ETD Collection

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Author: search.filters.author.Phaahlamohlaka, Tumelo Nathaniel

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    Fischer-Tropsch synthesis inside a nanoreactor
    (2017) Phaahlamohlaka, Tumelo Nathaniel
    Coal, biomass and natural gas are traditional energy carriers whose conversion via the Fischer-Tropsch process can be used to generate multiple hydrocarbon products such as fuels and fine chemicals. At the center of the Fischer-Tropsch process is the catalyst used for converting the syngas to hydrocarbons. Generally these catalysts using Co or Fe active sites are supported on high surface area inert materials such as silica and alumina. Our procedure in this thesis was to study some of the fundamental processes that affect Fischer-Tropsch catalysts (i.e. catalyst reduction, Ru as a reduction promoter and deactivation) using a so-called nanoreactor. This was done by loading metallic nanoparticles on either side of the nanoreactor surface. In this work hollow carbon sphere nanoreactors were mainly used as the catalyst support of choice to evaluate the processes mentioned above. First the effect of the hollow carbon sphere porosity on Fischer-Tropsch synthesis was evaluated using encapsulated Ru nanoparticles. Limited mass transfer limitations were observed on the mesoporous nanoreactor, thus suggesting the encapsulated nanoparticles were as accessible to reactants as the Ru nanoparticles loaded on the outside of the hollow spheres. Using mesoporous hollow carbon spheres the effect of hydrogen spillover on Co Fischer-Tropsch nanoparticles using a Ru promoter was evaluated by controlling the nanoparticle intimacy [of Ru and Co] by exploiting the hollow carbon sphere morphology. Primary hydrogen spillover was found to be more favorable in enhancing the Co nanoparticles extent of reduction and Fischer-Tropsch activity. However, secondary hydrogen spillover from the Ru nanoparticles to Co nanoparticles on the carbon shell was responsible for a complete reduction of the cobalt oxide when compared to an unpromoted Co Fischer-Tropsch catalyst on the hollow carbon spheres. It was also shown that the secondary hydrogen spillover led to the formation of highly hydrogenated products during Fischer-Tropsch synthesis. In terms of catalyst stability, the nanoparticles, by virtue of being embedded inside the carbon nanoreactor shell, showed good stability against sintering and re-oxidation by added water during process conditions. Furthermore a simple design of a highly sinter resistant catalyst is presented by making a compact nanoreactor on a titania supported Co Fischer-Tropsch catalyst. This study illustrates the benefit of rationally designing and characterizing materials for a comprehensive understanding of important catalytic reaction processes; in this case our focus was on the reduction behavior and stability of Fischer-Tropsch catalysts.
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    Synthesis of carbon nanofibers and their subsequent use as catalyst supports for Fischer-Tropsch synthesis
    (2014-07-07) Phaahlamohlaka, Tumelo Nathaniel
    In this study the synthesis and use of carbon nanofibers (CNFs) as catalysts supports for Fischer Tropsch synthesis is reported. The synthesis of carbon nanofibers with two distinct morphologies was optimized based on the reports in the literature that the straight (SCNF) and helical (CCNF) carbon nanofibers grow on Cu catalysts with different particle sizes. To selectively grow CNFs with a single morphology Cu catalysts were designed using different synthesis procedures (by using unsupported, coated and silica supported catalysts). The prepared copper oxide (CuO) nanoparticles were characterized by techniques such as TEM, XRD and nitrous oxide chemisorption. These techniques showed that the unsupported and coated CuO catalyst precursors has large particle sizes (range 100-300 nm) and thus had low Cu atomic surface area, while the supported CuO catalysts displayed low particles sizes in the nanoscale regime (<20 nm) and hence had high atomic surface area. Preparation of CNFs was carried out 300 using acetylene (C2H2) gas as the carbon source. Cu catalysts with large particle sizes resulted in straight CNFs and the small supported Cu nanoparticles grew helical CNFs because of the change in the nanoparticle surface energy during adsorption of the acetylene gas and the silica (SiO2) support effects that limited Cu nanoparticles from sintering (i.e. final particles size 60 nm). Soxhlet extraction proved to be an invaluable step in removing adsorbed polycyclic aromatic hydrocarbons. Because of the low thermal stability of these CNFs the materials were then annealed at higher temperatures ranging from 500-1400 in an inert environment (passing N2 gas). The helical CNFs snapped under high temperature annealing ( 900 ) resulting in shorter lengths in comparison to the straight CNFs. BET analysis of the annealed CNFs indicated that the CNFs annealed at 500 and 900 have increased surface area and have a mesoporous pore structure with the surface area ranging from 200-350 m2/g. Raman and Fourier transform IR spectroscopy indicated that the CNFs annealed at 500 and 900 , (which were the main material of interest because of their high surface area and thermal stability) had different hybridized carbon content. CNFs annealed at 500 contained both sp2 and sp3 hybridized carbon while annealing the CNFs at 900 resulted in a complete rehybridization of the carbon content to sp2. The carbon sp3 content in the CNFs annealed at 500 therefore implied that CNFs annealed at this temperature are more defective in comparison to the CNFs annealed at 900 . Since it is well known that material functionalities are affected by the amount of defects present inside the different CNFs were then applied as catalyst supports for Fischer Tropsch synthesis (FTS) to compare the support effects on cobalt active sites. The CNF surfaces were first modified by functionalization using concentrated HNO3 solution. The preparation of the catalyst systems was performed by a simple HDP method using urea. The CNFs and the FT catalysts were characterized using different techniques such as XRD, TEM, BET, TPR and Raman spectroscopy. Reactor studies performed at 220 (P = 8 bar, GHSV= 1200 mL.h-1. ) showed the catalysts had activities with CO conversion ranging from 25-45%. It was observed that catalysts supported on CNFs annealed at 500 displayed higher average activities of about 15% (based on the CO conversions) in relation to the catalysts supported on CNFs annealed at 900 . Catalysts showed minimal water gas shift reaction and high methane selectivity (i.e. 20-30%) which can be attributed to the small Co crystallite sizes and low pressure reaction conditions.