Catalysis

Source: Wikipedia, the free encyclopedia.
"Catalyst" redirects here. For other uses, see Catalyst (disambiguation).
Not to be confused with Catalist or Cathalistis.
For the stage of metabolism, see catabolism.

A range of industrial catalysts in pellet form
An air filter that uses a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air.

Catalysis (/kəˈtæləsɪs/) is the increase in rate of a chemical reaction due to an added substance known as a catalyst[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 form intermediates 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 either homogeneous, whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant, or heterogeneous, whose components are not in the same phase. Enzymes and other biocatalysts are often considered as a third category.

Catalysis is ubiquitous in chemical 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.

The term "catalyst" is derived from Greek καταλύειν, kataluein, meaning "loosen" or "untie". The concept of catalysis was invented by chemist Elizabeth Fulhame, based on her novel work in oxidation-reduction experiments.[7][8]

General principles

Example

An illustrative example is the effect of catalysts to speed the decomposition of hydrogen peroxide into water and oxygen:

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 as

effervescence of oxygen.[9] The catalyst is not consumed in the reaction, and may be recovered unchanged and re-used indefinitely. Accordingly, manganese dioxide is said to catalyze this reaction. In living organisms, this reaction is catalyzed by enzymes (proteins that serve as catalysts) such as catalase
.

Units

The SI derived unit for measuring the catalytic activity of a catalyst is the katal, which is quantified in moles per second. The productivity of a catalyst can be described by the turnover number (or TON) and the catalytic activity by the turn over frequency (TOF), which is the TON per time unit. The biochemical equivalent is the enzyme unit. For more information on the efficiency of enzymatic catalysis, see the article on enzymes.

Catalytic reaction mechanisms

Main article: catalytic cycle

In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction mechanism (reaction pathway) having a lower activation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst is regenerated.[10][11][12][13]

As a simple example occurring in the gas phase, the reaction 2 SO2 + O2 → 2 SO3 can be catalyzed by adding nitric oxide. The reaction occurs in two steps:

2 NO + O2 → 2 NO2 (rate-determining)
NO2 + SO2 → NO + SO3 (fast)

The NO catalyst is regenerated. The overall rate is the rate of the slow step[13]

v = 2k1[NO]2[O2].

An example of

Scanning tunneling microscopy showed that the molecules undergo adsorption and dissociation. The dissociated, surface-bound O and H atoms diffuse together. The intermediate reaction states are: HO2, H2O2, then H3O2 and the reaction product (water molecule dimers), after which the water molecule desorbs from the catalyst surface.[14][15]

Reaction energetics

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 lower activation energy. Consequently, more molecular collisions have the energy needed to reach the transition 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 an energy profile diagram.

In the catalyzed

perpetual motion machine, a contradiction to the laws of thermodynamics.[16] Thus, catalysts do 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-catalyzed hydrolysis of esters, where the produced carboxylic 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 the difference in energy between starting material and the transition state. It does 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).

Related concepts

Some so-called catalysts are really

precatalysts. 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 activated in situ. Because of this preactivation step, many catalytic reactions involve an induction period
.

In cooperative catalysis, chemical species that improve catalytic activity are called cocatalysts or promoters.

In

tandem catalysis
two or more different catalysts are coupled in a one-pot reaction.

In autocatalysis, the catalyst is 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 an ester such as aspirin to a carboxylic acid and an alcohol. In the absence of added acid catalysts, the carboxylic acid product catalyzes the hydrolysis.

A true catalyst can work in tandem with a sacrificial 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,

N-methylmorpholine N-oxide
(NMMO) regenerates OsO4, and only catalytic quantities of OsO4 are needed.

Classification

Catalysis may be classified as either

reactant's molecules. A heterogeneous 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.

Heterogeneous catalysis

Main article: Heterogeneous catalysis
The microporous molecular structure of the zeolite ZSM-5 is exploited in catalysts used in refineries
Zeolites are extruded as pellets for easy handling in catalytic reactors.

Heterogeneous catalysts act in a different

alumina,[17] higher-order oxides, graphitic carbon, transition metal oxides, metals such as Raney nickel for hydrogenation, and vanadium(V) oxide for oxidation of sulfur dioxide into sulfur trioxide by the contact process.[18]

Diverse mechanisms for

Eley-Rideal, and Mars-van Krevelen).[19]
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 has active 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 the Haber process for the synthesis of ammonia from nitrogen and hydrogen is often described as iron. But detailed studies and many optimizations have led to catalysts that are mixtures of iron-potassium-calcium-aluminum-oxide.[20] The reacting gases adsorb 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 strong triple 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.[21] Another place where a heterogeneous catalyst is applied is in the oxidation of sulfur dioxide on vanadium(V) oxide for the production of sulfuric acid.[18] Many heterogeneous catalysts are in fact nanomaterials.

Heterogeneous catalysts are typically "

zeolites or various kinds of activated carbon. Specialized supports include silicon dioxide, titanium dioxide, calcium carbonate, and barium sulfate.[22]

Electrocatalysts

Main article: Electrocatalyst

In the context of

nanoparticles of platinum that are supported on slightly larger carbon particles. When in contact with one of the electrodes in a fuel cell, this platinum increases the rate of oxygen reduction either to water or to hydroxide or hydrogen peroxide
.

