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)