JPH1033984A - Catalyst for purifying exhaust gas and method for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the durability by suppressing the decrease in durability due to particle growth of Pt by using Pt and Rh and preventing the NOX storage/release ability of an NOX storage material from being lowered. SOLUTION: This catalyst comprises a mixture of a first powder having Rh carried on porous particles and a second powder having Pt and NOX storage material carried on porous particles. At least one of the first and second powders carries at least one element selected from among Co, Fe, and Ni. Since Rh and Pt are carried separately, the oxidation ability of Pt is prevented from being lowered, the NOX storage material is not lowered in performance, and therefore a high NOX purification rate is retained from the initial stage to the duration life. Further, by causing Co and the like to be carried, generation of hydrogen is facilitated, NOX reduction is improved, and poisoning with sulfur can be suppressed.

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 an exhaust gas purifying method for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like. Carbon oxide (C
O), hydrogen (H ₂ ) and hydrocarbons (HC) such as nitrogen oxides (NO _x ) in exhaust gas containing excess oxygen in excess of the amount of oxygen necessary to completely oxidize reducing components. The present invention relates to an exhaust gas purifying catalyst capable of reduction purification and an exhaust gas purifying method.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst has been used as a catalyst for purifying exhaust gas of automobiles, which purifies by simultaneously oxidizing CO and HC and reducing NOx in exhaust gas at a stoichiometric air-fuel ratio (stoichiometric). As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), or the like is formed on the porous carrier layer. What carried the catalyst noble metal is widely known.

On the other hand, 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. As a solution, lean combustion in an oxygen-excess atmosphere has been proposed. 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.

On the other hand, in the conventional three-way catalyst, when the air-fuel ratio is the stoichiometric air-fuel ratio (stoichiometric), CO, H
It purifies by simultaneously oxidizing and reducing C and NO _x, and does not exhibit sufficient purification performance for the reduction and removal of NO _x under an atmosphere of excess oxygen in the exhaust gas during lean burn. Therefore, development of a catalyst and purification system has been desired can purify NO _x even in an oxygen rich atmosphere.

Accordingly, the applicant of the present application has previously described Al and others such as Ba.
Supports potassium earth metal and Pt on porous carriers such as alumina
Exhaust gas purifying catalysts (for example, see JP-A-5-317625)
Gazette). Using this exhaust gas purification catalyst
Air-fuel ratio is pulsed from lean to stoichiometric to rich
Control on the lean side, NO on the lean side
_xIs an alkaline earth metal (NO_xOcclusion material)
This leads to reduction of HC or CO on the stoichiometric or rich side.
Purified by reacting with the minute, even in lean burn
NO_xCan be efficiently purified.

[0006]

[0007] purification reaction of the NO _x in the catalyst for the exhaust gas purification, a first step of the NO _x is oxidized to NO in the exhaust gas, NO on _the NO _x storage material
_It is known that the method comprises a second step of storing _x and a third step of reducing NO _x released from the NO _x storage material on the catalyst.

[0007] However, the conventional exhaust gas purifying catalyst has a disadvantage in that Pt grains grow in a lean atmosphere, and the reactivity of the first and third steps is reduced due to a decrease in catalytic active points. On the other hand, Rh is known as a catalytic noble metal in which such grain growth hardly occurs in a lean atmosphere, but its oxidizing ability is inferior to Pt. Therefore, it has been considered to use Pt and Rh together.

However, when Pt and Rh are used together,
It is clear that there is a problem that the oxidizing ability decreases.
Was. Therefore, as the amount of Rh added increases, NO
Oxidizes NO_xThe reactivity of the first step decreases
NO in the second step _xStorage capacity also decreases.
Rh is NO_xPoor compatibility with occlusion material, Rh and NO_x
NO when coexisting with occlusion material_xThe characteristics of the storage material and Rh are sufficient
There is also a problem that it cannot be fully demonstrated.

Further, the sulfur (S) component contained in the fuel is
Oxidized to SO_TwoWhich is further oxidized on the catalyst.
To produce sulfate. This is NO_xOcclusion material and anti
NO_xNO of occlusion material_xThe occlusion effect disappears,
NO with sulfur poisoning _xReduction purification becomes difficult
There is also a defect. The present invention has been made in view of such circumstances.
The growth of Pt using Pt and Rh
Not only reduces the durability due to_xOcclusion material
NO_xPrevents a decrease in occlusion / release capacity, thereby improving durability
The purpose is to aim at the above.

