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Browsing by Author "Schulz, Kirk H."

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    Methanol decomposition on single-crystal Cu₂O
    Cox, David F.; Schulz, Kirk H. (American Institute of Physics, 1990-05-01)
    CH₃OD decomposition has been studied over the stoichiometric, nonpolar (111) and polar, copper_terminated (100) surfaces of Cu₂O. Thermal desorption studies for monolayer coverages of methanol at 90 K show similar conversions, but the product distributions vary significantly, indicating a structure sensitive reaction. The (111) surface shows a maximum selectivity for complete oxidative dehydrogenation to CO, while the (100) surface shows a maximum selectivity for partial dehydrogenation to CH₂O. Thermal desorptionspectroscopy (TDS) and x-ray photoelectron spectroscopy (XPS) data are consistent with a decomposition pathway which proceeds through a methoxy intermediate for both surfaces. Possible origins for the structure sensitivity are discussed.
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    The partial oxidation of propene to acrolein over single-crystal Cu₂O
    Schulz, Kirk H. (Virginia Tech, 1991)
    The partial oxidation of propene (CH₂=CHCH3) to acrolein (CH₂=CHCHO) has been studied over Cu₂0(100) and (111) single crystal surfaces. Propene adsorption under ultrahigh vacuum conditions yields no significant oxidation products, but the propene desorption temperatures are sensitive to the structural differences in the surfaces. Propene adsorption at atmospheric pressure followed by thermal desorption in ultrahigh vacuum demonstrates that propene may be activated at higher pressures. Over the nonpolar Cu₂0(111) surface, lattice oxygen insertion occurs at 300 K and 1 atm. with the formation of the σ-bonded allyl intermediate. Once formed, this specie is stable in ultrahigh vacuum and produces acrolein during TDS via a reaction-limited process. A comparison of these data with studies of allyl alcohol decomposition over Cu₂O surfaces indicate that the σ-bonded intermediate is surface allyloxy (CH₂=CH-CH₂0-) which dehydrogenates to acrolein via hydride elimination on the carbon α to the oxygen. Thus, oxygen insertion precedes the final hydrogen abstraction in the partial oxidation pathway. Propene is also observed during allyl alcohol decomposition indicating that the transformation of the π-allyl to the σ-allyl (allyloxy) during propene oxidation is reversible. The structure sensitivity of the propene oxidation reaction is demonstrated by the lack of acrolein production from the Cu-terminated, Cu₂0(100) surface following 1 atm. propene exposures. The origin of the structure sensitivity is related to the absence of coordinately-unsaturated lattice oxygen anions on the (100) surface.
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    Photoemission and low-energy-electron-diffraction study of clean and oxygen-dosed Cu₂O (111) and (100) surfaces
    Schulz, Kirk H.; Cox, David F. (American Physical Society, 1991-01-01)
    The geometric and electronic structure of clean and oxygen-dosed Cu₂O single-crystal surfaces was studied with x-ray and ultraviolet photoelectron (UPS) spectroscopies and low-energy electron diffraction. The nonpolar (111) surface can be prepared in a nearly stoichiometric (1×1) form by ion bombardment and annealing in vacuum. Oxygen adsorbs molecularly on the stoichiometric (111) surface at 300 K, but adsorbs dissociatively on a defective (111) surface prepared by ion bombardment. For the polar Cu₂O(100) face it was possible to prepare a reconstructed, Cu-terminated surface with a (3√2×√2)R45° periodicity by ion bombardment and annealing in vacuum. Preparation of an unreconstructed, (1×1), O-terminated (100) surface was possible by large (10⁹-L) oxygen exposures. UPS investigations of the O-terminated (100) surface suggest a mixture of incorporated (i.e., lattice) oxygen and adsorbed atomic oxygen (i.e., adatoms) in the terminating layer. The annealing behavior of the Cu₂O(100) surface was history dependent. Early in the sample history, bulk lattice oxygen diffused to the surface at temperatures above 800 K giving domains of (square-root 2×√2)R45° periodicity associated with half a terminating layer of oxygen atoms. After repeated ion bombardment and annealing cycles, heating above 800 K gave only a Cu-terminated surface, apparently because of a depletion of bulk lattice oxygen.