Exploring pretreatment effects in Co/SiO 2 Fischer-Tropsch catalysts: Different oxidizing gases applied to oxidation-reduction process

The influence of reduction-oxidation-reduction (ROR) pretreatment on 20% Co/SiO2 has been investigated using different oxidizing gases including water vapor and oxygen in the oxidation step. In this study, the evolution concerning the SiO2 structure and the cobalt phase and morphology is clearly elucidated at each step of reduction, oxidation and subsequent re-reduction. It is demonstrated that ROR treatment using both oxygen and water vapor decreases the average cobalt particle size. However, the catalytic performance affected in FTS is considerably different. ROR treatment in oxygen results in an increase in catalytic activity. In contrast, the water vapor applied in oxidation step obviously deactivates cobalt catalyst and enhances selectivity of methane. The resulting deactivation is ascribed to the promoted formation of irreducible cobalt silicate through the reaction between water vapor caused surface Si OH groups and oxidized cobalt (CoO) in spite of unchanged surface area and pore structure on SiO2. The characterization data reveals that the re-dispersion of cobalt particles occurs at oxidation step rather than the re-reduction step. In addition, the results indicate that the ROR treatment reducing the cobalt particle size depends on its initial size as no re-dispersion can be observed in the case of particles smaller than about 11 nm. Furthermore, the water vapor shows more effective re-dispersion in cobalt particles compared with the use of oxygen. This study provides fundamental insights into the control of catalytic activity, product selectivity, and catalyst stability over supported cobalt catalysts by understanding the evolution of catalyst structure through ROR treatment in different chemical environment.