Enhanced Oxygenates Formation in the Fischer-Tropsch Synthesis over Co- and/or Ni-Containing Fe Alloys: Characterization and 2D Gas Chromatographic Product Analysis

Transition metal alloys are receiving considerable attention in heterogeneous catalysis as they hold promise to combine advantageous properties of the constituting metals and, therefore, provide attractive avenues for targeted catalyst design. The present study concerns the effect of Co and Ni substituents in the ferrite (Fe3O4) structure used as a catalyst precursor for medium-temperature Fischer-Tropsch (MTFT) synthesis, in anticipation of enhanced oxygenate selectivities. The ferrites were synthesized by co-precipitation and characterized in detail before and after exposure to MTFT conditions, employing both conventional ex situ and state of the art in situ techniques. The complex product spectrum from the MTFT was analyzed by combining off-line one-dimensional and on-line two-dimensional gas chromatography. The latter was used specifically to investigate the formation of minority species, such as oxygenates, which are often disregarded in literature. In situ XRD and magnetometry showed no notable change in the reduction behavior of the ferrites with a cobalt substituent, but substituting with Ni decreased the reduction temperature drastically from 315 to 250 degrees C, most likely due to the increased hydrogen dissociation activity of Ni. The activity, CO conversion, in MTFT increased in the order Fe << CoFe < NiFe < CoNiFe. Incorporation of Co and Ni in the catalysts makes them less prone to deposition of inactive carbon. The addition of Ni specifically, also results in a significant shift in selectivity toward a shorter average chain length, lower olefinicity and higher water-gas shift activity. Interestingly, these shifts are paralleled by a 76% or 170% increase in C2+ oxygenates selectivity or yield, respectively. The increase in hydrogenation activity of substituted (i.e., Co and/or Ni) Fe-based catalysts, plays a critical role in the Fischer-Tropsch synthesis activity and selectivity to the different product classes (i.e., paraffins, olefins, and oxygenates) and the findings reported here provide valuable insights of key importance for further development and optimization of FT catalysts.