layer, which is prerequisite to observe the NEMCA effect.
Additionally, part of the H2 produced in the single chamber reacted with
other products such as CO, CO2, C2H4 and O2, lowering the faradaic
efficiency value below 1. It can be observed that an increase in the applied
current and the reaction temperature decreased the H2 faradaic Efficiency
value. At higher currents, more H2 and ethanol electro-oxidation products
were formed which reacted between them decreasing the amount of H2
obtained in the reactor (r) vs. the corresponding faradaic H2 production
rate via steam electrolysis (I/nF). For a fixed current, the rise in the
reaction temperature promoted the further reaction, in the single chamber
reactor, of H2 with the other obtained molecules, which also contributed to
the decrease in the Faradaic efficiency. On the other hand, a decrease in
the Faradaic efficiency can also be attributed to the oxygen evolution
reaction (reaction 1.9) due to an increase in the temperature. However, the
most interesting part of the experiment comes from the experimentally
measured modification in the overall H2/CO ratio of the obtained syngas.
Figure 1.10 shows the variation of the obtained H2/CO ratio of the
produced syngas vs. the applied potential for the experiment inFigure 1.8.
It can be again observed that the H2/CO ratio can be largely modified and
controlled by applied polarization. Values between 1.5-12 were obtained
depending on the conditions (applied potential and temperature). As
expected, the higher H2/CO ratio modification was attained at the lowest
explored temperature (600 ºC) where the higher H2 faradaic efficiencies
values were measured. However, at higher temperatures, where higher
rates of syngas were obtained (Figure 1.9), the H2/CO ratio can also be
controlled between 1.5-3, which is very interesting from the petrochemical
viewpoint [7].
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
73
0.0 0.5 1.0 1.5 2.0
0
2
4
6
8
10
12
H2/C
O
Potential / V
T = 600 ºC
T = 650 ºC
T = 700 ºC
Figure 1.10. Influence of the applied potential and reaction temperature on the
obtained H2/CO ratio of the produced syngas.
For instance, H2/CO ratio of 2 is typically required for the Fischer-
Tropsch synthesis while an optimal H2/CO ratio around 1 is required for
the oxo-synthesis process. On the other hand, large values of H2/CO ratio
are desired to reduce the purification steps of H2 streams to be used in fuel
cells. Finally, a comparison of the cost of the produced synthesis gas via
electrochemical reactions with that obtained with conventional processes
was performed. In this sense, the cost of synthesis gas in the
electrochemical process was set at 22 kW·h/kg syngas at 700 ºC and 2 V.
This value is very similar to those reported in literature, for instance for
the theoretically production of syngas from coal reforming (18.22 kW·h/kg
syngas) [32]. Hence, it can be suggested that the proposed reactor
configuration may be of great interest in biorefinery applications where
Chapter 1
74
high-quality H2 and syngas of flexible ratio may be required from
renewable resources such as biogas and bioethanol.
1.4 Conclusions
A novel solid electrolyte membrane reactor (Pt/YSZ/Pt) has been
developed for the direct production of synthesis gas with flexible H2/CO
ratio. The technique of cathodic arc deposition was used to obtain
nanocrystalline Pt catalyst/electrode films with excellent stability under
the explored reaction conditions.
The application of different polarizations at different temperatures
ranging from 600 to700 ºC allowed to produce syngas from an (ethanol-
water stream) with a flexible H2/CO ratio (between 1.5-12). The amount of
syngas increased with the applied potential and the reaction temperature
as a result of an increase in the rate of the electrocatalytic processes:
steam electrolysis at cathode and electrocatalytic partial oxidation of
ethanol at the anode. A decrease in the H2/CO ratio at fixed potentials was
achieved at higher temperatures due to the further reaction of the
produced H2 with the rest of species present in the gas phase, thus leading
to a decrease in the faradaic efficiency.
1.5. References
[1] G.J. Stiegel, M. Ramezan, International Journal of Coal Geology, 65 (2006) 173-
190.
[2] M. Conte, A. Iacobazzi, M. Ronchetti, R. Vellone, Journal of Power Sources, 100
(2001) 171-187.
[3] I. Dincer, International Journal of Hydrogen Energy, 27 (2002) 265-285.
[4] J. Comas, F. Marino, M. Laborde, N. Amadeo, Chemical Engineering Journal, 98
(2004) 61-68.
Direct Production of Flexible H2/CO Synthesis Gas via Steam Electrolysis and Ethanol Partial Oxidation
75
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4094-4123.
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[7] X. Song, Z. Guo, Energy Conversion and Management, 47 (2006) 560-569.
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Research (Ed.)Spain, 2015.
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313-318.
[10] G. Marnellos, M. Stoukides, Solid State Ionics, 175 (2004) 597-603.
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[12] A. Caravaca, A. de Lucas-Consuegra, J. González-Cobos, J.L. Valverde, F.
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[14] A. De Lucas-Consuegra, N. Gutiérrez-Guerra, A. Caravaca, J.C. Serrano-Ruiz,
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J.L. Valverde, F. Dorado, Applied Catalysis B: Environmental, 142–143 (2013) 298-
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[16] I. Garagounis, V. Kyriakou, C. Anagnostou, V. Bourganis, I. Papachristou, M.
Stoukides, Industrial and Engineering Chemistry Research, 50 (2011) 431-472.
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(2007) 507-513.
Chapter 1
76
[19] P. Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-
Consuegra, J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou,
E.A. Baranova, Chemical Reviews, 113 (2013) 8192-8260.
[20] C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides,
Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion, and
Metal-Support Interactions, 2001.
[21] A. de Lucas-Consuegra, J. González-Cobos, Y. Gacia-Rodriguez, J.L. Endrino,
J.L. Valverde, Electrochemistry Communications, 19 (2012) 55-58.
[22] A. Anders, Cathodic Arcs: From Fractal Spots to Energetic Condensation,
Springer Series on Atomic, Optical, and Plasma Physics, 2008.
[23] A. De Lucas-Consuegra, J. González-Cobos, V. Carcelén, C. Magén, J.L.
Endrino, J.L. Valverde, Journal of Catalysis, 307 (2013) 18-26.
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Journal of Catalysis, 251 (2007) 474-484.
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E9-E11.
[32] E.J. Bair, Connecting the Dots to Future Electric Power, AuthorHouse2010.
2.1. Introduction
2.2. Experimental
2.2.1 Catalytic activity measurements
2.2.2. Preparation of the solid
electrolyte cell
2.2.3. Characterization measurements
2.3. Results and discussion
2.4. Conclusions
2.5. References
CHAPTER 2:
Simultaneous Production and
Separation of H2 and C2 Hydrocarbons
via Steam Electrolysis and Methane
Partial Oxidation
INTRODUCTION
EXPERIMENTAL CHARACTERIZATION
SEM analysis
RESULTS
Outer chamber: CH4 = 1 %,
Inner chamber H2O = 3 %, N2 balance, T = 750 ºC
Hydrogen and syngas can be used in a wide range of industrial applications, such us
petrochemicals, ammonia, petroleum refining, and methanol synthesis
The thermal catalytic decomposition of methane (TCD) into hydrogen and carbon is
attracting much research interest since it appears to be a direct, mildly endothermic,
attractive way for producing highly “pure” hydrogen with reduced CO2 emissions
Thermo-catalytically decomposed of methane
CH4 (g) C (s) + 2 H2 (g)
H2O/N2
inner chamberPure H2
inner chamber
Fritted quartz
Double chamber
quartz reactor
Platinum deposited
in dense YSZ
Counter electrode
Platinum impregnated
in porous YSZ
Solid electrolyte
tube(YSZ)
Working electrode
H2, CO, CO2,
C2H4, C2H6
outer chamber
CH4/N2
outer chamber
Thermocouple
Au wires
Double chamber solid electrolyte cell reactor
- Porous layer thickness of around 60 μm.
- Pt impregnated particles were dispersed
on the YSZ porous layer by the presence of
lighter dots along the cross section
micrograph.
XRD analysis
20 30 40 50 60 70 80 90
YS
Z (
4 2
0)
YS
Z (
4 0
0)
YS
Z (
2 2
2)
Pt
(2 2
2)
Pt
(3 1
1)
Pt
(2 2
0)Pt
(2 0
0)
Pt
(1 1
1)
YS
Z (
3 3
1)
YS
Z (
2 2
0)
YS
Z (
2 0
0)
Inte
nsi
ty, a.
u.
2º
YS
Z (
1 1
1)
-Pt particle size on porous solid
electrolyte = 32 nm
-Porous YSZ interlayer over the dense
solid electrolyte strongly increase the
dispersion of the Pt particles.
Influence of the applied current
0
1
2
3
0
1
2
3
0 40 80 120 160 200 240 280
0.0
0.5
1.0
1.5
50 mA50 mA O.C.CO.C.CO.C.C
(rH
2oute
r /
mol·
s-1·
cm-2
) x108
50 mAO.C.C
0
6
12
(r
H2
inner
/ m
ol·
s-1·c
m-2
) x108
(r C
Oo
ute
r /
mol·
s-1·c
m-2
)x108
(rC
O2
ou
ter
/ m
ol·
s-1·c
m-2
) x108
0.0
0.6
1.2
Time / min
(rC
2H
6oute
r /
mol·
s-1·c
m-2
) x109
(rc 2
H4
oute
r /
mol·
s-1·c
m-2
) x101
0
0.0
0.3
0.6
Influence of the reaction atmosphere
Symbol Outer chamber Deposited Carbon
N2 No
1 vol.% CH4/N2 No
N2 Yes
1 vol.% CH4/N2 Yes
0 20 40 60 80
0.0
0.5
1.0
1.5
2.0
2.5
UW
C (V
)
I (mA)
3 vol.% H2O (deposited C)
3 vol.% H2O (non deposited C)
3 vol.% H2O / 1 vol.% CH
4 (non deposited C)
0 20 40 60 80
0.0
0.5
1.0
1.5
2.0
2.5
UW
C (V
)
I (mA)
3 vol.% H2O (deposited C)
3 vol.% H2O (non deposited C)
3 vol.% H2O / 1 vol.% CH
4 (non deposited C)
0 20 40 60 80
0.0
0.5
1.0
1.5
2.0
2.5
UW
C (V
)
I (mA)
3 vol.% H2O (deposited C)
3 vol.% H2O (non deposited C)
3 vol.% H2O / 1 vol.% CH
4 (non deposited C)
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
UW
C (V
)
I (mA)
3 vol.% H2O (deposited C)
3 vol.% H2O (non deposited C)
3 vol.% H2O / 1 vol.% CH
4 (non deposited C)
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
UW
C /
V
Current / mA
T = 700 ºC.
100 mVs-1
Reproducibility experiment
CH4 = 1 %, H2O = 3 %, N2 balance,
T = 750 ºC
0
10
20
30
40
50
60
0
2
4
6
0 100 200 300 400 500 600
0.0
0.5
1.0
1.5
2.0
(100 % N2) (100 % N2)
0% CH4
0% CH4
O.C.C
I / m
A
I = 50 mA
1% CH4
rCO
rCO
2
(rC
O, r
CO
2oute
r / m
ol·s
-1·c
m-2
) x1
08
(rH
2 oute
r /m
ol·s
-1·c
m-2)x
108
Time / min
T = 700 ºC.
- Stability of the Pt/YSZ/Pt catalyst exposed
to the reactions conditions
In this study, we propose a novel approach based on the ability of SEMRs to both, produce carbon
and H2, and to electrochemically oxidize the carbon deposited over an electrode. It consists of a
solid electrolyte membrane reactor that allows to electrochemically regenerate a Pt catalyst from
the carbon deposition in the TCD reaction.
0.0
0.4
0.8
1.2
1.6
2.0
0.00
0.15
0.30
0.45
0.60
0
1
2
3
4
5
0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5
0
5
10
15
20
25
30
35
(rH
2 o
ute
r/m
ol·
s-1·c
m-2
) x1
08
(r C
O2
ou
ter/
mol·
s-1
·cm
-2) x1
08
(rC
O o
ute
r/m
ol·
s-1
·cm
-2)
x1
08
0.0
0.3
0.6
0.9
1.2
1.5
1.8
(r C
2H
6ou
ter/
mol·
s-1·c
m-2
) x1
01
0
(rC
2H
4ou
ter/
mol·
s-1
·cm
-2)
x1
010
0
2
4
6
8
10
12
Cu
rren
t /
mA
Time / h
-0.85
-0.84
-0.83
-0.82
-0.81
-0.80
-0.79
UW
C (
V)
Gas de síntesis
Metanol
Nafta
IntermediosAmoniaco
Metano- Olefinas
- Aromáticos
- Tolueno
- Etilenglicol
- Isobutano
Haber-Bosch
- Urea
- Hidrazina
- Metilaminas
- Ácido Nítrico
- Acrilonitrilo
- Fertilizantes
H2
CO
Metanación
Proceso
MOBIL- Olefinas aromáticas
(gasolinas)
- Metanol
- Ácido acético
- Formaldehido
- Metilamina
CHAPTER 2. SIMULTANEOUS PRODUCTION AND
SEPARATION OF H2 AND C2 HYDROCARBONS VIA STEAM
ELECTROLYSIS AND METHANE PARTIAL OXIDATION
81
Abstract
his chapter reports the production of H2 via the catalytic
methane decomposition on Pt supported on yttria-stabilized
zirconia together with its electrochemical regeneration in a solid
electrolyte membrane reactor. Hence, a Pt-YSZporous/YSZ/Pt
double chamber solid electrolyte cell was prepared and tested under two
reaction regimes. In the first regime, under open circuit conditions,
hydrogen and carbon was produced on the catalytically active Pt-YSZ
porous catalyst via methane decomposition reaction (CH4 (g) C (s) + 2
H2 (g)). In the second regime, under polarization conditions, steam was
electrolysed at the Pt cathode of the cell (H2O + 2e- H2 + O2-) and the
produced O2- ions were simultaneously electrochemically pumped to the
Pt/YSZ porous catalyst (anode), thereby allowing removal of the previously
deposited carbon (C (s) + O2- CO2 (g)) and finally regenerating the
Pt/YSZ porous catalyst film. We demonstrated that the carbon generated in
the methane decomposition step served as a depolarizating agent in the
steam electrolysis process, thus decreasing the electrical energy input
required for electrochemically producing pure H2. In addition, during the
regeneration step, C2 hydrocarbons (e.g., ethane and ethylene) were obtained
as a result of the electro-catalytic methane oxidative coupling on the
Pt/YSZ porous catalyst film. The performance and durability of the system
was also verified for long operation times in view of the possible practical
application of this novel reactor configuration, which combines gas phase
catalysis and electro-catalysis for hydrogen production.
T
Chapter 2
82
2.1. Introduction
As already mentioned in Chapter 1, hydrogen and syngas can be used in
a wide range of processes, such as petrochemicals processing, ammonia
and methanol synthesis, and petroleum refining [1-3]. If attention is
focused on the use of hydrogen as clean fuel to produce energy by fuel cell
systems, the development of efficient processes for the production of “COx-
free” H2 streams becomes important in order to overcome hurdles
associated with the use of a conventional multi-step process for its further
separation and purification. In this sense, the thermal catalytic
decomposition of methane (TCD) into hydrogen and carbon is attracting
much research interest since it appears to be a direct, mildly endothermic,
attractive way for producing highly “pure” hydrogen with reduced CO2
emissions [4]. This process become more interesting taking into account
the reserves of natural gas and the recent advances in the technology for
the extraction of natural gas from impermeable shale formations
(commonly named shale gas) [5]. Hence, methane can be thermo-
catalytically decomposed into carbon and hydrogen without producing CO2
according to the following reaction:
CH4 (g) C (s) + 2 H2 (g) (2.1)
When compared to conventional methane reforming technologies, TCD
has a number of positive aspects, as recently stressed by some authors. For
instance, based on the life-cycle assessments, Dufour et al. [6] claimed
TCD as the most environmentally friendly process for hydrogen production
as it presented the lowest total environmental impact and CO2 emissions
in comparison with methane steam reforming coupled with CO2 capture
and methane thermal cracking technologies. Accordingly, different metal-
based catalysts have been widely used to decrease the high temperature
Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
83
required for methane decomposition [7-10]. These metal catalysts are
generally deactivated by deposited carbon during the course of the
reaction, thereby requiring frequent regeneration treatments for
recovering initial activity. This technical barrier has motivated researchers
to develop novel reactor designs and specifically membrane reactors [11-
13]. In this sense, Karagiannakis et al. reported the electrochemical double
chamber solid electrolyte membrane reactor (SEMR) using a proton
conducting ceramic membrane, SrCe0.95Yb0.05O3d (SCYb) [11]. In this
approach, the hydrogen produced by the catalytic hydrocarbon
decomposition is electrochemically transported through the proton
conducting membrane. In a similar work, the experimental verification
and evaluation of a barium cerate mixed conducting membrane perovskite
(BaCe0.9Y0.1O3−δ, BCYO) containing Pd catalyst was provided, leading to
high purity hydrogen and carbon production by TCD [14]. In addition, the
SEMRs have already been used in other related hydrogen production
processes, e.g., via steam reforming or partial oxidation reactions [15] and
have been described in detail by means of excellent reviews [16-19]. On the
other hand, the electrochemical oxidation of carbon in SEMRs (as Solid
Oxide Fuel Cells, SOFC) has been previously reported in literature [20-22].
In this chapter, a novel approach based on the ability of SEMRs to both
produce carbon and H2 and electrochemically oxidize the carbon deposited
over an electrode is proposed. It consists of a solid electrolyte membrane
reactor that allows to electrochemically regenerate a Pt catalyst from the
carbon deposition in the TCD reaction. Hence, a Pt-YSZporous/YSZ/Pt (YSZ =
Yttria-Stabilized Zirconia) double chamber cell operating under two
reaction regimes have been developed. In the first regime, under open
circuit conditions, hydrogen and carbon is produced on the catalytically
Chapter 2
84
active Pt/YSZ porous film via TCD. In the second regime, under close
circuit conditions, steam is electrolyzed at the Pt cathode of the cell
(reaction 2.2) thereby simultaneously generating O2- ions that are
electrochemically pumped to the Pt/YSZ porous catalyst film (anode)
assisting in the removal of the previously deposited carbon (reaction 2.3):
H2O + 2e- H2 + O2- (2.2)
C (s) + O2- CO2 (g) (2.3)
This way, the second reaction regime allows to electrochemically
regenerate the active Pt/YSZ porous film by a carbon assisted steam
electrolysis process leading to the further production of hydrogen. The use
of carbon containing molecules such as CH4 [23], CO [24] or Carbon [25] as
depolarizating agents is of great interest for the electrolytic production of
H2, since it allows to strongly decrease the required electrical power input
for the process. Consequently, the proposed double chamber configuration
allows the in-situ valorization of the produced carbon as a depolarizating
agent during the steam electrolysis process.
2.2. Experimental
2.2.1. Catalytic activity measurements
The reaction experiments were carried out in the experimental setup
described in Chapter 1. The main difference respect to the previous
chapter is the use of a double chamber solid electrolyte membrane reactor.
The reaction gases (Praxair, Inc.) were certified standards of 10% CH4/N2,
and N2 (99.999% purity) was used as the carrier gas. The gas flow was
controlled by a set of calibrated mass flow-meters (Brooks 5850 E and 5850
S) while water was introduced into the inner chamber feed stream, as
previously explained, by flowing N2 through a saturator. All lines placed
Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
85
downstream from the saturator were heated above 100 ºC to prevent
condensation. The inner Pt working electrode (W) was exposed to H2O/N2
mixture (3 %), whereas the outer Pt/YSZ porous catalyst film was exposed
to a CH4/N2 mixture (1%). The overall flow rate was 100 ml.min-1 in both
reaction chambers for all the performed experiments. Both reactions
atmospheres were completely isolated and no gas permeation occurred
between the two chambers of the electrochemical cell. The contributions of
both, the homogeneous reaction (activity without electrochemical catalyst)
and the catalytic rate on the current collector, were found to be negligible
under the operating conditions used. All the catalytic experiments were
carried out at atmospheric pressure and at reaction temperatures between
700-800 ºC. Hydrogen produced in the inner chamber was calculated on
the basis of the Faraday’s law from the applied current [15]. Reactant and
product gases of the outer chamber were analysed with a micro gas-
chromatograph (Varian CP-4900). All the products reaction rates were
normalized per catalyst-electrode area (2.01 cm2), thus being expressed in
units of mol s-1 cm-2. The main detectable products during the TCD
experiments in the outer side stream were H2 and CO (Open Circuit
Conditions) whereas during the regeneration step CO2, CO, C2H6 and
C2H4 were quantified in the outlet stream (Close Circuit Conditions).
2.2.2. Preparation of the solid electrolyte cell
The solid electrolyte cell consisted of an Yttria-Stabilized Zirconia (YSZ)
tube closed at one end, with 15 cm length, 1.8 cm internal diameter, and
1.5 mm thickness (supplied by CERECO). On both faces of the closed side
tube (Figure 2.1), two kind of Pt catalyst were prepared.
Chapter 2
86
H2O/N2
inner chamberPure H2
inner chamber
Fritted quartz
Double chamber
quartz reactor
Platinum deposited
in dense YSZ
Counter electrode
Platinum impregnated
in porous YSZ
Solid electrolyte
tube(YSZ)
Working electrode
H2, CO, CO2,
C2H4, C2H6
outer chamber
CH4/N2
outer chamber
Thermocouple
Au wires
Figure 2.1. Scheme of the double-chamber solid electrolyte cell reactor.
Firstly, in the inner side of the tube a continuous Pt catalyst film was
prepared on the dense YSZ by application of a thin coating of Pt paste
(METALOR), followed by two calcination steps, at 300 ºC (2 h) and 850 ºC
(2 h). The final Pt loading was around 5 mg Pt/cm2. A Pt/YSZ porous
catalyst film based on Pt nanoparticles supported on YSZ was furnished on
the outer side of the YSZ tube. For that purpose, a porous YSZ interlayer
was firstly deposited as follows. YSZ powder was mixed with an organic
binder (Decoflux, Zschwimmer and Schwartz) in a 1:1 (wt./wt.) ratio and
then spin-coated on the outer face of the tube. The assembly was dried at
100 °C for 1 h in an oven and then calcined at 850 °C for 6 h in order to
ensure good adherence. Then, the Pt active catalytic particles were
deposited on the porous YSZ interlayer. The Pt particles were prepared by
an impregnation technique consisting of successive steps of deposition and
Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
87
thermal decomposition of the 2-propanol solution of 0.1 M H2PtCl6.
Initially, 10 μl of precursor solution were deposited on the YSZ substrate
using a plastic circular mask in order to obtain a 2.01 cm2 geometric area
of the catalytic film. Then, evaporation of the solvent took place at 70 °C
for 10 min, followed by drying of the sample at 120 °C overnight and then
calcination at 850 °C for 2 h. Several successive steps of this deposition
procedure followed by drying and heating were repeated until a final metal
loading of 0.6 mg Pt was obtained. It led to a highly dispersed Pt/YSZ
electrode, with a poor electrical conductivity. Then, a catalytically inert Ag
current collector was added to the Pt/YSZ porous catalyst film for
polarization purposes. Hence, the electrical contact of the Pt electrode and
the Au current collector was carried out by gold wires, which were in turn
connected to a potentiostat-galvanostat Voltalab 21 (Radiometer
Analytical). The prepared solid electrolyte cell was placed on a quartz tube
with appropriate feed-through for both reaction sides as shown in Figure
2.1. Before the catalytic activity measurements, the Pt porous catalyst film
was reduced under H2 stream at 450 ºC for 1 h.
2.2.3. Characterization measurements
The Pt porous catalyst film was characterized by X-ray diffraction
(XRD) with a Philips PW 1710 instrument using Ni-filtered Cu Kα
radiation. The diffractograms were compared with the JCPDS-ICDD
references. The Pt porous catalyst film was also characterized via scanning
electron microscopy (SEM) with a BSE signal using a JEOL 6490 LV
microscope. Linear voltammetry measurements were also performed with
the potentiostat-galvanostat Voltalab 21 under different reaction
atmospheres and different Pt catalytic states (regenerated and
deactivated) and were recorded at a sweep rate of 100 mV/s.
Chapter 2
88
dense YSZ
pellet
porous
Pt/YSZ
catalyst
layer
2.3. Results and discussion
Figure 2.2 shows the cross section SEM analysis of the Pt-YSZ porous
catalyst film taken with BSE signal. The micrograph shows a porous layer
thickness of around 60 μm. Higher resolution images revealed good contact
between the porous layer and the dense electrolyte. No cracks or
delimitations of the porous layer were observed after preparation. On the
other hand, it is also interesting to note that the Pt impregnated particles
were dispersed on the YSZ porous layer as can be observed by the presence
of lighter dots along the cross section micrograph.
Figure 2.2. Cross-section SEM micrographs of the porous Pt-YSZ catalyst-
electrode supported over the YSZ solid electrolyte.
XRD analysis of the Pt-YSZ porous catalyst after reduction at 450 ºC is
shown in Figure 2.3. The peak assignments based on JCPDS standards
were consistent with a Pt face centered cubic (FCC) structure and the YSZ
Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
89
20 30 40 50 60 70 80 90
YS
Z (
4 2
0)
YS
Z (
4 0
0)
YS
Z (
2 2
2)
Pt
(2 2
2)
Pt
(3 1
1)
Pt
(2 2
0)
Pt
(2 0
0)
Pt
(1 1
1)
YS
Z (
3 3
1)Y
SZ
(2
2 0
)
YS
Z (
2 0
0)
Inte
nsi
ty /
a.u
.
2º
YS
Z (
1 1
1)
phase corresponding to the porous layer [26]. No patterns of platinum
oxides appeared in XRD analysis showing that all Pt particles were on its
reduced state after H2 reduction. According to previous studies [27], Pt
mean particle size can be estimated by using the Scherrer’s equation [28]
corresponding to the (1 1 1) peak resulting in a Pt crystal size of 32 nm.
Remarkably, this value is considerably lower than that typically obtained
by direct impregnation of the Pt precursor solution on dense solid
electrolytes (i.e., around 60 nm) [27], thereby demonstrating that the
addition of a porous YSZ interlayer over the dense solid electrolyte
strongly increased the dispersion of the Pt particles, positively affecting
its catalytic activity.
Figure 2.3. XRD pattern of the fresh porous Pt-YSZ catalyst electrode.
Figure 2.4 represents the rate of the different obtained products
reaction rates (e.g., H2 CO2, CO, C2H4 and C2H6) with the time on stream
under open circuit conditions (O.C.C) and current imposition of 50 mA
Chapter 2
90
0
1
2
3
0
1
2
3
0 40 80 120 160 200 240 280
0.0
0.5
1.0
1.5
50 mA50 mA O.C.CO.C.CO.C.C
(r
H2
ou
ter
/ m
ol·
s-1·
cm-2)
x1
08
50 mAO.C.C
0
6
12
(r
H2
inn
er
/ m
ol·
s-1·c
m-2)
x1
08
(r C
Oou
ter
/ m
ol·
s-1·c
m-2)x
10
8
(rC
O2
ou
ter
/ m
ol·
s-1·c
m-2)
x1
08
0.0
0.6
1.2
(rC
2H
4
ou
ter
/ m
ol·
s-1·c
m-2)
x1
010
Time / min
(rC
2H
6
ou
ter
/ m
ol·
s-1·c
m-2)
x1
09
0.0
0.3
0.6
a)
b)
c)
during the regeneration steps. The duration of each reaction regime was
40 min.
Figure 2.4. Influence of the applied current (from O.C.C to 50 mA) on the
dynamic value of the inner (pure H2) and the outer chamber products (H2, C2s and
COx) obtained in the double-chamber solid electrolyte membrane reactor.
Conditions: outer chamber CH4 = 1 %, inner chamber H2O = 3 %, N2 balance in
both cases, temperature T=750 ºC.
Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
91
Under O.C.C, TCD-derived hydrogen was produced at the outer side of
the solid electrolyte cell over porous Pt-YSZ catalyst. This result is in
agreement with previous works that have demonstrated the suitable
catalytic performance of Pt particles on the TCD reaction under similar
conditions [7]. During the O.C.C. regime, a strong deactivation of the
catalytic activity of the system caused by the deposition of Carbon on the
Pt active sites, which progressively decrease the activity of the catalyst
along the 40 min of duration of the experiment, was observed. On the other
hand, it can also be observed certain amount of CO production under open
circuit conditions during the TCD step. This observation could be mainly
attributed to the reaction between part of the carbon deposits and the
lattice oxygen of the YSZ porous catalyst film due to its oxygen storage
capacity and ionic conductivity at the temperature conditions of the
present study. The production of syngas in the absence of oxygen via TCD
over Pt supported on different oxygen storage materials such as CeO2 or
Gd-doped CeO2 has been recently reported [29]. In addition, the syngas
produced under open circuit conditions (H2/CO ≈ 2) suits with the
conditions required for liquid fuel production based on the Fischer–
Tropsch synthesis [30]. The possibility of syngas generation in the absence
of oxygen by methane decomposition offers an interesting route to decrease
reactor size and costs because methane is the only reactant in the gas
phase. Consequently, it could be considered a new application of ionically
conducting ceramics as active catalyst supports [31]. In the second reaction
regime at t= 40 min, a constant current of 50 mA was applied for another
40 min under the same reaction atmosphere. During this current
imposition step, CO2, CO, C2H4 and C2H6 were detected in the outer side of
the solid electrolyte cell. Furthermore, pure hydrogen was simultaneously
Chapter 2
92
obtained at the inner side of the solid electrolyte cell as a result of the
steam electrolysis process over the Pt cathode.
