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DONALD GRAHAM
Vol. 61
GEOMETRIC HETEROGENEITY IN THE ADSORPTION OF NITROGEN ON GRAPHITIZED CARBON SURFACES BY DONALD GRAHAM Contribution No. 886' from Jackson Labordory, Organic Chemicals Department, E . I . du Pont de Nemours and Company, Inc., Wilmington, Delaware Received February $6,1967
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The effects of small degrees of geometric (strong site) heterogeneit have been separated from isotherms for the adsor tion of nitrogen on two graphitized carbon blacks and measured. The surface of Graphon was found to contain 1.259/0 of strong sites and that of P-33 (heat treated a t 2700°), 0.10%. The isosteric heat of adsorption of nitrogen on these strong sites was about 4 kcal./mole or slightly less than twice that for adsorption on the predominant weaker sites. This suggests that the strong sites provide two bonds to each adsorbate molecule, as at an intersection of planes or a t an area of close approach between particles. There was no indication of the presence of more than one type of strong site. At very low cover.tge, and with the effects of the strong sites eliminated, nitrogen was adsorbed more strongly on Graphon than on P-33. However the contribution of interaytion between adsorbed molecules increased more rapid1 with increasing coverage on P-33 than on Graphon suggesting ,hat P-33 offers a lower energy barrier to movement of a&orbed molecules over the solid surface.
Introduction Isotherms for the adsorption of non-polar gases on carbon blacks, graphitized by heating in the absence of air, have indicated high degrees of surface uniformity. These adsorbents have therefore been useful for minimizing the effects of heterogeneity in the study of other factors influencing adsorption processes. Our knowledge regarding the small residual surface heterogeneity of these graphitized carbons has been limited to the concentration of H bonding sites in the carbon surface as determined from water vapor isotherm^.^^^ These sites which presumably result from traces of surface oxidation were found to constitute only a small fraction of 1% of the total surface. In van der Waals adsorption of non-polar molecules, such sites would be no stronger than the rest of the carbon surface and, t o the extent that they represent -OH groups, they would be much weaker. Their effect would therefore either be very small or would appear only in the upper part of the isotherm which is usually obscured by the beginning of second layer adsorption. A type of heterogeneity more important in physical adsorption on these very uniform adsorbents is that resulting from points of contact between adsorbent particles and from geometric irregularities in the solid surface which permit more than one point of interaction with an adsorbate molecule. Evidence of such heterogeneity first came to our attention in data for the adsorption of argon on a graphitized carbon (P-33).4 The lowest part of the isotherm was convex toward the pressure axis resulting in an upward displacement of the linear portion of the isotherm to a degree representing a small fraction of 1% of V m . (At low coverage, the isotherm for adsorption on uniform sites approaches linearity as 1 - 0 approaches unity.) Study of the adsorption of nitrogen at low coverage on two widely used graphitized carbons, Gra-
phon and P-33 (heat trebted at 2700"), was undertaken to measure the extent and strength of the strong sites and, by subtraction of their contribution from the total isotherms, t o obtain lowcoverage isotherms free from the effects of heterogeneity. Similar systems have been studied before but without the emphasis on precise measurements at very low coverage necessary for attainment of these objectives.
