JP2003097250A - Exhaust gas purifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purifier capable of reliably treating carbon monoxide generated by a partial imperfect combustion of a particulate. SOLUTION: While potassium carbonate is carried to the inlet side of the DPF 1 as a upstream side catalyst layer 4a, precious metals are carried to the outlet side as a downstream side catalyst layer 4b. While promptly burning collected particulates by the potassium carbonate in an upstream side catalyst layer 4a, carbon monoxide generated due to local oxygen deprivation during combustion is changed into carbon dioxide by oxidation function which precious metals in downstream side catalyst layer 4b performs.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an internal combustion engine (hereinafter,
The present invention relates to an exhaust gas purification device that collects particulates contained in exhaust gas of an engine) by a particulate filter and incinerates them.

[0002]

[Related Background Art] Exhaust gas emitted from a diesel engine includes hydrocarbons (HC) and carbon monoxide (C).
O), nitrogen oxides (NOx), and the like, and contains a large amount of particulates. As a post-treatment device for treating the particulates, exhaust gas purification in which particulates in exhaust gas are collected by a particulate filter. A device has been proposed. In such an exhaust gas purifying apparatus, the filter is regenerated by incinerating and removing the collected particulates, and continuous purification is possible. There is a demand for improving the regeneration efficiency of the filter by lowering the temperature at which the particulates start to burn because it is limited.

Alkali metals and alkaline earth metals are known as components that have the effect of lowering the combustion start temperature of particulates. For example, JP-A-1-1003.
In the technique described in Japanese Patent Publication No. 09, an auxiliary filter carrying an alkaline earth metal is provided on the upstream side of a main filter carrying a catalyst component to reduce the combustion starting temperature of particulates.

[0004]

However, in the technique described in the above publication, since the auxiliary filter carrying the alkaline earth metal is housed in a case different from the main filter, the size of the exhaust gas purification device becomes large. In addition to that, the heat of combustion of the particulates in the auxiliary filter is difficult to be transferred to the main filter in the subsequent stage, and it cannot be effectively used for activation of the main filter.

On the other hand, generally, when the burning start temperature of particulates is lowered by the action of alkali metal or alkaline earth metal, the burning rate tends to be increased accordingly, and as a result, when the particulates burn. Oxygen (O ₂ ) is locally deficient and incomplete combustion causes carbon monoxide (CO) to be produced. Therefore, in this type of exhaust emission control device, a measure for treating carbon monoxide generated due to the combustion of particulates is required.

The object of the present invention is to prevent the increase in size of the entire apparatus, to effectively utilize the combustion heat of the particulates for improving the purification performance, and further to cause the partial incomplete combustion of the particulates. An object of the present invention is to provide an exhaust emission control device capable of reliably treating carbon monoxide.

[0007]

In order to achieve the above object, according to the invention of claim 1, in an exhaust gas purifying device having a particulate filter in an exhaust passage of an engine, an alkali metal and an alkaline earth are provided at an upstream side portion of the filter. While supporting at least one selected from the group consisting of similar metals, a noble metal having at least an oxidizing function was carried on the downstream side portion of the filter.

Therefore, the particulates collected by the filter burn when the temperature of the filter rises, whereby the filter is regenerated. Here, since the combustion start temperature of the particulates is lowered in the upstream side portion by carrying the alkali metal or the alkaline earth metal, the particulates start the combustion promptly and contribute to the improvement of the regeneration efficiency of the filter.

Further, since the action of lowering the combustion starting temperature by the alkali metal or alkaline earth metal also causes the combustion speed to be increased at the same time, oxygen (O ₂ ) is locally insufficient when the particulates burn. Although carbon monoxide (CO) is easily generated due to incomplete combustion, the generated carbon monoxide is converted into carbon dioxide (CO ₂ ) by the oxidizing function of the noble metal at the downstream side and is rendered harmless. , Emission of harmful carbon monoxide is prevented.