The scheme shown in Figure 2.5 summarizes the main processes
occurring under the different reaction regimes.
Figure 2.5. Schematic representation of the main processes involved during
the different operation regimes.
The YSZ solid electrolyte could be considered as a pure O2- conductor
[31]. Thus, the application of positive currents (being the outer Pt porous
catalyst the working electrode of the cell) led to the steam electrolysis
process in the Pt inner electrode with the corresponding production of pure
gaseous H2, and O2- ions (reaction 2) [15]. The H2 production rate in the
inner chamber was calculated on the basis of the Faraday’s law by I/2 F
and normalized per catalyst-electrode area (2.01 cm2), where I is the
Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
93
applied current (50 mA), and F is the Faraday’s constant (see Figure 2.4a).
Simultaneously, the electrochemically supplied O2- ions to the outer Pt-
YSZ porous catalyst led to the formation of CO2, CO and C2s products (e.g.,
C2H4 and C2H6). The formation of these products was mainly attributed to
the removal of the previous deposited carbon along with the
electrocatalytic reaction of CH4 with the O2- ions, according to the reaction
scheme shown on Figure 2.5b. However, although the electrochemical
nature of these reactions will be explained later, the O2 evolution reaction,
as well as the catalytic methane oxidation, oxidative coupling and steam
reforming catalytic processes (see Figure 2.5b) is worth to consider. The
production of H2 and carbon derived products might be the sum of the
catalytic and electro-catalytic processes. On the other hand, it is evident
that the applied current allows to completely regenerate the Pt/YSZ porous
catalyst film from the previous deposited carbon since the catalytic activity
for the thermal decomposition of methane was again recovered in the next
OCC reaction step (Figure 2.4). A reproducible behavior in the hydrogen
production rate was obtained along the different cycles, being the slight
differences observed to be only attributed to the µGC injection delay time
after the current imposition (due to the fast nature of the deactivation
process). The observed deactivation process can be defined by the
deactivation index of the catalyst based on the H2 production. It refers to
the ratio between final and initial hydrogen production under O.C.C.
regime, (rH2f,O.C.C./ rH2o,O.C.C.)x100. In this chapter, the average deactivation
index of the catalyst based on the H2 production was around 55 %. Finally,
it is also interesting to note the formation of C2s products: ethane and
ethylene due to the activity of the Pt/YSZ porous catalyst in the methane
oxidative coupling reaction shown in Figure 2.5b. It is well known that
methane oxidative coupling reaction for C2s production is catalyzed by
Chapter 2
94
oxides materials and hence very likely most of the activity comes from the
porous YSZ interlayer [32]. However, as it can be observed in Figures 2.4 b
and c, due to the high catalytic activity of Pt in methane combustion
processes, the production of CO and CO2 (CO + CO2 yield ≈ 6.6 %) under
constant current imposition (50 mA, which corresponds to an O2- flux of
12.8 x 10-8 mol s-1 cm-2), is more than a magnitude order higher than that
of C2 hydrocarbons (C2s yield ≈ 0.44 %) coming from the oxidative coupling
reaction [33, 34].
In order to support the previous explained mechanism, a transient gas
atmosphere-potential experiment was carried (Figure 2.6). The experiment
was carried out at a constant temperature of 700 ºC. The outer porous
catalyst was initially kept under Open Circuit conditions (O.C.C) and
exposed to CH4 for 90 min. Subsequently, the outer chamber was purged
with N2 for 190 min in order to ensure the CH4 desorption from the
catalyst and to remove all the gaseous CH4 from the reactor. Finally, under
the same N2 atmosphere, a current of 50 mA was applied for 320 min. In
good agreement with the previous experiments, syngas was produced
during the initial TCD step. A strong deactivation was again observed due
to the deposition of Carbon on the Pt active sites. However, the final
application of 50 mA led again to the occurrence of CO and CO2 peaks as a
result of the removal of the previous deposited carbon adsorbed on the Pt
active sites. The absence of chemisorbed CH4 and CH4 coming from the gas
phase (after the purge state) evidenced the previous explained reaction
scheme (Figure 2.5). This experiment clearly demonstrated the ability of
the O2- ions to remove the previous deposited carbon from the Pt/YSZ
porous catalyst layer, leading to the electrochemical regeneration of the
catalyst.
Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
95
0
10
20
30
40
50
60
0
2
4
6
0 100 200 300 400 500 600
0.0
0.5
1.0
1.5
2.0
(100 % N2) (100 % N2)
0% CH4
0% CH4
O.C.C
I /
mA
I = 50 mA
1% CH4
rCO
rCO
2
(rC
O, r C
O2
ou
ter
/ m
ol·
s-1·c
m-2
) x
10
8
(rH
2 o
ute
r /m
ol·
s-1·c
m-2)x
10
8
Time / min
Figure 2.6. Polarization programmed oxidation experiment after the
deactivation of the porous Pt-YSZ catalyst. Conditions: Temperature = 700 ºC. a)
Deactivation stage (O.C.C.): Outer chamber CH4 = 1 %, Inner chamber H2O = 3 %,
N2 balance in both cases. b) Purge (O.C.C.): Outer chamber 100 % N2, Inner
chamber H2O = 3 % (N2 balance). c) Polarization: Current = 50 mA, Outer
chamber 100 % N2, Inner chamber H2O = 3 % (N2 balance).
Chapter 2
96
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
UW
C (V
)
Current / mA
Symbol Outer chamber Deposited carbon
N2 No
1 vol.% CH4/N2 No
N2 Yes
1 vol.% CH4/N2 Yes
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
U
U
U
U
UW
C (V
)
Current / mA
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
U
U
U
U
UW
C (V
)
Current / mA
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
U
U
U
U
UW
C (V
)
Current / mA
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
U
U
U
U
UW
C (V
)
Current / mA
The electrochemical nature of the previous mentioned reactions can be
further supported by the characterization of the electrochemical cell by
linear voltammetry under different gas exposed atmospheres of the Pt-YSZ
porous catalyst and different Pt-YSZ state (activated or deactivated).
Figure 2.7 shows the different current-potential curves obtained at 700 ºC.
All the lineal voltammograms were performed under 3 % H2O/N2
atmosphere at the inner chamber of the electrochemical cell while the
outer chamber was exposed to two different reaction atmospheres: N2 or 1
% CH4/N2.
Figure 2.7. Influence of the reaction atmosphere and the deactivation state on
the Current-Potential curves obtained during a linear voltammetry. Temperature
700 ºC. Sweep rate = 100 mVs-1
In addition, as shown in this figure, the Pt porous catalyst film was
initially either clean and regenerate from Carbon deposited (by applying
Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
97
50 mA for 30 min, denoted in Figure 2.7 as “Deposited Carbon: No”), or
previously deactivated under TCD conditions under the same conditions
shown in Figure 2.4 for 90 min (denoted in Figure 2.7 as “Deposited
Carbon: Yes”).
The trend of the current-potential curves clearly demonstrated the
depolarization effect of both CH4 and the carbon supported on the Pt-YSZ
porous catalyst layer. Accordingly, the presence of these species allows to
perform the steam electrolysis process with lower energy input (lower
potential is required for the same current). This fact confirms the
electrochemical nature of the previous mentioned reactions (as shown in
Figure 2.5), showing that these reactions involve the O2- ions (versus O2
coming from the gas phase) as a result of the O2- evolution reaction (2 O2-
O2 + 4 e-). Moreover, it is worth to note that the depolarization effect of
carbon deposited on the Pt-YSZ porous catalyst layer is higher than that of
chemisorbed CH4, although the highest depolarization effect took place
when both of them were present on the catalyst surface. The use of carbon
containing molecules such as: CH4 [23], CO [24] or Carbon [25] as
depolarizating agents is of great interest for the electrolytic production of
H2 since it allows to strongly decrease the required electrical power input.
On the other hand, it is worth to note that the enhanced electrochemical
properties of the system could be attributed to an improved electrical
conductivity of the anode due to the presence of carbon. However, further
experiments should be carried out to clarify the nature of the improved
electrochemical properties of the system. Anyway, this set of experiments
clearly demonstrated that the novel mode of operation proposed during the
regeneration stage of the Pt-YSZ porous catalyst (previously discussed on
Chapter 2
98
0.0
0.4
0.8
1.2
1.6
2.0
0.00
0.15
0.30
0.45
0.60
0
1
2
3
4
5
0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5
0
5
10
15
20
25
30
35
(rH
2 ou
ter/
mol
·s-1
·cm
-2)
x108
(rC
O2ou
ter/
mol
·s-1
·cm
-2) x
108
(rC
O o
ute
r/m
ol·s
-1·c
m-2
) x1
08
0.0
0.3
0.6
0.9
1.2
1.5
1.8
(r C
2H6ou
ter/
mol
·s-1
·cm
-2)
x1010
(rC
2H4ou
ter/
mol
·s-1
·cm
-2)
x1010
0
2
4
6
8
10
12
d)
c)
b)
Cur
rent
/ m
A
Time / h
a)
-0.85
-0.84
-0.83
-0.82
-0.81
-0.80
-0.79
UW
C (V
)
Figure 2.5b) allows the production of pure H2 by a high efficient C and CH4
assisted steam electrolysis process.
Finally, in order to check the reproducibility and durability of the solid
electrolyte electrochemical cell, a medium term operation experiment of 24
h was carried out at 750 ºC (Figure 2.8).
Figure 2.8. Reproducibility experiment under step changes in the applied current
(from O.C.C. to 30 mA). Conditions: Outer chamber CH4 = 1 %, Inner chamber
H2O = 3 %, N2 balance in both cases, Temperature = 750 ºC.
Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
99
A complete number of 18 cycles, which included the Open Circuit TCD
step and the steam electrolysis for the regeneration of the Pt-YSZ catalyst,
were carried out. A reproducible behavior was observed in both catalytic-
electrocatalytic activity and obtained potential curves during open and
close circuit conditions. It demonstrates the stability of the inner Pt
electrode (exposed to the steam electrolysis reaction atmosphere) and the
Pt-YSZ porous catalyst layer (exposed to the CH4 atmosphere) as well as
the YSZ solid electrolyte for the proposed mode of operation under the
explored reaction conditions. Finally, an average potential under open
circuit conditions of -0.82 V was observed in Figure 2.8d. This potential
difference could allow operating the electrochemical cell as a Solid Oxide
Fuel Cell, leading therefore to the co-generation of energy and reaction
products. Hence, further studies could be performed in order to develop the
multiple applications given by this system.
2.4 Conclusions
The system proposed in this chapter showed two different techniques
for hydrogen production: the catalytic methane decomposition and the
electro-catalytic steam electrolysis process, which can be successfully
coupled in a double solid electrolyte membrane reactor.
The study carried out in this chapter allows to regenerate a catalyst
from carbon deposition by an assisted steam electrolysis process, which led
to the production of produce hydrogen with a lower energy requirement
demand. Therefore, this configuration and mode of operation allowed the
in-situ valorization of the produced carbon as a depolarizating agent in the
steam electrolysis process. Furthermore, the regeneration step yielded C2s
hydrocarbons by the oxidative coupling of methane reaction on the Pt-YSZ
porous catalyst.
Chapter 2
100
Finally, the performance and durability of the system was also verified
for long operation times in view of the possible practical application of this
novel reactor configuration.
2.5. References
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Simultaneous Production and Separation of H2 and C2 Hydrocarbons via Steam and Methane Partial Oxidation
101
[13] X. Dong, Z. Liu, Y. He, W. Jin, N. Xu, Journal of Membrane Science, 331 (2009)
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J.L. Valverde, F. Dorado, Applied Catalysis B: Environmental, 142–143 (2013) 298-
306.
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Energy, 28 (2003) 483-490.
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Katsaounis, J.L. Valverde, Chemical Physics Letters, 519–520 (2012) 89-92.
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2121-2128.
Chapter 2
102
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340.
3.1. Introduction
3.2. Methodology
3.3. Process description
3.3.1. Catalytic steam reforming of
ethanol-water process
3.3.2. Electrochemical reforming of
ethanol-water process
3.4. Process simulation and energetic
evaluation
3.5. Conclusions
3.6. References
CHAPTER 3:
Electrochemical Reforming vs. Catalytic
Reforming of Ethanol: A Process Energy
Analysis for Hydrogen Production
INTRODUCTION
Catalytic steam reforming of ethanol
PROCESS DESCRIPTION
RESULTS
Hydrogen can be currently produced by a number of processes such as natural gas or biogas reforming, gasification of coal and biomass, water electrolysis,
photoelectrolysys and biological process. Traditionally, large-scale production of hydrogen is mainly based on the methane reforming process, however, this pathway
has a non-renewable nature as methane is mainly obtained from natural gas. In this sense, there is a growing interest in the search for effective alternatives to
produce hydrogen from renewable sources. In this regard, ethanol is very attractive because of its relatively high hydrogen content, broad availability, non-toxicity,
secure storage and handling. In addition, it can be obtained from the fermentation of biomass
Electrochemical reforming of ethanol
Energy consumption
Hydrogen yield and energy consumption
-Electrical energy consumption and process energy (electrical and
thermal energy) consumption were computed in the electrochemical
reforming process. In this case, the main energy requirement was the
electrical energy one.
- The electrochemical reforming process seemed to be less intensive in
feed stock material and energy consumption for H2 production than the
process based on the catalytic conventional route. In addition, the
calculated energy consumption of the overall electrochemical reforming
process was lower than that required by a traditional water electrolyzer
stack.
-For the catalytic steam reforming process electrical and thermal energy
consumption was considered. The highest energy consumption corresponded
to the stream e-3, which provided the energy required to heat the outlet
stream from the heat exchanger network to the reforming reactor
temperature (T = 800 ºC).
Catalytic steam reforming of ethanol is and endothermic process that requires external heat input and other process of hydrogen purification such as WGS and
COPROX reaction:
C2H5OH + 3H2O 2CO2 + 6H2 (∆H = 173.1 kJ mol-1) A novel process based on ethanol electro-oxidation (electrochemical reforming of ethanol) has recently attracted great interest since it allows the simultaneous
production of pure hydrogen and accomplished its separation in a single step:
C2H5OH + xH2O Cderived products + yH+ + ye- 2H+ + 2e- H2
The aims of this study is to compare the energetic performance of two technologies for renewable H2 production via reforming of ethanol-water mixtures:
electrochemical and catalytic ones. The energetic analysis was carried out by simulating these processes with Aspen HYSYS (Aspen Tech V.7.1). Finally, a
comparative study of the two processes was performed in order to evaluate the yield and energy consumption in the production of hydrogen.
Water
P-01
Ethanol
P-02
C-01
C-03HE-01 HE-02 HE-03 HE-04 HE-05
C-02
C-04 C-05 C-06 C-07 C-08HE-06 H-01
C-10
SR
B-01
C-13 HWGS
C-14 C-15
LWGS
C-16
C-17
COPROX
C-20
C-21
HE-07
C-22
C-23 C-24
H-03
SP-01K-02
K-01C-18
e-1
e-2
e-3
e-4
e-8
e-7
e-5
WATER GAS SHIFT
C-26
C-25Hydrogen
C-09
C-11
C-12
C-27
C-30
C-29
C-28C-32
C-31
H-02
C-19
e-6
P-01
P-02
H-01
Ethanol
Water
C-02
C-01
C-03
C-11
SP-01
C-08
A C
HE-01
e1
e-2
e-3
e-6
e-4
e-5
PEM
CELL
Hydrogen
C-05C-04 C-06
C-07
C-10
P-03C-09
e-1 e-2 e-3 e-4 e-5 e-6 e-7 e-8
0
8
16
60
80
100
120
140
En
ergy
/ k
J m
ol-1 H
2
e-1 e-2 e-3 e-4 e-5 e-6
0.0
0.3
0.6
0.9
1.2
30
60
90
120
150
180
210
En
ergy
/ k
J m
ol-1 H
2
e-1 e-2 e-80.00
0.01
0.02
0.03
0.04
0.05
0.06
En
ergy
/ k
J m
ol-1 H
2
e-1 e-20.00
0.03
0.06
0.09
0.12
0.15
0.18
0.21
En
ergy
/ k
J m
ol-1
H2
0.00
0.01
0.02
0.03
0.04
Catalytic steam reforming Electrochemical reforming
Yie
ld /
kg
H2
kg
-1
C2H
5O
H
0
10
20
30
En
erg
y c
on
sum
pti
on
/ k
W h
kg
-1
H2
Catalytic steam reforming of ethanol
Electrochemical reforming of ethanol
CHAPTER 3. ELECTROCHEMICAL REFORMING VS.
CATALYTIC REFORMING OF ETHANOL: A PROCESS
ENERGY ANALYSIS FOR HYDROGEN PRODUCTION
107
Abstract
n the last years the process of electrochemical reforming of alcohols
performed in low temperature Proton Exchange Membrane (PEM)
reactors have shown to be an interesting way of producing pure
hydrogen in a single reaction step. In addition, this technology allows to
produce hydrogen with lower electrical energy requirements compare to
other related technologies such as water electrolysis. This chapter reports
an energetic analysis of the hydrogen production via catalytic steam and
electrochemical ethanol reforming processes. For both systems, a complete
flow diagram process was proposed and simulated by Aspen HYSYS
according to literature data. Besides hydrogen, other byproducts such as
acetaldehyde (electrochemical reforming) and ethylene and methane
(catalytic reforming) were also considered. The energy requirement of the
different process units was calculated according to the operating
parameters. Material balances revealed electrochemical reforming to
present higher hydrogen yields (0.0436 vs. 0.0304 kg H2 / kg C2H5OH of the
classical catalytic reforming). In addition to its higher simplicity,
simulation results showed a lower energy consumption in the H2 production
by the electrochemical approach (29.2 vs. 32.70 kWh per Kg of H2). These
results demonstrated the interest of the electrochemical reforming of ethanol
to obtain high purity hydrogen in a single reaction/separation step, thereby
representing an interesting alternative to classical catalytic reforming.
I
Chapter 3
108
3.1. Introduction
As previously mentioned in previous chapters of this memory, hydrogen
can be currently produced by a number of processes such as natural gas or
biogas reforming [1], gasification of coal and biomass [2, 3], water
electrolysis [4], photoelectrolysis [5] and biological processes [6].
Traditionally, large-scale production of hydrogen is mainly based on the
methane reforming process. However, this pathway has a non-renewable
nature as methane is mainly obtained from natural gas. Furthermore,
together with hydrogen, other carbon derived products such as carbon
monoxide and carbon dioxide are formed as side products. In this sense,
there is a growing interest in the search for effective alternatives to
produce hydrogen from renewable sources. In this regard, ethanol is very
attractive because of its relatively high hydrogen content, broad
availability, non-toxicity, secure storage and handling. In addition, it can
be obtained from the fermentation of biomass [7].
The catalytic steam reforming of ethanol is an endothermic process
that requires external heat input which could be supplied from external
sources in order to maintain the system at a steady reaction temperature.
C2H5OH + 3H2O 2CO2 + 6H2 (∆H = 173.1 kJ mol-1) (3.1)
Heat can also be supplied externally by the combustion of part of the feed,
by burning combustible off gases or by a combination of both processes.
Additionally, the ethanol-steam mixture is catalytically converted to
carbon monoxide according to the following reaction:
C2H5OH + H2O 2CO + 4H2 (∆H = 298.5 kJ mol-1) (3.2)
Electrochemical reforming vs. Catalytic reforming of ethanol
109
Hydrogen production via catalytic reforming of ethanol involves two
more additional steps aimed at reducing the concentration of CO below the
stringent levels required by hydrogen fuel cells operating downstream: the
water gas shift reaction (WGS, reaction 3.3) and CO preferential oxidation
reaction (COPROX, reaction 3.4 and reaction 3.5). WGS (reaction 3.3) is an
important step in which CO is generally oxidized to CO2 in excess steam:
CO + H2O ↔ CO2 + H2 (∆H = -41 kJ mol-1) (3.3)
In the COPROX process, the following reactions occurs in the gas
phase:
CO + ½ O2 CO2 (∆H = -238 kJ mol-1) (3.4)
H2 + ½ O2 H2O (∆H = -242 kJ mol-1) (3.5)
The final step is the purification of the hydrogen from the gas stream
exiting the COPROX process, which can be accomplished using several
techniques. The most common methods are: pressure-swing-adsorption
(PSA) to separate CO2 followed by the condensation of the remaining H2O,
PSA to separate H2, and membrane separation of H2 [8]. Consequently,
production of high purity hydrogen by catalytic ethanol reforming is a
complicated process including several reaction and separation steps. A
novel process based on ethanol electrooxidation (also called electrochemical
reforming of ethanol) has recently attracted great interest since it allows
the simultaneous production of pure hydrogen and accomplished its
separation in a single step. In addition, the separation in a single-step of
the reaction product from the reaction chamber shifts the reaction
equilibrium to the right and thus, higher yields are achieved. This process
is based on a low temperature Proton Exchange Membrane (PEM) reactor
Chapter 3
110
configuration, consisting on a membrane electrode assembly (MEA) formed
by an Anode/membrane/Cathode, which allows simultaneous production
and separation of hydrogen. Consequently, this technology allows to
produce pure hydrogen in a quick and convenient way. In addition, this
process can be used to store electrical energy via H2 production [9]. Recent
studies have shown very promising results when the electrolysis of water-
alcohols mixtures proceeds. Thus, methanol [10-13], ethanol and bio-
ethanol [9, 14], glycerol [15, 16] and ethylene glycol [17] have been
successfully tested at lab scale. When compared to water electrolysis, the
electrochemical reforming of organics can be carried out at significantly
lower voltages, thus leading to a reduction of electricity consumptions. In
the electrochemical reforming process, an ethanol-water mixture is
supplied to the anode cell. The electro-oxidation of ethanol is carried out by
applying an electrical power, turning into the production of protons on the
anode catalyst-electrode according to the following reaction:
C2H5OH + xH2O Cderived products + yH+ + ye- (3.6)
The produced protons are selectively transported through the PEM
membrane to the cathode compartment, which leads to the production of
hydrogen:
2H+ + 2e- H2 (3.7)
In the previous chapter (Chapter 2) it was studied the simultaneous
production/separation of H2 and C2 hydrocarbons in a double chamber
SOEC configuration. It allowed to obtain pure hydrogen in the inner
chamber by the water electrolysis at the high reaction temperatures
necessary for the oxidative coupling reaction in the outer chamber. Then,
following the topic of the pure hydrogen production, the aims of this
Electrochemical reforming vs. Catalytic reforming of ethanol
111
chapter is to compare the energetic performance of two technologies for
renewable hydrogen production via reforming of ethanol-water mixtures:
electrochemical and catalytic ones. The performance of both processes was
evaluated by material and energy balances performed using conditions
reported in the literature. The energetic analysis was carried out by
simulating these processes with Aspen HYSYS (Aspen Tech V.7.1).
Finally, a comparative study of the two processes was performed in order
to evaluate the yield and energy consumption in the production of
hydrogen.
3.2. Methodology
The simulation of the catalytic steam reforming and electrochemical
reforming of ethanol-water mixtures processes was performed under
stationary conditions using the flowsheeting simulator Aspen HYSYS
(AspenTech V.7.1). Peng-Robinson equation was used to calculate the
thermodynamic properties of each flow stream. This equation of state is
widely used in reforming processes of ethanol and thus, it was used in this
chapter for comparison purposes [17, 18]. The component list was
restricted to C2H5OH, H2O, H2, CO, C2H4O, CO2, CH4, O2, N2 and C2H4 for
the catalytic steam reforming [21-24] and to C2H5OH, H2O, H2 and C2H4O
for the electrochemical reforming as experimentally confirmed [16]. The
reaction conditions for the catalytic steam reforming of ethanol were taken
from literature data for a Pt/Al2O3 catalyst (1 wt.% metal loading) [19]. In
the case of the electrochemical reforming of ethanol, catalysts based on Pt-
Ru (40 wt.% Pt-20 wt.% Ru) and Pt (20 wt.% Pt), both supported on carbon,
were used. The metal loading was of 1.5 mg·cm-2and 0.5 mg·cm-2 for the Pt-
Ru and Pt catalysts, respectively. As reported in literature, these metals
have been typically used as the anode (Pt-Ru) and cathode (Pt) electrodes
Chapter 3
112
in electrochemical reforming reactors [19]. The operating conditions and
the polarization curves for the process simulation were also taken from
literature [9, 17, 19-22].
The following assumptions were considered:
- Air, water and ethanol were injected into the process at room conditions
(T = 25 ºC and P = 1 atm).
- The composition of atmospheric air was fixed at 21 % O2 and 79 % N2.
- 10 % of excess air was used in the combustion furnace to heat the
reforming reactor unit.
- 10 % of excess air was used in the COPROX reactor unit for CO
oxidation.
- The adiabatic efficiency of pumps and compressor was considered to be
80 %.
- An average pressure drop of 9.807 kPa was considered for the heat
exchangers, heaters and splitter.
- The reforming reactor and PEM reactor units were simulated as a
conversion reactor using reported data.
- WGS and COPROX reactor were simulated as equilibrium reactors.
3.3. Process description
3.3.1. Catalytic steam reforming of ethanol-water process
Figure 3.1 shows the flow diagram of the catalytic steam reforming
process. The hydrogen production process began by pumping liquid water
(stream Water) and liquid ethanol (stream Ethanol). Both feedstocks were
separately fed at reference environmental conditions, i.e. 25 ºC and 1 atm
with a water/ethanol molar ratio= 6/1.
Electrochemical reforming vs. Catalytic reforming of ethanol
113
Wa
ter
P-0
1
Eth
an
ol
P-0
2
C-0
1
C-0
3H
E-0
1H
E-0
2H
E-0
3H
E-0
4H
E-0
5
C-0
2
C-0
4C
-05
C-0
6C
-07
C-0
8H
E-0
6H
-01
C-1
0
SR
B-0
1
C-1
3H
WG
S
C-1
4C
-15
LW
GS
C-1
6
C-1
7
CO
PR
OX
C-2
0
C-2
1
HE
-07
C-2
2
C-2
3C
-24
H-0
3
SP
-01
K-0
2
K-0
1C
-18
e-1
e-2
e-3
e-4
e-8
e-7
e-5
WA
TE
R G
AS
SH
IFT
C-2
6
C-2
5H
yd
rogen
C-0
9
C-1
1
C-1
2
C-2
7
C-3
0
C-2
9
C-2
8C
-32
C-3
1
H-0
2
C-1
9
e-6
Fig
ure
3.1
. F
low
dia
gra
m o
f th
e c
ata
lyti
c ste
am
refo
rmin
g p
roce
ss o
f eth
an
ol.
Chapter 3
114
The streams were later pumped by a centrifugal adiabatic pump to a
pressure of 2.5 atm and subsequently mixed (stream C-03). This pressure
was high enough to deal with the successive pressure drops originated in
downstream units. The water/ethanol mixture was then heated (stream C-
10) before entering the reformer reactor. The required heat was provided
by a heat exchanger network (from HE-01 to HE-06). The final
temperature of this stream (T = 800 ºC) was finally reached with heater H-
01 [22]. The hydrogen production in the ethanol reformer process was
carried out at T = 800 ºC and P = 1.8 atm with a molar flow of 736.6 mol h-1
for water and 122.7 mol h-1 for ethanol. The global conversion of ethanol in
this process was fixed at 60 % [19]. This conversion was the sum of all the
reactions taking place in the catalytic reforming reactor. The reformer
reactor (SR) was equipped with a furnace which provides the required heat
to perform the endothermic reforming reaction. 30 % of the outlet stream
coming from the reformer reactor was subsequently fed to the combustor
(B-01), which was modeled as an equilibrium reactor. This value was
optimized according to the energy consumption through different
simulations (not shown here). The minimum amount of supplied energy for
H2 production was 32.70 kWh per kg of H2. Then, stream C-27 was burnt
with 10 % of excess of air (Stream C-28) [23]. Next, the stream coming
from the reformer, which contained mainly H2 and CO, was conducted to
the water-gas shift (WGS) reactor, where a large amount of CO is
converted to CO2 and H2 (reaction 3.3). Water gas shift reaction is
considered to be reversible. At low temperatures, the reaction equilibrium
shifts to the right and favors the formation of H2 and CO2. On the other
hand, at high temperatures, the equilibrium shifts to the left, limiting the
complete conversion of CO [24]. As usual, this reaction was carried out in
two steps. This way, a high temperature shift reactor (HTS), which
Electrochemical reforming vs. Catalytic reforming of ethanol
115
operates at 400 ºC, and a low temperature shift reactor (LTS), which
operates at 250 ºC, were considered [20]. The remaining CO coming from
the latter reactor was oxidized with 10 % excess of air [20] (reaction 3.4) in
a COPROX reactor unit at 170 ºC. As expected, hydrogen was also oxidized
(reaction 3.5). The lack of inlet air in the COPROX reactor (operating at
170 ºC [21]) produce an incomplete conversion of CO, i.e. the CO
concentration did not reach the specified value (20 ppm). On the contrary,
an excess of air can produce an excessive oxidation of hydrogen, leading to
a decrease of its yield [25, 26]. In addition, an excess of air increases the
electric energy consumption in the compressor [23]. The temperature of the
reactor clearly influenced the performance of both reactions. A strong
temperature increase due to the high exothermicity of both reactions
involves a loss of selectivity towards CO oxidation since its activation
energy is lower than that of H2 oxidation.
