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Production of carbon molecular sieves by plasma treated activated carbon fibers☆
Fuel 82 (2003) 2045–2049 www.fuelfirst.com
Production of carbon molecular sieves by plasma treated activated carbon fibersq T. Orfanoudakia,b, G. Skodrasb,c, I. Doliosb, G.P. Sakellaropoulosa,b,* a
Chemical Process Engineering Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 1520, Thessaloniki 540 06, Greece b Laboratory of Solid Fuels and Environment, Chemical Process Engineering Research Institute, Thessaloniki, Greece c Centre for Solid Fuels Technology and Applications, Ptolemais, Greece Received 6 November 2002; accepted 27 February 2003; available online 11 June 2003
Abstract Carbon molecular sieves (CMS) are valuable materials for the separation and purification of gas mixtures. In this work, plasma deposition was used aiming to the formation of pore constrictions, by narrowing the surface pore system of commercial activated carbon fibers (ACF). For this reason propylene/nitrogen or ethylene/nitrogen discharges of 80 and 120 W were used. The molecular sieving properties of the plasma treated ACF were evaluated by measuring the adsorption of CO2 and CH4. The CO2/CH4 selectivity was significantly improved and depended on plasma treatment conditions (discharge gas and power). The optimum CO2/CH4 selectivity (26) was observed for C2H4/N2 plasma treated ACF at 80 W. Sample scanning electron microscopy (SEM) analysis after plasma treatment revealed an external film formation and X-ray photoelectron spectroscopy (XPS) analysis showed the incorporation of nitrogen functional groups in the film, which probably interact with CO2, thereby altering CO2/CH4 selectivity. q 2003 Elsevier Ltd. All rights reserved. Keywords: Plasma deposition; Activated carbon fibers; Carbon molecular sieves
1. Introduction Carbon molecular sieves (CMS) are high added value materials used in gas separation processes. Their unique ability to separate gases based on the different size and shape of molecules has been exploited in commercial applications such as pressure swing adsorption (PSA) [1 – 5]. The most important feature of CMS is their narrow pore size distribution accomplished either by controlled activation [6] or by employment of pore narrowing techniques on an inherent pore structure. This latter technique has been used on various carbon materials for the production of CMS suitable for air separation. Hu et al. [6] used 3-methylpentane as a source for carbon deposition on walnut shells, chemically activated by KOH. The best oxygen – nitrogen * Corresponding author. Address: Chemical Process Engineering Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 1520, Thessaloniki 540 06, Greece. Tel.: þ 302310-996271; fax: þ 30-2310-996168. E-mail address: [email protected] (G.P. Sakellaropoulos). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00172-8
separation selectivity reported in this study was 9.2. Freitas et al. [7] reported the modification of two activated carbons of different texture by the pyrolysis of benzene in an attempt to obtain CMS for O2/N2 separation. Their results showed that this objective can be attained when the carbon precursor has been activated only to a limited extent and when carbon deposition is carried out in the proper kinetic regime. Vyas et al. [8] obtained CMS by methane cracking on bituminous coal and coconut shells. The O2/N2 uptake ratio of the best CMS produced was 2.667. Cabrera et al. [9] described the preparation of CMS for air separation by a two-step hydrocarbon deposition with a single hydrocarbon. They found that the concentration of the carbon containing compound used in the first step should be larger than that of the second step, so that the pore openings of the micropores of the support narrowed gradually, avoiding pore plugging. CMS for CO2/CH4 separation has also been produced by the same method. Praseyto et al. [10] tailored the pore structure of activated carbon by benzene deposition, and improved the CO2/CH4 kinetic selectivity from 7 to 26. However, cobalt catalyst was used to enhance benzene
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cracking. Kawabuchi et al. [11] modified the pore size of several types of carbon adsorbents, suitable for CO2/CH2 separation, by chemical vapor deposition of benzene. They showed that pyrolytic carbon should be deposited only on the pore mouth in order to avoid pore plugging and to retain the CO2 adsorption capacity. Surface modification techniques, such as plasma, could be an alternative for CMS production. The unique ability of plasma to modify the surface of a material without changing the bulk properties is attractive for the modification of a carbon material to CMS by a one step treatment. Hydrocarbon plasma has been widely used to improve polymeric membrane efficiency [12,13]. However, studies concerning plasma deposition of a thin film on carbonaceous material in order to improve its molecular sieve properties are limited [14]. In this work commercial activated carbon fibers (ACF) based on phenol resin were modified by N2/propylene or N2/ethylene RF plasma discharges aiming to enhance the molecular sieve properties of the material. Plasma treated fibers were examined for possible surface modifications and for their selectivity towards adsorption of CO2 and CH4 gases.
