The Poisoning of Metallic Catalysts

The poisoning of catalysts is essentially a preferential adsorption effect dependent on the formation of abnormally strong adsorptive bonds between a catalyst and certain types of adsorbed species that are usually, but not always, foreign to the reacting system to be catalyzed. In most cases, the strong adsorptive bond by means of which the poison is held to the catalyst appears to be of a highly specific and chemical nature, the formation of such bonds being apparently dependent on definite types of electronic configuration both in the catalyst and in the poison. Although toxicity is of course a relative term, substances are in practice only regarded as poisons if they exert an appreciable inhibitive effect on catalysis even when they are present in very small concentrations. Although early examples of poisoning mainly concern the activity of platinum for oxidation or cognate reactions, including the conversion of sulfur dioxide to the trioxide, the Knallgas reaction, and the decomposition of hydrogen peroxide, the principal use of these metals as a class is probably as hydrogenation catalysts.