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Probing the transition state region in catalytic CO oxidation on Ru
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
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2015 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 347, no 6225, 978-982 p.Article in journal (Refereed) Published
Abstract [en]

Femtosecond x-ray laser pulses are used to probe the carbon monoxide (CO) oxidation reaction on ruthenium (Ru) initiated by an optical laser pulse. On a time scale of a few hundred femtoseconds, the optical laser pulse excites motions of CO and oxygen (O) on the surface, allowing the reactants to collide, and, with a transient close to a picosecond (ps), new electronic states appear in the OK-edge x-ray absorption spectrum. Density functional theory calculations indicate that these result from changes in the adsorption site and bond formation between CO and O with a distribution of OC-O bond lengths close to the transition state (TS). After 1 ps, 10% of the CO populate the TS region, which is consistent with predictions based on a quantum oscillator model.

Place, publisher, year, edition, pages
2015. Vol. 347, no 6225, 978-982 p.
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-115673DOI: 10.1126/science.1261747ISI: 000349958900034PubMedID: 25722407OAI: oai:DiVA.org:su-115673DiVA: diva2:800002
Note

AuthorCount:30;

Available from: 2015-04-01 Created: 2015-03-27 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Ultrafast Probing of CO Reactions on Metal Surfaces: Changes in the molecular orbitals during the catalysis process
Open this publication in new window or tab >>Ultrafast Probing of CO Reactions on Metal Surfaces: Changes in the molecular orbitals during the catalysis process
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents experimental studies of three different chemical reaction steps relevant for heterogeneous catalysis: dissociation, desorption, and oxidation. CO on single-crystal metal surfaces was chosen as the model systems.

X-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) provide information about the electronic structure, and were performed on CO/Fe to measure both a non-dissociative, and a pre-dissociative state. The measurement on the pre-dissociative state showed a π →  π* excitation, which implies a partly broken internal π bond in the molecule.

Ultrafast laser-induced reactions were used to examine the dynamic properties of desorption and oxidation. Here CO/Ru and CO/O/Ru were used as model systems. Desorption of CO from a Ru surface involve both hot electrons and phonons. In the case of CO oxidation from CO/O/Ru a pronounced wavelength dependence of the branching ratio between desorption and oxidation was observed. Excitation with 400 nm showed a factor of 3-4 higher selectivity towards oxidation than 800 nm. This was attributed to coupling to transiently excited, non-thermalized electrons.

Finally, by performing optical pump/x-ray probe XAS and XES changes in the electronic structure during the reaction could be followed, both for desorption and oxidation. In the CO/Ru experiment, two different transient excitation paths were observed, one leading to a precursor state, and one where CO moves into a more highly coordinated site. Using selective excitation in XES, these were shown to coexist on the surface. In the oxidation experiment, probing the reacting species located near the transition state region in an associative catalytic surface reaction was demonstrated for the very first time.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2017. 54 p.
Keyword
Hetrogenous catalysis, CO, transition metals, Ultrafast probing, oxidation, desorption, dissociation
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-132248 (URN)978-91-7649-441-7 (ISBN)978-91-7649-637-4 (ISBN)
Public defence
2017-03-30, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (Swedish)
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Available from: 2017-03-07 Created: 2016-08-02 Last updated: 2017-04-03Bibliographically approved

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