JP2010106799A - Exhaust emission control system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control system improving conversion efficiency of NOx by disposing a second catalyst producing reducing gas including H<SB>2</SB>by production of aromatic hydrocarbon and a dehydrogenation reaction in an upstream side of an NOx trap catalyst disposed in an exhaust passage, and effectively using the reducing gas. <P>SOLUTION: In the exhaust emission control system 100 disposing a first catalyst 34 eliminating NOx in exhaust emission in the exhaust passage 3 of an internal combustion engine (a system body 1), the second catalyst 33 producing aromatic hydrocarbon from hydrocarbon in the exhaust is equipped in an upstream side of the first catalyst 34, and a gas supplying means (e.g., a fuel injection valve 11 or the like) for supplying gas containing paraffin HC of C6 or more and/or olefin HC of C6 or more is equipped in the second catalyst 33. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust purification system capable of effectively purifying exhaust from an internal combustion engine.

In recent years, due to consideration for the global environment, reduction of carbon dioxide (CO ₂ ) emissions has been screamed, and lean burn (lean burn) has been achieved for the purpose of fuel efficiency of an internal combustion engine of an automobile.
The exhaust from gasoline lean burn engines, direct injection engines, and diesel engines is rich in oxygen, and conventional three-way catalysts cannot reduce and purify nitrogen oxides (NOx). . In particular, various technical developments for purifying exhaust gas from a diesel engine efficiently are underway.

  One effective method is the use of a NOx trap catalyst. In an engine equipped with a NOx trap catalyst, normally, lean combustion operation is performed, NOx generated during that time is accumulated in the NOx trap agent, and when a certain amount of NOx is deposited, the excess air ratio of the exhaust (hereinafter referred to as “λ”) In this case, NOx is desorbed from the absorbent to be reduced. Such control is referred to as rich spike control because λ is temporarily manipulated in the rich direction. A diesel engine is usually operated in a lean combustion state where λ is 1.2 to 3, and λ is changed to about 0.8 in the above-described rich spike control.

In the conventional method, when the rich spike control is performed, the reducing agent (hydrogen (H ₂ ), carbon monoxide (CO), hydrocarbon (HC)) in the exhaust gas is increased to desorb from the NOx trap catalyst. NOx is reduced, but excess reducing agent, especially excess HC, is released without being used for NOx reduction, and this may react with oxygen and increase CO ₂ emissions. .

Therefore, attempts have been made to use a more effective reducing agent, particularly hydrogen (H ₂ ), for NOx reduction, and a catalyst that generates H ₂ by steam reforming has been proposed (for example, Patent Document 1). reference).
Japanese Patent No. 3741303

However, the steam reforming reaction is an endothermic reaction, and it is necessary to supply an amount of heat in order to obtain a sufficient reaction rate, that is, it is necessary to set a high temperature condition. Therefore, for example, when the operation load is low, it is difficult to say that sufficient H ₂ is supplied so as to obtain a NOx purification effect under the conditions of the actual operation mode.
In addition, if the air-fuel ratio is made rich in order to increase the NOx conversion rate (reduction rate), the amount of unreacted HC emissions increases and causes exhaust deterioration, so a catalyst for removing the HC is added. There was a need.

As described above, in the conventional method, in order to improve the NOx purification efficiency, it is difficult to say that a reducing gas containing sufficient H ₂ , CO, or the like is supplied to the NOx catalyst. It cannot be said that it is effectively used to improve the purification efficiency of NOx.

The present invention has been made in view of the above-described problems of the prior art. The object of the present invention is to dehydrate together with the generation of aromatic hydrocarbons upstream of the NOx trap catalyst disposed in the exhaust passage. It is an object of the present invention to provide an exhaust purification system that arranges a second catalyst that generates a reducing gas containing H ₂ by an elementary reaction and effectively uses the reducing gas to improve the NOx purification efficiency.

  As a result of intensive studies to achieve the above object, the present inventors have arranged a second catalyst that generates aromatic hydrocarbons from hydrocarbons in the exhaust, upstream of the NOx trap catalyst, It has been found that the above object can be achieved by supplying a gas containing paraffin hydrocarbons having 6 or more and / or olefin hydrocarbons having 6 or more carbon atoms to the second catalyst.

  That is, the present invention provides an exhaust purification system in which a first catalyst for purifying NOx in exhaust gas is disposed in an exhaust passage of an internal combustion engine, and an aromatic from hydrocarbons in exhaust gas is disposed upstream of the first catalyst. A gas supply means comprising a second catalyst for generating hydrocarbons, and supplying gas containing paraffin hydrocarbons having 6 or more carbon atoms and / or olefin hydrocarbons having 6 or more carbon atoms to the second catalyst. An exhaust purification system provided.

In the present invention, a second catalyst that generates aromatic hydrocarbons from hydrocarbons in exhaust gas is disposed upstream of the first catalyst that deposits and desorbs NOx in exhaust gas, and the carbon number (C) is By providing a gas supply means for supplying a gas containing 6 or more olefin hydrocarbons and / or paraffin hydrocarbons having 6 or more carbon atoms, H ₂ can be produced by dehydrogenation along with the production of aromatic hydrocarbons. Can be generated. Therefore, according to the present invention, a reducing gas containing a large amount of H ₂ is generated by the second catalyst, and this reducing gas is supplied to the first catalyst arranged on the downstream side of the second catalyst. The NOx purification efficiency can be improved by effectively using the gas.

