J. Phys. Chem. C 2007, 111, 2679-2682
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Pd-Doped Double-Walled Silica Nanotubes as Hydrogen Storage Material at Room Temperature Jong Hwa Jung,*,† Jeong Ah Rim,‡ Soo Jin Lee,† Sung June Cho,§ Se Yune Kim,| Jeung Ku Kang,| Young Min Kim,‡ and Youn Joong Kim‡ Department of Chemistry and Research Institute of Natural Sciences, Gyeongsang National UniVersity, Chinju, 660-701, Korea, Korea Basic Science Institute (KBSI), 52 Yeoeun-dong, Yusung-gu, Daejeon, 305-333, Korea, Department of Applied Chemical Engineering, Chonnam National UniVersity, Gwangju, 500-757, Korea, and Department of Materials Science and Engineering, Korea AdVanced Institute of Science and Technology, Daejeon, 305-701, Korea ReceiVed: October 10, 2006; In Final Form: NoVember 24, 2006
Palladium-doped double-walled silica nanotubes (SNTs) were synthesized by sol-gel transcription, and H2 adsorption experiments were carried out at 298 K. The H2-storage capacities of the palladium-doped SNTs were 0.15 wt % for SNT-5, 1.90 wt % for SNT-10, and 0.75 wt % for SNT-15, showing that the palladiumdoped tubes are very effective as an H2-storage material even at room temperature. Remarkably, SNT-10 had the highest storage capacity. The results indicate that palladium nanoparticles acted as the primary receptor for adsorption of atomic hydrogen. Also, this finding suggests that a moderate amount of palladium nanoparticles is important to effectively uptake H2 in the nanotubes. We suggest that palladium-doped doublewalled SNTs are useful as H2-storage materials in fuel cells.
Introduction Hydrogen has received much interest as an energy source for the replacement of fossil fuels in fuel-cell vehicles and portable electronics due to its clean combustion and high heating value. The development of fuel-cell vehicles and portable electronics will require new materials that can store large amounts of hydrogen at ambient temperature and relatively low pressures while possessing a small volume, low weight, and fast kinetics for recharging.1 However, no practical means for hydrogen storage and transportation have yet been developed.2 Various materials including metal hydride, chemical hydride, carbon nanostructures, and metal-organic frameworks (MOFs) have been employed for hydrogen storage.3 This area has been dominated by announcements of high-storage capacities in carbon nanostructures over the past few years.4,5 However, a critical review shows that at room temperature and moderate pressure carbon nanostructures cannot store the amount of hydrogen required for automotive applications.6,7 Recently, MOFs have received much attention as a new approach for preparing porous materials because it allows more flexible and rational design of such materials.8-10 For example, R. T. Yang’s group11-13 reported that the hydrogen storage abilities of MOFs containing Pt or Pd on active carbon were dramatically enhanced by spillover effect, because Pt and Pd acted as the primary driving force for adsorption of atomic hydrogen. However, a serious shortcoming of these materials has been the framework’s instability, such as the collapse of the framework upon guest molecule removal. * Corresponding author. Telephone: 82-55-751-6027. Fax: 82-55-7586027. E-mail:
[email protected]. † Gyeongsang National University. ‡ KBSI. § Chonnam National University. | Korea Advanced Institute of Science and Technology.
