Apparent micropore volume

The nitrogen isotherm was replotted by Cases et al. (1992) in the usual BET coordinates and as a t plot. The derived BET area of 43.3 m2 g"1 appeared to be not far removed from the value of 45.9 m2 g 1 obtained from the amount adsorbed at Point B. The r-plot was constructed in the manner originally proposed by de Boer et al. (1966), which involved adopting a standard isotherm with the same value of C, which in this case was 485. It was not easy to interpret the /-plot, although three short linear sections were identified. From the initial slope, the total surface area appeared to be c. 50 m2 g-1. Back-extrapolation of a linear region at higher p/p° gave an apparent micropore volume of c. 0.01 cm3 g 1. 

The latter approach was adopted by Trillo et al. (1993) in their study of the effect of thermal and hydrothermal treatment on the accessible microporosity of alumina-pillared montmorillonite. This work revealed that X-ray measurements of the dmi spacing taken alone may give a misleading impression of the thermal stability of the PILC micropore structure. For example, after heat treatment of the Al-PILC at 300°C, it was found that the apparent micropore volume available for nitrogen adsorption amounted to only c. 30% of that indicated by the dm spacing. 

Table 12.7. Values of apparent micropore volume, vp, of VPI-5 evaluated from the adsorption capacities of different vapours at p/p° = 0.4 (Kenny et al., 1992).

To convert the micropore capacity into the micropore volume, it is usually assumed that the pores are filled with liquid adsorptive - as in the case of mesopore filling. This procedure does not allow for the dependency of molecular packing on both pore size and pore shape. For this reason, we recommend that the term apparent micropore volume should be adopted and that the gas and temperature should always be specified. 

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. 

Table III. Apparent Saturation Capacities and Micropore Volumes for Argon...

Porous carbon materials mostly consist of carbon and exhibit appreciable apparent surface area and micropore volume (MPV) [1-3], They are solids with a wide variety of pore size distributions (PSDs), which can be prepared in different forms, such as powders, granules, pellets, fibers, cloths,... 

The recent contribution by Kaneko et al. (1995) has revealed that it is possible to produce highly hydrophobic fluorinated microporous carbon fibres. Two fluorinated carbons were reported to have apparent surface areas of 420 and 340 m2g and micropore volumes of, respectively, 0.19 and 0.14cm3g 1. These materials gave Type I nitrogen and methanol isotherms, but the adsorption of water vapour was too small to measure at pjp° < 0.8 and the uptake was very low even at p/p° 1. 

Values of apparent specific micropore volume, vp, of VPI-5 evaluated from the uptakes determined by Kenny et al. (1992) of various adsorptives at p/pa = 0.4 are recorded in Table 12.7. As before, the adsorbate densities are assumed equal to the respective liquid densities at the operational temperatures. Also included in Table 12.7 are values of vp derived from the measurements of Davis etal. (1989b), Schmidt et al. (1992) and Reichert et al. (1994). The complexity of the behaviour of VPI-5 is... 

The results in Table 12.7 appear to show that a considerably larger micropore volume is available for the adsorption of water and methanol than for the other adsorp-tives. A somewhat larger uptake of water is to be expected in view of its molecular size and apparent ability to enter the narrow six-membered rings (Davis et al., 1989b), but the high uptake of methanol is unexpected and will require further investigation. 

For the ACs the data are representative of the samples after heat-treatment at all three temperatures since during their fabrication these materials have already been treated at temperatures in excess of 850°C. However, for the alumina and clay samples the surface areas and pore volumes are shown after treatment at each temperature as these materials undergo various phase transitions that lead to sintering of the samples and shifts in their relative pore size distributions with heat-treatment. The particle size was determined from the corresponding MIP curve for the powder raw material. The Sbet in the case of microporous ACs should be considered as an apparent surface area due to the micropore filling mechanism associated with these materials [15]. The external area and micropore volumes were calculated from the slope and intercept of the t-plots of the corresponding isotherms. The total pore volume was taken as the amount of gas adsorbed at a relative pressure of 0.96 on the desorption isotherm, equivalent to a pore diameter of 50 nm. The mesopore volume was calculated from the difference in the total pore volume and the micropore volume. 

As can be seen from Figure 2, CO2 adsorption data for all the samples are well described by the DR- equation. Consequently, the total micropore volume (apparent surface areas (SC02) according to the evaluation procedure described in detail elsewhere (14). 

The nitrogen adsorption isotherms are characteristic of microporous solids. The aging treatments cause a clear decrease in the micropore volume and in the microporous surface. The decreases are nevertheless smaller for the Cu-ZSM-5 solid (variation A = 0.04 after aging at 1173 K) than for the parent zeolite (A= 0.07) (Table 2). An apparent BET surface area has been reported though the BET theory is not applicable to microporous materials since the pore condensation isotherm is interfering with the multi-layer adsorption isotherm. 

The same procedure as for oxidation yield was applied to Sbet and to several porous textural parameters determined by the alpha method (total surface area, external surface area and micropore volume) on each sample oxidized in dry air. This led to the master curves at 673 K for the respective parameters shown on Fig. 21. All master curves for pore parameters were derived by using the same shift factors as for the oxidation yield (i.e., the same apparent activation energy). 

Apparent BET SA (m g ) Micropore volume (cm g ) Stability Special features Key references... 

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