WO2013027867A2

WO2013027867A2 - Exhaust gas purification catalyst

Info

Publication number: WO2013027867A2
Application number: PCT/JP2012/072088
Authority: WO
Inventors: Hirotaka Sone; Masahiko Takeuchi; Seiji Okawara; Takamitsu Asanuma; Norio Ishikawa
Original assignee: Kabushiki Kaisha Toyota Jidoshokki; Toyota Jidosha Kabushiki Kaisha
Priority date: 2011-08-24
Filing date: 2012-08-24
Publication date: 2013-02-28
Also published as: WO2013027867A3; JP2013044283A

Abstract

In an exhaust gas purification catalyst comprising noble metal, the amount of noble metal carried in an upstream portion of the carrier substrate of the catalyst on an upstream side in a direction of flow of exhaust gas is larger than the amount of noble metal carried in a downstream portion of the carrier substrate on a downstream side in the direction of flow of the exhaust gas. The amount of noble metal carried in the downstream portion is not less than an amount required for saturation of a purification rate of the exhaust gas in the downstream portion at the temperature of the exhaust gas flowing into the downstream portion. The temperature is the temperature of the exhaust gas when the exhaust gas having flowed into the upstream portion at a temperature of 168° C flows out of the upstream portion.

Description

DESCRIPTION

TITLE OF INVENTION EXHAUST GAS PURIFICATION CATALYST TECHNICAL FIELD

[0001]

The present invention relates to an exhaust gas purification catalyst.

BACKGROUND ART

[0002]

Exhaust gas discharged from an engine contains pollutants such as hydrocarbons (HC), nitrogen oxide (ΝΟχ), carbon monoxide (CO), and the like In order to purify them into water, carbon dioxide (CO2), nitrogen (N2), and the like and discharge them into air, a catalytic device is provided in an exhaust pipe in which the exhaust gas flows. As a catalytic component for purifying the pollutants, noble metals such as platinum (Pt) and palladium (Pd) are used in general. With recent strengthening of regulations in view of preventing air pollution, the need to increase the amount of noble metal used in a catalytic device has increased. However, noble metals are extremely expensive, and hence increases in the cost of catalytic devices are a problem. [0003]

Japanese Patent Application Laid-open No. H10-202105 describes an exhaust gas purification catalyst in which the amount of noble metal carried in a downstream portion of a carrier substrate is reduced to be smaller than the amount of the noble metal carried in an upstream portion thereof though it is not intended to reduce the amount of noble metal carried in the carrier substrate. In the exhaust gas purification catalyst, the amount of noble metal carried in the upstream portion of the carrier substrate is preferably not more than 80% of the amount of noble metal carried in the entire carrier substrate.

[0004]

However, since the exhaust gas purification catalyst described in Japanese Patent Application Laid-open No. H10-202105 is not an invention aimed at a reduction in the amount of noble metal carried in the carrier substrate, even when the amount of noble metal carried in the upstream portion of the carrier substrate is made to be different from the amount of noble metal carried in the downstream portion thereof, the amount of noble metal carried in the entire carrier substrate is not necessarily reduced. If the total amount of noble metal carried in the carrier substrate is to be reduced by setting the amount of noble metal carried in the upstream portion to 80% of the total amount thereof and setting the amount of noble metal carried in the downstream portion to 20% of the total amount thereof, the possibility of a reduction in the exhaust gas purification capability of the entire catalytic device is undeniable.

SUMMARY OF INVENTION

[0005]

The present invention has been achieved in order to solve the above- described problem, and an object thereof is to provide an exhaust gas purification catalyst that can reduce the amount of noble metal carried in the carrier substrate while preventing a reduction in exhaust gas purification capability.

[0006]

The present invention provides an exhaust gas purification catalyst comprising: a carrier substrate,^' a coating layer formed on a surface of the carrier substrate! and noble metal carried in the coating layer, wherein the amount of noble metal carried in an upstream portion of the carrier substrate on an upstream side in a direction of flow of exhaust gas is larger than the amount of noble metal carried in a downstream portion of the carrier substrate on a downstream side in the direction of flow of the exhaust gas, noble metal carried in the upstream portion is identical with noble metal carried in the downstream portion, the amount of noble metal carried in the downstream portion is not less than an amount required for saturation of a purification rate of the exhaust gas in the downstream portion at a temperature of the exhaust gas flowing into the downstream portion, and the temperature is the temperature of the exhaust gas when the exhaust gas having flowed into the upstream portion at a temperature of 168°C flows out of the upstream portion.

[0007]

According to the present invention, even when the amount of noble metal carried in the downstream portion is reduced to be smaller than the amount of noble metal carried in the upstream portion, by setting the amount of noble metal carried in the downstream portion to an amount not less than the amount required for saturation of a purification rate of the exhaust gas in the downstream portion at the temperature of the exhaust gas flowing into the downstream portion, the temperature of the exhaust gas flowing into the downstream portion is increased to be not less than a temperature at which the purification rate in the downstream portion is brought into a saturated state by heat of reaction of the purification reaction of the exhaust gas caused by noble metal carried in the upstream portion so that it is possible to reduce the amount of noble metal carried while preventing a reduction in exhaust gas purification capability.

BRIEF DESCRIPTION OF DRAWINGS

[0008]

FIG. 1 is a schematic view of the configuration of an exhaust gas purification device provided with an exhaust gas purification catalyst according to an embodiment of the present invention!

FIG. 2 is a cross-sectional view of a carrier substrate of the exhaust gas purification catalyst according to the embodiment;

FIG. 3 is a schematic view showing the relationship between temperature and purification rate in the exhaust gas purification catalyst according to the embodiment;

FIG. 4 is a schematic view showing the relationship between temperature and purification rate in which the procedure for determining the amount of noble metal carried in a downstream portion of the exhaust gas purification catalyst according to the embodiment is visualized;

FIG. 5 is a graph showing a change in the temperature of exhaust gas during an evaluation test of an HC purification rate and a CO purification rate of the exhaust gas purification catalyst according to the embodiment;

FIG. 6 is a graph showing the evaluation result of the HC

purification rate of the exhaust gas purification catalyst according to the embodiment;

FIG. 7 is a graph showing the evaluation result of the CO purification rate of the exhaust gas purification catalyst according to the embodiment;

FIG. 8 is a graph showing the relationship between temperature and HC purification rate in an exhaust gas purification catalyst of Example 1 according to the embodiment; and

FIG. 9 is a graph showing the relationship between temperature and CO purification rate in the exhaust gas purification catalyst of Example 1 according to the embodiment. DESCRIPTION OF EMBODIMENTS

[0009]

Hereinbelow, an embodiment of the present invention is described based on the accompanying drawings.

As shown in FIG. 1, an oxidation catalyst 3 is provided in an exhaust pipe 2 in which exhaust gas discharged from a diesel engine 1 as an internal combustion engine flows. The oxidation catalyst 3 includes a case 4 and a flow-through type (honeycomb carrier) carrier substrate 5 that is provided in the case 4 and made of a porous material. As the material for the carrier substrate 5, it is possible to use ceramic materials and metal materials used as normal honeycomb substrates such as cordierite, alumina, silicon carbide, and a metal foil of stainless steel or the like. In addition, the carrier substrate 5 has two regions of an upstream portion 6 as a portion on the upstream side in a direction of flow of the exhaust gas and a downstream portion 7 as a portion on the downstream side in the direction of flow of the exhaust gas. The ratio of the length between the upstream portion 6 and the downstream portion 7 is 1^*1.

