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 mu mol 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.