CA2492128A1 - Alloys of ti, ru and al and their use in the synthesis of sodium chlorate - Google Patents

Abstract

The invention relates to an alloy corresponding to the following formula: Ti 2 + t (Ru (1-x) Al (1 + x)) (1-u / 2) M u T y in which: t is a number between - 1 and +1 and preferably between -0.5 and + 0.5; x is a number between -0.95 and +0.95 and preferably between +0.5 and +0.95; 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 from Ag, Pd, Rh, Fe, Cr and V, the elements being preferably Ag, Pd or Rh, and T represents one or more elements selected from O, B, S, C, N, Si, P and H, the elements being preferably oxygen. This alloy is preferably in the nanocrystalline state and can be used as a cathode to electrochemically produce sodium chlorate.

Description

ALLOYS BASED ON Ti, Ru AND AI
AND
USE THEREOF FOR SYNTHESIS OF SODIUM CHLORATE
FIELD OF THE INVENTION
The subject of the present invention is new alloys based on Ti, Ru and HAVE. It also relates to the use of these new alloys for the electrochemical synthesis of sodium chlorate.
TECHNOLOGICAL BACKGROUND
Sodium chlorate (NaClO3) is a bleaching agent in the industry peas and paper. For environmental reasons, the trend towards recent years, has been to use chlorine dioxide (CI02) rather chlorine gas (CI2) to whiten the dough to reduce chlorinated organic products. Chlorine dioxide is obtained by reducing the sodium chlorate. This trend has had the effect of increasing significantly the demand for sodium chlorate.
The overall chemical reaction used in electrochemical cells of sodium chlorate synthesis is as follows NaCl + 3H 2 O - ~ NaClO 3 + 3H 2 The first step in the electrochemical manufacture of sodium chlorate is the anodic discharge of chlorine (CI-) at the anode with hydrogen production (H2) as a complementary reaction to the cathode. Nowadays, we use frequently steel or titanium as electrode material at the cathode. The cathodic overvoltage of steel electrodes is about 900 mV at a current density of 250 mA cm-2. This surge is practically an order

two of magnitude larger than the anode surge and is therefore even the main source of energy loss from the electrochemical process of synthesis of sodium chlorate. Substantial energy savings can be achieved by replacing the existing steel cathodes with better performing materials with lower surge cathode.
U.S. 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 about mV overvoltage at the cathode. This gain corresponds to an improvement of approximately 10% of the overall energy efficiency of the process. However, mention is made of in this patent that: "Whatever the case, the alloy must contain ruthenium.
However, the amount of ruthenium should not be too high because a the high price of this metal, and secondly and above all, its lack of stability when used in an electrolyte solution. ". It would be therefore desirable to find an element that can partly replace the Ru for reduce the cost of these new electrodes and improve their stability while in retaining their good electrocatalytic properties ie a low cathodic overvoltage of the order of 600 mV at a current density of 250mA / 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 the Ru in materials similar to those of US Pat. No. 5,662,834 and this in quantity significant without losing the advantageous properties of these new cathode materials.
To meet the requirements of sodium chlorate manufacturers, Cathode materials should ideally fulfill the following conditions

3 1) have a low surge, 2) be inactive against the decomposition of hypochlorite, 3) weakly reduce hypochlorite and chlorate at the cathode,

