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Langmuir 2005, 21, 7752-7759
Importance of Structural and Chemical Heterogeneity of Activated Carbon Surfaces for Adsorption of Dibenzothiophene Conchi O. Ania† and Teresa J. Bandosz*,†,‡ Department of Chemistry, The City College of New York, and The Graduate School of the City University of New York, New York, New York 10031 Received March 23, 2005. In Final Form: June 8, 2005 The performance of various activated carbons obtained from different carbon precursors (i.e., plastic waste, coal, and wood) as adsorbents for the desulfurization of liquid hydrocarbon fuels was evaluated. To increase surface heterogeneity, the carbon surface was modified by oxidation with ammonium persulfate. The results showed the importance of activated carbon pore sizes and surface chemistry for the adsorption of dibenzothiophene (DBT) from liquid phase. Adsorption of DBT on activated carbons is governed by two types of contributions: physical and chemical interactions. The former include dispersive interactions in the microporous network of the carbons. While the volume of micropores governs the amount physisorbed, mesopores control the kinetics of the process. On the other hand, introduction of surface functional groups enhances the performance of the activated carbons as a result of specific interactions between the acidic centers of the carbon and the basic structure of DBT molecule as well as sulfur-sulfur interactions.
Introduction Sulfur compounds constitute a leading cause of fuel fossils contamination. Besides being converted to sulfur oxides during combustion, thereby contributing to air pollution and acid rain,1 they have been shown to poison catalytic converters. Thus, when the sulfur content is lowered, an increase in the catalyst lifetime is achieved, along with a decrease in the amount of SO2 released to the atmosphere. Nowadays, deep desulfurization of fuels (gasoline and diesel fuels) is receiving increasing attention in the research community worldwide in order to satisfy the upcoming environmental regulations and fuel specifications. The U.S. Environmental Protection Agency (USEPA) has issued regulations that require a drastic cap for sulfur content in highway diesel from the current level of 500 ppmw to no more than 15 ppmw by 2006.2,3 The sulfur content of gasoline has to be reduced from a current average of 300 to 30 ppmw by 2006.4 In Europe, the former level of 350 ppmw has been decreased down to 50 ppmw since the beginning of 2005 and will be further lowered down to 10 ppmw by 2009.5,6 These requirements to produce “zero sulfur” fuels impose significant efforts in current desulfurization methods and in the development of new technologies. The conventional approach for desulfurization and deep desulfurization of fuel feedstocks in petroleum refining is based on a catalytic process called hydrodesulfurization (HDS).7 This technology is able to satisfy the current * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: (212) 650-6017. Fax: (212) 650-6107. † Department of Chemistry, The City College of New York. ‡ The Graduate School of the City University of New York. (1) Ho, T. C. Catal. Rev. 1988, 30, 117l. (2) U.S. EPA. Heavy-Duty Engines and Vehicle Standards and Highway Diesel Sulfur Control Requirements, January 2001. (3) U.S. EPA, Tier 2 Act, 1998. (4) U.S. EPA, Control of Air Pollution from New Motor Vehicles Amendment to the Tier-2/Gasoline Sulfur Regulations, April 2001. (5) Directive 98/70/CE, DOCE 350/L, 28/12/98. (6) Directive 2003/17/CE, DOCE 076/L, 22/03/2003. (7) Satterfeld, C. N. In Heterogeneous Catalysis in Industrial Practice; McGraw-Hill: New York, 1991; p. 378.
