JP2006231108A - Apparatus for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus for cleaning exhaust gas in which the activity of a catalyst at low temperature is made high without increasing the usage of an expensive noble metal. <P>SOLUTION: The apparatus for cleaning exhaust gas is constituted in such a way that an upstream-side catalyst and a downstream-side catalyst are arranged in an exhaust gas passage of an internal-combustion engine along the flow direction of exhaust gas and each of these catalysts has a coat layer which is formed on a base material layer and comprises particles of a noble metal-deposited carrier. The volume of the upstream-side catalyst is made smaller than that of the downstream-side catalyst and the mean particle diameter of particles of the carrier of the upstream-side catalyst is made smaller than that of particles of the carrier of the downstream-side catalyst. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

In general, an exhaust gas purifying apparatus for an internal combustion engine is a porous material made of a heat-resistant inorganic oxide such as alumina, on which a catalytic noble metal such as platinum is supported on the surface of a monolith-like heat-resistant carrier base material layer made of cordierite, for example. It has a structure that supports the carrier. Such an exhaust gas purification device oxidizes harmful carbon monoxide (CO) and hydrocarbon (HC) contained in the exhaust gas and converts them into harmless carbon dioxide (CO ₂ ) and water (H ₂ O) at the same time. Nitrogen oxide (NOx) can be reduced to be converted to harmless nitrogen (N ₂ ), so it is also called a three-way catalyst.

  The purification rate of the catalyst increases as the temperature increases. In other words, the purification rate of the catalyst decreases during warm-up immediately after starting the engine at a low temperature.

  The temperature of the exhaust gas generated even during normal operation varies depending on the operating state of the internal combustion engine, and generally the temperature of the exhaust gas decreases toward the downstream side in the flow direction of the exhaust gas purification device. On the other hand, when the surface perpendicular to the exhaust gas flow direction of the catalyst is taken as a reference, the gas entering the catalyst is cooled by the tube wall, and the temperature of the peripheral portion is lowered due to heat radiation from the mantle to the outside.

In order to optimize the purification of exhaust gas corresponding to these conditions, in Patent Document 1, a first catalyst layer is provided on the upstream side of the purification device, and a second catalyst layer is provided on the downstream side. A system is disclosed in which the precious metal components and amounts are changed between the second catalyst layers to enhance the catalyst purification ability. Further, Patent Document 2 discloses a configuration in which the catalyst purification performance is improved by providing a gradient of the loading amount of the noble metal and the transition metal oxide from the upstream side toward the downstream side. Further, Patent Document 3 discloses a method for increasing the catalyst purification ability by setting the precious metal coating ratio of the upstream catalyst and the downstream catalyst to 1 <front side / rear side ≦ 3. Further, Patent Document 4 discloses a configuration in which the catalyst warm-up property is improved by increasing the support density at the center of the catalyst.
Japanese Patent Laid-Open No. 2004-283692 JP-A-9-122443 JP 2003-245523 A JP 2004-275883 A

  However, in the configurations disclosed in each of the above patent documents, in order to improve the catalyst purification performance, a method of increasing the amount of noble metal supported (volume density) is used. There was a problem of increasing.

  Therefore, an object of the present invention is to provide an exhaust gas purifying apparatus that increases the low-temperature activity of a catalyst without increasing the amount of expensive noble metal used.

  In order to solve the above problems, the present invention takes the following method. That is, the invention described in claim 1 is provided in the exhaust gas passage of the internal combustion engine, and includes an upstream catalyst and a downstream catalyst along the flow direction of the exhaust gas. An exhaust gas purifying apparatus comprising a coating layer having carrier particles supporting noble metal on the upstream side, wherein the volume of the upstream catalyst is smaller than the volume of the downstream catalyst, and the average particle size of the carrier particles of the upstream catalyst is the downstream catalyst. The present invention is intended to solve the above-mentioned problem by an exhaust gas purifying apparatus characterized by being smaller than the average particle size of the carrier particles.

  Here, the “average particle size” means an average particle size measured by a laser diffraction particle size distribution measuring device.

  The invention according to claim 2 is the exhaust gas purification apparatus according to claim 1, wherein the amount of noble metal per volume of the upstream catalyst is the same as the amount of noble metal per volume of the downstream catalyst. .

  The invention according to claim 3 is the exhaust gas purifying apparatus according to claim 1 or 2, wherein a noble metal is previously supported on carrier particles, and the carrier particles are coated on a base material layer to form a coat layer. It is characterized by being.

