JP2005118742A - Manufacturing method of exhaust gas clarification catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing an exhaust gas clarification catalyst that is highly endurable and a NO<SB>x</SB>occlusion-reduction type. <P>SOLUTION: The manufacturing method of the exhaust gas clarification comprises the following processes; (a) a catalyst carrying layer is formed on a carrier substrate; (b) a noble metal catalyst is impregnated into and supported by the catalyst carrying layer; (c) a catalyst carrying layer is formed on the catalyst carrying layer by which the noble metal catalyst is supported; (d) the noble metal catalyst is impregnated into and supported by the catalyst carrying layer; and (e) the NO<SB>x</SB>occluding material entirely impregnated and supported on the layers. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing an exhaust gas purifying catalyst. More specifically, the present invention relates to a method for producing a NOx occlusion reduction type exhaust gas purifying catalyst that purifies exhaust gas discharged from an internal combustion engine such as an automobile.

  Conventionally, as a catalyst for exhaust gas purification of automobiles, a three-way catalyst for purifying by simultaneously oxidizing CO and HC in exhaust gas and reducing NOx has been used. As such a three-way catalyst, for example, a carrier layer made of γ-alumina is formed on a carrier substrate such as cordierite, and a catalytic noble metal such as platinum (Pt), palladium (Pd), rhodium (Rh) is formed on this carrier layer. A material that supports is widely known.

On the other hand, in recent years, from the viewpoint of protecting the global environment, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem. As a solution, so-called lean burn that burns fuel in an oxygen-excess atmosphere. Has been proposed. In this lean burn, since the fuel consumption is improved, the amount of fuel used is reduced, and as a result, the generation of CO ₂ as combustion exhaust gas can be suppressed.

  However, the conventional three-way catalyst is one that simultaneously oxidizes, reduces, and purifies CO, HC, NOx in the exhaust gas when the air-fuel ratio (A / F) is the stoichiometric air-fuel ratio (stoichiometric). In an oxygen-excess atmosphere, the oxidation reaction for purifying CO and HC is active, while the reduction reaction for purifying NOx is inactive, and NOx cannot be purified.

  Therefore, in lean burn, a system has been developed that purifies NOx using exhaust gas as a reducing atmosphere by always burning under lean conditions with excess oxygen and temporarily changing to stoichiometric to rich conditions. In this system, a NOx occlusion reduction type exhaust gas purification catalyst using a NOx occlusion material that occludes NOx in a lean atmosphere and releases NOx occluded in a stoichiometric to rich atmosphere has been proposed. If such a catalyst is used, the air-fuel ratio is controlled from the lean side so as to change from the lean side to the stoichiometric to rich side, so that NOx is occluded by the NOx occlusion material on the lean side and released on the stoichiometric to rich side. Further, since it is purified by reacting with reducing components such as HC and CO, NOx can be efficiently purified even with exhaust gas from a lean burn engine.

  However, the fuel contains a small amount of a sulfur component, which is oxidized during combustion or oxidized on the catalyst to generate SOx. This SOx is acidic, while the NOx occlusion material is basic, so SOx reacts with the NOx occlusion material to form sulfate. As a result, the NOx storage capacity of the NOx storage material is lost, and the NOx purification capacity is reduced. This phenomenon is known as sulfur poisoning of NOx storage materials.

  In order to solve this problem, a catalyst has been proposed in which the support layer has a two-layer structure of an upper layer and a lower layer, the Pt concentration in the upper layer is higher than the Pt concentration in the lower layer, and SOx is effectively supplemented in the upper layer (for example, , See Patent Document 1).

JP 2001-70790 A

  However, in the NOx occlusion reduction type exhaust gas purifying catalyst carrying the NOx occlusion material, the NOx occlusion material is initially carried on the entire carrying layer, but is scattered to the outside as the catalyst deteriorates, or the substrate It tends to move in the direction. This tendency is noticeable especially in potassium among NOx storage materials. On the other hand, since the noble metal catalyst is mainly supported in the vicinity of the surface of the supporting layer, the distance between the noble metal catalyst and the NOx storage material increases due to the movement of the deteriorated NOx storage material, and it is difficult to perform the NOx storage reduction reaction. There is a problem of becoming.

