JPH10356A - Catalyst of purification of exhaust gas and method for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the degradation in durability by the grain growth of Pt, to prevent the degradation in the NOx occluding and releasing capability of a NOx occluding material and to improve durability by allowing first power deposited with Rh on porous particles and second powder deposited with Pt and the NOx occluding material on the porous particles to coexist. SOLUTION: The catalyst for purification of exhaust gases which purifies the exhaust gases discharged from an automobile, etc., is produced by allowing the first power deposited with the Rh on the porous particles and the second powder deposited with the Pt and the NOx occluding material on the porous particles to coexist. At the time of purification of the exhaust gases, the catalyst is arranged in the exhaust gases to occlude NOx in the NOx occluding material in an oxygen excessive lean atmosphere. This atmosphere is temporarily changed from the stoichiometric to rich atmosphere, by which the NOx released from the NOx occluding material is reduced and the gases are purified. The grain sizes of the porous particles are preferably in the range of 1 to 100μm for both of the first and second powders. The amt. of the Rh deposited on the first powder is preferably in a range of 0.1 to 10g per 120g porous particles.

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.

[0009] The present invention has been made in view of such circumstances, and to suppress the deterioration of the durability due to the grain growth of Pt with Pt and Rh, NO _x storage material of the NO _x
An object of the present invention is to prevent a decrease in occlusion / release ability and thereby improve durability.

[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 And the mixed second powder. Similarly characteristic of the exhaust gas purifying method of the present invention for solving the above problems, and a second powder carrying Pt and _the NO _x storage material in the first powder and the porous particles carrying Rh on porous particles mixed comprising Te catalyst was placed in the exhaust gas, oxygen occludes NO _x in _the NO _x storage material in excess lean atmosphere, the NO _x released from _the NO _x storage material by temporarily varying the stoichiometric-rich atmosphere It is to purify by reduction.

[0011]

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 method of the present invention, HC and CO are oxidized and purified by Pt in a lean atmosphere. N 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 O _x . At this time, P
t and the _the NO _x storage material is close supported, Rh because it is carried separately from the Pt, there is no trouble such as oxidizing ability of Pt is lowered by the proximity of the Rh, the first step and second step It is performed smoothly.

[0014] Then By temporarily changed to the stoichiometric-rich atmosphere, NO _x, which have been stored in _the NO _x storage material is released, by reacting with HC and CO in the exhaust gas by the catalytic action of Pt and Rh , NO _x is reduced and purified, and HC and CO are oxidized and purified. As the porous particles, both the first powder and the second powder are made of alumina,
It can be selected from 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. Note that it is preferable to use alumina or zirconia for the first powder and to use alumina for the second powder because of the heat resistance and the good compatibility of Zr with Rh.

The particle size 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.

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 (first powder: second powder) in terms of the weight ratio of Rh and Pt. In addition, the first powder and the second powder are used as porous particles.
When alumina is used as the powder, the first powder: the second powder is preferably in the range of 0.1: 1 to 2: 1 in terms of the weight ratio of alumina. Outside of these ranges, R
In some cases, the same problem as in the case where h and Pt are excessive or insufficient may occur.

In the first powder, it is preferable that the porous particles carry a transition metal selected from Fe, Ni and Co together with Rh. The transition metal causes a water gas shift reaction between CO and water in the exhaust gas, and has a special effect that NO _x is reduced by the generated hydrogen gas. Therefore, the NO _x purification rate is further improved.

This transition metal can be supported on the porous particles of the second powder, but Rh particles are added to the porous particles of the first powder.
It is preferred to carry it together. The amount of the supported transition metal is preferably in the range of 0.01 to 0.5 mol per 120 g of the porous particles. The amount of the transition metal supported is 0.
If the amount is less than 01 mol / 120 g, the effect of supporting is not exhibited. If the amount is more than 0.5 mol / 120 g, the effect is saturated and the effect of the noble metal is reduced.

It is preferred that the porous particles carrying the transition metal further carry a co-catalyst comprising at least one of Si and Mg. By supporting the co-catalyst, an effect of accelerating the hydrogen generation reaction is added. The amount of the cocatalyst carried is desirably in the range of 0.01 to 0.5 mol per 120 g of the porous particles. If the supported amount of the cocatalyst is less than 0.01 mol / 120 g, the effect of supporting is not exhibited, and if the supported amount is more than 0.5 mol / 120 g, the effect is saturated.

The exhaust gas is purified from the 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.

In a stoichiometric-rich atmosphere,
NO _x from _the NO _x storage material is released, the released NO _x
Is HC and C in exhaust gas by the catalytic action of Pt and Rh.
It reacts with O to become N ₂ and is reduced and purified. At this time,
Even if Pt undergoes grain growth and the reducing ability is reduced,
Since Rh is excellent in NO _x reducing ability, NO _x is reduced and purified smoothly.

