Exhaust gas purification catalyst

Abstract

Exhaust gas purification catalyst in which a catalyst layer (2) containing at least one of Pd and Pt is formed on a substrate (1), characterized in that it comprises: a front section of the catalyst layer (21) which is provided in a region from an exhaust-stream end of the catalyst layer to a length position which is 50% or less of a total length of the catalyst layer, and which comprises a first OSC material having a pyrochlore structure and a second OSC material whose oxygen storage rate is faster than an oxygen storage rate of the first OSC material; wherein a content of the first OSC material in the front section of the catalyst layer (21) is 2 wt.% to 9 wt.%, based on the total content of the first OSC material and the second OSC material, and a rear catalyst layer section (22) which is arranged in a region downstream in a flow direction of an exhaust gas which is different from the front catalyst layer section and which comprises a second OSC material and at least one of Pd and Pt, wherein the rear catalyst layer section (22) does not contain the first OSC material having a pyrochlore structure.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The present invention relates to an exhaust gas purification catalyst or exhaust gas control catalyst for cleaning exhaust gas emitted from an internal combustion engine. 2. Description of the related prior art

[0002] Exhaust gas emitted from an internal combustion engine of a car or similar vehicle contains harmful components such as carbon dioxide (CO₂), hydrocarbons (HC), and nitrogen oxides (NOₓ). x These harmful components are released into the air after being cleaned by an exhaust gas purification catalyst. In the related prior art, a three-way catalyst is used for the exhaust gas purification catalyst, which simultaneously oxidizes CO and HC and reduces NO. xThe process is carried out as a three-way catalyst, a catalyst in which a precious metal such as platinum (Pt), palladium (Pd) or rhodium (Rh) is supported on a porous oxide support such as aluminum oxide (Al2O3), silicon dioxide (SiO2), zirconium dioxide (ZrO2) or titanium dioxide (TiO2).

[0003] To efficiently remove the harmful components described above from the exhaust gas using such a three-way catalytic converter, the air-fuel ratio (A / F), which is the ratio of air to fuel in an air-fuel mixture supplied to an internal combustion engine, is necessarily set close to the theoretical air-fuel ratio (stoichiometric ratio). However, depending on driving conditions and other factors of a vehicle, the actual air-fuel ratio will be either rich (fuel excess condition: A / F < 14.7) or lean (oxygen excess condition: A / F > 14.7), centered on the stoichiometric ratio, and the exhaust gas will be correspondingly rich or lean.

[0004] To improve the exhaust gas purification performance of a three-way catalytic converter, which varies depending on changes in the oxygen concentration in the exhaust gas, an OSC material is now being used in one of the catalyst layers. OSC is an inorganic material with oxygen storage capacity. When the air-fuel mixture is lean and the oxygen concentration in the exhaust gas is high (lean exhaust), the OSC material stores oxygen to reduce NOₓ. x to promote the oxidation of CO and HC in the exhaust gas. If the air-fuel mixture is rich and the oxygen concentration in the exhaust gas is low, the OSC material releases oxygen to promote the oxidation of CO and HC in the exhaust gas.

[0005] Japanese patent application publication no. 2012-152702 (JP 2012-152702 A) discloses an exhaust gas purification catalyst comprising: a substrate; a lower catalyst layer formed on the substrate and containing at least one of Pd and Pt; and an upper catalyst layer formed on the lower catalyst layer and containing Rh. In this exhaust gas purification catalyst, a region not containing the upper catalyst layer is located on an upstream side of the catalyst, the lower catalyst layer is formed from an upstream lower catalyst layer located on the upstream side and a downstream lower catalyst layer located on the downstream side, and the upstream lower catalyst layer contains an oxygen storage material.JP 2012-152702 A describes that with this configuration the growth of catalytic metal particles can be prevented when a Ce2Zr2O7 oxygen storage material with a pyrochlore phase is used, whose oxygen storage rate is slower than that of the other crystal structures.

[0006] Japanese patent application No. 2013-130146 (JP 2013-130146 A) discloses an exhaust gas control device with an exhaust gas purification catalyst in which a catalyst layer is formed on a substrate. This substrate includes a support containing an OSC material with oxygen storage capacity and a precious metal catalyst supported on the support. In this exhaust gas purification catalyst, the support contains, in a predetermined region downstream of a catalyst outlet end, an OSC material with a pyrochlore structure and an OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure.

