JP2009165929A - Catalyst for cleaning exhaust - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a HC trapping catalyst which increases the proportion of an exhausted gas passing through a cleaning catalyst layer being an upperlayer and diffusing to a HC absorbing layer being a lower layer to increase the HC amount absorbed in the HC absorbing layer being the lower layer to improve the HC cleaning rate during cold time. <P>SOLUTION: The catalyst for cleaning exhaust disposed in the exhaust passage of an engine and comprised of a honeycomb support 11 equipped with a plurality of cell channels 12... 12, a HC adsorbing layer 13 disposed on the honeycomb support 11, and a cleaning catalyst layer 14 laid over the HC adsorbing layer 13, is characterized in that a plurality of micro passages 21... 21 having a mean diameter greater than that of the particles constituting the cleaning catalyst layer 14 are formed and distributed in the cleaning catalyst layer 14, so as to form passages 31 that allow HC contained in the exhaust flowing in the plurality of cell channels 12... 12 of the honeycomb support 11 to flow from the cleaning catalyst layer 14 face disposed at the cell channel side to the cleaning catalyst layer 14 face disposed at the HC absorbing layer 13 side. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst, and more particularly to an exhaust gas purification catalyst that can suppress release of hydrocarbons contained in exhaust gas to the atmosphere when cold, and belongs to the technical field of engine emission reduction.

  Conventionally, in order to purify exhaust gas of an engine mounted on an automobile, for example, noble metals such as platinum (Pt), palladium (Pd), rhodium (Rh) as catalytic metals having a catalytic function, these noble metals As a high specific surface area oxide supporting oxygen, and a cerium (Ce) oxide as an oxygen storage material (OSC: Oxygen Storage Component) having an expansion function of an exhaust gas air-fuel ratio window by storing and releasing oxygen An exhaust gas purifying catalyst containing etc. is disposed in the exhaust passage of the engine.

  This type of exhaust gas purifying catalyst generally requires several tens of seconds until the catalyst metal reaches a temperature sufficient for activation when the engine is cold-started. There is a problem of being released to the atmosphere without being purified. Since the amount of fuel supply is usually increased when the engine is started, it is particularly important to suppress release of hydrocarbons (HC) contained in exhaust gas as unburned fuel into the atmosphere. .

  As a technique for dealing with this problem, a so-called HC trap catalyst is known. This HC trap catalyst is configured to temporarily adsorb HC at a low temperature, desorb the adsorbed HC when the catalyst temperature reaches a certain high temperature, and oxidize and purify the desorbed HC. It has been done.

  For example, Patent Document 1 discloses a low-temperature structure in which a honeycomb carrier has two upper and lower catalyst layers, a lower layer is an HC adsorption layer mainly composed of zeolite, and an upper layer is a purification catalyst layer supporting a catalyst metal. An HC trap catalyst is disclosed. According to this, the exhaust gas flowing into the carrier cell passage passes through the upper purification catalyst layer from the surface on the cell passage side to the surface on the lower HC adsorption layer side (that is, passes in the thickness direction of the layer). ) It diffuses into the HC adsorption layer, and as a result, HC in the exhaust gas is adsorbed by the zeolite constituting the HC adsorption layer at a low temperature.

If the temperature of the catalyst, more specifically, the temperature of the HC adsorption layer rises to a certain high temperature (for example, 120 ° C. to 200 ° C.) as the exhaust gas temperature rises, the adsorbed HC begins to desorb, The lower HC adsorption layer passes through the upper purification catalyst layer to the cell passage side (that is, passes in the layer thickness direction) and flows out of the carrier cell passage. At that time, HC is oxidized and purified into water (H ₂ O) and carbon dioxide (CO ₂ ) by the catalytic action of the catalytic metal when passing through the purification catalyst layer.

  Patent Document 2 discloses an exhaust gas having a structure in which a HC adsorption layer containing zeolite is provided on a honeycomb carrier, and a crack extending in the circumferential direction and a crack extending in the radial direction are formed on the HC adsorption layer in a manner satisfying a predetermined condition. A purification catalyst is disclosed. And according to this, it is described that the problem that the HC adsorbing layer and the purification catalyst layer laminated on the HC adsorbing layer is peeled off and dropped from the honeycomb carrier is suppressed.

