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Mechanism of the Chlorine Evolution on a Ruthenium Oxide/Titanium Oxide Electrode and on a Ruthenium Electrode

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Mechanism of the Chlorine Evolution on a Ruthenium Oxide/Titanium Oxide Electrode and on a Ruthenium Electrode Mechanism of the chlorine evolution on a ruthenium oxide/titanium oxide electrode and on a ruthenium electrode Citation for published version (APA): Janssen, L. J. J., Starmans, L. M. C., Visser, J. G., & Barendrecht, E. (1977). Mechanism of the chlorine evolution on a ruthenium oxide/titanium oxide electrode and on a ruthenium electrode. Electrochimica Acta, 22(10), 1093-1100. https://doi.org/10.1016/0013-4686(77)80045-5 DOI: 10.1016/0013-4686(77)80045-5 Document status and date: Published: 01/01/1977 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 27. Sep. 2021 MECHANISM OF THE CHLORINE EVOLUTION ON A RUTHENIUM OXIDE/TITANIUM OXIDE ELECTRODE AND ON A RUTHENIUM ELECTRODE L. J. J. JAN=,* L. M. C. ST.&S&W.,* J. G. Vrssmt and E. BARENDRECHT* * Department of Electrochemistry, Eindhoven University of Technology and t Computing Centre, Eindhoven University of Technology, Eindhoven, The Netherlands (Receioed 28 September 1976, in final form 7 December 1976) Abstract-The mechanism of the chlorine evolution on titanium electrodes coated with a layer of ruthenium oxide and titanium oxide under different experimental conditions, and on a ruthenium electrode, both in acidic chloride solution, has been investigated Potentiodynatnic current density- potential curves were recorded as a function of the time of anodic pre-polarisation, the composition of the solution and the temperature. Moreover, potential decay curves were determined. Theoretical potential decay curves were deduced for both the Tafel reaction (2C&,-rCls) and the Heyrovsky reaction (Cl- + Cl,,+ Cl, + e-) as a rate determining step in the formation of molecular chlorine. They were compared with those found experimentally. The intluence of possible diffusion of atomic chlorine out of the electrode was also taken into consideration. It was found that for all the electrodes investigated, molecular chlorine is formed both at anodic polarisation and on open circuit according to the Volmer-Heyrovsky mechanism, where the Heyrovsky reaction is the rate-determining step. The transfer coefficient is 0.5 for the chlorine evolution at an “ideal” ruthenium oxide titanium oxide electrode and at a ruthenium electrode. NOMENCLATURE overpotential on open circuit; q. = E, - E, 11. at t, = 0 Tafel slope of the E/log J curve degree of coverage with atomic chlorine concentration of atomic chlorine present in 0 at t. = 0; this being equal to the degree an electrode at a distance x at time r. after of coverage at the potential of the electrode switching off the polar&ion current on current flow c(x, 0) at x 2 0 or 403, t,) at t, > 0 co 6 0 at E, D diffusion coefficient of atomic chlorine E electrode potential us see . on open E” electrode potential circuit 1. lNTRODUCTlON J% reversible electrode potential as see F Faraday constant A few years ago the titanium electrode coated with k,; kr, the slope of the linear part of the a layer of mixed oxides of titanium, ruthenium and/or (q,(O) - q&log t. curve for respectively the other noble metals was introduced into the chlorine- Tafel and Heyrovsky reactions alkali industry. J geometry cd during anodic polar&ion JO geometric exchange cd Several investigators[lkI] have already examined j real cd; j = J/r the bebaviour of this anode material in concentrated In real exchange cd acidic chloride solutions. Moreover, the mechanism j0.T; &,A j, tor respectively the Tafel and the Hey- of the chlorine evolution has also been stud- rovsky reactions ied[2, 5,7]. reaction constant of respectively the Tafel The experimental results are often contradictory. and the Heyrovsky reactions For instance, the following Tafel slopes were found: k+ i32mV for a coating of oxides of Ti, Ir and Ru, 6 108 mV[4] and 40 mV[7] for a coating of RuO, and P constant factor; p = ejc R gas constant 35 mV for a coating of RuO, and TiO,[6]. The