Formula omitted] and [formula omitted] MAS NMR and [formula omitted] multiple quantum studies of Ti-doped NaAlH 4

Abstract

Previous X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) studies on Ti-doped NaAlH 4 revealed the reaction products of two heavily doped (33.3 at.%) samples. This investigation revealed that nano-crystalline or amorphous Al 2O 3 forms from the possible coordination of aluminum with the oxygen atom of the furan ring system from added tetrahydrofuran (THF) in solvent-mixed samples, and that TiAl 3 forms in mechanically-milled samples [E.H. Majzoub, J.L. Herberg, R. Stumpf, R.S. Maxwell, J. Alloys Compd. 394 (2005) 265], indicating the importance of understanding the processing conditions of these potentially important hydrogen storage materials. The present paper provides a more sophisticated NMR investigation of these materials and resolves some unanswered questions. On heavily doped (33.3 at.%) solvent-mixed samples, Al 27 Magic Angle Spinning (MAS) NMR Al 27 multiple quantum MAS (MQMAS) indicates the presence of an oxide layer of Al 2O 3 on the surfaces of potentially bulk nanocrystalline Ti, nanocrystalline TiAl 3, and/or metallic aluminum. The H 1 MAS NMR data also indicate the possible coordination of aluminum with oxygen atoms in the THF molecules. In addition, the H 1 MAS NMR and H 1 spin-lattice relaxation (T 1) measurements are consistent with the presence of TiH 2. These results are in agreement with recent XAFS measurements indicating both Al and H within the first few coordination shells of Ti in the doped alanate.