JP2004131346A - Metal oxide sol and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a metal oxide sol of tungsten or molybdenum comprising microparticles and giving a sol solution stable for a long time. <P>SOLUTION: The metal oxide sol contains tungsten oxide or molybdenum oxide and ascorbic acid and has an average particle size of 300 nm or less. The sol is prepared by adding ascorbic acid to a solution of pertungstic acid or permolybdic acid to react them. Since the sol comprises uniform fine particles of tungsten oxide or molybdenum oxide and gives a stable solution, it is advantageously used e.g. for various electronic materials. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
An object of the present invention is to provide a metal oxide sol of tungsten or molybdenum which is useful as a material for various catalysts, ceramics, electronics and the like, and relates to a metal oxide sol and a method for producing the same.
[0002]
[Prior art]
Tungsten oxide and molybdenum oxide are useful metal oxides used for electrochromic materials, catalysts, and the like, respectively. These materials are often used by being immobilized on a substrate or a carrier surface.
As one of the methods, there is a dry process method in which these metal oxide materials are fixed by a sputtering method, an evaporation method, or the like. Another method is a wet process in which a precursor substance such as a metal alkoxide or a metal acetate, a nitrate, or a complex is applied to a substrate or a carrier surface, impregnated, fixed, and then heat-treated to obtain a metal oxide. There is a law.
In the former dry process method, when a flat substrate is processed, a uniform and high-performance immobilized product can be obtained, but the processing cost is high and there is a problem that it is not suitable for processing inexpensive articles. Further, there is a problem that it cannot be used when immobilized on a carrier powder such as a catalyst or a molded article having a different shape.
On the other hand, in the latter wet process method, since the precursor substance generally does not become a metal oxide unless heat treatment is performed at a high temperature, there is a problem that its use range is limited.
[0003]
In addition to the above methods, as a method of immobilizing tungsten oxide and molybdenum oxide on a carrier by a wet process method, there is a method of coating or impregnating the carrier with a dispersion of metal oxide fine particles. Simply dispersing the powder in the solvent not only causes the metal oxide component to precipitate immediately, resulting in non-uniform immobilization, but also because the metal oxide particles are too large to be immobilized or have the desired material performance. Is difficult to obtain.
[0004]
In order to solve such a problem, the present applicant has proposed a novel molybdic acid solution characterized by forming molybdenum trioxide under a heat treatment condition of 90 ° C. for 2 hours. (See Patent Document 1)
In the invention of Patent Document 1, cations are removed from molybdate using an ion exchange resin to produce a molybdic acid solution. However, when the molybdic acid solution obtained by this method is used for the above-mentioned purpose, the service life of the solution is short, and particularly when the solution is used at a high concentration or for a long period of time, precipitation occurs in the solution. was there.
[0005]
On the other hand, a method is known in which a peroxytungstate solution is treated with an ion-exchange resin, reacted with hydrogen peroxide, dried, and then re-dissolved in ethanol to obtain a tungsten oxide precursor solution. (See Patent Document 2)
It discloses that a uniform tungsten oxide film can be obtained by applying a tungsten oxide precursor solution obtained by the method of Patent Document 2 to a substrate and heating it at a temperature of 300 ° C. or lower.
However, in the method of Patent Document 2, since the precursor is once dried and then redissolved in ethanol or the like in order to obtain a stable precursor, it is not necessarily an industrially advantageous method. There is a problem that the immobilized material becomes expensive. Further, in the re-dissolution step as described above, the obtained solution has a disadvantage that it is difficult to become a homogeneous solution and undissolved components are easily generated.
As described above, a solution in which particles of tungsten oxide and molybdenum oxide are stably dispersed has not yet been developed, and the development of a stable solution containing such metal oxides as fine particles has been desired. It is the current situation.
[0006]
[Patent Document 1]
Patent No. 2766990 [Patent Document 2]
JP 2000-510085 A
[Problems to be solved by the invention]
In view of such a current situation, the present inventors have proposed a sol solution in which oxides of tungsten and molybdenum are dispersed in a solution, particularly in the form of fine particles. It was thought that the above-mentioned problems could be solved by obtaining a sol, and the method was intensively studied.
