JP2009273989A - Exhaust gas cleaning device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To upgrade cleaning performance at a low temperature of PM, HC and NOx by supplying a large quantity of NO<SB>2</SB>to an NO<SB>x</SB>occlusion reduction catalyst etc. disposed downstream from an oxidation catalyst 1. <P>SOLUTION: The oxidation catalyst 1 comprises a first catalytic layer 11 containing no zeolite, a second catalytic layer 12 which contains zeolite but not a rare metal or contains a negligible amount of rare metal, and a third catalytic layer 13 containing a rare metal. As the chemical reaction to reduce NO<SB>2</SB>to NO is inhibited in the first catalytic layer 11, it is possible to supply a large amount of NO<SB>2</SB>to the NO<SB>x</SB>occlusion reduction catalyst etc. disposed downstream. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to an exhaust gas purifying apparatus excellent in HC and NO _x purification performance in a low temperature range. The exhaust gas purification apparatus of the present invention can be suitably used for purification of exhaust gas from a diesel engine.

In recent years, lean burn engines have become widespread in order to improve fuel efficiency and reduce CO ₂ emissions. A typical example is a diesel engine. As exhaust gas purification devices for diesel engines that have been developed so far, a trap type exhaust gas purification device (wall flow) and an open type exhaust gas purification device (straight flow) are known.

  Among these, as a trap type exhaust gas purification device, a ceramic plug-type honeycomb body (particulate filter (hereinafter referred to as DPF)) is known. This DPF is formed by alternately sealing both ends of the openings of the cells of the ceramic honeycomb structure, for example, in a checkered pattern, and is adjacent to the inflow side cells and the inflow side cells clogged on the exhaust gas downstream side. It consists of an outflow side cell clogged upstream of the exhaust gas and a cell partition partitioning the inflow side cell and the outflow side cell. The exhaust gas is filtered through the pores of the cell partition wall to collect PM, thereby suppressing emissions. To do.

However, since the exhaust gas from the lean burn engine has a lean atmosphere with an excess of oxygen, it is difficult to reduce and purify NO _x by using an ordinary three-way catalyst or the like. Further, exhaust gas from a diesel engine has a problem that it is difficult to purify exhaust gas because it is as low as 50 to 100 ° C. compared to exhaust gas from a gasoline engine and also contains particulates (PM).

Therefore, in recent years, as described in JP-A No. 09-173866 and the like, a filter catalyst in which a coating layer is formed from alumina or the like on the surface of the DPF cell partition walls and pores, and the coating layer carries Pt or the like. Has been developed. Japanese Patent Application Laid-Open No. 06-159037 describes a filter catalyst in which a NO _x storage material is further supported on a coat layer. Thus them can occlude NO _x in _the NO _x storage material if, by a reducing atmosphere with the addition of a reducing agent such as light oil into the exhaust gas, to purify by reduction the occluded NO _x It becomes possible.

However, in the case of a filter catalyst having a coat layer carrying a catalyst metal and an NO _x storage material, the amount of coat layer formed is limited in view of pressure loss. Therefore, in order to support the catalyst metal with high dispersion and suppress grain growth at high temperature, there is a problem that the amount of catalyst metal supported must be reduced, and the PM and NO _x purification performance is insufficient. . In addition, when the inflow of exhaust gas in a low temperature region is continuous, there is a problem that the pressure loss increases due to clogging due to a large amount of PM accumulated due to low PM oxidation activity.

Therefore, for example, in Japanese Patent Application Laid-Open No. 2002-021544, an oxidation catalyst or a NO _x storage reduction catalyst is disposed in front of a filter catalyst, and post-injection for injecting fuel into a combustion chamber or adding fuel to exhaust gas is performed. HC is supplied to the catalyst, and PM deposited on the DPF or filter catalyst is burned by the heat of reaction in the oxidation catalyst or NO _x storage reduction catalyst, and NO _x is reduced and purified.

