CN112823058A

CN112823058A - Exhaust gas purifying catalyst

Info

Publication number: CN112823058A
Application number: CN201980066707.3A
Authority: CN
Inventors: 岩知道均一
Original assignee: Mitsubishi Motors Corp
Current assignee: Mitsubishi Motors Corp
Priority date: 2018-10-04
Filing date: 2019-09-10
Publication date: 2021-05-18
Also published as: WO2020071065A1; JP7215487B2; JPWO2020071065A1

Abstract

The invention provides an exhaust gas purifying catalyst. The exhaust gas purifying catalyst has a catalyst layer containing at least rhodium and palladium, wherein rhodium is supported on at least a separator surface (14) of a carrier, and palladium is supported on at least a separator inside (13) of the carrier. The palladium supporting density is higher on the upstream side (11) than on the downstream side (12). Thereby, the exhaust gas purification performance can be effectively improved.

Description

Exhaust gas purifying catalyst

Technical Field

The present invention relates to an exhaust gas purifying catalyst having a catalyst layer containing at least rhodium and palladium.

Background

An exhaust gas purifying catalyst in which a catalyst is supported on a wall-flow type carrier has been known as an exhaust gas purifying catalyst for purifying an exhaust gas (exhaust gas) of an internal combustion engine. That is, a catalyst component is fixed inside a carrier (carrier) formed by penetrating many micropores to flow exhaust gas, and carbon monoxide, unburned combustion components, nitrogen oxides, Particulate Matter (hereinafter, referred to as PM), and the like contained in the exhaust gas are purified. The kind of the carrier includes a ceramic molded product, a metal product (metal carrier), and the like. The type of catalyst supported on the carrier can be variously selected depending on the substance to be purified (see patent documents 1 to 3).

[ patent document 1 ] International publication No. 2016/133086 of Japanese patent document

[ patent document 2 ] Japanese patent document International publication No. 2017/119101

[ patent document 3 ] Japanese patent document laid-open No. 2010-269205

Disclosure of Invention

Technical problem to be solved by the invention

However, to improve the exhaust gas purification performance of the exhaust gas purification catalyst at the time of cold start of the internal combustion engine, it is effective to shorten the distance from the internal combustion engine to the exhaust gas purification catalyst and to improve the temperature rise of the catalyst. However, the closer the exhaust gas purification catalyst is to the internal combustion engine, the higher the exhaust gas pressure near the exhaust manifold, and the lower the performance of the internal combustion engine may be. Therefore, in recent years, an exhaust gas purifying catalyst has been developed which reduces pressure loss using a carrier having a high porosity.

On the other hand, in the carrier having a high porosity, the catalyst can be supported not only on the surface of the rib (separator surface) which is in contact with the hollow portion of the chamber but also inside the rib (inside the separator). Further, the contact area and the contact probability with the exhaust gas are different between the rib surface and the rib interior. Therefore, it is preferable to carry an appropriate kind of catalyst on each position. These problems have not been sufficiently considered in the conventional exhaust gas purifying catalyst, and there is room for improvement in terms of effectively improving the exhaust gas purifying performance.

An object of the present invention is to provide an exhaust gas purifying catalyst which can effectively improve exhaust gas purifying performance in view of the above-mentioned problems. Further, not limited to this purpose, the operation and effect by each configuration shown in the following "detailed description" may be another object of the present application, while exerting the operation and effect which cannot be obtained by the conventional art.

Means for solving the problems

(1) The disclosed exhaust gas purifying catalyst has a catalyst layer including at least rhodium and palladium. Further, the rhodium is supported at least on the surface of the separator of the carrier, and the palladium is supported at least inside the separator of the carrier. Further, the supported density of palladium is higher on the upstream side than on the downstream side.

(2) Preferably, the apparatus includes an upstream portion containing the palladium and a downstream portion not containing the palladium.

