JPH07213906A - Exhaust gas purifying catalyst - Google Patents

Abstract

(57) [Abstract] [Purpose] The present invention efficiently makes nitrogen oxides discharged from an internal combustion engine such as an automobile, a petroleum stove, or a factory in a wide temperature range of 300 ° C to 600 ° C in an atmosphere of excess oxygen. Provided is a catalyst that can be removed. [Composition] A catalyst for purifying hydrocarbons, carbon monoxide and nitrogen oxides in exhaust gas in an atmosphere of excess oxygen, which is selected from a porous carrier and a transition metal oxide supported on the porous carrier. It is characterized in that it is composed of one or more kinds and one or more kinds selected from precious metals.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hydrocarbons (C) discharged from internal combustion engines such as automobiles, oil stoves or factories.
H), carbon monoxide (CO) and nitrogen oxides (NO _x ).
The present invention relates to a catalyst that efficiently removes R in a wide temperature range of 300 ° C to 600 ° C in an atmosphere of excess oxygen.

[0002]

2. Description of the Related Art Recently, the importance of maintaining the natural environment has been particularly emphasized, and the regulation of exhaust gas from automobiles has tended to be tightened. Therefore, a lean burn engine that burns lean in an oxygen-rich atmosphere is regarded as promising as one means for suppressing the generation of hydrocarbons (HC), carbon monoxide (CO), etc. in the combustion exhaust gas. Since the lean burn improves fuel consumption, the amount of fuel used is reduced, and as a result, generation of carbon dioxide (CO ₂ ) and the like in exhaust gas can be suppressed. However, nitrogen oxides (NO _x ) are present in the oxygen-excessive exhaust gas discharged from the lean burn engine.
Is contained in a large amount. Therefore, a catalyst capable of sufficiently reducing and purifying nitrogen oxides (NO _x ) under an atmosphere of excess oxygen has been extensively developed.

As such a catalyst, a catalyst in which a transition metal such as copper is supported on zeolite by ion exchange (JP-A-1
No. 139145), a catalyst in which a noble metal such as platinum is supported on a porous carrier such as alumina (Japanese Patent Laid-Open No. 16886/1993).
No. 0).

[0004]

[Problems to be Solved by the Invention] However, the catalyst of
There is a problem of poor durability. In addition, in the catalyst of 1, the nitrogen oxide purification start temperature is as low as 200 ° C., and the temperature range in which the nitrogen oxides can be sufficiently purified is 250 to 350.
Since it is as low as ℃, it is not suitable for purifying nitrogen oxides in exhaust gas discharged from internal combustion engines of automobiles.

The present inventors have conducted research to solve the above-mentioned problems and conducted various systematic experiments to complete the present invention. The present inventors have focused on cobalt oxide (Co ₃ O ₄ ) and platinum, which have been conventionally used as oxidation catalysts.
Oxygen is excessively present in the exhaust gas discharged from the lean burn engine, so that when the above-mentioned oxidation catalyst is used, the oxidation reaction further progresses, and the use of the oxidation catalyst reduces nitrogen oxides (NO _x ). It was common sense that a person skilled in the art would not obtain it. The inventors of the present invention dare to use the above-mentioned oxidation catalyst in an atmosphere of excess oxygen to remove nitrogen oxide (NO
_The present invention has been made based on the finding that _x ) can be selectively reduced.

[0006] The present invention is designed to remove hydrocarbons, carbon monoxide and nitrogen oxides discharged from internal combustion engines such as automobiles, petroleum stoves or factories in an atmosphere of excess oxygen.
An object of the present invention is to provide a catalyst that can be efficiently removed in a wide temperature range of ℃ to 600 ℃.

[0007]

The exhaust gas purifying catalyst of the present invention is a catalyst for purifying hydrocarbons, carbon monoxide and nitrogen oxides in exhaust gas in an oxygen-excess atmosphere, and is porous. It comprises a carrier and one or more kinds of transition metal oxides and one or more kinds of noble metals supported on the porous carrier.

[0008]

[Function] According to the present catalyst, in an atmosphere of excess oxygen,
Hydrocarbon (HC) in a wide temperature range of 600 ° C,
Carbon monoxide (CO) and nitrogen oxides (NO _x ) can be efficiently purified. In particular, it has made it possible to purify NO _x , which could not be sufficiently purified by the conventional catalyst.

Although the action of the catalyst is not clear, it is considered as follows. A conventional three-way catalyst adsorbs HC, CO, and NO _x , which are harmful components in exhaust gas, on a precious metal,
It is removed by the reaction shown below. CO + 1 / 2O ₂ → CO ₂ …… NO + H ₂ → 1 / 2N ₂ + H ₂ O …… NO + CO → CO ₂ + 1 / 2N ₂ …… C _n H _m + (n + m / 4) O ₂ → nCO ₂ + m / 2H ₂ O ...

