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) Özbek, Özüm; Kızılkaya, Ali CanFischer-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 Mechanistic Investigation of Carbon Dioxide Hydrogenation on Bimetallic Iron-Cobalt Surfaces by Density Functional Theory(2023) Tunçer, Dilan; Kızılkaya, Ali CanKüresel CO2 emisyonundaki artışa bağlı olarak iklim değişikliği, yalnızca CO2 üretiminin azaltılmasındaki önemi artırmakla kalmadı, aynı zamanda katalitik CO2 dönüşümü yoluyla kimyasalların ve yakıtların üretiminde kullanılmasının önemini de artırdı. Aktif ve seçici katalizörlerin rasyonel tasarımı, bu proseslerin endüstriyel uygulamalarına yönelik kritik öneme sahiptir. Bu tezde, C1 hidrokarbonların üretimi için CO2 hidrojenasyonunun mekanizmasını araştırmak ve FeCo bimetalik katalizörlerin (111) yüzeyindeki yapı-aktivite ilişkisinin atomik düzeyde anlaşılmasını sağlamak ve tasarıma rehberlik etmek için ilk prensiplere dayalı hesaplamalı bir çalışma yapıldı. Bu tezde, fcc-Co(111) ve Fe-katkılı Co(111) [FeCo(111)] yüzeyleri üzerinde CO2 hidrojenasyonunun C1 hidrokarbonlara verdiği temel reaksiyonların kinetiği, yoğunluk fonksiyonel teorisi (YFT) kullanılarak karşılaştırıldı. Araştırmamız Fe'nin Co(111) yüzeyine eklenmesi ile birlikte, CO2 aktivasyonunu desteklemesine rağmen genel reaksiyon hızını hafifçe azalttığını ortaya çıkardı. 1 ML Fe-katkılı Co(111) yüzeyinin daha düşük atomik H kapsamaları ve daha yüksek aktivasyon bariyerleri nedeniyle hidrojenasyon reaksiyonlarını engellemesi Fe'nin Lewis bazik karakterine atfedilmiştir. Fe'in katılması temel olarak kobalt yüzeylerinden oksijenin ayrılmasını engellemektedir. Bu nedenle, Fe katkısının, CO2 hidrojenasyonu sırasında bimetalik FeCo katalizörleri üzerinde oksidik fazların oluşumunu teşvik etmesi beklenmektedir.Master Thesis Design of Advanced Process Control System for Delayed Coker Unit(2023) Zihinli, İrem; Kızılkaya, Ali CanIt is essential for refineries to optimize the upgrading vacuum residue (VR) processes due to reducing of conventional light crude oil resources and increasing of fuel global demands. Delayed coking is a thermal cracking process used in refineries to upgrade and convert vacuum residuum into liquid and gas product streams including Light Coker Gas Oil (LCGO), Heavy Coker Gas Oil (HCGO), Sour Liquefied Petroleum Gas (LPG), Sour Coker Product Gas, Stabilized Naphtha and Petroleum Coke as a solid concentrated carbon material. Delayed coking is a semi-batch process where one or more pairs of coke drums are used for the thermal cracking and coking process. Simultaneously in each pair of coke drums, the feed stream is switched between two drums and one drum is online for the coking process while the other drum is offline undergoing decoking. The switching of the coke drums severely destabilizes the operation of the main fractionator and downstream process units. Applying advanced control concepts minimizes the disturbances and improves product quality and unit stability. Delayed coking is one of the most difficult refinery units to operate and control due to disturbances. Industrial chemical processes must operate at maximum efficiency and one of the ways to save energy and still obtain high quality product by using Advanced Process Control (APC) systems. The objective of thesis is to design an advanced process control system for main fractionator column of the delayed coker unit using Honeywell RMPCT. The aim of the APC is to decrease standard deviation of LCGO Final Boiling Point (FBP) quality in main fractionator column during steady state operation. The methods used in this thesis are the determination of the controller matrix and the application of pre-step and main tests to obtain process models for the advanced process control. According to obtained results, standard deviation for the LCGO FBP quality results are compared before and after APC implementation. It is shown that when the APC is turned on, the standard deviation of the LCGO product FBP quality is decreased by 3 ℃.Master Thesis The Influence of Inorganic and Organic Coating Layers on the Corrosion Behaviour of Coil Coated Aluminum Substrates(Izmir Institute of Technology, 2020) Uç, Merve; Kızılkaya, Ali CanOrganic coil coated aluminum substrates are one of the most widely used materials for construction industry. These coated substrates typically consist of 4 separate layers, the aluminum substrate, the inorganic conversion coating (resulting from the pretreatment application), and the organic coating layers that are called the primer and the topcoat. From industrial experience, it is not clearly and quantitatively determined how these layers contribute to the different processes that occur during corrosion, such as water uptake (diffusion of water through the coating) and the corrosion reactions that take place on the substrate. In this thesis, the formation of the inorganic conversion coating as a function of the pretreatment application parameters, immersion time, temperature and pH, and the individual effects of inorganic and organic coating layers on corrosion are investigated. The structure of the various surfaces formed after coating application and after corrosion testing by Neutral Salt Spray Test (NSST) is investigated by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), X-ray Fluorensence Spectrometer (XRF) and Atomic Force Microscopy (AFM) analysis, while their anticorrosive properties are determined by Electrochemical Impedance Spectroscopy (EIS). The results indicate that the formation of the pretreatment layer is enhanced continuously as function of temperature and immersion time, within the values investigated, while for pH an optimum is observed. The investigation of the corrosion resistance of substrates coated with different coating layers reveal that without organic coating layers, the substrates corrode much faster, indicating the importance of water diffusion on the corrosion reactions.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 Effect of Aging Electrolyte and Organic Coating Type on the Corrosion Mechanism of Tinplate(Izmir Institute of Technology, 2019) Yıldırım, Koray; Kızılkaya, Ali Can; Çağlar, BaşarMarket share of the can coating industry was 2 billion dollars in 2018 globally. A large portion of this market is focused on protecting valuable goods such as foods and cosmetics. Recent bans on the Bisphenol A(BPA) created a BPA-free coating demand due to the high portion of BPA based coatings in the industry such as epoxy. A conventional epoxy based (EP) can coating was compared with a new design of BPAfree polyester coating(PE). Atomic Force Microscopy(AFM), Scanning Electron Microscopy(SEM), Electron Dispersive Spectroscopy(EDX), Electrochemical Impedance Spectroscopy(EIS), Direct Current(DC) Polarization were used to investigate degradation. Industrial and model tinplate samples are compared with surface investigation methods. AFM results indicate the surface composition difference. To investigate bare tinplate corrosion, a mixture of acidic electrolyte was prepared and compared with other conventional aging electrolytes, containing mono acids. The electrolyte prepared from mixed acids was found to have the highest detinning abilities. Therefore, coated samples were aged with this mixed acid solution electrolyte. EIS results indicated that the type of electrolyte alters the corrosion mechanism, indicated by the observance of different time constants at different time scales. During immersion tests, EP based coatings showed better corrosion resistance as shown by higher impedance at low frequency, higher coating resistance and lower capacitance values. However, after sterilization, polyester coated samples showed better corrosion protective abilities (indicated by lower amount of corrosion products and higher impedance values) compared to the epoxy sample even though AFM measurements indicated that it had larger pore sizes after aging compared to epoxy sample. These results are attributed to the larger electrochemical area and higher diffusion properties of EP coatings, obtained after modelling of EIS data.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.