Au-Ag/TiO2: Nanocatalizadores más activos y estables para la oxidación
de CO
Rodolfo ZanellaCCADET-UNAM
Nanociencia y Nanotecnología en el Centro de Ciencias Aplicadas y Desarrollo Tecnológico
Objetivos:
Preparar materiales nanoestructurados con tamaño y forma controlada
Desarrollar metodologías de nano-ensamblado
Estudiar las propiedades ópticas, magnéticas, catalíticas y electrónicas de estos materiales
Desarrollar dispositivos basados en nanoestructuras (catalizadores, sensores, purificadores ambientales, etc.)
Desarrollar aplicaciones en áreas estratégicas (medio ambiente, salud, energía, alimentos,…)
Transferir las tecnologías desarrolladas N
N
N(CH2CH2O)7CH3
NO2
H3C
ACCEPTOR
DONOR
NN
N(CH2CH2O)7CH3
NO2
H3C
NN
N(CH2CH2O)7CH3
NO2
H3C
ACCEPTOR
DONOR (a)(a) (b)
NN
N(CH2CH2O)7CH3
NO2
H3C
ACCEPTOR
DONOR
NN
N(CH2CH2O)7CH3
NO2
H3C
NN
N(CH2CH2O)7CH3
NO2
H3C
ACCEPTOR
DONOR (a)(a) (b)
100 nm100 nm
5 nm5 nm
(100)
Materiales y Nanotecnología
Nanocompositos
Fotoacustica y luminiscencia
Soportes nanoestructurados
Aplicaciones biomédicas
Propiedades ópticas
Propiedades electrónicas
Propiedades magnéticas
Propiedades catalíticas
Remediacióndel medio ambiente
Películas delgadas
Nanopartículas de materialesmultifuncionales
Plasmas inducidos por láser
Sensoresde presión
4
1988: breakthrough by Haruta:small gold particles (<5nm) supported on oxides active in CO oxidation at T≤RTAu more active than Pd, Pt at low T
Now it is known that supported gold catalysts are active in several reactions. In many of them at lower temperatures than other noble metals
- DeNOx-Total oxidations- WGS- PROX- Selective Oxidations-Selective Hydrogenation- Dehydrogenation- Catalytic wet air oxidations- Chemical Processing
- Vinyl acetate synthesis- Nylon precursors
Supported gold catalysts
Fuel Cells Automotive pollution control
Instability of the Au nanoparticles
The Au NP agglomerate• when samples are exposed to environmental
conditions for long times• When they are exposed to heating-cooling
catalytic cycles
Weak interaction nanoparticle/support
It is necessary to overcome this problem
Time (days)
Particle Size (nm)
0 2.01 2.03 2.55 3.110 4.614 4.9
X. Bokhimi, R. Zanella, A. Morales, The Open Inorganic Chemistry Journal, 3 (2009) 69
Au/TiO2 (rutile) catalyst
Idea: The addition of a third element into the Au/oxide binary system, can act as nucleation center to anchor Au particles for stabilizing them
Selected System: Au-Ag/TiO2
Possibility of formation of bimetallic particles
Reaction Temperature (°C)
Con
vers
ion
(%)
Deposition-Precipitation with urea and NaOH
Drying 80 °C /vacuum
Thermal Treatment
H2Stirring at T =80 °Cfor ≠ times
Washing
urea hydrolysis: CO(NH2)2 + 3 H2O 2 NH4
+ + CO2 + 2 OH-
pH
TiO2 Degussa P25, CeO2 Alfa Aesar
012345678
0 50 100 150 200 250Time (min)
pH
% max. Au-Ag = 4 wt %
AgNO3 NaOH TiO2 + + HAuCl4 CO(NH2)2 CeO2
A. Sandoval, A. Aguilar, A. Traverse, R. Zanella. J. Catal. (2011) doi: 10.1016/j.jcat.2011.04.003R. Zanella, S. Giorgio, C.Henry, C. Louis, J. Phys. Chem. B, 106, (2002) 76343
0 100 200 300 400 5000
20
40
60
80
100
Con
vers
ion
of C
O (%
)
Reaction Temperature (°C)
650 °C 550 °C 450 °C 350 °C
0 100 200 300 400 5000
20
40
60
80
100
Con
vers
ion
of C
O (%
)
Reaction Temperature (°C)
1 : 0 1 : 0.15 0 : 0.37 1 : 0.66 1 : 0.76 1 : 0.90 0 : 1
Au-Ag/TiO2CO + ½ O2 CO2
- The optimization of Au/Ag relation is necessary to obtain synergetic effects- Small quantities of Ag are required to obtain synergetic effects- Optimum atomic relation Au:Ag 1:0.4
