Adsorption hysteresis isotherms

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 swelling of the adsorbent can be directly demonstrated as in the experiments of Fig. 4.27 where the solid was a compact made from coal powder and the adsorbate was n-butane. (Closely similar results were obtained with ethyl chloride.) Simultaneous measurements of linear expansion, amount adsorbed and electrical conductivity were made, and as is seen the three resultant isotherms are very similar the hysteresis in adsorption in Fig. 4.27(a), is associated with a corresponding hysteresis in swelling in (h) and in electrical conductivity in (c). The decrease in conductivity in (c) clearly points to an irreversible opening-up of interparticulate junctions this would produce narrow gaps which would function as constrictions in micropores and would thus lead to adsorption hysteresis (cf. Section 4.S). 

Nitrogen adsorption/desorption isotherms of all the activated carbons are of Type I, i.e. characteristic of basically microporous solids. There is a lack of adsorption/desorption hysteresis. More careful analysis permits to notice significant differences in the porous texture parameters depending on precursor origin. 

Figure 2 shows us the N2 adsorption-desorption isotherm of Beta/montmorillonite composite. At low relative pressure a sharp adsorption of nitrogen indicates the existence of large amount of micropore. The hysteresis shown in figure 2 is ascribed to type H4 which usually can be observed on layered clay and other materials [2], It is obvious that part of the pore structure in montmorillonite is still preserved after calcination under high temperature and the following hydrothermal crystallization. 

Adsorption of water by cellulose displays hysteresis. The adsorption isotherm is not identical to the desorption isotherm and the amount of adsorbed water in equilibrium with the atmosphere at a particular relative humidity is higher during desorption from a higher humidity than during adsorption from a lower humidity. A plot of the adsorption/desorption isotherm is shown in Figure 5.4. 

The shape of the hysteresis loop in the adsorption/desorption isotherms provides information about the nature of the pores. The loops have been classified according to shape as A, B and E (De Boer, 1958) or as HI - H4 by lUPAC (Sing et al, 1985). Ideally, the different loop shapes correspond to cylindrical, slit shaped and ink-bottle pores the loops in the isotherm IV and V of Figure 5.3 correspond to cylindrical pores. Wide loops indicate a broad pore size distribution (for an example see Fig. 14.9). The absence of such a loop may mean that the sample is either nonporous or microporous. These generalizations provide some initial assistance in assessing the porosity of a sample. In fact the adsorption/desorption isotherms are often more complicated than those shown in Figure 5.3 owing to a mixture of pore types and/or to a wide pore size distribution. 

Fig. 4 Nitrogen adsorption-desorption isotherms at 77 K of heat-treated NH4 -exchanged MSU-Ge-2 (solid circles, adsorption data open circles, desorption data). The hysteresis observed at P/Po > 0.8 is due to the voids between the agglomerated particles. (Inset) BJH pore size distribution calculated from the adsorption branch of the isotherm...

Nitrogen adsorption/desorption isotherm for calcined alumina sphere (Figure 4b) is a type IV with a large hysteresis. A steep increasing occurs in the isothem curve at a relative pressure 0.55

narrow pore size distribution at the mean value of 10.0 nm. Calcined alumina sphere has BET surface area of 360 m2/g and pore volume of 0.62 cm3/g. 

The N2 adsorption /desorption isotherms (Figure 2) for calcined STO shows abrupt step at P/Po = 0.1—0.4 and no obvious hysteresis loop at low relative pressure, indicating the pore size of this materials is uniform. The BET specific surface area and BJH pore diameter are 934 m2/g and 4.3nm, respectively. 

N2 adsorption-desorption isotherms and pore size distribution of sample II-IV are shown in Fig. 4. Its isotherm in Fig. 4a corresponds to a reversible type IV isotherm which is typical for mesoporous solids. Two definite steps occur at p/po = 0.18, and 0.3, which indicates the filling of the bimodal mesopores. Using the BJH procedure with the desorption isotherm, the pore diameter in Fig. 4a is approximately 1.74, and 2.5 nm. Furthermore, with the increasing of synthesis time, the isotherm in Fig. 4c presents the silicalite-1 material related to a reversible type I isotherm and mesoporous solids related to type IV isotherm, simultaneously. These isotherms reveals the gradual transition from type IV to type I. In addition, with the increase of microwave irradiation time, Fig. 4c shows a hysteresis loop indicating a partial disintegration of the mesopore structure. These results seem to show a gradual transformation... 

