JP4239864B2 - Diesel exhaust gas purification device - Google Patents

Description

  The present invention relates to an exhaust gas purification device that purifies exhaust gas from a diesel engine.

  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 JP-A-09-173866, for example, a coating layer is formed from alumina or the like on the surface of the cell partition of DPF, 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.

  However, even in a filter catalyst, when exhaust gas containing a large amount of PM flows in a short time or when it is cold such as at the time of start-up, its oxidation performance may be insufficient and PM may accumulate, increasing the exhaust pressure loss, The accumulated PM may burn at a high temperature, causing damage. Therefore, it has been proposed and put into practical use that an oxidation catalyst is arranged upstream of the exhaust gas of the filter catalyst, oxidizes HC and CO in the exhaust gas with the oxidation catalyst, and exhaust gas heated by the exothermic reaction flows into the filter catalyst. Yes.

For example, in Japanese Patent Laid-Open No. 60-043113, a catalyst having excellent HC oxidation performance is arranged in front of the DPF, and a large amount of HC is supplied by intake throttling or retarding control so that the reaction in the catalyst having excellent oxidation performance is achieved. A technique for burning PM deposited on DPF with heat is disclosed. Japanese Patent Laid-Open No. 2002-021544 discloses an HC by placing an oxidation catalyst or NO _x storage reduction catalyst in front of a DPF or filter catalyst, and injecting fuel into the combustion chamber by post injection or exhaust gas. Is supplied, and the PM deposited on the DPF or filter catalyst is combusted by the heat of reaction in the oxidation catalyst or NO _x storage reduction catalyst.

  In such technology, it is desirable to raise the temperature of the exhaust gas flowing into the subsequent DPF or filter catalyst to 600 to 650 ° C or higher. However, in the method of supplying HC by intake throttling or retarded angle control, it is difficult to raise the oxidation catalyst in the previous stage to the activation temperature when the exhaust gas temperature is low, such as when starting or running at low speed, and during that time, the latter stage PM accumulates on the DPF or filter catalyst. In particular, in the method of supplying HC by adding fuel to the exhaust gas, the oxidation catalyst in the previous stage is cooled because it is supplied as liquid fuel, and the DPF or filter catalyst in the subsequent stage is cooled in a low temperature range such as at the start. It becomes difficult to raise the temperature of the inflowing exhaust gas. Further, even if the exhaust gas containing fuel is ignited in the oxidation catalyst, it may be misfired on the way.

Further, the pre-stage oxidation catalyst carries a noble metal such as Pt as a catalyst component, but there is also a problem that the noble metal grain growth occurs and deteriorates at a high temperature range, the ignition temperature rises and the oxidation activity decreases. .
JP 09-173866 Japanese Patent Laid-Open No. 60-043113 JP 2002-021544

  The present invention has been made in view of the above-described circumstances, and has an object to improve the thermal characteristics of an oxidation catalyst arranged in the preceding stage and to increase the temperature early and to suppress grain growth of noble metal. .

The characteristics of the diesel exhaust gas purification apparatus of the present invention that solves the above-described problems are: an inflow side cell that is clogged on the exhaust gas downstream side; an outflow side cell that is adjacent to the inflow side cell and that is clogged on the exhaust gas upstream side; A porous cell partition wall having a large number of pores dividing the cell and the outflow side cell, and a catalyst formed by supporting a catalyst metal on a porous oxide formed on the surface of the cell partition wall and the inner surface of the pores A filter catalyst having a wall flow structure with a layer;
It is arranged on the upstream side of the exhaust gas of the filter catalyst and comprises a honeycomb structure having a straight flow structure and an oxidation catalyst layer having oxidation activity formed in the cell partition wall, and upstream of the upstream catalyst. A diesel exhaust gas purification device used in a system for adding a liquid reducing agent to exhaust gas,
The honeycomb structure of the pre-stage catalyst has a thermal conductivity of 10 W / m · ° C. or higher and a heat capacity of 0.4 J / cm ³ · ° C. or higher.

According to the diesel exhaust gas purification apparatus of the present invention, the honeycomb structure of the pre-catalyst has a high thermal conductivity and easily dissipates heat, so that the reaction heat is quickly transmitted to the flowing exhaust gas and the exhaust gas temperature is increased. Further, since the honeycomb structure of the pre-catalyst has a large heat capacity, it is suppressed from being cooled by the added liquid reducing agent even if the liquid reducing agent is added, and gasification of the liquid reducing agent is promoted. Therefore the PM deposited on the filter catalyst can be efficiently combusted, regeneration of the filter catalyst is efficiently. In addition, as a result of the reaction heat being quickly transmitted to the exhaust gas, the noble metal supported on the pre-stage catalyst is suppressed from being exposed to a high temperature for a long time, and the grain growth is difficult.

That is, according to the diesel exhaust gas purification apparatus of the present invention, even if a liquid reducing agent is added, the added reducing agent is reliably oxidized by the pre-stage catalyst, and the combustion heat is efficiently transmitted to the downstream filter catalyst and deposited. PM can be burned, and the bed temperature of the pre-catalyst can be lowered, so that noble metal grain growth can be suppressed.

