High-Pressure Hydrogen Storage in Zeolite-Templated Carbon

Feb 4, 2009 - Department, Electronics and Power Electronics Engineering DiVision, Nissan Motor Co., Ltd.,. Kanagawa 243-0123, Japan, and Carbon ...
1 downloads 0 Views 912KB Size
Download PDF
J. Phys. Chem. C 2009, 113, 3189–3196

3189

High-Pressure Hydrogen Storage in Zeolite-Templated Carbon Hirotomo Nishihara,*,† Peng-Xiang Hou,† Li-Xiang Li,†,‡ Masashi Ito,§ Makoto Uchiyama,§ Tomohiro Kaburagi,§ Ami Ikura,§ Junji Katamura,| Takayuki Kawarada,⊥ Kazuhiko Mizuuchi,⊥ and Takashi Kyotani† Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Sendai 980-8577, Japan, Chemical Engineering School, UniVersity of Science and Technology Liaoning, 185 Qianshan Zhong Road, Anshan, 114051, China, Frontier Research Group, Society and Frontier Laboratory, Nissan Research Center, Nissan Motor Co., Ltd., Kanagawa 237-8523, Japan, Battery Engineering Group, EV Energy DeVelopment Department, Electronics and Power Electronics Engineering DiVision, Nissan Motor Co., Ltd., Kanagawa 243-0123, Japan, and Carbon Materials Laboratories Technical Bureau, Nippon Steel Chemical Co., Ltd., Fukuoka 804-8503, Japan ReceiVed: October 8, 2008; ReVised Manuscript ReceiVed: December 12, 2008

High-pressure hydrogen storage in zeolite-templated carbon (ZTC) was investigated at room temperature (30 °C). Several types of ZTCs with different surface areas and a nitrogen-doped ZTC were prepared. Their hydrogen storage performance at room temperature was examined and the results were compared with those of commercial activated carbons. At pressures below 10 MPa, the hydrogen uptake capacity was simply proportional to specific surface areas of the carbons, and both ZTCs and activated carbon showed almost the same heat of adsorption (6∼8 kJ mol-1). On the other hand, at pressures above 10 MPa, uniform micropores with a diameter of 1.2 nm in ZTCs played a more important role in capacity increase than the specific surface area. As a result, the ZTC with the largest surface area (3370 m2 g-1) exhibited hydrogen uptake as high as 2.2 wt % at 34 MPa. This value is much larger than that of the activated carbon, and such a difference in the capacity between ZTC and activated carbon cannot be explained by the difference in specific surface area alone. Moreover, by loading only a small amount of Pt nanoparticles (ca. 0.2 wt %) onto ZTC, hydrogen uptake capacity was increased from 0.87 to 0.95 wt % at 10 MPa. The increase of hydrogen uptake capacity by Pt loading can be ascribed to hydrogen spillover through the supported Pt nanoparticles to the carbon surface. Introduction A fuel cell vehicle is a next-generation automobile that uses an energy source other than crude oil derivatives. However, there are several problems that should be solved before practical use. One of them is how to store hydrogen; that is, the development of a safe and efficient hydrogen storage system is required.1 There are several methods to store hydrogen: cryogenic liquid, compressed gas, metal hydrides, and adsorption. Each of them has advantages and disadvantages. Among these methods, adsorption of hydrogen in porous solids, especially carbon materials, is an attractive choice,2-12 They enjoy a long lifecycle, high durability, and hydrogen release at low temperature but lack sufficient storage capacity at room temperature. On the basis of physisorption, large surface area,7,8 large micropore volume,3,4,6 and a suitable pore size3,4,13 are highly desirable for carbon materials to store enough hydrogen at room temperature. Highly microporous carbons are therefore the most promising, rather than carbon nanotubes or carbon nanofibers. CazorlaAmoro´s and co-workers4 have indeed shown that a chemically activated carbon, KUA5, which they prepared, has such an appropriate pore structure, and hence exhibited the highest * Corresponding author: e-mail [email protected]. † Tohoku University. ‡ University of Science and Technology Liaoning. § Nissan Research Center, Nissan Motor Co., Ltd. | Electronics and Power Electronics Engineering Division, Nissan Motor Co., Ltd. ⊥ Nippon Steel Chemical Co., Ltd.

