Adsorption hysteresis classification

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... 

Classification and Modeling. Adsorption isotherms are classified by their shape, which also involves different sorption mechanisms. The isotherm classification of lUPAC (3) has been recently extended by Rouquerol and coworkers (4), subdividing types I, II, and IV (Fig. 4). The deviation of the adsorption and desorption curves (adsorption hysteresis) is associated with capillary condensation in mesopores. The shape of hysteresis loops (Fig. 5) also provides information about the texture of the mesoporous sorbent, including the geometry, size distribution, connectivity, and so on of the pores (3). 

The classification of adsorption hysteresis loops has been always stated In terms of the appearance of these curves, e.g. their shape or extension. Among the most Important classifications, that of de Boer (ref. I) is based on a combination between the steep or sloping character of the adsorption and desorption branches, while Everett s classification (ref. 2) emphasizes the extent of the region of relative pressures at which hysteresis occurs. A classification adopted by the lUPAC (ref. 3) considers four types of loops, which are Identified according to the slope of the boundary curves. It lias been Intended, a posteriori, to relate these shapes of hysteresis loops to some processes of filling by capillary condensate or evaporation of the liquid held in a pore, and in order to justify the existence of these mechanisms, several models of the pore geometry have been considered. 

The presence of adsorption hysteresis is the special feature of all adsorbents with a mesopore structure. The adsorption and desorption isotherms differ appreciably from one another and form a closed hysteresis loop. According to the lUPAC classification four main types of hysteresis loops can be distinguished HI, H2, H3 and H4 (ref. l). Experimental adsorption and desorption isotherms in the hysteresis region provide information for calculating the structural characteristics of porous materials-porosity, surface area and pore size distribution. Traditional methods for such calculations are based on the assumption of an unrelated system of pores of simple form, as a rule, cylindrical capillaries. The calculations are based on either the adsorption or the desorption isotherm, ignoring the existence of hysteresis in the adsorption process. This leads to two different pore size distributions. The question of which of these is to be preferred has been the subject of unending discussion. In this report a statistical theory of capillary hysteresis phenomena in porous media has been developed. The analysis is based on percolation theory and pore space networks models, which are widely used for the modeling of such processes by many authors (refs. 2-10). The new percolation methods for porous structure parameters computation are also proposed. 

The basis of the classification is that each of the size ranges corresponds to characteristic adsorption effects as manifested in the isotherm. In micropores, the interaction potential is significantly higher than in wider pores owing to the proximity of the walls, and the amount adsorbed (at a given relative pressure) is correspondingly enhanced. In mesopores, capillary condensation, with its characteristic hysteresis loop, takes place. In the macropore range the pores are so wide that it is virtually impossible to map out the isotherm in detail because the relative pressures are so close to unity. 

A new classification of hysteresis loops, as recommended in the lUPAC manual, consists of the four types shown in the Figure below. To avoid confusion with the original de Boer classification (p. 117), the characteristic types are now designated HI, H2, H3 and H4 but it is evident that the first three types correspond to types A, E and B, respectively, in the original classification. It will be noted that HI and H4 represent extreme types in the former the adsorption and desorption branches are almost vertical and nearly parallel over an appreciable range of gas uptake, whereas in the latter they are nearly horizontal and parallel over a wide range of relative pressure. Types H2 and H3 may be regarded as intermediate between the two extremes. 

Table 16-4 shows the IUPAC classification of pores by size. Micropores are small enough that a molecule is attracted to both of the opposing walls forming the pore. The potential energy functions for these walls superimpose to create a deep well, and strong adsorption results. Hysteresis is generally not observed. (However, water vapor adsorbed in the micropores of activated carbon shows a large hysteresis loop, and the desorption branch is sometimes used with the Kelvin equation to determine the pore size distribution.) Capillary condensation occurs in mesopores and a hysteresis loop is typically found. Macropores form important paths for molecules to diffuse into a par-... 

Figure 3. Classification of adsorption/desorption hysteresis loops.

