GCRIS

Now showing 1 - 3 of 3

Effect of Sulfur on the Elementary Reactions of Fischer-Tropsch Synthesis on Cobalt Surfaces
(Izmir Institute of Technology, 2020) Dağa, Yağmur; Kızılkaya, Ali Can
Industrial 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.
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, Haldun
The 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.
Structural and Magnetic Properties Os Si(100)/Ta Multilayers for Spintronics Applications
(Izmir Institute of Technology, 2007) Vahaplar, Kadir; Tarı, Süleyman
This thesis is concerned with the structural and magnetic properties of Si(100)/Ta/Co single and multilayer thin films grown by DC magnetron sputtering technique. The structural properties of the films have been studied by X-Ray Diffractometer (XRD), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). This study revealed that a single Co film grows amorphous on silicon substrate up to 50 nm at room temperature. After this thickness, Co starts crystallizing in hexagonal (002) plane. The same crystallinity was also observed for 25 nm amorphous Co which was annealed at 4500C at high vacuum for 30 minutes. The presence of a single crystalline tetragonal Ta phase (-Ta) with the orientation along (002) has been observed for 40 nm Ta growth on silicon substrate. The Si(100)/Ta/Co bilayers and multilayers show good crystallinity for both Ta and Co films. SEM and AFM results show that all the single and multilayers grew uniform, continuous and with very low surface roughness. The magnetic properties of the films were investigated using Vibrating Sample Magnetometer (VSM), by measuring hysteresis loops. The effects of the thickness and growth pressure on the magnetic properties of Co films were studied. The easy magnetization axis of the samples is found to be parallel to the Co film plane. As the Co film thickness increased from 4 nm to 15 nm, the coercivity (Hc) decreased from 72 G to 20 G and after a threshold thickness it increased almost linearly up to 180 G for 100 nm film while the magnetization decreased. Moreover, it has been observed that as the Co growth pressure increases, the Hc value of Co films increases. Finally, we obtained two different Hc values for our MTJ sandwich with the structure of Si(100)/Ta/Co/TaOx/Co/Ta.

Master Degree / Yüksek Lisans Tezleri

Browse

Filters

Settings

Sort By

Results per page

Search Results