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Thermodynamic study of high-pressure adsorption of methane on activated carbons: The effect of oxidation on pore structure and adsorption energy heterogeneity
$5IH)+ 04 OtMX-6223192 Copyright 0 1992Pergamon Press Ltd
CarbonVol. 30.No. 3, pp. 507-512,1992 Printedin GreatBritain.
THERMODYNAMIC STUDY OF HIGH-PRESSURE ADSORPTION OF METHANE ON ACTIVATED CARBONS: THE EFFECT OF OXIDATION ON PORE STRUCTURE AND ADSORPTION ENERGY HETEROGENEITY JACEK JAGIEuOt,
PRASHANT
and Department
SANGHANI,
JAMES A.
TERESA
J. BANDOSZ,
SCHWARZ*
of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244, U.S.A. (Received
15 July 1991; accepted in revised form 15 October 1991)
Abstract-The effects of oxidation of activated carbons are studied by means of high-pressure methane adsorption. Decreases in amounts adsorbed and adsorption enthalpies after oxidation are observed. Thermodynamic analysis of adsorption isotherms using the virial-type thermal equation of adsorption is presented. The calculated adsorption energy distributions are shifted to lower adsorption energies for oxidized carbons. The mean adsorption energies decrease, indicating that oxidized carbon surfaces have lower affinity for methane adsorption. Key Words-High bution.
pressure adsorption,
activated carbons, microstructure,
1. INTRODUCTION
The term activated carbon is given to carbonaceous materials having a large adsorption capacity. Activated carbons can be manufactured from many different materials, such as coconut shells, wood, peat, and coal. One might expect that the carbon’s performance as an adsorbent in a particular application will depend on its source. Indeed, this is found to be the case. The demands placed on activated carbon-based technologies have outpaced fundamental studies of the interplay between the carbon sources, the surface chemical properties of carbons, the accessible internal adsorption areas, and their performance as adsorbents. Unfortunately, preconceived notions have been established that attempt a priori to provide “prescriptions” for carbon selection in a technological application. The adsorption characteristics of activated carbon are affected by the type of carbon, that is, governed by the source of raw material, and the preparation procedures used during carbonization and activation. Any assessment of the adsorptive behavior of activated carbons based only on surface area is incomplete; carbons with equal density and equal surface area can exhibit different adsorptive characteristics when prepared by different methods. Some adsorptive properties can be explained by differences in pore size distribution. Surface chemical functionalities, derived
*Author to whom correspondence should be addressed. tpermanent address: Institute of Energochemistry of Coal and Physicochemistry of Sorbents, University of Mining and Metallurgy, 30-059 Krakow, Poland.
adsorption energy distri-
from activation or treatment procedures, also influence adsorption, but their role depends on their accessibility, which in turn depends on the carbon’s microporosity. The dichotomy described above is due, in part, to the chemical-physical properties of activated carbons. Microporous carbons are disordered solids in which the fundamental structural unit is graphitelike monocrystallites with cross-linked aliphatic bridging groups, incorporating such heteroatoms as H, 0, N, and S. The existence of functional groups such as carboxyls, phenols, lactones, aldehydes, quinones. hydroquinones, and ethereal structures have been postulated[l-41. The chemistry of a carbon surface has a profound influence on its adsorption properties. The nature of the activated carbon surface is also very complex from a physical[5,6] point of view. The porous structure and specific surface area[7,8] have great influence on the adsorption properties. In fact, the specific surface area and pore size distribution are proposed as fundamental quantities recommended by the International Union of Pure and Applied Chemistry (IUPAC)[9] for characterizing these adsorbents. They are usually determined from sorption isotherms of different gases[7,8]. In addition, surface chemical characteristics can also be obtained from investigations of the interactions between the solid surface and gas molecule probes. When only physical interactions occur, the study of adsorption phenomena allows a quantitative description of the energetic properties of the solid surface, especially the evaluation of surface energy heterogeneity, which can be connected with the microstructural het507
508
J. JAGIEEEOet al.
erogeneity, as well as with the chemical nature of the surface[lO]. In our previous work[ll] we have shown by using inverse gas chromatography the existence of a correlation between surface chemical properties of carbons and the energy of specific interactions with n bonds of unsaturated hydrocarbons. The dispersive component of surface energy, $, and the enthalpies of alkane adsorption decreased after oxidative pretreatment of carbons, which indicated that the oxidized carbons have less affinity for hydrocarbons than untreated carbon surfaces. In this paper we propose to separate the effects of microstructural heterogeneity of activated carbons and the chemical nature of their surfaces. The basis for our analysis is the hypothesis that the suppression in adsorption capacity for methane on oxidized carbons compared to their as-received counterpart is due either to surface chemical effects, microstructural effects, or combination of both. Our methodology, which is quite general and can be applied judiciously to other adsorption systems, is thermodynamic in nature and relies on a detailed examination of such quantities as adsorption isotherms, isosteric heats of adsorption, and adsorption energy heterogeneity for methane on several activated carbons.
