Theses supervised by Prof. Dr. Can Erkey
28 theses · Koç University
Preparation of Pt/Al2O3 and PtPd/Al2O3 diesel oxidation catalysts by supercritical deposition
Diesel oxidation catalysts (DOCs) are aftertreatment system parts responsible for the oxidation of CO, unburnt hydrocarbons and NO gases coming from heavy-duty diesel engines. In this study, DOCs were prepared, characterized, tested and compared with their commercial counterparts. In the first part of this study, four commercial monolithic DOCs with two different platinum group metal (PGM) loadings and Pt:Pd ratios of 1:0, 2:1 or 3:1 (w/w) were investigated systematically for NO, CO, and C3H6 oxidation, CO – C3H6 co-oxidation, and CO – C3H6 – NO oxidation reactions via transient activity measurements in a laboratory scale simulated diesel engine exhaust environment. As PGM loading increased, light-off curves shifted to lower temperatures for individual and co-oxidation reactions of CO and C3H6. CO and C3H6 were observed to inhibit the oxidation of themselves and each other. Addition of Pd to Pt was found to enhance CO and C3H6 oxidation performance of the catalysts while the presence and amount of Pd was found to increase the extent of self-inhibition of NO oxidation. NO inhibited CO and C3H6 oxidation reactions while NO oxidation performance was enhanced in the presence of CO and C3H6 due to the probable occurrence of reduced Pt and Pd sites during CO and C3H6 oxidations. The optimum Pt:Pd ratio for individual and co-oxidations of CO, C3H6 and NO was found to be Pt:Pd = 3:1 (w/w) in the range of experimental conditions investigated in this study. Pt/Al2O3 and bimetallic PtPd/Al2O3 catalysts were prepared via supercritical deposition (SCD) method using supercritical carbon dioxide. The effects of Pt loading of Pt/Al2O3 and Pd addition to Pt/Al2O3 on particle size, particle size distribution (PSD) and activity for NO and C3H6 oxidation and C3H6-selective catalytic reduction (C3H6-SCR) were investigated. Pt/Al2O3 catalysts were prepared with Pt loadings of 0.6, 1.2 and 2.1 wt% and a bimetallic PtPd/Al2O3 catalyst was prepared with total metal loading of 1.4 wt% and Pt:Pd molar ratio of 1.3:1. A small fraction of the particles agglomerated after calcination at 550 oC. Around 98% of the particles had an average particle size of 1 nm. The rest of the particles were larger and average size of these larger particles was 10 nm for monometallic catalysts and 6.5 nm for PtPd/Al2O3. All catalysts were found to be active for NO and C3H6 oxidation and C3H6-SCR reactions. NO oxidation performance of 1.2 wt% Pt/Al2O3 catalyst was the highest. C3H6 oxidation activity increased with increasing metal content. Light-off temperature for C3H6 oxidation shifted to higher temperature in the presence of NO, suggesting competitive oxidation of C3H6 and NO. Concentration profiles indicated that C3H6-SCR started when C3H6 conversion by oxidation reached 50%; C3H6 was consumed both by oxidation and C3H6-SCR at higher conversions. Morphologies of used and aged catalysts were investigated. HAADF-STEM imaging revealed that bimodality of the nanoparticles, sharp PSD and average nanoparticle sizes were maintained after usage in NO and C3H6 oxidation reactions. After thermal ageing at 800 oC, 1.2 wt% Pt/Al2O3 preserved the bimodality of the nanoparticles with slightly higher average nanoparticle size. 2.1 wt% Pt/Al2O3 prepared by SCD was compared with commercial monolithic Pt/Al2O3 DOC for NO oxidation performance. The DOC prepared by SCD performed better at temperatures lower than 325 oC. While maximum conversions achieved by both catalysts were more than 50%, commercial DOC achieved slightly higher values at a higher temperature. C3H6 oxidation kinetics of 2.1 wt% Pt/Al2O3 and bimetallic PtPd/Al2O3 prepared by SCD were investigated in a tubular flow reactor at differential conditions and in exhaust environment of diesel engines. Langmuir-Hinshelwood reaction mechanism with dissociative adsorption of O2 was assumed. C3H6-TPD results suggested dissociative adsorption of C3H6 as well. MATLAB was used to determine rate constants, adsorption equilibrium constants and exponents in the rate expressions by nonlinear regression. Pre-exponential factors and apparent activation energies were calculated using Arrhenius Law. For both catalysts, increase in C3H6 feed concentration decreased C3H6 oxidation reaction rate at isothermal conditions. The reaction rate increased with increasing O2 concentration for 2.1 wt% Pt/Al2O3 and it was zeroth order in terms of O2 for PtPd/Al2O3. Apparent activation energies of the oxidation reaction were calculated as 84.3 kJ mol-1 and 17.9 kJ mol-1 respectively for 2.1 wt% Pt/Al2O3 and PtPd/Al2O3. Average error between experimental and theoretical rates were calculated as 18.3% and 13.6% respectively. To our knowledge, this is the first study in which bimetallic PtPd/Al2O3 catalyst was prepared by SCD. This is also the first study on the kinetics of C3H6 oxidation over a DOC prepared by SCD and one of the very few studies where the presence of H2O in the feed mixture was taken into account during the investigation of C3H6 oxidation reaction kinetics.
Investigation of fluidization regimes and coating process for alginate aerogel particles in a wurster fluidized bed
Aerogels are solid materials with high porosities, open and inter-connected pore structures, and high surface areas. Over the last decade, there are an increasing number of research efforts concerning in coatings of aerogels since the combination of the porous structure of aerogels with the morphological, mechanical, and functional properties of a coating material leads to outstanding performances in different applications. In this study, coating process was investigated for highly porous alginate aerogel particles in a laboratory scale Wurster fluidized bed. In the first part of this study, fluidization regimes for alginate aerogel particles with two different particle sizes were characterized in the Wurster fluidized bed both in the annular zone and the tube zone without spraying. For both particle sizes, minimum fluidization and bubbling regimes were observed both in the zones whereas pneumatic regimes existed only in the tube zone. Moreover, a new regime which was horizontal circular motion was identified for large particles in the tube zone. There existed two other regimes in the bed which were turbulent and circulatory particle motion regimes. All fluidization regimes were mapped on Kunii and Levenspiel diagrams. Circulatory particle motion regime was given on these diagrams for the first time. Furthermore, effects of particle size, batch volume, Wurster tube size, perforated plate geometry and partition gap height on the boundaries of each fluidization regime were investigated by measuring superficial air velocities at the onset and the end of each regime. In general, increasing particle size and partition gap height led to an increase in superficial air velocities at the onset and the end of each regime. With increasing tube diameter and tube length, superficial air velocities at the onset and the end of pneumatic, turbulent and circulatory particle motion regimes increased. Increasing batch volume for small particles caused a decrease in superficial air velocities at the end of bubbling and pneumatic regimes and the onset of turbulent and pneumatic regimes. At the onset and the end of bubbling, turbulent and pneumatic regimes, superficial air velocities for large particles for 400 ml batch volume were higher than superficial air velocities for large particles for 200 ml batch volume. In this part, circulatory particle motion regime which is desirable for a successful coating process was particularly investigated in detailed. The results showed that there is an upper limit for each parameter to obtain a circulatory motion of the particles. It was found that the partition gap height should be 2 cm for proper particle circulation. Maximum batch volume for the tube with 10 cm diameter was found as 500 ml whereas maximum batch volume was 250 ml for the tube with 8 cm diameter. In the second part of this study, alginate aerogel particles were successfully coated with an aqueous polymer solution in a Wurster fluidized bed without damaging the pores of aerogels. The polymer was a mixture of hydroxypropyl cellulose (HPC) and copovidone and is generally used as a protective coating for pharmaceutical dosage forms. Variations occurring in coating layer thickness around alginate aerogel particles and changing coating layer surface morphology with coating time, bed temperature, atomizing air pressure and polymer rheology were investigated for the first time. Moreover, phase and rheological behavior of the coating polymer solution and individual solutions of HPC and copovidone were studied at different temperatures to understand their spreading and adhesion mechanisms on aerogel surfaces. Several sets of experiments were conducted at three different bed temperatures and atomizing air pressures. Coating time for all the runs ranged from 5 minutes to 40 minutes and the coating layer thickness ranged from 12.4 ± 4.6 µm to 170.6 ± 43.3 µm. The smoothest coating layer surface and the highest coating efficiency which was 69.2 ± 0.4 % with a linear increase in coating layer thickness were achieved at 50 °C with 1.7 bar. An increase in atomizing air pressure from 1.5 bar to 1.7 bar resulted in a smoother coating layer. A high mean coating polymer solution droplet velocity with a narrow droplet size distribution led to a homogeneous spreading and less variance in coating layer thickness at 1.7 bar. It was found that changing bed temperature led to more important changes in coating layer thickness compared to atomizing air pressure whereas both bed