Catalysts for exhaust gas purification

Abstract

The present invention provides a catalyst (10) for exhaust gas purification, which includes: a substrate (11); a reforming reaction layer (20), which is arranged on the substrate (11) and contains at least one of rare earth element oxides and transition metal oxides (but does not include precious metals other than Rh and Ru) as a hydrogen production catalyst for converting unburned components in exhaust gas into H2; and a catalyst layer (30), which contains precious metals for purifying the above-mentioned unburned components, and the content of the above-mentioned hydrogen production catalyst in the reforming reaction layer (20) is 5g / L or more per unit volume of the substrate (11).

Description

Technical Field

[0001] The present invention relates to a catalyst for exhaust gas purification. In particular, it relates to a catalyst for exhaust gas purification for purifying exhaust gas containing nitrogen oxides (NOx). This application claims priority based on Japanese patent application No. 2020-187482 filed on November 10, 2020, and the entire contents of the application are incorporated into this specification as a reference. Background Art

[0002] The exhaust gas discharged from the internal combustion engine of vehicles etc. contains unburned components (harmful components) such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). In order to efficiently react these unburned components and remove them from the exhaust gas, exhaust gas purification catalysts have been used. Typical exhaust gas purification catalysts have a catalyst layer containing precious metals such as Pt (platinum), Pd (palladium), and Rh (rhodium). NOx contained in the exhaust gas is reduced by the catalyst layer and converted into nitrogen. As related prior art documents for exhaust gas purification catalysts, Patent Documents 1 to 4 can be cited.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2002-282697

[0006] Patent Document 2: Japanese Patent Application Publication No. 2005-185966

[0007] Patent Document 3: Japanese Patent Application Publication No. 2011-183319

[0008] Patent Document 4: International Publication No. 2020 / 128786 Summary of the invention

[0009] Technical problem to be solved by the invention

[0010] However, in recent years, as represented by the European RDE (Real Driving Emission) regulations, even if the air / fuel ratio (A / F) of the exhaust gas is slightly deviated from the ideal state (A / F=14.5) when, for example, the vehicle suddenly accelerates or decelerates, it is required to properly purify the exhaust gas. Fig.14 As shown, when the exhaust gas air-fuel ratio is in a lean environment (14.5<A / F) with an excess of oxygen, the precious metal cannot fully exert NOx purification performance and the NOx purification rate decreases. Therefore, there is a demand for improving the NOx purification performance in a lean environment.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a novel exhaust gas purification catalyst having improved NOx purification performance under a lean environment.

[0012] Technical solutions for solving technical problems

[0013] According to the research of the inventors of the present invention, in a lean environment, the precious metal in the catalyst layer is usually in an oxide state. However, the activity of the precious metal in the oxide state is low and it cannot fully play the function of purifying exhaust gas. Therefore, the inventors of the present invention considered using the known strong reducing agent hydrogen to reduce the precious metal and quickly achieve metallization, thereby increasing the oxidation reaction rate of the unburned components in the exhaust gas. Then, after further repeated and careful research, the present invention was achieved.

[0014] According to the present invention, there is provided a catalyst for exhaust purification, which is arranged in the exhaust passage of an internal combustion engine to purify the unburned components in the exhaust gas discharged from the above-mentioned internal combustion engine. The above-mentioned catalyst for exhaust purification includes: a substrate; a reforming reaction layer, which is arranged on the above-mentioned substrate and contains at least one of rare earth element oxides and transition metal oxides (but not including precious metals other than Rh and Ru) as a hydrogen production catalyst for converting the above-mentioned unburned components in the above-mentioned exhaust gas into H2; and a catalyst layer, which contains precious metals for purifying the above-mentioned unburned components, and the content of the above-mentioned hydrogen production catalyst in the above-mentioned reforming reaction layer is 5g / L or more per unit volume of the above-mentioned substrate.

[0015] The exhaust gas purification catalyst includes a reforming reaction layer containing a hydrogen production catalyst. The reforming reaction layer can convert unburned components (such as HC, CO) in the exhaust gas into highly reactive hydrogen molecules (H2). In addition, through H2, the precious metal can be reduced from an oxide state to a highly active metal state. Moreover, since the reforming reaction layer does not contain precious metals other than Rh and Ru, the competition with the exhaust gas purification reaction caused by the oxidation reaction can be avoided, and the conversion reaction to the above-mentioned H2 can be promoted. As a result, the exhaust gas purification catalyst can increase the oxidation reaction rate of the exhaust gas and can improve the NOx purification performance under a lean environment. Therefore, even in a situation where the air-fuel ratio of the exhaust gas fluctuates slightly due to, for example, vehicle driving conditions, it is possible to stably exert excellent NOx purification performance.

[0016] In a preferred embodiment of the exhaust gas purification catalyst described in this specification, the reforming reaction layer further contains an OSC material having oxygen storage capacity. The synergistic effect of the hydrogen production catalyst and the OSC material can promote the conversion of unburned components in the reforming reaction layer into H2. Therefore, the NOx purification performance can be further improved.

[0017] In a preferred embodiment of the exhaust gas purification catalyst described in this specification, in the reforming reaction layer, the ratio of the content of the OSC material to the content of the hydrogen production catalyst is 0.5 to 10. This can not only more appropriately promote the conversion of unburned components into H2, but also control the coating amount of the reforming reaction layer, thereby reducing pressure loss and cost.

[0018] In a preferred embodiment of the exhaust gas purification catalyst described in this specification, the content of the hydrogen production catalyst is 20 g / L or less. This makes it possible to achieve both NOx purification performance under lean conditions and excellent low-temperature activity.

[0019] In a preferred embodiment of the exhaust gas purification catalyst described in this specification, the hydrogen production catalyst contains at least one of Pr, Nd, Co, and Ni. This can achieve the technical effects described in this specification at a high level.

[0020] In a preferred embodiment of the exhaust gas purification catalyst described in this specification, the reforming reaction layer does not contain precious metals, thereby reducing the amount of precious metals used, which are so expensive, and achieving cost reduction.

[0021] In a preferred embodiment of the exhaust gas purification catalyst described in this specification, the catalyst layer is arranged on the reforming reaction layer. This can prevent substances that are excessively reduced due to H2 generated by the reforming reaction layer (e.g., ammonia generated by NOx reduction) from being discharged from the exhaust gas purification catalyst without reoxidation. In addition, the exhaust gas can be brought into contact with the catalyst layer before the reforming reaction layer, thereby improving the durability of the reforming reaction layer.

[0022] In a preferred embodiment of the exhaust gas purification catalyst described in this specification, the catalyst layer comprises: a first catalyst layer containing an oxidation catalyst as the precious metal; and a second catalyst layer disposed on the first catalyst layer and containing Rh as the precious metal. With such a configuration, at least one of the following effects can be achieved: (1) suppressing the deactivation of the hydrogen production catalyst and suppressing the oxidation of Rh accompanying the use of the hydrogen production catalyst, thereby exerting the technical effects described in this specification for a long time; (2) using H2 generated by the hydrogen production catalyst to reduce and purify NOx into N2 and H2O, or converting the precious metal in the oxide state into metal. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a schematic diagram of an exhaust gas purification system according to an embodiment.

[0024] Figure 2 yes Figure 1 A three-dimensional schematic diagram of a catalyst for exhaust gas purification.

[0025] Figure 3 yes Figure 2 A cross-sectional view of an exhaust gas purification catalyst cut along the cylinder axis direction.

[0026] Figure 4 This is a diagram showing the relationship between the air-fuel ratio (A / F) and the NOx purification rate in Test Example 1.

[0027] Figure 5 This is a comparison chart of the NOx purification rate in the lean environment of Test Example 1.

[0028] Figure 6 This is a diagram showing the relationship between the air-fuel ratio (A / F) and the NOx purification rate in Test Example 2.

[0029] Figure 7 This is a comparison chart of the NOx purification rate in the lean environment of Test Example 2.

