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STEPWISE ISOTHERMS ON CARBON BLACK SURFACES
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DEVELOPMENT OF STEPWISE ISOTHERMS ON CARBON BLACK SURFACES' BY M. H. POLLEY, W. D. SCHAEFFER AND W. R. SMITH Research and Development Laboratory, Godfrey L . Cabot, Inc., Cambridge, Massachusetts Received November 4, 196.9
The adsorption isotherms of argon, nitrogen and oxygen on heat-treated carbon blacks a t - 195" reflect crystallographic structure changes induced in the black by heat treatment. Several types of carbon black were partially graphitized a t a series of temperatures up to 2700'. Heat treatment at these temperatures alters the carbon structure as evidenced by X-ray diffraction measurements. Electron micrographs indicate a transition from approximately spherical particles to irregular polyhedra for the graphitized samples. The original carbon samples exhibit the usual sigmoid type isotherms, whereas with increasing degree of graphitization as many as three ste s appear in the adsorption isotherms a t relative pressurcs of 0.35, 0.63 and 0.86. Qualitative agreement is obtained with t f e theoretical equations of Halsey for multilayer adsorption on uniform surfaces. It is believed that these isotherms reflect the transition from heterogeneous t o homogeneous surfaces.
Earlier studies of adsorption isotherms and calorimetric heats have demonstrated that there is a considerable degree of heterogeneity in the carbon black surface.2 Nitrogen adsorption isotherms were of the usual sigmoid type. The initial heats of adsorption were a little greater than three times the heat of vaporization and decreased rapidly as a function of the fraction of surface covered. It has been assumed that the observed multilayer adsorption is a consequence of surface heterogeneity plus cooperative condensation. Halsey3 has suggested that the sigmoid adsorption isotherm may really be the superposition of a number of stepwise adsorptions on a heterogeneous surface. On the other hand, according to the theories of Fowler and Guggenheim4 and the later developments by Halsey and Hill,3'6-7 multilayer adsorption on a homogeneous surface should give rise to an isotherm composed of a single series of steps. This is based on the hypothesis that the heat of adsorption in the first layer is greater than that in the second, and so on. Attractive interaction forces exist between neighboring molecules adsorbed on sites of nearly equal energy. When a certain critical pressure is reached, two-dimensional condensation will begin. This will produce an inflection or step in the isotherm. Additional steps should occur upon the completion of each successive layer. Orr,8with the adsorption of nonpolar gases on alkali halide crystals, was the first to obtain these steps experimentally. Recently Gulbransens has published isotherms up to P/Po = 0.5 for krypton on graphite which appear to be stepwise isotherms. Experimental The standard B.E.T.'O type apparatus was employed for (1) Presented before the Divibion of Colloid Chemistry at the 122nd National Meeting'of the American Chemical Society, which was held in Atlantic City, N. J., September, 1952. (2) R. A. Beebe, J. Biscoe, W. R . Smith and C. B. Wendell, Jr., J . A m . Chem. Xoc., 69, 95 (1947). (3) G . D. Halsey, Jr., J . Chem. Phys., 16, 931 (1948). (4) R. ' H. Fowler and E. A. Guggenheim, "Statistical Thermodynamics," Cambridge University Press, Teddington, England, 1939, pp. 426-444.
(5) G . D. Halsey, Jr., J . A m . Chem. Xoc.. 7 8 , 2693 (1951). (6) G.D . Halsey. Jr.. ibid., 74, 1082 (1952). (7) T. L. Hill, J . Chem. Phys., 16, 767 (1947). (8) W. J. C. Orr. PTOC. Roy. SOC.(London), Al78, 349 (1939). (9) E. A. Gulbransen and K. F. Andrew, I d . Eng. Chem., 44, 1039 (1952). (10) 8. Brunauer, P. H. Emmett and E. Teller, J . Am. Chem. SOC., 60, 310 (1938).
