JP2016079912A - Exhaust emission control system and emission control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify an exhaust emission control system while striking a balance between oxidation combustion of particulate and reduction cleanup of NOx.SOLUTION: In the exhaust emission control system, a filter 5 for collecting the particulate is provided in an exhaust gas passage 1 of an engine, and an SCR catalyst for selectively reducing NOx in the presence of a reductant agent and an alkaline-earth metal for combusting the particulate are supported to the filter 5.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an exhaust gas purification device and a purification method for an engine.

  NOx (nitrogen oxide) and particulates are contained in exhaust gas discharged from diesel engines and lean burn gasoline engines. Conventionally, various proposals have been made to process this NOx and / or particulates.

Patent Document 1 describes an example of an apparatus for reducing and purifying NOx. The purification device includes a porous filter manufactured from a catalyst, and an exhaust gas flow passage is formed in the filter. NH ₃ or hydrocarbon is supplied to the filter as a reducing agent, and NOx is selectively reduced and purified. As the catalyst, _a spinel having the general formula A _a B _b O ₄ is adopted, and it is preferable that this spinel has a catalytically active element such as Pd, Pt, Rh.

  Patent Document 2 describes that a particulate oxidation catalyst is supported on a particulate filter. As the oxidation catalyst, a Zr-based composite oxide containing rare earth metals other than Ce and Y, and a Ce-based composite oxide containing rare earth metals or alkaline earth metals other than Ce are employed. Pt is supported on both the Zr-based composite oxide and the Ce-based composite oxide. Patent Document 3 describes a particulate filter with catalyst containing Pt-supported alumina, CeZr-based composite oxide, and ZrNd-based composite oxide. In the catalysts described in Patent Documents 2 and 3, the combustion of particulates is promoted by the active oxygen released from the composite oxide.

  Patent Document 4 describes a particulate filter catalyst that supports a particulate material having a predetermined particle size distribution. The granular materials include inorganic ceramics such as alumina, zirconia, titania, silica, zeolite, alkali metal oxides, oxides of alkaline earth metals such as magnesia spinel and magnesia, and oxides of rare earth elements such as lantana and neodia. Is listed. The granular material preferably contains an alkali metal element and / or an alkaline earth metal element in order to impart the ability to absorb and reduce NOx. However, this document 4 does not disclose a specific example of a granular material containing an alkaline earth metal.

JP 09-173784 A JP 2007-54713 A JP 2009-39632 A JP 2005-152774 A

When an SCR catalyst using NH ₃ as a reducing agent is employed for NOx purification as in Patent Document 1, in order to prevent excess NH ₃ from being discharged into the atmosphere, the exhaust gas flow is more than that of the SCR catalyst. An NH ₃ oxidation catalyst is provided downstream in the direction. Then, in order to process NOx and particulates in the exhaust gas, it is necessary to sequentially arrange the filter with catalyst, the SCR catalyst, and the NH ₃ oxidation catalyst in the exhaust gas flow direction. Therefore, the entire exhaust gas purification device becomes large and its layout becomes difficult.

  In addition, in order to efficiently regenerate the filter (combustion removal of the collected particulates), an oxidation catalyst is provided on the upstream side of the filter, and the temperature of the filter is adjusted using the heat of reaction in this oxidation catalyst. It is desirable to raise it. In that case, the entire exhaust gas purification device becomes even larger.

  Therefore, the present inventor intends to integrate the particulate filter and the SCR catalyst in order to cope with the problem of increasing the size of the purifier and to make the SCR catalyst function at a position where the engine exhaust gas temperature is as high as possible. That is, it was examined that the SCR catalyst is supported on the filter. However, since the SCR catalyst is a reduction catalyst, the filter must also carry an oxidation catalyst for burning particulates. In that case, since the functions of the oxidation catalyst and the reduction catalyst are contradictory, compatibility between particulate oxidation combustion and NOx reduction purification becomes a problem. That is, when the reducing agent for purifying NOx to be supplied to the SCR catalyst is oxidized by the oxidation catalyst for particulate combustion, it interferes with the selective reduction of NOx by the SCR catalyst.

  It is an object of the present invention to simplify an exhaust gas purification device while achieving both oxidation and combustion of particulates and reduction and purification of NOx.

  As a result of experiments and examinations regarding the combustion of particulates, the present inventor burns particulates even when an alkaline earth metal is supported on a filter without using an oxidation catalyst metal such as Pt or Pd. As a result, the present invention has been completed.

That is, the exhaust gas purification device disclosed herein is a device suitable for processing NOx and particulates in exhaust gas discharged from an engine,
A filter for collecting the particulates is provided in the exhaust gas passage of the engine, and the SCR catalyst for selectively reducing the NOx in the presence of a reducing agent and an alkaline earth metal are supported on the filter. It is characterized by that.

