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Hydrogen Storage Properties of Low-Silica Type X Zeolites Lifeng Wang and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136
Hydrogen adsorption properties of low-silica type X zeolites (LSX, Si/Al ) 1) containing alkali or alkaliearth metal cations (Li+, Ca2+, and Mg2+) have been studied. It was found that the hydrogen adsorption capacities of LSX zeolites at 77 K were determined mainly by the porosity of the zeolite, while at 298 K, the storage capacities depended on both the H2-cation interactions and the porosity. Among the three exchanged zeolites, Li-LSX had the highest H2 capacity of 1.5 wt % at 77 K and 1 atm, and Ca-LSX had the highest capacity of 0.50 wt % at 298 K and 10 MPa. The hydrogen storage in LSX zeolites via spillover was also investigated. Three methods including bridge building with a catalyst, metal doping via incipient wetness impregnation and metal doping via chemical vapor deposition (CVD) were employed to induce hydrogen spillover, and enhance the storage capacities. Thus, the storage capacities were increased to 0.96-1.2 wt % on the Pt-doped zeolites at 298 K and 10 MPa. The differences between the three methods were compared and discussed. Furthermore, 5 and 10 wt % Ni were doped on Ca-LSX zeolite. The 10 wt % Ni-doped Ca-LSX zeolite showed a storage capacity of 1.15 wt % at 100 atm and 298 K. The important volumetric storage capacities of these zeolites were also estimated based on the densities of the densified zeolites. A 21 g/L portion was obtained for Pt-doped Ca-LSX, and 20 g/L was obtained for Ni-doped Ca-LSX, both at 298 K and 10 MPa. The high volumetric capacities were obtained because of the high densities of zeolites which are substantially higher (2-3 times higher) than that of carbons and metal-organic frameworks. 1. Introduction Hydrogen as a clean and efficient energy carrier has been proposed as an alternative fuel source due to the demands on environmental protection and concerns about global warming. The utilization of hydrogen as a substitute for fossil fuels leads to a number of issues related to hydrogen production, transportation, storage, and fuel cell technology. Storage is a most challenging one among the issues for the realization of fuelcell powered vehicles using hydrogen.1 Several candidate hydrogen storage methods including liquid or high-pressure H2 gas, chemical hydride, metal hydride, and porous adsorbents are being studied. Among these methods, storage by adsorption on porous adsorbents is considered as one potential technology for safe and efficient hydrogen storage. Nanostructured and porous adsorbents including carbon nanotubes, activated carbon, templated carbon, metal organic frameworks, and zeolites have been major candidates for hydrogen storage.2-9 Zeolites are a family of microporous materials widely used in the fields of catalysis, oil refining, and gas separation. Compared to other porous adsorbents, zeolites have the features of intricate structure, high thermal stability, high density, low cost, and adjustable composition.9-11 These features enable zeolites to be considered potential adsorbents for hydrogen storage. Hydrogen adsorption properties of various types of zeolites have been investigated.12-22 It was found that the amount of hydrogen adsorbed on zeolites can be affected by the framework structure, composition, acidic-basic nature, and charge-compensating cations. The effects of amount, size, charge, and concentration of cations on hydrogen adsorption can be compared on zeolites exchanged by various amounts or types of cations. Kazansky et al. studied the hydrogen adsorption on faujasite with different Si/Al ratios and found the highest adsorbed H2 molecules per Na+ was obtained at a Si/Al ratio of 1.05.14 Beyaz Kayiran et al. reported that the hydrogen * To whom correspondence should be addressed. Fax: (734) 7647453. E-mail address:
[email protected].
