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Ind. Eng. Chem. Res. 1999, 38, 1396-1399
Characterization of the Microporous Structure of Activated Carbons through Different Approaches Lourdes Dı´az, Rosario Herna´ ndez-Huesca, and Gelacio Aguilar-Armenta* Centro de Investigacio´ n de la Facultad de Ciencias Quı´micas, BUAP, 14 Sur y Av. San Claudio, 72570, Puebla, Pue., Me´ xico
The characterization of the porosity of three samples of commercial activated carbons, from adsorption isotherms of N2 (77 K), obtained by the volumetric method are presented. Special care was taken in the measurement of the isotherms in the range of low relative pressures (10-6) in order to characterize the microporosity of these carbons by different approaches: Dubinin-Radushkevich (D-R), R plots, Jaroniec-Choma (J-Ch), and Pfeifer et al. Microporosity characterization was completed by measurement of the specific surface area (BET) of the adsorbents in which different amounts of benzene at 295 K had been preadsorbed. It was established that the three samples are basically microporous, with a strongly pronounced structural and energetic heterogeneity. Introduction Physical adsorption of gases and vapors is one of the most widely used techniques for the characterization of porous solids (Gregg and Sing, 1982). The study of adsorption equilibrium in microporous solids such as activated carbons, zeolites, and pillared clays is of the utmost importance for their use in concrete applications. One of the peculiarities of activated carbons with a low mesopore content is that its adsorption capacity is principally concentrated in the volume of the wide micropores, whose sizes are usually not larger than 20 Å and which even extend up to molecular dimensions. As is generally known, the classical models of Langmuir and BET are used for the evaluation of the apparent specific surface area on the basis of these kinds of isotherms, although the former is more recommendable than the second, because it describes the experimental points of the isotherms of the adsorption of gases in a wider interval, even up to 0.4 (Gregg and Sing, 1982). Another model which has been widely applied to describe the adsorption in micropores is the one developed by Dubinin in his theory of the volume filling of micropores (TVFM), and together with the methods of R or t plots, it is used for the calculation of micropore volume. Here we present the study of the properties of porosity, principally microporosity, of three samples of industrial activated carbons (CG, C1, and C2) prepared by the company Nobrac, S.A., using coconut shells as raw material. On the basis of the measurement of the adsorption isotherms of N2 (77 K), starting at low relative pressures, the micropore volume, energetic and structural homogeneity of these adsorbents was assessed through approaches of the Dubinin-Radushkevich (D-R), R plots, Jaroniec-Choma (J-Ch), and Pfeifer et al. The objective of this paper was to compare these approaches for the evaluation of the mentioned above characteristics of these activated carbons prepared by different procedures. * To whom correspondence should be addressed. E-mail:
[email protected].
Figure 1. N2 adsorption isotherms (77 K).
Experimental Part The adsorption isotherms were measured in a highvacuum volumetric system. Prior to each adsorption run, the samples under study were outgassed in situ at 280 °C up to a residual pressure of less than 10-7 Torr (turbomolecular pump, Balzers). The pressures were measured by two types of pressure transducers (Balzers) of different range: TPR 017 (10-4 to 5 Torr), and APR 017 (1-1000 Torr). Results and Discussion Adsorption Isotherms. The adsorption isotherms of N2 (Figure 1) in the three samples are of type I, and they presented an extremely narrow hysteresis loop, which extended to P/P0 < 0.10. It was possible to detect, although not very well-defined, pore diameter distributions in the zone of narrow mesopores (23 Å). From the isotherm behavior (Figure 1), it can be deduced that the three samples have a high micropore content. r Plots. The isotherms of Figure 1 were transformed in the R plots (Figure 2), taking as the standard isotherm the one obtained by Rodriguez-Reinoso et al. (1987). The R plots for the three carbon samples show two straight lines situated at low and high relative pressures and one intermediate zone. The total micropore volume was evaluated through the extrapolation of the straight line at high pressures toward the
10.1021/ie9706166 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/17/1999
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1397
Figure 3. R plot (CG sample).
Figure 2. R plots, N2 adsorption (77 K). Table 1. Micropore Volume (V0r) and Mesopore Specific Surface Area (Sme) of Activated Carbonsa
CG C1 C2
a0R (mmol/g)
V0R (cm3/g)
a0.005 (mmol/g)
a0.005/a0R (%)
Sme (m2/g)
6.979 8.295 12.039
0.2420 0.2877 0.4175
5.54 6.92 9.84
79.4 83.4 81.7
23 16 31
a ba(R) ) 0.072 mmol/g; S(R) ) 4.4 m2/g), a 0.005 is the total amount adsorbed at P/P0 ) 0.005.