  According to this configuration, NOx in the exhaust gas is trapped (occluded) by the alkaline earth metal supported on the filter, and alkaline earth metal nitrate is generated. As the temperature of the filter rises, active oxygen is released by thermal decomposition of the nitrate. The active oxygen becomes an oxidizing agent and the particulates collected in the filter burn. In this case, the combustion of the particulates is advanced by the action of active oxygen even if no catalytic noble metal such as Pt or Pd is present.

  When a reducing agent is supplied to the filter, NOx in the exhaust gas is reduced and purified by the SCR catalyst in the presence of the reducing agent. Although the alkaline earth metal supported on the filter acts on the combustion of the particulates as described above, the alkaline earth metal itself does not have an oxidation catalyst function for oxidizing the reducing agent. Therefore, the reducing agent supplied to the filter is used for selective reduction of NOx by the SCR catalyst without being oxidized by the alkaline earth metal.

  As described above, according to the present invention, since the alkaline earth metal is used for burning the particulates, the burning of the particulates and the selective reduction of NOx by the SCR catalyst are compatible.

  Then, since the SCR catalyst is supported on the particulate collection filter, that is, the filter and the SCR catalyst are integrated, it is not necessary to separately provide a carrier dedicated to the SCR catalyst, and the exhaust gas purification device is simplified. Therefore, the layout to the exhaust gas passage becomes easy.

  Here, it is preferable that neither Pt nor Pd is supported on the filter so that the reducing agent supplied to the filter is not oxidized before being used for the selective reduction of NOx by the SCR catalyst.

  In a preferred aspect of the present invention, an oxidation catalyst containing Pt-supported alumina is disposed upstream of the filter in the exhaust gas passage in the exhaust gas flow direction.

According to this, when the engine exhaust gas contains a relatively large amount of NO, the NO can be oxidized to NO ₂ that is easily trapped by the alkaline earth metal by the oxidation catalyst and supplied to the filter. As a result, trapping of NOx by alkaline earth metal proceeds efficiently, which is advantageous for particulate combustion.

  In a preferred embodiment of the present invention, the filter further supports a rare earth metal other than Ce and a Zr composite oxide containing Zr, and the alkaline earth metal is supported on the Zr composite oxide. ing.

  Accordingly, the Zr-based composite oxide becomes a support material for the alkaline earth metal, and the alkaline earth metal can be supported on the filter with high dispersion. Since the Zr-based composite oxide releases active oxygen due to its oxygen ion conductivity, the combustion of particulates is promoted.

In a preferred aspect of the present invention, the apparatus comprises an injection means for injecting NH ₃ or urea as an NH ₃ precursor into the exhaust gas passage so as to supply NH ₃ as the reducing agent to the SCR catalyst,
An NH ₃ oxidation catalyst for oxidizing NH ₃ and / or a derivative thereof is disposed downstream of the filter in the exhaust gas passage in the exhaust gas flow direction.

In the selective reduction of NOx by the SCR catalyst, when NH ₃ is used as a reducing agent, an unpleasant odor is emitted when NH _{3 that} has not been used for selective reduction of NOx is discharged into the atmosphere. Therefore, an NH ₃ oxidation catalyst is disposed downstream of the filter in the exhaust gas flow direction to oxidize NH ₃ and / or a derivative thereof.

In a preferred aspect of the present invention, the apparatus comprises an injection means for injecting NH ₃ or urea as an NH ₃ precursor into the exhaust gas passage so as to supply NH ₃ as the reducing agent to the SCR catalyst,
An NH ₃ oxidation catalyst for oxidizing NH ₃ and / or a derivative thereof is supported on the downstream portion of the filter in the exhaust gas flow direction.

That is, by carrying the NH ₃ oxidation catalyst in the downstream portion of the exhaust gas flow direction in the filter, there is no need to separately provide a NH ₃ oxidation catalyst dedicated carrier. In other words, the filter, the SCR catalyst, and the NH ₃ oxidation catalyst are integrated. This simplifies the exhaust gas purification device and facilitates layout to the exhaust gas passage.

