J. Phys. Chem. C 2008, 112, 17423–17426
17423
Investigations on the Formation and Decomposition Behaviors of BaAlH5 and Ba2AlH7 Xiangfeng Liu, Kohta Asano, Kouji Sakaki, Yumiko Nakamura, H. Enoki, and Etsuo Akiba* National Institute of AdVanced Industrial Science and Technology (AIST), AIST Central-5, 1-1-1, Higashi, Tsukuba 305-8565, Japan ReceiVed: December 14, 2007; ReVised Manuscript ReceiVed: September 2, 2008
The formation and decomposition behaviors of BaAlH5 and Ba2AlH7 were investigated by means of X-ray diffraction and pressure-composition isotherms (PCT). The phase transitions, phase compositions, and lattice parameters were determined by Rietveld refinement. Significantly pure BaAlH5 with a purity of 99.1% was obtained through hydriding of Ba4Al5 alloy at 100 °C under 5 MPa H2. When the hydrogenation temperature increased to 250 °C, the main phase became Ba2AlH7 with some impurity phases of BaAl4 and BaH2. PCT measurements of Ba4Al5 showed that the plateau pressure at 150 °C was about 0.5 MPa, and the maximum absorption H2 content was 2.15wt %. The final products obtained after PCT measurements of Ba4Al5 at 150 °C, 280 and 350 °C were BaAlH5, a mixture of Ba2AlH7, BaH2 and BaAl4, a mixture of BaH2 and BaAl4, respectively. Independent decomposition experiments showed that BaAlH5 partly desorbed H2 to form a mixture of BaAlH5, Ba2AlH7, BaH2, and BaAl4 at 280 °C, whereas both BaAlH5 and Ba2AlH7 completely decomposed into BaH2 and BaAl4 at 350 °C. Introduction In recent years, complex metal hydrides as potential on-board hydrogen storage materials have attracted increasing interest for their high capacity.1-5 Complex hydrides have covalent bonds as well as ionic bonds. They usually have low density and high capacity but the high desorption temperature and low kinetics limit their practical applications in on-board vehicle. Bogdanovic and his co-worker6 discovered that the kinetics of sodium alanate (NaAlH4) could be greatly improved by adding some catalysts and the absoption-desorption reaction became reversible at a moderate temperature. This discovery also inspired the research and development of new complex hydrides such as borohidrides,7-9 amides,10-12 and alkaline earth metals (AE)-based hydrides.13-15 However, it is difficult to directly prepare AE-based alanates from AEH2 and aluminum. Mg(AlH4)2 and Ca(AlH4)2 are usually synthesized through ball milling MgCl2 and CaCl2 with NaAlH4 or LiAlH4.13 Varin et al. 16 attempted to synthesize Mg(AlH4)2 from Mg-2Al alloy by mechano-chemical methods but no Mg(AlH4)2 was achieved. Other AE-Al-based hydrides such as CaAlH5 were observed during the decomposition of Ca(AlH4)2 and have not been prepared directly.13,17 Gingl et al.18 first synthesized SrAl2H2 hydride containing a 2D polymer Zintl anion through hydrogenation reaction of SrAl2 alloy under 5 MPa H2 at about 473K. In this Zintl phase hydride, one hydrogen atom is covalently bonded to each aluminum atom. In the subsequent studies, Zhang et al.19,20 reported the synthesis and crystal structure of Sr2AlH7 consisting of [AlH6] octahedral and [HSr4] and BaAlH5 containing 1D zigzag chains of [AlH6]. Ba2AlH7 that is isostructural to Sr2AlH7 had been prepared through hydriding Ba7Al13 alloy.21 There have been plenty of studies on alanate consisted of alkali metals.22-25 However, the dehydriding behavior and thermodynamic characteristics of these new kinds of AE-Al-based hydrides have not been investigated so far. Knowing well about the hydriding-dehydriding behaviors of alkaline earth alanates will * To whom correspondence should be addressed. Tel./Fax: +81-29-8614541. E-mail:
[email protected].
