JP4470554B2 - Method for producing exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a method for producing an exhaust gas purification catalyst using a honeycomb-shaped substrate, and more particularly to a method for forming a coating layer in pores of a cell partition wall.

In order to suppress air pollution, exhaust gas purification catalysts such as oxidation catalyst, three-way catalyst, NO _x storage reduction catalyst are installed in the exhaust system of automobiles, and harmful substances such as HC, CO, NO _{x in} exhaust gas are removed. Purifying.

  For example, the three-way catalyst is a honeycomb substrate formed of heat-resistant ceramics such as cordierite and having a large number of cell passages, and a coating layer formed on the surface of the cell passages made of a porous oxide carrier such as alumina and ceria. And a noble metal such as Pt or Rh supported on the coating layer. This three-way catalyst can oxidize HC and CO in the exhaust gas burned at approximately the stoichiometric air-fuel ratio and simultaneously reduce NOx.

  In order to improve the purification performance by the exhaust gas purification catalyst, it is effective to increase the contact efficiency between the exhaust gas and the coat layer, and in the honeycomb base material, the cell density is increased. However, since the coat layer is generally formed on the surface of the cell wall, if the coat layer is formed thick, the exhaust pressure loss increases, and the cell density must be considered including the thickness of the coat layer. From the viewpoint of strength, there is a limit to reducing the thickness of the cell partition wall, and since there is a limit from the aspect of exhaust pressure loss, the cell density is already saturated.

  Therefore, Japanese Patent Laid-Open No. 2003-170043 uses a honeycomb substrate having a cell wall with a porosity of 40 to 75% and a D50 pore diameter of 10 to 50 μm, and at least 90% by mass of each of the catalyst carrier and the catalyst component is a cell. An exhaust gas purifying catalyst arranged in the pores of the wall has been proposed. According to this catalyst, the thickness of the catalyst layer formed on the surface of the cell wall can be extremely reduced while maintaining high purification performance, so that the geometric surface area is increased without further reducing the thickness of the cell wall. Therefore, exhaust pressure loss can be reduced. Moreover, since the catalyst layer formed in the pore has a larger surface area than that formed on the conventional cell wall surface, the contact efficiency with the exhaust gas is further improved.

  On the other hand, exhaust gas from a diesel engine contains PM, and a ceramic plug-type honeycomb body (diesel PM filter (hereinafter referred to as DPF)) is known for collecting the PM. This DPF is formed by alternately sealing both ends of the opening of the cell of the ceramic honeycomb structure, for example, in a checkered pattern, and is adjacent to the inflow side cell clogged on the exhaust gas downstream side and the inflow side cell. It consists of an outflow side cell clogged upstream of the exhaust gas, and a cell partition partitioning the inflow side cell and the outflow side cell. By filtering exhaust gas through the pores of the cell partition wall and collecting PM, the emission is suppressed. To do.

  In recent years, a continuous regeneration type DPF (exhaust gas purification filter catalyst) has been developed in which a coating layer is formed of alumina or the like on a cell partition of a DPF, and a catalytic metal such as Pt is supported on the coating layer. According to this continuous regeneration type DPF, since the collected PM is oxidized and burned by the catalytic reaction of the catalytic metal, the increase in exhaust pressure loss can be suppressed by burning simultaneously with the collection or continuously with the collection. Can be played. Since the catalytic reaction occurs at a relatively low temperature and can be burned while the amount collected is small, there is an advantage that the thermal stress acting on the DPF is small and damage is prevented.

  As such an exhaust gas purification filter catalyst, Japanese Patent Application Laid-Open No. 09-220423 discloses a porous oxide which has a cell partition wall porosity of 40 to 65% and an average pore diameter of 5 to 35 μm and constitutes a coating layer. A carrier having a structure in which a particle size smaller than the average pore size of the cell partition accounts for 90 wt% or more is disclosed. Thus, by coating the cell partition wall having pores with the porous oxide carrier powder having a high specific surface area, a coating layer can be formed not only on the surface of the cell partition wall but also on the inner surface of the pores. Further, if the coating amount is constant, the coating thickness can be reduced, so that an increase in exhaust pressure loss can be suppressed.

