WO2006072169A1

WO2006072169A1 - Ti5 ru and al-based alloys and use thereof for sodium chlorate synthesis

Info

Publication number: WO2006072169A1
Application number: PCT/CA2006/000003
Authority: WO
Inventors: Robert Schulz; Marie-Eve Bonneau; Lionel Roue; Daniel Guay
Original assignee: Hydro-Quebec; Eka Chimie Canada Inc.
Priority date: 2005-01-05
Filing date: 2006-01-04
Publication date: 2006-07-13
Also published as: CA2492128A1; UY29328A1

Abstract

The invention concerns a alloy of formula: Ti2+t(Ru(1-x)Al(1+x))(1-u/2)MuTy, wherein: t is a number between -1 and +1and preferably between -0.5 and +0.5; x is a number between -0.95 and +0.95 and preferably between +0.5 and +0.9; u is a number between 0 and +1, preferably less than 0.25; y is a number between 0 and +6, preferably equal to 2; M represents one or more elements selected among Ag, Pd, Rh, Fe, Cr and V, the elements being preferably, Ag, Pd or Rh; and T represents one or more elements selected among O, B, S, C, N, Si, P and H, the elements being preferably oxygen. Said alloy is preferably in nanocrystalline form and can be used as cathode for producing sodium chlorate by electrochemical process.

Description

ALLOYS BASED ON Ti, Ru AND Al

AND

USE THEREOF FOR SYNTHESIS OF SODIUM CHLORATE

FIELD OF THE INVENTION:

The present invention relates to new alloys based on Ti, Ru and Al. It also relates to the use of these new alloys for the electrochemical synthesis of sodium chlorate.

TECHNOLOGICAL BACKGROUND:

Sodium chlorate (NaCIO ₃ ) is a bleaching agent in the pulp and paper industry. For environmental reasons, the trend in recent years has been to use chlorine dioxide (CIO ₂ ^" ) rather than chlorine gas (Cl ₂ ) to whiten the pulp to reduce releases of chlorinated organic products. Chlorine dioxide is obtained by reduction of sodium chlorate, which has had the effect of significantly increasing the demand for sodium chlorate.

The overall chemical reaction implemented in electrochemical cells for the synthesis of sodium chlorate is as follows:

NaCI + 3 H ₂ O -> NaCIO ₃ + 3 H ₂

The first step in the electrochemical manufacture of sodium chlorate is the anodic discharge of chlorine (Cl ^" ) at the anode with production of hydrogen (H ₂ ) as a complementary reaction to the cathode. Nowadays, the steel or titanium as the electrode material at the cathode. The cathode overvoltage of steel electrodes is approximately 900 mV at a current density of 250 mA / cm ^2. This overvoltage is practically an order larger than the overvoltage at the anode and is therefore the main source of energy loss from the electrochemical process of synthesis of sodium chlorate. Substantial energy savings can therefore be achieved by replacing current steel cathodes with more efficient materials which have a lower cathode overvoltage.

US patent 5,662,834 and the corresponding Canadian application no. 2,154,428 on behalf of HYDRO-QUÉBEC tackles this problem by proposing a new alloy based on Ti, Ru, Fe and O which reduces the overvoltage at the cathode by around 300 mV. This gain corresponds to an improvement of around 10% in the overall energy efficiency of the process. However, it is mentioned in this patent that: "Whatever the case, the alloy must contain ruthenium. However, the amount of ruthenium should not be too high because, on the one hand, of the high price of this metal, and on the other hand and above all, of its lack of stability when it is used in an electrolyte solution. " It would therefore be desirable to find an element which can partially replace the Ru to reduce the cost of these new electrodes and improve their stability while retaining their good electrocatalytic properties, namely a low cathode overvoltage of the order of 600 mV at a density. current of 250 mA / cm ² .

SUMMARY OF THE INVENTION

It has been discovered in the context of the present invention that aluminum, which is a very inexpensive element, can precisely replace Ru in materials similar to those of US Pat. No. 5,662,834 and this, in significant quantity without losing the properties advantageous of these new cathode materials.

To meet the requirements of manufacturers of sodium chlorate, cathode materials should ideally meet the following conditions: 1) have a low overvoltage,

2) be inactive in the face of the decomposition of hypochlorite,

3) slightly reduce the hypochlorite and chlorate at the cathode, 4) do not interfere with the chromium hydroxide layer on the surface,

5) be stable in open circuit condition at operating temperatures,

6) be resistant to embrittlement by hydrogen,

7) have a lifespan of several years,

8) be able to tolerate periodic cleaning with nitric acid, and finally, 9) be inexpensive.