be helpful to develop novel complex metal hydrides. Especially, barium-based alanates are the most stable among alkali earth elements in our experience, and, therefore, we selected them to study formation and decomposition reactions of hydrides. Besides, in previous studies, Ba7Al13 alloys were used to synthesize BaAlH520 and Ba2AlH721 through a gas-solid reaction. In this study, Ba4Al5 whose Ba/Al ratio is nearer to that of hydrides than Ba7Al13 was used for the hydrogenation reaction and expected to obtain highly pure hydrides. The hydrogenation characteristics of Ba4Al5 alloy were investigated by pressure-composition isotherms (PCT) for the first time, and the phase transitions during the course of hydrogenation and decomposition were investigated. Experimental Section Ba4Al5 ingot was prepared by induction melting barium and aluminum metals with a purity of at least 99.9%. An extra 3 wt % of barium was added to compensate for the loss of barium during melting and annealing. The obtained Ba4Al5 ingot was annealed under high vacuum at 600 °C for 24 h. The alloy was then ground to powders with particle size smaller than 45 µm in a glovebox filled with high purity argon. Hydrogenation experiments were performed under a hydrogen pressure of 5 MPa at different temperatures for 5 days. X-ray diffractions were carried out on Rigaku Rint-2500 V diffractometer with Cu KR radiation at 50 kV and 200 mA. To avoid the oxidation and moisture a special sample holder filled with argon was used for XRD measurements. The crystal structures and phase compositions of the samples were analyzed through Rietveld refinement of XRD patterns with the FULLPROF program.26 Hydrogen absorption and desorption characteristics of Ba4Al5 alloy were assessed by a pressure-composition isotherms (PCT) apparatus (Suzuki Shokan Co. Ltd.). To further confirm the decomposition behavior the samples after PCT measurements were heated under different conditions and analyzed by X-ray diffractions.
10.1021/jp806138x CCC: $40.75 2008 American Chemical Society Published on Web 10/10/2008
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Liu et al.
Figure 1. X-ray diffraction patterns of as-cast Ba4Al5 ingot (A) and annealed Ba4Al5 (B).
Results and Discussion Preparation and Structure Analysis of Ba4Al5. Ba4Al5 was prepared by induction melting and subsequently annealed under vacuum at 600 °C for 24 h. X-ray diffraction was used to characterize the crystal structure of as-cast and annealed Ba4Al5 alloy. The phase compositions and lattice parameters were determined through Rietveld refinement. Figure 1 shows X-ray diffraction patterns of the as-cast and annealed Ba4Al5. The lattice parameters of the as-cast and annealed Ba4Al5 with P63/ mmc space group were listed in Table 1. As shown in Figure 1, the as-cast Ba4Al5 alloy contains Ba4Al5 (59.5 wt %), Ba3Al5 (35.8 wt %), and a small amount of Ba (4.7 wt %). After annealing, the content of Ba4Al5 reaches 94.8 wt % with only a small amount of Ba3Al5 (5.2 wt %). H2-Induced Phase Transitions of Ba4Al5 under Different Temperatures. Ba4Al5 was hydrogenated under 5 MPa H2 at different temperature for 5 days. Figure 2 shows X-ray diffraction patterns of the hydrogenated Ba4Al5 at different temperatures. Reitveld refinement was used to determine the phase compositions and lattice parameters. The calculated curve agreed well with the observed X-ray diffraction patterns as shown in Figure 3. The phase compositions derived from Rietveld refinements were listed in Table 2. When the hydrogenation temperature were 100, 150, and 200 °C, the main phases were identified to be BaAlH5 with orthorhombic structure and Pna21 space group, whereas the content of BaAlH5 decreased with increasing temperature. Under different hydrogenation temperature (100, 150, and 200 °C) the contents of BaAlH5 were 99.1, 98.6, and 79.5 wt %, respectively. Besides, BaAl4 and BaH2 appeared with the increase of hydrogenation temperature. When the hydrogenation temperature were 250, 300, and 350 °C, the main phase became Ba2AlH7 with a monoclinic structure and I2/a space group. The content of Ba2AlH7 also decreased with
Figure 2. X-ray diffraction patterns of the hydrogenated Ba4Al5 under 5 MPa H2 and different temperatures. A, B, C, D, and E represent the XRD patterns of hydrogenated Ba4Al5 at 100, 150, 200, 250, 300, and 350 °C, respectively.