When adding a reducing agent such as light oil in this manner in the exhaust gas, it is necessary to _the NO _x storage capacity of _the NO _x storage reduction catalyst to recover _the NO _x storage capacity by adding a reducing agent before the saturated is there. Therefore, it is necessary to add the reducing agent at relatively short intervals even during acceleration / deceleration at low speed. However, in such a case, the exhaust gas temperature is relatively low, and the exhaust gas temperature is further lowered by the addition of the reducing agent, so that the reaction between the reducing agent and NO _x hardly occurs. Therefore, the added reducing agent adheres to the filter catalyst in an unreacted state, and the supported catalytic metal is poisoned, resulting in a decrease in activity. Further, there is a problem that PM adheres to the attached reducing agent and the front end face of the cell is blocked. Further reducing agents, such as light oil for a polymer HC, characteristic of poor NO _{x storage-and-reduction} catalyst in the reaction activity of the NO _x is also a problem that not enough brought out.

It has also been proposed to reform the reducing agent added to the exhaust gas and reduce and purify NO _x by the modified HC. For example, JP 2002-295244 A discloses an exhaust gas in which a copper-zeolite catalyst that functions as an NO _x catalyst, a platinum-based catalyst that functions as an oxidation catalyst, and a DPF are arranged in this order from the exhaust gas upstream side to the downstream side. A purification device is disclosed. According to this exhaust gas purification apparatus, the reducing agent adsorbed on the copper-zeolite catalyst is reformed to become highly active HC, and HC and NO react on the platinum-based catalyst. This reduces NO _{x and} oxidizes and removes HC. Since the exhaust gas heated by the reaction heat at this time flows into the DPF, the PM accumulated on the DPF is gradually burned and removed.

  However, even if such an exhaust gas purification device is used, when the exhaust gas temperature is in a low temperature range such as at the time of start-up or acceleration / deceleration at a low speed, the reducing agent is not sufficiently reformed and desired performance cannot be obtained. There was a problem.

JP 2005-264868 proposes an exhaust gas purification apparatus in which an HC adsorption catalyst that adsorbs HC is disposed upstream of the exhaust gas of the filter catalyst, and an oxidation catalyst is further disposed upstream of the exhaust gas of the HC adsorption catalyst. Yes. According to this exhaust gas purifying apparatus, the reducing agent is partially oxidized and reformed by the oxidation catalyst even in the low temperature exhaust gas. Even if not modified, the reducing agent is adsorbed by the HC adsorption catalyst, so that the reducing agent is prevented from flowing directly into the filter catalyst in a liquid state or a polymer state, and the reducing agent adheres to the front end face. Is prevented. Accordingly, the end face of the filter catalyst can be prevented from being blocked. Then, the highly active HC released from the HC adsorption catalyst at the time of temperature rise reduces the NO _x efficiently in the filter catalyst, so that the NO _x purification rate is improved.
Japanese Patent Laid-Open No. 2002-021544 JP 2002-295244 A JP-A-2005-264868

In order to oxidize PM from a low temperature range in DPF or a filter catalyst, it is known that it is effective to supply NO ₂ having high oxidation activity. Further, in the NO _x storage reduction catalyst, NO in the exhaust gas becomes NO ₂ by oxidation and is stored in the NO _x storage material. In the NO _x selective reduction catalyst, NH ₃ generated from urea or the like reduces NO ₂ .

Therefore, in order to efficiently purify NO _x in the exhaust gas from a low temperature range, it is important to first increase the amount of NO ₂ in the exhaust gas. In the exhaust gas from automobile engines, the ratio of NO ₂ to NO is high in terms of thermal equilibrium in the low temperature range of about 200 ° C. That is, in theory, the above reaction is likely to occur due to a large amount of NO ₂ , so PM or NO _x should be efficiently purified at low temperatures.

However, in the current state of the exhaust gas purifying apparatus described in Patent Document 3, PM, HC and NO _x purification performance in a low temperature range of about 200 ° C. is low, and further improvement in low temperature purification performance is required.

