Chemical state of Ti atoms in NaAlH_4 doped with a Ti-based precursor by ball milling

摘要

X-ray absorption spectroscopy is used to investigate NaAlH_4 doped with 5 mol.% of either TiCl_3 or Ti_(13) · 6THF by ball milling. The Ti_(13) · 6THF is a highly dispersed Ti catalyst in the form of small colloidal Ti clusters. It consists of a few core Ti atoms, which are coordinated by O atoms from tetrahydrofuran (THF). XANES analysis shows that the TiCl_3 is reduced from Ti (+3) to Ti (0) whereas the valence state of the colloidal Ti does not change after the ball milling. Further reaction due to the release or the absorption of hydrogen does not affect the chemical state obtained after ball milling for TiCl_3. However, in the case of the colloidal Ti the transformation from the coordinated ('sub-oxide') to the metallic state is occurring during the hydrogenation reaction. EXAFS analysis shows that after ball milling a new local structure is formed with TiCl_3 whereas the local order around the Ti atoms is unchanged with Ti_(13) · 6THF. Subsequent reaction with hydrogen increases the order of the new structure around the Ti atoms with TiCl_3; in the case of the Ti clusters oxygen atoms are stripped off from the metallic core during the absorption reaction.