JPH09253491A - Catalyst for clarification of exhaust gas and its preparation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for clarification of exhaust gas for efficiently clarifying HC, CO and NOx and with low oxidation activity of SO2 . SOLUTION: A catalyst consisting of a porous oxide carrier incorporated with at least a basic oxide selected from alkali metals, alkaline earth metals and rare earth elements and a catalytic precious metal carried on the carrier is treated with an acid. It is estimated that the carrier or the catalytic precious metal is changed in properties with the acid treatment and the oxidation activity for SO2 is decreased without decreasing the oxidation activity for HC and CO.

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 discharged from a diesel engine, a lean burn gasoline engine, a boiler, etc. TECHNICAL FIELD The present invention relates to an exhaust gas purifying catalyst that can efficiently purify components, suppress the oxidation of sulfur dioxide in exhaust gas, and suppress the emission of sulfate, and a method for producing 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.

[0005]

Incidentally, Japanese Unexamined Patent Publication No.
When a conventional three-way catalyst is used in the exhaust system of a diesel engine or a lean-burn gasoline engine, as disclosed in Japanese Patent No. 36543, SO ₂ resulting from the sulfur content in the fuel is oxidized and the water content in the exhaust gas is increased. It reacts with and is discharged as sulfuric acid mist and sulfuric acid compound (sulfate). Since these sulfuric acid mists and sulfates are detected as particulates, there is a problem that the particulate component in the exhaust gas is increased by the mounting of the catalyst.

Further, since diesel fuel, which is a fuel for diesel engines, contains a large amount of sulfur, the exhaust gas from a diesel engine contains about 10 times more SO ₂ than the exhaust gas from a gasoline engine. It is normal. Therefore, when the three-way catalyst is used in the exhaust system of a diesel engine, there is a problem that the particulate component in the exhaust gas extremely increases.

The present invention has been made in view of the above circumstances, and an object thereof is to efficiently purify HC, CO and NO _x and to provide an exhaust gas purifying catalyst having a low SO ₂ oxidation activity. .

[0008]

The characteristics of the catalyst for purifying exhaust gas of the present invention which solves the above-mentioned problems are characterized by a porosity containing at least one basic element selected from alkali metals, alkaline earth metals and rare earth elements. A catalyst containing a carrier made of a porous oxide and a catalytic noble metal supported on the carrier is treated with an acid.

Further, the characteristic feature of the production method of the present invention for producing the above exhaust gas-purifying catalyst is that it comprises a porous oxide containing at least one basic element selected from the group consisting of alkali metals, alkaline earth metals and rare earth elements. Acid treatment is performed on the catalyst containing the carrier and the catalytic noble metal supported on the carrier. As described in claim 3, it is preferable that the acid treatment is performed by bringing the catalyst into contact with the sulfur dioxide gas and heating it in an oxidizing atmosphere containing the sulfur dioxide gas. In this case, as described in claim 4, the concentration of the sulfur dioxide gas is preferably 100 ppm to 10% in volume fraction.

It is also preferable that the acid treatment is carried out by bringing the catalyst into contact with a sulfuric acid solution. In this case, as described in claim 6, the concentration of the sulfuric acid solution is preferably 1% by weight to 50% by weight.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The carrier according to the present invention is composed of a porous oxide to which at least one basic element selected from alkali metals, alkaline earth metals and rare earth elements is added. Alkali metals include Li, Na, K,
Examples of Rb and Cs, examples of alkaline earth metals include Mg, Ca, Sr, and Ba, and examples of rare earth elements include La, Ce, Pr, Nd, Sm, Eu, and Gd.
And Tb are exemplified. The basic element is selected from at least one of these, and is added to the carrier as an oxide, for example.

