76
Langmuir 1987, 3, 76-81
rate constants ( t l l z= 8 s) and thus the literature values quoted in this paper should be considered lower limits to the reaction rate for the conditions employed here (1 N NaOH). Acid-Catalyzed Hydrolysis of PECO&HB. Hydrolysis was carried out by floating PE-COZCH3 (ester side down) on the surface of 50% (w/w) HzS04at 25 & 2 "C for the specified time.
The film was rinsed 4 times in water and once in acetone and air dried. Literature values for the hydrolysis rate were determined from the pseudo-first-order rate constants established under identical conditions.26 Registry No. polyethylene, 9002-88-4.
Use of N2 vs. C 0 2 in the Characterization of Activated Carbons J. Garrido, A. Linares-Solano, J. M. Mardn-Mardnez, M. Molina-Sabio, F. Rodriguez-Reinoso,* and R. Torregrosa Departamento Quimica Inorgbnica, Facultad de Ciencias, Universidad de Alicante, Alicante, Spain Received July 11, 1986. I n Final Form: September 29, 1986 The adsorption of Nz (77 K) and COz (273 and 298 K) has been studied on a series of activated carbons covering a wide range of burn-off. For carbons with low burn-off the characteristic curve is unique and a common value of micropore volume (Vo)is deduced from Nz (77 K) and COz (273K). As burn-off increases the micropore volumes are coincident only if low-pressure data for Nz are available; if not, Nz will yield larger Vovalues which are similar to those given by hydrocarbons such as benzene, because N z (and also hydrocarbons) measures the total micropore volume (including supermicropores) whereas COz-because of the higher temperature and much lower relative pressure range covered-would only measure the narrow microporosity. This means that Nz (77 K) and COZ (273 K) supplement each other and can provide more complete information about the microporous structure of activated carbons. The adsorption of COPat 298 K is complex because of the proximity to the critical temperature of COz (303 K) and the uncertainty of the density of the adsorbed phase: a value of 0.97 g ~ c m is - ~proposed so that the Vowould be similar to that measured at 273 K.
Introduction Several adsorptives have been proposed to characterize the porous structure of solids,' most of them having in common the following characteristics: (a) chemical inertness; (b) relatively large saturation pressure, so that a wide range of relative pressures can be covered a t the temperature of measurement; (c) a convenient adsorption temperature attainable with simple cryogenic systems (liquid N2 or Ar, solid C 0 2 ,ice, etc); and (d) a molecular shape as close as possible to a sphere to avoid uncertainties in the calculation of surface area due to the different possible orientations on the solid surface. N2at 77 K, the more widely used adsorptive, exhibits, in general terms, these characteristics although it has the problem of presenting a permanent quadrupole moment.' Also, when this adsorptive is used to characterize essentially microporous solids such as coals and some low activated carbons2v3it may present an additional problem because of the low temperature of measurements (77 K); if the mi: cropores are very narrow the entry of the N2molecules at 77 K may be kinetically restricted so that an increase in adsorption temperature would lead to an increase in the rate of diffusion of the molecules into the micropores and to an increase of the amount adsorbed. In order to avoid this problem, some authors4-' have proposed the use of COz as the adsorptive to characterize microporous carbons; since the critical dimensions of both molecules are very similar (0.28 nm for COPand 0.30 nm for N,) the higher temperature (usually 273 or 298 K) of *Author t o whom correspondence should be addressed.
