Mechanism of the Mn Promoter via CoMn Spinel for Morphology Control: Formation of Co₂C Nanoprisms for Fischer–Tropsch to Olefins Reaction

Abstract

The Fischer–Tropsch to olefins (FTO) reaction over Co2C catalysts is structure-sensitive, as the catalytic performance is strongly influenced by the surface structure of the active phase. The exposed facets determine the surface structure, and it remains a great challenge to precisely control the particle morphology of the FTO active phase. In this study, the controlling effect of the Mn promoter on the final morphology of the Co2C nanoparticles for the FTO reaction was investigated. The unpromoted catalyst and several promoted catalysts with Ce, La, and Al were also studied for comparison. For the Mn-promoted catalysts, the combination method of the Co and Mn components plays a crucial role in the final morphology of Co2C and thus the catalytic performance. For the CoMn catalyst prepared by coprecipitation, Co2C nanoprisms with specifically exposed facets of (101) and (020) can be obtained, which exhibit a promising FTO catalytic performance with high C2–4 = selectivity, low methane selectivity, and high activity under mild reaction conditions. However, for the Mn/Co catalyst prepared via impregnation, Co2C nanospheres are formed, which exhibit high methane selectivity, low C2–4 = selectivity, and low activity. For the unpromoted catalyst and the catalysts promoted by Ce and La, Co2C nanospheres are also obtained, with catalytic performance similar to that of the Mn/Co catalyst prepared via impregnation. Due to the high stability of the Co2AlO x composite oxide, no Co2C phase can be formed for the catalyst promoted by Al.

Tuning the Facet Proportion of Co2C Nanoprisms for Fischer‐Tropsch Synthesis to Olefins

“…For the reduced Co 3 O 4 catalyst without Mn addition, the peaks were mainly attributed to Co 0 and CoO (Figure a). Besides, the results for the spent Co 3 O 4 sample revealed that only Co 2 C phase at 2θ of 37.0°, 41.3°, 42.6°, 45.7° and 56.6° could be detected (Figure b), suggesting that Co species were all transformed into Co 2 C. However, all the reduced CoMn samples showed the characteristic peak of Co x Mn 1‐x O phase (Figure a), which was the precursor for the formation of Co 2 C nanoprisms . For the spent Co3Mn1, Co2Mn1 and Co1Mn1 catalysts, both MnCO 3 and Co 2 C phase co‐existed.…”

“…As for pure Co 3 O 4 , Co 0 and CoO phases were observed after reduction, while Co x Mn 1‐x O could be obtained after introducing Mn into Co‐based catalysts. Our previous work has already demonstrated that the Co x Mn 1‐x O spinel phase played a vital role for the formation of Co 2 C nanoprisms with exposed (101) and (020) facet . The absence of Mn promoter for Co 3 O 4 caused the formation of Co 2 C nanospheres with (111) as dominant facet via carburization of Co 0 and CoO route, which restrained the CO dissociation and favored the production of methane.…”

“…Based on theory study, HCP-Co was belonged to the D 3h point group and it exhibited a dihedral-like shape with two closed packed (0001) facets, while FCC-Co had an octahedron-like shape composed of (100), (110), (310) and (111) facets. In addition, the reaction rates for (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21), (10)(11), (10)(11)(12), and (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) facets of HCP-Co were higher than that for the exposed facets of FCC-Co. [5] During the FT reaction process, many products including paraffins, olefins and oxygenates are generated simultaneously. For the traditional FT catalysts, paraffins are the main products with ASF distribution.…”

“…Furthermore, we found that Na promoter could improve the formation rate of Co 2 C . In addition, it was found that Mn promoter was important in modifying the active metal properties, and the addition of Mn could increase the reaction rate and enhance the olefins selectivity and olefin/paraffin (O/P) ratio . We also investigated the effect of the Mn promoter and found that Mn could combine with Co element to form Co x Mn 1‐x O spinel, which was the essential precursor for the formation of Co 2 C nanoprisms under FTO reaction conditions .…”

Cobalt‐Iron‐Manganese Catalysts for the Conversion of End‐of‐Life‐Tire‐Derived Syngas into Light Terminal Olefins

“…[67] These authors found that MnO x affects the formation of Co 2 C, ap hase which has been recently shown to have ah igh selectivity towards lower olefins. [68] In our current work, it was furtherf ound that by doping a 5Co5Fe2.5Mn/g-Al 2 O 3 material with additional 1.2 wt.% of Na and 0.03 wt.% of S, the selectivity towards C 4 olefins could be significantly improved, whereas, compared to the Na/S-free catalysts, C 5 + and CH 4 fractionsa re reduced at ac omparable C 4 olefin productivity of 0.03 kg·kg cat À1 ·h À1 .F urthermore, the Na/S ratio has been identified as af actor influencing the isomerization of 1-alkenes. Thus, by adding Na as ap romoter,t he isomerization of 1-alkenes can be inhibited and the ratio of 1alkenes to internal and iso-alkenes is maintainede ven if Si s present.I no ther words, under S-rich conditions, the selectivity can be directed towards the target product 1-butene as the primary product in the C 4 olefin fraction and therefore the amount of waste streams that need to be recycled, treated or upgraded is reduced.…”