Japanese Patent Laid-Open No. 2-56247 (FIG. 1) JP2003-305369 (paragraph 0088, FIG. 2)

  By the way, as described above, the HC trap catalyst is intended to adsorb HC when the exhaust gas temperature is low and the catalytic metal is in an inactive state to suppress release to the atmosphere. The exhaust gas flowing into the carrier cell passage is more easily diffused to the lower HC adsorption layer through the upper purification catalyst layer in the thickness direction than in the rectified state when the exhaust gas is in a turbulent state. preferable.

  However, in general, the exhaust gas exhibits a turbulent flow state only within a limited range (for example, within a range of 10 mm on the carrier inlet side) from the carrier inlet end to the outlet side, and a large amount of the cell passage on the downstream side thereof. Within the range of the part, it is a layered rectification state. This is because when exhaust gas flows into the carrier cell passage, it collides with the wall surface at the end of the carrier inlet and becomes a turbulent state only for that moment. This is because it returns to a typical laminar flow. As a result, in the conventional HC trap catalyst, the proportion of the exhaust gas flowing in the cell passage that passes through the upper purification catalyst layer in the thickness direction and diffuses to the lower HC adsorption layer is relatively small. Therefore, the amount of HC adsorbed on the lower HC adsorption layer is relatively small, and therefore, there is a problem that the release of HC to the atmosphere cannot be sufficiently satisfactorily suppressed in the cold state.

  The present invention addresses the above-described problems in an exhaust gas purification catalyst, particularly an HC trap catalyst having a function of adsorbing HC in the exhaust gas when cold. It is an object to increase the ratio of exhaust gas that diffuses to the HC adsorption layer and to increase the amount of HC adsorbed to the lower HC adsorption layer, thereby improving the HC purification rate in the cold state.

  In order to solve the above problems, the present invention uses the following means.

  That is, the invention described in claim 1 of the present application is a honeycomb carrier that is disposed in an exhaust passage of an engine and has a large number of cell passages, an HC adsorption layer provided on the honeycomb carrier, and an HC adsorption layer on the honeycomb carrier. The exhaust gas purifying catalyst having a purification catalyst layer laminated on the HC adsorption layer side from the cell passage side surface of the purification catalyst layer to the purification catalyst layer. A plurality of fine paths having an average diameter larger than the average diameter of the particles constituting the purification catalyst layer are formed so as to generate a passage that can pass through to the surface.

  Here, the “passage through which HC in the exhaust gas flowing through the carrier cell passage can pass from the cell passage side surface of the purification catalyst layer to the HC adsorption layer side surface” is only a plurality of dispersed fine passages. Are generated by being connected to each other, or are formed by mutually connecting a plurality of finely formed fine paths and voids existing in the purification catalyst layer.

  Next, the invention according to claim 2 of the present application is the exhaust gas purifying catalyst according to claim 1, wherein the HC passing through the purifying catalyst layer is passed through the purifying catalyst layer to the HC adsorbing layer. A plurality of fine paths having an average diameter larger than the average diameter of the particles constituting the HC adsorption layer are formed in a dispersed manner so that a diffusible passage is generated from the surface on the layer side to the deep part of the HC adsorption layer. It is characterized by being.

  Here, the “passage through which the HC that has passed through the purification catalyst layer can diffuse from the surface of the HC adsorption layer on the purification catalyst layer side to the deep layer of the HC adsorption layer” is only a plurality of dispersed fine paths. A plurality of fine paths formed by being connected to each other, or the voids existing in the HC adsorption layer are connected to each other.

  Next, the invention according to claim 3 of the present application is the exhaust gas purifying catalyst according to claim 1 or 2, wherein the fine path is within a predetermined range from the carrier inlet end toward the outlet. It is characterized by not being formed.

  First, according to the first aspect of the present invention, a catalyst layer is laminated in two upper and lower layers on a honeycomb carrier having a large number of cell passages, the lower layer is an HC adsorption layer having HC adsorption capability, and the upper layer is HC purification capability. In an exhaust gas purifying catalyst having a structure having a purifying catalyst layer having HC, that is, an HC trap catalyst, constituent particles of the purifying catalyst layer (for example, alumina particles as a support, Ce-containing oxygen storage material particles, etc.) ) Is dispersed and formed (for example, purification catalyst) having a plurality of fine paths (for example, 5 to 10 μm in length) having an average diameter (for example, 1 to 5 μm) larger than the average diameter (for example, 0.1 to 0.9 μm). As a result, a passage through which HC in the exhaust gas flowing through the carrier cell passage can pass from the cell passage side surface of the purification catalyst layer to the HC adsorption layer side surface is generated. Exhaust gas Passing the purification catalyst layer in the thickness direction is to be squid.

