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Langmuir 1997, 13, 6502-6509
Comparative Study of Active Carbons from Different Precursors Attila Bo´ta,† Krisztina La´szlo´,*,† Lajos Gyo¨rgy Nagy,† and Thomas Copitzky‡ Department of Physical Chemistry, Technical University Budapest, H-1521 Budapest, Hungary, and Institute of Applied Physical Chemistry, Research Center Ju¨ lich, D-52425 Ju¨ lich, Germany Received January 28, 1997. In Final Form: July 28, 1997X The effect of the precursor on the morphology, the structure, and the adsorption properties of the activated carbon in the case of poly(ethylene terephthalate) and cellulose, was studied. Scanning tunnel microscopy, X-ray scattering methods, and adsorption from gas and liquid phases were applied to answer the problem. According to adsorption data, the micropore structure and the chemical character of the two activated samples were found to be practically independent of their origin, but essential structural dissimilarities were concluded from SAXS data. STM illustrated the effect of the precursor’s structure only in dimensions which are considerably larger than the size of the regions governing the microporous behavior. The morphological differences in the carbon correspond to the different links between the crystallites. Presumably these connections strongly affect the diffusion of the activating agent and thus the pore structure developing during the activation process.
Introduction The adsorption properties of microporous carbons show a very wide variety.1-3 The versatility of the adsorption behaviours derives from the complex structure of the carbon. In the case of microporous carbons the fundamental structural units are the more or less ordered graphite-like crystallites, the amorphous carbon, and other compounds, e.g. inorganic impurities. Structural complexity develops during the preparation and can be influenced by the combination of the activation and heat treatment processes.4-9 At the same time, the structure and the chemical composition of the precursor strongly affect the character of the resulting carbon.10-15 The compositions and the structures of the conventional starting materials are very different; e.g. a semicrystalline structure is already involved in the fossile carbonaceous materials, but in the organic polymers or phytogenic sources this structure only develops during the preparation. The size of the crystallites and that of the pores formed in the crystalline region are interrelated and very * To whom correspondence should be addressed. † Technical University Budapest. ‡ Research Center Ju ¨ lich. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Smisek, M.; Cerny, S. In Active Carbon; Elsevier: New York, 1970. (2) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988. (3) Dubinin, M. M. In Chemistry and Physics of Carbon, Walker, P. L., Jr., Ed.; Marcel Dekker, Inc.: New York, 1968; Volume 2, Chapter 2. (4) Hofmann, U.; Wilm, D. Z. Electrochem. 1936, 42, 504. (5) Warren, B. E. Phys. Rev. 1941, 59, 693. (6) Dubinin, M. M. In Characterisation of Porous Solids; Gregg, S. J., Sing, K. S. W., Stoeckli, H. F., Eds.; Society of Chemical Industry: London, 1979; p 1. (7) Evans, M.; Marsh, H. In Characterisation of Porous Solids; Gregg, S. J., Sing, K. W. S., Stoeckli, H. F., Eds.; Society of Chemical Industry: London, 1979; p 53. (8) Setoyama, N.; Ruike, M.; Kasu, T.; Suzuki, T.; Kaneko, K. Langmuir 1993, 9, 2612. (9) Kaneko, K.; Ishii, C.; Kuwabara, H. Carbon 1992, 30, 1075. (10) Juhola, A. J. Kem.-Kemi 1977, 11, 543. (11) Juhola, A. J. Kem.-Kemi 1977, 12, 653. (12) Wigmans, T. In Activated Carbon...a Fascinating Material; Capelle, A., de Vooys, F., Eds.; Norit N.V.: Amersfoort, The Netherlands, 1983; p 58. (13) Wigmans, T. Carbon 1989, 27, 13. (14) Rodrı´guez-Reinoso, F.; Molina-Sabio, M. Carbon 1992, 30, 1111. (15) Gergova, K.; Petrov, N.; Eser, S. Carbon 1994, 32, 693.
