JP2014213270A - Exhaust gas purification catalyst and exhaust gas purification device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purification catalyst and an exhaust gas purification device capable of particularly improving an NOx adsorbing rate in a lean atmosphere at 300°C or more while maintaining an HC purification rate, when an LNT and a noble metal/an acidic carrier are supported on the same support.SOLUTION: There is provided an exhaust gas purification catalyst for purifying an exhaust gas of an internal combustion engine which comprises a first catalyst and a second catalyst supported on the same support, wherein the first catalyst has a carrier having a solid acid and a noble metal supported on the carrier and the second catalyst has an NOx adsorbent which comprises: a CePr-based composite oxide and substantially contains neither an alkaline earth metal nor an alkali metal; and a noble metal.

Description

  The present invention relates to an exhaust purification catalyst and an exhaust purification device.

  Conventionally, stoichiometric combustion is performed on the high load side and lean combustion is performed on the low load side, such as an internal combustion engine (hereinafter referred to as “engine”) that performs premixed compression ignition combustion (hereinafter referred to as “HCCI”). Engines that perform are known. In such an engine, it is necessary to purify the exhaust gas both in a stoichiometric atmosphere and a lean atmosphere. In a stoichiometric atmosphere, exhaust gas can be purified by using a three-way catalyst, but in a lean atmosphere, exhaust gas cannot be sufficiently purified even if such a three-way catalyst is used. In this case, in particular, nitrogen oxides (hereinafter referred to as “NOx”) and hydrocarbons (hereinafter referred to as “HC”) contained in the exhaust gas cannot be sufficiently purified.

  Therefore, a lean NOx purification catalyst (hereinafter referred to as “LNT”) containing a NOx adsorbent composed of alkali metal or alkaline earth metal is known as a catalyst that can efficiently purify NOx even in a lean atmosphere (for example, “LNT”). , See Patent Document 1). According to this NOx purification catalyst, NOx in exhaust gas is adsorbed by a NOx adsorbent in a lean atmosphere, and the adsorbed NOx is released in a stoichiometric or rich atmosphere, so that NOx can be purified.

  Further, as a catalyst capable of efficiently purifying HC in a lean atmosphere, an exhaust purification catalyst (hereinafter referred to as “noble metal / acidic carrier”) in which a noble metal such as Pt is supported on a carrier having solid acidity is known (hereinafter referred to as “noble metal / acidic carrier”). For example, see Patent Document 2). According to this noble metal / acidic carrier, HC can be efficiently purified even in a lean atmosphere due to the high acidity of the acidic carrier.

  Therefore, by arranging the support supporting LNT and the support supporting the noble metal / acidic carrier in series in the exhaust passage, NOx and HC can be efficiently purified even in a lean atmosphere. However, it is desirable to support LNT and a noble metal / acidic carrier on the same support from the viewpoints of restrictions on the exhaust layout and cost increase due to the use of two general honeycomb supports as the support.

However, even if the conventional LNT and the noble metal / acidic carrier are simply supported on the same support, the effect of the combination is not sufficiently exhibited, and in particular, the improvement of the HC purification rate in a lean atmosphere is not recognized. was there. Therefore, the present applicant supports the first catalyst and the second catalyst on the same support, and includes the first catalyst, a support having solid acidity, and a noble metal supported on the support. And an exhaust purification catalyst comprising a NOx adsorbent containing CeO ₂ and substantially free of alkaline earth metal and alkali metal, and a noble metal. 2012-116289). According to this exhaust purification catalyst, both NOx and HC in a lean atmosphere can be efficiently purified on one support.

Japanese Patent No. 2600492 Special table 2010-506712 gazette Japanese Patent Application No. 2012-116289

Incidentally, in the above application, from using alkaline earth metal or alkali metal when HC purification rate of the first catalyst is reduced as the NOx adsorbent, it is selected CeO ₂ as an adsorbent. However, this CeO ₂ has poor heat resistance, and particularly when exposed to high temperatures containing water vapor, the NOx adsorption performance is lowered. Therefore, in order to suppress such CeO ₂ degradation, substituting a part of CeO ₂ to ZrO ₂ it is well known.

_CeO 2 -ZrO ₂ in which a part of CeO ₂ is replaced with ZrO _2, in a relatively low temperature range of about 200 ° C. to 300 ° C., NO is against NO ₂ produced is oxidized by the noble metal, excellent Has adsorption performance. On the other hand, at a more elevated temperature _range, the adsorption performance for NO ₂ of CeO 2 -ZrO ₂ is reduced. On the other hand, especially in the case of a gasoline engine, even a lean burn engine is operated with a stoichiometric exhaust temperature depending on the load, so it is excellent even in a temperature range of about 300 ° C to 400 ° C. NOx adsorption performance is required.

  As one method for improving NOx adsorption performance at 300 ° C. to 400 ° C., there is a method of adding La having a lower electronegativity than Ce to the catalyst. However, when La is added, the HC purification rate decreases as in the case where alkaline earth metal or alkali metal is added.

  The present invention has been made in view of the above, and its purpose is to maintain the HC purification rate particularly in a lean atmosphere of 300 ° C. or higher when LNT and noble metal / acidic carrier are supported on the same support. An object of the present invention is to provide an exhaust purification catalyst and an exhaust purification device that can improve the NOx adsorption rate.

  To achieve the above object, the present invention provides an exhaust purification catalyst for purifying exhaust gas from an internal combustion engine, comprising a first catalyst and a second catalyst carried on the same support, wherein the first catalyst is a solid catalyst. A NOx adsorbent comprising a support having acidity and a noble metal supported on the support, wherein the second catalyst contains a CePr composite oxide and is substantially free of alkaline earth metal and alkali metal. And an exhaust gas purification catalyst characterized by comprising a noble metal.

First, the reason why a high HC purification rate is obtained in a lean atmosphere when a noble metal / acidic carrier is used alone is as follows.
That is, in the noble metal / acidic carrier, the electron density on the surface of the noble metal is lowered by the electron withdrawing action based on the high acidity of the acidic carrier. Then, the binding force between oxygen and noble metal present in a large amount in the lean atmosphere is reduced, and adsorption of oxygen on the surface of the noble metal is suppressed. By suppressing the adsorption of oxygen, HC can be directly adsorbed on the surface of the noble metal, so that a high HC purification rate can be obtained.

