JP4740217B2 - Method for catalytic reduction of nitrogen oxides - Google Patents

Description

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. Specifically, the present invention provides fuel by supplying fuel to a combustion chamber of a diesel engine or a gasoline engine in a periodic rich / lean fuel supply process (excursion) and bringing the generated exhaust gas into contact with the catalyst. The present invention relates to a method for catalytic reduction of NOx in exhaust gas and a catalyst therefor. Such a method and a catalyst therefor are suitable, for example, for reducing and removing harmful nitrogen oxides contained in exhaust gas from automobile engines.

In particular, the present invention supplies and burns 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 method for catalytic reduction of NOx in exhaust gas produced by the combustion without deterioration of the catalyst, and a catalyst therefor.

  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.

  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.

  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.

  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.

  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.

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.

  Under such circumstances, development of a more highly active catalyst for NOx catalytic reduction has been demanded. Recently, a catalyst in which silver or silver oxide is supported on an inorganic oxide support material has been proposed. (For example, refer to Patent Documents 1 and 2). 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.

  In order to overcome the above-mentioned problems, a NOx storage-reduction system has recently been proposed as the most promising method (see, for example, Patent Documents 3 and 4). According to this system, 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.

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 absorbed by an alkali compound such as K ₂ CO ₃ or BaCO _3. Absorbed by the agent. 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 absorbed (stored) NO ₂ is released from the absorbent and is efficiently reduced to nitrogen by a hydrocarbon, carbon monoxide or hydrogen on a platinum or rhodium catalyst. The NOx storage - reduction system, if the absence of SOx, but well operated over a long period of time, if there SOx is, in the lean and rich any conditions, the NO ₂ absorption sites on the alkali compound Due to the irreversible absorption of SOx, the system has the problem of rapidly deteriorating.

Therefore, in order to alleviate or solve the problem of NOx storage-reduction system that the performance deteriorates in the presence of SOx,
(A) (a) ceria or (b) praseodymium oxide or (c) a mixture of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, gadolinium and lanthanum and a composite oxide of at least these two elements And (B) (a) at least one selected from platinum, rhodium, palladium and oxides thereof, and a surface catalyst layer having at least one selected from
(E) It has been proposed that a catalyst comprising an internal catalyst layer having a support as an internal catalyst component exhibits NOx purification ability close to a NOx storage-reduction system and high SOx durability (see Patent Document 5).

Further, a surface catalyst layer comprising a first catalyst component selected from rhodium, palladium and oxides thereof, and a second catalyst component selected from zirconia, cerium oxide, praseodymium oxide, neodymium oxide and a mixture thereof, A catalyst comprising an internal catalyst layer containing a third catalyst component selected from rhodium, palladium, platinum and a mixture thereof has been proposed as exhibiting high SOx durability (see Patent Document 6).
European Patent Application No. 526099 European Patent Application No. 6794427 International Publication No. 93/7363 Pamphlet International Publication No. 93/8383 Pamphlet International Publication No. 02/89977 Pamphlet WO02 / 22255 pamphlet

  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 the catalyst with high durability without deterioration of the catalyst and a catalyst therefor.

  In particular, in the present invention, in the presence of oxygen, sulfur oxides, or water, deterioration in the presence of sulfur oxides, which is a serious problem of NOx storage catalysts, and generation of harmful ammonia at the time of rich, in particular. It is another object of the present invention to provide a highly durable catalyst for reducing NOx in a lean process of periodic rich / lean combustion in a wide temperature range.

  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.

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) ceria or (b) praseodymium oxide or (c) a mixture and / or composite oxide of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum A catalyst component A comprising:
(B) (d) a noble metal catalyst component comprising at least one selected from platinum, rhodium, palladium and oxides thereof;
(E) a catalyst component B comprising a carrier;
(C) (f) a solid acid and (g) at least one selected from a solid acid supporting at least one metal oxide selected from vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese There is provided a method for catalytic reduction of nitrogen oxides in exhaust gas, characterized by comprising a catalyst component C comprising The catalyst used in this method is referred to as a single layer catalyst.

According to a preferred aspect of the present invention, in a method for supplying and burning fuel under periodic rich / lean conditions, bringing the produced exhaust gas into contact with a catalyst, and catalytically reducing nitrogen oxides in the exhaust gas, (A) (a) ceria or (b) praseodymium oxide or (c) a mixture of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum and / or A catalyst component A comprising a composite oxide;
(C) (f) a solid acid and (g) at least one selected from a solid acid supporting at least one metal oxide selected from vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese A surface catalyst layer having a catalyst component C comprising:
(B) (d) an internal catalyst layer having, as an internal catalyst component, a noble metal catalyst component consisting of at least one selected from platinum, rhodium, palladium and oxides thereof; and (e) a catalyst component B consisting of a carrier. A method for catalytically reducing nitrogen oxides in exhaust gas is provided. The catalyst used in this method is referred to as a first two-layer catalyst.

  In particular, according to the present invention, in the single-layer catalyst and the first two-layer catalyst described above, the catalyst component A carries a noble metal catalyst component consisting of at least one selected from platinum, rhodium, palladium and oxides thereof. It is preferable that

Furthermore, according to another preferred embodiment of the present invention, fuel is supplied and combusted under periodic rich / lean conditions, the generated exhaust gas is brought into contact with the catalyst, and nitrogen oxides in the exhaust gas are catalytically reduced. Wherein the catalyst is (A) (a) ceria or (b) praseodymium oxide or (c) an oxide of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum. A catalyst component A comprising a mixture and / or a composite oxide;
(C) (f) a solid acid and (g) at least one selected from a solid acid supporting at least one metal oxide selected from vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese A surface catalyst layer having a catalyst component C comprising:
(A) (a) ceria or (b) praseodymium oxide or (c) a mixture and / or composite oxide of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum A catalyst component A comprising:
(B) (d) an internal catalyst layer having, as an internal catalyst component, a noble metal catalyst component consisting of at least one selected from platinum, rhodium, palladium and oxides thereof; and (e) a catalyst component B consisting of a carrier. A method for catalytically reducing nitrogen oxides in exhaust gas is provided. The catalyst used in this method is referred to as a second two-layer catalyst.

  According to the present invention, in the second two-layer catalyst, at least one of the catalyst component A in the surface catalyst component and the catalyst component A in the internal catalyst component is selected from platinum, rhodium, palladium, and oxides thereof. It is preferable to carry at least one noble metal catalyst component.

  Furthermore, according to the present invention, as another aspect, for supplying and burning fuel under periodic rich / lean conditions, contacting the generated exhaust gas, and catalytically reducing nitrogen oxides in the exhaust gas The catalyst is provided as the catalyst.

  In particular, according to the present invention, as the most preferable aspect, fuel is supplied and burned under periodic rich / lean conditions, and the generated exhaust gas is brought into contact with the catalyst structure, so that nitrogen oxides in the exhaust gas are reduced. In the catalytic reduction method, a catalyst structure is provided in which the catalyst structure comprises the catalyst on an inert substrate.

  Further, according to the present invention, there is provided a catalyst structure for supplying and burning fuel under periodic rich / lean conditions, contacting exhaust gas to be generated, and catalytically reducing nitrogen oxides in the exhaust gas. A catalyst structure comprising the above-described catalyst on an inert base material is provided.

  According to the catalyst of the present invention, NOx in exhaust gas can be catalytically reduced with high durability without deterioration of the catalyst even in the presence of oxygen, sulfur oxides or water and in a wide range of reaction temperatures. Can do. In particular, according to the catalyst of the present invention, even in the presence of oxygen, sulfur oxides or water, deterioration and coexistence in the presence of sulfur oxides, which was a serious problem of the NOx storage-reduction system, were particularly important. NOx in the exhaust gas can be reduced and removed with high durability in a wide temperature range without generating harmful ammonia.