energies of the analyzer was calibrated according to the ASTM-E 902-88 standard method. Scanning electron microscopy (SEM) was also used for surface examination of the initial and the plasma treated ACF. The molecular sieving properties of samples were evaluated by measuring the adsorption of CO2 and CH4, volumetrically under ambient conditions.
3. Results and discussion 3.1. BET surface area measurements The N2 adsorption isotherm of untreated carbon fibers, Fig. 1, is of type I, according to BDDT classification and corresponds to a microporous material. Untreated samples have a BET surface area of 650 m2/g. Plasma treated carbon fibers gave negligible N2 adsorption. This indicates that the film deposited on ACF surface, due to plasma treatment, reduced significantly the surface pore entrance, possibly in the range of molecular dimensions. Hence, N2 is probably kinetically restricted to enter such narrow pores, and to diffuse in the interior pore structure of the ACF. Such a behavior is not unusual in CMS [6,15].
2. Experimental 3.2. Raman spectra The deposition apparatus used in this study consisted of a quartz reactor, an RF generator with an impedance matching network and a mechanical pump. The reactor was a quartz cylinder 1 m long, 65 mm diameter placed coaxially through a working coil. The coil was made of 9 turns of 1=4 in diameter copper tube. RF power was supplied from a 1 KW, 13.56 MHz generator. The system was also equipped with a water circulation unit, necessary for cooling the various RF plasma components. About 35– 40 mg of ACF (FR-10, Kuraray Chemical Co.) were introduced in the reactor in the middle of the coil and sealed therein by vacuum flanges. After system evacuation, a gas mixture, of 20% hydrocarbon and 80% nitrogen was introduced in the reactor, and plasma was ignited. The sample was always treated for 15 min at an 80 or 120 W plasma power. Since no additional heating was employed, the temperature of the ACF rose only by inductive heating and energy transfer from the plasma. The raw and treated ACF were characterized by N2 adsorption at 77 K, from which their BET surface area was estimated using the BET multiple point equation. Raman spectroscopy was employed to characterize the type of carbon– carbon bonds before and after plasma treatment. The 514 nm line spectra of an Arþ laser was used for excitation. XPS analysis was also employed to characterize the functional groups on the ACF surface before and after plasma treatment. The ionizing radiation, Mg Ka, was provided by a non-monochromatic X-ray source with characteristic energy 1253.6 eV. The range of kinetic
The Raman spectrum of the untreated carbon fibers is shown in Fig. 2(a). It consists of two peaks, characteristic of a graphite structure [16]. The presence of the Raman peak at 1350 cm21, in addition to the main one at 1580 cm21, suggests that small crystals are present [16]. In the Raman spectra of the plasma treated fibers the two peaks were replaced by a continuous line, Fig. 2(b). A similar result was also obtained by Hayashi and his co-workers, although the reason for such a behavior is still unclear [17].
Fig. 1. Nitrogen adsorption isotherm of commercial activated carbon fibers (FR-10) at 77 K. No nitrogen adsorption for plasma treated ACFs.
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Fig. 2. Raman spectrum of commercial activated carbon fibers (FR-10) (a) before plasma treatment (b) after plasma treatment.
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Fig. 4. XPS N1s spectrum of commercial activated carbon fibers treated by propylene–nitrogen plasma at 80 W.
