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 O^2– 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.