S0743-7463(97)00088-7 CCC: $14.00
often show similar values.16 Surface chemical functionalities derived from the precursor and the activation or treatment procedures may also influence adsorption.17-21 Therefore, the adsorption character of activated carbons are determined together by the matrix, the pore structure, and the chemical nature of the surface. The objective of the present paper is to recognize the effect of the precursor on the morphology, the structure, and the adsorption properties of the activated carbon in the case of two typical precursors. Therefore, activated carbons were produced from a linear polymer, poly(ethylene terephthalate) and an inhomogeneous natural phytogenic fiber, cellulose, by steam activation. The matrix, the pore structure, and the adsorption properties of the activated products were studied. The general method of characterization of porous adsorbents is by using low-temperature gas adsorption.22 Although helium (He) has several advantages as adsorbatesse.g., it is a small, spherical monoatomic gas, and when it adsorbs in the narrow, slit-like micropores, the adsorbentsadsorbate interaction is weak23-nitrogen is the most commonly used gas in the adsorption measurements. The micro- and the mesopore analyses based on the nitrogen adsorption isotherms can be used in the comparative study of the microporous carbons without considering the side effects of the nitrogen adsorption. Several procedures can be used to evaluate the lowtemperature (77.5 K) nitrogen adsorption data, e.g., micropore analysis,24 or the different variations of the theory for the volume filling of micropores developed by Dubinin and his co-workers.25-29 Besides, the processes (16) Bo´ta, A. J. Appl. Crystallogr. 1991, 24, 635. (17) Stoeckli, H. F. Carbon 1990, 28, 1. (18) Jagiello, J.; Schwarz, J. A. Langmuir 1993, 9, 2513. (19) Bandosz, T. J.; Jagiello, J.; Schwarz, J. A. Langmuir 1993, 9, 2518. (20) Xijun, Hu; Duong, D. D. Langmuir 1993, 9, 2530. (21) Heuchel, M.; Jaroniec, M.; Gilpin, R. K.; Bra¨uer, P.; von Szombathely, M. Langmuir 1993, 9, 2537. (22) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity; Academic Press: New York, 1982. (23) Kuwabara, H.; Suzuki, T.; Kaneko, K. J. Chem. Soc., Faraday Trans. 1991, 87, 1915. (24) Mikhail, R. S. H.; Brunauer, S.; Bodor, E. E. J. Colloid Interface Sci. 1968, 26, 54. (25) Dubinin, M. M.; Astakhov, V. A. Izv. AN SSSR. Ser. Khim. 1971, 5. (26) Dubinin, M. M. Prog. Surf. Membr. Sci. 1975, 9, 1.
© 1997 American Chemical Society
Active Carbons from Different Precursors
taking place in the transitional micro and mesopore range can be interpreted by several methods, e.g., by using the a- or the t-plots.30,31 The Polanyi-Dubinin theory is the starting point of numerous further methods and more sophisticated potential theories are also widely used.32-37 A comparison of the most frequently used methods was recently given by Rychlicki et al.38 Adsorption methods provide information about the pores available for the adsorbate.39 Applying the molecular probe technique gives an insight into the size distribution of the pores, i.e., a possible molecular sieve action, and the micropore wall thickness can be established.40-43 The pore and the matrix structures, however, can be simultaneously studied by the small-angle X-ray scattering (SAXS) method.44,45 Activated carbon scatters the traversing X-ray beam also in the small-angle regime, because the electron density shows heterogeneity in the colloidal region.46 The primary structural parameter generally obtained from SAXS results is the Guinier radius, which can only be calculated and interpreted under strict conditions. The average size of the pores and the matrix units can be derived from the moments of the scattering curves on less strict conditions.47 Microscopic pictures provide visual information on the structure of the probes. Though the resolution of the scanning tunnel microscopic pictures is rougher in the case of amorphous carbon samples than in the case of the samples showing ordered graphite structure, the resolution limit is correct compared with the roughness of the carbon matrix.48-50 The chemical (polar-nonpolar) character of the carbon surface can be tested by using an adequate binary liquid mixture.51-56 The heterogeneity of the activated carbons prepared from the previously (27) Dubinin, M. M.; Zaverina, E. D.; Radushkevich, L. V. Zh. Fiz. Khim. 1947, 21, 1351. (28) Izotova, T. I.; Dubinin, M. M. Zh. Fiz. Khim. 1965, 39, 2796. (29) Dubinin, M. M.; Stoeckli, H. F. J. Colloid Interface Sci. 1980, 75, 34. (30) Sing, K. S. W. Chem. Ind. 1968, 1528. (31) de Boer, J. H.; Linsen, B. G.; Van der Plas, Th.; Zondervan, G. J. J. Catal. 1965, 4, 649. (32) Jaroniec, M.; Choma, J. Mater. Chem. Phys. 1986, 15, 521. (33) Do, H. D.; Do, D. D. Langmuir 1995, 11, 2639. (34) Spitzer, Z.; Bı´ba, V.; Kadlec, O. Carbon 1976, 14, 151. (35) Dollimore, D.; Heal, G. R. J. Appl. Chem. 1964, 14, 109. (36) Everett, D. H.; Powl, J. C. J. Chem. Soc., Faraday Trans. 1 1976, 72, 619. (37) Horva´th, A.