Thermodynamic Consideration on Multi-Step Hydrogenation of Mg17Al12 Assisted by Phase Separation

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: Delta H-f = -38.5 +/- 0.9 kJ (mol(H))(-1) for the Mg-H system; Delta H-f = - 36 +/- 1 kJ (mol(H))(-1) for the first-step hydrogenation in Mg17Al12; Delta H-f = -31 +/- 1 kJ (mol(H))(-1) for the second-step hydrogenation in Mg17Al12.