Formation of Ca(BH₄)₂ from Hydrogenation of CaH₂+MgB₂ Composite

The hydrogenation of the CaH 2 +MgB 2 composite and the dehydrogenation of the resulting products are investigated in detail by in situ time-resolved synchrotron radiation powder X-ray diffraction, high-pressure differential scanning calorimetry, infrared, and thermovolumetric measurements. It is demonstrated that a Ca(BH 4 ) 2 +MgH 2 composite is formed by hydrogenating a CaH 2 +MgB 2 composite, at 350 °C and 140 bar of hydrogen. Two phases of Ca(BH 4 ) 2 were characterized: α- and β-Ca(BH 4 ) 2 . α-Ca(BH 4 ) 2 transforms to β-Ca(BH 4 ) 2 at about 130 °C. Under the conditions used in the present study, β-Ca(BH 4 ) 2 decomposes first to CaH 2, Ca 3 Mg 4 H 14, Mg, B (or MgB 2 depending on experimental conditions), and hydrogen at 360 °C, before complete decomposition to CaH 2, Mg, B (or MgB 2 ), and hydrogen at 400 °C. During hydrogenation under 140 bar of hydrogen, β-Ca(BH 4 ) 2 is formed at 250 °C, and α-Ca(BH 4 ) 2 is formed when the sample is cooled to less than 130 °C. Ti isopropoxide improves the kinetics of the reactions, during both hydrogenation and dehydrogenation. The dehydrogenation temperature decreases to 250 °C, with 1 wt % of this additive, and hydrogenation starts already at 200 °C. We propose that the improved kinetics of the above reactions with MgB 2 (compared to pure boron) can be explained by the different boron bonding within the crystal structure of MgB 2 and pure boron.