JP3250823B2 - NOx purification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lean burn engine.
NOx purification for purifying exhausted NOx in the presence of HC
About the method .

[0002]

2. Description of the Related Art To comply with fuel regulations for engines, lean-burn engines, so-called lean-burn engines, have been put to practical use. At the same time, among the gases emitted from the engine, NOx has an adverse effect on the human body and the ecosystem, so that it must be prevented from being emitted out of the device as much as possible.

[0003] There are several methods for preventing the emission, but in the case of a mobile engine, it is practical to remove NOx by means of a catalyst installed at the latter stage of the engine. However, since the conventional three-way catalyst has no NOx purifying function under a high oxygen concentration, it has not been possible to effectively remove NOx.

[0004] As a catalyst capable of effectively removing NOx, copper ion exchanged zeolite or γ-alumina is known. Furthermore, Japanese Patent Application Laid-Open No. 1-130735 describes a technique in which a catalyst obtained by wash-coating a zeolite containing a transition metal by ion conversion onto a support via a binder is used for removing NOx. .

[0005]

SUMMARY OF THE INVENTION As described above, NOx
Although the catalyst for removing NOx has a NOx purification rate of more than 90% at the laboratory level, in an actual mobile engine, (a) the temperature at which NOx can be removed (the active temperature) is determined by the actual vehicle exhaust gas. (B) The gas composition during the equipment test in the laboratory and the exhaust gas composition of the actual vehicle are slightly different. (C) The gas flow rate during the equipment test in the laboratory is lower than the exhaust gas flow rate of the actual vehicle. However, there is a problem that an excellent effect of removing NOx cannot be obtained due to the low temperature.

Further, when zeolite is used as a catalyst element for removing NOx, the temperature of exhaust gas from an actual vehicle may reach as high as 800 to 900 ° C. depending on the operating condition of the engine, and the heat resistance of zeolite may be reduced. In view of this, there is a problem in that measures must be taken to maintain the durability.

[0007] From the viewpoint of high heat resistance, use of γ-alumina as a catalyst has been considered. This γ
-Alumina has heretofore been used as a carrier for the three-way catalyst. On the other hand, γ-alumina having an increased specific surface area exhibits excellent NOx purification performance in the presence of a reducing agent (HC: hydrocarbon) contained in exhaust gas of an actual vehicle engine. It has been useful.

However, if γ-alumina having such a high specific surface area is wash-coated on the carrier simply by being mixed with an inorganic binder such as hydrated alumina, the γ-alumina cannot be reliably carried on the carrier. Can not.
If it is attempted to support the carrier to maintain the required strength, γ
-It is necessary to add an inorganic binder of 40% by weight or more of the alumina, and there is a problem that the addition of such a large amount of the inorganic binder lowers the NOx purification efficiency of γ-alumina.

[0009] In view of the above, the present invention is to provide a rich heat resistance, excellent γ- alumina functional air-fuel ratio of the engine to remove NOx in the exhaust gas if the lean state, ensures the support bearing NO using purified exhaust gas purification catalyst
x To provide a purification method .

[0010]

Means for Solving the Problems In order to achieve the above-mentioned object, the invention of claim 1 relates to a carrier obtained by adding an inorganic binder to γ-alumina having a high specific surface area having excellent NOx purifying ability. The amount of the inorganic binder to be added is set to a fine powder state, and the solidification is facilitated by drying, so that the NOx purification ability of the γ-alumina is not impaired. Notwithstanding, in which the γ- alumina was so that is securely supported on a carrier.

Specifically, a solution taken by the invention of claim 1 is that NOx discharged from a lean burn engine is shared with HC.
In the NOx purification method of purifying in the presence of
Γ-alumina of 50 m ² / g or more, with inorganic binder
5 to 20% by weight of hydrated aluminum of the γ-alumina
Add, mix and grind to obtain a fine powder.
After that, wash this fine powder as a slurry on the carrier.
Exhaust gas purification catalyst manufactured by coating
To NOx and HC exhausted from the lean burn engine
It is configured to contact and reduce and purify NOx .

Further, the invention of claim 2 is the same as that of claim 1
x In the purification method, the γ-alumina is
A zeolite is further added and mixed as a binder.

[0013]

According to the structure of the first aspect of the present invention, a required amount of γ-alumina having excellent characteristics for purifying NOx when the air-fuel ratio is in a lean state is supported on a carrier via a binder. In addition, a reliable binder carrying function exhibiting sufficient durability against external force such as vibration is exhibited.

[0014] No machine quality binder, when it is ground in a state of being mixed into γ- alumina, the fine powder of highly dispersed is sufficiently diffused into γ- alumina. Therefore, when this fine powder is made into a slurry, adhered to the surface of the carrier by immersion of the carrier or the like, and subjected to so-called wash coating and then fired, the inorganic binder has a uniform solidifying action in the γ-alumina adhered layer. After cooling, γ-alumina is sufficiently fixed to the carrier.

