| 1. |
- Evertsson, Jonas, et al.
(författare)
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The thickness of native oxides on aluminum alloys and single crystals
- 2015
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Ingår i: Applied Surface Science. - : Elsevier BV. - 1873-5584 .- 0169-4332. ; 349, s. 826-832
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Tidskriftsartikel (refereegranskat)abstract
- We present results from measurements of the native oxide film thickness on four different industrial aluminum alloys and three different aluminum single crystals. The thicknesses were determined using X-ray reflectivity, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. In addition, atomic force microscopy was used for micro-structural studies of the oxide surfaces. The reflectivity measurements were performed in ultra-high vacuum, vacuum, ambient, nitrogen and liquid water conditions. The results obtained using X-ray reflectivity and X-ray photoelectron spectroscopy demonstrate good agreement. However, the oxide thicknesses determined from the electrochemical impedance spectroscopy show a larger discrepancy from the above two methods. In the present contribution the reasons for this discrepancy are discussed. We also address the effect of the substrate type and the presence of water on the resultant oxide thickness.
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| 2. |
- Merte, Lindsay, et al.
(författare)
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Growth of Ultrathin Iron Oxide Films on Ag(100)
- 2015
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:5, s. 2572-2582
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Tidskriftsartikel (refereegranskat)abstract
- Ultrathin iron oxide films are useful model materials for fundamental studies of surface processes and exhibit intriguing properties as catalysts, as demonstrated recently in a number of studies utilizing platinum as a substrate. We report a study of the initial stages of iron oxide film growth on an Ag(100) surface using scanning tunneling microscopy, low energy electron diffraction, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy, with the goal of elucidating the effects of the substrate material on FeOx film growth and physical properties. We demonstrate that a well-ordered, monolayer-thick FeO(111) film can be prepared which is similar to the well-studied structure formed on Pt(111), though with a significantly expanded lattice constant indicative of smaller FeO buckling and weaker interactions with the substrate. Increased oxygen pressure during deposition leads to formation of a multilayer phase taking the form of well-ordered islands. Although superficially similar to FeO(111), spectroscopic measurements show a substantial proportion of Fe3+ in the phase. FeO(100) grains are observed upon deposition at elevated substrate temperatures, which is proposed to result from formation and oxidation of iron clusters embedded in the surface
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| 3. |
- Shipilin, Mikhail, et al.
(författare)
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Transient stuctures of PdO during CO oxidation over Pd(100)
- 2015
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:27, s. 15469-15476
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Tidskriftsartikel (refereegranskat)abstract
- In situ high-energy surface X-ray diffraction was employed to determine the surface structure dynamics of a Pd(100) single crystal surface acting as a model catalyst to promote CO oxidation. The measurements were performed under semi-realistic conditions, i.e. 100 mbar total gas pressure and 600 K sample temperature. The surface structure was studied in detail both in a steady gas ow and in a gradually changing gas composition with a time resolution of 0.5 sec. Our results show that \sqroot-PdO(101) surface oxide forms in a close to stoichiometric O2 and CO gas mixture as the mass-spectrometry indicates a transition to a highly active state with the reaction rate limited by the CO mass transfer to the Pd(100) surface. Using a low excess of O2 in the gas stoichiometry, islands of bulk oxide grow epitaxially in the same (101) crystallographic orientation of the bulk PdO unit cell according to a Stranski-Krastanov type of growth. The morphology of the islands is analyzed quantitatively. Upon further increase of the O2 partial pressure a polycrystalline Pd oxide forms on the surface.
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| 4. |
- Walter, Andrew L, et al.
(författare)
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X-ray photoemission analysis of clean and carbon monoxide-chemisorbed platinum(111) stepped surfaces using a curved crystal.
- 2015
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Surface chemistry and catalysis studies could significantly gain from the systematic variation of surface active sites, tested under the very same conditions. Curved crystals are excellent platforms to perform such systematics, which may in turn allow to better resolve fundamental properties and reveal new phenomena. This is demonstrated here for the carbon monoxide/platinum system. We curve a platinum crystal around the high-symmetry (111) direction and carry out photoemission scans on top. This renders the spatial core-level imaging of carbon monoxide adsorbed on a 'tunable' vicinal surface, allowing a straightforward visualization of the rich chemisorption phenomenology at steps and terraces. Through such photoemission images we probe a characteristic elastic strain variation at stepped surfaces, and unveil subtle stress-release effects on clean and covered vicinal surfaces. These results offer the prospect of applying the curved surface approach to rationally investigate the chemical activity of surfaces under real pressure conditions.
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| 5. |
- Zhang, Chu, et al.
(författare)
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Faceting of Rhodium(553) in Realistic Reaction Mixtures of Carbon Monoxide and Oxygen
- 2015
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:21, s. 11646-11652
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the gas composition-dependent faceting of a Rh(553) surface during catalytic CO oxidation under semirealistic reaction conditions using Surface X- ray Diffraction. We find that under stoichiometric CO and O2 conditions, the Rh(553) surface maintains its surface orientation without facet formation. In oxygen excess, the CO oxidation reaction becomes mass transfer limited by the CO diffusion, and the sur- face is observed to expose (331) or (11 ̄1) facets in coexistence with larger (111) terraces. The observed facet formation has previously been observed for pure O2 exposures of the Rh(553) surface, but at significantly lower O2 partial pressures. Surprisingly, in CO excess, which results in a CO poisoned surface with low activity, we instead find coexisting (110) and (111) facets. The reasons for and possible implications of the observed facetings are discussed.
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