temperature and atomizing air pressure affected coating layer surface morphology to a great extent. Finally, drug loaded, and unloaded alginate aerogel particles were successfully coated with a methacrylic acid-ethyl acrylate copolymer aqueous solution using a Wurster fluidized bed. For unloaded aerogels, atomizing air pressure was set to 1.7 bar depending on optimized conditions obtained in the protective coating. To prevent particle breakage and provide an increase in coating layer thickness, atomizing air pressure was changed during the coating process for drug loaded aerogels between 1.3 bar and 1.5 bar. The highest coating layer thickness was 50 ± 5 μm and was reached in 50 minutes for unloaded aerogels whereas coating layer thickness was found as 83.8 ± 11.9 μm in 3 hours for drug loaded aerogels. Two different coating layer surface morphologies on the coating layer were observed as bead and fiber for both unloaded and loaded aerogels. Rheology experiments showed that coating polymer stayed stable in a wide range of frequency domain and its viscosity was nearly constant in lower Newtonian region in applied atomizing air pressure range. Therefore, different coating mechanisms at different atomizing air pressures may lead to bead and fiber shaped surface morphologies. Subsequently, drug loaded, and coated alginate aerogels were used as drug carriers. Ibuprofen was used as a model drug and its release from coated and uncoated aerogels were investigated both in the acidic and basic mediums. It was first time shown in the literature, ibuprofen release in the acidic medium was prevented via synergetic effects of coating polymer with a proper coating layer thickness and alginic acid layer around the aerogels. In the basic medium, uncoated and coated aerogels provided different release profiles compared to the release profile of crystalline ibuprofen. Enteric coating led to a decrease in the release rate whereas ibuprofen release rate was increased with uncoated aerogels.
Novel MOF/aerogel composites ((MOFACs) for drug delivery purposes
Composites of MOFs and aerogels (MOFACs) are a new class of nanostructured materials attracting increasing attention due to their favorable properties. The combination of micro-/mesoporosities of MOFs with meso-/macroporosities of aerogels makes MOFACs hierarchically multimodal porous materials. MOFACs with their high surface areas, combined morphological, mechanical, physicochemical and functional properties of both MOFs and aerogels have demonstrated outstanding performances in various applications. In the field of drug delivery, MOFs and aerogels have both been investigated extensively. Combining bimodal porosity and functionality of both MOFs and aerogels is a promising area for drug delivery applications. In this study, novel Metal-Organic Frameworks/Aerogel composites (MOFACs) comprised of different types of MOFs (Fe-BTC, ZIF-8 or UiO-66) and calcium alginate were synthesized. Spherical bead shaped composites were obtained by incorporating microporous MOFs into meso- and macroporous aerogel matrix by simple mixing combined with dripping technique. The composites were characterized with nitrogen sorption, XRD, SEM and FTIR. The nitrogen adsorption study showed that the MOFACs synthesized had achieved hierarchically macro-, meso- and micro porosities and MOFs micropores were open and accessible. XRD analysis revealed that the MOFs were intact and in crystalline form. FTIR study showed that the MOF and alginate matrix formed a physical mixture in the composite. The synthesized composites were further loaded with two different drugs paracetamol (acetaminophen) or ibuprofen. Paracetamol was loaded from ethanolic solutions via SAS (supercritical antisolvent precipitation) inside the pores whereas ibuprofen was loaded from supercritical CO2 solutions. The factors that affect the loading amount and distribution of the drug inside the composite matrix were investigated. Paracetamol loadings were found to be highly dependent on and limited by the drug concentration in ethanolic loading solutions as well as having excess solutions in the extraction vessel prior to supercritical drying. Almost 70 wt% paracetamol loadings were achieved with 1.2 M loading solutions and having excess solutions. Around 20 wt% ibuprofen loadings were achieved from supercritical CO2 solutions. The release behavior of the drugs from the composite matrices to PBS buffer or distilled water were investigated. It was shown that the Fe-BTC/Alginate composites were promising for paracetamol delivery and ZIF-8/Alginate composites were found to be promising for ibuprofen delivery as both systems achieved delayed drug delivery with increasing amounts of MOF content in the composites. Lastly, all release kinetics were fitted into Korsmeyer-Peppas model. It was found that Fickian diffusion is the main transport mechanism while some PBS systems with excess solutions also showed swelling behavior.
Supercritical CO2 assisted preparation of Pt, PtCu, PtZn, PtCo and PtZnCo nanoparticles on various carbon aerogels as ORR electrocatalysts
Monometallic Pt, bimetallic PtCu, PtZn, PtCo and trimetallic PtZnCo nanoparticles on various carbon aerogels and N-doped carbon aerogels were prepared using supercritical deposition technique. Electrochemical oxygen reduction performance of the electrocatalysts was investigated along with material properties. For the first part of the thesis carbon aerogel (CA) and Vulcan supported PtCu electrocatalysts were prepared using the simultaneous and sequential in-situ supercritical deposition (SCD) method followed by thermal annealing and electrochemical dealloying. Effect of deposition technique to PtCu nanoparticle morphology and electrochemical performance was investigated. Highly dispersed PtCu alloy nanoparticles with small nanoparticle sizes were obtained by both routes. Simultaneous SCD resulted in a more uniform PtCu composition in PtCu alloy nanoparticles before dealloying, whereas sequential SCD led to Cu-rich surface on the PtCu alloy nanoparticles. After dealloying, PtCu/CA electrocatalyst prepared by simultaneous supercritical deposition had an enhanced electrochemical surface area of 159.4 m2/g due to the synergistic effects of PtCu nanoparticle size and PtCu composition in nanoparticles. All dealloyed electrocatalysts had higher mass activities and PtCu/CA electrocatalyst prepared by simultaneous SCD had a mass activity of 0.15 A/mgPt which was 2-fold of the mass activity of commercial Pt/C. PtCu/CA electrocatalyst prepared by sequential SCD showed a mass activity of 0.08 A/mgPt which was slightly higher than the mass activity of commercial Pt-C (0.07 A/mgPt) In the second part of the thesis effect of N-doping on electrochemical performance was investigated. Pt nanoparticles on polyamide aerogel (PA) derived CAs were prepared using SCD technique. PAs were pyrolyzed at 800 oC and some monoliths were subsequently etched with CO2 at 1000 oC to increase mesoporosity and surface area to demonstrate the effect of pore volume to Pt nanoparticle dispersion and electrocatalytic performance towards oxygen reduction. The N-rich backbone of PAs yielded homogenously distributed N atoms in the CA structure enabling homogenous distribution of Pt nanoparticles, efficient dispersion of the Nafion ionomer and possible ORR-active sites. Highly dispersed Pt nanoparticles with average size of 1.5 and 3.0 nm were obtained on carbon aerogels from polyamide aerogels resulting from pyrolysis (CPA) or pyrolysis and reactive CO2-etching (ECPA), respectively. Both electrocatalysts had only graphitic and pyridinic N-sites with the former being the dominant species. Oxygen reduction mass and specific activity of Pt-ECPA were 4- and 3-fold of the mass and specific activity of commercial Pt-C, respectively. Pt-CPA also showed similar mass and specific activity to that of commercial Pt-C due to lower mesopore volume and higher average Pt nanoparticle size. Accelerated stability tests (AST) revealed superior stability of Pt-CPA electrocatalyst due to favorable initial Pt nanoparticle size enabling successful immobilization Pt nanoparticles on CPA through the N-functionalities. In the final part of the thesis synergistic effects of alloying and nitrogen doping was investigated. Monometallic Zn, Co and bimetallic ZnCo-Alginate aerogels (AA) were prepared using sol-gel technique followed by crosslinking with polymeric 4,4'-Diphenylmethane diisocyanate (pMDI) and supercritical drying. Pyrolysis of the crosslinked AAs resulted in metal and nitrogen doped CAs (NCA). Three different N-species were present on the metal doped-NCAs; graphitic, pyridinic and pyridinic N-oxides with high loadings indicating crosslinking is an efficient N-doping technique. Pt nanoparticles were deposited on the Zn, Co and ZnCo-NCAs using SCD technique followed by thermal annealing. Bimetallic PtZn, PtCo and trimetallic PtZnCo alloy nanoparticles were obtained with different Pt:Co:Zn molar ratios. Average nanoparticle size of the electrocatalysts with 5 wt.% initial pMDI concentration was around 6 nm for trimetallic Pt-ZnCo-NCAs and around 9 nm for 10 wt.% initial concentration. Highest average nanoparticle sizes were obtained for Pt-Zn-NCAs (12.5 and 28.6 nm) and the lowest were obtained for Pt-Co-NCAs (5.9 nm and 4.1 nm). A volcano type structure-activity relationship was established with respect to Co and Zn mole ratios in the electrocatalysts. Higher mass activity was obtained for Pt:Co:Zn mole ratio of 68:24:8. The electrocatalyst showed 1.5-fold higher mass activity and 9-fold higher specific activity than commercial Pt-C at the half of the Pt loading (9.7 wt.% and 20 wt.%, respectively).This electrocatalyst also showed the highest stability over 2,500 potential cycles.
