Roles of Surface-Active Oxygen Species on 3DOM Cobalt-Based Spinel Catalysts MxCo3–xO4 (M = Zn and Ni) for NOx-Assisted Soot Oxidation

Co3O4 is a well-known catalyst in the oxidation reaction. In such a catalyst, the geometric and electronic structures of tetrahedrally coordinated Co2+ and octahedrally coordinated Co3+ can be regulated by directional metal ion substitution strategy, accompanied by the modification of catalytic activity. Herein, normal and inverse cobalt-based spinel catalysts MxCo3–xO4 (M = Zn and Ni) with a three-dimensionally ordered macroporous (3DOM) structure were successfully fabricated through the carboxy-modified colloidal crystal templating (CMCCT) method. The relationship between the dopant and activity during NOx-assisted soot oxidation was systematically studied by means of XPS, H2-TPR, soot-TPR, isothermal anaerobic titrations, NO-TPO, soot-TPO, and so on. The well-defined 3DOM structure for MxCo3–xO4 catalysts can improve the contact efficiency of soot and catalysts. 3DOM NiCo2O4 exhibits high catalytic activity for soot oxidation under a loose contact mode between soot and catalyst. For instance, its T50 and TOF values are 379 °C and 1.36 × 10–3 s–1, respectively. The doping of Ni to Co3O4 will induce the structural distortion, improve the density of oxygen vacancies, and enhance lattice oxygen mobility. It leads to more surface-active oxygen species. A vacancy-mediated pathway of NO oxidation on the spinel catalyst is proposed according to the experimental results of in situ DRIFT spectra, in situ Raman spectra, and the theoretical knowledge of coordination chemistry of metal–NO. The catalytic performance of soot oxidation is highly related to the capacity of a catalyst in oxidizing NO to NO2. Therefore, indirect NO2-assisted mechanism is proposed for soot oxidation under an NO/O2/N2 atmosphere.