Oxygen ceria, doped

Cerium-based catalysts have been successfully used in several processes. For example, ceria (Ce02) is used as an additive [ 1,2] in modem automotive exhaust catalysts. Ceria acts as an excellent oxygen store [3-5] in the catalyst, which is thus rendered a very effective catalyst for combustion [6]. Moreover, addition of ceria to the automotive exhaust catalysts minimises the thermally induced sintering of the alumina support and stabilises the noble metal dispersion [7]. Ceria also enhances nitric oxide dissociation when added to various supported metal catalysts [8], which is another important function of the automotive exhaust catalyst. Recent investigations by Harrison et al have shown that ceria doped with certain lanthanides and promoted with copper and chromium have catalytic activities comparable to that of the noble metal catalysts [9]... 

This tendency for reduction restricts the range of oxygen partial pressures over which the ionic transference number remains close to unity. For example, at 800°C, the oxygen partial pressure in the lower limit is restricted to partial pressures over 10 atm. This lower limit has been extended with no loss in conductivity to 10" atm at 700°C in ceria doped with 20 mol% GdaOs by replacing 3% of the gadolinium with praseodymium. 

At 1000°C, significant levels of electronic conduction in titania-doped YSZ, as in ceria-doped specimens, are foimd only under strongly reducing atmospheres. Data of oxygen permeability have been presented for the Zr02-Y203-Ti02 system by Arashi and Naito [127] (see also Table 10.1). By virtue of its... 

The most commonly used electrolyte materials in SOFCs are based on zirconia and ceria doped with a suitable cation, normally a rare earth (see Chapter 9). The properties that make these two materials attractive for use in fuel cells are discussed in Section 4.4.4, and it is sufficient to note that the most important feature is that they are good oxygen ion conductors. We will focus here on some recent investigations of these materials, with emphasis placed on their methods of preparation. 

The development of electrolytes that exhibit a higher conductivity at low temperature. Three candidates have emerged, namely doped ceria, doped lanthanum gallate, and doped barium zirconate. The first two of these are oxygen ion electrolytes, and the latter is a proton conductor. 

The conductivities of some ceria-containing compounds obtained from the literature have been compiled and shown as solid lines in Fig. 2.12. The main dopants for ceria belong to the alkaline earth or rare-earth metal series and the majority of doped samples exhibit conductivity values which fall into a rather limited band (gray band in Fig. 2.12), which points to a similar behaviour for all doped ceria samples. Exceptions are pure ceria and ceria doped with redox elements like Pr and Tb which give rise to electronic contribution to conductivity. An important requirement is that a homogeneous solid solution forms between the two oxides, which maintains the fluorite structure since the presence of a second phase or phase inhomogeneity due to insufficient solubility can affect ionic conductivity. The very low values of conductivity found for BaO and MgO-doped ceria were in fact attributed to the low solubility of these oxides into the lattice of CeOj . When the dopant element exists in one oxidation state, conduction is mainly ionic at atmospheric pressure of oxygen (for example ti=l for Bi , Ca, Sm and other rare-earth metals ). 

Due to the small association enthalpy between the dopant cation and oxygen vacancy in the fluorite lattice, the highest conductivity is achieved in ceria doped with Gd or Sm + [279,280]. The highest level of oxygen ionic transport is found in the solid solutions Cej j,.LrLx.02 s> where Ln = Gd or Sm, and x = 0.10-0.20. The lattice ionic conductivity of Ln-doped ceria is about 0.01 S cm at 500 °C however, the... 

Marnellos G, Stoukides M (1998) Ammonia synthesis at atmospheric pressure. Science 282 98-100 Marques FMB, Wirtz GP (1991) Electrical properties of ceria-doped yttria. J Am Ceram Soc 74 598-605 Marques FMB, Kharton W, Naumovich EN, Shaula AL, Kovalevsky AV, Yaremchenko AA (2006) Oxygen ion conductors for fuel cells and membranes selected developments. Sohd State Ionics 177 1697-1703 Marsal A, Comet A, Morante JR (2003a) Study of the CO and humidity interference in La doped tin oxide CO gas sensor. Sens Actuators B 94 324-329... 

Catalyst formulations containing ceria doped with cobalt have also been tested. Cobalt is generally present on the surface of ceria as Co304, - and the activity is associated with its reduction to CoO assisted by oxygen spillover on the ceria support. The redox activity of ceria can also enhance the reducibility of Co304, which in turn influences its oxidation activity. 

These results indicate that the full picture of ionic transport in doped ceria is still not complete and that there are additional parameters in play, such as deviations from the assumption of randomly distributed dopants, which may be influenced by powder synthesis and processing. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and EELS investigations have revealed the existence of dopant-rich nano-sized domains in ceria doped with La, Sm, Y, Yb, Ho, Dy, Tb, and Gd. Ordering of oxygen vacancies was observed to take place in these domains. Clustering of dopants and oxide ion vacancies was also indicated by analysis of extended X-ray absorption fine structure (EXAES) measurements for Y- and... 

Significantly enhanced electronic conductivity at elevated oxygen partial pressures has been observed in ceria doped with Pr or Tb compared to other rare-earth or alkaline-earth elements. - " This is attributed to the contribution of small polarons in the Pr or Tb 4f band. The electronic conductivity of Geo,8Pro.202 x is higher than that of Geo.gGdo.iOj 9g x for p02 > 10" atm, by as much as three... 

As expected from the discussion in the previous section, the electronic conductivity in acceptor-doped ceria (e.g. Sm-doped or Gd-doped ceria) and ceria doped with redox active elements such as Pr or Tb, as well as for materials with multiple substituents (e.g. Gd and Pr substitution) may be sufficient for use as an oxygen membrane. Several studies give measurements of the oxygen fluxes through dense samples of doped ceria. A subset of measured oxygen fluxes as well as the experimental conditions under which they were measured is collected in Table 12.6. The performance of the purely acceptor-doped materials (materials 17,18, and 19) was discussed in Section 12.6.4.3. The performance of membrane materials vdth redox active dopants (materials 15 and 16) and with redox active dopants -l- Co addition (materials 20 and 21) is discussed in this section. 

In the search of high-performance SOFC anode, doped ceria have been evaluated as possible anode materials [9,10]. Comparing Ni-samaria-doped ceria (SDC) with Ni-YSZ, the Ni-SDC anode exhibits higher open-circuit voltages and a lower degree of polarization with either methanol as the fuel, as shown in Fig. 5, or methane as the fuel, as shown in Fig. 6. It was found that the depolarization ability of the anode is associated with the catalytic activity, the electrical conductivity, and the oxygen ionic conductivity of the anode materials [9]. It was also found that the anodic polarization and electro-catalytic activity strongly depend on the Ni content in the anode, and the optimum result for the Ni-SDC anode is achieved with 60... 

For zirconia-doped ceria it was shown that redox cycling can enhance the oxygen storage capacity, see F. Fally, V. Perrichon, H. Vidal, J. Kaspar, G. Blanco, J. M. Pintado, S. Bernal, G. Colon, M. Daturi, J.C. Lavalley, Catalysis Today, 59, 373 (2000). 

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