JP5540521B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

Description

  The present invention relates to an exhaust gas purification catalyst suitable for purifying exhaust gas from an internal combustion engine and a method for producing the same.

In recent years, in order to remove harmful substances such as hydrocarbon compounds (HC), carbon monoxide (CO), and nitrogen oxides (NO _x ) contained in exhaust gas discharged from an internal combustion engine, alumina (Al ₂ Exhaust gas purification catalysts in which noble metal particles such as platinum (Pt) are supported on a metal oxide carrier such as O ₃ ) are widely used. In conventional exhaust gas purification catalysts, a large amount of noble metal particles is used in order to improve the durability of the noble metal particles against ambient fluctuations. However, using a large amount of noble metal particles is not desirable from the viewpoint of protecting earth resources.

  Against this background, recently, transition metals or transition metal compounds such as cerium (Ce) and manganese (Mn), which function as OSC (Oxygen Storage Component) materials, are disposed in the vicinity of noble metal particles by an impregnation method. An attempt has been made to improve the durability of the noble metal particles by suppressing changes in the atmosphere around the noble metal particles with a transition metal or a transition metal compound (see Patent Document 1).

Japanese Patent Laid-Open No. 2005-000830

  Due to the increasing demand for fuel-efficient vehicles due to the problem of depletion of petroleum resources and global warming, the ratio of air to gasoline is higher than that of conventional engines, and the engine is equipped with an engine that burns lean fuel. The development of combustion vehicles is drawing attention. In such a lean-burn vehicle, it is necessary to purify NOx produced by burning fuel in an atmosphere in which oxygen exceeds the stoichiometric air-fuel ratio. NOx purification action becomes insufficient due to the influence of oxygen present in the catalyst. Such an inadequate purification effect of NOx is not solved by the above prior art in which the durability of the noble metal particles is improved by arranging the OSC in the vicinity of the noble metal particles by the impregnation method. There is a need for a catalyst that can purify NOx in a thinner region, that is, a lean region.

  However, there are many catalysts that purify NOx in the lean region so far, but their lifetime is not sufficient, and the performance has deteriorated with the use of the exhaust gas purifying catalyst for a long period or at a high temperature.

In order to solve the above problems, an exhaust gas purification catalyst according to the present invention comprises a honeycomb carrier, a noble metal, a first compound supporting the noble metal, and alumina containing the noble metal and the first compound. And a catalyst layer formed on the honeycomb carrier, wherein the first compound contacts the noble metal and suppresses movement of the noble metal. The second compound suppresses aggregation of the first compound due to contact between the first compounds, and the first compound contains at least one element selected from alkali metals and alkaline earth metals. And an alkali metal contained in the first compound in the range of 0.05 to 0.3 [mol / L] in terms of molar amount per liter of honeycomb carrier capacity when an exhaust gas purification catalyst is formed on the honeycomb carrier. A At least one element of the potash earth metal is an element selected from K, Na, Cs, Mg, Ca and Ba, and the first compound is mixed with a complex oxide containing Zr and Ce in an alkaline earth. The gist is to contain Ba as a similar element.

  According to the exhaust gas purifying catalyst of the present invention, the NOx absorbent is present as a complex oxide in the first compound supporting a noble metal in the catalyst, thereby making the distance between the active point of the catalyst and the NOx absorbent closer. It is possible to maintain the above, and thereby the effect of containing the promoter component can be further exerted, so that the durability and activity of the noble metal particles in the catalyst for NOx purification in the lean region can be improved.

1 is a schematic diagram of an exhaust gas purification catalyst according to an embodiment of the present invention.

  Hereinafter, embodiments of the exhaust gas purification catalyst of the present invention will be described in more detail with reference to the drawings.

  FIG. 1 is a schematic view of an embodiment of the exhaust gas purification catalyst of the present invention. An exhaust gas purification catalyst 1 shown in FIG. 1 includes a noble metal 2, a first compound 3 supporting the noble metal 2, and a second compound made of an oxide containing the noble metal 2 and the first compound 3. 4 is provided.

  And this 1st compound 3 consists of a compound compound of the compound 3a which is one of the structural elements of the 1st compound 3, and the at least 1 sort (s) of element 3b chosen from this alkali metal and alkaline-earth metal. . In addition, the case where the compound 3a is a composite compound is also included.

