Chemical Engineering Seminar
Oxide catalysts are important materials for a number of important chemical transformations. Research in the Notestein group seeks to better control and understand the catalytic surface of these materials through novel syntheses, probe molecules, and reaction modes. Three short vignettes reflecting current materials development will be described.
In the first area, we describe continuing efforts at creating extremely highly dispersed, site-isolated metal oxide sites from groups IV-VI on silica and other supports, principally for selective epoxidation. 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, representing a collaboration with the Humboldt University, Berlin, we move away from typical metal oxide precursors, and instead use multinuclear, metal, siloxane clusters. Several reactions, such as oxidative dehydrogenations (ODH) that are long-sought in the petrochemicals industry, exhibit maxima in rate near monolayer coverage, reflecting the importance of small, redox-active MOx clusters in the mechanism. We demonstrate some of the first ODH by CuOx clusters, and show that the multinuclear precursor outperforms mononuclear Cu precursors by avoiding the formation of both isolated sites and larger aggregates. Similar reasoning helps explain improvements in the selective reduction of NO for emissions applications, when certain metal oxide precursors are used.
Finally, we demonstrate a technique to easily deposit thin (~2 nm) SiO2 shells on other oxides such as TiO2 or Al2O3. These core-shell materials help stabilize subsequently-deposited metal nanoparticles and also control access of molecules to the reactive surface. The latter trait, for example, minimizes readsorption and decomposition of H2O2 generated during alcohol photooxidation. This H2O2 can then be scavenged by a second oxidation catalyst, allowing for highly selective and environmentally benign tandem photo-thermo selective oxidation.
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.