| 1. |
- Merte, Lindsay R., et al.
(författare)
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Structure of two-dimensional Fe3O4
- 2020
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 152:11
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the structure of an ultrathin iron oxide phase grown on Ag(100) using surface X-ray diffraction in combination with Hubbard-corrected density functional theory (DFT+U) calculations. The film exhibits a novel structure composed of one close-packed layer of octahedrally coordinated Fe2+ sandwiched between two close-packed layers of tetrahedrally coordinated Fe3+ and an overall stoichiometry of Fe3O4. As the structure is distinct from bulk iron oxide phases and the coupling with the silver substrate is weak, we propose that the phase should be classified as a metastable two-dimensional oxide. The chemical and physical properties are potentially interesting, thanks to the predicted charge ordering between atomic layers, and analogy with bulk ferrite spinels suggests the possibility of synthesis of a whole class of two-dimensional ternary oxides with varying electronic, optical, and chemical properties.
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| 2. |
- Garcia-Martinez, Fernando, et al.
(författare)
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Catalytic oxidation of CO on a curved Pt(111) surface : simultaneous ignition at all facets through a transient CO-O complex.
- 2020
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Ingår i: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 59:45, s. 20037-20043
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Tidskriftsartikel (refereegranskat)abstract
- The catalytic oxidation of carbon monoxide (CO) on transition metals, such as platinum (Pt), is commonly viewed as a sharp transition from the CO-inhibited surface to the active metal, covered with oxygen (O). However, we find that minor amounts of O are present in the CO-poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near-ambient pressure X-ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. The systematic analysis of carbon and oxygen core levels across the curved crystal reveals that, right before light-off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces, as indicated by ab-initio theory: a CO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.
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| 3. |
- Rullik, L, et al.
(författare)
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Surface development of an aluminum brazing sheet during heating studied by XPEEM and XPS
- 2016
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Ingår i: Materials Research Express. - : IOP Publishing. - 2053-1591. ; 3:10
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Tidskriftsartikel (refereegranskat)abstract
- X-ray photoelectron emission microscopy (XPEEM) was used in combination with other microscopic and spectroscopic techniques to follow the surface development of an aluminum brazing sheet during heating. The studied aluminum alloy sheet is a composite material designed for vacuum brazing. Its surface is covered with a native aluminum oxide film. Changes in the chemical state of the alloying elements and the composition of the surface layer were detected during heating to the melting temperature. It was found that Mg segregates to the surface upon heating, and the measurements indicate the formation of magnesium aluminate. During the heating the aluminum oxide as well as the silicon is observed to disappear from the surface. Our measurements is in agreement with previous studies observing a break-up of the oxide and the outflow of the braze cladding onto the surface, a process assisted by the Mg segregation and reaction with surface oxygen. This study also demonstrates how XPEEM can be utilized to study complex industrial materials.
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| 4. |
- Schiller, Frederik, et al.
(författare)
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Catalytic Oxidation of Carbon Monoxide on a Curved Pd Crystal : Spatial Variation of Active and Poisoning Phases in Stationary Conditions
- 2018
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 140:47, s. 16245-16252
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Tidskriftsartikel (refereegranskat)abstract
- Understanding nanoparticle catalysis requires novel approaches in which adjoining crystal orientations can be studied under the same reactive conditions. Here we use a curved palladium crystal and near-ambient pressure X-ray photoemission spectroscopy to characterize chemical species during the catalytic oxidation of CO in a whole set of surfaces vicinal to the (111) direction simultaneously. By stabilizing the reaction at fixed temperatures around the ignition point, we observe a strong variation of the catalytic activity across the curved surface. Such spatial modulation of the reaction stage is straightforwardly mapped through the photoemission signal from active oxygen species and poisoning CO, which are shown to coexist in a transient regime that depends on the vicinal angle. Line-shape analysis and direct comparison with ultrahigh vacuum experiments help identifying and quantifying all such surface species, allowing us to reveal the presence of surface oxides during reaction ignition and cooling-off.
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