| 1. |
- Gustafson, Johan, et al.
(författare)
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One-dimensional PtO(2) at Pt steps : Formation and reaction with CO
- 2005
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 95:25, s. 1-4
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Tidskriftsartikel (refereegranskat)abstract
- Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) PtO(2) oxide structure forms at the steps of the Pt(332) surface after O(2) exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces.
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| 2. |
- Gustafson, J., et al.
(författare)
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Sensitivity of catalysis to surface structure: The example of CO oxidation on Rh under realistic conditions
- 2008
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 78:4
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Tidskriftsartikel (refereegranskat)abstract
- Using a combination of surface x-ray diffraction and mass spectrometry at realistic pressures, the CO oxidation reactivity of Rh(111) and Rh(100) model catalysts has been studied in conjunction with the surface structure. The measurements show that the presence of a specific thin surface oxide is crucial for the high activity of the Rh based CO oxidation. As this oxide is readily formed on all Rh facets, we conclude that the specific Rh crystal planes exposed during catalysis will not directly influence the reactivity. This is fortified by the very close similarity between the Rh(111) and the Rh(100) results.
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| 3. |
- Mikkelsen, Anders, et al.
(författare)
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A study of the surface structure and composition of annealed Ga0.96Mn0.04As(100)
- 2004
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Ingår i: Applied Surface Science. - 1873-5584. ; 222:1-4, s. 23-32
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Tidskriftsartikel (refereegranskat)abstract
- The surface structure and chemical composition of annealed Ga0.96Mn0.04As(1 0 0) have been studied by scanning tunneling microscopy (STM), auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The samples were As capped and subsequently transferred in-air from the MBE system to the STM chamber. After annealing to 600 K it is found that the Mn segregates to the surface and forms a compound, which is stable up to annealing temperatures of 790 K. For annealing temperatures above 825 K a well-ordered phase exists signified by a LEED pattern consisting of a superposition of a (1 x 6) and a (4 x 2) pattern. LEED and STM measurements demonstrate that the surface is dominated by (1 x 6) domains coexisting with small patches of (4 x 2) domains. By comparing the STM images of the high temperature phase found on Ga0.96Mn0.04As(1 0 0) with the high temperature phases found on ordinary GaAs(1 0 0), we demonstrate differences between annealed Ga0.96Mn0.04As(1 0 0) and GaAs(1 0 0) in both surface morphology and atomic structure. We argue that the Ga0.96Mn0.04As surface is more As rich than the GaAs surface prepared in a similar fashion. Reasons for these differences are discussed.
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| 4. |
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| 5. |
- Westerström, Rasmus, et al.
(författare)
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Stressing Pd atoms: Initial oxidation of the Pd(110) surface
- 2008
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 602:14, s. 2440-2447
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the oxygen induced structures of the Pd(1 1 0) surface in the pressure range of 10(-5)-10(-3) mbar of oxygen, at a sample temperature of around 300 degrees C. These structures, denoted as "(7 x root 3)" and "(9 x root 3)", are studied in detail by the use of a combination of low-energy electron diffraction, scanning tunneling microscopy, high-resolution core level spectroscopy, and ab-initio simulations. Based on our data a model is proposed for these structures containing segments of Pd atoms in the [1 (1) over bar0] direction, in which the Pd rows are decorated by O atoms in a zig-zag pattern. The segments are periodically separated by displaced Pd atoms. Density functional theory calculations show that the displacements reduce the oxygen induced stress significantly, as compared to a structure with no displacements. The calculations also suggest that the new structures are stabilized by domain formation. (C) 2008 Elsevier B.V. All rights reserved.
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| 6. |
- Westerström, Rasmus, et al.
(författare)
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Structure and reactivity of a model catalyst alloy under realistic conditions
- 2008
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 1361-648X .- 0953-8984. ; 20:18, s. 6-184018
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Tidskriftsartikel (refereegranskat)abstract
- Using a combined experimental and theoretical approach, we show that a thin RhO2 oxide film forms on a Pt25Rh75(100) surface at elevated oxygen pressures and temperatures prior to the bulk oxidation. By the use of in situ surface x-ray diffraction under realistic CO oxidation reaction conditions, we show that the onset of the growth of thin RhO2 oxide film coincides with an increase in CO2 production. During the reaction, the consumed oxide film is continuously re-grown by oxygen in the gas phase. Our theoretical results strongly suggest that the CO adsorbs on the metallic substrate but reacts with the O in the RhO2 oxide film at the border between the RhO2 oxide film and the metallic substrate. This scenario could explain the experimental observations of oxidation reactions on other late transition metal surfaces as well as on their corresponding nanoparticles under realistic conditions.