2. MATERIALS
AND METHODS
Three activated carbons from different sources (Westvaco, Norit, North American) were oxidized with 15N (73%) HNOs solution at 298K for 2.5 h according to the method described in our previous work[ll], in which the physico-chemical characteristics of these carbons were also reported. The high-pressure methane adsorption measurements were performed with a Cahn 1000 microbalante. The detailed procedure was described elsewhere[l2]. This balance can be operated between pressures of 10-l Pa and 10’ Pa. The pressures were measured by transducers. Pressures up to 1000 Torr were measured by an MKS capacitance manometer model 222B. Pressures higher than 1000 Torr were measured by Data Instruments Transducer model EA O-2000 psig range. All the weight measurements were carried out in the 1-mg range on the microbalance. The weight measurements were recorded on a Hewlett Packard model 7220 stripchart recorder. Before sorption experiments, samples were heated to 393K and evacuated to 10-l Pa at 393K until no further weight loss of the sample took place. This procedure ensured removal of preadsorbed gases. The measured weight of the sample at this stage was considered to represent its degassed weight. After this pretreatment, adsorption experiments were commenced. The isotherms were obtained at three different temperaturres between 283K and 310K. Each isotherm was comprised of 10 to 20 data points up to 60 atm. Selected pressures
were tested for reversibility in adsorption. In all cases the isotherms were reversible, indicating that adsorption equilibrium was achieved. In the adsorption experiments only the sample side was temperature controlled. The tare side was always at room temperature. This resulted in asymmetry in the balance operation. Buoyancy corrections were thus necessary for the sample holder, the sample, and the adsorbed phase. Blank runs were conducted to correct for the buoyancy of the Pyrex sample holder. The correction for the buoyancy of the sample was evaluated from the skeletal volume and the calculated gas density. The method of Ozawa et aZ.[13] was used to estimate the adsorbed phase buoyancy correction. Selected replicate runs were carried out; the reproducibility of the adsorption isotherms was within 2%. 3. RESULTS AND DISCUSSION
A convenient method to describe experimental isotherms at different temperatures is to use a virialtype thermal equation of adsorption, written in the following form In p = $8
I-0
c,,V’ + 2 c,,V’ + In V ,=o
(1)
where V, p, and T are amount adsorbed, pressure, and temperature, respectively, and c,, are empirical parameters. This equation was derived[l4] under the assumption that in the limited range of temperatures, adsorption isosteres are linear, which is equivalent to the assumption that the isosteric heat of adsorption, Q,, as written below, is temperature invariant $[($$),1
= -+y
= 0.
(2)
The linearity of adsorption isosteres in an experimental interval of temperatures is in fact the common basic assumption for the calculation of isosteric heats from adsorption data. In our formulation, the order of expansions, n, and n2, are adjusted on the basis of numerical analysis of fitting the equation to experimental data. In this equation and all other equations, where appropriate, the “pressure” used was the calculated fugacity to correct for gas non-ideality. A special case of eqn (1) was evaluated by Kiselev et al. [15] for n, = n2 from an exponential virial adsorption equation[l6]. It is seen that eqn (1) obeys Henry’s law for p + 0, and the Henry’s constant, KH, is given by K,=exp(+c,,). It was shown[ 141 that eqn (1) can describe, within experimental accuracy, different adsorption data
-
,.,. 1
509
Adsorption of methane on activated carbons
-
Norit
-
Norltox. NorthAm.
----a---_.___*._._.
Nmh Am. ex $&*s&eco
___*--
Westvaco
ox.
-I’ 0 .. 0
10
20
30
40
50
60
Fugaeity [atm]
Fig. 2. Methane adsorption isotherms generated from eqn (1) at 303K for North American (.~Xl.**) and Westvaco (...O...) as-received carbons and their oxidized counterparts (...a...), (.*.+*.s).
4,
I.
0
1
I,
I.
9.
r
2
3
4
5
“1
Fig. 1. Variation of the fitting error of eqn (1) vs the order of the series n, for fixed n2 = 0. Separately errors for n, = 3 and nz = 1 are displayed.
with a relatively small number of empirical parameters. In order to establish an optimal number of
parameters for our results, we have analyzed fitting errors, g,
CT=
j+$ \il
[hQxxp) - W:q)l*
the
(4)
as a function of n, and n,. The results are presented in Fig. 1. It is seen that for rz, = 3 and n2 = 0 the error associated with fitting the data for all samples drops essentially to the level of the experimental error, and that further improvement in the goodness of fit with increasing n, is rather flat. In Fig. 1 we also present, as separate points, values of u for n, = 3 and n2 = 1; this shows that adding one more parameter to the second part of eqn (1) has in our case little effect on the goodness of fit. On the other hand, we find that the uncertainty associated with this additional parameter is very high. Based on these results and on the fact that the influence of the experimental error and the uncertainty associated with adjustable parameters grows with their number, we have chosen n, = 3 and nz = 0 to be adequate to describe our adsorption isotherms Table 1. Parameters
within the experimental range. The fitting parameters and errors are listed in Table 1. Figure 2 presents isotherms of the four representative samples generated at a common temperature (303K) from eqn (1). The effect of oxidation on adsorption, which is seen by eye, manifests itself in depressing the isotherms at higher pressures, thus lowering the maximum adsorption amount. The knowledge of the temperature dependence of adsorption enables one to study the thermodynamic properties of the adsorption system. The isosteric heat of adsorption, Q,, which is the fundamental thermodynamic quantity of adsorption can be directly derived from eqn (1) and it takes the following form:
Q,i = -R
( 1 d Inp a(l/T)
y=
-R 2 C,,V’. c=l)
(5)
Figure 3 shows the variation of Q, versus V calculated from the above equation. The monotonic decline of Q,, reflects the energetic heterogeneity of all investigated carbons. It is also seen that the isosteric heat of methane adsorption on oxidized samples is considerably lower than those on nontreated ones. Consequently, values of the isosteric heat at zero coverage, QZ,, presented in Table 2, show the same trend. For the sake of compa~son we quote results of Sircar[l7], who obtained Q$ = 16.5 kJ/moi for methane adsorption on BPL activated carbon, which is within the range of our results. Isosteric heats are integrated quantities reflecting cooperative interaction of adsorbent sites with an
and errors obtained from the fit of eqn (1) to the ex~rimental amount adsorbed given in [atm] and [gig], repectiveiy
isotherms-Fugacity
and
Sample
c,o x 10-3
C,) x 10-3
c,z x lo-”
c,, x 10-S
CX
(T x 10:
Norit Norit ox. North Am. North Am. ox. Westvaco Westvaco ox.
