| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Degerman, David, 1989-
(författare)
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BUILDING AN X-RAY PHOTOELECTRON SPECTROSCOPY ENDSTATION FOR OPERANDO STUDIES OF CATALYTIC CO AND CO2 HYDROGENATION REACTIONS
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- X-ray Photoelectron Spectroscopy (XPS) provides an element-specific surface sensitive probe of the chemical composition in a system, and is consequently one of the workhorses of surface science and catalysis research. The obtained information on the chemical and physical state of the catalyst and adsorbates is essential in the endeavor to achieve a fundamental understanding how chemical reactions are facilitated by the catalyst. Due to the short mean free path of electrons in gaseous media most of the XPS experiments so far are done in the range between ultra-high vacuum (<10-7 mbar) and near-ambient pressure (1-10 mbar) regimes. For certain reactions, such as the hydrogenation of CO and CO2, higher pressures (comparable to one bar or higher) are needed in order to give a more realistic representation of the system.This thesis concerns the theoretical background, design, build-up and the first results of an instrument with the goal to bridge the pressure gap between operando conditions at the solid gas interface and surface science model systems. Thanks to a new design of the electron analyzer front cone we have built an instrument where the relevant length scales are reduced to match the electron inelastic mean free path in pressurized atmospheres above one bar. A number of key factors make this possible, but most prominently it is the unique sample environment using a “virtual pressure cell” in combination with a grazing incidence geometry below the critical angle of total reflection. Furthermore, the instrument utilizes hard x-rays to generate high-kinetic energy electrons and thereby increase the mean free path in the pressurized atmosphere. Lastly, the instrument uses a laser-based heating solution which removes the effect of electric and magnetic fields.With this we have been able to (1) record spectra of Rh above 2 bar of inert atmosphere, as well as with reaction mixtures of CO2 + O2 up to 1 bar and (2) probe surface species and observe temperature dependent chemistry during CO2 hydrogenation during ongoing reactions at 150 mbar.
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| 7. |
- Degerman, David, 1989-
(författare)
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CO, CO2 and N2 hydrogenation reactions probed by operando x-ray photoelectron spectroscopy
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Catalytic reactions are essential for generating the chemical products required by the modern society. In particular, reactions related to clean energy storage and generation as well as fertilizer production are facilitated by catalysts. However, the processes are often insufficiently understood at a mechanistic level. One of the main reasons is that a holistic investigation of heterogenous catalyst surfaces during reaction conditions requires experimental techniques that combine element specificity, surface sensitivity and can work under operando conditions. While excellent in terms of the first two criteria, x-ray photoelectron spectroscopy (XPS) has traditionally not been compatible with the high pressures and temperatures required for many catalytic reactions; a “pressure gap” opened between the obtainable conditions in the lab and the relevant conditions in a real catalytic reactor.We have built a scientific instrument, a synchrotron endstation, that addresses this issue and allows operando probing at 100x higher pressure than elsewhere. The POLARIS instrument is located at PETRA III in Hamburg. This work describes the instrumentation and the theoretical background for the technique. The main focus, however, is on the mechanistic discoveries made when operando XPS with POLARIS was applied to hydrogenation of CO, CO2 and N2 over single crystal catalysts. The surfaces examined in this work include Fe, Co, Ni, Cu-Zn, Rh and Ru.Regarding the CO hydrogenation reaction, this work describes how the Fe surfaces facilitate rapid CO dissociation, but slow adsorbate desorption. This combination results in carbide phases and a drastic accumulation of long-chain hydrocarbons. A similar behavior was noted in Ni catalysts at low temperatures, where a non-stoichiometric carbide was formed, but the hydrogenation rate of the carbide was dependent on the temperature and the partial pressure of the reactants. Co surfaces exhibit a mixture of CO and partly hydrogenated hydrocarbons, indicating a slower termination than observed on Ni, but without the drastic carburization noted for Fe. On Rh catalysts, a subset of the non-dissociated CO molecules may hydrogenate, and alkoxy intermediates co-exist with non-saturated hydrocarbons, allowing for selectivity towards oxygenated products. For the CO2 hydrogenation reaction on Rh, the residence time of CO2 was observed to be short and the coverage of dissociated intermediates was low in the 150 mbar pressure range. However, when switching the pressure rapidly it can be shown that pressures around 2 bar increase the coverage, and reveals other adsorbates than the static pressure study.A Cu catalyst with surficial Zn was examined in ternary reaction mixtures of CO2, CO and H2. Here we noted that CO kept the Zn reduced. In the N2 hydrogenation reaction, the rate of chemisorption and dissociation of N2 dictate two different rate limiting scenarios. On Ru the reaction is limited by the N2 dissociation and on Fe it is also limited by the hydrogenation of chemisorbed N.The significance of operando conditions is particularly manifested with regard to the hydrogen partial pressure and its interplay with the resulting adsorbate distribution.
