Fast and slow dehydrogenation of ball milled lithium alanate (LiAlH 4) catalyzed with manganese chloride (MnCl 2) as compared to nanometric nickel catalyst

Abstract

The results of the studies on the dehydrogenation behavior of the ball milled LiAlH 4 catalyzed with 5 wt.% of manganese chloride (MnCl 2) are reported. During ball milling for 15 min the LiAlH 4 + 5 wt.% MnCl 2 nanocomposite releases a miniscule amount of ∼0.25 wt.% H 2. However, no products of the possible reaction between LiAlH 4 and MnCl 2 (e.g. LiCl) are observed by X-ray diffraction (XRD). In a DSC test most of LiAlH 4 decomposes exothermically to Li 3AlH 6 in a solid state while a small fraction of retained LiAlH 4 melts and decomposes in a liquid state. During dehydrogenation at 100 °C under 0.1 MPa H 2 the ball milled LiAlH 4 + 5 wt.% MnCl 2 nanocomposite is able to desorb ∼4.6 wt.% H 2 within ∼30,000 s in a solid state but only in Stage I (reaction: LiAlH 4 (solid) → 1/3Li 3AlH 6 + 2/3Al + H 2). The apparent activation energy of dehydrogenation for this solid state reaction is equal to ∼80 kJ/mol as compared to ∼70 kJ/mol obtained for LiAlH 4+5 wt.% n-Ni [7]. However, during decomposition at 100 °C a chemical reaction occurs between LiAlH 4 and MnCl 2 producing LiCl and most likely an amorphous Mn metal catalyzing the reaction in Stage I. The ball milled LiAlH 4+5 wt.% MnCl 2 nanocomposite is capable of desorbing substantial quantities of H 2 during long term storage at room temperature (RT; ∼21 °C), 40 and 80 °C.