- 3.053 - 2.310 - 3.738 - 2.715 -3.18% - 1.933
11.69 3.593 9.950 5.816 16.02 1.821
-12.17 3.571 -7.098 - 1.619 - 15.33 6.783
9.724 1.266 4.417 3.317 7.536 - 1.704
13.71 11.74 15.91 12.88 14.16 11.48
5.1 3.6 5.6 5.5 4.3 4.3
J. JAGIEUOet al.
510
I
10 0
m?‘
0.04
0.06
0.08
0.10
012
Amount adsorbed, V [g/g]
Fig. 3. Isosteric heats of methane adsorption on Norit (**0..), North American (-..O-..), and Westvaco (**.O.*.) as-received carbons and their oxidized counterparts (..*a...), (...+...), (...D*..),
adsorbate. To study adsorption energy heterogeneity in a more rigorous way, we evaluate the so-called adsorption energy distribution function, x. The relation between the distribution x and the experimentally measured adsorption isotherm, V, is given by the following integral equation:
where tl(e, p, T) is the local isotherm of adsorption on sites with adso~tion energy E; E, and em are the minimum and maximum values of the adsorption energy of the adsorption system. We calculate x from the above adsorption integral equation using the local solution method[l8]. In this method, x is obtained directly from the isotherm V as the series X(E,) = i: (- l)n+’ (y:‘I;, n=O
.
s
c
(7)
where r is the order of approximation. The quantity E
0(c,p,
T) = [l
+ exp (%)I-’
sorbed molecules with molecules occupying neighboring sites, and V’ is the monolayer capacity. It was shown[l8,21] that for the Langmuir-like local isotherm series (7) gives an exact solution of the integral eqn (6) for r = ~0, whereas for a truncated series the approximation is obtained whose accuracy is dependent on temperature, T, and the shape of the distribution. For relatively broad distributions, a special case of eqn (7), the RudzinskiJagi&o approximation[Z2] (r = l), gives very good results(22,23], whereas the simplest case of this equation, the condensation approximation[24], CA (r = 0), is considered to be accurate only at low temperatures or for very broad distributions[25]. In our calculations we use approximation (7) with r = 2, having established that in the case of our adsorption systems, the higher terms of this series give no significant contribution to the calculated function x. It is known that the position of the distribution on the energy axis is dependent on the value of the Langmuir constant, K. There exist several empirical formulas in the literature for estimation of this parameter[26,27]. These formulas, however, take into account only individual properties of the adsorbate. Here we use a method based on a physical requirement of the adsorption energy distribution, which is that it should be temperature independent within the temperature interval where the solid surface does not change. This implies the following condition for the calculated dist~bution function:
W-9 Practically, we can obtain the effective value of the constant K for which the variation with temperature of the function x is the smallest by minimizing the sum of squares of the derivatives, ~?x/dT, evaluated for a certain number of points. Mathematically, this is expressed as
g
[in).,]’
= minimum.
(11)
We cross checked this method by calculation of the isosteric heat of adsorption at zero coverage, Qy,, from the resulting distribution function using the equation
(8) (12)
and le=
-RTlng-$ m
where K is the Langmuir constant, u is a parameter representing the mean interaction energy of ad-
and we obtained very good agreement with the empirical values calculated directly from eqn (5). These values along with calculated K constants are reported in Table 2. The lateral interaction parameter, U, was estimated from the van der Waals constants
511
Adsorption of methane on activated carbons Table 2. Resuits obtained
Sample Norit Norit ox. North Am. North Am. ox. Westvaco
Westvaco ox.
from thermodynamic analysis using the virial-type of adsorption
In K K in atm
QZ (eqn 5)
QY,(eqn 12)
E
~(60 atm)
kJlmo1
kJ/mol
kJ/mol
kJ/mol
10.7 8.5 13.2 9.8
25.4 19.2 31.4 22.6
27.1 19.3 33.3 23.1
20.1 15.2 26.1 17.5
12.4 8.2 18.8 9.9
11.5 8.1
26.5 16.0
29.4 15.8
20.5 12.6
13.2 7.4
using the method proposed by Ross and Oliver[28], which gives for methane u = 3.13 kJ/mol. Combining eqns (1) and (9), we obtain for E, a function of two variables, V and T, from which the analytical formulas can be evaluated for all derivatives in eqns (7) and (11). These formulas are used in our numerical calculations. The resuits are presented in Fig. 4. It is seen that the adsorption energy distributions for oxidized carbons are shifted to lower energies compared to those for the unoxidized carbons. The mean adsorption energy, Z (Table 2) decreases by 5 for Norit to 8 kJ/mol for Westvaco. To summarize our findings thus far, we find that both the isosteric heats of adsorption and mean adsorption energies decrease significantly upon oxidation of each carbon. This is consistent with our chromatographic results at infinite dilution[ 111, which have shown a decrease in alkane dispersive interactions with oxidized surfaces. This effect was correlated with acidic functional groups, which are created during the oxidation process. These decreases parallel the decreases, observed directly by experiment, in adsorbed amounts of methane on these samples. Although these results strongly suggest that the chemical nature of the activated carbon surface ptays a dominant role in observed changes of carbon adsorption properties, we cannot rule out either a more significant effect caused by microstructural changes or a combined effect caused by both of these factors. We seek an alternative approach to estimate the effects of oxidation on microstru~tural properties of our carbon samples. The effect of pore sizes on adsorption potential has been studied by direct model 1,
Adsorption
energy,
thermal equation
&[kJfmol]
Fig. 4. Adsorption energy distributions calculated using the local solution method (eqn 7 with r = 3). The notation is the same as for Fig. 3.
