JP2002336655A - Method for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning method by which exhaust gas can be purified by oxidizing methane even at low temperature and the lowering degree of methane oxidizing activity of a catalyst can be made small even at high temperature. SOLUTION: A catalyst obtained by depositing at least Pd on a carrier consisting of alumina-zirconia manufactured by a coprecipitation method is used for cleaning the exhaust gas the oxidizing component of which is more excessive than the reducing component and which is in a lean atmosphere. Through the reason is not clarified, hydrocarbon including methane can be oxidized efficiently even at a low temperature and the high temperature durability of the catalyst is enhanced remarkably.

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 method for oxidizing and purifying mainly methane in exhaust gas.

[0002]

2. Description of the Related Art Three-way catalysts have been widely used as exhaust gas purifying catalysts for purifying exhaust gas from automobiles.
This three-way catalyst uses platinum (P) on a porous carrier such as alumina.
t) and other precious metals.
O, it is possible to efficiently purify HC and NO _x.

[0003] Among HCs, olefinic hydrocarbons are relatively easy to purify, but saturated hydrocarbons are more difficult to purify than olefinic hydrocarbons, and methane is a saturated hydrocarbon that is particularly difficult to purify by oxidation. However, purification of methane is indispensable to comply with recent exhaust gas regulations.

However, when oxidizing and purifying methane using a three-way catalyst or the like, it is necessary to set the temperature to extremely high temperature, and it is difficult to purify methane in a low temperature range. For this reason, there is a problem in that the noble metal grains grow due to heat and the catalyst is deteriorated early.

Therefore, a catalyst capable of purifying methane has been developed, and it has been found that palladium (Pd) is effective as a catalyst metal. For example, JP-A-11-137998 discloses a catalyst in which Pd and at least one selected from Ru, Ir, and Cu are supported on an alumina carrier and exhibit methane purification ability. Also, JP-A-7-053976 discloses that
A catalyst for methane oxidation using a coprecipitate of Pd and Co as a catalyst metal is disclosed.

[0006]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open
The catalyst disclosed in -137998 has a problem that Pd undergoes large grain growth in a high temperature atmosphere of 700 ° C. or higher, and the activity is significantly deteriorated.

Further, the catalyst disclosed in Japanese Patent Application Laid-Open No. 7-053976 has a disadvantage that since no carrier is used, the particles in the coprecipitate of Pd and Co become coarse, and the activity decreases particularly after high-temperature durability.

In recent years, the maximum temperature of exhaust gas tends to be higher and higher, and even if methane can be oxidized and purified in a low temperature range before endurance (initial stage), a catalyst whose purification activity decreases after endurance at high temperature is not practical. .

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method of purifying exhaust gas which can oxidize and purify methane from a low temperature range and has a small degree of activity decrease even in a high temperature range. .

[0010]

The feature of the exhaust gas purifying method of the present invention which solves the above-mentioned problems is that a catalyst comprising at least Pd supported on a carrier made of alumina / zirconia produced by a coprecipitation method is converted from a reducing component. An oxidizing component is disposed in an exhaust gas in an excess lean atmosphere to oxidize and purify mainly methane in the exhaust gas.

The carrier made of alumina / zirconia preferably has a composition of Al ₂ O ₃ : ZrO ₂ = 99: 1 to 90:10 in a molar ratio.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying method of the present invention, a catalyst comprising at least Pd supported on an alumina / zirconia carrier produced by a coprecipitation method is used, and the catalyst is used in an exhaust gas in an oxygen-rich lean atmosphere. Used in Although the reason is not clear, hydrocarbons including methane can be efficiently purified from a low temperature range, and the high temperature durability is significantly improved.

The catalyst used in the present invention uses a carrier made of alumina / zirconia produced by a coprecipitation method. That is, for this carrier, a mixed aqueous solution of a water-soluble aluminum compound such as aluminum nitrate and a water-soluble zirconium compound such as zirconium oxynitrate is prepared, and an alkali aqueous solution is added thereto to coprecipitate the oxide precursor. , And dried and fired.

The support made of alumina / zirconia obtained by the coprecipitation method is considered to be a mixture of Al ₂ O ₃ and a composite oxide of ZrO ₂ and Al ₂ O ₃ -ZrO _2, but the detailed composition is unknown. is there.
However, by using alumina / zirconia obtained by the coprecipitation method as a carrier, the stability of at least Pd to be supported is improved, and the grain growth of at least Pd is suppressed even after high-temperature durability. It is possible to oxidize and purify methane from the area. Incidentally, alumina / zirconia can also be produced by the alkoxide method, but it has been found that a catalyst using alumina / zirconia obtained by the alkoxide method as a carrier has low durability at high temperatures. The reason why the difference in the high-temperature durability due to the difference in the production method is still unknown.

