JP3745988B2 - Method for catalytic reduction of nitrogen oxides and catalyst therefor - Google Patents

Description

【００８５】
更に、実施例１、比較例１及び２による触媒構造体をそれぞれ用いて、上記ガス条件、リッチ／リーン幅（秒／秒）を５５／５、反応温度３５０℃として、５０時間の耐久試験を行った。結果を表２に示す。表２から明らかなように、本発明による触媒は、硫黄酸化物に対しても、従来のＮＯx 貯蔵−還元システムに比べて、非常に高い耐性を有する。
【００８６】
【表２】

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for catalytic reduction of nitrogen oxide (mainly composed of NO and NO ₂ , hereinafter referred to as NOx), that is, a catalytic reduction. More specifically, the present invention catalytically reduces NOx in exhaust gas by supplying and burning fuel in a periodic rich / lean fuel supply process (excursion) and bringing the generated exhaust gas into contact with a catalyst. Regarding the method. This method is suitable, for example, for reducing and removing harmful nitrogen oxides contained in exhaust gas from automobiles.
[0002]
In addition, the present invention particularly supplies fuel in a periodic rich / lean fuel supply process in the presence of sulfur oxide (mainly composed of SO ₂ and SO ₃ , hereinafter referred to as SOx). The present invention relates to a catalyst for catalytically reducing NOx in exhaust gas produced by combustion.
[0003]
Here, the present invention
(A) a first catalyst component (provided that cerium oxide and (b) a mixture or complex oxide of at least one element selected from zirconium, gadolinium, terbium, samarium and lanthanum) Excluding mixtures of oxides and zirconium oxides or composite oxides);
(C) a surface catalyst layer having, as a surface catalyst component, a second catalyst component comprising at least one selected from rhodium, palladium, rhodium oxide and palladium oxide;
(B) (a) It comprises at least one catalyst component selected from platinum, rhodium, palladium, platinum oxide, rhodium oxide and palladium oxide, and (b) an internal catalyst layer having a carrier as an internal catalyst component. The present invention relates to a method for catalytic reduction of nitrogen oxides in exhaust gas using a catalyst and the catalyst.
[0004]
In the present invention, the term “excursion” means that the air / fuel ratio moves from its average value to both along the time axis, or such operation. The term “rich” means that the air / fuel ratio of the fuel in question is less than the stoichiometric air / fuel ratio, and the term “lean” means the air / fuel ratio of the fuel in question. It means that the fuel ratio is greater than the stoichiometric air / fuel ratio. In normal automobile gasoline, the stoichiometric air / fuel ratio is about 14.5. Also, in the present invention, “catalyst” means a catalyst or a structure including the catalyst that operates to remove NOx during fuel rich / lean combustion.
[0005]
Therefore, in the present invention, “supplying fuel in a periodic rich / lean fuel supply process” means that fuel combustion is mainly performed in lean conditions (in the exhaust gas after combustion) in the combustion chamber of a diesel engine or a gasoline engine. The oxygen concentration is normally about 5% to 10%.), And while adjusting the air / fuel ratio so as to periodically vibrate the atmosphere alternately between the rich condition and the lean condition, This refers to supplying, injecting or injecting fuel. Accordingly, the rich / lean stroke is synonymous with the rich / lean condition.
[0006]
Conventionally, NOx contained in exhaust gas is removed by, for example, a method of oxidizing this and then absorbing it into an alkali or a method of reducing to nitrogen using ammonia, hydrogen, carbon monoxide or hydrocarbon as a reducing agent. Has been. However, each of these conventional methods has drawbacks.
[0007]
That is, according to the former method, in order to prevent environmental problems, a means for treating the generated alkaline waste water is necessary. According to the latter method, for example, when ammonia is used as a reducing agent, ammonia reacts with SOx in the exhaust gas to form salts, and as a result, the catalytic activity is reduced at low temperatures. In particular, when NOx from a movement source such as an automobile is processed, its safety becomes a problem.
[0008]
On the other hand, if hydrogen, carbon monoxide or hydrocarbons are used as the reducing agent, these reducing agents will react preferentially with the oxygen because the exhaust gas contains oxygen at a higher concentration than NOx, thus If NOx is to be substantially reduced, a large amount of reducing agent is required, resulting in a significant reduction in fuel consumption.
[0009]
Therefore, it has been proposed to catalytically decompose NOx without using a reducing agent. However, in order to directly decompose NOx, a conventionally known catalyst has not been put into practical use because of its low decomposition activity. On the other hand, various zeolites have been proposed as NOx catalytic reduction catalysts using hydrocarbons or oxygen-containing organic compounds as reducing agents. In particular, copper ion exchange ZSM-5 and H-type (hydrogen type or acid type) zeolite ZSM-5 (SiO ₂ / Al ₂ O ₃ molar ratio = 30 to 40) are considered to be optimal. However, even H-type zeolite does not have sufficient reduction activity, and especially when the moisture is contained in the exhaust gas, the performance of the zeolite catalyst can be quickly reduced due to the dealumination of the zeolite structure. Are known.
