JP2006263582A - Exhaust gas purification catalyst - Google Patents

Abstract

【Task】
In an exhaust gas purification catalyst provided with a catalyst layer containing palladium, alumina, and an oxygen storage material as catalyst noble metals on a honeycomb carrier, when the catalyst noble metal is supported on both alumina and the oxygen storage material It is an object of the present invention to propose a loading ratio capable of expressing high purification performance by optimizing the catalytic reaction by the catalytic noble metal and the function of the oxygen storage material.
[Solution]
The catalyst layer 11b is composed of two upper and lower catalyst layers, and the loading amounts A and B of Pd per unit weight on the lower first alumina and the Ce · Zr · La · Y alumina composite are substantially the same. Further, the supported amounts C and D of Rh per unit specific surface area in the third alumina of the upper layer and the Ce.Zr.Nd double oxide are increased.
[Selection] Figure 6

Description

  The present invention relates to an exhaust gas purification catalyst and belongs to the technical field of automobile exhaust gas purification.

  Conventionally, hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) contained in automobile exhaust gas are simultaneously purified to carbon dioxide (CO2), water (H2O), nitrogen (N2), etc. Obtained three-way catalysts are known. In recent years, this three-way catalyst has been required to achieve a high purification rate immediately after the engine is started. As a countermeasure, the catalyst can be directly connected to the exhaust merging portion of the engine exhaust manifold (directly connected catalyst) or under the floor. The catalyst is arranged as upstream as possible in the exhaust passage as close to the engine as possible so that the temperature of the catalyst is raised above the catalyst reaction temperature in a short time after the engine is started. However, when the catalyst is arranged close to or relatively close to the engine in this way, the catalyst is exposed to extremely high temperature exhaust gas, and as a result, is supported on alumina or an oxygen storage material in the catalyst layer. Catalyst noble metal particles such as platinum (Pt), rhodium (Rh), palladium (Pd) move and collect on the alumina surface or oxygen storage material surface, causing sintering, reducing the total surface area of the catalyst noble metal, and purifying There is a problem that performance deteriorates. In particular, when different types of catalytic precious metals are sintered and alloyed, the catalytic activity itself is lost, and the purification performance is remarkably deteriorated.

Therefore, in order to prevent sintering and alloying of different types of catalytic noble metals, the technique disclosed in Patent Document 1 can be applied. That is, as shown in FIG. 12, two upper and lower catalyst layers are formed on the exhaust gas passage wall of the honeycomb carrier, and alumina, cerium (Ce), zirconium (Zr), neodymium (Nd) composites are formed on the upper layer. An oxide is contained, and the lower layer contains alumina, a Ce.Zr.Nd double oxide, and cerium oxide (ceria). Here, Ce · Zr · Nd double oxide and cerium oxide are oxygen storage materials having an oxygen storage capacity (OSC) and have a function of expanding the A / F window of the three-way catalyst. . Then, platinum is supported on the upper layer alumina, rhodium is supported on the upper layer oxygen storage material, and palladium is supported on the lower layer alumina, so that each catalyst noble metal is put on different alumina and oxygen storage materials. Since they are separated and supported, sintering and alloying of different kinds of catalyst noble metals can be prevented, and significant deterioration in purification performance can be avoided.
Japanese Patent Laid-Open No. 2003-170047

  However, in the technique disclosed in Patent Document 1, for example, platinum or palladium supported on alumina is not supported on the oxygen storage material, and rhodium supported on the oxygen storage material is not supported on alumina. Therefore, the state of the surface of the catalytic noble metal due to the interaction between the catalytic noble metal and the support material alumina or the oxygen storage material is not optimized, and furthermore, the degree of dispersion of the various types of catalytic noble metals on the support material is low. As a result, there is a problem that the reaction gas adsorption / purification characteristics deteriorate, and the purification characteristics per unit noble metal surface are not sufficiently exhibited. In addition, even when alumina or oxygen storage materials are sintered together, there is a problem that the degree of loss of the noble metal particles is increased and the purification performance is not sufficiently suppressed.