The present invention has been made in view of the above circumstances, and by supplying a large amount of NO ₂ to the downstream DPF, filter catalyst, NO _x storage reduction catalyst, or NO _x selective reduction catalyst, PM, HC and NO Improving the low-temperature purification performance of _x is a problem to be solved.

A feature of the exhaust gas purifying apparatus of the present invention that solves the above problems is that the first oxide support and the first oxide that are arranged in an exhaust gas flow path of an automobile and do not include a porous body having pores that can adsorb HC in the exhaust gas. A first catalyst comprising a noble metal supported on a carrier and having an oxidation activity;
A second catalyst including a second oxide support including a porous body having pores capable of adsorbing HC in the exhaust gas, and disposed on the exhaust gas downstream side of the first catalyst;
A third catalyst having an oxidation activity, which is composed of a third oxide support and a noble metal supported on the third oxide support and is disposed on the exhaust gas downstream side of the second catalyst;
Disposed in the exhaust gas downstream side of the third catalyst, NO _x storage-reduction catalyst, NO _x selective reduction catalyst, in that it comprises at least one and a is selected from the particulate filter, and the filter catalyst.

In the exhaust gas purifying apparatus in which the HC adsorption catalyst is disposed upstream of the DPF, filter catalyst, NO _x storage reduction catalyst, or NO _x selective reduction catalyst, the inventors of the present application use a PM or NO _x in a low temperature range of about 200 ° C. The cause of the difference in purification performance between the theory and the current situation was investigated. As a result, it became clear that NO ₂ was reduced to HC in the HC adsorption catalyst, resulting in a reaction that resulted in NO, and the amount of NO ₂ with high oxidation activity decreased.

Therefore, in the exhaust gas purification apparatus of the present invention, a first catalyst that is an oxidation catalyst that does not include a porous body (HC adsorbent), a second catalyst that includes a porous body (HC adsorbent), and a third catalyst that has oxidation activity, Are arranged in this order from the exhaust gas upstream side to the downstream side. Since the first catalyst does not include a porous body (HC adsorbent), a reaction in which NO ₂ is reduced to NO hardly occurs, and a reaction in which NO is oxidized to NO ₂ proceeds.

In the second catalyst, HC is adsorbed by the porous body (HC adsorbent), and the atmosphere of the exhaust gas becomes an oxidizing atmosphere. Therefore, a reaction in which NO ₂ is reduced to NO hardly occurs.

Since the reducing atmosphere is relaxed by the adsorption of HC to the second catalyst or the exhaust gas that has become an oxidizing atmosphere flows into the third catalyst, the oxidation reaction of NO further proceeds in the third catalyst. Even if NO ₂ is reduced in the second catalyst, the produced NO is oxidized again in the third catalyst to become NO ₂ . Therefore, a large amount of NO ₂ having high oxidation activity flows into at least one selected from NO _x storage reduction catalyst, NO _x selective reduction catalyst, DPF and filter catalyst.

When a DPF or a filter catalyst is arranged on the most downstream side, a large amount of NO ₂ with high oxidation activity flows in, so PM can be purified from a low temperature range. Further, when a liquid reducing agent is added to the exhaust gas, the reducing agent is partially oxidized and reformed by the first catalyst. Even if it is not modified, the reducing agent is adsorbed to the porous body (HC adsorbent) of the second catalyst, preventing the reducing agent from flowing directly into the DPF or filter catalyst in the liquid state or polymer state. Thus, the reducing agent is prevented from adhering to the front end face. Therefore, the end face of the DPF or the filter catalyst can be prevented from being blocked. And since high reformed by the generated active HC is consumed in the reduction of NO _x, also improves purification performance of HC.

Further, when the NO _x storage reduction catalyst is arranged on the most downstream side, NO ₂ that has flowed in a large amount is stored in the NO _x storage material, so that NO _x can be stored efficiently from a low temperature range. When the NO _x selective reduction catalyst is arranged on the most downstream side, NO ₂ that has flowed in a large amount is reduced by NH ₃ , so that NO _x can be efficiently purified from a low temperature range.

