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Oxygen Vacancy Chemistry in Ceria
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cerium(IV) oxide (CeO2), ceria, is an active metal oxide used in solid oxide fuel cells and for the purification of exhaust gases in vehicle emissions control. Behind these technically important applications of ceria lies one overriding feature, namely ceria's exceptional reduction-oxidation properties. These are enabled by the duality of the cerium ion which easily toggles between Ce4+ and Ce3+. Here the cerium 4f electrons and oxygen vacancies (missing oxygen ions in the structure) are key players. In this thesis, the nature of ceria's f electrons and oxygen vacancies are in focus, and examined with theoretical calculations.

It is shown that for single oxygen vacancies at ceria surfaces, the intimate coupling between geometrical structure and electron localisation gives a multitude of almost degenerate local energy mimima. With many vacancies, the situation becomes even more complex, and not even state-of-the-art quantum-mechanical calculations manage to predict the experimentally observed phenomenon of vacancy clustering. Instead, an alternative set of computer experiments managed to produce stable vacancy chains and trimers consistent with experimental findings from the literature and revealed a new general principle for surface vacancy clustering.

The rich surface chemistry of ceria involves not only oxygen vacancies but also other active oxygen species such as superoxide ions (O2). Experiments have shown that nanocrystalline ceria demonstrates an unusually large oxygen storage capacity (OSC) and an appreciable low-temperature redox activity, which have been ascribed to superoxide species. A mechanism explaining these phenomena is presented.

The ceria surface is also known to interact with SOx molecules, which is relevant both in the context of sulfur poisoning of ceria-based catalysts and sulfur recovery from them. In this thesis, the sulfur species and key mechanisms involved are identified.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2012. , 59 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 896
Keyword [en]
Ceria, Density Functional Theory, Oxygen storage, Nano crystals, Sulfur poisoning
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-167521ISBN: 978-91-554-8271-8 (print)OAI: oai:DiVA.org:uu-167521DiVA: diva2:491677
Public defence
2012-03-16, Å2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2012-02-24 Created: 2012-01-30 Last updated: 2012-03-01Bibliographically approved
List of papers
1. Tuning LDA+U for electron localization and structure at oxygen vacancies in ceria
Open this publication in new window or tab >>Tuning LDA+U for electron localization and structure at oxygen vacancies in ceria
2007 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, no 24, 244704-244704-11 p.Article in journal (Refereed) Published
Abstract [en]

We examine the real space structure and the electronic structure (particularly Ce4f electron localization) of oxygen vacancies in CeO2 (ceria) as a function of U in density functional theory studies with the rotationally invariant forms of the LDA+U and GGA+U functionals. The four nearest neighbor Ce ions always relax outwards, with those not carrying localized Ce4f charge moving furthest. Several quantification schemes show that the charge starts to become localized at U~3 eV and that the degree of localization reaches a maximum at ~6 eV for LDA+U or at ~5.5 eV for GGA+U. For higher U it decreases rapidly as charge is transferred onto second neighbor O ions and beyond. The localization is never into atomic corelike states; at maximum localization about 80-90% of the Ce4f charge is located on the two nearest neighboring Ce ions. However, if we look at the total atomic charge we find that the two ions only make a net gain of (0.2-0.4)e each, so localization is actually very incomplete, with localization of Ce4f electrons coming at the expense of moving other electrons off the Ce ions. We have also revisited some properties of defect-free ceria and find that with LDA+U the crystal structure is actually best described with U=3-4 eV, while the experimental band structure is obtained with U=7-8 eV. (For GGA+U the lattice parameters worsen for U>0 eV, but the band structure is similar to LDA+U.) The best overall choice is U~6 eV with LDA+U and ~5.5 eV for GGA+U, since the localization is most important, but a consistent choice for both CeO2 and Ce2O3, with and without vacancies, is hard to find.

