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Hydrogen Storage in Li3N: Deactivation Caused by a High Dehydrogenation Temperature Yun Hang Hu,* Nian Ying Yu, and Eli Ruckenstein Department of Chemical and Biological Engineering, State University of New York at Buffalo, Amherst, New York 14260
As a potential candidate for hydrogen storage, Li3N can absorb more than 9 wt % hydrogen. However, because of its incomplete dehydrogenation at a temperature of 280 °C, only about 5.5 wt % reversible hydrogen capacity could be reached. Although by increasing the temperature one can enhance dehydrogenation, this paper demonstrates that dehydrogenation of hydrogenated Li3N at the high temperature of 400 °C is followed by a very low (0.4 wt %) rehydrogenation capacity. Furthermore, scanning electron microscopy, Brunauer-Emmett-Teller surface area, and X-ray powder diffraction measurements have shown that both the sintering and the lattice structure change of Li2NH, which is a product of hydrogenation, might be responsible for such a major deactivation. 1. Introduction A hydrogen storage technology is critical for the hydrogen-powered automobiles to become competitive with the traditional vehicles. Three types of technologies are available to store hydrogen: the liquefying of hydrogen, the compression of hydrogen, and the hydrogen absorption/adsorption in a solid material. The former two are unsuitable for transportation because the liquid hydrogen is too expensive and the compressed hydrogen requires a too large volume.1 For this reason, the attention was focused on solid storage materials. As the most famous hydrogen storage material, MgH2 exhibits reversible hydrogen storage, but hydrogenation of magnesium to MgH2 occurs only under severe conditions (high temperatures, above 350 °C, and high pressures, above 50 atm) and only very slowly and incompletely. In addition, the rate of dehydrogenation of MgH2 is too low.2 The low-temperature reversible hydrides, such as LaNi5H6 and TiFeH2, exhibit suitable dehydrogenation kinetics;3 they have, however, very low hydrogen storage capacities (1.5 wt % for LaNi5H6 and 1.8 wt % for TiFeH2). To achieve high hydrogen storage capacities, the complex hydrides of light metals (Li, Na, and Al), such as LiAlH4 (10.5 wt % H2) and NaAlH4 (7.4 wt % H2), have been investigated.4 Although these complex hydrides are nonreversible, Bogdanovic et al.5-7 recently demonstrated that, upon their doping with titanium compounds, dehydrogenation of NaAlH4/Na3AlH6/Na2LiAlH6 was enhanced and rendered reversible under moderate conditions. This breakthrough was followed by progress in the development of catalysts for reversible dehydrogenation of NaAlH4.8-15 However, a reversible hydrogen storage capacity of greater than 6 wt % with a good cyclability is still a challenge for these catalyst-doped complex hydrides. Other promising materials for hydrogen storage are the nanostructured composites because they possess dramatically different chemical, physical, thermody* To whom correspondence should be addressed. Tel.: (716) 645-2911 ext. 2266. Fax: (716) 645-3822. E-mail: yhu@ buffalo.edu.
namic, and transport properties than their bulk counterparts. The ability to tailor pore and grain sizes and to intimately mix two or more phases at the nanometer scale may open a window to a greater hydrogen storage capacity than that of the coarse-grained materials.16,17 The hydrogen storage in carbon nanotubes attracted tremendous experimental and theoretical interest.18-26 However, their hydrogen storage is not as efficient as was expected.21-24 For this reason, the recent investigations have shifted away from them. Although other types of nanotubes, such as boron nitride,27 MoS2,28 and TiS229 nanotubes, can be used for hydrogen storage, their hydrogen capacities are lower than 3 wt % even at high pressures. Recently, Yaghi et al. reported about the hydrogen storage in microporous metal-organic frameworks.30 Although this observation opened an interesting direction of research, the current microporous metal-organic frameworks require a temperature as low as 78 K to yield a 4.5 wt % hydrogen storage capacity. At room temperature and 20 atm, their hydrogen storage capacity is 1 wt %. Very recently, the interest in hydrogenation of Li3N has been renewed.31-34 Dafert and Miklauz35 were the first to study hydrogenation and dehydrogenation of Li3N in 1910. They reported that the reaction between Li3N and H2 generates Li3NH4 (Li3N + 2H2 ) Li3NH4), that Li3N absorbs 10.4 wt % hydrogen, and that Li3NH4 can be decomposed to release H2.35 Furthermore, Ruff and Goeres36 noted that Li3NH4 is a