Critical Potential and Oxygen Evolution of the Chlorate Anode. Linda Nylén - PDF

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Critical Potential and Oxygen Evolution of the Chlorate Anode Linda Nylén Licentiate thesis KTH Chemical Science and Engineering Department of Chemical Engineering and Technology Applied Electrochemistry

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Critical Potential and Oxygen Evolution of the Chlorate Anode Linda Nylén Licentiate thesis KTH Chemical Science and Engineering Department of Chemical Engineering and Technology Applied Electrochemistry SE Stockholm, Sweden Linda Nylén TRITA-KET R228 ISSN ISRN KTH/KET/R SE Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknologie licentiatexamen tisdagen den 30:e maj 2006 kl i K2, Kungliga Tekniska Högskolan, Teknikringen 28, Stockholm ABSTRACT In the chlorate process, natural convection arises thanks to the hydrogen evolving cathode. This increases the mass transport of the different species in the chlorate electrolyte. There is a strong connection between mass transport and the kinetics of the electrode reactions. A better knowledge about these phenomena and their interactions is desirable in order to understand e.g. the reasons for deactivation of anode coatings and what process conditions give the longest lifetime and the highest efficiency. One of the aims of his work was to understand how the chlorate process has to be run to avoid exceeding the critical anode potential (E cr ) in order to keep the potential losses low and to achieve a long lifetime of the DSAs. At E cr anodic polarisation curves in chlorate electrolyte bend to higher Tafel slopes, causing increasing potential losses and accelerated ageing of the anode. Therefore the impact on the anode potential and on E cr of different electrolyte parameters and electrolyte impurities was investigated. Additionally, the work aimed to investigate the impact of an addition of chromate on oxygen evolution and concentration profiles under conditions reminiscent of those in the chlorate process (high ionic strength, 70 C, ruthenium based DSA, neutral ph), but without chloride in order to avoid hypochlorite formation. For this purpose a model, taking into account mass transport as well as potential- and concentration-dependent electrode reactions and homogeneous reactions was developed. Water oxidation is one of the side reactions considered to decrease the current efficiency in chlorate production. The results from the study increase the understanding of how a buffer/weak base affects a ph dependent electrode reaction in a ph neutral electrolyte in general. This could also throw light on the link between electrode reactions and homogeneous reactions in the chlorate process. It was found that the mechanism for chloride oxidation is likely to be the same for potentials below E cr as well as for potentials above E cr. This was based on the fact that the apparent reaction order as well as α a seem to be of the same values even if the anode potential exceeds E cr. The reason for the higher slope of the polarisation curve above E cr could then be a potential dependent deactivation of the active sites. Deactivation of active ruthenium sites could occur if ruthenium in a higher oxidation state were inactive for chloride oxidation. Concentration gradients of H +, OH -, CrO 2-4 and HCrO - 4 during oxygen evolution on a rotating disk electrode (RDE) were predicted by simulations. The ph dependent currents at varying potentials calculated by the model were verified in experiments. It was found that an important part of the chromate buffering effect at high current densities occurs in a thin (in the order of nanometers) reaction layer at the anode. From comparisons between the model and experiments a reaction for the chromate buffering has been proposed. Under conditions with bulk ph and chromate concentration similar to those in the chlorate process, the simulations show that the current density for oxygen evolution from OH - would be approximately 0.1 ka m -2, which corresponds to about 3% of the total current in chlorate production. Keywords: Chlorate, chloride oxidation, oxygen evolution, critical anode potential, chromate, DSA, mass transport, RDE SAMMANFATTNING I kloratprocessen ger den vätgasutvecklande katoden upphov till naturlig konvektion, vilket ökar materietransporten av de olika specierna i elektrolyten. Det är ett tydligt samband mellan materietransporten i cellen och kinetiken för elektrodreaktionerna. Ökad kunskap om dessa fenomen och deras samverkan kan öka förståelsen av t.ex. varför anodbeläggningar deaktiveras och vilka processbetingelser som ger längs livstid och högst strömutbyte. Ett av målen med detta arbete var att förstå hur kloratprocessen bör köras för att den kritiska anodpotentialen (E cr ) inte ska överskridas och därmed hålla spänningsförlusterna nere och öka livstiden för DSA-anoden. Vid E cr böjer anodiska polarisationskurvor av till högre Tafellutningar, vilket ger ökande spänningsförluster och ökat