Improved visible light photocatalytic nitrogen fixation activity using a FeII-rich MIL-101(Fe): breaking the scaling relationship by photoinduced FeII/FeIII cycling

The scaling relations between nitrogen adsorption and NHx destabilization are key challenges to the widespread adoption of the photocatalytic synthesis of ammonia. In this work, a FeII-rich MIL-101(Fe) (MIL-101(FeII/FeIII)) was synthesized using a one-step solvent thermal method with ethylene glycol (EG) as a reducing agent, which can break the scaling relationship by photoinduced FeII (high nitrogen adsorption ability) and FeIII (high NHz destabilization ability) cycling. XPS was used to detect the change in iron valence state in the MIL-101(FeII/FeIII) material. The photocatalytic nitrogen fixation efficiency of MIL-101(FeII/FeIII) under visible light without any sacrificial agent was 466.8 μmol h−1 g−1, five times that of MIL-101(Fe). After photocatalytic experiments, MIL-101(FeII/FeIII) retained an unchanged FeII/FeIII rate, indicating that this FeII/FeIII cycling can be maintained. DFT modeling of the FeII-rich MOF material showed that a FeII1 FeIII2 system has a higher N2 activation capacity than a FeIII3 system. The catalytic mechanism was further proved by in situ infrared spectra and N15 isotopic tracers. Therefore, the improvement of photocatalytic activity was mainly attributed to the change in the nitrogen adsorption capacity during the photoinduced FeII/FeIII cycling.