JP2015100787A - Production method of catalyst for exhaust gas purification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalyst having high exhaust gas purification performance by carrying Pt preferentially on an alumina layer with a small number of man-hours by a simple method, when carrying Pt on a laminate comprising a zeolite layer and the alumina layer.SOLUTION: A first catalyst later 2 containing zeolite is formed on a carrier 1, and a second catalyst later 3 containing basic alumina is formed on the first catalyst later 2. The first catalyst later 2 and the second catalyst later 3 are impregnated into dinitro diamine nitric acid solution containing Pt, to thereby carry Pt preferentially on the basic alumina.

Description

  The present invention relates to a method for manufacturing an exhaust gas purification catalyst, and more particularly to a method for manufacturing an exhaust gas purification catalyst in which a plurality of catalyst layers are stacked.

An exhaust gas treatment device of a diesel engine is generally constituted by an oxidation catalyst (DOC) and a diesel particulate filter (DPF) disposed downstream thereof. By DOC, hydrocarbon (HC) and carbon monoxide (CO) in the exhaust gas are oxidized and purified, and NO in the nitrogen oxide (NO _x ) is oxidized to NO ₂ . The catalytic reaction heat in the DOC can raise the temperature of the DPF, and the combustion of particulate matter (PM) deposited on the DPF is promoted. The combustion of PM at this time is caused by oxygen having higher oxidation activity than O ₂ in the exhaust gas supplied from the catalyst coated on the DPF. In addition, the combustion reaction is further promoted by the effect of NO ₂ having a strong oxidizing power that flows from the DOC or is generated in the catalyst coated on the DPF. Since the activity of DOC is low immediately after the engine is started, DOC contains zeolite as an HC trap material so that HC is not discharged without being purified.

On the other hand, in order to purify the NO _x, lean NO _x trap catalyst (LNT catalyst) are used in a lean-burn gasoline engines and diesel engines. The LNT catalyst occludes NO _x when the air-fuel ratio of the exhaust gas by _the NO _x storage material is lean. By rich purge modulating the air-fuel ratio of the engine to rich, NO _x emissions from _the NO _x storage materials, and unburned gas (HC, CO, _{H 2)} is reduced of the NO _x by performed. As the NO _x storage material, the use of alkaline earth metals such as barium (Ba), strontium (Sr), and magnesium (Mg) is the mainstream.

  When manufacturing an integrated catalyst in which the DOC catalyst layer and the LNT catalyst layer are laminated on a carrier as described above, a particulate material in which a catalyst metal is supported on a support material that is a material of each layer is prepared, and these are sequentially added. Each layer is formed by coating. In this case, since a process for supporting the catalyst metal on the support material is required for each layer, the number of steps increases.

  In order to solve this problem, a method of impregnating and supporting catalytic metals all at once after laminating a layer containing a support material is conceivable. For example, Patent Document 1 discloses dinitrodiamine containing Pt, which is a catalyst metal, in a laminate formed after laminating a layer containing a support material such as zeolite or metal oxide in the production of a laminated catalyst layer. A method for impregnating a Pt solution is presented.

JP 2010-149097 A

  In order to shorten the manufacturing process, when Pt is impregnated and supported on the laminate formed on the support using the method of Patent Document 1, Pt is supported almost uniformly on all the laminated layers. It becomes. When the support material supporting Pt is different, the catalyst performance is also different. For example, according to the present inventors' extensive research, it is better to use alumina as a support material supporting Pt which is a noble metal than zeolite. It is known that the performance is excellent. For this reason, in a catalyst having a laminated structure of a layer containing zeolite and a layer containing alumina, it is more advantageous as a catalyst that more Pt is supported on the layer containing alumina. However, when the method of Patent Document 1 is used, as described above, Pt is supported on both the zeolite layer and the alumina layer. That is, although Pt can be supported on each layer in one step, Pt cannot be preferentially supported on a desired layer that can sufficiently exhibit the performance of Pt.

  The present invention has been made in view of the above problems, and its purpose is to form a layer containing alumina by a simple method with less man-hours when Pt is supported on a laminate containing zeolite and alumina. An object is to preferentially support Pt so as to obtain a catalyst having high exhaust gas purification performance.

  In order to achieve the above object, in the present invention, basic alumina is used as the alumina as the material for the catalyst layer, and an acidic dinitrodiamine nitric acid solution is used as the raw material for the catalyst metal Pt.

  Specifically, in the method for producing an exhaust gas purifying catalyst according to the present invention, a step of forming a first catalyst layer containing zeolite on a support, and a second step of containing basic alumina on the first catalyst layer. A step of forming a catalyst layer, and a step of preferentially supporting Pt on the basic alumina by impregnating the first catalyst layer and the second catalyst layer with a dinitrodiamine nitric acid solution containing Pt. It is characterized by that.

