JP4823100B2 - New exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to an exhaust gas purifying catalyst for purifying NOx contained in exhaust gas of an internal combustion engine.

Conventionally, NOx, carbon monoxide, and hydrocarbons contained in exhaust gas from gasoline vehicles have been purified by a three-way catalyst composed of a platinum group element (see Patent Document 1). The platinum group catalyst which is the main component of the three-way catalyst has a high ability to oxidize NO in the rich burn exhaust gas having an oxygen concentration of 1% or less to NO ₂ , and NO ₂ is converted into N ₂ O and nitrogen by a reducing substance. The ability to reduce is also high, and the ability to completely oxidize a reducing substance with oxygen is also high. The three-way catalyst usually uses a cordierite monolith molded body as the catalyst support, and the activated alumina particles having a size of several μm to several tens of μm are applied to the inner wall of the gas flow path of the molded body. In this structure, platinum-palladium-rhodium particles having a size of several tens of nanometers to several hundreds of nanometers are supported. The purification method with a three-way catalyst is included in the exhaust gas by controlling the air-fuel ratio, which is the air: fuel weight mixing ratio, close to the theoretical air-fuel ratio (= 14.7) (this combustion is called rich burn). Since the oxygen concentration can be maintained at 1% or less, carbon monoxide and hydrocarbons contained in the exhaust gas can be used as a reducing agent for NOx. However, when the oxygen concentration in the exhaust gas exceeds several percent, the catalyst There is a problem that oxidation deterioration occurs.

  In addition, a so-called urea SCR method using a transition metal compound and / or a platinum group element as a catalyst and urea water as a reducing agent has been studied for exhaust gas treatment of large diesel vehicles such as trucks and buses that run on light oil fuel ( Patent Document 2). This method has the advantage of being able to efficiently purify NOx from a relatively low temperature region near 100 ° C. to a relatively high temperature region near 600 ° C., but it requires the mounting of expensive urea water as a reducing agent. There is a problem that most of the low temperature exhaust NOx below about 200 ° C. is discharged as ammonium nitrate, which causes water environmental pollution.

  As a method using a reducing agent other than urea water, a hydrocarbon SCR method using an automobile fuel (hydrocarbons such as ethylene and propylene contained in a small amount in the fuel) as a reducing agent has been studied. Although the method can obtain a high purification rate for rich burn exhaust NOx, there is a problem in fuel consumption because a large amount of reducing agent is required for lean burn exhaust NOx. Moreover, although the method of using methanol as a reducing agent is proposed (refer nonpatent literature 1), there exists a problem that reaction start temperature is 300 degreeC or more. Furthermore, recently, after reforming fuel with a reforming catalyst, a method for introducing exhaust gas into exhaust gas and treating exhaust NOx with an exhaust gas purifying catalyst has been proposed (see Patent Document 4 and Non-Patent Document 2). There exists a problem that reaction start temperature is 300 degreeC or more. Further, the zeolite-supported catalyst that has been studied for purifying exhaust NOx such as diesel exhaust gas has a problem that the activity is remarkably lowered by moisture and oxygen under hydrothermal conditions.