  In order to cope with such a problem, it is conceivable that a catalyst noble metal is supported on both the alumina and the oxygen storage material. However, considering the catalytic reaction of catalytic precious metals, it is necessary to optimize the relationship between oxygen storage and release by the oxygen storage material to achieve high purification performance. From this point of view, catalytic precious metals are combined with alumina and oxygen storage. There are no proposals for how to distribute the materials.

  The present invention addresses the above-described problems in a catalyst that is susceptible to a high heat load of engine exhaust gas, such as a catalyst disposed close to the engine or an underfloor catalyst disposed relatively close to the engine. Therefore, when catalyst noble metal is supported on both alumina and oxygen storage material, the problem is to propose a support ratio that can achieve high purification performance by optimizing the catalytic reaction of the catalyst noble metal and the function of the oxygen storage material. And

  That is, in order to solve the above-mentioned problem, the invention according to claim 1 of the present application is an exhaust gas purification device in which a catalyst layer containing palladium, alumina, and an oxygen storage material as catalyst noble metals is provided on a honeycomb carrier. The palladium is supported on one kind of alumina and one kind of oxygen storage material, and the supported amount of palladium per unit weight of the alumina and the oxygen storage material is substantially the same. It is characterized by being.

  Next, an invention according to claim 2 is an exhaust gas purification catalyst comprising a catalyst layer containing rhodium, alumina, and an oxygen storage material as catalyst noble metals on a honeycomb carrier, wherein the rhodium Is supported on one kind of alumina and one kind of oxygen storage material, and the specific surface area of the alumina and oxygen storage material is larger in the former, and the rhodium per unit specific surface area of the alumina and oxygen storage material The supported amount is larger in the latter.

  The invention according to claim 3 is the exhaust gas purifying catalyst according to claim 1 or 2, wherein the catalyst layer includes palladium and rhodium as catalyst noble metals, and the catalyst layer is divided into upper and lower portions. The palladium is supported on the lower layer, and the rhodium is supported on the upper layer.

  The invention according to claim 4 is the exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the catalyst layer further includes platinum as a catalyst noble metal, It is characterized by being supported on at least one of another alumina and oxygen storage material on which palladium and rhodium are not supported.

  First, according to the invention described in claim 1, since the supported amount of palladium per unit weight of alumina and the oxygen storage material is made substantially the same, the palladium is supported on alumina having a relatively high specific surface area and heat resistance. Then, palladium is separated and supported on both the alumina and the oxygen storage material, thereby increasing the active sites of palladium and improving the purification activity of hydrocarbons.

  In addition, since palladium is supported on an oxygen storage material that releases oxygen in a rich atmosphere, palladium is controlled to a good oxidation state by active oxygen supplied from the oxygen storage material (that is, palladium is Pd⇔PdO). The production ratio / coexistence ratio of ⇔PdO 2 ⇔ is a factor that has an important influence on the oxidation reaction of hydrocarbons and carbon monoxide, and the production ratio / coexistence ratio is optimized). The oxidation purification rate of carbon monoxide can be increased over a long period of time.

  Next, according to the invention described in claim 2, since a large amount of rhodium is supported per unit specific surface area in the oxygen storage material that releases oxygen in a rich atmosphere, the oxygen storage material is supported in the rich atmosphere. In addition to the purification performance by rhodium, the steam reforming reaction of hydrocarbons is promoted, and as a result, the generation degree of active hydrogen is increased, thereby contributing to the reduction purification rate of nitrogen oxides by this hydrogen. That is, rhodium supported on the oxygen storage material promotes a reduction reaction of NO + H2 → N2 + H2O by using active hydrogen generated by steam reforming reaction of HC + H2O → CO + H2 and CO + H2O → CO2 + H2.

  Also, when A / F is stoichiometric, the loading of Rh on the oxygen storage material is effective, and when A / F is accelerated to the rich side, the loading of Rh on alumina is effective. That is, when the A / F is stoichiometric, the oxygen storage / release capability of the oxygen storage material absorbs the fluctuation of the A / F, and Rh effectively purifies NOx, while the A / F swings to the rich side. At the time of acceleration, the Rh supported on the oxygen storage material has an oxygen excess state due to the released oxygen, and the purification performance tends to decrease. However, alumina has an oxygen storage / release capability. Therefore, Rh supported on alumina can effectively purify NOx without its surface being in an oxygen excess state. That is, according to the second aspect of the present invention, Rh is supported on the oxygen storage material and alumina, so that the NOx purification performance is enhanced.

