JPH1176819A - Catalyst for cleaning of exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst which is excellent in cleaning performance of nitrogen oxides under an excess oxygen atmosphere (lean atmosphere) of exhaust gas. SOLUTION: This catalyst removes nitrogen oxides from combustion exhaust gas containing nitrogen oxides and more oxygen than a theoretical reaction amount to a coexisting unburned constituent. In this case, the catalyst is provided wherein rhodium and palladium are contained, respective one parts are combined, a content ratio of rhodium: palladium is 1 : 0.1 to 0.5, and total content of rhodium and palladium is 15 to 200 g/cf. In order to manufacture such the catalyst, rhodium and palladium powders which are supported with a catalyst support are lastly burnt containing reducing gas and/or steam at 400 deg.C or higher.

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 and a method for producing the same, and more particularly to a stoichiometric atmosphere (hereinafter referred to as stoichiometric atmosphere).
It referred to as "stoichiometric atmosphere") excess oxygen atmosphere after passing (hereinafter, "lean atmosphere" hereinafter) nitrogen oxides under (hereinafter referred to as NO _x) an exhaust gas purifying catalyst and a manufacturing method thereof excellent in purification performance.

[0002]

2. Description of the Related Art Conventionally, as a catalyst for purifying exhaust gas discharged from an internal combustion engine such as an automobile, a noble metal such as platinum (Pt), palladium (Pd) and rhodium (Rh) is supported on alumina or cerium oxide. A monolithic carrier coated with this is used.
The catalyst for mainly primary purpose of improving the exhaust gas purification performance in the stoichiometric atmosphere, when used as a NO _x removal in the lean atmosphere, sufficient performance is not obtained.

Recently, a catalyst to improve _the NO _x purification performance in the lean atmosphere, have been reported. Examples include zeolite catalysts supporting alkali metals and / or alkaline earth metals, zeolite catalysts supporting cobalt and copper ion exchanged zeolites.

[0004]

For purifying catalyst used INVENTION to be solved challenges] lean atmosphere as described above, has already many proposals have been made, NO _x removal rate is not yet sufficient. Therefore, the development of a catalyst capable of improving the NO _x removal rate has been a major problem.

[0005]

SUMMARY OF THE INVENTION The present invention has been completed as a result of intensive studies to solve the above-mentioned problems, and the above-mentioned object is achieved by the present invention described below. That is, the present invention is an exhaust gas purifying catalyst for removing nitrogen oxides from a combustion exhaust gas containing a larger amount of oxygen than a theoretical reaction amount with respect to unburned components coexisting with nitrogen oxides, wherein the catalyst contains rhodium and palladium. And a part of each is composited, the content ratio of rhodium: palladium is 1: 0.1 to 1: 0.5, and the total content of rhodium and palladium is 15 to 200 g / cf. It is characterized by.

The present invention also relates to a method for producing an exhaust gas catalyst for removing nitrogen oxides from a combustion exhaust gas containing a larger amount of oxygen than a theoretical reaction amount of unburned components coexisting with the nitrogen oxides. Steps to (5): (1) Rhodium to palladium ion (1: 0.1 to
(A molar ratio of 0.5) with an aqueous solution containing a porous carrier, and (2) drying the obtained slurry to prepare a metal powder containing rhodium and palladium. And (3) a step of preparing a slurry by mixing the metal powder obtained in the step (2) with an inorganic ore sol; and (4) applying the slurry obtained in the step (3) to a catalyst carrier and drying the slurry. (5) obtaining the rhodium / palladium powder supported on the catalyst carrier by calcining the obtained rhodium / palladium powder on the catalyst carrier in a reducing gas and / or steam atmosphere at 400 ° C. or higher; The content ratio of palladium to 1: 0.1-0.5, and the total content of rhodium and palladium is 15-200 g.
The step of obtaining a rhodium-palladium composite catalyst of / cf is sequentially performed.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The exhaust gas purifying catalyst according to the present invention contains rhodium and palladium, and a part of each of them is combined. The composite in the present invention means a state in which rhodium atoms and palladium atoms are mutually dissolved (dispersed), that is, alloying. The content ratio of rhodium: palladium is 1: 0.1 to 1:
0.5, and the total content of rhodium and palladium is 15 to 200 g / cf. If the rhodium: palladium content ratio is lower than 1: 0.1, the degree of composite is small and the NO _x purification ability is extremely low. On the other hand, 1: 0.5
If it exceeds, the reaction is dominated by the properties of one of the metals, so that the NO _x purification ability is not improved. Further, even within the above range, the total content of rhodium and palladium is 1
If it is less than 5 g / cf, the number of complexed activities is too small,
By the reaction is dominated by large metal properties relatively content exceeds 200g / cf, NO _x purification performance is not improved.

