JP6748590B2 - Exhaust gas purification catalyst - Google Patents

Description

The present invention relates to an exhaust gas purifying catalyst that simultaneously purifies nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC) in engine exhaust gas, and particularly relates to an exhaust gas purifying catalyst that can obtain high NOx purification performance.

Exhaust gas discharged from an internal combustion engine such as an automobile contains carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and the like.
Of these, NOx such as NO and NO ₂ is an air pollutant that affects the human body, and N ₂ O is also a greenhouse gas that promotes global warming. NOx emission regulations have been enforced, and many exhaust gas purifying catalysts have been developed.

Noble metals such as Pt, Pd, and Rh are used as active species in the exhaust gas purifying catalyst, and these noble metals are used by being carried on a honeycomb substrate together with a promoter such as zirconium oxide.
In purification of NOx by Rh, it is considered that, for example, a steam reforming reaction or a CO+NO reaction promotes purification of NOx via the Rh component. The zirconium oxide promotes the steam reforming reaction and the CO+NO reaction together with the Rh component (see Patent Document 1).

For gasoline engines, stoichiometric air-fuel ratio (air/fuel ratio, A/F) conditions to reduce pollution by nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC) By operating the engine at or near it and installing a three-way catalyst (TWC) in the exhaust pipe at the engine outlet, these three types of pollutants can be reduced simultaneously and purified into carbon dioxide, water, and nitrogen. Has been done.

As a co-catalyst component, an alkaline earth metal such as a Ba component is known in addition to zirconium oxide (see Patent Document 2).

Generally, a large amount of NOx is generated when the amount of fuel supplied to the engine is small, the amount of air is large, and the combustion temperature is high, but the Ba component temporarily absorbs the generated NOx. The NOx absorbed in the Ba component is released from the Ba component when the NOx concentration in the exhaust gas is low and the carbon dioxide (CO ₂ ) concentration is high. This is because Ba(NO ₃ ) ₂ reacts with carbon dioxide in the presence of water vapor to become BaCO ₃ (a part also becomes barium oxide), which can be said to be in chemical equilibrium. The NOx released from the Ba component reacts with the reducing component on the surface of the Rh component and is reduced and purified as described above.

Two or more co-catalyst components may be used in combination. For example, TWC using a Ba component and cerium oxide is known (see Patent Document 3). However, depending on the combination of catalyst materials, the purification performance may be reduced, and for example, it has been reported that the NOx purification performance is degraded when the Rh component and the Ba component are present in the same composition (Patent Document 4). reference). The reason for this is that the alkaline earth metal component has a function of occluding NOx, so that the purifying action of NOx in the Rh component is disturbed and that the oxidized Rh structure is stabilized by the electron donating action from Ba to Rh. It seems that this is due to the cause. Therefore, by separating the Rh component and the Ba component and supporting them on alumina, NOx purification performance and heat resistance have been improved.

The catalyst excellent in purification of NOx functions to the maximum when the internal combustion engine (engine) operates under the condition that the ratio of oxygen to fuel in the exhaust gas (air-fuel ratio) is at or near the stoichiometric condition. However, since the actual air-fuel ratio changes with each passing, it is common to add a material having an oxygen storage capacity (oxygen storage capacity: hereinafter also referred to as “OSC”) to the noble metal. The oxygen storage material has a property of storing oxygen when the oxygen concentration is high and releasing oxygen when the oxygen concentration is low.

Therefore, when the air-fuel ratio of the internal combustion engine (engine) is biased to the lean side (excess oxygen side), the oxygen storage material stores oxygen, and when the air-fuel ratio is biased to the rich side (oxygen deficiency side), oxygen is absorbed. By releasing, purification components such as CO, HC, and NOx are efficiently purified by the noble metal component.

Generally, the exhaust gas purifying catalyst has two catalyst coat layers (see Patent Document 5). In Patent Document 5, Rh and an oxygen storage material are used as a catalyst component in the surface layer, and Pd and/or Pt and an oxygen storage material are used as a catalyst component in the inner layer.

The method of automobile exhaust gas test is uniquely set for each country. The European New European Driving Cycle (hereinafter referred to as NEDC) mode has higher temperature and more frequent stoppages than Japanese JC08 mode. This is a mode in which measurements are taken from a cold engine under severe conditions. Such a catalyst did not work well when the temperature of the catalyst bed was relatively low immediately after the engine was started, and the nitrogen oxide (NOx) purification performance in the high load region was not sufficient. It should be noted that the catalyst of Patent Document 5 is characterized in that it does not contain a noble metal catalyst in the pores of the cell wall, and nothing is mentioned regarding the storage of NOx.

Therefore, Kato et al. installed two or more honeycomb structure-type catalysts in which the surface of a honeycomb body substrate was coated with two or more layers of catalyst composition in the exhaust gas flow path, and the catalyst located upstream and the catalyst located downstream A catalyst in which an alkaline earth metal that is a NOx storage component is supported in the lower layer of each of the catalysts located in the above is proposed (see Patent Document 6). As a result, TWC performance has been improved, but exhaust gas regulations have been tightened worldwide year by year, and further improvement in NOx purification performance is required for exhaust gas purification catalysts.

Republished Patent 2000/027508, page 14 JP 2007-319768 A, paragraph 0003 JP, 03-106446, A JP 2002-326033 A, paragraph 0013 JP, 2013-85986, A JP, 2010-29752, A

In view of the above-mentioned conventional problems, an object of the present invention relates to an exhaust gas purification catalyst that simultaneously purifies nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC) in engine exhaust gas, and has particularly high NOx purification performance. An object of the present invention is to provide an exhaust gas purifying catalyst that can obtain COP

The present inventor has conducted extensive studies in order to solve the above-described problems of the prior art, and provides a NOx storage layer that adsorbs and stores Pt and NOx and a NOx reduction layer that contains Rh on a catalyst layer containing Pd. The present invention has been completed by finding that a three-layer structure in which the middle layer of the NOx storage layer and the upper layer of the NOx reduction layer are adjacent to each other exhibits excellent NOx purification performance.

