JP2010131526A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst capable of exhibiting the performance more excellent than before when used for cleaning exhaust gas even if the catalyst contains a small amount of a noble metal. <P>SOLUTION: The catalyst for cleaning exhaust gas includes: a carrier base material which has a plurality of through-holes in the flowing direction of the exhaust gas; one or a plurality of catalyst-deposited layers formed on the inner wall surface of each of the through-holes; and the noble metal deposited on each of the catalyst-deposited layers. The thickness of each of the catalyst-deposited layers is adjusted so that the pressure of the exhaust gas passing through the through-holes becomes uniform among all the through-holes. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst capable of more efficiently utilizing a noble metal supported on a catalyst support layer by adjusting the flow pressure of exhaust gas passing through each through hole.

  Conventionally, platinum, rhodium, palladium, etc. used as catalysts for exhaust gas purification as catalyst precious metals have been low in reserves and expensive, but demand in developing countries has increased rapidly in recent years, and as with other resources These prices are also rising. On the other hand, exhaust gas regulations in each country tend to be strengthened year by year due to the recent increase in environmental awareness, and the demand for exhaust gas purification performance of exhaust gas purification catalysts is also becoming stricter.

  For this reason, development of a catalyst capable of exhibiting better exhaust gas purification performance with a small amount of noble metal has been demanded. For example, Japanese Patent Application Laid-Open No. 2006-181487 proposes a catalyst in which a barrier layer is provided on the surface of a carrier and noble metal particles are arranged on the barrier layer via a fixed layer, thereby suppressing the use of the noble metal and increasing the catalytic activity. Has been.

JP 2006-181487 A

  This invention is made | formed in view of the said situation, and it is providing the exhaust gas purification catalyst which has a catalyst structure which can exhibit the exhaust gas purification performance superior to the past with little noble metal amount.

  The exhaust gas purification catalyst is required to have a large contact area with the exhaust gas and a small pressure pressure loss. Therefore, the exhaust gas purification catalyst is configured by using a carrier base material having a plurality of through-holes perpendicular to the exhaust gas flow. It is common.

  The carrier substrate of such an exhaust gas purifying catalyst can be adjusted by molding heat-resistant ceramics such as cordierite so as to have a shape having a large number of through holes. Alternatively, it can be formed into a shape having a large number of through-holes by overlapping a corrugated sheet and a flat plate made of metal foil in a roll shape (hereinafter referred to as a large number of through-holes for convenience in this specification). The carrier base material having the above is generically called “honeycomb structure”). In a general exhaust gas purifying catalyst, a catalyst support layer for coating a catalyst noble metal or the like is disposed on such a carrier substrate.

  The catalyst support layer is generally formed by coating the base material with a slurry containing alumina or the like by a wash coat method or a sol-gel method. In this case, the catalyst support layer is formed on the entire inner wall of the through hole in which the catalyst support layer exists. The thickness is usually uniform. In other words, each through hole in the honeycomb structure in the prior art has a substantially uniform opening area.

  When the present inventor investigated the flow of exhaust gas passing through a carrier substrate having a catalyst support layer formed by a general coating method, the exhaust gas does not flow uniformly in the honeycomb structure, but a specific portion. It turned out that it is flowing around. Here, when a general coating method is used, the catalyst metal is supported almost uniformly on the catalyst support layer regardless of the flow pressure of the exhaust gas passing through. As a result, the flow pressure of the exhaust gas is reduced. It is considered that the noble metal existing on the low through-hole is not effectively utilized compared with the portion where the fluid pressure is high. However, if the noble metal is intensively supported on the portion where the exhaust gas flow pressure is high, the density of the noble metal is increased in the portion, and sintering or the like is likely to occur, resulting in a decrease in purification performance.

  Therefore, the present inventors have solved the above problem by increasing the utilization efficiency of the catalyst noble metal supported on the catalyst support layer constituting the inner wall of the through hole in which the flow pressure of the exhaust gas passing therethrough is low among the catalyst noble metals. The present invention has been completed.