3.3. XPS analysis A typical XPS spectrum of raw ACF, before plasma treatment, is shown in Fig. 3. Similar spectra were obtained with plasma treated ACF. The main C1s peak of all samples can be deconvoluted to three components at around 284.6, 286 and 288 eV, Fig. 3, which probably correspond to C –C (sp2 or sp3), C –OH or bridged – CyO – H – OyC – , and COOH or COOR [18 –20]. A fourth peak, at the highest binding energy, is attributed to the filter used as substrate for the XPS analysis. However, C – N bonds show quite similar binding energies, 286– 288 eV [19]; hence, the assignment of peaks at 286 and 288 eV to C –O bonds, based on C1s spectra, is ambiguous. For this reason, N1s spectra of all samples were also obtained. Raw fibers before plasma treatment showed no N1s spectra, therefore, the peaks observed from 286 to 288 eV in the C1s spectra, Fig. 3, can be assigned to C – O groups as discussed earlier. The N1s XPS spectra of plasma treated samples are shown in Figs. 4 and 5. All N1s spectra after deconvolution show characteristic peaks around 399 and 400 eV, which correspond to pyridine and pyrole nitrogen groups, respectively, [21 –24]. The presence
Fig. 3. XPS C1s spectrum of raw (untreated) commercial activated carbon fibers (FR-10).
of NH2 groups cannot be excluded, whose characteristic binding energy is 399.3 eN [18,24]. In addition, to these two peaks, the N1 s spectrum of ethylene – nitrogen plasma treated ACF shows one additional peak at 402 eV, Fig. 5. The binding energy of this latter peak corresponds to oxidized nitrogen forms [24]. These results suggest that nitrogen present in the feed stream reacts and remains on the sample surface after plasma treatment. 3.4. SEM surface examination Fig. 6, shows the surface of the ACF, as observed by SEM. In Fig. 6(a) the pore structure of ACF surface can be identified, while Fig. 6(b) demonstrates that the fiber surface is rough and contains defects. From Fig. 6(c), it is evident that a thin film was deposited on the activated carbon surface after propylene –nitrogen plasma treatment. The film thickness along the fiber varies between 150 and 300 nm.
Fig. 5. XPS N1s spectrum of commercial activated carbon fibers treated by ethylene–nitrogen plasma at 80 W.
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Fig. 7. CO2 and CH4 uptake curves of commercial ACF treated by ethylene – nitrogen and propylene – nitrogen plasma at 80 W. closed symbols: CO2 open symbols: CH4 (VS FR-10, BA C2H4/N2, XW C3H6/N2).
Fig. 6. SEM characterisation of ACF (a,b) before plasma treatment (c) after propylene– nitrogen plasma treatment.
3.5. Molecular sieving properties The capacity of carbon fibers to act as molecular sieves for CO2 and CH4 separation was studied before and after plasma treatment. Fig. 7 shows the uptake curves of CO2 and CH4 for raw and plasma treated samples in a nitrogen– propylene and nitrogen– ethylene discharge at 80 W. In all cases the adsorption rate of both gases decreases by the plasma treatment, especially during the first minute of adsorption. This is probably due to film formation on
the fiber surface, which causes restrictions to gas diffusion. Although the adsorption of both CO2 and CH4 is suppressed by film formation, this is stronger in the case of CH4. The linear CO2 molecules probably diffuse easier through the film than the spherical CH4 molecules. Basic pyridine or amino surface groups present on plasma treated ACF may interact with acidic CO2 to enhance and improve its adsorption capacity. Pyridinic or amino groups were detected by N1 s XPS on the fiber surface after plasma treatment, as discussed previously. Therefore, an acid –base interaction between surface nitrogen groups and CO2 is very probable. This leads to a significant enhancement of CO2 diffusion and adsorption on the plasma treated ACF as compared to CH4. The difference in the adsorption rates of the two gases leads to a significant improvement of ideal selectivity (expressed as the ratio of the amount of CO2 adsorbed to that of CH4), as shown in Table 1. The CO2/CH4 ideal selectivity, measured at 60 s of adsorption, improves from 2.4, for the raw ACF, to 26 for C2H4/N2 and 18.5 for C3H6/N2 plasma treated ACF at 80 W. The selectivity decrease, observed at longer adsorption times, 120 s, is in agreement with the kinetic separation rules. The molecular sieving ability of the deposited film, Table 1, varies with the hydrocarbon used for the deposition process, due to the different deposition rates for different hydrocarbon sources Table 1 Ideal selectivity (CO2/CH4) of raw and plasma treated ACF (FR-10) at 60 and 120 s of adsorption Gas feed
Plasma power (W)
Ideal selectivity (CO2/CH4)-60 s
Ideal selectivity (CO2/CH4)-120 s
– C2H4/N2 C3H6/N2 C2H4/N2 C3H6/N2
– 80 80 120 120
2.4 26.0 18.5 19.5 15.6
2.4 20.0 12.5 10.0 9.2
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in the deposited material did not depend on hydrocarbon used. Ideal selectivity of ACF for CO2 and CH4 gas adsorption improved significantly after plasma treatment due to a film formation on the ACF surface. Although diffusion through the film of both CO2 and CH4 was hindered, compared to raw ACF, CO2 transport and adsorption was favored probably because of acid –base interaction between CO2 and basic pyridine or amino groups detected by XPS. Ideal selectivity differences at 80 and 120 W plasma power can be attributed to different film thicknesses obtained in the two cases.
