Effects of the Indirect Correlations of Interacting Particles on the Mono- and Bimolecular Adsorption and Desorption Rates during Chemisorption

The influence of indirect correlations between the nearest interacting chemisorbed particles on the rates of elementary mono- and bimolecular stages of adsorption and desorption was studied. The calculation was performed within the framework of the theory of absolute reaction rates, in which the difference between the interactions of particles in the ground states and the activated complex of the stage in the transition state is taken into account. The local distributions of particles were constructed within the framework of the cluster variation method (CVM), which allows us to go beyond the limits of the quasi-chemical approximation (QCA), reflecting the effects of only direct correlations or an isolated pair (2 × 1) in the CVM terminology. The type of cluster approximation is determined by the size of the basic cluster in the CVM. As an illustration, calculations of adsorption and desorption rates are given including and neglecting molecular dissociation on a homogeneous flat (100) face for the isothermal concentration dependences of the rates and for the thermal desorption curves. The rates calculated for a number of simple basic clusters (2 × 2, k1s, 3 × 3, 3 × 4) and in QCA were compared. The calculations were performed near the critical temperature in the corresponding cluster approximation without taking into account the appearance of ordered structures of chemisorbed particles. In all versions of calculations including indirect correlations, an increase in the size of the basic cluster increases the differences between this cluster approximation and the QCA. These differences are quantitative, leaving the type of concentration and thermal desorption curves of adsorption and desorption rates qualitatively unchanged.