[0010]
Under such circumstances, development of a more highly active catalyst for NOx catalytic reduction has been demanded. For example, EP-A1-526099 and EP-A1-679427 (corresponding to JP-A2-53117647) Recently, a catalyst in which silver or silver oxide is supported on an inorganic oxide support material has been proposed. Although this catalyst has high oxidation activity, it has been known that the rate of selective conversion of NOx to nitrogen is slow because of its low selective reduction activity for NOx. Moreover, this catalyst has a problem that its activity is quickly reduced in the presence of SOx. These catalysts have the catalytic action of selectively reducing NOx to some extent using hydrocarbons under fully lean conditions, but with a lower NOx removal rate and operating temperature window (3 Such a lean NOx catalyst is difficult to put into practical use because the temperature range is narrow. Thus, there is an urgent need for more heat resistant and highly active catalysts for NOx catalytic reduction.
[0011]
In order to overcome the above-mentioned problems, the NOx storage-reduction system has recently become the most popular, as described in “Society of Automotive Engineers” (SAE) (August 9, 1995). It has been proposed as a promising method. According to this proposal (Toyota Motor Corporation), fuel is periodically supplied to the combustion chamber in a short time in an amount exceeding the stoichiometric amount. An automobile equipped with a lean combustion engine can be driven with a very small fuel / air ratio, so that the fuel consumption rate can be lower than that of an automobile equipped with a conventional engine. Such a lean-burn engine NOx storage-reduction system reduces NOx by two periodic steps at intervals of 1-2 minutes.
[0012]
That is, in the first step, NO is oxidized to NO ₂ on a platinum or rhodium catalyst under (normal) lean conditions, and this NO ₂ is adsorbed by an alkali compound such as K ₂ CO ₃ or BaCO _3. The Then, a rich condition for the second step is formed and this rich condition is maintained for a few seconds. Under this rich condition, the adsorbed (stored) NO ₂ is efficiently reduced to nitrogen using a hydrocarbon, carbon monoxide or hydrogen on a platinum or rhodium catalyst. This NOx storage-reduction system works well over a long period of time in the absence of SOx. However, if there SOx is, even in the lean and rich any conditions, by irreversible adsorption of SOx in the NO ₂ adsorption sites on the alkali compound, the catalyst system deteriorates sharply.
[0013]
[Problems to be solved by the invention]
The present invention burns fuel under periodic rich / lean conditions in the presence of oxygen, sulfur oxides or water, and over a wide range of reaction temperatures, and NOx in the exhaust gas produced by this combustion. An object of the present invention is to provide a method for catalytically reducing a product with high durability.
[0014]
According to the method of the present invention, the noble metal catalyst component of platinum, rhodium or palladium in the inner catalyst layer oxidized in the lean process is reduced in a rich process, that is, under reducing conditions, and NOx is reduced on these noble metal catalysts. Reduced by contact.
[0015]
Further, in the rich process, what is important is that the first catalyst of the surface catalyst layer having OSC (oxygen storage capacity) is prepared with the aid of the high reducing power of the noble metal catalyst component of the inner catalyst layer in preparation for the next lean process. The catalyst component is rapidly reduced so that it can efficiently perform the oxygen storage function, that is, the oxygen in the exhaust gas is occluded during lean, and as a result, the reaction atmosphere of the noble metal catalyst component in the surface catalyst layer during lean Is reproduced so as to be kept in a rich or stoichiometric condition.
[0016]
Then, under the subsequent lean stroke, ie, oxidizing conditions, NOx is selectively reduced on the noble metal catalyst component of the surface catalyst layer. That is, as a result of preventing the first catalyst component of the surface catalyst layer reduced with the aid of the high reducing power of the noble metal catalyst component of the inner catalyst layer from being oxidized, such as rhodium, which is the second catalyst component, lean. The reduction reaction of NOx from the exhaust gas on the second catalyst component of the surface catalyst layer is continued with high efficiency.
[0017]
Further, the present invention does not deteriorate in the presence of oxygen, sulfur oxides, or water, particularly in the presence of sulfur oxides, which is a serious problem of NOx storage catalysts, and is periodic over a wide temperature range. An object of the present invention is to provide a highly durable catalyst for reducing NOx in a lean stroke of a rich / lean combustion. The reason why the catalyst according to the present invention exhibits high durability in the presence of sulfur oxide is that the coexisting SOx is adsorbed on the surface catalyst layer as SO ₂ when lean, and is desorbed into the gas phase when rich, and the NOx storage catalyst This is because it is not irreversibly occluded in the catalyst.
[0018]
It is another object of the present invention to provide a catalyst structure for NOx catalytic reduction in which a catalyst is supported on an inert supporting substrate.
[0019]
Thus, according to the present invention, in the method of supplying and burning fuel under periodic rich / lean conditions, bringing the generated exhaust gas into contact with the catalyst, and catalytically reducing the nitrogen oxides in the exhaust gas, A first catalyst component in which the catalyst comprises a mixture or composite oxide of (A) (a) cerium oxide and (b) an oxide of at least one element selected from zirconium, gadolinium, terbium, samarium and lanthanum (provided that , Excluding mixtures or composite oxides of cerium oxide and zirconium oxide),
(D) a surface catalyst layer having, as a surface catalyst component, a second catalyst component comprising at least one selected from rhodium, palladium, rhodium oxide and palladium oxide;
(B) (a) It comprises at least one catalyst component selected from platinum, rhodium, palladium, platinum oxide, rhodium oxide and palladium oxide, and (b) an internal catalyst layer having a carrier as an internal catalyst component. It is an object of the present invention to provide a method for catalytic reduction of nitrogen oxides in exhaust gas and the above catalyst.
