Keto-anthraquinone covalent organic framework for H2O2 photosynthesis with oxygen and alkaline water

Abstract Hydrogen peroxide (H2O2) photosynthesis is an attractive carbon-neutral process for decentralized applications, but suffers from insufficient activity of catalysts due to the high energy barrier of hydrogen extraction from H2O without sacrificial reagent. Herein, we report that mechanochemically synthesized keto-form anthraquinone covalent organic framework (Kf-AQ) is able to directly synthesize H2O2 with molecular oxygen and alkaline water (pH = 13) in the absence of any sacrificial reagents, with a superior production rate of 4784 µmol h− 1 g− 1 under visible light irradiation (λ > 400 nm) and an impressive apparent quantum yield (AQY) of 15.8% at 400 nm. Characterization results revealed that the strong alkalinity resulted in the formation of OH−(H2O)n clusters in water, which were first adsorbed on keto moieties of Kf-AQ and then more easily dissociated into molecular oxygen and active hydrogen with the injection of photoelectrons, because the energy barrier of hydrogen extraction from OH−(H2O)n was largely lowered by weakening the H-bonded networks of H2O molecules owing to the excessive electrons in OH−. The produced active hydrogen quickly diffused to react with anthraquinone to generate anthrahydroquinone, which was subsequently oxidized by molecular oxygen to selectively produce H2O2. This study provides a novel efficient H2O2 photosynthesis material, and also sheds light on the importance of hydrogen extraction from H2O for photocatalytic H2O2 synthesis.