Nitrogen-Doped Metal–Organic Frameworks for Boosting Photocatalytic Ammonia Synthesis

Abstract The efficiency of photocatalytic ammonia (NH 3 ) synthesis is severely limited by the extremely difficult activation of N 2 owing to its high N≡N triple bond energy. To address this challenge, we propose an N-doping strategy to facilitate the N 2 activation. Our strategy involves optimizing the electronic structure of the metal active sites by modulating the coordination element. First, we introduce five different N-coordination ligands with distinct steric hindrances and N electron densities (2-methylimidazole (MI), isoindolin-1-one (II), 1,2-benzisothiazolin-3-one (BIT), benzo[d]isoxazol-3-ol (BIX), and terephthalamide (TA)) into an amino-functionalized metal–organic framework (MOF), NH 2 -MIL-68 (NM), to construct the N-coordination via the partial replacement of the O-coordination in the metal clusters. Electrochemical impedance spectroscopy and photocurrent analysis demonstrate that N-doping enhances electron transfer and carrier separation. Moreover, incorporating ligands with moderate sizes and steric hindrances (II, BIT, and BIX) more effectively boosts the carrier separation efficiency than incorporating small (MI) or large (TA) ligands. Furthermore, the N-doped MOF modified with BIT (in which N exhibits a moderate electron density) exhibits the strongest carrier separation capability. Concurrently, the X-ray photoelectron spectroscopy, density functional theory, and N 2 temperature-programmed desorption results confirm that the established low-electronegativity N-coordination elevates the electron density of the metal active sites, which consequently enhances the N 2 activation process. The systematic optimization of the N-coordinating ligand species and doping concentrations allows the optimal NM-0.5BIT to achieve a NH 3 production rate of 175.5 μmol/(g·h). The proposed N-doping strategy offers several insights into the activation of inert molecules and the development of organic framework photocatalysts. Graphical Abstract