JPH08131849A - Production of honeycomb structure catalyst for purification of waste gas - Google Patents

Abstract

PURPOSE: To suppress the peeling and falling of a catalytic component by coating the surface of a honeycomb structure with an aq. slurry for coating having a specified viscosity and contg. a catalyst, a specified fixing agent, one or more kinds of specified viscosity reducing agents and a specified peeling preventing agent, and carrying out firing. CONSTITUTION: A catalyst is slurried with water, the surface of a honeycomb structure is coated with the resultant aq. slurry and the catalyst is fixed to obtain a honeycomb structure catalyst. At this time, a fixing agent made of a catalyst and one or more of sols of alumina, silica, titania and zirconia, one or more kinds of viscosity reducing agents selected from among acetic acid, acetic anhydride and alkali salts of acetic acid and a peeling preventing agent made of silica ceramic fibers are incorporated into the aq. slurry for coating and the viscosity of the slurry is regulated to <100cps. After the slurry is applied on the surface and the catalyst is fixed, firing is carried out.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a honeycomb structure catalyst for exhaust gas purification, which reduces nitrogen oxides (NO _x ) contained in exhaust gas having a high oxygen concentration from an internal combustion engine. NO _{x in} exhaust gas from internal combustion engines such as gasoline engines and diesel engines
The present invention relates to a method for producing a purifying honeycomb structure catalyst that reduces the amount of gas.

[0002]

2. Description of the Related Art In recent years, for the purpose of preventing air pollution, the exhaust gas from an internal combustion engine having a high oxygen concentration, such as exhaust gas from an internal combustion engine such as a vehicle equipped with a lean burn gasoline engine vehicle and a diesel engine or a cogeneration generator, has been developed. There is an urgent need to purify exhaust gas.

Since a catalyst for such an application is used at a very high gas hourly space velocity, it is necessary to reduce the pressure loss as much as possible. In general, a large number of direct tubular passages (cells and cells) formed in the gas flow direction are required. There is used a honeycomb structure catalyst in which a desired catalyst is applied in a thin film on the surface of each cell of a honeycomb structure made of a ceramic such as cordierite or mullite or having a plurality of metals.

Such a catalyst is manufactured by applying a desired catalyst component in the form of a thin film on the surface of each cell of a honeycomb structure as a base material. As a method of applying the catalyst component to each cell of the honeycomb structure, powdered alumina, silica, zeolite, silica-alumina, titania, a porous carrier which is an oxide or a composite oxide selected from zirconia, An active ingredient is supported and used as a catalyst, which is mixed with an appropriate fixing agent such as alumina sol or silica sol to form an aqueous slurry, and after dipping the above-mentioned honeycomb structure into this slurry, an excess slurry inside the cell is added. It is prepared by removing with compressed air, drying and then heat-treating (calcining).

A honeycomb structure for various reactions is obtained by applying a catalyst having a metal or metal oxide as an active ingredient, which is supported on a porous carrier such as alumina or silica, to the cell surface of the honeycomb structure by wash coating. When manufacturing a catalyst, in order to impart desired reaction characteristics and life to the honeycomb structure catalyst, as much catalyst component as possible should be coated on the cell surface of the honeycomb structure.

When a large amount of catalyst component is applied by the conventional method, the concentration of the catalyst component in the slurry is limited due to problems of viscosity and stability of the aqueous coating slurry used, and the number of coatings is significantly increased. Then, there was a problem in terms of economy that the manufacturing cost increased. In addition, when a large amount of the catalyst component is applied, the thickness of the catalyst component applied to the surface of each cell increases depending on the application amount. The increase in film thickness is caused by repeated thermal shocks in the coating film, which causes distortion and cracks in the coating film in the state of being used in the heat treatment step or reaction in the preparation of the honeycomb structure catalyst coated with the catalyst component. There is a problem that the applied catalyst component is easily peeled off, and as a result, the reaction characteristics and the life are reduced.

[0007]

SUMMARY OF THE INVENTION The present invention solves the problems of the prior art by economically applying a large amount of catalyst components with a small number of coatings and by depositing a large amount of catalyst components on the surface of each cell. An object of the present invention is to provide a method for manufacturing a honeycomb structure catalyst capable of suppressing the peeling and dropping of the catalyst component to a minimum even if the coat film thickness increases.