Kinetic study for hydrodesulfurization processes of diesel fuels over CoMo-based catalyst
Diesel is an important fuel for transportation vehicles and is to be produced in compliance with Euro V specifications. The sulfur content diesel fuels is limited to be maximum of 10 ppm and deep desulfurization process is required to reduce the sulfur content of the diesel feed. Refineries process various kinds of crude oils containing high amounts of nitrogen, aromatics, metals, and sulfur compounds, with complex structures. In addition, the quality of the crude oil is also being declining; the refineries operate at the commercial units including hydrodesulfurization (HDS) processes at more severe conditions. Several changes are also implemented by the refineries such as using more active catalysts, increasing hydrogen consumption, operating temperature and pressure, improving the reactors and understanding reactor and reaction modeling. However, severe operating conditions cause rapid catalyst deactivation and decrease the catalyst life. To optimize the operating conditions and maximize the catalyst performance, HDS kinetic models are important for reviewing the catalyst activity and comparing the activities of the catalysts in the case of selecting and evaluating new catalysts i.e., catalyst screening. Therefore, it is necessary to understand the nature of HDS process for diesel feeds with respect to different operating conditions. Apparent HDS kinetics applied for industrial real feeds depend significantly on feed properties having variety of different sulfur compounds, nature of the hydrocarbon matrix, operating conditions and type of catalysts. Therefore, kinetic parameters determined for a study is specific to the related process. The purpose of this study is to investigate apparent kinetic model for diesel HDS reaction using commercial CoMo/Al2O3 catalyst based on sulfur distribution data. In the first part of the work, different characterization methods are applied to commercial CoMo/Al2O3 catalyst. In the second part of the work, HDS performance tests were conducted at different operating conditions such as temperature, pressure and H2/oil ratio and LHSV. A pilot plant is designed, constructed and started-up during the course of the work to carry out the performance tests with continuous, safe, 24/7 unattended operations. The diesel feed and the obtained diesel products were analyzed with extensive methods to investigate the changes in the product properties and catalyst activity. In the third part of the work, apparent HDS kinetic models are derived with power law model based on total sulfur of the feed and the products as a reference point to compare with literature. In the fourth part of the work, apparent HDS kinetic model was investigated using the sulfur distribution of the feed and the liquid products by dividing into several sub-fractions using the sulfur distribution data from CNS-Simdis analysis. Then, power law model based on the sulfur contents of the related sub-fractions was applied to determine the apparent reaction rate order and activation energy of each sub-fraction. In the fifth part of the work, the feed and the products were separated physically into several fractions with distillation and the properties of the fractions were analyzed. Power law model was applied to each fraction to compare with the results obtained in the third part of the work.
Synthesis and characterization of novel sustainable dual crosslinked sodium carboxymethyl cellulose aerogels
Aerogels are remarkable nanoporous materials with unique properties such as low density, high porosity, high specific surface area, and interconnected pore networks. In addition, their ability to be synthesized from various precursors such as inorganics, organics, or hybrid, and the tunability of their properties make them very attractive for many applications such as adsorption, thermal insulation, catalysts, tissue engineering, and drug delivery. The physical and chemical properties and pore structure of aerogels are crucial in determining their application areas. Moreover, it is possible to tailor the aerogel properties to meet the specific requirements of each application. Synthesis of novel carboxymethyl cellulose (CMC) aerogels for thermal insulation applications is an essential area of research in finding alternative sustainable insulation materials instead of petroleum-based ones. Previous studies have been mainly focused on polysaccharide aerogels, especially cellulose aerogels. Although extensive research has been carried out on cellulose ether hydrogels or films, cellulose ethers and the effects of their functional groups on gelation mechanism, aerogels' morphology, and thermal properties have not been widely investigated. Moreover, when these cellulose ethers are used to synthesize aerogels, their functional groups can play a crucial role in controlling the morphology of the resulting aerogel. Modifying the synthesis parameters such as solvent choice, concentration, pH, and temperature makes it possible to tune the interactions between cellulose ethers and the solvent, leading to the formation of aerogels with tailored morphologies. Furthermore, adding crosslinking agents or other additives can also influence cellulose ether-based aerogels' gelation kinetics and final morphology. This tunability makes cellulose ether-based aerogels promising materials for various applications, including insulation, drug delivery, and tissue engineering. The objective of this study is to synthesize CMC aerogels and also, at the same time, preserve their 3D porous structure and shape. To obtain CMC hydrogels with a 3D porous structure, the freeze-thaw induced gelation method was used and effecting factors were investigated. Since CMC aerogels obtained via supercritical drying have not been studied extensively in the literature, solvent exchange conditions for CMC hydrogels were determined. We aimed to elucidate the pore size distribution, specific surface area, and porosity of these novel materials, thereby paving the way for future exploration of their potential applications in diverse fields. Our work expands the available synthesis methods for CMC aerogels and provides crucial insights into their fundamental pore characteristics, contributing to the ongoing development of sustainable and environmentally friendly aerogels. In this study, sustainable maleic anhydride crosslinked CMC aerogels were synthesized by freeze-thaw induced gelation method. The effect of precursor concentration, crosslinker concentration, and gelation parameters on the aerogel properties, such as density, volumetric shrinkage, surface area, pore diameter, and thermal properties, were investigated. The prepared aerogels exhibited low density between 0.051 g/cm3 and 0.204 g/cm3, high porosity (> 93%), and high specific surface areas up to 214 m2/g. The crosslinking mechanisms in CMC aerogels were examined by Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) analysis. The lowest thermal conductivity was measured as 0.038 W/mK. A theoretical equation was also developed to calculate the thermal conductivity of aerogels. The results indicated that it is possible to tune the aerogel properties by adjusting biopolymer or crosslinker concentration and changing the freeze-thaw parameters. PVA was incorporated into CMC aerogels to promote intermolecular crosslinking and, therefore, reduce the pore diameter. CMC/PVA hybrid aerogels from 4 and 6 wt% aqueous solutions were prepared, and the porous structures of hybrid aerogels were investigated. CMC/PVA hybrid aerogels had low bulk density and high porosity (> 95%). FTIR and NMR analyses were conducted to confirm the esterification reaction. The thermal conductivity of CMC/PVA hybrid aerogel was measured as 0.055 W/mK. Finally, sulfuric acid catalyzed aerogels were synthesized, and it was seen that it is possible to reduce the gelation time with an acid catalyst. The effect of acid concentration on the aerogel properties, such as density, volumetric shrinkage, surface area, pore diameter, and thermal properties, was investigated. The crosslinking reaction was confirmed by FTIR analysis. Contrary to expectations, sulfuric acid catalyzed aerogels had low porosity, low surface area, and high pore size. However, these results shed light on the potential to tailor aerogel properties using acid catalysts precisely. These findings provide valuable insights for optimizing aerogel synthesis tailored to specific applications.
