Exhaust gas purification catalyst

Abstract

To provide a novel exhaust-gas purification catalyst that has improved HC and CO purifying performance in a rich atmosphere.SOLUTION: The exhaust gas purification catalyst 10 disclosed herein includes a substrate 11, a catalyst layer 20 provided on the substrate 11, and a reforming reaction layer 30 provided on the substrate 11. The catalyst layer 20 contains a three-way catalyst. The reforming reaction layer 30 contains a hydrogen formation catalyst. The reforming reaction layer 30 is disposed in a position of downstream side of the catalyst layer 20 in the exhaust-gas flow direction.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a catalyst for purifying exhaust gases. [Background technology]

[0002] Exhaust gases emitted from internal combustion engines such as automobile engines contain harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Exhaust gas purification catalysts have traditionally been used to efficiently capture and remove these harmful components from exhaust gases.

[0003] Patent Document 1 is an example of a prior art document related to exhaust gas purification catalysts. As described in Patent Document 1, exhaust gas purification catalysts typically contain a three-way catalyst as a catalyst that can simultaneously and efficiently remove the above-mentioned harmful components. Patent Document 1 also discloses the use of a catalyst containing nickel or palladium as a fuel reforming catalyst in an exhaust gas treatment system. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2016-513198 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, exhaust gas regulations have become increasingly strict, as exemplified by regulations such as LEVIII and Euro 7. In order to comply with these regulations, it is necessary to reduce not only emissions during cold starts, but also emissions during hot starts (hereinafter also referred to as "hot emissions").

[0006] Three-way catalysts purify exhaust gases through three-way reactions (i.e., oxidation of HC, oxidation of CO, and reduction of NOx). However, because NOx reduction is difficult in lean atmospheres with excess oxygen, exhaust gas purification systems often employ feedback control (FB) to shift from stoichiometric to slightly rich atmospheres. However, during rich spikes after fuel cuts or high-load operation, the atmosphere shifts to the rich side. In rich atmospheres, oxygen is insufficient, preventing sufficient oxidation of HC and CO. As a result, some HC and CO may be emitted without being purified. Furthermore, because oxygen is consumed in large quantities upstream of the exhaust gas purification catalyst in the direction of exhaust gas flow, oxygen is likely to be insufficient downstream, especially in rich atmospheres. This also leads to some HC and CO being emitted without being purified. Therefore, to reduce hot emissions, there is a need for improved HC and CO purification performance in rich atmospheres.

[0007] 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 that has improved performance for purifying HC and CO in a rich atmosphere. [Means for solving the problem]

[0008] The exhaust gas purifying catalyst disclosed herein comprises a substrate, a catalyst layer provided on the substrate, and and a reforming reaction layer provided on the substrate, the catalyst layer containing a three-way catalyst, the reforming reaction layer containing a hydrogen generation catalyst, and the reforming reaction layer being disposed downstream of the catalyst layer in the flow direction of the exhaust gas.

[0009] The hydrogen generation catalyst uses water to convert HC and CO to H through a steam reforming reaction (HC + HO → CO + H) and a CO shift reaction (CO + HO → CO + H). Water is abundant in exhaust gas because it is produced during the combustion of fuel (typically gasoline) and the oxidation of HC. Therefore, with this configuration, the reforming reaction layer 30 can convert HC and CO to carbon dioxide and H using water instead of oxygen. Therefore, by locating the reforming reaction layer 30 downstream of the catalyst layer 20 in the exhaust gas flow direction in the exhaust gas purification catalyst 10, HC and CO can be removed at a high purification rate even under oxygen-deficient conditions. In other words, with this configuration, a novel exhaust gas purification catalyst with improved HC and CO purification performance in a rich atmosphere can be provided.

[0010] In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the hydrogen generation catalyst contains an oxide of at least one element selected from the group consisting of Pr, Nd, Co, and Ni. This configuration further improves the HC and CO purification performance of the exhaust gas purification catalyst in a rich atmosphere.

[0011] In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the hydrogen generation catalyst contains nickel oxide. With this configuration, the exhaust gas purification catalyst has particularly high HC and CO purification performance in a rich atmosphere.

[0012] In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the reforming reaction layer further contains an OSC material having oxygen storage capacity. With this configuration, the OSC material can promote the hydrogen generation reaction (steam reforming reaction and / or CO shift reaction) by the hydrogen generation catalyst, thereby particularly improving the HC and CO purification performance of the exhaust gas purification catalyst in a rich atmosphere.

[0013] In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the content of the hydrogen generation catalyst in the reforming reaction layer is 5 g / L or more per L of the volume of the portion of the substrate on which the reforming reaction layer is formed. With this configuration, the exhaust gas purification catalyst has particularly high HC and CO purification performance in a rich atmosphere.

[0014] In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the reforming reaction layer further contains Rh. With this configuration, CO, HC, and NOx can be removed in a balanced manner at a high purification rate. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic diagram showing an exhaust gas purification system in which an exhaust gas purification catalyst is used. [Figure 2] FIG. 2 is a perspective view schematically showing the exhaust gas purifying catalyst of FIG. 1. [Figure 3] 3 is a cross-sectional view of the exhaust gas purifying catalyst of FIG. 2 cut along the cylinder axis direction. [Figure 4] FIG. 10 is a cross-sectional view of a modified exhaust gas purifying catalyst taken along the cylinder axis direction. [Figure 5] 1 is a graph showing the relationship between the air-fuel ratio (A / F) and the CO purification rate in Test Example 1. [Figure 6] 1 is a graph showing the relationship between the air-fuel ratio (A / F) and the THC purification rate in Test Example 1. [Figure 7] 1 is a graph comparing the CO conversion rate and the THC conversion rate in Test Example 1 when the air-fuel ratio (A / F) is 14.2. [Figure 8] 1 is a graph comparing the CO conversion rate and the THC conversion rate at an air-fuel ratio (A / F) of 14.2 in Test Example 2. [Figure 9] 1 is a graph comparing the CO purification rate and the THC purification rate at an air-fuel ratio (A / F) of 14.2 in Test Example 3. [Figure 10] 1 is a graph comparing the CO purification rate and the THC purification rate at an air-fuel ratio (A / F) of 14.2 in Test Example 4. [Figure 11]1 is a graph comparing the CO conversion rate and the THC conversion rate at an air-fuel ratio (A / F) of 14.2 in Test Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0016] Preferred embodiments of the present invention will be described below with reference to the drawings. Matters necessary for implementing the present invention other than those specifically mentioned in this specification can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function are designated by the same reference numerals, and redundant explanations may be omitted or simplified. Dimensional relationships (length, width, thickness, etc.) in each drawing do not necessarily reflect actual dimensional relationships. Furthermore, in this specification, the notation "A to B" (A and B are arbitrary numbers) indicating a range means A or more and B or less.

