JP3874246B2 - Filter type catalyst for diesel exhaust gas purification - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a diesel exhaust gas purification catalyst that collects particulates (particulate matter) contained in exhaust gas from a diesel engine and purifies harmful components in the exhaust gas.
[0002]
[Prior art]
As for gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology that can cope with it. However, because diesel engines emit harmful components as particulates (particulate substances: carbon fine particles, sulfur fine particles such as sulfate, and high molecular weight hydrocarbon fine particles), regulations and technological progress are both gasoline. It is late compared to the engine.
[0003]
As exhaust gas purification devices for diesel engines that have been developed so far, a trap type exhaust gas purification device (wall flow) and an open type exhaust gas purification device (straight flow) are known. Among these, as a trap type exhaust gas purification device, a ceramic plug-type honeycomb body (diesel particulate filter (hereinafter referred to as DPF)) is known. This DPF includes a gas inflow side cell clogged with an opening at a cell downstream end of a ceramic honeycomb structure, a gas outflow side cell with a clogged opening at a cell upstream end, a gas inflow side cell, and a gas outflow side cell. And has a cell partition wall that serves as a filter during gas distribution, and the exhaust gas is filtered through the pores of the cell partition wall to collect particulates in the cell partition wall, thereby suppressing discharge.
[0004]
In recent years, a continuous regeneration type DPF has been developed in which a coating layer is formed from alumina or the like on the cell partition of the DPF, and an oxidation catalyst such as platinum (Pt) is supported on the coating layer. According to this continuous regeneration type DPF, the collected particulates are oxidized and burned at a relatively low temperature by the oxidation catalyst, so that the DPF is burned simultaneously with the collection or continuously with the collection. Can be played. This continuous regeneration type DPF has the advantage that the catalytic reaction occurs at a relatively low temperature and combustion is possible while the amount of collected particulates is small, so that the thermal stress acting on the DPF is small and damage due to heat is prevented. is there.
[0005]
As such a continuous regeneration type DPF, various types are known. For example, in Japanese Patent No. 3012249, a catalyst monolith body that generates NO _x from exhaust gas and a DPF are separately provided, the catalyst monolith body is disposed upstream of the filter, and the NO _x generated therein is collected by the downstream DPF. An apparatus for removing particulates that oxidizes particulates is disclosed. In Japanese Patent Publication No. 7-106290, a DPF is coated with an oxidation catalyst in which a platinum group metal is supported on an alkaline earth metal oxide, and a reaction between a solid and a solid occurring between the particulate and the catalyst metal is described. A utilized particulate continuous oxidation filter is disclosed.
[0006]
However, even in such a continuous regeneration type DPF, the oxidation rate of the particulates has not been sufficient yet.
[0007]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and an object of the present invention is to obtain a diesel exhaust gas purifying filter type catalyst having an improved particulate oxidation rate.
[0008]
[Means for Solving the Problems]
A filter type catalyst for purifying diesel exhaust gas according to the present invention that solves the above problems is a ceramic honeycomb structure, comprising a gas inflow hole, a gas outflow hole, the gas inflow hole, and the gas outflow hole. A filter body having a filter partition wall to be a filter at the time of,
The surface defining the at least the gas inlet of the filter septum, ZrO _2, zeolite, spinel, and at least one of the NO _X adsorbent is selected from the MgAl ₂ O _4, the oxidation catalyst material carried on the NO _X adsorbent And an exhaust gas purification layer containing at least a porous oxide containing Ce element.
[0009]
The exhaust gas purification layer constituting the diesel exhaust gas filter type catalyst has a NO _x adsorbent constituting the exhaust gas purification layer that adsorbs NO _x at a low temperature, desorbs NO _x at a high temperature, and the desorbed NO _x becomes a particulate. The curate is oxidized and removed, and NO _x is reduced to N ₂ and purified.
[0010]
Further, the oxidation catalyst material supported on the NO _x adsorbent is preferably a noble metal, and the noble metal is more preferably at least one selected from Pd and Pt. The noble metal is preferably colloidal, and the noble metal is preferably a Pd—Pt composite colloid.
[0011]
Furthermore, as the porous oxide containing at least the Ce element, an oxide containing the Ce element, a composite oxide, and a mixture thereof can be used. The oxidation catalyst material refers to a metal having an oxidation catalyst ability such as a noble metal or a transition metal, and the exhaust gas purification layer includes at least one selected from alkali metals, alkaline earth metals, and rare earth elements together with the oxidation catalyst material. It is preferable.
