Infrared study of CO adsorbed on a Co/Re/γAl2O3-based Fischer-Tropsch catalyst

The adsorption of CO(g) on a Co/Re/gamma-Al2O3 catalyst has been studied by diffuse reflectance infrared spectroscopy. The effects of pressure and temperature as well as addition of O-2(g) and H-2(g) have been investigated. The presence of Re as a promoter is shown to increase the degree of cobalt reduction, increase the amount of bridged species, and decrease the reduction temperature without any sign of electronic interaction between the two metals. Oxidation or hydrogenation reveals bands arising from rhenium carbonyl species, showing that the Re is located at the catalyst surface. It is proposed that Re either by direct blocking or by sterical hindrance inhibits the transportation of formate onto the active Co sites, and thereby hinders one of two possible reaction paths in the Fischer-Tropsch synthesis. When the catalyst is exposed to low pressures of CO for longer periods new band structures appear, suggesting that exposure to CO may lead to a reconstruction of the Co surface. This process involves creation/destruction of various adsorption shes and leads to a decrease in the long-range dipole-dipole interaction. Consequently, new absorptions are revealed in the spectrum. (C) 2000 Academic Press.