Examples of the porous oxide include alumina, silica, zirconia, titania, silica-alumina, zeolite and the like. The preferred combination of the basic element and the porous oxide constituting the carrier according to the present invention includes alkaline earth metals and rare earth elements such as La and C.
Particularly preferred is a combination of e, Pr, Nd and the like with a porous oxide such as alumina or silica, which easily obtains a high specific surface area. When an alkali metal such as K or Na is combined, the catalytic activity may decrease due to the interaction with the catalytic noble metal such as Pt.

The component ratio of the basic element and the porous oxide constituting the carrier according to the present invention is preferably about 0.01 to 50% as a weight fraction in terms of basic oxide. If the weight fraction in terms of basic oxide is less than 0.01%, the SO ₂ oxidation activity becomes high and sulfate is easily generated,
If the weight fraction in terms of basic oxide is higher than 50%, the catalytic activity is lowered, which is not preferable.

To add the basic element to the porous oxide, the sol-gel method, the coprecipitation method, or the porous oxide is impregnated with an aqueous solution of a salt of a metal selected from alkali metals, alkaline earth metals and rare earth elements. Examples include a method of supporting and firing, a method of mixing powders and firing. Examples of the catalytic noble metal supported on the carrier include Pt, Pd, and R.
h and Ir are exemplified, and among them, HC, CO and NO
Particularly preferred is Pt, which has a high activity of purifying x. The amount of the catalytic precious metal supported is 0.1 to 1 for the catalytic precious metal.
It can be arbitrarily selected within the range of 0% by weight. If the supported amount of the catalytic noble metal is less than 0.1% by weight, the purification performance of HC, CO, and NOx deteriorates, which is not practical. Even if the supported amount of more than 10% by weight saturates the purification performance and the cost rises. Invite.

The acid treatment referred to in the production method of the present invention includes:
A method of heat-treating the catalyst in an oxidizing atmosphere exhibiting acidity, a method of heat-treating the catalyst in contact with an aqueous solution of an acid such as sulfuric acid, or the like is used. When a sulfuric acid aqueous solution is used, the sulfuric acid concentration is preferably in the range of 1% by weight to 50% by weight. If it is less than 1% by weight, acid treatment becomes difficult, and if it exceeds 50% by weight, the catalytic activity is lowered.

In the former case, the oxidizing atmosphere is S
Examples thereof include a gas mixed with O ₂ , O ₂ and H ₂ , a gas mixed with HCl and H ₂ O, and a gas mixed with Cl ₂ , O ₂ and H ₂ O. When SO ₂ gas is used, the concentration of SO ₂ gas in the atmosphere is 10 in terms of volume fraction.
0 ppm to 10% is suitable, and 1000 ppm to
About 2% is particularly preferable. SO ₂ gas concentration is 100pp
If it is lower than m, acid treatment becomes difficult and it becomes difficult to prevent the formation of sulfate during use. Further, if the SO ₂ gas concentration is higher than 10%, the catalytic activity may be significantly reduced.

The oxygen gas concentration coexisting with the SO ₂ gas is preferably 1% or more in volume fraction in an oxidizing atmosphere, and particularly preferably 10% or more. When the oxygen gas concentration is lower than 1%, the acid treatment is difficult because the SO ₂ gas is less likely to be oxidized, and it is difficult to prevent the formation of sulfate during use. Further, the water content of water vapor is preferably 3% or more in volume fraction in an oxidizing atmosphere. If the water content is less than 3%, it becomes difficult to sulphate SO ₂ gas and thus acid treatment becomes difficult, and it becomes difficult to prevent the formation of sulfate during use.

Further, as the heat treatment temperature during the acid treatment,
300 to 800 ° C is suitable, and 400 to 600 ° C is particularly preferable. When the heat treatment temperature is lower than 300 ° C., the SO ₂ gas is less likely to be oxidized, so that the acid treatment becomes difficult and it becomes difficult to prevent the formation of sulfate during use.
Also, when the temperature is higher than 800 ° C, SO ₂ gas or sulfur oxides generated by the oxidation thereof scatters without being accumulated in the catalyst, which makes acid treatment similarly difficult and prevents the formation of sulfate during use. Will be difficult.