Experimental Materials.-The carbon adsorbents used were Graphon and P-33 (heat treated a t 270C1').~ Earlier measurements on these same adsorbents' have indicated that although both are much more uniform than previously available carbon blacks, P-33 is more uniform than Graphon. X-Ray data also show P-33 to be more crystalline than Graphon. The nitrogen employed in the adsorption measurements was obtained from Linde Air Products Co. as 99.99% highpurity dry nitrogen. The nitrogen and oxygen used in the gas thermometer were Linde's spectroscopic grade. Equipment.-The adsorption equipment was conventional except that mercury cut-offs (H. S. Martin Stock Valves) were used instead of stopcocks in the measuring system and three pressure ranges were covered separately. One McLeod gage was designed for measurement of pressures between 0.0001 and 0.2 mm.: a second McLeod gage covered from 0.1 to 8 mm., and a mercury manometer was used for higher pressures. The tubes of the gages and the manometer were ground to ensure uniformity of bore and to minimize sticking of the mercury meniscus. Within each pressure range, the precision of measurement varied from about 3% for the lowest values t o better than 1% for the highest. Method.-Adsorbent sample weights were adjusted to give an adsorbate monolayer volume of about 11 cc. of Nz a t S.T.P. for the total sample. The system, with sample in place, was conditioned by flushing with helium followed by a t least twenty nitrogenadsorption cycles. In desorption, the sample was heated to 120-150" for about two hours and the accessible parts of the measuring system were flamed during- evacuation to about 10-0 m k . To minimize accumulation of error, most of the data were taken sinnlv with desorDtion a t 120-150O and IO4 mm. after ea& *equilibration.^ (A few points in each pressure range were run with adsorbate accumulation as a check on equipment calibration .) Some desorption points were included. Pressure readings below 1 mm. were corrected for thermal transpiration by use of Liang's equation.0 The sample temperatures during adsorption were controlled by baths of liquid nitrogen or liquid oxygen with the
(1) M.H.Polley, W. D. Schaeffer and W. R . Smith, THISJOURNAL, 67, 469 (1953). (2) G.J. Young, J. J. Chessick, F. H. Healey and A. C. Zettlemoyer, ibid., 68, 313 (1954). (6) Both carbon samples were provided by Mr. W. D. Schaeffer, then of the Research and Development Department of Godfrey L. (3) F. H.Healey, Yung-Fang Yu and J. J. Chessick, ibid., 69, 399 Cabot. Inc., Cambridge, Massachusetts. (1955). (6) 8. C. Liang, THISJOURNAL, 67, 910 (1953). (4) M.L. Corrin, private communication of unpublished data.
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GEOMETRIC HETEROGENEITY IN ADSORPTION OF NITROGEN ON CARBON
Oct., 1957
o 0.9
0.01 I
o.oe I
0.03
0.04
0.03
I
I
I
0.06
0.07
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0.08
- 9.0 /
103 x RELATIVE PRESSURE.
of nitrogen on Graphon at 77.5"K.
Fig, 1.-Adsorption
ORIGINAL
0.a
I
LATER
ov
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I
I
I
I
I
I
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
lo 08
103 x RELATIVE PRESSURE.
Fig, 2.-Adsorption
of nitrogen on Graphon a t 90.3%.
DATA
RESULTS
/
,/
vi 0 . 7 D
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EXTRAPOLATION OF LINEAR PORTION OF LOW/
a 0.5
5:
/
0
/
k 0.3
/
/ / /LOW -COVERAGE ISOTHERM /+FOR UNIFORM SITES
/
0 ^ ^
3
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103 x RELATIVE PRESSURE.
///
Fig. 3.-Adsorption
, /
O k
o
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o.002
1
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0.004 0.006 0.006 0.010 0.012 103 x R E L A T I V E PRESSURE.
Fig. 1A.-Adsorption
l
0.014 O J
of nitrogen on Graphon a t 77.5"K. and very low coverage.
temperature (and PO)determined for each point with a gas thermometer containing nitrogen in the first case and oxygen in the second. There was some fluctuation of bath temperature due to changes in barometric pressure. The observed temperatures of the nitrogen bath were 77.5 f O.1"K. for the Graphon isotherm and 77.6 f 0.1"K. for the P-33 isotherm. The oxygen-bath temperatures were 90.3 i O.l°K. for Graphon and 90.4 f O.l°K. for P-33. The effects of these small temperature fluctuations were minimized by use of relative pressures and then largely averaged out by using a large number of experimental measurements. Volumes of adsorbate required for the first monolayer (V,) were determined by application of the BET equation to the nitrogen isotherms. The difficulties previously noted' in using the B E T equation for adsorption on very
of nitrogen on P-33 a t 77.6"IC.
uniform surfaces were avoided by use of the coverage limits suggested by MacIver and Emmett .'