increasing temperature. Under different hydrogenation temperature (250, 300, and 350 °C), the contents of Ba2AlH7 were determined to be 65.4, 59.0, and 55.5 wt %, respectively. It should be noticed that the content of BaAl4 increased with the increase of hydrogenation temperature. When the temperature was raised to 350 °C, the content of BaAl4 amounted to 37.1 wt %. The values of lattice parameters a, b, c, β, and unit cell volume V derived from Rietveld refinements of X-ray diffraction patterns were listed in Table 1. The lattice parameters a, b, c, β, and unit cell volume V of BaAlH5 and Ba2AlH7 main phases changed very slightly with increasing hydrogenation temperature. Pressure-Composition Isotherms (PCT) Measurements. To study the thermodynamic behavior and hydriding-dehydriding characteristics of Ba4Al5, the P-C isotherms measurements were performed at 150, 280, and 350 °C, respectively. Figure 4 shows the P-C isotherms of Ba4Al5 measured at 150, 280, and 350 °C, respectively. Similar to other hydrogen storage materials, there appeared an obvious absorption pressure plateau region. The maximum H2 absorption contents decreased with increasing measurement temperature. Under different measure-
TABLE 1: Lattice Parameters of Ba4Al5 Alloy and the Hydrides temp (°C) Ba4Al5 (As cast) Ba4Al5 (annealed) 100 150 200 250 300 350
main phase
a (Å)
6.0772(3) Ba4Al5 6.0839(2) Ba4Al5 BaAlH5 9.1764(4) BaAlH5 9.1738(4) BaAlH5 9.1670(8) Ba2AlH7 13.195(1) Ba2AlH7 13.208(2) Ba2AlH7 13.199(1) BaAlH5: Pna21 space group
b (Å) 6.0772(3) 6.0839(2) 7.0324(2) 7.0312(2) 7.0282(4) 10.239(1) 10.236(2) 10.231(1)
c (Å)
β (°)
17.7543(10) 17.7701(6) 5.1047(2) 5.1092(4) 5.1122(4) 8.508(1) 101.235(5) 8.503(1) 101.156(8) 8.491(1) 101.206(5) Ba2AlH7: I2/a space group
V (Å3) 567.87(5) 569.62(3) 329.42(2) 329.55(2) 329.36(6) 1127.4(2) 1127.9(3) 1124.7(1)
Behaviors of BaAlH5 and Ba2AlH7
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17425
Figure 5. X-ray diffraction patterns after PCT measurement at 150 °C (1), 280 °C (2), and 350 °C (3). Figure 3. Rietveld refinement results of hydrogenated Ba4Al5 under 150 °C (A) and 350 °C (B). The plus signs (+) represent the observed data. The solid line represents the calculated profile. The lowest curve is the difference between the observed and calculated patterns. For A, Rw ) 11.4%, Rp ) 14.3%, Rexp) 11.9%, χ2 )1.44; for B, Rw ) 9.75%, Rp ) 7.53%, Rexp) 8.40%, χ2 )1.35.
Figure 4. P-C isotherm of Ba4Al5 measured at 150, 280, and 350 °C.
TABLE 2: Phase Compositions of the Hydrogenated Ba4Al5 under 5 MPa H2 and Different Temperatures temp (°C) 100 150 200 250 300 350
BaAlH5 (wt %)
BaAl4 (wt %)
BaH2 (wt %)
Ba2AlH7 (wt %)
99.1 98.6 79.5 11.0 2.8
0.9 1.4 12.8 11.9 26.9 37.1
7.7 11.7 11.3 7.4
65.4 59.0 55.5
ment temperatures (150, 280, and 350 °C), the maximum H2 absorption contents were 2.15, 1.17, and 0.69 wt %, respectively. According to the above discussions and the following phase analysis, the main phase during the absorption at 150 °C should be BaAlH5, whereas the main phase during the absorption at 280 and 350 °C should be Ba2AlH7. For the absorption at 280 and 350 °C, the decrease of the maximum H2 absorption contents could be attributed to the decrease of Ba2AlH7 content, which is well in agreement with the results of phase composi-
tions at different temperature. From the P-C isotherms, it can be observed that the sample measured at 150 °C did not release hydrogen during the hydriding-dehydriding cycle. In contrast, the samples measured at 280 and 350 °C can release parts of H2. The desorbed H2 percentage of the samples measured at 280 and 350 °C were 0.14 and 0.45 wt %, respectively. In addition, owing to kinetic factor the absorption of the alloy has not saturated during the hydriding process and will continue to absorb a little amount of H2 in the initial desorption process as shown in PCT curves. It was also noticed that the increase of H2 capacity in the desorption process became less at higher temperature, which indicates that