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470. (38) Rychlicki, G.; Terzyk, A. P.; Łukaszewicz, J. P. Colloids Surf. A 1995, 96, 105. (39) La´szlo´, K.; Bo´ta, A.; Nagy, L. G.; Frischkorn, C. B. Carbon 1997, 35, 593. (40) Tsunoda, R.; Ando, J. J. Colloid Interface Sci. 1995, 171, 528. (41) Kipling, J. J.; Wilson, R. B. Trans. Faraday Soc. 1960, 56, 557. (42) Stoeckli, F.; Centeno, T. A.; Fuertes, A. B.; Muniz, J. Carbon 1996, 34, 1201. (43) Tsunoda, R. J. Colloid Interface Sci. 1992, 152, 571. (44) Ja´nosi, A.; Stoeckli, H. F. Carbon 1979, 17, 465. (45) Bo´ta, A.; La´szlo´, K.; Nagy, L. G.; Subklew, G.; Schlimper, H.; Schwuger, M. J. Adsorption 1996, 2, 81. (46) Guinier, A.; Fournet, G. Small Angle Scattering of X-Rays; J. Wiley & Sons, Inc.: New York, 1955. (47) Perret, R.; Ruland, W. J. Appl. Crystallogr. 1968, 1, 308. (48) Donnet, J.-B.; Quin, R.-Y. Carbon 1992, 30, 787. (49) Hoffman, W. P.; Fernandez, M. B.; Rao, M. B. Carbon 1994, 32, 1383. (50) Lei, H. N.; Me´trot, A.; Troyon, M. Carbon 1994, 32, 79. (51) Coltharp, M. T.; Hackerman, N. J. Colloid Interface Sci. 1973, 43, 176. (52) Schay, G.; Nagy, L. Gy. J. Chim. Phys. 1961, 149. (53) Dabrowski, A.; Jaroniec, M. J. Colloids Surf. 1980, 73, 475. (54) Coltharp, M. T.; Hackerman, N. J. Colloid Interface Sci. 1973, 43, 185. (55) Groszek, A. J.; Andrews, G. I. Third Conference on Industrial Carbons and Graphite; Society of Chemical Industry: London, 1971; p 156. (56) Groszek, A. J. In Proceedings International Symposium on Surface Area Determination; Everett, D. H., Ed.; Butterworths: London, 1969; p 313.
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mentioned precursors proved to be independent of their origin according to the liquid adsorption data. Experimental Section The granulated poly(ethylene terephthalate) (Qualon) was obtained from Mitsubishi (Singapore). The cellulose sheets were provided by the Paper Research Institute (Budapest, Hungary). The raw material was carbonized at 700 °C in a steel reactor flushed with nitrogen gas (50 dm3/h). The carbonized char samples were activated in a steam flow of 18 g/h at 900 °C in a rotary quartz reactor.57 The steam was diluted with nitrogen in a molar ratio of 1:1. The time of activation was optimized in order to obtain a burn-off of 50% related to the mass of the char. It took 90 min in the case of the poly(ethylene terephthalate) and 45 min in the case of the cellulose precursors. The activated samples will be abbreviated as PET (poly(ethylene terephthalate)) and CEL (cellulose). The surface morphology of the samples was imaged by using a Beetle scanning tunnelling microscope with an atomic Pt/Ir needle (developed by K. H. Besocke at the Research Center Ju¨lich, Germany). Probes pressed into platelets of about 2-mm thickness were imaged. The SAXS measurements were performed by a Kratky camera and a proportional counter (Anton Paar, Graz, Austria). The scattering of Ni-filtered Cu KR radiation (λ ) 1.542 Å) was recorded in the 6‚10-3-0.6 Å-1 range of the scattering variable, defined as h ) (4π sin Θ)/λ, where 2Θ and λ are the scattering angle and the wavelength, respectively. The primary beam was line focused. The intensity curves were corrected considering the geometry of the beam profile in order to obtain point-focused curves. SAXS measurements were completed by wide-angle X-ray scattering (WAXS), using a Seifert Diffractometer (Ahrensburg, Germany). The nitrogen adsorption/ desorption isotherms were measured at the boiling point of liquid nitrogen (77 K) from p/p0 ≈ 0.004 by AUTOSORB (Quantachrome, Syosset, NY) computer-controlled surface analyser. Samples were outgassed at 400 °C in a high vacuum (p < 5 × 10-7 Torr). The specific surface area, the total pore volume, and the pore size distribution were derived from the isotherms.58 Adsorption from benzene (1)-methanol (2) binary liquid mixtures were measured in order to characterize the chemical heterogeneity of the carbon surfaces. Isotherms were measured in the whole composition range at ambient temperature.
Results and Discussion STM Characterization of the Matrix. Due to the quite good electrical conductivity of the samples59 STM has been used for the observation of carbon samples. The surface morphology can be examined by STM from the atomic level up to a micrometer scale. The magnification available is limited by the quality of the surface. Hence, this method provides information about the carbons of amorphous and very rough surface in a range far from the atomic resolution. The 5100 × 5100 Å2 images show the different morphologies of PET and CEL samples. The PET sample consists of closely packed parallel anisotropic units of about 5000 Å in length and 1000 Å in diameter (Figure 1a). There is a finer arrangement along the units with a thickness of 200-300 Å. The long-shaped structural units may correspond to the formations developing from the linear poly(ethylene terephthalate) during the carbonization process. The CEL sample is also made of about 1000 Å thick units, but unlike in the case of the PET sample, their lengths are varied (Figure 1b). The fibrous units form a loose, irregular network, which resulted from the complex phytogenic structure of the precursor material. Along the fibers, which are alike in the case of PET sample, a 200-300 Å thick longitudinal pattern can be observed. The surface roughness of the (57) Noszko´, L.; Bo´ta, A.; Simay, AÄ .; Nagy, L. Gy. Period Polytech. 1984, 28, 293. (58) Bo´ta, A.; Valyon, J.; La´szlo´, K.; Nagy, L. Gy. Period Polytech. 1997, 41, 25. (59) La´szlo´, K. Unpublished results.
