does is to provide a path for the reaction to proceed swiftly and selectively to the desired products. Yet what about an operational definition of a catalyst?

A catalyst is a substance that transforms reactants into products, through an uninterrupted and repeated cycle of elementary steps, until the last step in the cycle regenerates the catalyst in its original form. Many types of materials can serve as catalysts. These include metals, compounds (e.g., metal oxides, sulfides, nitrides), organometallic complexes, and enzymes. Because not all portions of a catalyst participate in the transformation of reactants to products, those portions that do are referred to as active sites. Most industrial catalysts are used in the form of porous pellets, each of which contains typically 1018 catalytic sites.

The total amount of catalyst is small compared to the amount of reactants and products made during the life of the catalyst. The turnover frequency of the cycle is the quantity that defines the activity of a catalyst. Strictly speaking, the turnover frequency is the number of molecules of a given product made per catalytic site per unit time. In heterogeneous catalysis, the turnover frequency is typically of the order of one per second.

Who develops these catalysts? The development of catalysts is carried out by chemists and chemical engineers, often in large multidisciplinary teams that bring together expertise in the areas of physical, organic, and inorganic chemistry, as well as materials science and chemical reaction engineering. Such teams work on determining the optimal composition and physical structure of the catalyst, its activity and selectivity over the desired range of operating conditions, and its deactivation rate over time. Attention is also paid to developing methods for catalyst reactivation and recovery.

The generalities cited above can be illustrated by examples borrowed from the chemical, oil, and pharmaceutical industries, as well as environmental protection.

The first triumph of large-scale catalytic technology goes back to 1913 when the first industrial plant to synthesize ammonia from its constituents, nitrogen and hydrogen, 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. Small 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 severe 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



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