Catalysts for exhaust gas purification

Abstract

The present invention provides an exhaust gas purification catalyst capable of suppressing the movement of platinum group metals from the catalyst layer to the substrate during high-temperature durability. The exhaust gas purification catalyst disclosed herein includes a substrate, a catalyst layer, and an intermediate layer disposed between the substrate and the catalyst layer. The substrate contains SiC. The catalyst layer contains platinum group metals as catalyst components. The intermediate layer substantially does not contain platinum group metals. The product of the thickness (μm) of the intermediate layer and the specific surface area (m<supgt;2< / supgt> / g) of the intermediate layer is 1100 or more.

Description

Technical Field

[0001] The present invention relates to a catalyst for exhaust gas purification, and more particularly to a catalyst for exhaust gas purification suitable for use as an electrically heated catalyst. Furthermore, this application claims priority based on Japanese Patent Application No. 2019-236718 filed on December 26, 2019, the entire contents of which are incorporated herein by reference. Background Art

[0002] In recent years, electrically heated catalysts (EHCs) have attracted much attention as catalysts for purifying exhaust gas discharged from engines of automobiles and the like. In order for exhaust gas purification catalysts for internal combustion engines to fully exert purification performance, the temperature of the exhaust gas purification catalyst needs to be increased to a temperature at which the catalyst is activated. Conventional catalysts for exhaust gas purification are heated by the heat of the exhaust gas, and therefore cannot obtain high purification performance when the temperature of the exhaust gas is low, such as when the engine has just started. In contrast, when an electrically heated catalyst is used, even when the temperature of the exhaust gas is low, such as when the engine has just started, the temperature of the catalyst can be increased in a short time by electrical heating to activate the catalyst. Therefore, even when the temperature of the exhaust gas is low, sufficient purification performance can be obtained, thereby improving the purification efficiency of the exhaust gas.

[0003] A typical structure of an electrically heated catalyst includes a substrate and a catalyst layer. The substrate contains SiC that functions as a resistance heating element, and the catalyst layer contains a platinum group metal (PGM) as a catalyst component (see, for example, Patent Document 1).

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent No. 6052250 Summary of the invention

[0007] The inventors of the present invention have conducted in-depth research and found that the above-described prior art has a problem in which the platinum group metal of the catalyst layer moves to the substrate during high-temperature durability. Once the amount of platinum group metal moving to the substrate is large, the exhaust purification performance will be reduced. In addition, in the case where the exhaust purification catalyst is an electrically heated catalyst, if the amount of platinum group metal moving to the substrate is large, it will also have an adverse effect on the power-on state. As a result, the high-temperature durability of the exhaust purification performance deteriorates.

[0008] Therefore, an object of the present invention is to provide an exhaust gas purification catalyst capable of suppressing the migration of platinum group metals from the catalyst layer to the substrate during high temperature durability.

[0009] The exhaust gas purification catalyst disclosed herein includes a substrate, a catalyst layer, and an intermediate layer disposed between the substrate and the catalyst layer. The substrate contains SiC. The catalyst layer contains a platinum group metal as a catalyst component. The intermediate layer does not substantially contain a platinum group metal. The thickness (μm) of the intermediate layer is proportional to the specific surface area (m 2 / g) is greater than 1100.

[0010] According to such a structure, by making the intermediate layer have an appropriate thickness and an appropriate specific surface area, it can play an important role as a physical barrier that hinders the movement of platinum group metals. That is, the intermediate layer plays an effective barrier role and can inhibit the movement of platinum group metals from the catalyst layer to the substrate during high temperature durability. Therefore, the exhaust gas purification catalyst disclosed herein has excellent high temperature durability.

[0011] In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the thickness of the intermediate layer is 30 μm or more.

[0012] According to such a structure, the function of the intermediate layer as a barrier layer is improved. Therefore, the intermediate layer can further suppress the migration of the platinum group metal from the catalyst layer to the substrate during high-temperature durability, and the high-temperature durability of the exhaust gas purification catalyst is further improved.

