JPH05293376A - Catalyst for purifying exhaust gas and method therefor - Google Patents

Abstract

PURPOSE:To remove HC with high efficiency by introducing secondary air at the start of an engine when a large amt. of HC is generated by incorporating at least Pt or Pd into the uppermost surface of the upstream catalyst bed as a catalytically active component and at least Rh into the downstream catalyst bed. CONSTITUTION:The upstream side of a monolithic carrier 14 is coated with the upstream catalyst bed having hydrocarbon purifying activity and contg. at least Pt or Pd in the uppermost surface layer as a catalytically active component, and the downstream side is coated with a downstream catalyst bed having a ternary performance and contg. at least Rh as the catalytically active component. Consequently, secondary air is introduced to form a fuel-lean side to efficiently remove HC at the start of an engine when a large amt. of HC is generated, and the gas warming characteristic is improved by the heat of reaction generated at this time. Further, the ternary performance is appropriately exhibited even in the steady operation after the gas is warmed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying method, and more particularly to an exhaust gas purifying catalyst and an exhaust gas purifying method which can be preferably used for an internal combustion engine of an automobile. Hydrocarbons (HC) generated in large quantities at the cold start) can be suitably purified together with secondary air, and after warming up, HC, carbon monoxide (CO), nitrogen oxides ( The present invention relates to an exhaust gas purifying catalyst that can purify NO _x ) and an exhaust gas purifying method.

[0002]

2. Description of the Related Art Along with the tightening of exhaust gas regulations, a catalyst is installed in the vicinity of the manifold of an engine to improve the warm-up characteristics of the catalyst, or the temperature of the heater or the rear side of the heater is steeply raised by using an electric heating heater. For example, a technique for heating a main catalyst to purify a large amount of HC generated when the engine is started has been attracting attention. Furthermore, in addition to the technology to improve the warm-up of the catalyst at the engine start when the fuel becomes rich, secondary air is introduced and exhaust gas is introduced into the fuel lean range from near the stoichiometric air-fuel ratio (Stoichio) where the excess air ratio λ = 1. Techniques for purifying the atmosphere on the side have also been proposed.

As a catalyst used in such an exhaust gas purifying technique, for example, Japanese Patent Publication No. 38892/1993 has at least one catalyst component of platinum (Pt) and rhodium (Rh) contained on a catalyst carrier. Ternary provided with a catalyst layer and an alumina coating layer provided on the catalyst layer and containing 50 to 95% of cerium oxide (CeO ₂ ) acting as an oxygen storage capacity-imparting agent and a trace amount of palladium (Pd). A catalyst is disclosed.

Further, as a catalyst supported on an electric heating type heater, Japanese Utility Model Publication No. 63-67609 and Japanese Patent Publication No.
There are exhaust gas purifying catalysts described in JP-A-500911 and JP-A-3-72953. Further, Japanese Patent Application Laid-Open No. 2-56247 discloses a three-way catalyst having a particularly high hydrocarbon purification activity on the rich side from the vicinity of stoichio during cold start. This is a second catalyst having a redox ability in which a first catalyst layer containing zeolite as a main component is supported on a carrier, and a precious metal such as Pt, Pd, or Rh is supported on a coat layer of Al ₂ O ₃ or the like. An exhaust gas-purifying catalyst having a layer.

Furthermore, Japanese Patent Laid-Open No. 63-84635 discloses a three-way catalyst in which the kind and amount of catalytic metal supported on the exhaust gas inflow side and the exhaust gas outflow side are changed, for example, at low temperature at cold start. In order to improve the purification efficiency,
On the gas inlet side of the monolith carrier, 1 / of the total length of the catalyst carrier
An exhaust gas purifying catalyst in which Pd is carried in the range of 10 to 2/5 and Pt is carried in the range of 3/5 to 9/10 of the total length of the catalyst carrier on the gas outlet side. It is disclosed. In this case, Rh, which is effective in purifying NO _x , is
It is supported on the entire catalyst layer. That is, this catalyst has Pd and Rh loaded on the upstream side and Pt and Rh loaded on the downstream side.

Similarly, in Japanese Patent Laid-Open No. 3-101813, as a catalytic converter device for improving low temperature ignitability of the catalyst, a plurality of exhaust gas purifying catalysts are housed in one catalytic converter device. In the catalytic converter device, the exhaust gas inflow side catalyst contains only Pd, the exhaust gas outflow side catalyst contains Rh and Pt, or Rh, Pt and Pd, and the exhaust gas inflow side The volume ratio of the catalyst to the exhaust gas outflow side catalyst is 1: 8.
Disclosed is a catalytic converter device of ~ 3: 1.

[0007]

However, the three-way catalyst disclosed in Japanese Examined Patent Publication No. 3-38892 is devised for the purpose of improving the oxygen storage capacity and effectively acting near the stoichiometric air-fuel ratio (Stoichio). It is a catalyst that has not been considered for lean side performance or cold start performance. In addition, since the coating layer has a large thickness of 20 to 40 μm, which is an obstacle to the diffusion of gas into the catalyst layer containing Rh, which is most effective in reducing and removing NO _x , it was not completed as a three-way catalyst. ..

Further, the catalysts supported by the electric heating heaters disclosed in Japanese Utility Model Publication No. 63-67609, Japanese Patent Publication No. 3-500911 and Japanese Patent Application Laid-Open No. 3-72953 are all noble metals. The above-mentioned catalyst components are related to so-called conventional three-way catalysts supported on refractory metal oxides such as Al ₂ O ₃ and do not disclose any composition and structure suitable for electric heating. Further, the catalyst disclosed in Japanese Patent Laid-Open No. 2-56247 does not act on the lean side where HC can be most efficiently purified by oxidation,
The purification was inadequate.

Furthermore, in the exhaust gas purifying catalyst described in Japanese Patent Laid-Open No. 63-84635, the gas inlet side of a catalyst carrier having a wall coated with an alumina coat layer or the like is immersed in a solution of a Pd compound up to a predetermined length. Then, the catalyst carrier is turned upside down, the gas outlet side is immersed in a platinum compound solution up to a predetermined length to dry and calcinate, and finally, the Rh compound solution is applied to the entire catalyst layer for Rh. It is prepared by impregnating and drying and firing. Rh and Pt and Pd, which are easy to make an alloy, are supported without being separated, so they easily alloy and have poor heat resistance.