That is, the present invention includes the following inventions.
[1] A carrier base material having a plurality of through holes in the exhaust gas flow direction, one or more catalyst support layers formed on the inner wall surface of the through holes, a noble metal supported on the catalyst support layers, An exhaust gas purification catalyst having
An exhaust gas purifying catalyst in which the thickness of the catalyst support layer is adjusted so that the flow pressure of the exhaust gas passing therethrough is uniform among all through holes.
[2] Among all the through holes, the thickness of the catalyst support layer constituting the inner wall portion of the through hole in which the flow pressure of the exhaust gas passing therethrough is relatively high is increased according to the flow pressure and / or By reducing the thickness of the catalyst supporting layer constituting the inner wall portion of the through hole in which the flow pressure of the exhaust gas passing therethrough is relatively low among the through holes, the thickness of the catalyst supporting layer is reduced. The exhaust gas purifying catalyst according to [1], wherein adjustment is performed.
[3] The exhaust gas purifying catalyst according to [1] or [2], wherein an opening area of a through hole located in a central portion when viewed from an exhaust gas flow direction is smaller than an opening area of a through hole located outside the through hole.
[4] The exhaust gas purifying catalyst according to any one of [1] to [3], wherein the opening area of the through hole is reduced stepwise from the outside to the center as viewed from the exhaust gas flow direction.
[5] The exhaust gas purifying catalyst according to [1] to [4], wherein an opening area of the through hole located in the downstream portion when viewed from the axial direction of the catalyst is larger than an opening area of the through hole located in the upstream portion.
[6] The noble metal is supported only on the uppermost layer among the plurality of catalyst supporting layers, and the thickness of the catalyst supporting layer is adjusted by increasing or decreasing the thickness of the layers other than the uppermost layer. [1] The catalyst for exhaust gas purification of [5].

  According to the present invention, among all the through holes, the thickness of the one or more catalyst support layers constituting the inner wall portion of the through hole in which the flow pressure of the exhaust gas passing therethrough is relatively high is set to the height of the flow pressure. By increasing accordingly, the flow pressure of the exhaust gas passing therethrough becomes uniform as a whole. As a result, exhaust gas reaches every corner of all through holes, the noble metal supported on the catalyst support layer is utilized to the maximum extent, and exhaust gas purification performance is finally improved.

  In addition to the method of increasing / decreasing the thickness of the catalyst support layer, it is conceivable to adjust the size of the lattice itself to a desired size at the stage of preparing the honeycomb structure. However, taking a metal honeycomb as an example, a metal honeycomb structure is usually manufactured by winding a corrugated sheet and a flat plate made of metal foil in a roll shape as described above. When the diameter of the through hole is changed according to the flow pressure as in the present invention, a complicated manufacturing method is required. Even if it is possible, there is a problem in cost.

  The exhaust gas purifying catalyst of the present invention includes a carrier base material having a plurality of through holes in the exhaust gas flow direction, one or more catalyst support layers formed on the inner wall surface of the through holes, and supported on the catalyst support layer. An exhaust gas purifying catalyst comprising a noble metal, wherein the thickness of the catalyst support layer is adjusted so that the flow pressure of the exhaust gas passing therethrough is uniform between all through holes.

  As the carrier substrate used in the present invention, a carrier substrate having a large number of through-holes such as a honeycomb carrier substrate generally used in the industry can be used. It may be made of. Although not limited, ceramics that can be used for the carrier substrate include alumina, silica, cordierite, zirconia, silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia, mullite, and the like. Can be mentioned.

  Conventional so-called metal honeycomb bodies can also be used. Here, as described above, the metal honeycomb body is configured using a metal flat foil and corrugated foil. The metal foil constituting the metal honeycomb body preferably has a low heat capacity and excellent heat resistance, pressure resistance, and the like. Accordingly, stainless steel, heat resistant steel, and the like are preferable, but are not limited to these materials. Examples of stainless steel include steel materials such as ferritic stainless steel and austenitic stainless steel. In the present invention, these can be rolled into a foil shape and used.

