Structure and Reducibility of NiO-MoO3/γ-Al2O3 Catalysts: Effects of Loading and Molar Ratio

The effects of loading and molar ratio on the structure and reducibility of NiO-MoO3/gamma-Al2O3 catalysts have been investigated. Particular attention is given to the catalysts with excess of Mo. The catalysts were prepared by impregnation method and characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and temperature-programmed reduction techniques. Results from Raman, XPS, and XRD show that different surface species can be formed depending on the loading and molar ratio. At low loading spinel-like NiAl2O4 and dispersed MoO3 are the major species on gamma-Al2O3, With increasing the loading the aggregation of the Mo species occurs. Only at high loading are crystalline beta-NiMoO4 and MoO3 formed on the surface. For the first time, excess Mo was found to promote the formation of beta-NiMoO4 on gamma-Al2O3. The change in the structure of the catalysts has a large influence on their reducibility, suggesting that both the structure and oxidation-reduction properties of the NiO-MoO3/gamma-Al2O3 catalysts can be controlled by varying the loading and molar ratio.