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• Nitrogenase-inspired MIL-53(FeII/FeIII) is first synthesized for N 2 photo fixation. • Mixed-valence FeII/FeIII clusters mimic the active center in nitrogenase. • FeII are in-situ formed by ethylene glycol reductant in solvothermal process. • FeII/FeIII ratio is vital to coordinate catalytic activity and framework stability. • MIL-53(FeII/FeIII) stably yields 306 μmol h−1 g−1 NH 3 under visible light. Biological nitrogenases exhibit superior nitrogen fixation efficiency owing to their unique multi-iron metallocluster (Fe2+ 3 Fe3+ 4 M3+, M = Mo, V, Fe) coordinated by organic polypeptide. Herein, we design a kind of metal organic framework (MOF) photocatalyst, MIL-53(FeII/FeIII) (MIL = Material from Institute Lavoisier), in which the FeII and FeIII constitute the mixed-valence metalloclusters to mimic the Fe2+ active sites and high-valence metal ions in nitrogenases, respectively. Both the FeII and FeIII are coordinated by organic ligands (terephthalic acid), which afford the electron transfer chains as well as the support of monodispersed FeII active sites. The FeIII in MIL-53(FeII/FeIII) is partly in-situ reduced into FeII by ethylene glycol (EG) via one-step solvothermal method, and the FeII/FeIII ratio is regulated from 0.18:1 to 1.21:1 by varying the EG content. Our results show that the FeII/FeIII ratio can significantly affect the photocatalytic activity and structure stability, and MIL-53(FeII/FeIII)-1 with optimal FeII/FeIII ratio (1.06:1) achieves the highest ammonia evolution rate up to 306 μmol h−1 g−1, nearly 10-fold higher than that of other framework-based materials, while remaining stable after 24 h irradiation. Such nitrogenase-like design in MIL-53(FeII/FeIII) endows the efficient electron transfer, exposed active sites, and in particular rational synergy between the catalytic function and non-catalytic function. This work may open a new avenue to the rational design of nitrogen fixation photocatalysts based on framework materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • Active site engineering in MOFs is first explored for photocatalytic N 2 fixation. • Acetic acid is added to expose and regulate the (1 0 0) facet with more active sites. • The FeIII/FeII ratio is controlled to improve active site reduction ability. • The photocatalytic NH 3 production rate of optimal MOFs reaches 164 μmol h−1 g−1. Photocatalytic ammonia synthesis from N 2 is a carbon neutral approach, its efficiency is yet impeded by the lack of abundant and efficient active sites on catalyst surface. Herein, we explore active site engineering strategy via employing MOF-235-based heterovalent metal organic frameworks (HMOFs) with FeIII/FeII clusters to break this dilemma. Acetic acid was added in solvothermal process, which was adsorbed on (1 0 1) facet of HMOF(FeIII/FeII) to restrain its further growth. The (1 0 0)/(1 0 1) facet ratio of HMOFs was regulated by the acetic acid content. Meanwhile, the FeIII/FeII ratio was regulated by the Fe3+/Fe2+ feeding ratio. It is found that the (1 0 0) facet of HMOFs exists more abundant Fe active sites, exposing (1 0 0) facet can increase the amount of active sites. On the other hand, the FeIII active site is favourable of N 2 adsorption, while the FeII active site is favourable for N 2 activation. The excellent balance between N 2 adsorption and N 2 activation can promote the reduction ability of active sites. At the optimal FeIII/FeII ratio of 1:1 and (1 0 0)/(1 0 1) facet ratio of 4.84, the photocatalytic activity of HMOF(FeIII/FeII)-10 reached 164 μmol g−1 h−1, more than 20 times compared to HMOF(FeIII/FeII)-0 without exposing the (1 0 0) facet. Our strategy may provide a reference in exploiting advanced photocatalysts based on organic framework-based materials. [ABSTRACT FROM AUTHOR]