Furthermore, since the hydrated alumina as a non-machine quality binder is used, hydrated alumina to be well dispersed in the ground γ- alumina is a finely divided, yet crystal water of hydrated alumina by heating Is dissociated and the hydrated alumina solidifies by transferring from the gel state to alumina,
During this time, the γ-alumina is uniformly fixed to the carrier.

As a result, γ-alumina can be reliably supported on the carrier even when the fixation is sufficiently performed and the mixing amount of hydrated alumina with γ-alumina is relatively small.

According to the constitution of the second aspect of the present invention, the fine zeolite is mixed with the hydrated alumina in γ-alumina to form a slurry, and the γ-alumina is mixed with the hydrated alumina and zeolite by immersion of a carrier or the like. When the body adheres to the surface of the carrier and is subjected to so-called wash coating, the zeolite has a charge, and the zeolite itself has an average particle size of about 2 μm. In addition, they exhibit a behavior of assembling γ-alumina particles with each other.

When calcined in this state, CaO and SiO _{2 in} the composition of the zeolite produce a low-melting-point compound, and the γ-alumina particles are joined together by melting and solidification. Γ-alumina is fixed in a state of being sufficiently adhered to the carrier.

As a result, γ-Al is used as the inorganic binder.
By using mina and zeolite together, γ-Al
Ensures that the function of supporting mina on the carrier is provided
Becomes

[0020]

Next, embodiments of the present invention will be described specifically.

Example 1 A γ-alumina having a high specific surface area value of 250 m ² / g or more was prepared by a sol - gel method. To this γ-alumina, hydrated alumina as an inorganic binder was added in the range of 5 to 20% by weight as shown in Table 1, and mixed with a crusher such as a grinder. Hydrated alumina was sufficiently dispersed and mixed and finely pulverized to each particle diameter as shown in Table 1.

Next, the fine powder is combined with pure water and stirred using a stirrer to form a slurry. A 20 g of cordierite honeycomb as a carrier is immersed in the slurry and wash-coated to form a slurry. Carried on the substrate.

The honeycomb on which γ-alumina was supported using hydrated alumina as a binder was dried at 200 ° C., and then fired at 500 ° C. for about 1 hour. The weight of the fired body was measured, and the amount of γ-alumina supported was measured.

Γ having a high specific surface area value as described above
Mixing of alumina with hydrated alumina as a binder,
In the pulverization, the fineness of γ-alumina having a high specific surface area and the mixed amount of hydrated alumina, and the supported amount of γ-alumina supported on 20 g of each cordierite honeycomb corresponding to various cases are also shown in the table. It is shown in FIG.

Comparative Example 1 The same γ-alumina as in Example 1 prepared by the sol - gel method was combined with pure water in the obtained state and stirred sufficiently to form a slurry. Thereafter, a cordierite honeycomb having a weight of 20 g was immersed in the slurry and wash-coated, whereby γ-alumina was supported on the honeycomb.

Drying and firing are performed in the same manner as in Example 1,
Table 1 also shows the amount of γ-alumina supported on 20 g of cordierite honeycomb.

Comparative Example 2 The same γ-alumina prepared by the sol - gel method as in Example 1 was not ground, but was mixed with hydrated alumina in the range of 5 to 20% by weight and mixed with pure water. And sufficiently stirred to form a slurry. Thereafter, a weight of 2 was added to the slurry.
0 g of cordierite honeycomb was immersed and wash-coated to support γ-alumina on the honeycomb.

Drying and baking were performed in the same manner as in Example 1.
The amount of γ-alumina supported on 20 g of each cordierite honeycomb is also shown in Table 1 according to the amount of hydrated alumina mixed.

[0029]

[Table 1]

According to Example 1 and Comparative Examples 1 and 2, γ-alumina having a high specific surface area effective for NOx purification is mixed to be supported on a carrier such as a cordierite honeycomb. When the binder used is hydrated alumina, γ-alumina is sufficiently mixed with the binder and is pulverized to obtain a practical purification capacity (about 1% of the weight of the carrier).
It is clear that a supported amount of 0% or more is obtained.

In order that the mixture and pulverized body of γ-alumina and the binder be a fine powder sufficiently mixed as described above, a required amount of the binder is preliminarily mixed with γ-alumina.
Subsequent pulverization can easily achieve the above object.

It is needless to say that the above object can also be achieved by separately pulverizing γ-alumina or a required amount of a binder and then mixing.

Reference Example γ-alumina having a high specific surface area of 250 m ² / g or more was prepared by a sol - gel method. Although this γ-alumina contains a small amount of copper, it is mainly γ-alumina.

After such γ-alumina is dispersed in pure water, fine zeolite (in this embodiment, product name ZS
M-5, having a silica / alumina ratio of 30 was used. )
Were mixed with each other and stirred with a stirrer to obtain a well-mixed slurry.

In this slurry, a weight of 22 as a carrier was
g of cordierite honeycomb is immersed, pulled up after a sufficient time, and removed at 200 ° C. after removing excess slurry.
And dried.

