Modulation of Local Hydrogen Bonding for Highly Efficient Bi-MOFs Photocatalyzed Fixation of N₂ in Aqueous Solution

For photocatalytic reduction of dinitrogen in aqueous solution, water plays an important role as not only a reactant to supply protons via the water oxidation reaction, but also a solvent, which cannot be neglected. At the interface between the catalyst surface and local water molecules, modulation of the hydrogen bonds (HBs) is critical for improving the production rate of ammonia. Herein, both roles (i.e., reactant and solvent) were balanced through the formation of an interfacial HB network between organic ligands of Bi-MOFs and water with the aim of accelerating water oxidation to produce more protons and facilitating the adsorption of N₂ as well as the release of free water molecules. With this approach, the ammonia synthesis rate reached 258.86 µmol·g^-1·h^-1 at ambient conditions. As demonstrated, the empty 6p orbitals present in Bi³⁺(6s²6p⁰), can accept electrons from the ligands due to the ligand-to-metal charge transfer (LMCT) effect. These electrons are then transferred to the π* antibonding orbitals of N₂, thus significantly weakening the N≡N bond. The photogenerated holes on the ligands oxidize hydrogen-bonded water molecules, producing more protons, which can further promote the critical process, namely the proton-coupled electron transfer (PCET), for the multi-step hydrogenation of N₂. Therefore, the dynamic balance among N₂ adsorption and activation, proton supply capacity, and proton transfer was achieved through a local microenvironment modulation strategy on MOFs. As a further proof, a phototactically produced NH₄ ⁺ solution with a concentration of approx. 200 mg·L^-1 was concentrated and used as a fertilizer. Overall, this work provided a new design strategy for the photocatalytic reduction of N₂ to produce ammonia on MOFs by elucidating the key role of the HB network.