Exhaust gas purification catalyst

Abstract

The present disclosure provides an exhaust gas purification catalyst with improved exhaust gas purification performance. The exhaust gas purification catalyst has a substrate and a coating layer laminated on the substrate, wherein the coating layer has a first catalyst metal and a metal oxide support, the pore volume of the coating layer being greater than 0.44 mL / g, and the first catalyst metal being at least one selected from the group consisting of platinum and palladium and present in a diffused manner in the depthwise direction from the outer surface of the coating layer, while the ratio (Rb / Rs), as the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction, with respect to the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction, is 0.20 to 1.50.

Description

EXHAUST GAS PURIFICATION CATALYST

[0001] The present disclosure relates to an exhaust gas purification catalyst.

[0002] Exhaust gas emitted from internal combustion engines must adhere to various regulations enacted from the viewpoint of the environment. Exhaust gas is therefore emitted into the environment after having been purified by an exhaust gas purifying catalyst.

[0003] Removal performance for primarily hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) that are present in the components of exhaust gas has been an issue for exhaust gas purifying catalysts. Notably, hydrocarbons (HC) and carbon monoxide (CO) are purified by being converted to water (H2O) and carbon dioxide (CO2) by oxidation reaction, while nitrogen oxides (NOx) are purified by conversion to nitrogen (N2) by reduction reaction.

[0004] PTL 1 discloses an exhaust gas purification catalyst for a 2-stroke general-purpose engine having a substrate and a coat layer on the substrate, wherein the coat layer includes a noble metal and a metal oxide, the noble metal includes palladium and one or more metals selected from platinum and rhodium, the zirconia content of the coat layer is 0.07 mol or lower per 100 g of metal oxides in the coat layer, and the total content of one or more metal oxides selected from among lanthana and alkaline earth metal oxides in the coat layer is 0.05 mol or greater per 100 g of metal oxides in the coat layer.

[0005] PTL 2 discloses an exhaust gas purification catalyst with a catalyst layer containing at least rhodium and palladium, wherein the rhodium is supported at least on the wall surfaces of the carrier, the palladium is supported at least inside the walls of the carrier, and the loading density of the palladium is a higher density at the upstream end than at the downstream end.

[0006] International Patent Publication No. WO2017 / 200013International Patent Publication No. WO2020 / 071065 SUMMARYTechnical Problem

[0007] Provided herein is an exhaust gas purification catalyst with increased exhaust gas purification performance. Solution to Problem

[0008] The present inventors have found that the aforementioned object can be achieved by the following means: <Aspect 1> An exhaust gas purification catalyst having a substrate and a coating layer laminated on the substrate, wherein: the coating layer has a first catalyst metal and a metal oxide support, the pore volume of the coating layer being greater than 0.44 mL / g, and the first catalyst metal is at least one selected from the group consisting of platinum and palladium and is present in a diffused manner in the depthwise direction from the outer surface of the coating layer, and the ratio (Rb / Rs), as the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction, with respect to the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction, is 0.20 to 1.50.

[0009] <Aspect 2> The exhaust gas purification catalyst according to aspect 1, wherein the coating layer further has a second catalyst metal supported, the second catalyst metal being rhodium and being present in a diffuse manner in a range of within 20 μm from the outer surface of the coating layer in the depthwise direction.

[0010] <Aspect 3> The exhaust gas purification catalyst according to aspect 1 or 2, wherein the metal oxide support comprises alumina and ceria.

[0011] <Aspect 4> The exhaust gas purification catalyst according to aspect 3, wherein the molar ratio of ceria to alumina (CeO2 / Al2O3) in the metal oxide support is 0.90 to 1.10.

[0012] <Aspect 5> The exhaust gas purification catalyst according to any one of aspects 1 to 4, wherein the value of abundance ratio Rb / abundance ratio Rs is 0.45 to 1.00. Advantageous Effects of Invention

[0013] According to the disclosure it is possible to provide an exhaust gas purification catalyst with improved exhaust gas purification performance.