[0013] In another preferred embodiment of the exhaust gas purification catalyst disclosed herein, the specific surface area of ​​the intermediate layer is 40 m 2 / g or above.

[0014] According to such a structure, the function of the intermediate layer as a barrier layer is improved. Therefore, the intermediate layer can further suppress the migration of the platinum group metal from the catalyst layer to the substrate during high-temperature durability, and the high-temperature durability of the exhaust gas purification catalyst is further improved.

[0015] In another preferred embodiment of the exhaust gas purification catalyst disclosed herein, the platinum group metal contained in the catalyst layer is Pd.

[0016] In this case, among the platinum group metals, Pd is particularly likely to migrate from the catalyst layer to the substrate during high-temperature durability, and therefore the effect of improving high-temperature durability by the intermediate layer is particularly significant.

[0017] In another preferred embodiment of the exhaust gas purifying catalyst disclosed herein, the exhaust gas purifying catalyst further includes an electrode, and the exhaust gas purifying catalyst is an electrically heated catalyst.

[0018] According to such a structure, it is possible to provide an electrically heated catalyst having high high-temperature durability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1It is a perspective view schematically showing an example of an exhaust gas purification catalyst according to an embodiment.

[0020] Figure 2 It is schematically indicated Figure 1 A cross-sectional view of a layer structure of an exhaust gas purification catalyst.

[0021] Figure 3 This is an image showing the Pd mapping result of FE-EPMA of the exhaust gas purification catalyst of Comparative Example 1 before the endurance test.

[0022] Figure 4 This is an image showing the Pd mapping result of FE-EPMA after the endurance test of the exhaust gas purification catalyst of Comparative Example 1.

[0023] Figure 5 This is a graph showing the Pd mapping result of FE-EPMA after the endurance test of the exhaust gas purification catalyst of Example 5.

[0024] Figure 6 This is a graph showing the relationship between the product of the thickness and the specific surface area of ​​the intermediate layer and the Pd mobility, which is obtained based on the results of each example and each comparative example. DETAILED DESCRIPTION

[0025] Hereinafter, several preferred embodiments of the present invention will be described with appropriate reference to the accompanying drawings. Among them, matters other than those specifically mentioned in this specification and necessary for implementing the present invention can be understood as design matters of those skilled in the art based on the prior art in the field. The present invention can be implemented based on the contents disclosed in this specification and the technical knowledge in the field. In addition, the following Figure 1-2 The diagrams are provided for the purpose of understanding the contents of the present invention, and the dimensional relationships (length, width, thickness, etc.) in the diagrams do not reflect the actual dimensional relationships.

[0026] Figure 1 An example of the exhaust gas purification catalyst according to the present embodiment is schematically shown. Figure 1 This is an example in which the exhaust gas purification catalyst according to the present embodiment is configured as an electrically heated catalyst. Figure 2 Schematically Figure 1 Layer structure of an exhaust gas purification catalyst.

[0027] like Figure 1 and Figure 2 As shown, the exhaust gas purifying catalyst 10 includes a substrate 11 , a catalyst layer 20 , and an intermediate layer 30 disposed between the substrate 11 and the catalyst layer 20 .

[0028] <Base material>

[0029] The substrate 11 is a member that supports the catalyst layer 20 via the intermediate layer 30 .

[0030] The substrate 11 contains SiC (silicon carbide). Since the substrate 11 contains SiC, cracks are less likely to occur in the substrate due to heat cycles.

[0031] Furthermore, SiC can function as a resistance heating element, so the exhaust gas purification catalyst can be used as an electrically heated catalyst by including SiC in the substrate 11. The substrate 11 may include components other than SiC within a range that does not significantly impair the effects of the present invention.

[0032] The shape of the substrate 11 may be the same as that of a known exhaust gas purification catalyst. For example, the substrate 11 may be a straight-flow type or wall-flow type honeycomb substrate commonly used in exhaust gas purification catalysts.