Keyword
Other nonmetals, Point defects and defect clusters, Density functional theory, local density approximation, gradient and other corrections
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-12806 (URN)10.1063/1.2800015 (DOI)000251987800030 ()
Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2017-12-11Bibliographically approved
2. B3LYP calculations of cerium oxides
Open this publication in new window or tab >>B3LYP calculations of cerium oxides
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2010 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 132, no 5, 054110- p.Article in journal (Refereed) Published
Abstract [en]

In this paper we evaluate the performance of density functional theory with the B3LYP functional for calculations on ceria (CeO2) and cerium sesquioxide (Ce2O3). We demonstrate that B3LYP is able to describe CeO2 and Ce2O3 reasonably well. When compared to other functionals, B3LYP performs slightly better than the hybrid functional PBE0 for the electronic properties but slightly worse for the structural properties, although neither performs as well as LDA+U(U = 6 eV) or PBE+U(U = 5 eV). We also make an extensive comparison of atomic basis sets suitable for periodic calculations of these cerium oxides. Here we conclude that there is currently only one type of cerium basis set available in the literature that is able to give a reasonable description of the electronic structure of both CeO2 and Ce2O3. These basis sets are based on a 28 electron effective core potential (ECP) and 30 electrons are attributed to the valence space of cerium. Basis sets based on 46 electron ECPs fail for these materials

Place, publisher, year, edition, pages
American Institute of Physics, 2010
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-121560 (URN)10.1063/1.3253795 (DOI)000274319900011 ()
Available from: 2010-03-25 Created: 2010-03-25 Last updated: 2017-12-12Bibliographically approved
3. Many Competing Ceria (110) Oxygen Vacancy Structures: From Small to Large Supercells
Open this publication in new window or tab >>Many Competing Ceria (110) Oxygen Vacancy Structures: From Small to Large Supercells
2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, no 4, 044705- p.Article in journal (Refereed) Published
Abstract [en]

We present periodic "DFT+U" studies of single oxygen vacancies on the CeO2(110) surface using a number of different supercells, finding a range of different local minimum structures for the vacancy and its two accompanying Ce(III) ions. We find three different geometrical structures in combination with a variety of different Ce(III) localization patterns, several of which have not been studied before. The desired trapping of electrons was achieved in a two-stage optimization procedure. We find that the surface oxygen nearest to the vacancy either moves within the plane towards the vacancy, or rises out of the surface into either a symmetric or an unsymmetric bridge structure. Results are shown in seven slab geometry supercells, p(2 x 1), p(2 x 2), p(2 x 3), p(3 x 2), p(2 x 4), p(4 x 2), and p(3 x 3), and indicate that the choice of supercell can affect the results qualitatively and quantitatively. An unsymmetric bridge structure with one nearest and one next-nearest neighbour Ce(III) ion (a combination of localizations not previously found) is the ground state in all (but one) of the supercells studied here, and the relative stability of other structures depends strongly on supercell size. Within any one supercell the formation energies of the different vacancy structures differ by up to 0.5 eV, but the same structure can vary by up to similar to 1 eV between supercells. Furthermore, finite size scaling suggests that the remaining errors (compared to still larger supercells) can also be similar to 1 eV for some vacancy structures.

Keyword
DFT, Ceria, (110), Vacancies, Supercell approximation
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-167996 (URN)10.1063/1.4723867 (DOI)000307611500053 ()
Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2017-12-08Bibliographically approved
4. Oxygen Vacancy Clustering at the Ceria(111) surface
Open this publication in new window or tab >>Oxygen Vacancy Clustering at the Ceria(111) surface
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Oxygen vacancy clustering at the ceria(111) surface have been studied with both force-field and density function theory methods. Two of the methods predict that stable clusters of surface oxygen vacancies should form on these surface, as seen in numerous experimental studies. We propose that vacancy clustering of both surface and sub-surface vacancies follow the simple principle of sharing their Ce(III) neighbors. For surface oxygen vacancies this leads to compact clusters separated by one surface lattice constant. On the other hand for sub-surface this leads to sparse clusters separated by two surface lattice constants.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-168002 (URN)
Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2014-07-25
5. Supercharged Low-Temperature Oxygen Storage Capacity of Ceria at the Nanoscale
Open this publication in new window or tab >>Supercharged Low-Temperature Oxygen Storage Capacity of Ceria at the Nanoscale
2013 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 4, no 4, 604-608 p.Article in journal (Refereed) Published
Abstract [en]

