543
Langmuir 1995,11, 543-546
Heterogeneity of Synthetic Active Carbons? A. M. Puziy Institute for Sorption and Problems of Endoecology, Ukrainian Academy of Sciences, Palladin prospect 32 134, 252142 Kiev, Ukraine Received September 8, 1992. In Final Form: October 18, 1994@
This work is aims to reveal the heterogeneity of synthetic active carbons SCS at various extents of activation. For this purpose a comparison of various methods for analyzing the benzene adsorption isotherms (as, Dubinin-Astakhov, Dubinin-Stoeckli, Jaroniec-Choma) is carried out. It has been shown that micropores of carbons SCS are uniform. For description of adsorption data and energy distribution the Dubinin-Astakhov equation with exponent n close to or greater than 3 is most suitable. During the progress of activationthe total heterogeneity of SCS carbons changes from a broad to a one peak distribution function.
Introduction Synthetic active carbons are a new type of carbon adsorbents prepared from porous copolymers and spherical granulated resins by pyrolysis and subsequent activati0n.l This kind of active carbons due to improved properties including high mechanical strength and well-developed porosity has doubtless advantages in various sorption processes. The porous structure of synthetic active carbons consists of micropores and mesopores with different dimensions and low macropore volume. As any kind of carbonaceous adsorbent, synthetic active carbons have energetic heterogeneity which may be connected with nonuniformity of porous structure (structural heterogeneity) and inhomogeneity of chemical composition (“chemical” heterogeneity). The quantitative characteristic of adsorbent heterogeneity is a distribution of the adsorption potential. This distribution function can be determined from the experimental adsorption isotherm. Because the heterogeneity of active carbons plays an important role in the adsorption process and because there is no universal method for their characterization, a comparison of selected adsorption methods is made here by applying them to benzene adsorption isotherms measured for six synthetic active carbons SCS.
of pores (in micro- and mesopores)
where A = RT ln(P1PJ is the adsorption potential at temperature T and relative pressure PIP,. The adsorption potential distribution J(A) may be evaluated in terms of condensation approximation.2 Following this approach, the distribution of total adsorption potential will be the sum of distributions in micro- and mesopores: da J(A)= - =
dA
-(=+ dami
By using a specific form of adsorption equation for the different kinds of pores one can obtain, with subsequent differentiation, the equation of total adsorption potential distribution. For heterogeneous microporous solids several equations have been proposed. According to Dubinin’s theory for the volume filling of micropores the adsorption of gases and vapors may be described by the general equation of Dubinin-Astakh~v:~
(3)
Theoretical Section Physical adsorption on solid adsorbents having micro-, meso-, and macropores is a very complex process that cannot be described by any single current theory. Micropores are filled at low relative pressures by a volumefilling process. Due to enhanced adsorbent-adsorbate interactions in very narrow pores, the amount adsorbed in micropores gives a main contribution to the total adsorbed amount. At moderate relative pressures the micropore volume filling process is accompanied by layerby-layer adsorption on mesopore surface followed by capillary condensation at high relative pressures. The amount adsorbed on the macropore surface usually is negligible in comparison to that occurring in micropores and mesopores. The total adsorption on solid adsorbent may be expressed as a sum of adsorption in separate kinds + Presented at the International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, Kazimierz Dolny, Poland, July 1992. Abstract published in Advance ACS Abstracts, December 15, 1994. (1)Kartel, N. T.; Puziy, A. M.; Strelko, V. V. Characterization of Porous Solids; Elsevier Sci. Publ.: Amsterdam, 1991; p 439. @
O143-1463l95l24ll-O543$09.OOlO
dame
where a$ represents maximum amount adsorbed in micropores, which is proportional to micropore volume, p the affinity coefficient, E , the characteristic energy of adsorption, and n an exponent. This equation is suitable for describing adsorption on both homogeneous and heterogeneous microporous adsorbents. The degree of heterogeneity may be estimated by the value of exponent n. In case of active carbons, n varies from 3 to 1.5, as the microporous system becomes more heterogeneous.*g5 Homogeneous adsorbents tend to follow the DubininAstakhov equation with values of n close to 3.4,7This equation gives the following equation for adsorption (2) Rudzinski, W.; Everett, D. H.Adsorption of Gases on Heterogeneous Surfaces; Academic Press: New York, 1992. (3) Dubinin, M. M. In Progress in Surface and Membrane Science; Cadenhead, D. A,, Ed.; Academic Press: New York, 1975; Vol. 2, pp 1-70. (4) Dubinin, M. M.; Stoeckli, H. F. J.Colloid Interface Sei. 1980,75, 34. (5) Finger, G.; Bulow, M. Carbon 1979, 17, 8 7 . (6) Stoeckli, H.F. Carbon 1981, 19, 325. (7) Kraehenbuel, F.; Stoeckli, H. F.; Addoun, A.; Ehrburger, P.; Donnet, J. B. Carbon 1986,24, 483.
