JP6126858B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification device, an exhaust gas purification method, and an exhaust gas purification catalyst applicable to an internal combustion engine operated in a lean combustion region such as a diesel engine or a lean burn engine.

An exhaust gas containing a large amount of carbon monoxide (CO) and nitrogen oxides (NOx) is discharged from an internal combustion engine such as a diesel engine or a lean burn engine that burns fuel in an oxygen-rich state. Further, the exhaust gas of these internal combustion engines contains hydrocarbons (HC) that are unburned fuel. Further, in the exhaust gas of these internal combustion engines, ammonia (NH ₃ ) produced by thermal decomposition and hydrolysis of an aqueous urea solution added to the exhaust gas for N Ox purification is also included. Therefore, an exhaust gas purification device, an exhaust gas purification method, and an exhaust gas purification catalyst that can be applied to these internal combustion engines are required to be capable of highly oxidizing and purifying CO, NOx, HC, and NH ₃ in an oxygen-rich atmosphere.

  Conventionally, as techniques for oxidizing and purifying CO and HC, those using noble metal catalysts such as platinum (Pt) and palladium (Pd) are known and applied to exhaust gas purification of diesel engines (for example, (See pages 351 to 373 of Patent Document 1.) However, NOx cannot be purified with a noble metal catalyst.

On the other hand, as NOx purification technology, conventionally, a technology for purifying NOx by injecting an aqueous urea solution or NH ₃ into an exhaust gas passage and reacting NOx and NH ₃ with a NOx purification catalyst is known. NOx purification catalysts include honeycomb-shaped base metal oxides such as titania (TiO ₂ ) and vanadia (V ₂ O ₅ ), and ceramic honeycomb monoliths such as cordierite carrying vanadium catalysts and zeolite catalysts. Those with improved heat resistance are used. In addition, a technique in which an NH ₃ oxidation catalyst is installed after the NOx purification catalyst in order to prevent diffusion of the added NH ₃ component into the atmosphere is also known (for example, page 133 of Non-Patent Document 2). -See page 138).

Supervised by Yuichi Murakami, Catalyst Degradation Mechanism and Prevention Measures, Technical Information Association, 1995, pages 351-373 Issued by NTS, Trends in Elemental Technology for Development of Clean Diesel, 2008, pp. 133-138

As described above, since the exhaust gas of the internal combustion engine contains HC that is an unburned portion of the fuel, the NOx purification catalyst installed in the exhaust gas passage is exposed to HC. The inventors of the present application have studied the influence of the inflow of HC on the activity of the NOx purification catalyst. As a result, if the NOx purification catalyst contains a zeolite component, the NOx purification performance of the NOx purification catalyst is reduced and the HC It was found that CO was partially oxidized to generate CO. The exhaust gas purification devices described in Non-Patent Documents 1 and 2 do not consider anything about the effects of HC inflow, and highly oxidize and purify CO, NOx, HC and NH ₃ in the exhaust gas as they are. I can't.

The present invention has been made on the basis of the above-described knowledge, and an object thereof is an exhaust gas purifying apparatus for an internal combustion engine capable of highly purifying CO, NOx, HC and NH ₃ contained in exhaust gas containing excess oxygen. Another object is to provide an exhaust gas purification method and an exhaust gas purification catalyst.

The present invention to achieve the above object, relates to an exhaust gas purification system of an internal combustion engine, in the exhaust gas line of an internal combustion engine to discharge the exhaust gas containing excess oxygen than the stoichiometric amount in the oxidation reaction of CO and HC A NOx purification catalyst containing a zeolite component is installed, and a CO and HC purification catalyst for oxidizing and purifying CO and HC in the exhaust gas is installed upstream of the NOx purification catalyst with respect to the flow direction of the exhaust gas, and the flow of the exhaust gas An Ir-containing catalyst is installed downstream of the NOx purification catalyst with respect to the direction, and means for reducing the temperature of the exhaust gas that has passed through the NOx purification catalyst to a temperature that can increase the NOx-CO reaction activity of the Ir-containing catalyst is provided. It was configured as follows.

According to the present invention, CO, HC, NOx, and NH ₃ contained in exhaust gas discharged from an internal combustion engine that is operated by injecting a lean fuel such as a diesel engine into a combustion chamber can be efficiently purified. .

It is a figure which shows the NOx purification rate of a zeolite containing NOx purification catalyst. It is a figure which shows the HC conversion rate of a zeolite containing NOx purification catalyst. It is a figure which shows the NOx purification rate of Ir containing catalyst. It is a figure which shows the CO purification rate of an Ir containing catalyst. It is a diagram showing an NH ₃ purifying ratio Ir-containing catalysts. It is a figure which shows the 1st example of the exhaust gas purification apparatus installed in the exhaust gas flow path of an engine. It is a figure which shows the 2nd example of the exhaust gas purification apparatus installed in the exhaust gas flow path of an engine. It is a figure which shows the 3rd example of the exhaust gas purification apparatus installed in the exhaust gas flow path of an engine. It is a figure which shows the 4th example of the exhaust gas purification apparatus installed in the exhaust gas flow path of an engine. It is a figure which shows the change of the NOx purification rate before and behind the regeneration process of a zeolite containing NOx purification catalyst. It is a figure which shows the structure of the engine provided with the regeneration process means of the exhaust gas purification apparatus installed in an exhaust gas flow path.

  Hereinafter, embodiments of an exhaust gas purification apparatus, an exhaust gas purification method, and an exhaust gas purification catalyst of an internal combustion engine according to the present invention will be described in detail with reference to the drawings.