In the diesel exhaust gas purification apparatus of the present invention, the pre-stage catalyst and the filter catalyst are arranged in this order from the exhaust gas upstream side to the downstream side. Although the pre-stage catalyst and the filter catalyst can be arranged in separate converters, it is desirable to arrange both in a single converter in order to reduce heat loss of exhaust gas.

The pre-stage catalyst that characterizes the present invention includes a honeycomb structure having a straight flow structure, and an oxidation catalyst layer having oxidation activity formed on the cell partition wall. The honeycomb structure has a thermal conductivity of 10 W / m · ° C. or higher and a heat capacity of 0.4 J / cm ³ · ° C. or higher. For example, by forming a honeycomb structure from silicon carbide, silicon nitride or the like instead of conventional cordierite, a honeycomb structure having such characteristics can be obtained. When the thermal conductivity is lower than 10 W / m · ° C. or the heat capacity is lower than 0.4 J / cm ³ · ° C., the effect of the present invention cannot be obtained.

The oxidation catalyst layer is composed of a porous oxide powder and a catalytic metal supported on the porous oxide powder. As the porous oxide, oxides such as alumina, ceria, zirconia, and titania, or composite oxides composed of a plurality of these oxides 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 honeycomb body volume. 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. Further, the NO _x storage reduction catalyst further supporting a NO _x storage material selected from alkali metals, alkaline earth metals and rare earth elements may be used.

The filter catalyst is a plug- type honeycomb body similar to the conventional DPF. This honeycomb body 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, and the slurry is formed by extrusion or the like and fired. Instead of cordierite powder, powders 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 honeycomb body, combustible powder such as carbon powder, wood powder, starch and resin powder is mixed in the above slurry, and the combustible powder disappears during firing. Thus, the pores can be formed, and the distribution and opening area of the diameters of the surface pores and the internal pores can be controlled by adjusting the particle size and addition amount of the combustible powder.

  The pore distribution in the cell partition walls of the honeycomb body can be in the range of 40 to 80% porosity and 10 to 50 μm average pore diameter, as in the case of 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 filter catalyst includes a catalyst layer formed on the surface of the cell partition wall and the surface of the pores and having a catalyst metal supported 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 the precious metal supported is preferably 0.1 to 5 g per liter of the honeycomb body volume. 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 preferably further supports a NO _x storage material selected from alkali metals, alkaline earth metals, and rare earth elements. If the NO _x storage material is included 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. The amount of the NO _x storage material supported is preferably in the range of 0.05 to 1.00 mol per liter of honeycomb volume. 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 the catalyst layer on the honeycomb body, 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 fired, and then the precious metal and the NO _x storage material are added. What is necessary is just to carry. 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 of the honeycomb body volume. 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.

The constituent ratio of the pre-stage catalyst and the filter catalyst is not particularly limited, but is preferably in the range of the pre-stage catalyst: filter catalyst = 3: 1 to 1-3 in terms of volume ratio.

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

（実施例１）
図１に本実施例の排ガス浄化装置を示す。この排ガス浄化装置は、前段触媒１と、フィルタ触媒２とが、排ガス上流側から下流側に向かってこの順に隣接して触媒コンバータ３内に配置されてなる。触媒コンバータ３の上流側には、排ガス中に軽油を添加するインジェクタ４が備えられている。

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

  Hereinafter, the manufacturing method of the front | former stage catalyst 1 and the filter catalyst 2 is demonstrated, and it replaces with the detailed description of a structure.

<Preparation of pre-stage catalyst 1>
A honeycomb structure with a straight flow structure made of silicon carbide (SiC) was prepared. As shown 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 300 / inch ² and a thickness of 0.30 mm. The cell partition wall has a porosity of 30%, a core mass of 425 g / L, a thermal conductivity of 45 W / m · ° C., and a heat capacity of 0.45 J / cm ³ · ° C.

  Next, a mixed slurry in which each powder of alumina, titania, zirconia, and ceria was dispersed in water was prepared, and a coat layer was formed at 200 g / L on the cell partition wall surface of the honeycomb structure by a wash coat method. Thereafter, 3 g / L of Pt was supported and calcined by a water absorption supporting method to prepare a pre-stage catalyst 1.

<Preparation of filter catalyst 2>
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, a mixed slurry in which each powder of alumina, titania, zirconia and ceria is dispersed in water is prepared, and 120 g of a coating layer is applied to the surface of the cell partition walls of the honeycomb filter and the pore surfaces inside the cell partition walls by a wash coat method. / L formed. Thereafter, Pt was supported at 2 g / L by the water absorption method and baked, and 0.1 mol / L of Li, Ba, and K were each supported by the water absorption method and then calcined at 500 ° C. to prepare filter catalyst 2. .

(Examples 2-3, Comparative Examples 1-3)
Example 1 is the same as Example 1 except that the honeycomb structure of the pre-catalyst 1 is replaced with that shown in Table 1.