[0010]

Means for Solving the Problems The characteristics of the exhaust gas purifying catalyst of the present invention for solving the above-mentioned problems, carrying first powder carrying Rh on porous particles, Pt and _the NO _x storage material in the porous particles The second powder is mixed with at least one element selected from Co, Fe and Ni on at least one of the first powder and the second powder.

[0011] Similarly, the present invention solves the above-mentioned problems.
The feature of the gas purification method is that the porous particles support Rh
1 Pt and NO in powder and porous particles_xNo. supporting occlusion material
2 powders mixed together and at least the first powder and the second powder
On the other hand, at least one selected from Co, Fe and Ni
Placing a catalyst carrying one element in the exhaust gas,
NO in lean atmosphere with excess oxygen_xNO for occlusion material_x
Occlusion and temporarily change to stoichiometric to rich atmosphere
NO_xNO released from the storage material _xReduce
And purify it.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention,
Rh is present in the first powder, Pt and _the NO _x storage material is present in the second powder, the first powder and the second powder are mixed.
In other words, Pt and the NO _x storage material are carried in close proximity, and Rh and P
It is separated and carried from t. Therefore, the problem that the oxidizing ability of Pt decreases due to the proximity of Rh is prevented.
In addition, since Pt and the NO _x storage material are carried in close proximity, the first step in which NO in the exhaust gas is oxidized by the Pt to become NO _x, and the second step in which NO _x is stored in the NO _x storage material Is performed smoothly.

Since the first powder and the second powder are in a mixed state, Rh is somewhat close to the second powder even in a separated state. Thus _the NO _x storage NO released from material _x is purified is reduced by Rh. Further, Rh has a significantly smaller grain growth in a lean atmosphere than Pt. Therefore, the durability of ternary activity is improved by the presence of Rh. Further, since Rh is supported separately from the NO _x storage material, mutual incompatibility is eliminated, and the performance of the NO _x storage material and Rh is prevented from lowering.

Further, in the exhaust gas purifying catalyst of the present invention, at least one of the first powder and the second powder contains Co, Fe and N
At least one element selected from i is supported. These elements have a function of accelerating the steam reforming reaction represented by the formula (1) or the water gas shift reaction represented by the formula (2), thereby promoting the generation of hydrogen. The generated hydrogen is converted into N by its strong reducing power.
The O _x reduction, higher _the NO _x reduction performance is maintained even after endurance.

[0015] _{C n H m + n H 2} O → n CO + (n + m / 2) H 2 (1) CO + H 2 O → CO 2 + H 2 (2) were also generated near _the NO _x storage material Hydrogen reduces sulfate taken in the NO _x storage material, so that NO _x
Sulfur poisoning deterioration of the storage material can be suppressed.

In the exhaust gas purifying method of the present invention, HC and CO are oxidized and purified by Pt in a lean atmosphere. NO At the same time, a first step of NO in the exhaust gas is oxidized NO _x by Pt, to _the NO _x storage material
and a second step of storing _x . At this time, Pt
And the NO _x storage material are carried in close proximity to each other, and Rh is carried separately from Pt. Therefore, there is no problem that the oxidizing ability of Pt is reduced by the proximity of Rh, and the first step and the second
The steps are performed smoothly.

[0017] Then By temporarily changed to the stoichiometric-rich atmosphere, NO _x, which have been stored in _the NO _x storage material is released, reacts with HC and CO in the exhaust gas by the catalytic action of Pt and Rh, also By reacting with H ₂ generated by the reaction of the above formulas (1) and (2),
NO _x is reduced and purified. As the porous particles, both the first powder and the second powder can be selected from alumina, silica, zirconia, silica-alumina, zeolite, and the like. One of these may be used, or a plurality of them may be mixed or combined for use. The first heat resistance and the compatibility of Zr with Rh are good.
Preferably, alumina or zirconia is used for the powder, and alumina is used for the second powder.

The particle diameter of the porous particles is preferably in the range of 1 to 100 μm for both the first powder and the second powder. Particle size 1μm
If it is smaller, it is difficult to obtain the effect of separating and supporting Rh and Pt, and if it is larger than 100 μm, the action between the first powder and the second powder becomes small. Also, the particle size of the porous particles,
It is desirable that the first powder and the second powder have substantially the same particle size. If there is a large difference in particle size, small particles are densely packed between large particles, so that Rh and Pt and NOx
This is because the probability that the occlusion material approaches will increase.