0743-7463/87/2403-0076$01.50/0
adsorption for COz would facilitate the entry into the narrow micropores. It is generally admitted that adsorption in fine micropores takes place by a pore-filling mechanism rather than surface coverage; an adsorptive such as N2 at 77 K would fill these micropores in a liquidlike fashion at very low relative pressures (even below 0.01), but there are controversial opinions about the real state of the COzadsorbed in the micropores. For some a u t h o r COz ~ ~ adsorbed ~ ~ ~ ~ at around ambient temperature (273 or 298 K) would only form a monolayer on the walls of the pores whereas for others the C 0 2 adsorbed in fine micropores is a liquidlo or even close to a solid phase.'l However, both groups of authors use the adsorption of COzat 273 and/or 298 K to calculate the micropore volume from the Dubinin-Radushkevich equation12 which upon multiplication by the (1)Greg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (2) Walker, P. L., Jr.; Kini, K. S.Fuel 1965, 44, 453. (3) Rodriguez-Reinoso, F.; Lbpez-Gonzalez, J. D.; Berenguer, C. Carbon 1982,20,513. (4)Marsh, H.; Wynne-Jones, W. T. K. Carbon 1964,1, 269. ( 5 ) Lamond, T. G.; Metcalfe, J. E., 111;Walker, P. L., Jr. Carbon 1965, 3, 59. (6) Kipling, J. J.; Sherwood, J. N.; Shooter, P. V.; Thomson, H. R.; Young, R. N. Carbon 1966,4, 5. (7) Debelak, K. A.; Schrodt, J. T. Fuel 1979, 58, 732. (8) Lamond, T. G.; Marsh, H.Carbon 1964,1, 281. (9) Marsh, H.; Siemieniewska, T. Fuel 1966, 44, 355. (10) Walker, P. L., Jr.; Patel, R. L. Fuel 1970, 49, 91. (11) Mahajan, 0. P. Coal Structure; Academic Press: London, 1982; p 51. (12) Dubinin, M. M. Progress in Surface and Membrane Science; Cadenhead, D. A. Ed.; Academic Press: London, 1975; Vol. 9, p. 1.
0 1987 American Chemical Society
Langmuir, Vol. 3, No. 1, 1987 77
N2us. CO, in Activated Carbon Characterization cross-sectional area, Am, of a C 0 2 molecule gives the micropore surface area. The uncertainty about the state of the C02adsorbed in the micropores makes very confusing the selection of the appropriate value of Am to be used. Marsh et al.13have suggested the use of Am = 0.187 nm2 for COOadsorbed at 273 K and WalkerI4 and Toda1”17 the use of Am = 0.253 nm2 for C02 adsorbed at 298 K. However, values such as 0.174318and 0.204 nm2I9have also been used for C 0 2at 273 K and 0.17 nmZ2Oalso for COPat 298 K. Marsh et a1.21 have recently published some results for the adsorption of C 0 2 on commercially activated carbons with very large surface areas; they suggested that the adsorption of C 0 2 a t 195 K is taking place by a pore-filling mechanism whereas at 273 K the C 0 2 is just covering the surface of the pores with a monolayer. C 0 2 was then considered a good adsorptive to calculate the surface area of microporous carbons. However, the concept of surface area of microporous carbon adsorbents does not have much physical meaning if micropore filling is taking place and a t the same time an unambiguous value of density-and consequently of cross-sectional area-of the adsorbed molecule cannot be assigned. The comparison of the published results concerning the adsorption of N2and C02on different carbons is somewhat c o n t r a d i ~ t o r y , 2 ~In ~ ~an ~ Jattempt ~ to enlight this comparison, Rodriguez-Reinoso et al.3*22-32 have published a series of papers in which both the adsorption of N2 at 77 and 90 K and of C 0 2 at 273 and 298 K were included; the results concerning more than 100 different activated carbons prepared from lignocellulosic materids leds to three groups of carbons in respect to the adsorption of N2 (77 K) and C 0 2 (273 K), expressed either as micropore volume or apparent surface area.
