First-Principles Study Of The Interactions Between Small Molecules/Atoms And Magnesium Surfaces

The interaction of small adatoms/molecules with magnesium (magnesium alloy) surface is ubiquitous. Today, the molecule-substrate interfaces play a critical role in many environmental and modern technological processes. Yet as vital as these interfaces are, our knowledge of the interaction mechanism at molecular level is still very limited. In this work, the behaviors of H2, CO and water adsorbed on the Mg(0001) surface have been investigated at the atomic scale by using first-principles density functional theory (DFT) total energy calculations. The main content includes three parts: (i) the steric effects of hydrogen molecule on clean and doped Al Mg(0001) surfaces, and the incipient hydrogenation of the Mg(0001) surface; (ii) the behavior of CO molecule adsorbed on Mg(0001) surface; (iii) the growth mechanism of water molecules on Mg(0001) surface.In part I, we systematically investigated the interaction of hydrogen molecule/atoms dissociated and adsorbed the clean as well as the doped Al Mg(0001) surface. The preference of dissociation paths, the energy distribution, the steric effects and the corresponding electronic properties were obtained. The changes of geometry structure and electronic property of the Mg(0001) film due to the atomic Al doping are studied for the first time in a basic quantum mechanical viewpoint. The results show that the the hydrogen molecules shift into the preference of dissociation sites, along the vertical orientiation close to the surface. While getting closer to the transition state (TS), H2 begins to rotate due to two kinds of interactions between H and Mg: the orthogonalizations between molecular orbitals of H2 and electronic states of Mg; and the electrons transfer from the Mg(0001) surface to the anti-bonding orbital of H2.Then, the incipient hydrogenation of the dissociated hydrogen atoms are investigated on Mg(0001) surface at various coverage. In the coverage range 0<Φ≤1.0, the most stable among all possible adsorption sites is the on-surface fcc site followed by the hcp site, and the binding energy increases with the coverage. The on-surface diffusion path energetics of atomic hydrogen as well as the activation barriers for hydrogen penetration from the on-surface to the subsurface sites are also presented at low coverage. At high coverage of 1.0≤Φ<2.0, it is found that the coadsorption configuration with 1.0 monolayer of H residing on the surface fcc sites and the remaining 1.0 monolayer of H occupying the subsurface tetra-I sites is most energetically favorable. The resultant H-Mg-H sandwich structure for this most stable coadsorption configuration displays similar spectral features to the bulk hydride MgH2 in the density of states. The other properties of the H/Mg(0001) system including the charge distribution, the lattice relaxation, the work function, and the electronic density of states are also studied and discussed in detail. It is pointed out that the H-Mg chemical bonding during surface hydrogenation displays a mixed ionic/covalent character.And, for the doped Al Mg(0001) surface, our calculations show the surface relaxation around the doping layer changes from expansion of a clean Mg(0001) surface to contraction, due to the redistribution of electrons. After doping, the work function is enlarged, and the electronic states around the Fermi energy have a major distribution around the doping layer. For the dissociation of H2 molecules, we find that the energy barrier is enlarged for the doped surfaces. In particular, when the Al atom is doped at the first layer, the energy barrier is enlarged by 0.30 eV. For different doping lengths, however, the dissociation energy barrier decreases slowly to the value on a clean Mg(0001) surface when the doping layer is far away from the top surface. Our results well describe the electronic changes after Al doping for the Mg(0001) surface, and reveal some possible mechanisms for improving the resistance to corrosion of the Mg(0001) surface by doping of Al atoms.In part II, the adsorption behaviors of CO on the Mg(0001) surface were studied. The results show that CO molecularly adsorbs on the Mg surface with small energy barriers. The most stable adsorption state is found to be the one at the surface fcc hollow site, and the one at the surface top site is the adsorption state that has the smallest energy barrier. Based on electronic structure analysis, we further reveal that during the molecular adsorption, the 5σbonding and 2πantibonding orbitals of CO hybridize with s and pz electronic states of Mg, causing electrons to transfer from CO to Mg.In part III, the prototype water structures including monomers, clusters, and bilayers have been investigated on Mg(0001) surface. The adsorption water structures, energies and the charge density distribution are all obtained and compared with available experimental data. Calculated results show that all theoretical results are consistent with the experimental data. From these studies, a general picture has emerged regarding the water-surface interaction, the interwater hydrogen bonding, and the wetting order of the metal surfaces. This may lead to promising applications in hydrogen corrosion and fuel cells.