JP2010179200A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new catalyst for cleaning exhaust gas, wherein problems of the conventional techniques are solved. <P>SOLUTION: The catalyst for cleaning exhaust gas includes a carrier base material S, a palladium-deposited layer Pd, a low-OSC (oxygen storage/release capacity) rhodium-deposited layer Rh(low-OSC) and a high-OSC rhodium-deposited layer Rh(high-OSC). The concentration of the palladium deposited on the palladium-deposited layer is made higher than that of the palladium deposited on each of the low-OSC and high-OSC rhodium-deposited layers, and the concentration of the rhodium deposited on each of the low-OSC and high-OSC rhodium-deposited layers is made higher than that of the rhodium deposited on the palladium-deposited layer. The OSC of the high-OSC rhodium-deposited layer is made higher than those of other layers. On the carrier base material, the low-OSC rhodium-deposited layer is arranged at least partially on the downstream side of the palladium-deposited layer in an exhaust gas stream and the high-OSC rhodium-deposited layer is arranged at least partially on the downstream side of the low-OSC rhodium-deposited layer in the exhaust gas stream. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst, particularly an exhaust gas purification catalyst used for purifying exhaust gas from an internal combustion engine such as an automobile.

The exhaust gas from an internal combustion engine such as an automobile engine includes nitrogen oxide (NO _x ), carbon monoxide (CO), hydrocarbon (HC), and the like. These substances, at the same time to oxidize CO and HC, can be removed by an exhaust gas purifying catalyst for reducing NO _x. As a representative exhaust gas purification catalyst, a group consisting of a porous oxide carrier such as alumina, a noble metal such as platinum, rhodium and palladium, and an alkali metal and alkaline earth metal element such as barium and potassium. A NO _x storage reduction catalyst obtained by supporting a more selected NO _x storage material is known.

In the use of the NO _x storage reduction catalyst, NO _x is stored when the air-fuel ratio is in the fuel lean state, and when the air-fuel ratio is in the stoichiometric to fuel rich state, for example, when a rich spike is performed at regular intervals, The occluded NO _x is reduced to N ₂ . Such a NO _x storage reduction catalyst has the advantage of enabling exhaust gas purification in a lean bar engine.

However, for the use of such NO _{x storage-and-reduction} catalysts, in Patent Document 1, NO _x when reducing the occluded NO _x in storage material, _the NO _x storage material has been NO _x is all released into the gas phase from problem is discharged portion without being completely clean, and although the platinum is an effective noble metal _the NO _x storage-reduction reactions, have found that there is a problem of poor heat resistance at a lean atmosphere.

Patent Document 1 proposes that rhodium be supported at a high concentration on the downstream side of the exhaust gas stream in relation to the problem that all of the NO _x released from the NO _x storage material into the gas phase cannot be completely purified and is partially discharged. Yes. Patent Document 1 proposes that platinum and palladium are supported at a high concentration upstream of the exhaust gas stream in relation to the problem that platinum is inferior in heat resistance in a lean atmosphere.

JP 2006231120 A

As described in Patent Document 1, in recent years, it has been proposed to use rhodium for purification of exhaust gas from automobiles and the like. However, when rhodium is used as an exhaust gas purification catalyst in a fuel-lean atmosphere, there is a problem that rhodium is in an oxidized state, thereby reducing its activity as a catalyst. Further, the rhodium that has been oxidized in this way is difficult to be immediately reduced when the fuel lean atmosphere is changed to the fuel rich atmosphere, and therefore, NO _x cannot be sufficiently purified immediately after switching. there were.

  Therefore, the present invention provides a novel exhaust gas purification catalyst that solves such problems.

