Exhaust gas purification catalyst

Abstract

To provide an exhaust gas purification catalyst improved in warm-up performance while suppressing HC poisoning of a noble metal in an atmosphere in which an air-fuel ratio (A / F) is rich and the HC poisoning easily occurs.SOLUTION: There is provided an exhaust gas purification catalyst which has a substrate and a catalyst coating layer coated on the substrate, wherein the catalyst coating layer has a lower coating layer coated on the substrate and an upper coating layer coated on the lower coating layer, the lower coating layer contains a noble metal, the upper coating layer contains Pd and / or Pt, a certain amount or more of the Pd and / or Pt contained in the upper coating layer is supported on Al2O3 and a thickness of the upper coating layer is adjusted.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

[0002] Exhaust gas emitted from internal combustion engines for automobiles and the like, such as gasoline engines or diesel engines, contains harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx).

[0003] For this reason, internal combustion engines are generally equipped with exhaust gas purification devices to decompose and remove these harmful components, and these harmful components are almost entirely neutralized by exhaust gas purification catalysts installed in these exhaust gas purification devices.

[0004] For example, Patent Document 1 discloses an exhaust gas purification catalyst comprising a substrate and a catalyst coating layer formed on the surface of the substrate, the catalyst coating layer comprising upper and lower layers, with the lower layer being closer to the substrate surface and the upper layer being relatively farther away, the upper layer of the catalyst coating layer containing Rh and Pd and a carrier, the upper layer of the catalyst coating layer comprising a Pd outermost layer on the surface extending 20 mm or more downstream from the upstream end, the Pd concentration of which is relatively higher than in other parts of the upper layer, the lower layer of the catalyst coating layer containing at least one precious metal selected from Pd and Pt and a carrier, and 60 mass % or more of the Pd contained in the Pd outermost layer being present in a layer extending from the surface of the Pd outermost layer relatively far from the substrate surface to 50% of the thickness of the upper layer. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-069402 Summary of the Invention [Problem to be solved by the invention]

[0006] The amount of precious metal contained in each catalyst coating layer is required to be reduced from the viewpoint of resource risk. To reduce the amount of precious metal, it is necessary to prevent the catalytic activity of the precious metal from decreasing due to use of the exhaust gas purification catalyst. One way to prevent the catalytic activity of the precious metal from decreasing is, for example, to suppress the poisoning of the precious metal by HC in the exhaust gas (HC poisoning), which is one of the factors that causes the catalytic activity of the precious metal to decrease.

[0007] On the other hand, catalysts for purifying exhaust gases are also required to have good warm-up properties, that is, to purify exhaust gases in the early stages of exhaust gas purification (for example, immediately after starting the engine).

[0008] The exhaust gas purification catalyst of Patent Document 1 has a structure having two catalytic coating layers containing precious metals as catalytic metals on a substrate, and attempts to improve HC purification performance and warm-up performance by increasing the Pd concentration in the upper layer. However, Patent Document 1 does not specify the carrier on which Pd is supported, and there is room for improvement in the utilization efficiency of Pd due to HC poisoning of Pd.

[0009] Therefore, an object of the present invention is to provide an exhaust gas purification catalyst that has improved warm-up properties while suppressing HC poisoning of precious metals in an atmosphere with a rich air-fuel ratio (A / F) in which HC poisoning is likely to occur. [Means for solving the problem]

[0010] Each catalyst coating layer usually plays a role in purifying a different harmful component. Therefore, when an exhaust gas purification catalyst has a substrate, a lower coating layer coated on the substrate, and an upper coating layer coated on the lower coating layer, it is desirable that the upper coating layer have a structure that allows the lower coating layer and exhaust gas containing harmful components to come into contact with each other quickly, i.e., a structure that is excellent in gas diffusion properties.

[0011] Therefore, the present inventors have investigated various means for solving the above-mentioned problems, and as a result, they have found that, in an exhaust gas purification catalyst having a substrate and a catalyst coating layer coated on the substrate, a lower coating layer coated on the substrate as a catalyst coating layer and an upper coating layer coated on the lower coating layer are arranged, a precious metal is introduced into the lower coating layer, palladium (Pd) and / or platinum (Pt) supported on alumina (Al2O3) is introduced into the upper coating layer, and the thickness of the upper coating layer is further adjusted, thereby making it possible to first efficiently purify HC in the exhaust gas by Pd and / or Pt supported on Al2O3, which has high HC poisoning resistance, in the upper coating layer with which the exhaust gas first comes into contact, and then the exhaust gas from which HC has been purified can be quickly and efficiently purified in the lower coating layer, which is located beyond the upper coating layer, whose gas diffusion property has been improved by adjusting the thickness, and thereby completing the present invention.

