JP2010167381A - Exhaust gas cleaning catalyst - Google Patents

Abstract

A has an excellent catalytic activity for the reduction of NO _x, to provide an exhaust gas purifying catalyst containing oxygen-absorbing material.
The substrate includes a lower layer containing Rh and not containing an oxygen storage / release material, and an upper layer containing an oxygen storage / release material and Pd and / or Pt, and the upper layer is provided only on the upstream side of the exhaust gas. Thus, an exhaust gas purifying catalyst is provided.
[Selection] Figure 1

Description

  The present invention relates to an exhaust gas purification catalyst, and more particularly to an exhaust gas purification catalyst containing an oxygen storage / release material.

Conventionally, a three-way catalyst that simultaneously performs oxidation of carbon monoxide (CO) and hydrocarbon (HC) and reduction of nitrogen oxides (NO _x ) has been used as an exhaust gas purification catalyst for automobiles. As such a catalyst, a catalyst in which a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd) is supported on a porous oxide carrier such as alumina (Al ₂ O ₃ ) is widely known. Yes. Among these, Rh has high reduction activity of NO _x, which is an essential component in the exhaust gas purifying catalyst such as a three-way catalyst. In addition, in order to simultaneously and efficiently purify the three components of CO, HC, and NO _x by the action of the three-way catalyst, the air / fuel ratio (air-fuel ratio A / F) supplied to the engine of the automobile is theoretically reduced. It is important to control near the fuel ratio (stoichiometry).

However, since the actual air-fuel ratio fluctuates to the rich (excess fuel atmosphere) side or lean (fuel lean atmosphere) side with the stoichiometric centering on the driving conditions of the automobile, etc., the exhaust gas atmosphere is also on the rich side or lean side as well. fluctuate. Therefore, it is not always possible to ensure high purification performance with only a three-way catalyst. Therefore, in order to absorb the fluctuation of the oxygen concentration in the exhaust gas and enhance the exhaust gas purification ability of the three-way catalyst, oxygen is stored when the oxygen concentration in the exhaust gas is high, and oxygen is released when the oxygen concentration in the exhaust gas is low. An oxygen storage / release material such as ceria (CeO ₂ ) having a so-called oxygen storage capacity (OSC capacity) is used in an exhaust gas purification catalyst.

  In Patent Document 1, first and second exhaust purification catalysts are provided on the upstream side and the downstream side of the engine exhaust passage, respectively, and the content of ceria in the first catalyst is less than the content of ceria in the second catalyst. Thus, a catalytic converter device for an internal combustion engine is described in which the oxygen storage capacity of the first catalyst is smaller than the oxygen storage capacity of the second catalyst.

Patent Document 2 has an HC selective oxidation function that is provided in an exhaust passage of an internal combustion engine and selectively oxidizes HC over CO, and has a CO storage function that stores CO under a reducing atmosphere, and oxygen is stored under an oxidizing atmosphere. A three-way catalytic converter is described in which the oxygen storage capacity to be stored is lower than the CO storage capacity by the CO storage function, and further from an upstream part having a low oxygen storage capacity by the oxygen storage function and a downstream part having a high oxygen storage capacity A structured three-way catalytic converter is described. Further, in Patent Document 2, according to such a three-way catalytic converter, even during the rich air-fuel ratio operation, the amount of stored oxygen released from the upstream is small, so an excessive lean atmosphere is not generated, and NO _x spikes are generated. Is described as being able to be prevented well.

  In Patent Document 3, in an HC adsorption catalyst having a carrier and an HC adsorbent, an oxygen storage agent and a noble metal catalyst supported on the carrier, the oxygen storage agent moves from the inlet side to the outlet side of the HC adsorption catalyst. Thus, an HC adsorption catalyst supported so as to increase the supported concentration is described.

Japanese Patent No. 2623926 JP 2004-1000049 A JP 2006-006995 A

Rh is used as an essential component in the three-way catalyst because of its high reduction activity for NO _x . However, Rh is a rare metal that has a particularly small reserve and production volume compared to other noble metals, that is, Pt and Pd, and is expensive. For this reason, in the three-way catalyst, it is an important issue to effectively use Rh in a smaller amount. However, when Rh is exposed to a lean atmosphere containing excess oxygen, an oxide is formed and metallization becomes insufficient, and as a result, the performance of the catalyst may deteriorate.

