JP2009208045A - Exhaust gas cleaning catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst for efficiently cleaning an HC component even under the cold start of an engine. <P>SOLUTION: The exhaust gas cleaning catalyst 3 arranged on an exhaust passage of the engine includes: a honeycomb-like carrier; a zeolite layer 11 which is formed on the surface of a cell wall 5 of the honeycomb-like carrier and contains zeolite; a cleaning layer 13 containing a catalytic noble metal; and an oxide layer 12 which is formed between the zeolite layer 11 and the cleaning layer 13 and contains a Zr-based compound oxide containing zirconium (Zr) and alkali earth metal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying catalyst that purifies hydrocarbon (HC) and the like contained in exhaust gas.

  An exhaust gas purifying catalyst that purifies exhaust gas discharged from an engine is often disposed, for example, immediately below an engine exhaust manifold. This means that when the exhaust gas temperature is low, such as during cold start of the engine, the catalyst noble metal contained in the exhaust gas purification catalyst provided in the exhaust passage is not sufficiently activated, and HC and carbon monoxide ( This is because the exhaust gas is hardly purified by the oxidation reaction of CO) or the reduction reaction of nitrogen oxide (NOx). By disposing the exhaust gas purification catalyst at such a position, the catalyst precious metal in the exhaust gas purification catalyst can be activated as early as possible, and the purification of unburned exhaust gas such as HC is started early. Can be made. However, even if the exhaust gas purifying catalyst is disposed at such a position, the temperature reaches a temperature at which the precious metal in the exhaust gas purifying catalyst can oxidize HC components, etc. for several seconds to 20 seconds after the engine is started. Therefore, there is a problem that exhaust gas cannot be sufficiently purified.

  In order to solve this problem, it has been studied that the exhaust gas purifying catalyst contains a zeolite that adsorbs the HC component. Zeolite has pores of a predetermined size in the crystal structure in the range of several to several tens of centimeters, and can adsorb HC components into the pores, and further desorb the adsorbed HC components when the zeolite is warmed. It is known to do. Therefore, by incorporating zeolite into the exhaust gas purification catalyst, when the exhaust gas temperature is low, the zeolite adsorbs the HC component in the exhaust gas, and when the exhaust gas purification catalyst is warmed, it is adsorbed by the zeolite. It is expected that the HC component that had been gradually desorbed and purified by the activated catalytic noble metal. In practice, however, zeolite can adsorb HC components at a low temperature of about 60 ° C. or less, but when the exhaust gas temperature reaches about 250 ° C., most of the adsorbed HC components are desorbed. At an exhaust gas temperature of about 250 ° C., the catalyst noble metal in the exhaust gas purification catalyst is not sufficiently activated, and there is a problem that the desorbed HC component cannot be sufficiently purified.

  As an example of an exhaust gas purification catalyst that temporarily adsorbs HC components, for example, Patent Document 1 discloses a desorption containing at least one of an adsorbent composed of zeolite, Ag, Cu, Co, Ni, Sr, and Mg. An exhaust gas purifying catalyst in which a mixture of a catalyst composition A containing a temperature improving component and a combustion catalyst active component and a catalyst composition B containing the adsorbent and the combustion catalyst active component is supported on a honeycomb carrier Is disclosed.

Patent Document 2 has a catalyst containing a Zr oxide containing an alkaline earth metal selected from Mg, Ca, Sr, and Ba and Zr from the upstream side to the downstream side of the exhaust gas passage. An exhaust gas purification system in which a hydrogen enrichment means, a NOx catalyst, and an HC trap catalyst having a hydrocarbon adsorbent layer containing zeolite are sequentially arranged is disclosed.
JP 2004-8855 A JP 2002-364343 A

  The exhaust gas purification catalyst that temporarily adsorbs the HC component as described above is required to have the HC component adsorbed until it reaches a temperature at which the catalytic noble metal can purify the HC component.

  According to Patent Document 1, it is described that the desorption temperature of higher hydrocarbons can be increased by using a desorption temperature improving component such as Sr or Ba. However, since the desorption temperature improving component such as Sr or Ba here is supported on zeolite using a solution containing Sr, Ba or the like, the desorption temperature improving component supported on zeolite is It is presumed to be an oxide of an alkaline earth metal such as SrO or BaO. Such an alkaline earth metal oxide becomes a carbonate compound when the air-fuel ratio of the exhaust gas is rich, while it becomes a nitrate compound when the air-fuel ratio of the exhaust gas is lean. Therefore, depending on the air-fuel ratio of the exhaust gas when the engine is stopped, the compound type of the desorption temperature improving component varies. Depending on the compound type of the desorption temperature improving component, the desorption temperature may be improved at the next engine start. There are cases where it cannot be fully achieved.