Unlike the conventional three-way catalyst, the catalyst of the present invention has an action of reacting NO _x with HC in addition to CO and H _{2 as shown} in the formula to reduce it to N _2.
Ox reduction reaction was performed in a lean atmosphere with excess oxygen (A / F> 1).
It is characterized in that it has an action of advancing at 4.6, preferably 17 ≦ A / F ≦ 23). (2 n + m / 2) NO + C n H m → (n + m / 4) N 2 + nCO 2 + m / 2 H 2 O ......

Further, the NO _x reduction reaction does not occur with the noble metal alone or the transition metal alone, and appears only when the noble metal and the transition metal coexist. Therefore,
It is desirable that there are many interfaces so that the noble metal and the transition metal interact with each other.

[0012]

According to the exhaust gas purifying catalyst of the present invention,
Hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO _x ) emitted from internal combustion engines such as automobiles, oil stoves, or factories, etc., in a wide temperature range of 300 ° C to 600 ° C in an atmosphere of excess oxygen It can be removed efficiently in the range.

(Description of Other Inventions) The transition metal oxide used in the present invention is Co ₃ O _4, NiO, Fe.
₂ O ₃ , Cr ₂ O ₃ , Mn ₂ O _{3 and the} like can be mentioned, 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 purification activity cannot be exhibited. Preferably 100
Å below, 50m ² / g~200m ² / g is good.

Noble metals are rhodium (Rh), platinum (P
One or more of t) and palladium (Pd) are used. The amount of noble metal is 2 to 20 at% with respect to the metal component of the transition metal oxide.
And If it is 2 at% or less, the purifying activity will not be exhibited, and conversely, if it is 20 at% or more, the amount of noble metals will be too large and they will aggregate with each other, resulting in a decrease in activity. Desirably 2 to 1
0at% is good. It is desirable that the particle size of the noble metal be 500 Å or less.

These transition metal oxides and noble metals are mixed and used. This is because it is necessary that the noble metal and the transition metal oxide are in close contact with each other in order to remove and remove HC and the like on the surface of the noble metal. Further, if the interface where these two substances are in contact with each other is in solid solution, good purification characteristics are exhibited.

Further, the catalyst composed of the transition metal oxide and the noble metal of the present invention progresses the NO _x reduction reaction even in a lean atmosphere with excess oxygen and under a high space velocity (SV = 200000 hr ⁻¹ ). It has a characteristic that makes it attractive.

The catalyst according to the present invention is manufactured by the following method. A conventionally known method, a method of impregnating a nitrate metal salt of a transition metal, an organic metal salt or a metal chloride thermally decomposed with an aqueous solution of a noble metal salt, and hydrolyzing the various metal salts to obtain water. Any of a method of supporting a noble metal after firing an oxide, a method of coprecipitating from a mixed solution of a transition metal salt and a noble metal salt, and the like may be used.

The catalyst composed of a transition metal oxide and a noble metal may have any shape and structure such as powder, pellets and honeycomb. Further, a binder such as alumina sol, silica sol, zirconia sol, titania sol, etc. is added to the powdery catalyst to form it into a predetermined shape, or water is added to form a slurry into a refractory substrate such as alumina in the shape of a honeycomb or the like. You may apply.

[0019]

EXAMPLES to produce a catalyst according to the present invention, using a model gas of oxygen-rich atmosphere for the catalyst, NO _x, CO, HC
The purification activity was evaluated. The same activity evaluation was performed for the comparative catalyst.

Example 1 Cobalt Nitrate / 6 Aqueous Solution 54
g and 0.866 g of palladium nitrate, ion-exchanged water 2
What was dissolved in 1 (atomic ratio Pd: Co = 1: 50) was dripped at room temperature from a separating funnel at a rate of about 0.6 ml / sec into an alkaline aqueous solution prepared by dissolving 50 g of sodium carbonate in 2 l of ion-exchanged water. , Stir. The solution is left for one hour, then filtered or washed and dried under reduced pressure. Then, the obtained powder was fired in air at 350 ° C. for 3 hours. This fired product is pressed into a die shape of about 2 to 3 mm, and P
Example catalyst No. 1 composed of d and Co ₃ O ₄ . Got 1.

(Example 2) In the same manner as in Example 1, except that dinitrodiamino Pt was used instead of palladium nitrate (atomic ratio of Pt to Co was the same as 1:50) and sodium carbonate was used as the alkaline aqueous solution. Is an example catalyst consisting of Pt and Co ₃ O _{4 under the} same conditions. Got 2.

