CC BY
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ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
AZERBAIJAN CHEMICAL JOURNAL No 4 2020
43
UDC 5 54 544.654.2
ELECTROCHEMICAL REDUCTION OF SELENITE IONS FROM ORGANIC
SOLUTIONS
S.P.Javadova
M.Nagiyev institute of Catalysis and Inorganic Chemistry NAS of Azerbaijan
Received 20.06.2020 Accepted 17.08.2020
Due to the unique properties of metal dichalcogenides, they are wide used in various fields of nano- and optoelectronics. Bi2Se3 is one of the promising n-type semiconductor materials belonging to the Av - Bvi group, with a band gap of 0.3 eV. To obtain these compounds by co-electrodeposition, we study the electroreduction of the initial components separately. Therefore, the study is devoted to the electrochemical reduction of selenite ions from the ethylene glycol solution. By drawing cyclic and linear polarization curves on Pt electrodes, the kinetics, the mechanism of the process, and the influence of various factors on the electroreduction of selenite ions are studied. Using the obtained data on the influence of temperature, the effective activation energy was calculated by the Gorbachov method. The calculation results show that the electroreduction of selenite ions from ethylene glycol is accompanied by electrochemical kinetics closer to diffusion.
Keywords: selenite ions, ethylene glycol, electroreduction, semiconductors.
doi.org/10.32737/0005-2531-2020-4-43-48 Introduction
In recent years, thanks to intensive experimental and theoretical studies, significant progress has been made in the field of technology for producing thin semiconductor films [1-6]. They constitute a wide range of materials with differ from each other in a wide variety of electrical, physical and photoelectrochemical properties. This makes it possible to determine various purposes in their technical use. Given the valuable properties, there is always a need for high-quality semiconductor materials.
As you know, Se is one of the important components of thin semiconductor films used in the production of photosensitive, thermoelectric and other devices. Se is also an important trace element from an environmental and biological point of view. However, its intake as a nutrient should be limited to a very narrow range of concentrations: its consumption outside this range leads to deficiency or toxicity [7-10]. The purpose of the study is to study the kinetics and mechanism, to determine the potential range of the electroreduction process of selenite ions for the further preparation of Bi-Se on based semiconductor thin films.
In the literature, there are some works
devoted to electroreduction or deposition of selenium ions in various ways [11-19]. Among these methods, the electrochemical method is the most extended and it always attracts great interest of researchers.
The authors of [11] found that methyl vi-ologen (MV2+) can accelerate the recovery of water-soluble species Se(+4) and Se(+6). The highest acceleration value (more than 500 times) was obtained when Na2SeO3 was reduced to 0.1 mol/dm3 sodium phosphate buffer (PBS, pH 7) in the presence of 5 mmol/dm3 MV2+. Water-insoluble red Se is dispersed near the electrode surface as a reduced product. Red sediments or colloids of Se are formed as reduction products, which will be useful for recycling Se from wastewater containing Se. Dependence of the Michaelis-Menten type between the reduction current and the concentration of Na2SeO3 was obtained, what allows one suggests that MV2+ molecules mediate electron transfer from the electrode to HSeO3- or SeO dissolved in PBS. MV2+ is also effective in accelerating recovery of SeO2. High buffer capacity is believed to be important for achieving greater acceleration.
In work [12] the results of studies on the production of selenium powders upon polarization by a cathode pulsed current are presented. The potentiodynamic polarization curves of the recovery of selenite ions on a titanium electrode in a sulfuric acid medium were recorded depending on the scan rate. It was found that with an increase in the scan rate from 15 to 150 mV/s, there take place the current maxima increase and the potentials shift to the negative side. Studies of the obtained selenium powders showed that the average particle size alternates from 16 to 7 microns and depends on the density of the impulse current. It was shown that crystalline and amorphous forms of selenium powder are formed as a result of electrochemical polarization of selenium by a cathodic pulsed current in a sulfuric acid solution.
Besides, the kinetics and mechanism of the electrochemical reduction of selenite ions on a Pt electrode in an electrolyte containing selenium and tartaric acid were studied [13, 14]. Research shows that electroreduction proceeds in two stages. The influence of various factors on the cathodic reduction of selenite ions was also studied. Using polarization curves of temperature dependence on the process of electro-reduction of selenite ions, the effective activation energy was calculated. It was established that the studied process of electroreduction proceeds according to mixed kinetics.
