Carbon nanotube-supported Fe-Mn nanoparticles: A model catalyst for direct conversion of syngas to lower olefins

Carbon nanotube (CNT)-supported monodisperse Fe3-xMnxO4 (x = 0-0.5) nanoparticles in which Mn has an intimate contact with Fe were synthesized and used as a model catalyst for investigating the promotion effect of Mn oxide on the iron-based catalysts for Fischer-Tropsch synthesis (FTS) reaction. It was found that incorporation of Mn oxide (Mn/Fe = 0.024-0.2) into a Fe3O4/CNT catalyst promoted the reduction of Fe3O4 to FeO, but retarded the further reduction of FeO to metallic Fe. Incorporation of small amount of Mn (Mn/Fe <= 0.01) into the iron catalyst results in an increase in C5+ yield and C-2-C-4 olefin selectivity without any loss in FTS activity. However, the total selectivity of C-2-C-4 hydrocarbons is almost not affected by the addition of Mn oxide. An excess of Mn oxide in the catalysts (Mn/Fe > 0.024) can lead to a significant decrease in FTS activity with no further improvement in C-2-C-4 olefin selectivity. The results of temperature-dependent XRD study on the Fe3O4/CNT and Fe2.73Mn0.27O4/CNT catalysts under H-2/CO = 1 mixture suggest that the decreasing in FTS activity on the Fe3-xMnxO4/CNT catalysts with an excess of Mn may have resulted from that the rate of carburization of metallic Fe is retarded by the Mn oxide species. (C) 2013 Elsevier B.V. All rights reserved.