| Based on the PW-PP method of density functional theory,the hydrogen adsorption,dissociation and diffusion on the clean and modified Mg17Al12(110)surfaces are calculated by using the CASTEP module of Materials Studio software.We discuss the related geometries and electronic structures of the systems.Transition metals(Ni and V)are employed to add into Mg17Al12(110)surfaces.Among them,the studies about hydrogen absorption properties of Ni-adsorbed and Ni-doped Mg17Al12(110)surfaces with different concentrations are conducted.Besides,the effects of V and its compounds on the Mg17Al12(110)surface are studied.Among them,the addition of V consists of V-doped and V-adsorbed systems,while its oxides and carbides are only considered the adsorption on(110)plane.The main results are as follows:1.The adsorption and penetration of hydrogen on(in)Mg17Al12(110)surfaces have been investigated by using the DFT.Atomic and molecular hydrogen prefer to adsorb on the Mg-Mg bridge sites of the Mg17Al12(110)surface.The lowest barrier energy of H2 dissociation on the Mg17Al12(110)surface is 0.87eV.The H atom penetrates from the(110)surface into its subsurface with the minimum barrier of 0.63eV.In the sub-surface,the diffusion behaviors of hydrogen become much easier.PDOS exhibits that for H on the Mg-Mg bridge sites,the hybridization between the H s states and the Mg s states is a major interaction.For the H-adsorbed at the Mg-Al site,the Hs state is obviously hybridized with the s-orbital of the Mg and Al atoms.It is shown from the adsorption that the interaction between the H s state and the Mg s state is more favorable for the adsorption of H on the surface.2.The Ni-adsorbed and-substituted effects on the hydrogenation properties of Mg17Al12(110)surfaces are investigation by using DFT.Ni atoms prefer to adsorb on the Mg3-Mg3 bridge sites and the Ni dopants favor to occupy at the Mg3 sites in the two Ni-doped structures(Mg16NiAl12(1 10)and Mg15Ni2Al12(110)surfaces).The Ni-containing surfaces show enhanced hydrogen storage properties compared with pure Mg17Al12(110)surface.Adding 14.7 wt%Ni effectively improve the adsorption energies of atomic and molecular hydrogen on the Mg17Al12(110)surfaces.In addition,the Ni-adsorbed systems exhibit the best performances for hydrogen dissociation and permeation with the lowest barriers than the pure and the Ni-substituted surfaces.The improved hydrogenation properties of Mg17Al12(110)surface can be attributed to the ameliorated adsorption energies and the reduced dissociation and permeation barriers upon the catalytic role of Ni.The mechanisms of Ni effect on the Mg17Al12(110)surfaces are explained by using DFT,which can be employed to understand the activity of Ni in the reaction for Mg-Al alloy hydrogenation.3.The adsorption and dissociation of hydrogen on the Mg16VAl12(110)and Mg17Al12(110)/M(M=V,VC,VO)surfaces are studied by using density functional theory.The calculation results show that the H atom prefers to adsorb at Mg-Mg bridge sites(J and H)of cleaning Mg17Al12(110)surface,as well as the Ni doped Mg17Al12(11O).H2 dissociates automatically on the Mg17Al12(110)/VO surface,and the dissociation barrier energy of the Mg17Al12(110)/M(M=V,VC)surface is very small(O.11eV and 0.09eV).PDOS indicates that the hybridization between the H atom and the surface is related to the position,and the hybridization of H and Mg in the Mg17Al12(110)/VO system is more effective than other systems,and the adsorption energy(-0.87eV)means that it is the easiest to absorb hydrogen. |
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