WO2010131369A1

WO2010131369A1 - Exhaust purifying catalyst and method of manufacturing the same

Info

Publication number: WO2010131369A1
Application number: PCT/JP2009/059096
Authority: WO
Inventors: 智章砂田; 秀章植野
Original assignee: トヨタ自動車株式会社
Priority date: 2009-05-15
Filing date: 2009-05-15
Publication date: 2010-11-18

Abstract

An exhaust purifying catalyst (100) is provided with a catalyst layer (17) having a layer thickness gradually increasing from the center position (P) of a cell (11), which center position is equidistant from an exhaust inlet-side end surface (12) of the cell and from exhaust outlet-side end surface (13) of the cell, toward the end surfaces (12, 13) such that the layer thickness is Y, which is the smallest layer thickness, at the center position (P) and is X, which is greater by two or more times than the layer thickness Y, at the end surfaces (12, 13).

Description

Exhaust gas purification catalyst and method for producing the same

The present invention relates to an exhaust gas purification catalyst suitable for purifying gas such as NO _x contained in exhaust gas, and a method for producing the same.

Conventionally, various catalysts have been proposed as exhaust gas purification catalysts for purifying nitrogen oxides (NO _x ) contained in exhaust gas of internal combustion engines, and platinum (Pt), rhodium (Rh), palladium (Pd) as with _the NO _x storage material such as a noble metal or an alkali metal or an alkaline earth metal or the like and the like are generally utilized.

Also, as the structure of the purification catalyst, a purification catalyst having a two-layer structure, for example, in addition to a single layer structure in which a metal component is present in a single layer has been proposed. For example, a catalyst provided with two upper and lower catalyst layers in which Pt and Rh are included in different layers is known in consideration of the influence on catalytic activity due to solid solution of metals.

However, for example, when purifying exhaust gas discharged from an internal combustion engine mounted on an automobile or the like, the exhaust gas is not always discharged at a constant temperature or discharge amount. Therefore, when the purification catalyst does not reach a predetermined temperature when purifying the exhaust gas, there is a concern that unpurified exhaust gas flows out into the atmosphere until the catalytic activity of the purification catalyst increases.

In relation to the above, the exhaust gas purifying catalyst in which the pore volume of the cell partition wall is larger on the exhaust gas upstream side than on the exhaust gas downstream side so that the thickness of the catalyst support coat layer on the exhaust gas upstream side is thicker than that on the exhaust gas downstream side. Is disclosed (for example, see Patent Document 1).

JP 2007-330860 A

In the exhaust gas-purifying catalyst, heat on the upstream side is easily dissipated to the downstream side, and heat degradation on the upstream side can be suppressed, and a certain degree of improvement is expected in terms of warm-up characteristics. When the temperature of the exhaust gas is low, or when the gas temperature temporarily drops due to exhaust stop due to intermittent operation, etc., the heat retention performance that can maintain the purification activity may not be stably maintained. . For example, the catalyst temperature is low immediately after the engine is started, and unpurified exhaust gas tends to flow out until the catalyst is activated. Further, in a hybrid vehicle that has been attracting attention in recent years, the engine is intermittently stopped, so there are times when exhaust gas does not flow, the catalyst temperature tends to decrease, and the catalyst temperature may not be maintained above a certain temperature. Even in gasoline engine vehicles other than hybrid vehicles, when the vehicle speed is low, the exhaust gas temperature is lower than at high speeds, so the catalyst temperature tends to decrease and the catalyst temperature may not be kept above a certain temperature. .

The present invention has been made in view of the above situation. Under these circumstances, it is necessary to provide an exhaust gas purification catalyst that is hardly affected by the exhaust gas emission conditions, can maintain the catalytic activity stably, and maintain the purification performance, and a method for producing the same. ing.

The present invention provides the thinnest region (layer thickness Y) in the middle of the catalyst layer in the exhaust gas flow direction, and provides regions that are twice or more thicker than the layer thickness Y on the upstream side and downstream side thereof. The present inventors have obtained knowledge that the balance between heat transfer ability and heat capacity that can enhance early activation and heat retention of the catalyst from a low temperature can be optimized, and is based on such knowledge.

The exhaust gas purifying catalyst according to the first aspect of the present invention includes a region having a layer thickness X that is twice or more the thinnest layer thickness Y at the intermediate position between the upstream end surface and the downstream end surface in the exhaust gas flow direction. A catalyst layer having upstream and downstream of the position of the layer thickness Y in the exhaust gas flow direction is provided on the support base material.

In the first invention, the thickness Y of the layer is preferably in the range of 10 to 80 μm. In addition, the position of the layer thickness Y is based on the center position at an equal distance L from both ends of the exhaust gas flow direction of the catalyst layer, the position up to L × 3/10 on the upstream side in the exhaust gas flow direction and L × 3 on the downstream side. The case where it has between the position to / 10 is preferable.

Here, the intermediate position between the upstream end face and the downstream end face includes not only a position equidistant from the upstream end face and the downstream end face, but also an arbitrary position between the upstream end face and the downstream end face. .

In the exhaust gas purification catalyst according to the first aspect of the present invention, it is twice the thinnest layer thickness Y provided between the upstream end surface and the downstream end surface of the catalyst layer in the exhaust gas flow direction in which the exhaust gas flows. The regions having the above layer thickness X are provided on the upstream side and the downstream side in the exhaust gas flow direction as viewed from the position of the layer thickness Y, and the thickness of the catalyst layer once decreases from the exhaust gas inflow side and then increases. By having a region, for example, at the time of cold start of an engine where the catalyst temperature becomes low, the catalyst in a low temperature state is activated in a short time, and for example, the engine of a hybrid vehicle in which gas emission stops and the catalyst temperature decreases The heat of the catalyst can be maintained during intermittent stops or during low speed operation in which the catalyst temperature decreases as the exhaust gas temperature decreases.

