Artificial Photosynthesis of H₂O₂ through Reversible Photoredox Transformation between Catechol and o‑Benzoquinone on Polydopamine-Coated CdS

Artificial photosynthesis of H₂O₂ from water and O₂ only is a prospective alternative to the industrial anthraquinone process, which requires the development of cheap, efficient, and stable photocatalysts working without sacrificial reagents. Inspired by the industrial strategy, polydopamine (PDA)-coated CdS was designed for the photosynthesis of H₂O₂ and achieved a yield of 3.84 mM in 24 h under visible light from water and dioxygen, which is 13.7 times higher than that of CdS. The dehydrogenation of catechol to o-benzoquinone in PDA is coupled with two-electron O₂ reduction to produce H₂O₂, while the photoreductive hydrogenation of o-benzoquinone regenerates catechol in PDA concurrently. The reversible redox transformation between catechol and the o-benzoquinone moiety in PDA under visible light markedly increases both the production yield and the selectivity of H₂O₂ (from 30% on CdS to 82% on CdS–PDA). In addition, PDA coating increases the O₂ adsorption capacity, inhibits H₂O₂ decomposition, and enhances the stability of CdS against photocorrosion. This structural design enables artificial photosynthesis of H₂O₂ without sacrificial reagents.