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.