That is, in the exhaust gas purifying catalyst of the first invention, by providing a region where the layer thickness of the catalyst layer is increased to increase the pressure loss, heat transfer from the exhaust gas to the catalyst layer is accelerated, Since the heat capacity is reduced by providing the thin region having the layer thickness Y, the rate of temperature increase of the catalyst layer can be increased. Further, since the heat capacity is increased by increasing the upstream layer thickness and the downstream layer thickness of the region of the layer thickness Y, the heat is retained even when a relatively low temperature exhaust gas flows, Since the region of thickness Y is the thinnest, heat transfer to the downstream side in the exhaust gas flow direction is reduced, so that a temperature decrease on the downstream side of the region of layer thickness Y can be prevented.
As a result, when the catalyst temperature is low, the purification catalyst is activated in a short time, and when there is a possibility that the catalyst temperature is lowered, the heat is maintained and the catalyst activity is kept high, and the exhaust gas can be stably purified. The

In the first invention, the ratio of the layer thickness X to the layer thickness Y (X / Y) is preferably 3.0 or more. When X / Y is 3.0 or more, it is possible to expand the catalyst temperature range in which the exhaust gas purification performance can be maintained to the low temperature side. As a result, the influence of the catalyst temperature on the catalyst performance is suppressed, the catalyst activity is kept high, and the exhaust gas can be stably purified.

In particular, the catalyst layer is provided on the upstream side from an intermediate position between the upstream end surface and the downstream end surface in the exhaust gas flow direction, and the first catalyst layer region in which the layer thickness gradually increases from the layer thickness Y toward the upstream side. And a second catalyst layer region which is provided downstream from the intermediate position and whose layer thickness gradually increases from the layer thickness Y toward the downstream side. That is, the volume of the in-cell gas flow passage increases from the upstream end surface in the exhaust gas flow direction toward the region of the layer thickness Y, and the volume decreases from the position of the layer thickness Y toward the downstream end surface. In such a configuration, as described above, in the region of the catalyst layer having a layer thickness Y, the heat capacity is reduced, the temperature of the catalyst layer is increased rapidly, and the pressure loss increases toward the downstream side where the volume is reduced. Heat transfer to the catalyst layer is faster. Furthermore, since the heat capacity increases on the upstream side and the downstream side where the volume decreases from the position of the layer thickness Y, heat is retained even if a relatively low temperature exhaust gas flows. On the contrary, in the region of the thinnest layer thickness Y, heat transfer to the downstream side in the exhaust gas flow direction becomes small, so that a temperature decrease on the downstream side of the region of layer thickness Y is prevented. As a result, when the catalyst temperature is low, the purification catalyst can be activated quickly, and when the catalyst temperature falls, the heat is maintained and the catalyst activity is maintained, and the exhaust gas can be purified more stably.

Further, the catalyst layer can be configured in a two-layer structure, and when configured in a two-layer structure, the catalyst layer includes a region having a total thickness of two layers and a layer thickness X that is twice or more the layer thickness Y. be able to. For example, in the two-layer structure, the upper layer or the lower layer, or both the upper layer and the lower layer have a region having a layer thickness X that is twice or more the layer thickness Y, upstream of the position of the layer thickness Y in the exhaust gas flow direction It can be configured in a concave structure provided on the downstream side.

The method for producing an exhaust gas purification catalyst according to the second invention comprises a catalyst metal and a thickener, and the ratio of the viscosity b at a solid content of 80% by mass to the viscosity a at a solid content of 20% by mass (b / a ) Is provided at one end of the supporting base material, and the slurry is sucked from the other end, and the layer thickness is from one end of the supporting base material to the one end side from the middle position of the other end. A first step of forming a catalyst layer having a region including a layer thickness X that is twice or more the thinnest layer thickness Y at the intermediate position; and the other end of the support substrate on which the catalyst layer is formed Providing the slurry, sucking the slurry from the one end, and a catalyst layer having a region including a layer thickness X that is twice or more as large as the layer thickness Y from the intermediate position to the other end side; And a second step to be formed. At this time, the viscosity a at a solid content of 20% by mass is preferably in the range of 200 to 1000 mPa · s. As the thickener, a water-soluble polymer can be suitably used.

In the method for producing an exhaust gas purification catalyst according to the second invention, when the slurry is provided at one end (or the other end) of the support base material and sucked from the other end (or one end) to form the catalyst layer region, By using a slurry containing a thickener and having a ratio (b / a) of viscosity b at a solid content of 80% by mass to a viscosity a at a solid content of 20% by mass of 2.4% or more, suction of the slurry Sometimes when the solvent (especially moisture) contained in the slurry is gradually absorbed and the solid content increases, it can cause a sudden increase in viscosity after reaching the solid content in the slurry. ) To form a catalyst layer having a thickness that changes. Thereby, it has the area | region which has the layer thickness X more than twice the layer thickness Y from the intermediate position of each end surface of the upstream and downstream in the exhaust gas distribution direction when exhaust gas distribute | circulates to an upstream and downstream, respectively. A catalyst layer can be formed.

In the second invention, in the first step, the slurry provided at one end of the support substrate is sucked from the other end, and the layer thickness is the thinnest layer thickness at the intermediate position from the intermediate position toward the one end. In addition to forming a first catalyst layer region gradually increasing from Y, in the second step, the slurry is provided on the other end of the support substrate on which the catalyst layer is formed, and the slurry is sucked from the one end, A second catalyst layer region in which the layer thickness gradually increases from the layer thickness Y toward the other end from the intermediate position can be formed.