[0014] Fig. 1 is a schematic diagram showing an exhaust gas purification catalyst 1 according to an embodiment of the disclosure.Fig. 2 is a diagram showing Rb / Rs for exhaust gas purification catalyst 1 according to an embodiment of the disclosure.Fig. 3 is a schematic diagram showing Rb / Rs for exhaust gas purification catalyst 2 according to another embodiment of the disclosure.Fig. 4 is a schematic diagram showing Rb / Rs for an exhaust gas purification catalyst 3 different from the embodiments of the disclosure.Fig. 5 is a graph showing the relationship between Rb / Rs and temperature characteristic for the samples of Example 1 and Comparative Examples 1 to 11.Fig. 6 is a graph showing the relationship between Rb / Rs and temperature characteristic for the samples of Example 1 and Comparative Examples 3, 12 and 13. DESCRIPTION OF EMBODIMENTS

[0015] An embodiment of the disclosure will now be described in detail. The disclosure is not limited to the embodiment described below, however, and various modifications may be implemented which do not depart from the gist thereof.

[0016] 1. Exhaust gas purification catalyst The exhaust gas purification catalyst of the disclosure has a substrate and a coating layer laminated on the substrate, wherein the coating layer has a first catalyst metal and a metal oxide support, the pore volume of the coating layer being greater than 0.44 mL / g and the first catalyst metal being at least one selected from the group consisting of platinum and palladium and being present in a diffused manner in the depthwise direction from the outer surface of the coating layer, and the ratio (Rb / Rs), as the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction, with respect to the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction, is 0.20 to 1.50.

[0017] The exhaust gas purification catalyst of the disclosure has improved exhaust gas purification performance due to the construction described above. It is possible, although not essential, that the principle by which exhaust gas purification performance is improved by the exhaust gas purification catalyst of the disclosure is as follows.

[0018] The platinum and palladium catalyst metals used in an exhaust gas purification catalyst are generally susceptible to catalyst poisoning. These metals also tend to undergo sintering by heat more easily than rhodium. It is therefore desirable to increase the durability of an exhaust gas purification catalyst that uses platinum and palladium.

[0019] In the exhaust gas purification catalyst of the disclosure, the first catalyst metal which is at least one selected from the group consisting of platinum and palladium is present in a diffused manner in the depthwise direction from the outer surface of the coating layer, and the ratio (Rb / Rs), as the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction with respect to the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction, is 0.20 to 1.50. Having the first catalyst metal thus diffused in the coating layer will inhibit catalyst poisoning and heat-induced sintering.

[0020] If the pore volume of the coating layer is larger than 0.44 mL / g in the exhaust gas purification catalyst of the disclosure it will be possible for exhaust gas to permeate deep into the coating layer, thereby supplying exhaust gas to the deeply diffused first catalyst metal.

[0021] Fig. 1 is a schematic diagram showing an exhaust gas purification catalyst 1 according to an embodiment of the disclosure. The exhaust gas purification catalyst 1 in Fig. 1 has a coating layer 20 on a substrate 10. The first catalyst metal 30 is diffused in the coating layer 20.

[0022] Figs. 2 to 4 are schematic views showing the ratio (Rb / Rs), as the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction with respect to the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction, for an exhaust gas purification catalyst 1 according to one embodiment of the disclosure, an exhaust gas purification catalyst 2 according to another embodiment of the disclosure, and an exhaust gas purification catalyst 3 different from the embodiments of the disclosure. Figs. 2 to 4 are not intended to place any restrictions on the exhaust gas purification catalyst of the disclosure. Figs. 2 to 4 are for illustration and do not reflect accurate dimensions.

[0023] The exhaust gas purification catalysts 1 to 3 in Figs. 1 to 3 each have a coating layer 20 on a substrate 10. Exhaust gas purification catalysts 1 to 3 differ from each other in the distribution 21 of the first catalyst metal 30 in the coating layer 20. For exhaust gas purification catalyst 1, Rb / Rs is 0.20. For exhaust gas purification catalyst 2, Rb / Rs is 1.50. For exhaust gas purification catalyst 3, Rb / Rs is 0.10.

[0024] In Fig. 2, the distribution of the first catalyst metal 30 in the coating layer 20 decreases slightly from the surface layer toward the deep layer of the coating layer 20. In Fig. 3, the distribution of the first catalyst metal 30 in the coating layer 20 increases from the surface layer toward the deep layer of the coating layer 20. In Fig. 4, the distribution of the first catalyst metal 30 in the coating layer 20 decreases significantly from the surface layer toward the deep layer of the coating layer 20.

[0025] Since the Rb / Rs ratio is 0.20 to 1.50 in the exhaust gas purification catalysts 1 and 2, catalyst poisoning and heat-induced sintering of the first catalyst metal are inhibited, while it is also possible for exhaust gas to permeate deep into the coating layer, thereby supplying exhaust gas to the deeply diffused first catalyst metal.

