Langmuir 1995,11, 247-252
247
Micropore Structure of Activated Carbons Prepared from a Spanish Subbituminous Coal Studied by COa, Benzene, and Cyclohexane Adsorption C. Moreno-Castilla," F. Carrasco-Marin, and M. V. L6pez-Ram6n Departamento de Quimica Inorganica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain Received June 21, 1994@ Two series of activated carbons with different degrees of activation were prepared from a Spanish subbituminous coal by pyrolysis and steam activation. The micropore structure of the activated carbons so obtained was characterized by COz adsorption at 273 K by applying the Dubinin-Astakhov equation and by benzene and cyclohexane adsorption a t 303 K by applying the Dubinin-Stoeckli equation corrected with the Dubinin-Zaverina equation. This last method has allowed discovery ofthe real micropore volume and its distribution and dispersion, as well as the mesopore surface area o f the activated carbons. Results obtained also show that the Dubinin-Astakhov equation applied to COz adsorption isotherms on activated carbons with a medium to high degree of activation is affected by the adsorption on the mesopore surface.
Introduction It has been shown1s2that Spanish low-rank coals are good precursors to prepared activated carbons either upon COz or steam activation ofthe coal chars. It is well-known t h a t activated carbons are porous adsorbents with pores ranging from micropores (12.0 nm width) then to mesopores (2-50 nm width) and finally to macropores (250 nm width). Many activated carbons are essentially microporous, and to characterize the micropore structure of these materials, several methods of analysis have been One of the methods widely used6 is to apply the Dubinin-Radushkevich equation to the COZ adsorption isotherms obtained either at 273 or 298 Kon the activated carbons. However, for activated carbons with a medium to high degree of activation (or percentage burn-off), the Dubinin-Astakhov equation7s8has been showng to best fit the COZ adsorption data. However in these cases, due to the well-developed mesopore surface, the results obtained from the Dubinin- Astakhov equation might be affected in part by the adsorption on mesopores. This work aims to characterize the micropore structure of two series of activated carbons by using the COz, benzene, and cyclohexane adsorption isotherms and applying different methods of analysis to the results obtained.
Table 1. Characteristics of the Activated Carbons Wo
Eo
sample BO
(em3 (kJ ash mol-1) mol-')
AP-2.5 AP-5 AP-10 CP-5 CP-10
13.6 16.7 24.7 1.7 2.2
%
16 30 48 40 55
%
0.20 0.24 0.35 0.41 0.55
22.6 19.8 15.2 17.0 13.9
sco*
n (m2g-l) 2.00 530 1.85 633 1.55 920 1.55 1081 1.38 1423
SNn
(mZg-l) 490 633 828 905 1114
activationtreatment as described in detail e l s e ~ h e r e Briefly, .~~~ the original and the demineralized (with HCl and HF) coals were pyrolyzed at 1123 K (10 Wmin) in a Nz flow for 1 h. Samples so obtained will be referred to in the text as AF'(from the original coal) and CP (from the demineralized coal). These pyrolyzed samples were activated in steam at 1113 K for differentperiods of time (2.5,5, and 10 h). The percentages of burn-off (BO) and ashes are shown in Table 1. All activated carbons were characterized by Nz and COz adsorptionat 77 and 273 K, respectively, as explained in detail e l s e ~ h e r eThe . ~ ~Dubinin-Astakhov ~ (DA)equation was applied to the COZadsorption data. The DA equation reads7,*
w = w,exp[ -(+-)n] where W is the amount adsorbed at a given relative pressure
Experimental Section Activated carbons used in this work were obtained from a Spanish subbituminous coal by a two-stagepyrolysis and steam @
Abstract published in Advance A C S Abstracts, December 1,
1994. (1) Muiioz-Guillena, M. J.; IllAdXmez, M. J.;Martin-Martinez,J. M.; Linares-Solano, A.; Salinas-Martinez de Lecea, C. Energy Fuels 1992, 6 , 9. (2) L6pez-Ram6n, M. V.; Moreno-Castilla, C.; Rivera-Utrilla, J.; Hidalgo-Alvarez,R. Carbon 1993, 31, 815-819. (3) Dubinin, M. M. In Progress in Surface and Membrane Science; Cadenhead, D. A., Ed.; Academic Press: New York, 1975; Vol. 9, p 1. (4)Gregg, S.J.;Sing,K. S. W.Adsorption,Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (5) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker, Inc.: New York, 1988. (6) Rodriguez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon;Thrower, P. A., Ed.; Marcel Dekker, Inc.: New York, 1989; Vol. 21, p 1. (7) Dubinin, M. M.; Astakhov, V. A. Adu. Chem. Ser. 1970,102,69. (8) Dubinin, M. M.; Stoeckli,F. J.Colloid Interface Sci. 1980,75,34. (9) Carrasco-Marin, F.; L6pez-Ram6n, M. V.; Moreno-Castilla, C. Langmuir 1993, 9, 2758.
