First Principles Study On De/hydrogenation Of MA1H₄（m=li, Na） With Titanium And Graphyne Catalyst

The major obstacle of complex hydrides used as hydrogen storage media is their poor de/hydriding performance owing to the high kinetic barrier and/or the high thermodynamic driving force under the de/hydrogenation process. Suitable catalysts can help to improve the dehydrogenation reaction process if it is only hindered by the kinetic barrier. However, the catalysts cannot completely overcome the thermodynamic barrier, and therefore the material is impractical for reversible hydrogen storage applications until nowadays. In terms of the hydrogen storage capacity, Na Al H4 and Li Al H4 are undoubtedly a prototype for the study of high-capacity hydrogen storage materials. In this thesis we focus on the first step of the dehydrogenation of MAl H4(M=Li and Na) aiming to clarify the catalytic mechanisms of Ti dopant on the de/hydrogenation of MAl H4 and to propose an approach to overcome the bottleneck of the re-hydrogenation of Li Al H4. A reaction pathway mediating by Al H3 of the re-hydrogenation of Li Al H4 is proposed in this work.The influence of dopant Ti on the dehydrogenation of MAl H4 is studied based on the first principles calculations of total energy and electronic structure of Ti doped systems. Ti prefers to substitute for the Al in the Li Al H4 and weakens the stability of the [Al H4] group. Ti interacts with Al and H atoms and consequently cripples Al-H bonds, and thus promotes the releases of hydrogen from [Al H4] group.Studies on the influence of Ti- and Ni-doped Al H3 bulk and surface are carried out. High occupation energy is required by the doping of Ni causing it impractically to be achieved, although Ni does benefit the dissociation of hydrogen from Al H 3. In contrast, the Ti can promote hydrogen release from Al H3, and the effect is stronger if Ti is doped in surface than that in bulk.Based on the factor that the M3 Al H6 is one of the products of the dehydrogenation of MAl H4, the occupation behaviors of Ti in M3 Al H6 were studied in this thesis. The substitution and interstitial occupation are considered. Ti prefers to substitute Al in M3 Al H6. The co-existing between dopant and vacancy is also studied. For the Li3 Al H6, the formation of Li vacancy requires relatively large amount of energy and the doping of Ti cannot overcome such energy barrier. In the Na3 Al H6, a Na vacancy can be generated if Ti substitutes for Al. The formation of Na vacancy in Na3 Al H6 catalyzed by Ti is a high possibility to be the key factor driving the re-hydrogenation of Na Al H4 from Na3 Al H6. Further calculations considering the surface doping of Ti in M3 Al H6 are performed. The occupation energy of Ti in surface is lower than in the bulk and Ti intends to substitute the Al atom in the subsurface layer. Therefore, the Ti weakens the M-[Al H6] bonding in the surface doping systems.A reaction pathway of M3 Al H6+2Al H3â†'3MAl H4 is proposed. Thermodynamically, the reaction enthalpy of M3 Al H6+2Al H3â†'3MAl H4 is 5.5 k J/mol, which is more active than the reaction of M3 Al H6 + Al + H2 â†'MAl H4 with reaction enthalpy of 14.3k J/mol and more likely to regenerate MAl H4. The adsorption of Al H3 on Li3 Al H6(010) surface is studied kinetically. The diffusion barrier of a hydrogen atom dissociated from the Li3 Al H6(010) surface to the Al H3 group is 0.10 e V. The calculations of transition state reveal that the combination of an H atom from Li3 Al H6 surface with an Al H3 is not the bottleneck but the releasing of Li atoms, which acts as mediate caring H atoms to the Al H3 for the re-hydrogenation of Li Al H4 from Li3 Al H6. A model that Al H3 adsorbed on the Ti substituting Al in the subsurface layer is used to evaluate the catalytic effect of Ti on the escaping of Li atom from surface and the attracting of H atoms by Al H3. Ti promotes both processes and therefore catalyzes the reaction of M3 Al H6 + Al H3â†'MAl H4.Study on the dissociation of MAl H4 molecule catalyzed by Graphyne(GP) and Graphdiyne(GD) is carried out to study the intrinsic characteristics of the dissociation in molecular level. The results show that the alkali metal was pushed away from [Al H4] group by the affinity between carbon and alkali atoms, which lowers the stability of the [Al H4] group and therefore promotes the dissociation of the MAl H4 molecule.