Method for preparing Ti-doped NaAlH4 using Ti powder:: observation of an unusual reversible dehydrogenation behavior

Titanium powder can be directly used as dopant in the preparation of catalytically enhanced Ti-doped NaAlH4 upon mechanical milling under an atmosphere of hydrogen. The hydrogen storage performance of NaAlH4 that was doped through this method was found to be highly dependent on the milling time. A sample mechanically milled under hydrogen for 10 h was initially observed to discharge > 3.3 wt.% of hydrogen within 1 h at 150degreesC. Rehydrogenation was accomplished at 120degreesC under about 12 MPa H-2 within 10 h. The hydrogen capacity of this material was found to gradually decrease to 2.8 wt.% after eight cycles. The dehydriding kinetics and cycling properties of NaAlH4 that is doped through this novel method is clearly distinct from the material that is obtained through the utilization of Ti(III) or Ti(IV) dopant precursors and milling under argon. This suggests that the nature of active Ti-species may differ in these two varieties of doped NaAlH4. (C) 2004 Elsevier B.V. All rights reserved.