Beyond Promotion: Effects of Sodium on the Structural Evolution and Deactivation of Unsupported Iron(III) Oxide Catalysts during CO₂–Based Fischer–Tropsch Synthesis

The industrial production of liquid hydrocarbons through CO2-based Fischer–Tropsch synthesis is primarily hindered by the insufficient long-term stability of the corresponding Fe catalysts. To address this problem, we herein systematically investigated the role of Na promotion in the structural evolution and deactivation mechanisms of unsupported Fe2O3 catalysts during CO2 hydrogenation. Na–Fe2O3 catalysts with Na contents of 0–20 wt % were evaluated under industrially relevant conditions at times-on-stream of up to 1000 h, and the Na content was found to influence phase transformations, particle morphology, and carbon deposition. At optimal contents (5–10 wt %), Na promoted the formation and retention of the active χ-Fe5C2 phase while suppressing oxidation and particle pulverization. Unpromoted and low-Na catalysts suffered from severe deactivation due to oxidation and carbon buildup, whereas excess Na (20 wt %) suppressed coke formation but accelerated Fe carbide reoxidation due to nanoscale domain restructuring. This work demonstrates that Na not only acts as an electronic promoter in Fe catalysts but also modulates their spatiotemporal phase stability and surface reactivity, thereby enhancing durability.