Spatially resolved UV–vis microspectroscopy on the preparation of alumina-supported Co Fischer–Tropsch catalysts: Linking activity to Co distribution and speciation

The preparation steps of Co/γ-Al2O3 Fischer–Tropsch (FT) catalysts have been studied using spatially resolved UV–vis microspectroscopy. The speciation of the different Co phases inside the support bodies changed by varying the composition of the impregnation solutions and the applied treatment, that is, subsequent impregnation, aging, drying, and calcination. The activity in the FT synthesis reaction of the reduced catalysts has been correlated with the applied preparation procedure. The highest activity was observed when γ-Al2O3 was impregnated with an aqueous CoCO3 solution containing 1 equivalent citric acid. The formation of a Co citrate complex inside the support after impregnation was favored due to the higher pH of this solution compared with a similar solution involving Co(NO3)2. After calcination, Co3O4 was observed with an eggshell distribution, and no unfavorable CoAl2O4 was observed. The Co citrate complex formation is thought to restrict the extent of interaction of Co with alumina to form CoAl2O4, thereby allowing a higher degree of reduction and a concomitant higher catalytic activity. This study demonstrates the power of monitoring catalyst preparation with spatially resolved UV–vis microspectroscopy. The understanding of the physicochemical processes occurring at the different stages of the preparation process allows fine-tuning of the catalyst properties, which is ultimately expected to enable the development of improved supported catalysts.