[0008]

The above object of the present invention is achieved by incorporating silica-based ceramic fibers as an anti-stripping agent into an aqueous coating slurry.

That is, according to the present invention, on the surface of a honeycomb structure having a large number of cells formed in the gas flow direction,
A method for producing a honeycomb structure catalyst for exhaust gas purification, which comprises fixing a catalyst, wherein the catalyst, alumina sol, and silica sol are used as an aqueous slurry for coating when the catalyst is made into an aqueous slurry and the honeycomb structure surface is coated and fixed. ,
A fixing agent comprising at least one sol selected from titania sol and zirconia sol, and at least one selected from acetic acid, acetic anhydride, and an alkali salt of acetic acid.
Containing at least one viscosity reducing agent and a silica-based ceramic fiber exfoliation inhibitor, and having a viscosity of less than 100 cps, the aqueous coating slurry is applied to the surface of the honeycomb structure, and the catalyst is fixed to the surface of the honeycomb structure. The method for producing a honeycomb structure catalyst for exhaust gas purification is characterized in that the honeycomb structure catalyst for exhaust gas purification is characterized by firing (heat treatment).

The catalyst component used in the present invention is not particularly limited, but is preferably one in which at least one active component containing silver or a silver compound is supported on a porous heat-resistant carrier. Silver compounds such as silver oxide and silver chloride,
Examples thereof include silver halides such as silver bromide, silver carbonate, silver sulfate, and silver phosphate. From the viewpoint of catalytic properties, the amount of these active components is preferably 0.01 to 20.0% by weight, more preferably 0.05 to 15.0% by weight, in terms of silver. If the amount converted to silver is less than 0.01% by weight, the removal efficiency of nitrogen oxides is small, and if it exceeds 20.0% by weight, the removal efficiency of nitrogen oxides at high temperatures is poor.

Examples of the porous heat-resistant carrier include oxides of silica, alumina, titania, zirconia and the like, or composite oxides thereof such as silica-alumina and zeolite. This heat-resistant porous carrier is usually used in the state of an oxide, but is not particularly limited to the oxide, the silver or silver compound is supported on the hydrated oxide that has not undergone thermal decomposition, Finally, it can be heat-treated to form a catalyst.

The method for supporting the silver or silver compound on the heat-resistant porous carrier is not particularly limited and may be any method. For example, after suspending the carrier powder in an aqueous silver nitrate solution, An aqueous solution of a water-soluble chlorine compound such as ammonium chloride or sodium chloride may be added and supported as silver chloride on the carrier, followed by heat treatment to form a catalyst.

In the present invention, the catalyst thus obtained is used as an aqueous coating slurry. In addition to the catalyst, the aqueous slurry for coating contains a fixing agent, a viscosity reducing agent, and a peeling preventing agent.

The sticking agent is used to wash-coat the surface of each cell of the honeycomb structure with a slurry for coating and then firmly fix the catalyst particles to the cell surface of the honeycomb structure by heat treatment (calcination). Specifically, it is selected from at least one sol selected from alumina sol, silica sol, titania sol and zirconia sol.

As the viscosity reducing agent for reducing the viscosity of the aqueous slurry, at least one kind of acetic acid, acetic anhydride, or an alkali salt of acetic acid such as potassium acetate or sodium acetate is selected. In addition, carboxylic acids such as oxalic acid, tartaric acid and acrylic acid or derivatives thereof may be used. The content of the viscosity reducing agent is preferably 0.5 to 15% with respect to the weight of the catalyst in the aqueous slurry.
It is 0% by weight, more preferably 1.0 to 12.0% by weight. If the content of the viscosity reducing agent is less than 0.5% by weight, the effect of decreasing the viscosity cannot be exhibited, and if it exceeds 15.0% by weight, the carrier component of the catalyst is dissolved, which may deteriorate the catalyst performance. The economy is poor due to excess.

