Langmuir 1993,9, 2518-2522
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Effect of Surface Chemical Groups on Energetic Heterogeneity of Activated Carbons Teresa J. Bandosz, Jacek JagieUo,+and James A. Schwarz' Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244-1190 Received August 6,1992. I n Final Form: November 1 6 , 1 9 9 9 Activated carbons from different origins have been oxidized with nitric acid at different temperatures. Some of the oxidized samples were heat treated at 873 K in inert atmosphere in order to partially reduce their oxidized surface. The changes in the number of surface species were determined by the Boehm titration method and X-ray photoelectron spectroscopy. Gas chromatography at infinite dilution and finite concentration was used to study the effect of surface groups on adsorption properties of the carbon samples. Oxidation was followed by decreases in the enthalpies and free energies of alkane adsorption. The resulta obtained indicate that surface chemical groups may hinder and/or perturb occupancy of adsorbed molecules on their energetically most favorable positions.
Introduction Activated carbons are frequently used adsorbents in applications where their high adsorption capacity is required. The sorption properties of these materials are related to their pore structure which is created during the manufacturing processes consisting of carbonization, activation, and chemical modification.' The resulting activated carbons are heterogeneousfrom the point of view of their geometricaland chemical structure which is related to the source material and the particular preparation method. The geometrical structure of activated carbonsis usually characterized by its pore size distribution obtained from such techniques as adsorption experiments, mercury porosimetry, or small-angle X-ray scattering. The most common methods for studying micro- and mesopore structure are based on adsorption isotherms.2 However, these methods neglect the chemical composition of activated carbons which might influence the adsorption properties of the probe molecule. The chemical composition of these materials has been analyzed by chemical and instrumental methods such as Boehm titration, IR spectroscopy,and X-ray photoelectron spectroscopy, XPS.- It is found that the molecular structure of these materials is very complex from the viewpoint of packing of carbon atoms. In addition existing heteroatoms are present in the activated carbon network. The major chemical forma in which heteroatoms are present are functional groups and heterocyclic compounds analogous to those in organic compounds. Some of these groups are exposed to the surface; intuitively these surface chemical groups have the strongest influence on the adsorption process. If surface chemical modification processes are implemented during preparation, the quantity and quality of these groups are likely to be changed. One of the methods of carbon surface modification is oxidation with nitricacid. It is well known that this process
* To whom correspondenceshould be addressed.
+ Permanent address: Institute of Energochemistry of Coal and Physicochemistryof Sorbente,University of Mining and Metallurgy, 30-069 Krakbw, Poland. Abstract published in AdvanceACS Abstracts, August 15,1993. (1)B d ,R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988. (2) Gregg, 5.J.; Shg,K. 5.W. Adsorption, Surface Area, and Porosity; Academic Preas: New York, 1982. (3)h h m , H. P. Adv. Catal. 1966,16, 179. (4) Zawadzki, J. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 21. (5) Donnet, J. B.; Guilpain, G. Carbon 1989,27, 749.
increases the number of surface acidic group^.^*^ In our previousworks9JO we have studied the effect of controlled oxidation of activated carbons on their adsorption properties. The increase in acidic groups is accompanied by an increase of specific interaction of the carbon with alkene molecules. This effect wa8 expected since .rr bonds of alkenes interact in a specific way with acidic (electron acceptor) groups. On the other hand, the decrease of the adsorption enthalpy of methanegand longer alkanes" for oxidized carbons is also observed. The latter effect shows that the presence of surface acidic groups may have an influence on adsorption of alkanes even though these molecules are usually considered as nonspecific adsorbates and therefore should not be sensitive to the presence of specific adsorption centers. The results we cited have prompted a more detailed study of adsorption effects caused by introduction of acidic groups on the carbon surface. We apply gas chromatography at infinite dilution where Henry's law is obeyed as well as a t low concentrations of adsorbate which corresponds to the initial uptake in the adsorption isotherm. We show that surface chemical groups significantly affect energies of nonspecific alkane adsorption on activated carbons, and therefore they contribute to overall energetic heterogeneity of carbons.