the influence of kinetics was lessened with increasing temperature. The phase changes after P-C isotherms measurements were identified by X-ray diffraction. Figure 5 shows the X-ray diffraction patterns of Ba4Al5 after PCT measurement at 150, 280, and 350 °C. After PCT measurement at 150 °C, the phase was almost all BaAlH5, which also indicated that BaAlH5 could not desorb H2 at this temperature during the dehydriding process. The sample after hydriding-dehydriding cycle at 280 °C was composed of Ba2AlH7 41.6 wt %, BaH2 20.7 wt %, and BaAl4 37.7 wt %, indicating that a small part of Ba2AlH7 decomposed into BaH2 and BaAl4 in the dehydriding process. When the measurement temperature increased to 350 °C, the sample after hydriding-dehydriding cycle was only composed of BaH2 (51.9 wt %) and BaAl4 (48.1 wt %), which indicated that at this temperature the formed Ba2AlH7 during absorption process completely decomposed into BaH2 and BaAl4 in the course of desorption. Decomposition Bhaviors of BaAlH5 and Ba2AlH7. To further study the decomposition behavior and phase transitions of BaAlH5 and Ba2AlH7, the obtained samples after PCT measurement at 150 and 280 °C were heated under different temperature. BaAlH5 from PCT measurement at 150 °C was heated under argon (280 °C) and vacuum (350 °C) for 1 day, respectively. Ba2AlH7 obtained after PCT measurement at 280 °C was heated under vacuum at 350 °C for 1 day. Figure 6 shows the X-ray diffraction patterns of BaAlH5 and Ba2AlH7 under different decomposition conditions. The results of X-ray diffraction analysis indicated BaAlH5 partly decomposed into Ba2AlH7 (32.9 wt %), BaAl4 (16.3 wt %) and BaH2 (15.4 wt
17426 J. Phys. Chem. C, Vol. 112, No. 44, 2008
Liu et al. increasing hydrogenation temperature, the content of BaAlH5 decreased and impurity phases of BaAl4 and BaH2 appeared. When the hydrogenation temperatures rose to 250 °C, the main phase became Ba2AlH7 and the content of Ba2AlH7 also decreased with the increase of temperature. BaAlH5 could partly dehydride to form Ba2AlH7, BaH2, and BaAl4 at 280 °C, whereas both BaAlH5 and Ba2AlH7 completely decomposed into BaH2 and BaAl4 at 350 °C. Acknowledgment. This work has been supported by New Energy and Industrial Technology Development Organization (NEDO) under “Development for Safe Utilization and Infrastructure of Hydrogen”. The authors would like to thank Dr. Huaiyu Shao, Dr. J. Nakamura, Dr. Shibuya, and Mr. T. Yamada (National Institute of Advanced Industrial Science and Technology) for their fruitful discussion. References and Notes
Figure 6. X-ray diffraction patterns of BaAlH5 and Ba2AlH7 under different decomposition conditions. (a) BaAlH5 under Ar at 280 °C for 1 day, (b) BaAlH5 under vacuum at 350 °C for 1 day, and (c) Ba2AlH7 under vacuum at 350 °C for 1 day.
%) under 280 °C and 0.1 MPa argon, whereas it completely decomposed into BaH2 (45.5 wt %) and BaAl4 (54.5 wt %) at 350 °C. Ba2AlH7 completely decomposed into BaH2 (49.7 wt %) and BaAl4 (50.3 wt %) at 350 °C. On the basis of the above analysis of the decomposition behaviors, the decomposition process of BaAlH5 and Ba2AlH7 under different conditions can be proposed as follows: 280 °C, Ar
5 BaAlH5 98 Ba2AlH7+2 BaH2+BaAl4+ 7 H2(Partly)(1) 350 °C, Vac.
4 BaAlH5 98 3 BaH2+BaAl4+7 H2 350 °C, Vac.
4 Ba2AlH7 98 7 BaH2+BaAl4+7 H2
(2)
(3)
The decomposition behavior of BaAlH5 and Ba2AlH7 is different from what is observed in NaAlH46 or Ca(AlH4)2 and CaAlH513,27 systems. In previous studies, the two-step decomposition process of NaAlH4 was proposed as:
3 NaAlH4fNa3AlH6+2 Al + 3 H2f3 NaH + 3Al + 4.5 H2
(4) Mamatha et al.13 proposed the following decomposition paths of Ca(AlH4)2 based on the analysis of XRD, differential scanning calorimetry (DSC) NMR, and IR:
Ca(AlH4)2 f CaAlH5 + Al + 1.5 H2 f CaH2+2Al + 3 H2 f CaAl2 + 4 H2(5) Conclusions Ba4Al5 alloy was hydrogenated to significantly pure BaAlH5 with a purity of 99.1 (wt %) at 100 °C under 5 MPa H2. With
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JP806138X
