(89b) Oxide Nanostructures: Novel Supports, Active Sites, and Tandem Catalysts

Oxide catalysts are important materials for a number of important chemical transformations. Our research seeks to better control and understand the catalytic surface of these materials through novel syntheses, probe molecules, and reaction modes. Two short vignettes reflecting current materials development will be described.

In the first area, we describe continuing efforts at creating highly dispersed metal oxide sites on silica and other supports, principally for selective oxidation (epoxidation, hydroxylation, ODH). We deomonstrate that the use of bulky and/or charged precursor complexes helps maintain small clusters. For epoxidation, isolated Nb on silica derived from a calixarene ligand are exceptional catalysts, while for ODH, we demonstrate some of the first ODH by CuOx clusters, and show that a precursor with bulky siloxane ligands outperforms traditional Cu precursors like the nitrate or EDTA complexes by forming a particular type of reduced Cu site. Going further, we are able to quantify the fraction of kinetically-relevant sites of these and other oxide materials using a newly developed phosphonic acid titration technique. Combined, synthesis and site titration allows us to organize large families of supported catalysts into quantitative, predictive relationships.

In the second area, we demonstrate a technique to easily deposit thin (<2 nm), conformal SiO₂ shells on other oxides such as TiO₂ or Al₂O₃. These core-shell materials impart a number of new properties on the material, including the creation of acidity sufficiently strong to crack alkyl benzenes (SiO₂ on Al₂O₃), and the ability to act as supports that very effectively stabilize metal nanoparticles with presenting diffusion limitations or blocking active sites. We also have shown that we can control access of molecules to the reactive surface using these layers. This trait can, for example, minimize oxidation of one speices in a mixture due to size or relative strength of adsorption.

Overall, it will be shown how advances in oxide materials synthesis can lead to improved understanding and breakout reactivity even in this very mature field of research.