  On the other hand, the oxidized Rh can be recovered from an oxidized state to a highly active metal state by being exposed to a rich atmosphere with excess fuel. However, in the three-way catalyst including the oxygen storage / release material described above, when the atmosphere in the exhaust gas is switched from lean to rich, HC and CO, which are Rh reducing species, are released by oxygen released from the oxygen storage / release material. As a result, there are cases where these reducing gases cannot be effectively used for the reduction of Rh.

Accordingly, the present invention provides a novel structure has excellent catalytic activity for the reduction of NO _x, and an object thereof is to provide an exhaust gas purifying catalyst containing oxygen-absorbing material.

The present invention for solving the above problems is as follows.
(1) On a base material, it has a lower layer containing Rh and not containing an oxygen storage / release material, and an upper layer containing an oxygen storage / release material and Pd and / or Pt, and the upper layer is provided only on the exhaust gas upstream side. An exhaust gas purifying catalyst characterized by the above.
(2) The exhaust gas purifying catalyst according to (1), wherein the upper layer is further provided on the exhaust gas downstream side, and has a region not including the upper layer between the exhaust gas upstream side and the exhaust gas downstream side.
(3) The exhaust gas purifying catalyst as described in (1) or (2) above, wherein the oxygen storage / release material contains an oxide containing ceria or ceria-zirconia as a main component.
(4) The amount of ceria in the upper layer on the exhaust gas upstream side is 5 to 50 g / L with respect to the base material, and the upper layer on the exhaust gas upstream side is coated in a range of 10 to 40% of the total length of the base material. The exhaust gas-purifying catalyst as described in (3) above, wherein
(5) The amount of ceria in the upper layer on the exhaust gas downstream side is 10 to 100 g / L with respect to the substrate, and the upper layer on the exhaust gas downstream side is coated in a range of 10 to 40% of the total length of the substrate. The exhaust gas-purifying catalyst as described in (3) or (4) above,

According to the exhaust gas purifying catalyst of the present invention, the layer containing the oxygen storage / release material is provided only on the exhaust gas upstream side of the catalyst, so that the exhaust gas purifying catalyst including the oxygen storage / release material is contained in a rich atmosphere. In this case, the amount of oxygen released from the oxygen storage / release material can be reduced. As a result, to suppress the oxidation of HC and CO is a reducing species Rh, it is possible to increase the reducing activity of the NO _x to promote metalation of Rh. Further, by providing a layer containing an oxygen storage / release material further on the exhaust gas downstream side of the catalyst, and additionally providing a region not containing the oxygen storage / release material between the exhaust gas upstream side and the exhaust gas downstream side, NO as described above. _In addition to enhancing the reduction activity of _x , unburned HC and CO in the exhaust gas can be effectively oxidized by oxygen released from the oxygen storage / release material downstream of the exhaust gas. Therefore, according to such an exhaust gas purifying catalyst, it is possible to achieve high NO _x reduction activity and HC and CO oxidation activity as a whole catalyst.

It is a schematic diagram which shows the cross section of the catalyst layer in the exhaust gas purification catalyst of this invention. It is a schematic diagram which shows the catalyst layer cross section of the catalyst for exhaust gas purification by the preferable aspect of this invention. 6 is a graph showing the relationship between the upstream upper layer ceria amount (g / base material-L) and the NO _x concentration (ppm) in exhaust gas for each exhaust gas purifying catalyst of Example 2. FIG. 6 is a graph showing the relationship between the upstream upper layer coat length (%) and the NO _x concentration (ppm) in exhaust gas for each exhaust gas purifying catalyst of Example 3. 6 is a graph showing the relationship between the amount of ceria (g / base material-L) in the upper layer on the downstream side and the NO _x concentration (ppm) in the exhaust gas for each exhaust gas purifying catalyst of Example 4. 6 is a graph showing the relationship between the downstream upper layer coat length (%) and the CO concentration (ppm) in exhaust gas for each exhaust gas purifying catalyst of Example 5.

