A Local Structure Analysis of Defects in UiO-66: Insights from Solid-State Nuclear Magnetic Resonance and X-ray Absorption Fine Structure

Defect engineering in metal-organic frameworks (MOFs) offers a promising approach to modify material properties by introducing controlled structural imperfections. Zr-based MOFs, particularly the well-known UiO-66, hold significant potential for diverse applications. Defects in UiO-66 can be generated using monocarboxylic acids as modulators, among other methods. However, resolving the atomic-level local structures of these defects remains a considerable challenge. In this study, the local structures of these defects are carefully characterized by multinuclear solid-state NMR spectroscopy (SSNMR) in combination with X-ray absorption fine structure (XAFS). In situ heating XAFS analyses at Zr K-edge reveal critical changes in the local structure of Zr during the removal of trifluoroacetic acid (TFA), including the decreased Zr-O coordination numbers and alterations in Zr-Zr bond distances. Multinuclear ¹H, ¹³C, ¹⁹F, ^35/37Cl, ¹⁷O solid-state NMR methods are used to identify capping species and defect-associated species. Subsequently, the engineered defects are found to significantly improve the catalytic performance of Pt nanoparticles (NPs) integrated into the defective UiO-66 framework. Pt-UiO-66 with defects exhibits much improved hydrogen evolution reaction (HER) activity and stability compared to the Pt-UiO-66 without defects.