JP2608641B2 - Honeycomb catalyst for purifying exhaust gas and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a honeycomb catalyst for purifying exhaust gas and a method for producing the same, and more particularly, to a method in which black smoke particles in exhaust gas of a diesel engine or the like accumulate at an end of the catalyst to increase pressure loss. TECHNICAL FIELD The present invention relates to an exhaust gas purifying catalyst for a diesel engine or the like improved so as to prevent it, and a method for producing the same.

[0002]

2. Description of the Related Art Exhaust gas emitted from diesel engines and the like contains harmful components such as black smoke particles in addition to HC, CO, and NOx. It is known to use a filter to remove black smoke particles, but soluble organic matter among black smoke particles can be detoxified together with HC and CO by an oxidation catalyst. However, when a honeycomb-shaped catalyst having a large geometric surface area is used, conventionally, as shown in FIGS. 1 and 2,
In the honeycomb catalyst in which the catalyst layer 4 is applied to the inlet-side open end surface 2 and the end portion 3 of the honeycomb 1, the black smoke particles in the exhaust gas hit the inlet-side open end portion of the honeycomb catalyst, and the open end surface 2 and adjacent thereto. Channel 5 of honeycomb 1 at end 3
As a result, the effective diameter of the through-hole channel 5 of the honeycomb catalyst is reduced, and the pressure of the catalyst is increased. In particular, when the exhaust gas discharge speed is low, as in the case of low-speed running, the deposition is likely to proceed. As a result, there is a problem that the deposit is ignited, the temperature is rapidly increased, and the catalyst is damaged.

[0003] The application of activated alumina to the honeycomb catalyst is indispensable for supporting the catalytic noble metal. However, since the surface of the honeycomb catalyst is rough, the end face of the opening on the inlet side of the honeycomb catalyst and the end face adjacent to the end face thereof are inevitable. Black smoke particles that have collided or stalled are likely to be captured. Immediately after the application of the activated alumina slurry, the slurry flows to the edge to form a pool, which narrows the through-holes, which is a problem because it promotes the capture and accumulation of black smoke particles.

In order to solve such a problem, it is conceivable to expose the surface of the catalyst carrier without applying active alumina to the vicinity of the end portion. Although there is an advantage that the effective diameter can be increased, the above problem cannot be solved because the surface of the carrier is porous and rough.

[0005] As a prior art related to the present invention,
For the purpose of saving precious metal catalyst only in the vicinity of the outer wall of the honeycomb catalyst, a method of masking the relevant portion with a polymer film (Japanese Patent Application Laid-Open No. 63-84639) or a method of impregnating the relevant portion with oil (Japanese Patent Application No. 58-156352) is known,
In these methods, the polymer film and oil are removed in the baking step, and the surface of the porous carrier is only exposed as it is, so that there is no effect.

On the other hand, to reinforce the ceramic honeycomb structure, the outer peripheral wall thereof is coated with ceramic (Japanese Patent Publication No. 51-44713), and the glass component is impregnated with the ceramic (Japanese Kono Sho Jitsu).
53-34373) and filling the pores with a reinforcing material (Japanese Patent Publication No. 62-6855), and sealing the pores with a filler to suppress gas leakage in the rotary heat storage type heat exchanger (Japanese Patent Publication No. No. 62-27355), but the present invention differs in purpose, effect, and application site.

Japanese Patent Application Laid-Open No. 63-185425 discloses that in a filter for removing black smoke of a diesel engine, a cordierite layer is provided on the partition wall on the outlet side of the filter in order to partially limit the amount of black smoke particles deposited. It is disclosed that the wall thickness is increased by coating to reduce the amount of gas flowing to the outlet side, and to prevent the occurrence of cracks at the outlet side during regeneration due to the accumulation of black smoke particles in the gas. But this is also the purpose,
The effects and means are different from those of the present invention.

[0008]

SUMMARY OF THE INVENTION According to the present invention, an end portion including an inlet end surface of an exhaust gas purifying honeycomb catalyst is smoothed, thereby making it difficult to apply activated alumina, and an end portion including an inlet side open end surface. It is intended to prevent black smoke particles from being trapped and deposited on the surface.

Another object of the present invention is to provide a catalyst which is more thermally stable by preventing a decrease in thermal shock resistance due to uneven thermal expansion due to the application of different materials.

[0010]

SUMMARY OF THE INVENTION The honeycomb catalyst for purifying exhaust gas of the present invention comprises a honeycomb catalyst carrier comprising porous cordierite ceramic and a catalyst layer provided on the surface of the honeycomb catalyst carrier. The honeycomb catalyst carrier is characterized in that at least a part of an inlet-side open end face and an end part adjacent to the inlet-side end face is formed of a surface layer that is smoother than the catalyst layer.

