JPH08182928A - Nitrogen oxide occlusion composition and exhaust gas purification - Google Patents

Abstract

PURPOSE: To obtain an NOx occlusion material excellent in heat resistance in an excess oxygen atmosphere and exhibiting excellent NOx occluding power even at a high temp. by carrying each specific quantity of silver, at least one kind of alkaline earth metal elements and at least one kind of platinum group elements on an alumina based metal oxide carrier. CONSTITUTION: This nitrogen oxide occlusion composition used for a purifying device of nitrogen oxides contained in the exhaust gas discharged from internal engine or the like is prepared by carrying 5-40wt.% silver, 5-40wt.% at least one kind of alkaline earth metal elements and 0.05-8wt.% at least one kind of platinum group elements expressed in terms of metal per total weight of the carrier and the carried components on the alumina based metal oxide carrier. In the carried components, sum of the contents of silver and at least one kind of alkaline earth metal elements is controlled to 10-50wt.%. Nitrogen oxides in the exhaust gas is removed by providing an NOx occlusion material- containing layer on the upstream side of the exhaust gas flow and a ternary catalyst on the downstream side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitrogen oxide storage composition used in a device for purifying nitrogen oxide (NOx) contained in exhaust gas discharged from internal combustion engines such as automobiles, boilers and chemical plants. The present invention relates to a substance (hereinafter, also referred to as NOx storage substance) and an exhaust gas purification method using the same.

[0002]

2. Description of the Related Art In recent years, it has been possible to remove NOx in exhaust gas in which oxygen is present in excess of stoichiometric amount, such as exhaust gas emitted from lean burn engines for automobiles, diesel engines, gas turbines and chemical plants. It has become an important issue from the viewpoint of preventing environmental pollution. Conventionally, as a method of removing NOx contained in exhaust gas, a three-way catalyst such as platinum-rhodium / γ-alumina is used to control the exhaust gas while controlling the air-fuel ratio of the exhaust gas to near the stoichiometric amount. A three-way catalyst method has been used in which NOx therein is reduced and carbon monoxide (CO) and hydrocarbons (HC) are removed by oxidation.
On the other hand, recently, a method of occluding NOx in a metal oxide for the purpose of removing NOx in exhaust gas under an excess oxygen atmosphere (lean atmosphere) has been studied.
No. 317652 uses Pt-BaO / Al ₂ O ₃ to occlude NOx in an oxidizing atmosphere, and then releases the NOx in a reducing atmosphere, similar to a three-way catalyst for a gasoline vehicle in a reducing atmosphere. NOx is HC and C
A method of reducing and removing with O has been proposed. Other,
A method of using a Mn-Zr-based or Mn-Y-based composite oxide for the adsorption removal of NOx in an excess oxygen atmosphere is also known. (Arai, Eguchi et al. Proceedings of the 74th Catalysis Debate Conference 3A
05 1994)

On the other hand, although selective reduction of NOx by HC in exhaust gas under an excess oxygen atmosphere using a silver-supported alumina catalyst (Japanese Patent Laid-Open No. 4-281844) is known, NOx removal performance at low temperatures is not available. , And silver-loaded alumina is NO
It is not known that it can be used for storage-release of x.

[0004]

The above-mentioned three-way catalyst system is not effective in removing NOx in exhaust gas containing excess oxygen. Further, although the above-mentioned NOx storage substance has a NOx storage reduction ability at 200 to 300 ° C. in an excess oxygen atmosphere, it has a drawback that it cannot store a sufficient amount of NOx at 300 ° C. or higher. Further, the NOx storage substance has a poor heat resistance and has a drawback that it cannot store a sufficient amount of NOx after aging at a high temperature of 700 ° C. or higher. INDUSTRIAL APPLICABILITY The present invention has excellent heat resistance in an excess oxygen atmosphere, and can exhibit an excellent NOx storage capacity at 300 ° C. or higher.
An object is to provide an Ox storage material and an exhaust gas purification method using the same.