  Therefore, even if the exhaust gas flowing in the cell passage is in a straight layered rectified state, the exhaust gas flowing in the cell passage passes through the upper purification catalyst layer in the thickness direction and passes through the lower HC adsorption layer. As a result, the amount of HC adsorbed to the lower HC adsorption layer increases, and as a result, the amount of HC adsorbed on the lower HC adsorption layer can be increased, and the release of HC to the atmosphere can be suppressed sufficiently when cold. This improves the HC purification rate.

  In the present invention, the “average particle diameter” is a weighted average particle diameter, and the weighted average particle diameter is measured, for example, by the following method. First, each particle to be measured is sufficiently dispersed in ion-exchanged water using an ultrasonic vibration device or the like. A particle size distribution of each particle is obtained by measuring the obtained dispersion using a laser diffraction particle size distribution measuring apparatus. Then, the weighted average particle size is calculated from the obtained particle size distribution.

  Next, according to the invention described in claim 2, in addition to the upper purification catalyst layer, similarly to the lower HC adsorption layer, the constituent particles of the HC adsorption layer (for example, zeolite as an adsorbent) A plurality of fine paths (for example, 5 to 10 μm long) having an average diameter (for example, 1 to 5 μm) larger than the average diameter (for example, 0.1 to 0.9 μm) are dispersed (for example, HC adsorption layer) As a result, a passage through which HC passing through the purification catalyst layer can diffuse from the surface of the HC adsorption layer to the deep layer of the HC adsorption layer is generated. Thus, it becomes easy for the exhaust gas to permeate and diffuse the HC adsorption layer to the deep layer portion in the thickness direction.

  Therefore, more exhaust gas diffuses to the deep part of the HC adsorption layer, and as a result, the amount of HC adsorbed by the lower HC adsorption layer further increases, and thus the release of HC to the atmosphere when cold. This can be further suppressed, and the HC purification rate during cold can be further improved.

  According to the third aspect of the present invention, the exhaust gas is originally in a turbulent state and is in an environment in which the exhaust gas is likely to diffuse to the lower HC adsorption layer. Within the range (for example, within 1/20 of the total length of the carrier), the fine path is not formed. Therefore, the fine path is formed to improve the catalytic ability of the catalyst layer (HC adsorption layer, purification catalyst layer). Good diffusion of the exhaust gas to the HC adsorption layer is maintained while avoiding a reduction in the purification rate due to the reduction.

  Also, within a predetermined range from the carrier inlet end to the outlet side where foreign objects and deposits collide from the upstream side of the exhaust passage or are easily peeled and collapsed (wind erosion) due to exhaust pressure (for example, the total length of the carrier) In the range of 1/20 of the above), the fine path is not formed, so that the wind is generated by reducing the strength of the catalyst layer (HC adsorption layer, purification catalyst layer) by forming the fine path. Good diffusion of the exhaust gas to the HC adsorption layer is maintained while avoiding an increase in food damage.

  Hereinafter, the present invention will be described in more detail through the best mode and examples of the present invention.

As shown in FIG. 1, in this embodiment, the air-fuel mixture is combusted in the combustion chamber of the engine 1 of the automobile, and as a result, hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx), etc. The exhaust gas containing gas is released into the atmosphere through the exhaust passage 2, and the exhaust gas is purified into water (H ₂ O), carbon dioxide (CO ₂ ), nitrogen (N ₂ ), etc. in the exhaust passage 2. The exhaust gas purification catalyst 10 according to the present invention is accommodated in the catalytic converter 3.

  As shown in FIG. 2, the exhaust gas purifying catalyst 10 includes a cordierite honeycomb carrier 11 having a large number of cell passages 12... 12 and having a cylindrical outer shape. On the wall surface of the cell passage 12, an HC adsorption layer (sometimes referred to as HCT (HC Trap)) 13 having HC adsorption capability is provided. On this HC adsorption layer 13, HC, CO and NOx are purified. The purification catalyst layer 14 (which may be referred to as TWC (Three Way Catalyst)) 14 having a function is laminated. That is, the exhaust gas purifying catalyst 10 has catalyst layers 13 and 14 stacked on the upper and lower sides of the honeycomb carrier 11, temporarily adsorbs HC at a low temperature, and the catalyst temperature is high to some extent. This is an HC trap catalyst configured to desorb HC adsorbed when it reaches the limit and oxidize and purify the desorbed HC.