  The coating layer is formed by a wash coating method using a slurry in which an oxide carrier powder such as alumina is mixed with a binder and water. Then, after impregnating the slurry in the cell, a method of removing excess slurry by blowing off or sucking with high-pressure air, followed by drying and firing is performed. In order to form the coat layer even in the pores of the cell partition walls, a slurry having a low viscosity by adding a large amount of water is used, and it is easy to enter the pores by capillary action. In order to remove excess slurry, a suction method is used. For example, in the case of DPF, the slurry is impregnated from one end face of the honeycomb base material and sucked from the other end face, so that the slurry is put into the pores of the cell partition wall. Is forced to infiltrate.

However, when a slurry having a low viscosity is used, the slurry oozes out from the pores between the removal of the excess slurry and the drying, which may block the pores. If it becomes like this, while the gas diffusibility at the time of use will fall, utilization of the catalyst in the obstruct | occluded pore will become difficult, and purification performance will fall. When there is a large amount of oozing, the exhaust pressure loss may increase because the slurry is coated on the surface of the cell partition wall.
JP 09-220423 JP2003-170043

  The present invention has been made in view of the above-described circumstances, and an object to be solved is to reliably form a coating layer in the pores and to suppress the seepage of slurry from the pores.

  A feature of the method for producing an exhaust gas purifying catalyst of the present invention that solves the above problems is that a honeycomb base material having pores in cell partition walls that divide adjacent cells is used, and a slurry containing oxide carrier powder is used as the honeycomb base material. An impregnation step for adhering the slurry into the pores, a removal step for removing excess slurry, a thickening step for increasing the viscosity of the slurry in the pores, and drying and firing the adhered slurry And a firing step of forming a coating layer in the pores.

It is desirable that the viscosity of the slurry in the impregnation step is 5 m · Pa · s or less, and the viscosity of the slurry after the thickening step is 10 m · Pa · s or more.

  According to the production method of the present invention, since the viscosity of the slurry is low in the impregnation step, the slurry can be sufficiently infiltrated into the pores. After the excess slurry is removed, the slurry in the pores has increased in viscosity due to the thickening step, so that exudation from the pores is suppressed.

  Therefore, a coating layer can be sufficiently formed in the pores of the cell partition wall, and the coating layer on the surface of the cell partition wall can be made extremely thin or no, so that the cell density can be further increased and the purification performance is improved. improves.

  The honeycomb substrate has pores in cell partition walls that partition adjacent cells, and can be formed from heat-resistant ceramics such as cordierite, silicon carbide, and silicon nitride. Both a honeycomb substrate having a straight flow structure and a honeycomb substrate having a wall flow structure can be used. In the case of a honeycomb substrate having a straight flow structure, it is particularly preferable that the cell partition wall has a porosity of 40 to 75% and a D50 pore diameter of 10 to 50 μm. In this case, the pores of the cell partition walls are substantially non-through holes. In the case of a honeycomb substrate having a wall flow structure, the pores of the cell partition walls are substantially through-holes, and the pore distribution in the cell partition walls is 40 to 80% in the porosity as in the conventional DPF. The average pore diameter can be in the range of 10-50 μm. If the porosity or the average pore diameter is out of this range, the PM collection efficiency may decrease or the exhaust pressure loss may increase.

  By using the honeycomb substrate having such a pore distribution, the coating layer can be easily formed in the pores, and high gas diffusibility is exhibited. In order to form such pores, a raw material mixed with heat-resistant ceramic powder is controlled while controlling the particle size and amount of flammable powder such as carbon powder and resin powder, and formed into a honeycomb shape by extrusion or the like. And it can form by burning off combustible powder at the time of the baking. The pore distribution described above can be measured using a mercury porosimeter, and D50 pore diameter means a pore diameter having a cumulative volume of 50% in the measured pore diameter distribution.

  The slurry contains an oxide carrier powder, and a slurry composed of an oxide carrier powder, a binder, and water is generally used. As the oxide support, oxides such as alumina, zirconia, ceria, titania, etc. used for conventional oxidation catalysts and three-way catalysts, ceria-zirconia, alumina-ceria-zirconia, ceria-zirconia-yttria, A composite oxide such as zirconia-calcia can also be used. The average particle diameter of the oxide carrier powder is not particularly limited as long as it is smaller than the average pore diameter of the cell partition walls. The binder binds the oxide powders in the coat layer and binds the coat layer to the surface of the pores, and conventionally used ones such as the above-mentioned oxide sol can be used in the same manner as before. .