[0012]
The oxidation catalyst material supported on the porous oxide is more preferably a transition metal, and the NO _x adsorbent is further added with at least one selected from La, K, and Ca. preferable.
Further, the NO _x adsorbent is preferably ZrO ₂ .
The exhaust gas purification layer includes an oxidation catalyst layer formed on the filter body, and an NO _x adsorption catalyst layer formed on the oxidation catalyst layer. The oxidation catalyst layer includes the porous oxide and and a said oxidation catalyst material, NO _X adsorbing catalyst layer preferably contains the above-described NO _X adsorbent and the oxidizing catalyst material.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The filter catalyst for diesel exhaust gas purification of the present invention has a filter body and an exhaust gas purification layer.
[0014]
The filter main body is a ceramic honeycomb structure, and has a gas inflow hole, a gas outflow hole, and a filter partition wall that divides these and serves as a filter during gas flow.
[0015]
Here, the ceramic honeycomb structure is composed of a honeycomb-shaped cell composite having a cell diameter of about 31 to 62 cells / cm ² , and each cell of the cell composite is formed with cell holes surrounded by filter partition walls. Is done. The filter partition has a wall thickness of about 0.15 mm to 0.43 mm.
[0016]
The gas inflow hole is a cell hole that serves as an inlet when the exhaust gas enters the filter body. The cell upstream end located on the exhaust gas upstream side is opened, and the cell downstream end located on the exhaust gas downstream side is clogged. This is a closed cell hole. The gas outflow hole is a cell hole that becomes an outlet of the exhaust gas when the exhaust gas flows through the filter body, and is a cell hole in which the cell upstream end is clogged and closed, and the cell downstream end is opened. The exhaust gas that has entered the filter body from the gas inflow hole passes through the filter partition, is purified, and is discharged from the gas outflow hole.
[0017]
The filter body is formed of heat-resistant ceramics, and a filter body made by a conventional method such as extrusion molding can be used. Specifically, a commercially available honeycomb-type ceramic DPF may be used, and a commonly used heat-resistant ceramic raw material may be used as the raw material.
[0018]
Exhaust gas purification layer of the present invention is formed on a surface defining at least a gas inlet of the filter septum comprises a porous oxide, an oxidation catalyst material and a NO _X adsorbent.
[0019]
Exhaust gas purification layer, can either take the one-layer structure in which a catalyst layer is formed of one layer comprising a porous oxide oxidation catalyst material and NO _X adsorbent, the oxidation catalyst material is supported on a porous oxide It is also possible to take a multilayer structure of two or more layers in which the oxidation catalyst layer and the NO _x adsorption catalyst layer in which the oxidation catalyst material is supported on the NO _x adsorbent are laminated.
[0020]
The porous oxide contains at least a Ce element, and oxides, composite oxides, and mixtures thereof can be used. The porous oxide can form an oxidation catalyst layer by supporting an oxidation catalyst material. The porous oxide containing Ce oxidizes the particulate itself.
[0021]
The oxidation catalyst material supported by the porous oxide can be a metal having an oxidation catalytic ability such as a noble metal such as Pt, Pd, or Rh, or a transition metal such as Fe, Co, Mn, or Ni. It is preferable to use a metal.
[0022]
In addition to such oxidation catalyst material and the porous oxide in an exhaust gas purification layer, as _the NO _X storage material, an alkali metal, alkaline earth K, Na, Li, Cs, Ba, Ca, La, Y , etc. It is also preferable to include at least one selected from metals and rare earth elements to impart NO _x storage reduction catalytic action.
[0023]
The exhaust gas purification layer is preferably coated with 50 to 300 g per liter of the filter body. If it exceeds this, the pressure loss of the filter increases, and if it is less than this, the activity of the catalyst decreases.
[0024]
NO _X adsorbent, ZrO _2, zeolite, spinel, those having one or acidic and basic amphoteric with basic such as MgAl ₂ O _4, the NO _X in a high temperature range adsorbs NO _X in a low temperature range Those that desorb can be used. For example, ZrO ₂ has the property of adsorbing NO _{x in} the temperature range from room temperature to 300 ° C. and desorbing NO _{x in} the temperature range from 300 ° C. to 400 ° C. For reference, the NO _x desorption characteristics of ZrO ₂ are shown in FIG.
[0025]
Moreover, La in the NO _X absorbent, K, it is preferable to add at least one Ca. In particular, it is preferable to add to ZrO ₂ . By adding La, K, and Ca, the durability of the NO _x adsorbent is improved.
