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Ethylene on Cu(110) and Ni(110): Electronic structure and bonding derived from x-ray spectroscopy and theory
Stockholm University, Faculty of Science, Department of Physics.
Institut für Experimentalphysik, Universität Hamburg.
Stockholm University, Faculty of Science, Department of Physics.
Lehrstuhl für Festkörperphysik, Universität Erlangen-Nürnberg.
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2004 (English)In: Surface Science, ISSN 0039-6028, Vol. 559, no 2-3, 85-93 p.Article in journal (Refereed) Published
Abstract [en]

The bonding of ethylene to Cu(1 1 0) and Ni(1 1 0) is analyzed in detail using symmetry-resolved X-ray absorption (XAS) and emission (XES) spectroscopies in conjunction with density functional theory (DFT) calculations of geometric structure and spectra. XES, which probes the occupied valence states, reveals the formation of bonding and non-bonding orbitals of π-3d as well as π*-3d character. Additional mixing of σ and π states indicates rehybridization upon adsorption. The anti-bonding π-3d and π*-3d combinations are unoccupied and seen in XAS. A lower intensity of the π* transition for Ni is evidence of larger π* occupancy upon bonding. The position of the σ* shape-resonance indicates a 0.02 Å longer C–C bond on Ni than on Cu, in good agreement with the DFT structure optimizations. The XE spectra are well-reproduced both by specific spectrum calculations based on cluster models and by the carbon p-density of states calculated using periodic boundary conditions. The contribution of both π and π* levels to the new, surface-induced occupied states close to the Fermi-level lends support to the traditional Dewar–Chatt–Duncanson picture of the bonding. Theoretical charge-density difference plots support an alternative view of ethylene bonding in terms of the specific involvement of the excited molecular triplet state. Based on the variation in XE intensities the main difference between ethylene bonding to Cu and Ni is found to be an about two times larger occupancy of the π* orbital upon chemisorption on the transition metal, which comes along with C–C bond elongation and stronger σ–π rehybridization.

Place, publisher, year, edition, pages
Elsevier , 2004. Vol. 559, no 2-3, 85-93 p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-23164DOI: 10.1016/j.susc.2004.04.041OAI: oai:DiVA.org:su-23164DiVA: diva2:190460
Note
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.
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
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Supervisors
Available from: 2004-05-13 Created: 2004-05-13Bibliographically approved

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