Abstract
Carbon nanotubes (CNTs) were grown catalytically by a chemical vapor deposition
method and characterized by a range of techniques. Fe, Ru and Co catalysts supported on
the carbon nanotubes were prepared and investigated for their performances in the
Fischer-Tropsch synthesis.
CNTs were synthesized in a quartz tubular reactor at atmospheric pressure and at
temperatures of 700°C over iron supported on CaCO3 using C2H2 as carbon source. Prior
to CNT synthesis, the iron catalyst was first reduced under the same conditions (700°C
and atmospheric pressure) in a flow of 5% H2 balanced in Argon. The catalyst, for the
preparation of the CNTs, was prepared by the incipient wetness impregnation. The
purification of the CNTs was performed with 30 wt % HNO3. Characterization of the
CNTs using TEM, SEM, HRTEM, BET and TPR revealed that the crude product
contained solely CNTs, catalysts particles and support, while no amorphous carbon was
observed. The purified product is comprised of an interwoven matrix of tubes that were
shown to be multi-walled (MWCNTs).
CNT supported FT based catalysts were also prepared by an incipient wetness
impregnation method and tested in a plug flow reactor in Fischer-Tropsch synthesis. The
TEM images of the different FT catalysts supported on CNTs revealed that the catalyst
particles are well dispersed on the surface of the CNTs. The catalyst particles were very
iii
small, and some residual Fe catalyst material, not removed by the acid treatment, could
clearly be seen on the surface of the CNTs.
The reduction and metal dispersion properties of the catalysts were investigated through
TPR and chemisorption techniques. A TPR study showed three reduction steps for Co
catalysts, and addition of Ru to the catalyst decreased the reduction temperature of the
catalysts. Gasification of the CNTs was noted to occur at temperatures higher than
600°C.
The effect of metal catalyst loading and promoters on the activity and selectivity of CNT
supported FT synthesis catalysts was studied under condition of 275°C, 8 bar, CO/H2 =
1/2 and different flow rates. The FT catalysts supported on carbon nanotubes displayed a
high CO conversion and excellent stability with time on stream in the Fischer-Tropsch
synthesis. Fe catalysts displayed the lowest methane selectivity compared to all other FT
synthesis catalysts used in this study.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/4502 |
| Date | 29 February 2008 |
| Creators | Bahome, Munga Christian |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Format | 7858401 bytes, application/pdf, application/pdf |
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