Supercritical ion exchange synthesis and kinetic modeling ofselective catalytic reduction catalysts for diesel and hydrogenengine aftertreatment systems with engine control unitoriented implementation
This thesis investigates the synthesis, characterization, kinetic modeling, and control-oriented implementation of catalysts for selective catalytic reduction of NOx by ammonia (NH3-SCR) applications in advanced aftertreatment systems, including hydrogen internal combustion engines (H2-ICEs) and diesel engines. Various zeolite frameworks (SSZ-13, ZSM-5, and MOR) were ion exchanged with Copper(II)trifluoroacetylacetonate (Cu(tfa)2) using Supercritical and Aqueous Ion Exchange (SCIE and AIE) methods. Catalytic activity assessments revealed that Cu/MOR synthesized via SCIE exhibited superior NO conversion compared to AIE. Furthermore, SCIE enabled site-selective copper exchange, by varying synthesis temperature (40–80 °C) and Cu(tfa)2 concentration, tuning the distribution of ZCuOH and Z2Cu species located on the 8 membered-rings and 6 membered-rings of SSZ-13. Spectroscopic techniques (UV–Vis and ATR-FTIR) revealed that ZCuOH species dominated at high SCIE temperatures, while Z2Cu became more prevalent with increased Cu precursor concentration. The NH3-SCR performance of a commercial Cu/CHA catalyst was also evaluated under H2-ICE relevant conditions, including 175-760 ppm of NOx and NH3, 1–20% H2O, 1–14% O2, and 500 ppm H2 across 150–490 °C. NH3 uptake decreased by ~40% as H2O content increased from 1% to 20%. In Standard SCR conditions (NO/NOx=1), low-temperature NOx conversion decreased notably with increasing water content—dropping by up to 30% at 200 °C when H2O increased from 1% to 20%. However, at higher temperatures, water exerted a promoting effect: NOx conversion improved with increasing H2O and consistently exceeded 99% above 250 °C at 60,000 h⁻¹ gas hourly space velocity. In Fast SCR conditions (NO2/NOx=0.5), the impact of water was less pronounced, and high NOx conversion (>95%) was achieved even at 200 °C. Co-fed hydrogen (500 ppm) had minimal effect below 400 °C but slightly impacted high temperature NOx efficiency and N2 selectivity. To enable real-time application in Engine Control Units (ECUs), a Reduced Order Model (ROM) of the NH3-SCR process was developed. The model was calibrated using synthetic gas bench data and validated against dynamometer tests with 7.5 L (close coupled) and 41 L (underfloor) commercial SCR reactors. The ROM accurately predicted transient NO, NH3, and N2O behavior under World Harmonized Test Cycle (WHTC) conditions with less than 5% error as well as fluid/solid thermal behavior, offering a robust solution for ECU-integrated urea dosing strategies.
Yüksek sıcaklık uygulamaları için monolitik karbon siyah-silika aerojel kompozit malzemelerin gözenek yapılarının incelenmesi
The macroscopic properties of aerogels such as thermal, optical and mechanical are a result of the micro structures of these materials. In this thesis carbon black ? silica composite aerogel materials were prepared with different carbon contents and the effects of carbon content on the pore properties and the micro structure of silica aerogels are reported which were not investigated in literature. Aerogels were synthesized by two step sol - gel method using TEOS (Tetraethylorthosilicate) as precursor, HCl and NH4OH as catalysts, ethanol as solvent and dried by the supercritical CO2 extraction method. The effects of TEOS concentration, gelation and aging temperature on pore properties of aerogels were investigated. Increasing TEOS concentration in the sol shifted the pore size distributions to smaller pore diameters with a sharper peak. The alcogels were aged in water-ethanol solutions at different temperatures which caused the pore volume to increase dramatically. Cabot Vulcan XC72R was used for the synthesis of carbon black-silica aerogel composites. The powder premixed with ethanol was added to the silica sol just before the gelation step. It was shown that addition of carbon black to the sol at different ratios did not affect the porous structure of the silica matrix covering the carbon black particles. By this method silica aerogels with a pore volume of 5.3cc/g and carbon black silica composite aerogels with a pore volume of 4.6cc/g and surface areas on the order of 1000 m2/g were synthesized.
Karbon destekli tek metalli Pd, Pt ve iki metalli Pd-Pt nanoparçacıkların süperkritik CO2 depozisyon yöntemiyle hazırlanması
Metal nanoparticles supported on high surface area carbon substrates are used extensively as catalysts for a wide variety of reactions. Among these catalysts, carbon supported single Pd and bimetallic Pd-Pt catalysts are commonly used for hydrogenation reactions in the fine chemical industry. In order to prepare carbon supported catalysts, the supercritical fluid deposition technique has recently been receiving increased attention. This process involves the dissolution of an organometallic precursor (OM) in a supercritical fluid and the exposure of the carbon support to the solution. After adsorption of the precursor onto the support, the metallic precursor is converted to its metal form by chemical or thermal reduction. Although many studies have been conducted in order to prepare supported single metallic nanoparticles so far; there are very few studies regarding the preparation of supported bimetallic nanoparticles by scCO2 deposition in the literature.In this thesis the preparation of carbon black (Black Pearl 2000) supported single Pd and Pt nanoparticles by supercritical CO2 deposition was investigated. The scCO2 depostion was utilized also for the preparation of carbon supported bimetallic Pt-Pd by introduing two different deposition alternatives consisting of the preparation starting from the formation of carbon supported platinum (Pt/C) followed by deposition of Pd precursor and reduction; and the simultaneous adsorption and subsequent reduction of the precursors. Palladium (II) acetylacetonate (Pd(acac)2) and dimethyl (cyclooctadiene) platinum (II) (PtMe2COD) were utilized as metallic precursors. The adsorption isotherms of Pd(acac)2 and PtMe2COD on BP2000 in scCO2 at the experimental conditions; 20 MPa and 60 degrees centigrate was determined in order to understand the correlation between the concentration of the OMs in the supercritical phase and the uptake amount of the OMs on the substrate at the given experimental conditions. A mass transfer model was found to represent the experimental data on the kinetics of adsorption of Pd(acac)2 onto BP2000 in scCO2 fairly well with a fitting error of 5.6 %. With the help of the model obtained, the effect or process parameters like the tortuosity and particle size of support and isotherm constants on the adsorption kinetics was investigated. In order to reduce the OMs adsorbed on the substrate, chemical reduction with H2 in scCO2 was utilized. Increasing reduction temperature and metal loading caused an increase in Pd particle size. From the TEM image, it was seen that the Pd particles were irregularly distributed and the size range of the particles was 3-100 nm, whereas Pt nanoparticles were homogeneously distributed with a size range of 2-4 nm. From the TEM image of the supported bimetallic Pt-Pd nanoparticles, it was found that addition of Pt increased the homogeneity and reduced the particle size on the support compared to single Pd nanoparticles. From the EDS results, it was seen that there was a heterogeneous mixture of the bimetallic nanoparticles. Pt-rich nanoparticles had diameters of between 3-5 nm, whereas Pd-rich nanoparticles were larger with diameters of around 10 nm.Keywords: supercritical deposition, carbon, nanoparticles, platinum, palladium, organometallic.
