Phase transformation of unpromoted and promoted Fe catalysts and the formation of carbonaceous compounds during Fischer–Tropsch synthesis reaction Original Research Article

Precipitated Fe catalysts with or without additives (Zn, K, Cu) were prepared, pretreated by CO, and then time-dependent behaviors of their FTS activities were investigated at 503 K and 1.6 MPa in a fixed bed reactor. Over the catalyst without additives, CO conversion showed a maximum at ca. 8 h on stream and then decreased monotonically with increasing time on stream. When Zn, K and/or Cu were added, respectively, similar behaviors were observed. However, an increasing CO conversion was observed during on stream (29 h) when Zn, K and Cu were added simultaneously. X-ray diffraction (XRD) analysis of these catalysts showed the formation of Fe carbides (Fe5C2 and FexC) after the CO pretreatment irrespective of the catalyst composition. In the case of the catalyst without additives, these carbides were oxidized to Fe3O4 during on stream. The oxidation of the carbides was more significant on the catalyst charged near the outlet of the reactor. For the catalyst containing all the additives simultaneously, however, the Fe carbides were only phases detected by XRD analysis even after the long time on stream (29 h). In addition, the formation of amorphous carbonaceous compounds was observed by laser Raman spectroscopy (LRS) for the catalysts after the CO pretreatment and/or the FTS reaction. So-called a graphite-like carbonaceous compound was not observed by LRS, which was considered as one of reasons for the catalyst deactivation in the previous studies. Therefore, it is suggested that the oxidation of the Fe carbides by H2O vapor is a main reason for the deactivation of the Fe FTS catalyst. Because the Zn, K and Cu containing catalyst showed a higher CO2 selectivity, the combination of these additives improves the water-gas shift activity, thereby suppresses the oxidation of the Fe carbides.