Solid State NMR Studies of the Aluminum Hydride Phases
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0927-EE03-03
Solid State NMR Studies of the Aluminum Hydride Phases Son-Jong Hwang1, Robert C. Bowman2, Jason Graetz3, and J. J. Reilly3 1 Div. of Chem and Chem Eng, California Institute of Technology, Pasadena, CA, 91125 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109 3 Department of Energy Science and Technology, Brookhaven National Laboratory, Upton, NY, 11973 ABSTRACT Several solid state NMR techniques including magic-angle-spinning (MAS) and multiple-quantum (MQ) MAS experiments have been used to characterize various AlH3 samples. MAS-NMR spectra for the 1H and 27Al nuclei have been obtained on a variety of AlH3 samples that include the β- and γ-phases as well as the most stable α-phase. While the dominant components in these NMR spectra correspond to the aluminum hydride phases, other species were found that include Al metal, molecular hydrogen (H2), as well as peaks that can be assigned to Al-O species in different configurations. The occurrence and concentration of these extraneous components are dependent upon the initial AlH3 phase composition and preparation procedures. Both the β-AlH3 and γ-AlH3 phases were found to generate substantial amounts of Al metal when the materials were stored at room temperature while the α-phase materials do not exhibit these changes. INTRODUCTION In order to meet U.S. DOE hydrogen storage goals proposed for years 2010 and 2015, metal hydrides will need to be composed mainly of light elements (i.e., Li, B, Mg, Al, etc.). With a hydrogen gravimetric capacity over 10 wt.%, AlH3 would be an extremely attractive hydrogen storage material for low temperature fuel cells if its hydrogen absorption and desorption properties could be improved. At least three distinct polymorphic AlH3 phases can be produced by organometallic synthesis methods [1] where the most thoroughly investigated and stable polymorph is denoted as α-AlH3. Recently Sandrock, et al. [2] demonstrated that enhanced thermal decomposition from nearly 30-year old α-AlH3, which had been originally made by Dow Chemical, could be achieved by ball milling and doping this powder with LiH. Subsequently, various alane phases (e.g., α, β, and γ) were freshly prepared by Graetz, et al. [3,4] and their desorption kinetics measured over a range of temperatures. Phase composition was found to have significant impact on the stabilities and reaction rates for these samples [2 – 4]. Although the crystal structure for α-AlH3 had been determined previously [5], the structures for the β- and γ- phases are still unknown as are the locations of the Al and H atoms in these latter phases. In order to understand these kinetic properties as well as various thermodynamics and hydrogen storage behavior, additional fundamental structural and chemical bonding information is needed. One powerful and versatile technique for obtaining such insights in metal hydrides is solid state nuclear magnetic resonance (NMR) [6]. The only prior NMR measurements on AlH3 samples appears to have been performed over several decad
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