Kinetic and spectroscopic studies of heterogeneous catalytic reactions
Abstract
The selective oxidation of ethylene was studied on a high surface area colloidal silver catalyst. In 0.1 to 1 torr of 20% ethylene in oxygen, the catalyst produced ethylene oxide with a selectivity of 20 to 30% at temperatures between 473 and 573 K. Surface-enhanced Raman (SER) spectra exhibited a distinct feature at 995 cm$\sp{-1}$. This band was also observed for catalysts exposed to just oxygen, and it shifted to 980 cm$\sp{-1}$ after heating the catalyst to 873 K in $\sp{18}$O$\sb2$. No isotopic shift was observed with $\sp{18}$O$\sb2$ at temperatures below 573 K. Similar spectral results were obtained at atmospheric pressure. For silver catalysts that contain adsorbed chlorine, SER bands were observed at 240, 1015, and 1045 cm$\sp{-1}$ in oxygen-containing atmospheres; the 995 cm$\sp{-1}$ band was not evident. The extension of SERS to other metals that do not exhibit the surface-enhanced effect was examined by electrodepositing thin layers (2 to 3 monolayers) of platinum, rhodium, and ruthenium on a roughened gold substrate. At atmospheric pressure and temperatures between 298 and 473 K, metal-oxygen features were observed at 500 cm$\sp{-1}$, and 490 and 600 cm$\sp{-1}$ in SER spectra of rhodium and ruthenium surfaces exposed to oxygen, respectively. Bands attributed to adsorbed carbon monoxide were observed in SER spectra of platinum (470, 2060, 390, and 1890 cm$\sp{-1}$) and rhodium (465 and 2040 cm$\sp{-1}$) surfaces exposed to carbon monoxide and oxygen mixtures. The temporal replacement of adsorbed carbon monoxide by nitric oxide as well as temperature-induced changes in the surface composition were studied on a seconds timescale with a spectrograph-charge coupled device detector arrangement. The adsorption and oxidation of sulfur dioxide was also studied. A band assigned to the S-O stretch of molecularly adsorbed sulfur dioxide was observed at 1130 cm$\sp{-1}$ on unmodified gold and platinum-coated gold surfaces exposed to sulfur dioxide at 298 K. Dissociative chemisorption of sulfur dioxide on all these metals was inferred from a metal-sulfur stretch observed near 300 cm$\sp{-1}$. Oxidation of sulfur dioxide was observed at 298 K on platinum and unmodified gold surfaces after the addition of water to feeds containing sulfur dioxide, oxygen, and carbon monoxide. The oxidation products exhibited spectral features near 1010 and 1030 cm$\sp{-1}$.
Degree
Ph.D.
Advisors
Takoudis, Purdue University.
Subject Area
Chemical engineering|Chemistry|Condensation
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