In this Ph.D. thesis the reactivity of clean and silver precovered ceria surfaces is studied in the context of applications in catalysis of Diesel particulate soot abatement. In fact, ceria and in particular silver doped ceria show very good catalytic activity in decreasing the temperature of soot combustion. A key step in the mechanism of oxidation of the particulate, which is mainly amorphous carbon, is the activation of the oxygen molecule. In literature, experimental IR-Raman and EPR studies show the comparison of peroxo, O22-, and superoxo, O2-, species when O2 interacts with ceria. With theoretical calculations we firstly investigate the interaction of O2 with regular CeO2(111) and defective CeO2-x (111) surfaces. While with the regular surface we find no interaction and no activation of the oxygen molecule, on the oxygen vacancy a peroxo strongly linked to the surface is observed. The formation of O22- is due to a charge transfer from the two Ce3+ centers that are located on the surface when an O is removed. The formation of a superoxo, thus, is not found when such an interaction is considered. The hypothesis is then that the interaction of O2 with isolated Ce3+ could generate O2- by charge transfer. When O2- is studied, Electron Paramagnetic Resonance (EPR) properties can be evaluated. For superoxo formed on ceria the experimental spectra available in literature are not well resolved, so a comparison between calculated and experimental data is not trivial. Then, a model system is studied to validate the theoretical approach in reproducing EPR properties of superoxo. In fact, for O2- formed on Na precovered MgO surfaces, very well resolved EPR spectra are available. In this case we found that our theoretical approach can reproduce the EPR properties very well. Subsequently, we studied the interaction of O2 with ceria clusters and nanoparticles. At first, we considered the model system Ce2O3, that has two Ce3+ ions. This species is modeled at different theoretical levels, and we always find the expected result, the formation of O2- by charge transfer from one low-coordinated Ce3+ center. The evidences of this are in the O-O bond length, in the IR frequency and in the EPR properties, that are in very good agreement with the experimental ones. This result is confirmed also on bigger defective nanoparticles, where the oxygen vacancy leaves two Ce3+ centers that stabilize on low-coordinated sites. The presence of low-coordinated and, more important, isolated Ce3+ centers is thus responsible of the formation of superoxo ions on ceria. The role of silver in improving the catalytic activity of ceria is proved in literature by a decrease of the soot combustion temperature. Moreover, in literature, experimental EPR studies show a slightly increase of the signal due to the presence of superoxo ions. Experimental STM and XPS data of Ag nanoparticles deposited on stoichiometric and reduced thin CeO2 films grown on Pt(111) in UHV conditions show a reduction of ceria with comparison of Ce3+ centers. Theoretically, we studied the interaction of Ag atoms, Ag5 and Ag0 clusters and mono and bi-layers of silver adsorbed on CeO2(111) and CeO2-x(111) surfaces. We find a general tendency of silver to reduce ceria by charge transfer, with subsequent oxidation of the metal. However, the experimental data could also fit with an oxygen spillover mechanism. In this situation, oxygen atoms migrate from the surface to the metal particles, leaving a reduced support. Our calculations show that this mechanism is highly unfavourable from an energetic point of view. Therefore, both the experimental and the theoretical results agree with a reduction of ceria due to a charge transfer from silver.

(2012). Theoretical models of heterogeneous catalysts based on cerium oxide. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2012).

Theoretical models of heterogeneous catalysts based on cerium oxide

PREDA, GLORIA
2012

Abstract

In this Ph.D. thesis the reactivity of clean and silver precovered ceria surfaces is studied in the context of applications in catalysis of Diesel particulate soot abatement. In fact, ceria and in particular silver doped ceria show very good catalytic activity in decreasing the temperature of soot combustion. A key step in the mechanism of oxidation of the particulate, which is mainly amorphous carbon, is the activation of the oxygen molecule. In literature, experimental IR-Raman and EPR studies show the comparison of peroxo, O22-, and superoxo, O2-, species when O2 interacts with ceria. With theoretical calculations we firstly investigate the interaction of O2 with regular CeO2(111) and defective CeO2-x (111) surfaces. While with the regular surface we find no interaction and no activation of the oxygen molecule, on the oxygen vacancy a peroxo strongly linked to the surface is observed. The formation of O22- is due to a charge transfer from the two Ce3+ centers that are located on the surface when an O is removed. The formation of a superoxo, thus, is not found when such an interaction is considered. The hypothesis is then that the interaction of O2 with isolated Ce3+ could generate O2- by charge transfer. When O2- is studied, Electron Paramagnetic Resonance (EPR) properties can be evaluated. For superoxo formed on ceria the experimental spectra available in literature are not well resolved, so a comparison between calculated and experimental data is not trivial. Then, a model system is studied to validate the theoretical approach in reproducing EPR properties of superoxo. In fact, for O2- formed on Na precovered MgO surfaces, very well resolved EPR spectra are available. In this case we found that our theoretical approach can reproduce the EPR properties very well. Subsequently, we studied the interaction of O2 with ceria clusters and nanoparticles. At first, we considered the model system Ce2O3, that has two Ce3+ ions. This species is modeled at different theoretical levels, and we always find the expected result, the formation of O2- by charge transfer from one low-coordinated Ce3+ center. The evidences of this are in the O-O bond length, in the IR frequency and in the EPR properties, that are in very good agreement with the experimental ones. This result is confirmed also on bigger defective nanoparticles, where the oxygen vacancy leaves two Ce3+ centers that stabilize on low-coordinated sites. The presence of low-coordinated and, more important, isolated Ce3+ centers is thus responsible of the formation of superoxo ions on ceria. The role of silver in improving the catalytic activity of ceria is proved in literature by a decrease of the soot combustion temperature. Moreover, in literature, experimental EPR studies show a slightly increase of the signal due to the presence of superoxo ions. Experimental STM and XPS data of Ag nanoparticles deposited on stoichiometric and reduced thin CeO2 films grown on Pt(111) in UHV conditions show a reduction of ceria with comparison of Ce3+ centers. Theoretically, we studied the interaction of Ag atoms, Ag5 and Ag0 clusters and mono and bi-layers of silver adsorbed on CeO2(111) and CeO2-x(111) surfaces. We find a general tendency of silver to reduce ceria by charge transfer, with subsequent oxidation of the metal. However, the experimental data could also fit with an oxygen spillover mechanism. In this situation, oxygen atoms migrate from the surface to the metal particles, leaving a reduced support. Our calculations show that this mechanism is highly unfavourable from an energetic point of view. Therefore, both the experimental and the theoretical results agree with a reduction of ceria due to a charge transfer from silver.
PACCHIONI, GIANFRANCO
Ceria, calculations, catalysis, soot abatement, silver, surfaces
CHIM/03 - CHIMICA GENERALE E INORGANICA
English
13-gen-2012
Scuola di dottorato di Scienze
SCIENZA DEI MATERIALI - 08R
24
2010/2011
- Prof. Francesc Illas (Universitat de Barcelona): works in collaborations - Dott. Annapaola Migani (Universidad del País Vasco): calculations on ceria nanoparticles - Dott. Paola Luches, Prof. Sergio Valeri (Università di Modena e Reggio Emilia): STM and XPS experiments on ceria - Prof. Mario Chiesa, Prof. Elio Giamello: EPR experiments on magnesiun oxide
open
(2012). Theoretical models of heterogeneous catalysts based on cerium oxide. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2012).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/28402
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