(3) Preferably, the catalyst support further comprises a surface layer portion which is provided on a surface of the partition wall of the carrier in the upstream portion and contains the rhodium, and the downstream portion contains the rhodium in the interior of the partition wall of the carrier.

(4) Preferably, the first pores of the surface of the separator are larger than the second pores of the interior of the separator.

(5) Preferably, the first particle diameter of the substrate on which the catalyst is immobilized on the surface of the separator is larger than the second particle diameter of the substrate on which the catalyst is immobilized inside the separator.

Effects of the invention

By including rhodium in the surface of the partition, nitrogen oxides (NOx) can be effectively purified. Further, by including palladium in the separator, carbon monoxide and hydrocarbons can be effectively purified. Further, by increasing the palladium supporting density on the upstream side which is likely to contact with exhaust gas, the exhaust gas purification performance can be effectively improved.

Drawings

Fig. 1 is a schematic view showing the interior of a vehicle to which an exhaust gas purifying catalyst is applied.

Fig. 2 is a sectional view showing the structure of an exhaust gas purifying catalyst.

Fig. 3 is a sectional view showing an enlarged internal structure of the exhaust gas purifying catalyst.

Fig. 4 is a graph showing a relationship between the catalyst supporting density and the purification efficiency.

Fig. 5 is a graph showing a relationship between the pore diameter and the pore amount.

Fig. 6(a) to (C) are graphs showing changes in purification efficiency.

Fig. 7 is a schematic diagram illustrating a modification.

[ notation ] to show

1: engine (internal combustion engine)

2: exhaust passage

3: exhaust gas purifying catalyst

4: inlet chamber

5: outlet chamber

6: partition board

7: carrier

8: base material

9: catalyst and process for preparing same

10: vehicle with a steering wheel

11: upstream part

12: downstream part

13: upstream interior

14: upstream surface layer (surface layer part)

15: downstream interior

16: downstream skin

Detailed Description

[1. device Structure ]

Hereinafter, an exhaust gas purifying catalyst according to an embodiment will be described with reference to the drawings. As shown in fig. 1, an exhaust gas purifying catalyst 3 of the present embodiment is applied to a vehicle 10 equipped with an engine 1 (internal combustion engine). The engine 1 may be a diesel engine or a gasoline engine. An exhaust gas purifying catalyst 3 is inserted in an exhaust passage 2 of the engine 1. The exhaust gas purifying catalyst 3 is a catalyst device for effectively purifying various harmful components contained in exhaust gas, and has both a function of a three-way catalyst and a function of a PM removal filter. The mounting position of the exhaust gas purifying catalyst 3 should be such that catalytic reactivity is ensured, and is preferably set at a position where high-temperature exhaust gas passes. For example, as shown in fig. 1, it may be provided at a position close to the engine 1 (directly downstream of the exhaust manifold, directly downstream of the turbocharger, or the like).

A porous body in which many flow paths through which exhaust gas can pass are formed is provided inside the exhaust gas purification catalyst 3. The porous body supports a catalyst 9 on a porous support 7 through a base material 8, and is formed into a cylindrical or elliptic cylindrical, rectangular parallelepiped shape. The material of the carrier 7 may be ceramic such as silicon carbide or Cordierite (Cordierite), or may be metal foil. The higher the porosity of the support 7, the better. In the present embodiment, the porosity of the carrier 7 exceeds 50 [% ], and the pore volume of the pores having a diameter of 1[ μm ] or more exceeds 0.2[ mL/g ].

As the substrate 8 for supporting the catalyst 9, alumina (alumina, Al) can be mentioned₂O₃) Or cerium oxide (cerium oxide, CeO)₂) Or zirconium oxide (zirconium dioxide, ZrO)₂) And the like. Further, the substrate 8 may be made to contain an oxygen occluding material that occludes (occlusions) oxygen in an oxidizing atmosphere. As an example of the oxygen occluding material, a ceria-zirconia composite oxide (CeO) may be mentioned₂-ZrO₂Series substances) or rare earth oxysulfates (Ln)₂O₂SO₄) And the like.