  According to experiments and research by the present inventors, when Pt is supported on alumina, Pt is supported on alumina with high efficiency by impregnating basic alumina with a Pt solution using an acidic dinitrodiamine nitric acid solution as a solvent. It has been found that it can. For this reason, even if a dinitrodiamine Pt nitric acid solution is impregnated in a laminate in which a zeolite layer and a basic alumina layer are laminated, Pt can be preferentially supported on the basic alumina layer. That is, when the first catalyst layer containing zeolite and the second catalyst layer containing basic alumina are impregnated with dinitrodiamine Pt nitric acid solution, Pt can be preferentially supported on the basic alumina of the second catalyst layer. . As described above, since Pt-supported alumina has higher exhaust gas purification performance than Pt-supported zeolite, a catalyst having excellent exhaust gas purification performance can be obtained by the production method of the present invention. Moreover, in the manufacturing method of this invention, since the carrying | support process of Pt can be performed collectively, a man-hour can be reduced.

In the method for manufacturing the exhaust gas purifying catalyst according to the present invention, to grant NO _x trap function in the catalyst is in the process of supporting Pt on basic alumina, carrying an alkaline earth metal basic alumina with Pt It is preferable.

In this way, since the alkaline earth metal that is the NO _x storage material is impregnated with the dinitrodiamine Pt nitric acid solution, the NO _x trap layer can be produced without increasing the number of steps.

  In the method for producing an exhaust gas purifying catalyst according to the present invention, it is preferable to use alumina containing at least one of lanthanoid and Zr as basic alumina. In this case, it is more preferable to use alumina containing La as basic alumina. Since alumina containing La has particularly high heat resistance, the heat resistance of the catalyst can be improved by containing La.

  In the method for producing an exhaust gas purifying catalyst according to the present invention, the second catalyst layer includes a lower layer containing the basic alumina and ceria and an upper layer containing the basic alumina containing more Rh than the lower layer. It is preferable to have a laminated structure.

Rh contributes to the steam reforming reaction, and H ₂ is generated by this reaction. Therefore, the reduction and purification of NO _x can be promoted. Therefore, it is possible to obtain _the NO _x reduction purification performance high catalyst by the inclusion of Rh. In addition, when the alkaline earth metal is impregnated into the catalyst layer, a second catalyst layer functioning as a NO _x trap layer containing Rh is formed on the first catalyst layer functioning as an oxidation catalyst layer containing zeolite. be able to. In this catalyst, when the catalyst temperature is low, HC in the exhaust gas is adsorbed by the zeolite in the oxidation catalyst layer, and when the catalyst temperature rises, HC is released from the zeolite. The released HC is oxidized and purified by the catalytic metal whose activity is increased by the temperature rise together with the CO in the exhaust gas. Further, when the air-fuel ratio of the exhaust gas is lean, NO _x is occluded in _the NO _x storage material of the NO _x trap layer, from _the NO _x storage material when the air-fuel ratio becomes the stoichiometric air-fuel ratio near or rich NO _x is released. Rh passes the upper layer of the NO upper side of the _x trap layer, that is, provided in the exhaust gas passage side, NO _x trap layer when NO _x is released from _the NO _x storage material in an exhaust gas passage side, the upper layer Since Rh contains a large amount of Rh, the released NO _x can be efficiently reduced and purified.

  In the method for producing an exhaust gas purifying catalyst according to the present invention, the first catalyst layer includes a lower layer containing the basic alumina and ceria, and an upper layer containing a zeolite provided on the lower layer. It is preferable that

  If it does in this way, the lower layer containing basic alumina and ceria as an oxidation catalyst layer, and the upper layer containing the zeolite provided on this lower layer will be formed, and many Pt will be carry | supported by the lower layer. Thereby, since the layer which carries many Pt is provided in the upper and lower layers of the layer containing zeolite, HC released from the zeolite can be oxidized and purified more efficiently.

  According to the method for producing an exhaust gas purifying catalyst according to the present invention, when Pt is supported on a laminated structure of a zeolite layer and an alumina layer, Pt is preferentially applied to the alumina layer with a small number of steps and a simple method. It is possible to obtain a catalyst having high exhaust gas purification performance.

