Adsorption and Oxidation of CO on Ceria Nanoparticles Exposing Single-Atom Pd and Ag: A DFT Modelling

Various CO(x) species formed upon the adsorption and oxidation of CO on palladium and silver single atoms supported on a model ceria nanoparticle (NP) have been studied using density functional calculations. For both metals M, the ceria-supported MCO(x) moieties are found to be stabilised in the order MCO < MCO(2) < MCO(3), similar to the trend for CO(x) species adsorbed on M-free ceria NP. Nevertheless, the characteristics of the palladium and silver intermediates are different. Very weak CO adsorption and the small exothermicity of the CO to CO(2) transformation are found for O(4)Pd site of the Pd/Ce(21)O(42) model featuring a square-planar coordination of the Pd(2+) cation. The removal of one O atom and formation of the O(3)Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO(2) reaction. For the analogous ceria models with atomic Ag instead of atomic Pd, these two energies became twice as small in magnitude and basically independent of the presence of an O vacancy near the Ag atom. CO(2)-species are strongly bound in palladium carboxylate complexes, whereas the CO(2) molecule easily desorbs from oxide-supported AgCO(2) moieties. Opposite to metal-free ceria particle, the formation of neither PdCO(3) nor AgCO(3) carbonate intermediates before CO(2) desorption is predicted. Overall, CO oxidation is concluded to be more favourable at Ag centres atomically dispersed on ceria nanostructures than at the corresponding Pd centres. Calculated vibrational fingerprints of surface CO(x) moieties allow us to distinguish between CO adsorption on bare ceria NP (blue frequency shifts) and ceria-supported metal atoms (red frequency shifts). However, discrimination between the CO(2) and CO(3)(2−) species anchored to M-containing and bare ceria particles based solely on vibrational spectroscopy seems problematic. This computational modelling study provides guidance for the knowledge-driven design of more efficient ceria-based single-atom catalysts for the environmentally important CO oxidation reaction.