BET constant

A high value of the BET constant c is a useful preliminary indication of the presence of microporosity, but it does not enable one to estimate the micropore volume itself, that is in effect to break down the composite isotherm (iii) into its components (i) and (ii). 

V = volume of gas adsorbed at pressure P P = partial pressure of adsorbate Vm = volume of gas adsorbed in monolayer Po = saturation pressure of adsorbate at experimental temperature C = BET constant exponentially relating the heats of adsorption and condensation of the adsorbate... 

The monolayer of adsorbed gas (Vm) and the BET constant (C) are calculated from the slope and intercept ... 

A similar relationship between n-butane cross-sectional areas and the BET C constant was found by Lowell and Shields. A plot of the revised cross-sectional areas of n-butane versus the BET constant is given in Fig. 6.3. 

The easiest way to evaluate the BET constants is to rearrange Equation (71) into the following linear form ... 

The BET isotherm, like the isotherm developed by Langmuir (the first person to develop a rigorous model for gas adsorption), assumes that the adsorbing surface is energetically uniform, and that only one molecule could adsorb at each surface site. The BET isotherm is a generalized form of the Langmuir equation to account for multilayer adsorption, and assumes that after the adsorption of the first layer, the heat of condensation is equal to the heat of evaporation, and that the rates of adsorption for the second adsorbed layer and beyond are the same.29-31 From a practical perspective, variables in the equation must have specific values for the BET model to be valid, namely the y-intercept and BET constant, C, must be positive. Several excellent reviews of surface area measurement and gas adsorption can be found in References.6,32 34... 

Equation 6 will be valid for x values not more than 0.05 to 0.1, if the BET constant, c, is large compared to unity. 

Figure 1.27. -curve for (a) Nj isotherms on 22 different solid chlorides and (b) N2 isotherms on eight different organo-metallic compounds. The BET constant C is between 40 and 70 for cases (a) and between 20 and 30 for cases (b). (Redrawn from A. Lecloux. J.P. Pirard, J. Colloid Interface Sci.. 70 (1979) 265.)... 

Specific surface area from BET-method by N2/77K adsorption BET constant Total pore volume BJH Average pore diameter Pore wall thickness (A)... 

The first domain is used to determine the specific surface area (5 bet(H20)) and the BET constant values. Moreover, the large number of available experimental points allows the calculation of the corresponding adsorption energy distribution function of water on silica. 

The BET constant values (Cbet(H20)) are between 3.5 and 5, indicating that the isotherms are type II isotherms of the BET classification. Yet these values are rather low, indicating that the water molecules do not interact strongly with the silica surface. 

Davydov [4] describes similar behavior on Aerosil. He specifies that all physically adsorbed water may be removed by vacuum evacuation even at ambient temperature. This confirms that the water adsorption on silica is not as strong as initially supposed. This is also corroborated by the low BET constant values. 

The samples were conditioned following the D160 protocol (conditioning at 160 °C, under dry carrier gas stream). The methylene chloride adsorption isotherms were determined on each sample and used to compute the BET surface area (5bet(CH2C12)), the corresponding BET constant (Cbet), and the adsorption energy distribution functions (AEDF). 

The results of Table 5 indicate that the surface areas of the silica samples relative humidity increases, i.e., when the amount of physically adsorbed water increases. On the other hand, the BET constant values also decreases with increasing adsorbed water amounts. However, this decrease is not smooth like the one observed when the samples are heat-treated. Another interesting observation concerns the fact that in spite of the presence of water, the BET constant values remain above 4.5. In other words, the isotherms are still of type II. 

The actual adsorption curves obtained are not as simple as that depicted in Fig. 2.5 but, rather, fall into one of several general pattems, 0.ll one of which is depicted in Fig. 2.6. With these curves extrapolation to zero pressure is not unambiguous so monolayer coverage is frequently determined by applying the BET equation in the form shown in Eqn. 2.6,where Vgds is the volume of gas adsorbed at pressure P, is the saturation pressure, commonly 200-400 Torr, is the volume of gas adsorbed at monolayer coverage and C is the BET constant that is related to the enthalpy of adsorption. 

In the present section we discuss the mobile case making the above assumption, and consider pressures low enough so that multilayer adsorption is not important. (There are of course some systems which never form a monolayer in the usual sense but start on multilayers practically from the beginning, as in the adsorption of water vapor on graphite or other systems with BET constant c very small.) Also, we assume for simplicity a uniform surface. 

Surface concentration, JV/Ct or n1/nx in Sec. IV Surface or spreading pressure BET constant... 

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