Adsorption Brunauer classification

Brunauer classification, 1 591 and column performance, 1 604—606 gas adsorption, 1 622-623, 626-629 of nonionic surfactants, 24 142—143 predicting, 24 139-140 Adsorption kinetics... 

Brucine, 2 74 Brucite, 15 321, 323, 399 reserves of, 15 322 Brugnatelli, Luigi, 9 760 Brunauer classification of adsorption isotherms, 1 591... 

Adsorption of Cyclohexane. The adsorption of cyclohexane was determined on synthetic lepidocrocite and its decomposition products prepared by heating it in vacuo for varying intervals of time at 190°, 300°, 400°, and 500°, and also on synthetic goethite and its decomposition products obtained in a similar manner by heating at 150°, 180°, 250°, 300°, and 500°. The adsorption was found to be physical in nature, and the isotherms are Type II of the Brunauer classification (3) in all cases except on iron oxide prepared from goethite at 300° and 500° here the isotherms are Type III. This finding is in contrast to the Type IV isotherms common to ferric oxide gels the difference may be due to the crystalline nature of the parent material. 

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. 

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 complete nitrogen isotherms of dealuminated Y zeolites are reported in Fig. 1. The curve of the parent H-Y zeolite corresponded to type I in the Brunauer classification, which was typical for the crystalline microporous materials [17]. As expected, the starting material showed no evidence of mesopores. Fig. 1 shows that the AHFS-treated samples with dealumination levels equal or lower than 50% were characterised by a very flat adsorption-desorption isotherm with nearly no hysteresis loop [18]. 

The adsorption isotherms from GCMC simulation are shown in figure 1 as plots of absolute adsorbate density versus fiigacity. They are all of type I in the Brunauer classification, showing a... 

Adsorption equilibria determine the thermodynamic limits of the specific amounts of adsorption (mol/g) of a pure gas or the components of a fluid mixture (gas or liquid) under a given set of conditions [pressure (P), temperature (T), and mole function (y or Xi) of component /] of the bulk fluid phase. The simplest way to describe adsorption equilibria of pure gas i is in the form of adsorption isotherms where the amount adsorbed (n ) is plotted as a function of gas pressure (P) at a constant temperature (P). The pure gas adsorption isotherms can have various shapes (Types I-V) by Brunauer classification depending on the porosity of the adsorbent (microporous, mesoporous, or nonpo-rous) and the system temperature (below or above the critical temperature of the adsorbate). However, the most common isotherm shape is Type I, which is depicted by most microporous adsorbents of practical use. These isotherms exhibit a linear section in the very low-pressure region (Henry s law region) where the amount adsorbed is proportional to the gas pressure [ n ) = KiP]. The proportionality constant is called... 

The isotherms for adsorption of pure water vapour on activated aluminas are typically Type I (microporous) or Type IV (mesoporous) in shape according to the Brunauer classification [6]. Figure 1 shows several examples. Alcoa F-1 alumina has a type I shape while Alcan AA-300 and Alcoa H-156 exhibit type IV shapes. The plots represent the specific amount of water vapour adsorbed (n, g/g) as functions of the relative vapour pressure of water (x = P/P ) at 30°C. P(atm) is the water vapour pressure over the adsorbent... 

Figure 22.5(a) shows the variety of isotherms (Type I by Brunauer classification [19]) available for adsorption of pure CO2 on the activated carbons of Table 22.3 and on 5A zeolite [18]. Table 22.4 gives the corresponding Henry s law constants and selectivities for adsorption of CO2 and H2 at 303 K as well as the isosteric heats of adsorption of pure CO2 in the Henry s law regions [18]. These data show that (a) the strength of CO2 adsorption and the coadsorption of Hj from CO2 + H2 mixtures can vary significantly, and (b) the adsorption of COj on the 5A zeolite is too strong to be useful in a PSA process. 

Even limiting the goal to a purely numerical exercise, the study of the behaviours of eq. (51) is manifestly a difficult task, because of the numerous parameters it contains. Such an analysis, carried out in a preliminary form in ref. [74], shows however that eq. (51) is able to account for the Brunauer classification of adsorption isotherms. 