0 1995 American Chemical Society
544 Langmuir, Vol. 11, No. 2, 1995
Puziy
potential distribution in micropores:
It has been shown6that exponent n is related to the width of the distribution curve of the adsorption potential. It appears that the curve becomes sharper as n increases from 1.5 to 3 which implies an increase in the homogeneity of the micropore system. Characteristic energy of adsorptionE, providesthe basis for calculation of micropore size. In the simplest case the average pore size is inversely related to energy of adsorption:*
2 "
0.0
0.2
0.4
0.6
0.6
E o = -kv
(5)
*O
Thus, the Dubinin-Astakhov equation (3) allows the determination of the distribution of adsorption potential from eq 4 which is characteristic of energetic heterogeneity of the adsorbent. Also, the average size of micropores may be calculated on the base of eq 5. A second equation for heterogeneous micropores, associated with Gaussian distribution of micropore size, is the Dubinin-Stoeckli equation8
Figure 1. Experimental isotherms adsorption of benzene on synthetic active carbons SCS a t 298 K: open symbols, adsorption; closed symbols, desorption.
distribution:
Which of the adsorption potential distribution equations is suited best to experimental data will be analyzed later. For adsorption on a mesoporous carbon adsorbent at low relative pressures (0-0.3P/PO)Dubinin proposed equation8
ame= yOSmeexp - -
(
where m = l/(/3k)z, X, is the average dimension of micropores, and 0 denotes dispersion. The degree of heterogeneity may be estimated by the value of dispersion: the greater dispersion the more heterogeneous system. For homogeneous systems IT is close or equal to zero. This equation (6) gives the following form of adsorption potential distribution:
mx292
-
+
1 2mc?A2][
c?+
1
exp[ - 20%
+ 2mc?A2
mi
(10)
where yo = 9.16 x mmol/m2 is adsorption on square , - surface of mesopores. The corunit of surface, ,S responding equation of adsorption potential distribution in mesopores of carbon adsorbent is as follows: dame -YoSme --exp - -
dA
6.35
i
A51
(11)
Experimental Section
I}
+ 2mc?A2)
(7)
482)
a .=a'.(
6 3
The total adsorption potential distribution in solid adsorbent having micro- and mesopores is derivable by combining the proper equation representing adsorption potential distribution in micropores (eqs 4, 7, or 9)) with those in mesopores (eq 11).
Jaroniec and Choma for the same purpose have proposed a very simple equation associated with y distrib~tion:~
mi
1.0
P/P,
m+l
qP2 + A2
This equation gives the following adsorption potential ( 8 ) Dubinin, M. M. Carbon 1985,23,373.