Experimental Section Materials. Three activated carbons from different sourcea were used: B4x14 from Westvaco co., Sorbonorit 2-AS998 from Norit N. V., and Microsorbfrom Kansai Coke and ChemicalsCo. Ltd. Later we refer to these carbons as Westvaco, Norit, and Microsorb. They were derived from wood, peat, and petroleum residuum, respectively. Details as to their manufacture were not available. The Brunauel-Emmett-Teller (BET)surfacearea of the first two is about 1200 ma/g; the area of the third carbon is 2000 m2/g. Carbonswere oxidized with 15N (73% ) HNOa solutionat 293, 323, and 351 K for 2 h under continuous stirring. After treatment,
the samples were washed with distilledwater to zero acid removal and oven dried at 383 K. For an easier description the symbols (6) Puri, B. R.In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6. (7) Mataura, Y.;Hagiwara, S.; Takahaehi, H. Carbon 1976, 14, 163. (8) Hagiwara, S.; Tsutaumi, K.;Takahaehi, H. Carbon 1978,16, 89. (9) Jagiello, J.; Sanghani, P.; Bandosz, T. J.; Schwarz, J. A. Carbon 1992, 30, 507. (10) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. J. Colloidlnterface Sci. 1992,151, 433. (11)Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1992,30, 63.
0743-7463/93/2409-2518$04.00/00 1993 American Chemical Society
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Energetic Heterogeneity of Activated Carbons A, B, and C denote the following temperatures of oxidation: 293, 323, and 361 K. Some samples were reduced by heat treatment in a furnace at 873 K under a Na atmosphere for 6 h, and the symbol R is added in these cases. Methods. Gas Chromatography. The parameters which characterize gas-solid interactions are derived from the net retention volumes, VN,of molecular probes. These are injected into the stream of a carrier gas flowing through the chromatographic column filled with the solid to be investigated. The net retention volume is calculated from the measured net retention time, tN, which is the difference between the retention times of a probe and a nonadsorbing gas used as a reference:
whereFis the flowrate andj is the James-MartinBcompressibility factor which is dependent on the pressure drop along the chromatographic column. At infinitedilution,the net retentionvolumeis directlyrelated to the standard free energy of adsorption AGO:
where R and Tare the gas constant and temperature, m and S are the mass and specific surface area of the adsorbent, and C is a constant related to the standard states of gas and adsorbed phases. Values of VNmeasured at different temperatures are used to calculate the enthalpy of adsorption, AHo: (3)
Both AGO and AHo obtained under the conditions of infinite dilution are dependent only on the gas-solid interaction since interaction between adsorbed molecules can be neglected. The quantities such as enthalpy or free energy of adsorption, obtainedunder the conditionsof infinite dilution, for energetically heterogeneous surfaces are considered to be related to some average value of adsorption energy. The averaging formula follows from the relationship between Henry's constant and the adsorption energy and contains the weighting factor which is exponentiallydependent on adsorptionenergy.13 Physicallythis meansthat results obtainedat infinite dilution are a consequence of interactions of the probe with high-energy sites. In this work we use alkanes and alkenes to assess dispersive and specific gas-solid interactions. It is well known from the chromatographicliterature" that the logarithmsof VNat infinite dilution for n-alkanes vary linearly with their number of carbon atoms. Therefore,the quantity AGOCH~, which is defined as the difference in the AGO of two subsequent n-alkanes, represents the free energy of adsorption of a CHa group. This quantity is not related to any particular alkane molecule and, due to its incremental character, is not dependent on the choice of the reference state. Alkenes whose ?r bonds interact with electron acceptor sites were used to study specific interactions. Net retention volumes measured at finite concentrationsmay be used to calculate the adsorptionisotherm, V, accordingto the general relationshipla (4)
which is valid when under the prevailingexperimentalconditions the gas phase can be considered as ideal. If profiies of chromatographicpeaks correspondingto different injection sizes overlap,a singlepeak profile representsvariation of the retention volume as a function of pressure. In this case, an adsorption isotherm may be obtained by appropriate integration of one chromatographic peak. (12)Conder, J. R.;Young, C. L. PhysicochemicaZ Measurement by Gas Chromatography; John Wiley L Sons: New York, 1979. (13)Ross,S.; Olivier, J. P. On Physical Adsorption; Interscience Publishers: New York, 1964. (14)K i d e v , A. V.;Yashin, Y. I. Gas Adsorption Chromatography; Plenum Press: New York, 1969. (16)Conder, J. R.;Purnell, J. H.Trans. Faraday SOC.1968,64,3100.
