In situ fabrication of atomically adjacent dual-vacancy sites for nearly 100% selective CH₄ production

The photocatalytic CO₂-to-CH₄ conversion involves multiple consecutive proton-electron coupling transfer processes. Achieving high CH₄ selectivity with satisfactory conversion efficiency remains challenging since the inefficient proton and electron delivery path results in sluggish proton-electron transfer kinetics. Herein, we propose the fabrication of atomically adjacent anion-cation vacancy as paired redox active sites that could maximally promote the proton- and electron-donating efficiency to simultaneously enhance the oxidation and reduction half-reactions, achieving higher photocatalytic CO₂ reduction activity and CH₄ selectivity. Taking TiO₂ as a photocatalyst prototype, the operando electron paramagnetic resonance spectra, quasi in situ X-ray photoelectron spectroscopy measurements, and high-angle annular dark-field-scanning transmission electron microscopy image analysis prove that the V_Ti on TiO₂ as initial sites can induce electron redistribution and facilitate the escape of the adjacent oxygen atom, thereby triggering the dynamic creation of atomically adjacent dual-vacancy sites during photocatalytic reactions. The dual-vacancy sites not only promote the proton- and electron-donating efficiency for CO₂ activation and protonation but also modulate the coordination modes of surface-bound intermediate species, thus converting the endoergic protonation step to an exoergic reaction process and steering the CO₂ reduction pathway toward CH₄ production. As a result, these in situ created dual active sites enable nearly 100% CH₄ selectivity and evolution rate of 19.4 μmol g^-1 h^-1, about 80 times higher than that of pristine TiO₂. Thus, these insights into vacancy dynamics and structure-function relationship are valuable to atomic understanding and catalyst design for achieving highly selective catalysis.