Au-Ag/TiO 2 : Nanocatalizadores más activos y estables para la oxidación de CO Rodolfo Zanella

1. Au-Ag/TiO2: Nanocatalizadores más activos y estables para la oxidación de CO Rodolfo Zanella CCADET-UNAM

2. 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

3. Materiales y Nanotecnología Plasmas inducidos por láser Remediación del medio ambiente Propiedades catalíticas Películasdelgadas Nanocompositos Soportesnanoestructurados Fotoacustica y luminiscencia Nanopartículas de materiales multifuncionales Propiedades ópticas Propiedades magnéticas Propiedades electrónicas Sensores de presión Aplicaciones biomédicas

4. Supported gold catalysts 1988:breakthrough by Haruta: small gold particles (<5nm) supported on oxides active in CO oxidation at T≤RT Au 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 Automotive pollution control Fuel Cells • - DeNOx • Total oxidations • - WGS • PROX • Selective Oxidations • Selective Hydrogenation • Dehydrogenation • Catalytic wet air oxidations • Chemical Processing • Vinyl acetate synthesis • - Nylon precursors

5. Instability of the Au nanoparticles Au/TiO2 (rutile) catalyst • 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 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 Conversion (%) Reaction Temperature (°C) X. Bokhimi, R. Zanella, A. Morales, TheOpen Inorganic Chemistry Journal, 3 (2009) 69

6. Deposition-Precipitation with urea and NaOH TiO2 Degussa P25, CeO2 Alfa Aesar AgNO3NaOHTiO2 + + HAuCl4CO(NH2)2 CeO2 % max. Au-Ag = 4 wt % Washing Drying 80 °C /vacuum Thermal Treatment H2 Stirring at T =80 °C for ≠ times urea hydrolysis: CO(NH2)2 + 3 H2O  2 NH4+ + CO2 + 2 OH-  pH  A. Sandoval, A. Aguilar, A. Traverse, R. Zanella. J. Catal. (2011) doi: 10.1016/j.jcat.2011.04.003 R. Zanella, S. Giorgio, C.Henry, C. Louis, J. Phys. Chem. B, 106, (2002) 76343

7. Au-Ag/TiO2 CO + ½ O2 CO2 Thermal Treatment H2 Effect of Au-Ag relation Au:Ag • 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 A. Sandoval, A. Aguilar, A. Traverse, R. Zanella. J. Catal. (2011) doi: 10.1016/j.jcat.2011.04.003

8. HAADF (Z contrast) images 350 °C 450 °C 550 °C 650 °C

9. 0.1 6 5 0.08 4 0.06 3 0.04 2 0.02 1 0 0 50 100 150 200 250 300 350 400 T (°C) Thermal Treatment Au/TiO2 DP Urea Activity – Particle Size- % Au° = f(TTT) 0 100% Au Activity (molCO.molAu-1.s-1) Particle Size (nm) TThermal Treatment =200°C the highest activity  100% Au0 TThermal Treatment =200-400°C activity  y particle size R. Zanella, S. Giorgio, C. R. Henry, C Louis, J. Catal.222 (2004) 357

10. XANES Au-Ag/TiO2at the Au edge (11918 eV) Reduction in-situ in H2 Total reduction of gold in bimetallic catalyst at 103 °C Absorbance Au/TiO2at the Au edge Energy (eV) Total reduction of gold in monometallic catalyst at T > 150 °C Absorbance 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 Energy (eV)

11. EDS Au-Ag/TiO2

13. EXAFS of Au-Ag catalyst as a function of the T of thermal treatment in H2 Goldedge FCC Structure 350 °C 550 °C 450 °C

14. Au/TiO2 Au-Ag/TiO2 O2 activation on the support G. C. Bond and D. T. Thompson, Gold Bull., 2000, 33, 41 TiO2 A. Sandoval, C. Louis, A. Traverse, R. Zanella. J. Catal. (2011) doi: 10.1016/j.jcat.2011.04.003

15. Au/TiO2 vs Au-Ag/TiO2 Stability as a function of reaction time CO + ½ O2 → CO2 (1:2) Activity Reaction Time (h) Higher stability of Au-Ag/TiO2 compared to Au/TiO2

16. 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.

17. Thanks to: A. Sandoval (CCADET-UNAM) A. Aguilar (CCADET-UNAM) 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) Traverse(U. Paris Sud) S. Belin(Sincrotrón SOLEIL) Financial Support UNAM Nanoscience and Nanotechnology Project (PUNTA-IMPULSA 01) CONACYT 55154 grant PAPIIT-DGAPA-UNAM 108310 grant Mexico-France (CONACYT-CNRS) Bilateral Cooperation 5. Red de Nanociencia y Nanotecnología (CONACYT)