Acknowledgements Fig. 8. CO2 and CH4 uptake curves of commercial activated carbon fibers treated by ethylene–nitrogen and propylene–nitrogen plasma at 120 W. closed symbols: CO2 open symbols: CH4 (VS FR-10, BA C2H4/N2, XW C3H6/N2).
[25,26]. The rate of deposition depends on the energy input per gram of hydrocarbon used, which in turn depends on the molecular weight of the hydrocarbon employed (C2H4 and C3H6). Therefore, for the same energy supplied to C2H4 and C3H6 by plasma at a specific plasma power, e.g. 80 W, the energy input per gram of C2H4 is higher than that of C3H6, for the same flow rate. Thus, for the same treatment time (15 min) the film thickness is not the same when different hydrocarbon sources are used. Considering that film structure is related to the film thickness [27], it is reasonable to attribute the differences in CO2 and CH4 adsorption to the different film thickness obtained with ethylene or propylene. The capacity of ACF for CO2 and CH4 uptake increases with increasing plasma power, as shown in Fig. 8. At 120 W plasma power, CO2 gas uptake increases in the first 2 min by 15 –20%, compared with that of the 80 W treated samples. In order to explain this, one should consider not only the film formation but also possible film ablation [25,26]. The film ablation could be stronger in the case of 120 W plasma treatment [28] resulting in lower net deposition rates (net deposition rate ¼ deposition rate 2 ablation rate). Thus, for samples modified at 120 W plasma power and at the same treatment time, a lower net deposition rate has probably resulted in the formation of a thinner film than that of 80 W and, therefore, in lower restrictions to CO2 and CH4 adsorption. This is in good agreement with the reduced CO2/CH4 selectivity for ACF treated at 120 W, compared to those treated at 80 W.
4. Conclusions Carbon films were deposited on ACF by propylene – nitrogen and ethylene – nitrogen RF discharges. XPS analysis revealed that nitrogen reacted and remained on ACF surface during plasma treatment, but the nitrogen form
We thank the European Coal and Steel Community for financial support of this work. We also thank the Physics Division, School of Engineering of Aristotle University of Thessaloniki for Raman measurements, and the Institute of Chemical Engineering and High Temperature Processes of Patra for XPS measurements.
References [1] Gomes V, Hassan M. Sep Purif Technol 2001;24:189. [2] Dong F, Lou H, Kodama A, Goto M, Hirose T. Sep Purif Technol 1999;16:159. [3] Kapoor A, Yang R. Chem Eng Sci 1989;44:1723. [4] Stanciu V, Stefanescu D. Fuel Energ Abstr 1997;38:153. [5] Gaffney T. Curr Opin Solid State Mater Sci 1996;1:69. [6] Hu Z, Vansant E. Carbon 1995;33:561. [7] Freitas M, Figueiredo J. Fuel 2001;80:1. [8] Vyas S, Patwardhan S, Vijayalakshmi S, Gangadhar B. Fuel 1993;72: 551. [9] Cabrera A, Zehner J, Coe C, Gaffney T, Farris T. Carbon 1993;31:969. [10] Prasetyo I, Do D. Carbon 1999;37:1909. [11] Kawabuchi Y, Kawano S, Mochida I. Carbon 1996;34:711. [12] Liang L, Shi M, Viswanathan V, Peurrung LM, Young JS. J Membr Sci 2000;177:97. [13] Gorbig O, Nehlsen S, Muller J. J Membr Sci 1998;138:115. [14] Kunio Abe. US Patent 5.238.888 1993. [15] De La Casa-Lillo M, Alcaniz-Monge J, Raymundo-Pinero E, CazorlaAmoros D, Linares-Solano E. Carbon 1998;36:1353. [16] Abdelbasset H, Despax B. Thin Solid Films 2000;358:30. [17] Hayashi Y, Krishna K, Ebisu H, Soga T, Umeno M, Jimbo T. Diamond Relat Mater 2001;10:1002. [18] Zeng R, Pang Z, Zhu H. J Electrochem Soc 2000;490:102. [19] Biniak S, Szymanski G, Siedlewski J, Swiatkowski A. Carbon 1997; 35:1799. [20] Shewood PMA. J Electron Spectrosc Relat Phenom 1996;81:319. [21] Koh M, Nakajima T. Carbon 2000;38:1947. [22] Nakajima T, Koh M. Carbon 1997;35:203. [23] Kouvetakis J, Kaner B, Sattler L, Bartlett NJ. Chem Soc Chem Comm 1986;24:1758. [24] Pels R, Kapteijon F, Moulijin A, Zhu Q, Thomas M. Carbon 1995;33: 1641. [25] Biederman H, Osada Y. Plasma Polymerization Processes. Amsterdam: Elsevier; 1992. [26] Yasuda H. Plasma Polymerization. London: Academic Press; 1985. [27] Kramer P, Yeh Y, Yasuda H. J Membr Sci 1989;46(1):1. [28] Li K, Meichsner J. Surf Coat Technol 1999;116-119:841.
