A catalyst works by lowering the activation energy of a reaction, that is, the energy level at which reactants will convert to products. There are two theories about how catalysts lower the activation energy. Intermediate compound theory posits that a catalyst reacts with one or more reactant to give an intermediate compound which then more readily produces the desired products. Acid catalysts are believed to work in this way. Adsorption theory suggests that catalysts provide a "work surface" for reactants, allowing them to come together and react more easily. Enzymes work in this manner, as do catalysts based on transition metals.
There are two types of catalysts. Homogeneous catalysts work in the same phase as the reactants, while heterogeneous catalysts are in a different phase from the reactants. Adding a solution of sulfuric acid to a mixture of an alkylene and water is an example of homogeneous catalysis. The acid dissolves into the same solution as the reactants to initiate the hydration of unsaturated carbon bond in the alkylene to form an alcohol. The hydrogenation of alkylene using platinum supported on carbon is an example of heterogeneous catalysis. In this reaction, the hydrogen and alkylene reactants are in the vapor phase while the catalyst is in the solid phase.
Catalysts have a very wide range of scientific and industrial applications, but they are most commonly found in everyday life in the exhaust systems of automobiles. Internal combustion engines generate a range of acidic gases, including oxides of nitrogen. Exhaust gases also contain carbon monoxide, which is highly toxic. Catalytic converters are fitted to exhaust systems to reduce nitrogen oxides to nitrogen and oxygen and to oxidize carbon monoxide to carbon dioxide. These catalysts comprise rare earth metals such as rhodium, platinum and palladium fixed to aluminosilicate structures called zeolites. Catalytic converters are heterogeneous catalysts.
Catalysis is also found in biological systems in the form of enzymes. There are notable differences between catalysts and enzymes. Catalysts can facilitate a wide range of chemical reactions, while enzymes tend to be highly specific. Enzymes feature active sites that enable the joining or splitting of particular molecules. For example, the amylase enzyme, found in saliva, only facilitates the conversion of starch into glucose. Reactions involving catalysts are often conducted at high temperature. However, as they are found in biological systems enzymes work at around body temperature. Anything higher than 100 degrees Farenheit will destroy enzymes.