  In the exhaust gas catalyst of the present embodiment, the first compound 3 plays a role as a chemical anchor material of the noble metal 2 by supporting the noble metal 2, and contacts the noble metal 2 to move the noble metal. Is chemically suppressed. In addition, the second compound 4 includes the noble metal 2 and the first compound 3, thereby suppressing aggregation of the first compound due to contact between the first compounds and the noble metal 2 being the first compound 2. Even when the compound is removed from the top, the second compound 4 covering the first compound 3 serves as a wall, and the movement of the noble metal can be physically suppressed.

  The inventors conducted an endurance test to investigate the cause of the exhaust gas purification performance of the conventional NOx purification catalyst being lowered due to the use of the catalyst at a high temperature or for a long period of time. As a result, one of the reasons why the durability of the conventional NOx purification catalyst is not sufficient is that the distance between the NOx absorbent and the noble metal contained in the purification catalyst is such that noble metals or NOx absorbents agglomerate and move during durability. It turns out that it is possible to get far away. From this knowledge, it is important to prevent such agglomeration and migration, and that the active site is in close contact with a sufficient amount of NOx absorbent even after the endurance test, in order to prevent a decrease in the catalyst performance of the NOx purification catalyst. Something has been found by further research by the inventors.

Based on the above knowledge, as a point to improve the performance of a catalyst that absorbs NOx in a lean atmosphere (lean NOx trap; LNT catalyst)
(1) The required amount of NOx absorber is present,
(2) The presence of an NOx absorbent in the vicinity of the active point (particularly in the vicinity of the catalyst noble metal)
(3) Sufficient pore volume to ensure contact between the active site and NOx absorbent and the exhaust gas,
(4) The active site can be maintained with fine particles (there is no deterioration due to durability and a sufficient active surface area can be secured).

  And the exhaust gas purification catalyst of the present invention based on this point can maintain fine particles by adapting a specific catalyst structure in the LNT catalyst, and the pores necessary for contact with gas Capacity can be secured. In addition, NOx can be absorbed in the lean region of the exhaust gas atmosphere, and NOx absorbed and released in the rich region can be continued by placing a certain amount of NOx absorbent in the catalyst. Become.

  More specifically, the prevention of aggregation and movement of the noble metal is effectively achieved by supporting the noble metal with the first compound and enclosing the noble metal and the first compound with the second compound. It has been found by the inventors' research that this can be done.

In the present invention, the NOx absorbent is present as a composite oxide in the first compound supporting the noble metal in the nanocatalyst structure. As a result, the distance between the active point (the noble metal having catalytic activity) and the NOx absorbent can be kept close. Due to the close distance between the active point and the NOx absorbent, the exhaust gas purification catalyst of the present invention converts nitrogen oxides contained in the exhaust gas from the internal combustion engine into NO ₂ on the active point in a lean atmosphere. Oxidized, and this oxidized NO ₂ is combined with an alkali metal or alkaline earth metal contained in the first compound and having the function of a NOx absorbent, for example, by Ba as a compound called BaNO ₃ , Absorbed in the first compound. On the other hand, under a rich atmosphere, NO ₂ that was absorbed by the alkali metal or alkaline earth metal in the first compound is released, and on the first compound in the vicinity of the alkali metal or alkaline earth metal. It is reduced and purified by the noble metal supported. The fact that an alkali metal or alkaline earth metal having such an action of the NOx absorbent is contained in the first compound means that the NOx absorbent is impregnated in a catalyst layer of a honeycomb carrier, for example, Since the NOx absorbent can be disposed in the vicinity of the noble metal, it is possible to more effectively prevent a decrease in the catalyst performance after durability. In addition, it is advantageous for suppressing deterioration of the NOx absorbent. When an alkali metal or alkaline earth metal is impregnated in, for example, a catalyst layer of a honeycomb carrier, the alkali metal or alkaline earth metal is aggregated and deteriorated with durability. On the other hand, in this invention, since an alkali metal or alkaline-earth metal is contained in the 1st compound, it does not aggregate after durability. Accordingly, it is possible to keep a close distance from the noble metal.