these measurements. With a mercury manometer the pressure readings were accurate to h O . 1 mm. Pressure equilibration was assumed to have been attained when the value did not change more than 0.2 mm. in a 5-minute interval. The time for this equilibration depended both upon the sample and the isotherm region, varying from two minutes to one hour. The nitrogen, oxygen and argon adsorbates were obtained from Matheson Company and stated to be 99.9% pure. The following commercial grades of carbon black were used as raw materials: Spheron 6, a medium processing channel black; Sterling R, a semi-reinforcing furnace black; P-33, a fine thermal black; Vulcan 3, a high abrasion furnace black; and Thermax, a medium thermal black. Since the same type of data has been obtained for each black, on1 the first three will be discussed in the present paper. EacE of these carbon blacks had been heat-treated for 2-hour periods in an induction furnace a t temperatures ranging from 1000' to 3100°.11 The sample was contained in a graphite crucible. Virtually all air was driven off in the initial heating stages and excluded for the duration of the heating period. More complete details of the preparation, X-ray crystallite dimensions and other physical properties of these blacks, are discussed by Schaeffer.12
Results and Discussion Evacuation Temperature.-Prior to the detailed study of the adsorptive properties of heat-treated carbon blacks, it was essential to determine whether the standard outgassing temperature of 200" would be sufficient to obtain reproducible surfaces. Gulbranseng has reported that there is a marked difference in the isotherm for krypton on graphite if the sample had been outgassed at 950" rather than a t room temperature. To test for a similar effect on our samples, a quartz adsorption cell was designed in which the sample could be evacuated a t 1000°followed by the normal adsorption determination with no intermediate exposure of the sample to air. Since this adsorption isotherm checked the one following the normal procedure, the 200" evacuation temperature was retained. Adsorption-Desorption Comparison.-Several complete adsorption-desorption isotherms were carried out on the heat-treated carbon samples, Except in the high relative pressure region, i.e., P/Po > 0.75, the desorption curve is superimposed on the adsorption curve, indicating that the process is completely reversible. The small hysteresis loop may be due either to slow pressure equilibration or to interparticle condensation. (11) The heat-treated carbon black samples will be referred t o as P-33 (2700"); for example, to indicate a sample of P-33 which has been heated for two hours at 2700'. (12) W. D. Schaeffer. W. R. Smith and M. H. Polley. Ind. Eng. Chem., forthcoming publication.
M. H. POLLEY, W. D. SCHAEFFER AND W. R. SMITH
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Vol. 57
Duplication of Isotherms.-Check runs on two separate preparations of Spheron 6 (1000") indicated excellent duplication of samples and the sensitivity of adsorption measurements to changes in surface morphology. Agreement is estimated to better than = k l % . Surface Area of Samples.-Nitrogen surface area values are listed in Table I. It had been stated previously2 that I' devolatilization" at 927" had little effect on the surface area of carbon black. TABLE I KITROGENSURFACEAREAS OF HEAT-TREATED CARBON BLACKSIN m.2/g.a Carbon black
None
Treatment temperature, "C. 1500 2000 1000
2700
85.4 84.1 Spheron 6 114 91.1 88.0 25.2 24.9 25.5 26.0 Sterling R 27.9 12.5 12.9 12.6 13.1 P-33 15.5 a Determined from the isotherm "b" point, using 16.2 A.2 for the area of the nitrogen molecule. The B.E.T. plots are not linear.
c
0
Fig. 2.-Argon
0.2
0.4 PlPo.
0.6
0.8
1.0
isotherm a t -195' on heat-treated Sterling R (2700").
Upon heating the carbon black to 950", however the evolution of small amounts of hydrogen commences. This leaves a system of narrow pores which collapse on heating the sample to higher temperatures. This will produce a surface area increase a t first, followed by a sharp decrease. Electron micrographs of these heat-treated carbon samples indicate a decrease in particle diameter together with a transition from the spherical particle to irregular polyhedra. These two effects would produce a surface area increase, obviously in direct contradiction to the data in Table I. However, an increase in density with increasing temperature of heat treatment, together with an increased tendency toward agglomeration must counteract the effects of particle shrinkage and irregular shape. 0 0.2 0 0.2 0 0.2 0 0.2 0.4 0.6 0.8 1.0 Stepwise Isotherms-Typical argon isotherms PIPO. for the three types of carbon black are shown in Figs. 1-3. The blacks have a surface average Fig. 3.-Isotherins, argon on P-33 carbon black a t -195'. particle diameter of 354, 1222 and 2250 A. respec- original black and each treatment temperature. tively, and so provide a wide range of surface areas Thus on comparing the three blacks it may be seen (see Table I). Scale diagrams of the quasi-graphitic that not only is the growth of the crystallites a crystallites, which are randomly oriented within function of temperature but also a function of the the discrete carbon particles, are included for the carbon particle diameter. This crystallite growth is reffected in the shape of the argon isotherms. The slope of the isotherm in the region of the monolayer decreases, while a t the second layer an inflection or step appears. This occurs a t 0.35 relative pressure with the adsorbate argon. On the more highly heat-treated samples a second step appears a t P/Po = 0.63 and a third a t P/Po = 0.86. These steps correspond to the filling of the 2nd, 3rd and 4th adsorbed layers. Precisely, the steps occur at multiples of 0.85 Vm because either the molecules are not close-packed in the first layer or the presence of crystallite edges increases the surface area by 15%. These stepwise isotherms are believed to be a consequence of increasing surface homogeneity. The electron micrographs showed a gradual transi0 0.2 0 0.2 0 0.2 0 0.2 0.4 0.6 0.8 1.0 tion from the spherical to the irregular polyhedra. The X-ray diffraction measurements indicated PIPO. Fig. 1.-Isotherms, argon on Spheron 6 carbon black a t growth of the crystallites. In fact, the crystallites for P-33 (2700") are of the order of 200 A., and -195'.