hydrogen uptake capacity at room temperature among the microporous carbons that have been reported so far. If it is possible to prepare microporous carbons with larger surface area and larger micropore volume than those of KUA5, larger storage capacity will be expected. In the conventional chemical activation, larger surface area can be obtained under severer activation conditions, but at the same time, such activation significantly increases the amount of unnecessary mesopores. It is thus in principle impossible to further increase both the surface area and microporous volume simultaneously by conventional activation methods. The use of a zeolite template in carbon synthesis can overcome this problem and produce microporous carbons (hereinafter referred to as zeolite-templated carbon, ZTC) with very large surface area together with very large micropore volume.14-19 Very recently, several groups have reported the hydrogen storage performance of several types of ZTCs at -196 °C.20-24 It is noteworthy that Mokaya and co-workers23 showed that ZTC exhibits the highest hydrogen uptake capacity among porous materials, including porous carbons, carbon nanotubes, zeolites, and metal-organic frameworks. In addition, Lachawiec and Yang25 have recently reported noticeable large hydrogen uptake on ZTC at room temperature (up to 10 MPa). However, the relationship between the pore structure of ZTC and its high hydrogen uptake performance at room temperature is not fully understood. Moreover, its performance over 10 MPa is still unknown, though an actual hydrogen tank for the fuel cell vehicle requires superhigh pressure much more than 10

10.1021/jp808890x CCC: $40.75  2009 American Chemical Society Published on Web 02/04/2009

3190 J. Phys. Chem. C, Vol. 113, No. 8, 2009

Figure 1. XRD patterns of ZTCs.

MPa. In this work, we reveal key factors that make the performance of ZTC higher than that of other activated carbons, and we examine the behavior of ZTC as a high-pressure hydrogen storage medium at room temperature and ultrahigh pressure up to 34 MPa. The nanostructure of ZTC is unique compared with that of conventional activated carbons; the framework of ZTC comprises nanosized single and curved graphenes that assemble into three-dimensionally ordered network structure, and therefore, ZTC has extremely sharp pore size distribution.26 Here we synthesized several types of ZTCs with different specific surface areas by using different synthesis conditions and examined their hydrogen uptake capability. Moreover, we also synthesized ZTC containing nitrogen atoms in its framework to examine the effect of nitrogen doping. By introducing metal nanoparticles such as Pt, Pd, and Ni into microporous carbons, it is possible to enhance hydrogen uptake through chemisorption and/or spillover mechanism.25,27-33 Yang and co-workers25,32 have recently reported a remarkable increase of hydrogen uptake at room temperature by introducing Pt nanoparticles (6 wt %) onto ZTC (the BET surface area of the Pt-loaded ZTC was 2755 m2 g-1). They ascribed this increase to the effect of hydrogen spillover to the carbon surface. In addition, there are several other reports on the synthesis of Ptloaded ZTC for fuel cell application, but their surface areas are not high enough (3410 m2 g-1). For practical use, the amount of Pt should be as small as possible. Here we demonstrate that even a very small amount of Pt (only 0.2 wt %) can effectively increase hydrogen uptake capacity. Experimental Section Preparation of ZTCs. Zeolite Y (Na form, SiO2/Al2O3 ) 5.6, obtained from Tosoh Co., Ltd.) was used as a template. Dried zeolite was impregnated with furfuryl alcohol (FA) at room temperature under vacuum. After the sample was washed with mesitylene to remove FA from the external surface of the zeolite powder, the FA inside the zeolite channels was polymerized by heating the powder at 150 °C for 8 h under N2 flow. The resulting composite (referred to as PFA/zeolite composite) was then heated at 5 °C min-1 under N2 up to a temperature of 700 °C. When the temperature reached 700 °C, chemical vapor deposition (CVD) of propylene (7% in N2) was accomplished for 2 h to increase the amount of carbon deposition within the zeolite channels. After the CVD treatment, the carbon/zeolite composite was heat-treated at 900 °C for 3 h under a N2 flow. Finally, the zeolite Y template was dissolved by HF treatment (47% aqueous solution), and the resulting liberated carbon was