  The exhaust gas purifying catalyst of the present invention has, on a base material, a lower layer containing Rh and not containing an oxygen storage / release material, and an upper layer containing an oxygen storage / release material and Pd and / or Pt, the upper layer being an exhaust gas. It is characterized by being provided only on the upstream side.

  FIG. 1 is a schematic view showing a cross section of a catalyst layer in an exhaust gas purifying catalyst of the present invention. The exhaust gas purifying catalyst 10 of the present invention includes a lower layer 12 and an upper layer 13 as a catalyst layer on a substrate 11, has a region not including the upper layer 13 in a downstream portion of the catalyst, and further the lower layer 12 uses Rh as a catalyst metal. And the upper layer 13 contains Pd and / or Pt as the oxygen storage / release material and the catalyst metal.

When Rh is exposed to an oxygen-excess lean atmosphere, an oxide is formed and metallization becomes insufficient, and as a result, the performance of the catalyst may deteriorate. According to the present invention, by providing a layer containing an oxygen storage / release material only on the exhaust gas upstream side of the catalyst, the oxygen storage / release material in a rich atmosphere as compared with the exhaust gas purification catalyst including the oxygen storage / release material throughout the catalyst. The amount of oxygen released from can be reduced. As a result, it is considered that the reduction activity of NO _x can be enhanced by suppressing the oxidation of HC and CO, which are reducing species of Rh, and promoting the metalation of Rh.

According to the present invention, the substrate is not particularly limited, but any material generally used in exhaust gas purifying catalysts can be used. Specifically, a honeycomb-shaped material having a large number of cells can be used as the base material, such as cordierite (2MgO · 2Al ₂ O ₃ · 5SiO ₂ ), alumina, zirconia, silicon carbide, etc. A ceramic material having heat resistance or a metal material made of a metal foil such as stainless steel can be used.

  According to the present invention, as the catalyst carrier constituting the lower layer of the catalyst layer coated on the substrate, a porous carrier generally used as a catalyst carrier and not containing an oxygen storage / release material should be used. Can do. Preferred catalyst supports include metal oxides selected from the group consisting of zirconia, alumina, titania and combinations thereof.

  In the exhaust gas purifying catalyst of the present invention, Rh is supported as a catalyst metal on the lower layer containing the catalyst carrier. Note that the catalyst metal supported in the lower layer is not limited to Rh alone, but may support other metals such as Pd and Pt in addition to Rh. However, when Rh is used in the same catalyst layer as other metals, particularly Pt, it is generally possible that Rh and Pt react and partially alloy at high temperatures, reducing the activity of the catalyst. Are known. Therefore, in the exhaust gas purifying catalyst of the present invention, it is more preferable to support only Rh in the lower layer.

According to the present invention, the upper layer of the catalyst layer includes an oxygen storage / release material. The oxygen storage / release material is not particularly limited as long as it is a material having a so-called oxygen storage capacity (OSC capacity), but is preferably oxidized mainly with ceria (CeO ₂ ) or ceria-zirconia (CeO ₂ —ZrO ₂ ). Things can be used. The expression “main component” used in the present invention means that ceria or ceria-zirconia occupies more than 50% of the total mass of the material constituting the oxygen storage / release material.

The oxygen storage / release material according to the present invention may further include an additional oxide different from the oxides made of ceria or ceria-zirconia. For example, the oxygen storage / release material may further include an oxide of at least one metal selected from the group consisting of alkaline earth metals and rare earth elements. By adding such an additional oxide, the heat resistance of the oxide made of ceria or ceria-zirconia can be improved. Specific examples of such additional oxides include lanthanum oxide (La ₂ O ₃ ), yttria (Y ₂ O ₃ ), praseodymium oxide (Pr ₆ O ₁₁ ), neodymium oxide (Nd ₂ O ₃ ) and A combination of these is exemplified.

  As the above complex oxide, any complex oxide commercially available as an oxygen storage / release material can be used. Alternatively, such a composite oxide can be prepared by any method known to those skilled in the art. For example, a mixed oxide in which an alkaline substance such as aqueous ammonia is added to a mixed solution in which a salt of each metal constituting the complex oxide is dissolved and co-precipitated, and then heat-treated to form a complex oxide in which each metal oxide is dissolved. Can be prepared.

