Nonetheless, the influence of alkali metal and its counter-ion has not been considered for Co-based catalysts. S is generally perceived to be a poison for Co-based catalysts in terms of activity and selectivity towards long-chain hydrocarbons (C 5+) 30, however it was also shown to decrease chain growth probability and improve olefins selectivity depending on its concentration 31, 32, 33. ![]() The importance of counter-ions to alkali metal promoters was demonstrated previously for Fe-based catalysts 26, particularly the combination of Na and S was found to give a synergistic effect 27, 28, 29. These alkali metals including Na or K, exist as oxides Na 2O or K 2O during catalysis, yet the oxygen counter-ion was often overlooked. Besides acting as structural promoters, alkali metals were established to decrease activity for metallic Co-based catalysts and it was proposed to be correlated to the element electronegativity 24, 25. Adding alkali promoters to Co/MnO catalysts stimulates formation of Co-carbide, which inhibits methane, but promotes CO 2 production 23. Till now, Co-based catalysts for the direct conversion of syngas to lower olefins focused on MnO as promoter, but the product spectrum was still dictated by the ASF distribution 18, 19, 20, 21, 22. The direct production of lower olefins from H 2-rich syngas is advocated, but this poses two challenges, specifically the suppression of methane and of CO 2 formation during FTS. ![]() Metallic Co-based catalysts are used commercially for the gas-to-liquids process in which long-chain saturated hydrocarbon products are produced that are subsequently cracked to valuable transportation fuels in particular kerosene and diesel 14, 15, 16, 17. To be active for FTS but not for WGS, Co has to be in the metallic state during catalysis. in their development of phase pure, stable and low-CO 2 selective ε-iron carbide FT catalysts for the coal-to-liquids process 13. The importance of decreasing CO 2 production during the FT step was recently highlighted by Wang et al. Similarly, the bifunctional oxide-zeolite catalysts, which convert syngas directly to lower olefins, showed high activity for WGS and are thus only suitable for CO-rich syngas 11, 12. However, most carbide-based catalysts are also active for the water-gas-shift (WGS) reaction 10, thereby producing CO 2 and rendering them inefficient for methane-derived H 2-rich syngas. Deviation of the ASF distribution to suppress methane formation is critical to attain high fractions of lower olefins (ethylene, propylene, and butylenes), and this is possible with promoted Fe-carbide-based 5, 6, 7 and promoted Co-carbide-based catalysts 8, 9. FTS is a surface polymerization reaction so the product selectivity is governed by the Anderson–Schulz–Flory (ASF) distribution 4. ![]() Methane may be converted to synthesis gas (syngas, a mixture of H 2 and CO), which can then be used to produce chemicals and fuels via the Fischer-Tropsch synthesis (FTS) process 3. The abundant availability of methane feedstock due to the shale gas revolution decreases the dependence on crude oil, however new technologies have to be developed to utilize its potential 1, 2.
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