The impregnation step is a step of allowing the slurry to adhere to the honeycomb substrate and allowing the slurry to enter the pores. This step is desirably performed in a state where the viscosity of the slurry is lowered. If the viscosity is low, the slurry easily enters the pores by capillary action. The viscosity of the slurry in the impregnation step is preferably 5 m · Pa · s or less, for example. If the viscosity is higher than this, it may be difficult to enter the pores. In order to reduce the viscosity of the slurry, there are a heating method and a method of applying a shear stress. When the viscosity is reduced by heating, the slurry is attached to the honeycomb substrate in a state where the slurry is heated to a predetermined temperature or more, and the slurry is allowed to enter the pores. In this case, it is desirable to use a slurry having a large viscosity temperature dependency. The temperature dependence of the viscosity is greatly influenced by the particle size, hydrophilicity, density, specific surface area, solid content concentration of the slurry, and the like of the oxide carrier powder, so that an optimum temperature is set according to the composition of the slurry.

  In addition, when the viscosity is lowered by applying a shear stress, it is desirable that the slurry has thixotropic properties. For this purpose, various organic and inorganic thixotropy imparting agents such as bentonite, silicon oxide powder, and amide compounds may be added to the slurry.

  The impregnation step can be performed by a method of pouring the slurry into the cell, a method of immersing the honeycomb substrate in the slurry, or the like. When applying a shear stress, it is preferable that the honeycomb substrate is brought into contact with the slurry while being vibrated by ultrasonic waves or the like.

  The removing step is a step of removing excess slurry from the honeycomb substrate, and can be performed by blowing off with pressurized air or vacuum suction. In particular, in the case of DPF, it is desirable that the slurry be impregnated from one end face of the honeycomb base material and sucked from the other end face side, so that the slurry is forcibly entered into the pores of the cell partition walls.

The thickening step is a step of increasing the viscosity of the slurry in the pores. When the viscosity is lowered by heating in the impregnation step, the viscosity increases by cooling. In addition, when the viscosity is reduced by shear stress, the viscosity increases by standing. In the case of cooling, it may be left as it is, or forced cooling can be performed by circulating air in the cell in the removal step. The viscosity of the thickened slurry is desirably 10 m · Pa · s or more. If the viscosity is lower than this, the slurry may ooze out from the pores until drying.

  The thickening step may be performed before the removing step, the thickening step may be performed simultaneously with the removing step, or the thickening step may be performed after the removing step.

  The firing step is a step of forming a coating layer in the pores by drying and firing the slurry adhering in the pores. In order to prevent heat shock, the moisture can be dried at a relatively low temperature of 110 to 130 ° C and then fired at 250 to 300 ° C.

After the calcination step, depending on the type of oxide carrier, it may be used as a catalyst for exhaust gas purification as it is, but in general, catalytic metals such as noble metals, transition metals, NO _x storage materials are used depending on the type of catalyst. Supported. For this loading, a conventionally used method such as an adsorption loading method or an impregnation loading method can be used. The catalyst metal can also be supported by preparing a slurry using an oxide carrier powder on which the catalyst metal is previously supported, and then forming a coat layer.

  Hereinafter, the present invention will be specifically described with reference to test examples and examples.

(Test example)
A cordierite honeycomb base material (60% porosity, D50 pore diameter 23 μm, 4 mil 400 cell, diameter 30 mm, length 50 mm) having straight flow square cells was prepared.

  Also, a suspension composed of 100 parts by weight of ceria-zirconia composite oxide powder, 10 parts by weight of alumina powder, 10 parts by weight of alumina sol (solid content 10% by weight) and 30-40 parts by weight of ion-exchanged water was prepared. Milling was performed to prepare a slurry having an average particle size of 2.5 μm. Three types of slurries were prepared in a solid content concentration range of 50 wt% to 55 wt%.

  In the state where these three types of A, B, and C slurries having different solid content concentrations are heated to 70 ° C., the honeycomb base material is immersed in the slurry for 1 minute, and immediately after being pulled up, an excess slurry is obtained using a suction coating apparatus. Was left at room temperature for 30 minutes, dried at 120 ° C. for 1 hour, and calcined at 250 ° C. for 1 hour. Each coat layer was formed at 180 g / L. Thereafter, each coating state was observed with EPMA, and the thickness of the coating layer formed by oozing out on the cell partition wall surface was measured. The results are shown in FIG.

