Heterogeneous catalysis in solution. Part 22.—Oxidation–reduction reactions catalysed by electron transfer through the solid: theory for partial and complete mass-transport control

Abstract

Many redox reactions are so strongly catalysed at the surface of an electron-conducting solid that the catalysis becomes partly or wholly diffusion-controlled. By means of the electrochemical model (which allows one to calculate the catalytic rate v_cat from the current–voltage curves of the two interacting couples) we have derived kinetic equations for cases of mixed activation and diffusion control in the Tafel region. If the experimental results have been obtained at a rotating-disc catalyst, the intercept of a plot of 1/v_cat against [graphic omitted] (where ω is the angular velocity) will then lead to a value for the catalytic rate at the surface itself. We have also analysed situations in which the surface reaction is so fast as to be at equilibrium throughout. The kinetics then depend entirely upon mass transport of the reactants and products through the diffusion layer, and at a rotating disc catalyst v_cat will be directly proportional to [graphic omitted]. The theoretical equations predict that the reaction orders will depend only upon the stoichiometric coefficients of the reaction, that these orders will often be fractional and that they will change dramatically if product is added to the reaction mixture at the start.