[0005]

To achieve the above object, the present invention provides an alumina-based metal oxide carrier containing 5 to 4 silver in terms of metal, based on the total weight of the carrier and the supported component.
0 wt%, at least one of alkaline earth metal elements 5
-40 wt% and at least one of the platinum group elements is 0.0
Provided is a NOx storage substance loaded with 5 to 8 wt%.
In the present invention, the supported component has the above-mentioned composition, and the total amount of at least one of silver and alkaline earth metal elements is 10 or less.
Provide a NOx storage material that is ˜50 wt%.

According to the present invention, the NOx storage substance-containing layer or the NOx storage substance-containing layer of the present invention is provided on the upstream side of the exhaust gas flow, and the three-way catalyst is provided on the downstream side thereof so that the atmosphere is from lean to rich. Provided is a method of contacting with changing exhaust gas to remove nitrogen oxides in the exhaust gas. According to the present invention, the NOx storage substance-containing layer of the present invention is provided on the upstream side of the exhaust gas flow, and the excess oxygen-containing exhaust gas purification catalyst is provided on the downstream side thereof, and the exhaust gas temperature changes from a low temperature to a high temperature. Contact the exhaust gas of the atmosphere,
A method for removing NOx in the exhaust gas is provided.

Hereinafter, the present invention will be described in detail. Carrier As a carrier for the NOx storage substance of the present invention, an alumina-based metal oxide is used. As the alumina-based metal oxide, for example, activated alumina such as γ-alumina and η-alumina and alumina-based composite oxides such as silica-alumina, titania-alumina and zirconia-alumina can be used, and activated alumina is preferably used. .

Supporting Component The NOx storage substance of the present invention contains at least one of silver, an alkaline earth metal element and at least one of platinum group elements as a supporting component. The content of silver is 5 to 40 wt%, preferably 10 to 30 wt% in terms of metal, based on the total weight of the carrier and the supported component. If the content of silver is too small, the NOx storage amount at 300 ° C. or higher will decrease, and the heat resistance will not improve. On the contrary, if the amount is too large, the heat resistance tends to decrease. Strontium as an alkaline earth metal element,
At least one of barium, calcium and magnesium is used. Preferred is strontium and / or barium. When these elements are supported, they exist in an oxide state. The content of these elements is 5 to 40 wt% in terms of metal, based on the total weight of the carrier and the supported component,
Preferably, it is 10 to 30 wt%. If the amount is too small or too large, the NOx storage amount tends to decrease.

As platinum elements, rhodium, iridium,
At least one of platinum and palladium is used. Rhodium and / or platinum are preferable. The content of these platinum group elements is 0.05 to 8 wt% in terms of metal, preferably 0.1 to 6 with respect to the total weight of the carrier and supported components.
wt%. If it is too small, the NOx storage amount tends to decrease, and if it is too large, NOx is easily reduced by a reducing agent such as HC contained in the exhaust gas, which is an undesirable by-product nitrous oxide (N ₂ The production amount of O) increases. In a further preferred embodiment of the present invention, the total amount of at least one of the supported component silver and the alkaline earth metal element is 10 to 50 wt% in terms of metal based on the total weight of the carrier and the supported component.
%, And more preferably 15 to 40 wt%. The weight ratio of silver to alkaline earth metal element is not particularly limited, but is preferably 1: 0.1 to 1:20 in terms of metal, and more preferably 1: 0.2 to 1:10.

Next, a method for producing the NOx storage substance of the present invention will be described. The method for producing the NOx occluding substance of the present invention is not particularly limited, and an ordinary method for catalyst preparation such as an adsorption method, an impregnation method, an ion exchange method is applied. The metal source as a starting material is not limited and various salts and complexes are used. For example, silver nitrate, silver acetate, etc. are used as the silver source, strontium nitrate, strontium hydroxide, strontium oxide, barium nitrate, barium hydroxide, barium oxide, etc. as the alkaline earth metal source, and rhodium nitrate as the platinum group source. , Rhodium chloride, tetraammine platinum nitrate, chloroplatinic acid, tetraammine platinum chloride, palladium nitrate, palladium chloride, iridic acid chloride, iridium nitrate and the like are used.

