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Microbiology => Nitrogen Fixation => Catalyst
Catalyst
Catalysis, alteration of the speed of a chemical reaction, through the presence of an additional substance, known as a catalyst, that remains chemically unchanged by the reaction. Enzymes (see Enzyme), which are among the most powerful catalysts, play an essential role in living organisms, where they accelerate reactions that otherwise would require temperatures that would destroy most of the organic matter.
A catalyst in a solution with-or in the same phase as-the reactants is called a homogeneous catalyst. The catalyst combines with one of the reactants to form an intermediate compound that reacts more readily with the other reactant. The catalyst, however, does not influence the equilibrium of the reaction, because the decomposition of the products into the reactants is speeded up to a similar degree. An example of homogeneous catalysis is the formation of sulfur trioxide by the reaction of sulfur dioxide with oxygen, in which nitrogen dioxide serves as a catalyst. Under extreme heat, sulfur dioxide and nitrogen dioxide react to form sulfur trioxide and the intermediate compound nitric oxide, which then reacts with oxygen to re-form nitrogen dioxide. The same amount of nitrogen dioxide exists at both the beginning and end of the reaction.
A catalyst that is in a separate phase from the reactants is said to be a heterogeneous, or contact, catalyst. Contact catalysts are materials with the capability of adsorbing molecules of gases or liquids onto their surfaces. An example of heterogeneous catalysis is the use of finely divided platinum to catalyze the reaction of carbon monoxide with oxygen to form carbon dioxide. This reaction is used in catalytic converters mounted in automobiles to eliminate carbon monoxide from the exhaust gases.
Some substances, called promoters, do not have catalytic ability by themselves but increase the effectiveness of a catalyst. For example, if alumina is added to finely divided iron, it increases the ability of the iron to catalyze the formation of ammonia from a mixture of nitrogen and hydrogen. Materials that reduce the effectiveness of a catalyst, on the other hand, are referred to as poisons. Lead compounds reduce the ability of platinum to act as a catalyst; therefore, an automobile equipped with a catalytic converter for emission control must be fueled with unleaded gasoline.
Catalysts are of major importance in today's industrial world. It has been estimated that about 20 percent of the U.S. gross national product is generated through the use of catalytic processes. One current area of active research in catalysis is that of enzymes. Natural enzymes have long been used by a few industries, but fewer than 20 such enzymes are presently available in industrial amounts. Biotechnologists are seeking ways in which to expand this resource and also to develop semisynthetic enzymes for highly specific tasks.
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