The exhaust gas purification method disclosed herein is a method for treating NOx and particulates in exhaust gas exhausted from an engine,
The exhaust gas passage of the engine carries an SCR catalyst that selectively reduces the NOx in the presence of a reducing agent and an alkaline earth metal, and does not contain any of Pt and Pd. The catalyst is arranged so that the former is located downstream of the latter in the exhaust gas flow direction,
A step of oxidizing NO in the exhaust gas to NO ₂ by the oxidation catalyst and trapping the NO ₂ in the alkaline earth metal of the filter when the particulate collection amount of the filter does not reach a predetermined value When,
When the particulate collection amount of the filter does not reach a predetermined value and the temperature of the exhaust gas is equal to or higher than a predetermined value, a reducing agent or a reducing agent precursor is injected into the exhaust gas, Reducing and purifying NOx in exhaust gas by the SCR catalyst in the presence of the reducing agent;
When the particulate collection amount of the filter reaches a predetermined value, post injection is performed to inject fuel into the combustion chamber of the engine in an expansion stroke or an exhaust stroke, thereby reducing the amounts of HC and CO in the exhaust gas. Increasing the temperature of the filter by increasing, oxidizing the HC and CO with the oxidation catalyst, and allowing the exhaust gas whose temperature has been increased by the reaction heat to flow into the filter;
By reacting active oxygen generated by decomposition of the alkaline earth metal nitrate with the temperature rise of the filter and the particulates collected in the filter, the particulates are burned and removed. It is characterized by that.

  According to this method, it is possible to simplify the exhaust gas purification device while achieving both the oxidation combustion of particulates and the reduction purification of NOx, and it is advantageous for promoting the combustion of particulates.

  Here, the filter further supports a rare earth metal other than Ce and a Zr-based composite oxide containing Zr, and releases active oxygen from the Zr-based composite oxide to burn the particulates. It is preferable to promote.

  According to the present invention, the filter for collecting particulates carries the SCR catalyst for selectively reducing NOx in the presence of a reducing agent and the alkaline earth metal, so that the particulates are oxidized and burned and NOx is reduced and purified. In addition, the exhaust gas purification device can be simplified while the layout of the exhaust gas purification device in the engine exhaust gas passage is facilitated.

The block diagram of the exhaust-gas purification apparatus of an engine. The graph which shows the temperature at the time of DTA peak of the sample which mixed alkaline-earth metal and carbon. The graph which shows the DTA peak top temperature of the sample which mixed Zr type complex oxide and carbon. The graph which shows the carbon combustion performance before and behind the aging process of Zr type complex oxide. Sectional drawing which shows typically the support form of the catalyst with respect to a filter main body. Sectional drawing which shows typically another example of the support form of the catalyst with respect to a filter main body.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the invention, its application, or its use.

<Configuration of exhaust gas purification device>
The exhaust gas purification device shown in FIG. 1 is a device capable of processing NOx and particulates (hereinafter referred to as “PM”) in exhaust gas containing 5% or more of oxygen discharged from a lean burn engine. The engine of this example is a diesel engine, and an exhaust gas passage 1 includes an oxidation catalyst (DOC) 2, a reducing agent or reducing agent precursor injection means 3, a mixer 4, an SCR / filter 5 and an NH ₃ oxidation catalyst 6. They are arranged in order from the upstream side in the exhaust gas flow direction. In this specification, “upstream side” and “downstream side” are used in the exhaust gas flow direction. This exhaust gas purification apparatus includes a tank for storing a reducing agent or a reducing agent precursor and various sensors. Based on the signals of these sensors, the fuel injection control of the engine and the control of the injection means 3 are executed by an ECU (Engine Control Unit).

[About oxidation catalyst]
The oxidation catalyst 2 contains an HC trap material that traps HC in the exhaust gas, and a catalyst component that oxidizes HC trapped in the HC trap material and HC, CO, and NO in the exhaust gas. For example, it is preferable to employ zeolite as the HC trap material and a catalyst in which Pt and / or Pd is supported on a mixture of activated alumina and OSC material (Ce-containing oxide) as the oxidation catalyst component.

  Since the oxidation catalyst 2 contains an HC trap material, when the exhaust gas temperature is low (when the catalyst is not activated), HC in the exhaust gas is trapped, and when the exhaust gas temperature becomes high (catalyst) HC released from the HC trap material can be oxidized and purified, and the amount of HC discharged without being oxidized can be reduced.

[About SCR / Filter]
The SCR / filter 5 is for removing the SCR catalyst for selectively reducing and purifying NOx in the presence of a reducing agent in the filter body for collecting PM in the exhaust gas and burning the collected PM. The PM combustion catalyst is supported. The SCR / filter 5 does not contain Pt and Pd. The filter main body has a honeycomb structure in which an exhaust gas inflow passage whose downstream end is closed and an exhaust gas outflow passage whose upstream end is closed are alternately provided in parallel, and the exhaust gas flowing into the exhaust gas inflow passage is a passage. It is a wall flow type that flows out to the adjacent exhaust gas outflow passage through the pores of the partition walls. The filter body is formed from an inorganic porous material such as cordierite, SiC, Si ₃ N ₄ , sialon, AlTiO ₃ .

