J. Phys. Chem. 1987, 91, 6659-6663
6659
Perturbation of the Infrared Absorption Spectra of Aromatic Adsorbates in Microporous Carbons by the Christiansen Effect David D. Saperstein IBM Almaden Research Center, San Jose, California 951 20 (Received: April 27, 1987;
In Final Form: June 23, 1987)
The infrared spectra of aromatic adsorbates held in activated carbon can show large band asymmetries that appear to originate from index of refraction matching between adsorbate and the carbon surrounding the micropores. The magnitude of the effect is quite sensitive to carbon structure, adsorbate concentration, and wavelength but appears to be independent of the particle shape and surrounding medium, e.g., KBr or air. It is proposed that the effect can be used to differentiate microporous carbons and to probe the structure of their internal surfaces.
Introduction Optical studies of activated carbon (microporous and amorphous) are frustrated by strong molecular absorption and high scattering efficiency for the incident light beam. The IR spectrum, in particular, is quite diffuse and nearly featureless except for a very weak band near 1600 cm-’ that derives from an allowed in-plane mode of extremely small graphitic Other very small bands may be observed depending on the history of the sample and derive from impurities, carbon oxidation, and other structural domains. Recently, we have reported that activated carbon can be studied by the measurement of the IR absorption spectrum of retained toluene3Awhich is approximately proportional to the weight of the adsorbate over the range 3-30 wt %.4 Because the toluene absorption peaks were asymmetrically shaped, we hypothesized that the adsorbate spectrum is composed of overlapping bands of toluene in multilayer and monolayer environm e n t ~ . We ~ now show that a large contribution to the asymmetrical band shapes arises from changes in light scattering when an aromatic adsorbate is retained in some activated carbons. More than a century ago Christian~en~ described the enhanced transmission of small transparent particles suspended in a nonabsorbing medium due to index matching at a particular wavelength A,,. The enhancement can be quite sizeable and with proper selection of particles and media can be used to make band-pass filters and other devices.6 Examples of this phenomenon can be found when solids are suspended in gases,’ liquids,*-1° and solids.11J2 More complex examples of index matching have been reported such as the case of an adsorbed monolayer on submicron metallic particles in a thin film.I3 The case of activated carbon (1) Friedel, R. A.; Hofer, L. J. E. J . Phys. Chem. 1970, 74, 2921. (2) Bouwman, R.; Freriks, I. L. C.; Wife, R. L. J . C a r d . 1981, 67, 282. Ishizaki, C.; Marti, I. Carbon 1981, 19, 409. Friedel, R. L.; Carlson, L. J. Phys. Chem. 1971,75, 1149. Akhter, M. S.; Chughtai, A. R.; Smith, D. M. Appl. Spectrosc. 1985, 39, 143. Mattson, J. S.;Mark, H. B., Jr.; Weber, W. Anal. Chem. 1969, 41, 335. Tarkovskaya, I. A.; Tomashevskaya, A. N.; Rybachenko, V. I.; Chotii, K. Yu. Adsorbts. Adsorbenty 1980, 8, 43. (3) Saperstein, D. D. J . Phys. Chem. 1986, 90, 3883. (4) Saperstein, D. D. Langmuir 1987, 3, 81. (5) Christiansen, C. Ann. Phys. (Leipzig) 1884, 23, 298. Christiansen, C. Ann. Phys. (Leipzig) 1885, 24, 439. (6) Koester, L.; Washkowski, W.; Kluver, A. Physicu B C 1986, 137, 282. Gerritsen, H. J. Appl. Opt. 1986, 25, 2382. Craighead, H. G.; Cheng, J.; Hackwood, S . Appl. Phys. Lett. 1982,40,22. Yeh, P. Opt. Commun. 1980, 35, 9. (7) Carlon, H. R. Appl. Opt. 1979, 18, 3610. Carlon, H. R. Appl. Opt.
+
1980, 19, 1892. (8) Sakamoto, K.; Yoshida, R.; Hatano M.; Tachibana, T. J . Am. Chem. SOC.1978, 100, 6898. (9) Afghan, M.; Cable, M. J . Non-Cryst. Solids 1980, 38&39, 3. (10) Clarke, R. H. Appl. Opt. 1968, 7, 861. (1 1) Laufer. G.; Huneke. J. T.: Royce, B. S.H.: Teng, - Y. C. A..d . Phys. . Let;. 1980, 37, 517. (12) Julian, M. D.; Luty, F. Phys. Reu. B: Condens. Mutter 1980, 21, 1647.
