Russian Journal of Electrochemistry, Vol. 41, No. 9, 2005, pp. 971–978. Translated from Elektrokhimiya, Vol. 41, No. 9, 2005, pp. 1094–1101. Original Russian Text Copyright © 2005 by Gorodetskii, Neburchilov.

Titanium Anodes with Active Coatings Based on Iridium Oxides: The Corrosion Resistance and Electrochemical Behavior of Anodes Coated by Mixed Iridium, Ruthenium, and Titanium Oxides V. V. Gorodetskii and V. A. Neburchilov State Scientific Center of the Russian Federation Karpov Research Institute of Physical Chemistry, ul. Vorontsovo Pole 10, Moscow, 105064 Russia Received November 11, 2004

Abstract—A study of the corrosion resistance and electrochemical behavior of titanium anodes with active coatings prepared from mixed oxides iridium, ruthenium, and titanium (OIRTA) is continued. The dependence of the catalytic activity, selectivity, and corrosion resistance of these anodes with ı mol % RuO2 + (30 – ı) mol % IrO2 + 70 mol % TiO2 is studied in conditions of chlorine electrolysis on the ratio of concentrations of IrO2 and RuO2 in them at a constant loading of iridium in the coatings. It is established that the maximum corrosion resistance and selectivity is inherent in OIRTA with the RuO2 concentration close to 4 mol %. Partial curves, which describe the dependence of the rates of dissolution of iridium out of OIRTA and the evolution of chlorine and oxygen in them on the electrode potential, are obtained. The dependence of the rates of these processes on the solution pH, the concentration of NaCl in it, and the thickness of the active layer is studied. It is shown that the rate of dissolution of iridium out of OIRTA and the concentration of oxygen in chlorine at a constant potential increase approximately proportionally to the coating thickness, from whence it follows that the said processes proceed over the entire depth of the coating. An assumption is put forth that the chlorine evolution on OIRTA of the optimum composition, with a loading of iridium equal to 2.5 g m–2, at high anodic currents occurs in an outer-kinetics regime in the presence of diffusion limitations on the removal of chlorine out of the coating’s depth. Key words: metal oxide anodes, corrosion resistance, selectivity, catalytic activity, reaction distribution over the depth of a coating

INTRODUCTION In this work we continue with the studies of the corrosion resistance and electrochemical behavior of titanium anodes with active coatings comprising mixed oxides of iridium, ruthenium, and titanium (OIRTA) in conditions that are close to the conditions of chlorine electrolysis [1, 2]. EXPERIMENTAL All the anodes, which were investigated in this work, had been prepared in accordance with the standard procedure, which was described in detail in [2]. An active coating was deposited onto chemically polished titanium with a protective sublayer preliminarily formed on it, the sublayer in question consisting of metallic iridium and titanium and their oxides [3]. The formation of the active coating proper was realized by means of smearing the electrode with a corresponding volume (applied in a calculated amount of 0.0025 ml cm–2) of a covering solution (with a concentration of 30 g l–1 relative to IrO2), the covering solution was then dried up,

and thermolysis was performed in air at the temperature T = 350°ë for the duration of 15 min. This procedure was repeated as many times as was required for reaching the specified thickness of the coating. Thereafter there was performed a conclusive anneal of the anodes, also in air, at T = 450°ë, for the duration of one hour. The information concerning the catalytic activity of anodes was obtained by a method of polarization curves (PC) on vertically arranged stationary plate-like electrodes of size 10 by 10 by 1 mm. The measurements were conducted in conditions that were close to the conditions of chlorine electrolysis in a solution of 300 g l–1 of NaCl with pH 2 at a temperature of 87°ë, with the solution continuously bubbled with argon. All the potentials given in this work are referred to a normal hydrogen electrode and the polarization curves are corrected for the ohmic potential drop. The latter was determined from the curves of the potential decay after breaking the polarizing circuit off. The rate of dissolution of iridium out of the anodes was determined by means of a gamma-spectroscopic method of measurements of low corrosion rates, which had been devel-

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GORODETSKII, NEBURCHILOV E, V (NHE) 1 2 3 4

1.4

5 10 µm Fig. 1. An electron microscopy photograph, taken in secondary electrons, of the surface of OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2) with the iridium loading equal to 2.5 g m–2.

