JP2009019537A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance exhaust emission control performance, by early activating a three-way catalyst 32. <P>SOLUTION: A CO adsorbing member 31 is arranged on the upstream side of the three-way catalyst 32 of an exhaust passage of an engine 5. A metal carrier 36 having a heat conduction restraining part 35 by a slit is used in an intermediate position in the three-way catalyst 32. Pd/alumina and Rh/OSC are arranged in a catalyst part 32a on the upstream side of the heat conduction restraining part 35. Pt/alumina and the Rh/OSC are arranged in a downstream side catalyst part 32b. CeZr-based composite oxide having the ZrO<SB>2</SB>/CeO<SB>2</SB>mass ratio of 1 or more, is used as an oxygen storage material (an OSC material) of the upstream side catalyst part 32a. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

    Regarding engine exhaust gas purification devices, it is required to ensure low temperature purification performance when the exhaust gas temperature is low, such as when the engine is cold-started, and the catalyst when the exhaust gas temperature is high, such as during engine acceleration operation. It is required to suppress deterioration of the material. With regard to the low-temperature purification performance, the problem is how to suppress the emission of unpurified exhaust gas and how quickly the catalyst is activated at a low temperature of the exhaust gas where the catalyst is not sufficiently activated. On the other hand, regarding the thermal degradation of the catalyst, the specific surface area is reduced by suppressing the sintering of the catalyst metal, the oxygen storage / release material (OSC material) supporting the catalyst metal, and the crystal structure of other metal oxide support materials. It becomes a problem to suppress the decrease.

    Due to the development and advancement of the high-dispersion support technology of catalyst metal and the composite technology of support materials, the problem of thermal degradation of the catalyst is being solved. However, regarding the problem of unpurified exhaust gas emission at low temperatures, the use of HC trap materials has only been attempted, and for the early activation of the catalyst, by placing the catalyst close to the engine, etc. The catalyst is only heated to an early temperature.

    For example, Patent Document 1 discloses that an HC trap catalyst is disposed upstream of an engine exhaust passage, a three-way catalyst is disposed downstream thereof, and a honeycomb carrier of the HC trap catalyst is slit across the cell. Is described. This suppresses the heat conduction from the upstream side to the downstream side of the carrier by the slit, thereby delaying the temperature rise of the entire carrier of the HC trap catalyst. That is, by delaying the HC desorption timing, the amount of HC adsorption is increased, the temperature rise of the catalyst metal until HC desorption starts, and the purification efficiency of HC desorbed thereafter is increased. It is.

    Patent Document 2 describes that a CO low-temperature oxidation catalyst is disposed upstream of the three-way catalyst in the engine exhaust passage. This is to use the heat obtained by the oxidation of CO in the exhaust gas for the early activation of the three-way catalyst. As the CO low-temperature oxidation catalyst, a composite oxide of two or more base metals selected from the group consisting of Mn, Fe, Co, Ni and Cu (for example, a Cu—Mn composite oxide) is disclosed.

Patent Document 3 describes a CO selective oxidation catalyst in which a platinum component and cuprous oxide are supported on an inorganic carrier.
JP 2004-84517 A JP 2005-87963 A JP 2006-341206 A

    By the way, the engine is required to obtain high output with low fuel consumption. In order to meet these conflicting demands, engine control technologies such as fuel injection control, air-fuel ratio control, ignition control, etc., technology for reducing friction of valve system components, and technology related to the structure of the engine body are being developed. For example, a direct injection engine that directly injects fuel into an in-cylinder combustion chamber is used, and a high compression ratio engine is used in terms of the structure of the engine body.

    However, in the case of a direct injection engine, when the engine is cold, fuel vaporization in the cylinder tends to be insufficient, and the amount of HC emission increases. Also, if the compression ratio is high, fuel vaporization tends to be insufficient, and the fuel is pushed into an unburned area in the cylinder (such as a clevis defined between the upper surface of the cylinder block and the lower surface of the cylinder head). This increases the amount of HC emission.

    The engine output is also related to the structure of the exhaust system. That is, the exhaust gas purification catalyst is a factor that increases the back pressure of the engine by hindering the flow of exhaust gas. In addition, there is a problem that exhaust interference occurs in the exhaust manifold where the pressure of the exhaust gas discharged for each cylinder interferes with the exhaust of other cylinders. Therefore, it is required to reduce the back pressure (exhaust resistance) in order to improve engine output.

    For that purpose, for example, in the case of a four-cylinder engine, exhaust interference is obtained by collecting the four branch pipes of the exhaust manifold into two, and then combining the two into one. In addition, it is effective to increase the distance from the engine body to the exhaust manifold assembly (for example, 80 cm or more) and to provide a catalyst on the downstream side of the increase in back pressure.

    However, when the distance from the engine body to the catalyst is increased, it is advantageous for suppressing the sintering of the catalyst metal and the crystal structure of the support material, but the exhaust gas temperature decreases until reaching the catalyst. This is extremely disadvantageous from the viewpoint of improving the low-temperature purification performance (early catalyst activation and suppression of emission of unpurified exhaust gas).

    On the other hand, in the past, each problem has been individually addressed, such as suppression of unpurified HC emissions by the HC trap catalyst as described above, and early activation of the three-way catalyst by the CO oxidation catalyst. It is hard to say that has been made.

    Therefore, an object of the present invention is to increase the catalyst temperature efficiently at the time of low exhaust gas temperature to achieve early activation thereof.