  The honeycomb carrier 11 has, for example, a length of 20 cm, a cell wall thickness of 4.5 mil (4.5 μinch≈114 μm), a cell density of 400 cpsi (400 cells per square inch), and a cell shape of a regular hexagon. Can be preferably used.

  As shown in FIG. 3, the upper purification catalyst layer 14 has a two-layer structure of a purification catalyst layer lower layer 14 a in contact with the lower HC adsorption layer 13 and a purification catalyst layer upper layer 14 b that defines the carrier cell passage 12. ing.

  Next, the detailed chemical structure of each of these layers 13, 14a, 14b will be described.

First, as a preferable blending example of the purification catalyst layer upper layer 14b, binder (zirconyl nitrate) is 7.70 (as ZrO ₂ ) g / L (supported amount per 1 L of honeycomb carrier: the same applies hereinafter), oxygen storage material (Cerium oxide: CeO ₂ ) 3.70 g / L, palladium (Pd) -supported first powder (aluminum 45.00 g / L containing 4% by mass of lanthanum oxide (La ₂ O ₃ )) 70 g / L supported) and palladium-supported second powder (Ce / Zr (zirconium) / La / Y (yttrium) / alumina composite oxide 22.60 g / L with Pd supported at 0.30 g / L) It is the one that included.

In that case, the composition ratio of the Ce · Zr · La · Y · alumina composite oxide is preferably CeO ₂ : ZrO ₂ : La ₂ O ₃ : Y ₂ O ₃ : Al ₂ O ₃ = 10.5: 7.0: 1.5: 1.0: 80 (mass%) and the like.

In addition, as one of preferable blending examples of the purification catalyst layer lower layer 14a, the binder (zirconyl nitrate) is 11.80 (as ZrO ₂ ) g / L, and rhodium (Rh) -supported ceria (Zr · Ce · Nd ( Neodymium) composite oxide of 66.00 g / L with 0.15 g / L of Rh) and rhodium-supported alumina (42.00 g / L of alumina containing 10 mass% Zr with 0.05 g of Rh) / L loaded).

In that case, the composition ratio of the Zr · Ce · Nd composite oxide is preferably ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 80: 10: 10 (mass%).

Here, the cerium oxide, the Ce · Zr · La · Y · alumina composite oxide, and the Zr · Ce · Nd composite oxide all store oxygen in a lean atmosphere and release oxygen in a rich atmosphere. It functions as. The Zr (10% by mass) -containing alumina contained in the purification catalyst layer lower layer 14a is obtained by coating the surface of alumina with 10% by mass of zirconium oxide (ZrO ₂ ). It prevents solid solution in alumina at high temperatures.

  Moreover, as one of the preferable blending examples of the HC adsorption layer 13, β-zeolite as an adsorbent contains 160.00 g / L and an alumina binder.

  Here, the thickness of the HC adsorption layer 13 and the thickness of the purification catalyst layer 14 are each preferably about 50 μm from the viewpoint of the diameter of the remaining cell passage 12 and the balance between cost and catalytic ability.

  As shown in FIG. 3, the exhaust gas purifying catalyst 10 according to the present embodiment has a purifying catalyst layer on the upper purifying catalyst layer 14, that is, on both the purifying catalyst layer lower layer 14a and the purifying catalyst layer upper layer 14b. 14 is formed by dispersing a plurality of fine paths 21... 21 having an average diameter larger than the average diameter of particles constituting the particles 14 (that is, oxygen storage material particles, Pd-supported powder, Rh-supported ceria particles, Rh-supported alumina particles, etc.). Has been.

  As a result, only a plurality of finely-formed fine passages 21... 21 are connected to each other in the purification catalyst layer 14 or are present in the purification catalyst layer 14 together with the finely-formed fine passages 21. By connecting the gaps to each other, passages 31... 31 through which HC in the exhaust gas flowing through the carrier cell passage 12 can pass from the surface on the cell passage 12 side of the purification catalyst layer 14 to the surface on the HC adsorption layer 13 side. (One of which is shown thick in FIG. 3).

  Further, the exhaust gas purifying catalyst 10 according to the present embodiment also includes a plurality of lower HC adsorption layers 13 having a plurality of average diameters larger than the average diameter of particles (that is, zeolite or the like) constituting the HC adsorption layer 13. The fine paths 22 ... 22 are formed in a dispersed manner.