[0012] That is, the gist of the present invention is as follows. (1) A catalyst for purifying exhaust gases, comprising a substrate and a catalyst coating layer coated on the substrate, the catalyst coating layer has a lower coating layer coated on the substrate and an upper coating layer coated on the lower coating layer; the lower coating layer contains a precious metal; the upper coating layer contains Pd and / or Pt, 80 wt % or more of the total weight of Pd and / or Pt contained in the upper coating layer is supported on Al2O3, The ratio of the thickness of the upper coating layer to the thickness of the lower coating layer (upper coating layer / lower coating layer) is 0.2 or less; Catalyst for purifying exhaust gas. (2) The exhaust gas purifying catalyst according to (1), wherein 90% by weight or more of the total weight of Pd and / or Pt contained in the upper coating layer is supported on Al2O3. (3) The exhaust gas purifying catalyst according to (1) or (2), wherein 95% by weight or more of the total weight of Pd and / or Pt contained in the upper coating layer is supported on Al2O3. (4) The exhaust gas purifying catalyst according to any one of (1) to (3), wherein the thickness of the upper coating layer is 20 μm or less. [Effects of the Invention]

[0013] The present invention provides an exhaust gas purification catalyst that suppresses HC poisoning of precious metals and has improved warm-up properties in an atmosphere with a rich air-fuel ratio (A / F) where HC poisoning is likely to occur. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a photograph showing the arrangement of Pd, the arrangement of Rh, and the appearance of the catalyst coating layer of the exhaust gas purifying catalysts of Comparative Examples 1 and 2 and Example 1, taken by EPMA. [Figure 2] 1 is a diagram schematically illustrating embodiments of the catalyst coating layer of Comparative Examples 1 and 2. FIG. [Figure 3] 1 is a diagram schematically illustrating an embodiment of a catalyst coating layer according to Example 1. FIG. [Figure 4] FIG. 2 is a diagram schematically illustrating an evaluation layout for engine bench evaluation. [Figure 5] 1 is a graph showing the results of HC emission behavior in a rich steady state for exhaust gas purification catalysts of Comparative Examples 1(a) and 2(b) and Example 1(c). [Figure 6] 1 shows the results of NOx emission behaviors during rich steady state for exhaust gas purification catalysts of Comparative Examples 1(a) and 2(b) and Example 1(c). [Figure 7] 1 is a graph showing the relationship between the thickness of the upper coating layer of the exhaust gas purifying catalysts of Examples 1 to 4 and Comparative Examples 3 and 4 and the amount of HC emissions 3 minutes after switching to a rich atmosphere. [Figure 8] 1 is a graph showing the relationship between the thickness of the upper coating layer of the exhaust gas purifying catalysts of Examples 1 to 4 and Comparative Examples 3 and 4 and the NOx emissions 3 minutes after switching to a rich atmosphere. DETAILED DESCRIPTION OF THE INVENTION

[0015] Preferred embodiments of the present invention will now be described in detail. In this specification, the features of the present invention will be described with reference to the drawings as appropriate. In the drawings, the dimensions and shapes of each part are exaggerated for clarity, and the actual dimensions and shapes are not accurately depicted. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. Note that the exhaust gas purification catalyst of the present invention is not limited to the following embodiments, and can be embodied in various forms including modifications, improvements, etc. that can be made by those skilled in the art, without departing from the gist of the present invention.

[0016] The present invention relates to a catalyst for purifying exhaust gases, which has a substrate and a catalyst coating layer coated on the substrate, wherein the catalyst coating layer has a lower coating layer coated on the substrate and an upper coating layer coated on the lower coating layer, the lower coating layer contains a precious metal, the upper coating layer contains Pd and / or Pt, a certain amount or more of the Pd and / or Pt contained in the upper coating layer is supported on Al2O3, and the thickness of the upper coating layer is adjusted.

[0017] (base material) As the substrate, a substrate having a known honeycomb shape can be used, and specifically, a honeycomb-shaped monolith substrate (honeycomb filter, high-density honeycomb, etc.) is preferably used. The material of such a substrate is not particularly limited, and a substrate made of ceramics such as cordierite, silicon carbide, silica, alumina, mullite, etc., or a substrate made of metal such as stainless steel containing chromium and aluminum is preferably used. Among these, cordierite is preferred from the viewpoint of cost.

[0018] (catalyst coating layer) The catalyst coating layer has at least a lower coating layer coated on the substrate and an upper coating layer coated on the lower coating layer.

[0019] The undercoat layer is coated on the substrate.

[0020] The lower coating layer may be composed of one layer or multiple layers, i.e., two, three, or four or more layers. When the lower coating layer is composed of multiple layers, the composition and structure of each layer are not particularly limited. Furthermore, the lower coating layer does not necessarily have to be uniform over the entire substrate of the exhaust gas purification catalyst, and may have different compositions and structures in the upstream and downstream regions with respect to the exhaust gas flow direction.

[0021] The thickness of the lower coating layer (when the lower coating layer is composed of multiple layers, the thickness of the entire lower coating layer) is not limited as long as it satisfies the thickness ratio of the upper coating layer to the lower coating layer (upper coating layer / lower coating layer) described below, but is usually 20 μm to 100 μm, preferably 30 μm to 80 μm. In this specification, thickness refers to the average thickness, for example, the average value of thicknesses at 10 randomly selected points in an SEM or TEM image of the coating layer.