A = RT ) ; ( n I €'/PO; W Ois the micropore volume; A is the differential molar work, defined by (2);p is the affinity coefficient, takengas 0.48 for COz at 273 K; and Eo is the characteristic adsorption energy The value ofnreflects the width ofthe energy distribution,which is related in a complicated way with pore-size distribution.6a8J0J1 Values of n between 1 and 4 are observed for most carbon adsorbents, with a value of n > 2 for molecular sieve carbons or carbon adsorbents with highly homogeneous, small microp o r e ~ , lwhereas ~ - ~ ~ values of n < 2 are found for stronglyactivated carbons and heterogeneous carbons. The values of WO, Eo, and (10) Stoeckli, F. Carbon 1981, 19, 325. (11) Stoeckli, F. Carbon 1990,28, 1. (12) Dubinin, M. M.; Astakhov, V. A. Izu. Akad. Nauk SSSR. Ser. Khim. 1971,1, 5. (13) Rand, B. J. Colloid Interface Sci. 1976, 56, 337. (14) Finger, G.; Bulow, M. Carbon 1979,17, 87. (15) Kraehenbuehl, F.; Stoeckli, F.; Addoun, A.; Ehrburger, P.; Donnet, J. B. Carbon 1986,24, 483.
0743-7463/95/2411-0247$09.00/00 1995 American Chemical Society
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248 Langmuir, Vol. 11, No. 1, 1995
5.00
1
I
3.00
4.00
2.50
n c
L
I
?
2.00
3.00
EE
E f
W
4
2.00
0'
1'
'*0° 0.00 0.60
v
1.50
0
1.00
Oe50 I
0.lo
0.60
1 '
0.00 0.dQ
1.60
0.40
0.60
AP-2.5
1
,
I
0.20
7.00
6.00
0.60
0.40
P/Po
0.60
1.60
0.40
1.00
p/po
1 {
P
4.00
I
CP10
I
r
3.00
:I 0.00 0.00
8
2.00
0
I
, 0.20
,
,
,
0.40
, 0.60
,
, 0.80
,
, 1.00
P/Po
Figure 1. Benzene adsorption-desorption activated carbons at 303 K.
1
'*0° 0.00 1 0.00
I
,
0.20
,
I
0.60
0.60 P/PO
isotherms on
Figure 2. Cyclohexane adsorption-desorption isotherms on activated carbons at 303 K.
n obtained from eq 1are given in Table 1 and show that n and Eo decrease when the degree of activation increases because this increases the heterogeneity of the micropores and they are widened. The C02 liquid density at 273 K (to obtain W Ovalue) was taken6 as 1.03 g/cm3. It is well-known4that in microporous activated carbons the term surface area does not have much physical meaning and it is better to refer to pore volume. However,despite its limitations in these cases, surfacearea data are widely used,6J6and they are termed as apparent surface area. Thus, in order to calculate the apparent surface area with COz, Sco2,the cross-sectional area of C02 at 273 K was taken6 as 0.187 nm2. The BET equation was applied to the Nz adsorption data at 77 K, and the apparent surface area with N2, SN~, was obtained by using the value OP0.162 nm2 as cross-sectional area for the NZmolecule at 77 K. All these data are compiled in Table 1and have been discussed in preceeding works.2~9 Benzene and cyclohexane adsorption was carried out at 303 Kin a gravimetric equipment as described else~here.~'J~ Prior to carrying out the adsorption runs, samples were outgassed overnight at 383 K with a dynamic vacuum of Torr. The
adsorption isotherms of both adsorptives were obtained on different portions of the same sample.
(16) Barrer, R. M. In Structure and Properties of Porous Materials; Everett, D. H., Stone, F. S., Eds.; Butterworthe: London, 1958. (17) Domingo-Garcia,M.; Fernhdez-Morales,I.; Mpez-Garz6n,F. J.;Moreno-Castilla,C. Langmuir 1991, 7 , 339. (18) Moreno-Castilla, C.; Carrasco-Marin, F.; Utrera-Hidalgo,E.; Rivera-Utrilla,J. Langmuir 1993, 9, 1378.
