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

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