  Hereinafter, the second catalyst and the first catalyst of the exhaust purification system of the present invention will be described, and then the exhaust purification system of the present invention will be described with reference to the drawings. In the present specification, “%” for concentration, content, blending amount, etc. represents mass percentage unless otherwise specified.

FIG. 1 shows an example of an embodiment of an exhaust purification system of the present invention, and is an explanatory view showing a part of a simplified configuration of the exhaust purification system.
As shown in FIG. 1, the exhaust purification system of the present example includes a first catalyst 34 that deposits and desorbs NOx in exhaust gas on the downstream side of the exhaust passage 3 of the internal combustion engine (engine body) 1, A second catalyst 33 that generates aromatic HC from hydrocarbons in the exhaust (hereinafter abbreviated as “HC”) is provided upstream of the first catalyst 34.

  Although not shown in FIG. 1, the exhaust purification system includes means for performing rich spike control for desorbing NOx deposited on the first catalyst by adjusting the excess air ratio of the exhaust, There is provided a gas supply means for supplying a gas containing paraffin HC having 6 or more carbon (hereinafter abbreviated as “C”) and olefin HC having 6 or more carbon to the second catalyst.

First, the second catalyst 33 disposed on the upstream side of the exhaust passage 3 will be described.
FIG. 2 is a perspective view and a partially enlarged view showing an example of the second catalyst 33 used in the exhaust purification system.
In the second catalyst 33, a catalyst layer 33c containing an HC conversion catalyst and a catalyst layer 33b containing both an HC conversion catalyst and an OSC material are supported on a honeycomb monolithic carrier 33a made of, for example, cordierite. Is.

The second catalyst 33 is an HC conversion catalyst that converts C6 or higher paraffin HC and C6 or higher olefin HC in the exhaust gas into aromatic HC, and oxygen storage capacity (hereinafter abbreviated as “OSC”). ) Having a high OSC material.
The HC conversion catalyst preferably contains at least one noble metal element selected from the group consisting of platinum (Pt), rhodium (Rh) and palladium (Pd).
Examples of the OSC material include transition metal elements, specifically oxides including at least one transition metal element selected from the group consisting of cerium (Ce) and praseodymium (Pr).

The amount of the HC conversion catalyst supported on the second catalyst 33, specifically, the amount of noble metal element such as Pt, Rh or Pd supported is preferably 12.0 g / L or less, and the amount of noble metal element supported is 2. It is preferable that it is 8 g / L or more.
When the supported amount of the noble metal element to be supported on the second catalyst 33 is 2.8 to 12.0 g / L, C6 or more paraffin HC and / or C6 or more olefin HC contained in the exhaust gas is obtained by dehydrogenation reaction. Can be efficiently converted into aromatic HC, and a reducing gas containing a large amount of H ₂ can be generated by a dehydrogenation reaction.
The HC conversion catalyst utilizes a small amount of O ₂ in the exhaust gas having a low oxygen concentration of 0.8 to 1.5 vol% to convert unburned HC in the exhaust gas into aromatic HC, and also by dehydrogenation to produce H It is preferable that ₂ can be produced.
Further, the HC conversion catalyst is preferably activated at 200 ° C. or higher.

In the second catalyst 33, the HC conversion catalyst may have a content of the noble metal element constituting the HC conversion catalyst intermittently or continuously increased as the surface of the HC conversion catalyst is closer to the surface in contact with the exhaust gas. preferable.
In the second catalyst 33, as a method of increasing the content of the noble metal element constituting the HC conversion catalyst closer to the surface, for example, a plurality of layers of slurry in which the content of the noble metal element increases as the surface is closer to the surface, There is a method of forming a catalyst layer in which the content of the noble metal element increases as it is closer to the surface.

Moreover, it is preferable that the 2nd catalyst 33 contains both the noble metal element which comprises HC conversion catalyst, and the transition metal element which comprises OSC material in the same catalyst layer.
In the second catalyst 33, when both the noble metal element constituting the HC conversion catalyst and the transition metal element constituting the OSC material are included in the same catalyst layer, for example, when performing rich spike control, C6 or more Unburned HC such as paraffin HC takes oxygen stored in the OSC material and is easily converted to aromatic HC by the action of the HC conversion catalyst, and at the same time, H ₂ is also easily generated. That is, it is preferable that both the HC conversion catalyst and the OSC material are contained in the same catalyst layer because a dehydrogenation reaction that generates aromatic HC and H ₂ is likely to occur.

As the second catalyst 33, not only the catalyst layer including both the HC conversion catalyst and the OSC material but also the catalyst layer including only the HC conversion catalyst and the catalyst layer including only the OSC material are appropriately selected on the honeycomb carrier. Can be formed.
As the substrate for supporting the catalyst layer, a high specific surface area substrate such as alumina (Al ₂ O ₃ ), a cordierite honeycomb substrate, or the like can be used.

  The catalyst layer can be formed by a slurry containing a noble metal element such as Pt, Rh or Pd, a slurry containing an oxide containing a transition metal element of Ce or Pr as an OSC material, the noble metal element and the transition metal element. A method of forming a catalyst layer by adhering and drying a slurry containing both of these on a substrate.

  In addition, as the second catalyst 33, the HC conversion catalyst and / or the OSC material is granulated or pelletized, and the HC conversion catalyst and the OSC material are separately provided, or the HC conversion catalyst and the OSC material are combined. You may use what was filled in.