[0020]
[Means for Solving the Problems]
According to the present invention, in the method for supplying and burning fuel under periodic rich / lean conditions, bringing the produced exhaust gas into contact with the catalyst, and catalytically reducing the nitrogen oxides in the exhaust gas, the catalyst comprises: (A) a first catalyst component (provided that cerium oxide and (b) a mixture or complex oxide of at least one element selected from zirconium, gadolinium, terbium, samarium and lanthanum) Excluding mixtures of oxides and zirconium oxides or composite oxides);
(C) a surface catalyst layer having, as a surface catalyst component, a second catalyst component comprising at least one selected from rhodium, palladium, rhodium oxide and palladium oxide;
(B) (a) It comprises at least one catalyst component selected from platinum, rhodium, palladium, platinum oxide, rhodium oxide and palladium oxide, and (b) an internal catalyst layer having a carrier as an internal catalyst component. A method for catalytically reducing nitrogen oxides in exhaust gas is provided.
[0021]
Further, according to the present invention, there is provided a catalyst for supplying and burning fuel under periodic rich / lean conditions, bringing the generated exhaust gas into contact with the catalyst, and catalytically reducing nitrogen oxides in the exhaust gas. There,
(A) a first catalyst component (provided that cerium oxide and (b) a mixture or complex oxide of at least one element selected from zirconium, gadolinium, terbium, samarium and lanthanum) Excluding mixtures of oxides and zirconium oxides or composite oxides);
(C) a surface catalyst layer having, as a surface catalyst component, a second catalyst component comprising at least one selected from rhodium, palladium, rhodium oxide and palladium oxide;
(B) (a) It comprises at least one catalyst component selected from platinum, rhodium, palladium, platinum oxide, rhodium oxide and palladium oxide, and (b) an internal catalyst layer having a carrier as an internal catalyst component. A catalyst for catalytic reduction of nitrogen oxides in exhaust gas is provided.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
The catalyst for catalytically reducing nitrogen oxides according to the present invention has a surface catalyst layer and an internal catalyst layer, and the surface catalyst layer is exposed so as to be in direct contact with exhaust gas. According to the present invention, such a catalyst is preferably used as a catalyst structure in which an internal catalyst layer and a surface catalyst layer are laminated in this order on an inert base material.
[0023]
In the present invention, the catalytic reduction of nitrogen oxides means that most of NOx is directly decomposed into nitrogen and oxygen by a catalytic reaction. A part of NOx is converted into nitrogen and water, carbon monoxide, carbon dioxide and the like by reaction with a reducing agent.
[0024]
The surface catalyst layer of the catalyst according to the present invention comprises:
(A) a first catalyst component (provided that cerium oxide and (b) a mixture or complex oxide of at least one element selected from zirconium, gadolinium, terbium, samarium and lanthanum) Excluding mixtures of oxides and zirconium oxides or composite oxides);
(C) It has as a surface catalyst component the 2nd catalyst component which consists of at least 1 sort (s) chosen from rhodium, palladium, rhodium oxide, and palladium oxide.
[0025]
On the other hand, the internal catalyst layer is
(B) (a) It has at least one catalyst component selected from platinum, rhodium, palladium, platinum oxide, rhodium oxide and palladium oxide, and (b) a carrier as an internal catalyst component.
[0026]
The catalyst according to the present invention can have any catalyst structure as long as it has such a surface catalyst layer and an internal catalyst layer and operates according to the method described above. The catalyst of the present invention can also include additives to improve its efficiency.
[0027]
According to the invention, the surface catalyst layer comprises at least 50% by weight, particularly preferably at least 80% by weight, of the surface catalyst component. In the surface catalyst layer, when the ratio of the surface catalyst component is less than 50% by weight, the oxygen storage capacity of the surface catalyst layer obtained is reduced to the reducing atmosphere of the internal catalyst layer during the normal lean time (1 to 2 minutes). When it is lean, the NOx reducing ability on rhodium or palladium is lowered, and the SOx durability is lowered.
[0028]
According to the present invention, the first catalyst component of the surface catalyst layer contains the noble metal catalyst component such as platinum constituting the internal catalyst layer together with the second catalyst component of the surface catalyst layer under a short time rich condition . With the help of high reducing power , it is rapidly reduced and loses some of the oxygen in the oxide (eg ceria) while storing oxygen in the gas phase under lean conditions. That is, it functions as a substance having an oxygen storage capacity (hereinafter referred to as an OSC functional substance). Thus, under lean conditions, the first catalyst component of the surface catalyst layer, that is, the OSC functional substance, suppresses oxidation of rhodium or palladium, which is the second catalyst component of the surface catalyst layer, on rhodium or palladium. This makes it possible for the NOx reduction reaction to proceed with high efficiency.
[0029]
According to the present invention, as described above, in the surface catalyst layer, as the first catalyst component that functions as an OSC functional substance,
(A) a first catalyst component (provided that cerium oxide and (b) a mixture or complex oxide (solid solution) of an oxide of at least one element selected from zirconium, gadolinium, terbium, samarium and lanthanum Excluding mixtures of oxides and zirconium oxides or composite oxides).
[0030]
Here, the mixture is preferably a uniform mixture. However, according to the present invention, a complex oxide of the above element is preferably used rather than a mixture of the above oxides, and a binary or ternary complex oxide is particularly preferably used.