  As a result, only a plurality of finely formed fine paths 22... 22 are connected to each other in the HC adsorption layer 13, or both of the plurality of finely formed fine paths 22. HC that has passed through the purification catalyst layer 14 from the surface of the HC adsorption layer 13 on the purification catalyst layer 14 side to the deep layer portion of the HC adsorption layer 13 (that is, in the vicinity of the honeycomb carrier 11) Diffusable passages 32... 32 (one of which is shown thick in FIG. 3).

  In that case, the average diameter of the constituent particles of the purification catalyst layer 14 or the HC adsorption layer 13 is preferably 0.1 to 0.9 μm (or 1.0 μm), and the average diameter of the micro paths 21 and 22 is Preferably, the thickness is 1 to 5 μm (if it exceeds 5 μm, the catalytic ability and strength of the catalyst layers 13 and 14 are affected). The length of the fine path is preferably 5 to 10 μm (if it is less than 5 μm, the passages 31 and 32 are difficult to generate, and if it exceeds 10 μm, the catalytic performance and strength of the catalyst layers 13 and 14 are affected. The volume of the microchannel is preferably 1% by volume or less with respect to the purification catalyst layer 14 or the HC adsorption layer 13 (exceeding 1% by volume affects the catalytic performance and strength of the catalyst layers 13 and 14). Come out).

  The fine paths 21 and 22 are formed in the layers 13, 14a and 14b in the following manner.

  That is, for example, organic fine fibers (natural fibers, synthetic fibers, etc.) that are burned and burned below the firing temperature (eg, 500 ° C.) of each layer 13, 14a, 14b are dispersed and mixed in advance in the slurry of each layer 13, 14a, 14b. If the slurry is washed on the honeycomb carrier 11 and dried and fired as usual, the fine fibers are burned and burned out in the drying and firing stage, and the burned traces become elongated fine paths 21 and 22. Thus, the layers 13, 14 a, and 14 b remain dispersed at random.

  According to this method, there is no problem that, for example, the number of work steps is increased in order to form the micro paths 21 and 22, and the manufacture of the exhaust gas purification catalyst 10 is further complicated.

  Next, an example of a specific method for producing the exhaust gas purification catalyst 10 will be described.

  First, the lower HC adsorption layer 13 is formed in the following manner. That is, water and alumina sol as a binder are added to β-zeolite powder, and stirred using a disperser to obtain a slurry (at this time, when the fine paths 22... 22 are dispersedly formed in the HC adsorption layer 13 as well. The fine fibers are dispersed and mixed in the slurry). Then, the honeycomb carrier 11 made of cordierite is immersed in this slurry, and the operation of removing the excess slurry by air blow is repeated to wash coat a predetermined amount of the slurry on the honeycomb carrier 11. Thereafter, the honeycomb carrier 11 is heated from room temperature to 500 ° C. at a constant rate of temperature over 1.5 hours, held at 500 ° C. for 2 hours, dried and fired, whereby HC is deposited on the honeycomb carrier 11. The adsorption layer 13 is formed.

  Further, the purification catalyst layer lower layer 14a is formed in the following manner. That is, zirconyl nitrate as a binder, rhodium-carrying ceria powder, rhodium-carrying alumina powder, and fine fibers for dispersing and forming the fine paths 21... 21 are mixed, and water is added thereto to add a disperser. Is mixed and stirred to obtain a slurry. Then, the honeycomb carrier 11 with the HC adsorption layer 13 formed is immersed in this slurry, and the operation of removing the excess slurry by air blow is repeated to wash coat a predetermined amount of the slurry on the honeycomb carrier 11. Thereafter, the honeycomb carrier 11 is heated from room temperature to 500 ° C. at a constant rate of temperature over 1.5 hours, held at 500 ° C. for 2 hours, dried and fired, so that the honeycomb carrier 11 is placed on the HC adsorption layer 13. The purification catalyst layer lower layer 14a is laminated.

  Here, the rhodium-carrying ceria can be obtained by dropping a rhodium nitrate aqueous solution onto the Zr / Ce / Nd composite oxide and firing at 500 ° C.

  The rhodium-carrying alumina can be obtained by dropping an aqueous rhodium nitrate solution onto activated alumina powder coated with 10% by mass of zirconium oxide and firing at 500 ° C.