mixture of LiNH2 and 2LiH. Although Li3N can be a useful storage material, it did not attract attention probably because of the suspicion that it can generate NH3. Indeed, NH3 generation is a thermodynamically favorable process at temperatures below 400 °C.32 However, recent experiments revealed that no NH3 was formed during hydrogenation of Li3N and dehydrogenation of hydrogenated Li3N.31 Furthermore, our experiments32 demonstrated that the ultrafast reaction between NH3 and LiH enabled LiH to capture the entire NH3 generated during hydrogenation or dehydrogenation. One of the Li3N characteristics is its fast hydrogenation kinetics. However, a critical factor for practical applications is its dehydrogenation. Because of incomplete dehydrogenation of hydrogenated Li3N, its revers-
10.1021/ie0501834 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/14/2005
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ible hydrogen capacity at temperatures of 280 °C or lower is only about 5.5 wt %. In principle, to enhance the reversible hydrogen capacity, a high temperature appears to be required to increase dehydrogenation of hydrogenated Li3N. However, in this paper, it is reported that, although a high dehydrogenation temperature can increase dehydrogenation, the resulting solid has lost almost completely the ability to reabsorb hydrogen. 2. Experimental Section 2.1. Samples. Lithium nitride powder (about 80 mesh) was bought from Aldrich Chemical Co. Before it was used, Li3N was exposed to air at room temperature for several minutes in order for its surface layer to be partially oxidized. This partial oxidation is useful because the partially oxidized surface layer inhibits the sintering of Li3N during the initial hydrogenation, which being a highly exothermic reaction (∆H ) -116 kJ/mol) with fast kinetics generates hot spots.33 2.2. Volumetric Test of Hydrogen Storage. A volumetric method, described in a previous paper,33 was employed to accurately determine hydrogen absorption by Li3N. An amount of 0.25 g of Li3N was loaded in a reactor located inside an electrical tubular furnace. Before hydrogen absorption at an initial pressure of 7 atm, the sample was subjected to vacuum (p < 10-5 Torr). The change in the gas-phase pressure of H2 during absorption was measured using a digital pressure gauge. To examine rehydrogenation, the hydrogenated sample was exposed to vacuum to desorb hydrogen at a selected temperature, followed by reabsorption. An online mass spectrometer (HP quadrupole mass selective detector) equipped with a fast-response inlet capillary system was used to confirm that except for hydrogen no other compounds were present during hydrogenation and dehydrogenation. It should be noted that the temperature was measured outside the reactor. Therefore, the reaction temperature (230 °C) does not account for the hot spots generated during reaction. The hydrogen capacity is defined as the percentage of hydrogen absorbed based on the total weight of the solid and hydrogen. 2.2. X-ray Powder Diffraction (XRD). The XRD patterns of various hydrogenated and dehydrogenated samples were determined using a Siemens D500 XRD instrument, equipped with a Cu KR source, at 40 kV and 30 mA. The lattice parameter of Li2NH was calculated from the diffractions of (111) and (220) faces using the expression
Figure 1. Hydrogen absorption: (a) Li3N; (b) Li3N subjected to hydrogenation at 230 °C for 24 h and dehydrogenation at (230 °C) for 14 h; (c) Li3N subjected to hydrogenation at 230 °C for 24 h and dehydrogenation at 280 °C for 14 h; (d) Li3N subjected to hydrogenation at 230 °C for 24 h and dehydrogenation at 400 °C for 1 h; (e) Li3N subjected to hydrogenation at 230 °C for 24 h and dehydrogenation at 400 °C for 14 h. Absorption conditions: initial hydrogen pressure ) 7 atm and final pressure ≈ 4 atm; absorption temperature ) 230 °C.
a ) (h2 + k2 + l2)1/2λ/(2 sin θ) where θ and λ are the diffraction angle and the wavelength, respectively. 2.3. Scanning Electron Microscopy (SEM). A scanning electron microscope (Hitachi S-4000) was employed to examine the morphologies of the specimens after hydrogenation and dehydrogenation. The samples were coated with carbon before measurements. 2.4. Brunauer-Emmett-Teller (BET) Surface Area and Pore Size Distribution Measurements. A Micromeritics ASAP 2000 instrument was used to determine, via nitrogen adsorption at 77 K, the surface area and the pore size distribution of various specimens. The surface area was determined by the BET method, while the pore size distribution curve was obtained from
Figure 2. SEM micrographs of Li3N at two different scales.