adsorption on LTA zeolites varied when exchanged by cations of Na, Li, and K.16 Li and Yang studied the effect of monovalent cation exchange on the adsorption capacity of LSX and found that the capacity depended on the cationic radius.23 It is known that the cations in the zeolite create strong electric fields (and field gradients) that favor gas adsorption.24 High storage capacities could be expected on zeolites with high cation charge. In this work, we report the hydrogen adsorption properties of low silica type X zeolites (LSX, Si/Al ) 1) exchanged by alkali or alkali-earth metal cations (Li+, Ca2+, and Mg2+). Zeolite LSX was selected because it has the largest amount of chargecompensating cations per unit cell among all faujasites, and it is known as an excellent sorbent for gas separation and purification.25 In addition, the hydrogen storage via spillover was investigated on both Pt doped LSX and Ni doped LSX. Three methods including bridge building, doping via incipient wetness impregnation, and doping via chemical vapor deposition were employed to induce spillover and enhance storage capacities at 298 K. 2. Experimental Methods 2.1. Synthesis of Li+, Ca2+, and Mg2+ Exchanged LSX Zeolites. A (K,Na)-LSX zeolite (Praxair, No. 16193-42) with composition Na32K64Si96Al96O384 was used as the starting material. Li+, Ca2+, and Mg2+ exchanged LSX samples were prepared by conventional ion-exchange methods from (K,Na)LSX.26-29 The Li+ exchanged LSX was obtained by three consecutive ion exchanges with a 1.0 M LiCl solution at 368 K under reflux conditions. The pH value was controlled at around 9 with a 0.01 M LiOH solution. After each exchange, the solution was decanted, a fresh LiCl solution was added, and the procedure was repeated for a total of three exchanges. After the final exchange, the solid was washed with copious amounts of deionized water until no free ions were detected in the filter water. The resulting material was dried in vacuo prior to measurements. The Ca2+ and Mg2+ exchanged LSX samples
10.1021/ie1003152 2010 American Chemical Society Published on Web 03/17/2010
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were prepared by ion exchange, respectively, in 1.0 M Ca(NO3)2 and 1.0 M Mg(NO3)2 solutions six times under reflux conditions. The other treatments followed the same procedure as that of Li-LSX. 2.2. Preparation of 5 wt % Pt/Ca-LSX and 5 wt % Pt/Li-LSX via Incipient Wetness Impregnation. A 5 wt % portion Pt was directly doped on Ca-LSX or Li-LSX zeolites by incipient wetness impregnation of an aqueous solution of H2PtCl6 · xH2O. After drying in vacuo, the impregnated LSX sample was transferred to a horizontal quartz tube and reduced in H2 atmosphere at 573 K for 3 h. After being cooled to room temperature, the sample was purged with flowing He and stored under He atmosphere before further measurement. 2.3. Preparation of Pt-AC/Bridges/Ca-LSX via Bridge Building. The bridged Ca-LSX was prepared by building carbon bridges between a catalyst (Pt/AC: 5 wt % platinum supported on active carbon) and Ca-LSX zeolite by using sucrose as the carbon precursor according to previous method.30 Pt/AC was used as the hydrogen dissociation source. In the present case, active carbon was considered as the primary receptor for hydrogen atoms. Ca-LSX was the secondary spillover receptor. Carbon bridges between Pt/AC and Ca-LSX were formed by carbonization of sucrose that was previously introduced into a physical mixture of the two components. The receptor/bridgeprecursor/catalyst ratio was fixed at 8:1:1. The ternary mixture was ground together for 1 h. Subsequently, the mixture was heated at 473 K for 3 h and 673 K for 12 h in flowing He. The material was cooled to room temperature and stored under He atmosphere before further measurement. 2.4. Preparation of 5 wt % CVD-Pt/Ca-LSX via Chemical Vapor Deposition. A 5 wt % portion Pt was doped on CaLSX zeolites by chemical vapor deposition of a volatile platinum precursor (trimethyl)methylcyclopentadienyl platinum(IV). The Ca-LSX and (trimethyl)methylcyclopentadienyl platinum were placed in a tube separated by a glass frit and kept at 308 K for 12 h in a vacuum of Li, Ca > Cd, Ni > Mg.33 Yoshida et al. observed the partial collapse of Mg-LSX during the dehydration at 623 K.29 These and our results clearly showed that the Mg-exchanged LSX was partially collapsed. Hydrogen adsorption on LSX zeolites at 77 K and at pressures up to 1 atm are presented in Figure 3. The H2 adsorption on all samples increased with pressure. At around 1 atm, the hydrogen isotherms for Li-LSX and Ca-LSX samples did not show saturation, whereas Mg-LSX zeolite showed a plateau in the H2 uptake. These results imply the possibility of substantial additional increase of H2 adsorption at elevated pressures on the Li-LSX and Ca-LSX zeolites. The H2 uptakes at 1 atm are given in Table 1. Li-LSX zeolite had the highest storage capacity of 1.5 wt %, and the H2 capacity at 1 atm and 77 K followed the order of Li-LSX > Ca-LSX > Mg-LSX. This order followed the same trend as BET surface area or pore volume of exchanged LSX. This is in agreement with previous observation that the hydrogen adsorption at 77 K in porous adsorbent correlated with the BET surface area and the available pore volume.6-8,35 Hydrogen isotherms at 298 K and 0-1 atm for Li-LSX, CaLSX, and Mg-LSX samples are shown in Figure 4. In Figure 4, the H2 capacity at 298 K followed the order of Ca-LSX > Li-LSX > Mg-LSX. This indicates BET surface area or pore volume of the exchanged zeolites is not the only factor in determining the hydrogen adsorption capacity at room temperature. Hydrogen storages on these three zeolites were further
Figure 5. High-pressure hydrogen isotherms on Li-LSX (0), Ca-LSX(O), and Mg-LSX (4) at 298 K and 10 MPa.