Platinyum dimetil siklooktadiyenin resorsinol formaldehit aerojel destek üzerine süperkritik karbon dioksit ortaminda adsorplanmasinin termodinamik ve kinetiği
The thermodynamics and kinetics of adsorption of platinum dimethyl (cyclooctadiene) (PtMe2COD) on resorcinol-formaldehyde aerogel8 (RFA8) from supercritical carbon dioxide was investigated. An experimental technique was developed for the investigation of adsorption kinetics where the supercritical CO2 phase was analyzed directly.In order to investigate the thermodynamics of adsorption, adsorption isotherm of PtMe2COD-scCO2-RFA8 system at 20.7 MPa and 60°C was measured. The adsorption isotherm was represented by Langmuir Model.For the investigation of the kinetics of adsorption, a mass transfer model was developed. It was seen that the model represented the experimental data fairly well with a fitting error of 7.2 %. Spherical RFA8 particles with changing diameters were synthesized in order to investigate the effect of particle size on the adsorption kinetics. It was seen that the time to reach the adsorption equilibrium decreased by decreasing particle size. The effect the model parameter tortuosity on the adsorption kinetics was also investigated. As the model parameter tortuosity decreased, the time to reach equilibrium decreased.In this thesis, Pt nanoparticles supported on RFA8 was also prepared. Effect of metal load on the average particle size of the supported nanoparticles was investigated using XRD measurements. Three different metal loaded 10 wt. %, 22 wt. % and 34 wt % particles were prepared and the average particle sizes were found as 2.3 nm, 3.1 nm and 3.9 nm respectively. It was seen that the average particle sizes increased with increasing metal load.
Silika aerojellerin hexametildisilazan-karbon dioksit karişimlariyla yüzey modifikasyonu ve hexametildisilazan-karbon dioksit karişimlarinin faz davranişlari
There are several advantages to using supercritical CO2 (scCO2) as solvent in chemical modification of the metal oxide surfaces. In particular, the low viscosity and low surface tension of the super critical fluids leads to enhanced deposition kinetics over conventional solution methods. The application areas include polymer modification, construction of low-energy hydrophobic surfaces, preparation of stationary phases for reversed-phase chromatography, making biocompatible surfaces, and forming monolayers for lithography, micropatterning, and sensors where alkylsilanes are generally used as the modifying agent.In this study, we rendered the surface of monolithic silica aerogels hydrophobic by hexamethyldisilazane (HMDS)-carbon dioxide mixtures. The silica aerogels were prepared by two step (acid-base) sol-gel method using 50 wt. % tetraethylorthosilicate (TEOS) and ethanol as solvent. The effects of HMDS concentration in the fluid phase and the reaction time were investigated at a fixed temperature of 333.15 K and a fixed pressure of 20.68 MPa. The treatment led to hydrophobic silica aerogels which are as transparent as untreated aerogels. The extent of hydrophobicity was determined by contact angle test and contact angle was found to be 130° at different conditions. FTIR spectra confirmed the hydrophobicity by indicating a reduction in hydrophilic surface silanol groups and the emergence of hydrophobic methyl groups.The second part of the work deals with the vapor-liquid equilibria study of the binary mixture of HMDS-CO2. The bubble point pressures of the HMDS-CO2 system were obtained at temperatures 298.15 K, 313.15 K, 327.65 K and 341.95 K and at various concentrations. At a fixed temperature, the bubble point pressure decreased as the concentration of HMDS increased. At a fixed composition, bubble point pressure increased as the temperature increased. The bubble point pressures were modeled using the Peng-Robinson Stryjek-Vera equation of state (PRSVEOS) and compared well with the experimental data.
Mikro-çubukların gaz, sıvı ve süperkritik CO2 içindeki sıklık yanıtları
Micromechanical resonators have been extensively studied in recent years because of their potential for high-sensitive, low cost, compact device applications, such as in situ viscometers and densitometers. This thesis is the first study in the literature that investigates the response of microcantilevers in a supercritical fluid. In this study, the frequency response of microcantilevers in CO2 was investigated in order to relate the thermophysical properties such as density (?), viscosity (?) and isothermal compressibility (KT) to the resonance frequency (fR), quality factor (Q) and damping parameter (ß). 200 µm, 225 µm, 250 µm long, 20 µm wide and 1 µm thick multilayered Ni microcantilevers consisting thin Cr and Au films were fabricated by conventional microfabrication techniques. The cantilevers were placed in a high pressure view cell equipped with a custom designed Teflon housing holding the microcantilever chip and the electromagnetic coil which was actuated sinusoidally. A Laser Doppler Vibrometer (LDV) was used for detection. Experiments were carried out in the pressure range of 1 atm to 200 atm and at 25 °C, 35 °C, 45 °C and 55 °C. First, it was observed that fR decreased with increasing ? and Q decreased with increasing ?. The changes in fR and Q were larger in the gas phase than those in the liquid or supercritical (sc) phase. Second, fR was observed to vary linearly with ? as well as (??)0.5 in the gas phase which enabled the determination of ? and ? through calibration with percentage errors less than 1 %. A quadratic relationship was observed between Q and ?. Apart from that, the decreases of fR observed in successive measurements (i.e. drifts) at a constant temperature and pressure were found to correspond to the shifts of the maxima of the density multiplied by compressibility, defined as ? -?K ? _T=(??/?P)_T. Moreover, microscale bubbles were observed at a pressure less than the bubble point pressure at 25 °C due to heating of cantilevers by LDV.
N,N-dimetilasetamid-su-karbondioksit sisteminde dağılım katsayılarının ölçülmesi ve termodinamik modellemeleri
The partition coefficients of N,N-Dimethylacetamide (N,N-DMA) between the water and the supercritical carbon dioxide (scCO2) phases were measured in the temperature range of 298.15 K ? 318.15 K and the pressure range of 8.3 ? 24.1 MPa. The measurements were carried out in a 56 ml vessel by contacting the supercritical and the aqueous phases. The partition coefficients of N,N-DMA were found to increase from 0.05 to 0.150 with increasing pressure at a constant temperature and increase with temperature at a constant density.The bubble point pressures of N,N-DMA ? CO2 mixtures were measured at 298.15 K, 308.15 K and 318.15 K and were found to increase with increasing mole fraction of CO2. The partition coefficients were modeled using the Peng-Robinson Equation of State (PREOS) combined with modified van der Waals mixing rule ? The Panagiotopoulos and Reid mixing rule. The binary interaction parameters for the CO2 ? H2O pair were taken from the literature and were regressed for CO2 ? N,N-DMA and H2O ? N,N-DMA pairs by fitting partition coefficients data. The binary interaction parameter for CO2 ? N,N-DMA pair was found to depend linearly on temperature. The bubble point pressures of N,N-DMA and CO2 could be predicted fairly well using the regressed binary interaction parameters.