Results and Discussion Benzene and cyclohexane adsorption isotherms a t 303
K are depicted in Figures 1 and 2, respectively. The adsorption isotherms are closest to type I of the BDDT classification,4 in which the initial part of the adsorption branch of the isotherm represents micropore filling and the slope of the plateau at medium and high relative pressure is due t o multilayer adsorption on the nonmicroporous surface, i.e., in mesopores, in macropores, and on the external surface. The shape of the adsorption isotherms of benzene and cyclohexane on both series of activated carbons changes with the percentage BO. Thus, there is an opening of the knee and a n increase in the slope of the linear branch which indicates that there is progressive development of micro- and mesoporosity with the degree of activation.6 The adsorption-desorption isotherms of benzene and cyclohexane on all activated carbons, except those on M10 and CP-10,show a H4 type4 hysteresis loop, characteristic of slit-shaped pores; the adsorption and desorption branches are parallel. It is noteworthy that the less activated sample, AP-2.5, presents low-pressure hyster-
Micropore Structure of Activated Carbons
Langmuir, Vol. 11, No.1, 1995 249
esis; i.e., the hysteresis is maintained even at low relative (3): pressure. This phenomenon has been found in many microporous activated c a r b o n ~ ~ using J ~ - ~different ~ hyWO drocarbons as adsorptives. The low-pressure hysteresis (LPH)has been attributed to the irreversible intercalation of the adsorptive in pores to which it is not normally accessible due to more or less elastic distortions or deformations of the porous texture produced during the adsorption process. Such deformations will result in a n opening up of spaces previously inaccessible to the Where W and W Oare the adsorption values and are adsorptive molecules, in which they remain during the expressed in mmol/g or adsorbate volumes in cm3/g, as desorption process unless the outgassing temperature is necessary. A is defined by eq 2, LOis the micropore width, increased. This LPH will be more pronounced the m = (l/pk)z,k = L a d 2 , and p is the affinity cofficient, narrower the pore and the larger the molecular volume constant for a given vapor. of the adsorptive. Therefore, in LPH existence the However, when the activated carbon has a sufficiently micropore texture of the activated carbon and the modeveloped mesopore surface, the contribution of the lecular size ofthe adsorptive play an important role. Thus, adsorption on mesopores to the total adsorption is the LPH loop is greater in size in the case of cyclohexane considerable. In this case, the application of the DS desorption from AP-2.5 (Figure 2) than in the case of equation to the experimental vapor adsorption isotherm benzene desorption from the same sample (Figure 1).This yields the effective values of the parameters WO, LO,and is due to the different molecular size of both adsorptives. 6 . These effective parametersz6should be used to describe Thus, the minimum size for benzene is 0.37 nm and for the adsorption isotherms quantitatively, but cannot be cyclohexane 0.48 nm.18 used for the calculation of micropore volume distribution The increase in the degree of activation for sample Ap-5 with size. The experimental isotherm must, in this case, (with 30% BO) removes LPH for benzene (Figure 11, and be corrected for the adsorption on mesopores. Thus, the desorption branch of the isotherm presents a closing according to Dubinin,z6the experimental values of the point a t a relative pressure of approximately 0.25. This adsorption isotherms, aexp, are composed of the adsorption phenomenom was also found by McEnaneyzOwith cellulose in micropores, ami, and in mesopores, am,: carbons activated with steam in which, for BOs larger than 25%,the LPH dissapeared. However, LPH remains for cyclohexane (Figure 2) in the case of sample AP-5, which is again due to the greatest minimum size of Eq 4 can be converted to eq 