## 5 Conclusions  
The irreversible disproportionation (decomposition/recombination) reaction hidden in the reversible hydrogen desorption/absorption reaction of NaAlD(H)\({}_{4}\)-TiCl\({}_{3}\) was determined using pressure-composition isotherms, neutron/X-ray diffraction, anomalous X-ray scattering, X-ray absorption fine structure measurements, and DFT calculations. The reversible hydrogen desorption/re-absorption reactions of mechanically milled NaAlD\({}_{4}\) doped with 0.02 TiCl\({}_{3}\) were confirmed to be similar to those of the NaAlH\({}_{4}\)-TiCl\({}_{3}\) system. It is suggested that Ti is substituted at the Al sites in NaAlD(H)\({}_{4}\) before the first hydrogen desorption and that the amount of substituted Ti decreases after hydrogen re-absorption, as confirmed by the refinement of the neutron/X-ray diffraction profiles. The anomalous X-ray scattering results revealed that 1.3 \(\pm\) 0.3% of Ti was present at the Al site in NaAlH\({}_{4}\) before hydrogen desorption. Ti K-edge X-ray absorption fine structure curves indicated that mixed phases of Ti-substituted NaAlH\({}_{4}\) compound and Al-Ti alloys irreversibly changed to Al-Ti alloy, working as a catalyst for the reversible hydrogen desorption/absorption reaction. Highly dispersed Al-Ti nanoparticles prepared through the first decomposition of Ti-doped NaAlH\({}_{4}\) can possibly facilitate hydrogen storage. Therefore, the formation of Ti-substituted NaAlH\({}_{4}\) is an essential process for enhancing the catalytic effects of the Ti additives. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app11188349/s1](https://www.mdpi.com/article/10.3390/app11188349/s1)]([https://www.mdpi.com/article/10.3390/app11188349/s1](https://www.mdpi.com/article/10.3390/app11188349/s1)), Figure S1: X-ray powder diffraction profiles of NaAlD\({}_{4}\)-0.02 TiCl\({}_{3}\) before the desorption and after the re-absorption of deuterium. Figure S2: (a) Absorption edge energy and (b) relationship between the edge shift and the formal oxidation number of standard samples(Ti: 0, TiO: 2, Ti\({}_{2}\)O\({}_{3}\): 3, TiO\({}_{2}\): 4). Figure S3: Diffraction peaks of NaAlH\({}_{4}\)-0.02 TiCl\({}_{3}\) measured by using incident X-ray energies of (a) 4.6645 and (b) 4.9395 keV with background by linear function interpolation and integration range of \(d\) = 4.55 \(\pm\) 0.07 A and 3.005 \(\pm\) 0.035 A for 101 and 112 of each Bragg peak, respectively. Table S1: Oxidation numbers of Ti in NaAlH\({}_{4}\)-xTiCl\({}_{3}\) (\(x\) = 0.005, 0.010, 0.015, 0.020, 0.040, and 0.060) before the desorption and after the re-absorption of hydrogen with standard samples of Ti, TiO, Ti\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), TiH\({}_{2}\), TiCl\({}_{3}\), TiAl, Ti\({}_{3}\)Al, and TiAl\({}_{3}\). Conceptualization, K.I. and T.O. ; methodology, S.T. ; investigation, F.F., H.O., T.H., T.K., H.A. ([PERSON]), K.S. ([PERSON]), H.A. ([PERSON]), H.K., K.S. ([PERSON]), Y.N., A.M. and T.S. ; writing--original draft preparation, K.I. ; writing--review and editing, T.O. and S.-i.O. ; supervision, T.O. ; funding acquisition, K.I. and T.O. All authors have read and agreed to the published version of the manuscript. This work was partially supported by JSPS KAKENHI (Grant Numbers 23686101, 24241034, 15K13810, 19K12650, JP18H05518 (\"Hydrogenomics\"), grants from the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University (Proposal Number 15K0110). We acknowledge [PERSON] for his help in the anomalous X-ray scattering measurement. Synchrotron radiation X-ray diffraction experiments were performed using BL22 XU at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2014B3784) and under the Shared Use Program supported by JAEA Advanced Characterization Nanotechnology Platform under the remit of \"Nanotechnology Platform\" of MEXT Japan (Proposal No. JPMXP09A14 AE0042).