Hydrogen Storage Properties of N-Doped Microporous Carbon

Nov 24, 2009 - carbon, and a storage capacity of 1.26 wt % at 298 K and 10 MPa was obtained, showing an enhancement factor of 2.4 by spillover. In add...
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J. Phys. Chem. C 2009, 113, 21883–21888

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Hydrogen Storage Properties of N-Doped Microporous Carbon Lifeng Wang and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136 ReceiVed: August 24, 2009; ReVised Manuscript ReceiVed: October 28, 2009

A N-doped microporous carbon was synthesized by using NaY as a hard template and acetonitrile as the carbon and nitrogen precursor. The hydrogen storage measurements indicated that the N-doped microporous carbon had an 18% higher storage capacity than the pure carbon with a similar surface area. Furthermore, hydrogen storage via spillover was studied on a sample comprising Pt supported on N-doped microporous carbon, and a storage capacity of 1.26 wt % at 298 K and 10 MPa was obtained, showing an enhancement factor of 2.4 by spillover. In addition, the Pt/N-doped microporous carbon exhibited 1.46 times the storage capacity of Pt/microporous carbon. Significantly higher heats of adsorption were obtained on the N-doped microporous carbon samples than that on undoped carbons for both H2 adsorption and adsorption by spillover. The experimental results were consistent with the theoretical calculations from the literature. 1. Introduction With concerns with a potential energy crisis from the use of fossil fuels and increasing demands for environmental protection, hydrogen has been proposed as one of the best alternative energy sources for vehicles powered by fuel cells. Hydrogen storage plays a key role in the utilization of hydrogen as the energy carrier.1 Nanostructured and porous carbon materials, including carbon nanotubes (CNTs), graphite nanofibers, activated carbon, templated carbon, and graphene, are thought to be the promising candidates for hydrogen storage due to their high surface areas, light weight, and relative chemical stabilities.2-7 However, recent studies showed that these carbon materials cannot store a sufficient amount of H2 required for transportation applications merely by physical adsorption at ambient temperature.8,9 A promising approach for solving this problem has been shown by which hydrogen storage in an adsorbent could be enhanced significantly by hydrogen spillover at room temperature.10-16 Hydrogen spillover is defined as the dissociative chemisorption of hydrogen on metal nanoparticles and subsequent migration of hydrogen atoms onto adjacent surfaces of a receptor via spillover or surface diffusion.17-26 Enhancements in hydrogen storage capacities on carbon materials by doping transition metals have been recently studied.27-33 In the system of hydrogen storage in carbon via spillover, the hydrogen dissociation sources, the contact between the dissociation source and carbon receptor, and the nature of the carbon receptor are considered as main factors affecting the storage capacity. Different metals as hydrogen dissociation sources for hydrogen storage have been intensively studied.27-33 Bridge-building and plasma-assisted doping techniques have been applied to improve the contacts between the source and receptor and, hence, enhance the hydrogen storage capacity.12-16,34 Recently, developing receptors has received much research interest because enhanced storage capacities could be achieved with high surface area receptors or chemically modified receptors. For examples, templated carbons and MOFs with high surface areas have shown promising results.7,35 This can be understood because a receptor with a higher surface area would provide more * To whom correspondence should be addressed. Fax: (734) 764-7453. E-mail: [email protected].

