WO2004060541A1 - Method of catalytic reduction of nitrogen oxides and catalyst for use therein - Google Patents

Abstract

A method of catalytic reduction of nitrogen oxides involving supplying and burning a fuel under the periodical rich/lean condition described in the specification, and contacting the resultant exhaust gas with a catalyst, to thereby catalytically reduce nitrogen oxides in the exhaust gas, characterized in that the catalyst comprises: (A) a first catalyst component comprising (a) a ceria or (b) a praseodymium oxide or (c) at lest one selected from a mixture of oxides of at least two elements selected from among cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum, and a composite oxide of at lest two of these elements, and (B) a second catalyst composition comprising at least one selected from among platinum, rhodium, palladium, a platinum oxide, a rhodium oxide and a palladium oxide; an above catalyst; and a catalyst structure for use in the catalytic reduction of NOx which comprises an inert substrate and the above catalyst provided on the substrate as a catalyst layer.

Description

 Method for catalytic reduction of nitrogen oxides and catalyst therefor

The present invention, nitrogen oxides (mainly consisting of NO and N_〇 _2. Hereinafter referred to. N_〇_X) catalytic reduction decomposition by a catalyst, i.e., a method for the catalytic reduction. Specifically, the present invention relates to

A method for catalytically reducing NOx in exhaust gas by supplying and burning fuel in a periodic rich-no-lean fuel supply process (excursion: process) and bringing the generated exhaust gas into contact with a catalyst.

About. This method is suitable, for example, for reducing and removing harmful nitrogen oxides contained in exhaust gas from automobiles.

Further, the present invention is particularly made of sulfur oxides (mainly, so ₂ and S_〇 _3. Hereinafter, so

X In the presence of a), fuel is supplied by burning at periodic rich / lean fuel supply stroke, a catalyst for catalytic reduction of N_〇 _x in the exhaust gas produced by the combustion. In the context of the present invention, the term "excursion" means that the air-Z fuel ratio moves from its average value to both along the time axis or such an operation. The term "rich" means that the air-Z fuel ratio of the fuel in question is smaller than the stoichiometric air-Z 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. For ordinary motor gasoline, the stoichiometric air-fuel ratio is about 14.5. In the present invention, “catalyst” means a catalyst or a structure including the same that operates to remove Nx during fuel rich / lean combustion.

Therefore, in the present invention, "supplying the fuel in the periodic Ritchino lean fuel supply process" means that the combustion of fuel is mainly performed under lean conditions (oxygen concentration in exhaust gas after combustion) in a combustion chamber of a diesel engine or a gasoline engine. Is usually about 5% to 10%.) The fuel is adjusted while adjusting the air-Z fuel ratio so that the atmosphere is periodically vibrated alternately between the above-described rich condition and lean condition. Supply, injection or injection. Therefore, a rich lean process is equivalent to a rich / lean condition. Background art

 Conventionally, NOx contained in exhaust gas has been reduced, for example, by a method that absorbs it into an alkali after it has been neutralized, or a method that reduces ammonia to nitrogen using ammonia, hydrogen, carbon monoxide, or a hydrocarbon as a reducing agent. Has been removed. However, each of these conventional methods has drawbacks.

 That is, according to the former method, means for treating the generated alkaline wastewater is required to prevent environmental problems. According to the latter method, for example, when ammonia is used as a reducing agent, the ammonia reacts with S Ox in the exhaust gas to form salts, and as a result, the catalytic activity decreases at low temperatures. Also, especially when processing NOx from mobile sources such as automobiles, its safety is an issue.

 On the other hand, if a three-way catalyst is used as the catalyst and hydrogen, carbon monoxide or hydrocarbons are used as the reducing agent, those reducing agents contain oxygen at a higher concentration than N〇x. It reacts preferentially with oxygen, thus attempting to substantially reduce NOx would require large amounts of reducing agent, which would greatly reduce fuel consumption.

Therefore, it has been proposed to catalytically decompose N Ox without using a reducing agent. However, conventionally known catalysts for directly decomposing N〇x have not yet been put into practical use due to their low decomposition activity. On the other hand, various zeolites have been proposed as selective NO x catalytic reduction catalysts using hydrocarbons and oxygen-containing organic compounds as reducing agents. In particular, Di-exchange ZSM 5 or H-type (hydrogen type or acid type) Zeoraito ZSM 5 (S I_〇 ₂ / A 1 ₂ 0 ₃ molar ratio = 3 0-4 0) is to be optimal I have. However, even H-type zeolites do not have sufficient reducing activity.Particularly when the exhaust gas contains moisture, zeolite catalysts can quickly deteriorate due to dealumination of the zeolite structure. Are known. Under such circumstances, development of a catalyst having higher activity for NOx catalytic reduction has been demanded. Recently, a catalyst comprising silver or silver oxide on an inorganic oxide carrier material has been developed. It has been proposed (European Patent Application Publication No. 526,099 and European Patent Application Publication No. 699,427 (Japanese Patent Application Laid-Open No. 5-3176447)). It is known that this catalyst has a high selective reduction activity for NOx, but a low oxidizing activity, so that the conversion rate of NOx to nitrogen is slow, that is, the SV (space velocity) dependence is large. Moreover, this catalyst has a problem that its activity is rapidly reduced in the presence of SOx. These catalysts use NOx with hydrocarbons under completely lean conditions. 9

It has a catalytic action to selectively reduce 3 x to some extent, but it has such a low N〇 _x removal rate and a narrow temperature window (temperature range) that it operates compared to a three-way catalyst. This makes it difficult to commercialize Ox catalysts. Thus, there is an urgent need for a more heat-resistant and more active catalyst for N〇x catalytic reduction.

 In order to overcome the above-mentioned problems, Ox storage-reduction systems have recently been proposed as the most promising methods (Japanese Patent Application Laid-Open Nos. Hei 5-3-171652 and Hei 6-3-1113). No. 9). According to this proposal, fuel is periodically supplied to the combustion chamber in a short time in an amount exceeding the stoichiometric amount. Vehicles with lean-burn engines can be driven with very low fuel-to-air ratios, resulting in lower fuel consumption than vehicles with conventional engines. The NOx storage-reduction system of such a lean burn engine reduces NOx by two steps, a first step and a second step that are periodically performed at intervals of 1 to 2 minutes.

