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

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 COx. The highest methanol yield was obtained at a 2 : 1 molar ratio of oxidant to methane, at reaction temperatures of 250 degrees C and 350 degrees 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 alpha-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.