Surface structure and Fischer–Tropsch synthesis activity of highly active Co/SiO2 catalysts prepared from the impregnating solution modified with some chelating agents Original Research Article

The present study investigated the surface structure and Fischer–Tropsch synthesis (FTS) activity of highly active Co/SiO2 catalyst prepared from the aqueous Co nitrate solution modified with chelating agents, i.e. nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA) and/or trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CyDTA), in order to make clear the origin of their promoting effects reported previously. The catalyst was prepared by impregnating SiO2 support with the aqueous solution containing Co nitrate and the chelating agent followed by drying and calcination. Characterization results of the calcined catalysts by XRD, XPS and TPR showed that modification of the impregnating solution with different chelating agents results in the formation of Co species with different structures after the calcination. On one hand, dispersed Co oxide species was formed on the calcined catalyst modified with NTA, which was in more dispersed state compared with Co3O4-like species on the calcined catalyst without chelating agents. On the other hand, highly dispersed α-Co2SiO4-like species was mainly formed after the calcination when modified with EDTA and/or CyDTA. After H2 reduction, about 90% of Co oxide species over NTA-modified catalyst was reduced to create more metallic Co sites compared with the catalyst without chelating agents, which yielded C10–20 hydrocarbons from syngas at 503 K and 1.1 MPa ca. three times greater than the catalyst without chelating agents. The catalysts modified with EDTA and/or CyDTA showed relatively lower activities after the reduction because the reduction degree of highly dispersed α-Co2SiO4-like species was much lower than that of Co3O4-like species. Therefore, modification of the impregnating solution with chelating agents increases the number of surface metallic Co sites through modification of the structure of Co species after the calcination, leading to the higher FTS activity. The role of chelating agents during the preparation of the catalyst was also discussed here.