| 1. |
- Flege, Jan Ingo, et al.
(författare)
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Nanoscale analysis of the oxidation state and surface termination of praseodymium oxide ultrathin films on ruthenium(0001)
- 2017
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Ingår i: Ultramicroscopy. - : Elsevier BV. - 0304-3991. ; 183, s. 61-66
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Tidskriftsartikel (refereegranskat)abstract
- The complex structure and morphology of ultrathin praseodymia films deposited on a ruthenium(0001) single crystal substrate by reactive molecular beam epitaxy is analyzed by intensity-voltage low-energy electron microscopy in combination with theoretical calculations within an ab initio scattering theory. A rich coexistence of various nanoscale crystalline surface structures is identified for the as-grown samples, notably comprising two distinct oxygen-terminated hexagonal Pr2O3(0001) surface phases as well as a cubic Pr2O3(111) and a fluorite PrO2(111) surface component. Furthermore, scattering theory reveals a striking similarity between the electron reflectivity spectra of praseodymia and ceria due to very efficient screening of the nuclear charge by the extra 4f electron in the former case.
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| 2. |
- Höcker, Jan, et al.
(författare)
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Growth and structure of ultrathin praseodymium oxide layers on ruthenium(0001)
- 2016
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Ingår i: Physical Chemistry Chemical Physics. - 1463-9076.
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Tidskriftsartikel (refereegranskat)abstract
- The growth, morphology, structure, and stoichiometry of ultrathin praseodymium oxide layers on Ru(0001) were studied using low-energy electron microscopy and diffraction, photoemission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. At a growth temperature of 760 [degree]C, the oxide is shown to form hexagonally close-packed (A-type) Pr2O3(0001) islands that are up to 3 nm high. Depending on the local substrate step density, the islands either adopt a triangular shape on sufficiently large terraces or acquire a trapezoidal shape with the long base aligned along the substrate steps.
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| 3. |
- Schaefer, Andreas, 1981, et al.
(författare)
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Thermal reduction of ceria nanostructures on rhodium(111) and re-oxidation by CO2
- 2018
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 20:29, s. 19447-19457
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Tidskriftsartikel (refereegranskat)abstract
- The thermal reduction of cerium oxide nanostructures deposited on a rhodium(111) single crystal surface and the re-oxidation of the structures by exposure to CO2 were investigated. Two samples are compared: a rhodium surface covered to ≈60% by one to two O-Ce-O trilayer high islands and a surface covered to ≈65% by islands of four O-Ce-O trilayer thickness. Two main results stand out: (1) the thin islands reduce at a lower temperature (870-890 K) and very close to Ce2O3, while the thicker islands need higher temperature for reduction and only reduce to about CeO1.63 at a maximum temperature of 920 K. (2) Ceria is re-oxidized by CO2. The rhodium surface promotes the re-oxidation by splitting the CO2 and thus providing atomic oxygen. The process shows a clear temperature dependence. The maximum oxidation state of the oxide reached by re-oxidation with CO2 differs for the two samples, showing that the thinner structures require a higher temperature for re-oxidation with CO2. Adsorbed carbon species, potentially blocking reactive sites, desorb from both samples at the same temperature and cannot be the sole origin for the observed differences. Instead, an intrinsic property of the differently sized CeOx islands must be at the origin of the observed temperature dependence of the re-oxidation by CO2.
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| 4. |
- Merte, Lindsay R., et al.
(författare)
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Oxygen Storage by Tin Oxide Monolayers on Pt3Sn(111)
- 2023
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 127:6, s. 2988-2994
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Tidskriftsartikel (refereegranskat)abstract
- The high performance of platinum–tin catalysts for oxidation reactions has been linked to the formation of tin oxides at the metal surface, but little is known about the structure of these oxides or the chemical behavior that determines their catalytic properties. We show here how surface oxides on Pt3Sn(111) incorporate oxygen at the metal interface, which may be subsequently removed by reaction with CO. The storage mechanism, where oxygen uptake occurs without loss of interfacial Pt–Sn bonds, is enabled by the peculiar asymmetrical coordination state of Sn2+. O atoms are bound at pocket sites in the 2D oxide sheet between these outward-buckled Sn atoms and metallic Sn in the alloy surface below.
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| 5. |
- ul Hassan, Jawad, et al.
(författare)
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Surface evolution of 4H-SiC(0001) during in-situ surface preparationand its influence on graphene properties
- 2013
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Ingår i: Materials Science Forum (Volumes 740 - 742). - : Trans Tech Publications Inc.. ; , s. 157-160
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Konferensbidrag (refereegranskat)abstract
- The evolution of SiC surface morphology during graphene growth process has been studied through the comparison of substrate surface step structure after in-situ etching and graphene growth in vacuum. Influence of in-situ substrate surface preparation on the properties of graphene was studied through the comparison of graphene layers on etched and un-etched substrates grown under same conditions.
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| 6. |
- Wallander, Harald J., et al.
(författare)
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Dynamic Behavior of Tin at Platinum Surfaces during Catalytic CO Oxidation
- 2023
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 13:24, s. 16158-16167
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Tidskriftsartikel (refereegranskat)abstract
- Platinum-tin surfaces are active for CO oxidation, but their activity and the effects of tin oxide phases that form under reaction conditions are poorly understood. We have studied surface alloys of tin prepared on platinum single crystals during catalytic CO oxidation using near-ambient-pressure X-ray photoemission spectroscopy. On the flat terraces of Sn/Pt(111), a wetting layer of Sn(II) surface oxide forms, while on the stepped Sn/Pt(223) surface, 3D clusters of Sn(IV) oxide are formed. Oxidation of tin by O2 competes with the reduction of the oxides by CO under reaction conditions. Oxides that do not completely cover the surface can be reduced to metallic tin, while a fully covering layer of Sn(II) oxide cannot, showing the importance of oxide edge sites for the reduction process. The samples where 2D oxide layers are formed show a higher CO oxidation activity than for pure platinum at low temperatures, while the Sn(IV) oxide clusters on the stepped surfaces do not affect the measured CO oxidation rate. We therefore identify 2D Sn(II) oxide as an active phase for CO oxidation. While oxide island edges appear to make only minor contributions to conversion under these conditions, reactions at these sites play a major role in determining the phases present and their transformations.
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