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Surface Control of Activated Carbon Fiber by Growth of Carbon Nanofiber Seongyop Lim,† Seong-Ho Yoon,*,† Yoshiki Shimizu,† Hangi Jung,‡ and Isao Mochida† Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasugakoen, Kasuga, Fukuoka 816-8580, Japan, and Nexen Nanotech. Co. Ltd., 373-1 Guseong-Dong, Yusong-Gu, Taejon 305-701, Korea Received November 4, 2003. In Final Form: February 2, 2004 Carbon nanofiber/activated carbon fiber (CNF/ACF) composites with multifunctional surfaces were prepared through catalytic growth of CNFs on an ACF. Because of selective deposition of catalyst particles in ACF micropores, partial oxidation of ACF after catalyst impregnation was a critical step to control the surface area of the CNF/ACF composites, of which the surface functions can be synergistically performed by both the microporous surface of ACF and free edges of CNFs. CNF/ACF composites of this study are expected to provide an improved performance in SOx or NOx removal.
Introduction Activated carbon fiber (ACF) is a unique adsorbent with superhigh surface areas of 800-3000 m2/g, which basically originate from the micropores of the fiber skin.1,2 As a practical adsorbent, a carbon fiber has several advantages over powdered carbons: (1) low pressure drop because of fabricated form of fibers, (2) any fabricated forms as designed such as paper, felt, and mat, and (3) sufficient strength, modulus, flexibility, and low abrasion.1 Applications of ACF are now expanding into environmental cleaning, VOC recovery,3,4 energy saving and storage such as methane storage,5 electrodes for electric double-layer capacitor,6 CO2 capture, and air separation.7 An application specific to active carbon fibers is removal of SOx and NOx from the atmosphere as well as from flue gas.3,4,8 The basic principle of SOx removal is capture of SO2 in the form of aqueous H2SO4. Continuous recovery of concentrated aqueous H2SO4 is now being proved in a large scale test.9 Removal of NOx relies basically on the reduction into N2 with NH3 or its oxidation into NO2 and NO3, to be subsequently trapped by water or aqueous bases.8,10 The inhibition of humidity in the reaction at ambient temperature is a problem to be solved.10 * Corresponding author. Tel.: +81-92-583-7797. Fax: +81-92583-7798. E-mail address:
[email protected]. † Kyushu University. ‡ Nexen Nanotech. (1) Yoon, S.-H.; Korai, Y.; Mochida, I. In Sciences of Carbon Materials; Marsh, H., Rodriguez-Reinoso, F., Eds.; Universidad de Alicante: Alicante, 2000; pp 287-325. (2) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992, 30, 0 (7), 1075-1088. (3) Kisamori, S.; Kurod, K.; Kawano, S.; Mochida, I.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1994, 8 (6), 1337-1340. (4) Mochida, I.; Kisamori, S.; Hironaka, M.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1994, 8 (6), 1341-1344. (5) Lozano-Castello, D.; Cazorla-Amoros, D.; Linares-Solano, A. Energy Fuels 2002, 16 (5), 1321-1328. (6) Tanahashi, I.; Yoshida, A.; Nishino, A. Carbon 1990, 28, 8 (4), 477-482. (7) Kitagawa, H. Proceeding of Nippon Kagakukai, 65th Spring Meeting, 1993; p 153. (8) Mochida, I.; Sakanishi, K. Fuel 2000, 79 (3-4), 221-228. (9) Kisamori, S.; Mochida, I.; Fujitsu, H. Langmuir 1994, 10, 0 (4), 1241-1245. (10) Mochida, I.; Kawano, S.; Hironaka, M.; Kawabuchi, Y.; Korai, Y. Matsumura, Y.; Yoshikawa, M. Langmuir. 1997, 13 (20), 53165321.
