JPH1157477A - Exhaust gas cleaning catalyst and method of using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst and method of using the same, having an enhanced NOx cleaning capacity under a lean atmosphere in which catalysts of the prior arts cannot demonstrate sufficient activity and fully exhibiting a function as a tertiary catalyst. SOLUTION: At least two kinds of catalysts are provided against an exhaust gas steam, and the two kinds of the catalysts contain at least one kind of noble metal selected from the group consisting of platinum, palladium and rhodium and a complex oxide represented by the formula: (La1-x Ax )1-α BOδ (wherein 0<x<1, 0<α<0.2, and δ is oxygen amount which satisfies valence of each atom, A = barium or potassium, B = at least one kind selected from the group consisting of iron, cobalt, nickel and manganese) and the catalyst disposed in a front step with regard to an exhaust gas stream contains a larger amount of the composite oxide than a catalyst disposed in a rear step.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hydrocarbons (H) contained in exhaust gas discharged from an internal combustion engine such as an automobile and a boiler.
C), carbon monoxide (CO) and nitrogen oxides (NOx)
The present invention relates to an exhaust gas purifying catalyst for purifying NOx and a method of using the same, and more particularly to an exhaust gas purifying catalyst excellent in NOx purifying performance in an oxygen-excess atmosphere and a method of using the same.

[0002]

2. Description of the Related Art In recent years, in view of the problem of depletion of petroleum resources and the problem of global warming, realization of low fuel consumption vehicles is expected, and development of lean burn vehicles is particularly desired for gasoline vehicles. . In lean-burn vehicles, the exhaust gas atmosphere during lean-burn operation is based on the theoretical air-fuel condition
An oxygen-excess atmosphere (hereinafter, referred to as a “lean atmosphere”) as compared to “stoichiometric state”. When a conventional three-way catalyst is applied in a lean atmosphere, there has been a problem that the effect of excessive oxygen makes the NOx purification action insufficient. For this reason, NOx
There has been a demand for the development of a catalyst that can purify methane.

Conventionally, NOx in a lean atmosphere has been
Various catalysts for improving the purification performance have been proposed. For example, JP-A-5-168860 discloses a catalyst in which lanthanum or the like is supported on platinum (Pt) and lanthanum is used as a NOx absorbent. This is to absorb NOx in a lean atmosphere and to release and purify NOx in a stoichiometric or fuel-rich (rich) atmosphere.

However, the conventional NOx absorption catalyst (for example, a Pt-lanthanum catalyst) has a problem in that, when the vehicle is steadily driven in a lean atmosphere, the amount of absorbed NOx reaches saturation and the absorption action eventually disappears. , NOx purification performance is insufficient, the performance after durability is not sufficient, and NOx cannot be purified under a wide range of operating conditions.

[0005]

SUMMARY OF THE INVENTION
The object of the invention described is to improve the NOx purification performance in a lean atmosphere, which has not exhibited sufficient activity with a conventional catalyst, and to achieve exhaust gas purification capable of fully exhibiting a function as a three-way catalyst. To provide a catalyst for use.

It is another object of the present invention to provide a method of using an exhaust gas purifying catalyst capable of particularly effectively exhibiting its NOx purifying action of the exhaust gas purifying catalyst of the present invention.

[0007]

According to a first aspect of the present invention, there is provided a catalyst for purifying exhaust gas, wherein at least two kinds of catalysts are provided for an exhaust gas flow, each of the two kinds of catalysts comprising platinum, palladium and rhodium. At least one precious metal selected from the following general formula

(Equation 2) Wherein the catalyst disposed upstream of the exhaust flow contains more of the composite oxide than the catalyst disposed downstream.

According to a second aspect of the present invention, there is provided a catalyst for purifying exhaust gas, wherein the catalyst disposed in the first stage with respect to the exhaust flow contains the composite oxide in an amount of 30 to 80 g per liter of the catalyst, and the catalyst disposed in the second stage has the composite oxide. 2 to 40 g per 1 L of the catalyst.