  On the other hand, in the catalyst described in Patent Document 1, Pt is supported not only in the vicinity of the surface but also in the lower layer of the support layer on the side of the support base material, but this lower layer preloads Pt on alumina powder, The slurry containing the Pt-supported alumina powder is formed by wash-coating the substrate. However, since it is necessary to perform calcination when producing this Pt-supported alumina powder, there is a problem that the activity of Pt may not be sufficient in the obtained catalyst.

  Accordingly, an object of the present invention is to provide a method for producing a NOx occlusion reduction type exhaust gas purifying catalyst that exhibits sufficient performance even after deterioration.

In order to solve the above problems, according to the present invention, the following steps are performed.
(a) forming a catalyst support layer on a support substrate;
(b) impregnating and supporting a noble metal catalyst on the catalyst supporting layer,
(c) forming a catalyst supporting layer on the catalyst supporting layer supporting the noble metal catalyst;
(d) impregnating and supporting a noble metal catalyst on the catalyst supporting layer; and
(e) A method for producing an exhaust gas purifying catalyst comprising impregnating and supporting a NOx occlusion material as a whole is provided.

  According to the method of the present invention, since the noble metal catalyst is supported not only on the surface of the support layer but also on the inside by impregnation support, even if the NOx occlusion material moves in the direction of the substrate after the deterioration of the catalyst, The close arrangement is maintained, and NOx can be occluded and released.

  The present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the steps of the method of the present invention. In the method of the present invention, first, a catalyst support layer 12 is formed on a support substrate 11 (FIG. 1 (a)). As the carrier substrate 11, a monolith carrier substrate made of heat-resistant ceramics such as cordierite, a metal carrier substrate made of metal foil, and the like are used. The catalyst support layer 12 is composed of an oxide porous body. As such an oxide porous body, alumina, silica, zirconia, silica-alumina, zeolite, or the like is used. The catalyst support layer 12 can be formed by a conventional general method. For example, an oxide such as alumina is added with a binder such as alumina sol and water having an appropriate viscosity, and these are mixed. It is formed by preparing a slurry, immersing the carrier substrate in this slurry, wash-coating the oxide on the surface of the substrate, drying and firing. The thickness (coat amount) of the catalyst support layer 12 is preferably 50 to 200 g / L.

  Next, the noble metal catalyst 13 is supported on the catalyst support layer 12 thus formed (FIG. 1B). Examples of the noble metal catalyst include noble metals conventionally used as a three-way catalyst, such as platinum, rhodium, palladium, iridium, silver, and the like, and one or more of these can be used. The amount of the noble metal catalyst supported is preferably 0.1 to 10 wt% with respect to the catalyst supporting layer. If the amount is less than 0.1 wt%, sufficient catalytic activity cannot be obtained, and if it exceeds 10 wt%, the activity is only slightly improved and only expensive. The noble metal catalyst is supported, for example, by immersing the catalyst support layer 12 in an aqueous solution of a noble metal catalyst, such as platinum, an aqueous solution of a platinum compound such as dinitrodiamine platinum or platinum chloride, and drying and firing.

  Next, the catalyst supporting layer 14 is formed on the catalyst supporting layer 12 supporting the noble metal catalyst 13 in this manner (FIG. 1 (c)), and the noble metal catalyst 15 is supported on the catalyst supporting layer 14. (FIG. 1 (d)). The catalyst support layer 14 may be formed of the same material as the catalyst support layer 12 or may be formed of a different material, and the thickness (coating amount) is preferably 50 to 200 g / L. Further, the noble metal catalyst 15 may be the same as or different from the noble metal catalyst 13, and the supported amount is preferably 0.1 to 10 wt% with respect to the catalyst supporting layer.

