Photocatalytic Partial Water Oxidation Promoted by a Hydrogen Acceptor-Hydroxyl Mediator Couple

Hydrogen peroxide (H₂O₂) is an important chemical in synthetic chemistry with huge demands. Photocatalytic synthesis of H₂O₂ via oxygen reduction and water oxidation reactions (ORR and WOR) is considered as a promising and desirable solution for on-site applications. However, the efficiency of such a process is low due to the poor solubility of molecular oxygen and the rapid reverse reaction of hydroxyl radicals (^•OH) with hydrogen atoms (H). Here, a strategy is proposed to boost the H₂O₂ evolution via oxidation of water by employing a H acceptor (A, nitrocyclohexane), an ^•OH mediator (M, dioxane), and a photocatalyst (CdS nanosheets). While ^•OH radicals are stabilized by dioxane to produce ketyl radicals prior to the formation of H₂O₂, H atoms are effectively utilized in the generation of cyclohexanone oxime, an important intermediate in the production of Nylon 6. The system displays a rapid kinetic accumulation of H₂O₂ (0.13 min^-1) to a high concentration (6.6 mM). At optimum reaction conditions, a high quantum efficiency (16.6%) and light-to-chemical conversion efficiency (4.9%) can be achieved under 410 nm irradiation.