M.S. Theses

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A DFT study on interaction between Au, Re and Au-Re surfaces with species involved in WGS reaction
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2012., 2012.) Uzun, Ali.; Aksoylu, Ahmet Erhan.
In this study, the steps of WGS reaction was investigated through studying the adsorption/co-adsorption of its reactants, products and possible intermediate species on Au(111), Re(001) and Au-Re(001) surfaces at atomic scale via utilizing DFT modules of CASTEP. Firstly, the adsorption energies of CO, OH, CO2, H2O, HOCO molecules and H atoms on Au(111) and Re(001) surfaces, LDOS profiles of these adsorbates for their free and in adsorbed state, and the surface metal atoms on the adsorption sites of Au(111) and Re(001) surfaces for their bare and adsorbed state were analyzed. Additionally, CO-OH co-adsorption on Au(111) and Re(001) surfaces were carried out in order to analyze the reaction steps of WGS reaction on Au(111) and Re(001) surfaces. In the second part of the study, Au-Re surface alloys were generated by the addition of Au atoms on Re(001) surface as point defects. The adsorptions/co-adsorptions of CO and OH molecules on Au- Re(001) surface alloys having different Au concentration on Re(001) were carried out in order to understand the reason of high activity and selectivity of Au-Re/Ceria catalysts. The results revealed that monometallic Au and Au-Re surface alloy are active catalysts, whereas Re is not an active catalyst for WGS reaction. It was found that OH has stronger interaction with Au surface than that of CO in terms of its high binding energy, and it is inferred that the active sites on Au surface, which dominantly participate in WGS reaction steps, can be poisoned by OH molecules due to site competition. The results also showed that the binding energies of OH on Re are higher than that of OH on Au, and it is concluded that OH molecules can move from Au sites to Re sites, and the active sites on Au surface can be poisoned by OH molecules to a lesser degree. The co-adsorption of CO and OH molecules on Au(111), Re(001) and Au-Re surfaces revealed that WGS reaction takes place on Au sites of monometallic and Au-Re surface alloy, and the surface reaction between CO and OH molecules on Au is consistent with the carboxyl mechanism.
A mathematical model based analysis for designing demo scale fuel processor
(Thesis (M.A.) - Bogazici University. Institute for Graduate Studies in the Social Sciences, 2017., 2017.) Demirci, Barış.; Aksoylu, Ahmet Erhan.; Bedir, Hasan.
The aim of this study is to construct a FLUENT-based mathematical model of a Demo-scale fuel processor, which will be used as the base model to obtain the optimum operating parameters of a Demo-FP that produces sufficient amount of PEM-grade H2, having CO concentration below 100 ppm, from methane in amount enough to feed a 1 kW PEMFC. In the first part, the power law type kinetic expressions obtained from kinetic studies for the FP reactions (OSR-WGS-PROX) were corrected through minimizing the difference between experimentally obtained performance test results and the results obtained from the mathematical model formed in the current study for the same reaction conditions. In the second part, the fuel processor prototype (FPP) was modeled and its performance was simulated through the use of corrected kinetic models. 16 FPP simulations of the OSR reactor were performed by using the feed composition, temperature and W/F used in the first part. First, FP reactors having ¼ inch-OD were modeled in series. Then, the same simulations were performed for 1 inch-OD of the reactors, which is the OD of the reactors has been planned to be used in Demo-FP, while the catalyst bed height of each reactor were kept fixed as in the case of ¼ inch reactors; in those simulations, the catalyst amount and the flow rate of the OSR feed was increased such as to keep W/F of the OSR reactor the same as that of the ¼ inch reactor case. Finally, the reaction conditions yielding the best performance in terms of H2 and CO level at the PROX outlet (the simulations with 400 oC OSR reactor temperature) were selected. For the selected sets, the catalyst amounts in the reactors increased while keeping the W/F fixed such as to satisfy the H2 flow of 0.00272 moles/s and CO concentration of 100 ppm. In the last part, the operation of individual OSR reactor was further simulated for granule-size technical catalyst via introducing several levels of effectiveness factor to the power-law type OSR kinetic expression. The required catalyst amounts and the pressure drop values for the particle diameters and effectiveness factors studied were calculated. The bed density change with the change in particle size was almost insignificant, and consequently almost the same bed lengths were found. On the other hand, as the particle diameter was increased, the pressure drop was reduced sharply.
A preliminary work on design and development of sour water gas shift catalysts for synthetic natural gas process
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Özata, Hasretnur.; Aksoylu, Ahmet Erhan.
