JP2001224961A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a NOX occluding/reducing catalyst having high NOX cleaning capacity when exhaust gas has a small amount of HC content (a large amount of CO content). SOLUTION: This NOX occluding/reducing catalyst for cleaning exhaust gas contains furthermore Pt/TiO2 powder obtained by depositing platinum on titania. Since Pt/TiO2 is extremely high in water gas shift reaction activity, H2 high in reduction activity is generated from CO and H2O in the exhaust gas to reduce NOX efficiently and restrain a NOX occluding material from being contaminated by sulfur.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a catalyst for purifying exhaust gas for purifying exhaust gas of an automobile, and more particularly to a catalyst for purifying an exhaust gas of the NO _x storage reduction.

[0002]

2. Description of the Related Art In recent years, global warming due to carbon dioxide has become a problem, and reducing carbon dioxide emissions has become an issue. In automobiles, reduction of the amount of carbon dioxide in exhaust gas has become an issue, and lean burn engines have been developed in which fuel is burned lean in an oxygen-excess atmosphere. According to this lean burn engine, the emission of carbon dioxide can be suppressed by improving fuel efficiency.

[0003] In this lean burn engine, a system in which combustion is always carried out under an oxygen-excess fuel lean condition, and intermittent fuel stoichiometric to rich conditions are used to reduce and purify nitrogen oxides (NO _x ) using exhaust gas as a reducing atmosphere. Has been developed and put into practical use. And as the best catalyst for this system, it absorbs NO _x in a fuel-lean atmosphere,
NO _x storage-and-reduction type exhaust gas purifying catalyst of the occluded NO _x with _the NO _x storage material that releases fuel stoichiometric-rich atmosphere has been developed.

For example, Japanese Patent Application Laid-Open No. Hei 5-317652 proposes an exhaust gas purifying catalyst in which an alkaline earth metal such as Ba and Pt are supported on a porous oxide carrier such as γ-Al ₂ O ₃ . Also, JP-A-6-31139 discloses that an alkali metal such as K is used.
An exhaust gas purifying catalyst in which Pt is supported on a porous oxide carrier such as γ-Al ₂ O ₃ has been proposed. Further, Japanese Patent Application Laid-Open No. 5-168860 proposes an exhaust gas purifying catalyst in which a rare earth element such as La and Pt are supported on a porous oxide carrier such as γ-Al ₂ O ₃ .

[0005] By using this NO _x storage-and-reduction type catalyst, by controlling the air-fuel ratio so that the fuel stoichiometric-rich side from the fuel-lean side in a pulsed manner, the stoichiometric-rich atmosphere exhaust gas from a lean atmosphere in pulses Becomes Therefore, on the lean side, NO _x is stored in the NO _x storage material,
It is released on the stoichiometric-rich side and purified by reacting with reducing components such as hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gas in large amounts. can efficiently purify NO _x even. The HC and CO in the exhaust gas, since it is consumed in the reduction of the NO _x while being oxidized by the noble metal, HC
And CO is also purified efficiently. Controlling the air-fuel ratio so as to provide a stoichiometric to rich atmosphere in a pulsed manner as described above is called "introducing a rich spike".

[0006] in JP-A-10-356 is, NO _x storage reduction catalysts comprising Rh / ZrO ₂ powder carrying rhodium on zirconia have been disclosed. According to this catalyst, a steam reforming reaction of the following formula (1) occurs on the Rh / ZrO ₂ powder during a rich spike.

[0007] HC + H_TwoO → CO_Two + H_Two (1) Generated H_TwoHas a higher reducing activity than HC and CO, so NO
_x NO released from the storage material_x Is efficiently reduced. I
Therefore Rh / ZrO_TwoNO containing powder_x Using an occlusion reduction type catalyst
NO_x The purification rate is further improved. Also exhaust gas
SO contained in _x Is NO_x Sulfate generated by reaction with occlusion material
Also H_TwoNO is efficiently reduced by_x NO for occlusion material
_x NO after storage and recovery_x Suppress reduction in purification rate
Can be controlled.

