Question

A catalyst poison renders the catalyst ineffective because:

• A. It is preferentially adsorbed on the catalyst.
• B. It adsorbs the molecules of the reactants.
• C. It does not combine with the catalyst.
• D. It combine with one of the reactant.

Answer 1

The correct option is A It is preferentially adsorbed on the catalyst.

Catalytic poisons/ anti-catalysts are substances that are not catalysts by themselves​ but their presence decreases the activity of a catalyst in a chemical reaction.
The poisoning of catalyst is Poisoning is ​due to preferential adsorption of poison on the surface of the catalyst.​

Examples:

(i) $N_2\left(g\right)+3H_2\left(g\right)\underset{Co/H_{2}S\left(poison\right)}{\overset{Fe\left(catalyst\right)}{\to }}2N{H}_3\left(g\right)$

(ii)
$2S{O}_2\left(g\right)+O_2\left(g\right)\underset{A{s}_2{S}_3\left(poison\right)}{\overset{\text{Platinised asbestos}\left(catalyst\right)}{\to }}2S{O}_3$