The alloys of the prior art based on Ti and Ru described in US Pat. No. 5,662,834 contain Fe and one or more metals which can be substituted for iron, including V and Cr. In the context of the present invention, it has been discovered that Al can not only advantageously replace a good part of the Ru but also, when the Al is found in the presence of Ti and Ru in alloys of the TiaRuM type where M = AI, materials are less likely to absorb hydrogen during hydrogen evolution reactions than the corresponding alloys where M = Fe, V, and Cr. The aluminum-based materials of the present invention are therefore more resistant to embrittlement by hydrogen and therefore more stable than the materials of the prior art. More generally, it has been discovered that the alloys of the above-mentioned type where M = Al, Ag, Pd, Rh are less likely to absorb hydrogen than the alloys where M = Fe, Cr, V.

The first object of the invention is therefore a new alloy, characterized in that it corresponds to the following formula:

Ti2 ₊ t (RU (i _-X ) AI (i _{+ X} )) (i _-u / 2) M _u T _y

in which: t is a number between -1 and +1, preferably between -0.5 and + 0.5, and more preferably equal to 0; x is a number between -0.95 and +0.95, preferably between +0.5 and +

0.95, and more preferably equal to 0.75; u is a number between 0 and +1, preferably less than 0.25; y is a number between 0 and +6, preferably equal to 2 M represents one or more elements chosen from Ag, Pd, Rh, Fe, Cr and V, the element or elements preferably being Ag, Pd or Rh and T represents one or more elements chosen from O, B, S, C, N, Si, P and H, the element or elements preferably being oxygen.

This new alloy is preferably in the nanocrystalline state and can be advantageously used as a cathode to produce electrochemically sodium chlorate. By "nanocrystalline state" is meant a microstructure consisting of crystallites whose crystal size is less than 100 nm.

A second object of the present invention is also the use of this new alloy for the electrochemical synthesis of sodium chlorate. For this purpose, the new alloy can be in the nanocrystalline state or not. Preferably, it is in a nanocrystalline state to give rise to low cathode overvoltages.

Another subject of the present invention is also an electrode comprising a substrate and a coating of an alloy according to what is described and / or illustrated in the present application. Preferably, the substrate is a conductive metal, a valve metal, or any other suitable metal / material, as evident to a person skilled in the art.

The alloys according to the invention can be prepared in different ways. They can first be prepared in powder form by thoroughly grinding a mixture of elements chosen from Ti, Ru, Al and the elements M and T in adequate proportion to obtain the desired composition or a mixture of compounds chosen by combining various elements from Ti, Ru, Al, and the elements M and T. The grinding can be carried out in an inert atmosphere, in air or in a reactive atmosphere by including elements in the reaction medium chosen from the elements T. For example, it is possible to grind in a reactive medium, such as an oxygen (oxidizing) atmosphere, a hydrogen (reducing) atmosphere or even a nitrogen atmosphere. The temperature and pressure of the grinding medium gases can be adjusted as required. The powder thus produced, which may be in a nanocrystalline state or not dependent on the grinding conditions (such as for example the duration and intensity of the grinding), is then used to prepare electrodes which will serve as cathode to produce the chlorate sodium. Beforehand, this powder can be modified by various treatments, such as a heat treatment, an agglomeration treatment, etc. We can finally compress the powder or spray it on various substrates to make cathodes. Thermal spraying is particularly well suited for this purpose. For example, techniques such as plasma spraying can be used: APS (atmospheric plasma spray), HVOF (high velocity oxyfuel), VPS (vacuum plasma spray), LPPS (low pressure plasma spray) etc.

The substrates on which the powder is deposited may be of different types, but preferably steel, titanium or aluminum plates will be used. It may also be desirable to apply an undercoat at the interface between the substrate and the catalytic coating produced from the powder in order to improve the adhesion of the latter and the lifetime of the electrodes.

The alloys of the prior art described in US Patent 5,662,834 contain oxygen. Although the oxygen content had little effect on the electrochemical activity and therefore on the value of the overvoltage, its presence in the alloy is beneficial on the long term stability of the electrodes. In the context of the present invention, it has been discovered that this advantage was not only associated with oxygen but that several other elements among the T elements described above could provide better long term stability of Ti, Ru and Al based electrode materials.

The invention and its advantages will be better understood on reading the more detailed but nonlimiting description which follows, made with reference to the accompanying drawings.

