Mechanistic model of the oxygen reduction catalyzed by a metal-free porphyrin in one- and two-phase liquid systems

Abstract

Mechanistic model is presented, which allows simulating the experimentally observed catalytic effect of the 5,10,15,20-meso-tetraphenylporphyrin (H2TPP), as well as the effects of H+, tetrakis(pentafluorophenyl)borate anion (TB−) and water concentrations on the catalytic rate of the oxygen reduction reaction (ORR) with ferrocene (Fc) in 1,2-dichloroethane (DCE). Model calculations are based on the assumption that the electron transfer between the complex {(H4TPP2+)·(TB−)·O2} and Fc is the rate determining step (r.d.s.) in both the catalytic and electrocatalytic cycle. The model calculations are performed using the reported acid dissociation constants for mono- and diprotonated H2TPP forms, and the equilibrium constants of the extraction of the ligands L=O2, water, or TB− from the porphyrin complex {(H4TPP2+)·(TB−)·L} calculated by the DFT methods. The rate constant of r.d.s. evaluated from the initial rates is compared with that obtained by a numeric fit of the experimental Fc+ transients. Model predictions are relevant to electrocatalysis of ORR by protonated H2TPP at the polarized water|DCE interface.