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Experimental investigations of model catalytic surface reactions on metal and metal oxide surfaces
Stockholm University, Faculty of Science, Department of Physics.ORCID iD: 0000-0002-1805-4993
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the development of renewable energies catalysis plays an important role, for example in the production of H2 gas that drives fuel cells, or in the decomposition of annoying by-products of renewable energy production. Most catalysts and catalytic processes currently used in the industry have their roots in macroscopic empirical investigations and trial and error-based optimization. In order to be able to design novel catalytic processes more efficiently, detailed understanding of the catalyst-reactant interaction and the dynamics of the microscopic reaction steps is needed. The present thesis aims to contribute to the fundamental understanding of catalyst reactant systems by means of experiments using model systems in Ultra High Vacuum. For this purpose, several surface science techniques were employed such as vibrational sum-frequency generation (SFG), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and femtochemistry.

In the present thesis the results of three different projects are presented. The first concerns the adsorption and decomposition of naphthalene on Ni(111). Using scanning tunnelling microscopy (STM) and density functional theory (DFT) we identify the adsorption energy and geometry of the naphthalene molecule. Using SFG and TPD we investigate the temperature dependent breakdown of the naphthalene molecule and identify geometrical changes of the adsorbate as an intermediate step in the decomposition reaction. Additionally, we observe poisoning of the surface due to graphene growth using both STM and XPS and explore the possible effect of co-adsorption with oxygen on the reaction pathway and the poisoning of the catalyst.

The second section concerns the adsorption and decomposition of ethanol and methanol on cuprous oxide (Cu2O). Using mainly XPS and SFG we show that ethanol adsorbs dissociatively on Cu2O(100) and (111) and that methanol adsorbs dissociatively on the (100) but molecularly on the (111) surface. Furthermore, we identify intermediate surface species and products of the temperature dependent dehydrogenation of both alcohols and show that the (111) surface is the more effective catalyst for decomposition.

The third section explores the physics of non-thermal excitation methods and discusses CO oxidation on ruthenium (0001) induced by an optical laser and by X-rays from a free electron laser. Based on these femtochemistry experiments we discuss in particular the energy transfer both for direct excitation and for substrate mediated excitations. We show that we were able to control the branching ratios of competing mechanisms and understand the role of non-thermal electrons in the mechanisms of optical laser excitation. Furthermore, we show that it is possible to induce CO oxidation by direct X-ray core hole excitation and can rationalize the relaxation process that leads to CO oxidation.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2019. , p. 87
National Category
Other Physics Topics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-171385ISBN: 978-91-7797-706-3 (print)ISBN: 978-91-7797-707-0 (electronic)OAI: oai:DiVA.org:su-171385DiVA, id: diva2:1342563
Public defence
2019-09-26, FA31 sal, AlbaNova universitetscentrum, Roslagstullbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

Available from: 2019-09-03 Created: 2019-08-13 Last updated: 2019-08-30Bibliographically approved
List of papers
1. Naphthalene on Ni(111): Experimental and Theoretical Insights into Adsorption, Dehydrogenation, and Carbon Passivation
Open this publication in new window or tab >>Naphthalene on Ni(111): Experimental and Theoretical Insights into Adsorption, Dehydrogenation, and Carbon Passivation
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2017 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 40, p. 22199-22207Article in journal (Refereed) Published
Abstract [en]

An attractive solution to mitigate tars and also to decompose lighter hydrocarbons in biomass gasification is secondary catalytic reforming, converting hydrocarbons to useful permanent gases. Albeit that it has been in use for a long time in fossil feedstock catalytic steam reforming, understanding of the catalytic processes is still limited. Naphthalene is typically present in the biomass gasification gas and to further understand the elementary steps of naphthalene transformation, we investigated the temperature dependent naphthalene adsorption, dehydrogenation and passivation on Ni(111). TPD (temperature-programmed desorption) and STM (scanning tunneling microscopy) in ultrahigh vacuum environment from 110 K up to 780 K, combined with DFT (density functional theory) were used in the study. Room temperature adsorption results in a flat naphthalene monolayer. DFT favors the dibridge[7] geometry but the potential energy surface is rather smooth and other adsorption geometries may coexist. DFT also reveals a pronounced dearomatization and charge transfer from the adsorbed molecule into the nickel surface. Dehydrogenation occurs in two steps, with two desorption peaks at approximately 450 and 600 K. The first step is due to partial dehydrogenation generating active hydrocarbon species that at higher temperatures migrates over the surface forming graphene. The graphene formation is accompanied by desorption of hydrogen in the high temperature TPD peak. The formation of graphene effectively passivates the surface both for hydrogen adsorption and naphthalene dissociation. In conclusion, the obtained results on the model naphthalene and Ni(111) system, provides insight into elementary steps of naphthalene adsorption, dehydrogenation, and carbon passivation, which may serve as a good starting point for rational design, development and optimization of the Ni catalyst surface, as well as process conditions, for the aromatic hydrocarbon reforming process.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-149014 (URN)10.1021/acs.jpcc.7b07757 (DOI)000413131700047 ()
Available from: 2017-11-20 Created: 2017-11-20 Last updated: 2019-08-19Bibliographically approved
2. Investigation of the surface species during temperature dependent dehydrogenation of naphthalene on Ni(111)
Open this publication in new window or tab >>Investigation of the surface species during temperature dependent dehydrogenation of naphthalene on Ni(111)
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2019 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 150, no 24, article id 244704Article in journal (Refereed) Published
Abstract [en]

