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AZ9RBAYCAN KIMYA JURNALI № 4 2014
51
UDK 541.13
ELECTROCHEMICAL REDUCTION BEHAVIOR OF LEAD IN TETRAFLUOROBORIC SOLUTION ON PLATINUM ELECTRODE
A.Sh.Aliyev, Mahmoud Elrouby*, A.M.Aliyev, M.T.Abbasov, H.M.Tahirli
M.F.Nagiyev Institute of Catalysis and Inorganic Chemistry of Azerbaijan National AS
*
Chemistry Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt
chim.prob.tur@rambler.ru dr. mahmoudelrouby@gmail. com
Recived 20.09.2014
The electroreduction behavior of lead ions in aqueous solution of tetrafluoroboric acid on platinum electrode has been studied and investigated in this work. The cyclic voltammetry (CV) was used for studying the electrochemical reduction behavior of lead ions in certain conditions. The voltammetric responses of the investigated lead ions are found to be sensitive to temperature as well as the scan rate in these conditions. Interesting data and parameters were obtained and discussed in this work. The mechanism of the electrochemical reduction of lead at the optimal conditions on the electrode surface of platinum has been evaluated and proposed.
Keywords: electrochemical reduction, lead ions, tetrafluoroboric acid, platinum electrode.
Lead has high density and resistance from corrosion, so it is used for the ballast keel of sailboats. Because of its low relative cost, it can be used in many applications. One of the most important applications of lead is its use as alloying material for lead and tin and for lead antimony in lead-acid batteries [1], where the cathode and the anode reactions are as follows:
PbO2 + 4H+ + SO42" + 2e- ^ PbSO4 + 2H2O,
Pb + SO42- ^ PbSO4 + 2e- .
It is well known that the standard potential for lead in aqueous solutions is 0.12 V and it has a high hydrogen overpotential, which means that it is easily deposited electrochemically from strongly acidic solutions with a high cathodic efficiency. It is known that the values for the hydrogen over potential are dependent on the surface and structure of the electrode and are given in the literature as varying between 0.84 V for highly pure Pb to about 1.2 V for electrochemically deposited coatings [2]. The electrodeposition of lead is industrially applied using the Bett's process, which uses a fluorosilicate electrolyte containing PbSiF6 and H2SiF6 [3]. Lead also can be electrodeposited usually from acidic solutions of fluorosilicate, chloride [4], bromide, iodide [5], nitrate [6] and fluoborate [7]. Fluoroboric acid is a strong acid with a weakly coordinating, non-oxidizing conjugate base. Fluoroboric acid is used in plating circuits, metal finishing, elec-tropolishing of aluminium and its alloy and a component of galvanic baths. Inorganic fluorobo-rate salts are used as components of flux in and plating, as catalysts, in flame-retardant manufacture, in metal treatment, grain refining agents, active fillers in resin bonded abrasives and in the electrolytic generation of boron [8-10]. For that, we choose the fluoroboric acid as a blank solution of the electrodeposition bath and a solvent for antimony oxide.
In the work we are planning to use the electrodeposited antimony for electro-alloying with lead and tellurium (electrodeposition method) as a corrosion inhibitor of the steels, used in pipes of petroleum refining. For that, the electroreduction mechanism and kinetics of lead in fluoroboric acid aqueous solution on the platinum electrode was studied in the work.
EXPERIMENTAL
Apparatus. Electrochemical experiments were carried out using an IVIUM stat electrochemical interface potentiostat/galvanostat at a controlled thermostated desired temperature. A conventional three-electrode cell was used. The cell has four slots for purging the nitrogen gas and for the three electrodes. A silver/silver chloride [Ag/AgCl/KCJ(sat.)] and platinum sheet were used as reference and auxiliary, respectively. Platinum wire (0.25 cm2) was used as working electrode. The working electrode was cleaned with a solution contains 1:1 concentrated H2SO4 and H2O2 followed by acetone and rinsed with de-ionized water before performing the experiment. The electrochemical cell was set on a stirrer of varying velocities for mixing the solutions.
Reagents. Lead carbonate PbCO3Pb(OH)2, hydrofluoric acid and boric acid are of analytical grade from Merck, and BDH used without further purification. Aqueous solution of HBF4 is produced by dissolving boric acid in hydrofluoric acid [10].
RESULTS AND DISCUSSIONS
Cyclic voltammetric behavior. The cyclic voltammetry technique was used in this work in order to determine the electrode potential mechanism for the electrodeposition of lead and to determine the peak location of electroreduction.
A
Fig. 1. Cyclic voltammogram of 0.39 M Pb(BF4)2 solution in the presence of 1 g/l joiner's glue at room temperature and scan rate = 0.02 V/s.