  The amount of aromatic HC in the exhaust gas supplied from the second catalyst 33 to the first catalyst 34 is based on the total amount of non-methane hydrocarbon (hereinafter abbreviated as “NMHC”) in the exhaust gas. 2.0 mass% or more (Aromatic HC amount / NMHC total amount ≧ 0.02, Aromatic HC amount / NMHC total amount × 100 (%)).

When the amount of aromatic HC in the exhaust gas supplied from the second catalyst 33 to the first catalyst 34 is 2.0% by mass or more based on the total amount of NMHC in the exhaust gas, along with the generation of aromatic HC, Necessary and sufficient amount of H ₂ effective for NOx reduction can be generated. Therefore, the second catalyst 33 can supply the first catalyst 33 with a reducing gas containing a necessary and sufficient amount of H ₂ effective for NOx reduction, and the NOx conversion rate for reducing NOx to N ₂ is improved. Can be made.

Further, when the amount of aromatic HC supplied from the second catalyst 33 to the first catalyst 34 is 2.0 mass% with respect to the total amount of NMHC in the exhaust gas excluding methane having low optical activity, the photochemical smog The amount of NMHC that is likely to cause NO is reduced, and a reducing gas containing a sufficient amount of reducing agent (H ₂ ) necessary for NOx purification can be generated.

Next, the first catalyst 34 disposed on the downstream side of the second catalyst 33 will be described.
FIG. 3 is a perspective view and a partially enlarged view showing an example of the first catalyst 34 used in the exhaust purification system of this example.
As shown in FIG. 3, the first catalyst 34 of this example uses a honeycomb carrier 34a on which a NOx trap catalyst layer 34b including a NOx trap material and a purification catalyst is formed. As the first catalyst 34 in this example, a catalyst in which a zeolite layer 34c is further provided as an HC trap material layer between the NOx trap catalyst layer 34b and the honeycomb carrier 34a may be used.

The NOx trap material used for the first catalyst 34 is not particularly limited as long as NOx can be deposited and desorbed in accordance with fluctuations in the air-fuel ratio (exhaust air excess ratio) of the internal combustion engine.
Examples of the NOx trap material include alkali metal elements, alkaline earth metal elements, and oxides of rare earth elements such as barium (Ba), magnesium (Mg), sodium (Na), cerium (Ce), and samarium (Sm). And oxides containing these elements.

  As a purification catalyst, platinum (Pt), rhodium (Rh), palladium (Pd), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn) or zinc (Zn), and any of these A mixture etc. are mentioned.

  The HC trap material is not particularly limited as long as it can deposit and desorb HC, but it is preferable to use at least one of MFI zeolite and β zeolite having a silica / alumina ratio of 20 to less than 60. it can.

The first catalyst 34 for purifying NOx is not limited to this example, and the HC trap material, the NOx trap material, and the purification catalyst may be separately arranged in the exhaust gas flow path. Further, the HC trap material and the purification catalyst may be combined, or the three functions of the HC trap material, the NOx trap material, and the purification catalyst may be combined into one layer.
In order to fully exert the trap function of the NOx trap material, it is preferable to arrange the NOx trap material and the HC trap material separately. For example, the HC trap material is arranged upstream and the NOx trap material is arranged downstream. It is preferable to do.
When the NOx trap material and the HC trap material are stacked on the honeycomb carrier, it is preferable to arrange the HC trap material on the surface layer side and the NOx trap material on the inner layer side close to the honeycomb carrier. The purification catalyst is preferably arranged on the upstream side or the surface layer side, which is most easily in contact with the exhaust gas.

Next, the exhaust purification system of the present invention will be described.
FIG. 4 is a schematic configuration diagram illustrating an example of an embodiment of the present invention and illustrating an example of an exhaust gas purification system 100 of a turbocharged diesel engine to which the present invention is applicable.
As shown in FIG. 4, the engine body 1 of the exhaust purification system 100 includes a common rail type fuel injection device. The common rail type fuel injection device includes a common rail (accumulation chamber) 10 and fuel provided for each cylinder. An injection valve 11 is provided. Fuel is supplied from a common rail fuel injection system to the common rail fuel injection device.

  The common rail fuel injection system includes a fuel supply passage 12 for supplying fuel from the fuel tank 50, a supply pump 13 provided in the fuel supply passage 12, and a fuel return for return fuel (spill fuel) returning from the engine body 1 to the fuel tank 50. A passage 14 is provided.

The fuel supplied from the fuel tank 50 to the engine body 1 through the fuel supply passage 12 is pressurized by the supply pump 13 and then temporarily stored in the common rail 10, and then the high-pressure fuel in the common rail 10 is injected into each cylinder. The fuel is distributed to the valve 11 and supplied from the fuel injection valve 11 to the engine body 1.
The fuel injection valve 11 directly injects fuel into the combustion chamber of the engine main body 1. The fuel is injected under conditions such that pilot injection, main injection, and fuel in the cylinder of the engine main body 1 do not burn before main injection. The post-injection to be injected is variably controlled. Further, the fuel injection valve 11 is variably controlled by changing the fuel pressure in the common rail 10.

  In order to control the fuel pressure in the common rail 10, a part of the fuel discharged from the supply pump 13 is returned to the fuel supply passage 12 via an overflow passage (not shown) having a one-way valve. ing. More specifically, a pressure control valve (not shown) for changing the flow passage area of the overflow passage (not shown) is provided, and this pressure control valve responds to a duty signal from the engine control unit (ECU) 40 according to a duty signal. , The substantial fuel discharge amount from the supply pump 13 to the common rail 10 is adjusted, and the fuel pressure in the common rail 10 is controlled.