In the exhaust gas purification apparatus of the present invention, the first catalyst, the second catalyst, and the third catalyst are arranged in this order from the exhaust gas upstream side to the downstream side, and on the downstream side of the third catalyst, the NO _x storage reduction catalyst, NO _x selective reduction catalyst, DPF, and at least one selected from filter catalysts are arranged.

  The first catalyst is an oxidation catalyst that includes a first oxide support and a noble metal supported on the first oxide support and has an oxidation activity. As the first oxide carrier, a single product or a mixture of alumina, zirconia, titania, ceria, silica and the like can be used. A composite oxide composed of a plurality of these can also be used. The noble metal can be selected from Pt, Pd, Rh and the like, but Pt and Pd having high oxidation activity are particularly desirable.

  In the first catalyst, the first oxide support does not include a porous body having pores capable of adsorbing HC in the exhaust gas. A typical example of the porous body is zeolite. Examples of zeolites include ferrierite, ZSM-5, mordenite, Y-type zeolite, β-type zeolite, X-type zeolite, L-type zeolite, silicalite, silica sol, a template material added to form a gel, hydrothermal synthesis, and firing. Thus, zeolite such as synthetic zeolite produced can be used. Of these, mordenite, β-type zeolite and the like are preferable. Further, modified zeolite obtained by removing Al from these zeolites can also be used. As de-Al treatment, acid treatment, boiling water treatment, steam treatment and the like are known. Moreover, it is also possible to use zeolite carrying ion exchange on Fe, Ag, Cu, Mn or the like.

In the first catalyst, since the first oxide support does not contain a porous body, the oxidation of NO efficiently proceeds and most of NO can be converted to NO ₂ . If the first oxide support contains a porous material, as shown in a test example described later, a reduction reaction of NO ₂ occurs and NO is generated. Moreover, since the porous body adsorbs HC, if the porous body is contained in the first oxide carrier, the oxidation activity of the noble metal is reduced by HC coking (HC poisoning). However, by not including a porous material in the first oxide carrier, HC coking is suppressed and the oxidation reaction of NO can be further advanced.

  The 2nd catalyst contains the 2nd oxide support | carrier containing the porous body which has the pore which can adsorb | suck HC in waste gas. As this porous body, the above-mentioned zeolite, modified zeolite, ion-exchanged zeolite or the like can be used. The second oxide support may be composed only of a porous body, or a mixture of an oxide support such as alumina, zirconia, titania, ceria, silica, and the porous body may be used.

The second catalyst mainly reduces the amount of reducing components in the exhaust gas by adsorbing HC and suppresses NO ₂ from being reduced to NO. This function does not require precious metals. Therefore, it is desirable that the second catalyst does not contain a noble metal. By doing so, the amount of the noble metal of the first catalyst or the third catalyst can be increased correspondingly, and the oxidation activity of NO is further improved. The second catalyst may contain a noble metal in a form supported on the second oxide support. However, in this case, as shown in a test example to be described later, since the NO ₂ reduction reaction is likely to occur due to the characteristics of the zeolite and the noble metal, it is desirable to set the loading concentration lower than the loading concentration of the noble metal in the first catalyst. The type of noble metal is not particularly limited.

The third catalyst includes a third oxide support and a noble metal supported on the third oxide support. As the third oxide carrier, a single product or a mixture of alumina, zirconia, titania, ceria, silica, zeolite and the like can be used. A composite oxide composed of a plurality of these can also be used. Since HC is adsorbed by the second catalyst, the exhaust gas that has become an oxidizing atmosphere or a low-order reducing atmosphere flows into the third catalyst, so that NO ₂ is prevented from being reduced. Therefore, zeolite can also be used for the third oxide support.

  The noble metal can be selected from Pt, Pd, Rh and the like, but Pt and Pd having high oxidation activity are particularly desirable.

