JP3272464B2 - Exhaust gas purification catalyst structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst structure for purifying exhaust gas.

[0002]

2. Description of the Related Art As a catalyst for purifying exhaust gas of an engine,
Oxidation of CO (carbon monoxide) and HC (hydrocarbon) and N
Three-way catalysts that simultaneously reduce Ox (nitrogen oxide) are known. A three-way catalyst is known in which Pt (platinum), Pd (palladium) and Rh (rhodium) are supported on γ-alumina, and the air-fuel ratio (A
/ F) is around 14.7, which is the stoichiometric air-fuel ratio, whereby high purification efficiency is obtained.

On the other hand, in the field of automobiles, the fuel efficiency of the engine is reduced by increasing the air-fuel ratio. However, the exhaust gas in such a lean combustion state of the engine has a so-called lean atmosphere in which oxygen is highly concentrated. Therefore, in the above three-way catalyst, even if CO and HC can be oxidized and purified, N
Ox reduction purification is not possible.

On the other hand, as a catalyst capable of decomposing NOx even in the above-mentioned lean atmosphere, a zeolite catalyst in which a transition metal is supported by ion exchange is known. This zeolite catalyst, under the combustion of HC,
NOx is decomposed into N ₂ and O _2, and this zeolite catalyst has high activity at low temperature and NO
Various countermeasures, such as using together with a three-way catalyst, in order to obtain an exhaust gas purifying catalyst having excellent x-processing ability, are being studied.

[0005] For example, there is a catalyst described in Japanese Patent Application Laid-Open No. 1-139145 intended to complement the characteristics of a zeolite catalyst. In the catalyst container, the catalyst contains Cu and zeolite on the exhaust gas inflow side (upstream side).
A catalyst in which a transition metal such as Co or Ni is ion-exchanged is provided, and an oxidation catalyst or a three-way catalyst in which a noble metal such as Pt, Pd, Rh or the like is supported on alumina is provided on the exhaust gas outlet side (downstream side). Is provided. That is, NOx is supplied by the upstream catalyst, and CO, H is supplied by the downstream catalyst.
C or NOx, CO and HC.

[0006]

However, a zeolite catalyst having a transition metal ion-exchange supported thereon, such as the catalyst disclosed in the above-mentioned publication, has a capacity of 90 at the laboratory level.
%, The NOx purification rate is low in an actual vehicle engine.

In this regard, the present inventor has proposed the H-type ZSM-
5 (70 ratio of Cayban) with Pt, Rh and Ir 30:
The honeycomb carrier is supported at a ratio of 3: 1.
A study was conducted using a catalyst supported such that the total amount of Pt, Rh and Ir per liter was 6 g. FIG. 6 shows the A / F
= 22 model gas (SV = 55000 hr ^-1 ) to supply NO purification rate and NOx (total of NO and NO ₂ )
2 shows the results of measuring the purification rate of methane. According to the results in the figure,
Although NO exhibits a purification rate of about 30% even at 300 ° C. or higher, the purification rate of NOx is significantly reduced on the high temperature side. This is considered to be due to the reaction of NO → NO ₂ at high temperatures.

[0008] Incidentally, NOx in the zeolite catalyst
HC purification is involved in purification, and its purification mechanism is as follows:
NO is purified by the reaction between NO and an HC intermediate formed by incomplete oxidation of HC. NO is reacted with the HC intermediate via NO ₂ to purify NO.
There are two ways of thinking. In any case, it is necessary that HC is appropriately combusted to generate HC intermediates.

On the other hand, as described above, NOx
The reason why the purification rate has decreased is that, in an oxygen-excess atmosphere, at a catalyst active point at a high temperature, NO + 1/2 O ₂ → NO ₂ CnHm + (n + 1/4 m) O ₂ → mCO ₂ +1/2 mH ₂ O ( It is considered that the two reactions of (complete oxidation) occur independently, and the selective reduction of NO by HC is unlikely to occur.

That is, in the temperature range where the exhaust gas temperature is high, the HC required for the decomposition of NOx is completely burnt and disappears in the upstream portion of the exhaust gas flow in the catalyst, and the downstream portion effectively contributes to the purification of NOx. It is something that is no longer.

