Adsorption of Argon

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).

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. 2.25 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 °... 

The results of the accurate work of Orr (23) on the adsorption of argon on KC1 and Csl crystals have been mentioned above, and it was shown that when the surface was half covered, the adsorbate had an entropy equivalent to that of a two-dimensional gas. The curves given... 

The data shown in Figure 9.10 indicate both the kind of data that may be obtained by direct calorimetric study of gas adsorption and some evidence of the effect of preheating on the properties of surfaces. The figure shows the calorimetric heat of adsorption of argon on carbon black. The broken line indicates the behavior of the untreated black, and the solid line is the same adsorbent after heating at 2000°C in an inert atmosphere, a process known as graphitization. The horizontal line indicates the heat of vaporization of argon. 

Specific surface areas were determined by the volumetric method of adsorption of argon at liquid N2 temperature, using the BET equation (Parfitt and Sing, 1976). 

There have been numerous theoretical and experimental investigations on the adsorption of argon, oxygen, and nitrogen on potassium chloride (126-128) and in this connection we may refer to a survey in Brunauer s book on physical adsorption (129). There seems to be a general agreement that the most favorable positions for the adsorbed atoms or molecules will be found just above the center of a lattice cell. The electrostatic polarization is minimum at such spots, but the nonpolar van der Waals forces are at their maximum and dominate (130). Drain... 

Despite the somewhat larger heat of adsorption and the lower temperature, the time of adsorption of argon on charcoal therefore is practically the same as the corresponding figure for xenon on mercury. The higher the entropy, hence the more mobile the adsorbed molecule is, the longer is its time of adsorption, other quantities, such as heat of adsorption and temperature, being equal. 

Fio. 26. Heats of adsorption of argon on cesium iodide (curve A) and on potassium chloride (curve B) as a function of 6 (233). 

It is remarkable that the heats of adsorption of argon, oxygen, and nitrogen on rutile do not increase when the 9 value approaches unity, in other words, when the mutual distances of the molecules approach those of a completely filled unimolecular layer. Apparently the heterogeneity of the surface prevents this effect. In Fig. 24 it is shown that such an increase, caused by the mutual attraction of the adsorbed molecules, is found with the adsorption of nitrogen on copper. 

Wongkoblap A and Do DD. Characterization of Cabot non-graphitized carbon blacks with a defective surface model Adsorption of argon and nitrogen. Carbon, 2007 45(7) 1527-1534. 

Physical adsorption of argon at 80 K was performed on a Coulter, Omnisorp-lOO CX Analyzer. Chemisorption of hydrogen at room temperature was also performed on the Omnisorp Analyzer and was used to estimate the Pt cluster sizes. After reduction at 673 K for 8 h in flowing H2, samples were evacuated at 673 K for 1 h and then cooled to RT in vacuo before measuring isotherms. 

Figure 4. Isosteric heats of adsorption of argon on halozeolite and on Ca-chabazite (11)...

In the course of a study in this laboratory of the physical adsorption of argon and nitrogen on diamond and mineralogical graphite at 77° and 90° K and at low... 

Figure 4. Rates of adsorption of argon on diamond dust at 90° K after pretreatment...

It would appear from the above behavior that the actual physical adsorption of argon on a surface sufficiently aged at the temperature of the adsorption always takes place very rapidly. It is impossible at present to advance more than a con-... 

W have previously reported (3, 5) studies of the adsorption of argon at 77° and 90° K on muscovite mica which had been treated to replace the exchangeable surface potassium ions with other cations. The adsorption isotherms and thermodynamic functions evaluated from them showed significant differences among the various ion exchanged forms of mica. We have now obtained data for the adsorption of krypton on these substrates, and wish to discuss the differences in the behavior of the argon-mica and krypton-mica systems. 

Heat and Entropy Functions. Both differential and integral heats and entropies of adsorption were evaluated from the data for argon adsorption on muscovite (3). The variations in these functions with coverage for the potassium and barium muscovite were discussed in terms of localized adsorption on the various kinds of geometrical sites on the mica surface. On the cesium mica, however, the heat of adsorption of argon was higher and more uniform than on the potassium and barium micas, and the entropy functions showed a monotonic decrease with coverage. 

The similarity in the adsorption behavior of krypton on the three kinds of mica surfaces suggests that the adsorption here is primarily due to dispersion forces, with very little contribution from ion-induced dipole forces. The results of Barrer and Stuart (1) for the adsorption of argon on various ion-exchanged forms of faujasite are similar. They found that while calcium, strontium, and lithium faujasite—i.e., the materials containing cations with greater polarizing power—did show heat effects correlatable with ion-induced dipole interactions, no such effects were observed with sodium, potassium, or barium zeolites. With the latter materials, they also concluded that the adsorbed argon possessed appreciable mobility. 

Adsorption Data for Argon on Bone Mineral at —195°. In previous sections we have emphasized that the polarizability of the adsorbate on the polar bone mineral surface contributes to high heats of adsorption. For comparison we have made calorimetric measurements of the heat of adsorption of argon at —195° on the bare surface of bone mineral and on a methanol-covered surface. The data for differential heats of adsorption of argon at —195° are shown in Figure 3 and isotherms as measured on the pilot sample are recorded in Figure 4. 

In Figure 4, isotherms as determined on the pilot sample for the adsorption of argon on bone mineral surfaces arc shown for comparison with those of nitrogen. The shapes of the isotherms show the same relative behavior as the nitrogen iso-... 

Figure 2. Heats of adsorption of argon on single crystals of zinc...

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