0022-3654/8?/2091-6659$01.50/0
TABLE I: Carbon Opacity, Surface Area, and Density opacity“ surface area, density,* band distortion, carbon IR 32 IR 85 m2/g g/cmS 1590 cm-Ic
Engelhard Norit Kynol-20 Kynol-15
1.85
1.44
1000
2.16 1.50 0.99
1.28 1.01
1000
0.72
2000 1500
0.44 0.49 0.33 0.50
1.O
1.7 1.8 1.5d
“Log [SBS,/SBS] per mg. IR/85 values taken with an MCT det e ~ t o r .The ~ absolute difference between the absorbance values may be due to partial saturation of the MCT at the higher light values. bNominalvalues. Density for Engelhard CG-5 is stated as 0.42 to 0.46 g/cm3. Density for Norit RB-1 is from the manufacturer’s literature. Density for Kynols calculated from published23density of nonactivated carbons multiplied by the yield for the activation process. CRatioof the magnitude of the negative lobe below the base line to the positive lobe above the base line. dThe 1590 band is very weak-approximately the intensity of the same band in the other carbons. is another variation of the original idea-light scattering through a porous solid14 where the principal scattering occurs at the boundaries of the pores. For this case, the Christiansen wavelength should occur where the index of the adsorbed gas, liquid, or solid matches the host. In this paper we demonstrate the magnitude of the Christiansen effect in several high-area amorphous carbons and show that the carbons tested can be easily differentiated by the enhanced transmission associated with specific adsorbates. Experimental Section IR spectra were obtained in transmission with an IBM Instruments IR/32 FT-IR (Fourier transform infrared spectrometer, f/4.2 optics) equipped with a DTGS detector (4000-400 cm-I) at 2-cm-’ resolution. N o special accessory optics were used to collect the scattered light from the samples. The absorbance spectra are shown ratioed to air, Figure lA, B, and shown after subtraction of the appropriate carbon reference, Figures I C and 2. The measurement temperature was 26 0.5 OC. Four kinds of carbon, activated for gas adsorption, were used in the experiment: Norit RB-1, Engelhard CG-5, Kynol 509-15, and Kynol 509-20. These carbons have nitrogen BET surface areas ranging from 1000 to 2000 m2/gl5--see Table I. Although the physical appearances of the Norit and Engelhard carbons are different before grinding, they appear to be similar powders of 10-20-llm particles after grinding. The Engelhard CG-5 contains 0.5 w/w % ’ palladium which is not expected to affect the physical
*
(13) Bradshaw, A. M.; Pritchard, J. Proc. R. SOC.London, A 1970,316, 169. Bradshaw, A. M.; Pritchard, J. Surf. Sci. 1969, 17, 372. (14) Cloupeau, M.; Klarsfeld, S.Appl. Opt. 1973, 12, 198. (15) Engelhard CG-5 (Engelhard Catalysts, Newark, NJ); Kynol-15 and Kynol-20 (ACC509 from American Kynol, New York, NY); Norit RBI (American Norit, Jacksonville, FL).
0 1987 American Chemical Society
6660 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
-~ -~ -----
Y
_~_-7_---1
~1
7
loci
i o 0
2300
iico
YAIENUIBER Icn-:,
Figure 1. IR spectral comparison of Engelhard CG-5 and biphenyl (saturated) in Engelhard CG-5: 13-mm-diameter pellet; resolution 2 cm-l; 20000 scans coadded; x = adsorbed C 0 2 in KBr support. (A) 0.67 mg of Engelhard CG-5 in 240 mg of KBr. (B) 0.55 mg of Engelhard CG-5 with 0.19 mg of biphenyl in 245 mg of KBr. (C) (Spectrum b) - 0.945(spectrum a).
I
4 0-0,
,
,
,
I
,
.
