JP2012217950A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove NOx efficiently in such a transition period that a catalyst atmosphere transitions from an air-fuel ratio lean state to an air-fuel ratio rich state even in a state where a catalyst for cleaning exhaust gas has a low temperature and is packed in an air-fuel ratio lean atmosphere.SOLUTION: The catalyst for cleaning exhaust gas includes a Rh catalyst layer 3 arranged on a base material 1 and a non-Rh catalyst layer 2 arranged under the Rh catalyst layer 3. The Rh catalyst layer 3 is obtained by layering a plurality of layers 3a-3c that contain the Rh catalysts in which each kind of Rh catalysts is different from each other. An upper layer 3a has the NOx removal rate higher than those of lower layers 3b, 3c when the air-fuel ratio of exhaust gas is changed from the lean state to the rich state at the temperature lower than 290°C.

Description

  The present invention relates to an exhaust gas purification catalyst.

  In automobiles, the exhaust gas is an environmental load, so an improvement in the exhaust gas purification rate of the catalyst is required, and a reduction in fuel consumption is also required from the viewpoint of energy saving. In order to reduce fuel consumption, the combustion efficiency of the engine has been improved, and a hybrid using an engine and a motor as drive sources has also been performed.

  As for hybrid vehicles, there are known a series method in which an engine is exclusively used as a generator and the electric power is used for driving a vehicle by a motor, a parallel method in which driving by a motor and driving by an engine are combined. In any case, it is a feature of the engine in a hybrid vehicle that start and stop are frequently repeated. When the engine is stopped, only air passes through one of the cylinders and flows into the exhaust system after the fuel injection is stopped until the engine stops. The catalyst is exposed. As a result, the catalyst is enveloped in an air-fuel ratio lean atmosphere, and the next engine start is reached at a low temperature. Therefore, NOx (nitrogen oxide) purification by the catalyst is difficult to proceed, and there is a problem that NOx discharged into the atmosphere at the time of engine restart increases.

  The same problem is observed in automobiles that use only the engine as the drive source. Further, when fuel cut is performed during deceleration operation, only air is sent to the engine exhaust system during fuel cut. The purifying catalyst increases in the amount of accumulated oxygen and enters a low temperature state. Further, even in the case of starting the engine with an air-fuel ratio lean for low fuel consumption (lean start), the exhaust gas purifying catalyst is in a state of being wrapped in a low temperature and lean atmosphere, and thus has the same problem.

  On the other hand, Patent Document 1 describes that an exhaust gas purifying catalyst in which Pd is supported on a composite oxide of Zr and Pr is used to improve the exhaust gas purifying performance at the lean start of the engine. Has been.

  By the way, it is known that a catalytic metal effective for purifying NOx is Rh. Therefore, in the three-way catalyst, a catalyst layer containing Pt and / or Pd is provided on a base material (catalyst support), and an Rh catalyst layer containing Rh is provided on the catalyst layer. Yes (Patent Document 2). In this configuration, it is conceivable to improve NOx purification by providing Rh in the lower catalyst layer. Patent Document 3 discloses such a catalyst. This is composed of a first coat layer made of an alumina layer containing Pd and Rh on a substrate, and cerium oxide particles and activated alumina particles fixed on Rh and provided on the first coat layer. A second coat layer.

JP 2006-68728 A JP 2006-263582 A Japanese Patent Laid-Open No. 03-56137

  However, if only the Rh is provided in the lower catalyst layer, or only the Rh is provided in both the upper catalyst layer and the lower catalyst layer, if the catalyst is enveloped in a lean atmosphere, the air-fuel ratio rich exhaust gas is Even if it flows into the catalyst, a high NOx purification performance cannot be expected, and the NOx purification performance at a low temperature is particularly low.

  According to the inventor's research, the exhaust gas purification performance of the Rh catalyst is different depending on the oxide carrier supporting the Rh, and the NOx purification performance is enhanced by combining Rh catalysts supported on different oxide carriers. I understood it. However, the combination of different Rh catalysts has not been sufficiently studied so far, and there is no report on the effect of the combination of different Rh catalysts on the NOx purification performance.

  Based on the above, the present invention efficiently purifies NOx during a transition period in which exhaust gas rich in air-fuel ratio has flowed even when the exhaust gas purification catalyst is in a low temperature and air-fuel ratio lean atmosphere. In this way, for example, the amount of NOx that is exhausted without being purified into the atmosphere when the engine is started is reduced.

