Role of local configurations in a Langmuir–Hinshelwood surface reaction: Kinetics and compensation

Abstract

We have employed Monte Carlo sampling to calculate the rate coefficient of a Langmuir–Hinshelwood reaction between species A and B on a square lattice. The experimental situation that is simulated is the reaction between a preadsorbed overlayer of species A with species B. The preadsorbed overlayer of A is allowed to equilibrate prior to the adsorption of B. Upon adsorption of B, the initial reaction rate is calculated assuming that A is irreversibly adsorbed and immobile, and that the equilibrium between adsorbed B and gas-phase B is established much more rapidly than the time scale of the reaction between A and B. Reaction is allowed only between nearest-neighbor AB pairs. We examine the parametrization of the reaction rate coefficient into an effective activation energy and an effective preexponential factor. We find that correlations between nearest-neighbor particles affect the reaction rate coefficient significantly. We also find that if the distribution of local configurations of nearest-neighbor pairs of reactant particles changes with temperature, the corresponding Arrhenius plot is nonlinear. The effective activation energy and the effective preexponential factor vary strongly with the fractional coverage of A and show a large compensation effect, similar to that observed experimentally in many desorption and surface-reaction systems. We conclude that variations in the distribution of local configurations of pairs of reactant molecules is a function of temperature and fractional surface coverage can be responsible for these experimentally observed compensation effects.