According to a third aspect of the present invention, the exhaust gas purifying catalyst is characterized in that at least one selected from the group consisting of platinum, palladium and rhodium is carried on the composite oxide.

Further, in order to exhibit an effective NOx absorption / release cycle of the exhaust gas purifying catalyst of the present invention,
According to a fourth aspect of the present invention, there is provided the exhaust gas purifying catalyst of the present invention, wherein the exhaust gas purifying catalyst has an air-fuel ratio of 10 to 14.8.
And a range of 15 to 50.

[0011]

DETAILED DESCRIPTION OF THE INVENTION As the noble metal in the exhaust gas purifying catalyst of the present invention, at least one selected from the group consisting of platinum, palladium and rhodium is used. For example, Pt
And Rh, and various combinations such as Pd and Rh and Pd alone are possible. The content of the noble metal is not particularly limited as long as the NOx absorption capacity and the three-way catalyst performance are sufficiently obtained.
If the amount is less than 0.1 g, sufficient ternary performance cannot be obtained, and
g of the exhaust gas purifying catalyst of the present invention.
0 g is preferred.

The composite oxide contained in the exhaust gas purifying catalyst of the present invention has the following general formula:

(Equation 3) It is represented by

The composite oxide used in the exhaust gas purifying catalyst of the present invention contains a rare earth metal, an alkali metal and / or an alkaline earth metal, and at least one transition metal. Lanthanum is preferably used as the rare earth metal, potassium is used as the alkali metal, barium is used as the alkaline earth metal, and iron, cobalt, nickel and manganese can be suitably used as the transition metal.

A composite oxide such as the above-described perovskite-type oxide has characteristics of causing oxygen deficiency, easily adsorbing NOx through the generated oxygen deficiency, and absorbing NOx in a lean atmosphere. Utilization makes it possible to improve NOx purification performance.

Further, the perovskite-type oxide may cause a solid-phase reaction with the alumina-based oxide in the catalyst composition to deactivate the activity. In order to suppress this, there are a method of pre-coating lanthanum or the like on the alumina-based oxide and a method of using a material having low reactivity with perovskite such as zirconia. On the other hand, by slightly losing the A site of the perovskite-type oxide from the stoichiometric ratio as in the present invention, the solid-phase reaction between the perovskite-type oxide and another oxide (alumina or the like) in contact with the perovskite-type oxide To suppress
It has become possible to improve the thermal stability.

The substitution amount of the A site is 0 <x <1 and is not particularly limited. However, in order to obtain a sufficient NOx absorption capacity, it is particularly preferable that 0.2 ≦ x <1.

If the value of α exceeds 0.2, a single-phase perovskite structure is not formed, so that 0 <α <0.2 is preferable. The value of δ is the amount of oxygen that satisfies the valence of each atom, and is approximately 0 <δ <4.

The composite oxide used in the present invention, particularly the partially substituted perovskite oxide, exhibits the ability to absorb NOx in a lean atmosphere together with the partially substituted amount thereof. It is considered that NOx is oxidized to NO ₂ on the composite oxide and is absorbed in the vicinity of barium and / or potassium on the surface of the composite oxide in the form of a nitric acid group or a state close thereto. Therefore, the composition of the composite oxide for effectively absorbing NOx under a lean atmosphere is as follows:
Nitrate containing barium and / or potassium may be easily manufactured, and further, it is important to contain a transition metal element capable of oxidizing NOx to NO _2.

Each of the constituent elements of the composite oxide exerts the above-mentioned effects to the maximum when all of these elements contained in the catalyst are complexed, but at least a part thereof forms a complex. Even if it is possible, the above effect can be sufficiently obtained. Each constituent element of the composite oxide can exist as a composite oxide without being separated as a separate oxide even after thermal endurance, and this can be confirmed by, for example, X-ray diffraction measurement.

