CO₂hydrogenation into long-chain hydrocarbons offers a potential contribution towards achieving a sustainable carbon cycle. The reverse water gas shift (RWGS) process converts CO₂and H₂to CO and H₂O, enabling the use of CO as a carbon feedstock by utilizing existing syngas (CO and H₂) conversion technologies. Most RWGS processes operate at high temperatures (>600 °C) and ambient pressure due to favorable thermodynamics, whereas lower temperatures and higher pressures are preferred for subsequent syngas conversion via Fischer–Tropsch synthesis (FTS). H₂O is an inherent by-product of both processes with highly oxidizing properties and may influence the catalytic performance. This study investigates the effects of hydrophobic and hydrophilic carbon-supported Fe-based catalysts on RWGS and FTS. HNO₃reflux treatment of the pristine hydrophobic carbon support is performed to introduce hydrophilicity. The overall hydrophilicity of the catalysts depends on both the carbon support and the Fe loading, as Fe-based nanoparticles also exhibit hydrophilic characteristics. H₂O vapor sorption and contact angle measurements are employed to assess the catalysts’ H₂O affinity, which is linked to the catalytic properties, giving consistent results. Catalytic performance is evaluated at 300 °C, 11 bar, H₂/CO₂/Ar = 3/1/1, 600–500 000 mL g_cat⁻¹h⁻¹. RWGS is investigated at CO₂conversions below the equilibrium limit of 23%, and the more hydrophobic catalyst exhibits higher activity and CO selectivity compared to the more hydrophilic catalysts. Notably, the Sabatier reaction emerges as a competing pathway for 5 wt% Fe-based catalysts supported on more hydrophilic carbon. This higher CO₂methanation is likely facilitated by hydrogen transfer from the carbon support, and it can be suppressed by larger Fe nanoparticle size and higher Fe loading. No significant influence of support hydrophilicity on either the RWGS or FTS reactions is observed for 20 wt% Fe/C catalysts, likely due to their overall hydrophilic nature resulting from the high Fe loading.

CO₂ hydrogenation to chemicals and fuels has the potential to alleviate CO₂ emissions and displace fossil resources simultaneously via consecutive RWGS and FTS reactions, also known as CO₂-FTS. As Fe-based catalysts are active and selective for both reactions, their bifunctionality requires a delicate balance between the RWGS and FTS. In this work, we investigated the thermodynamic constraints of RWGS and CO₂-FTS, the influence of CO₂ conversion on selectivity and the influence of Fe nanoparticle size within the range of 4.7 to 10.3 nm. An inert carbon support was selected to rule out metal-support interaction and promoting effects of the support. Catalytic performance was evaluated at 300 °C, 11 bar, H₂/CO₂/Ar = 3/1/1, 600 to 72000 mL·g_cat^-1·h^-1. At a CO₂ conversion level below RWGS equilibrium conversion of 23 %, RWGS was found to be the primary and dominant reaction. No primary Sabatier reaction was observed. At higher CO₂ conversion till the CO₂-FTS threshold of 42 %, the secondary FTS reaction became dominant. Notably, a non-linear relation between CO₂ conversion and CO selectivity was discovered. Comparing two catalysts with identical 5 wt% Fe loading but different average Fe nanoparticle size (6.6 and 8.4 nm), the 8.4 nm Fe catalyst was at least two times more active than the 6.6 nm Fe catalyst. In situ Mössbauer spectroscopy suggested a positive correlation between particle size, carburization and selectivity towards long-chain hydrocarbons. For these potassium-promoted carbon-supported Fe-based catalysts, nanoparticles of at least 8 nm are required for the formation of Fe carbides and improved reactivity.