Uniform Surface-Induced Heterogeneity

In this model, all sites are identical but the activation energy for adsorption increases linearly with coverage 0 due to perturbation of the adsorbent uniformly over the whole of the surface, or to lateral (repulsive) interactions between adsorbed molecules (implicitly assumed to be mobile and hence uniformly distributed over the available sites). Thus 

Since the variation of (1 — 0) is negligible with respect to that of the exponential term then 

The parameter k contains the molecular collision number per unit area of adsorbent surface and hence varies as in contrast for site 

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Though both chemisorption and desorption kinetics can be explained by a model in which the surface is characterized by a fixed heterogeneity with a uniform energy distribution function, in chemisorption other explanations are possible, like induced heterogeneity or surface reconstruction. 

For induced heterogeneity on a uniform surface, the increase of activation energy is solely a function of the coverage q, and the adsorbate is always distributed uniformly over a constant adsorption potential surface, although this potential changes due to induced effects of coverage on the heat of adsorption. The rate equation, therefore, has the form... 

Although the obedience of the rate of adsorption to the Elovich equation is maintained, both for mobile and immobile layers, and for any functional relationship between — AH and E, when the heterogeneity is induced on a uniform surface, this is not so for the rate of desorption. 

Under sliding conditions, the measured op>en circuit potential is a mean value which depends on the electromotive force induced by the surface heterogeneity resulting from the coexistence of non-rubbed and rubbed areas which are in different electrochemical states, and on the areas of these zones and their spatial distribution that determines the non-uniform distribution of potential over the whole surface (Oltra et al, 1986). When applying for example a continuous sliding, this open circuit potential responds to the mechanical loading imposed, as shown in Table 1 and Figure 2. 

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