Electrocatalysis of oxygen reduction by water-soluble iron porphyrins: Thermodynamic and kinetic advantage studies

Abstract

Several structurally modified water-soluble iron porphyrins have been synthesized and investigated for their catalytic behaviour toward oxygen reduction. The redox potentials for the iron(III/II) couple of the metallated species were in the order Fe(2-TMPyP) > Fe(4-TMPyP) > Fe(3-TMPyP) ( n-TMPyP = tetrakis( N-methyl- n-pyridyl)porphyrin), and paralleled those of the non-metallated forms in nitrogen-purged 0.05 M H 2SO 4, solution. The normalized cyclic voltammetric peak currents for the catalyzed reduction of oxygen as a function of catalyst concentration were approximately equivalent for Fe(4-TMPyP) and Fe(3-TMPyP), whereas the peaks were considerably lower for Fe(2-TMPyP). Iron tetrakis (2-aminophenyl)porphyrin, Fe(2-TAPP), appeared to absorb strongly on a glassy carbon surface to catalyze an overall four-electron reduction of oxygen at a rate constant of 8(±2) × 10 5 M −1 s −1. The adsorption shifted the E °' and p K a compared with those of the bulk Fe(2-TAPP). Results from CV, chronoamperometry and RRDE experiments were in agreement with the proposed EC mechanism. Effects of adsorption on the thermodynamic potential as well as the kinetic rate have been discussed.