JP3538214B2 - Nitrogen oxide reduction catalyst and method of using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitrogen oxide in an exhaust gas discharged from an internal combustion engine such as an automobile (NO _X) under an oxygen-rich atmosphere, i.e. air-fuel ratio is the stoichiometric air-fuel ratio (14.
7) A nitrogen oxide reduction catalyst capable of reducing and purifying over a wide temperature range in a larger combustion atmosphere and a method for using the same.

[0002]

2. Description of the Related Art 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 protection of the global environment. Lean burn is promising. In this lean burn, the use of fuel is reduced to improve fuel efficiency, and the generation of CO ₂ , which is the combustion exhaust gas, can be suppressed.

Conventional three-way catalysts simultaneously oxidize and reduce carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NO _x ) in exhaust gas at a stoichiometric air-fuel ratio. And purify it. However, since the three-way catalyst does not exhibit sufficient purification ability for the reduction and removal of the NO _X in an oxygen excess atmosphere in the exhaust gas during the lean-burn, NO even under an oxygen rich atmosphere
Development of a catalyst and a purification system capable of purifying _X is desired.

As a catalyst exhibiting a relatively high purification ability for nitrogen oxides even under an excess of oxygen, for example, Japanese Patent Application Laid-Open No.
Copper (Cu) as disclosed in 139145
There is a zeolite catalyst in which a transition metal such as is supported on zeolite by ion exchange, and a platinum catalyst in which a noble metal such as platinum is supported on alumina (Japanese Patent Application Laid-Open No. 5-168860).

[0005]

However, zeolite-based catalysts have poor heat resistance and low nitrogen oxide purifying activity in an oxygen-lean atmosphere because of their stoichiometric air-fuel ratio.
There is a problem that the oxidation activity of HC and CO is low. On the other hand, in the case of a platinum-based catalyst, the above problems of the zeolite-based catalyst can be improved, but the nitrogen oxide purification start temperature is 200
The temperature range which is as low as about ℃ and which can purify nitrogen oxides is 2
Since it is a narrow range of about 100 ° C. from 50 ° C. to 350 ° C., it is necessary to accurately control the temperature of the actual exhaust gas to be within the purification activation temperature range when using the actual exhaust gas (100 ° C. to 700 ° C.). There is a problem.

The inventors of the present invention have conducted intensive studies to solve the above problems, and have conducted various systematic experiments. As a result, the present invention has been accomplished.

According to the present invention, the nitrogen oxide purification starting temperature is 28
It is an object of the present invention to provide a nitrogen oxide reduction catalyst in which a temperature range (a temperature range in which a purification rate is 10% or more) showing activity at a high temperature of 0 ° C. or more is as wide as 100 ° C. or more. Another object of the present invention is to provide a method for using a nitrogen oxide reduction catalyst which enables purification of nitrogen oxides over a wide range in which the purification start temperature is around 200 ° C. and the temperature range showing activity is 200 ° C. or more. .

[0008]

Means for Solving the Problems (Constitution of the First Invention) The nitrogen oxide reduction catalyst of the first invention is a porous carrier made of silica and an alkali metal supported on the porous carrier. At least three kinds of metals, platinum and molybdenum supported on the porous carrier, and the amount of molybdenum supported is at least 3 in molar ratio with respect to the amount of platinum supported, and the amount of supported alkali metal is the amount of platinum supported 0.001 in molar ratio to
３3.8 .

In the present invention, the porous carrier may be oxidized
Silicon (silica; SiO ₂ ) can be used. The amount of platinum carried is not particularly limited. Any amount that is normally carried may be used. When the supported amount of molybdenum to platinum is less than 3 in a molar ratio or the supported amount of alkali metal to platinum is less than 0.001 in a molar ratio, the interaction between platinum, molybdenum, and the alkali metal is weak, so that the nitrogen oxide It is not preferable because the purification start temperature drops below 280 ° C. or the temperature range showing the activity (temperature range where the purification rate is 10% or more) becomes narrower than 100 ° C.

When the amount of molybdenum supported on platinum is 3 or more in molar ratio and the amount of alkali metal supported on platinum is 0.001 or more in molar ratio, a large amount of Pt-Mo-alkali metal interaction sites on the catalyst surface. Therefore, the activity can be exhibited over a wide range of temperature, at which the nitrogen oxide purification starting temperature is as high as 280 ° C. or more, and the activity is as high as 100 ° C. or more.

When the amount of alkali metal carried exceeds 15,
It is not preferable because the structure of SiO ₂ is destroyed by the alkali metal and the specific surface area is estimated to be reduced. Although the molybdenum loading is effective even if the molar ratio with respect to platinum is 50 or more, the effect is hardly increased.