Production of carbon molecular sieves by plasma treated activated carbon fibersq T. Orfanoudakia,b, G. Skodrasb,c, I. Doliosb, G.P. Sakellaropoulosa,b,* a
Chemical Process Engineering Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 1520, Thessaloniki 540 06, Greece b Laboratory of Solid Fuels and Environment, Chemical Process Engineering Research Institute, Thessaloniki, Greece c Centre for Solid Fuels Technology and Applications, Ptolemais, Greece Received 6 November 2002; accepted 27 February 2003; available online 11 June 2003
Abstract Carbon molecular sieves (CMS) are valuable materials for the separation and purification of gas mixtures. In this work, plasma deposition was used aiming to the formation of pore constrictions, by narrowing the surface pore system of commercial activated carbon fibers (ACF). For this reason propylene/nitrogen or ethylene/nitrogen discharges of 80 and 120 W were used. The molecular sieving properties of the plasma treated ACF were evaluated by measuring the adsorption of CO2 and CH4. The CO2/CH4 selectivity was significantly improved and depended on plasma treatment conditions (discharge gas and power). The optimum CO2/CH4 selectivity (26) was observed for C2H4/N2 plasma treated ACF at 80 W. Sample scanning electron microscopy (SEM) analysis after plasma treatment revealed an external film formation and X-ray photoelectron spectroscopy (XPS) analysis showed the incorporation of nitrogen functional groups in the film, which probably interact with CO2, thereby altering CO2/CH4 selectivity. q 2003 Elsevier Ltd. All rights reserved. Keywords: Plasma deposition; Activated carbon fibers; Carbon molecular sieves
1. Introduction Carbon molecular sieves (CMS) are high added value materials used in gas separation processes. Their unique ability to separate gases based on the different size and shape of molecules has been exploited in commercial applications such as pressure swing adsorption (PSA) [1 – 5]. The most important feature of CMS is their narrow pore size distribution accomplished either by controlled activation [6] or by employment of pore narrowing techniques on an inherent pore structure. This latter technique has been used on various carbon materials for the production of CMS suitable for air separation. Hu et al. [6] used 3-methylpentane as a source for carbon deposition on walnut shells, chemically activated by KOH. The best oxygen – nitrogen * Corresponding author. Address: Chemical Process Engineering Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 1520, Thessaloniki 540 06, Greece. Tel.: þ 302310-996271; fax: þ 30-2310-996168. E-mail address: [email protected] (G.P. Sakellaropoulos). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00172-8
separation selectivity reported in this study was 9.2. Freitas et al. [7] reported the modification of two activated carbons of different texture by the pyrolysis of benzene in an attempt to obtain CMS for O2/N2 separation. Their results showed that this objective can be attained when the carbon precursor has been activated only to a limited extent and when carbon deposition is carried out in the proper kinetic regime. Vyas et al. [8] obtained CMS by methane cracking on bituminous coal and coconut shells. The O2/N2 uptake ratio of the best CMS produced was 2.667. Cabrera et al. [9] described the preparation of CMS for air separation by a two-step hydrocarbon deposition with a single hydrocarbon. They found that the concentration of the carbon containing compound used in the first step should be larger than that of the second step, so that the pore openings of the micropores of the support narrowed gradually, avoiding pore plugging. CMS for CO2/CH4 separation has also been produced by the same method. Praseyto et al. [10] tailored the pore structure of activated carbon by benzene deposition, and improved the CO2/CH4 kinetic selectivity from 7 to 26. However, cobalt catalyst was used to enhance benzene
