Further Studies of Adsorption on Graphitized Carbon Blacks

June, 1958

STUDIESOF ADSORPTION ON GRAPHITIZED CARBON BLACKS

hydrogenation to give surface layers of approximately the same composition), or on the extent of polymerization of the adsorbed radicals, but rather on the ability of the metal to catalyze the addition of hydrogen to dissociatively adsorbed hydrocarbon residues. On tungsten, and to a lesser extent on nickel, reaction between ethylene and hy-

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drogen is effectively limited to that of associatively adsorbed or gas phase ethylene with adsorbed hydrogen, while on palladium hydrogenation of dissociatively adsorbed molecules can occur as well. Acknowledgment.-The author wishes to express appreciation to E. E. Francois for making the mass spectrometric analyses.

FURTHER STUDIES OF ADSORPTION ON GRAPHITIZED CARBON BLACIW2 BY W. B. SPENCER, C.H.AMBERGAND R. A. BEEBE Department of Chemistry, Anzherst College, Aniherst, Mass. Received Februaru 11, 1968

Adsorption isotherms and calorimetric heats of adsorption have been determined a t -79” for carbon dioxide and ammonia on the graphitized carbon blacks Spheron (2700), Sterling FT (2700) and Sterling M T (3100). The results are compared with previous data on these adsorbents. Carbon dioxide adsorption is similar to that of the polar sulfur dioxide. The ammonia adsorption isotherms serve especially well to characterize the surfaces of these homogeneous adsorbents. Entropies of adsorption have been calculated for these systems, and are given together with a brief discussion of their significance.

Introduction A considerable body of experimental data has been built up on the adsorption of gases on samples of carbon black heat-treated a t successively higher temperatures up to 3000”. It has been shown that the extent of graphitization produced by high temperature heat treatment is a function of the type of carbon black initially used. This extent of graphitization, which can be evaluated by X-ray and electron diffraction studies, influences the degree of surface homogeneity of the graphitized carbon^.^^^ The adsorbates used for these investigations fall naturally into two classes: (1) non-polar substances such as nitrogen and the rare gases, (2) polar substances such as sulfur dioxide, water, ammonia and methylamine. Special interest has been attached to the investigations with polar adsorbates because the nature of the isotherms and of the heatcoverage curves is rather profoundly altered by the heat treatment of the carbon blacks. We have now included carbon dioxide in the list of adsorbates and have measured the heats of adsorption for this gas a t -79’ on two highly graphitized samples of carbon. Carbon dioxide is of interest because the molecules, although they have no permanent dipole, are highly polarizable. Their polarizability is about twice that of nitrogen5 Moreover they have a permanent electric quadrupole which is probably several times larger than that of nitrogen and which is bound t o play a significant role in the adsorption process.6 (1) The material in this paper was presented before the Division of Colloid Chemistry a t the 128th Meeting of the American Chemical Society, September 11, 1958, at Minneapolis, Minn. (2) This research was supported by the United States Signal Corps under Contracts No. DA 36-039-SC-56726 and 70124. We are also indebted for financial aid to Godfrey L. Cabot, Inc., of Boston, Mass. (3) C. Houska and B. E. Warren, J . A p p l . Phys., 2 6 , 1503 (1954). (4) R. A. Beebe and D. M. Young, THIS JOURNAL, 68, 93 (1954). (5) See, e.g., Landolt-Bornstein, “Zahlenwerte und Funktionen,” Vol. I, 3, Springer, 1951, p. 510.