(Constitution of the Second Invention) The method for using the nitrogen oxide reduction catalyst of the second invention is characterized in that the nitrogen oxide reduction catalyst of the first invention has a purifying active temperature range of 250 to 350 ° C. It is arranged upstream or downstream of a purification catalyst (auxiliary catalyst) and is brought into contact with nitrogen oxides (FIG. 1).

As the auxiliary catalyst, the purifying active temperature range is 25
There is no limitation as long as it is 0 to 350 ° C., for example, alumina,
A carrier in which a noble metal such as platinum, rhodium (Rh) or palladium (Pd) is supported on a silica-based porous carrier can be used. The type and physical properties of the porous carrier used as the auxiliary catalyst are not particularly limited, and any carrier conventionally used for a catalyst can be used.

[0014]

[Action]

(Operation of the First Invention) The reason why the nitrogen oxide reduction catalyst of the present invention exhibits excellent purification activity over a wide temperature range in a high temperature range is not clear, but is presumed to be as follows.

On the porous carrier, Pt is stabilized by being incorporated into Mo particles, and Mo near Pt is in a low valence state or zero valence, and exists in a bimetallic state with Pt. Here, the function of Mo is to change the electron density of Pt through the interaction shown in Chemical formula 1.

[0016]

Embedded image

By this interaction, Mo is adsorbed and activated on Mo as a reducing agent, and NO is reduced by the reaction between activated HC and NO on Pt. , Pt are considered to function effectively. It is presumed that such a cycle of the divided functions expresses a different catalytic activity from that of Pt alone.

The function of the alkali metal is to make the interaction between Pt and Mo close by ion exchange with the hydroxyl group on the porous carrier, or to reduce the electronic influence on the Pt-Mo interactant. Is to exert. Such interaction, oxidation of NO to NO ₂ is suppressed, since the reaction selectivity with HC is increased, NO _X at a high temperature
It is presumed to contribute to purification activity.

The amount of Mo to be supported is 3 or more in molar ratio with respect to the amount of Pt supported, and the amount of alkali metal supported is in the range of 0.001 to 15 in molar ratio to the amount of Pt supported. At this time, Pt can be stabilized in a form surrounded by Mo particles, and the effects of Pt, Mo, and alkali metal as described above are exerted. At this time, the formed Pt-
The binary reaction by the Mo bimetallic body proceeds effectively, and the nitrogen oxide purification start temperature is as high as 280 ° C. or higher, and the temperature range showing the activity is 100 ° C. or more, which shows excellent reduction purification action in a wide range. Presumed.

(Operation of the Second Invention) Since the nitrogen oxide reduction catalyst of the first invention has a high temperature of starting nitrogen oxide purification of 280 ° C. and has a wide temperature range of 100 ° C. or higher, it exhibits activity. When used in combination with a conventional nitrogen oxide purifying catalyst having a purifying activity temperature range of 250 to 350 ° C., nitrogen oxides can be reduced and purified over a wide temperature range. In this case, in the temperature range of 250 to 350 ° C., the activity of purifying nitrogen oxides is not different from that of the conventional nitrogen oxide purifying catalyst.

[0021]

According to the nitrogen oxide reduction catalyst of the first aspect of the present invention, the nitrogen oxide purification starting temperature is as high as 280 ° C. or higher, and the temperature range in which the activity is exhibited is 100 ° C. or higher. It can be reduced and purified. Further, according to the method for using the nitrogen oxide reduction catalyst of the second invention, the purification start temperature is around 200 ° C., and the temperature range showing the activity is 2
Nitrogen oxides can be effectively reduced and purified over a wide range of 00 ° C or higher.

[0022]

The present invention will be described below in detail with reference to examples.

(Preparation of Pt-Mo-alkali metal / SiO ₂ catalyst) An aqueous solution of lithium nitrate, sodium acetate, and potassium acetate was prepared at a predetermined concentration so that the amount of alkali metal supported was as shown in Table 1. , With powdered S
After immersing the iO ₂ carrier and evaporating it to dryness,
Firing for hours, lithium (Li), sodium (Na),
The amount of potassium (K) supported in Table 1 was No. 1 to No. 10
An alkali metal-supported catalyst having the following values was obtained.

Next, an aqueous solution of ammonium molybdate having a predetermined concentration was prepared so that the amount of molybdenum supported was as shown in Table 1, and the above-mentioned alkali metal supported catalyst was immersed in the aqueous solution and evaporated to dryness. After that, the mixture was calcined at 700 ° C. for 3 hours.
No. 1 to No. A Mo-alkali metal-supported catalyst of 3 to 50 shown in No. 10 was prepared.

