Ind. Eng. Chem. Res. 2003, 42, 5135-5139
5135
MATERIALS AND INTERFACES H2 Storage in Li3N. Temperature-Programmed Hydrogenation and Dehydrogenation Yun Hang Hu* and Eli Ruckenstein Department of Chemical Engineering, State University of New York at Buffalo, Amherst, New York 14260
Hydrogen storage in Li3N was investigated via temperature-programmed hydrogenation (TPH) and temperature-programmed dehydrogenation (TPD). TPH spectra indicated an initial hydrogenation temperature of Li3N of ∼150 °C, independent of the H2 partial pressure even below 0.5 atm. However, the increase in H2 partial pressure accelerated the hydrogenation rate. The TPD curves exhibited three H2 peaks located at 240, 270, and above 380 °C. The maximum amount of low-temperature released H2 corresponding to the two peaks at 240 and 270 °C was 6.0 wt %. XRD indicated that most of the low-temperature released hydrogen can be attributed to the transformation of LiNH2 to Li2NH. The amount and properties of the releasable hydrogen are also dependent on the duration and temperature of the Li3N hydrogenation. Introduction Hydrogen is viewed as the most promising clean fuel of the future, with vehicles using hydrogen propulsion either directly or through fuel cells. It has the advantage of avoiding the atmospheric pollution caused by the oxides of carbon (or sulfur) during the combustion of the hydrocarbons. There are, however, economical issues with the classical H2 storage methods, such as liquid hydrogen, compressed hydrogen, metal hydrides, and H2 adsorption on activated carbon. Storage as liquid hydrogen is unacceptable because the compression and cooling of the hydrogen to 20 K consumes almost 30% of the hydrogen energy.1 Even though compressed hydrogen is less expensive, it requires excessive storage space. H2 adsorption on activated carbon, which has a very high surface area (1500-2000 m2/g), requires a low temperature (77 K) and a high pressure (∼50 atm).1 Among all known metal hydrides, MgH2 is the most suitable for reversible hydrogen storage, because of its high theoretical capacity for storage (7.65 wt % hydrogen). However, the hydrogenation of magnesium to MgH2 occurs only under severe conditions (such as high temperatures, above 350 °C) and only very slowly and incompletely. Furthermore, the dehydrogenation rate of MgH2 is too low for a hydrogen storage material.2 The combination between Mg and transition metals as alloys can improve the kinetics of hydrogenation of Mg and of dehydrogenation of MgH2. However, the storage capacity of NiMgH4 is only 3.8 wt %. The low- and mediumtemperature reversible hydrides, such as LaNi5H6 and TiFeH2, exhibit suitable dehydrogenation kinetics;3 they are, however, expensive and have very low hydrogen storage capacities (1.5 wt % for LaNi5H6 and 1.8 wt % for TiFeH2). To increase the storage, complex hydrides of light metals (Li, Na, Al), such as LiAlH4 (10.5 wt % * To whom correspondence should be addressed. Phone: 716-6452911, ext 2266. Fax: 716-645-3822. E-mail: yhu@ buffalo.edu.