regulations. The HDS method used on the industrial scale is based on a catalytic reaction (usually on Co-Mo or NiMo catalysts) with hydrogen, to convert the organo-sulfur species into sulfur-free organic compounds.8 HDS requires high temperatures, high pressures, and large hydrogen consumption. To comply with the new regulations, this approach faces an important challenge: polyaromatic sulfur-containing compounds such as benzothiophene (BT), dibenzothiophene (DBT), and its alkyl derivatives (i.e., 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene) are difficult to remove. A deepest HDS requires the use of very high temperatures, pressures, and quantities of hydrogen, large reactor volumes, large quantity of active catalysts, and so forth. All of these involve an unavoidable and significant capital investment, thereby drastically increasing the costs of fuels.9 Another approach, the so-called Z-Sorb process for sulfur selective removal,10 also encounters difficulties with the most HDTrecalcitrant compounds as DBT and alkyl derivatives commonly present in gas oil and diesel. Therefore, extensive research is carried out to propose alternative technologies to obtain low-sulfur fuels. Among them, the selective adsorption of sulfur-containing compounds seems particularly interesting. This approach is an attractive field of research11-16 owing to such advantages as being a low-energy demanding process and having the availability of regeneration of the spent adsorbent and broad availability of adsorbents. The difficulty settles on finding an adsorbent that selectively adsorbs the sulfur compounds, but does not adsorb (or only weakly adsorb) (8) Whitehurst, D. D.; Isoda, I.; Mochida, I. Adv. Catal. 1998, 42, 345. (9) Speight, J. G. In The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1998. (10) Sughrue, E. L.; Khare, G. P.; Bertus, B. J.; Johnson, M. M. U.S. Patent 6.254.766, 2001. (11) Ma, X.; Sun, L.; Song, C. Catal. Today 2002, 77, 107. (12) Kobayashi, M.; Shirai, H.; Nunokawa, M. Energy Fuels 2002, 16, 1378. (13) Song C.; Ma, X. Appl. Catal. B 2002, 41, 207. (14) Yang, R. T.; Herna´ndez-Maldonado, A. J.; Yang, F. H. Science 2003, 301, 79. (15) Velu, S.; Ma, X.; Song, C. Ind. Eng. Chem. Res. 2003, 42, 5293. (16) Haji, S.; Erkey, C. Ind. Eng. Chem. Res. 2003, 42, 6933.
10.1021/la050772e CCC: $30.25 © 2005 American Chemical Society Published on Web 07/19/2005
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the coexisting aromatic hydrocarbons and olefins that are also present in the fuel in a large excess (i.e., their concentration usually exceeds 20 wt % in comparison with less than 1 wt %). Thus, a major challenge is to meet new sulfur specifications along with maintaining the aromatics contents of the fuels (octane number). The objective of this research is to acquire a deeper insight into the mechanism of desulfurization via reactive adsorption on carbonaceous materials. So far, this subject has not been well-described in the literature.13-16 To attain this goal, the adsorption capacity of activated carbons with different textural and chemical characteristics was investigated for the removal of sulfur-containing compounds from liquid hydrocarbon solutions. To study the performance of the carbonaceous materials, DBT dissolved in hexane was used as a model for the refractarious sulfurcontaining compounds present in the fuels. The effects of changes induced both in the texture and in the surface chemistry of the activated carbons were studied in relation to the changes in the capacity of the adsorbents to retain sulfur. Experimental Section Materials. Four activated carbons were chosen for this study. They are as follows: BX (Westvaco, wood-based, H3PO4 activation), BP (Calgon, bituminous coal-based, physical activation); PC (PET-waste derived, CO2 activation); PS (polystyrene polymerderived carbon). The procedures for the preparation of both PC and PS have been described elsewhere.17,18 To increase the surface heterogeneity, BP carbon was oxidized with (NH4)2S2O8 as follows. The sample was treated with a saturated solution of ammonium persulfate in 4 N sulfuric acid [ratio 1:10] and left overnight. The oxidized sample was washed with distilled water and dried at 373 K. It is referred to as BPS. Before the experiments, carbons were washed in a Soxhlet apparatus with distilled water. The carbon was then dried and the tests for adsorption from liquid phase were carried out. Methods. Adsorption of DBT from Solution. Adsorption of DBT was carried out at room temperature in a stirred batch system. Before these experiments, the kinetic studies were performed to determine the equilibration time of the system. Different amounts of carbons (from 25 mg to 1 g) were weighed and added to 15 bottles containing 40 mL of the sulfur-containing solution with an initial concentration of 1000 ppmw of DBT (ca. 178 ppmw of S). All of the solutions were prepared in hexane. The covered bottles were placed in a shaking bath and allowed to shake for 72 h at a constant temperature. After equilibration the concentration in the liquid was determined using a UV spectrophotometer at the corresponding wavelength. The amount adsorbed was calculated from the formula qe ) V(Co - Ce)/m, where qe is the amount adsorbed, V is the volume of the liquid phase, Co is the concentration of solute in the bulk phase before it comes into contact with the adsorbent, Ce is the concentration of the solute in the bulk phase at equilibrium, and m is the amount of the adsorbent. The equilibrium data was fitted to the so-called LangmuirFreundlich single solute isotherm,19 which has the equation:
ϑt ) qe/qo )
(KC)
n
1 + (KC)n
where qe is the adsorbed amount of the solute per unit gram of adsorbent, qo is its maximum adsorption per unit weight of the adsorbent, K is the Langmuir-type constant defined by the Van’t Hoff equation, and the exponential term n represents the heterogeneity of the site energies. The fitting range was from 0 to 250 mg of S/g of activated carbon (recalculated from its content in DBT). (17) Parra, J. B.; Ania, C. O.; Arenillas, A.; Pis, J. J. Stud. Surf. Sci. Catal. 2002, 144, 537. (18) Hines, D.; Bagreev, A.; Bandosz, T. J. Langmuir 2004, 20, 3388. (19) Derylo-Marczewska, A.; Jaroniec, M. Chem. Scr. 1984, 24, 239.
