CO2 Mixture Gas as

Herein, we report that the H2/CO2 gas mixture can be directly used as hydrogen source for hydrogen storage in LiNH2/Li3N. Furthermore, it was found th...
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Ind. Eng. Chem. Res. 2008, 47, 48-50

MATERIALS AND INTERFACES Hydrogen Storage in LiNH2/Li3N Material for H2/CO2 Mixture Gas as Hydrogen Source Yun Hang Hu*,† and Eli Ruckenstein‡ Department of Chemical and Biological Engineering, State UniVersity of New York at Buffalo, Buffalo, New York 14260

Our previous work showed that the preaddition of LiNH2 into Li3N increases the reversible hydrogen capacity up to about 6.8 wt % H2. Herein, we report that the H2/CO2 gas mixture can be directly used as hydrogen source for hydrogen storage in LiNH2/Li3N. Furthermore, it was found that 20% CO2 in H2 did not affect the high hydrogen capacity and the fast adsorption kinetics of LiNH2/Li3N. This material provides an opportunity to eliminate a preseparation step before hydrogen storage, which can greatly reduce the hydrogen storage cost. 1. Introduction Hydrogen is a promising fuel for the future. One of the important processes for hydrogen production is the steam reforming of natural gas,1,2 which can be represented by the reaction

CH4 + 2H2O ) 4H2 + CO2 The hydrogen produced during this process is accompanied by about 20% CO2. To obtain pure hydrogen for both hydrogen storage and its uses, a preseparation step is usually required. A low-cost hydrogen storage technology that provides a high storage capacity and fast kinetics is a critical factor in the development of a hydrogen economy for transportation. The compression of hydrogen for its storage requires a very high pressure to achieve enough hydrogen fuel for a driving distance of 400-500 km, and this generates a safety issue related to tank rupture in an accident.3 The liquefaction can also be used for hydrogen storage. However, the large amount of energy consumed during liquefaction and the continuous boil-off of hydrogen limit the use of the liquid hydrogen storage technology.4 For this reason, the current attention was focused on solid storage materials, such as lithium nitride,5-9 complexes of hydrides,10-13 microporous metal-organic frameworks,14-17 nanotubes,18-21 and metal hydrides.22,23 Very recently, we reported that a CO2/H2 mixture can be directly used as a hydrogen source for hydrogen storage in Li3N,24 its high hydrogen capacity (about 5.5 wt %) and fast hydrogen absorption kinetics being not affected by the presence of CO2 in H2. Furthermore, we found that the preaddition of LiNH2 into Li3N can increase the reversible hydrogen capacity of Li3N from about 5.5 to about 6.8 wt %6c. Herein, we report that the H2/CO2 mixture can also be directly employed as a * To whom correspondence should be addressed. Phone: 9064872261. Fax: 906-4872934. E-mail: [email protected]. † Present address: Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931. ‡ E-mail: [email protected].

hydrogen source for hydrogen storage in LiNH2/Li3N without affecting its high hydrogen capacity and fast hydrogen absorption kinetics. 2. Experimental Section 2.1. Material Preparation. A critical issue for hydrogen storage in Li3N is that its reversible hydrogen capacity is only about 5.5 wt %. This occurs because LiNH2 and 2LiH, which are the products of Li3N hydrogenation, dehydrogenate in two steps: LiH + LiNH2 ) Li2NH + H2 and LiH + Li2NH ) Li3N + H2. The first step, which provides about 5.5 wt % hydrogen capacity, takes place easily even at temperatures below 200 °C, whereas the second step requires high temperatures (>400 °C). The molar ratio of LiNH2/LiH of the hydrogenated Li3N free of added LiNH2 is 0.5. Consequently only half of the LiH releases hydrogen during the first step LiH + LiNH2 ) Li2NH + H2 at reasonable temperatures. In contrast, when the LiNH2 added to Li3N reaches 50 mol %, the entire LiH generated by the hydrogenation of Li3N can release hydrogen via the first step (LiH + LiNH2 ) Li2NH + H2). However, we found that, when the molar ratio of LiNH2 added to Li3N is slightly lower than the 50 mol % ratio, its hydrogen desorption kinetics is faster than that with a 50 mol % ratio. For this reason, in this work, 48 mol % LiNH2 was introduced into Li3N to obtain a LiNH2/Li3N mixture, which was prepared by mixing powders of LiNH2 and Li3N with an agate mortar and pestle by hand for 5 min. 2.2. Hydrogen Absorption. A barometric method was employed to accurately determine the hydrogen absorption in the sample. LiNH2/Li3N was first subjected to an in-situ pretreatment consisting of hydrogenation (at 230 °C for 24 h), followed by dehydrogenation (at 280 °C for 12 h) before the reversible hydrogen storage measurements. The solid sample (0.25 g) was loaded in a reactor located inside an electrical tubular furnace. The change of gas pressure during absorption was determined with a digital pressure gauge. An initial gas pressure of 7 atm was used in all absorption experiments. Before any re-absorption of hydrogen, the sample was subjected to vacuum (p < 10-5 Torr) at 230 °C. Two gases were used in