  In the present invention, in order to solve the above problems, in the exhaust gas purifying catalyst in which the Rh catalyst layer is provided on the upper side of the non-Rh catalyst layer on the base material, each of the Rh catalyst layers contains Rh and NOx. A laminated structure of a plurality of catalyst layers with different purification rates was used.

That is, the exhaust gas purifying catalyst disclosed herein includes a Rh catalyst layer containing Rh as a catalyst metal on a base material, and a non-Rh containing a catalyst metal other than Rh below the Rh catalyst layer. A catalyst layer is provided,
The Rh catalyst layer is formed by laminating a plurality of layers each containing Rh. The higher the layer is, the higher the NOx purification rate when the air-fuel ratio of the exhaust gas changes from lean to rich at a temperature lower than 290 ° C. It is large.

  Therefore, even if the exhaust gas purifying catalyst is enclosed in an air-fuel ratio lean atmosphere and the temperature is low, in the upper layer exposed to the exhaust gas of the Rh catalyst layer, the air-fuel ratio rich exhaust gas is applied to the catalyst. When flowing in, NOx is purified with relatively high efficiency as the air-fuel ratio of the atmosphere changes from lean to rich. Along with this, the catalytic reaction heat is transferred from the upper layer to the lower layer having a relatively low NOx purification rate, so that the lower layer is also easily purified of NOx.

  That is, with such an Rh catalyst layer, even if the NOx purification rate averaged over the entire Rh catalyst layer is the same as that of the single-layer Rh catalyst layer, NOx is efficiently purified in the upper layer, and the reaction Since the activation of the lower layer is promoted by heat, the NOx purification rate in the transition period in which the air-fuel ratio has changed from lean to rich is higher than that of the single-layer Rh catalyst layer.

  In addition, as described above, NOx is efficiently purified in the Rh catalyst layer, so that the NOx concentration of the exhaust gas diffusing into the lower non-Rh catalyst layer is lowered, and the HC and CO oxygen in the non-Rh catalyst layer is reduced. Oxidation purification becomes easier to proceed. That is, since oxygen works better for HC and CO oxidation than NOx, the higher the NOx concentration, the more disadvantageous it is for HC and CO purification. In the catalyst layer, the NOx concentration becomes low, and the purification of HC and CO easily proceeds.

In a preferred embodiment, the uppermost layer in the Rh catalyst layer includes ZrLa-based / Al ₂ O ₃ composite particles in which ZrLa-based composite oxide containing Zr and La is supported on activated alumina particles, and the composite particles Rh is supported on (Rh / ZrLa system / Al ₂ O ₃ catalyst),
The lower layer than the uppermost layer in the Rh catalyst layer includes a rare earth metal R other than Zr, La, and Ce, and ZrLaR-based composite oxide particles supporting Rh (Rh / ZrLaR-based catalyst); At least one of CeZr-based composite oxide particles (Rh / CeZr-based catalyst) containing Ce and Zr and supporting Rh is provided. In this case, the ZrLaR composite oxide is an oxygen ion conductive compound, and the CeZr composite oxide is a compound having an oxygen storage / release capability.

That is, according to the study by the present inventor, the NOx purification rate when the air-fuel ratio of the exhaust gas changes from lean to rich at a temperature of less than 290 ° C. is the Rh / ZrLa system / The Al ₂ O ₃ catalyst is the largest and the next largest is the Rh / ZrLaR catalyst, followed by the Rh / CeZr catalyst.

Since the Rh / ZrLa-based / Al ₂ O ₃ catalyst has a large specific surface area of activated alumina, it is considered that Rh can be highly dispersed and is advantageous for adsorption and purification of NOx by Rh. The ZrLa-based composite oxide is advantageous for NOx adsorption because it contains Zr and La, each of which shows basicity. In addition, in the case of a catalyst material in which Rh is directly supported on activated alumina, if it is used at high temperatures for many years, Rh dissolves in activated alumina and deactivates, but Rh / ZrLa system / Al ₂ O _In the case of the _three catalysts, at least a part of Rh is not directly supported on the activated alumina, but is supported on the activated alumina via the ZrLa-based composite oxide, so that the high temperature deactivation is also suppressed.

  The ZrLa-based composite oxide is not limited to containing only Zr and La as metals, and further containing a rare earth metal R other than Ce, or an alkaline earth metal such as Sr, Ca, Mg You may employ | adopt what contains. In the case of containing an alkaline earth metal, basic sites increase, which is advantageous for NOx adsorption and purification.