Langmuir, Vol. 21, No. 17, 2005 7753 Textural Characterization. Textural characterization was carried out by measuring the N2 adsorption isotherms at 77 K. Before the experiments, the samples were outgassed under vacuum at 393 K. The isotherms were used to calculate the specific surface area, SBET, total pore volume, VT, and pore size distributions. The pore size distributions were evaluated using density functional theory (DFT).20 Micropore distributions were divided into narrow microporosity (pore diameter, w < 0.7 nm) and medium-sized microporosity (0.7 < w < 2 nm). This subdivision of microporosity based on DFT calculations, although not in strict accordance with that established by IUPAC, is widely accepted.21 Thermal Analysis. Thermal analysis was carried out using a TA Instrument thermal analyzer. The instrument settings were heating rate 10 K/min and nitrogen atmosphere with 100 mL/ min flow rate. For each measurement about 25 mg of a ground carbon sample was used. Potentiometric Ttitration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm, Brinkmann Instruments, Westbury, NY). The instrument was set in the equilibrium mode when the pH was collected. Approximately 0.100-g samples were placed in a container thermostated at 298 K with 50 mL of 0.01 M NaNO3 and equilibrated overnight. To eliminate any interference by dissolved CO2, the suspension was continuously saturated with N2. The carbon suspension was stirred throughout the measurement. Each sample was titrated with 0.1M NaOH titrant using 0.001mL increments. Experiments were carried out in the pH range 3-10.22,23 Boehm Titration. The oxygenated surface groups were determined according to the method of Boehm.24 One gram of carbon sample was placed in 25 mL of the following 0.05 N solutions: sodium hydroxide, sodium carbonate, sodium bicarbonate, and hydrochloric acid. The vials were sealed and shaken for 24 h and then 5 mL of each filtrate was pipetted and the excess of base and acid was titrated with HCl and NaOH, respectively. The number of acidic sites was calculated under the assumption that NaOH neutralizes carboxyl, phenolic, and lactonic groups; Na2CO3- carboxyl and lactonic; and NaHCO3 carboxyl groups. The number of surface basic sites was calculated from the amount of hydrochloric acid. Surface pH. The pH of a carbon sample suspension provides information about the acidity and basicity of the surface. A sample of 0.4 g of dry carbon powder was added to 20 mL of water and the suspension was stirred overnight to reach equilibrium. Then the sample was filtered and the pH of solution was measured. XRF Analysis. X-ray fluorescence analysis was applied to study the content of sulfur in the carbons after DBT adsorption. For this purpose, SPECTRO Model 300T Benchtop Multi-Channel Analyzer from ASOMA Instruments, Inc. was used. It contains a titanium (Ti) target X-ray tube with Mo-2mil filter and highresolution detector with a filter. A home-developed method was selected to identify the sulfur and acquisition conditions were the following: voltage 9.0 kV, current 280 µA, count time 100 s, and warm-up 3 min. Instrument reference temperature was 293 K and background conditions were lower ROI 3200 and upper ROI 5750 keV. The amount of sulfur in the exhausted carbons was determined based on a calibration curve of sulfur in a carbonaceous matrix.
Results and Discussion Detailed characteristics of the pore structure of the activated carbons studied are presented in Table 1. Analysis of the data indicates differences in the porosity of the adsorbents. All the samples, except BX and PS, exhibit type I nitrogen adsorption isotherms of the BDDT classification.25 This indicates that the carbons are mainly (20) Olivier, J. Carbon 1998, 36, 1469. (21) Bandosz, T. J.; Biggs, M. J.; Gubbins, K. E.; Hattori, Y.; Liyama, Y.; Kaneko, K.; Pikunic, J.; Thomson, K. T. In Chemistry and Physics of Carbons; Marcel-Dekker: New York, 2003; Vol. 28, p 41. (22) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1994, 32, 1026. (23) Jagiello, J.; Bandosz, T. J.; Putyera, K.; Schwarz, J. A. J. Colloid Interface Sci. 1995, 172, 341. (24) Boehm, H. P. Adv. Catal. 1960, 16, 179.
