Dynamics of metal oxides surfaces and the connection to reactivity

Abstract: Many interesting chemical and physical phenomena which take place on surfaces of metaloxides rely on the material's ability to repeatedly alter the electronic surface structure. Formost metal oxides this functionality is connected to the degree of non-stoichiometry andthe presence of structural defects. In many catalytic applications, for example, the metaloxide surfaces display optimal performance at rather high temperatures, and it is thereforeessential to include thermal distortions in any investigation of the mechanisms behind thefunctionality. It can be expected that dynamical changes in the surface structure will drastically alter the electronic structure, and thereby also the catalytic characteristics of thesurface.For metal oxide surfaces, experimental as well as theoretical data on surface dynamics(at non-zero temperatures) are virtually non-existent. The very few MD simulation studiesthat exist in the literature have exclusively dealt with the effect of temperature on stability,structure and ordering.This thesis describes molecular dynamics simulations and subsequent quantum-chemicalcalculations of metal oxide surfaces, and how the surface properties vary with temperature,crystal face and depth from the surface for three structurally different materials. The surfaces studied are the low-index faces of MgO, α-Al2O3(0001), undoped and dopedlow-index CeO2 surfaces, and the α-Al2O3(0001┴,010Ⅱ)/CeO2(011┴,011Ⅱ) interface. The emphasis lieson dynamic quantities, and the ionic motion is analysed in terms of mean-square displacements, ion migrations, instantaneous ionic displacements, as well as vibrational densities-of-states. Other properties discussed include surface energies, interplanar relaxations, and pairdistribution functions. The thesis also discusses the implication of short-time atomic-leveldynamical changes on reactivity.