| 1. |
- Knudsen, Jan, et al.
(författare)
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Low-Temperature CO Oxidation on Ni(111) and on a Au/Ni(111) Surface Alloy
- 2010
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 4:8, s. 4380-4387
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Tidskriftsartikel (refereegranskat)abstract
- From an interplay between scanning tunneling microscopy, temperature programmed desorption, X-ray photoelectron spectroscopy, and density functional theory calculations we have studied low-temperature CO oxidation on Au/Ni(111) surface alloys and on Ni(111). We show that an oxide is formed on both the Ni(111) and the Au/Ni(111) surfaces when oxygen is dosed at 100 K, and that CO can be oxidized at 100 K on both of these surfaces in the presence of weakly bound oxygen. We suggest that low-temperature CO oxidation can be rationalized by CO oxidation on O-2-saturated NiO(111) surfaces, and show that the main effect of Au in the Au/Ni(111) surface alloy is to block the formation of carbonate and thereby increase the low-temperature CO2 production.
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| 2. |
- Merte, Lindsay, et al.
(författare)
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Water clustering on nanostructured iron oxide films
- 2014
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- The adhesion of water to solid surfaces is characterized by the tendency to balance competing molecule-molecule and molecule-surface interactions. Hydroxyl groups form strong hydrogen bonds to water molecules and are known to substantially influence the wetting behaviour of oxide surfaces, but it is not well-understood how these hydroxyl groups and their distribution on a surface affect the molecular-scale structure at the interface. Here we report a study of water clustering on a moire-structured iron oxide thin film with a controlled density of hydroxyl groups. While large amorphous monolayer islands form on the bare film, the hydroxylated iron oxide film acts as a hydrophilic nanotemplate, causing the formation of a regular array of ice-like hexameric nanoclusters. The formation of this ordered phase is localized at the nanometre scale; with increasing water coverage, ordered and amorphous water are found to coexist at adjacent hydroxylated and hydroxyl-free domains of the moire structure.
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| 3. |
- Schnadt, Joachim, et al.
(författare)
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Experimental and theoretical study of oxygen adsorption structures on Ag(111)
- 2009
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 80:7
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Tidskriftsartikel (refereegranskat)abstract
- The oxidized Ag(111) surface has been studied by a combination of experimental and theoretical methods, scanning tunneling microscopy, x-ray photoelectron spectroscopy, and density functional theory. A large variety of different surface structures is found, depending on the detailed preparation conditions. The observed structures fall into four classes: (a) individually chemisorbed atomic oxygen atoms, (b) three different oxygen overlayer structures, including the well-known p(4x4) phase, formed from the same Ag-6 and Ag-10 building blocks, (c) a c(4x8) structure not previously observed, and (d) at higher oxygen coverages structures characterized by stripes along the high-symmetry directions of the Ag(111) substrate. Our analysis provides a detailed explanation of the atomic-scale geometry of the Ag-6/Ag-10 building block structures and the c(4x8) and stripe structures are discussed in detail. The observation of many different and co-existing structures implies that the O/Ag(111) system is characterized by a significantly larger degree of complexity than previously anticipated, and this will impact our understanding of oxidation catalysis processes on Ag catalysts.
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| 4. |
- Schnadt, Joachim, et al.
(författare)
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Extended One-Dimensional Supramolecular Assembly on a Stepped Surface
- 2008
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Ingår i: Physical Review Letters. - 1079-7114. ; 100
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Tidskriftsartikel (refereegranskat)abstract
- 2,6-naphthalene-dicarboxylic acid was adsorbed on a Ag(110) surface with an average terrace width of only some tens of a nm. Scanning tunneling microscopy shows that the adsorbates self-assemble into one-dimensional mesoscale length chains. These extend over several hundred nanometers and thus the structure exhibits an unprecedented tolerance to monatomic surface steps. Density functional theory and x-ray photoelectron spectroscopy explain the behavior by a strong intermolecular hydrogen bond plus a distinct template-mediated directionality and a high degree of molecular backbone flexibility. ©2008 The American Physical Society
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| 5. |
- Schnadt, Joachim, et al.