BRIEF PRESENTATION OF THE DRAWINGS:

FIG. 1 represents polarization curves of alloys of the Ti ₂ RUi _-X Mi _{+ X type} for various elements M;

FIG. 2a represents an overvoltage curve as a function of the number of cycles of open circuit and of evolution of hydrogen for alloys of the Ti ₂ RuM type where M represents various elements;

FIG. 2b represents a curve similar to that of FIG. 2a for materials of the Ti2Rui type. _x AI _{1 + x} having various concentrations of aluminum;

FIG. 2c represents photographs of alloys of the Ti ₂ Rui _-x Ali _{+ x type} after 50 cycles of open circuit and of evolution of hydrogen;

FIG. 2d represents overvoltage curves as a function of the number of cycles for oxygenated and non-oxygenated materials, the photograph at the top right representing the oxygenated alloy after 50 cycles;

FIG. 3a represents an X-ray diffraction spectrum of a sample obtained after 40 hours of grinding a mixture of powders of two Ti, an Al and a Ru; FIG. 3b represents a series of X-ray diffraction spectra of a sample obtained after different grinding time of a mixture of powders of two TiO, one Al and one Ru;

FIG. 3c represents a series of X-ray diffraction spectra of a sample obtained after different grinding time of a mixture of powders of an AI ₂ O ₃ , three Ti, one TiO and two Ru; and

FIG. 3d represents a x-diffraction spectrum of Ti ₂ RuAIO ₂ obtained after 40 hours of grinding a powder mixture containing TiO (according to a manufacturing procedure corresponding to that of FIG. 3b), in comparison with that of a powder mixture containing AI ₂ O ₃ prepared according to a procedure corresponding to that of FIG. 3c;

FIG. 4 represents the evolution of the X-ray spectra of a mixture of powder of Ti and AI ₂ O ₃ as a function of the grinding time;

FIG. 5 represents X-ray spectra of various alloys of the Ti ₂ RuFeB _{x type} for different concentration of boron;

FIG. 6 represents micrographs of alloys of the Ti ₂ RuFeB _{x type} for different concentrations of boron;

FIG. 7 represents curves of evolution of the overvoltage as a function of the number of cycles of open circuit and of evolution of hydrogen for various alloys of the Ti ₂ RuFeB _{x type} with different concentrations of boron; and

FIG. 8 represents curves of evolution of the overvoltage as a function of the electrolysis time for three alloys of the Ti ₂ RuFeB _{x type} .

DETAILED DESCRIPTION OF THE INVENTION: As previously indicated, the alloy according to the invention corresponds to the following chemical formula:

Ti2 + t (RU (i- _X ) AI (i + χ)) (iu / 2) M _u Ty

in which:

t is a number between -1 and +1, preferably between -0.5 and + 0.5, and more preferably equal to 0;

x is a number between -0.95 and +0.95, preferably between +0.5 and +

0.95, and more preferably equal to 0.75;

u is a number between 0 and +1, preferably u less than 0.25;

y is a number between 0 and +6, preferably equal to 2

M represents one or more elements chosen from Ag, Pd, Rh, Fe, Cr and V, preferably Ag, Pd and Rh and

T represents one or more elements chosen from O, B, S, C, N, Si, P and H, preferably oxygen.

A special case is that where y = 0, u = 0 and t = 0 ie Ti ₂ Rui _-x AI _{1 + x} . For this particular case, FIG. 1 shows polarization curves obtained in a 4M NaOH solution at 25 ^° C. The cathode sample used for this test was a tablet of pressed powder of T ₂ Ru-ι _-x Mi + _x (or M = Al, Pd and Rh) produced by intensive grinding of a mixture of Ti, Ru and the element M.

The anode was a Pt wire and the reference Ag / AgCI. The correction for the ohmic drop was made by the current interruption method. Note that at a current density of 250 mA / cm ² , the voltage is the lowest for alloys of the Ti ₂ Ru ₁ AI ₁ and Ti ₂ RUo _J5 AIi _2S SUiVi type alloys of the Ti ₂ Ru _{0 type} . ₅ Ah. ₅ and finally, the alloys of the Ti ₂ RuiPdi and Ti ₂ Ru ₁ RlIi type have the highest tensions.

Table I below summarizes the corresponding overvoltage values:

TABLE 1

Ti ₂ RUi _-X Ah _{+ X} sample overvoltage values

This particular example shows that the alloys based on Ti, Ru and Al have low overvoltages and are therefore very effective as cathode materials if the Al concentration is chosen appropriately. The best values being obtained for Al ₁ and AIi ₂₅ .

