Adsorption in micropores

Statistical Thermodynamic Isotherm Models. These approaches were pioneered by Fowler and Guggenheim (21) and Hill (22). Examples of the appHcation of this approach to modeling of adsorption in microporous adsorbents are given in references 3, 23—27. Excellent reviews have been written (4,28). 

Recently, in the theoretical studies on the simulation for N2 adsorption in micropore, some researchers102-104 reported that the monolayer adsorption occurs even in the micropore whose pore width is greater than the bilayer thickness of N2 (about 0.7 nm). In addition, Kaneko et al. showed the presence of the orientational phase transition of N2 on the graphitic micropore wall, which is the same as the phase transition of the monolayer on the flat graphite surface,105 and gave an effective method for the surface area determination in the microporous system.106 Therefore, even for micropores whose width is greater than 0.7 nm, dV MS can be... 

Peculiarities of Adsorption in Microporous Carbons (the Polanyi Potential Theory Dubinin and Related)... 

The adsorption on microporous solids is not so well understood as in nonporous or mesoporous solids. When the pore size is similar to the size of the adsorbate molecule and the adsorption temperature is below the critical temperature, the adsorption does not take place by a progressive completion of a monolayer followed by multilayer adsorption, but by the filling of the MPV with the adsorbate in a liquid-like condition. There are a number of problems associated with adsorption in micropores including the following important points [23] ... 

All the aforementioned points about peculiarities of adsorption in micropores show that special attention is needed when microporous solids (i.e., activated carbons, ACFs, nanotubes, CMSs, charcoals, etc.) are characterized by the physical adsorption methods. 

As it can be seen in Figure 4.3, the characteristic curve for N2 superimposes over that for C02 up to a value of (A/p)2 lower than about 400 (kJ/mol)2, confirming what was previously explained. From this value, a large downward deviation is observed in the N2 characteristic curve. This behavior has been observed with other microporous carbon materials and, in fact, it is a common problematic feature of the characteristic curves for N2 adsorption in microporous carbons and hence, a limitation of its use in such low relative pressures (i.e., from 10-3 to 10-7) [10,11,17], This downward deviation... 

A remarkable adsorption capacity on high surface area ACs under a hydrogen pressure has been reported for the first time at the beginning of the 1980s [55,56], Whereas hydrogen is absorbed in the interstitial sites of metallic alloys, the main storage mechanism in carbon materials is the adsorption in micropores [57,58], Depending on the authors, theoretical studies found that the optimum pore... 

HOWARTH-KAWAZOE APPROACH FOR THE DESCRIPTION OF ADSORPTION IN MICROPOROUS MATERIALS FOR THE SLIT, CYLINDRICAL, AND SPHERICAL PORE GEOMETRIES... 

Isotherm I is typical of adsorption in micropores, which occurs, for instance, in zeolites and in activated carbons. Isotherm II represents multilayer physisorption on a flat surface (valid for many nonporous substances). Type HI and V isotherms are characteristics of weak gas-solid interactions (water adsorption on gold and bromine adsorption on silica are good examples). The type IV isotherm is frequently found in the study of heterogeneous catalysts its shape is characteristics... 

As can be seen, helium, in contrast to what was once assumed, is not necessarily the best gas to select for the determination of dead space. It is sometimes thought that helium allows dead space to be determined directly at 77 K in the presence of the sample, since it will not adsorb. However, since its virial coefficient is much smaller than that of most adsorptives (see Table 3.2), and because of the possibility of adsorption in micropores (see Chapter 9), its use cannot be recommended. This problem has been discussed recently by Neimark and Ravikovitch (1997). 

This is an indirect way of assessing the energetics of gas adsorption in micropores. The pre-adsorbed vapour can be that of the immersion liquid or it can be another adsorptive for instance, to study the water filling mechanism in microporous carbons, Stoeckli and Huguenin (1992) devised an experiment with water pre-adsorption prior to immersion calorimetry (in water or in benzene). 

The adsorption in micropores is due to overlap of the adsorption potentials from opposite walls until 12 A, and by co-operative process until a width of 20 A [3], which is the micropore width limit generally established [5]. 

Figure 2 contains water adsorption isotherms corresponding to some of the samples. Figure 3 presents the water adsorption isotherms for the series of heat treated CF. In Figure 2 it can be seen that water isotherms are type V, characteristic of water adsorption in microporous AC [8]. It must be indicated that, contrary to CO2 adsorption and to the adsorption of many other gases and vapours, where adsorption in the microporosity takes place at low relative pressures (see Figure 1), in the case of water, adsorption takes place in... 

ADSORPTION IN MICROPOROUS MATERIALS ANALYTICAL EQUATIONS FOR TYPE I ISOTHERMS AT HIGH PRESSURE... 

The concept of volume filling of micropores under review is based on a radical difference of adsorption in micropores as a limiting case as compared with the opposite limiting case of adsorption on the surface of nonporous adsorbents of identical chemical nature. The concept of adsorption in micropores is of a thermodynamical nature, and its main initial principle, which has the meaning of a rational approximation, con-... 

In the case of adsorption in micropores, when the condition of temperature invariance of the characteristic curve is fulfilled, the net differential molar heat of adsorption, q, and the differential molar entropy of adsorption, AS, may be expressed (2) as... 

The role of isosteric enthalpy of adsorption in micropore characterisation ... 

W. C. Conner, Physical adsorption in microporous solids. NATO Adv.Study Inst. C 33... 

A. Saito and H.C. Foley, Curvature and parametric sensitivity in models for adsorption in micropores. AIChE., 37 (1991) 429. 

J. D.F. Ramsay, Nitrogen adsorption in microporous silica compacts. /. Collloid Interface ScL, 51 (1975) 205. 

M. Molina-Sabio, F. Rodriguez-Reinoso, D. Valladares and G. Zgrablich, A lattice-gas model for adsorption in microporous solids, in J. Rouquerol, F. Rodriguez-Reinoso,... 

Murata and Kaneko [29] proposed a new equation of the absolute adsorption isotherm [30] for a supercritical gas in order to describe the adsorption of methane on activated carbons. The environmental aspects of supercritical gases confined in nanospaces have been reviewed by Kaneko [31]. The model assumes that the adsorption in micropores is slightly enhanced compared with that on a flat surface. The authors supported this assumption on the basis of comparison plots of experimental data obtained on activated carbons and flat surfaces like nonporous carbon black. 

Nguyen, C. and Do, D.D. (2001). Multicomponent supercritical adsorption in microporous activated carbon materials. Langmuir, 17, 1552-7. 

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