Investigation of the Dehydrogenation Reaction Pathway of Ca(BH4)2 and Reversibility of Intermediate Phases

The dehydrogenation reaction pathway of Ca(BH4)(2) has been investigated under an isobaric condition of 1 bar of hydrogen. beta-Ca(BH4)(2) gradually transforms into the (XRD invisible) amorphous phase at the initial stage. Upon further heating, it decomposes into CaB2Hx or CaB12H12, indicating that there is a subtle competition between two different dehydrogenation routes: (i) Ca(BH4)(2) -> amorphous phase -> CaB2Hx + (4 - x)/2H(2) -> 2/3CaH(2) + 1/3CaB(6) + 10/3H(2) and (ii) Ca(BH4)(2) -> amorphous phase -> 1/6CaB(12)H(12) + 5/6CaH(2) + 13/6H(2). It has also been concluded that CaB2Hx as well as the CaB6 + CaH2 mixture can be rehydrogenated at 330 degrees C under 90 bar of hydrogen without any catalytic additive, whereas CaB12H12 hardly rehydrogenated under the same conditions.