calculations[29-321. The results of Everett and Powl[31], who calculated the potential of a molecule in model cylindrical and slit-shaped pores, show that enhancement of adsorption energy in pores of width close to the diameter of the molecule can be by a factor of about 2 in slit-shaped pores, compared to its adsorption energy on a plane surface. This enhancement of adsorption energy decreases rapidly for wider pores and vanishes for pores of diameter 2 times larger than the diameter of the adsorbed molecule. Assuming that the minimum adsorption energy, e,, corresponds to the adsorption on a flat surface or in large pores, and the enhancement of adsorption energy is governed by the pore sizes, it follows that the pore size distribution governs the adsorption energy distribution x(E*) as a function of some net adsorption energy, E*, E* = E - E,
(13)
which is related to the enhancement of adsorption potential in micropores of given .sizes. Although it is difficult to evaluate exactly the minimum adsorption energies, E,, from our results. we can roughly estimate their upper bound. All distributions presented in Fig. 4 were calculated from isotherms in the same pressure interval from 0 to 60 atm. Therefore, the smallest adsorption energies of calculated distributions correspond to the same pressure, 60 atm. The values of these energies, ~(60 atm), listed in Table 2 can be taken as upper limits of E, values, since E, 5 ~(60 atm). From Fig. 4 and from changes in numerical values of E and ~(60 atm), one can deduce that energy distributions for oxidized samples are shifted almost parallel to lower adsorption energies compared to untreated carbons. Decreases in the height of peaks are explained by the fact that our distributions are not normalized; thus, as mentioned before, decreases in total adsorption capacity are reflected by depressing the distributions. From the observation that appropriate translations along the energy axis and normalizations of the X(E) functions for untreated and oxidized samples can generate approximately a common envelope for them, it follows that the distribution x(E*) as a function of the net adsorption energy, e*, is unchanged within the accuracy of this approximation. This implies that the corresponding micropore size distribution is practically unchanged due to oxidation.
J. JAGIEL~Oet al.
512
On the other hand, we observe that the initial part of the adsorption energy distribution, X(E), for oxidized samples is shifted to energy values well below the value of the heat of methane adsorption on graphitized carbon black. An energy of adsorption on activated carbon, which is below the value of the heat of adsorption on nonporous carbon black (12.7 kJ/mol[33]), can be explained on the basis of chemical changes of the carbon surface. The above considerations lead us to the conclusion that changes in the adsorption energy distribution that occur as a result of oxidation of the surface of activated carbons are due predominantly to the presence of surface chemical groups. Due to the electronic structure of the methane molecule, only dispersive interactions contribute to the value of the adsorption energy. The energy associated with dispersive interactions depends strongly on the distance between nuclei of adsorbent and adsorbed molecules. The surface chemical groups introduced onto the surface by oxidation may hinder the most favorable geometrical arrangement of adsorbed molecules over the surface, and thus cause the observed decrease in the adsorption energy. At the present stage of our research we are not able to present a more detailed quantitative analysis of the relationship between pore structure and surface chemistry of activated carbons. We show, however, the magnitude of the effect of surface chemical groups on adsorption energetics. This effect should be taken into account when the pore structure is studied by means of adsorption methods, even when nonspecific adsorbates are used. The structural and chemical effects on adsorption of gases on activated carbons remain a topic of continued research in our laboratory. Acknowledgemen&Support for this work comes from Brookhaven National Laboratories under Contract 466541. The authors wish to acknowledge helpful discussions with Dr. J. Wegrzyn. REFERENCES 1. B. R. Puri. In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 6, Marcel Dekker, New York (1970). 2. J. B. Donnet, Carbon 6, 161 (1968).
3. H. P. Boehm, Adv. Catalysis 16, 179 (1966). 4. J. Zawadzki. In Chemistry and Physics of Carbon (Edited by P. A. Thrower), Vol. 21, Marcel Dekker, New York (1989). 5. M. M: Dudinin. In Chemistry and Physics of Carbon (Edited bv P. L. Walker. Jr.). Vol. 2. Marcel Dekker. hew Yori (1966). ” 6. M. Jaroniec, Adv. Colloid Interface Sci. 18,149 (1983). 7. S. J. Gregg and K. S. Sing. In Adrorprion, Surface Area, and Porosity, Academic Press, New York (1982). 8. D. M. Young and A. D. Crowell. In Physical Adsorption of Gases. Butterworths, London (1962). 9. K. S.‘W. Sing, Colloids Surf. 38, 113 (1989j. 10. M. Jaroniec, R. Madey, X. Lu, J. Choma, Langmuir 4, 911 (1988).
11. J. JagieUo, T. J. Bandosz, and J. A. Schwarz, Carbon 30, 63 (1992).
12. R. K. Agarval and ;. A. Schwarz, Carbon 26, 873 (1988). 13. S. Ozawa, S. Kusumi, and Y. Ogino, J. Colloid Interface Sci. 79, 399 (1981). 14. L. Czepirski and J. Jagi&o, Chem. Eng. Sci. 44, 797
(1989). 15. N. N. Avgul and A. V. Kiselev. In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 6, Marcel Dekker, New York (1970). 16. R. S. Bradley, Phil. Msg. 11, 690 (1931). 17. S. Sircar, J. Colloid Interface Sci. 101, 452 (1984). 18. J. JagieUo, G. Ligner, and E. Papirer, /. Colloid Interface Sci. 137, 128 (1990).
