Theoretical Analysis on X-ray Absorption Spectra of Ti compounds as Catalysts in Lithium Amide-Imide reactions

Solid-state storage is conceptually efficient approach for on-board vehicular hydrogen storage. In this context, light-element materials such as lithium amide LiNH$_{2}$ and lithium imide Li$_{2}$NH have been attracted much attention due to their high gravimetric densities of hydrogen. Recently, various transition-metal compounds have been examined with ball-milling technique for exploring catalysts to improve the hydrogen storage and desorption kinetics, and it is found that a small amount (1mol\%) of titanium compounds revealed a superior effect in hydrogen desorption reaction LiNH$_{2}$ + LiH ${\to}$ Li$_{2}$NH + H$_{2}$ [1]. However, these catalysis mechanism and role of Ti in the reaction remain unanswered. Isobe $et$ $al$. have carried out measurements of X-ray absorption spectroscopy(XAS) at Ti $K$-edge to see the electronic states of Ti recently [2]. In this paper, we calculate the electronic structure of Ti metal and its compounds, and obtained theoretical spectra to compare with the measured spectra by using first-principles calculations based on the all-electron FLAPW method. We discuss chemical bonding and local geometry of catalytically active states in the reaction. [1] T. Ichikawa, S. Isobe, N. Hanada and H. Fujii, J. of Alloys and Comp. 365, 271 (2004) . [2] S. Isobe, T. Ichikawa, Y. Kojima and H. Fujii, J. of Alloys and Comp. 446-447, 360 (2007).