In addition, low reaction temperatures reduce the possibility of
formation of NOx (produced from the reaction of nitrogen and oxygen gases
in the air during combustion).
Finally, the hydrogen was separated by using a membrane process (SP-
01) at 300 ºC [8], which was modeled as a black box (split unit) with a
prescribed membrane effectiveness releasing a pure hydrogen stream
(Stream C-30). The membrane effectiveness was defined as the molar
percentage of the inlet hydrogen that was transferred to the product
stream [8]. In practice, a real membrane should be capable of producing
hydrogen with purity higher than 99% (Stream Hydrogen). The different
material and process energy are shown in Table 3.1.
Chapter 3
116
Table 3.1. Description of the material and process energy for the catalytic
steam reforming of ethanol-water mixtures.
Material
streams Description
Water Water input stream (room conditions)
Ethanol Ethanol input stream (room conditions)
C-01 Water stream pressurized at 2.5 atm
C-02 Ethanol stream pressurized at 2.5 atm
C-03 – C-08 Water/ethanol mixture input at heat exchanger network (HE-01 to HE-06)
C-09 Pre-heated water/ethanol mixture
C-10 Feed stream to the reforming reactor (SR)
C-11 Output stream to the reforming reactor (SR)
C-12 70 % outlet stream from the reformer reactor to heat exchanger (HE-02)
C-13 Feed stream to the high temperature water gas shift reactor (HWGS)
C-14 Output stream to the high temperature water gas shift reactor (HWGS)
C-15 Feed stream to the low temperature water gas shift reactor (LWGS)
C-16 Output stream to the low temperature water gas shift reactor (LWGS)
C-17 Feed stream to the CO preferential oxidation reactor (COPROX)
C-18 Air input stream (room conditions)
C-19 Air stream pressurized at 1.5 atm
C-20 Air input steam to the CO preferential oxidation reactor (COPROX)
C-21 Output stream from CO preferential oxidation reactor (COPROX)
C-22 Output stream from heat exchanger (HE-01) to heat exchanger (HE-07)
C-23 Output stream from heat exchanger (HE-07) to heater (H-03)
C-24 Input stream from the membrane process (SP-01)
C-25 Output stream from the membrane process (SP-01) to heat exchanger (HE-07)
C-26 Cool output stream from heat exchanger (HE-07)
C-27 30 % outlet stream from the reforming reactor to burner (B-01)
C-28 Air input stream (room conditions)
C-29 Air stream pressurized at 1.8
C-30 Output stream from the burner (B-01)
C-31 Cool output stream from heat exchanger (HE-06)
C-32 Hydrogen stream input at heat exchanger (HE-04)
Hydrogen Pure hydrogen outlet
Process
energy Description
e-01 Pump (P-01) electrical stream
e-02 Pump (P-02) electrical stream
e-03 Energy required to heat stream C-09
e-04 Reforming reactor energy stream
e-05 Compressor (K-01) electrical stream
e-06 CO preferential oxidation reactor (COPROX) energy stream
e-07 Energy required to heat stream C-22
e-08 Membrane process (SP-01) energy stream
Electrochemical reforming vs. Catalytic reforming of ethanol
117
P-0
1
P-0
2
H-0
1
Eth
an
ol
Wa
ter
C-0
2
C-0
1
C-0
3
C-1
1
SP
-01
C-0
8
A C
HE
-01
e1
e-2
e-3
e-6
e-4
e-5
PE
M
CE
LLH
yd
rogen
C-0
5C
-04
C-0
6
C-0
7
C-1
0 P-0
3C
-09
3.3.2. Electrochemical reforming of ethanol-water process
For the hydrogen production via electrochemical reforming of ethanol,
an integrated process was proposed according to a previous work focused
on the electrochemical reforming of ethylene glycol [17] (Figure 3.2).
Fig
ure
3.2
. . F
low
dia
gra
m o
f th
e e
lect
roch
em
ical
refo
rmin
g p
roce
ss o
f eth
an
ol.
Chapter 3
118
Aspen HYSYS cannot directly simulate an electrochemical reforming
PEM reactor unit but it could be modeled as a sequence of a conversion
reactor and a split separation unit, where the produced hydrogen was
completely separated from the reaction mixture. Table 3.2 summarizes the
description of the different material and process energy included in the
process.
Table 3.2. Description of the material and process energy for the
electrochemical reforming of ethanol-water mixtures.
The operating conditions of this process were optimized in a previous
work devoted to the electrochemical reforming of ethylene glycol [17].
Water and ethanol were mixed at 2 atm to form a 1 ml/min flow with a
concentration 6 M ethanol. Then, the mixture was preheated by a heat
exchanger (HE-01) using the sub-product stream C-10. This stream
Material streams Description
Ethanol Ethanol input (room conditions)
Water Water input (room conditions)
C-01 Ethanol stream pressurized at 2 atm
C-02 Water stream pressurized at 2 atm
C-03 Water/ethanol mixture input at heat exchanger (HE-01)
C-04 Pre-heated water/ethanol mixture
C-05 Water/ethanol mixture feed stream
C-06 PEM cell input stream
C-07 Liquid PEM cell output stream
C-08 Recirculation water/ethanol stream
C-09 Stream C-08 (adjusted pressure)
C-10 Vapor stream required to heat stream C-03
C-11 Cool output from heat exchanger (HE-01)
Hydrogen Pure hydrogen PEM cell output
Process energy Description
e-01 Pump (P-01) electrical energy
e-02 Pump (P-02) electrical energy
e-03 Energy required to heat stream C-05
e-04 Electrical energy applied to the system
e-05 Splitter energy stream
e-06 Pump (P-03) electrical energy
Electrochemical reforming vs. Catalytic reforming of ethanol
119
contained water and acetaldehyde produced by the electrochemical
reforming of ethanol according to the following overall reaction:
C2H5OH C2H4O + H2 (3. 8)
The resulting stream (C-04) was then mixed with the recycle stream
which was rich in ethanol (C-09). The stream (C-05) was heated to 90 ºC
(operating temperature of the PEM electrochemical reforming unit). The
electrochemical reforming of ethanol-water mixtures occurred at the anode
of the PEM (A), leading to the production of acetaldehyde (3.8) (C-07) and
pure hydrogen in the cathode (C) (3.7). Stream C-07 also contained an
important amount of unreacted ethanol, due to the low efficiency of the
process. Consequently, this stream (C-07) was separated in a splitter unit
(S-01) and then pumped and recycled (C-09) to the PEM electrochemical
reforming cell. The splitter unit (S-01) allowed to calculate, in a theoretical
way, the energy required for the separation of the unreacted ethanol.
3.4. Process simulation and energetic evaluation
Table 3.3 illustrates the main operation parameters used in the
simulation of both processes [9, 17, 19-22]. Firstly, the temperature of the
reformer reactor was established from a previous study of the catalytic
ethanol steam reforming. According to Khila et al. [20], the highest
hydrogen concentration was obtained at a temperature of 800 ºC.
Temperatures beyond 800 ºC would increase the heat required, thus
reducing the process efficiency. Thus, a steam reforming reactor
temperature of 800 ºC was selected. According to Liguras et al. [19], the
overall ethanol conversion for a reforming process carried out at 800 ºC
with a Pt catalyst is 60% under the explored reaction conditions.
Chapter 3
120
Ta
ble
3
.3.
Base
-ca
se e
xp
eri
men
tal
para
mete
rs f
or
cata
lyti
c st
eam
refo
rmin
g a
nd
ele
ctro
chem
ical
refo
rmin
g
Ca
taly
tic s
tea
m r
efo
rm
ing
pro
ce
ss
Refe
ren
ce
[21
, 2
4]
[22
]
[22
]
[23
]
Ele
ctr
och
em
ica
l refo
rm
ing
pro
cess
Refe
ren
ce
[11
,19
]
T /
ºC
80
0
40
0
25
0
17
0
T /
ºC
90
Ca
taly
st
Pt
Eq
uil
ibri
um
rea
ctor
Eq
uil
ibri
um
rea
ctor
Eq
uil
ibri
um
rea
ctor
Ca
taly
st
An
od
e P
t-R
u/C
Ca
thod
e P
t/C
Sele
ctiv
ity
/ %
14
28
15
13
30
Con
vers
ion
/
%
8.4
0
16
.80
9.0
0
7.8
0
18
.00
Con
vers
ion
/
%
2.0
5
Rea
ctio
n
C2H
5O
H +
3H
2O
→ 2
CO
2 +
6H
2
(3
.1)
C2H
5O
H +
H
2O
→ 2
CO
+
4H
2
(3
.2)
C2H
5O
H +
2H
2 →
2C
H4
+ H
2 O
(3.9
)
C2H
5O
H
→ C
2H
4 +
H
2 O
(3.1
0)
C2H
5O
H
→ C
2H
4O
+
H2
(3
.8)
CO
+ H
2O
→
CO
2 +
H
2
(
3.3
)
CO
+ H
2O
→
CO
2 +
H
2
(
3.3
)
CO
+ ½
O2 →
CO
2
(3
.4)
H2 +
½ O
2 →
H2O
(3
.5)
Rea
ctio
n
C2H
5O
H
→ C
2H
4O
+
H2
(3
.8)
Un
it
Ste
am
refo
rmin
g
rea
ctor
(SR
)
Hig
h t
em
pera
ture
wa
ter
gas s
hif
t
rea
ctor
(HW
GS
)
Low
tem
pera
ture
wa
ter
gas s
hif
t
rea
ctor
(LW
GS
)
CO
pre
fere
nti
al
oxid
ati
on
rea
ctor
(CO
PR
OX
)
Un
it
PE
M c
ell
Electrochemical reforming vs. Catalytic reforming of ethanol
121
In this chapter, the following side reactions in the reforming reactor
were also considered:
C2H5OH + H2O 2CO + 4H2 (3.2)
C2H5OH + 2H2 2CH4 + H2O (3.9)
C2H5OH C2H4 + H2O (3.10)
C2H5OH C2H4O + H2 (3.8)
The conversion of each reaction were established from the selectivity
values reported by Liguras et al. [19] (Table 3.3). 80 % of the CO was
converted in the high temperature water gas shift reactor (HTS) whereas
12 % of the remaining CO was converted in the lower temperature water
gas shift one (LTS). The gas leaving the WGS reactors contained about 1 %
of CO due to thermodynamics limitations [20]. Therefore, a second step for
removing CO was necessary. In the COPROX reactor, the remaining CO
was oxidized to CO2 at 170 ºC with a 10 % excess air (reaction 3.4). Giunta
el al., [21] analyzed different temperature control schemes for the
COPROX reaction and found that the most suitable temperature in terms
of selectivity towards CO was 170 ºC. However, hydrogen is partially
oxidized at these conditions (reaction 3.5) [23]. Reaction 3.5 was
thermodynamically favored because the hydrogen concentration in the
mixture was clearly higher than that of CO. For this reason, the catalyst
used should be highly selective to the total oxidation of CO (reaction 3.4).
The product stream was mainly composed by hydrogen, containing less
than 20 ppm of CO. The high purity of this stream made it suitable to be
used in PEM fuel cells [21]. The final step is the separation of the
hydrogen from the gas stream exiting the COPROX reactor, which is
mostly constituted by H2, H2O, C2H5O, CO2 and air, which can be
accomplished through a membrane. Membrane processes for hydrogen
Chapter 3
122
separation is a promising technology that is receiving high attention in the
last years, producing a high purity hydrogen stream (>99 %). The
membrane operated at 300 ºC and pure hydrogen was obtained at
atmospheric pressure [8]. Table 5.4 summarizes the values of the main
variables of the catalytic steam reforming process. It can be observed that
the hydrogen production rate was 85.09 mol h-1; i.e., 0.0236 mol s-1.
Additionally, several byproducts with industrial applications such as
ethylene, acetaldehyde and methane were also obtained. Acetaldehyde is
mainly used as a starting material in the synthesis of acetic acid, n-butyl
alcohol, ethyl acetate, and other chemical compounds. Ethylene is the raw
material used in the manufacture of polymers and methane can be used as
fuel and raw material for synthesizing other compounds.
As abovementioned, the operating conditions of the electrochemical
reforming unit were obtained from a previous work [9, 17]. The ethanol-
water mixture (6 M ethanol concentration) was fed at 90 ºC with a flow
rate of 1 ml min-1 (stream C-06). The electrical energy applied to the PEM
cell (e4) was set from the current-potential curves (current = 0.4 A and Vcell
= 0.9 V) [9, 17]. These parameters were directly related to the hydrogen
produced in the cathodic side of the PEM cell by the Faraday law, i.e. rH2 =
I/nF, where rH2 is the hydrogen production rate (mol s-1), n is the number of
transferred electrons, and F is the Faraday constant. This theoretical
hydrogen production rate value was experimentally confirmed by de
Lucas-Consuegra et al. [17]. As described by Caravaca et al. [14],
acetaldehyde is one of main byproducts from the electrochemical reforming
of ethanol. For this reason, acetaldehyde was assumed to be the unique by-
product of the reaction (reaction 3.8).
Electrochemical reforming vs. Catalytic reforming of ethanol
123
Table 3.4. Molar and energy balances of the catalytic steam reforming process.
F / mol h-1
Stream T
/ ºC
P
/ atm H2O C2H5OH H2 CO2 CH4 CO C2H4O C2H4 Air
Water 25 1 736.6 - - - - - - - -
Ethanol 25 1 - 122.7 - - - - - - -
C-01 25 2.5 736.6 - - - - - - - -
C-02 25 2.5 - 122.7 - - - - - - -
C-03 25 2.5 736.6 122.7 - - - - - - -
C-04 118 2.4 736.6 122.7 - - - - - - -
C-05 120 2.3 736.6 122.7 - - - - - - -
C-06 153 2.2 736.6 122.7 - - - - - - -
C-07 162 2.1 736.6 122.7 - - - - - - -
C-08 219 2.0 736.6 122.7 - - - - - - -
C-09 553 1.9 736.6 122.7 - - - - - - -
C-10 800 1.8 736.6 122.7 - - - - - - -
C-11 800 1.8 720.2 49.1 120.3 22.6 29.7 19.4 25.5 12.2 -
C-12 800 1.8 504.1 34.4 71.6 15.8 20.8 13.6 17.9 8.6 -
C-13 400 1.7 504.1 34.4 71.6 15.8 20.8 13.6 17.9 8.6 -
C-14 400 1.7 491.0 34.4 84.8 29 20.8 0.4 17.9 8.6 -
C-15 250 1.6 491.0 34.4 84.8 29 20.8 0.4 17.9 8.6 -
C-16 250 1.6 490.6 34.4 85.1 29.3 20.8 0.1 17.9 8.6 -
C-17 170 1.5 490.6 34.4 85.1 29.3 20.8 0.1 17.9 8.6 -
C-18 25 1 - - - - - - - - 502.6
C-19 73 1.5 - - - - - - - - 502.6
C-20 170 1.5 - - - - - - - - 502.6
C-21 170 1.5 490.7 34.4 85.1 29.4 20.8 - 17.9 8.6 502.6
C-22 45 1.4 490.7 34.4 85.1 29.4 20.8 - 17.9 8.6 502.6
C-23 257 1.3 490.7 34.4 85.1 29.4 20.8 - 17.9 8.6 502.6
C-24 300 1.2 490.7 34.4 85.1 29.4 20.8 - 17.9 8.6 502.6
C-25 300 1 490.7 34.4 - 29.4 20.8 - 17.9 8.6 502.6
C-26 45 1 490.7 34.4 - 29.4 20.8 - 17.9 8.6 502.6
C-27 800 1.8 216.1 14.7 30.7 6.8 8.9 5.8 7.7 3.7 -
C-28 25 1 - - - - - - - - 215.3
C-29 98 1.8 - - - - - - - - 215.3
C-30 906 1.8 233.9 14.7 30.7 15.7 - 5.8 7.7 3.7 197.5
C-31 230 1.7 233.9 14.7 30.7 15.7 - 5.8 7.7 3.7 197.5
C-32 300 1.1 - - 85.1 - - - - - -
Hydrogen 163 1 - - 85.1 - - - - - -
Table 5.5 shows the value of the overall molar balances of the
electrochemical reforming unit. Under these conditions the ethanol
Chapter 3
124
conversion obtained in the simulation process was 2.05 % for a hydrogen
production rate of 7.462·10-3 mol h-1 i.e. 2.072·10-6 mol s-1.
Table 3.5. Molar and energy balances of the simulation of the electrochemical
reforming process.
F / mol h-1
Stream T / ºC P / atm H2O C2H5OH H2 C2H4O
Water 25 1.0 0.648 - - -
Ethanol 25 1.0 - 0.007 - -
C-01 25 2.0 - 0.007 - -
C-02 25 2.0 0.648 - - -
C-03 25 2.0 0.648 0.007 - -
C-04 84 1.8 0.648 0.007 - -
C-05 88 1.8 2.160 0.363 - -
C-06 90 1.6 2.160 0.363 - -
C-07 90 1.3 2.160 0.355 - 0.007
C-08 90 1.3 1.512 0.355 - -
C-09 90 1.8 1.512 0.355 - -
C-10 90 1.3 0.648 - - 0.007
C-11 30 1.1 0.648 - - 0.007
Hydrogen 90 1 - - 0.007 -
Figure 3.3 summarizes the value of the overall energy consumption
per mol of hydrogen produced in the different units of the process of steam
reforming (a) and electrochemical reforming (b). It can be observed that
the highest energy input for the catalytic steam reforming was the energy
stream e-3 (unit H-01), which provided the additional energy to heat the
inlet stream (C-03) at the reforming reactor temperature (T = 800 ºC).
Energy stream e-4 accounted for the second highest energy consumption in
the process, which was used for providing the necessary energy to carry
out the endothermic reforming reaction. This result is in agreement with a
previous work of Tippawan et al., [27].
Electrochemical reforming vs. Catalytic reforming of ethanol
125
e-1 e-2 e-3 e-4 e-5 e-6 e-7 e-8
0
8
16
60
80
100
120
140
En
erg
y /
kJ
mol-1
H2
a)
b)
e-1 e-2 e-3 e-4 e-5 e-6
0.0
0.3
0.6
0.9
1.2
30
60
90
120
150
180
210
e-1 e-2 e-80.00
0.01
0.02
0.03
0.04
0.05
0.06
Ener
gy / k
J m
ol-1 H
2
En
erg
y /
kJ
mol-1
H2
e-1 e-20.00
0.03
0.06
0.09
0.12
0.15
0.18
0.21
En
erg
y / k
J m
ol-
1 H
2
Figure 3.3. Energy consumption of the different units of the (a) catalytic
steam reforming and (b) electrochemical reforming of ethanol-water mixtures.
Chapter 3
126
On the other hand, the lower energy consumption corresponded to the
energy required for pumping the feed streams (e-01 and e-02). In the
electrochemical reforming process, two kinds of energy streams can be
distinguished: the process energy (e-1, e-2, e-3, e-5, e-6), which includes all
the energy required in the process except the electrical energy needed for
the electrochemical reforming reaction and electrical ones (e-4). In this
case, the highest energy input corresponded to the electrical energy
consumption in the PEM cell (e-4 = 173.6 kJ mol-1 H2). This energy
consumption was similar to that recently reported (116.3 kJ mol-1 H2) for
the electrochemical reforming of ethylene glycol [17]. Both values were
obtained from experimental potential and current values, i.e. P = V·I (W).
This electrical energy value is more than twice the thermal energy
required for heating stream C-05 (e-3) and several orders of magnitude
higher than those required for the pumping of streams and the separation
of the unreacted ethanol.
Figure 3.4 displays for each process a comparison between the amount
of produced H2 in kg of hydrogen per kg of ethanol and the energy balance
in kW h per kg of hydrogen. For comparative purposes, the yield of
hydrogen production was referred to the amount of consumed ethanol
(Figure 3.4a). Regarding the catalytic steam reforming of ethanol, the
value obtained in this chapter (0.0301 kg H2 kg-1 C2H5OH) was similar to
that reported by Khila et al., [20] (0.0347 kg H2 kg-1 C2H5OH). Regarding
the case of the electrochemical reforming of ethanol, the obtained value
(0.0434 kg H2 kg-1 C2H5OH) was in the same range than those obtained by
de Lucas-Consuegra et al., [17] for the case of the electrochemical
reforming of ethylene glycol (0.0598 kg H2 kg-1 C2H6O2). From the feed
stock consumption point of view, it was observed that the electrochemical
Electrochemical reforming vs. Catalytic reforming of ethanol
127
reforming process was slightly more productive that the catalytic one. On
the other hand, the overall energy consumption for both processes was also
calculated (Figure 3.4b). It can be seen that the electrochemical reforming
process consumed less energy per kg of hydrogen produced, (29.2 kWh/kg
of H2) than the catalytic one (32.70 kWh/kg of H2). The higher energy
efficiency of the electrochemical reforming process for H2 production, if
compared to that of the catalytic one, can be related to the simplicity of
this process, which also operates at higher temperatures and pressures
and does not require of further purification and separation steps (e.g.
water gas shift reactors, COPROX reactor, etc). It should be mentioned
that a lower consumption energy value (12.18 kWh per kg of H2) have been
previously reported for the ethanol steam reforming process [22]. In the
present chapter, experimental catalytic results were used in the
simulation of the reforming unit, in contrast with reference [22], where the
authors assumed that the reforming reactor operated at equilibrium
conditions. Concerning the electrochemical reforming process, a value of
17.14 kWh per kg of H2 was recently reported for the electrochemical
reforming of ethylene glycol [17]. On the other hand, a value of 21.53 kWh
per kg of H2 was also reported for a methanol electrolyzer stack [28] where
the electrical energy was the only requirement considered. Unlike this
work where the electrical energy requirement (29.2 kWh/kg of H2) was set
from the deactivated current-potential curves, i.e. when the system
reaches the steady state value, references [17] and [28] set this
requirement from fresh MEA. In addition, it should be mentioned that the
electrical energy consumption value found here is lower than the amount
of energy required by a commercial water electrolyzer stacks, which are in
the range of 53.4 – 70.1 kWh per kg of H2 produced [24, 29].
Chapter 3
128
0.00
0.01
0.02
0.03
0.04
b)
Catalytic steam reforming Electrochemical reforming
Yie
ld /
kg
H2
kg
-1
C2H
5O
H
a)
0
10
20
30
En
erg
y c
on
sum
pti
on
/ k
W h
kg
-1
H2
Figure 3.4. Comparison of (a) H2 yield and (b) energy consumption in
catalytic steam reforming and electrochemical reforming processes.
In this sense, the US Department of Energy (DOE) has pointed out
that the electrical energy input to the electrolyzer stack should drop to 43
kWh per kg of H2 in 2020 [30]. Even considering the DOE output energy
for ethanol of 7.4 kWh per kg of H2 [31], the value of the required total
energy for the electrochemical reforming of ethanol make this technology
to be competitive. The use of ethanol is certainly promising due to its
possible renewable production (fermentation of biomass or steam
reforming of cellulosic materials) with reasonably low energy cost [32].
Ethanol allows electrolysis at potential < 1 V if compared to that of water
(1.23 V), leading to electrical power savings [14, 32].
Apart that the electrochemical reforming process seems to be more simple
and competitive that the most widely studied catalytic steam reforming,
the former process allows to produce H2 of high purity quickly and
Electrochemical reforming vs. Catalytic reforming of ethanol
129
conveniently (single reaction and a separation step at mild operating
conditions).
3.5. Conclusions
The following conclusions could be drawn from this chapter:
The electrochemical reforming process allowed to obtain pure hydrogen
in a single step while the catalytic process required additional steps of
purification and separation leading to a more complicated H2 production
plant.
For the catalytic steam reforming process electrical and thermal energy
consumption was considered. The highest energy consumption
corresponded to the stream e-3, which provided the energy required to heat
the outlet stream from the heat exchanger network to the reforming
reactor temperature (T = 800 ºC).
Electrical energy consumption and process energy (electrical and
thermal energy) consumption were computed in the electrochemical
reforming process. In this case, the main energy requirement was the
electrical energy one.
The electrochemical reforming process seemed to be less intensive in
feed stock material and energy consumption for H2 production than the
process based on the catalytic conventional route. In addition, the
calculated energy consumption of the overall electrochemical reforming
process was lower than that required by a traditional water electrolyzer
stack.
Chapter 3
130
3.6. References
[1] G. Iaquaniello, F. Giacobbe, B. Morico, S. Cosenza, A. Farace, International
Journal of Hydrogen Energy, 33 (2008) 6595-6601.
[2] F. Li, L. Zeng, L.-S. Fan, Industrial & Engineering Chemistry Research, 49 (2010)
11018-11028.
[3] R.C. Saxena, D. Seal, S. Kumar, H.B. Goyal, Renewable and Sustainable Energy
Reviews, 12 (2008) 1909-1927.
[4] W. Doenitz, R. Schmidberger, E. Steinheil, R. Streicher, International Journal of
Hydrogen Energy, 5 (1980) 55-63.
[5] Q. Huang, Q. Li, X. Xiao, Journal of Physical Chemistry C, 118 (2014) 2306-
2311.
[6] D. Das, T.N. Veziroǧlu, International Journal of Hydrogen Energy, 26 (2001) 13-
28.
[7] M. Ni, D.Y.C. Leung, M.K.H. Leung, International Journal of Hydrogen Energy,
32 (2007) 3238-3247.
[8] A.P. Simpson, A.E. Lutz, International Journal of Hydrogen Energy, 32 (2007)
4811-4820.
[9] A. Caravaca, A. De Lucas-Consuegra, A.B. Calcerrada, J. Lobato, J.L. Valverde,
F. Dorado, Applied Catalysis B: Environmental, 134-135 (2013) 302-309.
[10] G. Sasikumar, A. Muthumeenal, S.S. Pethaiah, N. Nachiappan, R. Balaji,
International Journal of Hydrogen Energy, 33 (2008) 5905-5910.
[11] C.R. Cloutier, D.P. Wilkinson, International Journal of Hydrogen Energy, 35
(2010) 3967-3984.
[12] T. Take, K. Tsurutani, M. Umeda, Journal of Power Sources, 164 (2007) 9-16.
[13] Z. Hu, M. Wu, Z. Wei, S. Song, P.K. Shen, Journal of Power Sources, 166 (2007)
458-461.
[14] A. Caravaca, F.M. Sapountzi, A. De Lucas-Consuegra, C. Molina-Mora, F.
Dorado, J.L. Valverde, Int. J. Hydrogen Energy, 37 (2012) 9504-9513.
Electrochemical reforming vs. Catalytic reforming of ethanol
131
[15] A.T. Marshall, R.G. Haverkamp, International Journal of Hydrogen Energy, 33
(2008) 4649-4654.
[16] S. Kongjao, S. Damronglerd, M. Hunsom, J Appl Electrochem, 41 (2011) 215-
222.
[17] A. de Lucas-Consuegra, A.B. Calcerrada, A.R. de la Osa, J.L. Valverde, Fuel
Processing Technology, 127 (2014) 13-19.
[18] S.M. Mousavi Ehteshami, S.H. Chan, Energy Technology & Policy, 1 (2014) 15-
22.
[19] D.K. Liguras, D.I. Kondarides, X.E. Verykios, Applied Catalysis B:
Environmental, 43 (2003) 345-354.
[20] Z. Khila, N. Hajjaji, M.-N. Pons, V. Renaudin, A. Houas, Fuel Processing
Technology, 112 (2013) 19-27.
[21] P. Giunta, M. Moreno, F. Mariño, N. Amadeo, M. Lobarde, Chemical
Engineering & Technology, 35 (2012) 1055-1063.
[22] D. Montané, E. Bolshak, S. Abelló, Chemical Engineering Journal, 175 (2011)
519-533.
[23] P. Giunta, C. Mosquera, N. Amadeo, M. Laborde, Journal of Power Sources, 164
(2007) 336-343.
[24] A. Haryanto, S. Fernando, N. Murali, S. Adhikari, Energy & Fuels, 19 (2005)
2098-2106.
[25] F. Mariño, C. Descorme, D. Duprez, Applied Catalysis B: Environmental, 54
(2004) 59-66.
[26] F. Mariño, C. Descorme, D. Duprez, Applied Catalysis B: Environmental, 58
(2005) 175-183.
[27] P. Tippawan, A. Arpornwichanop, Bioresource Technology, 157 (2014) 231-239.
[28] S.P. Sethu, S. Gangadharan, S.H. Chan, U. Stimming, Journal of Power Sources,
254 (2014) 161-167.
[29] V. Bambagioni, M. Bevilacqua, C. Bianchini, J. Filippi, A. Lavacchi, A.
Marchionni, F. Vizza, P.K. Shen, ChemSusChem, 3 (2010) 851-855.
Chapter 3
132
[30] Development and demonstration Plan of the US Department of Energy, in: D.o.
Energy (Ed.), Fuel Cell Technology Office Multi-Year Reserach, 2011.
[31] H. Shapouri, Energy Balance of the Corn-Ethanol Industry, in: USDA (Ed.),
2008.
[32] Y.X. Chen, A. Lavacchi, H.A. Miller, M. Bevilacqua, J. Filippi, M. Innocenti, A.
Marchionni, W. Oberhauser, L. Wang, F. Vizza, Nature Communications, 5 (2014).