As a result, these metal oxide sols obtained by adding and reacting ascorbic acid to a pertungstic acid or permolybdic acid solution are obtained as fine particle sols of tungsten oxide or molybdenum oxide having an average particle diameter of 300 nm or less. The present inventors have found that this sol is a material for solving the above-mentioned problems, and have completed the present invention based on such findings.
[0008]
[Means for Solving the Problems]
That is, the present invention relates to a metal oxide sol containing a metal oxide of tungsten or molybdenum and ascorbic acid and having an average particle diameter of 300 nm or less.
Furthermore, the present invention relates to a method for producing a metal oxide sol, wherein ascorbic acid is added to and reacted with a pertungstic acid or permolybdic acid solution.
According to the present invention, a metal oxide sol of tungsten oxide or molybdenum oxide is obtained by such a method, and this sol is used as various industrial materials.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
The metal oxide sol of the present invention is a metal oxide sol containing tungsten oxide (WO ₃ ) or molybdenum oxide (MoO ₃ ) and ascorbic acid. The average particle size of the sol is 300 nm or less. Due to such a particle size, the sol of the present invention does not settle in the solution and exhibits a homogeneous sol state for a long period of time. The average particle diameter mentioned here can be easily measured with a commercially available particle size distribution analyzer using a laser, but the particle diameter can also be measured by observation with an electron microscope or the like.
[0010]
Such a sol of the present invention will be described based on a method for producing the sol.
The sol of the present invention is characterized in that ascorbic acid is added to a pertungstic acid or permolybdic acid solution to cause a reaction.
Such a solution of pertungstic acid or permolybdic acid may be produced by any known method, for example, adding and dissolving tungsten oxide, tungstic acid or molybdenum oxide, molybdic acid directly in a hydrogen peroxide solution. Can be obtained by As another method, it can also be obtained by neutralizing and decomposing a chloride of tungsten or molybdenum, washing it to remove by-product salts, and then reacting with hydrogen peroxide.
In addition, salts such as ammonium tungstate, sodium tungstate, potassium tungstate, calcium tungstate, ammonium molybdate, sodium molybdate, potassium molybdate, calcium molybdate, lithium molybdate, barium molybdate, etc. It can be obtained by a method of removing cations by means and then contacting it with hydrogen peroxide, or a method of contacting with hydrogen peroxide and then removing cations by means such as ion exchange.
Further, it can also be obtained by a method in which after decomposing the above various salts using a strong acid, the by-product salts generated are washed away, and hydrogen peroxide is reacted with the salts. What is important is that the solution of pertungstic acid or permolybdic acid thus obtained is in the form of a transparent solution.
[0011]
The solution of pertungstic acid or permolybdic acid used in the production of the sol of the present invention coexists before the reaction with ascorbic acid if the solution contains hydrogen peroxide in the production. It is necessary to remove hydrogen peroxide.
The most convenient method for removing hydrogen peroxide is by heating. For example, by heating at 95 to 100 ° C. for 1 to 2 hours, coexisting hydrogen peroxide can be removed by decomposition.
For the decomposition and removal of hydrogen peroxide, a catalytic cracking catalyst can be used. For example, by bringing platinum or manganese dioxide into contact with a solution, coexisting hydrogen peroxide can be decomposed and removed.
[0012]
The solution of pertungstic acid or permolybdic acid obtained by such a method becomes a solution of 35% by mass or less as tungsten oxide (WO ₃ ) or molybdenum oxide (MoO ₃ ).
This solution of pertungstic acid or permolybdic acid is then added to and reacted with ascorbic acid. The concentration of pertungstic acid or permolybdic acid is 35% by mass or less as tungsten oxide (WO ₃ ) or molybdenum oxide (MoO ₃ ) as described above, but if this concentration exceeds 35% by mass, ascorbic acid The solution may be extremely thickened during the reaction with, and a uniform reaction may not be caused, which is not preferable. There is no particular lower limit, but if it is 5% by mass or less, the concentration of the obtained sol becomes low, which is uneconomical. Therefore, the lower limit is desirably 5% by mass or more. More preferably, it is desirable to carry out the reaction with ascorbic acid in the range of 15 to 25% by mass as WO ₃ or MoO ₃ .
[0013]
A solution of pertungstic acid or permolybdic acid temporarily shows a stable state even without reacting with ascorbic acid, but as the storage time elapses, aggregation occurs and precipitates. Is required.
