JP2009125707A - Exhaust gas cleaning catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst that can control emission of HC components in a low-temperature range by raising desorption temperature of the HC components adsorbed by zeolite. <P>SOLUTION: The exhaust gas cleaning catalyst 3 installed in a pathway of an engine exhaust gas has a honeycomb carrier and a catalyst layer 6 formed on the surface of a cell wall 5 of the honeycomb carrier. The catalyst layer 6 contains the zeolite 11 and a zirconium (Zr) composite oxide 12 containing Zr and an alkaline 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 when the engine is started, 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 (CO) This is because it is difficult to purify the exhaust gas by the oxidation reaction of NOx 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 catalyst precious metal in the exhaust gas purifying catalyst has reached a temperature at which the HC component can be oxidized 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, exhaust gas purifying catalysts containing zeolite that adsorbs HC components are known. Zeolite has pores of a predetermined size in the range of several to tens of thousands in the crystal structure, and can adsorb HC components in the pores, and when the zeolite is warmed, the adsorbed HC components are desorbed. It has been known. By containing zeolite, immediately after the engine is started, the zeolite adsorbs the HC component in the exhaust gas. When the exhaust gas purification catalyst is warmed, the HC component adsorbed on the zeolite is gradually desorbed and purified by the activated catalytic noble metal.

  As an example of such an exhaust gas purifying catalyst that temporarily adsorbs HC components, for example, Patent Document 1 discloses a first catalyst layer mainly composed of zeolite on a carrier and an oxidation-reduction capability thereon. A catalyst provided with a second catalyst layer comprising a precious metal catalyst as a main component is disclosed.

  Patent Document 2 discloses a zeolite adsorbent, a catalyst composition A containing a desorption temperature improving component containing at least one of Ag, Cu, Co, Ni, Sr and Mg, and a combustion catalyst active component, and the zeolite adsorption. An exhaust gas purifying catalyst in which a mixture of an agent and a catalyst composition B containing a combustion catalyst active component is supported on a honeycomb carrier is disclosed.

Patent Document 3 discloses a hydrogen enrichment having a Zr oxide containing an alkaline earth metal selected from the group consisting of 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 comprising a HC trap catalyst having a means, a NOx catalyst, and a hydrocarbon adsorbent layer containing zeolite is sequentially disclosed.
JP-A-2-56247 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. That is, an exhaust gas purification catalyst having a high HC component desorption temperature is required.

  Although the zeolite described in Patent Document 1 can adsorb HC components at a low temperature of about 60 ° C. or less, most of the adsorbed HC components are desorbed when the exhaust gas temperature is about 250 ° C. . At an exhaust gas temperature of about 250 ° C., the catalyst noble metal in the exhaust gas purification catalyst was not sufficiently activated, and the desorbed HC component could not be sufficiently purified.

  Patent Document 2 describes that the desorption temperature of higher hydrocarbons can be increased by using a desorption temperature improving component such as Sr or Ba. However, since desorption temperature improving components such as Sr and Ba are supported on zeolite using a solution containing Sr and Ba and the like, the desorption temperature improving components supported on zeolite are SrO and BaO. It is inferred that it is an oxide of an alkaline earth metal such as 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.

  According to Patent Document 3, it is described that NOx and HC in exhaust gas can be purified more efficiently under lean burn conditions. However, since the hydrogen enrichment means having the Zr oxide and the HC trap catalyst having the hydrocarbon adsorbent layer containing the zeolite are spaced apart via the NOx catalyst, the Zr oxide directly acts on the zeolite. Therefore, it is not considered to improve the HC desorption temperature of zeolite. Furthermore, the Zr oxide used in the hydrogen enrichment means has a function of generating hydrogen and does not improve the HC desorption temperature of zeolite.

  The present invention is to provide an exhaust gas purifying catalyst having a high desorption temperature of the adsorbed HC component.

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

  According to this configuration, since the Zr-based composite oxide includes an alkaline earth metal, the basicity is high. This highly basic Zr-based composite oxide acts on the HC component in contact with the catalyst layer, whereby the HC component in the exhaust gas can be changed to a state that is easily adsorbed by the zeolite. Then, the HC component that has been changed to a state in which it is easily adsorbed by the zeolite is firmly adsorbed by the nearby zeolite. 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.

  Therefore, regardless of the air-fuel ratio of the exhaust gas, the HC component is firmly adsorbed on the zeolite, so that an exhaust gas purification catalyst having a high desorption temperature of the adsorbed HC component can be obtained.