[0030] Figure 8 This is a diagram showing the relationship between the air-fuel ratio (A / F) and the NOx purification rate in Test Example 3.

[0031] Fig. 9 This is a graph showing the relationship between the nickel oxide content in Test Example 3 and the A / F of NOx T50.

[0032] Fig.10 This is a comparison diagram between Example 4 of Test Example 2 and Example 6 of Test Example 3.

[0033] Fig.11 This is a graph showing the relationship between the nickel oxide content and NOx T50 in Test Example 3.

[0034] Fig.12 This is a graph showing the relationship between the air-fuel ratio (A / F) and the NOx purification rate in Test Example 4.

[0035] Fig.13 This is a comparison chart of the NOx purification rate in the lean environment of Test Example 4.

[0036] Fig.14 It is a diagram showing the relationship between the air-fuel ratio (A / F) and the purification rate of unburned components (CO, HC, NOx).

[0037] Fig.15 This is a comparison chart of the NOx purification rates when the air-fuel ratio (A / F) of Test Example 3 is 14.6. DETAILED DESCRIPTION

[0038] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In addition, matters other than those specifically mentioned in this specification and necessary for the implementation of the present invention can be grasped as design matters for those skilled in the art based on the prior art in this field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in this field. In addition, in the following drawings, the same symbols are marked for components and parts that play the same role, and repeated descriptions are sometimes omitted or simplified. The dimensional relationships (length, width, thickness, etc.) in each figure do not necessarily reflect the actual dimensional relationships. In addition, the mark "A to B" (A and B are arbitrary numerical values) indicating the range in this specification means above A and below B, and includes the meanings of "preferably greater than A" and "preferably less than B".

[0039] 《Exhaust purification system》

[0040] Figure 1 Schematic diagram of an exhaust gas purification system 1. The exhaust gas purification system 1 includes an internal combustion engine (engine) 2, an exhaust gas purification device 3, and an engine control unit (ECU) 7. The exhaust gas purification system 1 is configured to purify unburned components such as HC, CO, and NOx contained in the exhaust gas discharged from the internal combustion engine 2 by using the exhaust gas purification device 3. It should be noted that Figure 1 The arrow in represents the flow direction of the exhaust gas. In the following description, the side of the exhaust gas flow close to the internal combustion engine 2 is referred to as the upstream side, and the side away from the internal combustion engine 2 is referred to as the downstream side.

[0041] In the present embodiment, the internal combustion engine 2 is mainly composed of a gasoline engine of a gasoline vehicle. However, the internal combustion engine 2 may be an engine other than a gasoline engine, such as a diesel engine or an engine mounted on a hybrid vehicle. The internal combustion engine 2 has a combustion chamber (not shown). The combustion chamber is connected to a fuel tank (not shown). In the fuel tank, gasoline is stored. Among them, the fuel stored in the fuel tank may be diesel fuel (light oil) or the like. The fuel supplied from the fuel tank is mixed with oxygen and burned in the combustion chamber. Thus, the combustion heat energy is converted into mechanical energy. The combustion chamber is connected to the exhaust port 2a. The exhaust port 2a is connected to the exhaust purification device 3. The fuel gas after combustion becomes exhaust gas, which is discharged to the exhaust purification device 3. The exhaust gas contains unburned components (harmful components).

[0042] The exhaust purification device 3 has an exhaust passage 4 connected to the internal combustion engine 2, an oxygen sensor 8, a first catalyst 10 and a second catalyst 9. The exhaust passage 4 is an exhaust flow passage through which the exhaust gas flows. In the present embodiment, the exhaust passage 4 has an exhaust manifold 5 and an exhaust pipe 6. The end on the upstream side of the exhaust manifold 5 is connected to the exhaust port 2a of the internal combustion engine 2. The end on the downstream side of the exhaust manifold 5 is connected to the exhaust pipe 6. On the way of the exhaust pipe 6, the first catalyst 10 and the second catalyst 9 are arranged in sequence from the upstream side. However, the arrangement of the first catalyst 10 and the second catalyst 9 can be changed arbitrarily. In addition, the number of the first catalyst 10 and the second catalyst 9 is not particularly limited, and a plurality of them can be respectively provided. In addition, the second catalyst 9 is not necessary and can be omitted in other embodiments.

[0043] Here, the first catalyst 10 is the catalyst that first contacts the exhaust gas. The first catalyst 10 has the function of purifying the unburned components in the exhaust gas while generating H2, and the details will be described below. The first catalyst 10 is an example of an "exhaust gas purification catalyst" described in this specification. It should be noted that, hereinafter, the first catalyst 10 is sometimes also referred to as an "exhaust gas purification catalyst." The structure of the first catalyst (exhaust gas purification catalyst) 10 will be described in detail later. The second catalyst 9 can be the same as the existing one and is not particularly limited. The second catalyst 9 can be, for example, a three-way catalyst that simultaneously purifies HC, CO and NOx contained in the exhaust gas; a gasoline particulate filter (GPF: Gasoline Particulate Filter) that removes PM contained in the exhaust gas, etc.

[0044] It should be noted that, on the upstream side of the first catalyst 10, there may also be arranged: a catalyst having a different structure from the first catalyst 10 and the second catalyst 9, for example, a diesel particulate filter (DPF: Diesel Particulate Filter) for removing PM contained in the exhaust gas; a diesel oxidation catalyst (DOC: Diesel Oxidation Catalyst) for purifying HC and CO contained in the exhaust gas; a NOx adsorption reduction (NSR: NOx Storage-Reduction) catalyst that absorbs NOx during normal operation (under lean conditions) and uses HC and CO as reducing agents to purify NOx when a large amount of fuel is injected (under rich conditions), etc.

[0045] ECU7 controls the internal combustion engine 2 and the exhaust purification device 3. ECU7 is electrically connected to the internal combustion engine 2 and sensors (such as oxygen sensor 8, temperature sensor not shown, pressure sensor, etc.) provided at various locations of the exhaust purification device 3. It should be noted that the structure of ECU7 can be the same as that of the prior art and is not particularly limited. For example, ECU7 is a processor or an integrated circuit. ECU7 has an input port (not shown) and an output port (not shown). ECU7 receives information such as the operating state of the vehicle, the flow rate, temperature, and pressure of the exhaust gas discharged from the internal combustion engine 2. ECU7 receives information detected by the sensor (such as the amount of oxygen measured by the oxygen sensor 8) through the input port. ECU7 sends a control signal through the output port based on the received information, for example. ECU7 controls the operation of the internal combustion engine 2, such as fuel injection control, ignition control, and intake air volume adjustment control. ECU7, for example, controls the operation of the internal combustion engine 2 so that the air-fuel ratio (A / F) of the exhaust gas reaches an ideal state. ECU7 controls the driving and stopping of the exhaust purification device 3 based on, for example, the operating state of the internal combustion engine 2 and the amount of exhaust gas discharged from the internal combustion engine 2.

[0046] 《Catalysts for exhaust gas purification》

[0047] Figure 2 It is a schematic perspective view of the exhaust gas purification catalyst 10 . Figure 3 2 is a schematic cross-sectional view showing a cross section of the exhaust gas purification catalyst 10 cut along the cylinder axis direction X. It should be noted that: Figure 2 , Figure 3 The arrows in the figure indicate the direction of exhaust gas flow. Figure 2 , Figure 3 In the figure, the left side represents the upstream side of the exhaust passage 4 relatively close to the internal combustion engine 2, and the right side represents the downstream side of the exhaust passage relatively far from the internal combustion engine 2. In addition, the symbol X represents the axial direction of the exhaust gas purification catalyst 10. The exhaust gas purification catalyst 10 is arranged in the exhaust passage 4 in such a way that the axial direction X is along the flow direction of the exhaust gas. In the following, one direction X1 in the axial direction X is referred to as the upstream side (also called the exhaust gas inflow side, the front side), and the other direction X2 is referred to as the downstream side (also called the exhaust gas outflow side, the rear side). However, this is only a direction specified for the convenience of explanation, and is not any limitation on the setting method of the exhaust gas purification catalyst 10. In addition, in this specification, "content" refers to the mass of the substrate per unit volume (1L).

[0048] The exhaust gas purification catalyst 10 has the function of generating H2 and purifying the unburned components in the exhaust gas. The end of the exhaust gas purification catalyst 10 in one direction X1 is the exhaust gas inlet 10a, and the end in the other direction X2 is the exhaust gas outlet 10b. In the present embodiment, the shape of the exhaust gas purification catalyst 10 is cylindrical. However, the shape of the exhaust gas purification catalyst 10 is not particularly limited, for example, it may also be an elliptical cylinder, a polygonal cylinder, a tubular, a foamed, a granular, a fibrous, etc. The exhaust gas purification catalyst 10 has a substrate 11 of a DC type structure, a reforming reaction layer 20 (refer to Figure 3 ) and the catalyst layer 30 (refer to Figure 3 ).

[0049] The substrate 11 is a member constituting the skeleton of the exhaust gas purification catalyst 10. The substrate 11 is not particularly limited, and various materials and forms currently used for such applications can be adopted. The substrate 11 can be a ceramic carrier composed of ceramics such as cordierite, aluminum titanate, and silicon carbide, or a metal carrier composed of stainless steel (SUS), Fe-Cr-Al alloy, Ni-Cr-Al alloy, etc. Figure 2 As shown, in the present embodiment, the substrate 11 has a honeycomb structure. The substrate 11 has a plurality of small chambers (cavities) 12 regularly arranged along the cylindrical axis direction X and partition walls (rib walls) 14 separating the plurality of small chambers 12. Although not a particularly limited matter, the length (average length) L of the substrate 11 along the cylindrical axis direction X can generally be 10 to 500 mm, for example, 50 to 300 mm. In addition, the volume of the substrate 11 can generally be 0.1 to 10 L, for example, 0.5 to 5 L. It should be noted that in this specification, the "volume of the substrate" refers to the apparent volume (bulk volume) of the volume of the internal small chambers 12 in addition to the volume (pure volume) of the substrate 11 itself.

[0050] The chamber 12 is a flow channel for the exhaust gas. The chamber 12 extends along the cylinder axis direction X. The chamber 12 is a through hole that penetrates the substrate 11 along the cylinder axis direction X. The shape, size, number, etc. of the chamber 12 can be designed, for example, in consideration of the flow rate and composition of the exhaust gas flowing through the exhaust purification catalyst 10. The cross-sectional shape of the chamber 12 that is orthogonal to the cylinder axis direction X is not particularly limited. The cross-sectional shape of the chamber 12 can be, for example, a square, parallelogram, rectangle, trapezoid, or other quadrilateral, and other polygonal shapes (such as triangle, hexagon, octagon), waveform, circle, or other geometric shapes. The partition wall 14 faces the chamber 12 and divides the adjacent chambers 12.

[0051] The reforming reaction layer 20 is a reaction site that converts unburned components in the exhaust gas into H2. Figure 3As shown, in the present embodiment, the reforming reaction layer 20 is provided between the substrate 11 and the catalyst layer 30 in the thickness direction orthogonal to the cylinder axis direction X. The reforming reaction layer 20 is specifically arranged on the substrate 11, and more specifically on the surface of the partition wall 14. This can suppress the situation where substances that are excessively reduced by H2 (for example, ammonia generated by the reduction of NOx) are discharged from the exhaust gas purification catalyst 10 without reoxidation. In addition, since the exhaust gas contacts the catalyst layer 30 before the reforming reaction layer 20, the active sites of the hydrogen production catalyst described later are not easily deactivated by coking, etc., and the durability of the reforming reaction layer 20 can be improved.

[0052] In the present embodiment, the reforming reaction layer 20 is arranged in a manner along the cylinder axis direction X from the exhaust gas inlet 10a toward the downstream side. However, in other embodiments, it can also be arranged in a manner along the cylinder axis direction X from the exhaust gas outlet 10b toward the upstream side. The reforming reaction layer 20 can be continuously arranged on the substrate 11, or it can be intermittently arranged. The overall coating width (average length) L1 of the reforming reaction layer 20 in the cylinder axis direction X can be designed after considering, for example, the size of the substrate 11, the flow rate of the exhaust gas flowing through the exhaust gas purification catalyst 10, etc. In some embodiments, when the total length of the exhaust gas purification catalyst 10 in the cylinder axis direction X is set to L, the coating width L1 of the reforming reaction layer 20 satisfies 0.3L≤L1<L, and preferably satisfies 0.5L≤L1≤0.8L. In this way, the reforming reaction layer 20 can be effectively utilized to exert the technical effects described in this specification at a higher level. From the viewpoint of durability and peeling resistance, the overall coating thickness of the reforming reaction layer 20 (average length in the thickness direction perpendicular to the cylinder axis direction X) is usually 1 to 100 μm, for example, about 5 to 100 μm.

[0053] The reforming reaction layer 20 must contain a hydrogen production catalyst. In the present embodiment, the reforming reaction layer 20 may also contain inorganic oxides other than the hydrogen production catalyst. Specifically, it includes OSC materials with oxygen storage capacity (Oxygen Storage Capacity: OSC) and non-OSC materials without oxygen storage capacity. However, inorganic oxides other than the hydrogen production catalyst (OSC materials and / or non-OSC materials) are not necessary and may be omitted in other embodiments. In the present embodiment, the reforming reaction layer 20 does not contain precious metals other than Rh and Ru. Thereby, the competition with the exhaust purification reaction caused by the oxidation reaction can be avoided, and the hydrogen production reaction can be better promoted. The reforming reaction layer 20 may contain Rh and / or Ru with high hydrogen production reaction activity.

[0054] A hydrogen production catalyst is a catalyst that initiates a hydrogen production reaction that converts unburned components in exhaust gas into H2. More specifically, it is a catalyst that converts HC and CO into H2 through, for example, a steam reforming reaction (HC+H2O→CO2+H2) and / or a CO shift reaction (CO+H2O→CO2+H2). The hydrogen production catalyst can also be a catalyst that converts precious metals in an oxide state into metals. It should be noted that the hydrogen production catalyst can be uniformly contained along the cylinder axis direction X, or the content can be gradually changed from the upstream side to the downstream side. For example, the content can be temporarily reduced from the upstream side to the downstream side.

[0055] The hydrogen production catalyst is in a granular form. In the present invention, the hydrogen production catalyst is mixed with the OSC material and / or the non-OSC material in the reforming reaction layer 20. The hydrogen production catalyst is not supported by the OSC material or the non-OSC material, for example, but is independent of the OSC material and the non-OSC material. This can improve the heat resistance of the hydrogen production catalyst and induce the hydrogen production reaction (steam reforming reaction and / or CO shift reaction) well. In addition, the dispersibility of the hydrogen production catalyst can also be improved to inhibit sintering. The average particle size of the hydrogen production catalyst (based on the number standard observed by electron microscope) is preferably smaller than the average particle size of the OSC material and / or the non-OSC material. This can prevent the situation where the reduction reaction is difficult to proceed and maintain stable activity.

[0056] In the technology described in this specification, the hydrogen production catalyst contains at least one of a rare earth element oxide and a transition metal oxide (but not including precious metals other than Rh and Ru). This can avoid the competition with the exhaust purification reaction caused by the oxidation reaction and promote the hydrogen production reaction. The hydrogen production catalyst may contain Rh and / or Ru with high hydrogen production reaction activity, or may not contain Rh and / or Ru. The hydrogen production catalyst, for example, contains at least one of a rare earth element oxide and a transition metal oxide (but not including precious metals) as a first hydrogen production catalyst, and may also contain Rh and / or Ru as a second hydrogen production catalyst.