For example, in order to support silver, an alkaline earth metal element and a platinum group element on an alumina carrier, the alumina carrier is brought into contact with a mixed aqueous solution in which a silver source, an alkaline earth metal source and a platinum group source are dissolved. . More specifically, it may be immersed in the aqueous solution, or the aqueous solution may be impregnated in an alumina carrier. However, when used as a mixed aqueous solution, it is necessary to select a combination that does not cause precipitation due to the reaction between the ligands. For example, the use of silver nitrate and rhodium chloride should be avoided as it will precipitate silver chloride. After all the supported components are supported in this way, they are dried and then baked. The firing temperature may usually be 500 to 700 ° C.

As another method, first, a silver source may be supported on alumina, followed by baking and fixation, then an alkaline earth metal source, and then a platinum group source, which are sequentially supported. The order of loading is arbitrary. The NO of the present invention
The shape and form of the x storage material are not particularly limited. It may be used in the form of powder or granules, or may be molded into various shapes such as spheres or pellets before use. In a preferred embodiment, the NOx storage material is a refractory support substrate having a large number of through holes that allow the exhaust gas to penetrate when placed in the exhaust gas, for example, a honeycomb monolith substrate made of cordierite or stainless steel. Examples include a form in which the powder is coated and used. When the surface of such a monolithic structure substrate is coated and used, it is possible to suppress the pressure loss in contact with the exhaust gas and increase the throughput of the exhaust gas.

Since the NOx storage material of the present invention contains silver and an alkaline earth element, NOx is stored in a lean atmosphere.
The oxide of silver and the alkaline earth element is converted into nitric oxide by coming into contact with, and is occluded in the silver and the alkaline earth element. The stored NOx is easily released by exposing it to a high temperature of 450 ° C. or higher, or in a stoichiometric or rich atmosphere. In particular, NOx released in a stoichio atmosphere or a rich atmosphere can be reduced and removed at the same time as being released by a reducing component such as HC by the platinum group contained in the NOx storage substance of the present invention. Since the NOx storage substance of the present invention contains silver in addition to the alkaline earth metal element, it suppresses poisoning of atmospheric gas such as carbon dioxide (CO2) with respect to NOx storage and has excellent heat resistance. That is, the conventional NOx storage material that depends on the NOx storage capacity for alkaline earth metal oxides and Mn oxides has poor heat resistance and is easily oxidized by aging in air at 700 ° C.
While the NOx storage capacity of the present invention is lost, the NOx storage material of the present invention has a heat resistance of more than 700 ° C to 800 ° C because the NOx storage capacity is maintained by silver in addition to the alkaline earth metal element. It has superior performance to other NOx storage materials.

Exhaust Gas Purification Method The present invention provides two types of methods for removing NOx in exhaust gas. First, the first method will be described. This method is a method of removing NOx contained in the exhaust gas whose atmosphere changes from lean to rich. The NOx storage substance-containing layer of the present invention is arranged in the flow of exhaust gas whose atmosphere changes from lean to rich. The term “containing layer” refers to a form in which a NOx storage substance is contained. For example, a powdery NOx storage substance may be filled in a certain space, or the NOx storage substance may be formed into a predetermined shape. It can be a thing. Alternatively, the NOx storage substance may be coated on the refractory support substrate.

