J. Phys. Chem. 1986, 90, 3883-3885
3883
Infrared Spectroscopy of Reversibly Adsorbed Gases Held by Activated Carbon David D. Saperstein IBM,' San Jose, California 951 93 (Received: April 23, 1986)
The adsorption of aromatics by silica and aluminosilicates is characterized by an upward frequency shift for the out-of-plane C-H wags. We now present in situ IR spectra of toluene adsorbed on high surface area activated carbon which show downward shifts for these out-of-plane modes. Changing temperature or partial pressure of toluene in the experiments leads to small changes in the frequency and bandwidth for the adsorbate. These observation are consistent with the adsorbate in two energetically favorable, distinct environments-monolayer (approximately 220 h 60 m2/g) and liquidlike. The latter is described by Dubinin and Radushkevich as the volume filling of pores.
Introduction Activated carbon is unique in its extremely high surface area, 1000-2000 m2/g4v5by nitrogen BET measurements, and affinity for organic adsorbates. Because of these properties, activated carbon is the choice for a number of applications, including removal of low-level organics from water, reduction of vapor contaminants in the home and in the workplace, and use in many industrial and lab p r o c e s ~ e s . ~The . ~ high affinity/surface area6 originates from the internal structure of activated carbon, which is composed largely of micropores (less than 2-nm channels), mesopores (2-50-nm channels), and macropores (greater than 50-nm channels). The adsorption of gases a t relatively moderate pressures not only coats the pore surfaces but fills the volume, especially the micropores, as weL3 This volume filling of the pores means that a liquid or liquidlike phase is formed during the adsorption of a gas. There is a very large body of physical evidence for the existence of this strongly held however, direct, spectroscopic evidence has not, to our knowledge, been published. IR spectroscopy is sensitive to molecular bonding and environment and has been used, along with Raman spectroscopy, to characterize some forms of carbon.*s9 In particular, although the spectrum of activated carbon as compared to diamond and graphite is not easily interpreted, IR techniques have been used to draw inferences about the nature of amorphous and activated carbon surfaces through chemical,10 pyrolytic,' and isotopic studies.12 Spectroscopic studies of the activated carbon surface holding physically adsorbed gases are sparse in the literature. The most often quoted is the work of Mattson and Mark,13 which reports the IR spectrum of phenols adsorbed to activated carbon. However, it is unclear from their work whether the phenol modified the surface in some way and whether their technique had sufficient sensitivity to measure the small absorbances reproducibly. The preparation of carbon samples for reproducible, quantitative I R measurement of adsorbates in activated carbon is not ~traightforward.'~Film samples, which can be better controlled for IR measurement^,'^ are not typically used for the adsorption of gas; powder samples, which are typical adsorbents, are often too heterogeneous in size and composition for quantitative IR m e a s ~ r e m e n t s . ' ~Because J~ we wished to examine the adsorbate on a typical activated carbon, powder samples were chosen for this initial study. Toluene was chosen for the adsorbate because (a) toluene readily adsorbs and desorbs over the temperature range 25-200 O C , (b) the in-phase C-H and ring, out-of-plane bending modes of aromatics are strong and show pronounced frequency shifts when bound to an (c) carbon powders show reduced scattering at the frequencies of interest, 650-900 cm- 1, relative to the higher frequencies, e.g., 2800-3200 cm-',the C-H stretching modes, and (d) it is readily available in most laboratories. Spectral evidence, 8 9 0 4 5 0 an-',for the toluene adsorbate is shown and compared to spectra of the neat gas and liquid. The area of the adsorbate bands is measured and compared to the
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'5600 Cottle Road, San Jose, CA 95193.
0022-3654/86/2090-3883501.50/0 , I
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adsorption isotherm measured from the weight gain of similarly prepared carbon.