The main goal of this study is to design and develop a model sour water gas shift catalyst showing high performance in terms of activity, stability & selectivity and conducting performance screening by mimicking the syngas composition of the potential coal gasifier under ideal, sulfur-free conditions. According to the literature, dry powder entrained-bed type gasifier seems potential gasifier for the Synthetic Natural Gas (SNG) production processes, and it contains low steam/CO ratio, below 0.8. On the other hand, classical sour water gas shift (SWGS) catalysts work under high steam/CO ratio, specifically, above 2 or 3. In this context, from literature, one of the potential SWGS catalysts which is suitable for dry-powder syngas composition was chosen as a reference. Activated carbon supported KCoRe as the model SWGS catalysts were developed and these catalysts were tested under ideal, sulfur-free syngas composition. The effect of reaction conditions (i.e., reaction temperature and steam to carbon monoxide ratio) and catalyst preparation & support pre treatment methods (i.e., impregnation method, air & nitric acid pre-treatment on the Activated Carbon support) were investigated on the prepared catalyst. Activity and selectivity were determined in terms of conversions of CO and H₂, respectively. Experimental results showed that reaction conditions are very important parameters on the performance of the catalysts. At low steam/CO ratios, it is necessary to increase the temperature in order to get high activity. The effect of the impregnation method may vary from support to support since impregnation depends on the surface chemistry of the support material. Co-impregnation method may not be suitable for air oxidized AC supported catalysts. Both air and nitric acid treatments are helpful to increase the performance of the catalysts by increasing the oxygen bearing surface groups on the AC support.
A QM study on determination of vibrational frequencies of the surface coordinated species involved in CDRM
(Thesis (M.A.) - Bogazici University. Institute for Graduate Studies in the Social Sciences, 2013., 2014.) Karagöz, Burcu.; Aksoylu, Ahmet Erhan.
The determination of the sites participating the reaction mechanism is of crucial importance in catalysis. Ni- and Co- based catalysts have been studied by our group for their activity in Catalytic Dry Reforming of Methane (CDRM). Although in-situ FTIRDRIFT tests have been conducted on those catalysts under adsorption and reaction conditions, and the vibrational spectra were obtained, the available information in the literature is not detailed and surface specific enough to allow the determination of the active sites on the catalyst. The aim of this study is to obtain a vibrational frequency database for the species involved in CDRM reaction mechanism occurs on the possible active faces of the Co- and Ni- based catalysts. In the formation of the database, quantum mechanical simulations utilizing Density Functional Theory (DFT) calculations are used. In the simulations, the metallic faces whose presences have been confirmed for our CDRM catalysts by XRD were considered. Vibrational frequencies of CHx(x=0-4), CO2, CO, O and H on Co(111) flat surface, Ni(220) and NiPt(220) terraced surfaces were calculated for each possible sites via DMol3 tool of Accelrys Materials Studio. Calculated vibrational frequencies of CO2, CO, CH3, and CH2 on Co(111) are in good agreement with the literature. Adsorption energies of CO2, CO, O, CH4, CH3, and CH2 on Co(111) are inversely related with their stretching frequencies whereas there is a linear relationship for adsorbed CH, C, and H on Co(111). Furthermore, calculated vibrational frequencies of CO2, CO and CH2 on Ni(220) are consistent with the literature, while stretching frequency of CH3 is far different from experimental findings, due to possible alloy formation, and geometry and/or support effect. Comparative analysis of the results obtained for Ni(220) and NiPt(220) revealed that for NiPt(220) surface, Pt existence affect the vibrational frequencies of all species. Moreover, CO2 and CO vibrational frequencies on NiPt(220) are considerably close to experimental findings. Vibrational frequency of CH4 is inconsistent with the literature for all surfaces, most probably because the weak CH4 adsorption on these surfaces may not be precisely calculated by DFT.
A study on biometallic water gas shift catalysts to be used ın fuel processing
(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2009., 2009.) Güven, Sabriye.; Aksoylu, Ahmet Erhan.
The aim of this study is to design and develop a Au-based, bimetallic WGS catalyst which has suitable activity and selectivity characteristics to be used in a fuel processor. The catalyst composition, preparation method, calcination temperature, reduction temperature and reduction mixture composition, and feed composition were the parameters used in the catalyst design and testing studies. Nine different catalyst samples were prepared, tested and they are characterized by SEM. The optimum catalyst composition for WGS reaction was found to be 1% Re-2% Au/ZrO2, prepared by impregnation of Re on ZrO2 followed by deposition precipitation of Au (Re+Au). Addition of Re in this sequence was observed to improve the catalytic activity significantly, suggesting that rhenium is a good promoter for bimetallic gold based WGS catalysts. Together with five other different realistic feed compositions, a reaction mixture of 6% CO, 36% H2O, 39% H2, 10% CO2, 9% He was used for most of the reaction tests performed. The WGS activities of the samples were determined for 250-350 ˚C range. Effect of W/F ratio (0.02, 0.03, 0.06) and H2O/CO ratio (5, 6 and 7) on the WGS performance of Re-Au bimetallic catalysts were also investigated, and as expected, the general trend observed was that the catalytic activity increases with increasing W/F and H2O/CO ratios, with the exception that the catalytic activity was found to be very close for all W/F ratios at 350 ˚C. For optimum catalyst composition and reaction conditions, the catalytic activity was observed to reach as high as 55% CO conversion at 350 ˚C. Trace amounts of methane formation was observed only for the samples prepared by Au+Re sequence at 350 ˚C. No methane activity was observed for the rest of the samples. The results of the current study indicate that the bimetallic Re-Au catalyst supported on zirconia has a real potential to be used in the WGS unit of a real fuel processor.
