JP2000300995A - Exhaust gas cleaning catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To further improve the durability of an exhaust gas cleaning catalyst of NOx occlusion reduction type by suppressing the sulfur poisoning of an NOx occlusion material, particularly the sulfur poisoning of an NOx occlusion material caused by the readsorption of SOx on a section positioned on the downstream side from the gas purifying catalyst in the exhaust gas stream. SOLUTION: The exhaust gas cleaning catalyst provided in a flow path of an exhaust from a lean-burn engine and cleaning exhaust gas is constituted of a base, a carrier composed of a porous oxide formed into the laminar shape on the surface of the base, a precious metal and an NOx occlusion material carried on the carrier and a coated layer composed of at least one kind of titania and silica coating the surface of the carrier on a position located at least on the downstream side of the exhaust gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for reducing the amount of N contained in an exhaust gas containing oxygen in excess of that required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) contained in the exhaust gas.
The present invention relates to an exhaust gas purifying catalyst capable of purifying O _x efficiently.

[0002]

BACKGROUND ART In a lean-burn engine, normally is burned with oxygen excess fuel lean condition, the system reduces and purifies NO _x exhaust gas as a reducing atmosphere is developed by the intermittent fuel stoichiometric-rich condition, Has been put to practical use. And as the best catalysts for this system, occludes NO _x in lean atmosphere, stoichiometric ~ _the NO _x storage-reduction type exhaust purifying catalyst is developing with _the NO _x storage material that releases occluded NO _x in a rich atmosphere Have been.

For example, JP-A-5-317652 proposes an exhaust gas purifying catalyst in which an alkaline earth metal such as Ba and Pt are supported on a porous oxide carrier such as alumina. Japanese Patent Application Laid-Open No. 6-31139 discloses K
There has been proposed an exhaust gas purifying catalyst in which an alkali metal such as Pt is supported on a porous oxide carrier such as alumina.
Further, Japanese Patent Application Laid-Open No. 5-168860 proposes an exhaust gas purifying catalyst in which a rare earth element such as La and Pt are supported on a porous oxide carrier such as alumina.

If this NO _x storage reduction catalyst is used, the exhaust gas changes from a lean atmosphere to a stoichiometric to rich atmosphere from a lean atmosphere by controlling the air-fuel ratio from a lean side to a stoichiometric to rich side. . Thus, NO _x is occluded in _the NO _x storage material in the lean side, because it is purified by reacting with reducing components, such as being released in the stoichiometric or rich side HC and CO, in exhaust gases from lean-burn engines can efficiently purify NO _x even. Further, HC and CO in the exhaust gas are oxidized by the noble metal and consumed for reduction of NO _x , so that HC and CO are also efficiently purified.

However, exhaust gas contains fuel.
SO generated by combustion of sulfur (S) _xIs included
Oxidized by precious metals in the exhaust gas of lean atmosphere
If SO_ThreeBecomes And that is also included in the exhaust gas
Sulfuric acid is easily converted to water vapor,_xOcclusion
Reacts with wood to form sulfites and sulfates,
O_xIt became clear that the occlusion material was poisoned and deteriorated. Toes
And thus NO_xThe storage material becomes sulfite or sulfate
NO after poisoning_xCan occlude
NO, NO after endurance_xThe problem that purification performance decreases
It is a title.

[0006]

[0007] NO _x storage-and-reduction type catalyst for purifying an exhaust gas, as described above, but is intended to release reducing the NO _x occluding the fuel stoichiometric-rich atmosphere, in the exhaust-gas upstream side hydrogen generation, the temperature rise or the like on the downstream side, release the reduction of the NO _x is a prone atmosphere part of the catalyst located downstream, catalyst N
_Ox purification performance is greatly affected by the state of the downstream catalyst.

The adsorption of SO _{x on} the NO _x storage material is performed with priority over NO _x , and tends to concentrate on the portion of the catalyst located on the upstream side. SO _x adsorbed on the upstream side, but not released in the fuel stoichiometric-rich atmosphere, once released SO _x has been found to be re-adsorbed on the portion located on the downstream side of the catalyst. Therefore, the SO _x re-adsorbed on the downstream side significantly deteriorates the NO _x purification performance of the catalyst.

