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

该项目由两个奖项资助：普渡大学CTS 90-17692和宾夕法尼亚州立大学CTS 90-19612。这是一项研究晶体大小、电学性质和高度分散的银颗粒在氧化铝或二氧化硅上的催化行为之间的相关性，颗粒直径为1至30纳米。所进行的实验包括在清洁银和覆盖有氧-16或氧-17的银上吸附含有碳-13的物质的固态核磁共振；小型（小于5 nm）银晶体的导电电子自旋共振乙烯在银上部分氧化生成环氧乙烷的同位素瞬态动力学研究。先前的研究表明，分散良好的硅负载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可与分散不良的银/ α -氧化铝催化剂相比较的选择性，但在低得多的促进剂（二氯乙烷）浓度下。环氧乙烷在银上形成的详细机理尚不清楚，但地下氧已被证明是存在的，并与反应动态耦合。本研究在一系列受控条件下考察了这些现象。乙烯的提纯是一项全球性的、竞争激烈的、价值数十亿美元的业务；即使是流程效率的适度改进也会对竞争地位产生重大影响。这项工作直接解决了这个问题，并且对其他过程也有影响。

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.