    Moreover, this invention makes it a subject to absorb the fluctuation | variation of the air fuel ratio A / F of exhaust gas efficiently, and to improve exhaust gas purification efficiency.

    In the present invention, an exhaust gas purification catalyst that employs a metal carrier with a heat conduction suppressing portion and a CO adsorbent are combined with respect to such problems.

That is, the present invention provides an exhaust gas purification device provided in an exhaust passage of an engine.
A CO adsorbent that adsorbs CO in the exhaust gas, and an exhaust gas purification catalyst disposed downstream of the CO adsorbent in the exhaust gas flow,
The exhaust gas purification catalyst is one in which a catalyst layer is formed on a cell passage wall of a honeycomb-shaped metal carrier having a large number of cell passages through which exhaust gas passes,
The metal carrier has a gap formed by partially cutting the cell passage wall at an intermediate position in the exhaust gas flow direction so that heat conduction from the carrier upstream side to the carrier downstream side is suppressed. ,
Pd / alumina in which Pd is supported on alumina particles and Rh / OSC in which Rh is supported on oxygen storage material particles are disposed in the catalyst portion upstream of the intermediate position of the metal carrier,
Pt / alumina in which Pt is supported on alumina particles and Rh / OSC in which Rh is supported on oxygen storage material particles are disposed in the catalyst portion downstream of the intermediate position of the metal carrier,
The oxygen storage component particles in the upstream catalyst unit is characterized in that the mass ratio _ZrO 2 / CeO ₂ in ZrO ₂ is formed of one or more composite oxide to and CeO ₂ containing Zr and Ce.

    Therefore, when the temperature of the exhaust gas, the CO adsorbent and the catalyst is low, such as when the engine is cold started, the CO in the exhaust gas is adsorbed by the upstream CO adsorbent, and the CO is discharged without being purified. It is prevented. In addition, the temperature of the catalyst rises as the exhaust gas temperature rises, but since the cell passage wall is partially cut and a gap is formed at the intermediate position of the catalyst metal carrier, Also, heat conduction from the upstream side to the downstream side is suppressed. For this reason, the temperature of the upstream catalyst portion is efficiently increased by the heat of the exhaust gas, that is, quickly rises, and the upstream catalyst portion can be activated early.

    Thus, Pd / alumina having a low light-off temperature related to the purification of HC and Rh / OSC having a low light-off temperature related to the purification of NOx are arranged in the upstream side catalyst portion. And NOx can be efficiently purified, which is advantageous in reducing the amount of HC and NOx discharged without purification. Further, Rh / OSC has a low light-off temperature for CO purification, and since this Rh / OSC is disposed in both the upstream catalyst portion and the downstream catalyst portion, the CO adsorbent desorbs CO as the temperature rises. Even if the amount of CO flowing into the catalyst increases, it can be efficiently purified.

In addition, since the oxygen storage material particles constituting the Rh / OSC of the upstream catalyst portion are composed of a composite oxide having a ZrO ₂ / CeO ₂ mass ratio of 1 or more, the oxygen storage material particles correspond to changes in the air-fuel ratio A / F of the exhaust gas. Responsibility when occluding and releasing oxygen is good, so A / F fluctuations are absorbed in the upstream catalyst part, and purification of HC, CO, and NOx not only in the upstream catalyst part but also in the downstream catalyst part To be advantageous.

    The gap in the cell passage wall at the intermediate position of the metal carrier may be a slit extending in the direction intersecting the exhaust gas flow direction (particularly the direction orthogonal), or a round hole penetrating the cell passage wall in the intersecting direction. It may be a hole such as a long hole extending. By providing a large number of such slits or holes in a direction intersecting the exhaust gas flow direction, the heat conduction suppressing portion can be formed. Or you may make it form the space | gap for heat conduction suppression by notching the said metal support | carrier from the outer periphery.

    Moreover, it is preferable to form the said space | gap in the position away from the upstream end of the said metal support | carrier to 1/4 downstream of the said carrier length from the upstream end to the downstream side.

Examples of the CO adsorbent include perovskite oxides such as SrTiO ₃ , or those in which Pd is supported thereon, Cu support materials in which Cu is supported on alumina, ceria, or the like, or Fe, Co, Ni, alumina What carried at least 1 type of W and Mo is preferable. In the present specification, claims, etc., the exhaust gas component is chemically or physically adsorbed or captured when the temperature is low, and the adsorbed or captured exhaust gas component is removed when the temperature rises. The material to be detached is referred to as “adsorbent”, but may be referred to as “occlusion material” or “trap material” by those skilled in the art.

Preferably, the upstream catalyst portion has a structure in which a plurality of catalyst layers are overlapped, the Rh / OSC is disposed in the upper layer, and the Pd / alumina is disposed in the lower layer.
The downstream catalyst portion has a structure in which a plurality of catalyst layers are overlapped, the Rh / OSC is disposed in the upper layer, and Pd / alumina is disposed in the lower layer together with the Pt / alumina.

    In other words, in both the upstream catalyst section and the downstream catalyst section, oxygen storage material particles (OSC material constituting Rh / OSC) that store and release oxygen according to the A / F fluctuation of the exhaust gas are disposed in the upper layer. Therefore, it is advantageous to absorb the A / F fluctuation and efficiently purify HC, CO and NOx. Further, since Rh / OSC is disposed in the upper layer that is easily heated by the exhaust gas, it is advantageous for purification of NOx, and when CO is desorbed from the CO adsorbent and a large amount of CO flows into the catalyst. This is advantageous for CO purification.