(9)Jaroniec, M.; Choma, J . Muter. Chem. Phys. 1987,18,103
Synthetic active carbons were prepared by carbonization, in argon atmosphere up to 900 "C, and subsequent activation, in a flow of steam a t 800 "C, for a periods of treatment of 0.5,1,2, 3 and 4 h, of porous copolymers of styrene-divinylbenzene. Textural characteristics of these carbons have been published elsewhere.' The adsorption isotherms of benzene were determined at 25 "C by using a conventional gravimetric adsorption system consisting of quartz springs with a constant of approximately 0.15-0.30 m d m g ; the spring extention was measured with a cathetometer to a precision of 0.01 mm. All samples were outgassed overnight under high vacuum at 400 "C.
Results and Discussion The porous structures of carbons under investigation were estimated from the adsorption isotherms of benzene. Experimental isotherms are presented in Figure 1. The surface of mesopores was calculated by the a,-method using a Carbon black-950 as reference material. Although
Langmuir, Vol. 11, No. 2, 1995 545
Heterogeneity of Synthetic Active Carbons Table 1. Adsorption Parameters of Synthetic Active Carbons SCS Calculated According to the %Method adsorbent
Sme,
scsc scs-0.5
&, mmoVg
m2/g
67.0 71.1 89.9 133.0 256.4 321.6
scs-1 scs-2 scs-3 scs-4
0.851 1.38 2.04 3.46 4.94 6.75
m 6
2 -
EE
Table 2. Adsorption Parameters of Synthetic Active Carbons SCS Calculated According to the Dubinin-Astakhov Equation (3) adsorbent
aLi,mmoVg
E,, kJ/mol
n
scsc scs-0.5
0.85 1.39 2.02 3.47 4.93 6.74
19.58 19.56 18.64 15.63 13.13 11.57
2.91 3.35 3.45 4.13 3.74 3.05
scs-1 scs-2 scs-3 scs-4
E, % X,,nm -
0.613 0.614 0.644 0.767 0.914 1.037
1.07 0.55 0.43 0.18 0.98 1.69
Table 3. Adsorption Parameters of Synthetic Active Carbons SCS Calculated According to the Dubinin-Stoeckli Equation (6) adsorbent
scsc scs-0.5
scs-1 scs-2 scs-3 scs-4
a t i , mmoVg
0.851 1.38 2.04 3.46 4.94 6.75
X,,nm 0.510 0.465 0.494 0.617 0.881 1.079
u, nm
0.003 0.003 0.003 0.003 0.003 0.003
E,
%
1.56 1.59 1.79 3.50 6.50 6.53
0 0.0
0.2
0.1
0.3
p/p,
Figure 2. Fit of adsorption equations of Dubinin-Astakhov (31,Dubinin-Stoeckli (61,and Jaroniec-Choma (8) to corrected experimental isotherm adsorption ofbenzene on synthetic active carbon SCS-4. P/P,
0.8
-
0.3 0.1
0.01
0.001
I
I
1
0.0001 1
0.00001 I
Table 4. Adsorption Parameters of Synthetic Active Carbons SCS Calculated According to the Jaroniec-Choma Equation (8) adsorbent
scsc scs-0.5
scs-1 scs-2 scs-3 scs-4
uLi, mmoVg
q, (kJ/mol)2
m
0.87 1.42 2.08 3.67 5.45 7.58
3493 4218 3724 2359 1134 753
5.45 5.43 5.43 5.43 5.48 5.65
E,
%
1.61 1.62 1.83 3.61 6.97 7.55 5
0
the adsorption at low relative pressures mainly occurs in micropores, for calculation of microporous structure parameters using adsorption equations (31, (61,and (8) the experimental isotherms were corrected for adsorption on mesopore surface using following equation
where ame(A)was calculated from eq 10 using mesopore surface area listed in Table 1. The correction was performed in the range O-O.3PIP0 where eq 10 remains valid. The approximation of corrected adsorption isotherms was made for different adsorption equations. The data obtained are summarized in Tables 1-4. It worthy ofnote that the porosity of synthetic active carbons steadily develops during activation (note increasing of aLi). For all carbons exponent n of Dubinin-Astakhov equation is close to or greater than 3 (Table 2), indicating that micropores of synthetic active carbons SCS tend toward uniform. In support of this conclusion the DubininStoeckli equation ( 6 ) also states that micropores of synthetic active carbons SCS are uniform ( a= 0). For all carbons the dispersion (T is actually equal to zero. In this case the distribution of micropores is a Dirac &function and the equation ( 6 )reduces to the core equation, i.e., to the original Dubinin-Radushkevich equation. It is necessary to stress that Jaroniec-Choma equation (8)is also based on the Dubinin-Radushkevich equation.
