Catalyst poisoning

Catalyst poisoning refers to the partial or total deactivation of a catalyst caused by exposure to a range of chemical compounds. Poisoning may be desirable when it results in improved selectivity (e.g. Lindlar's catalyst) but may be undesirable when the catalyst is rendered ineffective (e.g. Lead in catalytic converters). Poisoning refers specifically to chemical deactivation, rather than other mechanism of catalyst degradation such as thermal decomposition or physical damage.

Poisoning involves compounds which bonds chemically to the active surface sites of a catalyst. This may have two effects: the total number of catalytic sites or the fraction of the total surface area that has the capability of promoting reaction always decreases, and the average distance that a reactant molecule must diffuse through the pore structure before undergoing reaction may increase. Poisoned sites can no longer accelerate the reaction with which the catalyst was supposed to catalyze. Large scale production of substances such as ammonia in the Haber–Bosch process include steps to remove potential poisons from the product stream.

The poisoning reaction should be viewed like any other chemical reaction between a gas phase reactant and the solid surface, where the poisoned sites are distributed throughout the catalyst pore structure as a function of poison diffusion into the catalyst and the rate of the poisoning reaction. At the two extremes, this gives rise to two scenarios. First, when the poisoning reaction rate is slow relative to the rate of diffusion, the poison will be evenly distributed throughout the catalyst and will result in homogeneous poisoning of the catalyst. Conversely, if the reaction rate is fast compared to the rate of diffusion, a poisoned shell will form on the exterior layers of the catalyst, a situation known as "pore-mouth" poisoning, and the rate of catalytic reaction may become limited by the rate of diffusion through the inactive shell.

Organic functional groups and inorganic ions with lone pairs often have the ability to strongly adsorb to metal surfaces, thus prohibiting access to catalyst sites. Common catalyst poisons include the following: carbon monoxide, inorganic ions such as halide, cyanide, sulfide, sulfite, and phosphite and organic molecules such as nitriles, nitros, oximes and nitrogen-containing heterocycles. Agents vary their catalytic properties because of the nature of the transition metal.

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