  The intake passage 2 includes an air cleaner 21 at an upstream position, and an air flow meter 22 as intake air amount detection means on the downstream side of the air cleaner 21. A compressor 31 of the supercharger 30 is provided on the downstream side of the air flow meter 22 in the intake passage 2, and an intake collector 23 is provided on the downstream side of the compressor 31. In addition, an intake throttle valve 24 that is opened and closed by a stepping motor actuator, for example, is provided between the compressor 31 and the intake collector 23 in the intake passage 2. The intake throttle valve 24 adjusts the amount of air drawn into the engine body 1 according to the opening.

  The exhaust outlet passage 3 a includes an exhaust turbine 32 of the supercharger 30. The supercharger 30 rotates by exhaust from the engine body 1 and drives the compressor 31 of the supercharger 30 disposed in the intake passage 2.

  The exhaust outlet passage 3 a includes an EGR passage 4 that branches from between the engine body 1 and the exhaust turbine 32 and connects to the intake passage 2, and the EGR passage 4 includes an EGR valve 5. The EGR valve 5 is driven to open and close by, for example, a stepping motor type actuator, and its opening degree can be continuously controlled. The amount of exhaust gas recirculated to the intake side according to the opening degree, that is, the engine body 1 The amount of EGR that is inhaled is adjusted.

  A downstream side of the supercharger turbine 32 in the exhaust passage 3 is provided with a second catalyst 33 that generates aromatic coal HC from HC in the exhaust. On the downstream side of the second catalyst 33 in the exhaust passage 3, a first catalyst 34 that deposits and desorbs NOx in the exhaust (hereinafter, the first catalyst is also referred to as “NOx trap catalyst”) is provided. A downstream side of the catalyst 34 is provided with a DPF (diesel particulate filter) 35 that collects PM (particulate matter; particulate matter) in the exhaust gas.

  Air-fuel ratio sensors 36 and 37 serving as detection means for detecting the air-fuel ratio (excess air ratio) in the exhaust gas are provided at the respective inlet portions of the second catalyst 33 and the first catalyst 34 in the exhaust passage 3. Yes. The air-fuel ratio sensors 36 and 37 include, for example, an oxygen ion conductive solid electrolyte. The air-fuel ratio sensors 36 and 37 detect the oxygen concentration in the exhaust gas using this, and obtain the air-fuel ratio (excess air ratio) in the exhaust gas from the oxygen concentration. The excess air ratio of the exhaust means a value obtained by dividing the air-fuel ratio in the exhaust by the stoichiometric air-fuel ratio. The larger the air-fuel ratio, the richer, and the smaller the excess air ratio, the richer.

In the exhaust purification system 100 of this example, in addition to the air-fuel ratio sensors 36 and 37 and the air flow meter 22 for detecting the intake air amount as sensors for detecting various states, a rotational speed sensor (not shown) for detecting the engine rotational speed. ), An accelerator opening sensor (not shown) for detecting the accelerator opening, a water temperature sensor 15 for detecting the cooling water temperature of the engine, and the like.
Further, the exhaust purification system 100 of this example is provided with a pressure sensor 16 and a temperature sensor 17 for detecting the pressure and temperature of the fuel in the common rail 10 as sensors for detecting the state of the common rail 10. .

  An engine control unit (hereinafter abbreviated as “ECU”) 40 includes signals from various sensors, for example, an air flow meter signal for detecting an intake air amount, a water temperature sensor signal, a crank angle sensor signal for detecting a crank angle, and a cylinder. Crank angle sensor signal for discrimination, pressure sensor signal for detecting fuel pressure on common rail, temperature sensor signal for detecting fuel temperature, accelerator pedal position sensor signal for detecting accelerator pedal depression corresponding to load, empty Various signals such as a signal from the fuel ratio sensor are input.

  The ECU 40 controls the drive of the fuel injection valve 11 by determining the fuel injection pressure, setting the fuel injection amount and the injection timing based on the detection signals from the various sensors inputted.

  The fuel injection valve 11 is an electronic fuel injection valve 11 that is opened and closed by an ON-OFF signal from the ECU 40. The fuel injection valve 11 injects fuel into the fuel injection chamber by an ON signal and stops injection by an OFF signal. The longer the ON signal period applied to the fuel injection valve 11, the greater the fuel injection amount. The higher the fuel pressure in the common rail 10, the greater the fuel injection amount.

The fuel injection timing is appropriately determined based on a crank angle detection crank angle sensor signal input to the ECU 40, a cylinder discrimination crank angle sensor signal, and the like.
Under the control of the ECU 40, fuel is injected from the fuel injection valve 11 at any injection timing such as pilot injection, main injection, post injection, and the like.

In the exhaust purification system of this example, the rich spike control is performed by reducing the excess air ratio λ of the exhaust to 1.0 or less.
Specifically, in order to enrich the exhaust air excess ratio λ until it becomes 1.0 or less, the ECU 40 controls the opening of the intake throttle valve 24 and the EGR valve 5 to be reduced. Further, the fuel is injected from the fuel injection valve 11 at the injection timing of the pilot injection and the main injection, thereby enriching the exhaust gas.
In the lean fuel operation state in which the excess air ratio λ is large, NOx deposited on the first catalyst (NOx trap catalyst) 34 is desorbed from the first catalyst 34 when rich spike control is performed.

