| 1. |
- Blomberg, Sara, et al.
(författare)
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Combining synchrotron light with laser technology in catalysis research
- 2018
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Ingår i: Journal of Synchrotron Radiation. - 1600-5775 .- 0909-0495. ; 25:5, s. 1389-1394
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Tidskriftsartikel (refereegranskat)abstract
- High-energy surface X-ray diffraction (HESXRD) provides surface structural information with high temporal resolution, facilitating the understanding of the surface dynamics and structure of the active phase of catalytic surfaces. The surface structure detected during the reaction is sensitive to the composition of the gas phase close to the catalyst surface, and the catalytic activity of the sample itself may affect the surface structure, which in turn may complicate the assignment of the active phase. For this reason, planar laser-induced fluorescence (PLIF) and HESXRD have been combined during the oxidation of CO over a Pd(100) crystal. PLIF complements the structural studies with an instantaneous two-dimensional image of the CO2 gas phase in the vicinity of the active model catalyst. Here the combined HESXRD and PLIF operandomeasurements of CO oxidation over Pd(100) are presented, allowing for an improved assignment of the correlation between sample structure and the CO2distribution above the sample surface with sub-second time resolution.
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| 2. |
- Bounechada, Djamela, 1984, et al.
(författare)
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Mechanisms behind sulfur promoted oxidation of methane
- 2013
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 15:22, s. 8648-8661
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Tidskriftsartikel (refereegranskat)abstract
- The promoting effect of SO2 on the activity for methane oxidation over platinum supported on silica, alumina and ceria has been studied by flow-reactor, in situ infrared spectroscopy and in situ high-energy x-ray diffraction experiments under transient reaction conditions. The catalytic activity is clearly dependent on the support material and its interaction with the noble metal both in absence and presence of sulfur. On platinum, the competitive reactant adsorption favors oxygen dissociation such that oxygen self-poisoning is observed for Pt/silica and Pt/alumina. Contrarily for Pt/ceria, no oxygen self-poisoning is observed, which seems to be due to additional reaction channels via sites on the platinum-ceria boundary and/or ceria surface considerably far from the Pt crystallites. Addition of sulfur dioxide generally leads to the formation of ad-SOx species on the supports with a concomitant removal and/or blockage/rearrangement of surface hydroxyl groups. Thereby, the methane oxidation is inhibited for Pt/silica, enhanced for Pt/alumina and temporary enhanced followed by inhibition after long-term exposure to sulfur for Pt/ceria. The observations can be explained by competitive oxidation of SO2 and CH4 on Pt/silica, formation of new active sites at the noble metal-support interface promoting dissociative adsorption of methane on Pt/alumina, and in the case of Pt/ceria, formation of promoting interfacial surface sulfates followed by formation of deactivating bulk-like sulfate species. Furthermore, it can be excluded that reduction of detrimental high oxygen coverage and/or oxide formation on the platinum particles through SO2 oxidation is the main cause for the promotional effects observed.
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| 3. |
- Clemens, Anna, 1986, et al.
(författare)
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Reaction-driven Ion Exchange of Copper into Zeolite SSZ-13
- 2015
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 5:10, s. 6209-6218
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Tidskriftsartikel (refereegranskat)abstract
- We have used several techniques to characterize Cu-SSZ-13 before and after exposure to reaction conditions relevant for NO reduction by NH3-SCR to address the increase in NH3-SCR activity observed as a function of time on stream. Specifically, we focus on characterizing copper species in samples prepared with solid state ion exchange (SSIE) having varying Cu loadings from 0.7 to 5.2 wt %. X-ray diffraction shows that CuO species that likely remain from the SSIE synthesis on the outside of the zeolite crystallites are significantly reduced by exposure to reaction conditions. High-resolution X-ray diffraction (HR-XRD) further reveals a corresponding increase of Cu2O species. X-ray photoelectron, Auger electron, and ultraviolet -visible spectroscopy show an increase in both Cu(II) and Cu(I) species inside the zeolite pores, with a preserved long-range order of the zeolite structure as revealed by HR-XRD.
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| 4. |
- Di, Mengqiao, 1994, et al.
