JPH04215845A - Catalyst for exhaust gas purification - Google Patents

Abstract

PURPOSE:To efficiently remove hydrocarbons, carbon monoxides and nitrogen oxides in exhaust gas by contacting catalysts comprising transition metal oxides and precious metal (excluding gold) with exhaust gas of internal combustion engine or the like at a temperature of 250 deg.C or lower. CONSTITUTION:Metal nitrates, organic metal salts or metal chlorides of transition metal are thermally decomposed, which are impregnated with an aqueous solution of precious metal salts (excluding gold). As a result, catalysts for purifying exhaust gas comprising one or more kinds of transition metal oxides and one or more kinds of precious metals (excluding gold) are obtained. The transition metal oxides, for example, are Co3O4, NiO, Fe2O3, Cr2O3, Mn2O3, etc., and precious metals are Pt, Rh, Pd. The transition metal oxides discharge O2 occluded in transition metal itself at a temperature of 250 deg.C or lower and accelerate the reaction with CO adsorbed preferentially on precious metals and convert the CO to CO2, so that O2, HC and NO are adsorbed on the precious metals thereby enabling purification of HC and NO.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is applicable to hydrocarbons (HC) and carbon monoxide (C
The present invention relates to a catalyst that efficiently removes O) and nitrogen oxides (NOx) at a low temperature of 250°C or lower.

(Prior Art) Recently, the maintenance of the natural environment has become particularly important, and there is a tendency for regulations on exhaust gas from automobiles to be tightened. As part of tightening regulations, when starting a car (cold start)
There is a problem in purifying NOx, HC, and CO from exhaust gas.

The temperature of the catalyst layer during a cold start is as low as 300°C or less until it is heated by exhaust heat (about 2 minutes).
Conventional three-way catalysts for automobile exhaust consisting of catalyst metals made of noble metals such as platinum (Pt) and rhodium (Rh), rare earth oxides such as ceria (CeO2) as co-catalysts, and supports such as alumina (Al2O3) do not purify the exhaust gas. Since the temperature at which it fully exhibits its activity is as high as 300°C or higher, the catalyst layer is heated to 30°C by exhaust heat.
The harmful components discharged before being heated to 0°C or higher;
In particular, the current situation is that HC is not sufficiently purified. Therefore, ■ a catalytic converter with a built-in heater to heat the catalyst to a temperature at which it becomes active during a cold start, and ■ an adsorption trapper using zeolite to trap the harmful components until the catalyst becomes active. A method using a gold (Au)/iron oxide (Fe2O3) catalyst, which has the ability to oxidize CO from -70 DEG C., has been considered. However, these methods have cost and technical problems, namely, the problem of the capacity of the heater battery (i.e., extra power is required to heat the heater, and the capacity of conventional batteries is insufficient, so large-capacity batteries are not needed). It becomes necessary. In case (2), it is necessary to provide a very large adsorption trapper in order to adsorb all the HC discharged during a cold start. The major problem with (2) is that the HC purification ability is extremely poor.

In addition, HC generated at low temperatures in oil stoves, etc.
Similar problems exist regarding the removal of harmful gases such as

(Object of the invention) In order to eliminate the drawbacks of the above-mentioned prior art, the present invention aims to
It is an object of the present invention to provide a new catalyst for exhaust gas purification that can efficiently remove NOx, HC, CO, especially HC, from exhaust gas emitted from an internal combustion engine of an automobile in the following low temperature range.

(Description of the first invention) The first invention provides one or more transition metal oxides and a noble metal (A
The present invention relates to an exhaust gas purifying catalyst that removes hydrocarbons, carbon monoxide, and nitrogen oxides in exhaust gas from internal combustion engines, etc., at a temperature of 250° C. or lower.

According to this catalyst, HC in the low temperature range of 250℃ or less
, CO, and NOx can be efficiently purified. In particular, it has made it possible to purify HC, which could hardly be purified using conventional catalysts.

Although the effect of this catalyst is not clear, it is thought to be as follows.

The conventional three-way catalyst eliminates CO, a harmful component in exhaust gas.
, HC, and NOx are adsorbed onto the noble metal and removed by the reaction shown below.

In addition, the rare earth oxides represented by CeO2, which are co-catalysts, improve the thermal stability of the carrier and ensure high dispersion of precious metals. Even if CeO2 is O2
C due to the so-called O2 storage ability that supplies precious metals with
It has the function of advancing the oxidation of O and HC. However, 2
At low temperatures below 50°C, adsorption of CO to noble metals occurs preferentially, preventing the adsorption of O2, HC, etc., and removing almost no HC and NOx.
At low temperatures below 250°C, C
It was also impossible to purify HC and the like based on the O2 storage capacity of eO2.