  As the first catalyst, the second catalyst, and the third catalyst, independent catalysts may be used, but it is preferable to use a single carrier substrate and form each catalyst as a catalyst layer. That is, the first catalyst layer made of the first catalyst is formed within a predetermined length range from the exhaust gas inflow side end face of the honeycomb base material having the straight flow structure, and the second catalyst layer made of the first catalyst is formed downstream of the first catalyst layer. Is formed with a predetermined length, and a third catalyst layer made of the third catalyst is formed in a range from the downstream end of the second catalyst layer to the exhaust gas outflow side end face. If it does in this way, the number of honeycomb base materials will be reduced and the assembly man-hour will also be reduced.

  As the honeycomb substrate, the same material as that formed from cordierite, SiC metal or the like can be used. For forming each catalyst layer, a conventional washcoat method can be used.

The formation range of the first catalyst layer, the second catalyst layer, and the third catalyst layer is 1/4 to 1/2 of the first catalyst layer and 1 is the second catalyst layer with respect to the total length of the honeycomb substrate. The third catalyst layer is preferably formed in the range of 1/4 to 1/2 in the range of / 4 to 1/2. When the formation range of each catalyst layer is shorter than the respective lower limit value, the function of each catalyst layer is not sufficiently exhibited, and the amount of NO ₂ in the exhaust gas flowing out from the third catalyst layer is reduced.

  When the first catalyst, the second catalyst, and the third catalyst are individually manufactured, the capacity (bulk volume) of each catalyst may be set to the above ratio.

At least one selected from a NO _x storage reduction catalyst, a NO _x selective reduction catalyst, a DPF, and a filter catalyst is disposed on the exhaust gas downstream side of the third catalyst. The NO _x storage reduction catalyst is formed by supporting an NO _x storage material selected from alkali metals and alkaline earth metals and a noble metal such as Pt on an oxide carrier such as alumina, and the exhaust gas in a lean atmosphere. occluding NO _x in _the NO _x storage material, along with the release of NO _x from _the NO _x storage material in an intermittent manner to a rich atmosphere to reduce and purify the released NO _x. Since NO becomes NO ₂ and is stored in the NO _x storage material for the first time, the higher the NO ₂ / NO ratio in the inflowing exhaust gas, the greater the NO _x storage amount.

A typical example of the NO _x selective reduction catalyst is an NH ₃ denitration catalyst that reduces NO _x by NH ₃ produced by adding urea water or the like to exhaust gas. In this NH ₃ denitration catalyst, the NO ₂ / NO ratio is 1 or more and it is more likely to react with NH _3. Therefore, the higher the NO ₂ / NO ratio in the inflowing exhaust gas, the better the NO _x reduction efficiency.

The DPF and the filter catalyst are divided into an inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, and an inflow side cell and an outflow side cell. A wall flow structure having a porous cell partition wall having pores is formed, and PM is collected on the surface of the cell partition wall and the pores. The collected PM can be oxidized even at a low temperature by NO ₂ having high oxidation activity. Therefore, the higher the NO ₂ / NO ratio in the inflowing exhaust gas, the better the PM purification performance.

  Hereinafter, the present invention will be specifically described with reference to Examples, Comparative Examples, and Test Examples.

Example 1
FIG. 1 shows an exhaust gas purification apparatus of this embodiment. This exhaust gas purification device is mounted on the exhaust pipe of a 2L diesel engine 100, and an oxidation catalyst 1 and an NH ₃ denitration catalyst 2 are arranged in this order from the exhaust gas upstream side to the downstream side. Between the oxidation catalyst 1 and the NH ₃ denitration catalyst 2, a urea injector 101 for adding urea water to the exhaust gas is disposed. A filter catalyst (not shown) is disposed downstream of the NH ₃ denitration catalyst 2 so that PM can be purified.

