Unraveling the advantages of Pd/CeO₂ single-atom catalysts in the NO + CO reaction by model catalysts

Selective catalytic reduction of NO by CO is challenging in environmental catalysis but attractive owing to the advantage of simultaneous elimination of NO and CO. Here, model catalysts consisting of Pd nanoparticles (NPs) and single-atom Pd supported on a CeO₂ (111) film grown on Cu (111) (denoted as Pd NPs/CeO₂ and Pd₁/CeO₂, respectively) were successfully prepared and characterized by synchrotron radiation photoemission spectroscopy (SRPES) and infrared reflection absorption spectroscopy (IRAS). The NO + CO adsorption/reaction on the Pd₁/CeO₂ and Pd NPs/CeO₂ catalysts were carefully investigated using SRPES, temperature-programmed desorption (TPD), and IRAS. It is found that the reaction products on both model catalysts are in good agreement with those on real catalysts, demonstrating the good reliability of using these model catalysts to study the reaction mechanism of the NO + CO reaction. On the Pd NPs/CeO₂ surface, N₂ is formed by the combination of atomic N coming from the dissociation of NO on Pd NPs at higher temperatures. N₂O formation occurs probably via chemisorbed NO combined with atomic N on the surface. While on the single-atom Pd₁/CeO₂ surface, no N₂O is detected. The 100% N₂ selectivity may stem from the formation of O-N-N-O* intermediate on the surface. Through this study, direct experimental evidence for the reaction mechanisms of the NO + CO reaction is provided, which supports the previous density functional theory (DFT) calculations.