M.S. Theses

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Browsing M.S. Theses by Subject "Adsorption."

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    An investigation of on the properties of Pt-based surface alloys via computational QM methods
    (Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2004., 2004.) Sümer, Aslıhan.; Aksoylu, Ahmet Erhan.
    The adsorption properties of CO on stepped Pt3Sn(102) surface were investigated using quantum mechanical calculations. The two possible terminations of experimentally verified stepped surface, Pt3Sn (102), were generated and on these terminations all types of possible adsorption sites were determined. The adsorption energies and geometries of the CO molecule for all those sites were calculated. The most favorable sites for adsorption were determined as the short bridge site between SE and TA atoms of pure Pt row on the mixed atom ending termination, atop site on SE atom of the pure row of pure-Pt ending termination and atop site on SE atom of the pure row of the mixed atom ending termination. The results were compared with those for similar sites on the flat Pt3Sn(110) surface considering the fact that Pt3Sn(102) has terraces with (110) orientation. The adsorption energies of stable sites on Pt3Sn(102) surface are in general larger than those of the similar sites on Pt3Sn(110) surface. The difference in adsorption energies of those sites is a result of stepped structure of Pt3Sn(102). Aiming to understand the interactions between the adsorbate and surface metal atoms present at the adsorption sites in detail, the local density of states (LDOS) of the adsorbent Pt and C of adsorbed CO was utilized. The LDOS of the surface metal atoms with CO adsorbed atop and of its clean state were compared to see the effect of CO chemisorption on the electron distribution of the corresponding Pt atom. The downward shift in energy peak in the LDOS curves as well as changes in the electron densities of the corresponding energy levels indicate the orbital mixing between CO molecular orbitals and metal d states. The studies clearly showed that the adsorption strength of the sites has a direct relation with their LDOS profiles.
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    Carbon dioxide adsorption on flat Pt3Sn surfaces
    (Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2007., 2007.) Başaran, Duygu.; Aksoylu, Ahmet Erhan.
    The experimental studies have shown that the PROX activity of the Pt{Sn/AC catalyst is either unaffected or positively affected from the presence of carbon dioxide in the feed mixture. The catalyst characterization studies revealed that Pt{Sn/AC system has Pt3Sn alloy as one of the active metallic phases and the alloy plays a dominant role in the PROX activity. Thus, the unaffected or enhanced PROX activity levels by the presence of carbon dioxide can be explained by either the stabilization of carbon dioxide on AC surface or limited interaction between carbon dioxide and Pt3Sn alloy surface. In this computational work, the proposed mechanism on the alloy surface was investigated by reproducing the periodic vacuum slabs of the Pt3Sn alloy. Thus, carbon dioxide adsorption on the Pt3Sn (111), (110), and (001) surfaces was studied at atomic level by utilizing semi ab-initio quantum mechanical code, CASTEP. The surfaces of Pt3Sn alloy were generated with all possible bulk terminations, and all types of active sites on these terminations were tested for carbon dioxide adsorption. The adsorption energies of carbon dioxide molecule at all adsorption sites were calculated. The Local Density of States (LDOS) analysis was used for understanding the electronic interaction between the atoms of the adsorption sites and carbon dioxide molecule. In order to identify whether the carbon dioxide adsorption is stable, geometries of the carbon dioxide molecule as well as LDOS of both the carbon dioxide molecule and binding atom of the surface, Pt and/or Sn, were analyzed for all of the sites on each termination. The LDOS profiles for each active site were compared with those of the corresponding bare surfaces and free carbon dioxide to determine the new mixed orbitals. The study showed that (i) carbon dioxide adsorption on Pt3Sn surfaces is not thermodynamically favorable, (ii) the adsorbed carbon dioxide assumes two different geometries dependent on the presence of a Sn atom at the site of adsorption, (iii) the adsorbed carbon dioxide molecule does not dissociatively adsorb on Pt3Sn surfaces, and (iv) the adsorbed carbon dioxide molecule resembles the structure of the carbon dioxide anion (CO¡2 ).
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    Data mining for carbon dioxide adsorption over amine modified adsorbents from publications in literature
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2017., 2017.) Yıldız, Merve Gamze.; Yıldırım, Ramazan,; Davran, Tuğba..
    The aim of this thesis is to extract knowledge for carbon dioxide adsorption over amine modified adsorbents using two data mining techniques that are decision trees (DT) and artificial neural networks (ANN). The experimental data were collected from 30 papers published between 2002 and 2016. The data set consisted of 1356 data points with 26 attributes; the adsorption capacity (mmol CO2/g adsorbent) and amino efficiency (CO2/N ratio) were chosen as performance (output) variables. In DT analysis, the adsorption capacity and amino efficiency were classified in four groups, and the conditions leading to high adsorption capacity and amino efficiency were identified. Training and testing accuracies for adsorption capacity were 83.4% and 82%, respectively; while these values were 80.8 % and 77.3 % for the amino efficiency; from which the analysis for both output variables could be considered as successful. ANN analysis was also used to model the same data to develop predictive models; tansig function was used as the activation function and trainlm and trainbr were used as the training algorithms for training and testing, respectively. The optimal network topology was determined as 26-10-1 (10 neurons in one hidden layer) for adsorption capacity and 26-9-1 (nine neurons in hidden layer) for amino efficiency. The R2 and RMSE values for adsorption capacity were respectively found to be 0.97 and 0.118 for training, and 0.90 and 0.250 for testing. Similar results were obtained for the amino efficiency. The R2 and RMSE were 0.97 and 0.0230 respectively for training while R2 was 0.83 and RMSE was 0.053 for testing. These results indicate that ANN models were also successful. The relative significances of input variables were also calculated by using the optimal neural network topology and change of mean square error method. Operational variables had greater significance on both amino efficiency and adsorption capacity relative to the adsorbent properties.
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    Interaction between CO and PtSn alloy: a theoretical study on understanding selective CO oxidation
    (Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Duru, Birim.; Aksoylu, Ahmet Erhan.
    The adsorption properties of CO on PtSn(110) surface were investigated via quantum mechanical calculations. The adsorption energies and the geometries of CO molecule for all possible adsorption sites were calculated. The energetically most favourable CO adsorption site was found as Pt-Pt bridge site, site B. The adsorption properties of CO on PtSn(110) were compared with those on Pt3Sn(110) and Pt(110). The CO adsorption strength of bridge sites on three surfaces decreased with increasing Sn content. The adsorption properties of other PROX reactants and products, namely H2, CO2 and O2, were also investigated on PtSn(110) surface. Pt-Pt bridge site B was found as the only site adsorbing H atom stably. Stable CO2 adsorption was observed only on Pt-Sn bridge site C on PtSn(110) surface. For atomic oxygen adsorption, Sn-Sn bridge site F was the most favourable adsorption site on PtSn(110) surface. The adsorption properties of H2, CO2 and O2 gases were also compared for the three surfaces, namely PtSn(110), Pt3Sn(110) and Pt(110). In order to understand electronic interactions between the surface metals and the adsorbates, local density of states (LDOS) profiles were examined for bare and adsorbed states of the metal atoms and the adsorbate involved in the adsorption process. The common behaviour observed for all adsorbates was that electronic density around surface Pt atom increased with increasing Sn content in the alloy and the electronic interactions occurred in lower energy levels as Sn concentration in the alloy was increased.