It turns out that the ammonia isotherms on various heat-treated carbon blacks serve especially well as a means of characterizing the degree of graphitization of these adsorbents. For this reason we have measured the adsorption of ammonia on representative samples of heat-treated carbon blacks and these results are also presented. Experimental Materials.-The series of graphitized Spheron blacks for which adsorption data are given in Fig. I has been described in a previous publication? The numbers in parentheses represent the temperature of heat treatment, e.g., Spheron (lO000). The samples referred to as Sterling FT (2700) and Sterling M T (3100) were prepared by heat treatment of two thermal blacks to the temperatures indicated. It should be noted that Sterling FT (2700) is similar to the black P-33 (2700) used in previous work. The material designated as Shawinigan (3000) was prepared from Shawinigan acetylene black. All these samples of carbon black were kindly provided by Dr. W. R. Smith of Godfrey L. Cabot, Inc., Boston. For the conditions of heat treatment the reader should consult a paper by Schaeffer, et a1.B The three samples, Spheron (2700), Sterling FT (2700) and Sterling M T (3100), constitute a series of essenti?lly non-porous, non-polar adsorbents possessing an increasing degree of surface homogeneity in the order listed. The evidence for homogeneity of surface as based on adsorption, X-ray and electron micrograph data has been discussed previously.7 The growth of graphitic crystallites, which leads to surface homogeneity, is more extensive for particles of higher diameter. The three carbons listed above have average particle diameters of approximately 310, 2250 and 5600 8., respectively, with specific surface areas of 84.1, 12.5 and 6.3 sq. m. per g. based on nitrogen adsorption (BET). The corresponding values for the Shawinigan black are: diameter 430 8., and surface area 38.0 sq. m. per g. Carbon dioxide of 99.9% purity was obtained from the Matheson Company. This gas was solidified in a trap a t - 195’. It was permitted to warm up somewhat and about one-third of it was pumped out. The middle third was (6) (a) L. E. Drain, Trans. Faraday Soc., 49, 650 (1953); (b) R. W. Zwanzig, J . Chem. Phva., 26, 211 (1956). (7) M. H. Polley, W. D. Schaeffer and W. R. Smith, THIS JOURNAL, 67, 567 (1953). (8) W. D. Schaeffer, W. R. Smith and M. H. Polley, I n d . Ene. Chem., 4 6 , 1721 (1953).

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the saturation pressure PO,used in calculating relative pressures, was measured by means of an ammonia vapor thermometer, and run temperatures were estimated from appropriate vapor pressure data. In the large calorimeter Dewar we found apparent temperatures on this thermometer which varied from -78.7 to 80.4' for different experiments over a period of a year but never more than 0.5' during a given run. It seems probable that these apparent temperature differences indicated by the thermometer were much larger than those occurring in the calorimeter itself. We have therefore designated the temperature as -79" for all the experiments oreported here, with an uncertainty in absolute value of i l Because of the experimental difficulties involved and in particular because of the low adsorptive capacity of the carbon samples used in the present work we estimate that the calorimetric data may be in error by rt3% and possibly more in certain unfavorable circumstances.

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Fig. 1.-Adsorption Spheron 6 (orig.), run 1 AA, run 2 VV,run 3 Spheron 6 (lOOO")! . 0 Spheron 6 (2700'), nm. (Open symbols adsorption, closed desorption.) condensed into a trap at -195", and the last third discarded. From this trap, one-third was sublimed by pumping, and the middle third was expanded into the previously evacuated storage bulb for use in the present adsorption studies. Ammonia gas of 99.9% purity also was obtained from the Matheson Company. This gas was liquefied into a trap at -79". One-third of this was pumped out, and the middle third was condensed into a second trap at -79". From this trap one-third was evaporated by pumping, and the middle third was expanded into the storage bulbs as with