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Figure 1. 5100 × 5100 Å2 STM images of (a) PET and (b) CEL samples.
two samples are very different. More details are shown in the 1275 × 1275 Å2 images. The carbon skeleton of PET sample consists of isotropic units with a diameter of about 10-30 Å (Figure 2a). In the case of CEL sample further structural details have not been recognized. Occasionally 10-30 Å units were observed, but due to the surface heterogeneity, the surface could not be scanned in the nanometer range. CEL images of this resolution show a heterodispersive size distribution of the structural units (Figure 2b). SAXS Characterization of the Matrix. Both PET and CEL samples show scattering in the small-angle region (Figure 3), arising from their heterogeneous matrix structure. The dissimilarity of the SAXS curves suggests significant differences in the matrix structure. The most often derived parameter, the Guinier radius, RG, can be
Bo´ ta et al.
Figure 2. 1275 × 1275 Å2 STM images of (a) PET and (b) CEL samples.
calculated from the initial section of the scattering curve, assuming that RGh < 1
log I ) c1 +
RG2 2 h 3
(1)
where I is the corrected X-ray intensity and c1 is constant. The Guinier approximation (eq 1) is valid for monodispersive spherical particles but can be extended to any anisotropic form.60 If the assumption is fulfilled, the log I vs h2 plot is linear. According to Figure 4 none of the samples show a straight line in the initial section of this plot; that is the size of the scattering particles cannot be (60) Kakudo, M.; Kasai, N. X-ray Diffraction by Polymers; Kodashe LTD: Tokyo, 1992.
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Figure 5. The log I - log h2 representations of the SAXS data for PET and CEL. Table 1. Fitted Parameters of Maxwellian Distribution
Figure 3. Small-angle X-ray scattering curves of PET and CEL.
r0, in the small h range n, in the small h range r0, in the intermediate h range n, in the intermediate h range
PET
CEL
210 ( 1 6.00 ( 0.007 2.89 ( 0.01 14.076 ( 0.006
264 ( 2 7.020 ( 0.004
Figure 6. Guinier radius distributions for PET and CEL in the smaller region of the scattering variable. Figure 4. Guinier plot of the SAXS data of PET and CEL.
described by the Guinier radius. The deviation from the Guinier plot is due to the heterodispersive size distribution of the scattering particles. Shull and Roess described the scattering curves in the Guinier range by applying various types of size distribution functions.61 If a Maxwellian type of distribution is assumed, the first part of the scattering curve can be transformed into a straight line by a fitted shift (3/r02) according to eq 2,where c2 is constant, n and
log I ) c2 -
(n + 4) log(h2 + 3/r02) 2
(2)
r0 are the parameters of the Maxwellian distribution function in eq 3,and Γ((n + 1)/2) is the gamma function.
M(RG) )
2 rn exp(-RG2/r02) r0n+1Γ((n+1)/2)
(3)
M(RG) is normalized to the total mass of the scattering particles. The log I vs log h2 representation of both intensity curves (Figure 5) has declining sections in the very low log h2 range. The characteristic sizes of the larger scattering units were obtained from the linearization of this section of the curves. The PET carbon has a second (61) Shull, C. G.; Roess, L. C. J. Appl. Phys. 1947, 18, 295.
declination in the 0.05-0.5 range of the scattering variable, indicating the presence of scattering units with a smaller characteristic size. If the scattering of the larger units is neglected the characteristic size distribution was also computed to be in this range (Table 1). The distribution of the Guinier radii is shown in Figure 6. Assuming that the particles are narrow cylinders, as is supported by the STM pictures, the characteristic diameters calculated as 2Rcylinder ) 2(4/3)1/2RG are about 900 and 1160 Å in the case of PET and CEL, respectively. The size distribution in the latter case is more heterodispersive. The size and dispersity data are in good agreement with the STM observations. A size limit could not be detected for CEL particles either by STM or SAXS methods. In the case of PET sample the second deviation from the linearity at the higher region of the scattering variable was transformed and provided the size distribution of PET units (Figure 7). Assuming spherical particles, the average diameter is 2R ) 2(5/3)1/2RG ) 20 Å. As was mentioned previously, the strong small-angle scattering property of the active carbons is the consequence of the significant difference between the electron density of the matrix and that of the pores. The first is a constant positive value, while the latter is practically equal to zero. The characteristic sizes were computed by the method of moments. The correlation and the inhomogeneity length values were computed from the zeroth and first order
6506 Langmuir, Vol. 13, No. 24, 1997
Bo´ ta et al. Table 2. Structural Parameters and Specific Surface Area Values P
lc, Å lm, Å lp, Å f ) lc/2l Sx,a m2/g SBET,b m2/g
PET 0.55 15.1 CEL 0.42 35.7
7.0 24.3
8.6 17.6
1.95 1.47
2768 824
1190 760
a The specific surface area was derived from the SAXS data as Sx ) 4 × 104(1 - P)/lFHe. b Calculated from low-temperature nitrogen adsorption data by a multipoint equation.
