Electrocatalytic reduction of dioxygen by cobalt tetra(4-NN′N″-trimethylanilinium)porphyrin

Cobalt tetra(4-N N′N″-trimethylanilinium)porphyrin (CoTMAP) has been studied in aqueous solutions using cyclic voltammetry, differential pulse voltammetry as well as by RRDE and spectroelectrochemical methods. The Co^III/Co^IITMAP couple dissolved in 0.05 mol dm^-3 H₂SO₄ has a redox potential of +0.14 V. The λ_max of the Soret band are 427 and 412 nm for Co^III and Co^IITMAP, respectively. The rate constant for the reaction between Co^IITMAP and dioxygen in 0.05 mol dm^-3 H₂SO₄ was estimated to be 3 × 10⁷ dm³ mol^-1 s^-1. The Co^IIITMP has been found to absorb at monolayer levels on glassy carbon surfaces and to have a catalytic effect on dioxygen electroreduction in aqueous solutions. This catalytic effect is strongly dependent on electrolyte pH. The overvoltage of O₂ reduction is reduced by 480 mV and insignificant amounts of H₂O₂ are produced at pH 1, while an overvoltage decrease of 300 mV and an H₂O₂ yield of 70% are observed at pH 8.