  Further, the purification catalyst layer upper layer 14b is formed in the following manner. That is, zirconyl nitrate as a binder, cerium oxide powder as an oxygen storage material, palladium-carrying first powder, palladium-carrying second powder, and fine fibers for dispersing and forming fine paths 21. Mix, add water, mix and stir using a disperser to obtain a slurry. Then, by immersing the honeycomb carrier 11 on which the HC adsorption layer 13 and the purification catalyst layer lower layer 14a are formed in this slurry, and pulling up and removing excess slurry by air blow, a predetermined amount of slurry is applied to the honeycomb carrier 11. Wash coat. Thereafter, the honeycomb carrier 11 is heated from room temperature to 500 ° C. at a constant rate of heating over a period of 1.5 hours, held at 500 ° C. for 2 hours, dried and fired, so that the upper surface of the purification catalyst layer lower layer 14a The purification catalyst layer upper layer 14b is laminated. Here, the exhaust gas purification catalyst 10 is finally completed.

  Here, the first palladium-supported powder can be obtained by dropping a palladium nitrate aqueous solution into an activated alumina powder to which 4% by mass of lanthanum oxide is added and firing at 500 ° C.

  The palladium-supported second powder can be obtained by dropping a palladium nitrate aqueous solution onto a powder of Ce / Zr / La / Y / alumina composite oxide and firing at 500 ° C.

  In this case, the Ce / Zr / La / Y / alumina composite oxide can be prepared by either a hydrothermal synthesis method using an autoclave or a coprecipitation method using an acid-alkali neutralization treatment. In the coprecipitation method, first, nitrates of cerium, zirconium, lanthanum, yttrium, and aluminum are mixed, water is added, and the mixture is stirred at room temperature for about 1 hour. Next, the nitrate mixed solution and the alkaline solution (preferably 28% ammonia water) are mixed at room temperature to 80 ° C. using a disperser to perform neutralization treatment.

  The solution that has become cloudy due to this neutralization treatment is left for a whole day and night. The resulting precipitate cake is centrifuged and then thoroughly washed with water. The cake washed with water is dried at a temperature of about 150 ° C., held at a temperature of about 600 ° C. for about 5 hours, then held at a temperature of about 500 ° C. for 2 hours, baked, and then pulverized to obtain Ce.・ Zr / La / Y / alumina composite oxide powder is obtained.

  Thus, the exhaust gas purifying catalyst 10 of the present embodiment, as clearly shown in FIGS. 2 and 3, has catalyst layers 13 and 14 in two upper and lower layers on the honeycomb carrier 11 having a large number of cell passages 12. In which the lower layer is the HC adsorption layer 13 having the HC adsorption capability and the upper layer is the purification catalyst layer 14 having the HC purification capability, that is, the HC trap catalyst.

  A plurality of fine passages 21... 21 having an average diameter larger than the average diameter of the constituent particles of the purification catalyst layer 14 are dispersed and formed in the upper purification catalyst layer 14, thereby forming the carrier cell passage 12. Since the passage 31... 31 through which HC in the flowing exhaust gas can pass from the surface of the purification catalyst layer 14 on the cell passage 12 side to the surface of the HC adsorption layer 13 is generated, the exhaust gas passes through the purification catalyst layer 14. Passing in the thickness direction will be facilitated.

  Therefore, even if the exhaust gas flowing in the cell passage 12 is in a straight layered rectified state, the exhaust gas flowing in the cell passage 12 passes through the upper purification catalyst layer 14 in the thickness direction and passes through the lower purification catalyst layer 14. The ratio of the gas that diffuses to the HC adsorption layer 13 increases, and as a result, the amount of HC adsorbed by the lower HC adsorption layer 13 increases, and the release of HC to the atmosphere is sufficiently suppressed when cold. This can improve the HC purification rate when cold.

  Moreover, in addition to the upper purification catalyst layer 14, the lower HC adsorption layer 13 is similarly dispersed with a plurality of fine paths having an average diameter larger than the average diameter of the constituent particles of the HC adsorption layer 13. Thus, the passages 32... 32 that allow HC passing through the purification catalyst layer 14 to diffuse from the surface of the HC adsorption layer 13 on the purification catalyst layer 14 side to the deep layer portion of the HC adsorption layer 13 are generated. Therefore, it becomes easy for the exhaust gas to permeate and diffuse the HC adsorption layer 13 in the thickness direction to the deep layer portion.

  Therefore, more exhaust gas diffuses to the deep part of the HC adsorbing layer 13, and as a result, the amount of HC adsorbed by the lower HC adsorbing layer 13 further increases, so that the HC is released into the atmosphere when cold. The release can be further suppressed, and the HC purification rate in the cold state is further improved.