  This is presumably because 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.

  Moreover, it is preferable that the said catalyst layer consists of a mixture containing the said zeolite and the said Zr type complex oxide. According to this configuration, the catalyst layer in a state where the zeolite and the Zr-based composite oxide are close to each other can be easily formed. Further, in such a catalyst layer, the HC component that has been changed to a state in which it is easily adsorbed to the zeolite by the Zr-based composite oxide is easily adsorbed to the zeolite in the vicinity of the Zr-based composite oxide.

  The catalyst layer may be composed of a zeolite layer containing zeolite formed on the surface of the cell wall and an oxide layer containing the Zr-based composite oxide formed on the surface of the zeolite layer. preferable. According to this configuration, since the oxide layer is laminated on the zeolite layer, most of the HC components are adsorbed on the zeolite in the zeolite layer after coming into contact with the Zr-based composite oxide in the oxide layer. Therefore, the HC component is strongly adsorbed by the zeolite. Therefore, the desorption temperature of the adsorbed HC component can be further increased.

Also, in the Zr-based composite oxide, the content of the oxide of the alkaline earth metal to the total amount of the oxide of the alkaline earth metal and ZrO ₂ is preferably 1 mass% or more. According to this configuration, an exhaust gas purifying catalyst having a larger amount of HC desorption at a high temperature can be obtained.

  This is considered to be because the basicity of the Zr-based composite oxide is further increased, so that the HC component is easily changed to carbanion, and the HC component is strongly adsorbed by the zeolite.

  Moreover, it is preferable that the content rate of the said Zr type complex oxide with respect to the total amount of the said zeolite and the said Zr type complex oxide is 1-50 mass%. According to this configuration, since the relative amount of zeolite is large, an exhaust gas purification catalyst having a larger HC adsorption amount can be obtained.

  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, an exhaust gas purification catalyst having a high desorption temperature of the adsorbed HC component can be obtained.

  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 contained in the catalyst layer 6 to purify harmful components (HC, CO, NOx). ing.

  In the purification catalyst 3, for example, a cordierite ceramic carrier or a stainless steel metal carrier can be used as the honeycomb carrier. The catalyst layer 6 of the purification catalyst 3 is formed by coating the surface of the cell wall with zeolite, Zr-based composite oxide containing zirconium (Zr) and alkaline earth metal, and the like. That is, the catalyst layer 6 contains zeolite and a Zr-based composite oxide, and at least a part of the zeolite and at least a part of the Zr-based composite oxide are close to each other. In addition, the material contained in the catalyst layer 6 includes materials conventionally used as catalyst materials, for example, alumina and Ce, in addition to zeolite and Zr-based composite oxides, as long as the effects of the present invention are not impaired. Contains oxygen storage material composed of complex oxide, nickel (Ni) for suppressing poisoning by sulfur (S) component, barium (Ba) for suppressing poisoning by phosphorus (P) component, etc. Also good.

  Next, a specific configuration of the catalyst layer 6 will be described. FIG. 3 is a cross-sectional view schematically showing the catalyst layer 6 formed on the cell wall 5. As shown in FIG. 3 (a), the catalyst layer 6 contains a catalyst layer made of a mixture containing zeolite 11 and a Zr-based composite oxide 12, or contains zeolite 11 as shown in FIG. 3 (b). Examples thereof include a catalyst layer formed by laminating an oxide layer containing the Zr-based composite oxide 12 on the adsorbent layer. The catalyst layer 6 includes the zeolite 11 and the Zr-based composite oxide 12, and it is sufficient that at least a part of the zeolite 11 and at least a part of the Zr-based composite oxide 12 are close to each other. It is not limited to the configuration. Further, the catalyst layer 6 may contain a catalyst noble metal 13. For example, the catalyst noble metal 13 may be supported on the zeolite 11 or the Zr-based composite oxide 12. Further, the catalyst layer 6 includes an adsorption layer containing the zeolite 11 and the Zr-based composite oxide 12, and a three-way catalyst layer containing a catalyst noble metal formed on the exhaust gas downstream side from the adsorption layer. May be.