A study on dispersion improvement of active sites on supported metal catalysts
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Taş, Enes Emre.; Aksoylu, Ahmet Erhan.
The current work aims to enhance metal dispersion on supported metal catalysts synthesized via dry impregnation (DI) method by focusing on 10 wt.% Ni/γ-Al2O3 catalysts as a specific example. Impregnated samples underwent calcination at varying temperatures (500°C, 700°C, and 900°C) and subsequent reduction at 800°C for 1 h under pure H2 flow. Dispersion levels were determined by using the static H2 chemisorption method. Additionally, N2 physisorption, Raman, XRD and XPS analyses were performed on selected samples. The study includes the use of γ-Al2O3, precalcined Al2O3 at 920°C, and 1 wt.% La- doped Al2O3 as support materials, for which the PZC values were measured as 8.49, 8.42, and 7.82, respectively, using the salt addition method. pH shift models were utilized to determine the final pH values of impregnated slurry and to adjust the pH of the precursor solution. The behavior of nickel species in the impregnating solutions was modeled based on the changing pH, employing stability constants of potential compounds. Suitable complexing agents were selected according to the support material's surface charge across the pH scale to obtain Ni complexes/chelates. UV-Visible Spectrophotometry was used for the analysis of solutions. Results demonstrate that the surface distribution of metallic Ni is the primary factor determining dispersion levels at calcination temperature of 500°C. Conversely, at 700°C and 900°C, the formation of NiAl2O4 becomes the predominant factor, as confirmed by XPS and XRD analyses. NTA and CA complexes yield the highest dispersion results at lower calcination temperatures (500°C and 700°C), while the EDTA complex exhibits better results for sample calcined at 900°C. The highest dispersion level achieved using the Ni-CA complex for sample calcined at 500°C is 9.2624%, whereas the lowest dispersion is observed with the same support and solution when it is calcined at 900°C, with a dispersion of 2.5622%. These findings indicate that dispersion is influenced by a combination of different factors that should be evaluated holistically.
A study on enhancing thermal stability of alumina
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2021., 2021.) Çelebi, Gülten.; Aksoylu, Ahmet Erhan.
The use of temperature resistant support is a prerequisite for successful catalysts prepared to be used in reactions which need high temperature, like Dry and Mixed Reforming of Methane, catalytic oxidation, and combustion reactions. Alumina is a common and cheap catalyst support for achieving high metal dispersion, e.g., γ-Al2O3 has a surface area of 220 m2/g. However, as high temperature leads to phase change from high- to low-SA alumina phases, i.e. γδθα, preventing surface area loss requires to shift the phase change temperatures up to higher levels. In the current study, the thermal stability of commercial γ-Al2O3 samples was enhanced via doping with stabilizers La, Ba and Pt through application of various preparation and pre-treatment parameters such as; pre-doping temperature, vacuum pressure, drying temperature, pH of the precursor solution, and heat treatment applied upon doping. Total of 142 samples were prepared in 19 sets of samples, and all samples were characterized by their surface area, pore volume and pore size distribution. Selected samples chosen to represent each set were furtherly tested by XRD and Raman spectroscopy. Reference γ-Al2O3 calcined at the highest temperature applied, 1150 oC, for 2 hours had BET surface area of 8.81 m2/g. Under the same conditions, thermally resistant 1%La/Al2O3 and 1%Ba/Al2O3 samples prepared by incipient-to-wetness impregnation have BET surface areas of 61.74 m2/g and 38.26 m2/g, respectively. Results indicated that the effect of vacuum pressure and drying temperature applied during preparation were ambiguous. Changing only the pH of impregnation solution from pH1(acidic) to pH2(basic) led to decreased surface area values. While combination of pH regulation and post heat treatment significantly have a stabilizing effect leading 68.27 m2/g surface area for 1%La/Al2O3 upon calcination at 1150 °C for 2 hours, further elaboration of pre-doping temperature applied led to the highest BET surface area of 70 m2/g. XRD analysis confirmed the effect of pre-doping temperature on shifting the phase transfer temperature of alumina.
A study on kinetics of methane oxidative steam reforming (OSR) over PPt-Ni/δ-Al2O3 bimetallic catalysts
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2014., 2014.) Erdinç, Elif.; Aksoylu, Ahmet Erhan.