[0007] In JP 2013-130146 A, the OSC material with a pyrochlore structure and the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure are used together in a downstream section of the catalyst layer. However, since an oxygen storage and release reaction is actively occurring in an upstream section of the catalyst layer, oxygen in the exhaust gas is consumed in the upstream section and hardly reaches the downstream section of the catalyst layer. Therefore, an inactive catalytic reaction occurs in the downstream section of the catalyst layer.Furthermore, if the two OSC materials described above are used together and the amount of the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure is greater than that of the OSC material with a pyrochlore structure, the OSC material with a pyrochlore structure may not utilize oxygen efficiently, and its effect will decrease.

[0008] Furthermore, the published patent application US 2012 / 0021899A1 discloses an exhaust gas purification catalyst from the prior art.

[0009] Furthermore, to prevent catalyst deterioration, a decrease in the cleaning performance of a catalyst – called sulfur poisoning – must be reduced, and NO xTo reduce emissions, a catalyst is desirable that can maintain activity when the air-fuel mixture is rich, where sulfur poisoning is caused by a sulfur component in exhaust gas, with which a surface of a precious metal (for example, Pd) contained in an exhaust gas purification catalyst is coated, and the NO x -Emission is caused by fluctuations in the air-fuel ratio.

[0010] As described above, an exhaust gas purification catalyst is also required for the downstream section of the catalyst layer, which ensures that a catalytic reaction actively occurs. Providing an exhaust gas purification catalyst with a higher NO x -Reduction performance compared to the past is particularly necessary when an air-fuel mixture supplied to a machine is rich. SUMMARY OF THE INVENTION

[0011] The present invention provides an exhaust gas purification catalyst or exhaust gas control catalyst which causes a catalytic reaction to take place actively even in a downstream section of a catalyst layer, and which results in improved NO x -reduction capacity.

[0012] The inventors in this case have discovered that the NO x -Reduction performance of an exhaust gas purification catalyst is improved by a catalyst layer of the exhaust gas purification catalyst which contains in a predetermined area of ​​an upstream section of the exhaust gas stream a first OSC material with a pyrochlore structure and a second OSC material with a faster oxygen storage rate than that of the first OSC material, thereby completing the invention.

[0013] One aspect of the invention relates to an exhaust gas purification catalyst in which a catalyst layer containing at least one of Pd and Pt is formed on a substrate. This exhaust gas purification catalyst comprises a front section of the catalyst layer extending from an upstream end of the catalyst layer to a length position that is 50% or less of the total length of the catalyst layer. The exhaust gas purification catalyst includes a first OSC material with a pyrochlore structure and a second OSC material whose oxygen storage rate is faster than that of the first OSC material. The content of the first OSC material in the front section of the catalyst layer is 2 wt% to 9 wt%, based on the total content of the first OSC material and the second OSC material.The exhaust gas purification catalyst comprises a rear catalyst layer section located in a region downstream in the flow direction of an exhaust gas that is different from the front catalyst layer section, and comprising a second OSC material and at least one of Pd and Pt, wherein the rear catalyst layer section does not contain the first OSC material having a pyrochlore structure.

[0014] In the exhaust gas purification catalyst, the total content of the first OSC material and the second OSC material in the front catalyst layer section can be 80 g or less per 11 of the substrate.

[0015] The exhaust gas purification catalyst may also include a precious metal catalyst layer that is formed on the catalyst layer.

