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Catalysis () is the process of increasing the rate of a
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...

chemical reaction
by adding a substance known as a catalyst (). Catalysts are not consumed in the catalyzed reaction but can act repeatedly. Often only very small amounts of catalyst are required. The global demand for catalysts in 2010 was estimated at approximately US$29.5 billion.


General principles


Illustration

Illustrative is the
disproportionation In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, mo ...

disproportionation
of
hydrogen peroxide Hydrogen peroxide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by hav ...

hydrogen peroxide
to water and
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
: :2 HO → 2 HO + O This reaction proceeds because the reaction products are more stable than the starting material. The uncatalysed reaction is slow. In fact, the decomposition of hydrogen peroxide is so slow that hydrogen peroxide solutions are commercially available. This reaction is strongly affected by catalysts such as
manganese dioxide Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese Manganese is a chemical element Image:Simple Periodic Table Chart- ...

manganese dioxide
, or the enzyme
peroxidase Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act ...
in organisms. Upon the addition of a small amount of
manganese dioxide Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese Manganese is a chemical element Image:Simple Periodic Table Chart- ...

manganese dioxide
, the hydrogen peroxide reacts rapidly. This effect is readily seen by the
effervescence Image:Soda bubbles macro.jpg, Bubbles of carbon dioxide float to the surface of a Soft drink#Carbonated drinks, carbonated soft drink. Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from that rel ...
of oxygen. The manganese dioxide is not consumed in the reaction, and thus may be recovered unchanged, and re-used indefinitely. Accordingly, manganese dioxide ''catalyses'' this reaction.


Units

The
SI derived unit SI derived units are units of measurement derived from the seven SI base unit, base units specified by the International System of Units (SI). They are either dimensionless quantity, dimensionless or can be expressed as a product of one or more o ...
for measuring the catalytic activity of a catalyst is the
katal The katal (symbol: kat) is the unit of catalytic activity that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density abo ...
, which is quantified in moles per second. The productivity of a catalyst can be described by the
turnover numberTurnover number has two different meanings: In enzymology Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistr ...
(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 unitThe enzyme unit, or international unit for enzyme (symbol U, sometimes also IU) is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a ...
. For more information on the efficiency of enzymatic catalysis, see the article on ''
enzymes Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates int ...
''.


Typical mechanism

In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction pathway - or mechanism - with a lower
activation energy In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...

activation energy
than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst usually reacts to form an intermediate, which then regenerates the original catalyst in a process. Catalysts generally react with one or more reactants to form intermediates 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, X and Y are reactants, and Z is the product of the reaction of X and Y: Although the catalyst is consumed by reaction , it is subsequently produced by reaction . 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. The catalyst does often appear in the
rate equation The rate law or rate equation for a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical substance is a form of matter ha ...
. For example, if the
rate-determining step In chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with thermodynamics, which deals with the dire ...
in the above reaction scheme is the first step
X + C → XC, the catalyzed reaction will be second order with rate equation v = k C], which is proportional to the catalyst concentration However remains constant during the reaction so that the catalyzed reaction is pseudo-first order: v = k where k = k As an example of a detailed mechanism at the microscopic level, in 2008 Danish researchers first revealed the sequence of events when
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
and
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
combine on the surface of
titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical ...
(TiO, or ''titania'') to produce water. With a time-lapse series of
scanning tunneling microscopy A scanning tunneling microscope (STM) is a type of microscope used for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer, then at IBM Research – Zurich, IBM Zürich, the Nobel Pr ...
images, they determined the molecules undergo
adsorption Adsorption is the adhesion Adhesion is the tendency of dissimilar Particle, particles or interface (matter), surfaces to cling to one another (Cohesion (chemistry), cohesion refers to the tendency of similar or identical particles/surfaces ...

adsorption
, dissociation and
diffusion File:DiffusionMicroMacro.gif, 250px, Diffusion from a microscopic and macroscopic point of view. Initially, there are solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diff ...

diffusion
before reacting. The intermediate reaction states were: HO, HO, then HO and the final reaction product (), after which the water molecule from the catalyst surface.


Reaction energetics

Catalysts work by providing an (alternative) mechanism involving a different
transition state The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked with the double d ...

transition state
and lower
activation energy In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...