    Preferably, the response of the oxygen storage material particles arranged in the upstream catalyst section when storing and releasing oxygen in accordance with fluctuations in the air-fuel ratio A / F of the exhaust gas is the downstream catalyst. It is better than the responsiveness of the oxygen storage material particles arranged in the part.

    As a result, the A / F fluctuation is well absorbed in the upstream catalyst section, which is advantageous in efficiently purifying HC, CO, and NOx not only in the upstream catalyst section but also in the downstream catalyst section.

Preferably, the oxygen storage material particles in the downstream catalyst portion are composed of a composite oxide containing Zr and Ce,
The mass ratio ZrO ₂ / CeO ₂ of the oxygen storage material particles in the upstream catalyst part is larger than the mass ratio ZrO ₂ / CeO _{2 of the} oxygen storage material particles in the downstream catalyst part. .

That is, as will become clear from the experimental data described later, in the case of a composite oxide containing Zr and Ce, the larger the mass ratio ZrO ₂ / CeO ₂ is, the higher the oxygen storage / release response to the A / F fluctuation. Will be better. Therefore, the above configuration is advantageous in efficiently purifying exhaust gas by absorbing A / F fluctuations.

In this case, the oxygen storage material particles in the downstream catalyst portion have a relatively small mass ratio ZrO ₂ / CeO ₂ , that is, a large amount of Ce component, which means that the oxygen storage / release amount is upstream. It means more than the oxygen storage material particles in the catalyst part. Thus, when CO is desorbed from the CO adsorbent, a large amount of oxygen is required for the oxidation and purification of the CO, and the A / F of the exhaust gas is swung to the rich side along with the desorption of CO. Sometimes, a large amount of oxygen is released from the oxygen storage material particles in the downstream catalyst part, which is advantageous for CO oxidation purification during CO desorption.

As described above, according to the present invention, the CO adsorbent and the exhaust gas purification catalyst disposed on the downstream side thereof are provided, and the metal carrier of the catalyst is disposed at the intermediate position in the exhaust gas flow direction. Therefore, CO can be prevented from being discharged without being purified at a low temperature such as when the engine is cold started, and the upstream side catalyst portion can be activated earlier than the intermediate position. Pd / alumina and Rh / OSC are arranged in the side catalyst part, Pt / alumina and Rh / OSC are arranged in the downstream catalyst part, and the exhaust gas A is used as oxygen storage material particles in the upstream catalyst part. Since a composite oxide having a ZrO ₂ / CeO ₂ mass ratio of 1 or more effective for / F fluctuation absorption is employed, it is advantageous for efficient purification of HC, CO and NOx.

    Further, the catalyst layer of the upstream catalyst part is made into a laminated structure, Rh / OSC is placed in the upper layer, Pd / alumina is placed in the lower layer, the catalyst layer in the downstream catalyst part is made into a laminated structure, and Rh / OSC is placed in the upper layer. Arranging and disposing Pd / alumina together with Pt / alumina in the lower layer is advantageous for efficiently purifying HC, CO and NOx by absorbing the A / F fluctuation of the exhaust gas, and the exhaust gas temperature is low. This is advantageous for the purification of NOx and the purification of CO when CO is desorbed from the CO adsorbent.

    In addition, if oxygen storage material particles having better responsiveness to A / F fluctuations of the exhaust gas than the downstream catalyst part are arranged in the upstream catalyst part, the A / F fluctuation is absorbed well in the upstream catalyst part. In addition to the upstream catalyst portion, the downstream catalyst portion is advantageous for efficiently purifying exhaust gas.

Further, in the case where the mass ratio ZrO ₂ / CeO ₂ of the oxygen storage material particles in the upstream catalyst portion is larger than the mass ratio ZrO ₂ / CeO ₂ of the oxygen storage material particles in the downstream catalyst portion, the exhaust gas A This is advantageous for efficiently purifying exhaust gas by absorbing the / F fluctuation, and when the A / F of the exhaust gas moves to the rich side due to the desorption of CO from the CO adsorbent, the downstream side Since a large amount of oxygen is released from the oxygen storage material particles in the catalyst portion, it is advantageous for the oxidation purification of CO during the CO desorption.

    Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

    FIG. 1 shows an automobile gasoline engine and exhaust system. In the figure, 1 is a dash panel (cabinet front plate) that partitions the engine room 2 and the passenger compartment, 3 is a hood of the engine compartment 2, and 4 is a floor tunnel portion that extends in the vehicle longitudinal direction in the center of the floor panel of the passenger compartment. is there. In the engine room 2, 5 is a horizontal multi-cylinder engine body, 6 is a radiator, 7 is an intake manifold, and 8 is an exhaust manifold.

    In the engine main body 5, 11 is a cylinder, 12 is a piston, and 13 is a spark plug. The engine is a high-compression-ratio direct-injection gasoline engine having a side injector 14 to obtain high output with low fuel consumption. The exhaust manifold 8 extends rearward from the engine body 5, and a first catalytic converter 16 is coupled to a collecting portion 15 (see FIG. 2). A second catalytic converter 18 is provided in the front part of the exhaust pipe 17 extending rearward from the first catalytic converter 16.

<Exhaust manifold structure>
In the present embodiment, the collecting portion 15 of the exhaust manifold 8 is far from the engine body 5 (for example, 60 cm or more and 100 cm or less at the shortest pipe length portion) and is disposed at a position below the dash panel 1. Therefore, although the first catalytic converter 16 is directly connected to the exhaust manifold 8, the first catalytic converter 16 is disposed far from the engine body 5 and below the dash panel 1 or behind the dash panel 1. The first catalytic converter 16, the exhaust pipe 17 and the second catalytic converter 18 are disposed in the floor tunnel portion 4, and the exhaust pipe 17 extends rearward through the floor tunnel portion 4.