  Patent Document 2 describes that NOx and HC in exhaust gas can be purified more efficiently under lean burn conditions. However, the hydrogen enrichment means having a catalyst containing a Zr oxide containing an alkaline earth metal and Zr, and the HC trap catalyst having a hydrocarbon adsorbent layer containing zeolite are arranged in a spaced arrangement via a NOx catalyst. Therefore, it is difficult to improve the HC desorption temperature of zeolite due to Zr oxide acting on the HC component. Actually, the inventors of the present invention examined the catalyst system having the above-described configuration. Although the improvement of the HC desorption temperature was somewhat observed, the HC catalyst until the three-way catalyst provided in the HC trap catalyst was sufficiently activated was Many of the components were desorbed from the zeolite.

  Therefore, in the exhaust gas purification catalyst described in Patent Document 1 and the exhaust gas purification system described in Patent Document 2, when the exhaust gas temperature is low, such as during cold start of the engine, the HC component is sufficient. There was a problem that it could not be purified.

  The present invention is to provide an exhaust gas purifying catalyst that can efficiently purify HC components even when the engine is cold started.

  The exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst disposed in an exhaust passage of an engine, and contains a honeycomb-shaped carrier and zeolite formed on the cell wall surface of the honeycomb-shaped carrier. Zeolite layer, purification layer containing catalytic noble metal, and oxide layer containing Zr-based composite oxide containing zirconium (Zr) and alkaline earth metal formed between the zeolite layer and the purification layer It is characterized by providing.

  Since the Zr-based composite oxide contained in the oxide layer contains an alkaline earth metal, the basicity is high. When this highly basic Zr-based composite oxide acts on the HC component in the exhaust gas, the HC component in the exhaust gas can be changed to a state that is easily adsorbed by the zeolite. And by changing to the state in which HC component is easy to be adsorbed by zeolite, adsorption of HC component to zeolite in the zeolite layer becomes strong.

  Since the exhaust gas purifying catalyst of the present invention is provided with an oxide layer between the zeolite layer and the purifying layer, the HC component comes into contact with the Zr-based composite oxide in the oxide layer before the zeolite layer. Adsorbed on the zeolite in the layer. Therefore, since the HC component is strongly adsorbed on the zeolite, the temperature at which the HC component adsorbed on the zeolite is desorbed becomes high. In addition, since the chemical species of Zr-based composite oxides are less likely to change depending on the air-fuel ratio of the exhaust gas compared to the alkaline earth metal alone, the HC component in the exhaust gas is not affected by the air-fuel ratio of the exhaust gas. It can be changed to a state in which it is easily adsorbed on zeolite.

  Furthermore, since the oxide layer is provided between the zeolite layer and the purification layer, heat transfer from the purification layer to the zeolite layer can be suppressed, so that the temperature of the zeolite layer is less likely to rise than the purification layer. Therefore, when the temperature at which the HC component is released from the zeolite layer is reached, the catalytic noble metal is activated in the purification layer, and the HC component can be efficiently purified.

  According to the above configuration, since the HC component is firmly adsorbed to the zeolite regardless of the air-fuel ratio of the exhaust gas, the exhaust gas purifying catalyst having a high desorption temperature of the adsorbed HC component is provided. Even during cold start, the HC component can be purified efficiently.

  The reason why the HC component is strongly adsorbed on the zeolite is that the HC component in contact with the catalyst layer is changed to carbanion by the highly basic Zr-based composite oxide, and the carbanion is bonded to the Lewis acid point of the zeolite. It is thought that it is to do.

  In the exhaust gas purification catalyst, it is preferable that a heat transfer suppression layer containing a heat transfer suppression material is further provided between the purification layer and the oxide layer. According to such a configuration, heat transfer from the purification layer to the zeolite layer can be further suppressed, so that when the temperature reaches the HC component desorption from the zeolite layer, the temperature in the purification layer becomes higher. Therefore, when the HC component is desorbed, the catalytic noble metal is more activated and the HC component can be purified more efficiently.

  In the exhaust gas purification catalyst, it is preferable that the oxide layer further contains a heat transfer suppressing material. According to such a configuration, the oxide layer becomes a layer that can further suppress the heat transfer from the purification layer to the zeolite layer. Therefore, when the temperature reaches the temperature at which the HC component is desorbed from the zeolite layer, the oxide layer The temperature becomes higher. Therefore, when the HC component is desorbed, the catalyst noble metal is more activated, and the HC component can be purified more efficiently.

  The heat transfer inhibitor is preferably at least one selected from the group consisting of composite oxide particles containing zirconium (Zr) and a rare earth metal, and hollow alumina particles.

  According to the present invention, since the HC component is firmly adsorbed to the zeolite regardless of the air-fuel ratio of the exhaust gas, the exhaust gas purifying catalyst has a high desorption temperature of the adsorbed HC component, Even at the time of starting, the HC component can be efficiently purified.

