FBE- Malzeme Bilimi ve Mühendisliği Lisansüstü Programı
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Malzeme Bilimi ve Mühendisliği Ana Bilim Dalı altında bir lisansüstü programı olup, yüksek lisans ve doktora düzeyinde eğitim vermektedir.
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Sustainable Development Goal "none" ile FBE- Malzeme Bilimi ve Mühendisliği Lisansüstü Programı'a göz atma
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ÖgeInvestigation of the electrochemical co2 reduction mechanism on tin electrodes(Fen Bilimleri Enstitüsü, 2020) Yılmaz, Tuğçe ; Ürgen, Mustafa Kamil ; 633383 ; Malzeme Bilimi ve MühendisliğiIn today's world, one of the biggest concern is climate change. Even though fossil fuels are started to replaced by renewable energy sources in recent years, this solution turned out to be not sufficient to decrease the accumulated CO2 in the atmosphere Increased human population and energy demand escalated the rate of CO2 emission with a higher rate than environmentally friendly energy sources. The released CO2 gas to the atmosphere by human activities is the main factor causing climate change. The increased amount of CO2 in the atmosphere causes a greenhouse effect that led to an increment in temperature. Thus, the studies concentrate on the CO2 conversion and storage methods to get rid of the excess CO2 in the atmosphere. The conversion of CO2 is not only useful to eliminate the CO2 gas in the atmosphere but also the products of this conversion is used as a raw material to produce valuable materials. There are various methods for the conversion of CO2 such as chemical, thermochemical, electrochemical, biochemical, photochemical, and etc. Among these methods electrochemical CO2 reduction method has numerous advantages such as no need for heating or pressure, harmless reactants, the possibility to have a carbon- neutral chemical production by supplying energy from renewable energy sources, and convenience to scale-up. Electrochemical CO2 reduction is a method in which a CO2 is reduced on an electrode surface acts as a catalyst. The typical setup for this process involves a cell divided by a proton exchange membrane. The counter electrode and working electrode are located on different sides of the cell. While CO2 is reduced to various chemicals by taking + electrons and H , besides the hydrogen evolution reaction on the working electrode; water-splitting reaction occurs on the counter electrode. There are many products that can be produced by electrochemical CO2 reduction and their formation potentials are close to each other. In addition, CO2 is a highly stable molecule due to its linear molecular structures. These two conditions make the use of a catalyst inevitable. The metal catalysts capable of reducing CO2 are grouped according to their selectivity to specific products. These metals and the products selectively formed on them are Pb, Hg, Tl, In, Sn, Bi, Cd − formate, Au, Ag, Zn, Pd, Ga – CO, Cu − alcohols and hydrocarbons. The optimal binding energy between the catalyst surface and the key intermediate to produce a certain product led to high selectivity. The aim of this study is to produce selectively formate because of its high energy density and economical value, and its nontoxic nature. Among the metals that are selectively produced formate Sn stands out because of its relatively low price, high availability, innocuousness, and most importantly high selectivity towards formate. Even there are numerous studies published regarding design highly selective Sn-based catalyst to produce formate, a few of them discourse the mechanism providing the high selectivity. However, to design a highly selective and efficient catalyst, one should understand the factors favoring formate production. Thus, understanding the mechanism will be a breakthrough in electrochemical CO2 reduction. In this study, the mechanism of electrochemical CO2 reduction to formate on the Sn electrode is investigated. At first, the reliable setup was built-up to have replicable and trustworthy results. To achieve the reproducibility, adjustment of the position of the reference electrode and the working electrode with respect to the membrane, stabilization of temperature, the distance between the counter and working electrode, and anode area were done. The working electrode was masked to have an area of 4 2 cm . However, the results were not reproducible. Following this, the annealing at 150, 180, and 200oC and anode area studies were done. As a result, the annealing has no significant effect on faradaic efficiency in this study. The faradaic efficiencies obtained on 4 cm2 were not reproducible, but when the electrode area was decreased to 2 cm2 results were reproducible. The