JP5628487B2 - Nitrogen oxide purification method - Google Patents

Description

  The present invention relates to a nitrogen oxide purification method for purifying nitrogen oxide contained in exhaust gas discharged from an internal combustion engine using a nitrogen oxide purification catalyst.

In recent years, nitrogen oxides in exhaust gas discharged into the atmosphere from internal combustion engines such as generators and automobiles have been regarded as a problem from the viewpoint of suppressing emission of harmful substances. Since nitrogen oxides cause acid rain and photochemical smog, their emissions are regulated worldwide.
By the way, nitrogen oxides are purified by a reduction reaction. However, in an internal combustion engine such as a diesel engine or a gasoline lean burn engine where lean combustion is performed, a large amount of oxygen is present in the exhaust gas. Reduction of things is not easy. For this reason, various studies have been made to purify nitrogen oxides in lean combustion internal combustion engines.

For example, in a lean combustion internal combustion engine, after storing nitrogen oxides as nitrate radicals through precious metal active species in a state where oxygen is high, the fuel is injected to form a state where oxygen is temporarily low, thereby storing the stored nitric acid. Methods for reducing roots are being investigated.
However, this method has a problem that the occlusion reaction does not occur unless the temperature is at least about 200 ° C. because the occlusion of nitrogen oxides passes through the oxidation reaction by the noble metal active species. Further, in order to improve the purification rate, it is necessary to inject a large amount of fuel, which causes problems such as deterioration in fuel consumption and contamination due to an increase in hydrocarbon components in the exhaust.

In addition, selective reduction that injects a reducing agent such as hydrocarbon, urea, carbon monoxide, or hydrogen into exhaust containing a large amount of oxygen, and continuously purifies nitrogen oxides contained in the exhaust on the catalyst. Catalytic methods are being considered.
However, in this method, when hydrocarbon or urea (usually used after being decomposed into ammonia) is used as a reducing agent, a minimum of about 200 ° C. is required for the selective reduction reaction to proceed on the catalyst. (For example, see Non-Patent Documents 1 and 2).

In view of this, a selective reduction catalyst method using hydrogen as a reducing agent, which has good nitrogen oxide purification efficiency in a low temperature region, is being studied.
For example, in the method disclosed in Patent Document 1, excess oxygen and hydrogen are contained in a nitrogen oxide purification catalyst that includes a porous carrier and a noble metal element and molybdenum supported on the porous carrier. Nitrogen oxides are purified by contacting exhaust. Thereby, it is supposed that nitrogen oxides in the exhaust gas containing excess oxygen exceeding the stoichiometric ratio necessary for oxidizing the components to be oxidized in the exhaust gas can be reduced and purified.

Moreover, as a method of purifying nitrogen oxides using hydrogen as a reducing agent, for example, the method of Patent Document 2 can be mentioned. In the method of Patent Document 2, nitrogen oxides are reduced and removed using hydrogen as a reducing agent in an oxidizing atmosphere containing SO ₂ and 1% or more of oxygen. As the catalyst, a nitrogen oxide purification catalyst made of silica containing 0.05 to 10% by mass of rhodium is used.

  Moreover, as a method of purifying nitrogen oxides using hydrogen as a reducing agent, for example, the method of Patent Document 3 can be mentioned. In the method of Patent Document 3, oxygen-excess exhaust containing hydrogen is supplied to a nitrogen oxide purification catalyst in which platinum and cesium are supported on a porous carrier. And this nitrogen oxide purification catalyst and oxygen excess exhaust gas containing hydrogen are made to contact on the conditions of 250-600 degreeC. Thereby, it is supposed that the nitrogen oxide in oxygen excess exhaust can be purified.

On the other hand, studies on techniques for enhancing the functionality of a nitrogen oxide purification catalyst by, for example, laminating two or more catalyst layers are also being conducted.
For example, a nitrogen oxide purification catalyst (see Patent Document 4) in which a second catalyst layer containing iridium as an active metal is provided on a first catalyst layer containing a noble metal such as platinum, rhodium, and palladium, or two or more catalysts. The nitrogen oxide purification catalyst (refer patent document 5) which has arrange | positioned this in series with the exhaust passage is made | formed.
In addition, for example, by providing a catalyst layer with oxidation performance on the carbon monoxide shift layer, the reaction conditions under which the carbon monoxide shift reaction proceeds easily by reducing the oxygen concentration can be generated efficiently. Thus, a nitrogen oxide purification catalyst (see Patent Document 6) for purifying nitrogen oxide has been studied.
In addition, for example, nitrogen that efficiently reduces and purifies nitrogen oxides by providing an upper oxide layer that removes part or all of the carbon monoxide in the exhaust gas before the nitrogen oxide purifying catalyst contacts the exhaust gas. An oxide purification catalyst (see Patent Document 7) has been studied.

Japanese Patent No. 3382361 JP 2003-33654 A JP 2001-170454 A JP-A-8-52365 Japanese Patent Laid-Open No. 11-47605 Japanese Patent Laid-Open No. 2005-884 JP 2007-239616 A

Development of Urea-SCR System for Heavy-Duty Commercial Vehicles HC-SCR of NOx over Ag / alumina: a Combination of heterogeneous and homogeneous radical reaction? Inprovements in the N2 selectivity of Pt catalysts in the NO-H2-O2 reaction at low temperatures Lean NOx reduction with CO + H2 mixtures over Pt / Al2O3 and Pd / Al2O3 catalysts

By the way, in the nitrogen oxide purification method using hydrogen as a reducing agent, there is a problem that a temperature range (hereinafter referred to as a temperature window) in which the reduction reaction of nitrogen oxide occurs is narrow (for example, see Non-Patent Document 3).
In general, in a heterogeneous catalyst using a noble metal active species, hydrogen added as a reducing agent burns as the temperature increases in an atmosphere containing a large amount of oxygen. As a result, the hydrogen partial pressure decreases, the hydrogen component adsorbed on the catalyst (hydrogen component contributing to the reaction) decreases, and the purification rate of nitrogen oxides decreases. For this reason, there are restrictions on the temperature at which a high purification rate can be maintained on the high temperature side.
Further, on the noble metal active species, as the temperature increases in an atmosphere containing a large amount of oxygen, nitric oxide is likely to change to nitrogen dioxide. Further, in an atmosphere containing a large amount of oxygen, the reduction reaction of nitrogen dioxide with hydrogen is a reaction in which nitrogen monoxide is reduced to nitrogen, so the nitrogen oxide purification rate increases as the partial pressure of nitrogen dioxide increases. Will drop. Also from this point, the temperature at which a high purification rate can be maintained on the high temperature side is limited.
Furthermore, although heterogeneous catalysts using precious metal active species tend to adsorb nitrogen oxides and hydrogen at low temperatures, they lack the thermal energy (activation energy) necessary for the reaction between nitrogen oxides and hydrogen. Does not react. For this reason, the temperature at which the reduction reaction occurs is limited on the low temperature side.

