Intrinsic and Extrinsic Oxygen Diffusion and Surface Exchange Reaction in Cerium Oxide

Abstract

Fluorite structure oxides have a property of deviating from stoichiometry as a function of temperature and/or pressure. CeO 2 has a fluorite structure and a wide range of nonstoichiometry. 1 The material deviates from stoichiometry with increasing temperature and decreasing oxygen partial pressure, leading to a high concentration of defects. The concentration of defects can also be controlled by doping the oxide with impurities. Since CeO 2 exhibits a wide range of solubility 2 for rare earth elements, the concentration of defects may be controlled by dopant concentration. CeO 2 is a mixed conductor, and both electronic and ionic conductivities have been investigated in several studies. 3-10 It is generally agreed 11 that the electronic conductivity of undoped CeO 2 is of the n-type. Since the ionic conductivity in this material is directly related to oxygen diffusion, it is important to understand the diffusion characteristics. However, the characteristics of oxygen diffusion in CeO 2 remain unclear. In particular, the oxygen diffusion in undoped CeO 2 must be studied in detail, because it is useful in understanding the behavior of oxygen diffusion and electrical conductivity for doped CeO 2 .The oxygen self-diffusion coefficients in oxides having fluorite structure are summarized by Ando et al. 12 and Kamiya et al. 13 The characteristic in fluorite structure is that the absolute value of the oxygen diffusion coefficients in stoichiometric oxides, i.e., UO 2 , 14,15 PuO 2 , 16 ThO 2 , 17,18 and CeO 2 , 19 are lower than those of nonstoichiometric oxides, i.e., UO 2ϩx and CeO 2 doped with Y. In addition, the activation energy for oxygen diffusion in stoichiometric oxides is large (202 kJ mol Ϫ1 for PuO 2 and 248-273 kJ mol Ϫ1 for UO 2 ). However, the absolute values of the oxygen diffusion coefficients of nonstoichiometric oxides are higher than those of stoichiometric oxides, and the activation energy for oxygen diffusion in nonstoichiometric oxides is small (77-89 kJ mol Ϫ1 for CeO 2 doped with Y).In the literature, two results, one by Floyd 19 and the other by Kamiya et al., 13 have been presented for oxygen diffusion coefficients in undoped CeO 2 . The activation energy reported by Floyd for undoped CeO 2 was 104 kJ mol Ϫ1 and was close to the value for nonstoichiometric UO 2 and for CeO 2 doped with Y. In contrast, the activation energy indicated by the data by Kamiya et al. (322 kJ mol Ϫ1 ) was close to the value of stoichiometric UO 2 , ThO 2 , and PuO 2 and the absolute value of the oxygen diffusion coefficient of CeO 2.00 was found to be similar to the value of other stoichiometric oxides having fluorite structure. Consequently, Kamiya et al. concluded that their result corresponds to stoichiometric CeO 2 (Ce-1). In their study, the oxygen self-diffusion coefficient for stoichiometric cerium oxide was obtained using gas-phase analysis. Direct measurement of the diffusion coefficient can be performed via secondary ion mass spectroscopy (SIMS). One of the objectives of the present study was to investigate ...

Ceria: Recent Results on Dopant-Induced Surface Phenomena

“…The slope of the curve for nominally un-doped ceria in the Arrhenius plot in Figure 12 is small compared to the slope in the temperature range above 1000 °C, with T > 1000 °C representing the region of true intrinsic behaviour of un-doped CeO2 [26], and is similar to that of the Sm-doped ceria samples, yielding the same activation enthalpy. In the extrinsic region the experimentally obtained activation enthalpy comprises only the migration enthalpy because of the existence of extrinsic oxygen vacancies introduced by trivalent dopants.…”

“…Inorganics Oxygen tracer diffusion coefficients in ceria containing either trivalent dopants (Gd, Sm, Y) or a tetravalent dopant (Zr), and of nominally un-doped ceria samples in the temperature range from 700 to 900 °C from experiments conducted in 0.2 bar 18 O2 gas atmosphere are shown in Figure 12 together with literature data taken from [26]. Calculated activation enthalpies (numerical values in brackets) are also shown in the given temperature range of this work.…”

“…Previous studies of reaction kinetics comprise oxygen isotope exchange in chemical equilibrium and relaxation experiments based on oxygen potential changes. Depending on the microstructure of the sample material used in the different studies, the respective authors derived surface exchange coefficients, K, or diffusivities, D, of oxygen, or both of them [24][25][26][27][28][29][30].…”

“…Because of the lanthanum impurity in the nominally un-doped ceria samples it is reasonable to assume that the oxygen diffusion in the low temperature region T < 1000 °C is controlled by this trivalent dopant and therefore extrinsic, as was discussed by Kamiya et al [26]. A comparison with literature data taken from [26] and references therein as shown in Figure 12 confirms a good agreement of the measured oxygen tracer diffusion coefficients of nominally un-doped ceria in the temperature range 700 °C to 900 °C in this work with data taken from [26].…”

“…The difference in magnitude of the oxygen tracer diffusion coefficients for nominally un-doped ceria and Sm-doped ceria samples in Figure 12 is obviously partially due to the difference in the concentration of the trivalent dopant present on the cation sublattice, being 10 at % for the yttria-doped ceria samples and about 0.2 at % (La) for the nominally un-doped ceria samples. Oxygen tracer diffusion coefficients in ceria containing either trivalent dopants (Gd, Sm, Y) or a tetravalent dopant (Zr), and of nominally un-doped ceria samples in the temperature range from 700 to 900 • C from experiments conducted in 0.2 bar 18 O 2 gas atmosphere are shown in Figure 12 together with literature data taken from [26]. Calculated activation enthalpies (numerical values in brackets) are also shown in the given temperature range of this work.…”