10
15
20
25
30
A. kJ/mole
0.3
0.1
0.01
0.001
0.0001
0.00001
Figure 3. Adsorption potential distribution in micropores of synthetic active carbon SCS-4 calculated from experimental data (symbols) and from eqs 4 (solid line), 7 (dashed line), and 9 (dash-dot line).
Figure 2 demonstrates the applicability of various adsorption equations for the description of isotherm of benzene. From this figure it will be obvious that the Dubinin-Stoeckli and Jaroniec-Choma equations produce a similar form of adsorption isotherm and both have deviations from experimental data. In contrast, for the Dubinin-Astakhov equation, no departures are observed. Figure 3 illustrates these differences dramatically by comparing experimental and calculated energy distributions in synthetic active carbon SCS-4. Symbols are experimental points obtained by differentiating the adsorption data plotted as function of A. The DubininStoeckli and Jaroniec-Choma equations give analogous distributions, but both are in bad agreement with experimental data. It comes as no surprise that these equations are of limited utility for the description of peculiarities of the microporous system in a variety of active carbons, because both are derived from the generalized adsorption isotherm with the classical DubininRadushkevich equation as a core with an exponent equal 2. That is why both equations designed for adsorption on heterogeneousmicroporous solids (eqs 6 and 8)give worse
546 Langmuir, Vol. 11, No. 2, 1995 approximations than that of eq 3 (compare accuracy of approximation E ) , whereas from Table 2 it follows that approximation by Dubunin-Astakhov equation gives an exponent exceeding 3 for the majority of synthetic carbons. The assumption is made that the higher the exponent of Dubinin-Astakhov equation the more homogeneous adsorbent the homogeneity of synthetic active carbons increases in the early stages of activation up to carbon SCS-2 (increasing of exponent to value 4.13)and then decreases. For all cases extent of homogeneity is outside the capabilities of eq 6 and 8 based on the DubininRaduskevich equation. Figure 4 shows the total adsorption potential distribution in synthetic carbons SCS calculated from eq 2 and reflects the total energetic heterogeneity caused by both microporous and mesoporous systems. Nonactivated carbon SCS, shows a broad energy distribution. During the progress of the activation one peak distribution function becomes visible. The maximum value of this function steadily rises as the burn-off increases, indicating somewhat of an increase in total homogeneity of synthetic carbons SCS during activation. It is apparent that this fact takes place due to the enhancement of the role of microporosity as burn-off increases. The position of the maximum of total adsorption potential distribution shiRs to lesser values of A during activation. In conclusion, the following should be pointed out in the case of this set of synthetic active carbons SCS: (i) micropores of carbons SCS are uniform; (ii)for description of adsorption data and energy distribution the DubininAstakhov equation with exponent n close or greater than
Puziy
0.8 c 7 I
0
0.6
f0
-
i E 0.4
0.2
0.0
0
5
10
15
20
25
30
A, kJ/molr
Figure 4. Total adsorption potential distributionon synthetic active carbons SCScalculated from experimental data (symbols) and from eq 2 using eqs 4 and 11.
3is most suitable; and (iii)during the progress of activation the total heterogeneity of SCS carbons changes from a broad to a one peak distribution function. LA920499D
