ELECTROCHEMISTRY OF THE CU(II) COMPLEX OF 4,7-DIPHENYL-1,10-PHENANTHROLINEDISULFONATE ADSORBED ON GRAPHITE-ELECTRODES AND ITS BEHAVIOR AS AN ELECTROCATALYST FOR THE REDUCTION OF O2 AND H2O2

The complex formed between Cu2+ and 4,7-diphenyl-1,10-phenanthrolinedisulfonate, [Cu(II)L2]2-, is strongly and irreversibly adsorbed on graphite electrode surfaces. Both oxidation states of the adsorbed complex exist in two isomeric forms, believed to involve tetragonal or tetrahedral coordination geometry at the copper center. Formal potentials for the half-reactions connecting each pair of isomers are reported. Upon reduction, the adsorbed complex is an active electrocatalyst for the reduction of both O2 and H2O2. The catalyst accomplishes the four-electron reduction of O2 to H2O at a significantly greater rate than it catalyzes the two-electron reduction of H2O2 so that the latter cannot be an intermediate in the reduction of O2. The tetragonal form of the Cu(I) complex appears to be the more catalytically active and this isomer is unstable at the potential where the reductions proceed. As a result, catalytic currents gradually decrease as the adsorbed catalyst converts spontaneously from its active to its less active form.