  According to a fourth aspect of the present invention, in the exhaust gas purifying apparatus according to the first aspect, the upstream catalyst carries a noble metal on the coating layer after the carrier layer is coated on the base material layer to form a coating layer. In addition, the downstream catalyst is formed by previously supporting a noble metal on carrier particles, and coating the carrier particles on the base material layer to form a coat layer. The noble metal per volume of the upstream catalyst The amount is smaller than the amount of noble metal per volume of the downstream catalyst.

  In the invention according to any one of claims 1 to 4, the exhaust gas purifying device may be mounted under the floor of the vehicle. According to the configuration of the fourth aspect, since the low temperature activity of the catalyst can be improved, the exhaust gas can be kept clean even when the purification device cannot be disposed in the vicinity of the exhaust manifold.

  The invention according to claim 5 is an exhaust gas purifying apparatus comprising a coat layer having carrier particles disposed on an exhaust gas passage of an internal combustion engine and carrying a noble metal on a base material layer, the exhaust gas flowing direction The noble metal is supported on the surface of the coat layer only on the surface of the coat layer in the peripheral portion with respect to the surface orthogonal to the surface, and the noble metal is uniformly supported on the entire coat layer on the portion other than the periphery. The exhaust gas purifying apparatus characterized by the above is intended to solve the above-mentioned problem.

  In the invention according to claim 5, the thickness of the peripheral portion where the noble metal is supported only on the surface layer portion of the coat layer on the carrier particles is preferably less than 10 mm.

  Further, in the invention according to claim 5, the noble metal is supported on the surface layer after forming the base layer upper coat layer with the carrier particles in the peripheral portion, and the noble metal is previously supported in the portion other than the peripheral portion. It is preferable to coat the carrier particles on the base material layer to form a coating layer.

  The invention according to claim 6 is an exhaust gas purifying apparatus which is disposed in an exhaust gas passage of an internal combustion engine and has a coating layer formed by coating carrier particles on which a noble metal is previously supported on a base material layer. The exhaust gas purifying apparatus is characterized in that the average particle diameter of the carrier particles in the peripheral part is smaller than the average particle diameter of the carrier particles in the central part with respect to the plane orthogonal to the gas flow direction. It is.

  In the invention described in claim 6, it is preferable that the thickness of the peripheral portion where the particle size of the carrier particles is small is less than 10 mm.

  According to the first aspect of the present invention, the noble metal loading density (surface density) on the surface of the coat layer can be easily increased on the upstream side than on the downstream side, and thus the catalytic activity from a low temperature is improved. Further, since the volume of the downstream catalyst is larger than the volume of the upstream catalyst, a sufficient oxygen storage capacity (hereinafter referred to as “OSC”) is secured in the downstream catalyst, and the air-fuel ratio (hereinafter referred to as “A / F ratio”). "), Even if it fluctuates, a good purification rate can be maintained.

  According to the invention described in claim 2, since the carrying density of the noble metal is the same on both the upstream side and the downstream side, the amount of noble metal used can be suppressed.

  According to the third aspect of the present invention, a uniform noble metal carrying density can be obtained up to the inside in the depth direction of the coat layer.

  According to the invention described in claim 4, the surface density of the upstream noble metal can be further increased. For this reason, the surface activity is further improved. Further, if the surface density of the noble metal is increased, generally sintering due to heat tends to occur. However, since the carrying density per volume on the upstream side is reduced, sintering is prevented. Thus, it is possible to improve the low temperature activity while suppressing the amount of noble metal used.

  According to the fifth aspect of the present invention, the surface support density in the periphery of the catalyst is higher than that in the center, so that the catalytic activity from a low temperature is improved and the catalyst purification rate is increased. In addition, since the surface support density can be increased only in the peripheral portion where the temperature is lowered, the sintering of the noble metal due to heat can be suppressed to prevent deterioration of the catalyst.

Furthermore, in the invention according to claim 5,
(1) The noble metal support density in the catalyst peripheral part and the central part is set to the same level.
(2) The noble metal support density in the catalyst peripheral portion is made smaller than that in the central portion, and the noble metal support density in the coat layer surface layer portion is made larger in the peripheral portion than in the central portion.
(3) The noble metal supporting density in the periphery of the catalyst is made larger than that in the center.
It is possible to take any of the three aspects. In any aspect, it is possible to suppress the amount of noble metal used while maintaining a predetermined purification capacity.

