Catalysis



In rate) of a chemical reaction by means of a substance called a catalyst, which is itself not consumed by the overall reaction. More generally, one may at times call anything that accelerates a process, a "catalyst" (From the Greek καταλύειν, meaning to annul or to untie or to pick up).

A catalyst does not allow for a reaction to take place, but it provides an alternative route to products, the catalytic route being subject to lower reaction rate. Catalysts generally change in the course of a reaction but are regenerated.

A good example of a catalyst is in the oxygen:

2 H2O2 → 2 H2O + O2

This reaction is slow (hence one can buy solutions of hydrogen peroxide). Upon the addition of effervescence of oxygen. In demonstrations, the evolved oxygen is detectable by its effect on a glowing splint. The manganese dioxide may be recovered, and re-used indefinitely, thus it is a catalyst — it is not consumed by the reaction. (The H2O2 sold as a sterilizing agent in drugstores is too dilute for this to work dramatically.)

A promoter chemically modifies a catalyst but is not itself a catalyst. An inhibitor reduces the effectiveness of (or slows down the effect of) a catalyst.

History

The phrase catalysis was coined by Nobel Prize in Chemistry..

Definitions

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 → AC (1)
B + AC → ABC (2)
ABC → CD (3)
CD → C + D (4)

Although the catalyst (C) is consumed by reaction 1, it is subsequently produced by reaction 4, so for the overall reaction:

A + B → D

Catalytic cycles

Main article: catalytic cycle

A catalytic cycle is another term for mechanism. Catalytic cycles are central to any discussion of catalysis, be it in organometallic chemistry, or solid state chemistry.

Often, a so-called sacrificial catalyst is also part of the reaction system with the purpose of regenerating the true catalyst in each cycle. As the name implies the sacrificial catalyst is not regenerated and is instead irreversibly consumed. This sacrificial compound is also known as a stoichiometric catalyst when added in stoichiometric quantities compared to the main reactant. Usually the true catalyst is an expensive and complex molecule and added in quantities as small as possible. The stoichiometric catalyst on the other hand should be cheap and abundant.

Catalysts and reaction energetics

  Catalysts work by providing an (alternative) mechanism involving a different transition state and lower 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 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 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.

Autocatalysis

In autocatalysis, a reaction produces catalysts.

Types of catalysts

Catalysts can be either heterogeneous or homogeneous. Biocatalysts are often seen as a separate group.

Heterogeneous catalysts are present in different phases from the reactants (for example, a dissolved catalyst in a liquid reaction mixture).

Heterogeneous catalysts

Main article: Heterogeneous catalysis

A simple model for heterogeneous catalysis involves the catalyst providing a surface on which the reactants (or Eley-Rideal).

For example, in the 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 heterogeneous catalysts include Mesoporous silicates have found utility in heterogeneous reaction catalysis because their large accessible surface area allows for high catalyst loading.

In car engines, incomplete combustion of the fuel produces Carbon dioxide and nitrogen are desorbed from the surface and emitted as relatively harmless gases:

2CO + 2NO → 2CO2 + N2

Many catalysts used in refineries and in petrochemical applications are regenerated and reused multiple times to save costs and energy and to reduce environmental impact from recycling or disposal of spent catalysts.

Homogeneous catalysts

Main article: Homogeneous catalysis

Homogeneous catalysts are in the same phase as the reactants.

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 radiation on chlorofluorocarbons (CFCs). They react with ozone forming oxygen molecules and regenerating chlorine free radicals which then in turn destroys the thin layer that is the ozone.

Cl· + O3 → ClO· + O2
ClO· + O· → Cl· + O2

Biocatalysts

Main article: Biocatalysis

In nature deoxyribozymes. Biocatalysts can be thought of as a mixture of a homogenous and heterogeneous catalyst. This is because the enzyme is in solution itself, but the reaction takes place on the enzyme surface.

Electrocatalysts

In the context of hydrogen peroxide).

Significance

Catalysis is of paramount importance in the chemical industry. The production of most industrially important chemicals involves catalysis. The earliest commercial processes are the green chemistry due to the reduced amount of waste generated.

Notable examples

Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.[1]

oxygen and water.

Well-known applications of synthetic catalysts are:

  • Catalytic converters made from manganese break down some of the more harmful byproducts of automobile exhaust. There is a honey comb affect and the surface area is as big as a football pitch.
  • the iron is the catalyst.

Examples of catalysts that perform specific transformations on functional groups:

These given examples show that different catalysts perform other transformations on the same functional groups, where the reaction would not proceed, proceed very slowly, or proceed in an unselective manner without the presence of the catalyst.

The most common catalyst is the proton. Many complexes are used in catalysis as well.

New directions - organocatalysis

While transition metal catalysts are well established, a new trend is toward Organocatalysts of the "new generation" are competitive to traditional metal-containing catalysts and are owing to low product inhibition applicable in substoichiometric quantities. The chemical character of organocatalysts offers new and attractive perspectives and advantages to synthetically working chemists.

Catalytic processes

In 2005, Catalytic processes generated about $900 billion in products worldwide.(pdf)

See also

References

  1. ^ "Recognizing the Best in Innovation: Breakthrough Catalyst". R&D Magazine, September 2005, pg 20.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Catalysis". A list of authors is available in Wikipedia.