[0024]

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. Four types of first powders were prepared by selecting four levels of Rh loading from 0.1 g, 0.5 g, 1.0 g and 2.0 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 3 hours, and calcined at 500 ° C. for 2 hours to carry Ba. . The supported amount of Ba is as follows:
0.4 mol per 20 g. 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 a dinitrodiammineplatinum nitric acid aqueous solution 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. Thus, a second powder was prepared. <Preparation of Catalyst> Each of the first powder and the second powder was uniformly mixed in an equal weight ratio and pelletized by a conventional method to prepare four types of pellet catalysts. FIG. 1 is a schematic structural explanatory view of this pellet catalyst.

<Evaluation Test> Each of the obtained pellet catalysts was placed in an evaluation test apparatus, and the model gas shown in Table 1 was passed therethrough. That is, the rich model gas and the lean model gas are alternately flown at a rate of 2 liters / min every two minutes at an inlet gas temperature of 350 ° C., and the NO concentration in the catalyst-containing gas and the NO concentration in the catalyst-out gas at that time. , The transient purification rate of NO was measured for each catalyst. The results are shown in FIG.

[0028]

[Table 1] In addition, while changing the durability model gas shown in Table 2 between rich 1 minute and lean 4 minutes, 1 minute at an inlet gas temperature of 800 ° C.
An endurance test was performed for 0 hours. Thereafter, the transient NO purification rate was measured in the same manner as above, and the results are shown in FIG.

[0029]

[Table 2] (Example 2) Four kinds of first powders were prepared in the same manner as in Example 1 except that zirconia powder having an average particle size of 5 µm was used instead of alumina powder. And it mixed with the 2nd powder similar to Example 1, and prepared 4 types of pellet catalysts similarly.

2.0 g of the obtained pellet catalyst of Example 2
The NO purification rates at the initial stage and after the endurance were measured in the same manner as in Example 1, and the results are shown in FIG. 2 and FIG. (Comparative Example) FIG. 4 is a schematic structural explanatory view of an exhaust gas purifying catalyst of this comparative example. An alumina powder having an average particle size of 5 μm is impregnated with a predetermined amount of a barium acetate aqueous solution having a predetermined concentration.
After drying at 0 ° C. for 3 hours, it was baked at 500 ° C. for 2 hours to carry Ba. The amount of Ba supported is 0.2 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 a dinitrodiammineplatinum nitric acid aqueous solution having a predetermined concentration,
And dried at 250 ° C. for 2 hours to carry Pt. The amount of Pt supported was 1.0 per 120 g of alumina powder.
g.

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 alumina powder 12
0.05 g, 0.25 g, 0.5 g and 1.0 g per 0 g.
Four levels of 0 g were selected to prepare four types of catalyst powder. Then, each catalyst powder is pelletized by a standard method,
Various types of pellet catalysts were prepared. With respect to the obtained pellet catalyst of the comparative example, the NO purification rates at the initial stage and after the durability were measured in the same manner as in Example 1, and the results are shown in FIGS. 2 and 3.

(Evaluation) From FIG. 2 and FIG.
While the NO purification rate decreases as the supported amount of h increases, the NO purification rate increases in Examples 1 and 2 as the supported amount of Rh increases. This is apparently due to the effect of separating Rh from Pt and supporting it.

It can also be seen that zirconia powder as the porous particles of the first powder has a slightly higher NO purification rate than alumina powder. And although the difference is smaller in FIG. 3, the above tendency is the same in both FIG. 2 and FIG.
It is clear that the exhaust gas purifying catalyst of this embodiment shows a high NO purification rate both at the initial stage and after the endurance.

Although the pellet catalyst has been described in the above embodiment, a monolith 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.

[0036]

Effects of the Invention] That is, according to the exhaust gas purifying catalyst of the present invention, the initial showed both high _the NO _x purification performance after the durability test, has high durability. Further, according to the exhaust gas purifying method of the present invention, the generation of NO _x by the oxidation of NO and the NO _x
It can be the NO adsorption to _x storage material, and NO _x reduction and NO _x released from the storage material proceeds smoothly, to ensure high NO _x purification performance from the initial to after 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.

FIG. 2 is a graph showing a relationship between a Rh carrying amount and an initial NO purification rate.

FIG. 3 is a graph showing the relationship between the amount of Rh carried and the NO purification rate after durability.

FIG. 4 is a schematic enlarged explanatory view of an exhaust gas purifying catalyst of a comparative example.

Claims (2)

[Claims] 1. A first powder carrying the Rh on porous particles and porous particles Pt and _the NO _x storage material supported by the second powder and the mixed exhaust gas purification catalyst characterized by comprising a. 2. A first powder comprising Rh supported on porous particles.
Pt and NO on porous particles_xA second powder carrying an occluding material;
Is placed in the exhaust gas to reduce the excess oxygen.
NO in the atmosphere_xNO for occlusion material_xOcclude,
By temporarily changing to a stoichiometric-rich atmosphere
NO_xNO released from the storage material _xTo reduce and purify
Exhaust gas purification method characterized by performing.