FISCHER-TROPSCH SYNTHESIS OVER IRON-RHODIUM ALLOY CATALYSTS

To investigate the nature of iron-rhodium alloy catalysts during the Fischer-Tropsch synthesis, a combination of experimental techniques were applied. Infrared spectroscopy was mainly used to extract direct information on the surface of catalysts under the reaction conditions. In addition, Mossbauer spectroscopy was employed to study the iron alloy catalysts. Further characterization of the catalysts was performed by chemisorption measurements. Hydrocarbon products of the CO + H(,2) synthesis reaction were analyzed by gas chromatography. The working surface of a silica-supported rhodium catalyst was found to be saturated with molecular carbon monoxide. The intensity of the linear carbonyl absorption band remained constant compared to that for room temperature CO adsorption, while that of the bridge-bonded carbonyl absorption band was drastically reduced during the Fischer-Tropsch synthesis. The bridge-bonded adsorption sites are assumed to be the active sites for dissociating carbon monoxide. The hydrogenation rate of the linearly adsorbed carbon monoxide was much slower than the steady state reaction rate. The alloy catalyst did not form a bulk carbide, but the presence of surface carbon was suggested by the large shift of the linear carbonyl absorption band. On the other hand, infrared spectra on an iron catalyst showed only weak bands, indicating a high degree of CO dissociation. On a silica-supported iron-rhodium alloy catalyst, surface analysis by infrared spectroscopy presents evidence of well-mixed alloy formation. Three modes of carbon monoxide adsorption were identified. They are linear adsorption, bridge-bonded adsorption, and dicarbonyl adsorption. Since preoxidation produced dicarbonyls exclusively, the dicarbonyl species are assumed to form on oxidized rhodium sites. Thus, formation of dicarbonyls on the alloy surface suggests that iron stabilizes a partial oxidation of the surface. The dicarbonyl species had a very low thermal stability. There was a substantial amount of a shifted and asymmetric linearly adsorbed carbon monoxide peak under the reaction conditions, but bridge-bonded carbon monoxide disappeared. No adsorption bands other than molecular carbon monoxide were observed by infrared spectroscopy. If reaction intermediates such as hydrocarbon species or alcohol-like species exist on the surface, their concentration is low