Investigation on Catalytic Effect and Transformation Process in Mg/MgH2 System マグネシウム系水素貯蔵材料における触媒効果と変態の研究
Investigation on Catalytic Effect and Transformation Process in Mg/MgH2 System
Mg/MgH2 system is a promising candidate for hydrogen storage materials due to its high hydrogen capacity (7.6 wt%) and low cost; Yet the main obstacle impeding its application lies in the limitation of kinetics and thermodynamics. In respect to these problems, two significant issues, the catalytic effect and Mg→MgH2 transformation process, were studied in this thesis, as a contribution to further development on the system.First, the catalytic effect of Nb2O5 was investigated in the MgH2–Nb2O5 composites ball-milled for 0 (hand mixed), 0.02, 0.2, 2, and 20 h. An improvement on the desorption properties, in accordance with a decrease in the activation energy, was seen with the increase of ball-milling time. It was confirmed that the particle size of the additive was gradually refined during ballmilling, with the partial reduction occurred on the surface.Next, the state of the catalyst in MgH2–Nb2O5 composite was investigated during the full cycle. A transition of Nb2O5→NbH2→Nb→NbH was confirmed during ball-milling, dehydrogenation, and rehydrogenation, respectively. It is suggested that the catalytic effect of Nb2O5 follows the Nb-gateway model, in which Nb facilitates the hydrogen transportation from MgH2 to the outside, and accelerates the recombination of hydrogen molecules during the process. Nb crystals were observed to be highly dispersed in the sample, with 10–20 nm in size. As the essential catalyst, these tiny crystals worked as the gateway facilitating hydrogen transportation and hence improving dehydrogenation properties.Finally, Mg→MgH2 transformation process was observed by TEM. It was found that the hydrogenation took place along the specific orientation relationship, MgH2(101)
Mg(002). A structural model, in which the Mg–Mg distance is adjusted according to the introduction of H, and the Mg layers shift slightly, correspondingly, was proposed to demonstrate the transformation.
Hokkaido University（北海道大学）. 博士(工学)