[0026] In the exhaust gas purification catalyst 3, on the other hand, which has an Rb / Rs ratio of 0.10, the first catalyst metal is largely distributed in the surface layer of the coating layer, and most of the first catalyst metal is therefore susceptible to the effects of catalyst poisoning, with more of the first catalyst metal also being in mutual proximity which makes it more susceptible to sintering.

[0027] The abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction and the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction can be measured by the following method. The distribution of the catalyst metal in the coating layer of the exhaust gas purification catalyst is measured using a field emission-electron probe microanalyzer (FE-EPMA), measuring the coating layer at a location 10 mm from the exhaust gas inlet end of the exhaust gas purification catalyst, for example. The FE-EPMA is used for measurement of the thickest part of the coating layer of the sample, at an acceleration voltage of 20 kV, an irradiation current of 50 mA, an acquisition time of 30 seconds and a magnification of 150x. Based on image processing of the resulting FE-EPMA image, the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction and the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction are calculated to determine Rb / Rs.

[0028] 1-1. Substrate The substrate to be used for the exhaust gas purification catalyst of the disclosure is not particularly restricted so long as it is a substrate commonly used in exhaust gas purification catalysts. The substrate used may be a honeycomb substrate, for example, and more specifically a metal honeycomb substrate.

[0029] 1-2. Coating layer The coating layer of the exhaust gas purification catalyst of the disclosure has a first catalyst metal and a metal oxide support. The coating layer may also have a second catalyst metal. The coating layer may also have other optional components such as a binder, for example, which may be an alumina binder.

[0030] The pore volume of the coating layer is greater than 0.44 mL / g. The pore volume of the coating layer is greater than 0.44 mL / g, and may be 0.45 mL / g or greater, 0.46 mL / g or greater, 0.47 mL / g or greater or 0.48 mL / g or greater. The pore volume may also be 1.00 mL / g or less, 0.90 mL / g or less, 0.80 mL / g or less, 0.70 mL / g or less, 0.60 mL / g or less or 0.50 mL / g or less.

[0031] The pore volume of the coating layer can be measured by the BET method. More specifically, the pore volume of the coating layer can be measured using the BET method with powder obtained by thorough disaggregation of the coating layer.

[0032] The thickness of the coating layer may be 40 μm to 5000 μm, for example. The thickness of the coating layer may be 40 μm or greater, 50 μm or greater, 60 μm or greater, 70 μm or greater, 80 μm or greater, 90 μm or greater, 100 μm or greater or 200 μm or greater. The thickness of the coating layer may also be 5000 μm or smaller, 4000 μm or smaller, 3000 μm or smaller, 2000 μm or smaller, 1000 μm or smaller, 500 μm or smaller, 300 μm or smaller or 100 μm or smaller.

[0033] 1-2-1. Metal oxide support The metal oxide support is not particularly restricted so long as it is a metal oxide support that can be used for an exhaust gas purification catalyst. The metal oxide support may comprise alumina, ceria, zirconia and lanthana, for example.

[0034] The metal oxide support preferably comprises alumina and ceria. In this case, the molar ratio of cerium atoms to aluminum atoms (CeO2 / Al2O3) is preferably 0.90 to 1.10. If the molar ratio of cerium atoms to aluminum atoms is within this range the exhaust gas purification performance will be further enhanced. The value of CeO2 / Al2O3may also be 0.9 or higher, 0.95 or higher or 1.00 or higher. The value of CeO2 / Al2O3may also be 1.10 or lower, 1.05 or lower or 1.00 or lower.

[0035] The metal oxide support is able to support the first catalyst metal. When the coating layer further comprises a second catalyst metal, the metal oxide support is one that is also able to support the second catalyst metal.

[0036] 1-2-2. First catalyst metal The first catalyst metal is at least one selected from the group consisting of platinum and palladium. The first catalyst metal is present in a diffused manner in the depthwise direction from the outer surface of the coating layer. The first catalyst metal has a ratio (Rb / Rs) of 0.20 to 1.50, as the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction with respect to the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction.

[0037] The Rb / Rs value may be 0.20 or higher, 0.25 or higher, 0.30 or higher, 0.35 or higher, 0.40 or higher or 0.45 or higher. The Rb / Rs value may also be 1.50 or lower, 1.40 or lower, 1.30 or lower, 1.20 or lower, 1.10 or lower or 1.00 or lower, lower than 1.00, 0.90 or lower, 0.80 or lower, 0.70 or lower or 0.60 or lower.