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 reaction rate or selectivity, or enable the reaction at lower temperatures. This effect can be illustrated with an
energy profileFor a chemical reaction or process an energy profile (or reaction coordinate diagram) is a theoretical representation of a single energetic pathway, along the reaction coordinate, as the reactants are transformed into products. Reaction coordinate d ...
diagram. In the catalyzed
elementary reactionAn elementary reaction is a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical substance is a form of matter having cons ...
, catalysts do not change the extent of a reaction: they have no effect on the
chemical equilibrium In a chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes th ...
of a reaction because the rate of both the forward and the reverse reaction are both affected (see also
thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed b ...

thermodynamics
). The
second law of thermodynamics The second law of thermodynamics establishes the concept of entropy Entropy is a scientific concept, as well as a measurable physical property that is most commonly associated with a state of disorder, randomness, or uncertainty. The term an ...
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 if
Gibbs free energy In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quan ...
is produced, and if there is no energy barrier, there is no need for a catalyst. Then, removing the catalyst would also result in 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 a
perpetual motion machine 's 1618 "water screw" perpetual motion machine from a 1660 wood engraving. It is widely credited as the first attempt to describe such a device in order to produce useful work, that of driving millstones. Perpetual motion is the motion of bodies ...
, a contradiction to the laws of thermodynamics. Thus, catalyst does 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-catalysed
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord for ...

hydrolysis
of
ester An ester is a derived from an (organic or inorganic) in which at least one –OH group is replaced by an –O– () group, as in the substitution reaction of a and an . s are s of ; they are important in biology, being one of the main classe ...

ester
s, where the produced
carboxylic acid A carboxylic acid is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic 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 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 Wilkinson's catalyst is the common name for chloridotris(triphenylphosphine)rhodium(I), a coordination complex of rhodium Rhodium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic tabl ...
RhCl(PPh) loses one triphenylphosphine ligand before entering the true catalytic cycle. Precatalysts are easier to store but are easily activated
in situ ''In situ'' (; often not italicized in English) is a Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicar ...
. Because of this preactivation step, many catalytic reactions involve an
induction period An induction period in chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with thermodynamics, which ...
. Chemical species that improve catalytic activity are called co-catalysts (cocatalysts) or promoters in cooperative catalysis. In tandem catalysis two or more different catalysts are coupled in a one-pot reaction. In
autocatalysis A single chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical substance is a form of matter having constant chemical comp ...
, 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 An ester is a derived from an (organic or inorganic) in which at least one –OH group is replaced by an –O– () group, as in the substitution reaction of a and an . s are s of ; they are important in biology, being one of the main classe ...

ester
such as
aspirin Aspirin, also known as acetylsalicylic acid (ASA), is a medication used to reduce pain, fever, or inflammation. Specific inflammatory conditions which aspirin is used to treat include Kawasaki disease, pericarditis, and rheumatic fever. Aspirin ...

aspirin
to a
carboxylic acid A carboxylic acid is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acid ...
and an
alcohol In chemistry, alcohol is an organic compound that carries at least one hydroxyl functional group (−OH) bound to a Saturated and unsaturated compounds, saturated carbon atom. The term alcohol originally referred to the primary alcohol ethan ...

alcohol
. In the absence of added acid catalysts, the carboxylic acid product catalyzes the hydrolysis.


Classification

Catalysis may be classified as either homogeneous or heterogeneous. A
homogeneous catalysisIn chemistry, homogeneous catalysis is catalysis in a solution by a soluble catalyst. Homogeneous catalysis refers to reactions where the catalyst is in the same phase as the reactants, principally in solution. In contrast, heterogeneous catalysis ...
is one whose components are dispersed in the same phase (usually gaseous or liquid) as the
reactant 200px, Reactants, such as sulfur (''pictured''), are the starting materials that are used in chemical reactions. A reagent is a substance or compound added to a system to cause a chemical reaction A chemical reaction is a process that leads ...
's molecules. A
heterogeneous catalysis of ethene on a catalytic solid surface (1) Adsorption (2) Reaction (3) Desorption In chemistry, heterogeneous catalysis is catalysis where the Phase (matter), phase of catalysts differs from that of the reactants or product (chemistry), products ...
is one where the reaction components are not in the same phase.
Enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

Enzyme
s and other biocatalysts are often considered as a third category. Similar mechanistic principles apply to heterogeneous, homogeneous, and biocatalysis.