4) do not interfere with the surface chromium hydroxide layer,

5) be stable under open circuit conditions at operating temperatures,

6) be resistant to embrittlement by hydrogen,

7) have a life 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 the patent US
5,662,834 contain Fe and one or more metals that can be substituted for iron including V and Cr. In the context of the present invention, he has discovered that AI could not only good part of the Ru but also, when the AI is in the presence of the Ti and of Ru in alloys of the Ti2RuM type where M = Al, the materials have less tendency to absorb hydrogen during hydrogen evolution reactions than the corresponding alloys where M = Fe, V, and Cr. The materials based of the present invention are therefore more resistant to embrittlement by hydrogen and therefore more stable than the materials of the prior art. Of more generally it was discovered that alloys of the type above mentioned where M = AI, Ag, Pd, Rh are less likely to absorb hydrogen that alloys where M = Fe, Cr, V.
The invention therefore has for its first object a new alloy characterized in that it responds to the following formula Ti 2 + t ~ Ruc ~ -x ~ Al ~ 1 + X ~) ~ c ~ -ur) M ~ TV
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 2 M represents one or more elements selected from Ag, Pd, Rh, Fe, Cr and V, the element or elements being preferably Ag, Pd or Rh and T represents one or more elements selected from O, B, S, C, N, Si, P and H, the element or elements being preferably oxygen.
This new alloy is preferably in the nanocrystalline state and can be in use advantageously as a cathode for producing electrochemically the sodium chlorate. "Nanocrystalline state" means a microstructure consisting of crystallites whose crystal size is less than 100 nm.
Another subject of the present invention is the use of this new alloy for the electrochemical synthesis of sodium chlorate. For this end, the new alloy can be in the nanocrystalline state or not. Preferably, he is in a nanocrystalline state to give rise to small surges cathode.
The alloys according to the invention can be prepared in different ways.
We can first prepare them in powder form by grinding intensely a mixture of elements selected from Ti, Ru, Al and the elements M and T in adequate proportion to obtain the desired composition or mixture of compounds selected by combining various elements among Ti, Ru, Al, and elements M and T. The grinding can be carried out in an inert atmosphere, in air or in a reactive atmosphere including in the reaction medium of the elements chosen from among the elements T. For example, it is possible to grind in a reactive medium, such as an oxygen (oxidizing) atmosphere, an atmosphere hydrogen (reductive) or even a nitrogen atmosphere. The temperature and the pressure of the grinding media gases can be adjusted as needed. The powder thus manufactured, which can be in a nanocrystalline state or not depending on the grinding conditions (such as the duration and the intensity of grinding), is subsequently used to prepare electrodes that 5 will serve as the cathode for producing sodium chlorate. previously, this powder can be modified by various treatments, such as a treatment thermal, agglomeration treatment, etc ... We can finally compress the powder or project it on various substrates to make cathodes. The Thermal spraying is particularly well suited for this purpose. We can use for example techniques such as plasma projection: 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 nature but preferably, steel, titanium aluminum. It may also be desirable to apply an undercoat to the interface between the substrate and the catalytic coating made from of the powder in order to improve the adhesion of the latter and the life time of electrodes.
The alloys of the prior art described in US Pat. No. 5,662,834 contain oxygen. Although the oxygen content had little effect on the activity electrochemical and therefore, on the value of the surge, its presence in the alloy is beneficial on the long-term stability of the electrodes.
In the In the context of the present invention, it was discovered that this advantage was not not only associated with oxygen but that several other elements among the T elements described above could provide better long-term stability term of Ti, Ru and Al-based electrode materials.
The invention and its advantages will be better understood when reading the more detailed but non-limiting description which will follow, made in referring in the accompanying drawings.