On the other hand, even if LNT and noble metal / acidic carrier are simply supported on the same support, the effect of the combination is not sufficiently exhibited, and the reason why the improvement of the HC purification rate in a lean atmosphere is not particularly observed is It is as follows.
That is, conventionally, since the alkali metal and alkaline earth metal constituting the NOx adsorbent in LNT are basic, the electron donating action of these substances inhibits the electron withdrawing action of the acidic carrier, and the electron density on the surface of the noble metal Becomes higher. Then, the bonding force between the noble metal and oxygen is increased, and the adsorption of oxygen to the surface of the noble metal is promoted. In addition, for example, exhaust discharged from an engine during lean operation contains more paraffin as HC than during stoichiometric operation, and paraffin is structurally less reactive than olefins and aromas. For this reason, the purification rate of paraffin is greatly reduced. Thereby, the purification rate of HC falls.

Therefore, the present invention includes a first catalyst in which a noble metal is supported on a carrier having solid acidity as a noble metal / acidic carrier, and a CePr composite oxide as a LNT, which is substantially an alkaline earth metal and an alkali metal. And an NOx adsorbent that does not contain a second catalyst having a noble metal. That is, the first catalyst and the second catalyst are mixed or stacked, and the NOx adsorbent constituting the second catalyst as the LNT does not contain an alkaline earth metal and an alkali metal. An oxide is included.
Here, the CePr composite oxide is obtained by substituting part of Ce in CeO ₂ with Pr, and this CePr composite oxide is less basic than alkali metals and alkaline earth metals. The electron density of the noble metal surface can be reduced without significantly affecting the electron withdrawing action of the acidic carrier. As a result, since the bonding force between the noble metal and oxygen can be reduced and the adsorption of oxygen to the surface of the noble metal can be suppressed, the purification rate of paraffin can be particularly improved, and a high HC purification rate can be obtained.
The CePr composite oxide has a high oxygen storage capacity (OSC) in addition to the NOx adsorption capacity, and releases active oxygen at a predetermined temperature or higher. Since this active oxygen is very reactive, according to the present invention, oxidation purification of paraffin can be promoted even in a lean atmosphere, and the HC purification rate can be improved.

Furthermore, the exhaust purification catalyst according to the present invention corresponds to the exhaust purification catalyst according to the above-mentioned application, in which part of Ce in CeO ₂ constituting the NOx adsorbent is replaced with Pr. Even in a lean atmosphere of 300 ° C. or higher, the NOx adsorption rate can be improved while maintaining the HC purification rate. The reason is assumed to be as follows.
That is, NOx in the exhaust gas is oxidized from NO to NO ₂ by a noble metal and adsorbed on the NOx adsorbent by the spillover of NO ₂ . However, at 300 ° C. or higher, from the viewpoint of an equilibrium reaction, part of the oxidized NO ₂ is reduced to NO before being adsorbed. Therefore, sufficient adsorption performance cannot be obtained with CeO ₂ having a relatively weak adsorption force.
On the other hand, CePr composite oxide is superior to CeO ₂ in its ability to release active oxygen, and this active oxygen promotes the oxidation reaction of NO to NO ₂ . The reaction of NO → NO ₂ by active oxygen proceeds on the CePr composite oxide as the adsorbent. Thus, since NO is adsorbed to the CePr composite oxide immediately after being oxidized to NO ₂ , according to the present invention, a high NOx adsorption rate can be obtained even at a high temperature of 300 ° C. or higher.

Further, the NO ₂ oxidized on the noble metal is a reaction intermediate NO ₂ δ ⁻ , and the CePr composite oxide having an excellent active oxygen releasing ability has a function of increasing the ionization potential of the NOx adsorbent. As a result, the reaction intermediate NO ₂ δ ⁻ easily spills over the NOx adsorbent. As a result, the NOx adsorption rate is further improved.
As described above, according to the present invention, when the LNT and the noble metal / acidic carrier are supported on the same support, the NOx adsorption rate can be improved while maintaining the HC purification rate, particularly in a lean atmosphere of 300 ° C. or higher. .

  In this case, the ratio of the number of moles of Pr to the total number of moles of Ce and Pr in the CePr composite oxide is preferably 5% to 30%.

  In this invention, the ratio of the number of moles of Pr to the total number of moles of Ce and Pr in the CePr composite oxide is 5% to 30%. Thereby, it is possible to improve the NOx adsorption rate and the HC purification rate particularly in a lean atmosphere of 300 ° C. or higher.

  In this case, the ratio of the number of moles of Pr to the total number of moles of Ce and Pr in the CePr composite oxide is preferably 10% to 20%.

  In this invention, the ratio of the number of moles of Pr to the total number of moles of Ce and Pr in the CePr composite oxide is 10% to 20%. Thereby, the NOx adsorption rate and the HC purification rate can be further improved particularly in a lean atmosphere of 300 ° C. or higher.

  In this case, it is preferable to include a first catalyst layer formed by the first catalyst and a second catalyst layer adjacent to the first catalyst layer and formed by the second catalyst.

  In the present invention, the first catalyst layer is formed using the first catalyst, and the second catalyst layer is formed using the second catalyst adjacent to the first catalyst layer. As a result, the above-described effects are exhibited particularly at the interface between the first catalyst layer and the second catalyst layer.

  In this case, the first catalyst layer is preferably formed on the second catalyst layer.

In the present invention, the first catalyst layer is formed on the second catalyst layer. That is, the first catalyst layer is formed as an upper layer and the second catalyst layer is formed as a lower layer. Thereby, compared with the case where the first catalyst layer containing the noble metal / acidic carrier is used as the lower layer, the upper layer has better gas diffusibility, so that paraffin can be purified efficiently and the HC purification rate can be improved. Further, by using the first catalyst layer containing the noble metal / acidic carrier as the upper layer, NO in the exhaust is easily oxidized to NO ₂ and easily adsorbed to the LNT, so that the NOx purification rate is improved.

  In this case, the solid acid support is preferably at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide, ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica, and zeolite.

In the present invention, at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide, ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica, and zeolite is used as the carrier having solid acidity. Here, the ZrSi composite oxide is a composite oxide of ZrO ₂ and SiO ₂ , and the SiAl composite oxide is a composite oxide of SiO ₂ and Al ₂ O ₃ . The ZrSiYW composite oxide is one in which a part of Zr and Si in the ZrSi composite oxide is substituted with Y or W, and the ZrSiCeW composite oxide is a composition of Zr and Si in the ZrSi composite oxide. A part thereof is substituted with Ce or W.
These acidic carriers have both high acidity (the value obtained by subtracting 2.7 from the average value of the Pauling electronegativity of the constituent elements constituting the acidic carrier) and the specific surface area, and further reduce the electron density on the surface of the noble metal. Thus, the bonding strength between the noble metal and oxygen can be further reduced. Therefore, according to the present invention, the adsorption of oxygen to the surface of the noble metal can be further suppressed, and a higher HC purification rate can be obtained.