  According to the sixth aspect of the present invention, it is easy to make the surface support density at the periphery of the catalyst higher than that at the center, so that the catalytic activity from low temperature is improved and the catalyst purification rate is increased. In addition, since the precious metal is uniformly dispersed in the coat layer from the central portion to the peripheral portion, there is an advantage that OSC can be increased and it is resistant to catalyst poisoning such as phosphorus.

  Hereinafter, the best embodiment of the exhaust gas purifying apparatus of the present invention will be described more specifically with reference to the drawings.

  FIG. 1 is a system diagram showing an outline when the exhaust gas purifying apparatus of the present invention is used in a general gasoline engine. Usually, the system 100 is mounted on a vehicle. In FIG. 1, an air-fuel mixture supply unit 20 is shown on the left side of the engine 10, and an exhaust system is shown on the right side. In the air-fuel mixture supply unit 20, fuel and air mixed at a predetermined A / F ratio are supplied to the engine 10 and burned. Exhaust gas generated by combustion in each cylinder of the engine 10 is collected in the exhaust pipe 15 via the exhaust manifold 11 and guided to the exhaust gas purification device 30. Exhaust gas from which harmful substances have been purified by the exhaust gas purification device 30 is discharged from the exhaust pipe 16 into the atmosphere.

  The air-fuel mixture supply unit 20 is provided with a throttle valve 21, an air flow meter 22 is provided upstream thereof, and a throttle sensor 23 is provided in the vicinity of the throttle valve 21. Output signals from the meter 22 and the sensor 23 are sent to a motor electronic control unit (hereinafter referred to as “ECU”) 40.

An A / F sensor 24 and an O ₂ sensor 25 are respectively provided before and after the exhaust gas purification device 30 to determine the air-fuel ratio of the exhaust gas introduced into the purification device 30 and the oxygen concentration of the exhaust gas purified by the purification device 30. Detected.
Detection signals from the A / F sensor 24 and the O ₂ sensor 25 are also sent to the ECU 40.

The ECU 40 receives detection signals from the air flow meter 22, the throttle sensor 23, the A / F sensor 24, and the O ₂ sensor 25, and adds signals from other sensors as necessary to calculate optimum control conditions. The engine 10 is controlled.

  FIG. 2 is a diagram schematically showing the configuration of the exhaust gas purification device 31 according to the first embodiment and the exhaust gas purification device 32 according to the second embodiment. In FIG. 2, the exhaust gas purifying apparatus and the like of both embodiments are represented by reference numeral 30 for convenience. 2, the left side of the drawing is the exhaust pipe 15, that is, the upstream side in the exhaust gas flow direction, and the right side of the drawing is the exhaust pipe 16, that is, the downstream side in the exhaust gas flow direction. The illustrated exhaust gas purification device 30 includes a number of cells c1, c2, c3,... Penetrating from the upstream side to the downstream side. On the other hand, the exhaust gas purification device 30 has an upstream catalyst 30A on the upstream side and a downstream catalyst 30B on the downstream side, both of which have different catalyst configurations and are integrally formed. Now, let Qf be the average particle size of the carrier in each cell of the upstream catalyst 30A. On the other hand, the average particle diameter of the carrier in each cell of the downstream side catalyst 30B is defined as Qr.

In the exhaust gas purification device 31 of the first embodiment and the exhaust gas purification device 32 of the second embodiment, between these Qf and Qr,
Qf <Qr
This relationship is established.

The purification rate S of the catalyst is the product of the reaction rate R and the gas concentration C R × C
Is roughly determined by Here, when the number of active points is N, the activation energy is E, and the catalyst temperature is T,
S = R × C = C · N · exp (−E / T)
It is. That is, at the same temperature,
(1) Increase the number N of active points.
(2) Increase the gas concentration near the active point.
(3) The heat generated by the reaction is concentrated near the active point.
If measures such as these are taken, the temperature of the catalyst can be raised quickly, so that a synergistic effect of further increasing the purification rate can be expected.