In the exhaust gas purifying catalyst of the present invention, by coexisting the noble metal and the composite oxide, it is possible to obtain a NOx purifying action that cannot be obtained by each alone. That is, when the exhaust gas atmosphere becomes lean, high NOx purification performance can be obtained by the NOx absorbing action of the composite oxide in the exhaust gas purification catalyst of the present invention. When the composite oxide absorbs NOx and the exhaust gas atmosphere changes from lean to stoichiometric,
Ox is released, and high NOx purification performance is obtained. It is intended to obtain excellent NOx purification performance which cannot be obtained by simply mixing individual components of the composite oxide.

In order to smoothly carry out such an absorption and purification cycle, at least two types of catalysts are provided in the exhaust gas flow, and the catalyst arranged at the former stage with respect to the exhaust gas is more effective than the catalyst arranged at the latter stage with the composite oxide. It is necessary to contain a large amount. This is because the amount of reducing gas (HC, CO, H ₂ ) that both catalysts can contact at the time of stoichiometry is larger in the catalyst arranged in the front stage with respect to the exhaust gas flow, and less in the catalyst arranged in the latter stage with respect to the exhaust gas flow. This is because an appropriate value is generated in the NOx absorption capacity that allows the absorption and release cycle.

Further, the catalyst of the present invention has a high NOx absorption function even after heat endurance. This is because the composite oxide has a perovskite structure having a small proportion of A site, and has a high NOx absorption. This is because the solid-phase reaction was avoided.

In particular, the amount of the composite oxide contained in the catalyst disposed upstream of the exhaust gas flow is 3 per liter of the catalyst.
It is preferable that the amount of the composite oxide contained in the catalyst disposed in the subsequent stage be 0 to 80 g and 2 to 40 g per 1 L of the catalyst. When the amount of the composite oxide is smaller than the above, the NOx absorption function is not sufficient for any of the catalysts at the former stage and the latter stage with respect to the exhaust gas flow. Not.

Further, it is preferable that at least one selected from the group consisting of platinum, palladium and rhodium is carried on the composite oxide. As a result, high NOx absorption capacity can be obtained even after heat durability. This is because even after the heat endurance, the mutual contact is kept dense, and the smooth progress of the NOx absorption / release cycle can be maintained.

The exhaust gas purifying catalyst of the present invention can be used especially for lean burn engine vehicles in which the air-fuel ratio repeatedly ranges from 10 to 14.8 and from 15 to 50. With such a method of use, the cycle of NOx absorption / release is extremely effectively established, and particularly efficient NOx purification becomes possible.

The composite oxide used in the present invention is prepared by mixing nitrates, acetates or carbonates of the respective constituent elements of the composite oxide in a desired composition ratio of the composite oxide, calcining the mixture, and pulverizing the mixture. After the solid-phase reaction to be subjected to heat treatment and firing, nitrate, acetate or carbonate, hydrochloride, citrate, etc. of each constituent element of the composite oxide are mixed in a desired composition ratio of the composite oxide, and then dissolved in water. If necessary, an alkaline solution such as NH ₄ OH or NH ₃ CO ₃ may be added dropwise to form a precipitate, which can be prepared by a known method such as a coprecipitation method in which the precipitate is filtered, dried, and fired. By such a method, at least a part of each component constituting the composite oxide can be composited.

The raw material for preparing the catalyst used in the present invention may contain a trace amount of impurities as long as the above-mentioned action is not impaired. For example, strontium contained in barium, lanthanum, neodymium contained in cerium, And samarium.

The catalyst of the present invention is preferably used by being supported on a monolithic carrier. The catalyst of the present invention is pulverized into a slurry, coated on the catalyst carrier, and calcined at a temperature of 400 to 900 ° C. Thus, the exhaust gas purifying catalyst of the present invention can be obtained.