PEG-hidrojel kaplı silika aerojel-Yeni bir ilaç taşınım sistemi
A novel composite of silica aerogel and poly (ethylene glycol) PEG hydrogel was synthesized and its potential as a drug delivery system were investigated. The composite was synthesized by encapsulation of hydrophobic aerogels within PEG hydrogel via photoinitiated polymerization. Disks of aerogels were synthesized by the two step sol-gel method using tetraethylortosilicate (TEOS) as the silica precursor. After the gels were aged in ethanol, the alcogels were then contacted with a solution of eosin-Y, a photoinitiator, dissolved in ethanol. The adsorption of eosin-Y onto the surface of alcogel led to a reddish transparent composite of silica aerogel with eosin-Y. The surface of eosin functionalized silica aerogels was then rendered hydrophobic using hexamethyldisilazane (HMDS) as the surface modification agent, and supercritical carbon dioxide (ScCO2) as solvent. Hydrophobicity of aerogel was tuned by changing HMDS amount dissolved in ScCO2 phase which changes the contact angle between 0-128o. Hydrophobic or hydrophilic aerogels were then dipped into a PEG diacrylate prepolymer solution, and photopolymerization was carried out using visible light (514 nm). BET surface area and pore size distribution measurements show that both hydrogel encapsulation and eosin-Y loading did not affect the pore structure of the aerogel.The potential of this novel composite as a drug delivery system was tested by Ketoprofen as a model drug. The results demonstrate that both drug loading capacity and drug release profiles could be tuned by changing hydrophobicity of aerogels, and that drug loading capacity increases with decreased aerogel hydrophobicity while slower release rates are achieved with increased hydrophobicity from eosin functionalized aerogels. The effect of PEG concentration (0, 15 %, and 30 % w/w) in the prepolymer solution of the hydrogel coating on drug release rate from hydrophilic aerogel was also investigated. It was seen that as the PEG concentration increased, the drug release was retarded. The experimental results showed that drug release can be controlled with this novel aerogel-hydrogel composite system via changing hydrophobicity of the aerogel, and the concentration of the PEG in the hydrogel coating.
Silika aerojel polimer kompozitlerinin hazırlanması ve karakterize edilmesi
In this study, new nanotechnology-based high performance insulation systems for energy efficiency, nanostructured composites of silica aerogels with polymers are being developed as core materials for vacuum insulation panels in buildings. Monolithic composites of a wide variety of polymers with silica aerogels were synthesized by modification of the conventional sol-gel method to produce silica aerogels. The polymers used in the study are poly(ethylene block poly ethylene glycol) (PEPEG), poly(vinyl pyrrolidone) (PVP), poly(vinyl acetate) (PVAc) and poly(methyl vinyl ether) (PMVE). Characterization of the composite materials was performed by Fourier Transform Infrared - Attenuated Total Reflectance (FTIR-ATR) spectroscopy, Thermal Gravimetric Analysis (TGA) and by Nitrogen Physisorption using BET. Both transparent and opaque crack-free monolithic composites suitable for testing and use in VIPs were obtained. The presence of polymer in the composites was confirmed by IR spectroscopy and TGA. The composites were mesoporous materials with high surface areas around 800 m2/g and average pore sizes around 5 nm. Incorporation of polymers did not significantly change the pore size distribution and the specific surface area of the pure silica aerogels. The effects of the time of polymer addition at various stages of the conventional sol-gel process such as before/after the hydrolysis step and during the aging step, on the properties of the composites were investigated. Opacity was found to be correlated to the phase separation of the polymer from the reaction mixture. The effect of polymer content on the resulting properties was also investigated along with density, porosity and shrinkage calculations.
Süperkritik depozisyon tekniği ile tekli Ni ve çift-metalli Pt-Ni nanokatalizör sentezi
Monometallic nickel and bimetallic platinum-nickel nanoparticles were deposited on ?-Al2O3, Vulcan XC72R and carbon aerogel (CA) supports by supercritical carbon dioxide (scCO2) deposition (SCD). Characterization of the samples was carried out by BET, TPD, XRD, TEM, and EDXS. Nickel acetylacetonate (Ni(acac)2) was adsorbed on the supports from scCO2 solutions. The adsorbed Ni precursor was reduced to nickel nanoparticles by heat treatment under hydrogen. The resulting nanoparticles had an average size of 6 nm. Bimetallic Pt-Ni nanoparticles were synthesized by a two-step procedure termed sequential deposition. First, Pt nanoparticles were deposited on the supports by adsorption of platinum cyclooctadiene dimethyl Pt(cod)(me)2 from scCO2 solution followed by reduction of the adsorbed precursor to Pt by heat treatment at 200 oC under flowing nitrogen. Subsequently, Ni(acac)2 was adsorbed on Pt/support from scCO2. The adsorbed Ni precursor was then reduced to Ni by heat treatment under hydrogen. Spherical nanoalloys of Pt-Ni nanoparticles with uniform sizes of ~3nm were obtained over Vulcan XC 72R, and Pt-Ni nanoparticles with uniform sizes of ~1nm were successfully synthesized over carbon aerogel. As for the alumina support, supported Pt-Ni nanoparticles of ~3 nm in size were obtained. The small composition variations in EDXS measurements indicated a uniform distribution of the bimetallic nanoparticles. The effect of the order of sequential deposition of Pt and Ni on both morphology and activity of the synthesized materials were also investigated. It was found that the formation of Ni nanoparticles occurs on the previously deposited Pt islands resulting in bimetallic spherical nanoparticles having homogeneous chemical compositions with no phase separation when Pt deposition is carried out first and this is followed by Ni deposition. Interestingly, the size of the bimetallic nanoparticles was found to be smaller than the size of the monometallic Ni. However, pure Ni agglomerates were observed on TEM images when Ni deposition is achieved first and this was followed by Pt addition. Cyclic voltammograms of the carbon supported catalysts were also determined. Bimetallic nanocatalysts showed a higher activity than the monometallic nanocatalysts. Among the bimetallic nanocatalysts, higher activity was obtained when Pt deposition was carried out first.
Silika aerojellerin büyük ölçekli üretimi için sol-jel parametrelerinin, termal ve optik özelliklerinin incelenmesi
Transparency in thermal insulation systems provides new practical opportunities for insulation industry since transparent insulation materials can replace conventional window glazing. Silica aerogels are promising candidates to be used in such insulation systems because they are transparent and have very low thermal conductivity (10-15 mW/m.K). Silica alcogels are synthesized by sol-gel chemistry, gelled in a mold and dried by supercritical extraction with CO2 to get highly porous (porosity > 90%), low density (0.1-0.4 g/cm3) aerogels. The purpose of this study is to investigate the effects of synthesis parameters such as types and compositions of solvents on properties of silica aerogel to produce large scale silica aerogels with optimum thermal conductivity and transparency values and to use these large scale silica aerogels in a transparent vacuum insulation panel. Pore diameters, surface areas, and pore size distributions of the synthesized silica aerogels were measured with N2 adsorption-desorption technique. Optical properties were determined with using UV-VIS spectrophotometer. Transient Hot-Disk plane source and steady-state Guarded Hot Plate methods were used for analyses of thermal transport properties. It was found that decreasing the molar ratios of the solvents to precursor in the sol mixture increases transparency of resulting silica aerogel. Theoretical calculations on thermal conductivity showed that the lowest values can be reached when the density of the aerogel is between 100-200 kg/m3. Based on this information, a large scale (30x30x2.0 cm3) and transparent silica aerogel panel was produced with density of 180 kg/m3. Transparency ratio and thermal conductivity of the aerogel were measured as 88% (at 600 nm) and 16 mW/m.K, respectively.
