JP5556022B2 - Exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to an exhaust gas purification catalyst.

  A relatively large amount of NOx (nitrogen oxide) is contained in exhaust gas discharged from a diesel engine or a lean burn gasoline engine. In order to purify this NOx, NOx is occluded in an atmosphere where the air-fuel ratio of the exhaust gas is lean, and the NOx occluded when the atmosphere becomes rich (near the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio). A so-called lean NOx trap catalyst that has been released and reduced and purified by a reducing agent component such as HC (hydrocarbon) in exhaust gas is used.

  The lean NOx trap catalyst is generally a combination of a catalyst metal such as Pt and Rh, a NOx occlusion material made of alkali metal, alkaline earth metal or rare earth metal, and alumina supporting the catalyst metal, and In many cases, an oxygen storage material made of Ce-containing oxide is also combined.

  For example, Patent Document 1 describes that a NOx purification catalyst (lean NOx trap catalyst) is configured by combining a catalyst metal such as Pt, a NOx occlusion material such as an alkaline earth metal, and alumina. Further, in this document, hydrogen enrichment means is provided upstream of the exhaust gas flow from the NOx purification catalyst. As the hydrogen enrichment means, a solid acidic zirconium oxide represented by the general formula [Y] dZreOf is used. (Y in the formula is at least one element selected from the group consisting of Ti, Al, W, Mo and Zn, and d and e are atomic ratios of the respective elements, respectively). f represents the number of oxygen atoms necessary to satisfy the valences of Y and Zr, and d = 0.01 to 0.5, e = 0.5 to 0.99, and d + e = 1.

Here, the hydrogen enriching means supplies combustion gas or exhaust gas enriched with hydrogen to the NOx purification catalyst, and the solid acidic zirconium oxide suppresses the consumption of hydrogen, and produces a hydrogen production catalyst. In particular, by using together with Pt, the composition gas having a high H ₂ component ratio can be efficiently supplied to the NOx purification catalyst.

Further, Patent Document 2 discloses a composite that contains 0.2 to 5.0 mol% W, 0.2 to 5.0 mol% Cu, and 90 to 99.6 mol% Zr for purification of NOx in a lean burn engine. It is described that an oxide catalyst is used, and that the decomposition of NOx is promoted by using the catalyst to promote the decomposition of hydrocarbons.
JP 2007-132356 A Japanese Patent Laid-Open No. 7-204510

  As described above, it is known to use a composite oxide containing Zr and W (tungsten) for NOx purification in an oxygen-excess atmosphere. However, the composite oxide basically completely burns hydrocarbons. It is recognized that it is used to produce a reducing agent containing a large amount of hydrogen that becomes a reducing agent of NOx by decomposing moderately instead of. However, this only uses a composite oxide containing Zr and W as a reducing agent supply source.

  That is, the present invention uses a composite oxide containing Zr and W as an auxiliary agent for NOx occlusion material and catalytic metal, and promotes occlusion of NOx in the NOx occlusion material or reduction of NOx released from the NOx occlusion material. The challenge is to promote.

The present invention is an exhaust gas purifying catalyst in which a catalyst layer comprising at least two layers of an upper layer and a lower layer each containing a catalytic metal and a NOx storage material is formed on a carrier,
At least one of said upper and lower layers, a Zr ion and W ions, each in terms of ZrO ₂ amount and WO ₃ of WO ₃ weight relative to the total amount of the ZrO ₂ amount and WO ₃ weight The composite oxide is contained so that the ratio is 4% by mass or less, and the catalyst metal is supported on the composite oxide.

  The concept of the layered catalyst containing the catalyst metal and the NOx occlusion material is such that NOx is mainly occluded in the lower NOx occlusion material in the lean atmosphere, and NOx that is desorbed from the NOx occlusion material in the rich atmosphere is formed in the upper layer. The catalyst metal is surely reduced and purified. Of course, the upper NOx storage material also stores NOx, and the lower catalyst metal also reduces and purifies NOx.

  According to the inventor's research, the composite oxide containing Zr ions and W ions functions as an auxiliary agent for promoting NOx occlusion in exhaust gas in a lean atmosphere in relation to the NOx occlusion material. In relation to metal, it functions as an auxiliary agent that promotes reduction of NOx released from the NOx storage material in a rich atmosphere.

