| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- Goodwin, Christopher M., 1989-, et al.
(författare)
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Operando Probing of the Surface Chemistry During the Haber-Bosch Process
- 2024
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Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 625:7994, s. 282-286
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Tidskriftsartikel (refereegranskat)abstract
- The Haber-Bosch process produces NH3 from N2 and H21,2, typically with Fe and Ru3. HB has been proposed as the most important scientific invention in the 20th century4. The chemical state during reaction has been proposed as oxides5, nitrides2, metallic, or surface nitride6. The proposed rate-limiting step has been the dissociation of N27–9, reaction of adsorbed nitrogen10, or desorption of NH311. Due to the vacuum requirement for surface-sensitive techniques, studies at reaction conditions are limited to theory computations12–14. We determined the surface composition, during NH3 production, at pressures up to 1 bar and temperatures as high as 723 K on flat, stepped Fe, and stepped Ru single crystal surfaces using operando X-ray Photoelectron Spectroscopy15. We found that all surfaces remain metallic. On Fe only a small amount of adsorbed N remains, yet Ru’s surface is almost adsorbate free. At 523 K, high amines (NHx) coverages appear on the stepped Fe surface. The results show that the rate-limiting step on Ru is always N2 dissociation. Still, on Fe the hydrogenation step involving adsorbed N atoms is essential for the total rate, as predicted by theory13. If the temperature is lowered on Fe, the rate-limiting steps switch and become surface species’ hydrogenation.
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| 5. |
- Hejral, U., et al.
(författare)
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Composition-Dependent Alloy Nanoparticle Shape Changes under Reaction Conditions : Kinetic and Thermodynamic Effects
- 2024
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Ingår i: The Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 128:10, s. 4330-4342
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Tidskriftsartikel (refereegranskat)abstract
- Particle sintering and reshaping constitute the main cause for catalyst deactivation. An atomic-scale understanding of the correlation among the catalyst structure, its support, and the gas phase under realistic reaction conditions is required for its suppression. In this study, we combined high-energy grazing incidence X-ray diffraction, in situ mass spectrometry, ex situ scanning electron microscopy, and density functional theory calculations to unravel the driving force behind the composition-dependent particle shape changes and sintering processes of alpha-Al2O3(0001)-supported Pt-Pd alloy nanoparticles under realistic reaction conditions for CO oxidation. We find that pure Pt and Pt-rich particles, initially kinetically trapped in metastable flat particle shapes, undergo a strong reaction-induced height increase to adopt a more stable, theoretically predicted compact equilibrium shape. Contrarily, Pd-rich particles prove to be more resistant against shape changes, since they exhibit already a shape close to equilibrium. We thus conclude that the observed initial deviations in particle shape from the theoretical predictions are due to kinetic limitations during growth. Our data provide information on the segregation state of the alloy particles, indicating a Pt core and Pd shell structure under strongly reducing conditions and the alloying of Pt and Pd under the reaction conditions for CO oxidation close to stoichiometry.
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| 6. |
- Zhang, Chu, et al.
(författare)
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Steps and catalytic reactions : CO oxidation with preadsorbed O on Rh(553)
- 2022
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 715
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Tidskriftsartikel (refereegranskat)abstract
- Industrial catalysts are often comprised of nanoparticles supported on high-surface-area oxides, in order to maximise the catalytically active surface area and thereby utilise the active material better. These nanoparticles expose steps and corners that, due to low coordination to neighboring atoms, are more reactive and, as a consequence, are often assumed to have higher catalytic activity. We have investigated the reaction between CO and preadsorbed O on a stepped Rh(553) surface, and show that CO oxidation indeed occurs faster than on the flat Rh(111) surface at the same temperature. However, we do find that this is not a result of reactions at the step sites but rather at the terrace sites close to the steps, due to in-plane relaxation enabled by the step. This insight can provide ways to optimize the shape of the nanoparticles to further improve the activity of certain reactions.
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