ELECTRODE FOR GENERATING CHLORINE, AND METHOD FOR MANUFACTURING SAME

Abstract

 Provided is an electrode for generating chlorine, the electrode having high chlorine generating efficiency and excellent long-term durability even when used in electrolysis of low-concentration saltwater. A chlorine generating electrode comprising a conductive substrate, and a catalyst layer provided on the conductive substrate, the catalyst layer containing at least palladium oxide, ruthenium oxide and titanium oxide, the palladium oxide being in the form of particles having an average particle size of 5 µm or less.

Description

TECHNICAL FIELD

[0001] The present invention relates to a chlorine generating electrode, and particularly to an electrode to be used for on-site generation of sodium hypochlorite using dilute saltwater as in seawater electrolysis, and a method for producing the chlorine generating electrode.

BACKGROUND ART

[0002] A method for generating a hypochlorite by electrolysis of saltwater has been heretofore known, and use of a coating film of a mixed metal oxide as an electrode is widely known in the art.

[0003] For example, Patent Document 1 discloses an anode obtained by covering titanium or a titanium alloy with a mixture including a platinum group metal ternary mixture of platinum-palladium oxide-ruthenium dioxide having a composition of 3 to 42% by weight of platinum, 3 to 34% by weight of palladium oxide and 42 to 94% by weight of ruthenium dioxide; and 20 to 40% by weight of titanium dioxide based on the amount of the mixture.

[0004] In addition, in Patent Document 2, particularly an electrode for producing chlorine and a hypochlorite is a film of a mixed oxide of a platinum group metal oxide and a valve metal oxide, the mixed oxide includes platinum group metal oxides of ruthenium, palladium and iridium, and an oxide of titanium, the molar ratio of the platinum group metal oxides to the valve metal oxide is 90 : 10 to 40 : 60, the molar ratio of ruthenium to iridium is 90 : 10 to 50 : 50, and the molar ratio of palladium oxide to ruthenium oxide and iridium oxide is 5 : 95 to 40 : 60.

[0005] In addition, Patent Document 3 proposes a hypochlorite producing anode which includes a film containing 10 to 45% by weight of palladium oxide, 15 to 45% by weight of ruthenium oxide, 10 to 40% by weight of titanium dioxide, 10 to 20% by weight of platinum, and 2 to 10% by weight of an oxide of at least one metal selected from cobalt, lanthanum, cerium and yttrium.

PRIOR ART DOCUMENTS

PATENT DOCUMENTS

[0006] 

Patent Document 1: Japanese Patent Publication No. S59-24192

Patent Document 2: Japanese Translation of PCT International Patent Application No. 2008-528804

Patent Document 3: Japanese Patent No. 3319880

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] An electrode having a platinum group oxide as described in Patent Documents 1 to 3 has high oxidation efficiency of chloride ions, and is capable of generating high-concentration hypochlorite ions with a high chlorine generation efficiency of more than 90%, so that a high-concentration hypochlorite can be obtained with a lower electric power consumption rate as compared to a conventional anode.

[0008] However, this is based on the premise that saltwater (a sodium chloride aqueous solution) with a high concentration of 2.5 to 32% is used as an electrolytic solution, and when more dilute saltwater, e.g. saltwater with a concentration of 1% or less, which can be used for ballast water etc., is used as an electrolytic solution, there is the problem that chlorine generation efficiency is markedly reduced.

[0009] Further, when such low-concentration saltwater is used as an electrolytic solution, there is the problem that a high voltage is required for electrolysis of saltwater, and therefore there is the problem that a large burden is imposed on an electrode, so that the electrode has a reduced life.

[0010] In view of the circumstances described above, a main object of the present invention is to provide a chlorine generating electrode which has high chlorine generation efficiency, and is excellent in long-term durability even when used for electrolysis of low-concentration saltwater. Further, an object of the present invention is to provide a method for producing the chlorine generating electrode, a method for producing a hypochlorite using the chlorine generating electrode, and an electrolytic cell including the electrode.

MEANS FOR SOLVING THE PROBLEMS

[0011] The present inventors have extensively conducted studies for achieving the above-mentioned object. As a result, the present inventors have devised a chlorine generating electrode including a conductive substrate, and a catalyst layer provided on the conductive substrate, the catalyst layer containing at least palladium oxide, ruthenium oxide and titanium oxide, the palladium oxide being in the form of particles having an average particle size of 5 µm or less, and found that the chlorine generating electrode has high chlorine generation efficiency, and is excellent in long-term durability even when used for electrolysis of low-concentration saltwater. The present invention has been completed by further conducting studies based on the above-mentioned findings.

[0012] That is, the present invention provides an invention of the aspects described below.

[0013] Item 1. A chlorine generating electrode including a conductive substrate, and a catalyst layer provided on the conductive substrate,
the catalyst layer containing at least palladium oxide, ruthenium oxide and titanium oxide,
the palladium oxide being in the form of particles having an average particle size of 5 µm or less.

[0014] Item 2. The chlorine generating electrode according to item 1, wherein the catalyst layer has an X-ray diffraction peak intensity of 500 cps or more at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray.

