Protonated Imine-Linked Zirconium-Organic Cages for Enhanced Photocatalytic Hydrogen Peroxide Production

Zirconium-based metal-organic cages (Zr-MOCs) have garnered significant interest for their exceptional stability, tunable functionality, processability, and ease of synthesis. Herein, an imine-based zirconium-organic cage (designated ZrC-IM) was precisely synthesized via Schiff-base condensation between presynthesized aldehyde-functionalized zirconium-oxo clusters and p-phenylenediamine (PPD) linkers. To enhance light absorption capacity, protonation of the imine bonds in ZrC-IM was achieved via acetic acid treatment. Consequently, the protonated sample (ZrC-IM-AC) exhibits significant absorption red-shifts and improved charge separation efficiency, further demonstrating superior catalytic activity for H₂O₂ photogeneration, with a yield rate of 507.1 μmol g^-1 h^-1. Radical trapping experiments combined with electron spin resonance (ESR) spectroscopy demonstrated that both ZrC-IM and ZrC-IM-AC catalysts promote photocatalytic H₂O₂ generation through an indirect two-electron oxygen reduction pathway involving the sequential reactions: O₂ + e^- → ^•O₂^-, followed by ^•O₂^- + e^- + 2H⁺ → H₂O₂. This work not only expands the structural diversity of imine-based Zr-MOCs, but also offers new insights for the regulation of MOC-based photocatalysts with enhanced performance.