Further, the nitrogen oxide purification method using hydrogen as a reducing agent is greatly affected by carbon monoxide contained in the exhaust gas, and there is a problem that the above-described temperature window is further narrowed.
Normally, carbon monoxide is more easily adsorbed to the active sites on the noble metal surface than carbon, and carbon monoxide is preferentially adsorbed on the active sites on the noble metal surface. The purification reaction by hydrogen at the point becomes difficult to occur. The adsorption behavior of hydrogen and carbon monoxide is competitive adsorption. When hydrogen and carbon monoxide coexist, the carbon monoxide does not cover all the active sites on the surface of the noble metal, but the amount of carbon monoxide present When this increases, the purification reaction of nitrogen oxides by hydrogen is suppressed (for example, see Non-Patent Document 4).
On the other hand, it is considered that by increasing the hydrogen concentration, poisoning by carbon monoxide can be suppressed, and the temperature window in which nitrogen monoxide can be selectively reduced is widened. However, since hydrogen easily burns and has a low explosion limit, its concentration is limited. Further, in the exhaust gas containing excess oxygen, most of the hydrogen is consumed for the reaction with oxygen even if the hydrogen concentration is a certain value or higher in the temperature range where oxygen is activated.

Focusing on the above problems, for example, the method of Patent Document 1 does not have a function of removing carbon monoxide inevitably contained in the exhaust gas discharged from the internal combustion engine using hydrocarbon fuel. However, nitrogen oxides cannot be efficiently purified over a wide temperature window.
Further, in the method of Patent Document 2, carbon monoxide, which is an inevitable component in exhaust gas, is not considered at all, and nitrogen oxides cannot actually be purified in a low temperature range of 200 ° C. or lower. In particular, rhodium used in Patent Document 2 is more likely to adsorb carbon monoxide than platinum (for example, one molecule of carbon monoxide adsorbs to one platinum atom, whereas rhodium, Narrowing of the temperature window due to carbon monoxide poisoning is inevitable.
Further, in the method of Patent Document 3, when the exhaust temperature is 200 ° C. or higher, hydrogen is burned as described above, and the reduction reaction of nitrogen oxides is difficult to proceed, and nitrogen monoxide is changed to nitrogen dioxide. As a result, it becomes difficult to reduce with nitrogen. As a result, nitrogen oxides cannot be purified efficiently over a wide temperature window.

On the other hand, even in the technology in which the functions of the nitrogen oxide purification catalyst such as Patent Documents 4 to 7 are enhanced, for example, Patent Document 4 has a very high nitrogen oxide purification temperature, and Patent Document 5 has a large catalyst volume and a layout. There are problems such as restrictions on the temperature and a large heat capacity, resulting in a temperature difference between the catalysts, making it difficult to wrap the region that can be cleaned, and efficiently purifying nitrogen oxides over a wide temperature window. I can't.
Further, in Patent Document 6, the shift reaction is generally highly dependent on oxygen concentration, and the progress of the shift reaction is extremely difficult when several percent of oxygen is present with respect to about 0.1% of carbon monoxide in the exhaust gas. It is. Even if oxygen is removed by combustion, it is necessary to introduce a considerable amount of a reducing agent, and it is difficult to purify nitrogen oxides efficiently over a wide temperature window.
In Patent Document 7, hydrocarbons are trapped at the outermost layer in order to strongly adsorb on the catalyst in a low temperature range, and not only inhibit the combustion reaction of carbon monoxide but also selectively reduce nitrogen oxides in the lower layer. As a result, the nitrogen oxides cannot be efficiently purified over a wide temperature window.

  The present invention has been made in view of the above problems, and its purpose is to provide nitrogen oxides in exhaust gas containing excess oxygen in excess of the stoichiometric ratio necessary for the oxidation of the components to be oxidized contained in the exhaust gas. An object of the present invention is to provide a nitrogen oxide purification method capable of efficiently purifying hydrogen over a wide temperature window from a low temperature region to a high temperature region using hydrogen as a reducing agent.

  In order to achieve the above object, an invention according to claim 1 is directed to an exhaust gas discharged from an internal combustion engine using a nitrogen oxide purification catalyst in which a first catalyst layer is laminated on a second catalyst layer formed on a carrier. A method for purifying nitrogen oxides, wherein the first catalyst layer has at least a carbon monoxide removal function, and the second catalyst layer removes nitrogen oxides using hydrogen as a reducing agent. A nitrogen oxide purification catalyst having at least a function is used, and in the exhaust gas containing excess oxygen at a stoichiometric ratio or more necessary for oxidation of an oxidizable component contained in the exhaust gas, and hydrogen and carbon monoxide, By disposing the nitrogen oxide purification catalyst so that the stacking direction of the layers is orthogonal to the flow direction of the exhaust gas, the nitrogen oxide in the exhaust gas is selectively reduced and purified.

  The invention according to claim 2 is the nitrogen oxide purification method according to claim 1, wherein the temperature at which hydrogen starts to burn in the first catalyst layer is such that carbon monoxide starts to burn in the first catalyst layer. A nitrogen oxide purifying catalyst having a temperature higher than the temperature is used.

  According to a third aspect of the present invention, in the nitrogen oxide purification method according to the first or second aspect, the temperature at which carbon monoxide starts to burn in the second catalyst layer is the same as that in the first catalyst layer. A nitrogen oxide purification catalyst having a temperature equal to or higher than a temperature at which combustion is started is used.

  According to a fourth aspect of the present invention, in the nitrogen oxide purification method according to any one of the first to third aspects, the thickness of the first catalyst layer is smaller than the thickness of the second catalyst layer. It is characterized by using.

  The invention according to claim 5 is the nitrogen oxide purification method according to any one of claims 1 to 4, wherein the first catalyst layer and the second catalyst layer each contain at least one of platinum and palladium. An oxide purification catalyst is used.

  The invention according to claim 6 is the nitrogen oxide purification method according to any one of claims 1 to 5, wherein the first catalyst layer uses a nitrogen oxide purification catalyst containing at least one of cerium oxide and titanium oxide. It is characterized by that.

  The invention according to claim 7 is the nitrogen oxide purification method according to any one of claims 1 to 6, wherein the second catalyst layer is composed of zirconium oxide, alumina, silica, titanium oxide, zeolite, and cerium and zirconium. A nitrogen oxide purification catalyst containing at least one selected from the group consisting of complex oxides is used.