Homogeneous catalysis

Main article: Homogeneous catalysis

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 of

p-xylene to terephthalic acid
.

Organocatalysis

Main article: Organocatalysis

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 many

binding 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."[25]

Photocatalysts

Main article: Photocatalysis

Photocatalysis is the phenomenon where the catalyst can receive light to generate an excited state that effect redox reactions.[26] Singlet oxygen is usually produced by photocatalysis. Photocatalysts are components of dye-sensitized solar cells.

Enzymes and biocatalysts

Main article: Enzyme catalysis

In biology, enzymes are protein-based catalysts in metabolism and catabolism. Most biocatalysts are enzymes, but other non-protein-based classes of biomolecules also exhibit catalytic properties including ribozymes, and synthetic deoxyribozymes.[27]

Biocatalysts can be thought of as an intermediate between homogeneous and heterogeneous catalysts, although strictly speaking soluble enzymes are homogeneous catalysts and membrane-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.

In biocatalysis, enzymes are employed to prepare many commodity chemicals including high-fructose corn syrup and acrylamide.

Some

abzymes
".

Significance

cube sugar
, Right: burning cube sugar with ash as catalyst
A Ti-Cr-Pt tube (~40 μm long) releases oxygen bubbles when immersed in hydrogen peroxide (via catalytic decomposition), forming a micropump.[29]

Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.[30] In 2005, catalytic processes generated about $900 billion in products worldwide.[31] Catalysis is so pervasive that subareas are not readily classified. Some areas of particular concentration are surveyed below.

Energy processing

synthesis gas). Even the exhaust from the burning of fossil fuels is treated via catalysis: Catalytic converters, typically composed of platinum and rhodium
, break down some of the more harmful byproducts of automobile exhaust.

2 CO + 2 NO → 2 CO2 + N2

With regard to synthetic fuels, an old but still important process is the

water-gas shift reactions, catalyzed by iron. The Sabatier reaction produces methane from carbon dioxide and hydrogen. Biodiesel
and related biofuels require processing via both inorganic and biocatalysts.

Fuel cells rely on catalysts for both the anodic and cathodic reactions.

Catalytic heaters generate flameless heat from a supply of combustible fuel.

Bulk chemicals

Typical vanadium pentoxide catalyst used in sulfuric acid production for an intermediate reaction to convert sulfur dioxide to sulfur trioxide.
Typical vanadium pentoxide catalyst used in sulfuric acid production for an intermediate reaction to convert sulfur dioxide to sulfur trioxide.

Some of the largest-scale chemicals are produced via catalytic oxidation, often using oxygen. Examples include nitric acid (from ammonia), sulfuric acid (from sulfur dioxide to sulfur trioxide by the contact process), terephthalic acid from p-xylene, acrylic acid from propylene or propane and acrylonitrile from propane and ammonia.[19]

The production of ammonia is one of the largest-scale and most energy-intensive processes. In the Haber process nitrogen is combined with hydrogen over an iron oxide catalyst.[32] Methanol is prepared from carbon monoxide or carbon dioxide but using copper-zinc catalysts.

Bulk polymers derived from

acid-base catalysis
.

Most

Monsanto acetic acid process and hydroformylation
.

Fine chemicals

Many

Noyori asymmetric hydrogenation:[33]

levofloxaxin synthesis
levofloxaxin synthesis

Food processing

One of the most obvious applications of catalysis is the hydrogenation (reaction with hydrogen gas) of fats using nickel catalyst to produce margarine.[34] Many other foodstuffs are prepared via biocatalysis (see below).

Environment

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 of

free radicals in the breakdown of ozone. These radicals are formed by the action of ultraviolet radiation on chlorofluorocarbons
(CFCs).

Cl· + O3 → ClO· + O2
ClO· + O· → Cl· + O2

History

The term "catalyst", broadly defined as anything that increases the rate of a process, is derived from

lighter based on hydrogen and a platinum sponge, which became a commercial success in the 1820s that lives on today. Humphry Davy discovered the use of platinum in catalysis.[40] In the 1880s, Wilhelm Ostwald at Leipzig 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 1909 Nobel Prize in Chemistry.[41] Vladimir Ipatieff performed some of the earliest industrial scale reactions, including the discovery and commercialization of oligomerization and the development of catalysts for hydrogenation.[42]

Inhibitors, poisons, and promoters

An added substance that lowers the rate is called a reaction inhibitor if reversible and catalyst poisons if irreversible.[1] Promoters are substances that increase the catalytic activity, even though they are not catalysts by themselves.[43]

Inhibitors are sometimes referred to as "negative catalysts" since they decrease the reaction rate.

heterogeneous catalysis, coking inhibits the catalyst, which becomes covered by polymeric
side products.