The amount of Rh supported in the first powder is preferably in the range of 0.1 to 10 g per 120 g of the porous particles.
If the supported amount of Rh is less than 0.1 g / 120 g, the durability is reduced. If the supported amount is more than 10 g / 120 g, the effect is saturated and the cost is increased. The amount of Pt carried in the second powder was 0.1 g per 120 g of the porous particles.
A range of 1 to 10 g is desirable. 0.1 g of Pt carried
If it is less than / 120 g, the purification rate of HC, CO and NOx will decrease, and if it is carried more than 10 g / 120 g, the effect will be saturated and the cost will increase. In addition, Pd can be carried on the second powder together with Pt. In this case, it is desirable that the total of Pt and Pd be in the range of 0.1 to 10 g per 120 g of the porous particles.

As the NO _x occluding material, at least one element selected from alkali metals, alkaline earth metals and rare earth metals can be used. Examples of the alkali metal include lithium (Li), sodium (Na), potassium (K), and cesium (Cs). Alkaline earth metals refer to Group 2A elements of the periodic table, and include magnesium (Mg), calcium (Ca), strontium (S
r) and barium (Ba). Examples of the rare earth metal include lanthanum (La), cerium (Ce), and praseodymium (Pr).

The amount of the NO _x occluding material carried in the second powder is preferably in the range of 0.05 to 0.5 mol per 120 g of the porous particles. The loading amount of the NO _x storage material is 0.05
If the amount is less than mol / 120 g, the NO _x purification rate decreases,
Even if more than 0.5 mol / 120 g is supported, the effect is saturated. The mixing ratio of the first powder and the second powder is preferably in the range of 0.05: 1 to 1: 1 of the first powder and the second powder in terms of the weight ratio of Rh and Pt or the total of Pt and Pd. When alumina is used for both the first powder and the second powder as the porous particles, the ratio of the first powder to the second powder is preferably 0.1: 1 to 2: 1 in terms of the weight ratio of alumina. If the ratio is out of these ranges, the same problem as in the case of excess or deficiency of Rh and Pt described above may occur.

The amount of the element selected from Co, Fe and Ni is preferably in the range of 0.01 to 2.0 mol per 120 g of the porous particles, singly or in the case of plural kinds. The supported amount of this element is 0.01 mol / 12
When the amount is less than 0 g, the effect of supporting is not exhibited, and 2.0 mol /
Even if the amount is more than 120 g, the effect may be saturated and the effect of the noble metal may be reduced.

The elements selected from Co, Fe and Ni are:
It can be carried on both the porous particles of the first powder and the porous particles of the second powder. Further, it may be carried on both the first powder and the second powder. The porous particles supporting an element selected from Co, Fe and Ni are further provided with Si.
It is also preferred to carry a co-catalyst consisting of at least one of Mg and Mg. By supporting the co-catalyst, an effect of accelerating the hydrogen generation reaction is added. The amount of the co-catalyst to be carried is 0.005 to 120 g of the porous particles.
Desirably, the range is 2.0 mol. If the supported amount of the cocatalyst is less than 0.005 mol / 120 g, the effect of supporting is not exhibited, and if the supported amount is more than 2.0 mol / 120 g, the effect is saturated.

Exhaust gas purification from a mixture of the first powder and the second powder
The mixture is pelletized by a conventional method to form a catalyst for
Into a pellet catalyst. Also mix
Whether the main component slurry is cordierite or metal foil
Coated with a honeycomb support made of
You can also. The purification method of the present invention according to claim 2.
In the oxygen-rich lean atmosphere, the Pt of the second powder
NO is oxidized by NO_xAnd Pt and proximity carrying
NO _xNO for occlusion material_xIs quickly absorbed. This
Here, Pt is separated and supported from Rh, so that Pt is oxidized.
Function is prevented from being hindered, and NO_xTona
You. Also NO_xThe storage material is separated from Rh
And NO_xA decrease in storage capacity is prevented. Therefore
NO_xIs NO_xSmoothly absorbed by the storage material and released to the outside
Has been prevented. HC and CO in the exhaust gas are P
Reaction with excess oxygen present by catalysis of t and Rh
It is easily oxidized and purified.