[0026]
As the oxidation catalyst material supported on the NO _x adsorbent, it is preferable to use a noble metal having an oxidation catalyst ability such as Pt, Pd, Ag. Such as Pd, in turn have a NO _X adsorbing capacity in itself, if it is desired to further improve the NO _X adsorbing capacity is preferably used Pd. Further, the oxidation catalyst material is preferably used in the state of either one or a mixture of ammine-based Pt and nitric acid Pd, or a colloid Pt, a Pt-Pd composite colloid, or a mixture of Pt colloid and Pd nitrate. It is more preferable to use in this state.
[0027]
Colloidal Pt and Pd are supported in a state close to that of metal. Therefore, even when the Pt and Pd are supported and exposed to high temperature conditions such as by calcination, it is possible to avoid a problem that the catalytic activity decreases due to oxidation, so that higher activity can be maintained.
[0028]
FIG. 2 shows an example of the oxidation form of particulates by the oxidation catalyst material and the NO _x adsorbent in the exhaust gas purification layer of the present invention. Particulates in the exhaust gas flow into the filter body from the upstream end of the cell through the gas inflow hole 1 and are collected on the filter partition wall 2. On the other hand, NO in the exhaust gas is converted to NO ₂ by the oxidation reaction being promoted by the catalytic action of the oxidation catalyst material of the exhaust gas purification layer 3. The NO ₂ produced here is adsorbed by the NO _x adsorbent in the low temperature range and desorbed in the high temperature range. The desorbed NO ₂ acts as a gas oxidant that oxidizes the particulates, and reacts with the particulates to generate N ₂ and CO ₂ to oxidize and burn the particulates. N ₂ and CO ₂ produced here pass through the filter partition along the gas flow and are discharged from the gas outflow hole.
[0029]
Further, the oxidation catalyst material promotes the reaction of generating CO ₂ from the particulate in contact with the metal having oxidation catalytic ability under high temperature conditions and O ₂ to oxidize and burn the particulate. Moreover, the porous oxide containing Ce also oxidizes the particulates. The activity of the oxidation catalyst material is further increased by the heat generated at this time, and the oxidation combustion efficiency of the particulates is improved.
[0030]
Here, if _the NO _X storage reduction catalysis include _the NO _X storage material in an exhaust gas purification layer is applied, the oxidation catalyst material promotes the oxidation reactions that produce NO _x from NO, generated NO _X is stored in the NO _X storage material under lean air-fuel ratio atmosphere, it desorbed under the air-fuel ratio rich atmosphere. The NO _x desorbed at this time also becomes a gaseous oxidant that oxidizes the particulates.
[0031]
This arrangement, NO ₂ generated from NO in the exhaust gas in the temperature range in which the continuous oxidation less hard done particulate, adsorbed in the NO _X adsorbing layer by NO _X adsorbent is particulate continuous oxidation is performed Easily desorbs at a high temperature range. The desorbed NO ₂ is involved in particulate oxidation as a gaseous oxidant. Thus, the diesel exhaust gas purification filter catalyst of the present invention can efficiently utilize NO ₂ generated from NO in the exhaust gas.
[0032]
Here, since NO ₂ has a higher oxidizing power than O _2, it is effective in the particulate oxidation reaction to use NO ₂ as a gaseous oxidant.
[0033]
Further, it is advantageous to improve the particulate oxidation rate if the distance between NO ₂ which is a gas oxidant and the particulate is shorter. In the present invention, NO ₂ desorbed from the NO _X adsorbent is desorbed very close to the particulates, so the time from NO ₂ generation to particulate oxidation is shortened, and the particulates are efficiently oxidized. easy. In the present invention also has a porous oxide adjacent containing NO _X adsorbing and oxidizing catalyst material and Ce, between each, for the transmission of temperature and generated gas is rapidly performed, the particulate oxidation rate improves. Further, in the present invention, as an oxidant for burning particulates, not only a solid oxidation catalyst made of an oxidation catalyst material but also a gas oxidant made of NO ₂ is used. Both oxidation reactions between gases can be used. As a result, the particulate oxidation reaction is performed at a higher frequency.
[0034]
(Operation of the invention)
In the filter catalyst for exhaust gas purification according to the present invention, the NO _x adsorbent constituting the exhaust gas purification layer adsorbs NO _x at a low temperature, desorbs NO _x at a high temperature, and the desorbed NO _x oxidizes particulates. At the same time, the oxidation catalyst material and further the porous oxide containing Ce oxidize the particulates.