  In this case, the carrier having solid acidity is preferably at least one of ZrSiYW composite oxide and ZrSiCeW composite oxide.

  In the present invention, at least one of ZrSiYW composite oxide and ZrSiCeW composite oxide is used as the carrier having solid acidity. Both of these two acidic carriers have particularly high acidity (the value obtained by subtracting 2.7 from the average value of the Pauling electronegativity of the constituent elements constituting the acidic carrier) and the specific surface area, and the electron density on the surface of the noble metal is further increased. This can be reduced to further reduce the bonding strength between the noble metal and oxygen. Therefore, according to the present invention, the adsorption of oxygen to the surface of the noble metal can be further suppressed, and a higher HC purification rate can be obtained.

  In this case, it is preferable that the first catalyst and the second catalyst have Pt as the noble metal.

In the present invention, Pt is used as a noble metal contained in each of the first catalyst and the second catalyst. Pt has particularly high oxidation performance, and can efficiently oxidize NO to NO ₂ , so that the above-described effects are more reliably exhibited.

  Also provided is an exhaust gas purification apparatus for an internal combustion engine that is operated while controlling the air-fuel ratio to be lean and stoichiometric, and includes the above-described exhaust gas purification catalysts.

  According to this invention, the same effect as the invention of the exhaust purification catalyst is exerted.

  According to the present invention, when an LNT and a noble metal / acidic carrier are supported on the same support, an exhaust purification catalyst capable of improving the NOx adsorption rate while maintaining the HC purification rate, particularly in a lean atmosphere of 300 ° C. or higher. And an exhaust emission control device can be provided.

It is a figure which shows HC composition in exhaust_gas | exhaustion, (A) shows HC composition in exhaust_gas | exhaustion discharged | emitted from the engine in stoichiometric operation, (B) is in exhaust_gas | exhaustion discharged | emitted from the engine in lean (HCCI) operation | movement It is a figure which shows HC composition. It is a figure which shows the HC purification rate by a three way catalyst. It is a figure which shows the adsorption | suction behavior of HC with respect to the oxygen which adsorb | sucked to the noble metal surface, (A) shows the adsorption | suction behavior of an olefin and an aroma, (B) is a figure which shows the adsorption | suction behavior of a paraffin. Is a diagram showing the adsorption behavior of oxygen and paraffin in a noble metal surface, (A) is a case of a conventional exhaust gas purifying catalyst in which the noble metal is supported on Al ₂ O ₃ carrier, (B) is supported on the noble metal is acidic carrier It is the case of the exhaust gas purification catalyst which concerns on one Embodiment of this invention. It is a figure which shows the relationship of the acidity x specific surface area of an acidic support | carrier, and HC purification rate. It is a figure which shows the relationship between the electronegativity of an acidic support | carrier, and a specific surface area. It is a X-ray diffraction spectrum figure of ZrSi complex oxide, ZrSiW complex oxide, and ZrSiYW complex oxide. Is a diagram showing the results of performance evaluation al ₂ O ₃ carrier and ZrSiYW composite oxide support, (A) is a diagram showing the oxygen adsorption amount of supported noble metal surface on each carrier, (B) is according to the temperature It is a figure which shows the paraffin adsorption rate. It is a figure which shows the NOx purification rate of Examples 1-4 and the comparative example 1. FIG. It is a figure which shows the HC purification rate of Examples 1-4 and the comparative example 1. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
The exhaust purification catalyst according to an embodiment of the present invention is preferably used in an exhaust purification device of an internal combustion engine that is operated with the air-fuel ratio being controlled to be lean and stoichiometric. More specifically, it is preferably used for an exhaust purification device such as an HCCI engine or a diesel engine.
The exhaust purification catalyst according to the present embodiment includes a first catalyst and a second catalyst carried on the same support.

  As the support, for example, a cordierite honeycomb support is preferably used. In the present embodiment, the first catalyst and the second catalyst supported on different supports are not arranged in the exhaust passage in series, for example, but the first catalyst and the second catalyst are supported on the same support. It is.

The first catalyst and the second catalyst may be supported on the same support in a mixed state, and the first catalyst layer containing the first catalyst and the second catalyst layer containing the second catalyst are adjacent to each other. And may be supported in a laminated state. In the present embodiment, it is preferable that the first catalyst layer and the second catalyst layer are supported on the same support in a state where they are laminated adjacently.
Moreover, it is preferable that the 1st catalyst layer is formed on the 2nd catalyst layer. The reason will be described in detail later.

The first catalyst includes a carrier having solid acidity and a noble metal supported on the carrier.
As the carrier having solid acidity, that is, the acidic carrier, at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide, ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica and zeolite is preferably used. The reason why these are preferable will be described in detail later. Among these, at least one of ZrSiYW composite oxide and ZrSiCeW composite oxide is more preferably used.
As the noble metal, various noble metals such as Pt, Pd, and Rh may be used alone, or these may be used in combination. Among these, Pt is preferably used.

The second catalyst includes a NOP adsorbent containing a CePr composite oxide and substantially free of alkaline earth metal and alkali metal, and a noble metal.
The CePr composite oxide is obtained by substituting part of Ce in CeO ₂ with Pr. Further, “substantially free of alkaline earth metals and alkali metals” means that these alkaline earth metals and alkali metals are not actively added, and is excluded until they are inevitably contained. Not the purpose.
The noble metal is supported on a carrier such as alumina or the above NOx adsorbent. As the precious metal, similarly to the first catalyst, various precious metals such as Pt, Pd, and Rh may be used alone or in combination. Among these, Pt is preferably used.

  In the CePr composite oxide constituting the NOx adsorbent of the second catalyst, the ratio of the number of moles of Pr to the total number of moles of Ce and Pr is preferably 5% to 30%. When the ratio of the number of moles of Pr to the total number of moles of Ce and Pr is less than 5%, the effect of adding Pr cannot be sufficiently obtained. On the other hand, if it exceeds 30%, the heat resistance is lowered and the specific surface area of the CePr composite oxide is lowered, so that the HC purification rate and the NOx adsorption rate are lowered. A more preferable ratio of the number of moles of Pr to the total number of moles of Ce and Pr is 10% to 20%.

