Clean speed ahead with catalysts
Box 1 | Catalyst chemistry
Chemical reactions involve the breaking of bonds between reactants (the chemicals to be combined), then the formation of new bonds to create the product. To do this, the reactants need to collide with enough energy (the 'activation energy') and in the right orientation to break apart the original bonds and react. Catalysts provide a different pathway for the reaction that requires less activation energy than the reaction without the catalyst. By reducing the activation energy, catalysts can speed up a reaction.
The Haber-Bosch process is a dramatic example of the effect a catalyst can have on the activation energy requirements of a reaction (Box 2: The Haber-Bosch
process). In this case the catalyst works by bonding to the reactants which weakens their internal bonds making it easier for the reaction to proceed. A catalyst often reacts with one or more reactants to form an intermediate product. This then undergoes another reaction to give rise to the final product and, in the process, the catalyst returns to its original form.
There are three main types of catalysts: heterogeneous, homogeneous and enzymatic. A heterogeneous catalyst is one that is in a different phase to the reactants. For example, the Haber-Bosch process involves a solid-state catalyst, containing iron and gaseous reactants. In this instance, the iron-containing material is a heterogeneous catalyst. The catalysts in catalytic converters are also heterogeneous.
A material containing a homogeneous catalyst is one that is in the same phase as the reactants. Chlorine gas is a homogeneous catalyst in the degradation of ozone gas to oxygen gas, a process blamed for destroying the ozone layer in the Earth's upper atmosphere.
Both types have their advantages. Heterogeneous catalysts can be reused more easily because, being in a different state, they are generally easier to separate from the products. Homogeneous catalysts tend to be very specific and function at lower temperatures and pressures. They are, however, less easy to reuse and can't be used at the high temperatures often needed to achieve high rates of conversion.
Enzymes are the third kind of catalyst. Most enzymes are proteins manufactured inside living cells. Many have specific roles; for example, chymotrypsin digests protein in the digestive system at particular sites in the molecule. Like all catalysts, enzymes are ultimately unchanged by the reactions they catalyse.
Box
Box 2 The Haber-Bosch process
Related sites
The effect of catalysts on reaction rates (Chemguide, UK)
Types of catalysis (ChemGuide, UK)
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Posted November 2008.