pointed out that the Ti(OBu)\({}_{4}\)/Zr(OP)\({}_{4}\) alkoxide mechanical doped materials have low reversible capacities and release significant levels of hydrocarbon impurities [13]. Many types of Ti compounds, such as TiCl\({}_{4}\), TiN, TiF\({}_{3}\), TiH\({}_{2}\), and TiO\({}_{2}\), have shown this effect on some level [14; 15; 16; 17; 18; 19; 20; 21; 22]; however, TiCl\({}_{3}\) was found to show promising results [11]. The following reaction is assumed to proceed when TiCl\({}_{3}\) is added to NaAlH\({}_{4}\):  
\[\text{NaAlH}_{4}+\tau\text{TiCl}_{3}\ \rightarrow\ (1-3x)\text{NaAlH}_{4}+3x \text{NaCl}+3x\text{Al}+x\text{Ti}+6x\text{H}_{2}\ (x\ <\ 1/3) \tag{4}\]  
During this reaction, Al and Ti may form a stable Al-Ti alloy phase. The position of Ti in the doped material and the oxidation state of the Ti after doping remain highly controversial despite many studies and discussions so far [18]. Some computational works insist that Ti atoms or ions generally favor substitution over interstitial sites [23; 24], although no conclusion has been reached. This is attributed to the fact that the by-products vary depending on the species of the additives, and the phases involved in the disproportionation reaction are complicated. On the other hand, a common observation is the formation of an Al-Ti alloy after the hydrogen desorption/absorption cycles [25; 26; 27; 28; 29], and Ti compounds are not detected in the diffraction measurements immediately after the addition [25; 26; 30; 31]. It is suggested that the formation of TiAl\({}_{3}\) upon doping with TiCl\({}_{3}\) contributes to enhanced kinetics [32]. In fact, nanocrystalline Ti\({}_{3}\)Al with a longer milling time exhibits a catalytic function in the hydrogen re-absorption/desorption reaction of NaAlH\({}_{4}\)[33]. However, the direct addition of TiAl\({}_{3}\) or Ti\({}_{3}\)Al alloys to NaAlH\({}_{4}\) does not yield effective catalysis [14], and Ti-doped NaAlH\({}_{4}\) after 100 cycles includes additional phases that are apparently linked to decreased hydrogen desorption kinetics and total amount of released hydrogen [34]. These results imply that the microstructure of the Ti compound significantly contributes to the catalytic effect on NaAlH\({}_{4}\). A structural analysis of Ti(Obu)\({}_{4}\)-doped NaAlH\({}_{4}\) was performed using powder X-ray diffraction [25]. No phases other than NaAlH\({}_{4}\) were detected immediately after the addition, although NaAlH\({}_{4}\), Na\({}_{3}\)AlH\({}_{6}\), Al, NaH, and a small amount of unidentified phases were observed after hydrogen storage cycles. A shoulder on the high scattering angle side of the main Al diffraction peak (111) was noticed, the magnitude of which increased with the amount of the applied catalyst and which could be tentatively assigned to the Al-Ti alloy. The composition of the Al-Ti phase was estimated to be Al\({}_{0.85}\)Ti\({}_{0.15}\)[35] or Al\({}_{0.5}\)Ti\({}_{0.5}\)[36]. The formation of this phase may explain the reduction in capacity beyond the theoretical reduction from the dead weight of the additive and the reaction of Na\({}_{3}\)AlH\({}_{6}\) with Al and hydrogen to NaAlH\({}_{4}\)[25]. In addition, an extended X-ray absorption fine structure (EXAFS) analysis indicated the formation of TiAl\({}_{x}\) nanoparticles such as TiAl\({}_{3}\)[37; 38]. The possibility of Ti substitution into the NaAlH\({}_{4}\) host has been reported [30; 39], and it is implied that Ti is substituted at the Al site in NaAlH\({}_{4}\)[31; 40] or in Na\({}_{3}\)AlH\({}_{6}\)[41]. However, the solubility of Ti in NaAlH\({}_{4}\) is sometimes denied because of the lack of lattice parameter changes in the Rietveld refinement result [28].