Homogeneous redox catalysis of electrochemical reactions: Part I. Introduction

Abstract

In redox homogeneous catalysis the catalyst couple merely plays the role of an electron carrier by contrast with chemical catalysis which involves the transitory formation of catalyst-substrate adduct. A detailed kinetic analysis of redox catalysis in the case of an EC-type electrode reaction is given involving the following reaction sequence ▪ B → k C A + 1 e ⇌ B B → C } direct electrode reduction of the substrate The Hush-Marcus relationship between heterogeneous and homogeneous electron transfer rate constants is used to select the range of the parameter variations and to predict the magnitude of the catalytic efficiency as a function of the potential separation between the catalyst reduction and the direct substrate reduction, the ratio of the catalyst over the substrate concentration and the electrochemical standard rate constant of the substrate. Two situations are of particular interest for the kinetic analysis of the experimental data: (a) kinetic control of the direct substrate reduction by the follow-up chemical reaction with diffusion by charge transfer with activation or diffusion control of the solution electron exchange, the rate determining step of the catalytic sequence being A+Q→P+B. In these conditions, kinetic information on the substrate reduction process is obtainable which could not have been derived from a direct electrochemical analysis.