JP2005238168A - Exhaust gas purification catalyst and apparatus for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purification catalyst having excellent durability capable of maintaining a high NOx purification capacity by physically suppressing both movements via a gas phase of a NOx complementary component (especially, alkaline metal) and a solid phase. <P>SOLUTION: The gas purification catalyst is provided with a dense layer between a cordierite substrate and a support layer formed by carrying a catalytic active component. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a catalyst for purifying carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) contained in a gas discharged from an internal combustion engine such as an automobile, and maintains particularly high purification performance. The present invention relates to a catalyst that can be used and an exhaust gas purification apparatus using the catalyst. More specifically, the present invention relates to a lean NOx catalyst for purifying NOx in exhaust gas generated by combustion at an air-fuel ratio higher than the stoichiometric air-fuel ratio, and an exhaust gas purification apparatus using the same.

In recent years, lean burn vehicles, DI (Direct-Injection) vehicles driven by lean combustion, diesel engine vehicles, etc. are expected as fuel-efficient vehicles in the trend of resource saving and CO ₂ emission reduction. Since these engines are operated in a state where there is more air than fuel, the exhaust gas contains excessive oxygen.

  When such exhaust gas containing excessive oxygen is purified with a general three-way catalyst used for exhaust gas purification of automobile gasoline engines, HC and CO in the exhaust gas can be purified, but NOx is reduced. It cannot be purified. For this reason, development of a catalyst (lean NOx catalyst) capable of effectively purifying NOx with high efficiency at a lean combustion air-fuel ratio is underway. As such a lean NOx catalyst, for example, a catalyst that once traps NOx in an adsorbent and then reduces and purifies the trapped NOx is known.

  In general, an automobile exhaust gas purification catalyst such as a three-way catalyst or a lean NOx catalyst has a configuration in which a carrier carrying a catalytically active component is held on a cell inner surface of a honeycomb substrate mainly having a monolith structure. Here, the catalytically active component refers to a component involved in exhaust gas purification. The carrier is used for dispersing and holding the catalytically active component, and various metal oxides and composite oxides are applied, but alumina or a material mainly composed of alumina is frequently used. Cordierite is frequently used as a material for the monolith structure because of its excellent thermal shock resistance.

  As the lean NOx catalyst, a NOx trapping component mainly composed of an alkali metal such as Na or K or an alkaline earth metal such as Sr or Ba, and a redox component of NOx mainly composed of a noble metal such as Pt, Rh, Pd, Those composed of components for oxidizing HC, CO, etc. are known (for example, JP-A-8-141394, JP-A-11-114422, etc.).

  One of the important characteristics required for a catalyst used in an exhaust emission control device for automobiles is a characteristic that can withstand long-term use, that is, durability. In particular, even when exposed to high-temperature exhaust gas, characteristics that can withstand long-term use while maintaining a high purification rate, that is, heat durability are required.

  By the way, in the conventional lean NOx catalyst, when the NOx trapping component (especially alkali metal) is exposed to a high temperature exhaust gas, it moves through the catalyst layer along the solid surface or melts and evaporates in the gas phase. Diffusing and bonding with silica in the cordierite substrate, or covering the surface of a noble metal catalyst such as Pt, Rh, Pd, etc., reduces its redox activity. This characteristic is conspicuous for alkali metals such as Na and K, particularly elements having a large atomic number, such as K and Cs. For this reason, the conventional exhaust gas purification catalyst has a problem in its durability, for example, the NOx trapping component in the catalyst layer decreases with long-term use, resulting in a decrease in catalyst activity.

  In order to solve this problem, Japanese Patent Laid-Open No. 2002-361083 forms a microcavity in a carrier layer containing a NOx supplement component (alkali metal) to prevent the NOx supplement component from moving into or penetrating into the cordierite substrate. Is disclosed. This method is effective in suppressing the movement of the NOx supplement component to the substrate in the solid or via the solid surface, but it cannot prevent the movement due to gas phase diffusion.

  Therefore, as a catalyst for preventing vapor phase diffusion of alkali metal from the catalyst layer to the cordierite carrier, Japanese Patent Laid-Open No. 2001-129402 discloses a suppression layer (silica layer or titania layer) between the catalyst layer and the cordierite substrate. And a catalyst that blocks the alkali metal in the catalyst layer from vapor-phase diffusing into the cordierite substrate. However, this is not discussed from the viewpoint of porosity and affinity for alkali metals, and no countermeasures for suppression in the solid or via the solid surface are considered.

