| 1. |
- Braud, N., et al.
(author)
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Cleaning and tailoring the Pt3Sn(111) surface for surface experiments
- 2023
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In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 732
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Journal article (peer-reviewed)abstract
- The cleaning process of the bimetallic Pt3Sn(111) surface has been studied by means of low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (mu.-LEED), and X-ray photoemission electron microscopy (XPEEM). Different cleaning procedures, performed under ultra-high vacuum conditions (UHV), including sputtering with argon ions and repeated cycles of annealing up to 1500 K were investigated. In this work, we show that a clean Pt3Sn(111) surface of high structural quality with a sharp and brilliant (2 x 2) bulk reconstruction in LEED as well as a perfectly smooth surface with terraces of micron size can be achieved by sputtering, annealing at very high temperatures, followed by a subsequent slow (0.09 K/s) and careful cooling procedure. Additionally, we show the possibility of tailoring the Sn concentration in the topmost layers of Pt3Sn(111) as a function of annealing temperature and subsequent cooling rate. Structural changes of the surface are induced by Sn segregation combined with a surface order-disorder transition at 1340 K. Moreover, two new surface reconstructions depending on the cooling rate are reported.
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| 2. |
- Hejral, U., et al.
(author)
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High-energy x-ray diffraction from surfaces and nanoparticles
- 2017
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In: Physical Review B. - 2469-9950. ; 96:19
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Journal article (peer-reviewed)abstract
- High-energy surface-sensitive x-ray diffraction (HESXRD) is a powerful high-energy photon technique (E > 70 keV) that has in recent years proven to allow a fast data acquisition for the 3D structure determination of surfaces and nanoparticles under in situ and operando conditions. The use of a large-area detector facilitates the direct collection of nearly distortion-free diffraction patterns over a wide q range, including crystal truncation rods perpendicular to the surface and large-area reciprocal space maps from epitaxial nanoparticles, which is not possible in the conventional low-photon energy approach (E=10-20keV). Here, we present a comprehensive mathematical approach, explaining the working principle of HESXRD for both single-crystal surfaces and epitaxial nanostructures on single-crystal supports. The angular calculations used in conventional crystal truncation rod measurements at low-photon energies are adopted for the high-photon-energy regime, illustrating why and to which extent large reciprocal-space areas can be probed in stationary geometry with fixed sample rotation. We discuss how imperfections such as mosaicity and finite domain size aid in sampling a substantial part of reciprocal space without the need of rotating the sample. An exact account is given of the area probed in reciprocal space using such a stationary mode, which is essential for in situ or operando time-resolved experiments on surfaces and nanostructures.
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| 3. |
- Blomberg, Sara, et al.
(author)
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Strain Dependent Light-off Temperature in Catalysis Revealed by Planar Laser-Induced Fluorescence
- 2017
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In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 7:1, s. 110-114
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Journal article (peer-reviewed)abstract
- Understanding how specific atom sites on metal surfaces lower the energy barrier for chemical reactions is vital in catalysis. Studies on simplified model systems have shown that atoms arranged as steps on the surface play an important role in catalytic reactions, but a direct comparison of how the light-off temperature is affected by the atom orientation on the step has not yet been possible due to methodological constraints. Here we report in situ spatially resolved measurements of the CO2 production over a cylindrical-shaped Pd catalyst and show that the light-off temperature at different parts of the crystal depends on the step orientation of the two types of steps (named A and B). Our finding is supported by density functional theory calculations, revealing that the steps, in contrast to what has been previously reported in the literature, are not directly involved in the reaction onset but have the role of releasing stress.
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| 4. |
- Gericke, Sabrina M., et al.
(author)
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Effect of Different In 2 O 3 (111) Surface Terminations on CO 2 Adsorption
- 2023
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In: ACS Applied Materials & Interfaces. - 1944-8252 .- 1944-8244. ; 15:38, s. 45367-45377
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Journal article (peer-reviewed)abstract
- In2O3-based catalysts have shown high activity and selectivity for CO2 hydrogenation to methanol; however, the origin of the high performance of In2O3 is still unclear. To elucidate the initial steps of CO2 hydrogenation over In2O3, we have combined X-ray photoelectron spectroscopy and density functional theory calculations to study the adsorption of CO2 on the In2O3(111) crystalline surface with different terminations, namely, the stoichiometric, reduced, and hydroxylated surface. The combined approach confirms that the reduction of the surface results in the formation of In adatoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level shifts (using methanol and formic acid as benchmark molecules) suggests that CO2 adsorbs as a carbonate on all three surface terminations. We find that the adsorption of CO2 is hindered by hydroxyl groups on the hydroxylated surface.
