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, 5 (4). Multidisciplinary Digital Publishing Institute (MDPI). doi: 10.3390/inorganics5040076. 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/ | ||||||||||||||||||||||||||||||||||||||||
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Document Type: | Article | ||||||||||||||||||||||||||||||||||||||||
Additional Information: | Referierte Publikation | ||||||||||||||||||||||||||||||||||||||||
Title: | Ceria: Recent results on dopant-induced surface phenomena | ||||||||||||||||||||||||||||||||||||||||
Authors: |
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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 | ||||||||||||||||||||||||||||||||||||||||
Volume: | 5 | ||||||||||||||||||||||||||||||||||||||||
DOI: | 10.3390/inorganics5040076 | ||||||||||||||||||||||||||||||||||||||||
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 (old) | ||||||||||||||||||||||||||||||||||||||||
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: | 16 May 2022 09:15 |
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