Small Silver Particles: Characterization by Nuclear MagneticResonance, Conduction-Electron Spin Resonance, and CatalyticBehavior in Partial Oxidation Reactions

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This project is funded by two awards: CTS 90-17692 to Purdue University and CTS 90-19612 to the Pennsylvania State University. It is a study of the correlations between crystallite size, electrical properties, and catalytic behavior of highly dispersed silver particles on alumina or silica with particle diameters of 1 to 30 nm. Experiments performed include solid-state nuclear magnetic resonance of adsorbed species containing carbon-13 on clean silver and on silver covered with oxygen-16 or oxygen-17; conduction electron spin resonance of the conduction electrons of small (less than 5 nm) silver crystallites; and isotopic transient kinetic studies of the partial oxidation of ethylene (to ethylene oxide) over silver. Previous studies have shown that well-dispersed, silica-supported silver catalysts do not sinter during partial oxidation; these catalysts also exhibit selectivities comparable to poorly dispersed silver/alpha- alumina catalysts, but at much lower promoter (dichloroethane) concentrations. The detailed mechanism of ethylene oxide formation on silver is not known, but subsurface oxygen has been shown to be present and dynamically coupled to the reaction. This study examines these phenomena under a series of controlled conditions. The expoxidation of ethylene is a worldwide, very competitive, multibillion-dollar business; even modest improvements in process efficiency can have major impact on competitive position. This work addresses this issue directly, and also has implications for other processes.