Formation and Role of Iron-Containing Phases in CO₂ Hydrogenation to Higher Hydrocarbons

The hydrogenation of CO2 to higher hydrocarbons (C2+-hydrocarbons) over Fe-based catalysts represents a promising strategy for CO2 valorization. An in-depth understanding of the restructuring of Fe-containing phases under reaction conditions and their role in product formation is necessary to enable tailored catalyst design. To this end, we introduce a simple preparation method where Fe nanoparticles (NP) are simply mixed physically with Fe3O4 and/or a potassium-based promoter. To establish the direct influence of metallic Fe, Fe3O4 and/or a promoter on the catalyst restructuring and performance, the as-prepared physical mixtures are used in steady-state and time-resolved kinetic tests without any pretreatment. Combined with complementary in situ characterization methods, we demonstrate that Fe3O4 is not a necessary component in the initial catalyst or in the surface/bulk composition of the working catalyst to ensure efficient CO2 conversion to CO and subsequently C2+-hydrocarbons formation. Instead, metallic Fe reduces CO2 to gas-phase/surface CO species. Therefrom formed surface CO and/or C species should contribute to FeCx formation besides Fe carburization with the aid of gas-phase CO. This process is enhanced in the presence of a K-containing promoter. Its effectiveness depends on the ability of K species to migrate under reaction conditions, which is related to the type of K precursor. The catalysts based on physical mixtures of metallic Fe NP and K2CO3 developed in this study outperform the majority of previous catalysts in terms of higher hydrocarbon and C2–C4 olefin yield, yet suppressed CH4 formation. Thus, our strategy and results provide the basis for the development of efficient CO2 hydrogenation catalysts.