| 1. |
- Blomberg, Sara, et al.
(författare)
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Generation and oxidation of aerosol deposited PdAg nanoparticles
- 2013
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 616, s. 186-191
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Tidskriftsartikel (refereegranskat)abstract
- PdAg nanoparticles with a diameter of 10 nm have been generated by an aerosol particle method, and supported on a silica substrate. By using a combination of X-ray Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy it is shown that the size distribution of the particles is narrow and that the two metals form an alloy with a mixture of 75% Pd and 25% Ag. Under oxidizing conditions, Pd is found to segregate to the surface and a thin PdO like oxide is formed similar to the surface oxide previously reported on extended PdAg and pure Pd surfaces. (C) 2013 Elsevier B.V. All rights reserved.
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| 2. |
- Grönbeck, Henrik, 1966, et al.
(författare)
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Mechanism for reversed photoemission core-level shifts of oxidized Ag
- 2012
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121. ; 85:11
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations and high-resolution core-level spectroscopy are used to explore the remarkable observation of decreased Ag 3d binding energy upon silver oxidation. The shift in Ag 3d binding energy is investigated at different degrees of oxidation and compared to results for Pd 3d, which exhibits a normal shift. Analysis of initial-state effects and valence electronic structure shows that the onsite Ag core potential is insensitive to oxidation despite a clear metal-to-oxygen charge transfer. The substantial negative shift for oxidized Ag is instead attributed to final-state effects as screening of the core-hole occurs in metal s states of bonding character.
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| 3. |
- Gustafson, Johan, et al.
(författare)
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The Role of Oxides in Catalytic CO Oxidation over Rhodium and Palladium
- 2018
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 8:5, s. 4438-4445
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Tidskriftsartikel (refereegranskat)abstract
- Catalytic CO oxidation is a seemingly simple reaction between CO and O2 molecules, one of the reactions in automotive catalytic converters, and the fruit-fly reaction in model catalysis. Surprisingly, the phase responsible for the catalytic activity is still under debate, despite decades of investigations. We have performed a simple but yet conclusive study of single crystal Rh and Pd model catalysts, resolving this controversy. For Rh, the oxygen-covered metallic surface is more active than the oxide, while for Pd, thin oxidefilms are at least as active as the metallic surface, but a thicker oxide is less active. Apart from resolving a long-standing debate, our results pinpoint important design principles for oxidation catalysts as to prevent catalytic extinction at high oxygen exposures.
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| 4. |
- Hellman, Anders, 1974, et al.
(författare)
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The Active Phase of Palladium during Methane Oxidation
- 2012
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 3:6, s. 678-682
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Tidskriftsartikel (refereegranskat)abstract
- The active phase of Pd during methane oxidation is a long- standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts.
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| 5. |
- Klacar, Simon, 1984, et al.
(författare)
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Facile NOx interconversion over preoxidized Ag(111)
- 2013
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 617, s. 167-174
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Tidskriftsartikel (refereegranskat)abstract
- X-ray photoelectron spectroscopy and density functional theory calculations are used to investigate NO adsorption at low (100 K) and room temperature (RT) over preoxidized Ag(111). At 100 K, the data indicates presence of NO and N2O2, with little or no nitrite/nitrate formation. This is consistent with the calculated surface core level shifts and the pronounced barrier for nitrite formation. At RT, the recorded spectra indicate a complex interconversion between adsorbed species with an initial formation of a p(4 x 4) nitrate overlayer. With increasing NO pressure, the experimental results are best rationalized by partial nitrate decomposition into nitrites and subsequent NO physisorption, which leads to the formation of N2O3-like species.
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| 6. |
- Knudsen, Jan, et al.
(författare)
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Carbonate formation on p(4 x 4)-O/Ag(111)
- 2011
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121. ; 84:11
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution core-level spectroscopy and density-functional theory calculations have been used to investigate CO adsorption on the p(4 x 4) structure of oxidized Ag(111). CO adsorption with subsequent carbonate formation was observed at 100 K. The experimental results are consistent with calculations that reveal low activation barriers to form CO(2) and CO(3)(2-) from adsorbed CO. On the basis of a good match between calculated and experimental shifts in the Ag 3d and O 1s core-level binding energies, a model for a monolayer of carbonates on p(4 x 4)-O/Ag(111) is proposed.
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| 7. |
- Martin, Natalia, et al.
(författare)
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Dissociative Adsorption of Hydrogen on PdO(101) Studied by HRCLS and DFT
- 2013
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:26, s. 13510-13519
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution core-level spectroscopy (HRCLS) and density functional theory (DFT) calculations have been used to investigate the adsorption and dissociation of hydrogen on a PdO(101) film grown on Pd(111). Energy-dependent measurements of the O 1s and Pd 3d(5/2) binding energies enable identification of surface components that correspond to undercoordinated Pd and O atoms. HRCLS data obtained at 110 K, after hydrogen exposure at the same temperature, reveal hydrogen adsorption and formation of Pd-H and O-H groups. Adsorption at room temperature results instead in complete reduction of the oxide. The experimental results are supported by the DFT calculations of core-level shifts and barriers for water formation.
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| 8. |
- Martin, Natalia, et al.
(författare)
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High-Coverage Oxygen-Induced Surface Structures on Ag(111)
- 2014
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:28, s. 15324-15331
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Tidskriftsartikel (refereegranskat)abstract
- Surface structures formed by exposing Ag(111) to atomic oxygen have been studied by X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations. From the combination of the experimental and theoretical results, a model is proposed for the 0.5 ML oxygen coverage with a c(4 x 8) periodicity. Moreover, we find that a bulk-like Ag2O phase starts to form at coverages above 0.5 ML.
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| 9. |
- Martin, Natalia, et al.
(författare)
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High-resolution core-level spectroscopy study of the ultrathin aluminum oxide film on NiAl(110)
- 2011
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121. ; 83:12
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the ultrathin aluminum oxide film on NiAl(110) by a combination of high-resolution core-level spectroscopy and density functional theory calculations. Energy-dependent core-level data from the O 1s and Al 2p levels allows for a distinction between oxygen and aluminum atoms residing at the surface or inside the aluminum oxide film. A comparison to calculated core-level binding energies from the recent model by Kresse et al. [Science 308, 1440 (2005)] reveals good agreement with experiment, and the complex spectroscopic signature of the thin Al oxide on NiAl(110) can be explained. Our assignment of a shifted component in the O 1s spectra to oxygen atoms at the surface with a particular Al and oxygen coordination may have implications for the interpretation of photoelectron-diffraction experiments from similar ultrathin aluminum oxide films.
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| 10. |
- Martin, Natalia, et al.
(författare)
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Intrinsic Ligand Effect Governing the Catalytic Activity of Pd Oxide Thin Films
- 2014
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 4:10, s. 3330-3334
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Tidskriftsartikel (refereegranskat)abstract
- High-pressure X-ray photoelectron spectroscopy, mass spectrometry, and density functional theory calculations have been combined to study methane oxidation over Pd(100). The measurements reveal a high activity when a two-layer PdO(101) oriented film is formed. Although a one-layer PdO(101) film exhibits a similar surface structure, no or very little activity is observed. The calculations show that the presence of an oxygen atom directly below the coordinatively unsaturated Pd atom in the two-layer PdO(101) film is crucial for efficient methane dissociation, demonstrating a ligand effect that may be broadly important in determining the catalytic properties of oxide thin films.
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