[0031]
For example, in the case of a binary composite oxide, the oxide-based weight ratio of each element in the solid solution is preferably 80/20 to 60 if it is a ceria / terbium oxide composite oxide or a ceria / samarium oxide composite oxide. The range is / 40. In the case of a ternary composite oxide, the oxide-based weight ratio in the solid solution is ceria / gadolinium oxide / zirconia composite oxide, ceria / zirconia / gadolinium composite oxide, ceria / zirconia / lanthanum oxide composite oxide, In the case of ceria / zirconia / samarium oxide composite oxide, ceria / zirconia / terbium oxide composite oxide, etc., the range is preferably 45/30/30 to 75/20/5. However, in the present invention, the oxide basis weight ratio of the respective elements in these composite oxides, ceria, zirconia, terbium oxide, gadolinium oxide, CeO ₂ samarium oxide and lanthanum oxide, respectively, ZrO _2, Tb ₂ O ₃ , Ga ₂ O ₃ , Sm ₂ O ₃ and La ₂ O ₃ .
[0032]
According to the present invention, the surface catalyst layer, together with the first catalyst component, further
(C) having at least one second catalyst component selected from rhodium, palladium, rhodium oxide and palladium oxide, and according to the present invention, such second catalyst component is preferably It is supported on the first catalyst component which is the OSC functional material described above. According to this invention, such a 2nd catalyst component is contained in 0.05 to 3 weight% of the catalyst component of a surface catalyst layer.
[0033]
Such a second catalyst component increases the rate of reduction of the OSC functional substance when rich, and is itself reduced, functions as an active species of NOx decomposition reaction when lean, and plays a role of rapidly reducing and decomposing NOx. I'm in charge.
[0034]
The thickness of the surface catalyst layer has a great influence on the NOx reduction ability and SOx poisoning resistance in the rich / lean process of the catalyst according to the present invention. The optimum thickness of the surface catalyst layer depends on the reaction conditions such as temperature, oxygen concentration, space velocity (SV), etc., but a preferable surface catalyst layer capable of obtaining high NOx reduction ability in the rich / lean process. The thickness of is in the range of 20 μm to 80 μm. However, usually about 40 μm is preferable. Even if the thickness of the surface catalyst layer is set to 80 μm or more, the performance is not improved correspondingly, and rather the low performance is lowered. Further, when the thickness of the surface catalyst layer is made smaller than 20 μm, NOx is oxidized on the internal catalyst layer during lean, and the NOx reducing ability is lowered.
[0035]
Here, according to the present invention, the thickness of the catalyst layer is determined by calculation from the coating amount of the slurry containing the catalyst component on the base material with the apparent density of the catalyst layer being 1.5 g / cm ³ for convenience. Can do.
[0036]
In the present invention, the surface catalyst component forming the surface catalyst layer can be obtained, for example, by the following method. That is, first, a water-soluble salt of an element constituting the first catalyst component such as cerium, terbium, samarium, lanthanum, gadolinium, for example, an aqueous solution of nitrate is neutralized or hydrolyzed by heating to form a hydroxide. After forming, the obtained product is calcined at a temperature of 300 to 900 ° C. in an oxidizing or reducing atmosphere to obtain a first catalyst component. However, the first catalyst component can also be obtained by firing a hydroxide or oxide such as cerium, terbium, samarium, lanthanum, gadolinium and the like that is commercially available as described above. Next, a water-soluble salt of an element constituting the second catalyst component, for example, by an appropriate means such as an impregnation method or an ion exchange method conventionally known on the first catalyst component thus obtained, After carrying an aqueous solution of nitrate, the surface catalyst component composed of the first and second catalyst components can be obtained as a powder by firing at a temperature of 500 to 900 ° C. in an oxidizing or reducing atmosphere. .
[0037]
According to the method of the present invention, the reduction of nitrogen oxides is performed in the following manner under a rich stroke, that is, under reducing conditions. The noble metal catalyst component of the inner catalyst layer oxidized in the lean process is reduced, and NOx is reduced and decomposed on these catalysts. Furthermore, since oxygen is rapidly reduced by the reducing agent on the noble metal catalyst component, the first catalyst component in the surface catalyst layer, that is, the oxygen of the OSC functional material spills over to the internal catalyst layer, and as a result, The OSC functional material is efficiently partially reduced with the second catalyst component in the surface catalyst layer, and thus regenerated for the next lean stroke.
[0038]
Next, under the lean process following such a rich process, that is, under oxidizing conditions, nitrogen oxides are reduced as follows. That is, in the lean process, oxygen diffused into the surface catalyst layer is occluded by the first catalyst component of the surface catalyst layer, that is, the OSC functional substance, and thus the reaction of the second catalyst component in the surface catalyst layer. The atmosphere is maintained at rich or stoichiometric conditions. As a result, even under lean conditions, NOx is rapidly reduced and decomposed on the second catalyst component of the surface catalyst layer.
[0039]
Further, according to the present invention, SOx is trapped in the surface catalyst layer, and is desorbed and discharged when rich, and is difficult to be adsorbed by the promoter in the internal catalyst layer. It is very small compared to a storage-reduction system. Thus, the catalyst according to the present invention hardly deteriorates even when used in the presence of SOx. Further, the NOx adsorbed at the time of lean is easily regenerated at the time of rich.
