Brunauer-Deming-Teller isotherm types

Figure 7.2 The Brunauer-Deming-Teller classification of isotherm types I to VI (from top left to bottom right). In each case adsorption uptake is plotted against p/Po, where p is the adsorbate pressure and Po the saturated vapour pressure of the pure liquid adsorbate at the isotherm temperature.

Fig. I.l The five types of adsorption isotherm, I to V, in the classification of Brunauer, Deming, Deming and Teller (BDDT), together with Type VI, the stepped isotherm.

Five types of isotherms for adsorption according to Brunauer, Deming, Deming, and Teller (4). 

Brunauer, Deming, Deming and Teller, based upon an extensive literature survey, found that all adsorption isotherms fit into one of the five types shown in Fig. 3.1. 

Adsorption isotherms are plots of the amount of gas adsorbed at equilibrium as a function of the partial pressure p/p°, at constant temperature. The quantity of gas adsorbed is mainly expressed as the mass of gas (usually g) or the volume of gas reduced to STP (standard temperature and pressure). The majority of isotherms which result from physical adsorption may conveniently be grouped into five classes — the five types I to V included in the classification originally proposed by Brunauer, Deming, Deming and Teller — sometimes referred to simply as the Brunauer classification [2]. The essential features of these types are indicated in Fig. 12.1. 

An extension to the BET model was put forward by Brunauer, Deming, Deming and Teller (BDDT) in 1940. The BDDT equation contains four adjustable parameters and was designed to fit the isotherm Types I-V. From a theoretical standpoint, the BDDT treatment appears to offer very little more than the original BET theory and the cumbersome equation has very rarely been applied to experimental data. 

The majority of isotherms which result from physical adsorption may conveniently be grouped into five classes, i.e., the five types I to V included in the classification originally proposed by Brunauer, Deming, Deming and Teller [4]. The essential features of these types are indicated in Fig. 13.3. 

Fig. 13.3. The six types of adsorption isotherms following the Brunauer, Deming, Deming, Teller...

Brunauer, Deming, Deming, and Teller [3] later distinguished five different physical adsorption isotherms. The Type I adsorption isotherm is characteristic of chemisorption, for which the first layer is adsorbed much more strongly than subsequent layers. The type II isotherm is characteristic of the multilayer adsorption exhibited with physical adsorption near the boiling point of the adsorbate. Type III isotherms are obtained for multilayer physical adsorption with condensation of the adsorbate in narrow pores whereas Type IV isotherms are obtained when the first layer is adsorbed with a lower heat than the heat of condensation of the adsorbate. Finally, Type V isotherms are characteristic of adsorption according to Type IV on an adsorbent with narrow pores. 

Another important contribution by Brunauer, Deming, Deming and Teller [132] deals with the identification of five principal types of adsorption isotherms for gases and vapours. This identification is known as the BDDT classification and is recommended as the basis for a more complete classification introduced by lUPAC [15],... 

The nitrogen adsorption at 77 K of the samples is shown in Figure 1. The adsorption isotherms are of type I+II in the Brunauer, Deming, Deming and Teller (BDDT) classification... 

The majority of isotherms may be grouped in the six types shown in Figure 2.1. The first five types were described by Brunauer, Deming, Deming and Teller [1], Type 6 was identified later [2]. 

Experimental adsorption isotherms recorded in the literature, measured on a wide variety of gas-solid systems, have a wide variety of forms. Nevertheless, the majority of these isotherms which result from physical adsorption may conveniently be grouped into six classes in the IUPAC classification (cf. Figure 1.7). The first five types (I to V) of the classification were originally proposed by S. Brunauer, L.S. Deming, W.S. Denting and E. Teller as the BDDT classification (1940), sometimes referred to as the Brunauer classification (1945). 

Figure 3.1 The five types of van der Waals adsorption isotherms. Reproduced with permission from S. Brunauer, L S. Deming, W. E. Doming and E. Teller, J. Amer. Chem. Soc., 62 (1940) 1723 (Fig. 1), 1940 American Chemical Society.

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