  In the exhaust gas purification device 31 of the first embodiment and the exhaust gas purification device 32 of the second embodiment, which will be described below, the average particle diameter of the upstream carrier is made small, so the above (1) to (1) to It is possible to satisfy the condition (3) at the same time. By reducing the particle size, the gas diffusion distance between the active point and the gas passage can be shortened to increase the gas concentration. And the active point density of a part with high gas concentration can be made high. Therefore, a large amount of catalytic reaction can be obtained even at a low temperature. In addition, since the volume of the region where the catalytic reaction with active sites occurs can be reduced, the temperature rise near the active sites can be accelerated. Thus, the catalyst purification rate during warm-up can be increased.

Furthermore, in the exhaust gas purification device 31 of the first embodiment and the exhaust gas purification device 32 of the second embodiment, if the volume of the upstream catalyst 30A is Vf and the volume of the downstream catalyst 30B is Vr,
Vf <Vr
This relationship is established. That is, the volume Vr of the downstream catalyst 30B is larger than the volume Vf of the upstream catalyst 30A.

  When the average particle size of the carrier is reduced as described above, ceria having OSC and the like cannot be sufficiently supported on the coating material. For this reason, even after the catalyst is warmed up, if the variation in the A / F ratio of the exhaust gas is large, sufficient purification may not be performed. In order to improve this, in the exhaust gas purification device 31 of the first embodiment and the exhaust gas purification device 32 of the second embodiment, the volume on the upstream side of the catalyst is made half or less. By doing in this way, sufficient OSC can be ensured in the downstream catalyst, and a good purification rate can be maintained even if the A / F ratio varies.

  3A shows an exhaust gas purifying apparatus 31 according to the first embodiment, FIG. 3A shows a coat layer 52 formed on the base material layer 51 of the upstream catalyst 31A, and FIG. 3B shows a base of the downstream catalyst 31B. It is a figure which shows typically the cross section of thickness direction of the coating layer 62 formed by coating on the material layer 61. FIG. In each figure, the exhaust gas passage extends from the left side of the drawing to the right side above the coat layers 52 and 62.

  In the upstream catalyst 31 </ b> A shown in FIG. 3A, a carrier A <b> 1 having a small average particle diameter and a coat layer 52 made of a noble metal are formed on a base material layer 51. The carrier A1 is preliminarily impregnated with a noble metal, and the carrier A1 carrying the noble metal is coated on the base layer 51 to form a coat layer 52. Therefore, the noble metal is uniformly dispersed in the depth direction from the surface layer portion 53 of the coat layer 52 to a portion close to the inner base material layer 51.

  In the downstream catalyst 31 </ b> B shown in FIG. 3B, a carrier B <b> 1 having a large average particle diameter and a coat layer 62 made of a noble metal are formed on the base material layer 61. Also in the downstream side catalyst 31B, the support B1 is impregnated and supported in advance with the noble metal, and the support layer B1 supporting the noble metal is coated on the base layer 61 to form the coat layer 62. Therefore, also in the downstream catalyst 31 </ b> B, the noble metal is uniformly dispersed in the depth direction from the surface layer portion 63 of the coat layer 62 to a portion close to the inner base material layer 61.

  FIG. 4 is a diagram schematically showing a catalyst manufacturing process of the exhaust gas purification device 31 according to the first embodiment. A carrier powder A1 having a small average particle size is impregnated with a noble metal and coated on the upstream substrate layer 51. On the other hand, the noble metal is impregnated and supported on the carrier powder B1 having a large average particle diameter, and the downstream base layer 61 is coated. By baking such a base material, the catalyst of the exhaust gas purification device 31 according to the first embodiment can be obtained.

  In the exhaust gas purifying apparatus 31 according to the first embodiment configured as described above, the noble metal is uniformly dispersed in the carrier having a small average particle diameter in the upstream catalyst 31A having a small volume, and the downstream catalyst 31B having a large volume. A noble metal is uniformly dispersed in a carrier having a large average particle diameter. Therefore, since the noble metal carrying density (surface density) on the surface of the coat layer can be made higher on the upstream side than on the downstream side, the catalytic activity from a low temperature is improved. Further, since the volume of the downstream catalyst is larger than the volume of the upstream catalyst, sufficient OSC can be secured in the downstream catalyst, and a good purification rate can be maintained even if the A / F ratio varies. Moreover, the amount of noble metal used can be suppressed by making the loading density of the noble metal the same on the upstream side and the downstream side. In addition, since the base material layer is coated with the carrier in which the noble metal is impregnated in advance, a uniform noble metal loading density can be obtained up to the inside of the coating layer.