Similarly, in the catalytic converter device described in Japanese Patent Application Laid-Open No. 3-101813, Rh, Pt and Pd are also used.
No measures have been taken to prevent alloying. Further, this catalytic converter device is intended to improve the HC purifying ability on the side of rich air-fuel consumption, and is improved by introducing the secondary air into the HC.
It has not been completed for the purpose of improving the performance on the lean side, which is capable of highly efficiently removing carbon dioxide, and is not a proper catalyst for heaters.

The present invention has been made in view of such a situation, and an object thereof is to have an excellent purification action on the lean side capable of efficiently removing HC and to be carried by an electric heating heater. An object of the present invention is to provide an exhaust gas purification catalyst having a composition and structure suitable as a catalyst, and an exhaust gas purification method capable of highly efficiently purifying HC generated in large quantities at the time of engine start by introducing secondary air.

[0012]

According to the present invention, the upstream portion of at least one monolithic carrier is coated with the upstream catalyst layer consisting of at least one layer having a hydrocarbon purifying ability, and the downstream portion is ternary. An exhaust gas purifying catalyst coated with a downstream catalyst layer comprising at least one layer having performance, wherein the outermost surface layer of the upstream catalyst layer contains at least one of Pt and Pd as a catalytically active component, and the downstream catalyst layer There is provided an exhaust gas purifying catalyst, characterized in that at least Rh is included. Further, according to the present invention, the upstream portion of at least one monolith carrier is coated with the upstream catalyst layer comprising at least one layer having hydrocarbon purifying ability, and the downstream portion comprises at least one layer having three-way performance. An exhaust gas purifying catalyst comprising a first catalyst layer composed of and a downstream catalyst layer composed of a second catalyst layer coated on the surface of the first catalyst layer and having a hydrocarbon purifying ability. As the outermost surface layer of the upstream catalyst layer and the second catalyst layer of the downstream catalyst layer are Pt
An exhaust gas purifying catalyst is provided, which comprises one of Pd and Pd, and the first catalyst layer of the downstream catalyst layer contains at least Rh. The monolithic carrier used for the exhaust gas purifying catalyst of the present invention is made of a heat-resistant inorganic material and has a honeycomb structure and is an electric heating heater (honeycomb heater).
Is preferred.

Further, according to the present invention, at least one monolith carrier is coated with an upstream catalyst layer having hydrocarbon purifying ability on the upstream side and a downstream catalyst layer having three-way performance is coated on the downstream side. From the front of the exhaust gas purifying catalyst (exhaust gas upstream side) in which the outermost surface layer of the upstream catalyst layer contains at least one of Pt and Pd as a catalytically active component, and the downstream catalyst layer contains at least Rh. There is provided an exhaust gas purification method characterized by introducing secondary air into the exhaust gas. Furthermore, according to the invention, at least 1
An upstream catalyst layer composed of at least one layer having a hydrocarbon purifying ability is coated on the upstream part of the monolith carrier consisting of
A downstream catalyst layer including a first catalyst layer having at least one layer having three-way performance and a second catalyst layer having a hydrocarbon purifying ability and coated on the surface of the first catalyst layer is provided in the downstream portion. An exhaust gas purifying catalyst in which the outermost surface layer of the upstream catalyst layer and the second catalyst layer of the downstream catalyst layer contain Pt or Pd as the catalytically active component, and the first catalyst layer of the downstream catalyst layer contains at least Rh. There is provided an exhaust gas purification method characterized in that secondary air is introduced into the exhaust gas from the front side of the engine. In the exhaust gas purifying method of the present invention, it is preferable to introduce secondary air at the time of engine startup to create an atmosphere on the fuel lean side, and it is preferable to electrically heat the honeycomb heater together with the introduction of secondary air.

[0014]

The exhaust gas purifying catalyst and the exhaust gas purifying method of the present invention are configured as described above, and at the time of engine start (cold start) in which a large amount of hydrocarbons is generated, the upstream portion mainly having hydrocarbon purifying ability is provided. By operating the catalyst layer and introducing secondary air from the front side of the catalyst (exhaust gas upstream side) to make the air-fuel ratio lean, the HC can be purified with high efficiency, and the reaction heat generated at this time is generated in the downstream portion. The catalyst layer is heated to improve warmth and further improve purification efficiency.
On the other hand, in the steady operation after warming up, the downstream catalyst layer having mainly three-way performance effectively acts and preferably three-way performance is exhibited.

The present invention will be described in more detail below. In the present invention, the upstream catalyst layer having a hydrocarbon purifying ability is composed of one layer or a plurality of layers, and the outermost layer thereof contains at least Pt or Pd as a catalytically active component. Pt
Since Pt has an effect of purifying HC at high temperature and purification of unsaturated HC, Pt can also be preferably used. However, since Pd has particularly high HC purifying ability at low temperature, it is preferable that the outermost layer contains Pd as a main component. If necessary, both Pd and Pt can be used.

The upstream catalyst layer is a single layer consisting essentially of Pd as the catalytically active component, or the outermost layer is Pd.
at least R inside the outermost layer of Pd.
It may be a catalyst layer composed of a plurality of layers in which a catalyst layer containing h is arranged. In the former case, the low-temperature ignition characteristics are improved, so that the reaction heat generated in the upstream catalyst layer heats the downstream catalyst layer to improve the warm-up property of the catalyst, which improves the purification characteristics at the cold start, which is preferable. , in the latter case, the catalyst warm-up effect due to low activity of the outermost layer of Pd as described above, NO _X purifying at a high temperature by an inner layer of Rh is preferably expressed preferred. Further, Pd on the outermost layer, Rh on the inner layer,
Further, disposing Pt in the inner layer is also a preferable example.

