Selective oxidative coupling of methane to ethane with oxygen using an Au/Zn2Ti3O8 photocatalyst under mild conditions

Abstract

The photocatalytic selective oxidation of CH₄ to value-added higher hydrocarbons presents a promising avenue for the sustainable development of the chemical industry; however, the mild activation and conversion of CH₄ remain great challenges. Herein, a novel Au/Zn₂Ti₃O₈ hybrid photocatalyst is assembled from supporting Au nanoparticles (NPs) on the surface of Zn₂Ti₃O₈ nanospheres. The Zn₂Ti₃O₈ semiconductor with Zn²⁺ active sites drives the CH₄ coupling reaction, while the Au NPs promote the separation and migration of charge carriers. When irradiated with a 365 LED light, the 1.0%-Au/Zn₂Ti₃O₈ catalyst exhibits high activity and stability for selective CH₄ coupling with O₂, affording an optimal C₂H₆ yield of 609.49 μmol g⁻¹ h⁻¹ with 80.18% selectivity, which is among the state-of-the-art values under comparable conditions. Besides, the 1.0%-Au/Zn₂Ti₃O₈ sample affords a turnover number (TON) of 239.1 and an apparent quantum efficiency (AQE) of 1.05% at 365 nm. Studies reveal that the Schottky junction interface strongly promotes photoinduced electrons to be transferred to Au from Zn₂Ti₃O₈, realizing directed separation and migration of charge carriers for high photocatalytic activity. Various in situ spectroscopy analyses expose that the key ˙CH₃ species in CH₄-to-C₂H₆ conversion are stabilized by the surface Au sites for the subsequent coupling reaction to form C₂H₆, which prevents the undesirable overoxidation reaction to afford high C₂H₆ selectivity. A possible photocatalytic oxidative CH₄ coupling mechanism over the Au/Zn₂Ti₃O₈ hybrid is also proposed.