According to the present invention, Pd and / or Pt is supported as a catalyst metal on the upper layer of the catalyst layer. In general, CO is known to be more reducible than HC, and therefore it is preferable to contain a large amount of CO as a reducing species in order to metallize oxidized Rh. According to the configuration of the present invention, as described above, the amount of oxygen released from the oxygen storage / release material upstream of the exhaust gas in a rich atmosphere can be reduced, so that it depends on Pd and / or Pt supported on the upper layer. Oxidation of HC in the exhaust gas is also suppressed, that is, complete oxidation of HC in the exhaust gas to CO ₂ is suppressed, and HC can be selectively converted to highly reducible CO. The exhaust gas purifying catalyst of the present invention, the CO generated in this way, the metal of Rh is accelerated, it is believed to improve the reducing activity of the NO _x.

On the other hand, if the fuel-rich atmosphere continues for a long time, HC in the exhaust gas may adhere to Rh and reduce its activity. Regarding such Rh poisoning by HC, according to the exhaust gas purifying catalyst of the present invention, when the atmosphere in the exhaust gas is switched from rich to lean, oxygen absorbed by the oxygen storage / release material upstream of the catalyst is reduced. Since the amount is small, it is considered that oxygen not absorbed by the oxygen storage / release material oxidizes and removes HC adhering to Rh downstream thereof. No Without intending to be bound by any particular theory, the exhaust gas purifying performance of the exhaust gas purifying catalyst of the present invention, particularly with improved reduction activity of the NO _x is, the atmosphere in the exhaust gas from the lean Oxidation of HC and CO, which are reducing species of Rh, was suppressed when switching to rich, and recovery from HC poisoning of Rh when the atmosphere in exhaust gas was switched from rich to lean It is thought to be due to.

In addition to the configuration of the present invention described above, the inventor further provides a layer containing an oxygen storage / release material on the exhaust gas downstream side of the catalyst, and includes the oxygen storage / release material between the exhaust gas upstream side and the exhaust gas downstream side. by providing the free area, as well as increasing the reduction activity of the nO _x, as described above, effectively oxidized by oxygen released unburned HC and CO in the exhaust gas from the oxygen-absorbing material of the exhaust gas downstream side Found that you can.

  FIG. 2 is a schematic view showing a cross section of the catalyst layer of the exhaust gas purifying catalyst according to a preferred embodiment of the present invention. The exhaust gas purifying catalyst 10 of the present invention includes a lower layer 12 and an upper layer 13 as a catalyst layer on a base material 11, has a region not including the upper layer 13 in a middle portion of the catalyst, and further the lower layer 12 uses Rh as a catalyst metal. And the upper layer 13 contains Pd and / or Pt as the oxygen storage / release material and the catalyst metal.

Thus, by providing the upper layer containing the oxygen storage / release material further on the exhaust gas downstream side, it is possible to effectively oxidize HC and CO remaining without being sufficiently purified on the upstream side. Therefore, according to such an exhaust gas purifying catalyst, it is possible to simultaneously achieve high NO _x reduction activity and HC and CO oxidation activity as a whole catalyst.

According to a preferred embodiment of the present invention, the oxide containing ceria or ceria-zirconia as a main component has an amount of ceria in the upper layer on the exhaust gas upstream side of 5 to 50 g / L, more preferably 5 to 40 g. It is coated on the lower layer containing Rh in an amount such that / L. Moreover, it is preferable that the upper layer containing the ceria amount in the above range is coated on the exhaust gas upstream side in the range of 10 to 40% of the total length of the base material. By coating the exhaust gas upstream side with an oxide containing ceria or ceria-zirconia as the main component in these ranges, the Rh metallization under the rich atmosphere described above, particularly in the middle part of the catalyst, and under the lean atmosphere especially recovery from HC poisoning of Rh in the catalyst middle portion is promoted, as a result, it is possible to remarkably improve _the NO _x reduction activity of the resulting catalyst for purification of exhaust gas of.

