Fe-Based MOFs for Photocatalytic CO2 Reduction: Role of Coordination Unsaturated Sites and Dual Excitation Pathways

The utilization of solar energy for the conversion of CO2 into valuable organic products is one of the best solutions to solve the problems of global warming and energy shortage. The development of photocatalysts capable of reducing CO2 under visible light, especially those containing earth-abundant metals, is significant. Herein we report that a series of earth-abundant Fe-containing MOFs (MIL-101­(Fe), MIL-53­(Fe), MIL-88B­(Fe)) show photocatalytic activity for CO2 reduction to give formate under visible light irradiation. The direct excitation of the Fe–O clusters in these MOFs induces the electron transfer from O2– to Fe3+ to form Fe2+, which is responsible for the photocatalytic CO2 reduction. Among the three investigated Fe-based MOFs, MIL-101­(Fe) showed the best activity due to the existence of the coordination unsaturated Fe sites in its structure. All three amine-functionalized Fe-containing MOFs (NH2-MIL-101­(Fe), NH2-MIL-53­(Fe) and NH2-MIL-88B­(Fe)) showed enhanced photocatalytic activity in comparison to the unfunctionalized MOF, due to the existence of dual excitation pathways: i.e., excitation of an NH2 functionality followed by an electron transfer to the Fe center in addition to the direct excitation of Fe–O clusters.