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

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 N-2 adsorption-desorption, XRD, FT-IR, NOx-TPD, CO2-TPD, O-2-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-/O-2 -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 NOx storage capacities of Co3O4, and the NOx 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.