[0057] As rare earth element oxides, oxides of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) belonging to Group IIIB of the periodic table can be cited. Among them, oxides of lanthanide elements are preferred due to their high reactivity and good hydrogen production ability, and at least one of Pr and Nd is preferred from the viewpoints of durability, availability, etc.

[0058] As transition metal oxides (but not including precious metals other than Rh and Ru), for example, oxides of scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu) belonging to the fourth period of the periodic table, and oxides of yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), and technetium (Tc) belonging to the fifth period of the periodic table can be cited. Among them, oxides of metals belonging to the fourth period of the periodic table are preferred due to their high reactivity and good hydrogen production ability, and oxides of iron (Fe, Co, Ni) are more preferred, and Ni is particularly preferred. In addition, from the viewpoint of durability, at least one of Co and Ni is preferred.

[0059] In this embodiment, the content of the hydrogen production catalyst in the reforming reaction layer 20 is 5 g / L or more per 1L volume of the substrate, based on the oxide. This can give full play to the technical effects described in this specification, and quickly convert the unburned components (such as HC, CO) in the exhaust gas into highly reactive H2. In some embodiments, the content of the hydrogen production catalyst can be 10 g / L or more per 1L volume of the substrate.

[0060] In addition, from the viewpoint of suppressing excessive reduction of unburned components (for example, excessive reduction of NOx to generate ammonia), the content of the hydrogen production catalyst is generally 100 g / L or less, preferably 80 g / L or less, for example, 50 g / L or less, relative to each 1L volume of the substrate. In addition, from the viewpoint of having both NOx purification performance and excellent low-temperature activity (for example, warm-up performance, heat-insulating performance, etc.) under a lean environment, the content of the hydrogen production catalyst is preferably 30 g / L or less, particularly preferably 20 g / L or less, relative to each 1L volume of the substrate.

[0061] In certain embodiments, when the hydrogen production catalyst contains a rare earth element oxide and a transition metal oxide (but not including a noble metal) as a first hydrogen production catalyst, and Rh and / or Ru as a second hydrogen production catalyst, the content of the second hydrogen production catalyst is typically less than that of the first hydrogen production catalyst. The content of the second hydrogen production catalyst is generally 0.1 to 50% of the content of the first hydrogen production catalyst, for example 1 to 30%, preferably 2 to 20%.

[0062] In the technology described in this specification, the OSC material has the function of promoting the hydrogen production reaction (steam reforming reaction and / or CO conversion reaction). The OSC material can function as a catalyst promoter. More specifically, in the reforming reaction layer 20, the OSC material is arranged adjacent to the hydrogen production catalyst. Although not a restrictive interpretation, according to the inference of the inventor of the present invention, the OSC material is believed to have the function of dissociating H2O in the exhaust gas and supplying O atoms to the hydrogen production catalyst. Therefore, it is believed to be able to promote the hydrogen production reaction. In addition, as has been known for a long time, the OSC material has the function of serving as an absorbent material for absorbing and storing oxygen in a lean environment. Through these synergistic effects, due to the presence of the OSC material, the purification performance can be particularly stabilized even when the air-fuel ratio of the exhaust gas fluctuates from an ideal state. For example, at least one of the NOx purification performance and the HC purification performance (preferably both) can be improved.

[0063] As OSC materials, for example, metal oxides (Ce-containing oxides) containing cerium oxide (CeO2) with high oxygen storage capacity can be cited. Ce-containing oxides can be cerium oxide, or a composite oxide of cerium oxide and a metal oxide other than cerium oxide. From the viewpoint of improving heat resistance and durability, the Ce-containing oxide can be a composite oxide containing at least one of Zr and Al, such as cerium oxide (CeO2)-zirconium oxide (ZrO2) composite oxide (CZ composite oxide). From the viewpoint of improving heat resistance, the CZ composite oxide can also contain, for example, Nd2O3, La2O3, Y2O3, Pr6O 10 Rare earth metal oxides such as CZ composite oxides. The CZ composite oxide may be rich in Ce or Zr. In some embodiments, the mixing ratio of cerium oxide is generally 10 to 90% by mass, for example, 15 to 70% by mass, when the entire CZ composite oxide is 100% by mass. When the mixing ratio of cerium oxide is above the specified value, the conversion of unburned components into H2 can be promoted; when the mixing ratio of cerium dioxide is below the specified value, the heat resistance can be improved. When within the above range, the technical effects and heat resistance described in this specification can be achieved at a high level.

[0064] Although not particularly limited, the content of the OSC material in the reforming reaction layer 20 is generally 1 to 100 g / L per 1L volume of the substrate, for example, 5 to 50 g / L. In addition, in certain embodiments, the ratio of the content of the OSC material to the content of the hydrogen production catalyst (C2 / C1) is generally 0.1 or more, 0.2 or more, preferably 0.5 or more, for example, 1 or more, 2 or more, 3 or more, generally 30 or less, 20 or less, preferably 10 or less, for example, 5 or less. In this way, not only can the hydrogen production reaction be promoted, but also the coating amount of the reforming reaction layer 20 can be suppressed, and the pressure loss and cost can be reduced.

[0065] The non-OSC material has at least one of the functions of improving the heat resistance of the reforming reaction layer 20, improving the durability of the reforming reaction layer 20, and inhibiting the reforming reaction layer 20 from peeling off from the substrate 11. As non-OSC materials, for example, rare earth metal oxides such as aluminum oxide (Al2O3), titanium dioxide (TiO2), zirconium oxide (ZrO2), and silicon dioxide (SiO2) can be cited. Among them, metal oxides (containing Al oxides) containing aluminum oxide with high heat resistance and durability are preferred. The Al-containing oxide may be aluminum oxide, or it may be a composite oxide of aluminum oxide and a metal oxide other than aluminum oxide (such as a rare earth metal oxide). From the viewpoint of improving heat resistance and durability, the Al-containing oxide may be, for example, a La2O3-Al2O3 composite oxide (LA composite oxide). The LA composite oxide may be a La-rich oxide or an Al-rich oxide. In certain embodiments, from the viewpoint of suppressing aging degradation with use, the mixing ratio of the metal oxide other than aluminum oxide is usually less than 50 mass %, for example, 0.1 to 20 mass %, when the entire LA composite oxide is 100 mass %.

[0066] Although not particularly limited, the content of the non-OSC material in the reforming reaction layer 20 may be lower than that of the OSC material. The content of the non-OSC material may generally be 1 to 100 g / L, for example, 5 to 50 g / L, per 1 L volume of the substrate.

[0067] In the reforming reaction layer 20, the main body (accounting for more than 50% by mass of the total) of the hydrogen production catalyst can be formed, the main body can be formed by the OSC material, and the main body can also be formed by the non-OSC material. From the perspective of improving the dispersibility of the hydrogen production catalyst and inhibiting sintering, it is preferred that the main body of the reforming reaction layer 20 is formed by the OSC material and / or the non-OSC material (when both are contained, the total of the two can be used). Although there is no particular limitation, when the reforming reaction layer 20 as a whole is 100% by mass, from the perspective of being able to play the technical effects described in this specification for a long time, the content ratio of the hydrogen production catalyst is generally 5 to 90% by mass, preferably 10 to 70% by mass. In addition, from the perspective of promoting the hydrogen production reaction, the content ratio of the OSC material is generally 1 to 70% by mass, preferably 5 to 65% by mass. In addition, from the perspective of improving heat resistance, durability, peeling resistance, etc., the content ratio of the non-OSC material is generally 10% by mass or more, preferably 25% by mass or more, for example, 30 to 50% by mass.

[0068] The reforming reaction layer 20 may be composed of a hydrogen production catalyst, an OSC material, and a non-OSC material, and may also contain optional components. For example, when the reforming reaction layer 20 contains an OSC material, it may contain alkaline earth elements such as calcium (Ca), barium (Ba), and strontium (Sr). Thus, the oxygen storage capacity of the OSC material can be increased in a lean environment. In addition, (catalyst) poisoning in a rich environment can be suppressed, and the activity of the hydrogen production catalyst can be stably maintained.

