JP7186031B2 - honeycomb structure - Google Patents

Description

The present invention relates to a honeycomb structure.

Exhaust gases emitted from internal combustion engines such as automobiles contain harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC). Such an exhaust gas purifying catalyst that decomposes harmful gases is also called a three-way catalyst. things are common.

On the other hand, Patent Document 1 discloses an exhaust gas purifying catalyst in which a monolithic base material contains ceria-zirconia composite oxide particles and θ-phase alumina particles, and precious metal particles are supported on the monolithic base material.

JP 2015-85241 A

In the exhaust gas purifying catalyst described in Patent Document 1, cordierite is not used as the material of the monolith base material, and a material that itself has a catalyst carrier function and a co-catalyst function is used. It is said that the temperature of the catalyst can be easily increased and the warm-up performance of the catalyst can be improved.

However, in the flow-through type exhaust gas purifying catalyst, the exhaust gas tends to flow on the inner peripheral side of the honeycomb structure, which is a honeycomb-shaped monolith base material, that is, on the radially inner side of the honeycomb structure, while on the other hand, the outer periphery of the honeycomb structure Since it is difficult for the exhaust gas to flow to the side, that is, the radially outer side of the honeycomb structure, the flow of the exhaust gas tends to be non-uniform. Therefore, the catalyst supported on the outer peripheral side of the honeycomb structure may not be effectively used compared to the inner peripheral side.

Therefore, it is conceivable to reduce the amount of catalyst supported on the outer peripheral side of the honeycomb structure compared to the inner peripheral side thereof. However, in the loading method of immersing (dipping) the slurry containing the catalyst into the honeycomb structure, it is easy to change the amount of the catalyst loaded in the longitudinal direction of the honeycomb structure, whereas it is It is not easy to change the amount of catalyst supported in

The present invention has been made to solve the above problems, and an object of the present invention is to provide a honeycomb structure that can effectively use precious metals supported on the honeycomb fired body and that has excellent exhaust gas purification performance. .

A honeycomb structure of the present invention is a honeycomb structure comprising a honeycomb fired body in which a plurality of through holes are arranged in parallel in the longitudinal direction with partition walls interposed therebetween and an outer peripheral wall is provided on the outermost periphery, and the honeycomb fired body is , ceria-zirconia composite oxide particles and alumina particles, and the porosity in the outer 1/5 range in the radial direction of the honeycomb fired body is the porosity in the inner 4/5 range in the radial direction of the honeycomb fired body. characterized by being higher than the rate.

When a noble metal serving as a catalyst is carried on a honeycomb fired body constituting a honeycomb structure, a large amount of material having a large specific surface area is arranged in a portion with a low porosity. Noble metals are less likely to be carried in the high modulus portion. In the honeycomb structure of the present invention, the porosity on the outer peripheral side of the honeycomb fired body is higher than the porosity on the inner peripheral side. , the amount of noble metal supported on the inner peripheral side of the honeycomb fired body, which greatly contributes to exhaust gas purification, increases. Therefore, in the honeycomb structure of the present invention, even if the amount of precious metal supported on the entire honeycomb fired body is the same, the exhaust gas purification is more efficient than when the precious metal is supported on the honeycomb fired body having the same overall porosity. Better performance.

Further, in the honeycomb structure of the present invention, since the honeycomb fired body is composed of the ceria-zirconia composite oxide particles and the alumina particles, the heat capacity can be reduced. Therefore, the temperature tends to rise in a short time, and in particular, the temperature rises more easily on the inner peripheral side of the honeycomb fired body than on the outer peripheral side, where the exhaust gas tends to flow. As described above, in the honeycomb structure of the present invention, since the amount of noble metal supported on the inner peripheral side of the honeycomb fired body can be increased, high exhaust gas purification performance can be exhibited in a short period of time after starting.

In this specification, the radial direction of the honeycomb fired body is a direction perpendicular to the longitudinal direction in which the plurality of through holes extend.

