Formation and Decomposition of Magnesium Hydride

The absorption of hydrogen in magnesium (purity, 99.8%–99.94%) was studied in the temperature range 260–425 °C and at pressures from the equilibrium value to 2 MPa above equilibrium. At constant temperature the absorption rate depends on the pressure whereas total absorption is attained in approximately the same time regardless of the pressure.

The final composition is very close to stoichiometric MgH2 except at the lowest temperatures and highest pressures investigated when the reaction becomes extremely slow or ceases completely at 80%–90% of the stoichiometric composition. Accurate determination of the relative molar enthalpy gives a value of −70 kJ (mol H2)−1.

No hysteresis in the ordinary sense was observed, but although no change in the plateau pressure occurred desorption of from 5%–15% of the remaining hydrogen required almost zero pressure. The actual value appears to be dependent on the material.