[0017] <Exhaust gas purification system> FIG. 1 is a schematic diagram of an exhaust gas purification system 1. The exhaust gas purification system 1 includes an internal combustion 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 contained in exhaust gas emitted from the internal combustion engine 2, such as HC, CO, and NOx, using the exhaust gas purification device 3. Note that arrows in FIG. 1 indicate the flow direction of exhaust gas. In the following description, the side closer to the internal combustion engine 2 along the flow of exhaust gas will be referred to as the upstream side, and the side farther from the internal combustion engine 2 will be referred to as the downstream side.

[0018] Here, the internal combustion engine 2 is primarily configured as a gasoline engine for a gasoline vehicle. However, the internal combustion engine 2 may also be an engine other than a gasoline engine, such as a diesel engine or an engine installed in 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 this example, gasoline is stored in the fuel tank. However, the fuel stored in the fuel tank may be diesel fuel (light oil) or the like. In the combustion chamber, the fuel supplied from the fuel tank is mixed with oxygen and burned. This converts combustion energy into mechanical energy. The combustion chamber is connected to an exhaust port. The exhaust port is connected to an exhaust gas purification device 3. The burned fuel gas becomes exhaust gas and is discharged to the exhaust gas purification device 3. The exhaust gas contains unburned components (harmful components).

[0019] The exhaust gas purification device 3 includes an exhaust path 4 communicating with the internal combustion engine 2, an oxygen sensor 8, a first catalyst 10, and a second catalyst 9. The exhaust path 4 is an exhaust gas flow path through which exhaust gas flows. In this example, the exhaust path 4 includes an exhaust manifold 5 and an exhaust pipe 6. An upstream end of the exhaust manifold 5 is connected to an exhaust port of the internal combustion engine 2. A downstream end of the exhaust manifold 5 is connected to the exhaust pipe 6. The first catalyst 10 and the second catalyst 9 are arranged in this order from the upstream side in the exhaust pipe 6. However, the arrangement of the first catalyst 10 and the second catalyst 9 may be varied as desired. Furthermore, there is no particular limitation on the number of first catalysts 10 and second catalysts 9, and multiple catalysts of each may be provided. Furthermore, the second catalyst 9 is not essential and may be omitted in other embodiments.

[0020] Here, the first catalyst 10 is the catalyst that first comes into contact with the exhaust gas. As will be described in detail later, the first catalyst 10 has the function of purifying HC, CO, and NOx, which are harmful components in the exhaust gas. The first catalyst 10 is an example of the "catalyst for purifying exhaust gas" disclosed herein. Note that, hereinafter, the first catalyst 10 may also be referred to as the "catalyst for purifying exhaust gas." The configuration of the first catalyst (catalyst for purifying exhaust gas) 10 will be described in detail later. The second catalyst 9 may be the same as a conventional catalyst and is not particularly limited. The second catalyst 9 may be, for example, a three-way catalyst that simultaneously purifies HC, CO, and NOx contained in the exhaust gas; a gasoline particulate filter (GPF) that removes PM contained in the exhaust gas; or the like.

[0021] Furthermore, upstream of the first catalyst 10, there may be further arranged a catalyst having a configuration different from that of the first catalyst 10 and the second catalyst 9, such as a diesel particulate filter (DPF) that removes PM contained in exhaust gas; a diesel oxidation catalyst (DOC) that purifies HC and CO contained in exhaust gas; or a NOx storage-reduction (NSR) catalyst that stores NOx during normal operation (under lean conditions) and purifies NOx using HC and CO as reducing agents when a larger amount of fuel is injected (in a rich atmosphere).

[0022] As will be described later, hydrogen (H2) is produced when HC and CO are purified by the first catalyst 10. This hydrogen is typically emitted outside the exhaust gas purification system 1, and further outside the vehicle such as an automobile. However, although the hydrogen may be recovered and utilized, the exhaust gas containing the hydrogen is not returned to the internal combustion engine 2.

[0023] The ECU 7 controls the internal combustion engine 2 and the exhaust gas purification device 3. The ECU 7 is electrically connected to the internal combustion engine 2 and sensors (e.g., the oxygen sensor 8, and temperature and pressure sensors (not shown)) installed at various locations in the exhaust gas purification device 3. The configuration of the ECU 7 may be the same as that of a conventional device and is not particularly limited. The ECU 7 is, for example, a processor or an integrated circuit. The ECU 7 has an input port (not shown) and an output port (not shown). The ECU 7 receives information, for example, regarding the operating state of the vehicle and the amount, temperature, and pressure of exhaust gas emitted from the internal combustion engine 2. The ECU 7 receives information detected by a sensor (e.g., the amount of oxygen measured by the oxygen sensor 8) via the input port. The ECU 7 transmits a control signal via the output port based on, for example, the received information. The ECU 7 controls the operation of the internal combustion engine 2, such as fuel injection control, ignition control, and intake air amount adjustment control. The ECU 7 controls the operation of the internal combustion engine 2, for example, so that the air-fuel ratio (A / F) of the exhaust gas is stoichiometric. The ECU 7 controls the driving and stopping of the exhaust gas purification device 3 based on, for example, the operating state of the internal combustion engine 2, the amount of exhaust gas emitted from the internal combustion engine 2, and the like.

[0024] <Exhaust gas purification catalyst> FIG. 2 is a perspective view schematically showing the exhaust gas purifying catalyst 10 according to this embodiment. FIG. 3 is a cross-sectional view schematically showing a cross section of the exhaust gas purifying catalyst 10 cut along the cylinder axis direction X. The arrows in FIGS. 2 and 3 indicate the flow direction of exhaust gas. In FIGS. 2 and 3, the upstream side of the exhaust path 4 that is relatively close to the internal combustion engine 2 is shown on the left, and the downstream side of the exhaust path that is relatively far from the internal combustion engine 2 is shown on the right. The symbol X indicates the cylinder axis direction of the exhaust gas purifying catalyst 10. In the cylinder axis direction X, the X1 side is the upstream side (exhaust gas inflow side, also referred to as the front side), and the X2 side is the downstream side (exhaust gas outflow side, also referred to as the rear side). The exhaust gas purifying catalyst 10 is installed in the exhaust path 4 so that the cylinder axis direction X is along the flow direction of exhaust gas.

[0025] The exhaust gas purifying catalyst 10 has the function of purifying harmful components in exhaust gas. The X1 side end of the exhaust gas purifying catalyst 10 is an exhaust gas inlet 10a, and the X2 side end is an exhaust gas outlet 10b. The outer shape of the exhaust gas purifying catalyst 10 here is cylindrical. However, the outer shape of the exhaust gas purifying catalyst 10 is not particularly limited, and may be, for example, an elliptical cylindrical shape, a polygonal cylindrical shape, a pipe shape, a foam shape, a pellet shape, a fiber shape, or the like.

[0026] 3, the exhaust gas purifying catalyst 10 includes a substrate 11, a catalyst layer 20 formed on the substrate 11, and a reforming reaction layer 30 formed on the substrate 11. In the exhaust gas purifying catalyst 10, both the catalyst layer 20 and the reforming reaction layer 30 are formed on the single substrate 11, so the exhaust gas purifying catalyst 10 can achieve high purification performance for HC and CO in a rich atmosphere with high space efficiency.

