Chemical reactivity at the metal oxide interfaces

This dissertation considers atomic structure and chemical reactivity at metal oxide surfaces. In a study of water adsorption on the α-Al ₂O₃ (0001) it was found that molecular water adsorption energy is −23.40 kcal/mol, and dissociative adsorption energy is −31.57 kcal/mol. Changes in the surface atoms bonding are observed. At high hydroxylation levels of the α-Al₂O₃ (0001) surface water molecules that incorporate the Al₂O₃ crystal oxygen atoms can form. These water molecules can result in the oxygen isotopic scrambling. In a study of the molecular orbitals of water adsorbed on rutile TiO ₂ (110) surface we observed that the orbitals split and shift, resulting in a picture similar to dissociative adsorption of water. Qualitatively similar splitting occurs on MgO (100) as well, but the amount is not sufficient to interfere with observation of the 1π orbitals of the surface hydroxyls. We conclude that observation of the electronic states matching the position of the 1π hydroxyl molecular orbital is necessary, but not sufficient to prove water dissociation. Electronic excitations on the rutile TiO₂ surface with adsorbed water are discussed. Our results support formation of the hydroxyl radical as the active species on the rutile surface. Electronic effects of creation of a surface oxygen vacancy on rutile (110) surface and adsorption of H₂, O₂, CO and NO at the vacancy are analyzed. Upon formation of the vacancy the Ti3+ atoms form under the surface, rather than near the vacancy. H₂ and CO further reduce, while O₂ and NO oxidize the defective surface. Calculated adsorption energies are presented. A method for calculating solvent three dimensional distribution functions at the metal oxide-water interface is introduced. The 3D distribution functions of O and H are calculated for water at the MgO (100) surface. This work demonstrates how the use of theoretical methods can contribute to clarification or reconciliation of such results. The emphasis is on finding and applying cost-effective methods for theoretical investigation. In certain cases a feasibility analysis is provided. Reactive species and reaction mechanisms relevant to the chemistry of rutile TiO₂ (110) and α-Al ₂O₃ (0001) surfaces are suggested.