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MATERIALS AND INTERFACES Steam-Reforming Product (H2/CO2 Mixture) Used as a Hydrogen Source for Hydrogen Storage in Li3N Yun Hang Hu* and Eli Ruckenstein Department of Chemical and Biological Engineering, State UniVersity of New York at Buffalo, Buffalo, New York 14260
Hydrogen purification and storage are critical issues in the transportation application of hydrogen fuel. Herein, we report that hydrogen can be separated from other components during its storage in Li3N, when the steamreforming product of natural gas is employed as the hydrogen source. The kinetics and capacity of hydrogen absorption of Li3N are not affected by CO2, and only the kinetics is slightly affected by CO. During desorption, the hydrogen stored in Li3N can be released, thus providing a pure hydrogen source for fuel cells or other applications. The simultaneous separation of hydrogen during its storage tremendously reduces the cost of the hydrogen fuel for transportation and also provides a novel process for hydrogen purification. 1. Introduction Molecular hydrogen is considered a promising alternative fuel of the future. Although hydrogen comprises 80% of all known matter in the universe, it is not available in its free molecular form on earth. Obtaining molecular hydrogen as a source of fuel requires the development of methods to produce hydrogen molecules separate from other compounds. Currently, the available processes for preparing hydrogen do not produce hydrogen as the sole product. One of the important processes for hydrogen production is the steam reforming of natural gas,1 which can be represented by the reaction
CH4 + 2H2O ) 4H2 + CO2 The hydrogen produced during this process is accompanied by about 20% CO2 and some CO. Whereas much attention has recently been paid to H2 storage and fuel cell developments,2-5 H2 separation and purification remain important steps in the development of a hydrogen economy. So far, both the storage and use of hydrogen involve pure hydrogen, which is obtained via a very expensive separation process after hydrogen production. The technologies for storing hydrogen can be classified as compression, liquefaction, and storage in a solid material.6,7 The compression of hydrogen requires a very high pressure to achieve enough hydrogen fuel for a driving distance of 400500 km, and this entails a safety issue related to tank rupture in an accident.7 The large amount of energy consumed during liquefaction and the continuous boil-off of hydrogen limit the possible use of liquid hydrogen storage technology.6 Therefore, current attention has focused on solid storage materials, including metal hydrides,8,9 complexes hydrides,10-13 nanotubes,14-17 microporous metal-organic frameworks,18-21 and lithium nitride.22-26 So far, all of these materials have been investigated using only pure hydrogen as a source. * To whom correspondence should be addressed. Tel.: 716-6452911 ext. 2253. Fax: 716-645-3822. E-mail:
[email protected].
In this article, we report a novel process in which the product of the steam reforming of the natural gas (mainly methane) is employed as the hydrogen source for hydrogen storage in Li3N. No preseparation of H2 is required, as the hydrogen is separated from CO2 and CO during storage. Afterward, pure hydrogen can be obtained by desorbing the hydrogenated Li3N. 2. Experimental Section Li3N (purchased from the Aldrich Chemical Company) was first pretreated23b before gas absorption experiments as follows: Li3N powder was exposed to air for several minutes to generate a small amount of LiOH on the surface layer of the sample. This was followed by decomposition of the LiOH to Li2O on the surface layer of Li3N by heating in a vacuum at 230 °C. Furthermore, the sample was 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 partial oxidation and the hydrogenation-dehydrogenation pretreatments are important for the following reason: The hydrogenation of Li3N (Li3N + H2 ) Li2NH + LiH, ∆H ) -116 kJ/mol) is a highly exothermic reaction with fast kinetics, which, by generating hot spots, stimulates the sintering of Li3N. However, when part of the Li3N surface is transformed into LiOH in air at room temperature and is further decomposed into Li2O in a vacuum at a higher temperature, Li2O covers most of the surface active sites of Li3N, resulting in the lowering of the hydrogenation rate. During the hydrogenation-dehydrogenation pretreatment, Li2O diffuses into the bulk of the sample and no longer covers the surface. In addition, Li3N is transformed into Li2NH after the hydrogenation-dehydrogenation pretreatment. In other words, the hydrogen absorption of the hydrogenation-dehydrogenation-pretreated sample is provided by the reaction Li2NH + H2 ) LiNH2 + LiH, which has a much lower heat of reaction (-45 kJ/mol) than the hydrogenation of Li3N (-116 kJ/mol). In addition, the dispersed Li2O might play the role of a stabilizer. The combination of the lower reaction heat and
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Figure 1. Hydrogen absorption by pretreated Li3N at 230 °C: (a) pure H2 gas, (b) first gas absorption from 20% CO2/80% H2 gas, (c) reabsorption from 20% CO2/80% H2 gas after desorption, and (d) second reabsorption from 20% CO2/80% H2 gas after desorption.