  5A shows the exhaust gas purifying device 32 according to the second embodiment. FIG. 5A shows the coat layer 72 formed on the base material layer 71 of the upstream catalyst 32A, and FIG. 5B shows the base material of the downstream catalyst 32B. FIG. 3 is a diagram schematically showing a cross section in the thickness direction of a coat layer 82 formed on a layer 81. In each figure, the exhaust gas passage extends from the left side of the drawing to the right side above the coat layers 72 and 82.

  In the upstream catalyst 32 </ b> A shown in FIG. 5A, a carrier A <b> 2 having a small average particle diameter and a coat layer 72 made of a noble metal are formed on a base material layer 71. The carrier A2 is coated on the base material layer 71, and after such a coat layer is formed, the noble metal is impregnated and supported to form the coat layer 72. Accordingly, the surface layer portion 73 of the coat layer 72 has a higher dispersion density of the noble metal as compared with a portion close to the inner base material layer 71.

  In the downstream catalyst 32 </ b> B shown in FIG. 5B, a carrier B <b> 2 having a large average particle diameter and a coat layer 82 made of a noble metal are formed on the base material layer 81. In the downstream side catalyst 32B, the carrier B2 is impregnated with a noble metal in advance, and the carrier B2 carrying the noble metal is coated on the base material layer 81 to form a coat layer 82. Therefore, in the downstream catalyst 32B, the noble metal is uniformly dispersed in the depth direction from the surface layer portion 83 of the coat layer 82 to the portion close to the internal base material layer 81. In the exhaust gas purification device 32 according to the present embodiment, the amount of noble metal per volume of the upstream catalyst 32A is set smaller than the amount of noble metal per volume of the downstream catalyst 32B.

  FIG. 6 is a diagram schematically illustrating a catalyst manufacturing process of the exhaust gas purification device 32 according to the second embodiment. The carrier powder A2 having a small average particle diameter is coated on the upstream base material layer 71 without impregnating and supporting the noble metal. On the other hand, the noble metal is impregnated and supported on the carrier powder B2 having a large average particle size, and the downstream base material layer 81 is coated. After masking the site on the downstream side of the base material where the noble metal is already supported, the noble metal is impregnated and supported. As a result, the noble metal is supported only on the surface layer portion 73 of the upstream coat layer 72. By firing the base material thus treated, the catalyst of the exhaust gas purification device 32 according to the second embodiment can be obtained.

  In the exhaust gas purifying device 32 according to the second embodiment configured as described above, the upstream catalyst 31A having a small volume is coated with a carrier having a small average particle diameter, and the surface layer side of the coat layer has a high density of noble metal. The noble metal is uniformly dispersed in the carrier having a large average particle diameter in the downstream catalyst 31B having a large volume. Therefore, since the noble metal carrying density (surface density) on the surface of the coat layer is higher on the upstream side than on the downstream side, the catalytic activity from a low temperature is improved. Further, since the volume of the downstream catalyst is larger than the volume of the upstream catalyst, sufficient OSC can be secured in the downstream catalyst, and a good purification rate can be maintained even if the A / F ratio varies. Furthermore, since the surface density of the noble metal on the upstream side is increased, the surface activity is further improved. Further, if the surface density of the noble metal is increased, generally sintering due to heat tends to occur. However, sintering is prevented by reducing the carrying density per volume on the upstream side. Thus, it is possible to improve the low temperature activity while suppressing the amount of noble metal used.

  In the present embodiment, the noble metal support density on the surface layer can be maximized under the condition that the support density of the noble metal is constant. Therefore, the low temperature activity becomes high, and a good purification rate can be maintained even if an exhaust gas purification device is mounted under the floor. On the other hand, when the exhaust gas purifying device is disposed under the floor, the cooled gas enters even during a high load operation, so that the chance of the catalyst being exposed to high temperature is reduced. Therefore, it is possible to maintain a good purification rate over the life of the vehicle.

  FIG. 7 is a diagram schematically showing the configuration of the exhaust gas purification device 33 according to the third embodiment and the exhaust gas purification device 34 according to the fourth embodiment. In FIG. 7, the exhaust gas purifying apparatus and the like of both embodiments are represented by the reference numeral 300 for convenience. 7, the left side of the drawing is the exhaust pipe 15, that is, the upstream side in the exhaust gas flow direction, and the right side of the drawing is the exhaust pipe 16, that is, the downstream side in the exhaust gas flow direction. The illustrated exhaust gas purification apparatus 300 includes a large number of cells c1, c2, c3,... Penetrating from the upstream side to the downstream side. On the other hand, the exhaust gas purifying apparatus 300 has a central part as a central part catalyst 300A and a peripheral part as a peripheral part catalyst 300B with respect to a plane direction orthogonal to the flow direction of the exhaust gas. It is integrally formed.