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Apparatus and Procedure.-The adsorption isotherms were determined on a conventional volumetric apparatus and heat measurements were carried out in the calorimeter system of this Laboratory, which has been described in detail.9 The "constant" temperature-bath of -79" was obtained by using finely powdered Dry Ice without stirring in a large Dewar vessel surrounding the deeply immersed calorimeter. The heat capacity of the calorimeter and sample system was determined frequently by means of a small calibrated electric heater permanently incorporated in the center of the calorimeter. The temperature changes in the system produced duoring calibration and adsorption were of the order of 0.2 , and were measured to 0.002' by a single junction copper-constantan thermocouple whose e.m.f. was amplified with a d.c. breaker amplifier and fed into an automatic recorder. The method of calculation of the heat data has been outlined by Beebe and Dell.10 As might be expected, there was more evidence of variation in the bath temperature in the Dry Ice system than in earlier experiments using ice-water a t 0' and liquid nitrogen at -195'. We observed minor fluctuations in the timetemperature curves and drifts in the temperature of the calorimeter system which were in some instances as high as 0.02" in the 20 to 25 minute interval necessary for a calibration or the heat measurement for a given adsorption increment. I n the calorimetric measurements we were concerned with the temperature differential between the inner thermojunction and the heavily shielded reference junction which was immersed in the Dry Ice in the space outside the glass mantle of the calorimeter. An exact measurement of the absolute temperature of the bath was not essential for the calculation of the heat of adsorption. I n all experiments, (9) C. H. Amberg,

W. B. Spencer and R. A. Beebe,

Can. J . Chem.,

88, 305 (1955).

(IO) R. A. Beebe and R. M. Dell, THISJOURNAL, 69,746 (1955).

Results and Discussion Carbon Dioxide Adsorption.-The isotherms for the adsorption a t -79" of carbon dioxide on the heat-treated Spheron 6 carbon black series are given in Fig. 1. I n order to make allowance for any differences in the absolute surface areas, the results are plotted on the basis of volume adsorbed per square meter of surface as determined by the BET nitrogen method. It is seen from Fig. 1 that carbon dioxide behaves in a manner closely analogous to that of sulfur dioxide on this carbon series.1° Presumably the isotherms reflect the high oxygen content in the untreated surface and the reduction of the number of oxygen complexes in the heattreated surfaces. It should be noted that the untreated Spheron carbon black has a strong attraction for sulfur dioxide, ammonia and carbon dioxidelo,'' and that this attraction is greatly diminished] in the case of each of the three gases, upon heat treatment of the carbon. It may at first seem surprising that these gases, which are so dissimilar chemically, should exhibit such similar behavior with respect to adsorption on these carbon surfaces. The polar centers due to oxygen sites on the untreated carbon black may interact with permanent dipoles, quadrupoles and induced dipoles depending on the specific properties of the adsorbate. Thus the non-polar carbon dioxide molecule in the adsorbed state and especially in the field of a polar center may behave very much like a molecule with a permanent dipole. It should be noted that the adsorption and desorption isotherms for carbon dioxide on the untreated Spheron fail to coincide. This effect is possibly associated with some sort of interaction with the oxygen complexes on the surface, since it is completely removed by heat-treatment of the adsorbent even a t 1000". A similar effect has been observed with ammonia on untreated carbon blacksll?l 2 and it is discussed elsewhere. With Spheron (2700) as adsorbent the initial portion of the isotherm for carbon dioxide, like those for sulfur dioxide and ammonia, is somewhat convex to the pressure axis. This suggests a low heat of adsorption for the carbon dioxide-Spheroa (2700) system, which is indicated by the calorimetric results on similar blacks given in Figs. 3 and

4. (11) R . A. Beebe and R. M. Dell, (bid., 69, 754 (19.55). (12) (a) J. M. Holmes and R. A. Beebe, Can. J . Chem., 85, 1542 JOURNAL, 61, lt184 (1957). (1957); (b) THIS