hydrogen adsorption sites than one with lower surface area. As for the chemically modified receptors, it was reported that hydrogen storage capacity could be enhanced on boron-doped carbon and nitrogen-doped carbon.36-45 Hydrogen uptakes on boron-doped microporous carbon and carbon nanotubes have been studied, respectively, by Chung et al.38 and Viswanathan et al.39 Our recent results showed an enhanced storage capacity in a boron- and nitrogen-codoped carbon.40 Badzian et al. synthesized a carbon with 1% nitrogen contents and observed a 0.7-0.8 wt % uptake in the doped carbon.41 Lee et al. obtained a storage capacity of 0.28 wt % at 308 K by optimizing the nitrogen contents in carbon xerogel.42 Mokaya and co-workers investigated the hydrogen adsorption in nitrogen-doped carbon at 77 K.43 More recently, Zhu et al. theoretically investigated the interaction between hydrogen atoms and nitrogen-doped carbon materials and found that the doped nitrogen atoms increased the adsorption energy of hydrogen atoms at the neighboring C-atom sites.44 An ab initio study of hydrogen interaction with nitrogen-doped carbon nanotubes reported by Zhang and Cho showed that doping the CNTs with nitrogen reduced the energy barrier for hydrogen dissociation.45 From these theoretical calculations, one may expect, therefore, that a nitrogen-doped carbon receptor exhibiting stronger interaction with hydrogen or facilitating hydrogen dissociation would be favorable for hydrogen adsorption. However, hydrogen storage via spillover has not been studied on N-doped carbon, and an understanding of the effect of doped nitrogen atoms on hydrogen storage is needed. In this work, we prepared a N-doped microporous carbon and the same sample that was doped with Pt nanoparticles, and counterparts without N-doping, and investigated their hydrogen adsorption properties and the effects of nitrogen on hydrogen storage. 2. Experimental Methods 2.1. Synthesis. N-Doped Microporous Carbon. N-doped microporous carbon derived from zeolite NaY was prepared according to a procedure similar to that proposed by Mokaya.43 Typically, 2 g of NaY was degassed in a flask for 12 h at 473 K, then placed in a vertical quartz tube and heated to 1023 K under a N2 flow. When the temperature reached 1023 K, the N2 flow was switched to acetonitrile (saturated in a N2 flow

10.1021/jp908156v  2009 American Chemical Society Published on Web 11/24/2009

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rate of 150 cm3/min) to pass through the NaY for 4 h. After the CVD treatment, the composite was further heated at 1173 K for 2 h under a flow of N2. The obtained NaY/carbon composite was treated in HF solution (40%) for 24 h and subsequently refluxed by a concentrated HCl solution for 4 h to dissolve the NaY template. The resulting microporous carbon was collected by filtration and washing with distilled water. 6 wt % Pt Supported on N-Doped Microporous Carbon. Typically, 200 mg of well-dried N-doped microporous carbon was dispersed in 20 mL of acetone and was stirred for 0.5 h in a flask at room temperature. Five milliliters of acetone was mixed with 26 mg of H2PtCl6, which was slowly added to the above solution under vigorous stirring. The mixture was subjected to ultrasonication (100 W, 42 kHz) for 1 h and then magnetically stirred at room temperature for 24 h. After being dried in an oven at 333 K overnight, the impregnated sample was transferred to a horizontal quartz tube and further dried in a He flow at 393 K for 2 h to remove the residual acetone in the sample. The He flow was then switched to H2, and the temperature was increased to 573 K and held for 3 h. After cooling to room temperature in H2, the sample was purged with flowing He and was stored under He atmosphere before further measurement. Plain Microporous Carbon. Microporous carbon derived from zeolite NaY was prepared according to a procedure similar to that reported by Kyotani et al.46 NaY was degassed in a flask for 12 h at 473 K, then placed in a vertical quartz tube and heated to 1073 K under a N2 flow. When the temperature reached 1073 K, propylene gas (2% in N2 by volume, flow rate ) 150 cm3/min) was passed through the tube for 6 h. After the CVD treatment, the obtained NaY/carbon composite was treated in HF solution (40%) for 24 h and subsequently refluxed by concentrated HCl solution for 4 h to dissolve the NaY template. The resulting microporous carbon was collected by filtration and washing with distilled water. 6 wt % Pt Supported on Microporous Carbon. Pt/microporous carbon was prepared using the same procedure for preparing Pt/N-doped microporous carbon except that the plain microporous carbon was used as the support instead of N-doped microporous carbon. 2.2. Characterization. Powder X-ray diffraction (XRD) data were recorded on a Rigaku Miniflex diffractometer at 30 kV, 15 mA for Cu KR (λ ) 0.1543 nm) radiation, with a step size of 0.02° in 2θ. X-ray photoelectron spectroscopy was recorded on a Kratos Axis ultra XPS spectrometer. Nitrogen adsorption and low-pressure H2 adsorption isotherms (0-1 atm) were measured with a standard static volumetric technique (Micromeritics ASAP 2020). Hydrogen adsorption at 298 K and pressures greater than 0.1 MPa and up to 10 MPa were measured using a static volumetric technique with a specially designed Sieverts-type apparatus. The apparatus was previously tested and proven to be leak-free and accurate through calibration by using LaNi5, AX-21, zeolites, and MOFs at 298 K.47 Approximately 200 mg of sample was used for each high-pressure isotherm measurement in this study. 3. Results and Discussion N-Doped Microporous Carbon. Powder X-ray diffraction patterns of NaY zeolite and N-doped microporous carbon are shown in Figure 1. NaY zeolite exhibited typical peaks assigned to FAU structure (Figure 1a). The N-doped microporous carbon synthesized by using NaY as a hard template (Figure 1b) showed

