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The Influence of Catalytic Surfaces on the Barriers to Elementary Surface Reaction Steps

The transition states to elementary reaction steps on surfaces have received little attention because of the lack of experimental probes of their structure and properties. This lack of understanding of the transition states for surface reactions places severe constraints on our ability to predict the kinetics of catalytic reactions and other surface chemical processes. The use of substituent effects has provided one approach to probe the nature of transition states and a means for determining whether such transition states can be considered to occur early or late in the reaction coordinate. This has been applied to several well-defined elementary surface reactions. As examples, the transition state for ‚-hydride elimination in adsorbed alkyl and alkoxy groups is believed to occur late in the reaction coordinate while the transition state for dehalogenation reactions on surfaces is believed to occur early in the reaction coordinate. Combining this knowledge with a comprehensive review of the barriers to these reactions on a wide variety of surfaces has suggested a simple proposition for considering the effects of surfaces on the barriers to elementary reactions. The barriers to elementary reaction steps with late transition states are expected to be sensitive to the nature of the surface while the barriers to reactions with early transition states are expected to be relatively insensitive to the nature of the surface. This proposition is illustrated by first considering the trivial examples of molecular adsorption and desorption on surfaces and then by discussion of surface activated ‚-hydride elimination and dehalogenation reactions.