The carrier carrying rhodium and palladium is
Since the effect of increasing the contact area of the exhaust gas can be obtained, it is desirable to support the oxide or non-oxide carrier having a high specific surface area. The carrier may be any component as long as it satisfies the condition that rhodium and palladium can be stably supported for a long time.

As such a carrier component, for example, in addition to commonly used γ-alumina, a porous carrier of an alumina-based oxide such as silica-alumina and β-alumina is used. Further, a cocatalyst component or an alumina-based oxide such as lanthanum oxide can be contained as a component effective for stabilization. In addition, as a non-oxide, silicon nitride, aluminum nitride, silicon carbide, titanium nitride, and the like are stable for a long time, and thus can be preferably used as a carrier.

In the method of combining rhodium and palladium in the present invention, a salt (for example, nitrate) or an organic metal (for example, metal alkoxide) of rhodium and palladium is allowed to coexist in a solvent at the impregnation step, and this solution is then used. It is desirable to impregnate the carrier. Various methods such as an immersion method, an ion exchange method, and an alkoxide method can be used as the impregnation method. In the firing step, any atmosphere may be used in the preliminary firing, but in the final firing, the composite is fired by containing a reducing gas and / or steam at 400 ° C. or higher. A high temperature of 400 ° C. or higher is preferable because the compounding speed is high, but a temperature of 400 ° C. or lower is not preferable because the acid (eg, nitric acid) used in the preparation is difficult to decompose and hinders the compounding.

As the reducing gas, hydrocarbon, carbon monoxide, hydrogen, nitrogen, engine exhaust gas and the like can be used. Further, since the rate of compounding is further accelerated by using steam as a mixed gas, it is preferable to use the mixed gas in addition to the reducing gas.

For the catalyst of the present invention and the obtained catalyst of the present invention, the degree of rhodium and palladium complexation can be measured by X-ray diffraction. According to the X-ray diffraction, the lattice constant a was 3.820 to 3.875. When the lattice constant a is smaller than 3.820 or larger than 3.875, it is presumed that the composite of rhodium and palladium is not sufficient, and that rhodium or palladium alone has properties.

Although the shape of the catalyst carrier in the present invention is not particularly limited, it is usually preferable to use it in a honeycomb shape, and various honeycomb substrates are coated with a catalyst powder.

As the honeycomb material, for example, ceramics such as cordierite, mullite, alumina, magnesia, and spinel can be used, and cordierite-based materials are preferably used. On the other hand, for example, a honeycomb made of a metal material such as ferrite steel can be used, and the catalyst powder itself may be formed into a honeycomb shape. By making the shape of the catalyst into a honeycomb shape, the contact area between the catalyst and the exhaust gas is increased and the pressure loss is suppressed, so that it is extremely effective when used for automobiles.

The operation and effect of the present invention will be described below. To purify nitrogen oxides in an oxidizing atmosphere, first,
It is important to suppress the oxygen adsorption to the metal, which is the stage, and to facilitate the adsorption of nitrogen oxides. Rhodium alone is easier to adsorb oxygen than nitrogen oxides. It is considered that by using palladium, which consumes oxygen at a lower temperature than rhodium, to remove oxygen adsorbed on rhodium, nitrogen oxides are more likely to be adsorbed on rhodium and NO _x conversion is improved. However, the lattice constant a by X-ray diffraction must be 3.820 to 3.875. Palladium and rhodium do not achieve the above-mentioned effects merely by coexistence, but are alloyed by a production method described later, and the above-mentioned effects can be obtained unless their positions are close to each other at the atomic level. Absent.

