Designation and optimization of facets of photocatalysts is an effective strategy to address the issue of facet-dependent photocatalytic reactions. However, studies regarding the facet effect of metal–organic frameworks (MOFs) on the photocatalytic process are in infancy. In this study, NH2-MIL-125(Ti) with different ratios of {001} and {111} facets was exactly controlled and synthesized, and it was found that the activity in photoreduction of CO2 is enhanced with gradually increasing exposed proportion of {111} facets. The {111} facets exhibit photocatalytic activity with the maximal CO and CH4 yields of 8.25 and 1.01 μmol g–1 h–1, which are 9 and 5 times higher than those of {001} facets, respectively. Also, the {111} facets give the highest quantum yields of 0.14 and 0.07% for CO and CH4 production, respectively. Steady-state and time-resolved fluorescence spectra reveal the importance of inhibiting the recombination of photoinduced electrons and holes for the sample with {111} facets. Besides, TiIII formed during the reaction process exhibits strong reducibility for CO2. Starting from NH2-MIL-125(Ti), the photocatalytic performance can be enhanced by regulating exposed {111} facets. This work not only provides a strategy for further enhancing photocatalytic performance by tuning the exposed active facets of MOFs, but also provides a deep understanding of the factors for improving the photocatalytic reduction of CO2.
Visible light-driven photocatalytic reduction of CO 2 into value-added chemical fuel is considered as an up-and-coming pathway for CO 2 conversion utilizing green solar energy. Herein, we report heterostructures of NH 2 -MIL-101(Fe)/g-C 3 N 4 (g-C 3 N 4 = polymeric graphite-like carbon nitride) as prominent photocatalysts for the reduction of CO 2 via a solvent-free reaction. Among these heterogeneous photocatalysts, NH 2 -MIL-101(Fe)/g-C 3 N 4 -30 wt % referred to as MCN-3 shows superior catalytic activity for photocatalytic reduction of CO 2 to CO with a CO yield of 132.8 μmol g −1 , which is more than 3.6 times higher than that for pristine NH 2 -MIL-101(Fe) and 6.9 times higher than that for sole g-C 3 N 4 . In virtue of the elaborate designed photocatalysts and the gas−solid interfacial route, the heterostructure of NH 2 -MIL-101(Fe)/g-C 3 N 4 with efficient interfacial electron transfer between NH 2 -MIL-101(Fe) and g-C 3 N 4 results in the boosted photocatalytic reduction of CO 2 upon visible light irradiation.
It is deemed as a desired approach to utilize solar energy for the conversion of CO 2 into valuable products, and the majority of the MOFs-based photocatalytic reductions of CO 2 have focused on formic acid (HCOOH) production with an organic solvent as the reaction medium. Herein, we report a solvent-free reaction route for the photoreduction of CO 2 catalyzed by Fe-MOFs, namely, NH 2 -MIL-53(Fe)aminoterephthalic acid; G = guest and/or solvent molecules). Compared with the orthodox reaction route, the present out-of-the-way photocatalytic reduction of CO 2 with superior selectivity to CO occurs at the gas−solid interface. The reaction procedure is environmentally friendly and provides a possibility to address the CO 2 emission problem. Importantly, NH 2 -MIL-101(Fe) shows the highest photocatalytic activity among these Fe-MOFs due to its efficient charge separation and electron transfer.
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