Figure 7. Guinier radius distribution for PET in the higher region of the scattering variable.
moments of the SAXS curves. The correlation length, lc, was calculated as
∫hh
lc ) 2
max
min
∫hh
I(h) dh/
max
min
hI(h) dh
(4)
The correlation length is that maximum size where the scattered beams are in constructive interference. Its value gives the average extension of the homogeneous domains and is very sensitive to the corrections relating to multiple scattering occurring in the porous systems.47,62,63 At the same time, the multiple scattering is negligible in the h > 0.02 (1/Å) region. When calculating the zeroth moment in the 0 < h < 0.02 region, the assumption of I(h) ) constant ) I(0.02) was applied. In the case of CEL sample this correction resulted in a deviation of -10% compared to the uncorrected value. In the case of PET sample this deviation is less than 1%. The inhomogeneity length, l, was calculated as
l)
∫hh
max
min
hI(h) dh/lim Ih3
(5)
hf∞
This value represents the average length of all segments drawn across the matrix in all directions. From the inhomogeneity length the average wall thickness in the solid matrix and the width of the pores, lm and lp, respectively were derived as lm ) l/P and lp ) l(1 - P), where P and 1 - P are the volume fractions of the pores and the matrix, respectively. The volume fraction values have to be known in the regions corresponding to the coherent length. P can be calculated as P ) 1 - FHg/FHe, where FHg and FHe are the apparent and matrix density values, measured with mercury and helium, respectively.64 This approximation does not hold if there are closed pores in the sample or the mercury cannot penetrate into the pores having their size within the coherent region. (The highest pressure applied in mercury porosimeters is about 2000 bar. This pressure allows mercury penetration into pores with a diameter of lc ) 2rp ) 80 Å and is high enough to deform some of the pores as well.) Therefore, P was instead derived from the nitrogen adsorption isotherms, using the liquid nitrogen volume adsorbed in the pores of a diameter e lc ) 2rp. Data computed from the SAXS measurements are summarized in Table 2 as well as SBET calculated from nitrogen adsorption isotherms. Specific (62) Perret, R.; Ruland, W. J. Appl. Crystallogr. 1971, 4, 444. (63) Ruland, W.; Tompa, H. J. Appl. Crystallogr. 1972, 5, 1. (64) Kotlensky, W. V.; Walker, P. L. Crystallographic and Changes of Some Carbons upon Oxidation and Heat Treatment. In Proceedings of the Fourth Conference on Carbon; Pergamon Press: New York, 1960; p 423. (65) NORIT Testing Methods, NORIT N.V.-Holland, Special Publication.
surface area was also derived by the “t”-method and B-point method from nitrogen adsorption data and calculated from the iodine uptake from aqueous solution.65 All these data are conformable; therefore only the BET areas are reported. The two samples have different lc values, reflecting the different structure of the two matrices. Besides the coherent length, the average wall thickness and the pore diameter are also smaller in PET sample. The characteristic lengths in PET are small values, i.e., the PET sample unlike CEL is a loose formation of tiny units. The high dispersity results in high SAXS specific surface area values, SX. The geometry of the matrix units were concluded from the form factor, f ) lc/2l, f . 1, when the scattering units form a lamellar structure. Besides the f ) 1.95 value the lamellar structure of PET sample was also supported by the negative slope of the log(Ih2) vs h2 representation in the 0.3 < h < 0.5 range.66 The form factor for the CEL sample was found to be about 1.5, and at the same time, there is no linear section in the log(Ih2) vs h2 representation; hence, the lamellar structure of CEL units is not as positive as that of the PET units. From the WAXS curves the diffraction peak of a 002 Miller index is characteristic to the existence of graphite-like crystallites. According to the position of this peak, the average distance of the semicrystalline planes in both samples proved to be 3.60 ( 0.05 Å, which is larger than the basal plane distance in graphite. Pore Structure from Nitrogen Adsorption. Nitrogen adsorption data were used to study the pore structure of the samples. The isotherms are of type IV according to the IUPAC classification. The hysteresis loops are of types 3 and 4 in the case of CEL and PET, respectively.67 The maximal specific adsorptions of the two samples are similar, but the shapes of the isotherms are different (Figure 8). At p/p0 ) 0.2, 90% of the total adsorption is achieved in PET, but only 60% in CEL. As the adsorption in the low relative pressure range traces back to the presence of micropores, PET mostly contains micropores, while the CEL sample also contains mesopores in a considerable ratio. The contribution of the micropores is also reflected in the BET specific surface areas, SBET (Table 2). If the BET surface area values are compared with the corresponding SX values in Table 2, the latter ones are higher in both samples, but the SBET/SX ratios are different. This is the result of the dissimilar matrix structure of the two samples. Additional information was obtained by a prolonged activation process. At a conversion value of 77% the PET sample has SBET ) 1700 m2/g, showing a highly dispersive structure. This specific surface area is very high, considering that at this high conversion large pores necessarily are present as well. On the contrary, in the case of CEL sample SBET decreases above a conversion higher than 50%. SX gives the maximal specific surface area potentially available, but the actual value achieved basically depends on how the applied activation process opens up the inner structure of the precursor. (66) Glatter, O., Kratky, O., Eds. Small Angle X-Ray Scattering; Academic Press: London, 1982; Chapter 2. (67) Sing, K. S. W. Pure Appl. Chem. 1982, 54, 2201.
