Hysteresis, adsorption types

Another theory of adsorption hysteresis considers that there are two types of pores present, each having a size distribution. The first type are V-shaped, and these fill and empty reversibly. The second type have a narrow neck and a relatively wide interior. These ink-bottle pores are supposed to fill completely when a plp0 value corresponding to the relatively wide pore interior is reached, but once filled they retain their contents until plp0 is reduced to a value corresponding to the relatively small width of the pore neck. 

Let us examine one more simple three-step mechanism whose steady-state characteristics are also of the hysteresis type. In what follows we will show that their type differs considerably from the previous one. It is the mechanism including steps of "consecutive adsorption one gas-phase substance is adsorbed on unoccupied sites and is then joined by a second gaseous substance, whereupon the two intermediates interact. In the general form this... 

Accepting that the effects of various factors on adsorption hysteresis are not fully understood, experience has shown that there is probably a certain correlation between the structures of the pores and the shapes of hysteresis loops. Type HI has been identified with particles, from other evidence known to... 

In the following discussion we will consider the application of percolation theory to describing desorption of condensate from porous solids. In Sections III,A-III,C we briefly recall types of adsorption isotherms, types of hysteresis loops, and the Kelvin equation. The matter presented in these sections is treated in more detail in any textbook on adsorption [see, e.g., the excellent monographs written by Gregg and Sing (6) and by Lowell and Shields (49) Sections III,D-III,H are directly connected with percolation theory. In particular, general equations interpreting the hysteresis loop are... 

The type II isotherm is associated with solids with no apparent porosity or macropores (pore size > 50 nm). The adsorption phenomenon involved is interpreted in terms of single-layer adsorption up to an inversion point B, followed by a multi-layer type adsorption. The type IV isotherm is characteristic of solids with mesopores (2 nm < pore size < 50 nm). It has a hysteresis loop reflecting a capillary condensation type phenomenon. A phase transition occurs during which, under the eflcct of interactions with the surface of the solid, the gas phase abruptly condenses in the pore, accompanied by the formation of a meniscus at the liquid-gas interface. Modelling of this phenomenon, in the form of semi-empirical equations (BJH, Kelvin), can be used to ascertain the pore size distribution (cf. Paragr. 1.1.3.2). 

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). 

A selection of adsorption isotherms types (according to the lUPAC classification of physisorption isotherms) are schematically shown in Figure 1, where the adsorbed amount n is plotted against the relative pressure p/po of the adsorptive gas. Type I isotherms are typical for microporous materials, where the total pore volume of the adsorbent determines the saturation value. Reversible isotherms of type II are obtained for nonporous or macroporous materials, whereas type IV isotherms showing a hysteresis loop are characteristic of mesoporous materials, such as many practical catalytic materials. If the knee at point B of isotherm types II and IV is sufficiently sharp, the uptake at point B can be considered as the monolayer capacity of the material and its specific surface area can then be calculated assuming the formation of a close-packed monolayer of the test gas, provided its molecular area is known. For N2 the standard molecular area is 0.162 nm2. 

The whole adsorption / desorption isotherms at 300 K, 350 K, 400 K, 500 K, and 650 K are plotted in Fig 3 Oefi panel) (normalized to the sur ce area of the substrate exposed to water vapor, in pmol / m ). The result is given as a function of the chemicai potentiai of water imposed by the GCMC simulation instead of pressure to avoid the introduction of uncertainties in conversion. It is important to note that this chemical mtential does not contain the ideal rotational contribution -kT(-4.09+3/2ln1), witii T in Kelvin. The high temperature (650 K) isotherm is reversible (supercritical), whereas the low temperature cases present large hysteresis (type IV isotherm in tiie lUPAC classification). The steep rises are associated to the capillary condensation of water in tiie mesoporosity of the Vycor ass. These simulation results are in qualitative agreement with experimental data, witii an asymmetric adsorption / desorption hysteresis characteristic of disordered and intercormected pores. ... 

The reversible Type Ila isotherm is the normal form of nitrogen isotherm given by a non-porous or macroporous adsorbent and is indicative of unrestricted monolayer-multilayer adsorption. The adsorption branch of a Type Ilb isotherm appears to have the same characteristic Type II shape as a normal monolayer-multilayer isotherm, but the multilayer section of the desorption branch is quite different -giving rise to a form of adsorption hysteresis. Isotherms of this type are generally given by aggregates of platy particles or solids containing slit-shaped mesopores. 