5, cyclohexane. Finally, activated carbon CP-5, with a 40% BO, presents (5) a e x p = a m i + YSme high-pressure hysteresis for both benzene and cyclohexane, and samples AP-10 and CP-10, with 48 and 55%BO, where y is the adsorption of the vapor a t a given respectively, do not present a hysteresis loop, which temperature per unit mesoporous surface area of the indicates opening of the porosity when the degree of adsorbent and S,, is the surface area of the mesopores. activation of the samples increases. For carbonaceous adsorbents the value of y was The theory of volume filling of micropores ~TVFM)is experimentally obtained by Dubinin and Zaverinaz6*30-3z accepted worldwide to describe the porous structure of from the adsorption isotherm of benzene on a nonporous microporous activated Using TVFM, carbon black heat treated at 1223K, the results obtained three parameters of the activated carbon micropore were further extended for other organic adsorptives, and structure can be obtained: the micropore volume, WO; the this more general equation is known a s the modified characteristic adsorption energy, EO,or the micropore Dubinin-Zaverina equation (6): width at the distribution curve maximum, LO;and the dispersion,6, that characterizes the microporedistribution range. These three parameters can be determined from the experimental adsorption isotherms of different vapors by applying the Dubinin-Stoeckli (DS) e q u a t i ~ n ~ - ~ , ~ ~ - ~ O where, v is the millimolar volume of the adsorptive and (19)Bailey, A.;Cadenhead, D. A.; Davies, D.; Everett, D. H.; Miles, was taken as 0.0898 and 0.1094 for benzene and cycloA. Trans. Faraday SOC.1971,67,231. (20)McEnaney, B. Trans. Faraday SOC.1974,70,84. hexane,l8 respectively, and 6.35 is the characteristic (21)h e l l , J. C.; McDermott, H. L. Proc. Second Int. Cong. Surface adsorption energy, in kJ/mol, of a mesoporous surface. Activity, Butterworths: London, 1957;Vol. 11, p 113. According to the authors,z6,z8 this equation is valid in the (22)Martin-Martinez, J.M.; Linares-Solano, A.;Rodriguez-Reinoso, equilibrium relative pressure range between 1x and F.; L6pez-Gonzalez, J. D. Ads. Sci. Technol. 1984,1, 195. (23)Garrido, J.;Martin-Martfnez, J.M.; Molina-Sabio,M.; Rodriguez0.3. By using eqs 5 and 6 the plot of a,,!, against y should Reinoso, F.; Torregrosa, R. Carbon 1986,24,469. be a straight line the slope of which will be the value of (24)Dubinin, M. M. Dokl. Akad. Nauk SSSR 1984,275,1442. the mesopore surface area, Sme.Thus, Figure 3 shows, as (25)Kadlec, 0.In Characterization ofPorous Solids; Gregg, S. G., Chem. Industry: London, 1979; Sing, K. S. W., Stoeckli, F., Eds.; SOC. a n example, the application of the above equations to the p 13. cyclohexane adsorption isotherm obtained on activated (26)Dubinin, M. M. Carbon 1986,23,373. carbon CP-10, similar plots were obtained for other (27)Dubinin, M. M. Carbon 1989,27,457. activated carbons and adsorptives. (28)Dubinin, M. M.; Polyakov, N. S.;Kataeva, L. I. Carbon 1991,29, 481. Once S,, is known, the experimental adsorption iso(29)Stoeckli, F.;Huguenin, D.; Greppi, A. J. Chem. SOC. Faraday therms were corrected a t each adsorption point for Trans. 1993,89,2055. adsorption on mesopores acording to eqs 5 and 6 and the (30)Polyakov, N. S.;Dubinin, M. M.; Kataeva, L. I.; Petuhova, G. A. Pure Appl. Chem. 1993,65, 2189. DS equation (3) was applied to the corrected adsorption (31)Dubinin, M. M.;Izotova, T. M.; Kadlec, 0.;Krainova, 0. L. Izu. isotherms so obtained. This will yield the real parameters AN SSSR, Ser. Khim. 1976,1232. WO, LO,and 6 of the micropore structure of the activated (32)Dubinin, M. M.; Degtyarev, M. V.; Nikolaev, K. M.; Poyakov, N. carbons. S . Izu. Akad. Nauk SSSR, Ser. Khim. 1989,7, 1463.
Moreno-Castilla et al.
250 Langmuir, Vol.11, No. 1, 1995 0.4
AP-5
AP- 10
0.3
c
a, 3
5
" 0.2 0.1
0 0
0.5
1
1.5
2
2.5
1.5
2
2.5
L (nm)
0.5
Figure 3. Application of eqs 5 and 6 t o the cyclohexane
0.4
adsorption isotherm on activated carbon (CP-10).
