Towards sustainable chlorate production: The effect of permanganate addition on current efficiency

The oxygen evolution reaction (OER) and chlorine evolution reaction (CER) are electrochemical processes with high relevance to water splitting for (solar) energy conversion and industrial production of commodity chemicals, respectively. Carrying out the two reactions separately is challenging, since the catalytic intermediates are linked by scaling relations. Optimizing the efficiency of OER over CER in acidic media has proven especially difficult. In this regard, we have investigated the OER versus CER selectivity of manganese oxide (MnOx), a known OER catalyst. Thin films (∼5–20 nm) of MnOx were electrodeposited on glassy carbon-supported hydrous iridium oxide (IrOx/GC) in aqueous chloride solutions of pH ∼0.9. Using rotating ring–disk electrode voltammetry and online electrochemical mass spectrometry, it was found that deposition of MnOx onto IrOx decreases the CER selectivity of the system in the presence of 30 mM Cl– from 86% to less than 7%, making it a highly OER-selective catalyst. Detailed studies of the CER mechanism and ex-situ structure studies using SEM, TEM, and XPS suggest that the MnOx film is in fact not a catalytically active phase, but functions as a permeable overlayer that disfavors the transport of chloride ions.

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“…Interestingly, MnO x has been recently suggested as a promising alternative to the use of Cr(VI) for the selective hydrogen evolution. 101 …”

Section: Resultsmentioning

confidence: 99%

MnO_x/IrO_x as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

et al. 2018

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“…[3][4][5] As all chromium(VI) species are classified as carcinogenic, mutagenic and reprotoxic (CMR), it has been included in Annex XIV of REACH, and is aimed to be phased out from industrial use. An authorization must now be granted by the European Commission for continued industrial use in Europe and a search for alternatives 6 to chromium(VI) in the chlorate process is therefore of high concern. In this search, it is important to gain a better understanding of the functions of chromium(VI) in the process.…”

Section: Introductionmentioning

confidence: 99%

Suppressed oxygen evolution during chlorate formation from hypochlorite in the presence of chromium(VI)

Endrődi

Sandin

Wildlock

et al. 2019

J of Chemical Tech & Biotech

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Background Chromium(VI) is a crucial electrolyte component in industrial chlorate production. Due to its toxicity, it urgently needs to be abandoned and its functions fulfilled by new solutions. In the industrial production of sodium chlorate, homogeneous decomposition of the hypochlorite intermediate to chlorate is a key step. As a competing loss reaction, hypochlorite can decompose to oxygen. How chromium(VI) affects these reactions is not well understood. Results This work shows, for the first time, that chromium(VI) selectively accelerates the chlorate formation from hypochlorite both in dilute and concentrated, industrially relevant solutions. The effect of the ionic strength and the specific contribution of different electrolyte components were systematically studied. By simultaneously measuring the concentration decay of hypochlorite (UV–vis spectroscopy) and the oxygen formation (mass spectrometry), both the rate and the selectivity of the reactions were evaluated. Conclusion In the presence of chromium(VI) the hypochlorite decomposition is described by the sum of an uncatalyzed and a parallel catalyzed reaction, where oxygen only forms in the uncatalyzed reaction. When removing chromium(VI), the homogeneous oxygen formation increases, causing economic and safety concerns. The need for a catalyst selective for chlorate formation is emphasized. © 2018 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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“…Numerous alternative additives to dichromate have been proposed, including rare-earth metal (REM) salts [45]- [47], permanganate [55], [56], vanadate [57], and molybdate [49], [58]- [61]. Alloys of Fe and Cr were also investigated [43].…”

Section: Alternatives To Chromatementioning

confidence: 99%

“…Of the alternatives to chromate proposed [40], [45], [46], [55], [74] MnOx containing films seem very promising owing to the fact that they can be deposited in situ from non-toxic permanganate solutions, and have shown very similar selectivity towards H2 production [55], [56]. MnOx is also used as a selectivity-inducing film for the oxygen evolution reaction [75].…”

Section: Permanganatementioning

confidence: 99%

“…Unfortunately, contrary to chromate, film growth from a permanganate solution is not limited to the nanometer range. Unlimited film growth would cause various problems in an industrial electrochemical reactor, ranging from larger resistances and higher cell potentials to short-circuiting and cell clogging [55]. However, a recent publication shows that thermally deposited MnOx films also induce excellent selectivity towards the HER in solutions containing hypochlorite [56].…”

Section: Permanganatementioning

confidence: 99%

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Understanding selective thin films in the chlorate process

Smulders¹

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This chapter provides context and background information for the work in this thesis. First, chlorate is introduced and its applications are described. Then, an overview of the chlorate production process is provided, describing the reactions involved, the conditions in the industrial process, and a history of its development. 1.1.4 History of chlorate production and research Chlorate is one of the oldest electrochemical processes, with the first confirmed electrosynthesis dating back to 1802 [12]. The process was discovered by Wilhelm von Hisinger and Jöns Jakob Berzelius [12], hobbyist scientists, only two years after Alessandro Volta first invented the pile, a source of electrical current [13]. The first electrochemical plant opened in 1886 in Villers-St. Sépulcre, Switzerland, using a diaphragm setup in a reactor made of wood. It was not very successful due to cathodic losses, and production ended in 1898 [12]. Around the same year, plants opened in Buckingham and Niagara in Quebec, Canada. These had no diaphragm and used bipolar electrodes with a small cell gap (1.5 to 3 mm), already remarkably similar to today's designs [14]. At the turn of century there were also factories in Switzerland, Sweden, and France. Surprisingly, there were no factories in Germany, despite most chlorate research being performed there [12]. Research focused on developing an understanding of the formation of chlorate. Initially it was believed that the formation was a purely chemical process, but within a few years this theory was completely upended several times. The revisions were driven by thorough experiments and reportedly accompanied by "sometimes violent discussions" [12]. Several papers and discussions in rude tones were exchanged between Foerster and Wohlwill [15], [16] in particular. Foerster insisted there was no electrochemical aspect to chlorate formation, while Wohlwill, a student of Nernst, held that there was. The matter was eventually clarified in 1900 when Lorenz and Wehrlin published extensive experimental results that showed chlorate could be formed electrochemically under certain circumstances [17], although Foerster nevertheless maintained his stance [12]. Regardless of the exact mechanism, it was generally agreed that hypochlorite plays an important part in chlorate formation, and that its Finally, Chapter 7 summarizes my findings and provides a perspective on the future of this topic.

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Towards sustainable chlorate production: The effect of permanganate addition on current efficiency

Cited by 35 publications

References 48 publications

MnO_x/IrO_x as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

MnO_x/IrO_x as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

Suppressed oxygen evolution during chlorate formation from hypochlorite in the presence of chromium(VI)

Understanding selective thin films in the chlorate process

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Towards sustainable chlorate production: The effect of permanganate addition on current efficiency

Cited by 35 publications

References 48 publications

MnOx/IrOx as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

MnOx/IrOx as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

Suppressed oxygen evolution during chlorate formation from hypochlorite in the presence of chromium(VI)

Understanding selective thin films in the chlorate process

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Product

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MnO_x/IrO_x as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

MnO_x/IrO_x as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution