Enhancement And Mechanism Of Manganese Based Catalysts On Hydrogen Desorption And Absorption Properties For Magnesium Hydride

In recent years,the booming development of shipping industry consumes a large amount of fossil energy,which leads to more and more serious environmental problems.Therefore,it is urgent for all countries in the world to develop green energy.As a clean and green energy,hydrogen energy is becoming the focus of researchers due to its abundant reserves,high energy density and clean combustion products.However,how to store hydrogen safely,economically and efficiently becomes a key technology in the process of hydrogen energy utilization.Among many hydrogen storage materials,magnesium hydride（MgH₂）is regarded as one of the most promising hydrogen storage materials due to its high hydrogen storage capacity（110 g/L,7.6 wt%）,abundant reserves and good reversibility.However,high thermodynamic stability and reaction energy barrier lead to high dehydrogenation temperature and slow kinetic performance of MgH₂,which limits its practical application in marine hydrogen storage.In this thesis,on the basis of summarizing the research progress of MgH₂ at home and abroad,we successfully prepared three different Mn-based catalysts and systematically studied the catalytic modification effect and action mechanism of them.We first studied the modification of MgH₂ by MnCl₂,it was found that MgH₂+5wt%MnCl₂started to release hydrogen from 230℃,which was significantly lower than that of pure MgH₂.The microstructure analysis showed that Mn was found in the process of dehydrogenation.In order to further explore the effect of Mn on the hydrogen absorption and desorption properties of MgH₂,the submicron-Mn particles ranged about 500-800 nm could be prepared by a wet chemical method,and doping submicron-Mn into MgH₂ could significantly improve the hydrogen absorption and desorption properyties of MgH₂.The experimental results show that MgH₂ doped with 10 wt%submicron-Mn particles could release 6.6 wt%hydrogen within 8 min at a constant temperature of 300℃.Further characterization analysis showed that submicron-Mn particles would not react with Mg/MgH₂to form other substances in the process of dehydrogenation and rehydrogenation,which not only promoted the recombination and decomposition of hydrogen molecules,but also inhibited the thermal agglomeration of MgH₂,thus ensuring excellent hydrogen storage performance.Secondly,nano-sized Mn particles with particle size of 20-30 nm were prepared by adjusting the amount of buffering agent in the process of mechanical ball milling,and the modification effect of Mn particles on hydrogen storage performance of MgH₂ was explored.The results of desorption test showed that under the constant temperature of 300℃,6.7 wt%hydrogen could be rapidly released for MgH₂+10 wt%nano-Mn sample within 5 minutes.Compared with the previous study,the hydrogen desorption rate and amount of dehydrogenation have been significantly improved,which indicates that the reduction of particle size could further enhance the hydrogen desorption performance of MgH₂.Microstructure characterization exhibited that the Mn nanoparticles were evenly distributed on the surface of MgH₂,which increased the number of active sites and provided more channels for the diffusion and dissociation of hydrogen molecules,thus promoting the reaction of hydrogen absorption and desorption.In addition,the stable existence of Mn nanoparticles in the hydrogen storage process is also an important reason for the remarkable improvement of the hydrogen absorption and desorption performance for MgH₂.Finally,Mn₃O₄ nanoparticles were successfully prepared by a chemical method and doped as catalysts into MgH₂ to explore its catalytic effect.The particle size of prepared Mn₃O₄ particles ranged from 7 nm to 15 nm.After adding 10 wt%Mn₃O₄ nanoparticles into MgH₂,6.4 wt%hydrogen could be rapidly released under a constant temperature of 300℃.Furthermore,the activation energy value of absorption for the MgH₂+10wt%nano-Mn₃O₄sample decreased to 34.4±10.9 k J/mol.MgH₂+10wt%nano-Mn₃O₄also showed good cycling stability,a cycling test showed that the hydrogen storage capacity and reaction kinetics of the sample could be favourably preserved in 20 cycles.Further XRD and TEM results showed that Mn₃O₄ nanoparticles were uniformly covered on the surface of MgH₂and Mn₃O₄ reacts with MgH₂to form Mn nanoparticles in situ,which is also the reason for good cycling performance.Theoretical calculations proved that in-situ Mn particles weakened the Mg-H bond,which corresponds well to the decrease in apparent activation energy.