Understanding the effect of pH on protonated COF during photocatalytic H2O2 production by femtosecond transient absorption spectroscopy

Abstract

Covalent organic frameworks (COFs), recognized for their precisely tunable microstructures and high surface area, are promising photocatalysts for hydrogen peroxide (H₂O₂) production. However, the critical influence of pH on the stability of COF during the photocatalytic H₂O₂ production remains poorly understood. In this work, the photocatalytic H₂O₂ production performance of an imine-linked COF is significantly enhanced through a simple protonation strategy. Crucially, the protonated COF exhibits excellent stability under weakly acidic conditions (pH ≥ 3), but undergoes irreversible hydrolyzed under strongly acidic conditions (pH < 3). The protonation occurs specifically at the nitrogen atoms of imine units and serves a dual function: it suppresses ultrafast charge recombination (as revealed by femtosecond transient absorption spectroscopy) and directly provides a proton source for H₂O₂ generation. Moreover, fluoride ions (F⁻) are introduced into the photocatalytic system to further improve the photocatalytic H₂O₂ production rate. The strong electronegativity of F⁻ facilitates electron transfer from COF to F⁻, thus realizing the spatial separation of photogenerated carriers. Mechanistic studies confirm that H₂O₂ production follows a two-electron oxygen reduction reaction pathway. These findings elucidate the pH-dependent stability and activity of protonated COFs, provide fundamental insights into charge carrier dynamics, and establishe design principles to develop highly efficient and stable COF-based photocatalysts for solar-driven H₂O₂ generation.