[0069] In some specific embodiments, the coating amount (molding amount) of the reforming reaction layer 20 is generally 10 to 200 g / L, for example, 50 to 100 g / L, relative to each 1L volume of the substrate. By satisfying the above range, the technical effects described in this specification can be exerted at a stable high level for a long time. It should be noted that in this specification, "coating amount" refers to the mass of the solid component contained in each unit volume (1L) of the substrate.

[0070] The catalyst layer 30 is a reaction site for purifying unburned components in the exhaust gas. The exhaust gas flowing into the exhaust gas purification catalyst 10 contacts the catalyst layer 30 while flowing in the flow channel (cell 12) of the exhaust gas purification catalyst 10. In this way, the exhaust gas can be purified. For example, HC and CO contained in the exhaust gas are oxidized by the catalyst layer 30 and converted (purified) into water, carbon dioxide, etc. NOx contained in the exhaust gas is reduced by the catalyst layer 30 and converted (purified) into nitrogen.

[0071] like Figure 3 As shown, in the present embodiment, the catalyst layer 30 is arranged on the side farther from the substrate 11 than the reforming reaction layer 20 in the thickness direction orthogonal to the cylinder axis direction X. Specifically, the catalyst layer 30 is arranged on the surface of the reforming reaction layer 20 and is in contact with the reforming reaction layer 20. However, the catalyst layer 30 may also be arranged on the substrate 11, for example, between the substrate 11 and the reforming reaction layer 20 in the thickness direction.

[0072] The catalyst layer 30 may be arranged in a manner from the exhaust gas inlet 10a toward the downstream side along the cylinder axis direction X, or may be arranged in a manner from the exhaust gas outlet 10b toward the upstream side along the cylinder axis direction X. The catalyst layer 30 may be arranged continuously on the reforming reaction layer 20, or may be arranged intermittently. The overall coating width (average length) L2 of the catalyst layer 30 in the cylinder axis direction X may be designed, for example, after considering the size of the substrate 11, the flow rate of the exhaust gas flowing through the exhaust gas purification catalyst 10, etc. In some embodiments, when the total length of the exhaust gas purification catalyst 10 in the cylinder axis direction X is set to L, the coating width L2 of the catalyst layer 30 in the cylinder axis direction X satisfies 0.5L≤L2≤L, preferably satisfies 0.8L≤L2, for example, 0.9L≤L2. In addition, the overall coating thickness of the catalyst layer 30 (average length in the thickness direction orthogonal to the cylinder axis direction X) is generally 1 to 300 μm, for example, it may be about 5 to 100 μm.

[0073] Figure 3 The catalyst layer 30 has a laminated structure in which two catalyst coatings having different compositions are laminated in a thickness direction orthogonal to the barrel axial direction X. That is, in the present embodiment, the catalyst layer 30 has a lower layer 31 formed on the surface of the reforming reaction layer 20 and an upper layer 32 formed on the lower layer 31. The lower layer 31 is an example of the "first catalyst layer" described in this specification, and the upper layer 32 is an example of the "second catalyst layer" described in this specification. The length and thickness of the lower layer 31 and the upper layer 32 may be different from each other. In addition, the reforming reaction layer 20 may also be a single-layer structure. In addition, a third layer (intermediate layer) having different compositions and properties may be provided between the lower layer 31 and the upper layer 32. The catalyst layer 30 may have a lower layer 31, one or more intermediate layers, and an upper layer 32. In addition, in other embodiments, the catalyst layer 30 may have a front section located on the upstream side in the barrel axial direction X, and a rear section located on the downstream side of the front section and having a composition and properties different from the front section.

[0074] The lower layer 31 and the upper layer 32 each contain a noble metal as an essential component. In the present embodiment, the lower layer 31 and the upper layer 32 further contain a supporting material for supporting the noble metal. As the supporting material, for example, the above-mentioned materials as the OSC material or non-OSC material of the reforming reaction layer 20 can be used. As specific examples, Ce-containing oxides such as ceria and CZ composite oxides, and Al-containing oxides such as alumina can be cited.

[0075] Precious metals are catalysts for purifying unburned components in exhaust gas. There are no particular restrictions on the precious metals, and one or more of the various types of metals known to be used for this purpose and that act as oxidation catalysts or reduction catalysts in the purification of unburned components can be used. As specific examples, metals belonging to the platinum group (PGM), namely rhodium (Rh), palladium (Pd), platinum (Pt), ruthenium (Ru), osmium (Os) and iridium (Ir), or precious metals outside the platinum group, namely gold (Au) and silver (Ag), can be cited. Although not particularly limited, the content of precious metals in the catalyst layer 30 is typically less than the content of the hydrogen production catalyst in the reforming reaction layer 20. The content of precious metals is generally 0.1 to 10 g / L per 1L volume of the substrate, for example, it can be 0.5 to 5 g / L.

[0076] In some embodiments, the catalyst layer 30 includes an oxidation catalyst layer containing an oxidation catalyst with high oxidation activity (e.g., at least one of Pd and Pt) and a reduction catalyst layer containing a reduction catalyst with high reduction activity (e.g., Rh). In this embodiment, the lower layer 31 and the upper layer 32 contain different types of precious metals. The lower layer 31 contains a first type of precious metal, and the upper layer 32 contains a second type of precious metal different from the first type. This can suppress the sintering of the precious metal.

[0077] In the present embodiment, the lower layer 31 is an oxidation catalyst layer containing an oxidation catalyst (e.g., at least one of Pd and Pt). The upper layer 32 of the present embodiment is a reduction catalyst layer containing Rh. In the present embodiment, the upper layer 32 does not contain an oxidation catalyst (e.g., at least one of Pd and Pt). By bringing the lower layer 31 (oxidation catalyst layer) containing the oxidation catalyst into contact with the reforming reaction layer 20, the deactivation of the hydrogen production catalyst can be suppressed, and high activity can be maintained for a long time. In addition, by setting the upper layer 32 containing Rh at a position away from the reforming reaction layer 20, the oxidation of the reduction catalyst caused by the use of the hydrogen production catalyst can be suppressed, and high activity can be maintained for a long time. In addition, through such a structure, the H2 generated by the hydrogen production catalyst can be used to reduce NOx and purify it into N2 and H2O, or the precious metal in the oxide state can be reduced to metal.

[0078] The catalyst layer 30 may be composed of a noble metal and a supporting material, and may also contain optional components. For example, in addition to the noble metal, it may also contain metal species such as alkali metal elements, alkaline earth metal elements, and transition metal elements. When the catalyst layer 30 contains an oxidation catalyst and an OSC material, etc., as an optional component of the reforming reaction layer 20, it is preferred to contain the alkaline earth elements described above. In addition, the catalyst layer 30 may contain a NOx adsorbent having NOx storage capacity, a stabilizer, and the like. As a stabilizer, for example, rare earth elements such as yttrium (Y), lanthanum (La), and neodymium (Nd) can be cited.

[0079] In some specific embodiments, the coating amount (molding amount) of the catalyst layer 30 is generally 10 to 500 g / L per 1 L volume of the substrate, for example, 100 to 300 g / L. By satisfying the above range, it is possible to achieve a balance between the improvement of the purification performance of the unburned components and the reduction of the pressure loss at a higher level. In addition, it is also possible to improve durability and peeling resistance.

