Effects of Different Ti‐compounds on the Reversibility of NaAlH 4

SUMMARY TiCl 3 has been considered as the best catalyst for the hydrogen desorption/re‐absorption of NaAlH 4 in terms of kinetic enhancement. However, the formation of NaCl as a by‐product leads to the decrease in the reversible hydrogen capacity of NaAlH 4 . In this work, TiO 2 and metallic Ti were selected as catalysts for the reaction to avoid the formation of the by‐product. The comparison of the catalytic activity of Ti, TiCl 3 , TiO 2 and Ti(OBu) 4 on the hydrogen desorption/absorption NaAlH 4 were carried out. It was found that TiO 2 doped NaAlH 4 exhibits similar behavior as TiCl 3 doped NaAlH 4 with the reversible hydrogen capacity about 3.8 wt% (H/M). In addition, TiO 2 doped NaAlH 4 exhibits the superior hydrogen re‐absorption rate to the one doped with TiCl 3 . That may be due to the Ti 3+ defect sites on the surface of TiO 2 would facilitate the hydrogen dissociation. Moreover, high surface area of TiO 2 prevents the segregation and the morphological change of the desorbed substances (NaH and Al). This benefits to the mass transfer into the hydride system. However, doping with TiO 2 also produces sodium oxide and hydroxide as by‐products. Unexpectedly, metallic Ti doped NaAlH 4 shows the lowest hydrogen desorption/re‐absorption among the tested samples. Its hydrogen reversible capacity is around 1 wt% (H/M). The formation of TiH x (1 < x < 2) was detected in the sample after the hydrogen desorption/reabsorption. Copyright © 2011 John Wiley & Sons, Ltd.