Table 2. Textural Properties of Activated Carbons (N2, 77 K) specific surface area (m2/g) tot. pore vol mean pore V0R V0R/V∑ BET Langmuir V∑ (cm3/g)a radiusb (Å) (cm3/g) (%) CG 594 C1 727 C2 1050 a
688 814 1197
0.2825 0.3174 0.4671
9.5 8.7 9.0
P/P0 ) 0.994. rp ) 2VS/SBET ω(N2) ) 16.2 b
0.2420 0.2877 0.4175
85.7 90.6 89.4
Å2/molecule.
adsorption axis. It could be established that the increase of adsorption in the interval 0.005 e P/P0 e 0.94 is considerably less than that of the P/P0 e 0.005 zone, which means that the main occupation of the micropores is taking place at low relatively pressures. On the other hand, during a detailed analysis of the intermediate zone, it can be observed that for C2 this zone extends to P/P0 ≈ 0.60, whereas for C1 and CG it is prolonged to P/P0 ≈ 0.50. This is probably due to the fact that sample C2, when compared with the other two, has a wider micropore size distribution, as well as a larger specific surface of mesopores. The data in Table 1 illustrate the behavior of the R plots quantitatively: when the relative pressure reaches the value of 0.005, then approximately 80% of the total micropore volume is occupied, and the remaining 20% corresponds to the filling of wider micropores. It is therefore assumed, due to the existence of the considerably extended intermediate zone, that the three samples have a markedly heterogeneous micropore structure. In Table 2 the textural properties of activated carbons are concentrated. As a consequence of type I of the isotherms (Figure 1), the Langmuir model described the experimental points up to P/P0 e 0.40 perfectly, whereas the BET model did that only up to P/P0 e 0.10. According to the last two columns of Table 1 and the last column of Table 2, the sample C1 has a slightly larger proportion of micropores than the other two samples. In order to examine with more detail the intermediate zone of the R plots, the scale of the ordinates of Figure 2 was increased (Figure 3), and in three samples the presence of two “intermediate subzones” was observed,
Figure 4. Dubinin-Radushkevich plots. Table 3. Total Micropore Volume (cm3/g)
CG C1 C2
V0D1
V0D
V0R
(V0D -V0D1)/ V0D %
(V0R -V0D)/ V0R %
0.2213 0.2822 0.4021
0.2476 0.2918 0.4271
0.2484 0.3014 0.4314
10.6 3.3 5.8
0.3 3.2 1.0
a V 0D1 and V0D are the volume of the first substructure and the total volume of the micropore (D-R). V0a is the total micropore volume found by R - plots.
whose points submit to a straight line. The extrapolation of the straight lines toward the adsorption axis leads to the obtention of three values of micropore volume, which confirms the heterogeneity of the microporous structure of the samples. The Dubinin-Radushkevich Model. In order to describe the adsorption in microporous solids with bimodal structures, Dubinin and Stoeckli (1980) developed the following equation:
a ) a01 exp[-B1(A/β)2] + a02 exp[-B2(A/β)2] (1) in which A ) RT ln P0/P is the differential molar work of adsorption and (a01, B1) and (a02, B2) are the parameters which correspond to the micropores and supermicropores; i.e. to the first and second substructures, respectively. The adsorption isotherms of N2 at 77 K (Figure 1) were treated by eq 1, thus obtaining two straight lines: the first in the interval of approximately 10-4 e P/P0 e 0.01 and the second in the zone 0.01 e P/P0 e 0.30. For sample C1 this behavior was not as marked as the one observed for the other two samples (Figure 4). Table 3 contains the micropore volumes found on the basis of the values a01 and a02, and for purposes of comparison the ones deduced by the R plots have also been included. The next to the last column indicates that the contribution (%) of the micropore volume of the second substructure in the total micropore volume V0D increases in the
1398 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 Table 4. Parameters of Microporous Structure (N2, 77 K)
CG C1 C2
E01 (kJ/mol)
E02 (kJ/mol)
B1103 (mol/kJ)2
B2103 (mol/kJ)2
x1 (Å)
x2 (Å)
8.9 8.4 8.0
6.3 6.7 6.9
1.36 1.54 1.77
2.79 2.42 2.27
3.7 3.9 4.2
5.3 4.9 4.8
order C1 < C2 < CG. On the other hand, it can be established that a very good concordance exists between the values of micropore volume found by both methods (last column, Table 3). From the Dubinin-Radushkevich plots of the three carbons under study, the respective structural constants Bi were found by eq 1, and by the empirical relation B ) cx2 proposed by Dubinin and Stoeckli (1980), the micropore size x was evaluated, where c ) 0.01 (kJ nm/ mol)-2. The values of the characteristic adsorption energies E0i as well as those of Bi and xi are given in Table 4. Adsorption Potential Distributions. According to the results mentioned above (Figures 3 and 4), the activated carbon samples under study have a rather marked structural heterogeneity. For this reason we consider it interesting to evaluate the energetic heterogeneity of these carbons by the method proposed by Jaroniec et al. (1991), applying the next equation: n
∑ i)1
X(A) ) [
Figure 5. Adsorption potential distributions. Hollow symbols correspond to J-Ch; the full symbols correspond to D-R.