According to the first aspect of the present invention, the upper first catalyst layer has at least a carbon monoxide removing function, and the lower second catalyst layer has at least a nitrogen oxide removing function using hydrogen as a reducing agent. An oxide purification catalyst was used. Further, the nitrogen oxide purification catalyst is used in an exhaust gas containing excess oxygen at a stoichiometric ratio or more necessary for the oxidation of the components to be oxidized contained in the exhaust gas, and hydrogen and carbon monoxide as reducing agents. The stacking direction of the layers was arranged so as to be orthogonal to the flow direction of the exhaust gas.
That is, the present invention directly reduces nitrogen oxides in exhaust in an oxygen-excess atmosphere, and the oxygen concentration contained in the exhaust is much higher than that in exhaust during rich combustion such as a lean burn engine. Despite the large amount, nitrogen oxides can be purified at a high purification rate.
According to the present invention, the following three remarkable effects are exhibited.
As a first effect, it is possible to suppress a decrease in the nitrogen oxide purification rate due to carbon monoxide. In the present invention, since the upper first catalyst layer has a carbon monoxide removing function, the concentration of carbon monoxide in the exhaust gas that reaches the lower second catalyst layer can be reduced. For this reason, the fall of the nitrogen oxide purification rate by the above-mentioned carbon monoxide can be suppressed.
As a second effect, the maximum purification rate of nitrogen oxides can be improved as compared with the conventional case. In the present invention, since carbon monoxide can be reduced in the upper first catalyst layer, the reaction probability of the reduction reaction between nitrogen oxides and hydrogen can be increased in the lower second catalyst layer. For this reason, interference due to competitive adsorption of carbon monoxide on the catalytically active species can be suppressed, and the reduction reaction of nitrogen oxides can be promoted. As a result, the maximum purification rate of nitrogen oxides is improved.
As a third effect, a wider temperature window can be obtained than in the conventional case. As described above, this is mainly due to the fact that the reaction probability between nitrogen oxides and hydrogen is increased in the lower second catalyst layer, and the reduction reaction of nitrogen oxides is promoted. In addition, due to the temperature increase accompanying the oxidation reaction of carbon monoxide in the upper layer, the diffusion of nitrogen oxides becomes dominant and the reaction rate is also improved.
Due to the above three effects, according to the present invention, nitrogen oxides can be efficiently purified in a wide temperature window from a low temperature region to a high temperature region.

According to the second aspect of the present invention, the nitrogen oxide purification catalyst in which the temperature at which hydrogen starts to burn in the first catalyst layer is higher than the temperature at which carbon monoxide starts to burn in the first catalyst layer is used. .
Thereby, for example, in a low temperature range of 200 ° C. or lower, carbon monoxide can be preferentially reacted and removed over hydrogen in the upper first catalyst layer. For this reason, as a result of reducing the carbon monoxide concentration in the exhaust gas that reaches the lower second catalyst layer and increasing the hydrogen concentration, it is possible to suppress the adverse effect of the carbon monoxide on the nitrogen oxide purification rate. Therefore, according to the present invention, it is possible to efficiently purify nitrogen oxides in a wider temperature window from a low temperature region than in the past.

According to the invention of claim 3, a nitrogen oxide purification catalyst is used in which the temperature at which carbon monoxide starts burning in the second catalyst layer is equal to or higher than the temperature at which carbon monoxide starts burning in the first catalyst layer. It was.
Thereby, for example, in the temperature range where the reduction reaction of nitrogen oxide proceeds in the lower second catalyst layer, carbon monoxide can be preferentially reacted and removed in the upper first catalyst layer. As described above, carbon monoxide causes a reduction in the activity of the reduction reaction of nitrogen oxides. Therefore, by removing this carbon monoxide, the nitrogen oxide purification activity in the lower second catalyst layer can be maintained. Therefore, according to the present invention, nitrogen oxides can be purified more efficiently in a wider temperature window from a lower temperature range than in the past.

According to the fourth aspect of the present invention, the nitrogen oxide purification catalyst is used in which the thickness of the first catalyst layer is smaller than the thickness of the second catalyst layer.
As described above, the upper first catalyst layer plays a role of selectively removing carbon monoxide which causes a decrease in the catalytic activity of the lower second catalyst layer. Here, the oxidation reaction of carbon monoxide is a typical differential reaction, and the reaction can proceed efficiently even in a thin layer. From the viewpoint of reaction efficiency, it is important to reduce the thickness of the first catalyst layer and promote the diffusion of hydrogen and nitrogen oxides into the lower second catalyst layer.
Further, since the oxidation reaction of carbon monoxide is a differential reaction, it is not preferable to make the first catalyst layer too thick because a sufficient combustion temperature difference between carbon monoxide and hydrogen cannot be secured. Specifically, when the exhaust gas flow rate is large (in the case of a high space velocity), hydrogen that functions as a reducing agent in the second catalyst layer also burns. In particular, since the exhaust flow rate of an internal combustion engine used in an automobile is fluctuating, it is important to ensure the reduction ability even when the exhaust flow rate decreases.
In this regard, according to the present invention, the thickness of the first catalyst layer is designed to be smaller than the thickness of the second catalyst layer, so that it can be said that nitrogen oxides can be purified more efficiently.

According to the fifth aspect of the present invention, the first catalyst layer and the second catalyst layer each use a nitrogen oxide purification catalyst containing at least one of platinum and palladium.
As a result, it is possible to selectively remove carbon monoxide in the upper first catalyst layer and to reduce and purify nitrogen oxides with hydrogen in the lower second catalyst layer. Nitrogen oxide can be efficiently purified over a wide temperature window.

According to invention of Claim 6, the 1st catalyst layer used the nitrogen oxide purification catalyst containing at least one among cerium oxide and titanium oxide.
As a result, the selective removal of carbon monoxide in the upper first catalyst layer is more effectively performed. As a result, it is possible to efficiently purify nitrogen oxides in a wider temperature window from the low temperature range to the high temperature range as compared with the prior art. .

According to the seventh aspect of the invention, the second catalyst layer contains nitrogen oxide containing at least one selected from the group consisting of zirconium oxide, alumina, silica, titanium oxide, zeolite, and a composite oxide of cerium and zirconium. A material purification catalyst was used.
As a result, the reduction and purification of nitrogen oxides by hydrogen in the lower second catalyst layer is effectively performed. As a result, nitrogen oxides can be efficiently purified in a wider temperature window from the low temperature range to the high temperature range as compared with the conventional case. .

It is a schematic block diagram of the fixed bed catalyst reaction apparatus used by the present Example and the comparative example. It is a figure which shows the catalyst installation method of a fixed bed catalyst reaction apparatus. It is a figure which shows the evaluation result of Example 1. FIG. It is a figure which shows the evaluation result of Example 2. It is a figure which shows the evaluation result of Example 3. It is a figure which shows the evaluation result of the comparative example 1. It is a figure which shows the evaluation result of the comparative example 2. It is a figure which shows the evaluation result of the comparative example 3. It is a figure which shows the evaluation result of the comparative example 4. It is a figure which shows the evaluation result of the comparative example 5. It is a figure which shows the evaluation result of the comparative example 6. It is a figure which shows the evaluation result of the comparative example 7. It is a figure which shows the evaluation result of the reference example 1. FIG. It is a figure which shows the evaluation result of the reference example 2. FIG. It is a figure which shows the evaluation result of the reference example 3. It is a figure which shows the evaluation result of the reference example 4. FIG. It is the figure which compared the evaluation result of the reference examples 1-4.

