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Theoretical and Experimental Studies of Electrode and Electrolyte Processes in Industrial Electrosynthesis
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry. (Applied electrochemistry)ORCID iD: 0000-0002-3634-8856
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Heterogeneous electrocatalysis is the usage of solid materials to decrease the amount of energy needed to produce chemicals using electricity. It is of core importance for modern life, as it enables production of chemicals, such as chlorine gas and sodium chlorate, needed for e.g. materials and pharmaceuticals production. Furthermore, as the need to make a transition to usage of renewable energy sources is growing, the importance for electrocatalysis used for electrolytic production of clean fuels, such as hydrogen, is rising. In this thesis, work aimed at understanding and improving electrocatalysts used for these purposes is presented.

A main part of the work has been focused on the selectivity between chlorine gas, or sodium chlorate formation, and parasitic oxygen evolution. An activation of anode surface Ti cations by nearby Ru cations is suggested as a reason for the high chlorine selectivity of the “dimensionally stable anode” (DSA), the standard anode used in industrial chlorine and sodium chlorate production. Furthermore, theoretical methods have been used to screen for dopants that can be used to improve the activity and selectivity of DSA, and several promising candidates have been found. Moreover, the connection between the rate of chlorate formation and the rate of parasitic oxygen evolution, as well as the possible catalytic effects of electrolyte contaminants on parasitic oxygen evolution in the chlorate process, have been studied experimentally.

Additionally, the properties of a Co-doped DSA have been studied, and it is found that the doping makes the electrode more active for hydrogen evolution. Finally, the hydrogen evolution reaction on both RuO2 and the noble-metal-free electrocatalyst material MoS2 has been studied using a combination of experimental and theoretically calculated X-ray photoelectron chemical shifts. In this way, insight into structural changes accompanying hydrogen evolution on these materials is obtained.

Abstract [sv]

Heterogen elektrokatalys innebär användningen av fasta material för att minska energimängden som krävs för produktion av kemikalier med hjälp av elektricitet. Heterogen elektrokatalys har en central roll i det moderna samhället, eftersom det möjliggör produktionen av kemikalier såsom klorgas och natriumklorat, som i sin tur används för produktion av t ex konstruktionsmaterial och läkemedel. Vikten av användning av elektrokatalys för produktion av förnybara bränslen, såsom vätgas, växer dessutom i takt med att en övergång till användning av förnybar energi blir allt nödvändigare. I denna avhandling presenteras arbete som utförts för att förstå och förbättra sådana elektrokatalysatorer.

En stor del av arbetet har varit fokuserat på selektiviteten mellan klorgas och biprodukten syrgas i klor-alkali och kloratprocesserna. Inom ramen för detta arbete har teoretisk modellering av det dominerande anodmaterialet i dessa industriella processer, den så kallade “dimensionsstabila anoden” (DSA), använts för att föreslå en fundamental anledning till att detta material är speciellt klorselektivt. Vi föreslår att klorselektiviteten kan förklaras av en laddningsöverföring från ruteniumkatjoner i materialet till titankatjonerna i anodytan, vilket aktiverar titankatjonerna. Baserat på en bred studie av ett stort antal andra dopämnen föreslår vi dessutom vilka dopämnen som är bäst lämpade för produktion av aktiva och klorselektiva anoder. Med hjälp av experimentella studier föreslår vi dessutom en koppling mellan kloratbildning och oönskad syrgasbildning i kloratprocessen, och vidare har en bred studie av tänkbara elektrolytföroreningar utförts för att öka förståelsen för syrgasbildningen i denna process.

