Type I isotherms

When a solid contains very fine micropores that have pore dimensions only a few molecular diameters, the potential field of force from the neighboring walls of the pores will overlap causing an increase in the interaction energy between the solid surface and the gas molecules. This will result in an increase in adsorption, especially at low relative pressures. There is a possibility and considerable evidence that the 

The fact that the adsorption in Type 1 isotherms does not increase continuously as in lype II isotherms but attains a limiting value shown by the plateau is due to the pores being so narrow that they cannot accommodate more than a single molecular layer. The shape of the isotherm can be explained by the Langmuir model even though this model was derived for adsorption on an open surface or on a nonporous solid. 

FIGURE 2.14 Adsorption isotherms of nitrogen at 77 K on OX series activated carbons. (Source Parra, J.B., de Sousa, J.C., Bansal, R.C., Pis, J.J., and Pajares, J.J., Adsorption Sci. and TechnoL, 12, 51, 1995. With permission.) 

FIGURE 2.15 Five main types of adsorption isotherms. 

FIGURE 2.16 Adsorption isotherms of Ni(II) ions on different activated carbons. (Source Goyal, M., Rattan, V.K., and Bansal, R.C., Indian J. Chem. TechnoL, 6, 305, 1999. With 

In the simplest case, adsorption in a microporous solid leads to an isotherm of Type I consequently it is convenient to approach the subject by a discussion, from a classical standpoint, of Type I isotherms. 

Type I isotherms are characterized by a plateau which is nearly or quite horizontal, and which may cut the p/p° = 1 axis sharply or may show a tail as saturation pressure is approached (Fig. 4.1). The incidence of hysteresis varies many Type I isotherms exhibit no hysteresis at all (Fig. 4.1), others display a definite loop, and in others there is hysteresis which may or may not persist to the lowest pressures ( low-pressure hysteresis ) (Fig. 4.2). Type 1 isotherms are quite common, and are no longer restricted, as seemed at one time to be the case, to charcoals. Many solids, if suitably prepared, will yield Type 1 isotherms the xerogcls of silica, titania, alumina 

If relative pressure rather than pressure itself is used, the equation becomes 

In order to test the Langmuir isotherm against experimental data. Equation (4.1) may be rewritten in the form 

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This description is traditional, and some further comment is in order. The flat region of the type I isotherm has never been observed up to pressures approaching this type typically is observed in chemisorption, at pressures far below P. Types II and III approach the line asymptotically experimentally, such behavior is observed for adsorption on powdered samples, and the approach toward infinite film thickness is actually due to interparticle condensation [36] (see Section X-6B), although such behavior is expected even for adsorption on a flat surface if bulk liquid adsorbate wets the adsorbent. Types FV and V specifically refer to porous solids. There is a need to recognize at least the two additional isotherm types shown in Fig. XVII-8. These are two simple types possible for adsorption on a flat surface for the case where bulk liquid adsorbate rests on the adsorbent with a finite contact angle [37, 38]. 

Type 1 isotherms, as will be demonstrated in Chapter 4, are characteristic of microporous adsorbents. The detailed interpretation of such isotherms is controversial, but the majority of workers would probably agree that the very concept of the surface area of a microporous solid is of doubtful validity, and that whilst it is possible to obtain an estimate of the total micropore volume from a Type I isotherm, only the crudest guesses can be made as to the pore size distribution. 

Further evidence pointing in the same direction was provided by Pierce, Wiley and Smith, who found that on steam activation of a particular char at 900°C the saturation uptake increased three-fold, yet the isotherm was still of Type I. They argued that even if the width of the pores was only two molecular diameters before activation, it would increase, by removal of oxides, during the activation so that the second Type I isotherm would correspond to pores more than two molecular diameters wide. (The alternative explanation, that activation produced new pores of the same width as the old, seems unlikely.)... 

Evidence of a different kind is furnished by the fact that the Gurvitsch rule (p. 113) is often obeyed by systems showing Type I isotherms " the amounts of different adsorptives taken up by a given adsorbent, when expressed as a volume of liquid, agree within a few per cent. The order of agreement is illustrated by the typical examples in Table 4.1 for the adsorption of n-alkanes on ammonium phosphomolybdate, and in Table 4.2 which refers to a variety of adsorptives on a silica gel. It must be admitted, however, that there are cases where considerable deviations from the Gurvitsch mle are found, even though the isotherms are of Type 1. Thus, in Table 4.3 the variation in values of the saturation uptake is far outside... 

The uptake, expressed as a volume v, of liquid, of nitrogen and of butane by a number of coal samples, at a pressure slightly below saturation on the Type I isotherms ... 

It has already been noted (p. 195) that some Type I isotherms exhibit a kind of hysteresis which persists to the lowest pressures (cf. Fig. 4.2) some adsorbate is retained even after prolonged outgassing ( lO Torr) at the temperature of the isotherm determination, and can only be removed if the pumping is carried out at an elevated temperature. Further examples are shown in Fig. 4.25, as well as in Fig. 4.23. 

