Journal article

Synthesis Mechanism of Alkali Borohydrides by Heterolytic Diborane Splitting

Similar to alane in alanates, borane species are assumed to be the mass transport intermediate in the hydrogen storage reaction MH + B + 3/2H2 MBH4 with M = Li and Na. One possible substep of this reaction is the interaction of diborane with the alkali hydride. In this paper, we unravel the synthesis mechanism of alkali borohydrides by solid-gas reaction of alkali hydrides and diborane gas by H/D isotope labeling of the reaction educts (e.g., LiD + B2H 6). The labeling enables us to trace the hydrogen/deuterium atoms in the borohydride product by Raman scattering and in the gas by infrared spectrometry measurements. We conclude that, during the LiBH4 synthesis from LiH, the entire BH4- unit is transferred from the diborane to the Li+ cation. This provides clear evidence for the heterolytic splitting of diborane on alkali hydrides and implies exchange of BH4- with H- ions of the underlying hydride. The detection of Li-H bonds at the surface of newly formed LiBH4 confirms the importance of H- defects for the synthesis of borohydrides. © 2010 American Chemical Society.


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