  In the present invention, the catalytic reduction of nitrogen oxides means that NOx adsorbed on the catalyst at the time of lean is converted to ammonia by a catalytic reaction at the time of rich, and this ammonia is accumulated on the solid acid of the catalyst, and thus accumulated. This means that the ammonia that has been reacted reacts with NOx in the presence of oxygen during lean, and that NOx is converted into nitrogen, water, carbon monoxide, carbon dioxide, etc. with high efficiency throughout the lean / rich process.

  In the NOx storage-reduction system described in the above-mentioned WO 93/7363 and WO 93/8383, as shown in FIG. Since the NOx absorbed on a basic material such as NO is reduced by a reducing agent such as hydrogen, carbon monoxide, hydrocarbons, etc. under rich conditions, nitrogen is produced. Only allowed under rich conditions. On the other hand, in the method of the present invention, as shown in FIG. 1, nitrogen is generated only under lean conditions. This means that in the method of the present invention, ammonia produced on the catalyst under rich conditions is adsorbed on the solid acid catalyst component in the catalyst, and thus the ammonia adsorbed on the solid acid catalyst component is absorbed. This is because NOx is selectively reduced to nitrogen only under lean conditions. Therefore, according to the method of the present invention, NOx is purified by a reaction mechanism that is clearly different from the above-mentioned NOx storage-reduction system in which nitrogen is produced only under rich conditions.

In the presence of oxygen, the selective reduction reaction of NOx by ammonia is
NO + NH ₃ + (1/4) O ₂ → N ₂ + (3/2) H ₂ O
Therefore, if 50% of the NOx present in the exhaust gas is converted to ammonia, all of the NOx in the exhaust gas is converted to nitrogen.

The first catalyst for catalytic reduction of nitrogen oxides in exhaust gas according to the present invention supplies and burns fuel under periodic rich / lean conditions, contacts the generated exhaust gas, and A catalyst for catalytic reduction of nitrogen oxides,
(A) (a) ceria or (b) praseodymium oxide or (c) a mixture and / or composite oxide of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum A catalyst component A comprising:
(B) (d) a noble metal catalyst component comprising at least one selected from platinum, rhodium, palladium and oxides thereof; and (e) a catalyst component B comprising a support;
(C) (f) a solid acid and (g) at least one selected from a solid acid supporting at least one metal oxide selected from vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese And a catalyst component C. That is, the first catalyst according to the present invention is a single layer catalyst.

  In the present invention, the catalyst component A is sometimes referred to as an oxygen storage material, in some cases, focusing on the fact that it has an oxygen storage function.

  According to the present invention, in the above catalyst, the catalyst component A is 30 to 70% by weight, the catalyst component B is 20 to 50% by weight, and the catalyst component C is 10 to 25% by weight based on the weight of all catalyst components. It is preferable that Further, according to the present invention, the catalyst component B preferably comprises 0.5 to 5% by weight of the noble metal catalyst component and 95 to 99.5% by weight of the support.

  According to the invention, the catalyst component A is in one embodiment an oxide and / or complex oxide (solid solution) of at least two elements, as described as component (c), ie at least two The mixture may be at least one selected from a mixture of elements and a composite oxide (solid solution) of at least these two elements, and here, the mixture is preferably a uniform mixture. However, according to the present invention, the composite oxide of the at least two elements is preferably used rather than the mixture of the oxides of the at least two elements, and the binary or ternary composite oxide is particularly preferable. Used.

For example, in the case of a binary composite oxide, the oxide-based weight ratio of each element in the solid solution is ceria / praseodymium oxide composite oxide, ceria / zirconia composite oxide, ceria / terbium oxide composite oxide, ceria / samarium oxide. If it is a complex oxide or the like, it is preferably in the range of 80/20 to 60/40. 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 / neodymium oxide / zirconia composite oxide, ceria / zirconia / praseodymium oxide composite oxide, In the case of ceria / zirconia / lanthanum oxide composite oxide, ceria / zirconia / samarium oxide composite oxide, ceria / zirconia / terbium oxide composite oxide, etc., preferably in the range of 45/30/30 to 75/20/5 It is. However, in the present invention, the oxide basis weight ratio of the respective elements in these composite oxides, ceria, zirconia, terbium oxide, praseodymium oxide, gadolinium oxide, neodymium oxide, CeO ₂ samarium oxide and lanthanum oxide, respectively, ZrO ₂ , TbO ₂ , Pr ₆ O ₁₁ , Ga ₂ O ₃ , Nd ₂ O ₃ , Sm ₂ O ₃ and La ₂ O ₃ shall be calculated.

  The catalyst component A in the catalyst according to the present invention can be prepared, for example, by the following method. That is, first, a water-soluble salt of an element constituting the catalyst component, for example, an aqueous solution of nitrate is neutralized or hydrolyzed to form a hydroxide, and then the resulting product is oxidized or What is necessary is just to bake at the temperature of 300-900 degreeC in reducing atmosphere. However, the catalyst component A can also be obtained by firing a hydroxide or oxide of the above-mentioned element that is commercially available as described above.

  In the present invention, among the catalyst components C, solid acids include acid type zeolites such as H-Y zeolite, H-mordenite, H-β zeolite, H-ZSM-5, titanium oxide, zirconia, silica- Alumina or the like can be used. Among these, H-mordenite is most preferably used from the viewpoint of ammonia adsorption. The solid acid supporting the metal oxide is a catalyst component in which one or more metal oxides selected from vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese are supported on the solid acid as described above. is there. The metal oxide supported on the solid acid needs to be optimally selected according to the reaction temperature when treating the exhaust gas, but in the region where the reaction temperature is 200 to 300 ° C., oxides of vanadium and copper are preferably used. On the other hand, in the region where the reaction temperature is 300 ° C. or higher, oxides of tungsten, molybdenum, iron, cobalt, nickel or manganese are preferably used. In addition, by mixing and using a solid acid catalyst component carrying various metal oxides, an effective catalyst can be obtained in a wide temperature range. The catalyst component comprising these metal oxide-supported solid acids can be prepared by a conventionally known method for supporting a metal oxide, for example, an impregnation method, an ion exchange method, a kneading method and the like.

  According to the present invention, the supported amount of the metal oxide in the solid acid supporting the metal oxide is in the range of 0.1 to 10% by weight of the total weight of the solid acid and the metal oxide component. When the supported amount of metal oxide is less than 0.1% by weight, the selective reduction reaction of NOx by ammonia during lean operation is insufficient. On the other hand, when it exceeds 10% by weight, reoxidation of ammonia occurs. As a result, the NOx purification rate decreases.

  In particular, the catalyst according to the present invention is a two-layer catalyst having a surface catalyst layer and an internal catalyst layer, and the surface catalyst layer is preferably exposed so as to be in direct contact with exhaust gas.

Preferred first two-layer catalysts according to the present invention are:
(A) (a) ceria or (b) praseodymium oxide or (c) a mixture and / or composite oxide of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum A catalyst component A comprising:
(C) (f) a solid acid and (g) at least one selected from a solid acid supporting at least one metal oxide selected from vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese A surface catalyst layer having a catalyst component C comprising:
(B) (d) from an internal catalyst layer having, as an internal catalyst component, a noble metal catalyst component consisting of at least one selected from platinum, rhodium, palladium and oxides thereof, and (e) a catalyst component B consisting of a carrier. It will be.