[0010]

As shown in FIG. 2, a coating layer 8 made of alumina (AI2O3) is coated on a surface 5a of the carrier substrate 5, and noble metals of Pt and Pd as catalytically active components are carried in the coating layer 8. The amount of noble metal carried in the downstream portion 7 (see FIG. 1) is about 0.75 times the amount of noble metal carried in the upstream portion 6 (see FIG. l). With this, the amount of noble metal carried in the entire carrier substrate 5 is reduced as compared with the case where noble metal in the same amount as that of noble metal carried in the upper portion 6 is carried in the lower portion 7. Note that the same noble metals (the same composition of Pt and Pd) are carried in the upstream portion 6 and in the downstream portion 7, and only the carried amount on a per unit volume of the carrier substrate in the upstream portion 6 is different from that in the downstream portion 7. Herein, the material for the coating layer 8 may be any carrier that carries the noble metal component such as a mixture of titania (T1O2), zirconia (Zr02), and ceria (CeO2), or the like. In addition, as noble metal, Pt alone or Pd alone may be used, and other noble metals such as rhodium (Rh), iridium (Ir), silver (Ag), gold (Au), and ruthenium (Ru), and transition metals such as copper (Cu), iron (Fe), hafnium (Hf), tungsten (W), vanadium (V), lanthanum (La), and mixtures thereof may also be used.

[0011]

Next, a description is given of the operation of an exhaust gas purification device provided with the exhaust gas purification catalyst according to the embodiment.

As shown in FIG. 1, the exhaust gas discharged from the diesel engine 1 flows in the exhaust pipe 2. When the exhaust gas enters into the oxidation catalyst 3, in the upstream portion 6, HC and CO in the exhaust gas are oxidized by Pt and Pd carried in the coating layer 8 to be decomposed into water and CO2 (hereinbelow, this is referred to as a purification reaction). By heat of reaction caused by the purification reaction, the temperature of the exhaust gas is increased, and the exhaust gas flows into the downstream portion 7. HC and CO are also oxidized by the same purification reaction in the downstream portion 7, the exhaust gas flows out of the oxidation catalyst 3 into the exhaust pipe 2, and is then discharged into air.

[0012]

In general, if the relationship between the temperature and the purification rate (the rate of purification of HC/CO) in the purification reaction in each of the upstream and downstream portions 6 and 7 is schematically represented, FIG. 3 is obtained. In both of upstream and downstream portions 6 and 7, when the temperature is low, the purification reaction barely proceeds and the purification rate is close to 0. However, when the temperature reaches a certain temperature (temperature T₀ in the upstream portion 6 and temperature to in the downstream portion 7) or more, the purification reaction starts to proceed and the purification rate sharply increases with an increase in the temperature. In addition, when the temperature reaches a certain temperature (temperature Ti (> To) in the upstream portion 6 and temperature ti (> t₀) in the downstream portion 7) or more, the purification rate becomes substantially constant and is saturated. The relationships between the temperatures and the purification rates of the upstream and downstream portions 6 and 7 are represented by substantially the same curve. However, the amount of noble metal carried in the upstream portion 6 is larger than the amount of noble metal carried in the downstream portion 7 so that the purification rate of the upstream portion 6 is higher than that of the downstream portion 7 even at the same

temperature, and hence the curve of the upstream portion 6 is positioned on the left side of the curve of the downstream portion 7. In addition, with regard to the temperature at which the purification rate starts to sharply increase and the temperature at which the purification rate is saturated, the upstream portion 6 is positioned on the left side of the downstream portion 7. That is, as the amount of noble metal carried decreases, the curve indicative of the relationship between the temperature and the purification rate is shifted to the right.