In the honeycomb structure of the present invention, the porosity of the entire honeycomb fired body is 50 to 70%, and the porosity in the outer ⅕ range in the radial direction of the honeycomb fired body and the diameter of the honeycomb fired body It is desirable that the difference from the porosity in the inner 4/5 of the direction is 3% or more.
When the porosity of the honeycomb fired body is within the above range, the temperature of the honeycomb fired body tends to rise in a short period of time after starting, and thus the exhaust gas purification performance is excellent. In addition, when the difference between the porosity on the outer peripheral side and the inner peripheral side of the honeycomb fired body is 3% or more, the amount of noble metal supported on the inner peripheral side of the honeycomb fired body can be increased, which results in exhaust gas. Purification performance can be improved.

It should be noted that "the difference between the porosity in the outer 1/5 range in the radial direction of the honeycomb fired body and the porosity in the inner 4/5 range in the radial direction of the honeycomb fired body is 3% or more" means that the honeycomb fired body When P ₁ (%) is the porosity in the radially outer ⅕ range of the fired body, and P ₂ (%) is the porosity in the radially inner 4/5 range of the honeycomb fired body, P ₁ - means that P ₂ ≧3.

In the honeycomb structure of the present invention, it is desirable that the porosity of the outer peripheral wall of the honeycomb fired body is higher than the porosity of the outer ⅕ range in the radial direction of the honeycomb fired body.
By increasing the porosity of the outer peripheral wall of the honeycomb fired body, it is possible to reduce the amount of noble metal carried on the outer peripheral side of the honeycomb fired body, which contributes the least to exhaust gas purification.

In the honeycomb structure of the present invention, it is desirable that the through holes surrounded by the partition walls have the same size in a cross section perpendicular to the longitudinal direction of the honeycomb fired body.
There is also a means for improving the exhaust gas purification performance by changing the size of the through-hole to change the flow rate of the exhaust gas to make the flow rate of the exhaust gas approximately equal between the inner and outer circumferences, but in the present invention, the production is more efficient. Even if the sizes of the through-holes are the same for ease of use, the exhaust gas purifying performance can be improved as described above.
The through holes having the same size means that the through holes surrounded by the partition wall have the same shape in the cross section.

In the honeycomb structure of the present invention, it is desirable that the honeycomb fired body carries a noble metal.
By supporting a noble metal on the honeycomb fired body, the honeycomb structure can be used as a honeycomb catalyst for exhaust gas purification.

In the honeycomb structure of the present invention, the amount of the noble metal supported in the outer ⅕ range in the radial direction of the honeycomb fired body is equal to the amount of the noble metal supported in the inner 4/5 range in the radial direction of the honeycomb fired body. Less than the amount is desirable.
By increasing the amount of noble metal supported on the inner peripheral side of the honeycomb fired body, it is possible to exhibit high exhaust gas purification performance in a short time after starting.

In this specification, the supported amount of noble metal (g/L) refers to the weight (g) of noble metal per apparent volume (L) of the honeycomb fired body.

FIG. 1 is a perspective view schematically showing an example of the honeycomb structure of the present invention. FIG. 2 is a cross-sectional view taken along line AA in FIG.

(Detailed description of the invention)
[Honeycomb structure]
A honeycomb structure of the present invention will be described.

FIG. 1 is a perspective view schematically showing an example of the honeycomb structure of the present invention.
As shown in FIG. 1, a honeycomb structure 10 has a plurality of through holes 12 arranged side by side in the longitudinal direction (the direction indicated by the double arrow L in FIG. 1) with partition walls 13 interposed therebetween, and an outer peripheral wall 14 on the outermost periphery. It consists of a honeycomb fired body 11 provided.
The honeycomb fired body 11 is composed of ceria-zirconia composite oxide particles (hereinafter also referred to as CZ particles) and alumina particles.
As shown in FIG. 1, when the honeycomb structure 10 is composed of a single honeycomb fired body 11, the honeycomb fired body 11 is also the honeycomb structure itself.