[0027] [Base material] The substrate 11 forms the framework of the exhaust gas purification catalyst 10. The substrate 11 is not particularly limited, and various materials and shapes conventionally used for this type of application can be used. In the illustrated example, a substrate with a straight flow structure is used as the substrate 11. The substrate 11 may be, for example, a ceramic carrier made of ceramics such as cordierite, aluminum titanate, or silicon carbide, or a metal carrier made of stainless steel (SUS), an Fe-Cr-Al alloy, or an Ni-Cr-Al alloy. As shown in FIG. 2, the substrate 11 has a honeycomb structure. The substrate 11 includes a plurality of cells (cavities) 12 regularly arranged in the cylindrical axis direction X and partition walls (ribs) 14 separating the plurality of cells 12. Although not particularly limited, the length (average length) L of the substrate 11 along the cylindrical axis direction X may be approximately 10 to 500 mm, for example, 50 to 300 mm. The volume of the substrate 11 is, for example, 0.1 to 10 L, and may be 0.5 to 5 L. In this specification, the "volume of the substrate" refers to the apparent volume (bulk volume) including the volume of the substrate 11 itself (net volume) and the volume of the cells 12 inside.

[0028] The cells 12 are flow paths for exhaust gas. The cells 12 extend in the cylinder axis direction X. The cells 12 are through-holes that penetrate the substrate 11 in the cylinder axis direction X. The shape, size, number, etc. of the cells 12 may be designed, for example, taking into consideration the flow rate and components of the exhaust gas flowing through the exhaust gas purification catalyst 10. The shape of the cross section of the cells 12 perpendicular to the cylinder axis direction X is not particularly limited. The cross-sectional shape of the cells 12 may be, for example, a quadrangle such as a square, parallelogram, rectangle, or trapezoid, or any of various geometric shapes such as other polygons (e.g., triangle, hexagon, octagon), a wave shape, or a circle. The partition walls 14 face the cells 12 and separate adjacent cells 12.

[0029] [Catalyst layer] 3, in the exhaust gas purification catalyst 10, the catalyst layer 20 is disposed upstream of the reforming reaction layer 30 in the exhaust gas flow direction. In the illustrated example, the catalyst layer 20 is provided so as to be in contact with the reforming reaction layer 30. However, the catalyst layer 20 may be separated from the reforming reaction layer 30 as long as it is on the substrate 11 on which the reforming reaction layer 30 is formed.

[0030] In the catalyst layer 20, exhaust gas is purified by a three-way reaction. Therefore, the catalyst layer 20 contains a three-way catalyst. That is, the catalyst layer 20 contains a precious metal and a carrier that supports the precious metal. The catalyst layer 20 can be configured in the same manner as known catalyst layers that contain a three-way catalyst.

[0031] The noble metal is a catalytic metal that purifies harmful components in exhaust gas. The noble metal is not particularly limited, and known noble metals used in three-way catalysts may be used. Specific examples of noble metals include platinum group elements such as rhodium (Rh), palladium (Pd), platinum (Pt), ruthenium (Ru), osmium (Os), and iridium (Ir); gold (Au); and silver (Ag). These may be used alone or in combination of two or more. From the viewpoint of catalytic performance, at least one selected from the group consisting of Pt, Rh, Pd, Ir, and Ru is preferred, and at least one selected from the group consisting of Pt, Rh, and Pd is more preferred. When two or more of these metals are used, the noble metal may be an alloy of these two or more metal species.

[0032] It is preferable to use a combination of two or more of the above precious metals. A suitable example is a combination of Rh, which has high reduction activity, and Pd and / or Pt, which have high oxidation activity. In this specification, the expression "A and / or B" refers to either A or B, or both A and B.

[0033] The precious metal is preferably used as fine particles with a sufficiently small particle size. The average particle size of the precious metal particles (specifically, the average particle size of 20 or more precious metal particles determined based on a cross-sectional image of the catalyst layer taken by a transmission electron microscope) is usually about 1 to 15 nm, preferably 10 nm or less, more preferably 7 nm or less, and even more preferably 5 nm or less. This increases the contact area of ​​the precious metal with exhaust gas, thereby further improving purification performance.

[0034] The content of the precious metal is not particularly limited and may be, for example, 0.05 to 10 g / L, or 0.1 to 5 g / L per 1 L of the volume of the portion of the substrate on which the catalyst layer 20 is formed along the cylinder axis direction X.

[0035] As the carrier for supporting the precious metal, known inorganic compounds used in three-way catalysts can be used, and a porous carrier with a relatively large specific surface area is preferably used. In this specification, the "specific surface area" refers to the specific surface area measured by the BET method unless otherwise specified. Suitable carriers include, for example, alumina (Al2O3), ceria (CeO2), zirconia (ZrO2), silica (SiO2), titania (TiO2), and solid solutions thereof (e.g., ceria-zirconia composite oxide (CZ composite oxide)), or combinations thereof. The carrier is preferably in a particulate form (e.g., alumina powder, CZ powder, etc.). The carrier particles should have a specific surface area of ​​50 to 500 m 2 / g (especially 200-400m 2 / g) from the viewpoint of heat resistance and structural stability. The average particle size of the support particles (specifically, the average particle size of 20 or more support particles determined based on a cross-sectional image of the catalyst layer taken by a transmission electron microscope) is preferably 1 nm or more and 500 nm or less (particularly, 10 nm or more and 200 nm or less).

[0036] The catalyst layer 20 may contain optional components other than the three-way catalyst. For example, the catalyst layer 20 may contain, in addition to precious metals, metal species such as alkali metal elements, alkaline earth metal elements, transition metal elements, and rare earth elements. These elements (especially alkaline earth elements) may be contained in the form of oxides, hydroxides, carbonates, nitrates, sulfates, phosphates, acetates, formates, oxalates, halides, etc.

[0037] Regarding optional components, the catalyst layer 20 preferably further contains an oxygen storage / release material (OSC material) having oxygen storage / release capacity as a non-support material that does not support a precious metal. Known compounds known to have oxygen storage capacity may be used as the OSC material. Specific examples include metal oxides (Ce-containing oxides) containing ceria (CeO2), which has high oxygen storage capacity. The Ce-containing oxide may be ceria or a composite oxide of ceria and a metal oxide other than ceria. From the viewpoint of improving heat resistance and durability, the Ce-containing oxide may be a composite oxide containing at least one of Zr and Al, such as a ceria (CeO2)-zirconia (ZrO2) composite oxide (CZ composite oxide). From the viewpoint of improving heat resistance, the CZ composite oxide may be, for example, Nd2O3, La2O3, YO3, Pr6O 10 The oxide may further contain rare earth metal oxides such as:

[0038] Other optional components of the catalyst layer 20 include binders such as alumina sol and silica sol, and various additives.

