Influence of Oxygen Vacancy-Induced Coordination Change on Pd/CeO 2 for NO Reduction

The byproduct formation in environmental catalysis is strongly influenced by the chemical state and coordination of catalysts. Herein, two Pd/CeO catalysts (PdCe-350 and PdCe-800) with varying oxygen vacancies (O ) and coordination numbers (CN) of Pd were prepared to investigate the mechanism of N O and NH formation during NO reduction by CO. PdCe-350 exhibits a higher density of O 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 N O formation strongly correlated with the O 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 NH formation, its occurrence was closely associated with the activation of H O and C H , since a water-gas shift or hydrocarbon reforming could provide hydrogen. Both bridging and monodentate nitrates showed activity in NH 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 , and byproduct distribution.