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. 

Comparison of some adsorption hysteresis cycles, calculated from twofold distributions, for several types of porous structures. 

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. 

As also noted in the preceding chapter, it is customary to divide adsorption into two broad classes, namely, physical adsorption and chemisorption. Physical adsorption equilibrium is very rapid in attainment (except when limited by mass transport rates in the gas phase or within a porous adsorbent) and is reversible, the adsorbate being removable without change by lowering the pressure (there may be hysteresis in the case of a porous solid). It is supposed that this type of adsorption occurs as a result of the same type of relatively nonspecific intermolecular forces that are responsible for the condensation of a vapor to a liquid, and in physical adsorption the heat of adsorption should be in the range of heats of condensation. Physical adsorption is usually important only for gases below their critical temperature, that is, for vapors. 

Below the critical temperature of the adsorbate, adsorption is generally multilayer in type, and the presence of pores may have the effect not only of limiting the possible number of layers of adsorbate (see Eq. XVII-65) but also of introducing capillary condensation phenomena. A wide range of porous adsorbents is now involved and usually having a broad distribution of pore sizes and shapes, unlike the zeolites. The most general characteristic of such adsorption systems is that of hysteresis as illustrated in Fig. XVII-27 and, more gener-... 

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

A characteristic feature of a Type IV isotherm is its hysteresis loop. The exact shape of the loop varies from one adsorption system to another, but, as indicated in Fig. 3.1, the amount adsorbed is always greater at any given relative pressure along the desorption branch FJD than along the adsorption branch DEF. The loop is reproducible provided that the desorption run is started from a point beyond F which marks the upper limit of the loop. 

Closer examination reveals that the swing upwards in the Type IV isotherm not infrequently commences before the loop inception, showing that enhanced adsorption, not accompanied by hysteresis, can occur. The implications of this important fact are explored in Section 3.7. 

Fig. 3.28 The Kiselev method for calculation of specific surface from the Type IV isotherm of a compact of alumina powder prepared at 64 ton in". (a) Plot of log, (p7p) against n (showing the upper (n,) and lower (n,) limits of the hysteresis loop) for (i) the desorption branch, and (ii) the adsorption branch of the loop. Values of. 4(des) and /4(ads) are obtained from the area under curves (i) or (ii) respectively, between the limits II, and n,. (6) The relevant part of the isotherm.

Low-pressure hysteresis is not confined to Type I isotherms, however, and is frequently superimposed on the conventional hysteresis loop of the Type IV isotherm. In the region below the shoulder of the hysteresis loop the desorption branch runs parallel to the adsorption curve, as in Fig. 4.26, and in Fig. 4.2S(fi) and (d). It is usually found that the low-pressure hysteresis does not appear unless the desorption run commences from a relative pressure which is above some threshold value. In the study of butane adsorbed on powdered graphite referred to in Fig. 3.23, for example, the isotherm was reversible so long as the relative pressure was confined to the branch below the shoulder F. 

The adsorption of water on a fully hydroxylated silica involves hydrogen bonding but is essentially physical in nature and is completely reversible in the low pressure range the isotherm is of Type II on a nonporous sample (Fig. 5.17(a)), and of Type IV, with no low-pressure hysteresis, on a porous sample (Fig. 5.18). 

The effect of these factors on the adsorption isotherm may be elucidated by reference to specific examples. In the case of the isotherm of Fig. 5.17(a), the nonporous silica had not been re-heated after preparation, but had been exposed to near-saturated water vapour to ensure complete hydroxylation. The isotherm is of Type II and is completely reversible. On the sample outgassed at 1000°C (Fig. 5.17(h)) the isotherm is quite different the adsorption branch is very close to Type III, and there is extrensive hysteresis extending over the whole isotherm, with considerable retention of adsorbate on outgassing at 25°C at the end of the run. 

The first stage in the interpretation of a physisorption isotherm is to identify the isotherm type and hence the nature of the adsorption process(es) monolayer-multilayer adsorption, capillary condensation or micropore filling. If the isotherm exhibits low-pressure hysteresis (i.e. at p/p° < 0 4, with nitrogen at 77 K) the technique should be checked to establish the degree of accuracy and reproducibility of the measurements. In certain cases it is possible to relate the hysteresis loop to the morphology of the adsorbent (e.g. a Type B loop can be associated with slit-shaped pores or platey particles). 

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. 

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