slitage av DSA-beläggningen. Därför har påverkan på anodpotentialen och E cr av olika elektrolytparametrar och föroreningar undersökts. Vidare var målet att undersöka inverkan av kromat på syrgasutveckling och på koncentrationsprofiler under förhållanden liknande kloratprocessens (hög jonstyrka, 70 C, ruthenium-baserad DSA, neutralt ph), men utan tillsatts av klorid för att undvika hypokloritbildning. För detta har en modell utvecklats, som tar hänsyn till materietransport såväl som potential- och koncentrationsberoende elektrodreaktioner och homogena reaktioner. Vattenoxidation är en av de bireaktioner som anses minska strömutbytet vid kloratframställning. Resultaten från detta arbete ger en ökad förståelse av hur en buffert/svag bas påverkar en ph-beroende elektrodreaktion i en neutral elektrolyt. Detta kan även hjälpa till att förklara sambandet mellan elektrodreaktioner och homogena reaktioner i kloratprocessen. Detta arbete har visat att mekanismen för kloridoxidation troligen är samma för potentialer över likväl som under E cr. Detta baserades på att både den skenbara reaktionsordningen och α a inte verkar ändras då anodpotentialen överskrider E cr. Polarisationskurvans högre lutning vid potentialer högre än E cr skulle kunna ha sin orsak i en potentialberoende deaktivering av aktiva säten. Deaktivering av aktiva säten skulle kunna uppstå om ruteniums högre oxidationstillstånd vore inaktivt för kloridoxidation. Koncentrationsgradienter av H +, OH , CrO 4 och HCrO 4 under syrgasutveckling på en roterande skivelektrod (RDE) har uppskattats genom simuleringar. De ph-beroende strömmarna som räknats ut av modellen verifierades genom experiment. Det visade sig att en viktig del av kromatbuffringen vid höga strömtätheter sker i ett mycket tunt reaktionsskikt (nanometertjockt) vid anoden. Genom jämförelser mellan modell och experiment har en reaktion för kromatbuffringen föreslagits. Med ett ph och en kromatkoncentration som liknar de i kloratprocessen visade simuleringar att strömtätheten för syrgasutveckling ur OH - skulle vara ca 0,1 ka m -2, vilket skulle motsvara ungefär 3% av den totala strömmen vid kloratproduktion. Nyckelord: Klorat, kloridoxidation, syrgasutveckling, kritisk anodpotential, kromat, DSA, materietransport, RDE LIST OF PAPERS The thesis is a summary of the following papers: I. Critical Anode Potential in the Chlorate Process L. Nylén and A. Cornell J. Electrochem. Soc., 153, D14 (2006) II. Investigation of the Oxygen Evolving Electrode in ph Neutral Electrolytes. Modelling and Experiments of the RDE-cell. L. Nylén, M. Behm, A. Cornell and G. Lindbergh Manuscript, to be submitted to Electrochimica Acta CONTENT 1. PROJECT BACKGROUND AND GENERAL GOALS 1 2. INTRODUCTION The Chlorate Process Aim of the Papers 6 3. METHODS Experimental Model 8 4. RESULTS AND DISCUSSION The Critical Potential A Model for Oxygen Evolution in ph Neutral Electrolyte CONCLUSIONS The Critical Potential Mass Transport Modelling for the OER under Chlorate Condition ACKNOWLEDGEMENTS REFERENCES 32 1. PROJECT BACKGROUND AND GENERAL GOALS In the chlorate process, natural convection arises thanks to the hydrogen evolving cathode. The gas bubbles rise through the cell, and cause a gas-lift effect. There is a strong connection between mass transport and the kinetics of the electrode reactions. A better knowledge about these phenomena and their interactions is desirable in order to understand e.g. the reasons for deactivation of anode coatings and what process conditions give the longest lifetime and the highest efficiency. In earlier projects within FaxénLaboratoriet [1,2], efforts made to simulate the whole industrial chlorate cell have shown that the complexity of the system requires better understanding of the kinetics of the electrode reactions and the homogeneous reactions in the electrolyte bulk. This project, also partly carried out within FaxénLaboratoriet, has investigated the chloride oxidation (the desired electrode reaction in chlorate production) on the anode and the general connection between electrode reactions, mass transfer and homogeneous reactions. The project has been divided into two studies: An experimental study, which investigated the impact of different electrolyte parameters on the potential of the chlorate anode. The experiments aimed at understanding the critical potential (a potential where the polarisation curve for chloride oxidation bends to a higher slope), and how the process can be operated to avoid reaching/exceeding the critical potential. A study, in which mass transport simulations in an electrolyte where homogeneous reactions are coupled to electrode reactions has been carried out. Oxygen evolution (a side reaction in the chlorate process) was chosen as the electrode reaction, and the homogeneous reactions were chromate buffering and water dissociation. This system is, from a modelling prospective, less complex than the chlorate system. This facilitated convergence of the calculations as well as interpretation of the modelling results. The simulations aimed at understanding the coupling between homogeneous reactions, electrode reactions and mass transport by studying concentration profiles of the different species involved. The specific aims will be discussed below as a part of the introduction. 1 2. INTRODUCTION 2.1 The Chlorate Process Chlorate is today mostly produced in the form of sodium chlorate (NaClO 3 ). It is the raw material in production of chlorine dioxide (ClO 2 ), commonly used in pulp beaching [3]. In Sweden, Eka Chemicals is the only chlorate manufacturer, and has plants in Stockvik/Sundsvall and Alby. The development of electrochemical chlorate production in Sweden was induced by the need of potassium chlorate (KClO 3 ) in the 1880ties. The manufacturing of the Swedish invention the safety match, required large amounts of KClO 3. Nowadays, a very small part of the chlorate produced serves as raw material for matches, and the pulp industry is consuming most of the chlorate [3]. The chlorate process consumes significant amounts of electrical energy. In fact, up to 70% of the production costs are for electrical energy [4]. Due to high energy consumption even a small efficiency improvement can save large amounts of energy Chemistry Chlorate is produced in undivided cells, where the overall reaction (reaction 1) is sodium chloride and water forming sodium chlorate and hydrogen. The hydrogen gas bubbles formed on the cathode (reaction 2) give rise to a gas-lift effect, which enhances mass transport of reactants to the electrode surface. Chlorine formed on the anode (reaction 3) is dissolved in the electrolyte and reacts to form chlorate through a number of reaction steps (reactions 4-6). Overall reaction: NaCl s) + 3H O( l) NaClO ( s) + 3H ( ) (1) ( g Cathode reaction: + 2H + 2e H 2 (2) Anode reaction: 2Cl Cl2 + 2e (3) Chlorate is formed in a series of chemical reactions: Cl H O ClOH + Cl + H (4) + ClOH ClO + H (5) + 2ClOH + ClO ClO3 + 2H + 2Cl (6) At industrial conditions (ph 6-7) the side reaction oxygen evolution would be more pronounced if the electrode surface had the same ph as the bulk electrolyte. However, oxygen evolved from water containing electrolytes as well as chlorine reacting through reaction 4-5 produce protons causing an acidification at the electrode surface, which suppresses oxygen evolution. This acidification increases as the current density is increased. However, it is important to have a 2 higher ph (ph 6-7) in the bulk to achieve a high reaction rate of the chlorate formation (reaction 6) and a low amount of chlorine in the outlet gases Electrolyte Parameters To obtain an as high and energy efficient production rate as possible the process parameters such as electrolyte temperature, ph and chloride and chlorate concentrations have to be optimised. The text in this paragraph (2.1.2) is based on Ref. 5. The temperature of the chlorate process is typically C. An increase as well as a decrease in temperature would lead to a lower current efficiency because the rate of the side reaction oxygen evolution would increase. Increasing temperature results in a higher reaction rate of the chlorate forming reaction (reaction 6), but also increases the reaction rate of the oxygen forming side reactions (see below). A decrease in temperature would decrease the rate of chlorate formation (reaction 6) and the concentrations of HOCl and ClO - would build up in the electrolyte, and form oxygen (see reactions 7-11). A usual ph in the chlorate cell is approximately 6-7. As mentioned above, the chlorate forming reaction (reaction 6) has its highest reaction rate in this ph range, while the anode surface has a lower ph thanks to the acidifying reactions 4 and 5. The acidic anode surface suppresses the side reaction oxygen evolution, favouring chloride oxidation. A decrease in the bulk ph of the chlorate electrolyte would suppress the oxygen evolution even more, but would as well decrease the rate of reaction 6. Furthermore, the chlorine gas evolved in the electrode reaction would not dissolve in the electrolyte if ph were too low, but would escape with the cell gas. A ph increase would of course increase the oxygen evolution. To avoid mass transport limitations of chloride and to achieve a low reversible potential for chloride oxidation, a saturated chloride brine would be ideal. However, such a high chloride concentration would complicate the crystallisation of the chlorate salt, which is a later step in the production process. Therefore most of the chlorate plants have a sodium chloride concentration of g/l Side Reactions The current efficiency in the chlorate process is commonly 93-95% [6]. The deviation from 100% for the current efficiency is caused by the occurrence of side reactions in the bulk and on the electrodes as well as Cl 2 escaping with the cell gas. The major by-product is oxygen. Oxygen in the cell gas does not only affect the energy consumption, but is also considered as a safety risk. Too much oxygen could cause an explosion if reacting with hydrogen from the chlorate cathode. Oxygen Evolution Some suggested anode reactions giving oxygen are [7,8]: OCl + 6H 2 O 4ClO3 + 12H + 8Cl + 3O + 12e (7) ClOH + H 2e + 2 O 3H + Cl + O2 + (8) + 2H 2 O O2 + 4H + 4e (9) 4OH O2 + 2H 2O + 4e (10) 3 Instead of hypochlorite forming chlorate through reaction 6, as desired, it can decompose to oxygen and chloride [7,8]. 