In Situ FTIR Investigation of Metal Node Chemistry in Zr‑, Ce‑, and Zr-Ce Bimetallic UiO-66 Frameworks

Cerium-containing UiO-66 frameworks, including monometallic Ce-UiO-66 and bimetallic CeZr-UiO-66, were synthesized and investigated by in situ FTIR spectroscopy using probe molecules. FTIR analysis reveals the presence of accessible metal sites in both materials, which are attributed to structural defects within the UiO-66 framework. The dehydration-rehydration behavior was examined and found to differ markedly from that of Zr-UiO-66, with Ce-containing clusters undergoing dehydroxylation at lower temperatures and exhibiting hindered rehydrohylation. UV-Vis and FTIR spectroscopies indicated that the as-prepared Ce-containing materials contained negligible amounts of Ce³⁺. Dehydroxylation of Ce-UiO-66 leads to the formation of a fraction of Ce³⁺ cations, as evidenced by the appearance of a characteristic spin-orbit electronic transition band of Ce³⁺ (²F_5/2 → ²F_7/2). Hydrogen treatment provokes further reduction of Ce⁴⁺ to Ce³⁺. Subsequent oxidation leads to fading of the Ce³⁺ electronic transition band, demonstrating the reversibility of the Ce⁴⁺/Ce³⁺ redox process. These results provide molecular-level insight into the accessibility, dehydroxylation behavior, and redox activity of cerium sites in UiO-66-type frameworks and highlight their potential relevance for redox catalysis.