Structure-activity relationship and deactivation behavior of iron oxide during CO₂ hydrogenation

Identifying the dynamic structural evolution of iron during the thermocatalytic conversion of CO₂ into liquid hydrocarbons is a promising approach for understanding the deactivation behavior and to design an efficient catalyst. Despite the understanding of the oxidation of χ-Fe₅C₂ to high-valent iron oxides (e.g., Fe₃O₄, Fe₂O₃) caused by water formed as the byproduct, the deactivation mechanism depending on the particle size during a long-term reaction remains unclear. Herein, the structural evolution and deactivation mechanism of Na-promoted Fe₂O₃ catalysts with varying particle sizes were investigated. The catalyst activity and deactivation behavior were highly dependent on the initial morphology of the calcined catalysts. The reduced iron catalyst with an average particle size (d_ave) of 0.52 μm maintained its domain integrity, whereas that with a d_ave value of 2.18 μm was severely pulverized during the CO₂ hydrogenation. The pulverized particles exposed a new surface, making it highly susceptible to re-oxidation and catalyst deactivation. Conversely, maintenance of the iron integrity suppressed the re-oxidation, whereas the excess formation of graphitic carbon on the χ-Fe₅C₂ site was the main deactivation mechanism during long-term CO₂ hydrogenation.