Remarkably improved hydrogen storage properties of nanocrystalline TiO₂-modified NaAlH₄ and evolution of Ti-containing species during dehydrogenation/hydrogenation

Adding a small amount of nanocrystalline TiO₂@C (TiO₂ supported on nanoporous carbon) composite dramatically decreases the operating temperatures and improves the reaction kinetics for hydrogen storage in NaAlH₄. The nanocrystalline TiO₂@C composite synthesized at 900 °C (referred as TiO₂@C-900) exhibits superior catalytic activity to other catalyst-containing samples. The onset dehydrogenation temperature of the TiO₂@C-900-containing sample is lowered to 90 °C; this is 65 °C lower than that of the pristine sample. The dehydrogenated sample is completely hydrogenated at 115 °C and 100 bar of hydrogen pressure with a hydrogen capacity of 4.5 wt.%. Structural analyses reveal that the Ti undergoes a reduction process of Ti⁴⁺→Ti³⁺→Ti²⁺→Ti during the ball milling and heating processes, and further converts to Ti hydrides or forms Ti-Al species after rehydrogenation. The catalytic activities of Ti-based catalytic species decrease in the order Al-Ti-species > TiH_0.71 > TiH₂ > TiO₂. This understanding guides further improvement in hydrogen storage properties of metal alanates using nanocrystalline transition metal-based additives.