US20100205936A1

US20100205936A1 - Exhaust gas-purifying apparatus and exhaust gas-purifying method

Info

Publication number: US20100205936A1
Application number: US12/160,488
Authority: US
Inventors: Juji Suzuki; Masahiko Ogai
Original assignee: Individual
Current assignee: Toyota Motor Corp
Priority date: 2006-01-17
Filing date: 2007-01-10
Publication date: 2010-08-19
Also published as: KR20080085871A; RU2008133619A; EP1979585A1; WO2007083639A1; CN101371017A; RU2392456C2; JP2007192055A; BRPI0706870A2

Abstract

An exhaust gas purifying apparatus, including an NO_xocclusion reduction catalyst (2) and a filter catalyst (3) arranged in series, is used, and the exhaust gas is allowed to flow from the NO_xocclusion reduction catalyst (2) to the filter catalyst (3) in a normal flow process, and, in a recovery process for allowing exhaust gas added with a reducing agent to flow, the flow direction of the exhaust gas is reversed toward the NO_xocclusion reduction catalyst (2) from the filter catalyst (3). Since the exhaust gas is heated by reaction heat of the filter catalyst (3), the NO_xocclusion reduction catalyst (2) can recover from sulfur poisoning in the exhaust gas at a low temperature. Thereby, overheating of the filter catalyst (3) is prevented. Therefore, recovery from sulfur poisoning can be improved, and also deterioration and breakage of the filter catalyst can be prevented.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an exhaust gas-purifying apparatus, comprising at least an NO_xocclusion reduction catalyst and a filter catalyst arranged in series, and an exhaust gas purifying method using the same.
2. Description of the Related Art
A catalyst for efficiently removing NO_xfrom vehicle exhaust gas is known as an NO_xocclusion reduction catalyst. Such an NO_xocclusion reduction catalyst is formed by supporting an NO_xoccluding material, selected from among alkali metals, alkali earth metals on the like, and noble metal, on an oxide support such as alumina. In an oxygen-excess lean atmosphere, NO_xis adsorbed on an NO_xoccluding material in the form of nitrate or nitrite. The exhaust gas in a rich atmosphere is allowed to flow in a pulsing manner, thereby decomposing nitrate or nitrite. The emitted NO_xis reduced and purified by a reduction component that is abundantly present in the atmosphere.
However, under the NO_xocclusion reduction catalyst, sulfur oxide (SO_x) present in the exhaust gas reacts with the NO_xoccluding material, undesirably causing sulfur poisoning related to the deterioration of NO_xocclusion performance. The sulfur-poisoned NO_xoccluding material is present in the form of sulfate or sulfite, which has a higher decomposition temperature than nitrate or nitrite.
According to conventional technologies, treatment for recovering the NO_xocclusion function of the sulfur-poisoned NO_xoccluding material is performed. The recovery treatment is a process of allowing exhaust gas in a high-temperature rich atmosphere added with a reducing agent to flow in a pulsing manner to thereby reduce and decompose the sulfur-poisoned NO_xoccluding material.
For example, Japanese Patent No. 2605580 discloses a method of reducing and desorbing SO_xthrough the inflow of rich gas having a low oxygen concentration. According to this method, SO_xis found to be more easily removed under higher temperature conditions. Also, Japanese Patent Application Publication No. H08-061052 discloses a method of heating a catalyst to 800˜900° C. in order to emit SO_xfrom a sulfur-poisoned NO_xoccluding material. In addition, Japanese Patent Application Publication No. 2000-230447 discloses a method of decreasing the temperature of the recovery process by supplying a large amount of reduction gas such as CO.
However, in the case where the exhaust gas added with the reducing agent is supplied to the NO_xocclusion reduction catalyst, the temperature at the inlet of the catalyst is lower than that at the outlet thereof, and thus the NO_xocclusion function is insufficiently recovered near the inlet. Accordingly, Japanese Patent Application Publication No. 2002-013413 describes the reversed exhaust gas flow of the NO_xocclusion reduction catalyst upon recovery from sulfur poisoning. In a normal state, the temperature at the outlet of the catalyst is higher than that at the inlet thereof due to the reaction heat of the catalyst. Hence, when the exhaust gas flow direction is reversed upon recovery treatment, the original outlet having a high temperature becomes an inlet, and thus such heat is used to recover the NO_xocclusion function. On the other hand, when the original inlet becomes an outlet, it is brought into contact with the exhaust gas having a high temperature due to the reaction heat of the catalyst, and therefore the NO_xocclusion function may also be recovered at the original inlet.
Further, in the exhaust gas from a diesel engine, since harmful components are discharged in a particulate form (particulate material: carbon particles, sulfur particles such as sulfate, high-molecular-weight hydrocarbon particle and the like, hereinafter, referred to as “PM”), as compared with the gasoline engines, diesel engines have difficulty in purifying the exhaust gas.