The zeolite 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 ultra-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 Zr-based composite oxide 12 has a capability of increasing the desorption temperature of HC adsorbed on the zeolite 11 as will be described later. The Zr-based composite oxide 12 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 12, 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. Further, the higher the content, the higher the basicity of the Zr-based composite oxide 12, which is preferable in that the HC desorption temperature increases and the amount of HC desorption at a high temperature increases. The crystal structure is easily broken down. 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 12 is prepared, for example, as follows. 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. Then, the composite oxide precursor is washed with water, heated and dried at a predetermined temperature, and finally fired, whereby the Zr-based composite oxide 12 is obtained. The Zr-based composite oxide 12 may contain other metal oxides in addition to ZrO ₂ and alkaline earth metal oxides.

  Moreover, it is preferable that the content rate of Zr type complex oxide 12 with respect to the total amount of zeolite 11 and Zr type complex oxide 12 is 1-50 mass%. If the content is too small, there is a tendency that the effect of containing the Zr-based composite oxide 12 cannot be exhibited. If the content is too large, the amount of zeolite 11 decreases, so the maximum amount of adsorption of HC components decreases. End up.

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

  In the case of a catalyst layer made of a mixture containing zeolite 11 and Zr-based composite oxide 12 as shown in FIG. 3 (a), the zeolite 11, Zr-based composite oxide 12, water and a binder raw material are mixed to form a slurry. Is generated. Then, this slurry is coated on the honeycomb-shaped carrier, and excess slurry is removed by air blowing, followed by drying and firing. In this way, the catalyst layer 6 made of a fired product of the mixture containing the zeolite 11 and the Zr-based composite oxide 12 is formed on the cell wall 5 of the honeycomb-shaped carrier.

  In the case of a catalyst layer formed by laminating an oxide layer on an adsorbent layer as shown in FIG. 3 (b), first, the zeolite 11, water and a binder raw material are mixed to form an adsorbent layer forming slurry. Generate. 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, an adsorbent layer is formed on the cell walls 5 of the honeycomb-shaped carrier. Next, the Zr-based composite oxide 12, water, and a binder raw material are mixed to generate an oxide layer forming slurry. Then, this oxide layer forming slurry is coated on the honeycomb-shaped carrier on which the adsorbent layer is formed, and excess slurry is removed by air blowing, followed by drying and firing. In this way, the catalyst layer 6 formed by laminating the oxide layer on the adsorbent layer is formed on the cell wall 5 of the honeycomb-shaped carrier.

  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 12 included in the catalyst layer 6 has a performance of increasing the desorption temperature of HC adsorbed on the zeolite 11 (performance of improving 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 the exhaust gas is discharged from the engine, the HC component in the exhaust gas comes into contact with the catalyst layer 6 of the purification catalyst 3. Since the Zr-based composite oxide 12 in the catalyst layer 6 contains an alkaline earth metal, it is highly basic. Therefore, when the strongly basic Zr-based composite oxide 12 acts on the HC component in contact with the catalyst layer 6, a carbanion having a negative charge is generated on the carbon. Specifically, for example, when an olefin that is an HC component contacts the catalyst layer 6, as shown in FIG. 4A, H at the allylic position of the olefin is extracted as H ⁺ and becomes allyl carbanion.

Since the carbanion has a negative charge on the carbon, as shown in FIG. 4B, the carbanion interacts with the Lewis acid point (Al ⁺ ) of the zeolite 11 and is attracted. By this interaction, the Zr-based composite oxide 12 enhances the adsorption of the HC component to the zeolite 11. Therefore, it is estimated that the desorption temperature of the HC component adsorbed on the zeolite 11 increases.

  In FIG. 4, olefin (butene) is exemplified as the HC component. However, the HC component is not limited to the olefin, and a carbanion having a negative charge is generated on the carbon by the Zr-based composite oxide 12. It is estimated that the HC component desorption temperature increases by acting in the same manner as the above mechanism.

  Further, when the content of the alkaline earth metal oxide in the Zr-based composite oxide 12 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 11. It is guessed.

  Further, it seems that even if the alkaline earth metal alone is not the Zr-based composite oxide 12, the basic effect is strong, so that it seems that the same effect as the above mechanism can be achieved. Oxides, carbonate compounds, nitrate compounds, and chemical species are likely to change depending on the air-fuel ratio of the gas. On the other hand, since the Zr-based composite oxide 12 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 12 alone. From this, it can be seen that the Zr-based composite oxide 12 acts to improve the HC desorption temperature of the zeolite 11.

  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.

  Below, although the Example of the purification catalyst 3 which is embodiment of this invention is described, this invention is not limited to an Example at all.