The NOx occluding substance of the present invention occludes NOx in a lean atmosphere and reduces a part of the occluded NOx, but when the atmosphere changes from stoichiometric amount (hereinafter also referred to as stoichio) to rich, it occludes. The released NOx is released, and at least a part of the released NOx is reduced and decomposed by the reducing components such as CO and HC coexisting in the exhaust gas on the platinum group component contained in the NOx storage substance. To be done. More preferably, it is rich in NOx from the stoichio downstream of the NOx storage material-containing layer in the exhaust gas stream.
NOx released without reduction is removed by arranging a three-way catalyst that can reduce CO 2 and CO depending on HC. In the case of a gasoline lean burn engine for vehicle drive,
Usually, the atmosphere frequently changes from lean to stoichiometric or rich depending on the fluctuation of the load in the operating state. In this case, the fluctuation of the atmosphere due to the fluctuation of the load is directly used for the exhaust gas purification method without any special control, and the average NOx removal rate during operation is reduced by bringing the exhaust gas into contact with the catalyst system containing the NOx storage substance. improves. When the lean region continues beyond the NOx saturated storage amount of the NOx storage substance, a known method of changing the atmosphere of the exhaust gas from lean to stoichiometric or rich, for example, increasing the fuel supply amount, reducing the air supply amount, Alternatively, control may be performed by changing the atmosphere of the exhaust gas that is brought into contact with the catalyst system for a time required for NOx release or reduction according to additional addition of a reducing agent to the exhaust gas, and then returning to the original lean state.
The method of the present invention in which the NOx storage material contains silver is superior to the conventional method in that the heat resistance of the catalyst system can be improved.

Next, the second method will be described. The second method is applied to the removal of lean exhaust gases.
As shown in FIG. 1, the NOx storage material of the present invention is brought into contact with the NOx-containing exhaust gas in a lean atmosphere, and the gas temperature is from a low temperature range of normal temperature to 400 ° C to 450 ° C to 600 ° C.
When the temperature is raised to the high temperature region of NOx, NOx is occluded in the low temperature region of 250 to 400 ° C. and released in the high temperature region of 450 ° C. or higher. The NOx stored in the low temperature range is CO that coexists in the exhaust gas on the platinum group component contained in the NOx storage material of the present invention.
Although it is partially reductively decomposed depending on a reducing component such as HC, most of it is not reduced and is released when the temperature rises to a high temperature range. Furthermore, after releasing NOx in a high temperature range, the temperature of the gas that comes into contact with the NOx storage substance is lowered to 400 ° C. or lower, 200 ° C., or even to room temperature, so that NOx in the exhaust gas is saturated in the range of 500 to 200 ° C. Can continue to occlude up to the occluded amount. Thus, the N stored in the temperature decreasing process
Ox is partially reduced again in the subsequent temperature raising process, but most of it is released by the temperature rise to a high temperature region of 450 ° C. or higher. On the other hand, the conventional material as shown in FIG. 2 absorbs NOx in a low temperature range of 150 to 350 ° C. and raises the temperature to 400 ° C. when the gas temperature is raised from normal temperature while being brought into contact with NOx-containing exhaust gas in a lean atmosphere. It is released in the above high temperature range. After releasing NOx in the high temperature range and then lowering the gas temperature to room temperature, NOx in the exhaust gas can be continuously stored up to its saturated storage amount in the range of 500 to 200 ° C.

It is known that the noble metal catalyst has a NOx reducing action in a lean atmosphere, but since the NOx storage substance of the present invention and the conventional substance contain the noble metal,
A part of the reduction of NOx occurs together with the storage of NOx. N
Whether Ox removal contributes by occlusion or reduction can be estimated from the ratio of the NOx removal amount in the low temperature region to the NOx amount released in the high temperature region, and the contribution of reduction and net occlusion in the low temperature region. For example, in the initial temperature rising curve of FIG.
Stores or reduces NOx in the low temperature range of ~ 350 ° C
It is released in the high temperature range above 0 ° C. However, in the temperature rising curve after aging, NOx is occluded or reduced in the low temperature range of 150 to 350 ° C., but NOx is not released in the high temperature range of 400 ° C. or higher. From this, the conventional material is
It can be seen that the Ox storage capacity is lost and NOx is reduced. On the other hand, the NOx storage substance of the present invention in FIG. 1 stores or reduces NOx in a low temperature range of 300 to 450 ° C. and releases NOx in a high temperature range of 450 ° C. or higher in the temperature rising curve after aging. From this, the NO of the present invention
It can be seen that the x storage material maintains the NOx storage capacity even after aging.

