Unraveling the reversible formation of defective Ce3+ sites in the UiO-66(Ce) material: a multi-technique study

Ce³⁺ presence and formation in Ce-based UiO-66 Metal-Organic Framework (MOF) still presents a debated evaluation between the employed characterization techniques. In this work, we have prepared a defective UiO-66(Ce) and investigated the nature of Ce³⁺ sites on the CeO_x clusters. Laboratory techniques (EPR, XPS, UV–Vis and FTIR spectroscopy) were compared with operando Ce M₅-edge NEXAFS to study Ce³⁺ accessibility. All the employed techniques presented different degrees of accessibility or reliability (e.g., sample damage or not sufficient sensitivity). Among the obtained results, EPR, UV–Vis and NEXAFS spectroscopies unraveled Ce⁴⁺→Ce³⁺ conversion during the sample dehydration. The MOF structure was not damaged by neither water loss nor the beam, directly relating Ce oxidation state to the water content, opening a new route to both synthesis of stable and active MOFs and non-invasive characterization strategies. Finally, laboratory measurements considerations were exploited for studying Ce³⁺ formation in Zr-doped UiO-66(Ce) samples.