Heterogeneous catalysis

Heterogeneous catalysts act in a different
phase Phase or phases may refer to: Science * State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter) In the physical sciences, a phase is a region of space (a thermodynamic system A thermodynamic system is a ...
than the
reactants Image:SulfurReagent.jpg, 200px, Reactants, such as sulfur (''pictured''), are the starting materials that are used in chemical reactions. A reagent is a substance or compound added to a system to cause a chemical reaction, or added to test if a ...
. Most heterogeneous catalysts are
solid Solid is one of the four fundamental states of matter 4 (four) is a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is an ...

solid
s that act on substrates in a
liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, ...

liquid
or gaseous
reaction mixture A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...
. Important heterogeneous catalysts include
zeolite Zeolites are microporous, aluminosilicate Aluminosilicate mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Ear ...

zeolite
s,
alumina Aluminium oxide is a of and with the 23. It is the most commonly occurring of several , and specifically identified as aluminium(III) oxide. It is commonly called alumina and may also be called aloxide, aloxite, or alundum depending on part ...

alumina
, higher-order oxides, graphitic carbon,
transition metal In chemistry, the term transition metal (or transition element) has three possible definitions: * The IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations tha ...
oxide An oxide () is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by che ...
s, metals such as
Raney nickel Raney nickel , also called spongy nickel, is a fine-grained solid composed mostly of nickel derived from a nickel–aluminium alloy. Several grades are known, of which most are gray solids. Some are pyrophoric, but most are used as air-stable s ...

Raney nickel
for hydrogenation, and
vanadium(V) oxide Vanadium(V) oxide (''vanadia'') is the inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structur ...

vanadium(V) oxide
for oxidation of
sulfur dioxide Sulfur dioxide (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the autho ...
into
sulfur trioxide Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...
by the so-called contact process. Diverse mechanisms for
reactions on surfaces Reactions on surfaces are reactions in which at least one of the steps of the reaction mechanism is the adsorption of multilayer adsorption is a random distribution of molecules on the material surface. Adsorption is the adhesion Adhesion ...
are known, depending on how the adsorption takes place ( Langmuir-Hinshelwood, Eley-Rideal, and Mars- van Krevelen). The total surface area of 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 reaction actually occurs. Depending on the mechanism, the active site may be either a planar exposed metal surface, a crystal edge with imperfect metal valence or a complicated combination of the two. Thus, not only 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 requires technically challenging research. Thus, empirical research for finding out new metal combinations for catalysis continues. For example, in the
Haber process The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the ammonia production, production of ammonia today. It is named after its inventors, the German chemist ...

Haber process
, finely divided
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

iron
serves as a catalyst for the synthesis of
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
from
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
and
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
. The reacting
gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

gas
es
adsorb Adsorption is the of , s or s from a gas, liquid or dissolved solid to a . This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from , in which a (the ''absorbate'') is by or a liquid or ...
onto active sites on the iron particles. Once physically adsorbed, the reagents undergo
chemisorptionChemisorption is a kind of adsorption of multilayer adsorption is a random distribution of molecules on the material surface. Adsorption is the adhesion Adhesion is the tendency of dissimilar Particle, particles or interface (matter), surfac ...

chemisorption
that results in dissociation into adsorbed atomic species, and new bonds between the resulting fragments form in part due to their close proximity. In this way the particularly strong
triple bond A triple bond in chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they un ...