  According to a preferred embodiment of the present invention, the oxide containing ceria or ceria-zirconia as a main component is Rh in such an amount that the amount of ceria in the upper layer on the exhaust gas downstream side is 10 to 100 g / L with respect to the base material. Is coated on the lower layer containing Moreover, it is preferable that the upper layer containing the amount of ceria in the above range is coated on the exhaust gas downstream side in a range of 10 to 40% of the total length of the base material. In these ranges, the oxides mainly composed of ceria or ceria-zirconia are coated on the exhaust gas downstream side, so that unburned HC and CO in the exhaust gas can be reduced by oxygen released from the oxygen storage / release material on the exhaust gas downstream side. It can be oxidized particularly effectively.

  The exhaust gas purifying catalyst of the present invention having a catalyst layer including an upper layer and a lower layer on a substrate can be produced by any method known to those skilled in the art.

  For example, first, a layer containing a catalyst carrier and Rh is coated on a honeycomb substrate such as cordierite by a known wash coat method or the like, and then dried and fired to form a lower layer on the substrate. In addition, when forming the lower layer containing the catalyst carrier and Rh using the wash coat method, for example, after forming the catalyst carrier layer by the wash coat method, the obtained catalyst carrier layer is subjected to Rh by a known impregnation method or the like. Alternatively, a wash coat may be performed using a catalyst carrier powder on which Rh is previously supported by an impregnation method or the like. According to the latter method, Rh can be more uniformly dispersed and supported in the catalyst layer as compared with the case where Rh is impregnated and supported after wash coating.

  Next, an upper layer containing an oxygen storage / release material and Pd and / or Pt is formed on the obtained lower layer. In particular, in the case of the exhaust gas purifying catalyst of the present invention having an upper layer in each of the exhaust gas upstream portion and the exhaust gas downstream portion of the base material and having an area not including the upper layer therebetween, for example, first, it is formed first. On the lower layer, the catalyst carrier constituting the upper layer on the upstream side, that is, the slurry containing the oxygen absorbing / releasing material is introduced from the upper part of the base material by the wash coat method and sucked from the lower part, so that it is predetermined on the upstream side. Next, the slurry containing the oxygen storage / release material constituting the upper layer on the downstream side can be similarly coated on the predetermined range on the downstream side. In addition, when forming the upper layer containing the oxygen storage / release material and Pd and / or Pt using the wash coat method, for example, the upper layer of the oxygen storage / release material in the upstream portion and the downstream portion of the substrate is formed as described above. After that, Pd and / or Pt may be supported on each of the obtained upper layers by a known impregnation method or the like, or alternatively, an oxygen storage / release material powder that has previously supported Pd and / or Pt by an impregnation method or the like. You may perform a wash coat using. According to the latter method, Pd and / or Pt can be more uniformly dispersed and supported in the upper layer as compared with the case of impregnating and supporting Pd and / or Pt after wash coating. As another method, separate honeycomb base materials are prepared for the upstream portion, the midstream portion, and the downstream portion of the base material, respectively, and the upstream portion, the midstream portion, and the Each catalyst layer constituting the downstream portion may be formed and disposed in the upstream portion, the midstream portion, and the downstream portion of the exhaust gas.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples.

In this embodiment, it comprises Rh in the lower layer, ceria in the upper layer of the upstream side as Pd and oxygen-absorbing material - to prepare a catalyst for purification of exhaust gas containing an oxide mainly composed of zirconia, examined for its _the NO _x purification performance It was.

[Example 1]
First, zirconia powder (ZrO ₂ : 90 wt%, Y ₂ O ₃ : 10 wt%) made by Daiichi Rare Element Chemical Industry was impregnated and supported with Rh using a rhodium nitrate solution, and this was supported by γ-alumina (Sasole) Al ₂ O ₃ : 96 wt%, La ₂ O ₃ : 4 wt%) was mixed at a weight ratio of 9: 1 (mixed powder A). Next, this mixed powder A was mixed with an alumina binder at a weight ratio of 19: 1, and water was added to prepare a slurry for coating. Next, the obtained slurry was coated on a Denso ceramic honeycomb substrate (φ 103 mm, L 105 mm, wall thickness 3 mil, 600 cells / in 2) by a wash coat method, and then dried and fired to prepare honeycomb substrate cells. A lower layer having a coating amount of 150 g / base material-L was formed on the surface. The amount of Rh supported was 0.07 g.