  The carrier substrate has a large number of through-holes when viewed from the exhaust gas flow direction. A catalyst support layer is formed on the inner walls of these through holes. The catalyst-carrying layer can be formed by preparing a slurry containing ceramics such as alumina, adhering it to the inner wall of the through hole of the carrier substrate, and drying and firing (wash coat method). Examples of ceramics used in the method include slurries including alumina, silica, cordierite, zirconia, silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia, mullite, and the like. Although a catalyst noble metal can be included in advance, it is preferable to adjust the thickness of the catalyst support layer using a slurry containing no catalyst noble metal from the viewpoint of cost and catalyst performance.

  When providing a plurality of catalyst-carrying layers, a layer not containing a noble metal (hereinafter referred to as a “noble metal-free layer”) is disposed in the catalyst-carrying layer in the through hole where the flow pressure of the exhaust gas is relatively large, The flow pressure can be made uniform also by adjusting. The noble metal-free layer can be prepared by using only a general slurry such as alumina that does not contain a catalyst noble metal. Therefore, when the noble metal non-containing layer is arranged, it is possible to achieve an improvement in the exhaust gas purification performance by a cheaper means compared to the conventional one.

  In the present invention, among all the through holes, the thickness of the catalyst supporting layer constituting the inner wall portion of the through hole in which the flow pressure of the exhaust gas passing therethrough is relatively high is increased according to the height of the flow pressure. . Or you may reduce the thickness of the catalyst support layer which comprises the inner wall part of the through-hole in which the flow pressure of the exhaust gas which passes is relatively low according to the said flow pressure.

  The flow pressure of the exhaust gas is not limited, but can be determined using a general flow pressure gauge. As described as “relatively”, the increase or decrease in the thickness of the catalyst support layer is a comparison of the exhaust gas flow pressure between the through-holes. This is done by increasing the thickness of the catalyst-carrying layer of the through-hole in a certain region that is predicted not to be exhibited. Moreover, the through-hole in a fixed area | region with a high fluid pressure may reduce the thickness of the catalyst support layer. In this way, by adjusting the thickness of the catalyst support layer for each predetermined region according to the flow pressure of the exhaust gas, it is possible to achieve a uniform flow pressure of the exhaust gas that passes through, and thus improve the catalyst performance.

  Although it differs depending on the actual machine on which the catalyst of the present invention is mounted, in general, the exhaust gas has a high fluid pressure at the center portion of the carrier base material when viewed from the exhaust gas flow direction, so that the thickness of the catalyst support layer at the center portion is increased. Is preferred. Further, since the upstream portion of the catalyst in the axial length often has a high exhaust gas flow pressure, it is also preferable to increase the thickness of the catalyst support layer in the upstream portion.

  The thickness of the catalyst support layer may be changed stepwise from the center to the outer periphery or from the upstream to the downstream as viewed from the exhaust gas flow direction. The thickness of the catalyst support layer may be changed in a plurality of steps, for example, 2 to 3 steps. However, since it is not preferable from the viewpoint of cost to adjust the thickness by repeating the coating many times, it is preferable to change the thickness by about two steps.

  Although it differs depending on the situation of the catalyst of the present invention and the vehicle actually mounted, as shown in Example 2 below, it is located in the center portion when viewed from the exhaust gas flow direction and in the upstream portion in the axial length of the catalyst. It is preferred to increase the thickness of the catalyst support layer located (see FIG. 2).

  The thickness of the catalyst support layer can be adjusted by changing the composition, concentration, or number of coatings of the slurry. As a result, the inner diameter of the formed through hole, and thus the opening area of the through hole can be adjusted. From the viewpoint of material cost, it is preferable to adjust the thickness of the noble metal-free layer. The decrease in the opening area of the through hole can be relatively evaluated by the ratio (opening ratio) of the opening area after coating to the opening area of the through hole before coating.

  Although adjustment of the thickness of a catalyst carrying layer is not limited, it can be implemented as follows. For example, when adjusting the thickness of a catalyst support layer that constitutes the inner wall of a specific through hole, for example, the through hole existing in the center of the support substrate, as viewed from the exhaust gas flow direction, a slurry such as alumina is added to the support substrate. A catalyst-carrying layer with a desired thickness is formed by flowing the slurry through all the through holes from the inlet gas side, drying and firing, and then allowing the slurry to flow again only through the predetermined through holes where the thickness is to be increased. Can be done.