Further, since the upstream side portion and the downstream side portion are constituted as a single filter, the exhaust gas purifying apparatus as a whole can be downsized, and the heat of combustion of the particulates at the upstream side portion can be downstream. Since it is quickly transmitted to the side part, it is effectively utilized for activating the precious metal by promoting the temperature rise of the downstream part. Further, in the invention of claim 2, at least one selected from the group consisting of alkali metals and alkaline earth metals is carried on the inlet side of the filter, while the noble metal is carried on the outlet side of the filter.

The noble metal on the outlet side of the filter converts nitrogen oxide (NO) in the exhaust gas into nitrogen dioxide (N) due to its oxidizing function.
O ₂₎ to exhibit the effect of changing, nitrogen dioxide that is generated for burning particulates relatively reacts with particulates at a low temperature, contributing to promote the burning of particulates on the outlet side. Moreover, since the alkali metal or the alkaline earth metal on the inlet side has a function of occluding nitrogen oxides, the nitric oxide can be stably supplied to the precious metal on the outlet side, and the combustion of the particulates can be further ensured.

Further, in the invention of claim 3, at least one selected from the group consisting of an alkali metal and an alkaline earth metal is carried on the upstream side of the filter part of the filter while sandwiching the filter surface. A noble metal was supported on the downstream side part. Therefore, since the upstream side portion and the downstream side portion are completely partitioned by the filtration portion, the action of lowering the combustion start temperature exerted by the alkali metal or alkaline earth metal of the upstream side catalyst layer,
It is possible to avoid the situation of deterioration due to the influence of the noble metal on the downstream side part, and moreover, all the particulates are deposited on the upstream side part, and the action of the alkali metal or alkaline earth metal carried on the upstream side part is achieved. Since it is directly received, the burning of particulates is greatly promoted.

On the other hand, in the invention of claim 4, at least one selected from the group consisting of alkali metals and alkaline earth metals is supported as a carbonate. Carbonates are more excellent in the action of lowering the combustion starting temperature of particulates than, for example, nitrates or sulfates, and therefore, it becomes easier to burn particulates. Further, in the invention of claim 5, the amount of the noble metal carried is adjusted to 0 or a small amount in order to suppress the change of the carbonate carried on the upstream side portion on the filter into the nitrate or the sulfate. If carbonate reacts with precious metals,
The action of lowering the combustion start temperature is changed to nitrate or sulfate, which is weaker, but such a situation is prevented in advance, and the reaction of carbonate is mostly spent to reduce the combustion start temperature of particulates. As a result, the effect of lowering the combustion start temperature is maximized.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION [First Embodiment] A first embodiment of an exhaust emission control device embodying the present invention will be described below. The exhaust emission control device of the present embodiment is applied to a diesel engine, and the filter of the exhaust emission control device is provided as a diesel particulate filter (hereinafter referred to as DPF) in the exhaust passage of the engine. FIG. 1 is a cross-sectional view showing a DPF provided in the exhaust emission control device of the present embodiment, FIG. 2 is a front view showing the DPF, and FIG. 3 is a partially enlarged cross-sectional view showing a supported state of a catalyst layer of the DPF. As shown in these figures, the DPF 1 has a honeycomb (monolith) type carrier 2 composed of a large number of cells 2a. Each cell 2a of the carrier 2 is formed in a rectangular cross section, and the openings of the adjacent cells 2a on the upstream side and the downstream side are alternately closed by plugs 3.

The carrier 2 is cordierite (Mg ₂ Al ₄ Si ₅ O).
₁₈ ), for example, alumina source powder, silica source powder, and magnesia source powder are
A mixture of silica and magnesia having a cordierite composition is dispersed in water, the solid content thereof is formed into a honeycomb shape, and the honeycomb formed body is fired. When the carrier 2 is molded, the plug 3 is also integrally molded.

