Mn promotion effects in Co based Fischer-Tropsch production of light olefins
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In this work, Mn promotion of γ-Al2O3 supported Co catalysts was investigated for the purpose of producing light olefins via the Fischer-Tropsch synthesis process. CoMn/γ-Al2O3 catalysts were prepared by the incipient wetness impregnation route and compared to a Co/γ-Al2O3 catalyst. The effect of co-impregnation vs. sequential impregnation as well as the order of component addition was investigated. Physical characterisation was done by multiple methods, including TPR, XRD, chemisorption, SSITKA, XPS and in situ XRD/XANES. Catalyst activity and selectivity were investigated at Fischer-Tropsch conditions favouring light olefin formation. Moreover, joint noble metal and Mn promotion was investigated and finally, complementary, self-consistent DFT calculations were performed to further address the observed promotion effects. It was found that all Mn promoted catalysts displayed heightened intrinsic activity, heightened selectivity to light olefins and C5+ species as well as lowered selectivity to CH4 compared to unpromoted Co. The promotion effects on selectivity and intrinsic activity were found to be independent on catalyst preparation method. Depending on preparation method, Mn was shown to negatively affect the reducibility of Co by incorporating in the Co3O4 lattice. Physical characterisation revealed a complex, highly dynamic catalyst-promoter-support system. With in situ XANES-TPR, Mn was found to be reduced faster than Co independently of preparation method, suggesting an oxygen transfer from Mn to Co during the reduction process. The catalysts’ surface compositions were found with XPS to readily change with operating conditions. The surface concentrations of Mn relative to Co correlated with the reduction of Co’s oxidation state, indicating either a segregation of Mn and Co upon reduction, or an enrichment of Mn near the top of Co particles. The results indicate Mn to exist in two categories during operation, as MnO in close contact with Co which provides the promotion effects and also as non-contributing species. A saturated capacity for the former category may account for the promotion effects’ observed independence on catalyst preparation method, and as indicated by in situ XRD, large MnO particles and Co3−xMnxO4-type species may account for the latter. Joint promotion of noble metals and Mn was found to improve on the negative effect on Co reducibility observed with Mn promotion without compromising the positive effects on selectivity. Initial theoretical investigations predicted that Mn promotion of Co has a stabilizing effect on the adsorption of CO, C, O, H and CHx on Co. This in turn decreased the CO dissociation barrier, which presents a potential explanation for the experimentally observed increased intrinsic activity. Moreover, CH4 formation was found to be less energetically favourable with Mn addition, which would explain the observed lowered selectivity to CH4. Finally, Mn was found to give a net stabilising effect on olefin and paraffin precursors, predicting an increased CHx:H surface coverage ratio, something which was also observed experimentally with SSITKA. This possibly explains the observed increased selectivity towards olefins and C5+ species. Further theoretical investigations suggest that the Co-MnO interface is of particular catalytic importance. It was found that MnO exhibited attractive forces between Mn and species incorporating C and/or O. The binding energy CO was increased near the MnO in the adsorbed state due to attractive forced between Mn and the O of CO leading to CO adsorbing in a tilted fashion towards Mn. The MnO was further found to have a profound effect on CO dissociation, stabilising the transition state by approximately 0.4 eV by attracting the O atom. Finally, repulsive forces were observed between Mn and species containing H, lowering the binding energies of H and CHx near the interface. The decreased adsorption of H could explain the lowered hydrogenation activity observed experimentally, but calculations on selectivity effects requires further study.