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Ultrafast Probing of CO Reactions on Metal Surfaces: Changes in the molecular orbitals during the catalysis process
Stockholm University, Faculty of Science, Department of Physics. (Kemisk fysik)ORCID iD: 0000-0002-5389-5675
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents experimental studies of three different chemical reaction steps relevant for heterogeneous catalysis: dissociation, desorption, and oxidation. CO on single-crystal metal surfaces was chosen as the model systems.

X-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) provide information about the electronic structure, and were performed on CO/Fe to measure both a non-dissociative, and a pre-dissociative state. The measurement on the pre-dissociative state showed a π →  π* excitation, which implies a partly broken internal π bond in the molecule.

Ultrafast laser-induced reactions were used to examine the dynamic properties of desorption and oxidation. Here CO/Ru and CO/O/Ru were used as model systems. Desorption of CO from a Ru surface involve both hot electrons and phonons. In the case of CO oxidation from CO/O/Ru a pronounced wavelength dependence of the branching ratio between desorption and oxidation was observed. Excitation with 400 nm showed a factor of 3-4 higher selectivity towards oxidation than 800 nm. This was attributed to coupling to transiently excited, non-thermalized electrons.

Finally, by performing optical pump/x-ray probe XAS and XES changes in the electronic structure during the reaction could be followed, both for desorption and oxidation. In the CO/Ru experiment, two different transient excitation paths were observed, one leading to a precursor state, and one where CO moves into a more highly coordinated site. Using selective excitation in XES, these were shown to coexist on the surface. In the oxidation experiment, probing the reacting species located near the transition state region in an associative catalytic surface reaction was demonstrated for the very first time.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2017. , p. 54
Keyword [en]
Hetrogenous catalysis, CO, transition metals, Ultrafast probing, oxidation, desorption, dissociation
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-132248ISBN: 978-91-7649-441-7 (print)ISBN: 978-91-7649-637-4 (electronic)OAI: oai:DiVA.org:su-132248DiVA, id: diva2:951028
Public defence
2017-03-30, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (Swedish)
Opponent
Supervisors
Available from: 2017-03-07 Created: 2016-08-02 Last updated: 2017-04-03Bibliographically approved
List of papers
1. 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, article id 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
2. Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)
Open this publication in new window or tab >>Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)
2013 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 615, p. 65-71Article in journal (Refereed) Published
Abstract [en]

We studied femtosecond laser-induced desorption of CO from Ru(0001) using intense near-infrared and visible femtosecond laser pulses. We find a pronounced wavelength dependence with a factor 3-4 higher desorption yield at comparable fluence when desorption is induced via 400 nm light, compared to 800 nm and attribute this difference to the difference in penetration depth of the incident light. All our data can be described using empirical friction-modeling to determine the desorption mechanism with the same mechanism for both wavelengths. We find that both hot electrons and phonons contribute to the desorption process.

Keyword
Surface chemical reaction, Dynamics, Desorption, Carbon monoxide, Ruthenium
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-92614 (URN)10.1016/j.susc.2013.05.002 (DOI)000321409400009 ()
Note

AuthorCount:3;

Available from: 2013-08-19 Created: 2013-08-14 Last updated: 2017-12-06Bibliographically approved
3. Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser
Open this publication in new window or tab >>Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser
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2013 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 187, p. 9-14Article in journal (Refereed) Published
Abstract [en]

We report on an experimental system designed to probe chemical reactions on solid surfaces on a sub-picosecond timescale using soft X-ray emission spectroscopy at the Linac Coherent Light Source (LCLS) free electron laser (FEL) at the SLAC National Accelerator Laboratory. We analyzed the O 1s X-ray emission spectra recorded from atomic oxygen adsorbed on a Ru(0001) surface at a synchrotron beamline (SSRL, BL13-2) and an FEL beamline (LCLS, SXR). We have demonstrated conditions that provide negligible amount of FEL induced damage of the sample. In addition we show that the setup is capable of tracking the temporal evolution of electronic structure during a surface reaction of submonolayer quantities of CO molecules desorbing from the surface.