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. Another place where a heterogeneous catalyst is applied is in the oxidation of sulfur dioxide on
vanadium(V) oxide Vanadium(V) oxide (''vanadia'') is the inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structur ...

vanadium(V) oxide
for the production of
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
. Heterogeneous catalysts are typically " supported," which means that the catalyst is dispersed on a second material that enhances the effectiveness or minimizes their cost. Supports prevent or reduce agglomeration and sintering small catalyst particles, exposing more surface area, thus catalysts have a higher specific activity (per gram) on a 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 commonly
alumina Aluminium oxide is a of and with the 23. It is the most commonly occurring of several , and specifically identified as aluminium(III) oxide. It is commonly called alumina and may also be called aloxide, aloxite, or alundum depending on part ...

alumina
,
zeolites Zeolites are microporous, aluminosilicate mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (g ...
or various kinds of
activated carbon Activated carbon, also called activated charcoal, is a form of carbon Carbon (from la, carbo "coal") is a chemical element with the Symbol (chemistry), symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—making f ...

activated carbon
. Specialized supports include
silicon dioxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titanium and titanium forms six bonds to oxygen. An oxide () is a chemical compound that con ...
,
titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical ...
,
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togethe ...

calcium carbonate
, and
barium sulfate Barium sulfate (or sulphate) is the inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, p ...

barium sulfate
. In slurry reactions, heterogeneous catalysts can be lost by dissolving. Many heterogeneous catalysts are in fact nanomaterials. Nanomaterial-based catalysts with enzyme-mimicking activities are collectively called as nanozymes.


Electrocatalysts

In the context of
electrochemistry Electrochemistry is the branch of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying ...

electrochemistry
, specifically in
fuel cell A fuel cell is an electrochemical cell An electrochemical cell is a device capable of either generating electrical energy Electrical energy is energy derived from electric potential energy or kinetic energy. When used loosely, ''electri ...

fuel cell
engineering, various metal-containing catalysts are used to enhance the rates of the
half reaction A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction. Often, the concept of hal ...
s that comprise the fuel cell. One common type of fuel cell electrocatalyst is based upon
nanoparticles A nanoparticle or ultrafine particle is usually defined as a particle of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched ...

nanoparticles
of
platinum Platinum is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

platinum
that are supported on slightly larger
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
particles. When in contact with one of the
electrodes An electrode is an electrical conductor used to make contact with a nonmetallic part of a Electronic circuit, circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). The word was coined by William Whewell at the request of the scientist ...

electrodes
in a fuel cell, this platinum increases the rate of
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
reduction either to water, or to
hydroxide Hydroxide is a diatomic anion with chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other sym ...
or
hydrogen peroxide Hydrogen peroxide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by hav ...

hydrogen peroxide
.


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 on the
esterification An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH hydroxyl group is replaced by an –O– alkyl ( alkoxy) group, as in the substitution reaction of a carboxylic acid and an alcohol. Glycer ...

esterification
of carboxylic acids, such as the formation of
methyl acetate Methyl acetate, also known as MeOAc, acetic acid methyl ester or methyl ethanoate, is a carboxylate ester An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH hydroxyl group is replaced by an ...

methyl acetate
from
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
and
methanol Methanol, also known as methyl alcohol, amongst other names, is a chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. ...

methanol
. High-volume processes requiring a homogeneous catalyst include
hydroformylation Hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes from alkenes. This chemical reaction entails the net addition of a formyl group (CHO) and a hydrogen atom to a carbon-carbon do ...

hydroformylation
,
hydrosilylationHydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds."Hydrosilylation A Comprehensive Review on Recent Advances" B. Marciniec (ed.), Advances in Silicon Science, Springer Science, 2009. ...

hydrosilylation
,
hydrocyanation Hydrocyanation is the addition of hydrogen, H+ and cyanide, –CN to Substrate (chemistry), substrate. Usually the substrate is an alkene and the product is a nitrile. Hydrocyanation of unactivated alkenes Industrially, hydrocyanation is comm ...

hydrocyanation
. For inorganic chemists, homogeneous catalysis is often synonymous with organometallic catalysts. Many homogeneous catalysts are however not organometallic, illustrated by the use of cobalt salts that catalyze the oxidation of
p-xylene ''p''-Xylene ( ''para''-xylene) is an aromatic hydrocarbon Aromatic compounds are those chemical compounds (most commonly organic compound, organic) that contain one or more ring (chemistry), rings with pi electrons delocalized all the way aro ...

p-xylene
to
terephthalic acid Terephthalic acid is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reaction ...

terephthalic acid
.


