JP2005264868A - Diesel exhaust gas emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently purify NO<SB>x</SB>even in exhaust gas of a low temperature region and to prevent end face blocking of a filter catalyst. <P>SOLUTION: An oxidation catalyst 1, an HC adsorption reformed catalyst 2 having larger heat capacity than the oxidation catalyst 1, and a filter NO<SB>x</SB>catalyst 3 are arranged in this order from the upstream side of exhaust gas toward the downstream side. Oxidizing reaction of HC and reducing reaction of NO<SB>x</SB>occur following well the exhaust gas temperature changing every acceleration and deceleration, and a catalyst bed temperature can be maintained high against the drop of exhaust gas temperature in deceleration. An added reducing agent is thereby reformed efficiently to reduce NO<SB>x</SB>. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification device that purifies exhaust gas from a diesel engine, and more particularly to an exhaust gas purification device useful for a system that adds a reducing agent to exhaust gas.

  As for gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology that can cope with it. However, because diesel engines emit harmful components as particulates (particulate matter: carbon fine particles, sulfur fine particles such as sulfate, high molecular weight hydrocarbon fine particles, etc., hereinafter referred to as PM), regulations are also restricted. Technological progress is also slow compared to gasoline engines.

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

  However, in DPF, pressure loss increases due to PM accumulation, so it is necessary to periodically remove and regenerate PM accumulated by some means. Therefore, conventionally, when pressure loss increases, high temperature exhaust gas is circulated, or PM accumulated by burning with a burner or an electric heater is burned to regenerate DPF. However, in this case, the higher the amount of PM deposited, the higher the temperature during combustion, and the DPF may be damaged by the thermal stress.

  Therefore, in recent years, as described in, for example, JP-A-09-173866, a coating layer is formed from alumina or the like on the surface of the DPF cell partition wall, and a noble metal such as platinum (Pt) is supported on the coating layer. Continuous regeneration type DPF (filter catalyst) has been developed. According to this filter catalyst, the collected PM is oxidized and burned by the catalytic reaction of the noble metal, so that the filter function can be regenerated by burning the PM simultaneously with the collection or continuously with the collection. Since the catalytic reaction occurs at a relatively low temperature and PM can be burned while the amount of trapped is small, there is an advantage that the thermal stress acting on the DPF is small and damage is prevented.

There is also known a filter catalyst in which a coating layer further carries a NO _x storage material selected from alkali metals, alkaline earth metals and rare earth elements. According to this filter catalyst occludes NO _x in _the NO _x storage material in a lean atmosphere, so reducing the NO _x released in a rich atmosphere, NO _x purification performance is remarkably improved. Therefore, when purifying exhaust gas from a diesel engine using this filter catalyst, a system is used in which a reducing agent is intermittently added to the exhaust gas to create a rich atmosphere. However, there is a problem that a low oxidation activity of HC by the precious metal for carrying the _the NO _x storage material, devised to reduce the accumulation of deposits and PM of a reducing agent to the filter catalyst is required.

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

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

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

However, even if such an exhaust gas purification device is used, when the exhaust gas temperature is in a low temperature range such as at the time of starting or at low speed acceleration / deceleration, the reducing agent is not sufficiently reformed and desired performance cannot be obtained. There was a problem.
JP 09-173866 JP 2002-021544 JP 2002-295244 A

The present invention has been made in view of the above-described circumstances, and an object to be solved is to be able to efficiently purify NO _x even in an exhaust gas in a low temperature range and prevent blockage of the end face of the filter catalyst.

A feature of the diesel exhaust gas purification apparatus of the present invention that solves the above problems is a diesel exhaust gas purification apparatus used in a system in which a reducing agent is intermittently added to exhaust gas, the inflow side being clogged downstream of the exhaust gas A cell, an outflow side cell adjacent to the inflow side cell and clogged upstream of the exhaust gas, a porous cell partition wall that partitions the inflow side cell and the outflow side cell and has a large number of pores, a surface of the cell partition wall, and A catalyst layer formed on the inner surface of the pores and formed by supporting a catalyst metal and a NO _x storage material on a porous oxide, and a filter catalyst,
The present invention consists of an HC adsorption catalyst that is disposed on the exhaust gas upstream side of the filter catalyst and adsorbs HC in the exhaust gas, and an oxidation catalyst that is disposed on the exhaust gas upstream side of the HC adsorption catalyst and has oxidation activity.