investigated at 298 K and 10 MPa. As shown in Figure 5, CaLSX exhibited a hydrogen storage capacity of 0.50 wt %, which is higher than those of Li-LSX (0.43 wt %) and Mg-LSX (0.27 wt %). This followed the same storage capacity order observed within 1 atm. From Table 1, it can be seen that the BET surface area of Ca-LSX (669 m2/g) is lower than that of Li-LSX (764 m2/g), but the hydrogen capacity is higher than the latter. The differences were caused by the different cations exchanged in the zeolites. It is known that the cation sites represent the main adsorption sites for H2 and the interactions of H2 with cations can be related to the total sorbate-sorbent interaction potential energy (φ): φ ) φD + φR + φInd + φFµ + φFY´Q
(1)
where φD ) dispersion energy, φR ) repulsion energy, φInd ) induction energy (interaction between electric field and an induced dipole in H2), φFµ ) interactions between electric field (F) and a permanent dipole (µ), and φFY´Q ) interactions between ´ field gradient (FY) and the quadrupole of H2 (with quadrupole moment Q). These terms are affected by both the cation charge (q) and the equilibrium distance (r) between the interacting H2 cation. For H2-cation pairs, as the cation increases in size, the polarizability increases; hence (φD + φR) increases. It is known from Kirkwood-Mu¨ller formula that the dispersion constant (A) in φD also increases with the magnetic susceptibility (χ). Hence, the dispersion energies are higher for the divalent ions.
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Figure 6. TEM image of Pt/Ca-LSX. Scale bar: 20 nm.
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Figure 7. TEM image of Pt/Li-LSX. Scale bar: 20 nm.
The induction and the quadruple moment field gradient terms are also significant for H2. The H2 molecule has an average polarizability of 7.9 × 10-25 cm3, and a quadruple moment of 0.66 esu. The dependence of the two terms on cation charge and radius is φInd ∝
q2R r4
and
φFY´Q ∝ qQ r3
(2)
where q is the charge of the cation or anion. The ionic radii of the three cations are 0.068 (Li+), 0.099 (Ca2+), and 0.066 nm (Mg2+). Here, R is fixed for hydrogen. The Ca2+ is slightly bigger and Mg2+ is close to the size of Li+, but the divalent cations have twice the charge of Li+; hence, the induction energy and the field gradient-quadrupole interaction energy with the divalent cations (Ca2+ and Mg2+) are larger than that with Li+. The BET surface areas followed the order, Li-LSX > Ca-LSX . Mg-LSX. These results indicate the hydrogen storage in LSX at ambient temperature is governed by both the H2-cation interactions and the BET surface area. 3.2. Pt/Ca-LSX and Pt/Li-LSX. Recent studies showed that the hydrogen storage capacities at 298 K in nanostructured materials could be enhanced significantly by exploiting the hydrogen spillover phenomenon.36-52 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.6,36,53-56 Enhanced hydrogen storage capacity has been obtained by doping with metals (Pt, Ru, Ni, Pd, etc) as the dissociation source.36-52 It is expected that doping of Pt metal on LSX zeolite would further enhance the storage capacity due to hydrogen spillover. Herein we synthesized Pt/Ca-LSX and Pt/Li-LSX zeolites and investigated their hydrogen storage properties. A 5 wt % portion of Pt was directly doped on Ca-LSX or Li-LSX zeolites by incipient wetness impregnation of an aqueous solution of H2PtCl6 · xH2O. The TEM images of Pt/ Ca-LSX and Pt/Li-LSX zeolites are shown in Figures 6 and 7. It can be seen that black Pt particles with sizes around 1-3 nm were well-dispersed on the LSX zeolites. These results confirm the successful doping of Pt on LSX zeolites. By doping 5 wt % Pt, the hydrogen uptakes at 10 MPa were enhanced from 0.50 to 1.09 wt % on Ca-LSX, and from 0.43 to 0.96 wt % on Li-LSX, respectively (Figures 8 and 9). pThe enhanced
Figure 8. High-pressure hydrogen isotherms on Pt/Ca-LSX (4) and CaLSX(O) at 298 K and 10 MPa.