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Figure 1. X-ray diffraction patterns of NaY (a) and N-doped microporous carbon (b).

Figure 2. Nitrogen isotherm on N-doped microporous carbon.

a peak at 2θ ) 6.3°, indicating that the microstructure of the zeolite template had been replicated in the N-doped microporous carbon. Nitrogen adsorption at 77 K was employed to characterize the porosity in the N-doped microporous carbon. As shown in Figure 2, the isotherm of N-doped microporous carbon exhibited a sharp rise in the low relative pressure (P/P0 < 0.1) and a gradual rise in the high relative pressure, indicating the presence of microporosity and some mesoporosity. The presence of mesopores is due to the incomplete infiltration of carbon precursor into the channel of the NaY, which led to the mesopores after removal of the NaY. The BET surface area and pore volume of N-doped microporous carbon were 1663

H2 Storage Properties of N-Doped Microporous Carbon

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Figure 3. TEM image of N-doped microporous carbon.

Figure 5. High-pressure hydrogen isotherms at 298 K for N-doped microporous carbon (∆) and pure microporous carbon (]).

Figure 4. XPS spectrum of N-doped microporous carbon.

Figure 6. Low-pressure H2 adsorption isotherm for N-doped microporous carbon at 273 K (0), 298 K(]), and 323 K (∆). Inset: calculated isosteric heats of adsorption.

m2/g and 1.43 cm3/g, respectively. The surface area of the N-doped microporous carbon was higher than previously reported high surface area N-doped carbon xerogel (1602 m2/ g),42 N-doped mesoporous carbon (1271 m2/g),48 and N-doped carbon nanotubes (886 m2/g),49 indicating that the use of microporous NaY as a hard template is helpful for synthesizing high surface area N-doped microporous carbon. A high-resolution TEM image of the N-doped microporous carbon further showed the detailed microstructure. As shown in Figure 3, microporous channels (marked by arrows) could be observed at the edges of the N-doped microporous carbon sample. This indicates the successful structural transfer from the zeolite template, in agreement with the XRD and N2 isotherm observations. The X-ray photoelectron survey spectrum (Figure 4) of N-doped microporous carbon showed three sharp signals for C, N, and O elements. The elemental mass ratio of the N-doped microporous carbon was approximately 89% C, 7% N, and 4% O. This indicated that using the acetonitrile as both a carbon precursor and a nitrogen precursor was efficient for the synthesis of N-doped microporous carbon. Our results are in agreement with previous studies that showed acetonitrile can be also used for synthesis of N-doped carbon nanotubes,49-51 carbon nanofiber,52and mesoporous carbon.53 It was confirmed that the nitrogen doped in this manner was in the forms of pyridinelike nitrogen and quaternary nitrogen incorporated into graphene sheets.49-53 The high-pressure hydrogen isotherm at 298 K for the N-doped microporous carbon is presented in Figure 5, curve a.

