The role of oxidants in the activation of methane to methanol over a Ni–Mo/Al2O3 catalyst†

Abstract

A commercially available Ni–Mo/Al₂O₃ catalyst was evaluated for its effectiveness in the partial CH₄ oxidation to methanol by using various oxidants, including O₂, H₂O, and N₂O. The main products from the reactions were methanol, formaldehyde, hydrogen and carbon oxide gases. The study revealed that the one-step activation of CH₄ into oxygenates on the Ni–Mo/Al₂O₃ catalyst depended on the type of oxidant utilized. The research examined how the mobility and storage of lattice oxygen within the catalyst influenced its performance in methane conversion. High oxygen storage and release improved catalytic activity but reduced selectivity. Methane conversion without oxygenated products occurred when H₂O or N₂O was used, while O₂ promoted the formation of CO_x. The highest methanol yield was obtained at a 2 : 1 molar ratio of oxidant to methane, at reaction temperatures of 250 °C and 350 °C. When H₂O was used, significant quantities of H₂ and CO were produced, likely due to a simultaneous reforming reaction. Partial oxidation of nickel and molybdenum was observed under H₂O and N₂O conditions. Temperature-programmed reduction (TPR) indicated the transformation of higher-valence oxides into different sub-oxides. In temperature-programmed reduction–oxidation (TPRO), three peaks were detected, corresponding to oxygen surface sites and two framework locations. These peaks shifted to lower temperatures with N₂O, suggesting improved oxygen migration from the bulk to the surface. X-ray diffraction (XRD) analysis identified an active α-NiMoO₄ phase, which facilitated oxygen termination on molybdenum atoms. Under O₂ conditions, nickel also underwent oxidation. Overall, the Ni–Mo/Al₂O₃ catalyst showed notable methanol productivity, reaching up to 9.85 g of methanol per gram of catalyst per hour with N₂O as the oxidant, surpassing other catalysts reported in the literature.