[0035]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
<Example>
Diesel exhaust gas purifying filter catalyst of the present embodiment includes a filter body, and the oxidation catalyst layer formed on the filter body, the NO _X adsorbing catalyst layer formed on the oxidation catalyst layer.
[0036]
As the filter body, a commercially available DPF made of honeycomb cordierite was used. This filter body has a cross section of 30 mm × 30 mm and a length of 50 mm, a volume of 35 ml, a wall thickness of 304.8 μm, a porosity of 60%, and a cell density of 47 cells / cm ² .
[0037]
The oxidation catalyst layer contains CeO ₂ having an average particle size of 0.1 μm as a porous oxide and Fe as an oxidation catalyst material. This oxidation catalyst layer is formed in a filter partition that defines gas inflow holes of the filter body, and holds 150 g of CeO ₂ per liter of the filter body and 5 g of Fe per liter of the filter body.
[0038]
The NO _x adsorption catalyst layer contains ZrO ₂ as a NO _x adsorbent and a Pt—Pd composite noble metal as an oxidation catalyst material. This NO _x adsorption catalyst layer is formed on the oxidation catalyst layer, holds 120 g of ZrO ₂ per liter of filter body volume, and carries 1 g of Pt and Pd per liter of filter body volume.
[0039]
The manufacturing method of the filter type catalyst for diesel exhaust gas purification of an Example is shown below.
[0040]
A commercially available DPF made of honeycomb cordierite was prepared as the filter body. Then, a solution containing 40% by weight of CeO ₂ powder having an average particle size of 0.1 μm is prepared, injected into the gas inflow hole of the filter body, and then vacuum-suctioned from the gas outflow hole to thereby remove CeO ₂ in the filter partition wall. Held inside and on the surface. Thereafter, the filter partition walls were dried at 120 ° C. and baked at 500 ° C. for 120 minutes, whereby CeO ₂ was supported on the filter partition walls. Next, an Fe solution was prepared. The filter was immersed with this solution maintained at a temperature of 90 ° C., and Fe was supported on CeO ₂ of the filter partition wall. Then, it dried at 120 degreeC and baked at 500 degreeC for 120 minutes, and formed the oxidation catalyst layer.
[0041]
Next, a solution containing 40% by weight of ZrO ₂ powder was prepared, held in the oxidation catalyst layer of the filter partition wall as described above, dried at 120 ° C., and calcined at 500 ° C. for 120 minutes to form on the oxidation catalyst layer. ZrO ₂ was supported. Further, a Pt—Pd composite noble metal solution composed of colloid Pt and nitrate Pd was prepared, supported on ZrO ₂ in the same manner as described above, dried at 120 ° C., and calcined at 500 ° C. for 120 minutes for NO _x adsorption catalyst layer. Formed.
[0042]
By the above method, the filter type | mold catalyst for diesel exhaust gas purification of the Example was obtained. In addition, the filter type | mold catalyst for diesel exhaust gas purification of a present Example is a small thing for using for the evaluation test of the particulate oxidation rate mentioned later.
<Comparative example>
The filter type catalyst for purifying diesel exhaust gas of this comparative example was obtained by not forming the NO _x adsorption catalyst layer in the production process of the filter type catalyst for purifying diesel exhaust gas of the example. Is the same as the example. That is, the diesel exhaust gas purifying filter type catalyst of this comparative example has the same filter body as that of the example and an oxidation catalyst layer formed on the filter body. The oxidation catalyst layer contains CeO ₂ as a porous oxide and Fe as an oxidation catalyst material.
[0043]
<Test and evaluation>
An evaluation test of the particulate oxidation rate was performed using a particulate generator for the diesel exhaust gas purifying filter-type catalyst in the examples and comparative examples.
[0044]
First, each diesel exhaust gas filter type catalyst of Examples and Comparative Examples is attached to a particulate generator, containing 20% O ₂ , 3% H ₂ O and 600 ppm NO, and 100% N ₂ The gas was circulated at 100 ° C. for 30 minutes. After 30 minutes, the temperature was raised to 350 ° C. in N ₂ , and after the temperature was stabilized, it contained 20% O ₂ and 3% H ₂ O, and was made 100% with N ₂ . Particulate was added to the gas and allowed to flow through each of the diesel exhaust gas purifying filter catalysts.
[0045]
At this time, about 1 g of particulates per liter of the filter body is captured in advance in the filter type catalyst for purifying diesel exhaust gas of Examples and Comparative Examples, and then the particulates are added little by little. By doing so, the addition amount of particulates and the combustion amount were equal, and the pressure loss did not increase or decrease, and the pressure loss was balanced.