[0013]

For example, when exhaust gas having a temperature Ti flows into the upstream portion 6, as described above, the temperature of the exhaust gas is increased by the heat of reaction of the purification reaction in the upstream portion 6. If the temperature of the exhaust gas flowing out of the upstream portion 6 and flowing into the downstream portion 7 is increased to the temperature ti, the saturated purification rate can be obtained in the upstream portion 6 and also the saturated purification rate can be obtained in the downstream portion 7, and hence it is possible to prevent a reduction in the purification rate in the downstream portion 7 caused by reducing the amount of noble metal carried in the downstream portion 7 to be less than the amount of noble metal carried in the upstream portion 6. In other words, even when the mount of noble, metal carried in the downstream portion 7 is reduced to an amount that allows the saturation of the purification rate in the downstream portion 7 at the temperature of the exhaust gas flowing into the downstream portion 7, it is possible to prevent a reduction in the purification rate in the downstream portion 7.

[0014]

However, the temperature of the exhaust gas flowing into the downstream portion 7 depends on the temperature of the exhaust gas flowing into the upstream portion 6 and the amount of noble metal carried in the upstream portion 6, and hence it is not possible to determine the range of the amount of noble metal carried in the downstream portion 7 without considering these elements. Consequently, the temperature of the exhaust gas flowing into the upstream portion 6 is assumed to be 168°C which is within the temperature range of normal exhaust gas, and the amount of noble metal carried in the upstream portion 6 is assumed to be any amount. With this, the temperature of the exhaust gas flowing out of the upstream portion 6, i.e., the temperature of the exhaust gas flowing into the

downstream portion 7 corresponding to the amount of noble metal carried in the upstream portion 6 is determined. Subsequently, the amount of noble metal that allows the saturation of the purification rate in the downstream portion 7 when the exhaust gas flows into the downstream portion 7 at the determined temperature is set as the lower limit of the amount of noble metal carried in the downstream portion 7. Accordingly, in order to determine the amount of noble metal carried in the downstream portion 7, the amount of noble metal carried in the upstream portion 6 is predetermined and the temperature of the exhaust gas flowing into the downstream portion 7 after an exhaust gas having a temperature of 168°C has flowed into the upstream portion 6 is estimated under the condition of a predetermined amount of noble metal, whereby the amount of noble metal carried in the downstream portion 7 can be set to an amount not less than the amount required for the saturation of the purification rate in the downstream portion 7 at the estimated temperature.

[0015]

The procedure for determining the amount of noble metal carried in the downstream portion 7 is visually represented in FIG. 4. The

relationship between the temperature and the purification rate in the upstream portion 6 is represented by a solid line 10. On the right side of the solid line 10, the relationship between the temperature and the purification rate in the downstream portion 7 when the amount of noble metal carried in the downstream portion 7 is changed is illustratively represented by each of broken lines 11, 12, and 13. That is, the amount of noble metal carried in the downstream portion 7 in broken line 12 is smaller than the amount thereof in broken line 11, and the amount of noble metal carried in the downstream portion 7 in broken line 13 is smaller than the amount thereof in broken line 12. When it is assumed that the temperature of the exhaust gas flowing into the upstream portion 6 is 168°C and the temperature of the exhaust gas is increased by AT°C by the purification reaction while the exhaust gas flows in the upstream portion 6, the temperature of the exhaust gas flowing into the downstream portion 7 is represented by (168 + AT)°C. At this temperature, the purification rate in the downstream portion 7 is saturated in the cases of broken lines 11 and 12, and when the amount of noble metal carried in the downstream portion 7 is at least that of broken line 12, it is possible to prevent a reduction in the purification rate in the downstream portion 7.

[0016]

Thus, even when the amount of noble metal carried in the

downstream portion 7 is reduced to be smaller than the amount of noble metal carried in the upstream portion 6, by setting the amount of noble metal carried in the downstream portion 7 to an amount not less than the amount required for the saturation of the purification rate of the exhaust gas in the downstream portion 7 at the temperature of the exhaust gas flowing into the downstream portion 7, the temperature of the exhaust gas flowing into the downstream portion 7 is increased to be not less than a temperature at which the purification rate in the downstream portion 7 is brought into a saturated state by the heat of reaction of the purification reaction of the exhaust gas caused by noble metal carried in the upstream portion 6, and hence it is possible to reduce the amount of noble metal carried while preventing a reduction in exhaust gas purification capability.