That is, in the first step, or normal, Li Ichin conditions, NO on the platinum or rhodium catalyst is oxidized to N_〇 _2, the N 0 ₂ is K ₂ C_〇 ₃ and B a C_〇 ₃ It is adsorbed by alkaline compounds such as Next, a rich condition for the second step is formed, and the rich condition is maintained for several seconds. This rich conditions, N 0 ₂ where the adsorbed (storage) is reduced to efficiently nitrogen using desorbed from the adsorption sites, hydrocarbons on platinum or rhodium catalyst, the Ichisani匕炭group or hydrogen . This NOx storage-reduction system works well over a long period in the absence of SOx. However, if there is S Ox, even in the lean and rich any conditions, by irreversible adsorption of S_〇 _x in N_〇 ₂ adsorption sites on § alkali compound, the catalyst system will deteriorate rapidly. Disclosure of the invention

 The present invention relates to a method for burning fuel under periodic rich-noring conditions, even in the presence of oxygen, sulfur oxides, or water, and at a wide range of reaction temperatures. It is an object of the present invention to provide a method and a catalyst for reducing and decomposing N Ox with high durability, in particular, a catalyst structure.

According to the present invention, in the method for supplying and burning fuel under periodic lithium-lean conditions, contacting the generated exhaust gas with the catalyst, and catalytically reducing nitrogen oxides in the exhaust gas, (A) (a) ceria or

 (b) acid ft; praseodymium or

 (c) Cerium, zirconium, praseodymium, neodymium, terbium,

 A first catalyst component comprising a mixture of an oxide of at least two elements selected from samarium, gadolinium and lanthanum and at least one selected from a complex oxide of at least these two elements;

(B) at least one selected from platinum, rhodium, palladium and their oxides

 One kind of second catalyst component

And a method for catalytically reducing nitrogen oxides in exhaust gas. Further, according to the present invention, a catalyst for supplying and burning fuel under periodic rich Z-lean conditions, contacting the produced exhaust gas with the catalyst, and catalytically reducing nitrogen oxides in the exhaust gas And

 (A) (a) ceria or

 (b) Praseodymium oxide or

 (C) Cerium, zirconium, praseodymium, neodymium, terbium,

 A first catalyst component comprising a mixture of an oxide of at least two elements selected from samarium, gadolinium and lanthanum and at least one selected from a complex oxide of at least these two elements;

(B) at least one selected from platinum, rhodium, palladium and their oxides

 One kind of second catalyst component

The present invention provides a catalyst for catalytically reducing nitrogen oxides in exhaust gas, comprising:

 In particular, according to the present invention, there is provided a catalyst structure for NOx catalytic reduction in which such a catalyst is provided as a catalyst layer on an inert substrate. BEST MODE FOR CARRYING OUT THE INVENTION

 A catalyst for catalytically reducing nitrogen oxides in exhaust gas according to the present invention has a first catalyst component and a second catalyst component, wherein the first catalyst component is:

(A) (a) ceria or (b) Praseodymium oxide or

 (c) Cerium, zirconium, praseodymium, neodymium, terbium,

 It is at least one selected from a mixture of an oxide of at least two elements selected from the group consisting of summerium, gadolinium, and lanthanum and a complex oxide of at least the two elements.

 Hereinafter, focusing on this first catalyst component, it may be simply referred to as “ceria or the like”. In the first catalyst component, the mixture is preferably a homogeneous mixture. However, according to the present invention, a composite oxide of at least two elements is preferably used rather than a mixture of the oxides of at least two elements, and in particular, a binary or ternary composite oxide is used. Are preferably used.

For example, in the case of a binary composite oxide, the weight ratio of each element in the solid solution to the oxide is as follows: seria Z praseodymium oxide complex oxide, seria / zirconia complex oxide, seria Z terbium oxide complex oxide, ceria If it is a samarium oxide composite oxide or the like, it is preferably in the range of 80/20 to 60Z40. In the case of a ternary composite oxide, the weight ratio of the solid solution to the oxide is as follows: ceria Z gadolinium oxide / zirconia composite oxide, ceria neodymium oxide zirconia composite acid, ceria zirconia oxide praseodymium oxide If it is a cinderella, a ceria κ-zirconia / lanthanum oxide composite oxide, a ceria-zirconia oxide / summarium oxide composite oxide, a cerianozirconia oxide / terbium composite oxide, etc., preferably 45/5/30/3 It is in the range of 0 to 7520. However, in the present invention, the weight ratio of each element in these composite oxides to the oxide is Ceria, zirconia, terbium oxide, praseodymium oxide, gadolinium oxide, neodymium oxide, samarium oxide, and lanthanum oxide, respectively. E_〇 _2, Z R_〇 _2, T B_〇 _2, P r ₆ 〇_U, shall be calculated as the G a ₂ 〇 _3, N d ₂ 〇 _3, S m ₂ 〇 _3, and L a ₂ 0 ₃ And

In the present invention, such a first catalyst component can be obtained, for example, by the following method. That is, a water-soluble salt of an element such as cerium, praseodymium, neodymium, terbium, samarium, or gadolinium, for example, an aqueous solution of a nitrate is neutralized or heated and hydrolyzed to form a hydroxide. The product can be obtained by calcining the product in an oxidizing or reducing atmosphere at a temperature of 300 to 900 ° C. However, there are commercially available hydroxides and acids such as cerium, praseodymium, neodymium, terbium, samarium, and gadolinium. May be calcined as described above.

 According to the present invention, ceria and the like, which are the first catalyst components in the catalyst, are easily reduced under Licht conditions to lose a part of the oxygen in the oxide while Lean conditions, as described later. Below, store oxygen in the gas phase. That is, ceria and the like as the first catalyst component function as a substance having oxygen storage capacity (OSC). Therefore, in this specification, focusing on this function, cells and the like may be referred to as 〇SC functional substances or 0 SC materials.