The present inventors have found that supplying hydrogen to oxidized rhodium promotes the reduction of rhodium, and have arrived at the present invention as follows:
<1> It has a support substrate, and a palladium support layer, a low OSC ability rhodium support layer, and a high OSC ability rhodium support layer disposed on the support substrate;
The palladium carrying concentration in the palladium carrying layer is higher than the palladium carrying concentration in the low OSC ability rhodium carrying layer and the high OSC ability rhodium carrying layer, and the rhodium carrying in the low OSC ability rhodium carrying layer and the high OSC ability rhodium carrying layer. The concentration is higher than the rhodium loading concentration in the palladium loading layer;
The OSC capacity in the high OSC capacity rhodium support layer is greater than the OSC capacity in the palladium support layer and the low OSC capacity rhodium support layer;
On the support substrate, the low OSC-capable rhodium support layer is disposed at least partially downstream of the exhaust gas flow from the palladium support layer; and on the support substrate,
(A) the high OSC capacity rhodium support layer is disposed at least partially downstream of the low OSC capacity rhodium support layer, and / or (b) the low OSC capacity rhodium support layer; A high OSC capacity rhodium support layer is laminated at least partially, and the exhaust gas flow upstream of the high OSC capacity rhodium support layer is located downstream of the exhaust gas flow upstream end of the low OSC capacity rhodium support layer. The side edges are located,
Exhaust gas purification catalyst.
<2> further having a platinum support layer;
The platinum carrying concentration in the platinum carrying layer is higher than the platinum carrying concentration in the palladium carrying layer, the low OSC capacity rhodium carrying layer and the high OSC capacity rhodium carrying layer; and (a) at least partially the high OSC capacity rhodium. The platinum support layer is disposed downstream of the support layer with respect to the exhaust gas flow, and / or (b) the high OSC capacity rhodium support layer and the platinum support layer are at least partially laminated, and The exhaust gas flow upstream end of the platinum support layer is disposed downstream of the exhaust gas flow upstream end of the high OSC capacity rhodium support layer,
The exhaust gas purification catalyst as described in <1> above.
<3> a support substrate, and a palladium support layer and a high OSC-capable rhodium support layer disposed on the support substrate;
The palladium carrying concentration in the palladium carrying layer is higher than the palladium carrying concentration in the high OSC capable rhodium carrying layer, and the rhodium concentration in the high OSC capable rhodium carrying layer is higher than the rhodium concentration in the palladium carrying layer;
The OSC ability in the high OSC ability rhodium carrying layer is larger than the OSC ability in the palladium carrying layer; and the high OSC ability rhodium carrying layer is laminated at least partially on the palladium carrying layer on the carrier substrate. And where the palladium supporting layer and the high OSC-capable rhodium supporting layer are laminated, from the upstream side to the downstream side of the exhaust gas flow, (the thickness of the palladium supporting layer) / (the high The ratio of the OSC ability rhodium carrying layer) is small,
Exhaust gas purification catalyst.
<4> further having a platinum support layer;
The platinum support concentration in the platinum support layer is higher than the platinum support concentration in the palladium support layer and the high OSC rhodium support layer;
(A) The platinum support layer is disposed at least partially downstream of the exhaust gas flow from the high OSC capacity rhodium support layer, and / or (b) the high OSC capacity rhodium support layer and the platinum support layer. And the exhaust gas flow upstream end of the platinum support layer is disposed downstream of the exhaust gas flow upstream end of the high OSC capacity rhodium support layer.
The exhaust gas purifying catalyst as described in <3> above.
<5> The exhaust gas purification catalyst according to any one of <1> to <4>, wherein a NO _x storage material is further supported at least on the high OSC-capable rhodium support layer.

  In the present invention, the “supporting concentration” of a noble metal such as rhodium refers to the concentration per mass of the supporting layer. In the present invention, the magnitude of “OSC ability (oxygen storage and release ability)” refers to the magnitude of OSC ability per mass of the support layer. Here, “OSC ability” is the ability to occlude oxygen when the oxygen concentration in the exhaust gas is high and release oxygen when the oxygen concentration in the exhaust gas is low, and the exhaust gas purification catalyst has this ability. Thus, it is possible to suppress the atmospheric fluctuation of exhaust gas and / or realize on-board diagnosis (OBD) by measuring the deterioration state of the OSC ability of the catalyst.

It is a figure which shows the example of the exhaust gas purification catalyst of 1st this invention. It is a figure which shows the other example of the exhaust gas purification catalyst of 1st this invention. It is a figure which shows the other example of the exhaust gas purification catalyst of 1st this invention. It is a figure which shows the other example of the exhaust gas purification catalyst of 1st this invention. It is a figure which shows the example of the exhaust gas purification catalyst of 2nd this invention. It is a diagram showing a relationship between the hydrogen concentration and _the NO _x purification rate when using the rhodium catalyst. Is a diagram showing _the NO _x purification performance using a methane flame retardant hydrocarbons. It is a figure which illustrates the method of manufacturing the exhaust gas purification catalyst of 2nd this invention.

<Exhaust gas purification catalyst of the first invention>
The exhaust gas purifying catalyst of the first aspect of the present invention has a support substrate, a palladium supporting layer, a low OSC ability rhodium supporting layer and a high OSC ability rhodium supporting layer.

