Novel aspects of the physical chemistry of CO/SiO2 Fischer-Tropsch catalyst preparations - Cobalt oxide-induced silica migration during calcination of cobalt nitrate-impregnated high surface area silica

摘要

CO/SiO2 catalysts were prepared by aqueous cobalt nitrate impregnations of silicas with different surface areas to study the effect of the support surface area on the reactions occurring during impregnation and calcination and to define the stage and mode of metal-support interactions. TPR analyses of samples calcined in dry air showed the presence of various quantities of cobalt silicate species, while cobalt silicate formation was not discernible by other analytical techniques. Our conclusion, confirmed in our later studies, is that cobalt silicate does not form during impregnation or calcination, but is created during the reduction in the TPR instrument. Because of these and other ambiguities of the TPR analyses, in our continuing studies we preferred alternative analytical approaches. These studies on the calcination stage resulted in the following unusual findings: (1) X-ray photoelectron spectroscopy revealed drastic decreases in the surface cobalt concentration after calcination of high surface silicas impregnated with cobalt nitrate solutions. (2) Infrared spectroscopy indicated much less than expected Co3O4 formation upon calcination if high surface area silica was the support. (3) A method was devised to calculate the surface areas of individual components in mixtures. The calculations indicated about 20% surface area losses for the silica in calcined catalysts. (4) Scanning electron micrographs of a calcined catalyst on high surface area silica support showed smaller-sized decorations around the larger silica particles. Energy-dispersive X-ray analysis of the decorations showed Si as major, and Co as a minor component. Pure Co3O4 phases were not found by EDX analyses of these decorations. These four seemingly unrelated findings are attributed to a common cause: silica migration and weak bond formation between CoO and SiO2. The extent of surface area losses (i.e. the extent of silica migration) is about an order of magnitude greater in CoOx-SiO2 catalysts than in analogously treated SiO2. The migration of silica must have occurred in a relatively short time period during the thermal decomposition of cobalt nitrate, while simultaneous migration and oxidation of CoO to Co3O4 aggregates also occurred. The CoO species intercepted by SiO2 were unable to oxidize, resulting in reduced quantity of Co3O4 formation. The extensive migration of silica is attributed to strong attraction between SiO2 and CoO species, inducing the removal of silicic acid or silica molecules from the silica surface. (c) 2006 Elsevier B.V. All rights reserved.