Modifiye polianilin elektrodun metanol ve etilen glikol oksidasyonunda katalitik özelliklerinin incelenmesi
Başlık çevirisi mevcut değil.
- Tez No: 75068
- Danışmanlar: PROF. DR. FİGEN KADIRGAN
- Tez Türü: Doktora
- Konular: Kimya, Chemistry
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Kimyagerlik Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 96
Özet
ÖZET Son yıllarda iletken polimerlerin bir üyesi olan polianilin, elektrokimyasal olarak kolayca sentezlenebilmesi, asidik çözeltilerdeki dayanıklılığı ve havada yükseltgenmemesi gibi özellikleri nedeniyle elektrokatalizde uygulama alanı bulmuştur. Bu uygulamalardan biri de polianilinin yüzeyine elektroaktif metalik taneciklerin depolanarak büyük yüzeyli bir katalitik malzeme elde edilmesidir. Elde edilen bu büyük yüzeyli elektrotlar özellikle yakıt pili araştırmalarında organik moleküllerin elektrooksidasyonu için kullanılabilmektedir. Bu çalışmada önce; iletken polianilin film sentezinde ilk adım olduğu düşünülen, anilinin platin elektrot üzerindeki adsorpsiyonu değişik deneysel parametrelerin fonksiyonu olarak incelenmiş ve adsorpsiyon kinetiği belirlenmiştir. Ayrıca anilin konsantrasyonuna ve adsorpsiyon süresine bağlı olarak iki farklı adsorpsiyon modeli önerilmiştir. İkinci aşamada değişik kalınlıkta iletken polianilin filmler sentezlenerek, üzerine çok az miktarda (0.1 mg.cm"2 civarında) platin tanecikleri depolanmış ve elde edilen filmlerin yüzeylerinin kalitatif incelenmesi amacıyla taramalı elektron mikroprob cihazı ile fotoğrafları kaydedilmiştir. Elde edilen fotoğraflardan Pt dağılımının film kalınlığı ile değiştiği belirlenmiştir. Üçüncü aşamada ise elde edilen aktif elektrot, metanol ve etilen glikol gibi organik moleküllerin oksidasyonunda denenmiştir. Oluşturulan bu katalitik sistemlerin sözkonusu alkollerin oksidasyonunda aktif oldukları ve sahip oldukları katalitik aktivitenin depolanan platin miktarına ve platin depolama potansiyeline bağlı olduğu tespit edilmiştir. Metanol ve etilen glikol oksidasyonu için maksimum akım yoğunluğunu veren koşullar belirlenmiş ve bu koşullar kullanılarak değişik deneysel parametrelerin fonksiyonu olarak elektrooksidasyon çalışmaları yapılmıştır. Sonuç olarak, metanol-hava veya etilen glikol-hava yakıt pillerinde, saf platin elektroda göre daha ucuz ve avantajlı olan polianilin-platin elektrotların hazırlanmasının şartlan optimize edilmiştir. IX
Özet (Çeviri)
SUMMARY DETERMINATION OF CATALYTIC PROPERTIES OF MODIFIED POLYANILINE ELECTRODE FOR THE OXIDATION OF METHANOL AND ETHYLENE GLYCOL Until now electrocatalysis, i.e. heterogeneous catalysis of electrochemical reactions occuring at the electrode electrolyte interface, has mainly concerned technological investigations related either to energy storage (eg water electrolysis) or to energy conversion (eg fuel cells). 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. Moreover these alcohols are very interesting fuels due to several advantages: high solubility in aqueous electrolytes, relatively high reactivity, ease of storage and supply, low toxicity. Unlike electricity production by conventional heat cycles, the efficiency of energy production by fuel cells is not limited by a Carnot type relation and efficiencies of 60 percent have been obtained in practice at useful power lewels. Unlike conventional primary cells or batteries, the fuel cell continues to supply current as long as reactants are fed in and products are removed. Recharging is accomplished by refilling the fuel and the oxidant storage tanks while the cell is running, if need be. These two features are major advantages of fuel cells. Besides these advantages, a major problem which arises during the direct electrooxidation of organic molecules is the poisoning of the catalyst (platinum metal based) caused by the adsorption of residues, such as aldehyde, carboxylic acid and particularly CO. This poisoning phenomenon leads to a decrease in electrode activity. Due to these facts, fuel cell systems are still under development. Recent studies are focused mainly in two areas; improving the catalytic properties of the fuel cell anode and search for the alternative fuels, such as different organic molecules, to replace hydrogen. In order to enhance the performances of catalytic electrodes, one possibility is to increase the active surface area by dispersion of the catalyst as small particles on aconvenient conducting support. This procedure also allows the use of very small amounts of precious metals, typically a few mg.cm“2 To obtain efficient dispersion of the catalyst, it is necessary to use a convenient conducting support. Nation® and carbon are typical examples of such supports which have been extensively studied for the electrooxidation of methanol and formic acid. In recent years a new material namely conducting polymers, has been extensively studied in view of their potential applications. Considerable effort has been devoted to the preparation of conducting polymers such as polypyrrole, polythiophene and polyaniline. Metal microparticles