A proposed kinetic model for hydrogen sorption in MgH2.

The role of ternary Mg-Nb oxides on H₂ absorption and desorption properties of ball milled MgH₂ is discussed on the basis of a kinetic model involving the formation of reaction pathways of ternary oxide species with easier splitting of H₂ that facilitate the transport of H₂ into the solid structure. The kinetic model comprised three key steps: dissociation of molecular hydrogen into H atoms, diffusion of H into Mg to form MgH₂ during adsorption, and successive recombination of H-H on the surface of the additives during desorption to make a pathway for H₂ sorption in the structure. The presence of transition metal oxides remarkably increases the H₂ sorption rates of nano-structured MgH₂, as determined by a volumetric Sievert apparatus. The H₂ desorption rate increases with increasing temperature from 593 to 673 K and under the same conditions, the absorption rate decreases. The most promising results were obtained for Mg₃Nb₆O₁₁-doped MgH₂ nano-particles. The MgH₂/Mg₃Nb₆O₁₁ system is completely dehydrogenated at 673 K under 0.1 MPa H₂ and the samples fully rehydrogenated at 613 K under 2.5 MPa H₂ pressure. Activation energies were obtained for MgH₂/Mg₃Nb₆O₁₁ mixture, and without additive (ball milled MgH₂), to be 88 and 127 kJ mol^-1, respectively, underlying the effect of the ternary additive. [ABSTRACT FROM AUTHOR]