selectivity can be investigated by using spectroscopic techniques. Such promoters interact with the reactants and suppress the oxidation of sulfur dioxide to sulfur trioxide, an undesired side reaction.

In summary, investigations of reaction mechanisms and kinetics and, especially, in situ observations of catalytic reaction intermediates are essential for advancing the science of catalysis, inasmuch as the results of such studies provide an overall view of catalysis and help elucidate the relationships between catalyst structure and function. The current interest in developing catalysts for the production of environmentally benign gasoline, the abatement of air pollution from mobile and stationary sources, the synthesis of enantiomerically pure drugs, and the synthesis of novel polymers all benefit from studies of the relevant reaction mechanisms and kinetics. The successful advancement of knowledge in this area requires the further development of techniques for characterizing adsorbed species, particularly their structure and bonding; for identifying the connectivity between species in terms of a reaction network; and for characterizing the dynamics of elementary chemical transformations occurring under the influence of the catalyst.

THEORY OF CATALYSIS

The science of catalysis has traditionally advanced as a consequence of new experimental techniques, but theory has begun to play an increasingly important role. This change is the result of recent advances that have occurred in theoretical and computational chemistry, reaction engineering, and the availability of powerful supercomputers for extensive calculations, which enable graphical display of results. Information gained from theoretical studies is becoming helpful in guiding the design of novel catalysts, interpreting experimental measurements, and understanding the way in which catalyst composition and structure affect its activity and selectivity.

Considerable progress has been made in modeling and calculating the relative energies of intermediates in homogeneous transition metal catalysis. These systems are small and generally involve one metal and one constant ligand set. However, even for relatively simple systems, major approximations are required for ab initio calculations. The results obtained from these systems in which detailed molecular structures can be determined and systematic structural changes can be made will serve as guides for the more complex heterogeneous systems.

Theoretical calculations of catalyst structure can provide a useful basis for assessing the stability of particular structures as a function of composition and surrounding environment. For example, in the area of zeolite catalysis, it is possible to predict the stability of channel openings as a function of the ratio of silicon (Si) to aluminum (Al) and the preferred location of aluminum in the framework. Similarly, theoretical models of



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