A Study On Modification Of Mg-based Alloys And Hydrogenation/Dehydrogenation Properties Of The Organic Liquid Catalyzed By Mg-based Alloys

Hydrogen energy is thought to be one of the most promising substitutes of fossil fuel, and pursuit the compact, lightweight, cost-effective and safe means of storing and transporting hydrogen is always the target of researchers. The technology of storing hydrogen in a slurry system is a new type hydrogen storage system, in which the hydrogen storage alloy not only store hydrogen also act as the catalyst for hydrogenation/dehydrogenation of the organic liquid. The aim of the present investigation is to develop a liquid-solid two-phase hydrogen storage system with a high hydrogen storage capacity and a good kinetic property. Hence good hydrogen storage alloys, Magnesium-based alloys, and good organic compounds such as benzene/cyclohexane will be the better candidate. The hydriding/dehydriding properties of La_1.8Ca_0.2Mg₁₄Ni₃ alloy prepared by ball-milling in different environments and fluorizated with different concentration of NH4F aqueous respectively have been investigated. Then the hydrogenation/dehydrogenation properties of organic liquid catalyzed by the modified alloys were investigated. Meanwhile the hydrogenation properties of the slurry composed of Mg2Nio.95Sno.o5 alloy and benzene and the dehydrogenation properties of cyclohexane catalyzed by MINi₅ and Raney-Ni were also investigated.The alloy modified for 20h in THF has superior absorption/ desorption behaviors on hydrogen capacities and kinetics than those milled the same times under argon and in toluene, respectively. Though the F-treatment can also improve the hydrogen storage properties of the alloy, its hydrogen capacity is lower than that of modified by ball-milling for 20h. As for the alloy milled in THF for 20h, the hydrogen capacity in the initial time is 4.62 wt. % at 473 K, and after an activation treatment the alloy can absorb 5.1 wt. % hydrogen within 95 s at 600 K and desorb 5.05 wt. % hydrogen at 613 K. Even the alloy milled in THF for 10h shows a better improve of hydrogen storage properties.The X-ray diffraction (XRD) examination show that a broadening of diffraction peaks is clearly observed for all the ball-milled samples and the intensity of the peaks decreases dramatically. The diffraction intensity of the alloy milled 20 h in THF is the weakest. Meanwhile, a clearly amorphorized trend of alloys to some extent is observed after milling. The scanning electron microscope (SEM) examination show that ball-milling in different environments made the morphologies of the La_1.8Ca_0.2Mg₁₄Ni₃ alloy quite different. The particles of the alloy ball-milled under argon are separate each other, and the average particle size became smaller than that of the as-cast alloy, while the particles of the alloy that milled in the liquid media is easy to assemble together especially that milled in toluene. The particle size of the sample milled under argon is quite un-even, some became very smaller, but some seemed to assemble and came into being a group of particles, looking like so many little particles clinging to the larger one, and the particle shape is spherical. As for the alloy ball milled in toluene, the average particle size is much smaller than that milled under argon, and the shape of the particle is irregular but the trend to assembletogether is more distinct than that milled in other environments. The particle size of the alloy ball milled in THF for 20h is the smallest in all the alloys, and the shape of the particle is irregular, too. It indicates that all of the three methods can improve the hydrogen storage properties. It may be interpreted from the microstructure and chemical change of the alloy. Ball-milling can present the active sites. Besides, the electron donor-acceptor (EDA) complex was formed by charge-transfer between La_1.8Ca_0.2Mg₁₄Ni₃ alloy and the organic liquid when ball milled in toluene and THF. The existing of this complex can bring many sensitive surfaces, which are favorable for the conversion of hydrogen molecule to hydrogen atom and the nucleation of hydride. As for the F-treatment alloys, a new phase, MgF₂ is formed on its surface. And the concentration of the NH4F directly determines the structure and composition of the surface layer on the alloy particles. And this specific surface structure is capable of providing more channels for the diffusion of hydrogen from the surface into bulk of the sample.The hydrogen absorption measurements of the slurry formed by mixing benzene and Mg2Nio.95Sno.o5 and metal hydride respectively show that the metal hydride has better catalysis for benzene to cyclohexane. And the same phenomenon appears in the hydrogen absorption reaction of the slurry formed by modified La_1.8Ca_0.2Mg₁₄Ni₃ alloys and its hydride. The slurry composed of La_1.8Ca_0.2Mg₁₄Ni₃ hydride can absorb 6.61 wt. % hydrogen, and the conversion rate of benzene to cyclohexane in the liquid product is 99.7%. Its apparent activation energy is 26.3 KJ/mol.The dehydrogenation of cyclohexane catalyzed by Raney-Ni and MINi₅ was firstly investigated in "wet-dry multiphase" model. Then the dehydrogenation of cyclohexane catalyzed by modified La_1.8Ca_0.2Mg₁₄Ni₃ alloy and its hydride were investigated in this model, besides the closed-system high pressure reactor model, the inner-installing palladium tube in the high pressure reactor model and the external-connection circulating pump model were utilized for dehydrogenation of cyclohexane in turn. Among these different models, the first and the fourth are in favor of dehydrogenation of cyclohexane. But both have its advantages and disadvantages, e.g. for the first model, the conversion rate of cyclohexane to benzene is higher but the amount of catalysts used compared to cyclohexane injected is too large. While for the fourth model, the conversion rate is lower but the amount of cyclohexane injected is much larger. When the La_1.8Ca_0.2Mg₁₄Ni₃ alloy modified by ball-milling in THF for 20h act as catalyst, the conversion rate for the four reaction models in turn is 64%, 19.8%, 33.1% and 42.1%.