products, through an uninterrupted and repeated cycle of elementary steps, until the last step in the cycle regenerates the catalyst in its original form. More simply put, a catalyst is a substance that speeds up a chemical reaction without itself being consumed in the process. Many types of materials can serve as catalysts. These include metals, metal compounds (e.g., metal oxides, sulfides, nitrides), organometallic complexes, and enzymes.

CATALYTIC SCIENCE AND TECHNOLOGY

The first triumph of a large-scale catalytic technology goes back to 1913 when the first industrial plant to synthesize ammonia (NH3) from its constituents, elemental nitrogen (N2) and elemental hydrogen (H2), was inaugurated in Germany. From the outset, and until the present, the catalyst in such plants has consisted essentially of iron. The mechanism of the reaction is now well understood. Certain groups of iron atoms at the surface of the catalyst are capable of dissociating first a molecule of nitrogen and then a molecule of hydrogen, and finally of recombining the fragments to ultimately form a molecule of ammonia. The catalyst operates at high temperature to increase the speed of the catalytic cycle and at high pressure to increase the thermodynamic yield of ammonia. Under these conditions, the catalytic cycle turns over more than a billion times at each catalytic site, before the catalyst has to be replaced. This high productivity of the catalyst explains its low cost: the catalyst results in products worth 2000 times its own value during its useful life.

The next illustration of catalysis shows that industrial catalysts can be biomimetic in the sense that they imitate the ability of naturally occurring enzymes to produce optically active molecules. Many pharmaceuticals are known to be active in only one form, let us say the left-handed form. It is therefore critical to obtain the left-handed form with high purity. It is particularly important when the drug is toxic, even if only slightly so, and must be administered over many years. It is true of a molecule called L-Dopa used in the treatment of Parkinson's disease. The right-handed molecule is inactive. In ordinary synthesis, both forms are produced in equal amounts. Their separation is costly. Is it possible to produce only the left-handed form by means of a synthetic catalyst? The first successful industrial synthesis of this kind was achieved by Monsanto, and a patent for the selective synthesis of L-Dopa was granted in 1974. The catalytic process used to make L-Dopa today is an important achievement in industrial catalysis.

Finally, recent and current developments in catalytic technology are targeted at the protection of the environment. The best-known example deals with catalytic converters that remove pollutants from the exhaust gases of automobiles. Catalytic converters for automobiles were first installed in the United States in the early 1970s. These devices were subsequently intro-



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