  An exhaust gas purifying catalyst according to an embodiment of the present invention will be described.

  FIG. 1 is a cross-sectional view schematically showing a catalytic converter 1 including an exhaust gas purifying catalyst according to this embodiment. The catalytic converter 1 is disposed in an exhaust passage connected to the engine, and hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in the exhaust gas are purified in the catalytic converter 1. It is configured to be discharged. This catalytic converter 1 is configured by incorporating a purification catalyst (exhaust gas purification catalyst) 3 in a heat-resistant container 2. Although not shown, the heat-resistant container 2 may be provided with a temperature sensor that detects the temperature of the purification catalyst 3.

  FIG. 2 is an enlarged cross-sectional view showing a part of the purification catalyst 3. The purification catalyst 3 is configured by forming a catalyst layer 6 on the surface of a cell wall 5 of a substantially cylindrical honeycomb-shaped carrier having a large number of cells partitioned by partition walls, and exhaust gas flows from the upstream side toward the downstream side. While passing through the cell, it diffuses into the catalyst layer 6 formed on the cell surface and comes into contact with the catalyst noble metal in the catalyst layer 6 to purify harmful components (HC, CO, NOx). Yes.

  As the honeycomb-shaped carrier of the purification catalyst 3, for example, a cordierite ceramic carrier or a stainless steel metal carrier can be used.

  The catalyst layer 6 of the purification catalyst 3 has the following configuration. FIG. 3 is a cross-sectional view schematically showing the catalyst layer 6 formed on the cell wall 5. Examples of the catalyst layer 6 include a layer in which a zeolite layer 11, an oxide layer 12, and a purification layer 13 are sequentially laminated on the surface of the cell wall 5 as shown in FIG. The zeolite layer 11 is a layer containing zeolite formed on the surface of the cell wall 5 of the honeycomb-shaped carrier. The purification layer 13 is a layer containing a catalyst noble metal. The oxide layer 12 is a layer containing a Zr-based composite oxide containing zirconium (Zr) and an alkaline earth metal formed between the zeolite layer 11 and the purification layer 13.

  Further, the catalyst layer 6 is not limited to the configuration shown in FIG. 3A as long as the oxide layer 12 is laminated between the zeolite layer 11 and the purification layer 13. Other layers other than the layer 11, the oxide layer 12, and the purification layer 13 may be laminated. Examples of the catalyst layer 6 on which other layers are laminated include those in which a heat transfer suppression layer 14 is laminated between a zeolite layer 11 and an oxide layer 12 as shown in FIG. . By laminating the heat transfer suppression layer 14, heat transfer from the purification layer 13 to the zeolite layer 11 can be further suppressed. Therefore, when the temperature at which the HC component is desorbed from the zeolite layer 11 is reached, the temperature at the purification layer 13 is reached. Becomes a higher temperature. Therefore, when the HC component is desorbed, the catalytic noble metal is more activated and the HC component can be purified more efficiently.

The zeolite contained in the zeolite layer 11 is not particularly limited as long as it has a large number of pores to which HC components are adsorbed. For example, β-zeolite, MFI-zeolite (ZSM-5) and super Stabilized Y (USY) -zeolite and the like. Moreover, the one in which the Keiban (SiO ₂ / Al ₂ O ₃ ) ratio is 20 to 100 is preferable.

  The catalytic noble metal contained in the purification layer 13 oxidizes hydrocarbons (HC) and carbon monoxide (CO) and reduces nitrogen oxides (NOx). Specifically, for example, at least one selected from platinum (Pt), palladium (Pd), rhodium (Rh) and the like is exemplified, and the purification layer 13 contains two types of Pd and Rh. It is preferable that The purification layer 13 may contain the catalyst noble metal as it is, but may contain a support in which the catalyst noble metal is supported on a carrier such as alumina or a composite oxide. Examples of the method for supporting the catalyst noble metal on the carrier include the following methods. For example, as a method for supporting Pt, which is a catalyst noble metal, on alumina, which is a carrier, a dinitrodiamine platinum nitric acid solution is added to alumina and mixed, and then supported on alumina by an evaporation to dryness method. The amount of the catalyst noble metal supported on the support can be adjusted, for example, by adjusting the concentration and amount of the dinitrodiamine platinum nitrate solution with respect to Pt.

  The Zr-based composite oxide contained in the oxide layer 12 has the ability to increase the desorption temperature of HC adsorbed on the zeolite 11 as will be described later. The Zr-based composite oxide is not particularly limited as long as it contains Zr and an alkaline earth metal. Examples of the alkaline earth metal include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba), and Ca and Sr are preferably used. Specific examples of the Zr-based composite oxide include a ZrSr composite oxide composed of an oxide of Zr and Sr, a ZrCa composite oxide composed of an oxide of Zr and Ca, and the like.

