Continuous selective oxidation of methane to methanol by thermal-assisted photocatalysis over NiO/ZnO

The selective oxidation of methane to methanol under mild conditions remains challenging due to the delicate balance between methane activation and the prevention of overoxidation. Herein, we present a continuous thermal-assisted photocatalytic process for selective methane oxidation to methanol using NiO/ZnO catalysts in a flow fixed-bed reactor. Under optimal conditions, the process achieves a methanol yield of 47.5 μmol g^-1h^-1 with 94.7 % selectivity and long-term stability over 120 h. In situ EPR, time-resolved decay spectra, and transient photocurrent response analyses confirm that thermal assistance enhances the generation and migration of photogenerated carriers and promotes the activation of CH₄, resulting in a 16.49-fold increase in methanol yield compared to photocatalysis alone. Additionally, NiO modification of ZnO facilitates the formation of oxygen vacancies (O_Vs) in ZnO, boosting electron transfer, while NiO nanoparticles promote hole transfer. This dual-site synergistic charge transfer mechanism, combined with thermal assistance, significantly enhances the separation of photogenerated electron-hole pairs, thereby improving the overall performance of methane oxidation to methanol.