Difference between revisions of "Catalyst" - New World Encyclopedia

Revision as of 14:47, 4 October 2006

For a chemical reaction to take place, it requires a certain minimum amount of energy, called its activation energy. If a substance can lower this activation energy without itself being changed or consumed during the reaction, it is called a catalyst or catalytic agent (from the Greek word καταλύτης, catalytēs). The catalyst reduces the activation energy by providing an alternative pathway for the reaction to occur. In so doing, the catalytic agent increases the rate of the reaction—that is, it makes the reaction proceed faster.

More generally, the term catalyst may be applied to any agent (including a person or group) that brings about accelerated change. For example, someone may be called a "catalyst for political change."

A catalyst participates in one or more stages of a reaction, but it is not a reactant or product of the overall reaction that it catalyzes. [An exception to this rule is the process known as autocatalysis.] A substance that inhibits the action of a catalyst is called an inhibitor; one that accelerates the action of a catalyst is called a promoter.

A catalytic process

A catalyst often reacts with one or more reactants to form a chemical intermediate, and this intermediate subsequently reacts to form the final reaction product. In the overall process, the catalyst is regenerated.

Consider the following reaction scheme, in which C represents the catalyst, A and B are reactants, and D is the product of the reaction of A and B.

A + C → AC (1)

B + AC → ABC (2)

ABC → CD (3)

CD → C + D (4)

Here, the catalyst (C) is consumed by the reaction in stage 1, but it is regenerated in stage 4. Thus, the overall reaction can be written as:

A + B + C → D + C

Catalysts and reaction energetics

File:Catalyst effect.png

This generic graph shows the effect of a catalyst in a hypothetical chemical reaction. The initial reactants are on the extreme left of the graph, and the final products are on the extreme right. Notice that the catalyzed pathway (shown in red) has a lower activation energy, but it produces the same final result as the uncatalyzed pathway (shown in blue).

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 cannot make energetically unfavorable reactions possible — they have no effect on the chemical equilibrium of a reaction because the rate of both the forward and the reverse reaction are equally 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 degree of 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.

Types of catalysts

Catalysts can be either heterogeneous or homogeneous. Biocatalysis is often seen as a separate group.

A heterogeneous catalyst is one that is in a different phase from that of the reactants. For example, a solid catalyst may be used in a liquid reaction mixture. On the other hand, a homogeneous catalyst is one that is in the same phase as that of the reactants. For example, the catalyst may be dissolved in a liquid reaction mixture.

Heterogeneous catalysts

A simple model for heterogeneous catalysis involves the catalyst providing a surface on which the reactants (or substrates) temporarily become adsorbed.^[1] Chemical bonds in the substrate become sufficiently weakened for new bonds to be created. As the products are generated, they bind relatively weakly to the catalyst and are therefore released. Different possible mechanisms for reactions on surfaces are known, depending on how the adsorption takes place.

For example, consider the Haber process to manufacture ammonia from nitrogen and hydrogen. In this case, finely divided iron acts as a heterogeneous catalyst. As the reactant molecules (hydrogen and nitrogen) bind to the catalyst, this binding process has two effects: first, the molecules come closer together than they would be in the gas phase; and second, their internal bonds are weakened. In this manner, the catalyst makes it possible for the reactant molecules to react faster than they would if they had remained in the gas phase.

Homogeneous catalysts

In homogeneous catalysis, the catalyst itself may be transformed at an early stage of the reaction, and it is regenerated by the end of the reaction. An example is the breakdown of ozone by chlorine free radicals (free atoms of chlorine). Chlorine free radicals are formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs). These free radicals react with ozone to form oxygen molecules and regenerating chlorine free radicals:

Cl + O₃ → ClO + O₂

ClO + O → Cl + O₂

Biocatalysts

In nature, enzymes are catalysts in the metabolic pathway. In addition, catalysis is observed with abzymes, ribozymes and deoxyribozymes.

In biocatalysis enzymes are used as catalyst in organic chemistry.

Poisoning of a catalyst

A catalyst can be poisoned if another compound (similar to an inhibitor) alters it chemically or bonds to it and does not release it. Such interactions effectively destroy the usefulness of the catalyst, as it can no longer participate in the reaction that it was supposed to catalyze. Common catalyst poisons are lead, sulfur, zinc, manganese, and phosphorus.

Commonly used catalysts

Catalytic converter on a Saab 9-5.

According to some estimates, 60% of all commercially produced chemical products require catalysts at some stage during their manufacture.^[2] The most effective catalysts are usually transition metals or transition metal complexes.