An aqueous slurry containing the above-mentioned catalyst component, a fixing agent and a viscosity reducing agent is stirred and mixed, and the catalyst in the aqueous slurry is pulverized by a wet pulverizer so that the median diameter of the catalyst is preferably 0.1 to 10. The thickness is 15.0 μm, more preferably 0.5 to 12.0 μm. If the median diameter of the catalyst is less than 0.1 μm, the slurry viscosity increases, resulting in non-uniformity of catalyst adhesion to the honeycomb structure and increase in the rate of separation of the catalyst from the honeycomb structure catalyst. On the other hand, when the median diameter of the catalyst exceeds 15.0 μm, the precipitation of the catalyst in the aqueous slurry is remarkable and the stability is poor.

The aqueous slurry prepared by wet pulverization as described above is finally added with an anti-stripping agent to complete the aqueous slurry for coating. The peeling preventive agent is a silica-based ceramic fiber composed of silica or silica and alumina, calcia, etc., and is a heat-resistant fibrous substance, but the component is not particularly limited, and a heat-resistant agent containing silica as a main component is used. It may be any ceramic fiber material. The fiber diameter of the peeling preventive agent (silica-based ceramic fiber) is preferably 6.
0 to 15.0 μm, more preferably 8.0 to 13.0
μm. The average fiber length is preferably 30 to 30.
It is 0 μm, more preferably 60 to 200 μm. When the fiber diameter is less than 6.0 μm, the fiber diameter is too small, which causes a problem in strength, and there is a drawback that a desired fiber length cannot be maintained during preparation of the aqueous slurry. On the other hand, the fiber diameter is 1
If it exceeds 5.0 μm, the particle size of the catalyst component becomes remarkably larger than the particle size of the catalyst component in the aqueous slurry, which makes it difficult to uniformly mix the catalyst component particles and the peeling preventing agent. When the average fiber length is less than 30 μm, sufficient effect for preventing cracks generated in the coat film cannot be obtained, and when the average fiber length exceeds 300 μm, the aqueous slurry is wash-coated on each cell of the honeycomb structure. In addition, cell plugs are easily generated, which is not preferable. The peeling preventive agent having the fiber diameter and the length as described above is 1.0 to the catalyst component in the aqueous slurry.
20.0% by weight, more preferably 2.0 to 10.0% by weight is added. If the amount added is less than 1.0% by weight, the effect of preventing peeling decreases. Further, if the addition amount exceeds 20.0% by weight, economical efficiency is poor.

The production of the honeycomb structure catalyst according to the present invention is carried out by dipping the honeycomb structure into the aqueous slurry for coating prepared as described above and pulling it up, and then adding an extra slurry inside each cell of the honeycomb structure. Are removed by pressurized air (washcoat).

After further drying, the washcoat is repeated. In this way, a desired amount of the catalyst component is fixed on the cell surface of the honeycomb structure, and finally heat-treated (calcined) to obtain a honeycomb structure catalyst in which the catalyst component is firmly fixed.

[0020]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Example 1 250 g of pure water was placed in a beaker, and 4.14 g of silver nitrate was added and dissolved. Powdered alumina hydrate oxide (trade name: Pural SB, manufactured by Condea) was added to this liquid at 40 ° C.
Add 100 g, stir and suspend, and prepare an aqueous solution prepared by dissolving 1.4 g of separately prepared ammonium chloride in 50 g of pure water.
It was added in a minute to make silver chloride supported on the surface of the hydrated alumina oxide particles. After aging for 60 minutes, spray dry,
The obtained powder was further baked at 390 ° C. for 60 minutes to obtain a silver / alumina catalyst containing 2.97% by weight in terms of silver.

Next, 45 g of this catalyst, 100 g of pure water, 2.25 g of acetic acid and alumina sol (Al ₂ O ₃ content 1
0.5% by weight) (21.4 g) was added, and the mixture was stirred and mixed, and the median diameter of the catalyst was adjusted to 8.5 μm using a wet pulverizer to prepare an aqueous slurry having a solid content concentration of 28.0% by weight. The viscosity of this slurry was measured and found to be 15 cps. Fiber diameter of 1 as a peeling preventive agent in the wet-milled slurry
6.0 parts by weight of silica-based ceramic fibers having an average fiber length of 0 μm and an average fiber length of 100 μm were added to the catalyst weight and stirred,
Mixed.

