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Characterization of Nitrogen-Alloyed Activated Carbon Fiber Cheol-Min Yang,† M. El-Merraoui,† Hiroko Seki,‡ and Katsumi Kaneko*,† Graduate School of Science and Technology, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan, and Analytical Center, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan Received March 1, 2000. In Final Form: November 24, 2000 N-Alloyed activated carbon fibers (ACFs) were prepared by chemical vapor deposition (CVD) of pyridine on pitch-based ACF at 973-1273 K for 30-120 min. The N-alloyed ACFs were characterized by N2 adsorption at 77 K, elemental analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The nitrogen content increases due to the pyridine CVD. XPS examination showed that the percent of quaternary nitrogens of total nitrogen atoms deposited increases remarkably up to 91% by the CVD at 1273 K, even though the surface concentration of nitrogen atoms was only 2.0%. The morphological development and surface chemistry of N-alloyed ACFs treated by pyridine CVD depended on the CVD temperatures. The CVD treatment decreased the surface area and pore volume, while the average micropore width was not significantly changed. The pyridine deposition at 1273 K for 60 min was the best preparation method of nitrogen-deposited ACF (ACF/Py1273-60) with high microporosity and positively charged surface nitrogen atoms. The micropore width of ACF/Py1273-60 was smaller than that of ACF by 0.1 nm.
Introduction Porous carbons have gathered much attention from science and technology1,2 and they hold a great potential for application in gas storage3,4 and electronic devices5,6 and for environmental technologies such as removal of pollutants. A variety of porous carbons, which have different morphologies such as fiber,7 film,8 and monolith9,10 and have different pore widths from ultramicropores to mesopores, have been developed, inducing new science and technology. Activated carbon fiber (ACF) is one of the recently developed microporous carbons, and it has large microporosity and excellent adsorption characteristics for various vapors. The micropore structures of various kinds of ACFs have been actively studied.11-22 Since pitch-based ACFs of different pore † ‡
Graduate School of Science and Technology. Analytical Center.
(1) Iijima, S. Nature (London) 1991, 354, 56. (2) Kaneko, K.; Ishii, C.; Kanoh, H.; Hanzawa, Y.; Setoyama, N.; Suzuki, T. Adv. Colloid Interface Sci. 1998, 76-77, 295. (3) Chambers, A.; Park, C.; Baker, R. T. K.; Rodriguez, N. M. J. Phys. Chem. B 1998, 102, 4253. (4) Menon, V. C.; Komarrei, S. J. Porous Solids 1998, 5, 43. (5) Flandrois, S.; Simon, B. Carbon 1999, 37, 165. (6) Qu, D.; Shi, H. J. Power Sources 1998, 74, 99. (7) Kaneko, K. Stud. Surf. Sci. Catal. 1997, 104, 679. (8) Sato, M.; Isobe, H.; Yamamoto, K.; Iiyama, T.; Kaneko, K. Carbon 1995, 33, 1347. (9) Bekyarova, E.; Kaneko, K. Langmuir 1999, 15, 7119. (10) Hanzawa, Y.; Kaneko, K.; Pekala, R. W.; Dresselhaus, M. S. Langmuir 1996, 12, 6167. (11) Kobayashi, N.; Enoki, T.; Ishii, C.; Kaneko, K.; Endo, M. J. Chem. Phys. 1998, 109, 1983. (12) Miyawaki, J.; Kanda, T.; Suzuki, T.; Okui, T.; Maeda, Y.; Kaneko, K. J. Phys. Chem. B 1998, 102, 2187. (13) Wang, Z. M.; Kaneko, K. J. Phys. Chem. B 1998, 102, 2863. (14) Kaneko, Y.; Ohbu, K.; Uekawa, N.; Fujie, K.; Kaneko, K. Langmuir 1995, 11, 708. (15) El-Merraoui, M.; Tamai, H.; Yasuda, H.; Kanata, T.; Mondori, J.; Kadai, K.; Kaneko, K. Carbon 1998, 36, 1769. (16) Lozano-Castello, D.; Cazorla-Amoros, D.; Linares-Solano, A.; Hall, P. J.; Fernandez, J. J. Stud. Sci. Catal. 2000, 128, 523. (17) Ravikovitch, P. I.; Vishnyakov, A.; Russo, R.; Neimark, A. V. Langmuir 2000, 16, 2311. (18) Mangun, C. L.; Benak, K. R.; Daley, M. A. Economy, J. Chem. Mater. 1999, 11, 3476.
