Improved and Reduced Performance of Cu- and Ni-Substituted Co 3 O 4 Catalysts with Varying Co Oh /Co Td and Co 3+ /Co 2+ Ratios for the Complete Catalytic Oxidation of VOCs.

The Co 3 O 4 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 Co 3 O 4 spinel. In this study, Cu- and Ni-substituted Co 3 O 4 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 Co 3 O 4 spinel varied the valence (Co 3+ /Co 2+ ) and geometry (Co Oh /Co Td ) distributions of Co cations through different partial electron transfer and substitution sites. The higher Co 3+ /Co 2+ and Co Oh /Co Td ratios of the CuCo 2 O 4 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 NiCo 2 O 4 catalyst with lower Co 3+ /Co 2+ and Co Oh /Co Td ratios could not completely oxidize intermediates under the same conditions due to inferior redox properties. Therefore, the CuCo 2 O 4 catalyst showed superior catalytic activity and stability to the NiCo 2 O 4 catalyst for the catalytic oxidation of o -xylene. This work provides insights into the synthesis of substituted Co 3 O 4 catalysts from MMOFs and mechanism of substituting effects, which might guide the design of efficient TMO catalysts for VOC treatment.