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 + C-2 pathway was the major C-C coupling reaction pathway with an effective energy barrier of 0.97 eV. Ethane would be the major C-2 product from the HC(1)C2 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).