Cu mediated defect manipulation in MIL-88A(Fe) for boosting photocatalytic N2 fixation

The construction of metal–organic frameworks that possesses the tunable metal active sites and synergistic effect for the reactant activation is an attractive strategy for the optimization of photocatalytic performance. Herein, a series of MIL-88A(Fe 1-x Cu x ) were synthesized for photocatalytic nitrogen reduction reaction (NRR). It was characterized that the Fe 2+ and oxygen vacancies were produced in MIL-88A(Fe) due to the partly reduction of ligand fumaric acid in synthesis process. The substitution of Fe 3+ with Cu 2+ induces charge imbalance and lattice distortion, further increasing the content of Fe 2+ and oxygen vacancies. The optimal sample MIL-88A(Fe 0.95 Cu 0.05 ) (CMA-5 %) exhibits the highest nitrogen fixation performance of 68.6 μmol·g −1 ·h −1 , which is 8 times higher than that of the pristine material. It is attributable to the synergistic effect of the abundant Fe 2+ and oxygen vacancies as active sites to promote N 2 coordination and photogenerated carrier separation, resulting in the efficient conversion of activated N 2 to NH 3 . Furthermore, the in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy provide clear evidence of the behavior of the N 2 adsorption and activation on the catalyst. Finally, we propose a potential mechanism at molecular level regarding the relationship between the synergistic effect and the nitrogen fixation activity.