In a preferred embodiment of the present invention, it is preferable that the catalyst supports a porous ceramic honeycomb structure, and the smoothed portion is coated with a vitreous material.

The length of the smoothed end portion is preferably within 30 mm in the axial direction from the end face of the honeycomb opening.

The catalyst carrier serving as the substrate is a porous cordierite ceramic honeycomb, and the average thermal expansion coefficient (α ₁ ) from room temperature to 800 ° C. of the partition wall between the smoothed honeycomb channels and the activated alumina and the catalyst metal are determined. The average coefficient of thermal expansion (α
₂ ) The difference is preferably 0.7 × 10 ⁻⁶ / ° C. or less.

Further, according to the method for producing a honeycomb catalyst for purifying exhaust gas of the present invention, a material which becomes a low expansion glass by firing is applied to an open end face of a honeycomb catalyst carrier made of porous cordierite ceramic and an end portion adjacent to the open end face. After baking and impregnating to form a smooth surface layer, active alumina and a catalyst metal are applied to portions other than the surface layer to form a catalyst layer.

Further, the difference between the coefficient of thermal expansion (α ₁ ) of the smoothed partition wall and the coefficient of thermal expansion (α ₂ ) of the partition wall provided with the activated alumina is obtained by using a porous cordierite ceramic honeycomb as a catalyst carrier as a substrate. It is preferable to adjust the thickness and material of the glass layer and the thickness of the activated alumina layer so as to be 0.7 × 10 ⁻⁶ / ° C. or less.

[0016]

According to the present invention, a low-thermal-expansion glass material is applied to the inlet-side end face of the porous ceramic honeycomb and the end adjacent to the same and baked to make it smooth, and activated alumina and a catalytic noble metal are added to the slurry. After dipping and coating to remove excess slurry, baking is performed to obtain a honeycomb catalyst.

Although a slight amount of activated alumina adhered to the surface of the smoothed portion of the honeycomb catalyst, it was considered that the pressure loss did not substantially affect the pressure loss. As described above, the present invention can be applied to a case where an activated alumina layer is partially adhered to a portion which is smoothed within a range that does not substantially affect the pressure loss, for process variation, intentionally, or for another purpose. Of course.

The thus obtained honeycomb catalyst according to the present invention was mounted on a 2.8-liter diesel engine, and the rotation speed was 10
Under low-speed operation conditions of 00 rpm and an exhaust gas temperature of 240 ° C., the observation of the accumulation of black smoke particles and the measurement of the pressure rise were performed.

As a result, as shown in FIG. 11, the catalyst of the present invention showed a remarkable effect on the increase in pressure loss as compared with the conventional catalyst.

The effect on the pressure loss was confirmed by changing the length of one side of the honeycomb cell, that is, the distance from the end face of the smoothed portion to the honeycomb carriers having different through-hole diameters. As shown in Fig. 12, there is a difference due to the difference in the diameter of the through-hole.
It was found that the effect could be sufficiently exhibited by smoothing within mm. This is equivalent to about 20% of the length of a 152 mm long honeycomb. This effect is recognized as an effect of smoothing the upstream side, that is, the end on the inlet side with respect to the flow of the exhaust gas.

The downstream end may also be smoothed, whereby the downstream end can be effectively prevented from reducing pressure loss due to narrowing of the flow path due to activated alumina slurry.

It is advantageous in terms of manufacturing to smooth the open end over the entire cross section, but the end face is masked by various methods as described later to smooth the end face and part of the cross section of the end. Can also. The flow rate of the exhaust gas becomes slower from the center to the periphery, and the lower the flow rate, the more easily the black smoke particles are captured. Thus, only the outer peripheral portion of the cross section may be smoothed. In this case, the smoothing distance from the end does not need to be limited to 30 mm or less. It can be changed depending on the characteristics of the engine and the dimensions of the honeycomb catalyst.

Next, the type of the glass material to be smoothed was changed, and the thermal stability was evaluated. The thermal expansion coefficients of the cordierite ceramic carrier and the glass material, which are the bases, are not basically the same, so the smoothed part is the thickness of the partition walls of the carrier,
It depends on the thickness of the glass in the application area. Similarly, the activated alumina carrying portion also changes depending on the thickness of the carrier partition wall and the activated alumina layer.

It is desirable that the thermal shock resistance of the catalyst has no difference in thermal expansion coefficient inside the integrated catalyst. However, since the thermal expansion coefficient varies depending on the above-mentioned coating conditions, the thermal expansion is more affected by the supporting conditions and the partition wall thickness than by limiting the glass material. It is important to correct the coefficients.

According to the present invention, the thermal shock resistance of the catalyst can be enhanced by selecting the composition of the vitreous material and correcting the coefficient of thermal expansion and the coating thickness of the partition walls.