Stability tests were also carried out on these electrodes by alternately performing cycles of evolution of hydrogen HER (polarization for 10 minutes) followed by a period in open circuit OCP (for 10 minutes). These tests were carried out with a typical industrial chlorate solution at 70 ^° C. and at a pH of 6.5. The results are shown in Figure 2a. We can see in this figure that the overvoltage values at 250 mA / cm ² remain stable, low and between -0.55 and -0.65 during a period of more than 50 cycles for all alloys of the Ti ₂ RuM or M = Al type. , Ag, Rh and Pd. On the other hand, for the Ti ₂ RuFe type alloy corresponding to the prior art, the overvoltage begins to increase significantly after 22 cycles to deteriorate considerably around thirty cycles. Figure 2b shows similar curves for alloys of the Ti ₂ RUi- _x Ah + χ or x = 0, 0.25 and 0.50 respectively and Figure 2c shows the appearance of these latter electrodes after having undergone the 50 OCP / HER cycles. . The electrodes with the best overvoltages, ie the Ah and AI _{1 2} 5 electrodes (see above), are also the ones which seem to resist cycling best in the long term. In fact, as shown in FIG. 2c, the Ti ₂ RUc ₅ AIi ₅ alloy seems to have deteriorated considerably during the 50 OCP / HER cycles.

As was the case for the prior art alloys based on Ti, Ru and Fe, the addition of oxygen has a beneficial effect on the long-term stability of the electrodes based on Ti, Ru and Al Indeed, Figure 2d shows that the addition of oxygen to alloys with high aluminum content (Ti ₂ Ruo. ₂₅ AI1.7 ₅ O ₂ ), which as we have seen previously tend to deteriorate when cycling, stabilizes them. The image of the Ti ₂ Ruo electrode. ₂₅ AI ₁ . ₇₅ O ₂ after 50 cycles, which can be seen in FIG. 2d, shows that it resists this treatment very well if it contains oxygen.

It has also been discovered that the crystallographic structure of Ti ₂ RuAI is the same as that of the Ti ₂ RuFe alloy of the prior art. Indeed, Figure 3a shows the x-diffraction spectrum of a Ti ₂ RuAI alloy obtained by grinding intensively for 40 hours a mixture of powder of Ti, Ru and Al in the desired proportions. This figure reveals that the structure is simple cubic.

With regard to the corresponding oxygenated alloy, that is to say Ti ₂ RuAIO ₂ , several manufacturing avenues are open to us. It is in fact possible to use at the starting point a mixture of powder of Ru, Al and of TiO in the proportions 2 TiO + 1 Ru + 1 Al and to intensely grind the mixture for several hours. FIG. 3b shows the evolution of the x-diffraction spectra of such a mixture as a function of the grinding time. We see in this figure that it takes about twenty hours under the grinding conditions used for reveal the cubic structure characteristic of the materials of the invention. Furthermore, it is also possible to use starting from aluminum oxide AI ₂ Cb which is much cheaper than TiO to manufacture the alloy in question. Figure 3c shows a series of x-diffraction spectra obtained after different grinding times of a mixture of AbO ₃ + 3 Ti + 1 TiO + 2 Ru. After approximately 20 hours of grinding, we see the cubic phase appear as in the previous case. The equations corresponding to these two synthesis processes are indicated below

2 TiO + 1 Ru + 1 Al → Ti ₂ RuAIO ₂ (1)

AI ₂ O ₃ + 3 Ti + 1 TiO + 2 Ru → 2 Ti ₂ RuAIO ₂ (2)

It was very surprising to realize that aluminum oxide could be reduced by reaction with Ti during intense grinding. This discovery makes it possible to manufacture the materials of the present invention at low cost. Figure 3d shows the superposition of the x-diffraction spectra of the mixtures obtained after 40 hours of grinding in the case of the two synthesis experiments described above. We see in this figure that regardless of the manufacturing method, we get the same final product.

The ability of Ti to reduce alumina, that is to say aluminum oxide, is demonstrated in an exemplary manner in FIG. 4 where the evolution of the X-ray spectra of a mixture of Ti is presented. and AI ₂ O ₃ as a function of the grinding time. As the grinding takes place, the characteristic peaks of titanium and alumina disappear and the peaks of titanium oxide appear.

It is obvious that many other manufacturing methods (other than intense mechanical grinding) can be envisaged for manufacturing the materials of the present invention such as for example melting and rapid quenching, sintering etc. The above description concerning the methods of synthesis should therefore not be considered as limiting.

The prior US patent 5,662,834 describes that the addition of oxygen made it possible to stabilize the alloys of the Ti ₂ RuFe type within the framework of their use for the synthesis of sodium chlorate. It has now been discovered in the context of the present invention that this advantage was not unique to oxygen but that other elements also made it possible to stabilize not only the alloys of the Ti ₂ RuFe type but also the alloys of the Ti type. ₂ RuAI. These elements are B, S, C, N, Si, P and H in addition to oxygen.