  8A shows the exhaust gas purifying device 33 according to the third embodiment. FIG. 8A shows the coating layer 152 formed on the base material layer 151 of the central catalyst 33A, and FIG. 8B shows the base material of the peripheral catalyst 33B. FIG. 6 is a diagram schematically showing a cross section in the thickness direction of a coat layer 162 formed on the layer 161. In each figure, the exhaust gas passage is directed upward from the left side of the drawing above the coat layers 152 and 162.

  In the central catalyst 33A shown in FIG. 8A, a carrier layer A3 having a predetermined average particle diameter and a coat layer 152 made of a noble metal are formed on a base material layer 151. A carrier A3 is impregnated with a noble metal in advance, and the carrier A3 carrying the noble metal is coated on the base material layer 151 to form a coat layer 152. Therefore, the noble metal is uniformly dispersed in the depth direction from the surface layer portion 153 of the coat layer 152 to a portion close to the inner base material layer 151.

  In the peripheral portion catalyst 33B shown in FIG. 8B, a support layer B3 having an average particle size comparable to that of the central portion catalyst 33A and a coat layer 162 made of a noble metal are formed on the base material layer 161. In the peripheral catalyst 33B, the carrier B3 is coated on the base material layer 161, and after such a coat layer is formed, the noble metal is impregnated and supported to form the coat layer 162. Therefore, the surface layer portion 163 of the coat layer 162 has a higher dispersion density of the noble metal as compared to a portion close to the internal base material layer 161.

  FIG. 9 is a diagram schematically showing a catalyst manufacturing process of the exhaust gas purification device 33 according to the third embodiment. The carrier powder A3 is impregnated with a noble metal and coated on the base material layer 151 at the center. On the other hand, the carrier powder B3 is coated on the base material layer 161 in the peripheral portion without impregnating and supporting the noble metal. After masking a portion where the noble metal has already been supported at the center of the substrate, the noble metal is impregnated and supported. As a result, the noble metal is supported only on the surface layer portion 163 of the peripheral coat layer 162. By firing the base material thus treated, the catalyst of the exhaust gas purification device 33 according to the third embodiment can be obtained.

  In the exhaust gas purifying apparatus 33 according to the third embodiment configured as described above, the noble metal is uniformly distributed on the carrier in the central part of the surface orthogonal to the gas flow, and the carrier coating layer in the peripheral part. The noble metal density of the surface layer portion is high. Therefore, since the surface carrying density of the catalyst peripheral portion is higher than that of the central portion, the catalytic activity from a low temperature is improved and the catalyst purification rate is increased. In addition, since the surface support density can be increased only in the peripheral portion where the temperature is lowered, the sintering of the noble metal due to heat can be suppressed to prevent deterioration of the catalyst.

  In the peripheral catalyst, the amount of the noble metal on the surface is increased by supporting the noble metal on the surface portion of the coat layer as compared with the case where the noble metal is supported three-dimensionally like the central portion. For this reason, the number N of active points of the catalyst increases. Further, since the gas concentration on the surface is inversely proportional to the diffusion distance from the gas passage, the gas concentration C also increases.

As described above, the purification rate S of the catalyst is the product of the reaction rate R and the gas concentration C R × C
Is roughly determined by Here, when the number of active points is N, the activation energy is E, and the catalyst temperature is T,
S = R × C = C · N · exp (−E / T)
It is. That is, at the same temperature, the purification rate of the exhaust gas purification device 33 according to the present embodiment, which has a high number of active points N and a high gas concentration C, is increased. Furthermore, since reaction heat is concentrated on the surface layer when the reaction occurs, the temperature of the catalyst rises quickly, and the purification rate can be further improved.

  10A shows an exhaust gas purifying apparatus 34 according to the fourth embodiment. FIG. 10A shows a coat layer 172 formed on the base material layer 171 of the central catalyst 34A, and FIG. 10B shows a base of the peripheral catalyst 34B. It is a figure which shows typically the cross section of the thickness direction of the coating layer 182 coat-formed on the material layer 181. FIG. In each figure, the exhaust gas passage is directed from the left side of the drawing to the right above the coat layers 172 and 182.