Figure 9. High-pressure hydrogen isotherms on Pt/Li-LSX (]) and LiLSX (0) at 298 K and 10 MPa.
hydrogen storage capacity cannot be attributed to the differences in surface area because the doped samples have, in fact, lower surface areas than plain samples, as shown from nitrogen adsorption results (Table 1). The enhancement in hydrogen storage was mainly due to the spillover of atomic hydrogen from Pt particles to LSX. It can be seen that Pt/Ca-LSX exhibited a
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Figure 10. High-pressure hydrogen isotherms on Bridged Pt/Ca-LSX (4) at 298 K and 10 MPa. Figure 11. TEM image of CVD-Pt/Ca-LSX. Scale bar: 20 nm.
higher hydrogen adsorption capacity than Pt/Li-LSX. This indicates that Ca2+ was also favored for the adsorption of H atoms. To study the effects of the synthesis method on storage capacity, hydrogen storage properties of Pt-AC/bridges/Ca-LSX synthesized via bridge building and CVD-Pt/Ca-LSX synthesized via vapor deposition of a volatile platinum precursor were investigated. 3.3. Pt-AC/Bridges/Ca-LSX. The Yang group first proposed the bridge building technique, which involves mixing the sorbent with a small amount of catalyst (Pt/AC or Pd/AC) and carbon precursor (glucose or sucrose), followed by a temperature programmed heating protocol to carbonize the precursor.30,37 By using this technique, carbon “nanobridges” can be built between a spillover source particle and a secondary receptor particle. The bridges facilitate the hydrogen spillover from one material to another as well as intramaterial transfer and, hence, enhance the hydrogen storage capacity. In the present study, the Pt-AC/bridges/Ca-LSX was prepared by building carbon bridges between a catalyst (Pt/AC: 5 wt % platinum supported on active carbon) and Ca-LSX zeolite by using sucrose as the carbon precursor according to previous method. Pt was the hydrogen dissociation source. Pt/AC was the primary spillover source. Ca-LSX was the secondary spillover receptor. Carbon bridges between Pt/AC and Ca-LSX were formed by carbonization of sucrose. Nitrogen adsorption results showed that Pt-AC/bridges/CaLSX had a BET surface area of 530 m2/g, which is 80% of that of Ca-LSX. This indicates the porosity of Ca-LSX most stayed open after the bridge building process. Hydrogen adsorption isotherm at 298 K showed that Pt-AC/bridges/Ca-LSX had a storage capacity of 1.01 wt % at 10 MPa. Compared with that of plain Ca-LSX (0.50 wt %), the hydrogen uptake on Pt-AC/ bridges/Ca-LSX was enhanced by a factor of 2.0 (Figure 10). It is noted that Pt/AC had a storage capacity of 1.0 wt % at 10 MPa and the content of Pt/AC in Pt-AC/bridges/Ca-LSX is 10%. If the individual contributions of Pt/AC and Ca-LSX were counted additively, the expected storage capacity of the mixture would be less than 0.6 wt % at 10 MPa. The enhanced storage capacity on the Pt-AC/bridges/Ca-LSX (1.01 wt %) indicates the hydrogen spillover from Pt/AC to Ca-LSX. The storage capacity of Pt-AC/bridges/Ca-LSX is slightly less than that of Pt/Ca-LSX synthesized via incipient wetness impregnation. The incipient wetness impregnation led to the direct doping of Pt on the LSX sorbent, thus hydrogen spillover