dispersed in polymer modified electrodes have been recently recognized to have potential applications in electrocatalysis. Polyaniline, which is easy to synthesize in aqueous medium; is a very interesting material. It is generally homogeneous, strongly adherent to the support on which it is synthesized and chemically stable in acid medium. The possibility of dispersing metallic particles inside polyaniline gives electrocatalytic electrodes. Indeed, the polymer provides the possibility of higher surface areas and is conducting in the potential range where the organic molecule is oxidized. Polyaniline can be synthesized by two principal methods; the direct oxidation of aniline by chemical oxidants or by anodic oxidation on an inert electrode. Electrochemical methods generally employed for electrochemical polymerization of aniline are: constant potential or potential cycling polymerization. In previous works it has been observed that electrooxidation of aniline at constant potential produces a powder which adheres poorly to the electrode, whereas electrooxidation of aniline by continuously cycling the potential produces an even film which adheres strongly to the surface. Owing to the reasons given above, the method generally applied for polyaniline formation is potential cycling. The aim of the present work is to prepare a catalytic system using conducting polyaniline as a support for dispersion of platinum particles and to determine its catalytic activity in the electrooxidation of organic alcohols such as methanol and ethylene glycol. The experimental technique generally employed throughout of the thesis is cyclic voltammetry. Besides this, potential programmed voltammetry, rotating disc electrode technique and electron microprobe were also used as the auxiliary techniques. Cyclic voltammetry is usually performed by connecting a potentiostat to an electrochemical cell. The cell contains a test solution and three electrodes: working, reference and auxiliary. Special electronic circuitry within the potentiostat permits the working electrode potential to be controlled with respect to a reference electrode without any appreciable current flowing at the reference electrode. Thus; the current is forced to flow between the working electrode and the auxiliary electrode. In cyclic voltammetry, the working electrode potential is swept back and forth across the formal potential of the analyte. Repeated reduction and oxidation of the analyte causes alternating cathodic and anodic currents flow at the electrode. Cyclic xivoltammograms are recorded as current vs. potential graphs. It is possible to obtain detailed information about the system of interest from these voltammograms which are also called electrochemical spectrum. With cyclic voltammetry it is possible to study different experimental parameters such as potential sweep rate, concentration of electroactive species, pH of electrolyte and temperature. In that case it is possible to determine the mechanism of the reaction i.e. the number of exchanged electrons, transfer coefficient, reaction order and activation energy. Though there is much work aiming the elucidation of polyaniline film formation mechanism, there is very little knowledge on the adsorption characteristics and orientation of its monomer, aniline, on the electrode surface. It was found with the aid of labelled atoms that aniline is adsorbed on polyaniline and adsorption kinetics may play an important role in the process of polymer synthesis. Hence in the first part of the work, electroadsorption of aniline is studied on polycrystalline platinum electrode by potential programmed voltammetry and rotating disc electrode techniques. The calculated reaction order is approximately 0.24. This value shows that the reaction rate of aniline oxidation is not proportional to the concentration and adsorption may play a role under these conditions. However, by using rotating platinum disc electrode it was seen that the adsorption of aniline was determined by its diffusion rate to the surface of electrode. Adsorption of aniline is studied by potential programmed voltammetry using two different potential programs. It was observed that the oxidation charge of adsorption layer is exactly the same in both cases. This means that there is no electrochemical hydrogenation of the adsorbed material during potential excursion in the hydrogen region. According to our experimental results following facts are pointed out: In acidic media adsorbed aniline will probably be oxidized to CO2 at a platinum electrode. 