The aim of the current study is to obtain a reliable power law type rate expression for methane OSR over Pt-Ni/δ-Al2O3 catalyst valid for practical experimental condition ranges. In this context, 0.2wt.%Pt-10wt.%Ni/δ-Al2O3 and 0.3wt.%Pt-10wt.%Ni/δ-Al2O3 catalysts were prepared by sequential impregnation method. Methane OSR performance of these catalysts were tested for constant S/C feed ratio at 450 °C. The performance test results showed that increasing residence time and C/O2 feed ratio decreased OSR activity, whereas increase in temperature led to higher methane conversions. As the effect of Pt:Ni metal loading did not lead to significant changes in activity, the following preliminary kinetic tests were conducted over 0.2Pt-10Ni catalyst to determine kinetically controlled, mass transfer limitations free, experimental conditions. Based on the outcomes of the preliminary test results, the kinetic study was performed at 375 °C with feed ratio regions of 4.0 < C/O2 < 7.34 and 2.03 < S/C < 3.08. 17 pairs of kinetic experiments were conducted by changing partial pressures of reactants, methane, oxygen, and steam, and residence time, W/F. Reaction orders were estimated as 0.81, 1.60 and 0.44 in methane, oxygen and steam, respectively, by using multivariable non-linear optimization function of MATLABTM. The apparent activation energy of methane OSR was calculated as 31.99 kJ mol-1 and pre-exponential factor as 0.366 μmol mgcat-1 s-1 kPa-2.85 for the 350-425 °C temperature interval. The same analysis performed for a narrower temperature range, 350-400 °C, i.e. 375+ 25 °C, gave k0 and EA values as 0.956 μmol mgcat-1 s-1 kPa-2.85 and 39.05 kJ mol-1, respectively, confirming the high sensitivity of OSR pathway to temperature.
A theoretical study on dry reforming of methane (CDRM) over cobalt metal
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2011., 2011.) Çimenoğlu, Vasfiye.; Aksoylu, Ahmet Erhan.
The aim of this study is to analyze the adsorption and reaction steps of CDRM on the Co(111) surface by using DFT calculations. In this context, the adsorption/coadsorption of reactants, reaction intermediates, and the reaction were studied. LDOS analysis was also used whenever detailed analysis is necessary. In this study, adsorption behaviours of all reactants some products and intermediate products of CDRM reaction were investigated first. CO2 was adsorbed on all sites of Co(111) surface but adsorption energies and LDOS profiles show that the strengths of CO2 adsorption on those sites are very low. On the other hand, CH4 adsorption is either unstable or yields CH3 and H stabilized on the surface. CO adsorptions on all sites were found very strong with very similar adsorption energies. CH3 adsorption is stable on all sites, except on the bridge site, and the adsorption is strong. CH3 initially placed at bridge position assumed Hhcp position upon energy optimization. The most favorable sites for CH3 and CO adsorptions were Hhcp and atop sites, respectively. Simulation results show that oxygen adsorption is stable only at Hhcp site as O adsorbed on other sites assume Hhcp position upon energy optimization. In the second part of the work, 'C and O', and 'CO and CH3' coadsorption systems were studied for different surface concentration and adsorption configuration combinations. C and O coadsorption studies revealed that the energy optimized structures are very sensitive to relative surface coverages of the adsorbates as well as whether there is site competition between them; C and O interaction yield CO only when there is site competition. Finally, the TS searches of CH4 dehydrogenation and CO formation were carried out to obtain the reaction pathways and activation energies of elementary steps. CH4 dehydrogenation step was found as the rate-determining step of hydrogen and surface carbon production. Since CH dehydrogenation has high energy barrier and the strong endothermicity, CH dehydrogenation was found unfavorable both kinetically and thermodynamically. On the other hand, CO formation is an exothermic process and was found favorable.
A theoretical study on the interaction of bimetallic Pt-Ni surface and the reactants of oxidative steam reforming reaction
(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2009., 2009.) Ercan, Elif.; Aksoylu, Ahmet Erhan.
Pt-Ni bimetallic system is an efficient catalyst for Oxidative Steam Reforming (OSR) reaction; the detailed experimental test results obtained from Pt-Ni system have been already given in literature. In those research papers, the importance of understanding the bimetallic surface properties and the interaction between active sites and OSR reactants has been mentioned. In the current study (i) the properties of several bimetallic alloy surfaces having different Pt/Ni ratio, pseudomorphic Pt layers formed on Ni sublayers and monometallic Pt and Ni surfaces have been determined, (ii) the adsorption properties of all possible monometallic and bimetallic sites for oxygen and methane have been studied and (iii) the electronic interaction between metal atoms of the active sites and OSR reactant have been analyzed. The results showed that, Pt addition to Ni decreases the surface stress and the most stable site is found as 'five Ni layers containing 25 per cent Pt at topmost layer' surface alloy based on surface energy calculations. The adsorption studies indicate that oxygen adsorption energy does not only depend on surface concentrations of Pt and Ni but also the number of Pt-O and Ni-O bonds on the fcc site. Additionally, it was found that surface stress may be an important parameter in adsorption properties of the surface.
An experimental analysis on catalytic dry reforming of methane (CDRM) performance of Ni/SBA-15 system
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Türkmen, Mustafa Olcay.; Aksoylu, Ahmet Erhan.