(13)Iley, M.; Marsh, M.; Rodriguez-Reinoso,F. Carbon 1973,11,633. (14)Walker, P. L., Jr.; Austin, L. G.; Nandi, S. P. Phys. Carbon 1966, 2, 257. (15)Toda, Y.;Hatani, M.; Toyoda, S.; Yoshida, Y.; Honda, H. Fuel 1971,50, 187. (16)Toda, Y. Fuel 1972,51, 108. (17)Toda, Y.;Toyoda, S. Fuel 1972,51, 199. (18)Chiche, P.; Marsh, H.; Pregermain, S. Fuel 1967,44, 341. (19)Marsh, H.; Campbel, H. G. Carbon 1971,9,489. (20)Walker, P. L., Jr.; Shelef, M. Carbon 1967,5,7. (21)Marsh, H.; Crawford, D.; O’Grady,T. M.; Wennerberg, A. Carbon 1982,20,419. (22)Gonzilez-Vilchez, P.; Linares-Solano, A.; Lbpez-Gonzdez, J. D.; Rodriguez-Reinoso, F. Carbon 1979,17,441. (23)Linares-Solano, A.; Lbpez-GonzHlez, J. D.; Molina-Sabio, M.; Rodriguez-Reinoso, F. J. Chem. Technol. Bio Technol. 1980, 30, 65. (24)L6pez-GonzHlez, J. D.;Martinez-Vilchez, F.; Rodriguez-Reinoso, F. Carbon 1980,18,413. (25)Rodriguez-Reinoso, F.;Rodriguez-Ramos,I.; Moreno-Castilla,C.; Guerrero-Ruiz, A.; Ldpez-Gonzdez, J. D. J. Catal. 1986,99, 171. (26)Mpez-Godez, J. D.;Rodriguez-Reinoso, F.; Lmares-Solano,A.; Garrido-Torres-Puchol, A. An. Quim. 1982,78,173. (27)Rodriguez-Reinoso, F.; Linares-Solano, A.; Molina-Sabio, M.; L6pez-Gonzdez, J. D. Ads. Sei. Technol. 1984,1,211. (28)Rodriguez-Reinoso, F.; Lbpez-Gonzdez, J. D.; Berenguer, C. Carbon 1984,22,131. (29)Rodriguez-Reinoso, F.; Linares-Solano, A.; Mardn-Martinez, J. M.; Ldpez-Gonzaez, J. D. Carbon 1984,22,?23. (30)Rodriguez-Reinoso, F.; Guerrero-Ruiz, A.; Moreno-Castilla, C.; Rodriguez-Ramos, I.; Lbpez-Gonzdez, J. D. Appl. Catal. 1986,23,299. (31)Rodriguez-Reinoso,F.; Martin-Martinez, J. M.; Molina-Sabio, M.; PBrez-Lledb, I.; Prado-Burguete, C: Carbon 1985,23,19. (32)Rodriguez-Reinow,F.; Martin-Martinez, J. M.; Molina-Sabio, M.; Torregrosa-Maciii,R.; Garrido-Segovia,J. J. Colloid Interface Sei. 1985, 106, 315. (33)Rodriguez-Reinoso,F. Carbon and Coal Gasification; Figueiredo, J. L., Moulijn, J. A., Eds.; Martinus Nijhoffi Dordrecht, Netherland, 1986,p 601.
Carbonized
N;,(l123K)
C4 (1098K)
c
Olive stones
matwid
2 hours
Activated carbons
e 7-71 hours
(8-80’I.turn-off)
80
60 L
0 I
40
$
i-‘
20
0
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20
LO
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80
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Figure 1. Preparation of the carbons and evolution of burn-off as a function of activation time.
20
15
10
5
0 0
20
40
60
80
% burn-off
Figure 2. Development of microporosity during the activation
process.
A. N2 < C02: Carbonized materials, molecular sieve carbons, and activated carbons with very low burn-off ( -2
The novalues of Figure 2, expressed as cm3.g-l, yield the micropore volumes, Vo,which are included in Table I. The results refered to N2 at 77 K and C 0 2 at 273 K can be grouped as follows. (a) Carbon D-0: There are constrictions in the micropores producing the restricted diffusion of N2at 77 K (see Table I). Only if the adsorption temperature is increased (for instance, to 90 K) the Vowould become similar to that given by C02 at 273 K, as already described for similar
carbon^.^^^^ (b) Carbons D-8, D-19, and D-34 The constrictions have been eliminated by a ~ t i v a t i o n ~and ~ . the ~?~ micropores are narrow enough so that the adsorption of N2 and COP (either pore filling by both adsorbates or filling by N2and surface coverage by C02)yield similar Vovalues; the DR plots are linear over a large range of log2 (Po/P)for both adsorptives. (c) Carbons D-52, D-70, and D-80 The DR plots for N2 (77 K) are not as linear as those of COP at 273 K; the average microporosity has been widened by activation and the Vocalculated from N2 (77 K) is larger than Vocalculated from C02 at 273 K and even larger Vovalues for N2 may be obtained if the data at lower values of log2 (Po/P) are used for the extrapolation. It