The adsorption isotherm of N, on FSM-16 at 77 K had an explicit hysteresis. As to the adsorption hysteresis of N-, on regular mesoporous silica, the dependencies of adsorption hysteresis on the pore width and adsorbate were observed the adsorption hysteresis can be observed for pores of w 4.0nm. The reason has been studied by several approaches [5-8]. The adsorption isotherm of acetonitrile on FSM-16 at 303K is shown in Fig. 1. The adsorption isotherm has a clear hysteresis the adsorption and desorption branches close at PIP, = 0.38. The presence of the adsorption hysteresis coincides with the anticipation of the classical capillary condensation theory for the cylindrical pores whose both ends are open. The value of the BET monolayer capacity, nm, for acetonitrile was 3.9 mmol g. By assuming the surface area from the nitrogen isotherm to be available for the adsorption of acetonitrile, the apparent molecular area, am, of adsorbed acetonitrile can be obtained from nm. The value of am for adsorbed acetonitrile (0.35 nnr) was quite different from the value (0.22 nm2) from the liquid density under the assumption of the close packing. Acetonitrile molecules on the mesopore surface are packed more loosely than the close packing. The later IR data will show that acetonitrile molecules are adsorbed on the surface hydroxyls in... 

Figure 1 shows the adsorption-desorption isotherms of AC-ref, SC-100 and SC-155 materials, showing all of them a hysteresis loop that corresponds to the H3 type [19], and formed by very wide pores having narrow short openings, or by pores formed by parallel... 

Fleisch et al. (1984) measured the catalyst surface area and pore volume changes that occurred after severe deactivation of a 100- to 150-A pore catalyst. The results of these measurements are shown in Table XXVIII for various positions in the reactor bed. Catalyst surface area and pore volume are substantially reduced in the top of the bed due to the concentrated buildup of metals in this region. The pore volume distribution of Fig. 44 reveals the selective loss of the larger pores and an actual increase in smaller (<50-A) pores due to the buildup of deposits and constriction of the larger pores. Fleisch et al. (1984) also observed an increase in the hysteresis loop of the nitrogen adsorption-desorption isotherms between fresh and spent catalysts, which reflects the constrictions caused by pore... 

The capillary condensation theory provides a satisfactory explanation of the phenomenon of adsorption hysteresis, which is frequently observed for porous solids. Adsorption hysteresis is a term which is used when the desorption isotherm curve does not coincide with the adsorption isotherm curve (Figure 5.8). 

Although the BET equation is open to a great deal of criticism, because of the simplified adsorption model upon which it is based, it nevertheless fits many experimental multilayer adsorption isotherms particularly well at pressures between about 0.05 p0 and 0.35 pQ (within which range the monolayer capacity is usually reached). However, with porous solids (for which adsorption hysteresis is characteristic), or when point B on the isotherm (Figure 5.5) is not very well defined, the validity of values of Vm calculated using the BET equation is doubtful. 

Similar adsorption isotherms were obtained for samples PS and SPS, and for the lipase immobilized derivatives of silica gels (ADS, CB1, CB2, EN1, and EN2). Typical examples are shown in Fig. 2. We observed hysteresis loops in the adsorption-desorption isotherms, denoting materials with well-defined ordered structures, and the presence of mesopores, with hysteresis loops between isotherms types HI and H2 (8,16). Primarily on the mesopores of silica, a multilayer of adsorbate is formed, increasing the relative pressure, and depending on the mean pore diameter, at P/P0 0.4, capillary condensation takes place on the multilayer, resulting in a further increase in pore volume (1). 