  As shown in an enlarged view in FIG. 2, the oxidation catalyst 1 has a honeycomb base material 10 as a base material, a first catalyst layer 11 is formed in a range of 50 mm from the exhaust gas inflow side end face, and the downstream side of the first catalyst layer 10. The second catalyst layer 12 is formed in the range of 50 mm from the side end, and the third catalyst layer 13 is formed in the range of 50 mm from the downstream end of the second catalyst layer 12 to the exhaust gas outflow end surface.

  Hereafter, the manufacturing method of the oxidation catalyst 1 is demonstrated and it replaces with the detailed description of a structure.

First, a cordierite honeycomb substrate 10 (diameter 129 mm, length 150 mm, number of cells 400 / in ² , cell wall thickness 0.1 mm, 2 liters) was prepared.

Next, a slurry containing γ-Al ₂ O ₃ powder and alumina sol was prepared, and a range of 50 mm from the end face on the exhaust gas inflow side of the honeycomb base material 10 was immersed in the slurry, then pulled up, dried and fired after drying. An alumina coat layer corresponding to was formed. The alumina coat layer was formed in an amount of 75 g per liter of the honeycomb substrate 10.

Next, a slurry containing 100 parts by mass of γ-Al ₂ O ₃ powder, 100 parts by mass of β zeolite powder, and alumina sol was prepared, and a range of 100 mm from the exhaust gas outlet side end face of the honeycomb substrate 10 was immersed in the slurry and then pulled up Then, after baking, firing was performed to form an alumina / zeolite coat layer corresponding to the second catalyst layer 12 and the third catalyst layer 13. The alumina / zeolite coat layer was formed in an amount of 150 g per liter of the honeycomb substrate 10.

  Subsequently, a dinitrodiammine platinum chemical solution having a predetermined concentration was prepared, the range in which the alumina coat layer was formed was dipped, pulled up, dried and fired to carry Pt, thereby forming the first catalyst layer 11. The amount of Pt supported in the first catalyst layer 11 is 2.5 g per liter of the honeycomb substrate 10.

  Further, a dinitrodiammine platinum chemical solution having a predetermined concentration was prepared, immersed in a range of 50 mm from the exhaust gas outflow side end face of the honeycomb base material 10, and then dried and fired to carry Pt to form the third catalyst layer 13. The amount of Pt supported in the third catalyst layer 13 is 2.5 g per liter of the honeycomb substrate 10.

  Therefore, Pt is not supported on the second catalyst layer 12. As a whole, the oxidation catalyst 1 carries 4 g of Pt.

Next, a method for producing the NH ₃ denitration catalyst 2 will be described, and a detailed description of the configuration will be used.

  A catalyst coating layer was formed on the same honeycomb substrate 10 as that used in the oxidation catalyst 1 by using a slurry mainly composed of zeolite. The formation amount of the catalyst coat layer is 150 g per liter of the honeycomb substrate 10. Next, Fe was uniformly supported on the entire catalyst coat layer using an aqueous iron nitrate solution. The amount of Fe supported is 2 g per liter of the honeycomb substrate 10 as Fe.

(Example 2)
As shown in FIG. 3, it is the same as that of Example 1 except that a NO _x storage reduction catalyst 3 was used in place of the NH ₃ denitration catalyst 2. Note that a light oil injector 102 for adding light oil to the exhaust gas is disposed on the upstream side of the exhaust gas of the oxidation catalyst 1, and the urea injector 101 is not disposed.

The NO _x storage reduction catalyst is a coating layer made of a mixture of alumina, titania, zirconia, and ceria formed in an amount of 270 g per liter of the honeycomb substrate 10, and Pt, Li, Ba, K uniformly supported on the coating layer. Consists of. The supported amount of each catalyst metal per liter of the honeycomb substrate 10 is 2 g of Pt, 0.2 mol of Li, 0.1 mol of Ba, and 0.1 mol of K.

