Adsorption Isotherms I

Type I adsorption isotherms describe chemisorption, relating the amount of adsorbed molecules to pressure at a constant temperature.

According to the Langmuir isotherm for non-dissociative adsorption, the adsorbate forms a reversible monolayer on identical surface sites that do not interact with each other.

Consider gas A adsorbing reversibly on surface M, with adsorption governed by the rate constant k_a and desorption by k_d​. The adsorption rate depends on pressure and the number of vacant sites, while the desorption rate depends on the number of occupied sites.

At equilibrium, equating these rates yields the Langmuir equation, linking fractional coverage to pressure through the parameter α— the ratio of the adsorption and desorption rate constants.

The modified Langmuir isotherm explains dissociative adsorption, where diatomic molecules dissociate into atoms and occupy separate sites on the adsorbent surface. At equilibrium, equating the adsorption and desorption rates and rearranging the expression gives the modified Langmuir equation.

In both models, fractional coverage increases linearly at low pressures and approaches saturation only at very high pressures, where most surface sites are occupied.