As for the SCR catalyst, in this example, urea-SCR employing urea as a precursor of NH _{3 serving} as a reducing agent is employed. Therefore, urea water is stored in the tank. The SCR catalyst, the zeolite to trap NH _3, it is preferable to employ a catalyst component obtained by supporting a catalytic metal to reduce NOx and NH ₃ as a reducing agent. As the catalyst metal for NOx reduction, Fe, Ti, V, W, and the like are preferable, and it is not preferable to use Pt or Pd that easily oxidizes NH ₃ to NOx. The SCR catalyst is preferably contained in an amount of 60 g to 110 g per liter of the filter carrier.

  As the PM combustion catalyst, it is preferable to employ a catalyst in which an alkaline earth metal is supported on a Zr-based composite oxide, and does not contain a catalytic noble metal having a strong oxidation catalyst function such as Pt or Pd. The Zr-based composite oxide is a composite oxide containing Zr as a main component and a rare earth metal other than Ce, such as Nd, Pr, La, and Yb. A composite oxide containing Nd as a main component and containing Nd is preferable, and a composite oxide containing Pr in addition to Nd is particularly preferable. As the alkaline earth metal, it is preferable to employ at least one selected from Mg, Ca, Sr and Ba. Support materials other than Zr-based composite oxides, such as activated alumina or Rh-doped Ce-based composite oxides (compounds in which Rh is arranged between crystal lattice points or lattice points of Ce-based composite oxides) ). Here, when alkaline earth metal acetate is supported on a filter and fired, the alkaline earth metal becomes carbonate. By the reaction between the alkaline earth metal carbonate and NOx, the NOx becomes an alkaline earth metal nitrate and is trapped in the filter.

[About NH ₃ oxidation catalyst]
The NH ₃ oxidation catalyst 6 traps and oxidizes NH ₃ and its derivatives that pass (slip) through the SCR catalyst without reacting with NOx, and prevents the NH _{3 and the} like from slipping. The NH ₃ oxidation catalyst 6 preferably has a structure in which a Pt-supported zeolite in which Pt is supported on zeolite that traps NH ₃ and an OSC (Oxygen Storage capacity) material are supported on the cell walls of the honeycomb carrier.

[About injection means]
The injection means 3 can be constituted by an injection valve that supplies urea water in the tank to the exhaust gas passage 1 between the oxidation catalyst 2 and the mixer 4. The mixer 4 diffuses urea water into the exhaust gas in the exhaust gas passage 1.

[About sensor]
Next, various sensors arranged in the exhaust gas passage 1 will be described. Between the mixer 4 and the SCR / filter 5, a temperature sensor 11 for detecting the temperature of exhaust gas flowing into the SCR / filter 5 is disposed. Based on the exhaust gas temperature detected by the temperature sensor 11, the post injection amount for regenerating the filter is controlled. That is, in order to reliably increase the temperature of the filter to a temperature that promotes PM combustion, the post injection amount is controlled so that the exhaust gas temperature becomes a preset temperature. The post injection amount is controlled so that the exhaust gas temperature flowing into the SCR / filter 5 does not exceed a predetermined temperature, for example, 600 ° C. or higher, taking into consideration the heat resistance of the SCR catalyst zeolite.

Pressure sensors 12 and 13 for detecting a differential pressure Δ between exhaust gas upstream and downstream of the SCR / filter 5 are arranged upstream and downstream of the SCR / filter 5. The upstream pressure sensor 12 is disposed between the mixer 4 and the SCR / filter 5, and the downstream pressure sensor 13 is disposed between the SCR / filter 5 and the NH ₃ oxidation catalyst 6. The PM collection amount of the SCR / filter 5 is calculated based on the differential pressure Δ, and post injection is executed at a predetermined injection timing when the collection amount reaches a predetermined value.

An upstream NOx sensor 14 for detecting the NOx concentration of the exhaust gas flowing into the SCR catalyst is disposed between the oxidation catalyst 2 and the injection means 3. Between the SCR / filter 5 and the NH ₃ oxidation catalyst 6, a downstream NOx sensor 15 that detects the NOx concentration of the exhaust gas flowing out from the SCR / filter 5 is disposed.

  An injection means for purifying NOx with the SCR catalyst that the NOx concentration detected by the downstream NOx sensor 15 is equal to or higher than a predetermined value and that the exhaust gas temperature detected by the temperature sensor 11 is higher than a predetermined value. 3 is the urea water injection condition.