As the rare earth metal R that can be contained in the rare earth metal R of the Rh / ZrLaR catalyst or the ZrLa complex oxide of the Rh / ZrLa / Al ₂ O ₃ catalyst, Y, Pr, Nd, or the like is adopted. It is preferable.

  The CeZr-based composite oxide of the Rh / CeZr-based catalyst is not limited to the one containing only Ce and Zr as a metal, but may be one containing a rare earth metal other than Ce, such as Nd.

In another preferred embodiment, the uppermost layer in the Rh catalyst layer contains the rare earth metal R other than Zr, La, and Ce and supports the Rh-supported ZrLaR-based composite oxide particles (Rh / ZrLaR-based). Catalyst),
The layer below the uppermost layer in the Rh catalyst layer includes the CeZr-based composite oxide particles (Rh / CeZr-based catalyst) containing Ce and Zr and supporting Rh.

  The non-Rh catalyst layer contains active alumina particles and CeZr-based composite oxide particles having Ce and Zr-containing oxygen storage / release ability, and each of the active alumina particles and CeZr-based composite oxide particles includes the above-mentioned Pd is preferably supported as a catalyst metal. With such a Pd catalyst layer, HC and CO in the exhaust gas are efficiently purified.

  According to the present invention, an Rh catalyst layer and a non-Rh catalyst layer disposed below the Rh catalyst layer are provided on a substrate, and the Rh catalyst layer includes a plurality of layers each containing Rh. Therefore, the higher the layer, the higher the NOx purification rate when the air-fuel ratio of the exhaust gas changes from lean to rich at a temperature below 290 ° C., so that the exhaust gas-purifying catalyst enters the atmosphere of lean air-fuel ratio. Even if the temperature is low and the temperature is low, the NOx purification performance is excellent when the air-fuel ratio rich exhaust gas flows into the catalyst. For example, the amount of NOx that remains unpurified in the atmosphere when the engine is started This is advantageous for reducing.

It is sectional drawing which shows an example of the layer structure of the catalyst for exhaust gas purification | cleaning. It is sectional drawing which shows another example of the layer structure of the catalyst for exhaust gas purification. It is sectional drawing which shows a part of NOx purification performance evaluation apparatus. It is a NOx purification performance evaluation flowchart. It is a graph which shows an example of the time-dependent change of the air fuel ratio, NOx concentration, and NOx purification speed in a NOx purification performance evaluation test. It is a graph which shows the measurement result of the NOx purification rate of various sample catalysts.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the invention, its application, or its use.

  FIG. 1 shows an example of a layer structure of an exhaust gas purifying catalyst disposed in an exhaust passage of an automobile engine. In the figure, 1 is a cell wall of a honeycomb carrier as a substrate, 2 is a non-Rh catalyst layer formed on the surface of the cell wall 1, and 3 is an Rh catalyst layer superimposed on the non-Rh catalyst layer 2. . The Rh catalyst layer 3 includes a first layer (upper layer) 3a directly exposed to the exhaust gas, a second layer (intermediate layer) 3b provided below the first layer 3a, and a lower side of the second layer 3b. It has a three-layer structure with the third layer (lower layer) 3c provided.

The non-Rh catalyst layer 2 is a catalyst layer that does not contain Rh as a catalyst metal. In this embodiment, the non-Rh catalyst layer 2 contains two types of Pd catalysts having Pd as a catalyst metal. That is, a Pd / Al ₂ O ₃ catalyst in which Pd is supported on activated alumina particles, and a Pd / CeZr system in which Pd is supported on CeZr-based composite oxide particles containing Ce and Zr and having an oxygen storage / release capability. There are two types of catalysts. The Pd / Al ₂ O ₃ catalyst and the Pd / CeZr-based catalyst are mixed together with a binder and supported on the cell wall 1 to form a non-Rh catalyst layer 2.

  Each of the first layer 3a to the third layer 3c of the Rh catalyst layer 3 contains an Rh catalyst having Rh as a catalyst metal, but these Rh catalysts are different in the type of oxide supporting Rh. Therefore, the first layer 3a to the third layer 3c have different NOx purification rates under specific conditions (when the air-fuel ratio of the exhaust gas changes from lean to rich at a temperature lower than 290 ° C.). Specific contents of the specific condition will be described later. The magnitude relationship between the NOx purification rates is the largest for the Rh catalyst of the first layer 3a, the next largest is the Rh catalyst for the second layer 3b, and the next is the Rh catalyst for the third layer 3c. Yes.