In the honeycomb structure of the present invention, the porosity in the outer 1/5 range in the radial direction of the honeycomb fired body is higher than the porosity in the inner 4/5 range in the radial direction of the honeycomb fired body.
The radially outer ⅕ range of the honeycomb fired body and the radially inner 4/5 range of the honeycomb fired body will be described with reference to FIG.
FIG. 2 is a cross-sectional view taken along line AA in FIG.
In the honeycomb structure 10 shown in FIG. 2, the cross-sectional shape of the honeycomb fired body 11 perpendicular to the longitudinal direction is a circle with a radius R centered on the point O. As shown in FIG. The inner 4/5 range in the radial direction of the honeycomb fired body 11 is a range surrounded by an outline _S1 of a circle with a radius of 4/5R centered at the point O and having a length of 4/5 of the radius R ( _C1 ), and the radial outer ⅕ range of the honeycomb fired body 11 is the range surrounded by the contour S ₁ of a circle with a radius of 4/5R and the contour S ₂ of the honeycomb fired body 11 ( _C2 ).
The outline _S1 of the circle with a radius of 4/5R and the outline _S2 of the honeycomb fired body 11 are similar figures sharing the center of gravity, and the similarity ratio of the outline _S1 to the outline _S2 is 4/5.

In the honeycomb structure of the present invention, the cross-sectional shape perpendicular to the longitudinal direction of the honeycomb fired body is not limited to a circular shape. A similar figure of 4/5 (that is, a similar figure with an area of 64%) is assumed, and the range surrounded by the outline of this similar figure is defined as "the inner 4/5 range in the radial direction of the honeycomb fired body". The range surrounded by the outline of the similar figure and the outline of the honeycomb fired body is defined as "the radially outer ⅕ range of the honeycomb fired body".

In the honeycomb structure of the present invention, the porosity of the entire honeycomb fired body is 50 to 70%, and the porosity in the range of the outer ⅕ in the radial direction of the honeycomb fired body and the inner side in the radial direction of the honeycomb fired body It is desirable that the difference from the porosity in the range of 4/5 is 3% or more.
When the porosity of the honeycomb fired body is within the above range, the temperature of the honeycomb fired body tends to rise in a short period of time after starting, and thus the exhaust gas purification performance is excellent. In addition, when the difference between the porosity on the outer peripheral side and the inner peripheral side of the honeycomb fired body is 3% or more, the amount of noble metal supported on the inner peripheral side of the honeycomb fired body can be increased, which results in exhaust gas. Purification performance can be improved.

In the honeycomb structure of the present invention, it is desirable that the porosity of the outer peripheral wall of the honeycomb fired body is higher than the porosity of the outer ⅕ range in the radial direction of the honeycomb fired body.
By increasing the porosity of the outer peripheral wall of the honeycomb fired body, it is possible to reduce the amount of noble metal carried on the outer peripheral side of the honeycomb fired body, which contributes the least to exhaust gas purification.
The porosity of the outer peripheral wall of the honeycomb fired body can be measured by cutting out only the outer peripheral wall and using the gravimetric method described below. However, the size of the outer peripheral wall to be cut out is about 100 mm ³ .

The porosity of the honeycomb fired body is measured by a gravimetric method (also referred to as a gravimetric porosity measuring method).
For example, a honeycomb fired body located in a range where the porosity is to be measured is cut into a size of 10 cells×10 cells×10 mm, and the porosity of that portion is measured by the gravimetric method. When measuring the porosity in the outer ⅕ range in the radial direction of the honeycomb fired body, the honeycomb fired body is cut so as to include the outer peripheral wall, and the porosity of that portion is measured by the gravimetric method. do. When measuring the porosity in the radially inner 4/5 range of the honeycomb fired body, the honeycomb fired body in that portion is cut out and the porosity in that portion is measured by the gravimetric method.
The porosity of the entire honeycomb fired body is obtained by multiplying the porosity of the outer 1/5 range in the radial direction of the honeycomb fired body by 9 and the porosity of the inner 4/5 range of the honeycomb fired body in the radial direction. is multiplied by 16 and divided by 25.
The porosity of the honeycomb fired body measured here is the porosity that does not include the volume of the through holes of the honeycomb fired body, and specifically refers to the porosity of the partition walls and/or the outer peripheral wall.

