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

Abstract

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 (Fe 3 O 4 ) 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 °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 C 2+ 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.

Magnesium as a Methanation Suppressor for Iron- and Cobalt-Based Oxide Catalysts during the Preferential Oxidation of Carbon Monoxide

“…One of the advantages of targeting the spinel structure is the possibility of achieving high loadings of the dopant in the final catalyst [39]. Moreover, the uniform distribution of Mg in the catalysts may result in an optimized effect of Mg during CO-PrOx due to its close proximity to Fe or Co [33,39].…”

“…One of the advantages of targeting the spinel structure is the possibility of achieving high loadings of the dopant in the final catalyst [39]. Moreover, the uniform distribution of Mg in the catalysts may result in an optimized effect of Mg during CO-PrOx due to its close proximity to Fe or Co [33,39]. The CO-PrOx reaction is carried out under a reducing environment due to the high concentration of H2 (typically between 40 and 75 vol.%) [5,6,40,41].…”

“…The unsubstituted and Mg-substituted Fe-and Co-based oxide catalysts were synthesised via a co-precipitation method adopted from Fadlalla et al [33]. In the case of MgFe 2 O 4 , a magnesium nitrate solution (Mg(NO 3 ) 2 , Merck, (Darmstadt, Germany) 99%, Johannesburg, South Africa) and iron(III) nitrate solution (Fe(NO 3 ) 3 .9H 2 O, Merck, ≥98%, Johannesburg, South Africa) were mixed in a Fe:Mg molar ratio of 2.…”

“…The spinel structure allows for tailoring the properties of the final catalyst (based on the selection of the cation(s) in the A and B sites) and ensures uniform mixing of the cations in the spinel structure. Due to this, spinel-based catalysts are studied in different catalytic processes [32], including the Fischer-Tropsch synthesis (FTS) [33,34] and WGS [35]. In this study, we aimed at exclusively placing Mg in the A site of Fe-and Co-based oxide spinel structures to produce MgFe 2 O 4 and MgCo 2 O 4 , respectively, and thereafter study the effect of this inclusion on the physicochemical properties and CO-PrOx performance (under model/dry conditions) of the resulting Fe-and Co-based oxide catalysts.…”

MOF-Derived Porous Carbon-Supported Bimetallic Fischer–Tropsch Synthesis Catalysts

“…Hydrocarbon selectivity can be optimized by adjusting the geometric and electronic properties of the FTS catalyst. − In recent years, various types of strategies have been adopted to cope up with the defined challenges, such as core–shell structure, promoters, and supports . Bimetallic catalysts with an alloy or spinel structure were also studied by some other researchers. − Most research conducted on Fe and Co and a proper percentage of bimetallic catalysts showed enhanced activity compared to the monometallic Fe and Co catalysts . However, some other research shows reverse results .…”