(Comparative Example 1)
A slurry containing 100 parts by mass of γ-Al ₂ O ₃ powder, 100 parts by mass of β-zeolite powder and alumina sol is uniformly washed on the same honeycomb substrate 10 used in the oxidation catalyst 1 and fired after drying. Thus, an alumina / zeolite coat layer was formed. The alumina / zeolite coat layer was formed in an amount of 150 g per liter of the honeycomb substrate 10. This alumina / zeolite coat layer was uniformly loaded with Pt. The amount of Pt supported was 2 g per liter of the honeycomb substrate 10, and 4 g as a whole was supported in the same manner as in Example 1.

  An exhaust gas purification apparatus of Comparative Example 1 was obtained by replacing the oxidation catalyst with the oxidation catalyst 1 of Example 1.

(Comparative Example 2)
An exhaust gas purification apparatus of Comparative Example 2 was obtained by replacing the oxidation catalyst similar to that of Comparative Example 1 with the oxidation catalyst 1 of Example 2.

<Test Example 1>
A portion having a length of 50 mm having only the first catalyst layer 11 was cut out from the oxidation catalyst 1 in Example 1 to obtain a sample 1. Further, a part having a length of 50 mm having only the third catalyst layer 13 was cut out and used as sample 2.

  The catalysts of Sample 1 and Sample 2 were each placed on the engine bench of a diesel engine, and a predetermined amount of light oil was added to the exhaust gas for a predetermined time on the upstream side of the catalyst while circulating the exhaust gas at an inlet gas temperature of 400 ° C. The NO concentration in the inflowing exhaust gas (inflowing NO) and the NO concentration in the outflowing exhaust gas (outflowing NO) were measured continuously. The result of sample 1 is shown in FIG. 4, and the result of sample 2 is shown in FIG. 4 and 5 also show changes in the HC concentration in the inflowing exhaust gas and changes in the catalyst bed temperature.

4 and 5, paying attention to the difference between the NO concentration in the effluent exhaust gas and the NO concentration in the effluent exhaust gas (corresponding to the area of the shaded area in the figure), the catalyst bed temperature decreases to 200 ° C or less when light oil is added. It can be seen that the NO concentration in the outflowing exhaust gas is higher than the NO concentration in the inflowing exhaust gas. This means that NO ₂ has been reduced in the sample. Since this difference is larger in sample 2 than in sample 1, it is clear that in sample 1, the NO ₂ reduction reaction is suppressed more than in sample 2, and by not containing zeolite, NO in the low temperature range is clear. It is clear that the reduction reaction of ₂ can be suppressed.

<Test Example 2>
In the exhaust gas purifying apparatus of Example 1 and Comparative Example 1, first, the oxidation catalyst 1 was subjected to thermal durability for 20 hours at 700 ° C., and each oxidation catalyst 1 after thermal durability was disposed upstream of the NH ₃ denitration catalyst 2. .

Diesel engine 100 is operated at an engine speed of 2000 rpm, pre-treated for 10 minutes at an inlet gas temperature of 400 ° C, and then at an engine speed of 1800 rpm at a steady operation, the inlet gas temperature to the oxidation catalyst 1 is 180 ° C and the inlet gas. The HC concentration was set to 300 ppmC, and urea water having a concentration of 35% was added from the urea injector 101 while being added so as to be equivalent to NO ₂ in the gas and operated for 10 minutes. At that time, the gas emitted from the oxidation catalyst 1 was analyzed to calculate the NO ₂ / NO _x ratio. Further, the gas emitted from the NH ₃ denitration catalyst 2 was analyzed, and the HC purification rate and the NO _x purification rate were measured respectively. The results are shown in Table 1.

<Test Example 3>
In the exhaust gas purifying apparatuses of Example 2 and Comparative Example 2, the oxidation catalyst 1 was heat-endured in the same manner as in Test Example 2. Each oxidation catalyst 1 after heat endurance was arranged upstream of the NO _x storage reduction catalyst.