The catalytic converter of an automobile is a well-known example of the use of catalysts. In this device, platinum, palladium, or rhodium may be used as catalysts, as they help break down some of the more harmful byproducts of automobile exhaust. A "three-way" catalytic converter performs three tasks: (a) reduction of nitrogen oxides to nitrogen and oxygen; (b) oxidation of carbon monoxide to carbon dioxide; and (c) oxidation of unburnt hydrocarbons to carbon dioxide and water.

Other examples of catalysts and their applications are as follows.

Ordinary iron is used as a catalyst in the Haber process to synthesize ammonia from nitrogen and hydrogen, as mentioned above.

The mass production of a polymer such as polyethylene or polypropylene is catalyzed by an agent known as the Ziegler-Natta catalyst, which is based on titanium chloride and alkyl aluminum compounds.

Vanadium(V) oxide is a catalyst for the manufacture of sulfuric acid at high concentrations, by a method known as the contact process.

Nickel is used in the manufacture of margarine.

Alumina and silica are catalysts in the breakdown of large hydrocarbon molecules into simpler ones—a process known as cracking.

References
ISBN links support NWE through referral fees

↑ Note that the term "adsorption" means binding to the surface of a substance. In this case, the reactants bind to the surface of the catalyst.
↑ "Recognizing the Best in Innovation: Breakthrough Catalyst," R&D Magazine, September 2005, pg 20.

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

History of "Catalyst"

Note: Some restrictions may apply to use of individual images which are separately licensed.

[1] Note that the term "adsorption" means binding to the surface of a substance. In this case, the reactants bind to the surface of the catalyst.

[2] "Recognizing the Best in Innovation: Breakthrough Catalyst," R&D Magazine, September 2005, pg 20.

[1]

[2]