On the other hand, a three-way catalyst cannot be used for exhaust NOx treatment of small diesel vehicles such as diesel passenger cars. Because the air-fuel ratio is more than several times the air-fuel ratio of gasoline (diesel fuel combustion is lean burn), the oxygen concentration in diesel exhaust gas is usually over 5% and contains almost no reducing substances. It is. For the same reason, it is difficult to purify the exhaust gas from lean-burn gasoline vehicles using only a three-way catalyst. As described above, the platinum group catalyst that is the main component of the three-way catalyst has a high ability to oxidize NO to NO ₂ , but also has a high ability to reduce NO ₂ to N ₂ O and nitrogen by a reducing substance, Since the ability to completely oxidize reducing substances with oxygen is high, the catalyst structure in which a conventional platinum-based catalyst is in direct contact with the exhaust gas of lean burn with a high oxygen concentration is easy to burn into lean burn exhaust gas. Even when a reducing substance such as carbon or hydrocarbon is supplied, there is a problem that the processing temperature band of NOx is limited to a very narrow region of about 200 ° C to 250 ° C. On the other hand, although transition metal oxide catalysts have not been put to practical use, it is reported that the range of 250 ° C. to 400 ° C. is the NOx treatment temperature range because the redox power is lower than that of platinum. Conventionally, it has been studied to mix the platinum group catalyst and the transition metal oxide catalyst in order to expand the treatment temperature range, but the object could not be achieved by simply mixing them. The reason for this is considered that because the oxidizing power of the platinum catalyst in the mixed catalyst is too strong, most of the reducing substance contained in the exhaust gas component is consumed by the platinum catalyst. Currently, a so-called NOx occlusion reduction catalyst in which a NOx occlusion agent is added to a platinum group catalyst as a catalyst has been studied for the exhaust NOx treatment of lean burn gasoline vehicles and small diesel vehicles (see Patent Document 3). This method uses a combustion system in which the internal combustion engine alternately repeats lean burn and rich burn, absorbs lean burn exhaust NOx with a NOx storage agent, releases the absorbed NOx in a rich burn atmosphere, and releases the released NOx with rich burn exhaust gas. It is based on the idea of reducing with a platinum group catalyst using a large amount of reducing substances such as carbon monoxide, hydrogen and hydrocarbons present in the fuel or rich burn exhaust gas supplied therein. The combustion method that repeats lean burn and rich burn alternately and the purification method using NOx occlusion reduction catalyst are around 250 ° C even in a high-concentration oxygen atmosphere where the three-way catalyst used for exhaust gas treatment of gasoline passenger cars cannot be used. Has the advantage of being able to purify NOx over about 600 ° C., but there is a problem that NOx purification at about 200 ° C. or lower is very difficult, and the NOx occlusion agent due to moisture in the exhaust gas and a small amount of SOx Has a problem that catalyst regeneration by periodic high-temperature treatment is necessary. This is due to the chemical nature of the NOx storage reduction catalyst. NOx occlusion agents for absorbing NO _{2 in a} lean burn atmosphere are strongly basic alkali metal or alkaline earth metal oxides, hydroxides, carbonates, etc., so NO ₂ or nitric acid is one of these metals. It easily reacts with compounds at mild temperatures and is absorbed as nitrate. In a rich burn atmosphere, these nitrates decompose to generate the original metal compound and free NO ₂ . Therefore, the treatment temperature of the exhaust gas completely depends on the decomposition temperature of the nitrate of the metal compound that is the NOx storage agent. The decomposition temperature of the known NOx storage agent is 250 ° C. or higher, and no metal compound that operates at 200 ° C. or lower is known. Further, the metal compound is water-soluble and easily reacts with SOx to become an inactive sulfate.

By the way, in Japan, the temperature of exhaust gas discharged from diesel passenger cars is approximately 120 ° C. to 200 ° C. during transient travel and approximately 200 ° C. to 400 ° C. during stable travel, but about 80% of the NOx discharged is transient travel. Sometimes discharged.
From the above, the catalyst required for exhaust gas treatment of diesel passenger cars is desired to be a catalyst having high activity with respect to the lean burn exhaust NOx in the low temperature range of 120 ° C. to 200 ° C. However, an exhaust NOx purification catalyst effective for lean burn exhaust NOx at 250 ° C. or lower has not been found. Moreover, the problem that most of NOx purification in the low temperature to medium temperature region stops at the stage of dinitrogen monoxide (N ₂ O) having a very large warming coefficient has not been solved.

In general, industrial catalysts are often used in a state where they are supported on a porous material. The pores of the porous material are classified into micropores with a pore diameter of 2 nm or less, mesopores of 2 to 50 nm, and macropores of 50 nm or more according to IUPAC (International Pure and Applied Chemical Association). Yes. No single porous material other than activated carbon has a wide distribution ranging from the micro to meso range.
In recent years, silica, alumina, and silica-alumina-based mesoporous molecular sieves have been developed in which pores having a pore size of several nm are regularly arranged and the specific surface area has a very large value of 400 to 1100 m ² / g. These are disclosed in, for example, Patent Documents 1, 2, and 3, and are named crystalline mesoporous molecular sieves because the pore arrangement of pores is similar to the atomic arrangement of crystalline substances. .

