Kinetics and modeling study of magnesium hydride with various additives at constant pressure thermodynamic driving forces

In this research a comparison was made of the reaction kinetics of MgH2 ball milled with 4 mol% of TiH2, Nb2O5 or Mg2Ni. This comparison was made using a novel procedure in which the ratio of the equilibrium plateau pressure (P-m) to the opposing pressure (P-op) was the same in all cases. TPD analysis showed that the addition of 4 mol% of the various additives to MgH2 resulted in a reduction of the onset temperature of MgH2 by as much as 120 degrees C. Kinetic data showed that the reaction rates with additives are in the order: TiH2 < Mg2Ni < Nb2O5. These data were fitted to models for a moving boundary mechanism, a diffusion-controlled process, and nucleation and growth. It was found that a moving boundary mechanism best fit the data and most likely controlled the reaction rates. Kissinger plots showed that the activation energies for the mixtures were in the order: TiH2 > Mg2Ni > Nb2O5. (C) 2010 Elsevier B.V. All rights reserved.