[0015] Item 3. The chlorine generating electrode according to item 1 or 2, wherein the catalyst layer has an X-ray diffraction peak half-value width of 1.5 deg or less at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray.

[0016] Item 4. The chlorine generating electrode according to any one of items 1 to 3, wherein the ratio of palladium metal contained in the catalyst layer is 1 mol% or more where the content of metal elements in the catalyst layer is 100 mol%.

[0017] Item 5. The chlorine generating electrode according to any one of items 1 to 4, which is used for electrolysis of saltwater with a concentration of 1% or less.

[0018] Item 6. A method for producing a chlorine generating electrode including a conductive substrate, and a catalyst layer provided on the conductive substrate, the method including:

a coating step of coating a conductive substrate with a solution containing at least a palladium compound, a ruthenium compound and a titanium compound; and

a firing step of firing the conductive substrate coated with the solution,
the method including

using palladium oxide particles having an average particle size of 5 µm or less as the palladium compound.

[0019] Item 7. A method for producing a chlorine generating electrode including a conductive substrate, and a catalyst layer provided on the conductive substrate, the method including:

a coating step of coating a conductive substrate with a solution containing at least a palladium compound, a ruthenium compound and a titanium compound; and

a firing step of firing the conductive substrate coated with the solution, the method including

using at least one of palladium chloride and palladium nitrate as the palladium compound,

the firing step including heating the conductive substrate at a temperature of 400 to 600°C to generate palladium oxide particles having an average particle size of 5 µm or less from the palladium chloride.

[0020] Item 8. The method for producing a chlorine generating electrode according to item 6 or 7, wherein the catalyst layer formed by the firing step has an X-ray diffraction peak intensity of 500 cps or more at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray.

[0021] Item 9. The method for producing a chlorine generating electrode according to any one of items 6 to 8, wherein the catalyst layer formed by the firing step has an X-ray diffraction peak half-value width of 1.5 deg or less at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray.

[0022] Item 10. An electrolytic cell including the chlorine generating electrode according to any one of items 1 to 5.

[0023] Item 11. A method for producing a hypochlorite, the method including the step of electrolyzing a metal chloride aqueous solution using the chlorine generating electrode according to any one of items 1 to 5.

ADVANTAGES OF THE INVENTION

[0024] According to the present invention, there can be provided a chlorine generating electrode which has high chlorine generation efficiency, and is excellent in long-term durability even when used for electrolysis of low-concentration saltwater. Further, according to the present invention, there can be provided a method for producing the chlorine generating electrode, a method for producing a hypochlorite using the chlorine generating electrode, and an electrolytic cell including the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 

Fig. 1 shows a SEM image of a surface of a catalyst layer of an anode obtained in Example 1 (magnification: 5,000).

Fig. 2 shows a SEM image of a surface of a catalyst layer of an anode obtained in Example 2 (magnification: 5,000).

Fig. 3 shows a SEM image of a surface of a catalyst layer of an anode obtained in Example 3 (magnification: 5,000).

Fig. 4 shows a SEM image of a surface of a catalyst layer of an anode obtained in Example 4 (magnification: 10,000).

Fig. 5 shows a SEM image of a surface of a catalyst layer of an anode obtained in Example 5 (magnification: 10,000).

Fig. 6 shows a SEM image of a surface of a catalyst layer of an anode obtained in Comparative Example 1 (magnification: 5,000).

Fig. 7 shows a SEM image of a surface of a catalyst layer of an anode obtained in Comparative Example 2 (magnification: 5,000).

Fig. 8 shows a SEM image of a surface of a catalyst layer of an anode obtained in Comparative Example 3 (magnification: 5,000).

Fig. 9 shows a graph showing a relationship between a diffraction peak 2θ (°) and a peak intensity (cps) as measured by X-ray diffraction for the catalyst layer of the anode obtained in Example 2.

Fig. 10 shows a graph showing a relationship between a diffraction peak 2θ (°) and a peak intensity (cps) as measured by X-ray diffraction for the catalyst layer of the anode obtained in Example 3.

Fig. 11 shows a graph showing a relationship between a diffraction peak 2θ (°) and a peak intensity (cps) as measured by X-ray diffraction for the catalyst layer of the anode obtained in Example 4.

Fig. 12 shows a graph showing a relationship between a temperature and chlorine generation efficiency at a saltwater concentration of 0.1%.

Fig. 13 shows a graph showing a relationship between a temperature and chlorine generation efficiency at a saltwater concentration of 0.15%.

Fig. 14 shows a graph showing a relationship between a temperature and chlorine generation efficiency at a saltwater concentration of 0.5%.

Fig. 15 shows a graph showing an electrolysis time and a time-dependent change in chlorine generation efficiency in use of the anodes of Example 4 and Comparative Example 1.

