Ab initio study of the role of niobium oxides as catalysts in magnesium hydride

In this paper, in order to investigate the dependence of oxidation state of metal oxides, adsorption reaction of a MgH₂ on Nb₂O ₅ ((1 0 0), (0 1 0), (0 0 1)) and NbO {1 0 0} surfaces, in which the valence states of the Nb are +5 and +2, respectively, were mainly investigated using periodic Density Functional Theory (DFT) calculation. Mg-H bond length, adsorption energy and Local Density of States (Local DOS) were calculated. As a result, the Mg-H bond length in adsorption process on niobium oxides enlarged as compared with optimized MgH₂, because H atom came closer to Nb atom and created hybrid orbital. Moreover, Mg-H bond length of MgH₂ cluster with Nb₂O₅ became longer than that with NbO. The longer chemical bonds generally imply weaker bond strength. It is suggested that this might be affected by not only valence state of Nb but also a lattice configuration of catalysts. However, adsorption energy of MgH₂ with Nb₂O₅ was lower than that of NbO. It is considered that the interaction of MgH₂ with Nb₂O₅ was stronger than that with NbO like Local DOS results. Our results indicate that the lower oxidation state of Nb is catalytically active on the adsorption process of MgH₂.