Macroscopic Hexagonal Co3O4 Tubes Derived from Controllable Two-Dimensional Metal-Organic Layer Single Crystals: Formation Mechanism and Catalytic Activity

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 (Co_Oh/Co_Td) distributions of Co cations through different partial electron transfer and substitution sites. The higher Co³⁺/Co²⁺ and Co_Oh/Co_Td 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 Co_Oh/Co_Td 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.

Effect of Cu/Co ratio in CuaCo1-aOx (a = 0.1, 0.2, 0.4, 0.6) flower structure on its surface properties and catalytic performance for toluene oxidation

Catalytic oxidation is considered a high-efficient method to minimize efficiently toluene emission. It is still a challenge to improve the catalytic performance for toluene oxidation by modifying the surface properties to enhance the oxidation ability of catalyst. Herein, a series of Cu_aCo_1-aO_x (a = 0.1, 0.2, 0.4, 0.6) catalysts were synthesized via solvothermal method and applied for toluene oxidation. The effects of the Cu/Co ratio on the texture structure, morphology, redox property and surface properties were investigated by various characterization technologies. The Cu_0.4Co_0.6O_x catalyst with dumbbell-shaped flower structure exhibited much lower temperature of 50% and 100% toluene conversion and far higher reaction rate (13.96 × 10^-2 μmol·g^-1·s^-1) at 220 °C than the Co based oxides in previous reports. It is found that the good activity can be attributed to the fact that the proper Cu/Co ratio can significantly improve the formation of more surface adsorbed oxygen and Co³⁺ species, leading to the much higher oxidation ability came from the strong interaction between Cu and Co oxides. It is suggested that toluene should be oxidized more rapidly to CO₂ and H₂O over the Cu_0.4Co_0.6O_x catalyst than Co₃O₄ based on the results of in situ DRIFTS.

Enhancement of catalytic toluene combustion over Pt-Co3O4 catalyst through in-situ metal-organic template conversion

A Pt-Co₃O₄ catalyst named Pt-Co(OH)₂-O was prepared by metal-organic templates (MOTs) conversion and used for catalytic oxidation of toluene. Through the conversion, the morphology of catalysts transformed from rhombic dodecahedron to nanosheet and the coated Pt nanoparticles (NPs) were more exposed. The Binding energy shift in XPS test indicates that the strong metal-support strong interaction (SMSI) has enhanced, and the physicochemical changes caused by it are characterized by other techniques. At the same time, Pt-Co(OH)₂-O showed the best catalytic performance (T₅₀ = 157 °C, T₉₀ = 167 °C, E_a = 40.85 kJ mol^-1, TOF_Pt = 2.68 × 10^-3 s^-1) and good stability. In addition, the in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) studies have shown that because SMSI weakened the Co-O bond, the introduction of Pt NPs can make the migration of oxygen in the catalyst easier. The change of binding energy change and the content of various species in the quasi in situ XPS experiment further confirmed that the Pt-Co(OH)₂-O catalyst has stronger SMSI, resulting in its stronger electron transfer ability and oxygen migration ability, which is conducive to catalytic reactions. This work provides new ideas for the development of supported catalysts and provides a theoretical reference for the relevant verification of SMSI.