Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis

This chapter discusses the thermodynamic activity of an atomic species on the surface of a catalyst and its determination under steady-state conditions of a catalytic reaction. If the rate of an individual step of a catalytic reaction depends on the concentration of adsorbed atoms occurring as intermediates, it is imperative to determine this dependence. The determination of the surface concentration of molecules adsorbed on a catalyst is feasible under favorable conditions. In the case of atomic species, it has been found to be convenient to use the thermodynamic activity rather than the surface concentration as the relevant variable. The primary problem of chemical kinetics is the formulation of an empirical rate law that represents the rate of a reaction as a function of the concentrations or partial pressures of reactants, products, and catalysts present in a gaseous or liquid phase. The knowledge of the mechanism of a reaction may be used to formulate a rational rate law that may represent the empirical rate data in a more logical form than an empirical rate law.