.Mechanistic inquiries in surface catalysis seek to extract knowledge and insights about the identity and kinetic relevance of elementary steps and bound intermediates and about the nature and number of active sites. Such knowledge becomes especially relevant when derived from theory and experiments on realistic solids at relevant coverages through kinetic, isotopic, and spectroscopic methods rigorously interpreted with the aid of theory. The insights gained guide our choices of sites for specific reaction paths (and vice versa). These strategies are not limited to any type of site or reaction, as shown by the diverse reactions on metals, acids, acid-base pairs, and redox-active solids used as illustrations. In oxidation, reactivity descriptors bring together catalyst composition and structure to define the redox properties that determine the role and reactivity of electrophilic and nucleophilic O-atoms and how transition states sense the strength of the C-H bonds cleaved. For solid acids, this approach has led to more precise descriptions of acid strength and confinement effects, to predictions about their consequences for reactivity and selectivity, and specifically to assessments of the respective roles of the local properties of conjugate anions and their confining environment in favoring specific transition states based on their size and their charge distributions. This is demonstrated with C1 chemistry (carbonylation, methylative homologation) and in H-transfer, β-scission, and oligomerization of alkenes as the illustrative examples. Acid-base pairs of moderate strength in TiO2 and ZrO2 activate C-H bonds in condensation and dehydrogenation reactions through concerted interactions of these site pairs with relevant transition states, thus circumventing the less reactive intermediates and spectator species prevalent on strongly basic oxides or metal surfaces. On metals, Fischer-Tropsch synthesis turnovers occur at all only because dense CO adlayers destabilize the bound precursors that lead to the relevant C-O cleavage transition state, thus allowing facile H-assisted pathways unavailable on sparsely covered surfaces. Chain growth, the defining useful feature of this reaction, then occurs rapidly only because the dense adlayer is disrupted near a growing chain, causing monomers to form preferentially in the immediate vicinity of such chains.

Award Address (E. V. Murphree Award in Industrial and Engineering Chemistry sponsored by the ExxonMobil Research and Engineering Company). Unintended Consequences of Mechanistic Insights for the Practice of Catalysis: Active Sites for Purpose and Purposes for Active Sites

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.