  That is, in the first two-layer catalyst according to the present invention, the surface catalyst component of the surface catalyst layer is composed of the catalyst component A and the catalyst component C, and the internal catalyst component of the internal catalyst layer is composed of the catalyst component B. . According to the present invention, the weight ratio of the surface catalyst component / internal catalyst component is preferably in the range of 1 to 3, and the surface catalyst component of the surface catalyst layer is 50 to 90% by weight of the catalyst component A. And 10 to 50% by weight of catalyst component C. The internal catalyst component of the internal catalyst layer is the catalyst component B. As described above, the catalyst component B is composed of 0.5 to 5% by weight of the noble metal catalyst component and 95 to 99.5% by weight of the support. Is preferred.

  In particular, according to the present invention, the two-layer catalyst having the surface catalyst layer and the inner catalyst layer as described above has a catalyst structure in which the inner catalyst layer and the surface catalyst layer are laminated in this order on an inert base material. Used as a body.

Also, a preferred second two-layer catalyst according to the present invention is:
(A) (a) ceria or (b) praseodymium oxide or (c) a mixture and / or composite oxide of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum A catalyst component A comprising:
(C) (f) a solid acid and (g) at least one selected from a solid acid supporting at least one metal oxide selected from vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese A surface catalyst layer having a catalyst component C comprising:
(A) (a) ceria or (b) praseodymium oxide or (c) a mixture and / or composite oxide of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum A catalyst component A comprising:
(B) (d) an internal catalyst layer having, as an internal catalyst component, a noble metal catalyst component consisting of at least one selected from platinum, rhodium, palladium and oxides thereof; and (e) a catalyst component B consisting of a carrier. It is what you have.

  That is, in the second two-layer catalyst according to the present invention, the surface catalyst component of the surface catalyst layer is composed of the catalyst component A and the catalyst component C, and the internal catalyst component of the internal catalyst layer is the catalyst component A and the catalyst component. It consists of component B. This second two-layer catalyst differs from the first two-layer catalyst in that the internal catalyst component of the internal catalyst layer has the catalyst component B together with the catalyst component A.

  According to the present invention, also in this second two-layer catalyst, the weight ratio of the surface catalyst component / internal catalyst component is preferably in the range of 1 to 3, and the surface catalyst component of the surface catalyst layer is It is preferably composed of 50 to 90% by weight of catalyst component A and 10 to 50% by weight of catalyst component C. The internal catalyst component of the internal catalyst layer is preferably composed of 30 to 90% by weight of catalyst component A, 0.5 to 5% by weight of noble metal catalyst component, and 5 to 69.5% by weight of the carrier.

  Similarly to the first two-layer catalyst, the second two-layer catalyst having the surface catalyst layer and the inner catalyst layer as described above is also preferably provided with an inner catalyst layer and a surface catalyst layer on an inert base material. It is used as a catalyst structure laminated in this order.

  In particular, according to the present invention, in both the first and second two-layer catalysts, the surface catalyst layer comprises at least 75% by weight, preferably at least 75% by weight of the surface catalyst component comprising the catalyst component A and the catalyst component C. 90% by weight. In the surface catalyst layer, when the ratio of the surface catalyst component is less than 75% by weight, the NOx adsorption effect at the time of leaning of the obtained surface catalyst layer and the selective reduction reaction of NO by ammonia become insufficient, and SOx durability is also improved. Will be reduced.

  In both the first and second two-layer catalysts, the inner catalyst layer has at least 50% by weight, preferably at least 75% by weight, of the inner catalyst component composed of the catalyst component B (catalyst component A). . In the internal catalyst layer, when the ratio of the internal catalyst component is less than 50% by weight, the NO oxidation ability during lean and the ammonia production rate during rich decrease.

  In the second two-layer catalyst according to the present invention, the first catalyst component A of the inner catalyst layer is composed of an oxygen storage material, and this oxygen storage material functions as a NOx adsorbent. This oxygen storage material, that is, the catalyst component A occupies a range of 30 to 90% by weight in terms of metal oxide in the internal catalyst component. The second catalyst component B comprises a noble metal catalyst component and a carrier, and this noble metal catalyst component is preferably supported on the carrier and the catalyst component A. However, the noble metal catalyst component and the catalyst component A May be carried on a carrier.

  In the present invention, as the carrier in the catalyst component B, conventionally known ones such as alumina, silica, silica-alumina, zeolite, titania and the like are used. This support is preferably used in the range of 5 to 69.5% by weight in the internal catalyst component.

  In the inner catalyst layer of the second two-layer catalyst according to the present invention, the noble metal catalyst component in the second catalyst component B is included in the range of 0.5 to 5% by weight in terms of metal in the inner catalyst component. ing. Even when the ratio of the above precious metal catalyst component in the internal catalyst component exceeds 5% by weight in terms of metal, the ammonia production rate at the time of rich is not improved, and conversely, in some cases, it is adsorbed on the solid acid at the time of lean. The oxidation of ammonia is promoted, and the selectivity of the selective reduction reaction between NOx and ammonia during lean is lowered. On the other hand, when the ratio of the precious metal catalyst component in the internal catalyst component is less than 0.5% by weight in terms of metal, ammonia production by the reducing agent is reduced. When the noble metal catalyst component is supported on the support and the oxygen storage material, if the support used has an ion exchange capacity, the degree of dispersion can be increased, and therefore, the noble metal catalyst component is preferably ion exchange supported. However, even in this case, when the ions are supported on the carrier at a high loading rate exceeding 1%, the ion exchange ability of the carrier is limited, so that the element contains the ions and oxides. Is often carried in a mixed state.

  The internal catalyst component in the second two-layer catalyst according to the present invention is preferably, for example, by first supporting a noble metal catalyst component on a carrier such as alumina or an oxygen storage material by an appropriate means such as an impregnation method or an ion exchange method. Then, it can be obtained as a powder in which a noble metal catalyst component is supported on a support or an oxygen storage material by firing at a temperature of 500 to 900 ° C. in an oxidizing or reducing atmosphere. Of course, as described above, the internal catalyst component can be converted into a powder by preparing a material in which a noble metal catalyst component is supported on a support such as alumina and an oxygen storage material as described above and mixing them. Can be obtained as

  According to the present invention, as described above, the catalyst component A in the single-layer catalyst and the first two-layer catalyst carries a noble metal catalyst component consisting of at least one selected from platinum, rhodium, palladium and oxides thereof. Moreover, in the second two-layer catalyst, at least one of the catalyst component A in the surface catalyst component and the catalyst component A in the internal catalyst component is platinum, rhodium, palladium, and these. It is preferable to carry at least one noble metal catalyst component selected from oxides.

  Thus, when the catalyst component A carries a noble metal catalyst component consisting of at least one selected from platinum, rhodium, palladium, and oxides thereof, NOx to the catalyst component A that is an oxygen storage material. Is promoted over a wide temperature range, and as a result, the NOx purification rate is improved over a wide temperature range. Further, since the thermal deterioration of the catalyst component A due to NOx adsorption is suppressed, the heat resistance of the catalyst is also improved.

In the catalyst according to the present invention, the catalyst component A plays a role of mainly adsorbing NOx in the exhaust gas in the lean process. The catalyst component A has both NO adsorption sites and NO ₂ adsorption sites. Although it depends on the type of oxide used, generally the amount of NO ₂ adsorption sites is large. In this way, the catalyst component B including the noble metal catalyst component plays a role of highly efficiently reducing NOx adsorbed on the catalyst component A to ammonia in the rich process, and also oxidizes NO during the lean period, and the NOx adsorption rate. Fulfills the function of enhancing. Among various noble metal catalyst components, platinum is most preferably used from the viewpoint of ammonia generation efficiency and NO oxidizing power. However, when low temperature performance is desired for the catalyst, rhodium which is excellent in ammonia generation in a low temperature range or Palladium is preferably used. A combination of platinum and at least one selected from rhodium and palladium is also preferably used.

