BET Adsorption Isotherm Equation and Applications

BET adsorption isotherm proposed by Brunauer, Emmett, and Teller accounts for multilayer adsorption. According to the theory, adsorption takes place only on specific areas of the sample surface (one per molecule), and doesn’t stop at monolayer formation but first adsorbed gas molecules provide an adsorption site for subsequent gas molecules leading to the multilayer adsorption. i.e. after the monolayer has formed, the adsorption process proceeds to the formation of the multilayer forming second, third, and so forth layers. BET theory also considers that adsorption sites on the solid surface are homogenous and that adsorption at one site has no effect on adsorption at nearby sites.

Langmuir and Freundlich adsorption isotherm which fails to explain other isotherms except type I is well explained by the BET equation

BET Equation

On the basis of the above assumptions, BET proposed an equation called the BET equation.

If E₁>E_L, type II adsorption isotherm is obtained but if E₁<E_L, then type III adsorption isotherm is obtained. When adsorption occurs in pores and capillaries, type IV and V adsorption isotherms are formed.

For type IV adsorption isotherm, E₁must be greater than E_L i.e. E₁>E_L

For type V adsorption isotherm, E_L must be greater than E₁ i.e. E₁<E_L

Applications of BET adsorption isotherm

Determination of the surface area of the adsorbent is one of the major applications of the BET equation.

BET Equation Video

FAQs/MCQs

References

• Atkins, P. W.; De Paula, Julio; Keeler, James (2018). Atkins’ Physical chemistry (Eleventh ed.). Oxford, United Kingdom. ISBN 978-0-19-876986-6
• Arun Bahl, B. S. Bahl & G. D. Tuli, Essentials of Physical Chemistry, S. Chand and Company Ltd., New Delhi, 2012.
• Brunauer, Stephen; Emmett, P. H.; Teller, Edward (1938). “Adsorption of Gases in Multimolecular Layers”. Journal of the American Chemical Society. 60 (2): 309 – 319. doi:10.1021/ja01269a023