Oxidative-Branch Engineering for Radical Regulation in Methane-to-Methanol Photocatalysis

Selective methane-to-methanol photocatalysis requires efficient methane activation and control over the radical environment that determines whether methanol is preserved or further oxidized. Radical generation is therefore double-edged: an underdeveloped radical environment limits C-H activation and methanol-forming oxygenation, whereas an excessive oxidative environment promotes secondary oxidation. Here, the radical environment is regulated by engineering the oxidative branch of an In₂O₃/ZnO S-scheme heterojunction. Under ambient-pressure gas-solid flow conditions with O₂ and H₂O as co-reactants, the optimized catalyst delivers methanol at 638.2 µmol g^-1 h^-1 with 93.8% selectivity. The volcano-shaped dependence of methanol productivity on ZnO loading shows that methanol accumulation is governed by competition between partial and deep oxidation rather than oxidative strength alone. Spectroscopic analyses support an S-scheme charge-transfer pathway that preserves oxidative holes on ZnO and reductive electrons on In₂O₃, coupling methane activation, water oxidation and O₂ reduction within a branch-specific redox network. Increasing ZnO loading strengthens the methyl-, superoxide- and hydroxyl-radical probe responses, whereas methanol selectivity peaks at an intermediate loading. At higher ZnO loading, deep-oxidation signatures emerge without further enhancement of methanol-related surface chemistry, whereas the optimized heterojunction maintains a formation-dominant surface pathway. These findings identify a kinetic balance in the radical environment, rather than radical maximization, as the selectivity principle for methane-to-methanol photocatalysis.