  On the other hand, when coating from the inlet gas side to a specific part of the axial length of the carrier substrate in the through hole, the ceramic concentration is increased to increase the viscosity of the slurry, and only a predetermined range is coated. A sufficient amount of slurry is prepared. After setting the base material so that the honeycomb-shaped cross-section is parallel to the ground and the incoming gas side is up, the high-viscosity slurry is allowed to flow from the incoming gas side to a predetermined through hole, thereby being in a predetermined range. Only the inner wall of the through-hole can be coated, and the other part (outgas side) can be prevented from being coated.

  After the formation of the catalyst support layer, a desired noble metal is supported on the outermost surface of the layer, or the uppermost layer of the layer is formed in advance with a slurry containing the catalyst noble metal to produce the catalyst of the present invention. be able to. Although the catalyst noble metal to be used is not limited, the catalyst noble metal can be supported by coating a slurry obtained by preparing a catalyst powder in which a noble metal is previously supported on an oxide carrier powder such as alumina powder.

Example 1
In this embodiment, an exhaust gas purification catalyst is prepared in which a catalyst support layer having a thickness increased in three stages from the center to the outer periphery of the honeycomb structure as viewed from the exhaust gas flow direction. The catalyst noble metal is included only in the slurry constituting the uppermost layer, and is not included in the slurry constituting the other layer (noble metal-free layer).

  First, an alumina binder and alumina were mixed to prepare a slurry for a noble metal-free layer (slurry 1).

  Subsequently, a platinum nitrate solution and a cerium oxide were mixed, dried at 250 ° C. for 3 hours, and then fired at 500 ° C. for 2 hours to prepare a powder 1, and then a palladium nitrate solution and a zirconia oxide were mixed. The powder 2 was prepared by drying at 250 ° C. for 3 hours and then firing at 500 ° C. for 2 hours. By mixing these powders 1 and 2 with alumina, barium sulfate and an alumina binder, the final composition is 1.5 g / L of platinum, 0.4 g / L of rhodium, 135 g / L of cerium oxide, and 50 g / L of zirconia oxide. A slurry of L and alumina 75 g / L was prepared (slurry 2).

  A monolith honeycomb carrier base material is prepared, and the slurry 1 is poured into each through hole in the central portion A as shown in FIG. 1 and dried and fired. A noble metal-free layer was provided so as to be 80%.

  Subsequently, the slurry 1 is used in the same manner as the A portion for the through hole (corresponding to the B portion in FIG. 1) located around the through hole in which the noble metal-free layer is formed, and the opening ratio is about 90%. The noble metal-free layer was formed. At this time, the noble metal-free layer made of the slurry 1 is formed in all the through holes except for the C part (corresponding to the outer peripheral part of the B part).

  The entire monolith honeycomb carrier in which noble metal-free layers having different thicknesses were formed in the A part and the B part was immersed in the slurry 2. After the immersion, drying and calcination were performed to prepare a catalyst 1 having a plurality of catalyst supporting layers. When the opening ratio of the through holes before forming the catalyst support layer was 100%, the final opening ratio of the through holes was 70% for the A part, 80% for the B part, and 90% for the C part. In addition, A part occupies 10% of base-material cross-sectional areas, and B part occupies 40% of base-material cross-sectional areas when it sees from an exhaust gas flow direction.

Example 2
In this embodiment, an exhaust gas purification catalyst is prepared in which a catalyst-supporting layer is disposed on the inlet gas side portion of the through hole located in the central portion of the honeycomb structure when viewed from the exhaust gas flow direction. (See Figure 2).

  Desirable from below, with the inlet gas side of the base material facing downward so that the opening ratio of the through-holes in part D shown in FIG. 2 is 80% compared to the opening ratio of untreated through-holes (100%) After injecting the slurry (used in Example 1) into the through-hole located in the central part until the height reached, the excess slurry was removed by gravity by leaving the substrate as it was. Subsequently, a catalyst-free layer was formed only in part D by passing through the drying / firing process in the same manner as in Example 1.

