- Catalyst poisoning
Catalyst poisoning refers to the effect that a
catalystcan be 'poisoned' if it reacts with another compound that bonds chemically (similar to an inhibitor) but does not release, or chemically alters the catalyst. This effectively reduces the usefulness of the catalyst, (i. e. the number of active sites) as it cannot participate in the reaction with which it was supposed to catalyze.
An example can be seen with
Raney nickelcatalyst, which have reduced activity when it is in combination with mild steel. The loss in activity of catalyst can be overcome by having a lining of epoxy or other substances.
Poisoning of palladium and platinum catalysts has been extensively researched. As a rule of thumb,
platinum(as Adam's catalyst, finely divided on carbon) is less susceptible. Common poisons for these two metals are sulfur and nitrogen-heterocycles like pyridine and quinoline.
Catalyst poisoning to enhance selectivity
Usually, catalyst poisoning is undesirable as it leads to a loss of usefulness of expensive noble metals or their complexes. However, poisoning of catalysts can be used to improve selectivities of reactions.
In the classical "
Rosenmund reduction" of acyl chlorides to aldehydes, the palladiumcatalyst (over barium sulfateor calcium carbonate) is poisoned by the addition of sulfuror quinoline. This system reduces triple bonds faster than double bonds allowing for an especially selective reduction. Lindlar's catalystis another example — palladiumpoisoned with leadsalts.
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