  Note that the temperature of the slurry at the time when the slurry was removed by the suction coater was about 50 ° C. Therefore, the viscosity when each slurry was heated to 50 ° C. and 70 ° C. was measured. The results are shown in Table 1.

EPMAによる観察の結果、いずれもセル隔壁の細孔内にコート層が形成され、スラリーの粘度が５ｍ・Pa・ｓ以下であれば細孔内に十分に浸入することがわかった。また図１より、粘度が低いスラリーを用いた場合に滲み出しが多いことがわかり、スラリーの粘度が10ｍ・Pa・ｓ以上であれば滲み出しはほとんど生じないことがわかる。

As a result of observation by EPMA, it was found that in each case, a coating layer was formed in the pores of the cell partition walls, and if the viscosity of the slurry was 5 m · Pa · s or less, the coating penetrated sufficiently into the pores. Further, it can be seen from FIG. 1 that there is much oozing when using a slurry having a low viscosity, and that oozing hardly occurs if the viscosity of the slurry is 10 m · Pa · s or more.

(Example)
A suspension composed of 10 parts by weight of alumina powder, 100 parts by weight of ceria powder, 10 parts by weight of alumina sol (solid content 10% by weight) and 40 parts by weight of ion-exchanged water was prepared and milled with a ball mill to obtain an average particle size of 2.5 μm. A slurry was prepared. The solid content concentration of the slurry is 50% by weight. The temperature-viscosity curve of this slurry is shown in FIG.

<Impregnation process>
The slurry was heated to 70 ° C. with a heater, and a honeycomb substrate similar to that used in the test example was immersed for 1 minute while maintaining the temperature at 70 ° C.

<Removal process / Thickening process>
Immediately after pulling up, excess slurry was removed using a suction-type coater and forced cooling was performed, followed by standing at room temperature for 30 minutes. The temperature of the slurry at the time when the slurry was removed by the suction coater was about 40 ° C.

<Baking process>
Subsequently, it dried at 120 degreeC for 1 hour, and baked at 250 degreeC for 1 hour. The coat layer was formed at 180 g / L.

<Evaluation>
The obtained catalyst is shown in FIG. This catalyst is composed of a honeycomb substrate 1 having a straight flow structure and a coat layer 2 formed in the pores 11 of the cell partition walls 10 of the honeycomb substrate 1.

Since the slurry is heated to 70 ° C. in the impregnation step, the viscosity of the slurry is 3 m · Pa · s as shown in FIG. 2, and the slurry sufficiently penetrates into the pores. After the removal step, the temperature of the slurry is about 40 ° C., and the viscosity of the slurry in the pores 11 is about 13 m · Pa · s as shown in FIG. The slurry oozes out from the pores 11 at, and the cell partition wall 10 is not formed with a coating layer due to oozing.

  Therefore, according to the catalyst obtained in this example, since the exhaust pressure loss is low, the cell density can be further increased, and the contact area between the exhaust gas and the catalyst is increased. In addition, since the coat layer 2 is formed in the pores 11 and is not clogged and excellent in gas diffusibility, high purification performance is exhibited.

It is a graph which shows the result of a test example and shows the relationship between the viscosity at 50 ° C. and the coating thickness (exudation amount) on the cell partition wall surface. It is a graph which shows the temperature-viscosity curve of the slurry used in the Example. It is explanatory drawing which shows the catalyst manufactured in one Example of this invention.

Explanation of symbols

  1: Honeycomb substrate 2: Coat layer 10: Cell partition wall 11: Pore

Claims (3)

An impregnation step of using a honeycomb substrate having pores in cell partition walls that divide adjacent cells, attaching a slurry containing an oxide carrier powder to the honeycomb substrate, and infiltrating the slurry into the pores;
A removal step to remove excess slurry;
A thickening step to increase the viscosity of the slurry in the pores;
And a firing step of forming a coat layer in the pores by drying and firing the adhering slurry. The method for producing an exhaust gas purification catalyst according to claim 1, wherein the viscosity of the slurry in the impregnation step is 5 m · Pa · s or less, and the viscosity of the slurry after the thickening step is 10 m · Pa · s or more. .   The method for producing an exhaust gas purifying catalyst according to claim 1 or 2, wherein in the impregnation step, the slurry is heated to lower the viscosity, and in the thickening step, the honeycomb substrate containing the slurry is cooled.

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