Hydrophobic and Hydrophilic Carbon Supports for Iron-based CO2 Hydrogenation Catalysts: Impact on High-Pressure Low-Temperature Reverse Water Gas Shift and Fischer-Tropsch Synthesis

Abstract

CO2 hydrogenation into long-chain hydrocarbons offers a potential contribution towards achieving a sustainable carbon cycle. The reverse water gas shift (RWGS) process converts CO2 and H2 to 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. HNO3 reflux 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 - 500000 mL·gcat-1·h-1. RWGS is investigated at CO2 conversions 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 CO2 methanation 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.