The gravimetric method is the method shown below.
(1) Cut the honeycomb fired body into a size of 10 cells×10 cells×10 mm to obtain a measurement sample. After ultrasonically cleaning this measurement sample using ion-exchanged water and acetone, it is dried at 100° C. using an oven.
In addition, the measurement sample of 10 cells × 10 cells × 10 mm is a state in which 10 through-holes are arranged in the vertical direction and 10 in the horizontal direction, and the partition walls constituting the through-holes of the outermost through-holes are included, It refers to a sample cut out so that the length in the longitudinal direction is 100.
(2) Using a measuring microscope (Measuring Microscope MM-40 manufactured by Nikon, magnification: 100 times), measure the dimensions of the cross-sectional shape of the measurement sample, and determine the volume from geometric calculations (geometric calculations If the volume cannot be obtained from the above, measure the weight in saturated water and the weight in water to measure the volume).
(3) From the calculated volume and the true density of the measurement sample measured by the pycnometer, the weight of the measurement sample is calculated assuming that it is a completely dense body. The measurement procedure with the pycnometer is as shown in (4).
(4) Pulverize the honeycomb fired body to prepare 23.6 cc of powder. The powder obtained is dried at 200° C. for 8 hours. After that, the true density is measured according to JIS R 1620 (1995) using an Auto Pycnometer 1320 manufactured by Micromeritics. The evacuation time is 40 minutes.
(5) Measure the actual weight of the measurement sample with an electronic balance (HR202i manufactured by A&D).
(6) Obtain the porosity of the honeycomb fired body from the following equation.
(Porosity of honeycomb fired body) = 100 - (actual weight of measurement sample/weight assuming that measurement sample is a complete dense body) x 100 [%]
Even when the honeycomb structure of the present invention supports a noble metal, the change in the porosity of the honeycomb fired body due to the supporting of the noble metal is so small that it can be ignored.

In the honeycomb structure of the present invention, the honeycomb fired body is an extruded body containing CZ particles and alumina particles. That is, the honeycomb structure of the present invention is composed of a fired honeycomb body produced by firing a formed honeycomb body obtained by extruding a raw material paste containing CZ particles and alumina particles.
It can be confirmed by X-ray diffraction (XRD) that the honeycomb structure has the above components.

The honeycomb structure of the present invention may have a single honeycomb fired body, or may have a plurality of honeycomb fired bodies. When the honeycomb structure of the present invention includes a plurality of honeycomb fired bodies, it is desirable that the plurality of honeycomb fired bodies are bonded with an adhesive layer.

In the honeycomb structure of the present invention, the content of CZ particles constituting the honeycomb fired body is preferably 25 to 75% by weight.
When the content of CZ particles constituting the honeycomb fired body is 25 to 75% by weight, the oxygen storage capacity (OSC) of cerium can be increased.

In the honeycomb structure of the present invention, the alumina particles forming the honeycomb fired body are desirably θ-phase alumina particles.
If the alumina particles are θ-phase alumina particles, they have high heat resistance, so that noble metals can be supported and high exhaust gas purifying performance can be exhibited even after long-term use.

In the honeycomb structure of the present invention, the content of alumina particles constituting the honeycomb fired body is desirably 15 to 35% by weight.

Examples of the shape of the honeycomb structure of the present invention include cylindrical, prismatic, cylindric, long cylindrical, and chamfered prismatic shapes (for example, chamfered triangular prism shapes).

In the honeycomb structure of the present invention, it is desirable that the partition walls of the honeycomb fired body have a uniform thickness. Specifically, the thickness of the partition walls of the honeycomb fired body is desirably 0.05 to 0.50 mm, and more desirably 0.05 to 0.30 mm.

In the honeycomb structure of the present invention, the shape of the through-holes of the honeycomb fired body is not limited to a square pillar shape, and may be a triangular pillar shape, a hexagonal pillar shape, or the like.
Although the through-holes may have different shapes, it is desirable that all the through-holes have the same shape. That is, it is desirable that the sizes of the through holes surrounded by the partition walls are the same in the cross section perpendicular to the longitudinal direction of the honeycomb fired body.
There is also a means for improving the exhaust gas purification performance by changing the size of the through-hole to change the flow rate of the exhaust gas to make the flow rate of the exhaust gas approximately equal between the inner and outer circumferences, but in the present invention, the production is more efficient. Even if the sizes of the through-holes are the same for ease of use, the exhaust gas purifying performance can be improved as described above.

In the honeycomb structure of the present invention, it is desirable that the density of the through-holes in the cross section perpendicular to the longitudinal direction of the honeycomb fired body is 31 to 155/cm ² .

In the honeycomb structure of the present invention, the fired honeycomb body may further contain inorganic fibers and an inorganic binder.

