Interfacial kinetics in solution. Linear free-energy relationships applicable to heterogeneously catalysed reactions in solution

Abstract

The Brönsted and Hammett linear free-energy relations (LFER), widely used for homogeneous reactions, are applicable also to heterogeneously catalysed reactions in solution. When the reaction at the surface is sufficiently fast the kinetics become diffusion controlled. An analysis of this situation reveals a new LFER in which α(= d ln k/d ln K, where k is the rate constant and K the equilibrium constant) depends only on the stoichiometric coefficients of the reaction, as do the kinetic orders. Redox reactions catalysed by metals exhibit a different kind of diffusion control when the formal electrode potentials are far apart. If the two interacting redox couples are electrochemically reversible, the In k against In K diagram resembles the shape of a Brönsted–Eigen plot, although both the rising section and the plateau region are now controlled by diffusion processes.