May, 1956
OXIDATION OF GRAPHITIZED CARBON BLACK
689
THE OXIDATION OF GRAPHITIZED CARBON BLACK' BY W. R. SMITHAND M. H. POLLEY Research and Development Laboratory, Godfrey L . Cabot, Inc. Cambridge, Massachusetts Received December 14, 1866
The oxidation of Fine Thermal carbon black a t 600° increases its surface area about sixfold. This increase is not accompanied by a corresponding decrease in particle diameter and hence must be interpreted as development of .orosity. A sample of Fine Thermal black graphitized at 2700' does not develop porosity when similarly oxidized. The g g h t area increme obtained on oaidatiw of the graphitized sample is related to decrease in particle diameter. These results may be interpreted as preferential attack of oxygen on a heterogeneous surface in the first case and random attack on a u@form surface in the second. A true chemical process has been used to confirm the nature of the carbon black surface previously predicted by physical methods.
Partially graphitized carbon blacks prepared in this Laboratory have been used as adsorbents in a number of investigations concerned with the role of surface activity in absorptionmzOaGraphitized Fine Thermal blacks4 have been of most recent interest because of the remarkable uniformity of their surface.3 These blacks, graphitized6 a t 270O0, have been shown3 to provide stepwise isotherms for the adsorption of argon, krypton and nitrogen a t -195' rather than the smooth sigmoid type characteristic of ungraphitized blacks. While similar results have been observed with Graphon [Spheron 6 (2700°)],they are much more striking in the case of the graphitized Fine Thermal blacks. These experiments offer excellent confirmation of the theories of Hill' and of Halsep and, following their theories, indicate that the surface of this adsorbent is extremely uniform with regard to distribution of energy sites. It occurred to us that further confirmation of this uniformity, as well as an insight into the development of carbon black porosity, might be obtained by studying the high temperature oxidation of graphitized thermal blacks. Non-graphitized carbon blacks display a surface heterogeneity with regard to distribution of energy sites. This has been established both from the character of the adsorption isotherms as well as from heats of adsorption measurements.2" When a (1) Presented before the Division of Colloid Chemistry at the Meeting of the American Chemical Society, held in Minneapolis, Minnesota, September, 1955. (2) (a) R. A. Beebe, J. Biscoe, W. R. Smith and C. B. Wendell, J . A m . Chem. Soc., 89, 85 (1947); (b) R. A. Beebe and D. M. Young, THISJOURNAL, 18, 93 (1954); (c) L. G . Joyner and P. H. Emmett, J . Am. Chem. Soc., TO, 2353 (1948); (d) J. Mooi, C. Pierce and R. N. 87, 52 (1953). Smith, THIEJOURNAL, (3) (a3 C. H. Amberg, W. B. Spencer and R. A. Beebe, Can. J Chsm., 88, 305 (1955); (b) M. H. Polley, W. D . Schaeffer and W. R. JOURNAL, 87, 469 (1953); (c) 8. Ross and W. Winkler, Smith, THIS Baaed on Thesis presented by W. Winkler in partial fulfillment of the requirements for the degree of Doctor of Philosophy to the Department of Chemical Engineering, Renaselaer Polytechnic Institute, June 1956. ~ 88,330,1011 (1854). (d) J. Singletonand 0.D. Hakey, T H IJOURNAL, (4) Fine Thermal carbon blacks are designated as F T blacks in rubber technology. They are semi-reinforcing in rubber. Representative commercial grades are P-33 and Sterling FT. (6) In the past, it has been the habit to refer to increase in parallel layer group dimensions as increaeing "degree of graphitization." As Warren has pointed out,@there is considerable ambiguity asaociated widh this designation, since it appears that growth of these crystallites to a value of La of the order of 100 A. occura prior to any significant ordering of the layers into the graphite configuration. (6) C. Houakaand B. E. Warren, J . A p p l . Phya., 41, 1503 (1954). (7) T. L. Hill, J . Chem. Phya., 18,767 (1947). ( 8 ) (6) G. D. Haleey, Jr., J . Am. Chem. Soc., 78, 2698 (1951); (b) C;. D. Hakey, Jr., ibid., 74, 1082 (1952).
