Photoelectron spectroscopic studies of model catalyst surfaces: (1)~bonding interactions of 3d(10) metal ion sites; (2)~the chemistry of copper overlayers on zinc oxide single crystals

Variable photon energy photoelectron spectroscopy (PES) performed over the photon energy range of 16.7 to 1486.6 eV with both conventional radiation sources and synchrotron radiation is used to study the electronic structure and bonding properties of the surfaces of several copper and zinc complexes. The pertinent PES background concentrating on photoionization cross section effects is reviewed in Chapter 2. Valence band PES of three tetrahedral d{dollar}sp{lcub}10{rcub}{dollar} complexes: zinc oxide (ZnO); zinc(II) tetrachloride (ZnCl{dollar}sb4sp{lcub}-2{rcub}{dollar}); and cuprous chloride (CuCl); are presented in Chapter 3. The higher effective nuclear charge on zinc(II) lowers the 4s and 4p energies relative to copper(I), creating stronger {dollar}sigma{dollar}-donor, {dollar}pi{dollar}-donor covalent interactions and contributing to the greater thermodynamic stability of zinc(II) with donor ligands. The oxide ligands perturb the zinc 3d levels more than the chloride ligands with peak intensity and binding energy analyses indicating larger {dollar}sigma{dollar}-bonding interactions occurring in the oxide. In Chapter 4, copper overlayers are evaporated onto chemically different single crystal zinc oxide surfaces and studied in ultra-high vacuum as models for methanol synthesis catalysts. Intensity and binding energy changes of both substrate and overlayer core and valence PES peaks indicate predominantly two-dimensional overlayer growth at room temperature. Submonolayer copper on the zinc terminated (0001) surface is most prone toward three dimensional clustering upon heating to 200-250{dollar}spcirc{dollar}C and oxidation by molecular oxygen as evidenced by changes in copper PES features. Submonolayer (0.3 ML) copper on both the (0001) and (1010) ZnO surfaces chemisorbs carbon monoxide with similar affinity as copper metal ({dollar}Delta{dollar}H{dollar}sb{lcub}rm ads{rcub} sp{lcub}sim{rcub}{dollar} 15 kcal/mole), while dispersed copper on (0001) did not chemisorb CO. An oxidized copper(I) site with an apparent C{dollar}sb{lcub}rm 3v{rcub}{dollar} geometry which strongly chemisorbs CO ({dollar}Delta{dollar}H{dollar}sb{lcub}rm ads{rcub} sp{lcub}sim{rcub}{dollar} 21 kcal/mole) is prepared on (0001). In Chapter 5, the variable photon energy valence band PES spectra of three structurally different d{dollar}sp9{dollar} copper(II) tetrachlorides (CuCl{dollar}sb4sp{lcub}-2{rcub}{dollar}) are compared to cuprous chloride. Differences in valence band peak intensities and energy splittings are indicative of greater covalent mixing between the copper 3d and chloride 3p levels in D{dollar}sb{lcub}rm 4h{rcub}{dollar} and D{dollar}sb{lcub}rm 2d{rcub}{dollar} copper(II) salts. The PES data as well as SCF-X{dollar}alpha{dollar}-SW and Configuration Interaction analyses indicate that ionization of copper(II) d levels result in final states with holes predominantly in ligand based orbitals.