相分離を呈するMg₁₇Al₁₂-H系の2段階水素吸収反応に関する熱力学的考察

抄録

  The multi-step hydrogenation and dehydrogenation on the pressure-composition-temperature relations for the Mg₁₇Al₁₂-H system was thermodynamically discussed on the based upon the enthalpy for Mg-Al alloy recombination during corresponding reaction with hydrogen. Pressure-composition-temperature diagrams were measured Mg₁₇Al₁₂-H system by means of the volumetric technique in the temperature range of 598-648 K. The isotherms show two step hydrogenation and dehydrogenation indicating a disproportionate formation of MgH₂ via Mg₂Al₃ intermetallic phase. The decomposition of Mg from the intermetallic phase causes thermodynamic destabilisation for the formation of Mg hydrides. The molar enthalpies of hydride formation for the investigated systems are: ΔH_f=−38.5±0.9 kJ(mol_H)⁻¹ for the Mg-H system; ΔH_f=−36±1 kJ(mol_H)⁻¹ for the first-step hydrogenation in Mg₁₇Al₁₂; ΔH_f=−31±1 kJ(mol_H)⁻¹ for the second-step hydrogenation in Mg₁₇Al₁₂.