Since the catalytic reaction is a surface reaction, the larger the specific surface area of the catalyst, the higher the catalytic activity. Further, the carrier for supporting the catalyst is more likely to exhibit the catalytic activity as the specific surface area is larger. From this point of view, when looking at a three-way catalyst for automobiles, the monolith molded body as a support has a mesh-shaped cross section and is provided with gas flow paths partitioned by thin walls parallel to each other in the axial direction. It is a molded body, the specific surface area is about 0.2 m ² / g, the specific surface area of alumina particles as a support is 110 to 340 m ² / g, and the specific surface area of the catalyst is about ²⁰ to 40 m ² / g from the particle size. Presumed to be. Therefore, the surface area of the catalyst is 10 ^{2 of that of the} conventional three-way catalyst by supporting the nano-sized catalyst particles that are one to two orders of magnitude smaller than the particle diameter of the conventional catalyst particles in the pores of the mesoporous material. since 10 becomes ⁴ times larger, which is believed to enhance the catalytic activity for automotive exhaust by applying monolith forming body, this idea, for example, disclosed in Patent Document 5 to 10. However, it has been difficult to effectively remove low-temperature exhaust NOx around 120 ° C. to 200 ° C. discharged by diesel passenger cars and the like.

JP-A-5-254827 Special table hei 5-503499 published Special table hei 6-509374 publication WO2005 / 103461 US Pat. No. 5,143,707 JP-A-8-257407 JP 2001-9275 A JP 2002-210369 A JP 2002-320850 A JP 2003-135963 A Applied Catalysis B: Environmental 17 (1998) 115-129. Applied Catalysis B: Environmental 17 (1998) 333-345.

  In view of the above circumstances, an object of the present invention is to provide a novel catalyst for efficiently purifying lean burn exhaust NOx in a low temperature region, which has been difficult in the past.

As a result of intensive studies to achieve the above object, the present inventors have obtained a multilayer in which a porous NOx oxidation catalyst layer, a porous NOx adsorbent layer, and a porous NOx selective reduction catalyst layer are functionally laminated. The functional catalyst having the structure was found to be very effective for lean burn exhaust NOx, and the present invention was completed based on this finding.
That is, the present invention
(1) An exhaust gas purifying catalyst characterized by being a multi-layered functional catalyst comprising a porous NOx oxidation catalyst layer provided in the inner core and a porous NOx selective reduction catalyst layer provided thereon,
(2) In the exhaust gas purifying catalyst according to the above (1), further kite only set the porosity NOx adsorbent layer in the middle of the <br/> porous NOx oxidation catalyst layer and the porous NOx selective reduction catalyst layer An exhaust gas purifying catalyst characterized by
(3) The catalyst in the porous NOx oxidation catalyst layer is a micro mesoporous NOx oxidation catalyst in which a platinum group element is supported on a micro mesoporous material, and the catalyst in the porous NOx selective reduction catalyst layer is transition metal oxidized into a macro mesoporous material. A catalyst for purifying exhaust gas as described in (1) or (2) above, which is a macro-mesoporous NOx selective reduction catalyst carrying a substance,
(4) The exhaust gas purifying catalyst according to (2) or (3) above, wherein the adsorbent in the porous NOx adsorbent layer is a mesoporous adsorbent in which an indium alloy is supported on a mesoporous material,
About.

  The exhaust gas purifying catalyst of the present invention can perform lean burn exhaust NOx treatment, which could not be achieved conventionally, very efficiently even in a low temperature region. For example, although a three-way catalyst can hardly purify NOx in an atmosphere having an oxygen concentration of 10%, a three-layer structure in which a micro mesoporous platinum catalyst, a mesoporous indium alloy, and a macro mesoporous cobalt oxide catalyst according to the present invention are sequentially laminated. This functional catalyst can purify 60% or more of NOx at 180 ° C. to 500 ° C. with respect to lean burn exhaust gas having an oxygen concentration of 10%.