[0017]

In this embodiment, although a flow-through type carrier substrate is used, the carrier substrate is not limited to a flow-through type, and a wall-flow type carrier substrate may also be used. In addition, the number of coating layers 8 coated on the surface 5a of the carrier substrate 5 is not limited to one, and two or more layers made of the same carrier or made of different carriers may be coated. Further, in this embodiment, although the carrier substrate 5 has the upstream and downstream portions 6 and 7 that are integrated together, the present invention is not limited to this embodiment, and the upstream portion and the downstream portion may also be provided separately.

[0018]

In this embodiment, although the ratio of the length between the upstream portion 6 and the downstream portion 7 is l^'l, the present invention is not limited to this embodiment. The range of the present invention regarding the ratio of the length will become apparent in the Examples described later. In addition, in this embodiment, although the absolute value of the length of each of the upstream and downstream portions 6 and 7 is not described, the length of the upstream portion 6 needs to be at least 10 mm. When the exhaust gas flows into the internal portion of the carrier substrate 5, a turbulent flow occurs at an end surface of the carrier substrate 5. It is known that the range of occurrence of the turbulent flow is generally within about 10 mm from the end surface. The occurrence of the turbulent flow contributes to an improvement in

purification rate and, when the amount of noble metal is large in this range, the occurrence thereof significantly contributes to the improvement in purification rate. Consequently, the length of the upstream portion 6 where the amount of noble metal carried is large is preferably 10 mm or more.

[0019]

In this embodiment, although description is given by using the oxidation catalyst 3 as an example, the catalyst is not limited to the oxidation catalyst. A three-way catalyst, a DPF, an SCR (Selective

Catalytic Reduction), an ASC (Ammonia Slip Catalyst), or an integrated SCR/DPF can also similarly reduce the amount of noble metal carried in the downstream portion to be less than the amount of noble metal carried in the upstream portion. When one of these catalysts is used, the "purification reaction" corresponds to a chemical reaction caused by each of the catalysts.

EXAMPLE

[0020]

Next, the effect of the exhaust gas purification catalyst of the present invention is described based on Examples.

Table 1 shows, with regard to Example 1 as the exhaust gas purification catalyst according to the present invention and exhaust gas purification catalysts according to Comparative Examples 1 to 3, the composition of both the noble metal and the coating layer (the carrier), the mass of the noble metal carried in both the upstream and downstream portions, the ratio of the mass of the noble metal carried in the downstream portion to the mass of the noble metal carried in the upstream portion, and the mass of the noble metal carried in the entire flow-through type carrier substrate. Note that the volume of the carrier substrate in which the exhaust gas purification catalyst according to each of Example 1 and Comparative Examples 1 to 3 is 1.1 liters, and the length of both the upstream portion and the downstream portion is 65 mm.

[0021]

[Table l]

Table 1

[0022]

The carrier substrate in which the exhaust gas purification catalyst according to each of Example 1 and Comparative Examples 1 to 3 was used as the oxidation catalyst 3 of the exhaust gas purification device shown in FIG. 1, and the purification rate of the exhaust gas, particularly the purification rates of HC and CO in the exhaust gas were evaluated. The speed of the diesel engine 1 during the evaluation was 1600 rpm and the torque was changed in a range of 20 to 100 Nm. This is engine control simulating two to four cycles in a UDC mode of an EC mode. Exhaust gas was a result of this engine operation, and thus the average flow velocity of the exhaust gas was 13 g/second, the average concentration of HC in the exhaust gas flowing into the oxidation catalyst 3 was 264 ppm, and the average concentration of CO in the exhaust gas flowing into the oxidation catalyst 3 was 772 ppm. In the section of the two to four cycles in the UDC mode shown in FIG. 5, HC and CO purification rates were measured. In addition, a thermocouple was provided at the front end of the upstream portion 6 and at a boundary portion between the upstream portion 6 and the downstream portion 7, and the entrance temperature of the exhaust gas and the temperature of the exhaust gas at the boundary portion were measured. FIG. 5 shows a change in the temperature of the exhaust gas during the evaluation of the HC and CO purification rates (the temperature of the exhaust gas before the exhaust gas flows into the oxidation catalyst 3). The temperature of the exhaust gas indicated by the vertical axis is the entrance temperature of the exhaust gas at the entrance of the exhaust gas

purification catalyst.

