ELECTRON-TRANSFER KINETICS OF AQUATED FE+3 +2, EU+3/+2, AND V+3/+2 ATCARBON ELECTRODES - INNER-SPHERE CATALYSIS BY SURFACE OXIDES/

The electron-transfer kinetics for three aquated metal ions with low s elf-exchange-rates were examined on well-characterized carbon surfaces . All three ions exhibited slow kinetics on clean fractured glassy car bon (GC) and on the basal plane of highly ordered pyrolytic graphite ( HOPG). The reactions appeared to be outer sphere and had rates approxi mately consistent with those predicted from the self-exchange rates. E lectrochemical oxidation of either GC or HOPG greatly increased the el ectron-transfer rate constant to values significantly greater than pre dicted for outer-sphere reactions. The increase was greatest for Eu(aq )+3/+2 and was eliminated if the surf ace was silanized. Surface oxide s provide sites for inner-sphere catalysis of the aquated ions at carb on, perhaps involving a surface oxide structure similar to the common ligand acetoacetonate.