The exhaust gas purifier for diesel engines developed to date is largely classified into a trap type exhaust gas purifier (wall flow) and an open type exhaust gas purifier (straight flow). In particular, the trap type exhaust gas purifier is known to be a clogged ceramic honeycomb body (diesel PM filter, hereinafter referred to as “DPF”). The DPF, in which both open ends of the cells of a ceramic honeycomb structure (e.g., a checkered pattern) comprises inlet cells, outlet cells and cell partition walls. The inlet cells each are clogged at the downstream side of the exhaust gas flow direction. The outlet cells each adjoin the respective inlet cells and clogged at the upstream side of the exhaust gas flow direction. The cell partition walls each partition the respective inlet cells and outlet cells. Through the fine pores in the cell partitions, the exhaust gas is filtered and PM is thus captured, consequently making it possible to suppress the discharge of PM.
However, in the DPF, since the loss of exhaust gas pressure is increased due to the accumulation of PM, the accumulated PM needs to be periodically removed using any means in order to regenerate the DPF. Thus, in the case where the loss of exhaust pressure is increased, there are proposed conventional methods of flowing high-temperature exhaust gas or performing a heating process using a burner or an electrical heater to combust the accumulated PM in order to regenerate the DPF. In such a case, however, as the accumulated amount of the PM increases, the combustion temperature also increases, undesirably producing heat stress. This heat stress often causes damage to DPF.
Recently, a continuous regenerative type DPF (filter catalyst) has been developed by forming a coating layer such as an alumina on the surface of the cell partition of the DPF and supporting noble metal such as platinum (Pt) on the coating layer. According to this filter catalyst, since the captured PM is oxidized and combusted through the catalytic reaction of noble metal, the PM may be combusted simultaneously with or successively to the capture thereof, thereby making it possible to recover the filter function. Further, the catalytic reaction occurs at a relatively low temperature and the PM may be combusted even though the collected amount thereof is small. Thereby, the DPF is advantageously prevented from breakage due to low heat stress applied thereto.
In addition, There is known a filter catalyst, in which a coating layer is further supported with an NO_xoccluding material selected from among alkali metals, alkali earth metals, and rare earth elements. According to such a filter catalyst, in the same lean atmosphere as in the NO_xocclusion reduction catalyst, NO_xis occluded on the NO_xoccluding material and NO_xemitted in a rich atmosphere is reduced, thereby increasing NO_xpurification performance highly. Therefore, in the case where the exhaust gas of a diesel engine is purified using the above filter catalyst, a system in which a reducing agent is intermittently added to the exhaust gas to thereby form a rich atmosphere is adopted. However, this filter catalyst having the NO_xoccluding materials supported thereon has low HC oxidation activity of noble metal. Thus various methods of attaching a reducing agent to a filter catalyst or decreasing accumulation of PM need to be researched.
For example, Japanese Patent Application Publication No. 2002-021544 discloses a purification technologies, in which an oxidation catalyst or an NO_xocclusion reduction catalyst is disposed at the upstream side of a filter catalyst, and HC is supplied to the exhaust gas through the post spray of fuel into a combustion chamber or through the addition of fuel to the exhaust gas. It is also described that the reaction heat of the oxidation catalyst or NO_xocclusion reduction catalyst make PM accumulated in the DPF or filter catalyst combusted and NO_xreduced and purified.
In this way, in the case where the reducing agent, such as light oil, is added to the exhaust gas, the reducing agent need to be added before the NO_xocclusion capability of the NO_xocclusion reduction catalyst is saturated, in order to recover the ability to adsorb NO_x. Accordingly, even upon acceleration or deceleration at a low speed, the reducing agent need to be added at relatively short time intervals. In such a case, however, since the temperature of the exhaust gas is relatively low and decreases further by the addition of the reducing agent, it is thus difficult to react the reducing agent with NO_x. Hence, the added reducing agent is attached to the filter catalyst in an unreacted state, and the supported catalyst metal is poisoned, and thus the activity thereof is decreased. Furthermore, while the PM adheres to the attached reducing agent, front end cells are undesirably clogged.
In the case of the filter catalyst, ash is increasingly accumulated in the catalyst, undesirably increasing the loss of exhaust gas pressure.
In the case where the NO_xocclusion reduction catalyst is disposed at the upstream side of the filter catalyst, there is a need for recovery treatment from sulfur poisoning with respect to the NO_xocclusion reduction catalyst. When the temperature of exhaust gas flowing into the NO_xocclusion reduction catalyst is, for example, 300° C., the temperature distribution of each catalyst is as illustrated in FIG. 9. In order to perform sufficient recovery treatment, a temperature not lower than 650° C. is required. Thus, when such high-temperature exhaust gas flows into the upstream NO_xocclusion reduction catalyst, the temperature of the downstream filter catalyst is increased, and noble metal supported on the filter catalyst may be deteriorated due to grain growth. Further, the PM accumulated in the filter catalyst may be combusted all at once, undesirably causing heat stress. As a result, a filter catalyst may be damaged.

