Lanthanum, washcoat

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. 

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. 

It was noted in the introduction that the washcoat of the automobile catalysts contains several other oxides, mainly cerium and lanthanum oxide. 

Samples were than prepared to which was added 2% and 4% lanthanum oxide and 4% barium oxide some care was necessary as to when the dopant was introduced Into the preparation sequence These results were all very similar, and are summarised in Figure 11, It is seen chat at lower temperatures, surface areas are lower whereas at higher temperatures. surface area is maintained at the same relatively high level as washcoat alumina. 

The behaviour of a Pt-based catalyst on a metallic monolith support washcoated with alumina, with the addition of lanthanum and cerium, was studied by Musialik-Piotrowska and Mendyka. The activity of the catalyst was tested in the oxidation of ChB and DCE alone and in two-component mixtures with toluene, -hexane, acetone, ethanol, and ethyl acetate. The influence of non-chlorinated compounds on ChB oxidation differed from one compound to another. Over the whole range of reaction temperatures, ethanol enhanced the conversion of ChB by 10%. The addition of both hydrocarbons also slightly improved ChB destruction, while DCE conversion was inhibited in the presence of each non-chlorinated compound that was added. Both chlorinated hydrocarbons not only inhibited catalytic destruction of each of the non-chlorinated compounds added, but also increased the reaction selechvity and concentration of the intermediate yielded, the first of which was acetaldehyde. 

The surface of the honeycomb is covered by the alumina washcoat, which can be stabilized against sintering at high temperature by the addition of more refractory materials such as barium oxide, calcium oxide, magnesium oxide or lanthanum oxide. A very thin surface layer of the washcoat, up to about 10-30 xm, is applied although the thickness increases to about 150pm at the comers of the square channels. 

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