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X-ray emission spectroscopy and density functional study of CO/Fe(100)
Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
Visa övriga samt affilieringar
Antal upphovsmän: 92012 (Engelska)Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, nr 3, artikel-id 034702Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
2012. Vol. 136, nr 3, artikel-id 034702
Nationell ämneskategori
Fysik
Forskningsämne
kemisk fysik
Identifikatorer
URN: urn:nbn:se:su:diva-76343DOI: 10.1063/1.3675834ISI: 000299387700038OAI: oai:DiVA.org:su-76343DiVA, id: diva2:526404
Tillgänglig från: 2012-05-11 Skapad: 2012-05-10 Senast uppdaterad: 2017-12-07Bibliografiskt granskad
Ingår i avhandling
1. Surface reactions and chemical bonding in heterogeneous catalysis
Öppna denna publikation i ny flik eller fönster >>Surface reactions and chemical bonding in heterogeneous catalysis
2014 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Physics, Stockholm University, 2014. s. 66
Nationell ämneskategori
Fysik
Forskningsämne
kemisk fysik
Identifikatorer
urn:nbn:se:su:diva-102323 (URN)978-91-7447-893-8 (ISBN)
Disputation
2014-05-12, sal FP41, AlbaNova universitetscentrum, Roslagstullsbacken 33, Stockholm, 13:00 (Engelska)
Opponent
Handledare
Anmärkning

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 8: Manuscript.

Tillgänglig från: 2014-04-16 Skapad: 2014-04-01 Senast uppdaterad: 2014-04-22Bibliografiskt granskad
2. Ultrafast Probing of CO Reactions on Metal Surfaces: Changes in the molecular orbitals during the catalysis process
Öppna denna publikation i ny flik eller fönster >>Ultrafast Probing of CO Reactions on Metal Surfaces: Changes in the molecular orbitals during the catalysis process
2017 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Physics, Stockholm University, 2017. s. 54
Nyckelord
Hetrogenous catalysis, CO, transition metals, Ultrafast probing, oxidation, desorption, dissociation
Nationell ämneskategori
Atom- och molekylfysik och optik
Forskningsämne
kemisk fysik
Identifikatorer
urn:nbn:se:su:diva-132248 (URN)978-91-7649-441-7 (ISBN)978-91-7649-637-4 (ISBN)
Disputation
2017-03-30, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (Svenska)
Opponent
Handledare
Tillgänglig från: 2017-03-07 Skapad: 2016-08-02 Senast uppdaterad: 2017-04-03Bibliografiskt granskad

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