SUMMARY OF THE INVENTION

Accordingly, The present invention has been made to solve the above-described problems occurring in the prior art, and Embodiments of the present invention provide an exhaust gas purifying apparatus, comprising an NO_xocclusion reduction catalyst disposed upstream thereof and a filter catalyst disposed downstream thereof, thus improving recovery from sulfur poisoning, preventing deterioration and breakage of the filter catalyst.
The exhaust gas purifying apparatus for achieving the above object of the present invention is characterized in that it comprises a reducing agent supply device supplying a reducing agent to exhaust gas, a first catalyst comprising an NO_xocclusion reduction catalyst obtained by forming a catalyst layer on a surface of a honeycomb substrate having a straight flow structure, the catalyst layer including a porous oxide support and an NO_xoccluding material and noble metal supported thereon, a second catalyst obtained by forming a catalyst layer at least on a surface of filter substrate having a wall flow structure and including a porous oxide support and at least a noble metal supported thereon, a container having at least the first catalyst and the second catalyst arranged in series, and a direction change device changing the flow direction of the exhaust gas in the container between a normal flow direction and a reverse flow direction, wherein in the normal flow direction, the first catalyst is disposed upstream of the exhaust gas flow direction and the second catalyst is disposed downstream thereof, in the reverse flow direction, the second catalyst is disposed upstream of the exhaust gas flow direction and the first catalyst is disposed downstream thereof.
In the container, the first catalyst, the second catalyst, and a third catalyst obtained by forming a catalyst layer consisting of a porous oxide support and noble metal supported thereon on the surface of a honeycomb substrate having a straight flow structure are sequentially arranged in series, and the direction change device preferably changes (reverses) the flow direction of the exhaust gas between a normal flow direction in a sequence of the first catalyst, the second catalyst, and then the third catalyst and a reverse flow direction in a sequence of the third catalyst, the second catalyst, and then the first catalyst.
In addition, the exhaust gas purifying method according to the present invention is characterized in that it comprises a normal flow process for allowing exhaust gas to typically flow and a recovery process for allowing exhaust gas in a high-temperature rich atmosphere, added with a reducing agent, to flow in a pulsing manner so that a sulfur-poisoned NO_xoccluding material is reduced to thereby recover the NO_xocclusion function, using the exhaust gas purifying apparatus of the present invention. In the recovery process, the flow direction of the exhaust gas upon the normal flow process is changed (reversed).
Further, the method of the present invention also includes a regeneration process for allowing the exhaust gas in a lean atmosphere added with a reducing agent to flow in a pulsing manner to thereby generate combustion heat which is then used to combust PM accumulated in the second catalyst so as to regenerate the PM collection function. In the regeneration process, the flow direction of the exhaust gas upon the normal flow process is changed (reversed).
The exhaust gas purifying apparatus of the present invention comprises the direction change device changing the flow direction of the exhaust gas in the container having the first catalyst, composed of an NO_xocclusion reduction catalyst, and the second catalyst, serving as a filter catalyst, arranged in series, between the normal flow direction in which the first catalyst is disposed upstream of the exhaust gas flow direction and the second catalyst is disposed downstream thereof and the reverse flow direction in which the second catalyst is disposed upstream of the exhaust gas flow direction and the first catalyst is disposed downstream thereof. Further, in the recovery process of the exhaust gas purifying method of the present invention, the flow direction of the exhaust gas upon the normal flow process is changed (reversed).
Thus, in the case where the exhaust gas flows in a normal flow direction toward the second catalyst from the first catalyst in the normal flow process, the flow direction of the exhaust gas in the recovery process becomes the direction toward the first catalyst from the second catalyst. Upon the normal flow process, the amount of sulfur poisoning is increased toward the upstream side at a low temperature, and is larger in the first catalyst than in the second catalyst and also is larger at the upstream side of the first catalyst than at the downstream side thereof. Accordingly, in the case where the exhaust gas flow direction is changed (reversed) in the recovery process so that the second catalyst is provided upstream with respect to the exhaust gas flow direction, the exhaust gas is further heated by the reaction heat of the second catalyst, and therefore the temperature of the first catalyst becomes higher than that of the second catalyst. Consequently, in the NO_xocclusion reduction catalyst constituting the first catalyst, recovery from sulfur poisoning is improved.
In the normal flow process, the second catalyst, which is downstream, has a higher temperature than the first catalyst, and the temperature of the downstream side of the second catalyst is higher than that of the upstream side thereof. That is, even though the temperature of the exhaust gas flowing into the second catalyst is decreased to 650° C. or less upon the recovery process, the temperature of the exhaust gas flowing into the first catalyst is high, and thus the first catalyst may sufficiently recover from sulfur poisoning. Hence, it is possible to inhibit the deterioration and breakage of the second catalyst (filter catalyst) due to heat.
In the case where the second catalyst (filter catalyst) further includes the NO_xoccluding materials supported thereon, the amount of sulfur poisoning is larger at the upstream side thereof than at the downstream side thereof upon the normal flow process. When the exhaust gas flow direction is changed (reversed) upon the recovery process, the temperature of the exhaust gas at the outlet of the second catalyst is sufficiently increased even at the low temperature of the exhaust gas flowing into the second catalyst, and thus the recovery treatment of the second catalyst may be realized, and the deterioration and breakage of the second catalyst (filter catalyst) due to heat may be inhibited at the same time.
In the normal flow process, ash accumulated in the second catalyst (filter catalyst) is blown by the exhaust gas reversely flowing upon the recovery process, and is then passed through the first catalyst to thereby discharge it. Thereby, excessive ash accumulation in the second catalyst (filter catalyst) may be prevented, and the increase in the loss of exhaust pressure may be inhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments, given in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view illustrating the exhaust gas purifying apparatus according to a first embodiment of the present invention, in which exhaust gas flows in a normal flow direction,

FIG. 2 is a schematic view illustrating the exhaust gas purifying apparatus according to the first embodiment of the present invention, in which exhaust gas flows in a reverse flow direction,

FIG. 3 is a schematic view illustrating the exhaust gas purifying apparatus according to a second embodiment of the present invention, in which exhaust gas flows in a normal flow direction,

FIG. 4 is a schematic view illustrating the exhaust gas purifying apparatus according to the second embodiment of the present invention, in which exhaust gas flows in a reverse flow direction,

FIG. 5 is a graph illustrating the concentration distribution of sulfur,

FIG. 6 is a graph illustrating the concentration distribution of ash,

FIG. 7 is a schematic view illustrating the exhaust gas purifying apparatus according to a third embodiment of the present invention, in which exhaust gas flows in a normal flow direction,

FIG. 8 is a schematic view illustrating the exhaust gas purifying apparatus according to a fifth embodiment of the present invention, in which exhaust gas flows in a normal flow direction, and