<Exhaust gas purification catalyst of the first aspect of the present invention-supported concentration>
In the exhaust gas purification catalyst of the first aspect of the present invention, the palladium support concentration in the palladium support layer is higher than the palladium support concentration in the low OSC ability rhodium support layer and the high OSC ability rhodium support layer, for example, 2 times or more, 5 times or more. Or 10 times or more. Further, the rhodium carrying concentration in the low OSC ability rhodium carrying layer and the high OSC ability rhodium carrying layer is higher than the rhodium carrying concentration in the palladium carrying layer, for example, 2 times or more, 5 times or more, or 10 times or more. In addition, as long as said conditions are satisfy | filled, the palladium carrying layer may carry | support other catalyst metals, such as rhodium. The same applies to the high and low OSC-capable rhodium support layers. Further, the rhodium carrying concentration in the low OSC ability rhodium carrying layer and the rhodium carrying concentration in the high OSC ability rhodium carrying layer may be the same or higher.

  In this exhaust gas purification catalyst, a low OSC capacity rhodium support layer is disposed at least partially on the downstream side of the exhaust gas flow from the palladium support layer on the support substrate, and at least partially, the low OSC capacity rhodium. A high OSC capacity rhodium support layer may be disposed downstream of the support layer in the exhaust gas flow. Here, in this exhaust gas purification catalyst, as shown in FIG. 1, a palladium supporting layer (Pd), a low OSC ability rhodium supporting layer {Rh (low-OSC)}, and a high OSC ability rhodium supporting layer {Rh (high- OSC)} may be arranged, optionally at intervals, sequentially from the upstream side of the exhaust gas flow, without being substantially stacked on each other. In the figure, G indicates an arrow indicating the exhaust gas flow, and S indicates a carrier substrate.

  In this exhaust gas purification catalyst, the low OSC capacity rhodium support layer and the high OSC capacity rhodium support layer are at least partially laminated on the support base material, and the exhaust gas flow upstream of the low OSC capacity rhodium support layer. The exhaust gas flow upstream end of the high OSC capacity rhodium support layer may be arranged downstream of the end of the exhaust gas flow. Therefore, in this exhaust gas purification catalyst, as shown in FIG. 2, a low OSC ability rhodium supporting layer {Rh (low-OSC)} and a high OSC ability rhodium supporting layer {Rh (high-OSC)} are laminated. The exhaust gas flow upstream end of the high OSC capacity rhodium support layer is disposed downstream of the exhaust gas flow upstream end of the low OSC capacity rhodium support layer, and optionally the low OSC capacity rhodium support layer The downstream end of the exhaust gas flow with the OSC rhodium support layer may be aligned. In the figure, G indicates an arrow indicating the exhaust gas flow, and S indicates a carrier substrate.

  According to these configurations, since the low OSC capacity rhodium support layer and the high OSC capacity rhodium support layer are arranged downstream of the palladium support layer from the exhaust gas flow, the exhaust gas exhausted by the exhaust gas purification catalyst is emptied. When the fuel ratio is stoichiometric to fuel rich, hydrogen having high reducibility is generated in the palladium support layer, and this hydrogen is supplied to the low and high OSC capacity rhodium support layer downstream of the exhaust gas flow. Reduction can be performed effectively.

  Here, it is shown in the following Table 1 that hydrogen can be generated quickly when changing from the fuel lean state to the stoichiometric or fuel rich state by using the palladium supporting layer on the upstream side of the exhaust gas flow. Yes. More specifically, this Table 1 shows that the difference between T50 or T90 at the time of temperature increase and T50 or T90 at the time of the temperature decrease is small compared with rhodium, that is, palladium is compared by pretreatment in an oxidizing atmosphere. It is understood that the active activity is not immediately oxidized and / or palladium is quickly reduced in the stoichiometric atmosphere, thereby providing its original activity immediately after switching the exhaust gas from the fuel lean atmosphere to the stoichiometric atmosphere.

In the evaluation shown in Table 1, an exhaust gas purification catalyst for evaluation was obtained by supporting 1% by mass of palladium or rhodium on ceria-zirconia or alumina as a carrier, and this exhaust gas purification catalyst was oxidized at 400 ° C. The surface of palladium or rhodium was at least partially oxidized by pretreatment with atmospheric gas (O ₂ : 1.5%, N ₂ : balance) for 15 minutes. After the pretreatment, the evaluation gas in the stoichiometric atmosphere (NO: 1,500 ppm, C ₃ H ₆ : 3,000 ppmC, O ₂ : 0.7%, CO: 0.65%, CO with respect to the exhaust gas purification catalyst ₂ : 10%, H ₂ O: 3%, N ₂ : balance) were evaluated. In this evaluation, the evaluation gas flow rate is 20 liters / minute with respect to 3 g of the exhaust gas purification catalyst, the temperature is raised from 150 ° C. to 500 ° C., maintained at 500 ° C. for 15 minutes, and palladium or rhodium is reduced, and 500 ° C. To 150 ° C. The temperature (T50) at which the conversion rate of C ₃ H ₆ (T.HC) and carbon monoxide (CO) is 50%, and C ₃ H ₆ (T.HC) and monoxide during this temperature increase and decrease The temperature (T90) at which the carbon (CO) conversion was 90% was determined.