H) and NaAlH4 (7.4 wt % H), have been investigated.4 These complex hydrides, which were usually regarded nonreversible, became reversible by adding a transition or rare earth metal as catalyst.5,6 However, the constant storage capacity of hydrogen is ∼4 wt % even at high hydrogenation pressures (130-150 atm).7 In recent years, several reports regarding the high and reversible adsorption of H2 onto carbon nanotubes8-10 aroused tremendous interest, stimulating both experimental and theoretical work regarding their use for H2 storage.8-16 The U.S. Department of Energy (DOE)17 recommended a standard for reversible hydrogen adsorption, which requires a storage capacity of 6.5 wt % and a volumetric density of 63 kg of H2/m3. It is still unclear whether the hydrogen storage capability of carbon nanotubes can reach the U.S. DOE target.11-14 As early as 1910, Dafert and Miklauz18 reported that the reaction between Li3N and H2 generated Li3NH4 at 220-250 °C,
Li3N + 2H2 ) Li3NH4 Furthermore, they found that the Li3NH4 can be partially decomposed to release H2.18 Therefore, the hydrogenation of Li3N and the dehydrogenation of the hydrogenated Li3N constitute a reversible process, which can be used for H2 storage. The high theoretical hydrogen capacity (10.4 wt %) and relatively low hydrogenation temperatures could make Li3N an effective H2 storage material. In this paper, we investigated the H2 storage process in Li3N via temperature-programmed hydrogenation (TPH) and temperature-programmed dehydrogenation (TPD), as well as X-ray powder diffraction. Experiments Material. Lithium nitride powder (∼80 mesh) was bought from Aldrich Chemical Co. Its surface area and pore size distribution were determined through nitrogen
10.1021/ie030498o CCC: $25.00 © 2003 American Chemical Society Published on Web 09/23/2003
5136 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003
adsorption at 77 K, using a Micromeritics ASAP 2000 instrument. The surface area and the average pore size were 6 m2/g (determined by the BET method19) and 20 nm (obtained from the adsorption branch of the N2 isotherm by the BJH method20), respectively. Temperature-Programmed Hydrogenation. The TPH experiment was performed as follows: A hydrogen/ argon (5, 10, or 20 vol % H2) gas mixture was allowed to flow, at a rate of 20 mL/min and total pressure of 1 atm, through a 0.03-g sample of Li3N, which was loaded in a vertical quartz tube reactor (3-mm diameter), and through a thermal conductivity detector to determine the change of the H2 concentration. The quartz reactor was located in an electric tube furnace where it was heated at a rate of 3 K/min. An on-line mass spectrometer (HP Quadrupole, 5971 series mass selective detector), equipped with a fast-response inlet capillary system, was used to confirm that, except for hydrogen and argon, no other compounds were present in the gas stream after the reactor. Temperature-Programmed Dehydrogenation. A lithium nitride sample (0.03 g) was loaded into a vertical quartz tube reactor (3-mm diameter), which was located in an electric tube furnace. H2 (20 mL/min) was introduced into the reactor at room temperature, and then the reactor was heated to the reaction temperature (300 °C, unless otherwise mentioned) at a rate of 3 K/min. The reaction temperature was kept constant for selected times in order to hydrogenate Li3N and then was allowed to decrease naturally back to the room temperature. Then, the H2 stream was replaced by argon (20 mL/min). The argon stream carried out the H2, generated via the dehydrogenation of the hydrogenated Li3N by increasing the temperature at a rate of 3 K/min, into a thermal conductivity detector to determine the change of H2 concentration. An on-line mass spectrometer (HP Quadrupole, 5971 series mass selective detector), equipped with a fast-response inlet capillary system, was used to confirm that, except for hydrogen and argon, no other compounds were present in the gas stream after the reactor. X-ray Powder Diffraction (XRD). XRD was carried out with a Siemens D500 X-ray diffraction instrument, equipped with a Cu KR source, at 40 kV and 30 mA. The scanning speed was 1°/min. During the measurement, which lasted ∼70 min, the sample was exposed to air. As a result, some LiOH or Li2O had been formed because Li3N or its hydrogenated compounds can easily react with the water present in air. Results The hydrogenation of Li3N was examined by TPH for three H2 concentrations (5, 10, 20%). As shown in Figure 1, the initial hydrogenation temperature of Li3N, the same for all three H2 concentrations, is ∼150 °C. This indicates that the initial hydrogenation temperature is independent of the H2 concentration in the reactant stream. However, the three reactant streams have different temperatures at which the hydrogenation increases rapidly. These temperatures are about 260, 275, and 285 °C for the 20, 10, and 5% H2, respectively. This indicates that the increase in H2 concentration resulted in little change on the initial temperature needed for the hydrogenation of Li3N but accelerated the hydrogenation rate. The effect of Li3N hydrogenation temperature on the dehydrogenation of the hydrogenated Li3N was exam-
Figure 1. Temperature-programmed hydrogenation of Li3N.