[0025]

[Table 1]

Next, the Mo-alkali metal-supported catalyst is immersed in 200 cc of an aqueous solution of dinitrodiaminoplatinum having a predetermined concentration prepared so that 1.7 wt% of platinum is supported, and evaporated to dryness. Baked at 5 ° C. for 5 hours to support platinum, and the catalyst No. of the present example composed of Pt—Mo—alkali metal / SiO ₂ . 1 to No. 10 was obtained.

The catalyst No. of the comparative example was prepared in the same manner as in the above example except that the amount of molybdenum supported on platinum was 0, 1, and 2 shown in Table 1 in molar ratio. 11, No.
12, No. 13 was prepared.

Except that no alkali metal is supported, the catalyst of Comparative Example was prepared in the same manner as in the above example. 14
Was prepared.

The catalyst No. of the comparative example was prepared in the same manner as in the above example except that the amount of alkali metal supported on platinum was 18.9 in molar ratio. 15 were prepared.

(Preparation of Pt / Al ₂ O ₃ catalyst) Powdered alumina (Al ₂ O ₃ ) was added to an aqueous solution of dinitrodiaminoplatinum having a predetermined concentration prepared so that the amount of supported platinum was 1.7 wt%.
O ₃₎ was immersed and allowed to dryness, and calcined 5 hours at 500 ° C., the catalyst of Comparative Example consisting of Pt / Al ₂ O ₃ No.
16 was obtained.

(Preparation of Pt-Mo / Al ₂ O ₃ catalyst) The above-mentioned No. ₂ was prepared except that the carrier was changed to Al ₂ O ₃ . 14
According to the method of preparing the catalyst No. ₃ , the catalyst No. of Comparative Example composed of Pt-Mo / Al ₂ O ₃ was used. 17 was prepared.

(Preparation of Pt-Mo-Na / Al ₂ O ₃ catalyst) The above-described Pt was used except that the carrier was changed to Al ₂ O _3.
-Mo- analogously to preparation of alkali metal / SiO ₂ catalyst,
_{Pt-Mo-Na / Al 2} O consists of ₃ Comparative example catalyst N
o. 18 was prepared.

(Performance Evaluation Test 1) The catalysts shown in Table 1 were formed into a pellet having a diameter of 300 to 700 μm by powder compaction, and 0.5 g of the pelletized catalyst was packed in a quartz reaction tube. The air-fuel ratio (A / F) shown in Table 2 was added to this reaction tube.
A flow rate of 3.3 l of model gas having an exhaust composition equivalent to 18
/ Min, and the activity of reducing and purifying nitrogen oxides (NO) was examined by an exhaust gas analyzer.

[0034]

[Table 2]

(Performance Evaluation Test 2) The catalysts shown in Table 1 were formed into a pellet having a diameter of 300 to 700 μm by powder compaction, and 0.5 g of the pelletized catalyst was packed in a quartz reaction tube. A model gas having an exhaust gas composition having an air-fuel ratio close to the stoichiometric air-fuel ratio shown in Table 3 at a flow rate of 3.3 l / min shown in Table 3 was flown at 3.3 l / min, and nitrogen oxides (NO) were investigated by an exhaust gas analyzer. Then, the air-fuel ratio characteristics of the NO purification activity were investigated.

[0036]

[Table 3]

(Results of Performance Evaluation Test 1) FIG.
The catalyst of this example composed of Mo—Na / SiO ₂ (No.
4) and Pt-Na / Si, which is a catalyst of a comparative example
O ₂ (No. 11), Pt / Al ₂ O ₃ (No. 1)
6) and the results of performance evaluation tests on Pt-Mo / Al ₂ O ₃ (No. 17) catalyst. The horizontal axis shows the incoming gas temperature and the vertical axis shows the nitrogen oxide purification rate. From this result,
The catalyst of this embodiment having the Pt-Mo-Na / SiO ₂ is found to be capable of reducing purify nitrogen oxides over a temperature range in the range of 350 to 600 ° C..

FIG. 3 shows the catalysts of this example (No. 1 to No. 6) which consist of Pt--Mo--Na / SiO ₂ and in which the amount of Mo supported on Pt is 3 or more in molar ratio, and
It is composed of Pt-Mo-Na / SiO ₂ and has an M
The catalyst of Comparative Example (N
o. 12, No. 13), and Pt-Na / SiO
_The results of a performance evaluation test on the catalyst of Comparative Example No. ₂ (No. 11) are shown. From this result, the catalyst of the present embodiment has a high nitrogen oxide purification start temperature of 280 ° C. or higher,
It can be seen that the catalyst has excellent purification ability for nitrogen oxides over a temperature range of 100 ° C. or more in which the activity is exhibited.
In this example, the supported amount of Mo with respect to Pt was up to 50 in molar ratio, but even when it was 50 or more, still higher purification activity for nitrogen oxides was exhibited.

