Insight into CHx formation in Fischer–Tropsch synthesis on the hexahedron Co catalyst: Effect of surface structure on the preferential mechanism and existence form

Abstract

Spin-polarized density functional theory calculations have been performed to investigate the preferential mechanism of CHx(x=1–3) formation in Fischer-Tropsch synthesis on only the hexahedron Co(101¯0)-A and Co(10 1¯0) surfaces. Our results show that CO hydrogenation to CHO is favored compared to CO direct dissociation and hydrogenation to COH on these two surfaces. Starting from C and CHO, we further seek out the optimal pathways of CHx formation, suggesting that CHx is mainly formed through H-assisted CO dissociation pathways on Co(101¯0)-A surface, in which CH is form via CHO dissociation, CH2 and CH3 are formed through CH2O with the direct and H-assisted dissociation, respectively; meanwhile, CH2 hydrogenation also contributes to CH3 formation; CH2 and CH3 are the surface abundant species on Co(101¯0)-A surface. However, on Co(101¯1) surface, CHx species is formed through CO direct dissociation into C, followed by C successive hydrogenation, C and CH are the surface abundant species. Therefore, Co surface structure can affect the preferential formation pathways and the dominant existence form of CHx species. Moreover, CH3OH formation cannot compete with CHx formation on Co(101¯0)-A and Co(101¯1) surfaces, considering both surfaces covering 63% of the total surface area exposed of hexahedron Co surfaces, which depends on the reaction conditions, particle size, catalyst support, carbon deposition and many other factors, the contribution to the overall CHx sources from Co(101¯0)-A and Co(101¯1) surfaces even surpasses that of other hexahedron Co surfaces under the certain realistic conditions. As a result, the hexahedron Co surfaces exhibit high catalytic selectivity for CHx formation, and provide more CHx sources to participate into the F-T synthesis.