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Figure 1. Nitrogen adsorption isotherms at 77 K on the carbons studied. Table 1. Structural Features of the Carbon Studied, Evaluated from N2 Adsorption Isotherms at 77 K and the DFT Method
Figure 2. Pore size distributions (PSD) for the initial carbons.
DFT method SBET VT Vnarrow micropores Vmedium-micropores Vmesopores [m2 g-1] [cm3 g-1] [cm3 g-1] [cm3 g-1] [cm3 g-1] PS PC BX BP BPS
1737 1049 2271 1540 1345
0.974 0.410 1.285 0.712 0.603
0.146 0.092 0.072 0.051 0.103
0.368 0.245 0.431 0.366 0.269
0.288 0.051 0.765 0.292 0.230
microporous (Figure 1). In the case of PS, a hysteresis loop is observed at high relative pressures due to the presence of mesopores. Despite the upward swing at these p/po, the isotherm for this sample still shows a well-defined plateau characteristic for type I isotherm. On the contrary, the BX sample reveals a gradual increase in the slope of the isotherm at relative pressure, p/po > 0.3. This suggests a type IV isotherm and indicates textural heterogeneities. The conclusions obtained based on the shape of the nitrogen adsorption isotherms at 77 K are in good agreement with the pore size distributions calculated using the DFT method (Figure 2). Both PS and PC have large volumes of narrow micropores, while in BP and BX medium-size micropores and mesopores are predominant, especially for the latter sample. It is known that oxidation of activated carbons with ammonium persulfate is a mild treatment that usually preserves the physical morphology and textural properties for the majority of activated carbons.26,27 This treatment was applied only for BP commercial carbon due to the instability of the textural features of BX. The oxidized sample, BPS, showed a slight decrease in the nitrogen uptake, as compared to the initial counterpart, indicating minor modification in the pore volume and pore sizes. The apparent BET surface area and total pore volumes decreased by 13 and 15%, respectively, whereas the volume of medium-size micropores decreased by 26% (Table 1). On the other hand, the narrow microporosity increased 2-fold after oxidation, in comparison with the unmodified carbon. The changes in surface chemistry after oxidation were evaluated by potentiometric titration. The distributions (25) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723. (26) Ania, C. O.; Parra, J. B.; Pis, J. J. Adsorp. Sci. Technol. 2004, 22, 337. (27) Moreno-Castilla, C.; Carrasco-Marı´n, F.; Maldonado-Ho´dar, F. J.; Rivera-Utrilla, J. Carbon 1998, 36, 145.
Figure 3. pKa distributions for species present on the surface of the carbons studied. Table 2. pH and Number of Surface Groups [mequiv/g] Evaluated from Boehm Titration sample
pH
carboxylic
lactonic
phenolic
total acidic
total basic
PS PC BX BP BPS
3.9 6.9 6.5 7.8 3.4
0.345 0 0.243 0 0.923
0.365 0.099 0.140 0.025 0.764
0.503 0.365 0.347 0.163 0.454
1.213 0.464 0.730 0.188 2.141
0.298 0.543 0.363 0.450 0.472
of acidity constants of species present on the surface (f(pKa)), obtained by the numerical SAIEUS procedure (solution of adsorption integral equation using splines),28 are shown in Figure 3. The oxidized BP sample, BPS, becomes more acidic, as demonstrated from Boehm titration results and the surface pH value (Table 2). The differences in the surface chemistry before and after oxidation are clearly pronounced on the distribution curves. The oxidized carbon shows more peaks representing a high degree of surface chemical heterogeneity. The intensities of all the peaks also increased after oxidation, which indicates that the amount of both acidic and basic groups is larger than those in the unmodified carbon. An increase in the number of acidic groups is observed all over our experimental window (28) Jagiello, J. Langmuir 1994, 10, 2778.