FIG. 4 shows that Pt-Mo-Na / SiO ₂ was used, and the amount of Na supported on Pt was 0.41 in molar ratio,
The catalysts of this example, which are 0.19 and 3.8 (in order, No.
4, no. 7, no. 8), and a catalyst (N) in which the supported amount of Na with respect to Pt is 0,18.9 in molar ratio.
o. 14, No. 15 shows the performance evaluation test results for 15). From these results, the catalyst of the present example has an excellent purification ability for nitrogen oxides at a high temperature of starting nitrogen oxide purification of 280 ° C. or more, and over a temperature range of 200 ° C. or more showing activity. You can see that.

Table 4 shows the catalysts of this example (Nos. 1 to 1).
0) and the performance evaluation test results for the catalysts of Comparative Examples (Nos. 11 to 18). In this evaluation test, the measurement was performed up to a maximum of 600 ° C.

[0041]

[Table 4]

From these results, it can be seen that the catalyst of this embodiment has an excellent nitrogen oxide purifying ability over a temperature range of 100 ° C. or higher at a starting temperature of 280 ° C. or higher. You can see that it is doing.

(Results of Performance Evaluation Test 2) FIG.
The catalyst of this example composed of Mo—Na / SiO ₂ (No.
4, no. 7, no. 8) and Pt-Na / SiO ₂ (No. 11), Pt-Mo
The results of a performance evaluation test on the / SiO ₂ (No. 14) catalyst are shown (inlet gas temperature: 450 ° C.). In the figure, the horizontal axis L
ambda is the ratio of the air-fuel ratio to the stoichiometric air-fuel ratio of 14.7 (Lambda = (air-fuel ratio) /14.7), and the vertical axis is N
Shows the O purification rate.

From these results, it was found that Pt-Mo-Na / SiO
_It can be seen that the catalyst of Example ₂ composed of No. ₂ exhibits excellent purification activity in a wide air-fuel ratio range as compared with the catalyst of Comparative Example.

FIG. 6 shows that the catalyst (No. 4) of the present embodiment composed of Pt—Mo—Na / SiO ₂ was placed upstream of Pt—A
The result of an evaluation test performed by arranging a catalyst (No. 16) of Comparative Example made of l ₂ O _{3 on the} downstream side as an auxiliary catalyst is shown. From these results, by using the catalyst of the present example and the conventional catalyst of the comparative example on the upstream and downstream sides, respectively, the temperature range where the nitrogen oxide purification start temperature is around 200 ° C. and the activity is exhibited. It can be seen that the catalyst has excellent purification ability for nitrogen oxides over a wide range of 350 ° C. or more.

[Brief description of the drawings]

FIG. 1 is a schematic view showing the second invention.

FIG. 2 is a diagram showing the results of a performance evaluation test 1 of a catalyst of an example according to the first invention and a catalyst of a comparative example.

FIG. 3 is a diagram showing the results of a performance evaluation test 1 of a catalyst of an example according to the first invention and a catalyst of a comparative example.

FIG. 4 is a view showing the results of a performance evaluation test 1 of a catalyst of an example according to the first invention and a catalyst of a comparative example.

FIG. 5 is a diagram showing the results of a performance evaluation test 2 of a catalyst of an example according to the first invention and a catalyst of a comparative example.

FIG. 6 is a diagram showing the results of a performance evaluation test 1 of the catalyst of the example according to the second invention.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Shinichi Matsumoto               1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor               Dosha Co., Ltd.                (56) References JP-A-2-122831 (JP, A)

Claims (2)

(57) [Claims] 1. A porous carrier made of silica, and one selected from alkali metals supported on the porous carrier.
At least three kinds of metals, and platinum and molybdenum supported on the porous carrier, wherein the supported amount of molybdenum is 3 or more in molar ratio with respect to the supported amount of platinum, and the supported amount of the alkali metal is the supported amount of platinum. A nitrogen oxide reduction catalyst having a molar ratio of from 0.001 to 3.8 with respect to 2. The purification activity temperature range of the nitrogen oxide reduction catalyst according to claim 1 in an oxygen-excess atmosphere is from 250 to 350 ° C.
A method for using a nitrogen oxide reduction catalyst, wherein the catalyst is disposed upstream or downstream of the nitrogen oxide purification catalyst and brought into contact with nitrogen oxide.