In the porous body shown in fig. 2, a substrate 8 and a catalyst 9 are fixed to a support 7 having a honeycomb structure. The inside of the carrier 7 is divided into a plurality of chambers (small room-like passages) along the flow direction of the exhaust gas. The chambers are partitioned by a porous partition plate 6 (rib), and the inlet side or outlet side end is closed. Here, the chamber with the outlet side end closed is referred to as an inlet chamber 4. Similarly, the chamber with the inlet side end blocked is referred to as the outlet chamber 5. Based on the flow direction of the exhaust gas, an upstream side (inlet side) of the carrier 7 is referred to as an upstream portion 11, and a downstream side (outlet side) of the carrier 7 is referred to as a downstream portion 12.

Each inlet chamber 4 is arranged adjacent to at least more than one outlet chamber 5, preferably a number of outlet chambers 5. Thereby, the exhaust gas flowing in from the inlet of the exhaust gas purification catalyst 3 first enters the inlet chamber 4, passes through the partition plate 6, enters the outlet chamber 5, and flows out from the outlet of the exhaust gas purification catalyst 3. In the porous body of the present embodiment, the catalyst 9 is carried inside the partition plate 6 that partitions the inlet chamber 4 and the outlet chamber 5, and on the side facing the inlet chamber 4 among the surfaces of the partition plate 6. Hereinafter, the inside of the partition 6 is referred to as "partition inside", and the surface of the partition 6 facing the inlet chamber 4 is referred to as "partition surface".

The catalyst 9 contains at least rhodium (Rh) and palladium (Pd). Rhodium is supported on at least the separator surface of the carrier 7. On the other hand, palladium is supported at least inside the separator of the carrier 7. Further, the palladium supporting density is higher in the upstream portion 11 than in the downstream portion 12. For example, in the case where the separator surface does not contain palladium, the palladium carrying density in the inside of the separator of the upstream portion 11 is higher than that in the inside of the separator of the downstream portion 12. In addition, when the surface of the separator contains palladium, the supported density of palladium contained in the surface of the separator is higher on the upstream side than on the downstream side.

Fig. 3 is a sectional view showing a portion a of fig. 2 in an enlarged manner. The position of the carrier 7 to which the substrate 8 and the catalyst 9 are fixed may be divided into an upstream portion 11 and a downstream portion 12, and may be divided into a separator interior and a separator surface. Here, the upstream portion 11 and the inside of the separator are referred to as "upstream inside 13", and the upstream portion 11 and the surface of the separator are referred to as "upstream surface layer 14 (surface layer portion)". Similarly, the downstream portion 12 and the inside of the separator are referred to as "downstream inner portion 15", and the downstream portion 12 and the surface of the separator are referred to as "downstream skin 16".

Palladium is supported at a high concentration in the upstream inner portion 13, and palladium is not supported in the downstream inner portion 15. The supported density of palladium in the upstream inner portion 13 of this embodiment is 8.0[ g/L ]. Further, in the case where the downstream inner portion 15 is caused to support palladium, it is preferable that the supporting density thereof is lower than 8.0[ g/L ]. Rhodium is supported on the upstream surface layer 14 and the downstream inner portion 15. The rhodium carrier density in the upstream surface layer 14 was 1.0[ g/L ], and the rhodium carrier density in the downstream inner portion 15 was 0.4[ g/L ].

In the present embodiment, the downstream skin 16 shown by a broken line in fig. 3 is not formed. On the other hand, when the downstream skin 16 is formed, rhodium is preferably supported in the same manner as the upstream skin 14. In this case, the rhodium-carrying density in the downstream surface layer 16 is about 1.0[ g/L ].

