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Chemical Bonding of Hydrocarbons to Metal Surfaces
Stockholm University, Faculty of Science, Department of Physics.
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 [en]
Adsorption, hydrocarbon, core-level, spectroscopy
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-171ISBN: 91-7265-908-4 (print)OAI: oai:DiVA.org:su-171DiVA: diva2:190462
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
List of papers
1. XPS and XAS investigation of condensed and adsorbed n-octane on a Cu(110) surface
Open this publication in new window or tab >>XPS and XAS investigation of condensed and adsorbed n-octane on a Cu(110) surface
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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
Keyword
XPS; XAS; Condensed and adsorbed n-octane; Cu(110) surface
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-23160 (URN)10.1016/S0368-2048(02)00282-7 (DOI)
Note
Part of urn:nbn:se:su:diva-171Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2009-12-22Bibliographically approved
2. Orbital rehybridization in n-octane adsorbed on Cu(110)
Open this publication in new window or tab >>Orbital rehybridization in n-octane adsorbed on Cu(110)
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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
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-23161 (URN)DOI: 10.1063/1.1539866# (DOI)
Note
Part of urn:nbn:se:su:diva-171Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2009-12-22Bibliographically approved
3. Bonding of Saturated Hydrocarbons to Metal Surfaces
Open this publication in new window or tab >>Bonding of Saturated Hydrocarbons to Metal Surfaces
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2003 (English)In: Physical Review Letters, ISSN 0031-9007, Vol. 91, no 4, 046102- p.Article in journal (Refereed) Published
Abstract [en]

The adsorption of octane on Cu(110) was studied by x-ray absorption and x-ray emission spectroscopy, in combination with spectrum calculations in the framework of density functional theory, as a model system for alkane adsorption on transition metals. Significant electron sharing between the adsorbate and metal surface and involvement of both bonding and antibonding C-H molecular orbitals in the molecule-metal bond was found. The calculations were extended to the case of octane adsorbed on Ni(110), and the position of the metal d band was found to be important for the bonding. The results were generalized to show that this is important for the efficiency as an alkane dehydrogenation catalyst.

Place, publisher, year, edition, pages
The American Physical Society, 2003
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-23162 (URN)10.1103/PhysRevLett.91.046102 (DOI)
Note
Part of urn:nbn:se:su:diva-171Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2009-12-28Bibliographically approved
4. Electronic and geometric structure of methane adsorbed on a Pt surface
Open this publication in new window or tab >>Electronic and geometric structure of methane adsorbed on a Pt surface
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-23163 (URN)
Note
Part of urn:nbn:se:su:diva-171Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2010-01-13Bibliographically approved
5. Ethylene on Cu(110) and Ni(110): Electronic structure and bonding derived from x-ray spectroscopy and theory
Open this publication in new window or tab >>Ethylene on Cu(110) and Ni(110): Electronic structure and bonding derived from x-ray spectroscopy and theory
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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
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-23164 (URN)10.1016/j.susc.2004.04.041 (DOI)
Note
Part of urn:nbn:se:su:diva-171Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2009-12-28Bibliographically approved
6. Geometric structure and chemical bonding of acetylene adsorbed on Cu(110)
Open this publication in new window or tab >>Geometric structure and chemical bonding of acetylene adsorbed on Cu(110)
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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
Keyword
Alkynes; Copper; Carbon; Chemisorption; X-ray photoelectron spectroscopy; X-ray absorption spectroscopy
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-23165 (URN)10.1016/j.susc.2004.07.012 (DOI)
Note
Part of urn:nbn:se:su:diva-171Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2009-12-28Bibliographically approved

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