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:
Although the catalyst (C) is consumed by reaction 1, it is subsequently produced by reaction 2, so for the overall reaction:
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.