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

Abstract

Photocatalytic conversion of methane (CH 4 ) to ethane (C 2 H 6 ) has attracted extensive attention from academia and industry. Typically, the traditional oxidative coupling of CH 4 (OCM) reaches a high C 2 H 6 productivity, yet the inevitable overoxidation limits the target product selectivity. Although the traditional nonoxidative coupling of CH 4 (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 H 2 O 2 -triggered CH 4 coupling, where the H 2 O 2 -derived ·OH radicals are rapidly consumed for activating CH 4 into ·CH 3 radicals exothermically, which bypasses the endothermic steps of the direct CH 4 activation by photoholes and the interaction between ·CH 3 and ·OH radicals, affirmed by in situ characterization techniques, femtosecond transient absorption spectroscopy, and density-functional theory calculation. By this pathway, the designed Au-WO 3 nanosheets achieve unprecedented C 2 H 6 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-WO 3 nanosheets exhibit similar activity and selectivity toward C 2 H 6 production, showing the possibility for practical applications. Interestingly, this strategy can be applied to other various photocatalysts (Au-WO 3, Au-TiO 2, Au-CeO 2, Pd-WO 3, and Ag-WO 3 ), showing a certain universality. It is expected that the proposed mechanism adds another layer to our understanding of CH 4 -to-C 2 H 6 conversion.

Topics & Concepts