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

Materiales y Nanotecnología Nanocompositos Fotoacustica y luminiscencia Soportes nanoestructurados Aplicaciones biomédicas Propiedades ópticas Propiedades electrónicas Propiedades magnéticas Propiedades catalíticas Remediación del medio ambiente Películas delgadas Nanopartículas de materiales multifuncionales Plasmas inducidos por láser Sensores de presión

4 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 - DeNO x -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 The Au NP agglomerate when samples are exposed to environmental conditions for long times when samples are exposed to environmental conditions for long times When they are exposed to heating-cooling catalytic cyclesWhen they are exposed to heating-cooling catalytic cycles Weak interaction nanoparticle/support It is necessary to overcome this problem Time (days) Particle Size (nm) X. Bokhimi, R. Zanella, A. Morales, The Open Inorganic Chemistry Journal, 3 (2009) 69 Au/TiO 2 (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/TiO 2 Possibility of formation of bimetallic particles Reaction Temperature (°C) Conversion (%)

Deposition-Precipitation with urea and NaOH Drying 80 °C /vacuum Thermal Treatment H 2 Stirring at T =80 °C for ≠ times Washing urea hydrolysis: CO(NH 2 ) H 2 O  2 NH CO OH -  pH  TiO 2 Degussa P25, CeO 2 Alfa Aesar  % max. Au-Ag = 4 wt % AgNO 3 NaOH TiO HAuCl 4 CO(NH 2 ) 2 CeO 2 A. Sandoval, A. Aguilar, A. Traverse, R. Zanella. J. Catal. (2011) doi: /j.jcat R. Zanella, S. Giorgio, C.Henry, C. Louis, J. Phys. Chem. B, 106, (2002) 76343

Au-Ag/TiO 2 CO + ½ O 2  CO 2 - 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 °C °C °C 5.4 Thermal Treatment H 2 Effect of Au-Ag relation Au:Ag A. Sandoval, A. Aguilar, A. Traverse, R. Zanella. J. Catal. (2011) doi: /j.jcat

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

Activity – Particle Size- % Au ° = f(T TT ) T Thermal Treatment =200°C  the highest activity  100% Au 0 T Thermal Treatment = °C activity  y particle size  Au/TiO 2 DP Urea Activity (mol CO.mol Au -1.s -1 ) T Thermal Treatment (°C) 100% Au 0 Particle Size (nm) R. Zanella, S. Giorgio, C. R. Henry, C Louis, J. Catal. 222 (2004) 357

XANES Au-Ag/TiO 2 at the Au edge (11918 eV) Total reduction of gold in bimetallic catalyst at 103 °C Au/TiO 2 at the Au edge Total reduction of gold in monometallic catalyst at T > 150 °C Reduction in-situ in H 2 Energy (eV) Absorbance Differences in reduction temperature of Au/TiO 2 and Au-Ag/TiO 2 may indicate that the entourage of gold in Au-Ag catalyst is different to that of Au catalysts: Interaction Au-Ag

Au-Ag/TiO 2 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 ±

LayerBackscattererN 1Au8 Ag3 2 Au2 Ag4 3 Au9 Ag15 Layer Backscatterer N 1 Au8 Ag4 2Au2 Ag3 3Au16 Ag8 Layer Backscatterer N 1 Au8 Ag3 2Au2 Ag4 3Au18 Ag6 EXAFS of Au-Ag catalyst as a function of the T of thermal treatment in H °C 450 °C 550 °C Gold edge FCC Structure

O 2 activation on the support Au/TiO 2 Au-Ag/TiO 2 TiO 2 G. C. Bond and D. T. Thompson, Gold Bull., 2000, 33, 41 A. Sandoval, C. Louis, A. Traverse, R. Zanella. J. Catal. (2011) doi: /j.jcat

Higher stability of Au-Ag/TiO 2 compared to Au/TiO 2 Au/TiO 2 vs Au-Ag/TiO 2 Stability as a function of reaction time CO + ½ O 2 → CO 2 Reaction Time (h) Activity (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 The combination of Au-Ag shows synergetic effects in CO oxidation reaction and more stable and durable catalysts. 4. The characterization of Au-Ag/TiO 2 (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) A.Traverse (U. Paris Sud) S. Belin (Sincrotrón SOLEIL) Financial Support 1.UNAM Nanoscience and Nanotechnology Project (PUNTA-IMPULSA 01) 2.CONACYT grant 3.PAPIIT-DGAPA-UNAM grant 4.Mexico-France (CONACYT-CNRS) Bilateral Cooperation 5. Red de Nanociencia y Nanotecnología (CONACYT)