A Study On Hydrogen Storage Properties Of La₂Mg₁₆Ni Modified By Ball Milling And Dynamic Process Of Benzene Hydrogenation Reaction In Liquid Phase Catalyzed By La₂Mg₁₆Ni Hydride

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 and good organic compounds particularly the hydrocarbons will be the candidate. The hydriding/ dehydriding properties of La₂Mg₁6Ni alloy prepared by ball milling in different environments have been investigated. The kinetic model of liquid-phase hydrogenation of benzene is developed.The alloys prepared in organic liquid media have superior absorption/ desorption behaviors on capacities and kinetics than those milled under argon. At 623 K, the absorption and desorption hydrogen capacities are 4.2 wt.%, 4.0 wt.% for the alloys milled in organic liquid and 3.63 wt.%, 3.3 wt.% for the alloy milled under argon, respectively. Hydrogen is fully desorbed within 200 s for the alloy milled in organic solvents, but for the alloy milled under argon, 300 s was needed for full desorption of hydrogen. The sample milled in benzene is more easily activated than the others. The initial absorption can reach a hydrogen capacity of 3.60 wt.% at 473 K and 3.0 MPa. After 3 times hydriding-dehydriding cycles, the alloy can be completely activated. The improvement of the activity properties is probably due to a combination of two effects: the oxide nuclei served as nucleation centers and the formation of electron donor-acceptor (EDA) complexes. Hydrogen storage properties was improved when the milling time is prolonged. However, after activation, similar hydriding/ dehydriding properties of the samples milled in organic solvents are observed. We believe that similar surface activity and microstructure may all lead to the same hydrogen storage properties. It is of interest to note that La₂Mg₁6Ni alloy milled in organic solvents have both improved kinetic and thermodynamic properties. The possible explanations of the increasing entropy and enthalpy of the alloys milled in organic solvents may be due to the dependence of enthalpy on the hydrogen concentration and a higher binding energy of hydrogen in the milled alloy.The samples milled in organic solvents show a strong tendency to amorphization than that milled under argon. By X-ray diffraction (XRD) and scanning electron microscope (SEM) examination, we believe that the weakened and broadened diffraction peaks are not just because of particle size reduction; lattice deformation and micro-stress may also play a major role. More lattice deformations and structural defects occurred due to the increase of the balling time. Morphology differences between the samples prepared in different environments were exhibited by SEM. The particles milled in liquid media are smaller and more uniform in size (about 11 um). The shape of the particle is spherical and ellipsoidal for the sample milled under argon and in benzene, respectively. The particle of the sample milled in ethanol are angular and irregular in shape. The concentration changes of the elements on surface wereexamined with Auger electron spectroscopy (AES). Slight changes or none of elemental concentration are found in outmost surface region for the sample milled in ethanol and under argon. While drastic changes in elemental concentration are observed for the sample milled in benzene after 10 min sputtering with Argon ion. Enrichment of magnesium and depletion of lanthanum and nickel were found for the alloy milled in benzene. The chemical states of the component elements were revealed by X-ray photoelectron spectroscopy spectra (XPS). Depth profiling shows that the elements presented in the very surface regions are all largely in the hydroxide state. The sample milled under argon has the lowest level of oxidization. Metallic La appears at the surface after sputtering away 150 nm in depth. While the one milled in ethanol is oxidized throughout the entire sputtered depth (150 nm).The possibility of La₂Mg₁6Ni alloy hydride as a catalyst for benzene hydrogenation reaction and the effects of the reaction conditions in connection with the cataly...