[Example A]
First, the HC desorption temperature characteristics of the purification catalyst were examined.

Example 1
As the honeycomb-shaped carrier of this example, a cordierite carrier having a diameter of 25.4 mm, a length of 50 mm, a cell wall of 4 mil, and a cell count of 400 cpsi (400 cells per square inch) was used.

Next, a ZrSr composite oxide 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 nitrate of Zr and the nitrate of Sr in ion-exchanged water is such that, in the obtained ZrSr composite oxide, the content of Sr with respect to the total amount of Zr and Sr is 6.25 mol% (ZrO SrO content with respect to the total amount of ₂ and SrO was 5.31% by mass). The ZrSr composite oxide obtained here is hereinafter also referred to as ZrSr composite oxide (Sr / Zr + Sr = 6.25 mol%).

  On the other hand, β-zeolite (manufactured by Zeolis, Keiban ratio 40) was used as the zeolite.

  The obtained ZrSr composite oxide was mixed with β-zeolite, water, and zirconia nitrate as a binder raw material to produce a slurry. This slurry was coated on a honeycomb-shaped carrier, excess slurry was removed by air blowing, dried, and then fired at 500 ° C. for 2 hours. By doing so, a catalyst layer 6 made of a mixture containing β-zeolite and ZrSr composite oxide was formed on the cell wall 5 of the honeycomb-shaped carrier. Here, the mixing ratio of β-zeolite and ZrSr composite oxide was such that the content of ZrSr composite oxide with respect to the total amount of β-zeolite and ZrSr composite oxide was 25% by mass. Further, the supported amount of the catalyst layer 6 per liter of the honeycomb-shaped carrier was set to 160 g / L.

(Example 2)
Example 1 is the same as Example 1 except that a ZrCa composite oxide is used instead of the ZrSr composite oxide.

The ZrCa composite oxide used here was prepared as follows. Specifically, first, a predetermined amount of Zr nitrate and Ca 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 ZrCa composite oxide was prepared. Here, the predetermined amount for dissolving Zr nitrate and Ca nitrate in ion-exchanged water is such that, in the obtained ZrCa composite oxide, the Ca content relative to the total amount of Zr and Ca is 6.5 mol% (ZrO The content of CaO with respect to the total amount of ₂ and CaO was 2.9% by mass).

(Example 3)
Example 1 is the same as Example 1 except that a ZrSr composite oxide in which the ratio of Zr and Sr is changed is used.

In the ZrSr composite oxide used here, the Sr content relative to the total amount of Zr and Sr was 10.88 mol% (the content of SrO relative to the total amount of ZrO ₂ and SrO was 9.31% by mass). Thus, it was prepared in the same manner as in Example 1 except that Zr nitrate and Sr nitrate were dissolved in ion-exchanged water. The ZrSr composite oxide obtained here is hereinafter also referred to as ZrSr composite oxide (Sr / Zr + Sr = 10.88 mol%).

Example 4
It mixed with β-zeolite, water and zirconia nitrate 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 wall 5 of the honeycomb-shaped carrier.

  Next, a ZrSr composite oxide (Sr / Zr + Sr = 10.88 mol%), water and 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 ZrSr composite oxide layer was formed on the zeolite layer.

  As described above, the catalyst layer 6 formed by laminating the ZrSr composite oxide layer on the zeolite layer was formed on the cell wall 5 of the honeycomb-shaped carrier.

  Here, the ratio of the supported amount of β-zeolite and the supported amount of ZrSr composite oxide is such that the content of ZrSr composite oxide with respect to the total amount of β-zeolite and ZrSr composite oxide is 25% by mass. did. Further, the supported amount of the catalyst layer 6 per liter of the honeycomb-shaped carrier was set to 160 g / L.

(Comparative example)
It mixed with β-zeolite, water and zirconia nitrate as a binder raw material to produce a zeolite layer forming slurry. This slurry was coated on a honeycomb-shaped carrier, excess slurry was removed by air blowing, dried, and then fired at 500 ° C. for 2 hours. By doing so, a zeolite layer was formed on the cell wall 5 of the honeycomb-shaped carrier. Here, the supported amount of the zeolite layer per liter of the honeycomb-shaped carrier was set to 160 g / L.

[HC desorption temperature characteristics]
The purification catalysts according to Examples 1 to 4 and the comparative example were evaluated for HC desorption temperature characteristics as follows.