  Further, according to the invention described in claim 3, since palladium is supported in the lower layer, for example, when an oxygen storage material having a high oxygen storage capacity is disposed in the upper layer, the catalyst from upstream to downstream, When exhaust gas flows from the upper layer of the catalyst to the lower layer, A / F fluctuation is suppressed. As a result, the purification performance of the lower Pd support material (that is, alumina and oxygen storage material) is improved, and the catalyst performance is improved. In addition, sintering and alloying of the lower layer palladium and the upper layer catalyst noble metal such as platinum or rhodium are further suppressed. In addition, since palladium is protected by the upper layer, the suppression effect is expected also against S (sulfur) poisoning and P (phosphorus) poisoning. In addition, since rhodium is supported on the oxygen storage material and alumina in the upper layer, the reactivity of the oxygen storage material and the reactivity of the steam reforming reaction are enhanced, and the reduction reaction of nitrogen oxides can be promoted. .

  According to the invention described in claim 4, since platinum is supported on at least one of another alumina and oxygen storage material on which palladium and rhodium are not supported, the platinum is composed of palladium and rhodium. Is dispersed and supported, so that each catalyst noble metal is highly dispersed, and sintering and alloying are unlikely to occur between these different catalyst metals.

  FIG. 1 is a schematic configuration diagram of a spark ignition engine 1 of an automobile equipped with a three-way catalyst 11 according to the present embodiment. That is, this engine 1 has a plurality of cylinders 2... 2 (only one is shown in the figure), and includes an air supplied via the intake passage 3 and a fuel supplied via the fuel injection valve 4. In the combustion chamber 6 defined by the piston 5, the air-fuel mixture explodes and burns by spark ignition by the spark plug 7, and the exhaust gas is released to the atmosphere via the exhaust passage 8. In that case, the exhaust passage 8 is provided with a catalytic converter 10, and the catalytic converter 10 is filled with the three-way catalyst 11 according to the present invention. Here, the catalytic converter 10 is arranged as upstream as possible in the exhaust passage 8 so as to achieve a high purification rate immediately after the engine 1 is started, for example, directly connected to an exhaust merging portion of the exhaust manifold. However, as a result, the three-way catalyst 11 is exposed to an extremely high temperature exhaust gas, and heat resistance measures are required.

[Configuration of three-way catalyst]
As shown in FIG. 2, the three-way catalyst 11 has a configuration in which a catalyst layer 11b is formed on an exhaust gas passage wall of a cordierite honeycomb carrier 11a. The catalyst layer 11b may be a single layer or a multi-layer structure in which a lower catalyst layer and an upper catalyst layer are formed in layers. As shown in FIG. 3, the catalyst layer 11b is composed of a second alumina supporting platinum, a Ce.Zr.Nd double oxide supporting rhodium (its structure will be described later with reference to FIG. 7), rhodium. Supported third alumina, Ce / Zr / Nd double oxide, palladium-supported first alumina, palladium-supported Ce / Zr / La / Y / alumina composite (see FIG. 8 for the structure) And cerium oxide and a binder (zirconium oxide: ZrO2).

  Here, cerium oxide, Ce / Zr / Nd double oxide and Ce / Zr / La / Y / alumina composites all function as oxygen storage materials that store oxygen in a lean atmosphere and release oxygen in a rich atmosphere. To do. In addition, 4% by mass of lanthanum (La) is added to the first to third aluminas for thermal stabilization. In this case, the third alumina is obtained by coating the surface of the second alumina with 10% by mass of zirconium oxide, thereby preventing rhodium from dissolving in the alumina at a high temperature. And the 1st-3rd alumina differs in the state of a micropore (micropore) by the difference in manufacturing conditions, and has a different specific surface area and a different surface basicity. The reason why zirconium oxide is used as the binder is to improve the heat resistance of the catalyst layer 11b.

[Production method of three-way catalyst]
The method for producing the three-way catalyst 11 is as follows. However, the case where the catalyst layer 11b is composed of upper and lower two catalyst layers as shown in FIG. 3 will be described as an example.