The present invention has been made in view of the above circumstances, and has been made in view of the above circumstances. In particular, sulfur poisoning of the NO _x occluding material, particularly, sulfur poisoning caused by re-adsorption of SO _{x in} the portion of the catalyst located downstream of the exhaust gas. And to further improve the durability of the NO _x storage reduction type exhaust gas purifying catalyst.

[0009]

An exhaust gas purifying catalyst according to the present invention is disposed in a flow path of exhaust gas from a lean burn engine and purifies the exhaust gas. A carrier comprising a porous oxide formed in a layer on the surface of the substrate, a noble metal supported on the carrier and N
It is characterized by having an O _x occluding material and a coating layer formed of at least one kind of titania and silica formed by coating the surface of the carrier at least in a portion located on the downstream side of the exhaust gas.

[0010]

The effect of suppressing sulfur poisoning of the exhaust gas purifying catalyst of the present invention DETAILED DESCRIPTION OF THE INVENTION is based on the action of the coating layer coated on the carrier carrying surface _the NO _x storage material. Titania (TiO ₂ ), silica (Si
Since O ₂ ) is an acidic substance like SO _x , it is considered that the coating layer efficiently suppresses the adsorption of SO _x to the NO _x storage material inside this coating layer. Therefore,
By providing the coating layer on at least the downstream side of the catalyst, the sulfur poisoning resistance of the particularly important downstream side catalyst is improved. Hereinafter, embodiments of the present invention will be described in detail.

The exhaust gas purifying catalyst of the present invention can be particularly effectively applied to a so-called monolith type catalyst. In the case of a monolithic catalyst, the base material constituting the catalyst is
A honeycomb body made of ceramics such as cordierite or a heat-resistant alloy can be used. The carrier is obtained by forming a porous oxide in a layer on the surface of a substrate. As a porous oxide that can be used, for example, alumina (A
l ₂ O ₃ ), zeolite, silica, zirconia (Zr
O ₂ ), titania and the like, or a TiO ₂ -A
_{l 2 O 3, SiO 2 -Al} 2 O 3, ZrO 2 -Al 2 O 3 and the like. For the purpose of improving the purification performance, ceria (CeO ₂ ) or ceria stabilized with zirconia (ceria-zirconia composite oxide) may be added.

The noble metals supported on the carrier are mainly CO,
At the same time as oxidizing HC, it plays a role of oxidizing NO, which is the main component of NO _{x in} the exhaust gas, as NO _{2, so} as to be easily stored in the NO _x storage material. Examples of the noble metal that can be used include Pt, Rh, Pd, Ir, and Ru. Depending on the characteristics of the catalyst, one of these
The species or two or more species may be selectively carried. The loading method is not particularly limited, and may be carried out by a known method according to the noble metal used. If the amount of the noble metal carried is, for example, a monolithic catalyst formed, Pt, R
In the case of d, it is preferably 0.1 to 20 g, and 0.5 to
More preferably, it is 10 g. In the case of Rh,
It is preferably 0.01 to 10 g, and 0.05 to 5 g
More preferably,

[0013] _the NO _x storage material to be supported on the carrier, occludes NO _x under fuel-lean atmosphere, plays a role of releasing the NO _x under fuel stoichiometric-rich atmosphere, alkali metals, alkaline earth metals and rare earth At least one selected from elements can be used. Li, Na, K, Rb, Cs as alkali metals, Be, Mg, Ca, Sr, Ba as alkaline earth metals, and Sc, Y, La, Ce, Pr, Nd, as rare earth elements.
Dy, Yb, etc. can be exemplified. Among these,
Considering that alkalinity is excellent in high _the NO _x storage capacity, _the NO _x storage material are alkali metals, preferably selected from alkali earth metals.

The method of supporting the NO _x occluding material is not particularly limited, and may be performed by a known method according to each NO _x occluding material to be supported. Further, the loading amount of _the NO _x storage material, for example, in the case of monolithic catalysts, it is desirable to 0.05 mols relative volume of 1 liter monolith. When the supported amount is less than 0.05 mol, the NO _x storage capacity is small and the NO _x purification performance is reduced. When the supported amount exceeds 1 mol, the effect is saturated and, in addition, the problem due to the decrease in the amount of other components is caused. Is likely to occur.

The coating layer for coating the surface of the carrier is as described above.
SO_xNO_xSuppresses adsorption to occlusion materials
And NO_xSO coating by overcoating the occlusion material_xof
NO _xIt shows the function of inhibiting the arrival at the occlusion material. Suffered
The covering layer is made of less acidic substances such as titania and silica.
Both use one type. The method of forming the coating layer is particularly limited.
Although not specified, for example, a monolithic catalyst
Precious metals and NO _xAn object with an occlusion material supported on a carrier
Squirrel is milled with at least one fine powder of titania and silica
The powder is immersed in a sol, then dried and fired.
You can do it.

The thickness of the coating layer is 10 in consideration of the function of the coating layer to suppress the adsorption of SO _{x on} the NO _x storage material.
It is desirable that it is not less than μm. On the other hand, if the thickness is too large, even the adsorption of NO _{x to} the NO _x storage material is too impaired. Although it depends on the type of the NO _x occluding material, it is more preferable to be 20 μm or less.

The exhaust gas purifying catalyst of the present invention is preferably
_In order to suppress sulfur poisoning of the catalyst downstream of the exhaust gas, which is important for occlusion / reduction of _x , a coating layer is formed on at least the portion of the carrier surface located on the downstream side. FIG. 1 conceptually shows a portion where a coating layer is formed in a monolithic catalyst. In FIG. 1, a coating layer is formed on a portion of about 50% of the entire length of the monolith (full length of the base material) from the downstream end of the monolith (downstream end of the base material).

A coating layer may be formed on the entire monolith, but if the coating layer is formed on the entire monolith, NO
_Since the storage capacity of _x is inferior, it is desirable to provide a coating layer non-formed portion in a part of the upstream side as shown in the figure in order to secure a high NO _x storage capacity in the initial stage. In this case, depending on the characteristics of the catalyst to be produced, the range of the coating layer forming portion is 5 to 8 of the total length of the monolith from the downstream end.
It is desirable to set the range to 0%. The thickness of the coating layer does not need to be the same over the entire coating layer forming portion. For example, the coating layer is formed such that the coating layer becomes continuously thicker or gradually increases toward the downstream end. May be.

The exhaust gas purifying catalyst of the present invention having the above-described structure is incorporated in a catalytic converter, arranged in an exhaust gas flow path from a lean burn engine, and purifies the exhaust gas. The fuel-lean atmosphere, with HC and CO are purified by oxidation by the noble metal of the catalytic function, NO and the like contained in the exhaust gas is oxidized by the function of the noble metal, NO _x becomes, including NO _2, it It is stored in the NO _x storage material. Then, when the fuel stoichiometric-rich atmosphere is intermittently made, the stored NO _x is released, and this is converted to HC or CO on the catalyst.
To reduce NO _x .

In the above embodiment, one monolith is used, and a coating layer non-forming portion is provided in an upstream portion, and a coating layer forming portion is provided in a downstream portion. Instead of this embodiment, two monoliths are used, one of which has no coating layer, the other one has a coating layer formed entirely, and these two are one catalyst. It is also possible to adopt a mode in which the converter is incorporated so as to be upstream and downstream, respectively. In addition, a plurality of monoliths are used, and the coating layers having different thicknesses of the coating layers are provided on the whole, and these are arranged and incorporated into one catalytic converter in such an order that the coating layers become thicker toward the downstream side. Embodiments may also be used. Although the embodiment has been described on the premise of a monolithic catalyst,
A pellet-type catalyst may be used, or a mode in which pellets on which a coating layer is formed is produced, and these are incorporated in a catalytic converter so as to be disposed at least in a portion located on the downstream side.