The present invention maintains catalytic activity that is easily impaired by the influence of operating conditions such as cold engine start, intermittent engine stop in a hybrid vehicle, and low speed operation, and exhibits stable and excellent exhaust gas purification performance. A purifying system that can do this can be constructed.

According to the present invention, there is provided an exhaust gas purification catalyst that is hardly affected by exhaust gas emission conditions (particularly gas temperature) and that can stably maintain catalytic activity and maintain purification performance, and a method for producing the same. Can do.

1 is a perspective view showing a schematic structure of an exhaust gas purification catalyst according to a first embodiment of the present invention. It is a perspective view which expands and shows schematic structure of the cell which comprises the exhaust gas purification catalyst of FIG. FIG. 3 is a cross-sectional view taken along line A-A ′ of FIG. 2. It is process drawing for demonstrating the process order which forms the catalyst layer of the exhaust gas purification catalyst of this invention. It is a schematic sectional drawing which shows the internal structure of the cell in the exhaust gas purification catalyst of 2nd Embodiment of this invention. It is a schematic sectional drawing which shows the internal structure of the cell in the exhaust gas purification catalyst of 3rd Embodiment of this invention. It is a schematic sectional drawing which shows the internal structure of the cell in the exhaust gas purification catalyst of 4th Embodiment of this invention. It is a graph which shows the relationship between the solid content of a slurry, and a viscosity. It is a graph which shows the temperature rising conditions of the inflow gas temperature at the time of evaluating an exhaust gas purification catalyst. It is a graph which shows the temperature fall conditions of the inflow gas temperature at the time of evaluating an exhaust gas purification catalyst. It is a graph which shows the relationship between X / Y of an exhaust gas purification catalyst at the time of raising inflow gas temperature, and 50% purification temperature. It is a graph which shows the relationship between X / Y of an exhaust gas purification catalyst at the time of falling inflow gas temperature, and 50% purification temperature.

Hereinafter, embodiments of an exhaust gas purification catalyst and a method for producing the same of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following embodiment.

(First embodiment)
A first embodiment of the exhaust gas purification catalyst of the present invention will be described with reference to FIGS. The exhaust gas purification catalyst of the present embodiment is a catalyst layer having a single layer structure in which the layer thickness gradually increases from the center position of the catalyst layer that is equidistant from the inflow side end surface and the outflow side end surface of the exhaust gas toward each end surface. Is provided.

As shown in FIG. 1, the exhaust gas purification catalyst 100 of the present embodiment has a plurality of through-holes (cells; hereinafter referred to as “cells”) 11 that divide the interior with walls and penetrate from one end to the other end. And a cylindrical monolith substrate having a porous structure formed thereon. Exhaust gas (gas) flowing in the direction of the arrow flows from one end surface of the cylindrical shape, is purified when flowing through the inside, and the purified gas is discharged from the other end.

The inside of each cell constituting the exhaust gas purification catalyst 100 is in the half of the inflow side between the exhaust gas inflow end surface 12 and the outflow side end surface 13 with different thickness gradients of the catalyst layer in the cylindrical axial direction. The region 15 and the half 16 on the outflow side are configured. An example of a perspective configuration of each cell of the exhaust gas purification catalyst 100 is shown in FIG. As shown in FIG. 2, the cell 11 is formed in a cylindrical shape having a quadrangular cross section surrounded by walls on all sides, and exhaust gas can flow through a hollow portion inside the cylinder. The interior of the cell 11 is centered on a position P that is equidistant (distance L) from each of the inflow side end surface 12 and the outflow side end surface 13, toward the inflow side end surface 12 and the outflow side end surface 13. Thus, the catalyst layer 17 is formed so that the thickness gradually increases. That is, at the position P, the layer thickness is the thinnest.

The catalyst layer 17 is provided on the inner wall surface of the wall 14 forming each cell 11 as shown in FIG. FIG. 3 is a view showing a cross section taken along the line A-A ′ of FIG. 2. The catalyst layer 17 gradually starts from the layer thickness Y at the position P where the thickness is the smallest from the center position P that is equidistant from both end faces of the cell 11 (the inflow end face 12 and the outflow end face 13). The layer thickness is increased. That is, the catalyst layer 17 includes a first catalyst layer region 15a in which the layer thickness gradually increases from the layer thickness Y toward the upstream side from the position P on the upstream side of the position P in the flow direction of the exhaust gas (gas). On the downstream side of P, a second catalyst layer region 16a whose layer thickness gradually increases from the layer thickness Y from the position P toward the downstream side is provided. The layer thickness X of the catalyst layer 17 on both end faces of the cell 11 is formed to be twice or more (X / Y ≧ 2.0) with respect to the layer thickness Y at the position P. In the present embodiment, the first catalyst layer region 15a and the second catalyst layer region 16a have the same catalyst configuration.

In this way, when a structure in which the thickness of the catalyst layer 17 decreases and increases from the exhaust gas inflow side to the outflow side is adopted, the layer thickness increases from the position P where the layer thickness is the thinnest, and the pressure loss increases. By increasing the heat transfer from the exhaust gas to the catalyst layer and providing a thin region with a layer thickness Y, the heat capacity can be reduced, so that the heating rate of the catalyst layer can be increased. In addition to this, the heat capacity is increased by increasing the layer thickness from the layer thickness Y to twice or more in the first catalyst layer region 15a and the second catalyst layer region 16a, so that a relatively low temperature exhaust gas is generated. Even if it flows in, heat is retained. Furthermore, by providing a thin region with a layer thickness Y, heat transfer to the downstream side in the exhaust gas flow direction is reduced, thereby preventing a temperature decrease on the downstream side. As a result, for example, when the engine is cold started, the catalyst temperature tends to be low, but it is possible to activate the catalyst in a low temperature state in a short time. In addition, when the engine is intermittently stopped in a hybrid vehicle, the catalyst temperature is likely to drop when the gas emission is stopped, and during low speed operation, the exhaust gas temperature itself is low and the catalyst temperature is likely to drop. It is possible to maintain heat so that the catalytic activity can be purified.