On the other hand, in a temperature range where the exhaust gas temperature is low, even if HC is present in the exhaust gas, appropriate combustion (oxidation reaction) of HC does not occur in the downstream portion, and does not effectively contribute to NOx purification.

Due to the above, (a) the exhaust gas temperature fluctuates, (b) the exhaust gas composition is slightly different from the gas composition in the above equipment test, and (c) the exhaust gas flow rate fluctuates (the flow rate increases). It is recognized that the total NOx purification rate is particularly low in the engine of the actual vehicle.

In a zeolite catalyst supporting a transition metal, NOx can be removed by combustion of HC. On the other hand, coke is generated by combustion of HC, and this coke is finely divided into zeolite downstream of the catalyst. It is also known to deactivate the zeolite catalyst by closing the pores.

That is, the present invention addresses the problem that the downstream portion of the catalyst does not effectively contribute to NOx purification and the problem of coke, and provides a catalyst structure capable of obtaining a high NOx purification rate even in an actual vehicle. Is what you do.

[0015]

SUMMARY OF THE INVENTION The present invention provides
In order to solve such a problem, it is an object of the present invention to suppress HC combustion at an upstream portion of an exhaust gas flow in a catalyst or to enhance HC combustion at a downstream portion.

That is, a first means for solving the above-mentioned problem (the invention described in claim 1) is that the metal-containing silicate is
An exhaust gas purifying catalyst structure using a catalyst material carrying a catalytically active species that decomposes NOx in the presence of C, wherein a catalyst portion having low HC combustibility is located upstream of the exhaust gas flow. The catalyst parts having high HC combustibility are respectively arranged on the downstream side of the exhaust gas flow, and the downstream catalyst part is
Larger contact area with exhaust gas than the upstream catalyst part
Is formed, and the HC flammability is relatively higher than that of the upstream catalyst portion.
And it features that you have become to high.

In other words, conversely, the upstream catalyst section is exhausted.
The contact area with the gas is small, and the H
This means that the combustion of C is small. Thus, in this means, the HC in the upstream catalyst section is lower than that in the downstream catalyst section. Therefore, even if the exhaust gas is at a high temperature, the HC is not burned down in the upstream catalyst section without being burned in the upstream catalyst section. Is also supplied to the downstream catalyst section, and the NOx is detected not only in the upstream catalyst section but also in the downstream catalyst section.
This effectively contributes to the decomposition of x.

In addition, when the exhaust gas temperature is low, if the upstream catalyst section and the downstream catalyst section have the same HC flammability, it is almost impossible to expect NOx purification in the downstream catalyst section. In this case, since the HC flammability of the downstream catalyst portion is high, the downstream catalyst portion also effectively contributes to the purification of NOx. Then, the fact that HC combustion occurs also in the downstream-side catalyst section means that the catalyst temperature rises by that, so that the activity of the catalyst when the exhaust gas temperature is low, so-called low-temperature activity, is improved. With
Since coke is easily burned and removed in the downstream catalyst section,
Blockage of the pores of the metal-containing silicate is also prevented.

As the metal-containing silicate main body, aluminosilicate (zeolite) using Al as a metal forming a crystal skeleton is preferable. In addition, Ga, Ce, Mn, Tb, etc. may be used instead of Al or together with Al. A metal-containing silicate using another metal as a skeleton forming material can also be applied. A type, X as zeolite
Type, Y type, mordenite, ZSM-5 and the like are preferred.

As a transition metal serving as a catalytically active species, Cu is preferable. In addition to Cu, Co, Cr, Ni, Fe, Mn
And the like, and a noble metal such as Pt can also be preferably used.

The transition metal is supported on the metal-containing silicate by, for example, ion exchange to obtain a zeolite catalyst, and about 20% by weight of an inorganic binder such as hydrated alumina or silica sol is added thereto. Then, a monolithic NOx purification catalyst can be prepared by wash-coating the carrier.

When the catalyst material is wash-coated, the carrier is preferably a cordierite honeycomb, but other inorganic porous materials can also be used.

In order to allow the catalytically active species to be supported in a larger amount at the downstream portion of the catalyst than at the upstream portion, a plurality of catalyst materials having different amounts of the catalytically active species supported on the metal-containing silicate are prepared, and then these are increased in amount. For example, the catalyst material is filled in the catalyst container so as to be located on the downstream side, or a part of the honeycomb carrier on which the catalytically active species is uniformly supported is immersed in a solution of the active species prepared at a predetermined concentration. A method can be adopted.

