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Geometric structure and chemical bonding of acetylene adsorbed on Cu(110)
Stockholm University, Faculty of Science, Department of Physics.
Stanford Synchrotron Radiation Laboratory,.
Department of Physics, Uppsala University.
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2004 (English)In: Surface Science, ISSN 0039-6028, Vol. 565, no 2-3, 206-222 p.Article in journal (Refereed) Published
Abstract [en]

The chemical bonding and geometric structure of acetylene adsorbed on Cu(1 1 0) is analyzed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) in combination with density functional theory (DFT) total energy geometry optimizations and spectral calculations. XPS reveals two different adsorbed species at liquid nitrogen temperature. The molecular alignment is deduced from angle-resolved XAS, revealing that in one site the molecules are aligned with the C–C axis along the [0 0 1] direction and in the other site with an average angle of 35° to the Cu rows. The position of the shape resonance is used to deduce a C–C bond length of 1.35 Å, which is close to the values obtained from the DFT geometry optimizations. XES reveals strong σ–π mixing and new occupied states close to the Fermi level, originating from the out-of-plane π* orbital, which becomes occupied upon adsorption in agreement with the Dewar–Chatt–Duncanson model of the bonding.

Place, publisher, year, edition, pages
Elsevier , 2004. Vol. 565, no 2-3, 206-222 p.
Keyword [en]
Alkynes; Copper; Carbon; Chemisorption; X-ray photoelectron spectroscopy; X-ray absorption spectroscopy
National Category
Physical Sciences
URN: urn:nbn:se:su:diva-23165DOI: 10.1016/j.susc.2004.07.012OAI: diva2:190461
Part of urn:nbn:se:su:diva-171Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2009-12-28Bibliographically 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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