Brunauer s classification

Thus, in terms of a, the sorption isotherm for osmotically ideal solutions is of Type III in Brunauer s classification (29) and reduces to Henry s law for (Mi/M2)a< 1. 

Figure 5.5 Brunauer s classification of adsorption isotherms (pn saturated vapour pressure)...

Many adsorption isotherms are borderline cases between two or more of the above types. In addition, there are some isotherms which do not fit into Brunauer s classification at all, the most notable being the stepwise isotherms, an example of which is given in Figure 5.6. Stepwise isotherms are usually associated with adsorption on to uniform solid surfaces, each step corresponding to the formation of a complete monomolecular adsorbed layer (see page 133). 

The equilibrium isotherms for microporous adsorbents are generally of type I form in Brunauer s classification (Fig. 1). Such isotherms are commonly represented by the Langmuir model,... 

FIGURE 1 Brunauer s classification of equilibrium isotherms. P, sorbate pressure Ps, saturation vapor pressure. 

Intracrystalline sorption is normally of Type 1 in Brunauer s classification ( V7) and isotherm contours therefore resemble those according to Langmuir s isotherm equation. This can describe actual isotherms well enough (18) to be of value in predicting, through Equations 5 or 6, some features of zeolite chemistry. 

Two types of isotherms are most common. The first is type I of Brunauer s classification (6) with Langmuirs equation being the simplest example. The curves corresponding to this type are convex everywhere and have a v = 1 asymptote. This type was studied in the framework of the moving bed reactor by Viswanathan and Aris (1). According to the result of the previous section we cannot have more than one internal discontinuity. 

The adsorption-desorption isotherms of nitrogen at -196 °C obtained on all the catalysts under investigation were mainly of Type IV of Brunauer s classification [16], exhibiting hysteresis loops closed at P/Po ranging between 0.25 and 0.55. The adsorption data are summarized in Table 1, including BET-C constant, specific surface area(SBEj), total pore volume (Vp), estimated from the saturation values of the adsorption isotherms and average pore radius (r P), assuming cylindrical pore model for which superscript (cp) was used. 

The onset of capillary condensation generally coincides with an inflection in the equilibrium isotherm, that is, a type 4 isotherm of Brunauer s classification. A typical example is shown in Figure 2.11. 

Figure 4.24 shows plots of the BET Equation 4.58 for different c values, indicated in the figure. Note that as Py, diverges that is, condensation occurs. For c> 1 the plots correspond to type n in Brunauer s classification, whereas for c < 1 the type 111 is predicted. In the first case, the interaction of the adsorbate with the surface is stronger than with itself, thus the first layer is populated with some preference to subsequent layers, and so the curves show a hump around 6=1. For c < 1 the opposite preference occurs, the layers grow as soon as the first molecules adsorb, and thus no hump is seen in the plot. 

Moreover, even though the coordination of a molecule in a liquid is higher than at the surface of a solid, the secondary forces are usually strong enough to guarantee the following condition q/ < qo < qi. Adsorption described by isotherms of types III and V of Brunauer s classification does not run in this case. 

Most vapor adsorption isotherms for clays have a sigmoidal shape, which may be classified as Type II isotherms according to Brunauer s classification. In rare cases, Langmuir type isotherms are observed (van Olphen [1963, 1965]. A typical Type II isotherm, observed for lithium kaolinite, is shown in Figure 1. 

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