First-principles Study On The Catalytic Role Of An Isolated-Ti Atoms In The Hydrogenation Of Ti-doped NaAlH₄

NaAlH4 is considered to be a promising hydrogen storage material because of its relatively large reversible hydrogen capacity (theoretically:5.6wt.%) and moderately circle condition.The significance of the role of catalyze is well known,but the mechanism by which Ti enhance the hydrogenation kinetics of NaAlH4 system is still undefined, which hampered the development and practical usage of this system.In this paper,we studied the catalytic role of an isolated-Ti atoms in the hydrogenation of Ti-doped NaAlH4 by first-principles calculation. Details are as follows:In the chapter 3,we studied the isolated-Ti atom adsorbed on pure Al(100),(110) and (111) surface.The energy of adsorption,density of state and Milliken overlap population were calculated.The Milliken overlap population of Ti-Al showed that there are diversity interaction between Ti atom and Al atoms,which give rise to the formation of amorphous TiAln(n>1).In the chapter 4,we studied the hydrogen molecule adsorbed on pure Al(100),(110) and (111) surface.The adsorption energy and dissociate energy of hydrogen molecule were calculated.The dissociate path of hydrogen-adsorbed was investigated with transition state methed,and the activation energy of reaction is estimated as 0.51004eV,which indicated that the reaction of H2 with a clean Al surface is completely unfavorable.In the chapter 5,we studied the hydrogen molecule adsorbed on Ti-doped Al(100),(110) and (111) surface which based on the conclusion of chapter 3.The bond length,adsorption energy and dissociate energy of hydrogen molecule were calculated.We found the hydrogen molecule decomposed to hydrogen atoms on fcc site of Al(111) surface. The spatial configuration showed that Ti atom fall into the Al(111) surface then the Al atoms which around Ti protrude, which increased the contact area of Al and H2.The analysis of density of state and Milliken overlap population showed that there are covalence interaction between hydrogen atoms and Al atoms of surface, which give rise to the formation of Al-H, Na3AlH6 and NaAlH4. Analysis showed that the dissociation of H2 molecules is mainly the synergy of Al atoms and Ti atom, which is different from the traditional view that the dissociation of H2 molecules is only the role of Ti atoms. The role of Ti is mainly provide Al atoms for electronic then promote the interaction between Al-H. The results of transition state search showed that Ti-doped effectively reduce the H2 molecule decomposition activation energy required.In the chapter 5,we studied the hydrogen molecule adsorbed on pure and Ti-doped NaH(100) surface.We found the Ti@NaH(100) surface conducive to decomposition and accumulation of H2 molecules.These results contribute further towards understanding the improved observation in recycling of hydrogen with Ti-doped NaAlH4,meanwhile,these conclusion play a role in promoting the choice of catalyst and doping methods.