Concerted Proton-Coupled\nElectron Transfer by Mo<sup>5+</sup>/Mo<sup>6+</sup> Reversible Transformation\nfor CO<sub>2</sub> Photoreduction with Nearly 100% CH<sub>4</sub> Selectivity

Regulation of the proton-coupled electron transfer (PCET)\nprocess\nto avoid the unbalanced proton and electron regions on the reduction\nactive sites is key to dictating product selectivity in a photocatalytic\nCO<sub>2</sub> reduction reaction. Here, we show that reversible Mo<sup>5+</sup>/Mo<sup>6+</sup> as a mediator can regulate the proton and\nelectron transfer process at the Bi<sub>2</sub>MoO<sub>6</sub> nanosheet/In<sub>2</sub>O<sub>3</sub> microtube (BI) catalyst. The formed concerted\nproton-coupled electron transfer enables a champion solar-to-methane\nefficiency of 0.15%, resulting in nearly 100% CH<sub>4</sub> selectivity\nand a competitive CH<sub>4</sub> yield of 46.37 μmol g<sup>–1</sup> h<sup>–1</sup> in pure water. The experiments, together with\ntheoretical calculations, clearly validate that In sites as H<sub>2</sub>O oxidation centers provide protons, and the regulation of\nprotons and electrons by using Mo sites forms approximate electroneutral\nproton/electron pairs, which are conjointly transferred to Bi sites\nas CO<sub>2</sub> adsorption/reduction centers, thus achieving precise\nhydrogenation on Bi sites for binding of the *CH<sub>3</sub>O key\nintermediate to form CH<sub>4</sub>.