Iron catalyst supported on carbon nanotubes for Fischer-Tropsch synthesis : experimental and kinetic study
Malek Abbaslou, Mohammad Reza
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The main objectives of the present Ph.D. thesis are comprehensive studies on activity, selectivity and stability of iron catalysts supported on carbon nanotubes (CNTs) for Fischer-Tropsch (FT) reactions. In order to prepare iron catalyst supported on CNTs, it was necessary to study CNT synthesis in bulk scale. Therefore, a part of this research was devoted to the production and characterization of CNTs. High purity, aligned films of multi-walled carbon nanotubes were grown on quartz substrates by feeding a solution of ferrocene in toluene, in a carrier gas of Ar/H2, into a horizontal chemical vapour deposition (CVD) reactor. Results for CNTs synthesized using a wide range of toluene concentrations indicated that, for carbon concentrations higher than ~9.6 mol/m3, catalyst deactivation occurs due to encapsulation of iron metal particles. As the first step of catalyst development for FT reactions a fixed bed micro-reactor system was built and the effects of acid treatment on the activity, product selectivity and stability of iron Fischer-Tropsch catalysts supported on carbon nanotubes were studied. The results of Raman analysis showed that the acid treatment increased the number of functional groups as anchoring sites for metal particles. Fe catalysts supported on CNTs which were pre-treated with nitric acid at 110Â°C were more stable and active compared to the un-treated catalysts. In order to study the effects of catalytic metal site position on FT reactions, a method was developed to control the position of the deposited metal clusters on either the inner or outer surfaces of the CNTs. According to the results of the FT experiments, the catalyst with catalytic metal sites inside the pores exhibited higher selectivity (C5+ = 36 wt%) to heavier hydrocarbons compared to one with sites on the outer surfaces (C5+ = 24 wt%) . In addition, deposition of catalytic sites on the interior surfaces of the nanotubes resulted in a more stable catalyst. The effects of pore diameter and structure of iron catalysts supported on CNTs on Fischer-Tropsch reaction rates and selectivities were also studied. In order to examine the effects of pore diameter, two types of CNTs with similar surface areas and different average pore sizes (12 and 63 nm) were prepared. It was found that the deposition of metal particles on the CNT with narrow pore size (in the range of larger than 10-15 nm) resulted in more active and selective catalyst due to higher degree of reduction and higher metal dispersion. Promotion of the iron catalyst supported on CNTs with Molybdinium in the range of 0.5-1 wt % resulted in a more stable catalyst. Mo improves the stability of the iron catalyst by preventing the metal site agglomeration. Promotion of the iron catalysts with potassium increased the activity of FT and water-gas-shift reactions and the average molecular weight of the hydrocarbon products. Promotion of the iron catalyst supported on CNTs with 0.5% Cu and 1wt% K resulted in an active (5.6 mg HC/g-Fe.h), stable and selective catalyst (C5+ selectivity of 76%) which exhibited higher activity and better selectivity compared to the similar catalysts reported in the literature. Kinetic studies were conducted to evaluate reaction rate parameters using the developed potassium and copper promoted catalyst. It was found that the CO2 inhibition is not significant for FT reactions. On the other hand, water effects and presence of vacant sites should be considered in the kinetic models. A first-order reaction model verified that the iron catalyst supported on CNTs is more active than precipitated and commercial catalysts. The results of the present Ph.D. thesis research provide a map for designing catalysts using carbon nanotubes as a support. The key messages of the present thesis are as follows: 1- If the interaction of the metal site and support is strong, which poses negative effects on the catalytic performance, carbon nanotubes can be one solution. 2- Acid pre-treatments are required prior to impregnating nanotubes with metal salt solution. Also, the strong acid treatment should be used for deposition of catalytic sites inside the pores of nanotubes. 3- The structure and pore size of nanotubes have significant influence on the stability, activity and selectivity of the target catalyst. 4- The position of the catalytic sites has to be selected based on the type of reaction. In the case of Fischer-Tropsch reactions, the deposition of catalytic sites inside the pores of nanotubes results in higher activity, longer life span. The outcome of this Ph.D. thesis has been published/submitted in the form of 13 journal papers, one patent, one technical report and presented at 11 conferences.
DegreeDoctor of Philosophy (Ph.D.)
CommitteeHui Wang; Todd Pugsley; Richard Evitts; David Sumner; Ajay Ray
Copyright DateJune 2010