[0022] By setting the thickness of the lower coating layer within the above range, it is possible to suppress aggregation of catalytic metals due to an excessively thin thickness, for example, aggregation of catalytic metals due to an increased density of the catalytic metals, while increasing the frequency of contact between the catalytic metals contained in the lower coating layer and harmful components in the exhaust gas, such as NOx, thereby improving exhaust gas purification performance, and furthermore, it is possible to maintain a good balance between pressure loss, catalytic performance, and durability in the exhaust gas purification catalyst.

[0023] The upper coating layer is coated on the lower coating layer.

[0024] The thickness of the upper coating layer is not limited as long as the thickness ratio of the upper coating layer to the lower coating layer (upper coating layer / lower coating layer) described below is satisfied, but is usually 20 μm or less, preferably 1 μm to 20 μm, and more preferably 1 μm to 10 μm.

[0025] By setting the thickness of the upper coating layer within the above range, it is possible to suppress a decrease in the gas diffusion properties of exhaust gases due to an excessively thick layer, and to increase the frequency of contact between the exhaust gases and the lower coating layer that is located beyond the upper coating layer, thereby enabling harmful components in the exhaust gases that could not be completely purified by the upper coating layer, such as NOx, to be quickly and efficiently purified by the lower coating layer.

[0026] By having the catalyst coating layer comprise a lower coating layer and an upper coating layer, it is possible to efficiently purify HC in the upper coating layer and purify NOx in the lower coating layer.

[0027] The ratio of the thickness of the upper coating layer to the thickness of the lower coating layer (upper coating layer / lower coating layer) is 0.2 or less, and preferably 0.01 to 0.16.

[0028] When the thickness ratio of the upper coat layer to the lower coat layer (upper coat layer / lower coat layer) is within the above range, HC purification and NOx purification can be efficiently performed in a rich atmosphere.

[0029] (lower coat layer) The undercoat layer contains a noble metal as a catalytic metal.

[0030] Examples of the noble metal include catalytic metals commonly used in the technical field of exhaust gas purification catalysts, and are not limited as long as they can purify harmful components such as reduced HC contained in the exhaust gas that has passed through the upper coating layer, but examples include at least one selected from the group consisting of rhodium (Rh), Pt, Pd, gold (Au), silver (Ag), iridium (Ir), osmium (Os), and ruthenium (Ru). As the noble metal, Rh, which can efficiently purify NOx, is preferred.

[0031] By including a precious metal in the lower coating layer, in an atmosphere in which HC has been sufficiently purified by the upper coating layer, the precious metals, particularly Rh and the like which are easily poisoned by HC, can fully demonstrate NOx purification performance without being poisoned by HC.

[0032] The content of the precious metal contained in the lower coating layer is not limited, but is usually 0.05 g to 1.0 g, preferably 0.2 g to 0.8 g, in terms of precious metal per 1 L of substrate volume. The content of the precious metal that can be contained in the downstream coating layer depends on the amount of precious metal precursor added (excluding volatilized components) as a material during production of the exhaust gas purification catalyst.

[0033] By containing the above-mentioned amount of precious metal in the lower coating layer, harmful components other than HC that could not be purified by the upper coating layer, particularly NOx, can be efficiently and sufficiently purified in an atmosphere in which HC has been sufficiently purified by the upper coating layer.

[0034] The noble metal contained in the lower coating layer functions as a catalytic metal of the exhaust gas purifying catalyst as it is, but may also be supported on carrier particles.

[0035] Therefore, the lower coating layer may further contain carrier particles. Examples of carrier particles include any metal oxide commonly used in the technical field of exhaust gas purification catalysts, such as silica (SiO), magnesium oxide (MgO), zirconia (ZrO), ceria (CeO), alumina (AlO), titania (TiO), yttria (YO), neodymium oxide (NdO), lanthanum oxide (LaO), and composite oxides and solid solutions thereof, such as AlO-CeO-ZrO composite oxide (ACZ), CeO-ZrO composite oxide (CZ), AlO-ZrO composite oxide (AZ), and combinations of two or more thereof. The ratio of each oxide in the composite oxide, such as ACZ, CZ, and AZ, is not limited and may be any ratio commonly used in the technical field of exhaust gas purification catalysts.

[0036] Acidic supports, such as SiO2, are compatible with catalytic metals that reduce NOx. Basic supports, such as MgO, are compatible with potassium (K) and barium (Ba), which store NOx. ZrO2 suppresses sintering of other support particles at high temperatures that would otherwise cause sintering. When combined with, for example, Rh as a catalytic metal, ZrO2 induces a steam reforming reaction to generate H2, thereby efficiently reducing NOx. Acid-base amphoteric supports, such as Al2O3, have a high specific surface area and can be used to efficiently store and reduce NOx. TiO2 can effectively suppress sulfur poisoning of catalytic metals. ACZ and CZ are materials with oxygen storage capacity (OSC) that can maintain a constant oxygen concentration and maintain the purification performance of exhaust gas purification catalysts even when the air-fuel ratio fluctuates. Furthermore, the addition of Al2O3, ZrO2, and other metal oxides can increase the durability of the support.