the adsorption branch of the N2 isotherm by the Barrett-Joyner-Halenda method. The Li3N sample was degassed at 100 °C in high vacuum before the measurement. 3. Results Hydrogen absorption by Li3N was determined by the volumetric method. As shown in Figure 1, Li3N absorbed nearly 8.0 wt % hydrogen in 20 min at 230 °C and an
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Figure 3. SEM micrographs at two different scales of Li3N subjected to hydrogenation at 230 °C.
additional 1 wt % in the following 140 min. A Li3N sample subjected to hydrogenation at 230 °C, followed by dehydrogenation at 230 or 280 °C, reached 230 °C rapidly (in 8 min) a hydrogen uptake of 5.4 wt %. In contrast, the Li3N sample subjected to hydrogenation at 230 °C, followed by dehydrogenation at 400 °C for 1 and 14 h, could reabsorb only 0.6 and 0.4 wt % hydrogen in 140 min at 230 °C, respectively (parts d and e of Figure 1). Consequently, the high desorption temperature not only involved a higher energy cost but also destroyed the ability for hydrogen reabsorption. SEM was employed to examine the morphologies of the samples after hydrogenation and dehydrogenation. These morphologies are presented in Figures 2-5. In the initial Li3N, the sizes of the primary particles were about 0.5 µm (Figure 2b). However, these primary particles were not separated from each other but were aggregated in secondary particles of 1-60 µm, separated from each other (Figure 2a). After hydrogenation at 230 °C, the separated secondary particles formed larger blocks (see Figure 3a). Furthermore, the higher magnification image shows that the blocks consisted of primary particles of 0.5 µm (Figure 3b). After dehydrogenation at 280 °C of hydrogenated Li3N, one can still identify blocks but no longer separated particles (Figure 4a). However, in contrast to the previous blocks, the latter ones consisted of large secondary particles (Figure 4b). For Li3N subjected to hydrogenation at 230 °C, followed by dehydrogenation at 400 °C, one can identify various kinds of particles (small and large primary particles) (see Figure 5).
Figure 4. SEM micrographs at two different scales of Li3N subjected to hydrogenation at 230 °C, followed by dehydrogenation at 280 °C.
Furthermore, N2 adsorption was carried out at the temperature of liquid nitrogen (77 K) on various specimens. As shown in Table 1, the surface area of Li3N increased from 4.0 to 6.5 m2/g after its hydrogenation at 230 °C and further to 11.7 m2/g after subsequent dehydrogenation at 280 °C. Furthermore, the surface area (5.9 m2/g) of the sample, obtained during hydrogenation at 230 °C followed by dehydrogenation at 400 °C, is smaller than that (11.7 m2/g) of the sample obtained during hydrogenation at 230 °C and dehydrogenation at 280 °C. However, the surface area (5.9 m2/ g) of the sample produced in the latter case is still larger than that (4.0 m2/g) of the original Li3N. In other words, although hydrogenation of Li3N and dehydrogenation of hydrogenated Li3N generated blocks (Figures 2-5), these blocks possessed larger surface areas than that of the original Li3N. This occurred because of the pores they contain. Indeed, as shown in Figure 6, all samples, including the untreated Li3N, possess pores distributed in two ranges: 10-80 and 80-3000 Å. Furthermore, the number of 80-3000 Å mesopores increased after hydrogenation at 230 °C and even more after subsequent dehydrogenation at 280 °C. Only the Li3N sample, which was subjected to hydrogenation at 230 °C and dehydrogenation at 400 °C, possessed a smaller number of mesopores (80-3000 Å). XRD was employed to identify the phases present in the samples both after hydrogenation and after dehydrogenation (Figure 7). The XRD patterns revealed that the initial Li3N hydrogenated at 230 °C consisted of LiH
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Figure 6. Pore size distribution of Li3N: (a) without any treatment; (b) subjected to hydrogenation at 230 °C; (c) subjected to hydrogenation at 230 °C, followed by dehydrogenation at 280 °C; (d) subjected to hydrogenation at 230 °C, followed by dehydrogenation at 400 °C.
