Slow heterogeneous charge transfer kinetics for the ClO₂^- /CIO₂ redox couple at platinum, gold, and carbon electrodes. evidence for nonadiabatic electron transfer

Chlorite (CIO₂^-) is one of the few simple aqueous anions that forms a stable product on one-electron oxidation. The heterogeneous charge transfer rate constants at E° = 0.700 V vs SCE for the CIO₂^-/ClO₂ redox couple have been measured at Au, Pt, and glassy carbon electrodes in aqueous KNO₃ solutions using rotating disk and ac voltammetry techniques between 10 and 30 °C. At 25 °C in 1.0 M KNO₃ and pH = 7, the standard heterogeneous rate constants k_el were measured as 0.015 ± 0.001 cm/s and 0.014 ± 0.003 cm/s, respectively, at Au and reduced Pt electrodes. The transfer coefficient α on gold was measured to be 0.50 ± 0.01 using ac voltammetry. The heterogeneous rate constants obtained with the use of a glassy carbon electrode were slower (0.0079 ± 0.001 cm/s at 25 °C). The measured activation free energy for electron transfer at a gold electrode was 25 ± 3.3 kJ/mol with a preexponential factor of 310(+900/-230) cm/s. Heterogeneous rate constants of experiments performed in D₂O solvent were only slightly slower than those measured in H₂O, indicating that the redox couple does not exhibit differential hydrogen bonding. Digital simulation of cyclic voltammograms, with k_el determined from the ac voltammetry and rotating disk techniques, gave good agreement with experiment. Marcus theory was applied to understand the unexpected slow heterogeneous kinetics for this simple redox couple. Additionally, the preexponential factor in the semiclassical expression describing heterogeneous electron transfer was examined. The slow electron transfer process is primarily attributed to a small preexponential factor that may arise from a high degree of nonadiabaticity in the electron transfer process.