  In the exhaust gas purifying catalyst 10, the exhaust gas is within the carrier cell passage 12 within a predetermined range from the carrier inlet end to the outlet side (for example, within a range of 1/20 of the total length of the honeycomb carrier 11). Since the exhaust gas collides with the wall surface of the carrier inlet end when it flows into the gas, the exhaust gas is originally in a turbulent state, and the exhaust gas is easily diffused to the lower HC adsorption layer 13. Therefore, it is possible not to form the fine paths 21 and 22 within a predetermined range (same as above) from the carrier inlet end toward the outlet. With such a configuration, it is possible to improve the exhaust gas to the HC adsorption layer 13 while avoiding a reduction in the purification rate by forming the micro paths 21 and 22 and reducing the catalytic ability of the catalyst layers 13 and 14 correspondingly. Diffusion will be maintained.

  Further, in the exhaust gas purifying catalyst 10, foreign matter from the upstream side of the exhaust passage 2 is within a predetermined range from the carrier inlet end toward the outlet (for example, within a range of 1/20 of the total length of the honeycomb carrier 11). And deposits collide and receive exhaust pressure, so it is in an environment where peeling and collapse (wind erosion) are likely to occur. Therefore, also from this point of view, the fine paths 21 and 22 can be prevented from being formed within a predetermined range (same as above) from the carrier inlet end toward the outlet. With such a configuration, it is possible to improve the exhaust gas to the HC adsorption layer 13 while avoiding an increase in wind erosion damage by forming the micro paths 21 and 22 and reducing the strength of the catalyst layers 13 and 14 correspondingly. Diffusion will be maintained.

  Thus, in order to make the catalyst layer 13 and 14 with the fine paths 21 and 22 on the outlet side of the honeycomb carrier 11 and the catalyst layers 13 and 14 without the fine paths 21 and 22 on the inlet side, for example, the firing temperature Then, the slurry mixed with the burnable fine fiber is coated from the outlet side of the honeycomb carrier 11 (at this time, the inlet side of the honeycomb carrier 11 is left as an uncoated portion in a predetermined range), and then the burnable fine fiber is burned at the firing temperature. The slurry not mixed with the fiber may be coated from the inlet side of the honeycomb carrier 11 (the uncoated portion is coated).

  The above embodiment is the best embodiment of the present invention, but it goes without saying that various modifications and changes may be made without departing from the scope of the claims.

  For example, platinum (Pt) may be further added to the purification catalyst layer 14 as a catalyst metal. Alternatively, the purification catalyst layer 14 may be combined into one layer without being divided into the lower layer 14a and the upper layer 14b.

  As shown in FIG. 4, according to the method for manufacturing the exhaust gas purifying catalyst 10 and the chemical structure of each of the layers 13, 14a, 14b described above, the exhaust gas purifying catalysts of Examples 1 to 4 and Comparative Examples 1 and 2 are used. 10 was produced. FIG. 4 also shows the main specifications of the used honeycomb carrier 11.

  In Example 1, fine passages 21 and 22 are formed over the entire length of the catalyst 10 in both the purification catalyst layer 14 (both the upper layer 14b and the lower layer 14a) and the HC adsorption layer 13 (corresponding to claim 2). Configuration) In Example 2, fine paths 21 and 22 were formed in both layers of the purification catalyst layer 14 (both the upper layer 14b and the lower layer 14a) and the HC adsorption layer 13 except for a range of 10 mm on the inlet side of the catalyst 10. In Example 3, the purification catalyst layer 14 (both the upper layer 14b and the lower layer 14a) has a fine passage 21 formed over the entire length of the catalyst 10, and the HC adsorption layer 13 In the case where the fine path 22 was not formed (a configuration corresponding to claim 1), the fourth embodiment had a purification catalyst layer 14 (both the upper layer 14b and the lower layer 14a) except for a range of 10 mm on the inlet side of the catalyst 10. A fine path 21 is formed, and the HC adsorption layer 13 is fine. 22 is a what was not formed (structure corresponding to Claim 3).

  In contrast, in Comparative Example 1, the fine path 21 was not formed in the purification catalyst layer 14 (both the upper layer 14b and the lower layer 14a), and the fine path 22 was formed in the HC adsorption layer 13 over the entire length of the catalyst 10. In Comparative Example 2, the fine paths 21 and 22 were not formed in both the purification catalyst layer 14 (both the upper layer 14b and the lower layer 14a) and the HC adsorption layer 13.

