EFFECTS OF CARBON ELECTRODE SURFACE-STATES FOR THE ELECTRO-REDUCTION OF (HMC)COOOH2-2( WHICH IS AN INTERMEDIATE DURING THE CATALYTIC REDUCTION OF O)

The reduction of the cobalt(III) complex with a macrocyclic ligand 14, 14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (hmc) dissolved in sol ution or adsorbed on a graphite electrode in the presence of O-2 showe d two cathodic peaks. As discussed in earlier reports, an intermediate (hmc)CoOOH2+ produced by the first two-electron reduction of (hmc)Co3 + in the presence of O-2 was further reduced to (hmc)Co2+ and HOOH. Th is process appeared as a second cathodic wave and represents a barrier in the overall reduction of O-2. In relation to these studies, it was found that the second cathodic reduction was greatly affected by the surface states of the carbon electrode. The reduction potential of the (hmc)CoOOH2+ intermediate when it was adsorbed on a pyrolytic edge pl ane graphite electrode (EPG) surface was more positive than the corres ponding value of the dissolved species measured at a glassy carbon (GC ) electrode polished with alumina. It was also shown that the reductio n potential shifted in a positive direction when the solution pH was l owered or the surface of the glassy carbon electrode was heavily elect ro-oxidized. It is proposed that a proton transfer is involved in the electro-reduction of the (hmc)CoOOH2+ complex. The EPG surface which h as more surface functional groups has a more acidic environment than t he GC electrode and the reduction of (hmc)CoOOH2+ was facilitated. The oxidation of the glassy carbon electrode gives the same effect. (C) 1 998 Elsevier Science S.A. All rights reserved.