That is, according to the first aspect of the present invention, in an exhaust gas purifying catalyst having a catalyst layer in which a precious metal catalyst of Rh and Pt and/or Pd is supported on a honeycomb substrate,
Catalyst layer comprises three layers, the noble metal and oxygen storage material in the lower layer, and a NOx storage material comprising an alkaline earth metal compound of a noble metal and barium middle (Ba) and / or magnesium (Mg), a surface layer to the Rh of the precious metal, only including,
The lower precious metal is Pd,
The noble metal of the middle layer is Pt, or Pt and Pd,
The exhaust gas purifying catalyst is characterized in that the catalyst amount of the lower layer is 100 g to 200 g/L per unit volume of the honeycomb substrate .

According to a second aspect of the present invention, in the first aspect, the intermediate layer alkaline earth metal compound is an oxide of Ba, an oxide of Mg, barium carbonate, magnesium carbonate, barium sulfate, magnesium sulfate. An exhaust gas purifying catalyst is provided, which is at least one selected from the group consisting of, barium acetate, and magnesium acetate.

According to a third aspect of the present invention, there is provided the exhaust gas purifying catalyst according to the first or second aspect , wherein the lower layer oxygen storage material is a ceria-zirconia-based composite oxide. ..

Further, according to a fourth invention of the present invention, in the exhaust gas purifying catalyst according to any one of the first to third inventions, the surface layer further contains a ceria-zirconia-based composite oxide as an oxygen storage material. Will be provided.

Further, according to a fifth invention of the present invention, in any one of the first to fourth inventions, the Rh of the surface layer is supported on alumina and a ceria-zirconia-based composite oxide, A purification catalyst is provided.

According to the exhaust gas purifying catalyst of the present invention, CO, HC, and NOx contained in automobile exhaust gas can be purified at the same time, and particularly the NOx purifying characteristics are excellent. In particular, for a gasoline engine, the catalyst bed operates from a time when the temperature of the catalyst bed is relatively low immediately after starting, and a high NOx purification rate is achieved in a high load region.
By using the exhaust gas purifying catalyst of the present invention, it is possible to reduce the NOx emission amount after engine endurance (for example, 950° C., 50 hours) by 10% as a whole as compared with the conventional general catalyst.

It is explanatory drawing which showed the cross section typically about the three-layer catalyst (Ba and/or Mg compound in the middle layer) of this invention. It is explanatory drawing which showed typically the cross section about the conventional two-layer catalyst (A) and the three-layer catalyst (B) which has Mg compound and Ba compound for the lower layer for comparison. It is the result of testing the exhaust gas purification performance of the catalyst in the NEDC mode and measuring the NOx emission amount in the Cold region. It is the result of testing the exhaust gas purification performance of the catalyst in the NEDC mode and measuring the NOx emission amount in the high load range. It is the result of measuring the NOx emission amount by testing the exhaust gas purification performance of the catalyst in NEDC total mode.

Hereinafter, the exhaust gas purifying catalyst of the present invention will be described in detail with reference to the drawings. When used for purification of exhaust gas from a gasoline automobile or the like, it is possible to purify harmful components such as NOx with high performance, but the present invention is not limited to gasoline automobiles.

The exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst having a catalyst layer in which a precious metal catalyst of Rh, Pt and/or Pd is supported on a honeycomb substrate.
The catalyst layer is composed of three layers, the lower layer is a noble metal of Pt and/or Pd and an oxygen storage material, and the noble metal of Pt and/or Pd and an alkaline earth metal of barium (Ba) and/or magnesium (Mg) are in the middle layer. A NOx storage material containing a compound is included, and Rh of a noble metal is included in the surface layer.

1. Constituent Components of Catalyst The exhaust gas purifying catalyst of the present invention is composed of a carrier and a catalyst layer containing a noble metal component, an oxygen storage material, and a NOx storage material supported on the support. As shown in FIG. 1, the catalyst layer has a three-layer structure in which the honeycomb body base material of the carrier 1 is coated, and the lower layer 2 including the precious metal and the oxygen storage material formed on the honeycomb body base material, It has a middle layer 3 formed on the lower layer and containing a noble metal and a NOx adsorption/occlusion material, and a surface layer 4 formed on the middle layer and containing a noble metal and an oxygen storage material.

(1) Noble Metal Component The exhaust gas purifying catalyst of the present invention contains three types of Pt component, Pd component and Rh component as the noble metal component.

The Pt and Pd components are excellent in the oxidizing activity of O ₂ , such as HC and CO, and the Rh component is excellent in the reducing activity of NOx by the reducing components such as HC and CO. These three precious metal components work together. By doing so, purification of NOx, HC and CO is performed.

The Pt component, Pd component, and Rh component may be a metal or an oxide. The Pt component has a high activity for the oxidation of HC and CO, and the Pd component is effective for the oxidation of a specific HC species (high-boiling HC), and is used as an auxiliary to the Pt component.

Since the activity of the noble metal component depends on the size of its surface area, it is desirable that the noble metal component is stably dispersed in the catalyst composition in the form of fine particles and can maintain a stable and highly dispersed state even at high temperatures. Therefore, noble metal components such as alumina, zirconia, silica, silica-alumina, zeolite, titania, tungsten oxide, ceria, lanthanum oxide, neodymium oxide, praseodymium oxide, ceria-zirconia-based composite oxide having a high surface area of the support material itself. It is supported by a heat-resistant inorganic oxide. Of these, γ-alumina is preferable.

(2) Alkaline earth metal The catalyst of the present invention contains Ba component (for example, barium oxide) and/or Mg component (for example, magnesium oxide) which is an alkaline earth metal known as NOx storage component.
The alkaline earth metal may be either one of the Ba component and the Mg component, but preferably contains both.

The Ba compound as the Ba component mainly exists as barium oxide in the catalyst layer, but a part thereof may be barium sulfate or barium carbonate. Further, the Mg compound of the Mg component mainly exists as magnesium oxide in the catalyst layer, but a part thereof may be magnesium sulfate, magnesium carbonate, or the like.

In barium oxide, when oxygen is abundant (Lean), NOx is occluded by Ba component when it becomes barium nitrate, and when oxygen is abundant (Rich), barium nitrate is occluded by barium oxide and released NOx is released. To do. The NOx thus released is purified by the Rh component by utilizing HC and CO of the reducing agent and hydrogen generated by the steam reforming reaction. The same applies to magnesium oxide.