<Formation of lower catalyst layer>
A palladium nitrate aqueous solution is added dropwise to activated alumina powder (first alumina) to which 4% by mass of lanthanum has been added, and dried and fired at 500 ° C. to obtain first alumina carrying palladium. A first alumina carrying palladium, cerium oxide, a Ce / Zr / Nd double oxide, a Ce / Zr / La / Y / alumina composite containing palladium (the method of which will be described later), a binder, Are mixed with water, and mixed and stirred with a disperser to obtain a slurry. The carrier 11a is coated with a predetermined amount of slurry by repeating the operation of immersing the cordierite honeycomb carrier 11a in this slurry and pulling it up and removing excess slurry by air blow. Thereafter, the honeycomb carrier 11a is heated at a constant temperature increase rate from room temperature to 500 ° C. over 1.5 hours, held at that temperature for 2 hours, and dried and fired, whereby the lower catalyst Form a layer.

<Formation of upper catalyst layer>
An aqueous solution of dinitrodiamine platinum nitrate is added dropwise to activated alumina powder (second alumina) to which 4% by mass of lanthanum has been added, and dried and calcined at 500 ° C. to obtain second alumina carrying platinum. Also, rhodium is supported by adding 4% by mass of lanthanum and adding an aqueous rhodium nitrate solution to activated alumina powder (third alumina) coated with 10% by mass of zirconium oxide, followed by drying and firing at 500 ° C. The obtained third alumina is obtained. Further, a rhodium nitrate aqueous solution is dropped onto the Ce.Zr.Nd double oxide, and dried and fired at 500.degree. C. to obtain a Ce.Zr.Nd double oxide carrying rhodium. The second alumina supporting platinum, the third alumina supporting rhodium, the Ce · Zr · Nd double oxide supporting rhodium, and a binder are mixed, and water is added thereto, followed by mixing and stirring with a disperser. To obtain a slurry. The carrier 11a is coated with a predetermined amount of slurry by immersing the cordierite honeycomb carrier 11a having the lower catalyst layer formed thereon as described above and repeating the operation of lifting and removing excess slurry by air blow. . Thereafter, the honeycomb carrier 11a is heated at a constant heating rate from room temperature to 500 ° C. over a period of 1.5 hours, held at that temperature for 2 hours, and dried and fired, whereby the upper catalyst layer. Form.

<Production method of palladium-supported composite>
The palladium-supported Ce / Zr / La / Y / alumina composite contained in the lower catalyst layer should be prepared either by hydrothermal synthesis using an autoclave or by coprecipitation by acid-alkali neutralization. Can do. In the coprecipitation method, first, nitrates of cerium, zirconium, lanthanum, yttrium (Y), and aluminum are mixed, water is added, and the mixture is stirred at room temperature for about 1 hour. Next, this nitrate mixed solution and an alkaline solution (preferably 28% ammonia water) are mixed at room temperature to 80 ° C. to perform neutralization treatment. When this neutralization treatment is performed using a disperser, the rotational speed is set to approximately 4000 rpm to 6000 rpm. The addition rate of the nitrate mixed solution is preferably about 53 mL / min, and the addition rate of the alkaline solution is preferably about 3 mL / min.

  The solution that has become cloudy due to this neutralization treatment is left for a whole day and night. The resulting precipitate cake is centrifuged and then thoroughly washed with water. The cake washed with water is dried at a temperature of about 150 ° C., held at a temperature of about 600 ° C. for about 5 hours, then held at a temperature of about 500 ° C. for 2 hours, dried and fired, and then pulverized. Thereafter, a palladium nitrate solution was added to the obtained powder and evaporated to dryness, and then the obtained dried product was pulverized, heated, and calcined, thereby supporting Ce / Zr / La / Y carrying palladium.・ Alumina composite is obtained.

  In addition, the preferable composition ratio of each component is CeO2: ZrO2: La2O3: Y2O3: Al2O3 = 11.7: 7.7: 1.0: 0.4: When a component is converted into an oxide, it is a mass ratio. 79.2 mag.

[Example]
According to the said manufacturing method, the three-way catalyst of the following structure was manufactured (refer FIGS. 3-6). The carrying amount is the amount per 1 L of the honeycomb carrier.

