Turning Waste into Value: Photocatalytic Conversion of Methane to Methanol Using NiO/TiO2 Catalyst Derived from Spent Ni-Cd Batteries.

The selective conversion of methane to methanol under mild conditions remains a major challenge due to the high stability of the C-H bond and the propensity for overoxidation. Herein, NiO/TiO2 photocatalysts were synthesized using both commercial nickel precursors and nickel recovered from spent Ni-Cd batteries, providing a sustainable pathway that integrates methane valorization with battery-metal recycling. The battery-derived catalysts PB1 and TB1 achieved methanol formation rates of 2.66 and 6.36 mmol g-1·h-1, respectively, comparable to those of the commercial counterparts PC1 (2.38 mmol g-1·h-1) and TC1 (7.31 mmol g-1·h-1), demonstrating that recycled nickel can effectively replace commercial nickel without performance loss. Synthesized TiO2-based catalysts consistently outperformed P25-based materials, and increasing Ni loading correlated positively with methanol formation rate, underscoring the importance of Ni-induced defect engineering. Structural and spectroscopic analyses confirmed preservation of the anatase phase alongside the formation of Ti3+ and oxygen vacancies, which suppressed electron-hole recombination. Density functional theory calculations revealed that methane activation proceeds via a low-barrier homolytic C-H cleavage pathway on Ni-O sites in the presence of surface hydroxyl species, stabilizing methyl intermediates and favoring selective methanol formation. Ultimately, the combined experimental and theoretical results establish recycled Ni-modified TiO2 as a sustainable and mechanistically rational photocatalyst for selective methane-to-methanol conversion.