In the case of an engine for driving a vehicle, its exhaust gas temperature is 15 depending on the engine speed and the fluctuation of the load during operation.
Since the temperature frequently changes from a low temperature of about 0 ° C. to a high temperature of about 600 ° C. and from a high temperature to a low temperature, this temperature increase / decrease can be directly used for NOx storage / release. In the case of exhaust gas that maintains a high temperature range, the exhaust gas temperature is lowered by a known method that changes the exhaust gas temperature, such as introducing secondary air, in front of the catalyst system containing the NOx storage substance. When the storage temperature range is reached, a control method for canceling this is applied.
NOx released when the temperature rises is NO in the exhaust gas flow.
It is removed by bringing it into contact with a NOx reduction catalyst, which is arranged separately from the NOx storage material, and which is capable of selectively reducing NOx in the lean region, downstream of the x storage material.

Conventional NOx reduction catalysts have generally been limited in the effective temperature range. For example, Pt / zeolite catalyst at 150-300 ° C, copper / zeolite catalyst 350-5
Although it exhibited activity at 00 ° C, the NOx removal rate was not sufficient in other temperature regions, and NOx was discharged without being removed. On the other hand, if a NOx storage substance is placed in the front stage and a NOx reduction catalyst that is active in the temperature range of NOx release is combined in the rear stage, NOx purification in a wide temperature range becomes possible. As a NOx reduction catalyst, in a lean atmosphere, N
NO in the temperature range where NOx is released by the Ox storage material
There is no particular limitation as long as it is a catalyst having x-reducing ability. If the amount of the reducing component in the gas discharged from the NOx storage substance is insufficient for the selective reduction of NOx, the amount of N is downstream of the NOx storage substance and N
It is also possible to increase the conversion rate of NOx reduction by additionally adding the minimum necessary reducing agent such as HC within the range where the atmosphere is not stoichiometric to rich in front of the Ox reduction catalyst. As the NOx reduction catalyst, for example, a known Cu / zeolite catalyst (U.S. Pat. No. 4,297,328, Japanese Patent Laid-Open Publication No. Sho 6) is used.
3-100919) and silver / alumina catalyst (JP-A-4-1004).
281844), Ir / SiC catalyst (Japanese Patent Laid-Open No. 6-31)
No. 173) and the like can be used.

[0020]

EXAMPLES Next, examples of the present invention will be described, but the present invention is not limited to these examples.

Example 1 (a) 120.4 g of aluminum isopropoxide was dissolved in 30 ml of isopropanol, and heated to 60 ° C. under reflux in nitrogen. A mixed solution of 10 ml of deionized water and 20 ml of isopropanol was added dropwise to this solution, and the mixture was stirred until it solidified into a jelly form to obtain a solid. After cooling, the solid matter was filtered and washed, and dried in a dryer at 120 ° C. for 12 hours. The obtained solid was crushed and then calcined in an electric furnace at 600 ° C. for 3 hours to obtain an alumina carrier. (B) An aqueous solution of silver nitrate containing 20.7 g of the alumina carrier obtained in (a) and 6 g of silver as a starting material for the supporting component.
19.9 g of a mixed aqueous solution consisting of 8 g, 7.2 g of a strontium nitrate aqueous solution containing 3 g of strontium, and 3.9 g of a rhodium nitrate aqueous solution containing 0.3 g of rhodium was impregnated. Then, it was dried in a dryer at 120 ° C. for 12 hours, pulverized, and then calcined in an electric furnace at 600 ° C. for 3 hours to obtain a NOx storage substance powder. The composition of the supporting component of the obtained powder is 20 wt% of silver and 10 w of strontium in terms of metal.
It was t% and rhodium 1 wt%. (C) 30 g of the powder obtained in (b), alumina sol (1
10 g of 0 wt% alumina) and 30 g of deionized water were kneaded in a ball mill pot to obtain a slurry. A commercially available cordierite 400-cell honeycomb (1 inch in diameter, 2.5 inch in length) was dipped in this slurry and taken out. Then, the excess slurry was removed by an air knife to cover the honeycomb with the substance powder. Is dry base 100g / L
I adjusted it to be. Then 1 at 150 ° C in a dryer
Dried for an hour and baked in an electric furnace at 500 ° C for 30 minutes,
A NOx storage material-coated honeycomb (A-1) was obtained.