  Hereinafter, embodiments of the present invention will be described in detail.

[Nitrogen oxide purification catalyst]
In the nitrogen oxide purification method of this embodiment, a nitrogen oxide purification catalyst having a two-layer structure in which a first catalyst layer is laminated on a second catalyst layer formed on a carrier is used. Hereinafter, the nitrogen oxide purification catalyst used in the present embodiment will be described.

The first catalyst layer has at least a carbon monoxide removing function. Specifically, the first catalyst layer of the present embodiment has an oxidation function for oxidizing and burning carbon monoxide.
Further, in the first catalyst layer of the present embodiment, the temperature at which hydrogen starts to burn in the first catalyst layer is such that the carbon monoxide in the first catalyst layer is oxidized so that carbon monoxide is preferentially oxidized over hydrogen. Is preferably designed to be higher than the temperature at which combustion begins.
Moreover, in this embodiment, it is preferable that the temperature at which carbon monoxide starts burning in the first catalyst layer is equal to or lower than the temperature at which carbon monoxide starts burning in the second catalyst layer described later.

  The specific configuration of the first catalyst layer of the present embodiment is not particularly limited as long as it has the above-described function, but it is preferable that at least one of platinum and palladium is included as the catalytically active species. Moreover, it is preferable that at least one of cerium oxide and titanium oxide is included as a porous carrier for supporting these catalytically active species.

  In addition, the first catalyst layer of the present embodiment has a high hydrogen permeation rate and nitrogen oxide permeation rate in order to improve the reduction rate of nitrogen oxides using hydrogen as a reducing agent in the lower second catalyst layer. It is preferable. For this reason, it is preferable that the thickness of the first catalyst layer is smaller than the thickness of the second catalyst layer.

  The second catalyst layer has at least a nitrogen oxide removing function using hydrogen as a reducing agent. Moreover, it is preferable that the temperature at which carbon monoxide starts burning in the second catalyst layer is equal to or higher than the temperature at which carbon monoxide starts burning in the first catalyst layer.

  The specific configuration of the second catalyst layer of the present embodiment is not particularly limited as long as it has the above-described function, but it is preferable that at least one of platinum and palladium is included as the catalytically active species. Further, it is preferable that the porous carrier supporting these catalytically active species includes at least one selected from the group consisting of zirconium oxide, alumina, silica, titanium oxide, zeolite, and a composite oxide of cerium and zirconium.

  The second catalyst layer preferably includes a sufficient amount of catalyst so that a sufficient amount of nitrogen oxides can be reduced. The second catalyst layer preferably has good air permeability so that the exhaust gas that has permeated through the first catalyst layer is sufficiently diffused, and has a sufficient amount of catalyst within a range in which good air permeability is obtained. It is preferable.

  Further, in combining the first catalyst layer and the second catalyst layer, it is important that the thickness of the first catalyst layer is smaller than the thickness of the second catalyst layer. The amount needs to be able to cover the layer completely. This is because if the first catalyst layer is too thin and a part of the second catalyst layer is exposed, the reduction reaction in the second catalyst layer is not sufficiently performed due to the influence of carbon monoxide.

The nitrogen oxide purification catalyst of the present embodiment having the above-described configuration is manufactured by a conventionally known manufacturing method.
Specifically, first, after preparing a second catalyst slurry containing a catalyst material constituting the lower second catalyst layer, using this second catalyst slurry, for example, a conventionally known cordierite flow-through type A second catalyst layer is formed on the support made of the honeycomb structure by a wash coat method. Next, after preparing a first catalyst slurry containing a catalyst material constituting the upper first catalyst layer, the first catalyst slurry is used to form a first catalyst on the carrier on which the second catalyst layer is formed by a wash coat method. A catalyst layer is formed. Thereby, the nitrogen oxide purification catalyst according to the present embodiment having a two-layer structure is obtained.

[Nitrogen oxide purification method]
The nitrogen oxide purification method of the present embodiment purifies nitrogen oxides in exhaust gas discharged from an internal combustion engine. More specifically, the stoichiometry necessary for oxidizing the components to be oxidized contained in the exhaust gas. This is a method for purifying nitrogen oxides in exhaust gas containing excess oxygen exceeding a specific ratio, hydrogen and carbon monoxide.
Further, the nitrogen oxide purification method of the present embodiment purifies nitrogen oxide in exhaust using the above-described nitrogen oxide purification catalyst. Specifically, the stacking direction of the catalyst layer is the flow of exhaust. By disposing the above-described nitrogen oxide purification catalyst so as to be orthogonal to the direction, the nitrogen oxide in the exhaust gas is selectively reduced and purified.

The nitrogen oxide purification method of the present embodiment is executed using an exhaust gas purification apparatus for an internal combustion engine provided with the above-described nitrogen oxide purification catalyst.
As an internal combustion engine, it is preferable to apply the exhaust purification method of the present embodiment to a diesel engine that emits a large amount of nitrogen oxides. This is because the exhaust gas discharged from the diesel engine contains excess oxygen in excess of the stoichiometric ratio necessary for oxidizing the components to be oxidized.
Further, the exhaust purification device is not particularly limited as long as it includes the above-described nitrogen oxide purification catalyst. However, since the nitrogen oxide purification method of the present embodiment is for purifying nitrogen oxides in exhaust gas containing hydrogen and carbon monoxide as a reducing agent, a fuel reformer, an electrolysis device, a hydrogen cylinder, etc. It is preferable to provide an accompanying device for adding hydrogen as a reducing agent into the exhaust gas. Carbon monoxide is inevitably contained in the exhaust discharged from the internal combustion engine using hydrocarbon fuel.

Hereinafter, the effect of the nitrogen oxide purification method of the present embodiment will be described.
According to the present embodiment, the upper first catalyst layer has at least a carbon monoxide removing function, and the lower second catalyst layer has at least a nitrogen oxide removing function using hydrogen as a reducing agent. A catalyst was used. Further, the nitrogen oxide purification catalyst is used in an exhaust gas containing excess oxygen at a stoichiometric ratio or more necessary for the oxidation of the components to be oxidized contained in the exhaust gas, and hydrogen and carbon monoxide as reducing agents. The stacking direction of the layers was arranged so as to be orthogonal to the flow direction of the exhaust gas.
That is, the present embodiment directly reduces nitrogen oxides in exhaust gas in an oxygen-excess atmosphere, and the oxygen concentration contained in the exhaust gas is extremely low compared to exhaust gas in rich combustion such as a lean burn engine. However, it is characterized in that nitrogen oxides can be purified at a high purification rate despite being high.
According to this embodiment, the following three remarkable effects are produced.
As a first effect, it is possible to suppress a decrease in the nitrogen oxide purification rate due to carbon monoxide. In the present invention, since the upper first catalyst layer has a carbon monoxide removing function, the concentration of carbon monoxide in the exhaust gas that reaches the lower second catalyst layer can be reduced. For this reason, the fall of the nitrogen oxide purification rate by the above-mentioned carbon monoxide can be suppressed.
As a second effect, the maximum purification rate of nitrogen oxides can be improved as compared with the conventional case. In the present embodiment, since carbon monoxide can be reduced in the upper first catalyst layer, the reaction probability of the reduction reaction between nitrogen oxides and hydrogen can be increased in the lower second catalyst layer. For this reason, interference due to competitive adsorption of carbon monoxide on the catalytically active species can be suppressed, and the reduction reaction of nitrogen oxides can be promoted. As a result, the maximum purification rate of nitrogen oxides is improved.
As a third effect, a wider temperature window can be obtained than in the conventional case. As described above, this is mainly due to the fact that the reaction probability between nitrogen oxides and hydrogen is increased in the lower second catalyst layer, and the reduction reaction of nitrogen oxides is promoted. In addition, due to the temperature increase accompanying the oxidation reaction of carbon monoxide in the upper layer, the diffusion of nitrogen oxides becomes dominant and the reaction rate is also improved.
Due to the above three effects, according to the present embodiment, nitrogen oxides can be efficiently purified in a wide temperature window from a low temperature region to a high temperature region.