[0040]
According to the invention, the inner catalyst layer comprises at least 50% by weight, particularly preferably at least 80% by weight, of the inner catalyst component. In the internal catalyst layer, when the ratio of the internal catalyst component is less than 50% by weight, the NOx catalytic reduction ability when the internal catalyst layer is rich is reduced, and the reduction of the noble metal catalyst component and the OSC functional substance in the surface catalyst layer is facilitated. The effect is reduced.
[0041]
The internal catalyst layer has an internal catalyst component composed of at least one catalyst component selected from platinum, rhodium, palladium, platinum oxide, rhodium oxide, and palladium oxide and a carrier, and the catalyst component is an internal catalyst. In the layer, it is contained in the range of 0.05 to 5% by weight in terms of metal. When the ratio of the catalyst component in the internal catalyst layer exceeds 5% by weight in terms of metal, the selectivity of the reaction between NOx and the reducing agent at the time of richness decreases, and on the other hand, when it is less than 0.05% by weight When the catalytic reaction atmosphere changes from lean to rich, the conversion rate of the lean to rich reaction atmosphere decreases, and as a result, the NOx reduction resolution by the catalyst of the present invention decreases.
[0042]
According to the present invention, such a catalyst component is preferably supported on a conventionally known carrier such as alumina, silica, silica-alumina, zeolite, titania and the like.
[0043]
The internal catalyst component is one method, for example, adding a carrier such as alumina to an aqueous solution of a water-soluble salt such as platinum as a catalyst component to form a slurry, mixing, stirring, drying, and oxidizing or reducing atmosphere If it is fired at a temperature of 500 to 900 ° C., it can be obtained as a powder having a catalyst component on the carrier.
[0044]
In particular, as a preferable method, an aqueous solution of an ion-exchangeable aqueous salt such as platinum as a catalyst component (for example, tetraammineplatinum (II) nitrate (Pt (NH ₃ ) ₄ (NO ₃ ) ₂ ) or dinitrodiammine platinum (II)). (Pt (NO ₂ ) ₂ (NH ₃ ) ₂ ) The catalyst component is supported on a carrier such as alumina by an ion exchange method, dried, and calcined at a temperature of 500 to 900 ° C. in an oxidizing or reducing atmosphere. What has a component can be obtained as a powder.
[0045]
The internal catalyst layer has a high NOx reducing ability of the internal catalyst component. Therefore, the influence of the thickness on the NOx reducing ability in the rich / lean process is not as great as the thickness of the surface catalyst layer, but usually, A range of 10 μm to 50 μm is appropriate. Even if the thickness of the internal catalyst layer is larger than 50 μm, the performance is not improved correspondingly, and when the thickness of the internal catalyst layer is smaller than 10 μm, the NOx reducing ability is lowered.
[0046]
According to the present invention, the catalyst components for the surface catalyst layer and the internal catalyst layer can be obtained in various forms such as powder and granular materials as described above. Therefore, the internal catalyst layer is formed into various shapes such as a honeycomb, an annular material, a spherical material, and the like by using any of the internal catalyst components by a well-known method. By using a surface catalyst component and forming a surface catalyst layer on the surface of the internal catalyst layer, catalyst structures of various shapes can be obtained. In preparing such a catalyst structure, appropriate additives such as molding aids, reinforcing materials, inorganic fibers, organic binders, and the like can be used as necessary.
[0047]
In particular, the catalyst according to the present invention has an internal catalyst layer and a surface catalyst layer on the surface of an inert substrate for support of any shape, for example, by a wash coat method (for example, applied). It is advantageous to have a catalyst structure having The inert base material may be made of, for example, a clay mineral such as cordierite or a metal such as stainless steel, preferably a heat-resistant metal such as Fe-Cr-Al. The shape may be a honeycomb, an annular shape, a spherical structure, or the like.
[0048]
All of the catalysts according to the present invention are excellent not only in resistance to heat but also in resistance to sulfur oxides, so that the reduction of NOx in the exhaust gas of automobiles of diesel engines and lean gasoline engines, that is, denitration. It is suitable for using as a catalyst for this.
[0049]
In the present invention, the catalyst is preferably used in a catalytic reaction under a condition where the combustion atmosphere of the fuel vibrates between the rich condition and the lean condition as described above. Here, the period of the catalytic reaction (that is, the time from the rich atmosphere (or lean atmosphere) to the next rich atmosphere (or lean atmosphere)) is preferably 5 to 150 seconds, particularly preferably 30 to 90 seconds. is there. The rich / lean width, that is, the rich time (seconds) / lean time (seconds) is usually in the range of 0.5 / 5 to 10/150, preferably in the range of 2/30 to 5/90. It is.
[0050]
The rich condition is usually formed by periodically injecting fuel into the combustion chamber of the engine at an air / fuel ratio of 10 to 14 by weight when using gasoline as the fuel. Typical exhaust gases under rich conditions include hundreds of ppm by volume of NOx, 5-6% by volume of water, 2-3% by volume of CO, 2-3% by volume of hydrogen, thousands of ppm by volume of hydrocarbons and 0 Contains ~ 0.5% oxygen by volume. On the other hand, when using gasoline as fuel, the lean condition is usually formed by periodically injecting fuel into the combustion chamber of the engine at an air / fuel ratio of 20 to 40 by weight. Typical exhaust gases under lean conditions include hundreds of ppm by volume of NOx, 5-6% by volume of water, thousands of ppm by volume of CO, thousands of ppm by volume of hydrogen, thousands of ppm by volume of hydrocarbons, and 5-10 by volume. Contains oxygen by volume.