BR ~ VE PRESENTATION OF THE DRAWINGS
FIG. 1 represents polarization curves of alloys of the type Ti2Ru ~ _XM ~ + X for various elements M;
FIG. 2a represents an overvoltage curve as a function of the number of open circuit cycles and hydrogen evolution for alloys of the type Ti2RuM where M represents various elements;
FIG. 2b represents a curve similar to that of FIG. 2a for materials of the type Ti2Ru ~ _XAI ~ + X having various concentrations of aluminum;
FIG. 2c represents photographs of alloys of the Ti2Ru ~ _XAI ~ + x type.
after 50 cycles of open circuit and hydrogen evolution;
FIG. 2d represents overvoltage curves as a function of the number of cycles for oxygenated and non-oxygenated materials, the photograph at the top on the right, representing the oxygenated alloy after 50 cycles;
FIG. 3a shows an X-ray diffraction spectrum of a sample obtained after 40 hours of grinding a mixture of powders of two Ti, an AI and a Ru;
FIG. 3b represents a series of X-ray diffraction spectra of a sample obtained after different grinding times of a mixture of powders two TiO, one AI and one Ru;
FIG. 3c represents a series of X-ray diffraction spectra of a sample obtained after different grinding times of a mixture of powders one AI203, three Ti, one Ti0 and two Ru; and FIG. 3d represents a diffraction-x spectrum of the Ti2RuAlO2 obtained after 40h of grinding a powder mixture containing TiO (according to a manufacturing procedure corresponding to that of Figure 3b), in comparison to that of a powder mixture containing AIZ03 prepared according to a procedure corresponding to that of Figure 3c;
FIG. 4 represents the evolution of the X-ray spectra of a mixture of Ti and AI203 powder as a function of the grinding time;
FIG. 5 represents X-ray spectra of various alloys of the type Ti2RuFeBX for different boron concentration;
FIG. 6 shows micrographs of alloys of the Ti2RuFeBx type for different concentrations of boron;
Figure 7 shows the evolution curves of the overvoltage as a function of the number of open circuit cycles and hydrogen evolution for various alloys of the Ti2RuFeBx type with different boron concentrations; and FIG. 8 represents curves of evolution of the overvoltage as a function of electrolysis time for three alloys of the Ti2RuFeBx type.
DETAILED DESCRIPTION OF THE INVENTION
As previously indicated, the alloy according to the invention corresponds to the formula next chemical Ti 2 + t ~ ~ Rut -X AI ~ ~~ + X>) c ~ -urz) Ty M ~
in which oh t is a number between -1 and +1, preferably between -0.5 and + 0.5, and more preferentially 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 2 M represents one or more elements selected from Ag, Pd, Rh, Fe, Cr and V, preferably Ag, Pd and Rh and T represents one or more elements selected from O, B, S, C, N, Si, P and H, preferably oxygen.
A special case is where y = 0, u = 0 and t = 0 ie Ti2Ru _XAI ~ ~ + X. For this particular case Figure 1 shows polarization curves obtained in 4M NaOH solution at 25 ° C. The cathode sample used for this trial was a pressed powder pellet of Ti2Ru ~ _XM ~ + X (or M = AI, Pd and Rh) manufactured by intensely grinding a mixture of Ti, Ru and M element.
The anode was a Pt wire and the reference Ag / AgCl. The correction for the fall Ohmic was made by the current interruption method. We notice that a current density of 250 mAlcm 2, the voltage is the lowest for alloys of the type TI2RU ~ AI ~ and TI2RUp, 7 ~; I1.25 FOLLOWING alloys of the type Ti2Ruo.5Al ~ .5 and finally, the alloys of the type Ti2Ru ~ Pd ~ and Ti2Ru ~ Rh ~ possess the highest voltages. Table I below summarizes the values of corresponding surge Surge voltage of samples of type Ti2Ru ~ _xAl ~ + X
Ti2Ru ~ _XM ~ + X n 2so (V) AI ~ - 0.064 AI ~, 25 -0.067 AI ~, 5p (1) -0.108 AI ~, 5o (2) -0.105 This particular example shows that alloys based on Ti, Ru and AI
have low surges and are therefore very effective as cathode materials if the AI concentration is appropriately chosen. The better values being obtained for AI ~ and AI ,, 25.
Stability tests were also performed on these electrodes in performing alternating HER hydrogen evolution cycles (polarization for 10 minutes) followed by an OCP open circuit period (for 10 minutes). These tests were carried out with a chlorate solution typical industrial at 70 ° C and pH 6.5. The results are presented to Figure 2a. It can be seen in this figure that the surge values at 250 mA / cm2 remain stable, low and between -0.55 and -0.65 a period of more than 50 cycles for all alloys of the type Ti2RuM or M =
AI, Ag, Rh and Pd. On the other hand, for the alloy of the Ti2RuFe type corresponding to art previous, overvoltage starts to increase significantly after 22 cycles to degrade significantly around thirty or so cycles.
Figure 2b shows similar curves for alloys of the type Ti2Ru ~ _ XAI ~ + X or x = 0, 0.25 and 0.50 respectively and Figure 2c shows appearance of these last electrodes after having undergone the 50 cycles OCP / HER. The electrodes that have the best surges, ie the electrodes Ali and AI ~ .zS (see above), are also the ones that seem the best resist long-term cycling. Indeed, as shown in Figure 2c, the Ti2Ruo.5Al ~ .5 alloy seems to have deteriorated quite a bit during the 50's cycles OCP / H ER.

As was the case with Ti-based prior art alloys, Ru and of Fe, the addition of oxygen has a beneficial effect on long-term stability of the Ti, Ru and AI based electrodes. Indeed, Figure 2d shows that the addition of oxygen to alloys with a high aluminum content (TI2RUp, 25A11.75 ~ 2), which

10 as we have seen earlier tend to deteriorate at cycling, stabilizes them. The image of the electrode Ti2Ruo.25A1 ~. ~ 502 after 50 cycles, which can be seen in Figure 2d, shows that it resists very good to this treatment if it contains oxygen.
It has also been discovered that the crystallographic structure of Ti2RuAl is the same as that of the Ti2RuFe alloy of the prior art. Indeed, the figure 3a shows the diffraction-x spectrum of a Ti2RuAl alloy obtained by grinding intensively for 40 hours a mixture of Ti powder, Ru and AI in the desired proportions. This figure reveals that the structure is simple cubic.
With regard to the corresponding oxygenated alloy, that is to say the Ti2RuAl02, several avenues of manufacture are available to us. We can indeed use at starting point a mixture of powder of Ru, Al and TiO in the proportions 2 Ti0 + 1 Ru + 1 Al and grind the mixture intensively for several hours. FIG. 3b shows the evolution of the diffraction-x spectra of such a mixing according to the grinding time. We see on this figure that it is necessary about twenty hours under the grinding conditions used to bring out the characteristic cubic structure of the materials of the invention. Moreover, it is also possible to use from oxide aluminum AI203 which is much cheaper than the Ti0 to manufacture the alloy in question. Figure 3c shows a series of spectra of diffraction-x obtained after different grinding times of a mixture of Al 2 O 3 + 3 Ti + 1

