JP2001046835A - NOx absorption purification material and exhaust gas purification catalyst using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a NOx absorption / purification material which has excellent HC adsorbability, can improve the reaction selectivity between NOx and HC or CO, and can efficiently purify NOx even in an oxygen-excess atmosphere. To provide an exhaust gas purifying catalyst. A NOx containing a composite oxide containing zirconium with tungsten, silicon, titanium, sulfur and the like.
It is an absorption and purification material. The desorption temperature of NH ₃ of the composite oxide by NH ₃ thermal desorption method is 175 ° C. to 225 ° C., and the BET specific surface area is 50 to 200 m ² / g. This is an exhaust gas purifying catalyst in which a noble metal is supported on the above-mentioned NOx absorption / purification material. Further, an alkali metal, an alkaline earth metal, a rare earth element and the like may be added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a NOx absorbing and purifying material and an exhaust gas purifying catalyst using the same, and more particularly, to an exhaust gas containing excess oxygen, that is, carbon monoxide, hydrogen and carbon dioxide contained in the exhaust gas. The present invention relates to a NOx absorption and purification material capable of efficiently reducing and purifying nitrogen oxides in exhaust gas containing oxygen in excess of an amount of oxygen necessary to oxidize a reducing component such as hydrogen, and an exhaust gas purification catalyst using the same.

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas of an internal combustion engine such as an automobile, oxidation of carbon monoxide (CO) and hydrocarbon (HC) and reduction of nitrogen oxide (NOx) are simultaneously performed. Three-way catalysts are widely used. As such a three-way catalyst, for example, a porous substrate layer composed of γ-alumina is formed on a heat-resistant carrier composed of cordierite or the like, and platinum (Pt), rhodium (R) is formed on the porous substrate layer.
Catalysts supporting noble metals such as h) are widely known. Also known is a three-way catalyst in which ceria (cerium oxide) having oxygen storage capacity is used in combination to enhance low-temperature activity.

On the other hand, from the viewpoint of protecting the global environment in recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem. Lean burn is promising. According to this lean burn, fuel usage is reduced to fuel efficiency is improved, it is possible to suppress the generation of CO ₂ is the combustion exhaust gas.

[0004] On the other hand, in the conventional three-way catalyst, the CO, HC in the exhaust gas at the stoichiometric air-fuel ratio (stoichiometric) is used.
NOx and NOx are simultaneously oxidized and reduced to purify them, and do not exhibit sufficient purification performance for reducing and removing NOx under an oxygen-excess atmosphere of exhaust gas during lean burn. Therefore, development of a catalyst and a purification system capable of purifying NOx even in an oxygen-excess atmosphere is desired.

On the other hand, recently, an exhaust gas purifying catalyst in which an alkaline earth metal and Pt are supported on a multi-hard carrier such as alumina (JP-A-5-317652), or a lanthanum and Pt are converted into a porous material such as alumina. An exhaust gas purifying catalyst supported on a porous carrier (JP-A-5-168860) has been proposed. According to these exhaust gas purifying catalysts, NOx is stored on the lean side in an oxide of alkaline earth metal or an oxide of lanthanum (NOx storage material).
Is generated in the transition region or HC on the rich side
Since it is purified by reacting with reducing components such as CO and CO, as a result, excellent NOx purification performance can be obtained even on the lean side.

[0006]

However, in a catalyst simply combining a noble metal and an alkali or alkaline earth element, NOx stored on the lean side is reduced and purified into stoichiometric gas generated in a transient region or HC or CO on the rich side. At the same time, the alkali or alkaline earth element
Adsorption of reducing components such as C and CO to precious metals is hindered,
There has been a problem that NOx stored on the lean side cannot be purified as desired on the stoichiometric or rich side.

[0007] An exhaust gas purifying catalyst in which a noble metal is supported on a zirconia-alumina support and tungsten or the like is added has also been proposed (Japanese Patent Application Laid-Open No. 9-024274). Reduce
There has been a problem that excellent NOx purification performance cannot be maintained after durability.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to improve HC adsorption and improve the reaction selectivity between NOx and HC or CO. An object of the present invention is to provide a NOx absorption / purification material capable of efficiently purifying NOx even in an oxygen-excess atmosphere and an exhaust gas purification catalyst using the same.

