Langmuir Isotherm for Dissociative Adsorption

Adsorption of gases such as oxygen and hydrogen generally takes place by dissociation of the gas molecule on the surface of the solid. The adsorbed species are, therefore, atoms. In such cases the adsorption process may be considered as a reaction between the gas molecule and the two surface sites. The rate of adsorption in such cases is given by 

The process of desorption now involves interaction between two adsorbed atoms on neighboring sites and is, therefore, given by 

Following the earlier arguments for deriving the Langmuir adsorption isotherm equation and equating Equation 2.40 and Equation 2.41, we get 

In general, when the adsorbed molecule dissociates into n entities, each of which occupies a surface site, the Langmuir equation for dissociative adsorption can be written as 

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