Non-stoichiometry and electrical impedance of thin-film ceria-zirconia solid solutions at elevated temperatures

Titelangaben

Wollbrink, Alexander ; Steiner, Carsten ; Moos, Ralf ; Fritze, Holger:
Non-stoichiometry and electrical impedance of thin-film ceria-zirconia solid solutions at elevated temperatures.
2018
Veranstaltung: Materials Science and Engineering Congress (MSE) , 26.-28.9.2018 , Darmstadt, Germany.
(Veranstaltungsbeitrag: Kongress/Konferenz/Symposium/Tagung , Vorlesung )

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Abstract

Ceria (CeO2-δ) and ceria-zirconia solid solutions (Ce1-yZryO2-δ) are commonly used in three way catalysts (TWC) for exhaust gas treatment in gasoline engines. The TWC enable highly efficient reduction of exhaust emissions (HC, CO, and NOx) by the use of catalytic active materials, i.e. noble metals. As part of the TWC washcoat, ceria-based materials serve as reversible oxygen storage for buffering dynamic fluctuations of the air/fuel ratio around stoichiometric combustion in engines. The storage capacity of ceria-based materials results from the fast and reversible release of oxygen that is achieved by the formation of oxygen vacancies as a function of the composition Ce1-yZryO2-δ, temperature T and oxygen partial pressure pO2. Consequently, the understanding of kinetics and mechanisms of atomic transport processes related to the non-stoichiometry in ceria and also ceria-zirconia solid solutions is highly important and must be investigated in detail. In the present work, the non-stoichiometry of thin-film ceria and ceria-zirconia solid solutions with different compositions of Ce1-yZryO2-δ (y = 0, 0.33, 0.67) is studied. Thin films provide compared with i.) densely sintered polycrystalline samples the advantage of considerable lower time constants to achieve the equilibrium state, and ii.) nano-sized polycrystalline powders a higher thermal stability. The characterization of thin-film Ce1-yZryO2-δ is done by thermogravimetric analysis using piezoelectric resonators which provides direct access to the non-stoichiometry of the materials. In addition, the electric impedance of the thin films is studied and compared with the non-stoichiometry data. All samples are investigated for oxygen partial pressures pO2 in the range from 0.2 to 10-21 bar at elevated temperatures. Results of electric impedance measurements of thin-film pure ceria (CeO2-δ) as a function of pO2 and temperature in the range from 600 °C to 900 °C are in a good agreement with values given in the literature. The measured data show a dependency of conductivity σ ~ pO2-1/6 due to the formation of double ionized vacancies (VO˙˙) accompanied by the reduction of Ce4+ to Ce3+.