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| 8. |
- Degerman, David, 1989-, et al.
(författare)
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Operando Observation of Oxygenated Intermediates during CO Hydrogenation on Rh Single Crystals
- 2022
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:16, s. 7038-7042
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Tidskriftsartikel (refereegranskat)abstract
- The CO hydrogenation reaction over the Rh(111) and (211) surfaces has been investigated operando by X-ray photoelectron spectroscopy at a pressure of 150 mbar. Observations of the resting state of the catalyst give mechanistic insight into the selectivity of Rh for generating ethanol from CO hydrogenation. This study shows that the Rh(111) surface does not dissociate all CO molecules before hydrogenation of the O and C atoms, which allows methoxy and other both oxygenated and hydrogenated species to be visible in the photoelectron spectra.
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| 9. |
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| 10. |
- Degerman, David, 1989-, et al.
(författare)
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State of the Surface During CO Hydrogenation over Ni(111) and Ni(211) Probed by Operando X-ray Photoelectron Spectroscopy
- 2023
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 127:8, s. 4021-4032
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Tidskriftsartikel (refereegranskat)abstract
- The state of the surface near-region during CO hydro- genation of Ni(111) and Ni(211) single crystal surfaces was investigated using various gas mixtures between 150 and 500 mbar, 200 and 325 °C, by operando X-ray photoelectron spectroscopy. We report how higher temperatures and hydrogen content correlate with a movement of CO away from the on-top configurations and toward multicoordinated sites of the nickel surface and how a nickel carbide is formed in the surface near region, particularly at high partial pressures of CO and lower temperatures. The presence of the carbide affects the CO bonding and was observed to be reduced during hydrogen-rich conditions and temperatures above 250 °C.
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| 11. |
- Degerman, David, 1989-, et al.
(författare)
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Surface adsorbates during CO2 Hydrogenation on Rh(111) probed in-situ by x-ray photoelectron spectroscopy at 150 mbar
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The catalytic CO2 hydrogenation reaction was examined in situ by High Pressure X-ray Photoelectron Spectroscopy (HP-XPS) at 150 mbar and between 150 and 350°C. The results indicate two temperature regimes; the first one with temperature dependent desorption of carbon species between 150°C and 200°C. The second temperature regime is between 250 and 350 °C. In this interval, the carbon species are formed and immediately reacted away, resulting in a lower temperature dependence on surface coverage. The XPS coverage calculations and the component analysis indicate that water is the most abundant surface adsorbate, and that CHx fragments and CO are the most abundant carbon species. The hydrogenation state of the CHx species varies with temperature, where higher temperatures result in a larger population of more hydrogenated species.
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| 12. |
- Shipilin, Mikhail, et al.
(författare)
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In Situ Surface-Sensitive Investigation of Multiple Carbon Phases on Fe(110) in the Fischer-Tropsch Synthesis
- 2022
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 12:13, s. 7609-7621
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Tidskriftsartikel (refereegranskat)abstract
- Carbide formation on iron-based catalysts is an integral and, arguably, the most important part of the Fischer–Tropsch synthesis process, converting CO and H2 into synthetic fuels and numerous valuable chemicals. Here, we report an in situ surface-sensitive study of the effect of pressure, temperature, time, and gas feed composition on the growth dynamics of two distinct iron–carbon phases with the octahedral and trigonal prismatic coordination of carbon sites on an Fe(110) single crystal acting as a model catalyst. Using a combination of state-of-the-art X-ray photoelectron spectroscopy at an unprecedentedly high pressure, high-energy surface X-ray diffraction, mass spectrometry, and theoretical calculations, we reveal the details of iron surface carburization and product formation under semirealistic conditions. We provide a detailed insight into the state of the catalyst’s surface in relation to the reaction.
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| 13. |
- Wang, Hsin-Yi, et al.
(författare)
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Direct Evidence of Subsurface Oxygen Formation in Oxide-Derived Cu by X-ray Photoelectron Spectroscopy
- 2022
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Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 61:3
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Tidskriftsartikel (refereegranskat)abstract
- Subsurface oxygen has been proposed to be crucial in oxide-derived copper (OD-Cu) electrocatalysts for enhancing the binding of CO intermediates during CO2 reduction reaction (CO2RR). However, the presence of such oxygen species under reductive conditions still remains debated. In this work, the existence of subsurface oxygen is validated by grazing incident hard X-ray photoelectron spectroscopy, where OD-Cu was prepared by reduction of Cu oxide with H2 without exposing to air. The results suggest two types of subsurface oxygen embedded between the fully reduced metallic surface and the Cu2O buried beneath: (i) oxygen staying at lattice defects and/or vacancies in the surface-most region and (ii) interstitial oxygen intercalated in metal structure. This study adds convincing support to the presence of subsurface oxygen in OD-Cu, which previously has been suggested to play an important role to mitigate the σ-repulsion of Cu for CO intermediates in CO2RR.
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