FIG. 9 is a schematic view illustrating a general catalyst temperature distribution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
According to the present invention, the apparatus for purifying exhaust gas comprises a first catalyst and a second catalyst. The first catalyst comprises an NO_xocclusion reduction catalyst, that is, may be composed of only an NO_xocclusion reduction catalyst, or of a three-way catalyst or an oxidation catalyst and an NO_xocclusion reduction catalyst that have been applied and divided.
The NO_xocclusion reduction catalyst is obtained by forming a catalyst layer, consisting of a porous oxide support and an NO_xoccluding material and noble metal supported thereon, on the surface of a honeycomb substrate having a straight flow structure. Alternatively, a conventional NO_xocclusion reduction catalyst may be used. Examples of the honeycomb substrate include a monolithic honeycomb substrate, formed of heat resistant ceramics such as cordierite, or a metallic honeycomb substrate formed of a metal foil. The porous oxide is selected from among alumina, titania, zirconia, silica, ceria, composite oxides formed of a plurality of species thereof, and mixtures thereof.
The NO_xoccluding material is at least one selected from among alkali metals, alkali earth metals, and rare earth elements, and a mixture of alkali metal and alkali earth metal is preferably useful. The NO_xoccluding material is preferably supported in a range of an amount of 0.05˜1 mol per liter of the honeycomb substrate. The noble metal is selected from among Pt, Rh, Pd, Ru, and Ir, and Pt having high oxidation activity is particularly useful. The noble metal is preferably supported in a range of an amount of 0.1˜5 g per liter of the honeycomb substrate.
The second catalyst is a filter catalyst obtained by forming a catalyst layer consisting of a porous oxide support and at least a noble metal supported thereon on at least the surface of a filter substrate having a wall flow structure. The filter substrate is composed of inlet cells clogged at the downstream side of the exhaust gas flow direction, outlet cells adjacent to the inlet cells and clogged at the upstream side of the exhaust gas flow direction, and porous cell partitions having a plurality of fine pores and sectioning the inlet cells and the outlet cells. As the filter substrate, a conventional DPF made of heat resistant ceramics such as cordierite or silicon carbide may be used.
The fine pores of the cell partition of the filter substrate are preferably distributed to have porosity of 40˜80% and an average diameter of 10˜50 μm. In the case where the porosity or average diameter falls outside of the above range, PM capture efficiency is decreased and the loss of exhaust pressure may be increased.
On at least the surface of the filter substrate, a catalyst layer, including a porous oxide support and at least the noble metal supported thereon, is formed. Further, the catalyst layer is preferably formed on the inner surface of the fine pores of the cell partition. The porous oxide is selected from among alumina, titania, zirconia, silica, ceria, composite oxides formed of plurality of species thereof, and mixtures thereof.
The noble metal is one or more selected from among noble metals of a platinum group, including Pt, Rh, Pd, Ru, and Ir. The noble metal is preferably supported in a range of an amount of 0.1˜5 g per liter of the filter substrate. If the amount is less than the lower limit, the activity is very low and thus is unusable. On the other hand, if the amount exceeds the upper limit, the activity is saturated and the cost is increased.
Preferably, the catalyst layer of the second catalyst further includes a NO_xoccluding material which is selected from among alkali metals, alkali earth metals, and rare earth elements, and which is supported thereon, as in the first catalyst. Due to the NO_xoccluding material included in the catalyst layer, NO_xpurification activity is increased. The NO_xoccluding material is preferably supported in a range of an amount of 0.05˜1 mol per liter of the filter substrate. If the amount is less than the lower limit, the activity is very low and thus unusable. On the other hand, if the amount exceeds the upper limit, the catalyst metal is covered, and thus the activity thereof is decreased.
In order to form the catalyst layer on the filter substrate, porous oxide powder is formed into a slurry along with a binder component, such as alumina sol, and water. Subsequently, the slurry is attached to the partition and then burned. Subsequently, noble metal and an NO_xoccluding material are supported thereon. Although the attachment of the slurry to the cell partition may be realized using a typical dipping process, the slurry may be forcibly charged in the fine pores of the cell partition through air blowing or suction. Here, the slurry remaining after being charged in the fine pores is preferably removed.
The catalyst layer is formed in a range of an amount of 30˜200 g per liter of the filter substrate. If the amount of the catalyst layer is less than the lower limit, the durability of the noble metal or NO_xoccluding material is decreased. On the other hand, if the amount exceeds the upper limit, the loss of pressure is excessively increased and thus is unusable.
The exhaust gas purifying apparatus of the present invention comprises a reducing agent supply device supplying a reducing agent to the exhaust gas. For example, an injector directly supplying a liquid reducing agent, such as light oil, to the exhaust gas may be used. In addition, a reducing agent may be supplied to the exhaust gas under a fuel rich condition based on the air-to-fuel ratio in the combustion chamber. In addition, active HC, resulting from partial oxidation of a liquid reducing agent, such as light oil, may be supplied to the exhaust gas. To this end, it is preferred that an oxidation catalyst be disposed in the uppermost end of the exhaust gas flow direction in the recovery process.
Also, in the container, a third catalyst is further connected to the first catalyst and the second catalyst in series, and thus the first catalyst, the second catalyst, and the third catalyst, in that order, are preferably arranged. The third catalyst is preferably at least one selected from among an NO_xocclusion reduction catalyst, a three-way catalyst, and an oxidation catalyst, and preferably includes at least the NO_xocclusion reduction catalyst.
For example, in the case where the NO_xocclusion reduction catalyst is used as the third catalyst, the same purification performance may be assured even when the normal flow process is performed without changing the direction after the recovery process. Thus, in the normal flow process, since the second catalyst (filter catalyst) may be used in two directions, PM and ash accumulated in the fine pores of the cell partition are always discharged when the flow direction is changed, and an increase in the loss of pressure may be effectively inhibited.
In addition, in the case where the third catalyst is composed of the three-way catalyst or oxidation catalyst, the exhaust gas primarily flows into the third catalyst in the recovery process. Since the third catalyst has high oxidation activity, heat generated when part of the reducing agent in the exhaust gas is oxidized is responsible for drastically increasing the temperature of the exhaust gas. Ultimately, even though low-temperature exhaust gas is supplied, recovery from sulfur poisoning remains high, thereby improving such capability.
In addition, in the case where the first catalyst is provided upstream side of the exhaust gas flow direction, the first catalyst is preferably composed of an oxidation catalyst or a three-way catalyst disposed at the upstream side of the exhaust gas and an NO_xocclusion reduction catalyst disposed at the downstream side thereof. On the other hand, in the case where the third catalyst is provided upstream side of the exhaust gas flow direction, the third catalyst is preferably composed of an oxidation catalyst or three-way catalyst disposed at the upstream side of the exhaust gas and an NO_xocclusion reduction catalyst disposed at the downstream side thereof. In such cases, both functions are realized, so that the increase in the loss of exhaust pressure is inhibited and recovery from sulfur poisoning is improved.
In the exhaust gas purifying method of the present invention, the exhaust gas atmosphere in a normal flow process may be either a lean-burn atmosphere or an alternating lean/rich atmosphere. In the latter case, the rich atmosphere may also be provided in a recovery process or in a regeneration process, as described below.
In the exhaust gas purifying method of the present invention, the temperature of the exhaust gas containing the reducing agent upon the recovery process preferably ranges from 650 to 700° C. If the temperature is above the upper limit, grain growth of the noble metal in the downstream catalyst results, or PM accumulated in the second catalyst is rapidly combusted, undesirably resulting in a broken filter substrate. On the other hand, if the temperature is below the lower limit, recovery from sulfur poisoning is decreased.
In the exhaust gas purifying method of the present invention, a regeneration process is preferably further performed by allowing the exhaust gas in the lean atmosphere added with the reducing agent to flow in a pulsing manner to thereby generate combustion heat which is then used to combust the PM accumulated in the second catalyst, thus regenerating the PM capture function. Although the accumulated PM may be combusted through the recovery process, since the recovery process is not conducted in concurrence with the regeneration process all of the time, the regeneration process is preferably carried out in the case where the loss value of exhaust pressure, determined by continuous detection, falls within a predetermined range. As such, in the regeneration process, the reducing atmosphere is typically weaker than in the recovery process. Hence, it is preferred that the regeneration process precede the recovery process. In the case where the recovery process precedes the regeneration process, the temperature is drastically increased and thus the honeycomb substrate may be cracked, or may be melted and damaged. Consequently, the regeneration process, which acts to gradually increase the temperature, preferably precedes the recovery process.
Further, upon the regeneration process, the flow direction of the exhaust gas in the normal flow process is preferably changed. The amount of PM capture by the filter substrate is larger at the upstream side (inlet cell) of the exhaust gas flow direction upon the normal flow process. Also, a liquid reducing agent is attached to an opening portion of the passage of the inlet cell, and also a large amount of PM may adhere thereto. Therefore, in the case where the flow direction of exhaust gas is changed in the regeneration process, the PM and ash may be blown by the flow of exhaust gas, resulting in increased regeneration efficiency.

