Improving dehydrogenation properties of Mg/Nb composite films via tuning Nb distributions
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Borohydride
Magnesium hydride
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Abstract Dehydrogenation plays a very important role in both nature and human civilization. In chemical industry, dehydrogenations are used to produce propene, butene, butadiene, isobutene, and isopropene from the corresponding alkanes. In living organisms (both animals and plants), respiration is actually a process of oxidation wherein some steps involve dehydrogenation. Almost all dehydrogenation reactions require a catalyst. Catalysts for dehydrogenation can be classified into two main categories: conventional catalysts (including inorganic and organic) and enzymes. This article focuses on the application of biological catalysts in dehydrogenation and oxidation reactions occurring in nature. Biological dehydrogenation is illustrated from two aspects: chemistry of biocatalytic dehydrogenation and biocatalysts of dehydrogenation. Biological dehydrogenation reactions usually occur at very mild conditions and have very high selectivity. The catalysts for these processes are usually enzymes (or cells producing these enzymes). Enzymes having dehydrogenation capacities are usually dehydrogenases, oxidases, etc., and most of them need a coenzyme or a cofactor to work with them.
Propene
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Among metal hydrides, magnesium hydride (MgH2) would be a promising candidate for hydrogen storage materials because of its high hydrogen-storage capacity (7.6 mass%). However, the dehydrogenation temperature is too high for wide practical applications. Novel magnesium-based hydrides with better dehydrogenation properties than MgH2 can be prepared under hydrogen pressure of gigapascal (GPa). For instance, Mg7TiH16 (6.9 mass%) would release hydrogen at a lower temperature by 130 K than MgH2. The Mg-H ionic bonding distance in Mg7TiH16 is longer than that in MgH2. The observed lower dehydrogenation temperature seems to be consistent with the structure.
Magnesium hydride
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Potassium hydride (KH) was directly added to a Mg(NH2)2–2LiH system to improve the hydrogen storage properties; the corresponding mechanisms were elucidated. The Mg(NH2)2–2LiH–0.08KH composite displays optimized hydrogen-storage properties, reversibly storing approximately 5.2 wt% hydrogen through a two-stage reaction and a dehydrogenation onset at 70 °C. The 0.08KH-added sample fully dehydrogenated at 130 °C begins to absorb hydrogen at 50 °C, and takes up approximately 5.1 wt% of hydrogen at 140 °C. Adding KH significantly enhances the de-/hydrogenation kinetic properties; however, an overly rapid hydrogenation rate enlarges the particle size and raises the dehydrogenation temperature. A cycling evaluation reveals that the KH-added Mg(NH2)2–2LiH system possesses good reversible hydrogen storage abilities, although the operational temperatures for de-/hydrogenation increase during cycling. Detailed mechanistic investigations indicate that adding KH catalytically decreases the activation energy of the first dehydrogenation step and reduces the enthalpy of desorption during the second dehydrogenation step as a reactant, significantly improving the hydrogen storage properties of Mg(NH2)2–2LiH.
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Abstract This study investigated non-oxidative propane dehydrogenation over TiH2. It was found that H2 co-feeding positively affected dehydrogenation, improving the propylene formation rate. In situ spectroscopic characterization of TiH2 in the presence of H2 indicated that partially dehydrogenated titanium hydrides are active for dehydrogenation.
Propane
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