Arrhenius parameters for rate processes on solid surfaces

During the last decade, many attempts have been made to measure accurately the apparent Arrhenius parameters (i.e., the activation energy and pre-exponential factor) for various rate processes on monocrystalline surfaces. The main attention was directed to studying desorption of the simplest molecules such as CO, H2 and NO. Interesting results have also been derived for CO oxidation on different metals and for surface diffusion. The present review contains a detailed catalogue of the experimental data obtained in this field of surface science. The data collected indicate that the coverage dependence of the apparent Arrhenius parameters is, as a rule, rather strong. With increasing coverage, the change in the activation energy usually exceeds 10 kcal/mol and the pre-exponential factor often varies over three or more orders of magnitude. The direction of variation, as a rule, satisfies the compensation effect. The coverage dependence of the activation energy is customarily explained by lateral interactions between adsorbed particles, by adsorbate-induced changes in the surface, or by different types of adsorption sites. Various models collected predict also the coverage dependence of the pre-exponential factor for different rate processes in adsorbed overlayers. However, the predicted coverage dependences of the pre-exponential factor are usually not so strong as that measured in real systems.