A range of industrial catalysts in pellet formAnair filter that uses a low-temperature oxidation catalyst to convertcarbon monoxide to less toxiccarbon dioxide at room temperature. It can also removeformaldehyde from the air.
Catalysis (/kəˈtæləsɪs/) is the increase inrate of achemical reaction due to an added substance known as acatalyst[1][2] (/ˈkætəlɪst/). Catalysts are not consumed by the reaction and remain unchanged after it.[3] If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice;[4] mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to formintermediates that subsequently give the final reaction product, in the process of regenerating the catalyst.
The rate increase occurs because the catalyst allows the reaction to occur by an alternative mechanism which may be much faster than the non-catalyzed mechanism. However the non-catalyzed mechanism does remain possible, so that the total rate (catalyzed plus non-catalyzed) can only increase in the presence of the catalyst and never decrease.[5]
Catalysis may be classified as eitherhomogeneous, whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant, orheterogeneous, whose components are not in the same phase.Enzymes and other biocatalysts are often considered as a third category.
Catalysis is ubiquitous inchemical industry of all kinds.[6] Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.[7]
The term "catalyst" is derived fromGreekκαταλύειν,kataluein, meaning "loosen" or "untie". The concept of catalysis was invented by chemistElizabeth Fulhame, based on her novel work in oxidation-reduction experiments.[8][9]
An illustrative example is the effect of catalysts to speed the decomposition ofhydrogen peroxide into water andoxygen:
2 H2O2 → 2 H2O + O2
This reaction proceeds because the reaction products are more stable than the starting compound, but this decomposition is so slow that hydrogen peroxide solutions are commercially available. In the presence of a catalyst such asmanganese dioxide this reaction proceeds much more rapidly. This effect is readily seen by theeffervescence of oxygen.[10] The catalyst is not consumed in the reaction, and may be recovered unchanged and re-used indefinitely. Accordingly, manganese dioxide is said tocatalyze this reaction. In living organisms, this reaction is catalyzed byenzymes (proteins that serve as catalysts) such ascatalase.
Another example is the effect of catalysts on air pollution and reducing the amount of carbon monoxide. Development of active and selective catalysts for the conversion of carbon monoxide into desirable products is one of the most important roles of catalysts. Using catalysts for hydrogenation of carbon monoxide helps to remove this toxic gas and also attain useful materials.[11]
TheSI derived unit for measuring thecatalytic activity of a catalyst is thekatal, which is quantified in moles per second. The productivity of a catalyst can be described by theturnover number (or TON) and the catalytic activity by theturn over frequency (TOF), which is the TON per time unit. The biochemical equivalent is theenzyme unit. For more information on the efficiency of enzymatic catalysis, see the article onenzymes.
In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternativereaction mechanism (reaction pathway) having a loweractivation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst is regenerated.[12][13][14][15]
As a simple example occurring in the gas phase, the reaction 2 SO2 + O2 → 2 SO3 can be catalyzed by addingnitric oxide. The reaction occurs in two steps:
2NO + O2 → 2NO2 (rate-determining)
NO2 + SO2 → NO + SO3 (fast)
The NO catalyst is regenerated. The overall rate is the rate of the slow step[15]
Generic potential energy diagram showing the effect of a catalyst in a hypothetical exothermic chemical reaction X + Y to give Z. The presence of the catalyst opens a different reaction pathway (shown in red) with lower activation energy. The final result and the overall thermodynamics are the same.
Catalysts enable pathways that differ from the uncatalyzed reactions. These pathways have loweractivation energy. Consequently, more molecular collisions have the energy needed to reach thetransition state. Hence, catalysts can enable reactions that would otherwise be blocked or slowed by a kinetic barrier. The catalyst may increase the reaction rate or selectivity, or enable the reaction at lower temperatures. This effect can be illustrated with anenergy profile diagram.
