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Density Functional Theory Studies of Small Supported Gold Clusters and Related Questions: What a Difference an Atom Makes
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

During the last decades the specific manipulation of matter on the (sub-) nanometer scale, also known as nanoscience, became possible by technologies such as the scanning tunneling microscope. Nanocatalysts, i.e. catalytic active structures of up to a few nanometers in size, belong to this rather new class of materials. Unlike ordinary ’macroscopic’ catalytic materials, the performance of nanocatalysts does not simply scale, for instance, with the surface to volume ratio of the active material.

In this Thesis model nanocatalysts are investigated by means of ab-initio density functional theory calculations.

In paper I, we explain the experimentally observed catalytic characteristics of small gold clusters, Au1-4, on a regular magnesium oxide terrace towards the oxidation of carbon monoxide by thoroughly studying the adsorption of CO and O2 on these clusters.

In the subsequent paper II, we study the feasibility of a catalytic water-mediated CO oxidation reaction on Au1-4/MgO and find that this reaction mechanism is not assessable for Au2,4/MgO and unlikely for Au1,3/MgO.

Papers III and IV concentrate on the reactivity of clusters in the gas phase. Particularly, we focus on the relative stability of Au13 isomers and its potential for O2 dissociation (paper III). We find the lowest energy isomers, which contain a triangular prism at their center surrounded by a ring of the remaining seven atoms, to be generally stable upon O2 adsorption. The dissociation of O2 at certain sites of Au13 is found to be exothermic.

In paper IV we performed scans of the Born-Oppenheimer potential energy surfaces of neutral and charged Cu3, Ag3, and Au3 to explore the thermally excited vibrations of these trimers. While the Born-Oppenheimer surface of Cu3 exhibits one fairly deep energy minimum, it is comparatively flat with two shallow minima in the case of Ag3. Hence for Ag3 there exist many thermally accessible geometries in a wide range of angles and bond lengths. For Au3, two distinct energy minima appear, being well-separated by a barrier of 180 meV. Already at room temperature, we find bond lengths changes of up to 5% for the studied trimers. Choosing Au3 as a case study for the changed reactivity of thermally excited modes, we find CO to bind up to 150 meV stronger to the excited cluster.

Gold deposited on graphene and graphite was observed to form larger aggregates. In paper V, we study the electronic structures, high mobility, and substrate-mediated clustering processes of Au1-4 on graphene.

Already in the 1970s is was speculated that dispersion forces, i.e. van der Waals forces, significantly contribute to the adsorption energies of gold atoms on graphite. We accounted for van der Waals interactions in our density functional theory calculations (paper VI) and investigated the influence of these dispersion forces on the binding of copper, silver, and gold adatoms on graphene. While copper and gold show a mixed adsorption mechanism, i.e. chemical binding plus attraction due to the van der Waals forces, silver is purely physisorbed on graphene.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2010. , p. 85
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 788
Keywords [en]
Gold clusters, CO oxidation, graphene, spin-orbit coupling, van der Waals interactions, thermally excited vibrations
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-133246ISBN: 978-91-554-7946-6 (print)OAI: oai:DiVA.org:uu-133246DiVA, id: diva2:360638
Public defence
2010-12-17, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Note
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 719Available from: 2010-11-26 Created: 2010-11-04 Last updated: 2011-03-21Bibliographically approved
List of papers
1. Catalytic activity of small MgO-supported Au clusters towards CO oxidation: A density functional study
Open this publication in new window or tab >>Catalytic activity of small MgO-supported Au clusters towards CO oxidation: A density functional study
2010 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 81, no 19, p. 195443-Article in journal (Refereed) Published
Abstract [en]