[0080] <<Method for producing exhaust gas purification catalyst 10>>

[0081] The exhaust gas purification catalyst 10 can be manufactured by, for example, the following method. That is, first prepare a substrate 11, a slurry for forming a reforming reaction layer 20, a slurry for forming a lower layer 31, and a slurry for forming an upper layer 32. Regarding the slurry for forming the reforming reaction layer, it can be prepared by containing a hydrogen production catalyst (oxide) as an essential raw material component and dispersing other optional components such as inorganic oxides (OSC materials and / or non-OSC materials), binders, various additives, etc. in a dispersion medium. By using a hydrogen production catalyst in the form of an oxide, the hydrogen production catalyst can be included in the reforming reaction layer 20 in a state independent of the OSC material and the non-OSC material instead of being supported on the OSC material or the non-OSC material. As a binder, for example, alumina sol, silica sol, etc. can be used. As a dispersion medium, for example, water or an aqueous solvent can be used. In addition, regarding the slurry for forming the lower layer and the slurry for forming the upper layer, they are prepared by containing a noble metal source (for example, a solution containing a noble metal in ionic form) as an essential raw material component and dispersing other optional components such as a supporting material, a binder, various additives, etc. in a dispersion medium.

[0082] The formation of the reforming reaction layer 20 and the catalyst layer 30 (lower layer 31 and upper layer 32) can be carried out by existing methods such as impregnation method, washcoat method, etc. In one example, first, the slurry for forming the reforming reaction layer prepared above is flowed into the small chamber 12 from the end of the substrate 11 and supplied to a specified length along the cylindrical axis direction X. The slurry can flow in from any of the inlet 10a and the outlet 10b. At this time, the excess slurry can be sucked from the end on the opposite side. In addition, the excess slurry can also be discharged from the small chamber 12 by supplying air from the end on the opposite side. Then, the substrate 11 supplied with the slurry is fired at a specified temperature and time. The firing method can be the same as the existing one. In this way, the raw material components can be sintered on the substrate 11 to form the reforming reaction layer 20. The catalyst layer 30 can also be formed in the same way. In this way, the catalyst 10 for exhaust purification can be prepared.

[0083] 《Application of exhaust gas purification catalyst 10》

[0084] The exhaust gas purification catalyst 10 can be applied to the purification of exhaust gas discharged from vehicles such as cars or trucks, automatic two-wheeled vehicles or bicycles with prime movers, marine products such as ships, tankers, jet skis, personal watercraft, outboard motors, gardening products such as lawn mowers, electric saws, trimmers, leisure products such as golf carts, four-wheeled buggy racing cars, power generation equipment such as cogeneration systems, and internal combustion engines such as garbage incinerators. Among them, it can be applied to vehicles such as cars. In particular, the exhaust gas purification catalyst having both NOx purification performance in a poor environment and excellent low-temperature activity can be applied to environmentally friendly vehicles equipped with energy-saving mechanisms, in which the engine is frequently started and stopped during pauses such as driving or waiting for traffic lights.

[0085] Hereinafter, experimental examples related to the present invention will be described, but the present invention is not intended to be limited to the embodiments shown in the following experimental examples.

[0086] [Test Example 1] Study on the Types of Hydrogen Production Catalysts 1

[0087] (Example 1)

[0088] First, a cylindrical honeycomb substrate (made of cordierite, substrate volume: 1.3 L, substrate total length: 110 mm) was prepared, and then the following three slurries were prepared.

[0089] The slurry for forming the reforming reaction layer was 35 g / L of the ceria-zirconia composite oxide as an OSC material (17.5 g / L of ceria), 20 g / L of alumina as a non-OSC material, and 10 g / L of praseodymium oxide (Pr6O 11 ) is 10 g / L, a cerium oxide-zirconia composite oxide, aluminum oxide, a hydrogen production catalyst, and water are mixed to prepare a slurry for forming a reforming reaction layer.

[0090] ·Pd slurry for forming the lower layer: Cerium oxide-zirconia composite oxide, aluminum oxide, Pd nitrate, and water are mixed in such a manner that the content of the composite oxide is 50 g / L (including 20 g / L of cerium oxide), 50 g / L of aluminum oxide, 5 g / L of barium oxide, and 3 g / L of Pd as a precious metal to prepare a Pd slurry.

[0091] · Rh slurry for upper layer formation: Cerium oxide-zirconia composite oxide, aluminum oxide, Rh nitrate, and water are mixed in such a manner that the content of cerium oxide-zirconia composite oxide is 10 g / L (including cerium oxide of 2 g / L), aluminum oxide is 70 g / L, and Rh as a precious metal is 0.4 g / L to prepare Rh slurry.

[0092] Next, the slurry for forming the reforming reaction layer was supplied to the honeycomb substrate, and the material was washed and coated from the end of the exhaust gas inflow side to the portion corresponding to 80% of the total length of the substrate. Then, it was heated and dried in a dryer at 250°C for 1 hour to volatilize the water, and then fired in an electric furnace at 500°C for 1 hour. Thus, a reforming reaction layer was formed on the surface of the substrate.

[0093] Next, a catalyst layer with an upper and lower two-layer structure is formed on the reforming reaction layer. Specifically, Pd slurry is first supplied to the substrate forming the above-mentioned reforming reaction layer, and the material is washed from the end of the exhaust gas inlet side to the downstream side and from the end of the exhaust gas outlet side to the upstream side to the portion corresponding to 100% of the total length of the substrate. Then, after heating and drying in a dryer at 250°C for 1 hour to volatilize the water, it is fired in an electric furnace at 500°C for 1 hour. Thus, a lower layer is formed on the surface of the reforming reaction layer. Next, Rh slurry is supplied to the substrate forming the above-mentioned lower layer, and the material is washed from the end of the exhaust gas inlet side to the downstream side and from the end of the exhaust gas outlet side to the upstream side to the portion corresponding to 100% of the total length of the substrate. Then, after heating and drying in a dryer at 250°C for 1 hour to volatilize the water, it is fired in an electric furnace at 500°C for 1 hour. Thus, an upper layer is formed on the surface of the lower layer.

[0094] As described above, the exhaust gas purification catalyst of Example 1 was produced.

[0095] (Example 2)

[0096] In Example 2, an exhaust gas purification catalyst was produced in the same manner as in Example 1, except that neodymium oxide (Nd 2 O 3 ) was used as a hydrogen production catalyst when preparing the slurry for forming a reforming reaction layer.

[0097] (Comparative Example 1)

[0098] In Comparative Example 1, an exhaust gas purification catalyst was produced in the same manner as in Example 1 except that no reforming reaction layer was formed on the substrate, that is, a catalyst layer having only an upper and lower two-layer structure was formed on the substrate using Pd slurry for forming a lower layer and Rh slurry for forming an upper layer.

[0099] [Evaluation of NOx purification performance]

[0100] The NOx purification performance of each exhaust purification catalyst was evaluated using a catalyst evaluation device equipped with a gasoline engine. Specifically, each exhaust purification catalyst was set in the catalyst evaluation device, and a simulated exhaust gas with a changed air-fuel ratio (A / F) was supplied to the engine. The NOx purification rate was determined based on the ratio of the NOx concentration of the gas flowing into the exhaust purification catalyst to the NOx concentration of the gas flowing out of the exhaust purification catalyst. The results are shown in Figure 4In addition, the comparison of NOx purification rate under lean environment (A / F=14.57) is shown in Figure 5 .

[0101] like Figure 4 and Figure 5 As shown, the exhaust gas purification catalysts of Examples 1 and 2, which have a reforming reaction layer containing 5 g / L or more of rare earth element oxide as a hydrogen production catalyst, have relatively higher NOx purification performance under lean conditions than the exhaust gas purification catalyst of Comparative Example 1, which does not have a reforming reaction layer. It is believed that this is because the reforming reaction layer converts the unburned components (such as HC, CO) in the exhaust gas into highly reactive hydrogen molecules (H2), thereby increasing the oxidation reaction rate of the exhaust gas, and because the reforming reaction layer does not contain an oxidation catalyst, competition with the exhaust gas purification reaction caused by the oxidation catalyst is avoided, thereby achieving a synergistic effect.