Then, in a stoichiometric-rich atmosphere,
NO_xNO from occlusion material_xIs released and the released NO_x
Is HC and C in exhaust gas by the catalytic action of Pt and Rh.
Element that reacts with O and is selected from Co, Fe and Ni
Promotes the reactions of equations (1) and (2)
H_TwoAnd reduced to N_TwoIt is purified.
At this time, grain growth occurred in Pt, and the reducing ability was reduced.
Even if Rh is NO _xNO because of its excellent ability to reduce NO
_xIs reduced and purified smoothly.

[0026]

The present invention will be specifically described below with reference to examples and comparative examples. (Example 1) <Preparation of first powder> An alumina powder having an average particle diameter of 5 µm was impregnated with a predetermined amount of a rhodium nitrate aqueous solution having a predetermined concentration.
After drying at 0 ° C. for 3 hours, it was baked at 250 ° C. for 2 hours to carry Rh. The supported amount of Rh is 0.3 g per 120 g of alumina powder.

<Preparation of Second Powder> A predetermined amount of barium acetate aqueous solution having a predetermined concentration was impregnated into alumina powder having an average particle diameter of 5 μm, dried at 110 ° C. for 5 hours, and calcined at 500 ° C. for 2 hours to carry Ba. . The supported amount of Ba is as follows:
0.3 mol per 20 g. Next, the Ba-supported alumina powder obtained above was impregnated with 1.2 L of an aqueous solution of ammonium bicarbonate having a concentration of 15 g / L, and dried at 110 ° C. for 5 hours. As a result, Ba became barium carbonate and was uniformly supported on the alumina powder.

This Ba / alumina powder is impregnated with a predetermined amount of an aqueous solution of dinitrodiammineplatinic nitric acid having a predetermined concentration,
After drying at 110 ° C for 2 hours, baking at 400 ° C for 2 hours
Was carried. The supported amount of Pt is 2.0 g per 120 g of alumina powder. Thus, a second powder was prepared. <Support of transition metal> The first powder and the second powder are mixed in equal amounts to form a mixed powder.
Calcination was carried out at 2 ° C. for 2 hours to carry Co. The supported amount of Co is
0.1 mol per 120 g of the mixed powder.

<Preparation of Catalyst> The obtained powder was pelletized by a conventional method to prepare a catalyst of Example 1. FIG. 1 is a schematic structural explanatory view of this pellet catalyst. <Evaluation Test> The obtained pellet catalyst was placed in an evaluation test apparatus, and the model gas shown in Table 1 was passed therethrough. In other words, the rich model gas and the lean model gas are flowed under the condition of 2 liter / min at the inlet gas temperature of 350 ° C., respectively.
At that time, the NO concentration in the catalyst-containing gas and the N
From the difference in the O concentration, the amount of stored and reduced NO was measured for each catalyst. Table 3 shows the results.

[0030]

[Table 1] Further, while switching the endurance model gas shown in Table 2 between rich 1 minute and lean 4 minutes, 8
An endurance test was conducted for a long time. The sulfur poisoning amount (S poisoning amount) was determined from the elemental quantitative analysis of the catalyst after durability. The NO storage amount and the reduction amount of the catalyst after the durability test were measured in the same manner as described above, and the results are shown in Table 3.

[0031]

[Table 2] (Example 2) A first powder similar to that of Example 1 and a second powder prepared in the same manner as in Example 1 except that a mixed aqueous solution of barium acetate and cobalt nitrate was used instead of the aqueous barium acetate solution. And a mixed powder was prepared in the same manner as in Example 1. And the pellet catalyst of Example 2 was prepared using this mixed powder as it was. In addition, B of the second powder
The supported amounts of a and Co are 0.3 mol and 0.1 mol, respectively, per 120 g of alumina powder.

With respect to the obtained pellet catalyst of Example 2, the NO occlusion amount, the reduction amount, and the sulfur poisoning amount at the initial stage and after the endurance were measured in the same manner as in Example 1, and the results are shown in Table 3. Example 3 Alumina powder having an average particle size of 5 μm was impregnated with a predetermined amount of an aqueous solution of cobalt nitrate having a predetermined concentration, dried at 110 ° C. for 5 hours, and fired at 500 ° C. for 2 hours. The supported amount of Co is 0.1 mol per 120 g of alumina powder. This Co-supported alumina powder is impregnated with a predetermined amount of a rhodium nitrate aqueous solution having a predetermined concentration, dried at 110 ° C. for 3 hours, and then dried.
It was baked at 0 ° C. for 2 hours to prepare a first powder.