<Example>
-Lower catalyst layer-
Ce / Zr / Nd double oxide: supported amount 5.7 g / L
Pd / first alumina: supported amount 50.0 g / L (Pd supported amount 0.7 g / L)
Pd / Ce / Zr / La / Y alumina composite: supported amount 25.0 g / L (Pd supported amount 0.35 g / L)
-Cerium oxide: supported amount 5.7 g / L
-Zirconia binder: supported amount 8.5 g / L
-Upper catalyst layer-
-Pt / second alumina: supported amount 25.5 g / L (Pt supported amount 0.08 g / L)
Rh / Ce / Zr / Nd double oxide: supported amount 56.0 g / L (Rh supported amount 0.1 g / L)
Rh / third alumina: supported amount 17.0 g / L (Rh supported amount 0.04 g / L)
Zirconia binder: supported amount 11.0 g / L

  By configuring the upper and lower catalyst layers as described above, the supported amount of Pd per unit weight in the lower first alumina and Ce · Zr · La · Y alumina composite as shown in FIG. 5 and FIG. A and B are respectively A = 0.7 / 50.0 and B = 0.35 / 25.0, and the relational expression of A = B is established in the carrying amounts A and B per unit weight.

  And the carrying amounts C and D of Rh per unit specific surface area in the upper third alumina and Ce · Zr · Nd double oxide are C = 0.04 / 78 and D = 0.1 / 17, respectively. A relational expression of C <D holds for the loadings C and D per specific surface area. In addition, the aging conditions of FIG. 5 shall be 1100 degreeC and 24 hours in air | atmosphere.

<Comparative example 1>
The amount of Pd supported in the lower layer was the same as that of the example except that the first alumina and the Ce · Zr · La · Y alumina composite were distributed 1: 2.

<Comparative example 2>
The amount of Rh supported in the upper layer was the same as in the example except that the same amount per unit specific surface area was used for the third alumina and the Ce.Zr.Nd double oxide.

  Here, the Ce.Zr.Nd double oxide has a fluorite crystal structure as shown in FIG. 7 (in the figure, “M” represents a metal atom and “O” represents an oxygen atom). . On the other hand, as shown in FIG. 8, the Ce / Zr / La / Y / alumina composite has, for example, Ce / Zr double oxide, Ce / Zr / Y double oxide or La2O3 on the surface and inside of the alumina. It has a dispersed structure.

<Evaluation test 1>
For Examples, Comparative Examples 1 and 2, each catalyst was aged in an atmosphere of 2% O 2 and 10% H 2 O under the conditions of 1100 ° C. × 24 hours, and then the HC, CO of each catalyst was determined by a rig test. T50 (° C.) and C500 (%), which are indicators of NOx purification performance, were measured.

  The rig test was performed by cutting the aged catalyst into a cylindrical shape having a diameter of 2.54 cm and a length of 5 cm, and attaching this to a fixed bed flow reaction evaluation apparatus. The simulated exhaust gas (= mainstream gas + fluctuating gas) was A / F = 14.7 ± 0.9, and the flow rate of simulated exhaust gas to the catalyst was 25 L / min. That is, the A / F is forced at an amplitude of ± 0.9 by adding a predetermined amount of fluctuation gas in a pulse form at 1 Hz while constantly flowing the main stream gas of A / F = 14.7. Vibrated. As the fluctuating gas, O2 is used when the A / F is moved to the lean side (A / F = 15.6), and H2 is used when the A / F is moved to the rich side (A / F = 13.8). CO was used. The composition of the mainstream gas with A / F = 14.7 is as follows.

-Mainstream gas-
CO2: 13.9%, O2: 0.6%, CO: 0.6%, H2: 0.2%, C3H6: 0.056%, NO: 0.1%, H2O: 10%, remaining: N2

  T50 (° C.) gradually increases the simulated exhaust gas temperature, and the concentration of each component (HC, CO, and NOx) detected in the downstream of the catalyst is the concentration of each component (HC, CO, and NOx) that flows into the catalyst. ) The catalyst inlet gas temperature (light-off temperature) when the concentration becomes half (that is, when the purification rate reaches 50%), and represents the low-temperature purification performance of the catalyst.