A catalyst layer is supported on the cordierite carrier 2a. As shown in the supporting regions in FIGS. 1 and 3, in the present embodiment, the center of the cordierite carrier 3 is a boundary C (indicated by a chain line). ) As the catalyst layer on the upstream side (inlet side)
And the downstream side (outlet side), and the catalyst layers 4a and 4b have different compositions. That is, as the catalyst layer 4a on the upstream side (upstream side portion), potassium carbonate of alkali metal (K ₂ C
While O ₃ ) is supported, precious metal platinum (Pt) or cerium (Ce) is supported as the downstream catalyst layer 4b (downstream side portion). In the present embodiment, the amount of the alkali metal carried on the upstream catalyst layer 4a is set in the range of 1 to 100 g / l, and the amount of the noble metal carried on the downstream catalyst layer 4b is 0.1-1.
It is set in the range of 0 g / l.

The upstream and downstream catalyst layers 4a and 4b are formed on the surface of the cordierite carrier 3 as follows, for example. The upstream catalyst layer 4a is formed by immersing a region upstream of the boundary C of the cordierite carrier 2 in a slurry containing potassium carbonate, followed by drying and firing. Similarly, the downstream catalyst layer 4b has a boundary C of the cordierite carrier 2 in a slurry containing a noble metal such as platinum or cerium.
It is formed by immersing the region on the more downstream side and then drying and firing. The drying / firing treatment may be performed individually for each immersion of the upstream and downstream catalyst layers 4a, 4b, or may be performed simultaneously after both the catalyst layers 4a, 4b are immersed.

In the exhaust gas purifying apparatus constructed as described above, the exhaust gas from the engine passes through the filtering surface 2b which partitions each cell 2a as shown by the arrow in FIG.
The contained particulate PM is collected on the filtration surface 2b and then discharged into the atmosphere. The particulates PM deposited on the filtration surface 2b are incinerated and removed in an operating state where the DPF temperature reaches a predetermined temperature or higher, and the regeneration process is repeated, so that the DPF 1 continuously exerts the particulate PM purification action.

Such regeneration of the DPF 1 is naturally carried out along with an increase in the exhaust gas temperature in an operating region where engine load and rotation are high (continuous regeneration), while the particulate PM is trapped on the basis of the differential pressure across the DPF 1 and the like. When it is judged that the collection amount is at the limit, well-known intake / exhaust throttle and retard of injection timing, or in a common rail type diesel engine, post injection in the expansion stroke and exhaust stroke is executed to raise the exhaust gas temperature, and the particulate PM is increased. It is forcibly removed by incineration (forced regeneration).

Here, the present inventor conducted an experiment to find out the difference between the combustion starting temperatures of the particulate PM in the upstream and downstream catalyst layers 4a and 4b, and as shown in FIG. It was confirmed that the combustion start temperature was about 550 ° C. in the downstream side catalyst layer 4b supporting C, whereas it decreased to about 350 ° C. in the upstream side catalyst layer 4a supporting potassium carbonate. Therefore, due to the action of potassium carbonate, the particulate matter PM deposited on the upstream side catalyst layer 4a quickly starts to burn, and the burning rate is rapidly expanded to the particulate matter PM on the downstream side catalyst layer 4b, which results in As a result, the combustion of particulate PM as a whole is accelerated, which contributes to the improvement of the regeneration efficiency of the DPF 1.

Since the action of lowering the combustion starting temperature by potassium carbonate also causes a rapid increase in the burning rate,
When the particulate PM burns, oxygen (O
₂ ) is insufficient, carbon monoxide (CO) due to incomplete combustion
Are easily generated. Here, the generated carbon monoxide is transferred to the region of the downstream side catalyst layer 4b together with the exhaust gas, and is converted into carbon dioxide (CO ₂ ) by the oxidizing function of the noble metal of the downstream side catalyst layer 4b, so that it is rendered harmless. It is possible to prevent the emission of harmful carbon monoxide.