The exhaust purification catalyst according to the present embodiment can be obtained, for example, by preparing a slurry containing constituent materials of each catalyst, immersing the honeycomb support in the slurry, and drying and firing.
When the first catalyst layer is formed on the second catalyst layer, first, the honeycomb support is immersed in a slurry containing the constituent material of the second catalyst, and then dried and fired, whereby the second catalyst is formed. Form a layer. Next, the honeycomb support on which the second catalyst layer is formed is immersed in the slurry containing the constituent material of the first catalyst, and then dried and fired, whereby the first catalyst layer is formed on the second catalyst layer. The formed exhaust purification catalyst according to the present embodiment is obtained.

The CePr composite oxide can be prepared by the following method. However, the method for preparing the CePr composite oxide according to the present embodiment is not limited to the following method.
First, cerium nitrate and praseodymium nitrate are dissolved in pure water so as to have a desired Ce / Pr ratio. Then, a sodium hydroxide aqueous solution is dripped and pH of a solvent is set to 10, for example, and a precipitate is obtained. Thereafter, the solvent is evaporated by filtering the solution containing the precipitate under reduced pressure while being heated to 60 ° C., for example. Next, after extracting the residue, for example, calcination is performed at 500 ° C. for 2 hours in a muffle furnace to obtain a CePr composite oxide.

A ZrSiXW (X = Ce or Y) composite oxide can be prepared by the following method. However, the method for preparing the ZrSiXW composite oxide according to the present embodiment is not limited to the following method.
First, zirconium nitrate, cerium nitrate or yttrium nitrate is dissolved in pure water so as to have a desired Zr / X ratio. Thereafter, sodium silicate containing a desired amount of Si is added. Next, a precipitate is obtained by adding sodium hydroxide dropwise so that the pH becomes, for example, 10 if necessary. Thereafter, the solvent is evaporated by filtering the solution containing the precipitate under reduced pressure while being heated to 60 ° C., for example. Next, after extracting the residue, a ZrSiX composite oxide is obtained by performing calcination, for example, at 500 ° C. for 2 hours in a muffle furnace.
Next, the ZrSiX composite oxide obtained as described above is added to nitric acid to adjust the pH to 3, for example, and then sodium metatungstate (Na ₂ WO ₄ ) containing a desired amount of W is added. Next, sodium hydroxide is added dropwise so that the pH of the solution becomes 10, for example, to obtain a precipitate. Thereafter, the solvent is evaporated by filtering the solution containing the precipitate under reduced pressure while being heated to 60 ° C., for example. Next, after extracting the residue, calcination is performed at 500 ° C. for 2 hours in a muffle furnace to obtain a ZrSiXW composite oxide.

The effect of the exhaust purification catalyst according to the present embodiment having the above configuration will be described in detail below.
FIG. 1 is a diagram showing the HC composition in the exhaust, where (A) shows the HC composition in the exhaust discharged from the engine during stoichiometric operation, and (B) shows the exhaust from the engine during lean (HCCI) operation. It is a figure which shows HC composition in the exhaust_gas | exhaustion to be performed. In FIG. 1, the horizontal axis indicates the number of HC carbon atoms, and the vertical axis indicates the HC concentration (ppmC) in the exhaust gas.
As shown in FIG. 1 (A), the exhaust discharged from the engine during stoichiometric operation contains a lot of short carbon atoms, and most contains olefins. On the other hand, as shown in FIG. 1 (B), the exhaust discharged from the engine during lean operation contains a lot of carbon having a longer carbon number than during stoichiometric operation, It contains a lot of paraffin.

FIG. 2 is a graph showing the HC purification rate by the three-way catalyst. Specifically, the HC purification rate η350 when the exhaust discharged from the engine during the stoichiometric operation and the exhaust discharged from the engine during the HCCI lean operation are purified at 350 ° C. using a conventional three-way catalyst. It is a figure which shows (%).
As shown in FIG. 2, there is no difference between the stoichiometric atmosphere and the lean atmosphere with respect to the purification rates of olefin and aroma, which are almost the same. On the other hand, the purification rate of paraffin is greatly reduced in the lean atmosphere as compared with the stoichiometric atmosphere. From this, it can be seen that the cause of the decrease in the HC purification rate in the lean atmosphere is the decrease in the purification rate of paraffin.

FIG. 3 is a diagram showing the adsorption behavior of HC with respect to oxygen adsorbed on the surface of the noble metal (Pt), (A) shows the adsorption behavior of olefin and aroma, and (B) shows the adsorption behavior of paraffin. . FIG. 3 shows a state where a large amount of oxygen is adsorbed on the surface of the noble metal (Pt) in an oxygen-rich lean atmosphere.
As shown in FIG. 3A, since olefins and aromas have weak bonds, unstable and highly reactive π bonds, oxygen having low activity is adsorbed on the surface of the noble metal (Pt). Even so, the double bond is cleaved to react and adsorb. Thereby, the purification rate of olefin and aroma does not decrease even in a lean atmosphere.
On the other hand, as shown in FIG. 3B, paraffin has only a strong, stable, and low-reactivity σ bond, so that oxygen with low activity is adsorbed on the surface of the noble metal (Pt). In this case, the adsorbed oxygen inhibits the adsorption of paraffin to the noble metal, and the purification rate is greatly reduced in a lean atmosphere.

FIG. 4 is a view showing the adsorption behavior of oxygen and paraffin on the surface of the noble metal (Pt, Pd), and FIG. 4A is a case of a conventional exhaust purification catalyst in which the noble metal is supported on an Al ₂ O ₃ support. B) shows an exhaust purification catalyst according to an embodiment of the present invention in which a noble metal is supported on an acidic carrier.
As shown in FIG. 4 (A), precious metals (Pt, Pd) in the conventional exhaust gas purification catalyst supported on _Al 2 _{O 3} carrier, since the _Al 2 _{O 3} carrier is a carrier ampholyte, the noble metal surface electron There is no change in density, and a large amount of oxygen is adsorbed on the surface of the noble metal in a lean atmosphere. Therefore, the adsorption of paraffin to the noble metal is inhibited by the adsorbed oxygen, and the purification rate of paraffin is reduced.
On the other hand, as shown in FIG. 4B, in the exhaust purification catalyst according to this embodiment in which the noble metals (Pt, Pd) are supported on the acidic carrier, the electron withdrawing action based on the high acidity of the acidic carrier is used. , The electron density on the surface of the noble metal decreases. Then, the binding force between oxygen and noble metal present in a large amount in the lean atmosphere is reduced, and adsorption of oxygen on the surface of the noble metal is suppressed. Thereby, since the paraffin with low reactivity also reacts and adsorbs, the purification rate of paraffin improves.