    As described above, the reason why the distance from the engine body 5 to the manifold assembly portion 15 is long is to suppress exhaust interference between cylinders of the engine. That is, as shown in FIG. 2 as an example of a four-cylinder engine, the exhaust manifold 8 includes two branch pipes 21 at both ends of four branch pipes 21 to 24 extending from each cylinder (not shown in FIG. 2). 24 is assembled in one collecting pipe 25 on the way, while the central two branch pipes 22 and 23 are gathered on one collecting pipe 26 and the two collecting pipes 25 and 26 are assembled. The collecting unit 15 is used.

    With such an exhaust manifold structure, the ignition order of the four cylinders is determined so that exhaust is performed in the order of the branch pipe 21 → the branch pipe 23 → the branch pipe 24 → the branch pipe 22. Exhaust interference generated between the cylinders can be reduced.

    Further, by adopting the exhaust manifold structure, the exhaust passage from the engine body 5 to the first catalytic converter 16 becomes long (for example, 65 cm or more), so that a catalyst described later contained in the converter 16 can be used. The resistance to exhaust flow is reduced, which is advantageous for high fuel efficiency and high output of the engine. In this embodiment, the 1st catalytic converter 16 is arrange | positioned in the downward position of pedals, such as an accelerator and a brake which a driver | operator operates for the driving | operation of the said motor vehicle.

<About catalyst>
The first catalytic converter 16 accommodates a CO adsorbing member 31 and a three-way catalyst 32 so that the former is located upstream of the exhaust gas flow and the latter is located downstream thereof. An HC trap catalyst 33 is accommodated in the second catalytic converter 18.

    The CO adsorbing member 31 adsorbs the CO in the exhaust gas when the temperature is low (when the three-way catalyst 32 is inactive or low activity) on the cell wall of the honeycomb carrier, and adsorbs when the temperature is high. A CO adsorbent layer that begins to desorb CO is formed.

    The three-way catalyst 32 is a catalyst having a high purification rate near the stoichiometric air-fuel ratio for simultaneously purifying HC, CO, and NOx in the exhaust gas, and is a honeycomb-shaped metal carrier having a number of cell passages through which the exhaust gas passes. A catalyst layer is formed on the cell passage wall.

A heat conduction suppression unit that suppresses heat conduction from the upstream side of the carrier to the downstream side of the carrier by providing a gap formed by partially cutting the cell passage wall at an intermediate position in the exhaust gas flow direction of the metal carrier. 35 is formed. Pd / alumina in which Pd is supported on alumina particles and Rh / OSC in which Rh is supported on oxygen storage material particles are arranged in the catalyst portion 32a upstream of the heat conduction suppressing portion 35, Pt / alumina in which Pt is supported on alumina particles and Rh / OSC in which Rh is supported on oxygen storage material particles are disposed in the catalyst portion 32b on the downstream side of the heat conduction suppressing portion 35. . Oxygen storage component particles of the upstream catalyst unit 32a, the weight ratio _ZrO 2 / CeO ₂ in ZrO ₂ is formed of one or more composite oxide to and CeO ₂ containing Zr and Ce.

    The HC trap catalyst 33 is a honeycomb catalyst formed by forming a catalyst layer on the cell wall of the honeycomb carrier, and the catalyst layer is HC in the exhaust gas until the three-way catalyst 32 reaches a temperature near its light-off temperature. HC trap material (for example, zeolite) that traps HC and a catalyst that oxidizes and purifies HC released from the trap material when the temperature further rises.

-Suitable three-way catalyst 32-
As shown in FIG. 3, the preferred metal carrier 36 is formed by superposing a metal flat plate 37 and a metal corrugated plate 38 in a spiral shape, and a large number between the flat plate 37 and the corrugated plate 38. Cell passage 39 is formed. A large number of slit holes 41 are formed side by side in a direction perpendicular to the exhaust gas flow direction at an intermediate position in the exhaust gas flow direction (cell passage direction) of each of the flat plate 37 and the corrugated plate 38. In the example of FIG. 3, the slit holes 41 are provided in a plurality of rows, but may be a single row.

    The slit hole 41 is a gap formed by partially cutting the cell passage wall at an intermediate position of the metal carrier 36, and the slit hole 41 suppresses heat conduction from the carrier upstream side to the carrier downstream side. A conduction suppressing portion 35 is formed. The heat conduction suppressing portion 35 is preferably formed at a position away from the upstream end of the metal carrier 36 to the downstream side by about 1/3 of the carrier length.

    The upstream catalyst part 32a preferably has a laminated structure in which a plurality of catalyst layers are overlapped, and the Rh / OSC is disposed in the upper layer and the Pd / alumina is disposed in the lower layer. It is preferable that the downstream catalyst portion 32b has a laminated structure in which a plurality of catalyst layers are stacked, the Rh / OSC is disposed in the upper layer, and Pd / alumina is disposed in the lower layer together with the Pt / alumina.

    Thus, the oxygen storage material particles constituting the Rh / OSC of the upstream side catalyst part 32a are preferably those having good responsiveness to store and release oxygen in accordance with fluctuations in the air-fuel ratio A / F of the exhaust gas. About the oxygen storage material particle which comprises Rh / OSC of the downstream catalyst part 32b, a thing with a large oxygen storage-release amount is preferable.