4.1. Introduction
4.2. Experimental
4.2.1 Catalytic activity measurements
and EPOC parameters
4.2.2. Preparation of the solid
electrolyte cell
4.2.3. Characterization measurements
4.3. Results and discussion
4.3.1. Influence of the preparation
method of the catalyst film
4.3.2. Kinetic study and
electrochemical promotion experiments
4.4. Conclusions
4.5. References
CHAPTER 4:
Electrochemical Promotion of Ni with
Alkali Ions in the CO2 Hydrogenation
Toward CO and CH4
INTRODUCTION
EXPERIMENTAL CHARACTERIZATION
TPR analysis
RESULTS
H2/CO2 = 30 % /15 %, N2 balance, FT = 6 NL h-
1, T = 240 ºC
Hydrogenation of CO2 can be considered as one of the most important chemical conversion reactions not only
for the effective decrease of the overall CO2 emissions but also for the production of many possible renewable
fuels (hydrocarbons or alcohols). In the hydrogenation of CO2 process, depending on both the employed
catalytic system (metal/support) and on the reaction conditions, different products can be obtained including
CO, CH4, formic acid, formaldehyde, methanol or higher alcohols.
The reaction of hydrogenation of CO2 can be summarized according to the following scheme:
xCO2 + (2x – z + y/2)H2 → CxHyOz + (2x – z)H2O
Hence, the control of the catalyst selectivity toward different hydrogenation products in highly required. In
this sense the activity and selectivity can be modified by electrically polarizing the catalyst-electrode via the
effect of the electrochemical promotion of catalysis (EPOC).
Single chamber solid electrolyte cell reactor
- Final electrical resistance 0.3 Ω at 350 ºC
- Reduction to metallic Ni
Influence of the applied potential Effect of the temperature
H2/CO2 = 30 % /15 %, N2 balance, FT = 6 NL h-1
Effect of the applied potential and the H2 feed concentration
-The reaction rate of CO2 and production rate of CO
and CH4 were enhanced by increasing
temperatures
CO → electrophilic behaviour
CH4 → electrophobic behaviour
In this study, three different king of Ni-based catalysts were prepared on K-βAl2O3 solid electrolyte by combining the annealing of an organometallic paste and
the addition of a catalyst powder. The different catalysts films were tested in the CO2 hydrogenation reaction under electrochemical promotion by K+ ions, and
were characterized by XRD and SEM.
Outlet Inlet
Counter Electrode (CE)
Catalyst-working
Electrode (WE)
Reactor cap Cooling
Quartz
tube
Au wires
Alumina tube
with 4 bores
H2
e-
Kd+
Kd+
Kd+Kd+
d-
d-
d- d-
I < 0
e-
Ni
CO2
CO2
CO2
CO2
H2
Kd+
Kd+
Kd+
d-
d-
d-
(K-β-Al2O3)K+ K+K+K+
0
50
100
150
200
250
300
350
400
0 30 60 90 120 150 180 210
0
50
100
150
200
250
300
350
Tem
per
ature
/ º
C
0
20
40
60
80
100
120
140
Res
ista
nce
/
Tem
per
ature
/ º
C
Time / min
H2
consu
mpti
on /
a.u.
K-βAl2O3
Ni
Au
Ω
280
320
360
Tem
per
atu
re /
ºC
140 150 160 1700
5
10
15
20
Time / min
Res
ista
nce
/
SEM analysis
50 µm
b3) b4)
100 µm
100 µm 50 µm
a1) a2)
b1) b2)
c2)
Al Ni
50 µm
c1)
100 µm
0.0
0.3
0.6
0.9
1.2
0.0
0.2
0.4
0.6
0.8
1.0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0 20 40 60 80 100 120 140 160 180 200
-1000
-800
-600
-400
-200
0
200
400
600
800
UW
R /
V U
WR /
V
GNA NA N
UW
R /
V
(rC
O2/ m
ol C
O2·m
ol-1 N
i·s-1
)x10
3
a)
-2
-1
0
1
2
b)
(rC
O/ m
ol C
O·m
ol-1 N
i·s-1
)x10
3
-2
-1
0
1
2
c)
(rC
H4/ m
ol C
H4·m
ol-1 N
i·s-1
)x10
3
-2
-1
0
1
2
Current
Cur
rent
/ A
Time / min
d)
-3
-2
-1
0
1
2
UWR
UW
R/ V
-NA showed the higher catalytic activity
0
10
20
30
40
50
-2 -1 0 1 20
5
10
15
0
20
40
60
b)
(rC
O /
mol
CO
· m
ol-1 N
i·s-1
)x105
T = 240 ºC
T = 270 ºC
T = 300 ºC
c) 100 Nml min-1
UWR
/V
(rC
H4 /
mol
CH
4· m
ol-1 N
i·s-1
)x105
H2 / CO
2 = 30% / 1.5 %
Increasing
(rC
O2 /
mol
CO
2· m
ol-1 N
i·s-1
)x105
a)
0
10
20
30
40
50
60
-2 -1 0 1 2
40
50
60
70
80
90
T = 240 ºC
H2/CO
2 = 10
H2/CO
2 = 20
SC
H4
/%
H2/CO
2 = 2
H2/CO
2 = 6
SC
O/%
UWR / V
T = 240 ºC, FT = 6 NL h-1
2ºC/min, H2= 3 % (N2 balance) FT = 6 NL h-1
CHAPTER 4. ELECTROCHEMICAL PROMOTION
OF Ni WITH ALKALI IONS IN THE CO2
HYDROGENATION TOWARD CO AND CH4
137
Abstract
hree different kind of Ni-based catalysts were prepared on a K-
βAl2O3 solid electrolyte by combining the annealing of an
organometallic paste and the addition of a catalyst powder. The
different catalysts films were tested in the CO2 hydrogenation reaction
under electrochemical promotion by K+ ions, and were characterized by
XRD and SEM. The catalyst film derived from the addition of an α-Al2O3
powder to the Ni catalyst ink presented the highest catalytic activity as a
result of the increase in Ni catalyst film porosity. The influence of the
applied potential and other operation variables were evaluated on the Ni
catalytic activity and selectivity. Hence, the CO production rate was
enhanced either by decreasing the applied potential (with the consequent
supplied of K+ ions to the catalyst surface) or by increasing the CO2 (electron
acceptor) feed concentration. On the other hand, CH4 production rate was
favoured at positive potentials (removing K+ from the catalyst surface) or by
increasing the H2 (electron donor) feed concentration. The global CO2
consumption rate increased upon negative polarization in all experiments
and the Electrochemical Promotion of Catalysis (EPOC) effect showed to be
reversible and reproducible. Hence, the electrochemical promotion
phenomena demonstrated to be a very useful technique to in-situ modify
and control the catalytic activity and selectivity of a non-noble metal such
as Ni for the production of CH4 or syngas via CO2 valorisation.
T
Chapter 4
138
4.1. Introduction
As seen in previous chapters (Chapter 2 and Chapter 3) hydrogen
production free of CO2 emissions is a priority goal for the scientific
community. However, there are still many processes that emit CO2 causing
an overall increase in global warming. Therefore, different strategies are
being developed to mitigate the global warming and climate change, some
of them are focused on the separation, storage and utilization of the CO2.
In this sense, hydrogenation of CO2 can be considered as one of the most
important chemical conversion reactions not only for the effective decrease
of the overall CO2 emissions but also for the production of many possible
renewable fuels (hydrocarbons or alcohols) [1-3]. However, CO2 is a
thermodynamically stable compound and thus requires high activation
energy for its transformation into other chemicals [4, 5]. Although
numerous organic syntheses involve CO2 as the feedstock, only a few have
reached industrial commercialization, for instance, the production of urea
and its derivatives, salicylic acid, and carbonates [1, 5]. Most studies on
catalytic hydrogenation of CO2 have been performed using metal catalysts
(e.g. Pt, Rh, Pd, Ru, Cu, Fe, Co and Ni) supported on different oxides (e.g.
Nb2O3, ZrO2, Al2O3, SiO2, and MgO) [6-10]. Depending on both the
employed catalytic system (metal/support) and on the reaction conditions,
different products can be obtained including CO, CH4, formic acid,
formaldehyde, dimethyl ether, methanol or higher alcohols [1, 4-6, 11, 12].
These catalytic reactions can be summarized according to the following
scheme:
xCO2 + (2x – z + y/2)H2 → CxHyOz + (2x – z)H2O (4.1)
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
139
For instance, the reverse water gas shift (RWGS, eq. 3.2) and CO2
methanation (eq.3.3) reactions can be described as follows:
CO2 + H2 → CO + H2O (4.2)
CO2 + 4H2 → CH4 + 2H2O (4.3)
Hence, the control of the catalyst selectivity toward the different
hydrogenation products is highly required. In this sense, alkali promotion
has been described to be an effective approach to improve the activity
and/or selectivity of different metal catalysts [13]. For instance, the
promoter effect of potassium in the CO2 hydrogenation reaction has been
previously studied over conventional heterogeneous catalysts [14, 15].
Both the activity and the selectivity of a metal or metal oxide catalyst film
deposited on an ionic conductor can also be modified, in a significant,
controlled and reversible manner, by electrically polarizing the catalyst-
electrode via the effect of electrochemical promotion of catalysis (EPOC)
also known as NEMCA effect (non-faradaic electrochemical modification of
catalytic activity). This phenomenon was discovered by Stoukides and
Vayenas [16] and is based on the controlled migration of promoting ions,
e.g., O2-, Na+, K+ or H+ ions, from an electroactive support, such as β-Al2O3
(a Na+ or K+ conductor), YSZ (yttrium-stabilized-zirconia, an O2- conductor)
or CZI (CaZr0.9In0.1O3-α, a H+ conductor), to the catalytic metal/gas
interface. The EPOC phenomenon has been applied on a wide variety of
catalysts and in a large number of important industrial and environmental
catalytic reactions [17]. In particular, the electrochemical promotion has
been shown to enhance the catalyst activity and selectivity in the
hydrogenation of CO2 [2, 3, 6, 11, 12, 18-24]. Hence, the promotional effect
may allow to in-situ control the catalytic behavior of the system for syn-gas
Chapter 4
140
or CH4 production via CO2 hydrogenation. Most EPOC studies on CO2
hydrogenation (as shown on Table 4.1) have been performed with a YSZ
solid electrolyte and a noble metal catalyst such as Pt, Pd, Ru and Rh [2, 3,
6, 11, 18, 20, 21]. Nevertheless, only a few studies have been carried out by
using cationic solid electrolytes [3, 12, 19, 21-23] or non-noble metal
catalysts (Ni or Cu) [2, 11, 12, 20, 22]. However, for practical use, it is clear
that the development of low-cost and highly efficient catalyst is desired. In
this sense, we focused on the developing an efficient catalyst film of
suitable performance for the CO2 hydrogenation reaction via
electrochemical promotion by K+ ions. Hence, K-βAl2O3 was selected as the
solid electrolyte, and different catalyst film configurations were proposed
based on Ni catalyst: i) Ni catalyst film prepared by deposition of an
organometallic paste, ii) Ni catalyst film prepared by deposition of an
slurry made by mixing a Ni metal paste and α-Al2O3 powder and iii) and Ni
particles dispersed on Al2O3 powder and deposited on the solid electrolyte
by an slurry with Au paste. By this latter case, the possibility of exploring
the electro-promotional effect on dispersed metal nanoparticles was also
examined. Hence three different electrochemical catalysts (Ni/K-βAl2O3/Au,
Ni-α-Al2O3/K-βAl2O3/Au and Au-Ni(30%)-α-Al2O3/K-βAl2O3/Au) were
studied on the basis of different characterization techniques and catalytic
activity measurements under electrochemical promotion conditions.
Additionally, for the selected catalytic system a kinetic study under EPOC
conditions was carried out, as well as the possibility of controlling the Ni
catalytic activity and selectivity toward the different obtained products
(syn-gas or methane) via EPOC.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
141
Ta
ble
4.1
. E
PO
C s
tud
ies
of
the C
O2 h
yd
rogen
ati
on
react
ion
usi
ng d
iffe
ren
t ca
taly
sts
an
d s
oli
d e
lect
roly
tes
Refe
ren
ce
[2]
[3]
[18
]
[11
]
[6]
[19
]
EP
OC
beh
avio
ur
Invert
ed
volc
an
o
Invert
ed
volc
an
o
Ele
ctro
ph
ob
ic
(CH
4)/
Ele
ctro
ph
ibic
(CO
)
Ele
ctro
ph
ibic
Ele
ctro
ph
ob
ic
(CH
4)/
Ele
ctro
ph
ibic
(CO
)
Ele
ctro
ph
ob
ic
(CH
4)/
Ele
ctro
ph
ibic
(CO
). T
he
oth
ers
vary
wit
h e
ract
ion
con
dit
ion
s
Ma
in
rea
ctio
n
pro
du
cts
CO
, C
H4
C2H
4
CO
CO
, C
H4
CO
, C
H4
CO
, C
H4
CO
, C
H4,
CH
3O
H,
C2H
5O
H,
C2+
C3
FT
/ N
Lh
-1
60
2.3
-18
0.9
-3
6
12
90-5
22
H2
/ %
5.6
22.7
63
5.6
30
47.5
-76
CO
2
/ %
1
72.8
5.5
1
3
19-4
7.5
T
/ ºC
220
-380
533
-605
346
-477
200
-440
200
-300
400
Ca
taly
st f
ilm
pre
para
ion
tecn
iqu
e
Sp
utt
eri
ng
Org
an
om
eta
llic
pa
ste
Org
an
om
eta
llic
pa
ste
Imp
regn
ati
on
Imp
regn
ati
on
Dip
-coa
tin
g
Soli
d
ele
ctro
lyte
YS
Z
YS
Z,
Na
-βA
l 2O
3
YS
Z
YS
Z
YS
Z
K-β
Al 2
O3
Ca
taly
st
Rh
, P
t,C
u
Pd
Rh
Ni,
Ru
Ru
Pt
Chapter 4
142
Ta
ble
4.1
. (C
on
t.).
EP
OC
stu
die
s of
the C
O2 h
yd
rogen
ati
on
react
ion
usi
ng d
iffe
ren
t ca
taly
sts
an
d s
oli
d e
lect
roly
tes
Refe
ren
ce
[12
]
[20
]
[21
]
[22
]
[23
]
[24
]
EP
OC
beh
avio
ur
It v
ari
es
wit
h
rea
ctio
n
con
dii
tion
s
It v
ari
es
wit
h
rea
ctio
n
con
dii
tion
s
Ele
ctro
ph
obic
(CH
4)/
Ele
ctro
ph
ibic
(CO
)
Ele
ctro
ph
ibic
Ele
ctro
ph
obic
(CH
4)/
Ele
ctro
ph
ibic
(CO
)
Ele
ctro
ph
ibic
Ma
in
rea
ctio
n
pro
du
cts
CH
3O
H,
C2H
5O
H,
C2H
6O
CO
, C
H4,
CH
3O
H,C
2
H6,
C3H
6,
C2H
6O
CO
, C
H4
CO
CO
, C
H4
CO
FT
/ N
Lh
-1
90
-52
2
90
6
12
-1.8
6
1.2
H2
/ %
63
.3-7
6
47
.5-7
6
1-1
5
0.3
-4.9
30
1-1
0
CO
2
/ %
19
-31
19
-47
.5
0.0
25
-2
0.6
-
2.2
4
3
1-1
0
T
/ ºC
20
0-4
00
22
5-4
50
20
0-3
40
55
0-7
50
28
0-4
20
65
0-8
00
Ca
taly
st f
ilm
pre
para
ion
tecn
iqu
e
Ele
ctro
less
dep
osi
tion
Org
an
om
eta
llic
pa
ste /
Ele
ctro
less
Imp
regn
ati
on
Pd
Org
an
om
eta
llic
pa
ste/
Cu
pow
der
Imp
regn
ati
on
Org
an
om
eta
llic
pa
ste
Soli
d
ele
ctro
lyte
K-β
Al 2
O3
YS
Z
YS
Z,
K-β
Al 2
O3
SZ
Y
K-β
Al 2
O3
YS
Z
Ca
taly
st
Cu
Pt,
Ni,
Pd
Ru
Cu
Ru
Pt
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
143
4.2. Experimental
4.2.1. Catalytic activity measurements and EPOC parameters
The experimental setup was described in detail in a previous chapter
(Chapter 1). The reaction gases (Praxair, Inc.) were certified standards
(99.999 % purity) of CO2, H2 and N2, the latter being used as the carrier
gas, and the gas flow rates were controlled by a set of mass flowmeters
(Brooks 5850 E and 5850 S). The catalytic experiments were carried out at
atmospheric pressure with an overall gas flow rate, FT, ranging from 1.2 to
12 NL h-1, at three temperatures (T = 240, 270, and 300 ºC) and a feed
composition ranging from 3 to 30 % for H2 and from 1.5 to 10 % for CO2 (N2
balance).
Reactant and product gases were on-line analyzed by using a micro
gas-chromatograph (Varian CP-4900) as described in details in Chapter 1.
The detected reaction products were: CO, CH4 and H2O. The error in the
carbon atom balance did not exceed 5 %. In order to carry out the
electrochemical promotion (EPOC) experiments, the three electrodes
(working, counter and reference) were connected to a potentiostat-
galvanostat Voltalab PGZ 301 (Radiometer Analytical) using gold wires.
The inertness of both the gold counter/reference electrodes and the α-Al2O3
powder and Au-α-Al2O3 film was checked via blank experiments performed
under the studied reaction conditions.
The CO2 conversion and the CO and CH4 selectivities were calculated as
follows:
𝐶𝑂2 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 / % =𝐹0
𝐶𝑂2−𝐹𝐶𝑂2
𝐹0𝐶𝑂2
𝑥100 (eq. 4.1)
𝐶𝑂 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 / % =𝐹𝐶𝑂
𝐹0𝐶𝑂2−𝐹𝐶𝑂2
𝑥100 (eq. 4.2)
𝐶𝐻4 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 / % =𝐹𝐶𝐻4
𝐹0𝐶𝑂2−𝐹𝐶𝑂2
𝑥100 (eq. 4.3)
Chapter 4
144
where F0i and Fi are the molar flow rates of the i species at the inlet and at
the outlet of the reactor, respectively. On the other hand, the magnitude of
the electropromotional effect was quantified by two parameters commonly
used in this kind of studies:
- The rate enhancement ratio of each compound (ρi), defined by the
following equation:
𝜌i = 𝑟i
𝑟i,0 (eq. 4.4)
where ri and ri,0 are the promoted (UWR < 2 V) and reference state (UWR = 2
V) catalytic production rates, respectively, of the corresponding compound.
- The promotion index (PIK+), calculated by the following equation:
𝑃𝐼𝐾+ =
∆𝑟
𝑟0
𝜃𝐾+ (eq. 4.5)
where ∆r = r – r0 is the K-induced change in catalytic reaction rate and θK+
is the potassium coverage calculated from the integration of the current (I)
vs. time (t) curves via the Faraday law (eq. 4.6):
𝜃𝐾+ = ∫|I|dt
nF𝑁𝐺
t
0 (eq. 4.6)
where n is the potassium ion charge, i.e., +1, F is the Faraday constant
(96484.6 C), and NG is the active mol of the Ni catalysts, was calculated in
each case by means of the total amount of deposited Ni and the particle
diameter and dispersion values estimated from the (111) from XRD
analysis peak via Scherrer equation.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
145
4.2.2. Preparation of the solid electrolyte cell
Each electrochemical catalyst was based on a 19-mm-diameter, 1-mm-
thick K-βAl2O3 (Ionotec) pellet solid electrolyte. In each electrochemical
catalyst, the Au counter and reference electrodes were firstly deposited on
one side of the electrolyte by annealing a gold organometallic paste (Fuel
Cell Materials ref. 231001) at 800 ºC for 2 h (heating ramp of 5 ºC/min).
Then, three Ni-based catalyst were prepared by different preparation
methods. In the first sample (Ni/K-βAl2O3/Au, denoted as “N”), a coating of
a commercial Ni ink (Fuel Cell Material ref. 233001) was applied on the
electrolyte followed by calcination at 800 ºC for 2 h (heating ramp of 5
ºC/min). In the second sample (Ni-α-Al2O3/K-βAl2O3/Au, denoted as “NA”),
the catalyst film was prepared by mixing 40 mg of the Ni ink with 20 mg of
a commercial α-Al2O3, (Alfa Aesar) powder, and some ethylene glycol to
achieve an slurry with suitable viscosity. Then, the slurry deposited on the
K-βAl2O3 pellet was subjected to the same heat treatment as that
underwent by the previous electrode (800 ºC). In the third sample (Au-
Ni(30%)-α-Al2O3/K-βAl2O3/Au, denoted as “GNA”), 40 mg of the Au ink and
ethylene glycol were mixed with 20 mg of α-Al2O3 powder which was
previously impregnated with Ni (30 % in weight) through a conventional
impregnation method. The α-Al2O3 powder was impregnated with a
Ni(NO3)2·6H2O (Panreac) precursor water solution in a glass vessel under
vacuum at 90 ºC to yield 30 % weight of Ni. After drying over night at 120
ºC, the resulting powder was calcined at 450 ºC (heating ramp of 5ºC / min)
for 1 h. The slurry of 30% Ni/αAl2O3 + Au ink was deposited on the solid
electrolyte and annealed at 800 ºC for 2 h. In this way, the catalytic
contribution of the Ni dispersed particles on alumina support was isolated
for its catalytic evaluation due to the negligible activity of the Au particles
Chapter 4
146
coming from the ink. Table 4.2 summarizes the description and
nomenclature of each electrochemical catalyst along with the total Ni
weight, particle size and active surface area of all the catalyst electrodes.
Ta
ble
4.2
. S
um
mary
of
the d
iffe
ren
t N
i base
d e
lect
roch
em
ical
cata
lyst
NG /
Act
ive
mol
of
Ni
9.2
8·1
0-6
3.4
3·1
0-6
2.4
5·1
0-6
dm
eta
l aft
er
react
ion
/
nm
39.9
37.5
35.2
Tota
l N
i
weig
ht
/ m
g
17.8
0
6.1
7
4.1
3
Pow
der
ad
dit
ion
to
the s
lurr
y
-
αA
l 2O
3
30.7
3 %
Ni-
αA
l 2O
3
Meta
l
past
e
Ni
Ni
Ni
Desc
rip
tion
Ni-
K-β
Al 2
O3 /A
u
Ni-
αA
l 2O
3 /K
-βA
l 2O
3 /A
u
Au
-Ni(
30%
)-αA
l 2O
3 /K
-βA
l 2O
3 /A
u
Nom
en
clatu
re
N
NA
GN
A
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
147
The active surface area of the catalysts, NG, was calculated in each case
by means of the total amount of deposited mol of metal and the particle
diameter and dispersion values were estimated from the (111) peak via
Scherrer equation. It can be observed that the lower the amount of Ni
used, the lower the particle size. Additionally, blank experiments were
carried out on a pure gold electrode (Au-K-βAl2O3/Au) and a gold paste
electrode mixed with a non-impregnated alumina powder, (Au-α-Al2O3/K-
βAl2O3/Au) to confirm its negligible activity for the CO2 hydrogenation
reaction.
4.2.3. Characterization measurements
The different electrochemical catalysts were firstly reduced by an in-
situ temperature programmed reduction experiment, (TPR) under a H2
stream of 3 % (N2 balanced) with an overall flow rate of 6 NL·h-1 under
temperature programmed conditions (2ºC / min) from room temperature to
350 ºC. During the TPR, the H2 consumption was continuously monitored
by a thermal conductivity detector, as well as the in-plane Ni film surface
electrical resistance measured by a digital multimeter between two points
separated by 1 cm.
After reduction, the three Ni-based catalyst were characterized by X-
Ray Diffraction (XRD) analysis with a Philips PW-1710 instrument using
Ni-filtered Cu Kα radiation (λ = 1.5404 Å). Diffractograms were compared
with the JCPDS-ICDD references. All the catalysts were also characterized
after the catalytic activity experiments to evaluate their stability under
the studied reaction conditions. The morphology of the different catalyst
films was also evaluated by using a Quanta 250 scan electron microscopy
(SEM). This instrument is connected to an EDAX Apollo X (AMETEK),
which analyses the chemical compositions of the samples via X-Ray
Chapter 4
148
analyses (EDAX). The Ni metal loading in the impregnated α-Al2O3 was
determined by atomic absorption spectrophotometry, using a SPECTRA
220FS analyser. As shown on Table 4.2 the total amount of Ni on the
powder catalyst was 30.73 % in weight.
4.3. Results and discussion
4.3.1. Influence of the preparation method of the catalyst film
Figure 4.1 shows the variation of the surface electrical resistance of the
Ni catalyst film and the H2 consumption rate during the temperature
programmed reaction experiment (TPR) for sample N. During the TPR
experiments, no significant differences were appreciated between the two
electrochemical catalyst based on the Ni catalyst film (N, and NA). For
sample N, it can be observed that at the beginning of the experiment, the
as-deposited Ni catalyst film showed a very high value of the electrical
resistance (1.25 x 108 Ω). It seems to indicate that the catalyst preparation
method (involving calcination at high temperature under air atmosphere)
led to a nickel film on its oxidised state (as will be shown later by XRD).
However, during the TPR experiment the electrical resistance decreased
and finally stabilized at only 0.3 Ω after 1 hour at 350 ºC indicating the
complete reduction of the catalyst film to metallic Nickel (as will be shown
later by XRD), in agreement with previous works with Ni catalysts [25-27].
This reduction process can be confirmed by the H2 consumption peak
which started at around 250 ºC. The initial decrease in the electrical
resistance of the Ni catalyst film with temperature (25-125 ºC) with no H2
consumption could be attributed to the semiconducting properties of the
NiO [28]. On the other hand, the appearance of a local maximum value
around 300 ºC could be explained by the partial formation of different
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
149
K-βAl2O3
Ni
Au
Ω
Ni
β-Al2O3
Au
280
320
360
Tem
pera
ture
/ º
C
140 150 160 1700
5
10
15
20
Time / min
Resi
stan
ce /M
0
50
100
150
200
250
300
350
400
0 30 60 90 120 150 180 2100
50
100
150
200
250
300
350
Tem
pera
ture
/ º
C
0
20
40
60
80
100
120
140
Resi
stan
ce /
M
Tem
pera
ture
/ º
C
Time / min H
2 c
on
sum
pti
on
/a
.u.
a)
b)
intermediate Ni oxides of lower electrical conductivity. However, the
surface composition is unknown in the course of the TPR experiment.
Figure 4.1. Variation of Ni surface electrical resistance (a) and H2
consumption rate (b) during the TPR experiment for catalyst N. Inset of (a) shows
the final variation of Ni electrical resistance and temperature with time. Heating
rate of 2 ºC/min under a 3 % H2 stream (N2 balance).
Figure 4.2 shows the crystalline structure of the different Ni-based
catalysts, namely N (Figure 4.2.a), NA (Figure 4.2.b) and GNA (Figure
4.2.c), analysed by XRD both after the in-situ TPR up to 350 ºC (fresh
Chapter 4
150
20 30 40 50 60 70 80 90
-Al2O
3
b)
Inte
nsi
ty /
a.u
.
2º
Fresh
catalyst
Used
catalyst
-Al2O
3
Ni
(11
1)
(11
1)
(20
0)
(20
0)
(22
0)
(22
0)
NA
20 30 40 50 60 70 80 90
-Al2O
3
(00
6)
NiO2
(22
2)
(31
1)
(22
0)(20
0)
NiOBefore
reduction
a)
Inte
nsi
ty /
a.u
.
2/ º
Fresh
catalyst
Used
catalyst(1
11
)
Ni
(20
0)
(22
0)
(11
1)
(20
0)
(22
0)
N
20 30 40 50 60 70 80 90
2/ º
Inte
nsit
y / a
. u
.
Used
catalyst
c)
Fresh
catalyst
(111)
(220)
Au
Ni
(100)
(22
0)
GNA
(20
0)
(200)
-Al2O
3
-Al2O
3
catalyst, top spectra) and after exposure to all the studied reaction
conditions (used catalyst, bottom spectrum). For the case of sample N, the
XRD analysis before reduction (TPR) is also included in Figure 4.2a.
Figure 4.2. XRD analysis patterns of catalysts (a) N, (b) NA and (c) GNA before
and after exposure to reaction conditions. For catalyst N (a), the XRD analysis
patterns before reduction is also included.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
151
In this latter case it can be observed that the Ni catalyst film was mainly on
its oxidized state (NiO and NiO2, JCPDS, 78-0643 and JCPDS, 85-1977,
respectively), which is in agreement with the electrical resistance
measurements during TPR of Figure 4.1. However, the Ni catalyst films were
completely reduced during the TPR experiments since no patterns of nickel
oxides appeared in any of the fresh catalysts (after reduction and before
activity measurements). In all cases, the main diffraction peaks (111),
(200) and (220) which appeared at 2θ = 44.5º, 51.8º and 76.4º, respectively,
were associated with metallic nickel and exhibited a face-centered cubic
(FCC) crystalline structure (JCPDS, 87-0712). Diffraction peaks related to
the K-βAl2O3 solid electrolyte (JCPDS, 02-0921) were found in all the
spectra, and samples NA and GNA (Figures 4.2b and 2c, respectively) also
exhibited the peaks corresponding to α-Al2O3 (JCPDS, 01-1296). In
addition, reflections appearing at 38º, 44º, 64º and 82 º in Figure 4.2c
corresponded, respectively, to the (111), (200), (220) and (222) planes of
metallic gold (JCPDS, 01-1172) in sample GNA. Remarkably, the reduced
state of the Ni catalysts remained during catalytic experiments due to the
presence of H2 in the feed. Additionally, the intensity of the Ni XRD peaks
(111, 200, and 220) in used samples of catalyst N and NA (Figure 2.a and
2.b, respectively) slightly increased in relation to those obtained in the
fresh samples, as observed in post-reaction XRD spectra (denoted in all the
cases as used catalyst). However, the particle size of Ni estimated by
Scherrer equation before and after the experiments was practically the
same in all cases. This denoted the stability of the catalyst under
experimental conditions, also supported by the good reproducibility
observed in the catalytic performance under every application of the
reference estate (+2V), as will be shown later.