Hereinafter, the present invention will be described in detail.
As described above, a simple mixed catalyst of a three-way catalyst mainly composed of a platinum group catalyst and a transition metal oxide catalyst has a wide temperature range of lean burn exhaust NOx treatment ranging from a low temperature range of 200 ° C. or lower to an intermediate temperature range of 400 ° C. Could not expand into the area.
The catalyst of the present invention is devised in that functional separation of the catalyst is performed in order to solve the above problems. That is, the present invention provides a multilayer structure comprising a porous NOx oxidation catalyst layer provided in the inner core and a porous NOx selective reduction catalyst layer provided thereon in order to efficiently purify NOx discharged from a lean burn internal combustion engine. A functional catalyst having a structure is provided, and a functional catalyst having a three-layer structure in which a porous NOx adsorbent layer is provided between the NOx oxidation catalyst layer and the NOx selective reduction catalyst layer is provided as a more preferable functional catalyst. To do. The role of the NOx oxidation catalyst layer in the functional catalyst is to oxidize NOx (mainly NO) in the lean burn atmosphere to NOx (mainly NO ₂ ) having a higher degree of oxidation, and the role of the NOx adsorbent layer is NOx generated and reduced by the NOx oxidation catalyst layer is absorbed and desorbed, and the role of the NOx selective reduction catalyst layer is by selectively reducing NOx released from the NOx adsorbent layer to nitrogen and N ₂ O by a reducing substance. is there. Each layer is porous, and the upper NOx selective reduction catalyst layer is preferably macro-mesoporous in order to permeate most of the components in the exhaust gas. The middle NOx adsorbent layer is preferably mesoporous in order to selectively permeate NOx and oxygen and suppress permeation of the reducing substance. The lower NOx oxidation catalyst layer is preferably micro-mesoporous in order to selectively permeate NOx and oxygen and suppress permeation of the reducing substance. In the lower NOx oxidation catalyst layer, oxidation of NOx with oxygen is selectively performed, and wasteful consumption of reducing substances is suppressed. In the upper NOx selective reduction catalyst layer, NOx having higher oxidizing power (NO ₂ has higher oxidizing power than NO) is selectively reduced by the reducing substance. The intermediate NOx adsorbent layer adsorbs NOx until saturation adsorption, and releases NOx when the saturation adsorption or more is reached. Therefore, it has a function of adjusting the NOx concentration in the upper catalyst layer. That is, the catalyst of the present invention is a completely new functional catalyst having both a catalyst function for removing NOx and a function of selectively permeating exhaust gas components.

  Each of the above catalyst layers and adsorbent layers plays a role of selective permeability of exhaust gas components, so that the thickness of each layer is set in an appropriate range. In general, the thickness of the porous material when the gas passes through the porous material can be estimated by the pore diameter of the porous material, the gas diffusion coefficient, and the gas concentration gradient related to the gas diffusion rate. Is preferably in the range of 10 nm to 200 nm for the micro mesoporous NOx oxidation catalyst layer, preferably in the range of 10 nm to 500 nm for the mesoporous adsorbent layer, and the macro mesoporous NOx selective reduction catalyst layer. Is preferably in the range of 100 nm to 10 μm.

In the present invention, the selection of the catalyst in each catalyst layer is very important. The catalyst for oxidizing NO in the lower NOx oxidation catalyst layer to NO ₂ is preferably a platinum group catalyst, and the catalyst layer has a structure capable of selectively permeating oxygen and NO. The platinum group catalyst is a catalyst of a platinum group element which is a generic name of six elements of ruthenium, rhodium, palladium, osmium, iridium, and platinum. Among these, rhodium, palladium, iridium, and platinum are preferable, and platinum among them is preferable. Is most preferred. Normally, in the present invention, a platinum-based catalyst is used. The reason is that platinum, which is the main component of the catalyst, has a high ability to oxidize NO, which is the main component of exhaust NOx, into NO ₂ , even in a high-concentration oxygen atmosphere. This is because it is chemically stable and platinum is relatively active at a relatively low temperature among the platinum group elements.