The authors of [15] also studied the electrochemical reduction of selenium acid on gold, silver, and copper electrodes. It was found that the recovery of selenium acid is a very complex process and is highly dependent on the substrate applied. Voltammetric measurements show the range of potentials in which the recovery process of selenium acids on the supporting substrate is possible. Also, microgravimetric data confirm the precipitation of selenium and show the mechanism of the deposition process. Besides, microgravimetric data confirm the precipitation of selenium and show the mechanism of the deposition process.
The electrochemical formation of selenium (Se) nanoparticles in an amide-type ionic liquid containing selenium tetrachloride (SeCl4), was studied in the presence of an excess chlo-
ride anion on a glassy carbon electrode [16, 17]. The electrochemical reduction of [SeCl6]2- led to the deposition of Se on the electrode surface. At more negative potentials, the electrochemical deposition of Se decreased with the formation of [Se2]2-. Selenium nanoparticles scattered in an ionic liquid were formed by the proportionality reaction between [SeCl6]2- and [Se2]2. Using ionic liquids [18], selenium films with a controlled ca-thodic potential were obtained on a copper substrate by electrolysis at 600C.
The authors of [19, 20] from an electrolyte containing 0.005 M H2SeO3 and 1 M H2SO4, studied the electroreduction of selenite ions. It is established that recovery occurs in two stages. Starting from potentials of 0.45 V (h.s.e.), a selenium film forms on the surface of the Pt electrode, what corresponds to the reduction of selenite ions to Se0. Then, starting from 0.05 V (h.s.e.), they are deeply reduced to Se2-.
The electrochemical reduction of selenite ions was also carried out from citrate electrolytes [21]. By taking cyclic and linear polarization curves on Pt electrodes, the kinetics, the mechanism of the process, and the influence of various factors on the electroreduction of sele-nite ions are studied. Based on the data obtained, the effective activation energy is calculated. The calculation results show that the elec-troreduction of selenite ions from citrate solutions is accompanied by the electrochemical and concentration nature of polarization.
An analysis of the literature shows that the electroreduction of selenite ions was mainly studied from aqueous electrolytes. Therefore, the study of this process from non-aqueous electrolytes is very interesting.
Experimental part
An electrolyte for the electroreduction of selenite ions was prepared by dissolving the required amount of 0.03 M H2SeO3 in ethylene glycol at a temperature of 313-323 K. In the study, the temperature was regulated using a universal ultra-thermostat UTU-4. Polarization curves were taken in the IVIUMSTAT Electrochemical Interface potentiostat. An electrochemical three-electrode cell with a capacity of
100 ml was used. A Pt wire with an area of 0.610-2 cm2 and a Ni electrode with an area of 2 cm2 served as a working electrode. The silver chloride electrode served as the reference electrode, and the platinum plate with an area of 4 cm2 served as the auxiliary electrode. In the process of research, platinum electrodes need periodic cleaning. At the beginning of the experiments, Pt electrodes were purified in concentrated nitric acid and then washed with bidistilled water. Then they must be kept in boiling nitric acid, which contains a small amount of ferric chloride for 30 minutes. After they should be thoroughly washed with ordinary water, and then with distilled water, and finally rinsed with alcohol or acetone.
Results and discussion
Using the potentiodynamic polarization method investigated, the electrochemical reduction of ion selenite from organic solutions. Figure 1 shows the polarization curves of the electrochemical reduction of selenite ions from a solution of ethylene glycol.
As can be seen from the figure, the process of electroreduction occurs in a one-stage process with a potential range of 0.45-(-1.8) V. Compared to the aqueous electrolyte in non-aqueous media the reduction occurs at more negative potentials and lower current values [13, 14].