Hetrogeneity of Activated Carbon Surfaces
Figure 4. Examples of equilibration curves for the adsorption of DBT.
of pKa, indicating the formation of carboxylic acids (pKa lower than 7),29,30 as well as oxygenated basic functional groups (range of pKa >9)22,31 such as quinones and pyronelike structures. The surface chemistry of the carbons was also analyzed using thermal analysis. It is generally accepted that as a result of heat treatment oxygen-containing functional groups decompose into CO and CO2 and thus the weight loss can be linked to the chemical functionalities present on the surface. Evolution of CO2 between 473 and 873 K is due to decomposition of carboxylic acid whereas phenols and other basic groups (quinones and pyrone-like structures) decompose as CO at temperatures higher than 873 K.32 Based on the relative intensity of the peaks, the oxidation caused an increase in weight loss, due to decomposition of the oxygen functionalities introduced to the carbons surface during treatment with ammonium persulfate. The weight loss at T < 600 K indicates the decomposition of carboxylic acids while the weight loss at T > 800 K is related to the presence of phenols and other oxygen-based groups.32 These results are in good agreement with the pKa distributions obtained from potentiometric titration. In the case of BX, peaks at high temperatures (i.e., higher than 773 K) are linked to the technology of this production (low-temperature carbonization) and thus cannot be considered for interpretation of our results.33 The adsorptive capacity of low-ash content activated carbons obtained from polymeric precursors (PC and PS) for the removal of DBT from liquid hydrocarbon solution was compared with that of commercial carbons (BP and BX). Adsorption kinetics experiments were carried out to determine the time needed to reach equilibrium. As shown in Figure 4, it takes around 3 days to reach equilibrium for all the carbons tested, regardless of their nature. The concentration decay curves show different steps on the kinetic uptake, which suggests different mechanisms of adsorption/reactive adsorption. The porous structure and, more specifically, the mesoporosity of the carbons were found to play an important role in the kinetics of adsorption. The declining trend of sulfur removal was significantly much faster for the carbon BX, while BP and (29) Kortum, G.; Vogel, W.; Andrussow, K. In Dissociation Constants of Organic Acids in Aqueous Solutions; Butterworth: London, 1961. (30) Perdue, E. M.; Reuter, J. H.; Parrish, R. S. Geochim. Cosmochim. Acta 1984, 48, 1257. (31) Bandosz, T. J.; Jagiello, J.; Contescu, C.; Schwarz, J. A. Carbon 1993, 31, 1193. (32) Otake, Y.; Jenkins, R. G. Carbon 1993, 3, 109. (33) Jagtoyen, M.; Derbyshire, F. Carbon 1998, 36, 083.
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Figure 5. Adsorption isotherms for DBT at room temperature on the activated carbons studied. Symbols represent experimental data and solid lines fitting to LF equation.
PC have similar adsorption rates. These differences are linked to the mesopore volumes of the carbons, which seem to enhance the kinetics of the adsorption process. A welldeveloped mesoporosity in the BX carbon clearly increases the adsorption rate of DBT. As mentioned in the Experimental Section, the adsorption isotherms were obtained changing the absorbent dose and keeping constant the initial concentration of the adsorbate in solution. To avoid the effect of adsorbent dose on the adsorption capacity, the isotherms are plotted by normalizing the amount of the adsorbate in solution with the dose of adsorbent. Figure 5 illustrates the adsorption isotherms for DBT on the carbons studied. The shape of the DBT adsorption isotherms indicates that all of them are L type in the Giles classification.34 With an increase in the concentration of DBT in the solution (expressed as mg of sulfur/g), the amount adsorbed increases. A concavity toward the abscissa axis is displayed in all cases, indicating that as more sites in the substrate are filled, it becomes more difficult for a fresh solute molecule to find a vacant site. This implies either that the adsorbed solute molecule is not vertically oriented or that there is no strong competition with the solvent. At high equilibrium concentration of DBT in the solution, the adsorption isotherms show a tendency to reach the plateau (subgroups 1 and 2) while the amount adsorbed steadily increased. This suggests that the saturation limit is not attained. A great number of different isotherm equations were proposed in the literature to describe the single-solute adsorption from dilute solutions on energetically heterogeneous solids.19,35,36 As shown elsewhere,36 the adsorption equilibrium of single solutes from dilute solutions can be described by the so-called Langmuir-Freundlich (LF) equation. This equation presents the advantage versus the Freundlich and Langmuir model in its consideration of the nonuniformity of the energy of the solid surface (energetic heterogeneity of solid surfaces). Table 3 provides a compilation of the LF parameters obtained from fitting the experimental data to the model equation. The linear range and the correlation coefficients for the LF model are also summarized in this table. (34) Giles, C.; McEwan, T.; Nakhwa, S.; Smith, D. J. Chem. Soc. 1960, 3973. (35) Marczewski, A. W.; Derylo-Marczewska, A.; Jaroniec, M. J. Chem. Soc. Faraday Trans. 1988, 84, 2951. (36) Derylo-Marczewska, A.; Marczewski, A. W. In Adsorption Science and Technology: Proceedings 2nd Pacific Basin Conference Brisbane; Do, D. D., Ed.;World Scientific: Singapore, 2000; p 174.