A mixed powder was prepared in the same manner as in Example 1 by using the first powder and the second powder as in Example 1. And the pellet catalyst of Example 3 was prepared using this mixed powder as it was. With respect to the obtained pellet catalyst of Example 3, the NO storage amount, the reduction amount, and the sulfur poisoning amount at the initial stage and after the endurance were measured in the same manner as in Example 1, and the results are shown in Table 3.

(Comparative Example) A predetermined amount of a barium acetate aqueous solution having a predetermined concentration was impregnated into alumina powder having an average particle size of 5 μm.
After drying at 110 ° C. for 3 hours, baking at 500 ° C. for 2 hours and Ba
Was carried. The supported amount of Ba is 0.3 mol per 120 g of alumina powder. Next, the Ba-supported alumina powder obtained above was impregnated with an aqueous solution of ammonium bicarbonate having a concentration of 15 g / L, and dried at 110 ° C. for 3 hours. As a result, Ba became barium carbonate and was uniformly supported on the alumina powder.

This Ba / alumina powder is impregnated with a predetermined amount of an aqueous solution of dinitrodiammineplatinic nitric acid having a predetermined concentration,
After drying at 110 ° C for 3 hours, drying at 250 ° C for 2 hours, Pt
Was carried. The supported amount of Pt is 2.0 g per 120 g of alumina powder. Next, the obtained Pt-supported Ba / alumina powder was impregnated with a predetermined amount of a rhodium nitrate aqueous solution having a predetermined concentration, dried at 110 ° C. for 3 hours, and calcined at 250 ° C. for 2 hours to carry Rh. The supported amount of Rh is 0.3 g per 120 g of alumina powder.

The catalyst powder obtained by a conventional method was pelletized to prepare a pellet catalyst of Comparative Example. With respect to the obtained pellet catalyst of the comparative example, the NO occlusion amount, the reduction amount, and the sulfur poisoning amount at the initial stage and after the durability were measured in the same manner as in Example 1, and the results are shown in Table 3. (Evaluation)

[0037]

[Table 3] As shown in Table 3, each catalyst of the example has an improved NO occlusion amount and an NO reduction amount after the endurance as compared with the comparative example, and the improvement of the NO reduction amount is particularly large. It can also be seen that the sulfur poisoning of the catalyst of the example was suppressed as compared with the comparative example.

That is, it is clear that the exhaust gas purifying catalyst of this embodiment shows a high NO purifying rate both at the initial stage and after the endurance. In the above embodiment, the case of Co was described. However, it should be noted that the same result was obtained when Fe or Ni was used alone or in combination of plural kinds instead of Co.

Although a pellet catalyst has been described in the above embodiment, a monolithic catalyst in which a coat layer mainly composed of a mixed powder of the first powder and the second powder is formed on a honeycomb carrier made of cordierite or metal foil may also be used. Needless to say, the same operation and effects as those of the above-mentioned pellet catalyst are exhibited.

[0040]

According to the exhaust gas purifying catalyst of the present invention, high NO _x purification performance is exhibited both at the initial stage and after the endurance, and the sulfur poisoning of the NO _x occluding material is suppressed. doing. According to the exhaust gas purifying method of the present invention, the generation of the NO _x due to oxidation of NO, N of the NO _x
The storage in the O _x storage material and the reduction of the NO _x released from the NO _x storage material proceed smoothly, and high NO _x purification performance can be secured from the initial stage to the end of the durability.

[Brief description of the drawings]

FIG. 1 is a schematic structural explanatory view of an exhaust gas purifying catalyst according to one embodiment of the present invention.

Claims (2)

[Claims] 1. A first powder in which Rh is supported on porous particles and a second powder in which Pt and a NO _x storage material are supported on porous particles are mixed, and the first powder and the second powder are mixed. Characterized in that at least one of them carries at least one element selected from Co, Fe and Ni. Wherein the first powder and the porous first powder becomes a second and a powder mixture obtained by carrying Pt and _the NO _x storage material particles and the second powder carrying the Rh on porous particles of at least Meanwhile Co, the catalyst of at least one element selected from Fe and Ni is supported is disposed in the exhaust gas, the oxygen occluding NO _x in the _the NO _x storage material in excess lean atmosphere, temporarily exhaust gas purification method, characterized in that purifying by reducing the NO _x released from the _the NO _x storage material by changing the stoichiometric-rich atmosphere.