The main objective of this study is to design and develop an efficient Ni based and SBA-15 supported mono metallic catalyst in order to produce a high yield synthesis gas via catalytic dry reforming of methane. The priority was given to the inhibition of the coke deposition. In this context, first of all, the effect of the catalyst preparation & pre-treatment (i.e. impregnation technique, calcination and reduction temperatures) and reaction conditions (i.e. reaction temperature) was investigated on the reference catalyst, 10% Ni/SBA-15. Secondly, effect of the modifications (i.e. metal loadings as 7.5 and 15%, addition of CTAB and TMB to SBA-15, drying temperature) on the catalyst were investigated. During these tests, the activity and selectivity was determined in terms of CH4 and CO2 conversions, and of H2/CO ratio, respectively. The results of the performance tests suggested that the catalytic performance is highly dependent on the preparation route. It was seen that incipient wetness impregnation method favors catalytic performance in terms of both activity and selectivity. Additionally, higher calcination and reduction temperatures found to have a positive effect on the catalytic performance. Changing the metal loading (7.5 and 15%) led to an inverse impact in both activity and stability; whereas lower drying temperatures increased the catalytic activity and stability. Although modification of the SBA-15 by addition of CTAB and TMB enhanced the properties of the supporting material, Ni/CTAB_SBA-15 and Ni/TMB_SBA-15 catalysts showed lower reactant conversions with higher stabilities. Characterizations on the chosen samples indicated coke formation over the surface of the catalysts and the coke deposition could be suppressed with an appropriate preparation route. All in all, the results of the performance tests and characterization techniques suggested that 10% Ni/SBA-15 catalyst which was impregnated with the incipient wetness impregnation method and calcined at 750 °C for 6 hours with 2 °C/min ramping rate in air was the most promising CDRM catalyst. This idea was also supported by the stability test conducted at 750 °C for 72 hours.
An experimental study aiming to enhance the performance of OSR of a fuel processor
(Thesis (M.A.) - Bogazici University. Institute for Graduate Studies in the Social Sciences, 2017., 2017.) Öztepe, Cihat.; Aksoylu, Ahmet Erhan.
The aim of this study is to determine the optimum operating parameters of the methane OSR unit of the Fuel Processor Prototype-FPP yielding high methane conversion activity with suppressed H2/CO product selectivity through a Box-Behnken experimental design using temperature, S/C and O/C ratios of the feed, and the catalyst weight (or residence time, W/F) as the experimental parameters. The ranges of 350-450 °C, 3-5, 0.74-1.33, and 1.50-2.00 mg.min/ml were used as the experimental design parameter ranges for temperature, S/C feed ratio, O/C feed ratio and W/F, respectively. Additionally, a thermodynamic analysis was also performed for the same operation conditions in order to form a comparison basis for evaluating the performance test results, and for better understanding of the nature of OSR as well. Experimental CH4 conversion was increased by a rise in each of the parameters of temperature, S/C and O/C feed ratios; and this trend was confirmed via the thermodynamic analysis for the conditions used in the experiments. On the other hand, the observed effect of W/F on experimental CH4 conversion was nonconventional, i.e. an increase in W/F mostly resulted in a decrease in CH4 conversion. This was confirmed further by comparative analysis with the thermodynamic simulation results yielding lower CH4 conversion levels compared to those obtained in the experiments for W/F ratios of 1.50-2.00 mg.min/ml. In addition to CH4 conversion, product distribution of OSR varied with the changes in the reaction conditions. Consequently, as the trends in H2 production and H2/CO product ratio obtained as a function of W/F were -in general- opposite, the results reveal the existence of a Pareto optimal for W/F value, and strongly suggested the necessity of scrutinizing the relation between space time/space velocity (W/F) and reactor performance to find the practical operating conditions for the OSR reactor.
An experimental study on CO2 and butane adsorption on activated carbon
(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2009., 2009.) Oğur, Görkem.; Aksoylu, Ahmet Erhan.
The aim of this study is to investigate the effect of textural and surface chemical properties of activated carbon (AC) samples on their CO2 and butane adsorption capacities and adsorption behavior. In the first part of the study, the effects of surface chemistry, oxidation method, Na2CO3 impregnation and calcination temperature on selective CO2 adsorption behavior of activated carbon samples, which had been previously prepared by B. S. Çağlayan, was determined under dynamic conditions. Adsorption experiments were performed with a gravimetric analyzer and gas concentrations were analyzed with a mass spectrometer. Oxygen bearing surface groups of unimpregnated samples were determined via Temperature Programmed Surface Group Decomposition (TPSGD or TPD) deconvolutions. The results revealed that air oxidation increased the amounts of anhydrides and carbonyl/quinones whereas reduced the amounts of lactones and carboxyls. Total surface area and CO2 adsorption capacity increased with air oxidation. HNO3 oxidation had a strong influence on surface groups. Increase in the acidic carboxylic sites and reduction in the BET surface area upon HNO3 treatment, resulted in a decrease in the CO2 adsorption capacity. However, the results revealed that HNO3 oxidization has enhanced Na2CO3 dispersion more than air oxidation. This enhancement was attributed to increase in the carboxylic surface groups as well. It was discovered that Na2CO3 impregnation enhanced both reversible and irreversible CO2 adsorption. Calcination temperature, on the other hand, had no effect on the CO2 adsorption behavior up to 250 °C, however, sample calcined at 300 °C showed more superior adsorption capacity in comparison with other calcined samples. All samples showed selective CO2 adsorption behavior whereas Na2CO3 impregnated samples irreversibly adsorbed H2 and CO to a certain extent as the gases came into contact with the adsorbent. In the second part of the study, the relation between AC characteristics of commercial activated carbons (AC10, AC11 and AC12), which were provided by Ford Otosan A. Ş., and their butane adsorption capacity and characteristics were studied. For this purpose, butane adsorption isotherms were obtained at 40 °C, 70 °C and 100 °C by using a gravimetric analyzer in static mode. Oxygen bearing surface groups and total surface areas of the samples were determined via TPSGD/TPD and BET methods, respectively. Butane adsorption capacity of samples AC10, AC11 and AC12 was determined as 28.29%, 29.56% and 49.67% by weight at 1 bar and 40 °C respectively. Results revealed that there is a strong relationship between surface area and adsorption capacity. In addition, it was found out that lactone, phenol and carbonyl/quinone groups has no significant contribution on the adsorption capacity whereas, carboxyl and anhydride surface groups may have a limited positive contribution on the adsorption capacity. High correlation was obtained upon fitting Langmuir isotherm on the adsorption data showing butane adsorption on AC samples can be represented with Langmuir isotherm. The heat of adsorption was calculated as -13.2 kj/mol, -12.6 kj/mol and -16.5 kj/mol for samples AC10, AC11 and AC12 respectively from modified form of Clasius Clapeyron equation by using Langmuir constants obtained..
