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

Abstract

The utilization of solar energy for the conversion of CO 2 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 CO 2 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 CO 2 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 Fe 3+ to form Fe 2+, which is responsible for the photocatalytic CO 2 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 (NH 2 -MIL-101(Fe), NH 2 -MIL-53(Fe) and NH 2 -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 NH 2 functionality followed by an electron transfer to the Fe center in addition to the direct excitation of Fe–O clusters.