Breaking the Activity–Selectivity Trade-off for CH₄-to-C₂H₆ Photoconversion

Abstract

Photocatalytic conversion of methane (CH₄) to ethane (C₂H₆) has attracted extensive attention from academia and industry. Typically, the traditional oxidative coupling of CH₄ (OCM) reaches a high C₂H₆ productivity, yet the inevitable overoxidation limits the target product selectivity. Although the traditional nonoxidative coupling of CH₄ (NOCM) can improve the product selectivity, it still encounters unsatisfied activity, arising from being thermodynamically unfavorable. To break the activity-selectivity trade-off, we propose a conceptually new mechanism of H₂O₂-triggered CH₄ coupling, where the H₂O₂-derived ·OH radicals are rapidly consumed for activating CH₄ into ·CH₃ radicals exothermically, which bypasses the endothermic steps of the direct CH₄ activation by photoholes and the interaction between ·CH₃ and ·OH radicals, affirmed by <i>in situ</i> characterization techniques, femtosecond transient absorption spectroscopy, and density-functional theory calculation. By this pathway, the designed Au-WO₃ nanosheets achieve unprecedented C₂H₆ productivity of 76.3 mol mol_Au^-1 h^-1 with 95.2% selectivity, and TON of 1542.7 (TOF = 77.1 h^-1) in a self-designed flow reactor, outperforming previously reported photocatalysts regardless of OCM and NOCM pathways. Also, under outdoor natural sunlight irradiation, the Au-WO₃ nanosheets exhibit similar activity and selectivity toward C₂H₆ production, showing the possibility for practical applications. Interestingly, this strategy can be applied to other various photocatalysts (Au-WO₃, Au-TiO₂, Au-CeO₂, Pd-WO₃, and Ag-WO₃), showing a certain universality. It is expected that the proposed mechanism adds another layer to our understanding of CH₄-to-C₂H₆ conversion.