Investigating the CO activation mechanism on hcp-Fe7C3 (211) via density functional theory

Theoretically Predicted CO Adsorption and Activation on the Co-Doped hcp-Fe7C3 Catalyst

The Hcp-Fe7C3 phase has attracted more attention due to the high catalytic activity in Fischer–Tropsch synthesis (FTS) reactions. In this work, the adsorption and activation of CO on a Co-doped hcp-Fe7C3 catalyst were investigated by density functional theory (DFT) in order to understand the effect of Co doping on the initial step of FTS reactions on iron-based catalysts. Different Co-doped hcp-Fe7C3 001 and 11¯0 surfaces were constructed, and the CO adsorption configurations were studied. The calculated results show that the structure of the 001 surface remains basically unchanged after doping with Co atoms, while the replacement of Fe or C atoms on 11¯0 surfaces with Co atoms has a significant impact on the surface structure. The top sites on the doped Co atoms of hcp-Fe7C3 are disadvantages for the CO adsorption, whereas the T, 2F, or 3F sites around the doped Co atoms are beneficial for promoting the adsorption of CO. The CO direct dissociation pathways on the four types of Co-doped hcp-Fe7C3 001 surfaces are exothermic, while the H-assisted dissociation pathways of CO are endothermic. The H-assisted activation via HCO on the 3F1 site of the 2Co2-doped hcp-Fe7C3 001 surface shows the lowest energy barrier of 1.96 eV. For the Co-doped hcp-Fe7C3 11¯0 surfaces, the H-assisted activation via HCO is the preferred activation pathway for CO on the Co-doped surfaces with the energy barrier of approximately 1.30 eV.

Insight into the Fischer–Tropsch mechanism on hcp-Fe7C3 (211) by density functional theory: the roles of surface carbon and vacancies

Iron carbide phases discovered in the spent iron catalysts have proved to be active in the Fischer–Tropsch process. The surface carbon of the iron carbide played a key role in the Fischer–Tropsch mechanism. Since there are two surface carbons, C1 and C2, on the hcp-Fe₇C₃ (211), which are close to each other, their reaction mechanisms would be significant. Hence, the DFT calculations were performed to investigate the Fischer–Tropsch mechanism involving the surface carbon. It was found that the HC¹ + C² pathway was the major C–C coupling reaction pathway with an effective energy barrier of 0.97 eV. Ethane would be the major C₂ product from the HC¹C2 species through the stepwise hydrogenation pathway due to the high adsorption energy of ethylene (1.67 eV). After the desorption process of ethane, the carbon vacancy would form. The carbon vacancy was found to be the CO activation site through the CO direct dissociation pathway and the carbon vacancy would recover. It was concluded that the defect-hcp-Fe₇C₃ (211) is the high active facet of the Fischer–Tropsch synthesis, the carbon vacancy sites are the CO activation sites and the surface carbon sites are the C–C coupling sites. The surface carbons not only act as the chain initiation sites but also act as the chain growth sites in the Fischer–Tropsch mechanism on hcp-Fe₇C₃ (211).

The Fischer–Tropsch mechanism on hcp-Fe₇C₃ (211): the circulation of the surface carbon.