Table 2. Parameters Obtained from the Dubinin- Stoeckli Equation Applied to the Benzene Adsorption Isotherms effective parameters
WO
LO
6 sample (cm3 g-l) (nm) (nm) AP-2.5
AP-5 AP-10 CP-5 CP-10
0.17 0.25 0.36 0.33 0.45
1.20 1.36 1.77 1.32 1.55
0.144 0.145 0.242 0.118 0.160
real parameters Sme
wo
(m2g-l) (cm3g-l)
58 94 183 124 208
0.13 0.20 0.23 0.26 0.31
Lo 6 (nm) (nm) 0.71 1.14 1.37 1.11 1.34
0.107 0.115 0.136 0.116 0.139
Table 3. Parameters Obtained from the Dubinin-Stoeckli Equation Applied to the Cyclohexane Adsorption Isotherms
effective parameters WO LO 8 sample (cm3g1) (nm) AP-2.5 0.12 1.19 0.23 1.32 AP-5 AP-10 0.27 1.49 CP-5 0.31 1.38 CP-10 0.41 1.54
real parameters
wo
Lo 6 (nm) (m2g-l) (cm3g-l) (nm) (nm) 0.201 55 0.09 0.92 0.112 0.211 96 0.19 1.18 0.111 0.231 194 0.21 1.37 0.146 0.235 117 0.24 1.20 0.139 0.252 217 0.29 1.34 0.140 Sme
The effective and real parameter values obtained from the DS equation applied to the uncorrected and corrected benzene and cyclohexane adsorption isotherms are compiled in Tables 2 and 3, in which the S,, values are also included. In order to calculate the parameters of the DS equation applied to the adsorption isotherms, it was necessary to use a computer program with an interative method, which uses an approach based on minimizing the residual sum of square^.^,^^ Finally, the micropore size distribution was obtained from the real parameters WO,LO,and 6 by applyingz6Sz8 the normal distribution equation (7):
Micropore size distribution of activated carbons from AP and CP series obtained with both benzene and cyclohexane are depicted in Figures 4 and 5,respectively. Data from Tables 2 and 3 show that the values of the real parameters are lower than those obtained for effective parameters, which is a consequence of the increase in
0.1
0
0
0.5
1
L (nm) Figure 4. Micropore size distribution obtained from benzene adsorption data.
mesopore area with the percentage BO. For both series of activated carbons, AP and CP, activation increases the micropore volume, WO, the mean micropore width, LO, and the dispersion of the micropore size distribution, 6 , which can also be seen from Figures 4 and 5 . The mesoporous surface area, S,,, linearly increases with the degree of activation or percentage BO given to the activated carbon, as seen in Figure 6, and furthermore, for a given activated carbon, the S,, values obtained from benzene and cyclohexane adsorption are coincident, which indicates that both adsorptives, as expected, measure the same type of mesopores. These results indicate that steam activation of the activated carbons up to approximately 50% BO produced a widening of the microporosity and a n important development of the mesoporosity, which is in agreement with that found by other authors6 with activated carbons obtained from different precursors and steam activated. If W Ovalues (real parameter) for both benzene and cyclohexane are compared, it is seen that in the case of sample AP-2.5 (with 16% BO) the value obtained for benzene is somewhat greater than that obtained for cyclohexane; this indicates that this activated carbon presents a certain molecular sieve effect for this last molecule due to its larger minimum molecular dimension. For higher percentage BO, this molecular sieve effect is lost as a consequence of the microporosity widening, resulting in quite similar WOvalues for both benzene and cyclohexane. The DA equation (1) with the appropriate n value has been to fit the experimental COz adsorption data at both 273 and 298 K on activated carbons better than the Dubinin-Raduskevich equation (n = 2 in eq 1)
Langmuir, Vol. 11, No. 1, 1995 251
Micropore Structure of Activated Carbons
Table 4. W Oand S , . Values Obtained from the Dubinin-Stoeckli Equation Applied to the Second Adsorption Isotherms of Benzene and Cyclohexane benzene cyclohexane
1
0*4
g 2
0.1
0 0
0.5
1.5
1
2
2.5
2
2.5
L (nm)
OS5
1
4
A cp-io
0.4
d 0.2
B
0.1
0
0
0.5
1.5
1 L (nm)
Figure 5. Micropore size distribution obtained from cyclo-
hexane adsorption data. 250
200
-,
.