Experimental Results The physical constants of the adsorption systems studied, including the distribution of sites, are reported in Table I. Representative data for adsorption of nitrogen on Graphon and on P-33 (heat treated a t 2700"), each a t two different temperatures, are reported in Figs. 1 through 4. For the first case, the adsorption of nitrogen on Graphon at 77.5"K., the isotherm up to e 0.5 is shown in Fig. 1. The data will be used as explained later, to bring out the behavior of the isosteric heats of adsorption with the effects of geometric heterogeneity removed. The lowest part of this isotherm is expanded ten-fold in Fig. 1A to facilitate determination of the extent
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(7) D. 8.MacIver and P. H. Emmett, THIS JOURNAL, 60,824 (1956).
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Vol. 61
DONALD-GR AHAM
L
5001 103
0.1 I
RELATIVE PRESSURE.
of nitrogen on P-33 a t 90.4"K.
Fig. 4.-Adsorption
z
x
/
0.2 I
0.3 I
0.4 I
O.! I
FRACTIONAL COVERAGE ( 8 ) .
Fig. 6.-Equilibrium function plots for adsorption of nitrogen on Graphon and P-33 at 90.3 and 90.4"K., respectively. Solid lines represent uniform adsorption; dotted lines include effects of strong-site heterogeneity.
SI00
3
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1300
y
I
I
I
0.1
II
I
0.2 0.3 0.4 FRACTIONAL COVERAGE (81.
0.5
Fig, 5.-Equilibrium function plots for adsorption of nitrogen on Graphon and P-33 a t 77.5 and 77.6"K., respectively. Solid lines represent uniform adsorption; dotted lines include effects of strong-site heterogeneity.
TABLEI PHYSICAL CONSTANTS OF ADSORPTION SYSTEMS Adsorbent
Wt. adsorbent (g.) Crystallographic C parameter (A.1 Temp. of adsorption measurements, OK. Total content of first adsorbed monolayer, V m , in ml./g. a t
Graphon
- -0.5931 6.92
77.5
90.3
18.80
18.00
P-33
3.4825 6.82
77.6
90.4
2.863
2.740
0.0030
.0.0029
2,860
2.737
S.T.P. V , (strong sites only 1 Vm (predominant sites only)
0.234 18.57
0.224 17.78
2400'0
012 013 014 FRACTIONAL COVERAGE ( 8 ) .
011
01!
Fig. 7.-Isosteric heats of adsorption. Solid lines represent uniform adsorption; dotted lines include effects of strong-site heterogeneity.
of strong site heterogeneity. Data for the other three cases are reported in Figs. 2, 3 and 4. Each of these figures presents the isotherm to e 0.5 and also the expanded low coverage portion, by use of two sets of scales. (1) The Extent of Geometric Heterogeneity.Figure 1A shows how the extent of the strong site heterogeneity of Graphon was determined from the 77.5"K. isotherm. In adsorption on an assembly of identical sites, as the coverage (e) approaches zero, the quantity (1 - e) approaches unity and the isotherm approaches a straight line. The lowest part of this isotherm is convex toward the pressure axis and then becomes linear. If it is assumed that this linearity indicates saturation of the strong sites, the value of V a t which an extrapolation of this line reaches the V axis approximates the extent
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Oct., 1957
GEOMETRIC HETEROGENEITY IN ADSORPTION OF NITROGEN ON CARBON
of the strong sites as measured by the adsorption of nitrogen. (This extrapolation is represented in Figs. lA, 2, 3 and 4 by dotted lines.) The contribution of the strong sites to the measured isotherm is removed and an isotherm representing uniform adsorption is obtained by lowering the isotherm until the intercept of the extrapolation stands a t T‘ = 0. An isotherm representing the strong sites is obtained from the difference between the measured isotherm and that of the predominant weaker sitesa8 The strong sites in the Graphon surface were found t o represent 1.2401, of the total surface. The result obtained from the 90°K. isotherm was consistent with this value. This quantity was not, however, completely reproducible. The points shown in the isotherm of Fig. 1A were taken early in the investigation. Later measurements, following about 100 adsorption-desorption cycles, and shown in the same figure as (X), indicate a slightly lower degree of heterogeneity. This may be due either to a change in the carbon surface or possibly t o just a shift in the powdered adsorbent, modifying the areas of close approach between particles. The geometric heterogeneity of P-33 was less than that of Graphon, about O . l O ~ oof the total surface, and again, the isotherms a t different temperatures gave consistent results. (2) Strength of Adsorption Sites (Isosteric Heats) A. Strong Sites.