Effects of Different Ti-compounds on the Reversibility of NaAlH_4

摘要

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 byproduct. 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 3.8wt (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