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. 

If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed (converted to a liquid volume) in the plateau region, since the mesopore volume and the external surface are both relatively small. In the more usual case where the Type I isotherm has a finite slope at high relative pressures, both the external area and the micropore volume can be evaluated by the a,-method provided that a standard isotherm on a suitable non-porous reference solid is available. Alternatively, the nonane pre-adsorption method may be used in appropriate cases to separate the processes of micropore filling and surface coverage. At present, however, there is no reliable procedure for the computation of micropore size distribution from a single isotherm but if the size extends down to micropores of molecular dimensions, adsorptive molecules of selected size can be employed as molecular probes. 

As pointed out earlier (Section 3.5), certain shapes of hysteresis loops are associated with specific pore structures. Thus, type HI loops are often obtained with agglomerates or compacts of spheroidal particles of fairly uniform size and array. Some corpuscular systems (e.g. certain silica gels) tend to give H2 loops, but in these cases the distribution of pore size and shape is not well defined. Types H3 and H4 have been obtained with adsorbents having slit-shaped pores or plate-like particles (in the case of H3). The Type I isotherm character associated with H4 is, of course, indicative of microporosity. 

Eig. 4. The Bmnaner classification of isotherms (I V). Langmuir Isotherm. Type I isotherms are commonly represented by the ideal Langmuir model ... 

Five general types of isotherms have been observed, and the shapes of these eharaeteristic isotherms are shown in Fig. 6 [29]. The Type I isotherm represents systems where only monolayer adsorption occurs, while Type 11 indicates the... 

Here the phenomenon of capillary pore condensation comes into play. The adsorption on an infinitely extended, microporous material is described by the Type I isotherm of Fig. 5.20. Here the plateau measures the internal volume of the micropores. For mesoporous materials, one will first observe the filling of a monolayer at relatively low pressures, as in a Type II isotherm, followed by build up of multilayers until capillary condensation sets in and puts a limit to the amount of gas that can be accommodated in the material. Removal of the gas from the pores will show a hysteresis effect the gas leaves the pores at lower equilibrium pressures than at which it entered, because capillary forces have to be overcome. This Type IV isotherm. 

NaY yields a compietely reversible type I isotherm, characteristic of micropore filling common in many zeolites. However, USY-B and DAY yield an isotherm close to type IV. Similar differences in adsorption isotherms were observed for n-hexane, cyclohexane, n-pentane and benzene. Furthermore, many of the isotherms measured on DAY zeolites showed hysteresis loops (Figure 6). 

Adsorption Properties Type IV Isotherm Type IV Isotherm Type I 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. 

One of the most widely used methods for determining the pore size and surface area of zeolites is nitrogen physisorphon. From the shape of the nitrogen adsorption and desorption isotherm the presence and shape of the mesopores can be deduced. As shown in Figure 4.41 a faujasite without mesopores have a type I isotherm since the micropores fiU and empty reversibly, while the presence of mesopores results in a combination of type I and IV isotherms. The existence of a hysteresis loop in the isotherms indicates the presence of mesopores while the shape of this hysteresis loop is related to their geometric shape. 

The work of Collier at the University of Florida [14] produced the finding that a modified Brunauer type I isotherm, with a more modest degree of curvature to the isotherm, was the theoretical optimum for deep dehydration cycles that were expected to be used in open cycle desiccant cooling cycles. The adsorbent was dubbed a type IM (M for moderate). To understand this designation zeolite type X with its incredible steep isotherm is designated a type IE (E for extreme). 

The low-temperature physisorption (type I isotherm) of hydrogen in zeolites is in good agreement with the adsorption model mentioned above for nanostructured carbon. The desorption isotherm followed the same path as the adsorption, which indicates that no pore condensation occurred. The hydrogen adsorption in zeolites depends linearly on the specific surface areas of the materials and is in very good agreement with the results on carbon nanostructures [24]. 

Type I isotherms are encountered when adsorption is limited to, at most, only a few molecular layers. This condition is encountered in chemisorption where the asymptotic approach to a limiting quantity indicates that all of the surface sites are occupied. In the case of physical adsorption, type I isotherms are encountered with microporous powders whose pore size does not exceed a few adsorbate molecular diameters. A gas molecule, when inside pores of these small dimensions, encounters the overlapping potential from the pore walls which enhances the quantity adsorbed at low relative pressures. At higher pressures the pores are filled by adsorbed or condensed adsorbate leading to the plateau, indicating little or no additional adsorption after the micropores have been filled. Physical adsorption that produces the type I isotherm indicates that the pores are microporous and that the exposed surface resides almost exclusively within the micropores, which once filled with adsorbate, leave little or no external surface for additional adsorption. 

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