The transition metal oxide in the present invention differs from CeO2 in that the transition metal itself occludes O at a low temperature of 250°C or lower.
2 can be expelled to accelerate the reaction with CO preferentially adsorbed on precious metals, converting CO into CO2. Therefore, O2, HC, and NOx can also be adsorbed on precious metals, making it possible to purify HC and NOx. That is to say.

Furthermore, since the reactions (1) to (4) described above are carried out in this way, the temperature of the catalyst itself rises due to the reaction heat, and the catalytic activity is also increased.

(Description of other inventions of the first invention) In the first invention, the transition metal oxide is Co3O4, N
Examples include iO, Fe2O3, Cr2O3, Mn2O3, but these are merely examples. One or more of these are used. The size of the transition metal oxide is 2000 Å or less, and the specific surface area is 0.2 to 200 m 2 /g. If it exceeds this range, good purifying activity cannot be exhibited. The thickness is desirably 100 Å or less, preferably 50 m 2 /g to 200 m 2 /g.

As the noble metal, one or more of Rh, Pt, and Pd is used.

The amount of noble metal is 0.05 to 10 at for transition metal oxide.
%. If it is less than 0.05 at%, purification activity will not be exhibited, and conversely, if it is more than 10 at%, the amount of precious metals will be too large and will coagulate with each other, resulting in a decrease in activity. It is preferably 1 to 5 at%. The particle size of the noble metal is 500 Å or less.

These transition metal oxides and noble metals are used in a uniform mixture. As mentioned above, CO adsorbed on the surface of the noble metal is oxidized by O2 discharged from the transition metal oxide, and CO is
This is because the noble metal and the transition metal oxide need to be in close contact with each other in order to efficiently remove them from the surface of the noble metal. In addition, when these two substances are in solid solution at the interface where they come into contact, better purification properties are exhibited.

The catalyst according to the present invention is manufactured by the method shown below.

Conventionally known methods, (1) a method of impregnating thermally decomposed metal nitrates, organic metal salts, or metal chlorides of transition metals with an aqueous solution of noble metal salts, (2) hydrolyzing the various metal salts mentioned above, A method of supporting a noble metal after firing a hydroxide,
(2) Any method of producing by co-precipitation from a mixed solution consisting of a transition metal salt and a noble metal salt may be used.

The catalyst composed of a transition metal oxide and a noble metal may have any shape or structure, such as powder, pellet, or honeycomb shape. In addition, a binder such as alumina sol, silica sol, zirconia sol, titania sol, etc. is added to the powdered catalyst, and it is molded into a predetermined shape, or water is added to form a slurry that can be applied to a refractory substrate such as alumina in the shape of a honeycomb, etc. May be applied.

EXAMPLE A catalyst according to the present invention was manufactured, and its purification activity against NOx, CO, and HC was evaluated using a model gas at a stoichiometric air-fuel ratio. Similar activity evaluations were also conducted for comparative catalysts.

Example 1 54 g of cobalt nitrate and 0.433 g of palladium nitrate were dissolved in 2 μg of ion-exchanged water (atomic ratio Pd:Co
= 1:100) at room temperature, approximately 0.6 m from the separating funnel.
Add dropwise to an alkaline aqueous solution prepared by dissolving 50 g of sodium carbonate in 2 cm of ion-exchanged water at a rate of 1/sec and stir.

After leaving the solution for one hour, it is filtered, washed, and dried under reduced pressure. The obtained powder was then calcined in air at 350°C for 3 hours.

This baked product is compacted into a dice shape of approximately 2 to 3 mm,
Example catalyst No. consisting of Pd and Co3O4. I got 1.

Example 2 In the method of Example 1, dinitrodiamino Pt was used instead of palladium nitrate (the atomic ratio of Pt and Co was 1).
: Same as 100) Catalyst No. 1 consisting of Pt and Co3O4 under the same conditions except that ammonium carbonate was used as the alkaline aqueous solution. I got 2.

Example 3 A cobalt nitrate aqueous solution (50 g/■) was added dropwise to a sodium carbonate aqueous solution (50 g/■) and stirred to precipitate cobalt hydroxide, which was then filtered, washed, and further dried under reduced pressure.
Thereafter, the powder was baked at 350° C. for 3 hours in air, and the obtained powder was impregnated with an aqueous solution of Pd nitrate to have a Pd content of 1 at %, and then baked at 350° C. in air. This calcined product was compacted into a 2 to 3 mm dice shape, and the Example Catalyst No. 1 consisting of Pd and Co3O4 was prepared. I got 3.

Example 4 Pt and Co3O were produced under the same conditions as in Example 3 except that dinitrodiamino Pt was used instead of Pd nitrate.
Example catalyst No. 4 consisting of I got 4.

