Tuning Reaction Pathway with Surface Metal Cocatalyst for Ethylene Production via Photocatalytic Methane Conversion

Photocatalytic oxidative coupling of methane (POCM) provides a sustainable route to convert methane (CH₄) into value-added chemicals. However, the selectivity and production rate of high-value hydrocarbons, such as ethylene, are still bottlenecked by the ambiguous carbon-carbon (C-C) coupling mechanism and inefficient CH₄ activation processes. This work investigated the POCM process based on cocatalysts of palladium (Pd), gold (Au), and their alloy (PdAu) to elucidate the reaction pathway. By studying the intermediates and product selectivity, we reveal that the strong bonding between the metal cocatalyst and hydrocarbon intermediates of *CH_<i>x</i> (<i>x</i> = 2 or 3) is essential for achieving a rapid and selective CH₄ conversion process. The PdAu alloy facilitates the *CH₂ generation and lowers the energy barrier for *CH₂ coupling, thereby selectively tuning the reaction pathway toward ethylene generation. Using a classic TiO₂ photocatalyst loaded with PdAu cocatalysts, an ethylene production rate of 0.18 mmol g^-1 h^-1 and a CH₄ conversion rate of 13.73 mmol g^-1 h^-1 are achieved, corresponding to an apparent quantum efficiency (AQE) of 12% at a wavelength of 350 nm. Mechanistic studies establish that an effective cocatalyst for value-added product generation should perform three critical functions, including modulating CH₄ activation, stabilizing the *CH_<i>x</i> intermediates, and promoting C-C coupling. Our findings demonstrate that alloy engineering is an effective strategy to balance these three functions to advance methane valorization for the efficient and selective generation of multicarbon products.