Metastable dual-defect states drive deep protonation for selective CO₂ photomethanation

Efficient proton and charge carrier management is crucial in sustainable catalysis but is often constrained by the trade-off between proton supply kinetics and charge recombination, which limits selectivity. Here, we propose an in situ strategy to construct metastable adjacent dual-vacancy (MADV) sites, where dynamically tuned electronic states enable rapid electron transfer and spatial proximity ensures efficient mass transport, collectively enhancing proton-coupled electron transfer. Ti_3d-derived active electronic states promote H₂O dissociation, supplying abundant protons and forming hydroxylated surfaces for CO₂ activation. Concurrently, dual-vacancy adjacency induces bidentate coordination, lowering the CO₂ reduction barrier and steering selectivity toward CH₄. The engineered MADV sites achieve nearly 100% CO₂-to-CH₄ selectivity with a production rate of 251.85 μmol g^-1 h^-1, approximately 75 times higher than pristine TiO₂. These findings highlight the significance of adjacent sites with active electronic states in protonation processes and provide guidance for designing selective catalytic systems.