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Ceria: Recent results on dopant-induced surface phenomena

Knoblauch, Nicole and Simon, Heike and Dörrer, Lars and Uxa, Daniel and Fielitz, Peter and Wendelstorf, Jens and Spitzer, Karl-Heinz and Schmücker, Martin and Borchardt, Günter (2017) Ceria: Recent results on dopant-induced surface phenomena. Inorganics. Multidisciplinary Digital Publishing Institute (MDPI). ISSN 2304-6740

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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 vapor pressure is considerably higher than that of ZrO2. As a consequence surface segregation of zirconia occurs in (Ce,Zr)O2-d 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-d 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-d redox system. The kinetics investigations comprised the high temperature reduction step (T = 1600 K) and the "low" temperature (T = 1000 K) oxidation reaction with a carbon dioxide atmosphere. 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/(cm3s) 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 nonequilibrium 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.

Item URL in elib:https://elib.dlr.de/114826/
Document Type:Article
Additional Information:Referierte Publikation
Title:Ceria: Recent results on dopant-induced surface phenomena
Authors:
AuthorsInstitution or Email of AuthorsAuthors ORCID iD
Knoblauch, NicoleNicole.Knoblauch (at) dlr.deUNSPECIFIED
Simon, HeikeHeike.Simon (at) dlr.deUNSPECIFIED
Dörrer, Larstechnische universität clausthal, institut für metallurgieUNSPECIFIED
Uxa, Danieltechnische universität clausthal, institut für metallurgieUNSPECIFIED
Fielitz, Petertu clausthalUNSPECIFIED
Wendelstorf, Jenstechnische universität clausthal, institut für metallurgieUNSPECIFIED
Spitzer, Karl-Heinztechnische universität clausthal, institut für metallurgieUNSPECIFIED
Schmücker, Martinmartin.schmuecker (at) dlr.deUNSPECIFIED
Borchardt, Güntertu clausthalUNSPECIFIED
Date:November 2017
Journal or Publication Title:Inorganics
Refereed publication:Yes
Open Access:Yes
Gold Open Access:Yes
In SCOPUS:Yes
In ISI Web of Science:Yes
Publisher:Multidisciplinary Digital Publishing Institute (MDPI)
ISSN:2304-6740
Status:Published
Keywords:Ceria; thermal expansion; chemical expansion; oxygen diffusion; oxygen surface exchange;
HGF - Research field:Energy
HGF - Program:Renewable Energies
HGF - Program Themes:Solar Fuels
DLR - Research area:Energy
DLR - Program:E SW - Solar and Wind Energy
DLR - Research theme (Project):E - Solar Fuels
Location: Köln-Porz
Institutes and Institutions:Institute of Materials Research > Structural and Functional ceramics
Deposited By: Knoblauch, Nicole
Deposited On:09 Nov 2017 07:27
Last Modified:14 Dec 2019 04:24

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