Construction of MOF-on-MOF-derived composites with coexisting octahedrons and dodecahedrons for superior photocatalytic nitrogen fixation

The photocatalytic ammonia synthesis provides a promising alternative for the sustainable production of ammonia. However, poor separation efficiency of photoinduced carriers and insufficient activation of N₂ remain the key obstacles to high-performance N₂ reduction. Herein, a MOF-on-MOF-derived material with a Z-scheme heterojunction has been reported. The Brunauer-Emmett-Teller (BET) and electrochemical impedance spectroscopy (EIS) analyses indicate that the coexistence of the two MOFs accelerates the mass transfer process at the gas-solid-liquid interface and promotes the diffusion process of nitrogen molecules to the heterogeneous interface, thereby facilitating the adsorption of N₂. According to analyses of Diffuse Reflectance Spectroscopy (DRS), Mott-Schottky (M−S) and density functional theory (DFT), the MIL-101(Fe)/ZIF-67 derived material (MZ-600) complies with the charge transfer mechanism of the Z-scheme heterojunction, which realizes the effective separation of photogenerated carriers and preserves the strong redox capacity of electrons and holes. Meanwhile, it stimulates the activation of the N≡N bond and lowers the energy barrier of the rate determining step(*N₂→*N₂H). Therefore, the rationally designed MZ-600 heterojunction exhibits remarkable PNRR performance, with an NH₃ yield of 103.90 μmol·g⁻¹·h⁻¹ and an apparent quantum yield (AQY) of 1.17 % at 465 nm. Moreover, compared to catalyst powder, the MZ-600 catalyst supported on vertical carbon paper can decrease recontamination and is convenient to be recycled.