Electron Paramagnetic Resonance and X-ray Absorption Studies of Fluctuating Titanium Species During the Reversible Dehydrogenation of Ti-Doped Sodium Alanate

As part of our effort to characterize the active species in Ti-doped NaAlH$_{4}$ and elucidate its mechanism of action, we have carried out tandem electron paramagnetic resonance and X-ray absorption studies. We find that upon mechanical milling NaAlH$_{4}$ with 2 mol {\%} TiF$_{3}$, the majority of the titanium is present as a Ti(III) species. However, following a few cycles of dehydrogenation/re-hydrogenation, the majority of the titanium is converted to a Ti(0) species, \textbf{A} that is subsequently replaced by a different Ti(0) species,\textbf{ B} upon further hydrogen cycling. Hydride milled with TiCl$_{3}$, was found to contain mainly the Ti(0) species \textbf{A} and only a minor amount of a Ti(III) component. However, a parallel is seen with the TiF$_{3}$ doped hydride as after 10 cycles tthe Ti(0) species \textbf{A} is seen to completely convert to Ti(0) species \textbf{B}. These results will be presented and discussed in terms of their relationship to the mechanism of reversible elimination of hydrogen from the doped hydride.