This slurry has a diameter of 20 mm and a height of 16 m.
A cordierite honeycomb structure having a size of m and the number of cells of 200 cells / square inch was dipped, and after pulling up, excess slurry in the cells was removed by pressurized air. After preliminary drying at 50 ° C. for 5 minutes, the catalyst component dried at 190 to 200 ° C. was fixed and this operation was repeated twice. Finally, it was calcined in air at 600 ° C. for 90 minutes to obtain a honeycomb structure catalyst. The obtained honeycomb structure catalyst was 197 g / L.
Was coated with the catalyst component.

The peel test was carried out as follows. The honeycomb structure catalyst was dried at 180 ° C. for 2 hours, cooled in a desiccator, the weight (W ₁ ) of the honeycomb structure catalyst was measured, and the honeycomb structure catalyst was placed in an electric furnace maintained at 800 ° C. for 15 minutes. Hold, immediately leave at room temperature for 15 minutes. After repeating this operation twice, the honeycomb structure catalyst was put in water and subjected to ultrasonic treatment at 28 KHz for 10 minutes. The receipt (W ₂ ) after drying at 180 ° C. for 2 hours and cooling in a desiccator was measured, and the peeling rate was calculated from the following formula. In the following formula, W ₀ is the weight of only the honeycomb structure before wash coating.

Peeling rate (%) = {(W ₁ −W ₂ ) / (W ₁ −
W ₀ )} × 100 The peeling rate thus obtained is shown in Table 1 together with the number of coatings and the coating amount.

Example 2 1,000 g of pure water was placed in a beaker, and 5.7 g of silver nitrate was added and dissolved. Powder silica (trade name: Tokusil UR, manufactured by Tokuyama Corporation) 1
00 g was added and stirred and suspended, and an aqueous solution prepared by dissolving 2.0 g of separately prepared ammonium chloride in 50 g of pure water was added over 15 minutes to support silver chloride on the surface of silica particles. 6
After aging for 0 minutes, filtration and washing were repeated, it was dried by spray drying and further baked at 350 ° C. for 60 minutes to prepare a silver / silica catalyst containing 3.4% by weight in terms of silver.

Next, 45 g of this catalyst and potassium acetate 3.
6 g, 100 g of pure water and 17.6 g of silica sol (SiO ₂ content 20.5% by weight) were stirred and mixed, and the median diameter of the catalyst was adjusted to 6.5 μm using a wet pulverizer.
An aqueous slurry having a solid content concentration of 31.4% by weight was prepared. The viscosity of this slurry was 85 cps. Then, the wet-milled slurry was added with a fiber diameter of 8.5 as a peeling preventing agent.
12.0 parts by weight of silica-based ceramic fiber having an average fiber length of 180 μm was added to the catalyst weight, and the mixture was stirred and mixed.

A honeycomb structure catalyst was prepared by wash-coating the surface of the cells of the honeycomb structure with this aqueous slurry twice by the same method as described in Example 1. The obtained honeycomb structure catalyst was coated with 178 g / L of catalyst component.

A peeling test was conducted in the same manner as in Example 1, and the peeling rate is shown in Table 1 together with the number of coatings and the coating amount.

Example 3 250 g of pure water was placed in a beaker, and 7.4 g of silver nitrate was added and dissolved. To this liquid maintained at 40 ° C., 100 g of powder titania (trade name: C-2, manufactured by Ishihara Sangyo Co., Ltd.) was added, stirred, suspended, and separately prepared ammonium chloride 2.8.
An aqueous solution prepared by dissolving 50 g of pure water in 50 g of pure water was added over 15 minutes to support silver chloride on the surface of cyania particles. After aging for 60 minutes, filtration and washing were repeated, it was dried by spray drying and further baked at 350 ° C. for 60 minutes to prepare a silver / titania catalyst containing 4.4% by weight in terms of silver.

Next, 50 g of this catalyst, 2.5 g of acetic acid, 100 g of pure water and titania sol (TiO ₂ content of 30.
(0% by weight) was stirred and mixed, and a wet mill was used to adjust the median diameter of the catalyst to 1.2 μm to prepare an aqueous slurry having a solid content concentration of 32.6% by weight. The viscosity of this slurry was 36 cps. Then, the wet-milled slurry was added with a fiber diameter of 12.5 μm as an anti-peeling agent,
2.0 parts by weight of silica-based ceramic fiber having an average fiber length of 65 μm was added to the catalyst weight, and the mixture was stirred and mixed.

