Soot Oxidation over CeO2 or Ag/CeO2: Influences of Bulk Oxygen Vacancies and Surface Oxygen Vacancies on Activity and Stability of the Catalyst

Three types of CeO2 were synthesized and through impregnation of Ag employing these CeO2 types as supports, three kinds of Ag/CeO2 were prepared. Their morphologies and structures were characterized by SEM, HRTEM, nitrogen adsorption-desorption isotherms, and H-2-TPR reduction, indicating that the effective soot-catalyst contact and size distribution as well as the chemical state of Ag were identical. Surface oxygen vacancies (VO-s) and bulk oxygen vacancies (VO-b) were investigated by using XPS and Raman spectroscopy, respectively. The activities of the CeO2 catalysts are related to surface chemisorbed oxygen, while the activities of Ag/CeO2 are decided by VO-s near the Ag particles under oxygen-rich conditions. When switching the atmosphere to oxygen-poor conditions, the activities of Ag/CeO2 are controlled by VO-b instead. These results originate from the fact that the generation and delivery of active oxygen are affected by combinations of VO-s and VO-b rather than just one of them. The moderate concentration of VO-s and sufficient concentration of VO-b are favorable for increasing catalytic activity under oxygen-poor conditions for Ag/CeO2. In this work, Ag loaded on CeO2, prepared at 50 degrees C, (defined as Ag/CeO2-50) can be considered as a prospective catalyst for use in gasoline particulate filters.