[0023]

FIG. 6 shows the evaluation results of the average HC purification rate obtained by the above-described evaluation method at an entrance temperature of 172°C of the exhaust gas purification catalyst according to each of Example 1 and Comparative Examples 1 to 3, while FIG. 7 shows the evaluation results of the average CO purification rate. In Example 1, the mass of the noble metal carried in the downstream portion 7 was reduced by 25% relative to the mass of the noble metal carried in the upstream portion 6, i.e., the mass of the noble metal carried in the downstream portion 7 was 0.75 times the mass of the noble metal carried in the upstream portion 6, but the HC purification rate and the CO purification rate in Example 1 were substantially equal to those in Comparative Example 1 in which the mass of the noble metal carried in the upstream portion 6 was equal to that carried in the downstream portion 7. In contrast to Comparative Example 1, in Comparative Examples 2 and 3 in which the mass of the noble metal carried in the downstream portion 7 was reduced by 50% and 75% relative to the mass of the noble metal carried in the upstream portion 6, the HC

purification rates and the CO purification rates were lower than those in Example 1 and Comparative Example 1. From these results, it was found that, even when the mass of the noble metal carried in the downstream portion 7 was reduced to be less than the mass of the noble metal carried in the upstream portion 6 in the range of 25% or less, the HC purification rate and the CO purification rate obtained from the entire upstream and downstream portions 6 and 7 were not lowered. That is, it seems that, within this range of the reduction, the increase in the temperature of the exhaust gas by the heat of reaction generated by the purification reaction in the upstream portion 6 accelerates the purification reaction in the

downstream portion 7, whereby it is possible to compensate for the reduction in the mass of the noble metal carried in the downstream portion 7. In addition, the temperature at the boundary portion between the upstream portion 6 and the downstream portion 7 was higher than the entrance temperature of the exhaust gas at the entrance of the exhaust gas

purification catalyst by 4°C in each of Example 1 and Comparative Examples 1 to 3. That is, ΔΤ was 4°C.

[0024]

Herein, the results in Example 1 are examined. FIG. 8 shows the estimated relationship between the HC purification rate and temperatures (entrance temperatures) at the entrance of the upstream portion 6 in which 3.3 (g) of noble metal was carried under the same conditions as those of the test described above and the entrance of the downstream portion 7 in which 2.48 (g) of noble metal was carried under the same conditions as those of the test described above, and FIG. 9 shows the estimated relationship between the CO purification rate and the temperatures thereat. From the test, it is found that the temperature at the boundary between the upstream portion 6 and the downstream portion 7 is increased to be higher than the

temperature at the entrance of the upstream portion 6 by 4°C.

Consequently, an exhaust gas having a temperature of 168°C at the entrance of the upstream portion 6 has a temperature of 172°C when the exhaust gas flows into the boundary portion, i.e., the downstream portion 7. Herein, from FIGS. 8 and 9, it is seen that each purification rate of the catalyst in the downstream portion 7 at 172°C is saturated. Thus, the temperature of the exhaust gas is increased by the catalyst of the upstream portion 6 at the exit (the boundary portion) of the upstream portion 6 and the noble metal in the amount required for the saturation of the purification rate at the temperature is carried in the downstream portion 7 so that it was possible to prevent a reduction in purification rate.