[0021]

EXAMPLES Based on the above embodiment, various embodiments of the exhaust gas purifying catalyst of the present invention were produced as examples. Then, an exhaust gas purifying catalyst having no coating layer formed thereon was prepared as a comparative example, and an endurance test was performed on the exhaust gas purifying catalysts of the examples and comparative examples. NO after rich spike
By comparing the _x storage amounts, it was confirmed that the exhaust gas purifying catalyst of the present invention had excellent sulfur poisoning resistance. Less than,
This will be described.

<Exhaust Gas Purifying Catalyst of Example 1> The exhaust gas purifying catalyst of this example was prepared as follows. First,
Supported on a zirconia powder using a rhodium nitrate solution,
This was dried and fired to obtain a zirconia powder carrying Rh. The amount of Rh supported was 10% for Rh-supported zirconia powder.
1.0 wt% when 0 wt%. Next, a mixed powder of alumina and titania was mixed with the Rh-supported zirconia powder. The mixing ratio was 1: 1 by weight of alumina: titania in the alumina-titania mixed powder.
The weight ratio of alumina-titania mixed powder: Rh-supported zirconia powder was 4: 1. A ceria-zirconia composite oxide was further added to this mixed powder, and an alumina-titania mixed powder: ceria-zirconia composite oxide in a weight ratio of 10:
The components were mixed at a ratio of 1. This powder is slurried by a predetermined method, and 30 mmφ × 50 mm length (capacity 3
5 cc) of a ceramic honeycomb monolith substrate was coated to produce a monolith carrier. The coating amount was 270 g per liter of the monolith carrier.

This monolithic carrier is immersed in an aqueous solution of barium acetate at a predetermined concentration to support Ba,
After drying for 5 minutes, it was baked at 500 ° C. for 30 minutes. B
The amount of a carried was 0.2 mol per liter of the monolithic carrier. Next, this was immersed in an aqueous solution of ammonium bicarbonate having a concentration of 15 g / liter for 15 minutes, and then dried for 15 minutes. Next, Pt was supported thereon using a nitric acid solution of dinitrodiamine platinum, and dried and calcined at 300 ° C. for 15 minutes. The supported amount of Pt was 2 g per liter of the monolith carrier. Next, this is immersed in a mixed aqueous solution of potassium acetate and lithium acetate at a predetermined concentration to carry K and Li, dried at 250 ° C. for 15 minutes, and baked at 500 ° C. for 30 minutes to form a base. A monolith catalyst was prepared. The loading amounts of K and Li were each 0.1 mol per liter of the monolith carrier.

Finally, a portion of the monolith catalyst having a length of about 25 mm from the downstream end (about 50% of the total length) is immersed in a titania sol, dried at 250 ° C. for 30 minutes, and then dried at 500 ° C. for 60 minutes. The coating layer was formed by baking for minutes, and the exhaust gas purifying catalyst of this example was completed. The thickness of the coating layer is about 10 μm over the entire length of the coating layer forming portion.
Met.

<Exhaust Gas Purifying Catalyst of Example 2> This is an exhaust gas purifying catalyst in which the coating layer forming portion is formed to have a length of about 3 mm (about 6% of the entire length) from the downstream end. Other components and the manufacturing method were the same as those of the first embodiment. <Exhaust gas purifying catalyst of Example 3> This is an exhaust gas purifying catalyst in which the coating layer forming portion is formed to have a length of about 40 mm (about 80% of the entire length) from the downstream end. Other components and the manufacturing method were the same as those of the first embodiment.

<Exhaust Gas Purifying Catalyst of Example 4> This is an exhaust gas purifying catalyst in which a base monolith catalyst is immersed in silica sol to form a coating layer of silica. The length of the coating layer forming portion is about 25 mm from the downstream end (about 50 mm of the total length).
%), And the other components and the manufacturing method were the same as those in Example 1.

<Exhaust Gas Purifying Catalyst of Comparative Example> This is an exhaust gas purifying catalyst without a coating layer. The same monolithic catalyst as the base prepared in Example 1 is used. <Evaluation of resistance to sulfur poisoning> An endurance test was performed on the exhaust gas purifying catalysts of the above Examples and Comparative Examples. The conditions of the durability test are as follows: at a catalyst bed temperature of 400 ° C., a predetermined model gas, a durable gas, is introduced and a lean state is maintained for 55 seconds, and another durable gas, a model gas, is spiked (pulsed) for 5 seconds. ), And the cycle of making it rich is performed for one hour. The endurance gas for making the lean state and the rich state has a flow rate of 4.7 liter / mi.
n, the components of which are shown in Table 1 below.