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April, 1953
STEPWISE ISOTHERMS ON CARBON BLACK SURFACES
those oriented on the surface of the particle may be responsible for the "flat-edged" particles seen in the electron micrographs. From the determination of c, twice the interplanar distance within the crystallite, the degree of graphitization may be determined. Wherpas untreated carbon blacks have a c-vglue of 7.3 A., P-33 (2700") has a c-value of 6.84 A., nearly the same as the value for graphite of 6.74 A. Such an ordered system composed of large planar areas on the surface of the carbon black particles should be expected to provide a homogeneous surface for cooperative condensation. It was noted earlier that the magnitude as well as the number of steps appear to be a function of crystallite dimension for a given sample. If the isotherms are plotted as fraction of surface covered against relative pressure, samples having similar crystallite sizes should have similar isotherms. The two samples, Sterling R (2700") and P-33 (2000'), have vastly different particle diameters but almost. the same degree of graphitization. The two isotherms are coincident up to PIP0 = 0.4 and only slightly displaced a t higher relative pressures. The isotherm of a sample of P-33 (lOOOo), a less highly graphitized black, when similarly plotted, is nearly a sigmoid type isotherm. The slope of each of these isotherms in the region of P/Po = 0.35 illustrates the differences between the two states of graphitization. The value of the slope at the 0.35 relative pressure region was calculated for all the experimental argon isotherms and plotted as a function of both the La and L, crystallite dimensiop. This is a lineoar correlation as far as La = 46 A. and L, = 18 A., whereupon there is a shgrp break, forming a step that extends to L, = 93 A. and L, = 45 A. Further increase in crystallite dimensions has no effect upon the isotherm slope a t 0.35 P/Po. Use of such a correlation curve provides a rapid means of estimating crystallite dimensions for new carbon iamples. Adsorption of argon a t -195" in the region from PIPo = 0.25 to 0.50 is sufficient to determine the degree of graphitization of that sample. It had been called to our attention13 that these stepwise isotherms may be due to a freezing point lowering of the adsorbed argon, since the isotherms were determined well below the freezing point of the argon. Consequently, both nitrogen and oxygen isotherms a t -195" were determined on many of these same carbon samples. A typical set of isotherms for the three adsorbates on P-33 (2700") is shown in Fig. 4. The size of the first isotherm step is proportional to the interaction energy of the adsorbed gas, nitrogen having the lowest interaction energy. If the isotherm steps are a result of a freezing point phenomenon] the extent of surface homogeneity must play an important part. Comparison with Halsey's Equations.-Several equations have been developed by Halsey3Jn6 describing multilayer adsorption isotherms. With the assumption that exp { -AEo/Em) = 0.2 and A E J R T = 4 using equation 13 given in the second paper,6 or that AEBIAE, = 2 and AE,/RT = 4 using equation 8 of the latest paper,6 stepwise isotherms are obtained. A comparison of these (13) J A. Norcison, private communication.
0
0.2
0.4
47 1
0.6
0.8
1.0
PlPo. Fig. 4.-Isotherms
at
- 195'
on heat-treated P-33 @ -; nityogen,
(2700"): oxygen, - - - O - , -argon,* -0 --.
two theoretical equations with the experimental isotherm for argon on Thermax (3100') is shown in Fig. 5 . Correct choice of the variable parameters in the Halsey equations will produce a gradual transition from the sigmoid to the stepwise isotherm. Neither of the two theoretical equations precisely describes the experimental data but both predict steps a t the completion of each adsorbed layer occurring a t relative pressures of 0.35, 0.75 and 0.87.
4
a
s
3
2
1
0.2
0.4 0.6 0.8 1.0 P/Po. Fig. 5.--Comparison of theoretical and experimental isotherms: equation 13, - - - -; equation 8, -- - -., Thermax (3100°),-0 -. 0
Conclusions Several series of carbon black adsorbents have been prepared which demonstrate a gradual transition from a heterogeneous to a homogeneous surface. It is gratifying to observe that the degree of homogeneity developed is sufficient to provide experimental confirmation of the stepwise isotherms predicted for a uniform surface.