Also, here, thus it is reduced rhodium by using rhodium with hydrogen, that _the NO _x reduction performance due rhodium is improved is shown in FIG. 6, for example. More specifically, FIG. 6 shows that as the concentration of hydrogen (H ₂ ) increases, the purification rate of NO _x increases, that is, hydrogen is supplied to rhodium for the reduction of NO _x. It is understood that this is preferred.

In the evaluation shown in FIG. 6, the exhaust gas purification catalyst in which 0.024% by mass of rhodium is supported on the ceria-zirconia support is rich (5 minutes) / lean (5 minutes) (rich atmosphere: 2 % H ₂ /10% CO ₂ /3% H ₂ O / balance N ₂ , lean atmosphere: 1% O ₂ /10% CO ₂ /3% H ₂ O / balance N ₂ ) Endured for 5 hours, and then exposed to a lean gas (composition: 15% O ₂ / balance N ₂ ) having a gas temperature with catalyst of 500 ° C. for 20 minutes to oxidize rhodium, and then the rich composition shown in Table 2 at 500 ° C. Evaluation gas was supplied for evaluation.

  Here, λ represents an excess air ratio. When this value is 1, this indicates that the gas has a stoichiometric composition, and when this value is less than 1, this indicates that the gas has a fuel-rich composition. In addition, this value exceeding 1 indicates that the gas has a fuel lean composition. In addition, when increasing the concentration of hydrogen, the concentration of carbon monoxide was decreased and / or the concentration of oxygen was increased so that the following excess air ratio λ was constant:

λ = ([NO] +2 [O ₂ ]) / ([CO] + [H ₂ ] +3 [C ₃ H ₆ ])
{[NO]: NO concentration (%), [O ₂ ]: O ₂ concentration (%), [CO]: CO concentration (%), [H ₂ ]: H ₂ concentration (%), [ C ₃ H ₆ ]: Concentration of C ₃ H ₆ (%)}

<Exhaust Gas Purification Catalyst-OSC Capability of First Invention>
In the exhaust gas purification catalyst of the first aspect of the present invention, the OSC ability in the high OSC ability rhodium carrying layer is larger than the OSC ability in the palladium carrying layer and the low OSC ability rhodium carrying layer, for example, 2 times or more, 5 times or more, 10 times or more.

  According to this, since the OSC ability in the palladium supporting layer and the low OSC ability rhodium supporting layer is relatively small, hydrogen generated in the palladium supporting layer is oxidized by the OSC ability when the exhaust gas is in a fuel-rich atmosphere. Is less likely to be consumed. Therefore, according to this, since hydrogen can be effectively supplied to the low OSC capacity rhodium support layer, rhodium can be effectively reduced in the low OSC capacity rhodium support layer.

  Moreover, according to this, the high OSC ability rhodium carrying layer provides the OSC ability, thereby suppressing the atmospheric fluctuation of the exhaust gas and / or measuring the deterioration state of the OSC ability of the catalyst. Can be realized.

<Exhaust gas purification catalyst of the first aspect of the invention-platinum support layer>
The exhaust gas purification catalyst of the first aspect of the present invention can further have a platinum support layer. Here, the platinum supporting concentration in the platinum supporting layer is higher than the platinum supporting concentration in the palladium supporting layer, the low OSC ability rhodium supporting layer, and the high OSC ability rhodium supporting layer. In this case, if the platinum support layer is disposed at least partially downstream of the exhaust gas flow with respect to the high OSC capacity rhodium support layer, it is difficult to prevent methane or the like caused by the air-fuel ratio becoming stoichiometric or fuel-rich. Flammable hydrocarbons can be purified with this platinum support layer.

