Micropore filling

There has been fierce debate (see Refs. 232, 235-237) over the usefulness of the preceding methods and the matter is far from resolved. On the one hand, the use of algebraic models such as modified DR equations imposes artificial constraints, while on the other hand, the assumption of the validity of the /-plot in the MP method is least tenable just in the relatively low region where micropore filling should occur. 

I (curve D). Thus the micropores had been able to enhance the adsorbent-adsorbate interaction sufficiently to replace monolayer-multilayer formation by micropore filling and thereby change the isotherm from being convex to being concave to the pressure axis. 

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). 

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. 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

During the adsorption or occlusion of various molecules, the micropores fill and empty reversibly. Adsorption in zeoHtes is a matter of pore filling, and the usual surface area concepts are not appHcable. The pore volume of a dehydrated zeoHte and other microporous soHds which have type 1 isotherms may be related by the Gurvitch rule, ie, the quantity of material adsorbed is assumed to fill the micropores as a Hquid having its normal density. The total pore volume D is given by... 

One version of the microporous, filled polyethylene separator ( PowerSep ) [113], which is so successful in the lead-acid battery, is also being tested in nickel-cadmium batteries. This separator is manu-... 

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). 

Measuring the specific surface area, A, related to the mass of PS does not require a textural model (a morpho-independent parameter, i.e., one can apply an approach of partitioning and, correspondingly, the second statement of texturology, as we have already done for volume-related parameters). Let us consider the most widespread adsorption method based on proportionality of adsorption, Q. and the specific surface area in the absence of volumetric effects (capillaiy condensation, micropore filling, etc.) ... 

Next we analyze the sorption kinetics of a sorbate with constant aqueous concentration, C°, sorbing into a porous spherical aggregate with radius rQ. More precisely, the macroparticle is a homogeneous aggregate of microparticles which are separated by micropores filled with water (Fig. 19.17). The sorbate diffuses in these pores and sorbs to the microparticles. It is not relevant whether sorption occurs at the surface or in the interior of the microparticles as long as we can assume that sorption equilibrium between the solute concentration and the microparticles at each position within the aggregate is attained instantaneously. 

Figure 19.17 Spherical macroparticle with radius ra consisting of an aggregate of microparticles separated by micropores filled with water. A chemical with constant concentration C° diffuses into the pore volume of the macroparticle. The local dissolved pore concentration Cw is at instantaneous equilibrium with the local sorbed phase C ( K d is microscopic equilibrium coefficient). Note that the macroscopic distribution coefficient Kd is time dependent (see Eq. 19-78.)...

In this work, the relationship between micropore filling of supercritical Xe in micropores of ACF at 300 K and cluster size distribution by cluster analysis is described. 

Monolayer and multilayer adsorption, micropore filling and capillary condensation... 

Micropore filling is the process in which molecules are adsorbed in the adsorption space within micropores. 

Micropore filling, e.g, cations, water, amines, salts... 

Other -more complicated- models to evaluate the microporous volume exist. The Dubinin-Radushkevich model46,47,48,49 is based on thermodynamical considerations concerning the process of micropore filling. Full discussion of this model is beyond the scope of this book. The reader is referred to the standard work of Gregg and Sing50 on adsorption for a detailed treatment. 

The adsorption process in the micropores occurs by a volume-filling mechanism rather than by a surface-coverage mechanism. Thus, nm values obtained either by BET or Langmuir equation are similar to the limited uptake of the type I isotherms. Therefore, the amount adsorbed for different adsorptives (expressed as volume of liquid) at a relative pressure near unity is very similar, which implies that micropore filling occurs rather than the surface area coverage of the adsorbent. 

According to experimental data, and assuming that PSD is Gaussian, Dubinin and Radushkevich obtained an equation, which relates the degree of micropore filling (9) with the differential molar work of adsorption ... 

The adsorption of vapors in complex porous systems takes place approximately as follows [1-3] at first, micropore filling occurs, where the adsorption behavior is dominated nearly completely by the interactions of the adsorbate and the pore wall after this, at higher pressures, external surface coverage occurs, consisting of monolayer and multilayer adsorption on the walls of mesopores and open macropores, and, at last, capillary condensation occurs in the mesopores. 

These limits are to some extent arbitrary since the pore filling mechanisms are dependent on the pore shape and are influenced by the properties of the adsorptive and by the adsorbent-adsorbate interactions. The whole of the accessible volume present in micropores may be regarded as adsorption space and the process which then occurs is micropore filling, as distinct from surface coverage which takes place on the walls of open macropores or mesopores. Micropore filling may be regarded as a primary physisorption process (see Section 8) on the other hand, physisorption in mesopores takes place in two more or less distinct stages (monolayer-multilayer adsorption and capillary condensation). 

In monolayer adsorption all the adsorbed molecules are in contact with the surface layer of the adsorbent. In multilayer adsorption the adsorption space accommodates more than one layer of molecules so that not all adsorbed molecules are in direct contact with the surface layer of the adsorbent. In capillary condensation the residual pore space which remains after multilayer adsorption has occurred is filled with condensate separated from the gas phase by menisci. Capillary condensation is often accompanied by hysteresis. The term capillary condensation should not be used to describe micropore filling because this process does not involve the formation of liquid menisci. 

In the case of micropore filling, the interpretation of the adsorption isotherm in terms of surface coverage may lose its physical significance. It may then be convenient to define a monolayer equivalent area as the area, or specific area, respectively, which would result if the amount of adsorbate required to fill the micro-pores were spread in a close-packed monolayer of molecules (see Section 11.2.1.8). 

If the standard BET procedure is to be used, it should be established that monolayer-multilayer formation is operative and is not accompanied by micropore filling (Section 11.2.1.8.C), which is usually associated with an increase in the value of C (>200, say). It should be appreciated that the BET analysis does not take into account the possibility of micropore filling or penetration into cavities of molecular size. These effects can thus falsify the BET surface areas and in case of doubt their absence should be checked by means of an empirical method of isotherm analysis or by using surface area reference samples (see Section 11.2.1.6.B). 

The limiting dimensions of micropores are difficult to specify exactly, but the concept of micropore filling is especially useful when it is applied to the primary filling of pore space as distinct from the secondary process of capillary condensation in mesopores. 

In recent years a radical change has been taking place in the interpretation of the Type I isotherm for porous adsorbents. According to the classical Langmuir theory, the limiting adsorption w (at the plateau) represents completion of the monolayer and may therefore be used for the calculation of the surface area. The alternative view, which is now widely accepted, is that the initial (steep) part of the Type I isotherm represents micropore filling (rather than surface coverage) and that the low slope of the plateau is due to multilayer adsorption on the small external area. 

The O2 isotherms for these AlPO molecular sieves are essentially Type I, typical of micropore filling (12,13). In Figure 16 the O2 isotherms of AlP0i.-5, -11, -17 are compared with those of a typical zeolite, NaX, and a silica molecular sieve, silica-lite. A1P04-5 and -17 have saturation pore volumes of approximately 0.2 cm /g similar to that of silicalite, while A1P0 -11 has a relatively small O2 pore volume (ca. 0.1 cm3/g). 

The H2O adsorption isotherms for AIPO1.-5, -11, -17, and -20 are shown in Figure 17. For comparison, the hydrophilic NaX and the hydrophobic silicalite are included. The isotherm shape for A1P0 -11 and A1P0 -17, like that of NaX and silicalite, is Type I, typical of micropore filling. The isotherm shape of the AlPOi.-20... 

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