[0016] According to the present invention, an exhaust gas purification catalyst with improved NO x -Reduction performance provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Features, advantages, and the technical and industrial significance of exemplary embodiments of the invention are described below with reference to the accompanying drawings, in which the same symbols denote the same elements. The drawings show: Fig. Figure 1 is an enlarged cross-sectional view of an exhaust gas purification catalyst, illustrating an embodiment of an exhaust gas purification catalyst according to the present invention; Fig. Figure 2 is an enlarged cross-sectional view of an exhaust gas purification catalyst, illustrating another embodiment of the exhaust gas purification catalyst according to the present invention; Fig. Figure 3 is an enlarged cross-sectional view of an exhaust gas purification catalyst, illustrating an embodiment of an exhaust gas purification catalyst according to Example 1; Fig. 4 is a graph that shows the NOx -Reduction performance of exhaust gas purification catalysts from Example 1 and a comparison example illustrated; and Fig. Figure 5 is a graph showing the influence of the content of two OSC materials and the content of one OSC material with a pyrochlore structure in a front section of a lower catalyst layer of an exhaust gas purification catalyst on the NO x -Reduction performance illustrated. DETAILED DESCRIPTION OF EXECUTION FORMS

[0018] Preferred embodiments of the invention are described in detail below.

[0019] One embodiment of the invention relates to an exhaust gas purification catalyst or exhaust gas control catalyst. Fig.Figure 1 is an enlarged cross-sectional view of an exhaust gas purification catalyst, illustrating an embodiment of the exhaust gas purification catalyst according to the present invention. The exhaust gas purification catalyst according to the invention comprises a substrate 1 and a catalyst layer 2, which is formed by coating the substrate 1.

[0020] The substrate of the exhaust gas purification catalyst is not subject to any particular restrictions, and any material commonly used in an exhaust gas purification catalyst can be employed. Specifically, a honeycomb-shaped material with multiple cells can be used as the substrate, and examples include heat-resistant ceramic materials such as cordierite (2MgO·2Al2O3·5SiO2), aluminum oxide, zirconium dioxide, and silicon carbide; and metallic materials formed from a metal foil, such as stainless steel.

[0021] The catalyst layer of the exhaust gas purification catalyst is formed on the substrate. Exhaust gas fed to the catalyst comes into contact with the catalyst layer as it flows through a flow channel in the substrate. As a result, harmful substances are removed. For example, CO and HC contained in the exhaust gas are oxidized to water (H₂O), carbon dioxide (CO₂), and the like by a catalytic action of the catalyst layer, and NOₓ is removed. x is reduced to nitrogen (N2) by a catalytic function of the catalyst layer.

[0022] The total length of the catalyst layer is not subject to any special restrictions, but from the point of view of a suitable reduction of the harmful components in the exhaust gas, the manufacturing costs and the degree of freedom in the design of the apparatus, it is, for example, 2 cm to 30 cm, preferably 5 cm to 15 cm and more preferably about 10 cm.

[0023] The catalyst layer of the exhaust gas purification catalyst contains at least one catalytic metal (Pd) and one Pt, and includes an OSC material with a pyrochlore structure, as well as an OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure, in a region (front catalyst layer section) from an upstream end of the catalyst layer to a length position that is 50% or less of the total length of the catalyst layer. With an exhaust gas purification catalyst containing these two OSC materials with different crystal structures, oxygen even reaches a downstream section of the catalyst layer, and an active catalytic reaction takes place. Therefore, the proportion of NO can be reduced. x -Emission will be prevented.

[0024] The region of the catalyst layer containing the two OSC materials with different crystal structures extends from the exhaust gas stream end of the catalyst layer to a length position that is 50% or less of the total length of the catalyst layer. However, the length position could be, for example, 40% or less, or 30% or less of the total length of the catalyst layer.

[0025] In Fig.Figure 1, illustrating an embodiment of the exhaust gas purification catalyst according to the invention, contains at least one catalytic metal of Pd and Pt, an OSC material with a pyrochlore structure, and an OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure, in a region (front catalyst layer section 21) extending from an upstream end 2a of a catalyst layer 2 to a length position that is 50% or less of the total length of the catalyst layer 2. Furthermore, as described below, a downstream section (rear catalyst layer section 22) of the catalyst layer 2, which differs from the front catalyst layer section 21, contains at least one catalytic metal of Pd and Pt and also the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure.

[0026] The catalyst layer contains at least one of Pd and one of Pt as the catalytic metal. The catalytic metal contained in the catalyst layer is not limited to Pd and / or Pt. Optionally, the catalyst layer may contain other metals, such as rhodium, in addition to or instead of some of the aforementioned metals, provided this is done appropriately.