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STUDIES ON ADSORPTION OF GRAPHITIZED'CARBON BLACKS

I n Fig. 2 we have given the isotherms for carbon dioxide a t - 79" on several different heat-treated carbon black samples. All these isotherms are slightly convex to the pressure axis a t low coverage. From all that is known about these samples, we would conclude that the degree of homogeneity of surface would increase with increasing particle size and consequent decreasing surface area. On this basis the homogeneity would increase in the order Spheron (2700), Shawinigan (3000), Sterling FT (2700), Sterling MT (3100). Although the differences in the carbon dioxide isotherms are small there is a slight trend in the above order toward more convex isotherms at low pressures. This effect is much more pronounced with ammonia adsorption to be described below. Heat of Adsorption of Carbon Dioxide.-The calorimetrically measured heats of adsorption of carbon dioxide at -79" on Sterling FT (2700) and Sterling MT (3100) carbon blacks are given in Figs. 3 and 4, respectively. Heats of sublimation and vaporization of C 0 2a t 79" are indicated, the latter being calculated by extrapolation of appropriate liquid-vapor equilibrium data. As might be predicted from the isotherms, the heat curves for the adsorption of C02 on the FT (2700) and M T (3100) are very similar. The initial high heat on FT (2700) is not present with MT (3100), probably due to the greater surface homogeneity of the latter. Aside from this difference both curves lie between the heats of evaporation and sublimation of COZ, and exhibit the typical rlse to a maximum due to lateral interaction during formation of a monolayer as observed with noble gases on these adsorbents.5.9 It is noteworthy that the position of the maxinis in the heat-coverage curves of Figs. 3 and 4 is more consistent qith a cross-sectional area u of approximately 24 A.2, such as that reported for COz on anatase by Pickering and Eckstrom13 than with the lower values of 14.1 and 17.0 A.z calculated from an assumed hexagonal close-packing in solid and liquid monolayers respectively. l4 Thus on FT (2700) for Q values for C02 of 14.1, 17.0 and 24.4 the corresponding V,, values for COz based on the BET nitrogen surface area of 12.5 sq. m./g. are, respectively, 3.29, 2.73 and 1.89 cc./g. The corresponding Vm values for M T (3100) are 1.66, 1.37 and 0.95 cc./g. The smallest V , values, based on Q = 24.4 A.2,correspond most closely to the positions of the maxima in the heatcoverage curves, although it would appear that there is no sharp transition from the first to the second monolayer. It is of interest to compare these heat data for carbon dioxide with those for the gases previously studied, lo*ll although in making this comparison we must remember that the heat-treated carbon surfaces used in the present work are not identical with the Spheron (2700) used in the previous studies. As was the case with sulfur dioxide and ammonia, the adsorption of carbon dioxide on the highly graphitized carbon black surfaces produces heats which are not greatly in excess of the heats of (13) H. L. Piokering a n d H. C. Eckstrom, J . Am. Chem. Soc.. 74,

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4775 (1952).

(14) P. H. Emmett a n d S. Brunauer, obzd., 69, 1553 (1017).

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PIPO. Fig. 2d-Adsorption of COZ on graphitized carbon blacks a t -79 : Spheron 6 (2700"), 0.; Sterling FT (2700"), A A ; Sterling ICZT (3100°), run 1 V, run 2 V I ; Shawin.igan (3000°), OH.

(Open symbols adsorption, closed desorptlon.)

Fig. 3.-Calorimetric heats of adsorption of COZon Sterling FT (2700") a t -79": run 1 , o ; run 2, Q. I

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vaporization. As with sulfur dioxide, there is evidence of lateral interaction between the adsorbed molecules as is shown by the increase in heat of adsorption up to a maximum before the completion of the first monolayer. It is further noted that the heats shown in Fig. 3 on Sterling FT (2700) indicate that there are still some sites a t low coverage of somewhat higher than average energy with respect to carbon dioxide adsorption. Such sites appear to be absent on Sterling M T (3100) as seen from Fig. 4. The heat vaIues of Fig. 3 and 4, while in excess of the heat of vaporization, are definitely lower than the heat of sublimation, indicating that the properties of the absorbed layer approach those of a two-dimensional liquid. This phenomenon has

W. B. SPENCER, C. H. AMBERG AND R. A. BEEBE

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1 2 3 4 Volume adsorbed, cc./g. (S.T.P.). Fig. 5.-Calorimetric heats of adsorption of NH3, on Sterling MT (3100") a t -79". (Open symbols adsorption, closed desorption.)