According to the present invention, as described above, the catalyst is preferably a two-layer catalyst having a surface catalyst layer and an internal catalyst layer, but such a two-layer catalyst (first and second two-layer catalysts). ), The SO ₂ in the exhaust gas is adsorbed by the catalyst component A of the surface catalyst layer mainly as SO ₂ before being oxidized to the noble metal catalyst component in the internal catalyst layer. As a result, the catalyst of the present invention has an advantage that the catalyst performance does not deteriorate even in the presence of SOx. On the other hand, under the rich condition for a short time, NOx is rapidly reduced to ammonia, not nitrogen, with the help of the high reducing power of the noble metal catalyst component in the inner catalyst layer. As described above, ammonia generated by the reduction of NOx is almost completely trapped by the solid acid catalyst component C (supported by metal oxide) in the surface catalyst layer without desorbing into the gas phase. Further, in this way, the ammonia captured by the catalyst component C is used for the selective reduction reaction of NO by ammonia in the presence of oxygen in the next lean stroke. That is, ammonia captured by the catalyst component C is effectively used for selective reduction of NOx by ammonia on the catalyst component C without being re-oxidized by the noble metal catalyst component of the inner catalyst layer in the next lean process. . According to the catalyst of the present invention, as a result, a high NOx purification rate can be obtained in the lean / rich stroke.

According to the second two-layer catalyst of the present invention, the internal catalyst layer further includes the catalyst component B together with the catalyst component A. Therefore, since SO ₂ in the exhaust gas is mainly adsorbed by the catalyst component A in the surface catalyst layer as SO ₂ before being oxidized to the noble metal catalyst component in the internal catalyst layer, it is easily desorbed into the gas phase when rich. As a result, not only has the advantage that the catalyst performance does not deteriorate even in the presence of SOx, but also the adsorption rate of NO ₂ produced by NO oxidation on the noble metal catalyst component in the internal catalyst layer is improved. / A higher NOx purification rate can be obtained in the rich stroke.

In the present invention, when the catalyst is composed of a surface catalyst layer and an internal catalyst layer, the thickness of the surface catalyst layer greatly affects the NOx reduction ability and SOx poisoning resistance in the rich / lean process of the catalyst. 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 that can obtain high NOx reduction ability in the rich / lean stroke. The thickness ranges from 20 μm to 80 μm. However, usually about 40 μm is preferable. Even if the thickness of the surface catalyst layer is 80 μm or more, the performance is not improved correspondingly, and the diffusion of NOx and the reducing agent into the internal catalyst layer is inhibited, so the performance is rather lowered. Also, when the thickness of the surface catalyst layer is made smaller than 20 μm, the adsorption rate of NOx and SO ₂ at the time of lean is lowered, the capture rate of ammonia generated in the internal catalyst layer at the time of rich is lowered, and due to ammonia in the lean state The selective reduction reaction of NOx becomes insufficient, and as a result, the NOx purification ability decreases. Here, according to the present invention, the thickness of each catalyst layer is obtained 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 1.0 g / cm ³ for convenience. be able to.

  The internal catalyst layer has high NOx oxidation and ammonia generation ability of the noble metal component of the internal catalyst component. Therefore, the influence of the thickness on the NOx reduction ability in the rich / lean process is not as great as the thickness of the surface catalyst layer. However, a range of 10 μm to 50 μm is usually appropriate. Even if the thickness of the internal catalyst layer is greater than 50 μm, the performance does not improve accordingly. On the other hand, if the thickness of the internal catalyst layer is less than 10 μm, the total NOx reduction combined with the rich / lean process The performance drops.

  According to the present invention, as described above, the catalyst component can be obtained in various forms such as powder and granular materials. Therefore, it can be formed into various shapes such as, for example, a honeycomb, an annular material, a spherical material, and the like by using such a catalyst component by an arbitrary well-known method. 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.

  In particular, the catalyst according to the present invention is formed into a catalyst structure having a catalyst layer (for example, by coating), for example, by the wash coat method on the surface of an inert base material for support having an arbitrary shape. Is advantageous. 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.

  Further, in the case of the two-layer catalyst which is a preferred embodiment of the present invention, the catalyst components for the surface catalyst layer and the inner 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, 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.

  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.

  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 (second) / lean time (second) is usually in the range of 0.5 / 5 to 10 to 150, and preferably in the range of 2/30 to 5/90. It is.

  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 are hundreds of ppm by volume of NOx, 5-6% by volume of water, 2-3% by volume of carbon monoxide, 2-3% by volume of hydrogen, and thousands of ppm by volume of hydrocarbons. And 0 to 0.5 volume% oxygen. 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 carbon monoxide, thousands of volumes by volume of hydrogen, thousands of volumes by volume of hydrocarbons and 5 Contains -10 vol% oxygen.

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 that it has 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 150,000 h ⁻¹ .

  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.

  The catalyst according to the present invention can be used in combination with the NOx storage-reduction catalyst used in the above-described NOx storage-reduction system that reduces NOx stored during lean operation when necessary, as needed. That is, after the exhaust gas from the engine is treated using the NOx storage-reduction catalyst as the pre-stage catalyst, the catalyst according to the present invention can be treated as the post-stage catalyst.

  The NOx storage-reduction catalyst stores NOx in exhaust gas when lean, and reduces NOx to nitrogen by reducing components such as hydrocarbons and carbon monoxide contained in a large amount in exhaust gas when rich. Ammonia is also produced as a by-product. At this time, the NOx storage-reduction catalyst has strong alkalinity, and the ammonia thus generated is not adsorbed by the NOx storage-reduction catalyst and is discharged to the downstream of the catalyst, which may cause an environmental problem.

  On the other hand, the catalyst according to the present invention adsorbs NOx when lean, converts the adsorbed NOx to ammonia when rich, and adsorbs this ammonia, and then reacts this ammonia with gas phase NOx when lean, thus NOx is purified with high efficiency by selective reduction reaction with ammonia. Therefore, since the catalyst according to the present invention essentially comprises a component that adsorbs ammonia, ie a solid acid catalyst, it is an alkaline earth or alkali metal as used for NOx storage of NOx storage-reduction catalysts. Strong alkaline components such as cannot be used. As a result, in the treatment of exhaust gas using the catalyst according to the present invention, the NOx adsorption rate at high temperatures may be reduced, and NOx purification at high temperatures may not always be sufficient. Therefore, when performing NOx purification at a high temperature, it may be preferable to use a previously known NOx storage-reduction catalyst in combination.

  Therefore, as described above, the NOx storage-reduction catalyst is used as a pre-stage catalyst, and by using the catalyst according to the present invention as a post-stage catalyst, ammonia generated by the treatment of exhaust gas by the pre-stage catalyst is adsorbed on the post-stage catalyst, It can be used effectively for NOx reduction. Further, in the treatment of the exhaust gas by the front stage catalyst, by using a catalyst having a high NOx purification rate in a temperature range higher than that of the rear stage catalyst, a high NOx purification rate can be obtained in a wider temperature range.

  As the above-mentioned pre-stage catalyst, that is, NOx storage-reduction catalyst, for example, alumina is used as a support, and an alkali metal such as potassium, sodium, lithium and cesium and an alkaline earth such as barium and calcium are provided on this support. What is commonly referred to as a NOx storage-reduction catalyst, in which a metal and a noble metal component such as platinum are supported together with at least one component selected from rare earth metals such as lanthanum, cerium and yttrium, and alumina as a carrier On this support, for example, at least one selected from alkali metals and alkaline earth metals, at least one selected from rare earth metals, and at least one selected from noble metals such as platinum are supported, and if necessary, In general, it is called NOx adsorption catalyst containing titanium etc. It can be given to.