In the present invention, these barium compound and magnesium compound are used as they are for the honeycomb substrate, but may be used by supporting them on γ-alumina, ceria-alumina, and ceria-zirconia-based composite oxides. .. The preferred support material is γ-alumina.

(3) Oxygen storage material In the present invention, a ceria-zirconia-based composite oxide is used as the oxygen storage material. The ceria-zirconia-based composite oxide is an oxide containing ceria and zirconium as main components, but also functions as a support material for Pt component, Pd component, and Rh component. In order to support the noble metal component, it can be prepared by impregnating the support material with the raw material solution and firing the obtained impregnated powder in the air at 500 to 700° C. for 10 to 60 minutes.

Ceria in the ceria-zirconia-based composite oxide functions as an OSC, and changes from CeO ₂ to Ce ₂ O ₃ with the occlusion and release of oxygen.

On the other hand, zirconia promotes steam reforming reaction and CO+NO reaction when used together with Rh component, and in addition to ceria, additives such as lanthanum oxide, praseodymium oxide and neodymium oxide are further added in order to improve heat resistance of zirconia. Are added with additives such as calcium oxide, strontium oxide, magnesium oxide, and yttrium oxide, and are added to Ce-Zr-Sr composite oxide, Ce-Zr-La composite oxide, Ce-Zr-La-Pr composite oxide, Ce. Used in —Zr—La—Y composite oxide and the like.

Among them, in the case of Ce-Zr-La composite oxide, it is preferable that ceria is 5 to 30% by weight, zirconia is 50 to 90% by weight, and lanthanum oxide is 1 to 20% by weight. Further, in the case of Ce-Zr-La-Pr composite oxide, ceria is 20 to 50% by weight, zirconia is 20 to 50% by weight, lanthanum oxide is 2 to 20% by weight, and praseodymium oxide is 1 to 5% by weight. % Is preferably contained.

Among the ceria-zirconia-based composite oxides, those having a high surface area are preferable because they have excellent heat resistance and maintain a high dispersion of active species during long-term use. Therefore, BET specific surface area (measured by BET method. Hereinafter, the same) is preferably a 20~120m ² / g, more preferably 30~90m ² / g.
By using the above oxygen storage material, oxygen can be stored and released quickly, and the oxidation of HC and CO by the active component can be promoted.

(4) Alumina In the present invention, alumina is used for each layer of the catalyst layer and functions as a support material for rhodium (Rh), platinum (Pt), and palladium (Pd).

Examples of alumina include γ-alumina, δ-alumina, θ-alumina, α-alumina, and the like, but it is preferable to use γ-alumina, δ-alumina, and θ-alumina excluding α-alumina having a poor surface area. Among them, γ-alumina is inferior in durability at 1000° C. or higher compared to other aluminas, but has sufficient heat resistance as a catalyst used at 1000° C. or lower and has a surface area of all these aluminas. Since it is the highest among them, it is particularly preferable as a support material.

BET specific surface area of the alumina is preferably 50 to 250 m ² / g, more preferably 100 to 200 m ² / g. With such a surface area, not only a high surface area can be maintained even at high temperature, but also the noble metal component can be stabilized in a highly dispersed state. The average pore diameter is preferably 10 to 80 μm, more preferably 20 to 60 μm. With such average pore diameter, sufficiently high reactivity can be exhibited.

Further, in the present invention, in order to maintain the surface area of alumina at a high temperature, additives such as ceria, lanthanum oxide, silica, titania and iron may be added.

(5) Honeycomb Body Base Material The honeycomb body base material in the present invention is a material that supports the above catalyst component, and has a large number of through holes extending from one end surface to the other end surface. Together they form a honeycomb shape.

In the present invention, for example, a honeycomb-type ceramic carrier in which each cell has a rectangular cross section can be used. The material and structure of the carrier are not limited to this, and may be, for example, a metal carrier composed of a flat plate and a corrugated plate, and the shape of the cell may be hexagonal.

Further, the honeycomb body substrate is classified into a flow-through type and a wall-flow type according to its structure and function. The wall flow type is used for filtering out solid components such as soot and SOF in exhaust gas, and is used for a Diesel Particulate Filter (DPF). In the wall flow type, one end of a through hole made of a porous wall is alternately sealed and has a function as a filter for filtering out particulate components such as soot.
On the other hand, the flow-through type has a structure having a large number of through holes that open from one open end face to the other open end face, and is widely used in oxidation catalysts, reduction catalysts, and three-way catalysts (TWC). It is used. In the present invention, it is desirable to use a flow-through type honeycomb substrate.

In a honeycomb substrate such as a flow-through type carrier, its structural characteristic is represented by cell density. The honeycomb body substrate used in the present invention has a cell density of 100 to 1500 cell/inch ² (155k to 2325k/m ² ), particularly a flow-through type carrier of 200 to 900 cell/inch ² (310k to 1400k/m ² ). Is preferred. If the cell density is 10 cell/inch ² (155 k/m ² ) or more, the contact area between the exhaust gas and the catalyst required for purification can be secured, and the exhaust gas purification performance excellent in structural strength can be obtained. If the obtained cell density is 1500 cell/inch ² (2325 k/m ² ) or less, the contact area between the exhaust gas and the catalyst can be sufficiently ensured without significantly losing the pressure of the exhaust gas of the internal combustion engine. Particularly, in the catalyst of the present invention, a flow-through type carrier of 300 to 900 cell/inch ² (465k to 1400k/m ² ) is preferable from the viewpoint of suppressing pressure loss.

2. Layer Structure of Catalyst As shown in FIG. 1, the structure of the catalyst layer of the present invention is a three-layer structure composed of a honeycomb substrate as the carrier 1 and catalyst layers 2 to 4 containing the precious metal element carried thereon. Has become.

Each catalyst layer is composed of a support material, a noble metal, a cocatalyst component, and the like, and the support material is an alumina material or a ceria-zirconia-based composite oxide that is an oxygen storage material that also serves as a cocatalyst component. The alkaline earth metal compound of the NOx storage material is an essential component as the co-catalyst component.

(1) Lower layer In the lower layer 2 of the present invention, a catalyst layer contains a noble metal and an oxygen storage material. At least one selected from Pt, Pd, and Rh can be used as the noble metal, but it is desirable to use Pd in combination with the oxygen storage material.

