substances that induce the “poisoning” of catalysts (usually heterogeneous catalysts), that is, reduce their catalytic activity or completely stop the catalytic effect. The poisoning of heterogeneous catalysts occurs as a result of the adsorption of the poison or the product of its chemical transformation on the surface of the catalyst. The poisoning may be temporary or permanent. For example, during the synthesis of ammonia on an iron catalyst, oxygen and its derivatives temporarily poison Fe. In this case, the surface of the catalyst is freed of oxygen under the action of a pure N2 + H 2 mixture and the degree of poisoning is reduced. Sulfur compounds permanently poison Fe, since the activity of the catalyst is not successfully reduced by the action of the pure mixture. Thorough purification is required to prevent poisoning of the reacting mixture that passes onto the catalyst.
The most widespread catalyst poisons for metal catalysts include substances containing oxygen (H20, CO, C02), sulfur (H2S, CS2, C2H5SH), selenium, tellurium, nitrogen, phosphorus, arsenic, and antimony, as well as unsaturated hydrocarbons (C2H4, C2H2) and metal ions (Cu2+, Sn2+, Hg2+, Fe2+, Co2+, Ni2+). Acidic catalysts are usually poisoned by basic admixtures and basic catalysts, by acidic admixtures.