1 is a cross-sectional view showing a part of an exhaust gas purifying catalyst according to an embodiment of the present invention. It is sectional drawing which shows the catalyst layer structure of the exhaust gas purification catalyst which concerns on one Embodiment of this invention. It is sectional drawing which shows the modification of the catalyst layer structure of the exhaust gas purification catalyst which concerns on one Embodiment of this invention. It is a graph which shows the Pt crystallite diameter of Pt carry | supported by zeolite, basic alumina, and those mixtures. It is a graph which shows the change of the total HC density | concentration of the gas which flows out from a catalyst in the evaluation test of HC purification performance, and a catalyst inlet temperature. It is a graph which shows the HC purification rate of the Example and comparative example of this invention. Is a graph showing changes in the concentration of NO _x gases flowing out of the catalyst in the evaluation test of the NO _x purification performance. The _the NO _x purification rate of the Examples and Comparative Examples of the present invention is a graph showing.

  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 method of application, or its application.

(About catalyst structure)
First, the configuration of an exhaust gas purifying catalyst according to an embodiment of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a cross-sectional view showing a part of an exhaust gas purification catalyst according to the present embodiment, and FIG. 2 is a cross-sectional view showing a catalyst layer structure of the exhaust gas purification catalyst.

As shown in FIGS. 1 and 2, the exhaust gas purifying catalyst according to the present embodiment is an exhaust gas purifying catalyst discharged from a diesel engine (not shown), and is oxidized on the cell wall 1 of the honeycomb carrier. becomes DOC layer 2, and NO _x LNT layer 3 is a trapping layer as a catalyst layer are sequentially formed, the space inside is in the exhaust gas passage 4. The honeycomb carrier has a hexagonal cell honeycomb structure whose cell cross-sectional shape is a hexagon. In FIG. 1, the catalyst layer is drawn only in one cell for simplification of the drawing, but the catalyst layer is formed in all cells.

In the present embodiment, the DOC layer 2 is formed on the cell wall 1 of the carrier, which contains zeolite which is an HC trap material and contains Pt as a catalyst metal. LNT layer 3 formed on the DOC layer 2 comprises a basic alumina, further comprising a catalytic metal such as _the NO _x storage material and Pt, and Rh. In the LNT layer 3, the NO _x storage material and the catalyst metal are supported on basic alumina. The basic alumina may be a lanthanoid such as La, Ce, or Nd or a complex oxide with Zr.

Further, as shown in FIG. 2, the LNT layer 3 may have a two-layer structure. As described above, the lower LNT layer 3a and the upper LNT layer 3b contain a basic alumina material, and the upper LNT layer 3b has Rh as a catalyst metal in the basic alumina as compared with the lower LNT layer 3a. Contains more. As described above, Rh contributes to the steam reforming reaction, and H ₂ is generated by this reaction. Therefore, the reduction purification of NO _x can be promoted. Therefore, NO _x in which the air-fuel ratio is stored in the NO _x storage material of LNT layer 3 when the lean exhaust gas, the exhaust gas passage 4 through the upper layer 3b from LNT layer 3 when a rich state to be released, it is possible to efficiently reduce and purify NO _x by the action of Rh contained in the upper layer 3b of the NO _x trap layer.

  Here, the catalyst in which the DOC layer 2 has a single layer structure has been described. However, as shown in FIG. 3, the DOC layer 2 may have a two-layer structure of a lower DOC layer 2a and an upper DOC layer 2b. The lower DOC layer 2a contains a mixture of activated alumina and ceria-based material, and Pt is supported on them. The upper DOC layer 2b contains zeolite. If it does in this way, since zeolite is arrange | positioned above the DOC layer 2, it becomes advantageous to adsorption | suction of HC in exhaust gas. Further, HC desorbed from the zeolite is not only purified by the catalyst metal supported on the LNT layer, but also partially diffuses to the layer below the layer containing the zeolite. It is efficiently purified by the supported catalytic metal.

  In addition, it is preferable that the average particle diameter (D50) of the zeolite which is a structural component of the DOC layer 2 is 0.5 μm or more and 4.8 μm or less. If the average particle size of the zeolite is too large, the exposed surface area of the particles is reduced, and the amount of HC adsorbed is reduced.

(About catalyst production method)
Next, a method for manufacturing the exhaust gas purifying catalyst according to the present embodiment will be described.

  First, in order to form the DOC layer 2 on the cell wall 1 of the honeycomb carrier, zeolite is mixed with a binder and water to form a slurry. This slurry is coated on the cell wall 1 of the honeycomb carrier and dried. Thereby, the precursor layer (first catalyst layer) of the DOC layer 2 before the catalyst metal is supported is formed.

In addition, when forming the DOC layer 2 of the two-layer structure shown in FIG. 3, before forming the layer containing the zeolite, the basic alumina and the ceria-based material (Ce _0.9 Pr _0.1 O ₂ ) The mixture is prepared and slurried. First, the slurry is coated on the cell wall 1 of the support and dried, and then the slurry containing the zeolite is coated thereon and dried to dry the cell wall of the support. A precursor layer of the DOC layer 2 before the catalyst metal having a two-layer structure is supported on 1 is formed. As basic alumina, La-containing alumina containing 4% by mass of La ₂ O ₃ can be used.

