JPH09253498A - Exhaust gas purifying catalyst, method for producing the same, and method for using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shift a clarification temp. window for NOx to a high temp. side. SOLUTION: This catalyst for clarification of exhaust gas comprising a carrier comprising of a porous oxide contg. a rare earth element and a catalytic precious metal carried on the carrier is heat-treated with an oxidative atmosphere contg. an acidic gas. It is estimated that the carrier or the catalytic precious metal is changed in properties by this heat treatment and the clarification temp. window for NOx is shifted to the high temp. side.

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 exhaust gas containing excess oxygen discharged from a diesel engine, a lean burn gasoline engine, a boiler or the like, a method for producing the same, and a method for using the same. The present invention relates to an exhaust gas purifying catalyst capable of efficiently purifying NO _x in exhaust gas in a high temperature range, a method for producing the same, and a method for using the same.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by oxidizing CO and HC and reducing NO _x has been used as an exhaust gas purifying catalyst for automobiles. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat resistant base material made of cordierite, and platinum (Pt) and rhodium (R) are formed on the porous carrier layer.
A catalyst carrying a precious metal such as h) is widely known.

On the other hand, in recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem from the viewpoint of protecting the global environment. As a solution to this problem, so-called lean combustion in which oxygen is excessively burned is performed. Combustion (lean burn) is promising. In this lean-burn gasoline engine, the fuel consumption is improved so that the amount of fuel used is reduced, and as a result, the generation of CO ₂ which is combustion exhaust gas can be suppressed.

On the other hand, in the conventional three-way catalyst, when the air-fuel ratio is the stoichiometric air-fuel ratio (stoichiometric), CO, H in the exhaust gas
It purifies by oxidizing and reducing C and NO _x at the same time, and is sufficient for reducing and removing NO _x in exhaust gas in an oxygen excess atmosphere, such as exhaust gas during lean combustion and exhaust gas from a diesel engine. It shows no performance. Therefore, it is desired to develop a catalyst and a purification system that can efficiently purify NO _x even in an oxygen excess atmosphere.

Therefore, as described in, for example, Japanese Unexamined Patent Publication No. Hei 5-103985, a catalyst in which a catalytic precious metal such as platinum is carried on a carrier made of alumina or the like is used, and hydrocarbon is supplied to the catalyst, whereby the hydrocarbon Attempts have been made to purify NO _x by selective reduction of NO _x by. According to this method, it is possible to efficiently reduce and purify NO _x in exhaust gas in an oxygen excess atmosphere discharged from a diesel engine or a lean burn gasoline engine.

[0006]

By the way, according to the method described in the above publication, the temperature range in which NO _x is efficiently purified, for example, the inlet gas temperature at which the NO _x purification rate is 10% or more is about 220 to 320. The temperature is in the range of 0 ° C, and the temperature at which the NO _x purification rate is maximum is around 250 ° C. Therefore,
There is a problem that NO _x cannot be efficiently purified when the exhaust gas temperature becomes high, such as when the engine load is increased due to changes in operating conditions. Moreover, in recent years, the exhaust gas temperature has risen due to the fact that the vehicle often travels at high speed due to improvements in engine performance and maintenance of the highway network. This problem becomes more serious, and NO _{x in} the exhaust gas in the high temperature region can be efficiently removed. Development of a catalyst that can be reduced and purified is required.

The present invention has been made in view of the above circumstances, and has a temperature range in which NO _x can be reduced and purified, and NO.
_An object of the present invention is to provide an exhaust gas purifying catalyst in which the temperature at which the purification rate is maximized is shifted to a high temperature side, a method for producing the same, and a method for using the same.

[0008]

The features of the exhaust gas-purifying catalyst of the present invention for solving the above-mentioned problems include a carrier made of a porous oxide containing a rare earth element and a catalytic noble metal carried on the carrier. The exhaust gas-purifying catalyst is heat-treated in an oxidizing atmosphere containing an acidic gas.

