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BUILDING AN X-RAY PHOTOELECTRON SPECTROSCOPY ENDSTATION FOR OPERANDO STUDIES OF CATALYTIC CO AND COHYDROGENATION REACTIONS
Stockholm University, Faculty of Science, Department of Physics.ORCID iD: 0000-0001-6085-2916
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2019. , p. 52
National Category
Other Physics Topics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-168657OAI: oai:DiVA.org:su-168657DiVA, id: diva2:1313098
Presentation
2019-05-22, AlbaNova, FA 31, Roslagstullsbacken 21, Stockholm, 11:18 (English)
Opponent
Supervisors
Available from: 2019-06-11 Created: 2019-05-02 Last updated: 2019-06-11Bibliographically approved
List of papers
1. Surface adsorbates during CO2 Hydrogenation on Rh(111) probed in-situ by x-ray photoelectron spectroscopy at 150 mbar
Open this publication in new window or tab >>Surface adsorbates during CO2 Hydrogenation on Rh(111) probed in-situ by x-ray photoelectron spectroscopy at 150 mbar
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

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.

Keywords
HP-XPS, XPS, catalysis, in situ
National Category
Other Physics Topics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-168274 (URN)
Funder
Swedish Research Council
Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-05-02Bibliographically approved
2. A dedicated photoelectron spectroscopy instrument for studies of catalytic reactions at pressures exceeding 1 bar
Open this publication in new window or tab >>A dedicated photoelectron spectroscopy instrument for studies of catalytic reactions at pressures exceeding 1 bar
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

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.

National Category
Other Physics Topics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-168656 (URN)
Note

Version 8a of the manuscript.

Available from: 2019-05-02 Created: 2019-05-02 Last updated: 2019-05-02Bibliographically approved

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