Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Effect of Sulfur on the Elementary Reactions of Fischer-Tropsch Synthesis on Cobalt Surfaces(Izmir Institute of Technology, 2020) Dağa, Yağmur; Kızılkaya, Ali CanIndustrial observations indicate that sulfur acts a poison for Fischer-Tropsch Synthesis (FTS) and surface science studies show that sulfur blocks the adsorption sites for reactants on cobalt surfaces. However, various experimental studies have indicated conflicting results about the effect of sulfur on cobalt FTS catalyst activity and selectivity. This study aims to clarify the effect of sulfur on cobalt FTS catalysts by molecular modelling of the elementary reactions of FTS on surfaces that are present on sulfur covered cobalt surfaces that are present in fcc-Co nanoparticles, using Density Functional Theory (DFT). For 0.25 ML sulfur coverage, it is found that on bare, C and O covered surfaces, S is the main dissociation product, while HS can be present on low coverages. Atomic sulfur decreases the adsorption energies of all species investigated, while the decrease is more pronounced for CO compared to H2. The effect of S on the elementary FTS reactions direct and H-assisted CO dissociation, carbon hydrogenation, carbon coupling and oxygen removal are also investigated. The results indicate that S inhibits mainly the oxygen removal reaction, in terms of both H2O and CO2. CO dissociation is not inhibited but rather slowed down, due to increasing activation barriers. It is also found that carbon hydrogenation barriers are significantly decreased, while carbon coupling barriers are unaffected. These results indicate that the intrinsic effect of sulfur poisoning would be to increase methane selectivity, while decreasing the selectivity to long chain hydrocarbons.Master Thesis Effects of Manganese Promotion on Reactants and Intermediates of Fischer Tropsch Synthesis on a Model Cobalt Surface-A Density Functional Theory Investigation(Izmir Institute of Technology, 2019) Gençoğlu, Merve; Kızılkaya, Ali Can; Sevinçli, HaldunThe effects of manganese promotion on the adsorbates and specific elementary reactions of Fischer-Tropsch Synthesis (FTS) was investigated using periodic Density-Functional Theory (DFT) calculations on a close packed cobalt surface, Co(111). In particular the effects of MnO promotion on the adsorbates of CO, HCO, CH, CH2, C2H2, OH, H2O, C, O and on the reactions of direct CO dissociation, H-assisted CO dissociation and carbon hydrogenation were studied for MnO coverages of 0.25 ML and 0.11 ML. Mn was modeled in the chemical form of MnO. MnO was modeled as a singular monomer on the Co(111) surface, based on the findings from experimental studies. The results indicate that MnO promotion increases the adsorption energies of all adsorbates, except H and C2H2. In particular, CO and H2O adsorption energies increase significantly, which indicate that the selectivity increases to long chain hydrocarbons is mainly due to an increased surface coverage of CO with respect to H. The results also indicate that the relative effect of MnO on adsorption energies are strongly dependent on MnO coverage. MnO promotion is found to decrease the activation barriers for HCO and CH formation, while increasing the activation barriers for direct CO dissociation and HCO dissociation. The results point out that MnO does not promote the direct dissociation of CO and the activity increase due to Mn promotion is most probably due to a H or OH assisted CO dissociation pathway or another rate limiting step.