It is preferable that the rare earth element is lanthanum. Further, as described in claim 3, it is desirable that the porous oxide is alumina. Further, the feature of the method for producing an exhaust gas purifying catalyst of the present invention capable of producing the exhaust gas purifying catalyst is that the exhaust gas purifying comprises a carrier made of a porous oxide containing a rare earth element and a catalyst noble metal carried on the carrier. The catalyst is heat-treated in an oxidizing atmosphere containing an acidic gas.

As described in claim 5, the rare earth element is preferably lanthanum. Further, as described in claim 6, the porous oxide is preferably alumina. Further, as described in claim 7, it is desirable that the acidic gas contains at least sulfur dioxide, and the concentration of sulfur dioxide in the acidic gas is 1% by volume as described in claim 8.
It is desirable that this is the case.

A method of using the exhaust gas-purifying catalyst of the present invention is characterized in that the exhaust gas-purifying catalyst according to claim 1 is brought into contact with exhaust gas in a high temperature range.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst and the method for producing the same of the present invention, it is considered that the chemical properties of the carrier or the supported precious metal catalyst are changed by heat treatment in an oxidizing atmosphere containing an acidic gas. The mechanism is unknown,
Temperature at which the temperature range and _the NO _x purification rate becomes maximum which can reduce and purify NO _x is shifted to the high temperature side. Therefore, when the exhaust gas temperature becomes high, NO _x can be efficiently reduced and purified.

The carrier is made of a porous oxide containing a rare earth element, and the material thereof is, for example, at least one metal selected from 3B group, 4A group and 4B group of the periodic table of elements such as alumina, titania, zirconia and silica. Oxide of
Alternatively, zeolite is exemplified. A composite oxide such as silica-alumina can also be used. Of these, alumina having a high specific surface area and excellent activity is preferable. When using alumina, it is desirable that the carrier contains 50% by volume or more of alumina. If the content of alumina is less than 50% by volume, not only NO _{x but also} H
The purification rate of C and CO may also decrease.

This carrier contains a rare earth element as an additive component. La, Ce, P as rare earth elements
r, Nd, Sm, Eu, Gd and Tb are mentioned, and La is particularly preferable. _The NO _x adsorption action of the carrier by including La is estimated to be more improved, NO _x
The purification temperature range will shift to a higher temperature side. The rare earth element may be contained as a single oxide, but it is desirable that the rare earth element constitutes a composite oxide together with the porous oxide. By doing so, the oxidizing activity for SO ₂ in the exhaust gas is reduced, and the formation of sulfate can be suppressed.

The content of the rare earth element is preferably 0.01 to 10% by weight in terms of oxide in the carrier. If the content of the rare earth element is less than 0.01% by weight, the effect of addition will not be exhibited, and if the content exceeds 10% by weight, the effect may be saturated and the high temperature durability of the catalyst may be reduced.
The carrier carries a catalytic noble metal. Platinum (Pt) as well as rhodium (R
h), palladium (Pd), silver (Ag), gold (Au),
Iridium (Ir) etc. are illustrated.

The amount of the catalytic noble metal supported on the carrier can be arbitrarily selected within the range of 0.1 to 10% by weight of the catalytic noble metal. If the supported amount of the catalytic noble metal is less than 0.1% by weight, the NO _x purification performance is deteriorated and it is not practical.
Even if it is carried in an amount of more than wt%, the NO _x purification performance will be saturated and the cost will rise. In addition, P is contained in the catalytic precious metal.
It is desirable that t is contained in an amount of 0.1% by weight or more. P
When t the amount is less than this, NO with _x purification rate is lowered, the temperature range and NO _x that can reduce and purify NO _x
The shift width of the temperature at which the purification rate becomes maximum to the high temperature side becomes small.

Further, in the method for producing an exhaust gas purifying catalyst of the present invention, the exhaust gas purifying catalyst comprising a carrier made of a porous oxide containing a rare earth element and a catalytic noble metal supported on the carrier is an acidic gas. Heat treatment is carried out in an oxidizing atmosphere containing. Although the mechanism is unknown by this, it is presumed that the chemical properties of the carrier or the catalyst noble metal are changed.
_The temperature range where _x can be reduced and purified exceeds 220 ℃ and is about 40
The temperature spreads to around 0 ° C., and the temperature at which the NO _x purification rate becomes maximum shifts to a high temperature side up to around 350 ° C. at maximum.