[0038] The Rb / Rs value is most preferably 0.45 to 1.00. If the Rb / Rs value is within this range the first catalyst metal will be uniformly dispersed particularly in the depthwise direction of the coating layer, thus providing higher durability for the exhaust gas purification catalyst.

[0039] The value of Rs is greater than 0% and less than 50%. Rs may also be greater than 0%, and 5% or greater, 10% or greater, 15% or greater, 20% or greater, 25% or greater or 30% or greater. Rs may also be less than 50% and 45% or less, 40% or less or 35% or less.

[0040] The amount of the first catalyst metal with respect to 100 g of the metal oxide support in the coating layer may be 1.0 × 10-3g to 1.0 g. The amount of the first catalyst metal with respect to 100 g of the metal oxide support may also be 1.0 × 10-3g or greater, 2.0 × 10-3g or greater, 3.0 × 10-3g or greater, 4.0 × 10-3g or greater or 5.0 × 10-3g or greater, and 1.0 g or less, 1.0 × 10-1g or less, 1.0 × 10-2g or less, 9.0 × 10-3g or less, 8.0 × 10-3g or less, 7.0 × 10-3g or less or 6.0 × 10-3g or less.

[0041] 1-2-3. Second catalyst metal When the coating layer comprises a second catalyst metal, the second catalyst metal is preferably rhodium and is preferably present in a diffuse manner in a range of within 20 μm from the outer surface of the coating layer in the depthwise direction.

[0042] If the second catalyst metal is rhodium present in a diffuse manner in a range of within 20 μm from the outer surface of the coating layer in the depthwise direction, then the distribution will be different from the first catalyst metal so that alloying between the first catalyst metal and second catalyst metal will be inhibited. Rhodium used as the second catalyst metal is more susceptible to catalyst poisoning and heat, compared to the first catalyst metal. Therefore if the second catalyst metal is present in a diffuse manner in a range of within 20 μm from the outer surface of the coating layer in the depthwise direction, then it is possible to further improve the exhaust gas purification performance of the exhaust gas purification catalyst of the disclosure.

[0043] The amount of the second catalyst metal with respect to 100 g of the metal oxide support in the coating layer may be 1.0 × 10-4g to 1.0 g. The amount of the second catalyst metal with respect to 100 g of the metal oxide support may also be 1.0 × 10-4g or greater, 2.0 × 10-4g or greater, 3.0 × 10-4g or greater, 4.0 × 10-4g or greater or 5.0 × 10-4g or greater, and 1.0 g or less, 1.0 × 10-1g or less, 1.0 × 10-2g or less, 1.0 × 10-3g or less, 9.0 × 10-4g or less, 8.0 × 10-4g or less, 7.0 × 10-4g or less or 6.0 × 10-4g or less.

[0044] 2. Method for producing exhaust gas purification catalyst The production method of the disclosure can produce an exhaust gas purification catalyst of the disclosure.

[0045] The production method of the disclosure includes: (A) adhering a slurry comprising a metal oxide support and a first catalyst metal onto a substrate and drying it to form a coating layer precursor on the substrate, and (B) post-firing the coating layer precursor to form a coating layer on the substrate.

[0046] The first catalyst metal is at least one selected from the group consisting of platinum and palladium.

[0047] In the production method of the disclosure, step (A) may be repeated several times with different slurry concentrations before step (B), whereby the coating layer formed in step (B) has the first catalyst metal present in a diffused manner in the depthwise direction from the outer surface of the coating layer, and the ratio (Rb / Rs) of the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction with respect to the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction is 0.20 to 1.50.

[0048] The pore volume of the coating layer is adjusted so as to be greater than 0.44 mL / g, primarily by selecting the metal oxide support.

[0049] The production method of the disclosure preferably further includes, after step (B): (C) adhering a slurry comprising a second catalyst metal onto the coating layer. The second catalyst metal is loaded onto the coating layer in step (C).

[0050] The substrate, metal oxide support, first catalyst metal and second catalyst metal in the production method of the disclosure will be understood by referring to the description under “1. Exhaust gas purification catalyst” above.

[0051] The exhaust gas purification catalyst of the disclosure can also be produced by a method other than the production method of the disclosure. For example, in order to create a condition in which a noble metal is dispersed in the coating layer, first a slurry containing alumina A may be coated onto a substrate and fired, forming a coating layer without a noble metal. The coating layer without a noble metal may then be impregnated several times with a noble metal solution containing platinum and palladium, to produce an exhaust gas purification catalyst of the invention. EXAMPLES

[0052] 3. Examples 1 to 4 and Comparative Examples 1 to 14 Samples for Examples 1 to 4 and Comparative Examples 1 to 14 were prepared and tested in the following manner.

[0053] 3-1. Preparation of samples for Examples 3-1-1. Examples 1 and 2, and Comparative Examples 1 and 2 (1) Example 1 A metal honeycomb substrate with a diameter of 40 mm, a length of 60 mm and a volume of 400 cell / square inch (cpsi) was used as the substrate.

[0054] A slurry was prepared by mixing platinum, palladium, alumina A, CeZr complex oxide, alumina sol and purified water.

[0055] The slurry was coated onto the substrate and dried at 80℃, thereby layering a first coating layer precursor on the substrate. Next, a slurry with different platinum and palladium contents was coated onto the first coating layer precursor and dried at 80℃, to layer a second coating layer precursor on the first coating layer precursor. This procedure was repeated several times until the total thickness from the first coating layer precursor to the nth coating layer precursor reached 100 μm, and this was followed by firing at 500℃ for 1 hour to form a coating layer on the substrate.

[0056] A rhodium nitrate solution was then applied onto the coating layer and dried at 80℃, and further fired at 500℃ for 1 hour to prepare a sample.

[0057] The mass ratio of the sample of Example 1 was platinum:palladium:rhodium:alumina:CeZr complex oxide = 0.202:0.379:0.113:30:70.

[0058] The abundance ratio Rs for platinum and palladium in the sample of Example 1, in a range of within 20 μm from the surface of the coating layer in the thickness direction, was as shown in Table 1. The ratio (Rb / Rs), as the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction with respect to the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction, was as shown in Table 2. Rb and Rs were measured by the method described below under “3-4. Measurement of catalyst metal distribution”.

[0059] (2) Example 2 and Comparative Examples 1 and 2 Samples for Example 2 and Comparative Examples 1 and 2 were prepared in the same manner as Example 1, except that the amounts of platinum and palladium were changed for each slurry used for formation of the first coating layer precursor to the nth coating layer precursor, with Rs values as shown in Table 1.

[0060] 3-1-2. Comparative Examples 3 to 6 A sample for Comparative Example 3 was prepared in the same manner as Example 1, except that alumina B with a different pore volume was used instead of alumina A. Samples for Comparative Examples 4 to 6 were prepared in the same manner as Example 2 and Comparative Examples 1 and 2, except that alumina B with different pore volumes was used.

[0061] 3-1-3. Comparative Examples 7 to 10 A sample for Comparative Example 7 was prepared in the same manner as Example 1, except that alumina C with a different pore volume was used instead of alumina A. Samples for Comparative Examples 8 to 10 were prepared in the same manner as Example 2 and Comparative Examples 1 and 2, except that alumina C with different pore volumes was used.

[0062] 3-1-4. Comparative Examples 11 and 12 A sample for Comparative Example 11 was prepared in the same manner as Example 1, except that alumina B was used instead of alumina A, and the compositional ratio of the sample was platinum:palladium:rhodium:alumina:CeZr complex oxide:lanthanum carbonate = 0.202:0.379:0.113:48:45:4. A sample for Comparative Example 12 was prepared in the same manner as Comparative Example 2, except that alumina B was used instead of alumina A, and the compositional ratio of the sample was platinum:palladium:rhodium:alumina:CeZr complex oxide:lanthanum carbonate = 0.202:0.379:0.113:48:45:4.

[0063] 3-1-5.Examples 3 and 4 Samples for Examples 3 and 4 were prepared in the same manner as Example 1, except that the contents of alumina A and CeZr oxide in Example 1 were changed for the Ce / Al ratios as shown in Table 3.

[0064] The compositional ratio of the sample of Example 3 was platinum:palladium:rhodium:alumina:CeZr oxide = 0.202:0.379:0.113:22:75. The compositional ratio of the sample of Example 4 was platinum:palladium:rhodium:alumina:CeZr oxide = 0.202:0.379:0.113:25:83.

[0065] 3-2. Evaluation of temperature characteristic Each sample of the Examples was used in a gasoline engine with a cylinder capacity of 2000 cc, controlled to an average engine rotational speed of 3000 rpm and a theoretical air / fuel ratio of A / F = 14.55, and the catalyst inlet exhaust gas temperature was changed while determining the temperature (T50) at which the HC, CO and NOx in the exhaust gas was purified to 50%.

