Catalyst

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For other uses, see Catalyst (disambiguation).

A catalyst (Greek: καταλύτης, catalytēs) is a substance that accelerates the rate (speed) of a chemical reaction (see also catalysis). Chemical catalysts, the focus of this article, participate in reactions but are neither chemical reactants nor chemical products. More generally, one may sometimes call anything which accelerates a reaction without itself being consumed or transformed a "catalyst" (for example, a "catalyst for political change").

1 Catalysts and reaction energetics
2 Types of catalysts
- 2.1 Heterogeneous catalysts
- 2.2 Homogeneous catalysts
3 Poisoning a Catalyst
4 Commonly used catalysts
5 See also
6 References

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Catalysts and reaction energetics

Catalysts enable reactions to occur much faster or at lower temperatures because of changes that they induce in the reactants. Catalysts provide an alternative pathway of lower activation energy, for a reaction to proceed whilst remaining chemically unchanged themselves. 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. Molecules that would not have had the energy to react or that have such low energies that they probably would have taken a long time to react are able to react in the presence of a catalyst. Thus, more molecules that need to gain less energy to react will go through the 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.

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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Types of catalysts

Catalysts can be either heterogeneous or homogeneous. Heterogeneous catalysts are present in different phases from the reactants (e.g. a solid catalyst in a liquid reaction mixture), whereas homogeneous catalysts are in the same phase (e.g. a dissolved catalyst in a liquid reaction mixture). A simple model for heterogeneous catalysis involves the catalyst providing a surface on which the reactants (or substrates) temporarily become adsorbed. Bonds in the substrate become weakened sufficiently for new bonds to be created. The bonds between the products and the catalyst are weaker, so the products are released. Different possible mechanisms for reactions on surfaces are known, depending on how the adsorption takes place (Langmuir-Hinshelwood and Eley-Rideal).

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Heterogeneous catalysts

For example, in the Haber process to manufacture ammonia, finely divided iron acts as a heterogenous catalyst. The metal uses active sites to allow partial weak bonding to the reactant gases, 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.

Other heterogenous 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:

<math> 2CO + 2NO \rightarrow \; 2CO_2 + N_2</math>

Example of homogeneous catalysts are H+(aq) which acts as a catalyst in esterification and chlorine free radicals in the break down of ozone. Chlorine free radicals are formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs). They react with ozone forming oxygen molecules and regenerating chlorine free radicals:

Cl· + O₃ → ClO· + O₂

ClO· + O → Cl· + O₂

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Homogeneous catalysts

Homogeneous catalysts generally react with one or more reactants to form a chemical intermediate that subsequently reacts to form the final reaction product, in the process regenerating the catalyst. The following is a typical reaction scheme, where C represents the catalyst:

A + C → AC (1)

B + AC → AB + C (2)

Although the catalyst (C) is consumed by reaction 1, it is subsequently produced by reaction 2, so for the overall reaction:

A + B + C → AB + C

the catalyst is neither consumed nor produced. Enzymes are biocatalysts. Use of "catalyst" in a broader cultural sense is in rough analogy to the sense described here. Other biocatalysts are ribozymes and deoxyribozymes.

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Poisoning a Catalyst

A catalyst can be poisoned if another compound reacts with it and bonds chemically, but does not release. This effectively destroys the usefulness of the catalyst, as it cannot participate in the reaction with which it was supposed to catalyse, just like Raney nickel catalyst has reduced activity when it is in combination with mild steel. The loss in activity of catalyst can be overcome by having a lining of epoxy or other substances .

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Commonly used catalysts

Estimates are that 60% of all commerically 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> Some of the most famous catalysts ever developed are the Ziegler-Natta catalysts used to mass produce polyethylene and polypropylene. Probably the best-known catalytic reaction is the Haber process for ammonia synthesis, where ordinary iron is used as a catalyst. Catalytic converters made from platinum and rhodium break down some of the more harmful byproducts of automobile exhaust. The most effective catalysts are usually transition metals.