[0046]
The particulate oxidation rate was measured in the filter catalyst for purifying diesel exhaust gas of Examples and Comparative Examples in which the pressure loss was balanced.
[0047]
The particulate oxidation rate per 1 L of the volume of the filter body per hour (h) in the filter catalyst for diesel exhaust gas purification in a state where the pressure loss was balanced was defined as the particulate oxidation rate. The volume of the filter body represents the volume of the filter body. The particulate oxidation rate is
Ma = particulate oxidation rate (g / h · L),
e = Particulate collection efficiency (%)
M = Particulate inflow to the filter catalyst for purification of diesel exhaust gas when pressure loss is balanced (g / h · L)
Ma = e · M is satisfied.
[0048]
FIG. 3 shows the particulate oxidation rates of the filter catalysts for purifying diesel exhaust gas of Examples and Comparative Examples obtained by the above experiments. The filter exhaust catalyst for purifying diesel exhaust gas of the example in which the NO _x adsorption layer was formed on the oxidation catalyst layer had a particulate oxidation rate higher than the filter catalyst for purifying diesel exhaust gas of the comparative example in which only the oxidation catalyst layer was formed. Improved by about 30%. Efficiency This, NO ₂ adsorbed to ZrO ₂ of the NO _X adsorbent by the particulate continuous oxidation desorbed in performed easily temperature range, the generated NO ₂ is a gaseous oxidizing agent in the particulate matter oxidation reaction It seems to work well and improve the particulate oxidation rate.
[0049]
【The invention's effect】
According to the diesel exhaust gas purifying filter type catalyst of the present invention, it is possible to improve the particulate oxidation rate by oxidizing the particulates by efficiently using NO _x in the exhaust gas.
[Brief description of the drawings]
FIG. 1 is a graph showing NO _x desorption characteristics of a NO _x adsorbent.
FIG. 2 is an explanatory view schematically showing an example of an oxidation form of particulates by an oxidation catalyst material and a NO _x adsorbent in the diesel exhaust gas purification filter of the present invention.
FIG. 3 is a graph showing the particulate oxidation rate of a filter catalyst for purifying diesel exhaust gas in Examples and Comparative Examples.

Claims (11)

A ceramic honeycomb structure having a gas inflow hole, a gas outflow hole, a filter main body having a filter partition wall that divides the gas inflow hole and the gas outflow hole and serves as a filter during gas flow;
The surface defining the at least the gas inlet of the filter septum, ZrO _2, zeolite, spinel, and at least one of the NO _X adsorbent is selected from the MgAl ₂ O _4, the oxidation catalyst material carried on the NO _X adsorbent And a filter type catalyst for purifying diesel exhaust gas, comprising an exhaust gas purifying layer containing at least a porous oxide containing Ce element. The NO _X diesel exhaust gas purifying filter catalyst of claim 1 supported oxidation catalyst material adsorbent, which is a noble metal.   The filter catalyst for diesel exhaust gas purification according to claim 2, wherein the noble metal is at least one selected from Pd and Pt.   The filter catalyst for purifying diesel exhaust gas according to claim 3, wherein the noble metal is colloidal.   The filter catalyst for purifying diesel exhaust gas according to claim 4, wherein the noble metal is a Pd-Pt composite colloid.   The diesel exhaust gas purification filter catalyst according to any one of claims 1 to 5, wherein the exhaust gas purification layer further includes an oxidation catalyst material supported on the porous oxide. The filter catalyst for diesel exhaust gas purification according to claim 6, wherein the oxidation catalyst material supported on the porous oxide is made of Fe which is a transition metal. The NO _X in the adsorbent, La, K, at least one the claims 1 to diesel exhaust gas purifying filter catalyst according to claim 7, characterized in that it is added selected from Ca.   The diesel exhaust gas purification filter type according to any one of claims 1 to 8, wherein the exhaust gas purification layer further contains at least one selected from alkali metals, alkaline earth metals, and rare earth elements. catalyst. NO _X Adsorbent is ZrO ₂ The filter catalyst for purification of diesel exhaust gas according to any one of claims 1 to 9, wherein The exhaust gas purification layer includes an oxidation catalyst layer formed on the filter body and NO formed on the oxidation catalyst layer. _X An adsorption catalyst layer,
The oxidation catalyst layer includes the porous oxide and the oxidation catalyst material,
The NO _X The adsorption catalyst layer is the NO _X The filter type catalyst for diesel exhaust gas purification according to any one of claims 1 to 10, comprising an adsorbent and the oxidation catalyst material.

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