As the layer thickness of the catalyst layer, the ratio (X / Y) of the layer thickness X on the upstream side and downstream side of the position P of the thinnest layer thickness Y to the layer thickness Y in the exhaust gas flow direction is 2.0 or more. However, the ratio X / Y is preferably 3.0 or more. When the ratio X / Y is 3.0 or more, particularly when the temperature of the catalyst is raised, it is possible to maintain good purification performance even when the catalyst temperature is relatively low. Therefore, the temperature range in which the purification performance can be maintained is widened under both conditions of temperature rise and temperature fall, and a reduction in the purification performance due to the temperature fluctuation of the catalyst can be suppressed.
As shown in FIG. 10 and FIG. 11, the ratio X / Y is preferably 3.0 or more, 6.0 or less, more preferably 4.0 or more, and 6.0 or less, and still more preferably 5. It is 0 or more and 6.0 or less.

In the catalyst layer 17, the thinnest layer thickness Y is preferably in the range of 10 to 80 μm, more preferably in the range of 20 to 60 μm. If the thickness of the layer thickness Y is 10 μm or more, more preferably 20 μm or more, the desired performance can be maintained without impairing the purification efficiency of the exhaust gas flowing in, and if it is 80 μm or less, further 60 μm or less, the heat capacity This is advantageous in that becomes smaller.

The position of the thinnest layer thickness Y of the catalyst layer may be provided so as to be deviated from the center position equidistant from both end faces toward one end face side, and the same effect as when the layer thickness Y is at the position P is obtained. It is done. In particular, the position of the thinnest layer thickness Y is the direction of the exhaust gas flow in the catalyst layer from the viewpoint of rapidly activating the catalyst at low temperatures and enhancing the heat retention when conditions such as the temperature and discharge amount of the exhaust gas fluctuate. The center position P that is equidistant (distance L) from both ends is preferably between the position up to L × 3/10 on the upstream side and the position up to L × 3/10 on the downstream side.

In the present embodiment, the case where the layer thickness X in the first catalyst layer region 15a of the catalyst layer 17 and the layer thickness X in the second catalyst layer region 16a are the same has been described as an example. The catalyst layer region 15a and the second catalyst layer region 16a may be configured to have different thicknesses and ratios of increasing thickness.

Further, the catalyst layer 17 has a shape in which the thickness gradually increases from the position P toward both end faces as in the present embodiment, and the thickness is a stepped stepwise with a predetermined thickness from the position P toward both end faces. The thickness may be increased, or the thickness may be gradually or stepwise increased from the position P to a predetermined position that does not reach the end face, and after reaching that thickness, the end face may have the same thickness, or a catalyst layer may be provided. No shape is acceptable.

The catalyst layer 17 may be configured to be provided on the wall 14 by coating a noble metal having catalytic ability as a catalyst metal. The catalyst layer 17 stores, for example, NO _x in an oxygen-excess lean atmosphere, and releases the occluded NO _x by changing the exhaust gas atmosphere to a rich atmosphere with excess reducing components such as stoichiometric hydrogen. it can be a NO _x storage-and-reduction type of purification catalyst for reducing component is reacted with reduced purification such as HC and CO by the action of the noble metal. Examples of noble metals include platinum (Pt), palladium (Rh), rhodium (Rh), and the like. Further, the catalyst layer can contain a NO _x storage material such as an alkali metal such as Li, K, Na, Mg, Ca, St, or Ba, or an alkaline earth metal.

Pt is excellent in NO oxidation activity. The concentration of Pt in the catalyst layer is preferably in the range of 0.1 to 5 g / L from the viewpoint of NO oxidation efficiency. Further, Pd is higher _the NO _x reduction activity has oxidation activity of NO. Moreover, since Pd is highly stable in a lean atmosphere, the presence of Pd together with Pt can suppress the growth of Pt grains and keep the NO oxidation activity of Pt high.

Pt and Pd can be supported on a desired carrier and contained in the catalyst layer. As the support, zirconium dioxide (ZrO ₂ ), aluminum oxide (Al ₂ O ₃ ), silica, silica-alumina, ceria (CeO ₂ ), oxide particles such as zeolite, and mixed particles thereof can be used. .

When Pt and Pd are contained in the catalyst layer, Pt-supported oxide particles supporting Pt may be mixed with Pd-supported oxide particles supporting Pd, or both Pt and Pd are supported. Oxide particles may be used. The Pt-supported oxide is, for example, a mixture of an oxide powder or granular material such as ZrO ₂ powder and a diammineplatinum dinitrate solution, a platinum chloride solution, an ammineplatinum solution, and the like, followed by drying and firing (for example, 400 to For example, about 800 ° C.). The oxide particles supporting both Pt and Pd are, for example, oxide powder such as ZrO ₂ powder and granular materials, diammine platinum nitrate solution, platinum chloride solution, ammine platinum solution, and palladium nitrate. It can be obtained by mixing a solution, a palladium chloride solution and the like, drying, firing (for example, about 400 to 800 ° C.), and the like.

Rh is excellent in reducing activity of NO _x. The Rh to another layer from the layer containing Pt (preferably supported on a support) by the presence, is suppressed solid solution with Pt, favorably reducing NO _x in a rich atmosphere, to purify NO _x.

