The role of Ti in alanates and borohydrides: Catalysis and metathesis

Ti catalyzes the hydrogen sorption reaction in alanates and allows the measurement of pressure composition isotherms, that is, the reaction equilibrates at each point of the isotherm. Although some effects of Ti compounds addition to borohydrides have been shown, our measurements show that the hydrogen desorption reaction from borohydrides is not catalyzed by Ti, when the system is exposed to a gas flow. The reabsorption of hydrogen by the products of the desorption reaction requires high temperatures and high pressures. Furthermore, the reaction pathway for the hydrogen desorption is different from the absorption one and as a consequence the borohydrides do not equilibrate with the gas phase during the hydrogen sorption reactions. Ti in borohydrides leads to the formation of stable and volatile Ti-containing species, for example, (Ti(BH₄)₃). This is a metathesis reaction, that is, a bimolecular process involving the exchange of bonds between the two reacting chemical species. In this paper, we have investigated via spectroscopy measurements the hydrogen sorption reactions of NaAlH₄ and LiBH₄ with Ti catalyst in view of the changes in the solid phase as well as in the gas phase.