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cracking. Kawabuchi et al. [11] modified the pore size of several types of carbon adsorbents, suitable for CO2/CH2 separation, by chemical vapor deposition of benzene. They showed that pyrolytic carbon should be deposited only on the pore mouth in order to avoid pore plugging and to retain the CO2 adsorption capacity. Surface modification techniques, such as plasma, could be an alternative for CMS production. The unique ability of plasma to modify the surface of a material without changing the bulk properties is attractive for the modification of a carbon material to CMS by a one step treatment. Hydrocarbon plasma has been widely used to improve polymeric membrane efficiency [12,13]. However, studies concerning plasma deposition of a thin film on carbonaceous material in order to improve its molecular sieve properties are limited [14]. In this work commercial activated carbon fibers (ACF) based on phenol resin were modified by N2/propylene or N2/ethylene RF plasma discharges aiming to enhance the molecular sieve properties of the material. Plasma treated fibers were examined for possible surface modifications and for their selectivity towards adsorption of CO2 and CH4 gases.
energies of the analyzer was calibrated according to the ASTM-E 902-88 standard method. Scanning electron microscopy (SEM) was also used for surface examination of the initial and the plasma treated ACF. The molecular sieving properties of samples were evaluated by measuring the adsorption of CO2 and CH4, volumetrically under ambient conditions.
3. Results and discussion 3.1. BET surface area measurements The N2 adsorption isotherm of untreated carbon fibers, Fig. 1, is of type I, according to BDDT classification and corresponds to a microporous material. Untreated samples have a BET surface area of 650 m2/g. Plasma treated carbon fibers gave negligible N2 adsorption. This indicates that the film deposited on ACF surface, due to plasma treatment, reduced significantly the surface pore entrance, possibly in the range of molecular dimensions. Hence, N2 is probably kinetically restricted to enter such narrow pores, and to diffuse in the interior pore structure of the ACF. Such a behavior is not unusual in CMS [6,15].
2. Experimental 3.2. Raman spectra The deposition apparatus used in this study consisted of a quartz reactor, an RF generator with an impedance matching network and a mechanical pump. The reactor was a quartz cylinder 1 m long, 65 mm diameter placed coaxially through a working coil. The coil was made of 9 turns of 1=4 in diameter copper tube. RF power was supplied from a 1 KW, 13.56 MHz generator. The system was also equipped with a water circulation unit, necessary for cooling the various RF plasma components. About 35– 40 mg of ACF (FR-10, Kuraray Chemical Co.) were introduced in the reactor in the middle of the coil and sealed therein by vacuum flanges. After system evacuation, a gas mixture, of 20% hydrocarbon and 80% nitrogen was introduced in the reactor, and plasma was ignited. The sample was always treated for 15 min at an 80 or 120 W plasma power. Since no additional heating was employed, the temperature of the ACF rose only by inductive heating and energy transfer from the plasma. The raw and treated ACF were characterized by N2 adsorption at 77 K, from which their BET surface area was estimated using the BET multiple point equation. Raman spectroscopy was employed to characterize the type of carbon– carbon bonds before and after plasma treatment. The 514 nm line spectra of an Arþ laser was used for excitation. XPS analysis was also employed to characterize the functional groups on the ACF surface before and after plasma treatment. The ionizing radiation, Mg Ka, was provided by a non-monochromatic X-ray source with characteristic energy 1253.6 eV. The range of kinetic
The Raman spectrum of the untreated carbon fibers is shown in Fig. 2(a). It consists of two peaks, characteristic of a graphite structure [16]. The presence of the Raman peak at 1350 cm21, in addition to the main one at 1580 cm21, suggests that small crystals are present [16]. In the Raman spectra of the plasma treated fibers the two peaks were replaced by a continuous line, Fig. 2(b). A similar result was also obtained by Hayashi and his co-workers, although the reason for such a behavior is still unclear [17].