Since Rh easily forms an alloy with Pt and Pd and causes deactivation, it is necessary to support Rh separately. For example, it is preferable that an aggregate in which Rh and Pt are separately supported on a substrate in advance is arranged in a mixed phase as a catalyst layer, or a substrate containing Rh and a substrate containing Pt are arranged in layers. .. Of these, the layered structure is preferable because alloying can be substantially completely prevented.

The total amount of precious metal in the upstream catalyst layer is 30-
The range of 130 g / ft ³ is preferable in terms of cost performance. When Pd, Rh, Pt, etc. are arranged in multiple layers, for example, Pd is 5 to 60 g / ft ³ , Rh.
1 to 15 / ft ³ and Pt to 5 to 60 g / ft ³ . Rh is 1.5 because it is a particularly expensive precious metal.
It is particularly preferable to set it in the range of 10 to 10 g / ft ³ .

It is necessary that the upstream catalyst layer has a hydrocarbon purifying ability, and the substrate contains alumina and / or zeolite and, if necessary, zirconia as a main component. Alumina has a specific surface area of 50 m ² /
Those having a weight of g or more can be preferably used, and those having a weight of 100 m ² / g or more are preferable because they improve the dispersibility of the noble metal and the low temperature ignition performance. On the other hand, Rh can exhibit its performance sufficiently even when loaded on activated alumina, but has a relatively strong interaction with activated alumina and forms a solid solution on the lean side to cause deactivation, so Rh is 50 m ² / g or less. It is particularly preferred that the carrier be supported by alumina or zirconia. Zeolite selectively adsorbs HC at the time of cold start and releases it with warming up, thus improving the purification ability of HC. The amount of zeolite added can be in the range of 5 to 50 wt% with respect to activated alumina, and the composition has a Si / Al ratio of 40.
The above is preferable in terms of heat resistance.

A rare earth oxide such as CeO ₂ or La ₂ O ₃ having an oxygen storage capacity is not particularly required, but it is preferably added in an amount of 5 to 30 wt% with respect to the substrate from the viewpoint of catalytic performance during steady operation. Further, if necessary, Zr, Ni, Co, F
Transition metals such as e, Cu and Re may also be added as co-catalysts.

The thickness of the upstream catalyst layer is preferably in the range of 20 to 70 μm in terms of pressure loss and performance. For example, in the case of a catalyst layer composed of a plurality of layers, at least one of Pd and Pt is provided as the outermost layer. And a catalyst layer containing Rh on the inside, the outermost layer has a thickness of 2 to 20 μm.
A range of m is preferred. When it is less than 2 μm, it does not exhibit the desired HC purification ability, and when it exceeds 20 μm, the expensive Rh of the inner layer is high.
The NO _x reduction and removal ability of can not be expected.

The catalyst length of the upstream catalyst layer is preferably in the range of 1/10 to 3/5 of the total catalyst length. If it is less than 1/10, the HC purifying ability in the desired low temperature range cannot be expected, and the effect of heating the downstream catalyst layer by the reaction heat is insufficient. If the catalyst length exceeds 3/5, especially at high temperature. , The three-way reaction begins to react predominantly on the upstream side, and Rh
When is not included, for example, NO is not reduced and removed to N ₂ and NH ₃ as a by-product starts to be generated. A more preferable range of the catalyst length of the upstream catalyst layer is 1/10 to 3/10 when Rh is not contained in the upstream catalyst layer, and R
When h is contained, the range is 3/10 to 3/5.
All of these ranges are effective in reducing the amount of NH ₃ which is a by-product. The upstream catalyst layer is 1
It may be arranged on the upstream side of one monolith carrier, or may be arranged on the whole or a part of the upstream side of the monolith carrier on the upstream side of the plurality of monolith carriers.

The downstream catalyst layer is composed of one layer or a plurality of layers and contains at least Rh as a catalytically active component. The downstream catalyst layer, which is heated by the reaction heat of the upstream catalyst layer, starts predominantly proceeding with the three-way reaction, but Rh is indispensable especially for the purpose of reducing and removing NO _x . In order to achieve the above purpose, it is preferable to arrange Rh as a main component in the outermost surface layer of the downstream catalyst layer.

Further, if necessary, the downstream catalyst layer may include a first catalyst layer composed of at least one layer having three-way performance,
It is also possible to dispose a second catalyst layer (thin layer having a thickness of 2 to 20 μm) containing Pd and Pt on the first catalyst layer containing at least Rh, which is composed of a second catalyst layer having a hydrocarbon purifying ability. It is preferable from the viewpoint of improving the warm air property of the catalyst. Also in this case, Rh should be arranged as the main component in the outermost surface layer of the first catalyst layer.
_It is preferable for reducing and removing _X , and it is particularly effective for the second catalyst layer to contain Pd as a main component for improving warmth.

Further, Pt or Pd is provided inside the outermost surface layer of the downstream catalyst layer or the first catalyst layer containing Rh as a main component.
Disposing a layer containing a
It is preferable in that it exhibits desired catalyst performance. As with the upstream catalyst layer, Rh must be supported separately in order to prevent alloying with Pt and Pd.

The total amount of noble metals carried by the downstream catalyst layer, the preferred amount of each noble metal carried, and the thickness of the catalyst layer are the same as those of the upstream catalyst layer. However, the outermost surface layer of the downstream catalyst layer containing Rh or the outermost surface layer of the first catalyst layer has a thickness of 5 to 20.
The range of μm is particularly preferable in terms of NO _x purification ability.

The substrate of the downstream catalyst layer contains alumina and / or zirconia as a main component. Similar to the upstream catalyst layer, alumina has a specific surface area of 50 m ² / g or more, especially 100 m ² / g
Activated alumina can be preferably used, and Rh has a specific surface area of 50
It is particularly preferable that the particles are supported on alumina or zirconia of m ² / g or less.

Further, in order to appropriately exhibit the ternary performance, it is preferable to add a rare earth oxide such as CeO ₂ or La ₂ O ₃ having an oxygen storage capacity, and the addition amount is 5 to the substrate.
~ 30wt% widen the operating range (window) of the three-way catalyst,
It is also preferable because it improves the heat resistance of the substrate. In this case, since the Rh component easily forms a solid solution with CeO ₂ or the like and causes deactivation, it is preferable that Rh and CeO ₂ are contained in the catalyst layer in separate forms. Furthermore, CeO _{2 and the} like are Z
Forming a composite oxide with rO ₂ is more preferable because it further improves the oxygen storage capacity. If necessary, N
Transition metals such as i, Co, Fe, and Cu are 1 to 1 with respect to the substrate.
Addition of 0 wt% is preferable because various cocatalyst effects are exhibited.