Nishihara et al. washed with copious amounts of water and vacuum-dried at 150 °C for 8 h. ZTC thus obtained is referred to as P7(2)-H, where P, 7, (2), and H mean propylene CVD, CVD temperature (700 °C), CVD hours (2 h), and heat treatment at 900 °C, respectively. In addition to P7(2)-H, other types of ZTCs were also synthesized under different preparation conditions. One of them is P7(2), which was synthesized by the same CVD process as P7(2)-H but without the final heat treatment at 900 °C. The other is P8(4), which was synthesized at a higher CVD temperature (800 °C) and for a longer CVD time (4 h). Moreover, a nitrogen-doped ZTC was synthesized in accordance with the method previously reported.36 Briefly, the PFA/zeolite composite was heated at 5 °C min-1 under N2 up to a temperature of 850 °C and then CVD of acetonitrile (4.2% in N2) was accomplished for 2 h. After the CVD treatment, the carbon/zeolite composite was heat-treated at 900 °C for 1 h under a N2 flow. The liberated nitrogen-doped ZTC is referred to as AN8.5(2)-H. Four types of commercial activated carbons were used as references: MSC30, MSP20 (both from Kansai Coke and Chemicals Co., Ltd.), Norit A SUPRA EUR (Norit Japan Co., Ltd., referred to as ASE), and Diasorb (Calgon Mitsubishi Chemical Co.). Pt Loading on ZTC. About 100 mg of P7(2)-H was mixed with 6.68 mL of 0.19 M Pt(NH3)2(NO2)2/nitric acid solution, and the resulting mixture was stirred at 0 °C under vacuum for 30 min. Then the ZTC (P7(2)-H) was separated from the solution by centrifugation, followed by washing with pure water. Next, the Pt species adsorbed in ZTC were reduced by adding 66.7 mL of NaBH4 aqueous solution (0.0025 M) at 0 °C for 20 min. Finally, the reduced sample was washed with pure water several times and dried at 150 °C for 6 h under vacuum. The Pt-loaded ZTC is referred to as Pt/P7(2)-H. From analysis on an energy-dispersive X-ray spectroscope equipped with a scanning electron microscope (SEM; Hitachi, S-4800), the Pt content in the final product was found to be about 0.2%. To prepare an amount of sample large enough for the measurements of hydrogen adsorption isotherms, the above process was conducted five times and, as a result, 500 mg of Pt/P7(2)-H was obtained. Characterization. Powder X-ray diffraction (XRD) patterns of ZTCs were recorded with an X-ray diffractometer (Shimadzu, XRD-6100) with Cu KR radiation generated at 30 kV and 20 mA. Nitrogen physisorption measurements were carried out at -196 °C on a volumetric sorption analyzer (BEL Japan, BELSORP-max). The specific surface areas were calculated by two methods: a Brunauer-Emmett-Teller (BET) method (using the data in the relative pressure range of 0.01∼0.05) and a subtracting pore effect (SPE) method.37 The micropore volumes were calculated from the Dubinin-Radushkevich (DR) equation. Total pore volumes were calculated from the adsorbed amount at p/p0 ) 0.96. Pore size distribution was calculated by density functional theory (DFT). In the DFT calculations, the equilibrium model of carbon slit pores was used. Nitrogen content of AN8.5(2)-H was determined by elemental analysis. Pt nanoparticles in Pt/P7(2)-H were observed with a transmission electron microscope (TEM; JEOL, JEM-2010) equipped with an energy-dispersive analyzer. Hydrogen Adsorption Isotherm under High Pressure. Hydrogen adsorption isotherms were measured with a volumetric apparatus to perform high-pressure measurements up to 10.5 MPa. All measurements were carried out in accordance with a standard method (JIS H 7201) defined by the Japanese Standards Association. The temperature of a sample cell (its

High-Pressure Hydrogen Storage in ZTC

J. Phys. Chem. C, Vol. 113, No. 8, 2009 3191

TABLE 1: Surface Areas, Pore Volumes, Hydrogen Uptake Capacities, and Surface Coverage Ratios with Hydrogen for Microporous Carbons sample

SBETa (m2 g-1)

SRb (m2 g-1)

Vmicroc (cm3 g-1)

Vmesod (cm3 g-1)

Vtotale (cm3 g-1)

W10f (wt %)

φ10g (%)