-Suitable HC trap catalyst 33-
A honeycomb catalyst in which a catalyst layer is formed on the cell wall of the honeycomb carrier is used. The catalyst layer contains an HC trap material such as zeolite and an HC purification catalyst for oxidizing and purifying HC. Β-type zeolite is preferable as the HC trap material. The HC trap catalyst 33 may be a mixed type in which an HC trap material and an HC purification catalyst are mixed and supported so as to form a single catalyst layer on the cell wall of the honeycomb carrier, or a catalyst layer containing the HC trap material. And a catalyst layer containing an HC purification catalyst.

    In the case of a stacked type, the HC trap material is disposed in the lower layer, and the HC purification catalyst is disposed in the upper layer. For example, zeolite is disposed in the lower layer, and Pd / alumina as the HC purification catalyst is disposed in the upper layer. An Rh / OSC layer may be formed between the upper layer and the lower layer. Alternatively, the upper layer may be a mixed layer of Pd / alumina and Pd / OSC, and the Rh / OSC layer may be formed in the middle.

    Instead of providing the HC trap catalyst 33, an HC trap material may be provided in the downstream catalyst portion 32b of the three-way catalyst 32.

<Performance evaluation of oxygen storage materials>
−Oxygen storage amount−
As an oxygen storage material, a CeZrNd composite oxide (double oxide) containing Ce, Zr, and Nd and a CeZrLaPr composite oxide (double oxide) containing Ce, Zr, La, and Pr were prepared. The composition ratio of CeZrNd composite oxide is CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 10: 80: 10 (mass%), and the composition ratio of CeZrLaPr composite oxide is CeO ₂ : ZrO ₂ : La ₂ O ₃ : _{Pr 6 O 11 = 50: 40} : 5: a 5 (mass%).

    Then, the oxygen storage amount of each of these oxygen storage materials was measured by the measuring device shown in FIG. In this test apparatus, gas can be circulated through a test material (oxygen storage material) 48, and linear oxygen sensors 49 are provided on the inlet side and the outlet side of the test material 48 in the exhaust passage 47, respectively. It has been.

The test material was bench-aged at 900 ° C. for 100 hours in advance. In the measurement, first, 10% CO ₂ gas (remaining N ₂ ) was passed through the test material 48. Then, oxygen is added to the gas for 20 seconds (lean state), then the gas-free state (stoichiometric state) is continued for 20 seconds, then CO is added for 20 seconds (rich state), and then the gas-free state The cycle of continuing (stoichiometric state) for 20 seconds was repeated, and the output difference (inlet side-outlet side) of the linear oxygen sensor on the inlet side and the outlet side was measured. In the lean state, the specimen 48 occludes oxygen, so the output difference becomes positive. The oxygen storage amount was determined by integrating the output difference in the lean state. The oxygen storage amount was measured at 350 ° C, 400 ° C, 450 ° C and 500 ° C.

The measurement results are shown in FIG. In the figure, “Ce / Zr / Nd = 10/80/10” refers to the CeZrNd composite oxide, and “Ce / Zr / La / Pr = 50/40/5/5 /” refers to the CeZrLaPr composite oxide. That's it. In FIG. 5 and FIGS. 6 to 8 described later, “CeO ₂ ”, “ZrO ₂ ”, “La ₂ O ₃ ”, and “Pr ₆ O ₁₁ ” are respectively represented by “Ce”, “Zr”, “La”. And “Pr”. According to FIG. 5, the oxygen storage amount increases with increasing temperature. Further, the CeZrLaPr composite oxide has a larger oxygen storage capacity than the CeZrNd composite oxide.

−Oxygen release amount−
Four types of CeZrNd composite oxides having different ZrO ₂ / CeO ₂ mass ratios were prepared, and each oxygen release amount was measured by a measuring apparatus shown in FIG. The CeO ₂ : ZrO ₂ : Nd ₂ O ₃ (mass%) of each of the four types of CeZrNd composite oxides is 10:80:10 (ZrO ₂ / CeO ₂ mass ratio = 8), 20:70:10 (ZrO ₂ / CeO ₂ weight ratio = 7/2), 30: 60: a 10 _(ZrO 2 / CeO ₂ weight ratio = 2) and 50:40:10 _(ZrO 2 / CeO ₂ weight ratio = 4/5). Further, before the measurement, each specimen was subjected to aging for 24 hours in an atmosphere (temperature: 1100 ° C.) of 2% O ₂ , 10% H ₂ O and the remaining N ₂ .

    The measurement method is basically the same as in the case of the oxygen storage amount, but in the above cycle, the specimen 48 releases oxygen when it becomes rich, and the linear oxygen sensors on the inlet side and outlet side Since the output difference (inlet side−outlet side) becomes negative, the oxygen release amount was determined by integrating the output difference in this rich state. The measurement temperature was 500 ° C.

The measurement results are shown in FIG. The “Ce content” on the horizontal axis in the figure means the CeO ₂ content. According to the figure, it can be seen that as the CeO ₂ content increases, that is, as the ZrO ₂ / CeO ₂ mass ratio decreases, the oxygen release amount increases.

-Responsiveness to A / F fluctuation-
Responsiveness of occluding and releasing oxygen in accordance with fluctuations in the air-fuel ratio A / F of the exhaust gas of each of the four types of CeZrNd composite oxides having different mass ratios of the CeZrLaPr composite oxide and the ZrO ₂ / CeO ₂ (hereinafter, This was referred to as “A / F responsiveness” as appropriate.

