Gliserolun elektrooksidasyonunda Pd ve Pd bazlı elektrotların katalitik özelliklerinin belirlenmesi
Determination of catalytic effects od Pd and Pd based electrodes in electrooxidation of glycerol
- Tez No: 39411
- Danışmanlar: PROF. DR. FİGEN KADIRGAN
- Tez Türü: Doktora
- Konular: Kimya, Chemistry
- Anahtar Kelimeler: Elektrotlar, Gliserin, Katalizörler, Palladyum, Yakıt hücreleri, Electrodes, Glycerin, Catalysts, Palladium, Fuel cells
- Yıl: 1993
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Kimya Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
Bu çalışmada, gliserolün palladyum ye palladyum bazlı modifiye elektrotlar üzerinde, bazik ortamdaki el ek t rook s i das yon u incelenmişti r. Kullanılan yöntem döngülü voltametri yöntemidir. Ayrıca potansiyel programlı elektroliz yapılarak, elektroliz Ürünleri FTIRS İle tayin edilmiştir. Çalışma sırasında çeşitli reaksiyon kriterleri incelenerek, reaksiyon mertebesi, alınan verilen elektron sayısı, reaksiyon kinetiğini belirleyen adım tayin edilmiştir. Ayrıca çeşitli adatomların gliserolün elektrooksidasyonuna olan etkileri incelenmiştir. Platin-palladyum alaşım elektrotlar üzerinde de çalışılarak en iyi katalitik etkinin elde edildiği bileşim belirlenmişti r. Yapılan elekt rokimyasal ve spektroskopik incelemeler sonucunda, gliserolün. palladyum elektrot üzerindeki oksidasyonunda, reaksiyon hızını belirleyen adımın adsorpsiyon ve diffüzyon kontrollü ve başlıca reaksiyon ürününün gliserik asit olduğu belirlenmiştir. Çeşitli adatomlardan kurşun ve bizmutun katalitik etkiyi bir miktar arttırdığı tespit edilmiştir. Alaşım elektrotlar ile yapılan çalışmada sinerjik etki elde edilmiştir.
Özet (Çeviri)
The most of the energy consumed today is being obtained from the thermal combustion of coal, oil, and natural gas. These hydrocarbon fossil fuels which occured many millions of years ago are limited. This fact constitutes a starting point for thinking about the world energy situation. The other point is also the pollution of the atmosphere with products from internal -combustion reactions its possible effect on world temperature and sea levels. As an alternating energy supplies, the development and construction of electrochemical generators have begun since 1960's. Many types of fuel cells have been described in the literature. Among fuels already used in fuel cells, besides hydrogen, are many hydrocarbons, several lower alcohols, hydrazine, and ammonia. These fuels are generally used as anodically reacting materials in combination with an oxygen cathode. Single organic compounds, such as methanol, formaldehyde, formic acid, ethylene glycol have several advantages with regard to their use as fuels. They are non-toxic, easy to store and handle and they posses & high energy density as potential fuels. They can be generated from the biomass. Because of their simple structure, they should have the simplest and most straighforward reaction mechanisms of all the possible organic fuels. Glycerol is a potential fuel of practical interest whose electrooxidation has not been studied in detail although interest in this reaction is growing because of the possibilities to obtain it from the biomass extract. It is a complex reaction whose mechanism is far from being^ understood. The aim of this work is to obtain new data on glycerol oxidation on the palladium electrode. All the electrochemical measurements were made using a doublewall thermostated cell. The working electrode was a microsphere obtained by melting a 0.5 mm diameter wire of an high purity metal. viiiThe counter electrode was made a platinum wire with cylindrical geometry. The reference electrode was a Hg/HgSCU <MSE). The electrolytic solutions were prepared with ultrapure water obtained from Millipore Milli Q system, NaOH suprapur (Merck) and glycerol extra pure (Merck). The main problem in such a work is to maintain a clean system. All experiments were carried out in nitrogen atmosphere. Before each experiments a cleaning process was applied. Linear cyclic and programmed potential voltammetry were used throughout the work. Different parameters measured, together with other experimental data, allow to suggest of formulation of a mechanism. In our research, firstly, the electrooxidation of glycerol was investigated in alkaline medium, on palladium electrode. The kinetic parameters measured, together with other experimental data, allow to suggest of formulation of a mechanism. A systematic study was realized to determine the kinetic parameteres of reaction. Varying the experimental parameters, like the potential sweep rate, potential scan limits, glycerol and electrolyt concentrations allow us to appreciate the particular behaviour of glycerol on palladium. Glycerol oxidation at programmed potential was performed during prolonged electrolysis using a special potential -time programme. This potential programme consists of an oxidation potential plateau at -0.58 V (MSE) during a fixed time (30 s) at which the electrolysis of glycerol is realised. This plateau is then followed by a rapid voltammetric sweep between two given limits (adsorption and desorption) allowing. Samples obtained at different electrolysis times have been examined by FTIRS. These preliminary results confirm that the main oxidation product is glycerate on the palladium electrode in alkaline medium. Varying the potential limits shows that adsorbed hydrogen does not play a role in the poisoning of the surface. However, the presence of the poisoning species resulting from the adsorption of glycerol can be postulated because of the current densities vs. potential curves are not superimposed during the positive and negative going potential scans. The peak-current behaviour of glycerol oxidation shows a complex reaction mechanism in which a reversible diffusion and adsorption-control is involved. IXWhen varying the concentration of glycerol the reaction order obtained shows that as a first approximation» the