The catalyst of the present invention preferably contains alumina (palladium (Pd)-supported alumina) in the lower layer and ceria-zirconia-based composite oxide that is an oxygen storage material supporting palladium (Pd).

The Pd component may be a metal, but it may be oxidized in the firing process during the manufacturing process described later or in the exhaust gas purification process, and part of it may be palladium oxide. The amount of the Pd component supported on the alumina or ceria-zirconia-based composite oxide is not particularly limited, but in order to facilitate the oxidative purification reaction of O ₂ of HC and CO, it is calculated in terms of metal Pd. It can be 0.01 to 2 g/L, preferably 0.05 to 1 g/L per unit volume of the honeycomb body substrate. Further, it can be 0.3 to 1.5% by weight in total with respect to the lower layer. Preferred is 0.5 to 1.0% by weight.

In the present invention, it is relatively unlikely that the poisonous substance will adhere to the surface layer where the exhaust gas first comes into contact and the poisonous substance will diffuse and reach the lower layer, and a precious metal weak against poisoning should be arranged in the lower layer. You can Therefore, the cost of the catalyst itself can be reduced by disposing Pd, which is cheaper than Rh, in the lower layer.

In addition to the oxygen storage material, γ-alumina powder is preferably used as the support material supporting the noble metal. Since γ-alumina powder is generally higher in heat resistance than oxygen storage materials, it is effective in suppressing sintering of precious metals and can be expected to maintain performance after endurance.

The lower layer catalyst amount, particularly the lower layer alumina and ceria-zirconia-based composite oxide content is preferably 80 to 200 g/L, more preferably 100 to 150 g/L per unit volume of the honeycomb body substrate. .. Ceria carrying Pd is complexed with zirconia, and its oxygen storage/release capacity supports the oxidation performance of HC and CO by Pd.

Considering the control of the oxidation and purification reaction of O ₂ of HC and CO, the suppression of aggregation of ceria, the suppression of coarsening of Pd, etc., 20 to 50% by weight of ceria and 20 to 20% of zirconia are used as the ceria-zirconia-based composite oxide. It is preferable to use a Ce-Zr-La-Pr composite oxide containing 50% by weight, 2 to 20% by weight of lanthanum oxide, and 1 to 5% by weight of praseodymium oxide.

In the present invention, a binder component may be further added to all the catalyst layers to enhance the bonding between the honeycomb substrate and the catalyst layers or the bonding between the catalyst layers regardless of the use conditions.
As the binder component, various sols such as alumina sol, silica sol, zirconia sol, titania sol can be used. Also, soluble salts such as aluminum nitrate, aluminum acetate, zirconium nitrate, zirconium acetate can be used.

(2) Middle layer In the catalyst layer of the middle layer 3 in the present invention, a noble metal and a NOx storage material are used. As the noble metal, Pt or Pt and Pd are combined with the NOx storage material. Further, γ-alumina powder may be used as the support material for supporting the noble metal and the NOx storage material.

The catalyst of the present invention contains a NOx storage material containing Pt or a noble metal of Pt and Pd and an alkaline earth metal compound of barium (Ba) and/or magnesium (Mg) in the middle layer. The alkaline earth metal compound may be one of barium (Ba) or magnesium (Mg), but preferably contains both.
Examples of the NOx storage material include, in addition to alkaline earth metal compounds, inorganic oxides containing elements Ce and La having a NOx storage function, such as ceria, ceria-zirconia, La oxide, ceria-La composite oxide, and La. Examples thereof include alumina to which is added.

The Pt and Pd components may be metals, but may also be those that are partially oxidized to be oxidized in the firing process during the manufacturing process described later or in the exhaust gas purification process. The amount of the Pt component supported on alumina is not particularly limited, but can be 0.1 to 1 g/L in terms of metal Pt per unit volume of the honeycomb body substrate, and the amount of the reducing agent such as HC or CO can be reduced. 0.2 to 0.8 g/L is preferable because the oxidation purification reaction by O ₂ is sufficiently exhibited. The amount of the Pd component supported on the alumina is not particularly limited, but from the same viewpoint as in the case of Pt, it is 0.01 to 0.5 g/L per unit volume of the honeycomb substrate in terms of metal Pd. It is possible, and 0.05-0.3 g/L is preferable.

The amount of catalyst in the middle layer is preferably 30 to 150 g/L per unit volume of the honeycomb body substrate. In particular, the amount of alumina supporting the Pt and Pd components in the middle layer is preferably 50 to 150 g/L, and more preferably 50 to 100 g/L per unit volume of the honeycomb body substrate.

On the other hand, the content of the alkaline earth metal compound in the middle layer is 10 to 100 g/L in total per unit volume of the honeycomb substrate, because the reducing action of NOx by HC and CO is sufficiently exerted. Is preferable and 15-80 g/L is more preferable. The alkaline earth metal compound supports the NOx reduction performance of HC and CO due to its NOx adsorption and storage capacity.

In the present invention, a Ba component and/or a Mg component is used as the alkaline earth metal compound.
The Ba component and the Mg component are at least one selected from the group consisting of Ba oxide, Mg oxide, barium carbonate, magnesium carbonate, barium sulfate, magnesium sulfate, barium acetate, and magnesium acetate.

Although these are contained in the middle layer, they may be supported on a support material and used. For example, in the case of barium carbonate, when oxygen is abundant (Lean), NOx is occluded by Ba component when it becomes barium nitrate, and when it is abundant (Rich), barium nitrate is occluded by barium carbonate. Releases NOx. In such an atmosphere in which NOx is released, the exhaust gas contains abundant reducing components such as HC and CO. The released NOx utilizes HC, CO, and hydrogen generated by the steam reforming reaction, and is purified by the Rh component in the upper layer.

In the present invention, among the Ba component and the Mg component, since Ba and Mg do not aggregate and NOx storage performance can be maintained, 10-50 g/L of barium oxide and magnesium oxide per unit volume of the honeycomb body substrate. Is preferably contained in an amount of 2 g/L to 50 g/L.