Example 5 Co nitric acid was fired at 600°C in air for 5 hours to thermally decompose the C
Obtain o3O4. This Co3O4 powder was impregnated with a Pd nitrate content of 3.5 at%, and then
It was baked at 0°C for 3 hours. This calcined product was compacted into a 2 to 3 mm dice shape, and the Example Catalyst No. 1 consisting of Pd and Co3O4 was prepared. Got 5.

Example 6 In the method of Example 5, Pt and Co3 were prepared under the same conditions except that dinitrodiaminopt was used instead of Pd nitrate.
Example catalyst No. consisting of O4. I got 6.

Example 7 In the method of Example 5, Ni nitrate was used instead of Co nitrate.
Example catalyst No. 1 consisting of Pt and NiO was prepared under the same conditions except that catalyst No. 1 was used. I got a 7.

Example 8 Example catalyst No. 1 consisting of Pt and Fe2O3 was prepared under the same conditions as in Example 5 except that ferric nitrate was used instead of Co nitrate. I got 8.

Example 9 In the method of Example 1, 18 g of nickel nitrate was added in addition to 54 g of cobalt nitrate and 0.433 g of palladium nitrate, and the amount of sodium carbonate was 70 g. Example catalyst No. 1 consisting of Pd, NiO and Co3O4 was prepared under the same conditions except for the above. I got a 9.

Comparative Example 1 γ-A2O3 (with specific surface area of about 220 m2/gLa)
Ce in 120g of powder (La0.03mol/120g)
Impregnated with Ce nitric acid to contain 0.3 mol of O2 and fired in the atmosphere at 650°C for 3 hours, then impregnated with a dinitrodiamino Pt aqueous solution to contain 1.5 g of Pt in the air at 250°C for 3 hours. Calcined. Thereafter, it was impregnated with an aqueous solution of Rh nitric acid so that the amount of Rh contained was 0.3 g, and was dried in the air at 110° C. day and night.

This was compacted into a dice shape of about 2 to 3 mm, and the comparison catalyst No. Obtained C1. The catalyst of this comparative example is a Pt-Rh based three-way catalyst that is commonly used in automobile exhaust catalysts.

Comparative Example 2 Comparative Example Catalyst No. 1 consisting of only Co3O4 was prepared in the same manner as in Example 1 without adding palladium nitrate under the same conditions. Obtained C2.

Comparative Example 3 A γ-A2O3 pellet carrier (2 to 3 mmφ) with a specific surface area of about 140 m2/g was immersed in a palladium nitrate aqueous solution,
γ-A ■ Impregnated so that the amount of Pd was 0.14 wt% based on the weight of 2O3, dried in air at 110°C for 5 hours, and dried for 60 hours.
It was baked in air at 0°C for 3 hours. Approximately 2 to 3 m long of this fired material
Comparative Example Catalyst No. 1, which was compacted into a dice shape of m and made of Pd and A2O3. Obtained C3.

Comparative Example 4 In the method of Comparative Example 3, instead of palladium nitrate,
Pt under the same conditions except that dinitrodiamino Pt was used.
Comparative example catalyst No. consisting of and A2O3. C4 was obtained.

Catalyst activity evaluation This example catalyst No. 1 was diced. 1 to 9 and Comparative Example Catalyst No. Using C1 to C4, the heating purification characteristics of each component (NO, Co, HC) at 500°C to 600°C in a model gas having the stoichiometric air-fuel ratio shown in Table 1 was measured.

Among the results, Example catalyst No. Comparative Example Catalyst No. 1 is shown in FIG. C1 (conventional three-way catalyst for automobile exhaust)
Shown in the figure.

Example catalyst No. 1 is CO, HC, N at 200℃
It is possible to purify 100% of O. However, comparative example catalyst No. C1 has a purification rate that is only about 40 to 80% lower even at 200°C.

In addition, the measurement results for each of the above catalysts were
The results are shown in Table 2, sorted by % purification rate and temperature.

Example catalyst No. 1 is Comparative Example Catalyst No. It can be seen that compared to C1 etc. (comparison between Figures 1 and 2 and comparison in Table 2), it is significantly superior in low temperature activity, especially against HC.

[Brief explanation of the drawing]

FIG. 1 shows Example catalyst No. 1. Figure 2 shows comparative example catalyst No.
．． It is a figure which showed the relationship of the purification rate with respect to temperature regarding C1.

Claims (1)

[Claims] Comprising one or more transition metal oxides and one or more noble metals (excluding gold), hydrocarbons in exhaust gas from internal combustion engines, etc.
An exhaust gas purification catalyst that removes carbon monoxide and nitrogen oxides at temperatures below 250°C.