Hysteresis in the adsorption-desorption isotherms (Fig. 4) is a common observation for supports with a large fraction of small pores. It results from desorption from the meniscus at the end of a filled pore. The vapor pressure above the liquid at the pore mouth defines the pore radius in the Kelvin equation. Therefore, it is the desorption branch of the isotherm that is preferred in calculations of pore size distributions. 

The fluorinated carbon-coated AAO film has an interesting adsorption characteristic that has not been reported so far. Figure 3.12 shows N2 adsorption/desorption isotherms at -196°C for the pristine carbon-coated AAO film and the films fluorinated at different temperatures [119]. The isotherm of the pristine film is characterized by the presence of a sharp rise and a hysteresis in a high relative pressure range. Such a steep increase can be ascribed to the capillary condensation of N2 gas into the nanochannels of the AAO films, that is, the inner space of the nanotubes embedded in the AAO films. The amount of N2 adsorbed by the condensation into the fluorinated channels is lower than that of the pristine one. Moreover, the amount drastically decreases with an increase in the severity of fluorination. Since TEM observation revealed that the inner structure of the fluorinated CNTs was not different from that of the pristine nanotubes, the reason why the N2 isotherm was so changed as in Figure 3.12 cannot be attributed to the alteration of the pore texture upon the... 

The H20 adsorption-desorption isotherms of the above two carbons are plotted in Figure 3.22. Their isotherms are of type-V, and the shape is characterized by a sharp adsorption uptake accompanied by a clear hysteresis occurring over a medium relative pressure (P/P0) range. Such characteristics have often been observed in H20 isotherms of microporous carbons such as ACF [149,150], Mowla et al. found that the width of the hysteresis loop in H20 isotherms for microporous carbons depends on their pore size no hysteresis is observed for carbons with a pore size of less than 0.8 nm, but a wide loop exists for carbons having a larger pore size [151]. The latter is indeed the case for the present carbon samples. 

Figure 5 presents the N2 adsorption/desorption isotherms for calcined (650 °C) MSU-G silicas assembled from C H2 +,NH(CH2)2NH2 surfactants with n = 10, 12, and 14. The inset to the figure provides the framework pore distributions. The maxima in the Horvath-Kawazoe pore size distributions increase in the order 2.7, 3.2, 4.0 nm as the surfactant chain length increases. The textural porosity evident from the hysteresis loop at P/P0... 

The terms adsorption and desorption are often used to indicate the direction from which the equilibrium states have been approached. Adsorption hysteresis arises when the amount adsorbed is not brought to the same level by the adsorption and desorption approach to a given equilibrium pressure or bulk concentration. The relation, at constant temperature, between the amount adsorbed and the equilibrium pressure, or concentration, is known as the adsorption isotherm. 

A clearer picture of the sorption of water vapour by montmorillonite was obtained by Cases et al. (1992). Their adsorption-desorption isotherms of water on sodium montmorillonite are shown in Figure 11.7. The wavy nature of the adsorption and desorption branches (At and Dlr respectively) of the full hysteresis loop in Figure 11.7 is evidently similar to that of the water isotherm in Figure 11.6 and is indicative of a complex mechanism. However, it was established that the scale of the hysteresis loop depended on the maximum relative pressure reached before the pressure was reduced. This dependency is illustrated by the appearance of the partial sorption isotherms also plotted in Figure 11.7. Here, a small hysteresis loop (desorption branch D3) was the result of (p/p°)ma < 0.25, in contrast to much larger loop (desorption branch D2) when the adsorption was taken to (p/p°)max = 0.35. 

The more recent adsorption-desorption isotherms of some organic vapours of different molecular diameter are shown in Figure 12.15. These measurements were made on different samples of VPI-5, each having been outgassed for 16 hours at 673 K. Although essentially of Type I, the isotherms reveal some degree of thermodynamic irreversibility with hysteresis extending back to very low p/p°. Evidently,... 

There are many aggregated powders (clays, pigments, cements, etc.) that appear to give normal Type II adsorption isotherms, although their full adsorption-desorption isotherms exhibit Type H3 hysteresis. However, unlike Type IV isotherms, there is no plateau at high pjp°- These isotherms are now termed Type lib, as indicated in Figure 13.1. 

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