Next, ceria-zirconia powder (produced by Rhodia) as an oxygen storage / release material (CeO ₂ : 50 wt%, ZrO ₂ : 40 wt%, Y ₂ O ₃ : 3.5 wt%, La ₂ O ₃ : 3.5 wt%) ) Was impregnated with Pd using a palladium nitrate solution and mixed with γ-alumina (Al ₂ O ₃ : 96 wt%, La ₂ O ₃ : 4 wt%) manufactured by Sasol in a weight ratio of 9: 1. (Mixed powder B). Next, this mixed powder B was mixed with an alumina binder at a weight ratio of 19: 1, and water was added to prepare a slurry for coating. Next, a predetermined amount of the obtained slurry is introduced from above into the above honeycomb substrate on which the lower layer is formed, and sucked from the lower portion, so that 30% of the total length of the substrate is formed upstream of the substrate. Coated over range. Thereafter, this was dried and fired to obtain an exhaust gas purifying catalyst in which an upper layer (ceria amount 30 g / base material-L) was formed on the lower layer on the upstream side of the base material. The amount of Pd supported was 0.5 g.

[Comparative Example 1]
Except that the upper layer was formed over the entire surface of the lower layer, an exhaust gas purifying catalyst having the upper layer (ceria amount 80 g / base material-L) formed on the lower layer was obtained in the same manner as in Example 1. The amount of Pd supported was 0.5 g, the same as in Example 1.

[Comparative Example 2]
Exhaust gas purification catalyst in which a lower layer containing Rh was formed on the cell surface of the honeycomb substrate was obtained in the same manner as in Example 1 except that the upper layer was not formed.

[Evaluation of NO _x purification performance]
Each exhaust gas purification catalyst prepared above is subjected to an endurance test using an actual engine in order to confirm whether the catalytic activity is maintained even after a certain amount of travel. We were examined for _the NO _x purification performance. The durability test is performed by adjusting the engine speed / torque etc. so that the temperature of each exhaust gas purification catalyst becomes 950 ± 20 ° C., and repeating the air-fuel ratio A / F between 14 and 15 for a certain period of time. It was.

Each exhaust gas purifying catalyst subjected to the durability test was attached to the engine, and the engine speed / torque was adjusted so that the temperature at a position 10 mm before the catalyst was 500 ° C. The air-fuel ratio was maintained at A / F = 15 for 3 minutes, then switched to A / F = 14 and maintained for 120 seconds. The NO _x purification performance of each exhaust gas purification catalyst was evaluated by measuring the NO _x concentration in the exhaust gas on the catalyst outlet side 100 seconds after switching the A / F from 15 to 14. The results are shown in Table 1.

As is clear from Table 1, the exhaust gas purifying catalyst of Example 1 that contains a smaller amount of oxygen storage / release material only on the upstream side of the exhaust gas, and Comparative Example 1 that includes the oxygen storage / release material over the entire surface of the catalyst layer. Compared with the exhaust gas purifying catalysts of Comparative Example 2 that did not contain the release material, the NO _x concentration in the exhaust gas was greatly reduced.

[Example 2]
In this embodiment, an exhaust gas having an upper layer containing an oxygen storage / release material on both the exhaust gas upstream side and the exhaust gas downstream side of the catalyst, and a region not including the oxygen storage / release material provided between the exhaust gas upstream side and the exhaust gas downstream side the purification catalyst was prepared and tested for their _the NO _x purification performance.