[0028]

[Table 1] Then, the exhaust gas purifying catalyst of Examples and Comparative Examples after the durability test, not subjected to durability test, i.e. relative to the exhaust gas purifying catalysts of Examples and Comparative Examples of new, rich spike _the NO _x storage amount It was measured. Rich spike NO _x
Measurement conditions of the occlusion amount are as follows: at a catalyst bed temperature of 400 ° C., a predetermined model gas is introduced, a lean state is maintained for 5 minutes, then another model gas is introduced for 10 seconds in a spike form (pulse form) to obtain a rich state. This was repeated five times and an average was taken to measure the NO _x amount per test piece (35 cc). In this measurement, the model gas used for the lean state and the rich state has a flow rate of 30%.
It was introduced at a rate of 1 liter / min, and its components are shown in Table 2 below.

[0029]

[Table 2] FIG. 2 shows the amount of rich spike NO _x storage per catalyst of each of the durable and new exhaust gas purifying catalysts of Examples and Comparative Examples obtained by this measurement. As is apparent from this figure, the exhaust gas purifying catalyst of Comparative Example which is not to form a coating layer, in spite of _the NO _x storage amount is large in the case of new, in the case of durable goods of the NO _x storage amount The decrease is significant. On the other hand, it can be seen that the exhaust gas purifying catalyst of the embodiment in which the coating layer is formed has a small decrease in the NO _x storage amount even in the case of a durable product. Therefore, it can be confirmed that the exhaust gas purifying catalyst of the present invention having the coating layer formed thereon has excellent sulfur poisoning resistance.

Comparing the exhaust gas purifying catalysts of Examples 1 to 3, it can be seen that applying the coating layer over a wide range reduces the amount of NO _x occlusion in the case of a new product, that is, in the initial stage. In contrast, if after the durability test, (in this example about 50% of the total length) suitable range by applying a coating layer on it can be seen that to maintain a large _the NO _x storage amount. Therefore, it can be confirmed that the range in which the coating layer is formed may be selected according to the desired characteristics of the exhaust gas purifying catalyst.

Further, comparing the exhaust gas purifying catalysts of Example 1 and Example 4, there is no significant difference in the NO _x storage amount between the new and durable products. Therefore, the coating layer is formed of titania. It can also be confirmed that they may be formed by using silica.

[0032]

The exhaust gas purifying catalyst of the present invention exhibits, constituting the NO _x storage-and-reduction type catalyst, so as to form a coating layer comprising at least one of titania and silica on the carrier surface obtained by carrying _the NO _x storage material Is what you do. With this configuration, the exhaust gas purifying catalyst of the present invention, after the durability also _the NO _x storage amount is large, sulfur poisoning is effectively inhibited _the NO _x purification performance degradation small exhaust gas purifying catalyst .

[Brief description of the drawings]

FIG. 1 conceptually shows a portion where a coating layer is formed in a monolithic catalyst.

FIG. 2 shows the rich spike NO _x storage amounts of the durable and new exhaust gas purifying catalysts of Examples and Comparative Examples.

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Claims (2)

[Claims] An exhaust gas purifying catalyst disposed in a flow path of exhaust gas from a lean burn engine for purifying the exhaust gas, comprising: a substrate; and a porous oxidation layer formed on a surface of the substrate in a layered manner. And a noble metal and NO _x supported on the carrier
An exhaust gas purifying catalyst comprising: an occluding material; and a coating layer formed of at least one kind of titania and silica formed by coating a surface of the carrier at least in a portion located on the downstream side of the exhaust gas. 2. The exhaust gas purifying catalyst according to claim 1, wherein the coating layer is formed in a portion ranging from 5 to 80% of the entire length of the substrate from the downstream end of the substrate.