  C500 (%) is the purification rate of each component (HC, CO and NOx) of the gas when the simulated exhaust gas temperature at the catalyst inlet is 500 ° C., and represents the high temperature purification performance of the catalyst.

  The results for T50 (° C.) and C500 (%) are shown in FIG. For any of HC, CO, and NOx, both the T50 (° C.) and C500 (%) of the example were better than the comparative example 1.

  From the above results, since the loading amounts A and B of Pd per unit weight of the first alumina and the Ce · Zr · La · Y alumina composite were made substantially the same, the Pd has a relatively high specific surface area and heat resistance. It will be supported on high first alumina, and Pd will be supported separately on both alumina and Ce • Zr • La • Y alumina composite, thereby increasing the active sites of Pd, HC and It can be seen that high purification activity of CO can be obtained.

  In addition, since Pd is supported on the Ce · Zr · La · Y alumina composite that releases oxygen in a rich atmosphere, Pd is better because of the active oxygen supplied from the Ce · Zr · La · Y alumina composite. Controlled by the oxidation state, the oxidation purification rate by Pd is increased over a long period of time.

  Next, when the values of T50 (° C.) and C500 (%) of Example and Comparative Example 2 were examined for NOx, as shown in FIG. ) And C500 (%) were both good.

  From the above results, the Ce · Zr · Nd double oxide that releases oxygen in a rich atmosphere is loaded with a large amount of Rh per unit specific surface area. The supported Rh does not directly participate in the reduction reaction of NOx (because active oxygen is supplied from the oxygen storage material Ce · Zr · Nd double oxide), but rather HC steam reforming. It works to promote the reaction, and as a result, increases the degree of generation of active H2 and contributes to increasing the reduction and purification rate of NOx by this H2. That is, Rh supported on Ce · Zr · Nd double oxide can promote the reduction reaction of NO + H2 → N2 + H2O using active H2 generated by the steam reforming reaction of HC + H2O → CO + H2 and CO + H2O → CO2 + H2. Recognize.

  In addition, when A / F is stoichiometric, the loading of Rh on the oxygen storage material (Ce / Zr / Nd double oxide) is effective, and during acceleration when A / F swings to the rich side, alumina (third alumina) The loading of Rh on the surface is effective. That is, when the A / F is stoichiometric, the oxygen storage / release ability of the oxygen storage material (Ce / Zr / Nd double oxide) absorbs the fluctuation of the A / F, and Rh effectively purifies NOx. At the time of acceleration in which A / F swings to the rich side, in the Rh supported on the oxygen storage material (Ce / Zr / Nd double oxide), the Rh surface is in an oxygen-excess state due to the released oxygen. However, although the purification performance tends to decrease, alumina (third alumina) does not have oxygen storage / release ability, so the surface of Rh supported on the alumina (third alumina) is in an oxygen-excess state. Therefore, NOx can be effectively purified. That is, Rh is supported on both the oxygen storage material (Ce / Zr / Nd double oxide) and alumina (third alumina), so that the NOx purification performance is enhanced.

  Here, the amount of Rh supported will be considered. The third alumina has a very large specific surface area after aging as compared with the Ce · Zr · Nd double oxide. For this reason, if the amount of Rh supported per unit specific surface area of the third alumina and Ce · Zr · Nd double oxide is the same, the amount of Rh supported on the third alumina is increased, or Ce · Zr · Either the amount of Rh supported on the Nd double oxide is reduced.

  In the former case, that is, when the amount of Rh supported on the third alumina is increased, the amount of Rh required increases and the cost increases, and even if the third alumina is coated with ZrO 2, Rh is increased. It will be dissolved in the third alumina and the purification activity will be lost.

  On the other hand, in the latter case, that is, when the amount of Rh supported on the Ce · Zr · Nd double oxide is reduced, the A / F window in a rich atmosphere is difficult to expand, and improvement in purification performance cannot be expected.

  Therefore, in the loading amount of Rh per unit specific surface area on the third alumina and Ce · Zr · Nd double oxide, the loading amount D on the Ce · Zr · Nd double oxide is greater than the loading amount C on the third alumina. If it increases, it becomes possible to suppress the problem of the Rh cost increase and the problem of solid solution in the third alumina. And it becomes possible to expand the A / F window in the rich atmosphere, promote the steam reforming reaction, and reduce NOx.