Två studier relaterade till elektrokemisk vätgasproduktion har också gjorts. En experimentell studie av Co-dopad DSA har utförts, och detta elektrodmaterial visade sig vara mer aktivt för vätgasutveckling än en standard-DSA. Vidare har en kombination av experimentell och teoretisk röntgenfotoelektronspektroskopi använts för att öka förståelsen för strukturella förändringar som sker i RuO2 och i det ädelmetallfria elektrodmaterialet MoS2 under vätgasutveckling.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , x, 100 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2015:66
Keyword [en]
Electrocatalysis, metallic oxides, ruthenium dioxide, titanium dioxide, DSA, doping, selectivity, ab initio modeling, density functional theory
Keyword [sv]
Elektrokatalys, metalloxider, ruteniumdioxid, titandioxid, DSA, dopning, selektivitet, ab initio-modellering, täthetsfunktionalteori
National Category
Other Chemical Engineering Inorganic Chemistry Theoretical Chemistry Physical Chemistry Materials Chemistry Condensed Matter Physics
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-177025ISBN: 978-91-7595-781-4 (print)OAI: oai:DiVA.org:kth-177025DiVA: diva2:869860
Public defence
2015-12-18, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency, 33252-1
Note

QC 20151119

Available from: 2015-11-19 Created: 2015-11-13 Last updated: 2015-12-18Bibliographically approved
List of papers
1. Physical and electrochemical properties of cobalt doped (Ti,Ru)O-2 electrode coatings
Open this publication in new window or tab >>Physical and electrochemical properties of cobalt doped (Ti,Ru)O-2 electrode coatings
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2013 (English)In: Materials Science & Engineering: B. Solid-state Materials for Advanced Technology, ISSN 0921-5107, E-ISSN 1873-4944, Vol. 178, no 20, 1515-1522 p.Article in journal (Refereed) Published
Abstract [en]

The physical and electrochemical properties of ternary oxides Ti0.7Ru0.3-xCoxO2 (x = 0.093 and x = 0) have been investigated and compared. Samples of three different thicknesses were prepared by spin-coating onto polished titanium to achieve uniform and well-defined coatings. The resulting electrodes were characterized with a variety of methods, including both physical and electrochemical methods. Doping with cobalt led to a larger number of micrometer-sized cracks in the coating, and coating grains half the size compared to the undoped samples (10 instead of 20 nm across). This is in agreement with a voltammetric charge twice as high, as estimated from cyclic voltammetry. There is no evidence of a Co3O4 spinel phase, suggesting that the cobalt is mainly incorporated in the overall rutile structure of the (Ti,Ru)O-2. The doped electrodes exhibited a higher activity for cathodic hydrogen evolution compared to the undoped electrodes, despite the fact that one third of the active ruthenium was substituted with cobalt. For anodic chlorine evolution, the activity was similar for both electrode types.

Keyword
DSA (R), Cobaltdoping, XRD, Polarization curves, Hydrogen evolution, Chlorine evolution
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-139298 (URN)10.1016/j.mseb.2013.08.018 (DOI)000327830000024 ()2-s2.0-84887055267 (Scopus ID)
Funder
Swedish Energy AgencySwedish Research Council
Note

QC 20140108

Available from: 2014-01-08 Created: 2014-01-08 Last updated: 2017-12-06Bibliographically approved
2. Operando Characterization of an Amorphous Molybdenum Sulfide Nanoparticle Catalyst during the Hydrogen Evolution Reaction
Open this publication in new window or tab >>Operando Characterization of an Amorphous Molybdenum Sulfide Nanoparticle Catalyst during the Hydrogen Evolution Reaction
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2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 50, 29252-29259 p.Article in journal (Refereed) Published
Abstract [en]

Molybdenum sulfide structures, particularly amorphous MoS3 nanoparticles, are promising materials in the search for cost-effective and scalable water-splitting catalysts. Ex situ observations show that the nanoparticles exhibit a composition change from MoS3 to defective MoS2 when subjected to hydrogen evolution reaction (HER) conditions, raising questions regarding the active surface sites taking part in the reaction. We tracked the in situ transformation of amorphous MoS3 nanoparticles under HER conditions through ambient pressure X-ray photoelectron spectroscopy and performed density functional theory studies of model MoSx systems. We demonstrate that, under operating conditions, surface sites are converted from MoS3 to MoS2 in a gradual manner and that the electrolytic current densities are proportional to the extent of the transformation. We also posit that it is the MoS2 edge-like sites that are active during HER, with the high activity of the catalyst being attributed to the increase in surface MoS2 edge-like sites after the reduction of MoS3 sites.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-161156 (URN)10.1021/jp505394e (DOI)000346759300037 ()2-s2.0-84949116763 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20150319

