Mixtures of Hydrides and Reactive Additives

The value of the reaction enthalpy per mole H2 is lowered compared to the value of the formation enthalpy of MgH2- This can be understood by considering the following partial reactions for absorption  

Similarly, the magnesium-aluminum system shows several intermediate phases which react with hydrogen, such as Mg2Als [81] and Mgi7Ali2 [82]. 

While the dissociation pressure of pure MgH2 at 326 °C is 3 bar the respective dissociation pressure of the system 2MgH2 + 3 A1 is found to be increased to 10 bar. According to Bouaricha et al. [82] Mgi7Ali2 reacts vith hydrogen in tv o steps  

While Mg2Al3 absorbs only about 3 wt.% H2, Mgi7Ali2 can reversibly absorb 4.4 wt.% H2 with Mg2Al3 being an intermediate. 

Mgi7Ali2 has a formation enthalpy of about —102 kj mol, and Mg2Al3 —12.5 kj mol . Therefore, the addition of Al leads to a decrease in the value of the total reaction enthalpy of reactions (7.15a) and (7.15b) of about 6kJ (mol H2) each. 

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