Theoretical calculation study on effects of H coverage on CO activation in Fischer-Tropsch synthesis catalysts
Fe-based and Co-based catalysts are the most ideal Fischer-Tropsch synthesis catalysts.With the rapid development of computer simulation technology in the field of catalysis,it is possible to deeply understand the microscopic reaction mechanism on the surface of Fe-based and Co-based catalysts.Theoretical calculation method was used to study the effects of surface structures of Fe-based(reaction conditions:temperature 320 ℃,pressure 1 MPa and n(H2):n(CO)=1:1)and Co-based(reaction conditions:temperature 240 ℃,pressure 1 MPa and n(H2)∶n(CO)=2∶1)Fischer-Tropsch synthesis catalysts with different H coverages on the activation and dissociation of CO.The thermodynamically preferred H-covered surface models of Fe-based and Co-based Fischer-Tropsch synthesis catalysts were determined through the linear relationships between the hydrogen chemical potential and the Gibbs free energy of hydrogen adsorption,which is the x-Fe5C2(510)surface with H coverage of 11/42 and HCP Co(0001)surface with H coverage of 24/36,respectively.The CO dissociation mechanism and corresponding reaction energy barriers on the clean surface and the H-covered surface of catalysts were studied,and it is found that H coverage can change the reaction pathways of CO dissociation and increase the dissociation energy barrier.Compared with the clean surfaces ofx-Fe5C2(510)and HCP Co(0001),the dissociation energy barriers of CO on H-covered x-Fe5C2(510)and HCP Co(0001)surfaces increase by 5.4％and 20.3％,respectively.The origin reasons for the effects of H coverage on CO activation and dissociation were explained by calculating the density of states and crystal orbitals Hamiltonian population.
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