Experiment
IR spectra were obtained in transmission on an IBM Instruments IR/85 FT-IR (Fourier transform infrared spectrometer) equipped with an MCT detector (low-frequency cut-off, ca. 550 crn-') at 2-cm-' resolution. In all experiments the carbon powder was supported on a preroughened KBr pellet. The carbon (1) Galkin, G. A.; Kiselev, A. V.; Lygin, V. I. Russ. J. Phys. Chem. (Eng. Transl.) 1962,36(8), 951. Galkin, G. A.; Kiselev, A. V.; Lygin, V. I. Trans. Faraday Soc. 1964, 60, 431. Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; Kaner, N., translator; Halsted Press, Wiley: New York, 1975; e.g., p 211. (2) Angell, C. L.; Howell, M. V. J. Colloid Interface Sci. 1968, 28, 279. Pinnavaia, T. J.; Mortland, M. M. J. Phys. Chem. 1971, 75, 3957. Primet, M.; Garbowski, E.; Mathieu, M. V.; Imelik, B.J. Chem. Soc.,Faraday Trans. 1 1980, 76, 1942. Coughlan, B.; Carroll, W. M.; O'Malley, P.; Nunan, J. J. Chem. SOC.,Faraday Trans. 1 1981, 77, 3037. Datka, J. J. Chem. SOC., Faraday Trans. 1 1981, 77, 511. (3) Dubinin, M. M.; Zavarina, E. D.; Raduschkevich, L. V. Zh. Fir. Khim. 1947,21, 1351. Dubinin, M. M.; Zavarina, E. D. Zh. Fiz. Khim. 1949, 23, 1129. Dubinin, M. M. Prog. Surf.Memb. Sci. 1975, 9, 1-70. For a recent reference, see: Dubinin, M. M. Carbon 1985, 23, 373. (4) Emmett, P. H. Chem. Rev. 1948, 43, 69. (5) Mantell, C. L. Carbon and Graphite Handbook; Wiley: New York, 1968. Cheremisinoff, P.N., Ellerbush, F., Eds. Carbon Adsorption Handbook; Ann Arbor Science: Ann Arbor, MI, 1980. (6) Capelle, A., de Vooys, F. Eds. Activated Carbon ... A Fascinating Material; Norit: Amersfoort, The Netherlands, 1983; 13 ff. (7) 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. (8) IR: see, for example: Friedel, R. A.; Hofer, L. J. E. J. Phys. Chem. 1970, 74,2921. 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. (9) Raman: see, for example: Vidano, R.; Fischbach, D. B. J. Am. Ceram. Soc. 1978,61, 13. Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970,53, 1126. Nakamizo, M.; Kammereck, R.; Walker, P. L., Jr. Carbon 1974, 12, 259. Angell, C. L.;Lewis, I. C. Carbon 1978, 16, 431. Dillon, R. 0.;Woollam, J. A.; Katkanant, V. Phys. Rev. B Condens. Matter 1984,29,3482. Vidano, R. P.; Fischbach, D. B.; Willis, L. J.; Loehr, T. M. Solid State Commun. 1981, 39, 341. (10) Bouwman, R.; Freriks, I. L. C.; Wife, R. L. J . Catal. 1981,67,282. Ishizaki, C.; Marti, I. Carbon 1981,19,409. Zawadzki, J. Carbon 1981,19, 19. Zawadzki, J. Carbon 1978, 16, 491. Papirer, E.; Guyon, E.; Perol, N. Carbon 1978,26, 133. Ianniello, R. M.; Wieck, H. J.; Yacynych, A. M. Anal. Chem. 1983,55, 2067. (11) Morterra, C.; Low, M. J. D. Carbon 1983, 21, 283. Morterra, C.; Low, M. J. D. Lnngmuir 1985, I , 320. (12) Jassim, J. A,; Lu, H. P.; Chugtai, A. R.; Smith, D. M. Appl. Spectrow. 1986, 40, 113. (13) Mattson, J. S.;Mark, H. B., Jr.; Weber, W. J., Jr. Anal. Chem. 1969, 41, 355.'Mattson, J. S.;Mark, H. B., Jr.; Malbin, M. D.; Weber, W. J., Jr.; Crittenden, J. C. J . Colloid Interface Sci. 1969, 31, 116. Mattson, J. S.; Mark, H.B., Jr. J. Colloid Interface Sci. 1969, 31, 131. (14) Smith, D. M.; Griffin, J. G.; Goldberg, E. D. Anal. Chem. 1975, 47, 233. (15) Low, M. J. D.; Morterra, C. Carbon 1983, 21, 275
0 - 1986 American Chemical Societv
3884
Letters
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
0.01
c 850
1
800
750
700
650
abr. unit
1
wavenumbers
Figure 1. FT-IR spectra of toluene from 850 to 650 cm-' with a resolution of 2 cm-'. Gas-phase spectrum digitally subtracted: (a) adsorbate after 45 min in the presence of toluene at 0.7% of the saturation vapor pressure at 26 O C ; (b) same as (a) after a total of 75 min; (c) same as (a) after a total of 150 min; (d) same as (a) after a total of 240 min; (e) adsorbate after 35 min at 2% plus 240 min at 0.7% of the saturation vapor pressure at 26 O C ; (f) neat liquid, 23 OC (arbitrary scaling); (g) neat gas, 26 OC (arbitrary scaling).