In general, exhaust gas discharged from an internal combustion engine typified by a diesel engine mounted on a passenger car, construction machine, or the like often has an oxygen-excess atmosphere containing oxygen in excess of the stoichiometric amount. Further, the exhaust gas discharged from these internal combustion engines contains CO, HC, NOx, and in some cases also contains NH ₃ components. In the present invention, the stoichiometric amount means the amount of O ₂ , CO, and HC when O ₂ and CO, HC contained in the exhaust gas react with each other without excess or deficiency. Hereinafter, the meaning of the stoichiometric amount will be described in more detail.

When the exhaust gas contains O ₂ , CO, and HC, a chemical reaction represented by the following reaction formulas (1) and (2) occurs between these three gases in the exhaust gas flow path. .

2CO + O ₂ → 2CO ₂ (1)
CnHm + (n + m / 4) O ₂ → nCO ₂ + (m / 2) H ₂ O (2)
For example, when 300 ppm of CO and C ₃ H ₆ are present in the exhaust gas, respectively, in order for the chemical reaction represented by the reaction formulas (1) and (2) to proceed, 150 ppm and 1350 ppm of oxygen (O ₂ ) Is required. An oxygen-excess atmosphere means that O ₂ is present that exceeds the amount of oxygen that can oxidize CO and HC. That is, when 300 ppm of CO and C ₃ H ₆ are present in the exhaust gas as in the above example, this means a case where O ₂ is higher than 1500 ppm (= 150 ppm + 1350 ppm).

  The present invention relates to an exhaust gas purifying apparatus, an exhaust gas purifying method, and an exhaust gas purifying catalyst particularly suitable for purifying exhaust gas such as a diesel engine that exhausts exhaust gas containing oxygen in excess of the stoichiometric amount.

That is, the inventors of the present application, as a result of intensive studies, installed a NOx purification catalyst containing a zeolite component in the exhaust gas flow path, and a catalyst containing Ir in the subsequent stage with respect to the flow direction of the exhaust gas (Ir containing catalyst) It was clarified that CO, HC, NOx, and NH ₃ in the exhaust gas can be effectively purified.

When a NOx purification catalyst containing a zeolite component is installed in the exhaust gas flow path and NH ₃ component is added to the exhaust gas flowing into the NOx purification catalyst, NO and NH ₃ react with each other according to the following reaction formula (3), and NOx Purified.

4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (3)
Since the exhaust gas temperature of a diesel engine mounted on a passenger car or construction machine can be 400 ° C. or more, the NOx purification catalyst is required to have heat resistance. By containing a zeolite component as the NOx purification catalyst, the heat resistance performance of the NOx purification catalyst is improved, and further, the temperature range in which NOx can be purified can be increased, which is preferable.

However, if the NOx purification catalyst contains a zeolite component, it turns out that when HC flows into the NOx purification catalyst, the NOx purification performance deteriorates and the HC partially oxidizes and CO is generated. did. Further, the NH ₃ component added to the exhaust gas passage is not easily consumed on the NOx purification catalyst. That is, there is a possibility that NOx, CO, and NH ₃ are discharged from the latter stage of the NOx purification catalyst.

That is, when HC flows into the NOx purification catalyst, the NOx purification catalyst is poisoned by HC, and NH ₃ that is a reducing agent of NOx becomes difficult to be adsorbed to the NOx purification catalyst, so that the NOx purification reaction is reduced, and NH ₃ Is considered to be released. Further, it is considered that the adhering HC undergoes partial oxidation on the zeolite and CO is generated as well as CO ₂ . This phenomenon occurs particularly remarkably when the zeolite component is contained, and does not remarkably occur in the NOx purification catalyst not containing the zeolite component.

  The inventors of the present application have found that the above problem can be solved by installing an Ir-containing catalyst downstream of the NOx purification catalyst with respect to the flow direction of the exhaust gas. When an Ir-containing catalyst is used, even in an atmosphere containing oxygen in excess of the stoichiometric amount, NOx and CO react by a chemical reaction represented by the following reaction formula (4) to highly purify NOx and CO. be able to.

2NO + 2CO → N ₂ + 2CO ₂ (4)
Furthermore, the Ir-containing catalyst has the ability to oxidize NH ₃ . Therefore, by installing the Ir-containing catalysts downstream of the NOx purifying catalyst, it is possible to purify NOx flowing out from the front of the NOx purifying catalyst, CO, and NH _3. According to the combination of the zeolite-containing NOx purification catalyst and the Ir-containing catalyst, all nitrogen oxides such as NO, NO ₂ , N ₂ O, N ₂ O ₃ can be targeted for reduction. In addition, according to the combination of the zeolite-containing NOx purification catalyst and the Ir-containing catalyst, for example, all hydrocarbons such as CH ₄ , C ₃ H ₆ , C ₂ H ₄ , C ₂ H ₂ , C ₃ H _{8 and the} like are reduced. It can be. As the NH ₃ component for NOx reduction added to the exhaust gas flow path, urea, cyanuric acid, melamine, biuret and the like can be used in addition to the NH ₃ gas.

A CO and HC oxidation catalyst may be installed upstream of the NOx purification catalyst with respect to the flow direction of the exhaust gas. If the amount of HC flowing into the NOx purification catalyst is reduced by oxidizing and purifying HC, the purification of NOx and NH ₃ on the NOx purification catalyst containing zeolite proceeds, so that the NO on the Ir-containing catalyst in the latter stage is advanced. -CO reaction and further NH ₃ oxidation reaction proceed, and release of HC, CO, NOx, NH ₃ out of the system can be highly suppressed. For example, there are catalyst poisoning components such as SOx and soot in diesel engine exhaust gas, and it is possible that the HC oxidation performance deteriorates due to the poisoning of the HC oxidation catalyst. If it is installed at the rear stage of the catalyst, the release of HC, CO, NOx, NH ₃ out of the system can be highly suppressed even in that case.

