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Erratum published on 19 June 2018, see Inorganics 2018, 6(2), 62.

Open AccessArticle
Inorganics 2017, 5(4), 76; https://doi.org/10.3390/inorganics5040076

Ceria: Recent Results on Dopant-Induced Surface Phenomena

1
Deutsches Zentrum für Luft- und Raumfahrt, Institut für Werkstoff-Forschung, Linder Höhe, D-51147 Köln, Germany
2
Institut für Metallurgie, Technische Universität Clausthal, Robert-Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany
3
Institut für physikalische Chemie, RWTH Aachen University, Landoltweg 2, D-52074 Aachen, Germany
Dedicated to Professor Ilan Riess on the occasion of his 75th birthday.
*
Author to whom correspondence should be addressed.
Received: 27 September 2017 / Revised: 26 October 2017 / Accepted: 27 October 2017 / Published: 8 November 2017
(This article belongs to the Special Issue Cerium-based Materials for Energy Conversion)
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Abstract

Redox studies on dense zirconia-doped ceria pellets were carried out by thermogravimetric investigations and dilatometry. Up to 1600 K reduction parameters determined by both methods correspond to each other. At higher temperatures, however, thermogravimetry overestimates the degree of reduction since mass loss is not only due to oxygen exsolution but also to selective evaporation of CeO2 whose vapour pressure is considerably higher than that of ZrO2. As a consequence surface segregation of zirconia occurs in (Ce,Zr)O2−δ pellets leading to a porous surface zone of Ce2Zr2O7 pyrochlore which gradually grows in thickness. Surface enrichment of zirconia is detrimental for splitting CO2 or H2O since re-oxidation temperatures of (Ce,Zr)O2−δ are known to be shifted towards lower temperatures with increasing ZrO2 content. Thus, very harsh reduction conditions should be avoided for the (Ce,Zr)O2−δ redox system. The kinetics investigations comprised the high temperature reduction step (T ≅ 1600 K) and the “low” temperature oxidation reaction with a carbon dioxide atmosphere (T ≅ 1000 K). The reduction kinetics (at around 1600 K and an oxygen activity of 7 × 10−4 in the gas phase) directly yield the (reduction) equilibrium exchange rate of oxygen in the order of 10−7 mol·O/(cm3·s) as the kinetics are surface controlled. The oxidation step at around 1000 K, however, occurs in the mixed control or in the diffusion control regime, respectively. From oxygen isotope exchange in combination with SIMS depth profiling oxygen exchange coefficients, K, and oxygen diffusivities, D, were determined for so-called equilibrium experiments as well as for non-equilibrium measurements. From the obtained values for K and D the (oxidation) equilibrium exchange rates for differently doped ceria samples were determined. Their dependency on the oxygen activity and the nature and the concentrations of a tetravalent dopant (Zr) and trivalent dopants (La, Y, Sm) could be semi-quantitatively rationalised on the basis of a master equation for the equilibrium surface exchange rate. View Full-Text
Keywords: ceria; thermal expansion; chemical expansion; oxygen diffusion; oxygen surface exchange ceria; thermal expansion; chemical expansion; oxygen diffusion; oxygen surface exchange
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Knoblauch, N.; Simon, H.; Dörrer, L.; Uxa, D.; Beschnitt, S.; Fielitz, P.; Wendelstorf, J.; Spitzer, K.-H.; Schmücker, M.; Borchardt, G. Ceria: Recent Results on Dopant-Induced Surface Phenomena. Inorganics 2017, 5, 76.

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