[0009]

Means for Solving the Problems The inventors of the present invention have made intensive studies to solve the above-mentioned problems, and as a result, a zirconium-based composite oxide having a predetermined NH ₃ desorption temperature and a predetermined BET specific surface area has been obtained. It also has appropriate HC adsorption, NOx and H
They have found that the reaction selectivity with C and CO can be improved, and have completed the present invention.

That is, the NOx absorbing and purifying material of the present invention contains a composite oxide containing at least one element selected from the group consisting of tungsten, silicon, titanium and sulfur and zirconium. NH ₃ desorption temperature by NH ₃ thermal desorption method is 175 ° C. to 225 ° C., BET
It has a specific surface area of 50 to 200 m ² / g.

In a preferred embodiment of the NOx absorbing and purifying material of the present invention, the composite oxide contains at least one element selected from the group consisting of tungsten, silicon, titanium and sulfur in zirconia in an amount of 10 to 40 mol%. It is characterized by comprising.

Further, the exhaust gas purifying catalyst of the present invention is characterized in that a noble metal is carried on the NOx absorbing / purifying material as described above. In addition, at least one element selected from the group consisting of alkali metals, alkaline earth metals, and rare earth elements can be contained.

[0013]

The details of the fact that the NOx absorbing and purifying material of the present invention has excellent HC adsorbability and can improve the reaction selectivity between NOx and HC or CO as a reducing agent are not necessarily clear, but at present, It is inferred as follows. That is, the zirconium-based composite oxide contained in the NOx absorbing and purifying material of the present invention has a function of absorbing NOx, but has more acid sites than a conventional porous substrate such as alumina.
A larger amount of HC and CO can be adsorbed to the acid sites than in the past. Therefore, the absorbed NOx and HC can be made adjacent to each other in the zirconium composite oxide,
In addition, when purifying the NOx absorption amount, a sufficient amount of HC or the like can be supplied as compared with the conventional method, so that it is considered that the reaction selectivity between NOx and HC or CO can be improved.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the NOx absorbing and purifying material of the present invention will be described in detail. As described above, the NO of the present invention
The x-absorbing purification material is a predetermined NH ₃ by the NH ₃ thermal desorption method.
_{3 It} contains a zirconium (Zr) -based composite oxide having a desorption temperature and a predetermined BET specific surface area.

Here, the surface of the Zr-based composite oxide has an acid point and a base point.
Is adsorbed and activated. On the other hand, NOx such as NO
Due to the feature of adopting an electron configuration having one electron in the antibonding orbital, it is advantageously activated at a base site rather than an acid site. The above-mentioned Zr-based composite oxide can be used together with or instead of alumina conventionally used as a porous substrate in the use of an exhaust gas purifying catalyst, but is specified by the above-mentioned NH ₃ desorption temperature. Thus, it has more acid sites than alumina or the like, the ratio of the acid sites to the base sites is appropriate, and the acid sites and the base sites
Are adjacent to each other in the same material, ie, a composite oxide.

Therefore, in this Zr-based composite oxide, NO
Since x and HC or CO are activated in an adjacent state and an intermediate can be easily formed, the reaction selectivity between NOx and HC or CO can be improved. The reaction selectivity is promoted by adding a noble metal catalyst component such as Pt or an alkali metal, and the exhaust gas purifying catalyst of the present invention described below utilizes this fact. Further, since the Zr-based composite oxide does not adsorb SOx more easily than alumina or the like, it is possible to prevent the formation of sulfate due to the oxidation of SOx, and it is excellent in sulfur poisoning resistance.

[0017] The Zr-based composite oxide, and the acid strength desorption temperature of NH ₃ by NH ₃ TPD method is represented by 175 ° C. to 225 ° C., a BET specific surface area of 50 to 200 m ² / g Have. If the desorption temperature is lower than 175 ° C., the acid strength becomes too weak and the adsorbability of HC and CO is reduced, so that sufficient NOx reduction and purification performance in a stoichiometric to rich atmosphere cannot be obtained. On the other hand, if the desorption temperature exceeds 225 ° C., the acid strength becomes too strong, and the NOx absorption performance decreases, and even if an alkali metal described later is added, the desired NOx absorption performance may not be obtained.

[0018] In the present invention, NH ₃ measurements Atsushi Nobori by NH ₃ TPD method was carried out as follows.
A catalyst is filled in the center of a quartz reaction tube, and 500 ° C. He
After pretreatment in a stream of air for 2 hours, it was cooled,
_{The three} gases were adsorbed until saturated. Thereafter, He gas was passed at 100 ml / min, the temperature was raised at 10 ° C./min, and the desorbed gas was measured for desorption temperature using a thermal conductivity detector for gas chromatography.