This is due to the fact that the electrical conductivity of non-aqueous solutions is very
Potential v
Fig. 1. Polarization curve of electroreduction of selenite ions on a Pt electrode in a solution of ethylene glycol. Electrolyte (M): 0.03 ^SeOs+ CH2OH-CH2OH. T = 298 K, Ev = 0.02 V/c.
small, that prevents the rapid delivery of hydrogen ions to the electrode. Therefore, in our opinion, starting from the 0.6 V potential, the SeO ion is converted to Se0. That is, a deep
reduction of selenite ions to Se2- does not occur.
It can also be visually observed that with an increase in current after -0.55 V, the process of electroreduction is accelerated and the surface of the electrode is covered in golden red color. After clarifying the mechanism of the process, the kinetics of the electroreduction of selenite ions was further studied using the Gorbachev method [22].
For this, the first step is to study the effect of temperature on the electroreduction of selenite ions. The effect of temperature was studied by the potentiodynamic method at intervals of 288-338 K (Figure 2).
It can be seen from the recorded polarization curves, that despite the increase in current (from -2.177 10-4 A to -7.484 10-4 A) under the influence of temperature, the electroreduction potential does not shift to a more positive side.
Using these polarization curves, the dependence lgi^-1/r was constructed for a potential region of -0.5 - (-1.0) V (Figure 3). From the obtained lines, tga is calculated. The value of the effective activation energy at various potentials of the electrode process during the recovery of ion selenite is determined using the Aeff. = 2.3Rtga.
-1.S -1.0 -0.5 0 0 0.5
Potential v
Fig. 2. The effect of temperature on the process of electroreduction of selenite ions on a Pt electrode. Electrolyte (M): 0.03 H2SeO3 + CH2OH-CH2OH. Ev = 0.02 V/c, T (K); 1 - 288, 2 - 298, 3 - 308, 4 -318, 5 - 328, 6 - 338.
As can be seen from Figure 4, for the studied region of the kinetics of the process, the effective activation energy depends little on the potential. The established dependence gives grounds to conclude that the electrochemical reduction of selenite ions from non-aqueous electrolytes is accompanied by electrochemical kinetics closer to diffusion.
To find the optimal electrolysis mode and improve the quality of the deposited films, the influence of various factors on the electroreduc-tion of selenite ions from non-aqueous electro-
3.1 3.3 l/T-10', K
Fig. 3. Dependence of lgik-1/T. E(V): 1 - (-0.5), 2 - (-0.6), 3 - (-0.7), 4 - (-0.8), 5- (-0.9), 6 -(-1.0).
lytes was also studied. The effect of concentration on the process of electroreduction was studied in the range of 0.001-0.015 mol/l. Figure 5 shows the linear polarization curves of the effect of the concentration of selenite ions on the process of electroreduction. Polarization curves show that the effect of concentration on the process occurs with a small shift of the potential in a more positive direction (from 0.38 to 0.51 V). Since, the current at 0.001 M is -3.396 10-5 A, and at 0.015 M -1.91610-4 A.
Acfr, i k kj/mol
Fig. 4. The dependence of the effective activation energy.
Potential v
Fig. 5. The effect of the concentration of selenite ions on the process of electroreduction on the Pt electrode. Electrolyte (M): 1 - 0.01, 2 - 0.03, 3 -0.09, 4 - 0.15, H2SeO3 + CH2OH—H2OH. T = 298 K, EV = 0.02 V/s.
Potential v
Fig. 6. The effect of the scan rate on the process of electroreduction of ion selenite at the Pt electrode. Electrolyte (M): 0.03 H2SeO3 + CH2OH-
CH2OH. 1 - 0.005, 2 - 0.02, 3 - 0.04, 4 - 0.06, 5 - 0.08, 6 - 0.1, T = 298 K.
The influence of the potential sweep rate on the process of electroreduction of selenite ions was also investigated. Figure 6 shows the polarization curves taken linearly. As can be seen from Figure 6, with an increase in the scan rate, an increase in the current spent on the process of electroreduction is observed. So, like current at 0.005 V/s is -8.843 10-5 А, and at 0.1 V/s. -2.560 10-4 А. Potential displacement is not observed.
Also, by constructing the dependence between the peak of the current density on the
1/2
square root of the scan rate (ip-v ), we can determine the nature of the polarization of the elec-troreduction process of ion selenite (Figure 7).