In the catalyzedelementary reaction, catalysts donot change the extent of a reaction: they haveno effect on thechemical equilibrium of a reaction. The ratio of the forward and the reverse reaction rates is unaffected (see alsothermodynamics). Thesecond law of thermodynamics describes why a catalyst does not change the chemical equilibrium of a reaction. Suppose there was such a catalyst that shifted an equilibrium. Introducing the catalyst to the system would result in a reaction to move to the new equilibrium, producing energy. Production of energy is a necessary result since reactions are spontaneous only ifGibbs free energy is produced, and if there is no energy barrier, there is no need for a catalyst. Then, removing the catalyst would also result in a reaction, producing energy; i.e. the addition and its reverse process, removal, would both produce energy. Thus, a catalyst that could change the equilibrium would be aperpetual motion machine, a contradiction to the laws of thermodynamics.[18] Thus, catalystsdo not alter the equilibrium constant. (A catalyst can however change the equilibrium concentrations by reacting in a subsequent step. It is then consumed as the reaction proceeds, and thus it is also a reactant. Illustrative is the base-catalyzedhydrolysis ofesters, where the producedcarboxylic acid immediately reacts with the base catalyst and thus the reaction equilibrium is shifted towards hydrolysis.)
The catalyst stabilizes the transition state more than it stabilizes the starting material. It decreases the kinetic barrier by decreasing thedifference in energy between starting material and the transition state. Itdoes not change the energy difference between starting materials and products (thermodynamic barrier), or the available energy (this is provided by the environment as heat or light).
Some so-called catalysts are reallyprecatalysts. Precatalysts convert to catalysts in the reaction. For example,Wilkinson's catalyst RhCl(PPh3)3 loses one triphenylphosphine ligand before entering the true catalytic cycle. Precatalysts are easier to store but are easily activatedin situ. Because of this preactivation step, many catalytic reactions involve aninduction period.
Incooperative catalysis, chemical species that improve catalytic activity are calledcocatalysts orpromoters.
Intandem catalysis two or more different catalysts are coupled in a one-pot reaction.
Inautocatalysis, the catalystis a product of the overall reaction, in contrast to all other types of catalysis considered in this article. The simplest example of autocatalysis is a reaction of type A + B → 2 B, in one or in several steps. The overall reaction is just A → B, so that B is a product. But since B is also a reactant, it may be present in the rate equation and affect the reaction rate. As the reaction proceeds, the concentration of B increases and can accelerate the reaction as a catalyst. In effect, the reaction accelerates itself or is autocatalyzed. An example is the hydrolysis of anester such asaspirin to acarboxylic acid and analcohol. In the absence of added acid catalysts, the carboxylic acid product catalyzes the hydrolysis.
Switchable catalysis refers to a type of catalysis where the catalyst can be toggled between different ground states possessing distinct reactivity, typically by applying an external stimulus.[19] This ability to reversibly switch the catalyst allows for spatiotemporal control over catalytic activity and selectivity. The external stimuli used to switch the catalyst can include changes in temperature, pH, light,[20] electric fields, or the addition of chemical agents.
A true catalyst can work in tandem with asacrificial catalyst. The true catalyst is consumed in the elementary reaction and turned into a deactivated form.The sacrificial catalyst regenerates the true catalyst for another cycle. The sacrificial catalyst is consumed in the reaction, and as such, it is not really a catalyst, but a reagent. For example,osmium tetroxide (OsO4) is a good reagent for dihydroxylation, but it is highly toxic and expensive. InUpjohn dihydroxylation, the sacrificial catalystN-methylmorpholine N-oxide (NMMO) regenerates OsO4, and only catalytic quantities of OsO4 are needed.
Catalysis may be classified as eitherhomogeneous or heterogeneous. Ahomogeneous catalysis is one whose components are dispersed in the same phase (usually gaseous or liquid) as thereactant's molecules. Aheterogeneous catalysis is one where the reaction components are not in the same phase.Enzymes and other biocatalysts are often considered as a third category. Similar mechanistic principles apply to heterogeneous, homogeneous, and biocatalysis.