Also, in the Zr-based composite oxide, the content of the total amount oxides of alkaline earth metal to the oxide of ZrO ₂ and an alkaline earth metal, is preferably 1 mass% or more. In addition, the higher the content, the higher the basicity of the Zr-based composite oxide, which is preferable in that the HC desorption temperature increases and the amount of HC desorption at a high temperature increases. The structure tends to collapse. Therefore, the upper limit of the content is determined by the production limit of the Zr-based composite oxide. For example, in the case of a ZrSr composite oxide, the SrO content with respect to the total amount of ZrO ₂ and SrO is about 15% by mass (the content of Sr with respect to the total amount of Zr and Sr is about 21.0 mol%). It is a production limit and is the upper limit of the content.

The Zr-based composite oxide is prepared, for example, by the following ammonia coprecipitation method. A predetermined amount of Zr nitrate and Sr nitrate are dissolved in ion exchange water or the like to prepare a composite oxide precursor by ammonia coprecipitation. The composite oxide precursor is washed with water, dried by heating at a predetermined temperature, and finally fired to obtain a Zr-based composite oxide. The Zr-based composite oxide may contain other metal oxides in addition to ZrO ₂ and alkaline earth metal oxides.

  Moreover, the content rate of Zr type complex oxide with respect to the total amount of the zeolite contained in the zeolite layer 11, and the Zr type complex oxide contained in the oxide layer 12 is 3.5-38.5 mass%. preferable. If the content is too small, the effect of containing the Zr-based composite oxide tends to be unable to be exhibited. If the content is too large, the amount of zeolite decreases, so the maximum adsorption amount of the HC component decreases.

  The heat transfer suppressing material included in the heat transfer suppressing layer 14 has a performance of suppressing heat transfer from the purification layer to the zeolite layer. Specific examples include composite oxide particles containing zirconium (Zr) and a rare earth metal, and hollow alumina particles. These may be used alone or in combination of two or more.

  The hollow alumina particles are produced, for example, by a spray pyrolysis method. Specifically, first, a raw material solution is prepared by dissolving predetermined amounts of aluminum nitrate, lanthanum nitrate, and magnesium sulfate in ion-exchanged water. Then, the raw material solution is atomized by spraying the raw material solution into the heating furnace using air as a carrier gas. The droplets are heated in a heating furnace to become particles, and the particles are collected by a bag filter. The particles are dried after washing with water. By doing so, hollow alumina particles are obtained.

  Here, lanthanum nitrate is added in order to make La into a solid solution by forming a solid solution of La in the hollow alumina particles. The amount added is preferably about 5% by mass. Magnesium sulfate has the effect of suppressing sintering of crystallites when producing hollow alumina particles, and the effect of leading to a hollow structure while reducing the contact points between crystallites. The addition amount, concentration, and the like can be determined as appropriate.

  Moreover, although the thickness of each layer of the catalyst layer 6 varies depending on the composition of each layer, for example, the following is preferable so that the effects of each layer can be sufficiently exhibited. The thickness of the zeolite layer 11 is preferably such that the supported amount of zeolite is 80 to 200 g / L, for example, 50 to 125 μm. The thickness of the oxide layer 12 is preferably such that the supported amount of the Zr-based composite oxide is 10 to 50 g / L, for example, 15 to 40 μm. The thickness of the purification layer 13 is preferably 60 to 100 μm, for example. When the heat transfer suppression layer 14 is formed, the thickness is preferably such that the amount of the heat transfer suppression material carried is 10 to 50 g / L, for example, 15 to 40 μm.

  Further, in each layer of the catalyst layer 6, oxygen that is conventionally used as a catalyst material, for example, a composite oxide containing alumina and Ce, in addition to the above-described components, as long as the effects of the present invention are not impaired. It may contain occlusion material, nickel (Ni) for suppressing poisoning due to sulfur (S) component, barium (Ba) for suppressing poisoning due to phosphorus (P) component, and the like. For example, the oxide layer 12 may contain a heat transfer suppressing material similar to the heat transfer suppressing material included in the heat transfer suppressing layer 14. Since the heat transfer suppressing material is contained in the oxide layer 12, heat transfer from the purification layer 13 to the zeolite layer 11 can be further suppressed, so that when the temperature reaches the temperature at which the HC component is desorbed from the zeolite layer 11, the purification is performed. The temperature at the layer 13 is higher. Therefore, when the HC component is desorbed, the catalytic noble metal is more activated and the HC component can be purified more efficiently.

  In addition, composite oxides other than the Zr-based composite oxide used in the present invention, such as composite oxides used as heat transfer inhibitors and oxygen storage materials, can be used in the same way as in the method for preparing the Zr-based composite oxide. Prepared by precipitation method.