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1 2 3 Volume adsorbed, cc./g. (S.T.P.). Fig. 7.-Entropies of adsorption a t -79": (a) C02 011 Sterling FT (2700"); (b) C 0 2 on Sterling NIT (3100"); (c) NH, on Sterling M T (3100'). Net differential entropy ( SS - SI), ---------; net integral entropy ( SS - SL),

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Ammonia Isotherms as a Means of Characterizing Carbon Surfaces.-In Fig. 6 are shown the 0.3 isotherms for ammonia a t -79" on Spheron (2700) and Sterling M T (3100). As was pointed out i? 0.2 connection with the COZ isotherms these two adsorbents represent the extremes with respect to surface homogeneity among the heat treated carbon blacks, the Sterling M T (3100) being the 0.2 0.4 0.6 0.8 1.0 most homogeneous, and the Spheron (2700) the PIPO. Fig. 6.-Adsorption of NHa on graphitized carbon blacks least so. Of all the different means of charactera t -79": Spheron 6 (2700"), A; Sterling MT (3100°), 0. izing the surfaces of the heat treated carbon blacks the ammonia isotherms offer the advantages of (Open symbols adsorption.) being easy to obtain experimentally and of showing been observed in other gas-adsorbent systems near a wide variation with the different carbon surfaces. the freezing point of the bulk adsorbate.16 I n For instance the ammonia isotherms for Spheron contrast to this behavior of the two-dimensional (2700) and Sterling M T (3100) are markedly diffilm of adsorbed carbon dioxide, it should be noted ferent with much lower adsorption, up to 0.5 relathat this substance in the bulk phase can exist in tive pressure, on the latter more homogeneous the liquid state a t -79" only at pressures far in surface. Such differences while vaguely detectexcess of those encountered in the present work. able, are much less marked with carbon dioxide Heat of Adsorption of Ammonia.-The calori- as the adsorbate gas. It is interesting that the metric heat data.for ammonia a t -79" on Sterling isotherms for ammonia on these two surfaces can M T (3100) have been determined in the course of vary so widely while the heats are so nearly the the present work and are presented in Fig. 5. If same; this point is discussed below in connection due allowance is made for the difference in specific with entropies. . surface areas of the two adsorbents, this heatR. N. Smith, et u Z . , ' ~ have shown that Graphon, coverage curve is almost identical with that for which is essentially the same as Spheron (2700), ammonia-Spheron (2700) previously reported from has retained a low percentage of oxygen, presumthis 1aboratory.l' It is noteworthy that in Fig. 5 ably on its surface, and that this oxygen can be there is evidence, a t low coverage on the Sterling removed by reduction in hydrogen at 1000". I n MT (3100) surface, for high energy sites with re- connection with some other work in this laboratory, spect to ammonia adsorption but that such high the Sterling MT (3100) has been subjected to this energy sites with respect to carbon dioxide ad- hydrogen treatment at 1000" and the ammonia sorption are absent on the same adsorbent (Fig. 4). isotherm at - 79" has then been determined. This This may indicate that this initial adsorption in- isotherm has been reported elsewhere.lZb It is of volves some type of hydrogen bonding, possible interest here that the hydrogen reduction at 1000" with NH, but not COz. (16) R. N. Smith, J. Duffield, R. A. Pierotti and J. Mooi, THIS 5

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(15) R. A. Beebe, B. Millard and J. Cynarski, ibid., 76, 839 (1953).

JOURNAL,

60, 495 (1956).