@@ Line 40: / Line 40: @@
 ===Heterogeneous catalysts===
-A simple model for [[heterogeneous]]* catalysis involves the catalyst providing a [[surface]]* on which the reactants (or substrates) temporarily become [[adsorption|adsorbed]]*. [[Chemical bond]]s in the substrate become sufficiently weakened for new bonds to be created. As the products are generated, they bind relatively weakly to the catalyst and are therefore released. Different possible mechanisms for [[reactions on surfaces]]* are known, depending on how the adsorption takes place.
+A simple model for [[heterogeneous]]* catalysis involves the catalyst providing a [[surface]]* on which the reactants (or substrates) temporarily become [[adsorption|adsorbed]]*.<ref>Note that the term "adsorption" means binding to the surface of a substance. In this case, the reactants bind to the surface of the catalyst.</ref> [[Chemical bond]]s in the substrate become sufficiently weakened for new bonds to be created. As the products are generated, they bind relatively weakly to the catalyst and are therefore released. Different possible mechanisms for [[reactions on surfaces]]* are known, depending on how the adsorption takes place.
-For example, in the [[Haber process]]* to manufacture [[ammonia]], finely divided [[iron]] acts as a heterogeneous catalyst. Active sites on the metal allow partial weak bonding to the reactant [[gas]]es, which are [[adsorbed]] onto the metal surface. As a result, the bond within the molecule of a reactant is weakened and the reactant molecules are held in close proximity to each other. In this way the particularly strong [[triple bond]] in nitrogen is weakened and the hydrogen and nitrogen molecules are brought closer together than would be the case in the gas phase, so the rate of reaction increases.
+For example, consider the [[Haber process]]* to manufacture [[ammonia]] from [[nitrogen]] and [[hydrogen]]. In this case, finely divided [[iron]] acts as a heterogeneous catalyst. As the reactant molecules (hydrogen and nitrogen) bind to the catalyst, this binding process has two effects: first, the molecules come closer together than they would be in the gas phase; and second, their internal bonds are weakened. In this manner, the catalyst makes it possible for the reactant molecules to react faster than they would if they had remained in the gas phase.
-Other heterogeneous catalysts include [[vanadium(V) oxide]] in the [[Contact process]], [[nickel]] in the manufacture of [[margarine]], [[alumina]] and [[silica]] in the [[cracking]] of [[alkanes]] and [[platinum]] [[rhodium]] [[palladium]] in [[catalytic converters]].
-In car engines, incomplete [[combustion]] of the [[fuel]] produces [[carbon monoxide]], which is toxic. The electric spark and high temperatures also allow [[oxygen]] and [[nitrogen]] to react and form [[nitric oxide]] and [[nitrogen dioxide]], which are responsible for photochemical [[smog]] and [[acid rain]]. Catalytic converters reduce such emissions by adsorbing [[CO]] and [[NO]] onto catalytic surface, where the gases undergo a [[redox reaction]]. [[Carbon dioxide]] and nitrogen are desorbed from the surface and emitted as relatively harmless gases:
-:2CO + 2NO → 2CO<sub>2</sub> + N<sub>2</sub>'''
 ===Homogeneous catalysts===
-In homogeneous catalysis the catalyst is a [[molecule]] which facilitates the reaction.  The reactant(s) [[coordinate]] to the catalyst (or ''vice versa''), are transformed to product(s), which are then released from the catalyst.
-Examples of homogeneous catalysts are [[hydrogen|H]]<sup>+</sup>(aq) which acts as a catalyst in [[esterification]], and [[chlorine]] [[free radical]]s in the break down of [[ozone]]. Chlorine free radicals are formed by the action of [[ultraviolet]] [[radiation]] on [[chlorofluorocarbon]]s (CFCs). They react with ozone forming oxygen molecules and regenerating chlorine free radicals:
+In homogeneous catalysis, the catalyst itself may be transformed at an early stage of the reaction, and it is regenerated by the end of the reaction. An example is the breakdown of [[ozone]] by [[chlorine]] [[free radical]]*s (free atoms of chlorine). Chlorine free radicals are formed by the action of [[ultraviolet]] [[radiation]]* on [[chlorofluorocarbon]]*s (CFCs). These free radicals react with ozone to form oxygen molecules and regenerating chlorine free radicals:
 :Cl + O<sub>3</sub> → ClO + O<sub>2</sub>
@@ Line 60: / Line 53: @@
 ===Biocatalysts===
-In nature [[enzyme]]s are catalysts in the [[metabolic pathway]]. In [[biochemistry]] catalysis is also observed with [[abzyme]]s, [[ribozyme]]s and [[deoxyribozyme]]s.  In [[biocatalysis]] enzymes are used as catalyst in [[organic chemistry]].
+In nature, [[enzyme]]s are catalysts in the [[metabolic pathway]]. In addition, catalysis is observed with [[abzyme]]s, [[ribozyme]]s and [[deoxyribozyme]]s.
+* In [[biocatalysis]] enzymes are used as catalyst in [[organic chemistry]].
 == Poisoning of a catalyst==
-{{main|catalyst poisoning}}
 A catalyst can be poisoned if another compound (similar to an inhibitor) alters it chemically or bonds to it and does not release it. Such interactions effectively destroy the usefulness of the catalyst, as it can no longer participate in the reaction that it was supposed to catalyze. Common catalyst poisons are [[lead]], [[sulfur]], [[zinc]], [[manganese]], and [[phosphorus]].
@@ Line 74: / Line 68: @@
 The [[catalytic converter]]* of an automobile is a well-known example of the use of catalysts. In this device, [[platinum]], [[palladium]], or [[rhodium]] may be used as catalysts, as they help break down some of the more harmful byproducts of automobile exhaust. A "three-way" catalytic converter performs three tasks: (a) reduction of nitrogen oxides to [[nitrogen]] and [[oxygen]]; (b) oxidation of carbon monoxide to [[carbon dioxide]]; and (c) oxidation of unburnt hydrocarbons to carbon dioxide and [[water]].
-Ordinary [[iron]] is used as a catalyst in the [[Haber process]]*&mdash;a process for the synthesis of [[ammonia]] from [[nitrogen]] and [[hydrogen]]. The mass production of a polymer such as [[polyethylene]]* or [[polypropylene]]* is catalyzed by an agent known as the [[Ziegler-Natta catalyst]]*, which is based on titanium chloride and alkyl aluminum compounds.
+Other examples of catalysts and their applications are as follows.
+* Ordinary [[iron]] is used as a catalyst in the [[Haber process]]* to synthesize [[ammonia]] from [[nitrogen]] and [[hydrogen]], as mentioned above.
+* The mass production of a polymer such as [[polyethylene]]* or [[polypropylene]]* is catalyzed by an agent known as the [[Ziegler-Natta catalyst]]*, which is based on titanium chloride and alkyl aluminum compounds.
+* [[Vanadium(V) oxide]]* is a catalyst for the manufacture of [[sulfuric acid]] at high concentrations, by a method known as the ''[[contact process]]*''.
+* [[Nickel]] is used in the manufacture of [[margarine]]*.
+* [[Alumina]]* and [[silica]]* are catalysts in the breakdown of large [[hydrocarbon]] molecules into simpler ones&mdash;a process known as ''cracking''.
 == See also ==