Mechanistic Studies of Hydrogen Release from Solid Amine Borane Materials
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0927-EE02-04
Mechanistic Studies of Hydrogen Release from Solid Amine Borane Materials Mark Bowden1, Tim Kemmitt1, Wendy Shaw2, Nancy Hess2, John Linehan2, Maciej Gutowski2, Benjamin Schmid2, and Tom Autrey2 1 Industrial Research Ltd, Lower Hutt, 6009, New Zealand 2 Pacific Northwest National Laboratory, Richland, Washington, WA 99252
ABSTRACT Ammonia borane (NH3BH3) is a molecular solid with a high volumetric and gravimetric density of hydrogen. We report room temperature structural data which shows how the freely rotating NH3 and BH3 groups allow a N-H…H-B dihydrogen bond in which hydrogen atoms on adjacent molecules are separated by only 1.90Å. The initial decomposition of ammonia borane at 80-100°C into (NH2BH2)n and H2 has been studied by in-situ nmr spectroscopy and kinetic studies using isotopic substitution. The reaction proceeds by a bimolecular pathway involving a [NH3BH2NH3]+BH4- intermediate with an activation energy of 136kJmol-1. INTRODUCTION Ammonia borane (NH3BH3) has been the subject of a number of studies recently as research groups internationally intensify their interest in potential hydrogen storage materials. The weight fraction of hydrogen in ammonia borane (19.6wt% total) makes it an attractive candidate for hydrogen storage, surpassing the US DOE goals for gravimetric and volumetric hydrogen density. The structure undergoes a phase transformation from a low temperature orthorhombic arrangement to a tetragonal structure with similar unit cell dimensions at 225K. The low temperature structure [1] comprises NH3BH3 molecules arranged head-to-tail and linked by a network of N-H…H-B “dihydrogen” bonds. The H…H distances in this structure (2.02 and 2.21Å) are significantly shorter than the sum of van der Waals radii for two H atoms (2.4Å), as found in a number of similar compounds [2]. The high temperature structure is thought to differ principally through rotation of the NH3 and BH3 groups, leading to a higher symmetry. High hydrogen mobility has been observed by nmr [3], and although some structural data are available from powder studies [4], detailed crystallographic information has not yet been published. Hydrogen may be released from ammonia borane through hydrolysis [5,6] or thermolysis [7]. The hydrogen is released during thermolysis in three stages, each corresponding to one mole of H2 per NH3BH3 formula unit. The first two stages occur in distinct steps at approximately 110 and 150ºC, and yield polymeric products with formulae (NH2BH2)n and (NHBH)n respectively. The third mole of H2 is released over a broader temperature range to finally yield BN. This report concentrates on the first of these stages. Earlier work [8] has shown how the weight loss measured is dependent on the heating rate, with high heating rates promoting greater weight loss through volatile species such as monomeric NH2BH2 and B2H6. Smith et al. [9] have used isotope substitution to show how the generation of hydrogen is a bimolecular process. The
observation of HD in addition to H2 and D2 from an intimate mixture of
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