Effect of Site and Adsorbate Coordination Number

Since in chemisorption systems it is reasonable to suppose that the strong adsorbent-adsorbate interaction is associated with specific adsorption sites, a situation that may arise is that the adsorbate molecule occupies or blocks the occupancy of a second adjacent site. This means that each molecule effectively requires two adjacent sites. An analysis [106] suggests that in terms of the kinetic derivation of the Langmuir equation, the rate of adsorption should now be 

If the adsorbed molecule occupies two sites because it dissociates, the desorption rate takes on the form 

It should be cautioned that the correctness of the factors involving z has not really been verified experimentally and that the algebraic forms involved are modelistic. Additional analyses have been made by Ruckenstein and Dadybur-jor[lll]. 

Halsey and Yeates [112] add an interaction term, writing Eq. XVIII-14 in the form 

The preceding treatments are based on the concept of localized rather than mobile adsorption. The distinction may be difficult experimentally note Ref. 109 and the discussion in connection with Fig. XVII-25. There are also conceptual subtleties see Section XVIII-5. 

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