  The catalyst layer 6 is prepared as follows, for example.

  In the case of a catalyst layer in which a zeolite layer 11, an oxide layer 12, and a purification layer 13 are sequentially laminated on the surface of the cell wall 5 as shown in FIG. 3A, first, zeolite, water, and a binder raw material are mixed. An adsorbent layer forming slurry is produced. Then, this adsorbent layer forming slurry is coated on a honeycomb-shaped carrier, and excess slurry is removed by air blowing, followed by drying and firing. In this way, the zeolite layer 11 is formed on the cell wall 5 of the honeycomb-shaped carrier. Next, a Zr-based composite oxide, water, and a binder raw material are mixed to produce an oxide layer forming slurry. Then, this oxide layer forming slurry is coated on the honeycomb-shaped carrier on which the zeolite layer 11 is formed, and excess slurry is removed by air blowing, followed by drying and firing. In this way, the oxide layer 12 is formed on the zeolite layer 11. Next, a catalyst noble metal or a carrier supporting the catalyst noble metal, a composite oxide, water, and a binder raw material are mixed to produce a purification layer forming slurry. Then, this purification layer forming slurry is coated on the honeycomb-shaped carrier on which the oxide layer is formed, and excess slurry is removed by air blowing, followed by drying and firing. In this way, the purification layer 13 is formed on the oxide layer 12. As described above, the catalyst layer 6 as shown in FIG.

  In the case of a catalyst layer in which a heat transfer suppressing layer 14 is laminated between the zeolite layer 11 and the oxide layer 12 as shown in FIG. 3B, the zeolite in the catalyst layer as shown in FIG. After forming the layer 11, the heat transfer suppressing layer 14 is formed before the oxide layer 12 is formed. Specifically, a heat transfer suppressing material, water, and a binder raw material are mixed to produce a heat transfer suppressing layer forming slurry. Then, the heat transfer suppressing layer forming slurry is coated on the zeolite layer 11 formed on the cell wall 5 of the honeycomb-shaped carrier as described above, and the excess slurry is removed by air blowing, followed by drying and firing. To do. In this way, the heat transfer suppression layer 14 is formed on the zeolite layer 11. Then, the oxide layer 12 and the purification layer 13 are formed by the same method as described above. As described above, the catalyst layer 6 as shown in FIG. 3B is formed.

  The layer thickness of the catalyst layer 6 can be adjusted by the viscosity and concentration of the slurry. Further, as the binder raw material, for example, zirconyl nitrate, alumina sol or the like can be used, and the firing condition is preferably maintained at 450 to 550 ° C. in the atmosphere for about 1 to 3 hours.

  The Zr-based composite oxide contained in the oxide layer 12 has the ability to increase the desorption temperature of HC adsorbed on the zeolite contained in the zeolite layer 11 (performance to improve the HC desorption temperature). The mechanism that exhibits this performance is presumed as follows. FIG. 4 is an explanatory diagram showing a mechanism for improving the HC desorption temperature.

When exhaust gas is exhausted from the engine, HC components in the exhaust gas diffuse in the catalyst layer 6 of the purification catalyst 3. The exhaust gas diffused in the catalyst layer 6 reaches the zeolite layer after contacting the Zr-based composite oxide contained in the oxide layer 12 because the zeolite layer 11 is formed below the oxide layer 12. To do. And since the Zr system complex oxide contained in oxide layer 12 contains alkaline-earth metal, it is strong basic. Therefore, the strongly basic Zr-based composite oxide acts on the HC component in the exhaust gas diffused in the catalyst layer 6 to generate carbanions having a negative charge on the carbon. Specifically, for example, when toluene, which is an HC component, comes into contact with the Zr-based composite oxide, as shown in FIG. 4A, H is extracted as H ⁺ and becomes a carbanion of toluene.

Since the carbanion has a negative charge on the carbon, as shown in FIG. 4 (b), the carbanion interacts with the Lewis acid point (Si ⁺ ) of the zeolite 21 in the zeolite layer 11 and is attracted. By this interaction, the Zr-based composite oxide enhances the adsorption of the HC component to the zeolite. Therefore, it is estimated that the desorption temperature of the HC component adsorbed on the zeolite increases.

  In addition, when the content of the alkaline earth metal oxide in the Zr-based composite oxide is high, the basicity is further increased, so that the HC component is easily changed to carbanion, and the HC component is strongly adsorbed by the zeolite. Inferred.

  In addition, it seems that even if the alkaline earth metal alone is strong rather than the Zr-based complex oxide, it seems that the same effect as the above mechanism can be achieved. Depending on the air-fuel ratio, chemical species such as oxides, carbonate compounds and nitrate compounds are likely to change. On the other hand, since the Zr-based composite oxide is stable regardless of the air-fuel ratio of the exhaust gas, it is considered that the performance of improving the HC desorption temperature can be sufficiently exhibited.