  The alkali metal or alkaline earth metal is the first compound in an amount in the range of 0.05 to 0.3 [mol / L] in terms of molar amount per liter of honeycomb carrier capacity when the exhaust gas purification catalyst is formed on the honeycomb carrier. Included in. These NOx absorbents are effective for absorption, release and purification of N0x by being present even in a small amount, but it is preferable to contain a certain amount or more for improving the purification ability of the catalyst. Therefore, it is assumed that 0.05 [mol / L] or more is included in the molar amount per liter of the honeycomb carrier when the exhaust gas purification catalyst is formed on the honeycomb carrier, thereby maximizing these functions. Is possible. When the amount is less than 0.05 [mol / L], a sufficient amount of NOx cannot be absorbed in a lean atmosphere, and the catalyst performance decreases. On the other hand, when it is contained in a large amount so as to exceed 0.3 [mol / L], the amount of catalyst when the exhaust gas purification catalyst of the present invention is applied to the pores penetrating the honeycomb carrier is increased, and the catalyst layer Thickening may lead to pore clogging, and the contact between the active site and the exhaust gas is lowered, so the performance is lowered.

  The exhaust gas purification catalyst 1 according to the embodiment of the present invention has the above-described catalyst structure, so that the first compound 3 acts as an anchor material for chemical bonding and contacts the noble metal 2 to form the noble metal 2. The movement of the first compound 3 is chemically suppressed, the second compound 4 physically suppresses the movement of the noble metal 2 particles, and the aggregation of the first compound 3 accompanying the contact between the first compounds 3 is suppressed. The first compound 3 is prevented from moving beyond the section separated by the second compound 4 and coming into contact with each other to aggregate. From these facts, the exhaust gas purification catalyst having the structure of the catalyst 1 shown in FIG. 1 can prevent a decrease in catalytic activity due to agglomeration of two noble metal particles without increasing the manufacturing cost and environmental load. The activity improving effect of the noble metal 2 particles by the first compound 3 can be maintained. Therefore, an exhaust gas purification catalyst having high heat resistance and excellent durability over a long period of time can be obtained.

  The NOx absorbent contained in the first compound 3 is preferably an element selected from K, Na, Cs, Mg, Ca, and Ba among alkali metals and alkaline earth metals. By selecting these elements, NOx absorption under a lean atmosphere and NOx desorption / purification under a rich atmosphere can be more reliably advanced.

The first compound 3 is more preferably a composite oxide containing Ba as an alkaline earth metal. This is because Ba is a particularly effective element among NOx absorbents. The presence of Ba in the first compound 3 supporting the noble metal makes it possible to bring the active site and the NOx absorbent material into close contact with each other, that is, to reduce the distance between the active site and the NOx absorbent material as much as possible. Is possible. By closely contacting, the NOx species oxidized to NO ₂ are efficiently absorbed by the absorbent in a lean atmosphere. As a result, the lean NOx trap catalyst cycle of NOx absorption, release, and purification proceeds efficiently, improving the purification performance of the catalyst. Further, by combining Ba in the first compound (anchor material), it is also possible to prevent the movement and aggregation degradation of Ba accompanying durability. As a result, the functional degradation of the NOx absorbent is reduced and the purification performance of the catalyst is improved.

The first compounds 3, the composite oxide containing Zr and Ce, and a this is intended to include Ba as the alkaline earth element, and more preferable. In the first compound 3, it is effective to complex Ce and Zr together with Ba, and the above-described migration / aggregation deterioration of the NOx absorbent is suppressed by being a complex oxide containing Ce and Zr together with Ba. It becomes possible. Further, the presence of Ce in the first compound 3 is preferable because the movement of the noble metal can be suppressed by electronic interaction with the noble metal. However, when only Ce in addition to Ba is present in the first compound, the aggregation of CeO ₂ proceeds, and as a result, the aggregation of the noble metal supported on the first compound 3 proceeds, and with durability. Performance may be reduced. Therefore, it is effective to suppress the aggregation of CeO ₂ by further complexing Zr, and as a result, the aggregation of noble metal can be suppressed.

The exhaust gas purification catalyst preferably has a pore volume of 0.4 [cc / g] or more. By providing such a pore volume, it is possible to maintain the contact between the active site (noble metal) and gas for obtaining sufficient catalyst performance. This pore volume can be measured by N ₂ adsorption analysis of the exhaust gas catalyst powder.