  FIG. 2 shows a layer structure of an exhaust gas purifying catalyst according to another embodiment. Also in this catalyst, the non-Rh catalyst layer 2 is formed on the surface of the cell wall 1, and the Rh catalyst layer 3 is formed on the non-Rh catalyst layer 2. However, the non-Rh catalyst layer 2 is the same as the case of FIG. 1, but the Rh catalyst layer 3 is different from the case of FIG. 1 in that the first layer (upper layer) 3a directly exposed to the exhaust gas and the first layer It has a two-layer structure of a second layer (lower layer) 3b provided below the layer 3a. Each of the first layer 3a and the second layer 3b also contains an Rh catalyst, but these Rh catalysts have different types of oxides supporting Rh, and the NOx purification rate under the above specific conditions is the first layer 3a. This Rh catalyst is larger than the Rh catalyst of the second layer 3b.

  An important feature of the present invention is that the Rh catalyst layer 3 is composed of a plurality of layers having different NOx purification rates. This will be specifically described below.

<NOx purification rate of various Rh catalysts>
As sample catalysts (Rh catalyst) to be used for measurement, the following five types with different types of oxides supporting Rh were prepared.

[Preparation of sample catalyst]
-Rh / ZrLaO / Al ₂ O ₃ catalyst-
This is a kind of the Rh / ZrLa-based / Al ₂ O ₃ catalyst described above, and ZrLaO / Al ₂ O ₃ composite particles in which ZrLa composite oxide containing Zr and La is supported on activated alumina particles. And Rh is supported on the composite particles. The preparation method is as follows.

Activated alumina powder was dispersed in a mixed solution of zirconium nitrate and lanthanum nitrate, and ammonia water was added thereto to form a precipitate. The obtained precipitate was filtered, washed, dried at 200 ° C. for 2 hours, and fired at 500 ° C. for 2 hours to obtain ZrLa-based / Al ₂ O ₃ composite particles (powder). The composition is ZrO ₂ : La ₂ O ₃ : Al ₂ O ₃ = 38: 2: 60 (mass ratio). Rh was supported on the ZrLa-based / Al ₂ O ₃ composite particles (powder) by evaporation to dryness. For this evaporation to dryness, an aqueous Rh nitrate solution was employed as the Rh solution.

The obtained Rh / ZrLaO / Al ₂ O ₃ catalyst was supported on a honeycomb carrier to obtain a sample catalyst. The supported amount of Rh / ZrLaO / Al ₂ O ₃ catalyst per liter of support is 30 g (of which the supported amount of Rh is 0.1 g).

-Rh / ZrLaYO catalyst-
This is a kind of the above-mentioned Rh / ZrLaR-based catalyst, in which Rh is supported on ZrLaY composite oxide particles containing Zr, La and Y. The preparation method is as follows.

Zirconyl nitrate solution, lanthanum nitrate and yttrium nitrate are dissolved in ion exchange water. A coprecipitate is obtained by mixing and neutralizing this nitrate solution with an 8-fold diluted solution of 28% by mass ammonia water. The coprecipitate is washed with water by a centrifugal separation method, dried in air at a temperature of 150 ° C. for a whole day and night, pulverized, and then fired in air at a temperature of 500 ° C. for 2 hours. To obtain a powder. The composition is ZrO ₂ : La ₂ O ₃ : Y ₂ O ₃ = 84: 6: 10 (mass ratio). Rh was supported on this ZrLaY composite oxide powder by the same evaporation and drying method as the previous Rh catalyst.

  The obtained Rh / ZrLaYO catalyst was supported on a honeycomb carrier to obtain a sample catalyst. The loading amount of the Rh / ZrLaYO catalyst per liter of the carrier is 30 g (of which the loading amount of Rh is 0.1 g).

-Rh / ZrLaNdO catalyst-
This is a kind of the above-mentioned Rh / ZrLaR-based catalyst, in which Rh is supported on ZrLaNd composite oxide particles containing Zr, La and Nd. The preparation method is the same as the previous Rh / ZrLaYO catalyst, except that neodymium nitrate is used instead of yttrium nitrate. The obtained Rh / ZrLaNdO catalyst was supported on a honeycomb carrier to obtain a sample catalyst. The composition of the ZrLaNd composite oxide powder is ZrO ₂ : La ₂ O ₃ : Nd ₂ O ₃ = 84: 6: 10 (mass ratio), and the supported amount of Rh / ZrLaNdO catalyst per 1 L of support is 30 g (of which The amount of Rh supported is 0.1 g).