[0039] The catalyst layer 20 may have a single-layer structure or a multi-layer structure. In the example shown in FIG. 3, the catalyst layer 20 is a single layer. When the catalyst layer 20 has a single-layer structure, the catalyst layer 20 may be composed of multiple regions with different compositions, properties, etc. For example, the catalyst layer 20 may have a front stage portion located upstream in the cylindrical axis direction X and a rear stage portion located downstream of the front stage portion, and the front stage portion and the rear stage portion may have different compositions and / or properties. Specifically, for example, the front stage portion and the rear stage portion may contain different precious metals.

[0040] When the catalyst layer 20 has a multi-layer structure, the number of layers is not particularly limited. The catalyst layer 20 may have a two-layer structure having a layer (lower layer) on the substrate side and a layer (upper layer) on the surface side, or a three-layer or more structure having a layer (lower layer) on the substrate side, a layer (upper layer) on the surface side, and one or more intermediate layers positioned therebetween. In this multi-layer structure, each layer may contain a different precious metal.

[0041] As a modified example of the exhaust gas purification catalyst 10, a preferred example in which the catalyst layer 20 has a multi-layer structure is shown in FIG. 4. The arrows in FIG. 4 are the same as those in FIG. 3. In the modified exhaust gas purification catalyst 10′ shown in FIG. 4, the catalyst layer 20′ has a lower layer 22′, which is the layer on the substrate 11 side, and an upper layer 24′ provided on the lower layer 22′. The lower layer 22′ and the upper layer 24′ each contain a three-way catalyst. The lower layer 22′ contains Pd as the precious metal of the three-way catalyst. On the other hand, the upper layer 24′ contains Rh as the precious metal of the three-way catalyst. In this case, by supporting the oxidation catalyst and the reduction catalyst separately in the stacking direction, deterioration of the catalyst metal (e.g., sintering due to grain growth) can be suppressed, and the durability of the exhaust gas purification catalyst can be improved.

[0042] The coating amount (i.e., the formed amount) of the catalyst layer 20 (and catalyst layer 20') is not particularly limited. The coating amount is, for example, 10 to 500 g / L, or may be 100 to 300 g / L, per 1 L of the volume of the portion of the substrate on which the catalyst layer 20 is formed along the cylindrical axis direction X. By satisfying the above range, it is possible to achieve both an improvement in the purification performance of harmful components and a reduction in pressure loss at a high level. Furthermore, it is possible to improve durability and peeling resistance.

[0043] The thickness of the catalyst layer 20 is not particularly limited and may be appropriately designed taking into consideration durability, peeling resistance, etc. The thickness of the catalyst layer 20 is, for example, 1 to 100 μm, and may be 5 to 100 μm.

[0044] The coating width (average dimension in the cylindrical axis direction X) of the catalyst layer 20 (and catalyst layer 20') is not particularly limited, and may be appropriately designed taking into consideration the size of the substrate 11, the flow rate of exhaust gas flowing through the exhaust gas purification catalyst 10, etc. The coating width is, for example, 10% to 90% of the entire length of the substrate in the cylindrical axis direction X, preferably 20% to 80%, and more preferably 30% to 70%.

[0045] [Reforming reaction layer] The reforming reaction layer 30 is disposed in a position downstream of the catalyst layer 20 in the flow direction of exhaust gas in the exhaust gas purification catalyst 10. In the illustrated example, the catalyst layer 20 and the reforming reaction layer 30 are disposed in the cells 12 of the substrate 11 of the exhaust gas purification catalyst 10, so that the exhaust gas that has passed through the catalyst layer 20 can flow into the reforming reaction layer 30.

[0046] The reforming reaction layer 30 contains a hydrogen generation catalyst as an essential component. The hydrogen generation catalyst is a catalyst that causes a hydrogen generation reaction that converts HC and CO in the exhaust gas into carbon dioxide and H2. Specifically, the hydrogen generation catalyst is a catalyst that uses water to convert HC and CO into H2 through a steam reforming reaction (HC + H2O → CO2 + H2) and a CO shift reaction (CO + H2O → CO2 + H2).

[0047] As described above, in a rich atmosphere, oxygen may be insufficient, and therefore the catalyst layer 20 alone may not be able to sufficiently oxidize HC and CO. In addition, oxygen is consumed in large amounts in the upstream portion of the exhaust gas purification catalyst in the exhaust gas flow direction, resulting in a shortage of oxygen in the downstream portion, particularly in a rich atmosphere. On the other hand, water is produced during the combustion of fuel (typically gasoline) and the oxidation of HC, and is therefore abundant in exhaust gas. As described above, the reforming reaction layer 30 converts HC and CO into carbon dioxide and H2 using water instead of oxygen. Therefore, by locating the reforming reaction layer 30 downstream of the catalyst layer 20 in the exhaust gas flow direction in the exhaust gas purification catalyst 10, HC and CO can be removed at a high purification rate even in an oxygen-insufficient atmosphere. That is, the exhaust gas purification catalyst 10 according to this embodiment has improved HC and CO purification performance in a rich atmosphere, thereby reducing hot emissions.

[0048] The hydrogen generation catalyst may be contained uniformly along the cylinder axis direction X, or may be contained so that the content thereof changes stepwise from the upstream side to the downstream side, for example, so that the content thereof gradually decreases from the upstream side to the downstream side.

[0049] The hydrogen generation catalyst typically contains at least one of a rare earth oxide and a transition metal oxide (excluding oxides of noble metals other than Rh and Ru), which can prevent the oxidation reaction from competing with the exhaust gas purification reaction and promote the hydrogen generation reaction.

[0050] Examples of rare earth oxides include 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), which belong to Group 3 of the periodic table. Among these, lanthanide oxides are preferred because of their high reactivity and excellent hydrogen generation ability, and at least one of Pr and Nd is preferred from the viewpoints of durability, availability, etc.

[0051] Examples of transition metal oxides (excluding oxides of noble metals other than Rh and Ru) include 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. Among these, oxides of metals belonging to the fourth period of the periodic table are preferred due to their high reactivity and excellent hydrogen generation ability, and oxides of iron group elements (Fe, Co, Ni) are more preferred, with Ni being particularly preferred. Furthermore, from the viewpoint of durability, at least one of Co and Ni is preferred. For these reasons, the hydrogen generation catalyst preferably contains an oxide of at least one element selected from the group consisting of Pr, Nd, Co, and Ni, more preferably an oxide of Ni or Pr, and even more preferably nickel oxide.

[0052] The reforming reaction layer 30 may contain components other than the hydrogen generation catalyst as optional components. In this embodiment, the reforming reaction layer 30 contains inorganic oxides as optional components, specifically, an OSC material having oxygen storage capacity and a non-OSC material not having oxygen storage capacity.

