Hypochlorite Oxidation on RuO2-Based Electrodes: a Combined Electrochemical and In Situ Mass Spectroscopic Study
- 227 Downloads
The hypochlorite oxidation on RuO2 doped with Co, Mg, Ni and Zn was studied by means of voltammetry combined with mass spectroscopy. The hypochlorite oxidation takes place in two distinct steps separated with at least 200 mV. While the first step can be assigned to the direct hypochlorite oxidation, the second one seems to be connected with the oxidation of strongly adsorbed in situ formed hypochlorite. In the oxidation process of hypochlorite, oxygen is the main product but also hydrogen peroxide can be detected. The number of electrons used to produce one molecule of oxygen (z) is less than expected from an electrochemical point of view. This fact, together with an accumulation of chloride ions in solution, indicates that the reaction mechanism of the hypochlorite oxidation involves the formation of radicals that can be regenerated by chemical redox reactions.
KeywordsHypochlorite oxidation Chlorate process Ruthenium dioxide DEMS
It ought to be noted that oxygen production unifies all above summarized anodic parasitic reactions. A suppression of the parasitic oxygen formation is therefore of paramount importance to improve efficiency and safety of the process [7, 13, 18]. The active strategies mitigating the parasitic oxygen production are, however, hindered by limited knowledge of the true mechanism of the anodic hypochlorite oxidation and of its sensitivity to selective control via electrode material selection/optimization.
The oxygen formation accompanying hypochlorite oxidation on polycrystalline Pt was reported recently . A combination of linear sweep voltammetry with on-line mass spectroscopic detection of the produced oxygen revealed unusually high oxygen formation where the apparent number of electrons needed to produce an oxygen molecule dropped significantly below 4 encountered in conventional water oxidation. These results were interpreted in terms of a radical chain mechanism, where the initial hypochlorite oxidation yields the hypochlorite radical , which enters in radical-assisted water-splitting, yielding oxygen, hydrogen peroxide and protons . The results obtained on Pt, despite their fundamental importance, are rather departed from the industrial conditions encountered in chlorate production which predominantly uses DSA-based anodes.
Keeping in mind that the industrial dimensionally stable anodes (DSA) are based on oxides of Ti and Ru, where Ru oxides are believed to be responsible for DSA’s activity, one may obtain a better understanding of the hypochlorite oxidation behaviour under conditions relevant to the chlorate process by a systematic investigation of the hypochlorite oxidation on well-defined oxide electrodes based on Ru oxides and, therefore, related to DSA. The major advantage of such model system lies in their well-defined nature [20, 21] as well as in the known selectivity towards chlorine and oxygen evolution reactions [22, 23]. This paper, thus, extends the previous mechanistic studies of hypochlorite oxidation on polycrystalline Pt, by employing electrode materials based on RuO2 doped with different cations such as Ni, Co, Zn and Mg in a systematic study combining voltammetry with differential electrochemical mass spectroscopy (DEMS) detection of the reaction products.
The hypochlorite stock solution was prepared by letting chlorine gas into a 5 M NaOH (Scharlau, reagent grade, ACS ISO, Reag. Ph Eur). The hypochlorite concentration in the stock solution was 1.6 M, and the solution was cooled and stored cold in dark. Given the preparation technique, the hypochlorite stock solution contained an equimolar amount of NaCl. The oxidation of hypochlorite was studied in 0.1 M solution of NaClO4 (Aldrich, p.a) containing variable amounts of hypochlorite. The pH of the hypochlorite solutions was adjusted to 9 by an addition of 1 M solution of NaOH (Adrich, p.a.) in all experiments. The pH of the solution was checked using OK-104 conductometer (Radelkis, Hungary). The hypochlorite oxidation was studied by linear sweep voltammetry on RuO2-based nanocrystalline electrodes. The linear sweep voltammetry experiments were carried out in a three-electrode arrangement in a home-made single compartment Kel-F cell with Pt and Ag/AgCl auxiliary and reference electrode, respectively. All experiments were carried out at polarization rate of 5 mV/s in the potential range between 0.3 and 1.3 V vs. Ag/AgCl. The potential control was achieved using a PAR 263A potentiostat. The measured potentials were recalculated and are quoted in the reversible hydrogen electrode (RHE) scale to enable easy comparison of all samples.
The RuO2-based catalysts were supported by a Ti mesh (GoodFellow, electrode area of 1 cm2, open area 20%). The active catalysts were deposited on the Ti mesh by a procedure described in . A water-based suspension of nanocrystalline RuO2 catalyst (30 gL−1) was deposited by adding 25 μL aliquots to the Ti support and dried at 100 °C. The deposition procedure was repeated until the weight of the catalyst ranged between 1 and 2 mg. The total physical area of the catalyst corresponded to ca. 20 cm2. All electrodes were calcined at 400 °C in air for mechanical stability before electrochemical experiments. The electrochemical characterization of the hypochlorite oxidation on RuO2 catalysts was complemented by in situ spectroscopic detection of the volatile reaction products—namely of oxygen and chlorine. A differential electrochemical mass spectrometry (DEMS) apparatus consisting of a Prisma quadrupole mass spectrometer (QMS200, Balzers) connected with turbomolecular drag pump station (TSU071, Balzers) was used in these experiments.
The content of chlorides before and after electrochemical experiment was determined by argentometric titration with silver nitrate using potassium chromate as an indicator.
The Role of the Electrode Material
To obtain additional information regarding the mechanism of hypochlorite oxidation, the nature of possible reaction products needs to be assessed by an independent spectroscopic technique. Given the fact that the hypochlorite oxidation is known to produce large amounts of gaseous products, one may find the differential electrochemical mass spectroscopy (DEMS) as the most convenient tool for the reaction product detection.
Quantification of the Oxygen Formation
As shown in Fig. 9, gradual increase of z to values higher than 4 is obtained for materials showing high selectivity towards chlorine evolution (Ni- and Co-doped RuO2), which is consistent with blocking of the electrocatalytic hypochlorite oxidation or water oxidation due to a preference for chloride oxidation/adsorption. For the materials with known preference for oxygen evolution, the increase in initial hypochlorite concentration leads to formation of a plateau with z of ca. 4 in the potential interval 1.7–2.0 V (vs. RHE). In the case of the non-doped RuO2, the observed values of z remain significantly lower than 4 in the potential interval 1.7–2.0 V (vs. RHE), regardless of the initial hypochlorite concentration. The observed experimental trends confirm the surface sensitivity of the hypochlorite oxidation process at potentials positive to 1.7 V (vs. RHE). It needs to be stressed, however, that the observed behaviour results from complex interplay between the OER, hypochlorite oxidation and CER with apparent interdependence of hypochlorite and chloride adsorption/oxidation.
The Hypochlorite Oxidation Mechanism
The disagreement of the Foerster reaction stoichiometry with the quantitative measure of the oxygen production along with hydrogen peroxide production found for all RuO2-based anode materials forces us to reformulate the mechanism of the hypochlorite oxidation. Low apparent number of electrons needed for production of one molecule of oxygen, which indicates a mechanism with significant involvement of radicals, needs to be reconciled with (at least partial) electrochemical nature of the overall hypochlorite oxidation process as well as with the fact that it proceeds in two distinctive steps.
In short, the hypochlorite oxidation may proceed either as a sequence A, which encompasses reactions (11) through (15), or as a sequence B encompassing reactions steps (11), (12), (13), (14) and (16). It needs to be stressed that reaction (15) reforms the hypochlorite radical which can re-enter into step (12) as a reactant. The reaction sequence A then attains in part a radical chain nature which decreases the apparent number of electrons needed to evolve one molecule of oxygen significantly below 4 as observed in the experiments. The reaction sequence B changes the overall course of the process by regeneration of the hypochlorite anion instead of the hypochlorite radical (compare reactions (15) and (16)). These two processes may be distinguished by following the chloride concentration change during the hypochlorite oxidation process (see below).
The above-outlined mechanism can be applied to processes underlying both anodic peaks observed in voltammograms. Given that the hypochlorite oxidation-related peaks are separated with at least 200 mV, it is reasonable to assume that the initial radical formation proceeds in a different manner in each of them. While the onset of the first anodic peak is mainly surface-insensitive, i.e. the process is exclusively controlled by the electrode potential indicating an outer sphere nature of the electron transfer (the radical is formed from solution-based hypochlorite), the second process, however, is weakly dependent on the surface composition. It indicates that the hypochlorite anion is adsorbed on the surface to a certain extent, stabilising the anion and making it more difficult to oxidize compared with the hypochlorite anion in solution.