-Rh / ZrLaPrO catalyst-
This is a kind of the above-mentioned Rh / ZrLaR-based catalyst, in which Rh is supported on ZrLaPr composite oxide particles containing Zr, La and Pr. The preparation method is the same as the previous Rh / ZrLaYO catalyst, except that praseodymium nitrate is used instead of yttrium nitrate. The obtained Rh / ZrLaPrO catalyst was supported on a honeycomb carrier to obtain a sample catalyst. The composition of the ZrLaPr composite oxide powder is ZrO ₂ : La ₂ O ₃ : Pr ₂ O ₃ = 84: 6: 10 (mass ratio), and the supported amount of Rh / ZrLaPrO catalyst per 1 L of support is 30 g (of which The amount of Rh supported is 0.1 g).

-Rh / CeZrNdO catalyst-
This is a kind of the above-mentioned Rh / CeZr-based catalyst, in which Rh is supported on CeZrNd composite oxide particles containing Ce, Zr and Nd. The preparation method is the same as the previous Rh / ZrLaYO catalyst except that the starting materials for obtaining the composite oxide are cerium nitrate, a zirconyl nitrate solution, and neodymium nitrate. The obtained Rh / CeZrNdO catalyst was supported on a honeycomb carrier to obtain a sample catalyst. The composition of the CeZrNd composite oxide powder is CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 10: 80: 10 (mass ratio), and the supported amount of Rh / CeZrNdO catalyst per liter of support is 30 g (of which Rh is supported) The amount is 0.1 g).

[Measurement of NOx purification rate]
FIG. 3 shows a main configuration of the NOx purification performance evaluation apparatus for measuring the NOx purification rate. This evaluation apparatus is provided with a gas flow pipe 12 having an installation part for a sample catalyst 11 and allows the simulated exhaust gas to flow through the installed sample catalyst 11. NOx concentration detection means 13 is provided at the inlet and the outlet of the sample installation section, respectively.

  FIG. 4 shows an evaluation flow of NOx purification performance using the above evaluation apparatus. The sample catalyst 11 is installed in the sample installation part of the gas flow pipe 12 (step S1). While flowing the simulated exhaust gas having a rich air-fuel ratio (A / F) (for example, stoichiometric) to the sample catalyst 11 of the sample installation section, the temperature in the installation section is raised from room temperature to an evaluation temperature (for example, 250 ° C.) (step) S2).

  Next, the air-fuel ratio of the simulated exhaust gas is switched to lean (for example, A / F = 20), and detection of NOx concentrations on the upstream side and downstream side of the sample catalyst 11 is started (step S3). When a predetermined time (for example, 30 seconds) has elapsed since the air-fuel ratio was switched to lean, the air-fuel ratio is switched to rich (step S4). Next, when a predetermined time (for example, 30 seconds) elapses after switching to rich, the air-fuel ratio is switched to lean (step S5). Steps S4 and S5 are repeated as many times as necessary.

  Then, the NOx purification rate and the NOx purification amount in the air-fuel ratio rich period are obtained (step S6). The NOx purification rate in this case is the amount of decrease in the downstream NOx concentration during a predetermined time. For example, the maximum NOx purification rate or the average NOx purification rate in the entire air-fuel ratio rich period, or a predetermined period during the rich period The maximum NOx purification rate or the average NOx purification rate in the first half period, the second half period, or the intermediate period is obtained. The NOx purification amount in this case is the integrated amount of NOx purified during the entire air-fuel ratio rich period or the predetermined period. When the steps S4 and S5 are repeatedly executed, the NOx purification rate and the NOx purification amount in each air-fuel ratio rich period are obtained and their average values are taken.

  The above steps S1 to S6 are executed for each temperature to be evaluated (for example, in increments of 50 ° C from 250 ° C to 400 ° C) (step S7). The performance of the catalyst is evaluated based on the NOx purification rate and the NOx purification amount at each evaluation temperature (step S8).

