| The mechanism of the Fischer-Tropsch reaction on alumina and silica-supported ruthenium catalysts has been studied by using transient response methods. Gas phase kinetic study techniques and surface spectroscopic techniques were used to reveal surface phenomena and true reaction mechanism.; The gas phase transient study confirmed the existence of two reactive carbon speices located on metal surface. One of them is carbidic carbon, which is the direct result of CO dissociation on low coordination surface sites and occupies ca. 10% of the total surface area. This carbidic carbon contributes more than 90% of the total hydrocarbon production at normal steady state conditions. The products from the carbidic carbon contain mainly linear alkanes and alkenes and follow Schulz-Flory distribution. Another carbon species is a hydrogen containing species, which is surface alkylic in identity. This species contributes less than 10% of the total hydrocarbon production at steady state reaction conditions. The products from this species consist of linear and branched alkanes but no alkenes and do not follow Schulz-Flory distribution.; The surface IR transient study identified several different kinds of surface sites for CO adsorption. The majority of the surface CO are adsorbed in linear form which gives a band at 2040 cm{dollar}sp{lcub}-1{rcub}{dollar} vicinity. Without the coadsorption of hydrogen, several bands due to CO adsorbed on surface oxide and CO perturbed by surface oxygen were observed. Among these bands, 2145 cm{dollar}sp{lcub}-1{rcub}{dollar} is assigned to gem-dicarbonyls adsorbed on oxide, 2125 cm{dollar}sp{lcub}-1{rcub}{dollar} is assigned to CO adsorbed on oxide, 2070 cm{dollar}sp{lcub}-1{rcub}{dollar} is assigned to CO perturbed by surface oxygen. With the coadsorption of CO and hydrogen the surface oxygen contents decreased dramatically. In this case, the band at 2070 cm{dollar}sp{lcub}-1{rcub}{dollar} is considered to be due to gem-dicarbonyls adsorbed on low coordination surface sites. Fairly good correlation was found to correlate the formation of surface alkyls with the gem-dicarbonyls, suggesting a common surface site for both species. New evidence was found for the interaction of CO and hydrogen on the surface.; The surface IR transient study confirmed the low activity hydrogen containing surface species to be surface alkyls. These alkyls are located on the low coordination surface sites which are coordinated with carbonyls. The hydrocarbon production through these surface alkyls is identified as hydrogenolysis reaction of the alkyl groups. (Abstract shortened with permission of author.)... |