10.1021/ie071110i CCC: $40.75 © 2008 American Chemical Society Published on Web 12/01/2007

Ind. Eng. Chem. Res., Vol. 47, No. 1, 2008 49

Figure 1. Hydrogen absorption by LiNH2/Li3N at 230 °C: (a) pure H2 gas; (b) the first gas absorption from 20/80 CO2/H2 gas; (c) re-absorption from 20/80 CO2/H2 gas after desorption; (d) the second re-absorption from 20/80 CO2/H2 gas after desorption.

the experiments: pure hydrogen and a gas mixture consisting of 20% CO2 and 80% H2. Because the temperature was measured outside the reactor, the reaction temperature did not account for the hot spots generated during reaction. The hydrogen capacity is defined as the percentage of hydrogen absorbed on the basis of the total weight of the original solid sample.

Figure 2. Hydrogen absorption by LiNH2/Li3N at 230 °C from pure hydrogen: (a) hydrogen absorption without CO2 pretreatment; (b) hydrogen absorption after LiNH2/Li3N was exposed to a pure CO2 atmosphere.

However, LiNH2/Li3N has a higher reversible hydrogen capacity than Li3N. Furthermore, pure hydrogen can be obtained via desorption at 230 °C from a LiNH2/Li3N sample that was previously subjected to hydrogen absorption from the product of the steam re-forming of natural gas. Such a pure hydrogen source can be used in any application that requires pure hydrogen. 4. Conclusions

3. Results and Discussion Before we obtained the curve of hydrogen absorption versus reaction time of the LiNH2/Li3N sample for pure hydrogen gas, the sample was first used for hydrogen absorption and desorption cycles with pure hydrogen until the hydrogenation kinetics did not change with the cycle number. From Figure 1a, one can see that the amount of absorbed hydrogen reached 5.9 wt % in 10 min, 6.5 wt % in 60 min, and finally about 6.7 wt %. Importantly, we also employed a 20/80 CO2/H2 mixture gas (the main products of steam re-forming of natural gas) as the hydrogen source. As shown in Figure 1b, its absorption curve coincides with that obtained for pure hydrogen. Furthermore, the hydrogen re-absorption of the CO2/H2 mixture gas also coincides with the first gas absorption (curves b-d in Figure 1). This result indicates that 20% CO2 did not affect the hydrogen absorption in LiNH2/Li3N. In other words, LiNH2/ Li3N has a high hydrogen selectivity from a CO2/H2 gas mixture. Of course, the presence of 20% CO2 reduced the hydrogen partial pressure by 20%. However, the lack of effect of the 20% CO2 on the hydrogen absorption kinetics indicates that the 20% change of the hydrogen partial pressure did not affect the hydrogen absorption. This probably occurred because the bulk diffusion of hydrogen in the solid was the rate-determining step. In contrast, as shown in Figure 2, the CO2 pretreatment deactivated LiNH2/Li3N for hydrogen absorption. When LiNH2/ Li3N was first subjected to a pure CO2 atmosphere and then used for H2 absorption from pure H2, the material almost lost its ability to absorb hydrogen. However, when the hydrogen and CO2 were simultaneously contacted with LiNH2/Li3N, CO2 did not affect the hydrogen absorption (see Figure 1). This occurred because CO2 free of H2 oxidizes Li2NH (the main component in reversible hydrogen absorption), generating a Li2O layer that covers the Li2NH. As a result, hydrogen no longer has access to Li2NH for hydrogenation. In contrast, when hydrogen is also present, it has a higher activity to react with Li2NH than CO2. In other words, hydrogen inhibits the oxidation of Li2NH by CO2. Compared with Li3N24, LiNH2/Li3N has a similar performance for hydrogen storage when CO2/H2 is the hydrogen source.

