Hydrogen Back-Pressure Effects on the Dehydrogenation Reactions of Ca(BH_4)_2

Abstract

The dehydrogenation reactions of Ca(BH_4)_2 are investigated under different isobaric conditions using in situ synchrotron radiation powder X-ray diffraction and nuclear magnetic resonance measurements. Ca(BH_4)_2 dissociates in multiple steps, and several intermediate phases, such as an amorphous phase(s), CaB_2Hx, and CaB_(12)H_(12), are observed during dehydrogenation. Among the intermediate phases, it is known that CaB_2Hx is fully reversible, while the more stable CaB_(12)H_(12) with an icosahedral structure hinders reversible reactions. Here, we try to control the dehydrogenation reaction pathway of Ca(BH_4)_2 by applying different hydrogen back-pressures. The decomposition reaction of Ca(BH_4)_2 in the absence of a catalyst was found to be sensitive to the H_2 back-pressure. At p(H_2) = 1 bar, Ca(BH_4)_2 decomposes via two competitive dehydrogenation reaction routes to form CaB_2H_x or CaB_(12)H_(12). At p(H_2) = 10 bar, the overall dehydrogenation reaction remains unchanged. However, the formation of CaB_2H_x is reduced, and amorphous elemental boron is observed as a final dehydrogenation product. At p(H_2) = 20 bar, the elemental boron formation is significantly increased, and the formation of the CaB_2H_x phase is suppressed. Possible routes to form CaH_2 and elemental boron are discussed.

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