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-xCu_x) were synthesized for photocatalytic nitrogen reduction reaction (NRR). It was characterized that the Fe²⁺ and oxygen vacancies were produced in MIL-88A(Fe) due to the partly reduction of ligand fumaric acid in synthesis process. The substitution of Fe³⁺ with Cu²⁺ induces charge imbalance and lattice distortion, further increasing the content of Fe²⁺ and oxygen vacancies. The optimal sample MIL-88A(Fe_0.95Cu_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 Fe²⁺ and oxygen vacancies as active sites to promote N₂ coordination and photogenerated carrier separation, resulting in the efficient conversion of activated N₂ to NH₃. Furthermore, the in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy provide clear evidence of the behavior of the N₂ 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.