If the Zr-based composite oxide has a BET specific surface area of less than 50 m ² / g, the dispersibility of the noble metal carried on the exhaust gas purifying catalyst, which will be described later, deteriorates, and sufficient acid strength cannot be obtained. . On the other hand, if it exceeds 200 m ² / g, shrinkage of the surface area due to deterioration, particularly thermal deterioration, increases, and dispersibility of the noble metal deteriorates.

The above-mentioned Zr-based composite oxide contains Zr and elements related to tungsten (W), silicon (Si), titanium (Ti) or sulfur (S) and any combination thereof. Zirconia (ZrO)
₂ ) contains W, Si, Ti or S and a mixed element thereof at a ratio of 10 to 40 mol%. W
Is less than 10 mol%, there is no great difference from the case of ZrO ₂ alone, and since the acid strength is weak, adsorption of a reducing agent such as HC or CO is suppressed, and NOx is sufficiently reduced in a stoichiometric to rich atmosphere. May not be purified. Also, 4
If it exceeds 0 mol%, the effect of adding W or the like will be saturated, or if added, the NOx absorption reaction of the alkali element or alkaline earth element compound will be suppressed, and the NOx absorption capacity of the obtained exhaust gas purifying catalyst will decrease. May be done.

The method for producing such a Zr-based composite oxide is not particularly limited as long as it can form a composite oxide in which the above-mentioned element such as W and Zr are in a highly dispersed state. After mixing an aqueous solution of a metal salt of Zr, a coprecipitation method using ammonia water, ammonium carbonate or ammonium hydrogen carbonate and a mixture thereof as a precipitant,
Evaporation to dryness method after mixing the metal salt aqueous solution as above,
An alkoxide method can be mentioned. In addition, by manufacturing by such a method, the thermal durability of the obtained Zr-based composite oxide can be improved.

Next, the exhaust gas purifying catalyst of the present invention will be described. The exhaust gas purifying catalyst of the present invention is formed by supporting a noble metal on the above NOx absorbing and purifying material, and preferably further comprises adding an alkali metal, an alkaline earth metal or a rare earth element, and any mixed element thereof. By the addition of such a noble metal or alkali metal, the NOx absorption and purification material, in particular, the acid strength of the Zr-based composite oxide can be more appropriately adjusted, and when purifying exhaust gas containing HC, O ₂ and NOx, Reaction between NOx and HC and O ₂ and HC
Of exhaust gas purification, which can significantly improve the selectivity of the reaction between NOx and HC, effectively reduce and purify absorbed NOx in a stoichiometric to rich atmosphere, and exhibit a high NOx purification rate even in an oxygen-excess atmosphere. A catalyst is obtained.

Here, as the noble metal, Pt, Rh or Pd and an arbitrary mixed element thereof can be used, but Pt is preferable, and it is particularly preferable to use Pt supported on the Zr-based composite oxide. . By supporting Pt, the Zr-based composite oxide, which is an acidic material, can be used in combination with Pt, so that not only the acid strength of Pt is increased and HC is easily adsorbed, but also Pt NO The oxidation reaction from NO to NO ₂ is also promoted, and the NOx absorption can be further improved.

Examples of the alkali metal include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), sodium (Na), potassium (K), cesium (Cs), and any of these. Elements related to the combination can be used, and the addition of these elements and their oxides can further improve the NOx absorbing ability of the obtained exhaust gas purifying catalyst.

Although the exhaust gas purifying catalyst of the present invention can be used without a carrier, it is preferable to use a monolithic carrier composed of a heat-resistant material coated with a catalyst component, for example. By coating a catalyst component on such a monolithic carrier, the contact area between the catalyst and the exhaust gas can be increased, the pressure loss can be suppressed, and vibration and friction can be increased. This is advantageous when used as

As the monolithic carrier, cordierite-based honeycomb materials such as ceramics are generally used in many cases, but a honeycomb-shaped carrier made of a metal material such as ferritic stainless steel can also be used.
Further, the catalyst component itself can be formed into a honeycomb shape.

In the catalyst of the present invention, other additives can be added. For example, cerium (Ce)
Rare earth compounds such as La and La are added to improve the heat resistance of the carrier, or ceria (Ce
O ₂ ) can be added to enhance the three-way catalytic function.