-0.4 -0.2
0.4 0.6 0.8 1.0
E, V (vs. Ag/AgCl)
2.5-
2.0-
1.5-
1.0-
0.5-
0.0-
0.0
0.2
Fig. 1 shows the cyclic voltammogram of 0.39 M Pb(BF4)2 solution at room temperature and scan rate = 0.02 V/s in the potential range from 0.8 to -0.5 V, in the presence of 1 g/l joiner's glue. Joiner's glue, included mainly substances with adjacent systems of n electrons (aromatic compounds and their derivatives), which are adsorbed onto a positively charged metal surface [11, 12], and it has a high inhibiting effect. Moreover, it has a significantly lower impact in the deposition velocity of lead. It was found that by adding the joiner's glue, the resulting coatings of deposits are uniform, microcrystalline, and have a good quality.
The voltammetric profile shows a cathodic peak at -0.42 V and its anodic counterpart at -0.32 V approximately having some reversibility character. These peaks are related to lead elec-trodeposition and electro-dissolution of lead, respectively. The voltammogram clearly shows that the platinum electrode can be used as a good material substrate cathode for lead electrodeposi-tion in this electrolyte. Hydrogen evolution is appeared until potentials more negative than -0.6 V. From the voltammogram it is observed that on the down peak (cathodic direction), where the cathodic current density is lower than on the anodicdirection of the scan. This feature arises because of the need to form nuclei of the electrodeposited lead metal phase on the electrode surface of the platinum. On the other hand, the positive direction scan shows a sharp anodic peak due to the oxidation of the electrodeposited lead metal layer on the electrode surface. The lead deposition starts approximately at -0.36 V. A typical nucleation loop (crossover) appears at -0.37 V
attributed to the existence of a nucleation overvoltage due to the sluggishness of nuclei formation. After this, the current turns anodic, and the dissolution of deposited lead starts. Current density grows and then, suddenly, diminishes due to the removal of the deposit. Because of the presence of excess amount of BF4-, it can be adsorbed on the positively polarized electrode surface (at the positive potential range). The lead ions in the solution react with the adsorbed tetra-fluroborate ions on the electrode surface to form di-tetrafluoroborate complex Pb(BF4)2. The adsorbed formed complex begins to decompose into initial components on the electrode surface at about —0.02 V (the beginning of the negative potential range) and hence BF4- ions leave out the electrode surface and go into the solution. Adsorbed lead ions on the electrode surface (resulted from the decomposition of the adsorbed complex) and free lead ions in the solution are reduced at the main reduction peak start at potential = -0.34 V at which the lead ion gains two electrons converting into lead metal. The electroreduction mechanism of lead in tetra-fluoroboric acid can be abbreviated to the following main four steps:
a) adsorption of BF4- ions on the electrode surface,
b) 2BF ¡ads + Pb2+ = Pb(BF4)2 ads,
c) Pb(BF4)2 ads = 2BF 4 + Pb2+ads,
d) Pb2+(adsorbed and free) + 2e- = Pbads (main reduction peak).
Compared with literatures the electrodeposition potential peak of lead appeared at about -0.53 V [7], but in this work the deposition potential starts at -0.36 V, this depends on the catalytic activity of the electrode material type and the nature of blank solution.
■J 3.5 -
< 3.0 -2.5 2.0 1.5 1.0 0.5 -0.0 --0.5 -1.0 --1.5
Fig. 2. CV of 0.39 M Pb(BF4)2 in presence of 1 g/l joiner's glue at 220C at different scan rates of 0.01, 0.02, 0.05 and 0.10 V/s respectively labeled as labeled from 1-4.
—i—■—i—■—i—■—i—■—i—■—i—■—i—■—i—■—i
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
E, V (vs. Ag/AgCl)
The effect of the scan rate potential in the electroreduction process of lead in mentioned conditions was studied and reported in this work, in order to determine the nature of the process. The typical series of voltammograms were obtained at different scan rates from 0.01 to 0.10 V/s on the Pt electrode of 0.39 M Pb(BF4)2 at room temperature presented in Fig. 2. An increase in the potential scan rate causes an increase in the peak currents of reduction and oxidation. Following the Randles-Sevcik type equation [13-17],
jp = 2.686 • 105 nmACDV2 v1'2 . (1)
where, jp is the peak current density of the analyte in cyclic voltammetric (CV) or linear sweep voltammetric (LSV) behavior, n - the number of electrons appearing in the half-reaction for the redox couple, A - electrode surface area in cm , C - the analyte concentration in mol/cm, D - the analyte diffusion constant in cm2/s, v - the potential scan rate in mV/s.
2
3
4
E и
< 1.4
£L
1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6
-.1/2
Fig. 3. A correlation between jpc vs. V for the electroreduction of 0.26 M Pb(BF4)2 in presence of 1 g/l joiner's glue at t=220C, at different scan rates of as in Fig.2.