On the other hand, as is clear from the above description, the upstream catalyst layer 4a and the downstream catalyst layer 4b are formed on a single carrier 2, and these catalyst layers 4a and 4b are disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. Since the functions equivalent to the main filter and the auxiliary filter of the prior art disclosed in Japanese Patent Laid-Open No. 100309 are exhibited, the entire exhaust emission control device can be significantly downsized. Further, the combustion heat of the particulate matter PM on the upstream side catalyst layer 4a is rapidly transferred to the downstream side catalyst layer 4b located immediately downstream, so that the temperature rise of the downstream side catalyst layer 4a is promoted and activated. Therefore, the oxidizing function of the downstream side catalyst layer 4b and the combustion of the particulate PM in the downstream side catalyst layer 4b can be further promoted.

On the other hand, the noble metal carried on the downstream catalyst layer 4b is converted into nitrogen oxide (NO) in the exhaust gas by its oxidizing function.
Is converted into nitrogen dioxide, and the generated nitrogen dioxide reacts and burns with the particulate PM at a relatively low temperature, and this factor also contributes to the promotion of burning of the particulate PM in the downstream side catalyst layer 4b. . Moreover, since potassium carbonate in the upstream catalyst layer 4a has a function of storing nitrogen oxides, the nitrogen oxide is stably supplied to the downstream catalyst layer 4b, so that the combustion of the particulate PM can be further ensured.

Further, as shown in FIG. 4, in the case of potassium nitrate (KNO ₃ ) which is another oxide containing potassium, the combustion starting temperature of the particulate PM is about 400 ° C. and potassium sulfate (KNO ₃ ). 470 in the case of ₂ SO ₄ )
It can be seen that the temperature is about 0 ° C. and both of them have the effect of lowering the combustion start temperature, but they are not enough as potassium carbonate. That is, as compared with potassium nitrate or potassium nitrate, the particulate PM can be burned more easily, and its regeneration efficiency can be sufficiently increased.

When a noble metal such as platinum (Pt) is allowed to coexist with potassium carbonate in the upstream catalyst layer 4a, as in the case of adsorbing NOx (nitrogen oxide) on a so-called NOx storage catalyst,
Potassium carbonate reacts with NO in exhaust gas on precious metals,
A phenomenon occurs in which potassium nitrate is generated or potassium carbonate reacts with the sulfur component SO ₂ in the exhaust gas on the noble metal to generate potassium sulfate, as in the case of SOx (sulfur oxide) adsorption. That is, potassium carbonate is transformed into potassium nitrate or potassium sulfate having a weaker action of lowering the combustion starting temperature, but in the present embodiment, the upstream catalyst layer 4a is divided into regions with respect to the noble metal downstream catalyst layer 4b, and ,
Since the upstream catalyst layer 4a itself does not contain a noble metal, there is also an advantage that such a situation can be prevented. Further, by preventing the reaction with the noble metal, the reaction of potassium carbonate is entirely spent for lowering the combustion start temperature of the particulate PM, and as a result, the effect of lowering the combustion start temperature can be maximized. .

The solid line shown in FIG. 5 indicates the DP of this embodiment in which potassium carbonate is carried on the upstream side and noble metal is carried on the downstream side.
The purification characteristics of F1 are shown, but compared with the prior art in which a noble metal is entirely supported as shown by the broken line, the particulate P
It can be seen that the temperature range in which carbon dioxide (CO ₂ ) is generated decreases with the combustion of M, and the peak becomes much higher, and the particulate PM is actively burned and the regeneration efficiency of the DPF 1 is extremely high.

On the other hand, the structure of the catalyst layers 4a and 4b supported on the DPF 1 is simply changed as described above, and it is not necessary to change the existing exhaust system, which is also an advantage that it can be realized very easily. [Second Embodiment] Next, a second embodiment of the exhaust emission control device embodying the present invention will be described. The exhaust gas purification apparatus of the present embodiment is different from that of the first embodiment only in the loading state of the catalyst layers 5a and 5b of the DPF 1, and other configurations, such as the configuration of the carrier 2 itself, are different. It is the same. Therefore, the description of the common components will be omitted, and the differences will be mainly described.