  Although not shown, if necessary, the steps shown in FIG. 1 (c) and FIG. 1 (d) may be repeated to laminate three or more catalyst support layers.

  Finally, the NOx occlusion material is supported on the entire stack of the catalyst support layers thus formed. As the NOx storage material, at least one selected from alkali metals, alkaline earth metals, and rare earth elements can be used. Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium, and francium. Examples of the alkaline earth metal include beryllium, magnesium, calcium, strontium, and barium. Examples of rare earth elements include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, and the like. The amount of the NOx occlusion material supported is preferably 0.5 to 10 wt% with respect to the entire catalyst support layer. This NOx occlusion material can also be supported by a conventional method, for example, by immersing it in an aqueous solution of NOx occlusion material, drying and firing.

  By the method of the present invention, the noble metal catalyst can be supported not only on the surface of the support layer but also on the inside. The reason why the noble metal catalyst is supported in the support layer as described above is as follows.

  A conventional NOx occlusion reduction type exhaust gas purifying catalyst is provided with a support layer on which a NOx occlusion material and a catalyst noble metal are supported on a carrier base material. If this catalyst is arranged in the exhaust passage of the internal combustion engine, the carrier layer actually performs the trapping / releasing action of NOx, but there are some unclear parts about the detailed mechanism of the trapping / releasing action. However, it is considered that this capturing / releasing action is performed by the mechanism shown in FIG.

When the average air-fuel ratio of the inflowing exhaust gas becomes considerably leaner than the stoichiometric air-fuel ratio, the oxygen concentration in the inflowing exhaust gas greatly increases. As shown in FIG. 2 (a), these oxygen O ₂ is O ₂ ⁻ or O _2. It adheres to the surface of the noble metal catalyst 21 in the form of ^2- . On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the noble metal catalyst 21 to become NO ₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of the generated NO ₂ is further oxidized on the noble metal catalyst, diffuses in the form of nitrate ions NO ₃ ⁻ as shown in FIG. 2 (a), reacts with the NO x storage material 22, and becomes nitrate. NOx is stored in the NOx storage material.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is generated on the surface of the noble metal catalyst 21, and NO ₂ and nitrate ions NO ₃ ⁻ are generated unless the NOx storage capacity of the NOx storage material 22 is saturated. In contrast, when the oxygen concentration in the inflowing exhaust gas decreases and the amount of NO ₂ generated decreases, the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and thus the nitrate ion NO in the NOx storage material 22. ₃ ^- is released in the form of NO ₂ . That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx storage material 22. If the leanness of the inflowing exhaust gas is lowered, the oxygen concentration in the inflowing exhaust gas is lowered. Therefore, if the leanness of the inflowing exhaust gas is lowered, NOx is released from the NOx storage material 22.

On the other hand, if the inflow exhaust average air-fuel ratio is shifted to the rich side at this time, particularly if it is made richer than the stoichiometric air-fuel ratio, the exhaust contains a large amount of HC and CO. These HC and CO are oxygen on the noble metal catalyst 21. O ₂ ^- or O ^2- of reacting with oxidized. Further, when the inflow exhaust gas average air-fuel ratio is shifted to the rich side, particularly when the inflow exhaust gas average air-fuel ratio is made richer than the stoichiometric air-fuel ratio, the oxygen concentration in the inflowing exhaust gas extremely decreases, so that NO ₂ is released from the NOx storage material 22. As shown in FIG. 2 (b), NO ₂ reacts with HC and CO and is reduced. When NO ₂ no longer exists on the surface of the noble metal catalyst 21 in this way, NO ₂ is released from the NOx storage material 22 to the next. Therefore, when the inflow exhaust average air-fuel ratio is made richer than the stoichiometric air-fuel ratio, NOx is released from the carrier layer in a short time. Note that even if the inflow exhaust average air-fuel ratio is lean, NOx is released from the NOx storage material 22, and the released NOx can be reduced.

