Reaction stoichiometry between TiCl_3 and NaAlH_4 in Ti-doped alanate for hydrogen storage: The fate of the titanium species

摘要

In order to understand the final state of the TiCl_3 dopant during the dehydrogenation and rehydrogenation cycles of NaAlH_4 we determined the reaction stoichiometry between TiCl_3 and NaAlH_4 by measuring the amount of hydrogen evolution from NaAlH_4 with the varying TiCl_3 -load. We found that: (i) TiCl_3 reacted with 3 M equivalents of NaAlH_4 during the doping process of ball-milling, (ii) the Ti dopant continued to react with NaAlH_4 during the first dehydrogenation process until total six equivalents of NaAlH_4 were consumed, and (iii) Ti fixed Al, not NaH, so that Al became insufficient during the rehydrogenation process. These findings lead to the conclusion that the reaction stoichiometry between Ti and Al is 1:6, which probably yields TiAl_6 and plays a catalytic role in the hydrogen storage reactions of Ti-doped NaAlH_4.