As shown in fig. 4, the catalytic activity per supported amount of rhodium or palladium (conversion efficiency with respect to total hydrocarbons) after thermal endurance was higher than that of platinum. Therefore, by using rhodium and palladium, the exhaust gas purification performance can be effectively improved.

As for the pore diameters of the support 7 and the base material 8, it is preferable that the pore diameter of the separator surface (first pore diameter) is larger than the pore diameter of the separator interior (second pore diameter). As a method of making the pore diameters of the separator surface and the separator interior different, it is conceivable to make the particle diameter of the base material 8 different. That is, the particle diameter (first particle diameter) of the substrate on which the catalyst 9 is supported on the surface of the separator is preferably larger than the particle diameter (second particle diameter) of the substrate on which the catalyst 9 is supported inside the separator.

For example, the particle size of the base material of the upstream surface layer 14 is made larger than the particle size of the base material of the upstream inner portion 13. In the case where the catalyst 9 is also supported on the downstream surface layer 16, the particle diameter of the base material of the downstream surface layer 16 is made larger than the particle diameter of the base material of the downstream inner portion 15. The pore distribution of the present embodiment is shown in fig. 5. The solid line in fig. 5 shows the pore distribution on the surface of the separator, and the dotted line shows the pore distribution inside the separator. By making the pores on the surface of the separator larger than those in the interior of the separator, the flow of the exhaust gas can be improved and the pressure loss in the exhaust passage 2 can be reduced. In addition, the residence time of exhaust gas in the vicinity of the surface of the partition plate is prolonged, and the exhaust gas purification performance of the upstream surface layer is improved.

Mesopores (pores of 2 to 50[ nm ]) existing in the framework of the carrier 7 and the substrate 8 function as adsorption sites for catalytic reactions and target substances. In addition, macropores (pores of 50[ nm ] or more) function to eliminate the diffusion limitation of the target substance. Therefore, as shown in fig. 5, by forming both mesopores and macropores, exhaust gas can be diffused rapidly and the target substance is easily located at the active site, improving exhaust gas purification performance.

[2. action and Effect ]

(1) By including rhodium in the surface of the partition, nitrogen oxides (NOx) can be effectively purified. Further, by including palladium in the separator, carbon monoxide and hydrocarbons can be effectively purified. In particular, by increasing the palladium supporting density on the upstream side which is likely to contact exhaust gas, the exhaust gas purification performance can be effectively improved. Further, since the downstream-side palladium supporting density can be reduced, an increase in the production cost can be suppressed. As shown in fig. 4, rhodium has higher catalytic activity per supported amount than palladium or platinum. Therefore, by supporting rhodium on the upstream surface layer 14 which is most likely to come into contact with exhaust gas, the exhaust gas purification performance can be effectively improved.

The solid line charts shown in fig. 6(a) to (C) show the purification efficiency of rhodium immediately after the engine 1 is started, and the broken line charts show the purification efficiency of palladium. From this, it is understood that the exhaust gas purifying performance of palladium is high immediately after the engine 1 is started. Therefore, by using both rhodium and palladium, the exhaust gas purification performance can be improved even immediately after the engine 1 is started or even when the engine 1 is normally operated. In addition, the total manufacturing cost can be suppressed more than when only expensive rhodium is used, while inexpensive palladium is used.

(2) As shown in fig. 3, by providing the upstream portion 11 containing palladium and the downstream portion 12 containing no palladium, the exhaust gas purification catalyst can be formed with a simple structure. For example, the upstream inner portion 13 and the downstream inner portion 15 are easily made to support the catalyst 9 by the wash coating method. Further, since palladium is not supported on the downstream portion 12, the amount of palladium supported can be greatly reduced, and the manufacturing cost can be reduced.

(3) As shown in fig. 3, an upstream skin 14 containing rhodium is provided on the separator surface of the support 7 in the upstream portion 11. By providing such a simple structure, the upstream surface layer 14 can easily support the catalyst 9 by the wash coating method. In addition, in the case where the downstream surface layer 16 is not formed (that is, in the case where the downstream surface layer 16 is not caused to carry rhodium), the amount of rhodium carried can be greatly reduced, and the manufacturing cost can be reduced.