As long as the zeolite-containing NOx purification catalyst applied to the present invention is a zeolite-containing catalyst capable of purifying NOx, the catalytically active component is not particularly limited. However, as a NOx purification catalyst, at least one selected from vanadium (V), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), Ce, Zr on a zeolite carrier. If the catalyst active component is used, NOx in the exhaust gas is effectively purified. Furthermore, NOx is highly purified by blowing NH ₃ component into the exhaust gas flow path. This zeolite-containing NOx purification catalyst has high heat resistance and can effectively purify NOx even in a temperature range of 350 ° C. or higher. V, Mn, Fe, Co, Ni, Cu, Ce, and Zr used as the catalyst active component may be a combination of two or more elements. This is thought to be due to the fact that an interaction occurs by combining two or more catalytically active components, resulting in high performance.

Since the zeolite used as the carrier component has a high specific surface area, it has the effect of improving the NOx purification performance by improving the degree of dispersion of the catalytically active component. Moreover, it is thought that activity improves because an active ingredient can be carry | supported with an ionic state. No particular limitation is imposed on zeolite, heat resistance is increased by using the high-silica zeolite molar ratio of SiO ₂ and Al ₂ O ₃ is 5 or more. Examples of the zeolite species used include β zeolite, Y-type zeolite, ZSM-5, mordenite, and ferrierite.

  The total supported amount of the catalytically active components V, Mn, Fe, Co, Ni, Cu, Ce, and Zr is preferably 0.1 wt% or more and 30 wt% or less in terms of element with respect to the zeolite support, more preferably It is 1.0 wt% or more and 10 wt% or less. When the total amount of V, Mn, Fe, Co, Ni, Cu, Ce, and Zr is 0.1 wt% or less, the supporting effect becomes insufficient, and when it exceeds 30 wt%, the specific surface area of the active ingredient itself is reduced and the catalyst is reduced. This is because the cost becomes high.

On the other hand, the Ir-containing catalyst may be any catalyst containing Ir, and the other configurations are not particularly limited. As a preferred embodiment, Ir and Nb, Pt, Pd, Rh, Au as a catalytically active component are formed on a porous carrier made of an inorganic compound containing at least one selected from Al, Ce, Si, Ti, and Zr. And those carrying at least one selected from the above. By installing this Ir-containing catalyst downstream of the NOx purification catalyst, NOx, CO, and NH ₃ can be highly purified.

Ir has the capability of reacting NOx and CO in an oxygen-excess atmosphere containing oxygen in excess of the stoichiometric amount, and simultaneously purifying NOx and CO. Further, Ir has an oxidation ability of NH _3, NH ₃ can also be purified. Furthermore, Ir promotes the reaction between NOx and CO when SOx is contained in the reaction gas. This is probably because the presence of S in the reaction gas changes the electronic state of Ir.

  When Ir and at least one selected from Nb, Pt, Pd, Rh, and Au are combined as a catalyst active component, the activity and activation temperature of the catalyst change depending on the combination. Therefore, the optimal combination of components can be selected according to the properties of the exhaust gas to be applied. Moreover, the activity and activation temperature of a catalyst can be varied by including at least one selected from Al, Ce, Si, Ti, and Zr as a support. In particular, when Si oxide is used as a carrier, the activity of the Ir-containing catalyst can be improved.

  The total supported amount of the catalytically active component Ir and Nb, Pt, Pd, Rh, Au is preferably 0.00005 mol part to 10 mol part in terms of element with respect to 2 mol part of the porous carrier, more preferably , 0.0003 mol part to 0.3 mol part. If the total supported amount of Ir and Nb, Pt, Pd, Rh, and Au is less than 0.00005 mol part, the effect of supporting is insufficient, and if it exceeds 10 mol part, the specific surface area of the active ingredient itself decreases, resulting in a catalyst cost. Because it becomes higher. Here, "mol part" means the content ratio of each component in terms of mol number. For example, the loading amount of B component is 1 mol part with respect to 2 mol part of A component, regardless of the absolute amount of A component. Means that

  As described above, the temperature range in which the catalyst is activated can be changed in the Ir-containing catalyst by changing other active components combined with Ir or the carrier used. This is probably because Ir and other elements are alloyed or the electronic state of Ir changes. Therefore, by installing these two or more catalysts along the exhaust gas flow path, the temperature range in which NOx and CO can be purified can be expanded, and even if the exhaust gas temperature fluctuates, NOx and CO can be highly purified. Can do.

  As the CO and HC purification catalyst installed in the preceding stage of the zeolite-containing NOx purification catalyst, as a catalytically active component on a porous carrier made of an inorganic compound containing at least one selected from Al, Ce, Si, Ti and Zr A material carrying at least one selected from Pt, Pd, Rh, Au, Ir, ruthenium (Ru), and osmium (Os) can be used. By installing this CO, HC purification catalyst before the zeolite-containing NOx purification catalyst, the CO, HC purification performance can be enhanced. That is, by installing the CO and HC purification catalyst, the amount of HC flowing into the subsequent NOx purification catalyst can be reduced, so that high NOx purification performance can be exhibited as a whole.

The porous carrier of the CO, HC purification catalyst is made of an inorganic compound containing at least one selected from Al, Ce, Si, Ti, Zr by using an oxide having a high specific surface area. This is because Pt, Pd, Rh, Au, Ir, Ru, and Os are highly dispersed to enhance the CO and HC purification performance. In particular, when an oxide containing Al is used as the porous carrier, high HC purification performance can be stably obtained. The specific surface area of the porous carrier is preferably in the range of 30~800m ² / g, in particular in the range of 50 to 400 m ² / g are preferred. This is because if the specific surface area of the porous carrier is less than 30 m ² / g, the required HC purification performance cannot be obtained, and if it exceeds 800 m ² / g, the catalyst cost increases.