[0051]
The temperature suitable for NOx catalytic reduction using the catalyst according to the invention depends on the individual gas composition, but is usually 150 so as to have an effective catalytic reaction activity for NOx over a long period in the rich stroke. It is the range of -550 degreeC, Preferably, it is the range of 200-500 degreeC. In the reaction temperature range, the exhaust gas is preferably treated at a space velocity in the range of 5000 to 100,000 hr ⁻¹ .
[0052]
According to the method of the present invention, as described above, the exhaust gas containing NOx is brought into contact with the above-described catalyst in the periodic rich / lean process, so that the exhaust gas is present even in the presence of oxygen, sulfur oxide, or moisture. The NOx contained therein can be stably and efficiently catalytically reduced.
[0053]
【Example】
Hereinafter, the present invention will be described by way of examples of the production of the powder catalyst which is the internal catalyst component and the surface catalyst component, the production example of the honeycomb catalyst structure using them, and the nitrogen oxide reduction performance of the honeycomb catalyst structure as examples. Although described in detail, the present invention is not limited to these examples. In the following, all “parts” and “%” are by weight unless otherwise specified.
[0054]
(1) Preparation of internal catalyst component
Production Example 1
8.40 g of an aqueous solution of Pt (NH ₃ ) ₄ (NO ₃ ) ₂ (9.0% as platinum) is added to 100 mL of ion-exchanged water to obtain an aqueous solution, and γ-alumina (Sumitomo Chemical Co., Ltd. KC-) 501) 60 g was charged, dried at 100 ° C. with stirring, and then calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder having 1% platinum supported on γ-alumina. It was.
[0056]
Production Example 2
To 100 mL of ion exchange water, 8.40 g of an aqueous solution of Pt (NH ₃ ) ₄ (NO ₃ ) ₂ (9.0% as platinum) and 4.20 g of an aqueous rhodium nitrate solution (9.0% as rhodium) were added to obtain an aqueous solution. 60 g of γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) was added thereto, dried at 100 ° C. with stirring, and then calcined in air at 500 ° C. for 3 hours to obtain γ-alumina. A catalyst powder having 1% platinum and 0.5% rhodium supported thereon was obtained.
[0057]
Production Example 3
To 100 mL of ion-exchanged water, 4.20 g of a palladium nitrate aqueous solution (9.0% as palladium) and 8.40 g of Pt (NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) were added to obtain an aqueous solution. 60 g of γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) was added thereto, dried at 100 ° C. with stirring, and then calcined in air at 500 ° C. for 3 hours to obtain γ-alumina. A catalyst powder having 1% platinum and 0.5% palladium supported thereon was obtained.
[0058]
(2) Preparation of surface catalyst component
Production Example 4
In 100 mL of ion exchange water, 151.37 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O), 32.09 g of zirconium oxynitrate (ZrO (NO ₃ ) ₂ ), and gadolinium nitrate (Gd (NO ₃ ) ₃ .6H ₂ ) O) 21.41 g was added to make an aqueous solution, and 0.1 N ammonia water was added thereto to neutralize and hydrolyze the cerium ions, followed by aging for 1 hour. The obtained slurry was filtered, dried at 120 ° C. for 24 hours, and then calcined in air at 500 ° C. for 3 hours to obtain a ceria / zirconia / gadolinium oxide composite oxide powder (weight ratio of oxide based on 70/20 / 10 and a specific surface area of 182 m ² / g).
[0060]
To 100 mL of ion-exchanged water, 4.20 g of an aqueous rhodium nitrate solution (9.0% as rhodium) is added to obtain an aqueous solution, and 30 g of the above-mentioned ceria / zirconia / gadolinium oxide powder is added to the solution and dried at 100 ° C. with stirring. Then, it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 1% rhodium was supported on a ceria / zirconia / gadolinium oxide powder.
[0061]
Production Example 5
An aqueous solution was prepared by dissolving 103.77 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O) and 40.96 g of terbium nitrate (Tb (NO ₃ ) ₃ .6H ₂ O) in 100 mL of ion-exchanged water. . 0.1N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt and terbium salt, followed by aging for 1 hour. The product was separated from the obtained slurry by filtration, dried at 120 ° C. for 24 hours, and then calcined in air at 500 ° C. for 3 hours to obtain a ceria / terbium oxide composite oxide powder (oxide basis). A weight ratio of 70/30 and a specific surface area of 139 m ² / g) were obtained.
[0062]
2.100 g of rhodium nitrate aqueous solution (9.0% as rhodium) was added to 100 mL of ion-exchanged water to obtain an aqueous solution, and 30 g of the above-mentioned ceria / terbium oxide composite oxide powder was added to the solution at 100 ° C. with stirring. After drying, it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 0.5% rhodium was supported on the ceria / terbium oxide composite oxide.
[0063]
Production Example 6
In 1700 mL of ion-exchanged water, 34.59 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O), 84.45 g of zirconium oxynitrate (ZrO (NO ₃ ) ₂ ), and lanthanum nitrate (La (NO ₃ ) ₃ .6H ₂ ) O) 7.97 g was dissolved to prepare an aqueous solution. 0.1N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt, zirconium salt and lanthanum salt, followed by aging for 1 hour. The product was separated from the resulting slurry by filtration, dried at 120 ° C. for 24 hours, and then calcined in air at 500 ° C. for 3 hours to obtain a ceria / zirconia / lanthanum oxide composite oxide powder (oxidized). (Weight basis 22/73/5, specific surface area 80 m ² / g).