Figure 5. SEM micrographs at two different scales of Li3N subjected to hydrogenation at 230 °C, followed by dehydrogenation at 400 °C. Table 1. BET Surface Areas sample Li3N Li3N hydrogenated at 230 °C Li3N hydrogenated at 230 °C, followed by dehydrogenation at 280 °C Li3N hydrogenated at 230 °C, followed by dehydrogenation at 400 °C
BET surface area (m2/g) 4.0 6.5 11.7 5.9
and LiNH2 (Figure 7b). After dehydrogenation at 280 °C, LiNH2 disappeared from the XRD patterns, but Li2NH could be identified (Figure 7c). The diffraction peaks of LiNH2 and Li2NH above 2θ ) 30° are near one another. However, the two peaks at 17.4° and 19.6°, which are characteristic only for LiNH2, can be used to distinguish LiNH2 from Li2NH. In the sample obtained after hydrogenation at 230 °C and dehydrogenation at 400 °C, Li2NH and very small amounts of Li3N and LiH could be identified (Figure 7d). However, the positions of the Li2NH peaks for hydrogenated Li3N subjected to dehydrogenation at 400 °C are about 1° larger than those of hydrogenated Li3N subjected to dehydrogenation at 280 °C. In other words, the lattice parameter of Li2NH, which has a cubic structure, differs in the samples generated via low- and high-temperature dehydrogenations of hydrogenated Li3N. The lattice parameter of Li2NH has decreased from 5.026 to 4.951 Å when the dehydrogenation temperature was increased from 280 to 400 °C.
Figure 7. XRD patterns: (a) Li3N; (b) Li3N hydrogenated at 230 °C; (c) Li3N subjected to hydrogenation at 230 °C, followed by dehydrogenation at 280 °C; (d) Li3N subjected to hydrogenation at 230 °C, followed by dehydrogenation at 400 °C.
4. Discussions XRD revealed that hydrogenation of Li3N at 230 °C generated LiNH2/LiH, which were converted to Li2NH and LiH during the subsequent dehydrogenation at 280 °C and not to Li3N (Figure 7). Consequently, dehydrogenation of LiNH2 and LiH at 280 °C followed the reaction LiH + LiNH2 ) Li2NH + H2, thus providing about a 5.5 wt % reversible hydrogen capacity, and the second step of dehydrogenation, LiH + Li2NH ) Li3N + H2, did not take place. This explains why only about a 5.5 wt % hydrogen capacity was reached during hydrogenation at 230 °C of Li3N followed by dehydrogenation at 280 °C (or 230 °C) (Figure 1). One may be tempted to conclude that, to increase the reversible
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5. Conclusions
Figure 8. Hydrogen absorption of Li3N, which was previously subjected to hydrogenation at 230 °C for 24 h and dehydrogenation at 400 °C for 14 h, followed by hand grinding to a powder. Absorption conditions: initial hydrogen pressure ) 7 atm; absorption temperature ) 230 °C.
hydrogen capacity, higher temperatures are required to activate the second dehydrogenation step (LiH + Li2NH ) Li3N + H2). Indeed, a small amount of Li3N, besides Li2NH and LiH, could be detected by XRD in the sample generated via hydrogenation of Li3N at 230 °C and subsequent dehydrogenation at 400 °C. However, the material produced during the 400 °C dehydrogenation reabsorbed only 0.4 wt % hydrogen (see curve e in Figure 1), which is much lower than the about 5.5 wt % achieved during the lower temperature dehydrogenation (see curves b and c in Figure 1). The BET surface area measurements indicated that the material generated during the 400 °C dehydrogenation of hydrogenated Li3N had a smaller surface area (5.9 m2/g) than the one generated during the 280 °C dehydrogenation (11.7 m2/g). The higher temperature resulted in some sintering, which is consistent with the SEM micrographs, which revealed the presence of large particles in the material obtained after dehydrogenation at 400 °C. This sintering may be partially responsible for deactivation of the sample dehydrogenated at 400 °C. In addition, desorption at high temperatures may generate an inactive and impervious skin layer, which isolates the bulk material from hydrogen, resulting in deactivation of rehydrogenation. To verify the existence of such an effect, Li3N, which was subjected to hydrogenation at 230 °C followed by dehydrogenation at 400 °C for 14 h, was hand-ground as a powder in order to destroy the skin layer. As shown in Figure 8, the handground sample has a more rapid kinetics and higher reabsorption capacity (Figure 8) than the unground one (see curve e in Figure 1). Consequently, the formation of an impervious skin layer may play a role. Another effect may also be important. The amount of Li3N present in the sample dehydrogenated at high temperature is very small (Figure 7). Furthermore, in previous experiments,37 we found that Li3N subjected to heating at 400 °C was still active regarding its hydrogenation. Therefore, Li3N cannot play an important role in the deactivation process. In contrast, the amount of Li2NH present in the specimen is large. Furthermore, XRD showed that the lattice parameter of Li2NH (cubic structure) decreased from 5.026 to 4.951 Å as the dehydrogenation temperature increased from 280 to 400 °C. This lattice change might also be responsible for deactivation of Li2NH during the high-temperature dehydrogenation.