A second means for solving the above problem ( claim 2)
The invention described in ( 1 ) is obtained by developing the first means, wherein each of the upstream and downstream catalyst portions of the exhaust gas flow has a honeycomb structure, and the downstream catalyst portion comprises: It is characterized in that it is formed so that the number of cells per unit area is larger than that of the upstream catalyst section, and the contact area with the exhaust gas is relatively larger than that of the upstream catalyst section.

That is, in the honeycomb structure, the fact that the number of cells is large means that the contact area with the exhaust gas is small.

A third means for solving the above-mentioned problem ( claim 3)
The invention described in (1) is characterized in that a metal-containing silicate is added in the presence of HC.
Supported catalytically active species that decompose NOx
Exhaust gas purification catalyst structure using catalyst material
Therefore, the catalyst portion having low HC combustibility is located upstream of the exhaust gas flow.
In addition, a catalyst part with high HC combustibility is located downstream of the exhaust gas flow.
Be arranged respectively, the downstream catalyst unit has a smaller cross-sectional area than the upper <br/> downstream catalyst unit, is characterized and in that the passage length is made longer.

That is, in the case of this means, HC in the exhaust gas is activated (partial oxidation or incomplete combustion) in the upstream catalyst section and sent to the downstream catalyst section. Since the area is small, the flow velocity of the exhaust gas is high. Therefore, the HC activated on the upstream side is not burned out (completely burned), and the HC is activated in the downstream side catalyst section.
This will effectively contribute to the decomposition of Ox. Since the downstream catalyst section has a long passage length, the downstream catalyst section has a relatively wide temperature distribution in the exhaust gas flow direction.
x is advantageous for purification.

By the way, if the upstream catalyst portion is made thinner and longer like the downstream catalyst portion, the flow rate of the exhaust gas becomes too high, and it becomes impossible to activate HC sufficiently.

A fourth means for solving the above problem ( claim 4)
The invention described in (1) is characterized in that a metal-containing silicate is added in the presence of HC.
Supported catalytically active species that decompose NOx
Exhaust gas purification catalyst structure using catalyst material
Therefore, the catalyst portion having low HC combustibility is located upstream of the exhaust gas flow.
In addition, a catalyst part with high HC combustibility is located downstream of the exhaust gas flow.
Be disposed respectively on the upstream catalyst unit catalytically active species poisoning agent is carried, HC flammability of the lower <br/> downstream catalyst unit is relatively higher than the upstream-side catalytic portion The feature is that it has become.

That is, in the case of this means, the catalytic activity of the upstream catalytic portion is poisoned, so that the HC combustibility is low. Therefore, this means
It has the same effect as the first means.

A fifth means for solving the above-mentioned problem ( claim 5)
The invention described in (1) is characterized in that the poisoning agent is Pb.

That is, Pb poisons the catalytically active species, resulting in a decrease in the activity of the catalytically active species, so that the upstream catalyst section has a relatively lower HC flammability than the downstream catalyst section. It is.

[0033] Sixth means for solving the above-mentioned problems ( Claim 6)
The invention described in (1) is characterized in that a plurality of catalysts comprising a metal-containing silicate carrying a catalytically active species for decomposing NOx in the presence of HC are arranged at intervals in the flow direction of exhaust gas, This is an exhaust gas purifying catalyst structure in which a heat radiating material is disposed.

The combustion of HC by the catalyst becomes more active as the temperature of the exhaust gas becomes higher. In the case of this means, the temperature of the exhaust gas rises due to the combustion of HC in the upstream catalyst portion.
Before reaching the downstream-side catalyst portion, heat is taken away by the heat dissipating material, and the temperature is reduced. Therefore, it is advantageous in supplying HC in the exhaust gas without burning down to the downstream end of the downstream side catalyst.

A seventh means for solving the above-mentioned problem ( claim 7)
The invention described in (1) is a development of the first and sixth means, and is characterized in that the catalytically active species is Pt.

That is, as described above, Pt is H
C has a high combustion capacity and is optimal for decomposing NOx. Therefore, it is possible to purify NOx even when the exhaust gas temperature is low and low. A decrease in the NOx purification rate at high temperatures can be prevented.