  The heat capacity of the HC adsorption catalyst is desirably larger than the heat capacity of the oxidation catalyst. Further, it is desirable that the HC adsorption catalyst has an HC reforming ability for reforming HC.

According to the diesel exhaust gas purification apparatus of the present invention, it is possible to efficiently reduce NO _x even in the low temperature region of the exhaust gas, and it is possible to prevent the reducing agent or PM from adhering to the front end face of the filter catalyst, Blockage is prevented.

  If the heat capacity of the oxidation catalyst is reduced, the oxidation activity of HC and CO in a low temperature region is improved, and the partial oxidation reaction activity is also improved. Further, if the heat capacity of the HC adsorption catalyst is made larger than the heat capacity of the oxidation catalyst, the catalyst bed temperature can be kept high even when the exhaust gas temperature is lowered during deceleration, and the purification activity of the filter catalyst can be kept high.

Also if you have a HC reforming activity of the HC adsorption catalyst for reforming HC, since the reducing agent is further modified on the HC adsorption catalyst, with _the NO _x reduction activity in the filter catalyst is further improved, the filter catalyst End face obstruction can be further prevented.

In the diesel exhaust gas purification apparatus of the present invention, even if the exhaust gas is in a low temperature range, the reducing agent is partially oxidized and reformed by the oxidation catalyst. Even if not modified, the reducing agent is adsorbed by the HC adsorption catalyst, so that the reducing agent is prevented from flowing directly into the filter catalyst in a liquid state or a polymer state, and the reducing agent adheres to the front end face. Is prevented. Accordingly, the end face of the filter catalyst can be prevented from being blocked. Then, the highly active HC released from the HC adsorption catalyst at the time of temperature rise reduces the NO _x efficiently in the filter catalyst, so that the NO _x purification rate is improved.

The oxidation catalyst is composed of a carrier substrate and a catalyst layer formed on the carrier substrate. As the carrier substrate, a honeycomb substrate having a straight flow structure or a foam substrate can be used. As the material, ceramics such as cordierite or metal foil is used, and the heat capacity is preferably 0.25 J / cm ³ · ° C. or less.

  The catalyst layer is composed of a porous oxide powder and a catalyst metal supported on the porous oxide powder. As the porous oxide, an oxide such as alumina, ceria, zirconia, titania, or a composite oxide composed of a plurality of these can be used. The catalyst metal is preferably one or more selected from platinum group noble metals such as Pt, Rh, Pd, Ir, and Ru. The amount of the precious metal supported is preferably 0.1 to 5 g per liter of the carrier substrate. If the loading amount is less than this, the activity is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost is increased. In addition to noble metals, transition metals such as Fe, W, Co, Ni, and Cu can also be used.

As the HC adsorption catalyst, a catalyst capable of adsorbing and holding HC in exhaust gas such as zeolite is used. The zeolite powder may be used as it is or formed into a pellet, or the surface of the carrier base material may be coated with zeolite in the same manner as the oxidation catalyst. The HC adsorption catalyst preferably has a heat capacity larger than that of the oxidation catalyst, and is desirably 0.35 J / cm ³ · ° C. or higher. By making the heat capacity larger than the heat capacity of the oxidation catalyst, the catalyst bed temperature can be kept high even when the exhaust gas temperature is lowered during deceleration, and the purification activity of the filter catalyst can be kept high. In order to increase the heat capacity in this way, there are a method of using SiC or the like as the material of the carrier base material, or a method of mixing TiO ₂ , Al ₂ O ₃ , SiC or the like with zeolite.