    That is, a catalyst powder in which Rh was supported on each of the five types of composite oxide particles was prepared, and a test catalyst was prepared in which each was supported on a honeycomb carrier. The amount of catalyst powder supported per liter of honeycomb carrier was 70 g / L, and the amount of Rh supported was 0.3 g / L. Then, the NOx purification rate at each of the catalyst inlet gas temperatures of 400 ° C. and 300 ° C. of each test catalyst was measured by a rig test using various simulated exhaust gases having different A / F fluctuation rates.

    The A / F variation conditions are as shown in Table 1, and the gas composition at each A / F variation is as shown in Table 2. That is, the A / F was forcibly vibrated by adding a predetermined amount of fluctuation gas in a pulsed manner while constantly flowing the main stream gas of A / F = 14.7. The space velocity SV was 60000 / h.

    The A / F fluctuation speed (A / F / s) in Table 1 is d (A / F) / dT (T is time) shown in FIG. When changing the A / F fluctuation speed, the fluctuation amplitude was changed without changing only the fluctuation frequency, because the magnitude of fluctuation when the A / F was changed from 14.7 to plus or minus. This is because the A / F is quantitatively aligned by the relationship between the shake time and the amplitude when the A / F is positive (or negative).

The NOx purification rate C400 at 400 ° C. is shown in FIG. 8, and the NOx purification rate C300 at 300 ° C. is shown in FIG. 8 and 9, “CZN” refers to a CeZrNd composite oxide, and “CZLP” refers to a CeZrLaPr composite oxide. According to both figures, when the A / F fluctuation rate value is small, the complex oxide (the Ce content is large) having a small ZrO ₂ / CeO ₂ mass ratio has a larger NOx purification rate, but the A / F fluctuation is large. As the speed value increases, the magnitude relationship of the NOx purification rate between the composite oxides is reversed, and the composite oxide having a larger mass ratio has a higher NOx purification rate.

As is apparent from FIGS. 5 and 6, CeZr-based composite oxides such as CeZrNd composite oxide and CeZrLaPr composite oxide have a larger oxygen storage / release amount as the ZrO ₂ / CeO ₂ mass ratio is smaller. Therefore, when the A / F fluctuation speed value is small (FIGS. 8 and 9), the NOx purification rate increases as the ZrO ₂ / CeO ₂ mass ratio decreases (the Ce content increases). Is recognized.

If the responsiveness of oxygen storage / release to the A / F fluctuation of each composite oxide is the same, the NOx purification rate between the composite oxides even if the A / F fluctuation speed value increases. The magnitude relationship should not change. However, according to the results of FIGS. 8 and 9, when the A / F fluctuation speed value increases, the magnitude relationship of the NOx purification rate between the composite oxides is reversed. This is because the responsiveness of each composite oxide to A / F fluctuations is different, that is, the CeZr-based composite oxide has a higher oxygen storage / release response as the ZrO ₂ / CeO ₂ mass ratio increases. Means.

<Exhaust gas purification performance>
Based on the above, the exhaust gas purification devices of the following Examples 1 to 4 and Comparative Examples 1 to 3 having different configurations of the three-way catalyst were prepared, and the exhaust gas purification performance was evaluated.

Example 1
SrTiO ₃ was employed as the CO adsorbent for the CO adsorbing member 31. The catalyst metal is not supported. The carrier capacity of the CO adsorption member 31 is 0.3L. As the metal carrier 36 of the three-way catalyst 32, the heat conduction suppressing portion 35 is formed by the three rows of slit holes 41 at a position separated from the upstream end of the carrier to the downstream side of the length of 1/3 of the carrier length. It was adopted. The carrier capacity of the three-way catalyst 32 is 0.7L.

The upstream side catalyst portion 32a of the three-way catalyst 32 has Rh / OSC (OSC carrying amount = 70 g / L, Rh carrying amount = 0.3 g / L) disposed in the upper layer and Pd / alumina (alumina carrying amount = 50 g / L, Pd loading = 4.9 g / L). As the upper layer OSC, a CeZrNd composite oxide of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 10: 80: 10 (mass%) is adopted, and as the lower layer alumina, activated alumina containing 4 mass% of La is used. Adopted.

In the downstream side catalyst portion 32b of the three-way catalyst 32, Rh / OSC (OSC carrying amount = 70 g / L, Rh carrying amount = 0.1 g / L) is disposed in the upper layer, and Pt / alumina (alumina carrying amount = 25 g / L, Pt loading = 0.2 g / L) and Pd / alumina (alumina loading = 50 g / L, Pd loading = 1.4 g / L) were arranged. As the upper layer OSC, a CeZrLaPr composite oxide of CeO ₂ : ZrO ₂ : La ₂ O ₃ : Pr ₆ O ₁₁ = 50: 40: 5: 5 (mass%) is adopted, and the lower layer Pt / alumina and Pd / Activated alumina containing 4% by mass of La was used as the alumina.

    The blending ratio of the catalyst noble metal in the entire three-way catalyst 32 is Pt: Pd: Rh = 1: 30: 2 (mass ratio). Note that no HC trap catalyst is provided.

-Example 2-
The upstream side catalyst part 32a of the three-way catalyst 32 has a single layer structure in which Rh / OSC and Pd / alumina are mixed instead of the two-layer structure of Example 1, and the downstream side catalyst part 32b is also in Example 1. Instead of the two-layer structure, a single-layer structure in which Rh / OSC, Pt / alumina, and Pd / alumina are mixed is used. Other configurations are the same as those of the first embodiment.

