Argon isotherm

Useful information about micropore structures can be derived from nitrogen or argon isotherm data in terms of the C-constant (BET), t or as-plots and the Dubinin-Radushkevich models. 

The corresponding argon isotherms at 77 K also give a common reduced isotherm (Payne et al., 1973), but in this case monolayer completion cannot be identified as a single point on the isotherm (i.e. Point B is not well defined). Another difference is that at 77 K the argon multilayer development is restricted at highpjp° (see Figure 10.1). 

We turn now to the argon isotherms and as-plots, respectively, in Figures 10.10 and 10.11 (from the data of Payne et al., 1973). Here, the experimentally determined value of p°(i.e. p°(solid)) at 77 K has been used to calculate p/p°. It is of interest to compare the shapes of the argon isotherms with those of nitrogen on the same adsorbents (mesoporous gel B and microporous gel C). The nitrogen isotherm on gel B has... 

Table 10.9. Analysis of argon isotherms on silica gets.

The first systematic investigations of the adsorption of gases on dehydroxylated silicas were made by Kiselev and his co-workers (Kiselev, 1957,1958). In a study of the adsorption of argon and nitrogen, Aristov and Kiselev (1965) found that, in contrast to nitrogen, the reduced argon isotherm did not appear to depend on the degree of surface hydroxylation. 

In Figure 11.18 the argon isotherm is apparently a classical Type I isotherm, whereas the nitrogen isotherm has a well-defined hysteresis loop in the region of pjp° = 0.12-0.15. The nitrogen loop has upper and lower closure points and is quite stable and reproducible. This phenomenon must not be confused with the more common form of low-pressure hysteresis, which is much less well defined and persists to the lowest attainable pressures. However, a loop in this region of a nitrogen... 

H-K cumulative pore volume plots as a function of pore diameter for NaY and USY zeolites are presented on Figure 7 and 8 respectively micropore volumes calculated from nitrogen isotherm are overestimated compared to values calculated from argon isotherms. 

All the nitrogen and argon isotherms could be fitted with the Dubinin-Radushkevich equation between, typically, 10 and p/po 10. At higher pressures capillary condensation causes the isotherm to diverge. At lower pressures the typical deviation described in Fig 1 was observed. The carbon dioxide isotherm showed only a minor deviation from the DR-plot (one point) and was hence excluded from this study. 

The results of the fitting are shown in Figs 3-6. Figure 3 shows a Dubinin-Radushkevich plot for an activated carbon with an additional fitting of the deviation by Eq 2. Figure 4 shows the same three series (experimental isotherm, DR-plot and Eq 2) but on a linear scale. Figure 5 shows only the low pressure part of Fig 4. Finally, Fig 6 is similar to Fig 5 but for an argon isotherm on a zeolite. 

Argon isotherms were collected at pressures up to 850 torr at the six temperatures indicated in the Figures. Repeat determinations were performed in several instances. Approximately 0.1 Og of sample was used, and weighed after degassing. 

Textural characterization. The nitrogen and argon isotherms were obtained at liquid nitrogen and liquid argon temperature by using a Micromeritics ASAP 2010 apparatus (static volumetric technique). Before determination of adsorption-desorption isotherms the samples ( 0.2 g) were outgassed for 16 h at 350 °C under vacuum. 

Figure 4. Part a Nitrogen (O and %) and argon ( ) isotherms at 77 Kfor a mesoporous silica. Part b detailed low relative pressure data. Clear symbols denote adsorption dark symbols denote desorption.

The difference in shape of the nitrogen and argon isotherms, both determined at 77 K, on a mesoporous silica is illustrated in Figure 4a. In the middle range of relative pressure, the isotherms follow almost identical... 

While the IR and MAS NMR spectra are consistent with the delaminated structure of ITQ-2 and ITQ-6, the Argon isotherms represented as a function of log P/P are highly informative on the changes in topologies experienced during delamination (Fig. 3). It can be seen there that ITQ-2 does not show the adsorption corresponding the large 12 MR cavities characteristics of the MWW structure, while preserves that of the intralayer 10 MR circular channels. Meanwhile, in the case of ITQ-6, the adsorption at the 10 MR pores of ferrierite has practically disappeared. 

Channel width (long) x channel width (short) x diagonal length. Obtained from argon isotherm (at 87K). 

Levelt Sengers has examined the antisymmetry character of argon isotherms with respect to the critical isochore (p = p or and has found that... 

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