Model systems for the investigation of metal-support interactions in cobalt based Fischer-Tropsch

Master Thesis


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The catalyst used plays a pivotal role in the optimization of the Fischer Tropsch (FT) synthesis. Cobalt (Co)-based catalysts have been widely used in low temperature Fischer Tropsch synthesis for the production of longer chain olefins [1]. The interaction between the support and the active metal, Co, has been observed to affect the activity and selectivity of the FT synthesis [2]. In order to investigate metal support interactions independent of other support effects, the inverse method was used in the synthesis of model catalysts [3]. The inverse method mimics the interface bond between the support and the active metal, resulting in CoO-Si bond formation. By exposing Co nanoparticles (NPs) to a dilute solution of an alkoxide, tetraethyl orthosilicate (TEOS) or triphnyl ethoxy silane (TPEOS), Co-O-Si bond (ligand) is expected to form until it reaches equilibrium state. The concentration of the dilute alkoxide solution can be varied to observe its effect on the surface coverage of the Co NPs with the alkoxide. XRD analysis of the modified cobalt nanoparticles showed that there was no evidence of the formation of a crystalline silica phase as those characteristic diffraction lines were absent. The TPR profiles showed a shift to higher reduction temperatures with increasing silica loading suggesting that the surface modification results in retardation of reduction of the cobalt nanoparticles. CO-TPD studies of the model catalyst indicated that the total number of active sites available for CO adsorption and bond cleavage decreases upon surface modification, but it leads to a more facile CO bond cleavage as evidenced by decrease in energy of dissociation. The activity of these materials in the Fischer Tropsch synthesis decreased with increasing silicon content for the oxidized cobalt nanoparticles. This was attributed to the decline in the total number of active site available for CO bond cleavage with increasing silicon content. The methane selectivity passes a minimum with increasing silicon content. This may be related to the carbon coverage on the surface of the nanoparticles affecting the chain growth. It is recommended that Fischer Tropsch synthesis studies be conducted on the modified as-synthesized cobalt nanoparticles to compare the performance of the two catalysts.