The catalyst carrier used for the exhaust gas purifying catalyst of the present invention may be composed of one monolith carrier or a plurality of monolith carriers. When it is composed of one monolith carrier, the pressure loss is low, and when it is composed of a plurality of monolith carriers, gas mixing occurs between the monolith carriers to improve the purification performance. It is preferable in terms.

Since the exhaust gas purifying catalyst of the present invention is used under severe conditions such as a converter and an electric heating heater, the monolith carrier is preferably made of a heat resistant inorganic material and has a honeycomb structure. In addition, the electric heating heater is particularly suitable for using the present catalyst because it can rapidly raise the catalyst temperature and can maximize the purification at the cold start. As the electric heating heater, a conventionally used foil type heater can be used, but a heater made of a powder metallurgy method is preferable because it has high mechanical reliability and telescope and is highly reliable.

As a constituent material of the monolithic carrier, cordierite or a metallic honeycomb structure which generates heat by energization is preferably used, and a metallic honeycomb structure which generates heat by energization is particularly preferable because of its high mechanical strength. In the case of metallic material, for example, stainless steel, Fe-Cr-Al, Fe
-Cr, Fe-Al, Fe-Ni, W-Co, Ni-C
An example is one made of a material having a composition such as r. Of the above, Fe-Cr-Al, Fe-Cr, Fe-Al
Is excellent in heat resistance, oxidation resistance and corrosion resistance, and is inexpensive and preferable. The honeycomb structure may be porous or non-porous, but the porous honeycomb structure has strong adhesion to the catalyst layer, and catalyst peeling due to thermal expansion difference hardly occurs. Is preferred.

Next, an example of a method for manufacturing a metallic honeycomb structure out of a monolith carrier having a honeycomb structure will be described. First, to obtain a desired composition, for example, Fe powder,
A metal powder raw material is prepared from Al powder, Cr powder, or an alloy powder of these. Then, the metal powder raw material thus prepared, an organic binder such as methyl cellulose and polyvinyl alcohol, and water are mixed, and this mixture is extruded into a desired honeycomb shape.

Next, the extruded honeycomb molded body is
Baking is performed at 1000 to 1450 ° C. in a non-oxidizing atmosphere. Here, when firing is performed in a non-oxidizing atmosphere containing hydrogen,
It is preferable that the organic binder is decomposed and removed by using Fe or the like as a catalyst to obtain a good sintered body (honeycomb structure). If the firing temperature is lower than 1000 ° C, the molded body will not be sintered, and if the firing temperature exceeds 1450 ° C, the obtained sintered body will be deformed, which is not preferable. Desirably, the partition walls of the obtained structure and the surface of the mechanism are covered with a heat-resistant metal oxide.

The obtained honeycomb structure is preferably provided with a resistance adjusting mechanism between the electrodes to be described later in various ways. As the resistance adjusting mechanism provided in the honeycomb structure, for example, providing slits in various directions, positions, and lengths, changing the partition wall length in the penetrating axis direction,
Change the thickness of the partition walls (wall thickness) of the honeycomb structure,
Alternatively, changing the cell density of the through holes and providing slits in the partition walls of the honeycomb structure are preferable. Of these, the formation of slits is particularly preferable as a method for easily adjusting the heat generating portion.

The metallic honeycomb structure obtained as described above is usually provided with electrodes by means of brazing, welding or the like on the partition walls or inside of the outer peripheral portion thereof,
A honeycomb heater is manufactured. The term "electrode" used herein means a generic term for terminals for applying a voltage to the heater. When the heater is composed of a plurality of honeycomb structures (monolith carriers), they can be electrically connected in series or in parallel.

This heater is preferably formed so that the resistance value as a whole is in the range of 0.001Ω to 0.5Ω. The honeycomb shape of the honeycomb structure is not particularly limited, but specifically, for example, 6 to 1500.
Cell / inch ² (cpi ² ) (0.9 to 233 cell / cm
It is preferable to form it so as to have a cell density in the range of ² ). Further, the thickness of the partition wall is preferably in the range of 50 to 2000 μm.

As described above, the honeycomb structure may be porous or non-porous and the porosity thereof is not limited, but the strength is preferably in the range of 0 to 50%, preferably 5 to 25%. It is preferable in terms of characteristics, oxidation resistance, corrosion resistance, and adhesion with the catalyst layer. In the present invention, the honeycomb structure refers to an integral structure having a large number of through holes partitioned by partition walls, and for example, the cross-sectional shape (cell shape) of the through holes is circular, polygonal, corrugated, or any other arbitrary shape. Various shapes can be used.

Next, a method for preparing the upstream catalyst layer will be described. First, by using an impregnation method, a coprecipitation method, etc., an aqueous solution of a rare earth element such as Ce is added in an amount of 3 to 10 wt% in terms of oxide in advance to a substrate made of activated alumina, and the mixture is fired at a temperature of 500 to 950 ° C. to activate. A composite oxide of alumina-rare earth metal oxide is obtained. As a result, the noble metal supported in the subsequent step is obtained in a uniformly dispersed form. Then, the composite oxide is crushed by a wet method in the coexistence of a deflocculant such as an acid or an amine, and a rare earth metal is added in the form of an oxide, if necessary, and Rh, P
An aqueous solution of a noble metal such as d or Pt is added to obtain a desired supported slurry. This is coated on the upstream side of the monolith carrier (when the catalyst carrier is composed of a plurality of monolith carriers, the entire monolith carrier in the upstream portion or a part of the upstream side), and a drying step is performed to obtain 5
The upstream catalyst layer is obtained by firing at a temperature of 00 to 950 ° C.

