Effects of Different Ti-compounds on the Reversibility of NaAlH4

TiCl3 has been considered as the best catalyst for the hydrogen desorption/re-absorption of NaAlH4 in terms of kinetic enhancement. However, the formation of NaCl as a by-product leads to the decrease in the reversible hydrogen capacity of NaAlH4. In this work, TiO2 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, TiCl3, TiO2 and Ti(OBu)4 on the hydrogen desorption/absorption NaAlH4 were carried out. It was found that TiO2 doped NaAlH4 exhibits similar behavior as TiCl3 doped NaAlH4 with the reversible hydrogen capacity about 3.8wt% (H/M). In addition, TiO2 doped NaAlH4 exhibits the superior hydrogen re-absorption rate to the one doped with TiCl3. That may be due to the Ti3+ defect sites on the surface of TiO2 would facilitate the hydrogen dissociation. Moreover, high surface area of TiO2 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 TiO2 also produces sodium oxide and hydroxide as by-products. Unexpectedly, metallic Ti doped NaAlH4 shows the lowest hydrogen desorption/re-absorption among the tested samples. Its hydrogen reversible capacity is around 1wt% (H/M). The formation of TiHx (1<x<2) was detected in the sample after the hydrogen desorption/reabsorption. Copyright (c) 2011 John Wiley & Sons, Ltd.