The diesel engine 100 was operated at an engine speed of 2000 rpm, the inlet gas temperature was set to 300 ° C., and a pretreatment was performed to inject light oil from the light oil injector for 10 minutes so that the A / F was 14. After that, at an engine speed of 1800 rpm, the gas entering the oxidation catalyst 1 is set to 180 ° C and the HC concentration of the gas is set to 300 ppmC, and light oil is injected intermittently so that the added amount is about 0.5 cc / min. While driving for 10 minutes. At that time, the gas emitted from the oxidation catalyst 1 was analyzed to calculate the NO ₂ / NO _x ratio. Further, the gas emitted from the NO _x storage reduction catalyst 3 was analyzed, and the HC purification rate and the NO _x storage amount were measured. The results are shown in Table 1.

From Table 1, it can be seen that the output gas from the oxidation catalyst 1 according to Example 1 and Example 2 has a large NO ₂ / NO _x ratio, and the reaction in which NO ₂ is reduced to NO is suppressed. In view of the result of Test Example 1, it is clear that this is an effect obtained by arranging the first catalyst layer 11, the second catalyst layer 12, and the third catalyst layer in this order.

In the exhaust gas purification apparatuses of Example 1 and Example 2, HC and NO _x can be efficiently purified from a low temperature range, which is caused by the high concentration of NO ₂ in the exhaust gas from the oxidation catalyst 1. It is clear that

(Example 3)
FIG. 6 shows the exhaust gas purifying apparatus of this embodiment. The exhaust gas purification apparatus of this example is the same as that of Example 2 except that the filter catalyst 4 is used in place of the NO _x storage reduction catalyst 2.

According to the exhaust gas purifying apparatus of the present embodiment, the output gas from the oxidation catalyst 1 has a large NO ₂ / NO _x ratio, and a large amount of NO ₂ flows into the filter catalyst 4. Therefore, PM deposited on the filter catalyst 4 can be oxidized from a low temperature range.

  The exhaust gas purification apparatus of the present invention can be used not only in exhaust gas from a diesel engine but also in exhaust gas from a gasoline engine.

It is a block diagram showing typically an exhaust gas purification device of one example of the present invention. It is sectional drawing which shows the structure of the oxidation catalyst 1 used for the exhaust gas purification apparatus of one Example of this invention. It is a block diagram which shows typically the exhaust gas purification apparatus of 2nd Example of this invention. It is a graph which shows the relationship between time and the component density | concentration in waste gas. It is a graph which shows the relationship between time and the component density | concentration in waste gas. It is a block diagram which shows typically the exhaust gas purification apparatus of 3rd Example of this invention.

Explanation of symbols

1: Oxidation catalyst 2: NH ₃ denitration catalyst 10: Honeycomb substrate
11: First catalyst layer 12: Second catalyst layer 13: Third catalyst layer

Claims (5)

The first oxide carrier which is disposed in the exhaust gas flow path of an automobile and does not include a porous body having pores capable of adsorbing hydrocarbons in the exhaust gas and the noble metal supported on the first oxide carrier and has an oxidation activity A first catalyst;
A second catalyst comprising a second oxide support including a porous body having pores capable of adsorbing hydrocarbons in the exhaust gas, and disposed on the exhaust gas downstream side of the first catalyst;
A third catalyst having an oxidation activity, which comprises a third oxide support and a noble metal supported on the third oxide support, and is disposed on the exhaust gas downstream side of the second catalyst;
An exhaust gas purification apparatus, comprising at least one selected from a NO _x storage reduction catalyst, a NO _x selective reduction catalyst, a particulate filter, and a filter catalyst, disposed on the exhaust gas downstream side of the third catalyst.   The exhaust gas purification apparatus according to claim 1, wherein the porous body is zeolite.   3. The exhaust gas purifying apparatus according to claim 1, wherein the second catalyst includes a noble metal supported on the second oxide support at a support concentration lower than a support concentration of the noble metal in the first catalyst.   The exhaust gas purifying apparatus according to claim 1 or 2, wherein the second catalyst does not contain a noble metal.   The exhaust gas purification apparatus according to any one of claims 1 to 4, wherein the first catalyst, the second catalyst, and the third catalyst are formed in this order on one carrier base material.

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