  When two or more kinds selected from Pt, Pd, Rh, Au, Ir, Ru, and Os are contained as catalytic active components, the CO and HC purification performance after heat deterioration is enhanced. It is thought that the aggregation of the noble metal is suppressed by alloying the active components. In particular, when Pt and Pd or Pt and Rh are combined, the heat resistance performance is enhanced.

  The total supported amount of catalytically active components Pt, Pd, Rh, Au, Ir, Ru, and Os is preferably 0.00005 mol part to 10 mol part in terms of element with respect to 2 mol part of the porous carrier, and more preferably. Is from 0.0003 mol parts to 0.3 mol parts. When the total supported amount of Pt, Pd, Rh, Au and Ir is less than 0.00005 mol part, the effect of supporting is insufficient, and when it exceeds 10 mol part, the specific surface area of the active ingredient itself is reduced and the catalyst cost is reduced. Because it becomes high.

  The reaction between NOx and CO by the Ir-containing catalyst is optimally performed in a temperature range of about 350 ° C. or less. On the other hand, the temperature of the exhaust gas that has passed through the zeolite-containing NOx purification catalyst may be 350 ° C. or higher. In that case, the NOx-CO reaction activity on the Ir-containing catalyst can be increased by providing means for reducing the temperature of the exhaust gas that has passed through the zeolite-containing NOx purification catalyst. As a means for reducing the exhaust gas temperature, for example, a measure such as providing a cooler for adding moisture to the exhaust gas flow path can be considered. When water is added to the exhaust gas passage, the water flowing into the Ir-containing catalyst also increases, but the influence on the catalyst activity is small.

  If HC continues to flow into the zeolite-containing NOx purification catalyst, HC may adhere to the zeolite-containing NOx purification catalyst, and the NOx purification performance may be reduced. In that case, the adsorbed HC can be desorbed by flowing a gas substantially free of hydrocarbons into the NOx purification catalyst, and the NOx purification performance of the zeolite-containing NOx purification catalyst can be recovered. Note that if HC is desorbed by flowing a gas substantially free of hydrocarbons into the NOx purification catalyst, partial oxidation of HC may occur, and CO may be generated. However, even in that case, CO can be purified by the Ir-containing catalyst installed in the subsequent stage, so that release of CO and NOx to the outside of the system can be suppressed.

  Here, “substantially does not contain” means that the amount of HC contained in the reaction gas is small enough to desorb HC from the NOx purification catalyst, Specifically, the content is 100 ppm or less in terms of carbon element. Furthermore, the recovery of the NOx purification performance is promoted by increasing the temperature of the gas that does not substantially contain HC. The gas temperature is preferably 350 ° C. or higher and 550 ° C. or lower, and more preferably 400 ° C. or higher and 500 ° C. or lower. This is because the desorption reaction of HC does not proceed at 350 ° C. or lower, and the performance of the catalyst decreases due to heat load at 550 ° C. or higher.

  As for the method of causing the gas that does not substantially contain HC to flow into the exhaust gas flow passage, for example, a method of promoting combustion in the engine by controlling the engine operation with a controller and reducing the amount of HC in the exhaust gas is taken. (See FIG. 11). In this case, there is a possibility that the amount of NOx in the exhaust gas may increase. However, since the NOx in the exhaust gas can be highly purified by the zeolite-containing NOx purification catalyst and the Ir-containing catalyst, the amount of NOx released into the atmosphere increases. There is nothing.

  Furthermore, it is conceivable that the NOx purification performance of the zeolite-containing NOx purification catalyst deteriorates due to accumulation of S in the exhaust gas in the zeolite-containing NOx purification catalyst. However, even in this case, the temperature of the exhaust gas flowing into the zeolite-containing NOx purification catalyst is set to 350 ° C. or more and 550 ° C. or less, and the NOx purification catalyst is caused to flow into the zeolite-containing NOx purification catalyst. There is a possibility that S component can be desorbed from the catalyst, and as a result, the NOx purification performance can be recovered. When desorbing the S component from the CO and HC purification catalyst, if there is more CO than HC in the exhaust gas, desorption of the S component may be promoted.

  The porous carrier or catalytically active component used for the zeolite-containing NOx purification catalyst, Ir-containing catalyst, CO, HC purification catalyst may be supported on a substrate. As the base material, cordierite that has been conventionally used, ceramics made of Si—Al—O, heat resistant metal substrates such as stainless steel, and the like are suitable. When a substrate is used, the amount of the porous carrier supported is preferably 10 g or more and 300 g or less with respect to 1 liter of the substrate in order to improve the CO and HC purification performance. If it is 10 g or less, the dispersion of the noble metal is lowered, and the catalytic activity is lowered. On the other hand, when it is 300 g or more, when the substrate has a honeycomb shape, problems such as clogging in the gas flow path are likely to occur.

  As preparation methods of zeolite-containing NOx purification catalyst, Ir-containing catalyst, CO, HC purification catalyst, for example, physical preparation methods such as impregnation method, kneading method, coprecipitation method, sol-gel method, ion exchange method, vapor deposition method, A preparation method using a chemical reaction or the like can be used. In particular, by using a preparation method utilizing a chemical reaction, the contact between the raw material of the catalytically active component and the porous carrier is strengthened, and sintering (crystal growth) of the catalytically active component can be prevented.

