Catalyst poisoning refers to the partial or total deactivation of a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
by a chemical compound. Poisoning refers specifically to chemical deactivation, rather than other mechanisms of catalyst degradation such as
thermal decomposition
Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is re ...
or physical damage. Although usually undesirable, poisoning may be helpful when it results in improved catalyst selectivity (e.g.
Lindlar's catalyst
A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate or barium sulfate which is then poisoned with various forms of lead or sulfur. It is used for the hydrogenation of alkynes to alkenes (i.e ...
). An important historic example was the poisoning of
catalytic converters by
leaded fuel.
Poisoning of Pd catalysts
Organic functional groups and inorganic anions often have the ability to strongly adsorb to metal surfaces. Common catalyst poisons include carbon monoxide, halides, cyanides, sulfides, sulfites, phosphates, phosphites and organic molecules such as nitriles, nitro compounds, oximes, and nitrogen-containing heterocycles. Agents vary their catalytic properties because of the nature of the transition metal. Lindlar catalysts are prepared by the reduction of
palladium chloride
Palladium(II) chloride, also known as palladium dichloride and palladous chloride, are the chemical compounds with the formula PdCl2. PdCl2 is a common starting material in palladium chemistry – palladium-based catalysts are of particular value ...
in a slurry of calcium carbonate (CaCO
3) followed by poisoning with
lead acetate.
In a related case, the
Rosenmund reduction
The Rosenmund reduction is a hydrogenation process in which an acyl chloride is selectively reduced to an aldehyde. The reaction was named after Karl Wilhelm Rosenmund, who first reported it in 1918.
The reaction, a hydrogenolysis, is catalys ...
of
acyl halide
In organic chemistry, an acyl halide (also known as an acid halide) is a chemical compound derived from an oxoacid by replacing a hydroxyl group () with a halide group (, where X is a halogen).
If the acid is a carboxylic acid (), the compoun ...
s to
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...
s, the
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself ...
catalyst (over
barium sulfate
Barium sulfate (or sulphate) is the inorganic compound with the chemical formula Ba SO4. 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 ...
or
calcium carbonate) is intentionally poisoned by the addition of
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
or
quinoline in order to lower the catalyst activity and thereby prevent over-reduction of the aldehyde product to the primary alcohol.
Poisoning process
Poisoning often involves compounds that
chemically bond to a catalyst's active sites. Poisoning decreases the number of active sites, and the average distance that a reactant molecule must diffuse through the pore structure before undergoing reaction increases as a result.
[Charles G. Hill, ''An Introduction To Chemical Engine Design'', John Wiley & Sons Inc., 1977 , page 464] As a result, poisoned sites can no longer accelerate the reaction with which the catalyst was supposed to catalyze.
[Jens Hagen, ''Industrial catalysis: a practical approach '',Wiley-VCH, 2006 , page 197] Large scale production of substances such as
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
in the
Haber–Bosch process include steps to remove potential poisons from the product stream. 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
Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
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.
Selective poisoning
If the catalyst and reaction conditions are indicative of low effectiveness, selective poisoning may be observed, where poisoning of only a small fraction of the catalyst's surface gives a disproportionately large drop in activity.
If ''η'' is the effectiveness factor of the poisoned surface and ''h
p'' is the
Thiele modulus for the poisoned case:
:
When the ratio of the reaction rates of the poisoned pore to the unpoisoned pore is considered:
:
where ''F'' is the ratio of poisoned to unpoisoned pores, ''h''
T is the Thiele modulus for the unpoisoned case, and ''α'' is the fraction of the surface that is poisoned.
The above equation simplifies depending on the value of ''h''
T. When the surface is available, ''h''
T is negligible:
:
This represents the "classical case" of nonselective poisoning where the fraction of the activity remaining is equal to the fraction of the unpoisoned surface remaining.
When ''h''
T is very large, it becomes:
:
In this case, the catalyst effectiveness factors are considerably less than unity, and the effects of the portion of the poison adsorbed near the closed end of the pore are not as apparent as when ''h''
T is small.
The rate of diffusion of the reactant through the poisoned region is equal to the rate of reaction and is given by:
:
And the rate of reaction within a pore is given by:
:
The fraction of the catalyst surface available for reaction can be obtained from the ratio of the poisoned reaction rate to the unpoisoned reaction rate:
:
Benefits of selective poisoning
Usually, catalyst poisoning is undesirable as it leads to the wasting of expensive metals or their complexes. However, poisoning of catalysts can be used to improve selectivity of reactions. Poisoning can allow for selective intermediates to be isolated and desirable final products to be produced.
Hydrodesulfurization catalysts
In the purification of petroleum products, the process of hydrodesulfurization is utilized. Thiols, such as thiophene, are reduced using H
2 to produce H
2S and hydrocarbons of varying chain length. Common catalysts used are tungsten and molybdenum sulfide. Adding cobalt and nickel
to either edges or partially incorporating them into the crystal lattice structure can improve the catalyst's efficiency. The synthesis of the catalyst creates a supported hybrid that prevents poisoning of the cobalt nuclei.
Other examples
* In
catalytic converter
A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usual ...
s used on automobiles, the combustion of
leaded gasoline
Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb( C2H5)4. It is a fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster tha ...
produces elemental lead,
lead(II) oxide
Lead(II) oxide, also called lead monoxide, is the inorganic compound with the molecular formula Pb O. PbO occurs in two polymorphs: litharge having a tetragonal crystal structure, and massicot having an orthorhombic crystal structure. Modern ap ...
,
lead(II) chloride
Lead(II) chloride (PbCl2) is an inorganic compound which is a white solid under ambient conditions. It is poorly soluble in water. Lead(II) chloride is one of the most important lead-based reagents. It also occurs naturally in the form of the m ...
, and
lead(II) bromide
Lead(II) bromide is the inorganic compound with the formula PbBr2. It is a white powder. It is produced in the burning of typical leaded gasolines.Michael J. Dagani, Henry J. Barda, Theodore J. Benya, David C. Sanders "Bromine Compounds" in Ullman ...
. Lead alloys with the metals present in the catalyst, while lead oxides and halides coat the catalyst's surfaces, reducing the converter's ability to reduce NOx emissions.
* In
fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
s using platinum catalysts, the fuels must be free of sulfur and carbon monoxide, unless a desulfurization system is used.
*
Ziegler-Natta catalysts for the production of
polyolefin
A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More speciali ...
s (e.g.
polyethylene
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including b ...
,
polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins a ...
, etc) are poisoned by water and oxygen. This poisoning applies to both
homogeneous catalysts and
heterogeneous catalyst
In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. ...
s for olefin polymerization. This requires the monomers (
ethylene
Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).
Ethylene ...
,
propylene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petro ...
, etc.) to be purified.
See also
*
Hydrogen purity Hydrogen purity or hydrogen quality describes the presence of impurities in hydrogen when used as a fuel gas. Impurities in hydrogen can interfere with the proper functioning of equipment that stores, distributes, or uses hydrogen fuel.
Hydrogen ...
*
Reaction inhibitor
*
Enzyme inhibitor
An enzyme inhibitor is a molecule that binds to an enzyme and blocks its activity. Enzymes are proteins that speed up chemical reactions necessary for life, in which substrate molecules are converted into products. An enzyme facilitates a ...
References
{{DEFAULTSORT:Catalyst Poisoning
Catalysis
Fuel cells
fr:Poison de catalyseur