Fig. 1. Nitrogen adsorption isotherm of commercial activated carbon fibers (FR-10) at 77 K. No nitrogen adsorption for plasma treated ACFs.
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Fig. 2. Raman spectrum of commercial activated carbon fibers (FR-10) (a) before plasma treatment (b) after plasma treatment.
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Fig. 4. XPS N1s spectrum of commercial activated carbon fibers treated by propylene–nitrogen plasma at 80 W.
3.3. XPS analysis A typical XPS spectrum of raw ACF, before plasma treatment, is shown in Fig. 3. Similar spectra were obtained with plasma treated ACF. The main C1s peak of all samples can be deconvoluted to three components at around 284.6, 286 and 288 eV, Fig. 3, which probably correspond to C –C (sp2 or sp3), C –OH or bridged – CyO – H – OyC – , and COOH or COOR [18 –20]. A fourth peak, at the highest binding energy, is attributed to the filter used as substrate for the XPS analysis. However, C – N bonds show quite similar binding energies, 286– 288 eV [19]; hence, the assignment of peaks at 286 and 288 eV to C –O bonds, based on C1s spectra, is ambiguous. For this reason, N1s spectra of all samples were also obtained. Raw fibers before plasma treatment showed no N1s spectra, therefore, the peaks observed from 286 to 288 eV in the C1s spectra, Fig. 3, can be assigned to C – O groups as discussed earlier. The N1s XPS spectra of plasma treated samples are shown in Figs. 4 and 5. All N1s spectra after deconvolution show characteristic peaks around 399 and 400 eV, which correspond to pyridine and pyrole nitrogen groups, respectively, [21 –24]. The presence
Fig. 3. XPS C1s spectrum of raw (untreated) commercial activated carbon fibers (FR-10).
of NH2 groups cannot be excluded, whose characteristic binding energy is 399.3 eN [18,24]. In addition, to these two peaks, the N1 s spectrum of ethylene – nitrogen plasma treated ACF shows one additional peak at 402 eV, Fig. 5. The binding energy of this latter peak corresponds to oxidized nitrogen forms [24]. These results suggest that nitrogen present in the feed stream reacts and remains on the sample surface after plasma treatment. 3.4. SEM surface examination Fig. 6, shows the surface of the ACF, as observed by SEM. In Fig. 6(a) the pore structure of ACF surface can be identified, while Fig. 6(b) demonstrates that the fiber surface is rough and contains defects. From Fig. 6(c), it is evident that a thin film was deposited on the activated carbon surface after propylene –nitrogen plasma treatment. The film thickness along the fiber varies between 150 and 300 nm.
Fig. 5. XPS N1s spectrum of commercial activated carbon fibers treated by ethylene–nitrogen plasma at 80 W.
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Fig. 7. CO2 and CH4 uptake curves of commercial ACF treated by ethylene – nitrogen and propylene – nitrogen plasma at 80 W. closed symbols: CO2 open symbols: CH4 (VS FR-10, BA C2H4/N2, XW C3H6/N2).
Fig. 6. SEM characterisation of ACF (a,b) before plasma treatment (c) after propylene– nitrogen plasma treatment.