LiNH2-added Li3N can be employed for hydrogen storage by using a CO2/H2 mixture as hydrogen source without preseparation. Furthermore, the presence of CO2 in H2 does not affect the high reversible hydrogen capacity (about 6.7 wt %) and the fast hydrogen absorption kinetics of the LiNH2/Li3N material. This can reduce tremendously the cost of hydrogen fuel for transportation and also provides a novel process to obtain pure hydrogen. Literature Cited (1) Rostrup-Nielsen, J. R.; Sehested, J.; Norskov, J. K. Catalytic conversion of methane to synthesis gas by partial oxidation and CO2 reforming. AdV. Catal. 2002, 47, 65. (2) Hu, Y. H.; Ruckenstein, E. Catalytic conversion of methane to synthesis gas by partial oxidation and CO2 reforming. AdV. Catal. 2004, 48, 297. (3) Fichtner, M. Nanotechnological aspects in materials for hydrogen storage. AdV. Eng. Mater. 2005, 7, 443. (4) Zu¨ttel, A. Hydrogen storage methods. Naturwissenschaften 2004, 91, 157. (5) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Interaction of hydrogen with metal nitrides and imides. Nature 2002, 420, 302. (6) (a) Hu, Y. H.; Ruckenstein, E. Ultrafast reaction between LiH and NH3 during H2 storage in Li3N. J. Phys. Chem. A 2003, 107, 9737. (b) Hu, Y. H.; Ruckenstein, E. Highly effective Li2O/Li3N with ultrafast kinetics for H2 storage. Ind. Eng. Chem. Res. 2004, 43, 2464. (c). Hu, Y. H.; Ruckenstein, E. High reversible hydrogen capacity of LiNH2/Li3N mixtures. Ind. Eng. Chem. Res. 2000, 44, 1510. (7) Ichikawa, T.; Isobe, S.; Hanada, N.; Fujii, H. Lithium nitride for reversible hydrogen storage. J. Alloys Compd. 2004, 365, 271. (8) Noritake, T.; Nozaki, H.; Aoki, M.; Towata, S.; Kitahara, G.; Nakamori, Y.; Orimo, S. Crystal structure and charge density analysis of Li2NH by synchrotron X-ray diffraction. J. Alloys Compd. 2004, 393, 264. (9) Luo, W.; Ronnebro, E. Towards a viable hydrogen storage system for transportation application. J. Alloys Compd. 2005, 404, 392. (10) Bogdanovic, B.; Schwickardi, M. Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. J. Alloys Compd. 1997, 253, 1. (11) Bogdanovic, B.; Sandrock, G. Catalyzed complex metal hydrides. MRS Bull. 2002, 27, 712. (12) Jensen, C. M.; Zidan, R.; Mariels, N.; Hee, A.; Hagen, C. Advanced titanium doping of sodium aluminum hydride: Segue to a practical hydrogen storage material? Int. J. Hydrogen Energy 1999, 24, 461. (13) Majzoub, E. H.; Gross, K. J. Titanium-halide catalyst-precursors in sodium aluminum hydrides. J. Alloy. Compd. 2003, 356, 363.

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ReceiVed for reView August 14, 2007 ReVised manuscript receiVed October 5, 2007 Accepted October 10, 2007 IE071110I