width can be prepared by controlling the burnoff conditions, their hydrophobic micropore spaces are available for study by the confinement effect of molecular assemblies. Kaneko et al. showed an ordered structure or cluster formation of polar and nonpolar molecules in such micropore spaces even at 303 K.23-25 These adsorption properties of ACF are governed by the pore width and the pore wall chemistry. Hence the relationship between the pore width and adsorption characteristics has been studied. In the case of the por wall chemistry, dispersion of ultrafine metal oxide or metal particles or partial oxidation has been carried out in order to obtain better adsorbents or catalysts.26-31 However, the control of the pore wall chemistry by alloying with foreign atoms such as nitrogen or boron atoms has not been attempted. Porous carbon alloyed with nitrogen atoms should have a polar nature and, consequently, its physical properties will be different from unmodified ACFs. Kawabuchi et al.32,33 developed the chemical vapor deposition (CVD) technique (19) Endo, M.; Oshida, K.; Kobori, K.; Takeuchi, K.; Dresselhaus, M. S. J. Mater. Sci. 1995, 10, 1461. (20) Cazorla-Amoros, D.; Salinas-Martinez de Lecea, C.; AlcanizMonge, J.; Gardner, M.; North, A.; Dore, J. Carbon 1998, 36, 309. (21) Kawabuchi, Y.; Oka, H.; Kawano, S.; Mochida, I.; Yoshizawa, N. Carbon 1998, 36, 377. (22) Li, Z.; Kruk, M.; Jaroniec, M.; Ryu, S. K. J. Colloid Interface Sci. 1998, 204, 151. (23) Kaneko, K. Supramol. Sci. 1998, 5, 267. (24) Kaneko, K. Colloid Surf. 1996, 109, 319. (25) Ohkubo, T.; Iiyama, T.; Nishikawa, K.; Suzuki, T.; Kaneko, K. J. Phys. Chem. B 1999, 103, 1859. (26) Kaneko, K.; Kobayashi, Ai.; Suzuki, T.; Ozeki, S.; Kakei, K.; Kosugi, N.; Kuroda, H. J. Chem. Soc., Faraday Trans. 1 1988, 84, 1795. (27) Kaneko, K.; Kosugi, N.; Kuroda, H. J. Chem. Soc., Faraday Trans. 1 1989, 85, 869. (28) Nishi, Y.; Suzuki, T.; Kaneko, K. J. Phys. Chem. B 1997, 101, 1938. (29) Nishi, Y.; Suzuki, T.; Kaneko, K. Carbon 1998, 36, 1870. (30) Rodriguez-Reinoso, F.; Molina-Sabio, M.; Munecas, M. A. J. Phys. Chem. 1992, 96, 2707. (31) Marquez-Alvarez, C.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Carbon 1996, 34, 1509. (32) Kawabuchi, Y.; Sotowa, C.; Kishino, M.; Kawano, S.; Whitehurst, D. D.; Mochida, I. Langmuir 1997, 13, 2314. (33) Kawabuchi, Y.; Sotowa, C.; Kishino, M.; Kawano, S.; Whitehurst, D. D.; Mochida, I. Chem. Lett. 1996, 941.
10.1021/la000307b CCC: $20.00 © 2001 American Chemical Society Published on Web 01/06/2001
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Table 1. Elemental Composition of ACF and N-Alloyed ACFs, Obtained with XPS Spectra and C, H, and N Elemental Analysis XPS (%) sample
C
ACF ACF/Py1023-60 ACF/Py1073-60 ACF/Py1123-60 ACF/Py1173-60 ACF/Py1273-60
96.0 92.8 89.8 89.1 92.5 88.5
N
O
2.0 3.1 4.4 3.1 2.0
4.0 5.2 7.1 6.5 4.4 9.5
C, H, N elemental analysis (mol %) N/C
C
H
N
O
N/C
0.022 0.035 0.050 0.034 0.023
93.6 89.3 92.8 89.0 91.7 92.2
2.4 3.5 1.2 3.6 2.4 1.2
0.3 2.5 1.9 2.1 1.6 1.4
3.7 4.7 4.1 5.3 4.3 5.2
0.004 0.028 0.021 0.024 0.018 0.015
using the corresponding sensitivity factors in order to get quantitative data. Molar percentages of carbon, nitrogen, and oxygen of the measured layer were determined. The scanning electron microscope (SEM) was used to monitor the surface morphology change of the ACFs after pyridine CVD. SEM observations were carried out using a model S-4700 (Hitachi). The acceleration potential was 10 kV. Nitrogen adsorption isotherms were measured volumetrically with a commercial apparatus (Quantachrome AS-1-MP) at 77 K after preevacuation at 423 K for 2 h. The micropore structural parameters were obtained from high-resolution Rs-plots and Dubinin-Radushkevich (DR)-plots.
Results and Discussion Figure 1. Schematic diagram of CVD apparatus used to prepare N-alloyed ACFs.
with pyridine for modification of ACF, showing that the treated ACF has an excellent adsorption selectivity. However, they did not characterize sufficiently the modified ACFs. Their attempts shed a light on a new direction to develop highly functional porous carbon. Therefore, these authors prepared ACFs treated with pyridine CVD at various CVD conditions and characterized the treated ACFs in order to elucidate CVD roles in pore structure changes and surface chemistry on nitrogen-doped ACFs. Experimental Section Preparation of N-Alloyed Activated Carbon Fiber. Pitchbased ACF (A-20, Adohl Co.) was heated at 1273 K for 60 min in argon before deposition. Pyridine was used for preparing nitrogen-alloyed ACFs (N-alloyed ACFs). The CVD apparatus is schematically represented in Figure 1. The hot-wall reactor is a horizontal quartz glass tube (45 mm inner diameter) heated by a resistive electrical furnace. The helium carrier gas passes through a flask containing pyridine, and a mass flow controller controls its flow rate. The reactor was evacuated with a vacuum pump and then purged with helium before the heating treatment of the reactor. The deposition on 0.2 g of ACF was performed under the following conditions: flow rate of helium carrier gas, 200 mL min-1; time, 10-120 min; temperature, 973-1273 K. The treated ACF sample by pyridine CVD at a temperature T K for a duration t min is noted as ACF/Py T-t in this paper. For example, ACF treated with pyridine at 1023 K for 60 min is described ACF/Py1023-60. Characterization. The total chemical composition (C, H, N) of the CVD-treated ACFs was determined by elemental analysis using Perkin-Elmer-2400 after pretreatment at 423 K for 2 h. The surface composition and surface structure of the CVDtreated ACFs were examined by X-ray photoelectron spectroscopy (XPS) with an ESCA-850 electron spectrometer (Shimadzu). The measurements were performed with Mg KR under a vacuum pressure