Thus, in the catalyst of the present invention, ceria is the first catalyst component, Oite lean conditions, the noble metal catalyst component is a second catalyst component is rapidly oxidized, the reducing ability of N_〇 _x It acts as a buffering agent to prevent reduction, keeps high-efficiency NOx reduction on the catalyst, and also functions as a NOx reduction catalyst and NOx adsorbent during rich Z lean. The catalyst according to the present invention contains the above-mentioned first catalyst component in an amount of 40% by weight or more, preferably 60% by weight or more. In the catalyst according to the present invention, when the proportion of the first catalyst component is less than 40% by weight, the leanness of the first catalyst component decreases as the NOx adsorbing ability of the first catalyst component and the above-mentioned noble metal catalyst component under lean conditions. The buffering effect that prevents rapid oxidation of the catalyst decreases, leading to a decrease in the NOx purification ability of the catalyst.

 The second catalyst component of the catalyst according to the present invention is at least one selected from platinum, rhodium, palladium, and oxides thereof, and may be simply referred to as a noble metal catalyst component in view of its composition. The catalyst according to the present invention preferably contains such a noble metal catalyst component in the range of 0.01 to 2.5% by weight in terms of metal. Even if the ratio of the above-mentioned noble metal catalyst component in the catalyst exceeds 2.5% by weight in terms of metal, the N Ox purification ability of the obtained catalyst does not improve correspondingly. On the other hand, when the ratio of the noble metal catalyst component in the catalyst is less than 0.01% by weight in terms of metal, when the atmosphere of the catalytic reaction changes from lean to rich, the atmosphere of the reaction from lean to rich changes. The conversion rate is reduced, and as a result, the reducing ability of the catalyst is reduced, and the NO oxidizing ability during leaning is reduced. As a result, the NOx purification rate decreases.

According to the present invention, the noble metal catalyst component is preferably supported on ceria or the like as the first catalyst component. The reason is that the noble metal catalyst component can be highly dispersed and supported on ceria and the like by ion exchange, and when the noble metal catalyst component is highly dispersed and supported on ceria and the like, N adsorbed on the ceria and the like can be obtained. This is because _X can react with the reducing agent directly on the catalyst without desorbing from ceria or the like when rich, so that a very high N〇x purification rate can be obtained. That As described above, if ceria or the like is used as the N 吸着 x adsorbent, ceria or the like can be used as an alkali metal compound alkali earth compound, which has been conventionally used as an adsorbent in an occlusion-reduction system. Unlike metal compounds, it does not inhibit the reduction activity of Ν〇X. Therefore, a catalyst using ceria or the like as a thermal adsorbent has a low SV dependency, and thus can be used at a high SV.

However, when it is necessary to increase the heat resistance of the catalyst or, depending on the reaction conditions, to increase the oxidation of NO, a part or all of the noble metal catalyst component is converted to alumina, silica, silica-alumina, zeolite, It may be supported on a conventionally known carrier such as titania. Thus, in the present invention, when the catalyst contains a carrier as a catalyst component, the proportion of the carrier in the catalyst is preferably at most 60% by weight, particularly preferably at most 40% by weight. Good. During the proportion of carrier catalysts, when 6 greater than 0 wt%, the ratio of the first catalyst component in the catalyst is less than 4 0% by weight, as described above, N_〇 _x purification performance of the catalyst Causes a decrease in

 In a conventional N〇x storage-reduction system, an alkali metal compound or an alkaline earth metal compound is used as a NOx adsorbent.In such a case, these N〇x adsorbents are used as a carrier. As described above, the NOx adsorbent is preferably present in a state separated from the noble metal catalyst, because it reduces the heat resistance of the noble metal catalyst and, as described above, inhibits the N〇x reduction activity of the catalyst. Therefore, according to such a catalyst, NOx is adsorbed on the NOx adsorbent, then desorbed from the NOx adsorbent, and reduced on the noble metal catalyst component. Inevitably), and the NO x purification rate is reduced.

 On the other hand, ceria and the like have a smaller NOx adsorbing capacity compared to the NOx adsorbents used in the conventional NOx storage-reduction system, i.e., alkali metal compounds and alkaline earth metal compounds. It is known that a high NOX purification rate cannot be obtained even when used as a NOx adsorbent, except when used in combination with a compound with strong alkaline properties, including metal compounds and alkaline earth metal compounds. ing.

However, when ceria or the like is used as the NOx adsorbent according to the present invention, there is no such problem. That is, according to the present invention, in a catalyst in which ceria or the like is combined with a noble metal catalyst component, N〇x does not desorb from ceria or the like as an N〇x adsorbent, The catalyst is rapidly reduced on the noble metal catalyst component supported on the catalyst or on the noble metal catalyst component existing near the cell or the like. In addition, the use of highly alkaline alkali metal compounds or alkaline earth metal compounds as NOx adsorbents inhibits the NOx reduction activity of the catalyst, as well as heat resistance due to sintering of the carrier and precious metal catalyst. There is no problem of deterioration in the properties, and a catalyst having extremely important properties in practical use such as excellent heat resistance and low SV dependency can be obtained.

 Thus, according to the present invention, as described above, the ratio of the noble metal catalyst component in the catalyst is lower than when the alkaline metal compound or the alkaline earth metal compound is used as the N〇X adsorbent. It is possible to obtain a high-performance catalyst with extremely high NOx purification performance in a wide temperature range from 150 ° C to 550 ° C, while significantly lowering it.

 For example, the catalyst according to the present invention is, for example, a method in which ceria or the like is added to an aqueous solution of a water-soluble salt such as platinum, mixed and stirred to form a slurry in which the noble metal catalyst component is ion-exchanged and supported on ceria or the like. When dried and calcined in an oxidizing or reducing atmosphere at a temperature of 500 to 900 ° C., a powder obtained by supporting a noble metal catalyst component on ceria or the like can be obtained. Also, the catalyst according to the present invention can be obtained as a powder by mixing the thus obtained ceria or the like carrying the noble metal catalyst component with ceria or the like not carrying the noble metal catalyst component. be able to.

 Further, as another example, a catalyst in which a noble metal catalyst component is supported on a carrier such as alumina can be obtained as follows. That is, a carrier such as alumina is added to an aqueous solution of a water-soluble salt of a noble metal such as platinum, mixed and stirred to form a slurry in which a noble metal catalyst component is ion-exchanged and supported on a carrier, dried, and dried. Calcined at a temperature of 500 to 900 ° C. in a neutral or reducing atmosphere to obtain a powder having a noble metal catalyst component supported on a carrier, which is mixed with ceria, etc. The catalyst can be obtained as a powder.

According to the method of the present invention, under a rich step, that is, under a reducing condition, reduction of a nitrogen oxide is performed as follows. In the lean process, the oxidized noble metal catalyst components are reduced, and N Ox is reductively decomposed on these catalysts, and the N Ox adsorbed on ceria or the like is not desorbed from the noble metal catalyst components without being desorbed. Is reductively decomposed. Further, oxygen on the noble metal catalyst component is also rapidly reduced by the reducing agent. Furthermore, what is important in this rich process is that, in preparation for the next lean process, cerium etc. functioning as a 〇SC functional material in the catalyst is partially reduced efficiently, that is, during a certain time during lean operation. The oxygen in the exhaust gas The occlusion results in regeneration, depending on the reaction conditions, such that the reaction atmosphere in the catalyst layer is maintained at rich or stoichiometric conditions for some time during leaning.

 Next, under the lean process following the rich process, that is, under the oxidizing condition, the reduction of nitrogen oxides is performed as follows. That is, although depending on the conditions, oxygen in the exhaust gas is occluded by ceria or the like, which is an OSC functional substance, during a part of the lean process, so that the reaction atmosphere of the catalyst layer becomes rich or stoichiometric. Will be retained. As a result, NOx is rapidly reduced and decomposed on the noble metal catalyst component even during a part of the lean process. However, in the subsequent lean process, the noble metal catalyst component is gradually oxidized by oxygen generated by reductive decomposition of N Ox and oxygen in the exhaust gas. The efficiency of the reductive decomposition of X decreases. However, according to the present invention, since N〇x is adsorbed on ceria or the like in the catalyst layer, NOx in the exhaust gas is still continuously removed from the exhaust gas with high efficiency.

In particular, the catalyst according to the present invention has little degradation even in the presence of oxygen, sulfur oxides or water, especially in the presence of sulfur oxide, which is a serious problem in NOx storage-reduction catalysts, In addition, even if the catalyst has deteriorated, it can be regenerated under extremely mild conditions (ie, from 400 ° C. to 500 ° C.) as compared with the conventional NO x storage-reduction catalyst. The reason for the high durability of the catalyst according to the present invention in the presence of sulfur oxides is that the coexisting SO x is adsorbed on the catalyst as SO ₂ during re-injection, and desorbed in the gas phase when rich. This is because they are not irreversibly stored in the catalyst unlike the NOx storage-reduction catalyst.

Further, according to the method of the present invention, the first catalyst component composed of ceria and the like has a function of reducing and decomposing N Ox by a noble metal catalyst component, which is a second catalyst component, and also has an S Ox durability of the catalyst. Play an important role. That is, S Ox trapped ceria as S_〇 _2, it is desorbed emitted during rate Ji, the less likely to be occluded in the ceria or the like, N Ox adsorbent by S_〇_X, i.e., deterioration of ceria or the like, It is smaller than the NOx storage-reduction system described above. Also, S〇x adsorbed during leaning is easily regenerated under mild litz conditions (ie, at 400 ° C to 500 ° C) such that the NOx storage-reduction system is not regenerated at all.

The catalyst component according to the invention can be obtained in various forms such as powders and granules. Therefore, by using such a catalyst component by various methods well known in the art, for example, by forming into various shapes such as a honeycomb, a cyclic substance, a spherical substance, etc., various 16059

A catalyst structure having a 10 shape can be obtained. In preparing such a catalyst structure, an appropriate additive such as a molding aid, a reinforcing material, an inorganic fiber, an organic binder, or the like can be used as necessary.

 In particular, the catalyst according to the present invention is obtained by providing a catalyst layer on the surface of an inert substrate for support having an arbitrary shape, for example, by a wet coating method (for example, by coating). Advantageously, it is a catalyst structure. The inert substrate is 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—A1. The shape may be a honeycomb, a ring, a spherical structure, or the like.

In the present invention, the catalyst layer comprises at least 50 times the catalyst component comprising the above-described first catalyst component (such as seria), the second catalyst component (noble metal catalyst component), and, in some cases, the carrier. %, Preferably at least 80% by weight. If the ratio of the catalyst component in the catalyst layer is less than 50% by weight, the NOx adsorbing ability and the NOx reducing ability of the catalyst layer at the time of leaning decrease, and accordingly, the NOx purification ability decreases. Furthermore, reduced oxidation restraining force of the catalyst according to the oxygen storage capacity such as ceria, resulting scavenging as S_〇 ₂ S Ox catalyst layer is lowered, so that the inferior in S Ox durability.

Thus, in the present invention, when the catalyst structure is formed, the preferable thickness of the catalyst layer capable of obtaining high N〇x reduction property and S〇x durability in a rich / lean process is 20 μm. The range is from m to 8 Ο μπι. Usually, about 60 μπι is preferable. Even if the thickness of the catalyst layer is more than 8 μμη, the performance will not be improved correspondingly. On the other hand, if the thickness of the catalyst layer is less than 2 μμπ, the purification ability will decrease. Here, according to the present invention, the thickness of the catalyst layer is determined by calculating the apparent density of the catalyst layer as 1.0 Og / cm ³ and the amount of slurry containing the catalyst component applied to the base material for convenience. be able to.

 The catalyst according to the present invention is excellent not only in resistance to heat but also in resistance to sulfur oxides, and is a catalyst for reducing NOx in exhaust gas from automobiles of diesel engines and lean gasoline engines, that is, a catalyst for denitration. It is suitable for use as.