Example 3
For the OSC of the downstream side catalyst portion 32b of the three-way catalyst 32, CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 30: 60: 10 (mass%) CeZrNd composite oxidation instead of the CeZrLaPr composite oxide of Example 1 The thing was adopted. Other configurations are the same as those of the first embodiment.

Example 4
About the OSC of the downstream side catalyst part 32b of the three-way catalyst 32, instead of the CeZrLaPr composite oxide of Example 1, the same CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 10: 80: 10 (as in the upstream side catalyst part 32a) (Mass%) CeZrNd composite oxide was employed. Other configurations are the same as those of the first embodiment.

-Comparative Example 1-
The same configuration as that of Example 1 was adopted except that a three-way catalyst 32 using a metal carrier in which a heat conduction suppressing portion by a slit hole was not formed.

-Comparative Example 2-
For the OSC of the upstream side catalyst part 32a of the three-way catalyst 32, instead of the CeZrNd composite oxide of Example 1, the same CeZrLaPr composite oxide as that of the downstream side catalyst part 32b was adopted. Other configurations are the same as those of the first embodiment.

-Comparative Example 3-
As the OSC of the upstream side catalyst portion 32a of the three-way catalyst 32, CeO ₂ : ZrO ₂ : La ₂ O ₃ : Pr ₆ O ₁₁ = 50: 40: 5: 5 (mass%) CeZrLaPr composite oxide is adopted, A CeZrNd composite oxide of CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 10: 80: 10 (mass%) was employed as the OSC of the downstream catalyst portion 32b (the arrangement of two types of OSC materials is the same as in Example 1). Reversed.) Other configurations are the same as those of the first embodiment.

(Evaluation of exhaust gas purification performance)
About each exhaust-gas purification apparatus of the said Examples 1-4 and Comparative Examples 1-3, it set to the model exhaust-gas distribution | circulation reaction apparatus, and exhaust-gas purification performance (light-off temperature T50 and high temperature purification performance C400) was evaluated. The model exhaust gas was set to “A / F = 14.7 ± 0.9, 1.0 Hz” shown in Table 1. The space velocity SV was 60000 / h, and the heating rate was 30 ° C./min.

    In T50 (° C.), as the model exhaust gas temperature rises, the concentration of each component (HC and NOx) of the gas detected downstream of the catalyst becomes half of the concentration of each component (HC and NOx) of the gas flowing into the catalyst. This is the catalyst inlet gas temperature (light-off temperature) when the purification rate reaches 50% (that is, when the purification rate becomes 50%), and represents the low-temperature purification performance of the catalyst. C400 (%) is the purification rate of each component (HC, CO and NOx) of the gas when the model exhaust gas temperature at the catalyst inlet is 400 ° C., and represents the high temperature purification performance of the catalyst. The results are shown in Table 3.

    Looking at T50, all of Examples 1 to 4 have a lower light-off temperature than Comparative Examples 1 to 3 and excellent exhaust gas purification performance at low temperatures (the three-way catalyst is activated early. I understand). Looking at C400, in Example, the purification rate of CO and NOx in Example 4 is slightly lower, but in other cases, the purification rate of HC, CO and NOx is higher than in the comparative example, and at high temperatures It can be seen that the exhaust gas purification performance is also excellent.

    Hereinafter, specifically, Comparative Example 1 is different from Example 1 only in that there is no slit hole of the metal carrier, but the light-off temperature of Example 1 is more than a dozen degrees C. compared to Comparative Example 1. It is low. This is recognized as a result of the heat transfer from the upstream side catalyst part 32a to the downstream side catalyst part 32b being suppressed by the slit hole, so that the upstream side catalyst part 32a can be raised in temperature and activated early. Further, in the case of Example 1, since there is a slit hole, the upstream catalyst part 32a is warmed and activated early, and the amount of exhaust gas components flowing into the downstream catalyst part 32b is reduced without being purified. The reaction efficiency in the catalyst part 32b increases, and the time until the purification performance of 99% or 100% is exhibited is shortened. On the other hand, in the case of Comparative Example 1, since there is no slit hole, the effect as in Example 1 cannot be obtained, and the purification rate of 100% is not reached at 400 ° C.

    Comparative Example 2 is different from Example 1 in that CeZrLaPr composite oxide (the oxygen storage / release amount is large but the A / F responsiveness is low) is adopted for both the upstream and downstream catalyst parts 32a and 32b. The light-off temperature of Example 1 is lower than that of Comparative Example 2 by several tens of degrees Celsius. This is recognized to be due to the difference in the A / F responsiveness of the OSC material of the upstream side catalyst portion 32a. That is, in the case of Example 1, the CeZrNd composite oxide with good A / F response of the upstream side catalyst portion 32a effectively absorbs the A / F fluctuation of the exhaust gas, and the light-off temperature is lowered. it is conceivable that.

In Example 3, ZrO ₂ / CeO is more downstream than the OSC material of Example 1 (CeO ₂ : ZrO ₂ : La ₂ O ₃ : Pr ₆ O ₁₁ = 50: 40: 5: 5). _The OSC material (CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 30: 60: 10) having a large ₂ mass ratio is used, and in Example 4, the mass ratio of ZrO ₂ / CeO ₂ is further increased in the downstream side catalyst portion 32b. An OSC material (CeO ₂ : ZrO ₂ : Nd ₂ O ₃ = 10: 80: 10) is used. In other words, the oxygen storage / release amount of the OSC material in the downstream catalyst portion 32b increases in the order of Example 4 → Example 3 → Example 1.

