| Density functional theory which based on the first principles are used to investigate the adsorption of H2on the surface of Li3N(010), the reaction Mechanism LiH on the surface of Mg(NH2)2(001), the Bonding characteristics and thermodynamic properties of the Li-Mg-N-H complexes, in this paper. It provides theoretical guidance for the development of further experimental study of high-performance hydrogen storage materials. The main conclusions are shown as the followings:1. The investigation for the adsorption of H2on the surface of Li3N(010) shows that the most stable adsorption site is the initial position of the H2molecular on the N-N bridge for the surface of A-Li3N (010) which terminate with the Li and the N atoms. It is strong dissociation chemisorption with adsorption energy of4.967eV. The two H atoms of the H2moleculer fully dissociate, and tend to the tops of two N atoms, respectively, and form two NH. They are also strong dissociation chemisorption when the initial sites of the H2molecular are Li-Li-N-N hollow, N top and Li-N bridge which with adsorption energy of4.842eV,3.701eV and3.105eV, respectively, while they are physical adsorption when the initial sites of the H2molecular are Li top and Li-Li bridge, respectively. For the surface of B-Li3N (010) which Terminate with the Li atoms only, the most stable adsorption site is the initial position of the H2molecular on the Li-Li bridge. It is also strong dissociation chemisorption with adsorption energy of1.172eV. The two H atoms of the H2moleculer also fully dissociate, and tend to two hollow site, respectively, while they are physical adsorption when the initial sites of the H2molecular are Li top and Li-Li-Li-Li hollow, respectively. The main hydrogen storage methods are forming NH and NH2for A-Li3N (010) surface, while it is forming LiH for B-Li3N (010) surface.2. The result of investigation for the reaction Mechanism of LiH on the surface of Mg(NH2)2(001) shows that it has a certain metallic for the surface of Mg (NH2)2(001). The formation enthalpy of a unit of LiH interact with the surface of Mg(NH2)2(001) and form a stable system of Mg(NH2)2(001)/LiH is-6.253eV. It shows that it is more stable for the system of Mg(NH2)2(001)/LiH. The desorption energy which desorb H2from the system of Mg(NH2)2(001)/LiH is2.762eV. It is propably the reason why it need so high temperature before dehydrogenation. The interation between N and Mg atom is mainly ionic, while the interation between N and H atom is mainly covalent and the interation between N and Li atom is very weak. The transfer of electron occurs mainly between LiH molecular of H2atom during the reaction. It is easier that the electron move from HOMO of surface of Mg(NH2)2(001) to LUMO of LiH than from HOMO of LiH to LUMO of surface of Mg(NH2)2(001) when they are interacting each other. 3. The results of investigattion for the Bonding characteristics and thermodynamic properties of the Li-Mg-N-H complexes shows that Thermodynamic stability of Mg(NH2)2and LiH are better than LiNH2and MgH2, but the N-H bond from Mg(NH2)2is weaker than that from LiNH2. The Li-H bond from LiH is Mainly ionic bond. H atom and N atom from Li2Mg(NH)2form a cluster of [NH]2-, N-H bond is Covalent bond. The interaction between [NH]" and Li, Mg atoms are Mainly ionic bond. reaction enthalpy of reaction between Mg(NH2)2and LiH with a ratio of1:2,3:8and3:12are59.6KJ(mol.H2)-1、61.6KJ(mol.H2)-1、89.5KJ(mol.H2)-1, respectively, when the reaction temperature is298K. That explain why their reaction temperature in turn increase. |
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