The gas supply means supplies a gas containing C6 or higher paraffin HC and / or C6 or higher olefin HC to the second catalyst 33.
For example, when rich spike control is performed, when gas containing C6 or more paraffin HC and / or C6 or more olefin HC is supplied to the second catalyst 33 by the gas supply means, the second catalyst 33 uses C6. Aromatic HC is generated from the above HC, and H ₂ which is a reducing agent is generated by a dehydrogenation reaction accompanying this generation.

As described above, when the reducing gas containing a large amount of H _{2 serving} as the reducing agent is generated in the second catalyst 33 when the rich spike control is performed, the reducing gas is supplied to the NOx trap catalyst 34. The NOx conversion rate (reduction rate) of NOx desorbed from the catalyst can be increased.

  As a gas supply means for supplying a gas containing C6 or higher paraffin HC and / or C6 or higher olefin HC to the second catalyst 33, there is a fuel injection valve 11 controlled by the ECU 40 or the like.

  Specifically, the ECU 40 increases the fuel injection amount injected from the fuel injection valve 11 serving as the gas supply means, for example, during the rich spike control, and the fuel near the piston top dead center of the engine body 1. After the main injection for injecting the fuel, control is performed to perform post injection for injecting the fuel at a crank angle that does not burn the fuel in the cylinder of the engine body 1.

  In this way, by controlling the fuel injection amount and injection timing of the fuel injected from the fuel injection valve 11 and increasing the fuel injection amount and performing the post injection, C6 or higher paraffin HC and / or C6 or higher A gas containing olefin HC is supplied to the second catalyst 33.

When the rich spike control is performed, specifically, when the excess air ratio λ is 1.0 or less, preferably λ is 0.75 to 0.83, the oxygen concentration in the exhaust gas is preferably controlled by the oxygen concentration control means. Is controlled to 0.8 to 1.5 vol%, more preferably 1.1 to 1.4 vol%, still more preferably 1.1 to 1.2 vol%.
When the rich spike control is performed, if the oxygen concentration in the exhaust gas is as small as 0.8 to 1.5 vol%, the unburned HC in the exhaust gas from the OSC material of the second catalyst 34 in the second catalyst 33. Oxygen is deprived and the dehydrogenation reaction is facilitated by the action of the HC conversion catalyst, and H ₂ is easily produced along with the production of aromatic HC.
Therefore, when the rich spike control is performed, if the oxygen concentration in the exhaust gas is 0.8 to 1.5 vol%, the second catalyst 34 generates a sufficient amount of H ₂ to reduce NOx. The reducing gas containing a large amount of H ₂ can be supplied to the NOx trap catalyst 34.

  Examples of the oxygen concentration control means include an intake throttle valve 24 and an EGR valve 5 whose opening degree is controlled by the ECU 40. By controlling the opening degree of the intake throttle valve 24 and the EGR valve 5, the amount of air sucked into the engine body 1 is adjusted, and the oxygen concentration of the exhaust gas can be controlled.

Further, it is preferable that a temperature control means for controlling the temperature of the second catalyst 33 to 200 ° C. or more is provided near the second catalyst 33.
The temperature control means is not particularly limited, and examples thereof include a temperature sensor and a heater 38 disposed in the vicinity of the second catalyst 33. Using these temperature sensors, various heaters, etc., and a device having a CPU as necessary, for example, when the rich spike control is performed, the second catalyst 33 is controlled to be 200 ° C. or higher.

Oxygen absorbed by the OSC material of the second catalyst 33 is easily desorbed from the second catalyst 33 by being heated to a desorption temperature, typically 200 to 250 ° C. or higher.
When NOx is reduced by the first catalyst 34 (during the execution of rich spike control), if oxygen is easily desorbed by the second catalyst 33, the dehydrogenation reaction is promoted by the second catalyst 33, and the fragrance is reduced. Along with the generation of group HC, the amount of H ₂ generated increases, and a reducing gas containing a large amount of H ₂ can be supplied to the first catalyst 34.

  FIG. 5 is a schematic configuration diagram illustrating another example of the embodiment of the present invention and illustrating an example of an exhaust gas purification system 100 for a turbocharged diesel engine to which the present invention is applicable. In FIG. 5, the same members as those in FIG.

As shown in FIG. 5, the exhaust purification system 100 of this example includes a fuel supply system that supplies fuel directly from the fuel tank 50 to the exhaust passage 3 upstream of the second catalyst 33.
The fuel supply system includes a second fuel supply passage 6 and a second supply for supplying fuel directly from the fuel tank 50 to the exhaust passage 3 upstream of the second catalyst 33 (inlet side of the second catalyst 33). A pump 61 and an injector 62 are provided.
This fuel supply system is a gas supply means for supplying a gas containing C6 or higher paraffin HC and / or C6 or higher olefin HC to the second catalyst 33.

Specifically, the fuel is directly injected from the injector 62 into the exhaust passage 3 upstream of the second catalyst 33 through the second supply pump 61 under the control of the ECU 40 or the like, for example, when the rich spike control is performed. .
The second supply pump 61 adjusts the substantial fuel discharge amount injected from the injector 62 into the exhaust passage 3.

  In this way, by directly injecting fuel from the injector 62 into the exhaust passage 3 upstream of the second catalyst 33, gas containing C6 or higher paraffin HC and / or C6 or higher olefin HC is converted into the second catalyst. 33 can be supplied.