First, zirconia powder (ZrO ₂ : 90 wt%, Y ₂ O ₃ : 10 wt%) made by Daiichi Rare Element Chemical Industry was impregnated and supported with Rh using a rhodium nitrate solution, and this was supported by γ-alumina (Sasole) Al ₂ O ₃ : 96 wt%, La ₂ O ₃ : 4 wt%) was mixed at a weight ratio of 9: 1 (mixed powder A). Next, this mixed powder A was mixed with an alumina binder at a weight ratio of 19: 1, and water was added to prepare a slurry for coating. Next, the obtained slurry was coated on a Denso ceramic honeycomb substrate (φ 103 mm, L 105 mm, wall thickness 3 mil, 600 cells / in 2) by a wash coat method, and then dried and fired to prepare honeycomb substrate cells. A lower layer having a coating amount of 150 g / base material-L was formed on the surface. The amount of Rh supported was 0.07 g.

Next, ceria-zirconia powder (produced by Rhodia) as an oxygen storage / release material (CeO ₂ : 50 wt%, ZrO ₂ : 40 wt%, Y ₂ O ₃ : 3.5 wt%, La ₂ O ₃ : 3.5 wt%) ) Was impregnated with Pd using a palladium nitrate solution and mixed with γ-alumina (Al ₂ O ₃ : 96 wt%, La ₂ O ₃ : 4 wt%) manufactured by Sasol in a weight ratio of 9: 1. (Mixed powder B). Next, this mixed powder B was mixed with an alumina binder at a weight ratio of 19: 1, and water was added to prepare a slurry for coating. Next, a predetermined amount of the obtained slurry is introduced from above into the above honeycomb substrate on which the lower layer is formed, and sucked from the lower portion, so that 30% of the total length of the substrate is formed upstream of the substrate. Coated over range. Then, this was dried and baked to form an upper layer having an amount of ceria of 2 to 100 g / substrate-L on the lower layer on the upstream side of the substrate. The amount of Pd supported was 0.5 g.

  Next, in the same manner as above, the upper layer was coated on the downstream side of the base material over a range of 20% of the total length of the base material. Thereafter, this was dried and fired to obtain an exhaust gas purifying catalyst in which an upper layer (ceria amount 50 g / base material-L) was formed on the lower layer on the downstream side of the base material. The amount of Pd supported was 0.3 g.

Each exhaust gas purification catalyst prepared in Example 2 was subjected to the same durability test as described above, and the NO _x purification performance of each exhaust gas purification catalyst after the durability test was examined. The NO _x purification performance of each exhaust gas purification catalyst was evaluated by the same method as described above. The result is shown in FIG.

FIG. 3 is a graph showing the relationship between the upstream upper layer ceria amount (g / base material-L) and the NO _x concentration (ppm) in the exhaust gas for each exhaust gas purifying catalyst of Example 2. In FIG. 4, the horizontal axis indicates the amount of ceria, and the vertical axis indicates the NO _x concentration.

Referring to FIG. 3, for example, in a catalyst having a ceria amount of 80 g / base material-L, the NO _x concentration in the exhaust gas is 242 ppm, and the oxygen absorption / release material is included in the entire surface of the catalyst layer with the same ceria amount. It can be seen that the NO _x purification performance is improved compared to 360 ppm in Example 1. In addition, particularly high NO _x purification performance could be obtained in an exhaust gas purification catalyst having a ceria amount of about 5 to 40 g / base material-L.

[Example 3]
In this example, for the catalyst having a ceria amount of 20 g / base material-L in which high NO _x purification performance was obtained in Example 2, the coat length of the upper layer on the exhaust gas upstream side was 5 to 50% of the total length of the base material. The exhaust gas purifying catalyst varied between the two was prepared, and its NO _x purification performance was examined. The NO _x purification performance of each exhaust gas purification catalyst was evaluated by the same method as described above. The result is shown in FIG.

FIG. 4 is a graph showing the relationship between the upstream upper layer coat length (%) and the NO _x concentration (ppm) in the exhaust gas for each exhaust gas purifying catalyst of Example 3. In FIG. 4, the horizontal axis shows the length of the upper layer coat on the upstream side with respect to the total length of the substrate, and the vertical axis shows the NO _x concentration. From the results of FIG. 4, it was possible to obtain particularly high NO _x purification performance in the exhaust gas purification catalyst in which the upper layer on the upstream side was coated in the range of 10 to 40% with respect to the total length of the base material.

