Brunauer-Emmet-Teller adsorption modelling

The Brunauer-Emmet-Teller (BET) model of adsorption is one of the most common tools for surface analysis, and is also recommended by lUPAC [48] as a standard tool for surface area determination. According to the BET model, adsorption occurs... 

The determination of the specific surface area of a zeolite is not trivial. Providers of zeolites typically give surface areas for their products, which were calculated from gas adsorption measurements applying the Brunauer-Emmet-Teller (BET) method. The BET method is based on a model assuming the successive formation of several layers of gas molecules on a given surface (multilayer adsorption). The specific surface area is then calculated from the amount of adsorbed molecules in the first layer. The space occupied by one adsorbed molecule is multiplied by the number of molecules, thus resulting in an area, which is assumed to be the best estimate for the surface area of the solid. The BET method provides a tool to calculate the number of molecules in the first layer. Unfortunately, it is based on a model assuming multilayer formation. Yet, the formation of multilayers is impossible in the narrow pores of zeolites. Specific surface areas of zeolites calculated by the BET method (often termed BET surface area) are therefore erroneous and should not be mistaken as the real surface areas of a material. Such numbers are more related to the pore volume of a zeolite rather than to their surface areas. 

The most widespread method in determining the specific surface area of solid substrates is without doubt the Brunauer-Emmet-Teller (BET) method.3 It is based on a kinetic model of the adsorption process by Langmuir,7 in which the surface of the solid was regarded as an array of adsorption sites. A state of dynamic equilibrium... 

Brunauer, Emmet, Teller (BET) analysis of adsorption isotherms allows a surface area to be calculated [17,19]. The BET model is for multilayer adsorption on a flat surface, so when applied to a porous material the surface area obtained is an apparent value. Nevertheless, BET surface area provides a useful comparison between materials. PM-1 shows a higher BET surface area than the activated carbon in Eigure 2.2, a value of 780 m g" for PIM-1 as compared to 545 m g for the carbon. 

A number of models have been developed for the analysis of the adsorption data, including the most common Langmuir [49] and BET (Brunauer, Emmet, and Teller) [50] equations, and others such as t-plot [51], H-K (Horvath-Kawazoe) [52], and BJH (Barrett, Joyner, and Halenda) [53] methods. The BET model is often the method of choice, and is usually used for the measurement of total surface areas. In contrast, t-plots and the BJH method are best employed to calculate total micropore and mesopore volume, respectively [46], A combination of isothermal adsorption measurements can provide a fairly complete picture of the pore size distribution in sohd catalysts. Mary surface area analyzers and software based on this methodology are commercially available nowadays. 

Since this model was far too complex to serve any practical purpose, Brunauer, Emmet and Teller made some simplifying assumptions (the main one being that in all layers the evaporation-condensation mechanisms are identical) to derive their famous BET equation, to be used in the multilayer-adsorption region of the adsorption isotherm ... 

Brunauer, Emmet, and Teller have worked out a model for multilayer adsorption. They assumed that the first step in the adsorption is... 

The multilayer adsorption model, as shown in Fig. 2.11, was proposed by Brunauer, Emmet and Teller (BET) in 1938 to modify Langmuir s monolayer one. BET theory developed from the multilayer model can be applied to explain all types of isotherms. Based on the BET theory, a standard method for determining the specific surface area of solid catalysts was developed, which brought catalysis study into a new stage. 

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