An experimental study on cobalt based catalytic DRY reforming of methane catalysts
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2014., 2014.) Yassı, Cansu.; Aksoylu, Ahmet Erhan.
The aim of this study is to design Co-based CDRM (catalytic dry reforming of methane) catalysts and to understand the CDRM reaction kinetics over their surface. The thesis consists of two parts. In the first part, a 5 wt% Co-2 wt% Ce/δ-Al2O3 catalyst was designed and tested for its CDRM performance. Temperature, CH4/CO2 feed ratio and space velocity were used as experimental parameters. In the second part, 10 wt% Co-2 wt% Ce/ZrO2 catalyst was prepared and kinetic tests were conducted over the catalyst in order to obtain a power law type and plausible mechanistic rate expressions for CDRM. For both catalysts, Ce was used as a promoter in order to increase the oxygen storage capacity via controlling the electronic structure of the metals over the support. For Co-Ce/δ-Al2O3 the results have shown that the catalyst suffers from severe coke deposition and consequent activity loss. In the tests, increasing temperature increased the activity although at low temperatures catalyst deactivated relatively slower. CDRM did not occur on the catalyst for high space velocities (for W/F value of mL/h g-cat.) though for the space velocities of 10000 mL/h g-cat. and 20000 mL/h g-cat., almost the same initial performances were observed. The kinetic study over Co-Ce/ZrO2 catalyst revealed that the reaction could be expressed by a simple power-law rate equation, with reaction order of 1.0751 for CH4 and -0.0988 for CO2, indicating that the reaction rate is proportional to the partial pressure of methane while there is a very small inhibition effect of carbon dioxide. This inhibition effect may be due to CO2 and CH4 competing for the same active sites. An ER mechanism having the reaction of adsorbed CO2 with gas phase CH4 leading directly to products, and an LH mechanism having surface reaction of the adsorbed reactants to form products H2 and CO as the rate determining steps are found plausible. On the other hand, the results clearly indicate that further FTIR-DRIFT analysis is necessary for finding the mechanism of CDRM reaction on Co-Ce/ZrO2 system. In the mixed feed tests to determine effect of product, the correlation coefficient (R2) was found so small that any observation could not be possible.
An experimental study on design and characterization of CO2 adsorbents
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2016., 2016.) Acar, Burcu.; Aksoylu, Ahmet Erhan.
The aim of this study is to design and develop AC-based CO2 adsorbent(s) having both high and stable CO2 adsorption capacity, and ability to adsorb CO2 selectively from CO2-CH4 mixture. In this context, a commercial activated carbon, Norit ROX, was oxidized by air and HNO3 oxidation, and two series of adsorbents, AC8 and AC9 series, respectively, were prepared on those oxidized ACs by K2CO3 impregnation following by calcination at various temperatures. The adsorbent samples were characterized by SEM-EDX for analyzing their microstructural properties and alkali dispersion on their surface. The clusters formed on AC8 adsorbents were not homogeneously distributed over the adsorbent, whereas AC9 adsorbents had homogeneously dispersed K-formations on their surfaces. Adsorption/desorption and selective adsorption tests were conducted for 0-1000 mbar pressure range under 50 ml/min gas flow rate at room temperature (RT), 120 °C and 200 °C for pure CO2, pure CH4 and their mixtures, 50% CO2-50% CH4 and 10% CO2-90% CH4. The adsorption/selective adsorption performance of AC9 series, were inferior to the performance of the AC8 series adsorbents. The highest CO2 adsorption capacity, ca. 11 wt.%, was observed for AC8-300 sample at RT. CO2 adsorption was confirmed to be reversible, whereas CH4 adsorption was partially irreversible. Although AC8-200 has the highest mass based CO2:CH4 adsorption selectivity ratio, ca. 3.7, at RT for the 50% CO2-50% CH4 mixture, for the 10% CO2-90% CH4 mixture, mass based adsorption selectivity ratio was at its highest on AC8-250 with a value of 0.59. AC8-200 was further tested for 0-5000 mbar pressure range at RT under 50 ml/min pure CO2 and pure CH4 flow and 90 ml/min total flow of 50% CO2-50% CH4 and 10% CO2-90% CH4 gas mixtures. The CO2 adsorption capacity was measured as 19.7 wt.% at 5000 mbar. The experimental adsorption isotherm data were fitted to Langmuir, Freundlich and Dubinin-Radushkevich (D-R) models. D-R model was the most successful in explaining CO2 and CH4 adsorption behaviors of AC samples. Between the kinetic models, pseudo-first order kinetic model successfully explained both CO2 and CH4 adsorption kinetics at RT, whereas CH4 adsorption kinetics on AC8-200 and AC8-300 were more suitably explained by pseudo-second order kinetic model.
