Ceria high surface

In addition to platinum and related metals, the principal active component ia the multiflmctioaal systems is cerium oxide. Each catalytic coaverter coataias 50—100 g of finely divided ceria dispersed within the washcoat. Elucidatioa of the detailed behavior of cerium is difficult and compHcated by the presence of other additives, eg, lanthanum oxide, that perform related functions. Ceria acts as a stabilizer for the high surface area alumina, as a promoter of the water gas shift reaction, as an oxygen storage component, and as an enhancer of the NO reduction capability of rhodium. 

Alumina is used because it is relatively inert and provides the high surface area needed to efftciendy disperse the expensive active catalytic components. However, no one alumina phase possesses the thermal, physical, and chemical properties ideal for the perfect activated coating layer. A great deal of research has been carried out in search of modifications that can make one or more of the alumina crystalline phases more suitable. Eor instance, components such as ceria, baria, lanthana, or 2irconia are added to enhance the thermal characteristics of the alumina. Eigure 6 shows the thermal performance of an alumina-activated coating material. 

Imamura, Kaito, and coworkers—metal-support effects observed after calcination. Imamura et al 9X reported a strong metal-support interaction between Rh and Ce02, whereby high surface area ceria calcined at low temperature (550 °C) was able to transport Rh particles to the bulk, as measured by XPS. They suggested that despite the low degree of exposure of the Rh particle at the surface, the exposed Rh was highly active for the methanol decomposition reaction. 

Flytzani-Stephanopoulos and coworkers—urea method for preparing high surface area ceria/substituting noble metals with base metals/cationic active sites for Au and Pt-ceria catalysts/deactivation by hydroxycarbonates and improved stability with 02 co-feeding. Li et al.396 reported on low temperature water-gas shift catalysts in their search for a replacement catalyst for Cu/ZnO suitable for use in a fuel... 

There is an obvious overlap among various applications categories. An example of the overlap is alumina which is both a structural refractory ceramic as well as a catalyst support. The additives modify the interconversion of various AI2O3 phases and the high surface area of y-Al203 is maintained by the added 3 wt% ceria or lanthana. Additives like yttria stabilize zirconia with respect to inertness and mechanical stability. Addition of yttrium or lanthanide to Fe-Cr-Al alloys reduces the spallation of oxide film. 

Most of the current converters consist of a flow-through ceramic monolith with its channel walls covered with a high-surface-area 7-AI2O3 layer (the washcoat) which contains the active catalyst particles. The monolith is composed of cordicrite, a mineral with the composition 2MgO 2AI2O3 5Si02. The chemical composition of a modern TWC is quite complex. In addition to alumina, the washcoat contains up to 30 wt% base metal oxide additives, added for many purposes. The most common additives are ceria and lanthana in many formulations BaO and Zr02 are used, and in some converters NiO is present. The major active constituents of the washcoat are the noble metis Pt, Pd, and Rh (typically 1-3 g). Most of the TWC systems in use today are still based on Pt and Rh in a ratio of about 10 1. 

Mesoporous ceria and ceria-zirconia powders with high surface area have also been prepared using a surfactant-assisted method to prepare catalysts containing Ce02... 

High surface area ceria-zirconia (at. ratio CeiZr l.O) prepared by a microemulsion method has been studied in the same way [49]. The presence of both Zr -02 (features at g =2.03 7-2.032, gy=2.009 and g =2.002) and species (similar to those... 

Thermal deactivation involves processes such as diffusion and solid-state reaction. In early three-way catalysts where both the active metal and ceria were dispersed onto high-surface-area Y-A1203, loss of contact between them, due to sintering of either one or both, could effectively eliminate oxygen storage. The temperature required for ceria to sinter, somewhat above 800°C, was typically not attained under normal operating conditions, although relatively harsh conditions, with temperatures well in excess of 800°C under rich exhaust gas, did exist in heavy-duty truck operation, and in this case, reaction between ceria and alumina at times produced stable, inert cerium aluminate. 

As mentioned in Section 10.2 above, both ceria and ceria-zirconia contain relatively weakly-bound oxygen when freshly prepared, e.g., in high-surface-area form. The thermal stability of this oxygen may differ in the two materials, however, as shown in steady-state CO-oxidation measurements performed by Bunluesin et al. [11] on model planar catalysts. In these experiments, films of ceria and ceria-zirconia were subjected to calcination treatments over a wide range of temperature before noble... 

On the other hand Otsuka et al. found that for ceria both surface and bulk oxygen ions participate to the partial oxidation of methane [49]. When oxygen is missing there is syngas H2 + CO formation. However this reaction occurs only at high temperature close to 750°C ... 

Because of the promoting effects of ceria in many catalytic reactions, the preparation of high surface area and thermally stable ceria phases as well as the study of the parameters which control structural, textural and redox properties of the material are of particular interest [14-15],... 

Laosiripojana, N. and Assabumrungrat, S. Catalytic dry reforming of methane over high surface area ceria. Applied Catalysis. B, Environmental, 2005, 60 (1-2), 107. 

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