The chromatographic data were obtained with an ANTEK 3000 gas chromatograph using a flame ionization detector. Stainless steel columna 2.17 mm in diameter were used, 20 and 50 cm long for infinite dilution experiments and finite concen-
tration, respectively. Granulation of carbon samplesWBB in the range of 0.2-0.4 mm. Helium was used as a carrier gas with a flow rate of about 30 cmg/min. The samples were conditioned at 623 K in the chromatographiccolumn under helium gas flow for 16 h prior to the measurements. The hydrocarbons used for injections were HPLC grade (Aldrich Chemical Co.). The temperature was stabilized with an accuracy of fO.l K. The infiite dilution measurements were performed in the temperature range 473-623 K. When small amounts of solutes were injeded, the resultingpeakswere symmetzicaland retention times did not depend on the mount injected. These results confii that the experimentswere within the Henry's law region. The error in retention times was 5 9%. In the case of finite concentration, chromatographic peaks were recorded for solute injections rangingfrom 1to 90HLin the temperature interval of 673-623 K. Under these conditions the overlappingof peak profiieswas observed. This fact is explained by the strong curvature in adsorption isotherms on activated carbons at low valum of the adsorbate pressure. This situation enables us to integrate the profile of a single peak according to eq 4 in order to obtain an adsorption isotherm. BoehmTitration Methods. Toestimate the acidicproperties of each of the surface-modifiedactivated carbons, the method proposed by Boehm3 was used. This method is based on acid/ base titration of acidic or basic centers on the carbon by sodium hydroxide (NaOH), sodium carbonate (NasCOd, and sodium bicarbonate(NaHCOs). The number of acidic siteswas calculated under the assumptionthat NaOH neutralizescarboxyl,phenolic, and lactonic groups. The 1-g carbon samples were placed in vials with 25 mL of 0.05 N NaOH solutions. The vials were sealed and shaken for 24 h, and then the excess of base in the filtrate was titrated with 0.05 N HCl solution. To ensure the sametemperature pretreatment of the samples as in the case of the Chromatographicexperimenta,the as-received and oxidizedcarbonswere heated in inert (nitrogen)atmosphere at 623 K for 15 h prior to the Boehm titration. ESCA. A V.G. ScientificESCA 3 MK I1instrument was used when performing X-ray photoelectron spectroscopy (XPS) measurements. An Al K a (1486.6 eV) X-ray sourceprovidedthe excitation radiation. The unit was operated at a constant power level of 200 W, and the vacuum in the analyzer's section of the instrument was maintainedat 10-8Torr. Sampleswere mounted for 45O incidentradiation, and under these conditionsthe sample penetrationdepth was of the order of M A . Chemicalanalyses were performed by determiningthe position and area under the (XPS)peaks. For all high-resolution ( X P S )spectra, the carbon binding peak of 285.0 eV was establishedas an internal reference to set the calibration scale. No charging effects were observed for these samples.
Results and Discussion Chemical oxidation of activated carbons with nitric acid causes significant changes in carbon surface chemical properties. As a result of this treatment different types of acidic groups (carboxyl, phenolic, and lactonic) are created on the carbon surface.6 In Figure 1we show the total number of acidic groups obtained from the Boehm titration method. It is seen that oxidation increases the number of acidic groups for all carbons, and this effect is stronger for higher oxidation temperatures. The heat treatment of oxidized carbonsin inert atmosphere reduced the number of acidic groups. The surface of Norit which was oxidized at room temperature and then heat treated almost returned to its initial content of acidic groups (Figure 1). Heat treatment of carbons oxidized at higher temperatures considerably decreased the number of their acidic groups which, however, remained larger than that in their initial state. In Figure 2 we present examples of ESCA spectra obtained for Norit and Westvaco carbons. It is seen that
Bandosz et al.
2520 Langmuir, Vol. 9, No. 10,1993 0
structures. The behavior upon treatment is also different 3 0 0 for different carbons. In the case of Norit and Microsorb
WESTVACO
NORIT
MICROSORB
Figure 1. Number of acidic groups of investigated carbons. the effect of oxidation is stronger in the case of Westvaco carbon. After oxidation at room temperature, the main carbon peak of this sampleshiftsto higher binding energies and begins to lose its graphitic character. The more aggressive oxidation results in the more pronounced oxide region of the spectrum for Westvaco, which is related to different carbon oxide g r 0 u ~ s . l ~ Similar effects were observed for oxidized carbons fibers6J6J7 and activated carbons.la The effect of surface acidic groups on adsorption of specifically interacting molecules has been studied using alkene molecules.lOJ1 A correlation was foundlo between the number of these groups and the specific interaction parameter, e,, defined as a difference in the adsorption free energies of an alkaneand alkenewith the samenumber of carbon atoms. It is interesting to study the effect of surface groups on adsorption of alkanes which represent nonspecific adsorbates. In the case of our results which are obtained for microporous