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Orbital rehybridization in n-octane adsorbed on Cu(110)
Stockholm University, Faculty of Science, Department of Physics.
KTH Syd, Campus Haninge.
Department of Physics, Uppsala University.
Stockholm University, Faculty of Science, Department of Physics.
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2003 (English)In: Journal of Chemical Physics, ISSN 0021-9606, Vol. 118, no 8, 3782-3789 p.Article in journal (Refereed) Published
Abstract [en]

We have investigated the local electronic structure of n-octane adsorbed on the Cu(110) surface using symmetry-resolved x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) in combination with density functional theory (DFT) spectrum calculations. We found new adsorption-induced states in the XE spectra, which we assign to interaction between the bonding CH orbitals and the metal surface. By performing a systematic investigation of the influence of different structural parameters on the XA and XE spectra, we conclude that the molecular geometry is significantly distorted relative to the gas-phase structure. The bonding to the surface leads to a strengthening of the carbon–carbon bonds and a weakening of the carbon–hydrogen bonds, consistent with a rehybridization of the carbons from sp3 to sp2.8. ©2003 American Institute of Physics.

Place, publisher, year, edition, pages
American Institute of Physics , 2003. Vol. 118, no 8, 3782-3789 p.
National Category
Physical Sciences
Research subject
URN: urn:nbn:se:su:diva-23161DOI: DOI: 10.1063/1.1539866#OAI: diva2:190457
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.
Adsorption, hydrocarbon, core-level, spectroscopy
National Category
Physical Sciences
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
Available from: 2004-05-13 Created: 2004-05-13Bibliographically approved

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