JP2010216385A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device for purifying nitrogen oxides (NOx) in exhaust gas. <P>SOLUTION: The device includes an oxidizing catalyst and first and second NOx storage-reduction catalysts, arranged from the exhaust direction of an internal combustion engine which alternately exhausts lean exhaust gas and rich exhaust gas, wherein the first NOx storage-reduction catalyst is smaller in catalytically active metal amount than the second NOx storage-reduction catalyst. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

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

  Internal combustion engines (particularly diesel engines) employ a lean burn system in which combustion is performed with the air / fuel ratio (air / fuel) set to the lean side (air / fuel ratio> 1). In an internal combustion engine that employs this lean burn system, combustion is performed with the air-fuel ratio on the lean side, so the exhaust gas is in an oxygen-excess state. Nitrogen oxide (NOx) contained in exhaust gas containing excess oxygen is difficult to obtain with a three-way catalyst used in a gasoline engine.

  Conventionally, a NOx occlusion reduction type catalyst has been proposed for oxygen-excess exhaust gas (lean exhaust gas) (Patent Document 1: Japanese Patent Application Laid-Open No. 2007-285295). This is because NOx in the lean exhaust gas is occluded, and the exhaust gas (rich exhaust gas) rich in HC, CO, hydrogen, etc., which is a reducing agent, at the same time as being made oxygen deficient by post-injection of fuel, etc. The exhaust gas is purified by reducing and removing the stored NOx. In addition, an exhaust gas purification device is proposed in which an oxidation catalyst is disposed upstream of the NOx storage reduction catalyst (Patent Document 1). On the other hand, as a different method, a urea selective reduction type NOx catalyst is proposed in which a urea-based compound is used as a reducing agent in lean exhaust gas and NOx is selectively reduced and removed without producing rich exhaust gas ( Patent Document 2: JP 2008-19820).

JP2007-285295A JP2008-19820

  At the time of the present invention, the present inventors, in the present invention, in order of the oxidation catalyst and the first and second NOx occlusion reduction catalysts in the exhaust gas of the internal combustion engine that alternately exchanges lean exhaust gas and rich exhaust gas. In the disposed exhaust gas purification apparatus, a first NOx occlusion reduction type catalyst having a low concentration of catalytically active metal downstream of the oxidation catalyst, and a second NOx occlusion reduction type catalyst having a high concentration of catalytically active metal downstream thereof, High NOx purification rate by arranging a NOx occlusion reduction type catalyst adjusted so that the amount of catalytically active metal is gradually increased from the exhaust gas introduction side to the exhaust side. Obtained knowledge that can be obtained. The present invention is based on this finding.

Accordingly, the present invention is an apparatus for purifying nitrogen oxide (NOx) in exhaust gas discharged from an internal combustion engine,
The internal combustion engine exhausts lean exhaust gas and rich exhaust gas alternately,
From the exhaust direction of the internal combustion engine, comprising an oxidation catalyst and first and second NOx storage reduction catalysts,
The NOx occlusion reduction catalyst comprises a catalytically active metal;
An exhaust gas purification device is proposed in which the first NOx storage reduction catalyst has a smaller amount of catalytically active metal than the second NOx storage reduction catalyst.

Detailed Description of the Invention

I. Exhaust gas purifying apparatus The exhaust gas purifying apparatus according to the present invention comprises, from the exhaust direction of the internal combustion engine, 1) an oxidation catalyst, 2) a first NOx occlusion reduction type catalyst having a low concentration of catalytically active metal, and 3) catalytically active metal. And a second NOx occlusion reduction type catalyst having a high concentration of NO in this order.
The aspect of the exhaust gas purifying apparatus according to the present invention can be described with reference to FIGS. 1 and 2. The exhaust gas purifying apparatus of FIG. 1 includes an oxidation catalyst, a first NOx occlusion reduction type catalyst 1, and a second NOx occlusion reduction type catalyst 2 from the exhaust direction, and the first NOx occlusion reduction catalyst. A space is provided between the type catalyst 1 and the second NOx storage reduction type catalyst 2. On the other hand, the exhaust gas purifying apparatus of FIG. 2 is constituted by an oxidation catalyst, a first NOx storage reduction catalyst 1 and a second NOx storage reduction catalyst 2 from the exhaust direction. The occlusion reduction type catalyst 1 and the second NOx occlusion reduction type catalyst 2 are configured integrally. In any embodiment, the object of the present invention can be achieved. However, the present invention is not construed as being limited to that shown in FIGS.

Exhaust gas In the purification apparatus according to the present invention, the exhaust gas discharged from the internal combustion engine is one in which lean exhaust gas and rich exhaust gas are alternately discharged. In the present invention, it is preferable that the lean exhaust gas is alternately discharged in a period of 10 to 120 seconds and a rich exhaust gas is alternately discharged in a period of 1 to 5 seconds.
Lean exhaust gas is normal diesel engine exhaust gas, and contains O ₂ at a concentration of about 4% to the same level as the atmosphere. On the other hand, rich exhaust gas is exhaust gas in which the concentration of combustible gas is made rich by performing operations such as fuel post-injection and injection in the exhaust pipe, and when the combustible gas becomes COx by combustion, The exhaust gas is in such a state that the minimum concentration of O ₂ concentration coexisting with is 1 to 2% or less. The post-injection of fuel means that fuel is injected further after an engine combustion process of intake → compression → fuel injection / combustion, or fuel is injected at the time of subsequent →→ expansion → exhaust. . The exhaust pipe injection means that fuel is injected into the exhaust gas from a fuel injection device installed in the exhaust pipe.

