JPH10174844A - Exhaust gas cleaning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the deterioration of NOX-cleaning performance after endurance while securing the initial NOX purifying efficiency by constituting a carrier of a HC reducing means with a composite material of titania and silica or zirconia and alumina in an exhaust gas purifying device provided with a NOX occluding.reducing catalyst and the HC reducing means. SOLUTION: The HC reducing means 4 and the NOX occluding.reducing catalyst 5 are arranged from an upstream side to a downstream side of an exhaust gas passage 2 in this order. A ternary catalyst, an oxidizing catalyst, etc., are exemplified as the HC reducing means 4 and constituted by incorporating a carrier consisting of a heat resistant porous body and a catalytic noble metal. Then this carrier is composed of a composite material of at least one kind selected among the titania, silica and zirconia and the alumina. That is, the carrier of the HC reducing means 4 is an acidic material. Therefore, a formation of SOX is suppressed since a SO2 being the acidic material is hardly adsorbed to the carrier, and a sulfur poisoning of the NOX occluding material on the NOX occluding.reducing catalyst 5 arranged at downstream is suppressed.

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 apparatus and, more particularly, to carbon monoxide (CO) contained in exhaust gas.
Nitrogen oxides (N) in exhaust gas containing oxygen in excess of that required to oxidize hydrocarbons and hydrocarbons (HC)
The present invention relates to an exhaust gas purification device capable of efficiently purifying O _x ).

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NO _x has been used as a catalyst for purifying exhaust gas of automobiles. As such a catalyst, for example, a catalyst in which a support layer made of γ-alumina is formed on a heat-resistant carrier such as cordierite and a support noble metal such as Pt, Pd, and Rh is supported on the support layer is widely known. I have.

[0003] The purifying performance of such an exhaust gas purifying catalyst varies greatly depending on the air-fuel ratio (A / F) of the engine. That is, on the lean side where the air-fuel ratio is large, that is, the fuel concentration is lean, the amount of oxygen in the exhaust gas increases, and C
Oxidation reactions of purifying O and HC to purify although NO _x is active reducing reaction becomes inactive. Conversely small air-fuel ratio, that is the less oxygen content in the exhaust gas in the fuel concentration dark rich side, the oxidation reaction of CO and HC reduction reaction of but a inactive NO _x becomes active.

On the other hand, in the case of driving in a city, acceleration and deceleration are frequently performed in a car, and the air-fuel ratio frequently changes within a range from near stoichiometric (stoichiometric air-fuel ratio) to a rich state. In order to respond to such a demand for low fuel consumption during traveling, it is necessary to operate on a lean side that supplies an air-fuel mixture with an excess of oxygen as much as possible, and a lean burn engine has been used in recent years. Development of the catalyst is desirable to be able to sufficiently reduce and purify NO _x even Therefore lean burn.

Accordingly, the applicant of the present application has previously proposed an exhaust gas purifying catalyst (NO _x storage reduction catalyst) in which an alkaline earth metal and Pt are supported on a porous carrier such as alumina (Japanese Patent Laid-Open No. Hei 5-317652). ). According to this catalyst, NO _x
Is absorbed by the alkaline earth metal in a stoichiometric to rich atmosphere, and is released on the lean side and reacted with a reducing gas such as HC to be purified.
Excellent O _x purification performance.

In the catalyst disclosed in JP-A-5-317652, for example, barium is supported on a carrier as a single oxide, which reacts with NO _x to form barium nitrate (Ba).
It is considered that NO _x is occluded by generating (NO ₃ ) ₂ ). Further, the heat-resistant inorganic oxide comprising a zeolite or alumina, also _the NO _x storage material and the exhaust gas purifying catalyst in which platinum is supported such that a rare earth element typified by an alkaline earth metal or lanthanum typified by barium knowledge (JP-A-5-168860 and JP-A-6-31139).