In order to explain the experimentally found catalytic characteristics of Au1-4 /MgO (100) we have performed a comprehensive density functional study of these systems and their ability to (co)adsorb CO and O2 molecules. Starting from the carefully determined ground-state structures we have analyzed binding mechanisms, the influence of spin-orbit coupling, and charge redistributions in Au1-4 /MgO+CO (O2). Experimentally Au1,2 /MgO were found to be inactive under a mixed atmosphere. We show that O2 strongly binds to Au1 /MgO that prevents coadsorption. Although a catalytic reaction cycle towards CO oxidation, analogous to the gas phase reaction involving Au 2-, is energetically possible for Au2 /MgO, the cluster will get blocked by a strongly bound CO. On the other hand, the catalytic activity of Au3,4 /MgO could be explained by their ability to coadsorb CO and O2, hence indicating the occurrence of a Langmuir- Hinshelwood-type reaction mechanism for these clusters.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-133240 (URN)10.1103/PhysRevB.81.195443 (DOI)000278142000132 ()
Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved
2. Does H2O improve the catalytic activity of Au1−4/MgO towards CO oxidation?
Open this publication in new window or tab >>Does H2O improve the catalytic activity of Au1−4/MgO towards CO oxidation?
2010 (English)In: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694Article in journal (Other academic) Submitted
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-133241 (URN)
Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved
3. The relative stability of Au13 isomers and their potential for O2 dissociation
Open this publication in new window or tab >>The relative stability of Au13 isomers and their potential for O2 dissociation
(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-133242 (URN)
Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2012-03-29
4. Thermally Excited Vibrations in Copper, Silver, and Gold Trimers and Enhanced Binding of CO
Open this publication in new window or tab >>Thermally Excited Vibrations in Copper, Silver, and Gold Trimers and Enhanced Binding of CO
2010 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126Article in journal (Other academic) Submitted
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-133243 (URN)
Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved
5. Small gold clusters on graphene, their mobility and clustering: a DFT study
Open this publication in new window or tab >>Small gold clusters on graphene, their mobility and clustering: a DFT study
2011 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 23, no 20, p. 205301-Article in journal (Refereed) Published
Abstract [en]

Motivated by the experimentally observed high mobility of gold atoms on graphene and their tendency to form nanometer-sized clusters, we present a density functional theory study of the ground state structures of small gold clusters on graphene, their mobility and clustering. Our detailed analysis of the electronic structures identifies the opportunity to form strong gold-gold bonds and the graphene-mediated interaction of the pre-adsorbed fragments as the driving forces behind gold's tendency to aggregate on graphene. While clusters containing up to three gold atoms have one unambiguous ground state structure, both gas phase isomers of a cluster with four gold atoms can be found on graphene. In the gas phase the diamond-shaped Au-4(D) cluster is the ground state structure, whereas the Y-shaped Au-4(Y) becomes the actual ground state when adsorbed on graphene. As we show, both clusters can be produced on graphene by two distinct clustering processes. We also studied in detail the stepwise formation of a gold dimer out of two pre-adsorbed adatoms, as well as the formation of Au-3. All reactions are exothermic and no further activation barriers, apart from the diffusion barriers, were found. The diffusion barriers of all studied clusters range from 4 to 36 meV only, and are substantially exceeded by the adsorption energies of -0.1 to -0.59 eV. This explains the high mobility of Au1-4 on graphene along the C-C bonds.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-133244 (URN)10.1088/0953-8984/23/20/205301 (DOI)000290212300005 ()
Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved
6. Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions
Open this publication in new window or tab >>Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions
2011 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 39, article id 395001Article in journal (Refereed) Published
Abstract [en]

We performed a systematic density functional (DF) study of the adsorption of copper, silver, and gold adatoms on pristine graphene, especially accounting for van der Waals (vdW) interactions by the vdW-DF and PBE + D2 methods. In particular, we analyze the preferred adsorption site (among top, bridge, and hollow positions) together with the corresponding distortion of the graphene sheet and identify diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to the graphene sheet and that in some cases the buckling of the graphene layer can be significant. Only the results for silver are qualitatively at variance with those obtained with the generalized gradient approximation, which gives no binding in this case. However in all three cases, we observe some quantitative differences between the vdW-DF and PBE + D2 methods. For instance the adsorption energies calculated with the PBE + D2 method are systematically higher than the ones obtained with vdW-DF. Moreover, the equilibrium distances computed with PBE + D2 are shorter than those calculated with the vdW-DF method.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-133245 (URN)10.1088/0953-8984/23/39/395001 (DOI)000295035200010 ()
Available from: 2010-11-04 Created: 2010-11-04 Last updated: 2017-12-12Bibliographically approved

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