seems then that N2 is filling also the wider micropores whereas GO2 is either filling only narrow micropores or being adsorbed by a surface coverage mechanism. Consequently, these results seems to indicate that the two adsorptives (when the adsorption data are obtained in the usual relative pressure ranges (5 X 10-3)-1 for N2 at 77 K and (5 X 104)-(3 X for C02 at 273 K) do not measure the same type of microporosity in highly activated carbons (group c) so that the difference increases with burn-off. One could then conclude in principle that the deviations deduced for D-70 and D-80 in Figure 2 are due to both the nonlinearity of the N2 DR plots and the fact that N2and C02 do not measure the same type of microporosity under the experimental conditions described above. A more convenient way of comparing the adsorption of N2 and C02 would be to use the characteristic curves (as deduced from the basis of the micropore volume filling theory of D ~ b i n i n instead ~ ~ ) of the DR plots. If N2 and C 0 2 follow the same mechanism of adsorption, their characteristic curve should be unique for each carbon. Figure 5 includes the characteristic curves for carbons D-19, D-52, and D-80, taken as typical examples. The (36)Madn-Marthez, J. M.; Rodriguez-Reinoao, F.; Moline-Sabio, M.; McEnaney, B. Carbon 1986,24, 265. (37) Dubinin, M. M. Chem. Phys. of Carbon; Walker, P. L. Ed.; Marcel Dekker: New York, 1986; Vol. 2, p 51.
C
-4
0
-2
- L
Figure 5. Characteristic curves for some carbons: (A)Nza t 77 K; (A)N2a t 90 K; ( 0 )C 0 2 a t 273 K; (0) C 0 2 a t 298 K.
affinity coefficient p32,37has not been considered in these curves since it is very similar for N2 (0.33) and C02 (0.35). In order to cover a wider range of A2 ( A is the adsorption potential defied as A = R T In (Po/P)),the N2 (77 K) data from the gravimetric system was completed with that for N2(77 and 90 K) from the volumetric system using a lower range pressure transducer; in this way the relative pressure covers the range 104-1. It is noteworthy that the characteristic curve for D-19 (Figure 5a) is unique for C02 at 273 K and N2 at 77 and 90 K; the linearity extends in a wide range of A2 and the extrapolation to A2 = 0, for both adsorptives and the three temperatures used, leads to a common V,. The characteristic curve for carbon D-52 (Figure 5b) also fits the same type of data; however, the range of linearity is reduced (in respect to the D-19 sample) to higher A2 values. C 0 2 (273 K) and N2 (77 K) give the same value of Voonly if low-pressure data (either from 77 or 90 K in the volumetric system) are available; if the N2 (77 K) data obtained for PIPo > 5 X are used, their extrapolation would give a value of Volarger than the extrapolation of C02 (273 K) data. This phenomenon is enhanced in carbon D-80 (Figure 5c) for which the difference in micropore volume measured by N2 (77 K) and C 0 2 (273 K) is even larger (Table I). The analysis of many series of activated has shown that carbons from similar when the microporosity is relatively wide (burn-off above around 35%), the use of relative pressures above 0.01 for N2at 77 K (range covered in most experimental systems used in the characterization of activated carbons) would lead to values of Volarger than those of C 0 2at 273 K (see also Table I). The characteristic curve of Figure 5c shows an additional interesting feature. The region of A2 up to 30 (kJ/moU2 for N2 is completely curved in carbon D-80 so that the
80 Langmuir, Vol. 3, No. 1, 1987 A21R2
2m
0
0
,
,
,
,
,
( KJlmOl)Z
800
600
LOO ,
Garrido et al.
,
,
12m
tm I
,
,
,
?LOO ,
t
/
I
Figure 6. Characteristic curve for carbon D-52 (A) Nz(77 K); (A) n-butane (273 K); (0) COz (298 K); (A) Nz(90K); (v)benzene (298 K); (0)2,2-DMB (298 K); ( 0 )COz (273 K); isobutane (273 K); (0) isooctane (298 K).