Keyword
X-ray emission spectroscopy, Surface science, Free electron laser, Ultrafast
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-92519 (URN)10.1016/j.elspec.2013.03.006 (DOI)000320490500002 ()
Note

AuthorCount:18;

Available from: 2013-08-08 Created: 2013-08-07 Last updated: 2017-12-06Bibliographically approved
4. 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, p. 1302-1305Article 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
5. 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, article id 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
6. Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy
Open this publication in new window or tab >>Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy
2015 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 633, p. 77-81Article in journal (Refereed) Published
Abstract [en]

We demonstrate that temperature-programmed vibrational sum-frequency generation (SFG) spectroscopy has a unique sensitivity to certain adsorbate overlayer structural transitions. In the CO stretching vibration of co-adsorbed CO/O(2x1)/Ru(0001) we observe pronounced dips in the spectral intensity as the adsorbate overlayer undergoes structural transitions with temperature. Combining with temperature-programmed desorption (TPD) a more complete picture of temperature-dependent structural transitions is obtained. We extract kinetic parameters from the SFG data and obtain good agreement with TPD when both techniques see the same transition. Infrared-infrared visible SFG is used to determine changes in inter-adsorbate coupling that allow us to experimentally assign the structural transitions. Furthermore, density functional theory calculations of the proposed structures and energetics are performed to verify the experimental assignments.

Keyword
Vibrational spectroscopy, Sum-frequency generation, Surface science, Density functional theory calculations
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-114344 (URN)10.1016/j.susc.2014.11.006 (DOI)000348336000011 ()
Note

AuthorCount:4;

Available from: 2015-03-09 Created: 2015-03-02 Last updated: 2017-12-04Bibliographically approved
7. Indication of non-thermal contribution to visible femtosecond laser-induced CO oxidation on Ru(0001)
Open this publication in new window or tab >>Indication of non-thermal contribution to visible femtosecond laser-induced CO oxidation on Ru(0001)
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2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 7, article id 074701Article in journal (Refereed) Published
Abstract [en]

We studied CO oxidation on Ru(0001) induced by 400 nm and 800 nm femtosecond laser pulses where we find a branching ratio between CO oxidation and desorption of 1: 9 and 1: 31, respectively, showing higher selectivity towards CO oxidation for the shorter wavelength excitation. Activation energies computed with density functional theory show discrepancies with values extracted from the experiments, indicating both a mixture between different adsorbed phases and importance of non-adiabatic effects on the effective barrier for oxidation. We simulated the reactions using kinetic modeling based on the two-temperature model of laser-induced energy transfer in the substrate combined with a friction model for the coupling to adsorbate vibrations. This model gives an overall good agreement with experiment except for the substantial difference in yield ratio between CO oxidation and desorption at 400 nm and 800 nm. However, including also the initial, non-thermal effect of electrons transiently excited into antibonding states of the O-Ru bond yielded good agreement with all experimental results.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-121519 (URN)10.1063/1.4928646 (DOI)000360440400037 ()
Available from: 2015-10-09 Created: 2015-10-05 Last updated: 2018-03-06Bibliographically approved
8. Probing the transition state region in catalytic CO oxidation on Ru
Open this publication in new window or tab >>Probing the transition state region in catalytic CO oxidation on Ru
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2015 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 347, no 6225, p. 978-982Article in journal (Refereed) Published
Abstract [en]

Femtosecond x-ray laser pulses are used to probe the carbon monoxide (CO) oxidation reaction on ruthenium (Ru) initiated by an optical laser pulse. On a time scale of a few hundred femtoseconds, the optical laser pulse excites motions of CO and oxygen (O) on the surface, allowing the reactants to collide, and, with a transient close to a picosecond (ps), new electronic states appear in the OK-edge x-ray absorption spectrum. Density functional theory calculations indicate that these result from changes in the adsorption site and bond formation between CO and O with a distribution of OC-O bond lengths close to the transition state (TS). After 1 ps, 10% of the CO populate the TS region, which is consistent with predictions based on a quantum oscillator model.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-115673 (URN)10.1126/science.1261747 (DOI)000349958900034 ()25722407 (PubMedID)
Note

AuthorCount:30;

Available from: 2015-04-01 Created: 2015-03-27 Last updated: 2017-12-04Bibliographically approved

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