Hydroxyl-Functionalized Donor–Acceptor Covalent Organic Frameworks for Efficient Photocatalytic Hydrogen Peroxide Production under Visible Light

Abstract

Photocatalytic hydrogen peroxide (H₂O₂) production via the oxygen reduction reaction (ORR) provides a promising and energy-saving alternative to the traditional energy-intensive anthraquinone process. Nevertheless, how to decrease the energy barrier of the two-electron (2e^–) ORR process and photosynthesize H₂O₂ efficiently is still challenging. Herein, three hydroxyl-functionalized donor–acceptor covalent organic frameworks (COFs) are synthesized for photocatalytic H₂O₂ production under visible-light irradiation (420 ≤ λ ≤ 780 nm). It is observed that the dihydroxyl functionalization (2,5-DhaTph and 2,3-DhaTph) facilitates the transportation of photogenerated carriers between acceptor and donor units and accelerates the kinetics of the rate-limiting step of the ORR when comparing with the monohydroxyl functionalization (2-DhaTph). Further, 2,5-DhaTph with para-position hydroxyl functionalization shows higher H₂O₂ photosynthesis efficiency than 2,3-DhaTph (ortho-positioned hydroxyl), probably due to the greater exposure of catalytically active sites. This is supported by a better structural symmetry of 2,5-DhaTph, which contributes to higher crystallinity and higher specific surface areas. Electron paramagnetic resonance (EPR) spectra and theoretical calculations show that 2,5-DhaTph produces the *OOH intermediates with a reduced energy barrier, resulting in a high H₂O₂ production rate of 2103.1 μmol h^–1 g^–1. Regulating the amount of hydroxyl substituents and their location on the donor units of COFs is an effective strategy to boost photogenerated carrier transfer and reduce the energy barrier of O₂-to-H₂O₂ conversion.