In order to make this complex, the production method must finally contain a reducing gas and / or steam at 400 ° C. or higher. Since palladium and rhodium easily form independent oxides in an oxidizing gas, oxygen on the surface of the metal is removed using a reducing gas and the metal is turned into a metal state, so that palladium and rhodium are alloyed by the applied heat. It is. The firing temperature of 400 ° C. or higher is a temperature at which nitric acid and the like are easily decomposed, and it is considered that alloying is promoted because elements that hinder alloying are removed. Further, when steam is used, the NO _x purification ability is high, but it is considered that the decomposition of the acid (for example, nitric acid or the like) used in the catalyst preparation is assisted by the steam.

Hereinafter, the present invention will be described with reference to Examples and Comparative Examples.

[0018]

EXAMPLE 1 Commercially available γ-alumina powder (BET surface area: about 200 m ² /
g) (1000 g), a nitric acid solution of rhodium nitrate containing 8.5% by weight of Rh (30 ml), a nitric acid solution of dinitrodiaminopalladium containing 8.5% by weight of Pd (38 ml), and water were mixed and impregnated. . Then dried, 400-45
The powder was obtained by firing at a temperature of 0 degrees. This powder is mixed with nitric acid (3
0 ml) and boehmite sol were mixed to form a slurry, coated on a cordierite honeycomb (0.12 L, 400 cells), dried, and calcined at 400 ° C for 30 minutes in air.
Next, as a final calcination, the mixture was kept at 400 ° C. for 5 hours in a nitrogen gas stream containing 1% of propylene to obtain a catalyst A. The contents of palladium and rhodium in Catalyst A thus obtained are 15 g / cf palladium and 12 g / cf rhodium per 1 cf (= 28.3 L) of catalyst. Marble,
The lattice constant a of the obtained catalyst A was 3.820 as shown in Table 1 below.

Example 2 The content of palladium and rhodium in Example 1 was
25 g / c of palladium per f (= 28.3 L)
Catalyst B was obtained in the same manner as in Example 1 except that f and rhodium were set to 120 g / cf and the final firing step was maintained at 800 ° C. for 5 hours in a nitrogen stream. The lattice constant a of the obtained catalyst B was 3.823 as shown in Table 1 below.

Example 3 The content of palladium and rhodium in Example 1 was measured using catalyst 1c.
60 g / c of palladium per f (= 28.3 L)
Catalyst C was obtained in the same manner as in Example 1 except that f and rhodium were set to 120 g / cf and the final firing step was maintained at 400 ° C. for 1 hour with exhaust gas from a stoichiometric engine. The lattice constant a of the obtained catalyst C was 3.861 as shown in Table 1 below.
Met.

Example 4 The content of palladium and rhodium in Example 1 was measured using catalyst 1c.
10 g / c of palladium per f (= 28.3 L)
Catalyst D was obtained in the same manner as in Example 1 except that f and rhodium were set to 60 g / cf, and the final firing step was maintained at 400 ° C. for 1 hour with stoichiometric engine exhaust gas. The lattice constant a of the obtained catalyst D was 3.819 as shown in Table 1 below.

Example 5 The content of palladium and rhodium of Example 1 was measured using catalyst 1c.
20 g / c of palladium per f (= 28.3 L)
Catalyst E was obtained in the same manner as in Example 1 except that f and rhodium were 60 g / cf and the final calcination step was maintained at 800 ° C. for 5 hours in a nitrogen stream containing 10% of steam. The lattice constant a of the obtained catalyst E was 3.8 as shown in Table 1 below.
32.

EXAMPLE 6 The content of palladium and rhodium of Example 1 was
40 g / c of palladium per f (= 28.3 L)
Catalyst F was obtained in the same manner as in Example 1 except that f and rhodium were 60 g / cf, and the final calcination step was maintained at 800 ° C. for 5 hours in a nitrogen stream containing 10% of steam. The lattice constant a of the obtained catalyst F was 3.8 as shown in Table 1 below.
57.

