Preparation Of Mg-metal Oxide Nanocomposite Powders And Their Hydrolysis Properties After Hydrogenation

Due to comparatively stable thermodynamic properties(?H_d=76 kJ mol^-1 H₂)of MgH₂,the temperature of hydrogen release by pyrolysis method is extremely high?exceeding 200 ^oC despite lots of improvements?,which is far from practical applications.Compared to relatively harsh conditions for pyrolysis,hydrolysis can be carried out at room temperature to produce hydrogen.Moreover,the theoretical H₂yield of hydrolysis is 15.2 wt.%which is the twice amount of pyrolysis.However,the byproduct of hydrolysis,Mg?OH?₂,covering on the surface of the matrix,hinders the further hydrolysis reaction.Breaking the Mg?OH?₂ passivation layer and refining the powder are efficient ways to enhance the property of hydrolysis.In this work,common amphoteric metal oxides such as Al₂O₃,TiO₂ and Fe₂O₃ are selected as raw materials,which is mixed with Mg powders at a mass ratio of 1:9.After the Mg-MO_x?M=Ti,Al,Fe?nanocomposite powders are prepared by DC arc plasma method,the properties of hydrolysis of hydrogenated air oxidation and vacuum samples are investigated.XRD,TEM,SEM,EDS are used to analyze the phase compositions,microstructures and element distributions of samples.It is shown that new phases MgAl₂O₄ and Fe are formed during the preparation of Mg-Al₂O₃ and Mg-Fe₂O₃ nanopowders by DC arc plasma method.The content of TiO₂ in the sample is tiny because of the high melting point and no chemical reaction with Mg.For air oxidation samples,it is shown that the amphoteric metal oxide additives Al₂O₃,TiO₂ and Fe₂O₃ can effectively improve the hydrogen absorption and following hydrolysis properties of air oxidation Mg.For example,the maximum hydrogen absorption of pure Mg is 4.87wt.%and the time required for hydrogen absorption to reach 90%of the maximum is 662 s,but that of Mg-TiO₂ is 5.91 wt.%,200 s.The adhesion of TiO₂ and MgAl₂O₄ on the surface of Mg particles hinders the oxidation of Mg powders,which improves hydrogen absorption capacity.And the dislocations,some of which are perpendicular to?100?plane of Mg in Mg-TiO₂ particles,provide fast channels for the diffusion of H,resulting in fast hydrogen absorption kinetics.The acidic environment created by TiO₂ which has a good destructive effect on Mg?OH?₂ passivated layer,the primary cells formed between Fe and MgH₂ during reaction hindering the formation of Mg?OH?₂ passivated layer and the catalysis effect of MgAl₂O₄,significantly improve hydrolysis performance of the corresponding hydrogenated samples.Taking hydrogenated Mg-TiO₂ as an example,the hydrolysis hydrogen release increases from 1167 mL g^-1of hydrogenated pure Mg to 1525 mL g^-1,and the activation energy decreases from 53.42 kJ mol^-1 H₂ to 45.14 kJ mol^-1 H₂.For vacuum samples,it is shown that the addition of metal oxides Al₂O₃,TiO₂ and Fe₂O₃ reduces the hydrogen absorption of Mg powders and there is no positive effect on hydrogen absorption kinetics of Mg.The hydrolysis properties of hydrogenated samples in three aqueous solutions?deionized water,1 M NaCl,0.1 M MgCl₂?are enhanced by additives TiO₂ and Al₂O₃.The corresponding hydrolysis hydrogen yield is1482 mL g^-1,1440 mL g^-1,in 0.1 M MgCl₂ solution at room temperature,and that of pure magnesium is 1271 mL g^-1.There is no positive effect of Fe on the hydrolysis of hydrogenated samples in deionized water and 1 M NaCl solution,and the corresponding hydrolysis hydrogen yield is 171mL g^-1 and 552 mL g^-1,whereas Fe plays a significant role in the hydrolysis in 0.1 M MgCl₂ solution,and hydrolysis hydrogen yield is1271 mL g^-1.The primary cell is constituted between Fe and MgH₂,which enhances the hydrolysis properties of hydrogenated samples.