[0025]

Next, with regard to a catalyst according to each of Examples 2 and 3, Table 2 shows the composition of both the noble metal and the coating layer (carrier), the mass of the noble metal carried in each of the upstream and downstream portions, the ratio of the mass of the noble metal carried in the downstream portion to the mass of the noble metal carried in the upstream portion, and the mass of the noble metal carried in the entire carrier substrate. Note that, with regard to the carrier substrate in which the exhaust gas purification catalyst according to each of Examples 2 and 3 is carried, Table 3 shows the volume of the carrier substrate, the length of both the upstream and downstream portions thereof, and the ratio of the length between the upstream and downstream portions thereof.

[0026]

[Table 2]

Table 2

[0027]

[Table 3] Table 3

[0028]

On the exhaust gas purification catalyst according to both Examples 2 and 3, the evaluation of the purification rates of HC and CO in the exhaust gas was performed by the same method used for the exhaust gas purification catalyst according to each of Example 1 and Comparative Examples 1 to 3. Table 4 shows the evaluation results of the average HC and CO purification rates at entrance temperatures of the exhaust gas of 168°C, 169°C, and 175°C at the entrance of the exhaust gas purification catalyst.

[0029]

[Table 4]

Table 4

[0030]

Substantially the same HC and CO purification rates were obtained for Examples 2 and 3. From this result, it was found that the ratio of the length between the upstream and downstream portions did not influence the HC purification rate and the CO purification rate obtained from the entire upstream and downstream portions as long as the ratio thereof fell in the range from 1^1 of Example 2 to V4 of Example 3. In addition, it was found that the reduction in the mass of the noble metal carried in the downstream portion 7 did not influence the HC purification rate and the CO purification rate obtained from the entire upstream and downstream portions as long as the reduction in the mass thereof fell in the range from Example 2 in which the mass of the noble metal carried in the downstream portion 7 was reduced by 25% relative to the mass of the noble metal carried in the upstream portion 6 to Example 3 in which the mass thereof was reduced by 20% relative to the mass thereof (i.e., 0.8 times the mass of the noble metal carried in the upstream portion 6). When the mass of the noble metal in the downstream portion is made equal to the mass of the noble metal in the upstream portion, the mass of the noble metal is 2.6 (g), and it was possible to achieve a reduction of 0.32 (g) in Example 2, and a reduction of 0.26 (g) in Example 3.

Claims

1. An exhaust gas purification catalyst comprising:

a carrier substrate;

a coating layer formed on a surface of the carrier substrate! and noble metal carried in the coating layer, wherein

the amount of noble metal carried in an upstream portion of the carrier substrate on an upstream side in a direction of flow of exhaust gas is larger than the amount of noble metal carried in a downstream portion of the carrier substrate on a downstream side in the direction of flow of the exhaust gas, and noble metal carried in the upstream portion is identical with noble metal carried in the downstream portion,

the amount of noble metal carried in the downstream portion is not less than an amount required for saturation of a purification rate of the exhaust gas in the downstream portion at the temperature of the exhaust gas flowing into the downstream portion, and

the temperature is the temperature of the exhaust gas when the exhaust gas having flowed into the upstream portion at a temperature of 168°C flows out of the upstream portion.

2. The exhaust gas purification catalyst according to claim 1, wherein the exhaust gas purification catalyst is an oxidation catalyst.

3. The exhaust gas purification catalyst according to claim 1 or 2, wherein the amount of noble metal carried in the downstream portion is not less than 0.75 times the amount of noble metal carried in the upstream portion when a length of the downstream portion is set to be one to four times a length of the upstream portion in the direction of flow of the exhaust gas.

4. The exhaust gas purification catalyst according to claim 3, wherein the amount of noble metal carried in the downstream portion is not more than 0.8 times the amount of noble metal carried in the upstream portion.

5. The exhaust gas purification catalyst according to any of claims 1 to 4, wherein the length of the upstream portion is not less than 10 mm.