Figure 2. Temperature-programmed dehydrogenation of Li3N hydrogenated for 1 h at various temperatures: (a) 200, (b) 250, (c) 300, (d) 350, (e) 400, and (f) 500 °C.
ined by TPD. As shown in Figure 2, the amount of H2 generated via dehydrogenation was strongly dependent on the hydrogenation temperature. The amount of H2 released during the TPD increased with increasing hydrogenation temperature, reached a maximum at 300 °C, and then decreased with a further increase in the hydrogenation temperature. Three peaks of H2 are present in the TPD curve of the sample hydrogenated at 300 °C. The lower temperature peaks are located at about 240 and 270 °C, while the third one is very wide and is located above 380 °C. Only the two lower temperature peaks were observed for the samples hydrogenated at temperatures below 300 °C and only the third peak for the sample hydrogenated at 500 °C. This means that the hydrogen species corresponding to the third peak required higher activation temperatures. TPD was also employed to investigate the effect of thermal pretreatment of Li3N in an inert gas (argon) on its hydrogenation. As shown in Figure 3, the amounts of H2 released during the dehydrogenation of the samples, which were thermally pretreated in argon at 400 or 500 °C for 1 h and then hydrogenated at 300 °C for 1 h, were much smaller than that of the sample without any thermal pretreatment. The initial dehydrogenation temperatures were 170, 190, and 210 °C for the nonpretreated, and for the 400 and 500 °C
Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5137 Table 1. Amount of H2 Released from the TPD below 320 °Ca
Figure 3. Temperature-programmed dehydrogenation of Li3N hydrogenated at 300 °C for 1 h: Li3N without any pretreatment before hydrogenation (s), with pretreatment in argon at 400 (- ‚ -) and 500 °C (‚‚‚) before hydrogenation.
Figure 4. Temperature-programmed dehydrogenation of Li3N hydrogenated for (a) 1, (b) 2, (c) 4, (d) 16, and (e) 20 h.
thermally pretreated (in argon) samples, respectively. This indicates that the thermal pretreatment increases the initial dehydrogenation temperature. Figure 3 also shows that the high-temperature peak did not occur in the curves of the pretreated samples, which appear to have a single broad peak. Consequently, the thermal pretreatment has not only reduced the amount of dehydrogenated H2 but also changed the characteristics of the adsorption sites of Li3N. This reduced hydrogenation was probably caused by the sintering of the material during pretreatment. To examine the effect of Li3N hydrogenation time on dehydrogenation, the hydrogenation was carried out at 300 °C and 1 atm pressure for durations from 1 to 20 h. As shown in Figure 4, the two lower temperature peaks of the TPD curves were dependent on the hydrogenation time. When the adsorption time increased from 1 to 4 h, the first peak, located at ∼240 °C, increased. However, when the hydrogenation time was further increased, the first peak no longer increased, whereas the second peak located at ∼270 °C increased. Except for the sample hydrogenated for 20 h, the third wide peak located above 380 °C did not exhibit a significant change when the hydrogenation time was changed. Furthermore, one can see from Table 1 that the total amount of H2 desorbed corresponding to the first and second peaks increased with increasing hydrogenation time, becoming as large as 6.0 wt % (based on the total weight) for the 20-h-hydrogenated sample. This indicates that the longer the hydrogenation duration, the larger the low-temperature desorbed amount of H2 (below 300 °C).
TPD curve
adsorption time of H2 before the TPD (h)
total H2 due to the two low-temp peaks (240 and 270 °C) (wt %b)
a b c d e
1 2 4 16 20
3.0 4.0 4.5 5.8 6.0
a From the TPD curves of Figure 4. b Based on total weight, including Li3N and hydrogen.
