Structure and Electron Transfer Kinetics of of Ce3+ and Ce4+ Complexes in Acidic Electrolytes

The Ce³⁺/Ce⁴⁺ redox couple is used in many fields, including organic synthesis, waste remediation, and energy storage. Complexation of cerium ions with anions in acidic electrolytes greatly impacts its redox potential, however, it is unclear if anion complexation occurs with Ce³⁺, Ce⁴⁺ or both. Our recent work using experimental UV-Vis spectroscopy and DFT calculations shows that Ce³⁺ likely forms a complex with nine waters, whereas Ce⁴⁺ favorably undergoes single ligand exchange in seven acidic electrolytes (HCl, H₂SO₄, H₃NSO_3, CH₃SO₃H, HNO₃, CF₃SO₃H, and HClO₄). Here we report new experimental EXAFS and MD-EXAFS of cerium complexes to further elucidate the structures of the cerium complexes in solution. Clearly the Ce³⁺/Ce⁴⁺ redox couple provides a unique opportunity to study how changing ion complexation of a single oxidation state affects the charge transfer kinetics, while maintaining a constant structure of the ion in the other oxidation state. To understand what controls the Ce³⁺/Ce⁴⁺ mechanism we use theoretical charge transfer models based on the structure of the cerium complex in solution to predict kinetics and compare to experimental kinetic trends. We apply a methodology for extracting kinetic parameters of the outer sphere reaction using DFT calculations within the PCM framework. This research increases understanding of lanthanide ions using quantum chemical methods and tests theoretical paradigms for charge transfer.