J. Phys. Chem. 1987, 91, 515-516
515
A Standard Adsorption Isotherm for the Characterlzation of Activated Carbons Francisco Rodriguez-Reinoso,* Jose Miguel Martin-Martinez, Ceiia Prado-Burguete, and Brian McEnaney Department of Inorganic Chemistry, University of Alicante, Alicante, Spain and School of Materials Science, University of Bath, Bath, BA2 7AY, UK (Received: September 15, 1986)
The preparation and characterization of a new nonmicroporous carbon to be used as a reference material with a standard N2 adsorption isotherm for characterization of activated carbons are described.
Introduction
Activated carbons are porous adsorbents with pores ranging from micropores (C2.0 nm in width) through mesopores (2-50 nm in width) to macropores (>50 nm in width). For many activated carbons adsorption isotherms approximate closely to type I, since most of the adsorbed volume is contained in micropores. The heterogeneity of the porosity makes the interpretation of the adsorption isotherms very difficult.’ Empirical methods of analysis such as Dubinin-Radushkevich plots,2 t plots,3 and a plots4 have been used with conflicting results due to the essentially microporous nature of most activated carbons. It is now widely accepted that the initial part of the type I isotherm for activated carbons represents micropore filling and that the slope of the plateau at high relative pressure is due to multilayer adsorption on the nonmicroporous surface, i.e. in mesopores, in macropores, and on the external surface. The DR equation may be used to determine the micropore volume, but there are several types of deviation from the linear DR plot5 which can make the estimation of the micropore volume difficult. The t plots or a plots are methods for comparing the shape of a given isotherm with that of a standard isotherm on a nonporous solid which allow the micropore volume and nonmicroporous surface area to be estimated. When using t plots or a plots the choice of a standard isotherm is of great importance. It has been suggested6 that the reference isotherm should have the same BET C constant as the test material. However, IUPAC has recently recommended’ “that the standard isotherm should be obtained for the particular adsorption system and not by choosing a type I1 isotherm which happens to have the same C value as the isotherm on a particular microporous solid”. Previously published standard isotherms for carbon adsorbents (e.g., ref 8 and 9) are for adsorption of N2on graphitized carbon blacks. However, the structure of graphitized carbon black is quite different from that of activated carbon. Graphitized carbon blacks have a polycrystalline, three-dimensional graphite structure in which the basal planes are oriented parallel to the surface of the carbon black particles.I0 Graphitized carbon black surfaces are quite homogeneous so that adsorption of N2,Ar, or Kr often gives stepped isotherms. By contrast, the structure of activated carbon is highly disordered consisting of defective carbon layer planes in the form of twisted lamellae, cross-linked by an extended carbon network and with no extended, three-dimensional graphite structure (e.g., ref 11). Consequently, active carbons have a highly heterogeneous structure. For these reasons, new, nonmicroporous (1) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, Academic: London, 1982; 2nd ed., Chapter 4.
( 2 ) Dubinin, M. M. Progress in Surface and Membrane Science, Cadenhead, D. A., Ed.; Academic: New York, 1975; pp 1-70. (3) Lippens, B. C., Linsen, B. G.; de Boer, J. H. J . Catal. 1964, 3, 32. (4) Sing, K. S. W . Surface Area Determination, Everett, D. H.; Otewill, R. H., Eds.; Butterworths: London, 1970; pp 25-34. (5) Marsh, H.; Rand, B. J . Colloid Interface Sei. 1970, 33, 101. (6) Lecloux, A,; Pirard, J. P. J . Colloid Interface Sei. 1979, 70, 265. (7) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (8) de Boer, J. H.; Linsen, B. G.; Van der Plas, Th.; Zondervan, G. J. J . Catal. 1965, 4, 649. (9) Dubinin, M. M. Carbon 1985, 23, 373.
0022-3654/87/2091-05 15$01SO10
TABLE I: BET Surface Areas and C valus carbon heat treatment surface area. m2 E-’ V3G 3073 K, C12, Ar” 62 A 2073 K, Ar, 0.5 h 4.4 B 2073 K, Ar, 1.0 h 4.3
C
184
350 270
“Room temperature to 2000 O C in 4 h in an argon purge, 2000-2800 OC in 3 h in a chlorine-argon purge, hold at 2800-2810 “ C for 1 h in a chlorine-argon purge, and cool to room temperature in 3 days in an argon a t m 0 ~ p h e r e . I ~
. E
m
04
c6
a0
10
PIP.
Figure 1. Reduced isotherms for adsorption of N2 at 77 K on graphitized carbon black V3G (0)and heat-treated carbons: carbon A (A);carbon B ( X ) . The two lines indicate the range of the reduced isotherms for adsorption of N, on a variety of adsorbents collated by Pierce.I5
reference materials have been prepared by using an activated carbon as a starting material. Experimental Section
An activated carbon was prepared by carbonization of olive stones at 1123 K in Nzfor 2 h, followed by activation in COz at 1073 K for 0.5 h to produce only 2-376 burn-off. The activated carbon so produced has been shownI2 to have an essential!y bimodal pore size distribution consisting of macropores and micropores, having a narrow range of pores sizes, and with almost no mesopores. To produce nonmicroporous reference materials, the activated carbon was heat treated to 2073 K for 0.5 (carbon A) and 1 h (carbon B). Masters and McEnaneyI3 have shown that the heat treatment of a cellulose carbon in the range 1700-1800 K converts open micropores to closed micropores. A heat treatment temperature of 2073 K was chosen in the present (10) Heidenreich, H. D.; Hess, W. M.; Ban, L. L. J . Appl. Crystallogr. 1963, I , 1.
