Combined TEM-EDX and XAFS studies of Ti-doped sodium alanate

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Abstract
In context of investigations of doped sodium alanate as a hydrogen storage material, an investigation combining a TEM-EDX study and XAFS measurements has been carried out on doped sodium alanate, using titanium tetrabutylate (Ti(OBun)4), colloidal titanium nanoparticles (Ti*) or TiCl3 as doping agents. It was found that the dehydrogenated wet-chemically Ti(OBun)4 doped NaAlH4 consists of a crystalline Al and an amorphous NaH phase. The striking result of EDX analyses is that in each case the Ti-dopant is found to be present only in the Al phase. On the other hand, dehydrogenated NaAlH4 doped with Ti* or TiCl3via ball milling is an amorphous material, with diffuse boundaries between Al and NaH phases and a highly uniform distribution of titanium in the Al-phase. Both the use of the ball milling doping method and of Ti* nanoparticles as dopants, are probable reasons for the outstanding kinetics of the Ti* doped material (B. Bogdanović, M. Felderhoff, S. Kaskel, A. Pommerin, K. Schlichte and F. Schüth, Adv. Mater., 2003, 15, 1012; M. Fichtner, O. Fuhr, O. Kircher and J. Röthe, Nanotechnology, 2003, 14, 778). The course of hydrogen dis- and recharging reactions is discussed in light of the present TEM-EDX and the preceding XRD investigations of storage materials. Ti-EXAFS and XANES spectra of Ti-doped NaAlH4 after the doping, after several, and after 100 de- and rehydrogenation cycles are very close to each other. Apparently, after the Ti-doping, a zerovalent Ti species is formed which remains almost unchanged in the course of the cycle test. The XAFS spectra indicate that Ti is largely atomically dispersed in the Al phase, i.e. forms an Al–Ti alloy, thus confirming the earlier hypothesis (V. P. Balema, J. W. Wiench, K. W. M. Dennis, M. Pruski and V. K. Pecharsky, J. Alloys Compd., 2001, 329, 108; E. H. Majzoub and K. J. Gross, J. Alloys Compd., 2003, 356–357, 363) on this matter.