By the way, tungstic acid and molybdic acid are a kind of polyacid, and pertungstic acid and permolybdic acid obtained by reacting them with hydrogen peroxide also have a large oxygen content, but are considered to be a kind of this polyacid.
Polyacids consist of multiple metal element nuclei and oxygen and have a certain degree of molecular weight.However, in practice, particle size distribution analyzers cannot detect appropriate scattered light. It is considered to be present. It is considered that the solution of pertungstic acid or permolybdic acid precipitates over time and is not stable because the solution contains many fine units, that is, low molecular components. For example, when such a solution is circulated through an ultrafiltration module having a molecular weight cutoff of 10,000, about 30% by mass or more of the tungsten oxide or molybdenum oxide component is discharged together with the filtrate. It is thought that these components actually cause the instability of the solution.
However, the solution containing ascorbic acid of the present invention is a sol solution having excellent stability over time. Although it is not clear why the metal oxide sol of the present invention is stable, for example, after addition of ascorbic acid, after the reaction, the particle diameter can be easily measured by a measuring device using a laser, and as described above. Even if the solution is circulated through the ultrafiltration module, the amount of components leaking into the filtrate is 10% by weight or less.
After the reaction with ascorbic acid, no precipitation or thickening occurs even when stored for a long time. From these facts, it is presumed that ascorbic acid has some effect on pertungstic acid and permolybdic acid and contributes to formation of sol particles and stabilization of generated particles.
[0014]
The amount of ascorbic acid to be added is determined based on the reaction with the solution of pertungstic acid or permolybdic acid and the amount necessary for stabilization of the sol obtained by the reaction. The amount of ascorbic acid to be added to the acid solution is in the range of 0.1 to 1.5 mol of ascorbic acid per mol of pertungstic acid (WO ₃ ) or permolybdic acid (MoO ₃ ). .
The larger the amount of ascorbic acid added, the shorter the stabilization reaction time described below and the lower the heating temperature. However, the stability after the formation of the sol particles is not stable as the amount of ascorbic acid added is large, and is determined in consideration of the time required for the reaction, economy, and the like. On the other hand, if the amount of ascorbic acid to be added is less than 0.1 mol, it is not preferable because not only a long time is required for the reaction but also a precipitate is easily generated.
Conversely, if the amount of ascorbic acid exceeds 1.5 moles, an amount exceeding the purpose of stabilizing the sol will be added, which not only becomes uneconomical, but also in this region, at the time of production or over time after production. However, the amount of leakage due to the ultrafiltration increases, which causes instability.
[0015]
Ascorbic acid may be added by directly adding and dissolving ascorbic acid powder to a solution of pertungstic acid or permolybdic acid, or by dissolving ascorbic acid in water and adding it as an aqueous solution. The temperature at which ascorbic acid is added is not particularly limited, but preferably, the solution is added and reacted under stirring at a temperature of 60 to 100 ° C. while stirring a solution of pertungstic acid or permolybdic acid. The inventive metal oxide sol is stabilized in a short time. When ascorbic acid is added, the solution temporarily thickens and may generate heat. However, if the stirring is continued, the viscosity of the solution gradually decreases, and a tungsten or molybdenum metal oxide sol is formed.
[0016]
By the way, as mentioned above, it is considered that ascorbic acid acts to generate particles and stabilize the dispersion of the generated particles. However, once the particles are formed, the excess ascorbic acid remains due to the performance of the sol. May be adversely affected. Therefore, if necessary, it is desirable to remove excess ascorbic acid after the reaction. Of course, depending on the intended use of the sol, it can be used without removing the ascorbic acid.
The method of removing excess ascorbic acid is good by ultrafiltration washing.For example, ascorbic acid can be efficiently removed by washing while monitoring the electrical conductivity of the sol, but ascorbic acid is decomposed by heating, so thermal decomposition These methods may be used in combination.
[0017]
After removal of excess ascorbic acid, the sol according to the invention has an ascorbic acid content relative to the tungsten or molybdenum metal oxide of ascorbic acid per mole of tungsten or molybdenum metal oxide (WO ₃ or MoO ₃ ). It is necessary that the acid be between 0.03 and 0.6 mol. That is, when the molar ratio is less than 0.03, a stable sol which does not cause precipitation for a long period of time in the present invention is not obtained. On the other hand, when the ratio exceeds 0.6, the sol solution alone is stable, but the compatibility with other substances used at the time of use of the sol may be deteriorated, so that the use of the sol is limited.