rate determining step depends on the initial concentration of glycerol. This value is 1,14. The evidence for the participation of adsorbed hydroxyl ions in the oxidation of glycerol is indicated by the determination of reaction order owing to the hydroxyl ion concentration. This value obtained is close to 0,5. The oxidation of adsorbed glycerol (peak A) and further oxidation of glycerolic residues (peak B) is seen in cyclic voltammograms. After peak A the initial inhibition caused by the formation of oxide layer should be followed by the increase of the oxidation rate as the potential is shifted to more positive values. The preliminary data obtained by FTIRS, shows the formation of glyceric acid after 30 hours. These data obtained both by voltammetric and spectroscopic allowed us to write a general mechanism in this work. The mechanism of the glycerol oxidation on palladium electrode in alkaline solution appears to be complex. Electrochemical reactions are dependent on the catalytic properties of the electrode surface. So it is possible to control the overall reaction rate by modifying the electronic properties of electrode. e.g. the modification by adatoms and using alloy electrodes. The foreign metal adatoms may enhance the electrocatalytic activity of metal substrates in different ways. These arei -Formation of a bifunctional catalyst, providing different active sites for adsorption of molecules participating in the electrode reaction. -Modifying the electronic properties of the surface. -Preventing the poisoning of the surface of the electrode by strongly bounded intermediates. In second part of our research, the effect of six different adatoms are studied. These are Bi, Pb, Cu, Tl, Cd, Ru. The catalytic activity of these of glycerol oxidation was investigated on palladium electrode in alkaline medium. The experiments were carried out as a function of different adatom concentrations (10~ - 10~ > In the third part of our research the electrooxidation of glycerol was studied in alkaline medium on platinum- palladium codeposits were prepared by electrodeposition at 25°C and a controlled potential of 0.0S V vs. SCEThe alloy electrodes were prepared by using some different mixtures of 1 M hydrochloric acid solution i % chloroplatinic acid and 1 X palladium chloride and with different surface compositions alloys were obtained. These alloys costitute a continuous series of solid solutions which allows us to vary continuously the bulk composition and therefore the surface composition in the whole composition range. Although platinum and palladium have very similar properties (same group of the periodic table» same fee crystal structure, similar atomic size), they have a different electrochemical behaviour. Therefore, a systematic study of these alloys may be interesting in order to better understand the electrocatalytic mechanism of the glycerol oxidation. The cathodic potential limit of the sweep for palladium and platinum-palladium electrodes has been chosen to be more anodic than the hydrogen adsorption region in order to avoid any hydrogen absorption, which would change the electrode structure. Surface areas of pure metals and alloyed metals are evaluated from the quantity of electricity involved in the reduction of the oxygen layer previously adsorbed during the anodic sweep. Surface composition is estimated from the reduction peak potential of the oxygen layer adsorbed. These potentials are intermediate on alloy electrodes between those of pure metals. The surface composition on the alloy co-deposits is very often far from the bulk composition. The atomic palladium content is generally higher for the bulk alloy than for the surface. This could be due to a preferential chemical or electrochemical dissolution of palladium on the surface in alkaline media. The electrocatalytic activity of alloy electrodes for a certain surface composition is higher than that of pure metals. The anodic and cathodic sweeps are irreversible in the case of palladium-palladium electrodes, however these are superimposed on both platinum-platinum and alloy electrodes. The electrocatalytic activity of pure metal electrodes and alloy electrodes for the oxidation of glycerol has been systematically investigated in 0.1 M glycerol -0.1 M NaOH. xiAnalysis of the voltammograms in terms of Tafel plots has been made. These Tafel plots lead to relatively good straight lines. The equilibrium potential of glycerol oxidation is calculated from thermodynamic data. The extrapolation of Tafel plots to equilibrium potential gives the exchange current density. By this quantity great selectivity can be achieved through the choice of alloy electrodes, since the glycerol concentration is kept constant. In alkaline medium, the Tafel zone shows an average value an=0.5 When plotted vs the alloy surface composition the exchange current densities pass through a maximum at about 33 Vi atomic of palladium. This synergistic effect is relatively important since the exchange current densities obtained are greater than those for pure metals. In the present state of our knowledge and investigation it is hard to explain quantitatively the origin of the synergistic effect during the electrooxidation of glycerol. The first explanation is modification of the electronic properties. Since platinum and palladium have very similar electronic properties and the Fermi level does not change very much in alloy, this explanation may not be involved in the interpretatin of such effects. According to the second explanation, each atom playing a definite role in the overall oxidation process. The platinum-palladium alloy with higher palladium content shows less catalytic activity than platinum electrodes because of the inactivity of the palladium sites. At the end of these studies we were reached some important points. These fundamental results will be a light of with glycerol fuel cells.
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