Catalysis (pronounced cat-AL-uh-sis) is the process by which some substance is added to a reaction in order to make the reaction occur more quickly. The substance that is added to produce this result is the catalyst (pronounced CAT-uh-list).
You are probably familiar with the catalytic convertor, a device used in car exhaust systems to remove gases that cause air pollution. The catalytic convertor gets its name from the fact that certain metals (the catalysts) inside the device cause exhaust gases to break down. For example, when potentially dangerous nitrogen(II) oxide passes through a catalytic convertor, platinum and rhodium catalysts cause the oxide to break down into harmless nitrogen and oxygen. Nitrogen(II) oxide will break down into nitrogen and oxygen even without the presence of platinum and rhodium. However, that process takes place over hours, days, or weeks under natural circumstances. By that time, the dangerous gas is already in the atmosphere. In the catalytic convertor, the breakdown of nitrogen(II) oxide takes place within a matter of seconds.
Humans have known about catalysis for many centuries, even though they knew nothing about the chemical process that was involved. The making of soap, the fermentation of wine to vinegar, and the leavening of bread are all processes involving catalysis. One of the first formal experiments on catalysis occurred in 1812. Russian chemist Gottlieb Sigismund Constantin Kirchhof (1764–1833) studied the behavior of starch in boiling water. Under most circumstances, Kirchhof found, no change occurred when starch was simply boiled in water. But adding just a few drops of concentrated sulfuric acid to the boiling water had a profound effect on the starch. In very little time, the starch broke down to form the simple sugar known as glucose. When Kirchhof found that the sulfuric acid remained unchanged at the completion of the experiment, he concluded that it had simply played a helping role in the conversion of starch to sugar.
The name catalysis was actually proposed in 1835 by Swedish chemist Jöns Jakob Berzelius (1779–1848). The word comes from two Greek terms, kata (for "down") and lyein (for "loosen"). Berzelius used the term to emphasize that the process loosens the bonds by which chemical compounds are held together.
Catalysis reactions are usually categorized as either homogeneous or heterogeneous reactions. A homogeneous catalysis reaction is one in which both the catalyst and the substances on which it works are all in the same phase (solid, liquid, or gas). The reaction studied by Kirchhof is an example of a homogeneous catalysis. Both the sulfuric acid and the starch were in the same phase—a water solution—during the reaction.
A heterogeneous catalysis reaction is one in which the catalyst is in a different phase from the substances on which it acts. In a catalytic convertor, for example, the catalyst is a solid, usually a precious metal such as platinum or rhodium. The substances on which the catalyst acts, however, are gases, such as nitrogen(II) oxide and other gaseous products of combustion.
Some of the most interesting and important catalysts are those that occur in living systems: the enzymes. All of the reactions that take place within living bodies occur naturally, whether or not a catalyst is present. But they take place so slowly on their own that they are of no value to the survival of an organism. For example, if you place a sugar cube in a glass of water, it eventually breaks down into simpler molecules with the release of energy. But that process might take years. A person who ate a sugar cube and had to wait that long for the energy to be released in the body would die.
Fortunately, our bodies contain catalysts (enzymes) that speed up such reactions. They make it possible for the energy stored in sugar molecules to be released in a matter of minutes.
Today catalysts are used in untold numbers of industrial processes. For example, the commercially important gas ammonia is produced by combining nitrogen gas and hydrogen gas at a high temperature and pressure in the presence of a catalyst such as powdered iron. In the absence of the catalyst, the reaction between nitrogen and hydrogen would, for all practical purposes, not occur. In its presence, the reaction occurs quickly enough to produce ammonia gas in large quantities.