Microscopic Study of TiF3 as Hydrogen Storage Catalyst for MgH2

To understand the catalytic influence of TiF 3 on the de- and rehydrogenation of ball milled MgH 2 –TiF 3, a detailed study has been performed of the different crystallographic phases of the de- and rehydrogenated materials. Rietveld-refined XRD analysis of the dehydrogenated material indicates the presence of a rutile, nonstoichiometric MgF 2– x phase and a hexagonal Mg 1– y Ti y F z phase. After rehydrogenation three different fluorine environments were found by 19 F NMR, which can be attributed to MgF 2– x, MgF 2– x H x, and MgF 2– x dispersed with MgO nanodomains. The remarkable MgF 2– x H x phase, which is apparently formed from the nonstoichiometric MgF 2– x phase in the dehydrogenated material, may indicate a diffusion pathway of hydrogen within Mg/MgH 2– x . Furthermore, the previously found nonstoichiometric MgH 2– x phase appears to be metastable as is observed by XRD analysis after long-term storage. TEM with EDX measurements performed for both dehydrogenated and rehydrogenated samples provide information that the nanoparticles are highly crystalline and that the catalyst is homogenously distributed over the sample at a 100 nm length scale. More locally, the fluorine-containing phase is observed mainly at the surface of the Mg and MgH 2 particles and the Ti-containing species are present throughout the particles, likely as TiH 2 inclusions in the Mg structure and/or as Mg–Ti–F compound. Additionally, an about 2–5.5 nm MgO layer covers the Mg aggregates. A model of the hydrogen uptake and sample morphology is proposed.