carbons the effect of microporosity should be taken into account. It is well known that the adsorption potential in fine micropores is enhanced as compared to the flat surface.1g As a result of this potential enhancement, elevated initial heats of adsorption were observed for microporous carbons.20921 The measured free energy of adsorption a t infinite dilution is also related to the adsorption potential, and therefore the incremental quantity AGOCH~ which represents the free energy of adsorption of a CH2 group is dependent on the pore structure. In Figures 3 and 4 we present results obtained from infinite dilution chromatography. In order to quantify the effect of the enhancement of adsorption energy in micropores, we construct a relative quantity which is the ratio of the measured AG"CH~on our activated carbons and AGOCH~obtained for nonporous carbon black (3.52 kJ/mo1),22both at 473 K. The variation of this relative quantity as a function of the number of acidic groups is shown in Figure 3. The enhancement of AGOCH, values due to the adsorption potential in micropores is of the order of 2 which correspondsto -7 kJ/mol of the absolute value of the AGocw2 parameter. Such high values of this parameter have already been reported for activated carbons.23 The individual differences between the untreated carbons are ascribed to their different micropore (16) Kozlowski, C.; Sherwood, P. M. A. Carbon 1986,24, 357. (17) Nakayama, Y.; Soeda, F.; lehitani, A. Carbon 1990,28, 21. (18) Papirer, E.; Guyon, E. Carbon 1978,115, 133. (19) Everett,D. H.; Powl, J. C. J. Chem. Soc., Faradoy Trans. I , 1976, 72, 619. (20) Stoeckli, F. Helv. Chim. Acta 1974,57, 2192. (21) Carrott, P. J. M.; Sing, K. S. W . J. Chromatogr. 1978,406,139. (22) Eisen, 0.G.; Kieelev, A. V.; Pilt, A. E.;Rang, S. A.; Shcherbakova, K. D. Chromatographia 1971,4,448. (23) Domingo-Garcia,M;Fernandez-Morales,I,;Lopez-Ganon,F. J.; Moreno-Caatilla,C.;Prados-Ramirez,M. J. J. Colloid Interface Sci. 1990, 136, 160.
carbons the decreasing relationship between the number of acidic groups and the AG0cH2parameter is almostlinear. It is seen from Figure 3 that for these carbonsthe oxidation decreases the value of the AGOCH~parameter while subsequent heat treatment causes the reverse effect. The fact that the points for oxidized as well as heat treated Norit and Microsorb carbons lie on the same lines suggests that the micropore structure does not change significantly during their modifications. If this were otherwise, after heat treatment, we should obtain a deviation from the linear behavior due to the influence of changed microporosity. Indeed, in the cam of Westvaco the points for heat-treated samples lie above the line consisting of points related to the initial and oxidized samples. The elevated values of AGOCH~for Westvaco samples, which were heat treated after oxidation, indicate that some structural modification resulting in the creation of small pores took place. This observation when combined with ESCA results, which showed changes in the chemical structure of Westvaco carbon, indicates that this carbon undergoes complex structural changes after oxidation. Since our major concern here is the effect of surface chemical groups on adsorption energetics, we present in Figure 4 the variation of hexane adsorption enthalpies, AHo,as a function of the number of acidic groups only for Norit and Microsorb carbons whose micropore structures did not change during oxidation on the basis of our analysis of the AG0cwa results. We obtain a linearly decreasing relationship between the absolute value of AHo and the number of acidic groups whether the carbon sample was oxidized or obtained by subsequent heat treatment from the oxidized sample. The reversible character of this relationship is consistent with the results presented in Figure 3. The values of AHo for untreated samples of Norit and Microsorb are 1.8 and 1.6 times higher than AHo of hexane adsorption on nonporous carbon black (43 kJ/mo1).22A similar enhancement in AHo for microporous carbons compared to carbon black has been reported in the literature for various adsorbates and attributed to the enhanced adsorption potential in fine micropores.1S-21For hexane, in particular, the reported absolute values of AHo for microporous carbons were in the range of 70-90 kJ/ mo1.21 As an extension of infinite dilution chromatography we have recorded chromatographic peak profiles for liquid solute injections. We have found that under our experimental conditions and for injection sizes of hexane and hexene up to 90 p L the peak profiles overlapped. The resulting isotherms calculated by integration of a single peak profile according to eq 4 were in the relative pressure range below 0.005, which corresponds to a low relative surface coverage. The isotherms measured at different temperatures were used to calculate the isosteric heat of adsorption, Qet(V), as a function of the amount adsorbed. A convenient way to do it is to fit experimental isotherms a t different temperatures by a virial-typethermal equation of adsorption24 which is written in the following form:
where cji are the best fit parameters. This equation was derived under the assumption that in a limited range of temperatures the isosteric heat of adsorption is temperature invariant which is in fact the common basic (24) Czepireki, L.; JagidO, J. Chem. Eng. Sci. 1989,44,797.