Rh can be supported on a desired carrier and contained in the layer, and the same carrier as described above can be used. For example, an Rh-supported oxide carrying Rh is prepared by mixing oxide powder such as ZrO ₂ powder or granular material with a rhodium nitrate solution, a rhodium chloride solution, an ammine rhodium solution, and drying and firing (for example, For example, about 400 to 800 ° C.).

The first catalyst layer region 15a and the second catalyst layer region 16a of the catalyst layer 17 may have the same or different catalyst composition such as the type and ratio of the catalyst metal contained.

The supporting base material supports the catalyst layer, and a known material made of ceramics or metal can be selected depending on the purpose or circumstances. Specific examples include cordierite honeycomb substrates, SiC honeycomb substrates, metal honeycomb substrates, and the like.

The exhaust gas purification catalyst 100 includes a catalyst metal and a thickener, and the ratio (b / a) of the viscosity b at a solid content of 80% by mass to the viscosity a at a solid content of 20% by mass is 2.4 or more. Using the slurry, this slurry is provided on the exhaust gas inflow end surface 12 of the support base material, and the slurry is sucked from the outflow side end surface 13 which is the other end, whereby the inflow side end surface 12 and the outflow side end surface 13 of the support base material. Forming a first catalyst layer region 15a in which the layer thickness gradually increases from the thinnest layer thickness Y at the position P from the center position P that is equidistant to the inflow side end surface 12, respectively, The slurry is provided on the outflow side end surface 13 which is the other end of the support base in which the region 15a is formed, and the slurry is sucked from the inflow side end surface 12 of the exhaust gas, so that the center position P toward the outflow side end surface 13 Layer thickness It can be prepared by providing a step of forming a second catalyst layer region 16a which gradually increases from the thickness Y.

Specifically, as shown in FIG. 4, slurry is sequentially provided to one end (exhaust gas inflow end surface 12) and the other end (exhaust gas outflow end surface 13) of the support base material. It can be produced by performing an operation of sucking from the side where the slurry is not provided.
As shown in FIG. 4, a holding member 53 that holds a predetermined amount of slurry is provided at one end (an inflow side end surface 12 in FIG. 1) of a support base material 51 such as a monolith base material. After supplying the slurry 52 (I slurry installation; coat (1)), the slurry is sucked from the other end (outflow side end face 13 in FIG. 1) to be absorbed into the supporting substrate (II slurry suction). The holding member 53 covers the end of the support base 51 and holds the slurry supplied from a slurry supply member (not shown) on the end. There is no particular limitation on the timing of suction. For example, suction may be performed simultaneously when slurry is supplied to the end face, or suction may be started after a predetermined amount of slurry is supplied to the end face.

At this time, moisture in the slurry is absorbed by the base material, but the amount of moisture absorption increases in the direction closer to the inflow side end face 12 than in the suction direction, and the viscosity of the slurry increases accordingly as it approaches the inflow side end face. . Then, when a thickener is used in the slurry and the solid content changes, the ratio (b / a) of the viscosity b (solid content 80% by mass) to the viscosity a (solid content 20% by mass) is b / a ≧ 2. By adjusting the slurry so as to satisfy 4, the catalyst layer can be formed with a distribution in the coat thickness by utilizing the abrupt viscosity change caused by the increase in the solid content. Specifically, regions having a layer thickness X that is twice or more the layer thickness Y can be formed on the upstream side and the downstream side in the exhaust gas flow direction at the position of the layer thickness Y, respectively.
Thus, after forming the catalyst layer on the inflow side end surface 12 side of the support base material, as shown in FIG. 4, the outflow side end surface 13 that has been sucked is coated with slurry (III slurry installation; Coat (2)) and then sucking from the opposite outflow side end face 12 to absorb the slurry (IV slurry suction). Also at this time, a thickener is used, and a slurry in which the ratio of the viscosity b to the viscosity a (b / a) satisfies b / a ≧ 2.4 when the solid content changes is used.

The slurry includes at least a catalytic metal and a thickener, and generally includes a solvent such as water. Moreover, a slurry can be comprised using another component as needed.
Examples of the catalyst metal are as described above. Moreover, the said thickener is a compound which raises the viscosity of a slurry, For example, water-soluble polymers, such as hydroxyethyl cellulose (HEC), carboxymethylcellulose, methylcellulose, polyvinyl alcohol, etc. can be used. The solid content concentration of the slurry is generally adjusted to a range of 20 to 60% by mass.

In addition, the ratio of the viscosity b at a solid content of 80% by mass (b / a) to the viscosity a at a solid content of 20% by mass is 2.4 or more. If the ratio b / a is less than 2.4, the degree of thickening caused by the increase in the solid content when the slurry is sucked is too small, and 2 for the thinnest layer thickness Y of the catalyst layer. A catalyst layer having a region having a layer thickness more than twice cannot be formed.
In this case, the content of the thickener in the slurry is preferably 0.5 to 8.0% by mass in a range satisfying the b / a with respect to the total mass of the slurry.

As the viscosity (25 ° C.) of the slurry used for forming the catalyst layer, the viscosity a when the solid content is adjusted to 20% by mass is preferably 200 to 1000 mPa · s, and preferably 400 to 800 mPa · s. More preferred. The viscosity a on the low-viscosity side with a low solid content is 200 mPa · s or more, more preferably 400 mPa · s or more, because the amount of solvent (especially water) is not too much, so the viscosity increases rapidly due to moisture absorption. A viscosity change is obtained and the thickness of the catalyst layer is easily changed. Further, since the viscosity a is 1000 mPa · s or less, and further 800 mPa · s or less, the amount of solvent (especially water) is not small, so that the viscosity change due to moisture absorption is kept large, and from the substrate end face side to the inside It is easy to form a catalyst layer that is inclined toward the surface.