  Starting materials for zeolite-containing NOx purification catalyst, Ir-containing catalyst, CO and HC purification catalyst include various compounds such as nitric acid compounds, chlorides, acetic acid compounds, complex compounds, hydroxides, carbonic acid compounds, organic compounds, metals, Metal oxides can be used. For example, when two or more elements are combined as a catalyst active component, the catalyst component is uniformly supported by preparing it by a co-impregnation method using an impregnation solution in which the active component is present in the same solution. be able to.

  The shapes of the zeolite-containing NOx purification catalyst, Ir-containing catalyst, CO, and HC purification catalyst can be appropriately adjusted according to the application. For example, a honeycomb structure obtained by coating the purification catalyst of the present invention on a honeycomb structure made of various base materials such as cordierite, Si-Al-O, SiC, stainless steel, pellets, plates, granules, Examples include powder. In the case of a honeycomb shape, it is preferable to use a structure made of cordierite or Si—Al—O as the base material. It is preferable to use a material (for example, a base material such as a metal honeycomb mainly composed of Fe). Alternatively, the honeycomb may be formed with only the porous carrier and the catalytically active component. Furthermore, if a base material having a filter function is used, soot in the exhaust gas can be purified.

  The exhaust gas purification device, the exhaust gas purification method, and the exhaust gas purification catalyst according to the present invention are particularly effective for purifying exhaust gas containing oxygen in excess of the stoichiometric amount in the oxidation reaction of CO and HC, and are always effective in the exhaust gas. It is particularly suitable when the oxygen is in excess of the stoichiometric amount.

  Hereinafter, more specific examples of the exhaust gas purifying apparatus, the exhaust gas purifying method, and the exhaust gas purifying catalyst according to the present invention will be described.

<Preparation of zeolite-containing NOx purification catalyst>
A sample obtained by impregnating a Y-type zeolite (manufactured by Tosoh Corporation) with an Fe nitrate solution and drying it in the atmosphere at 150 ° C. for 10 hours was obtained using an electric furnace in the atmosphere at 600 ° C. × 1 By firing for a period of time, an Fe / zeolite powder carrying 2 wt% Fe in terms of metal element relative to zeolite was obtained. A slurry prepared by adding the obtained Fe / zeolite powder and alumina sol to water was coated on a cordierite honeycomb (300 cells / iNC ² ), and then dried by circulating hot air at 150 ° C. for 15 minutes. Furthermore, the obtained sample was baked at 600 ° C. for 1 hour in the air using an electric furnace to obtain a honeycomb catalyst coated with 250 g of Fe / zeolite per liter of apparent volume. This catalyst is referred to as Example catalyst 1.

<Performance evaluation of zeolite-containing NOx purification catalyst using reference gas>
The performance of the example catalyst 1 was evaluated under the following conditions. A honeycomb catalyst having a capacity of 6 cm ³ was fixed in a reaction tube made of quartz glass, and this reaction tube was installed in an electric furnace. The reaction gas introduced into the reaction tube has a composition simulating exhaust gas in an oxygen-excess atmosphere, NOx: 150 ppm, NH ₃ : 180 ppm, CO ₂ : 6%, O ₂ : 10%, H ₂ O: 6%, N ₂ : It was set as the residual. This gas is used as a reference gas. The NOx purification performance of Example Catalyst 1 was estimated by the NOx purification rate shown in the following equation. The volume space velocity was 45,000 / H. While the reaction gas was circulated, the gas temperature was controlled from 200 ° C. to 500 ° C., and the NOx purification performance was measured.

NOx purification rate (%) = ((NOx concentration flowing into the catalyst) − (NOx concentration flowing out from the catalyst))
÷ (NO concentration flowing into the catalyst) × 100
<Performance evaluation of zeolite-containing NOx purification catalyst taking into account the effect of C ₃ H ₆ >
The NOx purification performance of the zeolite-containing NOx purification catalyst was measured in the same manner as in the case of using the reference gas except that 300 ppm or 3333 ppm of C ₃ H ₆ was added to the reaction gas. Furthermore, the rate at which C ₃ H ₆ was converted to CO and CO ₂ on the catalyst was estimated by the HC conversion rate represented by the following formula.

HC → CO ₂ conversion rate = ((C ₃ H ₆ concentration flowing into the catalyst × 3) − (CO ₂ concentration flowing out from the catalyst))
÷ (C ₃ H ₆ concentration flowing into the catalyst × 3) × 100
HC → CO conversion rate = ((C ₃ H ₆ concentration flowing into the catalyst × 3) − (CO concentration flowing out from the catalyst))
÷ (C ₃ H ₆ concentration flowing into the catalyst × 3) × 100
<NOx purification rate of NOx purification catalyst containing zeolite>
1, the reference gas, gas obtained by adding _C 3 _{H 6} of 300ppm based gas, and in a case where the reference gas using the added gas _C 3 _{H 6} of 3333 ppm, the NOx purification rate of the catalyst of Example 1 Show. When the gas to which 300 ppm of C ₃ H ₆ was added was used, the NOx purification rate in the temperature range of 250 ° C. or lower and 400 ° C. or higher was lower than when the reference gas was used. Further, when the gas added with 3333 ppm of C ₃ H ₆ was used, the NOx purification rate was further greatly reduced. Focusing on the activity at 400 ° C., the NOx purification rate greatly decreased from 93% to 25% when compared with the reference gas. From this result, it is clear that the NOx purification rate decreases as the HC concentration in the exhaust gas increases.