[0064]
To 100 mL of ion-exchanged water, 2.10 g of an aqueous rhodium nitrate solution (9.0% as rhodium) was added to obtain an aqueous solution. To this, 30 g of the above-mentioned ceria / zirconia / lanthanum oxide composite oxide powder was added and stirred. After drying at 5 ° C., it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 0.5% rhodium was supported on the ceria / zirconia / lanthanum oxide composite oxide.
[0065]
Production Example 7
In 1700 mL of ion-exchanged water, 121.06 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O), 28.12 g of zirconium oxynitrate (ZrO (NO ₃ ) ₂ ) and samarium nitrate (Sm (NO ₃ ) ₃ · 6H ₂ O) 3.40 g was dissolved to prepare an aqueous solution. 0.1N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt, zirconium salt and samarium salt, followed by aging for 1 hour. The product was separated from the resulting slurry by filtration, dried at 120 ° C. for 24 hours, and then calcined in air at 500 ° C. for 3 hours to obtain a ceria / zirconia / samarium oxide composite oxide powder (oxidized). Weight basis weight ratio 72/24/4, specific surface area 187 m ² / g).
[0066]
To 100 mL of ion exchange water, 2.10 g of an aqueous rhodium nitrate solution (9.0% as rhodium) was added to obtain an aqueous solution, and 30 g of the above-mentioned ceria / zirconia / samarium oxide composite oxide powder was added to the solution while stirring. After drying at 5 ° C., it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 0.5% rhodium was supported on the ceria / zirconia / samarium oxide composite oxide.
[0067]
(3) Preparation of honeycomb catalyst
The catalyst layer thickness was determined by calculation with the apparent density of the catalyst layer being 1.5 g / cm ³ and the mechanical contact area of the honeycomb being 2500 m ² / g.
[0069]
Example 1
30 g of a powder catalyst obtained by supporting 1% platinum on the γ-alumina obtained in Production Example 1 was mixed with 3 g of silica sol (Nissan Chemical Industry Co., Ltd. Snowtex N, 20 wt% as silica) and an appropriate amount of water. Mixed. Using 50 g of zirconia balls as a grinding medium, this mixture was ground in a planetary mill for 5 minutes to obtain a washcoat slurry. The washcoat slurry was applied to a cordierite honeycomb substrate having 400 cells per square inch to obtain a honeycomb structure having an internal catalyst layer made of the powder catalyst and having a thickness of 30 μm.
[0070]
Next, 30 g of powder obtained by supporting 1% rhodium on the ceria / zirconia / gadolinium oxide composite oxide obtained in Production Example 4 was added to silica sol (Snowtex N, Nissan Chemical Industries, Ltd., 20% by weight as silica). ) 3 g and an appropriate amount of water were mixed, and a slurry was prepared in the same manner as described above. The slurry was coated on the internal catalyst layer to obtain a honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm made of a catalyst having 1% rhodium supported on a ceria / zirconia / gadolinium oxide composite oxide.
[0071]
Example 2
In the same manner as in Example 1, an internal catalyst layer having a thickness of 30 μm composed of a catalyst having 1% platinum supported on γ-alumina obtained in Production Example 1, and a ceria / terbium oxide composite oxide obtained in Production Example 5 A honeycomb catalyst structure having a surface catalyst layer with a thickness of 60 μm made of a catalyst in which 0.5% of rhodium was supported on (oxide basis weight ratio 70/30) was obtained.
[0072]
Example 3
In the same manner as in Example 1, an internal catalyst layer having a thickness of 30 μm composed of a catalyst in which 1% platinum and 0.5% rhodium were supported on the γ-alumina obtained in Production Example 2, and obtained in Production Example 6 were obtained. A honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm made of a catalyst in which 0.5% rhodium was supported on a ceria / zirconia / lanthanum oxide composite oxide (oxide basis weight ratio 22/73/5) was prepared. .
[0073]
Example 4
In the same manner as in Example 1, an internal catalyst layer having a thickness of 30 μm composed of a catalyst in which 1% platinum and 0.5% palladium were supported on the γ-alumina obtained in Production Example 3, and obtained in Production Example 7 were obtained. A honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm made of a catalyst having 0.5% rhodium supported on a ceria / zirconia / samarium oxide composite oxide (oxide basis weight ratio 72/24/4) was obtained.
[0074]
Comparative Example 1
15 g of the ceria powder obtained in Production Example 5 and 15 g of γ-alumina (KC-501, manufactured by Sumitomo Chemical Co., Ltd.) were dry-mixed, and 30 g of catalyst powder supporting 1% platinum on this mixture was used. In the same manner as in Example 1, a washcoat slurry was prepared. The slurry was coated on the same cordierite honeycomb substrate as in Example 1 in the same manner as in Example 1 to obtain a honeycomb catalyst structure having a catalyst layer with a thickness of 60 μm.
[0075]
Comparative Example 2
Barium carbonate was prepared from an aqueous caustic barium solution and an aqueous sodium carbonate solution, and platinum was mixed on the mixture (weight ratio 8/2) of this barium carbonate (BaCO ₃ ) and γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.). A catalyst powder was prepared by loading 1%. γ-Alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) was added to a potassium carbonate aqueous solution, mixed and dried, and then calcined in air at 1100 ° C. for 3 hours to obtain K ₂ O.12Al ₂ O _3. (Specific surface area 18 m ² / g) was prepared. Separately, γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) and the above K ₂ O.12Al ₂ O ₃ were mixed, and the weight ratio of γ-alumina / K ₂ O.12Al ₂ O _{3 was} 9/1. A mixture was prepared, and 1% platinum was supported thereon to obtain catalyst powder.
