Preparation of highly active CO/SiO2 fischer-tropsch synthesis catalyst with Chelating agents:: Effect of Chelating agents on structure of Co species during preparation steps

The present study investigated the structure of Co species formed during the preparation Of Co/SiO2 Fischer-Tropsch synthesis (FTS) catalysts with an aqueous Co nitrate solution modified with chelating agents (NTA and CyDTA), to clarify the origin of the promotion effects. Diffuse reflectance FTIR measurements showed that both NTA and CyDTA complexes formed in the impregnating solution are preserved on the SiO2 surface. These complexes were interacted with OH groups on SiO2 surface. During the subsequent drying step, some of the NTA-Co2+ complex were decomposed, whereas the CyDTA-Co2+ complex was completely preserved. Both types of complex are combusted during the calcination step at ca. 100 K higher temperature compared with Co nitrate. Furthermore, the combustion temperature was higher for the CyDTA-Co2+ complex than the NTA-Co2+ complex. After the calcination step, Co-3/O-4 and alpha-Co2SiO4-like structures with smaller cluster sizes were observed by Co K-edge EXAFS when modified with NTA and CyDTA, respectively. The FTS activity of the NTA-modified catalyst strongly depends on the calcination temperature. Higher FTS activities were obtained only when the catalyst was calcined above the combustion temperature of the NTA complex. Therefore, the interactions between the chelating agents, Co2+ and the SiO2 surface during the preparation steps is the crucial factor for the promotion effect of NTA. Modification with NTA results in moderate interactions with Co2+ and the SiO2 surface, leading to the higher FTS activity.