JPH04284824A - Method for purifying exhaust gas - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention deals with nitrogen oxides (NOx) in exhaust gas emitted from internal combustion engines of automobiles, etc.
More specifically, the present invention relates to an exhaust gas purification method for purifying NOx in exhaust gas in an oxygen-rich atmosphere at a practical temperature range of 300 to 500°C.

0002

[Prior Art] Exhaust gas emitted from internal combustion engines such as automobiles contains nitrogen oxides, etc., and in recent years, various studies have been conducted on purifying nitrogen oxides from exhaust gas. .

[0003] Conventionally, nitrogen oxides have been mainly purified using noble metals or metal reducing catalysts in the presence of reducing gases, but in recent years, nitrogen oxides have been purified in the presence of oxidizing gases. Various researches have been conducted on catalysts for this purpose. Such catalysts, especially in an excess oxygen atmosphere such as the exhaust from a lean-burn engine burning on the lean side with a high air/fuel ratio,
Cu/zeolite catalysts, Pt/Al2O3 catalysts, etc. are known as lean NOx catalysts that purify x. However, among these catalysts, the Cu/zeolite catalyst is highly efficient and the temperature at which it can purify NOx is 450~450℃.
550℃, and the Pt/Al2O3 catalyst is
It has a wide temperature range of 350 to 450 degrees Celsius and can purify NOx with high efficiency (for example, 300 to 500 degrees Celsius).
The catalyst was unknown.

[0004] Zeolites, also known as molecular sieves, have pores comparable in size to various molecules, and are used as catalysts in many reactions.
Zeolite contains cations to neutralize the negative charge of Al2O3, one of its constituent components, and this cation is easily exchanged with other cations in an aqueous solution, so it can also be used as an ion exchanger. It is well known that this is the case. Taking advantage of these characteristics, zeolite has recently been considered for use in exhaust gas purification catalysts, for example,
-135541 publication describes that zeolite contains Pt, Pd,
An exhaust gas purifying catalyst is disclosed in which a noble metal selected from Rh, Ir, and Ru is supported by ion exchange,
This catalyst exhibits excellent N even in lean atmospheres such as exhaust gases that are burned with excess oxygen relative to fuel.
It is stated that it exhibits Ox purification performance.

[0005]

[Problems to be Solved by the Invention] The above-mentioned Cu/zeolite catalyst in which Cu is supported as a cation on zeolite has excellent initial catalytic activity because Cu has a high adsorption capacity for NOx. Significant deterioration;
The temperature that shows high NOx purification ability is 450 to 550℃
The problem is that it is limited to a narrow area nearby. Furthermore, Pt/Al2O3, which is a typical example of a noble metal supported catalyst, exhibits a similarly high NOx purification ability at a temperature of 350 to 450°C.
This is a relatively narrow temperature range in the vicinity.

[0006] Therefore, the present invention solves the problems of the prior art described above, and in an oxygen-rich atmosphere,
The purpose of this project is to develop a method to efficiently purify NOx in exhaust gas over a wide practical temperature range of ℃.

[0007]

[Means for Solving the Problems] According to the present invention, an exhaust gas purifying catalyst comprising platinum and strontium supported on an alumina carrier is disposed in oxygen-excess exhaust gas to eliminate NOx in the exhaust gas. A method of purification is provided.

The catalyst used in the exhaust gas purification method according to the present invention is one in which γ-Al2O3 is supported with Pt and Sr as a second component element, for example, by an impregnation method.
Although the initial catalytic activity is inferior to that of u-supported zeolite catalysts (temperature range 400 to 550°C),
The deterioration after durability is small, so the decrease in NOx purification activity is small, and the activity below 450℃ after durability is C
The activity far exceeds that of u/zeolite catalyst.

The greatest feature of the present invention is that γ supporting Pt
-High NO by supporting Sr on Al2O3
x The temperature range in which purification activity is exhibited is expanded. The present inventors have so far prepared Pt-supported oxide catalysts including Pt-supported zeolite catalysts. However, these catalysts have the problem that the temperature range in which they exhibit high NOx purification activity is narrow, and we are currently investigating how to solve this problem and expand the temperature range in which they exhibit high NOx purification activity. However, S as a second component element
By adding a small amount of r, we succeeded in expanding the temperature range.