[0076]
48 g of catalyst powder supporting 1% platinum on the BaCO ₃ / γ-alumina and 12 g of catalyst powder supporting 1% platinum on the γ-alumina / K ₂ O.12Al ₂ O ₃ A dry coating slurry was prepared in the same manner as in Example 1 by dry mixing. This slurry was coated on the same cordierite honeycomb substrate as in Example 1 in the same manner as in Example 1 to obtain a honeycomb catalyst structure having a catalyst layer with a thickness of 80 μm.
[0077]
(4) Performance test
Using the catalyst structures according to the above examples and comparative examples, the gas containing nitrogen oxides was reduced under the following conditions. The conversion rate (removal rate) from nitrogen oxides to nitrogen was determined by the chemical luminescence method.
[0079]
Test method The composition of the mixed gas used in the NOx reduction experiment under rich conditions is as follows.
NO: 500ppm
SO ₂ : 20 ppm
O ₂ : 0.4%
CO: 2%
C ₃ H ₆ (propylene): 2000 ppm
H ₂ : 2%
H ₂ O: 6.0%
[0080]
The gas under lean conditions is prepared by injecting oxygen into a mixed gas under rich conditions, and the composition thereof is as follows.
NO: 500ppm
SO ₂ : 20ppm
O ₂ : 9.0%
CO: 0.2%
C ₃ H ₆ (propylene): 500 ppm
H ₂ : 0%
H ₂ O: 6.0%
[0081]
Catalytic reaction was conducted with the rich / lean width in the range of 3 to 30 (second / second) to 12/120 (second / second), and the performance of each catalyst was examined.
[0082]
(I) Space velocity:
70000 hr ^-1 (under lean conditions)
70000 hr ⁻¹ (under rich conditions)
[0083]
(Ii) Reaction temperature:
200, 250, 300, 350, 400, 450 and 500 ° C
The results are shown in Table 1. As is apparent from Table 1, the catalyst according to the present invention has a high nitrogen oxide removal rate. On the other hand, the catalyst according to the comparative example generally has a low nitrogen oxide removal rate.
[0084]
[Table 1]

[0085]
Furthermore, using the catalyst structures according to Example 1 and Comparative Examples 1 and 2, respectively, the above gas conditions, rich / lean width (seconds / second) was 55/5, reaction temperature was 350 ° C., and a 50 hour durability test was performed. went. The results are shown in Table 2. As is apparent from Table 2, the catalyst according to the present invention has a very high resistance to sulfur oxides as compared to the conventional NOx storage-reduction system.
[0086]
[Table 2]

Claims (6)

The rich time (seconds) / lean time (seconds) is set within a range of 0.5 / 5 to 10/150 , fuel is supplied and burned under periodic rich / lean conditions, and the generated exhaust gas is brought into contact with the catalyst. In the method for catalytically reducing nitrogen oxides in the exhaust gas, the catalyst is oxidized with at least one element selected from (A) (a) cerium oxide and (b) zirconium, gadolinium, terbium, samarium and lanthanum. A first catalyst component (excluding a mixture or composite oxide of cerium oxide and zirconium oxide) comprising a mixture or composite oxide of
(C) a surface catalyst layer having, as a surface catalyst component, at least one second catalyst component selected from rhodium, palladium, rhodium oxide and palladium oxide;
(B) (a) consisting of at least one catalyst component selected from platinum, rhodium, palladium, platinum oxide, rhodium oxide and palladium oxide, and (b) an internal catalyst layer having a carrier as an internal catalyst component. A method for catalytically reducing nitrogen oxides in exhaust gas. The method according to claim 1, wherein the rich time (seconds) / lean time (seconds) is in the range of 2/30 to 5/90. The method according to claim 1, wherein in the catalyst, the second catalyst component of the surface catalyst layer is supported on the first catalyst component. By setting the rich time (seconds) / lean time (seconds) to a range of 0.5 / 5 to 10/150, fuel is supplied and burned under periodic rich / lean conditions, and the generated exhaust gas is converted into the catalyst structure. In the method of contacting and reducing nitrogen oxides in the exhaust gas, (A) (a) cerium oxide and (b) zirconium, gadolinium, terbium, samarium on a substrate on which the catalyst structure is inert And a first catalyst component comprising a mixture or complex oxide with an oxide of at least one element selected from lanthanum (except for a mixture or complex oxide of cerium oxide and zirconium oxide);
(C) a surface catalyst layer having, as a surface catalyst component, at least one second catalyst component selected from rhodium, palladium, rhodium oxide and palladium oxide;
(B) (a) having at least one catalyst component selected from platinum, rhodium, palladium, platinum oxide, rhodium oxide and palladium oxide; and (b) an internal catalyst layer having a carrier as an internal catalyst component. A method for catalytic reduction of nitrogen oxides in exhaust gas. The method according to claim 4, wherein the rich time (seconds) / lean time (seconds) is in the range of 2/30 to 5/90. The method according to claim 4, wherein in the catalyst structure, the second catalyst component of the surface catalyst layer is supported on the first catalyst component.