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SPECTRAL CHARACTERIZATION OF ACTIVATED CARBON
Spectral Characterization of Activated Carbon
by R. A. Friedel U.S . Bureau of M i n e s , Pittsburgh, Pennsylvania
and L. J. E. Hofer' Mellon Institute of Carnegie-MeZZon University, Pittsburgh, Pennsyllvania
(Receiued February 2, 1970)
The infrared absorption spectrum of activated carbon has been observed for the first time. The method of sample preparation which involves very intense grinding is described in detail. The spectrum shows definite bands at 1735 cm-', 1590 cm-1, and 1215 cm-l. These are interpreted, respectively, in terms of a carbonyl, aromatic structures or unconjugated chelated carbonyl, and C-0 groups. The technique opens up the possibility for direct study both of the various functional groups on activated carbon itself and of various adsorbed species on activated carbon.
Infrared characterization of activated carbon is important for: (1) providing information on structure, such as the functional groups contained in the carbon, possibly mineral constituents, etc., (2) providing basic spectra of the activated carbon for comparison with spectra of the same carbon containing adsorbed material. Spectral studies of specific sorbates can provide much information on the molecular forces involved. No absorption spectra of activated carbon have appeared in the literature, presumably because of the difficulties involved in obtaining spectra. Coals and carbons are strong absorbers of visible radiation, but in nearly all cases transmission of infrared energy occurs. Usable coal sections several microns thick can be prepared rather easily;2asb however, for particulate samples such as carbons and cokes the thin section technique is, of course, not applicable. Even if sectioning were successful, the specimen would be full of holes and therefore unusable for spectral measurements. A successful approach involves the preparation of halide pellets (potassium bromide has been used in this work) in which finely divided carbonaceous samples are uniformly d i s t r i b ~ t e d . ~ ~Difficulties -~ are least with carbon blacks which are finely divided, but chars and activated carbons are very hard and are difficult to grind. This publication describes the preparation of halide pellets of activated carbon suitable for spectral characterization, the techniques necessary for scanning spectra, and the type of spectrum obtained. Mattson and coworkers have reported spectra of sorbates on activated carbons by means of the attenuated total reflectance (ATR) te~hnique.~-l For obtaining absorption spectra the preparation of KBr pellets of chars and activated carbons requires reducing the sample to a very finely divided form by grinding it for many hours in a ball mill. The ball mill used is a power-driven vibrator (frequency 3600 strokes/min, amplitude 5 mm) driving a stainless steel vial (i.d. 9 mm, inner length 15 mm with four l/s-in.
stainless steel balls). Total grinding time used is about 24 hr. It is important that the sample be no more than a few milligrams in order to ensure thorough grinding. If a large amount of sample is used in a small vial the sample serves as a cushion and thorough grinding is prevented. A standard KBr pellet is then prepared a t a concentration of about 0.5 wt The spectrometers used in this work were Perkin-Elmer Models 21 and 521. The absorption of carbonaceous materials may be so intense due to electronic absorptiong that the sample is completely opaque to visible and near-infrared light. This is not necessarily detrimental as transparency may exist for most carbonaceous materials in the infrared region of interest, particularly at lower frequencies. The spectral position of the initial transmittance, the so-called energy differs for different carbons and coals. For Pittsburgh CAL activated carbon the energy gap is about 2300 cm-', or 0.3 eV. If the first scan of the spectrum in the infrared shows complete or nearly complete absorption (Figure 1, spectrum a), one then rescans the spectrum with scale expansion or with a calibrated screen in the reference beam in order to produce a large magnification of the (1) Head, Adsorption Fellowship, sponsored by Pittsburgh Activated Carbon Division, a subsidiary of Calgon Corp. (2) (a) R.A. Friedel and M. G. Pelipetz, J. Opt. SOC.Amer., 43, 1051 (1953); (b) R. A. Friedel and J. A . Queiser, A n a l . Chem., 28, 22 (1956). (3) V. A, Garten and D. E. Weiss, A u s t . J . Chem., 10, 295 (1957). (4) R. A. Friedel, Proceedings of the Fourth Carbon Conference, University of Buffalo, Pergamon Press, New York, N. Y., 1960, p 321. (5) J. S. Mattson, H. B. Mark, Jr., M . D. Malbin, W. J. Weber, and J. C. Crittenden, J . Colloid Interface Sci., 31, 116 (1969). (6) J. S. Mattson and H. B. Mark, Jr., ibid., 31, 131 (1969). (7) J. S. Mattson and H. B. Mark, Jr., A n a l . Chem., 41, 355 (1969). (8) R. A. Friedel, R. A. Durie, and Y. Shewchyk, Carbon (Ozford), 5, 559 (1968). (9) R.A. Friedel, Fuel, 38, 541 (1959). (10) R.A. Friedel, Brennst.-Chem., 44, 23 (1963).
The Journal of Physical Chemistry, Vol. 74,No. 16,1970
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R. A. FRIEDEL AND L. J. E. HOFER
Figure 1. Infrared spectra of Pittsburgh activated carbon, type CAL. Spectrum a : KBr pellet, 0.640 mm thick and 12.7 mm in diameter, containing 0.5 wt % carbon in 200 mg of KBr; spectra b, c, d, e: scale expansions of spectrum a, a t 3 X , 5.5X, 14X, and 21 x, respectively.
transmitted energy. Wider slits and slower spectral scanning speeds must be used. If too great magnification is used, it is possible to reach a point where the apparent transmittance may consist of an appreciable amount of instrumental scattered energy; the usual check for scattered energy must be made. In a spectrum of Pittsburgh CAL activated carbon (Figure 1) some functional groups are observable, such as (1) the band at 1735 cm-I attributable to carbonyl groups, probably unconjugated ketone structures; (2) the band at 1590 cm-l, the well-known carbonaceous band which is variously attributed to aromatic structures and to unconjugated, chelated carbonyl groups;218,11and (3) the band at 1215 cm-l which is probably associated with a carbon-oxygen absorption and/or phenoxy absorption. The bottom curve on the spectrum is the original scan under ordinary instrument conditions, with no amplification of transmitted energy. The other curves are all amplifications of the transmitted energy. The actual transmittance values for all the curves are calculated from the parameters of the scaleexpansion or the calibrated screens. A spectrum of activated carbon obtained a t low noise level by the
The Journal of Physical Chemktry, Vol. 74,No. 16,1070
method described makes it possible to carry out spectral investigations of systems consisting of carbon and added materials. With this usable spectrum studies of sorbed species on activated carbon become possible. A point of difference appears between Mattson’s ATR results and the present results-little or no structure appears for the untreated, activated carbon by the ATR technique, whereas our absorption spectra demonstrate the presence of three distinct absorption bands. These bands are expected from the appearance of similar bands in spectra of coals and carbon blacks.2s12 We believe it is significant that the absorption technique appears to be more sensitive for observing the broad bands of activated carbons than the ATR technique. The fact that the absorption technique involves Mattson’s “difis not necessarily a disadvantage fuse since measurement of any spectrum, even of gases, suffers some loss of energy due to scatter, as the ATR method also does. (11) J. K.Brown, J. Chem. SOC.(London), 744 (1955). (12) V. A. Garten and D. E. Weiss in “Proceedings of the Third Conference on Carbon,” S. Mrozowski, Ed., Pergamon Press, New York, N. Y.,1959,p 295.