occurred through the contacting sites between the Pt and LSX. It is noted that 5 wt % Pt was used in Pt/Ca-LSX. When the loading of Pt was reduced to 0.5 wt %, the storage capacity of Pt/Ca-LSX was reduced to less than 0.8 wt % at 10 MPa due to the lower amount of Pt metals as hydrogen dissociation source. For Pt-AC/bridges/Ca-LSX, hydrogen spillover from the Pt/AC to LSX zeolite is via the carbon bridges between them. Only 0.5 wt % Pt was used in the Pt-AC/bridges/Ca-LSX. This indicates the effectiveness of the bridged building method. For using the same amount of Pt (0.5 wt %), bridge building is more efficient than incipient wetness impregnation. The carbon bridges not only facilitate spillover between the Pt/AC and LSX, also between different domains of zeolite particles. 3.4. CVD-Pt/Ca-LSX. Chemical vapor deposition of a volatile platinum precursor on Ca-LSX is another way to induce spiltover hydrogen.57-59 Pd and Pt metals have been doped on MOFs through the metal organic chemical vapor deposition for catalysis and hydrogen adsorption. In the present study, 5 wt % Pt was doped on Ca-LSX zeolites by vapor deposition of the platinum precursor (trimethyl)methylcyclopentadienyl platinum(IV) and subsequent reduction in a hydrogen atmosphere. The TEM image of VD-Pt/Ca-LSX showed nanosized Pt (∼1-3 nm) were well-dispersed on the particles of LSX (Figure 11). Nitrogen adsorption results showed that CVD-Pt/Ca-LSX had a surface area of 593 m2/g, which is 89% of the BET surface area of plain Ca-LSX. Compared with the Pt/Ca-LSX prepared via the incipient wetness impregnation method and Pt-AC/ bridges/Ca-LSX prepared via bridge building method, CVDPt/Ca-LSX had the highest surface area. These results indicate vapor deposition method is favorable for maintaining the integrity of LSX. The hydrogen adsorption isotherm at 298 K (Figure 12) showed that CVD-Pt/Ca-LSX had a storage capacity of 1.20 wt % at 10 MPa, enhanced by a factor of 2.4 compared with that of plain Ca-LSX (0.5 wt %). Reversibility was evaluated by measuring the desorption branch down to 1 atm. The desorption branch nearly followed the adsorption branch although there appeared to be a slight hysteresis. The second adsorption isotherm was in agreement with the first adsorption isotherm. These results indicated that hydrogen adsorption in the CVD-Pt/Ca-LSX was reversible at 298 K. Compared with the incipient wetness impregnation and bridge building methods, the process of chemical vapor deposition of a volatile platinum precursor is relatively complex and timeconsuming. However, the vapor deposition technique could lead to even distribution of Pt and the reduction can be carried out
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Figure 12. High-pressure hydrogen isotherms on CVD-Pt/Li-LSX (adsoption 2; desorption 4) at 298 K and 10 MPa.
at a mild condition. These attributes are beneficial for hydrogen spillover from Pt to the receptor and the maintenance of integrity of the receptor. It is noted that the volumetric storage capacity (in grams of H2 per liter) is a practically important measure for onboard hydrogen storage. Volumetric capacity could be estimated by multiplying the gravimetric capacity by the bulk density. The bulk density was obtained by measuring the density of the pelletized, dehydrated sample, which was 1.65 g/cm3 for CVDPt/Ca-LSX. Thus, the volumetric capacity of CVD-Pt/Ca-LSX was 21 g/L (Figure 12), which is close to the Department of Energy (DOE) target of 28 g/L. 3.5. Ni/Ca-LSX. Ni is also a good hydrogen dissociation catalyst.39-42 Enhanced storage capacity has been achieved by doping Ni on templated carbon, activated carbon, and carbon nanotubes. Compared with noble metals of Pt and Pd, the application of Ni as the hydrogen dissociation source reduces the synthesis cost. A 5 wt % portion of Ni was doped on CaLSX by incipient wetness impregnation of an aqueous solution of Ni(NO3)2 · 6H2O and subsequent reduction in a hydrogen
Figure 13. TEM image of 5 wt % Ni/Ca-LSX. Scale bar: 20 nm.