[0053] In the technology disclosed herein, the OSC material has the function of promoting the hydrogen generation reaction (steam reforming reaction and / or CO shift reaction). The OSC material can function as a promoter. Preferably, in the reforming reaction layer 30, the OSC material is disposed in close proximity to the hydrogen generation catalyst. Although this is not intended to be particularly restrictive, the inventors believe that the OSC material functions to dissociate HO in the exhaust gas and supply O atoms to the hydrogen generation catalyst. This is believed to promote the hydrogen generation reaction.

[0054] Examples of OSC materials include those exemplified as materials that can be used for the catalyst layer 20. The CZ composite oxide may be Ce-rich or Zr-rich. The ceria content in the CZ composite oxide is, for example, 10 to 90 mass%, and preferably 15 to 70 mass%, when the entire CZ composite oxide is taken as 100 mass%. A higher ceria content can promote the conversion of harmful components to H2. A higher zirconia content can improve heat resistance. When the ceria content is within the above range, it is possible to achieve both the effects of the technology disclosed herein and heat resistance at a high level.

[0055] Although not particularly limited, the content of the OSC material in the reforming reaction layer 30 is, for example, 1 to 100 g / L, and preferably 5 to 50 g / L, per 1 L of volume of the portion of the substrate on which the reforming reaction layer 30 is formed along the cylinder axis direction X. The ratio of the content of the OSC material to the content of the hydrogen generation catalyst is, for example, 0.1 or more, 0.2 or more, 0.5 or more, 1 or more, 2 or more, or 3 or more, and is, for example, 30 or less, 20 or less, 10 or less, or 5 or less. This promotes the hydrogen generation reaction and reduces the coating amount of the reforming reaction layer 30, thereby reducing pressure loss and costs.

[0056] The non-OSC material has at least one of the following functions: improving the heat resistance of the reforming reaction layer 30, improving the durability of the reforming reaction layer 30, and suppressing peeling of the reforming reaction layer 30 from the substrate 11. Examples of non-OSC materials include metal oxides such as alumina (Al2O3), titania (TiO2), zirconia (ZrO2), and silica (SiO2). Among these, metal oxides containing alumina (Al-containing oxides) are preferred because of their high heat resistance and durability. The Al-containing oxide may be alumina or a composite oxide of alumina and a metal oxide other than alumina (e.g., a rare earth metal oxide). From the viewpoint of improving heat resistance and durability, the Al-containing oxide is, for example, an La2O3-Al2O3 composite oxide (LA composite oxide). The LA composite oxide may be La-rich or Al-rich. The mixing ratio of the metal oxide other than alumina in the LA composite oxide may be, for example, less than 50 mass % and may be 0.1 to 20 mass % when the entire LA composite oxide is taken as 100 mass %, from the viewpoint of suppressing deterioration over time during use.

[0057] Although not particularly limited, the content of the non-OSC material in the reforming reaction layer 30 may be less than the content of the OSC material. The content of the non-OSC material per 1 L of volume of the portion of the substrate on which the reforming reaction layer 30 is formed along the cylinder axis direction X is, for example, 1 to 100 g / L, or may be 5 to 50 g / L.

[0058] The hydrogen generation catalyst is typically in the form of particles. It is preferable that the hydrogen generation catalyst particles (particularly nickel oxide (NiOx) particles) are not supported on a carrier. The carrier can be an optional component such as an OSC material or a non-OSC material. In other words, it is preferable that the hydrogen generation catalyst particles exist independently of the optional component such as an OSC material or a non-OSC material. When the hydrogen generation catalyst is supported on a carrier, it is susceptible to interaction with the carrier (particularly ceria in the OSC material). This makes it difficult for the metal elements (particularly Ni) of the hydrogen generation catalyst to be stabilized in the metallic state, making it difficult for the steam reforming reaction and the CO shift reaction to occur. Therefore, by not supporting the hydrogen generation catalyst particles on a carrier, it is easier to stabilize the metal elements (particularly Ni) of the hydrogen generation catalyst in the metallic state, thereby promoting the steam reforming reaction and the CO shift reaction.

[0059] The average particle size of the hydrogen generation catalyst (specifically, the average particle size of 20 or more particles determined by electron microscope observation) is not particularly limited, but is preferably 1 μm or more. In this case, the metal elements (particularly Ni) of the hydrogen generation catalyst are more easily stabilized in the metallic state, and the steam reforming reaction and the CO shift reaction can be further promoted. On the other hand, the average particle size of the hydrogen generation catalyst is preferably smaller than the average particle size of the OSC material and / or non-OSC material. This allows stable activity to be maintained. The average particle size of the hydrogen generation catalyst is more preferably 1 μm or more and 9 μm or less, and even more preferably 1 μm or more and 5 μm or less.

[0060] The content of the hydrogen generation catalyst in the reforming reaction layer 30 is not particularly limited. The higher the content of the hydrogen generation catalyst, the higher the purification performance for CO and HC tends to be. Therefore, the content of the hydrogen generation catalyst per 1 L of volume of the substrate in the portion where the reforming reaction layer 30 is formed along the cylinder axis direction X is preferably 5 g / L or more, more preferably 10 g / L or more, even more preferably 20 g / L or more, and most preferably 30 g / L or more.

[0061] On the other hand, from the viewpoint of suppressing excessive reduction of harmful substances (for example, excessive reduction of NOx to generate ammonia), the content of the hydrogen generation catalyst is, for example, 100 g / L or less, preferably 80 g / L or less, and more preferably 60 g / L or less per 1 L of volume of the substrate in the portion where the reforming reaction layer 30 is formed along the cylinder axis direction X.

[0062] The reforming reaction layer 30 may be composed of a hydrogen generation catalyst, an OSC material, and a non-OSC material, and may further contain optional components. For example, when the reforming reaction layer 30 contains an OSC material, it may contain alkaline earth elements such as calcium (Ca), barium (Ba), and strontium (Sr). This can improve the amount of oxygen stored in the OSC material in a lean atmosphere. In addition, poisoning in a rich atmosphere can be suppressed, and the activity of the hydrogen generation catalyst can be stably maintained.

[0063] In one preferred embodiment of the reforming reaction layer 30, the reforming reaction layer 30 does not contain a precious metal. If the reforming reaction layer 30 contains a precious metal, the effect of improving the purification performance of CO and HC of the exhaust gas purification catalyst 10 will be reduced. This is because CO and HC are adsorbed by the precious metal, which hinders their purification by the hydrogen generation catalyst.

[0064] Meanwhile, in another preferred embodiment of the reforming reaction layer 30, the reforming reaction layer 30 contains Rh. Rh is a precious metal, and although it reduces the effect of improving the CO and HC purification performance of the exhaust gas purification catalyst 10, the degree of reduction in this effect is very small for Rh. On the other hand, Rh can purify NOx. Therefore, when considering the purification performance of all harmful components (i.e., CO, HC, and NOx), that is, when removing CO, HC, and NOx in a balanced manner at a high purification rate, the reforming reaction layer 30 can contain Rh, even though Rh is a precious metal. Therefore, in this embodiment, it is preferable that the reforming reaction layer 30 does not contain any precious metals other than Rh.

