A catalyst in chemistry or biology is a substance that accelerates the rate of a chemical reaction, at some temperature, but without itself being transformed or consumed by the reaction (see also catalysis). A catalyst participates in the reaction but is neither a chemical reactant nor a chemical product.

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 with a lower activation energy, for a reaction to proceed. Molecules that would not have had the energy to react or that have such low energies that it is likely that they would take a long time to do so are able to react in the presence of a catalyst. A catalyst reduces the energy required for the reaction to occur.

The two main categories of catalysts are heterogeneous and homogeneous catalysts. Heterogeneous catalysts are present in different phases from the reactants in the reaction they are catalysing, whereas homogenous catalysts are in the same phase. 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 products to be created. The bonds between the products and the catalyst are weaker, so the products are released.

Homogenous 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 catalytic 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.

Fig. 1. Enthalpy profile for catalysed and uncatalysed reactions. AU is the activation energy for an uncatalysed reaction, AC is the reduced activation energy for the same reaction when catalysed. I represents the point at which a chemical intermediate has been formed, which then reacts to form the products.

Some of the most famous catalysts ever developed are the Ziegler-Natta catalysts used to mass produce polyethylene and polypropylene.

Catalytic converters break down some of the nastier byproducts of automobile exhaust. They are made from platinum and rhodium.