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Oxygen Storage Chemistry of Nanoceria
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.ORCID iD: 0000-0001-8245-6295
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The versatile redox chemistry of ceria (CeO2) originates from its Ce4f electron, which plays the key role in changing the oxidation state of Ce between +IV and +III. Ceria is, among other things, a material that can act as a powerful oxygen buffer with a high oxygen storage capacity (OSC). This is used in many technical applications, such as the three-way catalyst, cleaning exhausts from gasoline vehicles. This thesis is concerned with the dramatic OSC effect observed experimentally in the literature for very small ceria nanoparticles (NPs) at lower temperatures, where the effect was found to be accompanied by the formation of superoxide ions (O2).

The main aim of the thesis work was to develop strategies to allow us to discover the origin of the OSC phenomenon, and to simulate temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) experiments and collect useful mechanistic insight about these processes. Quantum-mechanical (DFT) calculations, partly with modified DFT functionals, and later augmented by microkinetic (MK) modelling building on the DFT-results, made it possible to model the large and complex NP systems needed to make detailed comparisons between theory and experiment feasible.

At first, a suitable DFT functional for nanoceria was needed. We turned to hybrid functionals, and more specifically, the non-local Fock exchange contribution within the hybrid functional HSE06 was explored. The amount that gave the best overall description was determined (15%, labeled HSE06' below) and was used in subsequent studies. Moreover, an accompanying HSE06'//PBE+U computational protocol was constructed (HSE06' energies calculated for pre-optimized structures at the PBE+U level); this made it possible to use the hybrid functional for large ceria systems.

With the modified HSE functional, we scrutinized a previously proposed OSC model, namely the "supercharge" model for nanoparticles loaded on the outside with superoxide ions at low-coordinated ridge sites, enabled by the oxidation of Ce3+ to Ce4+. In the previous study, adsorption energies were calculated using the PBE+U density functional, which does not give adsorption energies in agreement with experiment. With the new HSE06' functional, together with the Redhead equation, we obtained an estimated oxygen desorption peak at ca. 415 K, in much better agreement with the experimental TPD peak at 440 K. However, this calculation could still not explain the large broadening of the experimental TPD spectrum. An oxygen adsorption energy model was then formulated which took Ce coordination and superoxide ion coverage into account. With microkinetic simulations based in this energy model, we achieved a broad simulated TPD signal, which was largely in agreement with the experimental spectrum.

Finally, an improved “supercharge” model was assessed concerning its ability to mimic the temperature-programmed reduction (TPR) experiments reported in the literature for H2 interacting with ceria nanoparticles. We proposed that the reduction process follows a Langmuir-Hinshelwood reaction mechanism, which gave a simulated TPR spectrum in good agreement with the experimental results.

In summary, the goals listed above were achieved: we managed to simulate TPD and TPR spectra, using a DFT-based MK approach; the results were in good agreement with experiment and useful mechanistic insight about these processes and the OSC mechanism was derived from the MK simulations and the DFT analyses.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 58
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1813
Keywords [en]
Nanoceria, Density Functional Theory, Hybrid Functional, Oxygen Storage Capacity.
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-382396ISBN: 978-91-513-0666-7 (print)OAI: oai:DiVA.org:uu-382396DiVA, id: diva2:1306751
Public defence
2019-05-27, Å80127, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2019-05-10 Created: 2019-04-24 Last updated: 2019-06-17
List of papers
1. Screened hybrid functionals applied to ceria: Effect of Fock exchange
Open this publication in new window or tab >>Screened hybrid functionals applied to ceria: Effect of Fock exchange
2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 23, article id 235203Article in journal (Refereed) Published
Abstract [en]

We investigate how the redox properties of ceria are affected by the fraction of Fock exchange in screened HSE06-based hybrid density functionals, and we compare with PBE+U results, and with experiments when available. We find that using similar to 15% Fock exchange yields a good compromise with respect to structure, electronic structure, and calculated reduction energies, and represents a significant improvement over the PBE+U results. We also investigate the possibility to use a computationally cheaper HSE06//PBE+U protocol consisting of structure optimization with PBE+U, a subsequent lattice parameter rescaling step, and, finally, a single-point full hybrid calculation. We find that such a composite computational protocol works very well and yields results in close agreement with those where HSE06 was used also for the structure optimization.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-372692 (URN)10.1103/PhysRevB.97.235203 (DOI)000436035400005 ()
Available from: 2019-01-08 Created: 2019-01-08 Last updated: 2019-04-24Bibliographically approved
2. From Ceria Clusters to Nanoparticles: Superoxides and Supercharging
Open this publication in new window or tab >>From Ceria Clusters to Nanoparticles: Superoxides and Supercharging
2019 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, no 3, p. 1742-1750Article in journal (Refereed) Published
Abstract [en]

Several studies have reported a dramatically increased oxygen storage capacity (OSC) for small ceria nanoparticles (∼5 nm). Both experiments and theory have correlated this effect with superoxide ion formation. In previous studies, density functional theory (DFT) calculations with the PBE+U density functional have been used, and the obtained results were only in qualitative agreement with the experimental observations. One severe problem is the underbinding of the O2 molecule upon superoxide ion formation, which suggests that such species should not exist above room temperature. In this work, we use hybrid DFT functional to resolve this problem. We find that the discrepancy between theory and experiment originates from an incorrect estimate of the energy associated with the localized f-electrons with respect to the oxygen p-levels. By using average O2 adsorption energies from hybrid DFT calculations, extrapolated to large nanoparticles (3−10 nm), in conjunction with first-order desorption kinetics, we find that superoxide ions are indeed stable on nanosized ceria well above room temperature, in accordance with experiments.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2019
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-377356 (URN)10.1021/acs.jpcc.8b08977 (DOI)000457067500025 ()
Funder
Swedish Research CouncilÅForsk (Ångpanneföreningen's Foundation for Research and Development)eSSENCE - An eScience Collaboration
Available from: 2019-02-19 Created: 2019-02-19 Last updated: 2019-04-24Bibliographically approved
3. Simulated temperature programmed desorption experiments for calcined nanoceria powders
Open this publication in new window or tab >>Simulated temperature programmed desorption experiments for calcined nanoceria powders
(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-382449 (URN)
Available from: 2019-04-25 Created: 2019-04-25 Last updated: 2019-04-25
4. Simulated temperature programmed reduction by H2 — a key to understanding OSC on nanoceria
Open this publication in new window or tab >>Simulated temperature programmed reduction by H2 — a key to understanding OSC on nanoceria
(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-382450 (URN)
Available from: 2019-04-25 Created: 2019-04-25 Last updated: 2019-04-29

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