The noble metal 2 preferably contains at least one of Pt and Rh. The exhaust gas purifying catalyst of the present invention is particularly effective when the noble metal 2 contains at least one of Pt and Rh. In other words, when NOx is absorbed, the reaction of oxidizing NOx species to NO ₂ on Pt and / or Rh is rate-limiting, so it is effective to improve NOx purification performance by placing a large amount of NOx absorbent near Pt and / or Rh. It is. In order to dispose a large amount of NOx absorbent in the vicinity of Pt and / or Rh, a catalyst having a structure according to the present invention is particularly preferable, and therefore effective for improving NOx purification performance.

  Next, a method for manufacturing the exhaust gas purification catalyst according to the embodiment of the present invention described so far will be described. In the manufacturing method of one embodiment of the present invention, a first compound containing an alkali metal or an alkaline earth metal, a noble metal, and a second compound are prepared in advance. Next, a noble metal is impregnated and supported on the first compound containing the alkali metal or alkaline earth metal by an impregnation method. Next, the first compound carrying the noble metal is pulverized and refined by an impregnation method. Next, the first compound carrying the noble metal pulverized in this way is encapsulated with the second compound. In the manufacturing method of the present embodiment including such steps, a first compound including a NOx absorbent in a composite form is prepared in advance as the first compound, and the first compound including such a NOx absorbent is prepared. By using the compound as a raw material, it becomes possible to easily produce an exhaust gas purification catalyst having a structure in which the active site and the NOx absorbent are in close contact according to the present invention. By using the impregnation method, the noble metal can be easily supported on the first compound. By pulverizing the first compound carrying the noble metal particles, the amount of the noble metal in the compartment (unit) when the noble metal-carrying first compound is encapsulated by the second compound in a later step is set within a predetermined range. It becomes possible to adjust. By forming the second compound so as to cover the pulverized noble metal-carrying first compound, the first compound carrying the noble metal particle is encapsulated with the second compound, and the inside of the compartment separated by the second compound To be included. The conditions for performing these steps can be set to appropriate conditions. Moreover, about processes other than these processes, the exhaust-gas purification catalyst of this invention can be manufactured according to a well-known manufacturing process and manufacturing conditions.

The exhaust gas purification catalyst powder thus obtained is made into a slurry, and this slurry is applied to the inner wall surface of the catalyst honeycomb carrier, which is a refractory inorganic carrier, and is actually used in the form of a catalyst coat layer. Provided . Catalytic coat layer may include a plurality of kinds of exhaust gas purifying catalyst according to the present invention. The catalyst coat layer is not limited to a single layer, and may be composed of a plurality of layers having different catalyst component compositions. Further, an underlayer containing no catalyst exists under the catalyst coat layer. It may be. When it consists of multiple layers, it is necessary to provide at least one layer containing the catalyst according to the present invention. When the layer containing the catalyst according to the present invention is present in a plurality of layers, the metal species of the NOx absorbent in each layer may be the same or different.

  Hereinafter, the present invention will be specifically described based on examples.

Exhaust gas purification catalysts of Examples 1 to 8 and Comparative Examples 1 to 8 shown in Table 1 were prepared, and each was applied to the inner wall surface of the honeycomb carrier.

  The catalyst layer was a two-layer coat, the layer close to the space of the through-hole formed in the honeycomb carrier was the surface layer, and the side close to the base of the through-hole was the inner layer. Each Example and Comparative Example shown in Table 1 were produced as follows.

Example 1
Example 1 is an example in which the NOx absorbent contained in the first compound is Ba.

  A Ce-Zr-Ba composite oxide powder was impregnated with a predetermined amount of Pt so that the Pt support concentration was 0.5 wt%. The obtained powder was dried, calcined and pulverized in an air stream at 400 ° C. for 1 hour.

  Meanwhile, a slurry is prepared by mixing boehmite, nitric acid and pure water as the second compound raw material, and this slurry is mixed with the pulverized Pt-supported Ce-Zr-Ba composite oxide powder and stirred. Thus, a catalyst raw material slurry was obtained.

  The catalyst raw material slurry was dried and then calcined at 550 ° C. for 3 hours under an air stream to obtain Pt powder for the inner layer.