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| 5. |
- Grespi, A., et al.
(author)
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Probing the electrode-liquid interface using operando total-reflection X-ray absorption spectroscopy
- 2024
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In: Surface Science. - : Elsevier B.V.. - 0039-6028 .- 1879-2758. ; 748
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Journal article (peer-reviewed)abstract
- Traditional methods to study electrochemical (EC) processes, although successful, are based on current/voltage measurements, providing information about performances rather than offering a direct observation of chemical and structural changes occurring at the electrode surface. These processes are localized at the electrode-electrolyte interface, the structure of which is the main determinant of their behavior, but most surface sensitive experimental techniques are limited to ex situ conditions, owing to the need for an ultra-high vacuum environment. In this contribution, we report operando X-ray absorption spectroscopy in total external reflection geometry (Refle-XAFS) at P64 beamline (DESY, Hamburg), using a simple and versatile EC flow cell designed for multimodal surface sensitive studies with hard X-ray scattering and spectroscopy techniques. We show that the Refle-XAFS method can be used to study chemical surface changes of industrial alloys and model electrodes in harsh electrochemical environments, without being limited to thin film samples. The surface passive film development and breakdown of a corrosion-resistant Ni-Cr-Mo alloy and the electro-oxidation of polycrystalline gold (poly-Au), relevant for fundamental studies on water electrolysis, were investigated. Despite the strong attenuation of the beam by the electrolyte and the PEEK walls of the EC cell, nanoscale surface oxide films were detected using beam energies down to 8 keV. The passivity breakdown region of Ni alloy 59 in 1 M NaCl at pH 7 and pH 12 was identified, showing differences in the composition of the surface oxides during anodic polarization. The electro-oxidation of poly-Au in 0.05 M H2SO4 was observed, showing a progression from two-dimensional Au1+/3+ to three-dimensional thick Au3+ surface oxide/hydroxide during OER.
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| 6. |
- Klikovits, J., et al.
(author)
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Step-Orientation-Dependent Oxidation: From 1D to 2D Oxides
- 2008
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In: Physical Review Letters. - 1079-7114. ; 101:26
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Journal article (peer-reviewed)abstract
- Using scanning tunneling microscopy and density functional theory, we have studied the initial oxidation of Rh(111) surfaces with two types of straight steps, having {100} and {111} microfacets. The one-dimensional (1D) oxide initially formed at the steps acts as a barrier impeding formation of the 2D oxide on the (111) terrace behind it. We demonstrate that the details of the structure of the 1D oxide govern the rate of 2D oxidation and discuss implications for oxidation of nanoparticles.
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| 7. |
- Martin, Natalia Mihaela, 1984, et al.
(author)
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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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In: Catalysis Science and Technology. - 2044-4753 .- 2044-4761. ; 9:7, s. 1644-1653
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Journal article (peer-reviewed)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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| 8. |
- Shipilin, M., et al.
(author)
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Fe Oxides on Ag Surfaces : Structure and Reactivity
- 2017
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In: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 60:6-7
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Journal article (peer-reviewed)abstract
- One layer thick iron oxide films are attractive from both applied and fundamental science perspectives. The structural and chemical properties of these systems can be tuned by changing the substrate, making them promising materials for heterogeneous catalysis. In the present work, we investigate the structure of FeO(111) monolayer films grown on Ag(100) and Ag(111) substrates by means of microscopy and diffraction techniques and compare it with the structure of FeO(111) grown on other substrates reported in literature. We also study the NO adsorption properties of FeO(111)/Ag(100) and FeO(111)/Ag(111) systems utilizing different spectroscopic techniques. We discuss similarities and differences in the data obtained from adsorption experiments and compare it with previous results for FeO(111)/Pt(111).
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| 9. |
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