[0008]

By the way, HC and CO are contained in exhaust gas at the time of rich spike, but the ratio varies depending on conditions. Also generally, the included HC
The amount is often smaller than the amount of CO. However, when the NO _x storage reduction catalyst containing Rh / ZrO ₂ powder is used, there is no problem when the amount of HC contained in the exhaust gas is large, but when the amount of HC is small, the reaction of the above formula (1) causes There is a problem that the amount of H ₂ generated is also small, and the purification rate of NO _x is low.

[0009] The present invention has been made in view of such circumstances, and when the amount of HC contained in exhaust gas is small (CO
It is an object of the present invention to provide a NO _x storage-reduction type catalyst having a high NO _x purification ability in the case of a large amount of NO _x .

[0010]

Features of the exhaust gas purifying catalyst of the present invention to solve the above problems SUMMARY OF THE INVENTION becomes more a carrier consisting of porous oxide, a noble metal and NO _x storage material that is supported on a carrier NO _x An occlusion reduction type exhaust gas purifying catalyst is to further include a Pt / TiO ₂ powder obtained by supporting platinum on titania.

The amount of platinum supported on the Pt / TiO ₂ powder is 1/1/3 of the total amount of noble metal supported on the entire catalyst.
It is desirable that the number be 5 or more.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention,
It further includes a Pt / TiO ₂ powder obtained by supporting platinum on titania. According to the study by the inventors, a water gas shift reaction represented by the formula (2) occurs on Pt / TiO ₂ , and Pt / TiO ₂
TiO ₂ was found to have a very high reaction activity.

[0013] CO + H_TwoO → CO_Two + H_Two (2) Pt / TiO_TwoExhaust gas purification of the present invention further comprising powder
(2) occurs during rich spike
H_TwoIs generated. And in the exhaust gas at the time of rich spike
Since a large amount of CO is used in the reaction of equation (2),
H to form_TwoThe amount is large. Therefore, a large amount of H_TwoBy
Tte NO_x Is reduced efficiently, so NO _x Improved purification rate
I do. NO sulfated by sulfur poisoning_x Occlusion material
H_TwoIs reduced by the original NO_x Storage capacity is restored
NO after endurance_x The purification rate is improved.

Further, since the water gas shift reaction of the formula (2) occurs from a low temperature range, a large amount of H2 is generated from the low temperature range.
Since the reaction between H ₂ and NO _x also occurs in the low temperature range, the NO _x purification rate in the low temperature range is significantly improved.

As the porous oxide constituting the carrier, one or more of Al ₂ O ₃ , ZrO ₂ , TiO ₂ , SiO ₂ , SiO ₂ -Al ₂ O ₃ , zeolite and the like can be used as in the prior art. Can be.
Al ₂ O ₃ having high heat resistance is particularly preferred. In addition, when an acidic carrier such as TiO ₂ is used, sulfur oxides in exhaust gas are hardly adsorbed, so that the sulfur poisoning of the NO _x storage material can be further suppressed. However, since TiO ₂ alone has a problem that the purification activity is low from the initial stage, it is preferable to use it together with Al ₂ O ₃ or the like. Further, it is preferable that the carrier contains an oxygen storage / release material such as CeO ₂ or CeO ₂ —ZrO ₂ solid solution. Thereby, the purification performance is further improved.

As the noble metal supported on the carrier, Pt, R
One or more of h, Pd, Ir and Ru can be used. As the NO _x storage material, at least one selected from alkali metals, alkaline earth metals, and rare earth elements can be used. Above all, high alkalinity and NO _x
It is preferable to use at least one of an alkali metal and an alkaline earth metal having a high storage capacity.

Examples of the alkali metal include Li, Na, K and Cs. The alkaline earth metal refers to an element in Group 2A of the periodic table, and examples include Ba, Be, Mg, Ca, and Sr. Examples of the rare earth element include Sc, Y, La, Ce, Pr, Nd, Dy, and Yb.

