JP2921130B2 - Exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for purifying nitrogen oxides in exhaust gas discharged from an internal combustion engine of a car or the like or a nitric acid production plant. The present invention relates to an exhaust gas purifying catalyst having excellent durability and high NOx purifying activity in a wide temperature range.

[0002]

2. Description of the Related Art Exhaust gas discharged from internal combustion engines such as automobiles and nitric acid production plants contains nitrogen oxides (NOx). Various studies have been made. Conventionally, purification of nitrogen oxides has mainly involved the use of a noble metal or metal reducing catalyst in the presence of a reducing gas. Various studies have been made. Zeolite has pores comparable to the size of a molecule, also called a molecular sieve, and is used as an adsorbent and as a catalyst in many reactions. It also contains a cation to neutralize the negative charge of Al ₂ O ₃ , and this cation is used as an ion exchanger because it is easily exchanged with another cation in an aqueous solution.
Utilizing these characteristics, zeolite has recently been studied for use as a catalyst for purifying automobile exhaust gas. For example, a zeolite catalyst carrying Cu is known as an exhaust gas purifying catalyst (for example, see Japanese Patent Application Laid-Open No. 60-97047). JP-A-1-135541 discloses an exhaust gas purifying catalyst in which a noble metal selected from Pt, Pd, Rh, Ir or Ru is supported on zeolite by ion exchange. This catalyst is excellent in NOx purification performance even for combustion exhaust gas in a so-called lean atmosphere where oxygen is excessive with respect to fuel.

[0003]

The Cu-supported zeolite, which supports Cu as a cation, has excellent initial catalytic activity because Cu has a high adsorption capacity for NOx. There was a problem that deterioration after the catalyst durability test was large, and the NOx purification rate at low temperatures (300 ° C or lower) was low. The reason why the Cu-supported zeolite catalyst is inferior in durability is that at high temperatures of 600 ° C. or higher, Cu moves in the zeolite and aggregates, losing its catalytic effect. Also Cu
The activity of the supported zeolite catalyst at low temperatures is low because NOx is partially present as NO _{2 at} low temperatures, and the Cu-supported zeolite catalyst cannot reduce NO ₂ to N ₂ . On the other hand, the Pt-supported zeolite catalyst has a high purification ability at around 300 ° C., but has a problem that its activity is reduced at 400 ° C. or higher, and the effective temperature range is narrow.

Accordingly, an object of the present invention is to develop an exhaust gas purifying catalyst which has excellent durability even in a lean combustion exhaust gas atmosphere containing excess oxygen and has a wide temperature range with high NOx purifying activity.

[0005]

According to the present invention, Pt and
In an exhaust gas purification catalyst in which Rh is supported on zeolite, the amount of supported Rh is 1.5 to 100 parts by weight of supported Pt.
An exhaust gas purifying catalyst is provided which is 4.5 parts by weight, preferably 1.8 to 4.2 parts by weight.

The exhaust gas purifying catalyst according to the present invention can be supported on zeolite, for example, by ion-exchange of Pt and Rh with each ammine solution. Compared with the conventional Cu-supported zeolite, the catalyst activity in the initial stage is lower. Although inferior, the deterioration after durability is small, especially 300 ℃ ~ 400 after durability.
The activity at ℃ is higher, and it is extremely useful in practice, especially considering that the exhaust gas temperature during steady operation of the lean burn engine is about 300 ℃.

The most important feature of the present invention is that the ratio of the supported amount of Pt to Rh is in the range of 1.5 to 4.5 parts by weight, preferably 1.8 to 4.2 parts by weight, for 100 parts by weight of Pt.
x The expansion of the temperature range with high purification activity. The present inventors have been pursuing research and development on Pt-supported zeolite catalysts and Rh-supported zeolite catalysts.However, since the temperature range of high NOx purification activity is in a relatively narrow range, there is a problem in practical use. We have studied how to expand it, but succeeded in achieving the above object by supporting Pt and Rh in the above-mentioned ratio.

The zeolite used in the present invention is, as is well known, composed of a tetrahedral network of SiO ₂ and Al ₂ O ₃ , and the individual tetrahedral structures are bonded to each other through the corners by oxygen crosslinking. Exchangeable cations are introduced at the negatively charged exchange points (strong acid sites) of the lattice, with a network having channels and cavities penetrating therethrough. The molar ratio of the zeolite of SiO ₂ / Al ₂ O ₃ for use in the present invention is from 10 to 200 is preferable. If this ratio is less than 10, thermal stability will be poor at high temperatures of 600 ° C or higher, and if this ratio exceeds 200, the amount of Al ₂ O ₃ will decrease and the ion exchange point will decrease, that is, the catalytic activity will decrease. Will come. Since thermal deterioration is presumed to be mainly caused by structural changes around aluminum, it is preferable to use zeolite with a molar ratio of SiO ₂ / Al ₂ O ₃ of 20 or more, particularly when it is desired to ensure durability at high temperatures. Especially
A ZSM-5, Y or mordenite structure having a SiO ₂ / Al ₂ O ₃ molar ratio of 20 to 200 is desirable.

