Summary of Adsorption Isotherms for Pure Gases

In general, the adsorption isotherm for component i that dissociates into y submolecular fragments, with each fragment occupying one active site on the catalytic surface, is 

The vacant-site fraction 0 / is calculated from the requirement that all surface fractions must sum to unity. Hence, the population balance is 

The following relations between fractional surface coverage and gas pressure are useful for correlating experimental data on chemisorption. In most cases, there is theoretical justification for the functional form of the isotherm based on rates of adsorption and desorption. Langmuir (1918) and Sips (1948, 1950) proposed the following relation between a and pa - 

Previous examples in this chapter indicate that y should be interpreted as the number of sites required for one molecule of A to adsorb, after dissociation. Langmuir identifies C as (A a), where Ka is a temperature-dependent adsorption/desorption equilibrium constant. Equation (14-50) can be rearranged to express Pa in terms of a  

This represents the starting point for the Fowler-Guggenheim modification (1939), which includes ionic interactions between adsorbed species. If one site is required for one molecule of A to adsorb (i.e., y = 1), then  

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