Dynamic Valence-State-Adaptive Ta Single-Atom Sites for Artificial H₂O₂ Photosynthesis

Photocatalytic H₂O₂ synthesis via the two-electron O₂ reduction reaction (2e^- ORR) is attractive, yet the interplay between active-site-mediated excited-state electron behavior and in situ O₂ activation at the atomic scale remains unclear. Herein, we establish a mechanism-guided integrated strategy combining theoretical screening, experimental construction, and in situ characterization to identify Ta single-atom sites as the optimal 5d metal centers on carbon nitride for H₂O₂ photosynthesis. The resulting catalyst combines favorable excited-state charge localization with thermodynamic advantages for the 2e^- ORR, achieving an apparent quantum yield of 14.53% at 420 nm and a solar-to-chemical conversion efficiency of 1.12% in pure water. Multidimensional in situ spectroscopy measurements and theoretical calculations demonstrate that Ta single-atom sites act as dynamically adaptive catalytic centers through flexible in situ valence-state evolution (+4.16 → +4.37 → +3.14), facilitating initial O₂ adsorption and then accumulating and transferring excited-state electrons to drive end-on O₂ activation through Ta 5d-O 2p orbital coupling, thereby accelerating *OOH-mediated selective H₂O₂ formation. This work establishes a framework for understanding dynamic single-atom photocatalysis and guiding the design of adaptive active sites for artificial H₂O₂ photosynthesis.