[0037]

Therefore, according to the first means (the invention according to claim 1), a catalyst portion having a low HC flammability is provided upstream of the exhaust gas flow, and a catalyst portion having a high HC flammability is provided at the upstream side of the exhaust gas flow. Located downstream of the exhaust gas flow, i.e. downstream
The catalyst area has a larger contact area with the exhaust gas than the upstream catalyst area.
It is formed to be larger and has HC flammability higher than that of the upstream catalyst section.
Is relatively high , when the exhaust gas temperature is high, HC in the exhaust gas is supplied to the downstream catalyst portion without being burned by the upstream catalyst portion, and the downstream catalyst portion is used for decomposition of NOx. When the exhaust gas temperature is low, HC can be combusted in the downstream catalyst section to contribute to the purification of NOx, and the NOx purification rate can be improved. Further, clogging of pores of the metal-containing silicate by coke can be prevented.

According to the second means (invention of the second aspect), the downstream catalyst section has a unit area more than the upstream catalyst section.
Contact with the exhaust gas
The area is increased, and the effect of the first means is exhibited.
be able to.

[0039] According to the third means (claim 3), since the downstream catalyst part is formed longer smaller and path length the passage cross-sectional area than the upstream catalyst unit, upstream N at the downstream side without burning out the HC activated in
It can be used for the decomposition of Ox to improve the NOx purification rate.

According to the fourth means (invention of the fourth aspect ), since the poisoning agent of the catalytically active species is carried on the upstream catalyst section, the HC flammability of the upstream catalyst section is reduced. As a result, the NOx purification rate at high exhaust gas temperatures can be improved.

According to the fifth means (the invention as set forth in claim 5 ), the effect of the ninth means can be exerted because the poisoning agent is Pb.

According to the sixth means (invention of the sixth aspect ), since the heat radiating material is arranged between the plurality of catalysts in which the catalytically active species is carried on the metal-containing silicate, the exhaust gas contains HC can be supplied to the downstream end of the downstream catalyst without being burned down, and the same effect as the first means can be obtained.

According to the seventh means (invention of the seventh aspect ), since the catalytically active species is Pt, the entire catalyst is NOx
The catalyst can be effectively used for purification and the activation temperature range of the catalyst can be expanded.

[0044]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference examples of the present invention will be described below.
It will be described.

< Reference Examples 1 to 18> -Catalyst Structure- As shown in FIG. 1, a catalyst material in which a catalytically active species is supported on a metal-containing silicate is washed together with a binder into a cordierite honeycomb carrier in a catalyst container 1. A coated exhaust gas purifying catalyst 2 is accommodated. The exhaust gas flows in from the inlet 3 of the catalyst container 1, flows from the upstream side 4 to the downstream side 5 of the exhaust gas purifying catalyst 2, and flows out from the outlet 6.

The exhaust gas purifying catalyst 2 comprises an upstream catalyst section 9 disposed on the upstream side 4 of the exhaust gas flow and a downstream catalyst section 7 disposed on the downstream side 5. The downstream catalytic section 7 carries a higher concentration of catalytically active species having a large function of burning HC than the upstream catalytic section 9.

Preparation of Base Catalyst A base catalyst common to Reference Examples 1 to 18 was prepared as follows.

As a metal-containing silicate, Na-type ZSM-
5 was used. The ratio of SiO ₂ / Al ₂ O ₃ is preferably in the range of 20 to 200, and a ratio of 30 to 50 in that range was used. The cationic species may be other alkali metals, alkaline earth metals, H ⁺ or NH ₄ ⁺ in addition to Na.

After impregnating this metal-containing silicate powder with an aqueous solution of Cu nitrate or acetate, the mixture is stirred at room temperature to about 80 ° C., preferably 40 to 60 ° C. for 24 hours, washed with water, and washed with water.
After drying at 0 ° C. for 10 hours and baking in the air at 500 ° C. for 2 hours, Cu / ZSM-5 (hereinafter referred to as C
u / Z). As a supporting method of Cu, a general supporting method such as an ion exchange method, an impregnation method, and a coprecipitation method can be used. In this case, the supported amount of Cu in the catalyst material is preferably 1 to 10% by weight of the total weight of the material, but was adjusted to be more preferably 3 to 5% by weight. Cu loading is 1
If it is less than 10% by weight, the activity is low, and if it exceeds 10% by weight, the heat resistance is reduced.