It is worth noting that, in all TPHs of Li3N and TPDs of hydrogenated Li3N, no NH3 was detected by the online mass spectrometer. This indicates that NH3 did not form during the hydrogenation of Li3N and the dehydrogenation of hydrogenated Li3N. XRD showed that the hydrogenation of Li3N at 300 °C and atmospheric pressure for 1 h generated LiH, LiNH2, and Li2NH. A LiOH phase was also identified because the sample was exposed to air during the XRD measurements. Since Li3N could also be identified, it is obvious that 1 h of hydrogenation resulted in the transformation of only a fraction of Li3N. When Li3N was hydrogenated at 300 °C for 12 h, followed by vacuum desorption (∼10-3 Pa) at 300 °C for 12 h, neither Li3N nor LiNH2 could be identified. However, Li2NH could be detected. This indicates that, during dehydrogenation, LiNH2 was mainly transformed to Li2NH, but not back to Li3N. Discussion As early as 1910, Dafert and Miklauz18 reported that the reaction between Li3N and H2 generated Li3NH4,
Li3N + 2H2 ) Li3NH4 Li3NH4 was actually a mixture of LiNH2 and 2LiH.21 We found that, even if only a fraction of Li3N was hydrogenated, as happened during the 1-h hydrogenation at 300 °C, besides the lithium imide (Li2NH) and lithium hydride (LiH), lithium amide (LiNH2) was also generated(see Figure 5). This indicates that LiNH2 was formed before the entire amount of Li3N was transformed to Li2NH. Furthermore, we found that even when the Li3N hydrogenated at 300 °C was subjected to vacuum (∼10-3Pa) at 300 °C for 12 h, only Li2NH and LiH were detected (see Figure 5). This indicates that all the LiNH2 was transformed to Li2NH during the vacuum desorption, and not to Li3N. If Li3N could be completely transformed to LiNH2 and LH during the hydrogenation and these products could be completely recovered back to Li3N, the total theoretical reversible hydrogen storage capacity of Li3N would be 10.4 wt %. However, only a fraction of hydrogen can be released by the hydrogenated Li3N at dehydrogenation temperatures smaller or equal to 320 °C (see Figure 4). Furthermore, the maximum low-temperature releasable hydrogen is 6.0 wt % (Table 1). Because the XRD did not detect Li3N after the hydrogenated Li3N was dehydrogenated at 300 °C even for 12 h, most of the low-temperature released hydrogen can be attributed to the transformation of LiNH2 to Li2NH by the equation LiNH2 + LiH f Li2NH + H2. The hydrogen corresponding to the third peak in the TPD requires higher temperatures, above 350 °C, for release, and can be
5138 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003
Figure 5. XRD patterns: (a) Li3N hydrogenated at 300 °C for 1 h; (b) Li3N hydrogenated at 300 °C for 12 h and then dehydrogenated at 300 °C for 12 h.
attributed to the reaction Li2NH + LiH ) Li3N + H2. Therefore, the complete recovery of Li3N from the hydrogenated Li3N compounds is a difficult process, which requires high temperatures and long times. Furthermore, the use of such high temperatures produces sintering of the recovered Li3N and the material becomes ineffective (Figure 3). For this reason, the reversible storage capacity of Li3N is ∼6%. Although thermodynamic calculations indicated that NH3 formation from H2 and Li3N is a very favorable process at temperatures below 400 °C,22 no NH3 was detected during hydrogenation of Li3N and dehydrogenation of the hydrogenated Li3N. This occurred because there is an ultrafast reaction between NH3 and LiH, which prevents the NH3 generation during hydrogenation and dehydrogenation.22 During the temperature-programmed decomposition of a two-layer material, in which the carrier gas passed first through a LiNH2 layer and then through a LiH layer, no NH3 was detected even for very short contact times between the carrier gas and solid.22 In contrast, for the reverse two-