Further, the above-mentioned supporting slurry is dried as it is, and calcined at a temperature of 500 to 950 ° C. to obtain a composite oxide in which a noble metal is fixed in advance on a substrate, and this is again subjected to a wet method together with a peptizer. It is also possible to disintegrate and coat it on a monolith carrier as a supporting slurry. In this case, the noble metal and the substrate have an appropriate interaction, which is particularly preferable in terms of durability. If a multi-layer structure is used, the above-mentioned 2
The same method is applied to coat the monolithic carrier, but in this case, the intermediate baking step is not always necessary.

The upstream catalyst layer can be coated on the monolithic carrier by applying the above-mentioned method in the same manner when a layer containing Rh is provided. However, from the viewpoint of durability, the specific surface area of 50 m ² as a substrate. / G or less of alumina or zirconia is preferable, and direct contact with a rare earth oxide such as CeO ₂ is not preferable. Therefore, activated alumina, alumina or zirconia having a specific surface area of 50 m ² / g or less are preferable. Etc. were crushed by a wet method at a desired mixing ratio, and Rh
It is possible to coat the monolith carrier with the metal salt aqueous solution ( ₂₎ and CeO ₂ powder, if necessary, as a carrying slurry. Also in this case, similarly to the above, the supported slurry is dried as it is, and calcined at a temperature of 500 to 950 ° C. to obtain a composite oxide in which Rh is preliminarily immobilized on the substrate,
This can be crushed again by a wet method to obtain a supported slurry.

Further, as the most preferable example, Rh is previously fixed and supported on alumina or zirconia having a specific surface area of 50 m ² / g or less, or a composite oxide thereof, and this is supported on a monolith carrier, or active on this. A supported slurry can also be obtained by adding a necessary amount of alumina-rare earth composite oxide (rare earth may be added as an oxide). This dramatically improves the heat resistance of Rh, and Ce
Since it is possible to substantially prevent contact with Pt and other Pd and Pd, the heat resistance of Rh is improved, which is the most preferable.

The downstream catalyst layer is the same as the upstream catalyst layer except that the downstream catalyst layer is also coated and supported on the downstream side of the monolith carrier (when the catalyst carrier is composed of a plurality of monolith carriers, the entire monolith carrier or a part of the downstream side). Prepare using the method of preparation. In addition,
As a method of supporting the coating, when the catalyst carrier is composed of one monolithic carrier, the catalyst carrier is supported by, for example, reversing so that the upstream side and the downstream side are divided.

Next, an exhaust gas purification method using the above exhaust gas purification catalyst will be described. The present catalyst needs to introduce binary air from the front of the present catalyst at the time of engine start in order to maximize the HC purification performance at the cold start. That is, it is because HC and CO cannot be purified on the excessive rich side at the time of starting the engine, and the reaction heat does not lead to further improvement in warm-up.

The introduction position of the secondary air may be anywhere between the engine exhaust gas exhaust hole and the main catalyst and is not particularly limited, but the introduction position near the exhaust hole improves the mixing of the exhaust gas and the secondary air. Particularly preferred. The amount of dual air introduced depends on the engine displacement, but is generally 50 to 30.
It is 0 l / min. The air-fuel ratio at this time is from the vicinity of stoichiometry to the lean side (λ = about 0.9 to 1.5). Particularly, it is preferable to set the lean side to about λ = 1.0 to 1.3 because the HC purification performance is improved. The binary air is introduced from the time the engine is cranked to the time when the λ sensor generally operates, and is generally within 60 seconds.

In order to maximize the purification performance at cold start, it is most preferable to use the present catalyst in an energizing heat-generating heater, and energization and introduction of secondary air are started after engine cranking for about 60 seconds. Within each, energize and stop the supply. As a result, the ability to purify HC and CO becomes higher than when it is supported by an ordinary three-way catalyst, and the reaction heat can be utilized to the maximum, so that the power consumption to the heater can be greatly reduced, and Even in steady operation after warm-up, the three-way performance is suitably shown, so the effect is great. Regarding energization, before the engine crank (for example, 30
Even if it is carried out within 2 seconds, a suitable purification performance can be obtained.

[0046]

The present invention will be described in more detail based on the following examples, but the invention is not intended to be limited to these examples.

[Preparation of Honeycomb Heater] Pure Fe powder,
Pure Cr powder, Fe-50 wt% Al alloy powder, Fe-20
wt% B powder, Fe-75 wt% Si powder to Fe-20Cr
-5Al-1Si-0.05B (wt%) is mixed with the raw materials, and an organic binder (methylcellulose), an antioxidant (oleic acid), and water are added to the mixture to prepare a kneaded clay. Extrude a honeycomb consisting of cells,
After drying, baking at 1350 ° C in H ₂ atmosphere, rib thickness 4mil,
A honeycomb structure having a number of through holes of 400 cpi ² was obtained.

An outer diameter of 90 mmφ obtained by the above method,
As shown in FIG. 1, a honeycomb structure having a volume of 0.2 mm and a volume of 0.4 l of 120 mmφ 2 type, as shown in FIG.
According to No. 4, the slit 12 was inserted to adjust the resistance, and various catalysts were coated and carried on the obtained honeycomb structure with slits. Further, the outer peripheral portion 13 of the slit 12 has a heat resistant Z
An inorganic adhesive composed of rO ₂ was filled to form an insulating portion, an electrode 11 was set on the outer side surface 10, and two honeycomb structures having an outer diameter of 90 mmφ and a volume of 0.2 l were connected in series at intervals of 10 mm. A type A honeycomb heater having a total volume of 0.4 l and a type B honeycomb heater having an outer diameter of 120 mmφ and a volume 0.4 l of only one honeycomb structure having a volume of 0.4 l were manufactured. In the case of the A-type honeycomb heater, 0.2 l of the honeycomb structure on the upstream side is coated with the upstream catalyst layer, and 0.2 l of the honeycomb structure on the downstream side is coated with the downstream catalyst layer.