The microporous molecular structure of thezeolite ZSM-5 is exploited in catalysts used in refineriesZeolites are extruded as pellets for easy handling in catalytic reactors.
Diverse mechanisms forreactions on surfaces are known, depending on how the adsorption takes place (Langmuir-Hinshelwood,Eley-Rideal, and Mars-van Krevelen).[23] The total surface area of a solid has an important effect on the reaction rate. The smaller the catalyst particle size, the larger the surface area for a given mass of particles.
A heterogeneous catalyst hasactive sites, which are the atoms or crystal faces where the substrate actually binds. Active sites are atoms but are often described as a facet (edge, surface, step, etc.) of a solid. Most of the volume but also most of the surface of a heterogeneous catalyst may be catalytically inactive. Finding out the nature of the active site is technically challenging.
For example, the catalyst for theHaber process for the synthesis ofammonia fromnitrogen andhydrogen is often described asiron. But detailed studies and many optimizations have led to catalysts that are mixtures of iron-potassium-calcium-aluminum-oxide.[24] The reactinggasesadsorb onto active sites on the iron particles. Once physically adsorbed, the reagents partially or wholly dissociate and form new bonds. In this way the particularly strongtriple bond in nitrogen is broken, which would be extremely uncommon in the gas phase due to its high activation energy. Thus, the activation energy of the overall reaction is lowered, and the rate of reaction increases.[25] Another place where a heterogeneous catalyst is applied is in the oxidation of sulfur dioxide onvanadium(V) oxide for the production ofsulfuric acid.[22] Many heterogeneous catalysts are in fact nanomaterials.
Heterogeneous catalysts are typically "supported", which means that the catalyst is dispersed on a second material that enhances the effectiveness or minimizes its cost. Supports prevent or minimize agglomeration and sintering of small catalyst particles, exposing more surface area, thus catalysts have a higher specific activity (per gram) on support. Sometimes the support is merely a surface on which the catalyst is spread to increase the surface area. More often, the support and the catalyst interact, affecting the catalytic reaction. Supports can also be used in nanoparticle synthesis by providing sites for individual molecules of catalyst to chemically bind. Supports are porous materials with a high surface area, most commonlyalumina,zeolites, or various kinds ofactivated carbon. Specialized supports includesilicon dioxide,titanium dioxide,calcium carbonate, andbarium sulfate.[26]
In the context ofelectrochemistry, specifically infuel cell engineering, various metal-containing catalysts are used to enhance the rates of thehalf reactions that comprise the fuel cell. One common type of fuel cell electrocatalyst is based uponnanoparticles ofplatinum that are supported on slightly largercarbon particles. When in contact with one of theelectrodes in a fuel cell, this platinum increases the rate ofoxygen reduction either to water or tohydroxide orhydrogen peroxide.
Homogeneous catalysts function in the same phase as the reactants. Typically homogeneous catalysts are dissolved in a solvent with the substrates. One example of homogeneous catalysis involves the influence ofH+ on theesterification of carboxylic acids, such as the formation ofmethyl acetate fromacetic acid andmethanol.[27] High-volume processes requiring a homogeneous catalyst includehydroformylation,hydrosilylation,hydrocyanation. For inorganic chemists, homogeneous catalysis is often synonymous withorganometallic catalysts.[28] Many homogeneous catalysts are however not organometallic, illustrated by the use of cobalt salts that catalyze the oxidation ofp-xylene toterephthalic acid.
Whereas transition metals sometimes attract most of the attention in the study of catalysis, small organic molecules without metals can also exhibit catalytic properties, as is apparent from the fact that manyenzymes lack transition metals. Typically, organic catalysts require a higher loading (amount of catalyst per unit amount of reactant, expressed inmol%amount of substance) than transition metal(-ion)-based catalysts, but these catalysts are usually commercially available in bulk, helping to lower costs. In the early 2000s, these organocatalysts were considered "new generation" and are competitive to traditionalmetal(-ion)-containing catalysts.
