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Experimental femtosecond-laser based investigations of model catalytic surface reactions.
Stockholm University, Faculty of Science, Department of Physics. (Chemical Physics)ORCID iD: 0000-0002-1805-4993
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In order to be able to design novel catalytic processes more efficiently, detailed understanding of the catalyst-reactant interaction and the dynamics of the microscopic reaction steps is needed. The present thesis aims to contribute to the fundamental understanding of catalyst reactant systems by means of experiments using model systems in Ultra High Vacuum (UHV). The main body of work involves femtochemistry/mass spectrometry measurements as well as sum-frequency generation (SFG) measurements, which both make use of a femtosecond laser and a UHV sample environment. The results of two experimental investigations within the field of surface science are presented.

The first paper concerns CO oxidation on ruthenium (0001) and in particular the energy transfer from substrate to adsorbates upon laser excitation. For these experiments laser-induced desorption was performed. We were able to control the branching ratios of competing mechanisms and understand the role of non-thermal electrons in the mechanisms.

The second project aims to understand the adsorption and dehydrogenation of methanol on cuprous oxide (Cu2O) which is complicated by the fact that the cuprous oxide surface reconstructs differently under different conditions. The results presented in this part were acquired using mainly X-ray Photoelectron Spectroscopy and SFG. We were able to understand the restructuring of the Cu2O surface and to show that methanol adsorbs molecularly on Cu2O(111) instead of dissociatively as a methoxy and hydrogen species as it does on Cu2O(100).

Place, publisher, year, edition, pages
Stockholm University, 2018.
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-153830OAI: oai:DiVA.org:su-153830DiVA, id: diva2:1188094
Presentation
2018-03-28, FA31, Fysikum, Albanova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2018-03-15 Created: 2018-03-06 Last updated: 2018-03-15Bibliographically approved
List of papers
1. Indication of non-thermal contribution to visible femtosecond laser-induced CO oxidation on Ru(0001)
Open this publication in new window or tab >>Indication of non-thermal contribution to visible femtosecond laser-induced CO oxidation on Ru(0001)
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2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 7, article id 074701Article in journal (Refereed) Published
Abstract [en]

We studied CO oxidation on Ru(0001) induced by 400 nm and 800 nm femtosecond laser pulses where we find a branching ratio between CO oxidation and desorption of 1: 9 and 1: 31, respectively, showing higher selectivity towards CO oxidation for the shorter wavelength excitation. Activation energies computed with density functional theory show discrepancies with values extracted from the experiments, indicating both a mixture between different adsorbed phases and importance of non-adiabatic effects on the effective barrier for oxidation. We simulated the reactions using kinetic modeling based on the two-temperature model of laser-induced energy transfer in the substrate combined with a friction model for the coupling to adsorbate vibrations. This model gives an overall good agreement with experiment except for the substantial difference in yield ratio between CO oxidation and desorption at 400 nm and 800 nm. However, including also the initial, non-thermal effect of electrons transiently excited into antibonding states of the O-Ru bond yielded good agreement with all experimental results.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-121519 (URN)10.1063/1.4928646 (DOI)000360440400037 ()
Available from: 2015-10-09 Created: 2015-10-05 Last updated: 2018-03-06Bibliographically approved
2. Dehydrogenation of methanol on Cu2O(100) and (111)
Open this publication in new window or tab >>Dehydrogenation of methanol on Cu2O(100) and (111)
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2017 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 146, no 24, article id 244702Article in journal (Refereed) Published
Abstract [en]

Adsorption and desorption of methanol on the (111) and (100) surfaces of Cu2O have been studied using high-resolution photoelectron spectroscopy in the temperature range 120-620 K, in combination with density functional theory calculations and sum frequency generation spectroscopy. The bare (100) surface exhibits a (3,0; 1,1) reconstruction but restructures during the adsorption process into a Cu-dimer geometry stabilized by methoxy and hydrogen binding in Cu-bridge sites. During the restructuring process, oxygen atoms from the bulk that can host hydrogen appear on the surface. Heating transforms methoxy to formaldehyde, but further dehydrogenation is limited by the stability of the surface and the limited access to surface oxygen. The (root 3 x root 3)R30 degrees-reconstructed (111) surface is based on ordered surface oxygen and copper ions and vacancies, which offers a palette of adsorption and reaction sites. Already at 140 K, a mixed layer of methoxy, formaldehyde, and CHxOy is formed. Heating to room temperature leaves OCH and CHx. Thus both CH-bond breaking and CO-scission are active on this surface at low temperature. The higher ability to dehydrogenate methanol on (111) compared to (100) is explained by the multitude of adsorption sites and, in particular, the availability of surface oxygen.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-145191 (URN)10.1063/1.4989472 (DOI)000404302600033 ()
Available from: 2017-07-31 Created: 2017-07-31 Last updated: 2018-03-06Bibliographically approved

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