Dioxygen reduction and hydrogen peroxide dismutation using electropolymerized bilayers of cobalt + manganese tetrakis(o-aminophenyl)porphyrins

Bilayers of electropolymerized cobalt + manganese tetrakis(o-aminophenyl)porphyrins (poly[Co(o-NH₂)TPP] + poly[Mn(o-NH₂)TPP]) were used for the electrocatalytic reduction of dioxygen. The half-wave potential for O₂ reduction at these bilayer films follows that of O₂ reduction at single poly[Co(o-NH₂)TPP] films in the entire pH range, indicating that this process is governed by the potential of the Co(III)/Co(II) couple in both cases (+0.2 and -0.2 V vs. Ag/AgCl at pHs 2 and 14, respectively). The Co/Mn bilayer porphyrin films are more efficient catalysts than the cobalt porphyrin films in basic solutions, as deduced from the lower peroxide yields (30 and 75% at pH 12 for the Co/Mn and Co porphyrin films, respectively). The addition of 3x10^-3 M imidazole decreases the H₂O₂ yield obtained at the Co/Mn porphyrin bilayers (43 and 30% at pH 13, in the absence and presence of imidazole, respectively). This is attributed to a process in which H₂O₂ is dismutated by the imidazoleligated manganese porphyrin. The dismutation process is suggested to involve a μ-oxo-manganese(IV or V) porphyrin which is reduced by excess hydrogen peroxide to yield the original Mn(III) porphyrin. Increased stability of the Co/Mn porphyrin bilayer films was observed compared to that of the cobalt porphyrin films: the recorded current for O₂ reduction at glassy carbon/film electrodes held for 20 h at -0.3 V vs. Ag/AgCl at pH 10 decreased by 62% and only by 25% at Co and Co/Mn porphyrin films, respectively.