P7(2)-H P7(2) P8(4) AN8.5(2)-H Pt/P7(2)-H Diasorb ASE MSP20 MSC30

3800 2280 1610 2900 3410 1700 1710 2310 2680

3370 2160 1270 2820 3200 1700 1730 2390 2680

1.58 0.84 0.60 1.16 1.42 0.74 0.72 0.97 1.20

0.12 0.43 0.17 0.21 0.15 0.16 0.24 0.02 0.55

1.70 1.27 0.77 1.37 1.57 0.90 0.96 0.99 1.75

0.87 0.60 0.41 0.71 0.95 0.46 0.51 0.71 0.73

9 10 12 9 11 10 11 11 10

a BET surface area calculated at P/P0 ) 0.01∼0.05. b Surface area calculated by SPE method. c Micropore volume calculated by DR method. Mesopore volume calculated by Vtotal - Vmicro. e Total pore volume estimated from the adsorption amount of N2 at P/P0 ) 0.96. f Hydrogen adsorption amount at 10 MPa (30 °C). g Surface coverage ratio with physisorbed hydrogen molecules at 10 MPa (30 °C).

d

volume is ca. 3 cm3) was kept to a desired value in the range of 30∼150 °C ((0.2 °C). More than 0.4 g of sample was used for each high-pressure isotherm measurement. After a sample was set to the apparatus, the sample was degassed in vacuum at a measurement temperature for at least 4 h. Then a leak check was carried out by putting helium gas up to 10.5 MPa for 1 min, and it was ascertained that the pressure decrease during the checking was below a detectable value (0.6 nm) never have such an enhancement effect. On the other hand, the theoretical calculation has predicted that such pore size effect is quite small at 30 °C and below 10 MPa if a pore size is larger than 0.89 nm.13 Since all of the present porous carbons have larger pore size than 0.8

High-Pressure Hydrogen Storage in ZTC

Figure 5. Amount of hydrogen adsorbed at various pressures (30 °C) plotted against surface area calculated by the SPE method for ZTCs and activated carbons. (9, 0) 1 MPa; (2, 4) 5 MPa; (b, O) 10 MPa. Solid and open symbols represent ZTCs and activated carbons, respectively.

J. Phys. Chem. C, Vol. 113, No. 8, 2009 3193

Figure 7. Effect of H2 adsorption uptake on isosteric heat of adsorption (Q) for (b) P7(2)-H and (O) MSC30.

Figure 8. Hydrogen adsorption isotherms of (O) P7(2)-H and (0) MSC30 up to 34 MPa, measured at 30 °C by using an adsorption apparatus constructed for use under above 10 MPa. (b) Isotherm data of P7(2)-H shown in Figure 4a. Figure 6. Hydrogen adsorption isotherms of P7(2)-H and MSC30 measured at various temperatures: (b, O) 30, (9, 0) 80, (2, 4) 100, and ([, ]) 150 °C. Solid and open symbols represent P7(2)-H and MSC30, respectively.

nm (see Figure 3), the effect would be slight, and consequently, the hydrogen adsorption amount is governed by the specific surface area (SR) rather than micropore volume, as shown in Figure 5. In order to understand the amount of hydrogen molecules adsorbed on the carbon surface, we calculated the surface coverage ratio by physisorbed hydrogen molecules at 10 MPa, and the resulting ratios (φ10) are indicated in Table 1. In any carbon, only 10% of the surface is covered with physisorbed hydrogen molecules, indicating that at 10 MPa the micropores are not filled with hydrogen molecules at all. ZTCs [P7(2)-H, P7(2), P8(4), and AN8.5(2)-H] are characterized by completely different frameworks and pore structures than any conventional activated carbons; single and curved graphene sheet, large amount of edge, uniform pore size, ordered pore connection.26 However, the linearity in Figure 5 suggests that such unique structure has nothing to do with the hydrogen uptake capacity. Moreover, the results from AN8.5(2)-H indicates that nitrogen doping does not affect the hydrogen uptake. Though Thomas and co-workers39 have reported a negative effect of nitrogen functional groups at -196 °C and under a pressure below 0.1 MPa in the case of activated carbons, we did not observe such an effect. Figure 6 shows hydrogen adsorption isotherms of P7(2)-H and MSC30 measured at various temperatures. Since hydrogen storage is based on physisorption as described above, the adsorption amount decreases with increasing adsorption temperature. From the adsorption isotherms at different temperatures, the isosteric heat of adsorption (defined as Q) was calculated by use of the Clausius-Clapeyron equation.8,32 The

Figure 9. Plot of total volumetric storage amount of P7(2)-H and MSC30 against hydrogen pressure at 30 °C. (---) Amount for compressed hydrogen.