The role of chloride seems to be reflected also in the concentration dependence of z depicted in Fig. 9 The data presented in Fig. 9 show opposite trends attributable to hypochlorite and chloride present in the system. The increase of hypochlorite concentration ought to, as a rule, decrease the z due to the promotion of the radical chain reaction pathway and due to the buffering effect of the hypochlorite which keeps the pH more alkaline and consequently pushes the bulk chlorine evolution to higher potentials (vs. RHE). Such a behaviour is generally observable in the z vs. E curves in the potential interval 1.5–1.8 V (vs. RHE).1 The hypochlorite concentration increase, on the other hand, raises also the chloride concentration which increases the extent of the chlorine evolution leading in turn to an increase of the z. The increase of z is more pronounced for Ni- and Zn-doped RuO2 electrodes while in the case of the non-doped or Mg-doped electrode materials, the concentration dependence of z remains rather weak.
The surface sensitivity of the observed behaviour, therefore, suggests active role of the surface chemistry and consequently the role of the adsorption phenomena on the electrode’s activity in hypochlorite oxidation. It needs to be noted that this surface selectivity can be of a paramount importance in the actual chlorate electrolysis. The elucidation of the actual role of chlorides and hypochlorite anions in the anodic process related to the hypochlorite oxidation would, however, require more systematic approach with respect to the electrolyte composition.
Hypochlorite oxidation on RuO2-based electrodes occurs in two steps in the potential window above 1.2 V (vs. RHE). The initial hypochlorite oxidation seems to be independent of the employed electrode material, while the second oxidation process appearing at higher potentials is affected by the nature of the electrode. The latter process is attributed to an oxidation of surface-confined hypochlorite-like species. Regardless of the electrode material, the dominating product of the hypochlorite oxidation is oxygen. The oxygen production shows a maximum at potentials corresponding to the peak position of the second anodic process. In addition, the hypochlorite oxidation leads to the formation of hydrogen peroxide and eventually of chloride ions. The efficiency of the oxygen production in the electrochemical hypochlorite oxidation is high as shown by the apparent number of electrons needed to evolve one molecule of oxygen z. The low z values suggest a radical-based mechanism of the hypochlorite oxidation process. The radical nature of the hypochlorite oxidation is further confirmed by an accumulation of chlorides in the system observed during the hypochlorite oxidation.
The trends in the z values observed at potentials between 1.4 and 1.5 V (vs. RHE) should be taken with care since they are based on evaluation reading comparable with the resolution of the DEMS technique.
Financial support from Akzo Nobel Pulp and Performance Chemicals AB is gratefully acknowledged.
- 1.N. Ibl, H. Voght in Comprehensive Treatise of Electrochemistry, Electrochemical Processes, ed.By J. O. Bockris (Plenum Press, 1981), p. 167Google Scholar
- 3.J.E. Colman, AIChE Symp. Ser. 77, 244 (1981)Google Scholar
- 13.J.E. Colman, B.V. Tilak, Encyclopedia of chemical processing and design (M. Dekker, New York, 1995), p. 126Google Scholar
- 16.F. Foerster, Trans. Amer. Electrochem. Soc. 46, 23 (1924)Google Scholar
- 17.S. Kotowski, B. Busse, in Modern chlor-alkali technology, ed. By K. Wall (Ellis Horwood Ltd, Chichester, 1986), p. 310Google Scholar
- 22.V. Petrykin, K. Macounová, J. Franc, O. Shchlyakhtin, M. Klementová, S. Mukerjee, P. Krtil, Zn-Doped RuO2electrocatalyts for selective oxygen evolution: relationship between local structure and electrocatalytic behavior in chloride containing media. Chem. Mater. 23(2), 200–207 (2011)CrossRefGoogle Scholar
- 23.V. Petrykin, K. Macounová, O.A. Shlyakhtin, P. Krtil, Tailoring the selectivity for electrocatalytic oxygen evolution on ruthenium oxides by zinc substitution. Angew. Chem. Int. Ed. 49(28), 4813–4815 (2010)Google Scholar
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.