-Isosteric heats of adsorption of nitrogen on the strong sites only of Graphon and of P-33 were 4(* 10%) kcal./mole, with the level for Graphon slightly higher than that for. P-33. Although the precision of the low coverage isotherms from which the heats were calculated does not justify more detailed treatment, two tentative conclusions may be advanced. First, the magnitude of the heats is slightly less than twice that for the predominant sites. This suggests adsorption a t intersections of planes in the adsorbent surface or a t other regions where an adsorbed molecule can interact with two different points in the adsorbent surface. Second, the absence of any apparent variation in heat of adsorpt,ion with surface coverage indicates the probability that only one type of strong site is measurably involved. (8) The method used here is dso briefly outlined, without examples, in another paper by the writer, “Separation of Variables in Physical
Adsorption” presented at the Second International Congress of Surface Activity, London, April 8-12, 1957. This paper will appear in the proceedings of that meeting.
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B. Predominant Sites.-More precise results were obtained from the isotherms representing adsorption on the weaker, predominant sites, the effects of heterogeneity having been removed. The isotherms were first smoothed by the use of a plot of the equilibrium function, 6/[(l - 6)(P/Po)] vs. the coverage, 6, to exaggerate the scatter of the experimental measurements. These plots were linear within the limits treated, as shown in Figs. 5 and 6. (The dotted lines represent uncorrected low coverage data, including effects of strong-site heterogeneity.) The isosteric heats computed from these smoothed isotherms, reported in Fig. 7, show the effects of slight but increasing interaction, even a t the lowest coverage. It is particularly interesting to note that the lines cross (as do the EF vs. 6 plots of Figs. 5 and 6). We find that an isolated nitrogen molecule (coverage approaching zero) is more strongly bound t o Graphon than to P-33, but that the contribution of attractive interaction between adsorbed molecules to the total adsorption bond builds up more rapidly with increasing coverage on P-33. The first effect may result from to the more open structure of Graphon (larger crystallographic C parameter). Professor J. H. de Boer has suggested that this effect may be related to a previously observed decrease in C-C distance and increase in C parameter accompanying a decrease in the size of a graphite c r y ~ t a l . ~ ~ ~ ~ The more rapid increase in heat of adsorption with increasing coverage observed on P-33 suggests a lower energy barrier t o movement of an adsorbate molecule over the solid surface. This may be the result of a combination of two factors, a weaker bond to the surface, and fewer very strong sites which could act as centers of essentially complete localization. DISCUSSION MAX BENDER.--Were you referring specifically to only
two kinds of sites, the normal average sites and a series of sites of higher affinity or were there adsorption sites covering a spectrum of degrees of strength of adsorption?
DONALD GRAHAM.-AS explained in the paper, the data indicated the presence of only two kinds of sites with the minority strong sites almost twice as strong as the predominant sites. There was no evidence for any broad distribution or “spectrum” of site energies. (9) U. Hofmann and D. Wilm, 2. EEektrochem., 42, 504 (1936). (10) J. H. de Boer, Ree. trav. china., 59, 826 (1940).
