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.