[0065] Other optional components of the reforming reaction layer 30 include binders such as alumina sol and silica sol, and various additives.

[0066] The reforming reaction layer 30 may be mainly composed of a hydrogen generation catalyst (i.e., a component accounting for 50% or more by mass of the whole), an OSC material, or a non-OSC material. From the viewpoint of increasing the dispersibility of the hydrogen generation catalyst and suppressing sintering, the reforming reaction layer 30 may be mainly composed of an OSC material and / or a non-OSC material (if both are included, the total may be used). While not particularly limited, when the whole reforming reaction layer 30 is taken as 100% by mass, the content of the hydrogen generation catalyst is, for example, 5 to 90% by mass, preferably 10 to 70% by mass, from the viewpoint of suitably exerting the effects of the technology disclosed herein over a long period of time. Furthermore, the content of the OSC material is, for example, 1 to 70% by mass, preferably 5 to 65% by mass, from the viewpoint of promoting the hydrogen generation reaction. The content of the non-OSC material is, for example, 10 mass % or more, preferably 25 mass % or more, and more preferably 30 to 50 mass %, from the viewpoint of improving heat resistance, durability, and peeling resistance.

[0067] The coating amount (molding amount) of the reforming reaction layer 30 is not particularly limited. For example, it is 10 to 200 g / L, and may be 50 to 100 g / L per 1 L of volume of the portion of the substrate on which the reforming reaction layer 30 is formed along the cylinder axis direction X. By satisfying the above range, the effects of the technology disclosed herein can be stably exerted at a high level for a long period of time.

[0068] The average thickness of the reforming reaction layer 30 is not particularly limited and may be set appropriately taking into consideration durability, peeling resistance, etc. The average thickness of the reforming reaction layer 30 is, for example, 1 to 100 μm, and may be 5 to 100 μm.

[0069] The coating width (average dimension in the cylindrical axis direction X) of the reforming reaction layer 30 is not particularly limited, and may be appropriately designed taking into consideration the size of the substrate 11 and the flow rate of exhaust gas flowing through the exhaust gas purification catalyst 10. The coating width of the reforming reaction layer 30 is, for example, 10% to 90% of the entire length of the substrate in the cylindrical axis direction X, preferably 20% to 80%, and more preferably 30% to 70%.

[0070] <Method for manufacturing exhaust gas purification catalyst 10> The exhaust gas purification catalyst 10 can be manufactured, for example, by the following method. That is, first, a substrate 11, a catalyst layer forming slurry for forming the catalyst layer 20, and a reforming reaction layer forming slurry for forming the reforming reaction layer 30 are prepared. The catalyst layer forming slurry may be the same as a known catalyst layer forming slurry for forming a catalyst layer containing a three-way catalyst. For example, the catalyst layer forming slurry can be prepared by dispersing a precious metal source (e.g., a solution containing precious metal ions), a carrier, and optional components (binder, various additives, etc.) in a dispersion medium.

[0071] For the reforming reaction layer-forming slurry, the hydrogen generation catalyst source can be an oxide that serves as a hydrogen generation catalyst, or a compound that can be converted to an oxide that serves as a hydrogen generation catalyst by calcination (e.g., nitrates, carbonates, oxalates, hydroxides, etc. of rare earth metals and transition metals). For example, the catalyst layer-forming slurry can be prepared by dispersing the hydrogen generation catalyst source and optional components (OSC material, non-OSC material, binder, various additives, etc.) in a dispersion medium. Here, when the hydrogen generation catalyst is nickel oxide (NiOx), the average particle size is preferably 5 μm or more.

[0072] Next, the catalyst layer forming slurry and the reforming reaction layer forming slurry are applied to the substrate 11. These slurries can be applied by conventional methods, such as impregnation or washcoating. In one example, the catalyst layer forming slurry prepared above is first introduced into the cells 12 from the end of the substrate 11 on the inlet 10a side, supplied to a predetermined length along the cylindrical axis direction X, and dried. Next, the reforming reaction layer forming slurry is introduced into the cells 12 from the end of the substrate 11 on the outlet 10b side, supplied to a predetermined length along the cylindrical axis direction X, and dried. Next, the substrate 11 to which these slurries have been applied is calcined at a predetermined temperature and time. The calcination method may be the same as conventional. In this way, the catalyst layer 20 and the reforming reaction layer 30 can be formed on the substrate 11. In this manner, the exhaust gas purification catalyst 10 can be formed.

[0073] <10 uses of exhaust gas purification catalysts> The exhaust gas purification catalyst 10 can be suitably used to purify exhaust gases emitted from vehicles such as automobiles and trucks, motorcycles and mopeds, marine products such as ships, tankers, jet skis, personal watercraft and outboard motors, gardening products such as lawn mowers, chainsaws and trimmers, leisure products such as golf carts and all-terrain vehicles, power generation equipment such as cogeneration systems, and internal combustion engines of waste incinerators, etc. In particular, it can be suitably used for vehicles such as automobiles.

[0074] Test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to those shown in the following test examples. In the following test examples, the unit "g / L" indicates the content per 1 L of volume of the portion of the substrate on which a predetermined layer is formed in the axial direction of the cylinder.

[0075] [Test Example 1] Examination of reforming reaction layer containing hydrogen generation catalyst First, a cylindrical honeycomb substrate (made of cordierite, capacity: 1.2 L, substrate diameter: 118 mm, substrate total length: 112 mm) was prepared. Next, the following three types of slurries were prepared. For the OSC material, a ceria-zirconia composite oxide containing La, Pr, Nd, and Y as additive components was used. For the non-OSC material, an alumina composite oxide containing La was used. (A) Slurry for forming the reforming reaction layer: An OSC material, a non-OSC material, nickel oxide (average particle size: 8 μm) as a hydrogen generation catalyst, barium sulfate, and an alumina-based binder were mixed with distilled water. The mixture was milled to control the particle size, and a slurry for forming the reforming reaction layer was prepared. (B) Pd slurry for forming the catalyst underlayer: An OSC material, a non-OSC material, an aqueous solution of Pd nitrate, barium sulfate, and an alumina-based binder were mixed with distilled water. The mixture was milled to control the particle size, and a Pd slurry for forming the catalyst underlayer was prepared. (C) Rh slurry for forming the catalyst upper layer: An OSC material, a non-OSC material, a Rh hydrochloride aqueous solution, and an alumina-based binder were mixed with distilled water. The mixture was milled to control the particle size, and a Rh slurry for forming the catalyst upper layer was prepared.

[0076] [Comparative Example 1] The Pd slurry for forming the catalyst lower layer was poured into the front side of the substrate (the end side where the exhaust gas flows in) and sucked with a blower, coating the substrate over 100% of its entire length in the axial direction. The amount of Pd slurry coated was adjusted so that the Pd content was 0.14 g / L. This was then heated and dried in a dryer at 250°C for 1 hour, and then fired in an electric furnace at 500°C for 1 hour. This formed the lower layer of the catalyst layer.