Separately, after impregnating and supporting a predetermined amount of Pt in the Ce-Zr-Ba composite oxide powder so that the Pt supporting concentration is 1 wt%, the same procedure as the above-described process for producing the Pt powder for the inner layer is performed. Pt powder for the surface layer was obtained. Also, as another surface layer catalyst powder, after impregnating and supporting La-containing ZrO ₂ powder so that the Rh loading concentration is 0.7 wt%, the same procedure as in the above-described process for producing the inner layer Pt powder is used. Rh powder was obtained.

  180 g of the above Pt powder for the inner layer, 25 g of boehmite, 20 g of 10% nitric acid, and 280 g of ion-exchanged water were put into a magnetic pot, and shaken and pulverized with alumina balls to obtain a catalyst slurry for the inner layer.

  On the other hand, 180 g of Pt powder for the surface layer, 70 g of Rh powder for the surface layer, 35 g of boehmite, 30 g of 10% nitric acid, and 400 g of ion-exchanged water were put in a magnetic pot and shaken and ground with alumina balls to obtain a catalyst slurry for the surface layer. .

  The above inner layer catalyst slurry is put into a ceramic honeycomb support (400 cells / 6 mil, 0.12 L), excess slurry is removed by air flow, dried at 120 ° C., and fired at 400 ° C. under air flow. As a result, an inner catalyst layer was formed. Next, the above-mentioned catalyst slurry for the surface layer is put into a honeycomb carrier on which an inner catalyst layer is formed, excess slurry is removed with an air stream, dried at 120 ° C., and fired at 400 ° C. under an air stream, A surface catalyst layer was formed on the inner catalyst layer.

(Example 2)
Example 2 is an example in which the NOx absorbent contained in the first compound is Na.

  A Ce-Zr-Na composite oxide powder was impregnated with a predetermined amount of Pt so that the Pt loading concentration was 0.5 wt%. Thereafter, Pt powder for the inner layer was obtained by the same procedure as described in Example 1.

  On the other hand, after impregnating and supporting a predetermined amount of Pt in the Ce-Zr-Na composite oxide powder so that the Pt supporting concentration becomes 1 wt%, Pt for the surface layer is obtained in the same procedure as the manufacturing process of the Pt powder for the inner layer. A powder was obtained. In addition, Rh powder for the surface layer was obtained as another surface layer catalyst powder in the same procedure as in the production process of Example 1.

  Thereafter, the catalyst was prepared by the same procedure as in Example 1.

(Example 3)
Example 3 is an example in which the NOx absorbent contained in the first compound is Cs.

  A Ce-Zr-Cs composite oxide powder was impregnated with a predetermined amount of Pt so that the Pt support concentration was 0.5 wt%. Thereafter, Pt powder for the inner layer was obtained by the same procedure as described in Example 1.

  On the other hand, after impregnating and supporting a predetermined amount of Pt in the Ce-Zr-Cs composite oxide powder so that the Pt supporting concentration is 1 wt%, Pt for the surface layer is processed in the same procedure as the manufacturing process of Pt powder for the inner layer. A powder was obtained. In addition, Rh powder for the surface layer was obtained as another surface layer catalyst powder in the same procedure as in the production process of Example 1.

  Thereafter, the catalyst was prepared by the same procedure as in Example 1.

Example 4
Example 4 is an example in which the NOx absorbent contained in the first compound is Mg.

  A Ce-Zr-Mg composite oxide powder was impregnated with a predetermined amount of Pt so that the Pt support concentration was 0.5 wt%. Thereafter, Pt powder for the inner layer was obtained in the same procedure as described in Example 1.

  On the other hand, after impregnating and supporting a predetermined amount of Pt in the Ce-Zr-Mg composite oxide powder so that the Pt supporting concentration is 1 wt%, Pt for the surface layer is processed in the same procedure as the manufacturing process of the Pt powder for the inner layer. A powder was obtained. In addition, Rh powder for the surface layer was obtained as another surface layer catalyst powder in the same procedure as in the production process of Example 1.

  Thereafter, the catalyst was prepared by the same procedure as in Example 1.

(Example 5)
Example 5 is an example in which the NOx absorbent contained in the first compound is Ca.

  A Ce-Zr-Ca composite oxide powder was impregnated with a predetermined amount of Pt so that the Pt support concentration was 0.5 wt%. Thereafter, Pt powder for the inner layer was obtained in the same procedure as described in Example 1.