    Accordingly, when comparing the exhaust gas purification performance of Examples 1, 3, and 4, the light-off temperature decreases in the order of Example 4 → Example 3 → Example 1, and the C400 purification rate is also in Examples 1, 3 Is higher than Example 4. From this, it can be seen that the use of an OSC material having a large oxygen storage / release amount for the downstream catalyst portion 32b is advantageous in both light-off performance and high-temperature purification performance.

    Example 4 (OSC material with good A / F response was used for both upstream and downstream catalyst parts 32a and 32b) and Comparative Example 2 (both upstream and downstream catalyst parts 32a and 32b had oxygen storage and release amounts) In comparison with Comparative Example 2, the light-off temperature is lower in Example 4 than in Comparative Example 2. From this, the effect that the A / F responsiveness of the OSC material of the upstream side catalyst portion 32a has on the light-off performance is greater than the effect that the oxygen storage / release amount of the OSC material of the downstream side catalyst portion 32b has on the light-off performance. Recognize.

    Since Comparative Example 3 uses an OSC material having a low A / F response as in Comparative Example 2 for the upstream catalyst portion 32a, the light-off temperature is similarly low, and the downstream catalyst portion 32b Since the oxygen storage / release amount of the OSC material is small, the C400 purification rate is also lower than that of Comparative Example 2.

    In the second embodiment, each of the upstream and downstream catalyst portions 32a and 32b has a single layer structure in which catalyst components are mixed, unlike the two-layer structure of the first embodiment. It is a little higher than Example 1. Therefore, as in Example 1, each of the upstream and downstream catalyst portions 32a and 32b has a stacked structure in which a plurality of catalyst layers are overlapped, and the Rh / OSC is disposed on each upper layer to make exhaust gas efficient. It turns out that it is advantageous in purifying well.

It is a side view which shows the layout of the exhaust-gas purification apparatus of the engine in a motor vehicle front part. It is a perspective view which shows the exhaust-gas purification catalyst apparatus of an engine. It is a perspective view which shows an example of a metal carrier. It is a figure which shows the principal part structure of the apparatus which measures the oxygen storage-and-release amount of an oxygen storage material. It is a graph which shows the temperature and oxygen storage amount characteristic of an oxygen storage material. It is a graph which shows Ce content and oxygen release amount characteristic of an oxygen storage material. It is a figure which shows an A / F fluctuation characteristic. It is a graph which shows the relationship between the A / F fluctuation rate of the catalyst using various oxygen storage materials, and the NOx purification rate in the catalyst inlet temperature of 400 degreeC. It is a graph which shows the relationship between the A / F fluctuation rate of the catalyst using various oxygen storage materials, and the NOx purification rate in the catalyst inlet temperature of 300 degreeC.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Dash panel 4 Floor tunnel part 5 Engine main body 8 Exhaust manifold 15 Collecting part 16 Catalytic converter 17 Exhaust pipe 31 CO adsorption member 32 Three way catalyst 32a Upstream side catalyst part 32b Downstream side catalyst part 33 HC trap catalyst 35 Heat conduction suppression part 36 Metal carrier 39 Cell passage 41 Slit hole

Claims (4)

In the exhaust gas purification device provided in the exhaust passage of the engine,
A CO adsorbent that adsorbs CO in the exhaust gas, and an exhaust gas purification catalyst disposed downstream of the CO adsorbent in the exhaust gas flow,
The exhaust gas purification catalyst is one in which a catalyst layer is formed on a cell passage wall of a honeycomb-shaped metal carrier having a large number of cell passages through which exhaust gas passes,
The metal carrier has a gap formed by partially cutting the cell passage wall at an intermediate position in the exhaust gas flow direction so that heat conduction from the carrier upstream side to the carrier downstream side is suppressed. ,
Pd / alumina in which Pd is supported on alumina particles and Rh / OSC in which Rh is supported on oxygen storage material particles are disposed in the catalyst portion upstream of the intermediate position of the metal carrier,
Pt / alumina in which Pt is supported on alumina particles and Rh / OSC in which Rh is supported on oxygen storage material particles are disposed in the catalyst portion downstream of the intermediate position of the metal carrier,
The oxygen storage component particles in the upstream catalyst unit, exhaust gases, characterized in that the mass ratio _ZrO 2 / CeO ₂ in ZrO ₂ is formed of one or more composite oxide to and CeO ₂ containing Zr and Ce Purification equipment. In claim 1,
The upstream catalyst portion has a structure in which a plurality of catalyst layers are overlapped, the Rh / OSC is disposed in the upper layer, and the Pd / alumina is disposed in the lower layer.
The downstream catalyst portion has a structure in which a plurality of catalyst layers are overlapped, the Rh / OSC is disposed in an upper layer, and Pd / alumina is disposed in the lower layer together with the Pt / alumina. Exhaust gas purification device. In claim 1 or claim 2,
The oxygen storage material particles arranged in the upstream catalyst section are arranged in the downstream catalyst section so as to store and release oxygen in accordance with fluctuations in the air-fuel ratio A / F of the exhaust gas. An exhaust gas purifying apparatus characterized by being better than the responsiveness of the oxygen storage material particles. In any one of Claim 1 thru | or 3,
The oxygen storage material particles in the downstream catalyst portion are composed of a composite oxide containing Zr and Ce,
The mass ratio ZrO ₂ / CeO ₂ of the oxygen storage material particles in the upstream catalyst part is larger than the mass ratio ZrO ₂ / CeO _{2 of the} oxygen storage material particles in the downstream catalyst part. Exhaust gas purification device.

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