Available from: 2015-03-19 Created: 2015-03-09 Last updated: 2017-12-04Bibliographically approved
3. Ti atoms in Ru0.3Ti0.7O2 mixed oxides form active and selective sites for electrochemical chlorine evolution
Open this publication in new window or tab >>Ti atoms in Ru0.3Ti0.7O2 mixed oxides form active and selective sites for electrochemical chlorine evolution
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2014 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 146, 733-740 p.Article in journal (Refereed) Published
Abstract [en]

The electrocatalytic properties of the (1 1 0) surface of Ru-doped TiO2, Ti-doped RuO2 and the industrially important Dimensionally Stable Anode (DSA) composition Ru0.3Ti0.7O2 have been examined using density functional theory. It is found that the oxygen adsorption energy on a Ti site is strongly affected by the presence of small amounts of Ru dopant, whereas oxygen adsorption is relatively unaffected by Ti dopants in RuO2. The calculations also indicate that coordinatively unsaturated Ti sites on Ru-doped TiO2 and on Ru0.3Ti0.7O2 could form active and selective sites for Cl-2 evolution. These results suggest a reason for why DSA shows a higher chlorine selectivity than RuO2 and propose an experimental test of the hypothesis.

Keyword
Dimensionally stable anodes, Electrolysis, Oxygen, Chlorine, Selectivity, Density functional theory
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-158826 (URN)10.1016/j.electacta.2014.09.056 (DOI)000345226100091 ()2-s2.0-84908504059 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20150126

Available from: 2015-01-26 Created: 2015-01-12 Last updated: 2017-12-05Bibliographically approved
4. 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, 3767-3774 p.Article 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.

Keyword
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: 2017-12-04Bibliographically approved
5. Selectivity between oxygen and chlorine evolution in the chlor-alkali and chlorate processes: a comprehensive review
Open this publication in new window or tab >>Selectivity between oxygen and chlorine evolution in the chlor-alkali and chlorate processes: a comprehensive review
(English)Manuscript (preprint) (Other academic)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-177395 (URN)
Note

QS 2015

Available from: 2015-11-19 Created: 2015-11-19 Last updated: 2015-11-19Bibliographically approved
6. The electrocatalytic properties of doped TiO2
Open this publication in new window or tab >>The electrocatalytic properties of doped TiO2
2015 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 180, 514-527 p.Article in journal (Refereed) Published
Abstract [en]

To rationally control the catalytic properties of heterogeneous catalysts is the goal in heterogeneous (electro)catalysis research. Recent developments of theoretical methods based on density functional theory have enabled computational screening of catalysts, to achieve fundamental understanding of which catalyst is optimal for a certain reaction. In the present work, such screening is employed to elucidate the electrocatalytic properties of doped rutile TiO2. Electrodes based on this material are widely used in industrial production of, e.g., chlorine and sodium chlorate. The screening covers 38 different dopants, including all fourth, fifth and sixth row transition metals. Several dopants are predicted to activate TiO2, resulting in a material optimal either for the oxygen evolution reaction, or for selective chlorine evolution. The results can serve as a map for the rational design of electrocatalysts based on TiO2.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2015
Keyword
TiO2, Dopant effects, oxygen, chlorine, selectivity, density functional theory
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-176961 (URN)10.1016/j.electacta.2015.08.101 (DOI)000363345100062 ()2-s2.0-84940981192 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20151116

Available from: 2015-11-16 Created: 2015-11-13 Last updated: 2017-12-01Bibliographically approved
7. An improved force field for structures of mixed RuO2-TiO2 oxides
Open this publication in new window or tab >>An improved force field for structures of mixed RuO2-TiO2 oxides
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(English)Manuscript (preprint) (Other academic)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-177397 (URN)
Note

QS 2015

Available from: 2015-11-19 Created: 2015-11-19 Last updated: 2015-11-19Bibliographically approved
8. Structural changes in RuO2 during electrochemical hydrogen evolution
Open this publication in new window or tab >>Structural changes in RuO2 during electrochemical hydrogen evolution
(English)Manuscript (preprint) (Other academic)
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-177396 (URN)
Note

QS 2015

Available from: 2015-11-19 Created: 2015-11-19 Last updated: 2015-11-19Bibliographically approved

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