Water-Directed Radical Microenvironments at Au–TiO ₂ Nanointerfaces for Photothermal Conversion of Methane to Oxygenates with 100% Selectivity

Selective activation of CH4 under mild conditions to produce liquid oxygenates is central to low-carbon chemical manufacturing. While O2 or H2O2 are commonly used oxidants, the direct oxidizing role of H2O itself remains poorly defined. Here, we use Au–TiO2 as a prototypical nanointerface to disentangle the effects of H2O versus O2 on photocatalytic CH4 oxidation. In-situ EPR spin-trapping, high-pressure DRIFTS, and DFT reveal two distinct pathways. With H2O as the sole oxidant, photogenerated holes oxidize H2O to •OH, which abstracts H from CH4 to form •CH3; subsequent surface reactions yield CH3OH and HCHO. Computations indicate that roughly one-third of the generated •OH participates effectively in the productive sequence. In contrast, when O2 is present, interfacial electrons preferentially reduce O2 to •OOH; coupling of •OOH with •CH3 forms CH3OOH intermediates that evolve to CH3OH/HCHO, but the elevated •OOH flux also accelerates deep oxidation to CO2 and suppresses H2 evolution. These mechanistic differences in radical identity, intermediate pools, and apparent barriers translate into divergent selectivity. Under optimized, O2-free conditions (1 wt % Au–TiO2, 80 °C, 20 bar CH4, UV–vis; H2O as oxidant/medium), liquid C1 oxygenates are obtained at 7.7 mmol g–1 h–1 with 100% selectivity. The study reveals that tuning the oxidant and the interfacial radical landscape at a nanoscale metal–oxide interface governs the precision activation of CH4 and provides mechanistic insight into H2O-driven, selective methane oxidation on Au–TiO2.