FIG. 6 is a partially enlarged cross-sectional view showing a supported state of the DPF catalyst layer. As shown in this figure, in the present embodiment, the upstream catalyst layer 5a is carried on the surface of the cell 2a that is open toward the upstream side, that is, the surface of the cell 2a on which the particulate PM is deposited. On the contrary, the downstream catalyst layer 5b is carried on the surface of the cell 2a on the side opening toward the downstream side. The composition of both catalyst layers 5a and 5b is the same as that of the first embodiment, and potassium carbonate is carried as the upstream catalyst layer 5a and platinum or cerium is carried as the downstream catalyst layer 5b.

In the exhaust gas purification device thus constructed, potassium carbonate has the same effect of lowering the combustion start temperature of the particulate PM as in the first embodiment. In this embodiment, however, it is clear from FIG. As described above, since all the particulate matter PM is deposited on the upstream catalyst layer 5a, the action of the potassium carbonate directly acts on all the particulate matter PM, and the combustion of the particulate matter PM is greatly promoted. Thus, the regeneration efficiency of the entire DPF 1 can be improved.

Further, as in the first embodiment, carbon monoxide is produced in the upstream catalyst layer 5a by rapid combustion of particulate PM, and this carbon monoxide is used in the downstream catalyst layer 5a.
Since it can be made harmless by the oxidation function of the precious metal of b,
It can prevent the emission of harmful carbon monoxide. Further, since the upstream side catalyst layer 5a and the downstream side catalyst layer 5b are formed on the single carrier 2, it is possible to achieve the downsizing of the entire exhaust gas purification device and the particulates on the upstream side catalyst layer 5a. P
Since the combustion heat of M is rapidly transferred to the downstream side catalyst layer 5b located immediately downstream and can be effectively utilized for the temperature rise and activation of the downstream side catalyst layer 5b, the above-mentioned oxidation function of the downstream side catalyst layer 5b can be achieved. Can be promoted more.

Moreover, the upstream and downstream catalyst layers 5a and 5b of this embodiment are completely separated by the filtration surface 2b without being adjacent to each other via the boundary C as in the first embodiment. Therefore, it is possible to more reliably prevent the situation where potassium carbonate in the upstream catalyst layer 5a is changed to potassium nitrate or potassium sulfate due to the influence of the noble metal in the downstream catalyst layer 5b.

Although the embodiment has been described above, the aspect of the present invention is not limited to this embodiment. For example, in each of the above embodiments, cordierite was used as the material of the carrier 2, but the material is not limited to this as long as it is a material that can satisfy the strength and heat resistance required for the carrier 2 of the DPF 1. Carbide (SiC) carrier or metal fiber or ceramic fiber made of stainless steel or the like in a dispersed state so as to have a predetermined space filling rate.
You may comprise with the fixed filter.

In each of the above embodiments, potassium is supported on the carrier 2 in the form of carbonate, but it may be supported in the form of nitrate, sulfate, oxide, hydroxide or the like. For example, even in the case of nitrates or sulfates, as described with reference to FIG. 4, the combustion starting temperature of the particulate PM can be sufficiently lowered as compared with the case of supporting the noble metal. Further, an alkali metal other than potassium, for example, lithium (Li), sodium (Na), rubidium (Rb), or the like is supported,
Alkaline rare earth barium (Ba), beryllium (Be), magnesium (Mg), calcium (Ca),
Strontium (Sr), radium (Ra), etc. may be supported.

Further, in each of the above embodiments, the upstream catalyst layer 4
Although a and 5a were formed only from potassium carbonate, as long as the above-mentioned noble metal does not deteriorate potassium carbonate, it does not hinder the inclusion of the noble metal. For example, even if a small amount of platinum (Pt) or the like is contained. Alternatively, a catalyst material other than a noble metal, for example, a catalyst base material such as alumina, a transition metal, or a rare earth may be allowed to coexist with potassium carbonate.

