DBR Distinguished Speaker Series 2015-2016
"Nano-Structured Metal Catalysts for High-Temperature Applications"
Supported metal catalysts are ubiquitous in the chemical process industries and controlling the size and shape of the metal nanoparticles is important in maintaining activity and selectivity for many reactions. Maintaining metal dispersion is particularly challenging for applications requiring high temperatures where sintering of the metal nanoparticles and loss of active surface area can lead to deactivation. We have been investigating a novel approach to addressing this problem in which supported metal nanoparticles are synthesized in situ via exsolution from a perovskite type oxide support under reducing conditions. This approach has been used in some formulations of automotive emissions control catalysts where the ability to re-dissolve and exsolve the metal via redox cycling is exploited to maintain metal particle size. While this phenomenon is well known, the mechanism by which the transition metal is exsolved from the oxide host is still poorly understood. The relationships between the exsolution process and the resulting structure of the metal nanoparticles are also not well understood. In this talk I will discuss our recent mechanistic studies of the exsolution process. In this work we have used well-defined model systems and detailed structural analysis using electron microscopy and atomic force microscopy to characterize the nucleation and exsolution of Ni particles from Ni-doped strontium titanate. These studies show how exsolution produces unique surface structures consisting of metal particles partially submerged in pits on the oxide surface. We have shown that this particle-in-a-pit morphology imparts unusually high thermal stability relative to metal nanoparticles deposited on the same support via conventional methods, making the exsolved nanoparticles highly resistant to deactivation via sintering. The effect of the particle-in-a-pit morphology on catalytic activity will also be discussed. In particular we will show that while the metal particles maintain their high activity for oxidation reactions, they have significantly decreased activity for the formation of filamentous carbon deposits when exposed to hydrocarbons under reducing conditions. Additional examples of the use of exsolution to produce electrode catalysts for solid oxide fuel cells will also be discussed.
Professor John M. Vohs holds a Bachelor of Science Degree in Chemical Engineering from the University of Illinois, and a Ph.D. Degree, also in Chemical Engineering, from the University of Delaware. He joined the faculty at the University of Pennsylvania in 1989 where he is currently the Carl V. S. Patterson Professor of Chemical Engineering and Chair of the Department of Chemical and Biomolecular Engineering. Prof. Vohs has been the recipient of numerous awards including the American Chemical Society’s Victor K. LaMer Award, the National Science Foundation’s Presidential Young Investigator Award, the Catalysis Club of Philadelphia Award, and the University of Pennsylvania’s School of Engineering George H. Heilmeier Faculty Award for Excellence in Research. His research interests are in the areas of surface science, catalysis, and solid-state electrochemistry, and his group specializes in elucidating structure-activity relationships for a variety of catalytic materials, including metals and metal oxides, and the development of anodes and cathodes for solid oxide fuel cells and electrolyzers. Current research topics in his group include understanding the catalytic pathways for the reforming of bio-derived molecules, such as ethanol and glucose, to produce hydrogen and fuels, and using electrochemical techniques to characterize fuel cell and other catalysts. Prof. Vohs has authored 290 publications that have appeared in scientific journals and holds seven U.S. Patents.