Pure ceria

OSC of Ce Zr Oi mixed oxides is higher than that of pure ceria. It may also be higher than the theoretical OSC, which proves that several oxygen layers can be involved in the storage process. 

El Fallah, J., Boujani, S., Dexpert, H., Kiennemann, A., Majerus, J., Touret, O., Villain, F., and Le Normand, F. 1994. Redox processes on pure ceria and on Rh/ Ce02 catalyst monitored by x-ray absorption (fast acquisition mode)../. Phys. Chem. 98 5522-33. 

The pure ceria films were textured with out-of-plane (Aco) and in-plane (A) mosaic dispersions as low as 3.8° (Fig. 7.15a) and 6.1 ° (Fig. 7.15b), respectively. Although the out-of-plane angular dispersion increased from 3.8° to 5.10 when Sm doping was increased, a significant improvement was achieved in both out-of-plane and in-plane texture when compared with the Ni-based substrates i.e., Aco = 7.9° and A = 7.6° (Fig. 7.16). 

In pure ceria, taking the electroneutrality condition into consideration and using the reasonable approximation ... 

In the 1800s chemists searched for new elements by fractionating the oxides of rare-earths. Carl Gustaf Mosander s experiments indicated that pure ceria ores were actually contaminated with oxides of lanthanum, a new element. Mosander also fractionated the oxides of yttria into two new elements, erbium and terbium. In 1878 J. Louis Soret (1827—1890) and Marc Delafontaine (1837-1911), through spectroscopic analysis, found evidence of the element holmium, but it was contaminated by the rare-earth dysprosia. Since they could not isolate it and were unable to separate holmium as a pure rare-earth, they did not receive credit for its discovery. 

The [MSR 6] reactor type (see below) was applied for methanol steam reforming over Cu/Ce02/Al203 catalysts by Men et al. [34, 35], Wash coating of the alumina was performed, followed by subsequent impregnation steps with ceria and copper salt solutions. At 250 °C reaction temperature and a water/methanol molar ratio of 0.9, the copper/ceria atomic ratio was varied from 0 to 0.9, revealing the lowest conversion for pure ceria and a sharp maximum for a ratio of 0.1 (see Figure 2.13). 

The carbon monoxide selectivity was well below 2% for all samples. As a by-product, substantial amounts of dimethyl ether were found for all samples the highest selectivity of 23% was detected over pure ceria. Only traces of another by-product, methyl formate, were measured. The dimethyl ether formation was attributed to separate dehydration of the methanol on the alumina surface. 

Metallic gold has also been dissolved from pure ceria,51 from gadolinium-doped ceria,51 and from catalysts made by a urea gelation/coprecipitation method ,52 without significant effect on their activities for water-gas shift. The only problem that has been identified is the formation of a cerium hydroxycarbonate, which occurs during shut-down when water is present. The very extensive work performed on this system48-52 ensures that its conclusions are given full consideration. 

Related systems It should be noted that specific properties for applications could be enhanced by using solid solutions, doped materials, and composites, instead of pure ceria. For example, ceria-zirconia solid solution is a well known ceria based material for enhanced OSC and high ionic conductivity for solid state fuel cell components. It is also used in the three way catalysts for automobile waste gas cleaning, because of the improved thermal stability, surface area, and reducibility. The synthesis, structure, and properties of ceria-zirconia have been actively studied for a long time. Di Monte and Kaspar et al. presented feature articles on the nanostructured ceria—zirconia-mixed oxides. The studies on phase, structures, as well as the microstructures are discussed and reviewed (Di Monte et al., 2004). 

The chemical reagents used for the preparation of stock solutions were reagent grade and were used without further purifieation. The chemicals were in the form of the nitrate salts and were obtained eommercially from Aldrich or Fluka. Pure ceria samples were prepared by precipitation of ceria from aqueous solutions containing O.OIM Ce by adding 1 M NH3 solution. The precipitate was dried at 523K. The surface acid sites concentration was measured using Hammett titrations, with phenolphthalein as indicator, and a O.IM NaOH solution as base. 

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 When the... 

The mixture of these effects described above is obtained by the doping of rare earths elements with variable oxidation state. For example, incorporation of terbium or praseodymium increases both oxygen desorption at lower temperatures and the creation of oxygen vacancies than those of pure ceria. The former is due to the lower binding energy of a lattice oxygen in the mixed oxides and the latter is to the existence of irivalent terbium and praseodymium ions. In addition, a similar effect is also provided by the ternary oxides Cco6Zro4.iM 02. >2 (M = La, and Ga ), ... 

In conclusion, noble metals deposited on ceria-zirconia supports show both increased OSC and increased activity for NOx conversion, the clearer effect being obtained in dynamic conditions [102]. They are also more resistant than pure ceria to thermal sintering, since they are able to maintain a good activity even in a lOOO C durability test [102]. 

Figure 10J< Cfxjss-sectional SEM backscattering image (lower left) and composilional maps showing Ihe structure of a double-layer washcoat found in a currenl TWC Both pure ceria (dispersed throughout an alumina-rich inner layer) and ceria-zirconia (outer layer) are presenl. (J. Hangas)...

Even though OSC is an inherently transient phenomenon, it appears that there is a relationship between steady-state reaction rates and OSC [3,17J. For the CO-oxidation, WGS, and steam-reforming reactions, it has been shown that rates can be enhanced by contact between the precious metals and ceria. Furthermore, high-lemperature treatments, which are known to deactivate the OSC properties of pure ceria, also remove the promotional effects associated with ceria [3,20,221, Given that SO2 affects OSC, one should expect SOt to influence the steady-state behavior of ceria-supported catalysts, if OSC is related to these reactions. 

Expansion of reduction data for ceria with several Gd203 doping levels are given in Table 15.1, Results for pure ceria are included for comparison. The expansion on reduction is of great importance for the practical applications at temperatures... 

Pure ceria samples were prepared by precipitation of ceria from aqueous solutions by adding 1 M NHj solution. The effect of dilution on surface texture was studied, by varying the concentration of the reactants either by keeping the concentration of the base constant and changing the metal ion concentration between the 0.02 M < [Ce" ] < 1 M, or vice-versa by varying the base concentration between the limits 0.1 M < [NH,] < 1 M. In ail preparations, the precipitate was dried at 250 "C and subsequently calcined at 800 C. 

Figure 2 Nitrogen adsorption isotherms for pure ceria and ceria containing 2 mol% lithium...

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