FIG. 5 is a diagram showing the relationship between the acidity × specific surface area of the acidic carrier and the HC purification rate. Specifically, in FIG. 5, for a catalyst in which a noble metal (Pt) is supported on an acidic carrier, the HC purification rate at 300 ° C. and the HC at 400 ° C. when the acidity × specific surface area value of the acidic carrier is changed. The average purification rate η300 ° C.-400 ° C. (%) with the purification rate is shown.
Here, the acidity of the acidic carrier in the present embodiment means a value obtained by subtracting 2.7 from the average value of the Pauling electronegativity of the constituent elements constituting the acidic carrier. The specific surface area in the present embodiment is the ratio after aging is performed in an oxygen-excess atmosphere at 750 ° C. for 20 hours (O ₂ = 10% by volume, H ₂ O = 6% by volume, remaining N ₂ balance gas). It means surface area.
As shown in FIG. 5, there is a correlation between the acidity × specific surface area value of the acidic carrier and the HC purification rate, and the HC purification rate increases as the acidity × specific surface area value of the acidic carrier increases. Tend to be. From this result, it is understood that the product of the acidity and the specific surface area is an important index when selecting an acidic carrier.
It has been confirmed that there is not much correlation between the acidity of the acidic carrier and the HC purification rate, and there is not much correlation between the specific surface area of the acidic carrier and the HC purification rate.

FIG. 6 is a diagram showing the relationship between the electronegativity of the acidic carrier and the specific surface area. In FIG. 6, the average value of the Pauling electronegativity of the constituent elements is shown as the electronegativity of each acidic carrier.
As shown in FIG. 6, ZrO ₂ has both a small electronegativity and a small specific surface area, whereas Al ₂ O ₃ has a large specific surface area although it has a low electronegativity, while SiO ₂ has a small specific surface area but has an electronegativity. The degree is great. However, ZrSi composite oxide, which is a composite oxide of ZrO ₂ and SiO ₂ , and SiAl composite oxide, which is a composite oxide of SiO ₂ and Al ₂ O ₃ , are 2.7 from the average electronegativity of the constituent elements. It can be seen that large values can be obtained for both the acidity and the specific surface area, which are values obtained by subtracting. For this reason, ZrSi composite oxide or SiAl composite oxide is preferably used as an acidic carrier having a large value of acidity × specific surface area.
In addition, as shown in FIG. 6, a ZrSiYW composite oxide in which a part of Zr and Si in the ZrSi composite oxide is substituted with Y or W, or a part of Zr and Si in the ZrSi composite oxide is Ce or The ZrSiCeW composite oxide substituted with W is more preferably used as an acidic carrier having a larger value of acidity × specific surface area.

FIG. 7 is an X-ray diffraction spectrum diagram of a ZrSi composite oxide, a ZrSiW composite oxide, and a ZrSiYW composite oxide. The horizontal axis indicates the 2θ angle (°), and the vertical axis indicates the X-ray intensity.
As shown in FIG. 7, in the ZrSiW composite oxide formed by adding W having high electronegativity to the ZrSi composite oxide, in addition to the peak derived from the ZrSi composite oxide on the X-ray diffraction spectrum, , A peak derived from ZrO ₂ is confirmed. This means that a part of the crystal structure has collapsed.
Therefore, in the ZrSiYW composite oxide obtained by adding Y to the ZrSiW composite oxide, it was confirmed that the peaks derived from ZrO ₂ disappeared, while other new peaks were confirmed. Not. This means that the crystal structure is stabilized and the ZrSiYW composite oxide is well formed.
Therefore, from the results of FIGS. 6 and 7, it can be said that a ZrSiYW composite oxide having a stabilized structure in addition to a large value of acidity × specific surface area is particularly preferable as an acidic carrier. The same can be said about the ZrSiCeW composite oxide in which Y in the ZrSiYW composite oxide is substituted with Ce.

FIG. 8 is a view showing the performance evaluation results of the Al ₂ O ₃ support and the ZrSiYW composite oxide support, (A) is a view showing the oxygen adsorption amount on the surface of the noble metal supported on each support, and (B) These are figures which show the paraffin adsorption rate according to temperature.
The oxygen adsorption amount (μmol / g) in FIG. 8 (A) was raised after adsorbing oxygen by letting an exhaust gas in a lean atmosphere flow for a predetermined time with respect to each carrier on which noble metal (Pt) was supported. The amount of oxygen desorption is shown. Specifically, the oxygen desorption amount is measured as follows. First, each carrier on which a noble metal (Pt) is supported is heated to 500 ° C. in a He atmosphere. Next, oxygen is adsorbed by holding at 500 ° C. for 15 minutes in an atmosphere with an oxygen concentration of 7%. Next, after cooling to room temperature, the adsorbed oxygen is desorbed by heating to room temperature to 800 ° C. in a He atmosphere, and the amount of oxygen desorbed at this time is measured.

  Further, the paraffin adsorption rate (%) in FIG. 8B indicates the paraffin adsorption rate when exhaust gas in a lean atmosphere containing a lot of paraffin is circulated for a predetermined time at each temperature. Specifically, the paraffin adsorption rate is measured as follows. First, each support on which a noble metal (Pt) is supported is held for 15 minutes in an atmosphere having an oxygen concentration of 7% at each measurement temperature. Next, after replacing with a He atmosphere, 1000 ppmC of n-pentane is injected in pulses. At this time, the n-pentane concentration after passing through the catalyst (each carrier) is measured to determine the adsorption amount. A pulse is injected 6 times, and the paraffin adsorption rate is calculated using the total as the adsorption amount.

As shown in FIG. 8A, the oxygen adsorption amount on the surface of the noble metal supported on the ZrSiYW composite oxide support is approximately 20% of the oxygen adsorption amount on the surface of the noble metal supported on the Al ₂ O ₃ support. According to the composite oxide support, it can be seen that the oxygen adsorption amount is dramatically reduced.
Further, as shown in FIG. 8B, it can be seen that the paraffin adsorption rate of the ZrSiYW composite oxide carrier is larger than that of the Al ₂ O ₃ carrier, and this tendency becomes more remarkable as the temperature increases.
From these results, it can be seen that by using the ZrSiYW composite oxide support as the acidic support, a higher HC purification rate than in the prior art can be obtained in a lean atmosphere. The same can be said about the ZrSiCeW composite oxide in which Y in the ZrSiYW composite oxide is substituted with Ce.

The exhaust purification catalyst according to this embodiment has the following effects.
In the present embodiment, a first catalyst in which a noble metal is supported on a carrier having a solid acidity as a noble metal / acid carrier, and a CePr composite oxide as a LNT and substantially an alkaline earth metal and an alkali metal. An exhaust purification catalyst was configured including a NOx adsorbent not included and a second catalyst having a noble metal. That is, the first catalyst and the second catalyst are mixed or stacked, and the NOx adsorbent constituting the second catalyst as the LNT does not contain an alkaline earth metal and an alkali metal. An oxide was included.
Here, the CePr composite oxide is obtained by substituting part of Ce in CeO ₂ with Pr, and this CePr composite oxide is less basic than alkali metals and alkaline earth metals. The electron density of the noble metal surface can be reduced without significantly affecting the electron withdrawing action of the acidic carrier. As a result, since the bonding force between the noble metal and oxygen can be reduced and the adsorption of oxygen to the surface of the noble metal can be suppressed, the purification rate of paraffin can be particularly improved, and a high HC purification rate can be obtained.
The CePr composite oxide has a high oxygen storage capacity (OSC) in addition to the NOx adsorption capacity, and releases active oxygen at a predetermined temperature or higher. Since this active oxygen is very reactive, according to this embodiment, oxidation purification of paraffin can be promoted even in a lean atmosphere, and the HC purification rate can be improved.

Further, the exhaust purification catalyst according to the present embodiment corresponds to the exhaust purification catalyst according to the above-mentioned application, in which part of Ce in CeO ₂ constituting the NOx adsorbent is replaced with Pr. Thus, even in a lean atmosphere of 300 ° C. or higher, the NOx adsorption rate can be improved while maintaining the HC purification rate. The reason is assumed to be as follows.
That is, NOx in the exhaust gas is oxidized from NO to NO ₂ by a noble metal and adsorbed on the NOx adsorbent by the spillover of NO ₂ . However, at 300 ° C. or higher, from the viewpoint of an equilibrium reaction, part of the oxidized NO ₂ is reduced to NO before being adsorbed. Therefore, sufficient adsorption performance cannot be obtained with CeO ₂ having a relatively weak adsorption force.
On the other hand, CePr composite oxide is superior to CeO ₂ in its ability to release active oxygen, and this active oxygen promotes the oxidation reaction of NO to NO ₂ . The reaction of NO → NO ₂ by active oxygen proceeds on the CePr composite oxide as the adsorbent. Thus, since NO is adsorbed to the CePr composite oxide immediately after being oxidized to NO ₂ , according to the present embodiment, a high NOx adsorption rate can be obtained even at a high temperature of 300 ° C. or higher.

Further, the NO ₂ oxidized on the noble metal is a reaction intermediate NO ₂ δ ⁻ , and the CePr composite oxide having an excellent active oxygen releasing ability has a function of increasing the ionization potential of the NOx adsorbent. As a result, the reaction intermediate NO ₂ δ ⁻ easily spills over the NOx adsorbent. As a result, the NOx adsorption rate is further improved.
As described above, according to the present embodiment, when the LNT and the noble metal / acidic carrier are supported on the same support, the NOx adsorption rate is improved while maintaining the HC purification rate, particularly in a lean atmosphere of 300 ° C. or higher. it can.

  In this embodiment, the ratio of the number of moles of Pr to the total number of moles of Ce and Pr in the CePr composite oxide is 5% to 30%. Thereby, it is possible to improve the NOx adsorption rate and the HC purification rate particularly in a lean atmosphere of 300 ° C. or higher.

  In this embodiment, the ratio of the number of moles of Pr to the total number of moles of Ce and Pr in the CePr composite oxide is 10% to 20%. Thereby, the NOx adsorption rate and the HC purification rate can be further improved particularly in a lean atmosphere of 300 ° C. or higher.

  In the present embodiment, the first catalyst layer is formed using the first catalyst, and the second catalyst layer is formed using the second catalyst adjacent to the first catalyst layer. As a result, the above-described effects are exhibited particularly at the interface between the first catalyst layer and the second catalyst layer.

In the present embodiment, the first catalyst layer is formed on the second catalyst layer. That is, the first catalyst layer was formed as an upper layer and the second catalyst layer was formed as a lower layer. Thereby, compared with the case where the first catalyst layer containing the noble metal / acidic carrier is used as the lower layer, the upper layer has better gas diffusibility, so that paraffin can be purified efficiently and the HC purification rate can be improved. Further, by using the first catalyst layer containing the noble metal / acidic carrier as the upper layer, NO in the exhaust is easily oxidized to NO ₂ and easily adsorbed to the LNT, so that the NOx purification rate is improved.

In this embodiment, at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide, ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica and zeolite is used as the carrier having solid acidity. Among these, at least one of ZrSiYW composite oxide and ZrSiCeW composite oxide was more preferably used.
The above-described acidic carrier has both high acidity and specific surface area, and can further reduce the electron density on the surface of the noble metal and further reduce the bonding force between the noble metal and oxygen. Therefore, according to this embodiment, the adsorption of oxygen to the surface of the noble metal can be further suppressed, and a higher HC purification rate can be obtained.

In this embodiment, Pt is used as a noble metal contained in each of the first catalyst and the second catalyst. Pt has particularly high oxidation performance, and can efficiently oxidize NO to NO ₂ , so that the above-described effects are more reliably exhibited.

  Further, the present embodiment provides an exhaust gas purification apparatus for an internal combustion engine that is operated with the air-fuel ratio being controlled to be lean and stoichiometric, and includes the above-described exhaust gas purification catalysts. Thereby, the same effect as the invention of the exhaust purification catalyst is exhibited.

  It should be noted that the present invention is not limited to the above-described embodiment, and modifications and improvements within the scope that can achieve the object of the present invention are included in the present invention.

  Next, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.

<Example 1>
(1) A mixed solution obtained by mixing 100 g of γ-Al ₂ O ₃ powder with 43.0 g of a 5% dinitrodiammine platinum solution and 400 g of ion-exchanged water is dried at 200 ° C. for 12 hours after removing water with a rotary evaporator. I let you. Then, by performing the calcination of 500 ° C. × 2 hours in a muffle furnace to obtain a 2.1 wt% Pt supported _Al 2 _{O 3} powder A.

(2) First, cerium nitrate and praseodymium nitrate were dissolved in pure water so that the molar ratio of Ce to Pr was Ce: Pr = 80%: 20%. Thereafter, an aqueous sodium hydroxide solution was added dropwise to adjust the pH of the solvent to 10 to obtain a precipitate. Then, the solvent was evaporated by filtering under reduced pressure the solution containing a precipitate at 60 degreeC. Next, the residue was extracted, and then calcined at 500 ° C. for 2 hours in a muffle furnace to obtain a CePr composite oxide (Ce: Pr = 80%: 20% (molar ratio)).
Next, 2.1 mass% Pt-supported Al ₂ O ₃ powder A19 g obtained in (1) above and CePr composite oxide obtained as described above (Ce: Pr = 80%: 20% (molar ratio)) 19 g of powder was mixed with 10 g of Al ₂ O ₃ sol (Al ₂ O ₃ 20% by mass), 60 g of alumina balls, and 80 g of water. Next, slurry B was obtained by wet pulverization with a ball mill for 15 hours.

  (3) The slurry B obtained in the above (2) is immersed in a cordierite honeycomb support (diameter φ25.4 mm × length L30 mm (capacity 15 cc), 900 cells / inch, 2.5 mils), and extra Slurry was removed by air jet. This operation was repeated until a predetermined amount was supported, and the honeycomb support that had reached the predetermined amount was fired in a muffle furnace at 500 ° C. for 2 hours to obtain a lower layer coat catalyst C. The lower layer coating amount was 200 g / L, and the Pt carrying amount was 2 g / L.

(4) First, zirconium nitrate and cerium nitrate were dissolved in pure water so that the molar ratio of Zr to Ce was Zr: Ce = 77%: 10%. Thereafter, sodium silicate containing a desired amount of Si was added. Subsequently, sodium hydroxide was added dropwise so that the pH was 10 as necessary, thereby obtaining a precipitate. Then, the solvent was evaporated by filtering under reduced pressure the solution containing a precipitate at 60 degreeC. Next, after extracting the residue, a ZrSiCe composite oxide was obtained by calcining at 500 ° C. for 2 hours in a muffle furnace.
Next, the ZrSiCe composite oxide obtained as described above was added to nitric acid to adjust the pH to 3, and then sodium metatungstate (Na ₂ WO ₄ ) containing a desired amount of W was added. Subsequently, sodium hydroxide was added dropwise so that the pH of the solution was 10, thereby obtaining a precipitate. Then, the solvent was evaporated by filtering under reduced pressure the solution containing a precipitate at 60 degreeC. Next, after extracting the residue, by carrying out calcination at 500 ° C. for 2 hours in a muffle furnace, ZrSiCeW composite oxide (Zr: Si: Ce: W = 77%: 3%: 10%: 10% ( Molar ratio)).
Next, 42.3 g of the ZrSiCeW composite oxide (Zr: Si: Ce: W = 77%: 3%: 10%: 10% (molar ratio)) obtained as described above was added to dinitrodiammine platinum 5%. The mixed solution mixed with 44.5 g of the solution and 400 g of ion-exchanged water was dried with a rotary evaporator at 200 ° C. for 12 hours. Subsequently, the powder D of 5.3 mass% Pt carrying | support ZrSiYW complex oxide was obtained by baking at 500 degreeC * 2 hours in a muffle furnace.

(5) The powder D19 g of 5.3 mass% Pt-supported ZrSiCeW composite oxide obtained in the above (4) is mixed with 5 g of Al ₂ O ₃ sol (Al ₂ O ₃ 20 mass%), 30 g of alumina balls, and 80 g of water. Then, slurry E was obtained by wet pulverization with a ball mill for 15 hours.

  (6) The slurry E obtained in (5) above was immersed in the lower layer coat catalyst C, and excess slurry was removed by air injection. This operation is repeated until a predetermined amount is supported, and the honeycomb support that has reached the predetermined amount is fired in a muffle furnace at 500 ° C. for 2 hours to purify the exhaust gas of Example 1 in which the upper layer coat F is formed. A catalyst was obtained. The upper layer coat amount was 50 g / L, and the Pt carrying amount was 5 g / L.

<Example 2>
(1) The same operation as in Example 1 is performed except that the CePr compounding ratio of the CePr composite oxide added to the slurry B of Example 1 is replaced with Ce: Pr = 90%: 10% (molar ratio). Thus, an exhaust purification catalyst of Example 2 was obtained.

<Example 3>
(1) The same operation as in Example 1 is performed except that the CePr compounding ratio of the CePr composite oxide added to the slurry B of Example 1 is replaced with Ce: Pr = 95%: 5% (molar ratio). Thus, an exhaust purification catalyst of Example 3 was obtained.

<Example 4>
(1) The same operation as in Example 1 is performed except that the CePr compounding ratio of the CePr composite oxide added to the slurry B of Example 1 is replaced with Ce: Pr = 70%: 30% (molar ratio). Thus, an exhaust purification catalyst of Comparative Example 6 was obtained.

<Example 5>
(1) Implementation was performed except that the composite oxide of the powder D of Example 1 was replaced with a ZrSiYW composite oxide (Zr: Si: Y: W = 77%: 3%: 10%: 10% (molar ratio)). By performing the same operation as in Example 1, the exhaust purification catalyst of Example 5 was obtained.
The ZrSiYW composite oxide (Zr: Si: Y: W = 77%: 3%: 10%: 10% (molar ratio)) was used in Example 1 except that cerium nitrate was changed to yttrium nitrate. A ZrSiCeW composite oxide (Zr: Si: Ce: W = 77%: 3%: 10%: 10% (molar ratio)) was prepared by the same preparation method.

<Comparative Example 1>
(1) The same operation as in Example 1 was performed except that the CePr composite oxide added to the slurry B of Example 1 was replaced with CeO ₂ powder (purity of 99.9% or more). An exhaust purification catalyst was obtained.

<Comparative example 2>
(1) The same operation as in Example 1 was performed except that the CePr composite oxide added to the slurry B of Example 1 was replaced with a CeZr composite oxide (Ce: Zr = 90%: 10% (molar ratio)). As a result, an exhaust purification catalyst of Comparative Example 2 was obtained.
In the CeZr composite oxide (Ce: Zr = 90%: 10% (molar ratio)), praseodymium nitrate was changed to zirconium nitrate, and the molar ratio of Ce and Zr was set to Ce: Zr = 90%: 10%. Except for the above, it was prepared by the same preparation method as the CePr composite oxide used in Example 1.

<Comparative Example 3>
(1) The same operation as in Example 1 was performed except that the CePr composite oxide added to the slurry B of Example 1 was replaced with a CeLa composite oxide (Ce: La = 90%: 10% (molar ratio)). As a result, an exhaust purification catalyst of Comparative Example 3 was obtained.
In addition, CeLa composite oxide (Ce: La = 90%: 10% (molar ratio)) was changed from praseodymium nitrate to lanthanum nitrate, and the molar ratio of Ce and La was set to Ce: La = 90%: 10%. Except for the above, it was prepared by the same preparation method as the CePr composite oxide used in Example 1.

<NOx purification rate evaluation 1>
Aging was performed on each exhaust purification catalyst obtained in Example 2 and Comparative Examples 1 to 3. Aging was performed under an oxygen-excess atmosphere at 750 ° C. for 20 hours (O ₂ = 10% by volume, H ₂ O = 6% by volume, remaining N ₂ balance gas).
After the aging, the NOx purification rate of each exhaust purification catalyst was evaluated. Specifically, the NOx average adsorption rates η300 ° C (%) and η400 ° C (%) for 1 minute at 300 ° C and 400 ° C were determined. In addition, all NOx adsorbed on the catalyst was desorbed by circulating a gas in a rich atmosphere before the measurement. The evaluation conditions for the NOx purification rate were as follows. The evaluation results are shown in Table 1.

[Evaluation conditions]
Model gas composition: CO = 0.3%, C ₃ H ₈ = 1200 ppmC, NO = 70 ppm,
O ₂ = 7%, CO ₂ = 8%, H ₂ O = 7%, N ₂ = balance gas linear velocity SV: 100,000 / hour Temperature range: 300 ° C., 400 ° C.

<HC purification rate evaluation 1>
About each exhaust purification catalyst obtained in Example 2 and Comparative Examples 1-3, after implementing the above-mentioned aging, evaluation was implemented about the HC purification rate of each exhaust purification catalyst. Specifically, the average purification rate η300 ° C. (%) at 300 ° C., the average purification rate η400 ° C. (%) at 400 ° C., and the average purification rate η300 ° C.-400 obtained by averaging the HC purification rates at 300 ° C. and 400 ° C. ° C (%) was determined. The evaluation conditions for the HC purification rate were as follows. The evaluation results are shown in Table 2.

[Evaluation conditions]
Model gas composition: CO = 0.3%, C ₅ H ₁₂ = 1200 ppmC, NO = 50 ppm, O ₂ = 7%, CO ₂ = 8%, H ₂ O = 7%, N ₂ = balance gas linear velocity SV: 100,000 / hour Temperature range: Room temperature to 450 ° C
Temperature increase rate: 20 ° C / min

  As shown in Table 1 and Table 2, it was found that Example 2 was superior to Comparative Example 1 and Comparative Example 2 in both NOx purification rate and HC purification rate. Further, Example 2 was slightly inferior to Comparative Example 3 at the NOx purification rate of 400 ° C., but Comparative Example 3 was inferior to Comparative Examples 1 and 2 in terms of the HC purification rate. From this result, it was confirmed that the NOx purification rate (adsorption rate) and the HC purification rate in the lean atmosphere can be improved by the exhaust purification catalyst according to the present invention.

<NOx purification rate evaluation 2>
For each exhaust purification catalyst obtained in Examples 1 and 5, aging and NOx purification rate were evaluated. The aging condition and the NOx purification rate evaluation condition were the same as the NOx purification rate evaluation 1 described above. The evaluation results are shown in Table 3.

<HC purification rate evaluation 2>
Each exhaust purification catalyst obtained in Examples 1 and 5 was evaluated for aging and HC purification rate. The aging conditions and the HC purification rate evaluation conditions were the same as the HC purification rate evaluation 1 described above. The evaluation results are shown in Table 4.

  As shown in Tables 3 and 4, it has been found that Example 1 has almost the same NOx purification rate as Example 5, but improved HC purification rate. From this result, it was confirmed that the ZrSiCeW composite oxide is preferable to the ZrSiYW composite oxide as the carrier having the solid acidity of the first catalyst.

<NOx purification rate evaluation 3>
For each exhaust purification catalyst obtained in Examples 1 to 4 and Comparative Example 1, the aging and the NOx purification rate were evaluated. The aging condition and the NOx purification rate evaluation condition were the same as the NOx purification rate evaluation 1 described above. The evaluation results are shown in Table 5 and FIG.

<HC purification rate evaluation 3>
For each exhaust purification catalyst obtained in Examples 1 to 4 and Comparative Example 1, aging and HC purification rates were evaluated. The aging condition and the HC purification rate evaluation condition were the same as the HC purification rate evaluation 1. The evaluation results are shown in Table 6 and FIG.

  As shown in Tables 5 and 6 and FIGS. 9 and 10, Examples 1 to 4 were found to have higher NOx purification rates and HC purification rates than Comparative Example 1. From this result, when the ratio of the number of moles of Pr to the total number of moles of Ce and Pr in the CePr composite oxide of the second catalyst is Ce: Pr = 95%: 5% to 70%: 30%, It was confirmed that high NOx purification rate and HC purification rate can be obtained.

Claims (9)

An exhaust purification catalyst for purifying exhaust gas from an internal combustion engine,
Comprising a first catalyst and a second catalyst supported on the same support,
The first catalyst has a carrier having solid acidity and a noble metal supported on the carrier,
The second catalyst includes an NOx adsorbent containing a CePr composite oxide and substantially free of an alkaline earth metal and an alkali metal, and a noble metal.   The exhaust purification catalyst according to claim 1, wherein a ratio of the number of moles of Pr to the total number of moles of Ce and Pr in the CePr composite oxide is 5% to 30%.   The exhaust purification catalyst according to claim 1, wherein a ratio of the number of moles of Pr to the total number of moles of Ce and Pr in the CePr composite oxide is 10% to 20%.   The first catalyst layer formed by the first catalyst, and the second catalyst layer formed by the second catalyst adjacent to the first catalyst layer. The exhaust gas purification catalyst according to crab.   The exhaust purification catalyst according to claim 4, wherein the first catalyst layer is formed on the second catalyst layer.   The support having solid acidity is at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide, ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica, and zeolite. The exhaust purification catalyst according to any one of 1 to 5.   The exhaust purification catalyst according to any one of claims 1 to 5, wherein the carrier having solid acidity is at least one of a ZrSiYW composite oxide and a ZrSiCeW composite oxide.   The exhaust purification catalyst according to any one of claims 1 to 7, wherein the first catalyst and the second catalyst have Pt as the noble metal.   An exhaust emission control device for an internal combustion engine that is operated while controlling an air-fuel ratio to be lean and stoichiometric, comprising the exhaust emission control catalyst according to any one of claims 1 to 8.

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