  On the other hand, after impregnating and supporting a predetermined amount of Pt in the Ce-Zr-Ca composite oxide powder so that the Pt supporting concentration is 1 wt%, Pt for the surface layer is processed in the same procedure as the manufacturing process of the Pt powder for the inner layer. A powder was obtained. In addition, Rh powder for the surface layer was obtained as another surface layer catalyst powder in the same procedure as in the production process of Example 1.

  Thereafter, the catalyst was prepared by the same procedure as in Example 1.

(Example 6)
In Example 6, the NOx absorbent contained in the first compound is Ba, as in Example 1, but both the Pt-supported first compound and the Rh-supported first compound contained in the surface layer contain this Ba. It is an example that contains.

  A predetermined amount of Pt was impregnated in the Zr—Ce—Ba composite oxide powder so that the Pt support concentration was 0.6 wt%. Thereafter, the catalyst was prepared in the same procedure as described in Example 1 to obtain Pt powder for the inner layer.

  On the other hand, after a predetermined amount of Pt was impregnated and supported with Zr-Ce-Ba composite oxide powder so that the Pt supporting concentration was 1.2 wt%, the same procedure as the manufacturing process of the Pt powder for the inner layer of Example 1 was performed. Pt powder for the surface layer was obtained.

  Separately from this, after carrying a predetermined amount of Pt impregnated with Zr-Ce-Ba composite oxide powder so that the Rh loading concentration is 0.7 wt%, the same process as the production process of Pt powder for the inner layer of Example 1 Rh powder for the surface layer was obtained by the procedure.

  150 g of Pt powder for the inner layer, 20 g of boehmite, 17 g of 10% nitric acid, and 240 g of ion-exchanged water were put into a magnetic pot, and shaken and pulverized with alumina balls to obtain an inner layer catalyst slurry.

  Meanwhile, 150 g of Pt powder for the surface layer, 70 g of Rh powder for the surface layer, 30 g of boehmite, 27 g of 10% nitric acid, and 360 g of ion-exchanged water were put into a magnetic pot and shaken and ground with alumina balls to obtain a catalyst slurry for the surface layer. .

  The above inner layer catalyst slurry is put into a ceramic honeycomb support (400 cells / 6 mil, 0.12 L), excess slurry is removed by air flow, dried at 120 ° C., and fired at 400 ° C. under air flow. As a result, an inner catalyst layer was formed. Next, the surface layer catalyst slurry is put into a honeycomb carrier on which the inner catalyst layer is formed, excess slurry is removed by air flow, dried at 120 ° C., and fired at 400 ° C. under air flow. The surface catalyst layer was formed on the inner catalyst layer.

(Example 7)
Example 7 is an example in which the NOx absorbent contained in the first compound is Ba as in Example 1, but the Ba content per liter of the honeycomb carrier is reduced as compared with Example 1. .

  A predetermined amount of Pt was impregnated in a Zr—Ce—Ba composite oxide powder (the amount of Ba in the powder was half of the powder used in Example 1) so that the Pt support concentration was 0.5 wt%. Thereafter, the catalyst was prepared by the same procedure as described in Example 1 to obtain Pt powder for the inner layer.

  On the other hand, after impregnating and supporting a predetermined amount of Pt on Ce-Zr-Ba composite oxide powder (the amount of Ba in the powder is half of the powder used in Example 1) so that the Pt supporting concentration is 1 wt%, Pt powder for the surface layer was obtained in the same procedure as the production process of Pt powder for the inner layer of Example 1. In addition, Rh powder for the surface layer was obtained as another surface layer catalyst powder in the same procedure as in the production process of Example 1.

  Thereafter, the catalyst was prepared by the same procedure as in Example 1.

(Example 8)
Example 8 is an example in which the NOx absorbent contained in the first compound is Ba as in Example 1, but the Ba content per liter of honeycomb carrier is increased more than in Example 1. .

  A predetermined amount of Pt was impregnated in a Zr—Ce—Ba composite oxide powder (the amount of Ba in the powder was twice that of the powder used in Example 1) so that the Pt support concentration was 0.5 wt%. Thereafter, the catalyst was prepared by the same procedure as described in Example 1 to obtain Pt powder for the inner layer.

