HEATSOF ADSORPTION ON CARBON BLACKSURFACES
2321
Heats of Adsorption on Carbon Black Surfaces by P. A. Elkington and G. Curthoys Department of Chemistry, University of Newcastle, New South Wales, Australia
(Received November 26, 1968)
Heats of adsorption of a series of organic compounds on graphitized and oxidized carbon black were determined by means of gas chromatography. On graphitized carbon black the adsorption is nonspecific, whereas on the oxidized carbon it is specific. Absolute retention volumes (per unit surface) of the adsorbates on graphitized carbon black are reported.
The determination of heats of adsorption of adsorbate molecules on solid surfaces forms an important aspect of gas-solid investigations, since it is possible to obtain an insight into the nature of gas-solid interactions from heats of adsorption. Graphitized carbon black has been shown'-4 to be an adsorbent which possesses a homogeneous surface and can therefore be used to determine basic information about the nature of gas-solid interactions. Publications have described the adsorption of gases such as argon, krypton, nitrogen, oxygen, hydrogen, and other simple gases6r6as well as various polar molec u l e ~on ~ graphitized carbon black, and it has been demonstrated that, in general, adsorption is dependent mainly on the size and geometrical structure of the adsorbed molecule and not on its electronic configuration. I n this work the adsorption of a number of organic compounds onto carbon black was examined in order to study more fully the relative importance of electronic and geometric factors in adsorption. Since Cremer and Priors first suggested that differential heats of adsorption could be determined gas chromatographically, a number of papers have showngJo that good agreement exists between this method and heats of adsorption at low coverages determined by means of a calorimeter. ROSS,et al.," have shown that when log t,, where t, is the corrected retention time, is plotted against l / T c ,where T, is the column temperature, the slope of the line obtained is AH/2.303R, where AH is the isosteric heat of adsorption. This method was used to determine the heats of adsorption which are reported below. A linear relationship between log tm and l / T c was obtained over the temperature range studied. One significant feature of recent publication^'^-^^ has been to express the relative values of the specific retention volume (per gram of adsorbent in the column) in terms of absolute values (per unit surface), i.e., independent of the specific surface area of the adsorbent. I n this paper the retention volumes for adsorption on the well-characterized Sterling MT-D4 surface are reported in terms of absolute retention volumes. I n order to obtain a better understanding of the nature of adsorption on graphitized carbon black the
adsorption of a number of compounds on oxidized carbon black was investigated. Chemical groups on the surface of graphite are knownI6 to affect the properties of carbon surfaces. Attempts by V i l l a r ~ l ~to~ eluci'~ date the groups on oxidized carbon showed that the oxygen exists as both carbonyl groups and hydroxyl groups as well as some oxygen in an unreactive form. The existence of these groups as well as more complex lactones and phenolic hydroxyl groups have been found by other authors.18-20 Thus oxidized carbon black presents a surface which allows specific interactions to occur with the surface chemical groups. The effect of the electronic structure of the adsorbed molecules on the heats of adsorption is interpreted in terms of this specific interaction.
Experimental Section A Pye argon gas chromatograph was used to determine the heats of adsorption. A reproducible volume (1) M.H. Polley, W. D. Schaffer, and W. R. Smith, J. Phgs. Chem., 57, 169 (1953).
