Brunauer—Emmett—Teller theory

The adsorption isotherms provide information about the surface properties of the fibres or paper in the column. For example, the surface area in the column may be deduced from the shape of the isotherm according to the Brunauer-Emmett-Teller theory for multilayer adsorption (13, lJO. This theory gives the number of molecules, Nm, which forms a layer one molecule thick on the surface of the adsorbent. If the area, am, occupied by each vapour molecule adsorbed on the surface is known, then the total surface area, A, is simply given by... 

BET surface is related to the specific surface area according to Brunauer-Emmett-Teller theory... 

Surface areas of clay minerals may be derived from vapor-phase adsorption isotherms by applying either the B.E.T. (Brunauer, Emmett, Teller) theory (Brunauer et al. [1937, 1938, 1945]) or the method by Harkins and Jura [1944]. 

Popper and Bariska (1972) studied the moisture sorption properties of wood chemically modified with acetic (or phthalic) anhydride and analysed the results using Brunauer-Emmett-Teller (BET) theory and the H-H model. Acetylation was found to reduce the number of sorption sites, whereas little effect was noted with phthaloylation. By dividing the sorption isotherm into a monolayer component and a multilayer component using the H-H model, it was shown that there was a large reduction in the... 

Because the Langmuir isotherm is not an adequate description of most systems, Equation (2.9) is not used much for area measurement. A number of other isotherm formulations utilize adsorption in surface area measurements, however (cf. Young Crowell, 1962, for example). The best known and most widely used is the BET (Brunauer, Emmett Teller, 1938) theory, a generalization of the Langmuir model to multilayer absorption. Assuming that for the second and succeeding molecular... 

The Brunauer-Emmett-Teller (BET) gas adsorption method has become the most widely used standard procedure for the determination of the surface area of finely-divided and porous materials, in spite of the oversimplification of the model on which the theory is based. 

The appearance of Langmuir s comprehensive review of the nature of adsorption (1916, 1918) prompted several investigators to consider the possibility of using gas adsorption for surface area determination. Early attempts were made by Williams (1919) and Benton (1926), but these led to inconclusive findings. The first significant advances were made by Brunauer and Emmett (1935, 1937) and their work prepared the way for the development of the Brunauer-Emmett-Teller (BET) theory in 1938. 

The sigmoidal shape of the nitrogen isotherm of Figure 1 is quite amenable to analyses by the Brunauer-Emmett-Teller (BET) multilayer theory (8). The BET surface area calculated is 2.8 mVg. This value is quite consistent with the predominant l-to-10- xm size distribution observed on this sample with an optical microscope. Assuming cubic habits and density of about 2.3 g/cm, we find the range of specific surface area to be 0.26 to 2.6 m /g. Apparently the sorbed N2 does penetrate into the internal portion of the particles only to a limited extent. 

Brunauer-Emmett-Teller (BET) [20], Langmuir isotherm theory is applied when a monolayer of molecules is sorbed on a homogeneous surface where each site requires the same energy to adsorb molecules. The Freundlich isotherm also assumes a monolayer, but allows for an exponential distribution of adsorption energies. BET isotherms provide for multilayer adsorption. 

Gas adsorption is a suitable method for a fractal analysis because it is sensitive to the fine structure of the pores and has negligible adverse affects on the pore system. The results are usually analyzed by using fractal generalizations of the Brunauer-Emmett-Teller (BET) isotherm (30) or of the Frenkel-nalsey-TfiU (FHH) isotherm (31). The latter may also be seen as a fractal generalization of the Kelvin equation and is therefore also applicable in the capillary condensation regime (32). It has been claimed that the fractal BET theory is more appropriate for mass fractals (see sect. Fractals ), whereas surface fractals are to be analyzed using the fractal FHH theory (33). These methods have been applied to cellulose powders (34) and tablets (35). 

The Brunauer-Emmett-Teller (45) theory of physical adsorption was the first and is still probably the best and most useful theory of multilayer adsorption covering the complete range in p. This is not to say, however, that it is a really satisfactory theory. As will be made clear below, the assumptions of the theory are extremely crude, but they are still sufficiently good to contain a number of the important qualitative features actually observed experimentally. The role of the BET theory in physical adsorption is in fact rather analogous to that of the van der Waals equation in the theory of liquids it was the first important theory in the field it has stimulated virtually all the work that followed it it is still extremely useful as a qualitative guide but it is not quantitatively correct. 

Adsorption capacity related parameters are usually determined from gas adsorption measurements. The specific surface area is calculated by applying the Brunauer-Emmett-Teller (BET) equation [17] to the isotherms generated during the adsorption process. The adsorption of N2 at 77 K or CO2 at 273 K are the most commonly used to produce these isotherms. The BET theory is based upon the assumption that the monolayer is located on... 

Thus, it is most suitable for describing chemisorption (except possibly for assumption 1) and low-coverage physisorption where a single layer is probable. For higher-coverage physisorption, a theory that accounts for multiple layers is the Brunauer-Emmett-Teller (B-E-T) isotherm (see [20-24], [63]). 

The pore distribution functions in relation to their radii show that the materials studied can be classified as mesoporous adsorbents with the additional presence of micropores. The exception is the sample A2 in which a contribution from micropores on the surface is not observed. From the sorptometry data (Figure 13) the specific surface area (5), total pore volume (V) and pore diameter (Z> k) were calculated. These quantities were determined from the three Brunauer-Emmett-Teller (BET), Barrett-Joyner-Holenda (BJH) and Langmuir theories. 

NMR imaging of gas adsorption/desorption in nanoporous solids, such as Y-AI2O3 and ZnO powders and partially sintered ceramics of these materials, as well as Vycor porous glass was analysed using Brunauer-Emmett-Teller (BET) theory. Visualization of gaseous xenon and methane in the void spaces of aerogels offered unique information and insights into the pore structure and molecular diffusivities of occluded sorbates. ... 

Direct methods of measuring adsorption are essentially restricted to the measurements of adsorption at the solid surface from a gas phase. At sufficiently high specific surface area, 5i (m /g), the adsorption can be determined directly as the increase in mass. High-surface-area materials include highly disperse adsorbents with fine pores, such as activated charcoal, zeolites, and various catalysts for which the surface area is on the order of tens and hundreds of square meters per gram. The adsorption on such surfaces from a gas phase can also be determined by measuring a decrease in the gas (vapor) pressure, p, in a closed vessel. The multilayer adsorption of noncorrosive gasses is a commonly used method to determine the surface area of adsorbents on the basis of the Brunauer-Emmett-Teller (BET) theory included in all classic texts on physical chemistry. 

We used the Brunauer-Emmett-Teller (BET) theory [4], as a standard procedure, to determine specific surface area of samples, asEr and net heat of adsorption, AQ. For the various adsorbates abet was calculated at relative pressure, p/p (where p and p were the equilibrium and the saturation pressure, respectively), in the range 0.01-0.35, which was the validity domain of the linear BET plot. 

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