Silika aerojellerin hidroksil-son gruplu-poli(dimetilsilokzan) ile kompozitlerinin geliştirilmesi
Vacuum insulation panels (VIPs) with typical thermal conductivity values of 3 to 5 mW/mK are emerging as excellent systems nowadays for effective thermal insulation in buildings and household applications. The achievement of such low thermal conductivities in VIPs relies on the suppression of the gaseous convection by applying vacuum. A VIP structure is composed of a core insulation material and an envelope film covering the core material. Among different materials, fumed silica and glass fiber are the most commonly utilized core materials owing to their appreciably low thermal conductivity values, especially under vacuum conditions. However, these materials are not transparent and thus cannot be used in the development of transparent vacuum insulation panels. The idea of transparent VIPs has been recently established in order to replace the conventional window glazing. Since then, research about transparent core materials and barrier films that can be utilized in the development of transparent vacuum insulation panels has been gaining increasing interest. Silica aerogels appear as the most promising nanostructured materials to be implemented as filler materials in transparent vacuum insulation panels due to their transparency in addition to extremely low thermal conductivity. One drawback of silica aerogels is their poor mechanical properties which makes their utilization as monolithic and crack-free materials challenging. This problem can be solved by reinforcing aerogels with polymers which results in improved resilience that would allow for practical utilization. In this study, monolithic composites of silica aerogels with hydroxyl-terminated poly(dimethylsiloxane) (PDMS(OH)) were developed. The first route that was followed for the synthesis of the composites was the modification of the conventional two-step sol-gel process. The incorporation of the polymer in the synthesis was performed at different stages of the sol-gel process. Additionally, different co-solvents such as THF and toluene were used. The effects of several processing parameters such as polymer amount, type of co-solvent and the polymer addition step, on the properties of the composites were investigated. The composites synthesized with this route were obtained as opaque materials since PDMS(OH) was not soluble in the sol mixture. Utilization of THF and toluene as the co-solvents avoided the solubility problem, however the mechanical durability of the aerogel composites was very low which resulted in very high volumetric shrinkage during the supercritical drying. Reactive supercritical deposition technique was employed as the second route and the composites of silica aerogels with PDMS(OH) were developed by the deposition of the polymer from supercritical CO2. The technique is composed of two stages; the first stage includes the dissolution of PDMS(OH) in supercritical CO2 that results in a single phase binary mixture of PDMS(OH)-CO2 and the second stage is the exposure of the silica aerogel samples to the single phase binary mixture. Initially, the demixing pressures of PDMS(OH)-CO2 binary mixtures at various compositions were measured up to 24 MPa to determine the single phase region of the binary mixture. The demixing pressures were observed to decrease with increasing polymer content of the binary mixture. Subsequently, deposition experiments were performed and monolithic aerogel composites were obtained. The deposited samples were characterized by ATR-FTIR and BET analysis. It was revealed that during the course of the deposition, the polymer molecules react with the surface –OH groups of the aerogel. The effects of various parameters such as polymer concentration, deposition temperature, deposition time and polymer molecular weight on the properties of composites were investigated. The polymer uptake of the deposited aerogels increased with increasing PDMS(OH) concentration, deposition time and deposition temperature. It was found that the transparency of the aerogels can be controlled by the amount of the polymer loaded to the samples. It was also demonstrated that the deposition resulted in the coating of silica aerogel surface with a thin layer (~1-2 nm) of polymer molecules. According to the thermal conductivity model simulations, such a thin coating layer did not cause a noticeable increase in the thermal conductivity of the composites. Moreover, compression tests revealed a threefold improvement in the mechanical strength of the composites when compared to native silica aerogels. Hereby, this work presents silica aerogel-PDMS(OH) composite materials as novel candidates to be used as core insulation materials in transparent VIPs.
Süperkiritk depozisyon ve gözenek hacminde çözelti emdirme yöntemleriyle hazırlanmış katalizörlerin hidrakraking aktivitelerinin ve seçiciliklerinin karşılaştırılması
Hydrocracking is the most profitable operation in the petroleum refinery due to its role of converting heavy petroleum cuts, such as gas oil and asphaltenes, to the more valuable products like diesel and heavy naphtha. The process involves the cracking of long chained hydrocarbons followed by hydrogenation of the cracked chains. Thus, the catalyst used in hydrocracking has both cracking and hydrogenation function which are generally supplied from an acidic support and loaded noble or earth metal. The balance between these two functions makes the hydrocracking process very flexible for the production of wide range of products from variety of feed stocks. In this study, the aim was to compare hydrocracking activities and selectivities of catalysts prepared by two different methods as incipient wetness and supercritical deposition. For this purpose, three commercial were selected as reference materials. N-decane was chosen as a model feed in order to have better understanding of catalysts by tracking the product distributions using GC-MS for preliminary studies. Supports were prepared with different amounts silica-alumina, USY zeolite and Ludox binder. The characterization of synthesized materials was performed by XRF, N2 physisorption and TPD. Then, three different types of catalysts, commercial, prepared by impregnation and by supercritical deposition (SCD) methods, were tested for n-decane reactions in a laboratory scale plug flow trickle bed reactor which was developed during this study. Furthermore, heavy vacuum gas oil was replaced with n-decane and catalyst performance tests were repeated for this feed. The product analyses were done with ASTM standard distillation and GC Simdist analaysis, which was in agreement with the former. Finally, kinetic modelling of gas oil cracking was done using discrete lumping method in order to compare kinetic parameters of the catalysts. Results showed catalysts prepared with impregnation and supercritical deposition methods had comparable activities relative to the commercial catalysts for both n-decane and gas oil hydrocracking. Moreover, the catalysts prepared by SCD method had a superior activity and slightly higher middle distillate selectivity than the other catalysts. This finding was also supported by the results of the modeling studies.
Microçubukların frekans tepkisi ölçümleri kullanılarak yoğunluk ve ağdalık değerlerinin belirlenmesi
In this study, we investigate the resonant frequency and Q-factor of ferromagnetic nickel microcantilever immersed in Ar, N2, CO2 and CO2-N2 binary mixtures at 308.15 K and pressures up to 24 MPa. Measurements were performed using an experimental setup that includes a temperature controlled high-pressure vessel which can operate within a temperature range of 273 – 423 K and a pressure range of 1 – 30 MPa. A microchip containing microcantilevers and an electromagnet for microcantilever actuation were placed inside the vessel. The procedure followed for the determination of microcantilever frequency response then involved driving of microcantilevers with AC magnetic field at varying frequencies and detecting of the corresponding laser deflections from the driven cantilevers with a quadrant photodiode (QPD). At a constant temperature, resonant frequency and Q factor of microcantilever oscillations were found to decrease with increasing pressure as a result of increasing density and viscosity of the fluid. Experimental data was analyzed using Sader's model which describes oscillatory motion of immersed cantilevers incorporating the effects of density- and viscosity-dependent hydrodynamic forces. A very good agreement between the experimentally obtained cantilever resonant frequencies and quality factors and the predictions of Sader's model was observed. The low difference in experimental and theoretical values illustrates that our experimental approach is suitable for the density and viscosity measurement of gas and supercritical fluids and fluid mixtures at a wide range of pressures. Based on these results, we carried out simultaneous measurement of density and viscosity of nitrogen from the measured frequency responses of an oscillated microcantilever immersed in N2. To this end, using argon as a reference fluid of known density and viscosity, cantilever calibration parameters were obtained from nonlinear regression of cantilever resonant frequencies and quality factors recorded in argon. Subsequently, these calibration parameters were used in the model equations to determine the density and viscosity of nitrogen at the given experimental pressure and temperature. In the studied pressure range, the root-mean-square deviations of the measured density and viscosity of nitrogen from the reference values obtained from NIST database were 2.5% and 5.2%, respectively. Finally, we studied thermophysical properties of more complex fluid systems represented by fluid mixtures. In order to investigate the density and viscosity of fluid mixtures, CO2-N2 binary mixture was chosen as a model fluid. The average relative deviation range for different compositions of CO2-N2 binary mixture was found to be 0.96 % -13.27 % for density and 2.42 %- 15 % for viscosity when N2 was used as reference fluid.