<Test 1>
The pre-stage catalyst 1 and the filter catalyst 2 are arranged in the catalytic converter 3 adjacent to each other in this order from the exhaust gas upstream side to the downstream side to obtain the exhaust gas purification apparatus of this example. The catalytic converter 3 was mounted on the exhaust system of a diesel engine with a displacement of 2 L, and the torque of the diesel engine was adjusted at a rotational speed of 1900 rpm, so that the gas temperature entering the first catalyst 1 was 200 ° C. After operating in this state for 5 to 10 minutes, light oil is added to the exhaust gas from the injector 4 attached upstream of the catalytic converter 3, and the temperature and profile of the gas emitted from the pre-stage catalyst 1 are measured to detect ignition and misfire. Judgment was made. The results are shown in Table 2.

  The same test was conducted when the temperature of the gas entering the pre-stage catalyst 1 was 230 ° C and 270 ° C, and the results are shown in Table 2. In Table 2, the case where the gas emission temperature was about 350 ° C. and the ignition was good was evaluated as (◯). ), And those having an outlet gas temperature lower than the entering gas temperature were evaluated as (×).

<Test 2>
The catalytic converter 3 is installed in the exhaust system of a diesel engine with a displacement of 2L, and acceleration / deceleration from idling to 60km / hr and from 60km / hr to idling is performed 10 times, and acceleration from idling to 100km / hr One cycle of steady running at 100 km / hr was defined as one cycle, and this was performed for 100 cycles. During steady running at 100 km / hr, in the case of Comparative Example 1, light oil was added from the injector 4 in such an amount that the outgas temperature of the pre-stage catalyst 1 was 600 ° C. Incidentally, the temperature of the gas entering the pre-stage catalyst 1 is about 240 ° C. for 60 km / hr steady running and about 340 ° C. for 100 km / hr steady running.

  Then, after 100 cycles, the torque of the diesel engine was changed at 2000 rpm and the HC purification rate when the temperature was raised at a rate of 10 ° C./min was continuously measured to calculate the HC 50% purification temperature. The results are shown in Table 2. In addition, the HC50% purification temperature was measured in the same manner for a new purification device that was not subjected to the cycle test, and the results are shown in Table 2.

<Test 3>
Table 2 shows the results obtained by decomposing each filter catalyst after the cycle test and visually observing the PM deposition state. In Table 2, (○) indicates that there is almost no PM, (△) indicates that PM remains unburned only at the upstream or outer periphery, and (×) indicates that PM remains largely unburned as a whole. evaluated.

  <Evaluation>

表２からわかるように、実施例１〜３の浄化装置では、軽油の添加によっても冷却されることなく着火が円滑に進行しているが、比較例１〜３では着火が不安定となっている。これは、前段触媒１のハニカム構造体の熱容量の差に起因していると考えられる。

As can be seen from Table 2, in the purification apparatuses of Examples 1 to 3, the ignition proceeds smoothly without being cooled by the addition of light oil, but in Comparative Examples 1 to 3, the ignition becomes unstable. Yes. This is considered to be caused by the difference in the heat capacity of the honeycomb structure of the pre-stage catalyst 1.

  Moreover, about HC50% purification temperature, the purification apparatus of Comparative Examples 2-3 has a large raise degree after a cycle test, and its durability is low. However, according to the purification apparatuses of Examples 1 to 3, it can be seen that the degree of increase after the cycle test is small, and the Pt grain growth is suppressed. Since Comparative Example 1 has better durability than Comparative Examples 2 and 3, it is considered that this is due to the high thermal conductivity of the honeycomb structure of the pre-catalyst 1.

  Further, in Examples 1 to 3, the PM deposition state after the cycle test is better than in each comparative example, and this is an effect of using a honeycomb structure having a large heat capacity and high thermal conductivity as a pre-stage catalyst. Conceivable. That is, since the combustion heat of the added light oil is transmitted to the filter catalyst 2 through the exhaust gas without being trapped in the pre-stage catalyst 1, the Pt grain growth of the pre-stage catalyst 1 is suppressed, and the PM deposited on the filter catalyst 2 is efficient. It is thought that it burned well.

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

Explanation of symbols

  1: Pre-stage catalyst 2: Filter catalyst 3: Catalytic converter

Claims (1)

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 wall flow structure filter catalyst having a catalyst layer formed on the surface of the cell partition and the inner surface of the pores and having a catalyst metal supported on the porous oxide. ,
A pre-stage catalyst disposed on the exhaust gas upstream side of the filter catalyst and comprising a honeycomb structure having a straight flow structure and an oxidation catalyst layer having oxidation activity formed in the cell partition wall, upstream of the pre- stage catalyst A diesel exhaust gas purification device used in a system for adding a liquid reducing agent to exhaust gas on the side,
The diesel exhaust gas purification apparatus, wherein the honeycomb structure of the pre-stage catalyst has a thermal conductivity of 10 W / m · ° C or more and a heat capacity of 0.4 J / cm ³ · ° C or more.

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