As shown in Figure 5, curve a, the N-doped microporous carbon had a hydrogen storage capacity of 0.51 wt % at 298 K and 10 MPa. It has been suggested that doping of nitrogen into a carbon was favorable for hydrogen adsorption.37 For comparison, a pure microporous carbon was synthesized by using NaY as a hard template and propylene as the carbon precursor according to the literature.45 The obtained pure microporous carbon had a BET surface area of 1533 m2/g (Supporting Information, Figure 1). As shown in Figure 5, curve b, the pure microporous carbon had a storage capacity of 0.43 wt % at 298 K and 10 MPa. Therefore, the hydrogen uptake on the N-doped microporous carbon was 18% higher than that of the plain carbon with a similar surface area under the same conditions. DFT calculation results reported by Viswanathan et al. also showed that substitution of nitrogen in the carbon nanotube framework was favorable for hydrogen molecular adsorption.37 The heats of adsorption of H2 on the N-doped microporous carbon were calculated from the H2 adsorption isotherms at 273 and 298 K by using the Clausius-Clapeyron equation, as shown in Figure 6. The isosteric heats of adsorption were determined by evaluating the slope of the plot of ln(P) versus (1/T) at the same adsorption amount. It can be seen that the H2 adsorption amounts at all pressures up to 1 atm decreased with an increase in temperature. The inset in Figure 6 shows that the absolute values of the heats of adsorption decreased with adsorption amount for the N-doped microporous carbon. The heats of adsorption were ∼12 kJ/mol at low surface coverage and leveled

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Figure 7. X-ray diffraction pattern of Pt/N-doped microporous carbon.

off to ∼7.3 kJ/mol at relatively high surface coverage. The high values of heats of adsorption at low surface coverage were attributed to the adsorption of H2 on the more energetic sites on carbon. Defect sites and edge sites are strong sites for adsorption. The heats of adsorption (∼7.3 kJ/mol) at a high H2 adsorption amount on the N-doped microporous carbon were still significantly higher than that on pure carbon (∼5 kJ/mol).54,55 The relatively high heats of adsorption on N-doped microporous carbon were in qualitative agreement with theoretical predictions from the literature.39,40 Hydrogen Storage Properties of Pt/N-Doped Microporous Carbon. Recent studies showed that the hydrogen storage capacities at 298 K in nanostructured and porous materials, including carbon, zeolites, and metal-organic frameworks, could be enhanced by exploiting the hydrogen spillover phenomenon.10-16 In addition, theoretical calculations indicated that the doped nitrogen atoms could increase the adsorption energy of hydrogen atoms at the neighboring C atoms and facilitate hydrogen dissociation.43,44 Thus, we synthesized Pt/ N-doped microporous carbon and investigated its hydrogen storage properties. The powder X-ray diffraction pattern of Pt/N-doped microporous carbon is shown in Figure 7. In the low-angle XRD pattern, the Pt/N-doped microporous carbon sample exhibited a peak at 2θ ) 6.3°, similar to that of N-doped microporous carbon, indicating that the ordered microstructure of N-doped microporous carbon was kept after doping Pt metals. In addition, the wide-angle XRD pattern of Pt/N-doped microporous carbon exhibited two peaks at 39.8° (111) and 46.3° (200), characteristic of the metallic platinum (ICDD-JCPDS card no. 4-802). The size of the Pt particle calculated from the Scherrer equation was approximately 7 nm. These results confirmed that nanosized Pt metals had been successfully doped on the carbon supports by applying our doping method. Nitrogen isotherms at 77 K for Pt/N-doped microporous carbon are shown in Figure 8. Pt/N-doped microporous carbon exhibited a similar isotherm to the undoped microporous carbon, revealing the presence of microporosity and some mesoporosity. Pt/N-doped microporous carbon had a BET surface area of 1388 m2/g and a pore volume of 1.17 cm3/g, which were lower than those of N-doped microporous carbon (1663 m2/g and 1.43 cm3/ g). This was due to the increased weight and micropore blocking caused by the Pt metal particles. It is encouraging that the surface area decreased only slightly after doping of the Pt particles, indicating that most of the porosity of the carbon support remained open. The high-resolution TEM image of the Pt/N-doped microporous carbon is shown in Figure 9. The black spots of Pt

Wang and Yang

Figure 8. Nitrogen isotherms on N-doped microporous carbon (O) and Pt/N-doped microporous carbon (0).

