Difference between revisions of "Catalyst" - New World Encyclopedia
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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." | 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." | ||
| − | + | == General catalytic process == | |
| − | + | A catalyst participates in one or more stages of a reaction, but it is usually not a reactant or product of the overall reaction that it catalyzes. An exception to this rule is the process known as ''[[autocatalysis]]*'', in which the reaction product acts as a catalyst for the reaction. 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 catalyst | + | A catalyst may react 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. Alternatively, the catalyst may provide a surface to which the reactants bind, facilitating their reaction by bringing them close together. The products that are formed are released from the catalyst. |
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. | 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. | ||
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:A + B + C → D + C | :A + B + C → D + C | ||
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== Types of catalysts == | == Types of catalysts == | ||
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In nature, [[enzyme]]s are catalysts for biochemical reactions that take place within living organisms. Most enzymes are [[protein]]s, but some enzymes—called ''ribozymes''—are made of [[RNA]]. Some DNA molecules, called ''deoxyribozymes'', have been found to have catalytic activity. In addition, some antibodies, usually prepared artificially, have catalytic activity and are called ''abzymes''. | In nature, [[enzyme]]s are catalysts for biochemical reactions that take place within living organisms. Most enzymes are [[protein]]s, but some enzymes—called ''ribozymes''—are made of [[RNA]]. Some DNA molecules, called ''deoxyribozymes'', have been found to have catalytic activity. In addition, some antibodies, usually prepared artificially, have catalytic activity and are called ''abzymes''. | ||
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| + | == Catalysts and reaction energetics == | ||
| + | [[Image:Catalyst_effect.png|thumb|right|292px|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).]] | ||
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| + | 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. | ||
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| + | Catalysts cannot make energetically unfavorable reactions possible. They have ''no'' effect on the [[chemical equilibrium]]* of a reaction, because the rates of the forward and the reverse reactions are equally affected by the catalyst. 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. | ||
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| + | 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 ''[[turnover number]]*'' (TON), and the catalytic efficiency, by the ''turnover frequency'' (TOF). In biochemistry, the catalytic activity of an [[enzyme]] is measured in terms of [[enzyme unit]]s. | ||
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== Poisoning of a catalyst== | == Poisoning of a catalyst== | ||
Revision as of 15:47, 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."
General catalytic process
A catalyst participates in one or more stages of a reaction, but it is usually not a reactant or product of the overall reaction that it catalyzes. An exception to this rule is the process known as autocatalysis, in which the reaction product acts as a catalyst for the reaction. 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 catalyst may react 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. Alternatively, the catalyst may provide a surface to which the reactants bind, facilitating their reaction by bringing them close together. The products that are formed are released from the catalyst.
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
Types of catalysts
Catalysts can be either heterogeneous or homogeneous. Biological catalysts (or biocatalysts) are often considered 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 chlorine free radicals are regenerated. Some of the simplest reactions are as follows.
- Cl· + O3 → ClO· + O2
- ClO· + O3 → Cl· + 2 O2
Biological catalysts
In nature, enzymes are catalysts for biochemical reactions that take place within living organisms. Most enzymes are proteins, but some enzymes—called ribozymes—are made of RNA. Some DNA molecules, called deoxyribozymes, have been found to have catalytic activity. In addition, some antibodies, usually prepared artificially, have catalytic activity and are called abzymes.
Catalysts and reaction energetics
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 rates of the forward and the reverse reactions are equally affected by the catalyst. 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 turnover number (TON), and the catalytic efficiency, by the turnover frequency (TOF). In biochemistry, the catalytic activity of an enzyme is measured in terms of enzyme units.
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
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.
- A number of enzymes are used for chemical transformations of organic compounds. These enzymes are called biocatalysts and their action is called biocatalysis.
See also
ReferencesISBN links support NWE through referral fees
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