Chapter 4
152
As can be observed, a Ni particle size of 39.9 nm was estimated in
catalyst N (Table 4.2), which decreased in catalysts NA and GNA. These
values were of the same order than those reported in other EPOC studies
with Ni catalysts prepared by different techniques [11, 20] and should
imply a high catalytic activity [1, 19]. Aside from the approximate
character of this calculation, the addition of the alumina powder to the Ni
and Au pastes slightly decreased the metal particle size and very likely
increased the Ni particles dispersion. On the other hand, a very slight
increase in the particle size was observed in all cases after time on stream
(particle size before reaction of 38.9, 36.5 and 34.7 for catalysts N, NA and
GNA, respectively), which denoted a minimal thermal sintering effect
under the studied reaction conditions.
SEM micrographs and EDAX analysis of certain regions of the different
Ni catalyst-electrodes obtained after exposure to reaction conditions are
shown in Figure 4.3. It can be observed that all samples showed to be
porous, which facilitated the reactants and products diffusion. However, it
can also be observed that the catalyst preparation method (by the addition
of a powder to the ink) markedly influenced its surface composition and
morphology. From Figures 4.3.a1 and 4.3.a2 (sample N), the obtained Ni
film seems to resemble a typical homogeneous foam structure with big Ni
agglomerates ranging from 6.0 to 7.0 µm [25, 29]. However, two
components can be clearly distinguished in sample NA (Figures 4.3.b1 and
3.3.b2); Ni agglomerates with similar morphology than those present in
sample N (Figure 4.3.a2) and larger size particles with a smooth surface
which corresponded with the α-Al2O3 powder.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
153
50 µm
b3) b4)
100 µm
100 µm 50 µm
a1) a2)
b1) b2)
c2)
Al Ni
50 µm
c1)
100 µm
Figure 4.3. SEM micrographs of catalysts (a) N, (b) NA and (c) GNA after
exposure to reaction conditions. EDAX mapping is also included for catalysts NA.
These particles of α-Al2O3 were round shaped with a typical size
distribution ranging between 30 and 65 µm. Moreover, an elemental
Chapter 4
154
mapping of Figure 4.3.b2 was performed as shown in Figure 4.3.b3 and
3.3.b4 corresponding to Al and Ni, respectively. It was clear that the
previously mentioned largest particles were composed of Al (in green,
Figure 4.3.b3) and O (not shown), thus corresponding with the α-Al2O3
powder, while the smallest agglomerates mainly contained Ni (in blue,
Figure 4.3.b4). It can also be observed that sample NA exhibited a larger
roughness (porosity) and a higher number of valleys and holes in
comparison with sample N due to the presence of the heterogeneous
particles of α-Al2O3. Hence, it is very likely that the incorporation of α-
Al2O3 powder allowed to re-disperse the Ni particles coming from the ink,
leading to an increase of the catalytic surface area as already shown in a
previous study with Pt ink [30] and demonstrated by an increase in the
metal dispersion (3%, 3.4% and 3.5% for catalyst, N, NA and GNA,
respectively. Finally, sample GNA (Figures 4.3.c1 and 4.3.c2) showed a
similar porous structure than sample NA formed by the Au particles
coming from the Au ink and the α-Al2O3 powder.
In order to select the most suitable Ni catalyst preparation method for
CO2 hydrogenation via EPOC, all samples (N, NA and GNA) were tested
under certain reaction conditions and potentials (UWR) ranging from +2 to -
2 V. Figure 4.4 shows the dynamic response of the reaction rates for CO2
consumption, CO and CH4 formation for each catalyst (N, NA and GNA) and
the electrical current obtained upon the catalyst potential variation for catalyst
N. During the measurement of transient currents, no significant differences
were appreciated between the three electrochemical catalyst (N, NA and
GNA).
All reported rate values were normalized by the total mol of deposited
Ni. These experiments were carried out at 240 ºC with a feed composition
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
155
of H2/CO2 = 30 %/1.5 % (N2 balance) and an overall flow rate of 6 NL h-1.
The electropromotional behavior was the same as that of samples N and
NA (no electro-promotional effect was observed for sample GNA). Firstly,
the application of a positive potential UWR = + 2 V (reference state) at the
beginning and at the end of each experiment (as well as before each
negative polarization) ensured the removal of K+ ions from the catalyst
surface, which had previously migrated to the catalyst surface by thermal
migration [3, 31]. Thus, as shown in Figure 4.4.d, under an application of a
constant voltage, the generated current decreased with time to values close
to zero indicating that a steady state coverage of potassium was achieved
after certain time. In this way, a reference state was obtained and the
reversibility of the electropromoted effect was checked. At the reference
state, CO and CH4 were already produced by means of the reverse water-
gas shift (eq. 2) and CO2 methanation (eq. 3) reactions, respectively. Then,
the subsequent decrease in the applied catalyst potential increased the
amount of K+ ions that migrated electrochemically from the K-βAl2O3
pellet to the catalyst surface (promoted state), according to the obtained
negative current (Figure 4.4.d). As a consequence, the overall reaction rate
of CO2 increased as well as the production rate of CO while the production
rate of CH4 decreased for the case of samples N and NA. According to
previous EPOC studies on CO2 hydrogenation [6, 18, 19, 21], the RWGS
reaction is electrophilic, i.e., the CO production rate increases with
decreasing potential (with increasing K+ coverage). At the same time, an
electrophobic behaviour for the methanation reaction was typically
observed, i.e., the CH4 production rate increased with increasing potential
and decreasing the amount of K+ promoter supplied.
Chapter 4
156
0.0
0.3
0.6
0.9
1.2
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80 100 120 140 160 180 200
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
UW
R /
V U
WR /
V
GNA NA N
UW
R /
V
(rC
O2
/ m
ol C
O2·m
ol-1
Ni·
s-1)x
10
3
a)
-2
-1
0
1
2
b)
(rC
O/
mol C
O·m
ol-1
Ni·
s-1)x
10
3
-2
-1
0
1
2
Time / min
c)
(rC
H4
/ m
ol C
H4·m
ol-1
Ni·
s-1)x
10
3
-2
-1
0
1
2
Figure 4.4. Influence of the applied potential vs. time on the reaction rate values
of (a) CO2 consumption, (b) CO production and (c) CH4 production with the
different catalysts (N, NA and GNA) and obtained electric current (d) vs. time to
step changes in the applied catalyst potential UWR for catalyst N. Reaction
conditions: H2/CO2 = 30 %/1.5 % (N2 balance), FT = 6 NL·h-1, T = 240 ºC.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
157
The obtained results with samples N and NA were consistent with the
general rules of chemical and electrochemical promotion [17]. In these two
samples the migration of K+ onto the catalyst-working electrode would
weaken the nickel chemical bond with the electron donor adsorbates
(hydrogen) and strengthen that with the electron acceptors (CO2) [2, 3, 6,
18, 24]. The increase in the binding strength of CO2 on the Ni surface
would favour the dissociative adsorption of CO2 through RWGS reaction
rather than CO2 methanation. Hence, the presence of K+ ions on the
catalyst-film led to a strong promotional effect in the CO2 hydrogenation
mechanism toward CO formation. This effect has also been reported in
several previous studies on chemical and electrochemical promotion with
K+ ions on different metal catalysts [12, 19, 21, 32, 33]. Moreover, it should
be noted that for both samples: N and NA, after each polarization at +2 V
the electropromotional effect showed to be fully reversible and reproducible
in terms of catalytic activity. It demonstrates the great advantage of the
EPOC effect compared to the conventional chemical promotion by
providing full control and optimization of the promoted state of the
catalyst in the course of the reaction [13].
On the other hand, it is interesting to note that sample NA showed the
higher catalytic activity in terms of reaction rate normalized per amount of
deposited Ni for all the applied catalyst potentials between +2 and -2 V.
This catalytic enhancement derived from the powder addition can be
explained considering the increase in the Ni ink porosity, as mentioned
above. Catalyst NA was globally found to be the most active catalyst.
Regarding the catalytic results obtained with catalyst GNA, it must be
taken into account that both the pure gold electrode and the gold paste
mixed with non-impregnated alumina powder showed to be completely
inactive under all the studied reaction conditions (and negligible electro-
Chapter 4
158
promotional effect). Hence, the residual catalytic activity of catalyst GNA
can be attributed to the presence of Ni particles impregnated on the
powder since negligible activity was observed on a Au film prepared with
inert α-Al2O3 prepared for blank experiments (not shown). Hence, the
obtained results in Figure 4.4 would confirm the absence of any electro-
promotional effect related to Ni particles dispersed on the α-Al2O3. It can
be explained considering the absence of conduction pathways of K+ ions
from the solid electrolyte to achieve the Ni particles dispersed on the α-
Al2O3 support. Summarizing, Ni metal particles provided by the deposited
metal ink were the active catalyst (not the impregnated Ni particles) which
were also electrochemically promoted for the CO2 hydrogenation reaction.
According to all these previous results, all the electrochemical promotions
experiments in the next section were performed with catalyst NA.
4.3.2. Kinetic study and electrochemical promotion experiments
Figure 4.5 shows the effect of gas flow rate, FT, on the CO2 reaction rate,
CO2 conversion and selectivity to CO and CH4 at the reference state, UWR =
2 V, and different feed compositions. The reference catalytic rate initially
increased with flow rate and reached a plateau at high values (FT > 75
Nml·min-1). This indicates the absence of any mass transfer limitation
phenomena above this flow rate. Hence, all the catalytic experiments
hereafter were carried out at 100 Nml·min-1. As expected, the CO2
conversion values decreased continuously upon increasing the flow rate.
Very interestingly, the CH4 selectivity also followed a negative trend.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
159
20
25
30
35
40
45
50
55
60
0 20 40 60 80 100 120 140 160 180 200 220
3
4
5
6
7 H2 / CO2 = 10
H2 / CO2 = 20
H2 / CO2 = 2
H2 / CO2 = 6
C
H4 s
ele
ctiv
ity /
%
20
25
30
35
40
45
50
55
60
T = 240 ºC
CO
sele
ctiv
ity /
%
a)
b)( r C
O2 /m
ol
CO
2·m
ol-1
Ni
· s-1
) x
105
FT / NmL min-1
0.0
0.5
1.0
1.5
2.0
2.5
CO
2 c
on
vers
ion
/ %
Figure 4.5. Effect of the overall gas flow rate on the (a) CO2 consumption rate and
CO2 conversion, (b) CH4 and CO selectivity under +2 V polarization for catalyst
NA. Reaction conditions: H2/CO2 = 30 %/1.5 % (N2 balance), T = 240 ºC.
This seems to suggest that CH4 was produced, to a large extent, from
the CO hydrogenation (eq. 10), as a consecutive step of the RWGS reaction,
rather that from the direct CO2 methanation reaction (4.3).
CO + 3H2 → CH4 + H2O (4.4)
In fact, concerning the kinetics of the reactions, reaction rate of CO2
methanation is generally considered to be slower than that of CO one [34].
Furthermore, it can also be observed that an increase in the hydrogen
Chapter 4
160
concentration led to higher CO2 conversion and CH4 selectivity, which will
be better discussed below.
Figures 4.6 and 4.7 show the steady state effect of applied potential on
reaction rates of CO2, CO and CH4 at 240 ºC under different H2 feed
composition, H2 = 3 – 30 %, (Figure 4.6) and CO2 feed composition, CO2 =
1.5 – 10 % (Figure 4.7). In all cases, a cathodic (negative) polarization
increased the reaction rate of the CO production (electrophilic behaviour)
while an anodic (positive) polarization enhanced the reaction rate of CH4
formation (electrophobic behaviour). Since the potential-induced change in
CO production rate was more pronounced for all the experiments than that
in CH4 one, the overall reaction rate of CO2 was found to increase at
negative polarization (overally electrophilic behaviour). Besides, very
useful information on the reaction kinetics at the reference state can be
drawn from these two figures, which can be then correlated with the
electrochemical promotion behaviours observed in these experiments. In
the absence of K+ promoter (positive polarizations), the rate of CO
formation exhibited a zero to negative order with respect to H2
concentration (Figure 4.6) and a marked positive order with respect to the
CO2 concentration (Figure 4.7). On the contrary, the rate of CH4 formation
showed a positive order with respect to H2 and a negative to zero order
with respect to CO2. As a direct consequence, the CO2 reaction rate was
positive order with respect to both H2 and CO2.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
161
4
6
8
10
12
2
4
6
8
10
-2 -1 0 1 20.0
0.5
1.0
1.5
2.0
2.5
3.0
H2 = 3 %
H2 = 9 %
H2 = 15 %
H2 = 30 %
(rC
O2
/ m
ol
CO
2·
mol-1
Ni
·s-1
)x1
05
c)
b)
(rC
O /
mol
CO
· m
ol-1
Ni
·s-1
)x1
05
Increasing K+
T = 240 ºC
100 Nml min-1
(rC
H4
/ m
ol
CH
4·
mol-1
Ni
·s-1
)x1
05
UWR / V
CO2 = 1.5 %a)
Figure 4.6. Effect of the H2 concentration on the reaction rate values of (a) CO2
consumption, (b) CO production and (c) CH4 production under different applied
potentials (UWR) for catalyst NA. Reaction conditions: CO2 = 1.5 % (N2 balance), FT
= 6 NL·h-1, T = 240 ºC.
Chapter 4
162
0
5
10
15
20
-2 -1 0 1 20
1
2
3
5
10
15
20
25
c)
b)
(rC
O /
mol
CO
· m
ol-1
Ni·
s-1)x
10
5
CO2 = 1.5 %
CO2 = 3 %
CO2 = 5 %
CO2 = 10 %
UWR / V
(rC
H4
/ m
ol
CH
4·
mol-1
Ni·
s-1)x
10
5
H2 = 30 %
T = 240 ºC
100 NmL min-1
(rC
O2
/ m
ol
CO
2·
mol-1
Ni·
s-1)x
10
5
a)
Increasing K+
Figure 4.7. Effect of the CO2 concentration on the reaction rate values of (a) CO2
consumption, (b) CO production and (c) CH4 production under different applied
potentials (UWR) for catalyst NA. Reaction conditions: H2 = 30 % (N2 balance), FT =
6 NL·h-1, T = 240 ºC.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
163
Similar trends were observed in other EPOC studies on CO2
hydrogenation [6, 18]. Hence, in view of these results, one could easily
explain the EPOC behaviour found upon by varying the catalyst potential.
As mentioned above, CO production through RWGS was favoured by
increasing CO2 feed concentration, which is equivalent to increase the CO2
chemisorption strength on the Ni surface via alkali electrochemical
promotion. On the other hand, since methanation reaction under reference
state was already inhibited by increasing the CO2 feed concentration, it is
expected that the potassium backspillover at negative potentials and the
consequent strengthening of the CO2 chemisorption also derived in a
decreasing of the methane formation rate. Hence, RWGS is an electrophilic
reaction, i.e., it is positive order in the electron acceptor reactant (CO2) and
negative or zero order in the electron donor reactant (H2), while CO2
methanation is an electrophobic reaction, i.e., it is zero-to negative order in
the CO2 and positive order in the H2 [18, 21]. Then, it is clear that the
EPOC phenomena allows to in-situ control the competitive chemisorption
of the different adsorbates on the catalyst surface, which may allow to
control the selectivity requirements as will be shown later.
Figure 4.8 depicts the effect of temperature on the reaction rates of CO2,
CO and CH4 with a feed composition of H2/CO2 = 30 %/1.5 % (N2 balance).
At the reference state (UWR = 2 V) and electropromoted (UWR < 2 V)
conditions, it can be observed that the reaction rate of CO2, as well as also
the production rate of CO and CH4, were enhanced at increasing
temperatures. A similar positive effect of temperature was also observed in
other EPOC studies with different metal catalysts [11, 18, 22, 24].
Chapter 4
164
0
10
20
30
40
50
-2 -1 0 1 20
5
10
15
0
20
40
60
b)
(rC
O /
mol
CO
· m
ol-1
Ni·
s-1)x
10
5
T = 240 ºC
T = 270 ºC
T = 300 ºC
c) 100 Nml min-1
UWR
/V
(rC
H4 /
mol
CH
4·
mol-1
Ni·
s-1)x
10
5
H2 / CO
2 = 30% / 1.5 %
Increasing
(rC
O2 /
mol
CO
2·
mol-1
Ni·
s-1)x
10
5
a)
Figure 4.8. Effect of the temperature on the reaction rate values of (a) CO2
consumption, (b) CO production and (c) CH4 production rate under different
applied potentials (UWR) for catalyst NA. Reaction conditions: H2/CO2 = 30 %/1.5 %
(N2 balance), FT = 6 NL·h-1.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
165
At all temperatures, the same EPOC behavior of the RWGS and
methanation reactions was found (previously discussed at 240 ºC), i.e.;
electrophilic CO and electrophobic CH4 productions, respectively.
Moreover, it is important to note that the Ni catalyst was activated via
electrochemical promotion in such a way that a very similar production
rate of CO was achieved under promoted conditions at 240 and 270 ºC as
observed in the absence of promoter at 270 and 300 ºC, respectively.
Hence, the application of an electric potential which corresponded with a
current of the order of few µA may cause a similar modification in the
catalytic performance as an increase in temperature of 30 ºC. It
demonstrates the great practical interest of the EPOC phenomenon in
view of energy savings by activating metal catalyst at lower temperatures
[13, 35, 36]. Finally, the apparent activation energy calculated via
Arrhenius plot under reference state (UWR = 2 V) was 74.26 kJ·mol-1 for
CO2 consumption. This value is in the same range as those reported in
other works. For instance, an activation energy of 40-80 kJ·mol-1 was
reported for the hydrogenation of CO2 over Ni catalyst [11]. Peebles et al.
obtained activation energies of 88.7 kJ mol-1 and 72.8-82.4 kJ mol-2 for the
methanation and dissociation of CO2 on Ni (100) for producing CH4 and
CO, respectively [37]. On the other hand, an activation energy of 62.14 kJ
mol-1 under electro-promoted conditions (-2 V) for CO2 consumptions was
obtained. The activation energy was reduced from 74.26 kJ·mol-1 (reference
state) to 62.14 kJ mol-1 (promotion conditions), which demonstrates the
positive effect of K+ ions on the kinetics of the process as already concluded
in other previous works of potassium-electropromotion [38, 39].
Chapter 4
166
0.8
1.2
1.6
2.0
2.4
1.0
1.5
2.0
2.5
3.0
3.5
-2 -1 0 1 2
0.2
0.4
0.6
0.8
1.0PI
K+=-6.78
PIK
+=98.38
PI+
K=286.14
PIK
+=206.21
PIK
+=30.75
PI+
K=129.15
a)
CO
2
T = 240 ºC
T = 270 ºC
T = 300 ºC
PIK
+=105.13
b)
C
OIncreasing
PIK
+=-63.68
PIK
+=-80.68
c)
C
H4
UWR
/ V
Figure 4.9 shows the variation of the reaction rate enhancement ratio
(ρi), calculated for CO2, CO and CH4 through eq. 4.4, with the applied
potential, under a feed composition of H2/CO2 = 30% / 1.5% (N2 balance)
and three different reaction temperatures.
Figure 4.9. Effect of the applied potential (UWR) on the rate enhancement ratio
(ρi) of (a) CO2, (b) CO and (c) CH4 at different temperatures (T = 240 ºC, 270 ºC
and 300 ºC), with H2/CO2 = 30 %/1.5 % (N2 balance) and FT = 6 NL·h-1 for catalyst
NA. The promotional index (PIK+) values are also depicted for an applied potential
of -0.5 V. Data obtained from the experimental results shown in Fig. 4.7.
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
167
The corresponding promotional indexes (PIK+, eq. 3.8) are also indicated
for an applied potential of -0.5 V. At all explored temperatures, a clear
electrophilic EPOC effect can be observed in the global CO2 reaction and
the CO production (ρ > 1, PIK+ > 0), due to the promotional effect K+ ions
promoters previously explained at negative potentials. The electrophobic
behavior of the CH4 formation rate was also verified (ρ < 1, PIK+ < 0).
Hence, CO showed the highest promotion index values as well as the most
pronounced increase in the reaction rate with respect to the reference
state. The higher the reaction temperature, the lower the value of the rate
enhancement ratios is, which was associated to the increase in the
reference reaction rates (r0). Regarding the promotional index, the
maximum values were obtained at 270 ºC, since it depended on the relative
increase of the catalytic rates and potassium coverages. This fact could be
attributed to the increase in the ionic conductivity of the solid electrolyte,
and hence of the potassium coverage at fixed potential, with the
temperature.
Finally, the influence of the applied potential on the steady-state
variation of the selectivity toward CO and CH4 at different temperatures
(T = 240, 270 and 300 ºC) with different feed composition (H2/CO2 = 2-20) is
shown in Figure 4.10. At all temperatures, the decrease in the applied
potential led to a strong increase in the CO selectivity (Figures 4.10.a2, b2
and c2) and a decrease of the CH4 selectivity (Figures 4.10.a1, b1 and c1).
For instance, with a H2/CO2 ratio of 20, the selectivity of the Nickel
catalyst toward CO production was enhanced up to more than 95 % with a
potassium coverage, θK+, between 0.019 and 0.066, depending on the
reaction temperature.
Chapter 4
168
0
10
20
30
40
50
-2 -1 0 1 2
60
70
80
90
100
T = 300 ºC
H2/CO
2 = 10
H2/CO
2 = 20
SC
H4
/%
H2/CO
2 = 2
H2/CO
2 = 6
SC
O/%
UWR / V
C1)
C2)0
10
20
30
40
50
-2 -1 0 1 2
50
60
70
80
90
100
T = 270 ºC
H2/CO
2 = 10
H2/CO
2 = 20
SC
H4
/%
H2/CO
2 = 2
H2/CO
2 = 6
SC
O/%
UWR / V
b1)
b2)0
10
20
30
40
50
60
-2 -1 0 1 2
40
50
60
70
80
90
T = 240 ºC
H2/CO
2 = 10
H2/CO
2 = 20
SC
H4
/%
H2/CO
2 = 2
H2/CO
2 = 6
SC
O/%
UWR / V
a1)
a2)
Figure 4.10. Effect of the applied potential (UWR) and H2/CO2 ratio feed
concentration on the selectivity of CH4 and CO, at (a) 240 ºC, (b) 270 ºC and (c) 300
ºC for catalyst NA. Reactions conditions: FT = 6 NL·h-1.
As reported in previous EPOC studies on CO2 hydrogenation [12, 19,
21, 23], the activity and selectivity of other metallic catalysts were also in-
situ modified by pumping alkali ions. A value of 75 % in methane
selectivity at T = 300 ºC (H2/CO2 = 7) was reported by Theleritis et al., [21].
In the present work, a value of 50 % was obtained at the reference state.
Although the selectivity value obtained in this study was lower, the main
advantage to be highlighted here is the use of a non-noble metal catalyst
(Ni). Moreover, CH4 production rate was increased against CO production
at higher H2 concentrations, in good agreement with the previous
discussion and other CO2 hydrogenation studies [6, 19, 22, 40]. The
decrease in temperature also enhanced the methanation selectivity, as
reported in other works related to conventional chemical promotion [40]
and electrochemical promotion [6, 11, 21]. Depending on both the applied
Electrochemical Promotion of Ni with Alkali Ions in the CO2 Hydrogenation Toward CO and CH4
169
potential and the reactions conditions, one can control the Ni catalytic
activity and the selectivity toward CO and CH4 by means of the controlled
migration of K+ ions from a solid electrolyte.
4.4 Conclusions
Different Ni catalysts films were prepared on K-βAl2O3 by combining
the organometallic paste deposition and the addition of a powder, and
tested in the CO2 hydrogenation reaction. The addition of the α-Al2O3
powder to the Ni ink resulted in a slightly increase of the film porosity as
shown in SEM images which turned into a higher catalytic activity.
The Ni catalyst films were stable under the studied reaction conditions.
The Ni catalyst remained in their reduced state during the catalytic tests
and the Ni particle size was stable as confirmed by XRD analysis. This
suggested that the possible thermal sintering effect was negligible, leading
to a reversible EPOC behaviour between the different potentiostatic
transitions.
Both the activity and selectivity of the catalysts were in-situ modified by
the controlled electrochemical migration of K+ ions from the solid
electrolyte (electro-active catalyst support). The reverse water-gas shift
and methanation reactions presented electrophilic and electrophobic EPOC
behaviours, respectively, since negative polarizations promoted the
production of CO decreasing the CH4 production rate.
In good agreement with the obtained catalytic results and with the
rules of chemical and electrochemical promotion, the kinetics experiments
confirmed that CO production rate presented a positive order with respect
Chapter 4
170
to the electron acceptor reactant (CO2) and a negative to zero order with
respect to the electron donor (H2).
The Ni catalytic selectivity can be strongly modified and control by the
application of an electric potential. Depending on the reaction conditions,
CO and CH4 selectivity was enhanced up to more than 95 and 45 %
respectively via EPOC. Hence, one could in-situ control the preferential
formation of syngas or CH4 production rate via CO2 hydrogenation, which
may be of significant practical importance.
4.5. References
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5.1. Introduction
5.2. Experimental
5.2.1 Catalytic activity measurements
5.2.2. Preparation of the
electrochemical catalyst
5.2.3. Characterization measurements
5.3. Results and discussion
5.3.1. Characterization of the Cu
cathodic-catalyst and deposited
electrodes
5.3.2. Electrocatalytic experiments for
CO2 conversion
5.4. Conclusions
5.5. References
CHAPTER 5:
Gas Phase Electrocatalytic Conversion
of CO2 on Cu Carbon-based Catalyst-
Electrodes Toward Fuels
INTRODUCTION
EXPERIMENTAL CHARACTERIZATION
RESULTS
T = 90 ºC, FCO2,cathode = 0.5 NmL·min-1,
FCO2,anode = 6 NmL·min-1.
The shift from fossil fuel based economy towards a renewable energy one is a central strategy for achieving
sustainability and energy efficiency in the chemical industry. Meanwhile, reducing CO2 emissions is the key to proceed
effectively in this direction.
In this sense, convert CO2 into useful low-carbon fuels seems to be attractive and promising solution due to the energy
demand. One of the most interesting technology is the electrocatalytic conversion of CO2
The reaction of CO2 conversion can be summarized according to the following scheme:
xCO2 + 2(2x – z + y/2)H+ + 2(2x – z + y/2)e- → CxHyOz + (2x – z)H2O
This process is based on a low temperature Proton Exchange Membrane (PEM) reactor configuration, consisting on a
membrane electrode assemble (MEA) formed by an Anode/Membrane/Cathode.
Electrochemical reactor
Influence of the applied current Effect of the temperature
H2/CO2 = 30 % /15 %, N2 balance, FT = 6 NL h-1
Then main aim of this study was to carry out the electroreduction of CO2 in gas-phase by using the H+ produced from steam electrolysis in order to convert CO2
into high added-value compounds for an industrial and environmental point.
TEM analysis
0
5
10
15
20
25
30
35
40
Cu-CNFCu-AC
I = -10 mA
I = -20 mA
I = -30 mA
(rC
O2
m
ol·
h-1·m
g-1C
u)·
10
2
Cu-G
Influence of the applied current
T = 90 ºC, FCO2,cathode = 0.5 NmL·min-1,
FCO2,anode = 6 NmL·min-1.
-The increase in the applied current led to a rise in
the CO2 consumption
- Catalytic activity: Cu-AC > Cu-CNF > Cu-G
Cu-G Cu-AC CuCNF
0
5
10
15
20
25
30
35
40
45
50
55
60
(rC
O2
m
ol·
h-1·m
g-1C
u)·
10
2
T = 80 ºC
T = 90 ºC
I = -20 mA, FCO2,cathode = 0.5 NmL·min-1,
FCO2,anode = 6 NmL·min-1.
-The reaction rate of CO2 increased
with increasing the temperature
Effect of the selectivity
Cu-CNF Cu-AC Cu-G
Cu-CNF Cu-AC Cu-G
Cu
c)
CNF
I = -10 mA I = -20 mA I = -30 mA
0
10
20
30
40
50
60
70
80
90
100c)b)
Acetaldehyde
Methyl formate
CH4
Sel
ecti
vit
y /
%
Methanol
Acetone
a)
I = -10 mA I = -20 mA I = -30 mA
0
10
20
30
40
50
60
70
80
90
100 Methanol
Acetone Acetaldehyde
Methyl formate
Sel
ecti
vit
y /
%
CH4
I = -10 mA I = -20 mA I = -30 mA
0
10
20
30
40
50
60
70
80
90
100
Methyl formate
2-Propanol
n-Propanol
Methanol
Ethanol CO
Acetaldehyde
Sel
ecti
vit
y /
%
CH4
CHAPTER 5. GAS PHASE ELECTROCATALYTIC
CONVERSION OF CO2 ON Cu CARBON-BASED
CATALYST-ELECTRODES TOWARD FUELS
CO2 feeding N2 purge
O.C.C. I = -20 mA O.C.C.