The carrier comprising alumina and zirconia produced by the coprecipitation method used in the present invention has a molar ratio of Al ₂ O ₃ :
It is desirable to have a composition of ZrO ₂ = 99: 1 to 90:10. A
When l ₂ O ₃ exceeds this range, the low-temperature purification activity of methane before durability (initial stage) decreases, and when ZrO ₂ exceeds this range, the low-temperature purification activity of methane after high temperature durability decreases.

The alumina / zirconia carrier used in the present invention is most preferably made of alumina / zirconia alone, but is preferably a mixture of oxides such as Al ₂ O ₃ , ZrO ₂ , TiO ₂ , CeO ₂ and SiO _2. You can also. These other oxides can be used in a range that does not hinder the action of alumina / zirconia, and even in such a case, it is desirable that the content be in the range of 1 to 5 wt% in the catalyst. Also these other oxides
It may be physically mixed with alumina / zirconia, or in some cases, may be mixed by coprecipitation during coprecipitation for producing alumina / zirconia.

The carrier made of alumina / zirconia can be in the form of powder, pellets, or a monolith honeycomb substrate coated with alumina / zirconia powder.

At least Pd is supported on the carrier. By supporting at least Pd, high-temperature purification activity of methane at low temperature is exhibited. The amount of Pd supported in the catalyst
A range of 0.1 to 10% by weight is preferred. If the amount of Pd carried is less than 0.1% by weight, it becomes difficult to exhibit low-temperature purification activity of methane, and even if it exceeds 10% by weight, the activity will be saturated and the cost will increase.

In order to support Pd on a support made of alumina / zirconia, a conventional supporting method such as an adsorption supporting method, an impregnating supporting method, and a water absorbing supporting method can be used. Further, at the time of coprecipitation for producing a carrier composed of alumina / zirconia, it is also possible to carry the Pd by mixing a water-soluble compound of Pd into an aqueous solution.

In addition to Pd, noble metals such as Pt, Rh and Ir or base metals such as Ni, Co, Cu and Fe may be supported. The carried amount of the catalyst metal other than Pd may be within a range that does not decrease the low-temperature purification activity of methane by Pd,
Usually, it is preferably in the range of 1 to 3% by weight.

In the exhaust gas purifying method of the present invention, the above-described catalyst is disposed in the exhaust gas in a lean atmosphere containing methane and having an oxidizing component in excess of the reducing component. When the methane is not disposed in the exhaust gas in a lean atmosphere, it is difficult to purify methane by oxidation. Incidentally, the oxidizing component refers to O ₂ , NO _{x and the} like,
The reducing components HC containing methane (hydrocarbon), CO, refers to like H _2.

The degree of the lean atmosphere of the exhaust gas in the lean atmosphere is preferably in a range of 15 to 40 in terms of an air-fuel ratio (A / F), and particularly preferably in a range of 25 to 35. If this value is lower than 15, it is difficult to purify methane by oxidation, and if it is higher than 40, the catalytic metal may grow at high temperatures and its activity may decrease. It should be noted that the lean burn engine is burned in the range of A / F = 18 to 24, and is therefore most suitable for applying the present invention.

[0023]

The present invention will be specifically described below with reference to examples and comparative examples.

Example 1 Aluminum nitrate and zirconium oxynitrate were mixed at a molar ratio of Al ₂ O ₃ : ZrO ₂ = 99: 1, and the mixed powder was dissolved in distilled water to obtain a mixed aqueous solution. Was prepared. While stirring this mixed aqueous solution,
An aqueous solution of sodium hydrogen carbonate having a concentration of 5% by weight was gradually dropped, and the dropping was completed when the pH reached 7 to 8. Then, the obtained precipitate was centrifuged, washed several times with distilled water, and dried at 120 ° C. for 24 hours.

The obtained alumina-zirconia carrier powder is impregnated with a predetermined amount of an aqueous solution of palladium nitrate having a predetermined concentration,
After evaporation and drying, Pd was supported. Then, the mixture was calcined at 450 ° C. for 2 hours to prepare a catalyst powder. The supported amount of Pd in this catalyst powder was 2% by weight. The catalyst powder was formed into pellets by a conventional method to prepare a pellet catalyst.

A predetermined amount of the pellet catalyst is charged into an evaluation device, and the temperature of the model gas is raised from room temperature to 500 ° C. at a rate of 20 ° C./min while flowing the model gas (A / F = 32.4) shown in Table 1. The temperature was raised to ° C., during which the purification rate of CH ₄ was continuously measured. The methane 50% purification temperature was calculated from the obtained result, and the result is shown in FIG.

