Catalysis, << kuh TAL uh sihs, >> is a process in which a substance increases the speed of a chemical reaction without being consumed by the reaction. Any substance that accelerates a reaction in this way is called a catalyst. In industry, catalysts are used to speed up many chemical reactions that otherwise would take place too slowly to be practical. Enzymes serve as catalysts in many complex reactions that occur in all animals and plants (see Enzyme ).
In most cases, there are several possible sequences of steps by which a reaction can take place. A catalyst participates in some or all of the steps of a particular sequence. By doing so, the catalyst provides a chemical pathway along which the overall reaction can proceed far more rapidly than it otherwise could.
A typical example of catalysis is the effect of nitric oxide on the decomposition of ozone in the upper atmosphere of the earth. An oxygen atom and an ozone molecule combine slowly by themselves and produce two oxygen molecules. But in the presence of nitric oxide, a catalyst, a rapid two-step reaction takes place instead. First, a nitric oxide molecule combines with an oxygen atom, producing nitrogen dioxide. Then the nitrogen dioxide reacts with ozone and forms two molecules of oxygen and one molecule of nitric oxide. The second step of the reaction produces exactly as much nitric oxide as is consumed by the first step. Thus, the amount of nitric oxide does not change.
There are two types of catalysis, homogeneous and heterogeneous. In homogeneous catalysis, the catalyst and the reactants (reacting substances) are in the same physical state. For example, the catalytic decomposition of ozone is homogeneous because nitric oxide, oxygen, and ozone are all gases. On the other hand, heterogeneous catalysis involves two physical states, such as a solid catalyst affecting gaseous reactants.
Heterogeneous catalysts are generally used in industry because they can easily be separated from the products of reactions and then reused. Such catalysts are widely used in refining petroleum (see Petroleum (Conversion) ). In the production of ammonia, iron catalyzes the reaction of nitrogen with hydrogen. In the manufacture of nitric acid, platinum speeds the oxidation of ammonia.
Chemists also use simple organic (carbon-containing) molecules as catalysts. Such catalysts are typically less toxic and costly than metal catalysts.
