Vapor pressure measurements of Mg(BH4)2 using Knudsen torsion effusion thermo graphic method

• Published in: International journal of hydrogen energy 2014-02, Vol.39 (5), p.2175-2186
• Main Authors: Nforbi, L.-N.N., Talekar, A., Lau, K.H., Chellapa, R., Chien, W.-M., Chandra, D., Hagemann, H., Filinchuk, Y., Zhao, J.-C., Levchenko, Andre
• Format: Article
• Language: English
• Subjects: Alternative fuels. Production and utilization; Applied sciences; Crystal structure; Energy; Exact sciences and technology; Fuels; Hydrogen; Hydrogen desorption under dynamic vacuum; Mg(BH4)2; Torsion effusion vapor pressure measurements; Vaporization thermodynamics
• ISSN: 0360-3199; 1879-3487
• DOI: 10.1016/j.ijhydene.2013.11.071

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 PTotal (bar) = 9.2303 − 7286.2/T, logPMg(BH4)2(bar)=8.2515−7286.2/T, and logPH2(bar)=9.1821−7286.2/T. The partial pressures of the gaseous species were determined as PMg(BH4)2(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. •Vapor pressure measurements have been made for hydrogen desorption from Mg(BH4)2.•One amorphous and two crystalline α and β Mg(BH4)2 were used for these studies.•Hydrogen gas was evolved between 438 K and ∼533 K disproportionating Mg(BH4)2.•Amorphous Mg(BH4)2 behaved differently than the two crystalline phases (α and β).•Mechanism of phase transitions and disproportionation are proposed.