We provide an explanation for the experimental finding of a dramatically enhancedlow-temperature oxygen storage capacity for small ceria nanoparticles. At low temperature, small octahedral ceria nanoparticles will be understoichiometric at both oxidizing and reducing conditions without showing explicit oxygen vacancies. Instead, rather than becoming stoichiometric at oxidizing conditions, such particles are stabilized through oxygen adsorption forming superoxo (O-2(-)) ions and become in this way supercharged with oxygen. Thesupercharging effect is size-dependent and largest for small nanoparticles where it gives a direct increase in the oxygen storage capacity and simultaneously provides a source of active oxygenspecies at low temperatures.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-168004 (URN)10.1021/jz3020524 (DOI)000315432000010 ()
Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2017-12-08Bibliographically approved
6. SOx on ceria from adsorbed SO2
Open this publication in new window or tab >>SOx on ceria from adsorbed SO2
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2011 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 134, no 18, 184703- p.Article in journal (Refereed) Published
Abstract [en]

Results from first-principles calculations present a rather clear picture of the interaction of SO2 with unreduced and partially reduced (111) and (110) surfaces of ceria. The Ce3+/Ce4+ redox couple, together with many oxidation states of S, give rise to a multitude of SOx species, with oxidation states from + III to + VI. SO2 adsorbs either as a molecule or attaches via its S-atom to one or two surface oxygens to form sulfite (SO32-) and sulfate (SO42-) species, forming new S-O bonds but never any S-Ce bonds. Molecular adsorption is found on the (111) surface. SO32- structures are found on both the (111) and (110) surfaces of both stoichiometric and partially reduced ceria. SO42-structures are observed on the (110) surface together with the formation of two reduced Ce3+ surface cations. SO2 can also partially heal the ceria oxygen vacancies by weakening a S-O bond, when significant electron transfer from the surface (Ce4f) into the lowest unoccupied molecular orbital of the SO2 adsorbate takes place and oxidizes the surface Ce3+ cations. Furthermore, we propose a mechanism that could lead to monodentate sulfate formation at the (111) surface.

National Category
Chemical Sciences Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-154547 (URN)10.1063/1.3566998 (DOI)000290589900035 ()
Available from: 2011-06-07 Created: 2011-06-07 Last updated: 2017-12-11Bibliographically approved
7. Sulfidation of ceria surfaces from sulfur and sulfur diffusion
Open this publication in new window or tab >>Sulfidation of ceria surfaces from sulfur and sulfur diffusion
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 15, 8417-8425 p.Article in journal (Refereed) Published
Abstract [en]

Even very low levels of sulfur contaminants can degrade the catalytic performance of cerium oxide. Here, the interaction of atomic sulfur with the ceria (111) and (110) surfaces has been studied using first-principles methods. Two sulfoxy species are identified: oxido-sulfate(2-) species (SO2-) on both the CeO2 (111) and (110) surfaces and hyposulfite (SO22-) on the (110) surface. Sulfide (S2-) is formed when a surface or a subsurface oxygen atoms is replaced by sulfur. These sulfide species are most stable at the surface. Furthermore, sulfite (SO32-) structures are found when sulfur is made to replaces one Ce in the ceria (111) and (110) surfaces. The calculated sulfur diffusion barriers are larger than 1.4 eV for both surfaces and thus sulfur is essentially immobile, providing a possible explanation for the sulfidation phenomena of the ceria-based catalysis. Thus we find three different species from interaction of S with Ceria which are all, due to their strong binding, capable of poisoning the surface, reduced or unreduced. Our results suggest that under reducing conditions, sulfur is likely to be found in the (111) surface (replacing oxygen) but on the (110) surface (as SO22-).

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-167999 (URN)10.1021/jp2092913 (DOI)000302924900010 ()
Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2017-12-08Bibliographically approved
8. Sulfidation and Sulfur Recovery from SO2 over Ceria
Open this publication in new window or tab >>Sulfidation and Sulfur Recovery from SO2 over Ceria
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 31, 17499-17504 p.Article in journal (Refereed) Published
Abstract [en]

Sulfidation, sulfation and sulfur recovery of ceria(111) and ceria(110) surfaces are studied usingDensity Functional Theory(DFT) calculations. Under reducing atmosphere SO2 adsorption leadsto stable surface sulfate species on the (110) surface and sulfides on the (111) surface. A mechanismfor sulfur recovery from SO2 is also presented. In this mechanism SO2 reacts with a surface sulfideto form a thio-sulfite species. This thio-sulfite species is subsequently reduced by an oxygen vacancyto form a monodentate S2O structure. This structure can then be desorbed as S2 (g).

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-168001 (URN)10.1021/jp4094673 (DOI)000340222300036 ()
Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2017-12-08Bibliographically approved

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