Nitrogenase-inspired mixed-valence MIL-53(FeII/FeIII) for photocatalytic nitrogen fixation

Biological nitrogenases exhibit superior nitrogen fixation efficiency owing to their unique multi-iron metallocluster (Fe32+Fe43+M3+, M = Mo, V, Fe) coordinated by organic polypeptide. Herein, we design a kind of metal organic framework (MOF) photocatalyst, MIL-53(Fe-II/Fe-III) (MIL = Material from Institute Lavoisier), in which the FeII and FeIII constitute the mixed-valence metalloclusters to mimic the Fe2+ active sites and highvalence metal ions in nitrogenases, respectively. Both the Fe-II and Fe-III are coordinated by organic ligands (terephthalic acid), which afford the electron transfer chains as well as the support of monodispersed Fe-II active sites. The Fe-III in MIL-53(Fe-II/Fe-III) is partly in-situ reduced into Fe-II by ethylene glycol (EG) via one-step solvothermal method, and the Fe-II/Fe-III ratio is regulated from 0.18:1 to 1.21:1 by varying the EG content. Our results show that the Fe-II/Fe-III ratio can significantly affect the photocatalytic activity and structure stability, and MIL-53(Fe-II/Fe-III)(-1) with optimal Fe-II/Fe-III ratio (1.06:1) achieves the highest ammonia evolution rate up to 306 mu mol 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(Fe-II/Fe-III) endows the efficient electron transfer, exposed active sites, and in particular rational synergy between the catalytic function and non-catalytic function.