17448
J. Phys. Chem. C 2007, 111, 17448-17450
Supercritical Hydrogen Adsorption of Ultramicropore-Enriched Single-Wall Carbon Nanotube Sheet Dong Young Kim,† Cheol-Min Yang,† Masahiro Yamamoto,† Dae Ho Lee,† Yoshiyuki Hattori,‡ Kunimitsu Takahashi,§ Hirofumi Kanoh,† and Katsumi Kaneko*,† Graduate School of Science, Chiba UniVersity, 1-33 Yayoi, Inage, Chiba 263-8522, Japan, Faculty of Textile Science and Technology, Shinshu UniVersity, 3-15-1 Tokida, Ueda 386-8567, Japan, and Institute of Research and InnoVation, Laser Research Center, 1201 Takada, Kashiwa, Chiba, 277-0861, Japan ReceiVed: September 7, 2007
Ultramicropore-enriched single-wall carbon nanotube (SWCNT) sheet was prepared by a filtration process after acid treatment using HNO3/H2SO4 solution. Field emission scanning electron microscopic observation showed that a highly dense entangled structure of SWCNT bundles was formed by acid treatment. The pore structure of the SWCNT sheet was evaluated from adsorption isotherms of N2 at 77 K and CO2 at 273 K. The SWCNT sheet gave remarkably developed uniform ultramicroporosity and reduced mesoporosity through formation of the highly packed sheet. The acid treatment remarkably increased the adsorption amount of supercritical hydrogen at 77 K by about three times. The enhancement of hydrogen adsorption by acid treatment came mainly from the increase in the ultramicropores because the increase in isosteric heat of hydrogen adsorption by acid treatment was only 0.1-0.2 kJ mol-1.
Introduction Supercritical gas adsorption on nanostructured carbons has gathered great interest owing to their hopeful potential applications in clean energy storage.1-5 When the critical temperature of a gas is lower than the adsorption temperature, the gas is called a supercritical gas. Hydrogen, whose critical temperature is 33 K, is representative of clean fuels. Supercritical hydrogen gas cannot be condensed to liquid phase even by applying high pressure owing to a very weak intermolecular interaction against the thermal energy. Therefore, the saturated vapor pressure is not defined for a supercritical gas. It is not easy to get a highly dense adsorbed state of hydrogen using nanoporous solids under the supercritical condition. We need to design nanostructured materials fit for supercritical hydrogen adsorption. Single-wall carbon nanotubes (SWCNTs)1,6 in the bundle form have abundant adsorption sites of latent internal and interstitial spaces caused by hollow geometry and hexagonally packed bundle structure, respectively.7-10 Above all, all carbon atoms of SWCNT can contribute to adsorption directly, and thereby, it has been considered as a hopeful candidate for reversible storage of supercritical hydrogen.1,11-13 Extensive studies of supercritical hydrogen adsorption on the SWCNTs have been carried out; we do not sufficiently understand the relationship between the adsorption amount of supercritical hydrogen and the detailed nanostructure of the SWCNT bundle. As the interaction of a hydrogen molecule with the nanostructured carbon wall is not strong enough, we need to control the mutual orientation and stacking structure of the SWCNTs in the bundle to afford abundant ultramicropores (pore width < 0.7 nm) which are predicted to be better adsorption sites for supercritical hydrogen. Then, we tried to prepare ultramicropore-enriched SWCNTs. * To whom correspondence should be addressed. E-mail: kaneko@ pchem2.s.chiba-u.ac.jp. † Chiba University. ‡ Shinshu University. § Laser Research Center.
Figure 1. Photographs of flexible SWCNT-24 h sheet: (a) circular sheet form and (b) bent form held with tweezers.
Figure 2. Raman spectra of SWCNT samples: (a) RBM region, (b) D-band and G-band: (A) pristine SWCNTs; (B) SWCNT-24 h; (C) SWCNT-48 h.