19. T. L. Hill. In Statistical Mechanics, McGraw-Hill, New York, 1956. 20. T. L. Hill, J. Chem. Phys. 17, 762 (1949). 21. J. Jagiello and J. A. Schwarz, J. Colloid Interface Sci. 146,415 (1991). 22. W. Rudziriski, J. JagieIlo, and Y. Grillet, J. Colloid Interface Sci. 87, 478 (1982). 23. M. &I. Nederlof, W. H. Van Riemsdijk, and L. K. Koooal. J. Colloid Interface Sci. 135. 410 (1990). ’ 24. L. B’. Harris, Surface Sk 10, 128 (1968). ’ 25. G. F. Cerofolini, Thin Solid Films 23, 129 (1974). 26. J. P. Hobson, Canad. J. Phys. 43, 1934 (1965). 27. A. W. Adamson and I. Ling, Advan. Chem. 33, 51 (1961). 28. S. Ross and J. P. Olivier, On Physical Adsorption,
Interscience,
London, 1964.
29. J. H. de Boer and J. F. H. Custer. Z. Phvs. , Chem (Bj \
:;:
I
25, 225 (1934). F. Stoeckli, Helv. Chim. Acta, 57, 2195 (1974). D. H. Everett and J. C. Powl, J. Chem. Sot. Faraday
Trans. 172, 619 (1976). 32. V. Biba, Z. Spitzer, and 0. Kadlec, .I. Colloid Interface Sci. 69, 9 (1979). 33. G. Constabrais, J. R. Sams, Jr., and G. D. Halsey, Jr., J. Phys. Chem. 65, 367 (1961).
CarbonVol. 30.No. 3, pp. 507-512,1992 Printedin GreatBritain.
THERMODYNAMIC STUDY OF HIGH-PRESSURE ADSORPTION OF METHANE ON ACTIVATED CARBONS: THE EFFECT OF OXIDATION ON PORE STRUCTURE AND ADSORPTION ENERGY HETEROGENEITY JACEK JAGIEuOt,
PRASHANT
and Department
SANGHANI,
JAMES A.
TERESA
J. BANDOSZ,
SCHWARZ*
of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244, U.S.A. (Received
15 July 1991; accepted in revised form 15 October 1991)
Abstract-The effects of oxidation of activated carbons are studied by means of high-pressure methane adsorption. Decreases in amounts adsorbed and adsorption enthalpies after oxidation are observed. Thermodynamic analysis of adsorption isotherms using the virial-type thermal equation of adsorption is presented. The calculated adsorption energy distributions are shifted to lower adsorption energies for oxidized carbons. The mean adsorption energies decrease, indicating that oxidized carbon surfaces have lower affinity for methane adsorption. Key Words-High bution.
pressure adsorption,
activated carbons, microstructure,
1. INTRODUCTION
The term activated carbon is given to carbonaceous materials having a large adsorption capacity. Activated carbons can be manufactured from many different materials, such as coconut shells, wood, peat, and coal. One might expect that the carbon’s performance as an adsorbent in a particular application will depend on its source. Indeed, this is found to be the case. The demands placed on activated carbon-based technologies have outpaced fundamental studies of the interplay between the carbon sources, the surface chemical properties of carbons, the accessible internal adsorption areas, and their performance as adsorbents. Unfortunately, preconceived notions have been established that attempt a priori to provide “prescriptions” for carbon selection in a technological application. The adsorption characteristics of activated carbon are affected by the type of carbon, that is, governed by the source of raw material, and the preparation procedures used during carbonization and activation. Any assessment of the adsorptive behavior of activated carbons based only on surface area is incomplete; carbons with equal density and equal surface area can exhibit different adsorptive characteristics when prepared by different methods. Some adsorptive properties can be explained by differences in pore size distribution. Surface chemical functionalities, derived
*Author to whom correspondence should be addressed. tpermanent address: Institute of Energochemistry of Coal and Physicochemistry of Sorbents, University of Mining and Metallurgy, 30-059 Krakow, Poland.
adsorption energy distri-
from activation or treatment procedures, also influence adsorption, but their role depends on their accessibility, which in turn depends on the carbon’s microporosity. The dichotomy described above is due, in part, to the chemical-physical properties of activated carbons. Microporous carbons are disordered solids in which the fundamental structural unit is graphitelike monocrystallites with cross-linked aliphatic bridging groups, incorporating such heteroatoms as H, 0, N, and S. The existence of functional groups such as carboxyls, phenols, lactones, aldehydes, quinones. hydroquinones, and ethereal structures have been postulated[l-41. The chemistry of a carbon surface has a profound influence on its adsorption properties. The nature of the activated carbon surface is also very complex from a physical[5,6] point of view. The porous structure and specific surface area[7,8] have great influence on the adsorption properties. In fact, the specific surface area and pore size distribution are proposed as fundamental quantities recommended by the International Union of Pure and Applied Chemistry (IUPAC)[9] for characterizing these adsorbents. They are usually determined from sorption isotherms of different gases[7,8]. In addition, surface chemical characteristics can also be obtained from investigations of the interactions between the solid surface and gas molecule probes. When only physical interactions occur, the study of adsorption phenomena allows a quantitative description of the energetic properties of the solid surface, especially the evaluation of surface energy heterogeneity, which can be connected with the microstructural het507