One candidate for a hydrogen storage medium is based on a certain type of periodic porous silica material that appears to have the potential to store hydrogen under conditions of room temperature and moderate pressure.3,14-16 These materials possess the high surface area and porosity of carbon nanotubes and MOFs that may be important for adsorption of hydrogen. Moreover, the open channels in the nanomaterials are ordered and allow hydrogen effective access to the interior space.17,18 In view of their excellent properties, we chose to examine the porosity and hydrogen adsorption properties of Pd-doped double-walled silica nanotubes (SNTs). Pd nanoparticles would act as the primary receptor for atomic hydrogen. Also, SNTs would act as secondary spillover receptors. We have also examined the hydrogen adsorption capacities of SNTs without Pd nanoparticles, which play an important role in hydrogen uptake, as a comparison. Herein, we first report the adsorption of hydrogen at room temperature under moderate pressure by palladium-doped double-walled SNTs with ordered open channels. We found that the palladium-doped SNTs were very effective as H2-storage material, even at room temperature, at which the H2-storage capacity of SNT-10 was higher than that of other nanomaterials. Thus, these nanotubes may be suitable as H2-storage material. Experimental Section Hydrogen Adsorption Measurements. PCT adsorption apparatus equipped with a SETRA gauge Model 256 (-1 to 40 bar and (0.1% FS) was employed to measure the hydrogen adsorption at 298 K. Each cell was made of stainless steel and was calibrated before the sample measurement. Each stainless steel cell containing typically 500 mg was fully degassed overnight to remove the adsorbed impurities until the pressure dropped to 1.0 × 10-2 kPa. The hydrogen gas was 99.999% pure (Praxair Co.); after further purification the MnO/SiO2 trap
10.1021/jp066644w CCC: $37.00 © 2007 American Chemical Society Published on Web 01/19/2007
2680 J. Phys. Chem. C, Vol. 111, No. 6, 2007
Jung et al.
SCHEME 1: Overall Scheme for the Synthesis of Palladium-Doped Double-Walled Silica Nanotube Obtained from SolGel Transcriptiona
a (a) Organogelator 1, (b) self-assembled organogel tube 1, (c) organogel tube complex formation with Pd(II) ion, (d) sol-gel polymerization of TEOS, (e) double-walled silica nanotube after calcination, and (f) the palladium-doped silica nanotube after reduction of Pd(II) with ascorbic acid.
was charged into the sample cell for the hydrogen uptake measurement. Each point in the adsorption isotherm was recorded using the FLUKE 189 true RMS multimeter after 30 min equilibrium. Measurement of Adsorption Storage of SNT-10 by Temperature-Programmed Desorption and Gas Chromatography (GC)-Mass Spectrometry (MS). A gas chromatograph equipped with a TCD (thermal conductivity detector) and the selected capillary column (CARBOXEN 1006PLOT) was used to observe the hydrogen storage properties of 10 wt % palladium-doped silica nanotubes. A stainless steel reactor containing the silica nanotubes was degassed for 8 h at 573 K. Hydrogen (99.999%) was charged under 6 MPa at 300 K for 3 h after the degassing process. Then, the reactor was surrounded by a liquid nitrogen cooled cryostat and by a heating element with a programmable power supply. The injection port of the gas chromatograph was connected directly to the reactor and high-purity Ar (99.999%; 1 atm) was used as a carrier gas. The temperature scanning range and its rate were set to 140-350 K and 3.0-5.6 K/min, respectively. The gas chromatograph separated one gas species and ensured that only hydrogen is involved in the desorption peak. The hydrogen amount is determined by integrating the peak area. Also, the output gas from the gas chromatograph was captured in a Tedlar bag (SUPELCO) in the temperature range of 273-350 K. Gas constituents of this gas are analyzed by GC-MS equipped with a mass selective detector using He as a carrier gas. Synthesis of Palladium-Doped Double-Walled Silica Nanotubes. Compound 1 (0.1 g) was dissolved in acetic acid (2 mL) by heating. The gel sample was cooled to room temperature. A moderate amount of Pd(Ac)2 solution was added to the self-assembled organic nanotube 1. Then, TEOS (0.5 g) and water (0.25 g) were added to the gel sample. The sample was heated until a clear solution was obtained and then left at room temperature for 1 week. Subsequently, the sample was heated at 200 °C for 2 h and 500 °C for 2 h under a nitrogen atmosphere and then kept at 500 °C under aerobic conditions for 4 h. Finally, 1.5 equiv of ascorbic acid was added as a reducing agent to dope the palladium nanoparticles between the two layers. Results and Discussion The overall synthetic procedure is depicted in Scheme 1. The organogelator 1was synthesized by a method reported previously.19 The tubular structure of the organogel 1 as a template was prepared by using 1 in acetic acid (see Supporting Information, Figure S1). The Pd(Ac)2 solution was added to the self-assembled organogel nanotube 1 and adsorbed onto the tube surface. Then, TEOS and water were added to the organogel nanotube-Pd(II) complex. Finally, the reaction
mixture was maintained for 1 week at room temperature. After calcination, the Pd(II) ion was completely reduced with 1.5 equiv of ascorbic acid. SNT-5, SNT-10, and SNT-15 were prepared by the same method; the numerals indicate the respective weight percent of doped palladium on surface of the SNTs. Parts a and b of Figure 1 show scanning electron microscopic (SEM) and high-voltage transmission electron microscopic (HVTEM) images of SNTs obtained from the organogel 1 in the presence of 10 wt % Pd(II) ion, respectively. The resultant silica shows well-ordered tubular structures with ca. 500 nm outer diameter even before calcination. In addition, there are aggregates of the silica as bundle structures with a unidirectional arrangement. The bundle-type SNTs are the first example of unidirectional arrangement in the bulk sol-gel reaction, which can be maintained under weak acidic conditions. The unidirectional bundle structure of the SNTs may result from the high anionic charge on the surface. After calcination, the morphology of the SNTs did not change. In HV-TEM images, SNT-10 revealed inner hollow cavities (Figure 1b). A nanospace with 2-5 nm between the layers was observed. Also, it can be clearly seen that palladium nanoparticles with ca. 3 nm diameter mainly exist between layers in the silica tubes, strongly indicating that these nanotubes consist of double layers. The electron diffraction pattern, shown in the insert of Figure 1c, exhibits two spots, which could be assigned to the (111) and (200) reflections of the face-centered cubic (fcc) Pd. The X-ray powder diffraction pattern also confirmed the fcc structure of metallic Pd. In addition, a high-resolution TEM image of a single particle revealed an atomic lattice fringe with a 0.225nm distance for (111) (Figure 1d), demonstrating the crystalline nature of the nanoparticles. Furthermore, SNT-5 and SNT-15 were characterized by HV-TEM (see Supporting Information, Figure S2). They showed properties like those of SNT-10. The nanospaces between the nanotube layer and the layer in SNT-5 and SNT-10 were ca. 10 and 15 nm, respectively, and contained palladium particles with 7-10 nm diameter. The deposition of palladium nanoparticles (10 wt %) in the double-walled SNTs was confirmed independently by elemental
H2-Storage Capacities of Pd-Doped SNTs
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2681 TABLE 1: Structural Properties of Silica Nanotubes
Figure 1. (a) FE-SEM and (b) HV-TEM images of palladium-doped double-walled silica nanotubes after calcination. (c) Electron diffraction pattern and (d) HV-TEM images of doped palladium particles.
Figure 2. TEM images with electron energy loss spectroscopy (EELS) of palladium-doped double-walled silica nanotube. (a) Zero-loss image and (b) silicon component, (c) oxygen component, and (d) palladium component after calcination. The outer diameter of the nanotube is 500 nm.