[0007] More Hei 5-187230 discloses, place _the NO _x storage reduction catalyst as described above in an exhaust passage of a lean burn internal combustion engine capable, a three way catalyst or an oxidation catalyst on the upstream HC reduction An exhaust gas purifying apparatus in which means are arranged has been proposed. According to this exhaust gas purification device,
Since HC is oxidized and removed on the upstream side near the engine in the exhaust passage and at a high temperature, the HC purification performance in a low temperature range such as at the time of starting is excellent. Also since it is possible to suppress the HC to the catalyst noble metal is reduced the activity adsorbed, it is possible to maintain high _the NO _x purification rate over a long period of time.

[0008]

However, in the exhaust gas,
Represents sulfur generated by combustion of sulfur (S) contained in fuel.
O _TwoContained in the catalyst noble metal in an oxygen-rich atmosphere
More oxidized SO_ThreeBecomes And that is also exhaust gas
The sulfuric acid easily becomes sulfuric acid by the water vapor contained in
Reacts with barium etc. to produce sulfites and sulfates,
This makes NO_xIt is clear that the occlusion material is poisoned and deteriorated.
became. A porous carrier such as alumina is made of SO._xSuck
Due to the property of easy absorption, the above sulfur poisoning
There is a problem of being promoted.

Furthermore in the exhaust gas purifying device disclosed in JP-A 5-187230 discloses, often _the amount of SO _x to produce because the sulfur in the exhaust gas are oxidized by the HC reduction means upstream, downstream _the NO _x storage There is a problem that sulfur poisoning of the reduction catalyst is further promoted. And thus N
When the O _x storage material becomes a sulfite or a sulfate due to sulfur poisoning, it is no longer possible to store NO _x , and as a result, in the above-described catalyst and exhaust gas purification device, the purification performance of NO _x after durability is significantly reduced. There was a problem.

[0010] Since titania does not absorb SO ₂ , an experiment was conducted with the supposition of using a titania carrier. As a result, SO ₂ intact flows downstream without being absorbed by the titania, the degree of sulfur poisoning since only only SO ₂ in direct contact with the catalyst noble metal is oxidized to be small revealed. However low initial activity in the titania carrier, it has been found that there is a critical problem that the purification performance of the NO _x after the durability also remains low.

[0011] The present invention has been made in view of such circumstances, while maintaining the initial of the NO _x purification rate, and to prevent the deterioration of the NO _x purifying performance after endurance.

[0012]

The exhaust gas purifying apparatus of the present invention that solves the above-mentioned problems is characterized by an internal combustion engine capable of burning in a lean air-fuel ratio range, an exhaust passage thereof, and a lean air-fuel ratio range installed in the exhaust passage. of the NO _{x storage-and-reduction} catalyst capable purifying NO _x in the exhaust, and _the NO _x storage located upstream of the reduction catalyst heat resistance made of a porous body carrier and the catalyst noble metal supported on a carrier in the exhaust passage And an HC purifying means, wherein the carrier of the HC reducing means comprises a composite of alumina and at least one selected from titania, silica and zirconia.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the exhaust gas purifying apparatus of the present invention, H
The C reducing means and the NO _x storage reduction catalyst are arranged in this order from upstream to downstream of the exhaust passage. Examples of the HC reducing means include a three-way catalyst, an oxidation catalyst, and the like, and include a carrier made of a heat-resistant porous body and a catalytic noble metal. The greatest feature of the present invention is that the carrier is composed of a complex of at least one selected from titania (TiO ₂ ), silica (SiO ₂ ) and zirconia (ZrO ₂ ) and alumina (Al ₂ O ₃ ). Where it is.

That is, by combining at least one selected from titania, silica and zirconia with alumina, the carrier of the HC reducing means has an acid property. Therefore, sulfur (SO ₂ ), which is acidic, is less likely to be adsorbed on the carrier, so that the generation of SO _x is suppressed, and the sulfur poisoning of the NO _x storage material on the NO _x storage reduction catalyst disposed downstream is reduced. Is suppressed.

The carrier can have a smaller specific surface area as compared with the case of using only alumina, and in addition to the effect that the sulfur content is hardly adsorbed, the generation of SO _x is further suppressed, and the NO _x storage is suppressed. Sulfur poisoning of the material is further suppressed. The use of a carrier having a reduced specific surface area also has the effect of increasing thermal stability. The specific surface area of the carrier is desirably about 10 to 100.