[0066] 3-3. Measurement of catalyst metal distribution A field emission-electron probe microanalyzer (FE-EPMA) was used for measurement at a location 10 mm from the exhaust gas inlet end of the coating layer of each sample, to measure the distribution of the catalyst metal in the coating layer.

[0067] The FE-EPMA was used for measurement of the thickest part of the coating layer of the sample, at an acceleration voltage of 20 kV, an irradiation current of 50 mA, an acquisition time of 30 seconds and a magnification of 150x.

[0068] Based on image processing of the resulting FE-EPMA image, the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction and the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction were calculated to determine Rb / Rs.

[0069] 3-4. Measurement of pore volume The total pore volume for the alumina, CeZr complex oxide and alumina binder used was measured by the BET method. The pore volume of the coating layer was measured using the BET method with powder obtained by thorough disaggregation of the coating layer.

[0070] 3-5. Results The results are shown in Figs. 5 and 6 and in Tables 1 to 3.

[0071]

[0072] As shown in Table 1 and Fig. 5, in Example 1 in which the pore volume was 0.464 mL / g and the Rb / Rs ratio was 0.410, the temperature (℃) for 50% purification of HC, CO and NOx in the exhaust gas (T50) was a low value of 327℃. In other words, satisfactory exhaust gas purification performance was exhibited. Similarly, in Example 2 in which the pore volume was 0.493 mL / g and the Rb / Rs ratio was 0.310, T50 was 335℃, indicating satisfactory exhaust gas purification performance. In Comparative Examples 1 to 10, on the other hand, which had pore volumes of 0.44 mL / g or lower or Rb / Rs ratios of lower than 0.200, the T50 values were all higher than Examples 1 and 2.

[0073]

[0074] As shown in Table 2 and Fig. 6, in Example 1 the temperature (℃) for 50% purification of HC, CO and NOx in the exhaust gas (T50) was a low value of 327℃.

[0075] In contrast, Comparative Example 2 which had a pore volume of 0.488 and an Rb / Rs ratio of 0.005 exhibited a relatively high T50 value of 341℃. Similarly, Comparative Example 11 which had an Rb / Rs ratio of 0.420 and a pore volume of 0.397 also exhibited a relatively high T50 value of 363℃. Comparative Example 12 which had an Rb / Rs ratio of 0.006 and a pore volume of 0.397 likewise exhibited a relatively high T50 value of 371℃.

[0076]

[0077] As shown in Table 3, Examples 1, 3 and 4 which had a molar ratio of cerium atoms to aluminum atoms (Ce / Al) of 0.93 to 1.10 had temperatures for 50% purification of HC, CO and NOx in exhaust gas (T50) of 325 to 329 (℃), and thus all exhibited satisfactory exhaust gas purification performance. Example 2 which had a Ce / Al ratio of 1.00 exhibited especially satisfactory exhaust gas purification performance. REFERENCE SIGNS LIST

[0078] 1-3 Exhaust gas purification catalyst 10 Substrate 20 Coating layer 21 Distribution of first catalyst metal in coating layer 30 Catalyst metal

Claims

1. An exhaust gas purification catalyst having a substrate and a coating layer laminated on the substrate, wherein: the coating layer has a first catalyst metal and a metal oxide support, the pore volume of the coating layer being greater than 0.44 mL / g, and the first catalyst metal is at least one selected from the group consisting of platinum and palladium and is present in a diffused manner in the depthwise direction from the outer surface of the coating layer, and the ratio (Rb / Rs), as the abundance ratio Rb in a range of within 20 μm from the bottom of the coating layer in the thickness direction, with respect to the abundance ratio Rs in a range of within 20 μm from the surface of the coating layer in the thickness direction, is 0.20 to 1.50.

2. The exhaust gas purification catalyst according to claim 1, wherein the coating layer further has a second catalyst metal supported, the second catalyst metal being rhodium and being present in a diffuse manner in a range of within 20 μm from the outer surface of the coating layer in the depthwise direction.

3. The exhaust gas purification catalyst according to claim 1 or 2, wherein the metal oxide support comprises alumina and ceria.

4. The exhaust gas purification catalyst according to claim 3, wherein the molar ratio of ceria to alumina (CeO2 / Al2O3) in the metal oxide support is 0.90 to 1.10.

5. The exhaust gas purification catalyst according to claim 1 or 2, wherein the value of abundance ratio Rb / abundance ratio Rs is 0.45 to 1.00.

Images