Example 3 In the method of Example 1, except that dinitrodiamino Pt was used instead of palladium nitrate (the atomic ratio of Pt and Co was the same as 1:50), ammonium carbonate was used as the alkaline aqueous solution. Example catalyst No. consisting of Pt and Co ₃ O _{4 under the} same conditions. Got 3.

Example 4 Aqueous cobalt nitrate solution (54 g
/ L) was added dropwise to an aqueous solution of sodium carbonate (50 g / l) and stirred to precipitate cobalt hydroxide, which was then filtered, washed and dried under reduced pressure, and then 350 ° C. in air for 3 days.
The powder obtained by the hr calcination was impregnated with dinitrodiamino Pt at a Pt content of 2 at% and then calcinated in air at 350 ° C. The calcined product was compacted into a die having a diameter of 2 to 3 mm, and the catalyst of Example No. 1 composed of Pt and Co ₃ O ₄ was used. Got 4.

(Example 5) Co ₃ O ₄ was obtained by burning cobalt nitrate at 600 ° C for 5 hours in air and thermally decomposing it. This Co ₃ O ₄ powder was impregnated so that the content of palladium nitrate was 3.5 at%, and then calcined in air at 350 ° C. for 3 hours. The calcined product was compacted into a die having a diameter of 2 to 3 mm, and the catalyst of Example No. 1 containing Pd and Co ₃ O ₄ was used. Got 5.

(Example 6) In Example 5, except that dinitrodiamino Pt was used in place of palladium nitrate, the catalyst No. No. of Pt and Co ₃ O ₄ was used under the same conditions. Got 6.

(Example 7) In Example 5, except that cobalt acetate was used in place of cobalt nitrate, Example catalyst No. ₃ containing Pt and Co ₃ O ₄ was used under the same conditions. Got 7.

(Embodiment 8) An embodiment catalyst No. consisting of Pt and NiO was used under the same conditions except that nickel nitrate was used instead of cobalt nitrate in the method of Embodiment 5. Got 8.

(Embodiment 9) In the method of Embodiment 5, the catalyst No. of the embodiment of Pt and Fe ₂ O ₃ was used under the same conditions except that ferric nitrate was used instead of cobalt nitrate. Got 9.

(Comparative Example 1) In the method of Example 1, palladium nitrate was not added, and Co
Comparative example catalyst consisting of ₃ O ₄ only C1 was obtained.

(Comparative Example 2) A γ-Al ₂ O ₃ pellet carrier ( _{2 to 3} mmφ) having a specific surface area of about 140 m ² / g was immersed in an aqueous dinitrodiamino Pt solution, and the amount of Pt was adjusted with respect to the weight of γ-Al ₂ O _3. Impregnated to 0.14 wt% in air 11
It was dried at 0 ° C. for 5 hours and calcined in air at 600 ° C. for 3 hours. The calcined product was compacted into a dice shape of about 2 to 3 mm, and the catalyst of Comparative Example No. 1 composed of Pt and Al ₂ O ₃ was used. C2 was obtained.

(Evaluation of Catalytic Activity) The catalyst No. of this example in the form of a dice. 1 to 9 and Comparative Example Catalyst No. Using C1 to C2, SV in the model gas in the oxygen excess atmosphere shown in Table 1
= 200,000 hr ^-1 at 100 ° C to 600 ° C, the temperature rising purification characteristics of each component (NO, CO, HC) were measured. Among the results, Example catalyst No. No. 3 is shown in FIG. C1 is shown in FIG. C2 is shown in FIG.

[0032]

[Table 1]

Example catalyst No. 3 is approximately 100% for CO and HC and 30 to 80 for NO at 270 ° C to 600 ° C.
It can be purified with a high purification rate of%. However, the comparative catalyst No. Cl and No. C2 shows only a low value of about 20% in the NO purification rate at the maximum.

The measurement results of each of the above catalysts are shown as NO, CO,
The purification rates of HC at 400 ° C. are summarized and shown in Table 2. The example catalysts show excellent purification characteristics for all NO, CO and HC components.

[0035]

[Table 2]

Further, the catalysts of Examples showed excellent purifying characteristics at 250 ° C. as well as at 400 ° C.

[Brief description of drawings]

FIG. 1 Example catalyst No. It is a figure which shows the relationship of the purification rate with respect to the temperature about 3.

FIG. 2 Comparative catalyst No. It is a figure which shows the relationship of the purification rate with respect to the temperature about C1.

FIG. 3 Comparative example catalyst No. It is a figure which shows the relationship of the purification rate with respect to the temperature about C2.

Claims (1)

[Claims] 1. A catalyst for purifying hydrocarbons, carbon monoxide, and nitrogen oxides in exhaust gas in an oxygen-excess atmosphere, which is a porous carrier and a transition metal oxide supported on the porous carrier. An exhaust gas purifying catalyst comprising one or more kinds selected from among and one or more kinds selected from noble metals.