EMBODIMENTS OF THE INVENTION

[0026] A chlorine generating electrode of the present invention includes a conductive substrate, and a catalyst layer provided on the conductive substrate. In the chlorine generating electrode of the present invention, the catalyst layer contains at least palladium oxide, ruthenium oxide and titanium oxide, and the palladium oxide is in the form of particles having an average particle size of 5 µm or less. The chlorine generating electrode of the present invention includes such a specific catalyst layer, and is therefore capable of exhibiting high chlorine generation efficiency and excellent long-term durability even when used for electrolysis of low-concentration saltwater (e.g. saltwater with a concentration of 1% or less) (e.g. the electrolytic solution is low-concentration saltwater). Hereinafter, the chlorine generating electrode of the present invention will be described in detail.

[0027] The chlorine generating electrode of the present invention includes a conductive substrate and a catalyst layer. The material of the conductive substrate is not particularly limited, and examples thereof include materials that are used for known chlorine generating electrodes. Specific examples of the material of the conductive substrate include valve metals such as titanium, tantalum, zirconium and niobium, and alloys of two or more valve metals. The shape of the conductive substrate is not particularly limited, and examples thereof include a plate shape, a disk shape, a rod shape, a cylindrical shape, an expanded metal shape and a punching metal shape.

[0028] A surface of the conductive substrate may be subjected to a sandblasting treatment (surface roughening treatment) or the like as necessary for the purpose of, for example, exhibiting an anchor effect on the catalyst layer. The sandblasting treatment is a surface treatment method in which a high-pressure gas containing sand-like particles is sprayed to a surface of a material. The sandblasting treatment can be performed by a known method. For example, the surface roughness of the conductive substrate can be controlled by adjusting the type of a polishing agent to be used, the treatment time and the like. Examples of the material of the sand-like particles include alumina, glass and iron. Further, after the sandblasting treatment, a degreasing treatment or the like may be performed as necessary.

[0029] While depending on the size of particles that are used for the sandblasting treatment etc., the surface roughness Ra (arithmetic mean roughness) of the conductive substrate surface subjected to the roughening treatment is, for example, in a range of about 0.5 to 10 µm. By changing the size of particles that are used for the sandblasting treatment, the surface roughness Ra can be set outside the above-mentioned range.

[0030] The surface of the conductive substrate may be subjected to a surface treatment with an acid or the like. The acid is not particularly limited, and examples thereof include sulfuric acid, nitric acid, hydrochloric acid, oxalic acid and hydrofluoric acid.

[0031] The thickness of the conductive substrate is not particularly limited, and can be appropriately set according to, for example, a size of an electrolytic cell to be provided with the chlorine generating electrode. The thickness of the conductive substrate is, for example, about 0.5 to 10 mm.

[0032] In the chlorine generating electrode of the present invention, a catalyst layer is provided on the conductive substrate. The catalyst layer contains at least palladium oxide, ruthenium oxide and titanium oxide. More specifically, a surface of the conductive substrate is provided with a film including the catalyst layer.

[0033] In the chlorine generating electrode of the present invention, the average particle size of palladium oxide particles contained in the catalyst layer is 5 µm or less. As described above, a conventional chlorine generating electrode has the problem that, for example, when low-concentration saltwater is used as an electrolytic solution, chlorine generation efficiency is considerably reduced, and when low-concentration saltwater is used as an electrolytic solution, a high voltage is required for electrolysis of saltwater, and therefore a large burden is imposed on the electrode, so that the electrode has a reduced life. On the other hand, in the present invention, the catalyst layer contains palladium oxide, ruthenium oxide and titanium oxide, and the average particle size of palladium oxide is set to 5 µm or less, and thus the chlorine generating electrode is capable of exhibiting high chlorine generation efficiency and excellent long-term durability even when used for electrolysis of low-concentration saltwater.

[0034] The reason for this can be considered, for example, as follows. That is, in comparison of the surface areas of palladium oxide dispersed on the catalyst layer at the same molar ratio, the smaller the average particle size of palladium oxide, the larger the surface area thereof, and thus the larger the number of active points. Thus, it is considered that since the average particle size of palladium oxide contained in the catalyst layer is 5 µm or less, a function as a catalyst is improved. The chlorine generating electrode of the present invention is capable of exhibiting high chlorine generation efficiency particularly when used for electrolysis of a metal chloride aqueous solution (particularly saltwater) with a low concentration of about 0.1 to 1%.

[0035] The average particle size of palladium oxide may be 5 µm or less, but from the viewpoint of further improving the chlorine generation efficiency of the chlorine generating electrode of the present invention, and also further improving the long-term durability of the electrode, the average particle size of palladium oxide is preferably about 0.01 to 5 µm, more preferably about 0.01 to 2.5 µm, still more preferably about 0.1 to 1.8 µm.

[0036] When palladium oxide, ruthenium oxide, and titanium oxide are used as raw materials in the chlorine generating electrode of the present invention, the average particle size thereof is a value measured under the following conditions.

(Measurement of average particle size)

[0037] Measuring equipment: Laser scattering particle distribution measuring apparatus LA-950 (manufactured by HORIBA, Ltd.)

[0038] Measurement method: Suction is started for increasing the sample dispersion force. Thereafter, compressed air is supplied at 0.4 to 0.8 MPa to perform forced dispersion. A state with no sample is measured as a blank. The intensity of a feeder is adjusted, and measurement is started when the transmittance reaches 70 to 90%.