  Note that adsorption of HC components is not observed with the Zr-based composite oxide alone. This also indicates that the Zr-based composite oxide acts to improve the HC desorption temperature of zeolite.

  As described above, since the purification catalyst according to the present invention has a high desorption temperature of the adsorbed HC component, the desorbed HC component is purified by a catalytic noble metal that is sufficiently heated and activated. Can do.

  Next, as a comparative embodiment for comparison with the present invention, a purification catalyst 3 that does not include the oxide layer 12 will be described. The catalyst layer 6 of the purification catalyst 3 according to the comparative embodiment has a configuration as shown in FIG. FIG. 5 is a cross-sectional view schematically showing the catalyst layer 6 formed on the cell wall 5. For example, as shown in FIG. 5 (a), the zeolite layer 11 and the purification layer 13 are sequentially laminated on the surface of the cell wall 5, or the zeolite layer 11 and the heat transfer suppressing layer as shown in FIG. 5 (b). 14 and the purification layer 13 are sequentially laminated on the surface of the cell wall 5. Since the purification catalyst 3 having the catalyst layer 6 having such a configuration does not include the oxide layer 12, the HC desorption temperature of zeolite as described above cannot be improved. Therefore, before the catalytic noble metal contained in the purification layer 13 is activated, the HC component is desorbed from the zeolite, and the HC component cannot be efficiently purified.

  Examples of the exhaust gas purifying catalyst which is an embodiment of the present invention will be described below, but the present invention is not limited to the examples.

Example 1
As the honeycomb-shaped carrier of this example, a cordierite carrier having a cell wall of 4.5 mil and a cell count of 400 cpsi (400 cells per square inch) was used.

  First, materials used for each layer of the catalyst layer formed on the honeycomb-shaped carrier were prepared.

  As a zeolite used for the zeolite layer, β-zeolite (commercially available product, Keiban ratio 40) was used.

  A SrZr composite oxide used for the oxide layer was prepared. Specifically, first, a predetermined amount of Zr nitrate and Sr nitrate was dissolved in ion-exchanged water. And the basic solution adjusted with ammonia was dripped at the solution, the precipitation containing each metal element was produced | generated, it filtered, washed with water, dried, and baked at 500 degreeC for 2 hours. By doing so, a ZrSr composite oxide was prepared. Here, the predetermined amount for dissolving the Zr nitrate and the Sr nitrate in the ion-exchanged water is such that the Sr content in the obtained ZrSr composite oxide is 11.4 mol% with respect to the total amount of Zr and Sr. I did it.

  Hollow alumina particles used for the heat transfer suppression layer were prepared. Specifically, first, a predetermined amount of aluminum nitrate, lanthanum nitrate, and magnesium sulfate was dissolved in ion exchange water to obtain a raw material solution. Then, the raw material solution was atomized by spraying the raw material solution into the heating furnace using air as a carrier gas. These droplets were heated in a heating furnace to become particles, and these particles were collected by a bag filter. The temperature in the heating furnace was set to 700 ° C. ± 25 ° C. The particles were washed with water and dried. By doing so, hollow alumina particles were prepared.

Materials used for the purification layer include cerium oxide, ZrCeNd composite oxide (ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 55: 35: 10 (mass ratio)), Pd-supported La-containing alumina (La ₂ O ₃ content) : 4 mass%), Pd-supported CeZrLaY alumina composite oxide (CeO ₂ : ZrO ₂ : La ₂ O ₃ : Y ₂ O ₃ : Al ₂ O ₃ = 10: 7: 2: 1: 80 (mass ratio)), Rh-supported ZrCeNd composite oxide (ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 80: 10: 10 (mass ratio)), Rh-supported ZrLa alumina composite oxide (ZrO ₂ : La ₂ O ₃ : Al ₂ O ₃ = 38: 2: 60 (mass ratio)), La-containing alumina (La ₂ O ₃ content: 5.0 mass%) was used.

  Next, each layer of the catalyst layer was sequentially formed on the honeycomb-shaped carrier.

  It mixed with β-zeolite, water, and alumina sol as a binder raw material to produce a zeolite layer forming slurry. This zeolite layer forming slurry was coated on a honeycomb-shaped carrier, excess slurry was removed by air blowing, and the slurry was dried, followed by firing at 500 ° C. for 2 hours. By doing so, a zeolite layer was formed on the cell walls of the honeycomb-shaped carrier. The supported amount of zeolite per liter of honeycomb-shaped carrier was set to 160 g / L.