Further, according to a preferred aspect of the present embodiment, a nitrogen oxide purification catalyst in which the temperature at which hydrogen starts to burn in the first catalyst layer is higher than the temperature at which carbon monoxide starts to burn in the first catalyst layer. Using.
Thereby, for example, in a low temperature range of 200 ° C. or lower, carbon monoxide can be preferentially reacted and removed over hydrogen in the upper first catalyst layer. For this reason, as a result of reducing the carbon monoxide concentration in the exhaust gas that reaches the lower second catalyst layer and increasing the hydrogen concentration, it is possible to suppress the adverse effect of the carbon monoxide on the nitrogen oxide purification rate. Therefore, nitrogen oxides can be efficiently purified in a wider temperature window from a lower temperature range than in the past.

Further, according to a preferred aspect of the present embodiment, the nitrogen oxide purification catalyst in which the temperature at which carbon monoxide starts burning in the second catalyst layer is equal to or higher than the temperature at which carbon monoxide starts burning in the first catalyst layer. Was used.
Thereby, for example, in the temperature range where the reduction reaction of nitrogen oxide proceeds in the lower second catalyst layer, carbon monoxide can be preferentially reacted and removed in the upper first catalyst layer. As described above, carbon monoxide causes a reduction in the activity of the reduction reaction of nitrogen oxides. Therefore, by removing this carbon monoxide, the nitrogen oxide purification activity in the lower second catalyst layer can be maintained. Therefore, nitrogen oxides can be purified more efficiently in a wider temperature window from a lower temperature range than in the past.

Moreover, according to the preferable aspect of this embodiment, the nitrogen oxide purification catalyst whose thickness of the 1st catalyst layer is smaller than the thickness of the 2nd catalyst layer was used.
As described above, the upper first catalyst layer plays a role of selectively removing carbon monoxide which causes a decrease in the catalytic activity of the lower second catalyst layer. Here, the oxidation reaction of carbon monoxide is a typical differential reaction, and the reaction can proceed efficiently even in a thin layer. From the viewpoint of reaction efficiency, it is important to reduce the thickness of the first catalyst layer and promote the diffusion of hydrogen and nitrogen oxides into the lower second catalyst layer. In this regard, according to a preferred aspect of the present embodiment, it can be said that nitrogen oxides can be more efficiently purified because the thickness of the first catalyst layer is designed to be smaller than the thickness of the second catalyst layer.

Moreover, according to the preferable aspect of this embodiment, the 1st catalyst layer and the 2nd catalyst layer used the nitrogen oxide purification | cleaning catalyst which contains at least one among platinum and palladium, respectively.
As a result, it is possible to selectively remove carbon monoxide in the upper first catalyst layer and to reduce and purify nitrogen oxides with hydrogen in the lower second catalyst layer. Nitrogen oxide can be efficiently purified over a wide temperature window.

Moreover, according to the preferable aspect of this embodiment, the 1st catalyst layer used the nitrogen oxide purification catalyst which contains at least one among cerium oxide and titanium oxide.
As a result, the selective removal of carbon monoxide in the upper first catalyst layer is more effectively performed. As a result, it is possible to efficiently purify nitrogen oxides in a wider temperature window from the low temperature range to the high temperature range as compared with the prior art. .

According to a preferred aspect of the present embodiment, the second catalyst layer contains at least one selected from the group consisting of zirconium oxide, alumina, silica, titanium oxide, zeolite, and a composite oxide of cerium and zirconium. A nitrogen oxide purification catalyst was used.
As a result, the reduction and purification of nitrogen oxides by hydrogen in the lower second catalyst layer is effectively performed. As a result, nitrogen oxides can be efficiently purified in a wider temperature window from the low temperature range to the high temperature range as compared with the conventional case. .

  It should be noted that the present invention is not limited to the above-described embodiment, and modifications and improvements within the scope that can achieve the object of the present invention are included in the present invention.

  Examples of the present invention and comparative examples will be described in detail below, but the present invention is not limited to these examples.

<Catalyst preparation>
[Pt / CeO ₂ ]
A method for preparing the Pt / CeO ₂ catalyst powder used in Examples 1 to 3 and Comparative Example 1 is shown below.
A dinitrodiammine platinum nitrate acidic solution obtained by weighing 400 g of general ceria powder (BET specific surface area = 182 m ² / g) in 2000 mL of pure water to a platinum loading amount of 1% by mass was obtained. After diluting with 200 mL of pure water, it was added dropwise with stirring. After stirring for 2 hours, platinum was adsorbed and supported on ceria, filtered and washed, dried at 110 ° C. for 12 hours, and then calcined at 450 ° C. for 2 hours to obtain Pt / CeO ₂ catalyst powder.

[Pt / ZrO ₂ ]
The method for preparing the Pt / ZrO ₂ catalyst powder used in Example 1 and Comparative Examples 2 to 3 is shown below.
Acidic dinitrodiammine platinum nitrate measured in a slurry solution in which 400 g of general zirconia powder (BET specific surface area = 91.9 m ² / g) was dispersed in 2000 mL of pure water so that the amount of platinum supported was 1% by mass. The solution was diluted with 200 mL of pure water and then added dropwise with stirring. After stirring for 2 hours, platinum was adsorbed and supported on zirconia, filtered and washed, dried at 110 ° C. for 12 hours, and then calcined at 450 ° C. for 2 hours to obtain Pt / ZrO ₂ catalyst powder.

[Pt / Al ₂ O ₃ ]
The method for preparing the Pt / Al ₂ O ₃ catalyst powder used in Example 2 and Comparative Examples 4 to 5 is shown below.
A dinitrodiammine platinum nitrate acidic solution obtained by weighing 400 g of general alumina powder (BET specific surface area = 102 m ² / g) in 2000 mL of pure water so that the supported amount of platinum is 1% by mass is obtained. After diluting with 200 mL of pure water, it was added dropwise with stirring. After stirring for 2 hours, platinum was adsorbed and supported on alumina, filtered and washed, dried at 110 ° C. for 12 hours, and then calcined at 450 ° C. for 2 hours to obtain Pt / Al ₂ O ₃ catalyst powder.

