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Critical potential and oxygen evolution of the chlorate anode
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

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 (Ecr) in order to keep the potential losses low and to achieve a long lifetime of the DSAs. At Ecr 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 Ecr 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 Ecr as well as for potentials above Ecr. This was based on the fact that the apparent reaction order as well as αa seem to be of the same values even if the anode potential exceeds Ecr. The reason for the higher slope of the polarisation curve above Ecr 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-, CrO4 2- and HCrO4 - 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.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , p. 33
Series
Trita-KET, ISSN 1104-3466 ; 228
Keywords [en]
chlorate, chloride oxidation, oxygen evolution, critical anode potential, chromate, DSA, mass transport, RDE
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-3957OAI: oai:DiVA.org:kth-3957DiVA, id: diva2:10181
Presentation
2006-05-30, Sal K2, KTH, Teknikringen 28, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20101122Available from: 2006-05-11 Created: 2006-05-11 Last updated: 2010-11-22Bibliographically approved
List of papers
1. Critical Anode Potential in the Chlorate Process
Open this publication in new window or tab >>Critical Anode Potential in the Chlorate Process
2006 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 153, no 1, p. D14-D20Article in journal (Refereed) Published
Abstract [en]

Anodic polarization curves on dimensionally stable anodes (DSAs) of RuO2/TiO2 in chlorate electrolyte bend to a higher Tafel slope at the critical potential (E-cr) of approximately 1.2 V vs Ag/AgCl. Operating the chlorate process above E-cr leads to increased oxygen evolution and higher potential losses. In this study the impact of different electrolyte parameters and electrolyte impurities on the risk of reaching/exceeding E-cr was investigated. A dependency of Cl- concentration on E-cr of about -90 mV/dec C-Cl(-) was found at pH 2. An addition of Na2Cr2O7 to chlorate electrolyte is necessary in order to keep a high current efficiency on the cathode but was found to increase the anode potential and thereby increase the risk of exceeding E-cr at galvanostatic operation. Additions of impurities as 30 g/L Na2SO4 or 100 ppm Si (added as SiO2) resulted in increased anode potentials, but adding 1.4 g/L KH2PO4 or 1 g/L HF did not have significant short-term impact on the potential. The anode potential as well as E-cr decreased with increased temperature. A high temperature is beneficial in terms of the decreased anode potential, which outweighs the negative effect of a decrease in Ecr.

Keywords
Anodes, Anodic polarization, Electrolytes, Impurities, Oxygen, Thermal effects, Anode potential, Chlorate process, Electrolyte impurities
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8148 (URN)10.1149/1.2135216 (DOI)000234142400050 ()2-s2.0-33645515270 (Scopus ID)
Note
QC 20100831Available from: 2008-03-27 Created: 2008-03-27 Last updated: 2017-12-14Bibliographically approved
2. Investigation of the oxygen evolving electrode in pH-neutral electrolytes: Modelling and experiments of the RDE-cell
Open this publication in new window or tab >>Investigation of the oxygen evolving electrode in pH-neutral electrolytes: Modelling and experiments of the RDE-cell
2007 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 52, no 13, p. 4513-4524Article in journal (Refereed) Published
Abstract [en]

A model has been developed to illustrate the complex interplay between the acidifying electrode reactions for oxygen evolution, mass transport and homogeneous reactions in pH-neutral electrolytes. Modelled polarisation curves of the oxygen evolution reaction were verified by polarisation curves experimentally measured in 5 M NaClO4 on a RDE of DSA material. The conditions in the simulations and in the experiments were similar to those in the chlorate process (high ionic strength, 70 degrees C, chromate-containing electrolyte, DSA electrode), in which the oxygen evolution reaction is one of the possible side reactions. The model predicted the concentration gradients of H+, OH-, CrO42- and HCrO4- during oxygen evolution on the RDE. 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 buffering reaction has been proposed. The most likely reaction for the chromate buffering in the investigated system is CrO42- reacting with water to HCrO4- and OH-. In the chlorate process, where chromate is a buffer and oxygen evolution is a side reaction, it is likely that chromate promotes oxygen evolution from OH-.

Keywords
oxygen evolution, pH buffer, rotating disk electrode, mathematical modelling, chromate
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-5722 (URN)10.1016/j.electacta.2006.12.048 (DOI)000245476100028 ()2-s2.0-33847359224 (Scopus ID)
Note
QC 20100831. Uppdaterad från submitted till published (20100831).Available from: 2006-05-11 Created: 2006-05-11 Last updated: 2017-12-14Bibliographically approved

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