Dynamic reorganization of bimetallic nanoparticles under reaction depending on the support nanoshape: the case of RhPd over ceria nanocubes and nanorods under ethanol steam reforming

Abstract

Bimetallic catalysts exhibit high performance compared to that of their monometallic counterparts in a wide range of catalytic reactions. Both geometric and electronic phenomena yield unique properties that cannot be reproduced by other means. Bimetallic nanoparticles reorganize dynamically under reaction conditions, usually forming complex core-shell distributions of metals and oxidation states. This reorganization is even more complex in the presence of a reducible support. Here we show by operando ambient pressure X-ray photoelectron spectroscopy (AP-XPS) that the shape of the support also plays a crucial role in the reorganization of bimetallic nanoparticles, which has important consequences for catalytic performance. We have monitored the surface composition and oxidation states of preformed Rh0.5Pd0.5 model nanoparticles of 4 nm in size supported over CeO2 nanocubes and CeO2 nanorods during the catalytic steam reforming of ethanol (ESR) at 823 K. Over Rh0.5Pd0.5/CeO2-nanocubes, both rhodium and palladium become strongly oxidized and ethanol mainly dehydrogenates into acetaldehyde and H2. In contrast, over Rh0.5Pd0.5/CeO2-nanorods, there is an enrichment of Pd toward the surface, both palladium and rhodium undergo significant reduction, and ethanol is reformed efficiently.