| 1. |
- Albertin, Stefano, et al.
(författare)
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Oxidation and Reduction of Ir(100) Studied by High-Energy Surface X-ray Diffraction
- 2022
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:11, s. 5244-5255
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation and reduction of an Ir(100) surface using 2.5, 5, and 10 mbar O2 partial pressure and a sample temperature of 775 K have been studied by using high-energy surface X-ray diffraction (HESXRD) which allowed to record large volumes of reciprocal space in short time periods. The complex 3D diffraction patterns could be disentangled in a stepwise procedure. For the 2.5mbar experiment the measurements indicate the formation of an Ir(100)-O c(2 × 2) oxygen superstructure along with the onset of epitaxial IrO2(110) bulk oxide formation. For the 5 and 10 mbar O2 partial pressures the formation of additional IrO2 bulk oxide epitaxies with (100) and (101) orientations as well as of polycrystalline IrO2 was observed. Upon CO reduction, we found the IrO2 islands to be reduced into epitaxial and metallic Ir(111) and (221) oriented islands.
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| 2. |
- 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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| 3. |
- Gleißner, Robert, et al.
(författare)
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Role of Oxidation–Reduction Dynamics in the Application of Cu/ZnO-Based Catalysts
- 2023
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Ingår i: ACS Applied Nano Materials. - 2574-0970. ; 6:9, s. 8004-8016
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Tidskriftsartikel (refereegranskat)abstract
- We investigated Cu nanoparticles (NPs) on vicinal and basal ZnO supports to obtain an atomistic picture of the catalyst’s structure under in situ oxidizing and reducing conditions. The Cu/ZnO model catalysts were investigated at elevated gas pressures by high energy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu2O under atmospheric conditions at room temperature. As the nanoparticles swell during oxidation, they maintain their epitaxy on basal ZnO (000 ± 1) surfaces, whereas on the vicinal ZnO (101̅4) surface, the nanoparticles undergo a coherent tilt. We find that the oxidation process is fully reversible under H2 flow at 500 K, resulting in predominantly well-aligned nanoparticles on the basal surfaces, whereas the orientation of Cu NPs on vicinal ZnO was only partially restored. The analysis of the substrate crystal truncation rods evidences the stability of basal ZnO surfaces under all gas conditions. No Cu–Zn bulk alloy formation is observed. Under CO2 flow, no diffraction signal from the nanoparticles is detected, pointing to their completely disordered state. The AP-XPS results are in line with the formation of CuO. Scanning electron microscopy images show that massive mass transport has set in, leading to the formation of larger agglomerates.
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| 4. |
- 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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| 5. |
- Li, Tang, et al.
(författare)
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Dose-efficient scanning Compton X-ray microscopy
- 2023
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Ingår i: Light: Science and Applications. - 2095-5545. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- The highest resolution of images of soft matter and biological materials is ultimately limited by modification of the structure, induced by the necessarily high energy of short-wavelength radiation. Imaging the inelastically scattered X-rays at a photon energy of 60 keV (0.02 nm wavelength) offers greater signal per energy transferred to the sample than coherent-scattering techniques such as phase-contrast microscopy and projection holography. We present images of dried, unstained, and unfixed biological objects obtained by scanning Compton X-ray microscopy, at a resolution of about 70 nm. This microscope was realised using novel wedged multilayer Laue lenses that were fabricated to sub-ångström precision, a new wavefront measurement scheme for hard X rays, and efficient pixel-array detectors. The doses required to form these images were as little as 0.02% of the tolerable dose and 0.05% of that needed for phase-contrast imaging at similar resolution using 17 keV photon energy. The images obtained provide a quantitative map of the projected mass density in the sample, as confirmed by imaging a silicon wedge. Based on these results, we find that it should be possible to obtain radiation damage-free images of biological samples at a resolution below 10 nm.
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| 6. |
- Marlton, Frederick, et al.
(författare)
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A simple correction for the parallax effect in X-ray pair distribution function measurements
- 2019
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Ingår i: Journal of Applied Crystallography. - 0021-8898. ; 52, s. 1072-1076
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Tidskriftsartikel (refereegranskat)abstract
- Total scattering and pair distribution function (PDF) analysis has created new insights that traditional powder diffraction methods have been unable to achieve in understanding the local structures of materials exhibiting disorder or complex nanostructures. Care must be taken in such analyses as subtle and discrete features in the PDF can easily be artefacts generated in the measurement process, which can result in unphysical models and interpretation. The focus of this study is an artefact called the parallax effect, which can occur in area detectors with thick detection layers during the collection of X-ray PDF data. This effect results in high-Q peak offsets, which subsequently cause an r-dependent shift in the PDF peak positions in real space. Such effects should be accounted for if a truly accurate model is to be achieved, and a simple correction that can be conducted via a Rietveld refinement against the reference data is proposed.
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| 7. |
- Martin, Natalia Mihaela, 1984, et al.
