W-H Bond-Mediated "Anti-Overoxidation Valve": 100% Formaldehyde Selectivity Among Detected Products in Photocatalytic Methane-to-Formaldehyde Conversion.

Direct photocatalytic conversion of methane to formaldehyde is highly efficient, avoiding energy-intensive methanol oxidation. However, its selectivity and activity are severely limited by uncontrollable over-oxidation: Formaldehyde has lower C-H dissociation energy than methane, prone to deep overoxidation to CO2. Herein, we report H/WO2.72 hydrogenated nonstoichiometric tungsten oxide photocatalyst. At ambient temperature, it attains 100% HCHO selectivity over all detectable carbonaceous products at 30.94 µmol·h-1, with 11-fold higher activity than commercial WO3 and outperforming most conventional photocatalysts. Operando spectroscopic studies and density functional theory (DFT) calculations reveal that surface W-H bonds act as a kinetically gated "Anti-Overoxidation valve," W-H bonds are preferentially cleaved by reactive oxygen species (·OOH), as they are thermodynamically weaker than the C-H bonds in HCHO and its over-oxidation intermediate (*HCOOH). Then the generated hydrogen radicals (H*) drive a reverse reduction pathway to convert the *HCOOH back to *HCHO, interrupting deep oxidation. Concurrently, hydrogenation optimizes the electronic structure of WO2.72, lowering the HCHO desorption barrier to facilitate timely product release. This work provides a dynamic strategy for complete selectivity in catalytic CH4 conversion, emphasizing the value of precise kinetic pathway modulation for methane valorization.