Figure 5 shows x-diffraction spectra of a series of Ti ₂ RuFeB _x type alloys for different boron concentrations (x = 0, 0.1, 1, 2, 4, 6, 12) and Figure 6 shows micrographs of corresponding alloy powders. It can be seen in FIG. 7a that without adding boron, the overvoltage of the Ti ₂ RuFe alloy increases considerably after 20 cycles of OCP / HER whereas it remains rather stable and low in the case of Ti ₂ RuFeB _x alloys where x = 1, 4 and 6 (see figures 7c, 7d and 7e). When the boron content is too high, the properties deteriorate again (Figure 7f). FIG. 8 shows that under continuous electrolysis in an industrial chlorate solution, the Ti ₂ RuFeB ₆ alloy exhibits the best performance.

The alloys according to the present invention can therefore include stabilizing elements denoted by T according to the formula described above:

Ti ₂₊ t (RU ( _1-X ) AI (i _{+ X} )) (i _-u / ₂ ) M _u T _y

or

T represents one or more elements chosen from O, B, S, C, N, Si, P and H, this element preferably being oxygen and where y is a number between 0 and +6, preferably equal to 2

As mentioned previously, the present invention also relates to the use of these new alloys for the synthesis of sodium chlorate. While being as efficient (ie having a low overvoltage) as the materials of the prior art, the new alloys of the present invention are more stable and less expensive and therefore make it possible to synthesize sodium chlorate at a lower cost. cost. As indicated above, it is preferable that the microstructure of these new materials is nanocrystalline, that is to say that it consists of crystallites whose average size of the crystals is less than 100 nm.

The present invention also relates to a process aimed at manufacturing cathodes for the synthesis of sodium chlorate where, firstly, a powder of the alloy in question is manufactured by mechanically and intensely grinding (typically at powers greater than 0.2 kW / liter) a mixture of the constituents for a certain time (typically for durations greater than 20 hours) in a reactive medium or not, then, in a second stage, this powder which has or has not undergone a treatment is applied to a substrate ( typically a steel, titanium or aluminum plate) using one of the multiple thermal spraying techniques such as APS (atmospheric plasma spray), HVOF (high velocity oxyfuel), VPS (vacuum plasma spray) ) and LPPS (low pressure plasma spray).

The present invention also relates to an electrode comprising a substrate and a coating of an alloy according to what is described and / or illustrated in the present application. Preferably, the substrate is a conductive metal, a valve metal, or any other suitable metal / material, as evident to a person skilled in the art. It goes without saying that minor modifications could be made to what has just been described without departing from the scope of the present invention as defined in the appended claims.

Claims

1. An alloy of formula:

Ti2 + t (RU (i _-X ) AI (-i _{+ X} )) (i _-u / 2) MuTy

in which:

M represents one or more elements chosen from Ag, Pd, Rh, Fe, Cr and V;

T represents one or more elements chosen from O, B, S, C, N, Si, P and H;

t is a number between -1 and +1;

x is a number between -0.95 and +0.95;

u is a number between 0 and +1; and

y is a number between 0 and +6.

2. The alloy according to claim 1, in which M is chosen from Ag, Pd and Rh.

3. The alloy according to claim 1 or 2, wherein T is O.

4. The alloy according to any one of claims 1 to 3, in which t is between -0.5 and +0.5.

5. The alloy according to claim 4, in which t is equal to 0.

6. The alloy according to any one of claims 1 to 5, in which x is between +0.5 and +0.95.

7. The alloy according to claim 6, in which x is equal to 0.75.

8. The alloy according to any one of claims s 1 to 7, in which u is less than 0.25.

9. The alloy according to any one of claims 1 to 8, in which y is equal to 2.

10. A process for manufacturing a cathode for the synthesis of sodium chlorate, in which:

- Firstly, an alloy powder is made according to any one of Claims 1 to 9, by thoroughly grinding a mixture of the constituents for a certain time in a reactive medium or not and by optionally subjecting it to a treatment additional; and

- Secondly, the powder thus produced is projected onto a substrate using a thermal spraying technique.

11. Use of a cathode made from an alloy according to any one of claims 1 to 9 for the synthesis of sodium chlorate.

12. An electrode comprising a substrate and a coating of an alloy according to any one of claims 1 to 9.

13. An electrode according to claim 12, in which the substrate is a conductive metal.

14. The electrode of claim 12, wherein the substrate is a valve metal.