  Hereinafter, the present invention will be described in detail with examples of production of a powder catalyst as a catalyst component, examples of production of honeycomb catalyst structures using the catalyst, and nitrogen oxide reduction performance of the honeycomb catalyst structures as examples. The present invention is not limited to these examples. In the following, all “parts” and “%” are by weight unless otherwise specified.

(1) Preparation Example 1 of Catalyst Component A
Add 151.37 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O) to 1000 mL of ion-exchanged water to make an aqueous solution, and then add 0.1 N ammonia water to neutralize and hydrolyze cerium ions. Aged 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 ceria powder (specific surface area of 138 m ² / g).

Production Example 2
An aqueous solution was prepared by dissolving 103.77 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O) and praseodymium nitrate (Pr (NO ₃ ) ₃ .6H ₂ O) 35.77 g in 1000 mL of ion-exchanged water. . 0.1N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt and praseodymium 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 / praseodymium oxide composite oxide powder (oxide standard). (Weight ratio 60/40, specific surface area 112 m ² / g).

Production Example 3
In 1000 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, oxyzirconium 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).

Production Example 4
In 1000 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 gadolinium nitrate (Gd (NO ₃ ) ₃ .6H ₂ ) O) 7.48 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, oxyzirconium salt and gadolinium 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 ceria / zirconia / gadolinium oxide composite oxide powder (oxidized). Weight basis weight ratio 72/24/4, specific surface area 198 m ² / g).

Production Example 5
In 1000 mL of ion-exchanged water, 109.43 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O), 31.27 g of zirconium oxynitrate (ZrO (NO ₃ ) ₂ ) and neodymium nitrate (Nd (NO ₃ ) ₃ .6H ₂ ) O) 15.63 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, oxyzirconium salt and neodymium 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 / zirconia / neodymium oxide composite oxide powder (oxidized). (Weight basis weight ratio 70/20/10, specific surface area 171 m ² / g).

Production Example 6
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 1000 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.

Production Example 7
In 1000 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, oxyzirconium 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).
(2) Preparation of catalyst component B

Production Example 8
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, to which γ-alumina (KC-manufactured by Sumitomo Chemical Co., Ltd.) is added. 501 (hereinafter the same) The catalyst obtained by adding 60 g, drying at 100 ° C. with stirring, and calcining in air at 500 ° C. for 3 hours to carry 1% platinum on γ-alumina A powder was obtained.

Production Example 9
In Production Example 8, instead of 8.40 g of an aqueous solution of Pt (NH ₃ ) ₄ (NO ₃ ) ₂ (9.0% as platinum), 4.20 g of an aqueous rhodium nitrate solution (9.0% as rhodium) and Pt (NH ₃ ) 1% platinum and 0.5% rhodium were supported on γ-alumina in the same manner except that 8.40 g of ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) was used. A catalyst powder was obtained.

Production Example 10
In Production Example 8, instead of 8.40 g of an aqueous solution of Pt (NH ₃ ) ₄ (NO ₃ ) ₂ (9.0% as platinum), 4.20 g of an aqueous solution of palladium nitrate (9.0% as palladium) and Pt (NH ₃ ) 1% platinum and 0.5% palladium were supported on γ-alumina in the same manner except that 8.40 g of ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) was used. A catalyst powder was obtained.
(3) Preparation of catalyst component C

Production Example 11
Ammonia mordenite-10 (manufactured by Sud Chemical Co., silica / alumina ratio = 10) was calcined in air at 500 ° C. for 3 hours to obtain H-mordenite.

Production Example 12
Ammonia β-zeolite-25 (manufactured by Sud Chemical Co., silica / alumina ratio = 25) was calcined in air at 500 ° C. for 3 hours to obtain H-β-zeolite.

Production Example 13
60 g of metatitanic acid (TiO (OH) ₂ ) (manufactured by Chemilla Pigment) obtained from the sulfuric acid process was weighed in terms of titanium oxide, and an appropriate amount of water was added to form a slurry. The aqueous solution of ammonium metatungstate to the slurry (50% aqueous solution as WO ₃₎ 6.7 g was added in terms _of WO _3, with stirring, evaporated to dryness. This was further calcined in air at 500 ° C. for 3 hours to obtain titanium oxide powder carrying 10% of WO ₃ .

Production Example 14
60 g of zirconium oxide was weighed and an appropriate amount of water was added thereto to form a slurry. The 0.67g of 6.7g and vanadium oxalate solution in the aqueous solution of ammonium metatungstate to the slurry (WO ₃ as a 50% aqueous solution) WO ₃ in terms (V ₂ O ₅ as a 10% aqueous solution) in terms _of V ₂ O ₅ In addition, it was evaporated to dryness with stirring. This was further calcined in air at 500 ° C. for 3 hours to obtain zirconium oxide powder supporting 1% V ₂ O ₅ and 10% WO ₃ .

Production Example 15
60 g of ammonium β-zeolite-25 (manufactured by Sud Chemical Co., silica / alumina ratio = 25) was weighed, and an appropriate amount of water was added to form a slurry. To this slurry, 1.54 g of copper nitrate in terms of CuO was added, and the mixture was evaporated and dried with stirring. This was further calcined in air at 500 ° C. for 3 hours to obtain β-zeolite powder supporting CuO 2.5%.

Production Example 16
60 g of ammonium β-zeolite-25 (manufactured by Sud Chemical Co., silica / alumina ratio = 25) was weighed, and an appropriate amount of water was added to form a slurry. To this slurry, 1.54 g of iron nitrate in terms of Fe ₂ O ₃ was added, and the mixture was evaporated and dried with stirring. This was further calcined in air at 500 ° C. for 3 hours to obtain β-zeolite powder carrying 2.5% Fe ₂ O ₃ .
(4) Preparation of catalyst components A and B

Production Example 17
Add 151.37 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O) to 1000 mL of ion-exchanged water to make an aqueous solution. Aged 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 ceria powder (specific surface area of 138 m ² / g).

Pt (NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% 8.40 g as platinum was added to 100 mL of ion-exchanged water to obtain an aqueous solution. To this, 30 g of γ-alumina and 30 g of the above ceria powder were added, A catalyst powder comprising 1% platinum supported on γ-alumina / ceria (weight ratio 1/1) after being dried at 100 ° C. with stirring and calcined in air at 500 ° C. for 3 hours. Obtained.

  In the preparation of the catalyst powder having 1% platinum supported on γ-alumina / ceria (weight ratio 1/1), except that 10 g of γ-alumina and 50 g of ceria powder were used, γ A catalyst powder obtained by supporting 1% platinum on alumina / ceria (weight ratio 1/5) was obtained.

Production Example 18
Instead of 8.40 g of Pt (NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum), 4.20 g of rhodium nitrate aqueous solution (9.0% as rhodium) and Pt (NH ₃ ) ₄ (NO ₃ ) γ-alumina / ceria (weight ratio) in the same manner as in Production Example 17, except that 8.40 g of ₂ aqueous solution (9.0% as platinum) and 10 g of γ-alumina and 50 g of ceria powder were used. 1/5) A catalyst powder was obtained in which 1% platinum and 0.5% rhodium were supported.

Production Example 19
In Production Example 17, instead of 8.40 g of an aqueous Pt (NH ₃ ) ₄ (NO ₃ ) ₂ solution (9.0% as platinum), 4.20 g of an aqueous palladium nitrate solution (9.0% as palladium) and Pt (NH ₃ ) 1% platinum and palladium on γ-alumina / ceria (weight ratio 1/1) in the same manner except that 8.40 g of ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) was used. A catalyst powder having 0.5% supported thereon was obtained.

