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    A Step toward the Wet Surface Chemistry of Glycine and Alanine on Cu{110}: Destabilization and Decomposition in the Presence of Near-Ambient Water Vapor
    (AMER CHEMICAL SOC, 2011) Shavorskiy, Andrey; Aksoy, Funda; Grass, Michael E.; Liu, Zhi; Bluhm, Hendrik; Held, Georg
    The coadsorption of water with organic molecules under near-ambient pressure and temperature conditions opens up new reaction pathways on model catalyst surfaces that are not accessible in conventional ultrahigh-vacuum surface-science experiments. The surface chemistry of glycine and alanine at the water-exposed Cu{110} interface was studied in situ using ambient-pressure photoemission and X-ray absorption spectroscopy techniques. At water pressures above 10(-5) Torr a significant pressure-dependent decrease in the temperature for dissociative desorption was observed for both amino acids, accompanied by the appearance of a new CN intermediate, which is not observed for lower pressures. The most likely reaction mechanisms involve dehydrogenation induced by O and/or OH surface species resulting from the dissociative adsorption of water. The linear relationship between the inverse decomposition temperature and the logarithm of water pressure enables determination of the activation energy for the surface reaction, between 213 and 232 kJ/mol, and a prediction of the decomposition temperature at the solid-liquid interface by extrapolating toward the equilibrium vapor pressure. Such experiments near the equilibrium vapor pressure provide important information about elementary surface processes at the solid-liquid interface, which can be retrieved neither under ultrahigh vacuum conditions nor from interfaces immersed in a solution.
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    Deactivation of Ru Catalysts under Catalytic CO Oxidation by Formation of Bulk Ru Oxide Probed with Ambient Pressure XPS
    (AMER CHEMICAL SOC, 2013) Qadir, Kamran; Kim, Sun Mi; Seo, Hyungtak; Mun, Bongjin S.; Akgul, Funda Aksoy; Liu, Zhi; Park, Jeong Young
    The surface science approach of using model catalysts in conjunction with the development of in situ spectroscopic tools, such as ambient pressure X-ray photoelectron spectroscopy (AP-XPS), offers a synergistic strategy for obtaining a substantially better understanding of deactivation phenomena. In this study, we investigated the nature of Ru oxides on a Ru polycrystalline film under oxidizing, reducing, and catalytic CO oxidation reaction conditions. Thus, bulk Ru oxide was easily formed on such Ru catalysts, the growth of which was dependent on reaction temperature. Once formed, such an oxide is irreversible and cannot be completely removed even under reducing conditions at elevated temperatures (200 degrees C). Our reaction studies showed substantial deactivation of the Ru film during catalytic CO oxidation, and its activity could be partially recovered after reduction pretreatment. Such continuous deactivation of a Ru film is correlated with irreversibly formed bulk Ru oxide, as shown by AP-XPS. Such in situ spectroscopic evidence of the transition of oxides to a catalytically inactive state can enable more effective design of catalysts with less deactivation.
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    In Situ Observation of Water Dissociation with Lattice Incorporation at FeO Particle Edges Using Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy
    (AMER CHEMICAL SOC, 2011) Deng, Xingyi; Lee, Junseok; Wang, Congjun; Matranga, Christopher; Aksoy, Funda; Liu, Zhi
    The dissociation of H(2)O and formation of adsorbed hydroxyl groups, on FeO particles grown on Au(111) were identified with in situ,: X:ray photoelectron spectroscopy (XPS) at water pressures ranging from 3 x 10(-8) to 0.1 Torr. The facile dissociation of H(2)O takes place at FeO particle edges, and it was successfully observed in situ With atomically resolved scanning tunneling microscopy (STM). The in situ STM studies show that adsorbed hydroxyl groups were formed exclusively along the edges of the FeO particles with the 0 atom becoming directly incorporated into the oxide crystalline lattice The STM results are consistent with coordinatively unsaturated ferrous (CUF) sites along the FeO particle edge causing the observed reactivity with H(2)O. Our results also directly illustrate how structural defects and under.-coordinated sites participate in chemical reactions.