<HC conversion of zeolite-containing NOx purification catalyst>
2, the reference gas, the reference gas to a gas obtained by adding _C 3 _{H 6} of 300 ppm, and in the case of using a gas added _C 3 _{H 6} of 3333ppm based gas, the HC conversion rate of example catalyst 1 Show. As can be seen from this figure, CO is generated regardless of the C ₃ H ₆ concentration in the gas. In particular, in the case of a gas added with 3333 ppm of C ₃ H ₆ , the HC → CO conversion rate at 400 ° C. is 13%, and it can be seen that about 1300 ppm of CO is generated.

From the above examination results, it is clear that when C ₃ H ₆ is present in the exhaust gas, NOx and CO are discharged from the catalyst of Example 1.

<Preparation of Ir-containing catalyst>
A commercially available SiO ₂ powder (CarriAct G-3 manufactured by Fuji Silysia Chemical Co., Ltd.) was impregnated with an Ir nitrate solution, dried at 120 ° C., and then fired at 600 ° C. for 1 hour. The amount of Ir added was 0.05 wt% in terms of metal element with respect to SiO ₂ . This Ir-containing catalyst is referred to as Example catalyst 2. In addition, the SiO ₂ powder used for the production of Example catalyst 2 was previously treated in an electric furnace at 800 ° C. for 1 hour in the atmosphere, and an Ir-containing catalyst was produced in the same manner as Example catalyst 2 except that. This Ir-containing catalyst is referred to as Example catalyst 3. Further, each of Example Catalysts 2 and 3 was impregnated with an Nb ₂ O ₅ sol aqueous solution, dried at 120 ° C., and then calcined at 600 ° C. for 1 hour. The amount of Nb added was the same number of moles as the amount of Ir added. The Ir-containing catalysts are referred to as Example catalysts 4 and 5, respectively. Table 1 below shows a list of compositions of Ir-containing catalysts.

<Performance evaluation of Ir-containing catalyst>
With respect to Example Catalysts 2 to 5, performance evaluation regarding purification of NOx and CO was performed under the following conditions. A granular catalyst (diameter 0.75 mm to 1.5 mm) having a capacity of 0.9 cm ³ was fixed in a reaction tube made of quartz glass. This reaction tube was introduced into an electric furnace, and the temperature of the gas introduced into the reaction tube was controlled to be 200 ° C to 350 ° C. The reaction gas introduced into the reaction tube is a model gas simulating exhaust gas in an oxygen-excess atmosphere, and the composition thereof is NOx: 150 ppm, CO: 1500 ppm, O ₂ : 3%, SO ₂ : 3 ppm, N ₂ : residual It was. The volume space velocity (F / V) was 200,000 / H.

  The purification performance of Example Catalysts 2 to 5 was determined by determining the NOx and CO purification rates using the following formula.

NOx purification rate (%) = ((NOx amount flowing into the catalyst)-(NOx amount flowing out from the catalyst))
÷ (NOx amount flowing into the catalyst) × 100
CO purification rate (%) = ((CO amount flowing into the catalyst)-(CO amount flowing out from the catalyst))
÷ (CO amount flowing into the catalyst) x 100
In FIG. 3, the NOx purification rate of Example catalysts 2-5 is shown. As is clear from this figure, it was found that the NOx purification rate of about 40% was obtained for each catalyst, and NOx and CO reacted even in the presence of oxygen to purify NOx. The temperature ranges showing the highest activity were 300 ° C. for Example Catalysts 2 and 4, 280 ° C. to 300 ° C. for Example Catalyst 3, 260 ° C. to 270 ° C. for Example Catalyst 5, and the presence or absence of heat treatment on SiO ₂ It can be seen that the difference depends on whether or not Nb is added.

  In FIG. 4, the CO purification rate of Example catalysts 2-5 is shown. As is apparent from this figure, it is clear that in each catalyst, the CO purification rate exceeds 60% in the temperature range where the NOx purification rate reaches around 40%, and not only NOx but also CO is purified. .

  From the above evaluation results, it is clear that the Ir-containing catalyst can simultaneously purify NOx and CO in an oxygen-excess atmosphere. It is also clear that the NOx and CO purification rates can be further increased by using a catalyst containing Ir and Nb.

<Evaluation of NH ₃ oxidation rate of Ir-containing catalyst>
The oxidation rate of NH ₃ was evaluated for the Ir-containing catalyst. As the reaction gas, the above-mentioned reference gas excluding NOx was used, and the other test conditions were the same as the performance evaluation of the zeolite-containing NOx purification catalyst. The NH ₃ oxidation rate of the Ir-containing catalyst was estimated by the following equation.

NH ₃ purification rate (%)
= ((NH ₃ concentration flowing into the catalyst) − (NH ₃ concentration flowing out from the catalyst))
÷ (NH ₃ concentration flowing into the catalyst) × 100
FIG. 5 shows the NH ₃ purification rate of Example Catalysts 2 and ₃ . It can be seen that the NH ₃ purification rate exceeds 60% at 300 ° C. or higher for both catalysts, and shows high performance.

Hereinafter, the configuration of the exhaust gas purifying apparatus according to the present invention will be described in comparison with a comparative example .

<First comparative example of exhaust gas purification device>
As shown in FIG. 6, the first comparative example of the exhaust gas purifying apparatus is provided with the example catalyst 1 and the example catalyst 2 in this order from the upstream side in the exhaust gas flow direction in the exhaust gas flow path of the engine. An NH ₃ gas is added to the upstream side of the catalyst 1. As described above, in Example Catalyst 1, NOx and CO are discharged when C ₃ H ₆ is present in the exhaust gas, and when NH ₃ gas is added to the exhaust gas, unreacted. there is a possibility that the NH ₃ gas is discharged, but example catalyst 2, since NH ₃ can not only purify NOx and CO can also be purified, exhibit NOx, CO, a high purification performance for NH ₃ as a whole it can.