[0033] exist Figure 1 In the example shown, the substrate 11 has a cylindrical appearance. The appearance of the substrate 11 is not limited to this, and may be an elliptical cylinder, a polygonal cylinder, or the like.

[0034] Figure 1 and Figure 2 The arrow indicates the flow direction of the exhaust gas. The substrate 11 has a plurality of chambers 15 with openings at both ends on the exhaust gas inlet side and the exhaust gas outlet side, and partition walls 16 that divide the adjacent chambers 15. The shape of each chamber 15 is not particularly limited, and may be a square, parallelogram, rectangle, trapezoidal quadrilateral, triangle, hexagon, octagon, or other polygonal shape, circle, or the like. In the exhaust gas purification catalyst 10 having the substrate 11 shown in the figure, the exhaust gas flowing into the chamber 15 from the end on the exhaust gas inlet side flows out through the chamber 15 to the end on the exhaust gas outlet side.

[0035] A catalyst layer 20 and an intermediate layer 30 are formed on at least a portion of the substrate 11. Figure 1 Although not shown in the figure, the catalyst layer 20 and the intermediate layer 30 are formed in the cell 15 .

[0036] <Catalyst layer>

[0037] The catalyst layer 20 contains a platinum group metal as a catalyst component. That is, the catalyst layer 20 contains at least one metal selected from platinum (Pt), rhodium (Rh), palladium (Pd), ruthenium (Ru), osmium (Os) and iridium (Ir). Here, Pd and Pt have excellent purification performance (oxidation purification ability) of carbon monoxide (CO) and hydrocarbons (HC), and Rh has excellent NOx purification performance (reduction purification ability). Therefore, from the viewpoint of high purification ability, the platinum group metal contained in the catalyst layer 20 is preferably at least one selected from Pt, Rh and Pd. In addition, among the platinum group metals, Pd is particularly easy to move from the catalyst layer 20 to the substrate 11 during high temperature durability. Therefore, the high temperature durability improvement effect brought by the intermediate layer 30 is particularly good, and thus Pd is most preferably used as the platinum group metal contained in the catalyst layer 20.

[0038] From the viewpoint of increasing the contact area with the exhaust gas, the platinum group metal is preferably in the form of microparticles. Specifically, the average particle size of the platinum group metal is preferably 15 nm or less, more preferably 10 nm or less, further preferably 7 nm or less, and most preferably 5 nm or less. In addition, the average particle size of the platinum group metal is preferably 1 nm or more.

[0039] The average particle size of the platinum group metal is obtained by obtaining an electron microscope image (TEM image, etc.) of the platinum group metal and randomly selecting 20 or more particles from the image to obtain the average value of the particle sizes.

[0040] In addition, the catalyst layer 20 usually contains a carrier that supports a platinum group metal. The manner in which the carrier supports the platinum group metal is not particularly limited. As for the type of carrier, there is no particular limitation as long as it can support the platinum group metal, and a material known as a carrier of a catalyst layer for exhaust gas purification catalyst can be used. Examples of carriers include: inorganic materials with oxygen storage capacity (OSC) such as cerium oxide (ceria: CeO2), composite oxides containing the ceria (such as ceria-zirconia composite oxide (CZ or ZC composite oxide)), etc. (so-called OSC materials); oxides such as alumina (Al2O3), titanium dioxide (TiO2), zirconium oxide (ZrO2), silicon dioxide (SiO2), etc. They can be used alone or in combination of two or more. The OSC material can play the role of a catalyst promoter for exhaust gas purification, so it is preferred that the carrier contains an OSC material, and more preferably contains a ceria-zirconia composite oxide.

[0041] The catalyst layer 20 may also contain components other than the catalyst component and the carrier (e.g., a binder, an additive, etc.). Examples of the binder include alumina gel, silica gel, etc. Examples of the additive include NO x Adsorbents, stabilizers, etc.

