Oxygen storage effect

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... 

Lanthanum oxide is valence invariant, and does not exhibit any oxygen storage capacity, but it effectively stabilizes 5/-AI2O3. It spreads over the alumina surface and provides a barrier against dissolution of rhodium in the support. 

Larese, C., Lopez Granados, M., Mariscal, R. et al. (2005) The effect of calcination temperature on the oxygen storage and release properties of Ce02 and Ce—Zr—O metal oxides modified by phosphorus incorporation, Appl. Catal. B Environ., 59, 13. 

Meitzler, A. H. "Application of Exhaust-Gas-Oxygen Sensors to the Study of Storage Effects in Automotive Three-Way Catalysts" SAE Paper No. 800019, 1980. 

The hydraulic simulation tool AMESim (2006) has also been extended for exhaust aftertreatment simulation, by including routines developed together with IFP (2006). The software includes models for TWC, hydrocarbon (EIC) trap, NSRC, oxygen storage, DOC and DPF as well as pipes, etc. Catalysts are modeled via 0D approach, hence all transport effects are lumped into reaction kinetic parameters. These kinetic parameters can be adapted by the user. 

As mentioned above, both the point-mutation on the distal side of the Mb and the modification of the heme-propionate side chains are effective to convert the Mb into peroxidase and peroxygenase. Thus, one can imagine that the combination of an amino acid mutation and a modified-heme reconstitution may more effectively allow us a new catalyst by oxygen storage. Recently, two examples, which demonstrated the hybrid modification of Mb, have been reported. One is the T67R/S92D Mb reconstituted with the modified hemin where a histidine is linked at the terminal of the heme-propionate side chain (108). The peroxidase activities toward p-hydroxyphenylpropionic acid and tyramine oxidations by the reconstituted mutant Mb are increased by 24- and 2.3-fold, respectively, based on the kciJKm value compared to those observed for the native Mb. 

Lanthana is supposed to be superior to ceria as a washcoat stabilizer, but for oxygen storage, ceria is most effective. The market for auto exhaust catalysts is extensive with over several million units being produced each year in the world, but the rare earth content is only of the order of several grams. 

Answer 3.6 As with V2O5 in the Contact process for making sulfuric acid, the ability of cerium to adopt transition-metal-like behaviour in switching oxidation states means that the cerium oxide can effectively act as an oxygen-storage system. 

Figure 7.5. Evolution of the oxygen storage capacity (OSC) as a function of temperature. Effect of the metal (a tr. Rh) and the support (CeO broken lines. Ceo ftjZrojTOz full lines).

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. 

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