The HC adsorption catalyst preferably has an HC reforming ability for cracking and reforming HC. Since this reducing agent on the HC adsorption catalyst is further modified by, along with _the NO _x reduction activity in the filter catalyst is further improved, it is possible to further prevent the end surface blockage of the filter catalyst. Examples of the HC adsorption catalyst having the HC reforming ability include a catalyst in which a noble metal such as Pt is supported on zeolite, a copper-zeolite catalyst, and the like. When a noble metal such as Pt is supported, the supported amount is preferably in the range of 0.1 to 5 g per liter of the carrier substrate volume. When copper is supported, the supported amount is preferably in the range of 0.5 to 5% by weight.

  The filter catalyst is divided into an inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, and an inflow side cell and an outflow side cell. And a filter base material, and a catalyst layer formed on the surface of the cell partition wall and the inner surface of the pore and having a catalyst metal supported on a porous oxide.

  The filter substrate is a plug-type honeycomb body similar to a conventional DPF. This filter base material can be manufactured from heat-resistant ceramics such as cordierite and silicon carbide. For example, a clay-like slurry containing cordierite powder as a main component is prepared, formed by extrusion molding or the like, and fired. Instead of the cordierite powder, each powder of alumina, magnesia and silica can be blended so as to have a cordierite composition. Thereafter, the cell opening on one end face is sealed in a checkered pattern with the same clay-like slurry, and the cell opening of the cell adjacent to the cell sealed on the one end face is sealed on the other end face. Thereafter, it can be produced by fixing the sealing material by firing or the like.

  In order to form pores in the cell partition walls of the filter substrate, combustible powders such as carbon powder, wood powder, starch, and resin powder are mixed in the above slurry, and the combustible powder disappears during firing. Thus, pores can be formed, and by adjusting the particle size and addition amount of the combustible powder, it is possible to control the distribution of the surface pores and the diameters of the internal pores and the opening area.

  The pore distribution in the cell partition walls of the filter substrate can be in the range of 40 to 80% porosity and 10 to 50 μm average pore diameter, similar to the conventional DPF. If the porosity or the average pore diameter is out of this range, the PM collection efficiency may decrease or the exhaust pressure loss may increase.

  The catalyst layer is formed on the surface of the cell partition walls and the surface of the pores, and is formed by supporting a catalyst metal on a porous oxide. As the porous oxide, oxides such as alumina, ceria, zirconia, and titania, or a composite oxide composed of a plurality of these oxides can be used.

  As the noble metal, it is preferable to use one or more kinds selected from platinum group noble metals such as Pt, Rh, Pd, Ir, and Ru. The amount of noble metal supported is preferably 0.1 to 5 g per liter volume of the filter substrate. If the loading amount is less than this, the activity is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost is increased.

The catalyst layer further supports a NO _x storage material selected from alkali metals, alkaline earth metals, and rare earth elements. By including the NO _x storage material in the catalyst layer, NO ₂ generated by oxidation with the noble metal can be stored in the NO _x storage material, and the NO _x purification activity is improved. Loading amount of _the NO _x storage material, it is preferable that the volume per liter 0.05-1.00 mols of the filter substrate. If the supported amount is less than this, the activity is too low to be practical, and if the supported amount is more than this range, the catalyst metal is covered and the activity is lowered.

In order to form a catalyst layer on the filter substrate, the porous oxide powder is made into a slurry together with a binder component such as alumina sol and water, and the slurry is attached to the cell partition wall and then fired, and then the noble metal and NO _x storage material May be carried. A normal dipping method can be used to attach the slurry to the cell partition walls. However, the slurry is forcibly filled into the pores of the cell partition walls by air blowing or suction, and the excess slurry contained in the pores is filled. It is desirable to remove things.

The formation amount of the catalyst layer is preferably 30 to 200 g per liter volume of the filter substrate. If the catalyst layer is less than these ranges, the durability of the noble metal or NO _x storage material is inevitably lowered, and if it exceeds these ranges, the exhaust pressure loss becomes too high, which is not practical.

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

(Example 1)
FIG. 1 shows an exhaust gas purification apparatus of this embodiment. In this exhaust gas purifying apparatus, an oxidation catalyst 1, an HC adsorption reforming catalyst 2, and a filter catalyst 3 are arranged in the catalytic converter 4 adjacent in this order from the exhaust gas upstream side to the downstream side. An injector 5 for adding light oil to the exhaust gas is provided upstream of the catalytic converter 4.