Figure 9. TEM image of Pt/N-doped microporous carbon.

Figure 10. High-pressure hydrogen isotherms at 298 K for N-doped microporous carbon (∆) and Pt/N-doped microporous carbon (]).

(3-9 nm) were well-dispersed on the surface of the Pt/N-doped microporous carbon. The Pt size observed in TEM was in agreement with the calculated size from XRD data. This result further confirmed that Pt metals have been successfully doped on the N-doped microporous carbon support. High-pressure hydrogen isotherms at 298 K for the N-doped microporous carbon and the Pt/N-doped microporous carbon samples are compared in Figure 10. The N-doped microporous carbon had a hydrogen storage capacity of 0.51 wt % at 298 K and 10 MPa. When 6.0 wt % Pt metal was doped on the

H2 Storage Properties of N-Doped Microporous Carbon

J. Phys. Chem. C, Vol. 113, No. 52, 2009 21887 shown in the Supporting Information, Figure 3 inset, the heats of adsorption at high H2 adsorption amounts on Pt/microporous carbon were about 10 kJ/mol. The higher heats of adsorption on Pt/N-doped microporous carbon relative to Pt/microporous carbon suggest that more H atoms were favorably bonded to Pt/N-doped microporous carbon, in agreement with highpressure hydrogen adsorption results. It has been suggested that doping of nitrogen into carbon is favorable for hydrogen adsorption. 4. Conclusions

Figure 11. Low-pressure H2 adsorption isotherms for Pt/N-doped microporous carbon at 273 K (∆), 298 K (O), and 323 K (0). Inset: calculated isosteric heats of adsorption.

N-doped microporous carbon, the hydrogen uptake at 10 MPa was increased to 1.26 wt %, that is, by a factor of 2.4. The enhanced hydrogen storage capacity cannot be attributed to differences in surface area because the Pt/N-doped microporous carbon had a lower surface area than that of the N-doped microporous carbon. The enhancement in hydrogen storage was due to the spillover of atomic hydrogen from the Pt particles to the N-doped microporous carbon. Pt metals are known as hydrogen dissociation sources, and the enhanced hydrogen storage by metal doped on various carbon materials (CNTs, active carbon, carbon nanofiber, etc.) has been reported by many authors. In the present case, compared with the N-doped microporous carbon, it is remarkable that the hydrogen uptake on Pt/N-doped microporous carbon has been enhanced by a factor of 2.4. In addition, a hydrogen adsorption isotherm on Pt/microporous carbon synthesized by doping Pt on pure microporous carbon was also measured. As shown in the Supporting Information, Figure 2, Pt/microporous carbon with a surface area of 1308 m2/g had a storage capacity of 0.86 wt % at 298 K and 10 MPa. Thus, Pt/N-doped microporous carbon showed 46% higher adsorption than Pt/microporous carbon. These results indicated nitrogen doping enhanced significantly hydrogen adsorption by spillover. Use of the Clausius-Clapeyron equation would yield the overall heats of adsorption. The overall heats of adsorption of H2 on Pt/N-doped microporous carbon were calculated from the H2 isotherms at 273 and 298 K by using the ClausiusClapeyron equation. As shown in Figure 11, the H2 adsorption amount at all pressures up to 1 atm decreased with an increase in temperature. The inset in Figure 11 shows that the absolute values of heat of adsorption decreased sharply with the adsorption amount for each sample. The heats of adsorption were >20 kJ/mol at low surface coverage and leveled off to ∼11.8 kJ/mol at relatively high surface coverage. It is worth noting that the heats of adsorption on the Pt/N-doped microporous carbon were higher than that of hydrogen physisorption on N-doped microporous carbon (∼7.3 kJ/mol), reflecting the strong interactions between the spilt-over H and carbon supports. The high values of heats of adsorption at low surface coverages can be attributed to the strong adsorption of H atoms on the metal particles, as well as the H atoms on the strongest sites on carbon. As a first-order analysis, we take the heat of adsorption at high H2 adsorption amount as an indicator of the adsorption strength of hydrogen atoms on the receptor surface. The heats of adsorption of H2 on Pt/microporous carbon were also calculated from the H2 isotherms at 273 and 298 K. As