That is, in the exhaust gas purifying catalyst of the present invention,
Since the carrier is formed by the porous oxide to which the basic element is added, the basic component can control the acidity / basicity of the carrier, and the reactivity and absorbability to SO ₂ gas or sulfuric acid used for acid treatment. Is controlled. Therefore, it is presumed that the acid treatment of such a catalyst changes the properties of the carrier and the chemical properties of the supported catalytic noble metal, and as a result, HC, CO and NO in the exhaust gas are changed.
_x can be efficiently purified and SO ₂ oxidation activity can be reduced, and the production of sulfate can be suppressed.

[0020]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. (Example 1) First, aluminum nitrate and lanthanum nitrate were mixed so that 99.96 parts by weight of alumina and 0.04 parts by weight of lanthanum oxide (La ₂ O ₃ ) were mixed and precipitated by a coprecipitation method. It was Next, the obtained precipitate was calcined in the air at 900 ° C. for 5 hours to prepare an oxide carrier powder.

Next, 2 ml of a nitric acid aqueous solution of dinitrodiammineplatinum (concentration: 50 g / L) was added to 10 g of the carrier powder, 50 ml of distilled water was further added, and 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 amount of Pt supported is 100 g of carrier powder
On the other hand, the amount of metal Pt is 1 g.

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

[0023]

[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
SO ₂ in the exhaust gas at each temperature was changed between 0 ℃
The conversion of sulphate into sulfate was measured. The results are shown in FIG. Also, the conversion rate of HC to H ₂ O and CO ₂ was measured,
A comparison with the conversion of SO ₂ is shown in FIG.

[0024]

[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 1000 ppm. Then, in the same manner as in Example 1, the conversion rate of SO ₂ in the exhaust gas at each temperature was measured. The results are shown in FIG. The conversion of HC also measured, shows a comparison of the conversion of SO ₂ in FIG.

(Example 3) Heat treatment was carried out in the same manner as in Example 1 except that the same Pt-supported 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 5000 ppm. I went. Then, in the same manner as in Example 1, the conversion rate of SO ₂ in the exhaust gas at each temperature was measured. The results are shown in FIG. The conversion of HC also measured, shows a comparison of the conversion of SO ₂ in FIG.

Example 4 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% by volume. Was heat treated. Then, in the same manner as in Example 1, the conversion rate of SO ₂ in the exhaust gas at each temperature was measured. The results are shown in FIG. The conversion of HC also measured, shows a comparison of the conversion of SO ₂ 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, in the same manner as in Example 1, the conversion rate of SO ₂ in the exhaust gas at each temperature was measured. The results are shown in FIG. Also H
C conversion rate was measured, showing a comparison of the SO ₂ conversion in FIG.

Comparative Example 2 A Pt-supported powder was prepared in the same manner as in Example 1 except that alumina powder containing no basic element was used as the carrier powder. Then, as in Example 1, heat treatment was performed in an oxidizing atmosphere containing SO ₂ . The concentration of SO ₂ is 100 ppm, 1000 pp
m and 1%. Then, in the same manner as in Example 1, the conversion rate of SO ₂ in the exhaust gas at each temperature was measured, and the results are shown in FIG.

Example 5 First, 71 parts by weight of silica,
Magnesia (MgO) is adjusted to 29 parts by weight,
A mixture of laethoxysilane and magnesium acetate
The gel obtained by the gel method is heated in the air at 900 ° C. for 5 hours.
The carrier powder was prepared by firing for a period of time. With this carrier powder
A Pt-supported powder was prepared in the same manner as in Example 1, and SO_TwoMoth
For the same treatment as in Table 1 except that the concentration is 1% by volume
Heat treatment was performed in the same manner as in Example 1 using gas. Soshi
As in Example 1, SO in exhaust gas at each temperature_Twoof
The conversion was measured. FIG. 4 shows the results. In addition, conversion of HC
The rate is also measured, and SO _TwoThe comparison with the conversion rate is shown in FIG.