  In the center catalyst 34A shown in FIG. 10 (A), a carrier A4 having a large average particle diameter and a coat layer 172 made of a noble metal are formed on a base material layer 171. The carrier A4 is preliminarily impregnated with a noble metal, and the carrier A4 carrying the noble metal is coated on the base material layer 171 to form a coat layer 172. Therefore, the noble metal is uniformly dispersed in the depth direction from the surface layer portion 173 of the coat layer 172 to a portion close to the internal base material layer 171.

  In the peripheral catalyst 34B shown in FIG. 10B, a carrier B4 having a small average particle diameter and a coat layer 182 made of a noble metal are formed on a base material layer 181. In the peripheral portion catalyst 34B, the carrier B4 is impregnated with a noble metal in advance, and the carrier B4 carrying the noble metal is coated on the base material layer 181 to form a coat layer 182. Therefore, the noble metal is uniformly dispersed in the depth direction from the surface layer portion 183 of the coat layer 182 to a portion close to the internal base material layer 181.

  FIG. 11 is a view schematically showing a catalyst manufacturing process of the exhaust gas purification device 34 according to the fourth embodiment. The center part base material layer 171 is coated by impregnating and supporting a noble metal on the carrier powder A4 having a large average particle diameter. On the other hand, the carrier powder B4 having a small average particle size is impregnated and supported with a noble metal, and the peripheral base material layer 181 is coated. By firing such a base material, the catalyst of the exhaust gas purification device 34 according to the fourth embodiment can be obtained.

  In the exhaust gas purifying apparatus 34 according to the fourth embodiment configured as described above, the noble metal is uniformly distributed on the carrier having a large average particle diameter in the center portion of the surface orthogonal to the gas flow, and the peripheral portion. In No. 4, noble metals are uniformly distributed on a carrier having a small average particle diameter. Therefore, since the noble metal is uniformly dispersed in the coat layer from the central part to the peripheral part, there is an advantage that the OSC can be increased and it is resistant to catalyst poisoning such as phosphorus. Further, since the cell opening area in the peripheral portion can be increased, the pressure loss can be reduced.

  In the exhaust gas purification device 33 according to the third embodiment, the thickness of the peripheral portion where the noble metal is supported only on the surface portion of the coat layer, and in the exhaust gas purification device 34 according to the fourth embodiment, the particle size of the carrier particles It is preferable that the thickness of the peripheral part with a small is less than 10 mm. It is known that sintering of noble metal increases when the surface noble metal density (surface density) is increased. As shown in FIG. 1, the temperature around the exhaust gas purifying device 30 is lowered. This is because there is heat dissipation and the temperature of the exhaust gas flowing from the engine 10 side is low in the peripheral portion. For this reason, sintering is suppressed and deterioration of the catalyst is suppressed. Since the thickness of the low temperature portion is about 10 mm, it is preferable that the thickness of the peripheral portion where the particle size of the carrier particles is small is less than 10 mm.

  In the exhaust gas purifying apparatus of the present invention, the kind of noble metal is not particularly limited, but any one of platinum (Pt), rhodium (Rh), and palladium (Pd), or a combination thereof is used. be able to. Among these, Pt and / or Rh can be preferably used.

In the exhaust gas purifying apparatus of the present invention, the type of the carrier is not particularly limited, but a porous material having a large surface area such as ceria / alumina or zirconia can be preferably used.
In the present invention, the carrier particle diameters of the downstream catalysts 31B and 32B and the central catalyst 34A are preferably at least twice the carrier particle diameters of the upstream catalyst 31A and 32A and the peripheral catalyst 34B, respectively. By taking such a particle size ratio, the purification ability of the exhaust gas purification devices 31, 32, 34 of the present invention can be further enhanced.

