Since the formulation of the concept of catalysis, extensive research has been conducted to understand the nature of reaction intermediates and their interactions with catalytic active sites - the atom(s) that form bonds with the reaction intermediates. In heterogeneous catalysis, manipulating an active site to change catalytic properties would involve changing the nature, location, and density of these atoms and their immediate neighbors through ligand effects. Such strategies have been successfully deployed to develop many of the catalytic processes in practice to date.
Recent research has expanded to efforts that aim at manipulating the properties of an active site by controlling their environment. Molecular-size nanocages can create unusual environments. Examples from our laboratory demonstrate that catalytically active basic amino groups in these nanocages can remain free from protonation in an acidic medium, and highly unusual Co(I) species can be stabilized. Using the surrounding oxides to stabilize a tetrahedral Sn ion generates sites that can be Lewis and Brønsted acidic. Creating new active sites at the perimeter of the metal-oxide interfacial leads to catalytic selective production of acetone from propane or propanol. Extending beyond the active site, new observation suggests that catalysts can effect reactions to proceed at distances away from the active sites. In these cases, a messenger is generated at the active site, which transmits reactivity information to reactants far away from the active sites. Free radicals can serve as messengers, and there are evidence of catalytic generation of free radicals and their initiation of reactions away from the catalysts.
Speaker: Harold Kung, Northwestern Univ.
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