Organocatalysts are supposed to operate akin to metal-free enzymes utilizing, e.g., non-covalent interactions such ashydrogen bonding. The discipline organocatalysis is divided into the application of covalent (e.g.,proline,DMAP) and non-covalent (e.g.,thiourea organocatalysis) organocatalysts referring to the preferred catalyst-substratebinding and interaction, respectively. The Nobel Prize in Chemistry 2021 was awarded jointly to Benjamin List and David W.C. MacMillan "for the development of asymmetric organocatalysis."[29]
Photocatalysis is the phenomenon where the catalyst can receive light to generate anexcited state that effect redox reactions.[30]Singlet oxygen is usually produced by photocatalysis. Photocatalysts are components ofdye-sensitized solar cells.
In biology,enzymes are protein-based catalysts inmetabolism andcatabolism. Most biocatalysts are enzymes, but other non-protein-based classes of biomolecules also exhibit catalytic properties includingribozymes, and syntheticdeoxyribozymes.[31]
Biocatalysts can be thought of as an intermediate between homogeneous and heterogeneous catalysts, although strictly speaking soluble enzymes are homogeneous catalysts andmembrane-bound enzymes are heterogeneous. Several factors affect the activity of enzymes (and other catalysts) including temperature, pH, the concentration of enzymes, substrate, and products. A particularly important reagent in enzymatic reactions is water, which is the product of many bond-forming reactions and a reactant in many bond-breaking processes.
Somemonoclonal antibodies whose binding target is a stable molecule that resembles the transition state of a chemical reaction can function as weak catalysts for that chemical reaction by lowering its activation energy.[32] Such catalytic antibodies are sometimes called "abzymes".
Left: Partially caramelizedcube sugar, Right: burning cube sugar with ash as catalystA Ti-Cr-Pt tube (~40 μm long) releases oxygen bubbles when immersed inhydrogen peroxide (via catalytic decomposition), forming amicropump.[33]
Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture. In 2005, catalytic processes generated about $900 billion in products worldwide.[34] Catalysis is so pervasive that subareas are not readily classified. Some areas of particular concentration are surveyed below.
The production of ammonia is one of the largest-scale and most energy-intensive processes. In theHaber processnitrogen is combined with hydrogen over an iron oxide catalyst.[35]Methanol is prepared fromcarbon monoxide or carbon dioxide but using copper-zinc catalysts.
Manyfine chemicals are prepared via catalysis; methods include those of heavy industry as well as more specialized processes that would be prohibitively expensive on a large scale. Examples include theHeck reaction, andFriedel–Crafts reactions. Because most bioactive compounds arechiral, many pharmaceuticals are produced by enantioselective catalysis (catalyticasymmetric synthesis). (R)-1,2-Propandiol, the precursor to the antibacteriallevofloxacin, can be synthesized efficiently from hydroxyacetone by using catalysts based onBINAP-ruthenium complexes, inNoyori asymmetric hydrogenation:[36]
One of the most obvious applications of catalysis is thehydrogenation (reaction withhydrogen gas) of fats usingnickel catalyst to producemargarine.[37] Many other foodstuffs are prepared via biocatalysis (see below).
Catalysis affects the environment by increasing the efficiency of industrial processes, but catalysis also plays a direct role in the environment. A notable example is the catalytic role ofchlorinefree radicals in the breakdown ofozone. These radicals are formed by the action ofultravioletradiation onchlorofluorocarbons (CFCs).