  In the exhaust purification system of this example, it is preferable that a gas containing a large amount of C6 or higher paraffin HC and / or C6 or higher olefin HC can be supplied to the second catalyst 33 when the rich spike control is performed. . Therefore, the exhaust purification system of this example can be suitably used as an exhaust purification system of a diesel engine that uses light oil containing a large amount of HC having a large carbon number as fuel.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.

Example 1
<Manufacture of HC conversion catalyst (A)>
The γ-alumina powder was impregnated with an aqueous rhodium nitrate (Rh (NO ₃ ) ₃ ) solution having a rhodium (Rh) concentration of 6%, dried at 120 ° C. for a whole day and night, and baked at 450 ° C. for 1 hour. HC conversion catalyst powder a having a loading of 1% was obtained.
The catalyst powder a; 207 g, γ-alumina; 603 g, alumina sol; 90 g, and water; 900 g were charged into a magnetic ball mill, and wet mixed and pulverized until the average particle size became 3.8 μm. Got.

<Manufacture of HC conversion catalyst (B) containing OSC material>
A complex oxide powder of cerium: praseodymium = 0.7: 0.3 in a molar ratio was impregnated with an aqueous solution of palladium nitrate (Pd (NO ₃ ) ₃ ) having a palladium (Pd) concentration of 6%, and at 120 ° C. overnight. Drying, removing moisture, and firing at 600 ° C., an HC conversion catalyst powder b containing an OSC material with a Pd loading of 4% was obtained.
HC conversion catalyst powder b containing OSC material: 578 g, composite oxide powder having a molar ratio of cerium: praseodymium = 0.7: 0.3; 232 g, alumina sol; 90 g, water; 900 g are charged into a magnetic ball mill. Then, wet mixing and pulverization were performed until the average particle size became 3.8 μm, and HC conversion catalyst slurry B containing OSC material was obtained.

<Production of second catalyst (1)>
A cordierite honeycomb monolith support (0.92 L, 400 cpsi) is coated with HC conversion catalyst slurry A, excess slurry adhering to the cells is removed by a compressed air flow, dried at 130 ° C., and then 450 ° C. The HC conversion catalyst layer A having a coating amount of 100 g / L was formed on the honeycomb carrier by firing for 1 hour.
Next, the HC conversion catalyst slurry B containing the OSC material is coated on the HC conversion catalyst layer A, and as in the case of the HC conversion catalyst slurry A, excess slurry in the cell is removed by compressed air flow, and 130 ° C. After drying at 4 ° C., firing at 450 ° C. for 1 hour forms an HC conversion catalyst layer B containing an OSC material with a coating amount of 100 g / L on the HC conversion catalyst layer A, and HC conversion on the honeycomb carrier A second catalyst (1) having a catalyst layer A and an HC conversion catalyst layer B containing an OSC material was produced.
The amount of Rh supported on the HC conversion catalyst layer A supported on the second catalyst (1) is 0.23 g / L, and the amount of Pd supported on the HC conversion catalyst layer B containing the OSC material is 2.57 g. / L.

(Example 2)
<Manufacture of HC conversion catalyst (C)>
The γ-alumina powder was impregnated with an aqueous rhodium nitrate (Rh (NO ₃ ) ₃ ) solution having a rhodium (Rh) concentration of 6%, dried at 120 ° C. for a whole day and night, and baked at 450 ° C. for 1 hour. An HC conversion catalyst powder c having a loading of 4% was obtained.
HC conversion catalyst powder c: 207 g, γ-alumina: 603 g, alumina sol: 90 g, and water: 900 g were charged into a magnetic ball mill, and wet-mixed and pulverized until the average particle size became 3.8 μm. C was obtained.

<Manufacture of HC conversion catalyst (D) containing OSC material>
A complex oxide powder having a molar ratio of cerium: praseodymium = 0.7: 0.3 was impregnated with an aqueous solution of palladium nitrate (Pd (NO ₃ ) ₃ ) having a palladium (Pd) concentration of 6%, at 120 ° C. The HC conversion catalyst powder d containing the OSC material having a Pd loading of 16% was obtained by drying for a whole day and night, removing moisture, and firing at 600 ° C.
HC conversion catalyst d containing OSC material: 578 g, cerium: praseodymium = 0.7: 0.3 composite oxide powder in molar ratio; 232 g, alumina sol; 90 g, water; The mixture was wet-mixed and pulverized until the average particle size became 3.8 μm to obtain an HC conversion catalyst slurry D containing an OSC material.

<Production of second catalyst (2)>
HC conversion catalyst slurry C is coated on a cordierite honeycomb monolith support (0.92 L, 400 cpsi), excess slurry adhering to the cells is removed by compressed air flow, dried at 130 ° C., and then 450 ° C. Was fired for 1 hour to form an HC conversion catalyst layer C having a coating amount of 100 g / L on the honeycomb carrier.
Next, the HC conversion catalyst slurry D containing the OSC material is coated on the HC conversion catalyst layer C, and, as with the HC conversion catalyst slurry C, excess slurry in the cell is removed with a compressed air flow, and 130 ° C. And dried at 450 ° C. for 1 hour to form an HC conversion catalyst layer D containing an OSC material with a coating amount of 100 g / L on the HC conversion catalyst layer C, and an HC conversion on the honeycomb carrier. A second catalyst (2) having a catalyst layer C and an HC conversion catalyst layer D containing an OSC material was obtained.
The amount of Rh supported on the HC conversion catalyst layer C supported on the second catalyst (2) is 0.95 g / L, and the amount of Pd supported on the HC conversion catalyst layer D containing the OSC material is 10.3 g. / L.