0 50 100 150 200 250 300 350 400
0
2
4
6
8
10
12
14
16
18
20
22
0 50 100 150 200 250 300 350
0
20
40
60
80
100
120
140
160
0 50 100 150 200 250 300 350
0
2
4
6
8
10
12
14
16
18
20
22
CH4
Acetaldehyde
Methyl formate
Acetone
Methanol
(r /
mm
ol·
h-1·m
g-1C
u)·
10
3
Time / min
a)
CH4
Acetaldehyde
Methyl formate
Acetone
Methanol
(r /
mm
ol·
h-1·m
g-1C
u)·
10
3
Time / min
b)
(r /
mm
ol·
h-1·m
g-1C
u)·
10
3
Time / min
CH4
CO
Acetaldehyde
Methyl formate
Methyl acetate
Methanol
Ethanol
2-propanol
n-Propanol
c)
e-e-I < 0
H+
MEA
(Membrane Electrode Assembly)
Cathodic Catalyst
Cu-G, Cu-AC, Cu-CNFAnodic Catalyst
IrO2
Membrane
Sterion®
Furnace
ThermocoupleAu wire Au wire
25 % H2O (N2)
Vent
CO2
GC analysis
CO2 + H+ + e- → Products H2O → 2H+ + 2e- + ½ O2
H+
H+
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
177
Abstract
A novel electrocatalytic system based on a low temperature proton
exchange membrane (Sterion®) was developed for the gas phase
electrocatalytic conversion of CO2. This configuration allows the
introduction of renewable energy in the chemical production chain via fuels
production from direct CO2 electro-reduction at atmospheric pressure and
low temperatures (below 90 ºC). For that purpose, three different Membrane
Electrode Assemblies (MEAs) based on three different Cu based cathodic-
catalyst were prepared and characterized: Cu-G/Sterion/IrO2, Cu-
AC/Sterion/IrO2 and Cu-CNF/Sterion/IrO2; graphite (G), activated
carbon (AC) and carbon nanofibers (CNF). Thus, H2O was fed and
electrolyzed on the IrO2 anode of the cell, thereby supplying H+ across the
membrane to react with CO2 in the cathodic-catalyst and leading to the
production of a mixture of syn-fuels (syn-gas, methanol, acetaldehyde,
methane…). Remarkably, the nature of the cathodic-catalyst carbon support
had a strong influence on the electrocatalytic activity of the system, being
the surface area of the carbon support the most important parameter.
Hence, the Cu-AC-based cathodic-catalyst showed the highest CO2
electrocatalytic activity, due to the highest surface area of the AC support
and the larger metal dispersion of the Cu particles leading to acetaldehyde
and methanol as the main reaction products. Besides, the lower
conductivity of the AC support the Cu-AC cathodic catalyst also required
the lowest energy consumption for the electrocatalytic conversion of CO2.
A
Chapter 5
178
5.1. Introduction
As seen in previous chapter (Chapter 4), two main technologies have
been proposed to reduce CO2 emissions: (i) capture and geological
sequestration of CO2 [1] and (ii) conversion into useful low-chain carbon
fuels [2]. Sequestration still has certain barriers that make it unaffordable
from an industrial point of view, such as the high cost of CO2 capture,
separation, purification and transportation. On the other hand, conversion
into fuels seems to be a more attractive and promising solution that can
meet the growing energy demands. The chemical conversion of CO2 can be
effectively performed via hydrogenation reactions [3-5]. This conversion can
be achieved by chemical [6, 7], photocatalytic [8], electrocatalytic [4, 9-12],
biological [13] and reforming [14]. Among them, the electrochemical
pathway has been recognized as an efficient way to convert CO2 to energy-
rich products. The process possesses several advantages, namely: (i)
control of the process by electrode potentials and reaction temperature; (ii)
the supporting electrolytes can be fully recycled so that the overall
chemical consumption can be minimized to simply water or wastewater;
(iii) the electrochemical reaction systems are compact, modular, on-
demand, and easy for scale-up applications; (iv) the electricity used to
drive the process can be potentially obtained from a renewable source; (v)
no external H2 is required for the CO2 reduction process as H+ are in situ
generated within the process. Hence, the valorization of the CO2 molecule
by electrochemical reduction has attracted worldwide interest due to its
potential environmental and economic benefits [2, 15-17]. This technology,
when coupled to a renewable energy source such as solar and wind, could
generate carbon neutral fuels or high added-value chemicals that are
conventionally derived from petroleum at a competitive price. As a matter
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
179
of fact, the electrochemical reduction of CO2 using a liquid electrolyte,
either aqueous or organic, is being actively investigated in literature [18,
19]. However, the main drawback of these processes is the recovery of the
reaction product from the liquid electrolyte as the energy required to
separate the products is higher than the energy stored in the produced
molecules [10]. In this sense, the gas phase electrocatalytic conversion of
CO2 to liquid fuels allows easy product separation since there are no
problems of solubility of CO2 as in the case of liquid phase/electrolytes and
no needs to recover the products from a liquid phase. Thus, gas-phase
electroreduction of CO2, mainly developed by the group of Prof. Centi [4, 9-
12], represents a valuable opportunity to incorporate renewable energy
into the value chain of chemical industries. In this regard, the obtained
products contain a higher energy density and are easier to transport and
store.
The gas phase electrocatalytic conversion of CO2 is based on the use of a
low temperature Proton Exchange Membrane (PEM) reactor configuration,
consisting on a membrane electrode assembly (MEA) formed by an
Anode/Membrane/Cathode system. Water is electrolyzed at the anode of
the cell leading to the formation of O2 and H+ which are electrochemically
supplied across the membrane to the cathodic-catalyst where they react
with the adsorbed CO2, leading to the formation of different molecules
according to the following general reaction:
xCO2 + 2(2x – z + y/2)H+ + 2(2x – z + y/2)e- → CxHyOz + (2x – z)H2O (5.1)
Hence, the influence of the cathodic-catalyst may have strong
importance on the resultant electrocatalytic activity of the system.
Chapter 5
180
In contrast to previous studies [4, 9-12], this work reports for the first
time a systematic study based on three different Cu cathodic-catalysts
prepared on three different carbon supports: graphite (G), activated carbon
(AC) and carbon nanofibers (CNF). The use of carbon materials have
proven to be the best catalyst supports for such applications due to their
specific properties, such as acid and base resistance, porosity, conductivity
and the possibility of recovering the metals by combustion of the supports
[20]. As a result, carbon materials have been used as conductive substrates
for metal nanoparticles in electrocatalysis for the conversion of CO2 [4, 10,
11]. The use of carbon-based electrocatalysts, e.g. similar to those used in
PEM fuel cells, is critical to obtain good performances and control the
selectivity in CO2 conversion. In this sense, carbon support plays multiples
roles in these types of systems by allowing a good dispersion of metal
nanoparticles and especially facilitates the effectiveness of electrons and
protons transport due to a better distribution of the metal nanoparticles [4].
Hence, in this work the role of different carbon supports as well as the
influence of the applied current and reaction temperature on the catalytic
activity and selectivity of the gas phase electrocatalytic conversion of CO2
have been studied.
5.2. Experimental
5.2.1. Catalytic activity measurements
The catalytic activity measurements were carried out in an
experimental set-up as shown in Figure 5.1. The electrocatalytic
experiments were carried out at atmospheric pressure with an overall gas
flow rate of 0.5 NmL min-1 of CO2 for the cathode and 6 NmL min-1 for the
anodic stream (60 % H2O/N2), at different temperatures (T = 80 ºC and 90
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
181
FIC
FIC
FIC
CO2
N2
N2
TIC
Vent
Water
saturator
Reactor
Potentiostat-
Galvanostat
micro GC
Temperature
Controller
Vent
Flow
controller
Furnace
ºC, optimum values for the operation of the Sterion® membrane). Reactant
and products from the cathodic chamber of the cell were analyzed by using
a double channel gas chromatograph (Bruker 450-GC) equipped with
Hayesep Q-Molsieve 13X consecutive columns and flame ionization
detectors. The detected reaction products were a mixture of syn-fuels: syn-
gas (H2, CO), CH4, CO, methanol, acetaldehyde, acetone, methyl formate,
ethanol, 2-propanol and n-propanol. The error in the carbon atom balance
did not exceed 5 %. A potentiostat/galvanostat (Voltalab 21, Radiometer
Analytical) was used to supply a constant current (-10 to -30 mA) between
the electrodes which were connected using gold wires. The potentiostat-
galvanostat was also used to perform galvanostatic voltammetry
measurements under different reaction conditions.
Figure 5.1. Scheme of the experimental set-up for the CO2 electrochemical
conversion.
Chapter 5
182
e-e-I < 0
H+
MEA
(Membrane Electrode Assembly)
Cathodic Catalyst
Cu-G, Cu-AC, Cu-CNFAnodic Catalyst
IrO2
Membrane
Sterion®
Furnace
ThermocoupleAu wire Au wire
25 % H2O (N2)
Vent
CO2
GC analysis
CO2 + H+ + e- → Products H2O → 2H+ + 2e- + ½ O2
H+
H+
The electrocatalytic experiments of CO2 electro-reduction were carried
out in a lab-scale continuous electrocatalytic reactor operating at
atmospheric pressure. Figure 5.2 shows a schematic drawn of the
electrocatalytic reactor.
The cell reactor was made of two quartz tubes that act as cathodic and
anodic compartments, and included two inlets, for CO2 and H2O/N2,
respectively, and two outlets streams. The system was heated with a
furnace up to 90 ºC connected to a K-type thermocouple and a temperature
control system.
Figure 5.2. Schematic drawn of the electrochemical reactor used for the CO2
electrochemical conversion.
Water was introduced into the anode side of the cell by flowing N2
through a saturator in order to achieve liquid/vapor equilibrium. The
water content in the anodic chamber of reaction mixture (25 % H2O/N2)
was controlled by the vapor pressure of water at the temperature of the
saturator (65 ºC). All lines placed downstream from the saturator were
heated above 100 ºC to prevent condensation. In this side, the electrolysis
of water was produced with the IrO2 electrode in order to produce protons
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
183
across the Sterion® membrane. Furthermore, the fed water stream was
also used to hydrate the Sterion® membrane and keep its proton
conductivity properties [4]. The cathodic part of the cell operates in contact
with a gas flow of pure CO2 (Praxair, Inc. certified standards 99.999 %
purity). Both gas flow rates (N2 for the anode and CO2 for the cathode)
were controlled by a set of mass flowmeters (Brooks 5850 E and 5850 S).
The selectivity towards each compound was calculated by the following
equation:
𝑋𝑖 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 / % = 𝐹𝑋𝑖
𝐹𝐶𝑂2−0 𝐹𝐶𝑂2
𝑥 100 (5.2)
5.2.2. Preparation of the solid electrolyte cell
Copper catalysts supported on G, AC and functionalized CNF were used
as cathode materials of the electrocatalytic reactor. Iridium (IV) oxide
(IrO2) was used as the anode.
Commercial graphite (Aldrich), commercial active carbon (Panreac) and
synthesized functionalized carbon nanofibers were used as starting
materials. Carbon nanofibers were prepared by the catalytic chemical
vapor deposition method (CVD) in a fixed-bed reactor at atmospheric
pressure. The synthesis was conducted over a Ni/SiO2 (10%, w/w) catalyst
at 600 ºC to obtain fishbone type carbon nanofibers, employing ethylene as
the carbon source and hydrogen as the carrier gas (C2H4/H2, 4/1, v/v, 900
Ncm3 min-1). The carbon deposit obtained was demineralized using HF
(48 %, v/v) in order to remove the parent catalyst particles and to avoid
any residual Ni effect in later catalysts preparation/characterization steps.
Chapter 5
184
The material was dried for 12 h at 383 K in air to remove water prior to
characterization. Further details regarding the CNF synthesis are given in
a previous work [21]. The functionalization of the CNFs was performed by
an oxidative treatment in HNO3 to introduce oxygen functionalities on the
carbon surface. Metal nanoparticles were later deposited on the graphite,
active carbon and functionalized carbon nanofibers by the impregnation
method. The different supports (G, AC and CNF) were placed in a glass
vessel and kept under vacuum at room temperature for 2 h to remove
water and other compounds adsorbed on the structure. A known volume of
ethanolic solution of Cu(NO3)2·3H2O (Panreac) (the minimum amount
required to wet the solid) was then poured over the sample. After 2 h, the
solvent was removed by evaporation under vacuum at 90 ºC in a rotary
evaporator. The catalysts were dried at 120 ºC overnight, calcined in N2
atmosphere at 350 ºC using a heating ramp of 5ºC/min and kept at that
temperature for 4 h. Finally, they were reduced in H2 at 350ºC for 2 h
(heating rate 5 ºC· min-1). The total load of metal was around 50 wt.%.
The catalyst inks for the preparation of each electrode were prepared by
mixing appropriate amounts of the catalysts, IrO2 commercial catalysts
powders (Alfa Aesar, 99 %), Cu-graphite powder, Cu-activated-carbon
powder and Cu-carbon nanofibers powder with a Nafion solution (5 wt.%,
Aldrich chemistry, Nafion® 117 solution) and isopropanol (Sigma Aldrich)
with a binder/solvent volume ratio of 0.04. The selection of IrO2 as the
anode for the three explored MEAs has been done according to its unique
and superior ability for water oxidation reaction in conventional PEM
electrolyzers [22]. Then, the different inks were deposited on Carbon paper
(Fuel Cell Earth) substrates at 65 ºC until a metal loading of 0.5 mg·cm-2
for the anode was obtained after drying. For the cathodic catalyst, the final
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
185
loading was measured by atomic absorption spectrophotometry obtaining
0.37, 0.20 and 0.38 mg·cm-2 for the Cu-G, Cu-AC and Cu-CNF cathodic
catalysts, respectively. The geometric surface area of both electrodes was
12.56 cm2 (4 cm of circular diameter electrode). A proton conducting
Sterion® membrane of 185 µm thickness (supplied by Hydrogen works)
was used as the electrolyte (H+ conductor material). Prior to use, the
Sterion® membrane was treated by successive immersion at 100 ºC for 2 h
in H2O2 in order to remove organic impurities, in H2SO4 for activation and
in deionized water to remove traces of solutions. Finally, in order to
prepare the membrane electrode assembly (MEA), the membrane was
sandwiched between a couple of electrodes. Then, the whole system was
hot-pressed using a press (GRASEBY SPEAC) at 120 ºC and a pressure of
1 metric ton for 3 min.
5.2.3. Characterization measurements
Cu based electrodes were characterized before reaction tests by X-Ray
Diffraction (XRD) analysis with a Philips PW-1710 instrument, using Ni-
filtered Cu Kα radiation (λ = 1.5404 Å). The samples were scanned at a rate
of 0.02º·step-1 over the range 20º ≤ 2θ ≤ 90º (scan time 2 s・step-1) and the
diffractograms were compared with the JCPDS-ICDD references.
Cu metal loading, on the cathodic powdered catalyst and per area of
deposited electrode was determined by atomic absorption
spectrophotometry, using a SPECTRA 220FS analyzer. The sample (ca. 0.5
g) was treated in 2 mL HCl, 3 mL HF and 2 mL H2O2 followed by
microwave digestion (523 K).
Temperature programmed reduction (TPR) experiments were conducted
for the different cathodic powder catalysts in a commercial Micromeritics
Chapter 5
186
AutoChem 2950 HP unit equipped with a TCD detector. Samples (ca.
0.15 g) were loaded into a U-shaped tube and ramped from room
temperature up to 900 ºC (10 ºC min−1), using a reducing gas mixture of
17.5% v/v H2/Ar (60 cm3 min−1).
Transmission electron microscopy (TEM) analyses for the powder
cathodic catalysts were conducted on a JEOL JEM-4000EX unit with an
accelerating voltage of 400 kV. Samples were prepared by ultrasonic
dispersion in acetone with a drop of the resulting suspension evaporated
onto a holey carbon-supported grid.
Finally, surface area/porosity measurements of powder catalysts were
conducted using a Micromeritics ASAP 2010 for activated carbon and a
QUADRASORB 3SI sorptometer apparatus for graphite and carbon
nanofibers. In both cases, N2 was used as the sorbate at -193 ºC. The
samples were outgased at 180 ºC under vacuum (5x10-3 Torr) for 12 h prior
to the analysis. Specific surface areas were determined by the multi-point
BET method and Langmuir. The microporosity of the materials was
evaluated by Howath-Kawazoe (HK) method and the mesoporosity was
calculated by the Barret-Joyner-Halenda (BJH) method.
5.3. Results and discussion
5.3.1. Characterization of the Cu carbon-based cathodic catalyst
The three different cathodic-catalysts were characterized by N2
adsorption, atomic absorption spectrophotometry, temperature-
programmed reduction (TPR), transmission electron microscopy (TEM)
and X-ray diffraction (XRD). Physicochemical properties of the supports,
catalyst and fresh electrodes are shown in Table 5.1.
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
187
AC showed high values of Langmuir area and total pore volume, as
previously described in literature [23] (Table 5.1). In this case, the N2
adsorption/desorption isotherm (not shown) can be described as a
combination of types I and IV isotherms (IUPAC classification). On the
other hand, graphite (G) was characterized by a low surface area and
limited porosity showing a type IV adsorption/desorption isotherm (not
shown), with a very small volume of adsorbed N2 (Table 5.1). This fact is in
good agreement with the low porosity of these materials [23]. Finally,
carbon nanofibers (CNF) showed a type IV N2 adsorption/desorption
isotherm (not show). In this case, a mesoporous nature was observed and a
BET surface area in the range commonly observed for these materials was
obtained (10-300 m2 g-1) [24] (Table 5.1).
After the metal introduction, an important decrease of BET and
Langmuir surface area and pore volume took place in all the cases, which
can be attributed to the partial pore blockage by metal particles [25]. On
the other hand, the total amount of Cu measured by the atomic absorption
spectrophotometry was closed to 50 % in three synthesized catalysts
powders.
Chapter 5
188
Ta
ble
5.1
. P
hysi
coch
em
ica
l p
rop
ert
ies
of
the s
up
port
s, c
ata
lyst
s, a
nd
fre
sh
ele
ctro
des.
P
arti
cle
siz
e
fro
m T
EM
/
nm
- 96
- 40
- 74
Ele
ctr
od
e
me
tal
we
igh
t
/ m
g C
u c
m-2
-
0.3
7
-
0.2
0
-
0.3
8
Po
wd
er m
eta
l
loa
din
g / w
t.%
51.4
6
-
53.9
7
-
51.0
5
-
TP
R-T
ma
x
/ ºC
-
198
-
207
-
202
To
tal
po
re
vo
lum
e / c
m3 g
-1
0.0
68
0.0
46
0.3
73
0.2
11
0.5
55
0.2
33
Su
rfa
ce
are
a
/ m
2g
-1
10
7
906
836
95
47
Sa
mp
le
G
Cu
-G
AC
Cu
-AC
CN
F
Cu
-CN
F
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
189
100 200 300 400 500 600 700 800
TC
D S
ign
al
(a.u
.)
Temperature / ºC
Cu-G
Cu-AC
Cu-CNF
TPR profiles associated with each catalyst are given in Figure 5.3. For
the three catalysts the maximum reduction temperature was obtained at
around 200 ºC (Tmax), which was characterized by a sharp peak. Tmax
obtained for each catalyst, associated with the first hydrogen consumption
peak, are also given in Table 5.1. On other hand, an additional shoulder
centered at around 250 ºC was observed for the cathodic-catalysts Cu-G
and Cu-AC [26].
Figure 5.3. TPR profiles of the fresh catalyst.
This bimodal distribution of H2-TPR was described by Kargol et al. [27].
The first temperature maximum is due to the reduction of CuO and partial
reduction of Cu(II) ions to Cu(I). The presence of this compound has been
proved by the XRD technique on the powder catalyst (not shown here). The
second maximum corresponds to the reduction of Cu(I) species strongly
interacting with the support. On the other hand, the Cu-CNF cathodic-
Chapter 5
190
catalyst suggested one-step process of reduction. This process is ascribed to
the direct reduction of CuO to metallic copper [27]. Finally, a third peak
(appearing at temperature range between 520 and 600 ºC) associated to
the carbon support gasification was also observed for cathodic-catalyst Cu-
CNF [24]. According to the obtained results, 350 ºC was chosen as a
suitable reduction temperature to ensure the metal activation without
affecting the surface properties of the supports.
Transmission electron microscopy (TEM) was used to determine the metal
particle size and characteristics of the three prepared Cu-based cathodic-
catalysts. Representative TEM micrographs are shown in Figure 5.4 ((a) Cu-G
powder, (b) Cu-AC powder and (c) Cu-CNF powder). In the present work, the
mean Cu particle size, evaluated as the surface-are weighted diameter (𝑠) was
calculated according to:
𝑠 =∑ 𝑛𝑖𝑑𝑖
3𝑖
𝑛𝑖𝑑𝑖2 (5.3)
where ni represents the number of particles of diameter di.
The estimated particle size for the different catalyst is listed in Table
5.1. TEM micrographs for cathodic-catalysts, Cu-G (Figure 5.4.a) and Cu-
CNF (Figure 3.c) showed a high particle size value (96 and 74 nm,
respectively) and hence a lower dispersion of Cu particles is expected with
respect to the Cu-AC powder. For this latter case (Figure 5.4.b), the
corresponding TEM micrographs showed a smaller particle size (40 nm)
and consequently, a higher dispersion of Cu particles on the AC support.
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
191
Cu
Cu
Cu
a)
b)
c)
CNF
Figure 5.4. Representative TEM images of the powders of (a) Cu-G, (b) Cu-AC
and (c) Cu-CNF.
This observation is related to the high values of surface area and total
pore volume obtained for the activated carbon support as discussed above.
It is worth noting that for the case of cathodic-catalyst based on CNF
Chapter 5
192
(Figure 5.4.c), the carbon nanofibers can also be observed in the TEM
images.
Figure 5.5 shows the XRD analysis of the resultant Cu-based electrodes:
Cu-G (a) Cu-AC (b) and Cu-CNF (c) (after deposition of the Cu cathodic-
catalysts powders on the carbon paper substrates). The inset of Figure
5.5.a, 5.5.b and 5.5.c shows the magnification of XRD patterns. All samples
showed two peaks around 2θ = 25º and 55º, which could be associated with
the presence of the carbon paper substrate used as the current collector
and gas diffusion layer. This fact was corroborated by the XRD spectrum of
this material (not shown here). The main diffraction Cu peaks (111), (200),
(220) and (331) appeared in the three cases at 2θ = 43.3º, 50.4º, 74.1º and
90 º, respectively. These peaks are associated with metallic copper and
exhibited a face-centered cubic (FCC) crystalline structure (JCPDS, 85-
1326) (Inset of Figure 5.5.a, 5.5.b and 5.5.c). Additionally, the presence of
small CuO peaks (111) and (223) could be detected at 2θ = 32.5º and 86.6º,
respectively (JCPDS, 78-2076). However, the intensity of these peaks is
very low which indicates that the copper is almost completely reduced and
it has not been oxidized during the catalyst ink deposition on the carbon
paper support.
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
193
20 30 40 50 60 70 80 90
20 30 40 50 60 70 80 90
CuO
Inte
nsi
ty / a
.u.
2 / º
C
Cu
(111)
(200)
(220)
(331)
Inte
nsi
ty /
a.u
.
2 / º
b)
20 30 40 50 60 70 80 90
20 30 40 50 60 70 80 90
(331)
(220)(2
00)
(111)
Inte
nsi
ty / a
.u.
2 / º
CuO
C
Cu
Inte
nsi
ty /
a.u
.
2 / º
a)
20 30 40 50 60 70 80 90
20 30 40 50 60 70 80 90
CuO
Inte
nsit
y /
a.u
.
2 / º
C
Cu
(11
1)
(20
0)
(22
0)
(33
1)
Inte
nsi
ty /
a.u
.
2 / º
c)
Figure 5.5. XRD analysis patterns of cathodic-catalysts on carbon paper
substrates: (a) Cu-G, (b) Cu-AC and (c) Cu-CNF. Insets in (a), (b) and (c) show the
magnification of XRD patterns of cathodic-catalysts Cu-G, Cu-AC and Cu-CNF,
respectively.
Chapter 5
194
Prior to the catalytic activity measurements, the three different MEAs
(Cu-G/Sterion/IrO2 (Figure 5.6.a), Cu-AC/Sterion/IrO2 (Figure 5.6.b) and
Cu-CNF/Sterion/IrO2 (Figure 5.6.c) were in-situ characterized by a
galvanostatic voltammetry under two different reaction atmospheres fed to
the cathodic chamber: under presence of CO2 (FCO2, cathode = 0.5 NmL·min-1,
FH2O, anode = 6 NmL·min-1) and without feeding CO2 to the cell (FCO2, cathode =
0 NmL·min-1, FH2O, anode = 6 NmL·min-1) at 90 ºC.
In this latter case, N2 was fed to the cathodic side in order to purge the cell.
The potential (UWC) variation was recorded with the applied current (I)
between 0 and -20 mA with a scan rate of 80 µA s-1
. It can be observed that for
the three MEAs without feeding CO2 to the cathode, water electrolysis began
between -1.2 and -1.6 V [28], according to the following electrochemical
reaction:
H2O → H+ + ½ O2 + e
- (5.4)
An increase in the applied current led to higher negative potential values
and hence to an increase in the protons production rate. At the cathodic side,
hydrogen can be obtained due to the combination of protons that were
transported through the protonic membrane and the electrons transferred from
the external circuit:
2H+ + 2e
- H2 (5.5)
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
195
-3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0
-25
-20
-15
-10
-5
0
5
-3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0
-25
-20
-15
-10
-5
0
5
-3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0
-25
-20
-15
-10
-5
0
5
FCO
2,cathode
= 0.5 Nml·min-1 / F
H2O,anode
= 6 Nml·min-1
FCO
2,cathode
= 0 Nml·min-1 / F
H2O,anode
= 6 Nml·min-1
Cu
rren
t /
mA
Potential / V
a)
FCO
2,cathode
= 0.5 Nml·min-1 / F
H2O,anode
= 6 Nml·min-1
FCO
2,cathode
= 0 Nml·min-1 / F
H2O,anode
= 6 Nml·min-1
Cu
rren
t /
mA
Potential / V
b)
FCO
2,cathode
= 0.5 Nml·min-1 / F
H2O,anode
= 6 Nml·min-1
FCO
2,cathode
= 0 Nml·min-1 / F
H2O,anode
= 6 Nml·min-1
Cu
rren
t /
mA
Potential / V
c)
Figure 5.6. Influence of the reaction atmosphere on the current-potential
curves obtained during a galvanostatic voltammetry for cathodic-catalysts on
carbon paper substrates: (a) Cu-G, (b) Cu-AC and (c) Cu-CNF. Conditions:
temperature = 90 ºC, sweep rate = 80 µA·s-1.
Chapter 5
196
On the other hand, it can be observed that for the three MEAs, a higher
negative current values was obtained at fixed potential when CO2 was fed
to the cathode of the electrochemical system (FCO2, cathode = 0.5 Nml·min-1 /
FH2O,anode = 6 Nml·min-1). It demonstrates that the CO2 present in the gas
phase took part in the electrocatalytic process by its further adsorption
and reaction on the cathodic-catalyst with the electrochemically supplied
H+. In fact the presence of CO2 decreased the potential of the electrolysis
cell (for the same current), then acting as a depolarizating agent for the
electrochemical cathodic reaction [29]. The use of different molecules as
depolarizating agents has been studied for the electrolytic production of H2
at high temperatures in Solid Oxide Electrolysers. In this regard, the use
of CH4 [30, 31], CO [32] and C [29, 30] allowed to strongly decrease the
required electrical power input for the electrolysis process via reaction
with the O2- ions transported across the Anionic Conductor Electrolyte. As
can be observed in the Figure 5.6, the highest difference in the
voltammetry experiments performed with and without CO2 (highest
depolarization effect) occurred with Cu-AC cathodic-catalyst. This fact can
be attributed to the higher number of Cu active sites due to its higher
dispersion on the high surface area AC support exposed to the gas phase.
It probably facilitates the CO2 adsorption leading to faster reaction
kinetics with H+, increasing the current at fixed potential. Then, one may
expect a higher electrocatalytic activity of the cathodic-catalyst Cu-AC for
the CO2 electro-reduction (as will be shown later). Additionally, it can be
observed that cathodic-catalyst Cu-AC achieved the higher potential range
during voltammetry to reach the applied currents (up to -20 mA). It is due
to the lower electrical conductivity (around 250 Ω-1·m-1) of the AC support
vs. G and CNF which increase the overall electrical resistance of the MEA.
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
197
Hence, the lowest potential range was obtained with cathodic-catalyst Cu-
G, due to its highest electrical conductivity (around 1600 Ω-1·m-1) among
the carbonaceous supports. Finally, carbon nanofibers exhibited an
intermediate electrical conductivity value (around 900 Ω-1·m-1), leading to
intermediate vales of the current-potential curves.