The inhibitor may modify selectivity in addition to rate. For instance, in the hydrogenation of alkynes to alkenes, a palladium (Pd) catalyst partly "poisoned" with lead(II) acetate (Pb(CH3CO2)2) can be used(Lindlar catalyst).[45] Without the deactivation of the catalyst, the alkene produced would be further hydrogenated to alkane.[46][47]

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 of hydrogenolysis catalysis while sites catalyzing hydrogenation 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 in platforming). They can aid the dispersion of the catalytic material or bind to reagents.

See also

References

  1. ^
    ISBN 978-0-9678550-9-7
    .
  2. ISBN 0-471-24197-0
    .
  3. ISBN 0-13-737123-3
    . A catalyst is defined as a chemical substance which increases the rate of a chemical reaction without itself being consumed in the reaction.
  4. ^ Lerner, Louise (2011). "7 things you may not know about catalysis". Argonne National Laboratory.
  5. ISBN 0-8053-5682-7
    . 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
  6. S2CID 255047115
    .
  7. ^
    ISBN 9788437033280. Archived from the original
    (PDF) on January 23, 2015. Retrieved March 14, 2021.
  8. ^
    ISBN 978-0-8412-3522-9
    .
  9. ^ "Genie in a Bottle". University of Minnesota. March 2, 2005. Archived from the original on April 5, 2008.
  10. ^
    ISBN 0-618-12341-5
    .
  11. ISBN 0-8053-5682-7
    .
  12. ISBN 0-7167-8759-8
    . 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
  13. ^
    ISBN 0-13-737123-3
    . The catalyst concentration [C] appears in the rate expression, but not in the equilibrium ratio.
  14. ^ Jacoby, Mitch (February 16, 2009). "Making Water Step by Step". Chemical & Engineering News. p. 10.
  15. PMID 19309169
    .
  16. ^ Robertson, A.J.B. (1970) Catalysis of Gas Reactions by Metals. Logos Press, London.
  17. S2CID 225777024
    .
  18. ^
    ISBN 0130-39913-2
    .
  19. ^
    doi:10.1002/14356007.a05_313
  20. ISBN 3527306730
    .
  21. ^ "Chemistry of Vanadium". Chemistry LibreTexts. October 3, 2013. Retrieved July 8, 2022.
  22. ISSN 1687-8434
    .
  23. doi:10.1002/14356007.a18_215
  24. ISBN 978-3-527-29390-2
  25. ^ "The Nobel Prize in Chemistry 2021". NobelPrize.org.
  26. S2CID 246287799
    .
  27. ISBN 1-57259-153-6
    .
  28. ^ Catalytic Antibodies Simply Explained. Documentroot.com (2010-03-06). Retrieved on 2015-11-11.
  29. PMID 21505711. Archived
    (PDF) from the original on March 28, 2019.
  30. ^ "Recognizing the Best in Innovation: Breakthrough Catalyst". R&D Magazine, September 2005, p. 20.
  31. ^ 1.4.3 Iindustrial Process Efficiency Archived 2008-05-17 at the Wayback Machine. climatetechnology.gov
  32. ISBN 9780262693134
    .
  33. S2CID 106394152
    .
  34. ^ Clark, Jim (October 2013). "Types of catalysis". Chemguide.
  35. ^ Bård Lindström and Lars J. Petterson (2003) "A brief history of catalysis" Cattech, 7 (4) : 130–38.
  36. ^ 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 word katalys (catalysis) on p. 245:

    Original: Jag skall derföre, för att begagna en i kemien välkänd härledning, kalla den kroppars katalytiska kraft, sönderdelning genom denna kraft katalys, 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] the catalytic force and the decomposition of [other] bodies by this force catalysis, just as we signify by the word analysis the separation of the constituents of bodies by the usual chemical affinities.
  37. doi:10.1002/andp.18341071802
    .
  38. ^ Döbereiner (1822). "Glühendes Verbrennen des Alkohols durch verschiedene erhitzte Metalle und Metalloxyde" [Incandescent burning of alcohol by various heated metals and metal oxides]. Journal für Chemie und Physik. 34: 91–92.
  39. ^ Döbereiner (1823). "Neu entdeckte merkwürdige Eigenschaften des Platinsuboxyds, des oxydirten Schwefel-Platins und des metallischen Platinstaubes" [Newly discovered remarkable properties of platinum suboxide, oxidized platinum sulfide and metallic platinum dust]. Journal für Chemie und Physik. 38: 321–26.
  40. S2CID 97988261
    .
  41. S2CID 91507819
    .
  42. doi:10.1021/acscatal.8b02310
    .
  43. ISBN 9789352830688
    .
  44. ^
    ISBN 0-8053-5680-0
    .
  45. doi:10.15227/orgsyn.046.0089
    ; Collected Volumes, vol. 5, p. 880.
  46. ISBN 0-07-032305-4
  47. ISBN 0-471-05991-9

External links

Look up catalysis in Wiktionary, the free dictionary.
Wikimedia Commons has media related to Catalysis.
Wikisource has the text of the 1911 Encyclopædia Britannica article "Catalysis".
Retrieved from "https://en.wikipedia.org/w/index.php?title=Catalysis&oldid=1215315057"