EXAMPLE

Below, the present invention is described in detail through the following examples and comparative examples.

Example 1

FIGS. 1 and 2 schematically illustrate the exhaust gas purifying apparatus according to the present invention. In the exhaust gas purifying apparatus, an NO_xocclusion reduction catalyst 2 (hereinafter referred to as NSR2) as a first catalyst and a filter catalyst 3 (hereinafter, referred to as DPNR3) as a second catalyst are sequentially arranged in series in a catalytic converter 1. As such, DPNR3 is an NO_xocclusion reduction type filter catalyst. An exhaust pipe 100 from an exhaust manifold is divided into two passages, that is, a first passage 101 and a second passage 102, in front of the catalytic converter 1. Then, the first passage 101 and the second passage 102 are combined with each other again. That is, the first passage 101 and the second passage 102 are respectively disposed at either side of the catalytic converter 1, and are then connected to each other. Further, in the divided portion of the exhaust pipe 100, a first valve 200 for switching the exhaust gas from the exhaust pipe 100 to the first passage 101 or the second passage 102 is disposed. In addition, in the first passage 101, a second valve 201 for turning on or off communication between the opening of the catalytic converter 1 and the first passage 101 is disposed. In addition, in the second passage 102, a third valve 202 for turning on or off communication between the other opening of the catalytic converter 1 and the second passage 102 is disposed. In addition, in the exhaust pipe 100, an injector 103 for adding light oil to the exhaust gas is further provided.
The NSR2 comprises a cordierite-based honeycomb substrate (0.8 L, cell number 400/in²) having a straight flow structure and 270 g/L of a catalyst layer formed thereon, the catalyst layer including K, Ba, Li, and 5 g/L of Pt, supported thereon. Further, the DPNR3 comprises a cordierite-based filter substrate (2.0 L, cell number 300/in²) having a wall flow structure and 150 g/L of a catalyst layer, the catalyst layer including K, Ba, Li, and 5 g/L of Pt, supported thereon. The catalyst layer is formed not only on the surface of the cell partition but also on the inner surface of fine pores thereof.
<Test>
The exhaust gas purifying apparatus thus constructed was mounted to the exhaust system of a diesel engine for direct spray, having 2 L of exhaust air volume, on an engine bench. Further, while controlling lean and rich atmospheres to supply a rich spike for 0.2 sec at intervals of 30 sec, a normal flow process for 100 hours (about 5000 km) was conducted under conditions simulating 11 Lap driven by actual automobiles. At intervals of 10 hours during the normal flow process, a recovery process for adding light oil to the exhaust gas at a flow rate of 1000 cm³/min for 200 sec using the injector 103 was performed.
In the normal flow process, as illustrated in FIG. 1, the second passage 102 is closed by the first valve 200, the communication between the first passage 101 and the catalytic converter 1 is allowed by the second valve 201, and the communication between the second passage 102 and the catalytic converter 1 is blocked by the third valve 202. Accordingly, the exhaust gas flows into the catalytic converter 1 from the first passage 101, sequentially passes through the NSR2 and DPNR3, and is then discharged from the second passage 102.
In the recovery process, as illustrated in FIG. 2, the first passage 101 is closed by the first valve 200, communication between the second passage 102 and the catalytic converter 1 is allowed by the third valve 202, and communication between the first passage 101 and the catalytic converter 1 is blocked by the second valve 201. Thus, the exhaust gas flows into the catalytic converter 1 from the second passage 102, sequentially passes through the DPNR3 and NSR2, and is then discharged from the first passage 101.
In the case where the exhaust gas flow direction shown in FIG. 1 is set in a normal flow direction and the exhaust gas flow direction shown in FIG. 2 is set in a reverse flow direction, the exhaust gas flow direction in the present example is given in Table 1 below.