9 10 11
1/2 / , , -1*1/2 и , (mV s )
The linear response obtained for the correlation between the cathodic peak current density of lead at the mentioned conditions and the square root of the potential scan rate (jp vs. v12) shown in Fig. 3 reveals that, the electron transfer process on the particular Pt electrode for the electroreduction of lead is controlled via diffusion phenomena at these conditions.The diffusion coefficient (D) of lead ions was calculated, by the aid of Fig. 3 and equation (1) and it is found to
3 2
be 1.42-10- cm /s. The effect of temperature on electrochemical reduction of lead was investigated under two temperatures 22 and 300C. More than 300C is not studied, because the Pb(BF4)2 complex will be decomposed. The peak currents of reduction and oxidation were increased by increasing the temperature (Fig. 4). This indicates that the temperature increase causes an increasing in the rate of the electroreduction and the electro-oxidation process on the Pt-electrode surface, due to the decrease in the activation energy. The activation energy, Ea, can be calculated by using the following equation [18-20];
lg jp = const -
E„
2.303RT
(2)
where the rate of electrode processes can be expressed in terms of peak current density (jp) of the energy of the reactant, Ea- is the activation energy, R- the gas constant, and T- Kelvin temperature.
3.5 3.0 2.5 2.01.51.00.50.0 -0.5 -1.0 -1.5-2.0
Fig.4. CV of 0.39 M Pb(BF4)2 in presence of 1 g/l joiner's glue at scan rate 20 mV/s at different temperatures of 22 and 300C, respectively labeled as 1 and 2.
—I—
-0.6
—I—
-0.4
—I—
-0.2
—I—
0.0
—I—
0.2
—I—
0.4
—I—
0.6
—I—
0.8
1.0
E, V (vs. Ag/AgCl)
1.5
3
4
5
6
7
8
2
2
At ^=22 and t2=300C, the activation energy can be calculated by following equation 2. By replacing the two values of temperatures and their corresponding electroreduction peak currents densities, equation 2 will be as the following:
1 1
j = -£
jp2
■ (3)
2.303R (t2 + 273) (tx + 273)
The value of the effective activation energy Ea was found to be 18.025 kJ mol/K, at these conditions. This value is characteristic for typical diffusion process.
Thust, the electrochemical reduction behavior of lead in the solution of tetrafluoroboric acid on the platinum electrode has been achieved and reported in this work by cyclic voltammet-ric technique. The electroreduction (electrodeposition) peak of lead was detected and the electrochemical reduction process was found to be diffusion in nature. The rate of electroreduction process was found to be dependent upon and increased by increasing temperature, the scan rate. The mechanism of the electrode reactions for the investigated substance has been evaluated, proposed and summarized in four main processes, as mentioned above.
Authors thank the State Oil Company of Azerbaijan Republic "SOCAR" for its financial support for this work as a scientific grant.
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PLATÎN ELEKTRODUNDA FLUORBORAT ELEKTROLÎTL3RÎND3N QURGUÇUNUN
ELEKTROKÎMY3VÎ REDUKSÎYASI
A.§.3liyev, Mahmoud Elrouby, A.M.Qliyev, M.T.Abbasov, H.M.Tahirli
Bu içda platin elektrodunda fluorborat elektrolitlarindan qurguçunun elektrokimyavi reduksiya prosesi tadqiq edilmiçdir. Tsiklik voltampermetrik metodla qurguçun ionlarinin muayyan edilmiç çaraitda elktrokimyavi reduksiyasi oyranilmiçdir. Muayyan edilmiçdir ki, qurguçun ionlarinin reduksiya prosesinin surati elektrolitin temperaturundan va potensialin dayiçma suratindan asilidir.
Açar sôzldr: elektrokimyavi reduksiya, qurguçun ionlari, tetrafluorborat turçusu, platin elektrod.
ЭЛЕКТРОХИМИЧЕСКОE ВОССТАНОВЛЕНА СВИНЦА ИЗ ФТОРБОРАТНОГО ЭЛЕКТРОЛИТА НА ПЛАТИНОВОМ ЭЛЕКТРОДЕ
А.Ш.Алиев, Махмуд Эльруби, АЖАлиев, M.T. Abbasov, Г.M.Taгирли
Исследовано поведение электровосстановления ионов свинца в водном растворе тетрафторборной кислоте на платиновом электроде. Для изучения электрохимического поведения восстановления ионов свинца в различных условиях была использована циклическая вольтамперометрия (ЦВ). Выяснено, что электрохимическое восстановление свинца чувствительно к изменению температуры среды и скорости развертки потенциала в исследованных условиях.
Ключевые слова: электрохимическое восстоновление, ионы свинца, тетрафторборная кислота, платиновый электрод.