increased counter electrode to working electrode area ratio and the more similar sizes of the electrode and membrane, ease the charge transfer and resulted in uniform charge distribution. Once the reliable setup is achieved, a cyclic voltammetry analysis was done on the pure Sn electrode in CO2 saturated 0.1 M KHCO3 electrolyte with pH 6.8. As a result, the reduction peak of tin oxide appeared at 1.0 V vs. Ag/AgCl, and in the literature, the electrochemical CO2 reduction experiments were done at more cathodic potentials. However, there are many studies indicating that the oxide layer on the tin surface is the key factor governing the high selectivity for formate. To clear up this contradiction, the longtime electrochemical CO2 reduction experiment was designed to understand the effect of tin oxide on selectivity towards formate. In this experiment, the formate production rates are detected for every ten minutes. The results revealed that in the first ten minutes the produced formate amount was almost 4 times higher than the remaining time intervals. This could be concluded as after the initial reduction of tin oxide the formate production rate decreased dramatically. Thereafter, 6 different polarization experiments were conducted on pure Sn electrodes to have a better understanding of the relationship between the tin oxide and CO2 reduction reaction. In the beginning, the pure Sn electrodes were polarized from open circuit potential to -2 V vs. Ag/AgCl repeatedly until the polarization curves no longer changed. Firstly, this process was done in Ar saturated 0.1 M KHCO3 electrolyte with pH 8.5. The resulting curve preserved its oxide peak, however, it was slightly reduced. When the experiment was repeated in CO2 saturated 0.1 M KHCO3 with pH 6.8, the oxide was greatly reduced. This contrast between two experiments might be originating from the differences in CO2 presence or pH of the electrolytes. Thus, the final experiment was done in H2SO4 added 0.1 M KOH which has a pH of 6.8 but there was no CO2 or even HCO3- in the electrolyte. The eventuated curve preserved its oxide peak, and that was more similar to polarization curves obtained in Ar saturated 0.1 M KHCO3 than the CO2 saturated 0.1 M KHCO3. However, it is worth noting that after the first cycle the oxide is reduced even less than the Ar saturated 0.1 M KHCO3. The second set of polarization experiments was designed to reduce the tin oxide electrochemically before the polarization. At first, the pure Sn electrodes were reduced at -1.8 V vs. Ag/AgCl for 15 minutes to give enough time to have a full reduction and simulate the real experimental conditions. Then, the polarizations were started at -1 V rather than to open circuit potential to prevent the re-oxidation of the Sn surface, and it continued until the -2 V vs. Ag/AgCl. This process was repeated similar to the previous set of experiments until the polarization curves did not change anymore. The results were consistent with the experiments that do not involve prior electroreduction. The polarization curves obtained in Ar saturated 0.1 M KHCO3 with pH 8.5 and H2SO4 added 0.1 M KOH with pH 6.8 were similar, but the reduction of the oxide was less in the H2SO4 added 0.1 M KOH. In the polarization curve obtained in CO2 saturated 0.1 M KHCO3 with 6.8 pH, no oxide peak has appeared. Thus, it can be suggested that the oxide is more prone to be reduced in the presence of CO2 or HCO3-. This experiment proves that the electrochemical CO2 reduction proceeds with not only CO2 gas but also HCO3- ions in the electrolyte. Additionally, at more cathodic potentials than -1.6 V vs. Ag/AgCl, it was clear that both hydrogen evolution reaction and electrochemical CO2 reduction reactions become kinetically limited because the slope of the curves become steeper. The steepness of the curves at more negative potentials than -1.6 V vs. Ag/AgCl can be listed as higher to lower, CO2 saturated 0.1 M KHCO3, Ar saturated 0.1 M KHCO3, and H2SO4 added 0.1 M KOH. Since, as the CO2 and HCO3- amount increases in the electrolyte the reaction becomes more limited, it can be said that the electrochemical CO2 reduction reaction becomes more limited than the hydrogen evolution reaction. In conclusion, the two main outcomes of this study is that the formate production rate was decreased with the reduction of the tin oxide, and total reduction of tin oxide on the surface is only achieved by the presence of CO2 in the solution indicating a synergic interaction between tin oxide and CO2. Thus, this study reveals the strong relationship between the tin oxide reduction reaction and ECR. In addition, it reveals the missing point in the literature about how tin oxide is affected by the electrochemical CO2 reduction reaction.
