BDDT classification

Isotherms of the type now characterized as Type IV have played an essential part in the development of adsorption theory and practice, as being the first kind of isotherm to be studied in detail. Already in 1888, half a century before the BDDT classification had appeared, van Bemmelen had... 

When the BDDT classification was put forward in 1940 such isotherms were rare and although a number of well documented cases of both types have been reported in recent years they must still be regarded as somewhat uncommon. 

Figure 6 shows the isotherms of the samples using different alkanes as expander. Except the sample obtained with nonane, the adsorption-desorption isotherms of all other compounds are type IV, characteristic of mesoporous materials according to the BDDT classification [21], Isotherms can be decomposed in three parts the formation of the monolayer, a sharp increase characteristic of the capillary condensation of nitrogen within the mesopores and finally a plateau indicating the saturation of the samples. From pentane to decane the relative pressure at which the capillary condensation occurs, increases from 0 30 to 0.60, indicating that the value of the pore diameter increases when the alkane chain length is raised since the p/po position of the inflection point is related to the pore diameter. From undecane, this value decreases to reach 0.40 for dodecane We can conclude that the value of the pore diameter drops from decane to dodecane... 

The first systematic attempt to interpret adsorption isotherms for gas/solid equilibria was by Braunauer, Deming, Deming, and Teller (BDDT) in 1940.78 They classified the adsorption isotherms into five types, and the BDDT classification became the core of the modem International Union of Pure and Applied Chemistry... 

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). 

The adsorption isotherms of nitrogen at -196 °C for the samples studied are shown in Fig. 4. All the isotherms are type II in the BDDT classification [10]. The BET apparent surface areas values are in Fig. 4. The increase in the temperature of treatment produces a decrease in the amount of nitrogen adsorbed, with an important decrease in the BET areas due to an ordering in the sample structure. The great differences in the BET surface areas of the samples make it possible to measure a wide range of aetive surface areas. 

Figure 2 shows the Phe adsorption isotherms in terms of the steam percentage in the inlet gas. The adsorption isotherms are close to type 1 of the BDDT classification. Thus, the initial part of the adsorption of the isotherm represents micropore filling and the slope of the plateau at higher coneentrations is due to multilayer adsorption on the non-... 

Adsorption isotherm of Phe vapors on CA-3 sample were obtained at different CO2 percentages, 0%, 10%, 15% and 30% at 150°C (see Fig 3). The adsorption isotherms are close to type I of the BDDT classification. It is observed that the adsorption is mainly produced in the molecular size pores of Phe and the presence of CO2 in the gas stream seems not to avoid the multilayer adsorption. The decrease on the Phe adsorption capacity when CO2 is added to the gas stream is probably due to a competitive effect between both molecules for the adsorption sites. It is also observed that the increase of the CO2 percentage from 10% to 30% does not seem to affect to Phe adsorption capacities. This fact is congruent with the results obtained by Yong et al who reported that the CO2 adsorption at high temperature on non-chemically modified carbonaceous materials, follows Henry s law with a negligible influence of CO2 partial pressure on its adsorption capacity. 

The nitrogen gas adsorption-desorption isotherms of the metakaolins are classified as type II (BDDT classification [27]). They are almost reversible with a closed hysteresis cycle, indicating the absence of micropores. The samples obtained at room temperature show N2 isotherms similar to those from metakaolins. This treatment did not significantly modify the structure of the metakaolin and the porous properties of these samples are very close to those of the parent metakaolin. The samples obtained under reflux conditions for 6h show nitrogen gas adsorption-desorption isotherms different from those of the parent metakaolins. They show an increase of adsorption at low relative pressures and reach a plateau at intermediate values of P/Po. This kind of isotherm is classified as type I (BDDT classification, [27]) and it is characteristic of microporous materials. For treatment times higher than 6 h, the isotherms are analogous to those of metakaolins, classified as type II [27], which indicates the loss of the microporosity formed at lower times. 

Nitrogen adsorption-desorption isotherms measured for the studied samples were presented previously [1,2]. In all cases isotherms are a combination of type I for low pressure and type II for high pressure according to BDDT classification [5] and r-plots show a downward deviation, which is characteristic of micropores [5]. 

Figure la shows the nitrogen adsorption-desorption isotherms of the two pure materials as well as mixture X(50). According to BDDT classification, the adsorption-desorption isotherm obtained on the pure alumina sample, X(0), is of type IV with a type E hysteresis loop. This kind of isotherm is typical of a mesoporous material with cylindrical pores closed at one end. In contrast, the adsorption-desorption isotherm obtained on the pure Si-Ti co-gel, X(IOO), is of type I without hysteresis loop, which corresponds to an exclusively microporous material [ ] ... 

All the adsorption isotherms are type I on the BDDT classification [7], indicating that they are mainly microporous materials. 

As discussed in the previous section, the distribution between the adsorbate phase and the adsorbed phase can be described by an adsorption model, known as adsorption isotherms. Based on experimental data reported in the literature, Brunauer et al. [5] divided adsorption isotherms into five different types which are shown in Fig. 3.1 (BDDT classification). The first two types are by far the most frequently encountered in adsorption systems. The Type I isotherm is the well-known Langmuir isotherm which will be discussed in the next section. The... 

Fig. 3.1. The BDDT classification of the five types of adsorption isotherm.

Figure 3.1 Schematic representation of adsorption isotherms of (a) ii and (b) iV types according to BDDT classification.

The nitrogen adsorption isotherm of the carbon nanofibers (Fig. 2) is of Type II in the BDDT classification, which is normally obtained for carbons, which are predominantly mesoporous. The BET surface area (calculated with the data between 0.05 < p/po < 0.2) equals 91 m g and the BJH mesopore surface area equals 79 m g. The generd view is that a low surface area of the support would limit the metal loading if the purpose is to finely disperse the metal catalyst [22]. As was discussed in the introduetion the highly mesoporous structure of the carbon nanofibers significantly reduces diffusion limitations. 

Figure 3.3-5 BDDT classification of five isotherm shapes...

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... 

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