[0039] Measurement conditions: The transmittance is 70 to 90%, the number of repetitions is 30, and the particle size is on a volume basis.

[0040] On the other hand, when the average particle sizes of palladium oxide, ruthenium oxide, and titanium oxide are measured by observing the catalyst layer of the chlorine generating electrode of the present invention, each of the average particle sizes of the particles of these oxides is an average of the major axes of 20 particles present in a field of view of a SEM image of the catalyst layer.

[0041] In the present invention, the ratio of palladium oxide contained in the catalyst layer is not particularly limited, but from the viewpoint of further improving the chlorine generation efficiency of the chlorine generating electrode of the present invention, and also further improving the long-term durability of the electrode, the ratio of palladium metal contained in the catalyst layer is preferably 1 mol% or more, more preferably 1 to 90 mol%, further more preferably 3 to 75 mol%, especially preferably 5 to 75 mol% based on 100 mol% of metal elements contained in the catalyst layer.

[0042] From the same viewpoint as described above, the ratio of ruthenium metal contained in the catalyst layer is preferably 1 mol% or more, more preferably 1 to 90 mol%, further more preferably 3 to 70 mol%, especially preferably 3 to 50 mol%. From the same viewpoint as described above, the ratio of titanium metal contained in the catalyst layer is preferably 1 mol% or more, more preferably 5 to 90 mol%, further more preferably 10 to 70 mol%, especially preferably 15 to 60 mol%.

[0043] In addition to palladium oxide, ruthenium oxide and titanium oxide, other components may be contained in the catalyst layer in the present invention. The other components include platinum group metals or platinum group metal oxides, and specific examples thereof include platinum, iridium oxide and rhodium oxide. In addition, the catalyst layer may contain transition metal oxides such as manganese oxide, cobalt oxide and chromium oxide, valve metal oxides such as tantalum oxide, zirconium oxide and niobium oxide. The ratio of the other components is preferably 60 mol% or less, more preferably 5 to 50 mol%, further more preferably 5 to 40 mol% in terms of a ratio of metals contained in the catalyst layer.

[0044] Further, from the viewpoint of further improving the chlorine generation efficiency of the chlorine generating electrode of the present invention, and also further improving the long-term durability of the electrode, the X-ray diffraction peak intensity at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray is preferably 500 cps or more, more preferably 500 to 4000 cps, further more preferably 1000 to 4000 cps, especially preferably 1300 to 4000 cps in the present invention. When the peak intensity is 500 cps or more, stable chlorine generation efficiency can be suitably maintained. In addition, as the peak intensity increases, the crystallinity of palladium oxide becomes higher, so that a function as a catalyst can be suitably exhibited.

[0045] From the same viewpoint as described above, the X-ray diffraction peak half-value width at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray is preferably 1.5 deg or less, more preferably 0.1 to 1.0 deg, further more preferably 0.1 to 0.9 deg, especially preferably 0.1 to 0.8 deg. When the peak width is 1.5 deg or less, stable chlorine generation efficiency can be suitably maintained. In addition, as the peak width decreases, the crystallinity of palladium oxide becomes higher, so that a function as a catalyst can be suitably exhibited.

[0046] In the present invention, X-ray diffraction measurement of the catalyst layer is performed under the following conditions.

(X-Ray diffraction measurement)

[0047] Measuring equipment: Ultima IV manufactured by Rigaku Corporation

[0048] Measurement method: A measurement sample is placed in such a manner that the sample can be irradiated with an X-ray from the measurement equipment main body. A current and a voltage are applied to age the sample, and then the sample is irradiated with an X-ray to perform measurement.

[0049] X-ray source: CuKα ray

[0050] Output setting: 40 kV, 40 mA

[0051] Optical conditions in measurement:

divergence slit = 0.2 mm

scattering slit = 2°

photoreception slit = 0.15 mm

Position of diffraction peak: 2θ ≒ 34°

Measurement range: 5° to 90°

Scan speed: 20°/min

Preparation of sample: the electrode is cut to 35 mm × 50 mm × 1 mm.

[0052] The thickness of the catalyst layer is not particularly limited, and can be appropriately set according to, for example, a size of an electrolytic cell to be provided with the chlorine generating electrode. The thickness of the catalyst layer is, for example, about 0.1 to 10 µm.

[0053] In the present invention, the catalyst layer can be formed, for example, as follows. First, a coating step of coating a conductive substrate with a solution containing at least a palladium compound, a ruthenium compound and a titanium compound is carried out. Here, the ratio of the palladium compound, the ruthenium compound and the titanium compound is adjusted so as to coincide with the foregoing ratio of palladium metal, ruthenium metal and the titanium metal in the catalyst layer. The above-mentioned other components may be added to the solution.

