One of vital issues that inhibit photoactivity of metal-organic frameworks is the poor electrical conductivity. In this work, one-dimensional mixed-valence iron chains are used to improve this poor situation in MIL-53(Fe). A series of mixed-valence MIL-53(Fe) photocatalysts were obtained through heating at different temperatures in vacuum. The effect of FeII coordinatively unsaturated metal sites (CUS) and one-dimensional mixed-valence iron chains on their photocatalytic property was discussed. The experimental results indicated that mixed-valence MIL-53(Fe) with a reference FeII/FeIII ratio of 0.2725 displayed the best photocatalytic performance, which showed 96.28 and 95.01% removal efficiencies of RhB and TC-H in 100 min, respectively. Moreover, MIL-53(Fe) heated in vacuum displayed better catalytic activity than MIL-53(Fe) heated in air for RhB and TC-H degradation. Based on the analysis of various characterizations, the reinforced catalytic activity can be attributed to the charge mobilities in mixed-valence FeII/FeIII chains. It is worth mentioning that the method is also applicable to MIL-88(Fe) and MIL-101(Fe). Additionally, mixed-valence MIL-53(Fe) can also perform the catalysis reaction in the nighttime by activating persulfate (PS) to produce free radicals. Interestingly, it was found that the FeII CUS lost in activating PS can be supplemented by self-reduction of photogenerated electrons during illumination in the daytime, so as to achieve a more stable cycle. This work demonstrated that the photoactivity of MIL-53(Fe) can be improved by adjusting the ratio of FeII/FeIII and the feasibility of using as an all-day-active catalyst.
Keywords: all-day-active; bifunctional catalyst; mixed-valence MIL-53(Fe); persulfate; photocatalytic degradation.