The exhaust gas purifying catalyst of the present invention comprises the above-mentioned components, that is, a zirconia-based composite oxide, a noble metal and, if necessary, an alkali metal and other additives, in a conventional manner, typically by an impregnation method. It can be manufactured by coating on a monolithic structure carrier, drying and firing.

[0029]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

(Example 1) Zirconium hydroxide and tongue
After dissolving ammonium stenoate in water and stirring for 1 hour,
Dry at 150 ° C, and bake in air at 700 ° C for 3 hours
And WO₃/ ZrO ₂A composite oxide (powder I) was obtained. This
W in the powder I was 20 mol%. The resulting powder
Powder I is impregnated with an aqueous solution of dinitrodiamine Pt,
After drying at 400 ° C, it is fired in air at 400 ° C for 1 hour to
Holding WO₃/ ZrO₂A composite oxide (powder II) was obtained. This
The powder II had a Pt concentration of 8% by weight.

On the other hand, an activated alumina powder was impregnated with a Pd nitrate solution, dried at 150 ° C., and calcined at 400 ° C. for 1 hour in air to obtain a Pd-supported alumina powder (powder III).
The Pd concentration of this powder was 8% by weight. Also, nitric acid R
Activated alumina powder was impregnated with the aqueous solution, dried at 150 ° C., and calcined at 400 ° C. for 1 hour in the air to obtain Rh-supported alumina powder (powder IV). The Rh concentration of this powder was 2% by weight.

50 g of powder III, 200 g of alumina and 250 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. The average particle size of the slurry at this time was 3.
It was 2 μm. This slurry solution was attached to a cordierite type monolith carrier (1.3 L, 400 cells), excess slurry in the cells was removed by air flow, dried at 130 ° C., baked at 400 ° C. for 1 hour, and coated. Layer weight 125g
/ L of catalyst (A) was obtained.

Then, 100 g of powder II, 50 g of powder III, 50 g of powder IV, 50 g of alumina and 2 g of powder
50 g of water was put into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. At this time, the average particle size of the slurry was 3.2 μm.
Met. This slurry liquid was attached to the catalyst (A), excess slurry in the cell was removed by an air stream, dried at 130 ° C., and baked at 400 ° C. for 1 hour to obtain a total coat layer weight of 25.
0 g / L of catalyst (B) was obtained. Thereafter, the catalyst (B) was impregnated with an aqueous solution of Ba acetate in an amount of 35 g per 1 L of the catalyst in terms of oxide to obtain an exhaust gas purifying catalyst of this example.

Example 2 The procedure of Example 1 was repeated, except that W was changed to 10 mol% in the preparation of powder I, to obtain an exhaust gas purifying catalyst of this example.

Example 3 The same operation as in Example 1 was repeated except that W was changed to 40 mol% in the preparation of powder I, to obtain an exhaust gas purifying catalyst of this example.

Example 4 In preparing powder I, zirconium butoxide and tetraethyl orthosilicate were mixed with Si
Was adjusted to 20 mol%, and the same operation as in Example 1 was repeated, except that the SiO ₂ / ZrO ₂ powder was prepared by the alkoxide method, to obtain an exhaust gas purifying catalyst of this example.

Example 5 At the time of preparing powder I, zirconium hydroxide and sulfuric acid were mixed so that S was 20 mol%, filtered, dried and calcined to obtain SO ₄ ^2−.
The same operation as in Example 1 was repeated except that / ZrO ₂ powder was prepared, to obtain an exhaust gas purifying catalyst of this example.

Example 6 In preparing powder I, titanium tetrachloride (TiCl ₄ ) and zirconium oxide chloride (ZrC) were used.
l ₂ O · 8H ₂ O) was gradually added dropwise to water while stirring with ice cooling. With good stirring, 25% aqueous ammonia was gradually added dropwise until the pH reached about 7, and then left overnight to mature. The resulting gel is filtered and washed with water for 1 hour.
It was dried at 50 ° C., washed with water, and fired at 600 ° C. for 3 hours. T in the obtained TiO ₂ / ZrO ₂ composite oxide support
i was 20 mol%. Thus, TiO ₂
The same operation as in Example 1 was repeated except that / ZrO ₂ powder was prepared, to obtain an exhaust gas purifying catalyst of this example.

(Comparative Example 1) In preparing powder I, W and Z
In r and the atomic ratio 2: 8 such WO ₃ and ZrO ₂
The same operation as in Example 1 was repeated except that the mixed powder was used to obtain an exhaust gas purifying catalyst of this example.

