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XPS and XAS investigation of condensed and adsorbed n-octane on a Cu(110) surface
Department of Physics, Uppsala University.
Department of Physics, Uppsala University.
KTH Syd, Campus Haninge.
Department of Physics, Uppsala University.
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2003 (English)In: Journal of electron spectroscopy and related phenomena, ISSN 0368-2048, Vol. 128, no 2-3, 179-191 p.Article in journal (Refereed) Published
Abstract [en]

The electronic structure of n-octane adsorbed on Cu(110) is studied by using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) in combination with cluster model calculations in the framework of density functional theory (DFT). The molecule is found to be well oriented on the surface, which is seen from the high degree of XAS dichroism. Saturated hydrocarbons are commonly considered to physisorb on metals such as Cu(110), but still the C 1s XAS spectra reveal large changes in the electronic structure of the adsorbed octane relative to the free molecule. We find that the XAS resonances corresponding to the molecular Rydberg-valence states are strongly quenched upon adsorption and that there is a significant hybridization of the molecular valence orbitals with the metal bands. In addition to a precise interpretation of the XAS spectra, we present details on the molecular orbital structure of the adsorbed octane molecule. We also discuss shifts in the relative binding energies of the chemically inequivalent carbon atoms in octane upon adsorption, which lead to a narrower XPS spectrum for the adsorbate than the condensed phase spectrum due to the existence of a new relaxation channel.

Place, publisher, year, edition, pages
Elsevier , 2003. Vol. 128, no 2-3, 179-191 p.
Keyword [en]
XPS; XAS; Condensed and adsorbed n-octane; Cu(110) surface
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-23160DOI: 10.1016/S0368-2048(02)00282-7OAI: oai:DiVA.org:su-23160DiVA: diva2:190456
Note
Part of urn:nbn:se:su:diva-171Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2009-12-22Bibliographically approved
In thesis
1. Chemical Bonding of Hydrocarbons to Metal Surfaces
Open this publication in new window or tab >>Chemical Bonding of Hydrocarbons to Metal Surfaces
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Using x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) in combination with density functional theory (DFT) the changes in electronic and geometric structure of hydrocarbons upon adsorption are determined. The chemical bonding is analyzed and the results provide new insights in the mechanisms responsible for dehydrogenation in heterogeneous catalysis.

In the case of alkanes, n-octane and methane are studied. XAS and XES show significant changes in the electronic structure upon adsorption. XES shows new adsorption induced occupied states and XAS shows quenching of CH*/Rydberg states in n-octane. In methane the symmetry forbidden gas phase lowest unoccupied molecular orbital becomes allowed due to broken symmetry. New adsorption induced unoccupied features with mainly metal character appear just above the Fermi level in XA spectra of both adsorbed methane and n-octane. These changes are not observed in DFT total energy geometry optimizations. Comparison between experimental and computed spectra for different adsorbate geometries reveals that the molecular structures are significantly changed in both molecules. The C-C bonds in n-octane are shortened upon adsorption and the C-H bonds are elongated in both n-octane and methane.

In addition ethylene and acetylene are studied as model systems for unsaturated hydrocarbons. The validity of both the Dewar-Chatt-Duncanson chemisorption model and the alternative spin-uncoupling picture is confirmed, as well as C-C bond elongation and upward bending of the C-H bonds.

The bonding of ethylene to Cu(110) and Ni(110) are compared and the results show that the main difference is the amount of back-donation into the molecular π* orbital, which allows the molecule to desorb molecularly from the Cu(110) surface, whereas it is dehydrogenated upon heating on the Ni(110) surface.

Acetylene is found to adsorb in two different adsorption sites on the Cu(110) surface at liquid nitrogen temperature. Upon heating the molecules move into one of these sites due to attractive adsorbate-adsorbate interaction and only one adsorbed species is present at room temperature, at which point the molecules start reacting to form benzene. The bonding of the two species is very similar in both sites and the carbon atoms are rehybridized essentially to sp2.

Place, publisher, year, edition, pages
Stockholm: Fysikum, 2004. 60 p.
Keyword
Adsorption, hydrocarbon, core-level, spectroscopy
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-171 (URN)91-7265-908-4 (ISBN)
Public defence
2004-06-03, sal FD5, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00
Opponent
Supervisors
Available from: 2004-05-13 Created: 2004-05-13Bibliographically approved

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