The catalyst carrier can be appropriately selected from known catalyst carriers, and includes, for example, a honeycomb carrier and a metal carrier having a monolith structure made of a refractory material. Although the shape of the catalyst carrier is not particularly limited, it is usually preferable to use a honeycomb shape. As the honeycomb material, cordierite materials such as ceramics are generally used.
It is also possible to use a honeycomb made of a metal material such as a ferritic stainless steel, and further, the catalyst powder itself may be formed into a honeycomb shape. By making the shape of the catalyst into a honeycomb shape, the area of the catalyst and the exhaust gas becomes large, and the pressure loss can be suppressed.

Since the catalyst of the present invention also needs to function as a three-way catalyst during stoichiometry, at least one selected from the group consisting of Pt, Pd and Rh is at least partially supported on a heat-resistant carrier. It is particularly preferable that it is supported on alumina. The alumina used here is preferably one having high heat resistance, and particularly, having a specific surface area of 50%.
Activated alumina ~300m ² / g are preferred. In order to improve the heat resistance of alumina, a rare earth compound such as cerium or lanthanum or an additive such as zirconium may be further added as conventionally used in a three-way catalyst.

Further, since the catalyst used in the present invention also needs to function as a three-way catalyst during stoichiometry, additives conventionally used in three-way catalysts may be further added. H to ceria and precious metals
Barium, which alleviates C adsorption poisoning, and zirconia, which contributes to improving the heat resistance of Rh.

[0032]

The present invention will be described below with reference to the following examples and comparative examples. Example 1 Activated alumina powder was impregnated with a palladium nitrate solution, dried and calcined at 400 ° C. for 1 hour to obtain a Pd alumina powder (powder A). The concentration of the powder APd was 4.0% by weight.

Citric acid was added to a mixture of lanthanum carbonate, barium carbonate and cobalt carbonate, dried and calcined at 700 ° C. to obtain powder B. This powder B had a metal atom ratio of lanthanum / barium / cobalt = 3/6/10.

The powder A was 420 g and the powder B was 18
0 g, 300 g of activated alumina powder and 900 g of water were charged into a magnetic ball mill, and mixed and pulverized to obtain a slurry liquid.
This slurry liquid was attached to a cordierite-based monolithic carrier (1.0 L, 400 cells), excess slurry in the cells was removed by an air stream, dried at 130 ° C., and calcined at 400 ° C. for 1 hour. Coating layer weight 150g /
L-support catalyst 1 was obtained. The content of the powder B in the catalyst 1 was 30 g / L. 400 g of powder A, 60 g of powder B, 420 g of activated alumina powder, 900 g of water
Was put into a magnetic ball mill, mixed and pulverized to obtain a slurry liquid. This slurry liquid is adhered to a cordierite monolith carrier (1.0 L, 400 cells), excess slurry in the cells is removed by an air stream, dried at 130 ° C., baked at 400 ° C. for 1 hour, and coated. Layer weight 150g /
Catalyst 2 of L-support was obtained. The content of the powder B in the catalyst 2 was 10 g / L. With respect to the exhaust gas flow, the catalyst 1 was arranged at the front stage, and the catalyst 2 was arranged at the rear stage.

Example 2 A catalyst 3 was obtained in the same manner as in Example 1 except that the content of the powder B was changed to 70 g / L. The catalyst 3
At the front stage and the catalyst 1 at the rear stage.

Example 3 Catalysts 4 and 5 were obtained in the same manner as in Example 1 except that iron in the powder B was replaced with iron. The amounts of the composite oxide were 30 g / L and 10 g / L, respectively. With respect to the exhaust gas flow, the catalyst 4 was arranged at the front stage and the catalyst 5 was arranged at the rear stage.

Example 4 Example 1 was repeated except that cobalt in powder B was changed to nickel.
Catalysts 6 and 7 were obtained in the same manner as described above. The amounts of the composite oxide were 30 g / L and 10 g / L, respectively. The catalyst 6 was arranged at the front stage and the catalyst 7 was arranged at the rear stage with respect to the exhaust gas flow.