[0102] [Test Example 2] Study on the Types of Hydrogen Production Catalysts 2

[0103] (Example 3)

[0104] In Example 3, an exhaust gas purification catalyst was prepared in the same manner as in Example 1, except that cobalt oxide (CoO) was used as the hydrogen production catalyst when preparing the slurry for forming the reforming reaction layer.

[0105] (Example 4)

[0106] In Example 4, an exhaust gas purification catalyst was prepared in the same manner as in Example 1, except that nickel oxide (NiO) was used as the hydrogen production catalyst when preparing the slurry for forming the reforming reaction layer.

[0107] Then, the NOx purification rate was measured in the same manner as in Test Example 1. The results are shown together with the results of Comparative Example 1 which does not have a reforming reaction layer. Figure 6 In addition, the comparison of NOx purification rate under lean environment (A / F=14.6) is shown in Figure 7 .

[0108] like Figure 6 and Figure 7 As shown, the exhaust gas purification catalysts of Examples 3 and 4 having a reforming reaction layer containing 5 g / L or more of a transition metal oxide as a hydrogen production catalyst have relatively higher NOx purification performance under lean conditions than the exhaust gas purification catalyst of Comparative Example 1 having no reforming reaction layer. Among them, in Example 4 using nickel oxide (nickel as an active species), the NOx purification rate exceeds 99.9%, and the NOx purification performance under lean conditions is particularly high.

[0109] [Test Example 3] Study on the content of hydrogen production catalyst

[0110] (Examples 5 to 9, Comparative Example 2)

[0111] In Examples 5 to 9, when preparing the slurry for forming the reforming reaction layer, the contents of nickel oxide (NiO) as a hydrogen production catalyst and aluminum oxide as a non-OSC material were changed as shown in Table 1, and no OSC material was used. Except for this, an exhaust gas purification catalyst was prepared in the same manner as in Example 4.

[0112] [Table 1]

[0113]

[0114] Then, the NOx purification rate was measured in the same manner as in Test Example 1. The results are shown in Figure 8 In addition, the comparison chart of nickel oxide content and A / F of NOxT50 is shown in Fig. 9 .

[0115] like Figure 8 and Fig. 9 As shown, by increasing the content of the hydrogen production catalyst (nickel oxide), the NOx purification performance under lean environment is improved. In particular, it was found that by setting the hydrogen production catalyst (nickel oxide) to 10 g / L or more, the ideal operating range can be greatly expanded.

[0116] In addition, a comparison chart of the results of Example 6 not using an OSC material and the results of Example 4 containing an OSC material is shown in FIG. Fig.10 .like Fig.10 As shown, Example 4 containing OSC material has improved NOx purification performance under lean environment compared with Example 6 without OSC material. This is considered to be the effect that the OSC material acts as a catalyst promoter and promotes the hydrogen production reaction (steam reforming reaction and / or CO shift reaction).

[0117] [Evaluation of low temperature activity]

[0118] The exhaust gas purification catalyst of each example was assembled into the exhaust system of a V-type 8-cylinder (displacement: 4600cc) gasoline engine. Then, the engine was operated at an average engine speed of 3000 rpm, and a durability test was carried out for 45 hours at a catalyst input gas temperature of 850°C, with simulated exhaust gases of ideal conditions and lean environments alternately flowing through the exhaust gas purification catalyst for a specified time. After the durability test, the catalyst input gas temperature was raised from room temperature (25°C) to 500°C at a heating rate of 20°C / min while supplying simulated exhaust gas with A / F=14.6. The NOx purification rate was determined based on the ratio of the NOx concentration of the gas flowing into the exhaust gas purification catalyst to the NOx concentration of the gas flowing out of the exhaust gas purification catalyst at this time. Then the temperature at which the NOx purification rate is 50% (NOx T50) was calculated. The graph of nickel oxide content and NOx T50 is shown in Fig.11It should be noted that the smaller the value of NOx T50 is, the better the low-temperature activity (NOx purification performance at low temperatures) is.

[0119] like Fig.11 As shown in Figure 1, when the content of hydrogen production catalyst (nickel oxide) increases, NOx T50 increases and low temperature activity deteriorates. From this, it can be seen that from the perspective of combining NOx purification performance in a lean environment with excellent low temperature activity, it is preferred to set the content of hydrogen production catalyst to 30 g / L or less, or even 20 g / L or less.

[0120] [Test Example 4] Study on the content of OSC materials

[0121] (Examples 10 to 12, Comparative Example 3)

[0122] In Examples 10 to 12 and Comparative Example 3, when preparing the slurry for forming the reforming reaction layer, the contents of nickel oxide (NiO) as a hydrogen production catalyst, cerium oxide-zirconia composite oxide as an OSC material, and aluminum oxide as a non-OSC material were changed as shown in Table 2. Except for this, an exhaust gas purification catalyst was prepared in the same manner as in Example 4.

[0123] [Table 2]

[0124]

[0125] Then, the NOx purification rate was measured in the same manner as in Test Example 1. The results are shown in Fig.12 In addition, the comparison of NOx purification rate under lean environment (A / F=14.65) is shown in Fig.13 It should be noted that Fig.13 In the figure, the horizontal axis also represents the ratio of the content of the OSC material to the content of the hydrogen production catalyst.

[0126] like Fig.12 and Fig.13 As shown in the figure, by increasing the ratio of the content of OSC material to the content of hydrogen production catalyst, the NOx purification performance in lean environment is improved. It is believed that this is because the OSC material acts as a catalyst promoter to promote the hydrogen production reaction, and the OSC material acts as an absorbent material for absorbing and storing oxygen, thereby achieving a synergistic effect.

[0127] [Test Example 5] Study on the Types of Hydrogen Production Catalysts 3

[0128] Next, a reforming reaction layer was prepared to which nickel oxide as a hydrogen production catalyst was added and any metal belonging to the platinum group (PGM), specifically palladium (Pd), platinum (Pt), rhodium (Rh), and ruthenium (Ru) was added, and performance was evaluated.

[0129] (Example 13)

[0130] A cylindrical honeycomb substrate (made of cordierite, substrate volume: 1.3 L, substrate total length: 110 mm) similar to the material used in the above-mentioned test example was prepared, and the following three slurries were prepared.

[0131] Pd-containing reforming reaction layer forming slurry: Appropriate amounts of cerium oxide-zirconia composite oxide (CZ composite oxide), aluminum oxide, nickel oxide (NiO), palladium nitrate, and water are mixed to prepare a Pd-containing reforming reaction layer forming slurry.

[0132] Pd slurry for forming a lower layer: Appropriate amounts of cerium oxide-zirconium oxide composite oxide, aluminum oxide, barium oxide, palladium nitrate, and water are mixed to prepare Pd slurry.

[0133] Rh slurry for forming the upper layer: Appropriate amounts of ceria-zirconia composite oxide, alumina, rhodium nitrate, and water are mixed to prepare Rh slurry.

[0134] Next, the slurry for forming the Pd-containing reforming reaction layer was supplied to the honeycomb substrate, and the material was washed and coated from the end of the exhaust gas inflow side to a portion corresponding to 80% of the total length of the substrate. Then, it was heated and dried in a dryer at 250°C for 1 hour to volatilize the water, and then fired in an electric furnace at 500°C for 1 hour. Thus, a Pd-containing reforming reaction layer was formed on the surface of the substrate.

[0135] Next, on the Pd-containing reforming reaction layer, the same steps as in Example 1 were followed to form a catalyst layer having an upper and lower layer structure (the lower layer being a Pd layer and the upper layer being a Rh layer).

[0136] As described above, an exhaust gas purification catalyst of Example 13 was prepared. The contents of the components per 1 L volume of the catalyst were as follows.

