Catalysis
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{{redirect|Catalyst}}
'''Catalysis''' is the process in which the [[reaction rate|rate]] of a [[chemical reaction]] is increased by means of a [[chemical substance]] known as a '''catalyst'''. Unlike other [[reagent]]s that participate in the chemical reaction, a catalyst is not consumed. Thus, the catalyst may participate in multiple chemical transformations, although in practice catalysts are sometimes consumed in secondary processes.
==Background==
The production of most industrially important chemicals involves catalysis. Research into catalysis is a major field in [[applied science]] and involves many areas of chemistry, notably in [[organometallic chemistry]], and [[materials science]]. Catalysis is important in many aspects of [[environmental science]], from the [[catalytic converter]] in automobiles to the causes of the [[ozone hole]]. Catalytic reactions are preferred in environmentally friendly [[green chemistry]] due to the reduced amount of waste generated,<ref>{{cite web | title=The 12 Principles of Green Chemistry | work=[[United States Environmental Protection Agency]] | url=http://www.epa.gov/greenchemistry/pubs/principles.html | accessdate=2006-07-31}}</ref> as opposed to [[stoichiometric]] reactions in which all reactants are consumed and more side products are formed. The most common catalyst is the proton (H<sup>+</sup>). Many [[transition metal]]s and transition metal [[Complex (chemistry)|complexes]] are used in catalysis as well.
A catalyst works by providing an alternative reaction pathway to the reaction product. The rate of the reaction is increased as this alternative route has a lower [[activation energy]] than the reaction route not mediated by the catalyst. The [[disproportionation]] of [[hydrogen peroxide]] to give water and [[oxygen]] is a reaction that is strongly affected by catalysts:
:2 H<sub>2</sub>O<sub>2</sub> → 2 H<sub>2</sub>O + O<sub>2</sub>
This reaction is favoured in the sense that reaction products are more stable than the starting material, however the uncatalysed reaction is slow. The decomposition of hydrogen peroxide is in fact so slow that hydrogen peroxide solutions are commercially available. Upon the addition of a small amount of [[manganese dioxide]], the hydrogen peroxide rapid reacts according to the above equation. This effect is readily seen by the [[Effervescence (chemistry)|effervescence]] of oxygen.<ref>{{cite web | publisher = [[University of Minnesota]] | title = Genie in a Bottle | url = http://www.chem.umn.edu/services/lecturedemo/info/genie.htm | date = 2005-03-02}}</ref> The manganese dioxide may be recovered unchanged, and re-used indefinitely, and thus is not consumed in the reaction. Accordingly, manganese dioxide catalyses this reaction.<ref>Masel, Richard I. “Chemical Kinetics and Catalysis” Wiley-Interscience, New York, 2001. ISBN 0471241970.</ref>
==General principles of catalysis==
===Typical mechanism===
{{main|catalytic cycle}}
Catalysts generally react with one or more reactants to form an intermediate that subsequently give the final reaction product, in the process regenerating the catalyst. The following is a typical reaction scheme, where '''''C''''' represents the catalyst, A and B are reactants, and D is the product of the reaction of A and B:
:A + '''''C''''' → A'''''C''''' (1)
:B + A'''''C''''' → AB'''''C''''' (2)
:AB'''''C''''' → '''''C'''''D (3)
:'''''C'''''D → '''''C''''' + D (4)
Although the catalyst is consumed by reaction 1, it is subsequently produced by reaction 4, so for the overall reaction:
:A + B → D
As a catalyst is regenerated in a reaction, often only small amounts are needed to increase the rate of the reaction. In practice, however, catalysts are sometimes consumed in secondary processes.
===Catalysis and reaction energetics===
[[Image:Activation energy.svg|thumb|right|292px|Generic potential energy diagram showing the effect of a catalyst in an hypothetical exothermic chemical reaction. The presence of the catalyst opens a different reaction pathway (shown in red) with a lower activation energy. The final result and the overall thermodynamics are the same.]]
Catalysts work by providing an (alternative) mechanism involving a different transition state and lower [[activation energy]]. The effect of this is that more molecular collisions have the energy needed to reach the transition state. Hence, catalysts can perform reactions that, albeit thermodynamically feasible, would not run without the presence of a catalyst, or perform them much faster, more specific, or at lower temperatures. This can be observed on a [[Boltzmann distribution]] and [[energy profile diagram]]. This means that catalysts reduce the amount of energy needed to start a chemical reaction.
