Preliminary investigation on the catalytic mechanism of TiF₃ additive in MgH₂–TiF₃ H-storage system

A combined structure/property investigation is performed to understand the catalytic effect of TiF₃ additive on the absorption/desorption reactions of MgH₂. It was found that both TiH₂ and MgF₂ phases identified by x-ray diffraction cannot explain the observed kinetic enhancement in the MgH₂–TiF₃ system, whether they are incorporated in a direct or an in situ manner. In combination with the comparative investigation on the catalytic activity of TiF₃ and its analog TiCl₃, as well as the samples milled under inert and reactive atmospheres, we propose that the catalytically active species is a multicomponent metastable phase composed of host Mg, transition metal Ti, and F anion, the catalytic activity of which is dependent on its interaction with the surrounding chemical environment.