[Preparation of Catalyst] (1) Method A Commercially available γ-Al ₂ O ₃ (BET specific surface area 200 m ² / g)
Was impregnated and supported with an aqueous cerium nitrate solution so as to be 6 wt% in terms of ceria, and calcined at 600 ° C. for 3 hours to obtain alumina-
Obtain a ceria composite oxide. The obtained alumina-ceria composite oxide was crushed by a wet method, and ceria powder was γ-
24 wt% of Al ₂ O _{3 was} added, and a single precious metal and an appropriate amount of acetic acid were added to obtain a supporting slurry, which was coated and supported on a honeycomb structure, dried and then calcined at 550 ° C. for 3 hours to obtain a catalyst. obtain. Further, if necessary, supporting slurries containing different single noble metals prepared by the same method are coated and supported, dried and calcined at 550 ° C. for 3 hours to obtain a catalyst. In the subsequent catalyst preparation, dinitroamine platinum, palladium nitrate, and rhodium nitrate are used as noble metals. In addition, the precious metal is Rh
In the case of, the addition of the cerium component to the alumina is not performed.

(2) Method B The supported slurry obtained in Method A was dried at 100 ° C. for 15 hours and calcined at 550 ° C. for 3 hours to precious metal-alumina in which the precious metal was previously immobilized on the alumina-ceria composite oxide. −
Obtain a ceria composite oxide. Further, an appropriate amount of acetic acid is added, and the mixture is crushed by a wet method to obtain a supporting slurry, which is coated and supported on a honeycomb structure, dried and then calcined at 550 ° C. for 3 hours to obtain a catalyst. Further, if necessary, a different single noble metal prepared by the same method is coated and supported on the carrier slurry in which the alumina-ceria composite oxide is immobilized in advance, and the carrier slurry is dried at 550 ° C.
Calcination for 3 hours to obtain the catalyst. When the noble metal is Rh, the cerium component is not added to alumina.

(3) Method C At least two kinds of supporting slurries having different kinds of noble metals obtained in Method B are mixed to obtain a new supporting slurry, which is supported in the same manner as Method B.

(4) Method D Commercially available partially stabilized ZrO ₂ powder (containing ₃ mol% of Y ₂ O ₃ , BE
Rh was impregnated in ZrO ₂ using an aqueous rhodium nitrate solution with a T specific surface area of 16 m ² / g, dried at 100 ° C. for 15 hours, and then calcined at 550 ° C. for 3 hours to obtain Rh-containing ZrO ₂ powder. Proper acetic acid and γ-Al ₂ O ₃ powder (50 parts) are added to this, and the mixture is crushed by a wet method to obtain a supported slurry. In advance
In Method B, a honeycomb heater in which a platinum-alumina-ceria composite oxide is coated and supported on a honeycomb structure is provided with the above-mentioned Rh.
A supporting slurry composed of —ZrO ₂ —Al ₂ O ₃ is coated and supported, dried and then calcined at 550 ° C. for 3 hours to obtain a catalyst.

(5) Method E The catalyst obtained in Method D is impregnated with an aqueous palladium nitrate solution, dried and calcined (550 ° C. for 3 hours) to obtain a catalyst in which Pd is concentrated on the catalyst surface layer.

(6) Method F The same method as in Method A was used to obtain a supported slurry to which Pt and Rh were simultaneously added. In the obtained catalyst, Pt and Rh are uniformly mixed in the catalyst layer.

(7) Method G Using Method B, a catalyst was carried on the upstream side of one honeycomb structure for a certain length, dried and fired at 550 ° C. for 3 hours, and then the honeycomb structure was inverted to prepare Method B. A catalyst is supported on the downstream side (catalyst unsupported portion) by using the same, and similarly dried and calcined to obtain a catalyst.

The above-mentioned methods A to G were used alone or in combination to prepare the catalysts of the following Examples and Comparative Examples.

(Example 1) According to Method B, a catalyst layer having a film thickness of 40 μm and a Pd content of 40 g / ft ³ is coated and supported on the upstream side, and a film thickness of 30 μm and a Pt content of 35 g / f are provided on the downstream side.
t ³ catalyst layer (inner layer) and film thickness 10 μm, Rh content 5
A in which two layers of a catalyst layer (surface layer) of g / ft ³ were coated and supported
A type honeycomb heater was obtained.

Example 2 The same honeycomb heater as in Example 1 was obtained by the method B except that a catalyst layer having a film thickness of 40 μm and a Pt content of 40 g / ft ³ was coated and supported on the upstream side.

(Example 3) According to Method B, a catalyst layer (inner layer) having a film thickness of 30 μm and a Pt content of 35 g / ft ³ on the upstream side, a catalyst layer having a film thickness of 10 μm and an Rh content of 5 g / ft ³ (intermediate layer) ) And film thickness 10 μm, Pd content 40 g / ft
^The same type A honeycomb heater as in Example 1 was obtained except that 3 layers of 3 catalyst layers (surface layers) were coated and supported.

Example 4 The same type A honeycomb heater as in Example 3 was obtained except that the method A was used.

(Example 5) By the method C, the film thickness of 40 μm and the contents of Pd, Rh and Pt are 40 and 5, respectively on the upstream side.
A catalyst layer of 35 g / ft ³ was coated and supported, the film thickness was 40 μm on the downstream side, and the contents of Rh and Pt were ⁵ , 35 g / ft, respectively.
An A-type honeycomb heater having the catalyst layer ³ supported thereon was obtained.

Example 6 On the upstream side, according to Method D, a catalyst layer having a film thickness of 30 μm and a Pt content of 35 g / ft ³ (inner layer), a catalyst layer having a film thickness of 10 μm and a Rh content of 5 g / ft ³ (intermediate layer) was prepared. Layer) and method B, ^three layers of a catalyst layer (surface layer) having a film thickness of 10 μm and a Pd content of 40 g / ft ³ are coated and supported, and downstream, by method D, a film thickness of 30 μm and a Pt content of 35 g / ft ³ catalyst layer (inner layer), film thickness 10 μm,
An A-type honeycomb heater in which two layers of a catalyst layer (surface layer) having an Rh content of 5 g / ft ³ were coated and supported was obtained.

Example 7 On the upstream side, according to Method D, a catalyst layer having a film thickness of 30 μm and a Pt content of 35 g / ft ³ (inner layer), a catalyst layer having a film thickness of 10 μm and a Rh content of 5 g / ft ³ (surface layer) The same A-type honeycomb heater as in Example 6 was obtained except that two layers of (1) were coated and supported, and Pd was impregnated and supported by the method E on the upstream catalyst surface layer.

