Oxygen Reduction in Alkaline Media-a Discussion

We propose a complete reaction sequence for oxygen reduction in alkaline solutions, in which the first two steps occur in the outer sphere mode. The oxygen-oxygen bond is broken in the third step, which involves adsorption of OH, which is desorbed in the last step. We have investigated the sequence by quantum-chemical methods and determined the energies of activation. Whether the reaction follows a four- or a two-electron mechanism, depends critically on the energy of adsorption of OH. We surmise that our mechanism holds on all electrodes which interact weakly with oxygen, in particular on gold, silver, and graphite. We explain, why Au(100) is a better catalyst than Au(111), why at high overpotentials the reaction on Au(100) reverts to a two-electron mechanism, and why this does not happen on silver.