[Pt / ZSM-5]
A method for preparing the Pt / ZSM-5 catalyst powder used in Example 3 and Comparative Examples 6 to 7 is shown below.
Tetraammine platinum water weighed in a slurry liquid in which 400 g of zeolite (H-ZSM-5) powder (BET specific surface area = 442 m ² / g) was dispersed in 1200 mL of pure water so that the amount of platinum supported was 1% by mass. The acid salt solution was diluted with 1000 mL of pure water and then added with stirring. The platinum was supported by ion exchange with protons of H-ZSM-5 with stirring for 24 hours. Subsequently, after filtering and washing, drying was performed at 120 ° C. for 18 hours and 180 ° C. for 24 hours, and then calcined at 500 ° C. for 2 hours to obtain Pt / ZSM-5 catalyst powder.

<Example 1>
A method for producing the two-layer coated catalytic converter used in Example 1 is shown below.
181 g of the previously produced Pt / ZrO ₂ catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, and ball milling was performed for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / ZrO ₂ slurry.
Further, 181 g of the previously produced Pt / CeO ₂ catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, and ball milling was performed for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / CeO ₂ slurry.
Next, using a Pt / ZrO ₂ slurry, a supported amount of Pt / ZrO ₂ catalyst is 100 g on a cordierite honeycomb (1 inch (2.54 cm) φ × 60 mm, wall thickness 4 mil (0.1 mm), 600 cell)). A Pt / ZrO ₂ catalytic converter was obtained by wash-coating so as to be / L.
Furthermore, by using a Pt / CeO ₂ slurry, the upper layer Pt / CeO 2 was coated on the previously obtained Pt / ZrO ₂ catalytic converter so that the supported amount of the Pt / CeO ₂ catalyst was 100 g / L. _2. A two-layer coated catalytic converter of lower layer Pt / ZrO ₂ was obtained.

<Example 2>
A method for producing the two-layer coated catalytic converter used in Example 2 is shown below.
181 g of the previously produced Pt / Al ₂ O ₃ catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, and ball milling was performed for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / Al ₂ O ₃ slurry.
Further, 181 g of the previously produced Pt / CeO ₂ catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, followed by ball milling for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / CeO ₂ slurry.
Next, using a Pt / Al ₂ O ₃ slurry, a cordierite honeycomb (1 inch (2.54 cm) φ × 60 mm, wall thickness 4 mil (0.1 mm), 600 cells) was added to the Pt / Al ₂ O ₃ catalyst. A Pt / Al ₂ O ₃ catalytic converter was obtained by wash-coating so that the supported amount was 100 g / L.
Further, by using the Pt / CeO ₂ slurry, the upper layer Pt / CeO ₂ catalyst was washed on the previously obtained Pt / Al ₂ O ₃ catalytic converter so that the supported amount of the Pt / CeO ₂ catalyst was 100 g / L. A two-layer coated catalytic converter of / CeO ₂ and lower layer Pt / Al ₂ O ₃ was obtained.

<Example 3>
A method for producing the two-layer coated catalytic converter used in Example 3 is shown below.
181 g of the previously produced Pt / ZSM-5 catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, and ball milling was performed for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / ZSM-5 slurry.
Further, 181 g of the previously produced Pt / CeO ₂ catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, and ball milling was performed for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / CeO ₂ slurry.
Next, using Pt / ZSM-5 slurry, the amount of Pt / ZSM-5 catalyst supported on a cordierite honeycomb (1 inch (2.54 cm) φ × 60 mm, wall thickness 4 mil (0.1 mm), 600 cell)) Was Pt / ZSM-5 catalytic converter by wash-coating so as to be 100 g / L.
Further, the Pt / CeO ₂ slurry was used to wash coat the Pt / ZSM-5 catalytic converter obtained previously so that the supported amount of the Pt / CeO ₂ catalyst was 100 g / L, whereby the upper layer Pt / CeO _2, to obtain a 2-coat catalytic converter lower Pt / ZSM-5.

<Comparative Example 1>
A method for producing the single-layer coated catalytic converter used in Comparative Example 1 and Reference Example 3 is shown below.
181 g of the previously produced Pt / CeO ₂ catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, and ball milling was performed for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / CeO ₂ slurry.
Next, using a Pt / CeO ₂ slurry, the supported amount of Pt / CeO ₂ catalyst is 100 g on a cordierite honeycomb (1 inch (2.54 cm) φ × 60 mm, wall thickness 4 mil (0.1 mm), 600 cell)) A Pt / CeO ₂ catalytic converter was obtained by wash-coating to a / L level.
In Reference Examples 1, 2, and 4, wash coating was performed so that the supported amounts of the Pt / CeO ₂ catalyst were 50 g / L, 75 g / L, and 150 g / L, respectively.

<Comparative Examples 2-3>
A method for producing the single-layer coated catalytic converter used in Comparative Examples 2 to 3 is shown below.
181 g of the previously produced Pt / ZrO ₂ catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, and ball milling was performed for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / ZrO ₂ slurry.
Next, using a Pt / ZrO ₂ slurry, a supported amount of Pt / ZrO ₂ catalyst is 100 g on a cordierite honeycomb (1 inch (2.54 cm) φ × 60 mm, wall thickness 4 mil (0.1 mm), 600 cell)). A Pt / ZrO ₂ catalytic converter was obtained by wash-coating so as to be / L.

<Comparative Examples 4-5>
A method for producing the single-layer coated catalytic converter used in Comparative Examples 4 to 5 is shown below.
181 g of the previously produced Pt / Al ₂ O ₃ catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, and ball milling was performed for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / Al ₂ O ₃ slurry.
Using Pt / Al ₂ O ₃ slurry, supported amount of Pt / Al ₂ O ₃ catalyst on cordierite honeycomb (1 inch (2.54 cm) φ × 60 mm, wall thickness 4 mil (0.1 mm), 600 cell)) Was Pt / Al ₂ O ₃ catalytic converter by wash coating so as to be 100 g / L.

<Comparative Examples 6-7>
A method for producing the single-layer coated catalytic converter used in Comparative Examples 6 to 7 is shown below.
181 g of the previously produced Pt / ZSM-5 catalyst powder and 850 g of 1 mmφ zirconia balls were added to 314 g of pure water, and ball milling was performed for 16 hours. The obtained slurry and zirconia balls were separated, and methylcellulose was added to the slurry to obtain a Pt / ZSM-5 slurry.
Using Pt / ZSM-5 slurry, cordierite honeycomb (1 inch (2.54 cm) φ × 60 mm, wall thickness 4 mil (0.1 mm), 600 cells) on a Pt / ZSM-5 catalyst loading of 100 g The Pt / ZSM-5 catalytic converter was obtained by wash-coating to a / L level.