Selected from titania, silica and zirconia
And alumina to form a composite.
In such a case, it is preferable that the composition is combined at the ratio shown below. Less than
Bottom, TiO_Two, SiO_TwoAnd ZrO_TwoMO_TwoThat. MO_TwoWhen
Al_TwoO_ThreeThe compounding ratio with was converted to metal M and metal Al.
M / Al = 5/95 to 50/50 in molar ratio.
Preferably. When M / Al is smaller than 5/95
NO after endurance _xPurification rate decreases, greater than 50/50
When it comes to early NO_xPurification rate decreases and durable according to the value
Later NO_xThe purification rate is also low. Particularly desirable range
Is M / Al = 20/80 to 30/70.

It is desirable that MO ₂ and Al ₂ O ₃ are combined at the smallest possible level. This gives S
O _x generation can be further suppressed. For example, a composite oxide is more preferable than a simple mixture, and a composite at the atomic level is most preferable. In order to form a composite at the atomic level, there are methods such as a coprecipitation method and a sol-gel method.

The HC reducing means comprises Pt, Rh,
It can be configured to support a catalytic noble metal such as Pd and / or a base metal such as Fe, Mn, or Cu. For example, H
If the C reducing means is a three-way catalyst, it is desirable that 0.05 to 20 g of the catalytic noble metal be supported per liter of the carrier. If the amount is less than 0.05 g, it becomes difficult to oxidize HC.
It is not preferable that the amount exceeds g, since the oxidizing action is saturated and the cost increases.

The carrier for the HC reducing means may be a honeycomb shape or a pellet shape by molding the carrier itself, or a structure in which a carrier base material such as cordierite or a metal honeycomb shape is coated with a powder of the composite. It can also be. The NO _x storage-reduction catalyst may have a configuration in which a catalytic noble metal and a NO _x storage element are supported on a heat-resistant porous carrier as in the conventional case. Alumina, titania, silica, zirconia, silica-alumina and the like can be used as the heat-resistant porous carrier, and Pt, R
h, Pd and the like are used. Examples of NO _x storage elements include alkali metals such as Na, K, Cs and Rb, Ba,
Alkaline earth metals such as Ca and Sr, Y, Ce, La,
A rare earth element such as Pr can be used as needed.

The loading amount of the catalytic noble metal of the NO _x storage reduction catalyst is 0.05 to 2 per liter of the porous carrier.
It is desirable to carry 0 g. NO if less than 0.05g
It becomes difficult to reduce _x , and even if it exceeds 20 g, it is not preferable because the reducing action is saturated and the cost rises. The amount of the NO _x storage material to be carried is preferably in the range of 0.05 to 10 mol per liter of the porous carrier. If it is less than 0.05 mol, it is difficult to exhibit NO _x storage ability and it is difficult to reduce NO _x , and if it exceeds 10 mol, the heat resistance will be reduced.

[0021]

The present invention will be specifically described below with reference to examples. (Example 1) <Preparation of HC reducing means> A γ-alumina powder and a titania powder were mixed at a weight ratio of 1: 1 and wet-mixed with a ball mill for 5 hours to prepare a carrier powder. The specific surface area of this carrier powder is 130 m ² / g.

100 parts by weight of this carrier powder, 70 parts by weight of alumina sol (10% by weight of alumina), and 30 parts of water
A slurry was prepared by mixing parts by weight, and a cordierite honeycomb carrier substrate was immersed in the slurry, pulled out, blown off excess slurry, dried at 120 ° C. for 1 hour, and dried at 50 ° C. for 1 hour.
By firing at 0 ° C. for 1 hour, a coat layer composed of TiO ₂ —Al ₂ O ₃ was formed. The coating layer is 200 g per liter of the honeycomb carrier substrate.

Next, the honeycomb carrier having the coating layer formed thereon is immersed in an aqueous solution of palladium nitrate having a predetermined concentration, pulled up to blow off excess liquid droplets, and then dried at 120 ° C. for 1 hour.
The mixture was calcined at 0 ° C. for 1 hour to carry Pd to prepare a three-way catalyst as a means for reducing HC. The supported amount of Pd is 5 g per liter of the honeycomb carrier substrate. <Preparation of NO _x storage reduction catalyst> γ-alumina powder 100
A coating slurry was prepared by mixing parts by weight, 70 parts by weight of alumina sol (10% by weight of alumina), 15 parts by weight of a 40% by weight aqueous solution of aluminum nitrate, and 30 parts by weight of water.