The loading of Pt and Rh on the zeolite can be carried out by ion exchange or impregnation. A method commonly used for both the ion exchange method and the impregnation method can be used. For example, in the ion-exchange method, Na + or H + introduced at the ion-exchange point having a negative charge on the zeolite lattice is used.
Are exchanged for Pt and Rh ions. Loading by ion exchange is performed, for example, by the following steps. That is, Pt and Rh
An ion exchange step of immersing the zeolite in an aqueous solution of each ammine chloride or ammine hydroxide at room temperature to 80 ° C for 24 to 48 hours, and a drying step of drying this at a temperature of 100 to 120 ° C for about 10 hours, And a baking step of holding at 400 ° C. to 700 ° C. for several hours. The pH at the time of ion exchange is preferably 8 to 12.

The Pt / Rh-supported zeolite catalyst according to the present invention can be used as it is in the form of granules, pellets, honeycombs, etc., and can be used as cordierite, alumina, silica-alumina, spodumene. It can also be used by supporting it on an arbitrary porous carrier.

The exhaust gas purifying catalyst according to the present invention can be purified by a usual method. For example, the catalyst is disposed in a reactor, exhaust gas is introduced into the reactor, the catalyst is contacted with the exhaust gas to reduce and purify nitrogen oxides, and then the purified exhaust gas is discharged from the reactor. Can be carried out. The reaction temperature of the catalyst layer during purification is, for example, 200 to 800 ° C, preferably 250 to 550 ° C.
Can be suitably implemented. In addition, in the purification method according to the present invention, the space velocity (SV) when introducing exhaust gas into the catalyst layer is not particularly limited, but is preferably in a range of, for example, 10,000 to 400,000 / hour.

[0012]

According to the present invention, Pt 1 supported on a Pt-supported zeolite catalyst.
By supporting 1.5 to 4.5 parts by weight of Rh with respect to 00 parts by weight, excellent durability and 300 ° C to 5
A catalyst having a high purification activity in the practical range of 00 ° C. has been developed, and the above-mentioned problems of the prior art are solved.

[0013]

EXAMPLES The catalysts according to the present invention were prepared, and the purification activity of these catalysts against NOx was evaluated using a model gas in an excess oxygen state (lean state).

Example 1 Catalyst Preparation One of the zeolites, Na type ZSM-5 (SiO ₂ / Al ₂ O ₃ = 3)
9.4) (Mobile Oil Co., Ltd.) Powdered divalent Pt ammine solution and trivalent Rh ammine solution were used as starting materials. The zeolite was immersed in each of a divalent Pt ammine solution and a trivalent Rh ammine solution and stirred at room temperature for 24 hours. Then, the obtained mixture is filtered, washed with ion-exchanged water to remove unnecessary anions, dried at 100 ° C for 10 hours, and heated in air.
It was baked at 400 ° C. for 2 hours.

The catalyst for purifying the catalyst exhaust gas obtained in the analysis of the carried amount was analyzed by atomic absorption analysis. The results are shown in Table 1. The supported amount in Table 1 is the catalyst
The amount of noble metal (Pt, Rh) carried per 100 parts by weight.

[0016] The catalyst obtained in the evaluation of the catalytic activity, using the air-fuel ratio (A / F) = 18 model gases, measuring the NOx purification ratio after the durability test treatment in the range of the catalyst temperature 200 ° C. ~ 600 ° C. did. The sample size is
At 0.5 g, the catalyst is in the form of a pellet. The durability test of the catalyst was carried out at a temperature of 600 ° C. in a model gas (including 10% water vapor) atmosphere having an air-fuel ratio (A / F) = 18.
Exposure was carried out.

Experimental Results The relationship between the carried amount and the NOx purification rate after the durability test is shown in Table 1 and FIG.
3 are shown.

[0018]

[Table 1]

As is clear from the results shown in Table 1 and FIGS. 1 to 3, the temperature range showing high NOx purification activity in Pt.Rh / zeolite tends to spread to higher temperatures as compared with the Pt / zeolite catalyst. However, when Rh is further added, the catalyst purification activity on the low-temperature side decreases. Therefore, by adding Rh to the Pt-supported zeolite catalyst so that Rh / Pt becomes 1.8 to 4.2% (by weight), the temperature range showing high NOx purification activity could be remarkably expanded.

[0020]

As described above, according to the present invention, Pt
/ Rh is supported on a zeolite catalyst with 1.5 to 4.5 parts by weight of Rh per 100 parts by weight of supported Pt.
It is possible to obtain an exhaust gas purifying catalyst having excellent purifying ability after a durability test treatment at 5 ° C. × 5 hours and exhibiting good purifying activity in a wide temperature range.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between Rh / Pt (%) at 300 ° C. and a NOx purification rate (%).

FIG. 2 is a graph showing a relationship between Rh / Pt (%) at 400 ° C. and a NOx purification rate (%).

FIG. 3 is a graph showing the relationship between Rh / Pt (%) and NOx purification rate (%) at 500 ° C.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B01J 21/00-38/74 B01D 53/86-53/88

Claims (1)

(57) [Claims] 1. An exhaust gas purifying catalyst comprising Pt and Rh supported on a zeolite, wherein the amount of Rh supported is 1.5 to 4.5 parts by weight based on 100 parts by weight of supported Pt. .