This aqueous slurry was wash-coated with the catalyst component twice on the surface of the honeycomb structure cell in the same manner as in Example 1 to prepare a honeycomb structure catalyst. The obtained honeycomb structure catalyst was coated with 215 g / L of catalyst component.

A peeling test was conducted in the same manner as in Example 1, and the peeling rate is shown in Table 1 together with the number of coatings and the coating amount.

Example 4 250 g of pure water was placed in a beaker, and 1.59 g of silver nitrate was added and dissolved. Powder zirconia (trade name: RC, manufactured by Daiichi Rare Element Chemical Industry Co., Ltd.) 1 was added to this liquid maintained at 40 ° C.
An aqueous solution prepared by dissolving 0.55 g of sodium carbonate separately prepared in 50 g of pure water was added over 15 minutes to make silver carbonate supported on the surfaces of the zirconia particles. After aging for 60 minutes, filtration and washing were repeated, it was dried by spray drying, and further baked at 450 ° C. for 60 minutes to obtain silver containing 1.0% by weight in terms of silver /
A zirconia catalyst was prepared.

Next, 50 g of this catalyst and 5.5 of acetic anhydride were used.
g, 100 g of pure water and 22.0 g of zirconia sol (ZrO ₂ content 25.0% by weight) are stirred and mixed, and the median diameter of the catalyst is adjusted to 1.0 μm using a wet pulverizer, and the solid content concentration is 31. An aqueous slurry of 4% by weight was prepared. The viscosity of this slurry was 41 cps. Then, the wet-milled slurry was added with a fiber diameter of 1 as an anti-peeling agent.
4.0 parts by weight of silica-based ceramic fibers having a diameter of 2.5 μm and an average fiber length of 180 μm are added to the catalyst weight
Stir and mix.

A honeycomb structure catalyst was prepared by wash-coating the surface of the cells of the honeycomb structure with this aqueous slurry twice by the same method as described in Example 1. The obtained honeycomb structure catalyst was coated with 185 g / L of catalyst component.

A peeling test was conducted in the same manner as in Example 1, and the peeling rate is shown in Table 1 together with the number of coatings and the coating amount.

Example 5 250 g of pure water was placed in a beaker, and 14.9 g of silver nitrate was added and dissolved. Powdered alumina hydrate oxide (trade name: Pural SB, Con
(manufactured by Dea Co., Ltd.), stirred and suspended, and an aqueous solution prepared by dissolving separately prepared 5.1 g of sodium carbonate in 50 g of pure water was added over 15 minutes to support silver carbonate on the surface of the hydrated oxide oxide particles. . After aging for 60 minutes, filtration and washing are repeated, and then dried by spray drying.
Converted to silver by baking for 60 minutes at ℃ 10.0
A silver / alumina catalyst containing wt% was prepared.

Next, 65 g of this catalyst, 2.0 g of acetic acid, 100 g of pure water, and alumina sol (Al ₂ O ₃ content of 10.
5% by weight) (18.6 g) was stirred and mixed, and the median diameter of the catalyst was adjusted to 10.5 μm using a wet pulverizer to prepare an aqueous slurry having a solid content concentration of 36.1% by weight.
The viscosity of this slurry was 58 cps. Then, the wet-milled slurry was used as a peeling preventive agent with a fiber diameter of 10 μm.
m and an average fiber length of 180 μm, silica-based ceramic fibers were added in an amount of 8.0 parts by weight based on the weight of the catalyst, and the mixture was stirred and mixed.

A honeycomb structure catalyst was prepared by wash-coating the surface of the cells of the honeycomb structure with this aqueous slurry twice by the same method as described in Example 1. The obtained honeycomb structure catalyst was coated with 205 g / L of the catalyst component.

A peeling test was conducted in the same manner as in Example 1, and the peeling rate is shown in Table 1 together with the number of coatings and the coating amount.

Comparative Example 1 250 g of pure water was placed in a beaker and 4.14 g of silver nitrate was added and dissolved. Powdered alumina hydrate oxide (trade name: Pural SB, manufactured by Condea) was added to this liquid at 40 ° C.
Add 100 g, stir and suspend, and prepare an aqueous solution prepared by dissolving 1.4 g of separately prepared ammonium chloride in 50 g of pure water.
It was added in a minute to make silver chloride supported on the surface of the hydrated alumina oxide particles. After aging for 60 minutes, spray dry,
The obtained powder was further baked at 390 ° C. for 60 minutes to prepare a silver / alumina catalyst containing 2.97% by weight in terms of silver.

