A role of Ti dopants in catalyzing NaAlH$_4$ from x-ray absorption studies and first-principal density functional calculations.

We have performed Ti $K$-edge XAFS measurements on 2 and 4 mol$\% $ TiCl$_3$ doped sodium alanates. Ti does not enter substitutially or interstitially into the perfect NaAlH$_4$ lattice. A substance formed as a result of multiple hydrogen cycling is of close resemblance of an amorphous TiAl$_3$ alloy with local structure about the Ti atom given by a cluster expansion of Ti-H$_x$-Al$_{10}$-Ti$_2$-... Interatomic distances and Debye-Waller factors are determined for several structural models. These results are elaborated by Ti $K$-edge XANES measurements which are interpreted in terms of single-electron multiple scattering calculations. Main features of the absorption edge are reproduced reasonably well assuming that either 3-5 hydrogen atoms enter the tetrahedron positions of the bulk $I4/mmm$ phase or a few monolayer thick TiAl$_3$ clusters are formed. Structural properties and phase stability of hydrided Ti-Al alloys, NaAlH$_4$, and Na$_3$AlH$_6$, as well as several products of the decomposition reaction were determined at zero temperature within LDA approximation to DFT using LAPW method. The calculations reveal that partial decomposition of NaAlH$_4$ accompanied by formation of TiAl$_3$ alloy is preferred to Ti substitution for Na, in good agreement with our XAFS finding.