Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Molecular Modelling of the Effect of Alkali Promoters on Co Adsorption and Dissociation on the Co(111) Surface(2023) Kızılkaya, Ali Can; Kızılkaya, Ali Can; 03.02. Department of Chemical Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyFischer-Tropsch Synthesis (FTS) is a surface polymerization process that has been industrially used to convert non-petroleum feedstocks to synthetic transportation fuels. Modification with an alkali promoter of the Co-based catalysts provided promising results to obtain hydrocarbons with enhanced olefin content in FTS. Activation of CO is the key factor to achieve desired end products in FTS, yet the mechanism related to the CO dissociation behavior on alkali promoted cobalt surfaces remains unknown. This study aims to examine the impact of alkali promoters (Li, Na, K) on the adsorption and dissociation characteristics of CO on the Co(111) surface using Density Functional Theory (DFT). Our results revealed that CO adsorption energy increased by 32-37% with alkali addition, yet H adsorption energy remained relatively unchanged. The effect of alkali addition on CO dissociation routes were also examined. The high activation barrier (>200 kJ/mol) makes it improbable for direct CO dissociation to occur on alkali promoted Co(111) surfaces under FTS conditions. For H-assisted pathways, alkali addition increased the activation barrier for HCO and H2CO dissociation, overall reducing the H-assisted CO dissociation rate. It was found that alkali addition makes the surface more carbophilic since the C adsorption energy increased by 7-11% upon alkali addition. Also, with increasing C concentration on the Co(111) surface, subsurface carbon geometries became more stable. Ultimately, it is concluded that alkali promoters of Li, Na and K have similar effects on CO adsorption and dissociation on the Co(111) surface.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; Sevinçli, Haldun; Kızılkaya, Ali Can; Kızılkaya, Ali Can; Sevinçli, Haldun; 03.02. Department of Chemical Engineering; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyThe 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.