  Therefore, according to the catalyst configuration of the present invention, the composite oxide containing Zr ions and W ions can promote NOx occlusion by the NOx occlusion material, or promote NOx reduction by the catalyst metal, When the composite oxide is disposed in the lower layer, it is advantageous for promoting NOx occlusion, and when it is disposed in the upper layer, it is advantageous for promoting reduction of NOx.

Further, according to the study by the present inventors, it has been found that the composite oxide containing Zr ions and W ions has a composition ratio suitable for promoting NOx occlusion and a composition ratio suitable for promoting NOx reduction. doing. That is, when converted into each ZrO ₂ amount and WO ₃ weight content of Zr ions and W ions of the composite oxide, the ratio of the WO ₃ weight relative to the total amount of the ZrO ₂ amount and WO ₃ amount of 4 mass When the ratio is less than or equal to%, the effect of promoting NOx occlusion is enhanced, and when the proportion of the amount of WO ₃ is greater than 4% by mass, the effect of promoting NOx reduction is enhanced.

  Therefore, according to the present invention, the above WO ₃ Since the composite oxide having an amount of 4% by mass or less is disposed in at least one of the upper layer and the lower layer, the effect of promoting NOx occlusion by the NOx occlusion material particularly in a lean atmosphere is enhanced.

WO ₃ Concerning a composite oxide having an amount of 4% by mass or less, the WO ₃ The lower limit of the amount ratio may be about 1% by mass.

Of course, the composite oxide is a rate of the WO ₃ content ratio of more than 4 wt% complex oxide and the WO ₃ content is 4 mass% or less, can be arranged mixed in the upper layer, or It can be mixed and placed in the lower layer.

  WO ₃ The composite oxide with a proportion of more than 4% by weight, the WO ₃ The upper limit of the ratio of the amount may be about 12% by mass.

Preferably, the upper layer contains a composite oxide in which the proportion of the above WO ₃ amount is more than 4% by mass , the catalyst metal is supported on the composite oxide , and the lower layer has a proportion of the above WO ₃ amount of 4% by mass. % containing less is composite oxide, catalyst metal is Rukoto are carried to the composite oxide. As a result, the effect of promoting NOx occlusion by the NOx occlusion material in the lean atmosphere is enhanced, and reduction of NOx released from the NOx occlusion material in the rich atmosphere by the catalyst metal is promoted.

  Moreover, it is preferable that the lower layer contains a composite oxide containing Ce ions and Pr ions. This composite oxide has a function of adsorbing NOx at a low temperature, and is advantageous in suppressing discharge of unpurified NOx. In addition, this composite oxide has oxygen storage / release capability, and releases oxygen quickly when the atmosphere changes from lean to rich. A part of HC and CO is oxidized, and the NOx purification activity of the catalyst is increased by the heat of the oxidation reaction.

  Here, the complex oxide containing Ce ions and Pr ions is unique from the viewpoint of other oxygen storage materials. However, as described above, the oxygen release rate is fast, but the oxygen release amount itself is not so large. It is in.

  That is, HC and CO in the exhaust gas become NOx reducing agents. However, if the amount of oxygen released from the oxygen storage material is large, HC and CO, which are reducing agents, are oxidized and consumed by the released oxygen. It is disadvantageous for reduction purification. In this regard, although the above complex oxide has a high oxygen release rate, the oxygen release amount itself is not so large. Therefore, even if a relatively large amount is added to the catalyst to promote the activity of the catalyst, HC and CO that are reducing agents are not consumed excessively.

  Therefore, according to the composite oxide containing Ce ions and Pr ions, the catalyst is activated by its rapid oxygen releasing ability while avoiding excessive waste of HC and CO which are reducing agents of NOx. be able to.

  In another preferred embodiment of the present invention, there is provided an exhaust gas purification catalyst in which a catalyst layer having at least two layers of an upper layer and a lower layer each containing a catalyst metal and a NOx storage material is formed on a carrier,
  At least one of the upper layer and the lower layer contains a composite oxide containing Zr ions and W ions, and the catalyst metal is supported on the composite oxide,
  The lower layer contains a composite oxide containing Ce ions and Pr ions.