the dispersion of Li2O prevents the pretreated material from sintering during the hydrogen absorption that follows. A volumetric method was employed to accurately determine the hydrogen absorption in the Li3N samples. A 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 reabsorption of hydrogen, the sample was subjected to a vacuum (p < 10-5 Torr) at 230 °C. Two gases were used in the experiments: pure hydrogen and a model product of the steam reforming of natural gas (consisting of 20% CO2 and 80% H2). A GC-MS instrument was used to analyze the compositions before and after absorption. Because the temperature was measured outside the reactor, the reaction temperature did not account for any hot spots generated during reaction. The hydrogen capacity is defined as the percentage of hydrogen absorbed based on the total weight of the solid sample. 3. Results and Discussion The hydrogen absorption of Li3N was first determined for pure hydrogen. As shown in Figure 1a, the amount of hydrogen absorbed by Li3N reached about 5.5 wt % in 10 min and then remained constant, which is consistent with previous results.23b Further, the model product (20% CO2/80% H2) of the steam reforming of natural gas was employed as the hydrogen source. As shown in Figure 1b, its absorption curve coincides with that obtained for pure hydrogen. In addition, the gas reabsorption of the steam-reforming gas product also coincides with the first gas absorption (curves b-d in Figure 1). To verify whether only hydrogen was absorbed from the steam-reforming gas product (CO2/H2 gas mixture), GC-MS gas composition analyses were carried out before and after absorption using argon as an internal standard. The results showed that the CO2/Ar ratios were the same both before and after absorption. Because the inert argon gas cannot be absorbed by Li3N at 230 °C, the unchanged CO2/ Ar ratio during the absorption process indicates that CO2 was not absorbed. In other words, only H2 was absorbed when the CO2/H2 gas mixture was used as the hydrogen source. This result indicates that 20% CO2 does not affect hydrogen absorption in Li3N. Of course, when the gas contains 20% CO2, the hydrogen partial pressure is reduced by 20%. However, the 20% CO2 did not affect the hydrogen absorption kinetics, indicating that the 20% partial pressure change did not affect the hydrogen absorption. This probably occurred because the bulk diffusion of hydrogen in the solid material is the rate-determining step. When a Li3N sample was first subjected to a pure CO2 atmosphere and then used for H2 absorption with pure H2 gas,
Figure 2. Hydrogen absorption in Li3N at 230 °C from pure hydrogen: (a) without CO2 pretreatment and (b) after exposure of Li3N to a pure CO2 atmosphere.
Figure 3. Hydrogen absorption in Li3N at 230 °C from (a) H2, (b) 2.5 wt %CO/H2, (c) 5 wt % CO/H2, and (d) 10 wt % CO/H2.
the sample almost lost its ability to absorb hydrogen (Figure 2). This indicates that CO2 pretreatment deactivates Li3N for hydrogen absorption. In contrast, when hydrogen and CO2 were simultaneously contacted with Li3N, CO2 did not affect the hydrogen absorption (see Figure 1). This indicates that Li3N has a high hydrogen selectivity from a CO2/H2 gas mixture. Usually, the product of steam reforming also contains a small amount of CO. To examine whether CO affects the absorption of H2, we mixed CO with H2. As shown in Figure 3, the rate of hydrogen absorption was slightly affected by CO, as the time required to reach about 5.5 wt % hydrogen capacity changed from 10 to 15 min. However, an increase of the CO concentration from 2.5% to 10% did not increase the effect of CO on the hydrogen absorption kinetics. Pure hydrogen can be obtained via desorption at 230 °C from a Li3N sample that was previously subjected to hydrogen absorption from the product of the steam reforming of natural gas. Such a pure hydrogen source can be used in any application that requires pure hydrogen. 4. Conclusions In conclusion, the product of the steam reforming of natural gas (20% CO2/80% H2) can be directly used as a hydrogen source for hydrogen storage in Li3N without preseparation. This can tremendously reduce the cost of hydrogen fuel for transportation and also provides a novel process for hydrogen purification. 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.
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ReceiVed for reView April 30, 2007 ReVised manuscript receiVed June 5, 2007 Accepted June 6, 2007 IE070614D
