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

Abstract

Two Fe-Ce catalysts were prepared by wet impregnation of Ce onto iron oxyhydroxide (FeOOH) and hematite iron oxide (a-Fe 2 O 3 ), respectively. Their performance in the Fischer-Tropsch (FT) synthesis was investigated and compared with that obtained with a Ce-free a-Fe 2 O 3 catalyst. It was observed that the behavior of the different catalysts changed along the course of the FT reaction. The catalysts were 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 Fe 3 C species were transformed into x-Fe 2.5 C, which are suggested to be the carbide phase present when polymerized carbon species (C b ) are formed. The addition of Ce 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 C b species previous to the evolution of Fe 3 C into x-Fe 2.5 C, and therefore, promoted the FT synthesis reaction.

Iron-modified ceria and Au/ceria catalysts for Total and Preferential Oxidation of CO (TOX and PROX)

“…The decrease in the particle size and/or the modification of the lattice symmetry in the case of formation of solid solution may account for this behavior [15,[19][20][21]. On the other hand, the band due to oxygen vacancies disappears for the doped systems (Figure 2 inset).…”

“…Nevertheless, for Fe contents up to 25% the cell parameter decreases as the iron content. This suggests the formation of a solid solution, since it has been reported that the introduction of iron in the CeO 2 fluorite structure results in a contraction of the unit cell due to the smaller ionic radii of iron compared to that of cerium [19][20][21]27]. The coincidence in cell parameters of CeO 2 and CeFe50 catalysts is consistent with the presence of α-Fe 2 O 3 in the solids with the higher iron content, pointing out the existence of a solubility limit of iron in the ceria matrix.…”

“…Additionally, it is well known that this oxygen mobility can be improved by the introduction of doping agents, with a different chemical nature, throughout the generation of a mixed system with a strong interaction, preferably as a solid solution in order to promote electronic distortions in the oxide framework [17,18]. Iron is an interesting candidate as a dopant, especially because it has its own redox behavior (Fe 3+ /Fe 2+ ) [19][20][21].…”

Mars-van Krevelen-like Mechanism of CO Hydrogenation on an Iron Carbide Surface

“…With cobalt, a correlation between the presence of Co 2 C and a high selectivity to methane has been reported [15]. On iron-based catalysts, studies report a fast and reversible exchange of Fe 3 O 4 into Fe x C carbides and vice versa depending upon reaction environment, as a consequence of elementary FTS steps [16,17], whereas the activity can be correlated with the iron carbide surface area [18]. The facile transformation of the catalytic phases makes it likely that carbon atoms in the surface become incorporated into reaction products [19], in a way that is similar as in oxidation catalysis, where the Mars-van Krevelen mechanism is commonly invoked [20].…”

Fe-doped ceria solids synthesized by the microemulsion method for CO oxidation reactions

“…Fe-modified ceria systems present a remarkable improvement of their oxygen exchange abilities compared with the bare CeO 2 , because of the Ce-Fe synergy that is achieved by combining the redox behavior of the cerium (Ce Pérez-Alonso and coworkers have presented a complete study about the Ce-Fe interactions in several series of Ce-Fe mixed oxides prepared by coprecipitation [9,[14][15][16]. The catalytic performances of these materials were evaluated in the N 2 O decomposition and the Fisher-Tropsch synthesis.…”