As described above, according to the present invention, on the support, at least one of the upper layer and the lower layer each containing the catalytic metal and the NOx storage material contains Zr ions and W ions, and the amount of ZrO ₂ and A composite oxide containing the amount of WO ₃ in terms of the amount of WO _{3 in} terms of the total amount of ZrO ₂ and WO _{3 in} terms of the amount of WO ₃ is 4% by mass or less. since There has been supported, the composite oxide as a catalyst aid promotion of the NOx occlusion by NOx-absorbing material, or can promote the NOx reduction by the catalyst metal, that particularly high effect of promoting NOx occlusion .

In another preferred embodiment of the present invention, a composite oxide in which at least one of an upper layer and a lower layer each containing a catalytic metal and a NOx storage material includes Zr ions and W ions is formed on the support. And the above-mentioned catalytic metal is supported on the composite oxide, and the lower layer contains a composite oxide containing Ce ions and Pr ions, so that a composite oxide containing Zr ions and W ions is catalyzed. As an auxiliary agent, NOx occlusion can be promoted by a NOx occlusion material, or NOx reduction by a catalytic metal can be promoted, and further, the emission of unpurified NOx is suppressed by a composite oxide containing Ce ions and Pr ions. In addition, the catalyst can be activated by its rapid oxygen releasing ability while avoiding excessive waste of HC and CO which are reducing agents for NOx.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

  FIG. 1 shows the configuration of an exhaust gas purifying catalyst that purifies exhaust gas discharged from a diesel engine or a lean burn gasoline engine, particularly purifies NOx in the exhaust gas. In the figure, reference numeral 1 denotes a cell wall of a honeycomb carrier, and a catalyst layer having an upper layer (surface layer) 2 and a lower layer (deep layer) 3 is formed on the cell wall. The honeycomb carrier 1 can be formed of a heat resistant inorganic material such as cordierite.

  At least one of the upper layer 2 and the lower layer 3 contains a ZrW composite oxide containing Zr ions and W ions, and the lower layer 3 contains a CePr composite oxide containing Ce ions and Pr ions. Each of the upper layer 2 and the lower layer 3 may further include activated alumina. The upper layer 2 and the lower layer 3 can be formed by wash-coating the ZrW composite oxide, CePr composite oxide and activated alumina onto the honeycomb carrier 1. Each of the upper layer 2 and the lower layer 3 further contains a NOx storage material and a catalyst metal.

  As the NOx occlusion material, an alkali metal, an alkaline earth metal or a rare earth metal can be employed, and an alkaline earth metal such as Ba or Sr is particularly preferable. As the catalyst metal, noble metals such as Pt, Rh and Pd are preferable, and Pt and Rh are particularly preferable. The NOx storage material and the catalyst metal can be supported by impregnating the upper layer 2 and the lower layer 3 with the respective solutions. The ZrW composite oxide, CePr composite oxide, or activated alumina may be wash coated on the honeycomb carrier 1 in a state where a catalyst metal is previously supported by an evaporation-drying method or the like.

<Examples and Comparative Examples>
Examples of preferred catalysts of the present invention will be described below together with comparative examples.

Example 1
The upper layer 2 contains a Rh / ZrW composite oxide in which Rh as a catalyst metal is supported on a ZrW composite oxide. This Rh / ZrW composite oxide is a catalyst component that is wash-coated on the support 1 in a state where Rh is previously supported on the ZrW composite oxide. The lower layer 3 contains a mixture of activated alumina and CePr composite oxide. Further, the upper layer 2 and the lower layer 3 are impregnated with Ba and Sr as NOx storage materials and Pt and Rh as catalyst metals. Therefore, at least a part of the NOx storage material and the catalyst metal is in a state of being supported on the Rh / ZrW composite oxide, activated alumina, and CePr composite oxide.

ZrW composite oxide, the metal ions only contains Zr ions and W ions, the Zr ions and W ions when converted into ZrO ₂ amount and WO ₃ amount, the total amount of the ZrO ₂ amount and WO ₃ weight The proportion of the amount of WO ₃ occupied (WO ₃ / (ZrO ₂ + WO ₃ )) is 2% by mass. Therefore, it can be said that the ZrW composite oxide is a Zr-based composite oxide containing W because the ZrO ₂ component occupies the majority.

The CePr composite oxide contains only Ce ions and Pr ions as metal ions, and the CeO ₂ amount and Pr ₆ O ₁₁ amount when the Ce ions and Pr ions are converted into CeO ₂ amount and Pr ₆ O ₁₁ amount. The ratio of the amount of Pr ₆ O _{11 in} the total amount (Pr ₆ O ₁₁ / (CeO ₂ + Pr ₆ O ₁₁ )) is 10% by mass. Therefore, the CePr composite oxide is a Ce-based composite oxide containing Pr because the CeO ₂ component occupies most of the CePr composite oxide.

  The carrying amount of each of the above components per 1 L of carrier is as follows.

[Upper layer]
Rh / ZrW composite oxide; 50.4 g / L
(ZrW composite oxide; 50 g / L, Rh; 0.4 g / L)
[Underlayer]
Activated alumina; 135 g / L, CePr composite oxide; 135 g / L
[Impregnation component (amount of impregnation of both upper and lower layers)]
Rh; 0.1 g / L, Pt; 0.5 g / L, Ba; 30 g / L, Sr; 10 g / L

-Example 2-
The proportion of WO ₃ in the ZrW composite oxide in the upper layer 2 was 4.3% by mass, and the other catalyst configuration was the same as in Example 1.

-Example 3-
The proportion of WO ₃ in the ZrW composite oxide in the upper layer 2 was 6.5% by mass, and the other catalyst configuration was the same as in Example 1.

Example 4
The proportion of WO ₃ in the ZrW composite oxide in the upper layer 2 was 12% by mass, and the other catalyst configuration was the same as in Example 1.

-Example 5
The upper layer 2 was made to contain a mixture of Rh / ZrW composite oxide and Rh / activated alumina, and the same catalyst configuration as in Example 1 was used.

As in Example 1, the Rh / ZrW composite oxide of the upper layer 2 is obtained by supporting Rh on a ZrW composite oxide having a WO ₃ content ratio of 2 mass%. The supported amount of this Rh / ZrW composite oxide was 25.2 g / L (ZrW composite oxide; 25 g / L, Rh; 0.2 g / L). Rh / activated alumina is a catalyst component that is wash-coated on the support 1 in a state where Rh is supported on activated alumina in advance. The supported amount of Rh / activated alumina was 25.2 g / L (activated alumina; 25 g / L, Rh; 0.2 g / L).

-Example 6
The lower layer 3 contains, instead of activated alumina, a mixture of the ZrW composite oxide having the above-mentioned WO ₃ content of 2% by mass and CePr composite oxide, and the other catalyst is the same as in Example 2. Made the configuration. The ZrW composite oxide supported amount of the lower layer 3 is 135 g / L, and the CePr composite oxide supported amount is also 135 g / L.

-Example 7-
The lower layer 3 contains, instead of activated alumina, a mixture of the ZrW composite oxide having the above-mentioned WO ₃ content of 2% by mass and CePr composite oxide, and the other catalyst is the same as in Example 4. Made the configuration. The ZrW composite oxide supported amount of the lower layer 3 is 135 g / L, and the CePr composite oxide supported amount is also 135 g / L.

-Comparative Example 1-
The upper layer 2 contains Rh / (ZrO ₂ + WO ₃ ) in which Rh is supported on a mixture of ZrO ₂ and WO ₃ instead of the Rh / ZrW composite oxide, and the other catalyst is the same as in Example 1. Made the configuration. Rh / (ZrO ₂ + WO ₃ ) is a catalyst component that is wash-coated on the carrier 1 in a state where Rh is previously supported on a mixture of ZrO ₂ and WO ₃ .

Mixture of the ZrO ₂ and WO ₃ were the ratio of WO ₃ weight relative to the total amount of the ZrO ₂ amount and WO ₃ amount _{(WO 3 / (ZrO 2 +} WO 3)) and 2 wt%. The loading amount of this mixture is 50 g / L, and the loading amount of Rh with respect to this mixture is 0.4 g / L.

-Comparative Example 2-
The proportion of WO _{3 in} the mixture of ZrO ₂ and WO ₃ in the upper layer 2 was 4.3% by mass, and the other catalyst configuration was the same as in Example 1.

-Comparative Example 3-
The proportion of WO _{3 in} the mixture of ZrO ₂ and WO ₃ in the upper layer 2 was 6.5% by mass, and the other catalyst configuration was the same as in Example 1.

-Comparative Example 4-
The ratio of the above-mentioned WO ₃ amount in the mixture of ZrO ₂ and WO ₃ in the upper layer 2 was set to 12% by mass, and the other catalyst configuration was the same as in Example 1.

-Comparative Example 5-
The upper layer 2 was made to contain Rh / activated alumina in place of the Rh / ZrW composite oxide, and the other catalyst composition was the same as in Example 1. The amount of Rh / activated alumina supported was 50.4 g / L (activated alumina; 50 g / L, Rh; 0.4 g / L).

<Evaluation of exhaust gas purification performance>
About each catalyst of the said Examples 1-7 and Comparative Examples 1-5, after performing the aging hold | maintained for 24 hours in 750 degreeC air | atmosphere atmosphere, NOx purification performance is demonstrated using a model gas distribution reaction apparatus and an exhaust gas analyzer. Examined. First, the pretreatment gas shown in Table 1 was allowed to flow at a temperature of 600 ° C. for 10 minutes, and then lean model exhaust gas (A / F = 27.7 equivalent) was allowed to flow for 60 seconds, and the gas composition was rich in model exhaust gas. (Equivalent to A / F = 14.3) and the flow of flowing this for 60 seconds was repeated several times, then the lean NOx purification rate for 60 seconds from the time when the gas composition was switched from rich to lean, and from lean The rich NOx purification rate was measured for 60 seconds from the time of switching to rich. The composition of the lean model exhaust gas and the rich model exhaust gas is as shown in Table 1, and the space velocity was 25000 / h. Further, the model exhaust gas had three temperatures of 175 ° C., 200 ° C., and 250 ° C. The results are shown in Table 2, FIG. 2 and FIG.

FIG. 2 shows the lean NOx purification rate, and FIG. 3 shows the rich NOx purification rate. In both figures, the lines of “Al ₂ O ₃ at 175 ° C.”, “Al ₂ O ₃ at 200 ° C.” and “Al ₂ O ₃ at 250 ° C.” are 175 ° C., 200 ° C. and 250 ° C. of Comparative Example 5. Each NOx purification rate is shown.

When the lean NOx purification rates of Examples 1 to 4 shown in FIG. 2 are seen in relation to the purification rate line of Comparative Example 5, the Rh / ZrW combined oxidation of the upper layer 2 at any of 175 ° C., 200 ° C. and 250 ° C. When the ratio of the amount of WO _{3 in the} product is 5% by mass or less, particularly when it is 4.3% or less, the purification rate is higher than that of Comparative Example 5. On the other hand, in Comparative Examples 1 to 4 using Rh / (ZrO ₂ + WO ₃ ) for the upper layer 2, the purification rate tends to increase as the proportion of the amount of WO ₃ decreases, as in the examples. At any of the above temperatures, the purification rate is lower than that of the corresponding Examples 1 to 4, and is lower than that of Comparative Example 5.

From the above results, it can be seen that when a ZrW composite oxide having a WO ₃ content of 4% by mass or less is disposed in the upper layer 2, the lean NOx purification rate is increased. This is considered to be a result of the ZrW composite oxide promoting NOx occlusion by the NOx occlusion material (Ba, Sr). In this case, the lower limit of the proportion of WO ₃ may be about 1% by mass.

On the other hand, when the rich NOx purification rates of Examples 1 to 4 shown in FIG. 3 are seen in relation to the purification rate line of Comparative Example 5, the Rh / ZrW of the upper layer 2 at any of 175 ° C., 200 ° C. and 250 ° C. When the proportion of WO _{3 in the} composite oxide exceeds 4% by mass, the purification rate is higher than that in Comparative Example 5. In contrast, in Comparative Examples 1 to 4 using Rh / (ZrO ₂ + WO ₃ ) for the upper layer 2, the purification rate tends to increase as the proportion of the amount of WO ₃ increases as in the examples ( However, the purification rate cannot be improved even if the proportion of WO ₃ exceeds 12%). At any of the above temperatures, the purification rate is lower than the corresponding Examples 1 to 4.

From the above results, it can be seen that when the ZrW composite oxide in which the proportion of WO ₃ exceeds 4% by mass is disposed in the upper layer 2, the rich NOx purification rate is increased. This is considered to be a result of the ZrW composite oxide promoting reduction and purification of NOx by the catalyst metal (Rh, Pt). In this case, the upper limit of the proportion of WO ₃ may be about 12% by mass.

Next, other examples will be examined based on the results in Table 2. In Example 5, the amount of Rh / ZrW composite oxide in the upper layer 2 (the proportion of WO ₃ is 2% by mass) is half the amount of Example 1. The remaining half was replaced with Rh / activated alumina. The lean NOx purification rate is lower than that in the first embodiment, but is higher than those in the second to fourth embodiments. Therefore, when the Rh / ZrW composite oxide having a proportion of WO ₃ of 2% by mass is disposed in the upper layer 2, it can be seen that the effect of increasing the lean NOx purification rate is great even when the amount is relatively small.

In Examples 6 and 7, a ZrW composite oxide having a WO ₃ content ratio of 2 mass% is arranged in the lower layer 3 instead of activated alumina. The lean NOx purification rates of Examples 6 and 7 are higher than those of the corresponding Examples 2 and 4, respectively. As described above, in the case of a catalyst having an upper layer / lower layer two-layer structure, the NOx occlusion property of the lower layer 3 is preferably good. In Examples 6 and 7, it is considered that the above-described high lean NOx purification rate was obtained as a result of the ZrW composite oxide promoting NOx occlusion by the NOx occlusion material in the lower layer 3.

Further, the rich NOx purification rates of Examples 6 and 7 are relatively high as in Examples 2 and 4. Now, that the ratio of WO ₃ amount in the upper layer 2 is ZrW composite oxide exceeds 4 mass% place, the proportion of WO ₃ amount in the lower layer 3 is placed a 4 mass% of ZrW composite oxide, a lean NOx It turns out that it is effective in raising both a purification rate and a rich NOx purification rate.

1 is a cross-sectional view of an exhaust gas purifying catalyst according to the present invention. It is a graph which shows the lean NOx purification rate of an Example and a comparative example. It is a graph which shows the rich NOx purification rate of an Example and a comparative example.

1 carrier 2 upper layer 3 lower layer

Claims (5)

An exhaust gas purifying catalyst in which a catalyst layer having at least two layers of an upper layer and a lower layer each containing a catalytic metal and a NOx storage material is formed on a carrier,
The upper layer converts Zr ions and W ions into ZrO ₂ amount and WO ₃ amount , respectively , so that the proportion of the amount of WO _{3 in} the total amount of the ZrO ₂ amount and the WO ₃ amount is 4% by mass or less. composite oxide containing exhaust gas purifying catalyst in which the catalytic metal to the composite oxide is characterized by being carried, including a. An exhaust gas purifying catalyst in which a catalyst layer having at least two layers of an upper layer and a lower layer each containing a catalytic metal and a NOx storage material is formed on a carrier,
Said lower layer, a Zr ion and W ions, respectively so that the proportion of in terms of ZrO ₂ amount and WO ₃ weight WO ₃ weight relative to the total amount of the ZrO ₂ amount and WO ₃ amount of 4 wt% or less An exhaust gas purifying catalyst comprising the composite oxide contained in the catalyst, wherein the catalyst metal is supported on the composite oxide . In claim 2,
  The upper layer contains Zr ions and W ions, respectively, ZrO. ₂ Quantity and WO ₃ In terms of quantity, the ZrO ₂ Quantity and WO ₃ WO in the total amount ₃ An exhaust gas purifying catalyst, comprising: a composite oxide containing an amount of more than 4% by mass, wherein the catalyst metal is supported on the composite oxide. In any one of Claim 1 thru | or 3 ,
The exhaust gas purifying catalyst, wherein the lower layer contains a composite oxide containing Ce ions and Pr ions. An exhaust gas purifying catalyst in which a catalyst layer having at least two layers of an upper layer and a lower layer each containing a catalytic metal and a NOx storage material is formed on a carrier,
  At least one of the upper layer and the lower layer contains a composite oxide containing Zr ions and W ions, and the catalyst metal is supported on the composite oxide,
  The exhaust gas purifying catalyst, wherein the lower layer contains a composite oxide containing Ce ions and Pr ions.

Images