Characterization of titanium dopants in sodium alanate by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectra were obtained for samples of Ti-doped NaAlH₄ subjected to different numbers of cycles of dehydrogenation/re-hydrogenation. Ti is observed to evolve from its initial Ti(III) state through a series of Ti(0) species during the first 5 cycles. Although the conversion of Ti(III) to Ti(0) occurs much more readily for TiCl₃-doped samples than those prepared with TiF₃, in both cases the evolution of Ti follows the same sequence that involves 3 distinguishable Ti(0) species and ends in the predominance of the same single Ti(0) species. The spectrum of a sample of NaAlH₄ containing 2 mol% of cubic Al₃Ti is distinctly different than any of those observed for the Ti(0) species that arise during the hydrogen cycling of the hydride. The major changes in the nature of the predominant Ti species have little if any effect on the dehydrogenation kinetics, which strongly suggests that the profoundly enhanced hydrogen cycling kinetics of Ti-doped NaAlH₄ are due to a Ti species present in only a relatively minor amount.