Linking the Electrical Conductivity and Non-Stoichiometry of Thin Film Ce1-xZrxO2-δ by a Resonant Nanobalance Approach.

Iurii Kogut,Hendrik Wulfmeier,Carsten Steiner,Holger Fritze,Fatima-Ezzahrae El Azzouzi,Alexander Wollbrink,Ralf Moos

doi:10.3390/ma14040748

Iurii Kogut, Hendrik Wulfmeier + Show 5 more

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https://doi.org/10.3390/ma14040748

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Abstract

Bulk ceria-zirconia solid solutions (Ce1−xZrxO2−δ, CZO) are highly suited for application as oxygen storage materials in automotive three-way catalytic converters (TWC) due to the high levels of achievable oxygen non-stoichiometry δ. In thin film CZO, the oxygen storage properties are expected to be further enhanced. The present study addresses this aspect. CZO thin films with 0 ≤ x ≤ 1 were investigated. A unique nano-thermogravimetric method for thin films that is based on the resonant nanobalance approach for high-temperature characterization of oxygen non-stoichiometry in CZO was implemented. The high-temperature electrical conductivity and the non-stoichiometry δ of CZO were measured under oxygen partial pressures pO2 in the range of 10−24–0.2 bar. Markedly enhanced reducibility and electronic conductivity of CeO2-ZrO2 as compared to CeO2−δ and ZrO2 were observed. A comparison of temperature- and pO2-dependences of the non-stoichiometry of thin films with literature data for bulk Ce1−xZrxO2−δ shows enhanced reducibility in the former. The maximum conductivity was found for Ce0.8Zr0.2O2−δ, whereas Ce0.5Zr0.5O2-δ showed the highest non-stoichiometry, yielding δ = 0.16 at 900 °C and pO2 of 10−14 bar. The defect interactions in Ce1−xZrxO2−δ are analyzed in the framework of defect models for ceria and zirconia.

Highlights

Cerium oxide and its derivatives are of great technological interest for a variety of applications, of which the catalytic control of automobile emissions prevails [1,2,3,4]
We note the shift of the diffraction peaks towards larger 2θ in cubic Ce1−x Zrx O2−δ (CZO) (Figure 3)), which indicates the decrease of their lattice parameter with increasing zirconia fraction, as expected from literature on Ce1−x Zrx O2 [9]
Based on the 2θ positions of this broadening, in particular, as observed for CZO-50, the further analysis infers that this phase crystallizes in primitive P 21 3 cubic symmetry, as reported for metastable κ-CeZrO4 in [82,83] (Figure 3b)

Summary

Introduction

Cerium oxide (ceria, CeO2−δ ) and its derivatives are of great technological interest for a variety of applications, of which the catalytic control of automobile emissions prevails [1,2,3,4]. The operation of three-way catalytic converters (TWC), which was used for the exhaust gas aftertreatment of most gasoline-powered vehicles, relies on the capability of ceria-based catalyst to store and release oxygen This capability stems from achievable (and reversible) non-stoichiometry δ, which, in CeO2−δ at high temperatures and low oxygen partial pressures (pO2 ), can attain very high levels [5,6] up to the theoretical value of 0.5 (the Ce2 O3 limit) [7], while maintaining the cubic crystal structure of ceria (fluorite-type). Because oxygen vacancies are positively charged species at high temperature, the principle of electroneutrality requires the formation of opposite charge in the system—in this case, free electrons The latter are delivered by the reduction of cerium cations from the Ce4+ oxidation state to Ce3+. The non-stoichiometric ceria is considered to be a mixed conductor with ionic contribution being realized by vacancy transport mechanism and electronic part contributed by small polaron hopping localized at neighboring Ce4+

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