In the original IUPAC classification, the hysteresis loop was said to be a characteristic feature of a Type IV isotherm. It is now evident that this statement must be revised. Moreover, we can distinguish between two characteristic types of hysteresis loops. In the first case (a Type HI loop), the loop is relatively narrow, the adsorption and desorption branches being almost vertical and nearly parallel in the second case (a Type H2 loop), the loop is broad, the desorption branch being much steeper than the adsorption branch. These isotherms are illustrated in Figure 13.1 as Type IVa and Type IVb, respectively. Generally, the location of the adsorption branch of a Type IVa isotherm is governed by delayed condensation, whereas the steep desorption branch of a Type IVb isotherm is dependent on network-percolation effects. 

In fig. 1.31 the lUPAC-recommended classification is given for the most common types of hysteresis loops they are refinements of the general type IV in fig. 1.13. In practice, a wide variety of shapes may be encountered of which types HI and H4 are the extremes. In the former, the two branches are almost vertical and parallel over an appreciable range of V, whereas in the latter they remain more or less horizontal over a wide range of p/p(sat). Types H2 and H3 are intermediates. Many hysteresis loops have in common that the steep range of the desorption branch leads to a closure point that is almost independent of the nature of the porous sorbent and only depends on the temperature and the nature of the adsorptive. For example, it is at p/p(sat) = 0.42 for nitrogen at its boiling point (77 K) and at p/p(sat) = 0.28 for benzene at 25°C. 

The second set of data (Fig. 1, b) concerns a family of pillared clays Ali-xFe/)yVi ih systematically varied Al and Fe contents [16], The sorption data of the pillared clays samples exhibit at low pressure a high adsorption step and at higher relative pressure an H4-type (lUPAC classification) hysteresis loop that is transformed into an H2-type loop as the Fe content is being increased. Details of the preparation procedure and the results from the application of other characterization tests are provided in [16]. 

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. 

The texture properties of the ultrathin porous glass membranes prepared in our laboratory were initially characterized by the equilibrium based methods nitrogen gas adsorption and mercury porosimetry. The nitrogen sorption isotherms of two membranes are shown in Fig. 1. The fully reversible isotherm of the membrane in Fig. 1 (A) can be classified as a type I isotherm according to the lUPAC nomenclature which is characteristic for microporous materials. The membrane in Fig. 1 (B) shows a typical type IV isotherm shape with hysteresis of type FIl (lUPAC classification). This indicates the presence of fairly uniform mesopores. The texture characteristics of selected porous glass membranes are summarized in Tab. 1. The variable texture demanded the application of various characterization techniques and methods of evaluation. 

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 [ ] ... 

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]. 

For adsorbents with sufficiently laig e porosities (often referred to as mesoporous systems) isotherms can exhibit hysteresis between the adsorption and desorption branches as illustrated schematically in figure 1. A classification of the kinds of hysteresis loops has also been made. It is generally accepted that such behavior is related to the occurrence of capillary condensation - a phenomenon whereby the low density adsorbate condenses to a liquid like phase at a chemical potential (or bulk pressure) lower than that corresponding to bulk saturation. However, the exact relationship between the hysteresis loops and the capillary phase transition is not fully understood - especially for materials where adsorption cannot be described in terms of single pore behavior. 

Figure 1. An adsorption/desorption isotherm of density vs. relative pressure showing a hysteresis loop of type H2 in the lUPAC classification (Sing et al., 1985).

The adsorption isotherm of nitrogen (77 K) for the porous silica is shown in Figure 2. This has a type IV character in the lUPAC classification [17], exhibiting a hysteresis loop with a shape corresponding to a capillary condensation process in cylindrical pores [18]. The... 

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. 

Organically bridged ditin hexaalkynides 2 are therefore useful sol-gel precursors of mesoporous (or nanoporous) tin dioxide materials. Indeed, for each sample studied, theN2 adsorption-desorption isotherm is a type IV isotherm with a type H2 hysteresis loop, which is typical of mesoporous solids, according to the lUPAC classification (Figure 3.2.9). ... 

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