[0054] The palladium compound is not particularly limited as long as it forms palladium oxide in the catalyst layer after the later-described firing step, and examples of the palladium compound include palladium oxide, palladium chloride and palladium nitrate. Among them, palladium oxide and palladium chloride are preferable. The ruthenium compound is not particularly limited as long as it forms ruthenium oxide in the catalyst layer after the later-described firing step, and examples of the ruthenium compound include ruthenium oxide, ruthenium chloride and ruthenium nitrate. Among them, ruthenium oxide is preferable. The titanium compound is not particularly limited as long as it forms titanium oxide in the catalyst layer after the later-described firing step, and examples of the titanium compound include butyl titanate, titanium alcoholate and titanium trichloride. Among them, butyl titanate and titanium alcoholate are preferable. The liquid to be used for the solution is not particularly limited, and examples thereof include organic solvents such as n-butanol, propanol, and hexanol.

[0055] For example, for the catalyst layer to contain palladium oxide, it is preferable that the solution contain palladium oxide, but even when palladium chloride, palladium nitrate or the like is used, palladium chloride, palladium nitrate or the like may be converted into palladium oxide in the later-described firing step. For example, in the coating step, at least one of palladium chloride and palladium nitrate is used as a palladium compound, and in the later-described firing step, the palladium compound is heated at a temperature of 400 to 600°C to generate palladium oxide particles having an average particle size of 5 µm from the palladium chloride, palladium nitrate or the like.

[0056] Next, a firing step of firing the conductive substrate coated with the solution is carried out. Accordingly, a catalyst layer is formed on a surface of the conductive substrate. Preferably, the solution on the conductive substrate is evaporated before the conductive substrate is fired.

[0057] The coating step and the firing step may be repeated a plurality of times. By repeating these steps a plurality of times, the thickness of the catalyst layer can be increased.

[0058] The heating temperature in the firing step is not particularly limited, but is preferably about 400 to 650°C, more preferably about 450 to 650°C, further more preferably about 450 to 600°C. In addition, the firing time is preferably about 5 to 60 minutes, more preferably about 5 to 40 minutes, further more preferably about 5 to 30 minutes.

[0059] By carrying out the above-mentioned coating step and firing step, the chlorine generating electrode of the present invention can be suitably produced which includes a conductive substrate, and a catalyst layer provided on the conductive substrate.

[0060] Details of palladium oxide etc. that can be used in the method for producing a chlorine generating electrode according to the present invention are as described above. In addition, the X-ray diffraction peak intensity and peak width at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray in the catalyst layer formed by the firing step are as described above.

[0061] The chlorine generating electrode of the present invention can be placed in an electrolytic cell. That is, the electrolytic cell in the present invention includes the above-mentioned chlorine generating electrode. In the electrolytic cell of the present invention, the chlorine generating electrode serves as an anode, and further a cathode is provided. The material that forms the cathode is not particularly limited, and examples thereof include stainless steel and titanium.

[0062] In addition, a hypochlorite can be suitably produced by electrolyzing a metal chloride aqueous solution using the chlorine generating electrode of the present invention. As the metal chloride aqueous solution, saltwater (e.g. ballast water, seawater, or the like), a potassium chloride aqueous solution or the like is preferable. Examples of the hypochlorite include sodium hypochlorite and potassium hypochlorite.

[0063] As described above, the chlorine generating electrode of the present invention can be suitably used for electrolysis of, for example, saltwater with a low concentration of 1% or less. That is, in the method for producing a hypochlorite using the chlorine generating electrode of the present invention, the concentration of a metal chloride in a metal chloride aqueous solution is preferably 1% or less.

[0064] The temperature during electrolysis is not particularly limited, but is preferably about 2 to 35°C, more preferably about 5 to 30°C from the viewpoint of ensuring that the chlorine generating electrode has high chlorine generation efficiency, and improved long-term durability even when used for electrolysis of low-concentration saltwater.

[0065] The current density during electrolysis is not particularly limited, but is preferably about 1 to 20 A/dm², more preferably about 1 to 15 A/dm² from the viewpoint of ensuring that the chlorine generating electrode has high chlorine generation efficiency, and improved long-term durability even when used for electrolysis of low-concentration saltwater.

EXAMPLES

[0066] The present invention will be described in detail below by showing examples and comparative examples. It is to be noted that the present invention is not limited to the examples. In the following examples and comparative examples, measurement of the average particle size and X-ray diffraction measurement were performed under the following conditions.

Measurement of average particle size

[0067] The average particle size of palladium oxide used as a raw material was measured under the following conditions.
Measuring equipment: Laser scattering particle distribution measuring apparatus LA-950 (manufactured by HORIBA, Ltd.)
Measurement method: Suction is started for increasing the sample dispersion force. Thereafter, compressed air is supplied at 0.4 to 0.8 MPa to perform forced dispersion. A state with no sample is measured as a blank. The intensity of a feeder is adjusted, and measurement is started when the transmittance reaches 70 to 90%.
Measurement conditions: The transmittance is 70 to 90%, the number of repetitions is 30, and the particle size is on a volume basis.

[0068] For the average particle size of palladium oxide contained in the catalyst layer, a surface of the catalyst layer is observed with SEM, major axes of 20 particles observed in the field of view are measured, and the average thereof is determined. Palladium oxide is considerably different from ruthenium oxide and titanium oxide in particle size of particles used as a raw material, and therefore it is possible to distinguish palladium oxide particles from the other particles in a SEM image.

Measurement of X-rav diffraction

[0069] X-ray diffraction measurement of the catalyst layer was performed under the following conditions.
Measuring equipment: Ultima IV manufactured by Rigaku Corporation
Measurement method: A measurement sample is placed in such a manner that the sample can be irradiated with an X-ray from the measurement equipment main body. A current and a voltage are applied to age the sample, and then the sample is irradiated with an X-ray to perform measurement.
X-ray source: CuKα ray
Output setting: 40 kV, 40 mA
Optical conditions in measurement:
divergence slit = 0.2 mm
scattering slit = 2°
photoreception slit = 0.15 mm
Position of diffraction peak: 2θ ≒ 34°
Measurement range: 5° to 90°
Scan speed: 20°/min
Preparation of sample: the electrode is cut to 35 mm × 50 mm × 1 mm.

[Example 1]

[0070] A surface of a conductive substrate (thickness: 1 mm) composed of a titanium flat plate was subjected to a sandblasting treatment with #36 alumina. The thus-roughened surface of the conductive substrate was coated with a n-butanol solution of palladium oxide, ruthenium chloride and butyl titanate each having a predetermined average particle size as a raw material of a catalyst layer (the composition of the catalyst layer has values (mol%) as shown in Table 1), dried at 120°C for 10 minutes, and then fired at 500°C for 10 minutes. The coating-drying-firing process was repeated to prepare an anode with a catalyst layer provided on a surface of a conductive substrate. The average particle size of palladium oxide used as a raw material was 0.52 µm. In addition, for the catalyst layer of the anode, X-ray diffraction measurement using a CuKα ray was performed to determine the X-ray diffraction peak intensity and half width at a diffraction peak 2θ of about 34°. The results are shown in Table 1. Fig. 1 shows a SEM image of the surface of the catalyst layer of the anode obtained in Example 1 (magnification: 5,000).

[Example 2]

[0071] A surface of a conductive substrate (thickness: 1 mm) composed of a titanium flat plate was subjected to a sandblasting treatment with #36 alumina. The thus-roughened surface of the conductive substrate was coated with a n-butanol solution of palladium oxide, ruthenium chloride and butyl titanate each having a predetermined average particle size as a raw material of a catalyst layer (the composition of the catalyst layer has values (mol%) as shown in Table 1), dried at 120°C for 10 minutes, and then fired at 450°C for 10 minutes. The coating-drying-firing process was repeated to prepare an anode with a catalyst layer provided on a surface of a conductive substrate. The average particle size of palladium oxide used as a raw material was 0.17 µm. In addition, for the catalyst layer of the anode, X-ray diffraction measurement using a CuKα ray was performed to determine the X-ray diffraction peak intensity and half width at a diffraction peak 2θ of about 34°. The results are shown in Table 1. Fig. 2 shows a SEM image of the surface of the catalyst layer of the anode obtained in Example 2 (magnification: 5,000). In addition, Fig. 9 shows a graph showing a relationship between a diffraction peak 2θ (°) and a peak intensity (cps) as measured by X-ray diffraction for the catalyst layer of the anode obtained in Example 2.

[Example 3]

[0072] A surface of a conductive substrate (thickness: 1 mm) composed of a titanium flat plate was subjected to a sandblasting treatment with #36 alumina. The thus-roughened surface of the conductive substrate was coated with a n-butanol solution of palladium oxide, ruthenium chloride and butyl titanate each having a predetermined average particle size as a raw material of a catalyst layer (the composition of the catalyst layer has values (mol%) as shown in Table 1), dried at 120°C for 10 minutes, and then fired at 550°C for 10 minutes. The coating-drying-firing process was repeated to prepare an anode with a catalyst layer provided on a surface of a conductive substrate. The average particle size of palladium oxide used as a raw material was 1.53 µm. In addition, for the catalyst layer of the anode, X-ray diffraction measurement using a CuKα ray was performed to determine the X-ray diffraction peak intensity and half width at a diffraction peak 2θ of about 34°. The results are shown in Table 1. Fig. 3 shows a SEM image of the surface of the catalyst layer of the anode obtained in Example 3 (magnification: 5,000). In addition, Fig. 10 shows a graph showing a relationship between a diffraction peak 2θ (°) and a peak intensity (cps) as measured by X-ray diffraction for the catalyst layer of the anode obtained in Example 3.

[Example 4]

[0073] Except that as a raw material of a catalyst layer, cobalt nitrate was added so as to attain a composition as shown in Table 1, the same procedure as in Example 1 was carried out to prepare an anode. In addition, for the catalyst layer of the anode, X-ray diffraction measurement using a CuKα ray was performed to determine the X-ray diffraction peak intensity and half width at a diffraction peak 2θ of about 34°. The results are shown in Table 1. Fig. 4 shows a SEM image of the surface of the catalyst layer of the anode obtained in Example 4 (magnification: 10,000). In addition, Fig. 11 shows a graph showing a relationship between a diffraction peak 2θ (°) and a peak intensity (cps) as measured by X-ray diffraction for the catalyst layer of the anode obtained in Example 4.

[Example 5]

[0074] A surface of a conductive substrate (thickness: 1 mm) composed of a titanium flat plate was subjected to a sandblasting treatment with #36 alumina. The thus-roughened surface of the conductive substrate was coated with a n-butanol solution containing a predetermined amount of each of palladium chloride, ruthenium chloride and butyl titanate as a catalyst raw material (the composition of the catalyst layer has values (mol%) as shown in Table 1), dried at 120°C for 10 minutes, and then fired at 500°C for 10 minutes. The coating-drying-firing process was repeated to prepare an anode with a catalyst layer provided on a surface of a conductive substrate. Observation of the surface of the catalyst layer of the resulting anode with SEM revealed that palladium oxide particles of about 0.4 to 0.5 µm were present. The major axes of randomly selected 20 particles were measured by the above-mentioned method, and an average particle size was calculated. The average particle size was 0.15 µm. In addition, for the catalyst layer of the anode, X-ray diffraction measurement using a CuKα ray was performed to determine the X-ray diffraction peak intensity and half width at a diffraction peak 2θ of about 34°. The results are shown in Table 1. Fig. 5 shows a SEM image of the surface of the catalyst layer of the anode obtained in Example 5 (magnification: 10,000).

[Comparative Example 1]

[0075] A surface of a conductive substrate (thickness: 1 mm) composed of a titanium flat plate was subjected to a sandblasting treatment with #36 alumina. The thus-roughened surface of the conductive substrate was coated with a n-butanol solution containing a predetermined amount of each of palladium chloride, ruthenium chloride and butyl titanate as a catalyst raw material (the composition of the catalyst layer has values (mol%) as shown in Table 1), dried at 120°C for 10 minutes, and then fired at 400°C for 10 minutes. The coating-drying-firing process was repeated to prepare an anode with a catalyst layer provided on a surface of a conductive substrate. Observation of the surface of the catalyst layer of the resulting anode with SEM revealed that palladium oxide particles were not present. In addition, for the catalyst layer of the anode, X-ray diffraction measurement using a CuKα ray was performed to determine the X-ray diffraction peak intensity and half width at a diffraction peak 2θ of about 34°. The results are shown in Table 1. Fig. 6 shows a SEM image of the surface of the catalyst layer of the anode obtained in Comparative Example 1 (magnification: 5,000).

[Comparative Example 2]

[0076] A surface of a conductive substrate (thickness: 1 mm) composed of a titanium flat plate was subjected to a sandblasting treatment with #36 alumina. The thus-roughened surface of the conductive substrate was coated with a n-butanol solution containing a predetermined amount of each of ruthenium chloride and butyl titanate as a catalyst raw material (the composition of the catalyst layer has values (mol%) as shown in Table 1), dried at 120°C for 10 minutes, and then fired at 500°C for 10 minutes. The coating-drying-firing process was repeated to prepare an anode with a catalyst layer provided on a surface of a conductive substrate. For the catalyst layer of the resulting anode, X-ray diffraction measurement was performed, but since the catalyst layer did not contain palladium, there was no X-ray diffraction peak at a diffraction peak 2θ of about 34° as a matter of course. Fig. 7 shows a SEM image of the surface of the catalyst layer of the anode obtained in Comparative Example 2 (magnification: 5,000).

[Comparative Example 3]

[0077] Except that the average particle size of palladium oxide used as a raw material was 5.14 µm, the same procedure as in Example 1 was carried out to prepare an anode. In addition, for the catalyst layer of the anode, X-ray diffraction measurement using a CuKα ray was performed to determine the X-ray diffraction peak intensity and half width at a diffraction peak 2θ of about 34°. The results are shown in Table 1. Fig. 8 shows a SEM image of the surface of the catalyst layer of the anode obtained in Comparative Example 3 (magnification: 5,000).

<Measurement of chlorine generation efficiency from 0.1% saltwater >

[0078] 0.1% saltwater (sodium chloride aqueous solution) was electrolyzed at a current density of 3 A/dm² and an electrolysis temperature of 17°C using the anode obtained in each of Examples 1 to 5 and Comparative Examples 1 to 3, and stainless steel as a cathode, and chlorine generation efficiency (after lapse of 1,100 hours) was determined from an effective chlorine concentration. The results are shown in Table 1.

[Table 1]

	Composition of catalyst layer (mol%)				Average particle size of palladium oxide	X-ray diffraction peak intensity	X-ray diffraction peak half-value width	Chlorine generation efficiency
	Ru	Pd	Co	Ti	(µm)	(cps)	(deg)	(%)
Example 1	20	30	-	50	0.52	1,600	0.80	65
Example 2	20	40	-	40	0.17	2,800	0.35	66
Example 3	10	70	-	20	1.53	3,100	0.24	66
Example 4	20	30	10	40	0.52	1,400	0.45	66
Example 5	40	10	-	50	0.15*	1,700	0.75	65
Comparative Example 1	30	20	-	50	-	400	2.00	42
Comparative Example 2	50	-	-	50	-	-	-	38
Comparative Example 3	20	30	-	50	5.14	1,500	1.50	58

* In Table 1, the average particle size of palladium oxide in Example 5 is a value measured by observing a surface of the catalyst layer with SEM.

<Measurement of relationship between saltwater concentration and chlorine generation efficiency>

[0079] Saltwater with each saltwater concentration (0.1%, 0.15% or 0.5%) was electrolyzed using the anode obtained in Example 4, and stainless steel as a cathode, and chlorine generation efficiency was obtained from an effective chlorine concentration. Here, as shown in the graphs in Figs. 12 to 14, chlorine generation efficiency (after elapse of 1,100 hours) at a current density of 4.2 A/dm² was determined at each saltwater concentration. Figs. 12 to 14 show graphs showing a relationship between the temperature and the chlorine generation efficiency. As is evident from the graph shown in Fig. 12, the chlorine generation efficiency was 50% or more even at an electrolysis temperature of 10°C when the saltwater concentration was 0.1%. As is evident from the graph shown in Fig. 13, the chlorine generation efficiency was 50% or more even at an electrolysis temperature of 5°C when the saltwater concentration was 0.15%. As is evident from the graph shown in Fig. 14, the chlorine generation efficiency was 70% or more even at an electrolysis temperature of 2°C when the saltwater concentration was 0.5%.

<Electrolysis time and time-dependent change in chlorine generation efficiency>

[0080] 0.1% saltwater (sodium chloride aqueous solution) was electrolyzed at a current density of 3 A/dm² and an electrolysis temperature of 2°C using the anode obtained in each of Example 4 and Comparative Example 1, and stainless steel as a cathode, and chlorine generation efficiency was determined from an effective chlorine concentration. The electrolysis was continuously performed, and the electrolysis time and the time-dependent change in chlorine generation efficiency were measured. The obtained graph is shown in Fig. 15. As is evident from the graph shown in Fig. 15, chlorine generation efficiency was not significantly reduced even with an electrolysis time of more than 4,500 hours (chlorine generation efficiency was 46% with an electrolysis time of 0 hour, while chlorine generation efficiency was 42% with an electrolysis time of 4,573 hours) when the anode of Example 4 was used. On the other hand, when the anode of Comparative Example 1 was used, initial chlorine generation efficiency was high (chlorine generation efficiency was 46% with an electrolysis time of 0 hour), but chlorine generation efficiency was considerably reduced with an electrolysis time of more than 1,000 hours (chlorine generation efficiency was 39% with an electrolysis time of 1,176 hours).

Claims

1. A chlorine generating electrode comprising a conductive substrate, and a catalyst layer provided on the conductive substrate, the catalyst layer containing at least palladium oxide, ruthenium oxide and titanium oxide, the palladium oxide being in the form of particles having an average particle size of 5 µm or less.

2. The chlorine generating electrode according to claim 1, wherein the catalyst layer has an X-ray diffraction peak intensity of 500 cps or more at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray.

3. The chlorine generating electrode according to claim 1 or 2, wherein the catalyst layer has an X-ray diffraction peak half-value width of 1.5 deg or less at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray.

4. The chlorine generating electrode according to any one of claims 1 to 3, wherein the ratio of palladium metal contained in the catalyst layer is 1 mol% or more where the content of metal elements in the catalyst layer is 100 mol%.

5. The chlorine generating electrode according to any one of claims 1 to 4, which is used for electrolysis of saltwater with a concentration of 1% or less.

6. A method for producing a chlorine generating electrode including a conductive substrate, and a catalyst layer provided on the conductive substrate, the method comprising:

a coating step of coating a conductive substrate with a solution containing at least a palladium compound, a ruthenium compound and a titanium compound; and

a firing step of firing the conductive substrate coated with the solution,

the method including using palladium oxide particles having an average particle size of 5 µm or less as the palladium compound.

7. A method for producing a chlorine generating electrode including a conductive substrate, and a catalyst layer provided on the conductive substrate, the method comprising:

a coating step of coating a conductive substrate with a solution containing at least a palladium compound, a ruthenium compound and a titanium compound; and

a firing step of firing the conductive substrate coated with the solution, the method including using at least one of palladium chloride and palladium nitrate as the palladium compound, the firing step including heating the conductive substrate at a temperature of 400 to 600°C to generate palladium oxide particles having an average particle size of 5 µm or less from the palladium chloride.

8. The method for producing a chlorine generating electrode according to claim 6 or 7, wherein the catalyst layer formed by the firing step has an X-ray diffraction peak intensity of 500 cps or more at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray.

9. The method for producing a chlorine generating electrode according to any one of claims 6 to 8, wherein the catalyst layer formed by the firing step has an X-ray diffraction peak half-value width of 1.5 deg or less at a palladium oxide diffraction peak 2θ of 33° to 35° as measured by an X-ray diffraction method using a CuKα ray.

10. An electrolytic cell comprising the chlorine generating electrode according to any one of claims 1 to 5.

11. A method for producing a hypochlorite, the method comprising the step of electrolyzing a metal chloride aqueous solution using the chlorine generating electrode according to any one of claims 1 to 5.

Drawing