Example 2 Aqueous silver nitrate solution 1 containing 13.7 g of the same alumina carrier as in Example 1 (a) and 9 g of silver as a starting material for the supported component.
Barium nitrate aqueous solution 1 containing 3.3 g and 6 g of barium
Example 1 (b) except that 1.4 g, 12 g of a tetraammine platinum silver nitrate aqueous solution containing 1.2 g of platinum, and 38.7 g of a mixed aqueous solution containing 2.0 g of a rhodium nitrate aqueous solution containing 0.15 g of rhodium were used. A powder was obtained. The composition of the supporting component of the obtained powder was 30 wt% of silver in terms of metal,
Barium 20wt%, Platinum 4wt%, Rhodium 0.5w
It was t%. This powder was treated in the same manner as in Example 1 (c) to obtain a NOx storage material-coated honeycomb (A-2).

Example 3 Aqueous silver nitrate solution containing 21.9 g of the same alumina carrier as in Example 1 (a) and 3 g of silver as a starting material for the supported component.
21.1 g of a mixed aqueous solution containing 4 g, an aqueous solution of strontium nitrate containing 4.5 g of strontium and 10.7 g of an aqueous solution of tetraammine platinum nitrate containing 0.6 g of platinum
A powder was obtained in the same manner as in Example 1 (b) except that was used. The composition of the supported component of the obtained powder was 10 in terms of metal.
% by weight, 15% by weight of strontium, and 2% by weight of platinum. This powder was treated in the same manner as in Example 1 (c) to obtain a NOx storage material-coated honeycomb (A-3).

Comparative Example 1 23.4 g of the same alumina carrier as in Example 1 (a), 11.4 g of an aqueous barium nitrate solution containing 6.0 g of barium as a starting material for the supported component, and tetraammine platinum nitric acid containing 0.6 g of platinum. 17. A mixed aqueous solution containing 6.0 g of the aqueous solution.
A powder was obtained in the same manner as in Example 1 (b) except that 4 g was used. The composition of the supporting component of the obtained powder was barium 20 wt% and platinum 2 wt% in terms of metal. This powder was treated in the same manner as in Example 1 (c) with a powder-coated honeycomb (B-
1) was obtained. This catalyst system was prepared for comparison with a conventionally known catalyst system containing no silver (see JP-A-5-317652).

Comparative Example 2 26.7 g of the same alumina carrier as in Example 1 (a), 7.2 g of an aqueous strontium nitrate solution containing 3.0 g of strontium as a starting material for the supported component, and 0.3 g of rhodium.
Mixed aqueous solution 1 containing 3.9 g of aqueous rhodium nitrate solution containing
A powder was obtained in the same manner as in Example 1 (b) except that 1.1 g was used. The composition of the supported component of the obtained powder was 10 wt% strontium and 1 wt% rhodium in terms of metal. This powder was treated in the same manner as in Example 1 (c) to obtain a powder-coated honeycomb (B-2).

Comparative Example 3 Aqueous silver nitrate solution containing 21.0 g of the same alumina carrier as in Example 1 (a) and 6 g of silver as a starting material for the supported component.
Powder was obtained in the same manner as in Example 1 (b), except that 1 g of a mixed aqueous solution containing 7.2 g of an aqueous strontium nitrate solution containing 8 g and 3.0 g of strontium was used. The composition of the supporting component of the obtained powder was 20 wt% of silver in terms of metal,
It was 10 wt% of strontium. This powder was treated in the same manner as in Example 1 (c) to obtain a powder-coated honeycomb (B-3).

