M. Cargnello, A. Riscoe, C. Wrasman, A. Aitbekova, E. Goodman, A. Herzing, S. Bare
Stanford University,
United States
Keywords: enzymes, catalysts, nanocrystal
Summary:
Enzymes are ideal catalysts for many important reactions. They have evolved to contain confined metal centers within angstrom distances of amino acid groups with specific functionality to direct transport of species (reactants, products) as well as to shape the electronic landscape of adsorbates to impart high activity and selectivity for biologically relevant transformations. Most traditional catalytic approaches use extended metal surfaces as the active site for catalysis. Consequently, rational catalytic design is mostly limited to engineering metal electronic structure with alloying, surface structuring and metal oxide interface engineering. In this work we draw from enzymes to add another level of control to catalyst engineering by incorporating metal centers within polymers with tunable morphology and functionality. In this talk, we will highlight how we employ porous organic frameworks (POFs), a class of thermally stable, microporous, and chemically functionalizable polymers, to encapsulate colloidal metal nanoparticles as a catalytic platform. We first support the particles on POF, remove ligands with thermal treatment and then epitaxially grow porous polymer overlayers over the POF-metal composite to encapsulate the particles fully. This approach is tunable in the polymer chemistry, morphology as well as the metal particle composition. We synthesized an array of materials with differing POF chemistry, nanoparticle composition and morphology to demonstrate the wide range of possibilities with this system. In a second part, the catalytic properties of several materials will be discussed. We demonstrate that introducing the POF overlayer on Palladium particles increases intrinsic activity as well as induces catalytic oscillations under conditions that do not induce oscillations on supported Palladium catalysts for CO Oxidation, a test reaction. These catalytic changes have been demonstrated to be stable under oxidizing conditions for upwards of 40 hours. We discuss how the polymer overlayer induces changes to control transport and chemical environment of reactants, products, and intermediates at the metal active site much like in enzymes. These polymers open up avenues to explore catalytic materials with novel reactivity.