(V)
experimental points do not fit the straight portion of the characteristic curve (they did in some extention in carbons up to D-52); because of this, even lower relative pressures for N2 were used for carbons D-80 in order to try to find if such straight portion could be obtained. The corresponding experimental points have been included in the plot. A large deviation from the characteristic curve is observed for A2 > 30 (kJ/mol)2 similar to that described for the adsorption of N2 and Ar on some carbon^.^^^^ According to Rand3 this deviation indicates the existence of two distinct groups of micropores, the narrower being filled at higher A2 values and the wider being filled at low A2 values (the Vovalue deduced from the later being the generally accepted one). This deviation of experimental points below the extrapolation of C02 (273 K) data may be taken in our case 8s an indication of restricted diffusion of N2 at the very low adsorption temperature into very narrow micropores; in fact, the larger deviation corresponds to 77 K for which the restricted diffusion would be more important. The case of the adsorption of C02at 298 K is especially interesting since Table I shows that V, (273 K) is always lower than Vo(298 K) even although Figure 2 clearly shows that (i) there is not any problem of restricted diffusion for N2at 77 K and (ii) the amount adsorbed at the micropores (no,in mmol-g-') is always larger for 273 K. The reasons for these discrepancies could be (a) 298 K is too near the critical temperature of COz(303 K) and, consequently, its behavior toward the DR equation could be different to that at 273 K and/or (b) the density for COz adsorbed at 298 K could be higher than the value (0.700 commonly used. Again, a good way of solving this problem is to use the characteristic curves of COz at 273 and 298 K. Figure 5b includes the C02 (298 K) data for carbon D-52-taken as a typical example-calculated by using 0.700 g . ~ m as - ~the density; the data lie above-and relatively parallel to-the characteristic curve discussed above. Because of this, a simple change in the density of C02 adsorbed at 298 K could bring down the plot to the characteristic curve. This is so because, as Figure 6 shows, C 0 2at 273 K, N2at 77 and 90 K, and several hydrocarbons (n-butane, isobutane, benzene, 2,2-dimethylbutane, and isooctane) constitute an unique characteristic curve for carbon D-52 (taken as typical example), but C 0 2 at 298 K does not fit such a curve. The corrected density (0.97 ~) g.~m-~), which is similar to the value (1.02 g - ~ m -deduced (38) Marsh, H.; Rand, B. Proceedings of the 3rd Conference on Industrial Carbon and Graphite, London, 1970; p 172. (39) Rand, B.J. Colloid Interface Sci. 1976,56, 337. (40) Stoeckli, F.;Kraehenbuelhd, F.; Lavanchy, A.; Huber, U. Proceedings of the Conference International sur Les Carbones. Carbon'84, Bordeaux, 1984; p 785.
from D ~ b i n i n , seems ~' then more reasonable than 0.700 g . ~ m - ~When . the corrected value of density is used to calculate the micropore volume at 298 K, the values (see Table I) are very similar to the Voat 273 K for carbons D-8 to D-52; the values for D-70 and D-80, however, are lower at 298 K and, moreover, they are very similar one to another and similar to D-52. It may be that in the former carbons, with wide microporosity, the density of the C02 adsorbed at 298 K should have a different value (in fact, to bring down the plot to the characteristic curve, the density would be lower than the 0.97 which has been used for carbon D-52); it seems then that for C 0 2 at 298 K (temperature near the critical) the density is also a function of the width of the microporosity. In any case the value of Am for C 0 2 at 298 K cannot be as large as 0.253 nm215-17 and it should range from 0.195 to 0.220 nm2, the former applying to low-medium activated carbons; with these values the surface areas measured at 298 K will be closer to those determined at 273 K. All the above results lead to the question of which is the adsorptive to be used with more confidence in the characterization of activated carbons. At this stage it would be convenient to recall the results obtained in the adsorption of hydrocarbons (n-butane, isobutane, benzene, 2,2-dimethylbutane, and isooctane) on the same and similar carbon^^^?