An Analysis of Shear-Dependent Mechanochemical Reaction Kinetics

Abstract

The variation in the rate of a tribochemical reaction is calculated as a function of combined normal and shear stresses using Evans-Polanyi perturbation theory. The effect of perturbations such as stresses is obtained using transition-state theory from their influence on the equilibrium constant between the initial- and transition-state structures using the molar Gibbs free energy change. An advantage of this approach is it capability of calculating the effect of several perturbations, such as combined normal and shear stresses. Two effects have been identified. The first is that the effective activation volume contains contributions from both the normal and shear stresses. More importantly, the analysis predicts that the asymptote of this plot at zero stress is not equal to the thermal reaction rate; there is a change in the inherent tribochemical reaction rate that depends on velocity. This prediction is shown to be true for the shear-induced decomposition of ethyl thiolate species adsorbed on a Cu(100) single crystal substrate where this effect contributes to about two orders of magnitude increase in the reaction rate. This indicates that tribochemical reactions can be influenced by either just normal stresses or a combination of normal and shear stresses, but that the latter contribution is much larger. It is predicted that there is a linear relationship between the activation energy and the logarithm of the pr-exponential factor of this asymptotic rate constant, known as a compensation effect in catalysis. While this has not yet been seen for tribochemical reactions on surfaces, it has been found for reactions occurring in sheared fluids.