standard carbon black is treated with air or oxygen a t temperatures of from 300-650' a sixfold increase in area, as measured by nitrogen adsorption, may be noted without appreciable reduction in particle diameter. This development of porosity upon air It is interoxidation has long been recogni~ed.~~'O preted as arising from preferential attack of the oxygen a t high energy sites on the carbon surface. These sites may be associated with the edge atoms of the quasi-graphitic parallel layer groups composing the particle. Long and Sykesll have claimed that these edge atoms are more susceptible to chemical attack than are the atoms in the center of the basal plane. Since it has also been established that high energy sites are destroyed on high temperature (2700-3000°) graphitization,a" then preferred sites for oxygen attack must either be removed or greatly diminished in number. If the surface of the graphitized carbon particle is as uniform as has been suggested, then oxygen attack should occur uniformly over the surface, and the area increase on oxidation should be proportional to the decrease in particle diameter without development of porosity. The present paper presents data confirming these observations.
Experimental The Fine Thermal carbon black used in the present study was taken from commercial production. This grade of carbon black is prepared by thermal decomposition of natural gas diluted with flue gas consisting chiefly of hydrogen. The decomposition occurs in large preheated checker brick filled retorts a t about 1100". Samples of Fine Thermal black were gra hitized to varying degrees by heating in an inert atmospEere at temperatures of from 1000 to 2700". A 40 kw. input Ajax-Northrup converter was used for the heat source. This converter is a spark-gap unit utilizing 18 to 36 kc. at 4400 volts across the induction coil. Details of the "graphitizing" rocedure have been reported in an earlier publication.13 &idation of the carbon black was carried out by weighing samples into a 7 X 1 cm. platinum combustion boat placed in the center of an electric combustion furnace heated to the desired temperature. The flow of air over the sample was by natural draft, achieved by leaving the door of the furnace ajar by about 1 cm. Temperature was measured with a chromel-alumel thermocouple located in the center of the furnace about 3 inches over the carbon sample. After 15 minutes, the sample waA removed and cooled in a desiccator. The per cent. of carbon burned away was determined from loss in weight of the sample. The rate of oxi(9) W. R. Smith, F. 8. Thornbilland R. I. Bray, I n d . En#.Chem.. SS, 1303 (1941). (10) P. H. Emmett and M. Cines, THIS JOURNAL, 61, 1329 (1947). (11) F. J. Long and K. W. Sykes, Proc. Roy. Soc. (London), A198, 377 (1948). (12) W. D. Schaeffer, W. R. Smith and M. H. Polley, I n d . I n g . Chsm., 46, 1721 (1953).
W. R. SMITHAND M. H. POLLEY
690
dation decreased with increasing degree of graphitization of the carbon black. In order to produce the desired degree of oxidation in the 15-minute interval, the temperature of the furnace was increased. Ap roximately 600" sufficed for oxidizing the original FT blaci, while about 800-900" was required to produce a corresponding weight loss for the highly graphitized mmples. The per cent. weight loss on oxidation of the various samples a t increasing temperatures i8 summarized in Fig. 1.
700 800 900 Temp., 'C. Fig. 1.-The oxidation of Sterling FT (original), Sterling FT (lOOO'), Sterling FT (1500') and Sterling FT (2700'). 500
Results and Discussion Pertinent particle size, surface area and X-ray diffraction data for the Fine Thermal black, both original and heat treated at 1000, 1500 and 2700°, are reported in Table I. The nitrogen surface
Electron microscope
c.
None 1000 1500 2700
#
dA, a
A.
2094 2018 1984 1940
Calcd. area,b
Nitrogen surface area,
Parallel
layer
group
dimensions 0
A.
m.n/g.
m.l/g.