Japanese Patent Laid-Open No. 2002-361083 JP 2001-129402 A

  The present invention solves the above-mentioned problems, and physically suppresses both movement of the NOx supplement component (particularly alkali metal) via the gas phase and movement via the solid phase, thereby providing a high NOx purification capability. An object of the present invention is to provide an exhaust gas purifying catalyst having excellent durability that can be sustained.

  The present inventors have found that the above problem can be solved by providing a dense layer between a carrier supporting a catalytically active component and a substrate, and have completed the present invention.

That is, the present invention includes the following inventions.
(1) a catalyst active component, a carrier layer for supporting the catalyst active component, and a substrate having a monolith structure that holds the carrier, and a chemical reaction to an alkali metal between the carrier layer and the substrate. An exhaust for an internal combustion engine provided with a dense layer having a porosity lower than the chemical reactivity of the substrate with respect to the alkali metal and lower than the porosity of the substrate and suppressing the gas phase diffusion movement of the catalytically active component Gas purification catalyst.
(2) The exhaust gas purification catalyst for an internal combustion engine according to (1), wherein the catalytically active component contains at least one alkali metal.
(3) The dense layer is selected from the group consisting of TiO ₂ , Al ₂ O ₃ , SiC, rare earth metal oxides, alkaline earth metal oxides, transition metal oxides, and zeolites having a Si / Al ratio of 20 or more. The exhaust gas purifying catalyst for an internal combustion engine according to the above (2), characterized in that:
(4) The exhaust gas purification catalyst for an internal combustion engine according to any one of (1) to (3), wherein the catalytically active component is a lean NOx catalyst.
(5) The exhaust for an internal combustion engine according to any one of (1) to (4), wherein a barrier is further provided in the carrier layer to prevent movement of the catalytically active component in the carrier layer. Gas purification catalyst.
(6) The exhaust gas purification catalyst for an internal combustion engine according to (5), wherein the barrier is a microcavity, an anchor material, or a movement suppressing substance.
(7) An exhaust gas purification device for an internal combustion engine comprising the exhaust gas purification catalyst for an internal combustion engine according to any one of (1) to (6).

  According to the present invention, a carrier for supporting a catalytically active component and a substrate having a monolith structure for holding the carrier are provided, and gas phase diffusion of the catalytically active component to the substrate at the interface between the carrier and the substrate. By providing a dense layer for suppressing movement, an exhaust gas purification catalyst for an internal combustion engine that is excellent in thermal durability and can maintain a high NOx purification effect is provided.

Hereinafter, the present invention will be described in detail.
The catalyst of the present invention comprises a catalytically active component, a carrier for supporting the catalytically active component, and a substrate having a monolith structure that holds the carrier, and the catalyst layer is provided between the carrier layer and the substrate with respect to an alkali metal. An alkali layer is provided by providing a dense layer having a lower chemical reactivity than an alkali metal of the substrate and a lower porosity than that of the substrate, and suppressing the gas phase diffusion movement of the catalytically active component. Limiting the gas phase diffusion path and / or solid phase diffusion path of the metal component to suppress the reaction between the catalytically active component and the substrate, resulting in a decrease in the supported amount of the catalytically active component (especially alkali metal) in the support. The biggest feature is minimization.

  The catalytically active component used in the present invention is a component involved in exhaust gas purification, for example, a catalyst containing an alkali metal or an alkaline earth metal for supplementing (and / or storing) NOx, or oxidation / reduction of NOx. And catalysts containing noble metals such as Pt, Rh and Pd for oxidation of HC, CO and the like. Examples of the alkali metal include Li, Na, K, Rb, Cs, and Fr. Examples of the alkaline earth metal include Be, Mg, Ca, Sr, Ba, and Ra.

  The carrier used in the present invention is for dispersing and holding the catalytically active component, and the material is not particularly limited as long as it can disperse and hold the catalytically active component.

  Further, the material of the substrate having the monolith structure is not particularly limited, but cordierite is preferable because it has excellent thermal shock resistance.

Next, the dense layer used in the present invention will be described.
The dense layer used in the present invention is for suppressing the gas phase diffusion movement of the catalytically active component in the carrier to the substrate, and its chemical reactivity (affinity) with the alkali metal is between the substrate and the alkali metal. And its porosity is lower than that of the substrate. Here, the porosity is defined as the ratio of the pore (void) volume to the total volume of the material. A dense layer having a smaller porosity can effectively prevent gas phase diffusion of the catalytically active component. And as a material which comprises a dense layer, the material which has heat resistance and the reactivity with respect to a catalytically active component is lower than that of a base | substrate is preferable.

