Photoelectrochemical Conversion of Methane to Ethane and Hydrogen under Visible Light Using Functionalized Tungsten Trioxide Photoanodes with Proton Exchange Membrane.

Developing methane utilization technologies is desired to convert abundant and renewable carbon resources, such as natural gas and biogas, into value-added chemical products. This study provides insights into emerging photoelectrochemical (PEC) technology for the photocatalytic transformation of methane to CH and H using visible light at room temperature. The PEC conversion of methane to oxygenates has been investigated in aqueous electrolytes. Herein, we demonstrate the gas-phase PEC methane conversion using a proton exchange membrane (PEM) as a solid polymer electrolyte and a gas-diffusion photoanode for methane oxidation. Tungsten trioxide (WO), a semiconductor photocatalyst responsive to visible light, is utilized as the photoanode material. Ultraviolet light (∼365 nm) excitation predominantly results in CO production with lower CH selectivity in humidified methane. In contrast, visible light (∼453 nm) effectively promotes CH production over the WO photoanode, attributed to preferential hydroxyl radical (OH) formation compared to UV irradiation. Photogenerated holes formed near the valence band maximum of WO contribute to OH formation through a single-electron water oxidation. The photogenerated OH activates gaseous methane molecules to methyl radicals, subsequently coupled into CH at the gas-electrolyte-semiconductor boundary. H is concurrently formed on the cathode electrocatalyst. Improving the selectivity for the dehydrogenative coupling of methane is pivotal for enhancing the energy efficiency in the PEM-PEC system.