Microscopic study of TiF3 as hydrogen storage catalyst for MgH2

To understand the catalytic influence of TiF3 on the de- and rehydrogenation of ball milled MgH2–TiF3, 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 MgF2–x phase and a hexagonal Mg1–yTiyFz phase. After rehydrogenation three different fluorine environments were found by 19F NMR, which can be attributed to MgF2–x, MgF2–xHx, and MgF2–x dispersed with MgO nanodomains. The remarkable MgF2–xHx phase, which is apparently formed from the nonstoichiometric MgF2–x phase in the dehydrogenated material, may indicate a diffusion pathway of hydrogen within Mg/MgH2–x. Furthermore, the previously found nonstoichiometric MgH2–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 MgH2 particles and the Ti-containing species are present throughout the particles, likely as TiH2 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.