Recently, carbon nanofiber (CNF) has been extensively studied under recognition as a unique carbon material.11-14 CNF is characterized by the graphite-like structure in a nanoscale, where variable alignments of laminated hexagon layers along the fiber axis provide typically three types of CNFs such as platelet (alignment perpendicularly against the fiber axis), tubular (alignment parallel along the axis), and herringbone (alignment angled to the axis) CNF.14 In surface-related applications of carbon materials, it is important what surface morphology, structure, and chemistry is effective and how much they have effective sites on their surface. CNF can be of a high surface area from 50 to 800 m2/g as catalytically prepared or after chemically activated.15,16 Platelet and herringbone CNFs expose free edges of hexagons with few heteroatoms, differing from a negatively concaved surface of ACF based on a micropore-developing structure. Such a surface of CNF appears to be advantageous to providing active sites of exposed hydrophobic hexagon edges for high activity in DeSOx and DeNOx. However, the nano-ordered size is not adequate to be packed in a fixed-bed reactor due to difficulty in handling and pressure drop at rapid gas flow in processes.17 CNF/carbon fiber composites have been investigated (1) to improve adhesive properties at the interface between the fiber and matrix in carbon-fiber reinforced composites18-20 and (2) to give a macroscopic frame or support to be anchored by CNFs to solve handling difficulty and the pressure drop problem.17 (11) Iijima, S. Nature 1991, 354, 56-58. (12) Rodriguez, N. M. J. Mater. Res. 1993, 8 (12), 3233-3250. (13) Dekker: C. Phys. Today 1999, May (5), 22-28. (14) Rodriguez, N. M.; Chambers, A.; Baker, R. T. K. Langmuir 1995, 11 (10), 3862-3866. (15) Baker, R. T. K.; Rodriguez, N. M. US Patent No. 5618875, April 8, 1997. (16) Ota, Y.; Lee, S.; Tanaka, A.; Lim, S.; Yoon, S.-H.; Korai, Y.; Mochida, I. In Carbon’02: An International Conference on Carbon; Tsinghua University: Beijing, China, Sept 15-19, 2002; p 58 (Presentation no. 18.5). (17) Nhut, J. M.; Vieira, R.; Keller, N.; Pham-Huu, C.; Boll, W.; Ledoux, M. J. Stud. Surf. Sci. Catal. 2002, 143, 983-991. (18) Downs, W. B.; Baker, R. T. K. Carbon 1991, 29, 9 (8), 11731179. (19) McAllister, P.; Wolf, E. E. Carbon 1992, 30, 0 (2), 189-200. (20) Downs, W. B.; Baker, R. T. K. J. Mater. Res. 1996, 10 (3), 625633.
10.1021/la036077t CCC: $27.50 © 2004 American Chemical Society Published on Web 05/20/2004
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In this work, CNF/ACF composites were prepared to introduce multifunctions to the catalytic surface of ACF by catalytic growth of CNF. The surface properties of products, which can be contributed to by both porous surface of ACF and exposed edges of CNF, were expected to be controlled through (1) open/close of ACF micropores and (2) formation of new surface and pores via catalytic growth of CNF on ACF. Such composites of CNF/ACF were expected to be very useful adsorbents and catalysts, which may improve the performance of ACFs in such applications as DeSOx and DeNOx. Experimental Section Chemicals and Materials. Reagent grade iron nitrate (Fe(NO3)3‚9H2O), and nickel nitrate (Ni(NO3)2‚6H2O) were obtained from Wako Inc. (Japan). Hydrogen (>99.9999%), ethylene (99.9%), and helium (>99.9999%) were obtained from Nihon Sanso Inc. (Japan) and used without further purification. Activated carbon fiber (ACF) in this study was OG-15A, which involves in OG a series of pitch-based ACFs supplied by Osaka Gas Co. Impregnation of Catalyst into ACF. Prescribed amounts of nickel and iron nitrates were dissolved in ethanol, and then ACF dried in an oven at 65 °C was soaked in the solution. After ethanol evaporated, the wet ACF was dried in a vacuum oven at 105 °C overnight and stored. In this study, 5% w/w of Ni-Fe (8:2 w/w) was impregnated onto the ACF, which was named as NF82-ACF. Partial Oxidation of NF82-ACF. NF82-ACF was oxidized in air to be analyzed by thermogravimetric differential thermal analysis (TG-DTA) (SEIKO SSO/5200, SEIKO Co., Japan). NF82ACF was partially combusted in the furnace before CNF synthesis. The oxidation degree was controlled by the oxidation time at a prescribed temperature. The partially oxidized NF82ACF was named as PO-NF82-ACF. Synthesis of CNF over NF82-ACF. CNFs was synthesized over ACF in a quartz flow reactor (10 cm × 45 cm) heated by a conventional horizontal tube furnace. A prescribed amount of NF82-ACF or PO-NF82-ACF (250-300 mg) was placed in an alumina boat at the center of the reactor tube in the furnace. After reduction in a 20% H2/He mixture for 2 h at a prescribed temperature, helium was flushed for 30 min before introduction of a C2H4/H2 (1:1) (v/v) mixture (total flow rate ) 200 cm3/min) over the catalyst. All of the gas flows in the reactor were precisely monitored and regulated by MKS mass flow controllers. The total amount of carbon deposited during the time on stream was determined gravimetrically after cooling the product to ambient temperature. Characterization. Multipoint Brunauer, Emmett, and Teller (BET) surface area was measured with nitrogen adsorptiondesorption isotherm using a surface area analyzer (Sorptomatic 1990, FISONS Instruments). Prior to this measurement, the samples were degassed at 150 °C for 8 h. The pore distribution was calculated by the method of Barrett, Joyner, and Halenda.21 The nanostructure and morphology of the nanofibers were observed under both scanning electron microscopy (SEM) (JSM6320F, JEOL) and transmission electron microscopy (TEM) (JEM-2010F, JEOL).