Catalysts do ''not'' change the favorableness of a reaction: they have ''no'' effect on the [[chemical equilibrium]] of a reaction because the rate of both the forward and the reverse reaction are both affected (see also [[thermodynamics]]). The net free energy change of a reaction is the same whether a catalyst is used or not; the catalyst just makes it easier to activate.
The [[SI derived unit]] for measuring the '''catalytic activity''' of a catalyst is the [[katal]], which is moles per second. The activity of a catalyst can also be described by the [[turn over number]] (or TON) and the catalytic efficiency by the ''turn over frequency'' (TOF). The biochemical equivalent is the [[enzyme unit]]. For more information on the efficiency of enzymatic catalysis see the [[Enzyme#Kinetics]] section.
===Factors that affect catalytic rates===
Catalysis manifests itself in accelerated rates of reactions, and thus many catalytic systems are analyzed with attention to how those rates are affected, beyond the usual parameters that affect all reactions, e.g. temperature, pressure, and concentration. In [[autocatalysis]], a reaction produces catalysts, thus the rates of reactions subject to autocatalysis accelerate with time.
Some molecules inhibit catalysis by competing for the active sites. The strongest inhibitors are called [[Catalyst poisoning|poisons]]. Many catalysts used in petrochemical applications lose activity due to poisoning. Such catalysts are regenerated and reused multiple times to save costs and energy and to reduce environmental impact from disposal of spent catalysts.
In "product inhibition," the rate of catalysis is slowed by the presence of products. When the equilibrium constant for a reaction is very high, however, rates can appear unaffected by the presence of products. In the catalytic hydrogenation of alkenes, for example, one does not observe inhibition by alkanes.
===Typical catalytic materials===
The chemical nature of catalysts is as diverse as catalysis itself, although some generalizations can be made. Proton [[acids]] are probably the most widely used catalysts, especially for the many reactions involving water, including hydrolyses and its reverse. Multifunctional solids often are catalytically active, e.g. [[zeolite]]s, [[alumina]], certain forms of graphitic carbon. [[Transition metals]] are often used to catalyse [[redox]] reactions (oxidation, hydrogenation). Many catalytic processes, especially those involving hydrogen, require [[Platinum group|platinum metals]].
Some so-called catalysts are really "precatalysts." Precatalysts convert to catalysts in the reaction. For example, [[Wilkinson's catalyst]] RhCl(PPh<sub>3</sub>)<sub>3</sub> 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.
==Types of catalysis==
Catalysts can be either [[heterogeneous]] or [[wiktionary:Homogeneous|homogeneous]], depending on whether a catalyst exists in the same [[phase (matter)|phase]] as the [[substrate (chemistry)|substrate]]. [[Biocatalysts]] are often seen as a separate group.
===Heterogeneous catalysts===
{{main|Heterogeneous catalysis}}
Heterogeneous catalysts are present in different [[phase (matter)|phases]] from the [[reactants]]. Most heterogeneous catalysts are [[solid]]s that act on substrates in a [[liquid]] or gaseous [[reaction mixture]]. Diverse mechanisms for [[reactions on surfaces]] are known, depending on how the adsorption takes place ([[Reactions on surfaces#Langmuir-Hinshelwood mechanism|Langmuir-Hinshelwood]] and [[Reactions on surfaces#Eley-Rideal mechanism|Eley-Rideal]]).<ref>Helmut Knözinger, Karl Kochloefl “Heterogeneous Catalysis and Solid Catalysts” in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. {{DOI|10.1002/14356007.a05_313}}. Article Online Posting Date: January 15, 2003</ref>
For example, in the [[Haber process]], finely divided [[iron]] serves as a catalyst for the synthesis of [[ammonia]] from [[nitrogen]] and [[hydrogen]]. The reacting [[gas]]es [[adsorb]] onto "active sites" on the iron particles. Once adsorbed, the bonds within the reacting molecules are weakened, and new bonds between the resulting fragments form in part due to their close proximity. In this way the particularly strong [[triple bond]] in nitrogen is weakened and the hydrogen and nitrogen atoms combine faster than would be the case in the gas phase, so the rate of reaction increases.
Heterogeneous catalysts are typically “supported,” which means that the catalyst is dispersed on a second material that enhances the effectiveness or minimizes their cost. Sometimes the support is merely a surface upon which the catalyst is spread to increase the surface area. More often, the support and the catalyst interact, affecting the catalytic reaction.
[[Image:Heterogeneous cat.JPG|thumb|right|292px|Heterogeneous catalysts are often "supported" on complex structures to maximize surface area.]]
====Electrocatalysts====
In the context of [[electrochemistry]], specifically in [[fuel cell]] engineering, various metal-containing catalysts are used to enhance the rates of the [[half reaction]]s that comprise the fuel cell. One common type of fuel cell electrocatalyst is based upon [[nanoparticles]] of [[platinum]] that are supported on slightly larger [[carbon]] particles. When this platinum electrocatalyst is in contact with one of the [[electrodes]] in a fuel cell, it increases the rate of [[oxygen]] reduction to water (or [[hydroxide]] or [[hydrogen peroxide]]).
===Homogeneous catalysts===
{{main|Homogeneous catalysis}}
Homogeneous catalysts function in the same phase as the reactants, but the mechanistic principles invoked in heterogeneous catalysis are generally applicable. Typically homogeneous catalysts are dissolved in a solvent with the substrates. One example of homogeneous catalysis involves the influence of [[hydrogen|H]]<sup>+</sup> on the [[esterification]] of esters, e.g. methyl acetate from acetic acid and methanol.<ref>Arno Behr “Organometallic Compounds and Homogeneous Catalysis” Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. {{DOI|10.1002/14356007.a18_215}}. Article Online Posting Date: June 15, 2000</ref> For inorganic chemists, homogeneous catalysis is often synonymous with [[organometallic chemistry|organometallic catalysts]].
====Organocatalysis====
{{main|Organocatalysis}}
Whereas transition metals sometimes attract most of the attention in the study of catalysis, organic molecules without metals can also possess catalytic properties. Typically, organic catalysts require a higher loading (or amount of catalyst per unit amount of reactant) than transition metal-based catalysts, but these catalysts are usually commercially available in bulk, helping to reduce costs. In the early 2000s, organocatalysts were considered "new generation" and are competitive to traditional metal-containing catalysts. Enzymatic reactions operate via the principles of organic catalysis.
==Significance of catalysis==
Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.<ref>"Recognizing the Best in Innovation: Breakthrough Catalyst". ''R&D Magazine'', September 2005, pg 20.</ref> In 2005, catalytic processes generated about $900 billion in products worldwide.[http://www.climatetechnology.gov/library/2005/tech-options/tor2005-143.pdf (pdf)] Catalysis is so pervasive that subareas are not readily classified. Some areas of particular concentration are surveyed below.
===Energy processing===
[[Petroleum]] refining makes intensive use of catalysis for [[alkylation]], [[catalytic cracking]] (breaking long-chain hydrocarbons into smaller pieces), [[naphtha]] reforming, [[steam reforming]] (conversion of [[hydrocarbons]] into [[synthesis gas]]). Even the exhaust from the burning of fossil fuels are treated via catalysis: [[Catalytic converter]]s, typically composed of [[platinum]] and [[rhodium]], break down some of the more harmful byproducts of automobile exhaust.
:2 CO + 2 NO → 2 CO<sub>2</sub> + N<sub>2</sub>
With regards to synthetic fuels, an old but still important process is the [[Fischer-Tropsch synthesis]] of hydrocarbons from [[synthesis gas]], which itself is processed via [[water-gas shift reaction]]s, catalysed by iron. [[Biodiesel]] and related biofuels require processing via both inorganic and biocatalysts.
[[Fuel cells]] rely on catalysts for both the anodic and cathodic reactions.
===Heavy chemicals===
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 [[chamber process]]), [[terephthalic acid]] from p-xylene, and [[acrylonitrile]] from propane and ammonia.
Many other chemical products are generated by large-scale reduction, often via [[hydrogenation]]. The largest-scale example is [[ammonia]], which is prepared via the [[Haber process]] from [[nitrogen]]. [[Methanol]] is prepared from [[carbon monoxide]].
Bulk polymers derived from [[ethylene]] and [[propylene]] are often prepared via [[Ziegler-Natta catalysis]]. Polyesters, polyamides, and [[isocyanate]]s via [[acid-base catalysis]].
Most [[carbonylation]] processes require metal catalysts, examples include the [[Monsanto acetic acid process]] and [[hydroformylation]].
===Fine chemicals===
Many fine 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 [[olefin metathesis]] using [[Grubbs' catalyst]], the [[Heck reaction]], and [[Friedel-Crafts reaction]]s.
Because most bioactive compounds are [[chiral]], many pharmaceuticals are produced by enantioselective catalysis.
