Synthesis of ammine dual-metal (V, Mg) borohydrides with enhanced dehydrogenation properties

摘要

The penta-ammine vanadium (Ⅲ) borohydride, i.e. V(BH_4)_3·5NH_3, was successfully synthesized via ball-milling of VC1_3 · 5NH_3 and LiBH_4 in a molar ratio of 1:3. This compound was shown to release 11.5 wt% hydrogen with a H_2-purity of 85 mol% by 350 ℃. To improve the dehydrogenation purity of V(BH_4)_3·5NH_3, Mg(BH_4)_2 with various molar ratios was mixed with V(BH_4)_3·5NH_3 to synthesize expected ammine metal-mixed borohydrides, among which the formed VMg(BH_4)_5·5NH_3 was indexed to be a monoclinic unit cell with lattice parameters of a = 19.611 A, b = 14.468 A, c = 6.261 A, β = 93.678° and V = 1772.75 A3. Dehydrogenation results revealed that the Mg(BH_4)_2 modified V(BH_4)_3·5NH_3 system presents significantly enhanced dehydrogenation purity. For example, in the case of V(BH_4)_3·5NH_3/2Mg(BH_4)_2 sample, 12.4 wt% pure hydrogen can be released upon heating to 300 ℃. Further investigation on the dehydrogenation mechanism of the VMg(BH_4)_5·5NH_3 system by isotope tagging revealed that the interactions of homo-polar BH units also participated throughout the dehydrogenation process (onset at 75 ℃) as complementary to the prime combination of BH ··· HN.