Metalurjik olarak hazırlanan Au-Pd elektrotların katalitik özelliklerinin incelenmesi

Özden, Canpolat
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Fen Bilimleri Enstitüsü
Elektrodepozizyon yöntemi ile hazırlanan Au-Pd alaşımların, döngülü voltametri ile deney anında ( "in-situ" ) yapılan yüzey bileşimi analizlerinde, yüzeydeki altm oranın % 50' nin üzerinde olmadığı saptanmıştır. Yüzeyinde yüksek oranda altm içeren Au-Pd alaşımları hazırlamak, organik maddelerin elektrooksidasyonunda önemli olduğu kadar, bu kompozisyonlarda olası bir sinerjik etkinin varlığını araştırmak açısından önemlidir. Yukarıda belirtilen amaçlar doğrultusunda, elektrodepozisyon yöntemi ile hazırlanamamış Au-Pd alaşımları, metalurjik yöntemlerle geliştirilmeye çalışıldı. Bu amaçla mevcut alaşım hazırlama yöntemlerinden, bu amaca uygun olam seçildi. Her iki metalin faz diyagramları iyice incelendikten sonra gerekli atmosfer şartlarında ve ısıda döküm işlemleri yapıldı. Kullanılan metallerin birim fiyatlarının çok pahallı oluşu, çeşitli sayıda deneme yapmamıza izin vermedi. Kaynak taramalarından elde edilen bilgiler, genelde Au-Pd alaşımlarının herzaman için içyapı büeşimlerinin, yüzeye göre farklı olduğunu gösteriyor. Yüzey analiz işlemlerinde (asal metal alaşımları için) bugün kullanılan en önemli yöntemlerden birisi olan döngülü voltametrinin, avantajı, çalışma anında, bu işlemi yapabilmesidir. Sistemin gereği olarak üzerinde çalışma yapılan malzemenin yüzeyindeki ilk iki, üç atomik tabakadan bilgi almak mümkündür. Alaşım elektrotlarda yapının bileşimi hakkında bir bilgi edinmek için taramak elektron mikroskobu ile bir yan nicel analiz yaptık. Taramalı elektron mikroskobu ile alaşımların mikroyapılan hakkında da veri elde edildi. Elde edilen alaşımların, elektrokatalitik aktivitelerini saptamak amacı ile, etilen glikol ve glikozun elektrosidasyonlan incelenmiştir. Bunun için alaşımlar üzerinde, elektrooksidasyon reaksiyonlarına eşdeğer denge potansiyellerinde tafel analizleri yapılmıştır. Hesaplanan değişim akım yoğunlukları kullanılarak, yüzey bileşimine karşı bir grafik oluşturulmuştur. Bu grafikten, alaşımlar üzerinde sinerjik etki incelemesi yapılmıştır.
Fossil fuels are the main energy sources used in all over the world. But these reserves are so limited due to their rapid consumption. Researches, have been directed to new energy sources during last 40 years. At the begining of the Post War II, the first samples of the battery systems were produced, and they have found large application areas within the years. ( e.g. Leclanche zinc- carbon primary cell, Lead-acid, Edison nickel-iron secondary batteries) Electrochemical researches, devolopment, are onto fuel cells at recent times. The fuel cell is a battery except that one or both of the reactants are not permanently contained in the electrochemical cell but are fed into it from an external source when energy is desired. So, the cell can operate continuously as long as reactants are supplied and the internal cell electrodes and components remain unchanged. The anode materials, or fuel, is usually gaseous or liquid such as hydrogen, hydrazine, hydrocarbons, alcohols,... And oxygen or air is the oxidant. Although, fuel cell was investigated nearly 150 years ago, the first practical application was in the 1960's as a spacecraft power source for Gemini and Apollo missiles. Storage batteries were adequate for short space missions, fuel cells, however, were necessary to provide power (and energy) for the extend missions. The Carnot cycle limitation of thermal machines doesn't valid for fuel cells since it produces power by an electrochemical cycle rather than a thermal cycle. Sizes are taken not care in fuel cell power plants. Small plants can produce energy as much as big ones. They are silence and clean. They don't give hazard to enviroment therefore, the byproducts are water, carbondioxide and nitrogen. And they can be established anywhere desired. Fuel cell power plants operating on hydrogen and oxygen offer high energy density; that is relatively small weight and volume of total system can produce large energy outputs. vi Thus, fuel cells are preferred power genarators in remote applications where system weight and volume are important parameters. Fuel cell power plants operating on logistic fuels and air offer the potential for environmentally acceptable highly efficient, and low cost power generation. So, the fuel cells are being seriously considered for applications where these attributes are important, e.g., military and commercial electric power generation and possibly, vehicles. Hydrogen and oxygen electrodes were investigated in details for a long time. The electrochemical studies onto alcohols were showed that they have high solubility and low vapour pressure. The studies focused on nobel metal or alloy electrodes necessary for the electrooxidation of alcohols, poly-alcohols and carbohydrates. The direct oxidation, in a fuel cell, of organic compounds derived from biomass, such as alcohols, is a very attractive way of converting chemical energy into electrical energy. Methanol and ethylene glycol (EG) have mainly been used as fuels. In order to understand the catalytic role played by pure metals, e.g. Pt, Ni, Au or alloyed metals, or metals modified by foreign metal atoms, increasing numbers of fundamental studies have undertaken recently. The ethylene glycol and glucose elektrooxidations have chosen as a test reaction to study the electrocatalytic activity. Their electrocatalytic behaviours are systematicly determined. Alloys are of interest in the field of catalysis and surface science as typical examples of composite systems. Pd - Au alloys have been the subject of many investigations since basic interest related to, for example, % d character, the ligand and essemble effect, etc. The alloy system form continuous fee solid solutions without a miscibility gap over the entire composition range. The electrocatalytic efficiency of alloy electrodes was recently found to be often superior to electrodes consisting of the pure metals. However, on of the diffuculties with the use of alloy electrodes arises from a change