The viscosity in the present invention means a viscosity at 25 ° C. when a shear rate of 1 sec ⁻¹ is applied, and is measured by a TV33 viscometer (manufactured by Toki Sangyo Co., Ltd.).

(Second Embodiment)
A second embodiment of the exhaust gas purification catalyst of the present invention will be described with reference to FIG. In the present embodiment, on the first catalyst layer provided on the wall surface of the monolith substrate, the layer thickness is gradually increased from the center position of the catalyst layer toward the end surface as in the first embodiment. A second catalyst layer is provided to form a two-layer structure.

The support substrate and the catalyst layer 17 (second catalyst layer) can be configured in the same manner as in the first embodiment, and the same reference numerals are given to the same components as in the first embodiment, and the details thereof The detailed explanation is omitted.

As shown in FIG. 5, on the inner wall surface of the wall 14 forming the cell 21 constituting the exhaust gas purification catalyst, there is a catalyst layer 22 carrying a noble metal (catalyst metal) having catalytic ability as a first catalyst layer. Is provided. Examples of noble metals include platinum (Pt), palladium (Rh), rhodium (Rh), and the like. Further, the catalyst layer can contain a NO _x storage material such as an alkali metal such as Li, K, Na, Mg, Ca, St, or Ba, or an alkaline earth metal.

The thickness of the catalyst layer 22 is preferably in the range of 10 to 80 μm, more preferably in the range of 20 to 60 μm.

Then, as a second catalyst layer on the catalyst layer 22, the center position P is moved from the position P that is equidistant from both end faces (the inflow side end face 12 and the outflow side end face 13) of the cell 21 toward each end face. A catalyst layer 17 is formed in which the layer thickness gradually increases from the layer thickness Y to twice or more the layer thickness Y. As a result, the inside of each cell constituting the exhaust gas purification catalyst 100 has a first catalyst layer region 15a in which the thickness of the catalyst layer gradually increases from the layer thickness Y toward the upstream side from the position P in the cylindrical axial direction. And a second catalyst layer region 16a in which the thickness of the catalyst layer gradually increases from the layer thickness Y toward the downstream side from the position P.

Also in the present embodiment, the thickness gradually increases from the inflow side end surface 12 and the outflow side end surface 13 toward the inflow side end surface 12 and the outflow side end surface 13 around the position P that is equidistant (distance L). By forming the entire catalyst layer, as in the first embodiment, it is possible to quickly raise the temperature and activate the catalyst in a low temperature state, improve the heat retention of the catalyst, and improve the catalyst activity. It can be stably maintained in a purifiable range.

(Third embodiment)
A third embodiment of the exhaust gas purification catalyst of the present invention will be described with reference to FIG. In the present embodiment, as the first catalyst layer on the wall surface of the monolith substrate, a catalyst layer having a layer thickness that gradually increases from the center position of the catalyst layer toward the end surface as in the first embodiment, Furthermore, a second catalyst layer having no inclination in the layer thickness is provided to form a two-layer structure.

The support base material and the catalyst layer 17 can be configured in the same manner as in the first embodiment, and the catalyst layer 22 can be configured in the same manner as in the second embodiment. The same reference numerals are assigned and detailed description thereof is omitted.

As shown in FIG. 6, both end surfaces of the cell 31 (the inflow side end surface 12 and the outflow side end surface 13) are formed on the inner wall surface of the wall 14 forming the cell 31 constituting the exhaust gas purification catalyst as a first catalyst layer. A catalyst layer 17 is provided in which the layer thickness gradually increases from the layer thickness Y at the center position P to more than twice the layer thickness Y from the center position P that is equidistant from the center position toward each end face. On the catalyst layer 17, a catalyst layer 22 on which a noble metal (catalyst metal) having catalytic ability is supported is provided as a second catalyst layer. Examples of the noble metal and the NO _x storage material are as described above. As a result, the inside of each cell constituting the exhaust gas purification catalyst 100 has a first catalyst layer region 15a in which the thickness of the catalyst layer gradually increases from the layer thickness Y toward the upstream side from the position P in the cylindrical axial direction. And a second catalyst layer region 16a in which the thickness of the catalyst layer gradually increases from the layer thickness Y toward the downstream side from the position P.

(Fourth embodiment)
A fourth embodiment of the exhaust gas purification catalyst of the present invention will be described with reference to FIG. In the present embodiment, a catalyst layer having a non-inclined thickness is arranged in a partial region equidistant from the inflow side end surface and the outflow side end surface of the exhaust gas, and the layer thickness is directed to each end surface so as to sandwich the catalyst layer. Thus, a catalyst layer that gradually increases in thickness is provided to form a single layer structure.

The support base and the catalyst layer 17 can be configured in the same manner as in the first embodiment, and the same reference numerals are given to the same components as in the first embodiment, and detailed description thereof is omitted.

As shown in FIG. 7, the inner wall surface of the wall 14 forming the cell 41 constituting the exhaust gas purification catalyst has a center that is equidistant from both end surfaces (the inflow side end surface 12 and the outflow side end surface 13) of the cell 41. A catalyst layer 17a on which a noble metal (catalytic metal) having catalytic ability is supported is provided in the region. Examples of the noble metal and the NO _x storage material are as described above. Between the catalyst layer 17a, the inflow side end surface 12, and the outflow side end surface 13, a catalyst layer 17 having a layer thickness that gradually increases from the end of the catalyst layer 17a is provided. Here, the thickness of the catalyst layer 17a is the thinnest layer thickness Y in the cell, and the layer thickness gradually increases from the layer thickness Y toward both end surfaces of the cell. The layer thickness of the catalyst layer at the outflow side end face 13 is more than twice the layer thickness Y. As a result, the interior of each cell constituting the exhaust gas purification catalyst 100 has the first catalyst layer region 15a in which the thickness of the catalyst layer gradually increases from the layer thickness Y on the upstream side in the gas flow direction of the exhaust gas, and the downstream On the side, the second catalyst layer region 16a is formed such that the thickness of the catalyst layer gradually increases from the layer thickness Y. The layer thickness Y can be in the same range as in the first embodiment.