Yang et al. found that HNO3/H2SO4 treatment could provide a highly ultramicroporous SWCNT sheet due to a closely packed assembly structure and opening of SWCNTs for HiPco SWCNTs.14 Although HiPco SWCNTs15 are less crystalline due to plenty of defects and Fe impurities, SWCNTs produced by laser ablation are well crystalline from Raman spectroscopic examination. The highly ordered bundle of well-crystalline SWCNTs of less metallic impurities is expected to provide uniform ultramicropores due to the above HNO3/H2SO4 treat-
10.1021/jp0772023 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/25/2007
Single-Wall Carbon Nanotube Sheet
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17449
TABLE 1: Pore Structure Parameters of SWCNT Samples Determined by SPE and DR Methods N2 (77 K): subtracting pore effect (SPE) method microporosity
CO2 (273 K) mesoporosity
sample
at (m2 g-1)
Vt (mL g-1)
amicro (m2 g-1)
Vmicro (mL g-1)
aext (m2 g-1)
Vmeso (mL g-1)
VDR (mL g-1)
pristine SWCNTs SWCNT-24 h SWCNT-48 h
275 980 1010
0.92 0.68 0.58
115 770 880
0.10 0.37 0.38
160 210 130
0.82 0.31 0.20
0.06 0.31 0.34
TABLE 2: Adsorption Parameters Determined by Supercritical DR Plots for Hydrogen Adsorption at 77 K sample
WL (mL g-1)
P0q (MPa)
βE0 (kJ mol-1)
qst,φ)1/e (kJ mol-1)
pristine SWCNTs SWCNT-24 h SWCNT-48 h
9.2 20.4 21.4
1.0 ( 0.4 1.6 ( 0.4 1.6 ( 0.4
3.4 ( 0.2 3.5 ( 0.2 3.6 ( 0.2
4.3 ( 0.2 4.4 ( 0.2 4.5 ( 0.2
ment. In this study, we describe that a predominant enriching of ultramicropores is quite efficient for enhancement of supercritical hydrogen adsorption for the SWCNTs produced by laser ablation. Experimental Section SWCNTs were synthesized by the laser ablation process with Ni and Co as catalysts at 1423 K. The SWCNTs were sonicated for 2 min at a power of 700 W and an output frequency of 20 kHz after immersion in ethanol. The sample was dried at 383 K for 24 h, which was used as the pristine SWCNT in this study. The pristine SWCNTs (80 mg) were immersed and stirred at room temperature in 80 mL of a mixture of HNO3 (63%) and H2SO4 (98%) with a volume ratio of 1:3 for 24 and 48 h according to the method developed for the HiPco-SWCNT in the preceding study.14 The sample was filtrated through the membrane filter with a pore diameter of 10 µm. The precipitate was washed with distilled water several times. The washed sample was again filtrated after sonication in distilled water for 30 min. The sample was dried at 383 K for 24 h and annealed in Ar ambient at 1173 K for 1 h. The obtained SWCNTs had a slightly flexible sheet form. The acid-treated SWCNTs for 24 or 48 h are designated SWCNT-24 h or SWCNT-48 h, respectively. Thermogravimetric analysis (TGA) [SIIEXSTAR-6000] was done in air (N2:O2 ) 80:20) with a heating rate of 5 K min-1. The morphology of the SWCNT samples was observed by field emission scanning electron microscopy (FE-SEM; JEOL JSM6330F) at a 5 kV accelerating voltage. Raman spectra were measured at room temperature under ambient conditions using a microprobe Raman spectrometer (JASCO NRS-3100) with an excitation laser with a wavelength of 532 nm (YAG laser). Pore structures of the SWCNT samples were determined by N2 (77 K) and CO2 (273 K) adsorption volumetrically (Quantachrome Autosorb1-MP) after pre-evacuation at 423 K and 10-4 Pa for 2 h. Pore structure parameters were obtained by the subtracting pore effect (SPE) method and Dubinin-Radushkevich (DR) methods. The SPE method was performed using high-resolution Rs plots constructed for standard adsorption data on nonporous carbon black of high crystallinity.16 We also measured adsorption isotherms of supercritical hydrogen on the SWCNT samples at 77 K up to 0.1 MPa volumetrically. The samples were pre-evacuated at 423 K and 10-4 Pa for 2 h prior to the adsorption measurement. Results and Discussion Figure 1 shows photographs of SWCNT-24 h. The SWCNT24 h is flexible, and the thickness of the sheet is 40 µm according to FE-SEM observation. The FE-SEM observation shows that the pristine SWCNTs have an irregularly tangled network structure and large interbundle voids, while the acid-
