Mn promoted Co catalysts for Fischer-Tropsch production of light olefins – An experimental and theoretical study

Abstract

Three different CoMn/gamma-Al2O3 catalysts were prepared by the incipient wetness impregnation route and compared to a Co/gamma-Al2O3 catalyst. The effect of co-impregnation vs. sequential impregnation as well as the order of component addition was investigated. All catalysts were characterised by TPR, H2-chemisorption, XRD and XPS and their activity and selectivity in the Fischer-Tropsch reaction was investigated. Complementary, self-consistent DFT calculations were performed to further address the observed promotion effects. All Mn promoted catalysts displayed heightened intrinsic activity, heightened selectivity to light olefins and C5+ species and lowered selectivity to CH4 compared to Co. The promotion effects on selectivity and intrinsic activity were found to be independent on catalyst preparation method. The catalysts undergo a restructuring during operation, in which an excess of Mn saturates the catalytically relevant sites causing the similar behaviour. The Co-specific activity differed between the Mn promoted catalysts. This was attributed to varying degrees of Mn incorporation in the Co3O4 particles, causing different degrees of reduction limiting the available metallic Co surface area. The DFT calculations suggested that the binding energy for all investigated species increases on Co in the presence of Mn, facilitating CO dissociation which can explain the higher intrinsic activity. The affected selectivities for olefins, C5+ and CH4 can all be attributed to an inhibited hydrogenation activity demonstrated by the increased barriers for CH3 and CH4 formation.