Homogeneous catalysis of cathodic reduction of oxygen in copper-cystamine aqueous solutions

The copper cystamine complex is a suitable system for the homogeneous catalysis of oxygen cathodic reduction in aqueous solutions. A steady state cathodic current density of 3.2 mA/cm² at 0.2 V and an overvoltage decrease of 400 mV was observed in oxygen-saturated solutions of equimolar concentrations of Cu^II ions and cystamine (5 × 10^-2 mol dm^-3) at pH 4.5. For a given value of Cl^- ion concentration and pH the catalytic current can be expressed by: I_cat = k[Cu^II]₀[O₂]. The optical, magnetic and kinetic properties of the catalyst were investigated. The results suggest that the Cu^II ion, although initially reduced to Cu^I, is stabilized by the disulphide bridge. It is also suggested that because the bridge is a two electron mediator for the reduction of oxygen, the potential barrier of the first electron reduction can be overcome.