<Catalyst activity evaluation>
About each catalytic converter obtained by the Example and the comparative example, catalytic activity evaluation was implemented using the fixed bed catalyst reaction apparatus 10 made from quartz provided with a structure as shown in FIG.
Specifically, as shown in FIG. 2, the catalytic converter 11 (1 inch (2.54 cm) φ × 60 mm) was fixed in the fixed bed catalytic reaction tube 1 using a quartz tape 12. Next, a test gas necessary for the test was introduced from each gas cylinder 2A and pure water tank 2B by each mass flow controller 3, and the test gas was heated in the infrared gold image furnace 5 while being uniformly mixed by the gas mixer 4. Specifically, the temperature of the gas flowing into the catalytic converter 11 was increased from room temperature to 440 ° C. at 20 ° C./min. And the heated test gas was distribute | circulated from the upper direction of the fixed bed catalyst reaction tube 1 to the downward direction.
The test gas composition was NO = 100 ppm, CO ₂ = 6%, O ₂ = 10%, H ₂ = 5000 ppm, CO = 6000 ppm, H ₂ O = 7%, and N ₂ balance. The space velocity of the test gas was set to be 50,000 (1 / hr) (however, in Comparative Examples 3, 5, and 7, CO was excluded).
Moreover, as shown in FIG. 2, the thermocouple 6 was inserted into the upper 15 mm position of the catalytic converter 11, and the catalyst inlet temperature was measured.

Analysis of the gas composition using a chemiluminescence NOx meter (Ltd. Best Sokki Ltd. "BCL-121UV-M10") was measured NOx amount and the amount of NO, NO ₂ amount is the amount of NOx and NO amount Calculated as the difference. The amount of CO was measured using a non-dispersive infrared method (NDIR) type CO meter ("BAM-900U-M10" manufactured by Best Instrument Co., Ltd.). The amount of H ₂ was measured using a gas chromatograph (“GD-3200D” manufactured by GL Sciences Inc.).
The NOx purification rate (hereinafter referred to as the conversion rate) is measured by measuring the unreacted test gas by bypassing the catalytic converter 11 in the fixed bed catalytic reaction tube 1, and analyzing the gas that has reacted through the catalytic converter 11. It was calculated as the ratio of the difference from the value obtained. The purification rate of H _2, was calculated for the decrease of the peak area value of the gas chromatograph as consumption.

<Catalyst activity evaluation results>
[Example 1: Two-layer coat catalyst of upper layer Pt / CeO ₂ and lower layer Pt / ZrO ₂ ]
The results of the catalyst activity evaluation of Example 1 are shown in FIG. As shown in FIG. 3, the NOx conversion rate increased as the inflow gas temperature increased from room temperature, and showed an NOx conversion rate of 84.5% at an inflow gas temperature of 103.0 ° C. When the inflow gas temperature exceeded 103.0 ° C., the NOx conversion rate gradually decreased.
The CO conversion rate was similar to the result of the Pt / CeO ₂ catalyst of Comparative Example 1, and the inflow gas temperature at which the CO conversion rate of 50.0% was obtained was 85.0 ° C. (hereinafter, the conversion rate of 50% The inflow gas temperature at which is obtained is called the light-off temperature).
From this result, CO, which is a poisonous substance, burns at a low temperature in the upper layer Pt / CeO ₂ , and the lower layer Pt / ZrO ₂ exhibits excellent NOx purification activity in a wide temperature range from a low temperature range to a high temperature range. I found out.

[Example 2: Two-layer coat catalyst of upper layer Pt / CeO ₂ and lower layer Pt / Al ₂ O ₃ ]
The results of the catalyst activity evaluation of Example 2 are shown in FIG. As shown in FIG. 4, the NOx conversion rate increased as the inflow gas temperature increased from room temperature, and the NOx conversion rate was 78.2% at the inflow gas temperature of 88.0 ° C. When the inflow gas temperature exceeded 88.0 ° C., the NOx conversion rate gradually decreased.
The CO conversion was similar to the result of the Pt / CeO ₂ catalyst of Comparative Example 1, and the CO light-off temperature was 82.8 ° C.
From this result, as in Example 1, CO, which is a poisoning substance, burns at a low temperature in the upper layer Pt / CeO ₂ , so that the lower layer Pt / Al ₂ O ₃ has a wide temperature range from the low temperature range to the high temperature range. It was found that the NOx purification activity was excellent in the region.

[Example 3: Two-layer coat catalyst of upper layer Pt / CeO ₂ and lower layer Pt / ZSM-5]
The results of the catalyst activity evaluation of Example 3 are shown in FIG. As shown in FIG. 5, the NOx conversion rate increased as the inflow gas temperature increased from room temperature, and showed an NOx conversion rate of 64.8% at the inflow gas temperature of 109.5 ° C. When the inflow gas temperature exceeded 109.5 ° C., the NOx conversion rate gradually decreased.
The CO conversion was similar to the result of the Pt / CeO ₂ catalyst of Comparative Example 1, and the CO light-off temperature was 83.0 ° C.
From this result, as in Examples 1 and ₂ , CO, which is a poisoning substance, burns at a low temperature in Pt / CeO ₂ in the upper layer, so that Pt / ZSM-5 in the lower layer has a wide range from a low temperature region to a high temperature region. It was found that the NOx purification activity was excellent in the temperature range.

[Comparative Example 1: Pt / CeO ₂ one-layer coated catalyst]
The result of the catalytic activity evaluation of Comparative Example 1 is shown in FIG. As shown in FIG. 6, a high NOx conversion rate was temporarily obtained at an inflow gas temperature of about 80 ° C., but when the inflow gas temperature exceeded 80 ° C., a high NOx conversion rate was not obtained.
The CO light off temperature was 80.2 ° C. and the H ₂ light off temperature was 115.8 ° C. Further, there was a difference between the combustion start temperatures of CO and H ₂ , and it was confirmed that the temperature at which H ₂ starts combustion is higher than the temperature at which CO starts combustion.
From this result, it was found that an excellent NOx purification activity cannot be obtained in a wide temperature range with only one catalyst layer.

[Comparative Example 2: Pt / ZrO ₂ one-layer coated catalyst]
The result of the catalytic activity evaluation of Comparative Example 2 is shown in FIG. As shown in FIG. 7, although a high NOx conversion rate of 80% or more was temporarily obtained near the inflow gas temperature of 150 ° C., the NOx conversion rate gradually decreased when the temperature exceeded 150 ° C.
It was also found that CO, which is a poisonous substance, does not burn up to an inflow gas temperature of around 140 ° C.
Also from this result, it was found that excellent NOx purification activity could not be obtained in a wide temperature range with only one catalyst layer.