3.5. Molecular sieving properties The capacity of carbon fibers to act as molecular sieves for CO2 and CH4 separation was studied before and after plasma treatment. Fig. 7 shows the uptake curves of CO2 and CH4 for raw and plasma treated samples in a nitrogen– propylene and nitrogen– ethylene discharge at 80 W. In all cases the adsorption rate of both gases decreases by the plasma treatment, especially during the first minute of adsorption. This is probably due to film formation on
the fiber surface, which causes restrictions to gas diffusion. Although the adsorption of both CO2 and CH4 is suppressed by film formation, this is stronger in the case of CH4. The linear CO2 molecules probably diffuse easier through the film than the spherical CH4 molecules. Basic pyridine or amino surface groups present on plasma treated ACF may interact with acidic CO2 to enhance and improve its adsorption capacity. Pyridinic or amino groups were detected by N1 s XPS on the fiber surface after plasma treatment, as discussed previously. Therefore, an acid –base interaction between surface nitrogen groups and CO2 is very probable. This leads to a significant enhancement of CO2 diffusion and adsorption on the plasma treated ACF as compared to CH4. The difference in the adsorption rates of the two gases leads to a significant improvement of ideal selectivity (expressed as the ratio of the amount of CO2 adsorbed to that of CH4), as shown in Table 1. The CO2/CH4 ideal selectivity, measured at 60 s of adsorption, improves from 2.4, for the raw ACF, to 26 for C2H4/N2 and 18.5 for C3H6/N2 plasma treated ACF at 80 W. The selectivity decrease, observed at longer adsorption times, 120 s, is in agreement with the kinetic separation rules. The molecular sieving ability of the deposited film, Table 1, varies with the hydrocarbon used for the deposition process, due to the different deposition rates for different hydrocarbon sources Table 1 Ideal selectivity (CO2/CH4) of raw and plasma treated ACF (FR-10) at 60 and 120 s of adsorption Gas feed
Plasma power (W)
Ideal selectivity (CO2/CH4)-60 s
Ideal selectivity (CO2/CH4)-120 s
– C2H4/N2 C3H6/N2 C2H4/N2 C3H6/N2
– 80 80 120 120
2.4 26.0 18.5 19.5 15.6
2.4 20.0 12.5 10.0 9.2
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in the deposited material did not depend on hydrocarbon used. Ideal selectivity of ACF for CO2 and CH4 gas adsorption improved significantly after plasma treatment due to a film formation on the ACF surface. Although diffusion through the film of both CO2 and CH4 was hindered, compared to raw ACF, CO2 transport and adsorption was favored probably because of acid –base interaction between CO2 and basic pyridine or amino groups detected by XPS. Ideal selectivity differences at 80 and 120 W plasma power can be attributed to different film thicknesses obtained in the two cases.
Acknowledgements Fig. 8. CO2 and CH4 uptake curves of commercial activated carbon fibers treated by ethylene–nitrogen and propylene–nitrogen plasma at 120 W. closed symbols: CO2 open symbols: CH4 (VS FR-10, BA C2H4/N2, XW C3H6/N2).
[25,26]. The rate of deposition depends on the energy input per gram of hydrocarbon used, which in turn depends on the molecular weight of the hydrocarbon employed (C2H4 and C3H6). Therefore, for the same energy supplied to C2H4 and C3H6 by plasma at a specific plasma power, e.g. 80 W, the energy input per gram of C2H4 is higher than that of C3H6, for the same flow rate. Thus, for the same treatment time (15 min) the film thickness is not the same when different hydrocarbon sources are used. Considering that film structure is related to the film thickness [27], it is reasonable to attribute the differences in CO2 and CH4 adsorption to the different film thickness obtained with ethylene or propylene. The capacity of ACF for CO2 and CH4 uptake increases with increasing plasma power, as shown in Fig. 8. At 120 W plasma power, CO2 gas uptake increases in the first 2 min by 15 –20%, compared with that of the 80 W treated samples. In order to explain this, one should consider not only the film formation but also possible film ablation [25,26]. The film ablation could be stronger in the case of 120 W plasma treatment [28] resulting in lower net deposition rates (net deposition rate ¼ deposition rate 2 ablation rate). Thus, for samples modified at 120 W plasma power and at the same treatment time, a lower net deposition rate has probably resulted in the formation of a thinner film than that of 80 W and, therefore, in lower restrictions to CO2 and CH4 adsorption. This is in good agreement with the reduced CO2/CH4 selectivity for ACF treated at 120 W, compared to those treated at 80 W.
4. Conclusions Carbon films were deposited on ACF by propylene – nitrogen and ethylene – nitrogen RF discharges. XPS analysis revealed that nitrogen reacted and remained on ACF surface during plasma treatment, but the nitrogen form
We thank the European Coal and Steel Community for financial support of this work. We also thank the Physics Division, School of Engineering of Aristotle University of Thessaloniki for Raman measurements, and the Institute of Chemical Engineering and High Temperature Processes of Patra for XPS measurements.
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