  Further, in the present invention, the fact that the lean exhaust gas and the rich exhaust gas are alternately discharged is not only an act of changing from lean (or rich) to rich (or lean) at once, but also an action of changing gradually. included. “Gradually” means an action of changing from “lean” (or rich) to “rich” (or lean) “continuous”, “stepwise”, and “sequentially”.

1) Oxidation catalyst Although a normal thing can be used for an oxidation catalyst, you may be comprised by a catalytically active metal and aluminum oxide fundamentally.
Catalytically active metal The catalytically active metal comprises a noble metal, a base metal, or a mixture of these oxides, preferably a noble metal. Specific examples of the noble metal include platinum, palladium, rhodium, ruthenium, iridium, osmium, gold, or silver. The addition amount of the catalytically active metal is 4% by weight or less based on the total weight of the catalyst.

As the aluminum oxide, one represented by Al ₂ O ₃ is used, preferably one having a specific surface area of 70 g / cm ² or more. The addition amount is preferably about 50 to 99% by weight based on the total weight of the catalyst. Aluminum oxide disperses and supports catalytically active metals so that their activity can be fully exerted

Optional material The catalyst may comprise an optional material as required. For example, an oxygen storage material can be added. Specific examples of the oxygen storage material include oxides such as cerium, zirconium, neodymium, and lanthanum, and composite oxides thereof. The addition amount of the oxygen storage material is 5% by weight or more and less than 50% by weight with respect to the total weight of the catalyst, preferably the lower limit is 10% by weight or more and the upper limit is 30% by weight or less.

Specific examples of carriers carriers are alumina, monolith or honeycomb form made of metal such as ceramics or stainless such as cordierite and the like.

2) NOx occlusion reduction type catalyst In the present invention, the first NOx occlusion reduction type catalyst having a low concentration of catalytically active metal downstream of the oxidation catalyst and the second concentration of catalytically active metal having a high concentration downstream thereof. A NOx occlusion reduction type catalyst is arranged. The first NOx occlusion reduction type catalyst and the second NOx occlusion reduction type catalyst may be directly connected to each other, or may be arranged with a space between them. In the present invention, the first NOx occlusion reduction type catalyst and the second NOx occlusion reduction type catalyst are used. In the present invention, in one NOx occlusion reduction type catalyst, the downstream of the oxidation catalyst. From the side, it also includes those using a catalyst active metal whose concentration gradually increases.

  The NOx occlusion reduction type catalyst removes NOx in the exhaust gas by storing NOx in the exhaust gas in the catalyst during lean exhaust gas and reducing the NOx occluded in the rich exhaust. The catalyst having a low concentration of the catalytically active metal and the catalyst having a high concentration may be coated on separate supports or may be coated on a single support.

The NOx occlusion reduction type catalyst is basically composed of a NOx occlusion material, a catalytically active metal, and a catalyst auxiliary material.
Specific examples of the NOx storage agent NOx occluding agents are alkali metals, alkaline earth metals, rare earth elements, and those selected from the group consisting of a mixture thereof.
Specific examples of the alkali metal include those selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, francium, and mixtures thereof.
Specific examples of alkaline earth metals include those selected from the group consisting of beryllium, magnesium, calcium, strontium, barium, and mixtures thereof.
Specific examples of rare earth elements include those selected from scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, and mixtures thereof.
Further, the NOx storage agent can contain a base metal as necessary, and specific examples thereof include nickel, copper, manganese, iron, cobalt, zinc and the like.
The addition amount of the NOx storage agent is 1% by weight or more and 80% by weight or less with respect to the total amount of the NOx storage reduction catalyst component, preferably the upper limit is 75% by weight or less, preferably 70% by weight or less. The lower limit is 2% by weight or more, preferably 5% by weight or more.

Examples of catalytically active metals catalytically active metal, a noble metal, base metal and the like. Specific examples of noble metals include those selected from the group consisting of platinum, palladium, rhodium, ruthenium, iridium, osmium, gold, silver and mixtures thereof, preferably platinum, palladium, rhodium and mixtures thereof. Is mentioned.
Specific examples of the base metal include nickel, copper, manganese, iron, cobalt, tungsten, molybdenum, zinc, and mixtures thereof.
The addition amount of the catalytically active metal is 0% by weight and 50% by weight or less, preferably 45% by weight or less, preferably 40% by weight or less, with respect to the total amount of the NOx storage reduction catalyst component. is there.

Specific examples of the catalyst aids catalyst aids, aluminum oxide, cerium oxide, zirconium oxide, titanium oxide, include those selected from silica and the group consisting of composite oxides. The addition amount of the catalyst auxiliary is more than 5% by weight and not more than 95% by weight, preferably not more than 90% by weight, preferably not more than 60% by weight, based on the total amount of the NOx storage reduction type catalyst component. .

