Fischer-Tropsch synthesis on mono- and bimetallic Co and Fe catalysts supported on carbon nanotubes

An extensive study of Fischer-Tropsch synthesis (FTS) on carbon nanotubes (CNTs)-supported bimetallic cobalt/iron catalysts is reported. Up to 4 wt.% of iron is added to the 10 wt.% Co/CNT catalyst by co-impregnation. The physico-chemical properties, FTS activity and selectivity of the bimetallic catalysts were analyzed and compared with those of 10 wt.% monometallic cobalt and iron catalysts at similar operating conditions (H-2/CO = 2:1 molar ratio, P=2 MPa and T= 220 degrees C). The metal particles were distributed inside the tubes and the rest on the outer surface of the CNTs. For iron loadings higher than 2 wt.%, Co-Fe alloy was revealed by X-ray diffraction (XRD) techniques. 0.5 wt.% of Fe enhanced the reducibility and dispersion of the cobalt catalyst by 19 and 32.8%, respectively. Among the catalysts studied, cobalt catalyst with 0.5% Fe showed the highest FTS reaction rate and percentage CO conversion. The monometallic iron catalyst showed the minimum FTS and maximum water-gas shift (WGS) rates. The monometallic cobalt catalyst exhibited high selectivity (85.1%) toward C-5+ liquid hydrocarbons, while addition of small amounts of iron did not significantly change the product selectivity. Monometallic iron catalyst showed the lowest selectivity for 46.7% to C-5+ hydrocarbons. The olefin to paraffin ratio in the FTS products increased with the addition of iron, and monometallic iron catalyst exhibited maximum olefin to paraffin ratio of 1.95. The bimetallic Co-Fe/CNT catalysts proved to be attractive in terms of alcohol formation. The introduction of 4 wt.% iron in the cobalt catalyst increased the alcohol selectivity from 2.3 to 26.3%. The Co-Fe alloys appear to be responsible for the high selectivity toward alcohol formation. (C) 2009 Published by Elsevier B.V.