Synergistic enhancement of CO2 hydrogenation to C5+ hydrocarbons using mixed Fe5C2 and Na-Fe3O4 catalysts: Effects of oxide/carbide ratio, proximity, and reduction

Abstract

CO2 hydrogenation into sustainable chemical feedstocks and fuels is a pivotal focus of energy research. Fe-based catalysts have emerged as promising candidates due to their high efficiency in the CO2 hydrogenation toward C2–C4 olefins and C5+ hydrocarbons. Fe5C2 is known as the active phase of the CO2 hydrogenation. Therefore, it is imperative to discern the roles of active phases in product distribution. Accordingly, in the present study, Fe5C2 is synthesized separately and physically mixed with Na-Fe3O4. The effects of oxide/carbide ratio, the proximity of active phases, and reduction treatment are investigated. The results show that adding carbide to oxide in an appropriate ratio significantly increases CO2 conversion due to the controlled reduction of the oxide phase and enhanced CO2 adsorption. Additionally, a closer distance (mixed-powder pellets) between Na-Fe3O4 and Fe5C2 results in higher selectivity for C2-C4 olefins and C5+ hydrocarbons and less CH4. Furthermore, the graphitized layers of reduced catalyst (R-Na-Fe3O4/Fe5C2 (70/30 wt%)) enhance the selectivity to C2-C4 olefins (40 %) and C5+ hydrocarbons (35 %) at around 40 % CO2 conversion. In contrast, amorphous carbon in the non-reduced catalyst (NR-Na-Fe3O4/Fe5C2 (70/30 wt%)) favors more C2-C4 paraffins and CH4. Therefore, the interplay between the influential factors, i.e., oxide/carbide ratio, spatial distance, and reduction treatment, can result in tailoring an efficient CO2 hydrogenation catalyst toward the formation of valuable hydrocarbons.