Molecular electrocatalysis of the oxygen reduction at a polarised interface between two immiscible electrolyte solutions by Co(II) tetraphenylporphyrin

Cyclic voltammetry and the interfacial tension measurements are used to study electrochemistry and adsorption of 5,10,15,20-tetraphenyl-21H, 23H-porphine cobalt(II) (CoTPP) at the polarized water∣1,2-dichloroethane (1,2-DCE) interface. It is shown that CoTPP facilitates the successive transfer of two protons from water to 1,2-DCE, and that the formation of its protonated forms (CoTPPH+, CoTPPH22+) gives rise to an enhancement of the interfacial tension (negative surface pressure) at potentials positive to the potential of zero charge (pzc). This surprising effect is ascribed to the specific adsorption of CoTPPH+ and CoTPPH22+, which is accompanied by the formation of a dipolar surface layer and a positive shift of pzc. Evidence is provided that the presence of CoTPP leads to electrocatalysis of the interfacial reduction of molecular oxygen by decamethylferrocene dissolved in 1,2-DCE. It is proposed that the protonated CoTPP plays the role of the interfacial binding site for molecular oxygen and/or a mediator of electron transfer.