of surface composition during the catalysed process. This alteration may be caused by a specific dissolution of one of the components from the surface or by different affinities, as against chemical species like oxygen or carbon monoxide at the surface will be observed if the alloyed metals do not exhibit similar surface energies. vu In this study, the surface properties of pure gold, palladium metals and their alloys were investigated. In addition, their electrocatalytic effects during the electrooxidation reactions of ethylene glycol and glucose in alkaline medium are studied. Palladium-gold alloys exhibit interesting electrocatalytic behaviours during electrooxidation of organic substances. Gold-Palladium alloys are very suitable for investigation in the potential range as they are very resistant to corrosion. As the surface energies of gold and palladium are very similar, the surface composition of the alloys agrees well with the bulk composition. This was confirmed by many series of Auger electron spectroscopy studies. Moreover, electrochemical methods can be used to ascertain the surface composition. This can be performed even more exactlythan with other homogenous alloys as the separation of the important maxima is quite favourable. Alloys surface compositions show diferrences with their bulks during the " in-situ" cyclic voltammetry studies. It has been concluded that the electrodeposition technique was insufficient in order to make palladium- gold alloy has gold more than % 50 on its surface composition. A gold rich surface is important due to two reasons. One of them is electrocatalytic effect on electrooxidation or electrosynthesis of hydrocarbons e.g., ethylene glycol, glucose... The next one is any possible synergetic effect. The electrosynthesis of organic materials bring many advantegous. For example, glyoxlyic acid can be prepared from the chemical oxidation of ethanol, acetaldehyde, glycol etc. but is prepared mainly, industrially, by oxidizing glyoxal CHOCHO with concentrated nitric acid. This latter technique gives rise to many problems: it is diffucult to obtain chemically pure glyoxlyic acid since the oxidation of dilute solutions of glyoxal needs a great excess of nitric acid which has to be eliminated afterwards and, in addition, gives the undesirable oxalic acid. Glyoxylic acid can be prepared with good yields by electrochemical oxidation of aqueous solutions of glyoxal in three compartment cell separated with fritted glass. In the best conditions, about 80 % of the initial glyoxal was detected as glyoxalic acid, oxal.c and unoxidzed glyoxal, the other 20 % was partly diffused (3 %) or completely oxidized into carbon dioxide. The best electrolyte is chloride ion, particularly HCL, even though chloride ion is oxidized. Probably the adsorption of this ion favours the reaction: this means that a modification of the surface of the electrode will be great importance. viii Acording to the studies, a synergetic effect were found at gold- palladium alloys with the anodic oxidation of organic material such as methanol, sodium formate and carbon monoxide. It was thought that an oxide species must be formed at alloy surface. However, the mechanism of oxygen chemisorption on binary homogeneous alloys is still largely unknown. In electrochemical measurments, additional complications arise from the corrosion of the electrodes as quite positive potentials have to be applied. This involves a change of the surface compositionof the alloys, so that a reliable method for its determination has first to be found. The aim of this study was to improve an Au-Pd alloy which its surface rich in gold ( more than 50 % at.) by using metallurgical casting techniques. The alloys were casted in three different bulk composition. The purity of the metals were in analytical grade. The casting temperatures were sellected from phase diagram of gold and palladium. Event the gold - palladium alloys do not oxidize at open atmosphere conditions, the inside of the furnuce was purged by nitrogen gas during casting. At the first casting was made in a quartz pot. But the end of the experiment, the pot was crushed due possibly to high temperature. So, in order to prevent any crushing on pot, zirconia were used as pot material. After casting, they were analyzed by scanning electron microscope. The results gave informations about both compositions and microstructures of the alloys. Microstructure is important due to clearify it's physical properties. In the latter steps of the study, surface analysis of alloys was made by cyclic voltametry as "in-situ". Alloys surface clearness must taken into care before starting the experiment in order to take healthy and accurate results. Electrolyte should be prepaired by using top distilled water. After those laboratory studies, the results about the both surface and bulk compositions of the alloys were discussed. The electrocatalytic activities of the alloys were determined by means of electrooxidation of ethylene glycol and glucose. For this purpose, the tafel analysis were made onto alloys at equilibrium potentials. The calculated exchange current density values were plotted versus surface composition in order to see if there is or any synergetic effect. Also the two diferent alloy preparing tecniques were compared with each other. Since the surface roughness are considerably different even if they have same surface and bulk compositions. The metallurgically casted alloy electrodes show much less diffusion current densities during electrooxidation of organic materials because of their massive surface. IX Literature studies were showed that gold-palladium alloys exhibit differences between surface and bulk compositions during the in-situ cyclic voltametry studies. There may be many reasons for the problem. Among the reasons, the alloy preparation method must be taken into account. This study was made in order to bring an answer to the question.
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1994
Anahtar kelimeler
Kimya, Alaşımlar, Altın, Elektrotlar, Palladyum, Yakıt hücreleri, Chemistry, Alloys, Gold, Electrodes, Palladium, Fuel cells