High thermal treatment temperatures (550 °C) are needed to obtain high catalytic activity
T (°C) Particle Size (nm)350°C 2.6450°C 3.3550°C 3.9650°C 5.4
Thermal Treatment H2
Effect of Au-Ag relation
Au:Ag
A. Sandoval, A. Aguilar, A. Traverse, R. Zanella. J. Catal. (2011) doi: 10.1016/j.jcat.2011.04.003
HAADF (Z contrast) images350 °C
550 °C
450 °C
650 °C
Activity – Particle Size- % Au° = f(TTT)
TThermal Treatment =200°C the highest activity 100% Au0
TThermal Treatment =200-400°C activity y particle size
Au/TiO2 DP Urea
0
0.02
0.04
0.06
0.08
0.1
0
1
2
3
4
5
6
50 100 150 200 250 300 350 400
Acti
vity
(m
olCO
.mol
Au-1.s
-1)
TThermal Treatment
(°C)
100% Au 0
Part
icle
Siz
e (n
m)
R. Zanella, S. Giorgio, C. R. Henry, C Louis, J. Catal. 222 (2004) 357
XANES Au-Ag/TiO2 at the Au edge (11918 eV)
Total reduction of gold in bimetallic catalyst at 103 °C
Au/TiO2 at the Au edge
Total reduction of gold in monometallic catalyst at T > 150 °C
Reduction in-situ in H2
Energy (eV)
Energy (eV)
Abs
orba
nce
Abs
orba
nce
Differences in reduction temperature of Au/TiO2 and Au-Ag/TiO2 may indicate that the entourage of gold in Au-Ag catalyst is different to that of Au catalysts: Interaction Au-Ag
Au-Ag/TiO2
EDS
Temperature treatment (°C) Average
particle size (nm)
% of mixed particles
(containing both Au and Ag)
% of particles with Au-Ag ratio of 0.8
± 0.08
350 2.6 75 10550 3.9 90 30650 5.4 100 60
0 100 200 300 400 5000
20
40
60
80
100
Con
vers
ion
of C
O (%
)
Reaction Temperature (°C)
650 °C 550 °C 450 °C 350 °C
Layer Backscatterer
N
1 Au 8Ag 3
2 Au 2Ag 4
3 Au 9Ag 15
Layer
Backscatterer
N
1 Au 8Ag 4
2 Au 2Ag 3
3 Au 16Ag 8
Layer Backscatterer
N
1 Au 8Ag 3
2 Au 2Ag 4
3 Au 18
Ag 6
EXAFS of Au-Ag catalyst as a function of the T of thermal treatment in H2
350 °C 450 °C 550 °C
Gold edge
FCC Structure
O2 activation on the support
Au/TiO2
Au-Ag/TiO2
TiO2G. C. Bond and D. T. Thompson, Gold Bull., 2000, 33, 41
A. Sandoval, C. Louis, A. Traverse, R. Zanella. J. Catal. (2011) doi: 10.1016/j.jcat.2011.04.003
Higher stability of Au-Ag/TiO2 compared to Au/TiO2
0
50
100
150
200
250
300
350
400
450
500
0 5 10 15 20 25 30 35 40 45 50 55
Activ
idad
(mol
CO
mol
Au-1
h-1)
Tiempo de reacción (h)
Au/TiO2
Au-Ag/TiO2
Au/TiO2 vs Au-Ag/TiO2
Stability as a function of reaction time
CO + ½ O2 → CO2
Reaction Time (h)
Act
ivity
(1:2)
Summary
1. Deposition precipitation is an efficient method to prepare bimetallic Au-Ag catalysts with small metal particles well dispersed on the support surface.
2. The optimum Au/Ag ratio is about 2.5
3. The combination of Au-Ag shows synergetic effects in CO oxidation reaction and more stable and durable catalysts.
4. The characterization of Au-Ag/TiO2 (EDS, UV-Vis, XANES y EXAFS) show that Au and Ag atoms are combined, apparently as core-shell particles.
Thanks to: A. Sandoval (CCADET-UNAM)A. Aguilar (CCADET-UNAM)A. Soto (CCADET-UNAM)L. A. Calzada (CCADET, UNAM)J. M. Saniger (CCADET-UNAM)X. Bokhimi (IF-UNAM)P. Santiago (IF-UNAM)C. Ángeles (IMP)C. Louis (U. París VI)L. Delannoy (U. Paris VI)B. Traverse (U. Paris Sud)S. Belin (Sincrotrón SOLEIL)
Financial Support1. UNAM Nanoscience and Nanotechnology Project (PUNTA-
IMPULSA 01) 2. CONACYT 55154 grant3. PAPIIT-DGAPA-UNAM 108310 grant4. Mexico-France (CONACYT-CNRS) Bilateral Cooperation5. Red de Nanociencia y Nanotecnología (CONACYT)
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