| 1. |
- Albertin, S., et al.
(författare)
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Surface optical reflectance combined with x-ray techniques during gas-surface interactions
- 2020
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Ingår i: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 53:22
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Tidskriftsartikel (refereegranskat)abstract
- High energy surface x-ray diffraction (HESXRD), x-ray reflectivity (XRR), mass spectrometry (MS) and surface optical reflectance (SOR) have been combined to simultaneously obtain sub-second information on the surface structure and morphology from a Pd(100) model catalyst during in situ oxidation at elevated temperatures and pressures resulting in Pd bulk oxide formation. The results show a strong correlation between the HESXRD and SOR signal intensities during the experiment, enabling phase determination and a time-resolved thickness estimation of the oxide by HESXRD, complemented by XRR measurements. The experiments show a remarkable sensitivity of the SOR to changes in the surface phase and morphology, in particular to the initial stages of oxidation/reduction. The data imply that SOR can detect the formation of an ultrathin PdO surface oxide layer of only 2-3 angstrom thickness.
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| 2. |
- Amann, Peter, et al.
(författare)
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A dedicated photoelectron spectroscopy instrument for studies of catalytic reactions at pressures exceeding 1 bar
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Here, we present a new high-pressure x-ray photoelectron spectroscopy system dedicated to probing catalytic reactions under realistic conditions at pressures exceeding 1 bar. The instrument builds around the concept of a “virtual cell” in which a gasflow is directed onto the sample surface creating a local high pressure on top of the sample. This allows the instrument to maintain a low pressure of a few mbars in the main chamber, while simultaneously keeping a local pressure of around 1 bar. Synchrotron radiation based grazing incidence photoemission within ± 5° is used to enhance the surface sensitivity in the experiment. The aperture, separating the high-pressure region from the differential pumping of the electron spectrometer, consists of multiple, evenly spaced, mm sized holes matching the footprint of the x-ray beam on the sample surface. As the photo-emitted electrons are subject to strong scattering in the gas phase and the resulting signal is therefore highly dependent on the sample to aperture distance, the latter is controlled with high precision using a fully integrated manipulator that allows for sample movement with step sizes of 10 nm between 0 and –5 mm with very low vibrational amplitude. The instrumental features allows acquisition of metallic bulk spectra at He pressures up to 2.5 bar and also allows for following C1s spectra under realistic gas mixtures of CO + H2with various temperatures up to 500°C. This capability opens for studies of catalytic reactions in operandi.
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| 3. |
- Amann, Peter, et al.
(författare)
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A high-pressure x-ray photoelectron spectroscopy instrument for studies of industrially relevant catalytic reactions at pressures of several bars
- 2019
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Ingår i: Review of Scientific Instruments. - : American Institute of Physics (AIP). - 0034-6748 .- 1089-7623. ; 90:10
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Tidskriftsartikel (refereegranskat)abstract
- We present a new high-pressure x-ray photoelectron spectroscopy system dedicated to probing catalytic reactions under realistic conditions at pressures of multiple bars. The instrument builds around the novel concept of a "virtual cell" in which a gas flow onto the sample surface creates a localized high-pressure pillow. This allows the instrument to be operated with a low pressure of a few millibar in the main chamber, while simultaneously a local pressure exceeding 1 bar can be supplied at the sample surface. Synchrotron based hard x-ray excitation is used to increase the electron mean free path in the gas region between sample and analyzer while grazing incidence <5 degrees close to total external refection conditions enhances surface sensitivity. The aperture separating the high-pressure region from the differential pumping of the electron spectrometer consists of multiple, evenly spaced, micrometer sized holes matching the footprint of the x-ray beam on the sample. The resulting signal is highly dependent on the sample-to-aperture distance because photoemitted electrons are subject to strong scattering in the gas phase. Therefore, high precision control of the sample-to-aperture distance is crucial. A fully integrated manipulator allows for sample movement with step sizes of 10 nm between 0 and -5 mm with very low vibrational amplitude and also for sample heating up to 500 degrees C under reaction conditions. We demonstrate the performance of this novel instrument with bulk 2p spectra of a copper single crystal at He pressures of up to 2.5 bars and C1s spectra measured in gas mixtures of CO + H-2 at pressures of up to 790 mbar. The capability to detect emitted photoelectrons at several bars opens the prospect for studies of catalytic reactions under industrially relevant operando conditions.
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| 4. |
- Amann, Peter, et al.
(författare)
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The state of zinc in methanol synthesis over a Zn/ZnO/Cu(211) model catalyst
- 2022
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 376:6593, s. 603-608
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Tidskriftsartikel (refereegranskat)abstract
- The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO2/CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO2/CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO2 gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO2 methanol synthesis.
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| 5. |
- Blomberg, Sara, et al.