  FIG. 5A shows an example of changes in the air-fuel ratio (A / F) and changes in the downstream NOx concentration. In the sample catalyst 11, the NOx purification performance when the air-fuel ratio is lean is low, but when the air-fuel ratio becomes rich, the purification of NOx proceeds rapidly. Therefore, when the air-fuel ratio changes from lean to rich, the downstream NOx concentration decreases, and when it changes from rich to lean, the downstream NOx concentration suddenly increases. As shown in FIG. 5B, the NOx purification rate rapidly increases every time the air-fuel ratio is switched from lean to rich, and decreases with time.

[Measurement result of NOx purification rate]
FIG. 6 shows the measurement results of the NOx purification rate of the five types of test materials using the evaluation apparatus. This NOx purification rate is the maximum value in the air-fuel ratio rich period. Table 1 shows the compositions of the air-fuel ratio lean and rich simulated exhaust gases. The simulated exhaust gas has a stoichiometric composition when the air-fuel ratio is rich, and the air-fuel ratio becomes lean (A / F = 20) by adding 0.3% of O ₂ to the stoichiometric simulated exhaust gas. I did it. The simulated exhaust gas flow rate was set such that the SV value was 60000h- ¹ .

According to FIG. 6, the NOx purification rate is the largest for the Rh / ZrLaO / Al ₂ O ₃ catalyst at temperatures below 290 ° C., the next largest being the Rh / ZrLaYO catalyst and the Rh / ZrLaNdO catalyst, followed by The Rh / ZrLaPrO catalyst is large and the Rh / CeZrNdO catalyst is the smallest.

  Based on this result, exhaust gas purification catalysts of Examples 1 to 5 and Comparative Examples 1 to 3 shown in Table 2 were prepared, and their NOx purification performance was evaluated.

<Examples 1-5 and Comparative Examples 1-3>
Example 1
The non-Rh catalyst layer 2 was configured to contain the above-described Pd / Al ₂ O ₃ catalyst and Pd / CeZr-based catalyst in a mixed manner. As the Pd / Al ₂ O ₃ catalyst, an active alumina particle containing 4% by mass of La ₂ O ₃ and having Pd supported by the evaporation to dryness method was employed. The amount supported per liter of carrier is 45 g (of which the amount of Pd supported is 4.3 g). As the Pd / CeZr-based catalyst, a CeZrNd composite oxide particle having a composition of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 23: 67: 10 (mass ratio) is used in which Pd is supported by the evaporation to dryness method. did. The supported amount per liter of carrier is 35 g (of which the supported amount of Pd is 0.3 g).

The Rh catalyst layer 3 has a three-layer structure shown in FIG. The Rh / ZrLaO / Al ₂ O ₃ catalyst described above was employed as the Rh catalyst for the first layer 3a. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). The Rh / ZrLaYO catalyst described above was employed as the Rh catalyst for the second layer 3b. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). The Rh / CeZrNdO catalyst described above was employed as the Rh catalyst for the third layer 3c. The amount supported per liter of carrier is 60 g (of which the amount of Rh supported is 0.25 g). Therefore, as apparent from FIG. 6, the NOx purification rate at a temperature of less than 290 ° C. is the highest for the Rh catalyst of the first layer 3a, and the Rh catalyst of the second layer 3b and the Rh catalyst of the third layer 3c. Are in order.

-Example 2-
The non-Rh catalyst layer had the same configuration as in Example 1. The Rh catalyst layer 3 has a two-layer structure shown in FIG. The Rh / ZrLaO / Al ₂ O ₃ catalyst described above was employed as the Rh catalyst for the first layer 3a. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). In the second layer 3b, the aforementioned Rh / ZrLaYO catalyst and Rh / CeZrNdO catalyst were mixed and arranged. The amount of Rh / ZrLaYO catalyst supported per liter of carrier is 30 g (of which Rh is supported is 0.1 g), and the amount of Rh / CeZrNdO catalyst supported per liter of carrier is 60 g (of which Rh is supported is 0.25 g). ). Therefore, the NOx purification rate at a temperature of less than 290 ° C. is higher for the Rh catalyst of the first layer 3a than for the Rh catalyst mixture of the second layer 3b.

Example 3
The non-Rh catalyst layer had the same configuration as in Example 1. The Rh catalyst layer 3 has a two-layer structure shown in FIG. The Rh / ZrLaO / Al ₂ O ₃ catalyst described above was employed as the Rh catalyst for the first layer 3a. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). The Rh / ZrLaYO catalyst described above was employed as the Rh catalyst for the second layer 3b. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). Therefore, the NOx purification rate at a temperature lower than 290 ° C. is higher for the Rh catalyst of the first layer 3a than for the Rh catalyst of the second layer 3b.