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

<Preparation of oxidation catalyst 1>
A honeycomb structure with a straight flow structure made of cordierite was prepared. As summarized in Table 1, this honeycomb structure has a cell partition wall having a diameter of 129 mm, a length of 40 mm, a cell count of 400 / inch ² and a thickness of 0.10 mm. The cell partition wall has a porosity of 25%, a core mass of 284 g / L, and a heat capacity of 0.24 J / cm ³ · ° C.

  Next, a slurry in which 50 parts by weight of alumina powder, 50 parts by weight of titania powder, and a binder are dispersed in ion-exchanged water is prepared, and a coating layer is applied to the surface of the cell partition wall of the honeycomb structure by a wash coat method at 120 g / L. Formed. Thereafter, 2 g / L of Pt was supported and calcined by an impregnation supporting method to prepare an oxidation catalyst 1.

<Preparation of HC adsorption reforming catalyst 2>
A honeycomb structure with a straight flow structure made of cordierite was prepared. As summarized in Table 1, this honeycomb structure has a cell partition wall having a diameter of 129 mm, a length of 60 mm, a cell number of 400 / inch ² and a thickness of 0.15 mm. The cell partition wall has a porosity of 25%, a core mass of 417 g / L, and a heat capacity of 0.35 J / cm ³ · ° C.

  Next, a slurry in which 50 parts by weight of ZSM-5 powder, 90 parts by weight of titania powder, and a binder are dispersed in ion-exchanged water is prepared, and 150 g of a coating layer is applied to the cell partition wall surface of the honeycomb structure by a wash coat method. / L was formed. Thereafter, 2 g / L of Pt was supported and calcined by the impregnation supporting method to prepare HC adsorption reforming catalyst 2.

<Preparation of filter catalyst 3>
A honeycomb filter with a wall flow structure made of cordierite was prepared. This honeycomb filter has a cell partition wall having a diameter of 129 mm, a length of 150 mm, a cell count of 300 / inch ² and a thickness of 0.15 mm.

  Next, 50 parts by weight of alumina powder, 30 parts by weight of titania powder, 30 parts by weight of zirconia powder, 10 parts by weight of ceria powder, and a slurry in which a binder is dispersed in ion-exchanged water are prepared. A coating layer of 120 g / L was formed on the cell partition wall surface and the pore surface inside the cell partition wall. Thereafter, Pt was supported at 2 g / L by the impregnation supporting method and baked, and Li, Ba and K were each supported at 0.1 mol / L by the water absorption supporting method, and then calcined at 500 ° C. to prepare filter catalyst 3. .

  The oxidation catalyst 1, the HC adsorption reforming catalyst 2, and the filter catalyst 3 were arranged in the catalytic converter 4 adjacent in this order from the exhaust gas upstream side to the downstream side to obtain the exhaust gas purification device of this example.

(Example 2)
The honeycomb structure of the oxidation catalyst 1 is made of metal and has a cell partition with a diameter of 129 mm, a length of 40 mm, a cell count of 400 / inch ² and a thickness of 0.03 mm. The cell partition has a porosity of 0% and the core mass is The same as Example 1 except that 396 g / L and heat capacity of 0.2 J / cm ³ · ° C. were used.

(Example 3)
The honeycomb structure of the oxidation catalyst 1 is the same as that of Example 2, and the honeycomb structure of the HC adsorption reforming catalyst 2 is made of SiC with a diameter of 129 mm, a length of 60 mm, and a cell number of 400 / inch ² and a thickness. The same as Example 1 except that a 0.15 mm cell partition wall having a cell partition wall porosity of 30%, a core mass of 498 g / L, and a heat capacity of 0.52 J / cm ³ · ° C. was used. .

(Comparative Example 1)
The honeycomb structure of the oxidation catalyst 1 is made of cordierite, has a cell partition with a diameter of 129mm, a length of 60mm, a cell count of 400 / inch ² and a thickness of 0.15mm, a cell partition porosity of 25%, and a core mass. Is 417 g / L, the heat capacity is 0.35 J / cm ³ · ° C., and the HC adsorption reforming catalyst 2 is not used. This exhaust gas purification apparatus is shown in FIG.