Example 7 The content of palladium and rhodium in Example 1 was measured using catalyst 1c.
5 g / cf palladium per f (= 28.3 L),
Catalyst G was obtained in the same manner as in Example 1 except that the rhodium content was 10 g / cf and the final firing step was maintained at 400 ° C. for 1 hour with exhaust gas from a stoichiometric engine. Catalyst G obtained
Was 3.826 as shown in Table 1 below.

Comparative Example 1 The content of palladium and rhodium of Example 1 was
60 g / c of palladium per f (= 28.3 L)
f, rhodium and 0 g / cf, the final baking step, except that it has held for 5 hours at 800 ° C. in a stream of nitrogen containing 10% steam in the same manner as in Example 1 to obtain a catalyst b. Lattice constant a of the resulting catalyst i, as shown in the following Table 1 3.89
It was 0.

Comparative Example 2 The content of palladium and rhodium of Example 1 was
60 g / c of palladium per f (= 28.3 L)
A catalyst b was obtained in the same manner as in Example 1 except that f and rhodium were 5 g / cf and the final baking step was maintained at 800 ° C. for 5 hours in a nitrogen stream containing 10% of steam. The lattice constant a of the obtained catalyst b was 3.88 as shown in Table 1 below.
Nine.

Comparative Example 3 The content of palladium and rhodium of Example 1 was
0 g / cf palladium per f (= 28.3 L),
A catalyst C was obtained in the same manner as in Example 1, except that the rhodium content was 120 g / cf and the final firing step was maintained at 400 ° C. for 1 hour with exhaust gas from a stoichiometric engine. The resulting catalyst
The lattice constant a of C was 3.803 as shown in Table 1 below.

COMPARATIVE EXAMPLE 4 The content of palladium and rhodium in Example 1 was
5 g / cf palladium per f (= 28.3 L),
A catalyst d was obtained in the same manner as in Example 1 except that the rhodium content was 60 g / cf and the final firing step was maintained at 400 ° C. for 1 hour with exhaust gas from a stoichiometric engine. The lattice constant a of the obtained catalyst D was 3.807 as shown in Table 1 below.

Comparative Example 5 The content of palladium and rhodium in Example 1 was
5 g / cf palladium per f (= 28.3 L),
Example 1 except that the rhodium content was 120 g / cf and the final baking step was maintained at 800 ° C. for 5 hours in a nitrogen stream.
Catalyst e was obtained in the same manner as described above. The lattice constant a of the obtained catalyst E was 3.803 as shown in Table 1 below.

The catalysts obtained in the above Examples and Comparative Examples were evaluated for catalytic activity under the following conditions. In the activity evaluation, propylene and propane were reacted with nitrogen oxide using a model gas simulating exhaust gas from an automobile, and an automatic evaluation device equipped with a chemiluminescent nitrogen oxide analyzer was used. In addition, the L value used here is an oxidizing gas (N
O, O ₂ ) and reducing gas (CO, C ₃ H ₆ , C ₃ H ₈ )
And is defined by the following equation.

(Equation 1)

Activity Test Conditions Catalyst 0.05 L honeycomb coated catalyst Total gas flow rate 50 L / min Catalyst inlet gas temperature 250-350 ° C. Inlet gas composition Model gas composition equivalent to average air-fuel ratio 21.0 (L = 10.3) HC 2500 ppm _{C (C 3 H 6 + C} 3 H 8) NO 500ppm O 2 4.00% CO 2 10.0% H 2 O 10.0% N 2 balance A / F without amplitude

The catalyst activity evaluation value was determined by the following equation.

(Equation 2) Table 1 shows the obtained catalyst activity evaluation results.

[0033]

[Table 1]

For some of the catalysts used in Examples and Comparative Examples, the lattice constant a was calculated from X-ray diffraction measurement. The device used was RAD-B manufactured by Rigaku Denki, the X-ray source was Cu, and the acceleration voltage and current were 50 kv and 180 mA, respectively.
The value of the lattice constant a is shown together with the evaluation results of the catalyst activity in Table 1. The lattice constant a was obtained by calculating d (plane interval) from the measured value 2θ of the (111) plane as in Equation 1, and converting this d into a size per unit cell. In Expression 1, l represents the length per side of the unit cell of the crystal.