[0027] In one embodiment of the invention, the OSC material can be used as a support on which the catalytic metal is supported. The OSC material is an inorganic material with oxygen storage capacity and stores oxygen when lean exhaust gas is supplied to it and releases the stored oxygen when rich exhaust gas is supplied to it. Examples of OSC materials include cerium oxide (cerium dioxide: CeO2) and composite oxides containing cerium oxide (for example, cerium oxide-zirconium oxide composite oxide (CZ composite oxide)). Of these OSC materials, CZ composite oxide is preferably used due to its high oxygen storage capacity and relatively low price. A mixing ratio (CeO2 / ZrO2) of cerium oxide to zirconia in the CZ composite oxide is preferably 0.65 to 1.5 and more preferably 0.75 to 1.3.

[0028] In the embodiment of the invention, an OSC material with a pyrochlore structure and an OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure are used together in the front catalyst layer section. Since these two OSC materials with different oxygen storage rates are used together, oxygen can be stored in these OSC materials at a suitable rate. Therefore, oxygen itself reaches the downstream section of the catalyst layer, and an active catalytic reaction takes place.

[0029] Regarding the OSC material with a pyrochlore structure, the pyrochlore structure contains two metal elements A and B, is represented by A₂B₂O₇, where B is a transition metal element, a type of crystal structure consisting of a combination of A 3+ / B 4+ or A 2+ / B 5+This structure is formed when the ionic radius of A in the crystal structure with such a configuration is relatively small. When the CZ composite oxide is used as the OSC material, the chemical formula of the OSC material with a pyrochlore structure is represented by Ce₂Zr₂O₇, where Ce and Zr are arranged alternately and regularly with oxygen in between. The OSC material with a pyrochlore structure has a slower oxygen storage rate than an OSC material with a different crystal structure (for example, a fluorite structure) and can still release oxygen even after the OSC material with the other crystal structure has ceased releasing oxygen. That is, the OSC material with a pyrochlore structure can still exhibit oxygen storage capacity even after the peak oxygen storage capacity of the OSC material with the other structure has been reached.The reason for this is assumed to be that the crystal structure of the OSC material with a pyrochlore structure is complex, and therefore the pathways during oxygen storage are also complex. More precisely, in the OSC material with a pyrochlore structure, the total amount of oxygen released during a period from 10 seconds to 120 seconds after the start of oxygen release is, for example, 60% to 95%, preferably 70% to 90%, and more preferably 75% to 85%, based on 100% of the total amount of oxygen released during a period immediately from the start (0 seconds) to 120 seconds after the start of oxygen release.

[0030] Specific examples of a crystal structure of the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure include a fluorite structure. The OSC material with a fluorite structure has a faster oxygen storage rate than the OSC material with a pyrochlore structure. Therefore, the proportion of harmful components can be appropriately reduced even when exhaust gas is supplied at a high flow rate.

[0031] Preferably, the two OSC materials present together in the front catalyst layer section are formed from the same composite oxide and differ in their crystal structures. In this case, the oxygen storage rate of the OSC material with the higher oxygen storage rate can be further improved, since the two OSC materials can be suitably dispersed in the predetermined region within the support. Specifically, it is preferred that the two OSC materials present together in the region described above are a cerium oxide-zirconium oxide composite oxide.

[0032] In this embodiment of the invention, the front catalyst layer section can, in addition to the two OSC materials and the catalytic metal, further contain a support material different from the OSC materials. A porous metal oxide with superior heat resistance can be used as the support material different from the OSC materials, and examples include aluminum oxide (aluminum dioxide: Al₂O₃), zirconium oxide (zirconium dioxide (ZrO₂)), silicon oxide (silicon dioxide: SiO₂), and composite oxides containing the aforementioned metal oxides as a major component.

[0033] Furthermore, the front catalyst layer section can contain other materials (typically an inorganic oxide) as an additive. Examples of materials that can be added to the front catalyst layer section include rare earth elements such as lanthanum (La) and yttrium (Y); alkaline earth elements such as calcium; and other transition metal elements. Of these, rare earth elements such as lanthanum and yttrium are preferably used as stabilizers because they can improve specific surface area at high temperatures without impeding catalytic function. In addition, the additive content of the OSC materials is preferably 10 wt% or less and more preferably 5 wt% or less.

