Kang, Jeung Ku and Lee, Jai Young and Muller, Richard P. and Goddard, William A., III (2004) Hydrogen storage in LiAlH4: Predictions of the crystal structures and reaction mechanisms of intermediate phases from quantum mechanics. Journal of Chemical Physics, 121 (21). pp. 10623-10633. ISSN 0021-9606. http://resolver.caltech.edu/CaltechAUTHORS:KANjcp04
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We use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH4. First, we explore the decomposition mechanism of monoclinic LiAlH4 into monoclinic Li3AlH6 plus face-centered cubic (fcc) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH4 and Li3AlH6 agree well with experimental data. On the other hand, the alternative decomposition of LiAlH4 into LiAlH2 plus H-2 is predicted to be unstable with respect to that through Li3AlH6. Next, we investigate thermal decomposition of Li3AlH6 into fcc LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH6 to form the stable Li-H-Li-H complex. Then, two sequential H-2 desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li3AlH6(monoclinic)-->3 LiH(fcc)+Al(fcc)+3/2 H-2 is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s)+Al(s)-->LiAl(s)+1/2H(2)(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl.
|Additional Information:||©2004 American Institute of Physics. Received 23 February 2004; accepted 28 July 2004. This research was supported by General Motors Global Alternative Proposal Center (GM GAPC). The computational resources at the Materials and Process Simulation Center (MSC) in California Institute of Technology and Korea Advanced Institute of Science and Technology have been supported by grants from NSF-MRI, ARO-DURIP, and by a SUR grant from IBM. The authors would like to acknowledge the support from KISTI (Korea Institute of Science and Technology Information) under "the 5th Strategic Supercomputing Applications Support Program" with Dr. S. M. Lee as the technical supporter. The use of the computing system of the Supercomputing Center is also greatly appreciated.|
|Subject Keywords:||density functional theory; hydrogen storage; crystal structure; X-ray diffraction; neutron diffraction; pyrolysis; lithium compounds; dissociation energies; thermochemistry; enthalpy; reaction kinetics|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||22 Jun 2006|
|Last Modified:||26 Dec 2012 08:55|
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