Theoretical Study Of Uranium Surface And Reactions On The Surface

The interactions between uranium surface and small molecular have been received much attentions from the chemists/technologists/scientists during recent years.In this study,we use quantum mechanics(QM)methods to investigate bulk properties and surface properties of alpha uranium.We used density functional theory(DFT)to investigate the adsorptions and initial reactions of some atoms and small molecules on the uranium surface.We analysed the configurations of species on several alpha U(001)surface.The main research content is as follows:1.Density functional theory study of magnesium surfaces and uranium surfacesDFT calculations on seven magnesium surfaces of Mg(100),Mg(010),Mg(001),Mg(110),Mg(101),Mg(011)and Mg(111).The electronic state density was analyzed.The stabilities of the seven surfaces were established.The results indicated that the surface energies are in the range of 0.4610 to 1.0940 J/m2,which correspond to those of the Mg(001)and Mg(101)surfaces respectively.These data agree well with the available experimental results.In addition,it was found that the lower surface energy corresponds to more evenly distributed density of state(DOS),more number of DOS peaks but less height of it.First-principle calculation based on the GGA methods has been applied to prediction of the properties of bulk ?-uranium and seven ?-uranium surfaces.The number of layers in the slab has great effects on the simulated surface properties.The predicted surface properties are trustworthy when the slab number is nine or more.The surface energies of the seven low index uranium surfaces are in the range from 1.756 to 2.151 J/m2.The hybrid between the 5f orbital and 6d orbital also has somewhat impacts on the surface energies of uranium.2.Density functional study of H2 molecule and H atom adsorption on?-U(001)surfaceDFT method has been employed to investigate the adsorption of H2 molecule and H atom on ?-U(001)surface.There exist four initial sites(top(A),Triangle-centre(B),Long-bridge(C),Short-bridge(D))for H2 and H atom adsorptions on ?-U(001)surface.The Eads(adsorption energy)values on the top sites of H2-U(001)configurations are around-0.666 eV,and H2 molecule has been elongated but not broken into H atoms.For the other three sites,the Eads values are around-1.521 eV.The long-bridge site is the most reactive site for H2 decomposing.For the H-U(001)configurations,the Eads are around-2.904 eV.Top site and short-bridge site are the most reactive sites for the H atom reacts on the a-U(001)surface.Our work reveals that the different reactive sites play discrepant effects on hydrogenation process.Geometric deformations,diffusion paths and PDOS of H2-U(001)and H-U(001)configurations have also been analyzed.3.Density functional study of 02 molecule and O atom adsorption on a-U(001)surfaceFirst-principle calculations have been performed to study both O2 molecule and O atom adsorption on ?-U(001)surface and subsurface.The results show that the long-bridge site is the most stable adsorption site for O2-U(001)and O-U(001)surface adsorption.The tetrahedral(Tet)1 site is the most stable adsorption site for O-U(001)subsurface adsorption.The partial electronic density of states(PDOS)of O2-U(001)and O-U(001)show the hybridization of U and O atoms orbitals causes the electrons of 5f/6d of U more non-local and leads to the formation of O-U bond.The O-U bond is partly ionic and partly covalent.The analysis of the diffusion path of O atom on U(001)shows that O atom can easily transfer from hollow(hol)1 site to hollow 2 site with no energy barrier.O atom can hardly transfer from surface to the subsurface.4.Density functional study of CO adsorption on ?-U(001)surfaceThe adsorption,dissociation,and diffusion of CO on the ?-U(001)surface were studied using density functional theory with the generalized gradient approximation(GGA).The calculation results show that the CO is favored with adsorption energies of 1.78-1.99 eV.The U atoms of the surface layer are found to move upwards after adsorption coupled with a rumpling of the surface layer.The interaction between the U atoms and a CO molecule results mainly from the population of the CO 2?*LUMO by U electrons and the CO 2?*/5?/1?-U 6d orbital hybridization.Dissociative adsorption is energetically more favored than molecular adsorption with adsorption energies of 2.71 and 3.08 eV,respectively.The diffusion barriers of C and O atoms between two adjacent threefold hollow sites are found to be 0.57 and 0.14 eV,respectively,which indicates that the on-surface diffusion of O atoms is more easily achieved than that of C atoms.5.Density functional study of H2O molecule adsorption on ?-U(001)surfaceDFT calculations were performed to investigate the adsorption of H2O on U(001)surface.The metallic nature of uranium and different adsorption sites of U(001)surface play key roles in the H2O molecular dissociate reaction.The long-bridge site is the most favorable site of H2O-U(001)adsorption configuration.The triangle-centre site of H atom is the most favorable site of HOH-U(001)adsorption configuration.The interaction between H2O and U surface is more evident on the first layer than that on any others two sub-layers.The results also show that the dissociation energy of one hydrogen atom from H2O is-1.994 to-2.215 eV on U(001)surface,while the dissociating energy decreases to-3.351 to-3.394 eV with two hydrogen atom dissociating from H2O.These phenomena also indicate that the Oads can promote the dehydrogenation of H2O.A significant charge transfer from the first layer of the uranium surface to the H and 0 atoms is also found to occur,making the bonding partly ionic.