Examples of materials that satisfy the above conditions include TiO ₂ , Al ₂ O ₃ (particularly α-alumina), SiC, rare earth metal oxides such as CeO ₂ and La ₂ O ₃ , and alkalis such as SrO, BaO, and CaO. Examples include earth metal oxides, transition metal oxides such as Cr ₂ O ₃ , MnO ₂ , and Fe ₂ O ₃ , zeolites having a low Si / Al ratio (for example, 20 or more), and the like.

  Moreover, you may provide further the barrier which prevents the solid-phase transfer in the support | carrier layer of a catalyst active component (especially alkali metal component) in this support | carrier layer. As such a barrier for preventing the solid phase movement of the catalytically active component in the carrier layer, for example, a microcavity is provided in the carrier layer (that is, a porous carrier), an anchor material or a migration inhibitor is used. And may be added. Thereby, it is possible to achieve suppression of solid phase diffusion (prevention of alkali attack) as well as suppression of vapor phase diffusion of alkali metal in the catalytically active component.

  The exhaust gas purification catalyst of the present invention can be produced as follows. For example, a substrate such as a cordierite honeycomb is immersed or impregnated in a sol, solution or slurry containing the dense layer material (or a precursor thereof) as described above, or they are sprayed onto the substrate, and then as necessary. A dense layer is formed on the surface of the substrate by drying and firing it. Subsequently, the exhaust gas purification catalyst of the present invention can be obtained by holding the carrier carrying the catalytically active component on the dense layer by an ordinary method such as an impregnation method.

  For example, in the case of forming a dense titania layer on a cordierite honeycomb substrate, the cordierite honeycomb is immersed in a solution containing titania sol, pulled up, dried and fired to bind titania to the cordierite surface, and the honeycomb. A dense layer of a titania coat layer can be formed on the cell inner wall. Subsequently, catalyst components such as alkali metal components and noble metals can be supported by an impregnation method or the like to provide an exhaust gas purification catalyst.

  With the above method, an exhaust gas purification catalyst having a dense layer for suppressing movement of NOx trapping components can be produced. As a result, a catalyst capable of maintaining a high NOx purification rate for a long time and a purification device using the catalyst are realized.

Hereinafter, specific examples of the exhaust gas purification catalyst of the present invention will be described with reference to the drawings.
FIG. 1 shows a comparison between an exhaust gas purification catalyst not provided with a dense layer (A: conventional catalyst) and an exhaust gas purification catalyst provided with a titania dense layer 4 (B: catalyst of the present invention). . In the conventional catalyst (A), an alumina carrier layer 1 carrying a catalytically active component 3 such as a NOx supplement component (for example, an alkali metal or alkaline earth metal such as Na, K, or Cs) directly on the cordierite substrate 2 is directly provided. In contrast, in the catalyst (B) of the present invention, a dense layer 4 made of titania exists between the cordierite substrate 2 and the alumina support layer 1. In the catalyst (B) of the present invention, the gas phase diffusion of the catalytically active component 3 to the substrate 2 is suppressed by the titania dense layer 4. Therefore, the catalyst (B) of the present invention can maintain a high NOx purification rate for a long time.

  FIG. 2 is an example of an exhaust gas purification catalyst using calcia (CaO) or α-alumina instead of titania as the dense layer in the catalyst (B) of the present invention described in FIG.

  FIG. 3 shows an example of an exhaust gas purification catalyst having a dense layer between the substrate 2 and the carrier layer 1 and further provided with a void (microcavity) 7 in the carrier layer 1. As a result, not only gas phase diffusion of the catalytically active component to the substrate but also movement due to solid phase diffusion can be suppressed, and a high NOx purification rate can be maintained for a long time.

  In FIG. 4, instead of the gap 7 in FIG. 3, an anchor material or a movement suppressing substance 8, which is a component that suppresses the movement of the catalytically active component to the base 2 due to solid phase diffusion, is provided in the carrier layer 1. It is an example of an exhaust gas purification catalyst. As a result, not only gas phase diffusion of the catalytically active component to the substrate but also movement due to solid phase diffusion can be suppressed, and a high NOx purification rate can be maintained for a long time.

  Examples of the exhaust gas purifying catalyst of the present invention will be described below, but the present invention is not limited to these examples.