Also in the present embodiment, the entire catalyst layer is formed so that the thickness gradually increases toward the inflow side end surface 12 and the outflow side end surface 13, so that the catalyst in the low temperature state can be quickly removed as in the first embodiment. It is possible to increase the temperature and activate the catalyst at the same time, improve the heat retaining property of the catalyst, and maintain the catalyst activity in a more stable range.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples as long as the gist thereof is not exceeded.

[Examples 1 to 5, Comparative Example 1]
(Preparation of lower layer composite oxide)
0.3 mol / L zirconium nitrate, 0.3 mol / L cerium nitrate solution, 0.3 mol / L lanthanum nitrate solution, and 0.3 mol / L yttrium nitrate solution are oxidized A nitric acid-based precursor solution mixed so as to have a quantitative ratio of ZrO ₂ / CeO ₂ / La ₂ O ₃ / Y ₂ O ₃ = 60/30/5/5 was prepared, and this was dropped into aqueous ammonia. After completion of the dropwise addition, the resulting precipitate was baked at 850 ° C. to obtain a Zr, Ce, La, Y composite oxide.

(Preparation of upper layer composite oxide)
Zirconium nitrate, cerium nitrate solution, neodymium nitrate solution, aluminum nitrate solution, and lanthanum nitrate in terms of oxide mass ratio ZrO ₂ / CeO ₂ / Nd ₂ O ₃ / Al ₂ O ₃ / La ₂ O ₃ = A nitric acid-based precursor solution mixed so as to be 25.5 / 19.5 / 2.0 / 51 / 2.0 was prepared, and this was dropped into aqueous ammonia. After completion of the dropping, the produced precipitate was baked at 850 ° C. to obtain a Zr, Ce, Nd, Al, La composite oxide.

(Formation of lower catalyst coat layer)
The Zr, Ce, La, Y composite oxide obtained above was mixed with 0.5% diammineplatinum dinitrate solution, dried and fired at 500 ° C., and Pt 0.75 g / L was supported. Pt-supported oxide particle powder A was produced.

120 g / L of this Pt-supported oxide particle powder A, 40 g / L of commercially available γ-Al ₂ O ₃ powder, 5 g / L of barium sulfate, and 160 g / L of ion-exchanged water were mixed to obtain a slurry. The resulting slurry was washcoated onto an 875 ml cordierite monolith substrate. Thereafter, drying and firing were performed to form a catalyst coat layer. Furthermore, it baked at 500 degreeC and formed the lower side catalyst coat layer (1st catalyst layer). The amount of Pt supported was 0.75 g / L.

(Formation of upper catalyst coat layer)
Next, the Zr, Ce, Nd, Al, La composite oxide obtained above was mixed with a 0.1% rhodium nitrate solution, dried, fired at 500 ° C., and Rh 0.15 g / L was A supported Rh-supported oxide particle powder B was produced.

This Rh-supported oxide particle powder B was 60 g / L, a commercially available γ-Al ₂ O ₃ powder 25 g / L, and hydroxyethyl cellulose in the amount shown in Table 1 below (ratio to the total mass of the slurry [mass%]). (Thickener) and 85 g / L of ion exchange water were mixed to form a slurry. As shown in Table 1 below, the content ratio of the thickener was changed to prepare six types of slurries having a solid content of 20% by mass.

-Measurement of viscosity-
With respect to the six types of slurries prepared, each viscosity [mPa · s] at 25 ° C. when the solid content was adjusted from 20% by mass to 80% by mass every 20% by mass was given a shear rate of 1 sec ^−1. It was measured using a viscometer (manufactured by Toki Sangyo Co., Ltd.). The measurement results are shown in Table 1 below and FIG.

As shown in FIG. 8, the slurries 1 to 5 containing the thickener have a sharp increase in viscosity after the solid content exceeds 40% by mass, compared with the conventional slurry 6 containing no thickener. I understand.

A holding member 53 that holds a predetermined amount of slurry as shown in FIG. 4 is attached to one end of the monolith substrate on which the lower catalyst coat layer is formed, and the other end is opened in a substrate receiving portion that communicates with a decompression device (not shown). The inside of the base material can be sucked by decompression. The slurry obtained above was supplied to the attached holding member 53, and the inside of the cell was sucked at a linear velocity of 55 m / s by reducing the pressure. At this time, as shown in FIG. 5, the slurry spread from one end (inflow side end face 12 in FIG. 1) to the center position P by suction. Then, it was dried at 120 ° C. Subsequently, the holding member 53 is attached to the other end of the monolith substrate that has been sucked, and the other end is inserted into the opening of the substrate receiving portion that communicates with the decompression device so that the inside of the substrate can be sucked from the one end. I made it. The slurry was sequentially supplied to the holding member 53, and the inside of the cell was sucked at a linear velocity of 55 m / s by reducing the pressure. As shown in FIG. 5, the slurry spreads from the other end (outflow side end face 13 in FIG. 1) to the center position P. Then, it dried at 120 degreeC and further baked at 500 degreeC. In this way, an upper catalyst coat layer (second catalyst layer) having an X / Y gradient described in Table 1 below was formed. The amount of Rh supported was 0.15 g / L.