The amount of urea water injected is controlled to be an appropriate amount based on the amount of NH ₃ adsorbed on the zeolite of the SCR catalyst and the NOx concentration detected by the downstream NOx sensor 15. The amount of NH ₃ adsorbed on the zeolite is estimated based on the history of the NOx concentration and urea injection amount detected by the upstream and downstream NOx sensors 14 and 15.

  In addition, a sensor (not shown) for detecting the air-fuel ratio of the exhaust gas is provided in the exhaust gas passage. The air-fuel ratio of the exhaust gas may be estimated based on the operating state of the engine.

<PM combustion with alkaline earth metals>
Four types of Zr-based composite oxide samples supporting carbonates of Mg, Ca, Sr, and Ba and Zr-based composite oxide samples not supporting these carbonates were prepared. The amount of alkaline earth metal supported on the Zr-based composite oxide is 1% by mass. As the Zr-based composite oxide, a ZrNdPr composite oxide having a composition of ZrO ₂ : Nd ₂ O ₃ : Pr ₂ O ₃ = 70: 12: 18 (mol%) was employed. About each sample, the aging process hold | maintained at 800 degreeC for 24 hours in air | atmosphere was performed. Each sample after aging and carbon (carbon black) were weighed at a mass ratio of sample: carbon = 20: 1 and mixed for 1 minute in an agate mortar (tight contact condition). 5 mg of each mixed powder was weighed and placed in a TG-DTA apparatus. A temperature increase test was performed at 10 ° C./min with an air flow of “10% O ₂ +250 ppm NO ₂ / (N ₂ + Ar)”, and the temperature at the exothermic peak accompanying carbon combustion was read.

  The results are shown in FIG. The sample carrying the alkaline earth metal has a lower DTA peak top temperature than the sample not carrying the alkaline earth metal, and it can be seen that the PM combustibility is improved by loading the alkaline earth metal. The DTA peak top temperature is lower in the order of Mg → Sr → Ba → Ca, and it can be seen that the PM combustion start temperature is lowest when Ca is supported.

<Combustion of PM by Zr-based complex oxide>
The carbon combustion performance of the Zr-based composite oxide was also evaluated by the following method. The Zr-based composite oxide powder was subjected to an aging treatment that was held at 800 ° C. for 24 hours in the air. The aged Zr-based composite oxide powder and carbon black are mixed for 1 minute in an agate mortar (tight contact, Zr-based composite oxide powder: carbon black = 4: 1 (mass ratio)). Then, 5 mg of the mixed powder weighed was placed in a DTA apparatus using an alumina pan, and a temperature rise test was performed at 10 ° C./min in a 20% O ₂ / N ₂ +500 ppm NO ₂ stream (total flow rate 100 cc / min). . As a reference, a commercially available α-alumina powder was used. From the temperature at the exothermic peak accompanying carbon combustion (DTA peak top temperature), the influence of the Zr-based composite oxide on the combustion of PM was evaluated.

  Table 1 and FIG. 3 show the relationship between the composition and the DTA peak top temperature for various Zr-based composite oxides.

The specimen 1 is a Zr oxide containing no additive element, that is, ZrO ₂ . In ZrO ₂ , the DTA peak top temperature is 486.5 ° C., and as shown in FIG. 3, it can be seen that the DTA peak top temperature is higher than that of the Zr-based composite oxide containing the additive element.

  Here, the higher the DTA peak top temperature, the higher the exhaust gas temperature required for PM combustion. This increases the heat load associated with PM combustion and accelerates the deterioration of the filter. Therefore, the lower the peak top temperature, the more the PM combustion can be started at a lower temperature, which is preferable for suppressing the deterioration of the filter.

As shown in FIG. 3, the Zr-based composite oxide containing the rare earth metal other than Ce (samples 2 to 26) has a lower DTA peak top temperature than the ZrO _{2 of the} sample material 1, and the above-mentioned points It turns out that it is preferable. Among these, those containing Nd (samples 2 to 15) are particularly preferable. Furthermore, in addition to Nd, those containing Pr (test materials 5 to 15), especially those containing 12 mol% or more of Nd and Pr in terms of Nd ₂ O ₃ and Pr ₂ O ₃ (test materials 8 to 15) ) Is preferred.

This is probably because oxygen ion conductivity is increased by adding Nd and Pr as compared to ZrO ₂ , and the amount of active oxygen released during filter regeneration is increased.

  Further, as shown in FIG. 3, among the Zr-based composite oxides, Nd is not included, but those containing Y or Yb in addition to Pr (test materials 25 and 26) also have a low DTA peak top temperature. I understand. Therefore, these Zr-based composite oxides can also be preferably supported on the SCR / filter 5.

