Theory of activated adsorption

In the theory of activated adsorption the occurrence of a potential barrier between the surface and the gaseous molecules gives rise to a kinetic expression similar to Equation (25) with k defined by Equation (26). The energy term u in this case, however, is associated with the height of the potential barrier at the surface of the crystal instead of being associated with the heat of reaction between the defects according to Equation 12 or 16. 

Since 1916 when Langmuir published his fundamental paper on adsorption, the Theory of Activated Adsorption/Desorption Kinetics (TAAD) has, almost exclusively, been used for the interpretation of adsorption/desorption kinetics. However, contraiy to the success of Langmuir equation to represent the adsorption equilibria, a dramatic failure of TAAD was observed to represent by the Langmuir kinetic equation and its further modifications, [1-3] the monitored adsorption/desorption kinetics. 

For the kinetics of adsorption the bimolecular production of sites can be shown to 3deld either a rapid Langmuir kinetics determined by the rate at which molecules strike the surface or an adsorption proportional to the square root of the time. For a unimolecular production of sites the kinetic expression for adsorption is that typical of the so-called activated adsorption, k = exp (— Et,ct/RT). In the theory of activated adsorption a potential brrrier between adsorption sites and impinging molecule is assumed. In Volkenstein s treatment it is the number of sites which increases with temperature and proportionally to exp — ufRT). No potential barrier is assumed. An alternative method of stating the same is to say that the number of sites remains constant but the number of excited centres which are able to absorb increases as observed. 

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