[0077] Next, the Rh slurry for forming the upper layer was poured into the front side of the substrate and sucked with a blower, thereby coating the substrate over a portion corresponding to 100% of the entire length in the cylindrical axis direction. The amount of Rh slurry coated was set so that the Rh content was 0.07 g / L. This was then heated and dried in a dryer at 250°C for 1 hour, and then fired in an electric furnace at 500°C for 1 hour. This formed the lower layer of the catalyst layer. In this way, a catalyst for purifying exhaust gas of Comparative Example 1, which does not have a reforming reaction layer, was obtained.

[0078] Comparative Example 2 The slurry for forming the reforming reaction layer was poured into the front side of the substrate and coated by suction with a blower. The coating width was set to 50% of the total length of the substrate in the axial direction, and the coating amount was set so that the hydrogen generation catalyst content was 48 g / L in nickel oxide equivalent. This was then heated and dried in a dryer at 250°C for 1 hour, and then baked in an electric furnace at 500°C for 1 hour. This formed a reforming reaction layer on the surface of the front part of the substrate.

[0079] Next, the Pd slurry for forming the catalyst lower layer was poured into the rear side of the substrate (the end side on the exhaust gas outflow side) and coated by suction with a blower. The coating width was set to 50% of the total length of the substrate in the axial direction, and the coating amount was set to a Pd content of 0.14 g / L. This was then heated and dried in a dryer at 250°C for 1 hour, and then fired in an electric furnace at 500°C for 1 hour. This formed the lower layer of the catalyst layer on the surface of the rear part of the substrate.

[0080] Next, the Rh slurry for forming the upper layer was poured into the rear side of the substrate and coated by suction with a blower. The coating width was set to 50% of the total length of the substrate in the axial direction, and the coating amount was set so that the Rh content was 0.07 g / L. This was then heated and dried in a dryer at 250°C for 1 hour, and then fired in an electric furnace at 500°C for 1 hour. In this way, an upper layer of the catalyst layer was formed on the formed lower layer. In this way, an exhaust gas purification catalyst of Comparative Example 2 having a reforming reaction layer in the front portion was obtained.

[0081] [Example 1] The Pd slurry for forming the catalyst lower layer was poured into the front side of the substrate and coated by suction with a blower. The coating width was set to 50% of the total length of the substrate's cylindrical axis, and the coating amount was set to a Pd content of 0.14 g / L. This was then heated and dried in a dryer at 250°C for 1 hour, and then fired in an electric furnace at 500°C for 1 hour. This formed the lower layer of the catalyst layer on the surface of the front part of the substrate.

[0082] Next, the Rh slurry for forming the upper layer was poured into the front side of the substrate and coated by suction with a blower. The coating width was set to 50% of the total length of the substrate's cylindrical axis direction, and the coating amount was set to an Rh content of 0.07 g / L. This was then heated and dried in a dryer at 250 °C for 1 hour, and then fired in an electric furnace at 500 °C for 1 hour. This formed the upper layer of the catalyst layer on top of the formed lower layer.

[0083] Next, the reforming reaction layer forming slurry was poured into the rear side of the substrate and coated by suction with a blower. The coating width was set to 50% of the total length of the substrate, and the coating amount was set so that the hydrogen generation catalyst content was 48 g / L in terms of nickel oxide. The substrate was then heated and dried in a dryer at 250°C for 1 hour, and then fired in an electric furnace at 500°C for 1 hour. This formed a reforming reaction layer on the surface of the rear part of the substrate. In this way, the exhaust gas purification catalyst of Example 1, which has a reforming reaction layer on the rear part, was obtained.

[0084] [Evaluation of CO and HC purification performance] The CO and HC conversion performance of the exhaust gas purification catalyst prepared above was evaluated using a catalyst evaluation device equipped with a gasoline engine (2.5L NA engine). Specifically, the exhaust gas purification catalyst prepared above was installed in the catalyst evaluation device, the inlet gas temperature was set to 520°C, and simulated exhaust gas with various air-fuel ratios (A / F) was supplied to the engine. The CO and HC conversion efficiencies were calculated from the ratio of the CO or HC concentration in the inlet gas to the CO or HC concentration in the outlet gas from the exhaust gas purification catalyst. The HC conversion efficiencies were calculated as total hydrocarbon (THC) conversion efficiencies. The results are shown in Figures 5 and 6. Figure 7 shows a graph comparing the CO and THC conversion efficiencies in a rich atmosphere (A / F = 14.2).

[0085] 5 to 7, it can be seen that when a catalyst layer is provided in the front section and a reforming reaction layer is provided in the rear section (Example 1), a significantly high CO conversion rate and THC conversion rate can be obtained even in a rich atmosphere. This is because the O2 concentration is low in the rear section, and the reforming reaction (hydrogen generation reaction) proceeds efficiently in the reforming reaction layer in the rear section.

[0086] On the other hand, when the reforming reaction layer was provided in the front section and the catalyst layer was provided in the rear section (Example 1), the improvement in the CO conversion rate and THC conversion rate in a rich atmosphere was slight. This is because the O2 concentration in the front section was sufficiently high, and the oxidation reaction proceeded preferentially even in the reforming reaction layer in the front section, so the reforming reaction (hydrogen generation reaction) did not proceed efficiently.

[0087] [Test Example 2] Examination of the type of hydrogen generation catalyst [Example 2] The exhaust gas purification catalyst of Example 2 was obtained in the same manner as Example 1, except that the hydrogen generation catalyst source contained in the slurry for forming the reforming reaction layer was changed to Pr nitrate and the coating amount was changed to 30 g / L in terms of praseodymium oxide.

[0088] [Example 3] The exhaust gas purification catalyst of Example 3 was obtained in the same manner as Example 1, except that the hydrogen generation catalyst source contained in the slurry for forming the reforming reaction layer was changed to Cu nitrate and the coating amount was changed to 30 g / L in terms of copper oxide. [Example 4] The exhaust gas purification catalyst of Example 4 was obtained in the same manner as Example 1, except that the hydrogen generation catalyst source contained in the slurry for forming the reforming reaction layer was changed to Mg nitrate and the coating amount was changed to 30 g / L in terms of magnesium oxide.

[0089] [Evaluation of CO and HC purification performance] The CO conversion efficiency and THC conversion efficiency were measured in the same manner as described above for the exhaust gas purification catalysts of Examples 2 to 4. A graph comparing the CO conversion efficiency and THC conversion efficiency in a rich atmosphere (A / F=14.2) is shown in Figure 8. The graph also shows the results of Comparative Example 1 and Example 1.

[0090] As shown in Figure 8, when the metal species of the hydrogen generation catalyst was Ni or Pr, the CO conversion rate and THC conversion rate were particularly high. Between Ni and Pr, Ni had higher CO conversion rate and THC conversion rate.

