Dehydrogenation mechanism in catalyst-activatedMgH2

A small amount of ${\mathrm{Nb}}_{2}{\mathrm{O}}_{5}$ catalyst is known to substantially improve the desorption thermodynamics and kinetics of $\mathrm{Mg}{\mathrm{H}}_{2}$. Using density functional theory in combination with ab initio molecular dynamics simulation, we provide theoretical understanding of the mechanism of dehydrogenation in Nb doped $\mathrm{Mg}{\mathrm{H}}_{2}$. We show that the substitution of Nb at the Mg site followed by the clustering of H around Nb is a likely pathway for hydrogen desorption. We also find that dehydrogenation from the vicinity of Mg vacancies is exothermic. However, the vacancies are not likely to play a significant role in hydrogen desorption due to their high formation energy $(3.87\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$.