  In this exhaust gas purification catalyst, as shown in FIG. 3, the platinum support layer (Pt) is composed of a palladium support layer (Pd), a low OSC ability rhodium support layer {Rh (low-OSC)}, and a high OSC ability rhodium support layer. {Rh (high-OSC)} may be arranged on the downstream side of the exhaust gas flow at an optional interval. In the case where the platinum support layer (Pt) is disposed at a distance from another catalyst support layer, the platinum support layer (Pt) can be used by being disposed on a support substrate different from the other catalyst support layer. That is, for example, a palladium-carrying layer other than the platinum-carrying layer can be used as a start catalyst arranged directly under the engine, and the platinum-carrying layer can be used as an underfloor catalyst arranged downstream thereof.

  Further, in this exhaust gas purification catalyst, in addition to or instead of the platinum support layer disposed on the exhaust gas flow downstream side of the palladium support layer and the high OSC rhodium support layer, the platinum support layer is formed of the palladium support layer. , A low OSC capacity rhodium support layer, and a high OSC capacity rhodium support layer may be at least partially laminated with each other. For example, as shown in FIG. 4, the platinum support layer (Pt) has a high OSC capacity The platinum support layer {Rh (high-OSC)} is laminated, and the exhaust gas flow upstream end of the platinum support layer is disposed downstream of the exhaust gas flow upstream end of the high OSC capacity rhodium support layer. It may be.

Here, for example, FIG. 7 shows that the purification of the flame retardant hydrocarbon such as methane is promoted by arranging the platinum support layer in this way. More specifically, from FIG. 7, platinum (Pt) is compared with rhodium (Rh) and palladium (Pd) in relation to purification of NO _x using methane (CH ₄ ), which is a flame-retardant hydrocarbon. It is understood that it is preferable.

In the evaluation shown in FIG. 7, an exhaust gas purification catalyst in which rhodium (Rh), palladium (Pd), and platinum (Pt) are respectively supported on an alumina (Al ₂ O ₃ ) carrier is used in air. 2,000 ° C. for 5 hours, and then supply a rich evaluation gas {NO: 500 ppm, CH ₄ : 125 ppm, H ₂ O: 3%, N ₂ : balance} with a catalyst-containing gas temperature of 100 to 500 ° C. And evaluated.

<Exhaust Gas Purification Catalyst of Second Invention>
The exhaust gas purifying catalyst of the second aspect of the present invention has a support base, a palladium support layer disposed on the support base, and a high OSC-capable rhodium support layer.

<Exhaust gas purification catalyst of the second aspect of the present invention-supported concentration>
In the exhaust gas purification catalyst of the second aspect of the present invention, the palladium support concentration in the palladium support layer is higher than the palladium support concentration in the high OSC ability rhodium support layer, for example, 2 times or more, 5 times or more, or 10 times or more. Moreover, the rhodium carrying | support density | concentration in a high OSC ability rhodium carrying | support layer is higher than the rhodium carrying | support density | concentration in a palladium carrying layer, for example, 2 times or more, 5 times or more, or 10 times or more.

  In the exhaust gas purifying catalyst of the present invention, a high OSC capacity rhodium support layer is laminated at least partially on a palladium support layer on a support substrate, and the palladium support layer and the high OSC capacity rhodium support layer are provided. The ratio of (palladium supporting layer thickness) / (high OSC capacity rhodium supporting layer thickness) decreases from the upstream side to the downstream side of the exhaust gas flow in the stacked portion.

  Here, in this exhaust gas purification catalyst, as shown in FIG. 5, a high OSC capacity rhodium support layer {Rh (high-OSC)} is laminated on the palladium support layer (Pd), and the exhaust gas flow from the upstream side. The thickness of the palladium-carrying layer gradually decreases toward the downstream side, and the thickness of the high OSC ability rhodium-carrying layer may gradually increase.

  According to this exhaust gas purification catalyst, when the air-fuel ratio of the exhaust gas purified by the exhaust gas purification catalyst becomes a stoichiometric to fuel-rich state, highly reducible hydrogen is generated in the palladium-supporting layer, and this hydrogen has a high OSC capability. By supplying to the rhodium support layer, rhodium can be effectively reduced in the high OSC capacity rhodium support layer. Here, in this exhaust gas purification catalyst, the ratio of (palladium supporting layer thickness) / (high OSC capacity rhodium supporting layer) ratio is relatively large in the exhaust gas flow upstream portion, that is, in the exhaust gas flow upstream portion. Since the proportion of the palladium-carrying layer is relatively large, the amount of hydrogen consumed by the OSC ability of the high OSC ability rhodium-carrying layer in the upstream portion of the exhaust gas flow can be reduced, and rhodium can be effectively reduced. .