In the present invention, the catalyst is preferably used in a catalytic reaction under conditions where the combustion atmosphere of the fuel oscillates between the rich condition and the lean condition as described above. Here, the period of the catalyst reaction (that is, the rich atmosphere (or lean atmosphere) changes from the next rich atmosphere (or lean atmosphere). JP2003 / 016059

11) is preferably 5 to 150 seconds, particularly preferably 30 to 90 seconds. Also, the rich lean width, ie, the rich time (second), the Z lean time (second) is generally in the range of 0.5 / 5 to 10Zl50, preferably in the range of 2/30 to 5/90. . When gasoline is used as the fuel, the rich condition is usually formed by periodically injecting the fuel into the combustion chamber of the engine at an air / fuel ratio of 10 to 14 by weight. Typical exhaust gases under rich conditions are several hundred volumes of Ppm Ν, 5-6% by volume of water, 2-3% by volume of 〇, 2-3% by volume of hydrogen, thousands of Hydrocarbons and 0-0.5% by volume of oxygen. On the other hand, when gasoline is used as the fuel, the lean condition is usually formed by periodically injecting the fuel into the combustion chamber of the engine at an air-fuel ratio of 20 to 40 by weight. Typical emissions under lean conditions are hundreds of ppm of NOx, 5-6% by volume of water, thousands of ppm of CO, thousands of ppm of hydrogen, thousands of ppm of hydrocarbons and Contains ~ 10% by volume oxygen.

 Suitable temperatures for the catalytic reduction of N〇x with the catalyst according to the invention depend on the individual gas composition, but must be such that they have an effective catalytic activity on the N に わ た っ て x over a prolonged period in the rich process. The temperature is usually in the range of 150 to 550 ° C., and preferably in the range of 200 to 500. In the above reaction temperature range, the exhaust gas is preferably treated at a space velocity in the range of 5000 to 100000 h000.

 According to the method of the present invention, as described above, the exhaust gas containing 排 ガ ス is brought into contact with the above-described catalyst in the periodic rich-lean process, whereby the exhaust gas can be produced in the presence of oxygen, sulfur oxide, or moisture. However, NOx in exhaust gas can be stably and efficiently catalytically reduced. Example

 Hereinafter, the present invention will be described in detail with reference to Examples along with Production Examples of catalyst components, but the present invention is not limited to these Examples. In the following, all “parts” and “%” are by weight unless otherwise indicated.

 (1) Preparation of catalyst structure

Example 1

To 10 OmL of ion-exchanged water, add 151.37 g of cerium nitrate (Ce (Ν〇 ₃ ) ₃ · 6 交換₂と し) to make an aqueous solution, and add 0.1N aqueous ammonia to neutralize the cerium ion Addition 6059

12 Hydrolyzed and aged for 1 hour. The resulting slurry was filtered, dried at 120 ° C for 24 hours, and calcined in air at 500 ° C for 3 hours to obtain a ceria powder (specific surface area: 138m ² Zg). Ion-exchanged water 10 OML to _{P t (NH 3) 4 (} N0 3) 4 solution (0% 9. platinum) 2. adding 1 0 g, an aqueous solution, to which γ- alumina (Sumitomo Chemical (Co. 15) Add 15 g of KC-1 and dry it with stirring while drying at 100. Then, calcinate at 500 ° C for 3 hours in air to obtain a catalyst powder consisting of 1% platinum supported on alumina. Got a body. 10 g of this catalyst powder and 40 g of the above ceria powder were mixed to obtain a powder catalyst consisting of a mixture of the above 1% platinum Z alumina and the above ceria powder (weight ratio 25:75).

 To 50 g of this powder catalyst, 5 g of silica sol (Snowtex N, manufactured by Nissan Chemical Industries, Ltd., 20% as silica) and an appropriate amount of water were mixed. Using 100 g of zirconia pole as a grinding medium, this mixture was ground with a planetary mill for 5 minutes to obtain a slurry for wet coating. The above slurry for wet coating was applied to a cordierite honeycomb substrate having 400 cells per square inch to obtain a honeycomb structure having a catalyst layer having a thickness of 6 μππ composed of the powdered catalyst.

Example 2

 In the same manner as in Example 1, a powder catalyst composed of a mixture of 1% platinum alumina and ceria (weight ratio: 60:40) was obtained. Hereinafter, in the same manner as in Example 1, a honeycomb structure having a catalyst layer having a thickness of 60 μπι and made of the above-described powder catalyst was obtained.

Example 3

Ion-exchanged water 10 OML to P t (NH _{3) 4} (N_〇 ₃₎ 4 solution (9.0% as platinum) 8. added 4 O g, an aqueous solution, ceria powder obtained in Example thereto After charging 60 g of the powder and drying at 100 ° C with stirring, it was calcined in air at 500 ° C for 3 hours to obtain a catalyst powder comprising 1% platinum on ceria. Obtained. Thereafter, in the same manner as in Example 1, an 82-cam structure having a catalyst layer having a thickness of 6 μμπι and made of the above-mentioned powder catalyst was obtained.

Example 4

Cerium nitrate in deionized water _{170 OmL (C e (Ν0 3} ) 3 · 6Η 2 0) 34. 59 g and O carboxymethyl zirconium (Z R_〇 (NO 3) 2) 84. 45 g lanthanum nitrate (La ( Nyu_〇 _{3) 3} · _{6Η 2} 〇) 7. dissolved and 97 g, to prepare an aqueous solution. To this aqueous solution is added 0.1N aqueous ammonia to neutralize and hydrolyze the cerium salt, zirconium salt and lanthanum salt. 3016059

Aged for 13 hours. The product was separated from the obtained slurry by filtration, dried at 120 ° C for 24 hours, and calcined in air at 500 ° C for 3 hours to obtain a ceria / zirconia / lanthanum oxide composite oxide powder. A powder (oxide-based weight ratio 22 / 73,5, specific surface area 80 m2 g) was obtained. Ion-exchanged water 10 OML to _{P t (NH 3) 4 (} N0 3) 4 solution (9. platinum 0%) 2. addition of 1 0 g, an aqueous solution, to which the ceria / Jirukoniano lanthanum oxide composite oxide After charging 15 g of the powder and drying with stirring at 100 ° C, the mixture was calcined at 500 ° C for 3 hours in air to obtain 1% platinum on the ceria Z-zirconanoate / hylanthanum composite oxide. A supported catalyst powder was obtained. 10 g of this catalyst powder and 20 g of the above-mentioned celiano-zirconia Z lanthanum oxide composite oxide powder were mixed, and 1% platinum-supported seria Z-zirconia lanthanum oxide composite oxide and ceria Z zirconia Z lanthanum oxide composite oxide were mixed. (Weight ratio 33:67) was obtained. Thereafter, in the same manner as in Example 1, a honeycomb structure having a catalyst layer having a thickness of 6 μππ and made of the above powder catalyst was obtained.