Next, on the precursor layer of the DOC layer 2 formed as described above, the precursor layer (second catalyst layer) of the LNT layer 3 before supporting the catalyst metal to be the LNT layer 3 later is formed. . Here, formation of the precursor layer of the LNT layer 3 having the two-layer structure shown in FIGS. 2 and 3 will be described. In order to form the precursor layer of the LNT layer 3, first, basic alumina and a ceria-based material (Ce _0.9 Pr _0.1 O ₂ ) are mixed, and a binder and water are added to the mixture. Stir to form a slurry. This slurry is coated on the precursor layer of the DOC layer 2 and dried to form the precursor layer of the lower LNT layer 3a. Next, a binder and water are added to basic alumina and stirred to form a slurry. This slurry is coated on the precursor layer of the lower LNT layer 3a and dried. Thereby, the precursor layer of the upper LNT layer 3b is formed, and the precursor layer of the LNT layer 3 having a two-layer structure is formed. In order to contain more Rh in upper LNT layer 3b, it is preferable that Rh is contained in advance in basic alumina, for example, by coprecipitation.

Next, a mixed solution of Pt and Rh, which are catalytic metals, and an alkaline earth metal, which is a NO _x storage material, is prepared, and this carrier is impregnated with the carrier on which each of the precursor layers is formed. Specifically, dinitrodiamine Pt nitric acid solution and dinitrodiamine Rh nitric acid solution are used as raw materials for Pt and Rh. As alkaline earth metals, for example, Ba and Sr acetates can be used. These are mixed to prepare a mixed solution diluted with ion-exchanged water. The mixed solution is impregnated with the carrier on which each of the precursor layers is formed, and the carrier is dried and fired. Alkaline earth metals such as Ba and Sr have a high affinity with alumina, and many of them are supported on the precursor of the LNT layer 3. Thus, LNT layer 3 is NO _x trap layer from a precursor layer of the LNT layer 3 is formed. Further, the catalyst metal is supported on the precursor layer of the DOC layer 2 to form the DOC layer 2. As will be described in detail later, in this impregnation step, Pt is preferentially supported not on zeolite but on basic alumina. In the above production method, the drying can be performed by, for example, maintaining the temperature at about 100 ° C. to 250 ° C. for a predetermined time in an air atmosphere. Moreover, baking can be performed by hold | maintaining at the temperature of about 400 to 600 degreeC for several hours, for example in air | atmosphere atmosphere.

(Comparison between Pt-supported zeolite and Pt-supported alumina)
Next, the result of examining the difference between zeolite and basic alumina regarding the loading of Pt will be described.

  First, the crystallite diameter of Pt when Xt is supported on zeolite and basic alumina using tetraammine Pt hydrochloride solution (basic solution) or dinitrodiammine Pt nitric acid solution (acidic solution) Measured by diffraction method. The crystallite diameter was calculated from Scherrer's equation using the half width of the Pt diffraction peak. Note that β zeolite was used as the zeolite, and the above La-containing alumina was used as the basic alumina.

As preparation of the measured sample, first, equal amounts of β zeolite and basic alumina were prepared, and tetraammine Pt hydrochloride solution (basic solution) or dinitrodiammine Pt nitric acid solution (acidic solution) was used for them. Then, 2% by weight of Pt was supported by evaporation to dryness. Specifically, water was added to β zeolite or basic alumina and stirred to form a slurry, and a tetraammine Pt hydrochloride solution or a dinitrodiammine Pt nitric acid solution was added dropwise while stirring the slurry. Thereafter, stirring was continued while heating, and Pt was supported on β zeolite or basic alumina by completely evaporating water. After the obtained sample was coated on the carrier, an aging treatment was performed in which the sample was kept at a temperature of 800 ° C. for 24 hours in a gas atmosphere of 2% O ₂ , 10% H ₂ O, and the balance N ₂ . Thereafter, the Pt crystallite diameter was measured for each sample by the above method. The result is shown in FIG.

  As shown in FIG. 4, when Pt is supported using a tetraammine Pt hydrochloride solution, the Pt crystallite diameter is smaller when supported on β zeolite than when supported on basic alumina. I understand that. Also, when Pt is supported using a dinitrodiammine Pt nitric acid solution, the difference between them is reduced, but similarly, the crystallite size is better supported by β zeolite than supported by basic alumina. Get smaller. Therefore, Pt is supported on the mixture obtained by mixing β zeolite and basic alumina at a weight ratio of 1: 1 by the same method as described above, and the Pt crystallite diameter at that time is measured. It is indirect to determine whether more Pt is supported by β zeolite or basic alumina by comparing whether it is supported by β zeolite or when it is supported by basic alumina. Judged. The amount of the mixture was the same as that of the above β zeolite and basic alumina alone.