  An evaluation test of the cold HC purification rate was performed for Examples 1 to 4 and Comparative Examples 1 and 2. In this evaluation test, a core sample having a diameter of 25.4 mm, a length of 50 mm, and a volume of 25 ml was cut out from the honeycomb carrier having the specifications shown in FIG. 4 and used as a test sample.

First, a gas containing toluene (toluene concentration 2500 ppmC, residual N ₂ ) as simulated HC in the exhaust gas was allowed to flow through each test sample (test catalyst device) at a temperature of 50 ° C. for 900 seconds. Toluene was adsorbed on the catalyst device. At this time, the total amount of toluene flowed for 900 seconds is A, the amount of toluene adsorbed on the test catalyst device (more specifically, the HC adsorption layer 13) is B, and the toluene adsorption amount B is the total amount A and 900 It was determined from the amount of toluene passed through the test catalyst device when it was allowed to flow for 2 seconds.

Next, simulated exhaust gas with A / F = 1.47 (CO ₂ : 13.9%, O ₂ : 0.6%, CO: 0.6%, H ₂ : 0.2%, NO: 1000 ppm, H ₂ O: 10%, N ₂ : remaining, HC component: not contained), while increasing the gas temperature at a rate of 30 ° C./minute while flowing out from the test catalyst device. The amount of toluene C to be measured was measured. And the cold HC purification rate (%) of each test catalyst apparatus was calculated | required by [(BC) x100 / A].

  The evaluation results are shown in FIG. The greater the range in which the micro paths 21 and 22 are formed, the better the cold HC purification rate (%). By forming the micro paths 21 and 22, the HC in the exhaust gas flowing through the carrier cell path 12 is reduced. It is considered that passing and diffusing the purification catalyst layer 14 and the HC adsorption layer 13 in the thickness direction is facilitated.

  As described above in detail with reference to specific examples, the present invention provides an exhaust gas purification catalyst, particularly an HC trap catalyst having a function of adsorbing HC in exhaust gas when cold. Increase the proportion of exhaust gas that passes through and diffuses to the lower HC adsorption layer, and increases the amount of HC adsorbed to the lower HC adsorption layer, thereby improving the cold HC purification rate Since this is a possible technology, it is expected to have wide industrial applicability in the technical field of reduced engine emissions.

1 is a layout diagram of an engine of a catalytic converter filled with an exhaust gas purifying catalyst according to an embodiment of the present invention. FIG. It is sectional drawing by the II-II arrow line of FIG. 1, Comprising: It is an enlarged view which shows the physical structure of the said exhaust gas purification catalyst. 2 is a schematic diagram showing a detailed physical configuration of a catalyst layer (HC adsorption layer, purification catalyst layer) of Example 1. FIG. It is a table | surface which shows the structure of the catalyst layer of Examples 1-4 and Comparative Examples 1 and 2, and the evaluation result of a cold HC purification rate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine 2 Exhaust passage 3 Catalytic converter 10 Exhaust gas purification catalyst 11 Honeycomb carrier 12 Cell passage 13 HC adsorption layer 14 Purification catalyst layer 14a Purification catalyst layer lower layer 14b Purification catalyst layer upper layers 21, 22 Fine passages 31, 32 passage

Claims (3)

Exhaust gas purification having a honeycomb carrier having a large number of cell passages disposed in an exhaust passage of an engine, an HC adsorption layer provided on the honeycomb carrier, and a purification catalyst layer laminated on the HC adsorption layer Catalyst for
The purification catalyst layer is configured so that a passage through which the HC in the exhaust gas flowing through the carrier cell passage can pass from the cell passage side surface to the HC adsorption layer side surface of the purification catalyst layer is generated in the purification catalyst layer. An exhaust gas purifying catalyst characterized in that a plurality of fine paths having an average diameter larger than the average diameter of the particles are formed in a dispersed manner. The exhaust gas purifying catalyst according to claim 1,
Particles constituting the HC adsorption layer so that a passage through which the HC that has passed through the purification catalyst layer can diffuse from the surface of the HC adsorption layer on the purification catalyst layer side to the deep part of the HC adsorption layer is generated in the HC adsorption layer. A catalyst for purifying exhaust gas, wherein a plurality of fine passages having an average diameter larger than the average diameter is formed in a dispersed manner. The exhaust gas purifying catalyst according to claim 1 or 2,
The exhaust gas purifying catalyst, wherein the fine path is not formed within a predetermined range from the carrier inlet end toward the outlet.

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