| Due to serious problems caused by energy crisis and environmental pollution, much attention is focused on developing clean renewable energy. Hydrogen, for its renewable and pollution free characteristics, has become the most potential energy carrier. An important issue for the utilization of hydrogen energy is the hydrogen storage and transportation. In the solid hydrogen storage materials, alkali metals and alkaline earth metals-nitrogen-hydrogen (metal-N-H) systems possess quite high hydrogen storage capacity and good reversibility so that they are viewed as one of the most promising hydrogen storage materials. In this study, we carried out a systematic study of LiNH2 and Mg(NH2)2 prepared by ball milling, and TG analysis were employed to investigate thermo-decomposition of the two amides. Additionally, hydrogen storage properties of Mg(NH2)2-LiH system were also investigated. the results showed that:(1) When LiNH2 was synthesized by ball milling, with increasing the milling tank NH3 pressure and extending the ball milling time, the relative purity of ball milling product LiNH2 is increased. The optimization technique of synthesis Lithium amide is in condition of 0.3Mpa milling tank NH3 pressure,2h milling time. FTIR analysis showed that infrared absorption wave numbers of LiNH2 two N-H bonds are 3259cm-1 and 3312 cm-1 respectively, and no LiH but LiNH2 diffraction peak is detected in final synthesis product. At last, initial decomposition temperature of LiNH2 is within the limits of 340-370℃, and the apparent activation energy of the decomposition reaction is calculated and the value is about 75 kJ/mol;(2) When Mg(NH2)2 was prepared by two step method (ball milling then post heat-treatment), NH3 pressure, temperature and heating time in the post heat-treatment have not obvious influence on the synthesis of magnesium amide, but ball milling time has a obvious influence on the final product amide. The optimization technique of synthesis magnesium amide is in condition of 5atm primal milling NH3 pressure,8h primal milling time, and in 3atm NH3 pressure at 300oC for 3h post heat-treatment to crystallization. FTIR analysis showed that infrared absorption wave numbers of Mg(NH2)2 two N-H bonds are 3274cm-1and 3700 cm-1 respectively and this is consistent with XRD results. Finally, initial decomposition temperature of the magnesium amide is within the limits of 350-360℃.(3) Hydrogen storage performance of Mg(NH2)2-LiH (mol ratio 1:2.2) materials system was studied by an equipment of PCT(pressure-composition-Temperature).The results showed that initial dehydrogenation temperature of Mg(NH2)2-LiH Hydrogen Storage system is at about 150℃and its reversible hydrogen storage capacity is 4.6wt.% at above 200℃. Dehydrogenation equilibrium pressure of the hydrogen storage system is increasing with the increase of temperature. Equilibrium pressures and system compositions at different temperature meet the Van't Hoff equation, therefore, enthalpy and entropy of the dehydrogenation reaction could be calculated based on the Van't Hoff equation, and their value areΔH=42.8kJ/molH2,ΔS=149.2JK-1/molH2, and Gibbs free energyΔG= (42800-149.2×T) J/molH2 respectively.(4) Kinetics of hydrogen absorption and desorption showed that the Mg(NH2)2-LiH hydrogen storage system possess relatively fast kinetics, especially in dehydrogenation process desorption can be completed within 1 hour at above 200℃, and XRD result showed that hydrogenation products is Li2Mg(NH)2. Based on principle of kinetics, the apparent activation energy of the dehydrogenation reaction can be calculated by kinetics data of hydrogen desorption, and its value is 51.7kJ/mol.
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