JP2910278B2 - Exhaust gas purification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine of an automobile or the like, and more particularly, to a method for purifying nitrogen oxides (NOx) in an oxygen-excess atmosphere.
The present invention relates to an exhaust gas purification method for purifying NOx in exhaust gas in a practical temperature range of up to 500 ° C.

[0002]

2. Description of the Related Art Since exhaust gas discharged from an internal combustion engine of an automobile or the like contains nitrogen oxides and the like, various studies have been made on the purification of nitrogen oxides in exhaust gas in recent years. .

Conventionally, nitrogen oxides have been purified mainly by using a noble metal or metal reducing catalyst in the presence of a reducing gas. In recent years, nitrogen oxides have been purified in the presence of an oxidizing gas. Various catalysts have been studied. Such catalysts, particularly lean NOx catalysts that purify NOx in an excess oxygen atmosphere such as exhaust from a lean burn engine burned on the lean side with a high air-fuel ratio, include Cu / zeolite catalysts,
Pt / Al ₂ O ₃ catalysts and the like are known. However, among these catalysts, the Cu / zeolite catalyst has a high-efficiency NOx purification temperature of 450 to 550 ° C, and Pt / Al ₂ O
_{(3) The} catalyst has a temperature of 350 to 450 ° C, and a catalyst with a wide temperature range (for example, 300 to 500 ° C) that can purify NOx with high efficiency has not been known.

[0004] That is, zeolite, which is also called a molecular sieve, has pores comparable to the size of various molecules and is used as a catalyst in many reactions.
Zeolite contains cations to neutralize the negative charge of one of its constituents, Al ₂ O ₃ , and these cations are easily exchanged with other cations in an aqueous solution, so the ion exchanger It is well known that it is also used.
Taking advantage of such features, the use of zeolite as a catalyst for purifying exhaust gas has been studied in recent years.
-135541 discloses that zeolites include Pt, Pd, Rh, Ir and
Disclosed is an exhaust gas purification catalyst in which a noble metal selected from Ru is carried by ion exchange, and this catalyst can be used even in a lean atmosphere such as exhaust gas in which oxygen is burned in an excessive state with respect to fuel. It states that it exhibits excellent NOx purification performance.

[0005]

The Cu / zeolite catalyst in which zeolite supports Cu as a cation is Cu
Has high adsorption capacity for NOx, so its initial catalytic activity is excellent, but its deterioration after durability is large, and there is a problem that the temperature showing high NOx purification capacity is limited to a narrow range around 450 to 550 ° C . Pt / Al ₂ O _3, which is a typical example of a noble metal-supported catalyst, also has a high NOx purification ability at a temperature of 35 ° C.
It is a relatively narrow temperature range around 0 to 450 ° C.

Accordingly, the present invention is to solve the above-mentioned problems of the prior art and to develop a method for efficiently purifying NOx in exhaust gas in a wide practical temperature range of 300 to 500 ° C. in an oxygen excess atmosphere. With the goal.

[0007]

According to the present invention, there is provided, on an alumina carrier, 0.5 to 2% by weight of platinum based on the weight of the alumina carrier and 5 to 20% by weight of strontium based on the weight of platinum. Of the exhaust gas purifying catalyst carrying the air-fuel ratio (A /
A method is provided for purifying nitrogen oxides (NO _x ) in exhaust gas by disposing F) in an exhaust gas containing 18 or more oxygen.

[0008] The catalyst used in the exhaust gas purification method according to the present invention comprises Pt in γ-Al ₂ O ₃ and Sr as a second component element.
For example, the catalyst is supported by the impregnation method and has a lower initial catalyst activity (temperature range: 400 to 550 ° C.), but its deterioration after durability is small, and therefore a decrease in NOx purification activity is lower than that of the above-mentioned Cu-supported zeolite catalyst. Small and durable
The activity below 450 ° C is much higher than that of the Cu / zeolite catalyst.

The most important feature of the present invention is that γ-
By supporting Sr on Al ₂ O ₃ , the temperature range showing high NOx purification activity is expanded. The present inventors have been preparing Pt-supported oxide catalysts including Pt-supported zeolite catalysts. However, these catalysts have a problem that the temperature range showing high NOx purification activity is narrow, and we are studying how to solve this problem and expand the temperature range showing high NOx purification activity. However, by adding a small amount of Sr as the second component element, the temperature range was successfully expanded.

