CO2 formation mechanism in Fischer–Tropsch synthesis over iron-based catalysts: a combined experimental and theoretical study

Abstract

Fischer–Tropsch synthesis (FTS) is one of the most attractive routes to convert syngas (CO + H₂) into liquid fuels and high value-added chemicals. However, FTS over Fe-based catalysts generates and emits large amounts of CO₂, which reduces the carbon atom economy and causes huge greenhouse gas emission. In this work, CO₂ formation mechanisms in FTS over Fe-based catalysts were systematically investigated by combining experiments and DFT calculations, aiming to provide atomic-scale insights into the CO₂ formation process. Our results indicate that the Boudouard mechanism, in which the surface O* species formed by CO* dissociation reacts with another CO* to form CO₂, plays a predominant role in CO₂ formation on the active χ-Fe₅C₂ phase, while the hydrogenation of surface O* species to form H₂O is hindered. The existence of the Fe₃O₄ phase is favorable for the reverse water-gas shift (RWGS) reaction, leading to the decrease of CO₂ selectivity and increase of the amount of generated H₂O. The modification by the potassium promoter does not alter the predominant reaction pathway for CO₂ formation over Fe-based FTS catalysts and the Boudouard mechanism still plays the dominant role. The potassium promoter can increase CO₂ selectivity and decrease the amount of H₂O mainly through the following two ways: (1) potassium largely increases the proportion of the χ-Fe₅C₂ phase and thus increases the amount of active sites for the Boudouard reaction; (2) potassium leads to the disappearance of the Fe₃O₄ phase and thus suppresses the RWGS reaction. The electronic structures were systematically analyzed to shed light on the nature of the potassium effect. On the one hand, the potassium promoter makes the d-band center of the χ-Fe₅C₂(510) surface atoms shift toward the Fermi level, facilitating the back-donation of electrons from the χ-Fe₅C₂(510) surface to the adsorbed CO* antibonding orbital; on the other hand, the direct interaction between K₂O and adsorbed CO* weakens the C–O bond by decreasing its electron density, which also contributes to the promoted CO dissociation.