The archetypical example NaAlH4 decomposes (and reabsorbs) hydrogen in two distinct reaction steps as follows:

3NaAlH4 $ Na3AlH6 + 2Al + 3H2

3.7 wt.%, AH = 38 kJmoP1 (8Л)

Na3AlH6 $ 3NaH + Al + 1.5 H2

3 6 2 (8.2)

1.7 wt.%, AH = 47 kJmoP1

Additives such as TiCl3, ScCl3, or CeCl3 enable nearly complete reversible conversion and rehydrogenation at 373 K and 100 bar H2. Undoped NaAlH4, on the other hand, exhibits negligible reversibility under these conditions. The function of the catalyst is still being debated, and while numerous different materials have been identified to be able to catalyze these reactions [4—7], a unified picture for their activity is still lacking.

Neutron powder diffraction measurements have identified the structure of NaAlD4 as body-centred tetragonal I41/a (NaAlH4) [8]. The intermediate decomposition product, Na3AlH6 occurs in two modifications, i. e. low temperature monoclinic a-Na3AlH6 (P21/n) [9, 10], and P-Na3AlH6 with a cubic structure (Fm 3m) [11] is observed above 525 K. The reversible hydrogen exchange reaction is greatly facil­itated by Ti additives and numerous attempts have been made to localize the additive and to unravel its catalytic mode of operation. Among the suggested mechanism are: (a) Ti acts as a surface catalyst that facilitates the splitting of the AlH4/AlH3 bonds [12, 13], (b) Ti initiates vacancy formation and hence promotes H diffusion [11, 14], (c) Ti weakens the Al-H bond [15], or (d) Ti acts as a grain refiner [16]. While neutron powder diffraction measurements did not show any indication of a solid solution with Ti-based additives directly after ball milling, the formation of Al1-xTix species were observed upon cycling [17, 18]. Synchrotron X-ray diffraction studies of Ti-doped NaAlH4 suggested that Ti could substitute in Al-sites [19] while density functional theory (DFT) calculations show that both the substitution of Na or Al in the NaAlH4 structure should be possible in the bulk [20-23] or small clusters [24]. The influence of Ti-doping on the native vacancy formation was also investigated by DFT methods. Ti-doping in NaAlH4 can yield the formation of hydrogen vacancies and interstitials [25]. In contrast, the same paper suggests that in Ti-doped LiBH4 only a reorientation of the BH4 units occurs. Since structural characterization of the Ti sites proved to be difficult, the localization of the additive and/or its induced vacancy diffusion mech­anisms was studied by neutron spectroscopy techniques. QENS data on Ti-doped NaAlH4 at temperatures below 350 K indicate that in both NaAlH4 and Na3AlH6, the hydrogen mobility is vacancy mediated [26, 27] but the relative amount of mobile hydrogen at these temperatures is less than 1 % in NaAlH4 even at 390 K and there is no significant difference on the bulk diffusion induced by TiCl3 additions. Similarly, neutron vibrational spectroscopy showed no distinct differences for pure NaAlH4 and Ti-doped NaAlH4 [24, 28] while significant changes of the vibrational density-of- states were observed after thermal treatment of both NaAlH4 and Na3AlH6 [29]. These were attributed to the presence of different ionic species resulting from the partial decomposition of the samples. Neutron spectroscopy data (Fig. 8.2) suggest the formation of AlH3 and its oligomers (AlH3)n [28] during reabsorption of the H depleted mixture NaH/Al (after 0.5 h at 140 bar H2, 403 K).

Here, the measured inelastic neutron scattering (INS) spectra are compared with calculated spectra of AlH3 and of Al4H12. Volatile species of this kind had been suggested to be an intermediate species responsible for mass-transfer processes during the hydrogen exchange reaction [30] and neutron spectroscopy is a unique tool to detect these intermediate species.