Next, a cordierite honeycomb carrier substrate is prepared, immersed in the slurry, pulled up and blow off excess slurry, dried, and fired at 600 ° C. for 1 hour to form an alumina coat layer. Thus, a carrier was prepared. The coating amount is 120 per liter of the volume of the honeycomb carrier substrate.
g. This carrier was immersed in an aqueous solution of dinitrodiammine platinum, pulled up and blow off excess water droplets,
It was dried at ℃ to support Pt. Then, the sample was immersed in an aqueous rhodium nitrate solution, and Rh was supported in the same manner. Pt and Rh
Are 2 g and 0.1 g per liter of the carrier, respectively.

Next, an aqueous solution of barium acetate having a predetermined concentration was prepared, the above-mentioned Pt-Rh carrier was immersed, pulled up, dried by blowing off excess water droplets, and calcined at 600 ° C. for 1 hour to carry Ba. . Next, immerse in a predetermined concentration of lithium acetate aqueous solution, pull up, blow off excess water drops, and dry.
It was fired at 600 ° C. for 1 hour to carry Li. Further, it was immersed in an aqueous solution of potassium acetate having a predetermined concentration, pulled up, dried by blowing off excess water droplets, and baked at 600 ° C. for 1 hour to carry K. The loading amount of each NO _x storage element was such that 0.3 mol of Ba was contained per liter of
Is 0.1 mol and K is 0.1 mol.

<Catalyst Device and Evaluation Test> As shown in FIG. 1, the three-way catalyst 4 is disposed near the exhaust manifold 3 in the exhaust passage 2 of the vehicle equipped with the lean burn engine 1, and is located 1 m away from the downstream side. NO _x storage-reduction catalyst 5 to
Was placed. And NO _x operating at 10 · 15 mode
The purification rate was measured, and the results are shown in Table 1 as the initial purification rate.

In the same manner as described above, a durability test was conducted in which both catalysts were arranged in a vehicle equipped with a lean burn engine and exhaust gas was circulated for 200 hours under operating conditions simulating running in an urban area.
Thereafter, the NO _x purification rate was measured in the same manner as above, and the results are shown in Table 1 as the post-durability purification rate. (Example 2) γ-alumina powder and titania powder were mixed at a weight ratio of 1: 1 and wet-mixed in a ball mill for 5 hours, and then calcined at 800 ° C for 3 hours to prepare a carrier powder. The specific surface area of this carrier powder is 100 m ² / g.

Example 1 except that this carrier powder was used
Similarly prepared a three-way catalyst and, arranged in the same manner in an exhaust passage of a lean-burn engine equipped vehicles with similar NO _x storage reduction catalyst as in Example 1, likewise initial and NO after the endurance
_{x The} purification rate was measured. Table 1 shows the results. Example 3 A γ-alumina powder and a titania powder were mixed at a weight ratio of 1: 1 and wet-mixed in a ball mill for 5 hours, and then calcined at 1000 ° C. for 3 hours to prepare a carrier powder. The specific surface area of this carrier powder is 60 m ² / g.

Example 1 except that this carrier powder was used.
Similarly prepared a three-way catalyst and, arranged in the same manner in an exhaust passage of a lean-burn engine equipped vehicles with similar NO _x storage reduction catalyst as in Example 1, likewise initial and NO after the endurance
_{x The} purification rate was measured. Table 1 shows the results. Example 4 γ-alumina powder and silica powder were mixed at a weight ratio of 1: 1 and wet-mixed in a ball mill for 5 hours, and then calcined at 1000 ° C. for 3 hours to prepare a carrier powder. The specific surface area of this carrier powder is 60 m ² / g.