(Witcarb 96516) was ground in the presence of reagent-grade chloroform, and a slurry of the CHC13and carbon was dripped onto a 1-cm-diameter KBr pellet. The slurry was spread mechanically to make a quasi-uniform (speckled) coating weighing approximately 1 mg (dry). The excess chloroform was allowed to evaporate before oven drying a t approximately 100 OC. After drying for more than 1 h, there was no spectral evidence for any chloroform or perturbed carbon bands, Le., less than 0.0005 AU, which was the limit of our sensitivity. Weight measurements, before the addition and after the removal of the chloroform, show less than a 0.5% change. Two different techniques, vapor adsorption and liquid desorption, were used to obtain the spectra. Gas-phase adsorption was accomplished with an I R environmental chamber" (Spectra-Tech, Stamford, CT) that permitted the carbon on KBr to be heated in an enclosed cell (approximately 20 cm3) with controlled flow (1 atm, 50-150 cm3/min). Because the path length of the cell is relatively long, ca. 7 cm,the absorption of IR radiation by the gas was readily apparent when toluene was in the chamber. The contribution of the gas-phase toluene was digitally subtracted from the spectra prior to plotting. Spectrograde toluene and nitrogen (99.999%) were separately metered and mixed continuously at the desired temperature upstream from the sample chamber. Spectra of the toluene adsorbed to carbon were recorded over a 50-fold range of relative pressures and from 25 to 122 OC. A total of 1000-16 000 scans was co-added and ratioed to a previously recorded spectrum of the carbon without toluene to produce the spectra shown. Liquid-phase adsorption also starts with the dried carbon on KBr, to which liquid toluene is added to produce a wet carbon. The sample is air dried a t room temperature, ca. 22 OC, and then further dried at 65 "C for selected periods, 5,25,75,and 900 min, and shows the expected band intensity decreases associated with toluene desorption. When liquid benzene was used in place of toluene, a single band at 673 cm-' is observed, which loses its intensity with applied heat much more rapidly than do the observed (16) Witco Chemical Co., New York, NY: Surface Area, 1300 m2/g determined from N2BET measurements; density, 0.45 g/cm'; average pore size, 3 nm (with more than 9G% of the surface area in pores of 2.5 nm or less); CCll activity number (equivalent to gas adsorption at 36% of saturation at 25 "C), 0.65 g/g. (17) Saperstein, D. D. Appl. Spectrosc. 1985,39, 615.
wavenumbers
Figure 2. FT-IR spectra of toluene from 850 to 650 cm-I with a resolution of 2 cm-I. Happ-Genzel apodization: (a) neat liquid, 23 O C (arbitrary scaling); (b) residual adsorbate after saturated gas at 30 O C ; (c) residual adsorbate after gas at 7% of the saturation vapor pressure at 30 O C ; (d) residual adsorbate after gas at 4.5% of the saturation vapor pressure at 72 "C;(e) residual adsorbate after gas at 1% of the saturation
vapor pressure at 120 O C ; (f) transmission spectrum of Witco carbon, 2-cm-' resolution, support is a KBr disk (offset 0.71 AU and then X I/*). toluene bands. The liquid samples are easy to prepare and may be useful for qualitative analysis of the carbon. TGA (thermal gravimetric analysis) measurements were recorded with a Du Pont 990 analyzer. Dry-ground carbon (2 mg) was wetted with chloroform and then dried a t 110 OC for 1 h. This chloroform treatment mimics the preparation of the carbon in the IR experiments. Toluene and nitrogen were mixed as before and allowed to flow past the carbon on the microbalance. The recorded weights (0-0.45g/g adsorption and 0.45-0.035 g/g desorption) show the expected3 adsorption/desorpfion behavior for the range of temperatures, 25-122 OC, studied.