Alumina fibers are desirable as the inorganic fibers.
Using alumina fibers as the inorganic fibers can improve the mechanical properties of the honeycomb structure.

Boehmite is desirable as the inorganic binder.
This is because most of the boehmite becomes γ-alumina through the firing process.

In the honeycomb structure of the present invention, it is desirable that the honeycomb fired body supports a noble metal.
Examples of noble metals include platinum group metals such as platinum, palladium and rhodium.
The amount of noble metal supported on the entire honeycomb fired body is desirably 0.1 to 15 g/L, and more desirably 0.5 to 10 g/L.
In the present specification, the supported amount of noble metal refers to the weight of the noble metal per apparent volume of the honeycomb structure. The apparent volume of the honeycomb structure is the volume including the volume of the voids, and when the adhesive layer is included, the volume of the adhesive layer is included.

In the honeycomb structure of the present invention, the amount of noble metal supported in the outer ⅕ range in the radial direction of the honeycomb fired body is smaller than the amount of noble metal supported in the inner 4/5 range in the radial direction of the honeycomb fired body. is desirable.
By increasing the amount of noble metal supported on the inner peripheral side of the honeycomb fired body, it is possible to exhibit high exhaust gas purification performance in a short time after starting.

In the honeycomb structure of the present invention, an outer peripheral coat layer may be formed on the outer peripheral surface of the honeycomb fired body.
The thickness of the peripheral coat layer is preferably 0.1 to 2.0 mm.

[Manufacturing method of honeycomb structure]
Next, a method for manufacturing the honeycomb structure of the present invention will be described.
The honeycomb structure of the present invention is a honeycomb formed body in which a plurality of through-holes are arranged in parallel in the longitudinal direction with partition walls interposed therebetween by molding a raw material paste containing, for example, CZ particles, alumina particles, inorganic fibers, and an inorganic binder. a drying step of drying the honeycomb formed body formed in the forming step; and a firing step of producing a honeycomb fired body by firing the honeycomb formed body dried in the drying step; It can be produced by

(Molding process)
In the molding step, first, CZ particles and alumina particles are mixed to prepare a raw material paste.
The raw material paste may further contain inorganic fibers, inorganic binders, organic binders, pore-forming agents, molding aids, dispersion media, and the like.

CZ particles are a material used as a co-catalyst (oxygen storage material) for an exhaust gas purification catalyst. As the CZ particles, those in which ceria and zirconia form a solid solution are desirable.

The CZ particles may further contain rare earth elements other than cerium. Rare earth elements include scandium (Sc), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), Ytterbium (Yb), ruthenium (Lu) and the like can be mentioned.

The CZ particles preferably contain 30% by weight or more of ceria, more preferably 40% by weight or more, and preferably contain 90% by weight or less of ceria, more preferably 80% by weight or less. Also, the CZ particles desirably contain zirconia in an amount of 60% by weight or less, more preferably 50% by weight or less. Since such CZ particles have a small heat capacity, the temperature of the honeycomb structure can be easily increased, and the warm-up performance can be improved.

From the viewpoint of improving the thermal shock resistance, the average particle size of the CZ particles is preferably 1 to 50 μm. Further, it is more desirable that the CZ particles have an average particle size of 1 to 30 μm. When the average particle diameter of the CZ particles is 1 to 50 μm, the honeycomb structure has a large surface area, so that the oxygen storage capacity can be increased.

Although the type of alumina particles is not particularly limited, θ-phase alumina particles (hereinafter also referred to as θ-alumina particles) are desirable.
By using the θ-phase alumina particles as a partition material for the CZ particles, it is possible to suppress the alumina particles from sintering each other due to heat during use, so that the catalytic function can be maintained. Furthermore, the heat resistance can be enhanced by making the alumina particles the θ phase.

Although the average particle size of the alumina particles is not particularly limited, it is preferably 1 to 10 μm, more preferably 1 to 5 μm, from the viewpoint of improving gas purification performance and warming up performance.

The average particle size of CZ particles and alumina particles can be determined by a laser diffraction particle size distribution analyzer (MASTERSIZER 2000 manufactured by MALVERN).

Materials constituting the inorganic fibers are not particularly limited, but examples thereof include alumina, silica, silicon carbide, silica-alumina, glass, potassium titanate, and aluminum borate, and two or more of them may be used in combination. Among these, alumina fibers are desirable.