Figure 5. How the C M equation accounts for Brunauer classification of adsorption isotherms.

Figure 4 shows a type of isotherm shape that is seen with crystalhne ion exchangers such as molecular sieves and clay minerals, but is nevertheless relatively uncommon. The shape resembles the type II vapour adsorption isotherm of the Brunauer classification, having a clear plateau region and inflexion point. An example is the Na/K exchange in zeohte P [48] that was found to be reversible over the whole range of equivalent fraction of potassium in the crystal Zeohte P has the gismondine-type structvue (CIS [49]). More commonly, isotherms of this type are found to be partiaUy irreversible in the plateau... 

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.

The advantage of equation 17.14 is that it may be fitted to all known shapes of adsorption isotherm. In 1938, a classification of isotherms was proposed which consisted of the five shapes shown in Figure 17.5 which is taken from the work of Brunauer et alSu Only gas-solid systems provide examples of all the shapes, and not all occur frequently. It is not possible to predict the shape of an isotherm for a given system, although it has been observed that some shapes are often associated with a particular adsorbent or adsorbate properties. Charcoal, with pores just a few molecules in diameter, almost always gives a Type I isotherm. A non-porous solid is likely to give a Type II isotherm. If the cohesive forces between adsorbate molecules are greater than the adhesive forces between adsorbate and adsorbent, a Type V isotherm is likely to be obtained for a porous adsorbent and a Type III isotherm for a non-porous one. 

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 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. 

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. 

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... 

We studied the surface characteristics of palygorskite and its organocomplexes first by nitrogen adsorption at standard temperature and pressure (STP). The isotherms are shown in Fig. 15. According to Brunauer s, Emmet s and Teller s classification, the isotherm determined on palygorskite is of type IV and exhibits adsorption hysteresis. An... 

The classification of the low concentration region of the sample isotherm into linear, concave, and convex isotherms has been discussed (Section 2-4). For adsorption of a compound from the gas phase, Brunauer et al. (30) have described the five basic isotherm types shown in Fig. 3-5. Type I isotherms occur when monolayer adsorption is distinctly favored over multilayer adsorption. After an initial rapid uptake of adsorbate by the adsorbent, the surface monolayer is completed and further adsorption does not occur. If additional layers begin to adsorb prior to completion of the first monolayer, isotherms of type II result. These are typical of physical adsorption. Type III isotherms are less common and occur in adsorption systems where the attraction between adsorbed molecules are strong and adsorbent-adsorbate interactions are either relatively weak or are independent of surface coverage (e.g., uniform surface graphites). As a result the total attraction of an adsorbed molecule... 

The analysis of physical adsorption in general, and that used to approach this particular problem, derives from a classification later summarized by Brunauer [S. Brunauer, The Adsorption of Gases and Vapors, Princeton University Press, Princeton, NJ, (1945)]. He classified the isotherms for physical adsorption of gases on surfaces into five general types, as shown in Figure 3.7. Isotherm I is already... 

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.

Physical adsorption is the basis for the various techniques to measure surface area of ceramic powders. The surface area is determined in terms of the amount of the gas adsorbed by a given mass of solid powder at a given temperature, under different gas pressures p. In practice, gases with a fixed volume are used for the powder, so that the amount of gas adsorbed can be identified according to the decrease in pressure of the gas. The amount of gas adsorbed versus p, or p/po, when the gas is at pressures below its saturation vapor pressure po, can be plotted as a graph, which is known as the adsorption isotherm. Figure 4.3 shows the types of these isotherms, according to Brunauer, Emmett and Teller (BET) classification [35-38]. The Type VI isotherm is called stepped isotherm, which is relatively rarely observed, but has special theoretical interest. This isotherm offers the possibility to determine the monolayer capacity of a solid, which is defined as the amount of gas that is required to cover the surface of the unit mass of the solid with a monolayer, so as to calculate the specific surface area of the solid. 

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