mapping with electron energy loss spectroscopy, as shown in Figure 2. The material (Figure 2a) contains silicon (Figure 2b), oxygen (Figure 2c), and palladium (Figure 2d) components after calcination. The distribution of the palladium nanoparticles
sample
BET surf. area (m2/g)
Langmuir surf. area (m2/g)
BJH pore diam (nm)
SNT SNT-5 SNT-10 SNT-15
45.9 160.6 264.3 190.3
62.6 218.9 440.2 314.7
13.8 1.8 3.0, 6.2 3.0, 6.2
shows a one-dimensional structure, strongly implying again that palladium nanoparticles were homogeneously deposited between the layers in the double-walled SNTs. Nitrogen adsorption-desorption isotherms and the pore diameter distribution of the SNTs were measured using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods. The BET surface area and BJH pore diameters of the palladium-doped SNTs with different amounts of palladium nanoparticles are shown in Figure S3 (see Supporting Information) and Table 1. The SNT, SNT-5, SNT-10, and SNT15 have BET surface areas of 45, 160, 264, and 190 m2/g, respectively. Interestingly, the surface area of the palladium-doped SNTs was dramatically larger than that of a pure SNT without palladium nanoparticles. Among palladium-doped SNTs, the surface area of SNT-10 was the largest. The nitrogen adsorption-desorption isotherm of the pure SNT exhibited a typical type II, whereas the nitrogen adsorption-desorption isotherms of palladium-doped SNT-5, SNT-10, and SNT-15 exhibited a typical type IV, characteristic of mesoporosity. The BJH pore diameters of three palladium-doped SNTs showed a typical mesoporous size (Figure S4B). Among these three, SNT-5 had a 1.8 nm pore diameter with a narrow monodispersion. On the other hand, SNT-10 and SNT-15 showed two pore size distributions. The pore size of SNT-10 was 3.0 nm with a larger area and 6.2 nm with a smaller area, whereas the pore size of SNT-15 was 3.0 nm with a smaller area and 6.2 nm with a larger area. Furthermore, the palladium particles were collected to measure particle size after the silica nanotubes were removed by dissolution in HF. Palladium particle size was measured using the dynamic light scattering method. The palladium diameters for SNT-5, SNT-10, and SNT-15 were 1.8, 3.5, and 6.0 nm with a narrow distribution, respectively (see Supporting Information, Figure S4). These palladium particle sizes are quite similar to the pore size of SNTs. The small size of the palladium nanoparticles induced the small pore size. Also, among the three palladium-doped double-walled SNTs, SNT-15 had the largest palladium nanoparticles, supporting the view that a high concentration of palladium results in large size particles with aggregation in the reduction process. We carried out H2 adsorption experiments using three SNTs doped with different amounts of palladium as well as using a pure SNT without palladium nanoparticles as a reference at 298 K. As shown in Figure 3, the H2-storage capacities of the palladium-doped SNTs were 0.15 wt % for SNT-5, 1.90 wt % for SNT-10, and 0.75 wt % for SNT-15, respectively, showing that the palladium-doped tubes are very effective as an H2storage material even at room temperature. Remarkably, SNT10 had the highest storage capacity. The results indicate that the palladium nanoparticles acted as the primary receptor for adsorption of atomic hydrogen.7 Also, this finding suggests that a moderate amount of palladium nanoparticles is important to effectively uptake H2 in the nanotubes. Once again, the palladium nanoparticles acted as a driving force to bind hydrogen molecules.13 Then, transport of atomic hydrogen from the palladium nanoparticle to the double-walled silica nanotube is spillover, as shown in Figure S6. In addition, a similar
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Jung et al. Conclusion This study clearly demonstrates that an H2-storage material must have an indispensable effective adsorption site, approximately nanometer pore size, and a large surface area with narrow distribution. Furthermore, both ends of the nanotube should be open. We suggest that the palladium-doped doublewalled SNTs are useful as H2-storage materials in fuel cells. In particular, the H2 adsorption capacity of 1.90 wt % was achieved at 298 K under moderately low pressure using the palladiumdoped double-walled SNTs obtained from the self-assembling superstructure of an organogel by sol-gel reaction. We also anticipate that further increases in performance can be expected with new metal-doped inorganic nanotubes that have a large surface area and a nanometer pore size.
Figure 3. H2 adsorption curves for (a) SNT, (b) SNT-5, (c) SNT-10, and (d) SNT-15 at 298 K.