A porous cordierite ceramic honeycomb is used as a catalyst carrier as a substrate, and an average thermal expansion coefficient (α ₁ ) from room temperature to 800 ° C. of a partition wall between honeycomb channels in a densified portion and activated alumina and a catalyst metal are provided. Average coefficient of thermal expansion (α) from room temperature to 800 ° C
_By setting the difference in ₂ ) to 0.7 × 10 ⁻⁶ / ° C. or less, cracks can be prevented up to high temperatures.

[0027]

3 to 6 show an embodiment of the present invention, in which a glass material having a low thermal expansion is applied to an opening end face 2 on the inlet side of a porous ceramic honeycomb 1 and an end 3 adjacent thereto, and baked. After smoothing, the entire porous ceramic honeycomb 1 is dipped and coated in a slurry of activated alumina and a catalytic noble metal to remove excess slurry and then baked to provide a catalyst layer.

According to the present invention, black smoke particles are hardly deposited at the opening end of the inlet side. Even when black smoke particles are deposited, the amount of the black smoke particles is small as indicated by the imaginary line 7.
The channel 5 does not become so narrow as to be problematic.

FIGS. 7 and 8 show examples in which the central circular portion of the inlet-side open end face 2 of the porous ceramic honeycomb 1 is masked to provide a smooth surface layer only on the outer peripheral portion 10 of the cross section.

FIGS. 9 and 10 show that the end portion 3 of the porous ceramic honeycomb 1 is first smoothed using a large-diameter circular mask at the center of the inlet-side open end surface 2 and then to a small diameter. An example in which the smoothing process is performed again using the circular mask shown in FIG.

Example 1 An outer diameter of 118 mm was used as a porous ceramic honeycomb catalyst carrier.
A fired product of a cordierite-based honeycomb structure having a length of 152 mm, a partition wall thickness of 150 μm, and 62 cells per square centimeter was prepared. In the smoothing treatment, water and a dispersant were added to the coating material powder A shown in the Zegel formula in Table 1 to obtain a slurry having a water content of 50%.

[0032]

5 mm from one open end face of the honeycomb structure
Was immersed in the slurry for 1 minute, excess slurry was scattered and removed with compressed air, dried at 120 ° C. for 2 hours, and calcined at 1300 ° C. for 3 hours. Next, the entire honeycomb was immersed in a slurry containing a catalytic noble metal and containing an active alumina as a main component for 2 minutes, and the excess activated alumina slurry was carefully blown out with compressed air at 120 ° C. for 2 minutes.
Hr dried. After performing the steps of immersion and drying of the slurry twice, 70
It was fired at 0 ° C.

Activated alumina was hardly adhered to the smoothed portion, and no protrusion of activated alumina slurry from the honeycomb end face was observed. The smoothing-treated honeycomb structure had small clogging due to the loading of activated alumina and a small decrease in the opening area.

A honeycomb structure B (active alumina supported) manufactured by the conventional method and a honeycomb structure C before the active alumina support were prepared at the same time as the end-smoothed honeycomb structure A according to the present invention thus prepared. As an example, each of the honeycomb structures is accommodated in a holding container, a 2.8-liter diesel engine, an engine speed of 1,000 rpm, and a torque of 7 kg.
FIG. 11 shows the result of observing the change in pressure loss at −m and the exhaust gas temperature of 240 ° C. It was confirmed that the increase in the pressure loss of the honeycomb structure having the smoothed end portions was extremely low.

Example 2 Subsequently, a fired product of a cordierite-based honeycomb structure shown in Table 2 was prepared as a carrier to be subjected to an edge smoothing treatment.

[0037]

In the same manner as in the first embodiment, the end portions were smoothed, and the smoothed end portions of the respective honeycomb structures shown in Table 2 had lengths of 2 mm, 5 mm, 10 mm, 20 mm, and 30 mm.
mm, 50 mm, 70 mm samples were prepared, and then activated alumina was applied in the same manner as in the first embodiment.
Next, the change in pressure loss of each honeycomb catalyst was investigated.

FIG. 12 shows the test results described above. Although there is a difference depending on the cell structure of the honeycomb structure, an effect of suppressing a remarkable increase in pressure loss as the length is increased up to a length of 30 mm from the end face of the smoothing end is recognized. Even if the length of the smoothing process from the end face exceeded 30 mm, only the same effect as 30 mm was obtained.

Example 3 Further, an edge smoothing treatment was carried out in the same manner as in Example 1 using the smoothing treatment material shown in the Zegel formula in Table 3 and the slurry concentration and the number of times of loading were changed in the activated alumina coating treatment. Various evaluation samples were produced.

