<b> Constructing bimetallization and hydroxylation in metal–organic framework for efficient Cr( <scp>vi</scp> ) photoreduction </b>

Metal-organic frameworks (MOFs), as promising photocatalysts, have been widely used in Cr(VI) removal due to their high porosity and structural tunability, but the simple hybridization of MOFs is not beneficial for the separation of photogenerated carriers. In this study, a novel Al-doped and hydroxyl-functionalized Zr-based MOF (named Al/Zr-UiO-66-(OH)₂) was synthesized <i>via</i> the dual strategies of bimetallization and hydroxylation. Characterization results confirmed that the incorporation of Al and hydroxyl groups effectively reduced the bandgap and enhanced the separation efficiency of photogenerated charges. Al/Zr-UiO-66-(OH)₂ (1 : 5) exhibited outstanding photocatalytic reduction performance towards Cr(VI) (99.3% within 2 h) under visible light. The photocatalytic rate of Al/Zr-UiO-66-(OH)₂ (<i>K</i> = 0.0148 min^-1) was 74 times higher than that of UiO-66 (<i>K</i> = 0.0002 min^-1). Trapping experiments, EPR and XPS analysis revealed that the highly efficient photoreduction of Cr(VI) into Cr(III) was attributed to electrons and ·O₂^- radicals. These findings provide a feasible strategy for designing MOF-based photocatalysts for practical environmental remediation.