  On the other hand, after impregnating and supporting a predetermined amount of Pt on Ce-Zr-Ba composite oxide powder (the amount of Ba in the powder is twice that of the powder used in Example 1) so that the concentration of Pt supported is 1 wt%. Then, Pt powder for the surface layer was obtained in the same procedure as in the production process of Pt powder for the inner layer in Example 1. In addition, Rh powder for the surface layer was obtained as another surface layer catalyst powder in the same procedure as in the production process of Example 1.

  Thereafter, the catalyst was prepared by the same procedure as in Example 1.

(Comparative Example 1)
Comparative Example 1 is an example in which the first compound supporting a noble metal does not contain an alkali metal or an alkaline earth metal as a NOx absorbent.

A predetermined amount of Pt was impregnated into the Zr-containing CeO ₂ powder so that the Pt loading concentration was 0.5 wt%. Thereafter, Pt powder for the inner layer was obtained by the same procedure as described in Example 1.

On the other hand, after impregnating and supporting a predetermined amount of Pt in a Zr-containing CeO ₂ powder so that the Pt supporting concentration was 1 wt%, a Pt powder for the surface layer was obtained in the same procedure as the manufacturing process of the Pt powder for the inner layer . In addition, Rh powder for the surface layer was obtained as another surface layer catalyst powder in the same procedure as in the production process of Example 1.

  Thereafter, the catalyst was prepared by the same procedure as in Example 1.

(Comparative Example 2)
Comparative Example 2 is an example in which Ba as an NOx absorbent was not impregnated in the first compound as in Example 1, but was impregnated in the catalyst layer formed on the honeycomb carrier.

  The catalyst was prepared according to the manufacturing process described in Comparative Example 1 above. Thereafter, the catalyst on which the surface and inner catalyst layers were formed was impregnated with Ba acetate as Ba to a concentration of 0.13 mol / L, and calcined at 400 ° C. in an air stream.

(Comparative Example 3)
Comparative Example 3 is an example in which Na as a NOx absorbent is impregnated in a catalyst layer formed on a honeycomb carrier.

  The catalyst was prepared according to the manufacturing process described in Comparative Example 1 above. Thereafter, the catalyst on which the surface and inner catalyst layers were formed was impregnated with Na acetate as Na to a concentration of 0.13 mol / L, and calcined at 400 ° C. in an air stream.

(Comparative Example 4)
Comparative Example 4 is an example in which Cs as a NOx absorbent is impregnated in a catalyst layer formed on a honeycomb carrier.

  The catalyst was prepared according to the manufacturing process described in Comparative Example 1 above. Thereafter, the catalyst on which the surface and inner catalyst layers were formed was impregnated with Cs acetate to give 0.13 mol / L as Cs, and calcined at 400 ° C. in an air stream.

(Comparative Example 5)
Comparative Example 5 is an example in which Mg as a NOx absorbent is impregnated in a catalyst layer formed on a honeycomb carrier.

  The catalyst was prepared according to the manufacturing process described in Comparative Example 1 above. Thereafter, the catalyst on which the catalyst layer of the surface layer and the inner layer was formed was impregnated with Mg acetate as 0.13 mol / L as Mg, and calcined at 400 ° C. in an air stream.

(Comparative Example 6)
Comparative Example 6 is an example in which Ca as a NOx absorbent is impregnated in a catalyst layer formed on a honeycomb carrier.

  The catalyst was prepared according to the manufacturing process described in Comparative Example 1 above. Thereafter, the catalyst on which the surface and inner catalyst layers were formed was impregnated with Ca acetate as Ca to a concentration of 0.13 mol / L, and calcined at 400 ° C. in an air stream.

(Comparative Example 7)
Comparative Example 7 is an example in which Ba as a NOx absorbent is impregnated in a catalyst layer formed on a honeycomb carrier. The amount of Ba in the catalyst was reduced as compared with Comparative Example 2 .

  The catalyst was prepared according to the manufacturing process described in Comparative Example 1 above. Thereafter, the catalyst on which the catalyst layer of the surface layer and the inner layer was formed was impregnated with Ba acetate as Ba at 0.06 mol / L and calcined at 400 ° C. in an air stream.

(Comparative Example 8)
Comparative Example 8 is an example in which Ba as a NOx absorbent is impregnated in a catalyst layer formed on a honeycomb carrier. The amount of Ba in the catalyst was increased as compared with Comparative Example 2 .

