Water Vapor-Assisted Photocatalytic Methane Conversion over Calcined Zn–Fe Layered Double Hydroxides

Direct conversion of methane to liquid oxygenates under ambient conditions remains a formidable challenge due to its inherent inertness. Herein, we report a gas–solid photocatalytic system using water vapor as an oxygen source over calcined Zn–Fe layered double hydroxides (ZnFe-CLDHs). The optimized Zn4Fe-400 catalyst achieves a remarkable formaldehyde production rate of 553 μmol·g–1·h–1 with 83% selectivity in the liquid products at ambient conditions, outperforming conventional aqueous-phase systems. Mechanistic studies reveal that higher Zn/Fe ratios enhance the charge separation efficiency, facilitating methane activation via photogenerated holes. Crucially, increased surface charge density strengthens the confinement of adsorbed hydroxyl radicals (*OH), promoting rapid oxidation of the generated methanol to formaldehyde. In situ DRIFTS, H-D isotope labeling, and electron paramagnetic resonance (EPR) spin-trapping experiments collectively confirm the reaction pathway: CH4 → *CH3 → *CH3OH → HCHO, with surface-bound *OH dictating the product distribution. This work demonstrates that the surface states of catalysts play a modulating role of reaction intermediate species, thereby significantly influencing catalytic activity.