Dehydrogenation in catalyst activated MgH$_{2}$*

Dehydrogenation in catalyst activated magnesium hydride (MgH$_{2})$ has been investigated using \textit{ab initio} Molecular Dynamics (MD) simulation and Nudged Elastic Band (NEB) method. Our calculation explains why small amount of Nb$_{2}$O$_{5}$ catalyst can substantially improve the thermodynamics and kinetics of MgH$_{2}$. We show that Nb$_{2}$O$_{5}$ promotes the creation of Mg vacancies and that the hydrogen desorption from the vicinity of Mg vacancies occurs in molecular form and is exothermic. The activation energy barrier for H$_{2}$ desorption in vacancy containing magnesium hydride (1.02 eV) is much lower than that in the pure magnesium hydride (3.30 eV). Therefore, the effective catalyst for dehydrogenation in MgH$_{2}$ will be one that can easily facilitate MgO formation.