Promotion effects of iron in carbon nanotubes synthesis to make supports for Fischer-Tropsch synthesis catalysts

摘要

Iron (Fe), catalysts were prepared by incipient wetness impregnation method using CaCO3 as audsupporter. 0.6% Cu, 0.4% K, 0.6% Cu/0.4% K, 0.1 ml of Tetraethyl orthosilane (TEOS) in 2mludwater were used as promoters for the Fe catalysts. The promoted catalysts were then employed inudthe synthesis of carbon nanotubes (CNTs) using a chemical vapor deposition (CVD) method.udTransmission electron microscopy (TEM) revealed that Cu increased the diameter of the CNTsudand caused them to be coiled, thereby altering the shape of the CNTs. The addition of K resultedudin no recognizable change in the microstructure of these tubes. The CNTs obtained lookedudsimilar to those obtained when Fe was used without a promoter. However, the diameter of theudtubes also increased due to K addition.udThermol Gravimetric Analysis (TGA) results showed that the addition of K to the Fe catalystud(Figure 20, p56, 57) resulted in the enhancement in thermal stability of the CNTs. BET analysisudrevealed that Cu increased the surface area while K decreased the surface area of this solidudmaterial. The Cu/K promoted catalyst produced CNTs with diameters of 60 nm which was theudsame as unpromoted catalyst. However, the CNT diameter distribution was quiet different for theudtwo catalysts. The surface area was less than that of the unpromoted catalyst. The Fe/Cu/Kudsynthesized CNTs were coiled and they looked more like the product produced from Fe/CuudCNTs. Thus the Cu catalyst has the dominant effect in determining the CNT morphology. WhenudSiO2 was used as a promoter no change in the diameter distribution of the CNTs could beuddetected. Thus while changes to the carbon surface may have occurred at the atomic level, theudchanges were not detected at the TEM resolution used. The surface area was also less than that ofudCNTs produced over the unpromoted catalyst.udBoth promoted and unpromoted Fe/CNT were tested for FT synthesis. The activity, selectivityudand CO conversions were recorded. The K was found to strongly influence the production of theudhydrocarbon yield and increased the CO conversion. The K promoted catalyst increased the COudreaction rate, and increased the olefinity and the alpha value. The effect of K on the olefinity ofudthe C2 hydrocarbon ranged from 0.04 to 0.26. An increase in FTS activity is also observed for theudK promoted catalyst. Cu decreased the reduction temperature of Fe oxides as noted by TPR studies. The Cu promoted catalyst showed a high selectivity to methane and a decrease of C5+udhydrocarbons, the C2 olefins also decreased.