Hydration-Dependent Working-Surface Chemistry in Photocatalytic Methane Coupling

Selective photocatalytic methane coupling requires directing methane-derived intermediates toward C–C bond formation while limiting their oxidation to CO2, yet how the reaction environment controls this competition through surface chemistry remains unclear. Water is ubiquitous in photocatalytic methane conversion, but its influence is commonly interpreted through specific chemical functions rather than through its broader effects on working-surface chemistry. Here, Au/ZnO is used as a compositionally defined, coupling-competent model catalyst to reveal how hydration regulates carbon transformation pathways by reshaping the working-surface chemistry. Systematic variation of water availability reveals three hydration-dependent catalytic regimes with markedly different product distributions. Moderate hydration increases the ethane production rate from 1590 to 8407 μmol g–1 h–1 and enables ethylene formation, whereas excessive hydration increases CO2 production and lowers C2 selectivity. In situ spectroscopies show that these catalytic regimes are accompanied by hydration-dependent changes in methane adsorption, methane-derived and coupling-related surface intermediates, and oxygen-deficient surface states within a reaction network involving both carbon- and oxygen-centered radical chemistry. Moderate hydration establishes surface conditions that favor C–C coupling and C2 hydrocarbon accumulation, whereas excessive hydration favors oxidized surface species and carbonate formation. These results identify hydration as a key determinant of the steady-state surface chemistry governing carbon partitioning between C–C coupling and oxidation during photocatalytic methane conversion.