  In addition, since Pd is supported on the lower layer of the upper and lower two catalyst layers, the oxygen storage material having a high Rh-supporting oxygen storage capacity disposed on the upper layer causes the upstream to downstream of the catalyst and from the upper layer of the catalyst. When the exhaust gas flows in the lower layer, A / F fluctuation is suppressed. As a result, the purification performance of the lower layer Pd support material (that is, alumina and oxygen storage material) is improved, and the catalyst performance is improved. Further, sintering and alloying between the lower layer palladium and the upper layer catalyst noble metal, particularly rhodium, are further suppressed. In addition, since palladium is protected by the upper layer, the suppression effect is expected also against S (sulfur) poisoning and P (phosphorus) poisoning. On the other hand, since rhodium is supported on the Ce · Zr · Nd double oxide and the third alumina in the upper layer, the reactivity of the oxygen storage material (Ce · Zr · Nd double oxide) and the reactivity of the steam reforming reaction. And the reduction reaction of nitrogen oxide (NOx) can be promoted.

  Since Pt is supported on the second alumina on which Pd and Rh are not supported, Pt is supported in a dispersed manner with Pd and Rh. Therefore, sintering and alloying hardly occur between these different kinds of catalytic metals.

<Evaluation test 2>
(Test evaluation catalyst)
Experimental Example 1 A catalyst (ZrO ₂ -coated third alumina = 100 g / L, Rh = 0.5 g / L) in which only a Rh / ZrO ₂ -coated third alumina was supported on a honeycomb carrier was prepared.
Experimental Example 2: A catalyst (Ce · Zr · Nd double oxide = 100 g / L, Rh = 0.5 g / L) in which only a Rh / Ce · Zr · Nd double oxide was supported on a honeycomb carrier was prepared.
Experimental Example 3: A catalyst (third alumina = 100 g / L, Rh = 0.5 g / L) in which only Rh / third alumina was supported on a honeycomb carrier was prepared.

(Test evaluation method)
Each catalyst of Experimental Examples 1, 2, and 3 was subjected to thermal aging at 1100 ° C. for 24 hours, and then the NOx purification rate of the simulated gas was measured using the test apparatus shown in FIG. That is, the test apparatus shown in FIG. 10 simulates a state in which the three-way catalyst 21 is interposed in the exhaust passage 20 and allows the simulation gas to flow. Linear oxygen sensors 22, 22 are provided on the inlet side and the outlet side of the catalyst 21. As the catalyst 21, the catalysts of Experimental Examples 1, 2, and 3 after the thermal aging are attached, respectively, and stoichiometric (air / fuel ratio A / F = 14.7) → lean (air / fuel ratio A / F = 17.0) → rich. (Air-fuel ratio A / F = 14.3) → stoichiometric → lean → rich →... Simulated exhaust gas assuming cyclic traveling is circulated to the test apparatus, and the simulated gas is stoichiometric and rich. The NOx purification rate was measured.

(Test evaluation results)
The results are shown in FIG. As illustrated, the NOx purification rate at the time of stoichiometry was in the order of Experimental Example 2> Experimental Example 1> Experimental Example 3. The rich NOx purification rate was in the order of experimental example 1> experimental example 3> experimental example 2.

Considering the result at the time of stoichiometry, in Experimental Example 2, fluctuations in the air-fuel ratio A / F are absorbed by the high oxygen storage and oxygen release ability of the Ce · Zr · Nd double oxide, so that Rh is effectively NOx. While the purification performance is exhibited, in the experimental examples 1 and 3, the NOx purification performance is lower than the experimental example 2 because the ZrO ₂ -coated third alumina or the third alumina does not have oxygen storage and oxygen release ability. This is probably because of this. On the other hand, considering the result at the time of rich, in Experimental Example 2, oxygen is released through the Rh surface due to the oxygen releasing ability of the Ce · Zr · Nd double oxide, the Rh surface becomes oxygen excess, and NOx However, in Experimental Examples 1 and 3, the ZrO ₂ -coated third alumina or the third alumina does not have oxygen storage and oxygen releasing ability, so there is no obstacle as in Experimental Example 2. This is considered to be because the NOx purification performance is not affected.