11 Ti0 + 2 Ru. After about 20 hours of grinding, the phase appears cubic as in the previous case. The equations corresponding to these two synthesis processes are shown below 2 Ti0 + 1 Ru + 1 Al - + Ti2RuA102 (1) AI203 + 3 Ti + 1 Ti0 + 2 Ru - ~ 2 TizRuAl02 (2) It was very surprising to realize that aluminum oxide could be reduced by reaction with Ti during intense grinding. This discovery makes kind it is possible to manufacture at a low cost the materials of the present invention. Figure 3d shows the superposition of the diffraction-x spectra of the mixtures obtained after 40 hours of grinding in the case of both synthesis experiments previously described. We see on this figure that Regardless of the method of manufacture, the same final product is obtained.
The ability of Ti to reduce alumina, ie aluminum oxide is exemplary in Figure 4, where the evolution of X-ray spectra of a mixture of Ti and Al 2 O 3 as a function of the time of grinding. As the grinding takes place, we see the disappearance of peaks features of titanium and alumina and sees the peaks of the oxide of titanium.
It is obvious that many other manufacturing methods (other than grinding intense mechanics) can be envisaged to manufacture the materials of the the present invention, such as, for example, melting and quenching, sintering etc. The foregoing description of methods of synthesis should therefore not to be considered as limiting.
US Pat. No. 5,662,834 discloses that the addition of oxygen allows stabilize alloys of the Ti2RuFe type as part of their use for the synthesis of sodium chlorate. It has now been discovered as part of

12 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 Ti2RuFe type but also alloys of the Ti2RuAl type. These elements are B, S, C, N, Si, P and H in addition to oxygen.
S
Figure 5 shows x-ray diffraction spectra of a series of alloys of type Ti2RuFeBX for different boron concentrations (x = 0, 0.1, 1, 2, 4, 6, 12) and Figure 6 shows micrographs of powders of corresponding alloys. We can see in Figure 7a that without addition of boron, the overvoltage of the alloy Ti2RuFe increases considerably after 20 cycles of OCP / HER then that it remains fairly stable and low in the case of Ti2RuFeBx alloys where x = 1, 4 and 6 (see Figures 7c, 7d and 7e). When the boron content is too much big the properties deteriorate again (Figure 7f). Figure 8 shows that continuous electrolysis condition in an industrial chlorate solution, alloy Ti2RuFeB6 has the best performance.
Alloys according to the present invention may therefore include elements stabilizers denoted by T according to the formula described above Ti2 + t (Ru ~~ ~ _x + Al ~~ X ~) c ~ - '~ M i2 "Ty or T represents one or more elements selected from O, B, S, C, N, Si, P and H, this element being preferably oxygen and where y is a number between 0 and +6, preferably 2 As mentioned above, the present invention also relates to the use of these new alloys for the synthesis of sodium chlorate. All by being as efficient (ie having a small surge) that the materials of the prior art, the new alloys of the present invention are

13 more stable and cheaper and therefore allow to synthesize the sodium chlorate at a lower cost. As noted above, it is it is preferable that the microstructure of these new materials be nanocrystalline that is to say, it consists of crystallites whose average size of the crystals is less than 100 nm.
Another subject of the present invention is a process for manufacturing cathodes for the synthesis of sodium chlorate where at first, one manufactures a powder of the alloy in question by mechanically grinding and intensively (typically at powers greater than 0.2 kW / liter) a mixture of constituents for a period of time (typically for durations longer than 20 hours) in a reactive medium or not, then in a second time, we project this powder that has or has not been treated on a substrate (typically a steel, titanium or aluminum plate) to ugly of one of the many thermal projection techniques such as APS (atmospheric plasma spray), HVOF (high velocity oxyfuel), VPS
(vacuum plasma spray) and LPPS (low pressure plasma spray).
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 such as defined in the appended claims.

Claims (11)

1. An alloy of formula:
Ti2+t(Ru(1-x)Al(1+x))(1-u/2)M u T y 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, in which 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 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. Process for manufacturing a cathode for the synthesis of chlorate sodium, in which - initially, we manufacture a powder of an alloy according to one any of claims 1 to 9, by intensively grinding a mixture constituents for a certain time in a reactive or non-reactive medium and by optionally subjecting him to additional treatment; And - secondly, the powder thus produced is projected onto a substrate using a thermal spray technique. 11. Use of a cathode made from an alloy according to one any of claims 1 to 9 for the synthesis of sodium chlorate.