Rhodium-on-Palladium Nanocatalysts for Selective Methanation of Carbon Dioxide

Nanocatalysts with a Rh shell and Pd core (Rh-on-Pd) were synthesized. These Rh-on-Pd catalysts showed the highest selectivity for methane in the methanation of carbon dioxide by hydrogen among Rh, Pd and Rh-on-Pd bimetallic core–shell structures tested when the Rh shell became about two atom-layers thick. In situ environmental transmission electron microscopy (ETEM) study was used to investigate the dynamic structural change of these Rh-on-Pd core–shell nanoparticles under hydrogen atmosphere at variable temperatures, showing the feasibility of controlling the surface compositions. Thickening of the Rh shell was observed by nano beam diffraction (NBD) at elevated temperatures during the sintering, resulting in the change of selectivity in methane formation over Rh-on-Pd catalysts. This selectivity became similar to that of Rh catalyst at high temperatures due to the disappearance of ligand effects for thick Rh shells. Density functional theory (DFT) calculations suggest CO dissociation on the (111) surface of monolayers or double layers of Rh on Pd structure is energetically favored over either Rh or Pd, showing the potential for producing hydrocarbons through hydrogenation of carbon dioxide.