[Example 4]
In this example, the ceria amount in the upper layer on the upstream side and the coating length relative to the total length of the base material are fixed to 10 g / base material-L and 30%, respectively, and the amount of ceria in the upper layer on the downstream side is changed to purify each exhaust gas. A catalyst was prepared, and its CO purification performance was examined. The coating length with respect to the entire length of the base material of the upper layer on the downstream side was 30%. Other conditions such as the amount of Pd supported were the same as in Examples 2 and 3.

  Each catalyst was subjected to the same durability test as described above. Each catalyst after the durability test was attached to the engine, and the engine speed / torque was adjusted so that the temperature at a position 10 mm before the catalyst was 500 ° C. The air-fuel ratio was maintained at A / F = 15 for 3 minutes, then switched to A / F = 14 and maintained for 120 seconds. Next, it was further switched to A / F = 15 and held for 120 seconds. Note that the CO purification performance of each exhaust gas purification catalyst was evaluated by measuring the CO concentration in the exhaust gas on the catalyst outlet side 60 seconds after the A / F was switched to 15. The result is shown in FIG.

FIG. 5 is a graph showing the relationship between the amount of ceria (g / base material-L) in the downstream upper layer and the NO _x concentration (ppm) in the exhaust gas for each exhaust gas purifying catalyst of Example 4. FIG. 5 shows the amount of ceria on the horizontal axis and the NO _x concentration on the vertical axis. Referring to FIG. 5, by including an oxygen storage / release material in the downstream catalyst layer, the CO purification performance of the catalyst is greatly improved, and in particular, the exhaust gas whose downstream upper layer has a ceria amount of about 10 to 100 g / base material-L. High CO _x purification performance could be obtained in the purification catalyst.

[Example 5]
In this example, for the catalyst having a ceria amount of 50 g / base material-L in the downstream upper layer where high CO purification performance was obtained in Example 4, the coating length of the upper layer on the exhaust gas downstream side is 5 to 5 times the total length of the base material. An exhaust gas purification catalyst that was varied between 50% was prepared, and its CO purification performance was examined. The CO purification performance of each exhaust gas purification catalyst was evaluated by the same method as described in Example 4. The result is shown in FIG.

  FIG. 6 is a graph showing the relationship between the downstream upper layer coat length (%) and the CO concentration (ppm) in the exhaust gas for each exhaust gas purifying catalyst of Example 5. In FIG. 6, the horizontal axis indicates the length of the upper layer coat on the downstream side with respect to the total length of the substrate, and the vertical axis indicates the CO concentration. From the results of FIG. 6, it was possible to obtain particularly high CO purification performance in the exhaust gas purification catalyst in which the upper layer on the downstream side was coated in the range of 10 to 40% with respect to the total length of the base material.

10 Exhaust gas purifying catalyst 11 Base material 12 Lower layer 13 Upper layer

Claims (5)

  It has a lower layer containing Rh and not containing an oxygen storage / release material and an upper layer containing an oxygen storage / release material and Pd and / or Pt on the base material, and the upper layer is provided only on the exhaust gas upstream side. An exhaust gas purifying catalyst that is characterized.   The exhaust gas purification catalyst according to claim 1, wherein the upper layer is further provided on the exhaust gas downstream side, and has a region not including the upper layer between the exhaust gas upstream side and the exhaust gas downstream side.   The exhaust gas purifying catalyst according to claim 1 or 2, wherein the oxygen storage / release material contains an oxide mainly composed of ceria or ceria-zirconia.   The amount of ceria in the upper layer on the exhaust gas upstream side is 5 to 50 g / L with respect to the base material, and the upper layer on the exhaust gas upstream side is coated in a range of 10 to 40% of the total length of the base material. The exhaust gas-purifying catalyst according to claim 3, wherein the catalyst is an exhaust gas-purifying catalyst.   The amount of ceria in the upper layer on the exhaust gas downstream side is 10 to 100 g / L with respect to the substrate, and the upper layer on the exhaust gas downstream side is coated in a range of 10 to 40% of the total length of the substrate. The exhaust gas-purifying catalyst according to any one of claims 3 and 4, wherein the exhaust gas-purifying catalyst is characterized by the following.

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