  First, toluene was adsorbed on each of the obtained purification catalysts until saturation. Specifically, a catalytic converter was produced by disposing each purification catalyst in a heat-resistant container. Then, while the temperature of the purification catalyst was heated to 50 ° C., a gas having a toluene concentration of 2000 ppmC was introduced from the upstream side of the catalytic converter until the toluene concentration at the downstream side of the catalytic converter reached 2000 ppmC. By doing so, toluene was adsorbed on the purification catalyst until it was saturated.

  After adsorbing toluene until saturation, a gas not containing toluene was introduced from the upstream side of the catalytic converter. At that time, the temperature of the purification catalyst was raised at a rate of 30 ° C./min, and the toluene concentration (desorbed toluene concentration) of the gas discharged from the downstream side of the catalytic converter was measured.

  5 and 6 are graphs showing the relationship between the temperature of the purification catalyst and the desorbed toluene concentration. The horizontal axis indicates the temperature (° C.) of the purification catalyst, and the vertical axis indicates the desorbed toluene concentration (ppmC). The results of Example 1 are shown in graph 21 of FIG. 5, the results of Example 2 are shown in graph 22 of FIG. 5, the results of Example 3 are shown in graph 24 of FIG. The results are shown in a graph 25 of FIG. 6, and the results of the comparative example are shown in a graph 23 of FIG.

  From FIG. 5, Example 1 and Example 2 in which a catalyst layer containing zeolite and a Zr-based composite oxide was formed were compared with a comparative example in which a catalyst layer not containing a Zr-based composite oxide was formed. It can be seen that the amount of toluene desorbed from the purification catalyst is high at a high temperature, for example, at 300 ° C. or higher. From this, it can be seen that Example 1 and Example 2 have higher toluene desorption temperatures than Comparative Examples.

  Furthermore, from FIG. 6, in Example 4 in which a catalyst layer in which a ZrSr composite oxide layer was laminated on a zeolite layer was formed, a catalyst layer made of a sintered product of a mixture containing zeolite and ZrSr composite oxide was formed. From Example 3, it can be seen that the amount of desorbed toluene at a high temperature is large. This is presumably due to the fact that a large amount of toluene is adsorbed on the zeolite after contacting the ZrSr composite oxide because the ZrSr composite oxide layer is laminated on the zeolite layer.

  In Examples 1 to 4 in which the catalyst layer containing zeolite and the Zr-based composite oxide was formed, the ratio of the amount of toluene desorbed from the purification catalyst at 300 ° C. or higher to the total amount of toluene desorbed from the purification catalyst was 15% or higher. On the other hand, in the comparative example in which the catalyst layer containing no Zr-based composite oxide was formed, the ratio of the amount of toluene desorbed from the purification catalyst at 300 ° C. or higher to the total amount of toluene desorbed from the purification catalyst was 1 As low as 5%. That is, in Examples 1 to 4, the amount of toluene desorbed from the purification catalyst at a high temperature at 300 ° C. or higher is higher than that in the comparative example.

  Further, in Example 4 in which the catalyst layer in which the ZrSr composite oxide layer was laminated on the zeolite layer was formed, the ratio of the amount of toluene desorbed from the purification catalyst at 300 ° C. or higher to the total amount of toluene desorbed from the purification catalyst was 46.1%, very high.

[Example B]
Next, the influence of the composition of the Zr-based composite oxide on the HC desorption amount at 300 ° C. or higher of the purification catalyst was examined.

As the Zr-based composite oxide, a purification catalyst was obtained in the same manner as in Example A, except that a ZrSr composite oxide prepared so that the SrO content with respect to the total amount of ZrO ₂ and SrO had a predetermined ratio was used. Was prepared.

FIG. 7 is a graph showing the influence of the composition of the ZrSr composite oxide on the toluene desorption amount at 300 ° C. or higher. The abscissa indicates the SrO content (% by mass) with respect to the total amount of ZrO ₂ and SrO, and the ordinate indicates the toluene desorption amount at 300 ° C. or higher. The result of the purification catalyst in which a catalyst layer made of a mixture containing zeolite and a ZrSr composite oxide was formed is shown in graph 31, and the result of the purification catalyst in which the catalyst layer in which the ZrSr composite oxide layer was laminated on the zeolite layer was formed. The results are shown in graph 32.