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    In Situ Oxidation Study of Pt(110) and Its Interaction with CO
    (AMER CHEMICAL SOC, 2011) Butcher, Derek R.; Grass, Michael E.; Zeng, Zhenhua; Aksoy, Funda; Bluhm, Hendrik; Li, Wei-Xue; Liu, Zhi
    Many interesting structures have been observed for O(2)-exposed Pt(110). These structures, along with their stability and reactivity toward CO, provide insights into catalytic processes on open Pt surfaces, which have similarities to Pt nanoparticle catalysts. In this study, we present results from ambient-pressure X-ray photoelectron spectroscopy, high-pressure scanning tunneling microscopy, and density functional theory calculations. At low oxygen pressure, only chemisorbed oxygen is observed on the Pt(110) surface. At higher pressure (0.5 Torr of O(2)), nanometer-sized islands of multilayered alpha-PtO(2)-like surface oxide form along with chemisorbed oxygen. Both chemisorbed oxygen and the surface oxide are removed in the presence of CO, and the rate of disappearance of the surface oxide is dose to that of the chemisorbed oxygen at 270 K. The spectroscopic features of the surface oxide are similar to the oxide observed on Pt nanoparticles of a similar size, which provides us an extra incentive to revisit some single-crystal model catalyst surfaces under elevated pressure using in situ tools.
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    Promoter Effect of Early Stage Grown Surface Oxides: A Near-Ambient-Pressure XPS Study of CO Oxidation on PtSn Bimetallics
    (AMER CHEMICAL SOC, 2012) Jugnet, Yvette; Loffreda, David; Dupont, Celine; Delbecq, Francoise; Ehret, Eric; Aires, Francisco J. Cadete Santos; Liu, Zhi
    The knowledge of the catalyst active phase on the atomic scale under realistic working conditions is the key for designing new and more efficient materials. In this context, the investigation of CO oxidation on the bimetallic Pt3Sn(111) surfaces by near-ambient-pressure X-ray photoelectron spectroscopy and density functional theory calculations illustrates how combining advanced methodologies allows the determination of the nature of the active phase. Starting from 300 K and 500 mTorr of oxygen, the progressive formation of surface oxides is observed with increasing temperature: SnO, PtO units first, and SnO2, PtO2 units afterward. For CO oxidation on the (2 X 2) surface, the activity gain is assigned to the build-up of ultrathin domains composed of SnO and SnO2 units. The formation of these early stage surface oxides is entirely supported by a density functional theory analysis. More generally, this study demonstrates how the catalyst surface oxidation and transformation can be better controlled by a relevant choice of environmental conditions.
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    Structural and electronic properties of SnO2
    (ELSEVIER SCIENCE SA, 2013) Akgul, Funda Aksoy; Gumus, Cebrail; Er, Ali O.; Farha, Ashraf H.; Akgul, Guvenc; Ufuktepe, Yuksel; Liu, Zhi
    Highly transparent polycrystalline thin film of SnO2 (tin dioxide) was deposited using a simple and low cost spray pyrolysis method. The film was prepared from an aqueous solution of tin tetrachloride (stannic chloride) onto glass substrates at 400 degrees C. A range of diagnostic techniques including X-ray diffraction (XRD), UV-visible absorption, atomic force microscopy (AFM), scanning electron microscopy (SEM), and synchrotron-based X-ray photoelectron spectroscopy (XPS) were used to investigate structural, optical, and electronic properties of the resulting film. Deposited film was found to be polycrystalline. A mixture of SnO and SnO2 phases was observed. The average crystallite size of similar to 21.3 nm for SnO2 was calculated by Rietveld method using XRD data. The oxidation states of the SnO2 thin film were confirmed by the shape analysis of corresponding XPS O 1s, Sn 3d, and Sn 4d peaks using the decomposition procedure. The analysis of the XPS core level peaks showed that the chemical component is non-stoichiometric and the ratio of oxygen to tin (O/Sn) is 1.85 which is slightly under stoichiometry. (c) 2013 Elsevier B.V. All rights reserved.