How catalysis works
Every chemical reaction has an activation energy (Eₐ) — the minimum energy that colliding particles must have for a reaction to occur. A catalyst does not change the energy of the reactants or products; it provides an alternative reaction mechanism with a lower activation energy.
Because more colliding molecules exceed the lower activation energy, the reaction rate increases. At a higher temperature the same effect can be achieved by increasing the kinetic energy of molecules — but a catalyst achieves it without any energy input.
Crucially, the catalyst is regenerated at the end of the reaction. It participates in the mechanism (forming intermediate species) but is not consumed as a net result.
For biological catalysts, see our enzyme glossary entry. For the role of bonds in determining reaction energy, see our chemical bonding guide.
Homogeneous vs heterogeneous catalysis
The most important practical distinction in catalysis is the phase relationship between catalyst and reactants:
- Homogeneous catalysis — catalyst and reactants are in the same phase (usually aqueous solution). The catalyst often forms an unstable intermediate. Example: acid-catalysed ester hydrolysis.
- Heterogeneous catalysis — catalyst and reactants are in different phases (usually a solid surface with gaseous or liquid reactants). Reactants adsorb onto the catalyst surface, react, and the products desorb. Example: the iron catalyst in the Haber process for ammonia.
Heterogeneous catalysts can be poisoned by impurities that bind irreversibly to active sites. The iron catalyst in the Haber process must be protected from sulfur compounds, which is why the feed gases are purified.
Catalyst examples
Industrial and environmental importance
Catalysis underpins most of the chemical industry. The Haber–Bosch process — which uses an iron catalyst to synthesise ammonia from nitrogen and hydrogen — produces the fertilisers that feed roughly half the world's population.
Catalytic converters fitted to vehicles since the 1970s have dramatically reduced air pollution in cities. Zeolite catalysts are used in petroleum refining to crack heavy hydrocarbons into petrol and diesel.
In research, the development of better catalysts for nitrogen fixation at lower temperature and pressure is seen as one of the highest-priority targets in green chemistry — a problem that quantum computing may eventually help crack. See our quantum computing and chemistry article.