  Here, neither Pt nor Pd is supported on the Zr-based composite oxide shown in Table 1. This is because the SCR / filter 5 is also used for selective reduction of NOx as an SCR catalyst, and therefore it is inappropriate to support Pt and Pd. However, as described above, the Zr-based composite oxide shown in Table 1 shows a good PM combustion performance even when Pt and Pd are not supported. This is considered to be due to the high activity of active oxygen, so that PM combustion proceeds even in the absence of a catalytic noble metal such as Pt or Pd.

(Durability of Zr complex oxide)
For Table 1 shows test piece _{9 (ZrO2-12mol% Nd 2 O 3} -18mol% Pr 2 O 3), subjected to an aging treatment for 24 hours at 800 ° C. in air from its TG-DTA thermal analysis results before and after The thermal stability of the specimen 9 was evaluated. The preparation method and the measurement method of the specimen were the same as the above DTA thermal analysis, and the weight reduction rate of the evaluation sample accompanying carbon combustion was examined. The results are shown in FIG.

  In FIG. 4, the solid line indicates the measurement result before the aging process, and the broken line indicates the measurement result after the aging process. As shown in FIG. 4, it can be seen that the measurement results hardly change before and after the aging treatment. This means that even when high-temperature exhaust gas is introduced during filter regeneration and PM is burned, the specimen 9 hardly deteriorates. Therefore, for example, a filter carrying the specimen 9 is considered to exhibit high durability as a PM filter.

<Supported amount of alkaline earth metal and Zr-based composite oxide>
The alkaline earth metal is preferably contained in an amount of 0.01 to 0.12 g per liter of filter. The Zr-based composite oxide is preferably contained in an amount of 10 g to 60 g per liter of filter.

<Purification of exhaust gas>
[NOx trap and PM collection by SCR / filter 5]
When the air-fuel ratio of the exhaust gas is lean, NOx in the exhaust gas is trapped by the alkaline earth metal of the SCR / filter 5 and PM is collected by the filter. When the oxidation catalyst 2 is active, NO in the exhaust gas reacts with O ₂ in the exhaust gas in the oxidation catalyst 2 and is oxidized to NO ₂ . Therefore, NOx is efficiently trapped in the alkaline earth metal of the SCR / filter 5, and nitrate of the alkaline earth metal is generated. When, for example, Ba is adopted as the alkaline earth metal, NO ₂ reacts with BaCO _{3 in} the presence of oxygen and is trapped. That is, NO ₂ becomes NO ₃ ⁻ and binds to Ba to generate Ba (NO ₃ ) ₂ , and CO ₂ is desorbed and released from BaCO ₃ .

[PM combustion in SCR / filter 5]
When it is detected that the PM collection amount of the filter has reached a predetermined value based on the differential pressure Δ between the exhaust gas upstream and downstream of the SCR / filter 5 in a state where the air-fuel ratio of the exhaust gas is lean In addition, post injection is executed based on the exhaust gas temperature flowing into the SCR / filter 5. Thereby, the PM collected by the filter is burned and removed, and the PM collecting ability of the filter is recovered (filter regeneration). This will be specifically described below.

Post injection increases HC and CO in the exhaust gas discharged from the engine 1. The HC and CO react with oxygen in the exhaust gas in the oxidation catalyst 2. Thereby, CO ₂ and H ₂ O are generated and discharged. The temperature of the exhaust gas flowing into the SCR / filter 5 rises due to the oxidation reaction heat generated at this time. Along with this, the temperature of the SCR / filter 5 rises, and the PM combustion rate is greatly improved.

As the temperature of the SCR / filter 5 increases, the alkaline earth metal nitrate (Ba (NO ₃ ) ₂ ) is thermally decomposed to release active oxygen. Further, active oxygen is released from the Zr-based composite oxide constituting the PM combustion catalyst. And this active oxygen becomes an oxidizing agent and PM combustion proceeds. That is, PM reacts with active oxygen.

Further, SCR by the oxidation catalyst 2, NO in the exhaust gas reacts with oxygen _{(O 2)} in the exhaust gas NO ₂ is produced, as an oxidizing agent with oxygen _{(O 2)} of the NO ₂ is in the exhaust gas / Supplied to the filter 5 Therefore, PM combustion is promoted. PM is discharged as CO ₂ by reaction with oxygen and NO ₂ .

[NOx selective reduction of SCR / filter 5 by SCR catalyst]
When the PM collection amount of the filter does not reach the predetermined value, the NOx concentration of the exhaust gas flowing out from the SCR / filter 5 is equal to or higher than the predetermined value, and the exhaust gas temperature flowing into the SCR / filter 5 is a predetermined value ( For example, the selective reduction of NOx by the SCR catalyst is performed on the condition that the temperature is 200 ° C. or higher. That is, urea water is injected into the exhaust gas passage 1 by the injection means 3, NH ₃ (reducing agent) is generated by thermal decomposition and hydrolysis of the urea, and is adsorbed on the zeolite of the SCR catalyst. NOx (NO, NO ₂ ) flowing into the SCR / filter 5 is reduced and purified to N ₂ by NH ₃ adsorbed on the zeolite, and is discharged together with H ₂ O generated at that time.