treated SWCNT-24 h has an exceedingly massed network structure without the voids (Supporting Information, Figure S1). The distinguished change of the morphology suggests that acid treatment gives rise to a more efficient packing in the mutual SWCNT alignment and the bundle structure. The residual catalyst contents of the pristine SWCNTs and SWCNT-24 h after burning up to 1100 K were 16.1 and 6.4 wt %, respectively (Supporting Information, Figure S2). Accordingly, one-step HNO3/H2SO4 treatment can give two marked effects on removal of metallic catalysts and change in the intra- and interbundle structures. The main burning temperature of SWCNT-24 h shifts to a higher temperature by 170 K compared to that of pristine SWCNTs, which should be attributed to reduction of metal catalysts and increment of the bundle size. Figure 2a and b shows Raman spectra in the radial breathing mode (RBM) and G-band regions, respectively. Raman spectrum in the RBM region of the pristine SWCNTs indicates a narrow distribution of the tube diameters in the range of 1.35 and 1.52 nm (corresponding to 186 and 167 cm-1, respectively).17 Acid treatment gives no significant changes in the RBM band. Also, the spectrum in the G-band region does not vary due to acid treatment. Hence, acid treatment does not change the primary tube structure but the intra- and interbundle structures. Figure 3a shows N2 adsorption isotherms of the pristine SWCNTs and acid-treated SWCNTs at 77 K. The pristine SWCNTs show a typical IUPAC type II adsorption isotherm with a gradual uptake of N2 in the medium relative pressure range. The predominant adsorption of N2 at higher P/P0 originates from multilayer adsorption on the external surface and in larger mesopores and macropores formed at spaces between SWCNT bundles. On the other hand, the N2 adsorption isotherms of the acid-treated SWCNTs are close to type I with a predominant uptake below P/P0 ) 0.1, showing the highly microporous nature. The N2 adsorption amount below P/P0 ) 0.1 dramatically increases after HNO3/H2SO4 treatments, as shown in the inset of Figure 3a. This is ascribed to enriching of micropores and reduction of mesopores, suggesting a development of closely packed assembly structures. Figure 3b shows high-resolution Rs plots for N2 adsorption isotherms of the pristine SWCNTs and acidtreated SWCNTs. The SPE method using Rs plot has been used as an effective method for determining the correct microporosity.16 At a low Rs region, the Rs plot of acid-treated SWCNTs shows a significant upward deviation due to the presence of uniform micropores. The pore structure parameters of the SWCNT samples evaluated from the SPE method are summarized in Table 1. HNO3/H2SO4 treatment remarkably increases the total surface area and micropore volume. In particular, acid treatment for 48 h noticeably increases the micropore surface area and micropore volume from 115 to 880
17450 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Kim et al. where WL is the saturated amount of hydrogen adsorption which can be determined from the Langmuir plot of the hydrogen adsorption isotherm, W is the adsorbed amount at adsorption pressure P, P0q is the quasi-saturated vapor pressure, and β and E0 are an affinity coefficient and characteristic adsorption energy, respectively. The βE0 value can lead to the isosteric heat of adsorption at fractional filling of 1/e using the relation
qst,φ)1/e ) ∆Hv + βE0 Figure 3. (a) N2 adsorption isotherms of SWCNT samples at 77 K. The open and filled symbols indicate adsorption and desorption branches, respectively. The inset shows N2 adsorption isotherms on logarithmic scale. (b) Rs plots of the N2 adsorption isotherms for SWCNT samples: pristine SWCNTs (O), SWCNT-24 h (4), and SWCNT-48 h (0).
Figure 4. Hydrogen adsorption isotherms of SWCNT samples at 77 K. The open and filled symbols indicate adsorption and desorption branches, respectively: pristine SWCNTs (O), SWCNT-24 h (4), and SWCNT-48 h (0).