Examples of the acidic gas include sulfur dioxide gas and chlorine gas, and sulfur dioxide gas is particularly preferable. Sulfur dioxide gas is considered to undergo the catalytic action of the catalytic precious metal due to the presence of oxygen and water to become sulfuric acid, which acts on the carrier and the catalytic precious metal to change the chemical properties,
The temperature at which the temperature range and _the NO _x purification rate becomes maximum which can reduce and purify NO _x can be further shifted to the high temperature side. Therefore, it is desirable that oxygen gas and water vapor coexist with sulfur dioxide gas.

The amount of sulfur dioxide gas in the oxidizing atmosphere is 1
It is desirable that the content be at least volume%. 1 sulfur dioxide gas
If less than the volume%, the temperature range and _the NO _x purification rate can be reduced and purified NO _x is small shift width in the high-temperature side of the temperature at which the maximum. When the content is 1% by volume or more, the shift width to the high temperature side becomes large, and for example, the temperature at which the NO _x purification rate becomes maximum can be shifted to the high temperature side by about 50 ° C. or more. Further, in order to form an oxidizing atmosphere, oxygen may be present in an amount equal to or more than the amount required to oxidize the component to be oxidized such as contained sulfur dioxide gas.

The oxygen gas concentration which is preferably present together with the sulfur dioxide gas is 1 to 20 in the oxidizing atmosphere.
The range of volume% is preferable, and 3 to 10 volume% is particularly preferable. Further, as the water content, a water content of about 3 to 10% by volume is suitable. The action of sulfur dioxide gas and oxygen gas concentration and the steam concentration is less than this is reduced, the temperature range and _the NO _x purification rate can be reduced and purified NO _x shift width is reduced to the high temperature side of the temperature at which the maximum . Further, if the oxygen gas concentration and the water vapor concentration are too high, the activity is remarkably reduced and NO _x cannot be purified.

The heat treatment temperature is preferably in the range of 300 to 800 ° C, particularly preferably 400 to 600 ° C. It is considered that when the heat treatment temperature is lower than 300 ° C., the reaction due to the acidic gas does not occur, and when the heat treatment temperature exceeds 800 ° C., the acidic gas or the acid generated by the oxidation is scattered.
It becomes difficult to shift the temperature range in which O _x can be reduced and purified and the temperature at which the NO _x purification rate is maximum to the high temperature side.

[0022]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. Example 1 Using an alumina carrier containing 0.04% by weight of La ₂ O ₃ , 2 ml of an aqueous nitric acid solution of dinitrodiammineplatinum (concentration: 50 g / L) was added to 10 g of this carrier powder, and further 50 ml of distilled water was added. The mixture was stirred at room temperature for about 5 hours. The obtained suspension was heated at 110 ° C. overnight to be dried, and calcined in the air at 500 ° C. for 3 hours to prepare a Pt-supported powder. The supported amount of Pt is 1 g as metallic Pt with respect to 100 g of the carrier powder.

A quartz tube was filled with 0.5 g of the Pt-supported powder obtained, and 500 to 100 processing gas having the composition shown in Table 1 was used.
While circulating at a flow rate of 0 ml / min, 3 at 500 ° C
A heat treatment of heating for a time was performed.

[0024]

[Table 1] 2 g of the obtained heat-treated Pt-supported powder was loaded into a normal atmospheric fixed bed flow reactor, and a model gas simulating exhaust gas from a diesel engine shown in Table 2 was supplied at 10 L / m.
circulating at an inflow rate of 100 to 50
The NO _x purification rate at each temperature was measured while varying between 0 ° C. The results are shown in FIG.

[0025]

[Table 2] (Example 2) The same Pt-supported powder as in Example 1 was used, and S
Heat treatment was performed in the same manner as in Example 1 using the same processing gas as in Table 1 except that the O ₂ gas concentration was 1% by volume. Then, the NO _x purification rate at each temperature was measured in the same manner as in Example 1, and the results are shown in FIG.

