Bakırın alt potansiyelde depolanması ile Pt-Pd alaşım elektrotların karekterizasyonu
Characterization of Pt-Pd alloy electrodes by under potential deposition of copper
- Tez No: 22016
- Danışmanlar: PROF. DR. FİGEN KADIRGAN
- Tez Türü: Yüksek Lisans
- Konular: Kimya, Chemistry
- Anahtar Kelimeler: Adsorpsiyon, Bakır, Palladyum, Platin, Adsorption, Copper, Palladium, Platinum
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
Yaptığı mız çalışmada Pt-Pd alaşım elektrotlarının yüzeyinde alt potansiyellerde balcı r depolayarak alaşımın yüzey özelliklerinin karakterızasyonu amaçlanmıştır. Kullandığımız yöntem siklik voltametri yöntemidir. Bu yöntemin en önemli avantajı elektrot yüzeyinde oluşabilecek faradaik yada non-faradaik herhangi bir reaksiyonu tespit etmesidir. Bu yöntemin yanışı ra yardımcı yöntem olarak taramalı elektron mikroskop kullanıl mı ştır. Elde edilen sonuçlar bakırın düşük konsantrasyonları için alt potansiyellerde elektrot yüzeyinde bir tek tabaka oluşumunun mümkün olduğunu göstermiştir. Daha yüksek bakır konsantrasyonları için ise elektrot yüzeyinde bir tek tabaka oluşumundan önce kütlesel bir depo oluştuğu tespit edilmiştir. Bu sonuçlardan hareket ederek alaşım elektrodun gerçek yüzeyinin tayini için bir yöntem önerilmiştir.
Özet (Çeviri)
In recent years, the electrocatalysis.studies have been focused in the field of fuel cells. The most important features of fuel cells are that they are silent, clean i.e. they cause no environmental pollution and their efficiency is higher than the other energy converters. In fuel cells mainly hydrocarbones, such as, f ormaidehide. ethylene glycol. formic acid and some other alcohols are used as a. fuel. In the case of the hydrocarbones, low solubility in water and low diffusion current causes the low electric current. Contrary to hydrocarbones the work with the fuels such as hydrogen and alcohols gives us a very high current- density. One of the main problems in elect rocatalysis is the poisoning effect of the strongly bound intermediates, i.e. the intermediate product formed during the electrochemical reaction will occupy the active sites on electrode surface and this will cause to a decrease in the resulting current density. We can overcome this problem by using alloy catalysts. The electrochemical oxidation of methanol and ethylene glycol was studied on platinum-palladium alloy electrodes and the synergistic effect was observed for the surface concentrations of %15-20 at.Pd. Platinum and palladium can be in electrocatalysis. Palladium absorb hydrogen, so when we work electrode the cathodic limit shoul the beginning of hydrogen adsorpti the oxygen adsorption begins at mo in alkaline medium with respect palladium. So it's difficult to d the hydrogen adsorption and begi adsorption. used to a great extend has the ability to with the palladium d be chosen just to on region. Besides re negative potentials to acidic medium on istinguish the end of nning of the oxygenBecause" of the reasons mentioned above. it ls impossible to use the hydrogen adsorption properties for determining the real surface area of platinum-palladium alloys. Woods and co-workers proposed the use of oxygen desorption peak: for determining the real surface area of palladium electrodes assuming 1:1 stoichiometry i.e. one oxygen atom occupies one site on the electrode surface. But in alkaline medium the higher oxides such as PdO_ was formed in the same region. Obviously it remains dubious to use the oxygen adsorption region for a direct measure of real surface area. A different technique used for characterisation of electrode surfaces is so called underpotential deposition >UFD>. The basic feature of UPD i s to form submonoiayers from foreign metal adatoms on electrode surface in a potential range positive than the reversible Nernst potential, that is, before bulk deposition can occur. Due to the simple atomic. nature of the adsorbed species, these modified electrodes provide useful model systems for studying the influence of surface area and electronic structure on heterogeneous catalysis by the metal s. The foreign metal adatoms may enhance the electro- catalytic activity of metal substrates in different wa y s, i. e. by: -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 bound intermediates. There are many studies concerning the thermodinamic and structural effects of UPD metal adatoms on the elect rocataiytic reactions. However only few of them are