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| 7. |
- Gladys, M. J., et al.
(författare)
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Chemical composition and reactivity of water on clean and oxygen-covered Pd{111}
- 2008
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 602:22, s. 3540-3549
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Tidskriftsartikel (refereegranskat)abstract
- The chemical composition and dissociation behaviour of water adsorbed on clean and oxygen pre-covered Pd{111} was studied using high-resolution time-resolved and temperature-programmed X-ray photoelectron spectroscopy. We find that water remains intact at all temperatures up to desorption on the clean surface and at high oxygen coverage(0.69 ML) when a surface oxide is formed. The highest desorption peaks occur at 163 K from the clean surface and at 172 K from the surface oxide. At the intermediate coverage of 0.20 ML oxygen reacts with coadsorbed water at 155 K, to generate a mixed H2O/OH layer exhibiting a (root 3- x root 3)R30 degrees diffraction pattern, which is stable up to 177 K. The measured ratio between intact water and the hydroxyl species in this layer varies between 1.5 and 2 depending on temperature. (C) 2008 Elsevier B.V. All rights reserved.
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| 8. |
- Gladys, M. J., et al.
(författare)
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Modifying the adsorption characteristics of water on Ru{0001} with preadsorbed oxygen
- 2008
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 78:3
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Tidskriftsartikel (refereegranskat)abstract
- The coadsorption of water and preadsorbed oxygen on Ru{0001) was studied by synchrotron-based high-resolution x-ray photoelectron spectroscopy. A dramatic change was observed in the interaction of water with oxygen between low and high oxygen precoverages. Low oxygen coverages below 0.18 ML induce partial dissociation, which leads to an adsorbed layer of H2O and OH. Around half the oxygen atoms take part in this reaction. All OH recombines upon heating to 200 K and desorbs together with H2O. Oxygen coverages between 0.20 and 0.50 ML inhibit dissociation, instead a highly stable intact water species is observed, which desorbs at 220 K. This species is significantly more stable than intact water on the clean surface. The stabilization is most likely due to the formation of hydrogen bonds with neighboring oxygen atoms. For intermediate oxygen coverages around 0.18 ML, the dissociation behavior depends on the preparation conditions, which points toward possible mechanisms and pathways for partial dissociation of water on Ru{0001}.
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| 9. |
- Gustafson, Johan, et al.
(författare)
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Self-limited growth of a thin oxide layer on Rh(111)
- 2004
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Ingår i: Physical Review Letters. - 1079-7114. ; 92:12
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of the Rh(111) surface at oxygen pressures from 10(-10) mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures.
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| 10. |
- Gustafson, Johan, et al.
(författare)
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Structure and catalytic reactivity of Rh oxides
- 2009
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 145:3-4, s. 227-235
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Konferensbidrag (refereegranskat)abstract
- Using a combination of experimental and theoretical techniques, we show that a thin RhO2 surface oxide film forms prior to the bulk Rh2O3 corundum oxide on all close-packed single crystal Rh surfaces. Based on previous reports, we argue that the RhO2 surface oxide also forms on vicinal Rh surfaces as well as on Rh nanoparticles. The detailed structure of this film was previously determined using UHV based techniques and density functional theory. In the present paper, we also examine the structure of the bulk Rh2O3 corundum oxide using surface X-ray diffraction. Being armed with this structural information, we have explored the CO oxidation reaction over Rh(1 1 1), Rh(1 0 0) and Pt25Rh75(1 0 0) at realistic pressures using in situ surface X-ray diffraction and online mass spectrometry. In all three cases we find that an increase of the CO2 production coincides with the formation of the thin RhO2 surface oxide film. In the case of Pt25Rh75(1 0 0), our measurements demonstrate that the formation of bulk Rh2O3 corundum oxide poisons the reaction, and argue that this is also valid for all other Rh surfaces. Our study implies that the CO oxidation reaction over Rh surfaces at realistic conditions is insensitive to the exact Rh substrate orientation, but is rather governed by the formation of a specific surface oxide phase. (C) 2008 Elsevier B.V. All rights reserved.
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