508
J. JAGIEEEOet al.
erogeneity, as well as with the chemical nature of the surface[lO]. In our previous work[ll] we have shown by using inverse gas chromatography the existence of a correlation between surface chemical properties of carbons and the energy of specific interactions with n bonds of unsaturated hydrocarbons. The dispersive component of surface energy, $, and the enthalpies of alkane adsorption decreased after oxidative pretreatment of carbons, which indicated that the oxidized carbons have less affinity for hydrocarbons than untreated carbon surfaces. In this paper we propose to separate the effects of microstructural heterogeneity of activated carbons and the chemical nature of their surfaces. The basis for our analysis is the hypothesis that the suppression in adsorption capacity for methane on oxidized carbons compared to their as-received counterpart is due either to surface chemical effects, microstructural effects, or combination of both. Our methodology, which is quite general and can be applied judiciously to other adsorption systems, is thermodynamic in nature and relies on a detailed examination of such quantities as adsorption isotherms, isosteric heats of adsorption, and adsorption energy heterogeneity for methane on several activated carbons.
2. MATERIALS
AND METHODS
Three activated carbons from different sources (Westvaco, Norit, North American) were oxidized with 15N (73%) HNOs solution at 298K for 2.5 h according to the method described in our previous work[ll], in which the physico-chemical characteristics of these carbons were also reported. The high-pressure methane adsorption measurements were performed with a Cahn 1000 microbalante. The detailed procedure was described elsewhere[l2]. This balance can be operated between pressures of 10-l Pa and 10’ Pa. The pressures were measured by transducers. Pressures up to 1000 Torr were measured by an MKS capacitance manometer model 222B. Pressures higher than 1000 Torr were measured by Data Instruments Transducer model EA O-2000 psig range. All the weight measurements were carried out in the 1-mg range on the microbalance. The weight measurements were recorded on a Hewlett Packard model 7220 stripchart recorder. Before sorption experiments, samples were heated to 393K and evacuated to 10-l Pa at 393K until no further weight loss of the sample took place. This procedure ensured removal of preadsorbed gases. The measured weight of the sample at this stage was considered to represent its degassed weight. After this pretreatment, adsorption experiments were commenced. The isotherms were obtained at three different temperaturres between 283K and 310K. Each isotherm was comprised of 10 to 20 data points up to 60 atm. Selected pressures
were tested for reversibility in adsorption. In all cases the isotherms were reversible, indicating that adsorption equilibrium was achieved. In the adsorption experiments only the sample side was temperature controlled. The tare side was always at room temperature. This resulted in asymmetry in the balance operation. Buoyancy corrections were thus necessary for the sample holder, the sample, and the adsorbed phase. Blank runs were conducted to correct for the buoyancy of the Pyrex sample holder. The correction for the buoyancy of the sample was evaluated from the skeletal volume and the calculated gas density. The method of Ozawa et aZ.[13] was used to estimate the adsorbed phase buoyancy correction. Selected replicate runs were carried out; the reproducibility of the adsorption isotherms was within 2%. 3. RESULTS AND DISCUSSION
A convenient method to describe experimental isotherms at different temperatures is to use a virialtype thermal equation of adsorption, written in the following form In p = $8
I-0
c,,V’ + 2 c,,V’ + In V ,=o
(1)
where V, p, and T are amount adsorbed, pressure, and temperature, respectively, and c,, are empirical parameters. This equation was derived[l4] under the assumption that in the limited range of temperatures, adsorption isosteres are linear, which is equivalent to the assumption that the isosteric heat of adsorption, Q,, as written below, is temperature invariant $[($$),1
= -+y
= 0.
(2)
The linearity of adsorption isosteres in an experimental interval of temperatures is in fact the common basic assumption for the calculation of isosteric heats from adsorption data. In our formulation, the order of expansions, n, and n2, are adjusted on the basis of numerical analysis of fitting the equation to experimental data. In this equation and all other equations, where appropriate, the “pressure” used was the calculated fugacity to correct for gas non-ideality. A special case of eqn (1) was evaluated by Kiselev et al. [15] for n, = n2 from an exponential virial adsorption equation[l6]. It is seen that eqn (1) obeys Henry’s law for p + 0, and the Henry’s constant, KH, is given by K,=exp(+c,,). It was shown[ 141 that eqn (1) can describe, within experimental accuracy, different adsorption data
-
,.,. 1
509
Adsorption of methane on activated carbons
-
Norit
-
Norltox. NorthAm.
----a---_.___*._._.
Nmh Am. ex $&*s&eco
___*--
Westvaco
ox.
-I’ 0 .. 0
10
20
30
40
50
60
Fugaeity [atm]
Fig. 2. Methane adsorption isotherms generated from eqn (1) at 303K for North American (.~Xl.**) and Westvaco (...O...) as-received carbons and their oxidized counterparts (...a...), (.*.+*.s).
4,
I.
0
1
I,
I.
