Preparation of Fischer–Tropsch cobalt catalysts supported on carbon nanofibers and silica using homogeneous deposition-precipitation

Homogeneous deposition-precipitation on either a silica or carbon nanofiber (CNF) support of cobalt from basic solution using ammonia evaporation was studied and compared with conventional deposition from an acidic solution using urea hydrolysis. In the low-pH experiment, the interaction between precipitate and silica was too high; cobalt hydrosilicates were formed requiring a reduction temperature of 600 °C, resulting in low cobalt dispersion. Lower interaction in experiments performed in a basic environment yielded a well-dispersed Co3O4 phase on silica, and after reduction at only 500 °C, a catalyst with 13-nm cobalt particles was obtained. On CNF from an acidic solution, cobalt hydroxy carbonate precipitated and displayed a low interaction with the support resulting after reduction at 350 °C in a catalyst with 25-nm particles. From basic solution we obtained high dispersion of cobalt on the CNF, probably related to the greater ion adsorption. After drying, Co3O4 crystallites were obtained that, after reduction at 350 °C, resulted in a catalyst with 8-nm Co particles. Samples prepared in the high-pH experiment had 2–4 times higher cobalt-specific activity in the Fischer–Tropsch reaction than their low-pH counterparts. CNF support materials combined with the high-pH deposition-precipitation technique hold considerable potential for cobalt-based Fischer–Tropsch catalysis.