Abstract
Metal-borohydrides M(BH 4) n (M = Ca, Sc, Ti, V, Cr, Mn, Zn (fourth period in periodic table) and Al; n = 2–4) were synthesized by a mechanical milling process according to the following reaction: MCl n + nLiBH 4 → M(BH 4) n + nLiCl. The thermal desorption properties of M(BH 4) n were investigated by gas-chromatography and mass-spectroscopy combined with thermogravimetry. The results indicate that the hydrogen desorption temperature T d of M(BH 4) n correlates with the Pauling electronegativity χ P of M; that is, T d decreases with increasing values of χ P. The components of desorbed gas for M = Ca, Sc, Ti, V and Cr ( χ P ≤ 1.5) are hydrogen only, while those for M = Mn, Zn and Al ( χ P ≥ 1.5) contain borane and hydrogen. Therefore, the appropriate χ P in M(BH 4) n for hydrogen storage is expected to be smaller than 1.5, and the resulting predicted relationship between the heat of formation of M(BH 4) n and χ P is a useful indicator for estimating the enthalpy change for the desorption reaction for M(BH 4) n .
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