  As shown in FIG. 4, when Pt is supported on a mixture of β zeolite and basic alumina using a tetraammine Pt hydrochloride solution, the Pt crystallite size is equal to the Pt crystallite size supported on β zeolite. It became almost the same. Therefore, it can be seen that Pt is preferentially supported on the β zeolite when a tetraammine Pt hydrochloride solution is used.

  On the other hand, when Pt was supported on a mixture of β zeolite and basic alumina using a dinitrodiammine Pt nitric acid solution, the Pt crystallite diameter was larger than the Pt crystallite diameter supported on basic alumina. became. From this result, it is considered that Pt is preferentially supported on basic alumina instead of β zeolite, and it is included in the mixture that the crystallite size is larger than that of single basic alumina. Since basic alumina is mixed with β-zeolite at a ratio of 1: 1, it is considered that the basic alumina is supported in a small area because it is half the amount of basic alumina.

  From the above, it was suggested that when a dinitrodiammine Pt nitric acid solution is used, Pt can be preferentially supported on basic alumina instead of β zeolite.

  Next, Table 1 shows the results of measuring the specific surface areas of the zeolite powder and the acidic alumina and basic alumina powders. Note that β zeolite was used as the zeolite, pure alumina not containing La was used as the acidic alumina, and the above La-containing alumina was used as the basic alumina.

  As shown in Table 1, when the specific surface area of β zeolite was compared with the specific surface area of acidic and basic alumina, the specific surface area of β zeolite was smaller. Further, there was no difference in specific surface area between acidic alumina and basic alumina. It is conceivable that the alumina with a large specific surface area tends to have a smaller Pt crystallite diameter, but it is not only the physical characteristics such as the specific surface area that are smaller than the zeolite, but the details are not clear. Although it is not, it is considered that the chemical interaction of Pt has an influence on zeolite and alumina having different acidity and basicity.

  Next, each light-off temperature (T50 (° C)) for CO and HC purification is measured for Pt-supported zeolite with Pt supported on β zeolite and Pt-supported alumina with Pt supported on basic alumina. The results will be described. T50 (° C.) is the catalyst inlet gas temperature when the temperature of the model gas flowing into the catalyst is gradually increased from room temperature and the purification rate of each component of CO and HC reaches 50%.

Pt-supported zeolite (Pt / β zeolite) and Pt-supported alumina (Pt / Al ₂ O ₃ ) are obtained by supporting Pt on zeolite or basic alumina using a dinitrodiamine Pt nitric acid solution by evaporation to dryness. It was. Specifically, water was added to β zeolite or basic alumina and stirred to form a slurry, and the dinitrodiamine Pt nitric acid solution was added dropwise while stirring the slurry. Thereafter, stirring was continued while heating, and Pt was supported on β zeolite or basic alumina by completely evaporating water. The supported amount of Pt was 4 g / L. The obtained sample was a cordierite hexagonal cell honeycomb having a cell wall thickness of 4.5 mil (1.143 × 10 ⁻¹ mm) and 400 cells per square inch (645.16 mm ² ). After coating on a carrier (diameter 25.4 mm, length 50 mm) (wash coat amount was 100 g / L), O ₂ was 2%, H ₂ O was 10%, and the balance was N _2. An aging treatment was performed at a temperature of 0 ° C. for 24 hours. Thereafter, the catalyst was attached to a model gas flow reactor, and model gas for evaluating CO and HC purification performance was introduced into the honeycomb catalyst. The composition of the model gas was C ₃ H ₆ ; 200 ppmC, CO; 400 ppm, NO; 100 ppm O ₂ ; 10%, H ₂ O; 10%, the remaining N ₂ , and the space velocity was 72000 / h. The measurement results are shown in Table 2.

As shown in Table 2, when T50 of Pt / β zeolite and Pt / Al ₂ O ₃ were compared, both CO and HC were lower in Pt / Al ₂ O ₃ . That is, it can be seen that Pt / Al ₂ O ₃ can purify CO and HC at a lower temperature than Pt / β zeolite, and therefore has higher exhaust gas purification performance.

  As described above, according to the method of this embodiment in which Pt is impregnated and supported in a lump using a dinitrodiamine Pt nitric acid solution on a laminate of a layer containing zeolite and a layer containing basic alumina, Since Pt can be preferentially supported on basic alumina having excellent oxidation activity, a catalyst with high purification performance of exhaust gas components can be obtained with a small number of man-hours.