In the present invention, the oxide used for the carrier is γ
-Al ₂ O ₃ is mainly used, and γ-Al ₂ O ₃ alone may be used.However, in order to secure further durability at a high temperature, La-added γ-Al ₂ O ₃ added with La Use is preferred. The amount of strontium added is preferably about 5 to 20% by weight of the amount of supported Pt, and the amount of Pt supported on the carrier alumina is preferably 0.5 to 2% by weight.

The loading of Pt and Sr on alumina can be carried out, for example, by a usual impregnation method. To explain an example of loading by the impregnation method, an alumina carrier is immersed in a solution containing a supported metal component of Pt and Sr at room temperature for 24 to 48 hours (immersion step), and then, for example, at a temperature of 100 to 110 ° C. It consists of drying for about 10 hours (drying step) and finally firing at a temperature of 400 to 800 ° C. for several hours (firing step). As the platinum compound to be contained in the impregnation liquid, for example, a dinitrodiamine platinum nitrate solution, a chloroplatinic acid solution, or the like can be used. As the strontium compound, strontium nitrate, strontium acetate, or the like can be used.

[0012]

According to the present invention, although the reason is not clear, by adding Sr as a second component element to the Pt / alumina catalyst, as shown in Table 1 and FIG. NOx can be purified at a high purification rate after an endurance test in an oxygen-excess atmosphere.

[0013]

EXAMPLES The following are preparation examples of the catalyst according to the present invention and the comparative example catalyst, and NOx using a model gas in an excess oxygen state.
An evaluation example of the purification activity of the catalyst will be described below, but 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 solution and nitrates of Sr, Ca, K or Ba or sulfates of Mg were used as starting materials. First, a commercially available La-added γ-alumina (MI-386 manufactured by WR GRACE) was immersed in a 0.1 mol% aqueous solution of dinitrodiamine platinum nitrate, and kept at room temperature with stirring for 24 hours. Next, it was filtered and washed, dried at a temperature of 110 ° C. for 10 hours, and calcined at a temperature of 500 ° C. for 3 hours. The catalyst thus obtained is referred to as Catalyst A. Further, catalysts B to F in which Sr, Ca, K, Ba or Mg were further supported on catalyst A were prepared in the same procedure as catalyst A.

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

Activity evaluation conditions For each of the obtained catalysts, the NOx conversion rate after endurance treatment was determined using a model gas having an air-fuel ratio (A / F) of 18 and a catalyst temperature of 300 to
Steady state evaluation was performed every 50 ° C in the range of 500 ° C. The catalyst used was 0.5 g of pellets. The durability treatment was carried out by exposing to a temperature of 600 ° C. for 5 hours in an atmosphere of a model gas (including 10% water vapor) corresponding to an air-fuel ratio (A / F) = 18.

Experimental Results Table 1 and FIG. 1 show the results of the supported amount of each catalyst and the NOx conversion. The Cu-supported catalyst had 2.6% of zeolite (ZSM-5).
It was prepared by supporting Cu by a conventional method.

[0018]

[Table 1]

[0019]

As is clear from the results shown in Table 1 and FIG. 1, even when Ca, Mg, K or Ba is added as a second component element to the Pt-supported alumina catalyst, there are some catalysts whose low-temperature activity is improved. However, at 400 ° C or higher, the NOx conversion rate conversely decreases, whereas for a catalyst in which Sr is added to Pt as a second component element,
Not only is the activity at 300 ° C improved, but the NOx conversion is hardly reduced at high temperatures. That is, by adding Sr to the Pt / alumina catalyst, the temperature range in which the NOx conversion rate is high can be expanded even after durability in an oxygen-excess atmosphere.

[Brief description of the drawings]

1 is a graph showing the NOx conversion at 300 to 500 ° C. of a Pt + Sr / alumina catalyst, a catalyst having Pt, Pt + Ca, Pt + Mg, Pt + K and Pt + Ba supported on alumina, and a Cu / zeolite catalyst according to the present invention.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B01D 53/94 B01J 23/58

Claims (1)

(57) [Claims] 1. An alumina carrier, based on the weight of the alumina carrier.
0.5 to 2% by weight of platinum and 5 to 20 % by weight of platinum
A method for purifying nitrogen oxides in exhaust gas by arranging an exhaust gas purifying catalyst carrying strontium in an amount of 18 % or more in an oxygen-excess gas having an air-fuel ratio (A / F) of 18 or more. A method for purifying exhaust gas.