[0034] The total content of the two OSC materials (the OSC material with a pyrochlore structure and the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure) in the front catalyst layer section is 80 g or less per 1 liter of substrate. If the total content of the two OSC materials in the front catalyst layer section is 80 g or less per 1 liter of substrate, the proportion of NO can be x -Emission will be reduced compared to the case where the total content is greater than 80 g / 1 1 substrate.

[0035] The content of the OSC material with a pyrochlore structure in the front catalyst layer section is 2 wt% to 9 wt%, and preferably 6 wt% to 9 wt%, based on the total content of both OSC materials (the OSC material with a pyrochlore structure and the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure) in that region. If the content of the OSC material with a pyrochlore structure in the front catalyst layer section is higher than the total content of both OSC materials in that region, the proportion of NO can be reduced. x Emissions will be reduced.

[0036] The ratio of the two OSC materials present together in the front catalyst layer can be determined by measuring peak intensity using X-ray diffraction analysis. Specifically, when X-ray diffraction analysis is performed on constituent materials in the predetermined region, characteristic peaks appear near 2θ / θ=14° and near 2θ / θ=29°. Of these peaks, one near 2θ / θ=14° originates from the pyrochlore structure, and one near 2θ / θ=29° originates from another crystal structure (for example, a fluorite structure). Accordingly, the ratio of a composite oxide with a pyrochlore structure to a composite oxide with a different crystal structure can be changed, that is, by adjusting a value I. 14 / 29, which is obtained by dividing a peak intensity near 2θ / θ=14° by a peak intensity near 2θ / θ=29°, an exhaust gas purification catalyst is obtained in which the two OSC materials are present together in a suitable ratio in the front catalyst layer section.

[0037] In the catalyst layer of the exhaust gas purification catalyst according to an embodiment of the invention, a downstream section (rear catalyst layer section), distinct from the front catalyst layer section, contains at least one Pd and Pt material and furthermore the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure. As in the case of the front catalyst layer section, the rear catalyst layer section can contain a support material different from the OSC materials and other materials as an additive component. According to a preferred embodiment of the invention, the rear catalyst layer section contains at least one Pd and Pt material and the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure.

[0038] The front and rear catalyst layer sections can be formed on the substrate by coating using a process well known to those skilled in the art. For example, at least one Pd and Pt, the two OSC materials, and optionally other catalyst layer components are applied to a predetermined area of ​​an upstream section of the substrate using a well-known washing-coating process, followed by drying and firing at a predetermined temperature for a predetermined time. As a result, the front catalyst layer section is formed on the substrate.Next, using the same procedure as above, the rear catalyst layer section, containing at least one Pd and Pt component as well as other components of the rear catalyst layer section, such as the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure, can be formed on a downstream side of the obtained front catalyst layer section. If each catalyst layer of the exhaust gas purification catalyst is formed using a wash-coating process, a method can be employed, for example, in which, after the formation of a layer of the OSC materials and / or another support using a wash-coating process, at least one Pd and Pt component is impregnated onto the obtained layer using a well-known impregnation process or the like from the related prior art.Alternatively, a wash coating can be applied using a powder of the OSC materials and / or another carrier on which the catalytic metal is pre-coated using an impregnation process or the like.

[0039] The exhaust gas purification catalyst can also contain a precious metal catalyst layer (also referred to as the "upper catalyst layer"), which is formed by coating the catalyst layer (also referred to as the "lower catalyst layer"). The inclusion of this precious metal catalyst layer further improves the exhaust gas purification performance of the catalyst.

[0040] The precious metal catalyst layer can contain a catalytic metal and a support on which the catalytic metal is mounted. A catalytic metal well-known in the prior art can be used as a precious metal catalyst for an exhaust gas purification catalyst. Specifically, the precious metal catalyst is not subject to any particular restrictions, provided it has a catalytic function with respect to harmful components contained in exhaust gas, and precious metal particles formed from various precious metal elements can be used. For example, any metal belonging to the platinum group or an alloy containing a platinum group metal as a major component can be preferably used as the metal that can be used in the precious metal catalyst. Examples of platinum group metals include platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os).The support on which the catalytic metal is supported is not subject to any particular restrictions, and examples include aluminium oxide (aluminium dioxide: Al2O3), zirconium oxide (zirconium dioxide (ZrO2)), silicon oxide (silicon dioxide: SiO2) and composite oxides containing the aforementioned oxides as a major component.

