Argon adsorption

Calculate A52 at = 0.1 for argon at 77 K that forms a weak adsorption bond with the adsorbent, having three vibrational degrees of freedom. 

Such isothemis are shown in figure B 1,26.4 for the physical adsorption of krypton and argon on graphitized carbon black at 77 K [13] and are examples of type VI isothemis (figure B 1.26.3 ). Equation (B1.26.7)) further... 

Figure Bl.26.4. The adsorption of argon and krypton on graphitized carbon black at 77 K (Eggers D F Jr, Gregory N W, Halsey G D Jr and Rabinovitch B S 1964 Physical Chemistry (New York Wiley) eh 18).

A study of Table 1.1 reveals interesting features as to the mobility of the adsorbed atoms. Thus, for an argon atom on the (100) face, the easiest path from one preferred site S to the next is over the saddle point P, so that the energy barrier which must be surmounted is (1251 — 855) or 396 X 10 J/molecule. Since the mean thermal energy kT at 78 K is only 108 J/molecule, the argon molecule will have severely limited mobility at this temperature and will spend nearly all of its time in the close vicinity of site S its adsorption will be localized. On the other hand, for helium on the... 

From these various examples, it is clear that the adsorption energy for a given kind of site can vary quite markedly from one crystal face of the adsorbent to another. For argon on solid xenon (Table 1.1), for example, the most favourable site has a o value of —1251 x 10" J on the (100) face but only -1072 on the (111) face. Such differences are in no way surprising, and they have been found also with ionic crystals. 

Similar results with graphitized carbon blacks have been obtained for the heat of adsorption of argon,krypton,and a number of hydrocarbons (Fig. 2.12). In all these cases the heat of adsorption falls to a level only slightly above the molar heat of condensation, in the vicinity of the point where n = n . 

Fig. 2.14 The isosteric heat of adsorption ( ) of argon, nitrogen and oxygen of rutile at 95 K, plotted as a function of the amount adsorbed (expressed in cm (stp). The uptake of each gas corresponding to the completion of a monolayer is marked. Note the more rapid decrease in as the amount adsorbed approaches monolayer completion. (After Drain.)...

Fig. 2.18 Isotherms of argon (A) and of nitrogen (B) at 78 K on a non-porous silica (TK800-III). Open circles, adsorption solid circles,...

The survey in the present section shows quite clearly that it is not possible to assign a fixed value of a to a given adsorptive, which will remain valid for its adsorption on ail adsorbents. As demonstrated in Section 2.7, nitrogen and argon would seem to provide the best approximation to a constant effective molecular area, with = 16-2 A and a, (Ar) = 16-6 A. ... 

When it is desired to evaluate the specific surfaces of a set of closely related samples of solid, however, only one of the samples needs to be calibrated against nitrogen (or argon), provided that all the isotherms of the alternative adsorptive can be shown to have indentical shape. A simple device for testing this identity, by use of the a,-plot, is described in Section 2.13 by means of the a,-plot it is also possible to proceed directly to calculation of the specific surface without having to assign a value to or to evaluate the BET monolayer capacity, of the alternative adsorptive. 

Fig. 2.22 Adsorption isotherms of argon on graphitized carbon black at a number of temperatures," plotted as fractional coverage 0 against relative pressure p/p°. (Courtesy Prenzlow and Halsey.)...

Fig. 2.23 Adsorption isotherms on graphitized carbon black at 77 K. (A) argon (B) krypton. (Courtesy Dash.)...

Fig. 2.24 Adsorption isotherms of argon at 78 K on Spheron-6 carbon black, heated to various temperatures indicated in °C on each isotherm. (After Polley, Schaeffer and Smith. )...

Fig. 2.25 The differential heat of adsorption of argon on carbon blacks at 78 K, before and after graphitizalion.. Spheron O, Graphon. , and El denote molar heat of sublimation and of evaporation respectively.

Standard data for the adsorption of argon at 77 K on nonporous hydroxylated silica °... 

When other adsorptives, such as those detailed in Section 2.9, are employed for surface area determination, calibration against nitrogen or argon is strongly recommended, so long as the specific surface exceeds lm g . For areas below this figure the calibration becomes too inaccurate, and an alternative adsorptive, usually krypton, has to be used. 

As explained in Section 2.13, the use of iz,-plots makes it possible to avoid the involvement of either n or when an alternative adsorptive is being used for evaluating the surface areas of a set of related solids. It is then no longer necessary to exclude the use of isotherms having a low value of c, consequently the method is applicable even if the isotherm of the alternative adsorptive is of Type III (cf. Chapter 5). Calibration of one sample by nitrogen or argon adsorption is still required. 

Fig. 3.2 Adsorption isotherms for argon and nitrogen at 78 K and for n-butane at 273 K on porous glass No. 3. Open symbols, adsorption solid symbols, desorption (courtesy Emmett and Cines). The uptake at saturation (calculate as volume of liquid) was as follows argon at 78 K, 00452 nitrogen at 78 K, 00455 butane at 273 K, 00434cm g .

Fig. 4.4 Plot of the logarithm of the Langmuir quotient 0/p( - 0) against 0 for adsorption on H-chabasite at various temperatures." (a) Argon (h) carbon dioxide. (After Barrer and Davies.)...

It would clearly be of interest to discover how far the nonane method can be used with adsorbates other than nitrogen. A study along these lines has been carried out by Tayyab, but a discussion of his rather unexpected results is best deferred until the role of fine constrictions has been considered (p. 228). Meanwhile it may be noted that the applicability of the technique seems to be limited to adsorptives such as nitrogen or argon which have negligible solubility in solid or supercooled liquid n-nonane. 

Final purification of argon is readily accompHshed by several methods. Purification by passage over heated active metals or by selective adsorption (76) is practiced. More commonly argon is purified by the addition of a small excess of hydrogen, catalytic combustion to water, and finally redistiHation to remove both the excess hydrogen and any traces of nitrogen (see Fig. 5) (see Exhaust control, industrial). With careful control, argon purities exceed 99.999%. 

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