SynchronousPhotocatalytic Redox Conversion of Chromium(VI)and Arsenic(III) by Bimetallic Fe/Ti Metal–Organic Frameworks

In this work, bimetallic organic frameworks NH₂-MOFs­(Fe, Ti) with different Fe³⁺/Ti⁴⁺ molar ratios were prepared by a hydrothermal method for the synchronous redox transformation of Cr­(VI) and As­(III). These results showed that NH₂-MIL-125­(Ti) was less effective in the photocatalytic removal of Cr­(VI), whereas NH₂-MIL-88B­(Fe) was less effective in the photocatalytic oxidative removal of As­(III). Due to the introduction of Fe³⁺, the photocatalytic reduction removal of Cr­(VI) (23.04 → 42.56%) and the photocatalytic oxidation removal of As­(III) (5.58 → 26.09%) by NH₂-MOFs­(Fe, Ti) were significantly enhanced. Among them, NH₂-MIL-88B­(Fe_0.6Ti_0.4) exhibited the best performance in the photoreduction of Cr­(VI) and photo-oxidation of As­(III), which balanced the insufficiency of monometallic MOFs­(Fe/Ti). In this case, the total removal of Cr­(VI) and As­(III) by NH₂-MIL-88B­(Fe_0.6Ti_0.4) was found to be 94.19 and 83.54%, respectively. The excellent photocatalytic property could account for the ligand-to-metal charge transfer (LMCT), where photogenerated <i>e</i>^– generated by −NH₂ group excitation was transferred to the active site Fe–O clusters, as well as <i>e</i>^– transported along Ti → O → Fe (metal-to-metal charge transfer, MMCT). Therefore, the synergetic effect of LMCT and MMCT could efficiently inhibit the recombination of <i>e</i>^––<i>h</i>^<i>+</i> pairs, improving the photocatalytic performance of NH₂-MOFs­(Fe, Ti).