(2) N. N. Avgul, A. V. Kiselev, and I. A. Lygina, Kolloid. Zh., 23, 396 (1961). (3) W. R. Smith and D. G. Ford, J. Phys. Chem., 69, 3587 (1965). (4) R. A. Beebe and D. M. Young, ibid., 58, 93 (1954). (5) J. M.Holmes, "The Solid-Gas Interface," Vol. I, E. A. Flood, Ed., Arnold, London, 1967,p 127. (6) A. V. Kiselev, Quart. Rev. (London), 15, 99 (1961). (7) A. V. Kiselev, Russ. J. Phys. Chem., 38, 1501 (1964). (8) E.Cremer and F. Prior, 2. Elektroehem., 55, 66 (1951). (9) 8. A. Greene and H. Pust, J. Phys. Chem., 62, 55 (1958). (10) R. A. Beebe and P. H. Emmett, ibid., 65, 184 (1961). (11) S.'Ross, J. K. Saelens, and J. P. Olivier, ibid., 66, 696 (1962). (12) R. 8. Petrova, E. V. Xhrapova, and K. D. Shcherbakova, Kinet. Katal., 5,456 (1964). (13) A. V. Kiselev, E. A. Paskonova, R. S. Petrova, and K. D. Shcherbakova, Russ. J . Phys. Chem., 38, 84 (1964). (14) L.D.Belyakova, A. V. Kiselev, and N. V. Kovaleva, ibid., 40, 228 (1966). (15) H.-P. Boehm, E. Wehl, W. Heck, and R. Sappok, Angew. Chem. Intern. Ed. Enol., 3, 669 (1964). (16) D.S. Villars, J . Amer, Chem. Soc., 69, 214 (1947). (17) D.S. Villars, ibid., 70, 3655 (1948). (18) V. A. Garten and D. E. Weiss, Auat. J . Chem., 8, 68 (1955). (19) V. A. Garten, D. E. Weiss, and J. B. Willis, ibid., 10, 295 (1957). (20) J. B. Donnet, F. Hueber, G. Reitzer, J. Oddux, and G. Reiss, Mem. SOC.Chem., 50, 1727 (1962). Volume 78, Number 7 July 1969
2322 of vapor (0.01 cma) was introduced onto the glass columns which were packed with 37-72 mesh size (BSS) adsorbent particles. The graphitized carbon black was provided by Dr. W. R. Smith of the Cabot Laboratories, U. S. A., and was designated as Sterling MTD4. It has been graphitized at a temperature of 3100". This adsorbent was obtained as a very fine powder and was pelletized by rolling in a ball mill without the addition of water or a binder. Sterling MTD4 has been found to have a specific surface area of 7.6 m2 g-laZ1 Oxidation of Sterling MTD4 was accomplished by refluxing in concentrated nitric acid. Heats of adsorption of two oxidized samples were investigated. The first sample (14.8 g) was one which had been oxidized for 48 hr and the second (4.3 g) was one which had been oxidized for 160 hr. After oxidization each sample was carefully washed with distilled water a number of times and evaporated to dryness, then ground, and the same size particles as used with the Sterling MTD4 column were used to pack the columns. Microanalysis of the two oxidized samples by the Australian Microanalytical Service, Melbourne, showed that the concentrations of hydrogen and oxygen in the less oxidized Sterling MTD4 was 0.83 and 0.27%, respectively and in the more severely oxidized sample were 1.09 and 0.87%, respectively. These two adsorbents were used t o investigate the nature of adsorption on surfaces containing polar groups which are capable of specific interaction with absorbed molecules. Adsorbate solutions were of the purest available quality and were fractionally distilled before use. Precautions were taken to ensure that all traces of water were removed from samples. Results and Discussion The peaks obtained using Sterling MTD4 as an adsorbent were relatively symmetrical. This symmetry is not customary in gas-solid chromatography and is an indication of the close approach t o uniformity of the adsorbent surface, and that it is the linear (Henry's law) region of the isotherm that is being investigated. Typical results for the plots of the logarithm of the retention times against the reciprocal of the absolute temperature are shown in Figure 1. Calculation of Absolute Retention Volumes on Graphitized Carbon Black. The concept of calculating an absolute value for the retention volume of an adsorbate molecule on graphitized carbon black has been suggested by Kiselev, et a1.18z21 I n this way it was expected to obtain values which might provide thermodynamic dataz1 or values which might be used to identify an adsorbed molecule. The formula used t o evaluate the absolute retention volume V , is
P. A. ELKINGTON AND G. CURTHOYS
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.
2.5
2 pl
0 -1
2.0
1.5
1.9
2.0
2.1
2.2
2.3
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