(1 1) Oberlin, A,; Villey, M.; Comraz, A. Carbon 1980, 18, 347. (1 2) Rodriguez-Reinoso, F.; Martin-Martinez, J. M.; Molina-Sabio, M.; Torregrosa, R.; Garrido-Segovia, J. J. Colloid Interface Sei. 1985, 106, 315. (13) Masters, K. J.; McEnaney, B. Carbon 1984, 22, 595.
0 1987 American Chemical Societv
516 The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 TABLE 11: Standard Data for the Adsorption of Nitrogen at 77 K on CarbonA PIP, nln, CY PIP, nln, CY 0.005 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28
0.82 0.87 0.92 0.95 0.98 1.00 1.02 1.03 1.05 1.09 1.12 1.14 1.17 1.21 1.24 1.27 1.30 1.33 1.37
0.51 0.54 0.57 0.59 0.61 0.63 0.64 0.65 0.66 0.68 0.70 0.71 0.73 0.75 0.78 0.79 0.81 0.83 0.85
0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.50 0.54 0.60 0.64 0.70 0.74 0.80 0.84 0.90 0.94
1.41 1.44 1.48 1.52 1.56 1.60 1.64 1.68 1.71 1.79 1.88 2.02 2.13 2.32 2.46 2.71 2.87 3.29 3.91
0.88 0.90 0.93 0.95 0.98 1 1.02 1.05 1.07 1.11 1.17 1.26 1.33 1.45 1.54 1.69 1.79 2.05 2.44
work in an attempt to ensure complete micropore closure. Adsorption of N2 at 77 K on the carbons which had been outgassed at 383 K for 16 h was measured in a high-precision volumetric system. The monolayer values, n,, were obtained by using the BET equation with Po = 757.0 Torr; the linear ranges of the BET plots were from PIPo = 0.05 to 0.26; specific surface areas, A,, were calculated from n,,, by using a cross-sectional area for the N2 molecule of 0.162 nm.2
Results and Discussion Values of A, and the BET constant C for the heat-treated carbons are compared in Table I with those for a graphitized carbon black, V3G,I4using the data of Salinas-Martinez de k e a et al.Is The A, and BET Cvalues for carbons A and B are similar, suggesting that heat treatment for 0.5 h at 2073 K is sufficient to ensure complete micropore closure. This carbon A has been chosen as the new standard material. The adsorption isotherms for the heat-treated carbons A and B are plotted in reduced coordinates (Le., n/n, against PIP,) in Figure 1 where they are compared with the range of reduced isotherms collated by PierceI6 using N, adsorption data of a
(14) Ehrburger, P.; Mahajan, 0. P.; Walker, P. L. J . Catal. 1976,411, 61. (15) Salinas-Martinez de Lecea, C.; Linares-Solano, A.; Lopez-Gonzales, J. D.; Rodriguez-Reinoso, F. Carbon 1981, 19, 6 5 . (16) Pierce, C. J . Phys. Chem. 1968, 72, 3673.
Letters
1 I 01,
.;' 05
10
Is
2s
U
Figure 2. An CY plot for adsorption of N, at 77 K on an olive stone carbon activated to 34% burn-off and using carbon A as reference adsorbent.
number of workers on a variety of adsorbents; Pierce's collation extends over the range PIPo = 0.2 to 0.9, whereas the present data extend from PIPo = 0.005 to 0.94. The reduced isotherms for the heat-treated carbons fall within the range found by Pierce. In the range PIPo from 0.005 to about 0.3 the reduced isotherm for V3G is very close to those for the heat-treated carbons. However, at about PIPo = 0.3 there is a clear step in the reduced isotherm for V3G, Figure 1, and, at higher PIP,, the reduced isotherm for V3G lies above the range found by Pierce. As noted above, stepped isotherms are often found for such homogeneous adsorbents. Steps in the isotherm of a reference adsorbent are undesirable, since they can complicate the interpretation o f t plots and cy plots. Therefore, the new reference material, carbon A, appears to be superior to V3G in this respect. To facilitate the use of the new reference material, Table I1 contains the standard adsorption data in the reduced form appropriate to the t plot and cy plot methods. Figure 2 shows the result of applying the cy plot method to adsorption of N2 at 77 K on an active carbon using the new standard reference material. The active carbon was prepared by carbonization of olive stones in N2 at 1123 K followed by activation in C 0 2at 1098 K to 34% burn-off.I2 The micropore volume deduced from the cy plot is 0.39 cm3 g-' and the nonmicroporous surface area is 21 m2 g-I. More extensive application of the new standard material to activated carbons will be published elsewhere. Acknowledgment. Financial support from CAICYT (Project No. 795/8 1) and the Anglo-Spanish Joint Research Programme is gratefully acknowledged.