The reversible hydrogen capacity of Li3N is affected by the dehydrogenation temperature. For a dehydrogenation temperature of 280 °C, the reversible hydrogen capacity was about 5.5 wt % because of incomplete dehydrogenation of hydrogenated Li3N. Although a high temperature (400 °C) can increase dehydrogenation of hydrogenated Li3N, the solid thus regenerated had a very low (0.4 wt %) rehydrogenation capacity. Furthermore, SEM and BET surface area measurements showed that some sintering occurred during dehydrogenation at 400 °C, which may be partially responsible for deactivation. Furthermore, XRD measurements showed that the lattice parameter of Li2NH decreased from 5.026 to 4.951 Å as the dehydrogenation temperature increased from 280 to 400 °C. This change of the Li2NH lattice parameter might also play a role in deactivation. Literature Cited (1) Trudeau, M. L. Advanced materials for energy storage. MRS Bull. 1999, 24, 23. (2) Genossar, J.; Rudman, P. S. Catalytic role of Mg2Cu in the hydriding and dehydriding of Mg. Z Phys. Chem., Neue Folge 1979, 116, 215. (3) Buchner, H. Energiespeichrung in Metallhydriden; Springer: Wien, Austria, 1982. (4) Wiswall, R. In Hydrogen in Metals II; Alefeld, G., Vo¨lkl, J., Eds.; Springer-Verlag: New York, 1978; p 201. (5) Bogdanovic, B.; Schwickardi, M. Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. J. Alloys Compd. 1997, 253, 1. (6) Bogdanovic, B.; Sandrock, G. Catalyzed complex metal hydrides. MRS Bull. 2002, 27, 712. (7) Bogdanovic, B.; Brand, R. A.; Marjanovic, A.; Schwickardi, M.; Tolle, J. Metal-doped sodium aluminium hydrides as potential new hydrogen storage materials. J. Alloys Compd. 2000, 302, 36. (8) Jensen, C. M.; Zidan, R.; Mariels, N.; Hee, A.; Hagen, C. Advanced titanium doped of sodium aluminum hydride: segue to a practical hydrogen storage material? Int. J. Hydrogen Energy 1999, 24, 461. (9) Zidan, R. A.; Takara, S.; Hee, A. G.; Jensen, M. C. Hydrogen cycling behavior of zirconium and titanium-zirconium hydride doped aluminum. J. Alloys Compd. 1999, 285, 119. (10) Jensen, C. M.; Gross, K. J. Development of catalytically enhanced sodium aluminum as a hydrogen-storage material. J. Appl. Phys. A 2001, 72, 213. (11) Anton, D. L. Hydrogen desorption kinetics in transition metal modified NaAlH4. J. Alloys Compd. 2003, 356, 400. (12) Majzoub, E. H.; Gross, K. J. Titanium-halide catalystprecursors in sodium aluminum hydrides. J. Alloys Compd. 2003, 356, 363. (13) Balogh, M. P.; Tibbetts, G. G.; Pinkerton, F. E.; Meisner, G. P.; Olk, C. H. Phase changes and hydrogen release during decomposition of sodium alanates. J. Alloys Compd. 2003, 350, 136. (14) Sandrock, G.; Gross, K.; Thomas, G.; Jensen, C.; Meeker, D.; Takara, S. Engineering considerations in the use of catalyzed sodium alanates for hydrogen storage. J. Alloys Compd. 2002, 330, 696. (15) Gross, K. J.; Thomas, G. J.; Jensen, C. M. Catalyzed alanates for hydrogen storage. J. Alloys Compd. 2002, 330-332, 683. (16) Seayad, A. N.; Antonelli, D. M. Recent advances in hydrogen storage in metal-containing inorganic nanostructures and related materials. Adv. Mater. 2004, 16, 765. (17) Zaluska, A.; Zaluski, L.; Strom-Olsen, J. O. Sodium alanates for reversible hydrogen storage. J. Alloys Compd. 2000, 298, 125. (18) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethuune D. S.; Heben, M. J. Storage of hydrogen in singlewalled carbon nanotubes. Nature 1997, 386, 377. (19) Ye, Y.; Ahn, C. C.; Witham, C.; Fultz, B.; Liu, J.; Rinzler, A. G.; Colbert, D.; Smith, K. A.; Smalley, R. E. Hydrogen
Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4309 adsorption and cohesive energy of single-walled carbon nanotubes. Appl. Phys. Lett. 1999, 74, 2307. (20) Liu, C.; Fan, Y. Y.; Cheng, H. M.; Dresselhaus, M. S. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999, 286, 1127. (21) Dagani, R. Tempest in a tiny tube. Chem. Eng. News 2002, 80 (2), 25. (22) Yang, R. T. Hydrogen storage by alkali-doped carbon nanotubes-revisited. Carbon 2000, 38, 623. (23) Hirscher, M.; Becher, M.; Haluska, M.; Dethlaff-Weglikowska, U.; Quintel, A.; Duesberg, G. S. Hydrogen storage in sonicated carbon materials. Appl. Phys. A 2001, 72, 129. (24) Cheng, H.; Pez, G. P.; Cooper, A. C. Electrochemical hydrogen storage in MoS2 nanotubes. J. Am. Chem. Soc. 2001, 123, 5845. (25) Wang, Q.; Karl Johnson, J. Computer Simulations of Hydrogen Adsorption on Graphite Nanofibers. J. Phys. Chem. B 1999, 103, 277. (26) Dresselhaus, M. S.; Williams, K. A.; Eklund, P. C. Hydrogen adsorption in carbon materials. MRS Bull. 1999, 24, 45. (27) Ma, R.; Bando, Y.; Zhu, H.; Sato, T.; Xu, C.; Wu, D. Hydrogen Uptake in Boron Nitride Nanotubes at Room Temperature. J. Am. Chem. Soc. 2002, 124, 7672. (28) Chen, J.; Kuriyama, N.; Yuan, H.; Takeshita, H. T.; Sakai, T. Electrochemical Hydrogen Storage in MoS2 Nanotubes. J. Am. Chem. Soc. 2001, 123, 11813. (29) Chen, J.; Li, S. L.; Tao, Z. L.; Shen, Y. T.; Cui, C. X. Titanium Disulfide Nanotubes as Hydrogen-Storage Materials. J. Am. Chem. Soc. 2003, 125, 5284.
(30) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keefe, M.; Yaghi, O. M. Hydrogen storage in microporous metal-organic frameworks. Science 2003, 300, 1127. (31) Hu, Y. H.; Ruckenstein, E. H2 storage in Li3N. Temperature-programmed hydrogenation and dehydrogenation. Ind. Eng. Chem. Res. 2003, 42, 5135. (32) Hu, Y. H.; Ruckenstein, E. Ultrafast reaction between LiH and NH3 during H2 storage in Li3N. J. Phys. Chem. A 2003, 107, 9737. (33) (a) Hu, Y. H.; Ruckenstein, E. Highly Effective Li2O/Li3N with Ultrafast Kinetics for H2 Storage. Ind. Eng. Chem. Res. 2004, 43, 2464. (b) Hu, Y. H.; Ruckenstein, E. Highly reversible hydrogen capacity of LiNH2/Li3N. Ind. Eng. Chem. Res. 2005, 44, 1510. (34) Ichikawa, T.; Isobe, S.; Hanada, N.; Fujii, H. Lithium nitride for reversible hydrogen storage. J. Alloys Compd. 2004, 365, 271. (35) Dafert, F. W.; Miklauz, R. New compounds of nitrogen and hydrogen with lithium. Monatsh. Chem. 1910, 31, 981. (36) Ruff, O.; Goeres, H. Li imide and some compounds of N, H, and Li. Ber. Dtsch. Chem. Ges. 1910, 44, 502. (37) Hu, Y. H.; Yu, N. Y.; Ruckenstein, E. Effect of the heat pretreatments on hydrogen storage in Li3N-based materials. Ind. Eng. Chem. Res. 2004, 43, 4174.
Received for review February 15, 2005 Revised manuscript received April 18, 2005 Accepted April 22, 2005 IE0501834
