Improved and Reduced Performance of Cu- and Ni-Substituted Co3O4 Catalysts with Varying CoOh/CoTd and Co3+/Co2+ Ratios for the Complete Catalytic Oxidation of VOCs

The Co3O4 spinel is one of the most promising transition metal oxide (TMO) catalysts for volatile organic compound (VOC) treatment. Substituting effects have usually been utilized to improve the catalytic performance of the Co3O4 spinel. In this study, Cu- and Ni-substituted Co3O4 catalysts derived from mixed metal-organic frameworks (MMOFs) retained similar spinel structures but exhibited improved and reduced performance for o-xylene oxidation, respectively. Physicochemical characterization and DFT calculations revealed that Cu and Ni substitution into the Co3O4 spinel varied the valence (Co3+/Co2+) and geometry (CoOh/CoTd) distributions of Co cations through different partial electron transfer and substitution sites. The higher Co3+/Co2+ and CoOh/CoTd ratios of the CuCo2O4 catalyst contributed to the superior reducibility and oxygen mobility, which facilitated the oxidation of intermediates at lower temperatures in the catalytic oxidation of o-xylene. Meanwhile, the NiCo2O4 catalyst with lower Co3+/Co2+ and CoOh/CoTd ratios could not completely oxidize intermediates under the same conditions due to inferior redox properties. Therefore, the CuCo2O4 catalyst showed superior catalytic activity and stability to the NiCo2O4 catalyst for the catalytic oxidation of o-xylene. This work provides insights into the synthesis of substituted Co3O4 catalysts from MMOFs and mechanism of substituting effects, which might guide the design of efficient TMO catalysts for VOC treatment.