[0035]

As described above, according to the exhaust gas purifying apparatus of the invention of claim 1, it is possible to effectively use the combustion heat of the particulates to improve the purifying performance while preventing the entire apparatus from becoming large in size. Further, it is possible to reliably treat the carbon monoxide generated by the local incomplete combustion of particulates.

According to the exhaust gas purifying apparatus of the invention of claim 2, in addition to the invention of claim 1, the noble metal on the outlet side of the filter changes nitrogen oxide in the exhaust gas into nitrogen dioxide,
Combustion of particulates can be promoted by using this carbon dioxide. Further, according to the exhaust gas purifying apparatus of the invention of claim 3, in addition to the invention of claim 1, deterioration of alkali metal or alkaline earth metal due to noble metal can be avoided and the action of alkali metal or alkaline earth metal can be prevented. It maximizes and greatly promotes the burning of particulates,
Therefore, the regeneration efficiency of the filter can be further improved.

On the other hand, according to the exhaust gas purifying apparatus of the fourth aspect of the present invention, in addition to the first aspect of the invention, since it carries a carbonate excellent in the action of lowering the combustion starting temperature of particulates,
The regeneration efficiency of the filter can be further improved. According to the exhaust gas purifying apparatus of the invention of claim 5, in addition to the invention of claim 4, since the reaction between the carbonate and the noble metal is prevented, the regeneration efficiency of the filter can be further improved.

[Brief description of drawings]

FIG. 1 is a DP provided in an exhaust emission control device according to a first embodiment.
It is sectional drawing which shows F.

FIG. 2 is a front view showing a DPF.

FIG. 3 is a partially enlarged cross-sectional view showing a supported state of a DPF catalyst layer.

FIG. 4 is an explanatory diagram showing the relationship between the composition of the catalyst layer and the combustion start temperature.

FIG. 5 is an explanatory diagram comparing the purification characteristics between the first embodiment and the prior art.

FIG. 6 is a partially enlarged cross-sectional view showing a supported state of a catalyst layer of the DPF of the second embodiment.

[Explanation of symbols] 1 DPF (filter) 3 Filtering surface (filtering part) 4a, 5a upstream catalyst layer (upstream site) 4b, 5b Downstream catalyst layer (downstream site)

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F01N 3/24 B01D 53/36 ZAB F term (reference) 3G090 AA02 AA03 BA01 CA00 EA04 3G091 AA18 AA28 AB02 AB13 BA00 CB02 CB03 GA06 GB01W GB02W GB03W GB05W GB17X 4D019 AA01 BA05 BA06 BC07 CA01 4D048 AA13 AA14 AB01 BA01X BA02X BA10X BA14X BA15X BA30X BB02 CC32 CD05

Claims (5)

[Claims] 1. An exhaust gas purification device having a particulate filter in an exhaust passage of an engine, wherein at least one selected from the group consisting of alkali metals and alkaline earth metals is carried on an upstream side portion of the filter, An exhaust gas purification device, characterized in that at least a noble metal having an oxidizing function is carried on a downstream side portion of the filter. 2. The inlet side of the filter carries at least one selected from the group consisting of the alkali metals and alkaline earth metals, while the outlet side of the filter carries the noble metal. The exhaust emission control device according to claim 1. 3. The filter carries at least one selected from the group consisting of the alkali metals and alkaline earth metals at the upstream side of the filter section while the filter section sandwiches the downstream side of the filter section. The exhaust emission control device according to claim 1, which carries a noble metal. 4. The exhaust emission control device according to claim 1, wherein at least one selected from the group consisting of the alkali metal and the alkaline earth metal is carried as a carbonate. 5. The amount of noble metal carried in the upstream side portion of the filter is adjusted to 0 or a small amount in order to suppress the change of the carried carbonate into nitrate or sulfate. The exhaust emission control device according to claim 4.