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Table 3. Fitting Parameters of DBT Adsorption Isotherms to Langmuir-Freundlich Equation
PS PC BX BP BPS
qm [mg of S/g]
n
R2
65 54 53 47 82
0.37 0.35 0.35 0.36 0.30
0.9939 0.9996 0.9976 0.9975 0.9981
The results obtained from adsorption of DBT show that the activated carbons chosen for this study have favorable features for the removal of DBT from liquid solutions. Adsorptive capacities in the nonmodified carbons are much larger than those reported in the literature for activated carbons and zeolites37-41 In comparison, for Cu-Y zeolites a capacity of 18.9 mg of S/g was reported,39 and on more expensive materials such as carbon aerogels 15.1 mg of S/g was retained on the surface.16 The excellent goodness of the fit (in all cases correlation coefficients > 0.99 were attained) proved that the LF equation is suitable for application for the systems studied. The value of exponent n is related to the degree of carbons surface heterogeneity.42 Values close to 1 (where the equation becomes the general Langmuir model) mean that the surface is energetically homogeneous and all the active sites have the same energy toward DBT adsorption. On the other hand, values close to 0 indicate the presence of a wide distribution of energetic sites on the carbon surfaces. For all carbons studied, n < 1 values were obtained, which indicates a high degree of the surface heterogeneity from the point of view of adsorption energy. Moreover, exponent n decreased significantly for BP after oxidation (from an initial value of 0.36 for BP to 0.30 for BPS). This indicates that formation of surface functionalities results in the appearance of new energetic active sites for DBT adsorption. The lowest adsorption capacity was found for the BP sample, which has both high surface area and high total pore volume. In contrast, the PC sample showed a higher uptake than that of BP, despite its lowest surface area and total pore volume. These results suggest that the adsorption capacity is rather related to the pore size distribution than to the extent of the surface area. Thus, the adsorption capacity was correlated with the microporosity of the samples, and a linear correlation was found between the capacity and the volume of narrow micropores (w < 0.7 nm) (Figure 6). Taking into account the size of the DBT molecule (i.e., diameter 0.65 nm), these pores were chosen assuming the highest adsorption potential since the adsorbate molecule is able to enter them only in the parallel to the wall surface position. In the case of PS, the high uptake might be linked to the large volume of narrow micropores. Nevertheless, this uptake is much larger than that for PC, although the differences in the volume of narrow micropores are not very pronounced. This suggests that in the case of PS samples there is another specific contribution to DBT adsorption besides only pure physisorption. It is well-known that the surface chemistry plays a role in the adsorption processes, and both aspects (porous (37) Jiang, Z.; Liu, Y.; Sun, X.; Tian, F.; Xun, F.; Liang, C.; You, W.; Han, C.; Li, C. Langmuir 2003, 19, 731. (38) Richardeau, D.; Joly, G.; Canaff, C.; Magnoux, P.; Guisnet, M.; Thomas, M.; Nicolaos, A. Appl. Catal., A 2004, 263, 49. (39) Hernandez-Maldonado, A. J.; Yang, R. T. Ind Eng Chem Res. 2003, 42, 3103. (40) Mikhail, S.; Zaki, T.; Khalil, L. Appl. Catal., A 2002, 277, 265. (41) Hernandez-Maldonado, A. J.; Yang, R. T. Ind. Eng. Chem. Res. 2003, 42, 122. (42) Derylo-Marczewska, A.; Marczewski, A. W. Appl. Surf. Sci. 2002, 7845, 1.
Figure 6. Correlation between the adsorption capacities and the volume of narrow micropores (w < 0.7 nm).
structure and chemical nature) should not be considered separately. In fact, the adsorptive capacity of the oxidized carbon, BPS, significantly increased in comparison to the as-received sample, BP. This effect has also been reported by Jiang and co-workers after oxidation of activated carbons with sulfuric acid.37 Since the porosity of BP carbon does not change significantly after oxidation (Table 1), it is plausible to link the increase in the adsorption capacity to an increase in the number of surface oxygen groups formed on the surface as a result of oxidation. It also has to be taken into account that the volume of narrow micropores (w