Example 5

Cerium nitrate in deionized water 170 OmL (C e (Nyu_〇 _{3) 3 · 6Η 2 0)} 77. 83 g and O carboxymethyl zirconium _{(Z rO (N0 3) 2} ) 36. 03 g and praseodymium nitrate (P r _{(NO 3) 3 · 6Η 2} 0) 35. was dissolved and 26 g, 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 praseodymium salt, and then aged 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 composite oxide of ceria Z-zirconia and oxidized praseodymium. A powdery product (47 33/22 based on oxide weight, specific surface area 205 m2 / g) was obtained.

Ion-exchanged water 10 OML to _{P t (NH 3) 4 (} N0 3) 4 solution (9.0% as platinum) 4. Make 2 Og, an aqueous solution, to which the above-mentioned ceria Jirukonia / praseodymium oxide composite oxides Pour 15 g of powder, dry with stirring at 100 ^, and calcinate at 500 ° C for 3 hours in air to obtain 2% platinum on ceria / zirconia praseodymium composite oxide. A supported catalyst powder was obtained. A mixture of 10 g of this catalyst powder and 40 g of the above-mentioned ceria "zirconano zirconium oxide braseodymium powder was mixed with 2% platinum / celia zirconia Z praseodymium oxide composite oxide and ceriano zirconia / acidil praseodymium composite oxide. (Weight ratio: 25:75) A powder catalyst comprising the above-mentioned powder catalyst was obtained in the same manner as in Example 1. 2003/016059

An 82 cam structure having a catalyst layer of 14 μΠΙ was obtained.

Example 6

 In Example 5, a powder catalyst having a weight ratio of 50:50 of a mixture of 2% platinum-seria / zirconia-praseodymium oxide composite oxide and ceria / zirconia / praseodymium oxide composite oxide was obtained. . Except for this point, a honeycomb structure having a catalyst layer having a thickness of 60 μπΐ and made of the above-described powder catalyst was obtained in the same manner as in Example 5.

Example 7

 In Example 5, a powder catalyst having a weight ratio of 75:25 of a mixture of 2% platinum / ceria / zirconia / praseodymium oxide composite oxide and ceria / zirconia / praseodymium oxide composite oxide was obtained. Except for this, in the same manner as in Example 5, a honeycomb structure having a catalyst layer having a thickness of 6 μππ and made of the above-described powder catalyst was obtained.

Example 8

Cerium nitrate in deionized water _{10 OmL (Ce (Ν0 3)} 3 · 6Η 2 0) 103. 77 g and nitric acid praseodymium (P r (Nyu_〇 _{3) 3} · _{6Η 2} 0) dissolved and 35. 77 g Thus, an aqueous solution was prepared. 0.1N aqueous ammonia was added to this aqueous solution to neutralize and hydrolyze the cerium salt and praseodymium salt, and then aged for 1 hour. The product was separated from the obtained slurry by filtration, dried at 120 ° C for 24 hours, and calcined in air at 500 ° C for 3 hours to obtain praseodymium oxide ceria oxide powder (oxide A reference weight ratio of 60/40 and a specific surface area of 112 m ² Zg) were obtained.

On the other hand, ion-exchanged water 10 OML to _{P t (NH 3) 4 (} N0 3) 4 solution (9.0% as platinum) 4. added 20 g, an aqueous solution, to which γ- alumina (Sumitomo Chemical ( 500 g of KC-5001) was added, dried at 100 ° C with stirring, and calcined in air at 500 ° C for 3 hours to support 2% platinum on alumina The resulting catalyst powder was obtained. 10 g of this catalyst powder and 40 g of the above-mentioned ceria-praseodymium oxide composite oxide powder are mixed, and a powder catalyst comprising a mixture of 2% platinum alumina and ceria / praseodymium oxide composite oxide (weight ratio 25:75) Got.

To 50 g of this powder catalyst, 5 g of silica sol (Snowtex N, manufactured by Nissan Chemical Industries, Ltd., 20% as silica) and an appropriate amount of water were mixed. Using 100 g of zirconia pole as a grinding medium, this mixture was ground with a planetary mill for 5 minutes to obtain a slurry for wet coating. One The above slurry for polish coating was applied to a cordierite honeycomb substrate having 400 cells per square inch to obtain a honeycomb structure having a catalyst layer having a thickness of 6 μμη and made of the above powder catalyst.

Example 9

Cerium nitrate in deionized water 170 OmL (C e (Nyu_〇 _{3) 3 · 6Η 2 0)} 121. 06 and Okishi zirconium (Z R_〇 _{(N0 3) 2) 28. 12} g and gadolinium nitrate (Gd (N 0 ₃ ) ₃ · 6Η ₂ 〇) 7.48 g was dissolved in water to prepare an aqueous solution. 0.1 N aqueous ammonia was added to this aqueous solution to neutralize and hydrolyze the cerium salt, zirconium salt and gadolinium salt, and then aged for 1 hour. The product was separated from the obtained slurry by filtration, dried at 120 ° C for 24 hours, and calcined in air at 500 ° C for 3 hours to obtain ceria Z zirconian oxide gadolinium composite oxide powder ( An oxide-based weight ratio of 72/24/4 and a specific surface area of 198 m ² g) were obtained.

 Hereinafter, the same as Example 8 except that a powder catalyst composed of a mixture of the 2% platinum alumina obtained in Example 8 and the above-mentioned Seria Z praseodymium oxide composite oxide powder (weight ratio: 25:75) was used. Thus, a honeycomb structure having a catalyst layer having a thickness of 6 μμη was obtained.

