Synthesis Of Mg（AlH₄）₂and Hydrogen Storage Propetries Of Its Catalytic And Composite Systems

The development situation, classification and features of hydrogen storage materials werereviewed in this thesis, with emphasis on the research evolution of alkaline–earth metalaluminium hydride represented by Mg（AlH₄）₂. For the purpose to prompt the development ofMg（AlH₄）₂, some catalytic and composite systems based on Mg（AlH₄）₂were constructed, onthe basis of the study of Mg（AlH₄）₂preparation via mechanochemical method. The hydrogenstorage properties of Mg（AlH₄）₂and its catalytic and composite systems were characterized byPressure–Composition–Temperature （P–C–T） testing apparatus. The microstructure and de-/hydriding mechanism were analyzed by means of X–ray diffraction （XRD）, scanning electronmicroscope/energy dispersive X–ray spectrometer （SEM/EDX） and thermal analysis.Mg（AlH₄）₂was synthesized from Na（Li）AlH₄and halogen salts （MgCl₂and MgF₂） throughmechanochemical method. It is indicated that Mg（AlH₄）₂could be synthesized by ball millingthe2Na（Li）AlH₄+MgCl₂mixtures for25h. But the2Na（Li）AlH₄+MgF₂mixtures could not beused to synthesize Mg（AlH₄）₂even ball milling for duration of40h, which may be hindered bythe strong bond energy of MgF₂. On the basis of Mg（AlH₄）₂synthesis, the hydrogen storageproperties of Mg（AlH₄）₂modified by catalysts were investigated. It is found that the addition ofTi, MgF₂and Nb2O5could not improve the dehydriding property of Mg（AlH₄）₂. While TiF₃could decrease the dehydriding temperature of Mg（AlH₄）₂obviously. For instance, thedehydriding temperature of Mg（AlH₄）₂–4mol%TiF₃composite was decreased by40°C ascompared with the case without doping TiF₃.The de-/hydriding properties and mechanism of xMg（AlH₄）₂–LiNH₂（x=0.5,1and2）composites were studied. The results show that the dehydriding process of xMg（AlH₄）₂–LiNH₂（x=0.5,1and2） composites consists of two stages. Firstly, the compound Mg（AlH₄）₂dehydrogenated to form MgH₂and Al at140¹50°C, and then MgH₂reacted with LiNH₂at280°C. The actual hydrogen amounts desorbed for the xMg（AlH₄）₂–LiNH₂（x=0.5,1and2）composites are4.22,4.45and4.22wt.%, respectively. With the change of x value, thexMg（AlH₄）₂–LiNH₂（x=0.5,1and2） composites have different dehydriding products. For theMg（AlH₄）₂–2LiNH₂composite, the dehydriding product is composed of Mg₃N₂, Li2NH and Al.In comparison, the dehydriding product of Mg（AlH₄）₂–LiNH₂composite consists of Mg₃N₂,LiH and Al. Whereas, Al₃Mg₂exists in the the dehydriding product of2Mg（AlH₄）₂–LiNH₂composite. Li2Mg（NH2）2was not formed in the dehydriding products of xMg（AlH₄）₂–LiNH₂（x=0.5,1and2） composites.The Mg（AlH₄）₂–yLiBH₄（y=2,4and6） composites were prepared to investigate thedehydriding and hydriding properties. It is found that the hydrogen amounts desorbed at400°C for the Mg（AlH₄）₂–yLiBH₄（y=2,4and6） composites changes from6.50wt.%to8.69wt.%with increasing y value from2to6. However, the relative dehydriding rate decreases with theincrease of y value. The hydrogen amounts absorbed at400°C for the Mg（AlH₄）₂–yLiBH₄composites are3.16,5.32and4.72wt.%, respectively. In terms of hydriding capacity andrelative hydriding rate, the Mg（AlH₄）₂–4LiBH₄composite has the best hydriding property.Moreover, the addition TiF₃could decrease the dehydriding temperature of Mg（AlH₄）₂–yLiBH₄（y=2,4and6） composites.The dehydriding mechanism of Mg（AlH₄）₂–yLiBH₄（y=2,4and6） composites was revealed.Firstly, the compound Mg（AlH₄）₂was self-decomposed to form Al₃Mg₂and Al, and thenAl₃Mg₂reacted with LiBH₄to form AlMgB₄, LiH, Al and H2. Finally, the reation betweenelement Al and the residual LiBH₄happened to generate AlB₂, LiH and H2. The investigationson the hydriding mechanism of Mg（AlH₄）₂–yLiBH₄（y=2,4and6） composites show thatAlMgB₄, LiH and Al （or AlB₂） could absorb hydrogen to form Al₃Mg₂and LiBH₄firstly, andthen Al₃Mg₂absorbed hydrogen to generate MgH₂and Al at400°C. It is also found that thedehydriding property of Mg（AlH₄）₂–6LiBH₄composite is better than those of the MgH₂–2LiBH₄and Al–2LiBH₄composites. Based on the SEM/EDX analysis, the elements Mg, Aland B are well dispersed in the dehydriding product for the Mg（AlH₄）₂–6LiBH₄composite. TheAl₃Mg₂and Al in situ developed by the decomposition of Mg（AlH₄）₂have a large contact areawith LiBH₄, thus contributing to the kinetic enhancement of the de-/hydriding reactions.