After adding hydrated alumina, silica sol and the like as a binder to the above-mentioned Cu / ZSM-5 so as to be 20% by weight, an appropriate amount of water was added to prepare a slurry for coating. A commercially available cordierite honeycomb (400 cells per square inch) was immersed in the slurry, excess slurry was blown off by air blow, dried, and then fired with air columns. The firing temperature is 500 ° C. and the firing time is 2 hours. The amount of the catalyst material carried on the honeycomb carrier was adjusted to 20% by weight of the honeycomb carrier.

Preparation of Downstream Catalytic Unit (High Concentration Supporting Unit of Catalytically Active Species) ( Reference Examples 1 to 5) Pt-ammine crystal, which is a compound of Pt and ammonia, is added to an appropriate amount of water and dissolved. The downstream end face of the honeycomb carrier supporting Cu / Z was impregnated. At this time, the length and the amount of Pt carried were adjusted according to the Pt solution concentration and the amount of water. Thereafter, baking was performed at 500 ° C. for 2 hours in the air to prepare Reference Examples 1 to 5 as shown in Table 1. In Table 1, the length of the high-concentration carrier is indicated by a ratio to the entire length of the honeycomb carrier, and the amount of Pt supported is indicated by the number of grams of Pt per gram of the Cu / Z (this point will be described below). other
The same applies to Reference Examples and Examples).

[0052]

[Table 1]

Reference Examples 6 to 10 A solution obtained by adding an appropriate amount of water to a rhodium nitrate solution was mixed with the Cu /
The end face on the downstream side of the honeycomb carrier supporting Z was impregnated. At that time, the length and the Rh carrying amount of the Rh high concentration carrying portion on the downstream side of the honeycomb carrier were adjusted depending on the Rh solution concentration and the amount of water. Thereafter, baking was performed in the air at 500 ° C. for 2 hours to prepare Reference Examples 6 to 10 as shown in Table 2.

[0054]

[Table 2]

Reference Examples 11 to 15 A solution obtained by adding an appropriate amount of water to a palladium nitrate solution was mixed with the above Cu
/ ZSM-5 catalyst (Cu / Z) was impregnated on the downstream end face of the honeycomb carrier. At that time, the length and the amount of Pd supported on the downstream side of the honeycomb carrier were controlled by the Pd solution concentration and the amount of water. After a while
By baking in air at 500 ° C. for 2 hours, Reference Examples 11 to 15 as shown in Table 3 were prepared.

[0056]

[Table 3]

( Reference Examples 16 to 18) Copper nitrate or copper acetate was added to an appropriate amount of water and dissolved,
The end face on the downstream side of the honeycomb carrier carrying / Z was impregnated. At that time, the length and the amount of Cu supported on the high-concentration Cu support portion on the downstream side of the honeycomb carrier were adjusted by the Cu solution concentration and the amount of water. Thereafter, firing was performed in the air at 500 ° C. for 2 hours to obtain Reference Examples 16 to 18 as shown in Table 4.
Was prepared.

[0058]

[Table 4]

(Conventional Example) The above-described example using only the base catalyst was used as the conventional example.

-Purification Test- The exhaust gas purifying catalysts of Reference Examples 1 to 18 and the conventional example were mounted on a normal-pressure fixed-bed flow reactor, and the NOx purification rate was measured to evaluate the NOx purification activity. That is, NO: 20
00 ppm, HC: 5500 ppm (carbon content), O
_{2: 8%, H 2:} 650ppm, CO: 0.2%, C
Using a model gas having a composition of O ₂ : 10% and N ₂ : residual amount, this gas is flowed from the upstream side to the downstream side of each of the above catalysts so as to have an SV of 25000 hr ^−1, and the NOx purification rate in each temperature range (%) Was measured, and the results are shown in Table 5.

[0061]

[Table 5]

According to the results shown in Table 5, the conventional C
Compared with the u / ZSM-5 catalyst, each of the exhaust gas purifying catalysts of the reference example is superior in NOx purification rate particularly in a low temperature range of 300 to 350 ° C. In addition, a catalyst in which Rh or Pd is supported at a high concentration at a downstream portion of the Cu / ZSM-5 catalyst is the above P
The NOx purification rate in a low temperature range of 300 to 350 ° C. is inferior to that of a catalyst in which t or Cu is supported at a high concentration in the downstream portion.