Next, 45 g of this catalyst, 100 g of pure water, 2.25 g of acetic acid, and alumina sol (Al ₂ O ₃ content 1
0.5% by weight) (21.4 g) was added, and the mixture was stirred and mixed, and the wet mill was used to adjust the median diameter of the catalyst to 8.5 μm to prepare an aqueous slurry having a solid content concentration of 28.0% by weight. The viscosity of this slurry was measured and found to be 15 cps.

A honeycomb structure catalyst was prepared by wash-coating the surface of the cells of the honeycomb structure with this aqueous slurry 5 times in the same manner as in Example 1. The obtained honeycomb structure catalyst was coated with 153 g / L of catalyst component.

A peeling test was conducted in the same manner as in Example 1, and the peeling rate is shown in Table 1 together with the number of coatings and the coating amount.

[0046]

[Table 1]

[0047]

According to the production method of the present invention, by adding silica-based ceramic fibers as an anti-stripping agent to the aqueous slurry used for washcoating, more catalyst components can be applied to the honeycomb structure surface with a smaller number of coatings. It is possible to coat. Further, since the coated film of the applied catalyst component is firmly held by the added silica-based ceramic fiber, even if the honeycomb structure catalyst has a significantly large coating amount, the separation of the catalyst component during use is extremely small. Since the honeycomb structure catalyst obtained by the present invention has advantages such as durability of the catalyst, economic efficiency in production, and environmental improvement during use due to a small amount of peeling, the catalyst for exhaust gas purification is applicable. Is extremely advantageous.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01D 53/94 B01J 23/50 ZAB A 23/58 ZAB A 35/04 301 Z (72) Inventor Mitsuo Kojima 1-26, Takitanihonmachi, Niitsu, Niigata Prefecture JGC Chemicals Co., Ltd. Niitsu Plant

Claims (5)

[Claims] 1. A method for producing a catalyst for a honeycomb structure for exhaust gas purification, which comprises fixing a catalyst to the surface of a honeycomb structure having a large number of cells formed in a gas flow direction, wherein the catalyst is an aqueous slurry. When it is solidified and coated on the surface of the honeycomb structure to be fixed, the coating aqueous slurry is a catalyst and a fixing agent composed of at least one sol selected from alumina sol, silica sol, titania sol and zirconia sol, acetic acid, and anhydrous. An aqueous slurry for coating, which contains at least one or more viscosity reducing agents selected from acetic acid and alkali salts of acetic acid and a peeling-preventing agent composed of silica-based ceramic fibers, and has a viscosity of less than 100 cps, is used for the honeycomb structure. An exhaust gas purifying Hanika characterized in that it is applied to the surface, the catalyst is fixed to the surface of the honeycomb structure, and then fired. Method for producing a structure catalyst. 2. The catalyst has at least one active ingredient selected from silver or a silver compound supported on a porous heat-resistant carrier, and the active ingredient is converted into silver in an amount of 0.01 to 20.
The method for producing a honeycomb structure catalyst for exhaust gas purification according to claim 1, containing 0% by weight. 3. The exhaust gas-purifying honeycomb structure catalyst according to claim 1, wherein the viscosity reducing agent is contained in an amount of 0.5 to 15.0% by weight based on the amount of the catalyst in the coating aqueous slurry. Production method. 4. The median diameter of the catalyst in the coating aqueous slurry is 0.1 to 15.0 μm.
Alternatively, the method for producing a honeycomb structure catalyst for exhaust gas purification according to Item 3. 5. A peeling-preventing agent is contained in an amount of 1.0 to 20.0 parts by weight with respect to the amount of catalyst in the aqueous slurry for coating, and the fiber diameter of the silica-based ceramic fiber as the peeling-preventing agent is 6 0.0 to 15.0 μm, average fiber length 30 to 3
The method for producing an exhaust gas-purifying honeycomb structure catalyst according to claim 1, wherein the thickness is 00 μm.