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June, 1958

VANDER WAALSINTERACTION OF GASESAND SOLIDS

makes virtually no diffeience in the shape of the ammonia isotherm for Sterling M T (3100) except that the nearly vertical portion of the curve rises somewhat more steeply after the hydrogen treatment. I n a previous publication on Spheron blacks1’ it was concluded, on the basis of the isotherm shape and in particular on the basis of the calorimetrically measured heats of adsorption for ammonia, that patches of an adsorbed monolayer are initiated a t ‘(active” centers on the surface and that these patches grow and finally merge by virtue of lateral hydrogen bonding between adsorbed ammonia molecules. I n order to explain the more extreme form of the isotherm on Sterling M T (3100) as compared to Spheron (2700), it may be postulated that fewer “active” centers are present here to initiate patch formation and that the patches, once initiated, grow and merge at about 0.55 to 0.60 relative pressure. The relatively small effect of the hydrogen treatment of Sterling MT (3100) on the ammonia isotherm on this material suggests that the “active” centers for initiating ammonia adsorption are not oxygen complexes since these probably are removed by the hydrogen treatment. It may well be that the sites for initiating the ammonia patches are topographical irregularities such as graphite crystal defects, inter-crystalline boundaries, or edges. l 7 Entropies of Adsorption.-In Fig. 7a,b are shown integral and differential molar entropies of adsorption for COz on FT (2700) and M T (3100), respectively, referred to the liquid state. They were calculated by the method of Drain and Morrison’s from appropriate isotherm and heat data. It will be noted that they both lie in the main between Xvap and &ubl for COz, indicating that the adsorbed film approaches a two-dimensional liquid. The minimum of each curve, indicative of monolayer formation, lies close to the V , values indicated by a molecular area of 24.4 A.z. There is a (17) J. G. Aston and J. Greyson, ibzd., 61, G13 (1957). (18) L. E. Drain and J. A. Morrison, Tiand. Faraday SOC., 48, 840 (1952).

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fairly definite minimum a t low coverage for FT (2700), indicating a comparatively strong interaction with part of the surface, and the completion of a pseudo-monolayer similar to that observed by Corrin for the system heptane-ferric oxide.lg This minimum is much less pronounced with NIT (3100), due probably to the fewer high energy sites. In Fig. 7c are similarly calculated curves for NH, on M T (3100). These curves confirm what has been inferred earlier about the adsorption process. Thus the sharp minimum at low coverage arises from a comparatively strong interaction with the more “active” portion of the surface, a portion on M T (3100) which affects adsorption of NHB but apparently not COz. With NH, these initially adsorbed molecules could act as nuclei for patch formation, and the entropy of the ammonia film at higher coverages is consistent with the proposed picture of the growth of patches in that it varies little from and gradually approaches that of the bulk liquid. This is the region where the isotherm indicates quasicondensation at a relative pressure of 0.5-0.6. As previously stated, it is interesting that the heat curves are similar for ammonia on Spheron G (2700)” and M T (3100) while the isotherms are quite different. A direct consequence of this must be a difference in the entropy curves. On comparison of these curves, it appears that entropies are higher for Spheron 6 (2700), which probably indicates a less liquid film. The initial minimum is less pronounced and the approach to the nionolayer minimum at the liquid “level” is steeper, all of which might mean a state somewhere between true patch formation and adsorption on a heterogeneous surface with a more normal energy distribution. Thus, while the MT (3100) would have two pseudo-surfaces which are widely separated energy-wise (one might think of them perhaps as the nuclei and regions of growth of the patches), the Spheron 6 (2700) would show a more gradual transition from one to the other. (19) M. L. Corrin, J. P h w Chem.. 59, 313 (1955).

THE NATURE OF THE VAN DER WAALS INTERACTION OF GASES AND SOLIDS. I. SECOND-ORDER INTERACTION BY MARKP. FREEMAN Contribution from the Department of Chenzistry, University of California, Berkeley, California Receiued Februaru l & 1968

I n Part I of this paper, the model for high temperature interaction of gases and surfaces first proposed by Halsey, et al.9 is identified with more conventional adsorption theories by showing its thermodynamic equivalence t o the Gibbs boundary condition. An improved model for gas-surface interaction energy is explored (using inverse 9th power repulsion and inverse cube attraction) and is shown to fit the experimental data very well. It is also shown that a consequence of this fit is that these high area surfaces are probably more nearly homogeneoua than is commonly supposed.

Although he had a very clear picture of what the nature of an interface is, Gibbs was able to show that the true situation must be thermodynamically

up to a discontinuous geometric surface. All of the excess (or deficient) molecules and other thermodynamic quantities are then arbitrarily assigned to

phases running continuously and homogeneously

geneous Substances.