In this study, the hydrogen storage properties of N-doped microporous carbon and Pt/N-doped microporous carbon were investigated. It was found that nitrogen doping is favorable for hydrogen adsorption. N-doped microporous carbon had a storage capacity of 0.51 wt % at 298 K and 100 atm, which was 18% higher than that of the pure microporous carbon with similar surface area. Furthermore, hydrogen storage in Pt/N-doped microporous carbon via spillover was studied, which showed a storage capacity of 1.26 wt %, an enhancement factor of 2.4 compared with the N-doped microporous carbon. It is remarkable that Pt/N-doped microporous carbon showed 1.46 times adsorption compared with Pt/microporous carbon (without N doping). These results were interpreted by the results on increased heats of adsorption by N-doping, in agreement with theoretical predictions. Acknowledgment. The authors acknowledge NSF Grant No. CBET-0753008 and the funding provided by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy within the Hydrogen Sorption Center of Excellence (HSCoE). Supporting Information Available: Nitrogen isotherm on pure microporous carbon and high- and low-pressure hydrogen isotherms for Pt/microporous carbon. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353. (2) Dillon, A. C.; Johns, K. M.; Bekkedahl, T. A.; Klang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (3) Chambers, A.; Park, C.; Baker, R. T. K.; Rodriguez, N. M. J. Phys. Chem. B 1998, 102, 4253. (4) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth: London, U.K., 1987; Chapter 4. (5) Dillon, A. C.; Heben, M. J. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 133. (6) Benard, P.; Chahine, R. Scr. Mater. 2007, 56, 803. (7) Yang, Z.; Xia, Y.; Mokaya, R. J. Am. Chem. Soc. 2007, 129, 1673. (8) Yang, R. T. Carbon 2000, 38, 623. (9) Shiraishi, M.; Takenobu, T.; Kataura, H.; Ata, M. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 947. (10) Lueking, A.; Yang, R. T. J. Catal. 2002, 206, 165. (11) Lueking, A.; Yang, R. T. Appl. Catal., A 2004, 265, 259. (12) Lachawiec, A. J.; Qi, G. S.; Yang, R. T. Langmuir 2005, 21, 11418. (13) Li, Y. W.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 8136. (14) Li, Y. W.; Yang, R. T. J. Phys. Chem. B 2006, 110, 17175. (15) Wang, L.; Yang, R. T. Energy EnViron. Sci. 2008, 1, 268. (16) Wang, Y.; Yang, R. T. J. Catal. 2008, 260, 198. (17) Robell, A. J.; Ballou, E. V.; Boudart, M. J. Phys. Chem. 1964, 68, 2748. (18) Srinivas, S. T.; Rao, P. K. J. Catal. 1994, 148, 470. (19) Pajonk, G. M. Appl. Catal., A 2000, 202, 157. (20) Sinfelt, J. H.; Lucchesi, P. J. J. Am. Chem. Soc. 1963, 85, 3365. (21) Conner, W. C., Jr.; Falconer, J. L. Chem. ReV. 1995, 95, 759. (22) Mitchell, P. C. H.; Ramirez-Cuesta, A. J.; Parker, S. F.; Tomkinson, J.; Thompsett, D. J. Phys. Chem. B 2003, 107, 6838. (23) Mitchell, P. C. H.; Ramirez-Cuesta, A. J.; Parker, S. F.; Tomkinson, J. J. Mol. Struct. 2003, 651-653, 781. (24) Pamitar, A. P.; Yates, J. T., Jr. J. Phys. Chem. C 2007, 111, 2959.