[0041] The noble metal catalyst layer may contain other materials (typically an inorganic oxide) as an additive component. Examples of materials that can be added to the noble metal catalyst layer include rare earth elements such as lanthanum (La) and yttrium (Y); alkaline earth elements such as calcium; and other transition metal elements. Of these, rare earth elements such as lanthanum and yttrium are preferred as stabilizers because they can improve specific surface area at high temperatures without impeding catalytic function.

[0042] The precious metal catalyst can be formed, as in the case of the catalyst layer, by applying a layer containing the catalytic metal and the support to a predetermined area on the catalyst layer formed on the substrate using a washing coating process or the like, followed by drying and firing at a predetermined temperature for a predetermined time.

[0043] Fig.Figure 2 illustrates a preferred embodiment of the exhaust gas purification catalyst according to the invention. The exhaust gas purification catalyst comprises an upper catalyst layer 3 (precious metal catalyst layer) formed by coating the front section 21 and the rear section 22 of the lower catalyst layer. In the preferred embodiment of the invention, the front section 21 of the lower catalyst layer extends from the exhaust gas-stream end 2a of the catalyst layer 2 to a length position that is 50% or less of the total length of the catalyst layer 2 and contains at least one catalytic metal of Pd and Pt, the OSC material with a pyrochlore structure, and the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure.The rear section 22 of the lower catalyst layer contains at least one catalytic metal from Pd and Pt, as well as the OSC material, whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure. The upper catalyst layer 3 contains any catalytic metal belonging to the platinum group.

[0044] The invention is described in more detail below using examples. However, the technical scope of the invention is not limited to these examples. Example 1: Exhaust gas purification catalyst

[0045] CeO2-ZrO2 composite oxide was used as the OSC material. [Preparation of OSC material with pyrochlore structure]

[0046] 49.1 g of an aqueous cerium nitride solution with a concentration of 28 wt% based on CeO₂, 54.7 g of an aqueous zirconium oxynitrate solution with a concentration of 18 wt% based on ZrO₂, and a commercially available surfactant were dissolved in 90 ml of ion-exchange water. An ammonia solution containing 25 wt% NH₃ was added in an amount of 1.2 equivalents based on anions to prepare a coprecipitate, and the resulting coprecipitate was filtered and washed. Next, the coprecipitate was dried at 110°C and fired in air at 500°C for 5 hours to obtain a solid solution of cerium and zirconium. Next, the obtained solid solution was comminuted to a mean particle size of 1000 nm using a comminution device to obtain a powder of solid CeO2-ZrO2 solution, in which a molar ratio (CeO2 / ZrO2) of the CeO2 to ZrO2 content was 1.09.Next, a polyethylene bag was filled with this solid CeO₂-ZrO₂ solution powder, the contents were degassed, and the bag was then sealed by heating. The solid CeO₂-ZrO₂ solution powder was then compressed using an isostatic press at a pressure of 300 MPa for 1 minute to obtain a solid raw material. This raw material was then placed in a graphite crucible, which was covered with a graphite lid, followed by 5 hours of reduction in argon gas at 1700°C. The reduced material was then pulverized using a grinding device to obtain a CeO₂-ZrO₂ composite oxide powder with a pyrochlore structure and a mean particle size of approximately 5 µm. [Formation of a front section of the lower catalyst layer]

[0047] Palladium was supported by impregnation using a palladium nitrate solution such that the ratio of metallic palladium to 40 g / l of aluminum oxide substrate with added lanthanum (La₂O₃ / Al₂O₃ = 4 / 96 wt%) was 1 g / l of substrate. The substrate was dried for 30 minutes at 120°C and then fired for 2 hours at 500°C to obtain a palladium-supported powder. The obtained Pd-supported powder (41 g / l substrate), the obtained OSC material with a pyrochlore structure (4.8 g / l substrate), the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure (35.2 g / 1 l substrate), water and a binder (5 g / 1 l substrate) were mixed, and their pH and viscosity were adjusted using acetic acid or the like to obtain a slurry for the front section of the lower catalyst layer.