Fig. 7. The dependence of the current density in the same potentials on the square root of the scan rate. Electrolyte (M): 0.05 ^SeOs + CH2OH-CH2OH. Scan of potential (V/с): 1 -0.005, 2 - 0.02, 3 - 0.04, 4 - 0.06, 5 - 0.08; 6 - 0.1. Т = 298 К.
As can be seen from the figure, with an increase in the scan rate of the potential, the speed of the cathodic process increases. The polarization curves obtained at different scan
rate speeds confirm the diffusion nature of the
1/2
process since the dependence between i and v i straightforward. That is, with an increase in the scan rate, i also increases. As noted above, with an interval of potential in which the effective activation energy is calculated (-0.5) - (-1.0)V, electroreduction is accompanied by electrochemical kinetics closer to diffusion. And the
construction of the relationship between i and
1/2
v1/2 corresponds to the fact that after -1.0 V po-
tential only the diffusion nature of polarization is observed.
Conclusion
Using the potentiodynamic method from a solution of ethylene glycol on a Pt electrode, the kinetics and mechanism of the process of cathodic electroreduction of selenite ions were studied. The influence of various factors on the process is studied. The results showed that the electroreduction of SeO ions occur at a potential range of 0.5 - (-1.5) V. Using the Gorbachev method, it was determined that the process of electroreduction proceeds mainly with concentration polarization.
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UZVI MOHLULLARDAN SELENIT IONLARININ ELEKTROKIMYOVI REDUKSIYASI
S.P.Cavadova
Metal dixalkogenidlari öz banzarsiz xassalarina göra nano- va optoelektronikamn müxtalif sahalarinda geni§ tatbiq olunurlar. Bi2Se3, AV-BVI qrupuna aid, qadaga olunmu§ zolaginin eni 0.3 eV olan perspektivli n-tip yarimkegirici materiallardan biridir. Elektrokimyavi üsul ila bu birla§malari birga gökdürmak ügün ilkin olaraq ba§langic komponentlarin elektroreduksiya proseslari ayriliqda tadqiq edilir. Buna göra tadqiqat etilen qlikol mahlulundan selenit ionlarinin elektrokimyavi reduksiyasina hasr edilmi§dir. Pt elektrodu üzarinda tsiklik va xatti polyarizasiya ayrilarinin gakilmasi ila selenit ionlannin elektrokimyavi reduksiya prosesinin kinetika, mexanizmi va müxtalif amillarin prosesa tasiri öyranilmiijdir Temperaturun tasirinin naticalarina asasan Qorbagov metodu ila effektiv aktivla§ma enerjisi hesablanmi§dir. Hesablamalann naticalari göstarir ki, etilenqlikol mahlullanndan selenit ionlannin elektrokimyavi reduksiya prosesi qatiliq kinetikasina yaxin olan elektrokimyavi kinetika ila mü§ayiat olunur.
Agar sözlar: selenit ionlari, etilenqlikol, elektrokimyavi reduksiya, yarimkegirici.
ЭЛЕКТРОХИМИЧЕСКОЕ ВОССТАНОВЛЕНИЕ СЕЛЕНИТ-ИОНОВ ИЗ ОРГАНИЧЕСКИХ
РАСТВОРОВ
С.П.Джавадова
Дихалькогениды металлов благодаря уникальным свойствам широко применяются в различных областях нано-и оптоэлектроники. Bi2Se3 является одним из перспективных полупроводниковых материалов п-типа, относящихся к группе с шириной запрещенной зоны 0.3 эВ. Для получения этих соединений
совместным электроосаждением, изучается электровосстановление исходных компонентов по отдельности. Поэтому исследование посвящено электрохимическому восстановлению селенит-ионов из раствора этиленгликоля. Снятием циклических и линейных поляризационных кривых на Pt электродах изучена кинетика, механизм процесса и влияние различных факторов на процесс электровосстановления селенит-ионов. По полученным данным влияния температуры рассчитана эффективная энергия активации по методу Горбачова. Результаты вычисления показали, что процесс электровосстановления селенит - ионов из этиленгликоля сопровождается электрохимической кинетикой ближе к диффузионной.
Ключевые слова: селенит-ионы, этиленгликоль, электровосстановление, полупроводники.