Here, in this way it is reduced rhodium by using rhodium with hydrogen, that _the NO _x reduction performance due rhodium is improved with regard to the exhaust gas purifying catalyst of the first aspect of the present invention are described above.

<Exhaust gas purification catalyst of the second aspect of the present invention-OSC ability>
In the exhaust gas purification catalyst of the second aspect of the present invention, the OSC ability in the high OSC ability rhodium carrying layer is larger than the OSC ability in the palladium carrying layer, for example, 2 times or more, 5 times or more, or 10 times or more.

  According to this, since the OSC ability in the palladium supporting layer is relatively small, when the exhaust gas is made into a fuel-rich atmosphere, the hydrogen generated in the palladium supporting layer is less likely to be oxidized and consumed by the OSC ability. . Therefore, according to this, since hydrogen can be effectively supplied to the high OSC capacity rhodium support layer, rhodium can be effectively reduced in the high OSC capacity rhodium support layer.

  Moreover, according to this, the high OSC ability rhodium carrying layer provides the OSC ability, thereby suppressing the atmospheric fluctuation of the exhaust gas and / or measuring the deterioration state of the OSC ability of the catalyst. Can be realized.

  Further, according to the exhaust gas purification catalyst, the high OSC-capable rhodium support layer is laminated at least partially on the palladium support layer, so that the OSC capability of the high OSC-capacity rhodium support layer makes the palladium support relatively small in OSC. The layer can be protected from changes in the atmosphere of the exhaust gas.

<Exhaust Gas Purification Catalyst-Platinum Supporting Layer of Second Invention>
In addition, the exhaust gas purification catalyst of the second aspect of the present invention can further have a platinum support layer. Here, the platinum carrying concentration in the platinum carrying layer is higher than the platinum carrying concentration in the palladium carrying layer and the high OSC rhodium carrying layer. In this case, if the platinum support layer is disposed at least partially downstream of the exhaust gas flow with respect to the high OSC capacity rhodium support layer, it is difficult to prevent methane or the like caused by the air-fuel ratio becoming stoichiometric or fuel-rich. Flammable hydrocarbons can be purified with this platinum support layer.

  In this exhaust gas purification catalyst, the platinum support layer may be disposed at an optional interval on the downstream side of the exhaust gas flow of the palladium support layer and the high OSC capacity rhodium support layer. Further, in this exhaust gas purification catalyst, in addition to or instead of the platinum support layer disposed on the exhaust gas flow downstream side of the palladium support layer and the high OSC rhodium support layer, the platinum support layer is formed of the palladium support layer. And any of the high OSC-capable rhodium support layers may be at least partially stacked on each other. In addition, about the structure of this exhaust gas purification catalyst, FIG. 3 and 4 regarding the exhaust gas purification catalyst of 1st this invention can be referred.

  Here, it has been described above with respect to the exhaust gas purification catalyst of the first aspect of the present invention that the purification of the flame-retardant hydrocarbon such as methane is promoted by arranging the platinum support layer in this way.

<Details of Each Part of Exhaust Gas Purification Catalyst of First and Second Inventions>
For details of each part of the exhaust gas purification catalyst of the present invention, for example, the above-mentioned Patent Document 1 can be referred to, and this will be described below.

<Base material>
As the carrier base material used in the exhaust gas purification catalyst of the present invention, any carrier base material capable of carrying a catalyst carrier layer for exhaust gas purification can be used. A honeycomb structure.

<Palladium support layer>
As the palladium supporting layer used in the exhaust gas purification catalyst of the present invention, palladium (Pd) as a catalyst metal is supported on an arbitrary catalyst carrier such as alumina, ceria, zirconia, ceria-zirconia solid solution, particularly a powdery catalyst carrier. Any catalyst support layer that has been described can be used. The palladium-supporting layer may further support a catalyst metal other than palladium, for example, other noble metals such as platinum and rhodium. Furthermore, this palladium carrying layer may further carry a NO _x storage material selected from the group consisting of alkali metals and alkaline earth metal elements such as barium and potassium.

  In the exhaust gas purification catalyst of the present invention, the OSC ability in the high OSC ability rhodium carrying layer needs to be larger than the OSC ability in the palladium carrying layer. Such OSC capability is generally provided by ceria in the catalyst support. Therefore, in order to adjust the OSC ability in each catalyst supporting layer, the amount of ceria in the catalyst supporting layer can be increased or decreased. That is, in the palladium supporting layer, ceria-zirconia solid solution, alumina, zirconia, etc. having a relatively small content of ceria can be used as the catalyst support, in particular, zirconia, in order to relatively reduce or substantially eliminate the OSC ability. .

In order to form a palladium support layer on a support substrate, for example, a honeycomb structure is washed with a slurry containing a catalyst support such as alumina and fired to form a catalyst support layer. Palladium can be supported on the catalyst support layer by a method, a water absorption support method, or the like. In addition, in order to form a palladium support layer on a support substrate, palladium is supported on a catalyst support such as alumina in advance, and then a slurry containing the catalyst support supporting palladium is washed on the support substrate. It can also be coated. Incidentally, NO _x storage material, for the carrying of such other catalytic metals, can be performed in the same manner as loading of palladium.

<Low OSC ability rhodium support layer>
As the low OSC-capable rhodium carrying layer used in the exhaust gas purification catalyst of the present invention, an arbitrary catalyst carrier such as alumina, ceria, zirconia, ceria-zirconia solid solution or the like is supported by rhodium (Rh) as a catalyst metal. The catalyst support layer can be used. The low OSC-capable rhodium support layer may further support a catalyst metal other than rhodium, for example, another noble metal such as platinum or palladium. Furthermore, the low OSC-capable rhodium carrying layer may further carry a NO _x storage material selected from the group consisting of alkali metals and alkaline earth metal elements such as barium and potassium.

  In the exhaust gas purifying catalyst of the present invention, the OSC ability in the high OSC ability rhodium supporting layer needs to be larger than the OSC ability in the low OSC ability rhodium supporting layer. Therefore, as in the case of the above-mentioned palladium-supported layer, the low OSC-capable rhodium-supported layer has a ceria-zirconia having a relatively low content of ceria as a catalyst support in order to relatively reduce or substantially eliminate OSC ability Solid solution, alumina, zirconia, etc., especially zirconia can be used.

  For the formation of the low OSC-capable rhodium support layer on the support substrate, the description regarding the palladium support layer can be referred to.

<High OSC capacity rhodium support layer>
As the high OSC-capable rhodium carrying layer used in the exhaust gas purification catalyst of the present invention, any catalyst carrier such as alumina, ceria, zirconia, ceria-zirconia solid solution, etc., carrying rhodium (Rh) as a catalyst metal is arbitrarily used. The catalyst support layer can be used. The high OSC-capable rhodium carrying layer may further carry a catalyst metal other than rhodium, for example, other noble metals such as platinum and palladium. Furthermore, the high OSC-capable rhodium carrying layer may further carry a NO _x storage material selected from the group consisting of alkali metals and alkaline earth metal elements such as barium and potassium.

  In the exhaust gas purifying catalyst of the present invention, it is necessary that the OSC ability in the high OSC ability rhodium carrying layer is larger than the OSC ability in the palladium carrying layer and the low OSC ability rhodium carrying layer. Therefore, in the high OSC ability rhodium supporting layer, in order to relatively increase the OSC ability, ceria, ceria-zirconia solid solution, such as ceria-zirconia solid solution, in particular, can be used as a catalyst carrier.

  For the formation of the high OSC-capable rhodium support layer on the support substrate, the description regarding the palladium support layer can be referred to.

<Platinum support layer>
As a platinum carrying layer optionally used in the exhaust gas purification catalyst of the present invention, any catalyst carrier such as alumina, ceria, zirconia, ceria-zirconia solid solution or the like is supported with platinum (Pt) as a catalyst metal. A catalyst support layer can be used. The platinum-supporting layer may further support a catalyst metal other than platinum, for example, other noble metals such as rhodium and palladium. Furthermore, the platinum support layer may further support a NO _x storage material selected from the group consisting of alkali metals and alkaline earth metal elements such as barium and potassium.

  Any catalyst carrier can be used for the platinum support layer, but alumina can be used as the catalyst carrier in particular in order to maintain a large surface area of platinum.

  For the formation of the platinum-supporting layer on the support substrate, the description regarding the palladium-supporting layer can be referred to.

<Changes in the ratio of the layer structure and thickness of the catalyst support layer>
In the exhaust gas purifying catalyst of the present invention, when at least partially stacking another catalyst supporting layer on one catalyst supporting layer, for example, when stacking a high OSC capacity rhodium supporting layer on a palladium supporting layer, One catalyst supporting layer can be formed by wash coating, firing or the like, and then another catalyst supporting layer can be formed on the formed catalyst supporting layer by wash coating, firing or the like.

  In the exhaust gas purification catalyst of the present invention, when the ratio of the thicknesses of the two catalyst support layers is changed from the exhaust gas flow upstream side to the downstream side, that is, for example, from the exhaust gas flow upstream side to the downstream side ( When the ratio of (palladium support layer thickness) / (high OSC capacity rhodium support layer thickness) ratio is to be reduced, this means that the thickness of at least one catalyst support layer is increased from the exhaust gas flow upstream side to the downstream side. Can be achieved by changing.

  In this way, changing the thickness of the catalyst support layer from the upstream side to the downstream side of the exhaust gas flow is, for example, divided into the same catalyst support layer multiple times as described above for the laminated structure of the catalyst support layer. Can be achieved by forming a stepped slope. That is, as shown in FIG. 8, first, the lowermost catalyst support layer 1 is formed, then the second catalyst support layer 2 is formed, and finally the third catalyst support layer 3 is formed. Can be achieved.

G Arrow indicating exhaust gas flow S Support substrate Pd Palladium support layer Rh (low-OSC) Low OSC capacity rhodium support layer Rh (high-OSC) High OSC capacity rhodium support layer Pt Platinum support layer

Claims (5)

A support substrate, and a palladium support layer, a low OSC ability rhodium support layer and a high OSC ability rhodium support layer disposed on the support substrate;
The palladium support concentration in the palladium support layer is higher than the palladium support concentration in the low OSC ability rhodium support layer and the high OSC ability rhodium support layer, and the rhodium support in the low OSC ability rhodium support layer and the high OSC ability rhodium support layer. The concentration is higher than the rhodium loading concentration in the palladium loading layer;
The OSC ability in the high OSC ability rhodium support layer is greater than the OSC ability in the palladium support layer and the low OSC ability rhodium support layer;
On the support substrate, the low OSC-capable rhodium support layer is disposed at least partially downstream of the exhaust gas flow from the palladium support layer; and on the support substrate,
(A) the high OSC capacity rhodium support layer is disposed at least partially downstream of the exhaust gas flow from the low OSC capacity rhodium support layer, and / or (b) the low OSC capacity rhodium support layer; A high OSC capacity rhodium support layer is laminated at least partially, and the exhaust gas flow upstream of the high OSC capacity rhodium support layer is located downstream of the exhaust gas flow upstream end of the low OSC capacity rhodium support layer. The side edges are located,
Exhaust gas purification catalyst. A platinum support layer;
A platinum support concentration in the platinum support layer is higher than a platinum support concentration in the palladium support layer, the low OSC ability rhodium support layer and the high OSC ability rhodium support layer; and (a) at least partially the high OSC ability rhodium The platinum support layer is disposed downstream of the support layer with respect to the exhaust gas flow, and / or (b) the high OSC capacity rhodium support layer and the platinum support layer are at least partially laminated, and The exhaust gas flow upstream end of the platinum support layer is disposed downstream of the exhaust gas flow upstream end of the high OSC capacity rhodium support layer,
The exhaust gas purification catalyst according to claim 1. A support substrate, and a palladium support layer and a high OSC capacity rhodium support layer disposed on the support substrate;
The palladium carrying concentration in the palladium carrying layer is higher than the palladium carrying concentration in the high OSC capable rhodium carrying layer, and the rhodium concentration in the high OSC capable rhodium carrying layer is higher than the rhodium concentration in the palladium carrying layer;
The OSC ability in the high OSC ability rhodium carrying layer is larger than the OSC ability in the palladium carrying layer; and the high OSC ability rhodium carrying layer is laminated at least partially on the palladium carrying layer on the carrier substrate. And where the palladium supporting layer and the high OSC-capable rhodium supporting layer are laminated, from the upstream side to the downstream side of the exhaust gas flow, (the thickness of the palladium supporting layer) / (the high The ratio of the OSC ability rhodium carrying layer) is small,
Exhaust gas purification catalyst. A platinum support layer;
A platinum support concentration in the platinum support layer is higher than a platinum support concentration in the palladium support layer and the high OSC capacity rhodium support layer; and (a) an exhaust gas flow at least partially than the high OSC capacity rhodium support layer. The platinum support layer is disposed downstream, and / or (b) the high OSC capacity rhodium support layer and the platinum support layer are at least partially laminated, and the high OSC capacity rhodium support layer. The exhaust gas flow upstream end of the platinum-supported layer is disposed on the exhaust gas flow downstream side of the exhaust gas flow upstream end of
The exhaust gas purification catalyst according to claim 3. The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein a NO _x storage material is further supported on at least the high OSC-capable rhodium support layer.

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