Example 8 The same type A honeycomb heater as in Example 5 was obtained except that the method F was used.

(Example 9) According to Method B, a catalyst layer having a film thickness of 40 μm and a Pd content of 40 g / ft ³ is coated and supported on the upstream side, and a film thickness of 30 μm and a Pt content of 35 g / f are provided on the downstream side.
t ³ catalyst layer (inner layer), film thickness 10 μm, Rh content 5 g
An A-type honeycomb heater was obtained in which three layers of a catalyst layer (intermediate layer) of / ft ³ , a film thickness of 10 μm, and a catalyst layer (surface layer) having a Pd content of 40 g / ft ³ were coated and supported.

(Example 10) By Method G, the film thickness was 40 μm and the Pd content was 40 within the range of the catalyst length 1/2 from the upstream side.
A catalyst layer of g / ft ³ is coated and supported, and a film thickness of 30 μm, a Pt content of 35 g / ft ³ of a catalyst layer (inner layer), a film thickness of 10 μm, and a Rh content of 5 g / ft ³ of the catalyst layer ( A B-type honeycomb heater in which two layers (surface layer) were coated and supported was obtained.

(Comparative Example 1) According to Method B, a catalyst layer having a film thickness of 40 μm and a Rh content of 5 g / ft ³ is coated and supported on the upstream side, and a film thickness of 30 μm and a Pt content of 35 g / ft ³ are provided on the downstream side.
³ catalyst layer (inner layer), film thickness 10 μm, Pd content 5 g /
An A-type honeycomb heater in which two catalyst layers (surface layers) of ft ³ were coated and supported was obtained.

(Comparative Example 2) According to Method B, a catalyst layer having a film thickness of 40 μm and an Rh content of 5 g / ft ³ is coated and supported on the upstream side, and a film thickness of 30 μm and a Pt content of 35 g / ft ³ are provided on the downstream side.
³ catalyst layer (inner layer), film thickness 10 μm, Rh content 5 g /
An A-type honeycomb heater in which two catalyst layers (surface layers) of ft ³ were coated and supported was obtained.

[Evaluation Method] (1) Catalyst Endurance Test In order to estimate the long-term life of the catalysts obtained from the above Examples and Comparative Examples by using the exhaust gas of an actual engine, the catalyst temperature was set to 750 ° C. And the fuel cut mode was incorporated and aged for a total of 100 hours.

(2) Evaluation of characteristics of catalyst at cold start Using the sample after the above durability test, exhaust gas purification characteristics at engine start were evaluated (Bag 1A test in FTP). The sample was set at a position 200 mm from the engine exhaust hole, and energized for 60 seconds by on-off control so that the heater temperature during energization was 450 ° C. Further, the secondary air is introduced at a position of 100 mm from the engine exhaust hole at 200 l / min for 40 seconds after the engine is started, and λ = 1.0.
The atmosphere of ~ 1.3 was maintained. The exhaust gas purification characteristics when the heater was not electrically heated were also evaluated. The results obtained are shown in Table 1.

[0071]

[Table 1]

It can be seen from Table 1 that the sample of this example has lower emission values of CO and HC than the sample of the comparative example and is superior in exhaust gas purification ability. In particular, in the case of heating with energization, the sample of the example has a high HC purification performance because the upstream catalyst layer having hydrocarbon purification performance is arranged. Also,
The performance as the manifold converter (maniverter) can be estimated from the performance when the secondary air is introduced (the heater is not electrically heated), but all of the examples have high purification efficiency. The emission values when secondary air is not introduced are 1.8 g of HC and 25
g, NO _x 1.4 g, and the effect is exhibited as in the embodiment only after the secondary air is introduced. In addition, when the samples after the endurance were cut out so as to have the same configuration and the ignition performance (two points of lean side and stoichio) and the steady-state characteristics of the catalyst were evaluated using an engine simulation gas composed of synthetic gas, an example was obtained. It was confirmed that the sample of No. 3 showed excellent ternary performance (low temperature activity, high purification rate at 400 ° C.) as compared with the sample of Comparative Example, and it was also confirmed that the sample of No. 3 also suitably operates in steady operation after warming.

[0073]

As described above, according to the exhaust gas purifying catalyst and the exhaust gas purifying method of the present invention, the hydrocarbon (H
At the time of engine start (cold start) in which a large amount of C) is generated (upstream), the upstream catalyst layer mainly having hydrocarbon purifying ability acts to introduce secondary air to most efficiently purify HC. By setting it to the side, HC can be purified with high efficiency. In addition, the reaction heat generated at this time improves the warm-up characteristics, and especially when used for an electric heating heater,
Since the reaction heat can be used to the maximum extent, the power consumption to the heater can be greatly reduced. Further, in the steady operation after the warm-up, the downstream catalyst layer having the three-way performance effectively acts, and the three-way performance is preferably expressed.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an example of a honeycomb heater.

[Explanation of symbols]

 10 Outer Side Surface 11 Electrode 12 Slit 13 Outer Part of Slit 14 Honeycomb Heater

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical indication location F01N 3/22 321 P 3/28 301 P (72) Inventor Fuminori Yamanashi Takara-cho, Kanagawa-ku, Yokohama-shi, Kanagawa No. 2 in Nissan Motor Co., Ltd. (72) Inventor Katsuhiro Shibata No. 2 Takara-cho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd. (72) Fumio Abe 29 (72) Inventor, No. 1 Aga-cho, Handa-shi, Aichi Tomoji Kondo 4-43, Izuminishi-machi, Toki City, Gifu Prefecture (72) Inventor Jun-ichi Suzuki, Kuwana City, Mie Prefecture 13 (72) Yanagihara, Kogaisu, Mie Prefecture (72) Inventor Naomi Noda, Yamato-machi, Ichinomiya-shi, Aichi Prefecture 13 back

Claims (25)

