Thermal stability of Vale Inco nanonometric nickel as a catalytic additive for magnesium hydride (MgH_2)

摘要

The structure stability of nanometric-Ni (n-Ni) produced by Vale Inco Ltd. Canada as a catalytic additive for MgH_2 has been investigated. Each n-Ni filament is composed of nearly spherical interconnected particles having a mean diameter of 42 ± 16 nm. After ball milling of the MgH_2 + 5 wt.%n-Ni mixture for 15 min the n-Ni particles are found to be uniformly embedded within the particles of MgH_2 and at their surfaces. Neither during ball milling of the MgH_2 + 5 wt.%n-Ni mixture nor its first decomposition at temperatures of 300, 325, 350 and 375 ℃ the elemental n-Ni reacts with the elemental Mg to form the Mg_2Ni intermetallic phase (and eventually the Mg2_NiH_4 hydride). The n-Ni additive acts as a strong catalyst accelerating the kinetics of desorption. From the Arrhenius and Johnson-Mehl-Avrami-Kolmogorov theory the activation energy for the first desorption is determined to be ～94kJ/mol. After cycling at 300 C the activation energy for desorption is determined to be ～ 99 kJ/mol. This is much lower than ～ 160 kJ/mol observed for the undoped and ball milled MgH_2. During cycling at 275 and 300 ℃ the n-Ni additive is converted into Mg_2Ni (Mg_2NiH_4). The newly formed Mg_2NiH_4 has a nanosized grain on the order of 20 nm. Its catalytic potency seems to be similar to its n-Ni precursor. The formation of Mg_2Ni (Mg_2NiH_4) may be one of the factors responsible for the systematic decrease of hydrogen capacity observed upon cycling at 275 and 300 ℃.