Boosting Photocatalytic N₂ Reduction via Ligand-Engineered MOFs: Regulated Hole Utilization and Interfacial Water...

Photocatalytic nitrogen reduction requires the concerted transfer of six electrons and six protons, where inefficient electron utilization and proton supply represent the critical limiting factors for proton-coupled electron transfer in this process. A key challenge lies in hole accumulation within photocatalysts, as hole transfer typically proceeds 2 to 3 orders of magnitude more slowly than electron transfer. This imbalance leads to charge recombination and sluggish water oxidation, limiting proton availability. In this work, we address these issues by engineering the ligands of metal-organic frameworks (MOFs) to serve as oxidation sites, enabling the simultaneous enhancement of hole extraction and water activation. In sulfonic-acid-functionalized MIL-101(Fe) (MIL-101(Fe)-SO₃H), transient absorption spectroscopy (TAS) elucidates that photoinduced electrons are either efficiently injected to the metal center (∼986 ps) or trapped at defect states (∼9 ps), while the holes remain on organic ligands to drive water oxidation. Crucially, -SO₃H groups restructure the interfacial water coordination to asymmetric configurations, facilitating the water dissociation and enhancing proton generation. Hence, MIL-101(Fe)-SO₃H achieves an impressive NH₃ selectivity (98.8%), surpassing that of pristine MIL-101(Fe). This work provides a rational strategy for boosting nitrogen reduction by engineering oxidation sites to promote the proton supply via coordinated control of charge transfer and water activation.