Therefore, in the catalyst according to the present invention including both elements of Experimental Examples 1 and 2, a high NOx purification performance is obtained by the Rh / Ce · Zr · Nd double oxide during stoichiometry, and when rich, Rh / ZrO ₂ -coated third alumina compensates for the decrease in NOx purification performance of Rh / Ce · Zr · Nd double oxide, which in combination results in high NOx purification performance under a wide range of operating conditions. Can be obtained.

Further, comparing Experimental Example 1 with Experimental Example 3, it can be seen that Experimental Example 1 has a higher NOx purification rate both in stoichiometric and rich conditions. Therefore, it can be seen that the third alumina supporting Rh is preferably coated with ZrO ₂ .

  As described above in detail with reference to specific examples, the present invention is a catalyst that is easily subjected to a high heat load of engine exhaust gas, for example, a catalyst disposed close to the engine or a position relatively close to the engine. In an underfloor catalyst, etc., when a catalyst noble metal is supported on both alumina and an oxygen storage material, a high purification performance can be obtained by optimizing the catalytic reaction by the catalyst noble metal and the function of the oxygen storage material. It has wide industrial applicability in the technical field of exhaust gas purification.

It is a schematic block diagram of the spark ignition type engine of the motor vehicle carrying the three way catalyst which concerns on the best embodiment of this invention. It is the perspective view and partial enlarged view which show the structure of the said three-way catalyst. It is a schematic diagram which shows the structure of the catalyst layer of an Example. It is a detailed table | surface of the structure of the catalyst layer of the said Example. It is a table | surface of the specific surface area before and behind aging of a 1st-3rd alumina and Ce * Zr * Nd double oxide. The amount of Pd supported per unit weight on the first alumina and Ce.Zr.La.Y alumina composite, and the amount of Rh supported per unit specific surface area on the third alumina and Ce.Zr.Nd composite oxide. It is a table | surface which shows. It is a figure which shows the crystal structure of Ce * Zr * Nd double oxide. It is an enlarged view which shows the structure of Ce * Zr * La * Y alumina composite. It is a table | surface which shows the evaluation result of the purification performance of the said Example, the comparative example 1, and the comparative example 2. FIG. It is a schematic diagram of the test apparatus used with the test evaluation method of the evaluation test 2. FIG. It is a time chart which similarly shows the test evaluation result of the evaluation test 2. FIG. 2 is a schematic diagram illustrating a configuration of a catalyst layer disclosed in Patent Document 1. FIG.

Explanation of symbols

1 Engine 10 Catalytic converter 11 Three-way catalyst (exhaust gas purification catalyst)
11a honeycomb carrier 11b catalyst layer

Claims (4)

An exhaust gas purifying catalyst provided on a honeycomb carrier with a catalyst layer containing palladium, alumina, and an oxygen storage material as catalyst noble metals,
The palladium is supported on one kind of alumina and one kind of oxygen storage material,
The exhaust gas purifying catalyst, wherein the supported amount of palladium per unit weight of the alumina and the oxygen storage material is substantially the same. An exhaust gas purifying catalyst provided with a catalyst layer containing rhodium, alumina, and an oxygen storage material as catalyst noble metals on a honeycomb carrier,
The rhodium is supported on one kind of alumina and one kind of oxygen storage material,
The specific surface area of the alumina and oxygen storage material is larger in the former,
The catalyst for purifying exhaust gas, wherein the latter has a larger load of rhodium per unit specific surface area of the alumina and oxygen storage material. In the exhaust gas purifying catalyst according to claim 1 or 2,
The catalyst layer comprises palladium and rhodium as catalyst noble metals,
The catalyst layer is composed of two upper and lower layers,
An exhaust gas purifying catalyst, wherein the palladium is supported on a lower layer and the rhodium is supported on an upper layer. In the exhaust gas purifying catalyst according to any one of claims 1 to 3,
The catalyst layer further comprises platinum as a catalyst noble metal,
The exhaust gas purifying catalyst, wherein the platinum is supported on at least one of another alumina and oxygen storage material on which palladium and rhodium are not supported.

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