Attempts to acclerate the rates of oxidation of ruthenium and osmium tetramethylcyclam complexes at graphite and glassy carbon electrodes and within nafion coatings

The oxidation of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane complexes of ruthenium(III) and (IV) and osmium(III) proceeds much more slowly than the reduction of the corresponding oxidation products at platinum, gold and untreated glassy carbon electrodes. The oxidations at oxidized glassy carbon and pyrolytic graphite electrode surfaces are more rapid. The suggestion (Cabaniss et al., J. Am. Chem. Soc., 107 (1985) 1845) that quinone-like functional groups on the oxidized electrode surfaces are responsible for the higher reaction rates was tested by adsorbing various quinones on the surfaces of unactivated glassy carbon electrodes. Rate enhancements resulted in some, but not all, cases. When they were incorporated in Nafion coatings, the oxidation of the ruthenium and osmium complexes was slow at all electrode surfaces. The pH dependences of the oxidation rates on both bare electrodes and within Nafion indicated that the rate limiting step was the proton loss that accompanies the electron transfer during the oxidation. One attempt to utilize a powerfully oxidizing, proton-accepting complex as a homogeneous catalyst for the oxidations was unsuccessful. A possible reason is suggested for the apparently superior catalytic activity of oxidized surfaces.