	TABLE 1

	Process

Normal		Normal		Normal
Flow	Recovery	Flow	Recovery	Flow	. . .

Flow	Normal	Reverse	Normal	Reverse	Normal	. . .
Direction

After the completion of the operation, the NO_xpurification efficiency in the normal flow process was measured. The results are given in Table 5 below.

Example 2

As illustrated in FIGS. 3 and 4, the exhaust gas purifying apparatus of the present example is the same exhaust gas purifying apparatus of Example 1, with the exception that NSR2, DPNR3, and NSR2 are sequentially arranged in a catalytic converter 1. The NSR2 provided at each of the two sides of DPNR3 is the same as the case in which the NSR2 of Example 1 is halved. As in Example 1, such an exhaust gas purifying apparatus was mounted to the 2 L exhaust system of a diesel engine for direct spray, and the same normal flow process and recovery process were conducted.
After the recovery process, the normal flow process was conducted under unchanged valve set conditions, and then the recovery process was conducted with the exhaust gas flow direction changed. That is, in the state shown in FIG. 3, the normal flow process was performed for 10 hours, after which the recovery process was performed in the state shown in FIG. 4, in which the valves were converted. Thereafter, the normal flow process was conducted for 10 hours under unchanged conditions, and then the recovery process was conducted in the state shown in FIG. 3, in which the valves were converted again, after which the normal flow process was conducted for 10 hours under unchanged conditions. These procedures were repeated.
In the case where the exhaust gas flow direction shown in FIG. 3 is set in a normal flow direction and the exhaust gas flow direction shown in FIG. 4 is set in a reverse flow direction, the exhaust gas flow direction of the present example is given in Table 2 below.

	TABLE 2

	Process

Normal		Normal		Normal		Normal
Flow	Recovery	Flow	Recovery	Flow	Recovery	Flow	. . .

Flow	Normal	Reverse	Reverse	Normal	Normal	Reverse	Reverse	. . .
Direction

Further, in the recovery process, when the exhaust gas flow direction is any one among the normal flow direction and the reverse flow direction, the exhaust gas heated in the NSR2 flows into the DPNR3. Thus, in the recovery process, a regeneration process for combusting the PM accumulated in the DPNR3 may be further performed. Accordingly, Table 2 is also given like Table 3 below.

	TABLE 3

	Process

Normal	Regener-		Normal	Regener-		Normal
Flow	ation	Recovery	Flow	ation	Recovery	Flow	. . .

Flow	Normal	Reverse	Reverse	Reverse	Normal	Normal	Normal	. . .
Direction

Thereafter, the NO_xpurification efficiency was measured using the method of Example 1. Further, after measurement of the NO_xpurification efficiency, the NSR2 and DPNR3 were decomposed and the amount of sulfur poisoning was determined through element analysis. In addition, from the difference in weight compared to before the test, the amount of ash after the combustion of PM was calculated. The amounts of sulfur poisoning and ash were measured at the ends of the inlets and outlets of the catalysts. The results are given Table 5 and FIGS. 5 and 6.

Comparative Example 1

The normal flow process and recovery process were conducted in the same manner as in Example 2 using that the exhaust gas purifying apparatus of Example 2, with the exception the state shown in FIG. 3 was maintained and thus the exhaust gas flow was not reversed.
In the case where the exhaust gas flow direction of FIG. 3 is set in a normal flow direction and the exhaust gas flow direction of FIG. 4 is set in a reverse flow direction, the exhaust gas flow direction in the present comparative example is given in Table 4 below. That is, in the present comparative example, in any one among the normal flow process and the recovery process, the exhaust gas flow direction is maintained in a normal flow direction.