related to the subject whether it's possible to use UPD for characterisation of surface properties of e 1 ec t rodes o r not, As a result in our work we used UPD of copper for VIcharacterization of platinum-palladium alloy electrodes. We choose the copper adatoms to determine the real surface of the platinum-palladium alloys because of two main reasons. Firstly, copper was extensively studied as adatom on different metal i c substrates and its adsorption mechanism is well established. Secondly. some recent works on the characterization by LEED. AE3 and work function methodes of a copper layer on a palladium single crystal electrode have shown that copper forms a monolayer on substrate before bulk deposition. We used cyclic voltammetry as an experimental technique in our work. Here the potential of the working electrode. controlled by a potentiostat is changed continuously with a constant scan rate <dE/dt) by a ramp potential from a sweep generator. Any reaction on the electrode surface, faradaic or non fara- daic will usually be detected in the cyclic current potential curves as a current superimposed to the base current due to double layer charging. The electrochemical cell was designed for the purposeof use. This is a classical three electrode, double wall cell. The electrodes are working, counter and referance electrode respectively. Electrochemical reaction occurs on the working electrode, counter electrode is responsible of the current passing across the system while referance electrode controls the potential. The main problem in such a work is to maintain a clean system. For this purpose we used triply distilled water throughout our experiments. Water was obtained from the Millipore Milli -Q suprapure water system. Platinum-palladium co-deposits were prepared by electrodeposition at 25 C and a controlled potential of 0.05 V vs. SCE from different mixtures of 1 M hydrochloric acid solution 1% chloroplatonic acid and IX palladium chloride. For the electrochemical measurements the metals were deposited on small platinum bead (about 1.5 mm diameter) obtained by melting a platinum wire of 0.5 mm diameter. To decrease the roughness factor, the alloy electrodes were then annealed by warming them to red heat in a hydrogen flame. The electrolyte was 0. 1 M NaOH and was deoxygenated by bubbling pure nytrogen through the cell before each experiment. VXlAs we mentioned before we used underpotenti al deposited copper adatoms for the char-act eri zat ion of platinum-palladium alloy electrodes. For this purpose copper was allowed to deposit on the wording electrode in the potential -time sequences. The charges associated with thus formed copper deposits were determined from anodic I -E curves observed by a single triangular pulse applied at the end of the potential -time sequence. It was seen that the quantity of adsorbed copper adatoms reaches a stationary maximum value at about td = 15 min. indicating that adsorption desorption equilibrium is attained. Using this equilibrium value the experiments were carried out as* a function of different adatom concentrations (10 M - 5x10 ' M CuSO) and different deposition potentials. Anodic charge values obtained were plotted vs. deposition potentials. The amount of adsorbed copper reaches a plateau at about -0.8 - 0.7 V. MSE for low CuSÖ4 concentrations which indicates the completion of a monolayer on the electrode surface. For higher adatom concentrations the plateau disappears. This can be deduced that in this last case the deposition of bulk. copper is starting before the completion of a monolayer copper adatoms. Electrode surface was observed using scanning electron microscopy. It was seen from the photographs taken that platinum-palla.dium alloy electrode has an intermediate surface structure between platinum and palladium smooth electrodes. The electrode on which copper was allowed to deposit for 35 min. was also observed and it was seen that the porosity factor of electrode surface was increased to a great extend after deposition. The true surface area of platinum palladium alloy electrode was calculated from the surface estimation based on the oxygen monolayer and copper monolayer and obtained values were compared. Thus, from this comparison, the estimation of the viiitrue surface area of a platinum-palladium alloy electrode using ÜPD of copper adatoms appears as the more appropriate method.
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