Oxygen storage

Oxygea is a commodity sold either as a gas uader pressure or a very low temperature Hquid close to its boiling poiat. Because of the expease of the coataiaers required, it caimot be stored ia more than modest amounts nor can it be shipped profitably more than a few hundred kilometers. Liquid oxygen storage faciHties are limited to volumes on the order of 4000 m. ... 

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. 

An oxygen storage component stores oxygen under lean operating conditions, ie, fuel-poor/air-rich, and releases it under fuel-rich, air-poor... 

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... 

Fuel sulfur is also responsible for a phenomena known as storage and release of sulfur compounds. Sulfur oxides (S02,S02) easily react with ceria, an oxygen storage compound incorporated into most TWC catalysts, and also with alumina. When the air/fuel mixture temporarily goes rich and the catalyst temperature is in a certain range, the stored sulfur is released as H2S yielding a rotten egg odor to the exhaust. A small amount of nickel oxide incorporated into the TWC removes the H2S and releases it later as SO2 (75—79). 

This process increases the percent removals of BOD and TSS to a minimum of 85 percent. A secondary treatment facility can be comprised of Oxygenation Tanks, Pure Oxygen Generating Plant, Liquid Oxygen Storage Tanks, Secondary Clarifiers,... 

High levels of sulfur not only form dangerous oxides, but they also tend to poison the catalyst in the catalytic converter. As it flows over the catalyst in the exliaust system, the sulfur decreases conversion efficiency and limits the catalyst s oxygen storage capacity. With the converter working at less than maximum efficiency, the exhaust entering the atmosphere contains increased concentrations, not only of the sulfur oxides but also, of hydrocarbons, nitrogen oxides, carbon monoxides, toxic metals, and particulate matter. 

Ceria is another type of mixed conducting oxide which has been shown already to induce electrochemical promotion.71 Ceria is a catalyst support of increasing technological importance.73 Due to its nonstoichiometry and significant oxygen storage capacity it is also often used as a promoting additive on other supports (e.g. y-A Cb) in automobile exhaust catalysts.79 It is a fluorite type oxide with predominant n-type semiconductivity. The contribution of its ionic conductivity has been estimated to be 1-3% at 350°C.71... 

A washcoat, which provides a high surface area onto which the active catalyst is impregnated. The washcoat typically consists of a mixture of zirconium, cerium and aluminium oxides. Apart from providing high surface area the washcoat also acts as an oxygen storage system (see below). 

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. 

Catalyst Source Aging Pd concentration (%) Oxygen storage component ... 

Dynamometer-aged (120 h) catalysts showed no evidence for oxygen storage via rare earth oxides. 

Formation of bulk PdO is the primary oxygen storage mechanism in the dynamometer-aged Pd-based catalysts. 

A threshold level of oxygen storage (via bulk PdO) is required to reach Ngh CO/NOx conversion levels in dynamometer sweep tests Pd loading, rather than dispersion or surface area, is the most impoirtant factor affecting oxygen uptakes. 

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. 

Fally, F., Perrichon, V., Vidal, H. et al. (2000) Modification of the oxygen storage capacity of Ce02—Zr02 mixed oxides after redox cycling aging, Catal. Today, 59, 373. 

THE ROLE OF CERIUM-BASED OXIDES USED AS OXYGEN STORAGE MATERIALS IN DeNO CATALYSIS... 

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