Example 3 The same Pt-supporting powder as in Example 1 was used, and the same processing gas as in Table 1 was used except that the SO ₂ gas concentration was 1.5% by volume. Heat treatment was performed in the same manner as in. Then, the NO _x purification rate at each temperature was measured in the same manner as in Example 1, and the results are shown in FIG.

(Comparative Example 1) Using the same Pt-supported powder as in Example 1 and using the same processing gas as in Table 1 except that SO ₂ gas was not included, heat treatment was performed in the same manner as in Example 1. It was Then, the NO _x purification rate at each temperature was measured in the same manner as in Example 1, and the results are shown in FIG. (Comparative Example 2) P was prepared in the same manner as in Example 2 except that a carrier powder made of only alumina and not containing lanthanum oxide was used.
A t-supported powder was prepared. Then, using the same processing gas as in Table 1 except that the SO ₂ gas concentration is 1% by volume,
Heat treatment was performed in the same manner as in Example 2. Then, the NO _x purification rate at each temperature was measured in the same manner as in Example 2, and the results are shown in FIG.
Shown in

(Evaluation) From FIG. 1, it is understood that in the exhaust gas purifying catalysts of the respective examples, the temperature window for NO _x purification shifts to the high temperature side as the SO ₂ gas concentration in the processing gas increases. . However, the SO ₂ gas concentration is 500
It can be seen that the shift width is small up to 0 ppm, and that the SO ₂ gas concentration is preferably 1% by volume or more.

Further, as shown in FIG. 2, Comparative Example 2 and Example 2
From the comparison of Example 2, the temperature window of NO _x purification shifts to a higher temperature side in Example 2 in which La oxide is contained in the carrier,
It is clear that it is desirable to use an alumina carrier containing La.

[0030]

That is, according to the catalyst for purifying exhaust gas and the method of using the same of the present invention, N in exhaust gas in an oxygen excess atmosphere is
O _x and exhaust gas can be reduced and purified efficiently be hot. Further, according to the method for producing an exhaust gas purifying catalyst of the present invention, the exhaust gas purifying catalyst capable of efficiently reducing and purifying NO _x even when the exhaust gas is at a high temperature can be easily heat-treated in an oxidizing atmosphere containing an acidic gas. And it can be manufactured reliably.

[Brief description of drawings]

FIG. 1 is a graph showing the NO _x purification rate at each temperature.

FIG. 2 is a graph showing the relationship between the incoming gas temperature and the NO _x purification rate with and without La.

Claims (9)

[Claims] 1. An exhaust gas-purifying catalyst comprising a carrier made of a porous oxide containing a rare earth element and a catalytic noble metal supported on the carrier, the catalyst being heat-treated in an oxidizing atmosphere containing an acidic gas. An exhaust gas purifying catalyst characterized by: 2. The exhaust gas-purifying catalyst according to claim 1, wherein the rare earth element is lanthanum. 3. The exhaust gas-purifying catalyst according to claim 1, wherein the porous oxide is alumina. 4. An exhaust gas purifying catalyst comprising a carrier made of a porous oxide containing a rare earth element and a catalytic noble metal supported on the carrier is heat-treated in an oxidizing atmosphere containing an acidic gas. And a method for producing a catalyst for purifying exhaust gas. 5. The method for producing a catalyst for purifying exhaust gas according to claim 4, wherein the rare earth element is lanthanum. 6. The method for producing an exhaust gas purifying catalyst according to claim 4, wherein the porous oxide is alumina. 7. The method for producing a catalyst for purifying exhaust gas according to claim 4, wherein the acid gas contains at least sulfur dioxide. 8. The concentration of sulfur dioxide in the acid gas is 1
The method for producing an exhaust gas purifying catalyst according to claim 7, wherein the content is at least volume%. 9. A method of using an exhaust gas purification catalyst for purifying nitrogen oxides in a high temperature range contained in exhaust gas, comprising a carrier made of a porous oxide containing a rare earth element and a catalyst noble metal supported on the carrier. A method for using an exhaust gas purifying catalyst, which comprises contacting an exhaust gas purifying catalyst, which is heat-treated in an oxidizing atmosphere containing an acid gas and containing, with exhaust gas in a high temperature range.