Rare-earth-containing perovskite-type catalysts

Let us review first the state of the art concerning nitric oxide decomposition. In the last 10 years the search for NO decomposition catalysts sharply increased in an effort to comply with more severe regulations which reduce the allowed emission level of this contaminant. Particularly during the 1990s quite a few papers have been published concerning the use of different rare-earth-containing perovskites and K2NiF4-type mixed oxides. Ni, Cu and Co are the most commonly used transition metals. 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

The most important catalyst systems involving rare earth elements are the oxides and intermetallics. Catalytic properties of rare earth oxides are described in section 4 and those of intermetallic compounds in section 6. Reports on surface reactivities of other binary rare earth compounds are only sparse, and this is mentioned in section 5. A very interesting class of catalyst systems comprises the mixed oxides of the perovskite structure type. As catalysis on these oxides is mainly determined by the d transition metal component and the rare earth cations can be regarded essentially as spectator cations from the catalytic viewpoint, these materials have not been included in this chapter. Instead, we refer the interested reader to a review by Voorhoeve (1977). Catalytic properties of rare earth containing zeolites are, in our opinion, more adequately treated in the general context of zeolite catalysis (see e.g. Rabo, 1976 Katzer, 1977 Haynes, 1978) and have therefore been omitted here. 

Screening of Perovskite-type Catalysts Containing Rare Earths... 

The experimental results indicate that the order of the oxidation activities of the three perovskite-type catalysts containing rare earths is La 7Sr 3Co03 > LaCoOj > > LaMnO,. This order agrees well with the evaluation using a catalytic reactor [22,23]. 

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