Examples for explaining in detail the method for producing an exhaust gas purifying catalyst according to the present invention are shown below. In this example, the cordierite hexagonal cell honeycomb carrier having a cell wall thickness of 4.5 mil (1.143 × 10 ⁻¹ mm) and 400 cells per square inch (645.16 mm ² ). An exhaust gas purification catalyst was prepared by the above method for producing an exhaust gas purification catalyst using (diameter 25.4 mm, length 50 mm). The prepared catalyst was evaluated for HC purification performance and NO _x purification performance.

Below, the structure of the exhaust gas purifying catalyst which concerns on Example 1 and Comparative Examples 1-5 is demonstrated. In Example 1, a catalyst layer having both a DOC layer and an LNT layer as shown in FIG. For forming the lower DOC layer, a mixed powder of 30 g / L of basic alumina material and 30 g / L of ceria-based material (Ce _0.9 Pr _0.1 O ₂ ) is prepared, and a binder is added to this powder. In addition, it was coated on a carrier. In order to form the upper DOC layer, a binder was added to 100 g / L of the zeolite material and coated on the lower DOC layer. In order to form the lower LNT layer, a mixed powder of 30 g / L basic alumina material and 30 g / L ceria-based material (Ce _0.9 Pr _0.1 O ₂ ) is prepared, and a binder is added to this powder. In addition, this was coated over the upper DOC layer. In order to form an upper LNT layer, a precursor containing 0.4 g / L of Rh in 2 g / L of CeO ₂ by coprecipitation is formed, and 18 g / L of basic alumina material is added thereto, After sufficiently stirring, the mixture was centrifuged, and the resulting gel cake was dried and baked to obtain Rh-doped CeO ₂ / alumina powder. A binder was added to the powder and this was coated on the lower LNT layer. After preparing these four layers, dinitrodiamine Pt nitric acid solution and dinitrodiamine Rh nitric acid solution for supporting catalytic metals 0.38 g / L Pt and 0.01 g / L Rh were diluted with ion-exchanged water. And a solution obtained by diluting each acetate for supporting 7.5 g / L Ba and 2.5 g / L Sr, which are NO _x storage materials, with ion-exchanged water. Prepared. After impregnating the support having the above four-layer structure in this impregnation solution, the support was dried at 150 ° C. and calcined at 500 ° C. for 2 hours to obtain the catalyst of Example 1.

Unlike Example 1, the catalysts of Comparative Examples 1 to 5 using tetraammine Pt hydrochloride solution (basic solution) and tetraammine Rh hydrochloride solution (basic solution) as raw materials for Pt and Rh were used. Produced. In Example 1, the catalyst metal and the NO _x storage material were impregnated in the respective layers at once, but in Comparative Examples 1 to 4, the material of each layer was impregnated and supported before coating of the respective layers.

Specifically, in Comparative Example 1, the lower DOC layer is prepared by mixing 30 g / L of basic alumina material and 30 g / L of ceria-based material (Ce _0.9 Pr _0.1 O ₂ ), A binder was added to this powder, and this was coated on a carrier. The upper DOC layer is a powder in which 4.0 g / L Pt and 0.1 g / L Rh are previously supported on a zeolite material of 100 g / L by evaporation to dryness. did. The support by the evaporation to dryness method was obtained by mixing each tetraammine hydrochloride solution (basic solution) of Pt and Rh with zeolite to form a slurry, followed by evaporation to dryness and baking at 500 ° C. for 2 hours. The lower LNT layer was prepared by mixing 30 g / L of basic alumina material and 30 g / L of ceria-based material (Ce _0.9 Pr _0.1 O ₂ ), and adding 22.5 g / L to this powder. Ba and 7.5 g / L of Sr were supported by evaporation to dryness, and a binder was added to this powder to coat the upper DOC layer. For the loading by the evaporation to dryness method, Ba acetate and Sr acetate were dissolved in ion exchange water, mixed with the mixed powder to form a slurry, evaporated to dryness, and fired at 500 ° C. for 2 hours. The upper LNT layer was formed by forming a precursor containing 0.4 g / L of Rh in 2 g / L of CeO ₂ by coprecipitation, and 18 g / L of basic alumina material was added thereto and stirred sufficiently. The gel-like cake obtained after centrifugation was dried and fired to obtain Rh-doped CeO ₂ / alumina powder. The powder was loaded with 7.5 g / L Ba and 2.5 g / L Sr by evaporation to dryness in the same manner as described above. A binder was added to the obtained powder, and this was coated on the lower LNT layer. did. After providing the above four layers on the carrier, the catalyst of Comparative Example 1 was obtained by calcining at 500 ° C. for 2 hours. That is, the catalyst of Comparative Example 1 is “amount of noble metal supported on zeolite”: “alumina material + amount of noble metal supported on ceria material” = 1: 0.