values of Q thus obtained were plotted against the hydrogen adsorption amount in Figure 7, where Q decreases from 8 to 6 kJ mol-1 when H2 uptake is increased up to 0.5 wt %. This behavior of Q for ZTC is similar to that reported by Mokaya’s group,23 despite the large difference in adsorption temperature (30 to 150 °C in the present study; -196 to -186 °C in the literature). Though they reported that Q of ZTC is higher than those of activated carbons and metal-organic frameworks at very low H2 uptake, we could not observe such noticeable differences between P7(2)-H and MSC-30. Since ZTC comprises a single graphene sheet,26 its potential is anticipated to be lowered by the low atom density of the material but may be increased in some places by the curvature of the graphene sheet. As a result, ZTC may show a heat of adsorption similar to that of the activated carbon. Hydrogen Storage up to 34 MPa. At pressures above 30 MPa, it is theoretically predicted that relatively large micropores

3194 J. Phys. Chem. C, Vol. 113, No. 8, 2009

Nishihara et al.

Figure 10. TEM images of Pt-loaded ZTC [Pt/P7(2)-H]. Pt nanoparticles are highlighted by arrows in panel b.

(>0.65 nm) can enhance the hydrogen uptake capacity.13 Therefore, ZTC (its pore size is ca. 1.2 nm) is expected to exhibit such an enhancement effect. Figure 8 shows hydrogen adsorption isotherms of P7(2)-H and MSC30 measured by using the adsorption apparatus that was specially tailored for use up to 34 MPa. Note that we have carefully examined the reproducibility of isotherms in both samples, as explained in the Experimental Section. Moreover, the good agreement in the data up to 10 MPa between Figures 4 and 8 also supports the reliability of this high-pressure apparatus. In MSC30, its isotherm apparently levels off above 20 MPa, and it almost reaches the ceiling at 30 MPa. A similar behavior of MSC30 has been reported also by Kojima et al.8 However, P7(2)-H does not level off up to 34 MPa. Cazorla-Amoro´s and co-workers3 have measured high-pressure hydrogen isotherms of various kinds of activated carbons at room temperature up to 70 MPa. They reported that most of the activated carbons reached a plateau above 20 MPa, but some showed almost no leveling off even at 70 MPa. They explained that there is an optimal pore size that can enhance the adsorption. The optimal pore size changes with the storage pressure. Kowalczyk et al.13 have theoretically predicted that, in the case of carbon slit pores at 30 °C, 0.65 nm is optimum below 30 MPa and the optimal pore size is increased to 2.13 nm at 50 MPa. Thanks to the zeolite template, ZTC contains only such suitable-sized micropores (about 1.2 nm) for the enhancement effect. ZTC thus can exhibit one of the highest storage amounts (2.2 wt % at 30 °C and 34 MPa) among any pure carbon materials.3-8 Next, we examine the potential of ZTC as a hydrogen storage medium at room temperature and under high pressure in comparison with compressed hydrogen. In this purpose, a total storage capacity, which is the sum of physisorbed hydrogen and compressed hydrogen gas in the pores of carbon, should be considered. The weight of hydrogen gas per unit volume (Wgas, in grams of H2 per cubic centimeter) at each temperature and pressure can be theoretically estimated by using a modified Benedict-Webb-Rubin equation.40 Hydrogen amount that is compressed in the pores of carbons, Wcp, can be calculated by Wcp ) (1 - Fapp/Ftrue)Wgas, where Fapp, and Ftrue are the apparent density (grams per cubic centimeter) of the carbons and the true density (grams per cubic centimeter) of the carbon framework, respectively. The value of Fapp was calculated by Fapp ) (1/Ftrue + Vtotal)-1, while Ftrue was determined by helium pycnometry [Ftrue ) 1.9 g cm-3 for P7(2)-H and 2.0 g cm-3 for MSC30]. From the physisorbed hydrogen amount, Wphy, the total storage amount of hydrogen per unit volume, Wtotal, can be calculated