Production Example 20
In 1000 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, oxyzirconium 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).

  Thereafter, in the same manner as in Production Example 17, a catalyst powder in which 2% of platinum was supported on a mixture (weight ratio 1/2) of γ-alumina and ceria / zirconia / lanthanum oxide composite oxide was obtained.

Production Example 21
In 1000 mL of ion-exchanged water, 77.83 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O), 36.03 g of zirconium oxynitrate (ZrO (NO ₃ ) ₂ ) and praseodymium nitrate (Pr (NO ₃ ) ₃ · 6H ₂ ) O) 35.26 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, oxyzirconium salt and praseodymium 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 / zirconia / praseodymium oxide composite oxide powder (oxidized). Weight basis weight ratio 47/33/22, specific surface area 205 m ² / g).

Thereafter, in the same manner as in Production Example 8, a catalyst powder in which 2% platinum was supported on a mixture (weight ratio 1/2) of γ-alumina and ceria / zirconia / praseodymium oxide composite oxide was obtained.
(5) Preparation of catalyst component A carrying a noble metal catalyst component

Production Example 22
8.40 g of Pt (NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) was added to 100 mL of ion-exchanged water to obtain an aqueous solution, and 60 g of ceria powder prepared in Production Example 1 was added thereto. The mixture was 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 ceria.

Production Example 23
_4. Pt (NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) 8.40 g and palladium nitrate (Pd (NO ₃ ) ₃ ) aqueous solution (9.0% as palladium) in 100 mL of ion-exchanged water 20 g was added to prepare an aqueous solution, and 60 g of the ceria powder prepared in Production Example 1 was added thereto, and the mixture was dried at 100 ° C. with stirring, and then calcined in air at 500 ° C. for 3 hours. To obtain a catalyst powder in which 1% platinum and 0.5% palladium were supported.

Production Example 24
_3. Pt (NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) 8.40 g and rhodium nitrate (Rh (NO ₃ ) ₃ ) aqueous solution (9.0% as rhodium) in 100 mL of ion-exchanged water 20 g was added to prepare an aqueous solution, and 60 g of the ceria powder prepared in Production Example 1 was added thereto, and the mixture was dried at 100 ° C. with stirring, and then calcined in air at 500 ° C. for 3 hours. To obtain a catalyst powder in which 1% platinum and 0.5% rhodium were supported.

(6) Preparation of honeycomb catalyst structure The thickness of the catalyst layer was calculated by assuming that the apparent density of the catalyst layer was 1.0 g / cm ³ and the mechanical contact area of the honeycomb was 2500 m ² / g.

Example 1
30 g of the ceria powder catalyst obtained in Production Example 1, 15 g of catalyst powder obtained by supporting 1% platinum on the γ-alumina obtained in Production Example 8, 15 g of H-mordenite obtained in Production Example 11, and silica sol (Nissan Chemical Industry Co., Ltd. Snowtex N, 20% as silica, hereinafter the same) 6 g and an appropriate amount of water were 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 wash coat slurry. The wash coat slurry was applied to a cordierite honeycomb substrate having 400 cells per square inch to obtain a honeycomb catalyst structure having a catalyst layer having a thickness of 60 μm made of the catalyst.

Example 2
In the same manner as in Example 1, 1% platinum and 0.5% rhodium were supported on the ceria / praseodymium oxide composite oxide powder obtained in Production Example 2 and the γ-alumina obtained in Production Example 9. A honeycomb catalyst structure having a catalyst layer with a thickness of 60 μm was obtained using the powder comprising the catalyst powder and the H-β-zeolite obtained in Production Example 12.

Example 3
In the same manner as in Example 1, on the ceria / zirconia / lanthanum oxide composite oxide powder obtained in Production Example 3 and on the γ-alumina obtained in Production Example 10, 1% platinum and 0.5% palladium were produced. A honeycomb catalyst structure having a catalyst layer with a thickness of 60 μm was obtained using the zirconium oxide powder supporting 1% of V ₂ O ₅ and 10% of WO ₃ obtained in the above.

Example 4
3 g of silica sol and an appropriate amount of water were mixed with 30 g of catalyst powder obtained by supporting 1% platinum on the γ-alumina obtained in Production Example 8. Using 50 g of zirconia balls as a grinding medium, this mixture was ground in a planetary mill for 5 minutes to obtain a wash coat slurry. The wash coat slurry was applied to a cordierite honeycomb substrate having 400 cells per square inch to obtain a honeycomb structure having an internal catalyst layer having a thickness of 30 μm made of the catalyst.

Next, ₃ g of silica sol and an appropriate amount of water were mixed with 25 g of the ceria / zirconia / gadolinium oxide composite oxide powder obtained in Production Example 4 and 5 g of titanium oxide powder with 10% WO ₃ supported thereon. Using 50 g of zirconia balls as a grinding medium, this mixture was ground in a planetary mill for 5 minutes to obtain a wash coat slurry. Using this slurry, the wash coat slurry was applied onto the internal catalyst layer previously prepared to obtain a honeycomb structure having a surface layer catalyst layer having a thickness of 60 μm made of the catalyst.

Example 5
In the same manner as in Example 4, an internal catalyst layer having a thickness of 30 μm composed of a catalyst in which 1% of platinum was supported on the γ-alumina obtained in Production Example 8 was obtained. Next, 25 g of the ceria / zirconia / neodymium oxide composite oxide powder obtained in Production Example 5 and 5 g of β-zeolite powder supporting 2.5% of CuO obtained in Production Example 15 were used. A honeycomb catalyst structure having a surface catalyst layer was obtained.

Example 6
In the same manner as in Example 5, an internal catalyst layer having a thickness of 30 μm made of a catalyst in which 1% of platinum was supported on the γ-alumina obtained in Production Example 8 was obtained. Next, a honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm is obtained by using 25 g of the ceria / terbium oxide composite oxide powder obtained in Production Example 6 and 5 g of the H-mordenite powder obtained in Production Example 11. It was.

Example 7
In the same manner as in Example 5, an internal catalyst layer having a thickness of 30 μm made of a catalyst in which 1% of platinum was supported on the γ-alumina obtained in Production Example 8 was obtained. Next, using the ceria / zirconia / samarium oxide composite oxide powder 25 g obtained in Production Example 8 and the H-mordenite powder 5 g obtained in Production Example 11, a honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm Got.

Example 8
In the same manner as in Example 5, an internal catalyst layer having a thickness of 30 μm made of a catalyst in which 1% of platinum was supported on the γ-alumina obtained in Production Example 8 was obtained. Next, a honeycomb catalyst structure having a surface catalyst layer with a thickness of 60 μm was obtained using 25 g of the ceria powder obtained in Production Example 1 and 5 g of the H-mordenite powder obtained in Production Example 11.

Example 9
In the same manner as in Example 5, an internal catalyst layer having a thickness of 30 μm made of a catalyst in which 1% of platinum was supported on the γ-alumina obtained in Production Example 8 was obtained. Next, a honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm was obtained using 20 g of the ceria powder obtained in Production Example 1 and 10 g of the H-mordenite powder obtained in Production Example 11.

Example 10
In the same manner as in Example 5, an internal catalyst layer having a thickness of 30 μm made of a catalyst in which 1% of platinum was supported on the γ-alumina obtained in Production Example 8 was obtained. Next, a honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm was obtained using 25 g of the ceria powder obtained in Production Example 1 and 25 g of the H-mordenite powder obtained in Production Example 11.