Example 10

Cerium nitrate in deionized water 170 OML (Ce (Nyu_〇 _{3) 3 · 6Η 2 0)} 109. 43 g and Okishi zirconium (Z R_〇 _{(N0 3) 2) 31. 27} g and neodymium nitrate (Nd (NO _{3) 3 - 6H 2 0)} 15. was dissolved and 63 g, to prepare an aqueous solution. To this aqueous solution was added 0.1 N aqueous ammonia to neutralize and hydrolyze the above-mentioned cerium salt, zirconium salt and neodymium salt, followed by aging for 1 hour. The product was separated from the resulting slurry by filtration, dried at 120 for 24 hours, and calcined in air at 500 ° C for 3 hours to obtain ceria / "zirconia / neodymium oxide composite oxide powder. (A weight ratio of 70 Z 20 Z 10 and a specific surface area of 171 m ² g based on the oxide) 2% platinum Z alumina obtained in Example 8 and the above-described seria / zirconia neodymium oxide composite oxide powder were obtained. A honeycomb structure having a catalyst layer having a thickness of 6 μm was obtained in the same manner as in Example 8, except that a powder catalyst composed of a mixture with a catalyst (weight ratio 25:75) was used.

Example 11

Cerium nitrate in deionized water 10 OmL (Ce (Nyu_〇 _{3) 3} · _{6Η 2} 〇) 103. 77 g and nitric acid terbium (Tb (Nyu_〇 _{3) 3 · 6Η 2 0)} 40. dissolved and 96 g Thus, an aqueous solution was prepared. 16 0.1 N aqueous ammonia was added to this aqueous solution to neutralize and hydrolyze the cerium salt and terbium salt, and then aged 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 Z terbium oxide composite oxide powder ( An oxide-based weight ratio of 70Z30 and a specific surface area of 139 m ² / g) were obtained. The same procedures as in Example 8 were carried out except that a powder catalyst comprising a mixture of the 2% platinum / alumina obtained in Example 8 and the ceria Z terbium oxide double composite oxide powder (weight ratio: 25:75) was used. In the same manner, an 82 cam structure having a catalyst layer having a thickness of 6 μμπι was obtained.

Example 12

Cerium nitrate in deionized water 170 OmL (C e (Nyu_〇 _{3) 3 · 6Η 2 0)} 121. 06 g and Okishi zirconium (Z R_〇 _{(N0 3) 2) 28. 12} g and samarium nitrate (Sm ( NO ₃ ) ₃ · 6Η ₂ 〇) 3.40 g was dissolved to prepare an aqueous solution. 0.1 N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt, oxyzirconium salt and samarium salt, and then aged for 1 hour. The product was separated from the obtained slurry by filtration, dried at 120 ° C for 24 hours, and calcined in air at 500 ° C for 3 hours to obtain a ceria / zirconium Z oxide summary oxide composite oxide. A powder (oxide standard weight ratio: 72 24/4, specific surface area: 187 m ² Zg) was obtained.

 Hereinafter, except that a powder catalyst consisting of a mixture of the 2% platinum alumina obtained in Example 8 and the above-mentioned Seria / Zirconia Z-summarium oxide composite oxide powder (weight ratio 25:75) was used, In the same manner as in Example 8, a honeycomb structure having a catalyst layer having a thickness of 6 μm was obtained.

Comparative Example 1

Barium carbonate was prepared from aqueous solution of sodium hydroxide and sodium carbonate. 48 g of γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) and 12 g of the above barium carbonate were added to 10 OmL of ion-exchanged water, dried at 10 Ot with stirring, and then reduced to 50 in air. And baked for 3 hours to obtain γ-alumina (Al ₂ 〇 ₃ ) Z barium carbonate (BaC ₃ ) (weight ratio: 80 20) powder.

The γ- alumina (Al ₂ 〇 ₃₎ Z barium carbonate (BaC0 ₃₎ a powder 48 g and Jitsuzorei ceria powder 12 g obtained in 1 was dry-mixed to obtain mixed powder. An aqueous solution of 8.40 g of Pt (NH ₃ ) 4 (N〇 ₃ ) 2 aqueous solution (9.0% as platinum) in 10 OmL of ion-exchanged water, 60 g of the above mixed powder was added thereto, and stirred While drying at 100 ° C, then in air at 500 ° C And calcined for 3 hours to obtain a γ- alumina (Alpha 1 ₂ 0 ₃₎ Zeta barium carbonate (B AC0 ₃₎ / ceria powder catalyst composed by supporting 1% of platinum (by weight 60/20/20) .

 Using 60 g of this catalyst powder, a slurry for wet coating was prepared in the same manner as in Example 1. 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 having a thickness of 8 μm.

Comparative Example 2

Barium carbonate was prepared from aqueous solution of sodium hydroxide and sodium carbonate. Ion-exchanged water l O OML to _{P t (NH 3) 4 (} N0 3) (9. 0% of platinum) 2 aqueous solution was added to 8. 40 g, an aqueous solution, to which γ- alumina (Sumitomo Chemical (Co. ) -501) 48 g and 12 g of the above-mentioned parium carbonate are charged, dried at 100 ° C with stirring, and calcined at 500 ° C for 3 hours in air to obtain γ- to obtain an alumina (Al ₂ 〇 ₃₎ Z barium carbonate (BaC_〇 ₃₎ powder catalyst composed by supporting 1% of platinum on (weight ratio 80Z20).

Separately, a mixture of a γ- alumina and aqueous potassium carbonate, dried, and then calcined in the air at 3 hours at _{110 0, Κ 2 0 · 12} A 1 2 0 3 (specific surface area: 18m2 / g) Prepared. I O-exchange water l O OML to _{P t (NH 3) 4 (} N0 3) 4 solution (9.0% as platinum) was added 8. 40 g, an aqueous solution, in which γ- alumina (Sumitomo Chemical Ltd. KC 501) 54 g and the kappa ₂ 〇 · 12 a 1 ₂ 0 ₃ to 6 g was charged, with stirring, followed by drying at 100 ° C, in air, 3 at 500 ° C and baking time, to obtain a catalyst powder comprising γ- alumina / kappa ₂ 〇 · 12 a 1 ₂ 0 ₃ (weight ratio 90/1 0) by supporting 1% of platinum on.

The above-mentioned γ-alumina (Al ₂ 〇 ₃ ) / barium carbonate (BaC〇 ₃ ) (weight ratio: 80 20) and 48 g of catalyst powder in which 1% of platinum is supported on the γ-alumina Κ ₂₀ · 12 Α ₁₂ 〇 ₃ (weight ratio 90/10) and 12 g of catalyst powder carrying 1% of platinum were dry-mixed, and a slurry for wet coating was prepared in the same manner as in Example 1. 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 having a thickness of 8 μm.