As described above, six types of exhaust gas provided with a catalyst layer having a two-layer structure of a lower catalyst coat layer and an upper catalyst coat layer by sequentially using six types of slurries having different contents of the thickener. A purification catalyst was prepared.

[Evaluation]
The exhaust gas purification catalyst was placed in the start converter downstream of the engine, the engine was started, and the HC concentration [ppm] in the exhaust gas discharged was measured by MOTOR EXHAUST GAS ANALYZER [manufactured by Horiba, Ltd.]. The ratio of the HC gas concentration in the exhaust gas (outflow gas) after purification treatment (purification rate [%]) to the HC gas concentration in the introduced exhaust gas (inflow gas) is calculated from the following formula, and this is the gas purification capacity. The following (1) was evaluated as an index for evaluating.
Purification rate [%] = 100-{HC gas concentration in outflow gas / HC gas concentration in inflow gas x 100}
The evaluation was performed by adopting the temperature rise evaluation pattern shown in FIG. 9A and the temperature drop evaluation pattern shown in FIG. 9B. The former can evaluate the early activation of the catalyst, and the latter can evaluate the storage stability of the catalyst.

(1) 50% purification temperature For the six exhaust gas purification catalysts produced, the ratio of the HC concentration in the outflow gas to the HC gas concentration in the inflow gas (purification rate [%]) is required to reach 50%. The temperature was determined. The results are shown in FIGS.
The “50% purification temperature” is the temperature of the inflowing gas when the purification rate becomes 50% in the temperature increase evaluation pattern and the temperature decrease evaluation pattern.

As shown in FIGS. 10 to 11, in the embodiment, the temperature range in which the purification performance can be maintained is widened to the low temperature side as compared with the comparative (conventional) purification catalyst, and the purification performance is reduced due to the temperature drop. Suppressed. For this reason, it is possible to suppress the influence of operating conditions such as when the engine is cold started, when the engine is intermittently stopped in a hybrid vehicle, and during low-speed operation, and to stably exhibit excellent exhaust gas purification performance. Further, as shown in FIG. 8, the slurry prepared to a predetermined b / a value using a thickener has a large viscosity due to an increase in the solid content, and the formation of a catalyst layer having a desired X / Y can be achieved. I was able to do it.

11, 21, 31, 41 ... cell 12 ... exhaust gas inflow side end surface 13 ... exhaust gas outflow

side end surface

17, 17a, 22 ... catalyst layer 52 ... slurry 100 ... Exhaust gas purification catalyst

Claims

An area having a layer thickness X that is twice or more the thinnest layer thickness Y at the intermediate position between the upstream end face and the downstream end face in the exhaust gas flow direction is the position of the layer thickness Y in the exhaust gas flow direction. An exhaust gas purification catalyst comprising a catalyst layer having upstream and downstream sides on a support substrate.
The exhaust gas purification catalyst according to claim 1, wherein the ratio (X / Y) of the layer thickness X to the layer thickness Y is 3.0 or more.
The catalyst layer is provided upstream of the intermediate position in the exhaust gas flow direction, and a first catalyst layer region whose layer thickness gradually increases from the layer thickness Y toward the upstream side, and downstream of the intermediate position. The exhaust gas purifying catalyst according to claim 1, further comprising a second catalyst layer region that is provided and has a layer thickness that gradually increases from the layer thickness Y toward the downstream side.
The catalyst layer has a two-layer structure, and at least one of the upper layer and the lower layer of the two-layer structure has a region having a layer thickness X that is twice or more the layer thickness Y in the exhaust gas flow direction. The exhaust gas purification catalyst according to claim 1, which is respectively provided on the upstream side and the downstream side of the position of the layer thickness Y.
The exhaust gas purification catalyst according to claim 1, wherein the layer thickness Y is in the range of 10 to 80 µm.
The position of the layer thickness Y is based on a central position equidistant (L) from both ends of the catalyst layer in the exhaust gas flow direction, with a position up to L × 3/10 on the upstream side in the exhaust gas flow direction and L × on the downstream side. The exhaust gas purifying catalyst according to claim 1, wherein the exhaust gas purifying catalyst is located between the position and up to 3/10.
A slurry containing a catalyst metal and a thickener and having a ratio (b / a) of viscosity b at a solid content of 80% by mass to a viscosity at a solid content of 20% by mass of 2.4% or more is one end of a supporting substrate. The slurry is sucked from the other end, and the layer thickness is more than twice the thinnest layer thickness Y at the intermediate position from the intermediate position of the one end and the other end of the support base to the one end side. A first step of forming a catalyst layer having a region including a layer thickness X of
The slurry is provided on the other end of the support base on which the catalyst layer is formed, the slurry is sucked from the one end, and the layer thickness is from the intermediate position to the other end side with respect to the layer thickness Y. A second step of forming a catalyst layer having a region including a layer thickness X of twice or more;
A method for producing an exhaust gas purification catalyst having
The first step forms, by the suction, a first catalyst layer region in which the layer thickness gradually increases from the thinnest layer thickness Y at the intermediate position from the intermediate position toward the one end. The method for producing an exhaust gas purification catalyst according to claim 7, wherein a second catalyst layer region whose layer thickness gradually increases from the layer thickness Y toward the other end from the intermediate position is formed by the suction.
The method for producing an exhaust gas purification catalyst according to claim 7, wherein the thickener is a water-soluble polymer.
The method for producing an exhaust gas purification catalyst according to claim 7, wherein the viscosity a at the solid content of 20% by mass is 200 to 1000 mPa · s.