Vapor Pressure Measurements of Mg(BH4)(2) Using Knudsen Torsion Effusion Thermo Graphic Method

Abstract

The vapor pressure and molecular weight of effusing vapors of α, β, and amorphous Mg(BH4)2 were determined by Torsion-effusion gravimetric method, under dynamic vacuum. A Cahn balance in the system yielded the rate of the weight loss. Molecular weights measured revealed if the effusion was congruent or there was disproportionation. The vaporization behavior of crystalline Mg(BH4)2, was measured up to 533 K at pressures of ∼10−5 torr. It was found that Mg(BH4)2 disproportionates to form predominantly H2 gas (∼95%) with a small amount of Mg(BH4)2 (∼5%) in the gas phase. The combined average molecular weight measured is 4.16 g/mol. The equations for vapor pressures for crystalline Mg(BH4)2 are given by: log P  Total (bar) = 9.2303 − 7286.2/T  ,  [View the MathML source] , and  [View the MathML source] . The partial pressures of the gaseous species were determined as PMg2(4BH)(g)/PT=0.105 and PH2(g)/PT=0.895. Enthalpies of vaporization for the effusing gases were calculated to be ΔH = +558.0 kJ/mol H2 and ΔH = +135 kJ/mol Mg(BH4)2. The standard Gibbs free energy changes, ΔG°(kJ/mol), for the complete decomposition reaction (Mg(BH4)2(s) → Mg(s) + 2B(s) + 4H2(g)), sublimation reaction (Mg(BH4)2(s) → Mg(BH4)2(g)) and the disproportionation reaction for Mg(BH4)2 are reported in this paper. The decomposition pathway of amorphous Mg(BH4)2 was also carried out between 388.2 K and 712.8 K showing multistep decomposition of a-Mg(BH4)2 Different reaction products were obtained depending on the method used in the vaporization experiment. The behavior of the amorphous Mg(BH4)2(s) is very different from those for the two crystalline phases (α and β). The vapor pressure behavior and thermodynamics of vaporization of different phases of Mg(BH4)2 are presented.