Here, since the SCR / filter 5 does not contain Pt and Pd, NH ₃ (reducing agent) is oxidized by oxygen in the exhaust gas before being adsorbed on the zeolite or used for reducing NOx. Can be avoided. Therefore, the SCR catalyst carried on the filter works effectively for NOx reduction purification.

[Oxidation of NH ₃ etc. by NH ₃ oxidation catalyst 6]
NH ₃ and its derivatives passing through the SCR catalyst without reacting with NOx are trapped in the zeolite of the NH ₃ oxidation catalyst 6. Thus, NH ₃ and its derivatives are prevented from being discharged into the atmosphere. NH ₃ and its derivative trapped in the zeolite are desorbed and oxidized by the Pt catalyst and discharged when the temperature of the zeolite rises. In the present embodiment, the temperature of the zeolite of the NH ₃ oxidation catalyst 6 increases due to heat during filter regeneration, and NH ₃ is desorbed from the zeolite.

<Support form of PM combustion catalyst and SCR catalyst in filter>
The PM combustion catalyst and the SCR catalyst can be supported on the filter body in at least one form selected from the following A, B, and C.
A As shown in FIGS. 5A and 5B, the PM combustion catalyst 21 and the SCR catalyst 22 are formed in the exhaust gas passage (exhaust gas inflow passage, exhaust gas outflow passage, and passage partition wall) of the filter body 23. ) Is disposed on the upstream side in the exhaust gas flow direction, and the other is disposed on the downstream side.
B As shown in FIGS. 5 (c) and 5 (d), the PM combustion catalyst 21 and the SCR catalyst 22 are layered such that one is disposed on the wall surface of the filter body 23 and one is disposed closer to the space through which the exhaust gas passes than the other. It is carried on.
C PM combustion catalyst and SCR catalyst are mixed and supported on the wall surface of the filter body.

[Supported form A]
5A, the PM combustion catalyst 21 is disposed on the upstream side, and the SCR catalyst 22 is disposed on the downstream side. In the carrying form shown in FIG. 2B, the SCR catalyst 22 is disposed on the upstream side, and the PM combustion catalyst 21 is disposed on the downstream side.

The support form shown in FIG. 5B is preferred in which the SCR catalyst 22 is arranged on the upstream side. In this supported form, NO ₂ is not consumed due to the reaction with PM 24 on the upstream side where the SCR catalyst 22 is supported. Therefore, the NOx purification reaction by NH ₃ on the SCR catalyst proceeds most efficiently NO: NO _{It is} easy to approach ₂ = 1: 1 exhaust gas conditions, and a high NOx purification rate can be expected. Further, since the temperature on the upstream side is higher, PM is expected to burn even on the SCR catalyst 22 having a relatively low PM combustion performance. Moreover, since the upstream side of the SCR catalyst 22 does not have the ability to trap NO _2, can be accumulated to trap the NO ₂ to alkaline earth metal of the downstream side of the PM combustion catalyst 21 in the absence of the SCR reaction, When regenerating the filter, it is advantageous for the combustion of PM24 by active oxygen generated by the decomposition of alkaline earth metal nitrates.

[Supported form B]
5C, the PM combustion catalyst 21 is disposed on the upper side, that is, the space side through which the exhaust gas passes, and the SCR catalyst 22 is disposed on the lower side. In the carrying form shown in FIG. 4D, the SCR catalyst 22 is disposed on the upper side and the PM combustion catalyst 21 is disposed on the lower side.

  The support form shown in FIG. 5C in which the PM combustion catalyst 21 is arranged on the upper side is preferable. With this support form, the contact between the PM combustion catalyst 21 and the PM 24 becomes good, and the combustion of PM easily proceeds. Further, since PM 24 on the PM combustion catalyst 21 is easily burned and removed, NOx in the exhaust gas is likely to diffuse into the lower SCR catalyst 22. Therefore, it is advantageous for selective reduction of NOx.

