Catalyst poisoning specific surface effect

This paper will put together facts and ideas in Alfin polymerization which are related to the surface effects. The specific components of the reagent, their proportions, and their probable arrangement will be described. Polymerization will be interpreted as a phenomenon which occurs only on the surface and actually only on certain areas of that surface. Once started, chain growth does not spread to other areas or into solution, but remains on that particular area. In line with other catalysts, the Alfin reagent can be spread upon certain noncatalytic surfaces and can be poisoned by various salts or ions. The special features are further exemplified by the fact that the action is specific for extremely few monomers. Finally, a manner by which polymerization actually occurs on such a surface will be suggested. 

Catalyst poisons are materials that significantly alter, reduce, or completely destroy the activity of a given catalyst. Such materials generally function by binding strongly and (effectively) irreversibly to the specific surface sites necessary for the functioning of the desired process. Particularly troublesome materials in that sense are sulfur-containing compounds, especially thiols and thioethers. For example, the catalytic converters used to oxidize hydrocarbon residues in automobile exhausts will rapidly lose their effectiveness if exposed to such materials. 

The intentional design of model systems can be envisioned, as for instance binary or multiple assemblies (clusters) of active components and poisons, for the examination of their activity in chemisorption, or specific reactions. The results can then be compared with respective clusters containing the active species only. Perhaps, such model systems will be amenable to computational methods capable of predicting their chemisorptive behavior and their surface reactivity. Such approaches are now employed for the design of improved multicomponent catalysts and can, obviously, be used to study the reverse effect, i.e., the mutual deactivation of the cluster components. 

A Pt-Rh three way catalyst used in natural gas-fueled engine systems for 21,000 h showed specific deactivation characteristics, including a decrease in the selectivity of NO reduction, which can neither be reproduced by heat treatment nor explained by physical poisoning such as the blockage of micropores. Through chemieal analyses, EPMA, and activity tests of the used catalyst and model-poisoned catalysts, it was found that the activities of Rh on the used eatalyst were decreased by chemical poisoning due to Pb, causing a decrease in the NO reduction selectivity, and that the absolute rates of NO reduction and other reactions are considerably reduced by a decrease in the effective surface area of the catalyst due to accumulated compounds on the wash coat surface, in addition to thermal effects. 

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