[Claims] 1. A downstream portion comprising at least one upstream layer of a monolithic carrier, which is coated with an upstream catalyst layer comprising at least one layer having hydrocarbon purifying ability, and a downstream portion comprising at least one layer having three-way performance. An exhaust gas purifying catalyst coated with a catalyst layer, characterized in that the outermost surface layer of the upstream catalyst layer contains at least one of Pt and Pd as a catalytically active component, and the downstream catalyst layer contains at least Rh. Exhaust gas purification catalyst. 2. The exhaust gas purifying catalyst according to claim 1, wherein the outermost surface layer of the upstream catalyst layer contains Pd as a main component. 3. The upstream catalyst layer is a single layer composed of only Pd or a plurality of layers in which the outermost surface layer is composed of Pd and a layer containing at least Rh is provided inside the outermost surface layer. Alternatively, the exhaust gas-purifying catalyst according to item 2. 4. The exhaust gas-purifying catalyst according to claim 1, wherein the outermost surface layer of the downstream catalyst layer contains Rh as a main component. 5. The downstream catalyst layer according to claim 1, wherein the outermost surface layer is composed of Rh, and the outermost surface layer is formed of a plurality of layers in which a layer containing at least one of Pt and Pd is provided. The exhaust gas-purifying catalyst according to Crab. 6. The exhaust gas purifying catalyst according to claim 1, wherein Rh contained in the upstream catalyst layer and / or the downstream catalyst layer is separated so as not to come into contact with Pt and Pd. 7. The upstream catalyst layer comprises alumina and / or zeolite, optionally a base material containing zirconia as a main component, and optionally a rare earth oxide.
The exhaust gas-purifying catalyst according to any one of 1. 8. The exhaust gas-purifying catalyst according to claim 1, wherein the downstream catalyst layer comprises a substrate containing alumina and / or zirconia as a main component and, if necessary, a rare earth oxide. 9. A first catalyst comprising at least one monolithic carrier coated with an upstream catalyst layer consisting of at least one layer having hydrocarbon purifying ability, and a downstream part comprising at least one layer having three-way performance. An exhaust gas purifying catalyst comprising a downstream catalyst layer comprising one catalyst layer and a second catalyst layer which is coated on the surface of the first catalyst layer and has a hydrocarbon purifying ability, the upstream portion serving as a catalytically active component. The outermost surface layer of the catalyst layer and the second catalyst layer of the downstream catalyst layer include one of Pt and Pd,
An exhaust gas-purifying catalyst, characterized in that the first catalyst layer of the downstream catalyst layer contains at least Rh. 10. The exhaust gas-purifying catalyst according to claim 9, wherein the outermost surface layer of the upstream catalyst layer contains Pd as a main component. 11. The upstream catalyst layer comprises a single layer consisting of Pd alone, or a plurality of layers in which the outermost layer is composed of Pd and a layer containing at least Rh is provided inside the outermost layer. Exhaust gas purification catalyst described. 12. The exhaust gas-purifying catalyst according to claim 9, wherein the second catalyst layer of the downstream catalyst layer contains Pd as a main component. 13. The outermost layer of the first catalyst layer of the downstream catalyst layer contains Rh as a main component, and a layer containing at least one of Pt and Pd is provided inside the outermost layer.
2. The exhaust gas purifying catalyst according to any one of 2 above. 14. The exhaust gas purifying catalyst according to claim 9, wherein Rh contained in the upstream catalyst layer and / or the downstream catalyst layer is separated so as not to come into contact with Pt and Pd. 15. The exhaust gas according to claim 9, wherein the upstream catalyst layer is composed of alumina and / or zeolite, optionally a substrate containing zirconia as a main component, and optionally a rare earth oxide. Purification catalyst. 16. The exhaust gas-purifying catalyst according to claim 9, wherein the downstream catalyst layer comprises a substrate containing alumina and / or zirconia as a main component and, if necessary, a rare earth oxide. 17. The exhaust gas-purifying catalyst according to claim 1, wherein the monolith carrier is made of a heat resistant inorganic material and has a honeycomb structure. 18. The monolithic carrier has a honeycomb structure, has at least one electrode, and the monolithic carrier is connected as necessary to generate heat by energization.
7. The exhaust gas purifying catalyst according to any one of 7. 19. The exhaust gas purifying catalyst according to claim 18, wherein a resistance adjusting mechanism is provided between the electrodes of the monolith carrier having a honeycomb structure. 20. An upstream catalyst layer having hydrocarbon purifying ability is coated on the upstream portion of at least one monolithic carrier, and a downstream catalyst layer having three-way performance is coated on the downstream portion, and as a catalytically active component. Secondary air is introduced into the exhaust gas at engine startup from the front of the exhaust gas purifying catalyst in which the outermost surface layer of the upstream catalyst layer contains at least one of Pt and Pd and the downstream catalyst layer contains at least Rh. Exhaust gas purification method. 21. A first catalyst comprising at least one monolithic carrier coated with an upstream catalyst layer consisting of at least one layer having hydrocarbon purifying ability, and a downstream part comprising at least one layer having three-way performance. A downstream catalyst layer consisting of one catalyst layer and a second catalyst layer having a hydrocarbon purifying ability that is coated on the surface of the first catalyst layer is provided, and the outermost surface layer and the downstream portion of the upstream catalyst layer serve as catalytically active components. The second catalyst layer of the catalyst layer contains either Pt or Pd, and the first catalyst layer of the downstream catalyst layer contains at least Rh. From the front of the exhaust gas purifying catalyst, secondary air is introduced into the exhaust gas at engine start. An exhaust gas purification method characterized by the above. 22. The exhaust gas is made leaner by introducing secondary air when the engine is started.
The exhaust gas purification method described in 1. 23. The exhaust gas purification method according to claim 20, wherein the monolith carrier is made of a heat resistant inorganic material and has a honeycomb structure. 24. The monolith carrier has a honeycomb structure, has at least one electrode, and introduces secondary air at the time of engine start and generates heat by energization.
23. The exhaust gas purification method according to any one of 0 to 22. 25. The exhaust gas purification method according to claim 24, wherein a resistance adjusting mechanism is provided between the electrodes of the monolith carrier having a honeycomb structure.