The existing states of potassium species in K-doped Co3O4 catalysts and their influence on the activities for NO and soot oxidation

Abstract

Potassium-doped Co₃O₄, prepared via the sol–gel method, was pretreated by water washing and subsequently the fresh, washed catalysts were compared with pure Co₃O₄ in order to investigate the existing states of potassium species and their influence on the activities for NO and soot oxidation. The samples were characterized by N₂ adsorption–desorption, XRD, FT-IR, NO_x-TPD, CO₂-TPD, O₂-TPD and XPS. It was found that two types of K species existed in K-Co₃O₄, i.e., the “free” K and the “stable” K. The “free” K species, which were present as carbonates and nitrates with high mobility, could improve the contact state of catalyst-soot and thus significantly accelerate soot oxidation, but they also greatly covered the active surface of Co₃O₄ and were adverse for NO oxidation. The “stable” K species, which interacted strongly with Co₃O₄ and acted as electronic and structural modifiers, facilitated the formation of O⁻/O₂⁻ species and oxygen vacancies, exposed more surface Co³⁺ sites and increased the amount of the medium/strong basic sites, and as a result, they enhanced the intrinsic activities of Co₃O₄ for soot and NO oxidation. Both K species improved the NO_x storage capacities of Co₃O₄, and the NO_x species adsorbed on medium/strong basic sites with appropriate stability, caused by the effect of the “stable” K, were more beneficial for soot oxidation than those adsorbed on “free” K with high stability.