Electron Transfer Kinetics of Aquated Fe §247 Eu +3/§ and V §247 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-char-acterized 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 actions. The increase was greatest for Eu~+~/+2 and was eliminated if the surface 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. Due principally to their widespread use in electrosynthe-sis and electroanalysis, carbon electrodes have been exam-ined extensively, with particular attention to factors affect-ing electrode kinetics and background currents. 14 Most previous studies have been dominated by characteristics of the carbon surface and by pretreatments wh ich alter th