Buffered Hydroxyl Radical for Photocatalytic Non-Oxidative Methane Coupling

Hydroxy radical (⋅OH) is a prestigious oxidant that allows the cleavage of strong chemical bonds of methane but is untamed, leading to over-oxidation of methane and waste of oxidants, especially at high methane conversion. Here, we managed to buffer ⋅OH in an aqueous solution of photo-irradiated Fe³⁺, where ⋅OH almost participates in methane oxidation. Due to the interaction between Fe³⁺ and SO₄ ^2-, the electron transfer from OH^- to excited-state Fe³⁺ for ⋅OH generation is retarded, while excessive ⋅OH is consumed by generated Fe²⁺ to restore Fe³⁺. When combined with a Ru/SrTiO₃:Rh photocatalyst, the buffered ⋅OH converts methane to C₂₊ hydrocarbons and H₂ with formation rates of 246 and 418 μmol h^-1, respectively. The apparent quantum efficiency reaches 13.0±0.2 %, along with 10.2 % methane conversion and 81 % C₂₊ selectivity after 80 hours of reaction. Overall, this work presents a strategy for controlling active radicals for selective and efficient photocatalysis.