[0041]

The amount of activated alumina carried on the sample, the thermal expansion coefficient of the smoothed end portion after carrying the activated alumina and the thermal expansion coefficient of the coated portion of activated alumina, and the thickness of the partition wall, and the thermal shock resistance of the produced sample from an electric furnace by a rapid cooling method The sex was evaluated. The results are shown in Table 4.

[0043]

[Table 4]

When the difference between the coefficient of thermal expansion (α ₂ ) of the partition wall on the activated alumina coated portion and the coefficient of thermal expansion (α ₁ ) of the partition wall provided with the end face coating becomes large, after the thermal shock resistance test at which the electric furnace is taken out to the boundary surface. , Cracks were observed, 65
Α ₁ and α ₂ to withstand the thermal shock temperature difference of 0 ℃ or more
It is found that the difference is preferably within 0.7 × 10 ⁻⁶ / ° C.

[0045]

According to the present invention, it is possible to prevent a sudden increase in pressure loss due to the accumulation of black smoke, to maintain a high catalytic performance continuously, and to have a high reliability even at high temperatures. A high exhaust gas purifying catalyst can be provided, which is extremely useful from the viewpoint of preventing pollution.

[Brief description of the drawings]

FIG. 1 is a partial longitudinal sectional view of an inlet end of a conventional exhaust gas purifying honeycomb catalyst.

FIG. 2 is a cross-sectional view taken along line II-II of FIG.

FIG. 3 is a partial longitudinal sectional view of an end portion on the inlet side of the honeycomb catalyst for purifying exhaust gas according to the present invention.

FIG. 4 is a sectional view taken along line IV-IV in FIG.

5 is a schematic diagram of the inlet end of the honeycomb catalyst having the smoothed end shown in FIG. 3, in which the smoothed end is hatched.

FIG. 6 is a plan view of the honeycomb catalyst shown in FIG. 5, in which an end face of the smoothing treatment is hatched.

FIG. 7 is a view similar to FIG. 3, showing another embodiment of the present invention.

FIG. 8 is a plan view of the honeycomb catalyst shown in FIG. 7, in which an end face of the smoothing treatment is hatched.

FIG. 9 is a view similar to FIG. 3, showing another embodiment of the present invention.

FIG. 10 is a plan view of the honeycomb catalyst shown in FIG. 9, in which an end face of the smoothing treatment is hatched.

FIG. 11 is a graph showing the relationship between the operating time and the pressure loss of the honeycomb catalyst according to the present invention in comparison with a conventional product.

FIG. 12 is a graph showing a relationship between a length of an end portion smoothing process and a pressure loss.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Honeycomb 2 Inlet side opening end surface 3 End part 4 Catalyst layer 5 Flow path 6 Partition wall 7 Virtual line showing accumulation of black smoke particles 10 Outer peripheral part

Claims (6)

(57) [Claims] 1. An exhaust gas purifying honeycomb catalyst comprising a honeycomb catalyst carrier made of porous cordierite ceramic and a catalyst layer provided on the surface of the honeycomb catalyst carrier, wherein the honeycomb catalyst carrier has an inlet-side open end face, A honeycomb catalyst for purifying exhaust gas, wherein at least a part of an end portion adjacent to the catalyst layer has a surface layer that is smoother than the catalyst layer. 2. The exhaust gas purifying honeycomb catalyst according to claim 1, wherein said surface layer is coated with a vitreous material. 3. The honeycomb catalyst for purifying exhaust gas according to claim 1, wherein the length of the surface layer is within 30 mm in the axial direction from the end face of the opening of the honeycomb catalyst carrier. 4. An average coefficient of thermal expansion (α ₁ ) of the partition wall provided with the surface layer from room temperature to 800 ° C. and an average thermal expansion coefficient of the partition wall provided with the catalyst layer from room temperature to 800 ° C. The honeycomb catalyst for purifying exhaust gas according to any one of claims 1 to 3, wherein a difference of α ₂ ) is 0.7 × 10 ⁻⁶ / ° C. or less. 5. An open end surface of a honeycomb catalyst carrier made of porous cordierite ceramic and an end portion adjacent to the open end surface are coated with a material that becomes low expansion glass by firing, and baked and impregnated to form a smooth surface layer. A method for producing a honeycomb catalyst for purifying exhaust gas, comprising forming a catalyst layer by applying active alumina and a catalyst metal to portions other than the surface layer. 6. The coefficient of thermal expansion (α ₁ ) of the partition wall where the surface layer is formed after the surface layer is formed, and the coefficient of thermal expansion (α ₂ ) of the partition wall where the catalyst layer is formed in the portion where the catalyst layer is formed. 6. The exhaust gas according to claim 5, wherein the thickness and material of the low expansion glass forming the surface layer and the thickness of the catalyst layer are adjusted so that the difference between them is 0.7 × 10 ⁻⁶ / ° C. or less. A method for producing a purifying honeycomb catalyst.