Specific examples of carriers carriers are alumina, monolith or honeycomb form made of metal such as ceramics or stainless such as cordierite and the like.

Internal combustion engine In this specification, an internal combustion engine means a heat engine that explosively burns air and fuel in a cylinder to give a reciprocating motion to a piston. As a typical example of an internal combustion engine, a 2-cycle or 4-cycle engine classified by piston stroke and operation; a spark ignition engine classified by a fuel ignition device, a compression ignition engine; or a fuel used Examples include gasoline engines, diesel engines, oil engines, and alcohol engines. The exhaust gas purification method of the present invention can be preferably used for a diesel engine among internal combustion engines.

Application of the exhaust gas purification device The device according to the invention can preferably be used in internal combustion engines, in particular diesel engines. Further, according to a preferred aspect of the present invention, the apparatus according to the present invention can be used for an exhaust system of a vehicle equipped with an internal combustion engine. Further, these engines may be engines that adjust the air-fuel ratio and burn fuel, and preferred examples thereof include lean burn engines, direct injection engines, and preferably combinations of these engines (ie, direct injection). Type lean burn engine). The direct injection engine is an engine that employs a fuel supply system that can achieve a high compression ratio, an improvement in combustion efficiency, and a reduction in exhaust gas.

  The exhaust gas purification apparatus according to the present invention is used in an exhaust system of an internal combustion engine mounted on a transporter, a machine, or the like. Specific examples of transporters and machines include transporters and transporters of machines such as automobiles, buses, trucks, dump trucks, airway cars, motorcycles, motorbikes, ships, tankers, motor boats, airplanes, etc. Agricultural and forestry industrial machines such as cultivators, tractors, combine harvesters, chain saws, timber carriers, etc .; fisheries and fishery machines such as fishing boats; For example, in the case of an exhaust system of a vehicle, the exhaust gas catalyst according to the present invention can be installed as a start catalyst, an under floor, or a manifold converter.

Embodiments for carrying out the present invention

  The content of the present invention will be described in more detail with reference to examples. However, the contents of the present invention are not construed as being limited by the examples.

Example
Catalyst Preparation 1) Oxidation Catalyst A catalyst layer in which aluminum oxide and Pt and Pd as precious metals were mixed was formed on the surface of a cordierite honeycomb substrate by a PC coating process and a 500 ° C. firing process in air.
2) NOx storage reduction catalyst A catalyst layer made of a metal or oxide of aluminum oxide, barium, cerium, zirconium, Pt, Rh was formed on the surface of a cordierite honeycomb substrate by a PC coating process and a firing process. .
NOx occlusion reduction type catalysts were prepared so that the concentration ratios and catalyst length ratios of Pt and Rh, which are catalytically active metals, were distributed as shown in Tables (A) to (E). The amount of Pt and Rh is the same for both catalysts.

Construction of Exhaust Gas Purifying Device In the same manner as the exhaust gas purifying device shown in FIG. 1, from the engine exhaust side, an oxidation catalyst, an upstream NOx occlusion reduction type catalyst, and a downstream NOx reduction are placed in a metal pipe. An exhaust gas purification device was constructed in which the catalyst was installed in this order.

An exhaust gas purification device was installed in the exhaust system of a diesel engine installed and adjusted in the evaluation test engine bench. Before evaluating the performance of the catalyst, the engine was operated in advance so that the temperature in the system was 150 ° C. or higher.
The catalyst performance was evaluated by operating so that the temperature of the catalyst was in the range of 150 ° C to 250 ° C. NOx in the exhaust gas flowing out from the conventional system or the system of the present case was measured and evaluated by the NOx purification rate calculated from the following equation.
NOx purification rate calculation NOx purification rate = (AB) / A x 100
(Where
A: NOx in exhaust gas at NOx occlusion reduction type catalyst inlet
B: NOx in exhaust gas at NOx occlusion reduction catalyst outlet)

Comparison Results When the NOx purification rate of the NOx occlusion reduction type catalyst as shown in Tables 1A to 1E was measured, the NOx occlusion reduction type catalyst shown in claim 1 was obtained as shown in Table 2 below. , (C) and (E) showed a high NOx purification rate.

FIG. 1 shows an example of an exhaust gas purifying apparatus according to the present invention, in which a space is provided between a first NOx storage reduction catalyst and a second NOx storage reduction catalyst. FIG. 2 shows an example of the exhaust gas purifying apparatus of the present invention, which is constructed by integrating a first NOx storage reduction catalyst and a second NOx storage reduction catalyst.

Claims (1)

An apparatus for purifying nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine,
The internal combustion engine exhausts lean exhaust gas and rich exhaust gas alternately,
From the exhaust direction of the internal combustion engine, comprising an oxidation catalyst and first and second NOx storage reduction catalysts,
The NOx occlusion reduction catalyst comprises a catalytically active metal;
The exhaust gas purifying apparatus, wherein the first NOx storage reduction catalyst has a smaller amount of catalytically active metal than the second NOx storage reduction catalyst.

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