Investigation of promoter effects of manganese oxide on carbon nanofiber-supported cobalt catalysts for Fischer-Tropsch synthesis

The effects of the addition of MnO were studied on a carbon nanofiber-supported cobalt catalyst. The starting sample, cobalt loading 9.5 wt% and 8% cobalt dispersion, was promoted by impregnation with small amounts of MnO (0.03, 0.1, 0.3, 0.6, and 1.1 wt%). XPS and STEM-EELS showed MnO to be associated with Co in both dried and reduced catalyst. In the drying step, MnO was deposited on the passivated cobalt particles, because of the tendency of both metals to form stable mixed compounds. After reduction, the MnO remained close to the cobalt particles, because the support material lacked sites with significant interaction with MnO. The promoter suppressed both the hydrogen chemisorption uptake and the cobalt reducibility even with the lowest MnO loading. At 1 bar, large improvements in the selectivity toward C5+ products (from 31 to 45 wt%) were found with MnO loadings of 0.3 wt% and higher. At 20 bar, the addition of only 0.03 wt% MnO improved the C5+ selectivity from 74 to 78 wt%, but larger amounts decreased the selectivity to 52 wt% at 1.1 wt% MnO. The surface-specific activity (TOF) first increased with MnO loading from 26 to 60 x 10(-3) s(-1) for 0.3 wt% MnO, whereas it decreased, probably as a consequence of excessive Co surface coverage at MnO loadings > 0.3 wt%. Product analysis (paraffin:olefin ratio) indicates that a major role of MnO involves moderation of hydrogenation reactions. (c) 2005 Elsevier Inc. All rights reserved.