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ÖgePartial Replacement Of EPDM By Devulcanized Rubber In Thermoplastic Vulcanizates Based On PP / EPDM: Effect Of Devulcanized Rubber, EPDM / PP Ratio, And Compatibilizer(Institute of Science and Technology, 2020-07-15) Pamukoğulları, Beste ; Özkal, Burak ; 521181002 ; Material Science and Engineering ; Malzeme Bilimi ve MühendisliğiIn recent years, as the consumption frenzy increases in the world, the demands for materials that may be advantageous for both usage and mass production are increasing. Polymeric materials were born out of this need and replaced traditional materials. Unfortunately, the rapid consumption of polymeric materials brings with it the problem of waste management. When it comes to applications that require flexibility and durability, the first materials that come to mind are rubbers. But the structure that provides these features makes its recycling impossible. The production of rubber materials includes vulcanization, an irreversible reaction between elastomer, sulfur and other chemicals. Consequently, vulcanization procures cross-links in elastomer chains leading to the formation of a three-dimensional chemical network. The three-dimensional network of rubbers causes them to be solid and insoluble materials, so recycling of such materials is a current technological problem. One of the biggest challenges of 21st-century waste management is the recycling of rubber produced for various purposes. Approximately 70% of the rubber is used in tires, and the range of waste tires disposed of annually is about 800 million. Today, the most common approach to get rid of waste rubbers is to collect them in waste landfills. This leads to an accumulating scrap stock. This creates a fire hazard and also creates a favorable environment for rodents, mosquitoes, and other living things that cause health and environmental problems. Other approaches used to solve the problem of waste rubber are to grind the rubber into small pieces and re-use them in the production of low-performance products such as sports and play surfaces, floors, etc. or use rubbers as fuel. With these approaches, generally, low-quality rubber products are produced or additional pollution problems arise. It is stated in the literature that recycling is more efficient than other options. The breakdown of three-dimensional structures of rubbers is one of the most environmentally friendly options for recycling. The devulcanization methods are one of the most environmentally friendly recycling methods which allow the selective disintegration of sulfur-sulfur and carbon-sulfur chemical bonds formed as a result of vulcanization, without causing degradation in the polymer main chains of rubber. It is aimed to reuse the waste rubber formed after the devulcanization process applied by different methods such as chemical, ultrasonic, microwave, biological, thermomechanical, in the production of high-performance rubbers such as virgin rubber. In this study, recycled rubber obtained by thermomechanical and ultrasonic devulcanization of the waste of washing machine gaskets were use in the production of thermoplastic vulcanizates (TPVs). Devulcanized waste rubber was provided by the Fraunhofer Applied Research and Development Association. Thermoplastic elastomers are defined as a family of polymeric materials that can be processed and recycled in the same manner as thermoplastic materials but also exhibit several features associated with conventional thermoset rubbers. TPVs belong to this family and they are high-performance blends of thermoplastics and rubbers which prepared by dynamic vulcanization. In recent years, ethylene propylene diene rubber (EPDM) / polypropylene (PP) TPVs have attracted the attention of industry and academia with their commercial advantages, high performances and various application areas. EPDM / PP TPVs are widely used in automotive, electronic and electrical, construction and sports equipment, as they have excellent weathering, ozone and ultraviolet resistance and processing advantages. The devulcanization process is largely restricted in practice due to its high cost. Also, since the crosslinking structure of the rubber phase is required in EPDM / PP TPVs, it is not necessary to form the rubber phase from fully devulcanized rubber. Therefore, in this study, it was preferred to prepare TPVs by partially replacing the EPDM phase with devulcanized rubber. The purpose of this study is to produce EPDM / PP TPVs which devulcanized washing machine gaskets wastes are partially replaced with the EPDM phase without causing a serious change. EPDM phase in TPVs, in which 5%, 10%, and 20% of EPDM is replaced by devulcanized EPDM, was prepared using Banbury, two roll mill, and grinding devices, respectively. Then EPDM / PP TPVs are prepared by using twin-screw extruder with EPDM:PP ratios of 90/10 and 85/15 by weight. In addition, 2% and 5% by weight of maleic anhydride grafted polypropylene (PP-g-MA) was added to examine the effect of the compatibilizer in EPDM / PP TPVs. Briefly, in this study, the effect of partial replacement of EPDM with devulcanized waste EPDM replacement at different rates, the use of compatibilizers in EPDM / PP TPVs, and the effect of two different EPDM: PP weight ratios on EPDM / PP TPVs were investigated. To achieve this goal, formulations were developed, samples were manufactured, and their properties such as mechanical, physical, thermal, morphological, and aging were tested. The properties of the produced EPDM / PP TPVs are compared with the TPVs used in commercial products. We hypothesize that the devulcanized EPDM waste can be replaced with EPDM in EPDM / PP TPVs without a serious change in TPVs properties and have appropriate properties with commercial TPVs properties. This will create a remarkable, environmentally friendly approach for rubbers that are difficult to recycle.