Example 5 Example 1 was repeated except that cobalt in powder B was changed to manganese.
Catalysts 8 and 9 were obtained in the same manner as described above. The amounts of the composite oxide were 30 g / L and 10 g / L, respectively. The catalyst 8 was arranged at the front stage and the catalyst 9 was arranged at the rear stage with respect to the exhaust gas flow.

Example 6 The powder B obtained in Example 1 was impregnated with an aqueous solution of palladium nitrate, dried and calcined at 400 ° C. for 1 hour to obtain a palladium-supported composite oxide powder (powder C). The palladium concentration of this powder C was 4.0% by weight.

108 g of the powder A obtained in Example 1, 180 g of the powder C, 480 g of the activated alumina powder, and 900 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry liquid was adhered to a cordierite-based monolithic carrier (1.0 L, 400 cells), excess slurry in the cells was removed by air flow, dried at 130 ° C., and calcined at 400 ° C. for 1 hour. Coat layer weight 1
Catalyst 10 of 50 g / L-support was obtained. The content of the powder C in the catalyst 10 was 30 g / L.

108 g of the powder A obtained in Example 1, 60 g of the powder C, 480 g of the activated alumina powder and 9 g of water
00g was charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry liquid was adhered to a cordierite-based monolithic carrier (1.0 L, 400 cells), excess slurry in the cells was removed by air flow, dried at 130 ° C., and calcined at 400 ° C. for 1 hour. Coat layer weight 15
0 g / L-support of catalyst 11 was obtained. Powder C in catalyst 11
Was 10 g / L. The catalyst 10 was arranged at the front stage and the catalyst 11 was arranged at the rear stage with respect to the exhaust flow.

Example 7 Activated alumina powder was impregnated with an aqueous rhodium nitrate solution, dried and calcined at 400 ° C. for 1 hour in air to obtain a rhodium-supported alumina powder (powder D). The rhodium concentration of this powder D was 2.0% by weight.

240 g of powder A obtained in Example 1, 180 g of powder C obtained in Example 6, and 84 g of powder D were obtained.
g, activated alumina powder (396 g) and water (900 g) were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry liquid was used as a cordierite monolithic carrier (1.
(0 L, 400 cells), and the excess slurry in the cells was removed by an air stream and dried at 130 ° C.
The mixture was calcined at 0 ° C. for 1 hour to obtain 150 g / L of a coat layer and a catalyst 12 of a carrier. The content of powder C in this is 30 g
/ L.

360 g of powder A obtained in Example 1, 60 g of powder C obtained in Example 6, and 84 g of powder D were obtained.
g, activated alumina powder (396 g) and water (900 g) were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry liquid. This slurry liquid was used as a cordierite monolithic carrier (1.
(0 L, 400 cells), and the excess slurry in the cells was removed by an air stream and dried at 130 ° C.
The mixture was calcined at 0 ° C. for 1 hour to obtain a catalyst 13 having a coat layer weight of 150 g / L-carrier. The content of the powder C in the catalyst 13 is 10
g / L. The catalyst 12 was arranged at the front stage and the catalyst 13 was arranged at the rear stage with respect to the exhaust gas flow.

Example 8 Catalysts 14 and 15 were obtained in the same manner as in Example 7, except that Pd was replaced with Pt in Powder A and Powder C. The catalyst 14 was arranged at the front stage and the catalyst 13 was arranged at the rear stage with respect to the exhaust gas flow.

Example 9 The amount of Pt per liter of catalyst was 2.8 g and the amount of Pd was 2.8.
Catalysts 15 and 10 g were prepared in the same manner as in Example 7, except that the amounts of g and Rh were changed to 0.3 g. With respect to the exhaust gas flow, the catalyst 16 was arranged before the catalyst 15 and the catalyst 16 was arranged after.

