Influence of Oxygen Vacancy-Induced Coordination Change on Pd/CeO₂ for NO Reduction

The byproduct formation in environmental catalysis is strongly influenced by the chemical state and coordination of catalysts. Herein, two Pd/CeO₂ catalysts (PdCe-350 and PdCe-800) with varying oxygen vacancies (O_v) and coordination numbers (CN) of Pd were prepared to investigate the mechanism of N₂O and NH₃ formation during NO reduction by CO. PdCe-350 exhibits a higher density of O_v and Pd sites with higher CN, leading to an enhanced metal-support interaction by electron transformation from the support to Pd. Consequently, PdCe-350 displayed increased levels of byproduct formation. In situ spectroscopies under dry and wet conditions revealed that at low temperatures, the N₂O formation strongly correlated with the O_v density through the decomposition of chelating nitro species on PdCe-350. Conversely, at high temperatures, it was linked to the reactivity of Pd species, primarily facilitated by monodentate nitrates on PdCe-800. In terms of NH₃ formation, its occurrence was closely associated with the activation of H₂O and C₃H₆, since a water-gas shift or hydrocarbon reforming could provide hydrogen. Both bridging and monodentate nitrates showed activity in NH₃ formation, while hyponitrites were identified as key intermediates for both catalysts. The insights provide a fundamental understanding of the intricate relationship among the local coordination of Pd, surface O_v, and byproduct distribution.