	TABLE 4

	Process

Flow	Normal	Normal	Normal	Normal	Normal	Normal	Normal	. . .
Direction

Further, the NO_xpurification efficiency was measured through the method of Example 1 and the amounts of sulfur poisoning and ash were measured through the method of Example 2. The results are given in Table 5 and FIGS. 5 and 6.
<Evaluation>

	TABLE 5

	No.	NO_xPurification (%)

	Example 1	80
	Example 2	85
	Comparative Example 1	64

According to the exhaust gas purifying method in the examples, even after the diesel engine was driven for 100 hours, the NO_xpurification efficiency was higher than in Comparative Example 1. In Example 2, the amount of sulfur poisoning was much lower than in Comparative Example 1. From this, in the examples, high NO_xpurification efficiency is considered to have been realized by virtue of high recovery from sulfur poisoning.
Further, in Comparative Example 1, related to the conventional exhaust gas purifying method, the amount of sulfur poisoning was higher toward the upstream side, and the sulfur poisoning of NSR2 in the uppermost end was not recovered even through the recovery process. As in Example 2, the exhaust gas flow direction was changed to thereby perform the recovery process, whereby the NSR2 on both sides of DPNR3 and the DPNR3 equally recovered from sulfur poisoning, and the ash amount of the DPNR3 was decreased.

Example 3

As illustrated in FIG. 7, the exhaust gas purifying apparatus of the present invention is the same exhaust gas purifying apparatus as in Example 1, with the exception that NSR2, DPNR3, and an oxidation catalyst 4 (hereinafter referred to “CCo4”) are sequentially arranged in a catalytic converter 1. As such, the NSR2 is the same as the case in which the NSR2 of Example 1 is halved. Further, CCo4 is composed of a cordierite-based honeycomb substrate (2.0 L, cell number of 400/in²) having a straight flow structure and 160 g/L of a catalyst layer formed thereon, the catalyst layer including 5 g/L of Pt supported thereon.
Such an exhaust gas purifying apparatus was mounted to the 2 L exhaust system of a diesel engine for direct spray, as in Example 1, and a normal flow process and a recovery process were performed, as in Example 2.
In the case where the exhaust gas flow direction shown in FIG. 7 is set in a normal flow direction and the exhaust gas flow direction shown in FIG. 4 is set in a reverse flow direction, the exhaust gas flow direction of the present example is the same as in Example 2, and is given in Table 6 below.

	TABLE 6

	Process

Flow	Normal	Reverse	Reverse	Normal	Normal	Reverse	Reverse	. . .
Direction

Using the method of Example 1, the NO_xpurification efficiency was measured, and also the HC purification efficiency was measured in the normal flow process. Further, using the method of Example 2, the amounts of sulfur poisoning and ash were measured. The results are shown in Table 10 below. Since the amounts of sulfur poisoning and ash are the same as in Example 2, a figure related thereto is omitted.

Example 4

The normal flow process and recovery process were conducted in the same manner as in Example 2 using the exhaust gas purifying apparatus of Example 3, with the exception that the exhaust gas flow direction was changed only in the recovery process. That is, after the recovery process, the exhaust gas flow direction was changed again and thus the normal flow process was conducted. In the subsequent recovery process, the exhaust gas flow direction was changed. That is, in the state shown in FIG. 7, a normal flow process was performed for 10 hours. Thereafter, respective valves were converted to thereby set the state corresponding to FIG. 4, and thus the recovery process was performed, and then the normal flow process was performed for 10 hours in the state shown in FIG. 7. These procedures were repeated.
In the case where the exhaust gas flow direction shown in FIG. 7 is set in a normal flow direction and the exhaust gas flow direction shown in FIG. 4 is set in a reverse flow direction, the exhaust gas flow direction of the present example is shown in Table 7 below.

	TABLE 7

	Process

Flow	Normal	Reverse	Normal	Reverse	Normal	Reverse	Normal	. . .
Direction

Using the method of Example 3, the NO_xpurification efficiency and the HC purification efficiency were measured, and the amounts of sulfur poisoning and ash were measured using the method of Example 2. The results are shown in Table 10 below. Also, since the amounts of sulfur poisoning and ash are the same as in Example 2, a figure related thereto is omitted.

Example 5

As illustrated in FIG. 8, the exhaust gas purifying apparatus of the present example is the same exhaust gas purifying apparatus as in Example 1, with the exception that CCo4, NSR2, DPNR3, NSR2, and CCo4 are sequentially arranged in a catalytic converter 1. The NSR2 and CCo4 are the same as the case in which each of the NSR2 and CCo4 of Example 4 is halved. In the present example, although the NSR2 and CCo4 are separately formed, the respective catalyst layers of NSR and CCo4 may be formed on a single honeycomb substrate.
Such an exhaust gas purifying apparatus was mounted to a 2 L exhaust system of a diesel engine for direct spray as in Example 1, and a normal flow process and a recovery process were conducted according to the method of Example 3.
In the case where the exhaust gas flow direction shown in FIG. 8 is set in a normal flow direction and the exhaust gas flow direction shown in FIG. 4 is set in a reverse flow direction, the exhaust gas flow direction of the present example is shown in Table 8 below.

	TABLE 8

	Process

Flow	Normal	Reverse	Reverse	Normal	Normal	Reverse	Reverse	. . .
Direction

Using the method of Example 3, NO_xpurification efficiency and the HC purification efficiency were measured, and the amounts of sulfur poisoning and ash were measured using the method of Example 2. The results are shown in Table 10 below. Also, since the amounts of sulfur poisoning and ash are the same as in Example 2, a figure related thereto is omitted.

