X-ray Absorption Spectroscopic Study on Valence State and Local Atomic Structure of Transition Metal Oxides Doped in MgH₂

Abstract

A valence state and a local structure of transition metals (Nb, V, and Ti) in MgH 2 doped with metal oxides (Nb 2 O 5 , V 2 O 5 , and TiO 2 nano ) by ball milling were examined by X-ray absorption spectroscopy (XAS). The main edge regions of the Nb, V, and Ti K-edges in the X-ray absorption near edge structure (XANES) profiles are located between 0 and +5 in the oxidation states. Since these spectra coincide with those of NbO, VO, and Ti 2 O 3 , respectively, the additives are reduced by MgH 2 to the metal oxides, which have lower oxidation states than those of the starting materials. Furthermore, in order to examine the local structures around the transition metal atoms, the extended X-ray absorption fine structure (EXAFS) spectra were analyzed. In the Fourier transformation curves of the EXAFS spectra, all samples doped with the metal oxides show two peaks corresponding to metal-oxygen and metal-metal bonds, being the same as the references of NbO, VO, and Ti 2 O 3 . The local structure formed after ball milling or dehydrogenation is close to that of each of the reduced metal oxides (NbO, VO, and Ti 2 O 3 ) but in a more disarrangement state.

Formation of Mg–Nb–O rock salt structures in a series of mechanochemically activated MgH2 + nNb2O5 (n = 0.083–1.50) mixtures

“…In order to understand the mechanistic role of Nb 2 O 5 additive, researchers have devised extensive experimental and theoretic strategies but the problem remains unsolved [20e25]. Now it is clear that, after ball milling with MgH 2 , at least a partial reduction of Nb 2 O 5 occurs [24,26,27] but little is so far known about this transformation. However, it is very clear that the existing reduced/chemically modified Nb containing species has dramatic effect to enhance the hydrogen storage performance of the remaining MgH 2 in the system.…”

Catalytic Effect of Nb2O5 in MgH2-Nb2O5 Ball-Milled Composites

“…Hanada et al performed X-ray absorption spectroscopy for investigating the valences of metal oxide additives. The results showed that Nb 2 O 5 had been reduced by ball-milling with MgH 2 for 20 h. Therefore, they suggested that the improvement of desorption properties was caused by the catalytic effect of reduced niobium oxides [16]. Meanwhile, some complex compounds, such as MgNb 2 O 3.67 , have been discovered by another group [15].…”

Correlation between kinetics and chemical bonding state of catalyst surface in catalyzed magnesium hydride

“…Additionally, they performed X-ray absorption spectroscopy for investigating the valence of metal oxide additives such as Nb 2 O 5 and V 2 O 5 . The results showed that the metal oxide had been reduced by ball-milling with MgH 2 for 20 h. Therefore, they suggested that the improvement of hydrogen sorption was caused by the catalytic effect of reduced metal oxides [13] …”