The amount of the noble metal carried on the carrier is preferably 0.1 to 10 g, particularly preferably 0.5 to 10 g, for Pt and Pd per liter of the carrier substrate. Also 0 for Rh.
It is preferably from 01 to 10 g, particularly preferably from 0.05 to 5 g. Also
The loading amount of the NO _x storage material is 0.
A range of 05 to 1.0 mole is desirable. NO _x
When the amount is less than 0.05 mol / L, the NO _x storage ability is reduced, and when the amount is more than 1.0 mol / L, grain growth of the noble metal is promoted.

The Pt / TiO ₂ powder which is a feature of the present invention is TiO ₂
It can be prepared by supporting Pt on a powder. Pt
/ Pt loading in TiO ₂ powder and Pt /
Although the content of the TiO ₂ powder is not particularly limited, it is preferable that the amount of Pt supported on the Pt / TiO ₂ powder be 1/5 or more of the total supported amount of the noble metal supported on the entire catalyst. desirable. If the amount of Pt supported on the Pt / TiO ₂ powder is less than 1/5 of the total amount of the noble metal supported on the entire catalyst, the water gas shift reaction of the formula (2) does not proceed sufficiently,
The amount of H ₂ generated is extremely small, and the effect of adding the Pt / TiO ₂ powder cannot be obtained.

If the loading density of Pt carried on the Pt / TiO ₂ powder is too high, there is a problem that Pt tends to grow at high temperatures, so that the Pt carried on the Pt / TiO ₂ powder has a disadvantage. The loading density is desirably 10% by weight or less. Range of TiO ₂ content These restrictions also determined inevitably, TiO ₂ derived from Pt / TiO ₂ powder catalyst as a whole 10-50
It is preferred to be volume%.

Pt / TiO_TwoTo include the powder, porous oxidation
Powder and Pt / TiO_TwoMix the powder to make the carrier
No. For example, porous oxide powder and Pt / TiO_TwoMixing powder
A slurry is prepared from the powder and coated on a honeycomb-shaped substrate.
To form a coat layer. After that, the precious metal
And NO_x By supporting the occlusion material, honeycomb-shaped NO _x
An occlusion reduction type catalyst can be prepared.

Alternatively, Pt may be used as a support made of a porous oxide.
/ TiO ₂ powder can also be supported. In this case, for example, a coat layer is formed from a porous oxide powder, and a Pt / TiO ₂ powder can be adhered and fixed to the coat layer and carried.

It should be noted that a powder of a porous oxide such as Al ₂ O ₃
A coat layer is formed from the mixed powder with TiO ₂ powder, and Pt
Can be used to form a Pt / TiO ₂ powder. However, in this case, Pt is supported in a large amount on the surface layer of the coat layer, so that it becomes difficult to support Pt on TiO ₂ inside the coat layer. Therefore it is difficult to utilize the TiO ₂ of the inner coating layer as Pt / TiO ₂ powder, the amount of H ₂ becomes small. In the method of supporting Pt / TiO ₂ powder on a support made of a porous oxide, the Pt / TiO ₂ powder may fall off. Therefore, Pt is supported on TiO ₂ powder in advance and Pt / TiO ₂
It is desirable to prepare a powder and mix it with a porous oxide powder to form a coat layer.

[0024]

The present invention will be specifically described below with reference to Test Examples, Examples and Comparative Examples.

(Test Example) 40 g of TiO ₂ powder was dispersed in 200 g of water, a predetermined amount of an aqueous solution of a dinitrodiammine platinum complex having a predetermined concentration was added thereto, and the mixture was stirred for 30 minutes, filtered and heated at 110 ° C.
And then calcined at 450 ° C. for 2 hours to prepare a Pt / TiO ₂ powder. The concentration of Pt in this Pt / TiO ₂ powder is 1.0% by weight
It is.

Instead of the TiO ₂ powder, a ZrO ₂ powder and an Al ₂ O ₃ powder were used, and Pt was carried in the same manner, and Pt / ZrO ₂
Powder and Pt / Al ₂ O ₃ powder were prepared respectively. The concentration of Pt in each powder is 1.0% by weight.

In the same manner as above, except that rhodium nitrate was used instead of the aqueous dinitrodiammine platinum complex solution.
And Rh / TiO ₂ powder, Rh / ZrO ₂ powder and Rh / Al ₂ O ₃
Each powder was prepared. The concentration of Rh in each powder is 1.0% by weight.