[0042] The content of the platinum group metal in the catalyst layer 20 is not particularly limited, but is, for example, 0.01% by mass to 8% by mass, or 0.1% by mass to 5% by mass, relative to the total mass of the carrier contained in the catalyst layer.

[0043] The catalyst layer 20 may be a single-layer structure or a multilayer structure having two or more layers containing a platinum group metal. When the catalyst layer 20 has a multilayer structure, the platinum group metals contained in each layer may be the same or different. For example, the catalyst layer 20 may have a multilayer structure in which a layer containing Rh is formed on a layer containing Pd. In addition, the catalyst layer 20 may also contain different platinum group catalysts in the front and rear portions.

[0044] The thickness and length of the catalyst layer 20 can be appropriately determined based on the size of the cells 15 of the substrate 11, the flow rate of exhaust gas supplied to the exhaust gas purifying catalyst 10, etc. The thickness of the catalyst layer 20 is not particularly limited, and is, for example, 1 μm to 500 μm or 5 μm to 200 μm.

[0045] <Middle layer>

[0046] The intermediate layer 30 does not substantially contain a platinum group metal. In the present specification, "substantially free of" means that the platinum group metal is not intentionally contained for use as a catalyst component, but the mixing of the platinum group metal during the manufacture of the intermediate layer or the mixing of the platinum group metal due to impurities is allowed. Alternatively, in the present specification, "substantially free of" means that the content of the platinum group metal in the intermediate layer 30 (i.e., the content of the platinum group metal relative to the total weight of the intermediate layer 30) is less than 0.001% by mass, and the content of the platinum group metal in the intermediate layer 30 is preferably less than 0.0005% by mass, and more preferably 0% (i.e., the intermediate layer 30 does not contain a platinum group metal).

[0047] The components contained in the intermediate layer 30 are not particularly limited as long as they do not substantially contain platinum group metals. The intermediate layer, for example, contains a carrier that can be used in the catalyst layer 20 (i.e., the OSC material, oxide, etc. exemplified above). The carrier may contain alkaline earth metals such as Ba and Sr. For example, the intermediate layer 30 contains a carrier of the same type as the carrier contained in the catalyst layer 20. The intermediate layer 30 has the same composition as the catalyst layer except that it does not substantially contain platinum group metals. That is, the intermediate layer 30 does not substantially contain platinum group metals, but may contain the carrier, additives, etc. actually contained in the catalyst layer 20 in the same composition ratio as the catalyst layer 20.

[0048] The thickness (μm) of the intermediate layer 30 and the specific surface area (m 2 / g) (i.e., the product of the thickness of the intermediate layer 30 (μm)×the specific surface area of ​​the intermediate layer 30 (m 2In this case, the intermediate layer 30 functions excellently as a physical barrier that hinders the movement of the platinum group metal. That is, the intermediate layer 30 functions as an effective barrier layer, and can inhibit the movement of the platinum group metal from the catalyst layer 20 to the substrate 11 during high temperature durability. Therefore, the high temperature durability of the exhaust gas purification catalyst 10 is excellent.

[0049] The thickness (μm) of the intermediate layer 30 and the specific surface area (m 2 Specifically, the product is preferably 1200 or more, more preferably 1500 or more, further preferably 2000 or more, and most preferably 2500 or more. The upper limit of the product is not particularly limited, and is, for example, 50000 or less.

[0050] In order to further improve the function of the intermediate layer 30 as a barrier layer, the thickness of the intermediate layer 30 is preferably greater than 20 μm. At this time, the intermediate layer 30 can further inhibit the migration of platinum group metals from the catalyst layer 20 to the substrate 11 during high-temperature durability, and the high-temperature durability of the exhaust purification catalyst 10 is more excellent. The thickness of the intermediate layer 30 is more preferably greater than 30 μm, and more preferably greater than 50 μm. On the other hand, when the thickness of the intermediate layer 30 is too large, it will lead to increased resistance. Therefore, the thickness of the intermediate layer 30 is preferably less than 200 μm, more preferably less than 100 μm, and more preferably less than 80 μm.

[0051] The thickness of the intermediate layer 30 can be measured according to a known method. For example, an electron microscope image of a cross section of the exhaust purification catalyst 10 can be obtained, and the thickness of the intermediate layer 30 can be obtained by measuring the thickness. When it is difficult to determine the intermediate layer 30 using an electron microscope, an electron probe microanalyzer (EPMA) can be used to observe the cross section of the exhaust purification catalyst 10 and obtain an EPMA observation image, and the intermediate layer can be determined by elemental analysis, and the thickness can be obtained based on the EPMA observation image.

[0052] In order to further enhance the function of the intermediate layer 30 as a barrier layer, the specific surface area of ​​the intermediate layer 30 is preferably 40 m 2 In this case, the intermediate layer 30 can further suppress the migration of the platinum group metal from the catalyst layer 20 to the substrate 11 during high temperature durability, and the high temperature durability of the exhaust gas purification catalyst 10 is more excellent. The specific surface area of ​​the intermediate layer 30 is more preferably 50 m 2 / g or more, more preferably 70m 2 The upper limit of the specific surface area of ​​the intermediate layer 30 is not particularly limited, but is, for example, 1000 m 2 / g or less or 500m 2 / g.

[0053] In this specification, the specific surface area of ​​the intermediate layer 30 refers to a value measured by a BET one-point method using nitrogen as an adsorbent by a flow method.

[0054] In addition, the thickness (μm) of the intermediate layer 30 and the specific surface area (m 2 The value of the product of 100 g / g) can be controlled by adjusting the coating amount of the slurry for forming the intermediate layer, the particle size of the constituent material of the intermediate layer contained in the slurry, and the like.

[0055] The intermediate layer 30 may have a single layer structure or a multilayer structure of two or more layers substantially free of platinum group metals. From the viewpoint of easy production, the intermediate layer 30 preferably has a single layer structure.

[0056] The exhaust purification catalyst 10 may further include elements other than the substrate 11 , the catalyst layer 20 and the intermediate layer 30 .

[0057] exist Figure 1 In the example shown, the exhaust purification catalyst 10 is configured as an electrically heated catalyst. Therefore, the exhaust purification catalyst 10 also has an electrode 40. The electrode 40 can be the same as the electrode of a known electrically heated catalyst, and the electrode 40 can also be a metal electrode, a carbon electrode, etc. In the example shown in the figure, the electrode 40 has an electrode layer 42 and an electrode terminal 44 provided on the outer surface of the substrate 11. The electrode layer 42 has the function of diffusing the current. However, as long as the substrate 11 can be energized, the structure of the electrode 40 is not limited thereto.

[0058] The exhaust purification catalyst 10 can be manufactured, for example, as follows.

[0059] First, prepare the substrate 11. And prepare a slurry containing the constituent components of the intermediate layer 30 (such as OSC materials, oxides, etc.). Apply the slurry in the small chamber 15 of the substrate 11, dry it, and fire it as needed. Thus, the intermediate layer 30 is formed first. Then, the catalyst layer 20 is formed according to a known method. Specifically, for example, a platinum group metal source (such as a solution containing ions of platinum group metals), its carrier (such as OSC materials, oxides, etc.) and a dispersion medium are mixed to prepare a slurry for forming the catalyst layer 20. Apply the slurry on the intermediate layer 30 formed in the small chamber 15 of the substrate 11, dry it, and then fire it. Thus, the catalyst layer 20 is formed on the intermediate layer 30.

