refers to the effect that a catalyst
can 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 nickel
catalyst, 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.
A catalytic converter
for an automobile can be poisoned if the vehicle is operated on gasoline
containing lead additives.
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
, the palladium
catalyst (over barium sulfate
or calcium carbonate
) is poisoned by the addition of sulfur
. This system reduces triple bonds... Read More