The term "catalyst", broadly defined as anything that increases the rate of a process, is derived fromGreekκαταλύειν, meaning "to annul", or "to untie", or "to pick up". The concept of catalysis was invented by chemistElizabeth Fulhame and described in a 1794 book, based on her novel work in oxidation–reduction reactions.[8][9][38] The first chemical reaction in organic chemistry that knowingly used a catalyst was studied in 1811 byGottlieb Kirchhoff, who discovered the acid-catalyzed conversion of starch to glucose. The termcatalysis was later used byJöns Jakob Berzelius in 1835[39] to describe reactions that are accelerated by substances that remain unchanged after the reaction.Fulhame, who predated Berzelius, did work with water as opposed to metals in her reduction experiments. Other 18th century chemists who worked in catalysis wereEilhard Mitscherlich[40] who referred to it ascontact processes, andJohann Wolfgang Döbereiner[41][42] who spoke ofcontact action.He developedDöbereiner's lamp, alighter based onhydrogen and aplatinum sponge, which became a commercial success in the 1820s that lives on today.Humphry Davy discovered the use of platinum in catalysis.[43] In the 1880s,Wilhelm Ostwald atLeipzig University started a systematic investigation into reactions that were catalyzed by the presence of acids and bases, and found that chemical reactions occur at finite rates and that these rates can be used to determine the strengths of acids and bases. For this work, Ostwald was awarded the 1909Nobel Prize in Chemistry.[44]Vladimir Ipatieff performed some of the earliest industrial scale reactions, including the discovery and commercialization of oligomerization and the development of catalysts for hydrogenation.[45]
An added substance that lowers the rate is called areaction inhibitor if reversible andcatalyst poisons if irreversible.[1] Promoters are substances that increase the catalytic activity, even though they are not catalysts by themselves.[46]
Inhibitors are sometimes referred to as "negative catalysts" since they decrease the reaction rate.[47] However the term inhibitor is preferred since they do not work by introducing a reaction path with higher activation energy; this would not lower the rate since the reaction would continue to occur by the non-catalyzed path. Instead, they act either by deactivating catalysts or by removing reaction intermediates such as free radicals.[47][12] Inheterogeneous catalysis,coking inhibits the catalyst, which becomes covered bypolymeric side products.
The inhibitor may modify selectivity in addition to rate. For instance, in the hydrogenation ofalkynes toalkenes, apalladium (Pd) catalyst partly "poisoned" withlead(II) acetate (Pb(CH3CO2)2) can be used (Lindlar catalyst).[48] Without the deactivation of the catalyst, the alkene produced would be further hydrogenated toalkane.[49][50]
The inhibitor can produce this effect by, e.g., selectively poisoning only certain types of active sites. Another mechanism is the modification of surface geometry. For instance, in hydrogenation operations, large planes of metal surface function as sites ofhydrogenolysis catalysis while sites catalyzinghydrogenation of unsaturates are smaller. Thus, a poison that covers the surface randomly will tend to lower the number of uncontaminated large planes but leave proportionally smaller sites free, thus changing the hydrogenation vs. hydrogenolysis selectivity. Many other mechanisms are also possible.
Promoters can cover up the surface to prevent the production of a mat of coke, or even actively remove such material (e.g., rhenium on platinum inplatforming). They can aid the dispersion of the catalytic material or bind to reagents.
Life is based on an interplay between information processing and catalytic activity carried out by biological polymers.[51] A possibleevolutionary pathway for the emergence of catalytic functions in prebiotic information codingpolymers was proposed.[51] It has also been proposed that life emerged as anRNA-protein system in which the two components cross catalyzed the formation of each other.[52]
^Masel, Richard I (2001).Chemical Kinetics and Catalysis. New York: Wiley-Interscience.ISBN0-471-24197-0.
^Steinfeld, Jeffrey I.; Francisco, Joseph S.; Hase, William L. (1999).Chemical Kinetics and Dynamics (2nd ed.). Prentice Hall. p. 147.ISBN0-13-737123-3.A catalyst is defined as a chemical substance which increases the rate of a chemical reaction without itself being consumed in the reaction.
