Activity and stability of Co 3 O 4 -based catalysts for soot oxidation: The enhanced effect of Bi 2 O 3 on activation and transfer of oxygen

Bi2O3-Co3O4 catalysts were prepared by sol-gel method and tested for soot oxidation by O2. The composite oxides showed excellent activity under both tight and loose contact when compared with individual Co3O4 or Bi2O3, and the maximum activity was obtained over catalyst with Bi/Co molar ratio of 0.2. The samples were characterized by means of XRD, N2 adsorption, FE-SEM, XPS, FT-IR, C-TPR and O2-TPD. It was found that Bi2O3 with low melting point deposited on Co3O4 surface could not only promote the contact state between soot and catalyst, but also produce more oxygen species with high mobility and reactivity at Bi-Co interface layer. Oxygen activation channel and reaction pathway were discussed based on the results of isothermal anaerobic titrations and 18O-isotopic tests, which confirmed that soot was more likely to react with lattice oxygen species rather than O2, especially at low temperatures. The high mobility of lattice oxygen species was attributed to a combination of the O2− conductivity of Bi2O3 and the accelerative formation of oxygen vacancies at Bi-Co interface. A feasible reaction mechanism over the binary catalysts for soot oxidation was proposed. The stability tests were also studied and the results indicated that Bi-modified Co3O4 showed prominent tolerance against thermal shock, H2O and SO2, thus being a promising active component for practical application.