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

The dehydrogenation reactions of Ca(BH4)(2) are investigated under different isobaric conditions using in situ synchrotron radiation powder X-ray diffraction and nuclear magnetic resonance measurements. Ca(BH4)(2) dissociates in multiple steps, and several intermediate phases, such as an amorphous phase(s), CaB2Hx and CaB12H12, are observed during dehydrogenation. Among the intermediate phases, it is known that CaB2Hx is fully reversible, while the more stable CaB12H12, with an icosahedral structure hinders reversible reactions. Here, we try to control the dehydrogenation reaction pathway of Ca(BH4)(2) by applying different hydrogen back-pressures. The decomposition reaction of Ca(BH4)(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(BH4)(2) decomposes via two competitive dehydrogenation reaction routes to form CaB2Hx or CaB12H12. At p(H-2) = 10 bar, the overall dehydrogenation reaction remains unchanged. However, the formation of CaB2Hx 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 CaB2Hx phase is suppressed. Possible routes to form CaH2 and elemental boron are discussed.