The Study On Active Sites Of Mo-Based Metal Oxide Catalysts In Oxidative Dehydrogenation Of Propane

This work focuses on the structure and catalytic active sites of Mo-based metal oxide catalysts in propane oxidative dehydrogenation. The effects of loading and molar ratio on the structure and reducibility of MoO₃/γ-Al₂O₃ and NiO-MoO₃/γ-Al₂O₃ catalysts has been investigated. The active sites of NiO-MoO₃/γ-Al₂O₃ catalysts for the oxidative dehydrogenation of propane were studied using in situ UV Raman spectroscopic technique.The catalysts used in this study were prepared by impregnation method, and characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Temperature-programmed reduction and Fixed-bed catalytic reactor techniques. For MoO₃/γ-Al₂O₃ samples the isolated MoO3 is the major structure at low loading. As the loading increases the polymerized molybdate is formed on the surface. At high loading, the formation of crystalline MoO₃ occurs on the surface. The two-dimentional polymerized MoO₃ shows the highest reducibility among the MoO₃/γ-Al₂O₃ catalysts with different loadinds.For NiO-MoO₃/γ-Al₂O₃ catalysts, different surface species can be formed depending on the loading and molar ratio. At low loading spinel-like NiAl₂O₄ and dispersed MoO₃ are major species onγ-Al₂O₃, and the reducibility of Ni and Mo species is promoted by each other. The interaction between Ni and the support is stronger than that between Mo and the support. With increasing the loading the aggregation of the Mo species occurs, which leads to a decrease of the reduction temperature. At high loading crystallineβ-NiMoO₄ and MoO₃ can be formed on the surface, which results in a peak shift to high temperature side. In particular, excess Mo was found to promote the formation ofβ-NiMoO₄ onγ-Al₂O₃.For 40wt.% NiO-MoO₃/γ-Al₂O₃ catalyst, the results from in situ UV Raman spectroscopic study shows that their reducibility decreases in the sequence: Mo-O-Mo>Mo=O in NiMoO₄ lattice>Mo=O in MoO₃ lattice. A correlation between the catalytic activity/selectivity and the structure proposes that Mo-O- Mo bond and Mo=O in NiMoO₄ are crucial active sites for the oxidative dehydrogenation of propane.