(författare)
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Chasing PtO x species in ceria supported platinum during CO oxidation extinction with correlative operando spectroscopic techniques
- 2022
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 409, s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- Industrially relevant, highly dispersed, Pt/ceria and reference Pt/alumina catalysts with narrow Pt particle size distributions have been prepared, characterised ex situ and studied for CO oxidation by operando infrared and X-ray absorption spectroscopy. At high CO conversions, spectator CO ad-species on ionic platinum are observed while the CO oxidation proceeds on Pt particles in a high oxidation state exhibiting significant Pt[sbnd]O coordination. During the protracted catalytic extinction, the CO coverage builds up gradually while the Pt oxidation state and Pt[sbnd]O coordination remain high because of interactions with ceria. The observed CO oxidation at high CO coverage is suggested to involve sites at the platinum-ceria boundary that cannot be CO self-poisoned. This behaviour is in stark contrast to that of Pt/alumina, which shows removal of platinum oxides formed during CO oxidation and the classical drop in catalytic activity caused by rapid CO self-poisoning when reaching a critical temperature.
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| 5. |
- Gajdek, Dorotea, et al.
(författare)
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Structural Changes in Monolayer Cobalt Oxides under Ambient Pressure CO and O 2 Studied by In Situ Grazing-Incidence X-ray Absorption Fine Structure Spectroscopy
- 2022
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:7, s. 3411-3418
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Tidskriftsartikel (refereegranskat)abstract
- We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O2, with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O2-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co3O4 under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.
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| 6. |
- Gustafson, Johan, et al.
(författare)
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High-energy surface X-ray diffraction for fast surface structure determination
- 2014
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 343:6172, s. 758-761
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the interaction between surfaces and their surroundings is crucial in many materials-science fields such as catalysis, corrosion, and thin-film electronics, but existing characterization methods have not been capable of fully determining the structure of surfaces during dynamic processes, such as catalytic reactions, in a reasonable time frame. We demonstrate an x-ray-diffraction–based characterization method that uses high-energy photons (85 kiloelectron volts) to provide unexpected gains in data acquisition speed by several orders of magnitude and enables structural determinations of surfaces on time scales suitable for in situ studies. We illustrate the potential of high-energy surface x-ray diffraction by determining the structure of a Pd surface in situ during catalytic CO oxidation and follow dynamic restructuring of the surface with subsecond time resolution.
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| 7. |
- 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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| 8. |
- 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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| 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 Mihaela, 1984, et al.
(författare)
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Characterization of Surface Structure and Oxidation/Reduction Behaviour of Pd-Pt/Al2O3 Model Catalysts
- 2016
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:49, s. 28009-28020
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Tidskriftsartikel (refereegranskat)abstract
- Structural and morphological characterisation of bimetallic Pd-Pt/Al2O3 model cat- alysts are performed using X-ray diffraction, X-ray absorption spectroscopy, transmis- sion electron microscopy and CO chemisorption. Further, the catalysts were studied under oxidising and reducing conditions using both X-ray absorption spectroscopy and low-energy ion scattering spectroscopy. For the as-prepared catalysts, the existence of alloyed bimetallic Pd-Pt particles and of (tetragonal) PdO were found for the samples calcined at 800 C. PdO is present in form of crystals at the surface of the Pd-Pt par- ticles or as isolated PdO crystals on the support oxide. Bimetallic Pd-Pt nanoparticles were only formed on the Pd-Pt catalysts after calcination at 800 C. The results show that the Pd-Pt nanoparticles undergo reversible changes in surface structure composi- tion and chemical state in response to oxidising or reducing conditions. Under oxidising conditions Pd segregates to the shell and oxidises forming PdO, while under reducing conditions regions with metallic Pd and Pd-Pt alloys were observed at the surface. No bimetallic Pd-Pt nanoparticles were observed for the sample initially calcined at 500 C, but instead isolated monometallic particles, where small Pt particles are easily oxidised under O2 treatment. In the monometallic catalysts, the Pd is found to be com- pletely oxidised already after calcination and to consist of metallic Pd after reductive treatment.
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