spillover effect was also observed in a Pd-containing zerolite system, whose atomic hydrogen is adsorbed inside the zeolite structure by spillover effect.20 The surface area and the pore volume of SNT-10 were much larger than those of SNT-5 and SNT-15. These results suggest that the surface area, the pore size, and the pore volume of the nanotube are important factors in the uptake of high amounts of H2 in inorganic nanomaterials. The significant enhancement for the H2 uptake of SNT-10 was clear evidence for spillover effect.7 To confirm whether water exists in the adsorption process of H2, first, the adsorption of H2 in SNT-10 was determined with temperature-programmed-desorption spectroscopy in an ultrahigh-vacuum chamber equipped with a liquid nitrogen cooled crystat. As shown in Figure S5, the temperatureprogrammed-desorption peak appeared around 300 K. The amount of H2 was about 1.85 wt %, indicating that the adsorption-desorption process of H2 is reversible. Then, as described, we tried to determine whether water exists in the desorption process by GC-MS (Figure S7). As a result, we confirmed no peak corresponding to water content (Figure S7a). Therefore, the results strongly suggest that the H2 stored in the Pd-doped double-walled silica nanotube does not contain water. On the other hand, the H2-storage capacity of the pure SNT without palladium nanoparticles was only 0.1 wt % (Figure 3, curve a), supporting the view that the palladium nanoparticle acted as a driving force in absorbing H2 in the palladium-doped tubes. Also, this finding suggests that a small inner diameter and large surface area of the palladium-doped tubes are crucial for obtaining high storage capacities for H2. Repeated adsorption experiments, carried out using identical experimental conditions, showed that the H2 storage is fully constant, with the H2-storage capacity remaining almost unchanged over several repeats of adsorption. Once again, the H2-storage capacity value obtained for SNT-10 is much higher than those obtained using activated carbon (0.6 wt %) or other nanomaterials (less than 1.0 wt %).9
Acknowledgment. This work was partially supported by grants from the Korea Energy Management Corporation and the Korea Research Foundation (KRF-2005-005-J09703 and KRF-2005-070-C00068). Supporting Information Available: TEM and SEM observations for organogel 1; SEM and TEM images of SNT-5 and SNT-15; BET isotherms and pore size distributions of SNT, SNT-5, SNT-10, and SNT-15; chromatogram obtained in H2 desorption process of SNT-10 by GC-MS. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127 and references therein. (2) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353. (3) Seayad, A. M.; Antonelli, D. M. AdV. Mater. 2004, 16, 765 and references therein. (4) Schimmel, H. G.; Kearley, G. J.; Nijkamp, M. G.; Visser, C. T.; de Jong, K. P.; Mulder, F. M. Chem.sEur. J. 2003, 9, 4764. (5) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127. (6) Hirscher, M.; Becher, M. J. Nanosci. Nanotechnol. 2003, 3, 3. (7) Schimmel, H. G.; Kearley, G. J.; Nijkamp, M. G.; Visser, C. T.; de Jong, K. P.; Mulder, F. M. Chem.sEur. J. 2003, 9, 4764. (8) Sudik, A. C.; Millward, A. R.; Ockwig, N. W.; Coˆte´, A. P.; Kim, J.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 7110. (9) Kubota, Y.; Takata, M.; Matsuda, R.; Kitaura, R.; Kitagawa, S.; Kato, K.; Sakata, M.; Kobayashi, T. C. Angew. Chem., Int. Ed. 2005, 44, 920. (10) Rowsell, J. L. C.; Millward, A. R.; Park, K. S.; Yaghi, O. M. J. Am. Chem. Soc. 2004, 126, 5666. (11) Li, Y.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 726. (12) Li, Y.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 8136. (13) Lachawiec, A. J.; Qi, G. S.; Yang, R. T. Langmuir 2005, 21, 11418. (14) Pang, J.; John, V. T.; Loy, D. A.; Yang, Z.; Lu, Y. AdV. Mater. 2005, 17, 704. (15) Kapoor, M. P.; Setoyama, N.; Yang, Q.; Ohashi, M.; Inagaki, S. Langmuir 2005, 21, 443. (16) Matsumoto, A.; Misran, H.; Tsutsumi, K. Langmuir 2004, 20, 7139. (17) Stein, A. AdV. Mater. 2003, 15, 763. (18) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403. (19) Jung, J. H.; Rim, J. A.; Lee, S. J.; Lee, S. S. Chem. Commun. 2005, 468. (20) Scarano, D.; Bordiga, S.; Lamberti, C.; Ricchiardi, G.; Bertarione, S. Spoto, G. Appl. Catal., A 2006, 307, 3.