[0037] It should be understood that, according to the above-described characteristics of the support particles, the exhaust gas purification performance, particularly the NOx purification performance, of the exhaust gas purification catalyst of the present invention may be improved depending on the selected type, composition, combination, ratio, and / or amount of the support particles.

[0038] When the precious metal is supported on the support particles, the large specific surface area of ​​the support particles allows the contact area between the exhaust gas and the precious metal to be increased, thereby improving the performance of the exhaust gas purification catalyst.

[0039] The noble metal can be supported on the carrier particles by any method generally used in the technical field of exhaust gas purification catalysts.

[0040] The content of the carrier particles in the lower coating layer is not limited, but is usually 25 g to 170 g, and preferably 100 g to 140 g, per 1 L of the substrate volume. The content of the carrier particles that can be contained in the lower coating layer depends on the amount of the carrier particles added as a material when producing the exhaust gas purifying catalyst.

[0041] The undercoat layer is composed of a precious metal and, optionally, carrier particles supporting the precious metal, but may further contain other components within the scope of the present invention. Examples of other components include other metal oxides and additives used in catalyst coatings for this type of application, specifically alkali metals such as potassium (K), sodium (Na), lithium (Li), and cesium (Cs), alkaline earth metals such as barium (Ba) and strontium (Sr), rare earth elements such as lanthanum (La), yttrium (Y), and cerium (Ce), transition metals such as iron (Fe), and one or more of the metal oxides listed above as carrier particles (i.e., metal oxides not supporting a precious metal). The other components may be in their original form or, like the precious metal, supported on carrier particles.

[0042] The content of the other components in the lower coating layer is not limited, but is usually 20 g to 120 g, and preferably 80 g to 120 g, per 1 L of the substrate volume. The content of the other components that can be contained in the downstream coating layer depends on the amount of the other components (excluding volatilizable components) added as materials during production of the exhaust gas purifying catalyst.

[0043] The precious metal contained in the lower coating layer may be present in a homogeneously dispersed state in the lower coating (for example, the precious metal may be uniformly supported on all of the materials constituting the lower coating layer), or may be locally unevenly distributed (for example, the precious metal may be supported only on specific carrier particles among the materials constituting the lower coating layer).

[0044] The coating amount of the lower coating layer is not limited, but is usually 45 g to 250 g, and preferably 160 g to 250 g, per 1 L of the substrate volume. The coating amount of the lower coating layer depends on the total weight of materials (excluding volatilized components) used in producing the exhaust gas purifying catalyst.

[0045] When the amount (coating amount) of each material in the lower coating layer falls within the above range, a good balance can be maintained between pressure loss, catalytic performance, and durability in the exhaust gas purifying catalyst.

[0046] (Upper coat layer) The upper coating layer contains Pd and / or Pt as a catalytic metal.

[0047] By including Pd and / or Pt in the upper coating layer, exhaust gas, particularly HC, immediately after being discharged from an engine or the like comes into contact with Pd and / or Pt, thereby improving the HC purification performance in the exhaust gas.

[0048] The content (total weight) of Pd and / or Pt contained in the upper coating layer is not limited, but is usually 0.2 g to 5.0 g, preferably 2.0 g to 5.0 g, calculated as the metal of the catalytic metal per 1 L of the substrate volume. The content of Pd and / or Pt contained in the upper coating layer depends on the amount of Pd and / or Pt precursor added (excluding volatilized components) as a material during production of the exhaust gas purification catalyst.

[0049] When the upper coating layer contains Pd and / or Pt in the above amount, the ignition performance is improved due to the increased density of Pd and / or Pt, and the performance of purifying exhaust gas, especially HC, can be improved.

[0050] The upper coating layer further contains Al2O3 for supporting the Pd and / or Pt contained in the upper coating layer.

[0051] The type and size of Al2O3 particles carrying Pd and / or Pt contained in the upper coating layer are not limited, but it is desirable that the average particle size be 5 μm or less.

[0052] By including Al2O3 in the upper coating layer, it is possible to improve the HC poisoning resistance of Pd and / or Pt supported on Al2O3.

[0053] The content of Al2O3 supporting Pd and / or Pt contained in the upper coating layer is not limited, but is usually 5 g to 40 g, and preferably 10 g to 30 g, per 1 L of the substrate volume. The content of Al2O3 supporting Pd and / or Pt contained in the upper coating layer depends on the amount of Al2O3 added to support Pd and / or Pt as a material during production of the exhaust gas purifying catalyst.

[0054] When the upper coating layer contains Al2O3 in the above amount, the HC purification performance can be improved by improving the ignition ability due to the high density of Pd and / or Pt, while the HC poisoning resistance of Pd and / or Pt supported on Al2O3 can be sufficiently improved.