^^ because all the results may be summarized as follows: (i) For carbons with low burn-off the characteristic curve is unique and linear for the hydrocarbons (if accessible to the microporosity), N2 (77 and 90 K), and C 0 2 (273 K); C02 at 298 K clearly deviates from the curve if the density is not corrected. Consequently all the adsorptives (except C02 at 298 K) yield the same micropore volume (Table I shows that benzene is not completely accessible to carbons D-8 and D-19). (ii) For carbons with larger burn-off (3540%) of which D-52 is a typical example, the characteristic curve is like the one shown in Figure 6; there is a unique curve (which is not linear in the whole range of A2/P2)and only if low-pressure data for N2 are available the micropore volume will be coincident with that of COP at 273 K; if not, the micropore volume will be slightly larger and similar to that obtained from the hydrocarbons. Again, C02at 298 K clearly deviates from the characteristic curve. (iii) For carbons with very large burn-off (i.e., D-70 and D-80) the characteristic curves are also unique except for C02 at 298 K (for the same reason as above), the curvature in the region defined by hydrocarbons and Nz being more pronounced; the N2(77 K) data show two straight portions (as in the DR plots of Figure 3) and the hydrocarbons and Nz (Table I) yield the same value of Voonly if experimental points for the latter in the relative pressure range 0.05-0.25 are used. This micropore volume is larger than that calculated from N2 at lower relative pressures (straight portion at larger log2 (Po/P) in Figure 3), which at the same time is larger than the Vo calculated from C02 at 273 K. Therefore, in these carbons with widely distributed microporosity, hydrocarbons and N2 (relative pressure range 0.05-0.25 for the latter) are measuring the total micropore volume (including supermicropores and perhaps part of the lower mesoporosity) whereas C02-because of the higher temperature and much lower relative pressure range covered-would only measure the narrow m i c r o p ~ r o s i t y . ~ ~ , ~ ~ (41) Dubinin, M. M. Chem. Rev. 1960,60, 235. (42) Garrido-Segovia,J.; Martin-Martmez, J. M.; Molina-Sabio, M.; Rodriguez-Reinoso, F.; Torregrosa-Macii, R. Carbon 1986,24, 469. (43) SepSveda-Escribano, A.; Linares-Solano,A,; Rodriguez-Reinoso, F.; Salinas-Martinezde Lecea, C.; Ahela-Alarc6nn,M. Proceedings of the 4th International Carbon Conference, Baden-Baden, 1986.
Langmuir 1987, 3, 81-85 The conclusion is then that the use of N2 at 77 K (or hydrocarbons such as benzene at 298 K) should be complemented with the adsorption of C 0 2 at 273 K in order to obtain a better knowledge of the whole porosity range of the activated carbons.
81
Acknowledgment. The study was sponsored by C. A.I.C.Y.T. (project no. 795/81) and, in part, by Caja de Ahorros Provincial (Alicante). Registry No. N2, 1721-31-9; COP, 124-38-9; C, 7440-44-0.
IR Spectrophotometric Method for the Measurement of Toluene Adsorptivity in Activated Carbon? David D. Saperstein IBM, San Jose, California 95193 Received July 15, 1986. I n Final Form: October 17, 1986 IR spectra of toluene adsorbed to high surface area (activated) carbons show downward shifts for both the ring and C-H out-of-plane modes. The band shapes are consistent with the adsorbate in two distinct environments-monolayer and liquidlike. Progress has now been made toward developing an estimate of carbon adsorptivity based on IR measurement of retained toluene (3-30 w/w %). IR intensity correlates directly (*15%) with uptake and highlights surface area differences among four types of activated carbon.
Introduction Activated carbon readily adsorbs toluene and other organics because of its very high internal surface areal-l consisting of micropores (less than 2-nm channels), mesopores (2-50-nm channels), and macropores (greater than 50-nm channels). In general, most of the surface is associated with the micropores. IR spectra of activated carbons show a very weak and characteristic band at approximately 1580 cm-' due to residual graphitic structure5 (Figure 1B). Other features in the 3200-2800 cm-' (C-H stretches), 1720 cm-' (C=O stretch), and 1220 cm-' (C-0 stretch) may not be observed in all carbon^.^,^ Likewise, low-frequency bands in the region 800 cm-l and below may be due to metal-oxygen modes from impurities or arise from ring motions.' Unlike the carbon spectrum, toluene adsorbed to activated carbon (Figure 1A) shows two prominent peaks (Figure 1C) in the 600-4000-cm~' region which arise from the C-H (approximately 726 cm-') and ring (approximately 692 cm-') out-of-plane modes.s The corresponding peaks in liquid toluene, observed at 730 and 695 cm-', indicate that the toluene is physically adsorbed to the carbons as expected.' Each of the toluene peaks in the adsorbed sample is composed of two overlapping bands separated by about 8 cm-'. The more intense bands in each peak are consistent with IR absorption by toluene in multilayer environments; the low-energy shoulders are consistent with IR absorption by the first adsorbed monolayer. Other toluene bands can be observed;8however, they exhibit lower signal to noise. When benzene is used instead of toluene, its out-of-plane C-H vibration can be observeds at 673-675 cm-l, which is red-shifted from the liquid frequency of 677 cm-'. Activated carbon is physically and chemically heterogeneous,3p4and therefore, the preparation of carbon samples for reproducible, quantitative IR measurement is not ~ t r a i g h t f o r w a r d . Typical ~ ~ ~ ~ ~ adsorbents such as fiber and powder samples are very different in shape and thus may show apparent differences in their carbon and adsorbate This work was presented in part at the 192nd National Meeting of the American Chemical Society, Anaheim, CA, Sept 1986.