Comparative Example 2

The normal flow process and recovery process were conducted in the same manner as in Example 3 using the exhaust gas purifying apparatus of Example 3, with the exception that the state shown in FIG. 7 was maintained and thus the exhaust gas flow direction was not reversed.
In the case where the exhaust gas flow direction shown in FIG. 7 is set in a normal flow direction and the exhaust gas flow direction shown in FIG. 4 is set in a reverse flow direction, the exhaust gas flow direction of the present comparative example is given in Table 9 below. In the present comparative example, in any one of the normal flow process and the recovery process, the exhaust gas flow direction was maintained in a normal flow direction.

	TABLE 9

	Process

Flow	Normal	Normal	Normal	Normal	Normal	Normal	Normal	. . .
Direction

Using the method of Example 3, the NO_xpurification efficiency and the HC purification efficiency were measured, and the amounts of sulfur poisoning and ash were measured using the method of Example 2. The results are shown in Table 10 below. Also, since the amounts of sulfur poisoning and ash are the same as in Comparative Example 1, a figure related thereto is omitted.
<Evaluation>

TABLE 10

No.	NO_xPurification (%)	HC Purification (%)

Example 3	75	71
Example 4	82	75
Example 5	85	74
Comparative Example 2	64	70

According to the exhaust gas purifying method of the examples, even after the diesel engine was driven for 100 hours, the NO_xpurification efficiency and HC purification efficiency were higher than in Comparative Example 2. In respective examples, the amount of sulfur poisoning was much lower than in Comparative Example 2. Consequently, high NO_xpurification efficiency is considered to have been realized due to high recovery from sulfur poisoning in the examples.
In Comparative Example 2, since the amount of sulfur poisoning was increased toward the upstream side, the sulfur poisoning of NSR2 was not recovered even through the recovery process. However, when the exhaust gas flow direction was reversed to perform the recovery process as in Example 3, the NSR2 and DPNR3 equally recovered from sulfur poisoning, and the ash amount of the DPNR3 was decreased.
However, when comparing the examples, the NO_xpurification efficiency of Example 3 was slightly lower. This is because the rich spike is consumed in the CCo4 due to the flow of the exhaust gas in the sequence of CCo4, DPNR3, NSR2 in the normal flow process upon measurement. Thus, it is preferred that the NSR2 be disposed on both sides of the DPNR3.
As described hereinbefore, the exhaust gas purifying apparatus and the exhaust gas purifying method, according to the present invention, can be applied not only to the purification of exhaust gases from diesel engines but also to the purification of exhaust gases from gasoline engines, gas engines, boilers, etc.
While the invention has been shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modification may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims

1. An exhaust gas purifying apparatus, comprising:

a reducing agent supply device supplying a reducing agent to exhaust gas;

a first catalyst comprising an NO_xocclusion reduction catalyst obtained by forming a catalyst layer on a surface of a honeycomb substrate having a straight flow structure, the catalyst layer including a porous oxide support and an NO_xoccluding material and noble metal supported thereon;

a second catalyst obtained by forming a catalyst layer at least on a surface of a filter substrate having a wall flow structure and including a porous oxide support and at least a noble metal supported thereon;

a container having at least the first catalyst and the second catalyst arranged in series; and

a direction change device changing a flow direction of exhaust gas in the container between a normal flow direction and a reverse flow direction, wherein the normal flow direction, the first catalyst is provided upstream side of an exhaust gas flow direction and the second catalyst is provided downstream side thereof, and in the reverse flow direction, the second catalyst is provided upstream side of the exhaust gas flow direction and the first catalyst is provided downstream side thereof.

2. The apparatus according to claim 1, wherein the second catalyst further comprises an NO_xoccluding material supported on the catalyst layer.

3. The apparatus according to claim 1, wherein, in the container, the first catalyst, the second catalyst, and a third catalyst are arranged in sequence, the third catalyst obtained by forming a catalyst layer on a surface of a honeycomb substrate having a straight flow structure, the catalyst layer including a porous oxide support and a noble metal supported thereon, and

the direction change device changes the flow direction of the exhaust gas between a normal flow direction in a sequence of the first catalyst, the second catalyst, and then the third catalyst and a reverse flow direction in a sequence of the third catalyst, the second catalyst, and then the first catalyst.

4. The apparatus according to claim 3, wherein the third catalyst is at least one selected from among an NO_xocclusion reduction catalyst, a three-way catalyst, and an oxidation catalyst.

5. The apparatus according to claim 3, wherein the first catalyst comprises an oxidation catalyst or a three-way catalyst disposed at an upstream side thereof and a NO_xocclusion reduction catalyst disposed at a downstream side thereof when the first catalyst is provided upstream side of the exhaust gas flow direction, and

the third catalyst comprises an oxidation catalyst or a three-way catalyst disposed at an upstream side thereof and an NO_xocclusion reduction catalyst disposed at a downstream side thereof when the third catalyst is provided upstream side of the exhaust gas flow direction.

6. An exhaust gas purifying method using the exhaust gas purifying apparatus of claim 1, comprising a normal flow process for allowing exhaust gas to normally flow and a recovery process for allowing exhaust gas in a high-temperature rich atmosphere added with a reducing agent to flow in a pulsing manner so that a sulfur-poisoned NO_xoccluding material is reduced to thereby recover an NO_xocclusion function, in which a flow direction of the exhaust gas in the normal flow process is changed in the recovery process.

7. The method according to claim 6, further comprising a regeneration process for allowing the exhaust gas in a lean atmosphere added with the reducing agent to flow in a pulsing manner to thereby generate combustion heat which is then used to combust PM accumulated in the second catalyst, thus regenerating a PM capture function, in which the flow direction of the exhaust gas in the normal flow process is changed in the regeneration process.

8. The method according to claim 7, wherein the regeneration process precedes the recovery process.