Catalyst poisoning

When a catalyst is completely or partially rendered inactive by a chemical substance, the condition is known as catalyst poisoning. Unlike other catalyst degrading processes like heat breakdown or physical damage, poisoning only pertains to chemical deactivation. When it leads to increased catalyst selectivity (such as with Lindlar's catalyst), poisoning—while often undesirable—can be advantageous. Leaded fuel's toxicity toward catalytic converters serves as a significant historical illustration. The capacity to strongly adsorb on metal surfaces is frequently possessed by organic functional groups and inorganic anions. Typical catalyst poisons include carbon monoxide, halides, cyanides, sulphides, sulfites, phosphates, and phosphites, as well as organic molecules including nitriles, nitro compounds, oximes, and heterocycles that include nitrogen. As a result of the transition metal's composition, different agents have different catalytic characteristics. A slurry of calcium carbonate (CaCO3) is reduced with palladium chloride to create Lindlar catalysts, which are then poisoned with lead acetate. In a related instance, the Rosenmund reduction of acyl halides to aldehydes, the palladium catalyst (over barium sulphate or calcium carbonate) is purposefully poisoned by the addition of sulphur or quinoline in order to lower the catalyst activity and thereby prevent over-reduction of the aldehyde product to the primary alcohol.