<Second comparative example of exhaust gas purification device>
As shown in FIG. 7, in the second comparative example of the exhaust gas purifying apparatus, Example catalyst 1, Example catalyst 2 and Example catalyst 5 are installed in this order from the upstream side in the exhaust gas flow direction in the exhaust gas flow path of the engine. In addition, NH ₃ gas is added to the upstream side of the catalyst 1 of the embodiment. As is apparent from the experimental data in FIG. 3, the catalyst examples 2 and 5 show NOx purification rates of around 40% at around 300 ° C. and around 260 ° C., respectively. Therefore, by disposing the example catalyst 2 and the example catalyst 5 at the subsequent stage of the example catalyst 1, the temperature range showing a high NOx purification rate can be expanded, and NOx and CO are effectively purified even if the exhaust gas temperature changes. can do.

<Third comparative example of exhaust gas purification device>
As shown in FIG. 8, in the third comparative example of the exhaust gas purifying apparatus, Example catalyst 1 and Example catalyst 2 are installed in this order from the upstream side in the exhaust gas flow direction in the exhaust gas flow path of the engine. In addition, NH ₃ gas is added to the upstream side of No. 1 and a cooling device for spraying water in the exhaust gas flow path is installed between the Example catalyst 1 and the Example catalyst 2. As is apparent from the experimental data in FIG. 2, the generation of CO from HC in the example catalyst 1 is about 350 ° C. or higher. On the other hand, as is clear from the experimental data of FIG. 3, the NOx and CO purification progresses at 300 ° C. or less in the example catalyst 2. Therefore, the NOx and CO purification reaction on the example catalyst 2 can be promoted by spraying the cooling water from the cooling water supply device and lowering the exhaust gas temperature at the subsequent stage of the example catalyst 1. When the interval between the example catalyst 1 and the example catalyst 2 is sufficiently large, the exhaust gas temperature decreases during this period, and the temperature range is optimal for both catalysts. Therefore, the cooling water supply device may be omitted. it can.

<Example of the exhaust gas purifying device>
As shown in FIG. 9, the embodiment of the exhaust gas purifying apparatus is configured such that the HC and CO oxidation catalyst, the embodiment catalyst 1 and the embodiment catalyst 2 are arranged in this order from the upstream side in the exhaust gas flow direction in the exhaust gas flow path of the engine. While being installed, NH ₃ gas is added between the HC, CO oxidation catalyst and the example catalyst 1. In the exhaust gas purification apparatus of this configuration, the HC and CO oxidation catalyst is installed in the preceding stage of the example catalyst 1, so that HC and CO flowing into the example catalyst 1 can be reduced, and NOx on the example catalyst 1 can be reduced. The purification reaction can be promoted, and NOx released to the outside of the system can be reduced. Furthermore, even if the HC and CO oxidation catalyst is poisoned by SOx or the like in the exhaust gas and the HC and CO oxidation performance is reduced, NOx and CO can be purified on the example catalyst 2.

<Preparation of HC, CO oxidation catalyst>
A slurry prepared by adding alumina sol and water to Al ₂ O ₃ powder obtained by calcining boehmite powder in an electric furnace at 600 ° C. for 5 hours in the atmosphere was used as a cordierite honeycomb (300 cells / cell). inc ² ) and then dried by flowing hot air at 150 ° C. for 15 minutes. Furthermore, the obtained sample was fired at 600 ° C. for 1 hour in an electric furnace to coat 200 g of Al ₂ O ₃ powder per liter of apparent volume of the honeycomb. The resulting alumina-coated honeycomb is impregnated with a mixed solution of dinitrodiammine Pt nitric acid solution and Pd nitric acid solution, dried by flowing hot air at 150 ° C. for 15 minutes, and then fired at 600 ° C. for 1 hour in an electric furnace. went. By this method, PtPd / Al ₂ O ₃ honeycomb catalyst powder containing 1.5 g and 0.5 g of Pt and Pd per liter of honeycomb in terms of element was obtained.

<Regeneration treatment of NOx purification catalyst containing zeolite: Part 1>
After evaluating the NOx purification activity using the reference gas with respect to the catalyst of Example 1, the gas added with 3333 ppm of C ₃ H ₆ was circulated for 30 minutes while maintaining the catalyst inlet temperature at 400 ° C. It was poisoned with C ₃ H ₆ . Thereafter, the catalyst inlet temperature was lowered to 200 ° C., and the NOx purification activity was evaluated again with the reference gas. Further, the catalyst regeneration process was performed by maintaining the catalyst inlet temperature at 450 ° C. for 60 minutes while the reference gas was circulated. Thereafter, the catalyst inlet temperature was lowered to 200 ° C., and the NOx purification activity was evaluated again with the reference gas. The test results are shown in FIG.

As is clear from FIG. 10, the catalyst of Example 1 has approximately 20 points lower activity in the entire temperature range after the C ₃ H ₆ poisoning compared to the new one. Restore activity. Therefore, it can be seen that even if HC flows into the example catalyst 1 and the NOx purification activity decreases, the activity of the zeolite-containing NOx purification catalyst can be recovered by circulating the gas having a reduced HC concentration.

