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Rare earth metal salts as potential alternatives to Cr(VI) in the chlorate process
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0001-5816-2924
2010 (English)In: Journal of Applied Electrochemistry, ISSN 0021-891X, E-ISSN 1572-8838, Vol. 40, no 8, 1529-1536 p.Article in journal (Refereed) Published
Abstract [en]

Chromate is today added to industrial chlorate electrolyte, where it forms a thin cathode film of chromium hydroxide that hinders unwanted reduction of hypochlorite and chlorate. The aim of this study was to investigate rare earth metal (REM) ions as an environmentally friendly alternative to the toxic chromate addition. Potential sweeps and iR-corrected polarisation curves were recorded using rotating disc electrodes of iron and gold. Addition of Y(III), La(III) or Sm(III) to 5 M NaCl at 70 A degrees C suppressed hypochlorite reduction. Activation of hydrogen evolution by REM ion addition to 0.5 M NaCl was more significant at 25 A degrees C than at 50 and 70 A degrees C. Increasing the chloride concentration to 5 M had a detrimental effect on this activation. The major problem in replacing chromate with REM salts is the poor solubility of REM ions at normal chlorate process conditions, and therefore REM salts are not a realistic alternative to chromate addition.

Place, publisher, year, edition, pages
2010. Vol. 40, no 8, 1529-1536 p.
Keyword [en]
Rare earth metals, Chlorate production, Yttrium, Hydrogen evolution, Hypochlorite, In situ electrodeposition
National Category
Inorganic Chemistry
URN: urn:nbn:se:kth:diva-27266DOI: 10.1007/s10800-010-0136-4ISI: 000279033200010ScopusID: 2-s2.0-77956907518OAI: diva2:378742

QC 20101216

Available from: 2010-12-16 Created: 2010-12-09 Last updated: 2013-07-02Bibliographically approved
In thesis
1. In-situ activated hydrogen evolution from pH-neutral electrolytes
Open this publication in new window or tab >>In-situ activated hydrogen evolution from pH-neutral electrolytes
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The goal of this work was to better understand how molybdate and trivalent cations can be used as additives to pH neutral electrolytes to activate the Hydrogen Evolution Reaction (HER). Special emphasis was laid on the chlorate process and therefore also to some of the other effects that the additives may have in that particular process.

Cathode films formed from the molybdate and trivalent cations have been investigated with electrochemical and surface analytical methods such as polarization curves, potential sweep, Electrochemical Impedance Spectroscopy (EIS), current efficiency measurements, Scanning Electron Microscope (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), X-Ray Fluorescence (XRF) and Inductively Coupled Plasma (ICP) analysis.

Trivalent cations and molybdate both activate the HER, although in different ways. Ligand water bound to the trivalent cations replaces water as reactant in the HER. Since the ligand water has a lower pKa than free water, it is more easily electrochemically deprotonated than free water and thus catalyzes the HER. Sodium molybdate, on the other hand, is electrochemically reduced on the cathode and form films which catalyze the HER (on cathode materials with poor activity for HER). Molybdate forms films of molybdenum oxides on the electrode surface, while trivalent cation additions form hydroxide films. There is a risk for both types of films that their ohmic resistance increases and the activity of the HER decreases during their growth. Lab-scale experiments show that for films formed from molybdate, these negative effects become less pronounced when the molybdate concentration is reduced.

Both types of films can also increase the selectivity of the HER by hindering unwanted side reactions, but none of them as efficiently as the toxic additive Cr(VI) used today in the chlorate process. Trivalent cations are not soluble in chlorate electrolyte and thus not suitable for the chlorate process, whereas molybdate, over a wide pH range, can activate the HER on catalytically poor cathode materials such as titanium.

Abstract [sv]

Målsättningen med detta doktorsarbete har varit att bättre förstå hur trivalenta katjoner och molybdat lösta i elektrolyten kan effektivisera elektrokemisk vätgasproduktion. Tillämpningen av dessa tillsatser i kloratprocessen och eventuella sidoeffekter har undersökts.

De filmer som bildas på katoden av tillsatserna har undersökts med både elektrokemiska och fysikaliska ytanalysmetoder: polarisationskurvor, potentialsvep, elektrokemisk impedansspektroskopi (EIS), strömutbytesmätningar, svepelektronmikroskopi (SEM), energidispersiv röntgenspektroskopi (EDS), röntgenfotoelektronspektroskopi (XPS), röntgenfluorensens (XRF) och induktivt kopplat plasma (ICP).

Både trivalenta katjoner och molybdat kan aktivera elektrokemisk vätgasutveckling, men på olika sätt. Vatten bundet till trivalenta katjoner ersätter fritt vatten som reaktant vid vätgasutveckling. Eftersom vatten bundet till trivalenta katjoner har lägre pKa-värde, går det lättare att producera vätgas från dessa komplex än från fritt vatten. Natriummolybdat däremot reduceras på katoden och bildar filmer som kan katalysera vätgasutvecklingen på substratmaterial som har låg katalytisk aktivitet för reaktionen. Molybdat bildar molybdenoxider på ytan medan trivalenta katjoner bildar metallhydroxider. Båda typerna av film riskerar att bilda filmer som är resistiva och deaktiverar vätgasutvecklingen. Laboratorieexperiment visar att problemen minskar med minskad molybdathalt.

Båda filmerna kan öka selektiviteten för vätgasutveckling genom att hindra sidoreaktioner. Filmerna är dock inte lika effektiva som de filmer som bildas från den ohälsosamma tillsatsen Cr(VI), vilken används i kloratprocessen idag. Trivalenta katjoner är inte lösliga i kloratelektrolyt och är därför inte en lämplig tillsats i kloratprocessen. Molybdat har god löslighet och kan aktivera vätgasutveckling i ett stort pH‑intervall på titan och andra substratmaterial som själva har betydlig sämre aktivitet för vätgasutveckling.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. 45 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2012:26
molybdate, trivalent cations, electrolysis, hypochlorite reduction, films, electrolysis, chlorate process, molybdat, trivalenta katjoner, elektrolys, hypokloritreduktion, filmer, kloratprocessen
National Category
Engineering and Technology
Research subject
SRA - Energy
urn:nbn:se:kth:diva-95369 (URN)978-91-7501-391-6 (ISBN)
Public defence
2012-06-15, E3, Lindstedtsvägen 3, entréplan, KTH, Stockholm, 10:00 (English)

QC 20120530

Available from: 2012-05-30 Created: 2012-05-23 Last updated: 2013-04-18Bibliographically approved

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