Further, Pd was supported in the same manner except that palladium nitrate was used instead of the aqueous solution of the dinitrodiammine platinum complex, and Pd / TiO ₂ powder, Pd / ZrO ₂ powder and Pd / Al
₂ O ₃ powder was prepared respectively. The concentration of Pd in each powder is 1.0% by weight.

Each of the obtained powders has a particle size of 1 to 3 mm.
, And placed in a laboratory reactor. Then, a model exhaust gas containing 1.0% of CO and 3.0% of H ₂ O was introduced at a gas space velocity of 100,000 h ⁻¹ . The outgassed gas was analyzed by gas chromatography at a catalyst bed temperature in the range of 200 to 600 ° C., and the amount of generated hydrogen was measured. The results are shown in FIGS.

As can be seen from FIGS. 1 and 2, Pt / TiO ₂
Powder produces the largest amount of hydrogen compared to other powders, 300 ° C
It can be seen that the activity is remarkably high in the following low temperature range. It can also be seen that the amount of hydrogen generated by the reaction of CO + H ₂ O → CO ₂ + H ₂ in Rh / ZrO ₂ powder conventionally used for hydrogen generation is inferior to that of Pt / TiO ₂ powder.

Example 1 Prepared in the above test example
Pt / TiO ₂ powder, TiO ₂ powder, Al ₂ O ₃ powder and CeO ₂ -Zr
The O ₂ composite oxide powder is mixed in a weight ratio of Pt / TiO ₂ : TiO ₂ : Al
_A mixed powder was prepared by mixing such that ₂ O ₃ : CeO ₂ -ZrO ₂ = 5: 10: 10: 2. This mixed powder was dispersed in water together with alumina sol to prepare a slurry, which was coated on a ceramic honeycomb substrate having a capacity of 35 cm ³ and dried at 250 ° C. for 15 minutes to form a coat layer. The coating layer was formed in an amount of 9.4 g per honeycomb substrate.

A predetermined amount of a barium acetate aqueous solution having a predetermined concentration was absorbed by a honeycomb substrate having a coat layer, dried at 250 ° C. for 15 minutes, and baked at 500 ° C. for 30 minutes to carry Ba. The amount of Ba supported is 0.007 mol per honeycomb substrate. This was immersed in a 15 g / L aqueous solution of ammonium bicarbonate for 15 minutes.
After drying at 250 ° C. for 15 minutes, Ba supported on the coat layer was converted to barium carbonate.

A predetermined amount of a dinitrodiammine platinum complex nitric acid aqueous solution having a predetermined concentration was absorbed into the solution, dried and calcined at 300 ° C. for 15 minutes to carry Pt. The supported amount of Pt was 0.07 g per honeycomb substrate.

Then, a predetermined amount of an aqueous solution containing potassium nitrate and lithium nitrate at predetermined concentrations was absorbed thereinto,
After drying at 150 ° C. for 15 minutes, it was baked at 500 ° C. for 30 minutes to carry K and Li. The loading amounts of K and Li are 0.0035 mol per one honeycomb substrate. Thus, the NO _x storage reduction type catalyst of Example 1 was prepared.

(Comparative Example 1) Prepared in the test example described above.
Rh / ZrO ₂ powder, TiO ₂ powder, Al ₂ O ₃ powder and CeO ₂ -Zr
The O ₂ composite oxide powder is mixed in a weight ratio of Rh / ZrO ₂ : TiO ₂ : Al
_A mixed powder was prepared by mixing such that ₂ O ₃ : CeO ₂ -ZrO ₂ = 5: 10: 10: 2. This mixed powder was dispersed in water together with alumina sol to prepare a slurry, which was coated on a ceramic honeycomb substrate having a capacity of 35 cm ³ and dried at 250 ° C. for 15 minutes to form a coat layer. The coating layer was formed in an amount of 9.4 g per honeycomb substrate.