oped previously in [4, 5]. The radioisotope 192Ir, with a half-value period of 79 days, which was used for this purpose, formed in a coating as a result of irradiation of electrodes in an atomic reactor by a neutron flux equal to 3 × 1013 n cm–2 s–1, the irradiation being performed for the duration of ten hours. The prolonged and continuous experiments, which were necessary for reaching steady-state rates of dissolution and the concentration of oxygen in the chlorine gas, were conducted in a cell of a flow-through type, which was described at length in [6]. The selectivity of the anodes was judged upon from the concentration of oxygen in the chlorine gas, which was measured with the aid of a chromatographic procedure [7]. The investigation of the morphology of the surface of the anodes was realized with the aid of a JSM-35CF JOEL scanning electron microscope. RESULTS AND DISCUSSION It was established that the anodes of the type OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2), which were prepared in a standard manner, with an overall iridium loading equal to 2.5 g m–2 over the microrelief of its surface (Fig. 1), hardly differ from the anodes with a coating of IrO2 or IrO2 + TiO2 (OITA) (see Fig. 1 in [3]), which had been manufactured in a similar manner and which possessed a hilly relief of their surface, which was close to the relief of chemically polished titanium [2]. At the same time, as opposed to the latter, in an OIRTA coating there appears a tremendous number of pores less than 0.5 µm in diameter and short cracks. Polarization curves for the chlorine evolution on OIRTA of the composition (30 – x) mol % IrO2 + x mol % RuO2 + 70 mol % TiO2, obtained at various values of x and a constant iridium loading in the coating, which was equal to 2.5 g m–2, are presented in Fig. 2. It was shown that, at all the RuO2 concentrations in the coating x = 1, 2, 4, and 15 mol %, in the region of low cur-

1.3

3

1 –log i [A cm–2]

2

Fig. 2. PC for the chlorine evolution in a solution of 300 g l–1 NaCl with pH 2 at T = 87°ë continuously blown with argon, obtained on OIRTA of the composition (30 – x) IrO2 + x RuO2 + 70 mol % TiO2, at x equal to (1) 1, (2) 2, (3) 4, and (4) 15 mol % and a constant iridium loading of 2.5 g m–2 in the coating; curve 5 is a similar curve, which had been obtained on ORTA with a ruthenium loading in the coating equal to 6.5 g m–2 [8].

rents, the PC possessed an extended region with a slope that was equal to approximately 0.034 V. Following an increase in the RuO2 concentration from 1 to 4 mol %, the position of that region in the PC practically did not vary (curves 1–3), while at currents in excess of 5 × 10−2 A cm–2 there was observed deviation from a linear dependence in the direction of a higher slope. This deviation revealed itself the stronger at higher current densities and lower concentrations of RuO2 in the coating. At the RuO2 concentration that was equal to 15 mol % the linear portion of the PC with b = 0.034 V shifted in parallel to itself by approximately 0.008 V in the direction of lower potentials (curve 4) and in so doing coincided with the linear “Nernstian” portion of the PC that had been obtained in the same conditions on ORTA [8] (curve 5). Such a coincidence of the linear regions exhibited by polarization curves 4 and 5 points to that, in this region of potentials, the chlorine evolution rate on OIRTA, as well as on ORTA, is limited by the diffusion of chlorine away from the electrode [8–10], with the chlorine evolving predominantly out of a thin surface layer of the coating [10–12]. The decrease in the slope of PC, observed in both curves 4 and 5 at higher currents, may be explained by an increase in so doing of the rate of removal of chlorine away from the electrode as a result of the appearance of an additional path for its removal in the form of bubbles of gas [8–10]. Following a further increase in the current, there occurs divergence of curves 4 and 5, which is caused by that, while on ORTA there occurs the exit of PC onto a practically horizontal segment of low polarizability (SOLP) [13–15], on OIRTA there is observed an increase in the

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tective sublayer and titanium because of the emergence of a semiconducting film of TiO2 on titanium.

–logqIr [g cm–2 h–1]

7

8

4

1 2

9 0

973

3 500

τ, h

Fig. 3. Variation of the rate of the iridium dissolution out of OIRTA with time in the course of their anodic polarization by a current of 0.2 A cm–2 in a solution of NaCl (300 g l–1) of pH 2 at T = 87°ë for the same compositions of coatings as in Fig. 2.

slope of PC, which results from that the electrode process starts to be limited by electrochemical stage of discharge of chloride ions. The latter is caused by a somewhat lower catalytic activity of OIRTA as compared with ORTA, as well as by the fact that on OIRTA (as well as on IrO2 and OITA [16]), as opposed to ORTA, the chlorine evolution occurs not with a transfer coefficient that is equal to 2 but with a transfer coefficient that is close to 1.5 (considered are apparent transfer coefficients). This must lead to that, with increasing current, there decreases the contribution made by the subsequent diffusion stage to the measured rate of the process and, in the long run, it must begin to be limited by the stage of discharge of chloride ions [17]. In a kinetic regime there occurs, most probably, the chlorine evolution at high currents also on OIRTA with 1, 2, and 4 mol % RuO2 (Fig. 2; curves 1–3). In so doing, at the highest currents, there emerges probably additional polarization, which is connected with the emergence of an ohmic potential drop at the interface between the proRUSSIAN JOURNAL OF ELECTROCHEMISTRY

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Studied was the effect the concentration of RuO2 in an OIRTA coating exerts on the corrosion resistance of anodes in conditions that are close to the conditions of chlorine electrolysis (Fig. 3). Shown was that the steady-state rate of dissolution of iridium out of such anodes (qIr, g cm–2 h–1) is reached only 200–300 h after the beginning of electrolysis. Following an increase in the RuO2 concentration in the coating first from 1 to 2 mol % and then to 4 mol %, the steady-state rate of dissolution of iridium decreases in the series 2.8 × 10–9, 2 × 10–9, and 1 × 10–9 g cm–2 h–1 and after an even further increase in the RuO2 concentration to 15 mol % it, conversely, increases to 3.9 × 10–9 g cm–2 h–1. The potentials of those OIRTA were increasing with time even up to the reaching of their steady-state values, altering in the intervals 1.38–1.470, 1.365–1.470, 1.360–1.380, and 1.337–1.382 V. In so doing, for the RuO2 concentrations in the coatings equal to 4 and 15 mol % RuO2, there were established approximately identical steady-state values of potentials. The obtained nonmonotonic character of the dependence of the iridium dissolution rate out of OIRTA on the RuO2 concentration may be explained by assuming that, following its increase from 1 to 4 mol %, the rate of the iridium dissolution decreases because of a decrease in so doing of the steady-state potential of the anode. The subsequent increase in the iridium dissolution rate following an increase in the RuO2 concentration to 15 mol % is probably caused by that, with increasing the RuO2 concentration in excess of 4 mol %, there occurs a substantial increase in the content, inside the coating, of a rutile phase of a solution on the basis of IrO2 and RuO2 with the RuO2 concentration increasing in it1 at a low concentration of TiO2. The lower corrosion resistance of RuO2 as compared with IrO2 [1] and the selective dissolution of RuO2 out of this phase lead to an increase in its defectness and an increase, as a result of this [18], of the rate of dissolution out of the coating of iridium. At a low concentration in the coating of RuO2 and a high concentration of IrO2, iridium is present in the coating mainly in the form of a solid solution on the basis of IrO2 and TiO2 with a low concentration of RuO2 in it, which provides for the high corrosion resistance of the coating. Increasing the ratio of the RuO2 concentration in the coating to that of IrO2 also leads to a decrease in the selectivity of anodes because of a higher catalytic activity of RuO2 as compared with IrO2 in the oxygen evolution reaction. All this makes it not worthwhile to increase the concentration of RuO2 in an OIRTA coating above 4–6 mol %. 1 This

conclusion was drawn on the basis of x-ray structure measurements that had been performed by V.I. Marchenko of the Donetsk State University.

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E, V (NHE)

E, V (NHE) 2

1 1.4

1.4

1.3

1.3

3

9 8 4 1 3 2 –logqIr [g cm–2 h–1] –logiO , –logiCl [A cm–2] 2

2

Fig. 4. Partial curves of the dependence of (1) iridium dissolution rate, (2) oxygen evolution rate, and (3) chlorine evolution rate on the potential of OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2) in a solution of NaCl (300 g l–1) of pH 2 at T = 87°ë; the iridium loading in the coating is as follows: (1) 10 and (2, 3) 2.5 g m–2.

Obtained were steady-state partial PC for the dependence of rates of the dissolution of iridium out of these OIRTA (qIr) and the evolution of chlorine ( i Cl2 ) and oxygen ( i O2 ) on them on the electrode potential (Fig. 4). Established was that these curves (E– log q Ir , E– log i O2 , E– log i Cl2 ) are almost linear, with slopes equal to, respectively, 0.077, 0.081, and 0.060 V2 in the same range of potentials studied, whence it follows that all the three said processes are limited in this range of potentials by electrochemical stages. Taking into account that the slope of the curve for the chlorine evolution is substantially lower than the slopes of the curves pertaining to the oxygen evolution and the iridium dissolution, we have come to the conclusion that on OIRTA, as well as on ORTA [19] and IrO2 [1], both the consumption of precious metal per metric ton of chlorine and the inefficient consumption of electric energy for the side process of oxygen evolution decrease. The practical coincidence of slopes of partial curves E, log q Ir - and E, log i O2 points to that on OIRTA, as opposed to ORTA [20, 21], there is observed a parallelism between processes of dissolution of precious metal (iridium) and evolution of oxygen. This difference is caused probably by that, in the case of OIRTA, there were obtained steady-state partial curves of dissolution of iridium and evolution of oxygen.3 2 Strictly

speaking, the slope of PC equal to 0.06 mV is not a Tafel slope. In this range of potentials, a Tafel slope equal to ~0.034 V is distorted by an ohmic potential drop (see above). 3 The assumption that the partial curve for the oxygen evolution on ORTA [22] was non-steady-state had been aired earlier in [21].

Studied was the dependence of the corrosion resistance of OIRTA on the solution pH. To this end obtained were curves of variation in the rate of the iridium dissolution out of OITA with time at their anodic polarization in a solution of 300 g l–1 NaCl (T = 87°ë, i = 0.2 A cm–2) and a successive variation of pH in the interval from 1 to 5.9 (Fig. 5a). Shown was that, as well as on IrO2 [3] (Fig. 5b; curve 1), in a broad region of pH from 1 to 5.1 the steady-state rate of the iridium dissolution out of OIRTA depends very weakly on the solution acidity (Fig. 5b; curve 2), and in conditions that are close to the conditions of chlorine electrolysis, at pH lying in the interval 2–4, amounts to the quantity 1.1 × 10–9 g cm–2 h–1. As opposed to IrO2, it increases by almost twofold to 1.9 × 10–9 g cm–2 h–1 following an increase in pH to 5.9, which is probably connected with an increase in so doing of the degree of the defectness of the coating because of selective dissolution of the ruthenium component out of it. Following a change in the solution pH from 2 to 4, the OIRTA potential practically did not alter, remaining close to 1.380 V, and increased up to 1.410 V at pH 1. This points to that on OIRTA, as also on ORTA and RuO2 [23–25], there occurs the hindering of the reaction of chlorine evolution with increasing the solution acidity. In order to imitate an emergency situation in conditions of a membrane chlorine electrolysis, where, following the emergence of defects in the membrane, there can occur a severe alkalization of electrolyte, performed were tests of OIRTA in a solution of 200 g l–1 NaOH at T = 87°ë and i = 0.4 A cm–2 (Fig. 5a, curve DE). Shown was that, following such a severe increase in the solution pH, there is observed an almost sixtyfold increase in the rate of the iridium dissolution out of OIRTA up to the quantity 6 × 10–8 g cm–2 h–1. The lifetime of the OIRTA operation prognosticated in so doing (until a complete working-out of the entire coating) amounts to approximately 4200 h, which is substantially higher than on ORTA and the OIRTA patented in [26], which consisted of 15 mol % IrO2 + 15 mol % RuO2 + 70 mol % TiO2. Indeed, as follows from [27], in analogous conditions, the rate of the ruthenium dissolution out of the coating on ORTA with a ruthenium loading equal to 6 g m–2 is equal to 3.4 × 10–6 g cm–2 h–1 (which corresponds to a lifetime of 176 h) and the rate of the iridium dissolution out of OIRTA with 15 mol % RuO2 and 15 mol % IrO2 with a loading of iridium in them equal to 6 g m–2 is equal to 4.5 × 10–7 g cm–2 h–1 (which corresponds to a lifetime of 1350 h). These results points to that the OIRTA we have created and patented, with a lowered content of RuO2 in the coating [28], may be successfully used for producing chlorine and alkali by a membrane method. The latter was confirmed by the results of industrial tests that were conducted with our participation. Established was that the corrosion resistance and selectivity of OIRTA in conditions of the chlorine evolution on them are substantially dependent on the concentration of chloride ions in solution. For example,

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–logqIr [g cm–2 h–1]

–logqIr [g cm–2 h–1] 7

(b)

8

1 2 pH

9

0

(a)

975

4

8

8

9 O 0

A 400

B

C

D

800

1200

E

F τ, h

1600

Fig. 5. (a) Variation of the rate of the iridium dissolution out of OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2) with time in the course of its anodic polarization by a current of 0.2 A cm–2 in a solution of NaCl (300 g l–1) at T = 87°ë following a successive change in the solution pH as follows: (OA) 2, (AB) 1, (BC) 3.8, (CD) 5.1, and (DE) 5.9; curve EF is a similar dependence, which was obtained in a solution of 200 g l–1 NaOH at T = 87°C and i = 0.4 A cm–2; and (b) dependences of the steady-state rate of the iridium dissolution in the above-stated conditions on the solution pH, obtained at (1) IrO2 and (2) OIRTA.

following a decrease in the concentration of NaCl from 300 to 50 g l–1, in the course of an anodic polarization of OIRTA by a current density of 0.2 A cm–2 at pH 2 and T = 87°ë, the potentials of anodes increases from 1.380 to 1.460 V, and the steady-state rate of the iridium dissolution out of the coating increases from 1.1 × 10–9 to 3.5 × 10–9 g cm–2 h–1. With decreasing concentration of sodium chloride in solution, there also increases the concentration of oxygen in the chlorine gas that evolves on OIRTA (Table 1). At a constant chloride concentration, the oxygen concentration in question increases with increasing the solution pH.

tion in the “Tafel” region of potentials is limited by the chlorine diffusion away from the electrode, with the chlorine evolving predominantly out of a thin surface layer of the coating. In so doing, a quasi-steady state with respect to the reaction of discharge and ionization of chlorine is realized inside the porous coating [10–12, 29]. As to a slight increase in the chlorine evolution rate, which is observed in the “Tafel” region of potentials with increasing the thickness of the coating, it may be explained by an increase in so doing of the rate of the

Studied was the effect the thickness of an active coating of OIRTA has on the corrosion resistance and selectivity of anodes. Established was that, following an increase in the iridium loading in the active coating of OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2) from 2.5 to 10 and then to 15 g m–2, the extended segment with a slope on the order of 0.034 V persists (Fig. 6), and the PC itself successively shifts in the negative direction by, respectively, 0.006 and 0.008 V. In so doing, a not-extended, practically horizontal, SOLP emerges at high currents on an anode with an iridium loading equal to 15 g m–2 (curve 3), and the curve itself practically coincides with a similar curve obtained in the same conditions on ORTA (curve 4). It follows that, on the anode OIRTA with an iridium loading equal to 15 g m–2 pointed out in the foregoing, as also on OIRTA [10–12], the rate of the chlorine evolu-

Table 1. Dependence of the concentration of oxygen in the chlorine gas ( c O2 ) on the concentration, in solution, of NaCl (cNaCl) and pH in the course of anodic polarization of OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2) at i = 0.2 A cm–2 (the iridium loading in the coating is equal to 2.5 g m–2)

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cNaCl, g l–1

pH

c O2 , vol %

300

3.2 3.8 4.6 3.2 3.8 4.6

0.05 0.12 0.54 0.14 0.67 1.39

200

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GORODETSKII, NEBURCHILOV –logqIr [g cm–2 h–1]

E, V (NHE) 1.4 1 2

7 3 4 1.3

3

2

1 –logi [A cm–2] 8

Fig. 6. PC for the chlorine evolution in a solution of 300 g l–1 NaCl with pH 2 at T = 87°ë continuously blown with argon, obtained on OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2) with different iridium loadings in the coating as follows: (1) 2.5, (2) 10, and (3) 15 g m–2. Curve 4 is a similar curve, which had been obtained on ORTA with a ruthenium loading in the coating equal to 6.5 g m–2 [8].

3 2

chlorine diffusion away from the anode, occurring because of an increase in the external surface area of the electrode at the expense of the crackedness of coatings. The latter must lead also to a decrease in the overvoltage at which there will begin, on the anode, gas evolution [30]. In contradistinction to the catalytic activity, the corrosion resistance of OIRTA decreases with increasing the coating thickness (Fig. 7). For example, in the case of OIRTA anodically polarized in a solution of 300 g l–1 NaCl with pH 2 at T = 87°ë and i = 0.2 A cm–2, with increasing the iridium loading in the coating from 2.5 to 4.5 and then to 10 g m–2, the steady-state rate of the iridium dissolution out of the coating in so doing increases respectively from 1 × 10–9 to 1.8 × 10–9 g cm−2 h–1 and then to 3 × 10–9 g cm–2 h–1, i.e. following a fourfold increase in the iridium loading, the rate of the iridium dissolution increased threefold. In so doing, the steadystate potential of the anode increased by 0.008– 0.010 V. On the basis of the obtained results, there was made the conclusion that the dissolution of iridium out of OIRTA in the region of high currents, as well as the dissolution of ruthenium out of ORTA [31], occurs in conditions of chlorine electrolysis out of the entire depth of the coating. The steady-state concentration of oxygen in the chlorine gas increases with increasing thickness of the coating far weaker than the rate of the iridium dissolution does. For example, in the course of anodic polarization of OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2) in conditions that were close to the conditions of a membrane chlorine electrolysis in a

9

1

0

τ, h

500

Fig. 7. Variation of the rate of the iridium dissolution out of OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2) with time at iridium loadings in the coating equal to (1) 2.5, (2) 4.5, and (3) 10 g m–2 in the course of anodic polarization in a solution of 300 g l–1 NaCl of pH 2 at T = 87°ë and i = 0.2 A cm–2.

solution of 200 g l–1 NaCl at pH 4 and i = 0.4 A cm–2, the concentration of oxygen in chlorine increased by a mere 1.7 times following an increase in the iridium loading from 2.5 to 30 g m–2, i.e. by approximately ten times (Table 2). However, it should be taken into account that the values of the concentration of oxygen in the chlorine gas, which were measured experimentally, refer to substantially different potentials of anodes. Because of this, to explain these results, these were recalculated to a constant potential E = 1.375 V under the condition that the partial polarization curve for the oxygen evolution in this region of potentials, as well as the PC for the oxygen evolution in [23], possesses a slope that is equal to nearly 0.110 V at pH 4. The corresponding quantities are presented in Table 2 (last column). As follows from an analysis of these results, the concentration of oxygen in chlorine and, consequently, the oxygen evolution rate, increased at a constant potential by approximately five times, i.e.

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Table 2. Dependence of the concentration of oxygen in the chlorine gas ( c O2 ) and the electrode potential (E) on the iridium loading in the active coating of OIRTA (with 26 mol % IrO2 + 4 mol % RuO2 + 70 mol % TiO2) in the course of their anodic polarization in a solution of 200 g l–1 NaCl with pH 4 at T = 87°C and i = 0.4 A cm–2 The iridium loading in the coating, g m–2

c O2 vol % (experiment)

E, V (NHE)

Polarization duration, h

c O2 at E = 1.375 V, vol % (calculation)

2.5

1.01 ± 0.15

1.425

550

0.3

4.5

1.22 ± 0.26

1.390

410

0.88

15.5

1.77 ± 0.45

1.375

510

1.77

30.0

1.75 ± 0.29

1.385

560

–

approximately directly proportional to the thickness of the coating, following an increase in the iridium loading from 2.5 to 15.5 g m–2, i.e. by 6.2 times. Following a further increase in the iridium loading from 15.5 to 30 g m–2, the electrode potential and the oxygen concentration in chlorine remained practically unchanged. The obtained results made it possible to put forth the assumption that the evolution of oxygen in conditions of a chlorine electrolysis on OIRTA with a thickness of the coating that does not exceed 15.5 g m–2 occurs, probably, from the entire depth of the coating. However, following an increase in the iridium loading to above 15 g m–2, this dependence breaks down. Thus, as a result of performed investigations, created were new metal oxide anodes of the type OIRTA [28], which exceeded, by their corrosion resistance and selectivity, the best analogues existing in the world [26, 32]. This achievement has proved possible owing to the fact that authors radically altered the ratio between concentrations of IrO2 and RuO2 in the active coating. Instead of anodes with a low concentration of IrO2 and a high concentration of RuO2, there were manufactured the latter, in whose active coating the concentration of IrO2 substantially exceeded the concentration of RuO2. This made it possible to considerably raise the corrosion resistance of the coating while simultaneously decreasing its thickness, thus providing for a substantial increase in the lifetime of anodes at their reduced cost. The decrease of the concentration of RuO2 in the coating, its thickness and density, made it possible to also decrease the rate of the occurrence of the side reaction of evolution of oxygen on such anodes, thus providing for a high selectivity of anodes. The dramatic decrease of the thickness of the coating became possible not only as a consequence of an increase in its corrosion resistance but also owing to the deposition, in between the active coating and titanium, of a specially designed dense protective sublayer [3, 28], which prevents the possibility of oxidation of titanium in the process of manufacture and exploitation of anodes. Sadly enough, at this particular stage of work, the created OIRTA were somewhat inferior to ORTA [9, 10] and to OIRTA developed previously [26, 32] as RUSSIAN JOURNAL OF ELECTROCHEMISTRY

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to their catalytic activity. This deficiency of theirs has now been removed. REFERENCES 1. Gorodetskii, V.V., Neburchilov, V.A., and Pecherskii, M.M., Elektrokhimiya, 1994, vol. 30, p. 1013. 2. Gorodetskii, V.V. and Neburchilov, V.A., Elektrokhimiya, 2003, vol. 39, p. 1243. 3. Gorodetskii, V.V. and Neburchilov, V.A., Elektrokhimiya, 2003, vol. 39, p. 1249. 4. Gorodetskii, V.V., Dembrovskii, M.A., and Losev, V.V., Zh. Prikl. Khim., 1963, vol. 36, p. 1543. 5. Chemodanov, A.N. and Kolotyrkin, Ya.M., Itogi Nauki Tekh., Ser: Elektrokhim., 1981, vol. 8, p. 102. 6. Gorodetskii, V.V., Pecherskii, M.M., Yanke, V.B., Shub, D.M., and Losev, V.V., Elektrokhimiya, 1979, vol. 15, p. 559. 7. Bune, N.Ya., Pecherskii, M.M., and Losev, V.V., Elektrokhimiya, 1975, vol. 11, p. 1382. 8. Pecherskii, M.M., Gorodetskii, V.V., Evdokimov, S.V., and Losev, S.V., Elektrokhimiya, 1981, vol. 17, p. 1087. 9. Evdokimov, S.V., Gorodetskii, V.V., and Losev, V.V., Elektrokhimiya, 1985, vol. 21, p. 1427. 10. Gorodetskii, V.V., Evdokimov, S.V., and Kolotyrkin, Ya.M., Itogi Nauki Tekh., Ser: Elektrokhim., 1991, vol. 34, p. 84. 11. Evdokimov, S.V., Yanovskaya, M.I., Roginskaya, Yu.E., Lubnin, E.N., and Gorodetskii, V.V., Elektrokhimiya, 1987, vol. 23, p. 1509. 12. Evdokimov, S.V., Gorodetskii, V.V., Yanovskaya, M.I., and Roginskaya, Yu.E., Elektrokhimiya, 1987, vol. 23, p. 1516. 13. Losev, V.V., Elektrokhimiya, 1981, vol. 17, p. 733. 14. Losev, V.V. and Selina, L.E., Elektrokhimiya, 1989, vol. 25, p. 1155. 15. Gorodetskii, V.V., Elektrokhimiya, 2003, vol. 39, p. 722. 16. Gorodetskii, V.V., Neburchilov, V.A., and Alyab’eva, V.I., Elektrokhimiya, 2005, vol. 41, p. 964. 17. Losev, V.V., Molodov, A.I., and Gorodetskii, V.V., Elektrokhimiya, 1965, vol. 1, p. 572. 18. Kasparova, O.V. and Kolotyrkin, Ya.M., Itogi Nauki Tekh., Ser: Elektrokhim., 1981, vol. 8, p. 51. No. 9

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