(författare)
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Bridging the Pressure Gap in CO Oxidation
- 2021
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 11:15, s. 9128-9135
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Tidskriftsartikel (refereegranskat)abstract
- Performing fundamental operando catalysis studies under realistic conditions is a key to further develop and increase the efficiency of industrial catalysts. Operando X-ray photoelectron spectroscopy (XPS) experiments have been limited to pressures, and the relevance for industrial applications has been questioned. Herein, we report on the CO oxidation experiment on Pd(100) performed at a total pressure of 1 bar using XPS. We investigate the light-off regime and the surface chemical composition at the atomistic level in the highly active phase. Furthermore, the observed gas-phase photoemission peaks of CO2, CO, and O2 indicate that the kinetics of the reaction during the light-off regime can be followed operando, and by studying the reaction rate of the reaction, the activation energy is calculated. The reaction was preceded by an in situ oxidation study in 7% O2 in He and a total pressure of 70 mbar to confirm the surface sensitivity and assignment of the oxygen-induced photoemission peaks. However, oxygen-induced photoemission peaks were not observed during the reaction studies, but instead, a metallic Pd phase is present in the highly active regime under the conditions applied. The novel XPS setup utilizes hard X-rays to enable high-pressure studies, combined with a grazing incident angle to increase the surface sensitivity of the measurement. Our findings demonstrate the possibilities of achieving chemical information of the catalyst, operando, on an atomistic level, under industrially relevant conditions.
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| 6. |
- 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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| 7. |
- Blomberg, Sara, et al.
(författare)
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Strain Dependent Light-off Temperature in Catalysis Revealed by Planar Laser-Induced Fluorescence
- 2017
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 7:1, s. 110-114
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Busch, Michael, 1983, et al.
(författare)
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Adsorption of NO on Fe3O4(111)
- 2018
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 693, s. 84-87
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Tidskriftsartikel (refereegranskat)abstract
- Adsorption of NO on Fe 3 O 4 (111) is studied by density functional theory (DFT) calculations. NO is preferably adsorbed atop the octahedral site which has a clearly higher adsorption energy than the tetrahedral site. The difference in adsorption energy correlates with differences in adsorption geometries and N–O stretch vibrations. The results are in good agreement with temperature programmed desorption (TPD) and reflection-adsorption IR spectroscopy (RAIRS) measurements and provide an explanation of the observation of only one vibrational mode despite two distinct TPD features.
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| 9. |
- Degerman, David, 1989-, et al.
(författare)
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Demonstrating Pressure Jumping as a Tool to Address the Pressure Gap in High Pressure Photoelectron Spectroscopy of CO and CO2 Hydrogenation on Rh(211)
- 2024
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Ingår i: ChemPhysChem. - 1439-4235 .- 1439-7641. ; 25:1
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Tidskriftsartikel (refereegranskat)abstract
- Operando probing by x-ray photoelectron spectroscopy (XPS) of certain hydrogenation reactions are often limited by the scattering of photoelectrons in the gas phase. This work describes a method designed to partially circumvent this so called pressure gap. By performing a rapid switch from a high pressure (where acquisition is impossible) to a lower pressure we can for a short while probe a remnant of the high pressure surface as well as the time dynamics during the re-equilibration to the new pressure. This methodology is demonstrated using the CO2 and the CO hydrogenation reaction over Rh(211). In the CO2 hydrogenation reaction, the remnant surface of a 2 bar pressure shows an adsorbate distribution which favors chemisorbed CHx adsorbates over chemisorbed CO. This contrasts against previous static operando spectra acquired at lower pressures. Furthermore, the pressure jumping method yields a faster acquisition and more detailed spectra than static operando measurements above 1 bar. In the CO hydrogenation reaction, we observe that CHx accumulated faster during the 275 mbar low pressure regime, and different hypotheses are presented regarding this observation.
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| 10. |
- Degerman, David, 1989-, et al.
(författare)
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Effect of CO2-Rich Syngas on the Chemical State of Fe(110) during Fischer-Tropsch Synthesis
- 2024
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Ingår i: The Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 128:13, s. 5542-5552
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Tidskriftsartikel (refereegranskat)abstract
- We have used in situ X-ray photoelectron spectroscopy to obtain information about the chemical state of a Fe single-crystal catalyst upon addition of CO2 in the syngas feed during Fischer–Tropsch synthesis (FTS) between 85 and 550 mbar. We found that at certain temperatures, the ternary mixture of CO, CO2, and H2 yields a chemical state which is resemblant of neither the CO hydrogenation nor the CO2 hydrogenation reaction mixtures in isolation. The addition of CO2 to a CO + H2 reaction mixture mostly affects the chemical state at low-temperature FTS conditions (i.e., below 254 °C). In this temperature span, the ternary reaction mixture resulted in a carburized surface, whereas the CO + H2 reaction led to surface oxidation. We propose a hypothesis, where a carbonate intermediate produced by CO2 interaction with Fe oxide aids the reduction of the Fe oxide, paving the way for the carburization of the Fe by dissociated CO. Very small differences in the spectra of the CO + H2 and the CO + CO2 + H2 reaction mixtures were observed above 254 °C, suggesting that the CO2 is a spectator in these conditions. Changing the total pressure of both the CO hydrogenation and ternary reaction mixture causes quantitative changes in the spectra at both low- and high-temperature FTS conditions, the degree of oxidation/carburization was affected in the low-temperature-FTS regime, and the degree of hydrocarbon build-up was affected in the high-temperature-FTS.
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