Evolution of the bulk structure and surface species on Fe-Ce catalysts during the Fischer-Tropsch synthesis

Two Fe-Ce catalysts were prepared by wet impregnation of Ce onto iron oxyhydroxide (FeOOH) and hematite iron oxide (alpha-Fe2O3) respectively. Their performance in the Fischer-Tropsch (FT) synthesis was investigated and compared with that obtained with a Ce-free alpha-Fe2O3 catalyst. It was observed that the behavior of the different catalysts changed along the course of the FT reaction. The catalysts mere tested for different periods of time, carefully passivated, recovered from the reactor and characterized by different techniques. The FT activity of the Ce-loaded and Ce-free catalysts decreased initially, but at a certain point the catalytic activity started to increase. The time needed to reach this inflection point depended on the catalyst composition, being shorter for the Ce-promoted catalysts. The catalytic activity of the Ce-free catalyst increased when the Fe3C species were transformed into chi-Fe2.5C, which are suggested to be the carbide phase present when polymerized carbon species (C beta) are formed. The addition of Cc to the iron oxyhydroxide developed solids with a higher BET surface area. Besides, these samples displayed a higher FT activity at long time-on-stream (TOS). Moreover, Ce addition also facilitated the formation of the Cp species previous to the evolution of Fe3C into chi-Fe2.5C, and therefore, promoted the FT synthesis reaction.