An experimental study on design and development of efficient catalysts for dry reforming of methane (CDRM)
(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2010., 2010.) Paksoy, Aysun İpek.; Aksoylu, Ahmet Erhan.
The overall purpose of this research study is to design and develop effective Cobased bimetallic catalyst supported on zirconia for the production of synthesis gas via CO2 reforming of methane (CDRM). In order to increase the oxygen storage capacity of the catalyst, Ce was used as a promoter. The type of carbon deposited on catalysts during reactions performed under various reaction conditions was investigated to understand the effect of those conditions on type and abundance of coke formation. The reaction temperature, feed ratio and space velocity were the parameters used in activity and selectivity tests; activity was determined in terms of CH4 and CO2 conversions and selectivity was determined in terms of H2/CO product ratio in the product stream. Characterization of the 5 % Co-2 % Ce/ZrO2 catalyst indicated uniformly distributed metal particles allowing reactants to access the appropriate sites. The type of carbon deposited on catalysts under different reaction conditions was all filamentous. It was also clearly observed that the parts of the catalyst particles having high Ce/Co ratios have less carbon deposition, remarking the effect of ceria on the catalyst activity. On the basis of the performance test results, it was concluded that increasing temperature increased CH4 and CO2 conversions and H2/CO product ratio. CH4/CO2 feed ratio of 2/1 gave the lowest methane conversion but the highest values for carbon dioxide conversion and H2/CO product ratio. On the other hand, CH4/CO2 feed ratio of 1/1 presented similar results for carbon dioxide conversion and H2/CO product ratio with increased methane conversion. Additionally, it was found out that increasing space velocity decreased the conversion values and H2/CO product ratio without affecting the stability profile. Considering all of these parameters, it was suggested that CH4/CO2 ratio of 1/1 with high temperature and low space velocity were the optimum reaction conditions for CDRM over Co-Ce/ZrO2 system.
An experimental study on design, development and perormance evaluation of catalytic DRY reforming of methane catalysts
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2014., 2014.) Bal, Hazal.; Aksoylu, Ahmet Erhan.
The overall purpose of this research study is to design and develop effective non-noble catalysts having high and stable activity with suitable H2/CO product selectivity for the production of synthesis gas via CO2 reforming of methane (CDRM). In this context, bimetallic Co-La/ZrO2, and trimetallic Co-Ni-La/ZrO2 and Co-Ni-Ce/ZrO2 catalysts were prepared, characterized and tested for their CDRM performance. The freshly reduced and spent samples were characterized by SEM-EDX. The reaction temperature, feed composition, space velocity, reaction time and nickel loading were the parameters used in activity and selectivity tests. In the catalysts, Co and promoter, i.e. La and Ce, loadings were kept fixed as 5 wt.% and 2 wt.%, respectively, while two Ni loading levels, 3 wt.% and 5 wt.%, was used in trimetallic samples. The results have shown that Co-La catalyst exhibited high stability, but only has moderate activity in CDRM. The introduction of nickel led to an increase in the activity which may be related with high dehydrogenation activity of Ni and strong and extensive Co-Ni surface interaction. The results show that Co:Ni loading ratio in the trimetallic catalyst has a significant effect on CDRM performance for the La promoted samples. There was no significant change in terms of activity and stability occur Co-Ni-Ce/ZrO2 system in response change in Ni:Co ratio. Characterization of the catalysts indicated uniform distribution of metal on both Ce and La promoted trimetallic catalysts. The type of carbon deposited on trimetallic catalysts was all filamentous. It was also clearly observed that Ce-promoted catalyst have less carbon deposition than La-promoted one, remarking the effect of ceria on the stable activity of the catalysts through CeOx redox cycle regulating surface oxygen transfer. On the basis of the performance test results, it was concluded that increasing temperature increased CH4 and CO2 conversions and H2/CO product ratio. The results have shown that when the CH4:CO2 feed ratio is greater than 1.0, the lowest methane conversion, the highest carbon dioxide conversion and H2/CO product ratio were observed for all catalysts. It was found out that increasing space velocity decreased the conversion values and H2/CO product ratio.
An experimental study on mixed reforming performance of Co-based catalysts
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2014., 2014.) Erşahin, Serhat.; Aksoylu, Ahmet Erhan.