(Production of first catalyst)
First, alumina is put into a mixed aqueous solution of cerium acetate (Ce (CH ₃ CO ₂ ) ₃ ) and barium acetate (Ba (CH ₃ CO ₂ ) ₂ ), and stirred at room temperature for about 1 hour. After drying all day and night to remove moisture, it was baked in the atmosphere at 600 ° C. for about 1 hour to obtain a baked powder. This calcined powder is impregnated with a tetraammine platinum hydrochloride solution (pH = 10.5) having a platinum concentration of 2%, dried at 120 ° C. for one day and night, removed moisture, and then calcined at 450 ° C. for 1 hour. As a result, a catalyst powder e having a Pt loading of 1%, a Ba loading of 8% as BaO, and a Ce loading of 20% as CeO ₂ was obtained.

Next, alumina was put into an aqueous solution of zirconium acetate (Zr (CH ₃ CO ₂ ) ₄ ), stirred for about 1 hour at room temperature, dried at 120 ° C. overnight, removed water, and then at 900 ° C. for 1 hour. Firing was performed to obtain a calcined powder. This calcined powder is impregnated with an aqueous rhodium nitrate (Rh (NO ₃ ) ₃ ) solution having a rhodium concentration of 6%, dried at 120 ° C. for a whole day and night, after removing moisture, and then calcined at 450 ° C. for 1 hour, thereby reducing the amount of Rh supported. A catalyst powder f having 2% and a Zr loading of 3% was obtained.

In addition, alumina was put into a mixed aqueous solution of cerium acetate (Ce (CH ₃ CO ₂ ) ₃ ) and barium acetate (Ba (CH ₃ CO ₂ ) ₂ ), and the mixture was stirred at room temperature for about 1 hour at 120 ° C. After drying all day and night to remove moisture, it was baked in the atmosphere at 600 ° C. for about 1 hour to obtain a baked powder. This calcined powder is impregnated with a tetraammine platinum hydrochloride solution (pH = 10.5) having a platinum concentration of 2%, dried at 120 ° C. for one day and night, removed moisture, and then calcined at 450 ° C. for 1 hour. As a result, catalyst powder g having a Pt loading of 3.5%, a Ba loading of 8% as BaO, and a Ce loading of 20% as CeO ₂ was obtained.

  Also, proton type β zeolite having a silica / alumina ratio of about 25: 720 g, silica sol: 180 g, water: 900 g are charged into an alumina magnetic ball mill, and wet mixed and pulverized until the average particle size becomes 3.8 μm. As a result, zeolite slurry H was obtained.

The above catalyst powder e: 555 g, alumina: 25 g, β zeolite: 230 g, alumina sol: 90 g, water: 900 g are charged into a magnetic ball mill and wet-mixed and pulverized until the average particle size becomes 3.2 μm to obtain catalyst slurry E. It was.
The above catalyst powder f: 317 g, catalyst powder g: 454 g, alumina: 38 g, alumina sol: 90 g, water: 900 g were charged into a magnetic ball mill, and wet mixed and pulverized until the average particle size became 3.0 μm. Obtained.

Cordierite honeycomb monolith support (1.2 L, 400 cpsi) is coated with zeolite slurry H, excess slurry adhering to the cells is removed by compressed air flow, dried at 130 ° C., and then fired at 450 ° C. for 1 hour. As a result, a zeolite layer (first layer: HC trap material) having a coating amount of 80 g / L was formed on the honeycomb carrier.
Next, the catalyst layer E is coated on the zeolite layer, similarly, the excess slurry in the cell is removed with a compressed air flow, dried at 130 ° C., and then calcined at 450 ° C. for 1 hour. A catalyst layer having a coating amount of 220 g / L (second layer: coexistence layer of HC trap material and NOx trap catalyst) was formed on the layer.
Further, the catalyst slurry F is coated on the catalyst layer, similarly, excess slurry in the cell is removed with a compressed air flow, dried at 130 ° C., and then calcined at 450 ° C. for 1 hour. A catalyst layer (third layer: NOx trap catalyst layer) having a coating amount of 100 g / L was formed on the layer (second layer).

<Construction of exhaust gas purification system and performance test I>
As shown in FIG. 5, the second catalyst 33 (Example 1 <second catalyst (1)> or implementation is implemented in the exhaust passage of an in-line four-cylinder 2500 cc direct injection diesel engine (engine body 1) manufactured by Nissan Motor Co., Ltd. Example 2 <Second catalyst (2)> was mounted, and the first catalyst 34 was mounted downstream of the second catalyst 33.
Further, as a comparative example, an exhaust purification system was formed in which only the first catalyst 34 was attached to the exhaust passage without attaching the second catalyst 33.
And after driving for 40 seconds by lean (A / F = 30), operation which performs rich spike control which operates for 2 seconds by rich (A / F = 11.7) was repeated.
When this rich spike control is performed, fuel is directly injected from the gas supply means (injector 62) into the exhaust passage 3 on the inlet side of the second catalyst 33, and C6 or higher paraffin HC and / or C6 or higher olefin HC is produced. Gas containing was supplied.
The excess air ratio λ of the exhaust when the rich spike control is performed is controlled to 1.0 or less, and the oxygen concentration of the exhaust is set to 0.8 to 1.2 vol% according to the method disclosed in Japanese Patent No. 3918402. Controlled.
The fuel used was a commercially available JIS No. 2 light oil, and the inlet temperature of the second catalyst 4 was set to 220 ° C.

