Defect modulation of MIL-68(Fe) MOFs by Cu doping for boosting photocatalytic N2 fixation

Photocatalytic N2 fixation is a green process for ammonia synthesis by converting N2 and H2O to NH3 and O2 directly using solar energy. Its efficiency is largely limited by the chemisorption and activation of N2. This work adopts defect engineering strategy to develop a series of MIL-68(Fe) MOFs with varying concentrations of defects by doping Cu2+ on the metal nodes of MOFs. Upon Cu2+ doping, a larger number of oxygen vacancy defects are created due to the induced crystal distortion to form coordinatively unsaturated Fe2+ sites, which can serve as the active sites for promoting the chemisorption and activation of N2. The photogenerated holes oxidize H2O to O2 and H+, and the photogenerated electrons combine with formed H+ to reduce the activated N2 to NH3. It was found that the sample with 10 mol% Cu2+ doping shows the highest NH3 production rate (21.0 μmol·g−1·h−1), which is 8.4 times higher than that (2.50 μmol·g−1·h−1) of the pristine MIL-68(Fe). The excellent performance is ascribed to the adequate Fe2+ active sites to chemisorb and activate N2 and the optimal mobility of photogenerated charges. Finally, a mechanism is proposed to illustrate how the number of defects affects the photocatalytic N2 fixation performance at the molecular level.