(Comparative Example 3) The same heat treatment as in Example 1 was performed using the same Pt-supporting powder as in Example 5 and the same processing gas as in Table 1 except that SO ₂ gas was not included. It was Then, in the same manner as in Example 1, the conversion rate of SO ₂ in the exhaust gas at each temperature was measured. FIG. 4 shows the results. Also H
The conversion rate of C was also measured, and the comparison with the conversion rate of SO ₂ is shown in FIG.

As can be seen from the (evaluation) diagram, the exhaust gas purifying catalysts of the examples are prevented from oxidizing SO ₂ at each temperature as compared with the comparative examples.
Further, it is clear that the reaction selectivity of HC with respect to SO ₂ is significantly improved. Further, from comparison of each example, as the SO ₂ gas concentration in the processing gas becomes higher, the SO ₂
It can also be seen that the conversion rate of is decreased and the SO ₂ gas concentration is particularly preferably 1% by volume or more.

From the comparison between Example 4 and Comparative Example 2,
It can also be seen that the acid treatment is effective by using the carrier to which the basic element is added. Further, as shown in FIG. 6 and FIG. 7, it was found that the catalysts of this example both had the same HC and NOx purification activities as the catalyst of Comparative Example 1, and the acid treatment adversely affected the purification performance. Clearly not.

[0033]

[Effects of the Invention] That is, according to the exhaust gas purifying catalyst of the present invention, HC, CO and NO in exhaust gas in an oxygen excess atmosphere
_It is possible to efficiently purify _x, and it is possible to prevent the oxidation of SO _{2 in} the exhaust gas and suppress the production of sulfate. Further, according to the method for producing an exhaust gas purifying catalyst of the present invention, the above exhaust gas purifying catalyst can be easily and reliably produced.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between temperature and SO ₂ conversion rate.

FIG. 2 is a graph showing the relationship between SO ₂ conversion rate and HC conversion rate.

FIG. 3 is a graph showing the relationship between temperature and SO ₂ conversion rate.

FIG. 4 is a graph showing the relationship between temperature and SO ₂ conversion rate.

FIG. 5 is a graph showing the relationship between SO ₂ conversion rate and HC conversion rate.

FIG. 6 is a graph showing the relationship between the treated SO ₂ concentration and the maximum NO _x purification rate.

FIG. 7 is a graph showing the relationship between the treated SO ₂ concentration and the 50% purification temperature of HC.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location B01D 53/36 104A

Claims (6)

[Claims] 1. A catalyst comprising a carrier made of a porous oxide to which at least one basic element selected from alkali metals, alkaline earth metals and rare earth elements is added, and a catalytic noble metal carried on the carrier. An exhaust gas-purifying catalyst characterized by being treated with an acid. 2. A catalyst comprising a carrier made of a porous oxide to which at least one basic element selected from alkali metals, alkaline earth metals and rare earth elements is added, and a catalytic noble metal supported on the carrier. A method for producing an exhaust gas-purifying catalyst, characterized by performing an acid treatment. 3. The method for producing a catalyst for purifying exhaust gas according to claim 2, wherein the acid treatment is performed by bringing the catalyst into contact with the sulfur dioxide gas in an oxidizing atmosphere containing the sulfur dioxide gas and heating the catalyst. 4. The concentration of sulfur dioxide gas is 10 in terms of volume fraction.
The method for producing an exhaust gas purifying catalyst according to claim 3, wherein the content is 0 ppm to 10%. 5. The method for producing an exhaust gas purifying catalyst according to claim 2, wherein the acid treatment is performed by bringing the catalyst into contact with a sulfuric acid solution. 6. The sulfuric acid solution has a concentration of 1% by weight to 50% by weight.
The method for producing a catalyst for purifying exhaust gas according to claim 5, wherein