Mechanistic Insight into Hydrocarbon Synthesis via CO₂Hydrogenation on χ-Fe₅C₂Catalysts

Converting CO₂into value-added chemicals and fuels is one of the promising approaches to alleviate CO₂emissions, reduce the dependence on nonrenewable energy resources, and minimize the negative environmental effect of fossil fuels. This work used density functional theory (DFT) calculations combined with microkinetic modeling to provide fundamental insight into the mechanisms of CO₂hydrogenation to hydrocarbons over the iron carbide catalyst, with a focus on understanding the energetically favorable pathways and kinetic controlling factors for selective hydrocarbon production. The crystal orbital Hamiltonian population analysis demonstrated that the transition states associated with O-H bond formation steps within the path are less stable than those of C-H bond formation, accounting for the observed higher barriers in O-H bond formation from DFT. Energetically favorable pathways for CO₂hydrogenation to CH₄and C₂H₄products were identified which go through an HCOO intermediate, while the CH∗ species was found to be the key C₁intermediate over χ-Fe₅C₂(510). The microkinetic modeling results showed that the relative selectivity to CH₄is higher than C₂H₄in CO₂hydrogenation, but the trend is opposite under CO hydrogenation conditions. The major impact on C₂hydrocarbon production is attributed to the high surface coverage of O∗ from CO₂conversion, which occupies crucial active sites and impedes C-C couplings to C₂species over χ-Fe₅C₂(510). The coexistence of iron oxide and carbide phases was proposed and the interfacial sites created between the two phases impact CO₂surface chemistry. Adding potassium into the Fe₅C₂catalyst accelerates O∗ removal from the carbide surface, enhances the stability of the iron carbide catalyst, thus, promotes C-C couplings to hydrocarbons.