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distribution function is based on the relation r ∝ E1/s (where s ≈ 3) and has the form
dW/dr ) W0(k/E)n(ns/rns+1)exp[-(k/E)nr-ns]
Figure 8. Adsorption isotherms of N2 on PET and CEL samples. Symbols show measured data.
The adsorption in the micropores were interpreted according to the pore filling model.26,68 A special form of the Dubinin-Astakhov equation, the Dubinin-Radushkevich (D-R) equation was applied:
[(
W ) W0 exp -
)]
RT p0 ln βE p
2
(6)
E is the energy characteristic to the adsorption occurring in the micropores, β is the affinity coefficient, W is the volume of the partially filled micropores, and W0 is the total volume of the micropores. For N2 adsorption β ) 0.33 was used.69,70 In the 0 < p/p0 < 0.2 range, our curves were approximated with the two-term D-R equation:28
[(
W ) W1 + W2 ) W01 exp -
)]
p0 RT ln βE1 p
[(
W02 exp -
2
+
)]
p0 RT ln βE2 p
2
(7)
From the fitting functions two different micropore ranges were distinguished and characterized by the W01 and W02 pore volumes and the E1 and E2 adsorption energies (Table 3). The ratios of the pore volumes in the two micropore size ranges are similar in both samples; i.e. the 50% conversion resulted in similar micropore structures in both samples. It is possible to estimate the characteristic dimensions of micropores from the knowledge of E by various semiempirical relations17,29,34,71-76 most often established for slitlike micropores. The D-R method modified by Spitzer et al.34 was used in this paper. The (68) Marsh, H.; Rand, B. J. Colloid Interface Sci. 1970, 33, 101. (69) Garrido, J.; Linares-Solano, A.; Martin-Martinez, J. M.; MolinaSabio, M.; Rodriguez-Reinoso, F.; Torregrosa, R. Langmuir 1987, 3, 76. (70) Ehrburger-Dolle, F.; Holz, M.; Lahaye, J. Pure Appl. Chem. 1993, 65(10), 2223. (71) Dubinin, M. M. Carbon 1989, 27, 457. (72) McEnaney, B. Carbon 1987, 25, 69. (73) Stoeckli, H. F.; Kraehenbuehl, F.; Ballerini, L.; De Bernardini, S. Carbon 1989, 27, 125. (74) Stoeckli, H. F.; Ballerini, L.; De Bernardini, S. Carbon 1989, 27, 501. (75) Dubinin, M. M. Carbon 1985, 23, 593. (76) Dubinin, M. M. Pure Appl. Chem. 1989, 61, 1841.
(8)
where r is the micropore radius and k ) 0.695 kJ nm3/mol, calculated for laterally ended pores and the nitrogencarbon interaction from the Kirkwood-Mu¨ller equation; n ) 2 according to the D-R equation, and s ) 3 is the exponent of the pore radius in the de Boer-Clusters equation.34,77,78 Nevertheless, the value n ) 2, hypothetically corresponding to a homogeneous system of micropores, has been used by different authors to describe heterogeneous micropore systems.73 The pore size distributions of PET and CEL samples are shown in parts a and b of Figure 9. The distributions in the two pore size ranges are also displayed. The shapes of the corresponding curves are similar. The average pore radii calculated are 3.1 and 4.5 Å in the polymer and 2.9 and 4.5 Å in the cellulose-derived carbons; i.e., two characteristic pore sizes have been found in both samples. In the semicrystalline layer structure those micropores are abundant, which have been yielded by the burn-off of one or two adjacent layers, respectively. It is presumable that, in the crystallite units, the narrower pores develop when a single layer burns off (2rN ≈ 2r1) and the wider ones are formed when two adjacent layers are gasified (2rW ≈ 2r2).6,79 The distance between the adjoining crystallites is slightly larger than the graphitic carbon layer distance. The outermost layers of two adjacent microcrystallites may form narrow pores, which contrary to the space between the graphite layers are already available for nitrogen molecules. If the burnoff does not affect the outer layers of the adjoining microcrystallites, their distance, which is not far from that of the graphite layers, shifts the average pore size toward smaller values when compared to WAXS data. The very similar micropore size distributions of the two samples indicate similar fine structures of the carbon skeleton, as similar layer distances were also computed from the WAXS results. The morphology of the precursor material affects the matrix and pore structure of the activated carbon samples only in dimensions beyond the crystallite size. According to SAXS data measured in the case of CEL (Table 2), the mesopores, which are responsible for the adsorption in the 0.2 < p/p0 < 1 range, are formed by the complete burn-off of the larger structural units. Fractal Analysis. The concepts of fractal geometry elaborated