by Wtotal ) Wcp + Wphy. Note that Wtotal means the total hydrogen amount that can be stored inside a single carbon particle; that is, this value represents the maximum potential of hydrogen uptake in a porous carbon, independent of its packing density. Thus Wtotal is interesting from a fundamental point of view, while the packing density or the density of the monolith must be considered in construction of the actual hydrogen tank containing carbon sorbents. Figure 9 shows the total storage amounts of P7(2)-H and MSC30 together with the amount of compressed hydrogen (Wgas). At around 10 MPa, both P7(2)-H and MSC30 have an advantage over compressed hydrogen. However, the storage amount of MSC30 becomes close to the value of compressed hydrogen above 30 MPa. On the other hand, P7(2)-H can still keep its high priority over 30 MPa. Thus P7(2)-H can in principle store 1.2 times larger amount of hydrogen than compressed hydrogen even at 34 MPa. Increase of Hydrogen Uptake by Pt Doping. So far we have demonstrated that ZTC is a promising candidate for a hydrogen storage medium. In addition to its inherent large physisorption capacity, we tried to add hydrogen spillover function into ZTC by putting Pt nanoparticles in ZTC. Figure 10 shows TEM images of such Pt-loaded ZTC [Pt/P7(2)-H]. We confirmed with energy-dispersive spectroscopy that the black spheres shown in Figure 10 are Pt. Figure 10a shows that Pt nanoparticles are uniformly dispersed in ZTC particles. From the enlarged image (Figure 10b), the size of Pt nanoparticles is estimated to be about 1∼3 nm, similar to those of the previously reported ones (1.4∼3.8 nm) in Pt-loaded ZTCs.34,35 Figure 11 shows XRD patterns of P7(2)-H before and after Pt loading. Generally, Pt shows diffraction peaks at 2θ ) 39.8° and 46.2° that correspond to (111) and (200) planes, respectively. However, no evident peak from Pt was observed in the XRD pattern of Pt/P7(2)-H, due to both the very small size of Pt particles and the very small amount of Pt (only 0.2 wt %). Only a broad carbon (10) peak appeared around 2θ ) 44°. It is noteworthy that the lattice pattern of ZTC can be observed in Figure 10b, indicating that the present Pt loading process did not destroy the ordered structure of ZTC. Indeed, the intensity of a sharp XRD peak around 2θ ) 6.3° was not reduced by Pt loading, as shown in Figure 11. Pt/P7(2)-H can therefore still retain very high surface area (SBET ) 3410 m2 g-1). Nitrogen isotherms before and after Pt loading are presented in Supporting Information. Figure 12 shows high-pressure hydrogen isotherms of P7(2)-H before and after Pt loading. Generally, Pt-loaded activated carbons with a sufficient amount of Pt show rapid hydrogen

High-Pressure Hydrogen Storage in ZTC

J. Phys. Chem. C, Vol. 113, No. 8, 2009 3195 hydrogen-spillover is a reversible process at room temperature, as pointed out by Lachawiec and Yang.25 Conclusions

Figure 11. XRD patterns of Pt-loaded and unloaded P7(2)-H. Positions of Pt (111) and (200) peaks are indicated by arrows.

High-pressure hydrogen uptake capacity of ordered microporous carbons, zeolite-templated carbons (ZTCs), was examined at 30 °C. ZTCs with different surface area and nitrogen-doped ZTC were synthesized and used for experiments, together with several types of commercial activated carbons as references. At pressures below 10 MPa, the hydrogen storage amount correlated well with the specific surface area determined by the subtracting pore effect method, rather than the BET surface area and the micropore volume. The ZTC with the largest specific surface area (>3300 m2 g-1) exhibited the highest hydrogen uptake capacity of 0.87 wt % at 30 °C and 10 MPa. In the pressure range up to 34 MPa, ZTC showed outstanding hydrogen capability, because the pore size of ZTC (ca. 1.2 nm) greatly enhanced the hydrogen uptake capacity. Since ZTC has a very sharp pore size distribution, ZTC is promising as a physisorption-based hydrogen storage medium at room temperature under such high pressure. Its hydrogen adsorption amount, 2.2 wt % (30 °C and 34 MPa) is one of the highest values among any pure porous carbons. Moreover, by introducing only a very small amount of Pt nanoparticles (ca. 0.2 wt %) onto ZTC, the hydrogen uptake capacity was further increased to 0.95 wt % at 30 °C and 10 MPa, probably due to the hydrogen spillover mechanism.

Figure 12. Hydrogen isotherms measured at 30 °C up to 10.5 MPa for Pt-loaded and unloaded P7(2)-H. When isotherm measurements were repeated several times for both samples, each datum was placed within the range of the error bar ((0.03 wt %).

