Chapter 5. Statistical rate theory and the material properties controlling adsorption kinetics, on well defined surfaces

Abstract

The Statistical Rate Theory approach for predicting the rate of kinetic processes is developed from a simple quantum mechanical model of an isolated, thermodynamic system. The approach leads to an interpretation of irreversibility and to an expression for the rate in terms of molecular or material, properties, of the system. The approach is evaluated by examining the predictions that follow when it is used to examine two types of reactions: those, occurring homogeneously within one phase and those occurring when a non-dissociating, diatomic molecule is exposed to a single crystal solid surface with one type of binding site. In the former case, the material properties are known. In the latter, it is found that they can be evaluated from the equilibrium adsorption isotherms. As evidenced by the agreement between the predictions and the measurements for both types of reactions, the expression for the rates obtained from Statistical Rate Theory appears, to contain explicitly the concentration dependence. The adsorption, kinetics, of CO-Ni(111) was the adsorption system considered. When examined with Absolute Rate Theory, it was found that, as with many other systems, agreement between the measurements and the predictions can not be, obtained without introducing, coverage dependence into the rate expression through the introduction of the concepts of precursor states and of a coverage dependent sticking probability. Since the Statistical Rate Theory expression for the adsorption kinetics does not require the concepts of precursor states or of sticking probability, but nonetheless leads to the correct prediction of the adsorption kinetics for this system, it is not clear that these concepts need to be introduced. Other systems need to be examined before this possibility could be established.