It is a system diagram showing an outline when the exhaust gas purification device of the present invention is used in a general gasoline engine. It is a figure which shows roughly the structure of the exhaust-gas purification apparatus concerning 1st embodiment and 2nd embodiment. FIG. 1 schematically shows a cross section in the thickness direction of a coating layer formed on a base material layer of an upstream catalyst, and (B) of a downstream catalyst of the exhaust gas purification apparatus according to the first embodiment. It is. It is a figure which shows typically the manufacturing process of the exhaust-gas purification apparatus catalyst concerning 1st embodiment. (A) of the exhaust gas purifying apparatus according to the second embodiment is a diagram schematically showing a cross section in the thickness direction of the coat layer formed on the base material layer of the upstream catalyst, and (B) of the downstream catalyst. It is. It is a figure which shows typically the manufacturing process of the exhaust-gas purification apparatus catalyst concerning 2nd embodiment. It is a figure which shows roughly the structure of the exhaust-gas purification apparatus concerning 3rd embodiment and 4th embodiment. (A) of the exhaust gas purifying apparatus according to the third embodiment is a diagram schematically showing a cross section in the thickness direction of a coat layer formed on the base material layer of the central catalyst, and (B) of the peripheral catalyst. It is. It is a figure which shows typically the manufacturing process of the exhaust-gas purification apparatus catalyst concerning 3rd embodiment. (A) of exhaust gas purifying apparatus according to the fourth embodiment is a diagram schematically showing a cross section in the thickness direction of a coating layer formed on a base material layer of a central catalyst, and (B) of a peripheral catalyst. It is. It is a figure which shows typically the manufacturing process of the exhaust-gas purification apparatus catalyst concerning 4th embodiment.

Explanation of symbols

c1, c2, c3,... Cell 10 Engine 11 Exhaust Manifold 15 Exhaust Pipe 16 Exhaust Pipe 20 Mixture Supply Unit 21 Throttle Valve 22 Air Flow Meter 23 Throttle Sensor 24 A / F Sensor 25 O2 Sensor 30, 31, 32, 33, 34 , 300 Exhaust gas purification devices 30A, 31A, 32A Upstream catalyst 30B, 31B, 32B Downstream catalyst 300A, 33A, 34A Center catalyst 300B, 33B, 34B Peripheral catalyst 40 ECU
51, 61, 71, 81, 151, 161, 171, 181 Base material layer 52, 62, 72, 82, 152, 162, 172, 182 Coat layer 53, 63, 73, 83, 153, 163, 173, 183 Surface layer

Claims (6)

A coating which is disposed in an exhaust gas passage of an internal combustion engine, and includes an upstream catalyst and a downstream catalyst along the flow direction of the exhaust gas, and each catalyst has carrier particles carrying a noble metal on a base material layer. An exhaust gas purification device comprising a layer,
The volume of the upstream catalyst is smaller than the volume of the downstream catalyst,
The exhaust gas purifying apparatus according to claim 1, wherein an average particle diameter of the carrier particles of the upstream catalyst is smaller than an average particle diameter of the carrier particles of the downstream catalyst. The exhaust gas purifying apparatus according to claim 1, wherein the amount of the noble metal per volume of the upstream catalyst is the same as the amount of the noble metal per volume of the downstream catalyst. The exhaust gas purifying apparatus according to claim 1 or 2, wherein the noble metal is previously supported on carrier particles, and the carrier particles are coated on the base material layer to form the coat layer. The upstream catalyst is formed by coating the carrier particles on the base material layer to form the coat layer and then supporting the noble metal on the coat layer, and the downstream catalyst includes the noble metal. Is previously supported on carrier particles, and the carrier particles are coated on the base material layer to form the coat layer,
The exhaust gas purification device according to claim 1, wherein the amount of the noble metal per volume of the upstream catalyst is smaller than the amount of the noble metal per volume of the downstream catalyst. An exhaust gas purification apparatus comprising a coating layer disposed in an exhaust gas passage of an internal combustion engine and having carrier particles carrying a noble metal on a base material layer,
With respect to the plane perpendicular to the flow direction of the exhaust gas, the noble metal is supported only on the surface layer of the coat layer in the periphery in the periphery, and the noble metal is in the support particle in a portion other than the periphery. An exhaust gas purifying apparatus, wherein the exhaust gas purifying apparatus is uniformly supported on the entire coating layer. An exhaust gas purifying device that is disposed in an exhaust gas passage of an internal combustion engine and has a coating layer formed by coating carrier particles on which a noble metal is previously supported on a base material layer,
The exhaust gas purifying apparatus according to claim 1, wherein the average particle diameter of the carrier particles in the peripheral part is smaller than the average particle diameter of the carrier particles in the central part with respect to a plane orthogonal to the flow direction of the exhaust gas.

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