Graphitic carbons

The above methods for obtaining D, as well as other ones, are reviewed in Refs. 3-12, and Refs. 7-9 give tables of D values for various adsorbents. For example, D is close to 3 for the highly porous silica gels and close to 2 for nonporous fumed silica and for graphitized carbon black coconut charcoal and alumina were found to have D values of 2.67 and 2.79, respectively [7]. 

Some general points are the following. One precondition for a vertical step in an isotherm is presumably that the surface be sufficiently uniform that the transition does not occur at different pressures on different portions, with a resulting smearing out of the step feature. It is partly on this basis that graphitized carbon, BN, MgO, and certain other adsorbents have been considered to have rather uniform surfaces. Sharp LEED patterns are another indication. 

Fig. XVII-21. Continued) (c) Isosteric heats of adsorption of n-hexane on ice powder Vm = 0.073 cm STP. (From Ref. 125). (d) Isosteric heats of adsorption of Ar on graphitized carbon black having the indicated number of preadsorbed layers of ethylene. (From Ref. 126.)...

It is noted in Sections XVII-10 and 11 that phase transformations may occur, especially in the case of simple gases on uniform surfaces. Such transformations show up in q plots, as illustrated in Fig. XVU-22 for Kr adsorbed on a graphitized carbon black. The two plots are obtained from data just below and just above the limit of stability of a solid phase that is in registry with the graphite lattice [131]. 

Fig. XVII-22. Isosteric heats of adsorption for Kr on graphitized carbon black. Solid line calculated from isotherms at 110.14, 114.14, and 117.14 K dashed line calculated from isotherms at 122.02, 125.05, and 129.00 K. Point A reflects the transition from a fluid to an in-registry solid phase points B and C relate to the transition from the in-registry to and out-of-registry solid phase. The normal monolayer point is about 124 mol/g. [Reprinted with permission from T. P. Vo and T. Fort, Jr., J. Phys. Chem., 91, 6638 (1987) (Ref. 131). Copyright 1987, American Chemical Society.]...

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).

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 . 

Molecular area, a (Ar) of argon at 77 K on graphitized carbon blacks (Argon BET plots constructed with p (liquid) surface areas determined by BET-nitrogen, with a (Na)... 

Extrapolation from these two sets of values to C4 gives a (C4H,o) = 40-5 A for n-butane, which may be compared with the figure of 44-4 A (with a standard deviation of 4 A ) given in McClellan and Harnsberger s final list of recommended values and the value of 40 A found by Davis for n-butane on graphitized carbon at 195 K. A contributory factor to the variation in a -values is that the value of c tends to be low, e.g. on CaCOj (p. 251) c for butane is around 25. 

An example of a stepped isotherm, for krypton at 90 K, is shown in Fig. 2.21(a), where the adsorbent is graphitized carbon black, which is known to possess a very uniform surface. Figure 2.21(h) shows the steps obtained, also with krypton, on cadmium bromide. 

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. 5.10 The adsorption isotherms of n-hexane (A) and of water (B) on graphitized carbon black.Solid symbols denote desorption. (After...

CARBON - CARBON AND ARTIFICIALGRAPHITE - APPLICATIONS OF BAKED AND GRAPHITIZED CARBON] (Vol 4)... 

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