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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 > TiH0.71 > TiH2 > TiO2. This understanding guides further improvement in hydrogen storage properties of metal alanates using nanocrystalline transition metal-based additives.
We gratefully acknowledge the financial support received from the National Natural Science Foundation of China (Nos. 51222101, 51171170, and 51025102), the Ministry of Science and Technology of China (No. 2010CB631304), the Research Fund for the Doctoral Program of Higher Education of China (Nos. 20130101110080 and 20130101130007), the Program for Innovative Research Teams in Universities of the Ministry of Education of China (No. IRT13037), and the Fundamental Research Funds for the Central Universities (No. 2014XZZX003-08).