From the graph 31, it can be seen that the higher the SrO content relative to the total amount of ZrO ₂ and SrO, the greater the amount of toluene desorption at 300 ° C. or higher. This is presumably because the higher the SrO content, the higher the basicity of the ZrSr composite oxide, so that toluene is easily changed to carbanion, and toluene is strongly adsorbed by the zeolite.

The higher the SrO content, the more easily the crystal structure of the ZrSr composite oxide is destroyed, and the SrO content relative to the total amount of ZrO ₂ and SrO is about 15% by mass (relative to the total amount of Zr and Sr). Sr content is about 21.0 mol%) is the production limit.

  Further, by comparing the graph 31 and the graph 32, the purification catalyst in which the catalyst layer in which the ZrSr composite oxide layer is laminated on the zeolite layer is formed by mixing the zeolite and the ZrSr composite oxide. It can be seen that the amount of desorbed toluene at a high temperature is larger than that of the purification catalyst in which is formed.

[Example C]
Next, the influence of the content ratio of zeolite and Zr-based composite oxide on the HC adsorption amount of the purification catalyst was examined.

  Example A, except that a ZrSr composite oxide (Sr / Zr + Sr = 6.25 mol%) was used as the Zr-based composite oxide, and the content ratio of zeolite to the ZrSr composite oxide was set to a predetermined ratio. In the same manner as above, a purification catalyst was prepared.

  FIG. 8 is a graph showing the influence of the content ratio of zeolite and ZrSr composite oxide on toluene adsorption. The horizontal axis represents the content (mass%) of the ZrSr composite oxide with respect to the total amount of β-zeolite and ZrSr composite oxide, and the vertical axis represents the toluene adsorption amount. The result of the purification catalyst in which the catalyst layer made of the mixture containing zeolite and the ZrSr composite oxide was formed is shown in graph 41, and the result of the purification catalyst in which the catalyst layer in which the ZrSr composite oxide layer was stacked on the zeolite layer was formed. The results are shown in graph 42.

  From graph 41, it can be seen that the lower the content of ZrSr composite oxide with respect to the total amount of β-zeolite and ZrSr composite oxide, the greater the amount of toluene adsorbed. This is thought to be due to a decrease in the amount of zeolite. This indicates that the content of the ZrSr composite oxide with respect to the total amount of β-zeolite and ZrSr composite oxide is preferably 50% by mass or more.

  Further, by comparing the graph 41 and the graph 42, the purification catalyst in which the catalyst layer in which the ZrSr composite oxide layer is stacked on the zeolite layer is formed is a catalyst layer made of a mixture containing zeolite and ZrSr composite oxide. It can be seen that the amount of toluene adsorbed is larger than that of the purification catalyst on which is formed.

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. It is the graph which showed the relationship between the temperature of a purification catalyst, and desorption toluene concentration. It is the graph which showed the relationship between the temperature of a purification catalyst, and desorption toluene concentration. It is the graph which showed the influence of the composition of ZrSr complex oxide with respect to toluene desorption amount at 300 degreeC or more. It is the graph which showed the influence of the content ratio of a zeolite and ZrSr complex oxide with respect to toluene adsorption amount.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Catalytic converter 2 Heat-resistant container 3 Purification catalyst 5 Cell wall 6 Catalyst layer 11 Zeolite 12 Zr system complex oxide 13 Catalyst noble metal

Claims (5)

An exhaust gas purifying catalyst disposed in an exhaust passage of an engine,
A honeycomb carrier, and a catalyst layer formed on the cell wall surface of the honeycomb carrier,
The exhaust gas purification catalyst, wherein the catalyst layer contains zeolite and a Zr-based composite oxide containing zirconium (Zr) and an alkaline earth metal.   The exhaust gas purifying catalyst according to claim 1, wherein the catalyst layer is made of a mixture containing the zeolite and the Zr-based composite oxide.   The catalyst layer comprises a zeolite layer containing the zeolite formed on the surface of the cell wall and an oxide layer containing the Zr-based composite oxide formed on the surface of the zeolite layer. The exhaust gas purifying catalyst according to 1. 4. The content of the alkaline earth metal oxide with respect to the total amount of ZrO ₂ and the alkaline earth metal oxide in the Zr-based composite oxide is 1% by mass or more. The exhaust gas purifying catalyst according to claim 1.   The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein a content ratio of the Zr-based composite oxide with respect to a total amount of the zeolite and the Zr-based composite oxide is 1 to 50% by mass. .

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