Next, an example of performance evaluation will be described. Samples A-1 to A-3 and Comparative Examples B-1 to B-3 and aging treatment at 700 ° C. for 5 hours in air containing 10% steam were prepared. Performance evaluation example 1 (NOx with lean-rich model gas
(Evaluation of Storage and Removal Performances) The samples of A-1 and B-1 in the initial stage and in the aged state were used as model gases of lean gas and rich gas having the compositions shown in Table 1 below.

[0028]

[Table 1]

While the temperature of the honeycomb inlet gas was kept constant at 350 ° C., while supplying the model gas of Table 1 at SV38,000 / hr, the gas atmosphere was switched between lean and rich every 5 minutes, and NOx in the honeycomb outlet gas was changed. The concentration was measured by a chemiluminescence type NOx meter, and the removal performance by NOx occlusion in a lean atmosphere and the NOx removal performance by reduction in a rich atmosphere were evaluated. As the NOx occlusion rate and the removal rate, the values calculated by the following calculation formulas were used.

[0030]

[Equation 1]

Table 2 shows changes with time in the NOx occlusion rate and removal rate. Furthermore, A-1 is made half the length and is filled in the upstream side in the gas flow, and the same length of conventional Pt-Rh / Al is used.
_The bonded catalyst system in which the ₂ O ₃ ternary catalyst-coated honeycomb was arranged on the downstream side and B-1 were similarly evaluated for the bonded catalyst system. The results are also shown in Table 2.

[0032]

[Table 2]

As can be seen from Table 2, when the lean gas and the rich gas are switched, A-1 of the first embodiment of the present invention is used.
It can be seen that, compared with B-1 of the comparative example, shows a high NOx removal performance on average even in the initial stage and after aging.

Performance evaluation 2 (NO with lean model gas
x Storage and Removal Performance Evaluation) As the sample, the initial and aged products of Examples and Comparative Examples were used, and the gas having the composition shown in Table 3 below was used as a model gas.

[0035]

[Table 3]

The model gas of Table 3 was SV38,000 / h.
While supplying at r, the honeycomb inlet gas temperature is raised from 100 ° C. to 600 ° C. at a temperature rising rate of 30 ° C./min,
Further, the temperature was lowered from 600 ° C. to 100 ° C. at a temperature lowering rate of 30 ° C./min, and the NOx occlusion and removal performance was evaluated. Honeycomb inlet temperature 300 during heating with lean model gas
Table 4 shows the NOx occlusion and removal performance evaluation at ˜450 ° C.

[0037]

[Table 4]

As is clear from Table 4, the NOx storage substance-coated honeycombs A-1 to A-3 of the example are 300 to 400 even after initial or aging, as compared with the substance-coated honeycombs B-1 to B-3 of the comparative example. It can be seen that the NOx occlusion rate and removal rate at ° C are excellent.

FIG. 1 shows Example A-1 and FIG. 2 shows Comparative Example B-1.
The relation between the NOx occlusion rate and removal rate at the time of initial temperature rise and temperature decrease after aging and the honeycomb inlet gas temperature are shown below. From FIG. 1, NO of Example 1 of the present invention
The x storage material-covered honeycomb A-1 stores NOx at a temperature of the inlet gas between 150 ° C. and 400 ° C. at a temperature rise,
When the temperature is raised above 0 ° C, the stored NOx is released, but when the temperature is lowered from 600 ° C, the NOx is stored over a wide temperature range up to 100 ° C.
x It can be seen that it is an occlusion substance. N of Example 1 of the present invention
The Ox storage material-coated honeycomb A-1 is 300 even after endurance.
Since NOx stored at ˜400 ° C. is released at 400 ° C. or higher, it can be seen that the NOx storage capacity is retained even after the endurance.