[0055] At least 80 wt % of the total weight of Pd and / or Pt contained in the upper coating layer is supported on Al2O3. In one embodiment, at least 90 wt % of the total weight of Pd and / or Pt contained in the upper coating layer is supported on Al2O3. In one embodiment, at least 95 wt % of the total weight of Pd and / or Pt contained in the upper coating layer is supported on Al2O3. In one embodiment, at least 99 wt % of the total weight of Pd and / or Pt contained in the upper coating layer is supported on Al2O3. In one embodiment, approximately 100 wt % of the total weight of Pd and / or Pt contained in the upper coating layer is supported on Al2O3.

[0056] The method for supporting Pd and / or Pt on Al2O3 can be a method generally used in the technical field of exhaust gas purification catalysts, such as a method in which Pd and / or Pt, Al2O3, and a solvent, such as water, are mixed with stirring in a vessel, and the resulting mixture is optionally filtered, dried, and calcined.

[0057] By supporting most of the Pd and / or Pt contained in the upper coating layer on Al2O3, Pd and / or Pt can efficiently purify HC while suppressing HC poisoning by Al2O3, which has excellent resistance to HC poisoning.

[0058] In addition to Pd and / or Pt and Al2O3 supporting Pd and / or Pt, the upper coating layer may further contain other components to the extent that the effects of the present invention are not impaired. These other components include precious metals other than Pd and / or Pt commonly used in the technical field of exhaust gas purification catalysts, specifically at least one selected from the group consisting of Rh, Au, Ag, Ir, Os, and Ru (preferably, the precious metal is also supported on Al2O3, which has excellent HC poisoning resistance), metal oxides such as SiO2, MgO, ZrO2, CeO2, TiO2, YO3, Nd2O3, La2O3, Al2O3 not supporting Pd and / or Pt, and composite oxides and solid solutions thereof such as ACZ, CZ, and AZ, as well as combinations of two or more thereof, alkali metals, alkaline earth metals, rare earth elements, and transition metals. The other components may be in their original form or supported on carrier particles.

[0059] The content of the other components in the upper coating layer is not limited, but is usually 5 g to 20 g, and preferably 10 g to 20 g, per 1 L of the substrate volume. The content of the other components that can be contained in the upper coating layer depends on the amount of the other components (excluding volatilizable components) added as materials during production of the exhaust gas purifying catalyst.

[0060] The coating amount of the upper coating layer is not limited, but is usually 5 g to 60 g, preferably 10 g to 60 g, and more preferably 20 g to 60 g per 1 L of the substrate volume. The coating amount of the upper coating layer depends on the total weight of the materials (excluding the components to be volatilized) when producing the exhaust gas purifying catalyst.

[0061] By ensuring that the amount (coating amount) of each material in the upper coating layer falls within the above range, the gas diffusion in the upper coating layer is improved, and further, the warm-up performance is improved by increasing the frequency of contact of exhaust gas with the lower coating layer, and the purification performance of exhaust gas, particularly HC, is improved by improving ignition performance due to the high density of Pd and / or Pt, and the purification performance of harmful components other than HC, such as NOx, due to the precious metals contained in the lower coating layer can be improved.

[0062] The upper coating layer is preferably composed of Pd and / or Pt and Al2O3 carrying Pd and / or Pt.

[0063] Since the upper coating layer is composed of Pd and / or Pt and Al2O3 carrying Pd and / or Pt, HC in the exhaust gas can be efficiently purified while avoiding HC poisoning by the Pd and / or Pt, and further, the exhaust gas from which HC has been purified can be quickly guided to the lower coating layer located below the upper coating layer, and can be efficiently purified in the lower coating layer.

[0064] (Method of manufacturing exhaust gas purification catalyst) The exhaust gas purifying catalyst of the present invention can be produced by using known coating techniques, except for using the above-described constituent components of the exhaust gas purifying catalyst.

[0065] The exhaust gas purification catalyst of the present invention can be produced, for example, as follows. First, a catalyst coating layer slurry for the underlayer coating layer containing materials constituting the underlayer coating layer, such as a catalyst metal precursor, for example, a salt containing Rh, such as a nitrate salt, a solvent (for example, water, alcohol, a mixture of water and alcohol, etc.), and optionally support particles, such as an alumina-ceria-zirconia composite oxide and / or a ceria-zirconia composite oxide, and additives, such as a thickener, is applied by suction to an area on the substrate where the underlayer coating layer is to be formed. After blowing off excess slurry with a blower or the like, the mixture is dried, for example, in the air, typically at 100°C to 150°C, for typically 1 hour to 3 hours to remove the solvent, and then calcined in the air, typically at 450°C to 550°C, for typically 1 hour to 3 hours, to form the underlayer coating layer. Next, on the substrate on which the undercoat layer has been formed, the materials constituting the upper coat layer, i.e., a catalyst coating layer slurry for the upper coat layer containing a precursor of Pd and / or Pt, for example, a salt containing Pd and / or Pt, such as a nitrate salt, Al2O3, a solvent (for example, water, alcohol, a mixture of water and alcohol, etc.), and optionally an additive, for example, a thickener, etc., is coated by suction. After blowing off excess slurry with a blower or the like, the solvent is removed by drying, for example, in the air, typically at 100°C to 150°C, for typically 1 hour to 3 hours, and then calcining in the air, typically at 450°C to 550°C, for typically 1 hour to 3 hours, to form the upper coat layer.

[0066] (Use of catalyst for purifying exhaust gas) The exhaust gas purification catalyst of the present invention can exhibit a significant effect in exhaust gas purification performance in a rich atmosphere, and can be used even in an environment in which excess HC and the like are adsorbed to the exhaust gas purification catalyst in a rich atmosphere and can poison the exhaust gas purification catalyst, and can be used as an exhaust gas purification catalyst that exhibits a high HC poisoning suppression effect and is excellent in warm-up properties. [Example]

[0067] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to those shown in these examples.

[0068] 1. Preparation of catalyst for exhaust gas purification Comparative Example 1 (1) First, Al2O3-CeO2-ZrO2 composite oxide (Al2O3: 30 wt%, CeO2: 20 wt%, ZrO2: 50 wt%), Al2O3-ZrO2 composite oxide (Al2O3: 30 wt%, ZrO2: 70 wt%), Al2O3, Al2O3 binder, thickener (hydroxyethyl cellulose), organic fiber, and rhodium nitrate were added to water with stirring, and the mixture was further stirred and mixed to prepare a catalyst coating layer slurry for the undercoat layer.

[0069] (2) Next, the catalyst coating layer slurry for the undercoat layer was poured into a substrate (an NGK honeycomb substrate with an 875 cc (600 cells, hexagonal, and 2.5 mil wall thickness) size) by suction, coating the substrate wall with the material to prepare a precursor layer for the undercoat layer. For each coating material, the amounts of Al2O3-CeO2-ZrO2 composite oxide, Al2O3 ...

[0070] (3) The substrate coated with the precursor layer of the undercoat layer was dried in a dryer maintained at 120°C for 2 hours to remove moisture, and then fired in an electric furnace maintained at 500°C for 2 hours to prepare the undercoat layer.

[0071] (4) Next, an aqueous solution of Pd nitrate was absorbed onto the outermost surface of the lower coating layer of the substrate provided with the lower coating layer. The amount of Pd was adjusted to 0.24 g (0.28 g / L) in terms of metal per 1 L of substrate volume. The substrate with Pd supported on the lower coating layer was then dried in a dryer maintained at 120°C for 2 hours to remove moisture, and then calcined in an electric furnace maintained at 500°C for 2 hours to prepare a catalyst for purifying exhaust gases.

[0072] Comparative Example 2 An exhaust gas purifying catalyst was prepared in the same manner as in Comparative Example 1, except that the step (4) in Comparative Example 1 was changed to the following steps (4') and (5').

[0073] (4') Next, a Pd nitrate aqueous solution was absorbed into the outermost surface of the lower coating layer of the substrate provided with the lower coating layer. The Pd content was adjusted to 0.06 g (0.07 g / L) in terms of metal per 1 L of substrate volume. The substrate with Pd supported on the lower coating layer was then dried in a dryer maintained at 120°C for 2 hours to remove moisture, and then fired in an electric furnace maintained at 500°C for 2 hours.

[0074] (5') (4') Steps were repeated four times to prepare a catalyst for purifying exhaust gas. The final catalyst for purifying exhaust gas contained 0.24 g of Pd metal per 1 L of substrate volume (0.28 g / L).

[0075] Example 1 An exhaust gas purifying catalyst was prepared in the same manner as in Comparative Example 1, except that the step (4) in Comparative Example 1 was changed to the following steps (4″) to (6″).

[0076] (4'') Next, Al2O3, a thickener (hydroxyethyl cellulose), and Pd nitrate were added to the water while stirring, and further stirred and mixed to prepare a catalyst coating layer slurry for the upper coating layer.

[0077] (5'') Next, the catalyst coating layer slurry for the upper coating layer was poured onto the lower coating layer of the substrate with the lower coating layer by suction in the same manner as the catalyst coating layer slurry for the lower coating layer, coating the material onto the lower coating layer of the substrate to prepare a precursor layer for the upper coating layer. At this time, for each coating material, 5.25 g (6.0 g / L) of Al2O3 and 0.24 g (0.28 g / L) of Pd in ​​metal equivalent per 1 L of substrate volume were used. In addition, the precursor layer for the upper coating layer was applied from the upstream end to the downstream end in the exhaust gas flow direction of the exhaust gas purification catalyst, i.e., along the entire length of the substrate in the exhaust gas purification catalyst.

[0078] (6'') The substrate coated with the precursor layer of the upper coating layer was dried in a dryer maintained at 120°C for 2 hours to remove moisture, and then baked in an electric furnace maintained at 500°C for 2 hours to prepare the upper coating layer and the catalyst for purifying exhaust gases.

[0079] 2. Electron probe microanalyzer (EPMA) analysis The surfaces of the exhaust gas purifying catalysts of Comparative Examples 1 and 2 and Example 1 were analyzed by EPMA, and the results are shown in FIG.

[0080] 1, it was found that in Comparative Examples 1 and 2 and Example 1, the thickness of the catalyst coating layer (lower coating layer) was approximately 100 μm. Furthermore, it was found that in Comparative Example 1, Rh was uniformly present throughout the catalyst coating layer, while Pd was present only near the surface of the catalyst coating layer. Furthermore, it was found that in Comparative Example 2, Rh was uniformly present throughout the catalyst coating layer, while Pd was present only near the surface of the catalyst coating layer, particularly only nearer the surface than in Comparative Example 1. Additionally, it was found that in Example 1, Rh was uniformly present throughout the catalyst coating layer, while Pd was present only near the outermost surface of the catalyst coating layer, particularly only in the range from the outermost surface to approximately 5 μm (i.e., in the exhaust gas purification catalyst of Example 1, the thickness of the upper coating layer was approximately 5 μm). Note that in the catalyst coating layers of Comparative Examples 1 and 2, the lower coating layer and the upper coating layer were not clearly distinguished from each other. This is due to the method of forming the catalyst coating layers of Comparative Examples 1 and 2.

[0081] FIG. 2 shows a schematic representation of the catalyst coating layer 2 of Comparative Examples 1 and 2, and FIG. 3 shows a schematic representation of the catalyst coating layer 2 of Example 1. As shown in FIG.

[0082] In Fig. 2, the catalyst coating layer 2 of Comparative Examples 1 and 2 is formed on a substrate 1. Furthermore, Rh in the catalyst coating layer 2 of Comparative Examples 1 and 2 is uniformly present throughout the catalyst coating layer 2. On the other hand, Pd in ​​the catalyst coating layer 2 of Comparative Examples 1 and 2 is present only near the surface of the catalyst coating layer 2. Note that in the catalyst coating layer 2 of Comparative Examples 1 and 2, Pd is not supported only on specific carrier particles, such as alumina.

[0083] In Fig. 3, the catalyst coating layer 2 of Example 1 is formed on a substrate 1. The catalyst coating layer 2 of Example 1 is composed of a lower coating layer 3 formed on the substrate 1 and an upper coating layer 4 formed on the lower coating layer 3. Rh in the catalyst coating layer 2 of Example 1 is uniformly present throughout the lower coating layer 3. On the other hand, Pd in ​​the catalyst coating layer 2 of Example 1 is supported on Al2O3 and is present only in the upper coating layer 4.

[0084] 3. Durability test The exhaust gas purifying catalysts of Comparative Examples 1 and 2 and Example 1 were subjected to the following durability test using an actual engine.

[0085] Each exhaust gas purification catalyst was installed in the exhaust system of a V8 engine, and exhaust gases in stoichiometric and lean atmospheres were repeatedly passed through for a fixed period of time (3:1 ratio) for 50 hours at a catalyst bed temperature of 950°C.

[0086] 4. Engine bench evaluation 3. Durability Tests We performed an evaluation of the emission behavior of HC and NOx in a rich atmosphere using an actual engine for the exhaust gas purification catalysts of Comparative Examples 1 and 2 and Example 1. Figure 4 shows a schematic layout of the engine bench evaluation.

[0087] (1) First, the L-type four-cylinder engine 5 was started, and the engine conditions were adjusted so that the inlet gas temperature was 550°C. (2) Each exhaust gas purification catalyst 6 was installed in the exhaust system of the engine 5, and once stabilized, the air-fuel ratio (A / F) was alternately varied at 3-minute intervals between an A / F of 14.1 (rich condition) and an A / F of 15.1 (lean condition). (3) After the third A / F change from 15.1 (lean) to 14.1 (rich), the HC and NOx emission behaviors were evaluated for three minutes under the A / F condition of 14.1.

[0088] 5. Engine bench evaluation results FIG. 5 shows the results of HC emission behavior in a rich steady state for the exhaust gas purification catalysts of Comparative Examples 1(a) and 2(b) and Example 1(c), and FIG. 6 shows the results of NOx emission behavior in a rich steady state for the exhaust gas purification catalysts of Comparative Examples 1(a) and 2(b) and Example 1(c).

[0089] 5, it was found that in Example 1, the amount of HC emissions was significantly reduced compared to Comparative Examples 1 and 2. In Example 1, Pd, which is disposed in the upper coating layer close to the flow of exhaust gas, i.e., which first comes into contact with exhaust gas, is supported on Al2O3, and this is thought to be because Al2O3, which has excellent resistance to HC poisoning, suppresses HC poisoning of Pd, improving the utilization efficiency of Pd. It is thought that this effect can also be obtained when Pt, which is a catalytic metal equivalent to Pd, is used.

[0090] 6, it was found that NOx emissions were also suppressed in Example 1 compared to Comparative Examples 1 and 2. This is thought to be because, in Example 1, the HC purification ability of the upper coating layer was improved, which relatively suppressed HC poisoning of Rh in the lower coating layer, and improved the utilization efficiency of Rh. It is thought that this effect can be obtained even when Rh is replaced with another precious metal.

[0091] 6. Evaluation of the ratio of the thickness of the upper coating layer to the lower coating layer (upper coating layer / lower coating layer) in exhaust gas purification catalysts 6-1. Preparation of catalyst for exhaust gas purification Example 2 An exhaust gas purification catalyst was prepared in the same manner as in Example 1 (5''), except that the amount of Al2O3 supporting Pd was changed to 1.05 g (1.20 g / L) to make the thickness of the upper coating layer 1 μm.

[0092] Example 3 An exhaust gas purification catalyst was prepared in the same manner as in Example 1 (5''), except that the amount of Al2O3 supporting Pd was changed to 10.5 g (12.0 g / L) to make the thickness of the upper coating layer 9.6 μm.

[0093] Example 4 An exhaust gas purification catalyst was prepared in the same manner as in Example 1 (5''), except that the amount of Al2O3 supporting Pd was changed to 15.75 g (18.0 g / L) to make the thickness of the upper coating layer 15.8 μm.

[0094] Comparative Example 3 An exhaust gas purification catalyst was prepared in the same manner as in Example 1 (5''), except that the amount of Al2O3 supporting Pd was changed to 26.25 g (30.0 g / L) to make the thickness of the upper coating layer 23.4 μm.

[0095] Comparative Example 4 An exhaust gas purification catalyst was prepared in the same manner as in Example 1 (5''), except that the amount of Al2O3 supporting Pd was changed to 31.5 g (36.0 g / L) to make the thickness of the upper coating layer 29.9 μm.

[0096] Evaluation The exhaust gas purification catalysts of Examples 1 to 4 and Comparative Examples 3 and 4 were subjected to the above-mentioned 3. Durability test and 4. Engine bench evaluation, and the HC emissions and NOx emissions were measured 3 minutes after switching to a rich atmosphere in 4. Engine bench evaluation (3).

[0097] FIG. 7 shows the relationship between the thickness of the upper coating layer of the exhaust gas purification catalysts of Examples 1 to 4 and Comparative Examples 3 and 4 and the amount of HC emissions 3 minutes after switching to a rich atmosphere, and FIG. 8 shows the relationship between the thickness of the upper coating layer of the exhaust gas purification catalysts of Examples 1 to 4 and Comparative Examples 3 and 4 and the amount of NOx emissions 3 minutes after switching to a rich atmosphere.

[0098] 7 and 8, it was found that when the thickness of the lower coating layer of the exhaust gas purification catalyst is 100 μm, and the thickness of the upper coating layer is 20 μm or less, preferably 1 μm to 16 μm, both the HC emissions and NOx emissions 3 minutes after switching to a rich atmosphere are good. Therefore, it was found that in the exhaust gas purification catalyst of the present invention, when the thickness ratio of the upper coating layer to the lower coating layer (upper coating layer / lower coating layer) is 0.2 or less, preferably 0.01 to 0.16, HC purification and NOx purification in a rich atmosphere can be efficiently performed. [Explanation of symbols]

[0099] 1: Substrate, 2: Catalyst coating layer, 3: Lower coating layer, 4: Upper coating layer, 5: Engine, 6: S / C (exhaust gas purification catalyst installed directly below the engine)

Claims

1. A catalyst for purifying exhaust gases, comprising a substrate and a catalyst coating layer coated on the substrate, the catalyst coating layer has a lower coating layer coated on the substrate and an upper coating layer coated on the lower coating layer; the lower coating layer contains a precious metal; the upper coating layer is made of Pd and / or Pt and Al 2 O 3 ; 80% by weight or more of the total weight of Pd and / or Pt contained in the upper coating layer is Al ２ O ３ It is supported on the content of Al 2 O 3 carrying Pd and / or Pt contained in the upper coating layer is 1.2 g to 18 g per 1 L of the volume of the substrate; the ratio of the thickness of the upper coating layer to the thickness of the lower coating layer (upper coating layer / lower coating layer) is 0.2 or less; Catalyst for purifying exhaust gas.

2. 90% by weight or more of the total weight of Pd and / or Pt contained in the upper coating layer is Al ２ O ３ The exhaust gas purifying catalyst according to claim 1, wherein the catalyst is supported on a carbon material.

3. 95% by weight or more of the total weight of Pd and / or Pt contained in the upper coating layer is Al ２ O ３ 3. The exhaust gas purifying catalyst according to claim 1, wherein the catalyst is supported on a carbon material.

4. 4. The exhaust gas purifying catalyst according to claim 1, wherein the upper coating layer has a thickness of 20 μm or less.

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