In the present invention, the Rh component, the Pt component, and the Pd component are used in a catalyst in which the lower layer, the middle layer, and the surface layer are multi-layered. In the multi-layered catalyst, the surface layer has more opportunities to come into contact with harmful gases such as HC, CO, and NOx in the exhaust gas than the middle layer and the lower layer, so that the upper layer tends to have higher purification activity than the lower layer. Further, the upper layer may be directly exposed to high temperature exhaust gas, or locally exposed to high temperature due to heat generated during the redox reaction.

Therefore, the support material, precious metal component, co-catalyst component, etc. of the upper layer may be more likely to sinter or grow particles than the lower layer, and further, in the upper layer, poisonous substances represented by sulfur are mixed in the exhaust gas. Since they flow to the catalyst and adhere to the upper layer first, they are easily affected by these poisonous substances.

On the other hand, since the poisonous substance adheres to the upper layer in the middle layer, the risk that the poisonous substance diffuses and arrives is relatively low. By arranging Pt and Pd, which are cheaper than Rh, the unit price of the catalyst itself is reduced. Can be lowered.

In the present invention, the intermediate layer contains a Ba and/or Mg compound of an alkaline earth metal which is a NOx storage material. Thereby, the NOx storage layer functions to reduce the NOx emission amount. Since the catalyst bed temperature in the high load region exceeds 600° C., the NOx occlusion action is unlikely to occur, but it is considered that NOx adsorption/desorption occurs in Ba, Mg, etc. in a short time, and the NOx emission amount is reduced.

Further, since the middle layer is sandwiched between the upper layer and the lower layer, and both the upper layer and the lower layer are made of a relatively dense material, the middle layer has a NOx retention function, and the NOx purification performance is improved. .. That is, when the middle layer contains a specific NOx storage material, the NOx retention property is improved as compared with the case where the NOx storage material is not contained. This is because HC and CO that have flowed in the NOx staying in the middle layer from the surface layer are reduced. It is considered that the NOx emission amount is reduced. Since the NOx purification rate is higher in the middle layer than in the lower layer, it is considered that the NOx storage layer is effective in the middle layer. Further, if the NOx storage layer is arranged in the lower layer and a relatively dense material containing Pd is arranged in the upper intermediate layer, the diffusion of NOx and the reducing agents HC and CO is prevented and the reactivity is not improved. It is considered to be a thing.

(3) Surface layer The noble metal Rh is used for the catalyst layer of the surface layer 4 of the present invention, and alumina and an oxygen storage material are used in combination as a support material supporting the precious metal. That is, the catalyst in the uppermost layer contains alumina carrying rhodium (Rh) and ceria-zirconia-based composite oxide carrying rhodium (Rh).

As described above, the surface layer has more opportunities to come into contact with harmful gases such as HC, CO, and NOx in the exhaust gas than the lower layer, so that the surface layer tends to have higher purification activity. Further, the surface layer may be directly exposed to high-temperature exhaust gas, or locally exposed to high temperature due to heat generated during the redox reaction. Therefore, the support material, the noble metal component, the cocatalyst component, and the like on the surface layer may be more likely to undergo sintering and particle growth than the lower layer. Further, in the surface layer, poisonous substances represented by sulfur are mixed in the exhaust gas and flow to the catalyst, and are attached first, so that they are easily affected by these poisoning substances.

As described above, in the present invention, the intermediate layer contains the Ba and Mg compounds of the alkaline earth metal as the NOx storage material. On the other hand, when a certain amount of the alkaline earth metal Ba or Mg compound, which is the NOx storage material, is also included in the surface layer, the NOx storage amount increases, but the alkaline earth metal Ba or Mg compound adsorbs NOx. Since the force is high, it is difficult for NOx to be released at low temperatures, so reduction may not proceed and the performance may deteriorate.

The Rh component may be present in the form of metal Rh, but it may be contained in the form of rhodium oxide by being oxidized in the firing in the manufacturing process described later or in the exhaust gas purification process.

Since Rh is a highly active active species, if the particle size is small and the dispersion state is good, the activity can be exhibited even in a small amount. However, since Rh has a small amount of resources and is expensive, it is desired to maintain a high dispersion state. Therefore, in the present invention, the Rh component is used for the surface layer directly exposed to NOx and reactive HC component.

The amount of the Rh component is not particularly limited, but it suppresses the enlargement of the Rh particles during preparation and secures the absolute value of the Rh surface area, while suppressing the reduction of the NOx purification action due to sintering at high temperature. In terms of Rh, it can be 0.01 to 5 g/L per unit volume of the honeycomb body substrate, and 0.1 to 1 g/L is preferable.

The ceria-zirconia-based composite oxide in the surface layer preferably contains 2 to 30% by weight of lanthanum oxide. More preferably, the ceria-zirconia-based composite oxide contains 5 to 20% by weight of lanthanum oxide.

The content of alumina and ceria-zirconia-based composite oxide in the surface layer maintains the dispersibility of Rh particles, Pt and/or Pd and does not reduce the activity, and increases exhaust pressure due to the thickness of the catalyst layer and output of the internal combustion engine. From the viewpoint of preventing the decrease, the honeycomb body substrate has a unit volume of 30 to 150 g/L, preferably 40 to 100 g/L.

In the catalyst as a whole, the amount of the essential constituents is such that, from the viewpoint that the active species can be stably dispersed without clogging by the catalyst composition, the total coating amount of the catalyst composition per unit volume of the honeycomb body substrate. Is 150 to 500 g/L, preferably 200 to 400 g/L.

In the catalyst of the present invention, the surface layer, the middle layer, and the lower layer have the minimum constituent units of the catalyst composition. However, unless the above-mentioned object of the present invention is impaired, a separate binder layer between the honeycomb body substrate and the lower layer, between the lower layer and the upper layer, and further above the upper layer, a suppression for suppressing the migration of the catalyst component. A layer, a coating layer, a different catalyst composition layer, and the like may be appropriately provided.

In the surface layer, the middle layer, the lower layer of the present invention, and other layers provided as necessary, in addition to the above essential components, transition metals such as silver, copper, nickel, tungsten, vanadium, silicon, titanium, and tungsten, potassium, etc. The oxides of alkali metals and alkaline earth metals such as calcium may be used alone or as a composite oxide.