5.3.2. Electrocatalytic experiments for CO2 conversion
Figure 5.7 shows the time-on-stream evolution of the different
measured products rates for a constant applied current of -20 mA at 90 ºC
for the three different cathodes under study at Temperature = 90 ºC,
FCO2,cathode = 0.5 NmL·min-1, FH2O,anode = 6 NmL·min-1. Initially, under open
circuit conditions (O.C.C, no current application), no products were
obtained at any case. Then, a constant current of -20 mA was applied for
approximately 300 min under the same reaction atmosphere. During this
current imposition step, hydrogen (not shown here), and different products
such as methanol, acetaldehyde, acetone, methane, methyl formate, carbon
monoxide, methyl acetate, ethanol, 2-propanol and n-propanol were
obtained via CO2 electro-reduction (reactions 5.6-5.15).
Most of these products have already been identified in similar previous
studies of electrocatalytic conversion of CO2 [4, 9-12] using Fe, Co, Cu and Pt
as cathodic-catalyst over CNT according to the following electrochemical
reactions.
CO2 + 6H+ + 6e- → CH3OH + H2O (5.6)
2CO2 + 10H+ + 10e- → CH3CHO + 3H2O (5.7)
3CO2 + 16H+ + 16e- → CH3COCH3 + 5H2O (5.8)
Chapter 5
198
CO2 + 8H+ + 8e- → CH4 + 2H2O (5.9)
2CO2 + 8H+ + 8e- → HCOOCH3 + 2H2O (5.10)
CO2 + 2H+ + 2e- → CO + H2O (5.11)
3CO2 + 14H+ + 14e- → CH3COOCH3 + 4H2O (5.12)
2CO2 + 12H+ + 12e- → CH3CH2OH + 3H2O (5.13)
3CO2 + 18H+ + 18e- → CH3CH(OH)CH3 + 5H2O (5.14)
3CO2 + 18H+ + 18e- → CH3CH2CH2OH + 5H2O (5.15)
Finally, in all the experiments, the cathodic side of the cell was purged
with N2 (30 Nml·min-1) under open circuit conditions in order to remove all
the products for subsequent reaction experiments.
In first place it should be mentioned that the electrochemical nature of
the different obtained products vs. the catalytic route (CO2 hydrogenation
via previous H2 evolution reaction) was confirmed with further
experiments performed under open circuit conditions by co-feeding CO2
and H2 at the same temperature to the cathode (not shown here). Under
these explored reaction conditions, no reaction products were detected
confirming the electrochemical nature of the products observed on Figure
5.7. It is further supported by the modification of the current-potential
curves previously shown under presence of CO2 during voltammetry
experiments which confirms CO2 adsorption and further reaction with H+.
This is in good agreement with previous works of catalytic CO2
hydrogenation on Cu-based catalyst which showed that temperatures
typically above 250 ºC are required [33, 34] for the process.
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
199
CO2 feeding N2 purge
O.C.C. I = -20 mA O.C.C.
0 50 100 150 200 250 300 350 400
0
2
4
6
8
10
12
14
16
18
20
22
0 50 100 150 200 250 300 350
0
20
40
60
80
100
120
140
160
0 50 100 150 200 250 300 350
0
2
4
6
8
10
12
14
16
18
20
22
CH4
Acetaldehyde
Methyl formate
Acetone
Methanol
(r /
mm
ol·
h-1·m
g-1C
u)·
10
3
Time / min
a)
CH4
Acetaldehyde
Methyl formate
Acetone
Methanol
(r /
mm
ol·
h-1·m
g-1C
u)·
10
3
Time / min
b)
(r /
mm
ol·
h-1·m
g-1C
u)·
10
3
Time / min
CH4
CO
Acetaldehyde
Methyl formate
Methyl acetate
Methanol
Ethanol
2-propanol
n-Propanol
c)
Figure 5.7. Time-on-stream evolution of different products for a constant current
of -20 mA. Cathodic-catalysts on carbon paper substrates: (a) Cu-G, (b) Cu-AC
and (c) Cu-CNF. Conditions: temperature = 90 ºC, FCO2,cathode = 0.5 NmL·min-1,
FH2O,anode = 6 NmL·min-1
Chapter 5
200
The coupling of the steam electrolysis process in the proposed
configuration allows to directly supplying H+ to the cathodic catalyst, of
higher reactivity, which allows working at lower reaction temperatures vs.
catalytic CO2 hydrogenation processes.
On the other hand it can be observed that among the three investigated
cathodic-catalyst, the Cu-AC system showed the higher electrocatalytic
activity (around five times higher) in comparison with the other two
cathodic-catalysts: Cu-G and Cu-CNF. This observation is in good
agreement with the galvanostatic voltammetry curves previously shown
which demonstrates the higher depolarization effect caused by CO2 on the
MEA based on Cu-AC. This higher electrocatalytic activity could be related
to the higher surface area of the AC support leading to a higher metal
dispersion of the Cu particles, which is a key factor for CO2 adsorption and
further reaction via the electrochemical reactions (5.6-5.10). It should be
mentioned that on these experiments the effect of the conductivity of the
different carbon supports it is not affecting the electrocatalytic activity of
the system since the three MEAS are compared under the same current
application (-20 mA), i.e., under the same H+ supplied rate to the cathode
(r=I/(nF) = 1.036·10-7 mol s-1). However, is clear that a high potential will
be required to achieve the current of -20 mA for the case of the Cu-AC
cathodic catalyst (-2.72 V) vs. the Cu-G and Cu-CNF (-1.54 V and -2.11 V
respectively), therefore leading to a higher electrical energy consumption.
Concerning the different obtained products it can be observed that
methanol was the main reaction product for the case of the Cu-G cathodic-
catalyst while acetaldehyde was the main one for the case of Cu-AC and
Cu-CNF systems. This observation can be explained according to previous
studies of catalytic CO2 hydrogenation that have shown that Cu particles
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
201
a) b)
0
5
10
15
20
25
30
35
40
Cu-CNFCu-AC
I = -10 mA
I = -20 mA
I = -30 mA
(rC
O2
/mm
ol·
h-1·m
g-1C
u)·
10
2
Cu-G
T = 90 ºC
Cu-G Cu-AC CuCNF0
5
10
15
20
25
30
35
40
45
50
55
60
(rC
O2
/mm
ol·
h-1·m
g-1C
u)·
10
2
T = 80 ºC
T = 90 ºCI = -20 mA
of higher size are more selective for methanol production rather than for
acetaldehyde [35, 36]. On the other it should be mentioned that a wider
variety of reaction products with higher number of carbon atoms was
obtained for the case of Cu-CNF cathodic-catalyst, which could be related
to the presence of the functional oxygen groups in the CNF support. As
reported by Genovese et al, [4], the nature of functional groups on the
carbon surface when CNF is used as a catalyst support has a significant
influence on determining the possibility to form > C1 products from CO2.
Figure 5.8.a and 5.8.b shows the effect of the applied current and
reaction temperature, respectively, in the normalized CO2 electrocatalytic
steady state reaction rate (per mg of deposited Cu) after 300 min of
polarization at each current.
Figure 5.8. (a) Effect of the current at 90 ºC and (b) temperature (T = 80 and 90
ºC) at I = -20 mA on the steady state CO2 consumption rate for Cu/Sterion/IrO2
and Cu-C/Sterion/IrO2 electrodes. Conditions: FCO2,cathode = 0.5 NmL·min-1,
FH2O,anode = 6 NmL·min-1.
In agreement with the previous experiment, it can be observed that for
all the explored reaction conditions (applied currents and temperatures),
the electrocatalytic activity of the cathodic-catalyst Cu-AC is higher than
Chapter 5
202
that of Cu-G and Cu-CNF. Additionally, it can be observed that the higher
the applied current, the higher the CO2 reaction rate, attributed to the
higher amount of H+ ions electrochemically supplied. On the other hand,
the higher the reaction temperature, the higher the electrocatalytic
activity of the system. This fact can be attributed to the enhanced kinetics
of electrochemical reactions at higher reaction temperature [37]. However,
90 ºC was the highest explored temperature, which ensures the stability
and conductivity of the protonic membrane under suitable humidity
conditions.
The influence of the applied current (I = -10, -20 and -30 mA) on the
steady state variation of the selectivity of the different products was
studied at 90 ºC (Figure 5.9) after the polarization at each current for 300
min: for cathodic-catalysts Cu-G, Cu-AC and Cu-CNF, Figures 5.9.a, 5.9.b
and 5.9.c, respectively.
Figure 5.9. Effect of the applied current on the steady state selectivity toward the
different products on Cu cathodic-catalysts: (a) Cu-G, (b) Cu-AC and (c) Cu-CNF.
Conditions: Temperature = 90 ºC, FCO2,cathode = 0.5 NmL·min-1, FH2O,anode = 6
NmL·min-1.
I = -10 mA I = -20 mA I = -30 mA
0
10
20
30
40
50
60
70
80
90
100c)b)
Acetaldehyde
Methyl formate
CH4
Sele
ctiv
ity /
%
Methanol
Acetone
a)
I = -10 mA I = -20 mA I = -30 mA
0
10
20
30
40
50
60
70
80
90
100 Methanol
Acetone Acetaldehyde
Methyl formate
Sele
ctiv
ity /
%
CH4
I = -10 mA I = -20 mA I = -30 mA
0
10
20
30
40
50
60
70
80
90
100
Methyl formate
2-Propanol
n-Propanol
Methanol
Ethanol CO
Acetaldehyde
Sel
ecti
vit
y /
%
CH4
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
203
As shown before, methanol was the main obtained product on cathodic-
catalyst Cu-G. In this case, the selectivity to methanol varied between 60
% under I = -10 mA and 75 % for an applied current of I = -30 mA. On the
other hand for the cathodic-catalysts Cu-AC and Cu-CNF, acetaldehyde
was the main reaction product with selectivity values around 60 %. It can
be observed that for the case of Cu-G and Cu-AC (Figure 5.9.a and 5.9.b)
an increase in the applied cathodic current (from -10 mA to -30 mA) led to
an increase in the methanol selectivity at the expense of a decrease in the
acetaldehyde. In this case is clear that at higher supplied rate of protons at
higher intensity values the selectivity shift to lighter and more saturated
compounds in agreement with previous studies [38, 39], i.e. toward
methanol production (reaction 5.6). However, for the Cu-CNF cathodic-
catalyst, the variation of product selectivity with the applied current does
not seem to follow a clear trend, probably due to the high number of
obtained products obtained and to the complex number of reactions that
occurred on the system.
In order to finally address, the best electrocatalytic system in terms of
activity but also energy consumption, Table 5.2 shows a comparison
between the three different MEAs at T = 90 ºC and I = -30 mA for the
overall energy consumption for CO2 conversion (kWh mol-1 CO2) as well as
energy consumption for the production of methanol (kWh mol-1 CH3OH) and
acetaldehyde (kWh mol-1 CH3CHO).
Chapter 5
204
Table 5.2. Comparison of energy consumption for different membrane
electrode assemblies in the electrocatalytic conversion of CO2 at 90 ºC and I
= -30 mA
Sample kWh mol-1CO2
kWh mol-1CH3OH kWh mol-1
CH3CHO
Cu-G 199.8 260.6 2574.7
Cu-AC 92.4 197.9 185.9
Cu-CNF 189.1 23785.8 274.2
Even though, the AC support has the lower electrical conductivity value
(as shown on the voltammetry curves of Figure 5.6) its higher activity at
the same current conditions shown on Figures 5.7 and Figures 5.8 led to
the lower energy consumption values for CO2 conversion and methanol and
acetaldehyde production. Hence, the MEA based on the Cu-AC cathodic-
catalyst consumed less energy per kg of methanol produced and per kg of
acetaldehyde produced than the other two MEAs based on Cu-G and Cu-
CNF cathodic-catalyst. For each system the lower energy consumption is
evidently obtained with the most selective product: methanol for the case
of Cu-G cathodic catalyst and acetaldehyde for the case of Cu-AC and Cu-
CNF. These results allow to conclude that among the three different
explored cathodic catalyst, the higher activity of the Cu-AC cathodic
catalyst due to the higher dispersion of Cu particles on the high surface
area AC support is the most important parameter besides its lower
electrical conductivity. However, is clear that suitable values of both
parameters: activity and conductivity should be found for the design of
efficient cathodic catalyst for the CO2 electrochemical conversion. Then the
use of novel supports (e.g. reduced graphene powder, carbon black,
molybdenum-carbide-derived carbon…) of high surface area and high
electrical conductivity may decrease the overall energy consumption values
reported on Table 5.2 in view of the practical application of this
Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
205
configuration for renewable production of solar fuels via electrochemical
conversion of CO2.
5.4. Conclusions
Three different Cu-based cathodic-catalysts (metal loading closed to 50
%) supported on Graphite (G), Activated Carbon (AC) and Carbon
Nanofibers (CNF) have been prepared via impregnation technique and
characterized by different techniques.
Among them, the Cu-AC-based cathodic-catalyst showed the highest
CO2 electrocatalytic activity under all the explored reaction conditions, due
to the highest surface area of the AC support and the larger metal
dispersion of the Cu particles. Methanol was the main reaction product for
the case of the Cu-G cathodic-catalyst while acetaldehyde was the main
one for the case of Cu-AC and Cu-CNF systems which can be attributed to
the higher size of Cu particle sizes in these two latter cases more selective
for methanol formation.
Concerning the product selectivity variation with the current, for the
cased of Cu-G and Cu-AC cathodic catalyst, an increase in the applied
cathodic current led to an increase in the methanol selectivity at the
expense of a decrease in the acetaldehyde. In this case is clear that at
higher supplied rate of protons at higher intensity values the selectivity
shift to lighter and more saturated compounds. However, for the Cu-CNF
cathodic-catalyst, the variation of product selectivity with the applied
current does not seem to follow a clear trend, probably due to the high
number of obtained products obtained and to the complex number of
reactions that occurred on the system.
Chapter 5
206
Besides the lower conductivity of the AC support, the energy
consumption values for CO2 conversion and methanol and acetaldehyde
production was achieved with the MEA based on Cu-AC cathodic catalyst.
Therefore, the higher activity of the Cu-AC cathodic catalyst due to the
higher dispersion of Cu particles on the high surface area AC support is
the most important parameter besides its lower electrical conductivity
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Gas phase electrocatalytic conversion of CO2 to syn-fuels on Cu based catalyst-electrodes
209
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6.1. General Conclusions
6.2. Recommendations
CHAPTER 6:
General Conclusions
and Recommendations
Chapter 6
213
The present work has been aimed at the study of novel electrocatalytic
systems for energetic and environmental applications. This chapter lists
the main conclusions derived from the research performed in this Doctoral
Thesis. In addition, some recommendations are suggested to be taken into
account in further studies.
6.1. Conclusions
The results obtained from the present research work support the
following main conclusions:
o The developed Pt/YSZ/Pt solid electrolyte cell allowed the
production of syngas of flexible H2/CO ratios from ethanol-water
streams by the application of different polarizations. In this way,
the proposed configuration may be of great interest especially for
biorefinery applications where H2, syngas and electricity may be
produced from bioethanol.
o The proposed double chamber Pt-YSZporous/YSZ/Pt solid electrolyte
cell allowed to simultaneously produce H2 and C2 hydrocarbons
from methane. In this regard, the carbon generated in the methane
decomposition step served as a depolarizating agent in the steam
electrolysis process decreasing the electrical energy consumption.
o The study reported in Chapter 3 evaluated the energetic analysis
for the hydrogen production via catalytic steam and electrochemical
ethanol reforming processes by Aspen HYSYS simulation. The
highest yield in the hydrogen production and the lowest energy
consumption were obtained in the electrochemical reforming of
General Conclusions and Recommendations
214
ethanol. These results demonstrated the potential of this process for
obtaining high purity hydrogen in a single reaction/separation step.
o The Ni-αAl2O3/K-βAl2O3/Au solid electrolyte cell can be
electrochemically promoted for the reverse water gas shift reaction
demonstrating the occurrence of the EPOC phenomenon.
Additionally, this system allowed the possibility of control the Ni
catalytic activity and selectivity toward CO and CH4 by means of
the controlled migration of K+ ions from a solid electrolyte. Thus, it
may have significant importance for the practical use of
electrochemistry to activate non noble metal catalyst for the CO2
hydrogenation process.
o The developed Cu-G/Sterion/IrO2, Cu-AC/Sterion/IrO2 and Cu-
CNF/Sterion/IrO2 MEAs allowed, without feeding H2, obtaining a
wide range of compounds such as methanol, acetaldehyde and
methane via electrocatalytic reduction of CO2. These results
demonstrate the interest of these PEM reactors configurations to
obtain high added-value compounds from an industrial and
environmental point of view.
6.2. Recommendations
The following proposals can be stated in order to complete and extend
this research work:
o To investigate the behaviour of non-noble metals in the steam
electrolysis process and partial oxidation of ethanol as well as the
simultaneous production/separation of H2 and C2 hydrocarbons.
This way, the economy of this process could be improved in view of
its practical application.
Chapter 6
215
o To perform an exergetic analysis to select the optimum operating
parameters for the catalytic steam reforming and the
electrochemical reforming of ethanol, allowing to evaluate the
location, cause and true magnitude of energy wastes and losses in
the system.
o To use YSZ as a solid electrolyte in order to improve the activity
towards CH4 production in the CO2 hydrogenation via EPOC
phenomenon.
o To evaluate the impact of different catalysts and preparation
techniques on the performance of the proposed PEM cell
configuration in the CO2 valorisation process. Additionally, the
temperature of the reaction process could be increased by using a
Sterion® membrane doped with H3PO4.
List of Publications and
Conferences
List of publications and conferences
219
Publications
1. Coupling Catalysis and Electrocatalysis for Hydrogen Production
in a Solid Electrolite Membrane Reactor. A. de Lucas-Consuegra,
N. Gutiérrez-Guerra, A. Caravaca, J.C. Serrano-Ruiz, J. L.
Valverde. Applied Catalysis A: General, 483, 25-30 (2014).
2. Electrochemical Reforming vs. Catalytic Reforming of Ethanol: A
process Energy Analysis for Hydrogen Production. N. Gutiérrez-
Guerra, M. Jiménez-Vázquez, J.C. Serrano-Ruiz, J. L. Valverde,
A. de Lucas-Consuegra. Chemical Engineering and
Processing: Process Intensification, 95, 9-16 (2015).
3. Direct Production of flexible H2/CO Synthesis Gas in a Solid
Electrolyte Membrane Reactor. A. de Lucas-Consuegra, N.
Gutiérrez-Guerra, J.L. Endrino, J.C. Serrano-Ruiz, J. L.
Valverde. Journal of Solid State Electrochemistry. DOI:
10.1007/s10008-015-2922-8.
4. Electrochemical activation of Ni catalyst with potassium ionic
conductors for CO2 hydrogenation. N. Gutiérrez-Guerra, J.
González-Cobos, J.C. Serrano-Ruiz, J. L. Valverde, A. de Lucas-
Consuegra. Topics in Catalysis. In press.
5. A gas phase electrocatalytic conversion of CO2 to liquid fuels at
lower temperature using different carbon-based support. N.
Gutiérrez-Guerra, L. Moreno-López, J.C. Serrano-Ruiz, J. L.
Valverde, A. de Lucas-Consuegra. Submitted to Applied Catalysis
B.
Patents
1. Procedimiento de Obtención de Gas de Síntesis. Antonio de Lucas
Consuegra, Nuria Gutiérrez Guerra, Jesús González Cobos, Carmen
Jiménez Borja, José Luis Valverde Palomino, P201330975, 28
Junio 2013.
2. Procedimiento de Obtención de Metanol a partir de CO2 y Sistema
Electroquímico para Realizarlo. Juan Carlos Serrano Ruíz, Antonio
de Lucas Consuegra, Nuria Gutiérrez-Guerra, José Luis Valverde
Palomino. Submitted
List of publications and conferences
220
Conferences
Oral presentations in congress:
1. Conversión electrocatalítica de CO2 en compuestos de interés
industrial. N. Gutiérrez-Guerra, C. Marchante, J.C. Serrano-
Ruíz, J.L. Valverde, A. de Lucas-Consuegra. JJ. II. SECAT’
2014. I Encuentro de Jóvenes Investigadores de la SECAT,
Málaga (España), Junio 2014.
2. Promoción electroquímica de catalizadores de Ni para la
hidrogenación de CO2. N. Gutiérrez-Guerra, J. González-Cobos, J.C.
Serrano-Ruiz, J. L. Valverde, A. de Lucas-Consuegra. SECAT’
2015, Barcelona (España), Julio 2015.
Poster presentations in congress:
1. Nuevas perspectivas de la electrocatálisis en fase gas. A. de Lucas-
Consuegra, N. Gutiérrez, C. Jiménez-Borja, J. González-Cobos,
J.L. Endrino, J.L. Valverde. SECAT’13. Sevilla (España), Junio,
2013.
2. Análisis energético y exergético de los procesos de reformado
catalítico y electro-reformado de etanol para la producción de
hidrógeno. N. Gutiérrez, J.L. Valverde, J.C. Serrano-Ruíz, A. de
Lucas-Consuegra. I WORKSHOP EN INGENIERÍA QUÍMICA
(FEIQ), Ciudad Real (España), Noviembre 2013
3. Electrochemical Regeneration of Pt catalyst for hydrogen
production in a solid electrolyte membrane reactor. A. de Lucas-
Consuegra, N. Gutiérrez-Guerra, A. Caravaca, J.C. Serrano-Ruíz,
J.L. Valverde. ISE 2014, Laussane (Switzerland), Septiembre
2014.
4. Regeneración electroquímica para la producción de hidrógeno en
un reactor de membrana. . N. Gutiérrez, J.L. Valverde, J.C.
Serrano-Ruíz, A. de Lucas-Consuegra. IV Jornadas Doctorales.
Cuenca (España), Octubre, 2014.
5. Electrochemical modification of Ni catalyst with alkali ionic
conductors for CO2 hydrogenation. N. Gutiérrez-Guerra, J.
González-Cobos, J.C. Serrano-Ruiz, J. L. Valverde, A. de Lucas-
Consuegra. ISE 2015, Taipei, (Taiwan), October 2015.
Anexo
Anexo
i
Anexo
ii
Anexo
iii
Anexo
iv
Anexo
v
Anexo
vi
Anexo
vii
Procedimiento de obtención de gas de síntesis
DESCRIPCIÓN
La presente invención se refiere a un procedimiento de obtención de gas de
síntesis (H2/CO) de ratio controlable mediante un proceso catalítico y
electroquímico que emplea una celda electroquímica formada por 5
electrolitos sólidos conductores aniónicos o catiónicos. El control del ratio
H2/CO se lleva a cabo en una única etapa bajo condiciones constantes de
operación, es decir, a temperatura constante de la celda electroquímica y
condiciones constantes de composición y concentración de la corriente de
entrada. 10
Por tanto, la presente invención se engloba en el campo técnico de la
producción de gas de síntesis y para su utilización en la industria
petroquímica o en la producción de combustibles.
15
ESTADO DE LA TÉCNICA ANTERIOR
El gas de síntesis (mezcla de H2/CO) es conocido por tener una gran
variedad de aplicaciones en la industria petroquímica. Por ejemplo, el gas
de síntesis puede ser empleado en la producción de amoníaco o metanol. 20
Además, el gas de síntesis se puede utilizar como producto intermedio en
la producción de gasolinas sintéticas, para su uso como combustible o
lubricante a través de la síntesis de Fischer-Tropsch. Para estas
aplicaciones el ratio H2/CO requerido es típicamente de 2. Sin embargo,
existen otros procesos dentro de la industria petroquímica como son los 25
procesos de oxosíntesis que requieren ratios de H2/CO menores,
Anexo
viii
comprendidos entre 1 y 2, o incluso monóxido de carbono (CO) puro, como
ocurre en los procesos de carbonilación. Por otro lado, en la industria
petroquímica también existen muchos procesos donde se requiere
hidrógeno (H2) de alta pureza, tales procesos son por ejemplo reacciones de
hidrogenación, interesando en este caso obtener un ratio H2/CO mayor de 5
2, lo más alto posible.
El gas de síntesis se obtiene generalmente a nivel industrial mediante
procesos catalíticos de reformado o de oxidación parcial de hidrocarburos,
principalmente a partir de metano (EP0168892 A2). Éste tipo de procesos 10
permite obtener un ratio H2/CO fijo y típicamente de 3. Para obtener un
ratio H2/CO distinto son necesarias etapas adicionales de purificación,
separación y conversión como por ejemplo: reacciones de desplazamiento
del agua en estado gaseoso (denominadas en inglés water gas shift),
procesos de adsorción a presión u oxidación preferencial de CO. Estas 15
etapas adicionales, previas al proceso de síntesis, implican una mayor
complejidad del proceso así como mayores costes de producción del
producto final.
Otra posibilidad conocida de variar el ratio H2/CO de forma controlada se 20
realiza mediante la adición controlada de oxígeno (O2) puro a la atmósfera
donde se lleva a cabo la reacción de síntesis [Cao, Y. et al Energ. Fuel.
2008, 22, 1720-1730], donde se produce una oxidación parcial o reformado
autotérmico que modifica la concentración de CO producido. La adición de
O2 puro en este tipo de procesos implica etapas previas y adicionales de 25
separación del mismo del nitrógeno (N2) del aire que implica una mayor
complejidad del proceso, al añadir más etapas al proceso. Por otro lado, se
puede controlar el ratio H2/CO en estos procesos ajustando las condiciones
Anexo
ix
de operación tales como la temperatura a la que se lleva a cabo la síntesis o
la relación entre el hidrocarburo de partida y el O2 añadido. La complejidad
y los costes de estos procesos son altos porque las temperaturas utilizadas
suelen ser altas, mayores de 1000 ºC y se requieren de reactores de dos
entradas de gases y dos salidas de gases para poder trabajar en doble 5
atmósfera [US47993904].
Por tanto, para superar todos los problemas técnicos mencionados es
necesario desarrollar un nuevo proceso de obtención de gas de síntesis de
ratio controlable de H2/CO. 10
DESCRIPCION DE LA INVENCIÓN
La presente invención se refiere a un procedimiento de obtención de gas de
síntesis (H2/CO) de ratio controlable mediante un proceso catalítico y
electroquímico que emplea una celda electroquímica formada por 15
electrolitos sólidos conductores iónicos, aniónicos o catiónicos. El control
del ratio H2/CO se lleva a cabo bajo condiciones constantes de operación, es
decir, a temperatura constante de la celda electroquímica y condiciones
constantes de composición y concentración de la corriente de entrada.
20
En la presente invención la corriente de entrada se selecciona de entre una
corriente gaseosa de hidrocarburos ligeros junto con una corriente de vapor
de agua, o una corriente gaseosa que contiene al menos un alcohol (C1-C3).
Por “hidrocarburos ligeros” se entiende a aquellos compuestos químicos 25
orgánicos formados únicamente de hidrógeno y carbono (C1-C4), incluyendo
al gas natural.
Anexo
x
El gas natural es un gas combustible que proviene de formaciones
geológicas, por lo que constituye una fuente de energía no renovable.
Además de metano, el gas natural puede contener dióxido de carbono,
etano, propano, butano y nitrógeno, entre otros gases.
5
Por tanto, en la presente invención, los hidrocarburos ligeros se
seleccionan de la lista que comprende metano, etano, propano, butano, gas
natural o cualquiera de sus combinaciones.
En el caso de que el conductor electrolito sólido sea un material conductor 10
aniónico, por ejemplo conductor de iones oxígeno (O2-), en la presente
invención éste comprende al menos un electrodo selectivo a la electrólisis
del agua y al menos un contraelectrodo selectivo a la reacción de reformado
y a la oxidación parcial de la corriente de entrada de la celda
electroquímica. 15
Por tanto, en la presente invención, cuando se emplean conductores
aniónicos, la adición de corrientes gaseosas de hidrocarburos
humidificadas o de corrientes gaseosas alcohólicas, junto con o sin una
corriente de vapor de agua, va a permitir que además del gas de síntesis 20
obtenido por reformado convencional catalítico en el catalizador
electroquímico se produzcan procesos adicionales electrocatalíticos que
permitan controlar el ratio H2/CO final bajo condiciones constantes, es
decir, a temperatura constante de la celda electroquímica y condiciones
constantes de composición y concentración de la corriente de entrada. 25
Estos procesos adicionales son principalmente el proceso de electrólisis de
vapor de agua (H2O --> H2 + O2-) que permite producir una mayor cantidad
de H2 y la oxidación electroquímica y catalítica del hidrocarburo o alcohol
que no haya reaccionado y del CO producido a partir de los iones O2- y las
Anexo
xi
moléculas de O2, ambos generados en el proceso de electrólisis anterior. De
este modo el ajuste final del ratio H2/CO se lleva a cabo en una sola etapa.
El H2 adicional producido en el proceso de electrólisis así como la oxidación
de parte del CO producido a dióxido de carbono (CO2) permite modificar
considerablemente el ratio del gas de síntesis. 5
Por tanto, cuando la celda electroquímica se encuentra a una temperatura
de entre 300 ºC y 980 ºC, y bajo unas condiciones constantes de
composición y concentración de la corriente de entrada, tiene lugar el
proceso catalítico de reformado sobre el contraelectrodo selectivo a este 10
proceso. Adicionalmente, bajo la aplicación de corriente eléctrica ocurre la
reacción de electrólisis con la consecuente producción de H2.