  Subsequently, the entire monolith honeycomb carrier was immersed in the slurry 2. The catalyst 2 as shown in FIG. 2 was prepared by performing drying and baking after immersion. When the opening ratio of the untreated through hole was 100%, the final opening ratio of the through hole was 70% for the D portion and 90% for the E portion. Note that the portion D has a length of 40% of the entire length of the base material, and occupies 25% of the cross-sectional area of the base material when viewed from the exhaust gas flow direction.

Comparative Example 1
Using the same monolith honeycomb carrier base material as in the example and applying the slurry 2 to the entire carrier under the same conditions as in Example 1, one catalyst supporting layer containing a catalyst noble metal was formed.

(Purification performance measurement)
The catalyst was endured for 40 hours at a catalyst bed temperature of 1050 ° C. with an engine with a displacement of 4.3 L, and then mounted on an actual vehicle having an engine with a displacement of 2.2 L. The vehicle was driven with the operation mode set to LA # 4 mode, and the NOx emission effect of the catalyst was measured. The results for NOx emissions are shown in FIG.

  As shown in FIG. 3, the NOx emissions of the catalysts of Examples 1 and 2 were reduced by about 10% compared to the catalyst of Comparative Example 1, even though the catalyst noble metal amount was the same as that of Comparative Example 1. did.

  According to the exhaust gas purification catalyst of the present invention, the exhaust gas purification performance can be improved without increasing the amount of the catalyst noble metal. The present invention can achieve such an effect only by appropriately increasing or decreasing the coat layer thickness in accordance with the flow pressure of the exhaust gas, and can be applied to any existing honeycomb catalyst. Therefore, the present invention is more suitable than conventional catalysts. .

FIG. 1 shows a cross-sectional view of the catalyst of Example 1. The upper diagram (a) is a cross-sectional view in the direction parallel to the flow of exhaust gas, and the lower diagram (b) is a cross-sectional view in the direction perpendicular to the flow of exhaust gas. FIG. 2 shows a cross-sectional view of the catalyst of Example 2. The upper diagram (a) is a cross-sectional view in the direction parallel to the flow of exhaust gas, and the lower diagram (b) is a cross-sectional view in the direction perpendicular to the flow of exhaust gas. FIG. 3 shows the NOx emission (gpm) of the catalysts of Examples 1 and 2 and Comparative Example 1.

Claims (6)

Exhaust gas comprising a carrier substrate having a plurality of through holes in the exhaust gas flow direction, one or more catalyst support layers formed on the inner wall surface of the through holes, and a noble metal supported on the catalyst support layers. A purification catalyst,
An exhaust gas purifying catalyst in which the thickness of the catalyst support layer is adjusted so that the flow pressure of the exhaust gas passing therethrough is uniform among all through holes. Among all the through holes, the thickness of the catalyst supporting layer constituting the inner wall portion of the through hole in which the flow pressure of the exhaust gas passing therethrough is relatively high is increased according to the flow pressure, and / or Among them, the thickness of the catalyst support layer is adjusted by reducing the thickness of the catalyst support layer constituting the inner wall portion of the through hole where the flow pressure of the exhaust gas passing therethrough is relatively low. The exhaust gas-purifying catalyst according to claim 1.   The exhaust gas purifying catalyst according to claim 1 or 2, wherein an opening area of a through hole located in a central portion when viewed from an exhaust gas flow direction is smaller than an opening area of a through hole located outside thereof.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein an opening area of the through hole is reduced stepwise from an outer side to a central part as viewed from an exhaust gas flow direction.   The exhaust gas purifying apparatus according to any one of claims 1 to 4, wherein an opening area of a through hole located in a downstream portion when viewed from the axial direction of the catalyst is larger than an opening area of a through hole located in an upstream portion. catalyst.   The noble metal is supported only on the uppermost layer among the plurality of catalyst-supporting layers, and the thickness of the catalyst-supporting layer is adjusted by increasing or decreasing the thickness of the layers other than the uppermost layer. The exhaust gas-purifying catalyst according to any one of the above items.

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