(Example 1)
The exhaust gas purification catalyst of the present invention was produced as follows.
The titania sol was supported on a cordierite honeycomb until the supported amount reached 20 g / L. After that, an alumina slurry composed of alumina, nitric acid, an alumina precursor and purified water is coated on a cordierite honeycomb until the supported amount of alumina becomes 190 g / L, dried and fired to produce an alumina-coated honeycomb having a dense layer. did. The honeycomb thus prepared was impregnated and fired with a noble metal solution mainly composed of dinitrodiammine Pt solution and dinitrodiammine Pd solution, and further impregnated with a NOx trapping component solution mainly composed of Na and K.

(Comparative Example 1)
An exhaust gas purification catalyst was produced in the same manner as in Example 1 except that the dense layer was not provided.
Thermal durability test About each honeycomb catalyst of the said Example 1 and the comparative example 1, the thermal durability process of 830 degreeC x 60 hr was performed in the air atmosphere furnace. Each catalyst after the thermal endurance test was evaluated for NOx purification performance using a model gas test apparatus. A model gas simulating an actual vehicle stoichiometric combustion exhaust gas was circulated in the catalyst, and then switched to a model gas simulating lean burn combustion. After switching from stoichiometric gas to lean gas, the NOx purification rate of the catalyst after 1 minute was calculated by the following equation.
NOx purification rate (%) = (1− (NOx concentration at catalyst outlet / NOx concentration at catalyst inlet)) × 100

  At this time, the gas temperature was changed between 300-500 degreeC, and the temperature dependence of catalyst performance was evaluated. An example of the evaluation is shown in FIG. As is clear from FIG. 5, the NOx purification rate when the catalyst of Example 1 was used exceeded the NOx purification rate when the catalyst of Comparative Example 1 was used in all temperature regions of 300 to 500 ° C. . An improvement in purification rate of up to about 15% was observed. This is a result of suppressing the movement of the alkali metal component to the monolith substrate. Thus, a catalyst having excellent heat durability can be obtained by a relatively easy method.

  The exhaust gas purifying catalyst of the present invention is useful as a catalyst for purifying carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) contained in gas discharged from an internal combustion engine. In particular, it is useful as a lean NOx catalyst for purifying NOx in exhaust gas of an internal combustion engine that is burned at an air / fuel ratio higher than the stoichiometric air / fuel ratio.

It is a figure which shows the difference in a structure of the conventional exhaust-gas purification catalyst and the exhaust-gas purification catalyst of this invention. It is a figure which shows an example of the exhaust-gas purification catalyst of this invention. It is a figure which shows an example of the exhaust-gas purification catalyst of this invention. It is a figure which shows an example of the exhaust-gas purification catalyst of this invention. It is a figure showing the purification characteristic of the catalyst by this invention.

Explanation of symbols

1 ... Support for supporting catalytically active components (alumina layer, etc.)
2 ... Substrate (cordierite, etc.)
3 ... Catalyst active component (NOx trapping component, etc.)
4 ... Dense layer (titania layer)
5 ... Dense layer (calcia layer)
6 ... dense layer (α-alumina layer)
7 ... Void (microcavity)
8 ... Anchor material

Claims (7)

  A catalytic active component; a carrier layer for supporting the catalytic active component; and a substrate having a monolith structure for holding the carrier, wherein the chemical reactivity of the alkali metal is between the carrier layer and the substrate. Exhaust gas for internal combustion engines provided with a dense layer having a porosity lower than the chemical reactivity of the base and the alkali metal and lower than the porosity of the base and suppressing the gas phase diffusion movement of the catalytically active component Purification catalyst.   The exhaust gas purification catalyst for an internal combustion engine according to claim 1, wherein the catalytically active component contains at least one alkali metal. The dense layer is selected from the group consisting of TiO ₂ , Al ₂ O ₃ , SiC, rare earth metal oxides, alkaline earth metal oxides, transition metal oxides, and zeolites having a Si / Al ratio of 20 or more. The exhaust gas purification catalyst for an internal combustion engine according to claim 2,   The exhaust gas purification catalyst for an internal combustion engine according to any one of claims 1 to 3, wherein the catalytically active component is a lean NOx catalyst.   The exhaust gas purification catalyst for an internal combustion engine according to any one of claims 1 to 4, further comprising a barrier in the carrier layer that hinders movement of the catalytically active component in the carrier layer.   6. The exhaust gas purifying catalyst for an internal combustion engine according to claim 5, wherein the barrier is a microcavity, an anchor material or a movement suppressing substance.   An exhaust gas purification apparatus for an internal combustion engine, comprising the exhaust gas purification catalyst for an internal combustion engine according to any one of claims 1 to 6.

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