[0063] The structure of <Examples 1 and 2> Examples 1 and 2 is shown in Figure 2, the more the upstream catalyst 11 and the downstream catalyst unit 12. Each catalyst unit 11,
In each case, a honeycomb carrier made of cordierite is used, and the capacity is 12.5 cc in each case.

-Preparation of Catalyst Material- Divalent platinum ammine crystals, iridium trichloride and rhodium nitrate were weighed so that the molar ratio of Pt, Ir and Rh was 30: 10: 1. The divalent platinum ammine crystals and rhodium nitrate are dissolved in water (ion-exchanged water), and iridium trichloride is dispersed in ethanol. Thereafter, the two are mixed and further mixed with H at a carbane ratio of 144.
Type ZSM-5 powder was added. After stirring at room temperature for 2 hours, the mixture is heated at 80 ° C. for about 3 hours to evaporate the liquid component,
Further, after drying in a thermostat at 150 ° C. for about 6 hours, Pt, I
catalyst material Pt in which r and Rh are supported on ZSM-5
-Ir-Rh / Z was obtained.

( Example 1 ) In this example, 200 cells / 6.45 cm were placed in the upstream catalyst portion 11.
² honeycomb carriers, and 400 cells / 6.45 cm ² honeycomb carriers were used in the downstream catalyst section 12. In each,
The above Pt-Ir-Rh / Z is a binder (hydrated alumina)
At the same time, wash coating is performed so that the total amount of Pt, Ir, and Rh is 6 g per liter.

Example 2 In this example, a 400-cell / 6.45 cm ² honeycomb was also used for the upstream catalyst section 11 and the downstream catalyst section 12, and the above-mentioned Pt-Ir-Rh / Z was used together with a binder. , Pt,
Wash coating was performed so that the total amount of Ir and Rh was 6 g per liter, and Pb was carried on the upstream catalyst portion 11. The loading of Pb was carried out by impregnating the wash-coated honeycomb with an aqueous solution of lead acetate. The carried amount of Pb was set to 2 g per liter.

(Comparative Example 1) Pt-Ir-Rh / Z was added to a cordierite honeycomb carrier (400 cells / 6.45 cm ² ) having a capacity of 25 cc together with a binder, and the total amount of Pt, Ir and Rh was 6 g per liter. Wash coat so that it becomes.

Comparative Example 2 A 12.5 cc capacity cordierite honeycomb carrier (400 cells / 6.45 cm ² ) that does not carry catalytically active species is loaded as a dummy in the upstream portion of the catalyst container 1. In the downstream portion, the downstream catalyst portion 12 of Example 19 is provided.
A catalyst having the same configuration as that of the above was loaded.

-Purification Test- The exhaust gas purifying catalysts of Examples 1 and 2 and Comparative Examples 1 and 2 were mounted on a normal-pressure fixed-bed flow reactor to measure the NOx purification rate, and the NOx purification activity was measured. evaluated. That is, A
/ F = 22 using a model gas,
The catalyst was flowed from the upstream side to the downstream side of each of the above catalysts so as to have a flow rate of 5000 hr ^-1, and the NOx purification rate (%) was measured when the catalyst inlet temperature was 250 ° C. and the catalyst inlet temperature was 350 ° C. It was shown to.

[0070]

[Table 6]

According to the results shown in Table 6, the NOx purification rates when the exhaust gas temperature is low are shown in Examples 1 and 2.
Although there is no significant difference between this embodiment and Comparative Example 1, the NOx purification rate when the exhaust gas temperature is high is much higher in the embodiment, so that the downstream side catalyst unit 12 performs NOx purification at the high temperature. It can be seen that it is effectively contributing.

In the case of Comparative Example 2, NOx when the catalyst inlet gas temperature (dummy inlet gas temperature) was 350 ° C.
Although the purification rate is high, it is recognized that the exhaust gas is cooled by the dummy and supplied to the catalyst.

