Critical limitations in applying MgH₂ as a hydrogen-storage medium include the high H₂ desorption temperature and slow reaction kinetics. In this study, we synthesized hierarchical porous TiNb₂O₇ spheres in micrometer scale built with 20-50 nm nanospheres, which showed stable activity to catalyze hydrogen storage in MgH₂ as precursors. The addition of 7 wt.% TiNb₂O₇ in MgH₂ reduced the dehydrogenation onset temperature from 300 to 177 °C. At 250 °C, approximately 5.5 wt.% H₂ was rapidly released in 10 min. Hydrogen uptake was detected even at room temperature under 50 bar hydrogen; 4.5 wt.% H₂ was absorbed in 3 min at 150 °C, exhibiting a superior low-temperature hydrogenation performance. Moreover, nearly constant capacity was observed from the second cycle onward, demonstrating stable cyclability. During the ball milling and initial de/hydrogenation process, the high-valent Ti and Nb of TiNb₂O₇ were reduced to the lower-valent species or even zero-valent metal, which in situ created multivalent multielement catalytic surroundings. A strong synergistic effect was obtained for hybrid oxides of Nb and Ti by density functional theory (DFT) calculations, which largely weakens the Mg-H bonding and results in a large reduction in kinetic barriers for hydrogen storage reactions of MgH₂. Our findings may guide the further design and development of high-performance complex catalysts for the reversible hydrogen storage of hydrides.

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 ℃ (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 ℃; this is 65 ℃ lower than that of the pristine sample. The dehydrogenated sample is completely hydrogenated at 115 ℃ 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.