(författare)
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Structure-function relationship during CO2 methanation over Rh/Al2O3 and Rh/SiO2 catalysts at atmospheric pressure conditions
- 2018
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 8:10, s. 2686-2696
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Tidskriftsartikel (refereegranskat)abstract
- The effect of support material and chemical state of Rh for Rh/Al2O3 and Rh/SiO2 model catalysts during CO2 hydrogenation were studied by a combined array of in situ characterisation techniques including diffuse reflectance infrared Fourier transform spectroscopy, energy-dispersive X-ray absorption spectroscopy and high-energy X-ray diffraction at 250-350 °C and atmospheric pressure. The CO2 methanation proceeds via intermediate formation of adsorbed CO species on metallic Rh likely followed by their hydrogenation to methane. Linearly-bonded CO species is suggested to be a more active precursor in the hydrogenation compared to the bridge-bonded species, which seems to relate to particle size effects: for larger particles mainly the formation of inactive bridge-bonded CO species takes place. Further, analysis of the chemical state of Rh during reaction conditions reveal a minor formation of RhOx from dissociation of CO2 , which is a consequence of the increased activity observed over Rh/Al2O3 catalyst.
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| 8. |
- Martin, Natalia M., et al.
(författare)
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Structure-function relationship for CO2 methanation over ceria supported Rh and Ni catalysts under atmospheric pressure conditions
- 2019
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Ingår i: Catalysis Science & Technology. - : Royal Society of Chemistry. - 2044-4753 .- 2044-4761. ; 9:7, s. 1644-1653
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Tidskriftsartikel (refereegranskat)abstract
- In situ structural and chemical state characterization of Rh/CeO2 and Ni/CeO2 catalysts during atmospheric pressure CO2 methanation has been performed by a combined array of time-resolved analytical techniques including ambient-pressure X-ray photoelectron spectroscopy, high-energy X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy. The ceria phase is partially reduced during the CO2 methanation and in particular Ce3+ species seem to facilitate activation of CO2 molecules. The activated CO2 molecules then react with atomic hydrogen provided from H-2 dissociation on Rh and Ni sites to form formate species. For the most active catalyst (Rh/CeO2), transmission electron microscopy measurements show that the Rh nanoparticles are small (average 4 nm, but with a long tail towards smaller particles) due to a strong interaction between Rh particles and the ceria phase. In contrast, larger nanoparticles were observed for the Ni/CeO2 catalyst (average 6 nm, with no crystallites below 5 nm found), suggesting a weaker interaction with the ceria phase. The higher selectivity towards methane of Rh/CeO2 is proposed to be due to the stronger metal-support interaction.
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| 9. |
- Martin, Natalia Mihaela, 1984, et al.
(författare)
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Structure-function relationship for CO2 methanation over ceria supported Rh and Ni catalysts under atmospheric pressure conditions
- 2019
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Ingår i: Catalysis Science and Technology. - 2044-4753 .- 2044-4761. ; 9:7, s. 1644-1653
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Tidskriftsartikel (refereegranskat)abstract
- In situ structural and chemical state characterization of Rh/CeO 2 and Ni/CeO 2 catalysts during atmospheric pressure CO 2 methanation has been performed by a combined array of time-resolved analytical techniques including ambient-pressure X-ray photoelectron spectroscopy, high-energy X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy. The ceria phase is partially reduced during the CO 2 methanation and in particular Ce 3+ species seem to facilitate activation of CO 2 molecules. The activated CO 2 molecules then react with atomic hydrogen provided from H 2 dissociation on Rh and Ni sites to form formate species. For the most active catalyst (Rh/CeO 2 ), transmission electron microscopy measurements show that the Rh nanoparticles are small (average 4 nm, but with a long tail towards smaller particles) due to a strong interaction between Rh particles and the ceria phase. In contrast, larger nanoparticles were observed for the Ni/CeO 2 catalyst (average 6 nm, with no crystallites below 5 nm found), suggesting a weaker interaction with the ceria phase. The higher selectivity towards methane of Rh/CeO 2 is proposed to be due to the stronger metal-support interaction.
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| 10. |
- Shipilin, Mikhail, et al.
(författare)
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Quantitative surface structure determination using in situ high-energy SXRD: Surface oxide formation on Pd(100) during catalytic CO oxidation
- 2014
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 630, s. 229-235
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Tidskriftsartikel (refereegranskat)abstract
- We have performed a quantitative structure determination of the SQRT(5×5)R27° surface oxide, formed on Pd(100) under semi-realistic conditions for catalytic CO oxidation, using in situ high-energy surface X-ray diffraction. We describe the experiment and the extraction of quantitative data in detail. The structural results are in agreement with previous reports of a system consisting of a single layer of PdO(101) formed in pure O2 on top of Pd(100) and studied under ultra high vacuum conditions.
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| 11. |
- 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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| 12. |
- Simutis, Gediminas, et al.
(författare)
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In situ uniaxial pressure cell for x-ray and neutron scattering experiments
- 2023
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 94:1
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Tidskriftsartikel (refereegranskat)abstract
- We present an in situ uniaxial pressure device optimized for small angle x-ray and neutron scattering experiments at low-temperatures and high magnetic fields. A stepper motor generates force, which is transmitted to the sample via a rod with an integrated transducer that continuously monitors the force. The device has been designed to generate forces up to 200 N in both compressive and tensile configurations, and a feedback control allows operating the system in a continuous-pressure mode as the temperature is changed. The uniaxial pressure device can be used for various instruments and multiple cryostats through simple and exchangeable adapters. It is compatible with multiple sample holders, which can be easily changed depending on the sample properties and the desired experiment and allow rapid sample changes.
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