9.
r
2
3
4
5
“1
Fig. 1. Variation of the fitting error of eqn (1) vs the order of the series n, for fixed n2 = 0. Separately errors for n, = 3 and nz = 1 are displayed.
with a relatively small number of empirical parameters. In order to establish an optimal number of
parameters for our results, we have analyzed fitting errors, g,
CT=
j+$ \il
[hQxxp) - W:q)l*
the
(4)
as a function of n, and n,. The results are presented in Fig. 1. It is seen that for rz, = 3 and n2 = 0 the error associated with fitting the data for all samples drops essentially to the level of the experimental error, and that further improvement in the goodness of fit with increasing n, is rather flat. In Fig. 1 we also present, as separate points, values of u for n, = 3 and n2 = 1; this shows that adding one more parameter to the second part of eqn (1) has in our case little effect on the goodness of fit. On the other hand, we find that the uncertainty associated with this additional parameter is very high. Based on these results and on the fact that the influence of the experimental error and the uncertainty associated with adjustable parameters grows with their number, we have chosen n, = 3 and nz = 0 to be adequate to describe our adsorption isotherms Table 1. Parameters
within the experimental range. The fitting parameters and errors are listed in Table 1. Figure 2 presents isotherms of the four representative samples generated at a common temperature (303K) from eqn (1). The effect of oxidation on adsorption, which is seen by eye, manifests itself in depressing the isotherms at higher pressures, thus lowering the maximum adsorption amount. The knowledge of the temperature dependence of adsorption enables one to study the thermodynamic properties of the adsorption system. The isosteric heat of adsorption, Q,, which is the fundamental thermodynamic quantity of adsorption can be directly derived from eqn (1) and it takes the following form:
Q,i = -R
( 1 d Inp a(l/T)
y=
-R 2 C,,V’. c=l)
(5)
Figure 3 shows the variation of Q, versus V calculated from the above equation. The monotonic decline of Q,, reflects the energetic heterogeneity of all investigated carbons. It is also seen that the isosteric heat of methane adsorption on oxidized samples is considerably lower than those on nontreated ones. Consequently, values of the isosteric heat at zero coverage, QZ,, presented in Table 2, show the same trend. For the sake of compa~son we quote results of Sircar[l7], who obtained Q$ = 16.5 kJ/moi for methane adsorption on BPL activated carbon, which is within the range of our results. Isosteric heats are integrated quantities reflecting cooperative interaction of adsorbent sites with an
and errors obtained from the fit of eqn (1) to the ex~rimental amount adsorbed given in [atm] and [gig], repectiveiy
isotherms-Fugacity
and
Sample
c,o x 10-3
C,) x 10-3
c,z x lo-”
c,, x 10-S
CX
(T x 10:
Norit Norit ox. North Am. North Am. ox. Westvaco Westvaco ox.
- 3.053 - 2.310 - 3.738 - 2.715 -3.18% - 1.933
11.69 3.593 9.950 5.816 16.02 1.821
-12.17 3.571 -7.098 - 1.619 - 15.33 6.783
9.724 1.266 4.417 3.317 7.536 - 1.704
13.71 11.74 15.91 12.88 14.16 11.48
5.1 3.6 5.6 5.5 4.3 4.3
J. JAGIEUOet al.
510
I
10 0
m?‘
0.04
0.06
0.08
0.10
012
Amount adsorbed, V [g/g]
Fig. 3. Isosteric heats of methane adsorption on Norit (**0..), North American (-..O-..), and Westvaco (**.O.*.) as-received carbons and their oxidized counterparts (..*a...), (...+...), (...D*..),
adsorbate. To study adsorption energy heterogeneity in a more rigorous way, we evaluate the so-called adsorption energy distribution function, x. The relation between the distribution x and the experimentally measured adsorption isotherm, V, is given by the following integral equation:
where tl(e, p, T) is the local isotherm of adsorption on sites with adso~tion energy E; E, and em are the minimum and maximum values of the adsorption energy of the adsorption system. We calculate x from the above adsorption integral equation using the local solution method[l8]. In this method, x is obtained directly from the isotherm V as the series X(E,) = i: (- l)n+’ (y:‘I;, n=O
.
s
c
(7)
where r is the order of approximation. The quantity E
0(c,p,
T) = [l
+ exp (%)I-’
sorbed molecules with molecules occupying neighboring sites, and V’ is the monolayer capacity. It was shown[l8,21] that for the Langmuir-like local isotherm series (7) gives an exact solution of the integral eqn (6) for r = ~0, whereas for a truncated series the approximation is obtained whose accuracy is dependent on temperature, T, and the shape of the distribution. For relatively broad distributions, a special case of eqn (7), the RudzinskiJagi&o approximation[Z2] (r = l), gives very good results(22,23], whereas the simplest case of this equation, the condensation approximation[24], CA (r = 0), is considered to be accurate only at low temperatures or for very broad distributions[25]. In our calculations we use approximation (7) with r = 2, having established that in the case of our adsorption systems, the higher terms of this series give no significant contribution to the calculated function x. It is known that the position of the distribution on the energy axis is dependent on the value of the Langmuir constant, K. There exist several empirical formulas in the literature for estimation of this parameter[26,27]. These formulas, however, take into account only individual properties of the adsorbate. Here we use a method based on a physical requirement of the adsorption energy distribution, which is that it should be temperature independent within the temperature interval where the solid surface does not change. This implies the following condition for the calculated dist~bution function:
W-9 Practically, we can obtain the effective value of the constant K for which the variation with temperature of the function x is the smallest by minimizing the sum of squares of the derivatives, ~?x/dT, evaluated for a certain number of points. Mathematically, this is expressed as
g
[in).,]’
= minimum.