On the other hand, according to the substance B-1 of the comparative example, as shown in FIG. 2, the NOx occlusion rate and the removal rate at a substance inlet temperature of around 200 ° C. are high, but 300 to 4 compared with the substance A-1 of the example.
The storage rate and removal rate at 50 ° C are low. Further, in the substance B-1 of the comparative example, it was confirmed that the stored NOx was released at 400 ° C. or higher in the initial stage, but after the aging, the release of NOx almost disappeared. From this fact, the material B-1 of the comparative example was not subjected to N by aging at 700 ° C.
It can be seen that the Ox storage performance is lost and the NOx removal rate from 200 ° C to 400 ° C is mainly due to the reduction of NOx by HC. Thus, since the NOx storage material of the present invention contains silver, an alkaline earth metal element, and a platinum group element in the alumina-based metal oxide carrier, it has excellent heat resistance in an excess oxygen atmosphere and even in the presence of HC. It was found that excellent NOx storage performance is exhibited at 300 ° C or higher.

[0041]

INDUSTRIAL APPLICABILITY According to the present invention, the alumina-based metal oxide carrier contains 5 to 40% by weight of silver in terms of metal, and at least one of alkaline earth metal elements, based on the total weight of the carrier and supported components. A nitrogen oxide storage composition containing 5 to 40 wt% and at least one platinum group element of 0.05 to 8 wt% stores and releases NOx in exhaust gas, thereby making the atmosphere lean to rich. It is possible to efficiently treat the exhaust gas that changes to the exhaust gas and the NOx in the exhaust gas in the lean atmosphere in which the exhaust gas temperature changes from low temperature to high temperature. The nitrogen oxide storage composition of the present invention is excellent in heat resistance and has heat resistance of 700 ° C to 800 ° C. Further, by providing a nitrogen oxide reduction catalyst on the downstream side of the composition of the present invention, it is possible to remove nitrogen oxides in the exhaust gas in a wide range of oxygen content in the exhaust gas and in the exhaust gas temperature range. .

[Brief description of drawings]

FIG. 1 shows the relationship between the NOx removal rate and the honeycomb inlet gas temperature at the time of temperature increase / decrease after initial aging and after aging of the NOx storage substance of Example A-1.

FIG. 2 shows the relationship between the NOx removal rate and the honeycomb inlet gas temperature at the time of temperature increase and temperature decrease of the substance of Comparative Example B-1 at the initial stage and after aging.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location B01D 53/81 53/86 ZAB 53/94 B01D 53/36 ZAB 102 B 102 H

Claims (6)

[Claims] 1. Alumina-based metal oxide carrier containing 5 to 5 silver, respectively, in terms of metal, based on the total weight of the carrier and supported components.
40 wt%, 5-40 wt% of at least one alkaline earth metal element, and at least one platinum group element of 0.
A composition for occluding nitrogen oxides, which is supported by 05 to 8 wt%. 2. The nitrogen oxide storage composition according to claim 1, wherein the total amount of at least one of silver and an alkaline earth metal element is 10 to 50 wt%. 3. The nitrogen oxide storage composition according to claim 1, which is molded into a certain shape or coated on a refractory support substrate. 4. Exhaust gas whose atmosphere changes from lean to rich is brought into contact with the nitrogen oxide storage composition according to claim 1 to remove nitrogen oxide in the exhaust gas. A method of occluding and removing when leaning. 5. The exhaust gas in a lean atmosphere, the exhaust gas temperature of which changes from a low temperature to a high temperature, is brought into contact with the nitrogen oxide storage composition according to claim 1, and the exhaust gas is discharged. A method of occluding and removing nitrogen oxides in gas at low temperatures. 6. A nitrogen oxide reduction catalyst is provided on the downstream side of the exhaust gas to reduce the nitrogen oxides released from the nitrogen oxide storage composition according to any one of claims 1 to 3. A method of removing nitrogen oxides comprising: