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

• Magnesium is of interest as a possible hydrogen storage material.
• Catalytic effect of transition metals oxides was dependent on oxidation state.
• Nb2O5 ((1 0 0), (0 1 0), (0 0 1)) and NbO {1 0 0} surfaces were mainly investigated.
• The Mg–H bond length of MgH2 in adsorption process on niobium oxides enlarged.
• The lower oxidation state of Nb is catalytically active on the adsorption process.

In this paper, in order to investigate the dependence of oxidation state of metal oxides, adsorption reaction of a MgH2 on Nb2O5 ((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.