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 < MCO₂ < MCO₃, 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 CO₂ transformation are found for O₄Pd site of the Pd/Ce₂₁O₄₂ model featuring a square-planar coordination of the Pd²⁺ cation. The removal of one O atom and formation of the O₃Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO₂ 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. CO₂-species are strongly bound in palladium carboxylate complexes, whereas the CO₂ molecule easily desorbs from oxide-supported AgCO₂ moieties. Opposite to metal-free ceria particle, the formation of neither PdCO₃ nor AgCO₃ carbonate intermediates before CO₂ 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 CO₂ and CO₃^2- 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.