Catalytic poisons , or contact poisons, are substances that cause “poisoning” of a catalyst , that is, reducing its catalytic activity or completely ceasing its catalytic effect.
Reasons and Action
The cause of poisoning is the adsorption of catalytic poison on the surface of the catalyst. The following types of catalytic poisoning are distinguished:
- Catalytic poison can be adsorbed on the active center of the catalyst, preventing the interaction of reagents with this active center. In this case, a linear dependence of the decrease in catalytic activity on the amount of poison introduced into the system is observed.
Example: the effect of quinoline on aluminosilicate catalysts during cracking .
- Screening - poison can greatly poison the surface of the catalyst, which will lead to a sharp decrease in catalytic activity. In this case, the introduction of additional doses of the poison will reduce activity slowly.
Example: decomposition of hydrogen peroxide on a platinum catalyst during the adsorption of a poison - mercury dichloride .
It happens that in microdoses a certain catalytic poison can also serve as a promoter , when in large quantities it begins to act as a poison. This type of action is called modification of the catalyst. In the case of complex catalytic reactions, catalytic poisons can act selectively - inhibit catalytic activity in some catalysts and not affect others. In some cases, the same substances can be catalytic poisons in relation to one reaction and not affect others.
Examples of Catalytic Poisons
Among the most common catalytic poisons are H 2 O , CO , CO 2 , H 2 S , N , P , As , Sb , etc.
Literature
Catalytic poisons // Brief chemical encyclopedia / Knunyants I. L. (chief editor) - M: Soviet Encyclopedia, 1961-1967. T.2, S.488