by Mandelbrot can be applied successfully to the study of solid surfaces. Traditionally, the fractal dimension D is calculated from adsorption measurements, based either on the monolayer capacities of different adsorbates or on the adsorption model.80,81 Developments in the theory of gas adsorption on structurally heterogeneous microporous solids and extensive experimental studies improve evaluation of the micropore-size distribution, which permits a direct estimation of the fractal dimension.82-84 The analysis of micrographs obtained by scanning tunneling electron microscopy may also be applied to characterize the fractal behavior of open surfaces at the nanometer level.85 (77) de Boer, J. H.; Custers, J. F. Z. Phys. Chem. 1934, 25, 225. (78) London, F. Z. Phys. Chem. 1931, 11B, 246. (79) Ehrburger, P.; Pusset, N.; Dziedzinl, P. Carbon 1992, 30, 1105. (80) Avnir, D.; Farin, D. In The Fractal Approach to Heterogeneous Chemistry; Avnir, D., Ed.; Wiley: New York, 1989; Chapter 4. (81) Avnir, D.; Farin, D.; Pfeifer, P. New J. Chem. 1992, 16, 439. (82) Jaroniec, M. Fuel 1990, 69, 1573. (83) Terzyk, A. P.; Wojsz, R.; Rychlicki, G.; Gauden, P. A. Colloid Surf. A 1996, 119, 175. (84) Ismail, I. M. K.; Pfeifer, P. Langmuir 1994, 10, 1532. (85) Donnet, J. B.; Custodero, E. Carbon 1992, 30, 813.
6508 Langmuir, Vol. 13, No. 24, 1997
Bo´ ta et al.
Table 3. Micropore Volumes and Characteristic Adsorption Energies from Two-Term D-R Analysis and Mean Micropore Width from the Spitzer Equation34 PET CEL
W0,N, cm3/g
W0,W, cm3/g
E1, kJ/mol
E2, kJ/mol
2rN, nm
2rW, nm
0.418 ( 0.001 0.256 ( 0.001
0.199 ( 0.001 0.118 ( 0.001
29.91 ( 0.17 33.70 ( 0.27
9.06 ( 0.11 9.03 ( 0.30
0.56 0.54
0.84 0.82
Adsorption from Benzene (1)-Methanol (2) Binary Liquid Mixture. As the pore structure of the two samples are similar, excess isotherms can be used to describe the character of the surface. The surface groups on carbon have a large impact on the liquid-phase adsorption of organic molecules from such mixtures. Surface oxides impart polar character to carbons, and as a result preferential adsorption for the more polar component occurs from binary mixtures of alcoholbenzene.88,89 Carbons containing surface oxides appear to preferentially adsorb alcohol in the binary mixture. However, as the relative concentrations of the polar and nonpolar components in the binary mixture change, the organic molecule that is preferentially adsorbed may also change. The preferential adsorption of benzene is attributed to the interaction between π-electrons in the benzene ring and the carbon surface.90 The behavior in which both components in the binary mixture adsorb, depending on the relative concentration of each component, is ascribed to the acidic CO2 complexes on the carbon surface. Part of the oxygen present in CO2 complexes is thought to render the surface “benzophilic”, while part of the oxygen renders the surface polar and enhances its interaction with the polar component (methanol) in the binary mixture. Excess isotherms (n1σ(n) vs x1 functions) were calculated from the primary experimental data:
a
b Figure 9. Micropore size distributions of PET (a) and CEL (b) by N2 adsorption.
The SAXS provides an excellent possibility to follow the fractal behavior of the activated carbon.80,86,87 Both mass and surface fractal dimensions, Dm and Ds, respectively, were computed from the scattering data
Dm ) |S1|
(9a)
Ds ) 6 - |S2|
(9b)
and
where S1 and S2 are the slopes of the log I vs log h scattering curves. S1 and S2 were determined in the medium and the Porod regime of the scattering variable, respectively. Ds ) 2.05 ( 0.1 for the PET sample, which is practically equal to the dimension of a Euclidean surface. This value shows that units in the PET sample are nearly monodisperse. Ds ) 2.11 ( 0.1 for CEL. This slightly higher value is supported by the wide RG distribution; i.e., there is no characteristic unit size in the CEL sample. The shape of the formations in the solid matrix is well reflected by the Dm values. A very compact structure has a Dm ) 3, while the stacked ones have a value less than 3, in extreme cases close to 1, if the structure is chain-like. Dm ) 1.26 ( 0.1 and 2.76 ( 0.1 in the case of PET and CEL, respectively. Supposedly, this extreme difference in the matrix formations results in different opening of the pores during the activation process. (86) Reich, M. H.; Russo, S. P.; Snook, I. K.; Wagenfeld, H. K. J. Colloid Interface Sci. 1990, 135, 353. (87) Snook, I. K.; McMahon, P. Langmuir 1993, 9, 2726.