2. Preparation of catalyst A preferable mode of the method for producing the exhaust gas purifying catalyst will be described. The exhaust gas purifying catalyst can be manufactured by performing a slurry coating step, a drying step, and a firing step.
Prior to the slurry coating step, the catalyst component used for each layer is prepared, and a catalyst composition slurry containing this catalyst component is formed.

Pd-supported γ-alumina and Pd-supported Ce-Zr-La-Pr oxide, Pt-Pd-supported γ-alumina, Rh-supported γ-alumina and Rh-supported Ce-Zr-La system, which are constituents used in the catalyst of the present invention. The composite oxide and the like can be produced by a known method.

That is, in order to support rhodium (Rh), platinum (Pt), and palladium (Pd) on the support material, it is possible to prepare a raw material solution and perform it by a known method such as an impregnation method, an ion exchange method, or a kneading method.
As the raw material of the noble metal component, compounds such as nitrates, sulfates, carbonates and acetates and ethanolamine solutions can be used, and nitrates and ethanolamine compounds are preferably used. At this time, an acid or alkali for pH adjustment, a surfactant for adjusting viscosity or improving slurry dispersibility, a dispersing resin, or the like can be added to the raw material solution, if necessary. Then, the heat-resistant inorganic oxide containing a noble metal component is dried at 50 to 200° C. to remove the solvent.

Here, as a method for mixing the catalyst support material containing the noble metal component and the heat resistant inorganic oxide, pulverization and mixing by a ball mill or the like can be applied, but other pulverization or mixing methods may be applied. The crushing condition is preferably such that the particle size of the catalyst support material is 0.1 to 50 μm. If necessary, the mixture of catalyst support materials is calcined to obtain a catalyst composition. The firing temperature is preferably 200 to 650°C, more preferably 250 to 600°C. As for the heating means, a known heating means such as an electric furnace or a gas furnace can be used.

On the other hand, the same applies to the case where alumina supported as an oxide is prepared using barium sulfate, barium carbonate, magnesium acetate, or magnesium carbonate.
The Ba component exists as barium oxide in the honeycomb structure type catalyst, but in the production of the catalyst composition slurry, barium sulfate or barium carbonate can be finely pulverized and then supported on alumina as a support material. .. In order to highly disperse barium, it is preferable to add it in the form of a soluble salt, and barium chloride, barium hydroxide, barium acetate, barium nitrate or the like may be used in combination. Further, the Mg component also exists as magnesium oxide in the honeycomb structure type catalyst, but in producing the catalyst composition slurry, not only magnesium acetate and magnesium carbonate but also soluble salts such as magnesium chloride, magnesium nitrate and magnesium sulfate. Can be used.

Note that barium oxide, barium sulfate, barium carbonate, and magnesium oxide, magnesium sulfate, and magnesium carbonate are difficult to dissolve in water, so barium and magnesium are unlikely to dissolve into the aqueous solution during slurry formation. ..

A uniform solution containing the Ba component and/or the Mg component is supported on the alumina powder by an impregnation method or the like, and then baked on an electric furnace or the like to be fixed on the alumina.

The catalyst component of each layer thus prepared is made into a catalyst composition slurry containing an aqueous medium, and the exhaust gas purifying catalyst is manufactured by applying the catalyst composition slurry to a carrier made of a cordierite honeycomb substrate in layers. That is, in the present invention, a honeycomb base material is coated with the catalyst composition slurry by a known wash coating method to obtain a honeycomb catalyst.

First, the lower layer material slurry is coated, the middle layer material slurry is coated thereon, and finally the surface layer material slurry is coated. The wash coating method is to obtain a honeycomb structure type catalyst coated with a catalyst composition by applying a catalyst composition to a honeycomb substrate and then drying and firing. The catalyst composition slurry is used as a viscosity suitable for coating. The viscosity has a value measured by a B-type viscometer of 5 to 2000 mPa·s, and preferably 10 to 1000 mPa·s so that the entire inside of the honeycomb substrate can be coated.

The drying temperature is preferably 50 to 200°C, more preferably 70 to 150°C. The firing temperature is preferably 300 to 700°C, and particularly preferably 350 to 650°C. As for the heating means, a known heating means such as an electric furnace or a gas furnace can be used. In the above-mentioned production method, the slurry may be applied and the catalyst layer may be fired every time a catalyst layer is added, but if the catalyst layer can be formed with a desired composition, the firing may be performed collectively at the end.

The exhaust gas purifying catalyst is used by being arranged in the exhaust gas flow path, that is, in the exhaust gas flow of an automobile. Especially, when it is arranged just below the gasoline engine, the discharged NOx is occluded by the Ba component and/or the Mg component contained in the middle layer of the present catalyst, and the occluded NOx is desorbed at a high temperature, and HC in the exhaust gas is desorbed. It is effectively reduced by contact with CO and CO.

Although the catalyst of the present invention may be arranged alone in the exhaust gas flow, a plurality of catalysts may be used or a catalyst having a different action may be used in combination. When using a plurality of catalysts, the catalysts may be arranged adjacent to each other, or may be arranged immediately below the engine and below the floor of the chassis.

Hereinafter, examples for explaining the exhaust gas purifying catalyst according to the present invention in detail will be shown. However, the present invention is not construed as being limited to these examples.

<Engine durability processing>
The obtained honeycomb catalyst was heat-treated at 950° C. for 50 hours in a stoichiometric gasoline engine operating near the stoichiometric air-fuel ratio.

<Catalyst performance test with actual vehicle>
The honeycomb catalyst after the engine durability treatment was mounted on a catalytic converter, and the exhaust gas purification performance of the catalyst was examined in a NEDC mode using a gasoline vehicle equipped with a European specification naturally-aspirated engine having a displacement of 1.0 L.

(Example 1)
In the following procedure, first, the catalyst component used for each layer is prepared, and the catalyst composition slurry containing the catalyst component is applied in layers to a carrier composed of a cordierite honeycomb structure to produce the exhaust gas purifying catalyst shown in FIG. 1. did.