The aspect ratio of the inorganic fibers is desirably 5-300, more desirably 10-200, even more desirably 10-100.
Inorganic fibers refer to those having an aspect ratio of 5 or more.

Boehmite is desirable as the inorganic binder.
Boehmite is an alumina monohydrate having a composition of AlOOH, and is well dispersed in a medium such as water. Therefore, it is desirable to use boehmite as an alumina binder.
Moreover, by using boehmite, the moisture content in the raw material paste can be lowered, and the moldability can be improved.

Examples of organic binders include, but are not limited to, methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, polyethylene glycol, phenolic resins, epoxy resins, and the like, and two or more of them may be used in combination.

Examples of the pore-forming agent include, but are not particularly limited to, acrylic resin, coke, starch, and the like.
A pore-forming agent is used to introduce pores into the interior of a honeycomb fired body when manufacturing the honeycomb fired body.

Examples of molding aids include, but are not limited to, ethylene glycol, dextrin, fatty acids, fatty acid soaps, polyalcohols, and the like, and two or more of them may be used in combination.

Examples of the dispersion medium include, but are not limited to, water, organic solvents such as benzene, and alcohols such as methanol, and two or more of them may be used in combination.

When CZ particles, alumina particles, alumina fibers and alumina binder are used as the raw materials described above, the mixing ratio of these is, relative to the total solid content remaining after the firing process in the raw materials, CZ particles: 25 to 75% by weight, alumina Desirably, particles: 15 to 35% by weight, alumina fibers: 5 to 15% by weight, and alumina binder: 5 to 20% by weight.

When preparing the raw material paste, it is desirable to mix and knead, and the mixture may be mixed using a mixer, an attritor or the like, or may be kneaded using a kneader or the like.

In the forming step, the raw material paste containing the CZ particles and the alumina particles is extruded to obtain a formed honeycomb body in which a plurality of through holes are arranged in parallel in the longitudinal direction with partition walls interposed therebetween.
At this time, as a method for increasing the porosity in the outer ⅕ range in the radial direction of the honeycomb fired body after the firing process to the porosity in the inner 4/5 range in the radial direction of the honeycomb fired body, for example, In the molding process, there is a method of adjusting the molding conditions and/or the shape of the mold so that the extrusion speed of the raw material paste near the outer peripheral wall is faster than the extrusion speed of the raw material paste at other locations.
Since the weight of the raw material paste supplied from the mold does not change between the vicinity of the outer circumference of the mold and the rest of the mold, the extrusion speed of the raw material paste near the outer peripheral wall is made faster than the extrusion speed of the raw material paste at other locations. As a result, the density of the honeycomb molded body in the vicinity of the outer peripheral wall can be lowered, so that the porosity in the vicinity of the outer peripheral wall can be increased in the honeycomb fired body after the firing step.

The shape of the formed honeycomb body is not particularly limited, but a columnar shape is desirable. Moreover, it is desirable that the diameter is 150 mm or less in the case of a cylindrical shape.
Moreover, the honeycomb formed body may have a prismatic shape, and in the case of a prismatic shape, it is preferably a square prismatic shape.

(Drying process)
Subsequently, a drying step of drying the formed honeycomb body to obtain a dried honeycomb body is performed.
In the drying step, the formed honeycomb body is dried using a dryer such as a microwave dryer, a hot air dryer, a dielectric dryer, a reduced pressure dryer, a vacuum dryer, or a freeze dryer to produce a dried honeycomb body.

(Baking process)
In the firing step, a fired honeycomb body is produced by firing the dried honeycomb body dried in the drying step. Note that this step can be called a “degreasing/firing step” because the dried honeycomb body is degreased and fired, but for the sake of convenience, it will be referred to as a “firing step”.

The temperature of the firing step is desirably 800-1300.degree. C., more desirably 900-1200.degree. In addition, the time of the firing process is preferably 1 to 24 hours,
More preferably 3 to 18 hours. Although the atmosphere of the firing process is not particularly limited, it is desirable that the oxygen concentration is 1 to 20%.

Through the above steps, the honeycomb structure of the present invention can be manufactured.