[0137] <Pd-Containing Reforming Reaction Layer>

[0138]

[0139] <Catalyst layer lower layer>

[0140]

[0141] <Catalyst layer upper layer>

[0142] Rh 0.4g / L-cat

[0143] CZ composite oxide 10g / L-cat (cerium oxide 2g / L-cat)

[0144] Alumina 70g / L-cat

[0145] (Example 14)

[0146] In Example 14, a slurry for forming a reforming reaction layer containing Pt was prepared in the same manner as in Example 13, except that a dinitrodiamine platinum nitrate solution was used instead of the above-mentioned palladium nitrate. Then, the exhaust gas purification catalyst of Example 14 was prepared using the slurry for forming a reforming reaction layer containing Pt, and the same Pd slurry for forming a lower catalyst layer and Rh slurry for forming an upper catalyst layer as in Example 13. It should be noted that the content of each component per 1L volume of the catalyst is as follows.

[0147] <Pt-Containing Reforming Reaction Layer>

[0148]

[0149]

[0150] <Catalyst layer lower layer> and <Catalyst layer upper layer> are the same as in Example 13.

[0151] (Example 15)

[0152] In Example 15, a slurry for forming a reforming reaction layer containing Rh was prepared in the same manner as in Example 13, except that rhodium nitrate was used instead of the palladium nitrate. Then, the exhaust gas purification catalyst of Example 15 was prepared using the slurry for forming a reforming reaction layer containing Rh, and the Pd slurry for forming a lower catalyst layer and the Rh slurry for forming an upper catalyst layer, which were the same as in Example 13. It should be noted that the content of each component per 1L volume of the catalyst is as follows.

[0153] <Rh-containing reforming reaction layer>

[0154]

[0155] <Catalyst layer lower layer> and <Catalyst layer upper layer> are the same as in Example 13.

[0156] (Example 16)

[0157] In Example 16, a Ru-containing reforming reaction layer forming slurry was prepared in the same manner as in Example 13, except that ruthenium nitrate was used instead of the above-mentioned palladium nitrate. Then, the exhaust gas purification catalyst of Example 16 was prepared using the Ru-containing reforming reaction layer forming slurry, the same Pd slurry for forming a catalyst lower layer and the same Rh slurry for forming a catalyst upper layer as in Example 13. It should be noted that the content of each component per 1L volume of the catalyst is as follows.

[0158] <Rh-containing reforming reaction layer>

[0159]

[0160] <Catalyst layer lower layer> and <Catalyst layer upper layer> are the same as in Example 13.

[0161] Table 3 summarizes the structures of the reforming reaction layers of the exhaust gas purifying catalysts of Examples 13 to 16 prepared in this manner.

[0162] [Table 3]

[0163]

[0164] [Evaluation of NOx purification performance]

[0165] Then, using the obtained exhaust gas purification catalysts of Examples 13 to 16, the exhaust gas purification catalyst of Example 4 (i.e., the reforming reaction layer does not contain PGM. See Table 3) and the exhaust gas purification catalyst of Comparative Example 2 (i.e., no reforming reaction layer. See Table 1), the endurance test was carried out in the same manner as in Test Example 1, and the NOx purification rate was measured. The results of the comparison of the NOx purification rate at A / F: 14.6 are shown in FIG. Fig.15 .

[0166] like Fig.15 As shown, regarding the NOx purification rate under the above conditions, the exhaust gas purification catalyst of Example 4 has the highest NOx purification rate of 94.1%. Secondly, the exhaust gas purification catalysts of Example 15 and Example 16 have NOx purification rates of 87.2% and 85.2%, respectively, and maintain good NOx purification rates even when Rh or Ru is added to the reforming reaction layer.

[0167] On the other hand, the NOx purification rates of the exhaust gas purification catalyst of Example 13 and the exhaust gas purification catalyst of Example 14 were 80.6% and 79.1%, respectively, and the NOx purification rate in the case where Pd or Pt was added to the reforming reaction layer was lower than the NOx purification rate in the case where Rh or Ru was added to the reforming reaction layer. However, it showed a value higher than the NOx purification rate (78.0%) of the exhaust gas purification catalyst of Comparative Example 2 without a reforming reaction layer.

[0168] This result is believed to be due to the fact that when PGM is added to the reforming reaction layer, competitive adsorption with the hydrogen production catalyst (nickel oxide in this embodiment) occurs, and the reforming reaction is inhibited. However, when Rh or Ru is added as PGM, such inhibition is not observed, and a more favorable effect is achieved compared to the comparative example without the reforming reaction layer. It is believed that when Rh or Ru is added, these PGMs can cause the same reforming reaction as nickel.

[0169] Alternatively, by adding these PGMs, the reforming reaction layer can be endowed with a so-called three-way catalyst function.

[0170] Some embodiments of the present invention have been described above, but the above embodiment is only an example. The present invention can be implemented in various other ways. The present invention can be implemented based on the contents described in this specification and the technical common sense in the field. The technology described in the scope of protection of the present invention includes various deformations and modified modes of the above-mentioned embodiments. For example, part of the above embodiment can be replaced by other modified embodiments, and other modified embodiments can be added to the above embodiment. In addition, its technical features can be deleted as appropriate as long as they are not described as mandatory items.

[0171] For example, in the above-mentioned exhaust purification catalyst 10, the reforming reaction layer 20 is arranged on the substrate 11, and the catalyst layer 30 is arranged on the reforming reaction layer 20. However, it is not limited to this. In other embodiments, the exhaust purification catalyst may also be configured, for example, on the substrate 11, and the catalyst layer 30 is configured on the catalyst layer 30. Alternatively, the reforming reaction layer 20 and the catalyst layer 30 may not be stacked in the thickness direction, but may be arranged in parallel along the cylinder axis direction X. For example, the reforming reaction layer 20 and the reforming reaction layer 20 may be configured to be divided into the upstream side and the downstream side of the cylinder axis direction X. In addition, the reforming reaction layer 20 and the reforming reaction layer 20 may be respectively configured on different substrates.

Claims

1. An exhaust gas purification catalyst, which is arranged in an exhaust passage of an internal combustion engine and purifies unburned components in the exhaust gas discharged from the internal combustion engine, wherein the exhaust gas purification catalyst comprises: Base material; a reforming reaction layer, which is disposed on the substrate and contains at least one of a rare earth element oxide and a transition metal oxide as a hydrogen production catalyst for converting the unburned components in the exhaust gas into H2, wherein the transition metal oxide does not include precious metals other than Rh and Ru; and a catalyst layer containing a noble metal for purifying the unburned components, The content of the hydrogen production catalyst in the reforming reaction layer is 5 g / L or more per unit volume of the substrate. The catalyst layer is disposed on the reforming reaction layer. The reforming reaction layer is provided shorter than the substrate along the extension direction from the end portion of the substrate on the exhaust gas inflow side. The catalyst layer is provided longer than the reforming reaction layer along an extension direction from an end portion of the substrate on the exhaust gas inflow side. The reforming reaction layer further contains an OSC material having oxygen storage capacity. The hydrogen production catalyst is not supported by the OSC material but is independent of the OSC material.

2. The exhaust gas purification catalyst according to claim 1, characterized in that: In the reforming reaction layer, the ratio of the content of the OSC material to the content of the hydrogen production catalyst is 0.5-10.

3. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that: The content of the hydrogen production catalyst is less than 20 g / L.

4. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that: The hydrogen production catalyst contains at least one of Pr, Nd, Co and Ni.

5. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that: The reforming reaction layer does not contain precious metals.

6. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that: The catalyst layer has: a first catalyst layer containing an oxidation catalyst as the noble metal; and A second catalyst layer is provided on the first catalyst layer and contains Rh as the noble metal.

7. The exhaust gas purification catalyst according to claim 1, characterized in that: The catalyst layer is provided to have the same length as that of the substrate.

8. The exhaust gas purification catalyst according to claim 1, characterized in that: The catalyst layer constitutes the outermost layer.

9. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that: The coating amount of the reforming reaction layer is 50 g / L or more and 100 g / L or less per 1 L volume of the substrate.

10. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that: The hydrogen production catalyst is the rare earth element oxide.

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