===Food processing===
One of the most obvious applicatoins of catalysis is the hydrogenation (reaction with [[hydrogen]] gas) of fats using [[nickel]] catalyst to give [[margarine]].<ref>{{cite web | publisher = Chemguide | title = Types of catalysis | url = http://www.chemguide.co.uk/physical/catalysis/introduction.html | accessdate = 2008-07-09}}</ref> Many other foodstuffs are prepared via biocatalysis (see below).
===Biology===
{{main|Biocatalysis}}
In nature, [[enzyme]]s are catalysts in [[metabolism]] and [[catabolism]]. Most biocatalysts are protein-based, i.e. enzymes, but other classes of biomolecules also exhibit catalytic properties including [[abzyme]]s, [[ribozyme]]s, and synthetic [[deoxyribozyme]]s.
Biocatalysts can be thought of as intermediate between homogenous 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, concentration of enzyme, 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.
Enzymes are employed to prepare many commodity chemicals including [[high-fructose corn syrup]] and [[acrylamide]].
===In the environment===
Catalysis impacts the environment by increasing the efficiency of industrial processes, but catalysis also directly plays a direct role in the environment. A notable example is the catalytic role of [[Chlorine]] [[free radical]]s in the break down of [[ozone]]. These radicals are formed by the action of [[ultraviolet]] [[radiation]] on [[chlorofluorocarbon]]s (CFCs).
:Cl<sup>'''·'''</sup> + O<sub>3</sub> → ClO<sup>'''·'''</sup> + O<sub>2</sub>
:ClO<sup>'''·'''</sup> + O<sup>·</sup> → Cl<sup>'''·'''</sup> + O<sub>2</sub>
==History==
In a general sense, anything that increases the rate of any process is often called a "catalyst,"a term derived from [[Greek language|Greek]] [[wikt:καταλύω|καταλύειν]], meaning "to annul," or "to untie," or "to pick up." The phrase ''catalysed processes'' was coined by [[Jöns Jakob Berzelius]] in 1836<ref>K.J. Laidler and J.H. Meiser, Physical Chemistry, Benjamin/Cummings (1982), p.423</ref> to describe reactions that are accelerated by substances that remain unchanged after the reaction. Other early chemists involved in catalysis were [[Alexander Mitscherlich]] who in 1831 {{Fact|date=August 2007}}<!-- he was born in 1836 --> referred to ''contact processes'' and [[Johann Wolfgang Döbereiner]] who spoke of ''contact action'' and whose [[Lighter (fire starter)|lighter]] based on [[hydrogen]] and a [[platinum]] sponge became a huge commercial success in the 1820’s. [[Humphrey Davy]] discovered the use of platinum in catalysis. In the 1880s, [[Wilhelm Ostwald]] at [[Leipzig University]] started a series of systematic investigations into reactions that were catalyzed by the presence of [[acids]] and bases, and found both 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]].<ref>{{cite journal
| author = M.W. Roberts
| title = Birth of the catalytic concept (1800-1900)
| journal = [[Catalysis Letters]]
| volume = 67
| issue = 1
| year = 2000
| doi = 10.1023/A:1016622806065
| pages= 1–4
| url =http://www.springerlink.com/content/qm3732u7x7577224/fulltext.pdf}}</ref>
==See also==
{{Portal|Chemistry|Nuvola apps edu science.png}}
{{Portal|Biology|EscherichiaColi_NIAID.jpg}}
<div class="references-small" style="-moz-column-count:2; column-count:2;">
* [[Autocatalysis]]
* [[BIG-NSE]] (Berlin Graduate School of Natural Sciences and Engineering)
* ''[[Catalysts and Catalysed Reactions]]'' (a chemistry journal)
* [[Environmental triggers]]
* [[Enzyme]]
* [[Enzyme catalysis]]
* [[Phase Boundary Catalysis]]
* [[Phase transfer catalyst]]
* [[Ribozyme]] (RNA Biocatalysis)
* [[SUMO enzymes]]
</div>
==References==
<references/>
==External links==
{{Wiktionary}}
* [http://scienceaid.co.uk/chemistry/inorganic/catalysis.html Science Aid: Catalysts] Page for high school level science
* W.A. Herrmann Technische Universität presentation [http://aci.anorg.chemie.tu-muenchen.de/wah/vortraege/catalysis.pdf]
* [http://www.inorganic-chemistry-and-catalysis.eu/ Inorganic Chemistry and Catalysis Group, Utrecht University, The Netherlands]
* [http://www.biw.kuleuven.be/ifc/cok/home.htm Centre for Surface Chemistry and Catalysis]
* [http://www.udec.cl/~carbocat Carbons & Catalysts Group, University of Concepcion, Chile]
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