The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H-2 gas, as well as the formation of carbidic and graphitic surface carbon. Published under license by AIP Publishing.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-170840 (URN)10.1063/1.5098533 (DOI)000473303200040 ()31255092 (PubMedID)
Available from: 2019-07-29 Created: 2019-07-29 Last updated: 2019-08-19Bibliographically approved
3. Adsorption and decomposition of naphthalene on oxygen pre-covered Ni(111)
Open this publication in new window or tab >>Adsorption and decomposition of naphthalene on oxygen pre-covered Ni(111)
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(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-171509 (URN)
Available from: 2019-08-13 Created: 2019-08-13 Last updated: 2019-08-19Bibliographically approved
4. Adsorption and decoposition of ethanol on Cu2O(111) and (100)
Open this publication in new window or tab >>Adsorption and decoposition of ethanol on Cu2O(111) and (100)
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2019 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, no 33, p. 20384-20392Article in journal (Refereed) Published
Abstract [en]

Ethanol dehydrogenation on metal oxides such as Cu2O is an important reaction for the production of renewable energy by fuel cells both via the production of H2 fuel and applied in direct alcohol fuel cells. To better understand this reaction we studied the adsorption, dissociation and desorption of ethanol on Cu2O(111) and (100) surfaces using high-resolution photoelectron spectroscopy (PES), vibrational sum frequency generation spectroscopy (SFG), and temperature programmed desorption (TPD) accompanied by density functional theory (DFT) calculations. On Cu2O(100) the first layer consists primarily of dissociatively adsorbed ethoxy. Second and third layers of ethanol physisorb at low temperature and desorb below 200 K. On the Cu2O(111) surface, adsorption is mixed as ethoxy, ethanol and the products following C-C cleavage, CHx and OCHx, are found in the first layer. Upon heating, products following both C-C and C-O bond breaking are observed on both surfaces and continued heating accentuates the molecular cracking. C-O cleavage occurs more on the (100) surface, whereas on the Cu2O(111) C-C cleavage dominates and occurs at lower temperatures than on the (100) surface. The increased ability of Cu2O(111) to crack ethanol is explained by the varied surface structure including both surface oxygen, electron rich O-vacancies and Cu.

National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-171369 (URN)10.1021/acs.jpcc.9b05394 (DOI)000482545700035 ()
Available from: 2019-08-06 Created: 2019-08-06 Last updated: 2019-09-16Bibliographically approved
5. Dehydrogenation of methanol on Cu2O(100) and (111)
Open this publication in new window or tab >>Dehydrogenation of methanol on Cu2O(100) and (111)
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2017 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 146, no 24, article id 244702Article in journal (Refereed) Published
Abstract [en]

Adsorption and desorption of methanol on the (111) and (100) surfaces of Cu2O have been studied using high-resolution photoelectron spectroscopy in the temperature range 120-620 K, in combination with density functional theory calculations and sum frequency generation spectroscopy. The bare (100) surface exhibits a (3,0; 1,1) reconstruction but restructures during the adsorption process into a Cu-dimer geometry stabilized by methoxy and hydrogen binding in Cu-bridge sites. During the restructuring process, oxygen atoms from the bulk that can host hydrogen appear on the surface. Heating transforms methoxy to formaldehyde, but further dehydrogenation is limited by the stability of the surface and the limited access to surface oxygen. The (root 3 x root 3)R30 degrees-reconstructed (111) surface is based on ordered surface oxygen and copper ions and vacancies, which offers a palette of adsorption and reaction sites. Already at 140 K, a mixed layer of methoxy, formaldehyde, and CHxOy is formed. Heating to room temperature leaves OCH and CHx. Thus both CH-bond breaking and CO-scission are active on this surface at low temperature. The higher ability to dehydrogenate methanol on (111) compared to (100) is explained by the multitude of adsorption sites and, in particular, the availability of surface oxygen.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-145191 (URN)10.1063/1.4989472 (DOI)000404302600033 ()
Available from: 2017-07-31 Created: 2017-07-31 Last updated: 2019-08-19Bibliographically approved
6. 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: 2019-08-13Bibliographically approved
7. Atom-specific activation in CO oxidation
Open this publication in new window or tab >>Atom-specific activation in CO oxidation
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2018 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 149, no 23, article id 234707Article in journal (Refereed) Published
Abstract [en]

We report on atom-specific activation of CO oxidation on Ru(0001) via resonant X-ray excitation. We show that resonant 1s core-level excitation of atomically adsorbed oxygen in the co-adsorbed phase of CO and oxygen directly drives CO oxidation. We separate this direct resonant channel from indirectly driven oxidation via X-ray induced substrate heating. Based on density functional theory calculations, we identify the valence-excited state created by the Auger decay as the driving electronic state for direct CO oxidation. We utilized the fresh-slice multi-pulse mode at the Linac Coherent Light Source that provided time-overlapped and 30 fs delayed pairs of soft X-ray pulses and discuss the prospects of femtosecond X-ray pump X-ray spectroscopy probe, as well as X-ray two-pulse correlation measurements for fundamental investigations of chemical reactions via selective X-ray excitation.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-163527 (URN)10.1063/1.5044579 (DOI)000454102600024 ()30579301 (PubMedID)
Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2019-08-19Bibliographically approved

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