Highly Selective Photocatalytic Methane Coupling by Au-Modified Bi₂WO₆

Photocatalytic oxidative coupling of methane (OCM) to ethane promises a route to value-added C₂ products from an abundant and low-cost feedstock. However, selective activation of C-H bond of CH₄ without overoxidation to CO₂ has been a major challenge. An important reason for overactivation of CH₄ is the fact that the BDEs of the subsequent C-H bonds are lower than that of the initial C-H bond. Thus, when thermal energy is applied to overcome the activation barrier, it is often exceedingly difficult to activate only the first C-H bond. Photocatalysis can be performed at significantly lower temperatures and addresses this concern. However, the cooperative interactions between the metal co-catalyst and the oxide photocatalyst, which are necessary for elucidating the variations in reactivity observed on different oxide photocatalysts, remain relatively unexplored. Another inspiration for our work revolves around the correlation between the nature of the oxide support (and/or catalyst) and overoxidation in selective oxidative reactions. It has been acknowledged that the surface lattice oxygen replenished from oxygen dissociation acts as a relatively mild oxidant, preventing overoxidation in preferential oxidation of carbon monoxide (PROX). Inspired by previous studies, we established a correlation between improved C₂H₆ selectivity and production with the nature of the photocatalyst, which should feature more facile release of surface lattice oxygen and, consequently, the easier formation of oxygen vacancies (O_v) on the oxide surface. Specifically, we present the use of Au-modified Bi₂WO₆ as a prototypical photocatalyst, demonstrating high performance OCM through photocatalysis. A C₂H₆ production rate of 1.69×10³ µmolg^-1h^-1 with approximately 85% selectivity was achieved which ranks among the top-performing photocatalytic OCM systems. Efforts were also made in establishing the correlation between improved OCM performance and photocatalyst system by examining the nature of the oxide photocatalyst. Our findings indicated that the oxygen within the oxide surface, likely contributed and/or later regenerated from the adsorbed oxygen at the vacancy sites, afforded a desired reactivity to selectively activate the C-H bond without significant overoxidation. Surprisingly, it was revealed that the Au co-catalyst plays dual roles of activating the oxide photocatalyst for enhanced CH₄ activation and promoting C-C coupling to yield C₂H₆ as the main product.