[0035]

(Equation 3)

The above-mentioned catalysts A to G have a rhodium: palladium content ratio of 1: 0.1 to 0.5, a total content of rhodium and palladium of 15 to 200 g / cf, and a final production method. a is obtained by sintering comprise 400 ° C. or more reducing gases and / or steam, a high NO _x conversion than the comparative catalyst b ~ e is outside this range. In particular, the NO _x conversion at high temperatures is high. Such an effect of the present invention is brought about by the composite of a part of each of rhodium and palladium constituting the catalyst of the present invention. Further, the above catalysts A to G are:
The extent of conjugation of rhodium and palladium, can be measured by X-ray diffraction, the lattice constant a is from 3.820 to 3.875, a high NO _x conversion than the comparative catalyst b ~ e is outside this range.

[0037]

As described above, the catalyst of the present invention contains rhodium and palladium, and a part of each of them is complexed, and the content ratio of rhodium: palladium is 1: 0.
1 to 0.5, and the total content of rhodium and palladium is 15 to 200 g / cf. The method for producing a catalyst according to the present invention is characterized in that the catalyst is finally calcined with a reducing gas and / or steam at 400 ° C. or higher. Since the catalyst of the present invention and the method of the present invention are configured as described above, nitrogen oxides are extremely efficiently removed from combustion exhaust gas containing nitrogen oxides and oxygen in excess of the theoretical reaction amount for unburned components coexisting. This has the effect of doing so.

Claims (8)

[Claims] 1. An exhaust gas purifying catalyst for removing nitrogen oxides from a combustion exhaust gas containing a larger amount of oxygen than a theoretical reaction amount of unburned components coexisting with the nitrogen oxides, the catalyst comprising rhodium and palladium. And a part of each is complexed, the rhodium: barium content ratio is 1: 0.1 to 1: 0.5, and the total content of rhodium and palladium is 15 to 200 g / cf. The exhaust gas purifying catalyst described above. 2. An exhaust gas purifying catalyst comprising the catalyst according to claim 1 provided as a coat layer on a catalyst carrier. 3. The exhaust gas purifying catalyst according to claim 2, wherein the catalyst carrier is a honeycomb monolith carrier substrate. 4. A crystal lattice a measured by X-ray diffraction,
The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the catalyst has a particle size of 3.820 to 3.875. 5. A method for producing an exhaust gas catalyst for removing nitrogen oxides from a combustion exhaust gas containing a larger amount of oxygen than a theoretical reaction amount of unburned components coexisting with the nitrogen oxides, comprising the following (1) to ( Step 5): (1) Rhodium to palladium ion (1: 0.1 to 0.1)
(A molar ratio of 0.5) with an aqueous solution containing a porous carrier, and (2) drying the obtained slurry to prepare a metal powder containing rhodium and palladium. And (3) a step of preparing a slurry by mixing the metal powder obtained in the step (2) with an inorganic ore sol; and (4) applying the slurry obtained in the step (3) to a catalyst carrier and drying the slurry. (5) sintering the obtained rhodium / palladium powder supported on the catalyst carrier in a reducing gas and / or steam atmosphere at 400 ° C. or higher to obtain rhodium / palladium powder; The content ratio of palladium to 1: 0.1-0.5, and the total content of rhodium and palladium is 15-200 g.
Wherein the step of obtaining a rhodium-palladium composite catalyst of / cf is sequentially performed. 6. After the step (4) and before the step (5), the rhodium-palladium powder supported on the dried catalyst carrier is calcined in air at 400 ° C. for 30 minutes. The manufacturing method as described. 7. The method according to claim 4, wherein the catalyst carrier is a honeycomb monolith carrier substrate. 8. The method according to claim 1, wherein the reducing gas is at least one selected from the group consisting of hydrocarbons, hydrogen, nitrogen, and engine exhaust gas.
6. The method of claim 4 or 5, which is a species.