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

Selective activation of CH 4 under mild conditions to produce liquid oxygenates is central to low-carbon chemical manufacturing. While O 2 or H 2 O 2 are commonly used oxidants, the direct oxidizing role of H 2 O itself remains poorly defined. Here, we use Au–TiO 2 as a prototypical nanointerface to disentangle the effects of H 2 O versus O 2 on photocatalytic CH 4 oxidation. In-situ EPR spin-trapping, high-pressure DRIFTS, and DFT reveal two distinct pathways. With H 2 O as the sole oxidant, photogenerated holes oxidize H 2 O to • OH, which abstracts H from CH 4 to form • CH 3; subsequent surface reactions yield CH 3 OH and HCHO. Computations indicate that roughly one-third of the generated • OH participates effectively in the productive sequence. In contrast, when O 2 is present, interfacial electrons preferentially reduce O 2 to • OOH; coupling of • OOH with • CH 3 forms CH 3 OOH intermediates that evolve to CH 3 OH/HCHO, but the elevated • OOH flux also accelerates deep oxidation to CO 2 and suppresses H 2 evolution. These mechanistic differences in radical identity, intermediate pools, and apparent barriers translate into divergent selectivity. Under optimized, O 2 -free conditions (1 wt % Au–TiO 2, 80 °C, 20 bar CH 4, UV–vis; H 2 O 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 CH 4 and provides mechanistic insight into H 2 O-driven, selective methane oxidation on Au–TiO 2 .