Bet isotherm 1. BET Isotherm An isotherm that takes account of the possibility that the monolayer in the Langmuir adsorption isotherm can act as a substrate for further adsorption. The BET isotherm named after S. Brunauer, P. Emmett, and E. In the BET isotherm, the isotherm rises indefinitely at high pressures in contrast to the Langmuir isotherm. It provides a useful approximation over some ranges of pressure but underestimates adsorption for low pressures and overestimates adsorption for high pressures.
Type of Adsorption Isotherm Five different types of adsorption isotherm and their characteristics are explained below. This explains formation of monolayer followed by multilayer. This can be explained on the basis of a possibility of gases getting condensed in the tiny capillary pores of adsorbent at pressure below the saturation pressure PS of the gas. Theory BET theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of a material.
The concept of the theory is an extension of the Langmuir theory, which is a theory for monolayer molecular adsorption, to multilayer adsorption with the following hypotheses: a gas molecules physically adsorb on a solid in layers infinitely; b there is no interaction between each adsorption layer; and c the Langmuir theory can be applied to each layer. The resulting BET equation is expressed by 1 : P and P0 are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption, v is the adsorbed gas quantity for example, in volume units , and vm is the monolayer adsorbed gas quantity.
This plot is called a BET plot. The linear relationship of this equation is maintained only in the range of 0. The value of the slope A and the y-intercept I of the line are used to calculate the monolayer adsorbed gas quantity vm and the BET constant c.
The following equations can be used: The BET method is widely used in surface science for the calculation of surface areas of solids by physical adsorption of gas molecules. A total surface area Stotal and a specific surface area S are evaluated by the following equations: where vm is in units of volume which are also the units of the molar volume of the adsorbate gas 7.
Rate of Evaporation Rate of Condensation. Rate of Condensation Rate of Condensation. If the quantity of adsorbed water vapor is measured at different levels of relative humidity a BET plot is obtained.
From the slope A and y-intersection I on the plot it is possible to calculate 9. Activated Carbon For example, activated carbon, which is a strong adsorbate and usually has an adsorption cross section s of 0. Moreover, in the field of solid catalysis, the surface area of catalysts is an important factor in catalytic activity.
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Clipping is a handy way to collect important slides you want to go back to later. Now customize the name of a clipboard to store your clips. Visibility Others can see my Clipboard. Cancel Save. Emmett, and E. Teller has the form:. In the BET isotherm, the isotherm rises indefinitely at high pressures in contrast to the Langmuir isotherm.
It provides a useful approximation over some ranges of pressure but underestimates adsorption for low pressures and overestimates adsorption for high pressures. Subjects: Science and technology — Chemistry. View all related items in Oxford Reference ». All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single entry from a reference work in OR for personal use for details see Privacy Policy and Legal Notice.
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