Using a honeycomb substrate having this coat layer,
In the same manner as in Example 1, the same amount of Ba, Pt, K and Li was supported to prepare the NO _x storage reduction type catalyst of Comparative Example 1.

Comparative Example 2 TiO ₂ powder, Al ₂ O ₃ powder and
CeO ₂ -ZrO ₂ composite oxide powder is mixed with TiO ₂ : Al ₂ O ₃ :
A mixed powder was prepared by mixing CeO ₂ -ZrO ₂ = 15: 10: 2. This mixed powder was dispersed in water together with alumina sol to prepare a slurry, which was coated on a ceramic honeycomb substrate having a capacity of 35 cm ³ and dried at 250 ° C. for 15 minutes to form a coat layer. Coating layer per honeycomb substrate
9.4 g were formed.

Using a honeycomb substrate having this coat layer,
In the same manner as in Example 1, the same amount of Ba, Pt, K and Li was supported to prepare the NO _x storage reduction type catalyst of Comparative Example 2. Regarding Pt, in Example 1, 0.017 g (20% of the total amount) was previously supported on TiO ₂ , and the remaining 0.070 g was supported after forming the coat layer, whereas in Comparative Example 2, 0.087 g was supported. Pt is carried at once after the formation of the coat layer.

The NO _x storage-reduction type catalyst of Comparative Example 2 had the same composition as the catalyst of Example 1 except for the presence or absence of Pt / TiO ₂ powder. Is the same as

<Test / Evaluation> The catalysts of Example 1, Comparative Example 1 and Comparative Example 2 were respectively placed in a laboratory reactor. Then, the model exhaust gas shown in Table 1 was subjected to a gas space velocity of 50,000 hours.
^It was introduced under the condition of ^-1 . In the range of the catalyst entering gas temperature 200 to 600 ° C., by switching the gas from the Rich gas steady state Lean gas steady state, NO _x amount which the catalyst is occluded until the concentration of NO _x emissions is constant (saturated NO _x Occlusion amount) was measured. Rich gas was introduced for 4 seconds from the lean gas steady state, and the NO _x storage amount (rich spike NO _x storage amount) when switching to lean gas again was measured. The results of each of these initial (new) catalysts are shown in FIGS.

[0041]

[Table 1]

Further, the catalysts of Example 1, Comparative Examples 1 and 2 were placed in a laboratory reactor, respectively, and the conditions shown in Table 1 were obtained under the conditions of a catalyst-containing gas temperature of 300 ° C. and a gas space velocity of 50,000 h ^−1. A sulfur poisoning durability test was conducted in which a model gas obtained by further adding 100 ppm of SO ₂ to the model gas was repeatedly introduced for 4 hours at a cycle of Lean gas of 55 seconds and Rich gas of 5 seconds. And for each of the catalyst after sulfur poisoning durability test, saturated _the NO _x storage amount in the same manner as described above and the rich spike _the NO _x storage amounts were measured, respectively, indicates the results in FIGS. 7 to 10.

As can be seen from FIGS. 3 to 10, the catalyst of Example 1 has a larger rich spike NO _x storage amount than Comparative Examples 1 and 2 both at the initial stage and after the durability test. large. This is, H ₂ is generated from CO and H ₂ O contained much in Rich model gas, that it is effectively used for the reduction of released NO _x the H ₂ from _the NO _x storage material This shows that the effect of adding a Pt / TiO ₂ powder having a high water gas shift reaction activity is apparent.

7 to 10, the fact that the catalyst of Example 1 has a large saturated NO _x storage amount after the durability test means that the sulfur-poisoned NO _x storage material is reduced by the sulfur poisoning and the NO _x storage capacity is restored. the possible means, catalyst of example 1 is considered to sulfur poisoning with H ₂ to produce is suppressed.

The reason why higher _the NO _x storage amount of the catalyst of the catalyst Comparative Example 2 Example 1 despite the composition of each component are the same, considered as follows. That is, in the catalyst of Example 1, since Pt is previously supported on TiO ₂ , hydrogen generation active sites are widely distributed inside the coat layer, and the water gas shift reaction efficiency is high. On the other hand, in the catalyst of Comparative Example 2, a large amount of Pt was carried near the surface layer of the coat layer, so that Ti
It is considered that Pt is not supported on O _2, and the active site for hydrogen generation is smaller than that in Example 1.