  The catalyst of Comparative Example 2 differs from Comparative Example 1 only in that Pt and Rh are supported on each layer other than the upper DOC layer (layer containing zeolite). Specifically, 0.86 g / L Pt and 0.021 g / L Rh are supported on the lower DOC layer, 2.0 g / L Pt and 0.05 g / L Rh are supported on the upper DOC layer, The lower LNT layer supported 0.86 g / L Pt and 0.021 g / L Rh, and the upper LNT layer supported 0.29 g / L Pt and 0.007 g / L Rh. That is, the catalyst of Comparative Example 2 is “amount of noble metal supported on zeolite”: “amount of noble metal supported on alumina material + ceria material” = 1: 1.

  The catalyst of Comparative Example 3 also differs from Comparative Example 1 only in that Pt and Rh were supported on each layer other than the upper DOC layer (layer containing zeolite). Specifically, 1.14 g / L Pt and 0.029 g / L Rh are supported on the lower DOC layer, 1.33 g / L Pt and 0.03 g / L Rh are supported on the upper DOC layer, The lower LNT layer supported 1.14 g / L Pt and 0.029 g / L Rh, and the upper LNT layer supported 0.38 g / L Pt and 0.01 g / L Rh. That is, the catalyst of Comparative Example 3 is “Noble metal loading on zeolite”: “Alumina material + Noble metal loading on ceria material” = 1: 2.

  The catalyst of Comparative Example 4 also differs from Comparative Example 1 only in that Pt and Rh are not supported on the upper DOC layer (the layer containing zeolite) but are supported on each layer other than the upper DOC layer. Specifically, 1.71 g / L of Pt and 0.043 g / L of Rh are carried on the lower DOC layer, 1.71 g / L of Pt and 0.043 g / L of Rh are carried on the lower LNT layer, The upper LNT layer was loaded with 0.57 g / L Pt and 0.014 g / L Rh. That is, in the catalyst of Comparative Example 4, “amount of noble metal supported on zeolite”: “alumina material + amount of noble metal supported on ceria material” = 0: 1.

The catalyst of Comparative Example 5 was obtained by forming a four-layer structure on a carrier in the same manner as in Example 1, and then impregnating and supporting the catalyst metal and NO _x storage material in each layer. However, compared with Example 1, only the tetraammine Pt hydrochloride solution (basic solution) and the tetraammine Rh hydrochloride solution (basic solution) were used as raw materials for Pt and Rh.

The HC purification performance measurement test and the NO _x purification rate measurement test performed on the catalyst of Example 1 and the catalysts of Comparative Examples 1 to 5 and the results thereof will be described below.

In the measurement test of the HC purification performance, first, for each of the honeycomb catalysts of Example 1 and Comparative Examples 1 to 5, 750 ° C. in a gas atmosphere of 2% O ₂ , 10% H ₂ O and the balance N _2. An aging treatment was carried out at this temperature for 24 hours. The honeycomb catalyst was attached to a model gas flow reactor, the catalyst inlet gas temperature was maintained at 100 ° C. with N ₂ gas flowing through the honeycomb catalyst, and then a model gas for evaluating HC purification performance was introduced.

Model gas composition, n- octane 600PpmC, ethylene 150PpmC, propylene 50PpmC, CO is 1500 ppm, NO is 30 ppm, _{O 2} 10%, _{H 2} O 10%, balance being _{N 2,} space velocity 72000 / h.

  The catalyst inlet gas temperature was raised from the time when 2 minutes passed after the start of the model gas introduction, and the total HC concentration (THC) of the gas flowing out from the honeycomb catalyst was measured. An example of the result is shown in FIG.

  Since the catalyst temperature is low for a while after the introduction of the model gas, HC in the model gas is adsorbed on the zeolite. Therefore, the THC of the outflow gas is lower than 800 ppmC, which is the THC of the model gas. Thus, as the catalyst temperature rises, the amount of HC adsorbed by the zeolite gradually decreases. When the catalyst inlet gas temperature is close to 200 ° C., the amount of HC desorbed is larger than the amount of HC adsorbed on the zeolite, and THC increases rapidly and becomes higher than 800 ppmC. As the catalyst temperature rises, the catalyst becomes active, and purification of the desorbed HC by the DOC layer starts. For this reason, THC decreases rapidly and becomes lower than 800 ppmC.