[0027]

[Table 1]

Further, a predetermined amount of the above-mentioned pellet catalyst was charged into an evaluation device, and while the rich model gas and the lean model gas shown in Table 2 were alternately passed at a space velocity of 170,000 / hr for 1 minute, the temperature of the gas containing the catalyst was 900. A high-temperature endurance test of holding at 5 ° C. for 5 hours was performed. Then, for the pellet catalyst after the high-temperature durability test, the methane 50% purification temperature was measured in the same manner as above, and the result is shown in FIG.

[0029]

[Table 2]

Example 2 Alumina-zirconia was prepared in the same manner as in Example 1 except that aluminum nitrate and zirconium oxynitrate were mixed at a molar ratio of Al ₂ O ₃ : ZrO ₂ = 95: 5. A carrier powder was prepared and used as a pellet catalyst in the same manner as in Example 1. The methane 50% purification temperatures before and after the durability test were measured in the same manner as in Example 1, and the results are shown in FIG.

Example 3 Alumina / zirconia was prepared in the same manner as in Example 1 except that aluminum nitrate and zirconium oxynitrate were mixed at a molar ratio of Al ₂ O ₃ : ZrO ₂ = 90: 10. A carrier powder was prepared and used as a pellet catalyst in the same manner as in Example 1. The methane 50% purification temperatures before and after the durability test were measured in the same manner as in Example 1, and the results are shown in FIG.

Example 4 Alumina-zirconia was prepared in the same manner as in Example 1 except that aluminum nitrate and zirconium oxynitrate were mixed at a molar ratio of Al ₂ O ₃ : ZrO ₂ = 80: 20. A carrier powder was prepared and used as a pellet catalyst in the same manner as in Example 1. The methane 50% purification temperatures before and after the durability test were measured in the same manner as in Example 1, and the results are shown in FIG.

Comparative Example 1 A carrier powder was prepared in the same manner as in Example 1 except that zirconium oxynitrate was not used, and a pellet catalyst was obtained in the same manner as in Example 1. Then, in the same manner as in Example 1, methane 50 before and after endurance was used.
% Purification temperature was measured, and the results are shown in FIG.

[0034] (Comparative Example ₂₎ γ-Al 2 O ₃ powder was prepared to prepare a catalyst powder and 2 wt% on the Pd in the same manner as in Example 1. This was used as a pellet catalyst in the same manner as in Example 1,
The 50% purification temperature of methane before and after endurance was measured in the same manner as in Example 1, and the results are shown in FIG.

<Evaluation> From FIG. 1, it can be seen that in each of the examples, the methane 50% purification temperature after durability was lower than that in Comparative Example 2, and methane was oxidized and purified from a low temperature region even after durability. In other words, it is clear that the use of the alumina-zirconia support manufactured by the coprecipitation method improves the high-temperature durability as compared with the case where the γ-Al ₂ O ₃ support is used.

Further, from the comparison between the respective embodiments, it was found that the second embodiment
Has the highest methane purification activity at low temperatures before endurance, and almost no difference in activity between before and after endurance.
Then, as the amount of ZrO ₂ becomes smaller than that in Example 2, the difference in activity between before and after endurance increases, and in Comparative Example 1 containing no ZrO ₂ , the difference in activity between before and after endurance is extremely large and durability is low. You can see that. On the other hand, in Example 4, the methane 50% purification temperature becomes high both before and after endurance.

Therefore, the composition of the carrier is expressed as a molar ratio of Al ₂ O ₃ :
It is apparent that by setting ZrO ₂ = 90: 1 to 90:10, the difference between before and after endurance becomes small, and methane can be efficiently purified from a low temperature region both before and after endurance.

[0038]

According to the exhaust gas purification method of the present invention, methane can be efficiently oxidized and purified from a low temperature range, and the high-temperature durability can be improved.

[Brief description of the drawings]

FIG. 1 is a graph showing methane 50% purification temperatures in Examples and Comparative Examples.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Akira Nishimura 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F-term (reference) 3G091 AA12 AB02 BA03 BA07 BA15 FA02 FA04 FA12 FA13 FA14 FB02 FB03 FB10 GA01 GB01X GB07W GB10X GB16X 4D048 AA18 AB01 AB07 BA03X BA08X BA31X BA42X BB01 BB02 4G069 AA03 AA08 BA01A BA01B BA05A BA05B BA20A BA20B BB02A BB02B BC72A BC72B CA03 CA07 CA15 EA01Y EA02Y EA19 FA01FB09

Claims (2)

[Claims] 1. A catalyst comprising at least Pd supported on a support made of alumina / zirconia produced by a coprecipitation method is disposed in an exhaust gas in a lean atmosphere in which an oxidizing component is more excess than a reducing component, and An exhaust gas purification method comprising oxidizing and purifying methane. 2. The exhaust gas purifying method according to claim 1, wherein the carrier made of alumina / zirconia has a composition of Al ₂ O ₃ : ZrO ₂ = 99: 1 to 90:10 in a molar ratio.