Example 1 except that this carrier powder was used.
Similarly prepared a three-way catalyst and, arranged in the same manner in an exhaust passage of a lean-burn engine equipped vehicles with similar NO _x storage reduction catalyst as in Example 1, likewise initial and NO after the endurance
_{x The} purification rate was measured. Table 1 shows the results. Example 5 A γ-alumina powder and a zirconia powder were mixed at a weight ratio of 1: 1 and wet-mixed in a ball mill for 5 hours, and then calcined at 1000 ° C. for 3 hours to prepare a carrier powder. The specific surface area of this carrier powder is 60 m ² / g.

Example 1 except that this carrier powder was used
In the same manner as in Example 1, a three-way catalyst was prepared._xSucking
Emissions of vehicles equipped with lean burn engines together with the storage reduction catalyst
NO in the initial stage and after endurance
_xThe purification rate was measured. Table 1 shows the results. Comparative Example 1 Only the γ-alumina powder was used as the carrier powder.
The specific surface area of this carrier powder is 180 m ^Two/ G. Soshi
In the same manner as in Example 1 except that this carrier powder was used
A three-way catalyst was prepared, and the same NO_xOcclusion reduction
In the exhaust passage of vehicles equipped with a lean burn engine together with the medium
In the same manner, the NO after initial and endurance_xPurification rate
Was measured. Table 1 shows the results.

Comparative Example 2 Example 1 without using a three-way catalyst
Only the same NO _x storage reduction catalyst was placed in the exhaust passage of the vehicle equipped with a lean burn engine as in Example 1, and the initial and endurance NO _x purification rates were measured. Table 1 shows the results.

[0033]

[Table 1] According to Table 1, the exhaust gas purifying apparatus of each embodiment has a smaller decrease in the NO _x purification rate after the endurance as compared with the initial stage compared with the comparative example, which indicates that the TiO ₂ -Al ₂ O ₃ , SiO ₂ -A
it is clear that the l ₂ O ₃ or ZrO ₂ -Al effect using a complex consisting of ₂ O _3.

From the comparison between Examples 1 to 3 and Comparative Example 1, it can be seen that as the specific surface area of the carrier of the three-way catalyst decreases, the NO _x purification rate after durability increases. The specific surface area decreases as the sintering temperature increases,
At the same time, the amount of the composite oxide of titania and alumina increases, so this effect is considered to be a combination of the effect of the specific surface area and the effect of the composite oxide compounded at the atomic level.

Further, in Comparative Example 2, since the initial NO _x purification rate is lower than that of the embodiment, the effect of arranging the three-way catalyst upstream of the NO _x storage reduction catalyst is apparent.

[0036]

According to the exhaust gas purifying apparatus of the present invention, even if the air-fuel ratio changes from lean to stoichiometric or rich and the concentration of HC in the exhaust gas increases, it can be sufficiently reduced by the HC reducing means. In addition, HC poisoning of the catalytic noble metal of the NO _x storage reduction catalyst can be suppressed, and durability can be improved.

Since the oxidation of SO ₂ by the HC reducing means is suppressed, the frequency of contact between the NO _x storage reduction catalyst and SO _x is reduced, and the sulfur poisoning of the NO _x storage element is suppressed. Therefore, it is possible to suppress the decrease in the NO _x purification rate after the durability while securing the initial NO _x purification rate, and the durability is improved.

[Brief description of the drawings]

FIG. 1 is a system diagram of an exhaust gas purifying apparatus according to one embodiment of the present invention.

[Explanation of symbols]

1: engine 2: exhaust passage 3: exhaust manifold 4: three-way catalyst (HC reducing means) 5: NO _x
Storage reduction catalyst

Claims (1)

[Claims] 1. A combustion possible internal combustion engine and an exhaust passage lean air fuel ratio zone, and can purify _the NO _x storage reduction catalyst the NO _x in the exhaust gas of lean air fuel ratio zone which is installed in the exhaust passage,
In the exhaust gas purifying apparatus and a HC reduction means including a said _the NO _x storage located upstream of the reduction catalyst heat resistance made of porous carrier and carrier carried on the catalyst noble metal of the exhaust passage, The exhaust gas purifying apparatus, wherein the carrier of the HC reducing means is composed of a composite of alumina and at least one selected from titania, silica and zirconia.