[0091] [Test Example 3] Examination of the amount of hydrogen generation catalyst added [Example 5] An exhaust gas purifying catalyst of Example 5 was obtained in the same manner as in Example 1, except that the coating amount of the slurry for forming the reforming reaction layer was changed to 10 g / L in terms of nickel oxide.

[0092] [Example 6] An exhaust gas purifying catalyst of Example 6 was obtained in the same manner as in Example 1, except that the coating amount of the slurry for forming the reforming reaction layer was changed to 30 g / L in terms of nickel oxide.

[0093] [Example 7] An exhaust gas purifying catalyst of Example 7 was obtained in the same manner as in Example 1, except that the coating amount of the slurry for forming the reforming reaction layer was changed to 50 g / L in terms of nickel oxide.

[0094] [Evaluation of CO and HC purification performance] The CO conversion efficiency and THC conversion efficiency were measured in the same manner as described above for the exhaust gas purification catalysts of Examples 5 to 7. A graph comparing the CO conversion efficiency and THC conversion efficiency in a rich atmosphere (A / F=14.2) is shown in Figure 9. The graph also shows the results of Comparative Example 1.

[0095] The results in Figure 9 show that the greater the amount of hydrogen generation catalyst coated, the higher the CO conversion rate and THC conversion rate, and that a coating amount of 5 g / L or more provides a significantly improved effect on the CO conversion rate and THC conversion rate.

[0096] [Test Example 4] Study of hydrogen generation catalyst source [Example 8] The exhaust gas purifying catalyst of Example 8 was obtained in the same manner as in Example 1, except that Ni nitrate was used instead of nickel oxide as the hydrogen generation catalyst source in the slurry for forming the reforming reaction layer. In Example 8 and Example 1, the coating amount of the hydrogen generation catalyst converted into nickel oxide was the same.

[0097] [Evaluation of CO and HC purification performance] The CO conversion rate and THC conversion rate were measured in the same manner as above for the exhaust gas purification catalyst of Example 8. A graph comparing the CO conversion rate and THC conversion rate in a rich atmosphere (A / F=14.2) for Example 1, Comparative Example 1, and Comparative Example 8 is shown in Figure 10.

[0098] The results in FIG. 10 show that the CO conversion rate and THC conversion rate are higher when nickel oxide particles are used as the hydrogen generation catalyst source in the reforming reaction layer-forming slurry than when nickel nitrate particles are used. When nickel nitrate is used as the hydrogen generation catalyst source (Example 8), the hydrogen generation catalyst is highly dispersed and supported on the support. Therefore, the hydrogen generation catalyst is susceptible to interaction with the support (especially ceria in the OSC material). This facilitates the oxidation reaction, but makes it difficult for Ni to stabilize in a metallic state, and the reforming reaction does not proceed. On the other hand, when nickel oxide is used as the hydrogen generation catalyst source (Example 1), there is no interaction with the support, making Ni more likely to stabilize in a metallic state, and the reforming reaction proceeds more easily. For these reasons, the CO conversion rate and THC conversion rate are higher when nickel oxide particles are used as the hydrogen generation catalyst source. Furthermore, since larger nickel oxide particles make it easier for Ni to stabilize in a metallic state, it can be said that a larger median diameter of nickel oxide particles is preferable (specifically, 1 μm or more and 9 μm or less, particularly 1 μm or more and 5 μm or less).

[0099] [Test Example 5] Examination of the combined use of hydrogen generation catalyst and precious metal [Example 9] An exhaust gas purifying catalyst of Example 9 was obtained in the same manner as in Example 1, except that Pd nitrate was added to the slurry for forming the reforming reaction layer so that the coating amount was 0.14 g / L in terms of Pd.

[0100] [Example 10] An exhaust gas purifying catalyst of Example 9 was obtained in the same manner as in Example 1, except that dinitrodiammine platinum was added to the slurry for forming the reforming reaction layer so that the coating amount was 0.14 g / L in terms of Pt.

[0101] [Example 11] An exhaust gas purifying catalyst of Example 9 was obtained in the same manner as in Example 1, except that Rh hydrochloride was added to the reforming reaction layer forming slurry so that the coating amount was 0.07 g / L in terms of Rh.

[0102] [Evaluation of CO and HC purification performance] The CO conversion efficiency and THC conversion efficiency were measured in the same manner as described above for the exhaust gas purification catalysts of Examples 9 to 11. A graph comparing the CO conversion efficiency and THC conversion efficiency in a rich atmosphere (A / F=14.2) is shown in Figure 11. The graph also shows the results of Comparative Example 1 and Example 1.

[0103] 11, when Pd and Pt coexist in the reforming reaction layer (Examples 9 and 10), the improvement in the CO conversion rate and THC conversion rate is smaller than when they do not coexist (Example 1). This is because large amounts of CO and HC are adsorbed on Pd and Pt, inhibiting the reforming reaction by the hydrogen generation catalyst.

[0104] When Rh was present in the reforming reaction layer, the decline in CO and THC conversion rates was small. This is because, although Rh adsorbs CO and HC, it promotes the reforming reaction in a rich atmosphere and does not significantly inhibit the reforming reaction overall. Therefore, when a precious metal is present, the improvement in CO and THC conversion rates is small, but when the precious metal is Rh, it can be said that it can coexist with the hydrogen generation catalyst.

[0105] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Explanation of symbols]

[0106] 1 Exhaust gas purification system 2. Internal combustion engine 3. Exhaust gas purification equipment 10 Second catalyst (exhaust gas purification catalyst) 11 Base material 20 Catalyst layer 30 Reforming reaction layer

Claims

1. A substrate; a catalyst layer provided on the substrate; a reforming reaction layer provided on the substrate; An exhaust gas purifying catalyst comprising: the catalyst layer contains a three-way catalyst, the reforming reaction layer contains a hydrogen generation catalyst; the hydrogen generation catalyst comprises nickel oxide; the hydrogen generation catalyst is in particulate form; All of the hydrogen generation catalyst particles are unsupported; the average particle size of the hydrogen generation catalyst is 1 μm or more and 9 μm or less; the reforming reaction layer is disposed downstream of the catalyst layer in the flow direction of the exhaust gas. Catalyst for purifying exhaust gas.

2. The reforming reaction layer further contains an OSC material having oxygen storage capacity. The exhaust gas purifying catalyst according to claim 1.

3. 3. The exhaust gas purification catalyst according to claim 1, wherein the content of the hydrogen generation catalyst in the reforming reaction layer is 5 g / L or more per 1 L of volume of the portion of the substrate on which the reforming reaction layer is formed.

4. A catalyst for exhaust gas purification as described in claim 1 or 2, wherein the content of the hydrogen generation catalyst in the reforming reaction layer is 30 g / L or more and 60 g / L or less per 1 L of volume of the portion of the substrate on which the reforming reaction layer is formed.

5. 5. The exhaust gas purifying catalyst according to claim 1, wherein the reforming reaction layer further contains Rh.

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