Thermodynamic consideration on multi-step hydrogenation of Mg 17Al12 assisted by phase separation

Abstract

The multi-step hydrogenation and dehydrogenation on the pressure-composition-temperature relations for the Mg17Al 12-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 Mg 17Al12-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: ΔHf = -38.5 ± 0.9kJ(molH)-1 for the Mg-H system; ΔHf = -36 ± 1 kJ (molH) -1 for the first-step hydrogenation in Mg17Al 12; ΔHf = -31 ± 1 kJ (molH) -1 for the second-step hydrogenation in Mg17Al 12. © 2011 The Japan Institute of Metals.