Ceria, bulk preparation

Ce02/Al203 supports were studied in detail by Shyu et al. [108], who examined specimens with different ceria loadings prepared by impregnating AI2O3 with Ce nitrate and calcination at 1073 K. From XPS-measured Ce/Al ratios they concluded that transition from highly dispersed ceria species to bulk phases occurred for loads... 

Many groups have reported XPS data on this type of systems. Before any further discussion it is important to note that, although a number of effects may be induced by metal-ceria interactions, they can be also provoked, or at least enhanced, by the presence of chlorine introduced with the metal. The strong retention at the surface (or/and in the bulk) of ceria and related oxides, even after redox treatments, of chlorine coming from the transition metal salt used in preparing the catalyst is well documented [109, 138, 147, 148, 40, 149, 150, 151]. It has been verified that Cl... 

It is considered that the bulk area specific resistance i o must be lower than l o = k/<7 = 0.15 Qcm, where L is the electrolyte thickness and a is its total conductivity, predominantly ionic [39]. At present, fabrication technology allows the preparation of reliable supported structures with film thicknesses in the range 10-15 pm consequently, the electrolyte ionic conductivity must be higher than 10 Scm. As shown in Figure 12.9, a few electrolytes (ceria-based oxides, stabihzed zirconias, and doped gallates) exceed this minimum ionic conductivity above 500 °C. 

Another approach to preparing model catalysts is the preparation of inverse supported catalysts . In this approach, the catalytically active metal (usually single crystal) is used as a substrate upon which an oxide is deposited, presumably leaving patches of exposed metal. This approach has been used to study reduction of ceria, and methanation kinetics on Rh as promoted by deposited ceria, and chemisorption of various molecules. As stated above, it is generally assumed that thick enough ceria layers will continuously cover the metal substrate, placing a limit on the thickness of the ceria islands that can be achieved for an inverse supported catalyst. The different procedures used for the inverse and metal particle on bulk oxide model catalysts is expected to produce differences in thermal stability, morphology and surface structure which may have consequences for the reactivity of the model catalyst. 

Figure 3-10. Schematic diagram of bulk ceria preparation...

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