Elementary Processes and their Models

Changes in the state of the adsorbent-adsorbate system which, at the atomic-molecular level, is described by the lattice-gas model are caused by variations in the occupancy of its individual sites as a result of the elementary processes. The following elementary processes occur on the adsorbent surface adsorption and desorption of the gaseous phase molecules, reaction between the adspecies, migration of the adspecies over the surface and their dissolution inside the solid. The solid s atoms are capable of participating in the chemical reactions with the gaseous phase molecules, as well as migrating inside the solid or on its surface. 

Within the frameworks of the lattice gas model it is reasonable to classify the elementary processes by the number of sites m, which a given process occurs on, i.e., one- and two-site cases. In the first case the changing parameter is the occupancy state of one site. The processes such as these include isomerization associated with changes in the internal degrees of freedom of the adspecies (ZA- ZB, i.e., transition of the adspecies from state A to state B), adsorption-desorption of the atoms and nondissociating molecules (A + Z- ZA), reaction according to the collision mechanism (A + ZB - ZD + C, Eley-Rideal s-type mechanism). It should be remembered that ZA, Z and A denote adspecies A, empty lattice site and species A in the gaseous phase, respectively. 

The two-site processes involve a simultaneous change in the occupancy state of both sites. They include reactions between the adspecies (Z/A + ZgB- Z/C + ZffD Langmuir-Hinshelwood s-type mechanism), 

The rate of the elementary process is the most important characteristic. If the conditions of the acting masses law are regarded fulfilled, the rate t/A of the mono- (ZA - Product) and t/AB of the bimolecular (ZA + ZB — Product) elementary processes on the homogeneous surface can be written down as follows [58]  

Here Ka and Kab are the constants of the mono- and bimolecular reaction, respectively. When writing Eq. (5), it is assumed that the reagents are 

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