Ceria components

As a result of the mentioned interactions, both the activity for catalytic CO oxidation (even in the absence of oscillations in the gas phase composition) and the oxygen buffering efficiency, are improved by the existence of such contacts. Therefore, when catalyst aging produces sintering of both metal and ceria components on the alumina support, with the consequent decrease in the number of those contacts, catalyst performance is degraded, more than what would be expected on the basis of the decrease in the surface areas alone. 

It was observed that, when supported R catalysts reach temperatures as high as -1273 K, the phase transition occurring in the major support component (y- -> 8,0-Al2Oj) has lethal effects on the adsorptive properties of the supported noble metal. It was also observed that, when a supported R catalyst reaches temperatures higher than those at which the catalyst was first fired and/or reduced, but still lower than those needed for the y—> 8,0-Al2O3 phase transition, the R° adsorptive capacity of pure-alumina-supported catalysts is somewhat increased, whereas the capacity of ceria-containing catalysts is appreciably decreased. This effect was ascribed to an increased strong interaction between R particles and the ceria component of the support. 

In conclusion, in order to get the TWC (a composite catalyst containing different catalytic functionalities with distinct catalytic roles) to operate effectively (to remove NO, CO and VOC), changes needed to be made to the process stream (via the sensor) and to the catalyst formulation (via the addition of a ceria component). 

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. 

In an actual exhaust system controlled by the signal of the oxygen sensor, stoichiometry is never maintained, rather, it cycles periodically rich and lean one to three times per second, ie, one-half of the time there is too much oxygen and one-half of the time there is too Httle. Incorporation of cerium oxide or other oxygen storage components solves this problem. The ceria adsorbs O2 that would otherwise escape during the lean half cycle, and during the rich half cycle the CO reacts with the adsorbed O2 (32,44,59—63). The TWC catalyst effectiveness is dependent on the use of Rh to reduce NO and... 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

Since nitrous oxide, NjO, is a designated "greenhouse" gas, and may contribute to depletion of the ozone layer, its removal from emissions to atmosphere is desirable [1]. However, there are several reports that NjO can be formed at low selectivity as an undesirable by-product of NO+CO conversions during the initial warm-up-from-cold periods in three-way-catalytic (TWC) converters or components thereof [1-3]. TWC s commonly contain Rhodium and Ceria and although N,0 dissociation over RhjO, has been extensively studied [4], the following are among mechanistic possibilities as yet... 

Cerium oxides are outstanding oxide materials for catalytic purposes, and they are used in many catalytic applications, for example, for the oxidation of CO, the removal of SOx from fluid catalytic cracking flue gases, the water gas shift reaction, or in the oxidative coupling reaction of methane [155, 156]. Ceria is also widely used as an active component in the three-way catalyst for automotive exhaust pollution control,... 

Panagiotopoulou and Kondarides—YSZ, like Ce, categorized as active partially reducible oxide component. Yttrium-stabilized Zr02 was also tested by Panagiotopoulou and Kondarides, as mentioned previously in the section on ceria.454 It was found to exhibit better activity over the less reducible oxides. 

The catalytic activity of the anode toward oxidation reactions is a dominant factor in determining SOFC performance, particularly with hydrocarbon fuels. In Cu—cermet anodes, the only role played by Cu is that of electronic conductor. The Cu does not appear to have any catalytic function, and the oxidation reaction in the TPB relies on the addition of other components, primarily ceria. The evidence for this is as follows. First, Cu—YSZ anodes that do not contain ceria exhibit very low performance, even though they are stable in hydrocarbon fuels. Second, substitution of Cu with Au has essentially no effect on anode performance. Since Au is usually thought to be catalytically inert, it seems unlikely that Cu and Au would perform in a similar manner if Cu had a catalytic function. 

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