Acknowledgment. We are grateful to Dr. F. Sua´rez-Garcı´a for helpful discussions. This work was performed as one of the researches in the Materials Science and Technology Research Centre for Industrial Creation at the Institute of Multidisciplinary Research for Advanced Materials.

uptake at very low relative pressure near 0, because of direct hydrogen chemisorption onto Pt surface.29,32 However, in this work, such uptake was not observed due to the very small amount of Pt. From the theoretical calculation, chemisorption can be estimated to be only 0.0013 wt % for Pt/P7(2)-H, if we assume that the size of the Pt particle is 1 nm and one hydrogen atom is trapped by one Pt atom on the surface. Despite its very small amount of Pt and lower surface area (SR) than pristine P7(2)-H, Pt/P7(2)-H shows higher hydrogen uptake around 10 MPa. Since hydrogen chemisorption on the Pt surface is negligible, such uptake is most likely to be due to the spillover phenomenon, as Li and Yang32 reported in a Pt-loaded activated carbon. Uptake enhancement by spillover in Figure 12 is not so remarkable compared with the results of Li and Yang, probably because of the very small amount of Pt in our case. However, the difference is evident when the very small experimental error ((0.03 wt %, see Supporting Information) in the present isotherm measurement is considered. In the spillover mechanism, hydrogen molecules dissociatively adsorb on the surface of Pt particles and then atomic hydrogen is moved to the carbon surface. Our results indicate that even very small amounts of Pt can give rise to hydrogen spillover and can contribute to increasing hydrogen uptake onto microporous carbons. If the spilled-over hydrogen atoms are stabilized by interaction with carbon active sites, some of the hydrogen would be irreversibly adsorbed and, as a result, not be desorbed anymore. However, we found that this is not the case in Pt/ P7(2)-H. The second isotherms agree well with the first isotherms, still keeping a larger uptake amount than P7(2)-H at 10 MPa (see Supporting Information). This result indicates that

Supporting Information Available: Elemental compositions of P7(2)-H and AN8.5(2)-H (Table S1); logarithmic plots of N2 adsorption isotherms at extremely low pressure (Figure S1); plots of hydrogen uptake capacity versus BET surface area and micropore volume (Figures S2 and S3 and Table S2); N2 adsorption isotherms of P7(2)-H and Pt/P7(2)-H (Figure S4); and repeatability of hydrogen isotherms (Figure S5). This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353–358. (2) Strobel, R.; Garche, J.; Moseley, P. T.; Jorissen, L.; Wolf, G. J. Power Sources 2006, 159, 781–801. (3) de la Casa-Lillo, M. A.; Lamari-Darkrim, F.; Cazorla-Amoros, D.; Linares-Solano, A. J. Phys. Chem. B 2002, 106, 10930–10934. (4) Jorda-Beneyto, M.; Suarez-Garcia, F.; Lozano-Castello, D.; CazorlaAmoros, D.; Linares-Solano, A. Carbon 2007, 45, 293–303. (5) Kiyobayashi, T.; Takeshita, H. T.; Tanaka, H.; Takeichi, N.; Zuttel, A.; Schlapbach, L.; Kuriyama, N. J. Alloys Compd. 2002, 330, 666–669. (6) Takagi, H.; Hatori, H.; Soneda, Y.; Yoshizawa, N.; Yamada, Y. Mater. Sci. Eng., B 2004, 108, 143–147. (7) Haas, M. K.; Zielinski, J. M.; Dantsin, G.; Coe, C. G.; Pez, G. P.; Cooper, A. C. J. Mater. Res. 2005, 20, 3214–3223. (8) Kojima, Y.; Kawai, Y.; Koiwai, A.; Suzuki, N.; Haga, T.; Hioki, T.; Tange, K. J. Alloys Compd. 2006, 421, 204–208. (9) Dillon, A. C.; Heben, M. J. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 133–142. (10) Nijkamp, M. G.; Raaymakers, J.; van Dillen, A. J.; de Jong, K. P. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 619–623. (11) Poirier, E.; Chahine, R.; Bose, T. K. Int. J. Hydrogen Energy 2001, 26, 831–835. (12) Texier-Mandoki, N.; Dentzer, J.; Piquero, T.; Saadallah, S.; David, P.; Vix-Guterl, C. Carbon 2004, 42, 2744–2747.