[0048] Next, the obtained slurry was applied using a wash-coating process to an upstream section of a ceramic honeycomb substrate (φ 103 mm, L 105 mm, volume 875 cc, cordierite), in which several cells were separated by a partition, over a width equal to 50% of the total length of the honeycomb substrate, followed by drying and firing. As a result, a front section of the lower catalyst layer was formed on one cell surface of the honeycomb substrate. [Formation of a rear section of the lower catalyst layer]

[0049] A slurry was prepared using the same procedure as the front section of the lower catalyst layer, except that the OSC material with a pyrochlore structure was not used. Next, the resulting slurry was applied, using a wash coating process, to a downstream section of the honeycomb substrate on which the front section of the lower catalyst layer had been formed, in a width equal to 50% of the total length of the honeycomb substrate, followed by drying and firing. As a result, a rear section of the lower catalyst layer was formed on the cell surface of the honeycomb substrate. [Formation of the upper catalyst layer]

[0050] Next, Rh (0.2 g / L substrate) was supported by impregnation using a rhodium nitrate solution onto 40 g / L substrate of the OSC material, whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure. The substrate was dried for 30 minutes at 120°C and then fired for 2 hours at 500°C to obtain an Rh-supported powder. This Rh-supported powder (40.2 g / L substrate) was then mixed with the aluminum oxide used in the front section of the lower catalyst layer, along with added lanthanum (40 g / L substrate), water, and a binder (5 g / L substrate). The pH and viscosity of this mixture were adjusted using acetic acid or the like to obtain a slurry for the front section of the upper catalyst layer.Next, the resulting slurry was applied to the entire area of ​​the honeycomb structure, where the front and rear sections of the lower catalyst layer were formed, using a washing coating process, followed by drying and firing. As a result, an exhaust gas purification catalyst was obtained in which the upper catalyst layer was formed on top of the lower catalyst layer, which comprised the front and rear sections of the lower catalyst layer.

[0051] Fig. Figure 3 illustrates the exhaust gas purification catalyst obtained in Example 1. Fig. 3 represents the conventional OSC material, the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure.

[0052] A catalyst of a comparison example was prepared using the same procedure as in Example 1, except that the OSC material with a pyrochlore structure was removed from the front section of the lower catalyst layer of Example 1. Example 2: Assessment of NO x -Reduction performance of the exhaust gas purification catalyst

[0053] With regard to the exhaust aftertreatment catalyst of Example 1 and the exhaust aftertreatment catalyst of the comparison example, an exhaust gas test equivalent to 150,000 miles was performed. Next, each of the exhaust aftertreatment catalysts was fitted to a 2.5-liter L4 engine, and exhaust gas was supplied to the engine for 15 seconds at an intake air flow rate (Ga) of 20 g / s. In this case, the temperature of exhaust gas entering the catalyst was 600°C, and the air-fuel ratio (A / F) entering the catalyst was 14.6. Next, exhaust gas was supplied to the engine for 30 seconds at an air-fuel ratio of 14.1, and the NOₓ fraction was measured. x Emissions were measured on one side of the catalytic converter outlet to determine NOₓ emissions. x -to assess the reduction performance of each of the exhaust gas purification catalysts. The results are in Fig. 4 shown. In Fig. 4 represents a solid line, the proportion of NO x-Emission from the exhaust gas purification catalyst of example 1, a dashed line represents the proportion of NO x -Emission of the exhaust gas purification catalyst of the comparison example, and a dashed line represents an air-fuel ratio (A / F).

[0054] As from Fig. As is clearly evident in Figure 4, the exhaust gas purification catalyst of Example 1 exhibited an extremely much higher NO under the condition of a rich air-fuel ratio of the exhaust gas. x -Reduction performance is greater than that of the exhaust gas purification catalyst of the comparison example. Example 3: Influence of the total content of OSC materials and the content of OSC material with pyrochlore structure on NO x -Reduction performance

[0055] Regarding the exhaust gas purification catalysts, the proportion of NOₓ was x-Emission was measured while the total amount of the two OSC materials (the OSC material with a pyrochlore structure and the OSC material whose oxygen storage rate is faster than that of the OSC material with a pyrochlore structure) in the front section of the lower catalyst layer was changed, and the proportion of NO x -Emission was measured while the content of the OSC material with a pyrochlore structure in the front section of the lower catalyst layer was changed relative to the total content of the two OSC materials.