layer material, in which the carrier gas passed first through a LiH layer and then through a LiNH2 layer, NH3 was detected in the broad temperature range between 60 and 500 °C. This means that NH3 was formed through the decomposition of LiNH2 but was captured by LiH. The complete hydrogenation of Li3N generated a mixture (Li3NH4) of 2LiH and LiNH2. Thus, for each LiNH2 molecule generated, two LiH molecules are formed. Because LiH reacted fast with NH3, the LiH generated via the Li3N hydrogenation could capture all the NH3 formed through the LiNH2 decomposition, and NH3 could not escape into the H2 stream during the H2 release process. On the other hand, the fast reaction between NH3 and LiH affected the product composition and also prevented the formation of NH3 impurity during the hydrogenation process of Li3N. Because Li2NH was detected during the hydrogenation of Li3N (Figure 5), the hydrogenation of Li3N is a multistep process: Li3N was first hydrogenated to Li2NH and LiH. Further, Li2NH was hydrogenated to LiNH2 and LiH, and LiNH2 hydrogenated to LiH and NH3. This means
Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5139
that, for each NH3 molecule generated, three LiH molecules are formed. Because the reaction between LiH and NH3 to LiNH2 is ultrafast, all NH3 was transformed back to LiNH2. Conclusion Li3N has a low initial hydrogenation temperature of ∼150 °C, which is independent of the H2 partial pressure. However, the increase in H2 partial pressure accelerated the hydrogenation rate. The TPD spectra showed that there are three H2 peaks located at 240, 270, and above 380 °C. The maximum amount of lowtemperature released hydrogen, which corresponds to the first and second peaks (240 and 270 °C) in the TPD spectra, was 6.0 wt %. Most of the low-temperature released hydrogen can be attributed to the transformation of LiNH2 to Li2NH. 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. Zf. Phys. Chem., Neue Folge 1979, 116, 215. (3) Buchner, H. Energiespeichrung in Metallhydriden; SpringerVerlag: Wein, 1982. (4) Wiswall, R. In Hydrogen in Metals II; Alefeld, G., Vo¨lkl, J., Eds.; Springer-Verlag: Wein, 1978; p 201. (5) Bogdanovic, B.; Schwickardi, M. Ti-doped Alkali Metal Aluminium Hydrides as Potential Novel Reversible Hydrogen Storage Material. 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) 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.
(9) Ye, C. C.; Ahn, C.; Witham, B.; Fultz, J.; Liu, A. G.; Rinzler, D.; Smalley, R. E. Hydrogen Adsorption and Cohesive Energy of Single-walled Carbon Nanotubes. Appl. Phys. Lett. 1999, 74, 2307. (10) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Hydrogen Storage in Single-walled Carbon Nanotubes at Room Temperature. Science 1999, 286, 1127. (11) Dagani, R. Tempest in a Tiny Tube. Chem. Eng. News 2002, 80 (2), 25. (12) Yang, R. T. Hydrogen Storage by Alkali-doped Carbon Nanotubes-revisited. Carbon 2000, 38, 623. (13) 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. (14) Cheng, H.; Pez, G. P.; Cooper, A. C. Mechanism of Hydrogen Sorption in Single-walled Carbon Nanotubes. J. Am. Chem. Soc. 2001, 123, 5845. (15) Wang, Q.; Johnson, J. K. Computer Simulations of Hydrogen Adsorption on Graphite Nanofibers. J. Phys. Chem. B 1999, 103, 277. (16) Dresselhaus, M. S.; Williams, K. A.; Eklund, P. C. Hydrogen Adsorption in Carbon Material. MRS Bull. 1999, 24, 45. (17) Hynek, S.; Fuller, W.; Bentley, J. Hydrogen Storage by Carbon Sorption. Int. J. Hydrogen Energy 1997, 22, 601. (18) Dafert, F. W.; Miklauz, R. New Compounds of Nitrogen and Hydrogen with Lithium. Monatsh. Chem. 1910, 31, 981. (19) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 308. (20) Barret, E. P.; Joyner, L. G.; Halenda, P. H. The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherm. J. Am. Chem. Soc. 1951, 73, 373. (21) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Interaction between hydrogen and metal nitrides and imides. Nature 2003, 420, 302. (22) Hu, Y. H.; Ruckenstein, E. Ultra-Fast Reaction Between LiH and NH3 Impurity during H2 Storage in Li3N. J. Phys. Chem. B. Submitted.
Received for review June 19, 2003 Revised manuscript received August 1, 2003 Accepted August 7, 2003 IE030498O
