Modulating N-Heterocyclic Microenvironment in β-Ketoenamine Covalent Organic Frameworks to Boost Overall Photosynthesis of H₂O₂

Covalent Organic Frameworks (COFs) have emerged as promising platforms for photocatalytic synthesis of hydrogen peroxide (H₂O₂) due to their tunable chemical compositions and efficient catalytic functionalities. Inspired by the role of the microenvironment in enzyme catalysis, this study introduces various N-heterocyclic species into β-ketoenamine COFs (N_x-COFs, where N_x represents the number of nitrogen atoms in the N-heterocycle) to regulate the microenvironment around catalytic sites on acceptor-donor-acceptor (A-D-A) COFs foroverall H₂O₂ photosynthesis in pure water. The N_x-COFs exhibit distinct H₂O₂ photosynthetic rates following the number of nitrogen atoms sequence of N₃-COF > N₂-COF > N₁-COF > N₀-COF, with N₃-COF with triazine structure showing the highest H₂O₂ generation rate (4881 µmol h⁻¹ g⁻¹) and the decent solar-to-chemical conversion (SCC) efficiency (0.413%), surpassing many existing COF-based catalysts. In situ characterization and theoretical calculations support the experimental results, revealing that N-heterocyclic species promote the photosynthesis of H₂O₂ through both an indirect stepwise single-electron oxygen reduction reaction (1e⁻ ORR) mechanism and a direct two-electron water oxidation (2e⁻ WOR) pathway. This study advances the design paradigm of photocatalysts by modulating the microenvironment within A-D-A COFs, paving the way for the development of more efficient and robust photocatalytic systems for the overall photosynthesis of H₂O₂.