Hydrogen-Bond–Coordination Coupling on ZnO as Inhibitory Interfacial Structures in Photocatalytic Methane Oxidation

Abstract Understanding the role of interfacial water is essential for improving gas-phase photocatalytic methane (CH4) oxidation under realistic conditions. Here, ZnO is used as a model photocatalyst to examine how humidity influences CH4 oxidation pathways. Photocatalytic tests show that water markedly suppresses CH4 conversion and CO2 formation, while diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirms that CH4 uptake is only weakly affected. Operando DRIFTS reveals that humid conditions do not prevent formation of oxygenated intermediates but instead promote the accumulation of HCOOH/HCOO– species and suppress their onward oxidation to CO2. Formic acid probe experiments further show that coadsorbed water strengthens interfacial hydrogen bonding and drives formate to a stable bridging bidentate coordination mode. Density functional theory calculations and temperature-programmed DRIFTS confirm enhanced stability and persistence of the hydrogen-bond-assisted bridging bidentate formate. Together, these results identify hydrogen-bond–coordination coupling as a previously unrecognized interfacial inhibition mechanism in humid gas-phase methane photooxidation on ZnO, in which coadsorbed water simultaneously reinforces the hydrogen-bond network and drives formate toward persistent bridging bidentate coordination, retarding onward oxidation to CO2. These findings provide molecular-level insight into the inhibitory role of water in ZnO photocatalysis and highlight intermediate stabilization as a key design parameter for tuning CH4 oxidation pathways under ambient humidity.