3196 J. Phys. Chem. C, Vol. 113, No. 8, 2009 (13) Kowalczyk, P.; Tanaka, H.; Holyst, R.; Kaneko, K.; Ohmori, T.; Miyamoto, J. J. Phys. Chem. B 2005, 109, 17174–17183. (14) Kyotani, T.; Nagai, T.; Inoue, S.; Tomita, A. Chem. Mater. 1997, 9, 609–615. (15) Ma, Z. X.; Kyotani, T.; Liu, Z.; Terasaki, O.; Tomita, A. Chem. Mater. 2001, 13, 4413–4415. (16) Ma, Z. X.; Kyotani, T.; Tomita, A. Chem. Commun. 2000, 2365– 2366. (17) Ma, Z. X.; Kyotani, T.; Tomita, A. Carbon 2002, 40, 2367–2374. (18) Kyotani, T.; Ma, Z. X.; Tomita, A. Carbon 2003, 41, 1451–1459. (19) Matsuoka, K.; Yamagishi, Y.; Yamazaki, T.; Setoyama, N.; Tomita, A.; Kyotani, T. Carbon 2005, 43, 876–879. (20) Chen, L.; Singh, R. K.; Webley, P. Microporous Mesoporous Mater. 2007, 102, 159–170. (21) Pacula, A.; Mokaya, R. J. Phys. Chem. C 2008, 112, 2764–2769. (22) Roussel, T.; Didion, A.; Pellenq, R. J. M.; Gadiou, R.; Bichara, C.; Vix-Guterl, C. J. Phys. Chem. C 2007, 111, 15863–15876. (23) Yang, Z. X.; Xia, Y. D.; Mokaya, R. J. Am. Chem. Soc. 2007, 129, 1673–1679. (24) Yang, Z. X.; Xia, Y. D.; Sun, X. Z.; Mokaya, R. J. Phys. Chem. B 2006, 110, 18424–18431. (25) Lachawiec, A. J.; Yang, R. T. Langmuir 2008, 24, 6159–6165. (26) Nishihara, H.; Yang, Q. H.; Hou, P. X.; Unno, M.; Yamauchi, S.; Saito, R.; Paredes, J. I.; Martínez-Alonso, A.; Tascón, J. M. D.; Sato, Y.; Terauchi, M.; Kyotani, T. Carbon 2009, in press, doi: 10.1016/j.carbon.2008.12.040. (27) Lueking, A. D.; Yang, R. T. Appl. Catal., A 2004, 265, 259–268.

Nishihara et al. (28) Takagi, H.; Hatori, H.; Yamada, Y. Chem. Lett. 2004, 33, 1220– 1221. (29) Takagi, H.; Hatori, H.; Yamada, Y.; Matsuo, S.; Shiraishi, M. J. Alloys Compd. 2004, 385, 257–263. (30) Lachawiec, A. J.; Qi, G. S.; Yang, R. T. Langmuir 2005, 21, 11418– 11424. (31) Takagi, H.; Hatori, H.; Yamada, Y. Carbon 2005, 43, 3037–3039. (32) Li, Y. W.; Yang, R. T. J. Phys. Chem. C 2007, 111, 11086–11094. (33) Zielinski, M.; Wojcieszak, R.; Monteverdi, S.; Mercy, M.; Bettahar, M. M. Catal. Commun. 2005, 6, 777–783. (34) Su, F. N.; Zeng, H. J.; Yu, Y. J.; Lv, L.; Lee, J. Y.; Zhao, X. S. Carbon 2005, 43, 2368–2373. (35) Coker, E. N.; Steen, W. A.; Miller, J. T.; Kropf, A. J.; Miller, J. E. J. Mater. Chem. 2007, 17, 3330–3340. (36) Hou, P. X.; Orikasa, H.; Yamazaki, T.; Matsuoka, K.; Tomita, A.; Setoyama, N.; Fukushima, Y.; Kyotani, T. Chem. Mater. 2005, 17, 5187– 5193. (37) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992, 30, 1075–1088. (38) Vanmal, H. H.; Buschow, K. H. J.; Miedema, A. R. J. Less-Common Met. 1974, 35, 65–76. (39) Zhao, X. B.; Xiao, B.; Fletcher, A. J.; Thomas, K. M. J. Phys. Chem. B 2005, 109, 8880–8888. (40) Younglove, B. A. J. Phys. Chem. Ref. Data 1982, 11, 1–349.

JP808890X