< Embodiment 3 > This embodiment is shown in FIG.
And a catalyst structure in which heat radiating portions 16 and 16 are disposed between the middle catalyst portion 14 and the downstream catalyst portion 15, respectively.

The upstream, middle and downstream catalyst units 1
Nos. 3, 14, and 15 show that the total amount of Pt, Ir and Rh together with a binder (hydrated alumina) is 6 g per liter in a honeycomb carrier of 400 cells / 6.45 cm ² , together with the binder (hydrated alumina). It will be wash-coated. The dummy referred to in Comparative Example 2 was used for the heat radiating portion 16. The catalyst capacity of each of the catalysts 13 to 15 and the radiator 16 is 5 cc.

The exhaust gas purifying catalyst of this embodiment was mounted on a normal-pressure fixed-bed flow reactor, and the NOx purification rate was measured.
Purification activity was evaluated. That is, a model gas corresponding to A / F = 22 is used, and this gas is flowed from the upstream side to the downstream side of each of the above-mentioned catalysts so as to have an SV of 55000 hr ^-1 . NO at the time
x The purification rate was measured.

As a result, the NOx purification rate was 47.5% when the catalyst inlet temperature was 250 ° C., and the NOx purification rate was 350 ° C.
At 18.degree. C., it was 18.2%. This is because, although the temperature of the exhaust gas rises due to the combustion of HC in the upstream catalyst section 13, the heat is taken away by the heat radiating section 16 before reaching the middle catalyst section 14, and the temperature of the exhaust gas falls. The same applies to the relationship between the middle-stream catalyst section 14 and the downstream-side catalyst section 15. Therefore, HC in the exhaust gas can be supplied to the middle-stream catalyst section 14 and the downstream-side catalyst section 15 without being burned out, and the middle-stream catalyst section 14 and the downstream-side catalyst section 15
This will effectively contribute to purification.

< Embodiment 4 > This embodiment is shown in FIG. 4, in which a large-diameter upstream catalyst section 17 and a small-diameter downstream catalyst section 18 are connected. The passage length of the downstream catalyst section 18 is longer than that of the upstream catalyst section 17. That is, the upstream catalyst section 17 has a diameter of 2.54 cm,
Using a 2.5 cm long honeycomb carrier, the downstream catalyst section 18
A honeycomb carrier having a diameter of 2.03 cm and a length of 4 cm was used. Each of the upstream and downstream catalyst portions 17, 18 has a capacity of 12.5 cc.
t-Ir-Rh / Z together with the binder (hydrated alumina), the total amount of Pt, Ir and Rh is 6 g per liter.
Wash-coated so that

The exhaust gas purifying catalyst of this embodiment was mounted on a normal pressure fixed bed flow type reactor, and the NOx purification rate was measured.
Purification activity was evaluated. That is, a model gas corresponding to A / F = 22 is used, and this gas is flowed from the upstream side to the downstream side of each of the above-mentioned catalysts so as to have an SV of 55000 hr ^-1 . NO at the time
x The purification rate was measured.

As a result, the NOx purification rate was 52.5% when the catalyst inlet temperature was 250 ° C.
At the time of ° C. was 25.5%. The reason why such a high purification rate was obtained is that HC in the exhaust gas was
And is sent to the downstream catalyst section 18, and is considered to have contributed to the decomposition of NOx effectively without being burned down in the downstream catalyst section 18. In this case, in the downstream catalyst section 18, since the passage cross-sectional area is small and the exhaust gas flow is fast, it is recognized that the burnout of HC is prevented, and the passage length is long and the exhaust gas flow direction is long. It is recognized that it has a relatively wide temperature distribution, which is advantageous for NOx purification.

Incidentally, the above Pt-Ir-Rh was placed on a honeycomb carrier (catalyst capacity 25 cc) having a diameter of 2.03 cm and a length of 8 cm.
In the case of a catalyst which was wash-coated with / Z together with a binder (hydrated alumina) so that the total amount of Pt, Ir and Rh was 6 g per liter, the NOx purification rate was measured under the same conditions as in the above example. The catalyst inlet temperature was 52.5% when the temperature was 250 ° C, and 13% when the catalyst inlet temperature was 350 ° C.

< Embodiment 5 > FIG. 5 shows that the exhaust gas purifying catalyst 2 is divided into three catalytic units, an upstream catalytic unit 9, a midstream catalytic unit 8, and a downstream catalytic unit 7, which have different catalytic active species support modes. An example of the configuration is shown. That is, the upstream-side catalyst unit 9 is composed of only the base catalyst described in the paragraphs of Reference Examples 1 to 18, the middle-stream catalyst unit 8 is obtained by impregnating and supporting a small amount of Pt on the base catalyst, and the downstream-side catalyst unit 7 Is obtained by impregnating and supporting a large amount of Pt on the base catalyst. The catalyst 2 is prepared by separately preparing the above-described catalyst units 7 to 9 and loading the catalyst unit 1.

[Brief description of the drawings]

FIG. 1 is a partially cutaway front view showing a catalyst structure according to Reference Examples 1 to 18.

FIG. 2 is a front view showing a catalyst structure according to Examples 1 and 2 .

FIG. 3 is a front view showing a catalyst structure according to a third embodiment .

FIG. 4 is a front view showing a catalyst structure according to Example 4 .

FIG. 5 is a front view showing a catalyst structure according to Example 5 .

FIG. 6 is a graph showing the relationship between the NO purification rate, the NOx purification rate, and the exhaust gas temperature.

[Explanation of symbols]

 2 Exhaust gas purification catalyst 4 Upstream side 5 Downstream side 7, 12, 15, 18 Downstream catalyst section 9, 11, 13, 17 Upstream catalyst section 16 Heat radiating section

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshishi Kamioka 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd. (72) Masahiko Shigetsu 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Matsu (72) Inventor Akihide Takami 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Corporation (56) References JP-A-5-269350 (JP, A) JP-A-5-38452 ( JP, A) JP-A-4-256445 (JP, A) JP-A-63-100919 (JP, A) JP-A-1-139145 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , (DB name) B01J 21/00-37/36 B01D 53/86

Claims (7)

(57) [Claims] An exhaust gas purifying catalyst structure using a catalyst material in which a catalytically active species that decomposes NOx in the presence of HC is supported on a metal-containing silicate, wherein the catalytic portion has low HC flammability. Is located upstream of the exhaust gas flow.
C higher catalyst portion flammability has been arranged respectively on the downstream side of the exhaust gas flow, the downstream catalyst unit, exhaust gases than the upstream catalyst unit
The contact area with the upstream catalyst section is increased.
Remote HC flammability catalyst structure for purifying exhaust gases, characterized that you have relatively high. Wherein each catalyst section of the upstream and downstream is a honeycomb structure, the downstream catalyst unit is formed so that the number of cells per unit area than the upstream-side catalyst unit increases the exhaust gas purifying catalyst structure of claim 1, the contact area is relatively large and the exhaust gas than the upstream-side catalyst unit. 3. A method for producing a metal-containing silicate in the presence of HC.
Catalyst carrying a catalytically active species that decomposes NOx
An exhaust gas purifying catalyst structure using a material, wherein a catalyst portion having low HC combustibility is provided on the upstream side of the exhaust gas flow.
The highly flammable C catalyst section is located downstream of the exhaust gas flow.
Are be arranged, the downstream catalyst unit has a smaller cross-sectional area than the upstream-side catalytic portion, and that the passage length is made longer
Characteristic catalyst structure for exhaust gas purification. 4. A method for producing a metal-containing silicate in the presence of HC.
Catalyst carrying a catalytically active species that decomposes NOx
An exhaust gas purifying catalyst structure using a material, wherein a catalyst portion having low HC combustibility is provided with an H
The highly flammable C catalyst section is located downstream of the exhaust gas flow.
The poisoning agent of the catalytically active species is carried on the upstream catalyst portion,
Exhaust gas purifying catalyst structure, wherein the HC combustion of the downstream catalyst unit than the upstream-side catalytic portion is relatively high. 5. The exhaust gas purifying catalyst structure according to claim 4 , wherein the poisoning agent is Pb. 6. A plurality of catalysts comprising a metal-containing silicate carrying a catalytically active species that decomposes NOx in the presence of HC are arranged at intervals in the direction of exhaust gas flow,
An exhaust gas purifying catalyst structure, wherein a heat radiating material is arranged between the adjacent catalysts. 7. The catalytic active species according to claim 1 or a Pt
Is a catalyst structure for purifying exhaust gas according to claim 6 .