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Figure 14. High-pressure hydrogen isotherms on 5 wt % Ni/Ca-LSX (0) at 298 K and 10 MPa.
atmosphere at 723 K. The TEM image showed that black Ni particles were dispersed on the LSX zeolites with sizes around 4-10 nm (Figure 13). Nitrogen adsorption results showed the Ni/Ca-LSX had a surface area of 601 m2/g and a pore volume of 0.44 cm3/g. It is noted that the isotherm of Ni/Ca-LSX showed an obvious hysteresis loop at relative pressure from 0.4 to 0.95 (Supporting Information Figure 1). This indicates that some mesoporosities were generated in Ni/Ca-LSX during the treatment at high temperatures. Hydrogen adsorption isotherm at 298 K showed that Ni/Ca-LSX had a storage capacity of 1.06 wt % at 10 MPa (Figure 14). This represented an enhancement factor of 2.1 in comparison with plain Ca-LSX. The loading amount of Ni on Ca-LSX was further increased from 5 to 10 wt % to increase the dissociation sites. As shown in Figure 15, the 10 wt % Ni/Ca-LSX had a storage capacity of 1.15 wt %. Although the loading amount of Ni was doubled, the storage capacity was not increased much. The increase in loading amount of Ni led to the lower surface area of the doped sample (Table 1), which indicates there was a trade-off between
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Figure 15. High-pressure hydrogen isotherms on 10 wt % Ni/Ca-LSX (O) at 298 K and 10 MPa.
the increased dissociation sites induced by more Ni and the decreased surface area and partial pore blocking caused by Ni. In addition, the volumetric storage capacity of Ni/Ca-LSX was estimated to be 20 g/L (Figure 15), based on the pellet density of 1.7 g/cm3. This indicates Ni/Ca-LSX is a promising sorbent for onboard hydrogen storage. 4. Conclusions In this study, the effects of exchangeable cations (Li+, Ca2+, and Mg2+) on hydrogen storage properties of LSX zeolite were investigated. It was found that, the storage capacities of LSX at 77 K were mainly affected by the porosity. At 298 K, the storage capacities depended both on the H2-cation interactions and the porosity. Li-LSX had the highest H2 capacity of 1.5 wt % at 77 K and 1 atm, and Ca-LSX had the highest capacity of 0.50 wt % at 298 K and 10 MPa. These results can be explained by the total sorbate-sorbent interaction potential energy. Three methods were employed to induce hydrogen spillover to increase the storage capacities at 298 K. They were bridge building to a catalyst, incipient wetness impregnation of metal, and chemical vapor deposition, to dope Pt as a hydrogen dissociation source. The storage capacities were increased to 0.96-1.20 wt % at 298 K and 10 MPa. The CVD-Pt/Ca-LSX showed a promising volumetric capacity of 21 g/L. Incipient wetness impregnation is a simple process for doping Pt. Bridge building method worked at the lowest loading of Pt. Chemical vapor deposition has the advantages of even distribution of Pt and preparation under mild conditions. Ni was also used as the hydrogen dissociation source instead of Pt. 5 and 10 wt % Ni were doped on Ca-LSX zeolite. The 10 wt % Ni doped Ca-LSX zeolite showed a storage capacity of 1.15 wt % at 100 atm and 298 K and a volumetric storage capacity of 20 g/L. Acknowledgment The authors acknowledge the funding provided by the US Department of Energy’s Office of Energy Efficiency and Renewable Energy within the Hydrogen Sorption Center of Excellence (HSCoE) and NSF. Supporting Information Available: Supporting Figure 1. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Schlapbach, L.; Zu¨ttel, A. Hydrogen-storage materials for mobile applications. Nature 2001, 414, 353.
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ReceiVed for reView February 9, 2010 ReVised manuscript receiVed March 4, 2010 Accepted March 5, 2010 IE1003152
