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Oxygen formation in the chlorate process and preparation and deactivation of ozone selective anodes
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Applied Electrochemistry.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents experimental studies concerning two differ-ent electrolytic processes. One part deals with the electrochemicalformation of ozone and focuses on the preparation and deactiva-tion of a highly ozone-selective metal oxide anode (NATO - nickeland antimony doped tin oxide). The preparation of this anode bythermal decomposition of metal chloride salts was investigated anddifficulties and complications of common procedures were identi-fied. The same anodes were also studied regarding the deactivationof their ozone selective properties, identifying possible underlyingmechanisms for this as well as providing indications of the ozoneformation mechanism on the anode. When preparing these anodesby thermal decomposition, the volatility of the precursor salt usedfor the different components needs to be considered. For instanceextensive evaporation of the precursors of tin and antimony canlead to an unreliable preparation process resulting in the difficultyof controlling the properties of the prepared electrodes and a poorreproducibility of the process. The deactivation of the NATOelectrodes was investigated using the ozone current efficiency as amain indicator. The electrodes and the electrolyte were examinedusing electrochemical as well as physical techniques after differentperiods of galvanostatic polarization. The main mechanism behindthe deactivation was identified as the dissolution of antimony fromthe electrode surface. Also contributing, but not as detrimental,seems to be the dissolution of nickel. Both dopants, Ni and Sb, arepresent at the surface of the oxide anode and both seem equallyimportant for enabling the electrochemical ozone formation.The second part of this thesis concerns the decomposition ofhypochlorite, an important intermediate in the industrial chlorateprocess. A connection was found between the formation of chlorateand oxygen, both occurring according to 3rd order kinetics withregard to hypochlorite and both having their highest rates atpH 6 - 7. In the presence of chromium(VI) the hypochloritedecomposition can be modelled as the sum of two parallel reactions:one catalyzed by chromium(VI) and one uncatalyzed reaction.The byproduct oxygen seems to be formed only in the latter. Thusvaddition of chromium(VI) in the electrolyte increases both therate and the selectivity of chlorate formation. These findings areimportant as chromium(VI) needs to be removed from the processdue to its toxicity and in its absence the uncatalyzed decompositionpath would lead to an increase in oxygen formation, resultingin efficiency losses as well as potentially explosive gas mixtures.There is a need for a catalyst that can replace chromium(VI) inthis function.

Abstract [sv]

I denna avhandling presenteras experimentella studier sombehandlar två olika elektrolytiska processer. Den första delenhandlar om tillverkning och deaktivering av en högselektiv met-alloxidanod för elektrokemisk ozonbildning (NATO - nickel- ochantimondopad tennoxid). Tillverkningen av dessa elektroder medtermisk oxidation av metall-kloridsalter medför vissa komplika-tioner, vidare har elektroderna en alltför kort livslängd för indus-triellt bruk. Såväl produktion av NATO som dess deaktiveringhar undersökts i detta arbete. Vid den termiska oxidationenomvandlas lösta metallsalter av tenn, antimon och nickel till ox-ider. Resultaten visar dock att både tenn och antimon avgår tillstor del i gasfas vid upphettningen under tillverkningsprocessen.Detta kan resultera i en opålitlig och svårkontrollerad produk-tion vilket också påverkar reproducerbarheten för processen ochegenskaperna hos elektroderna. Deaktiveringen av dessa NATOelektroder undersöktes med avseende på det uppmätta strömut-bytet för ozonbildning. Elektroder och elektrolyt undersöktes medelektrokemiska såväl som fysikaliska, analystekniker efter olikatidsperioder av galvanostatisk polarisation. Resultaten indikeraratt den huvudsakliga mekanismen bakom deaktiveringen är up-plösning av antimon från elektrodytan. Även nickel löses uppfrån elektrodytan i elektrolyten och bidrar, dock inte till lika storutsträckning, till deaktiveringen av anoderna. Antimon och nickelfinns vid elektrodytan och verkar båda vara viktiga för elektrodensozonbildande egenskaper.I den andra delen av denna avhandling behandlas det kemiskasönderfallet av hypoklorit, en viktig intermediär i den industriellaproduktionen av klorat. Resultaten indikerar en koppling mellanklorat- och syrgasbildning, då båda följer 3:e ordningens kinetikmed avseende på hypoklorit och båda har högst reaktionshastighetvid pH 6 - 7. Vid närvaro av krom(VI) i elektrolyten kan hypok-loritsönderfallet ses som summan av två parallella reaktioner: enkatalyserad som innefattar krom(VI) och en okatalyserad reak-tion. Biprodukten syrgas verkar endast bildas i den okatalyseradereaktionen. Krom(VI) ökar således både hastigheten och selek-tiviteten för den önskade kloratbildningen. Dessa resultat ärviiviktiga, då krom(VI) på grund av dess toxicitet inte längre skaanvändas i kloratprocessen. Utan krom(VI) ökar syrgasbildningoch därmed effektivitetsförluster i systemet, samt också risken förpotentiellt explosiva gasblandningar. Detta understryker behovetav en ersättare till krom(VI), som selektivt ökar hastigheten förkloratbildning.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. , p. 76
Series
TRITA-CBH-FOU ; 2019:32
Keywords [en]
NATO, EOP, ozone, electrode preparation, elec- trode deactivation, hypochlorite, chlorate, oxygen, chromium(VI), sodium dichromate, electrocatalysis, catalysis
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-251424ISBN: 978-91-7873-212-8 (electronic)OAI: oai:DiVA.org:kth-251424DiVA, id: diva2:1315586
Public defence
2019-06-05, K1, Teknikringen 56, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 2019-05-13

Available from: 2019-05-15 Created: 2019-05-14 Last updated: 2019-05-16Bibliographically approved
List of papers
1. Deposition efficiency in the preparation of ozone-producing nickel and antimony doped tin oxide anodes
Open this publication in new window or tab >>Deposition efficiency in the preparation of ozone-producing nickel and antimony doped tin oxide anodes
2017 (English)In: Journal of Electrochemical Science and Engineering, ISSN 1847-9286, Vol. 7, no 1, p. 51-64Article in journal (Refereed) Published
Abstract [en]

The influence of precursor salts in the synthesis of nickel and antimony doped tin oxide (NATO) electrodes using thermal decomposition from dissolved chloride salts was investigated. The salts investigated were SnCl4 center dot 5H(2)O, SnCl2 center dot 2H(2)O, SbCl3 and NiCl2 center dot 6H(2)O. It was shown that the use of SnCl4 center dot 5H(2)0 in the preparation process leads to a tin loss of more than 85 %. The loss of Sb can be as high as 90 % while no indications of Ni loss was observed. As a consequence, the concentration of Ni in the NATO coating will be much higher than in the precursor solution. This high and uncontrolled loss of precursors during the preparation process will lead to an unpredictable composition in the NATO coating and will have negative economic and environmental effects. It was found that using SnCl2 center dot 2H(2)0 instead of SnCl4 center dot 5H(2)O can reduce the tin loss to less than 50 %. This tin loss occurs at higher temperatures than when using SnCl4 center dot 5H(2)O where the tin loss occurs from 56 - 147 degrees C causing the composition to change both during the drying (80 - 110 degrees C) and calcination (460 - 550 degrees C) steps of the preparation process. Electrodes coated with NATO based on the two different tin salts were investigated for morphology, composition, structure, and ozone electrocatalytic properties.

Place, publisher, year, edition, pages
INT ASSOC PHYSICAL CHEMISTS-IAPC, 2017
Keywords
NATO, ATO, tin chloride precursor, thermal decomposition, TGA, deposition, efficiency, dopant enrichment, ozone electrocatalysis
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-214910 (URN)10.5599/jese.374 (DOI)000409210400005 ()
Note

QC 20171020

Available from: 2017-10-20 Created: 2017-10-20 Last updated: 2019-05-14Bibliographically approved
2. Deactivation and selectivity for the electrochemical ozone production at Ni- and Sb- doped SnO2 / Ti electrodes
Open this publication in new window or tab >>Deactivation and selectivity for the electrochemical ozone production at Ni- and Sb- doped SnO2 / Ti electrodes
Show others...
(English)Manuscript (preprint) (Other academic)
Keywords
Ozone, EOP, ATO, NATO, selective electrocatalyst, deactivation, nickel and antimony doped tin oxide
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-251612 (URN)
Note

Manuscript in preparation

QC 20190523

Available from: 2019-05-15 Created: 2019-05-15 Last updated: 2019-05-23Bibliographically approved
3. Catalyzed and uncatalyzed decomposition of hypochlorite in dilute solutions
Open this publication in new window or tab >>Catalyzed and uncatalyzed decomposition of hypochlorite in dilute solutions
2015 (English)In: Industrial & Engineering Chemistry Research, ISSN 0888-5885, E-ISSN 1520-5045, Vol. 54, no 15, p. 3767-3774Article in journal (Refereed) Published
Abstract [en]

Hypochlorite decomposition has been investigated by the combined measurement of aqueous concentrations of total hypochlorite, chlorate, and chloride, as well as that of evolved oxygen. In all experiments, the initial concentrations of NaOCl and NaCl were 80 mM, and the temperature was 80°C. The pH was kept constant in the range 5-10.5. The uncatalyzed decomposition of hypochlorite and the formation of chlorate and oxygen were all found to be third order of the form r<inf>i</inf> = k<inf>i</inf>[HOCl]2[OCl-], and k<inf>O</inf><inf>2</inf> was determined to be 0.046 M-2 s-1. A reaction mechanism in which oxygen and chlorate formation share an intermediate is proposed. Several compounds were tested for catalytic effects. The addition of chloride salts of cobalt and iridium showed catalytic effects on oxygen formation. The addition of iridium chloride also catalyzed the formation of chlorate with increasing selectivity for chlorate with increasing pH.

Keywords
Catalysis, Chlorine compounds, Iridium, Reaction intermediates, Aqueous concentrations, Catalytic effects, Chlorate formation, Chloride salts, Combined measurements, Dilute solution, Initial concentration, Reaction mechanism, Oxygen
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-167744 (URN)10.1021/ie504890a (DOI)000353929300005 ()2-s2.0-84928485256 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20150601

Available from: 2015-06-01 Created: 2015-05-22 Last updated: 2019-05-14Bibliographically approved
4. Suppressed oxygen evolution during chlorateformation from hypochlorite in the presenceof chromium(VI)
Open this publication in new window or tab >>Suppressed oxygen evolution during chlorateformation from hypochlorite in the presenceof chromium(VI)
Show others...
2019 (English)In: Journal of chemical technology and biotechnology (1986), ISSN 0268-2575, E-ISSN 1097-4660, Vol. 94, no 5, p. 1520-1527Article in journal (Refereed) Published
Abstract [en]

BACKGROUND: Chromium(VI) is a crucial electrolyte component in industrial chlorate production. Due to its toxicity, iturgently 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 hypochloriteboth in dilute and concentrated, industrially relevant solutions. The effect of the ionic strength and the specific contributionof different electrolyte components were systematically studied. By simultaneously measuring the concentration decayof hypochlorite (UV–vis spectroscopy) and the oxygen formation (mass spectrometry), both the rate and the selectivity of thereactions were evaluated.

CONCLUSION: In the presence of chromium(VI) the hypochlorite decomposition is described by the sum of an uncatalyzedand 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 selectivefor chlorate formation is emphasized.

Place, publisher, year, edition, pages
Wiley, 2019
Keywords
sodium chlorate; chemical engineering; industrial electrochemistry; sodium dichromate; hypochlorite decomposition; selective catalysis
National Category
Chemical Sciences Other Chemical Engineering Chemical Process Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-251421 (URN)10.1002/jctb.5911 (DOI)000467986900019 ()2-s2.0-85061035820 (Scopus ID)
Note

QC 20190611

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-06-11Bibliographically approved

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