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Surface reactions and chemical bonding in heterogeneous catalysis
Stockholm University, Faculty of Science, Department of Physics.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis summarizes studies which focus on addressing, using both theoretical and experimental methods, fundamental questions about surface phenomena, such as chemical reactions and bonding, related to processes in heterogeneous catalysis. The main focus is on the theoretical approach and this aspect of the results. The included articles are collected into three categories of which the first contains detailed studies of model systems in heterogeneous catalysis. For example, the trimerization of acetylene adsorbed on Cu(110) is measured using vibrational spectroscopy and modeled within the framework of Density Functional Theory (DFT) and quantitative agreement of the reaction barriers is obtained. In the second category, aspects of fuel cell catalysis are discussed. O2 dissociation is rate-limiting for the reduction of oxygen (ORR) under certain conditions and we find that adsorbate-adsorbate interactions are decisive when modeling this reaction step. Oxidation of Pt(111) (Pt is the electrocatalyst), which may alter the overall activity of the catalyst, is found to start via a PtO-like surface oxide while formation of α-PtO2 trilayers precedes bulk oxidation. When considering alternative catalyst materials for the ORR, their stability needs to be investigated in detail under realistic conditions. The Pt/Cu(111) skin alloy offers a promising candidate but segregation of Cu atoms to the surface is induced by O adsorption. This is confirmed by modeling oxygen x-ray emission (XES) and absorption spectra of the segregated system and near-perfect agreement with experiment is obtained when vibrational interference effects are included in the computed XES. The last category shows results from femtosecond laser measurements of processes involving CO on Ru(0001). Using free-electron x-ray laser experiments a precursor state to desorption is detected and also found in simulations if van der Waals effects are included. Resonant XES can be used to distinguish two different species of CO on the surface; vibrationally hot, chemisorbed CO and CO in the precursor state. Laser-induced CO oxidation on Ru(0001) is modeled and three competing mechanisms are found. Kinetic modeling reproduces the experiment qualitatively.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2014. , 66 p.
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-102323ISBN: 978-91-7447-893-8 (print)OAI: oai:DiVA.org:su-102323DiVA: diva2:709397
Public defence
2014-05-12, sal FP41, AlbaNova universitetscentrum, Roslagstullsbacken 33, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 8: Manuscript.

Available from: 2014-04-16 Created: 2014-04-01 Last updated: 2014-04-22Bibliographically approved
List of papers
1. Adsorption and Cyclotrimerization Kinetics of C2H2 at a Cu(110) Surface
Open this publication in new window or tab >>Adsorption and Cyclotrimerization Kinetics of C2H2 at a Cu(110) Surface
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2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 17, 9550-9560 p.Article in journal (Refereed) Published
Abstract [en]

The kinetics of acetylene adsorption and cyclotrimerization was studied by vibrational sum-frequency generation spectroscopy (SFG) and density functional theory (DFT) calculations. At low temperature, SFG shows two resonances corresponding to acetylene adsorbed in two different sites. Upon heating, two new vibrational resonances appear. We interpret these resonances as being due to C2H2 island formation and adsorbed C4H4, which is the intermediate in the subsequent cyclotrimerization reaction to form benzene. A kinetic model is applied, which allows determination of the relevant activation barriers. The barrier for C2H2 diffusion is determined to be 43 +/- 1 kJ/mol. The activation barrier for formation of the C4H4 intermediate is found to be 84 +/- 6 kJ/mol and the barrier for benzene formation 5 +/- 3 kJ/mol lower. Barriers to diffusion and formation of C4H4 and C6H6 obtained from DFT calculations are in quantitative agreement with the experiments once the locally high coverage in C2H2 islands is included.

National Category
Physical Sciences Physical Chemistry Materials Engineering
Identifiers
urn:nbn:se:su:diva-80097 (URN)10.1021/jp300514f (DOI)000303426500053 ()
Note

AuthorCount:7;

Available from: 2012-09-19 Created: 2012-09-12 Last updated: 2017-12-07Bibliographically approved
2. X-ray emission spectroscopy and density functional study of CO/Fe(100)
Open this publication in new window or tab >>X-ray emission spectroscopy and density functional study of CO/Fe(100)
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2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 3, 034702Article in journal (Refereed) Published
Abstract [en]

We report x-ray emission and absorption spectroscopy studies of the electronic structure of the pre-dissociative alpha(3) phase of CO bound at hollow sites of Fe(100) as well as of the on-top bound species in the high-coverage alpha(1) phase. The analysis is supported by density functional calculations of structures and spectra. The bonding of lying down CO in the hollow site is well described in terms of pi to pi* charge transfer made possible through bonding interaction also at the oxygen in the minority spin-channel. The on-top CO in the mixed, high-coverage alpha(1) phase is found to be tilted due to adsorbate-adsorbate interaction, but still with bonding mainly characteristic of vertical on-top adsorbed CO similar to other transition-metal surfaces.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-76343 (URN)10.1063/1.3675834 (DOI)000299387700038 ()
Available from: 2012-05-11 Created: 2012-05-10 Last updated: 2017-12-07Bibliographically approved
3. Low O-2 dissociation barrier on Pt(111) due to adsorbate-adsorbate interactions
Open this publication in new window or tab >>Low O-2 dissociation barrier on Pt(111) due to adsorbate-adsorbate interactions
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2010 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 133, no 22, 224701- p.Article in journal (Refereed) Published
Abstract [en]

O2 dissociation on Pt(111) has been followed at low and saturation coverage using temperature-programmed x-ray photoelectron spectroscopy and simulated with mean-field kinetic modeling, yielding dissociation (Ea) and desorption (Ed) barriers of 0.32 and 0.36 eV, respectively. DFT calculations show that Ea is strongly influenced by the O–O interatomic potential in the atomic final state: of the supercells considered, that which maximizes attractive third-nearest-neighbor interactions in the atomic final state yields both the lowest computed dissociation barrier (0.24 eV) and the best agreement with experiment. It is proposed that the effect of adsorbate-adsorbate interactions must be considered when modeling catalytic processes involving dissociative steps.

Place, publisher, year, edition, pages
American Institute of Physics, 2010
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-47119 (URN)10.1063/1.3512618 (DOI)000285477800016 ()
Funder
Swedish Research Council
Available from: 2010-11-29 Created: 2010-11-29 Last updated: 2017-12-12Bibliographically approved
4. Oxidation of Pt(111) under Near-Ambient Conditions
Open this publication in new window or tab >>Oxidation of Pt(111) under Near-Ambient Conditions
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2011 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 19, 195502- p.Article in journal (Refereed) Published
Abstract [en]

The oxidation of Pt(111) at near-ambient O(2) pressures has been followed in situ using x-ray photoelectron spectroscopy (XPS) and ex situ using x-ray absorption spectroscopy (XAS). Polarization-dependent XAS signatures at the O K edge reveal significant temperature-and pressure-dependent changes of the Pt-O interaction. Oxide growth commences via a PtO-like surface oxide that coexists with chemisorbed oxygen, while an ultrathin alpha-PtO(2) trilayer is identified as the precursor to bulk oxidation. These results have important implications for understanding the chemical state of Pt in catalysis.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-70675 (URN)10.1103/PhysRevLett.107.195502 (DOI)000297006700011 ()
Note
authorCount :10Available from: 2012-01-23 Created: 2012-01-23 Last updated: 2017-12-08Bibliographically approved
5. Stability of Pt-Modified Cu(111) in the Presence of Oxygen and Its Implication on the Overall Electronic Structure
Open this publication in new window or tab >>Stability of Pt-Modified Cu(111) in the Presence of Oxygen and Its Implication on the Overall Electronic Structure
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2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 32, 16371-16380 p.Article in journal (Refereed) Published
Abstract [en]

The electronic structure and stability of Cu(111)-hosted Pt overlayers with and without the presence of atomic oxygen have been studied by means of core-level spectroscopy and density functional theory (DFT). Because of lattice mismatch, Pt(111) overlayers grown on Cu(111) are compressively strained, and hard X-ray photoelectron spectroscopy together with Pt L-3-edge X-ray absorption spectroscopy (XAS) reveals a pronounced downshift of the Pt d-band owing to the increased overlap of the d-orbitals, an effect also reproduced theoretically. Exposure to oxygen severely alters the surface composition; the O-Cu binding energy largely exceeds that of O-Pt, and DFT calculations predict surface segregation of Cu atoms. Comparing the adsorbate electronic structure for O on unstrained Pt(111) with that of O on Pt-modified Cu(111) using O K-edge XAS and X-ray emission spectroscopy salient differences are observed and calculations show that Cu-segregation to the topmost layer is required to reproduce the measured spectra. It is proposed that O is binding in a hollow site constituted by at least two Cu atoms and that up to 75% of the Pt atoms migrate below the surface.

National Category
Physical Chemistry Materials Chemistry Nano Technology
Identifiers
urn:nbn:se:su:diva-93562 (URN)10.1021/jp400486r (DOI)000323301100012 ()
Note

AuthorCount:13;

Funding agencies:

Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering  DE-AC02-76SF00515;  Swedish Research Council (VR);   Ministry of Education, Culture, Sports, Science and Technology  2009B1751/BL-47XU;

 U.S. Department of Energy  DE-AC02-76SF00515;  Alexander von Humboldt Foundation  

Available from: 2013-09-11 Created: 2013-09-10 Last updated: 2017-12-06Bibliographically approved
6. Real-Time Observation of Surface Bond Breaking with an X-ray Laser
Open this publication in new window or tab >>Real-Time Observation of Surface Bond Breaking with an X-ray Laser
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2013 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 339, no 6125, 1302-1305 p.Article in journal (Refereed) Published
Abstract [en]

We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells-chemisorbed and precursor state separated by an entropy barrier-explain the anomalously high prefactors often observed in desorption of molecules from metals.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-89556 (URN)10.1126/science.1231711 (DOI)000316053400033 ()
Funder
Swedish Research Council
Note

AuthorCount:24;

Available from: 2013-05-02 Created: 2013-04-29 Last updated: 2017-12-06Bibliographically approved
7. Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)
Open this publication in new window or tab >>Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)
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2013 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 18, 186101Article in journal (Refereed) Published
Abstract [en]

We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-91297 (URN)10.1103/PhysRevLett.110.186101 (DOI)000319019300011 ()
Funder
Swedish Research Council
Note

AuthorCount:23;

Available from: 2013-06-27 Created: 2013-06-24 Last updated: 2017-12-06Bibliographically approved
8. CO oxidation on Ru(0001) modeled from first-principles and femtosecond laser measurements
Open this publication in new window or tab >>CO oxidation on Ru(0001) modeled from first-principles and femtosecond laser measurements
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-102320 (URN)
Available from: 2014-04-01 Created: 2014-04-01 Last updated: 2014-04-01Bibliographically approved

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