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
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s lack transition metals. Typically, organic catalysts require a higher loading (amount of catalyst per unit amount of reactant, expressed in mol% amount of substance) than transition metal(-ion)-based catalysts, but these catalysts are usually commercially available in bulk, helping to reduce costs. In the early 2000s, these organocatalysts were considered "new generation" and are competitive to traditional metal(-ion)-containing catalysts. Organocatalysts are supposed to operate akin to metal-free enzymes utilizing, e.g., non-covalent interactions such as hydrogen bonding. The discipline organocatalysis is divided in the application of covalent (e.g., proline, 4-Dimethylaminopyridine, DMAP) and non-covalent (e.g., thiourea organocatalysis) organocatalysts referring to the preferred catalyst-Substrate (chemistry), substrate binding (molecular), binding and interaction, respectively.


Photocatalysts

Photocatalysis is the phenomenon where the catalyst can receive light (such as visible light), be promoted to an excited state, and then undergo intersystem crossing with the starting material, returning to ground state without being consumed. The excited state of the starting material will then undergo reactions it ordinarily could not if directly illuminated. For example, singlet oxygen is usually produced by photocatalysis. Photocatalysts are also the main ingredient in dye-sensitized solar cells.


Enzymes and biocatalysts

In biology,
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s 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. Biocatalysts can be thought of as intermediate between homogeneous and heterogeneous catalysts, although strictly speaking soluble enzymes are homogeneous catalysts and Biological membrane, 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. In biocatalysis, enzymes are employed to prepare many commodity chemicals including high-fructose corn syrup and acrylamide. Some monoclonal antibodies whose binding target is a stable molecule which resembles the transition state of a chemical reaction can function as weak catalysts for that chemical reaction by lowering its activation energy. Such catalytic antibodies are sometimes called "abzymes".


Significance

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. 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), Petroleum naphtha, naphtha reforming and steam reforming (conversion of hydrocarbons into synthesis gas). Even the exhaust from the burning of fossil fuels is treated via catalysis: Catalytic converters, typically composed of
platinum Platinum is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

platinum
and rhodium, break down some of the more harmful byproducts of automobile exhaust. :2 CO + 2 NO → 2 CO + N With regard 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, water-gas shift reactions, 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. Catalytic heaters generate flameless heat from a supply of combustible fuel.


Bulk chemicals

Some of the largest-scale chemicals are produced via catalytic oxidation, often using
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
. Examples include nitric acid (from ammonia),
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
(from
sulfur dioxide Sulfur dioxide (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the autho ...
to
sulfur trioxide Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...
by the contact process),
terephthalic acid Terephthalic acid is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reaction ...

terephthalic acid
from p-xylene, acrylic acid from propylene or propane and acrylonitrile from propane and ammonia. The production of ammonia is one of the largest-scale and most energy-intensive processes. In the
Haber process The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the ammonia production, production of ammonia today. It is named after its inventors, the German chemist ...

Haber process
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
is combined with hydrogen over an iron oxide catalyst. Methanol is prepared from carbon monoxide or carbon dioxide but using copper-zinc catalysts. Bulk polymers derived from ethylene and propylene are often prepared via Ziegler-Natta catalysis. Polyesters, polyamides, and isocyanates are derived via acid-base catalysis. Most carbonylation processes require metal catalysts, examples include the Monsanto acetic acid process and
hydroformylation Hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes from alkenes. This chemical reaction entails the net addition of a formyl group (CHO) and a hydrogen atom to a carbon-carbon do ...

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 the Heck reaction, and Friedel–Crafts reactions. Because most bioactive compounds are Chirality (chemistry), chiral, many pharmaceuticals are produced by enantioselective catalysis (catalytic asymmetric synthesis).(R)-1,2-Propandiol, precursor to the antibacterial levofloxacin, can be efficiently synthesized from hydroxyacetone using Noyori asymmetric hydrogenation:


Food processing

One of the most obvious applications of catalysis is the hydrogenation (reaction with
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
gas) of fats using nickel catalyst to produce margarine. Many other foodstuffs are prepared via biocatalysis (see below).