  When the rich spike control was performed, the amount of aromatic HC relative to the total amount of NMHC in the exhaust gas supplied from the second catalyst 33 to the first catalyst 34 was measured with a gas chromatograph mass spectrometer. Further, when the rich spike control was performed, the NOx conversion rate of the first catalyst 34 was obtained by measuring the NOx concentration before and after the catalyst using a chemiluminescence NOx analyzer. The results are shown in FIG.

In addition, when the rich spike control is performed, the concentration (ppm) of aromatic HC in the exhaust gas supplied from the second catalyst 33 to the first catalyst 34 and the amount of H _{2 with} respect to 100% of the total exhaust gas in the same exhaust gas ( %) Was measured with a gas chromatograph mass spectrometer. The results are shown in FIG.

As shown in FIG. 6, the NOx conversion rate (reduction rate) in the first catalyst increases as the proportion of the amount of aromatic HC in the exhaust gas supplied from the second catalyst to the first catalyst increases. It was confirmed that the rate was high. In particular, when the total amount of NMHC contained in exhaust gas is 100%, the amount of aromatic HC is 2.0% or more (aromatic HC amount / HC total amount ≧ 0.02). Obtained. In the exhaust gas purification system (comparative example) not equipped with the second catalyst, the NOx conversion was less than 80%.
From this result, when the rich spike control is performed, a gas containing C6 or higher paraffin HC and / or C6 or higher olefin HC is supplied to the second catalyst 33, whereby the second catalyst 33 generates aromatic HC. It can be presumed that the conversion rate of NOx was improved by generating H ₂ by the dehydrogenation reaction along with the generation.

This can also be confirmed from the results shown in FIG. That is, as shown in FIG. 7, when aromatic HC was generated by the second catalyst 33, the amount of H ₂ generated increased with the increase of the amount of aromatic HC generated.

It is explanatory drawing which shows an example of embodiment of the exhaust gas purification system of this invention, and shows a part of simplified structure of an exhaust gas purification system. It is the perspective view and partial enlarged view which show an example of the 2nd catalyst used for an exhaust gas purification system. It is the perspective view and partial enlarged view which show an example of the 1st catalyst used for an exhaust gas purification system. It is the schematic block diagram which showed an example of the exhaust gas purification system of this invention. It is the schematic block diagram which showed the other example of the exhaust gas purification system of this invention. It is a graph which shows the relationship between the amount of aromatic HC / NMHC in exhaust_gas | exhaustion supplied to a 1st catalyst from a 2nd catalyst, and NOx conversion rate. It is a graph showing the relationship between the concentration of the aromatic HC in the exhaust gas supplied from the second catalyst in the first catalyst (ppm) and H _{2 (%).}

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine main body 2 Intake passage 3 Exhaust passage 3a Exhaust outlet passage 4 EGR passage 5 EGR valve 6 2nd fuel supply passage 10 Common rail 11 Fuel injection valve 12 Fuel supply passage 13 Supply pump 14 Fuel return passage 15 Water temperature sensor 16 Pressure sensor 17 Temperature sensor 21 Air cleaner 22 Air flow meter 23 Intake collector 24 Intake throttle valve 30 Supercharger 31 Compressor 32 Exhaust turbine 33 Second catalyst 34 First catalyst 35 Diesel particulate filter (DPF)
36 Air-fuel ratio sensor 37 Air-fuel ratio sensor 38 Heater 40 Engine control unit (ECU)
50 Fuel tank 61 Second supply pump 62 Injector

Claims (9)

In an exhaust gas purification system in which a first catalyst that purifies NOx in exhaust gas is disposed in an exhaust passage of an internal combustion engine,
A second catalyst that generates aromatic hydrocarbons from hydrocarbons in the exhaust gas upstream of the first catalyst;
An exhaust purification system comprising gas supply means for supplying a gas containing paraffin hydrocarbons having 6 or more carbon atoms and / or olefin hydrocarbons having 6 or more carbon atoms to the second catalyst. Means for performing rich spike control for desorbing NOx deposited on the first catalyst by adjusting an excess air ratio of exhaust;
The exhaust gas purification system according to claim 1, further comprising oxygen concentration control means for controlling the oxygen concentration in the exhaust gas to 0.8 to 1.5 vol% when the rich spike control is performed.   The amount of aromatic hydrocarbons in the exhaust gas supplied from the second catalyst to the first catalyst with respect to the total amount of non-methane hydrocarbons in the exhaust gas is 2.0% by mass or more. Item 3. The exhaust gas purification system according to Item 1 or 2.   The second catalyst includes a transition metal constituting an oxide having an ability to occlude oxygen and at least one noble metal element selected from the group consisting of platinum, rhodium and palladium in a catalyst layer provided on a support. The exhaust purification system according to any one of claims 1 to 3, comprising at least one transition metal element among the elements.   The exhaust purification system according to claim 4, wherein the second catalyst contains the noble metal element and the transition metal element in the same catalyst layer.   6. The exhaust gas purification system according to claim 4, wherein the second catalyst has a precious metal element loading of 12.0 g / L or less.   The exhaust gas purification system according to any one of claims 4 to 6, wherein the second catalyst increases the content of the noble metal element as being closer to the surface in contact with the exhaust gas.   The exhaust purification system according to any one of claims 1 to 7, further comprising temperature control means for setting the temperature of the second catalyst to 200 ° C or higher when the rich spike control is performed.   The exhaust gas purification system according to any one of claims 1 to 8, wherein the internal combustion engine is a diesel engine.

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