Example 11
In the same manner as in Example 5, an internal catalyst layer having a thickness of 30 μm made of a catalyst in which 1% of platinum was supported on the γ-alumina obtained in Production Example 8 was obtained. Next, a surface catalyst layer having a thickness of 60 μm is obtained using 25 g of the ceria powder obtained in Production Example 1 and 5 g of β-zeolite powder carrying 2.5% of Fe ₂ O ₃ obtained in Production Example 16. A honeycomb catalyst structure was obtained.

Example 12
In the same manner as in Example 4, first, an internal catalyst having a thickness of 30 μm was used by using a catalyst powder in which 1% of platinum was supported on γ-alumina / ceria (weight ratio 1/1) obtained in Production Example 17. Next, a honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm was obtained using 25 g of the ceria powder obtained in Production Example 1 and 5 g of the H-mordenite powder obtained in Production Example 11.

Example 13
In the same manner as in Example 4, the catalyst powder obtained by supporting 1% platinum and 0.5% rhodium on the γ-alumina / ceria (weight ratio 1/5) obtained in Production Example 18 was used. A honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm prepared by preparing an internal catalyst layer of 30 μm and then using 25 g of ceria powder obtained in Production Example 1 and 5 g of H-mordenite powder obtained in Production Example 11 Got.

Example 14
In the same manner as in Example 4, the catalyst powder obtained by supporting 1% platinum and 0.5% palladium on the γ-alumina / ceria (weight ratio 1/1) obtained in Production Example 19 was used. A honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm prepared by preparing an internal catalyst layer of 30 μm and then using 25 g of ceria powder obtained in Production Example 1 and 5 g of H-mordenite powder obtained in Production Example 11 Got.

Example 15
In the same manner as in Example 4, catalyst powder obtained by supporting 2% platinum on the mixture (weight ratio 1/2) of γ-alumina obtained in Production Example 20 and ceria / zirconia / lanthanum oxide composite oxide. An internal catalyst layer having a thickness of 30 μm was prepared, and then 25 g of ceria / zirconia / lanthanum oxide composite oxide powder obtained in Production Example 3 and 5 g of H-mordenite powder obtained in Production Example 11 were used. A honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm was obtained.

Example 16
In the same manner as in Example 4, catalyst powder obtained by supporting 2% platinum on a mixture (weight ratio 1/2) of γ-alumina obtained in Production Example 21 and ceria / zirconia / praseodymium oxide composite oxide. An internal catalyst layer having a thickness of 30 μm was prepared, and then 25 g of the ceria / praseodymium oxide composite oxide powder obtained in Production Example 2 and 5 g of the H-mordenite powder obtained in Production Example 11 were used. A honeycomb catalyst structure having a surface catalyst layer of 60 μm was obtained.

Example 17
Catalyst powder obtained by supporting platinum 2% on the mixture (weight ratio 1/2) of γ-alumina obtained in Production Example 21 and ceria / zirconia / praseodymium oxide composite oxide in the same manner as in Example 16. Was used to prepare an internal catalyst layer having a thickness of 15 μm, and then 5 g of the H-mordenite powder obtained in Production Example 11 was used to obtain a honeycomb catalyst structure having a surface catalyst layer having a thickness of 40 μm.

Example 18
In Production Example 11, 30 g of catalyst powder obtained by supporting 1% platinum on ceria obtained in Production Example 22 and 15 g of catalyst powder obtained by carrying 1% platinum on γ-alumina obtained in Production Example 8 15 g of the obtained H-mordenite and 6 g of silica sol were mixed with an appropriate amount of water. Using 50 g of zirconia balls as a grinding medium, this mixture was ground in a planetary mill for 5 minutes to obtain a wash coat slurry. The wash coat slurry was applied to a cordierite honeycomb substrate having 400 cells per square inch to obtain a honeycomb catalyst structure having a catalyst layer having a thickness of 60 μm made of the catalyst.

Example 19
6 g of silica sol and an appropriate amount of water were mixed with 30 g of catalyst powder obtained by supporting 1% platinum on the γ-alumina obtained in Production Example 8. Using 50 g of zirconia balls as a grinding medium, this mixture was ground in a planetary mill for 5 minutes to obtain a wash coat slurry. The wash coat slurry was applied to a cordierite honeycomb substrate having 400 cells per square inch to obtain a honeycomb structure having an internal catalyst layer having a thickness of 30 μm made of the catalyst.

  Next, 25 g of catalyst powder obtained by supporting 1% platinum and 0.5% palladium on the ceria obtained in Production Example 23 and 5 g of H-mordenite powder obtained in Production Example 11 were mixed with 3 g of silica sol and an appropriate amount of water. Mixing was carried out to obtain a wash coat slurry in the same manner as described above. The slurry was coated on the honeycomb structure to obtain a honeycomb catalyst structure having a surface catalyst layer made of the catalyst and having a thickness of 60 μm.

Example 20
In the same manner as in Example 4, first, an internal catalyst having a thickness of 30 μm was used by using a catalyst powder in which 1% of platinum was supported on γ-alumina / ceria (weight ratio 1/1) obtained in Production Example 17. Next, 25 g of catalyst powder obtained by supporting 1% platinum and 0.5% rhodium on the ceria obtained in Production Example 24 and 5 g of H-mordenite powder obtained in Production Example 11, A honeycomb catalyst structure having a surface catalyst layer having a thickness of 60 μm was obtained.
Comparative Example 1

8.100 g of Pt (NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) was added to 100 mL of ion-exchanged water to obtain an aqueous solution. After drying at 0 ° C., it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 1% of platinum was supported on γ-alumina.

  Hereinafter, in the same manner as in Example 1, from an internal catalyst layer having a thickness of 30 μm made of a catalyst having 1% platinum supported on the γ-alumina, and the ceria / zirconia / lanthanum oxide composite oxide obtained in Production Example 16. A honeycomb catalyst structure having a surface catalyst layer with a thickness of 60 μm was obtained.

Comparative Example 2
8.100 g of Pt (NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) was added to 100 mL of ion-exchanged water to obtain an aqueous solution. After drying at 0 ° C., it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 1% of platinum was supported on γ-alumina.

Add 151.37 g of cerium nitrate (Ce (NO ₃ ) ₃ .6H ₂ O) to 1000 mL of ion-exchanged water to make an aqueous solution, and then add 0.1 N ammonia water to neutralize and hydrolyze cerium ions. Aged 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 ceria powder (specific surface area of 138 m ² / g).

  To 1000 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 ceria powder is added to the solution and dried at 100 ° C. with stirring. And calcining at 500 ° C. for 3 hours to obtain a catalyst powder having 1% rhodium supported on ceria.

  Hereinafter, 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 the γ-alumina and a catalyst having a thickness of 60 μm composed of a catalyst having 1% rhodium supported on ceria. A honeycomb catalyst structure having a surface catalyst layer was obtained.

Comparative Example 3
A barium carbonate is prepared from a caustic barium aqueous solution and a sodium carbonate aqueous solution, and 1% platinum is supported on a mixture (weight ratio 8/2) of this barium carbonate (BaCO ₃ ) and γ-alumina to prepare a catalyst powder. did. γ-alumina was charged into an aqueous potassium carbonate solution, mixed, dried, and then calcined in air at 1100 ° C. for 3 hours to prepare K ₂ O.12Al ₂ O ₃ (specific surface area 18 m ² / g). . Separately, γ-alumina and the above K ₂ O · 12Al ₂ O ₃ were mixed to prepare a γ-alumina / K ₂ O · 12Al ₂ O ₃ (weight ratio 9/1) mixture, which was mixed with 1% platinum. Was supported to obtain a catalyst powder.

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 Dry-mixing was carried out to prepare a wash coat slurry 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 with a thickness of 80 μm.

(7) 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.