(Other processes)
The method for manufacturing the honeycomb structure of the present invention may further include a supporting step of supporting a noble metal on the honeycomb fired body, if necessary.
As a method for supporting a noble metal on a honeycomb fired body, for example, a method of immersing a honeycomb fired body or a honeycomb structure in a solution containing noble metal particles or a complex, then pulling out and heating the body, or the like can be mentioned.
When the honeycomb structure has a peripheral coat layer, the noble metal may be carried on the honeycomb fired body before forming the peripheral coat layer, or the noble metal may be loaded on the honeycomb fired body or the honeycomb structure after forming the peripheral coat layer. You can carry it.

In the method for manufacturing a honeycomb structure of the present invention, the amount of the noble metal supported in the supporting step is desirably 0.1 to 15 g/L, more desirably 0.5 to 10 g/L.

In the method of manufacturing the honeycomb structure of the present invention, when forming the outer peripheral coat layer on the outer peripheral surface of the honeycomb fired body, the outer peripheral coat layer is applied to the outer peripheral surface excluding both end surfaces of the honeycomb fired body by applying a paste for the outer peripheral coat layer. After that, it can be formed by drying and solidifying. As the outer peripheral coat layer paste, one having the same composition as the raw material paste can be used.

(Example)
Examples that more specifically disclose the present invention are shown below. In addition, the present invention is not limited only to the following examples.

[Fabrication of honeycomb structure]
(Example 1)
26.4% by weight of CZ particles (average particle diameter: 2 μm), 13.2% by weight of θ-alumina particles (average particle diameter: 2 μm), alumina fibers (average fiber diameter: 3 μm, average fiber length: 60 μm) 5.3% by weight of boehmite as an alumina binder, 5.3% by weight of methyl cellulose as an organic binder, 2.1% by weight of acrylic resin as a pore-forming agent, and 2.1% by weight of coke as a pore-forming agent. 6% by weight, 4.2% by weight of polyoxyethylene oleyl ether, which is a surface active agent as a forming aid, and 29.6% by weight of deionized water were mixed and kneaded to prepare a raw material paste.

Using an extruder, the raw material paste was extruded to produce a cylindrical honeycomb molded body.
At this time, the extrusion conditions were adjusted so that the extrusion speed of the raw material paste in the vicinity of the outer peripheral wall was higher than the extrusion speed of the raw material paste in other locations.
Then, using a vacuum microwave dryer, the honeycomb formed body was dried for 12 minutes at an output of 1.74 kW and a reduced pressure of 6.7 kPa.

[Baking process]
The honeycomb fired body according to Example 1 was produced by degreasing and firing the obtained dried honeycomb body at 1100° C. for 10 hours. ^The honeycomb fired body had a cylindrical shape with a diameter of 103 mm and a length of 105 mm.

[Measurement of porosity]
From the honeycomb fired body according to Example 1, a sample for porosity measurement (10 cells×10 cells×10 mm) was cut out from the radially inner 4/5 range and the radially outer 1/5 range, and the outer peripheral wall Cut the chisel into a size of about 100 mm ³ , measure the porosity of each sample, measure the porosity in the range of the outer 1/5, the porosity in the range of the inner 4/5, and the porosity of the outer peripheral wall. The total porosity was calculated from the porosity in the 1/5 range and the porosity in the inner 4/5 range. Table 1 shows the results.

[Supporting step]
Dinitrodiammine palladium nitrate solution ([Pd( _NH3 ) ₂ ( _NO2 ) ₂ ] _HNO3 , palladium concentration 100 g/L) and rhodium nitrate solution ([Rh( _NO3 ) ₃ ], rhodium concentration 50 g/L) : 1 volume ratio to prepare a mixed solution. The honeycomb fired body manufactured by the above steps was immersed in this mixed solution and held for 24 hours. Thereafter, the honeycomb fired body was dried at 110° C. for 2 hours and fired in a nitrogen atmosphere at 500° C. for 1 hour to obtain a honeycomb structure in which palladium and rhodium catalysts were supported on the honeycomb fired body.
The amount of noble metals supported was set to 3.0 g/L per apparent volume of the honeycomb fired body in total of palladium and rhodium.

