Investigation of Ti-doped NaAlH4 by solid-state NMR

Abstract

ABSTRACTIn recent years, the development of Ti-doped NaAlH4 as a hydrogen storage material has gained attention because of its large weight percentage of hydrogen (∼5 %) compared to traditional interstitial hydrides. The addition of transition-metal dopants, in the form of Ti-halides, such as TiCl3, dramatically improves the kinetics of the absorption and desorption of hydrogen from NaAlH4. However, the role that Ti plays in enhancing the absorption and desorption of H2 is still unknown. In the present study, 27Al, 23Na, and 1H MAS (Magic Angle Spinning) NMR (Nuclear Magnetic Resonance) has been performed to understand the titanium speciation in Ti-doped NaAlH4. All experiments were performed on a sample of crushed single crystals exposed to Ti during growth, a sample of solvent-mixed 4TiCl3 + 112NaAlH4, a reacted sample of solvent-mixed TiCl3 + 3NaAlH4 with THF, and a reacted sample of ball-milled TiCl3 + 3NaAlH4. The 27Al MAS NMR has shown differences in compound formation between solvent-mixed TiCl3 + 3NaAlH4 with THF and the ball-milled TiCl3 + 3NaAlH4. 27Al MAS NMR of the ball-milled mixture of fully-reacted TiCl3 + 3NaAlH4 showed spectral signatures of TiAl3 while, the solvent-mixed 4TiCl3 + 112NaAlH4, which is totally reacted, does not show the presence of TiAl3, but suggests the existence of Al2O3.