Nanoyapılı aerojellerin ilaç taşıyıcı sistemi olarak incelenmesi
Over the past few years, nanoporous aerogels have shown a great promise as drug delivery vehicles. In this study, inorganic and organic aerogels such as silica aerogel and alginate aerogel were investigated as drug delivery systems. Moreover, a novel layered material consisting of these two distinct aerogels was prepared for the first time by encapsulating a silica aerogel with an alginate aerogel layer. To investigate aerogels as drug delivery vehicles, different methods are utilized to load drugs into them such as the addition of the drug to the reaction mixture in one of the steps before the gel formation or by supercritical deposition to the aerogels. These techniques have disadvantages such as low solubility of pharmaceutical compounds in scCO2 and possible reactions of pharmaceutical compounds with reactants used to form gels. An alternative technique is to load the drug after gel formation by contacting the gel with a solution of the drug. The drug diffuses into the liquid inside the pores. When this drug-loaded gel is subjected to supercritical drying, scCO2 not only removes the solvent from the pores but also acts as an antisolvent, which causes the precipitation of the drug in the pores of the aerogel. This is similar to the gas antisolvent crystallization (GAS) process but in this case the process takes place inside the pores. In this study, this technique was used to load paracetamol into silica and alginate aerogels with densities ranging 0.13 to 0.15 g/cm3 and 0.055 to 0.062 g/cm3 respectively. The initial concentration of the paracetamol-ethanol solution ranged from 0.25 M to 0.8 M. The factors affecting the amount and distribution of the drug inside the aerogel matrix were investigated. The Attenuated Total Reflectance (ATR) results indicated that the paracetamol diffused inside of the aerogel and X-ray Diffraction (XRD) analysis demonstrated that the paracetamol was in crystalline form. It was possible to control loading by changing the concentration of the drug in the contacting solution and also by changing the pore volume of the alcogel matrix which in turn depended on the reactant concentrations. Moreover, the process enabled very high amount of loadings such as 40 wt. % for silica aerogels and 75 wt.% for alginate aerogels compared to conventional systems and also offers an advantage for aerogels as it combines two processes such as drying and loading in a single one reducing the time and the operating expenses. On the other hand, diffusion of paracetamol inside the pores was also theoretically modeled by solving partial differential equations. Experimental paracetamol uptake was shown to be in good agreement with the model predictions without any adjustable parameters. Finally, the release of paracetamol from loaded aerogel supports in PBS buffer was investigated showing the promise of these materials as drug delivery vehicles. It was shown that native aerogels could be used as carriers for fast acting medications whereas the hybrid aerogel was suitable to achieve a retarded release profile. A mathematical model was also developed to describe the release kinetics of paracetamol from alginate aerogels comprising the swelling of the aerogel matrix, the dissolution of the drug into the pore liquid followed by its diffusion from the pores into the surrounding medium. The inclusion of these physical phenomena in the mathematical model had greatly reduced the deviations from the experimental data and resulted in good model predictions.
Aerojel kullanımı ile ılık hisli malzemelerin geliştirilmesi
The warm sensation is related to the amount of heat extracted from the human skin when it is in contact with a material. This heat extraction can be described by the time dependent heat flux which occurs from the skin to the object. The heat flux depends on the material properties such as thermal conductivity, density and heat capacity. Increasing warm sensation requires decreasing these properties. Urea formaldehyde is a thermoplastic polymer which is most commonly used to make toilet seats. Improving warm sensation of toilet seat has become a good marketing strategy. To enhance the thermal comfort of the seats electric heaters are currently in use. However it raises safety issues because of high possibility of water contact with the heater. This project aims to give the urea formaldehyde based toilet seats to warm sensation by incorporating aerogels which exhibits high resistance to heat flow. Urea formaldehyde grains were mixed with aerogels (silica and resorcinol formaldehyde aerogels) and hot pressed together to obtain an aerogel incorporated composite. Composites were manufactured by hot pressing this mixture at elevated pressures and temperatures in the industrial size hot press molds at the manufacture site of toilet seats. Thermal conductivity and density of the composites were measured. Changes in aerogels' density and pore properties were determined at different pressures with a laboratory scale cold press. Denser aerogels were synthesized to make aerogels more resistant to the composite manufacture pressure. Composites with dense aerogels (0.361-0.48 g/cm3) successfully reduced the thermal conductivity of native urea formaldehyde by up to 40% by using volume fractions up to 0.38. With reduced contact coefficient parameters, up to 45% reduction in the heat flux was obtained. Thermal conductivity of composites was also predicted with effective thermal conductivity models such as Maxwell and effective medium theory (EMT). At lower volume fractions Maxwell and Rayleigh models gave good predictions for the composite thermal conductivity. Effective thermal conductivity of composites with dense silica aerogels were best predicted with EMT.
Karbon dioksit ve metanın çeşitli aerojeller üzerine adzorpsiyonunun termodinamiği araştırma
Thermodynamics of carbon dioxide and methane adsorption on various aerogels was investigated to evaluate their potential as adsorbents for carbon dioxide capture and methane storage. Excess CO2 and CH4 adsorption isotherms on aerogels such as silica, resorcinol-formaldehyde, carbon and wheat starch were measured using a volumetric method in the temperature range of 298-328 K and pressures up to 120 bar. Total or absolute adsorption isotherms were calculated from experimentally obtained excess adsorption isotherms using the pore volume of each adsorbent. It was determined that silica aerogel had maximum absolute CO2 uptake of 14 mmol/g at 308 K and 35 bar and resorcinol-formaldehyde aerogel (RFA-17) had maximum absolute CH4 uptake of 22 mmol/g at 298 K and 100 bar. CO2 and CH4 adsorption isotherms for silica aerogel were well represented with the Langmuir model. CO2 isotherms for resorcinol-formaldehyde, carbon and wheat starch aerogel and CH4 isotherms for carbon and resorcinol-formaldehyde aerogels were fitted with the Freundlich model. It was also found that the excess CO2 uptake correlated well with the mesopore surface area of each aerogel at various pressures while micropore surface area significantly influenced CH4 uptake in aerogels. The isosteric heat of adorption for each aerogel was also determined from the variation of pressure with temperature at a constant excess uptake and was found to be dependent on the surface coverage. Among all aerogels, wheat starch aerogel had highest heats of CO2 adsorption and resorcinol-formaldehyde aerogel had highest heats of CH4 adsorption. Adsorption capacities of aerogels determined were comparable with other classes of adsorbents and presents an opportunity for further investigation of aerogels as potential materials for applications in carbon dioxide capture and methane storage.
Pt-Cu atom topaklarının genetik algoritma ve yoğunluk fonksiyonel teorisi ile global optimizasyonu
The atomic arrangements and structures of monometallic Pt and Cu clusters and bimetallic PtCu clusters composed of 2-40 atoms were determined using a genetic algorithm (GA) with multiple runs between 100 and 400. GA was designed associated with the Gupta potential energy with the aim to define atomic interactions within cluster regime. The algorithm was developed and implemented in MATLAB [1] environment. After a large number of GA runs on Pt, Cu and PtCu clusters, the algorithm successfully found the minimum energy structures. To improve our understanding of the structural and energetic properties of the mono- and bimetallic Pt-Cu clusters, a wide range of energetic and structural properties (i.e., excess, binding energy and second difference in energy, effective coordination number, average weighted bond length, and second order parameter) were calculated. Energetic analysis provide strong evidence that the lowest energy structures of Pt, Cu and PtCu clusters were stable and energetically favorable. The optimum structures for every size of Pt and Cu clusters were symmetric, regular and mainly based on icosahedron structures while PtCu clusters above 20 atoms were distorted. The lowest energy structures of PtCu clusters were found as mixed distorted icosahedrons with Pt segregation in core region whereas Cu atoms were located on the surface. Remarkably, 38 atoms of Pt, Cu and PtCu alloy clusters tend to be perfect truncated octahedron which was the similar face centered cubic packing as in bulk Pt and Cu. Further, global optimization of each composition of 10 atoms of PtCu clusters were carried out with DFT approach using Gaussian 09. Energetically the lowest energy composition was obtained as Pt3Cu7. Hydrogen and OH adsorption was studied to investigate how hydrogen interacts with Pt−Cu clusters for improving our understanding of the factors determining the reactivity. Hydrogen and OH adsorption energy and adsorption properties were found in the different charge states and in different adsorption sites as top, bridge and hollow of Pt10, Cu10 and Pt3Cu7 clusters. More negative adsorption energy indicates the stronger the adsorption since the adsorption energy measures the magnitude of the binding energy of the species to the cluster. Based on our calculations, the lowest energy structures for H and OH adsorbed on Pt3Cu7 cluster were reached in the neutral state. The most favorable adsorption site of hydrogen was found as the top site for Pt10 and bridge site for Cu10 cluster. For the case of bimetallic Pt3Cu7 cluster, the hollow site are more favorable for hydrogen adsorption but Pt-hollow site is more energetically favorable. This result indicates that H atoms show a preference for Pt atoms rather than Cu in Pt3Cu7 cluster.