Results and Discussion We show two sets of IR spectra to help elucidate the nature of the binding of toluene to activated carbon. Figure 1 shows the increase in the toluene absorption with time and partial pressure of the gas at a fixed temperature. The spectra of residual toluene on carbon, which were obtained after drying the liquid-immersed carbon, show essentially the same results and thus are not plotted here. The spectra of liquid and gaseous toluene are included for visual comparison purposes. Figure 2 shows the effect of temperature, 30,70, and 120 OC, on the I R spectra of the adsorbed toluene. Two bands are observed that are not in the spectrum of the fresh carbon, Figure 2f. For the neat liquid and gas, the higher frequency band a t 730 cm-' has been assigned to the in-phase, out-of-plane C-H wag and the band at 695 cm-' has been assigned to an out-of-plane ring sextant bend.I8 Although these bands show no measurable shift between the gas and liquid phases, the C-H wag has been described by PinnavaiaZas being sensitive to its environment, and 5-10-cm-l upward shifts have been observed for the out-of-plane C-H wag of physically sorbed toluene and benzene on silica' and molecular sieves2 Our spectra clearly show a downward frequency shift of the out-of-plane bands (18) Colthup, N. B.; Daly, L. H.; Wiberley, S . E. Introduction to Infrored and Raman Spectroscopy, 2nd ed.; Academic: New York, 1975; p 262 ff.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3885
Letters
TABLE I: Summary of IR Absorption and Carbon Adsorption Data for Toluene
spectra no. If 2b le
temp, OC 22 30
2d 2e
26 26 26 72 122
la
26
IC
Id
re1 press." liquid 0.95 0.05 0.02 0.007 0.007
desorbd desorbd desorbdafter 2e
wt toluene adsorb, g/g 0.45 0.41
0.34 0.34 0. l e
0.043 0.043
out-of-plane C-H wagb peak frep peak ht peak area 730 lC le 727 0.01 14 0.133 727 0.0097 0.122 726 0.0074 0.096 726 0.0083 0.106 726 0.0046 0.079 721 0.0035 0.066 726 0.0032 0.043
out-of-plane sextant ring bendb peak freq peak ht peak area 695 0.63 0.33 694 0.005 1 0.042 693
0.0063
0.043
693
0.0037 0.0048 0.0022 0.0018 0.002
0.037 0.038
692
692 688 693
0.017
0.015 0.016
"Vapor pressure divided by the saturation vapor pressure. bReference 18, frequency in cm-I, intensity in absorbance units, and area in cm-I. erntensity and area separately normalized to 1 in comparison to the 695-cm-I band. dDesorption of toluene from the carbon with 50 cm3/min nitrogen. Weights are approximate and show the TGA determined g/g for an equivalent time period as the IR measurement. CEstimatebased on a calculation according to ref 3. TABLE II: Comparison of Band Frequencies (cui1) for Toluene Adsorbed to Carbon, Silica,and a Copper-Doped Aluminosilicate carbon copper high
liquid
loading
695 694 730 727 1495 1492-1494 1604 1602-1604
montmorilloniteb ligand
low
loading silica" sorbed 688 721
736 1495
695 738 1495
695 764 1487 1593
assign' oop ring w 00pC-H b
ipCC st ipCC st
Reference 1. Peak values adjusted so that reported liquid values match this experiment. bReference 2, Pinnavaia and Mortland. 'Reference 18, cop, out-of-plane; ip, in plane; st, stretch; b, bend; w, wag.