  Next, a slurry for forming an oxide layer was formed by mixing SrZr composite oxide, water, and zirconyl nitrate as a binder raw material. This oxide layer forming slurry was coated on a honeycomb-shaped carrier on which a zeolite layer was formed, and excess slurry was removed by air blowing, followed by firing at 500 ° C. for 2 hours. By doing so, a SrZr composite oxide layer was formed on the zeolite layer. Note that the amount of the SrZr composite oxide supported per liter of the honeycomb-shaped carrier was set to 30 g / L.

  Next, hollow alumina particles, water, and zirconyl nitrate as a binder raw material were mixed to produce a slurry for forming a heat transfer suppression layer. This heat transfer suppressing layer forming slurry was coated on a honeycomb-shaped carrier on which an oxide layer was formed, and excess slurry was removed by air blowing, followed by firing at 500 ° C. for 2 hours. By doing so, a hollow alumina particle layer was formed on the oxide layer. The carrying amount of the hollow alumina particles per liter of the honeycomb-shaped carrier was set to 20 g / L.

  Next, cerium oxide, ZrCeNd composite oxide, Pd-supported La-containing alumina, Pd-supported CeZrLaY alumina composite oxide, Rh-supported ZrCeNd composite oxide, Rh-supported ZrLa alumina composite oxide, La-containing alumina, water and binder materials Zirconyl nitrate was mixed to produce a purification layer forming slurry. This purification layer forming slurry was coated on a honeycomb-like carrier on which a heat transfer suppressing layer was formed, and excess slurry was removed by air blowing, followed by firing at 500 ° C. for 2 hours. By doing so, the purification layer was formed on the heat transfer suppression layer. In addition, cerium oxide per liter of honeycomb-shaped carrier, ZrCeNd composite oxide, Pd-supported La-containing alumina, Pd-supported CeZrLaY alumina composite oxide, Rh-supported ZrCeNd composite oxide, Rh-supported ZrLa alumina composite oxide, La-containing alumina Each supported amount was set to 5.5 g / L, 5.5 g / L, 45 g / L, 23 g / L, 70 g / L, 30 g / L, and 13 g / L. The supported amounts of Pd of Pd-supported La-containing alumina, Pd of Pd-supported CeZrLaY alumina composite oxide, Rh of Rh-supported ZrCeNd composite oxide, and Rh of Rh-supported ZrLa alumina composite oxide were 0.45 g / L, 0.25 g / L, 0.1 g / L, and 0.05 g / L.

  As described above, an exhaust gas purification catalyst as shown in FIG. 3B was manufactured.

(Example 2)
Instead of forming a hollow alumina particle layer as a heat transfer suppression layer, the same procedure as in Example 1 is performed except that a ZrY composite oxide layer is formed as follows.

  A ZrY composite oxide (Y: 3 mol% contained, Zr: remainder), water and zirconyl nitrate as a binder raw material were mixed to produce a slurry for forming a heat transfer suppression layer. This heat transfer suppressing layer forming slurry was coated on a honeycomb-shaped carrier on which an oxide layer was formed, and excess slurry was removed by air blowing, followed by firing at 500 ° C. for 2 hours. By doing so, a ZrY composite oxide layer was formed on the oxide layer. The amount of ZrY composite oxide supported per liter of honeycomb-shaped carrier was set to 20 g / L.

(Example 3)
Instead of forming the SrZr composite oxide layer as the oxide layer, it is the same as in Example 2 except that the BaZr composite oxide layer is formed as follows.

  First, a BaZr composite oxide was prepared. Specifically, first, a predetermined amount of Zr nitrate and Ba nitrate was dissolved in ion-exchanged water. And the basic solution adjusted with ammonia was dripped at the solution, the precipitation containing each metal element was produced | generated, it filtered, washed with water, dried, and baked at 500 degreeC for 2 hours. By doing so, BaZr composite oxide was prepared. Here, the predetermined amount for dissolving Zr nitrate and Ba nitrate in ion-exchanged water is such that the Ba content relative to the total amount of Zr and Ba is 11.4 mol% in the obtained BaZr composite oxide. I did it.

  Next, a BaZr composite oxide, water, and zirconyl nitrate as a binder raw material were mixed to produce an oxide layer forming slurry. This oxide layer forming slurry was coated on a honeycomb-shaped carrier on which a zeolite layer was formed, and excess slurry was removed by air blowing, followed by firing at 500 ° C. for 2 hours. By doing so, a BaZr composite oxide layer was formed on the zeolite layer. Note that the supported amount of BaZr composite oxide per liter of the honeycomb-shaped carrier was set to 30 g / L.

Example 4
Except not forming a heat transfer suppression layer, it is the same as that of Example 1. That is, an exhaust gas purification catalyst as shown in FIG.

(Comparative Example 1)
This example is the same as Example 1 except that the heat transfer suppression layer and the oxide layer are not formed. That is, an exhaust gas purification catalyst as shown in FIG.

(Comparative Example 2)
Example 1 is the same as Example 1 except that no oxide layer is formed. That is, an exhaust gas purification catalyst as shown in FIG. 5B was manufactured.