 (2) Performance test

Using the catalyst structures according to Examples 1 to 10 and Comparative Examples 1 and 2, a 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. Test method

 The composition of the mixed gas used in the NOx reduction experiment under the rich conditions is as follows.

NO: 500 ppm

S〇 ₂ : 40 ρ pm

0 ₂ : 0.4%

CO: 2%

C ₃ H ₆ (propylene): 2000 ppm

 H2

Η ₂₀ : 9.0%

 The gas under lean conditions was prepared by injecting oxygen into the mixed gas under rich conditions, and its composition is as follows.

 NO: 456 p pm

 SO2: 37 p pm

0 ₂ : 9.2%

CO: 1.8%

C ₃ H ₆ (propylene): 1822 p pm

h.2: 1.8 / 0

H ₂ 〇: 8.2%

 The catalytic reaction was performed with a rich / lean width of 5/55 (second Z seconds), and the performance of each catalyst was examined.

 (i) Space velocity:

 70000 h r'i (lean conditions)

 69312 h r-i (Rich condition)

 (ϋ) Reaction temperature:

Table 1 shows the results of 150, 200, 250, 300, 350, 400, 450, 500 ° C and 550 ° C. As is clear from Table 1, the catalyst according to the present invention has a high nitrogen oxide removal rate. In contrast, the catalyst according to the comparative example generally has a low nitrogen oxide removal rate. Table 1

 NOx removal rate)

 Temperature (° C)

150 200 250 300 350 400 450.500 550 Example 1 76.4 88.6 94.6 96.2 96.6 95.3 93.6 87.0 75.6 Example 2 81.4 92.2 93.7 94.7 93.5 91.9 87.9 79.0 67.1 Example 3 86.1 93.0 96.7 97.3 97.0 96.6 94.8 88.6 79.1 Example 4 68.0 82.4 93.6 98.3 98.1 98.4 98.2 93.4 86.4 Example 5 84.5 91.0 95.6 99.3 99.2 99.0 98.1 94.1 86.3 Example 6 85.2 .93.2 97.5 98.1 98.5 97.7 93.1 87.7 78.9 Example 7 87.5 94.8 98.6 99.0 98.2 97.1 93.4 86.0 75.9 Example 8 73.6 87.6 95.1 98.3 99.6 99.8 99.0 89.3 80.7 Example 9 66.8 82.8 92.7 95.9 95.6 94.2 92.7 86.8 75.7 Example 10 68.7 84.5 93.6 95.8 96.6 96.2 94.4 88.8 80.2 Comparative example 1 62.5 83.5 94.8 97.4 94.2 85.2 77.0 63.7 44.9 Comparative example 2 48.6 71.8 84.8 95.4 97.5 97.8 94.0 91.2 85.4

Further, the above embodiment; Using the catalyst structures according to Comparative Examples 1 and 2, the above gas conditions, the rich / lean width (second Z second) was set to 5Z55, and the reaction temperature was set to 350 ° C. A time durability test was performed. · The results are shown in Table 2. As is clear from Table 2, the catalyst according to the present invention has a very high resistance to sulfur oxides as well as the conventional NOx storage-reduction catalyst. Table 2

また、 上記耐久試験において劣化した実施例 1、 実施例 5及び比較例 1による触媒構造 体を 3 %の水素の存在下、 4 0 0 °Cで 5分間加熱処理して、 劣化触媒が再生するかどうか を調べた。 結果を第 3表に示す。 第 3表から明らかなように、 本発明による触媒は、 硫黄 酸化物によって劣化しても、 上記再生条件にて再度、 賦活することができたが、 従来の N Ox貯蔵一還元触媒は、 これを賦活することができなかった。 2003/016059

In addition, the catalyst structures according to Example 1, Example 5, and Comparative Example 1, which were deteriorated in the above durability test, were heated at 400 ° C. for 5 minutes in the presence of 3% hydrogen to regenerate the deteriorated catalyst. I checked whether or not. Table 3 shows the results. As is evident from Table 3, the catalyst according to the present invention was able to be activated again under the above-mentioned regeneration conditions even if it was deteriorated by sulfur oxides. Could not be activated. 2003/016059

twenty one

Table 3

 NOx removal rate (%)

 Temperature (° c)

 200 250 300 350 400 450 500 Example 1 88.6 95.0 96.2 96.3 95.5 92.9 85.8 Example 5 90.4 95.2 99.3 99.5 99.0 98.2 93.3 Comparative example 1 24.4 30.2 12.1 2.1 1.8 0.9 0.5

Claims

The scope of the claims 1. In a method in which fuel is supplied and burned under a periodic rich Z-lean condition, and the generated exhaust gas is brought into contact with the catalyst, and the nitrogen oxides in the exhaust gas are catalytically reduced, the catalyst comprises (A ) (a) ceria or  (b) Praseodymium oxide or  (c) Cerium, zirconium, praseodymium, neodymium, terbium,  A first catalyst component consisting of a mixture of an oxide of at least two elements selected from samarium, gadolinium and lanthanum and at least one selected from a complex oxide of at least these two elements; (B) at least one selected from platinum, rhodium, palladium and their oxides  One kind of second catalyst component And a method for catalytically reducing nitrogen oxides in exhaust gas. 2. The method according to claim 1, wherein the catalyst is used as a catalyst structure provided as a catalyst layer on an inert substrate. 3. A catalyst for supplying and burning fuel under periodic rich / lean conditions, contacting the generated exhaust gas, and catalytically reducing nitrogen oxides in the exhaust gas.  (A) (a) ceria or  (b) Praseodymium oxide or  (c) Cerium, zirconium, praseodymium, neodymium, terbium,  A first catalyst component consisting of a mixture of an oxide of at least two elements selected from samarium, gadolinium and lanthanum and at least one selected from a complex oxide of at least these two elements; (B) at least one selected from platinum, rhodium, palladium and their oxides  One kind of second catalyst component A catalyst for catalytically reducing nitrogen oxides in exhaust gas, comprising: 4. A catalyst structure comprising the catalyst according to claim 3 provided as a catalyst layer on an inert substrate.