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Monday, October 18, 2004 - 3:00 PM
STO 2.3

Characterization and Mechanistic Studies of the Active Titanium Species in the Reversible Dehydrogenation of Ti-Doped Sodium Aluminum Hydride

C. M. Jensen, M. Sulic, M. Kuba, S. Srinivasan, University of Hawaii, Honolulu, HI; S. Eaton, University of Denver, Denver, CO; J. T. Rijssenbeek, General Electric Company, Schenectady, NY; Y. Gao, South China University of Technology, Guangzhou, China; H. Brinks, B. Hauback, Institute for energy Technology, Kjeller, Norway; K. Yvon, H. Hans, D. S. Gomes, University of Geneva, Geneva, Switzerland

In 1997, Bogdanovic and Schwickardi reported that the elimination of hydrogen from solid NaAlH4 is markedly accelerated and rendered reversible under moderate conditions upon mixing the hydride with a few mole percent of selected transition metal complexes. We found that doping the hydride through an alternative, mechanical milling method leads to considerable improvements in the practical hydrogen cycling performance of the hydride. It now appears that a variation of the doped hydride could possibly be developed as a viable means for the onboard storage of hydrogen. However, no dopant precursors have been found that give a greater kinetic enhancement than those cataloged in Bogdanovic’s original, 1995 patent. Similarly, only the sodium and mixed sodium, lithium salts of the alanates have been found undergo largely reversible dehydrogenation under moderate conditions upon doping. This lack of progress is surprising in view of the recent "gold rush" flurry of activity that has been direct towards the development of alanates as practical onboard hydrogen carriers. Clearly, these efforts have been handicapped by a lack of understanding of the nature and mechanism of action the dopants. We have therefore initiated efforts to elucidate the fundamental basis of the remarkable hydrogen storage properties of this material. Towards this end, we have recently conducted infra red, Raman, and electron paramagnetic resonance spectroscopic studies as well as synchrotron X-ray and neutron diffraction ; X-ray absorption fine structure (XAFS) : and kinetic investigations of the doped hydride. The results of these investigations and the emerging understanding of the nature of the active titanium species will be presented.

Summary: Ti-doped sodium aluminum hydride has attracted considerable attention as the prototype of a promising new class of hydrogen storage materials based on complex hydrides. However, in the seven years since Bogdanovic’s original report of the remarkable hydrogen cycling properties of this material, the nature and mechanism of action the Ti-dopant has remained an enigma. This talk will report on fundamental insights about Ti-doped NaAlH4 that we have recently gained through infra red and electron paramagnetic resonance spectroscopy; synchrontron X-ray and neutron diffraction, X-ray absorption fine structure (XAFS), and kinetic investigations.