^Laidler, Keith J.; Meiser, John H. (1982).Physical Chemistry. Benjamin/Cummings. p. 425.ISBN0-8053-5682-7.Inhibitors do not work by introducing a higher reaction path; this would not reduce the rate, since the reaction would continue to occur by the alternative mechanism
^abLaidler, K.J. and Meiser, J.H. (1982)Physical Chemistry, Benjamin/Cummings, p. 425.ISBN0-618-12341-5.
^Laidler, Keith J.; Meiser, John H. (1982).Physical Chemistry. Benjamin/Cummings. pp. 424–425.ISBN0-8053-5682-7.
^Atkins, Peter; de Paula, Julio (2006).Atkins' Physical Chemistry (8th ed.). W.H.Freeman. p. 839.ISBN0-7167-8759-8.The catalyst lowers the activation energy of the reaction by providing an alternative path that avoids the slow, rate-determining step of the uncatalyzed reaction
^abSteinfeld, Jeffrey I.; Francisco, Joseph S.; Hase, William L. (1999).Chemical Kinetics and Dynamics (2nd ed.). Prentice Hall. pp. 147–150.ISBN0-13-737123-3.The catalyst concentration [C] appears in the rate expression, but not in the equilibrium ratio.
^Matthiesen J, Wendt S, Hansen JØ, Madsen GK, Lira E, Galliker P, Vestergaard EK, Schaub R, Laegsgaard E, Hammer B, Besenbacher F (2009). "Observation of All the Intermediate Steps of a Chemical Reaction on an Oxide Surface by Scanning Tunneling Microscopy".ACS Nano.3 (3):517–26.CiteSeerX10.1.1.711.974.doi:10.1021/nn8008245.ISSN1520-605X.PMID19309169.
^Robertson, A.J.B. (1970)Catalysis of Gas Reactions by Metals. Logos Press, London.
^Shafiq, Iqrash; Shafique, Sumeer; Akhter, Parveen; Yang, Wenshu; Hussain, Murid (June 23, 2020). "Recent developments in alumina supported hydrodesulfurization catalysts for the production of sulfur-free refinery products: A technical review".Catalysis Reviews.64 (1):1–86.doi:10.1080/01614940.2020.1780824.ISSN0161-4940.S2CID225777024.
^abHousecroft, Catherine E.; Sharpe, Alan G. (2005).Inorganic Chemistry (2nd ed.). Pearson Prentice-Hall. p. 805.ISBN0130-39913-2.
^abKnözinger, Helmut and Kochloefl, Karl (2002) "Heterogeneous Catalysis and Solid Catalysts" in Ullmann'sEncyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim.doi:10.1002/14356007.a05_313
^Behr, Arno (2002) "Organometallic Compounds and Homogeneous Catalysis" in Ullmann'sEncyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim.doi:10.1002/14356007.a18_215
^Smil, Vaclav (2004).Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production (1st ed.). Cambridge, MA: MIT.ISBN9780262693134.
^Dub, Pavel A.; Gordon, John C. (2018). "The role of the metal-bound N–H functionality in Noyori-type molecular catalysts".Nature Reviews Chemistry.2 (12):396–408.doi:10.1038/s41570-018-0049-z.S2CID106394152.
^Berzelius, J.J. (1835)Årsberättelsen om framsteg i fysik och kemi [Annual report on progress in physics and chemistry]. Stockholm, Sweden: Royal Swedish Academy of Sciences. After reviewing Eilhard Mitscherlich's research on the formation of ether, Berzelius coins the wordkatalys (catalysis) onp. 245:
Original:Jag skall derföre, för att begagna en i kemien välkänd härledning, kalla den kroppars katalytiska kraft,sönderdelning genom denna kraftkatalys,likasom vi med ordet analys beteckna åtskiljandet af kroppars beståndsdelar medelst den vanliga kemiska frändskapen.
Translation: I shall, therefore, to employ a well-known derivation in chemistry, call [the catalytic] bodies [i.e., substances] thecatalytic force and the decomposition of [other] bodies by this forcecatalysis, just as we signify by the wordanalysis the separation of the constituents of bodies by the usual chemical affinities.