<Pd-supporting γ-alumina and Pd-supporting Ce-Zr-La-Pr composite oxide>
A commercially available γ-alumina powder (BET specific surface area of 141 m ² /g, average pore diameter of 41 nm) was used as a commercially available solution of Pd nitrate diluted with pure water (Pd content: 20% by weight), and Ce-Zr-La-Pr composite oxide. The powder (impregnated with a content ratio of Ce-Zr-La-Pr oxide of 45:45:8:2, BET specific surface area of 69 m ² /g, average pore diameter of 52 nm) was impregnated and supported. These impregnated powders were calcined in air at 600° C. for 30 minutes. Thereby, a mixture of Pd-supporting γ-alumina and Pd-supporting Ce-Zr-La-Pr composite oxide for the lower layer (total Pd content 0.794% by weight) was prepared.

<γ-alumina carrying Pt and Pd>
A solution of Pd nitrate diluted with pure water (Pd content 20% by weight) and a Pt monoethanolamine solution (Pt content 13% by weight) were mixed with γ-alumina powder (BET specific surface area 141 m ² /g, average pore diameter 41 nm). Was impregnated and supported. This impregnated powder was calcined in air at 600° C. for 30 minutes to prepare a Pt—Pd-supported alumina powder (0.602 wt% Pt, 0.061 wt% Pd) for the middle layer.

<Rh-supporting γ-alumina and Rh-supporting Ce-Zr-La composite oxide>
A commercially available γ-alumina powder (BET specific surface area 141 m ² /g, average pore diameter 41 nm) and a Ce-Zr-La composite oxide powder ( It was impregnated and supported with a Ce-Zr-La oxide conversion content ratio of 10:84:6, a BET specific surface area of 59 m ² /g, and an average pore diameter of 36 nm. By firing these impregnated powders in air at 600° C. for 30 minutes, Rh-supporting alumina powder for the surface layer and Rh-supporting Ce-Zr-La-based composite oxide (total Rh content 0.595% by weight). Was prepared.

<Three-layer catalyst (Ba and/or Mg compound in the middle layer)>
Next, the prepared Pd-supported alumina powder for the lower layer and the Pd-supported Ce-Zr-La-Pr composite oxide (total Pd content 0.794% by weight) were mixed and pulverized with a pot mill, and pure water was added. Was slurried and coated on a cordierite honeycomb carrier at 107 g/L. The Pd-supported alumina powder and the Pd-supported Ce-Zr-La-Pr composite oxide were used in amounts of 34.5% by weight and 65.5% by weight, respectively, with respect to 100% by weight of the entire lower layer catalyst composition. %.
Then, the prepared Pt-Pd-supported alumina powder for the middle layer (0.602 wt% Pt, 0.061 wt% Pd) was mixed with a Mg compound (substance name: magnesium acetate, manufactured by Kishida Chemical Co., purity: 98%). And Ba compound (substance name: Ba sulfate, manufactured by Sakai Chemical Co., Ltd., purity: 97% or more), pulverized, slurried with pure water, and applied on a cordierite honeycomb carrier at 83 g/L. The Pt-Pd-supported alumina powder, the Mg compound, and the Ba compound were used in an amount of 65.1% by weight of Pt-Pd-supported alumina powder and 6% by weight of Mg compound based on 100% by weight of the total catalyst composition for the middle layer. 0.0 wt% and Ba compound were 28.9 wt %.
Subsequently, the prepared Rh-supported alumina powder for the surface layer and the Rh-supported CeO ₂ -ZrO ₂ -based composite oxide (total Rh content of 0.595 wt %) were mixed and pulverized to form a slurry with pure water, and then a cordier 86 g/L was coated on a honeycomb carrier made of Wright to manufacture the three-layer catalyst of the present invention. The Rh-supported alumina powder and the Rh-supported CeO ₂ —ZrO ₂ -based composite oxide were used in amounts of 76.7% by weight and 23.3% by weight, respectively, based on 100% by weight of the entire surface layer catalyst composition. And
After that, the honeycomb catalyst obtained was subjected to engine durability treatment in the above-described manner, and a catalyst performance test was carried out using an actual vehicle. The results are shown in FIGS.

(Comparative example 1)
In contrast to Example 1, a two-layer catalyst shown in FIG. 2 having no Pd layer (lower layer) and Rh layer (surface layer) was prepared in the following manner.

<Two-layer catalyst>
A mixture of Pd-supporting γ-alumina and Pd-supporting Ce-Zr-La-Pr composite oxide (total Pd-containing content) was carried out in the same manner as in Example 1 except that a Pd nitric acid solution (Pd content 20% by weight) was used. Amount 1.453% by weight) was prepared. The prepared Pd-supported alumina powder and the Pd-supported Ce-Zr-La-Pr composite oxide (total Pd content: 1.453% by weight) were mixed and pulverized in a pot mill, slurried with pure water, and made of cordierite. A honeycomb carrier was coated with 190 g/L. The amounts of the Pd-supported alumina powder and the Pd-supported Ce-Zr-La-Pr composite oxide used were 22.6% by weight and 77.4% by weight, respectively, with respect to 100% by weight of the entire lower layer catalyst composition. %.
Then, Rh/Pt loading was carried out in the same manner as in Example 1 except that a Pt monoethanolamine solution (Pt content 13% by weight) was used in addition to the Rh nitrate solution (Rh content 6% by weight). alumina powder, and Rh / Pt-supported CeO2-ZrO ₂ composite oxide was prepared. The prepared Rh/Pt-supported alumina powder and the Rh/Pt-supported CeO2-ZrO ₂ -based composite oxide (total Rh content 0.195% by weight, total Pt content 0.061% by weight) were mixed and pulverized. A slurry of pure water was applied to a cordierite honeycomb carrier at 90 g/L to produce a two-layer catalyst. The amounts of the Rh/Pt-supported alumina powder and the Rh/Pt-supported CeO ₂ —ZrO ₂ -based composite oxide used were 50.0% by weight and 50%, respectively, with respect to 100% by weight of the entire upper layer catalyst composition. It was set to 0.0% by weight.
After that, the honeycomb catalyst thus obtained was subjected to an engine durability treatment in the above-described manner, and a catalyst performance test was carried out using an actual vehicle. The results are shown in FIGS.

(Comparative example 2)
A three-layer catalyst as shown in FIG. 3 in which the intermediate layer and the lower layer were arranged opposite to Example 1 was prepared in the following manner.

<Three-layer catalyst (Ba and/or Mg compound in lower layer)>
Pt and/or Pd-supporting γ-alumina was prepared in the same manner as in Example 1 except that a Pt monoethanolamine solution (Pt content 13 wt%) was used in addition to the Pd nitric acid solution (Pd content 20 wt%). And a mixture of Pt and/or Pd-supported Ce-Zr-La-Pr composite oxide was prepared. The prepared Pt-Pd-supported alumina powder (Pt 0.599 wt%, Pd 0.060 wt%) was mixed with a Mg compound (substance name: Mg acetate, manufactured by Kishida Chemical Co., Ltd., purity: 98%) and a Ba compound (substance name: Ba sulfate, Sakai). Chemically-produced, purity: 97% or more) to form a slurry, which was coated on a cordierite honeycomb carrier at 91.5 g/L. The amounts of the Pt-Pd-supporting alumina powder, Mg compound and Ba compound used were 83.6% by weight of the Pt-Pd-supporting alumina powder and 5. 5% of the Mg compound, respectively, based on 100% by weight of the entire lower layer catalyst composition. 5 wt% and Ba compound 10.9 wt%.
Next, a mixture of Pd-supporting γ-alumina and Pd-supporting Ce-Zr-La-Pr composite oxide (total Pd content 0.894% by weight) was prepared. The prepared Pd-supported alumina powder and the Pd-supported Ce-Zr-La-Pr composite oxide (total Pd content 0.894% by weight) were mixed and pulverized in a pot mill to form a slurry with pure water, and a cordierite honeycomb was prepared. The carrier was coated with 120 g/L. The amounts of the Pd-supported alumina powder and the Pd-supported Ce-Zr-La-Pr composite oxide used were 58.3% by weight and 41.7% by weight, respectively, with respect to 100% by weight of the total catalyst composition for the middle layer. %.
Thereafter, a Rh-supported alumina powder and a Rh-supported Ce-Zr-La-based composite oxide were prepared in the same manner as in Example 1 except that a low-concentration Rh nitric acid solution (Rh content 6% by weight) was used. The prepared Rh-supported alumina powder and Rh-supported Ce-Zr-La-based composite oxide (total Rh content 0.359% by weight) were slurried, and 86 g/L was applied to a cordierite honeycomb carrier to form a lower layer. A three-layer catalyst containing Ba and/or Mg compounds was prepared. The amounts of the Rh-supported alumina powder and the Rh-supported Ce-Zr-La-based composite oxide used were 76.7% by weight and 23.3% by weight, respectively, with respect to 100% by weight of the entire surface layer catalyst composition. And
After that, the honeycomb catalyst thus obtained was subjected to an engine durability treatment in the above-described manner, and a catalyst performance test was carried out using an actual vehicle. The results are shown in FIGS.

<Evaluation>
FIG. 3 is a result of the NEDC mode Cold region, FIG. 4 is a high load region, and FIG. 5 is a result of NOx emission amount in the mode total.
From FIG. 3, the NOx storage layer was arranged in the lower layer in Comparative Example 2 as compared with the two-layer Comparative Example 1 in which the NOx storage layer was not arranged, but the NOx emission amount in the Cold region was not reduced. On the other hand, in Example 1 in which the NOx storage layer is arranged in the intermediate layer and has three layers, the NOx emission amount in the cold region is reduced.
When the NOx storage layer is arranged in the lower layer of the three-layer structure as in Comparative Example 2, it is considered that the gas diffusivity of NOx in the exhaust gas is lowered in the Cold region, and NOx is not adsorbed to the NOx storage material.

Further, from FIGS. 4 and 5, in Comparative Example 2 and Example 1 having the NOx storage layer in the high load region, the NOx emission amount is smaller than that in Comparative Example 1, and the catalyst bed temperature in the region is Since the temperature exceeds 600° C., it is considered that the NOx occlusion action does not occur, but it is considered that the NOx adsorption/desorption to Ba, Mg, etc. in a short time occurs.
That is, the comparative example 2 and the example 1 including the NOx adsorbent have improved NOx retention properties as compared with the comparative example 1 not including the NOx storage material, and the NOx retained in the middle layer flows from the surface layer to the HC and Since CO is reduced, it is considered that the NOx emission amount is reduced. The NOx retention is greater in Example 1 in which the NOx storage layer is disposed in the middle layer than in Comparative Example 2 in which the NOx storage layer is disposed in the lower layer, and it can be said that the NOx storage layer is effective when disposed in the middle layer.

When the exhaust gas purifying catalyst of the present invention is used for purifying NOx-containing exhaust gas discharged from a gasoline engine, remarkable effects can be obtained. In addition to diesel engines, it can also be used for exhaust gas purification from combustion engines that use fossil fuels such as gas turbines.

1 carrier 2 lower layer 3 middle layer 4 surface layer (upper layer)

Claims (5)

In an exhaust gas purifying catalyst having a catalyst layer in which a precious metal catalyst of Rh and Pt and/or Pd is supported on a honeycomb substrate,
Catalyst layer comprises three layers, the noble metal and oxygen storage material in the lower layer, and a NOx storage material comprising an alkaline earth metal compound of a noble metal and barium middle (Ba) and / or magnesium (Mg), a surface layer to the Rh of the precious metal, only including,
The lower precious metal is Pd,
The noble metal of the middle layer is Pt, or Pt and Pd,
An exhaust gas purifying catalyst , wherein the catalyst amount of the lower layer is 100 g to 200 g/L per unit volume of the honeycomb body substrate . The alkaline earth metal compound of the middle layer is at least one selected from the group consisting of Ba oxide, Mg oxide, barium carbonate, magnesium carbonate, barium sulfate, magnesium sulfate, barium acetate, and magnesium acetate. The exhaust gas purifying catalyst according to claim 1. The exhaust gas purifying catalyst according to claim 1 or 2, wherein the lower layer oxygen storage material is a ceria-zirconia-based composite oxide. The surface layer, the exhaust gas purifying catalyst according to any one of claims 1 to 3 further characterized in that it comprises a ceria-zirconia composite oxide of the oxygen storage material. The surface layer of Rh is, the exhaust gas purifying catalyst according to any one of claims 1 to 4, characterized in that it is supported on alumina and ceria-zirconia composite oxide.

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