Comparative Example 2 In preparing powder I, W was 6
The same operation as in Example 1 was repeated except that the amount was set to 0 mol%, to obtain an exhaust gas purifying catalyst of this example.

Comparative Example 3 In preparing powder I, BET
The same operation as in Example 1 was repeated, except that the powder had a specific surface area of 26 m ² / g, to obtain an exhaust gas purifying catalyst of this example.

(Comparative Example 4) In preparing powder I, W was 5
The same operation as in Example 1 was repeated except that the content was mol%, to obtain an exhaust gas purifying catalyst of this example.

(Comparative Example 5) In preparing powder I, BET
The same operation as in Example 1 was repeated, except that the powder had a specific surface area of 276 m ² / g, to obtain an exhaust gas purifying catalyst.

Table 1 shows the composition of the noble metal contained, the element added with the Zr composite oxide, the NH ₃ desorption temperature, and the BET specific surface area of the exhaust gas purifying catalyst obtained in each of the Examples and Comparative Examples.

[0045]

[Table 1]

(Performance evaluation) The following performance evaluation was carried out on the exhaust gas purifying catalysts of the above examples, and the obtained results were shown in Table 2.
It was shown to.

[Endurance Method] A catalyst is mounted on the exhaust system of an engine with a displacement of 4400 cc, and the catalyst inlet temperature at the preceding stage is set
It was brought to 0 ° C. and operated for 50 hours. [Evaluation method] Displacement 20
A 1.3L catalyst was installed in the exhaust system of a 00cc engine,
A / F = 14.6 for 60 seconds, A / F = 22 for 20 seconds, A
The operation was repeated in the order of / F = 50 for 20 seconds. The inlet temperature of the pre-stage catalyst was set to 350 ° C. The initial and post-durable total conversion rates of HC, CO and NOx in the exhaust gas of one cycle of this switching operation were determined.

[0048]

[Table 2]

From Table 2, it can be seen that the exhaust gas purifying catalysts obtained in Examples 1 to 6 included in the scope of the present invention have a HC and NOx conversion rate of about 10% after the initial durability. On the other hand, in the exhaust gas purifying catalysts obtained in Comparative Examples 1 to 5, it can be seen that the conversion after the durability is reduced by about 30%. Also, it can be seen that the CO conversion rate in the comparative example is significantly lower than that in the example.

[0050]

As described above, according to the present invention, since the zirconium-based composite oxide having a predetermined desorption temperature of NH ₃ and a predetermined BET specific surface area is used, the HC adsorption is improved. It is possible to provide a NOx absorption / purification material capable of improving the reaction selectivity between NOx and HC or CO and purifying NOx efficiently even in an oxygen-excess atmosphere, and an exhaust gas purification catalyst using the same.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/10 B01J 23/56 301A F-term (Reference) 3G091 AA02 AA12 AB03 AB06 AB09 AB10 BA01 BA14 BA15 BA19 BA39 CB02 EA30 FB10 FB11 FB12 GA06 GB01W GB01X GB01Y GB02W GB02Y GB03W GB03Y GB04W GB04Y GB05W GB10W GB10X GB10Y GB14W HA18 4D002 AA12 BA04 BA06 DA01 DA04 DA11 DA15 DA21 DA25 DA46 DA70 EA02 GA01 GA02 GB02B03B08 GB08 ABBA08 GB06 GB08 ABA BC08A BC38A BC50A BC50B BC51B BC60A BC60B BC72B BC75B BD05A BD05B BD08A CA13 EC02X EC03X FC08

Claims (4)

[Claims] 1. A composite oxide containing at least one element selected from the group consisting of tungsten, silicon, titanium and sulfur and zirconium, wherein the composite oxide is formed by NH ₃ thermal desorption. _{3 has} a desorption temperature of 175 ° C to 225 ° C and a BET specific surface area of 50 to 2
A NOx absorption / purification material characterized by being 00 m ² / g. 2. The composite oxide according to claim 1, wherein the zirconia contains 10 to 40 mol% of at least one element selected from the group consisting of tungsten, silicon, titanium and sulfur. The NOx absorbing and purifying material according to the above. 3. An exhaust gas purifying catalyst comprising the NOx absorbing and purifying material according to claim 1 and a noble metal supported thereon. 4. The exhaust gas purifying catalyst according to claim 3, further comprising at least one element selected from the group consisting of an alkali metal, an alkaline earth metal and a rare earth element.