Pore-filling mechanism

Because of their ordered stmcture, molecular sieves have high capacity at low water concentrations and do not exhibit a capiHary condensation pore-filling mechanism at high water concentrations. The desiccating properties of the material are stiU good at elevated temperatures (Fig. 10). A dew point of —75° C can be obtained in a gas dried at 90°C with a molecular sieve that adsorbs water to the level of 1 wt %. In normal operations at ambient temperature, dew points of < — 100° C have been measured. 

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). 

As indicated earlier, an extensive range of linearity of a DR plot is usually associated with primary micropore filling. However, it must be kept in mind that the micro-pore filling mechanism is dependent on the nature of adsorption system and the temperature as well as on the pore size. Since it contains an additional adjustable parameter, the DA equation is obviously more adaptable than the simple DR... 

These limits are somewhat arbitrary pore filling mechanisms also depend on the shapes of the pores and on the size of the adsorptive molecule. Despite this Inherent vagueness, the classification has its use as a first means of discrimination because it points to different pore filling mechanisms macropores are so wide that they behave as "virtually flat" surfaces, mesopores are mainly responsible for capillary condensation, whereas micropores are so narrow that one cannot speak of a macroscopic fluid in them. Because in micropore Jilling adsorbates are only a few layers thick, an adsorption plateau is found suggesting monolayer filling and applicability of the Langmuir or Volmer premises. This mechanism Is distinct from that in meso- and macropores. 

In all examined cases, equation 2 provided a very good fit of the first part of the isotherm. As this equation represents a layer-by-layer adsorption rather than a pore filling mechanism, this strongly suggests a different adsorption behaviour for the very first part of the isotherms. The fact of Um being proportional to Wo shows this type of adsorption to be exclusively related to the micropore system. 

It is by now clear that several mechanisms and phases are present in the adsorption process in micropores, due to the interplay between gas-gas and gas-solid interactions, depending on their geometry and size. For this reason all those methods assuming a particular pore-filling mechanism, or adsorption model, should show shortcomings in some regions of the relevant parameters and their predictions should be compared with those based on more fundamental formulations of the adsorption process, like Density Functional Theory (DFT) [13] or Monte Carlo simulation [13,15]. Then, one question that arises is how the adsorption model affects the determination of the MSD ... 

The empirical equations dealt with so far, Freundlich, Sips, Toth, Unilan and Keller et al., are applicable to supercritical as well as subcritical vapors. In this section we present briefly a semi-empirical equation which was developed originally by Dubinin and his co-workers for sub critical vapors in microporous solids, where the adsorption process follows a pore filling mechanism. More details about the Dubinin equation will be discussed in Chapter 4. 

These authors assumed simple structures for porosity and simulated pore-filling mechanisms to create theoretical isotherms. Monte Carlo computer simulations were reported for the adsorption of argon at 90 K in different pores modeled as perfectly flat graphitic planes assembled to form rectangular cross-sections so allowing four layers of argon in the full pore (Figure 3.23). 

This type of analysis also indicates that values of Awsm > 0.2 result from adsorption in porosity which is not slit shaped, but may be cylindrical or spherical. Such adsorptions processes have been known for several decades with so many articles discussing pore filling mechanisms at values of p/p > 0.5. The shapes of mesoporosity need not be as symmetrical as a cylinder or a sphere as micrographs of later chapters will show. But this is as far as such modeling can go at the moment. 

Often, from the same isotherm, the three adsorption equations, described above, predict different values of ri. It is not a matter of debate as to which equation is the correct one. It is that different parts of the isotherm give different informations. A further complication, for the same adsorbent, is that different adsorbates provide different isotherms and so provide different values of n - The answer to these problems is the same, namely that the carbon is exhibiting a surface area for adsorbate A1 and a different surface area for adsorbate A2 because surface coverage and/or pore filling mechanisms are different. There is no right or wrong in this issue. 

These transitions in pore filling mechanisms are dependent on the size of the adsorbate molecule and so the above definition (>2 nm and <50 nm) is arbitrary being a function of the adsorbate used. However, as nitrogen dominantly is the characterizing adsorptive, these definitions must suffice. From an adsorption point of view, mesoporosity is not that all-important. Its role is as a transport pore (a means of passage), particularly for adsorptions from solutions, to permit rapid access to the retaining microporosity of the adsorbate of the system. 

Concentration changes of acetic acid were measured by titration with a standard sodium hydroxide solution. Extents of adsorption of acetic acid on 850 °C PVDC carbons (0,41 and 70 wt% bum-off) are plotted against acetic acid concentration in Figure 8.3. Extents of adsorption of acetic acid by the carbons reached a maximum in about 4M acetic acid (mole fraction = 0.09). The maximum amounts of acetic acid plotted in these isotherms (Figure 8.3) correspond more to capillary condensation than pore filling mechanisms (Table 8.1) with the possibility of capillary condensation of hydrated acetic acid molecules. Thus, no unequivocal conclusions are possible. The conclusions of this study, relevant to understanding adsorptions from solution are as follows ... 

Adsorption of iodine from aqueous I2/KI solution resembles adsorption of CO2 at 195 K and N2 at 77 K. That is, adsorption by iodine proceeds in the microporosity by a pore filling mechanism, and by capillary condensation in the mesoporosity, as does nitrogen at 77 K. The adsorption of iodine is apparently not influenced by the solvent, in this case, water which is an inert diluent. 

For nitrogen adsorbing on porous carbon this critical pore size corresponds roughly to the conventional boundary between micropores and mesopores at 2nm [9]. The pore filling mechanism is not accounted for in the thermodynamic methods, which are therefore incapable of determining pore size distributions in the micropore range. 

The Polanyi-Manes model is postulated to follow a pore-filling mechanism, which was first applied by Xia and Ball [35], and later applied by other research groups [34, 36] to describe sorption of several HOCs by selected natural soils and sediments. The Polanyi adsorption model originally was set up for the quantification of the adsorption of gas molecules to energetically heterogeneous solids, and was extended to a wide range of vapor and liquid phase systems by Manes and his co-workers. The Polanyi theory considers that, for a molecule located within the attractive force field of a micro-porous solid, there exists an... 

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