m2 g-1 and from 0.10 to 0.38 mL g-1, respectively. On the other hand, both the external surface area and the mesopore volume decrease with HNO3/H2SO4 treatment. This diminution in the mesoporosity should originate from orientational repacking of the intra- and interbundle structures of SWCNTs to provide micropores. The total surface area of the acid-treated SWCNTs is more than three times larger than that of the pristine SWCNTs. In particular, the micropore surface area after acid treatment increases up to a seven or eight times larger value. The micropore volume by acid-treated SWCNTs becomes almost four times larger. CO2 adsorption at 273 K leads to information on narrow micropores, mainly ultramicropores with a width of less than 0.7 nm.18 The CO2 adsorption isotherms of the acid-treated SWCNTs at 273 K are of Langmurian, suggesting the presence of abundant ultramicropores (Supporting Information, Figure S3). The ultramicropore volume determined by the DR plot of the CO2 adsorption isotherm is shown in Table 1. The CO2 adsorption data indicates that the ultramicropore volume of the acid-treated SWCNTs is about five times larger than that of pristine SWCNTs. Accordingly acid treatment is quite efficient for production of ultramicropores that are expected to be fit for supercritical hydrogen adsorption. Figure 4 shows adsorption isotherms of supercritical hydrogen of the pristine SWCNTs and acid-treated SWCNTs at 77 K up to 0.1 MPa. The hydrogen adsorption amount is dramatically enhanced by acid treatment, corresponding to the increase of the ultramicroporosity. The hydrogen adsorption is completely reversible. The following supercritical DR equation has been widely used to describe supercritical gas adsorption in micropores19,20
[ln(WL/W)]1/2 ) (RT/βE0)(ln P0q - ln P)
(1)
(2)
where ∆Hv is the enthalpy of vaporization. The supercritical DR plots are almost linear below 10-2 kPa (Supporting Information, Figure S4). WL, P0q, βE0, and qst,φ)1/e determined from the linear part of supercritical DR plots are summarized in Table 2. As the qst,φ)1/e value is more reliable than the P0q one, the qst,φ)1/e values can be compared with each other. The qst,φ)1/e values of the acid-treated SWCNT samples are slightly larger than that of the pristine SWCNTs. The qst,φ)1/e difference is only 0.1-0.2 kJ mol-1, suggesting that the width of the ultramicropores added by acid treatment is almost similar to that of the pristine SWCNTs. Thus, acid treatment is an efficient method to increase the ultramicroporosity, which is fit for supercritical hydrogen adsorption. Acknowledgment. This work was partially supported by a Grant-in-Aid for Fundamental Scientific Research (S) (grant no. 15101003) and by the Evaluation of Hydrogen Storage on Nanocarbons, NEDO. Supporting Information Available: FE-SEM images of pristine SWCNTs and SWCNT-24 h, TGA curves of pristine SWCNTs and SWCNT-24 h, CO2 adsorption isotherms of SWCNT samples at 273 K, supercritical DR plots of the hydrogen adsorption isotherms of SWCNT samples at 77 K. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (2) Kowalczyk, P.; Hołyst, R.; Terrones, M.; Terrones, H. Phys. Chem. Chem. Phys. 2007, 9, 1786. (3) Cao, D.; Zhang, X.; Chen, J.; Wang, W.; Yun, J. J. Phys. Chem. B 2003, 107, 13286. (4) Chambers, A.; Park, C.; Baker, R. T. K.; Roddriguez, N. M. J. Phys. Chem. B 1998, 102, 4253. (5) Murata, K.; Miyawaki, J.; Yudasaka, M.; Iijima, S.; Kaneko, K. Carbon 2005, 43, 2826. (6) Iijima, S. Nature 1991, 354, 56. (7) Yoo, D.-H.; Rue, G.-H.; Chan, M. H. W.; Hwang, Y.-H.; Kim, H.-K. J. Phys. Chem. B 2003, 107, 1540. (8) Cinke, M.; Li, J.; Chen, B.; Cassell, A.; Delzeit, L.; Han, J.; Meyyappan, M. Chem. Phys. Lett. 2002, 365, 69. (9) Burde, J. T.; Calbi, M. M. J. Phys. Chem. C 2007, 111, 5057. (10) Yang, C.-M.; Kaneko, K.; Yudasaka, M.; Iijima, S. Nano Lett. 2002, 2, 385. (11) Du, W.-F.; Wilson, L.; Ripmeester, J.; Dutrisac, R.; Simard, B.; Denommee, S. Nano Lett. 2002, 2, 343. (12) Yildirim, T.; Ciraci, S. Phys. ReV. Lett. 2005, 94, 175501. (13) Darkrim, F. L.; Malbrunot, P.; Tartaglia, G. P. Int. J. Hydrogen Energy 2002, 27, 193. (14) Yang, C.-M.; Kim, D. Y.; Lee, Y. H. Chem. Mater. 2005, 17, 6422. (15) Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A.; Bronikowski, M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H. J. Phys. Chem. B 2001, 105, 8297. (16) Ohba, T.; Kaneko, K. J. Phys. Chem. B 2002, 106, 7171. (17) Kramberger, Ch.; Pfeiffer, R.; Kuzmany, H.; Zo´lyomi, V.; Ku¨rti, J. Phys. ReV. B 2003, 68, 235404. (18) Cazorla-Amoros, D.; Alcaniz-Monge, J.; Linares-Solano, A. Langmuir 1996, 12, 2820. (19) Kaneko, K. Colloids Surf. 1989, 37, 115. (20) Kaneko, K.; Murata, K. Adsorption 1997, 3, 197.