Environment

Catalysis impacts 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 chlorine free radicals in the breakdown of ozone. These radicals are formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs). :Cl + O → ClO + O :ClO + O → Cl + O


History

Generally speaking, anything that increases the rate of a process is a "catalyst", a term derived from Greek language, Greek wikt:καταλύω, καταλύειν, meaning "to annul," or "to untie," or "to pick up." The concept of catalysis was invented by chemist Elizabeth Fulhame and described in a 1794 book, based on her novel work in oxidation-reduction experiments. The first chemical reaction in organic chemistry that utilized a catalyst was studied in 1811 by Gottlieb Kirchhoff who discovered the acid-catalyzed conversion of starch to glucose. The term ''catalysis'' was later used by Jöns Jakob Berzelius in 1835 to describe reactions that are accelerated by substances that remain unchanged after the reaction. Elizabeth Fulhame, Fulhame, who predated Berzelius, did work with water as opposed to metals in her reduction experiments. Other 18th century chemists who worked in catalysis were Eilhard Mitscherlich who referred to it as ''contact'' processes, and Johann Wolfgang Döbereiner who spoke of ''contact action. ''He developed Döbereiner's lamp, a Lighter (fire starter), lighter based on
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
and a
platinum Platinum is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

platinum
sponge, which became a commercial success in the 1820s that lives on today. Humphry Davy discovered the use of platinum in catalysis. 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. Vladimir Ipatieff performed some of the earliest industrial scale reactions, including the discovery and commercialization of oligomerization and the development of catalysts for hydrogenation.


Inhibitors, poisons, and promoters

An added substance which does reduce the reaction rate is a reaction inhibitor if reversible and Catalyst poisoning, catalyst poisons if irreversible. Promoters are substances that increase the catalytic activity, even though they are not catalysts by themselves. Inhibitors are sometimes referred to as "negative catalysts" since they decrease the reaction rate. However the term inhibitor is preferred since they do not work by introducing a reaction path with higher activation energy; this would not reduce 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.Laidler, K.J. (1978) ''Physical Chemistry with Biological Applications'', Benjamin/Cummings. pp. 415–17. .Laidler, K.J. and Meiser, J.H. (1982) ''Physical Chemistry'', Benjamin/Cummings, p. 425. . In 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 reduction of alkynes to alkenes, a palladium (Pd) catalyst partly "poisoned" with lead(II) acetate (Pb(CHCO)) can be used. Without the deactivation of the catalyst, the alkene produced would be further reduced to alkane.Bender, Myron L; Komiyama, Makoto and Bergeron, Raymond J (1984) ''The Bioorganic Chemistry of Enzymatic Catalysis'' Wiley-Interscience, Hoboken, U.S. 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 surface randomly will tend to reduce the number of uncontaminated large planes but leave proportionally more smaller sites free, thus changing the hydrogenation vs. hydrogenolysis selectivity. Many other mechanisms are also possible. Promoters can cover up surface to prevent production of a mat of coke, or even actively remove such material (e.g., rhenium on platinum in catalytic reforming, platforming). They can aid the dispersion of the catalytic material or bind to reagents.


See also

* Chemical reaction ** Substrate (chemistry), Substrate ** Reagent **
Enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

Enzyme
** Product (chemistry), Product * Abzyme * Acid catalysis (includes Base catalysis) * Autocatalysis * BIG-NSE (Berlin Graduate School of Natural Sciences and Engineering) * ''Catalysis Science & Technology'' (a chemistry journal) * Catalytic resonance theory * Electrocatalyst * Environmental triggers * Enzyme catalysis * Industrial catalysts * Kelvin probe force microscope * Limiting reagent * Pharmaceutic adjuvant * Phase-boundary catalysis * Phase transfer catalyst * Photocatalysis * Ribozyme (RNA biocatalyst) * SUMO enzymes * Temperature-programmed reduction * Thermal desorption spectroscopy


References

*


External links


Science Aid: Catalysts
Page for high school level science
W.A. Herrmann Technische Universität presentation

Alumite Catalyst, Kameyama-Sakurai Laboratory, Japan

Inorganic Chemistry and Catalysis Group, Utrecht University, The Netherlands



Carbons & Catalysts Group, University of Concepcion, Chile

Center for Enabling New Technologies Through Catalysis, An NSF Center for Chemical Innovation, USA

"Bubbles turn on chemical catalysts"
Science News magazine online, April 6, 2009. {{Authority control Catalysis, Chemical kinetics Articles containing video clips