  The catalyst was prepared according to the manufacturing process described in Comparative Example 1 above. Thereafter, the catalyst on which the surface and inner catalyst layers were formed was impregnated with Ba acetate as Ba at 0.26 mol / L and calcined at 400 ° C. in an air stream.

[Evaluation]
The honeycomb carrier on which the exhaust gas purifying catalysts of Examples 1 to 8 and Comparative Examples 1 to 8 described above are formed is arranged in the exhaust system of a V-type 6-cylinder 3.5L engine, and the catalyst inlet temperature is adjusted to 800 ° C. The endurance treatment was performed for 100 hours in the exhaust gas atmosphere. Unleaded gasoline was used as the fuel.

After this endurance test, the gas type and concentration were adjusted so that the gas conditions shown in Table 2 were obtained using a laboratory evaluation apparatus, and the NOx purification rate was evaluated by switching to a lean atmosphere (30 seconds) and a rich atmosphere (2 seconds). The evaluation temperature (catalyst inlet temperature) at this time was 300 ° C.

  The NOx purification rate (ηNOx) was calculated using the following formula.

ηNOx (%) = {inlet NOx amount (rich + lean)-outlet NOx amount (rich + lean)} / inlet NOx amount (rich + lean) x 100
Further, the pore volume of the catalyst was examined. The pore volume was measured by scraping 0.07 g of the catalyst powder from the catalyst carrier after 100 hours of durability treatment at an inlet temperature of 800 ° C., and measuring the pore volume in the catalyst powder by the N2 adsorption method. Is. Among the measurement results, the pore capacities obtained in the region where the pore diameter was 100 nm or less were summed to obtain the pore capacities of the respective catalysts.

  Table 1 shows the pore volume and NOx purification rate of each catalyst. As is clear from Table 1, Examples 1 to 8 containing an alkali metal or an alkaline earth metal in the first compound have a markedly improved NOx purification rate compared to Comparative Example 1 not containing a NOx absorbent. And showed excellent performance as an LNT catalyst. Moreover, these Examples 1-8 showed the NOx purification rate improved compared with the comparative examples 3-8 which impregnated the NOx absorber in the catalyst layer, and showed the outstanding performance as a LNT catalyst. . Furthermore, from the results of Example 1 and Examples 7 and 8, the NOx absorbent is included in the range of 0.05 to 0.3 [mol / L], so that it has particularly excellent NOx catalyst purification performance. Recognize.

  As mentioned above, although the embodiment to which the invention made by the present inventors was applied has been described, the present invention is not limited by the description and the drawings that form part of the disclosure of the present invention according to this embodiment. That is, it should be added that other embodiments, examples, operation techniques, and the like made by those skilled in the art based on the above embodiments are all included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Exhaust gas purification catalyst 2 Noble metal 3 1st compound 3b At least 1 element chosen from alkali metal and alkaline-earth metal 4 2nd compound

Claims (4)

A honeycomb carrier;
A catalyst powder having a noble metal, a first compound supporting the noble metal, and a second compound made of alumina enclosing the noble metal and the first compound, and formed on the honeycomb carrier; A catalyst layer,
With
The first compound is in contact with the noble metal to suppress the movement of the noble metal, the second compound is to suppress the aggregation of the first compound accompanying the contact between the first compounds,
The first compound contains at least one element selected from alkali metals and alkaline earth metals in a molar amount per liter of honeycomb carrier capacity when an exhaust gas purification catalyst is formed on the honeycomb carrier. In the range of ~ 0.3 [mol / L],
At least one element of alkali metal and alkaline earth metal contained in the first compound is an element selected from K, Na, Cs, Mg, Ca and Ba,
The exhaust gas purification catalyst, wherein the first compound contains Ba as an alkaline earth element in a complex oxide containing Zr and Ce. The exhaust gas purification catalyst according to claim 1, wherein the catalyst powder has a pore volume of 0.4 [cc / g] or more.   The exhaust gas purifying catalyst according to claim 1, wherein the noble metal includes at least one of Pt and Rh. A method for producing the exhaust gas purification catalyst according to any one of claims 1 to 3 ,
Impregnating and supporting a noble metal on a first compound containing an alkali metal or an alkaline earth metal;
Crush and refine the first compound carrying the noble metal,
A method for producing an exhaust gas purifying catalyst, comprising a step of encapsulating a pulverized first compound carrying a noble metal with a second compound.

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