A proposed kinetic model for hydrogen sorption in MgH2

The role of ternary Mg-Nb oxides on H-2 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 H-2 that facilitate the transport of H-2 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 H-2 sorption in the structure. The presence of transition metal oxides remarkably increases the H-2 sorption rates of nano-structured MgH2, as determined by a volumetric Sievert apparatus. The H-2 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 H-2 and the samples fully rehydrogenated at 613 K under 2.5 MPa H-2 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.