Constructing Mimic-Enzyme Catalyst: Polyoxometalates Regulating Carrier Dynamics of Metal–Organic Frameworks to Promote Photocatalytic Nitrogen Fixation

In nature, nitrogenase can achieve efficient environmental fixation of nitrogen (N2) molecules through π-backbonding mechanism, which inspires us to simulate the N2 fixation process of nitrogenase to overcome the bottleneck in existing artificial synthetic ammonia (NH3), so as to achieve efficient artificial conversion of N2 to NH3 under mild conditions. Herein, we highlight a strategy of constructing a mimic-enzyme catalyst for photocatalytic N2 fixation based on metal–organic frameworks (MOFs) and polyoxometalates (POMs) in which POMs can efficiently regulate carrier dynamics of MOFs to simulate the π-backbonding mechanism of nitrogenase. Oxygen-rich group POMs with strong electronegativity attract electrons from transition-metal atom in MOFs, thus reducing the 3d orbitals' electron density of transition-metal atom and enhancing unoccupied d-orbitals in favor of adsorbing N2. Simultaneously, the photoexcited electrons in MOFs are efficiently transferred by POMs into the N–N π* antibonding system to activate N2 via π-backbonding mechanism. Taking MIL-88A (C12H6O13Fe) and PMo10V2 (H5PMo10V2O40) as an example, we have obtained a significantly enhanced NH3 production rate of 50.82 μmol g–1 h–1, which is enhanced by 6 times for MIL-88A and 14 times for PMo10V2. Our results make an important contribution to further guidance for artificial N2 fixation under mild conditions.