The g-C ₃ N ₄ with dual defect sites exhibits excellent photocatalytic H ₂ O ₂ generation activity and selectivity, and the key role of each defect site in the surface reaction mechanism is revealed.

Photocatalytic two-electron oxygen reduction to produce high-value hydrogen peroxide (H₂O₂) is gaining popularity as a promising avenue of research. However, structural evolution mechanisms of catalytically active sites in the entire photosynthetic H₂O₂ system remains unclear and seriously hinders the development of highly-active and stable H₂O₂ photocatalysts. Herein, we report a high-loading Ni single-atom photocatalyst for efficient H₂O₂ synthesis in pure water, achieving an apparent quantum yield of 10.9% at 420 nm and a solar-to-chemical conversion efficiency of 0.82%. Importantly, using in situ synchrotron X-ray absorption spectroscopy and Raman spectroscopy we directly observe that initial Ni-N₃ sites dynamically transform into high-valent O₁-Ni-N₂ sites after O₂ adsorption and further evolve to form a key *OOH intermediate before finally forming HOO-Ni-N₂. Theoretical calculations and experiments further reveal that the evolution of the active sites structure reduces the formation energy barrier of *OOH and suppresses the O=O bond dissociation, leading to improved H₂O₂ production activity and selectivity.