2NH”-C^He +24H O o N“ +12CO”+ 62H+ + 62e“ 2 6 5 2 2 2 Within the overall reaction 31 electrons per aniline molecule will be exchanged. Thus the teoretical number of electrons per site should be approximately five if the oxidation of chemisorbed species is completed. Two possible models for adsorption of aniline at the metal solution interface are as follows: a) at the stationary time of adsorption the flat orientation in which the ring is more or less parallel to the electrode surface plane; xub) in the case of high aniline concentration (higher than 10'5M) and short adsorption time the ring is perpendicular to the surface. However, the adsorbed layer may be a mixture of adsorbed states in which some molecules are attached through the phenyl ring, whereas depending on the conditions others are attached solely through both the amine and the phenyl groups at the same time. In the second part of the work electrooxidation of methanol and ethylene glycol was studied on platinum doped polyaniline electrodes. Polyaniline films are formed by electrochemical cycling on a platinum foil electrode. In order to obtain catalytic surfaces, further modification was carried out since polyaniline films do not exhibit any catalytic current towards oxidation of these organic molecules. Polyaniline films were modified by dispersing platinum microparticles on the surface. Platinum microparticles were deposited into polyaniline films by constant potential electrolysis. Amount of deposited platinum was determined using the.cathodic charge consumed during electrodeposition assuming that Pt4+ to Pt° reduction is 100% efficient. It was found that these modified polyaniline films are electroactive towards oxidation of methanol and ethylene glycol. Electrooxidation of methanol on modified polyaniline films was studied as a function of platinum deposition potential. Maximum current density for the oxidation of methanol was obtained with -0.365 V/MSE deposition potential. It was found that platinum is well dispersed, modifiying the polyaniline surface and it is not possible to observe hydrogen adsorption-desorption regions corresponding to bulk platinum. The current densities vs. potential curves during the positive and negative sweeps in the methanol oxidation voltammogram are not superimposed because of the different intensities but the shift of peak potentials is not significant i.e. strongly chemisorbed poisoning species are not formed to any extent on platinum modified polymer electrodes.This means that strongly chemisorbed species require an arrangement of active sites on the catalyst surface which is not provided by the platinum deposition conditions applied in this work. The effect of temperature on the electrooxidation of methanol is studied on platinum doped polyaniline electrodes of two different polyaniline film thicknesses. Activation energies of 12.1 and 6.0 kcaLmol”1 were calculated for polyaniline film thicknesses of 0.11 and 0.38 um respectively. For a 0.11 um thick film the activation energy value of 12. 1 kcal.mol“1 might indicate that the rate determining step is the adsorption, whereas an activation energy value of 6.0 kcal.mol”1 for the polyaniline film of 0.38 um thickness suggests that the reaction is diffusion controlled. This may be explained by the higher surface area provided by the thicker film. In that case increased porosity with increasing film thickness allows the platinum particles to be deposited in regular style giving the maximum possible surface area for the oxidation of methanol. XlllOxidation of ethylene glycol at modified polyaniline electrode was studied as a function of amount and deposition potential of platinum. Maximum current density was obtained for 150ug.cm“2 platinum and -0.565 V/MSE deposition potential. Following experiments are carried out using these values constant. Electrooxidation kinetics of ethylene glycol was studied as a function of ethylene glycol concentration, potential sweep rate and temperature. Effect of temperature was studied for two different polyaniline film thicknesses. Activation energy values were calculated as 9.4 and 10.4 kcal.mol'1 for 0.11 um and 0.38 um film respectively. It can be deduced that film thickness has almost no effect on the oxidation mechanism and in that oxidation of ethylene glycol on 0.11 um film is more feasible. The effect of varying the sweep rate of the voltage was examined at constant potential limits. The current densities vary linearly with increasing scan rate until 200 mV.s”1, therein they become independent of sweep rate for values > 200 mV.s'1. A linear relationship between peak current and square root of scan rate values < 200 mV.s“1 indicates an irreversible diffusion controlled process. Effect of concentration of ethylene glycol on the oxidation process was based on current-potential curves taken at sweep rate”of 10 mV.s"1. Calculated reaction order with respect to the initial concentration of ethylene glycol is 0.44 i.e. reaction mechanism is dependent on the concentration of electroactive species. As a result it was found that polyaniline films can be used as a convenient conducting support for the dispersion of platinum particles for oxidation of organic alcohols. The electroactivity of the catalytic system depends on the deposition potential and amount of deposited platinum. Moreover, current densities obtained with modified polyaniline electrodes in acidic media are comparable to the bulk platinum. XIV
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