<SCR catalyst / NH ₃ oxidation catalyst / filter integrated type>
The above embodiment includes the NH ₃ oxidation catalyst 6 independent of the SCR / filter 5. However, as shown in FIG. 6, the NH ₃ oxidation catalyst 25 is disposed downstream of the SCR / filter 5 in the exhaust gas flow direction. May be supported. In the example shown in FIG. 6, the catalysts 21 and 22 are supported on the upstream portion of the filter body 23 so that the PM combustion catalyst 21 is on the upper side and the SCR catalyst 22 is on the lower side (see FIG. 5C). (Supporting form), the NH ₃ oxidation catalyst 25 is supported downstream of the filter body 23. Thereby, it is not necessary to separately provide a carrier dedicated for the NH ₃ oxidation catalyst, so that the exhaust gas purification device is simplified and the layout to the exhaust gas passage is facilitated.

Note that, in supported form in FIG. 5 (b), and the downstream portion has a two-layer structure of the PM combustion catalyst and NH ₃ oxidation catalyst arranged PM combustion catalyst in the upper (space side where the exhaust gas passes) of the NH ₃ oxidation catalyst You may make it do. Alternatively, in the carrying form of FIG. 5B, the downstream portion may be a mixed layer of PM combustion catalyst and NH ₃ oxidation catalyst.

1 Engine exhaust gas passage 2 Oxidation catalyst 3 Injection means 4 Mixer 5 SCR / filter 6 NH ₃ oxidation catalyst 21 PM combustion catalyst 22 SCR catalyst 23 Filter body

Claims (8)

An exhaust gas purification device capable of treating NOx and particulates in exhaust gas discharged from an engine,
A filter for collecting the particulates is provided in the exhaust gas passage of the engine, and the SCR catalyst for selectively reducing the NOx in the presence of a reducing agent and an alkaline earth metal are supported on the filter. An exhaust gas purification apparatus characterized by the above. In claim 1,
An exhaust gas purifying apparatus, wherein neither Pt nor Pd is supported on the filter. In claim 1 or claim 2,
An exhaust gas purification apparatus, wherein an oxidation catalyst containing Pt-supported alumina is disposed upstream of the filter in the exhaust gas passage in the exhaust gas flow direction. In any one of Claim 1 thru | or 3,
The filter further comprises a rare earth metal other than Ce and a Zr composite oxide containing Zr, and the alkaline earth metal is supported on the Zr composite oxide. Exhaust gas purification device. In any one of Claims 1 thru | or 4,
Injecting means for injecting NH ₃ or urea as an NH ₃ precursor into the exhaust gas passage to supply NH ₃ as the reducing agent to the SCR catalyst,
An exhaust gas purification device, wherein an NH ₃ oxidation catalyst for oxidizing NH ₃ and / or a derivative thereof is disposed downstream of the filter in the exhaust gas passage in the exhaust gas flow direction. In any one of Claims 1 thru | or 4,
Injecting means for injecting NH ₃ or urea as an NH ₃ precursor into the exhaust gas passage to supply NH ₃ as the reducing agent to the SCR catalyst,
An exhaust gas purifying apparatus, wherein an NH ₃ oxidation catalyst for oxidizing NH ₃ and / or a derivative thereof is supported at a downstream portion of the filter in the exhaust gas flow direction. An exhaust gas purification method for treating NOx and particulates in exhaust gas discharged from an engine,
The exhaust gas passage of the engine carries an SCR catalyst that selectively reduces the NOx in the presence of a reducing agent and an alkaline earth metal, and does not contain any of Pt and Pd. The catalyst is arranged so that the former is located downstream of the latter in the exhaust gas flow direction,
A step of oxidizing NO in the exhaust gas to NO ₂ by the oxidation catalyst and trapping the NO ₂ in the alkaline earth metal of the filter when the particulate collection amount of the filter does not reach a predetermined value When,
When the particulate collection amount of the filter does not reach a predetermined value and the temperature of the exhaust gas is equal to or higher than a predetermined value, a reducing agent or a reducing agent precursor is injected into the exhaust gas, Reducing and purifying NOx in exhaust gas by the SCR catalyst in the presence of the reducing agent;
When the particulate collection amount of the filter reaches a predetermined value, post injection is performed to inject fuel into the combustion chamber of the engine in an expansion stroke or an exhaust stroke, thereby reducing the amounts of HC and CO in the exhaust gas. Increasing the temperature of the filter by increasing, oxidizing the HC and CO with the oxidation catalyst, and allowing the exhaust gas whose temperature has been increased by the reaction heat to flow into the filter;
By reacting active oxygen produced by decomposition of the alkaline earth metal nitrate with the temperature rise of the filter and the particulates collected in the filter, the particulates are burned and removed. An exhaust gas purification method characterized by the above. In claim 7,
The filter further supports a rare earth metal other than Ce and a Zr-based composite oxide containing Zr, and releases active oxygen from the Zr-based composite oxide to promote combustion of the particulates. An exhaust gas purification method characterized by the above.

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