Catalysts can be divided into two main types - heterogeneous and homogeneous. In a heterogeneous reaction, the catalyst is in a different phase from the reactants. In a homogeneous reaction, the catalyst is in the same phase as the reactants. A catalyst is any substance (including light) that directly alters the rate of a chemical reaction without entering into the net chemical reaction itself. Note that by this definition, a catalyst might increase or decrease the rate, but most commonly, we think of catalysts as increasing the rate and reserve the word "inhibitor" for those that decrease the rate. The idea that catalysts exits at all was slow in coming, and it took the work of a brilliant chemist from the early part of the 1800s to make the discovery. That chemist was the Swede Jöns Jacob Berzelius, shown below, who also made many other fundamental contributions to chemistry. You can click his picture to read more about him. A heterogeneous catalyst is one whose molecules are not in the same phase as the reactant's, which are typically gases or liquids that are adsorbed onto the surface of the solid catalyst. Enzymes and other biocatalysts are often considered as a third category. When using a catalyst, it is possible to replace a polluting chemical reaction with a more environmentally friendly alternative. Today, and in the future, this can be vital for the chemical industry. In addition, it’s important for a company/researcher to pay attention to market development. If a company’s catalyst is not continually improved, another company can make progress in research on that particular catalyst and gain market share.