ACID-BASE PROPERTIES OF MODEL MGO SURFACES

The acid-base properties of three model MgO surfaces, a MgO thin film surface formed on the Mg(0001) surface by oxidation and sputtered and annealed MgO(100) single crystal planes, have been studied by adsorption experiments under ultrahigh vacuum conditions. The nature and coverage of the species derived from a variety of molecules, including formic and acetic acids, methanol, ethanol, acetylene, and water, on these surfaces have been examined by X-ray photoelectron spectroscopy at different temperatures. All of the above molecules underwent heterolytic dissociation on these MgO model surfaces. Ion scattering spectroscopy showed that the conjugate base anions are bound on surface Mg2+ sites. Three classes of molecules can be distinguished by their different surface coverages and their behavior in displacement/blocking experiments. Coverage measurements demonstrate that MgO thin film and sputtered MgO(100) surfaces possess higher populations of defect sites than the annealed MgO(100) surface and are thus better models for the surfaces exposed by MgO powders.