  Then, the HC purification rate from the start of model gas introduction until the gas temperature reached 300 ° C. was determined for each of the honeycomb catalysts of Example 1 and Comparative Examples 1 to 5. The HC purification rate was calculated by subtracting the HC desorption amount (C) from the sum of the THC reduction amount (A) accompanying HC adsorption and the THC reduction amount (B) accompanying HC purification shown in FIG. The result is shown in FIG.

  As shown in FIG. 6, when Example 1 and Comparative Examples 1 to 5 are compared, it can be seen that the catalyst of Example 1 has a higher HC purification rate than the catalysts of Comparative Examples 1 to 5.

On the other hand, in the measurement test of NO _x storage performance, the honeycomb catalyst of Example 1 and Comparative Examples 1 to 5 was subjected to the same aging treatment as in the case of the HC purification rate measurement, and then the honeycomb catalyst was used as a model gas. Attached to the flow reactor. The catalyst inlet gas temperature is maintained at 200 ° C. with the air-fuel ratio rich model gas flowing through the honeycomb catalyst, and the model gas is switched to the air-fuel ratio lean model gas while maintaining the temperature. After passing, the air-fuel ratio rich model gas was switched.

An example of the result of measuring the NO _x concentration of the gas flowing out of the honeycomb catalyst is shown in FIG. Immediately after the model gas is switched from rich to lean, the NO _x concentration increases with time, and the NO _x concentration of the model gas gradually approaches 220 ppm as the NO _x storage amount approaches saturation. When the model gas is switched from lean to rich, NO _x is released from the NO _x storage material, but the reduction of NO _x by Pt and Rh is abruptly caused by the supply of reducing agents (HC and CO) due to the switching to rich. to proceed, NO _x concentration of the effluent gas decreases rapidly.

Based on the NO _x reduction amount by _the NO _x storage lean 180 seconds and (A) NO _x reduction amount by _the NO _x reduction rich 10 seconds and (B) shown in FIG. 7, the _the NO _x purification rate at 190 seconds total Asked. Similarly, the catalyst inlet temperature was set to 250 ° C., and the average NO _x purification rate in total for 190 seconds was obtained.

The composition of the rich model gas is 220 ppm NO, 3400 ppm HC, 1.0% CO, 0.5% O ₂ , 6% CO ₂ , 10% H ₂ O and the balance N ₂ . The composition of the lean model gas is 220 ppm NO, 400 ppm HC, 0.15% CO, 10% O ₂ , 6% CO ₂ , 10% H ₂ O, and the balance N ₂ . The results of the NO _x storage performance measurement test are shown in FIG.

As shown in FIG. 8, when comparing the Comparative Examples 1-5 Example 1, towards the catalyst in Example 1, it can be seen that higher _the NO _x purification rate than the catalyst of Comparative Example 1-5.

  From the above results, the catalyst of Example 1 obtained by impregnating and supporting a dinitrodiamine Pt nitric acid solution, a dinitrodiamine Rh nitric acid solution and an alkaline earth metal all together on a four-layer structure containing basic alumina was obtained from Pt and Rh. It was suggested that the exhaust gas component purification performance is higher than the catalysts of Comparative Examples 1 to 5 using these tetraammine hydrochloride solutions as raw materials.

1 Carrier (cell wall)
2 DOC (the DOC layer) (oxidation catalyst) layer 2a (the DOC layer) underlying 2b (the LNT layer) layer 3 LNT (lean NO _x traps) layer 3a (the LNT layer) lower 3b layer 4 exhaust gas passage

Claims (6)

Forming a first catalyst layer containing zeolite on a support;
Forming a second catalyst layer containing basic alumina on the first catalyst layer;
And a step of preferentially supporting the Pt on the basic alumina by impregnating the first catalyst layer and the second catalyst layer with a dinitrodiamine nitric acid solution containing Pt. A method for producing a gas purification catalyst.   The method for producing an exhaust gas purifying catalyst according to claim 1, wherein, in the step of supporting the Pt on the basic alumina, an alkaline earth metal is supported on the basic alumina together with the Pt.   The method for producing an exhaust gas purification catalyst according to claim 1 or 2, wherein alumina containing at least one of lanthanoid and Zr is used as the basic alumina.   The method for producing an exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein alumina containing La is used as the basic alumina.   The said 2nd catalyst layer is taken as the laminated structure which has the lower layer containing the said basic alumina and ceria, and the upper layer containing the said basic alumina in which Rh contained more than this lower layer. The manufacturing method of the exhaust gas purification catalyst of any one of -4.   The said 1st catalyst layer is taken as the laminated structure containing the lower layer containing the said basic alumina and ceria, and the upper layer containing the zeolite provided on this lower layer of Claim 1-5 characterized by the above-mentioned. A method for producing an exhaust gas purifying catalyst according to any one of the preceding claims.

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