(753e) Shape- and Size-Specific Chemistry of Ag Nanostructures In Catalytic Ethylene Epoxidation

Phillip Christopher and Suljo Linic

Department of Chemical Engineering, University of Michigan,

Ann Arbor, Michigan 48109-2136  

Catalytically active metallic nanoparticles typically exhibit surface structure dominated by the most thermodynamically stable surface facet of the metal. This is a result of catalyst synthesis procedures that allow thermodynamics to control the resulting size and shape of the metal particles. Consequently, it has remained challenging to tune the catalytic function of metallic nanoparticles by engineering the exposed surface facet.  Recent progress in solution-based synthesis of metallic nanoparticles (exploiting kinetics to control nanoparticle growth) coupled with advances in molecular scale modeling of catalytic processes is allowing, for the first time, the rational design of catalytic metallic nanostructures with targeted geometric (size and shape) features. 

In this work, shape and size controlled synthesis of silver (Ag) nanoparticles was utilized in concert with Density Functional Theory (DFT) calculations to design Ag catalysts that are more selective in the oxidation of ethylene to form ethylene oxide (EO) than Ag catalysts prepared using conventional methods.  DFT calculations showed that the Ag(100) surface facets should be inherently more selective than the Ag(111) facet towards EO. The Ag(111) facet is the most abundant surface facet on spherical Ag particles, that are the standard form of Ag catalysts used in ethylene epoxidation. Ag nanoparticles dominated by the (100) facet (Ag nanocubes and nanowires) and the (111) facet (nanospheres) were synthesized using a modified polyol process. Steady-state reactor studies showed that the shape and size of catalytic Ag nanoparticles play important roles in governing the selectivity to EO, as predicted by the DFT calculations. Furthermore, these studies have allowed us to develop a physically transparent model, which describes the effect of shape, size and external conditions on activity and selectivity in the industrially important ethylene epoxidation reaction. (Christopher, P., Linic, S., ChemCatChem, 2010, 2, 78.  Christopher, P., Linic, S., JACS, 2008, 130, 11264.)