Promotional Effects of Sm/Ce/La Doping on Soot Oxidation over MnCo2O4 Spinel Catalysts

Sm/La/Ce-doped spinel catalysts were prepared by a sol–gel method, and their catalytic performance in soot oxidation was investigated. The catalytic activity of the samples was ranked as follows: Sm0.1Mn0.9Co2O4 > Ce0.1Mn0.9Co2O4 > La0.1Mn0.9Co2O4 > MnCo2O4. Numerous analytical techniques including X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), O2-temperature-programmed desorption (TPD), H2-temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) were performed to investigate the structural and physicochemical properties of the catalysts. The XRD patterns confirmed that the doped ions had been incorporated into the lattice of the spinel oxides. The crystallite size decreased after doping, resulting in an increase in SBET, as proved by the results of SEM and BET. The characterization results (H2-TPR, O2-TPD, and XPS) revealed that the amounts of Co3+ and surface-active oxygen species of catalysts were increased after doping. The Co3+ and Oα ratios of the catalysts were linearly correlated with the catalytic activity, indicating that Co3+ and Oα played important roles in soot combustion. Co3+ was deemed to promote soot oxidation by generating more NO2, which could react with soot to form surface oxygen complexes (SOCs), and then the Oα could oxidize the SOCs to generate CO2. This study provides rational insights into the influence of heteroatom doping on soot oxidation over spinel oxide catalysts.