(11)
We cross checked this method by calculation of the isosteric heat of adsorption at zero coverage, Qy,, from the resulting distribution function using the equation
(8) (12)
and le=
-RTlng-$ m
where K is the Langmuir constant, u is a parameter representing the mean interaction energy of ad-
and we obtained very good agreement with the empirical values calculated directly from eqn (5). These values along with calculated K constants are reported in Table 2. The lateral interaction parameter, U, was estimated from the van der Waals constants
511
Adsorption of methane on activated carbons Table 2. Resuits obtained
Sample Norit Norit ox. North Am. North Am. ox. Westvaco
Westvaco ox.
from thermodynamic analysis using the virial-type of adsorption
In K K in atm
QZ (eqn 5)
QY,(eqn 12)
E
~(60 atm)
kJlmo1
kJ/mol
kJ/mol
kJ/mol
10.7 8.5 13.2 9.8
25.4 19.2 31.4 22.6
27.1 19.3 33.3 23.1
20.1 15.2 26.1 17.5
12.4 8.2 18.8 9.9
11.5 8.1
26.5 16.0
29.4 15.8
20.5 12.6
13.2 7.4
using the method proposed by Ross and Oliver[28], which gives for methane u = 3.13 kJ/mol. Combining eqns (1) and (9), we obtain for E, a function of two variables, V and T, from which the analytical formulas can be evaluated for all derivatives in eqns (7) and (11). These formulas are used in our numerical calculations. The resuits are presented in Fig. 4. It is seen that the adsorption energy distributions for oxidized carbons are shifted to lower energies compared to those for the unoxidized carbons. The mean adsorption energy, Z (Table 2) decreases by 5 for Norit to 8 kJ/mol for Westvaco. To summarize our findings thus far, we find that both the isosteric heats of adsorption and mean adsorption energies decrease significantly upon oxidation of each carbon. This is consistent with our chromatographic results at infinite dilution[ 111, which have shown a decrease in alkane dispersive interactions with oxidized surfaces. This effect was correlated with acidic functional groups, which are created during the oxidation process. These decreases parallel the decreases, observed directly by experiment, in adsorbed amounts of methane on these samples. Although these results strongly suggest that the chemical nature of the activated carbon surface ptays a dominant role in observed changes of carbon adsorption properties, we cannot rule out either a more significant effect caused by microstructural changes or a combined effect caused by both of these factors. We seek an alternative approach to estimate the effects of oxidation on microstru~tural properties of our carbon samples. The effect of pore sizes on adsorption potential has been studied by direct model 1,
Adsorption
energy,
thermal equation
&[kJfmol]
Fig. 4. Adsorption energy distributions calculated using the local solution method (eqn 7 with r = 3). The notation is the same as for Fig. 3.
calculations[29-321. The results of Everett and Powl[31], who calculated the potential of a molecule in model cylindrical and slit-shaped pores, show that enhancement of adsorption energy in pores of width close to the diameter of the molecule can be by a factor of about 2 in slit-shaped pores, compared to its adsorption energy on a plane surface. This enhancement of adsorption energy decreases rapidly for wider pores and vanishes for pores of diameter 2 times larger than the diameter of the adsorbed molecule. Assuming that the minimum adsorption energy, e,, corresponds to the adsorption on a flat surface or in large pores, and the enhancement of adsorption energy is governed by the pore sizes, it follows that the pore size distribution governs the adsorption energy distribution x(E*) as a function of some net adsorption energy, E*, E* = E - E,
(13)
which is related to the enhancement of adsorption potential in micropores of given .sizes. Although it is difficult to evaluate exactly the minimum adsorption energies, E,, from our results. we can roughly estimate their upper bound. All distributions presented in Fig. 4 were calculated from isotherms in the same pressure interval from 0 to 60 atm. Therefore, the smallest adsorption energies of calculated distributions correspond to the same pressure, 60 atm. The values of these energies, ~(60 atm), listed in Table 2 can be taken as upper limits of E, values, since E, 5 ~(60 atm). From Fig. 4 and from changes in numerical values of E and ~(60 atm), one can deduce that energy distributions for oxidized samples are shifted almost parallel to lower adsorption energies compared to untreated carbons. Decreases in the height of peaks are explained by the fact that our distributions are not normalized; thus, as mentioned before, decreases in total adsorption capacity are reflected by depressing the distributions. From the observation that appropriate translations along the energy axis and normalizations of the X(E) functions for untreated and oxidized samples can generate approximately a common envelope for them, it follows that the distribution x(E*) as a function of the net adsorption energy, e*, is unchanged within the accuracy of this approximation. This implies that the corresponding micropore size distribution is practically unchanged due to oxidation.
J. JAGIEL~Oet al.
512
On the other hand, we observe that the initial part of the adsorption energy distribution, X(E), for oxidized samples is shifted to energy values well below the value of the heat of methane adsorption on graphitized carbon black. An energy of adsorption on activated carbon, which is below the value of the heat of adsorption on nonporous carbon black (12.7 kJ/mol[33]), can be explained on the basis of chemical changes of the carbon surface. The above considerations lead us to the conclusion that changes in the adsorption energy distribution that occur as a result of oxidation of the surface of activated carbons are due predominantly to the presence of surface chemical groups. Due to the electronic structure of the methane molecule, only dispersive interactions contribute to the value of the adsorption energy. The energy associated with dispersive interactions depends strongly on the distance between nuclei of adsorbent and adsorbed molecules. The surface chemical groups introduced onto the surface by oxidation may hinder the most favorable geometrical arrangement of adsorbed molecules over the surface, and thus cause the observed decrease in the adsorption energy. At the present stage of our research we are not able to present a more detailed quantitative analysis of the relationship between pore structure and surface chemistry of activated carbons. We show, however, the magnitude of the effect of surface chemical groups on adsorption energetics. This effect should be taken into account when the pore structure is studied by means of adsorption methods, even when nonspecific adsorbates are used. The structural and chemical effects on adsorption of gases on activated carbons remain a topic of continued research in our laboratory. Acknowledgemen&Support for this work comes from Brookhaven National Laboratories under Contract 466541. The authors wish to acknowledge helpful discussions with Dr. J. Wegrzyn. REFERENCES 1. B. R. Puri. In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 6, Marcel Dekker, New York (1970). 2. J. B. Donnet, Carbon 6, 161 (1968).
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