2OCl O2 + 2Cl (11) Kotowski et al. and Hardee et al. [7,8] claimed that reactions 7, 8 and 11 are responsible for the major part of the oxygen evolved in a chlorate cell. However, Tilak et al. [5] argued that the primary source of oxygen is anodic discharge from water molecules, and that hypochlorite contributes to additional oxygen. Additionally, they pointed out the difficulties in separating different contributions generating oxygen. Oxygen Evolution from Water and OH - Oxidation As mentioned above, the oxygen by-product in chlorate electrolysis could come from different sources. In Paper II, oxygen evolution from reactions 9 and 10 is considered. To be to able to separate these reactions from other oxygen evolving ones, chloride free electrolytes were used. Of course, the oxygen evolution from water and OH - in chlorate electrolyte cannot be assumed to be identical to that in chloride free solutions. The intention is however that the knowledge gained in this work would facilitate the general understanding of the interaction between mass transfer, electrode reactions and homogeneous reactions. The focus has been on how the oxygen evolving electrode reactions, mass transfer and the homogeneous reactions, such as chromate buffering and water dissociation control each other. This has been done by studying the modelled concentration profiles for the different species of the electrolyte. Most work on the oxygen evolving reaction (OER) in chloride free solutions is reported for either strongly acidic or strongly alkaline electrolytes [9,10]. The electrode reaction differs between the two cases, the reactant being water or hydroxide respectively (reactions 9 and 10). The electrode reactions are not affected by mass transport limitations except for at very high current densities in alkaline electrolytes. Sato et al. [11] studied the OER on nickel electrodes in electrolytes of varying alkalinity and found limiting current densities that depended on OH - concentration. An overlimiting current was attributed to OER from water (reaction 9). This mechanism is thus less kinetically favoured at sufficient OH - concentrations but will be predominate when the OH - concentration is too low. Both oxygen evolving reactions decrease ph in a region close to the anode, reaction 9 by direct production of H +, while the consumption of OH - in reaction 10 affects the water dissociation equilibrium to produce H + (reaction 12). + OH + H H 2O (12) If the kinetics of this reaction were sufficiently high, a limiting current density would not appear, since the water dissociation would supply OH - to reaction 10 at a sufficiently high rate even in the presence of mass transport limitations in the bulk of the electrolyte. When a buffer/weak base is added to the electrolyte, the acid-base reaction can also serve as source of OH - next to the surface. Chromate, a component in chlorate electrolyte, forms such a buffer system The Chlorate Cell Most chlorate producers have developed their own cell technology [5]. According to Tilak et al. [5] in 1999, there were 14 chlorate cell technologies to be found in the world. The chlorate 4 cathodes are commonly made of steel [12], but can also be of titanium [13]. The anodes are made of titanium and with a noble metal coating [12]. The coating of the anodes can be based on platinum, iridium and ruthenium. The high price of the platinum-iridium based anodes have made the ruthenium based dimensionally stable anodes (DSAs) the most widely used today. In this work DSAs of TiO 2 /RuO 2 have been used. The design of the chlorate cells can either be bipolar or monopolar. The monopolar cells have the advantage of being simple to construct and their price is relatively low. The disadvantage is that they attain a higher cell voltage than the bipolar cells [13]. A higher production rate can be achieved with bipolar cells [13] The Critical Potential It has been shown [14,15,16] that anodic polarisation curves on dimensionally stable anodes (DSAs) of RuO 2 /TiO 2 bend to a higher Tafel slope at approximately 1.2 V vs Ag/AgCl. The change in Tafel slope is, according to Cornell et al. [15] neither related to mass-transport limitations nor to an ohmic drop and would therefore indicate a change in electrode kinetics. The potential and the current density where the polarisation curve bends have been referred to as the critical potential (E cr ) and the critical current density, respectively [14]. The current densities used in industrial chlorate production are usually around 3 ka/m 2, which would result in an anode potential close to E cr. Operating the chlorate process at the current densities, for which
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