<Regeneration treatment of zeolite-containing NOx purification catalyst: Part 2>
FIG. 11 shows the configuration of the exhaust gas purifying apparatus according to the present invention. The exhaust gas purifying apparatus of this example purifies exhaust gas of the diesel engine 1 and includes a controller 9 that performs drive control of the engine 1. The diesel engine 1 obtains power by compressing the air in the combustion chamber (cylinder) 1a with a piston 1b to a high temperature, supplying fuel from the fuel injection device 2 to the compressed air, and causing spontaneous ignition. The diesel engine 1 includes an intake valve 1c between the intake pipe 4 and the combustion chamber 1a, and an exhaust valve 1d between the combustion chamber 1a and the exhaust pipe 3. In FIG. 11, one intake / exhaust valve 1c, 1d is shown, but the number of valves is not limited to this, and a plurality of intake valves 1c and exhaust valves 1d are provided as necessary. Can do. Purifying catalysts 5, 6, and 7 are installed on the outlet side of the exhaust pipe 3, and exhaust gas discharged from the diesel engine 1 is supplied to the purifying catalysts 5, 6, and 7 and purified by the purifying catalysts 5, 6, and 7. Is released to the outside. A CO and HC oxidation catalyst is installed as the purification catalyst 5, a zeolite-containing NOx purification catalyst is installed as the purification catalyst 6, and an Ir-containing catalyst is installed as the purification catalyst 7. In the figure, reference numeral 11 is an NH ₃ tank, reference numeral 8 is an NH ₃ injection nozzle for injecting NH ₃ between the CO and HC oxidation catalyst 5 and the zeolite-containing NOx purification catalyst 6, and reference numeral 10 is HC, CO, NOx. The sensor is shown.

  When the controller 9 determines the degree of decrease in the HC, CO, NOx purification performance based on the output signal of the sensor 10, and determines that the HC, CO, NOx purification performance has decreased below a predetermined value, The combustion in the engine 1 is promoted to increase the temperature of the exhaust gas flowing into the purification catalysts 5 and 6, and the HC concentration in the exhaust gas flowing into the purification catalysts 5 and 6 is reduced.

As described above, the exhaust gas purification apparatus shown in FIG. 11 has the CO, the HC oxidation catalyst 5, the zeolite-containing NOx purification catalyst 6 and the Ir-containing catalyst 7 installed in this order from the upstream side of the exhaust gas flow passage. HC, NOx, NH ₃ can be removed to a high degree, and the controller 9 controls the combustion state of the engine 1 even when the CO, HC oxidation catalyst 5 and the zeolite-containing NOx purification catalyst 6 are poisoned by HC and S. By doing so, the HC and CO purification performance of the CO and HC oxidation catalyst 5 and the zeolite-containing NOx purification catalyst 6 can be recovered.

DESCRIPTION OF SYMBOLS 1 ... Engine, 1a ... Combustion chamber, 1b ... Piston, 1c ... Intake valve, 1d ... Exhaust valve, 2 ... Fuel injection device, 3 ... Exhaust pipe, 4 ... Intake pipe, 5 ... HC, CO oxidation catalyst, 6 ... Zeolite containing NOx purification catalyst, 7 ... Ir-containing catalysts 2, 8 ... NH ₃ inlet, 9 ... controller, 10 ... HC, CO, NOx sensor, 11 ... NH ₃ tank

Claims (9)

A NOx purification catalyst containing a zeolite component is installed in an exhaust gas passage of an internal combustion engine that exhausts exhaust gas containing oxygen in excess of the stoichiometric amount in the oxidation reaction of CO and HC, and the NOx is related to the flow direction of the exhaust gas. A CO and HC purification catalyst that oxidizes and purifies CO and HC in the exhaust gas is installed before the purification catalyst, an Ir-containing catalyst is installed after the NOx purification catalyst with respect to the flow direction of the exhaust gas, and the NOx purification catalyst is installed. An exhaust gas purification apparatus for an internal combustion engine, comprising means for reducing the temperature of the exhaust gas that has passed to a temperature at which the NOx-CO reaction activity of the Ir-containing catalyst can be increased . 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein an urea aqueous solution or NH ₃ is added to the exhaust gas between the installation position of the NOx purification catalyst and the installation position of the CO and HC purification catalyst. An exhaust gas purification apparatus for an internal combustion engine characterized by the above.   3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the NOx purification catalyst includes a zeolite carrier and V, Mn, Fe supported on the zeolite carrier as a catalytically active component. An exhaust gas purifying apparatus for an internal combustion engine, comprising at least one selected from Co, Ni, Cu, Ce, and Zr.   The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the Ir-containing catalyst includes a porous carrier made of an inorganic compound and a catalytically active component carried on the porous carrier. The porous support includes at least one selected from Al, Ce, Si, Ti, and Zr, and the catalytically active component is selected from Ir and Nb, Pt, Pd, Rh, and Au. An exhaust gas purifying device for an internal combustion engine, comprising:   The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the Ir-containing catalyst installed at a subsequent stage of the NOx purification catalyst includes two or more kinds having different active components or different carriers. An exhaust gas purification apparatus for an internal combustion engine, characterized in that a catalyst is installed along the exhaust gas flow path.   6. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the CO and HC purification catalyst is a porous carrier made of an inorganic compound, and a catalyst carried on the porous carrier. The porous support includes at least one selected from Al, Ce, Si, Ti, and Zr, and the catalytic active component includes Pt, Pd, Rh, Au, Ir, Ru, An exhaust gas purifying apparatus for an internal combustion engine, comprising at least one selected from Os.   The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 6, wherein the NOx purification performance of the NOx purification catalyst is restored by reducing the HC concentration of the exhaust gas flowing into the NOx purification catalyst. An exhaust gas purifying device for an internal combustion engine, characterized by comprising:   The exhaust gas purification apparatus for an internal combustion engine according to claim 7, wherein the temperature of the exhaust gas flowing into the NOx purification catalyst is set to 350 ° C or higher and 550 ° C or lower.   The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 8, wherein the exhaust gas flowing into the NOx purification catalyst is always oxygen in excess of the stoichiometric amount in the oxidation reaction of CO and HC. An exhaust gas purifying apparatus for an internal combustion engine, comprising: