Thermographic studies of spatial effects on metallic catalysts

摘要

This work is a combined theoretical and experimental study of the spatial behavior of polycrystalline metallic catalyst systems. Theoretically, the gauze catalyst was examined in both a differential reactor and a CSTR. The simplest gauze system studied in a differential reactor was a plus-shaped catalyst, a system consisting of four catalyst elements (ribbons), all connected at one end to form a plus. For an N-element system; there were 3$sp{rm N}$ steady-state solutions found, of which 2$sp{rm N}$ were stable. A parameter study was carried out, revealing all 81 possible steady states for the plus, 78 of which were spatially nonuniform. The nonuniform states fell consistently into three isola-type categories; however, when the thermal conductivity approached that of solid platinum, many of these solution states disappeared. Similar results were found for the larger gauze systems studied, including a tic-tac-toe configuration and a ten-by-ten gauze. Additional work involving spatially distributed parameters revealed the spatial communication of the polycrystalline catalyst to be strong enough to inhibit extremely spatially nonuniform catalyst behavior. Generally, a distributed activity encourages a single active area on the catalyst, which dominated the catalyst behavior, while a distributed transfer rate encourages multiple hot spots which could vary spatially during a parameter step change. Successively more complex catalyst systems revealed that additional crosslinking of the catalyst also discouraged nonuniform behavior. The examination of the plus in the CSTR revealed that the backmixing of the reactor environment allowed for added stable nonuniformity on a uniform catalyst.;The experimental study involved polycrystalline ribbons in a CSTR environment. Surface temperatures on the catalyst were measured using an in-situ infrared imaging technique whereby local surface temperatures are related to local reactions rates. The studies included both single ribbon studies as well as a plus-shaped catalyst study, where two ribbons were crossed and forced to touch at the center. When the ribbons were studied separately, this study revealed virtually identical catalyst elements. When the catalyst ribbons were crossed, only a single ribbon appeared active, and spatial interaction occurred primarily during an oscillatory phase before ignition.