,
.
t
Saperstein For the IR measurement the carbon sample was mixed with a preweighed amount of KBr until a uniform grey color was obtained. Most samples contained 0.5-1.5 mg of carbon in 200-300 mg of KBr. The powder was pressed into a 13-mmdiameter pellet that transmitted between 1% and 10%of the light, corresponding to absorbance values (log [SBSo/SBS]; see below and ref 4) values of 2.0-1 -0, respectively. The usual method for normalizing an IR spectrum, dividing the absorbance values by the sample weight, does not account sufficiently for the sample opacity when different carbons are compared! For example, 1 mg of the Engelhard CG-5 is sufficient to produce the same light extinction as 2 mg of the Kynol 509-15. To account for the effect of the optical extinction of the carbon on the measured intensity of the absorbate, we note both the sample weight and total optical extinction of the pellet, log [SBSo/SBS]. Log [SBSo/SBS] is obtained by ratioing the peak intensity of the carbon interferogram, SBS, with the peak intensity of the interferogram, SBSo, collected without carbon. Table I shows the range of opacities for 1 mg of the different carbons used in these experiments. For comparison with the IR/32, we also measured opacity data4 on an IR/85 FT-IR equipped with an MCT detector. Although the two instruments produce nearly identical adsorbate spectra from the same sample, the carbon background measured with the IR/85 shows approximately 30% lower opacity values; see Table I. The lower carbon absorbance probably arises from nonlinearities in the MCT detector.
Results A typical IR spectrum of a sample of Engelhard CG-5 activated carbon is shown from 4000 to 450 cm-' in Figure 1A and is nearly featureless except for small peaks at 3400 cm-I (HzO in the KBr support), 1590, 1385 (adsorbed COz in the KBr support), and 1150 cm-I. The large offset from the abscissa, greater than 1 absorbance unit over most of the mid-infrared, shows the high opacity of less than 1 mg of an amorphous carbon sample. Figure 1B shows the spectrum of the Engelhard CG-5 after biphenyl vapor at 66 OC is allowed to equilibrate with the carbon in the vial for one day. The features in the spectrum of the adsorbate and carbon, Figure 1B, which are not present in the spectrum of the carbon alone, Figure lA, arise from the absorption bands for the biphenyl whose peak frequencies are only slightly shifted from their values in the melt spectrum. In particular, the presence of the weak 780-cm-' out-of-plane mode in the biphenyl spectrum indicates that the two rings are twisted with respect to each other.I6 Because this band is observed in the melt and not in the room temperature crystal spectrum, its presence is further evidence3s4for the retention of the adsorbates in a liquidlike state in activated carbons. There is an indication in Figure 1B that the absorption bands for the biphenyl in Engelhard CG-5 are asymmetric. This asymmetry is unexpected because isolated bands of biphenyl in the absence of carbon are symmetric. To magnify the biphenyl absorption asymmetries, we create the difference spectrum (Figure 1B minus Figure 1A) in Figure lC, thereby eliminating the carbon background. The asymmetrical band shapes are now more clearly resolved and show pronounced dips below the base line (enhanced transmission) for the in-plane mode at 1480 cm-I and the outof-plane modes at 780, 730, and 695 cm-'. Examination of the spectra in Figure 1A,B shows that the weak in-plane graphitic mode at 1590 cm-I also has a distinct asymmetry to the band with a pronounced dip below the base line on the high wavenumber side of the band. When these spectra are magnified, we can calculate a relative distortion of the carbon based on the ratio of the peak magnitude of the negative lobe to the peak magnitude of the positive feature in the band. Estimates of these band distortions are shown in Table I for the four carbons tested (16) Zerbi, G.; Sandroni, S. Spectrochim. Acta, Part A 1968, 24A, 483. Zerbi, G . ;Sandroni, S. Spectrochim. Acta, Part A 1968; 24A, 5 1 1. Almenningen, A.; Bastiansen, 0.; Fernholt, 0.; Cyvin, B. N.; Cyvin, S. J.; Samdal, S . J . Mol. Struct. 1985, 128, 59. Barrett, R. M.; Steele, D. J . Mol. Struct. 1972, 1 1 , 105. F'ulham, R. J.;Steele, D. J . RamanSpectrosc. 1984, IS, 217. Bree, A.; Edelson, M. Chem. Phys. L e f t . 1977, 46, 500.
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6661
I p Absorption Spectra of Aromatic Adsorbates TABLE II: Comparison of Apparent Enhanced Transmissions'
carbon Norit Kynol- 15 Engelhard Kynol-20 Kynol-20 Kynol-20
biphenyl, n = 1.475* wt % 730 cm-' 1480 cm-I 38 62 34 93 18 5
