Dehydrogenation reactions of 2NaBH_4 + MgH_2 system

摘要

Reactive Hydride Composites (RHCs), ball-milled composites of two or more different hydrides, are suggested as an alternative for solid state hydrogen storage. In this work, dehydrogenation of 2NaBH_4 + MgH_2 system under vacuum was investigated using complementary characterization techniques. At first, thermal programmed desorption of as-milled composite and single compounds was used to identify the temperature range of hydrogen release. RHC samples annealed at various temperatures up to 500 ℃ were characterized by X-ray diffraction, infrared spectroscopy and scanning electron microscopy. It was found that the dehydrogenation reaction under vacuum is likely to proceed as follows: 2NaBH_4 + MgH_2 (>250 ℃) → 2NaBH_4 + l/2MgH_2 + l/2Mg + 1/2H2 (>350 ℃) (←→) 3/2NaBH_4 + l/4MgB_2 + l/2NaH + 3/4Mg + 7/4H2 (>450 ℃) → 2Na + B + l/2Mg + l/2MgB_2 + 5H_2. In addition, presence of NaMgH_3 phase suggests the occurrence of secondary reactions.