[Comparative Example 3: Pt / ZrO ₂ one-layer coated catalyst]
FIG. 8 shows the results of Comparative Example 3 in which the Pt / ZrO ₂ single-layer coated catalyst was tested by removing CO from the test gas. As shown in FIG. 8, a high NOx conversion rate was exhibited from a low temperature range, and a high NOx conversion rate of 80% or more was observed from the inflowing gas temperature of about 73 ° C. to about 150 ° C.
From this result, it was found that if NO, which is a poisonous substance, is not present in the exhaust gas, a high NOx conversion rate can be obtained in a wide temperature range from a low temperature range to a high temperature range. That is, it was confirmed that it is effective to provide the first catalyst layer having a carbon monoxide removing function as in the present invention.

[Comparative Example 4: Pt / Al ₂ O ₃ one-layer coat catalyst]
The results of the catalyst activity evaluation of Comparative Example 4 are shown in FIG. As shown in FIG. 9, a high NOx conversion rate of 80% was temporarily obtained near the inflowing gas temperature of 180 ° C., but a high NOx conversion rate was not obtained in a temperature range lower than 180 ° C.
Further, it was found that CO, which is a poisonous substance, does not burn up to an inflow gas temperature of around 165 ° C.

[Comparative Example 5: Pt / Al ₂ O ₃ one-layer coated catalyst]
FIG. 10 shows the results of Comparative Example 5 in which the Pt / Al ₂ O ₃ single-layer coated catalyst was tested by removing CO from the test gas. As shown in FIG. 10, a high NOx conversion rate was exhibited from a low temperature range, and a 66.4% NOx conversion rate was obtained at an inflow gas temperature of 67.0 ° C. The NOx conversion rate once decreased near the inflow gas temperature of 80 ° C., but the NOx conversion rate increased again near 130 ° C.
From this result, it was also found that a high NOx conversion rate can be obtained in a wide temperature range from a low temperature range to a high temperature range if CO, which is a poisoning substance, is not present in the exhaust gas. That is, it was confirmed that it is effective to provide the first catalyst layer having a carbon monoxide removing function as in the present invention.

[Comparative Example 6: Pt / ZSM-5 one-layer coat catalyst]
The result of the catalyst activity evaluation of Comparative Example 6 is shown in FIG. As shown in FIG. 11, NOx was hardly converted in the range of the inflow gas temperature of 50 to 200 ° C.
It was also found that CO hardly burns in the range of the inflow gas temperature of 50 to 200 ° C.

[Comparative Example 7: Pt / ZSM-5 one-layer coated catalyst]
FIG. 12 shows the result of Comparative Example 5 in which the Pt / ZSM-5 single-layer coated catalyst was tested by removing CO from the test gas. As shown in FIG. 12, a high NOx conversion rate was exhibited in the range of the inflow gas temperature of 50 to 200 ° C. Specifically, the NOx conversion rate gradually increased from room temperature to around 160 ° C., and a high NOx conversion rate of 85.1% was obtained at the inflow gas temperature of 162.7 ° C.
From this result, it was also found that a high NOx conversion rate can be obtained in a wide temperature range from a low temperature range to a high temperature range if CO, which is a poisoning substance, is not present in the exhaust gas. That is, it was confirmed that it is effective to provide the first catalyst layer having a carbon monoxide removing function as in the present invention.

[Reference Examples 1 to 4: Pt / CeO ₂ single layer coat catalyst]
Reference Examples 1 to 4 are each a single-layer coated catalyst in which the Pt / CeO ₂ catalyst coating amount was changed stepwise to 50, 75, 100, and 150 g / L. 16 shows. A summary of these results is shown in FIG.
As shown in these figures, the amount of catalyst coat does not have a significant effect on the NOx conversion rate, but increasing the amount of catalyst coat lowers the reaction between CO and H ₂ , and in particular reduces the reaction of H ₂ at a lower temperature. The effect to make it effective is produced. Therefore, in comparison with a case small catalyst coating amount, it was found that the reaction temperature difference between the H ₂ and CO is reduced. Further, it was found that there was not much change even when the catalyst coat amount was 100 g / L or more, and this was considered to indicate that the reaction gas was diffusion controlled in the catalyst coat thickness direction.

Table 1 shows a summary of the results of the above catalytic activity evaluation for the catalyst used in the upper layer in this example. Moreover, about the catalyst used for a lower layer, it excluded the CO from test gas, and the result of having performed said catalytic activity evaluation (average purification rate of the nitrogen oxide from catalyst inlet gas temperature 50 to 200 degreeC) was put together. Those are shown in Table 2.
As shown in these tables, the catalyst-supported oxides used were TiO ₂ and CeO _{2 in the} upper layer, ZrO ₂ , Al ₂ O ₃ , TiO ₂ , SiO ₂ , zeolite, CeO ₂ —ZrO ₂ in the lower layer, It was found that Pt or Pd is effective for the supported active metal in both the upper layer and the lower layer.

（平均ＮＯｘ転化率は、触媒入口ガス温度５０〜２００℃の範囲の平均浄化率）

(The average NOx conversion rate is the average purification rate in the range of the catalyst inlet gas temperature of 50 to 200 ° C.)

DESCRIPTION OF SYMBOLS 1 Fixed bed catalyst reaction tube 2A Gas cylinder 2B Pure water tank 3 Mass flow controller 4 Gas mixer 5 Infrared gold image furnace 6 Thermocouple 7 Vaporization part 8 Analyzer 10 Fixed bed catalyst reaction apparatus 11 Catalytic converter 12 Quartz tape

Claims (2)

A nitrogen oxide purification method for purifying nitrogen oxide in exhaust gas discharged from an internal combustion engine using a nitrogen oxide purification catalyst in which a first catalyst layer is laminated on a second catalyst layer formed on a carrier. There,
The first catalyst layer includes cerium oxide supporting platinum, titanium oxide supporting platinum, or titanium oxide supporting palladium to oxidize carbon monoxide in the exhaust. At least a carbon monoxide removing function to remove, and
The second catalyst layer is zirconium oxide having platinum supported silica having platinum supported, platinum titanium oxide is supported, the zeolite having platinum supported, platinum composite oxide supported cerium and zirconium, Zirconium oxide supporting palladium, titanium oxide supporting palladium, zeolite supporting palladium, or a composite oxide of cerium and zirconium supporting palladium is used to form hydrogen as a reducing agent. Using a nitrogen oxide purification catalyst having at least a nitrogen oxide removal function,
Hydrogen is added by an accompanying device that adds hydrogen as a reducing agent to the exhaust gas containing excess oxygen in excess of the stoichiometric ratio necessary for the oxidation of the components to be oxidized contained in the exhaust gas and carbon monoxide, A nitrogen oxide purification method characterized by selectively reducing and purifying nitrogen oxide in exhaust gas by disposing the nitrogen oxide purification catalyst so that the stacking direction of the catalyst layer is orthogonal to the flow direction of exhaust gas .   The nitrogen oxide purification method according to claim 1, wherein a nitrogen oxide purification catalyst having a thickness of the first catalyst layer smaller than a thickness of the second catalyst layer is used.

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