Pore width

The table convincingly demonstrates how the unsuspected presence of micropores can lead to an erroneous value of the specific surface calculated from a Type II isotherm by application of the standard BET procedure. According to the foregoing analysis, the external specific surface of the solid is 114m g" the micropore volume (from the vertical separation of isotherms A and E) is 105 mm g but since the average pore width is not precisely known, the area of the micropore walls cannot be calculated. Thus the BET figure of 360m g calculated from isotherm E represents merely an apparent and not a true surface area. 

The enhancement of interaction energy in micropores was discussed in some detail in Chapter 4. It was emphasized that the critical pore width d at which the enhancement first appears increases with increasing diameter a of the adsorbate molecule, since the relevant parameter is the ratio d/a rather than d itself. The quantity a is involved because the magnitude of the dispersion interaction increases as the polarizability, and therefore the size, of the molecule increases (cf. p. 5). 

The stmcture of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aHphatic bridging groups (6). X-ray diffraction patterns of activated carbon reveal that it is nongraphitic, remaining amorphous because the randomly cross-linked network inhibits reordering of the stmcture even when heated to 3000°C (7). This property of activated carbon contributes to its most unique feature, namely, the highly developed and accessible internal pore stmcture. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of carbonization and activation. Pore sizes are classified (8) by the International Union of Pure and AppHed Chemistry (lUPAC) as micropores (pore width <2 nm), mesopores (pore width 2—50 nm), and macropores (pore width >50 nm) (see Adsorption). 

Adsorption of supercritical gases takes place predominantly in pores which are less than four or five molecular diameters in width. As the pore width increases, the forces responsible for the adsorption process decrease rapidly such that the equilibrium adsorption diminishes to that of a plane surface. Thus, any pores with widths greater than 2 nm (meso- and macropores) are not useful for enhancement of methane storage, but may be necessary for transport into and out of the adsorbent micropores. To maximize adsorption storage of methane, it is necessary to maximize the fractional volume of the micropores (<2 nm pore wall separation) per unit volume of adsorbent. Macropore volume and void volume in a storage system (adsorbent packed storage vessel) should be minimized [18, 19]. 

The models of Matranga, Myers and Glandt [22] and Tan and Gubbins [23] for supercritical methane adsorption on carbon using a slit shaped pore have shown the importance of pore width on adsorbate density. An estimate of the pore width distribution has been recognized as a valuable tool in evaluating adsorbents. Several methods have been reported for obtaining pore size distributions, (PSDs), some of which are discussed below. 

We now proceed with the study of the phase behavior of associating fluids in pores. To elucidate the effects of changes of the association energy, again we have considered = 0, 7, and 10. The pore width was fixed and set to H = 6a. Figs. 14(a) and 14(b) show some examples of the... 

Poren-fiiller, m. pore filler, primer, -gehalt, tn. content of pores or voids, -grosse, /. size of pore(a) or voids, -raum, m. pore space, -volumen, n. volume of pores or voids, -weg, n. pore passage, -weite, /. pore width, pore diameter,... 

The temperature, pore width and average pore densities were the same as those used by Snook and van Megen In their Monte Carlo simulations, which were performed for a constant chemical potential (12.). Periodic boundary conditions were used In the y and z directions. The periodic length was chosen to be twice r. Newton s equations of motion were solved using the predictor-corrector method developed by Beeman (14). The local fluid density was computed form... 

Figure 2. Density profiles Illustrating effect of pore width on layering structure. Theory with 6 - oo LJ fluid.

Molecular dynamics results for restricted pore average density versus pore width. 

Figure 5. Pore dlfifuslvlty versus pore width. Theory Is for 6-oo LJ fluid. Units of dlfifuslvlty are (3a/8)(kBT/7rm) ...

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... 

The average pore size of PS structures covers four orders of magnitude, from nanometers to tens of micrometers. The pore size, or more precisely the pore width d, is defined as the distance between two opposite walls of the pore. It so happens that the different size regimes of PS characterized by different pore morphologies and different formation mechanisms closely match the classification of porous media, as laid down in the IUPAC convention [Iu2]. Therefore the PS structures discussed in the next three chapters will be ordered according to the pore diameters as mostly microporous (d<2 nm), mostly mesoporous (2 nm50 rim). Note that the term nanoporous is sometimes used in the literature for the microporous size regime. 

Microporous substances with pore widths smaller than twenty Angstroms exhibit type 1 isotherms (lUPAC classification) in the absence ofmesopores, as illustrated in Figure 13.1 [4]. The steep uptake of N2 at very low relative pressures is due to the capillary condensation in micropores. Following the filling of micropores, adsorption isotherms become nearly fiat because further N2 uptake can now only occur at the external surface area, which is typically much lower relative to the total surface area. 

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