Example 4
The non-Rh catalyst layer had the same configuration as in Example 1. The Rh catalyst layer 3 has a two-layer structure shown in FIG. The Rh / ZrLaO / Al ₂ O ₃ catalyst described above was employed as the Rh catalyst for the first layer 3a. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). The Rh / CeZrNdO catalyst described above was employed as the Rh catalyst for the second layer 3b. The amount supported per liter of carrier is 60 g (of which the amount of Rh supported is 0.25 g). Therefore, the NOx purification rate at a temperature lower than 290 ° C. is higher for the Rh catalyst of the first layer 3a than for the Rh catalyst of the second layer 3b.

-Example 5
The non-Rh catalyst layer had the same configuration as in Example 1. The Rh catalyst layer 3 has a two-layer structure shown in FIG. The Rh / ZrLaYO catalyst described above was employed as the Rh catalyst for the first layer 3a. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). The Rh / CeZrNdO catalyst described above was employed as the Rh catalyst for the second layer 3b. The amount supported per liter of carrier is 60 g (of which the amount of Rh supported is 0.25 g). Therefore, the NOx purification rate at a temperature lower than 290 ° C. is higher for the Rh catalyst of the first layer 3a than for the Rh catalyst of the second layer 3b.

-Comparative Example 1-
This is a comparative example corresponding to Examples 1 and 2, and the non-Rh catalyst layer has the same configuration as Example 1. The Rh catalyst layer 3 has a single layer structure. That is, as the Rh catalyst, a mixture of the aforementioned Rh / ZrLaO / Al ₂ O ₃ catalyst, Rh / ZrLaYO catalyst, and Rh / CeZrNdO catalyst was employed. The amount supported per liter of the carrier was 30 g of Rh / ZrLaO / Al ₂ O ₃ catalyst (of which Rg was 0.1 g), 30 g of Rh / ZrLaYO catalyst (of which Rh was 0.1 g), Rh / CeZrNdO catalyst is 60 g (of which Rh loading is 0.25 g).

-Comparative Example 2-
This is a comparative example corresponding to Example 3, and the non-Rh catalyst layer has the same configuration as Example 1. The Rh catalyst layer 3 has a two-layer structure shown in FIG. The Rh / ZrLaYO catalyst described above was employed as the Rh catalyst for the first layer 3a. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). The Rh / ZrLaO / Al ₂ O ₃ catalyst described above was employed as the Rh catalyst for the second layer 3b. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). Therefore, the NOx purification rate at a temperature of less than 290 ° C. is smaller in the Rh catalyst of the first layer 3a than the Rh catalyst of the second layer 3b, contrary to the third embodiment.

-Comparative Example 3-
This is a comparative example corresponding to Example 4, and the non-Rh catalyst layer has the same configuration as that of Example 1. The Rh catalyst layer 3 has a two-layer structure shown in FIG. The Rh / CeZrNdO catalyst described above was employed as the Rh catalyst for the first layer 3a. The amount supported per liter of carrier is 60 g (of which the amount of Rh supported is 0.25 g). The Rh / ZrLaO / Al ₂ O ₃ catalyst described above was employed as the Rh catalyst for the second layer 3b. The amount supported per liter of carrier is 30 g (of which the amount of Rh supported is 0.1 g). Therefore, the NOx purification rate at a temperature of less than 290 ° C. is smaller in the Rh catalyst of the first layer 3a than the Rh catalyst of the second layer 3b, contrary to the fourth embodiment.

<Evaluation of NOx purification performance>
For each exhaust gas purification catalyst of Examples 1 to 5 and Comparative Examples 1 to 3, the maximum NOx purification rate at 250 ° C. and the rich period using the simulated exhaust gas of Table 1 above by the evaluation method described above. The total NOx purification amount was measured. The results are shown in Table 2.

  Examples 1 and 2 and Comparative Example 1 have the same catalyst material structure. In Examples 1 and 2, the Rh catalyst layer has a three-layer structure or a two-layer structure. Then, it is different in that it is a mixed layer. When these are compared, the NOx purification rate and the NOx purification amount are much larger in Examples 1 and 2 than in Comparative Example 1. From this, it is effective for NOx purification to arrange the Rh catalyst layer in a multilayer structure in which the Rh catalyst having the highest NOx purification rate at a temperature of less than 290 ° C. is arranged in the first layer directly exposed to the exhaust gas. Is inferred.

