The nature of K promotion of χ-Fe₅C₂ for high chain-growth probability in the Fischer–Tropsch reaction

Potassium (K) is widely used as a promoter in industrial Fe-based Fischer-Tropsch (FT) catalysts to enhance CO conversion and increase the chain-growth probability while decreasing CH₄ selectivity. We deployed density functional theory calculations and microkinetics simulations to elucidate the mechanistic role of K promotion on χ-Fe₅C₂ (Hägg carbide). The preferred state of K under reaction conditions, K₂O, on the reactive (010) surface strengthens the adsorption of CO, H, C, and CH. This results in faster C-O bond dissociation and slower hydrogenation reactions. The main FT reaction mechanism involves HCO dissociation, O removal as H₂O, CH+CR (R = alkyl group) coupling reactions, and termination by α-CH hydrogenation to olefins. Microkinetics were determined to study the impact of K on the FT reaction catalyzed by the χ-Fe₅C₂(010) surface. Irrespective of the presence of K, the FT reaction operates at the interface of chain growth-limiting and oxygen removal-limiting regimes, with CCH₂ hydrogenation and OH disproportionation controlling the overall CO conversion rate. On the unpromoted surface, CO hydrogenation and CHO dissociation control the chain-growth process and slightly inhibit the overall CO conversion rate. Simulations of the K-promoted surface demonstrate that K promotion removes the kinetic limitation of CHO dissociation in the chain-growth process. Overall, K₂O increases the chain-growth probability while decreasing the CH₄ selectivity because the impact of the promoter is stronger for CH₄ formation than the hydrogenation reactions of olefinic surface precursors. The main effect of K₂O on the intrinsic FT chemistry is electronic, involving electron transfer to Fe surface atoms. The main kinetic consequences are a lower overall barrier of CHO dissociation and higher barriers for the hydrogenation of carbon-containing surface intermediates.