Langmuir, Vol. 9, No.10, 1993 2521
Energetic Heterogeneity of Activated Carbons I
I
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l
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!
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'
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28 3
291 287 BINDING ENERGY, aV
Figure 2. ESCA (Cls) spectra: (A) Norit, (B) Westvaco carbons. Samples: as received (- - -), oxidized at 293 K (- - -), oxidized at 323 K (---). 2.5 0 U
0
2.3. 2.1
-
-
NORlT N0RIT.R WESTVACO
P 0.6 0
E 0.5
I
A
p 0.4 %4 0.3 m 5 0.2
MICROSORB-R
-
1.9-
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603 K Open symbols - Norit as received
E" 4 0.1
1.7. 1.54
0
0.7 ,
Filled symbols - Norit oxidized - Virial equation fit
0
100
200
300
Number of acidic groups [meq/lOOg]
NORlT
A MICROSORB A MICROSORB-R
75
20
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In Figure 5 we show hexane adsorption isotherms for Norit as-received and oxidized samples. In this figure we also present the calculated curves obtained from the virial equation. It is seen that the isotherms of the oxidized sample lie slightly below those obtained for this carbon as received. A similar situation was observed in the case of hexene adsorption isotherms on these samples. According to the Clausius-Clapeyron law, the isosteric heat of adsorption follows directly from eq 5:
-)
wn
I 100
200
300
Number of acidic groups [meq/lOOg]
Figure 4. Variation of AHo of hexane adsorption of Norit and Microsorb versus the number of acidic groups. The notation is as in Figure 3.
assumption for the calculation of isosteric heats from adsorption data. It is seen that eq 5 obeys Henry's law forp 0. The order of polynomials, nl and n2, is adjusted on the basis of numerical analysis of fitting the equation to experimental data. In our case we have found a sufficient goodness of fit (error less than 3% ) for nl = 3 and n2 = 0.
-.
100
Figure 5. Hexane adsorption isotherms on Norit and Norit-A.
Qst = -R( d h P
0
80
60
Pressure [Torr]
Figure 3. Variation of AGoanverswthe number of acidicgroups. A, B, and C denote temperaturesof oxidation (293,323,and 361 K respectively), 0 denotes the carbon sample as received, and open symbols correspond to the samples which were heat treated after oxidation. 0
0
= -R&cljV .
(6)
a-
The calculated variations of Qat of hexane and hexene for Norit samples versus V in the region where experimental data were available are presented in Figure 6. The monotonic decline of Qat reflects the energetic heterogeneity of these samples. It is also seen that the isosteric heats on the oxidized sample are lower than that on the as-received one, which is consistent with the results obtained at infinite dilution. This difference is, however, smaller in the case of hexene adsorption. These observations should be considered in terms of two effects. One is the decrease in nonspecific hydrocarbon interactions with oxidized carbon surfaces which is seen in the case of hexane adsorption, and the other is the contribution of the specific interaction of the alkene ?r bond with surface acidic sites which contributes to the former effect in the case of hexene adsorption.
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2622 Langmuir, Vol. 9, No.10,1993
variations of QBtversusthe amount adsorbed for as-received and oxidized samples (Figure 6)suggest that sites (pores) of different adsorption energies are uniformly affected by surface chemical groups generated during the oxidation process.
-
Hexene/oxidized
- Hexane/Carbon black 0
01
0.2
0.3
04
0.5
0.6
0.7
Amount adsorbed [mmol/g]
Figure 6. Isosteric heat of hexane adsorptionon Norit and NoritA.
The fact that oxidation decreases the dispersive gassolid interactions as shown by decreases in both AGOCH~ and QBt cannot be explained on the basis of any specific interaction of alkanes which are nonspecificallyinteracting molecules. It may be attributed to the fact that the oxidized carbon surface is covered by different chemical species which may hinder and/or perturb occupancy of adsorbed molecules on their energetically most favorable positions. Furthermore, parallel lines corresponding to
Conclusion Analysis of the results of alkane adsorption on carbon samples subjected to controlled oxidation, where the pore structure was not affected, shows that adsorption energies of nonspecific interactions may be changed (decreased) by the presence of surface chemical groups. These groups may hinder and/or perturb occupancy of adsorbed molecules on their energetically most favorable positions. This finding has the i m p o h t implication that surface chemistry should be taken into account when pore structure is studied by adsorption methods, especially when the number of surface groups is relatively high. Acknowledgment. The work was supported by the New York State Energy Research and Development Authority under Contract 139-ERER-POP-90. The authors wish to acknowledge ESCA results from Brookhaven National Laboratories and helpful discussions with Dr. J. Wegrzyn.