Results Growth of CNF on ACF. Critical factors such as the nature of the catalyst, the hydrocarbon reactant, and the growth conditions were based on the preliminary work22 with Ni-Fe (8:2) supported on carbon black, which produced herringbone-type CNFs with 5-40 nm diameter at a high yield of 100-150 g of CNFs/1 g of alloys from ethylene at 480-600 °C. (21) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73 (1), 373-380. (22) Lim, S.; Yoon, S. H.; Korai, Y.; Mochida, I. In Carbon’03: An International Conference on Carbon, Auditorio Principe Felipe: Oviedo (Spain), July 6-10, 2003; p 77 (Presentation no. 8.3).
Figure 1. Dependence of the specific surface area on CNF growth from ethylene/H2 (1:1) on PO-NF82-ACF at 600 °C. Table 1. Carbon Yield and Surface Area According to the Synthetic Condition pore product/ CNF/ surface specific temp time starting product area volume (°C) (min) (g/g) (wt/wt %) (m2/g) (cm3/g) reaction
sample no. ACF A-15 NF82-ACF
480 530 600 600 PO-NF82-ACF 600 600 600
0 60 60 60 20 0 20 40 60
1.11 1.27 2.05 1.08
10 21 51 7
1.12 1.44 1.79
10 31 44
1014 118 114
0.73 0.08 0.08
131
0.09
1625 1099 582 479
0.92 0.61 0.38 0.35
The carbon yields of CNF over the activated carbon fiber supported catalyst were summarized in Table 1. In the case of NF82-ACF, the CNF content in the product, which corresponds to the carbon yield, was gravimetrically measured from 10 wt % at 480 °C to 50% at 600 °C for 1 h of reaction time. The weight loss in preparation of PONF82-ACF (in air at 350 °C for 1 h) ranged 20-30 wt % of NF82-ACF. The partial oxidation appears to affect hardly the catalytic activity of Ni-Fe catalyst on the ACF, since the amounts of carbon deposition on the PO-NF82ACF and on the NF82-ACF were very similar. Surface Area and Pore Distribution of CNF/ACF Composites. The BET specific surface area and the specific pore volume are summarized in Table 1. The ACF in this study had a high surface area over 1000 m2/g, while CNF from ethylene on Ni-Fe catalyst showed a surface area of 150-220 m2/g. After impregnation of Ni-Fe catalyst into the original ACF, the surface area markedly decreased to 118 m2/g. Most of ACF micropores must be blocked by the catalyst deposition. Growth of carbon nanofiber on NF82-ACF increased little surface area, irrespective of the amount of the produced CNF according to synthetic temperature and time (Table 1). The micropores which contributed to the large surface area must be still closed at CNF growth on the catalyst particles. The partial oxidation of the NF82-ACF in air restored the surface area and the pore volume over those of the original ACF (Table 1). CNF growing on the PO-NF82ACF decreased the surface area according to its amount as shown in Figure 1. Figure 2b shows the pore size distribution in original and CNF-grown ACFs calculated from adsorption/desorption profiles of Figure 2a. The micropores with radii of 0.6-1.2 nm in the ACF appear to provide its large surface area. The catalyst impregnated on the ACF blocked
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Figure 2. Surface properties of ACF, NF82-ACF, PO-NF82ACF, and CNF/ACF composites: (a) N2 adsorption/desorption isotherms, and (b) pore size distributions calculated by the BJH procedure. Table 2. Elemental Analysis of the Products elemental analysis C
H
N
ACF 93.9 0.9 0.7 NF82-ACF 88.6 0.6 1.0 PO-NF82-ACF-600 °C-60 m 97.5 0.7 0.0
O(diff) O/C ratio 4.1 9.8 1.8
0.044 0.111 0.018
most of the micropores, indicating that the catalyst particles were selectively deposited in the pores of the ACF. The partial oxidation recovered the surface area by regeneration of micropores with radii of 0.9-1.8 nm, which are larger than those of the original ACF. Formation of CNF created new pores with radii of 0.6-0.9 nm, although the catalyst particles still blocked partially the pores present on the surface of the ACF. Elemental Analysis. Table 2 shows the elemental analysis results of several samples. The ACF as prepared showed the atomic ratio of oxygen and carbon, O/C ) 0.044. Various functional groups containing oxygen and nitrogen appear to be located on the surface of ACF. Such heteroatoms were found to be introduced through impregnation of iron and nickel catalysts. The ratio O/C decreased to 0.018, and nitrogen was not found in the CNF/ACF composites synthesized. Microscopic Analyses of CNF/ACF Composites. Figure 3 shows SEM photographs of ACF (Figure 3a), NF82-ACF (Figure 3b), PO-NF82-ACF (Figure 3c), and
Figure 3. SEM images of (a) the ACF, OG-15A, (b) NF82ACF, (c) PO-NF82-ACF, (d) NF82-ACF-600 °C-60 m, (e) PONF82-ACF-600 °C-20 m, and (f) PO-NF82-ACF-600 °C-60 m.
