Sustainable Fuels from CO₂-Rich Synthesis Gas via Fischer-Tropsch Technology

CO₂-containing synthesis gas is a relevant feedstock for the production of synthetic fuels using Fischer-Tropsch synthesis (FTS). We report the role of CO₂ in CO₂, CO, and H₂ mixed feeds over a cobalt-based catalyst at 220 °C and 21 bar in a packed bed reactor and define the process boundary conditions where CO₂ switches from an inert to a reactive gas in FTS. The C₅₊ selectivity remains above 78% even for CO₂-rich synthesis gas with 75% CO₂/(CO + CO₂). Using ¹³CO₂ isotopic labeling, the increase in methane selectivity is attributed to both CO and CO₂ methanation, which is limited by maintaining a H₂/CO outlet ratio below 10 and an outlet CO partial pressure above 0.2 bar, respectively. CO and CO₂ are proposed to adsorb on the same Co sites, and with CO adsorption being more favorable, we postulate that sufficient CO prevents CO₂ adsorption and reaction. These results overturn the prevailing assumption that C₅₊ selectivity is dictated by the CO₂ partial pressure or the CO₂/CO ratios. In situ modulated diffuse reflectance infrared Fourier transform spectroscopy confirms a positive relationship between CO surface coverage and CO partial pressure. From DFT and microkinetic modeling, enhanced CO and CO₂ methanation could be attributed to a lower surface coverage and a higher H₂ surface coverage. This work identifies boundaries for efficient cobalt-catalyzed mixed-feed FTS for synthetic fuel production.