~ a ,
15.4 16.0 16.2 16.6
14.5 13.1 12.9 12.6
27.6 37.5 62.0 132
16
x4
14 12
\
g 10
n $ 8 8
-g 4 9
6
2
TABLE I PROPERTIES OF TME STERLINQ FT SERIES temp.,
somewhat larger electron microscope values for the area over those calculated from nitrogen adsorption. However, these variations are not of major significance in the present studies since changes due to porosity will be several times larger. In the last two columns of Table I are dimensions of the parallel layer groups or quasi-graphitic crystallites within the particle. l 2 Attention is called to the remarkable increase in the dimensions of these crystallites with increasing temperature of heat treatment. It is our opinion that the unusual surface uniformity of the 2700" material arises from the dimensions of the crystallites composing the particle. The stepwise isotherm characteristic of this uniform surface is shown in Fig. 2. The smooth sigmoid isotherm obtained for the original Fine Thermal black prior to graphitization is included for comparison. Isotherms for the 1000 and 1500' heat treated samples we found to be intermediate between those of the original and 2700" material.
600
Surface areas were calculated both from adsorption isotherms of nitrogen a t - 195O and from particle size memurements obtained with the RCA-EMU electron microscope.
Treatment
Vol. 60
LO,
A.
16.8 17.6 38.6 88
a d~ = surface average diameter = Z n d 3 / 2 n d Z . E. M. surface area = (6 X 104)/(1.86 X d A ) m.l/g. L. = dimension along parallel plane; L, = dimension perpendicular to layer normal. See ref. 6, and also B. E. Warren "Proceedings of 2nd Bi-Annual Carbon Conference,University of Buffalo, June 10, 1955" (in press).
areas of the samples agree sufficiently well with that calculated from the surface average diameter, as measured by the electron microscope, to indicate that the particles are essentially non-porous. There is a decrease in nitrogen surface area with increasing temperature of heat treatment, due perhaps to some degree of particle sintering. The area, as measured by the electron microscope, appears to increase slightly. However, with increasing degree of graphitization, the departure of particle shape from spherical to rather regular polyhedra becomes marked. This not only increases the difficulty in measuring the particle but it also introduces uncertainty in tke computed area since the expression employed is derived for a system of spheres. This may account for the
0 0.2 0.4 0 0.2 0.4 0.6 0.8 1.0 PIPO. Fig. 2.-Argon isotherms at -195' on Sterling FT (original), Sterling FT (2700'), and S4erling FT (2700") oxidized 77% with suggested configurations of the surface. 0.2 0.4
The effect of oxygen attack on the surface area of the carbon blacks described in Table I is evident from data of Fig. 3. The original Fine Thermal black undergoes an area increase, as measured by nitrogen adsorption, of about 63.5 m.2/g. when some 39% of the black has been burned away. In contrast, the area of the 2700" sample increases only 4 m.2/g. for the same weight loss. Particle size data and areas, as computed from electron microscope measurements of the oxidized carbon blacks are presented in Table 11. Electron microscope measurements show that oxidation does produce some decrease in particle diameter. For example, when 39% of the original FT black was burned away the diameter is reduced about 320 8. This reduction in particle size would account for an increase of only 2.8 m.2/g. It is quite evident that the increase of 63.5 m.2/g. found by nitrogen adsorption must be accounted for principally in the development of an internal porosity, arising from preferential attack of oxygen at certain sites on the carbon black surface. Close examination of the electron micrographs of this oxidized material re-
4
May, 1956
OXIDATIONOF GRAPHITIZED CARBON BLACK
69 1
veals some degree of surface roughness suggestive of porosity.la However, the nature of oxygen attack 70 after the Fine Thermal black has been graphitized at 2700" is quite different. In this case, there is no development of porosity. The small increase in nitrogen area after oxidation, as shown in Table 11, is in agreement with that anticipated on the basis of the decrease in particle diameter as measured in the electron microscope. From purely geometric considerations, assuming uniform removal of carbon from the particle surface, it is possible to calculate the diameter decrease from weight loss of the sample. The areas so calculated are shown in the last column of Table I1 and in the dotted curve of Fig. 3. The calculated values and the areas meas0 10 20 30 40 50 60 70 80 ured by nitrogen adsorption are in good agreement % oxidation. for the FT (2700") sample, demonstrating that Fig. 3.