Example 10 The lanthanum / barium / cobalt ratio in Example 1 was changed to 4
Catalyst 17 was prepared in the same manner as in Example 1, except that the ratio was changed to /4.5/10. The catalyst 17 was arranged at the front stage and the catalyst 2 was arranged at the rear stage with respect to the exhaust gas flow.

COMPARATIVE EXAMPLE 1 The catalyst 2 was arranged at the front stage and the catalyst 1 was arranged at the latter stage with respect to the exhaust gas flow.

COMPARATIVE EXAMPLE 2 The catalyst 1 was arranged at the front stage and the catalyst 3 was arranged at the rear stage with respect to the exhaust gas flow.

Table 1 shows the catalyst compositions of the exhaust gas purifying catalysts obtained in Examples 1 to 10 and Comparative Examples 1 and 2.

[0051]

[Table 1]

Test Examples The exhaust gas purifying catalysts obtained in Examples 1 to 10 and Comparative Examples 1 and 2 were evaluated for initial and endurance catalytic activities under the following conditions. For the activity evaluation, an automatic evaluation device using a model gas simulating the exhaust gas of an automobile was used.

Endurance conditions A catalyst was attached to the exhaust system of a 4400 cc engine, and operation was performed at a catalyst inlet temperature of 700 ° C. for 50 hours to endurance.

Evaluation Conditions The catalyst activity was evaluated by mounting each catalyst in an exhaust system of an engine with a displacement of 2000 cc, A / F = 14.6 (stoichiometric state) for 30 seconds, and then A / F = 22 (lean atmosphere).
For 30 seconds, then 3 at A / F = 50 (lean atmosphere)
One cycle of operation for 0 seconds was performed, and the average conversion was measured. The average conversion when A / F = 14.6 (stoichiometric state) and the average conversion when A / F = 22 (lean atmosphere). And the average conversion in the case of A / F = 50 (lean atmosphere) was averaged to obtain the total conversion. This evaluation was performed at the initial stage and after the endurance test, and the catalytic activity evaluation value was determined by the following equation. However, the catalyst inlet temperature of the former stage is 400 ° C.
And

[0055]

(Equation 4)

Table 2 shows the catalytic activity evaluation results obtained as the total conversion. The catalytic activity of the example was higher than that of the comparative example, and the effect of the present invention described later could be confirmed.

[0057]

[Table 2]

[0058]

The exhaust gas purifying catalyst according to claims 1 and 2 improves the NOx purifying performance in a lean atmosphere, which has not exhibited sufficient activity with the conventional catalyst, and has a sufficient function as a three-way catalyst. And can exhibit excellent NOx purification performance even after heat endurance.

The exhaust gas purifying catalyst according to the third aspect can obtain higher heat durability in addition to the above effects.

The exhaust gas purifying catalyst according to the fourth aspect is capable of exhibiting the effective NOx absorption / release cycle of the exhaust gas purifying catalyst of the present invention particularly efficiently.

Claims (4)

[Claims] 1. An exhaust stream provided with at least two catalysts, each of which comprises at least one noble metal selected from the group consisting of platinum, palladium and rhodium, and the following general formula: 1) A catalyst for exhaust gas purification, comprising a composite oxide represented by the formula: wherein the catalyst disposed upstream of the exhaust stream contains more of the composite oxide than the catalyst disposed downstream. . 2. The catalyst disposed at the front stage with respect to the exhaust gas flow contains 30 to 80 g of the composite oxide per 1 L of the catalyst, and the catalyst disposed at the rear stage contains 2 to 40 g of the composite oxide per 1 L of the catalyst. The exhaust gas purifying catalyst according to claim 1, characterized in that: 3. The exhaust gas purifying catalyst according to claim 1, wherein at least one selected from the group consisting of platinum, palladium and rhodium is carried on the composite oxide. 4. An exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the air-fuel ratio is 10 to 14.8 and 15 to 5
A method for using an exhaust gas purifying catalyst, which is used for a lean burn engine vehicle that repeats a range of 0.