Improving metal catalysts through nanoalloying and supporting on reducible oxides
Résumé
A major application field for supported nanoparticles is heterogeneous catalysis, which plays a key role in strategic domains such as depollution, oil refining, and synthesis of chemicals. It has been long known that combining several metals can increase the performance of metal-based catalysts through, e.g., synergistic alloying effects. In recent years, nanoalloys have been the subject of renewed interest due to the recent progress in atomic-scale characterization, which has stimulated the development of new synthesis and structural modelling methods leading to novel applications. In this presentation, catalytic nanoalloying effects will be illustrated by two examples of bulk-immiscible bimetallic systems.
Ir-Pd nanoparticles were prepared by incipient wetness impregnation on alumina or amorphous silica-alumina. These catalysts present a positive alloying effect for the preferential oxidation of CO in the presence of hydrogen (PROX), a reaction used to purify hydrogen for proton-exchange-membrane fuel cells. Both experiments and calculations show the formation of Ir@Pd core-shell nanoparticles. The synergy in PROX has been correlated to iridium-induced inhibition of hydrogen absorption by palladium, Pd hydrides being unselective for this reaction.
Au-Rh nanoparticles were synthesized using colloidal chemical co-reduction, and immobilized on rutile TiO2 nanorods. The resulting model catalysts exhibit improved stability for the hydrogenation of tetralin in the presence of H2S in the context of fuel upgrading, and enhanced selectivity for the hydrodeoxygenation of guaiacol for lignin valorisation. STEM and DFT investigations suggest that under such reductive conditions, the Au-Rh nanoalloys adopt a Janus configuration with Au/Rh/TiO2 stacking, which is held responsible for the observed synergy.