Test Method The composition of the mixed gas used in the NOx reduction experiment under rich conditions is as follows.
NO: 100ppm
SO ₂ : 50 ppm
O ₂ : 0.4%
CO: 2%
C ₃ H ₆ (propylene): 2000 ppm
H ₂ : 2%
H ₂ O: 9.0%

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: 100ppm
SO ₂ : 50 ppm
O ₂ : 9.0%
CO: 0.2%
C ₃ H ₆ (propylene): 500 ppm
H ₂ : 0%
H ₂ O: 6.0%

  Catalytic reaction was performed 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.

(I) Space velocity:
100000 hr ^-1 (lean condition)
100000 hr ^-1 (rich condition)

(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.

  Furthermore, using the catalyst structures according to Example 15 and Comparative Examples 1 and 3, respectively, the above gas condition, 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 is very resistant to sulfur oxides as compared to conventional NOx storage-reduction systems.

(8) Nitrogen production confirmation test (i) Preparation of catalyst packed bed using catalyst according to the present invention 10 g of catalyst powder (catalyst component B) prepared in Production Example 8 and 24 g of silica sol were mixed with an appropriate amount of water, After evaporating and drying, baking was performed in air at 500 ° C. for 1 hour. The fired product was sieved to have a particle size of 0.25 to 0.5 mm to prepare an internal catalyst component (catalyst component B) composed of a granular material having the particle size.

  Separately, 3 g of the catalyst powder (catalyst component C) prepared in Production Example 11 and 7 g of the catalyst powder (catalyst component A) prepared in Production Example 3 and 24 g of silica sol are mixed with an appropriate amount of water and pulverized with a planetary mill. Thus, a slurry of the surface catalyst components (catalyst components A and C) was prepared. The slurry is sprayed onto the internal catalyst component made of the above granular material, dried, and calcined in air at 500 ° C. for 1 hour, and the catalyst powder made of the internal catalyst component having a coating made of the surface catalyst component on the surface And weighed. Thereafter, the slurry of the surface catalyst component is sprayed on the surface of the catalyst powder thus obtained, and the operation of drying and firing in the same manner as described above is carried out by the operation of the surface catalyst component and the internal catalyst. Repeated until the weight ratio of the components was 4: 1. The catalyst thus obtained (0.5 g) was filled in a quartz reaction tube to form a catalyst packed bed.

(Ii) Preparation of catalyst packed bed using catalyst according to Comparative Example The wash coat slurry prepared in Comparative Example 3 was evaporated and dried, and then calcined in air at 500 ° C. for 1 hour. The fired product was sieved to have a particle size of 0.25 to 0.5 mm to prepare a catalyst component composed of a granular material having the particle size. Thus obtained catalyst component (0.5 g) was filled in a quartz reaction tube to form a catalyst packed bed.

(Iii) Reaction test Using each of the catalyst packed beds, a test for confirming the generation of nitrogen was performed under the following test gas treatment conditions. That is, the test gas composition under lean conditions was NO 2000 ppm, oxygen 9 volume%, and the balance was helium. A test gas under rich conditions was prepared by periodically injecting 5 volume% hydrogen into this test gas. The lean / rich width was set to 120 seconds / 30 seconds, and nitrogen and NOx in the treated gas that passed through the catalyst packed bed under rich conditions were measured using a quadrupole mass spectrometer (Omnistar manufactured by Balzer).

  As shown in FIG. 1, the result of treating the test gas in the catalyst packed bed using the catalyst according to the present invention, nitrogen is generated under lean conditions over a reaction temperature range of 250 to 400 ° C. Is recognized. This is because, under rich conditions, ammonia produced on the catalyst is adsorbed by the solid acid component in the catalyst, and the ammonia thus adsorbed selectively reduces NOx to nitrogen only under lean conditions. It is. On the other hand, since NOx is adsorbed on the catalyst component A after the generation of nitrogen is completed, almost no NOx is recognized in the gas that has passed through the catalyst packed bed in most periods under lean conditions.

On the other hand, when the test gas was treated in the catalyst packed bed using the catalyst according to the comparative example, the result is shown in FIG. Nitrogen formation is observed only immediately after the change from lean to rich conditions. This is because NO ₂ adsorbed by the alkali compound (NOx adsorbent) is reduced only by the reducing agent present in the gas when rich. On the other hand, since NOx is adsorbed on the catalyst under lean conditions, it is not recognized in the initial stage of leaning in the gas that has passed through the catalyst packed bed, but the concentration gradually increases after the adsorption breakthrough. . This is because an alkaline compound (NOx adsorbent) needs to adsorb NOx over the entire period under lean conditions, and therefore requires a larger amount of NOx adsorption than the catalyst packed bed according to the present invention described above. is there.

FIG. 1 shows the change of nitrogen oxides and nitrogen concentration in a gas with time (rich / lean time) when exhaust gas is treated over a reaction temperature of 250 to 400 ° C. using an example of the catalyst according to the present invention. . FIG. 2 shows the change of nitrogen oxide and nitrogen concentration in the gas with time (rich / lean time) when exhaust gas is treated over a reaction temperature of 250 to 400 ° C. using an example of a catalyst according to a comparative example. .

Claims (8)

In a method in which fuel is supplied and combusted under periodic rich / lean conditions, the generated exhaust gas is brought into contact with the catalyst, and nitrogen oxides in the exhaust gas are contact-reduced.

(A) (a) Ceria or

(B) praseodymium oxide or

(C) a catalyst component A comprising a mixture of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum and / or a composite oxide;
(C) (g) a surface having, as a surface catalyst component, a catalyst component C made of a solid acid carrying at least one metal oxide selected from vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese A catalyst layer;
(A) (a) ceria or (b) praseodymium oxide or (c) a mixture and / or composite oxide of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum A catalyst component A comprising:
(B) (d) a noble metal catalyst component comprising at least one selected from platinum, rhodium, palladium and oxides thereof;
A catalyst component B consisting of (e) a carrier possess the inner catalyst layer having the inner catalyst component, the proportion of the catalyst component A in the outer catalyst component is 50 to 90 wt%, the proportion of the catalyst component C is 10 To 50 wt%, the proportion of catalyst component A in the internal catalyst component is 30 to 90 wt%, the proportion of noble metal catalyst component is 0.5 to 5 wt%, and the support is 5 to 69.5 wt%. %, A method for catalytically reducing nitrogen oxides in exhaust gas, The process according to claim 1, wherein the solid acid is acid-type zeolite, titanium oxide, zirconia or silica-alumina. The solid acid is acid-type zeolite, titanium oxide, zirconia or silica-alumina, and the metal oxide is one or more oxides selected from vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese. The method according to 1. The method according to claim 1, wherein the solid acid is acid-type zeolite, titanium oxide or zirconia, and the metal oxide is one or more oxides selected from vanadium, tungsten, molybdenum, copper and iron. At least one of the catalyst component A in the surface catalyst component and the catalyst component A in the internal catalyst component carries a noble metal catalyst component consisting of at least one selected from platinum, rhodium, palladium and oxides thereof. The method of claim 1 . The method according to claim 1, wherein the surface catalyst layer has a thickness of 20 µm to 80 µm, and the inner catalyst layer has a thickness of 10 µm to 50 µm. The method according to claim 1, wherein the surface catalyst layer has a thickness of 40 µm to 80 µm, and the inner catalyst layer has a thickness of 10 µm to 50 µm. In a method for supplying and burning fuel under periodic rich / lean conditions, bringing the generated exhaust gas into contact with the catalyst structure, and catalytically reducing the nitrogen oxides in the exhaust gas, the catalyst structure is inactive A method for catalytically reducing nitrogen oxides in exhaust gas comprising the catalyst according to any one of claims 1 to 7 on a simple substrate.

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