n1σ(n) ≡ n0(x1,0 - x1) ) nsx1s - nsx1
(10)
where n1σ(n) is the specific surface excess amount (mmol/ g) of benzene in the interfacial layer, n0 is the specific amount of the initial bulk liquid phase (mmol/g), x1,0 and x1 are the initial and the equilibrium molar fractions of the benzene, respectively, in the bulk phase, ns is the specific amount of the adsorbed liquid in the interfacial layer and x1s is its composition in equilibrium. When 0 < x1 < 1 and n1σ(n) ) 0, x1s ) x1 ) x1,a. This is the so called adsorption azeotropic composition, where the composition of the bulk and interfacial phases are equal. Both isotherms (Figure 10) are of type IV according to the Schay-Nagy classification.52 x1,a ) 0.62 and 0.67 in PET and CEL samples, respectively. In the case of both samples in the 0 < x1 < x1,a interval, the nonpolar benzene adsorbs preferentially while in the x1,a < x1 < 1 range the adsorption of the polar methanol is preferred, showing that the surfaces contain both nonpolar and polar structure sites. Both isotherms have a relatively long linear section. In this section the composition of the interfacial layer is practically constant52,91-94 and equals x1,a. The (88) Cookson, J. T., Jr. In Carbon Adsorption Handbook; Cheremisinoff, P. N.; Ellerbusch, F., Eds., Ann Arbor Science: Ann Arbor, MI, 1978; p 270. (89) Kipling, J. J. Adsorption from Solutions of Non-Electrolytes; Academic Press: London, 1965; Chapter 4. (90) Gasser, G. C.; Kipling, J. J. Proceedings of the Fourth Conference on Carbon, Pergamon Press: New York, 1960; p 55. (91) Schay, G.; Nagy, L. Gy. Elegyadszorpcio´ folyade´ k/szila´ rd e´ s folyade´ k/go´ z hata´ rfelu¨ leten; Akade´miai, Kiado´: Budapest, 1978. (92) De´ka´ny, I.; Zsednai, A Ä .; Kira´ly, Z.; La´szlo´, K.; Nagy, L. G. Colloids Surf. 1986, 19, 47. (93) De´ka´ny, I.; Zsednai, AÄ .; La´szlo´, K.; Nagy, L. G. Colloids Surf. 1987, 23, 41. (94) De´ka´ny, I.; A Ä braha´m, T.; Nagy, L. G.; La´szlo´, K. Colloids Surf. 1987, 23, 57. (95) La´szlo´, K. Ph.D. Thesis, Technical University Budapest, 1994.
Active Carbons from Different Precursors
Langmuir, Vol. 13, No. 24, 1997 6509
Figure 10. Adsorption excess isotherms from benzene (1)methanol (2) mixtures.
specific free enthalpy (∆G) vs. activity function was also computed
∆G ) -RT
n1σ(n)(x1)
∫a )0 (1 - x )a a1 1
1
da1
∆G SBET
Conclusion
(11)
1
where a1 is the activity of the benzene. The activity was derived from x1 by using vapor/liquid equilibrium data.95 ∆G was related to the BET specific surface area
∆G′ )
Figure 11. Free enthalpy functions from benzene (1)methanol (2) adsorption.
(12)
and the ∆G′ vs x1 functions are displayed in Figure 11. Both samples exhibit a wide range, where ∆G′ is practically constant and equals 19.5 and 22.3 mJ/m2 for PET and CEL, respectively. From the similarity of the length of the linear section and the corresponding x1,a and ∆G′ values, we conclude that the surfaces are alike and are inhomogeneous. They contain both nonpolar, i.e., benzophilic, and polar regions. The latter increases with the number of carbon atoms being at the edge of the crystallites.51 As the x1,a values, i.e., the ratio of the polar/ nonpolar regions are very similar in the two samples, we presume that the ratios of the outer polar and inner nonpolar regions of the crystallites and thus their lateral extensions are almost identical in the two samples.
The micropore structure and the chemical character of the two samples studied were found to be practically independent of their origin. As these statements were derived from adsorption data, they hold only for regions available for the applied adsorbates. Considering the whole structure, there are essential structural dissimilarities, as was concluded from SAXS data. STM imaged the effect of the precursor’s structure only in dimensions which are considerably larger than the size of the regions governing the microporous behavior. These morphological differences correspond to the different links between the crystallites. Presumably these connections strongly affect the diffusion of the activating agent and thus the ratio of the micropores in the pore structure developing during the activation process. Acknowledgment. The research described in this paper was supported by the OTKA Funds (Hungary), Grant Nos. T 017019 and T 017039. True credit is due to our co-workers Emese Fu¨lo¨p, Gyo¨rgy Bosznai, and Miklo´s Ko¨vi for their experimental work. LA9700883
