JP2010179204A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new catalyst for cleaning exhaust gas to remove NO<SB>x</SB>, 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 rhodium-deposited layer Rh and a platinum-deposited layer Pt. The concentration of the palladium deposited on the palladium-deposited layer is made higher than that of the palladium deposited on each of the rhodium-deposited layer and the platinum-deposited layer. The concentration of the rhodium deposited on the rhodium-deposited layer is made higher than that of the rhodium deposited on each of the palladium-deposited layer and the platinum-deposited layer and the concentration of the platinum deposited on the platinum-deposited layer is made higher than that of the platinum deposited on each of the palladium-deposited layer and the rhodium-deposited layer. On the carrier base material, the rhodium-deposited layer is arranged at least partially on the downstream side of the palladium-deposited layer in an exhaust gas current and/or at least partially on the downside of the palladium-deposited layer. The platinum-deposited layer is arranged at least partially on the downstream side of each of the palladium-deposited layer and the 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.

Accordingly, the present invention provides a novel NO _x exhaust gas purifying catalyst to solve such a problem.

The inventors have found that supplying hydrogen to rhodium in an oxidized state promotes reduction of rhodium, and that flame retardant carbonization such as methane is satisfactorily oxidized in the platinum-supported layer. Invented the invention:
<1> having a carrier substrate, a palladium carrying layer, a rhodium carrying layer, and a platinum carrying layer;
The palladium carrying concentration in the palladium carrying layer is higher than the palladium carrying concentration in the rhodium carrying layer and the platinum carrying layer, and the rhodium carrying concentration in the rhodium carrying layer is the rhodium carrying concentration in the palladium carrying layer and the platinum carrying layer. And the platinum carrying concentration in the platinum carrying layer is higher than the platinum carrying concentration in the palladium carrying layer and the rhodium carrying layer;
On the carrier substrate, a rhodium support layer is disposed at least partially downstream of the exhaust gas flow from the palladium support layer, and / or at least partially below the palladium support layer. And at least partially, the platinum support layer is disposed downstream of the exhaust gas flow from the palladium support layer and the rhodium support layer.
Exhaust gas purification catalyst.
<2> The exhaust gas purifying catalyst according to <1>, wherein a NO _x storage material is further supported on at least one of the palladium supporting layer, the rhodium supporting layer, and the platinum supporting 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.

It is a figure which shows the example of the exhaust gas purification catalyst of this invention. It is a figure which shows the other example of the exhaust gas purification catalyst of this invention. It is a figure which shows the other example of the exhaust gas purification catalyst of 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.

<Exhaust gas purification catalyst of the present invention>
The exhaust gas purification catalyst of the present invention has a support substrate, a palladium support layer, a rhodium support layer, and a platinum support layer.

  In the exhaust gas purification catalyst of the present invention, the palladium carrying concentration in the palladium carrying layer is higher than the palladium carrying concentration in the rhodium carrying layer and the platinum carrying layer, for example, 2 times or more, 5 times or more, or 10 times or more. The rhodium carrying concentration in the rhodium carrying layer is higher than the rhodium carrying concentration in the palladium carrying layer and the platinum carrying layer, for example, 2 times or more, 5 times or more, or 10 times or more. Furthermore, the platinum carrying concentration in the platinum carrying layer is higher than the platinum carrying concentration in the palladium carrying layer and the rhodium 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 rhodium support layer and the platinum support layer.

  In this exhaust gas purifying catalyst, a rhodium supporting layer may be disposed on the carrier substrate at least partially downstream of the exhaust gas flow from the palladium supporting layer. Here, in this exhaust gas purification catalyst, as shown in FIG. 1, the palladium support layer (Pd) and the rhodium support layer (Rh) are optionally spaced apart from each other in order from the exhaust gas flow upstream side. It may be arranged with a gap. In the figure, G indicates an arrow indicating the exhaust gas flow, and S indicates a carrier substrate.

  In this exhaust gas purification catalyst, a rhodium support layer may be disposed at least partially below the palladium support layer on the support substrate. Therefore, in this exhaust gas purification catalyst, as shown in FIG. 2, the palladium carrying layer (Pd) and the rhodium carrying layer (Rh) may be laminated. In the figure, G indicates an arrow indicating the exhaust gas flow, and S indicates a carrier substrate.

  According to the configuration of the exhaust gas purification catalyst of the present invention, the exhaust gas treated in the palladium support layer flows into the rhodium support layer, so that the air-fuel ratio of the exhaust gas to be purified in the exhaust gas purification catalyst becomes a stoichiometric to fuel rich state. Occasionally, highly reducible hydrogen is generated in the palladium-supporting layer, and rhodium in the rhodium-supporting layer can be reduced, that is, activated by this hydrogen.

  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) reaches 50%, and C3H6 (T.HC) and carbon monoxide (CO The temperature (T90) at which the conversion of) becomes 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. 4, for example. More specifically, FIG. 4 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. 4, 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 decreased 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 (%)}

  Further, according to the configuration of the exhaust gas purification catalyst of the present invention, the air-fuel ratio is in a stoichiometric to fuel-rich state because the platinum support layer is arranged downstream of the exhaust gas flow from the palladium support layer and the rhodium support layer. Flame retardant hydrocarbons such as methane, which are sometimes generated in the palladium support layer and the rhodium support layer, can be purified by this platinum support layer.

  In the exhaust gas purification catalyst of the present invention, as shown in FIGS. 1 and 2, the platinum support layer (Pt) is optionally spaced on the downstream side of the exhaust gas flow of the palladium support layer (Pd) and the rhodium support layer (Rh). And may be disposed on the same carrier substrate. Further, in the exhaust gas purification catalyst of the present invention, as shown in FIG. 3, the platinum support layer (Pt) is optionally spaced on the downstream side of the exhaust gas flow of the palladium support layer (Pd) and the rhodium support layer (Rh). The catalyst support layer can be used on another carrier substrate. 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.

Here, for example, FIG. 5 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. 5, regarding the purification of NO _x using methane (CH ₄ ), which is a flame retardant hydrocarbon, platinum (Pt) is compared with rhodium (Rh) and palladium (Pd). It is understood that it is preferable.

In the evaluation shown in FIG. 5, 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. , is durable for 5 hours at 000 ° C., then entering the catalyst gas temperature 100 to 500 ° C. rich evaluation gas _{{NO: 500ppm, CH 4:} 125ppm, H 2 O: 3%, N 2: balance} supplies And evaluated.

<Details of Each Part of Exhaust Gas Purification Catalyst of the Present Invention>
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.

  The palladium-supporting layer can optionally have OSC capability. 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.

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.

<Rhodium support layer>
As a rhodium carrying layer used in the exhaust gas purification catalyst of the present invention, an arbitrary catalyst carrying rhodium (Rh) as a catalytic metal on an arbitrary catalyst carrier such as alumina, ceria, zirconia, ceria-zirconia solid solution, etc. Layers can be used. The rhodium-supporting layer may further support a catalyst metal other than rhodium, for example, other noble metals such as platinum and palladium. Furthermore, this 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.

  For the OSC ability of the rhodium-carrying layer and the formation of the rhodium-carrying layer on the support substrate, the description regarding the palladium-carrying 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.

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

G Arrow indicating exhaust gas flow S Support substrate Pd Palladium support layer Rh Rhodium support layer Pt Platinum support layer

Claims (2)

Having a support substrate, a palladium support layer, a rhodium support layer, and a platinum support layer;
The palladium carrying concentration in the palladium carrying layer is higher than the palladium carrying concentration in the rhodium carrying layer and the platinum carrying layer, and the rhodium carrying concentration in the rhodium carrying layer is the rhodium carrying concentration in the palladium carrying layer and the platinum carrying layer. And the platinum carrying concentration in the platinum carrying layer is higher than the platinum carrying concentration in the palladium carrying layer and the rhodium carrying layer;
On the carrier substrate, a rhodium support layer is disposed at least partially downstream of the exhaust gas flow from the palladium support layer, and / or at least partially below the palladium support layer. And at least partially, the platinum support layer is disposed downstream of the palladium support layer and the rhodium support layer in the exhaust gas flow.
Exhaust gas purification catalyst. 2. The exhaust gas purification catalyst according to claim 1, wherein a NO _x storage material is further supported on at least one of the palladium supporting layer, the rhodium supporting layer, and the platinum supporting layer.

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