CNF/ACF composites (Figures 3d-f). ACF showed a smooth but porous surface, which did not change by the impregnation of catalyst. The surface became rough after partial oxidation in air. CNFs with very thin diameter of about 10 nm were formed uniformly both on the NF82ACF and the PO-NF82-ACF. CNFs on the surface of ACF became dense according to the reaction time. Figure 4 shows TEM images of CNF/ACF composites, which were prepared from ethylene/H2 (1:1v/v) on PONF82-ACF at 600 °C for 20 min. A number of very thin CNFs about 10 nm in diameter were grown on the surface of ACF. Many CNFs are entangled like a bird’s nest. Such thin CNFs are not of a tubular type but are close to herringbone type (Figure 4b).
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Figure 4. TEM images of PO-NF82-ACF-600 °C-20 m. The image (b) is a high-resolution image of one part in the image (a).
Near the surface of ACF, it cannot be clearly distinguished whether the structure involves in the structure of CNF or ACF (Figure 4b). A kind of epitaxial growth of CNF on an ACF surface can be suspected, even though the CNF grows on the catalyst particles dispersed on the ACF surface. Discussion CNF/ACF composites were successfully prepared through catalytic growth of CNF on the ACF and the partially oxidized ACF after the catalyst impregnation. However, the surface areas of CNF/ACF products were limited below 200 m2/g without partial oxidation prior to the CNF growth. Hence the partial oxidation step was found to be critical to express both surfaces of CNF and ACF. Figure 5 illustrates the schematic model of the overall step from catalyst deposition on ACF, partial oxidation of ACF containing the catalyst, and growth of CNF on the ACF surface. In this model, the ACF micropores, which have been known to be open directly to the outer surface
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in comparison to the complex pore structure of active carbon granules,23 are simply depicted as a rectangular shape which can be obtained by a side view of a cylinder, even though the real morphology and structure of micropores may be more complicated. The micropore size of the present ACF distributes from 0.6 to 1.2 nm hydraulic radii as shown in Figure 2b. To explain a series of the results in this study, the catalyst dispersion on ACF may be the first subject to discuss. The state of catalysts deposited on ACF can be thought in three types such as (a) catalyst particles in the pore, (b) ones on the pore or at the entrance of pore, and (c) one on the flat surface of ACF as shown in Figure 5. After impregnation of catalyst into ACF, the surface area markedly decreased simultaneously with disappearance of micropores, indicating that the catalyst blocked most of the micropores. Thus, ACF cannot contribute to the surface properties of CNF/ACF composites in which ACF appears only as a catalyst support or macroscopic frame. The CNF yield with catalyst on the ACF was much lower than that obtained on carbon black support in a previous work.22 Such a low yield may be ascribed to the location of catalyst in the micropores of ACF like the state (a) where the catalyst is difficult to work for CNF yield by contacting the gas-phase carbon source on proper surface of catalyst.12,24,25 The surface area decreased by catalyst deposition was hardly recovered by the CNF growth. A simple numerical calculation from NF82-ACF of 118 m2/g surface area and CNF of 200 m2/g provides 133-159 m2/g surface area of CNF/ACF products at a CNF yield of 1.5-2.0 product per starting (wt/wt). Although experimental surface areas of CNF/ACF products without partial oxidation were slightly smaller than the calculated ones, the recovery of surface area was found to be difficult in this case as in the experimental results. The partial oxidation in air after catalyst deposition on ACF resulted in sharp increase of surface area with development of slightly larger pores than those of the
Figure 5. Illustration of formation of multi surface CNF/ACF composites through catalyst impregnation and CNF synthesis with or without partial oxidation.