(4) By making the pore diameter (first pore diameter) on the surface of the separator larger than the pore diameter (second pore diameter) inside the separator, the exhaust gas flow rate can be improved and the pressure loss of the exhaust passage 2 can be reduced. This makes it difficult for the output of the engine 1 to be reduced due to the pressure loss, and improves the fuel economy. Further, since the pores of the upstream inner portion 13 are smaller than the upstream surface layer 14, the exhaust gas retention time in the vicinity of the upstream surface layer 14 can be extended. This can improve the exhaust gas purification performance of the upstream surface layer 14.

(5) Further, by making the particle diameter (first particle diameter) of the substrate on which the catalyst is immobilized on the surface of the separator larger than the second particle diameter of the substrate on which the catalyst is immobilized inside the separator, a structure in which the pore diameters are different for each support 7 portion can be easily formed. For example, when the catalyst 9 is supported by a wash coat method, the pore diameter can be controlled by making the particle diameters of alumina and ceria added as the base material 8 different. That is, the substrate 8 having a large particle size may be used for forming the upstream inner portion 13, and the substrate 8 having a small particle size may be used for forming the upstream surface layer 14.

[3. modification ]

The above embodiment is merely an example, and various modifications or techniques not explicitly described in the present embodiment are not excluded from application. The respective configurations of the present embodiment can be variously modified within a range not departing from the above-described gist. Further, they may be selected as necessary or appropriately combined.

In the above embodiment, the upstream portion 11 is the portion on the upstream side (inlet side) of the carrier 7, and the downstream portion 12 is the portion on the downstream side (outlet side), but the method of distinguishing the upstream portion 11 from the downstream portion 12 is not limited to this. For example, as shown in fig. 7, the upstream portion 11 and the downstream portion 12 may also be defined by dividing regions in the thickness direction of the partition plate 6 based on the flow direction of the exhaust gas passing through the partition plate 6 that partitions the inlet chamber 4 and the outlet chamber 5. The exhaust gas purifying catalyst 3 having such a structure also exhibits the same effects as those of the above-described embodiment.

In the above embodiment, the portion of the separator surface included in the upstream portion 11 is defined as the upstream skin 14 (skin portion), and rhodium is supported on the upstream skin 14, but the position of the upstream skin 14 may be changed. For example, the downstream skin 16 shown by a dotted line in fig. 3 may be formed so as to carry rhodium in the same manner as the upstream skin 14. Alternatively, rhodium may be supported only by forming the downstream surface layer 16, or any number of rhodium-containing layers may be formed at any position on the surface of the separator. The same effects as those of the above embodiment can be obtained by supporting rhodium on at least the surface of the separator of the support.

Claims

1. An exhaust gas purification catalyst having a catalyst layer comprising at least rhodium and palladium, characterized in that,

The rhodium is carried at least on the surface of the separator of the carrier,

The palladium is carried at least inside the separator of the carrier,

The support density of the palladium is higher on the upstream side than on the downstream side.

2 . The exhaust gas purification catalyst according to claim 1 , further comprising an upstream portion containing the palladium and a downstream portion not containing the palladium. 3 .

3 . The exhaust gas purification catalyst according to claim 2 , further comprising a surface layer portion provided on a separator surface of the carrier in the upstream portion and containing the rhodium, and the downstream portion is provided on the carrier. 4 . The interior of the separator contains the rhodium.

4 . The exhaust gas purification catalyst according to claim 1 , wherein the first pore diameter on the surface of the separator is larger than the second pore diameter inside the separator. 5 .

5. The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein the first particle diameter of the base material that fixes the catalyst on the surface of the separator is larger than the base material that fixes the catalyst inside the separator. The second particle size of the material is large.