  During normal operation, the air-fuel ratio of the air-fuel mixture combusted in the internal combustion engine is maintained lean, so that the air-fuel ratio of the exhaust gas flowing into the catalyst at this time is lean. As a result, NOx in the exhaust is trapped in the NOx storage material 22 and is prevented from being discharged into the atmosphere.

  However, it has been found that NOx occlusion materials, particularly potassium, move in the support layer in the direction of the substrate as the catalyst deteriorates. On the other hand, the noble metal catalyst is supported on the outermost layer (20 to 50 μm) of the support layer in order to increase the contact efficiency with the exhaust gas. Therefore, the distance between the noble metal catalyst and the NOx storage material increases with deterioration. . As a result, the NOx occlusion reduction reaction shown in FIG. 2 becomes difficult to perform.

  Therefore, in order to solve this problem, according to the method of the present invention, since the noble metal catalyst is supported not only on the surface of the support layer but also on the inside, the NOx occlusion material passes through the support layer on the substrate side due to deterioration. Even if it moves to, it can maintain the proximity arrangement with the noble metal catalyst and can maintain the NOx occlusion reduction reaction.

Example 1
200 g of ion-exchanged water was added to 200 g of alumina powder to prepare a coating slurry. A cordierite monolith substrate was immersed in this slurry and taken out, and then the excess slurry was blown off, dried at a temperature of 250 ° C., and then fired at 500 ° C. for 2 hours to form a support layer. The amount of alumina coated was 100 g per liter of monolith substrate. Next, this supporting layer was immersed in a dinitroammine platinum nitric acid aqueous solution to support platinum. The amount of platinum supported was 0.5 wt%.

  Next, in the same manner as described above, an alumina catalyst support layer was formed at 100 g / L, and 1 wt% of platinum was supported on this support layer.

  Finally, the support layer formed on this base material is immersed in an aqueous potassium acetate solution and taken out. Then, excess slurry is blown off, dried at a temperature of 250 ° C., and then baked at 500 ° C. for 2 hours to store NOx. Potassium as a material was supported throughout. The amount of potassium supported was 5 wt%.

Comparative Example 1
Commercially available activated alumina powder was dispersed in an aqueous solution of Pt dinitrodiammine and allowed to stand for 2 hours with stirring to carry 1.8 g of platinum per 240 g of alumina powder. The powder was dried at 120 ° C. and then calcined in the air at 500 ° C. for 1 hour. The same monolith substrate as in Example 1 was wash coated with the slurry containing the platinum-supported alumina powder. The amount of coating was 200 g per liter of catalyst volume, and the platinum concentration was 1.5 wt%. This monolith catalyst was further loaded with 5 wt% potassium per liter of catalyst volume as a NOx storage material.

Evaluation of purification performance The exhaust gas purification catalyst (deteriorated equivalent to 50,000 km) obtained in this way is loaded into a multi-converter and installed in the exhaust system of a lean-burn engine. Exhaust temperature (inlet gas temperature) 400 ° C, lean-burn The NOx purification rate when a rich spike (corresponding to A / F = 11.5) was driven once in 60 seconds at medium A / F = 30 was measured. The result is shown in FIG.

  As is clear from the results shown in FIG. 3, the exhaust gas purification catalyst obtained by the method of the present invention showed a better NOx purification rate than the conventional catalyst.

It is a figure which shows the process of the method of this invention. It is a figure explaining the supplement and release mechanism of NOx. It is a graph which shows a NOx purification rate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Carrier base material 12, 14 ... Catalyst support layer 13, 15 ... Precious metal catalyst 21 ... Precious metal catalyst 22 ... NOx storage material

Claims (1)

A method for producing an exhaust gas purification catalyst comprising the following steps:
(a) forming a catalyst support layer on a support substrate;
(b) impregnating and supporting a noble metal catalyst on the catalyst supporting layer,
(c) forming a catalyst supporting layer on the catalyst supporting layer supporting the noble metal catalyst;
(d) impregnating and supporting a noble metal catalyst on the catalyst supporting layer; and
(e) A method for producing an exhaust gas purifying catalyst, comprising impregnating and supporting the entire NOx storage material.

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