[Measurement of noble metal concentration]
The honeycomb structure was cut in a direction perpendicular to the longitudinal direction, and the partition walls exposed in the radial inner 4/5 range and the partition walls exposed in the radial outer 1/5 range of the cut surface were observed by EPMA. Then, the noble metal (total of Pd and Rh) concentration on the surface of the partition wall was measured. Table 1 shows the results.
Specifically, a honeycomb structure was processed into a size of 3 cells×3 cells×10 mm, hardened with an epoxy resin, mirror-polished, and platinum was vapor-deposited on the observation surface to obtain an observation sample. The device used was JXA8500F manufactured by JEOL. The acceleration voltage was 25 kV, the irradiation current was 4×10 ⁻⁸ A, the beam diameter was 10 μm, and the irradiation time was 40 ms.
The observation sample of 3 cells x 3 cells x 10 mm includes the outermost through-hole and the partition wall that constitutes the through-hole in a state in which three through-holes are arranged in the vertical direction and three in the horizontal direction, It refers to a sample cut out so that the length in the longitudinal direction is 10 mm.

[Evaluation of warm-up performance]
The honeycomb catalyst according to Example 1 was set in a V-type 6-cylinder 3.5L engine, and the HC concentration ((HC inflow - HC outflow)/(HC inflow) x 100) was observed from the start of the stoichiometric engine. The time until 50% or less (HC light-off time) was measured to evaluate the warm-up performance of the honeycomb catalyst. Table 1 shows the results.
A shorter HC light-off time means better warm-up performance.

(Example 2 and Comparative Example 1)
A honeycomb structure according to Example 2 was produced in the same procedure as in Example 1, except that the conditions for extrusion molding were changed so that the extrusion speed of the raw material paste near the outer peripheral wall was higher than in Example 1.
In addition, a honeycomb structure according to Comparative Example 1 was produced in the same manner as in Example 1, except that the conditions for extrusion molding were changed so that the extrusion rate of the raw material paste near the outer peripheral wall was the same as in other locations. did.
As for the honeycomb structures according to Example 2 and Comparative Example 1, the porosity and noble metal concentration were measured in the same manner as in Example 1, and the warm-up performance was evaluated. Table 1 shows the results.

As shown in Table 1, it was found that the honeycomb structure of the present invention exhibits higher warm-up performance when the same amount of noble metal is supported.

10 Honeycomb structure 11 Honeycomb fired body 12 Through hole 13 Partition wall 14 Peripheral wall C ₁ Range of inner 4/5 in radial direction of honeycomb fired body C ₂ Range of outer 1/5 in radial direction of honeycomb fired body L Longitudinal direction O Center of honeycomb fired body R Radius of honeycomb fired body S ₁ Outline of circle with radius 4/5R S ₂ Outline of honeycomb fired body

Claims (5)

A honeycomb structure made of a honeycomb fired body in which a plurality of through holes are arranged in parallel in the longitudinal direction with partition walls interposed therebetween, and an outer peripheral wall is provided on the outermost periphery,
The honeycomb fired body comprises ceria-zirconia composite oxide particles and alumina particles,
The porosity of the entire honeycomb fired body is 50 to 70%,
The porosity in the outer 1/5 range in the radial direction of the honeycomb fired body is higher than the porosity in the inner 4/5 range in the radial direction of the honeycomb fired body, and the honeycomb fired body has a porosity in the radial direction. A honeycomb structure, wherein the difference between the porosity in the outer ⅕ range of the honeycomb fired body and the porosity in the inner ⅕ range in the radial direction of the honeycomb fired body is 3% or more. 2. The honeycomb structure according to claim 1 , wherein the porosity of the outer peripheral wall of the honeycomb fired body is higher than the porosity of the outer ⅕ range in the radial direction of the honeycomb fired body. The honeycomb structure according to claim 1 or 2 , wherein the through holes surrounded by the partition walls have the same size in a cross section perpendicular to the longitudinal direction of the honeycomb fired body. The honeycomb structure according to any one of claims 1 to 3 , wherein the honeycomb fired body supports a noble metal. 5. The method according to claim 4 , wherein the amount of the noble metal supported in the outer ⅕ range in the radial direction of the honeycomb fired body is smaller than the amount of the noble metal supported in the inner 4/5 range in the radial direction of the honeycomb fired body. honeycomb structure.

Images