[0060] Alternatively, as another example of a manufacturing method, a substrate 11 is prepared, and a slurry containing constituent components of the intermediate layer 30 is prepared. The slurry is applied to the chamber 15 of the substrate 11, dried, and fired as needed. Thus, a layer is formed on the substrate 11. Thereafter, a solution containing ions of a platinum group metal is infiltrated into the upper portion of the formed layer, and the solution is dried and fired. At this time, the lower portion of the formed layer becomes the intermediate layer 30 that does not substantially contain a platinum group metal, and the upper portion of the formed layer becomes the catalyst layer 20 that carries the platinum group metal. Therefore, according to such a method, the intermediate layer 30 can be formed on the substrate 11, and the catalyst layer 20 can be formed on the intermediate layer 30 at the same time.

[0061] In the above, an example in which the exhaust purification catalyst involved in the present embodiment is configured as an electrically heated catalyst is described. However, the exhaust purification catalyst involved in the present embodiment is not limited to this method as long as it can be used for exhaust purification of internal combustion engines. For example, the exhaust purification catalyst involved in the present embodiment can also be configured as an exhaust purification catalyst for a diesel engine. The exhaust purification catalyst for a diesel engine is heated during the regeneration process. In the case where the exhaust purification catalyst involved in the present embodiment is an exhaust purification catalyst for a diesel engine, it can exhibit high high-temperature durability during the heating, and can inhibit the movement of platinum group metals from the catalyst layer to the substrate.

[0062] The exhaust gas purification catalyst according to the present embodiment can suppress the migration of platinum group metals from the catalyst layer to the substrate during high temperature (eg, 900° C. or 1000° C.) durability. Therefore, the exhaust gas purification catalyst according to the present embodiment is excellent in high temperature durability.

[0063] The exhaust gas purification catalyst involved in this embodiment can be used for exhaust gas purification of an internal combustion engine according to a known method. Specifically, for example, the exhaust gas purification catalyst involved in this embodiment is configured in an exhaust system (particularly an exhaust pipe) of an automobile engine for use. Here, the exhaust gas purification catalyst shown in this embodiment is, for example, electrically connected to a car to be powered by a battery of the car when it is an ETC as shown in the example.

[0064] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not intended to be limited to these examples.

[0065] <Manufacturing of exhaust gas purification catalyst>

[0066] (Comparative Example 1)

[0067] As a substrate, a honeycomb substrate made of silicon carbide (volume 547 mL, number of cells 600 cpsi, partition wall thickness 5 mm, cell shape quadrilateral, substrate length 500 mm) was prepared.

[0068] A palladium nitrate solution, aluminum oxide powder, CZ composite oxide powder, barium sulfate and ion exchange water were mixed to prepare a Pd-containing slurry.

[0069] Rhodium nitrate solution, aluminum oxide powder, ZC composite oxide powder and ion exchange water are mixed to prepare Rh-containing slurry.

[0070] The Pd-containing slurry was poured into the substrate, and the unnecessary slurry was blown off by a blower, thereby coating the substrate wall with the slurry. Next, the substrate was placed in a dryer set at 120°C and dried for 2 hours. Thereafter, the substrate was fired in an electric furnace at 500°C for 2 hours to form a Pd catalyst layer.

[0071] Next, the Rh-containing slurry was poured into the substrate, and the unnecessary slurry was blown off by a blower, thereby coating the substrate wall with the slurry. Next, the substrate was placed in a dryer set at 120°C and dried for 2 hours. Thereafter, the substrate was fired in an electric furnace at 500°C for 2 hours to form a Rh catalyst layer.

[0072] The coating amounts of the Pd-containing slurry and the Rh-containing slurry were 80 g / L, respectively, relative to the substrate.

[0073] As described above, an exhaust gas purification catalyst of Comparative Example 1 in which a Pd catalyst layer and a Rh catalyst layer were sequentially laminated on a substrate was obtained.

[0074] [Each Example and Other Comparative Examples]

[0075] As a substrate, a honeycomb substrate made of silicon carbide (volume 547 mL, number of cells 600 cpsi, partition wall thickness 5 mm, cell shape quadrilateral, substrate length 500 mm) was prepared.

[0076] A palladium nitrate solution, aluminum oxide powder, CZ composite oxide powder, barium sulfate and ion exchange water were mixed to prepare a Pd-containing slurry.

[0077] Rhodium nitrate solution, aluminum oxide powder, ZC composite oxide powder and ion exchange water are mixed to prepare Rh-containing slurry.

[0078] Alumina powder, CZ composite oxide powder, and ion-exchanged water are mixed to prepare an intermediate layer slurry.

[0079] The slurry for the intermediate layer is poured into the above substrate, and the unnecessary slurry is blown away by a blower, thereby coating the wall surface of the substrate with the slurry. Next, the substrate is placed in a dryer set at 120°C and dried for 2 hours. Thereafter, the substrate is fired in an electric furnace at 500°C for 2 hours to form an intermediate layer.

[0080] Next, the Pd-containing slurry was poured into the substrate, and the unnecessary slurry was blown off by a blower, thereby coating the substrate wall with the slurry. Next, the substrate was placed in a dryer set at 120°C and dried for 2 hours. Thereafter, the substrate was fired in an electric furnace at 500°C for 2 hours to form a Pd catalyst layer.

[0081] Then, the Rh-containing slurry was poured into the substrate, and the unnecessary slurry was blown away by a blower, thereby coating the substrate wall with the slurry. Next, the substrate was placed in a dryer set at 120°C and dried for 2 hours. After that, the substrate was fired in an electric furnace at 500°C for 2 hours to form a Rh catalyst layer.

[0082] In these steps, the specific surface area and thickness of the intermediate layer were changed by changing the particle size and surface area of ​​the alumina powder and the CZ composite oxide powder used for the intermediate layer and changing the coating amount of the intermediate layer slurry.

[0083] As described above, exhaust gas purification catalysts of each example and other comparative examples were obtained in which the intermediate layer, the Pd catalyst layer and the Rh catalyst layer were sequentially laminated on the substrate.

[0084] <Measurement of thickness of intermediate layer>

[0085] The cross section of the exhaust gas purification catalyst of each example and each comparative example was observed using an electron probe microanalyzer (EPMA) and a scanning electron microscope (SEM) to measure the thickness of the intermediate layer.

[0086] <Measurement of specific surface area of ​​intermediate layer>

[0087] The intermediate layer is cut out from the exhaust purification catalyst of each embodiment and each comparative example, or the powder is scraped from the intermediate layer of the exhaust purification catalyst of each embodiment and each comparative example to prepare a sample. For this sample, nitrogen is used as a purge gas and a gas purge method is used to perform pretreatment under the conditions of a gas flow rate of 25 mL / min, a heating temperature of 250°C, and a treatment time of about 23 minutes. Next, a commercially available fully automatic specific surface area measuring device "Macsorb Model HM-1230" (manufactured by MOUNTECH Co., Ltd.) is used to measure the specific surface area of ​​the sample by the BET1 point method using a flow method using nitrogen as the adsorbent. In this measurement, a mixed gas of N2 (30%) and He (70%) is used, and the adsorption temperature is the boiling point of liquid nitrogen.

[0088] <High temperature durability test>

[0089] The exhaust gas purification catalysts of the embodiments and comparative examples were installed in the exhaust system of a V-type 8-cylinder gasoline engine installed on a table, and a durability test was carried out for 50 hours at a catalyst bed temperature of 1000° C. under conditions including a predetermined fuel cut.

[0090] <Evaluation of Pd mobility>

[0091] The stacked portion of the exhaust gas purification catalyst of each embodiment and each comparative example before and after the durability test was cut into a predetermined size, embedded in a resin, and then the cross section was ground. Carbon was vapor-deposited on the cross section, and the cross section was analyzed using a FE-EPMA device "JXA-8530F" (manufactured by JEOL).

[0092] The amount of Pd present in the catalyst layer before the durability test was set to 100, and the ratio of the amount of Pd that moved to the substrate side was determined as the Pd mobility (%).

[0093] [Table 1]

[0094] Table 1

[0095]

[0096] Figure 3 The results of Pd mapping of FE-EPMA of the exhaust gas purification catalyst of Comparative Example 1 before the endurance test are shown. Figure 4 The results of Pd mapping of FE-EPMA of the exhaust gas purification catalyst of Comparative Example 1 after the endurance test are shown. Figure 5 The results of Pd mapping of FE-EPMA of the exhaust gas purification catalyst of Example 5 after the endurance test are shown.

[0097] Figure 3 to Figure 5 This is the Pd mapping result of the corner of the quadrilateral cell of the substrate. In this Pd mapping image, the part where Pd exists is displayed brighter. At the corner of the quadrilateral cell of the substrate, the catalyst layer is formed in an L shape. Figure 3 A bright L-shaped region was observed in the . Therefore, it is known that Pd existed in the L-shaped catalyst layer before the durability test. Figure 4 In the figure, bright areas appeared on the left and bottom of the L-shaped brightly displayed area. This shows that in Comparative Example 1 without an intermediate layer, a large amount of Pd moved from the catalyst layer to the substrate side after the durability test. Figure 5 In the example, bright areas also appear on the left and bottom of the L-shaped brightly displayed area. However, the area of ​​the newly appeared bright area is smaller than Figure 4 This shows that the provision of the intermediate layer significantly suppresses the movement of Pd from the catalyst layer to the substrate side.

[0098] Based on the results shown in Table 1, the Pd mobility is plotted against the product of the thickness and specific surface area of ​​the intermediate layer. Figure 6 Among them, Figure 6 In the figure, the results of Comparative Example 1 and Examples 3 to 5 are not plotted.

[0099] Depend on Figure 6 It can be seen that when the product of the thickness of the intermediate layer and the specific surface area increases, the Pd mobility decreases. If the experimental error is taken into account, the movement of Pd can be greatly suppressed when the product of the thickness of the intermediate layer and the specific surface area is greater than 1100. Therefore, according to the above results, it can be seen that the exhaust gas purification catalyst disclosed herein can suppress the migration of platinum group metals from the catalyst layer to the substrate during high temperature durability.

[0100] according to Figure 6 As a result, it can be said that the product is preferably 1200 or more, more preferably 1500 or more, further preferably 2000 or more, and most preferably 2500 or more.

[0101] And, according to Figure 6 It can be seen that the greater the thickness of the intermediate layer, the smaller the Pd mobility. Figure 6 As a result, it can be said that the thickness of the intermediate layer is preferably 20 μm or more, more preferably 30 μm or more, and further preferably 50 μm or more.

[0102] And, by Figure 6 It can be seen that the larger the specific surface area of ​​the intermediate layer, the smaller the Pd mobility. Figure 6 The results show that the specific surface area of ​​the intermediate layer is preferably 40 m 2 / g or more, more preferably 50m 2 / g or more, more preferably 70m 2 / g or above.

[0103] Although the specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the scope of the claims includes various modifications and changes of the specific examples described above.

Claims

1. A catalyst for exhaust gas purification, characterized in that: It is an electrically heated catalyst for exhaust purification. The invention comprises a substrate, a catalyst layer, an intermediate layer and an electrode arranged between the substrate and the catalyst layer. The substrate contains SiC, The catalyst layer contains Pd as a catalyst component, The intermediate layer contains substantially no platinum group metals, The thickness μm of the intermediate layer and the specific surface area m of the intermediate layer 2 The product of 1 / g is greater than 1100.

2. The exhaust gas purification catalyst according to claim 1, characterized in that: The thickness of the intermediate layer is greater than 20 μm.

3. The exhaust gas purification catalyst according to claim 1, characterized in that The specific surface area of ​​the intermediate layer is 40 m 2 / g or above.

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