[0056] The exhaust gas purification catalysts, catalysts 1 to 10 shown in Table 1 below, and the catalyst from Example 1, were prepared using the same method as above, wherein the total content of the two OSC materials in the front section of the lower catalyst layer was 80 g / L substrate and 100 g / L substrate, respectively, and the content of the OSC material with a pyrochlore structure was 0, 3, 6, 9, and 12 wt.%, respectively, based on the total content of the two OSC materials in each of the catalysts. In Table 1, all OSC materials represent the two OSC materials contained in a region (front section of the lower catalyst layer) extending from the upstream end of the lower catalyst layer to a length position that is 50% or less of the total length of the lower catalyst layer. Catalysts 1, 5, 6, and 10 are not part of the invention. [Table 1] Ratio of OSC material with a pyrochlore structure / all OSC materials Content of all OSC materials (g / L) Catalyst 1 0 80 Catalyst 2 3 Catalyst 3 6 Catalyst 4 9 Catalyst 5 12 Catalyst 6 0 100 Catalyst 7 3 Catalyst 8 6 Catalyst 9 9 Catalyst 10 12

[0057] Regarding catalysts 1 to 10, the same test was performed as for NO. x -Reduction performance test of example 2 performed, and the proportion of NO x CO2 emissions were measured 30 seconds after changing the air-fuel ratio to 14.1. The results are in Fig. 5 shown. In Fig. 5 represents the proportion of NO. x -Emission, which was measured when the total content of the two OSC materials in the front section of the lower catalyst layer was 80 g / 1 l substrate (catalysts 1 to 5), and the black triangle represents the proportion of NO x -Emission measured when the total content of the two OSC materials in the front section of the lower catalyst layer was 100 g / 1 l substrate (catalysts 6 to 10).

[0058] In Fig. 5 was the proportion of NO xEmissions were reduced when the total content of both OSC materials in the front section of the lower catalyst layer was 80 g / l substrate compared to 100 g / l substrate. Furthermore, the proportion of NO x Emissions were reduced when the content of the OSC material with a pyrochlore structure in the front section of the lower catalyst layer was between 2 wt% and 10 wt% based on the total content of both OSC materials. When the content of the OSC material with a pyrochlore structure is in this range, the OSC material with a pyrochlore structure can utilize oxygen efficiently. Therefore, it is assumed that an active catalytic reaction occurred and the exhaust gas purification performance of the catalyst was improved.

[0059] By using the exhaust gas purification catalyst according to the present invention, an exhaust gas purification catalyst with improved NOₓ efficiency can be achieved.x -Reduction power will be provided.

Claims

[1] Exhaust gas purification catalyst in which a catalyst layer (2) containing at least one of Pd and Pt is formed on a substrate (1), characterized in that it comprises: a front section of the catalyst layer (21) which is provided in a region from an exhaust-stream end of the catalyst layer to a length position which is 50% or less of a total length of the catalyst layer, and which comprises a first OSC material having a pyrochlore structure and a second OSC material whose oxygen storage rate is faster than an oxygen storage rate of the first OSC material; wherein a content of the first OSC material in the front section of the catalyst layer (21) is 2 wt.% to 9 wt.%, based on the total content of the first OSC material and the second OSC material, and a rear catalyst layer section (22) which is arranged in a region downstream in a flow direction of an exhaust gas which is different from the front catalyst layer section and which comprises a second OSC material and at least one of Pd and Pt, wherein the rear catalyst layer section (22) does not contain the first OSC material having a pyrochlore structure. [2] Exhaust gas purification catalyst according to claim 1, characterized by , that the total content of the first OSC material and the second OSC material in the front section of the catalyst layer (21) is 80 g or less per 1 liter of substrate. [3] Exhaust gas purification catalyst according to claim 1 or 2, characterized in that it further comprises: a precious metal catalyst layer (3) formed on the catalyst layer (2).

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