Small Silver Particles: Characterization by NMR Maganetic Resonance Catalytic Behavior in Partial Oxidation Reactions

该项目由两个奖项资助：普渡大学的CTS90-17692和宾夕法尼亚州立大学的CTS90-19612。研究了高度分散的银颗粒在粒径为1~30 nm的氧化铝或二氧化硅上的微晶尺寸、电学性质和催化行为之间的关系。实验包括含碳-13吸附物种在纯银和包覆氧-16或氧-17的银上的固体核磁共振，小(小于5 nm)银晶体传导电子的传导电子自旋共振；以及银上乙烯部分氧化为环氧乙烷的同位素瞬时动力学研究。以前的研究表明，分散良好的二氧化硅载体银催化剂在部分氧化过程中不会烧结；这些催化剂也表现出与分散较差的银/α-氧化铝催化剂相当的选择性，但助剂(二氯乙烷)浓度要低得多。环氧乙烷在银表面形成的详细机制尚不清楚，但已证明地下氧存在并动态耦合到反应中。这项研究在一系列受控条件下考察了这些现象。乙烯环氧化是一项世界性的、竞争激烈、价值数十亿美元的业务；即使工艺效率略有提高，也可能对竞争地位产生重大影响。这项工作直接解决了这个问题，也对其他过程有影响。

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.