SELECTIVE CATHODE FOR USE IN ELECTROLYTIC CHLORATE PROCESS

Abstract

The present disclosure relates to a process for the production of alkali metal chlorate in a single compartment electrolytic cell, which avoids the need for addition of sodium dichromate to the process, in which unwanted side-reactions are reduced by using a cathode having an electrocatalytic top layer on a substrate that optionally also has one or more intermediate layers. The top electrocatalytic layer comprises an oxide of manganese and/or cerium.

Claims

1. A process for producing alkali metal chlorate, comprising introducing an electrolyte solution, free of added chromium, said solution comprising alkali metal chloride to a non-divided electrolytic cell comprising at least one anode and at least one cathode, and electrolyzing the electrolyte solution to produce an electrolyzed solution enriched in chlorate, wherein at least one cathode comprises a conductive electrode substrate which may be coated with one or more intermediate conductive layers, and an electrocatalytic top layer applied onto said substrate or onto intermediate layers, said top layer comprising cerium oxide and/or manganese oxide.

2. A process according to claim 1, in which the one or more intermediate layers comprising at least one of titanium suboxide, titanium nitride (TiNX), MAX phase, silicon carbide, titanium carbide, titanium aluminium carbide, titanium silicon carbide, graphite, glassy carbon or mixtures thereof.

3. A process according to claim 1, wherein the top layer comprises cerium and/or manganese oxide in their +4 oxidation state.

4. A process according to claim 1, wherein the conductive substrate is titanium, or titanium provided with a layer of titanium suboxide.

5. A process according to claim 1, wherein electrocatalytic layer is deposited by thermal decomposition.

6. A process according to claim 1, wherein the electrodeposited layer is deposited by thermal decomposition and heat treated between about 400 and about 500° C.

7. A process according to claim 1, wherein the surface coverage of the electrocatalytic layer is in the range of between about 0.1 and about 4.0 mg/cm.sup.2.

8. A process according claim 1, wherein the electro-catalytic layer provides a cerium and/or manganese content in an amount of between about 1 and about 3 mg/cm.sup.2.

9. A process according to claim 2, wherein the top layer comprises cerium and/or manganese oxide in their +4 oxidation state.

10. A process according to claim 9, wherein the conductive substrate is titanium, or titanium provided with a layer of titanium suboxide.

11. A process according to claim 10, wherein electrocatalytic layer is deposited by thermal decomposition.

12. A process according to claim 11, wherein the electrodeposited layer is deposited by thermal decomposition and heat treated between about 400 and about 500° C.

13. A process according to claim 12, wherein the surface coverage of the electrocatalytic layer is in the range of between about 0.1 and about 4.0 mg/cm.sup.2.

14. A process according claim 13, wherein the electro-catalytic layer provides a cerium and/or manganese content in an amount of between about 1 and about 3 mg/cm.sup.2.

15. A process according to claim 3, wherein the conductive substrate is titanium, or titanium provided with a layer of titanium suboxide; wherein electrocatalytic layer is deposited by thermal decomposition; wherein the electrodeposited layer is deposited by thermal decomposition and heat treated between about 400 and about 500° C.; and wherein the surface coverage of the electrocatalytic layer is in the range of between about 0.1 and about 4.0 mg/cm.sup.2.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0033] FIG. 1. XRD pattern of the MnO.sub.x samples, formed from the thermal decomposition of Mn(NO.sub.3).sub.2 at different annealing temperatures.

[0034] FIG. 2. Raman spectra of cerium oxide development from cerium nitrate at different annealing temperatures.

EXAMPLES

Example 1

Electrode Preparation and Characterization

[0035] In typical preparations of electrodes for example 2, described hereafter, titanium substrates were cleaned and subsequently etched in boiling 1:1 mixture of 37% hydrochloric acid and deionized water for 20 minutes. The electrodes were rinsed with an excess amount of deionized water and ethanol and were dried by air. V≈50 μl of 1M ethanol-based solution of Mn(NO.sub.3).sub.2 or Ce(NO.sub.3).sub.2 was spread homogeneously using a short-haired brush. The electrodes were dried at T.sub.1=60° C. for 10 minutes and subsequently annealed at T.sub.2=200-500° C. for 10 minutes in air atmosphere. The catalyst loading of the different electrodes shown in example 2 was controlled by the repetition of this coating cycle. After casting the last layer of the coating, the electrodes were annealed at T.sub.2 for an extra 60 minutes.

[0036] Electrode Characterization:

[0037] XRD (FIG. 1) measurements were performed to verify the phase composition of the manganese oxides formed from a Mn(NO.sub.3).sub.2 precursor at different annealing temperatures. The electrocatalytic top layer formed at T.sub.2=200° C. can be identified as mostly Mn.sub.2O.sub.3 with β-MnO.sub.2 minority, based on the XRD measurement (FIG. 1). At higher annealing temperatures the Mn.sub.2O.sub.3 phase is still present, but the β-MnO.sub.2 phase becomes dominant. The XRD patterns recorded for the two highest annealing temperatures are very similar, indicating a similar phase composition for these cases.

[0038] Raman analysis was used to verify the phase composition of the top layer comprising cerium oxides. FIG. 2 show the spectra taken of the samples formed at 250° C. respectively 500° C. show that both layers mostly consist of CeO.sub.2 (Ce+4 oxidation state). Some Ce-nitrate residues can be seen in the 250° C. samples.

Example 2

Current Efficiency Measurements

[0039] The selectivity towards HER was determined as Cathodic Current Efficiency, CCE (%), by analysis of gases evolved from an electrochemical set-up. The current efficiency measurements were performed in a custom-designed electrochemical setup. It consisted of a sealed, jacketed cell which had two openings on a tightly fitting lid—an inlet for the continuous Ar gas purging and an outlet connected to a mass spectrometer through a silica gel filled gas drying column. The pH of the solution was regulated using NaOH and HCl solutions. The temperature of the electrolyte was controlled by circulating water from an external heater bath in the jacket of the cell. The H.sub.2 production-rate and the Faradaic efficiency values were calculated from the composition of the cell gas outlet. UV-vis spectroscopy was used to determine the hypochlorite concentration of the solutions. For the analysis, 200 μl liquid aliquots were taken, and immediately added to 0.5 M NaOH. The hypochlorite concentration was calculated from the absorbance maximum at λ=292 nm, (ε.sub.292 nm=350 dm.sup.3 mol.sup.−1 cm.sup.−1).

[0040] The evolved hydrogen (c.f. reaction 1) is compared with the theoretical amount of hydrogen that can be formed at a certain current density. In the presence of hypochlorite any other reaction not producing hydrogen is seen as a loss according to reaction 7.

[0041] The selectivity of an electrode with a top layer produced from Ce(NO.sub.3).sub.2 at different annealing temperatures is reflected in Table 1.