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(25) Chen, L.; Cooper, A. C.; Pez, G. P.; Cheng, H. S. J. Phys. Chem. C 2007, 111, 18995. (26) Lachawiec, A. J., Jr.; Yang, R. T. Langmuir 2008, 24, 6159. (27) Lupu, D.; Biris, A. R.; Misan, I.; Jianu, A.; Holzhuter, G.; Burkel, E. Int. J. Hydrogen Energy 2004, 29, 97. (28) Zacharia, R.; Kim, K. Y.; Fazle Kibria, A. K. M.; Nahm, K. S. Chem. Phys. Lett. 2005, 412, 369. (29) Li, Y.; Yang, R. T. J. Phys. Chem. C 2007, 111, 11086. (30) Zielinski, M.; Wojcieszak, R.; Monteverdi, S.; Mercy, M.; Bettahar, M. M. Catal. Commun. 2005, 6, 777. (31) Back, C.; Sandi, G.; Prakash, J.; Hranisavljevic, J. J. Phys. Chem. B 2006, 110, 16225. (32) Anson, A.; Lafuente, E.; Urriolabeitia, E.; Navarro, R.; Benito, A. M.; Maser, W. K.; Martinez, M. T. J. Phys. Chem. B 2006, 110, 6643. (33) Wang, L.; Yang, R. T. J. Phys. Chem. C 2008, 112, 12486. (34) Li, Y.; Yang, R. T.; Liu, C.-J.; Wang, Z. Ind. Eng. Chem. Res. 2007, 46, 8277. (35) Wong-Foy, A. G.; Matzger, A. J.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 3494. (36) Zhu, Z. H.; Lu, G. Q.; Hatori, H. J. Phys. Chem. B 2006, 110, 1249. (37) Sankaran, M.; Viswanathan, B. Carbon 2006, 44, 2816. (38) Chung, T. C. M.; Jeong, Y.; Che, Q.; Kleinhammes, A.; Wu, Y. J. Am. Chem. Soc. 2008, 130, 6668. (39) Viswanathan, B.; Sankaran, M. Carbon 2007, 45, 1628. (40) Wang, L.; Yang, F. H.; Yang, R. T. AIChE J. 2009, 55, 1823.

Wang and Yang (41) Badzian, A.; Badzian, T.; Breval, E.; Piotrowski, A. Thin Solid Films 2001, 398-399, 170. (42) Kang, K. Y.; Lee, B. I.; Lee, J. S. Carbon 2009, 47, 1171. (43) Yang, Z.; Xia, Y.; Sun, X.; Mokaya, R. J. Phys. Chem. B 2006, 110, 18424. (44) Zhu, Z. H.; Hatori, H.; Wang, S. B.; Lu, G. Q. J. Phys. Chem. B 2005, 109, 16744. (45) Zhang, Z.; Cho, K. Phys. ReV. B 2007, 75, 075420. (46) Kyotani, T.; Nagai, T.; Inoue, S.; Tomita, A. Chem. Mater. 1997, 9, 609. (47) Lachawiec, A. J.; DiRaimondo, T. R.; Yang, R. T. ReV. Sci. Instrum. 2008, 79, 063906. (48) Kima, N. D.; Kima, W.; Jooa, J. B.; Oha, S.; Kimb, P.; Kimc, Y.; Yia, J. J. Power Sources 2008, 180, 671. (49) Yang, Z.; Xia, Y.; Mokaya, R. Chem. Mater. 2005, 17, 4502. (50) He, M.; Zhou, S.; Zhang, J.; Liu, Z.; Robinson, C. J. Phys. Chem. B 2005, 109, 9275. (51) Yang, Q.; Xu, W.; Tomita, A.; Kyotani, T. Chem. Mater. 2005, 17, 2940. (52) Lim, S.; Yoon, S.-H.; Mochida, I.; Jung, D.-H. Langmuir 2009, 25, 8268. (53) Xia, Y.; Mokaya, R. Chem. Mater. 2005, 17, 1553. (54) Be´nard, P.; Chahine, R. Langmuir 2001, 17, 1950. (55) Zhou, L.; Zhou, Y.; Sun, Y. Int. J. Hydrogen Energy 2004, 29, 475.

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