In chemistry and biology, catalysis (in Greek meaning to annul) is the acceleration of the rate of a chemical reaction by means of a substance, called a catalyst, that is itself unchanged chemically by the overall reaction. The opposite of a catalyst is an inhibitor, which slows down the rate of a chemical reaction.

A common misunderstanding is that catalysis "makes the reaction happen", and that the reaction would not otherwise proceed without the presence of the catalyst. In biologically- or industrially-useful timescales, this may be true in a limited sense; however, a catalyst cannot make a thermodynamically unfavorable reaction proceed. Rather, it can only speed up a reaction that is already thermodynamically favorable. Such a reaction in the absence of a catalyst would proceed, even without the catalyst, although perhaps too slowly to be observed or of use in a given context.

The mechanisms by which a catalyst speeds up a reaction are many, but they are all based on the reduction of the activation energy that is necessary to initiate the reaction. If you think of a reaction as a hill to be passed, the activation energy is the uphill part from the reactants' energy level to the energy level of the activated complex. The activated complex can then descend on either side of the reaction, either returning to the reactants or becoming products. In the hill example, catalysis functions as a tunnel, providing an easier way to the other side of the hill.

Catalysts accelerate the chemical reaction by providing a lower energy pathway between the reactants and the products. This usually involves the formation of one or more intermediates, which cannot be formed without the catalyst. The formation of this intermediate and subsequent reaction generally has a much lower activation energy barrier than is required for the direct reaction of reactants to products. The SI derived unit for measuring catalytic activity is the katal, which is moles per second.

Catalysis is a very important process from an industrial point of view since the production of most industrially important chemicals involve catalysis. Research into catalysis is a major field in applied science, and involves many fields of chemistry and physics.

Two types of catalysis are generally distinguished. In homogeneous catalysis the reactants and catalyst are in the same phase. For example acids (H⁺ ion donors) are common catalysts in many aqueous reactions. In this case both the reactants and the catalysts are in the aqueous phase. In heterogeneous catalysis the catalyst is in a different phase than the reactants and products. Usually, the catalyst is a solid and the reactants and products are gases or liquids. In order for the reaction to occur one or more of the reactants must diffuse to the catalyst surface and adsorb onto it. After reaction, the products must desorb from the surface and diffuse away from the solid surface. Frequently, this transport of reactants and products from one phase to another plays a dominant role in limiting the rate of reaction. Understanding these transport phenomena is an important area of heterogeneous catalyst research.

Important catalytic processes

• The Haber process for ammonia synthesis
• Steam reforming of hydrocarbons to produce synthesis gas
• Methanol synthesis
• Fischer-Tropsch synthesis
• Hydrogenation/dehydrogenation of organic compounds
• Sulfuric acid production
• Nitric acid production
• Maleic anhydride production
• Petroleum refining and processing
  • Hydrotreating - hydrogenation of hydrocarbons and removal of organic sulfur, nitrogen, oxygen, and metals
  • Catalytic cracking - breaking long-chain hydrocarbons into smaller pieces
  • Naptha reforming
  • Alkylation
• Industrial and automotive abatement of NO_x, CO, and hydrocarbons
• Nearly every chemical process associated with life!

See also

• Autocatalysis
• Metabolic pathway
• Surface chemistry
• Abzyme
• Dispersion (materials science)
• EC number
• Ozone hole
• Catalyst
• Albert Szent-Györgyi
• Ribozyme
• Enzyme
• Michaelis-Menten kinetics
• Catalytic converter (automobiles)