Catalyst

A catalyst (Greek: καταλύτης, catalytis) 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.
In some rare situations, one may describe an atomic nucleus as a catalyst in a nuclear reaction (see, for example, the CNO cycle). More generally, one may sometimes call anything which accelerates a reaction without itself being consumed or transformed a catalyst. This article will focus on chemical catalysts.
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. This means that catalysts reduce the amount of energy needed to start a chemical reaction. Molecules that would not have had the energy to react or that have such low energies that they probably would have taken a long time to react are able to react in the presence of a catalyst. Thus, more molecules that need to gain less energy to react will go through the chemical reaction.
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.
Some of the most famous catalysts ever developed are the Ziegler-Natta catalysts used to mass produce polyethylene and polypropylene. Probably the best-known catalytic reaction is the Haber process for ammonia synthesis, where ordinary iron is used as a catalyst. Catalytic converters break down some of the nastier byproducts of automobile exhaust. They are made from platinum and rhodium.
Although catalysts will change rate of a reaction, they will not shift an equilibrium reaction. This is because a catalyst affects the rate of reaction equally in both the forward and reverse directions.