Fe-based MOFs for photocatalytic N2 reduction: Key role of transition metal iron in nitrogen activation

The nitrogen activation is the Achilles’ heel of photocatalytic nitrogen fixation. Herein, metal-organic frameworks (MOFs), for first time, are proposed to solve the above bottleneck. A series of Fe-Based MOFs (MIL-101(Fe), MIL-100(Fe), MIL-88(Fe)) demonstrated excellent photocatalytic nitrogen fixation activity. To gain insight into the reaction mechanism, MIL-101(Cr) and MIL-101(Fe), with same molecular structure except the central metal ions, selected for system comparison. Photocatalytic nitrogen fixation activity of MIL-101(Fe) and MIL-101(Cr) are 50.355 ​μmol ​L−1 ​h−1 and inactive, respectively. This suggested that the nitrogen fixation activity originated from the Fe catalytic center. The results of in situ FTIR and quantum computation verifies Fe catalytic center have a higher electron density, a lower reaction activation energy and more generation of N–H bond than those of Cr catalytic center. In view of the analogous outer electron configuration of Cr and Fe, it was suspect that two extra d-orbitals electronics of Fe element are the basic reason of the excellent photocatalytic nitrogen fixation activity. This work provides a new application of MOFs in photocatalytic nitrogen fixation field and emphasizes the vital of d-orbitals electronics during the nitrogen activation process.