About 10% of γ-
After confirming that the mixture of alumina and zeolite was supported, the mixture was calcined at 500 ° C. for about 1 hour.

The honeycomb catalyst thus obtained is
When the NOx purification rate was measured using an actual vehicle simulated gas test device, the NOx purification rate was 27 to 30%. The reaction conditions during this test were as follows: NOx: 2000 pp
m, HC: 6000 ppm C, O ₂ : 8.0%, CO:
_{0.18%, CO 2: 8.4%} , H 2: 650ppm, S
V: 25,000 hr ^-1 (N ₂ balance).

The γ-alumina containing a little copper was converted into a slurry without mixing with zeolite and supported on a honeycomb as the carrier. As can be seen, only 0.4 g was loaded per 22 g carrier weight. Thereafter, the same operation was repeated several times, but it was practically impossible to reach the supported amount of about 2 g required for evaluating the characteristics of the catalyst.

Therefore, zeolite is used as a binder in 10
The slurry was mixed with wt% to form a slurry, and the slurry was supported on a honeycomb as a carrier having a weight of 22 g. The weight reached 2 g as shown in Table 2 in four immersion-drying steps.

From the results shown in Table 2, it can be seen that the mixing of zeolite is effective for loading γ-alumina on the carrier.

[0041]

[Table 2]

Next, the relationship between the mixed amount of zeolite and the amount supported on the carrier was examined. As shown in Table 3, if the mixed amount of zeolite was 5 wt% or more, it was possible to secure the initial supported amount. Was. Each numerical value shown in Table 3 was immersed in the slurry and pulled up to remove excess slurry,
At the time after drying, the time after firing at 500 ° C. for one hour, and the time after applying an external force such as vibration after firing.
It is each supported amount of the mixture of γ-alumina and zeolite with respect to g.

According to the results shown in Table 3, the amount of zeolite supported increased as the amount of zeolite increased, but when the amount of zeolite exceeded 50 wt%, the NOx purification rate tended to decrease. This means that when the zeolite is in an excessive state, the fine zeolite powder enters the γ-alumina particles and inhibits the diffusion of the gas into the γ-alumina particles. This is considered to be due to the relative decrease in the supported amount.

[0044]

[Table 3]

The amount of the carrier carried by the carrier was re-measured after the NOx purification rate was evaluated by an actual vehicle simulated gas test apparatus. It was found that the catalyst was partially blown off.

Further, with respect to zeolite mixed with γ-alumina as a binder, the effect of the properties on the amount supported on the carrier was examined. The results are shown in Table 4. In this case, the mixed amount of zeolite was 10 wt% with respect to γ-alumina. According to the results shown in Table 4, various characteristics such as presence or absence of metal modification or ratio of silica to alumina in the zeolite composition were used. Does not significantly affect the amount of γ-alumina supported on the carrier using zeolite as a binder.

[0047]

[Table 4]

The above, in supporting the in γ- alumina support from Example 1, a γ- alumina with the fine powder, it is mixed with hydrated alumina fine powder, also participated
From the examples, when supporting γ-alumina on a carrier,
Mixing zeolite with γ-alumina is an early N
It is clear that this is an effective means for obtaining a preferable loading amount for obtaining the Ox purification rate.

From the above, when γ-alumina is supported on a carrier, the combined use of hydrated alumina and zeolite as a binder can be evaluated as a means for providing a reliable supporting function.

[0050]

As described above, according to the NOx purification method according to the first aspect of the present invention, since the inorganic binder mixed with γ-alumina is made into a fine powder, the sinterability of the inorganic binder is improved. Γ-alumina prepared to a high specific surface area value having excellent NOx purification characteristics,
Good loading on the carrier can be achieved. Moreover, inorganic
Hydrated alumina was used as the porous binder,
Sintered due to low melting point and fine powder of mina
Performance, and the amount of inorganic binder used is relatively small.
While supporting γ-alumina in a stable state
You can have.

According to the second aspect of the present invention, the inorganic material
Γ-alumina and zeolite are used together as an indah
Ensures the function of supporting γ-alumina on a carrier
Can be brought to

[0052]

Continuation of the front page (56) References JP-A-54-148187 (JP, A) JP-A-55-20699 (JP, A) JP-A-62-221445 (JP, A) JP-A-55-162466 (JP) , A) JP-A-4-180840 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01D 53/94 B01J 21/00-38/00

Claims (2)

(57) [Claims] 1. NOx emitted from a lean burn engine
In a NOx purification method for purifying γ-alumina having a specific surface area of 250 m ² / g or more,
5-20 wt% of the γ-alumina as an inorganic binder
Hydrated alumina is added, mixed and pulverized
Powder and then use this fine powder as a slurry
Exhaust gas produced by washcoating
NOx discharged from lean-burn engine
NOx is reduced and purified by contacting it with HC and HC
NOx purification method. 2. The method for purifying NOx according to claim 1,
In addition, zeo is further added to the above-mentioned γ-alumina as an inorganic binder.
NOx purification characterized by adding light and mixing
Method.