n
(iB*i/β)(A/β)i-1] exp[-
B*i(A/β)i] ∑ i)1
(2)
Figure 5 contains the distributions found by eq 2, as well as the classical distributions of Dubinin. This figure shows that, contrary to the distribution curves of Dubinin, those obtained by eq 2 of Jaroniec are narrow, their maximums are very well defined, and they are shifted toward smaller values of A. The distribution curves do not reach zero because of B1* * 0, which is in concordance with eq 2, i.e. lim X(A) ) B1*β-1, when A ) 0 (P/P0 f 1). Taking into account the width as well as the position of the maximums of these distributions, the energetic heterogeneity of the microporous structure increases in the order C1 < C2 < CG. Comparison of the adsorption potential distributions (Figure 5) reveals that the application of eq 2 is more appropriate than the Dubinin’s classical equation. Micropore Size Distribution by the Preadsorption Method. After preadsorbing a certain amount of benzene (295 K), the adsorbent was cooled to 77 K, and the adsorption isotherm of N2 was measured at this temperature, up to relative pressures of 0.3-0.4. Each series of preadsorption of benzene-adsorption of N2 was preceded by a period of thermal activation (250 °C) of the adsorbent, until a vacuum of less than 6 × 10-4 Torr was reached. As expected, when preadsorption of benzene is increased, adsorption of N2 decreases. It was established that while the total micropore volume of C1 and C2 is not occupied by benzene at approximately one-third, i.e., for θ ) V/V0 < 0.33, adsorption of nitrogen is relatively fast. However, for degrees of coverage in the interval 0.33 < θ < 0.60, a drastic decrease was observed, and for θ > 0.60 a considerable increase of the adsorption speed of nitrogen was registered, which was nearly instantaneous for θ > 0.90. These results indicate that the more the thickness z of the benzene film is increased, considering that this film is formed on both walls of the slit-shaped micropore, the space in which
Figure 6. Benzene film thickness distributions.
the N2 molecules are supposed to spread out is ever more reduced. When adsorption begins to slow, i.e., when the blocking of the micropores begins, the specific surface area (S) of both carbons begins to fall abruptly. The tendency which was observed is that as the quantity of preadsorbed benzene increases, the description of the adsorption isotherms of N2 by the BET model is constantly improving, whereas the contrary can be observed for the Langmuir model; i.e., the isotherms are changing from type I to type II or IV. It was established that the function log S vs log a cannot be represented by a straight line, and therefore, according to the theory of Pfeifer et al. (1991), the surfaces of both carbons cannot be considered as fractal. This means that the thickness z of the film and the pore size distribution must be evaluated, respectively, by the following equations:
zi ) zi-1 + 0.5(Vi - Vi-1)(1/Si + 1/Si-1)
(3)
(dVpore/dz)i ) -zi(Si - Si-1)/(zi - zi-1)
(4)
The distribution curves of pore size or, in other words, of the thickness of the preadsorbed benzene film are shown in Figure 6. As expected, the maximum for C2 is found at higher z values than for C1. Besides, the first sample shows a wider distribution than the second, and we can therefore be assured that the microporous structure of C1 is more homogeneous than that of C2. Due to the fact that the pores of activated carbons are slit-shaped, their actual diameter (d) can be evaluated by d ) 2zmax + σcr, where σcr ) 4 Å is the critical diameter of the N2 molecule. Conclusions Although there is no total quantitative coincidence between the values of micropore size evaluated on the basis of the adsorption isotherms at very low relative
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1399
pressures, by means of the different approach methods used, it could be established that C1 is the sample with a narrower microporosity than C2 and CG, and that the three samples are basically microporous, with a rather marked energetic and structural heterogeneity. According to results we recommend the Jaroniec model for evaluation of the energetic heterogeneity and Pfeifer method for evaluation of the pore size distribution of activated carbons. The D-R and R methods lead to micropore volumes which are in very good agreement. Acknowledgment We thank the Consejo Nacional de Ciencia y Tecnologı´a (CONACYT) of Me´xico for the financial support (master scholarship, No. reg. 93792 and 93793). Literature Cited Dubinin, M. M.; Stoeckli, H. F. Homogeneous and Heterogeneous Micropore Structures in Carbonaceous Adsorbents. J. Colloid Interface Sci. 1980, 75, 34-42.
Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press, Inc.: New York, 1982. Jaroniec, M.; Gilpin, R. K.; Kaneko, K.; Choma, J. Evaluation of Energetic Heterogeneity and Microporosity of Activated Carbon Fibers on the Basis of gas Adsorption Isotherm. Langmuir 1991, 7, 2719-2722. Pfeifer, P.; Johnston, G. P.; Deshpande, R.; Smith, D. M.; Hurd, A. J. Structure Analysis of Porous Solids from Preadsorbed Films. Langmuir, 1991, 7, 2833-2843. Rodrı´guez-Reinoso, F.; Martı´n-Martı´nez, J. M.; Prado-Burguete, C.; McEnaney, B. A. Standard Adsorption Isotherm for the Characterization of Activated Carbons. J. Phys. Chem. 1987, 91, 515-516.
Received for review September 2, 1997 Revised manuscript received October 7, 1998 Accepted December 8, 1998
IE9706166