The major purpose of this research study is to investigate the mixed reforming, both CDRM + SR and CDRM + POX, activity over Co-Ce/ZrO2 bimetallic catalyst, and to experimentally determine the optimal mixture of oxygen sources for the improved catalytic performance. 10 wt.%Co-2 wt.%Ce/ZrO2 catalyst was prepared and tested for its mixed reforming performance. Temperature, CH4/CO2 feed ratio and additional oxygen concentration levels in the feed during mixed reforming tests were used as experimental parameters. The results have shown that, mixed reforming, CDRM + SR and CDRM + POX, activity increase with an increase in temperature. The overall performance and stability in mixed reforming compared to those in individual CDRM has improved with the addition of steam and oxygen. It is observed that the lower CH4/CO2 feed ratios have resulted in higher CO2 conversion and better selectivity. It is concluded that stable activity can be achieved and H2/CO product ratio can be controlled by changing additional oxygen source concentration in the feed. Coke deposition in reactor is decreased with increasing O2 concentration levels in the feed. It is also shown that the additional oxygen source, steam or oxygen, doesn’t significantly affect activity, but the H2/CO product selectivity is closer to unity in CDRM + POX mixed reforming. XPS analysis of freshly reduced and spent catalysts have shown that CO2 conversion is positively affected by surface Co2+/Co3+ ratio. In CDRM+POX mixed reforming, Ce3+/Ce4+ ratio decreases at higher conversion values, due to high utilization of oxygen on active sites. The optimal mixed reforming condition is found as high temperature, low CH4/CO2 feed ratio and addition of O2 with concentration level enough to prevent coke deposition.
An experimental study on optimization of Pt-based trimetallic WGS catalysts
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2017., 2017.) Kesim, Bahar.; Aksoylu, Ahmet Erhan.
The overall purpose of this study is to design and develop Pt-based, CeO2 supported trimetallic Pt-Re-V/CeO2 catalysts(s) having high WGS activity, selectivity and stability with suppressed methanation under realistic feed (i.e. feeds simulating typical reformer outlet) flow in HTS-LTS transition temperature region, allowing the use of a single WGS reactor in fuel processors. In this context trimetallic Pt-Re-V/CeO2 catalysts prepared by incipient to wetness impregnation were characterized and tested for their WGS reaction performance. In the performance tests, metal loading levels, feed composition and temperature were used as the experimental parameters. Both freshly reduced and spent catalysts were characterized by XPS and Raman spectroscopy analyses. The results revealed that 1Pt-0.5Re-1V/CeO2 and 1Pt-0.5Re-0.5V/CeO2 have great potentials to be used in a fuel processor as the WGS catalyst owing to their high activity, stability and selectivity. Another important point is that those catalysts can make the use of a single stage WGS unit in an FP instead of conventional two stage (HTS and LTS) WGS unit. XPS analysis reported Ce3+ content is slightly higher in the case of the catalysts with higher WGS activity since Ce3+ plays a crucial role in increasing electron transfer ability from support to metallic sites and Ce3+ ions are highly active towards reactants due to excess electrons that are left behind when an oxygen atom is removed. Raman spectroscopy results showed no detectable bands for bulk V2O5 crystals in any of the samples. This result indicates that vanadium is highly dispersed at the surface without bulk V2O5 formation. XPS and Raman spectroscopy results showed the formation of CeVO4 species and VO2 compound, which has been reported to have promotion effect in the WGS activity, were present on the catalyst. Raman results also revealed the existence of polyvanadate surface species in the catalyst. Moreover, no coke formation was observed on any spent samples.
An experimental study on Pt-based bimetallic oxidative steam reforming (OSR) catalysts
(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2009., 2010.) Göçmen, Bayram Ali.; Aksoylu, Ahmet Erhan.
In the current study, Pt-Ni/Al2O3 system was optimized for catalytic hydrogen production via oxidative steam reforming of propane. Oxidative steam reforming of propane was tested over four Pt-Ni/d-Al2O3 bimetallic and one Pt-Au-Ni/d-Al2O3 trimetallic catalysts aiming to investigate the effect of Ni:Pt loading ratio on catalyst performance. Catalyst types and reaction conditions were firstly arranged to identify the effect of reaction temperature, carbon to oxygen ratio, residence time, and catalyst loadings on hydrogen production. The catalysts were prepared as having two different levels of Pt loadings (0.2 wt % and 0.3 wt %) and two different levels of Ni loadings (10 wt % and 15 wt %) on alumina support. Gold was added to the Pt-Ni system, aiming to see its affect both on OSR performance of the catalyst as well as its secondary WGS activity. The effect of C/O2 ratio on the hydrogen production and H2/CO selectivity were found dependent on the Pt and Ni loadings, and their loading ratio (Ni:Pt) as well. The highest hydrogen production and H2/CO ratio levels were obtained for the highest C/O2 ratio tested. Lower Pt loadings have made the catalysts more sensitive to the changes in C/O2 ratio. An optimum Ni:Pt ratio was found around 50 by weight due to suppressed methanation and enhanced hydrogen production activities of these catalysts. The presence of gold in the trimetallic catalyst caused poor activity and selectivity in comparison to bimetallic catalysts.