(Comparative Example 2)
The same honeycomb structure as that of Comparative Example 1 is used as the honeycomb structure of the oxidation catalyst 1, and a coating layer similar to that of the HC adsorption reforming catalyst 2 of Example 1 is formed thereon, and Pt is supported in the same manner. The same as Example 1 except that 2 was not used.

<Test and evaluation>
Each catalyst used in the examples and comparative examples was subjected to an endurance test that was held at 700 ° C. for 5 hours in an electric furnace. Each catalyst after the durability test was placed in the catalytic converter 4 and mounted on the exhaust system of a diesel engine having a displacement of 2 L. The emission during EU mode travel was measured, and the NO _x and HC purification rates were measured. During traveling, a predetermined amount of light oil was added to the exhaust gas at a predetermined timing from the injector 4 attached to the upstream side of the catalytic converter 3. The example and the comparative example have the same conditions. The results are shown in Table 1.

表１より、各実施例ではNO_x 及びHC共に、比較例より高い浄化率が発現されている。これは、各実施例で用いた酸化触媒１は比較例に比べて熱容量が小さいため、加減速毎に変化する排気温によく追随してHCの酸化反応とNO_x の還元反応が起きたと考えられる。一方、比較例では、酸化触媒１の熱容量が大きいために、加速時の昇温に追随できす、添加された軽油に起因するHCが未反応のまま排出されたと考えられる。

From Table 1, NO _x and HC together in each example, higher than the comparative example purification rate is expressed. This is because the oxidation catalyst 1 used in each example has a smaller heat capacity than the comparative example, and therefore, the HC oxidation reaction and the NO _x reduction reaction occurred following the exhaust temperature changing at each acceleration / deceleration well. It is done. On the other hand, in the comparative example, since the heat capacity of the oxidation catalyst 1 is large, it is considered that the HC caused by the added light oil that can follow the temperature increase during acceleration was discharged without being reacted.

In Comparative Example 2, since the purification rate is improved as compared with Comparative Example 1, it can be seen that it is effective to include zeolite in the coating layer of the oxidation catalyst 1. It is clear that it is preferable to arrange the HC adsorption reforming catalyst 2 containing zeolite on the downstream side. That is, in the respective embodiments, are modified into active HC with light oil is trapped in the HC adsorbing reforming catalyst 2, the filter catalyst 3 thus NO _x is considered to have been effectively reduced.

  Furthermore, the comparison between the examples shows that the larger the difference between the heat capacity of the oxidation catalyst 1 and the heat capacity of the HC adsorption reforming catalyst 2, the higher the purification rate. By using it, it is considered that the catalyst bed temperature can be kept high with respect to the exhaust temperature drop during deceleration and the purification rate is improved.

It is sectional drawing which shows the structure of the exhaust gas purification apparatus of one Example of this invention. It is sectional drawing which shows the structure of the exhaust gas purification apparatus of one comparative example of this invention.

Explanation of symbols

1: Oxidation catalyst 2: HC adsorption reforming catalyst 3: Filter catalyst 4: Catalytic converter 5: Injector

Claims (3)

A diesel exhaust gas purification device used in a system in which a reducing agent is intermittently added to exhaust gas,
An inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, the inflow side cell and the outflow side cell are partitioned, and a large number of pores are formed. A filter catalyst having a porous cell partition wall, and a catalyst layer formed on the surface of the cell partition wall and the inner surface of the pores and having a catalyst metal and a NO _x storage material supported on the porous oxide,
An HC adsorption catalyst arranged on the exhaust gas upstream side of the filter catalyst and adsorbing HC in the exhaust gas,
A diesel exhaust gas purification device comprising an oxidation catalyst disposed on the exhaust gas upstream side of the HC adsorption catalyst and having oxidation activity.   The diesel exhaust gas purification device according to claim 1, wherein a heat capacity of the HC adsorption catalyst is larger than a heat capacity of the oxidation catalyst. The diesel exhaust gas purification apparatus according to claim 1 or 2, wherein the HC adsorption catalyst has an HC reforming ability to reform HC.

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