Incidentally, the content of ascorbic acid contained in the sol of the present invention may be directly obtained by analyzing the carbon content, or may be analyzed after the sol is gelled with an alkali agent or the like to release ascorbic acid. More conveniently, the weight loss after thermal decomposition and burning at 100 to 600 ° C. by thermal analysis can be regarded as the amount of ascorbic acid.
[0018]
The metal oxide sol of the present invention obtained in this way is stable for a long time without causing precipitation or thickening. Further, the sol of the present invention can be used by mixing with other fine particle dispersions such as silica sol, alumina sol, titania sol and the like, depending on the intended use.
Further, in the production process of the sol of the present invention, other metal element components such as Y, Ti, Zr, V, Nb, Ta, Fe, Ru, Co, Al, Si, Sn, P, A small amount of peroxides or basic salts of elements such as Sb, La, Ce, Nd and Sm may be added. In addition, in the production process of the present invention, the addition and use of other sol components may be carried out before or after the addition of ascorbic acid, so that a stable sol component partially comprising a mixture with another metal oxide is used. An inventive sol can be obtained.
[0019]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. All percentages are by mass unless otherwise specified.
[0020]
[Example 1]
Tungstic acid (manufactured by Wako Pure Chemical Industries, Ltd. _Reagent, WO 3 92.8%) to 100 g, of 35% aqueous hydrogen peroxide 466.8g was added and after stirring for 4 hours, further 1 hour under heating at 90 ° C. Stirring yielded a clear yellow solution. To this solution, 7.0 g of ascorbic acid (ascorbic acid / WO, ₃ molar ratio: 0.1) was added, and the mixture was further stirred under heating at 90 ° C. for 1 hour to obtain a yellow sol solution.
This sol solution was circulated and washed with an ultrafiltration module having a molecular weight cut-off of 10,000 (Asahi Kasei Corp. Microosa @ UF).
As a result of measuring the average particle diameter of the obtained sol solution of the present invention by a particle size distribution diameter using dynamic scattering theory, it was 70 nm. As a result of a composition analysis of this sol, WO _{3 was} 20% and the ascorbic acid / WO ₃ molar ratio was 0.05.
Further, when the sol solution of the present invention was allowed to stand at room temperature for 10 days, a precipitate was slightly generated at the bottom of the container, but as a result of analysis, the concentration at the upper portion of the solution did not change, and the viscosity was the same as that immediately after production. Met.
[0021]
[Example 2]
To 100 g of tungstic acid (a reagent manufactured by Wako Pure Chemical Industries, Ltd., WO ₃ 92.8%), 466.8 g of 35% aqueous hydrogen peroxide was added, and the mixture was stirred for 2 hours, and further stirred at 90 ° C. for 1 hour under heating. This gave a clear yellow solution. 21.1 g of ascorbic acid (a reagent manufactured by Wako Pure Chemical Industries, Ltd.) (ascorbic acid / WO ₃ molar ratio: 0.3) was added to the solution, and the mixture was further stirred at a temperature of 90 ° C. for 1 hour to give a blue sol solution. Got.
The sol solution was circulated and washed with an ultrafiltration module having a molecular weight cutoff of 10,000 (Asahi Kasei Corporation, Microza UF).
The average particle size of the obtained sol solution of the present invention was measured by a particle size distribution diameter (manufactured by Pacific Scientific Inc., Nikon 370) using dynamic scattering theory. As a result, it was 40 nm.
As a result of a composition analysis of this sol, WO _{3 was} 20% and the ascorbic acid / WO ₃ molar ratio was 0.06.
Further, this sol solution was allowed to stand at room temperature for 10 days, but no precipitation occurred in the sol solution, and no change in appearance such as thickening was observed.
[0022]
[Example 3]
Tungstic acid (manufactured by Wako Pure Chemical Industries, Ltd. _Reagent, WO 3 92.8%) to 100 g, 35% aqueous hydrogen peroxide 233.4g of deionized water 150g was added and after stirring for 4 hours, heating of 90 ° C. Stirred underneath for an additional hour to give a clear yellow solution. To this solution, 35.2 g of ascorbic acid (a reagent manufactured by Wako Pure Chemical Industries, Ltd.) (ascorbic acid / WO ₃ molar ratio: 0.5) was added, and the mixture was further stirred at a temperature of 90 ° C. for 1 hour. Obtained. As a result of measuring the average particle diameter of the obtained sol solution of the present invention by a particle size distribution diameter using dynamic scattering theory, it was 52 nm.
As a result of a composition analysis of this sol, WO _{3 was} 25% and the ascorbic acid / WO ₃ molar ratio was 0.4.
Further, this sol solution was allowed to stand at room temperature for 10 days, but no precipitation occurred in the sol solution, and no change in appearance such as thickening was observed.
[0023]
[Example 4]
Tungstate in (manufactured by Wako Pure Chemical Industries, Ltd. _Reagent, WO 3 92.8%)) 100 g, 35% aqueous hydrogen peroxide 466.8g was added and after stirring for 4 hours, further under heating at 90 ° C. 1 After stirring for an hour, a clear yellow solution was obtained. To this solution, 70.5 g of ascorbic acid (ascorbic acid / WO ₃ molar ratio: 1.0) was added, and the mixture was further stirred under heating at 90 ° C. for 1 hour to obtain a blue sol solution.
The sol was circulated and washed with an ultrafiltration module having a molecular weight cutoff of 10,000 (Asahi Kasei Corporation, Microza UF).
As a result of analyzing the sol after washing, WO _{3 was} 19.5%, the ascorbic acid / WO ₃ molar ratio was 0.1, and the yield was 85%.
Furthermore, as a result of measuring the average particle diameter of this sol by a particle size distribution diameter using dynamic scattering theory, it was 43 nm.
The sol solution was allowed to stand at room temperature for 10 days, but no precipitation occurred in the sol solution, and no change in appearance such as thickening was observed.
[0024]
[Example 5]
To 50 g of ammonium tungstate (a reagent manufactured by Wako Pure Chemical Industries, Ltd., WO ₃ 88.8%), 200 g of ion-exchanged water was added, and after stirring and dissolving, the solution was passed through a column of a cation exchange resin to give a pale yellow color. An acidic solution was obtained. 111.7 g of 35% aqueous hydrogen peroxide was added to this solution, and the mixture was stirred for 1 hour, and further stirred at 90 ° C. for 1 hour. 20.2 g of ascorbic acid (ascorbic acid / WO ₃ mol / 0.6 ratio) was added to the obtained yellow transparent solution, and the mixture was further stirred under heating at 90 ° C. for 1 hour to obtain a blue sol. As a result of measuring the average particle diameter of the obtained sol of the present invention by a particle size distribution diameter using dynamic scattering theory, it was 48 nm.
As a result of a composition analysis of this sol, WO _{3 was} 25% and the ascorbic acid / WO ₃ molar ratio was 0.5.
Further, when the sol of the present invention was allowed to stand at room temperature for 10 days, no precipitation occurred in the sol solution, and no change in appearance such as thickening was observed.
[0025]
[Example 6]
To 100 g of molybdic acid (reagent manufactured by Wako Pure Chemical Industries, Ltd., 80% of MoO ₃ ), 162.1 g of 35% hydrogen peroxide solution and 200 g of ion-exchanged water were added, and after stirring for 1 hour, the mixture was heated at 90 ° C. The mixture was further stirred for 1 hour to obtain a clear yellow solution. 19.6 g of ascorbic acid (ascorbic acid / MoO ₃ molar ratio: 0.2) was added to this solution, and the mixture was further stirred under heating at 90 ° C. for 1 hour to obtain a blue sol.
As a result of measuring the average particle diameter of the obtained sol of the present invention by a particle size distribution diameter using dynamic scattering theory, it was 35 nm.
Further, as a result of composition analysis of this sol, MoO _{3 was} 15% and ascorbic acid / MoO ₃ molar ratio was 0.2.
Further, when the sol of the present invention was allowed to stand at room temperature for 10 days, no precipitation occurred in the sol solution, and no change in appearance such as thickening was observed.
[0026]
[Example 7]
To 100 g of molybdenum oxide (first grade reagent, manufactured by Wako Pure Chemical Industries, Ltd., 99% MoO ₃ ), 202.6 g of 35% hydrogen peroxide solution and 200 g of ion-exchanged water were added, and the mixture was stirred for 16 hours and then heated at 90 ° C. And further stirred for 1 hour to obtain a yellow transparent solution. 36.7 g of ascorbic acid (ascorbic acid / MoO ₃ molar ratio: 0.3) was added to this solution, and the mixture was further stirred under heating at 90 ° C. for 1 hour to obtain a blue sol.
As a result of measuring the average particle diameter of the obtained sol of the present invention by a particle size distribution diameter using dynamic scattering theory, it was 25 nm.
As a result of a composition analysis of this sol, MoO _{3 was} 18% and the ascorbic acid / MoO ₃ molar ratio was 0.3.
Further, when the sol of the present invention was allowed to stand at room temperature for 10 days, no precipitation occurred in the sol solution, and no change in appearance such as thickening was observed.
[0027]
[Comparative Example 1]
To 100 g of tungstic acid (a reagent manufactured by Wako Pure Chemical Industries, Ltd., WO ₃ 92.8%), 466.8 g of 35% hydrogen peroxide solution was added, and the mixture was stirred for 4 hours, and further heated at 90 ° C. for 1 hour. The mixture was stirred to obtain a yellow transparent solution containing 20% of tungsten oxide (WO ₃ ).
This solution was circulated and washed with an ultrafiltration module having a molecular weight cutoff of 10,000. As a result of analyzing the solution after washing, the yield of tungsten oxide was 41%.
In addition, as a result of leaving this solution at room temperature for 5 days, the viscosity of the solution was almost the same as immediately after the production, but the concentration at the top of the solution was reduced, and a pale yellow precipitate was generated at the bottom of the container.
[0028]
[Comparative Example 2]
Tungstic acid (manufactured by Wako Pure Chemical Industries, Ltd. _Reagent, WO 3 92.8%) to 100 g, of 35% aqueous hydrogen peroxide 466.8g was added and after stirring for 2 hours, further 1 hour under heating at 90 ° C. Stirring yielded a clear yellow solution.
To this solution, 25.2 g of oxalic acid (a reagent manufactured by Wako Pure Chemical Industries, Ltd.) (oxalic acid / WO, ₃ molar ratio: 0.5) was added, and the mixture was further stirred under heating at 90 ° C. for 1 hour to obtain tungsten oxide ( A pale yellow slurry solution containing 20% of WO ₃ ) was obtained.
Further, as a result of leaving this solution at room temperature for 2 days, the viscosity of the solution was almost the same as that immediately after the production, but the concentration at the top of the solution was reduced and a pale yellow precipitate was generated at the bottom of the container.
[0029]
【The invention's effect】
The tungsten or molybdenum metal oxide sol of the present invention is composed of uniform fine particles of tungsten oxide or molybdenum oxide and shows a homogeneous sol state for a long period of time.
Therefore, the sol of the present invention can be used as a tungsten oxide or molybdenum oxide raw material for various electronic materials to form a thin film by coating and baking the substrate. Further, it can be used as a catalyst after being supported and immobilized on a carrier such as titanium oxide, aluminum oxide, silicon oxide, and aluminosilicate by a method such as impregnation and coprecipitation with a different metal, and used as a catalyst. Therefore, the sol of the present invention is a sol material which is extremely easy to handle in such applications.

Claims (4)

A metal oxide sol containing a metal oxide of tungsten or molybdenum and ascorbic acid and having an average particle diameter of 300 nm or less. The content of ascorbic acid to the metal oxide of tungsten or molybdenum, a metal oxide of tungsten or molybdenum (WO ₃ or _MoO 3) according to claim 1, wherein ascorbic acid 0.03 to 0.6 moles per mole of Metal oxide sol. A method for producing a metal oxide sol, wherein ascorbic acid is added to a pertungstic acid or permolybdic acid solution to cause a reaction. The amount of ascorbic acid added to the pertungstic acid or permolybdic acid solution is 0.1 to 1.5 mol of ascorbic acid per 1 mol of pertungstic acid (WO ₃ ) or permolybdic acid (MoO ₃ ). 4. The method for producing a metal oxide sol according to 3.