Simultáneamente los iones O2- generados en la reacción electroquímica son
transportados por el electrolito sólido conductor hasta el contraelectrodo
que actúa como catalizador de la oxidación electrocatalítica de la corriente 15
de entrada y del CO con la consecuente producción de gas de síntesis
(H2/CO). Además parte del CO, puede ser oxidado a CO2 por oxidación
electroquímica, lo que permite un control neto del ratio H2/CO del gas de
síntesis producido al controlar la velocidad de cada uno de los procesos con
la intensidad eléctrica. Es la intensidad de voltaje aplicada la que permite 20
controlar la velocidad electroquímica de los procesos mencionados.
En el caso de que el conductor electrolito sólido sea un material conductor
catiónico, por ejemplo conductor de iones sodio Na+ y potasio K+, en la
presente invención éste comprende un electrodo catalizador selectivo a 25
proceso de reformado de la corriente de entrada de la celda electroquímica
y un contraelectrodo metálico. Esta configuración permite promocionar por
vía electroquímica el proceso de reformado de la corriente de entrada,
Anexo
xii
mediante el envío de iones promotores desde el material conductor
catiónico al electrodo selectivo del proceso de reformado.
De este modo mediante el conocido fenómeno de promoción electroquímica
de catalizadores heterogéneos o efecto NEMCA (del acrónimo inglés Non 5
Faradaic Electrochemical Modification of Catalitic Activity), la presencia
de los iones electropositivos por ejemplo de iones Na+ y K+ en el electrodo
catalizador favorece la quimisorción de moléculas electronegativas como es
el agua frente al hidrocarburo o el alcohol que forman la corriente de
entrada. En la presente invención, la adsorción controlable de la corriente 10
de entrada se realiza mediante la variación del potencial eléctrico que
varía el contenido de promotor enviado al electrodo catalizador
permitiendo controlar el grado de descomposición de la corriente de
entrada y el grado del proceso de reformado, con ello el ratio de H2/CO del
gas de síntesis resultante obtenido. 15
En el caso de utilizar, en la presente invención un conductor electrolito
sólido catiónico (conductor Na+ o K+), el ajuste final del ratio H2/CO se
produce por el envío de promotores iónicos (Na+ o K+) mediante corriente
eléctrica al electrodo catalizador que modifican la adsorción del agua en los 20
centros activos y con ello la cinética del proceso catalítico. Esto ocurre bajo
una concentración constante de la corriente de entrada y una temperatura
específica de operación de la celda electroquímica.
Por tanto, las principales ventajas con respecto a las técnicas 25
convencionales de reformado es, primeramente, que no es necesaria la
incorporación de una corriente de O2 puro en la celda electroquímica ya que
éste se produce in situ durante el proceso de electrólisis, evitándose de este
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xiii
modo etapas previas adicionales como por ejemplo 20 la separación previa
adicional de N2 del aire.
En la presente invención, además del gas de síntesis obtenido por
reformado convencional catalítico en el catalizador electroquímico se 5
produzcan procesos adicionales electrocatalíticos que permitan controlar el
ratio H2/CO, es decir, en una 25 única etapa se produce gas de síntesis de
ratio H2/CO controlable.
Además, la celda electroquímica que se puede utilizar es sencilla, no 10
requiere de complejas cámaras de separación de atmósferas. En la
presente invención, el control del ratio H2/CO se lleva a cabo mediante una
variación del voltaje aplicado a los electrodos que permite el control de los
procesos electroquímicos. Las condiciones de operación son constantes, la
síntesis se realiza a una temperatura constante y a una temperatura baja 15
en comparación con las utilizadas en las técnicas convencionales.
La corriente de entrada no sólo se limita a hidrocarburos ligeros gaseosos
húmedos, también se pueden utilizar corrientes gaseosas de alcoholes
puras o humedecidas. 20
Por tanto, un primer aspecto de la invención se refiere a un procedimiento
para producir gas de síntesis, de ratio H2/CO controlable, que comprende el
paso de una corriente de entrada seleccionada de entre una corriente
gaseosa de hidrocarburos ligeros y una corriente de vapor de agua, o una 25
corriente gaseosa que contiene al menos un alcohol (C1-C3) a una celda
electroquímica que se encuentra a una temperatura de entre 300ºC y
980ºC, caracterizado porque dicha celda electroquímica contiene un
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xiv
conductor electrolito sólido iónico al que se le aplica un potencial de entre -
3 y +3 voltios.
Por “celda electroquímica” se entiende en la presente invención un
dispositivo capaz de transformar una corriente eléctrica en una reacción 5
química de oxidación-reducción que no tiene lugar de modo espontáneo. La
celda electroquímica también se refiere en la presente invención a un
reactor electroquímico adecuado para su uso a nivel industrial en cualquier
configuración conocida por cualquier experto en la materia, como por
ejemplo, un reactor electroquímico con configuración tubular o de tipo 10
monolítico.
Preferiblemente la corriente de entrada no contiene O2 puro.
Preferiblemente, la corriente de entrada está diluida en una corriente de 15
gas inerte, donde el gas inerte se selecciona de la lista que comprende
nitrógeno (N2), helio (He), neón (Ne), argón (Ar), kriptón (Kr) y xenón (Xe).
Preferiblemente la corriente de gas inerte es de N2. La corriente de entrada
se puede diluir hasta en un 98% en volumen en dicha corriente de gas
inerte. 20
En una realización preferida, la celda electroquímica se encuentra a una
temperatura 30 de entre 500ºC y 900ºC.
En otra realización preferida, el potencial aplicado es de entre -2,5 y +2,5 25
voltios. Más preferiblemente, de entre -2 y +2 voltios.
Preferiblemente, en la presente invención los hidrocarburos ligeros
gaseosos se seleccionan de la lista que comprende metano, etano, propano,
Anexo
xv
butano, gas natural o cualquiera de sus combinaciones. En una realización
más preferida el hidrocarburo ligero es una combinación de hidrocarburos
ligeros que comprende al menos metano. En otra realización preferida, el
hidrocarburo ligero es gas natural.
5
La proporción de hidrocarburo ligero y vapor de agua depende del
hidrocarburo utilizado, por ejemplo, en el caso de que el hidrocarburo sea
metano, la proporción preferida será de aproximadamente 1:3.
Preferiblemente, en la presente invención el alcohol se selecciona de la 10
lista que comprende metanol, etanol, propanol o cualquiera de sus
combinaciones. Más preferiblemente, el alcohol es metanol o etanol.
En la presente invención, los alcoholes pueden ser bioalcoholes obtenidos
por la acción de un microorganismo o por algún otro procedimiento 15
biotecnológico. Por ejemplo, se pueden utilizar alcoholes procedentes de
corrientes residuales de alcoholeras con elevada graduación, de hasta 90ºC.
En otra realización preferida, al paso de la corriente gaseosa que contiene
al menos un alcohol (C1-C3) además se añade una corriente de vapor de 20
agua.
Las corrientes de entrada gaseosas de hidrocarburos o de alcohol y vapor
de agua se pueden mezclar antes de pasar a la celda electroquímica o
pueden pasar sin 25 mezclarse previamente. 25
Preferiblemente las corrientes gaseosas de hidrocarburo o alcohol y la
corriente de vapor de agua se mezclan antes de pasar a la celda
electroquímica.
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Por otro lado, en una realización preferida, el conductor electrolito sólido
iónico es un 30 conductor aniónico que conduce iones oxígeno (O2-).
En otra realización preferida, el conductor aniónico comprende un
electrolito sólido que se selecciona de entre óxido de zirconio, óxidos de 5
titanio, óxido de itrio estabilizado con óxido de zirconio, óxido de zirconio
estabilizado con calcio, perovskitas con conductividad mixta o cualquiera
de sus combinaciones.
Además, preferiblemente, el conductor aniónico comprende al menos un
electrodo selectivo a la electrólisis del agua y al menos un contraelectrodo 10
selectivo a la reacción de reformado y al proceso de oxidación parcial de la
corriente de entrada.
En otra realización preferida, el electrodo selectivo a la electrólisis del
agua es de platino (Pt). 15
Preferiblemente, el contraelectrodo selectivo a la reacción de reformado de
la corriente de entrada se selecciona de entre níquel (Ni), platino (Pt),
paladio (Pd) o cualquiera de sus combinaciones.
En otra realización preferida, la celda electroquímica que contiene un 20
conductor aniónico como el descrito anteriormente se encuentra a una
temperatura de entre 700 y 900 ºC cuando la corriente de entrada es una
corriente gaseosa de hidrocarburos ligeros y una corriente de vapor de
agua.
25
Anexo
xvii
En otra realización preferida, la celda electroquímica que contiene un
conductor aniónico como el descrito anteriormente se encuentra a una
temperatura de entre 500 y 750 ºC cuando la corriente de entrada es una
corriente gaseosa que contiene al menos un alcohol (C1-C3).
5
Por otro lado, en otra realización preferida, el conductor electrolito sólido
iónico es un conductor catiónico. La corriente de entrada en este caso
también está formada por una corriente gaseosa de hidrocarburos ligeros y
una corriente de vapor de agua, o una corriente gaseosa que contiene al
menos un alcohol (C1-C3). Sin embargo, en el caso de que se quiera utilizar 10
el conductor electrolito iónico catiónico, la corriente gaseosa que contiene al
menos un alcohol (C1-C3) debe contener también una 30 corriente de vapor
de agua.
Preferiblemente, el conductor catiónico conduce iones sodio Na+ o potasio 15
K+.
En otra realización preferida, el electrolito catiónico se selecciona de entre
Na-β-Al2O3, K-β-Al2O3, NASICON, LISICON o cualquiera de sus
combinaciones.
20
En otra realización preferida, el conductor catiónico además comprende al
menos un electrodo metálico selectivo al proceso de reformado de la
corriente de entrada y al menos un contraelectrodo metálico.
Preferiblemente, el electrodo metálico selectivo al proceso de reformado de
la corriente de entrada es de platino (Pt). 25
Preferiblemente, el contraelectrodo metálico es de oro (Au).
Anexo
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En otra realización preferida, la celda electroquímica, que contiene un
conductor catiónico como el descrito anteriormente, se encuentra a una
temperatura de entre 700 y 900 ºC cuando la corriente de entrada es una
corriente gaseosa de hidrocarburos 15 ligeros y una corriente de vapor de
agua. 5
En otra realización preferida, la celda electroquímica, que contiene un
conductor catiónico como el descrito anteriormente, se encuentra a una
temperatura de entre 500 y 750 ºC cuando la corriente de entrada es una
corriente gaseosa que contiene al 20 menos un alcohol (C1-C3) y que añade 10
una corriente de vapor de agua.
La fuente de voltaje utilizada para aplicar el voltaje mencionado
anteriormente a la celda electroquímica puede ser una fuente convencional
con procedencia de energía fósil o nuclear o una fuente renovable que 15
utiliza por ejemplo, energías hidráulica, solar, eólica, geotérmica, marina
y/o biomasa. Por lo que, por último, en una realización preferida, el
procedimiento utiliza una fuente convencional o renovable para la
aplicación del potencial. Más referiblemente, el procedimiento utiliza una
fuente convencional. 20
A lo largo de la descripción y las reivindicaciones la palabra "comprende" y
sus variantes no pretenden excluir otras características técnicas, aditivos,
componentes o pasos. Para los expertos en la materia, otros objetos,
ventajas y características de la invención se desprenderán en parte de la 25
descripción y en parte de la práctica de la invención. Los siguientes
ejemplos y figuras se proporcionan a modo de ilustración, y no se pretende
que sean limitativos de la presente invención.
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xix
BREVE DESCRIPCIÓN DE LAS FIGURAS
FIG. 1. Representación de una celda electroquímica que utiliza un
conductor electrolito sólido iónico, que puede ser tanto aniónico como
catiónico. 5
FIG. 2. Representación esquemática de un conductor electrolito sólido que
actúa como conductor aniónico que comprende un electrolito sólido
aniónico, un electrodo selectivo a la electrólisis del agua y un
contraelectrodo selectivo a la reacción de reformado y a la oxidación parcial 10
de la corriente de entrada.
FIG. 3. Variación del ratio H2/CO en función del potencial aplicado para
diferentes temperaturas de reacción. Condiciones: [CH4] = 1%, [H2O] = 3%,
[N2] = 96%, F = 100 ml/min, con conductor aniónico. 15
FIG. 4. Gráfica dinámica donde se muestras los distintos ratios H2/CO
obtenidos en función del potencial aplicado en un determinado periodo de
tiempo.
20
FIG. 5. Representación esquemática de un conductor electrolito sólido que
actúa como conductor catiónico que comprende un electrolito sólido
catiónico, un electrodo metálico selectivo al proceso de reformado de la
corriente de entrada y un contraelectrodo metálico.
25
FIG. 6. Variación del ratio H2/CO en función del potencial aplicado para
diferentes temperaturas de reacción. Condiciones: [CH4] = 1%, [H2O] = 3%,
[N2] = 96%, F = 100 ml/min, con conductor catiónico.
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FIG. 7. Variación del ratio H2/CO en función del potencial aplicado para
diferentes temperaturas de reacción. Condiciones: [CH3OH] = 0,7%, [H2O]
= 2%, [N2] = 97,3%, F = 100 ml/min, con conductor aniónico.
EJEMPLO 5
FIG. 1. muestra la configuración de una celda electroquímica de
laboratorio que utiliza conductores electrolitos sólidos iónicos, tanto
aniónicos (9) como catiónicos (10). Esta celda consta de los siguientes
elementos: 10
(1) Electrodo de trabajo
(2) Contralectrodo
(3) Salida de gases
(4) Tapa metálica
(5) Serpentín de refrigeración 15
(5) Tubo de alúmina perforado
(5) Hilos de oro
(5) Tubo de cuarzo
(9) Conductor electrolito sólido aniónico
(10) Conductor electrolito sólido catiónico 20
(11) Entrada de gases
En la celda electroquímica que se muestra en FIG. 1., los conductores
electrolitos sólidos aniónico (9) y catiónico (10) están dentro del tubo de
cuarzo (8) que limita la zona donde se produce el gas de síntesis de ratio
H2/CO controlado y los procesos adicionales electrocatalíticos en fase 25
Anexo
xxi
gaseosa. Este tubo de cuarzo (8) está cerrado por una tapa metálica (4) que
acopla un tubo de alúmina perforado (6). El electrodo de trabajo (1) y el
contraelectrodo (2) se conectan con los conductores electrolitos sólidos
aniónico (9) o catiónico (10) a través de este tubo de alúmina perforado (6),
el potencial se aplica a estos electrodos (1) y (2). Las distintas 5
temperaturas que se alcanzan en la celda electroquímica se consiguen
mediante el serpentín de refrigeración (5) acoplada a la tapa metálica (4) y
mediante hilos de oro (7) que actúan como conductores térmicos y que se
introducen dentro de la celda a través del tubo de alúmina. La corriente de
entrada se introduce por la entrada de gases (11) y el gas de síntesis 10
producido se recoge a través de la salida de gases (3).
Ejemplo 1:
En este primer ejemplo, para producir gas de síntesis de ratio controlable 15
se utilizó una celda electroquímica como la que se representa en FIG. 1.
que comprende un conductor electrolito sólido aniónico como el que se
muestra en FIG. 2.
El conductor electrolito sólido aniónico representado en FIG. 2. consta de 20
los siguientes elementos:
(12) Contraelectrodo selectivo al proceso de reformado y oxidación parcial
de la corriente de entrada (por ejemplo CH4 → CH2, CO, CO2) es platino
(Pt) 25
(13) Electrodo de trabajo selectivo a la electrólisis del agua (H2O + 2e- →
H2) es platino (Pt)
Anexo
xxii
(14) Electrolito sólido aniónico tipo YSZ (óxido de ytrio estabilizado con
óxido de zirconio)
FIG. 2. muestra una representación esquemática de un conductor aniónico
que comprende un electrolito sólido aniónico que actúa como conductor tipo 5
YSZ de iones O2- y que además comprende un electrodo de trabajo selectivo
a la electrólisis del agua (13) y un contraelectrodo selectivo a la reacción de
reformado y oxidación parcial 20 de la corriente de entrada (12).
Ambos electrodos son conectados a una fuente de alimentación que permite 10
la aplicación de la intensidad eléctrica al sistema y que, por tanto, permite
el control de la composición del gas de síntesis en condiciones fijas de
operación (temperatura de operación de la celda electroquímica) y de
reacción (composición y concentración de la corriente de entrada de la
celda electroquímica). 15
El acoplamiento del proceso de electrólisis al proceso catalítico de
reformado permite llevar a cabo la producción adicional de H2 así como la
oxidación parcial del hidrocarburo (y por tanto el ajuste del ratio) sin
necesidad de alimentar oxígeno puro al reactor electroquímico (evitando 20
etapas previas de separación del mismo del aire). El O2 es generado in-situ
en el propio proceso lo que permite la presencia de reacciones secundarias
de oxidación total y parcial.
Para llevar a cabo la producción de gas de síntesis de ratio variable 25
utilizando esta celda electroquímica y el electrolito sólido aniónico descrito,
se introdujo una corriente de entrada compuesta por metano [CH4] = 1 %,
Anexo
xxiii
vapor de agua [H2O] = 3 % y nitrógeno como gas inerte [N2] = 96%, siendo
el caudal de esta corriente de F = 100 ml/min.
En FIG. 3. se puede observar cómo se puede variar el ratio H2/CO obtenido
en función del potencial y las diferentes temperaturas de reacción 5
aplicadas. Esta figura demuestra que la variación del voltaje permite el
control del ratio H2/CO. Los valores del ratio H2/CO obtenidos en esta celda
electroquímica que comprende un conductor electrolito sólido aniónico
oscilan de entre 1,5 y 11 para un rango de temperaturas de entre 750 ºC y
800 ºC. 10
Por otro lado FIG. 4. muestra los ratios de H2, CO y CO2 frente al tiempo
obtenidos usando la configuración de la celda electroquímica descrita en
este ejemplo. La palabra OCP se refiere al estado de circuito abierto en el
que se encuentra la celda electroquímica cuando no se aplica ningún 15
potencial. Durante los periodos de tiempo durante los cuales la celda
electroquímica está en circuito abierto, el ratio H2/CO se mantiene
prácticamente constante. En los periodos de tiempo donde se aplica un
potencial de 2,5 voltios se observa una respuesta inmediata, menor de 5
minutos, que se corresponde con el aumento drástico del ratio H2/CO en 20
función del voltaje aplicado.
Ejemplo 2:
En este ejemplo se utiliza una celda electroquímica como la que se 25
representa en FIG. 1. con un conductor electrolito sólido catiónico como el
que se muestra en FIG. 5. para producir gas de síntesis.
Anexo
xxiv
El electrolito sólido catiónico representado en la figura 5 consta de los
siguientes elementos:
(15) Electrodo de trabajo de platino selectivo al proceso de reformado de la
corriente de entrada (por ejemplo CH4 + H2O → H2, CO, CO2) 5
(16) Contraelectrodo de Au
(17) Electrolito sólido catiónico tipo Na-β-Al2O3
FIG. 5. muestra una representación esquemática de un electrolito sólido
catiónico (17) que actúa como conductor tipo Na-β-Al2O3 de iones Na+ y que
comprende un electrodo selectivo al proceso de reformado de la corriente de 10
entrada (15) y un contraelectrodo de Au (16).
Al igual que en el ejemplo 1, los electrodos son conectados a una fuente de
alimentación que permite la aplicación de la intensidad eléctrica al sistema
y que, por tanto, permite el control de la composición del gas de síntesis en 15
condiciones fijas de operación (temperatura de operación de la celda
electroquímica) y de reacción (composición y concentración de la corriente
de entrada de la celda electroquímica).
Para llevar a cabo la producción de gas de síntesis de ratio variable 20
utilizando la celda electroquímica y el electrolito sólido aniónico descrito,
se introdujo una corriente de entrada compuesta por metano [CH4] = 1%,
vapor de agua [H2O]= 3% y nitrógeno como gas inerte [N2] = 96%, siendo el
caudal de esta corriente de F = 100 ml/min.
25
El control de la cantidad de iones sodio Na+ promotores enviados al
electrodo catalizador se lleva a cabo mediante la aplicación controlada de
Anexo
xxv
corriente eléctrica que permite controlar la adsorción de las especies que
participan en el proceso catalítico y por ende el ratio del gas de síntesis
producido.
En FIG. 6. se muestra la variación del ratio H2/CO en función del potencial 5
aplicado para diferentes temperaturas de reacción a partir de una
corriente humidificada de metano. Esta figura demuestra que la variación
del voltaje permite el control del ratio H2/CO. Los valores del ratio H2/CO
obtenidos en esta celda electroquímica que comprende un conductor
electrolito sólido catiónico oscilan de entre valores de 6 y 30 para un rango 10
de temperaturas de entre 450 ºC y 550 ºC.
Ejemplo 3:
En este ejemplo se utiliza una celda electroquímica como la que se 15
representa en FIG. 1. con el conductor electrolito sólido aniónico que se
describe en el ejemplo 1 y se representa en FIG. 2 para producir gas de
síntesis.
Para llevar a cabo la producción de gas de síntesis de ratio controlable 20
utilizando esta celda electroquímica y el electrolito sólido aniónico descrito,
se introdujo una corriente de entrada compuesta por metanol [CH3OH] =
0,7 %, vapor de agua [H2O] = 2 % y nitrógeno como gas inerte [N2] = 97,3%,
siendo el caudal de esta corriente de F = 100 ml/min.
25
En FIG. 7. se puede observar cómo se puede variar el ratio H2/CO obtenido
en función del potencial y las diferentes temperaturas de reacción
Anexo
xxvi
aplicadas. Esta figura demuestra que la variación del voltaje permite el
control del ratio H2/CO. Los valores del ratio H2/CO obtenidos en esta celda
electroquímica que comprende un conductor electrolito sólido aniónico
oscilan de entre 2,4 y 9,13 para un rango de temperaturas de entre 500 ºC
y 600 ºC. 5
Los ejemplos 1 a 3 proporcionados a modo de ilustración no pretenden ser
limitativos de la presente invención. Aunque se refieran a una celda
electroquímica tamaño laboratorio, esta celda podría ser sustituida por
configuraciones tubulares o configuraciones de tipo reactor monolítico 10
(denominado en inglés Monolithic Electro20 promoted reactor) a escala
industrial.
REIVINDICACIONES
1. Un procedimiento para producir gas de síntesis, de ratio H2/CO 15
controlable, que comprende el paso de una corriente de entrada
seleccionada de entre una corriente gaseosa de hidrocarburos ligeros y
una corriente de vapor de agua, o una corriente gaseosa que contiene al
menos un alcohol (C1-C3) a una celda electroquímica que se encuentra a
una temperatura de entre 300ºC y 980ºC, caracterizado porque dicha 20
celda electroquímica contiene un conductor electrolito sólido iónico al
que se le aplica un potencial de entre -3 y +3 voltios.
2. El procedimiento, según la reivindicación 1, donde la corriente de
entrada está diluida en una corriente de gas inerte seleccionado de la 25
lista que comprende nitrógeno, helio, neón, argón, kriptón y xenón.
Anexo
xxvii
3. El procedimiento, según cualquiera de las reivindicaciones 1 o 2, donde
la celda electroquímica se encuentra a una temperatura de entre 500ºC
y 900ºC.
4. El procedimiento, según cualquiera de las reivindicaciones 1 a 3, donde 5
el potencial aplicado es de entre -2,5 y +2,5 voltios.
5. El procedimiento, según la reivindicación 4, donde el potencial aplicado
es de entre -2 y +2 voltios.
6. El procedimiento, según cualquiera de las reivindicaciones 1 a 5, donde 10
los 25 hidrocarburos ligeros gaseosos se seleccionan de la lista que
comprende metano, etano, propano, butano, gas natural o cualquiera de
sus combinaciones.
7. El procedimiento, según la reivindicación 6, donde el hidrocarburo ligero
es una combinación de hidrocarburos ligeros que comprende al menos 15
metano o es gas natural.
8. El procedimiento, según cualquiera de las reivindicaciones 1 a 5, donde
el alcohol se selecciona de la lista que comprende metanol, etanol,
propanol o cualquiera de sus combinaciones. 20
9. El procedimiento, según la reivindicación 8, donde el alcohol es metanol
o etanol.
Anexo
xxviii
10. El procedimiento, según cualquiera de las reivindicaciones 1 a 5, 8 o 9,
donde al paso de la corriente gaseosa que contiene al menos un alcohol
(C1-C3) además se añade una corriente de vapor de agua.
11. El procedimiento, según cualquiera de las reivindicaciones 1 a 10,
donde las corrientes gaseosas de hidrocarburo o alcohol y la corriente de 5
vapor de agua se 10 mezclan antes de pasar a la celda electroquímica.
12. El procedimiento, según cualquiera de las reivindicaciones 1 a 11,
donde el conductor electrolito sólido iónico es un conductor aniónico que
conduce iones oxígeno.
10
13. El procedimiento, según la reivindicación 12, donde el conductor
aniónico comprende un electrolito sólido que se selecciona de entre óxido
de zirconio, óxidos de titanio, óxido de itrio estabilizado con óxido de
zirconio, óxido de zirconio estabilizado con calcio, perovskitas con
conductividad mixta o cualquiera de sus combinaciones. 15
14. El procedimiento, según cualquiera de las reivindicaciones 12 o 13,
donde el conductor aniónico comprende al menos un electrodo selectivo
a la electrólisis del agua y al menos un contraelectrodo selectivo a la
reacción de reformado y a la 25 oxidación parcial de la corriente de
entrada. 20
15. El procedimiento, según la reivindicación 14, donde el electrodo
selectivo a la electrólisis del agua es de platino.
16. El procedimiento según cualquiera de las reivindicaciones 14 o 15,
donde el contraelectrodo catalítico poroso selectivo a la reacción de
Anexo
xxix
reformado y la oxidación parcial de la corriente de entrada se selecciona
de entre níquel, platino, paladio o cualquiera de sus combinaciones.
17. El procedimiento, según cualquiera de las reivindicaciones 12 a 16,
donde la celda electroquímica se encuentra a una temperatura de entre 5
700 y 900 ºC cuando la corriente de entrada es una corriente gaseosa de
hidrocarburos ligeros y una corriente de vapor de agua.
18. El procedimiento, según cualquiera de las reivindicaciones 12 a 16,
donde la celda electroquímica se encuentra a una temperatura de entre
500 y 750 ºC cuando la corriente de entrada es una corriente gaseosa 10
que contiene al menos un alcohol (C1-C3).
19. El procedimiento, según cualquiera de las reivindicaciones 10 o 11,
donde el conductor electrolito sólido iónico es un conductor catiónico.
20. El procedimiento, según la reivindicación 19, donde el conductor 15
catiónico conduceiones sodio Na+ o potasio K+.
21. El procedimiento, según cualquiera de las reivindicaciones 19 o 20,
donde el conductor catiónico comprende un electrolito sólido que se
selecciona de entre Naβ-Al2O3, K-β-Al2O3, NASICON, LISICON o
cualquiera de sus combinaciones. 20
22. El procedimiento, según cualquiera de las reivindicaciones 19 a 21,
donde el conductor catiónico además comprende al menos un electrodo
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metálico selectivo al proceso de reformado de la corriente de entrada y
al menos un contraelectrodo metálico.
23. El procedimiento, según la reivindicación 22, donde el electrodo
metálico selectivo al proceso de reformado de la corriente de entrada es 5
de platino.
24. El procedimiento, según cualquiera de las reivindicaciones 22 o 23,
donde el 30 contraelectrodo metálico es de oro.
25. El procedimiento, según cualquiera de las reivindicaciones 19 a 24,
donde la celda electroquímica se encuentra a una temperatura de entre 10
700 y 900 ºC cuando la corriente de entrada es una corriente gaseosa de
hidrocarburos ligeros y una corriente de vapor de agua.
26. El procedimiento, según cualquiera de las reivindicaciones 19 a 24,
donde la celda electroquímica se encuentra a una temperatura de entre
500 y 750 ºC cuando la corriente de entrada es una corriente gaseosa 15
que contiene al menos un alcohol (C1-C3) y una corriente de vapor de
agua.
27. El procedimiento, según cualquiera de las reivindicaciones 1 a 26,
donde se utiliza 10 una fuente convencional o renovable para la
aplicación del potencial. 20
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DIBUJOS
FIG. 1
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FIG. 2
FIG. 3
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FIG. 4
0 10 20 30 40 50 60 70 80 90 100 110
0.0
0.5
1.0
1.5
2.0
H2/CO = 20
H2/CO = 40
I= +2,5 V OCPI= -2.5 VOCP rH
2
rCO
rCO2
r H2, r C
O, r C
O2 (
mol/
s m
g P
t) 1
0-8
Time (min)
OCP
H2/CO = 1
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FIG. 5
FIG. 6
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FIG. 7
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RESUMEN
Procedimiento de obtención de gas de síntesis
5
La presente invención se refiere a un procedimiento de gas de síntesis
(H2/CO) de ratio controlable mediante un proceso catalítico y electroquímio
que emplea una celda electroquímica formada por electrolitos sólidos
conductores iónicos, aniónicos o catiónicos. El control del ratio H2/Co se
lleva a cabo en una única etapa bajo condiciones constantes de operación, 10
es decir, a temperatura constante de la celda electroquímica y condiciones
constantes de composición y concentración de la corriente de entrada. En
la presente invención la corriente de entrada se selecciona de entre una
corriente de hidrocarburos ligeros y una corriente de vapor de agua, o una
corriente gaseosa que contiene al menos un alcohol (C1-C3). 15
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