upon adsorption of toluene to activated carbon. Benzene, not shown, also exhibits this downward shift of ca. 2-4 cm-' from the liquid frequency of 677 cm-I. Two more features of the data are apparent. Within the limit of our sensitivity, no other absorption bands are observed, and the measured band shapes and peak frequencies depend upon the amount of toluene sorbed. The frequency decrease can be as much as 6-8 cm-I for the adsorbate spectra at relatively low pressure and/or high temperature. These shifts correspond to the lowest toluene loadings achieved in our experiments, Table I, spectra 2d, 2e, and la. The trends in Figures 1 and 2 shows bands sharpening and a very small frequency increase of the peaks as the toluene loading increases. These data are consistent with two overlapping bands contributing to each of the observed peaks. The higher frequency band in each peak dominates at high concentration, is shifted downward by only 2-4 cm-' from the liquid peak frequencies, and may arise from a perturbed liquid environment, i.e., a multilayer2 or volume filling of the pores.3 This small shift is due to decreased repulsion or increased attraction between the adsorbate and the carbon surface even in this liquidlike environment. The lower frequency band in each peak is more distinct at the lower loadings of our experiments, is shifted by as much as 10 cm-' from its liquid value, and may derive from more strongly held adsorbate-probably the first adsorbed monolayer. The width of the monolayer bands, 20 cm-', e.g., Figure 2e, compared to 10 cm-' for the adsorbate a t high loading, e.g., Figure 2b, may indicate that the toluene is not held in a unique site on the carbon but rather is dispersed among a variety of sites of slightly varying energy. Table I summarizes these data, listing peak frequencies, heights, and areas for the out-of-plane absorption bands, and the weight (g/g) increases, for several different toluene pressures and temperatures. Spectra c and d of Figure 1 were sequentially recorded a t the same temperature and partial pressure of toluene. Their differences show the variation of intensity and frequency measurements in the experiment: approximately f0.005 AU in height, f0.005cm-' in area, and 0.5 cm-' in frequency. A comparison of adsorption weights to the corresponding IR peak heights and areas shows a direct relation. The samples that correspond to the highest intensities measured, Figures 2b and le, show the most
weight gain, Table I, and the samples that correspond to the smallest IR intensities, Figures 2e and l a , show the least weight gain. The weight gains corresponding to the samples shown in Figures lc,d and 2d are of intermediate value. The overall trend of the data is that the IR absorption intensities are a direct measure of the quantity of toluene adsorbed. A more quantitative comparison of the weights and intensities indicates that the IR absorption strengths for the monolayer may be greater than that of the liquidlike adsorbate. However, this hypothesis cannot be substantiated because the apparent error in the intensities could account for the nonlinearity between intensities and weights. It is instructive to estimate the toluene weight corresponding to a monolayer. A least-squares procedure was applied to the data in Table I and yielded an adsorbed monolayer weight of 0.1 1 f 0.03 g/g for toluene. With the assumption of a planar area of 0.30 nm2/molecule for toluene, this weight is equivalent to 220 f 60 m2/g. This estimate is a factor of 5 less than the nominal N2 BET surface area for this carbonI6 and shows that the bulk of the adsorbate at high loading is in a liquidlike environment. Table I1 liits several peak frequencies measured for toluene neat and adsorbed to carbon, silica,' and an aluminosilicate.2 Toluene and benzene on hydrophilic surfaces show in-plane frequencies that are generally unaffected by adsorption while their out-of-plane modes shift to higher energy. These observations have been explained in terms of increased repulsion between the aromatic and the sorbent that would mainly affect the out-of-plane modes because they most overlap the surface of the sorbent. Carbon is more lipophilic than silica or molecular sieves, and the toluene adsorption spectra might be expected to show a different pattern of frequency shifts as we observe. However, it is unclear from our limited spectral range whether downward shifts of the outof-plane modes are due to a steric repulsion decrease or to electron withdrawal from the ring, which would affect the in-plane frequencies as well. That we see a small shift of the 1495-cm-' band to lower energy at high toluene loading is certainly intriguing and warrants further investigation.
Conclusion The spectra of reversibly adsorbed aromatics held by activated carbon can be investigated with IR transmission spectroscopy. Even with a relatively long path length and high volume cell, we are able to show downward frequency shifts and bandwidth increases for the adsorbate when compared to the neat liquid. Toluene adsorbed to carbon shows overlapping bands for the monolayer and the multilayer, indicating a difference in their configurations and binding energies. Acknowledgment. I am grateful to Charles A. Brown, Edward Engler, Heinrich Hunziker, and Russ Wendt for the many insights about activated carbon that they shared with me. I a m also indebted to Leo Volpe for his technical insights and suggestions for improvement of this manuscript. Registry No. Toluene, 108-88-3;carbon, 7440-44-0.
