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

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 <= lambda <= 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 mu mol 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.