0743-7463/81/2403-0081$01.50/0
IR spectra. We previously reporteds that IR spectra of adsorbed toluene gas in high-area carbons might be used to estimate adsorbate quantities. That report, however, was based only on the analysis of one type of carbon. Because we wish to determine whether adsorptivity of activated carbon can be quantified spectroscopically, the desorption of toluene from three different kinds of powder and one fiber has now been investigated. Initial results show that with IR techniques an estimate of the carbon area (>lo0 m2/g) covered by adsorbed toluene can be obtained on nonuniform samples.
Experimental Section IR spectra were obtained in transmission with an IBM Instruments IR/85 FT-IR (Fourier transform infrared spectrometer, fJ4.2 optics) equipped with an MCT detector (low-frequency cutoff, ca. 550 cm-') at 2-cm-' resolution. No special accessory optics were used to collect the scattered light from the samples. Before the start of data collection each day, the detector was allowed to settle for approximately90 min to equilibratethe MCT element to the liquid nitrogen temperature. A wire screen was (1)Dubinin, M. M.;Zavarina, E. D.; Radushkevich, L. V. Zh. Fiz. Khim. 1947,21,1351. Dubinin, M.M.;Zavarina, E. D. Zh. Fiz. Khim. 1949,23,1129. Dubinin, M.M.Q. Rev., Chem. Soc. 1955,9,101. Dubinin, M.M.; Plavnik, G. M.; Zavarina, E. D. Carbon 1964,2,261. Dubinin, M.M.; Plavnik, G. M. Carbon 1968,6,183. Dubinin, M.M. Bog. Surf. Membr. Sci. 1975,9,1-70. (2)Emmett, P. H.Chem. Rev. 1948,43,69. (3)Mantell, C. L.Carbon and Graphite Handbook; Wiley: New York, 1968. Carbon Adsorption Handbook; Cheremisinoff, P. N.; Ellerbush,
F., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980. (4)Activated Carbon A Fascinating Material; Capelle, A., de Vooys, F., Eds.; Norit: Amersfoort, The Netherlands, 1983;p 13 ff. (5)See, for example: Friedel, R. A.; Hofer, L. J. E.; J. Phys. Chem. 1970,74,2921. Friedel, R.A.; Carlson, G. L. Fuel 1972,51,194.Rockley, M. G.; Devlin, J. P. Appl. Spectrosc. 1980,34,405,407.Akhter, M. S.; Chughtai, A. R.; Smith, D. M. Appl. Spectrosc. 1985,39,143. (6)Smith, D. M.; Griffin, J. G.; Goldberg, E. D. Anal. Chem. 1975,47, 233. Low,M. J. D.; Morterra, C. Carbon 1983,21, 275. (7)Colthup, N. B.; Daly, L. H.;Wiberley, S. E. Introduction t o Infrared and Raman Spectroscopy, 2nd ed.; Academic Press: New York, 1975;p 262 ff. (8)Saperstein, D. D. J. Phys. Chem. 1986,90,3883. (9)For a recent discussion of quantitation problems associated with diffuse reflection of carbon black, see: Brimmer, P. J.; Griffiths, P. R. Anal. Chem. 1986,58,2179.
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0 1987 American Chemical Society