[Cold HC purification rate]
The exhaust gas purification catalysts according to Examples 1 to 4 and Comparative Examples 1 and 2 were measured for the cold HC purification rate as follows.

  First, a cylindrical shape (volume: 25 ml) having a diameter of 25.4 mm and a length of 50 mm was cut out from each of the obtained exhaust gas purification catalysts.

Next, toluene was adsorbed on the purification catalyst. Specifically, the catalyst device was produced by arranging the purification catalyst in a heat-resistant container. While each purification catalyst is heated to 50 ° C., a gas having a toluene concentration of 2000 ppmC (remainder: N ₂ ) flows in for 900 seconds from the upstream side of the catalytic device, and the toluene concentration (desorption) of the gas flowing out from the downstream side of the catalytic device. (Toluene concentration) was measured. At that time, the total amount of toluene A that has flowed into the catalyst device is calculated from the toluene concentration of the gas that has flowed into the catalyst device, and the purification catalyst is further calculated from the toluene concentration and desorbed toluene concentration of the gas that has flowed into the catalyst device. The amount of toluene B adsorbed on was calculated.

Then, A / F = 14.7 for simulated exhaust gas _(CO 2: 13.9 _vol%, O 2: 0.6 vol%, CO: 0.6 _vol%, H 2: 0.2 vol%, NO : 1000 ppm, H ₂ O: 10% by volume, N ₂ : balance (without HC) circulated through the catalyst device, and the temperature was raised at a rate of 30 ° C./min, and the concentration of toluene flowing out from the catalyst device was measured. did. At that time, the amount of toluene C flowing out from the catalyst device during the heating was calculated from the concentration of toluene flowing out from the catalyst device.

  Next, the cold HC purification rate was calculated by the following equation (I).

Cold HC purification rate (%) = [(BC) / A] × 100 (I)
The obtained cold HC purification rate is shown in Table 1 below.

  As shown in Table 1, Examples 1-4 in which an oxide layer containing a Zr-based composite oxide containing zirconium (Zr) and an alkaline earth metal is formed between the zeolite layer and the purification layer, Compared with Comparative Examples 1 and 2 in which no oxide layer is formed, the cold HC purification rate is low. In Comparative Examples 1 and 2 where no oxide layer exists, the HC desorption temperature of the zeolite cannot be increased, and the HC component is desorbed from the zeolite before the catalytic noble metal of the purification layer is activated. This seems to be because.

  Further, as in Comparative Example 2, even if a hollow alumina particle layer is formed between the zeolite layer and the purification layer as a heat transfer suppression layer and the temperature rise of the zeolite layer is suppressed, the catalyst noble metal of the purification layer It can be seen that it is difficult to suppress desorption of the HC component from the zeolite until it is activated.

  From the above, a zeolite layer containing zeolite, a purification layer containing a catalytic noble metal, and a Zr-based complex oxidation containing zirconium (Zr) and an alkaline earth metal formed between the zeolite layer and the purification layer. By providing the oxide layer containing the substance, the HC component can be efficiently purified even during the cold start of the engine.

It is sectional drawing which shows typically the catalytic converter 1 provided with the exhaust gas purification catalyst which concerns on this embodiment. 2 is an enlarged sectional view showing a part of a purification catalyst 3. FIG. 2 is a cross-sectional view schematically showing a catalyst layer 6 formed on a cell wall 5. FIG. It is explanatory drawing which shows the mechanism which improves HC desorption temperature. 2 is a cross-sectional view schematically showing a catalyst layer 6 formed on a cell wall 5. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Catalytic converter 2 Heat-resistant container 3 Purification catalyst 5 Cell wall 6 Catalyst layer 11 Zeolite layer 12 Oxide layer 13 Purification layer 14 Heat transfer suppression layer

Claims (4)

An exhaust gas purifying catalyst disposed in an exhaust passage of an engine,
A honeycomb carrier;
A zeolite layer containing zeolite formed on the cell wall surface of the honeycomb-shaped carrier;
A purification layer containing a catalytic noble metal;
An exhaust gas purification catalyst comprising an oxide layer containing a Zr-based composite oxide containing zirconium (Zr) and an alkaline earth metal, formed between the zeolite layer and the purification layer .   The exhaust gas purifying catalyst according to claim 1, further comprising a heat transfer suppressing layer containing a heat transfer suppressing material between the purification layer and the oxide layer.   The exhaust gas purifying catalyst according to claim 1 or 2, wherein the oxide layer further contains a heat transfer suppressing material.   4. The exhaust gas purification according to claim 2, wherein the heat transfer inhibitor is at least one selected from the group consisting of composite oxide particles containing zirconium (Zr) and a rare earth metal, and hollow alumina particles. Catalyst.

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