Catalyst poisoning
Encyclopedia
Catalyst poisoning refers to the effect that a catalyst can be 'poisoned' if it reacts with another compound
that bonds chemically to its active surface sites. This effectively reduces the usefulness of the catalyst. 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.
catalyst, which have reduced activity when it is in combination with mild steel. The loss in activity of catalyst can be overcome by having a lining of epoxy or other substances.
Poisoning of palladium and platinum catalysts has been extensively researched. As a rule of thumb, platinum
(as Adam's catalyst, finely divided on carbon) is less susceptible. Common poisons for these two metals are sulfur and nitrogen-heterocycles like pyridine and quinoline.
A catalytic converter
for an automobile can be poisoned if the vehicle is operated on gasoline
containing lead additives. Fuel cell
s running on hydrogen must use very pure reactants, free of sulfur and carbon compounds.
In the classical "Rosenmund reduction
" of acyl chloride
s to aldehyde
s, the palladium
catalyst (over barium sulfate
or calcium carbonate
) is poisoned by the addition of sulfur
or quinoline
. This system reduces triple bonds faster than double bonds allowing for an especially selective reduction. Lindlar's catalyst is another example — palladium
poisoned with lead
salts.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
that bonds chemically to its active surface sites. This effectively reduces the usefulness of the catalyst. Poisoned sites can no longer accelerate the reaction with which the catalyst was supposed to catalyze.
Large scale production of substances such as ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
in the Haber-Bosch process include steps to remove potential poisons from the product stream.
Examples
An example can be seen with Raney nickelRaney nickel
Raney nickel is a solid catalyst composed of fine grains of a nickel-aluminium alloy, used in many industrial processes. It was developed in 1926 by American]] engineer Murray Raney as an alternative catalyst for the hydrogenation of vegetable oils in industrial processes...
catalyst, which have reduced activity when it is in combination with mild steel. The loss in activity of catalyst can be overcome by having a lining of epoxy or other substances.
Poisoning of palladium and platinum catalysts has been extensively researched. As a rule of thumb, platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
(as Adam's catalyst, finely divided on carbon) is less susceptible. Common poisons for these two metals are sulfur and nitrogen-heterocycles like pyridine and quinoline.
A catalytic converter
Catalytic converter
A catalytic converter is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint...
for an automobile can be poisoned if the vehicle is operated on gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
containing lead additives. Fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
s running on hydrogen must use very pure reactants, free of sulfur and carbon compounds.
Catalyst poisoning to enhance selectivity
Usually, catalyst poisoning is undesirable as it leads to a loss of usefulness of expensive noble metals or their complexes. However, poisoning of catalysts can be used to improve selectivities of reactions.In the classical "Rosenmund reduction
Rosenmund reduction
The Rosenmund reduction is a chemical reaction that reduces an acid halide to an aldehyde using hydrogen gas over palladium-on-carbon poisoned with barium sulfate...
" of acyl chloride
Acyl chloride
In organic chemistry, an acyl chloride is an organic compound with the functional group -CO-Cl. Their formula is usually written RCOCl, where R is a side chain. They are usually considered to be reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride,...
s to aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s, the palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
catalyst (over barium sulfate
Barium sulfate
Barium sulfate is the inorganic compound with the chemical formula BaSO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and materials prepared from it...
or calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) is poisoned by the addition of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
or quinoline
Quinoline
Quinoline is a heterocyclic aromatic organic compound. It has the formula C9H7N and is a colourless hygroscopic liquid with a strong odour. Aged samples, if exposed to light, become yellow and later brown...
. This system reduces triple bonds faster than double bonds allowing for an especially selective reduction. Lindlar's catalyst is another example — palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
poisoned with lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
salts.