-Change in surface area with increasing oxidation the oxidation has occurred uniformly over the sur- on the Sterling FT series. The dotted line is the calculated area assuming uniform particle diameter decrease. face. a t the surface. They occur oniy on edges facing TABLEI1 toward or away from the aperture. This means PROPERTIES OF THE STERLING FT SERIES AFTEROXIDATION that the crystallites are oriented so that diffractions Electron NitroArea from these two sides are directed through the darkmicroscope gen calod. sursurfrom field aperture, while diffractions from the other Diamface faoe wt. Oxieter, area, area, ~OEE, sides are lost against the dark-field mask. Since Sample dation A. m.'/g. m.'/g. m.r/g. electron diffraction diagrams of the 2700" material Original FT None 2094 15.4 14.5 .. indicate that most of the diffracted intensity is in Original FT 6 1950 16.6 48.8 14.8 the 002 reflection, we have concluded that the great Original FT 39 1771 18.2 78.0 17.1 20.2 20.5 FT(2700°) 77 1155 27.9 preponderance of crystal images seen in dark field are from the 002 reflection of crystallites oriented It is interesting to note in Fig. 2 that after oxida- parallel to the carbon surface. tion of the 2700" graphitized material, the steps Electron micrographs of the FT black graphitized originally observed in the argon isotherm become a t 2700" reveal a striking change from the spherical less pronounced. particles of the original material to regular polyThese data appear to confirm our original opin- hedra with sharply defined faces. These faces are ion that oxygen attack on standard carbon black most evident if the black is dispersed and embedded occurs preferentially at specific high energy sites in polybutyl methacrylate prior to examination. on the surface. These sites may be edge carbon Microtome section^'^ examined under the elecatoms in the layer lattice. Upon heat treatment at tron microscope clearly reveal quite uniform faces 2700", the number of edge sites is greatly reduced. on the graphitized black surface. The area of the For the Fine Thermal blacks studied here, the X-ray majority of these faces is between 1 and 2 X los diffraction data quoted in Table I reveal an increase of about fivefold in the size of the parallel square Angstrom units. Since the average diamelayer groups. This growth occurs presumably ter of the particles measured is about 1600 b. there at the expense of the smaller and less-ordered crys- are of the order of 40 of these faces per particle. tallites within the particle. Since the size of the The first atoms or molecules adsorbed may orient carbon black particle as revealed by the electron preferentially along the intersection of these faces. microscope does not change significantly upon However, since this "edge" area is comparatively graphitization, it is obvious that the surface of the small, it will be filled rapidly, and subsequent adparticle must become more uniform with regard to sorption will then be random on what is, in essence, a surface composed of a single crystal face. the crystal faces exposed. Perhaps the most interesting conclusion to be Recent experimental evidence from dark-field electron microscopy indicates that the surface of the drawn from the present study is that from evalua2700" material is composed primarily of oriented tions of the nature of a surface by physical adsorp002 planes. Dark-field electron micrographs of tion it has been possible to predict and confirm its the FT (2700") sample show well-defined dif- behavior toward a true chemical process. Acknowledgments.-The authors wish to acfraction images at the surface of the graphitized carbon particles. They do not occur symmetric- knowledge the assistance of Mrs. M. M. Chappuis ally around the particle circumference as they would for the electron micrographs and Mr. A. F. Cosman if the parallel layer groups were randomly oriented for the adsorption measurements. We are also indebted to Professor B. E. Warren for providing (13) Electron micrographs have been omitted from this publication X-ray diffraction data. since they do not reproduce clearly on the stock presently used in
L
THIS JOURNAL. request.
The authors will be happy to supply prints upon
(14) M. M. Chappuis, M. H. Polley, and R . A. Schula, AuObes WorZd, 130, 507 (1954).