  In Example 1 and Example 2, the Rh / ZrLaYO catalyst and the Rh / CeZrNdO catalyst are arranged separately in the second layer and the third layer in the former, whereas the latter is mixed. It is different in point. When both are compared, the NOx purification rate and the NOx purification amount are larger in Example 1 than in Example 2. From this, it can be seen that it is effective for NOx purification to arrange an Rh catalyst having a high NOx purification rate on the upper side of the layer below the first layer.

Example 3 and Comparative Example 2 have the same material composition of the catalyst, but the Rh / ZrLaO / Al ₂ O ₃ catalyst having a large NOx purification rate is arranged in the upper first layer in Example 3. On the other hand, Comparative Example 3 is different in that it is arranged in the lower second layer. When both are compared, the NOx purification rate and the NOx purification amount are greater in Example 3 than in Comparative Example 2. This also shows that it is advantageous for NOx purification to arrange the Rh catalyst having a high NOx purification rate in the first layer that is directly exposed to the exhaust gas.

The relationship between Example 4 and Comparative Example 3 is also the case where the material composition of the catalyst is the same, and the first and second layers of the Rh catalyst layer are reversed. The NOx catalysts arranged oppositely in both are Rh / ZrLaO / Al ₂ O ₃ catalyst and Rh / CeZrNdO catalyst. Even in this case, Example 4 in which the Rh / ZrLaO / Al ₂ O ₃ catalyst having a large NOx purification rate is arranged in the first layer is more NOx than Comparative Example 3 in which the Rh / CeZrNdO catalyst is arranged in the first layer. The purification rate and the NOx purification amount are increased.

In Example 5, the Rh / ZrLaYO catalyst having a high NOx purification rate is arranged in the first layer, and the Rh / CeZrNdO catalyst having a relatively low NOx purification rate is arranged in the second layer. Compared with Example 4 in which the Rh / ZrLaO / Al ₂ O ₃ catalyst is arranged in the first layer, the NOx purification rate is larger in Example 4, but the NOx purification amount is larger in Example 5. . From this, the Rh / ZrLaYO catalyst has a lower NOx purification rate when the air-fuel ratio changes from lean to rich than the Rh / ZrLaO / Al ₂ O ₃ catalyst, but NOx purification when the air-fuel ratio rich state continues. It can be said that the rate of decrease in speed is small (the NOx purification rate does not decrease significantly).

1 Cell wall of honeycomb carrier (base material)
2 Non-Rh catalyst layer 3 Rh catalyst layer 3a 1st layer 3b 2nd layer 3c 3rd layer 11 Sample catalyst 12 Gas flow pipe 13 NOx concentration detection means

Claims (4)

An exhaust gas purifying catalyst comprising a Rh catalyst layer containing Rh as a catalyst metal on a base material, and a non-Rh catalyst layer containing a catalyst metal other than Rh provided below the Rh catalyst layer ,
The Rh catalyst layer is formed by laminating a plurality of layers each containing Rh. The higher the layer is, the higher the NOx purification rate when the air-fuel ratio of the exhaust gas changes from lean to rich at a temperature lower than 290 ° C. An exhaust gas purifying catalyst characterized by being large. In claim 1,
The uppermost layer in the Rh catalyst layer includes composite particles in which a composite oxide containing Zr and La is supported on activated alumina particles, and Rh is supported on the composite particles.
The lower layer of the Rh catalyst layer than the uppermost layer contains a rare earth metal R other than Zr, La, and Ce and contains ZrLaR-based composite oxide particles supporting Rh, Ce and Zr, and An exhaust gas purification catalyst comprising at least one of CeZr-based composite oxide particles supporting Rh. In claim 1,
The uppermost layer in the Rh catalyst layer includes ZrLaR-based composite oxide particles containing a rare earth metal R other than Zr, La, and Ce and supporting Rh,
The exhaust gas purifying catalyst characterized in that the lower layer than the uppermost layer in the Rh catalyst layer includes CeZr-based composite oxide particles containing Ce and Zr and supporting Rh. In any one of Claim 1 thru | or 3,
The non-Rh catalyst layer contains activated alumina particles and CeZr-based composite oxide particles containing Ce and Zr, and Pd is supported as the catalyst metal on each of the activated alumina particles and CeZr-based composite oxide particles. An exhaust gas purifying catalyst characterized by that.

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