1
CP-10
/
wo
sample
(cm3 g-1)
Sme (m2 g-1)
AP-2.5 AP-5 AP-10 CP-5 CP-10
0.09 0.17 0.23 0.20 0.29
66 100 183 121 218
wo
(cm3 g-1)
Sme (m2 9-1)
0.07 0.16 0.20 0.22
65 93 205 124
(Table 1) with those obtained from either benzene or cyclohexane adsorption (Tables 2 and 3, respectively) by applying the DS equation (3). Thus, WO from C02 adsorption is similar or higher than the effectiveparameter WO from benzene or cyclohexane, and in general, the greater the difference, the higher the degree of activation ofthe activated carbon. In the case ofAP-2.5 the difference found between W O((202and ) the effective parameter W O (benzene) might be due to the differences in molecular size of COz and benzene. Thus, C02 might penetrate pores with smaller dimensions than benzene. After the first benzene and cyclohexane adsorption isotherm was completed, the samples were outgassed at 383 K overnight under a dynamic vacuum of loT6Torr and a second adsorption isotherm a t 303 K was obtained. This new adsorptionisotherm was corrected for adsorption on mesopores and the DS equation (3)was applied to the corrected adsorption isotherms, with the new real parameter values WO, LO,and d so calculated, the micropore size distribution was again obtained by applying equation 7. Results obtained are given in Figures 7 and 8 for benzene and cyclohexane,respectively, and Table 4 shows the real parameter W Oobtained from the corrected benzene and cyclohexane second adsorption isotherms and S,,,,. These results clearly show that, with the exception of the most activated samples from both series, AP-10and CP10, some of the adsorptive molecules remain in the micropores making the W Ovalue obtained from the second adsorption isotherm lower than the WO value obtained from the first adsorption isotherm. It should be noted however that the outgassing method followed to remove benzene and cyclohexane from the samples (commonly used with activated carbons) was not enough in the case of the activated carbons with low and medium degrees of activation. Finally, S,, values obtained were, in general, fairly close to those obtained from the first adsorption isotherms.
Conclusions
0
lb
2b
1
J'O
40
,
1
50
1
1
60
I BO
Figure 6. Relationship between the mesoporous surface area of the activated carbons and their percentage burn-off.
especially on those with medium to high percentage activation. In these cases, the SCOz values (obtained from WOby the DA equation) are equal or higher than SNZ (Table 11, which indicates that the value of W Oobtained from eq 1 not only gives the ultra- and supermicropore v ~ l u m ebut ~ , ~is ~also affected by the adsorption on the mesopore surface. An indication of this is given when comparing the WOvalues obtained from COz adsorption (33) Salas-Peregn'n,M. A,; Carrasco-Marin, F.; L6pez-Garz6n, F. J.; Moreno-Castilla, C. Energy Fuels 1994, 8, 239.
This work clearly shows that the WOvalues obtained from the Dubinin-Astakhov equation applied to COZ adsorption isotherms at 273 K on activated carbons with medium to high degree of activation are affected by the adsorptionon the mesopore surface, because in these cases there is an increase in this surface when the degree of activation is increased. The real micropore volume and its distribution and dispersion, as well as the mesopore surface area of the activated carbons, were obtained from both benzene and cyclohexane adsorption by applyingthe Dubinin-Stoeckli equation corrected with the DubininZaverina equation. Benzene and cyclohexane adsorption-desorption isotherms obtained on the less activated carbon present lowpressure hysteresis. This was more marked in the case of cyclohexane due to its greater minimum dimension. This phenomenon disappeared when the degree of activation was increased.
Moreno-Castilla et al.
252 Langmuir, Vol. 11,No. 1, 1995
0.3
z
AP-10
AP-5
AP-2.5 1st
0.2
"
s
0 0
0.5
0
2
1.5
1
0.5
0
2
1.5
1
0.5
0.5
2.5
0.5 1
1st D
0
2
CP-10
CP-5
0.5 1
1.5
1
L (nm)
L (nm)
L (nm)
1
1.5
0.3
0
2
1
0.5
1.5
2
2.5
L (nm)
L (nm)
Figure 7. Micropore size distribution obtained from the first and second adsorption isotherm of benzene. AP-2.5
AP-5
AP-10
CP-5
0.3
p 0.2 2 nd
0
0.5
1
L (nm)
1.5
2
0
0.5
1
2nd
s d 0.1
2nd
1.5
2
0.1
0
0.5
L (nm)
1
1.5
2
2.5
0
0.5
L (nm)
1
1.5
2
2.5
L (nm)
Figure 8. Micropore size distribution obtained from the first and second adsorption isotherm of cyclohexane.
Finally, the method of outgassing in which the samples were heated overnight a t 383 K and under a dynamic vacuum of Torr after the first adsorption isotherm was completed was not sufficient to remove all the benzene or cyclohexane molecules from the activated carbons with low and medium degrees of activation.
Acknowledgment. We thank CICYT (Project AMB921032),DGICYT(ProjectCE91-0011),CECA(Project 7220EC/758), and OCICARBON (Project C-23,275) for the financial support of this study. LA9404999