[0010] The oxide used for the carrier in the present invention is γ
-Al2O3 is the main component, and γ-Al2O
3 may be used alone, but in order to ensure further durability at high temperatures, it is preferable to use La-added γ-Al2O3. In addition, the amount of strontium added is
The amount of Pt supported on the alumina carrier is preferably 0.5 to 2% by weight.

[0011] Pt and Sr can be supported on alumina by, for example, a conventional impregnation method. To describe an example of supporting by an impregnation method, an alumina support is immersed in a solution containing supported metal components of Pt and Sr at room temperature for 24 to 48 hours (immersion step), and then the alumina support is then heated to a temperature of, for example, 100 to 1.
Dry at 10℃ for about 10 hours (drying process), and finally
It consists of holding and firing at 400 to 800°C for several hours (firing step). As the platinum compound to be contained in the liquid to be impregnated, for example, dinitrodiamine platinum nitrate solution, chloroplatinic acid solution, etc. can be used, and as the strontium compound, strontium nitrate, strontium acetate, etc. can be used.

0012

[Operation] According to the present invention, although the reason is unknown, Pt/
By adding Sr as a second component element to the alumina catalyst, as shown in Table 1 and FIG.
It is possible to purify NOx with a high purification rate after a durability test in an oxygen-rich atmosphere over a wide temperature range of ℃.

[0013]

[Example] Below, preparation examples of catalysts according to the present invention and comparative catalysts, and N
An example of evaluation of the purification activity of the catalyst against Ox will be explained.
It goes without saying that the technical scope of the present invention is not limited to the following examples.

Preparation of the Catalyst Commercially available γ-alumina, dinitrodiamine Pt nitrate solutions and nitrates of Sr, Ca, K or Ba or sulfates of Mg were used as starting materials. First, commercially available La-added γ
- Alumina (MI-386 manufactured by W.R. GRACE)
was immersed in a 0.1 mol% dinitrodiamine platinum nitrate aqueous solution and kept under stirring at room temperature for 24 hours. Next, it was filtered and washed, dried at a temperature of 110°C for 10 hours, and further baked at a temperature of 500°C for 3 hours. The catalyst thus obtained is referred to as catalyst A. Catalysts B to F, in which Sr, Ca, K, Ba, or Mg was further supported on catalyst A, were prepared in the same manner as catalyst A.

Supported Amount Analysis The obtained exhaust gas purifying catalyst was analyzed by atomic absorption spectrometry. The results are shown in Table 1. The indicated value is the amount of supported metal relative to 100 parts by weight of catalyst.

Conditions for evaluating activity The NOx conversion rate of each of the obtained catalysts after durability treatment was evaluated at regular intervals of 50°C within the catalyst temperature range of 300 to 500°C using a model gas with an air-fuel ratio (A/F) of 18. I did it. The catalyst used was 0.5 g of pellets. The durability treatment was performed at a temperature of 6 in a model gas atmosphere (containing 10% water vapor) equivalent to an air-fuel ratio (A/F) of 18.
This was done by exposing to 00°C for 5 hours.

[0017] Experimental results The supported amount of each catalyst and the NOx conversion rate are shown in Table 1 and Figure 1.
Shown below. Note that the Cu-supported catalyst is zeolite (ZSM-5)
It was prepared by supporting 2.6% Cu on it by a conventional method.

[0018]

[Table 1]

[0019]

[Effects of the Invention] As is clear from the results in Table 1 and Figure 1, low-temperature activity can be improved in some Pt-supported alumina catalysts when Ca, Mg, K, or Ba is added as a second component element. However, at temperatures above 400°C, the NOx conversion rate decreases; however, when Sr is added to Pt as a second component element,
The catalyst containing NOx not only has improved activity at 300°C, but also shows almost no decrease in NOx conversion even at high temperatures. That is, by adding Sr to the Pt/alumina catalyst, NO
x The temperature range where the conversion rate is high can be expanded.

[Brief explanation of the drawing]

FIG. 1: Pt+Sr/alumina catalyst according to the invention and alumina containing Pt, Pt+Ca, Pt+Mg, Pt+K
It is a graph diagram showing the NOx conversion rate at 300 to 500°C of a catalyst supporting Pt+Ba and a Cu/zeolite catalyst.

Claims (1)

[Claims] Claim 1: An exhaust gas purifying catalyst comprising platinum and strontium supported on an alumina carrier is disposed in oxygen-excess exhaust gas to purify nitrogen oxides in the exhaust gas. Exhaust gas purification method.