differ- i- roughness factor ences in Tafel slopes cannot be explained only in T temperature in “C or K terms of chemical composition of the oxide layer. TR the firing temperature Possible important factors may be the conditions tA time of anodic polarisation used in the preparation and the electrochemical pre- CR the firing time treatment of the electrode. The influence of these fac- t. time after switching off the polarization cur- tors are presented in this paper for two different types rent of electrodes, namely for a titanium electrode coated rate of chlorine evolution for respectively the Tafel and the Heyrovsky reaction with an oxide mixture composed of 30mole% RuO, distance in the electrode measured from the and 7Omole% TiO, and for a pure ruthenium eleo boundary electrode surface/electrolyte trode. The mechanisms proposed in the literature are transfer coefficient mainly based on Ejlog J curves. In addition, we also a for the Heyrovsky reaction examined potential decay curves for elucidating the overpotential during anodic polar&ion mechanism of the chlorine evolution. 1093 1094 L. J. J. JANSSEN, L. M. C. STARMAN& J. G. VISER AND E. BARENDRECHT t.EXPERlMENTAL cedure was applied to minimise the change of the composition of the electrolyte during the long con- Preparation of the Ru0,fli02-electrode and the 2.1 tinued electrolysis. Ru-electrode The RuO,/TiO, electrodes were prepared by coat- ing titanium sheets (40 x 5 x 0.5 mm, purity of Ti 3. RESULTS 9849%) with a layer of ruthenium oxide and titanium 3.1 Roughness of the electrodes oxide. The RuO, content of the coating was 30% The capacitance determined by the potential-pulse The titanium sheets were cleaned with acetone and method[8] was used as a measure for the roughness subsequently etched in concentrated hydrochloric of the RuOfliOL electrode. acid at 80” for IO min. It appeared that in the investigated range from 800 The coating was achieved by painting the titanium to IO00 mV, both the direction and the amplitude of sheet with an aqueous solution consisting of 1.4ml the potential-pulse had no influence upon the capaci- TiCl,, 3 ml H20, 0.5 ml 30% H,O, and 120mg tance. Assuming that the capacitance measured is RuCl, .3 Hz0 and then by drying in air at about SO”. equal to the double layer capacitance (viz 17.4 pF/cm2 Thereafter the coated sheet was fired at 500” for 1 h, determined at a mercurcy electrode in a 4M except when otherwise stated. In applying multilayer NaCL + 1 M HCL solution), the roughness factor of coatings the procedure was repeated. Unless other- the different Ru02fli0, electrodes varied between 20 wise stated RuO,/IIO, electrodes with two coating and 40. No systematic increase of the roughness of layers were used. the electrodes was found with increasing numbers of For comparison, a pure ruthenium rod was used; coating layers. a disc having a geometric area of l.OOcm’ was the _- exposed electrode surface; the cylinder part of the rod 3.2 Influence of the sweeprate and of the scanning was completely isolated with a Perspex cylinder. potential range on the E/J curves To investigate the effect of the sweeprate on the 2.2 Experimental techniques and apparatus morphology of the E/J curve, this curve was Two different electrolytic cells were used. The measured at a sweeprate varying between 0.5 and measuring cell, a normal H-shaped diaphragm cell 50 mV/s. A typical E/J curve for a Ru03fliOz elec- (volume of both compartments was lOOmI), was trode is represented in Fig. 1. For these electrodes applied for the determination of the potential-current hysteresis occurs; however, at sweeprates lower than curves, the potential-time curves and of the capaci- 2 mV/s the hysteresis disappeared practically. The E/J tance of the electrodes. This cell was thermostatted curves were determined in the potential range from (unless otherwise stated at 2Y). The other cell, a ves- 1050 to 1500mV. This potential range was chosen sel divided into an anodic compartment of 5000 ml to prevent a change of the nature of the &&rode and a cathodic compartment of 2COOml was used for surface during the sweeping of the potential.
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