It has also been found that Pt carried on TiO ₂ is more likely to cause grain growth by heat than Pt carried on Al ₂ O ₃ . In the catalyst of Comparative Example 2, Pt is supported at a high concentration on the surface layer of the coat layer, so that the TiO ₂ of the surface layer has a high Pt support density and tends to cause grain growth. However, since 20 wt% of Pt in the catalyst of Example 1 is uniformly supported on TiO _2, the loading density of Pt on TiO ₂ in the catalyst of Example 1,
The catalyst of Comparative Example 2 is lower than TiO ₂ in the surface layer, so that grain growth hardly occurs. Therefore, it is considered that the catalyst of Example 1 suppresses the growth of Pt particles as compared with the catalyst of Comparative Example 2 as a whole, and as a result, shows high NO _x storage ability even after durability.

In each catalyst after the sulfur poisoning durability test,
The sulfur content was measured by elemental analysis, and the results are shown in FIG.
NO_x The storage material is BaSO due to sulfur poisoning._Four, K_TwoSO_FourPassing
And Li _TwoSO_FourIn FIG. 11, Ba, K and
2 shows the equivalent ratio of S to the total of Li and Li.

It is apparent from FIG. 11 that the catalyst of Example 1 contained less sulfur than the catalysts of Comparative Examples 1 and 2, and the sulfur poisoning was suppressed.
It is clear that this is the effect of adding the TiO ₂ powder.

[0049]

According to the exhaust gas purifying catalyst of the Effect of the Invention] The present invention, together with higher _the NO _x purification performance by using in exhaust gas containing a large amount of CO is expressed, after durability for sulfur poisoning is suppressed Even high NO _x purification ability is maintained.
In particular, a high NO _x purification ability is exhibited in a low temperature range of 400 ° C. or lower.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the temperature of each powder and the amount of generated hydrogen used in a test example.

FIG. 2 is a graph showing the relationship between the temperature of each powder used in a test example and the amount of hydrogen generated.

FIG. 3 shows the temperature and initial saturated NO of the catalysts of Example 1 and Comparative Example 1.
₆ is a graph showing a relationship with _x storage amount.

FIG. 4 is a graph showing the relationship between the temperature of the catalyst and the initial rich spike NO _x storage amount of Example 1 and Comparative Example 1.

FIG. 5 shows the temperature and initial saturation NO of the catalysts of Example 1 and Comparative Example 2.
₆ is a graph showing a relationship with _x storage amount.

FIG. 6 is a graph showing the relationship between the temperature of the catalysts of Example 1 and Comparative Example 2 and the initial rich spike NO _x storage amount.

FIG. 7 shows the temperatures of the catalysts of Example 1 and Comparative Example 1 and saturation after endurance.
4 is a graph showing a relationship with an NO _x storage amount.

FIG. 8 is a graph showing the relationship between the temperature of the catalyst of Example 1 and Comparative Example 1 and the amount of occluded rich spike NO _x after endurance.

FIG. 9 shows the temperatures of the catalysts of Example 1 and Comparative Example 2 and saturation after endurance.
4 is a graph showing a relationship with an NO _x storage amount.

FIG. 10 is a graph showing the relationship between the temperature of the catalyst of Example 1 and Comparative Example 2 and the amount of stored rich spike NO _x after endurance.

FIG. 11 is a graph showing the sulfur amount after durability of the catalysts of Example 1, Comparative Examples 1 and 2.

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Claims (2)

[Claims] A carrier consisting of 1. A porous oxide, comprising the further comprising NO _x storage-and-reduction type exhaust gas purifying catalyst is supported noble metal and NO _x storage material to the carrier, platinum supported on titania Pt A catalyst for purifying exhaust gas, further comprising / TiO ₂ powder. 2. The method according to claim 1, wherein the amount of platinum supported on the Pt / TiO ₂ powder is at least の of the total amount of noble metal supported on the entire catalyst. Exhaust gas purification catalyst.