(författare)
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Interplay of adsorbate-adsorbate and adsorbate-substrate interactions in self-assembled molecular surface nanostructures
- 2010
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Ingår i: Nano Reseach. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 3:7, s. 459-471
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of 2,6-naphthalenedicarboxylic acid (NDCA) molecules on the Ag(110), Cu(110), and Ag(111) surfaces at room temperature has been studied by means of scanning tunnelling microscopy (STM). Further supporting results were obtained using X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS). On the Ag(110) support, which had an average terrace width of only 15 nm, the NDCA molecules form extended one-dimensional (1-D) assemblies, which are oriented perpendicular to the step edges and have lengths of several hundred nanometres. This shows that the assemblies have a large tolerance to monatomic surface steps on the Ag(110) surface. The observed behaviour is explained in terms of strong intermolecular hydrogen bonding and a strong surface-mediated directionality, assisted by a sufficient degree of molecular backbone flexibility. In contrast, the same kind of step-edge crossing is not observed when the molecules are adsorbed on the isotropic Ag(111) or more reactive Cu(110) surfaces. On Ag(111), similar 1-D assemblies are formed to those on Ag(110), but they are oriented along the step edges. On Cu(110), the carboxylic groups of NDCA are deprotonated and form covalent bonds to the surface, a situation which is also achieved on Ag(110) by annealing to 200 degrees C. These results show that the formation of particular self-assembled molecular nanostructures depends significantly on a subtle balance between the adsorbate-adsorbate and adsorbate-substrate interactions and that kinetic factors play an important role.
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| 6. |
- Schnadt, Joachim, et al.
(författare)
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Revisiting the structure of the p(4x4) surface oxide on Ag(111)
- 2006
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Ingår i: Physical Review Letters. - 1079-7114. ; 96
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Tidskriftsartikel (refereegranskat)abstract
- Scanning tunneling microscopy (STM) and density-functional theory are used to reexamine the structure of the renowned p(4×4)-O/Ag(111) surface oxide. The accepted structural model [C. I. Carlisle et al., Phys. Rev. Lett. 84, 3899 (2000)] is incompatible with the enhanced resolution of the current STM measurements. An "Ag6 model" is proposed that is more stable than its predecessor and accounts for the coexistence of the p(4×4) and a novel c(3×5sqrt(3))rect phase. This coexistence is an indication of the dynamic complexity of the system that until now has not been appreciated.
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| 7. |
- Vang, Ronnie T., et al.
(författare)
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Ethylene dissociation on flat and stepped Ni(111): A combined STM and DFT study
- 2006
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 600, s. 66-77
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Tidskriftsartikel (refereegranskat)abstract
- The dissociative adsorption of ethylene (C2H4) on Ni(1 1 1) was studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM studies reveal that ethylene decomposes exclusively at the step edges at room temperature. However, the step edge sites are poisoned by the reaction products and thus only a small brim of decomposed ethylene is formed. At 500 K decomposition on the (1 1 1) facets leads to a continuous growth of carbidic islands, which nucleate along the step edges. DFT calculations were performed for several intermediate steps in the decomposition of ethylene on both Ni(1 1 1) and the stepped Ni(2 1 1) surface. In general the Ni(2 1 1) surface is found to have a higher reactivity than the Ni(1 1 1) surface. Furthermore, the calculations show that the influence of step edge atoms is very different for the different reaction pathways. In particular the barrier for dissociation is lowered significantly more than the barrier for dehydrogenation, and this is of great importance for the bond-breaking selectivity of Ni surfaces. The influence of step edges was also probed by evaporating Ag onto the Ni(1 1 1) surface. STM shows that the room temperature evaporation leads to a step flow growth of Ag islands, and a subsequent annealing at 800 K causes the Ag atoms to completely wet the step edges of Ni(1 1 1). The blocking of the step edges is shown to prevent all decomposition of ethylene at room temperature, whereas the terrace site decomposition at 500 K is confirmed to be unaffected by the Ag atoms. Finally a high surface area NiAg alloy catalyst supported on MgAl2O4 was synthesized and tested in flow reactor measurements. The NiAg catalyst has a much lower activity for ethane hydrogenolysis than a similar Ni catalyst, which can be rationalized by the STM and DFT results.
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| 8. |
- Weigelt, Sigrid, et al.
(författare)
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Formation of trioctylamine from octylamine on Au(111)
- 2008
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 130:16, s. 5388-5388
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of octylamine on Au(111) under ultrahigh vacuum conditions is investigated. The molecules surprisingly undergo a thermally activated chemical reaction, resulting in formation of trioctylamine as confirmed both by X-ray photoelectron spectroscopy (XPS) and by comparison to the scanning tunneling microscopy (STM) signature of trioctylamine deposited directly onto the surface.
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