Carbon Nanofiber/Fiber Composites
original ACF (see Figure 2b). From the results, the carbon consumption by oxidation around catalysts appeared to open partially the pores blocked by the catalyst particles, forming a different pore system where the pore structure and morphology were complicated in the three-dimensional view by formation of new spaces among the carbon surface and catalyst particles, as illustrated in Figure 5. The surface area of CNF/ACF composites produced after the partial oxidation decreased depending on the content of CNF grown on ACF. However, surface areas measured in the experiment were much lower than the values calculated for simple mixtures of PO-ACF (1625 m2/g) and CNF (200 m2/g). In the calculation, the CNF yield of 1.1, 1.5, and 2.0 (product per starting, w/w) should provide CNF/ACF products of 1495, 1150, and 912 m2/g surface areas, respectively. Such an inconsistency in surface area of calculation and measurement suggests a progressive blockage of the micropores of the ACF according to CNF growth, especially over the catalyst located in the pores. As shown in SEM or TEM, the CNFs grown on ACF were not straight, had entanglements, and exhibited nodes along their axes. Such a knotted CNF is likely to fill more open space in the pores of ACF. However, high surface areas of 500-1000 m2/g achieved by partial oxidation prior to CNF growth on the ACF suggest that a large amount of micropores on ACF still contribute the surface properties of the composites. Thus, free edges of CNF and micropores of ACF are expected to cooperatively perform surface functions. CNFs grown on ACF had the diameters distributing quite uniformly around 10 nm. The diameter of CNF has been known to depend on the catalyst size.12 The catalyst particles dispersed on the carbon surface can be sintered with near particles, forming bigger ones through catalyst reduction in hydrogen at a high temperatures such as 600 °C.26,27 The micropores of the ACF appear to be effective to finely disperse the catalyst precursors in the impregnation step and then to suppress severe sintering of the active catalysts reduced in hydrogen, providing CNFs of uniformly thin diameters. On the other hand, CNF of around 1.0 nm corresponding to the micropore dimension was not found in the products. Very fine catalyst particles present in the micropores of ACF may be inactive or easily deactivated by loss of adsorption surface to carbon sources.12,24,25 The growth of CNF appears to occur on the
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catalyst particles of favorable sizes with interaction of carbon sources, as described by Otsuka et al.28,29 and Ermakova et al.30 Concluding Remarks In this article, we have described a series of preparations of CNF/ACF composites on the purpose of introduction of a new catalytic surface (CNF) on ACF surface with the active sites of ACF exposed after the procedure. Partial oxidation prior to growing CNF was necessary for exposing ACF surface in CNF/ACF composites. The conclusion is summarized as follows. (1) CNF/ACF composites in this work provide a free surface of CNF as well as microporous surface of ACF. Such a protruded free surface is likely to be advantageous to continuous H2SO4 recovery in DeSOx process. Fixation of CNFs on macroscopic frames may release handling and pressure drop problems in practical processes. (2) The pore size was found to be controlled through growth of CNF on ACF through partial oxidation. (3) More hydrophobic catalytic surface was prepared through removal of heteroatoms through preparation of CNF/ACF composites, which may be also preferential for such application as DeSOx and DeNOx. Detailed examination on the products in this study is in progress in several applications, especially DeSOx. Acknowledgment. This study was carried out within the framework of the CREST program. The present authors acknowledge financial support of the Japan Science and Technology Corporation (JST) of Japan. LA036077T (23) Sanada, Y.; Suzuki, M.; Fujimoto, K. Active Carbon; Koudansha: Tokyo, 1992; p 190. (24) Goodman, D. W.; Kelley, R. D.; Madey, T. E.; Yates Jr., J. T. J. Catal. 1980, 63 (1), 226-234. (25) Nakamura, J.; Hirano, H.; Xie, M.; Matuo, I.; Yamada, T.; Tanaka, K. Surf. Sci. 1989, 222 (1), L809-L817. (26) Guerrero-Ruiz, A.; Sepulveda-Escribano, A.; Rodriguez-Ramos, I. Appl. Catal., A 1992, 81, 81-100. (27) Gleiter, H. Prog. Mater. Sci. 1989, 33 (4), 223-315. (28) Otsuka, K.; Ogihara, H.; Takenaka, S. Carbon 2003, 41, 223233. (29) Takenaka, S.; Ogihara, H.; Otsuka, K. J. Catal. 2002, 208, 5463. (30) Ermakova, M. A.; Ermakov, D. Y.; Chuvilin, A. L.; Kuvshinov, G. G. J. Catal. 2001, 201, 183-197.