As a catalyst for reducing NO ₂ in the upper NOx selective reduction catalyst layer with a reducing substance, various heteropolyphosphates, peroxides, hydrides, carbides, nitrides, borides, metallic transition elements Among these, transition metal oxides are preferable, and selective reduction of NO ₂ can be performed rather than oxidation of NO. The provision of the transition metal oxide catalyst in the upper layer, because it was found that the main component of the NOx to exhibit catalytic performance over a high-temperature region of remarkably 400 ° C. reduction reaction at around 180 ° C. If NO ₂ . Examples of the transition element of the transition metal oxide catalyst include elements of groups 3 to 11 in the periodic table. Among these, vanadium, chromium, manganese, iron, cobalt, nickel, copper, niobium, molybdenum, silver , Tungsten, and rhenium are preferable, and iron, cobalt, and tungsten are most preferable. These oxides may be used as a single component, but may be a double oxide composed of a plurality of elements or a double oxide having a perovskite structure containing a lanthanoid element. By adding a co-catalytic component having a different function to the transition metal oxide catalyst, the catalyst performance can be improved by the synergy effect. As such components, for example, elements of Group 1 to Group 3 can be cited, and among these, metals, oxides, hydroxides, carbonates, and heavy metals of alkali elements or alkaline earth elements are included. Carbonate and the like are preferable when alcohol is used as the reducing agent because it has an effect of promoting the alcohol reforming reaction. The added mass of these promoter components is usually about 0.01 to 100 times that of platinum, but may be 100 times or more as required.

Moreover, the provision of the NOx adsorbent layer in the middle of the NOx oxidation catalyst layer and a NOx selective reduction catalyst layer, realizing enlargement things NOx treatment temperature range for performing the density adjustment of NO ₂ to be supplied to the NOx selective reduction catalyst layer It is because it discovered that it was preferable in order to do. The adsorption and desorption properties of NOx adsorbent, like the properties of adsorbents such as activated carbon, depend on the concentration of the adsorbent, but not on the temperature. Therefore, the NOx adsorbent layer has a lean burn atmosphere from around room temperature. NOx can be easily desorbed in an atmosphere where NOx adsorption starts and the NOx concentration is reduced by the treatment with the NOx reduction catalyst. The desorbed NOx is reduced by the NOx reduction catalyst. Therefore, the exhaust gas temperature at which the purification catalyst of the present invention is effective largely depends on the activation temperature of the NOx oxidation and selective reduction catalyst, and its lower limit temperature is about 160 ° C. Further, the NOx adsorbent adsorbs a small amount of SOx contained in the exhaust gas, but easily desorbs, so there is almost no catalyst poisoning by SOx. NOx adsorbents include carbon nitride compounds, boron carbonitride compounds, boron nitride, silicon carbonitride, boron carbide, nitrides, carbides, borides, activated carbons, metals and alloys, and zeolites. As the carbon nitride compound, a carbon nitride having a planar structure in which the 1,3,5 positions of the triazine ring are bonded with NH groups, and a carbon nitride having a planar structure in which the 1,5,9 positions of the tristriazine ring are bonded with NH groups are used. Ride can be mentioned. Examples of the boron carbonitride compound include carbon boron nitride having a structure similar to graphite. As nitrides, silicon nitride, silicon nitride oxide (= silicon oxynitride), aluminum nitride, transition metal nitride (titanium nitride, titanium nitride oxide, vanadium nitride, zirconium nitride, chromium nitride, tungsten nitride, molybdenum nitride, nitride) Examples thereof include nitrides of rare earth elements such as iron, manganese nitride, cobalt nitride, nickel nitride), scandium nitride, hafnium nitride, niobium nitride, tantalum nitride, and cerium nitride. Examples of the metal and alloy include metals and alloys of elements of Groups 1 to 14 in the periodic table. Examples of zeolite include natural zeolite and synthetic zeolite (generic name for zeolite A, zeolite X, zeolite Y, zeolite L, mordenite, ZSM-5, and other compounds having zeolite-like properties and structures). it can. Among these, carbon nitride compounds, boron carbonitride compounds, transition metal nitrides, group 13 element metals and their alloys, and zeolite are preferred adsorbents. Among the group 13 element metals and alloys thereof, metal indium and indium alloys are most preferred. Since the nitrogen atom which comprises a carbon nitride compound and a boron carbonitride compound has a lone pair, it is thought that this is a NOx adsorption site. The transition metal nitride is considered to easily adsorb NOx because the bond electron density is biased toward nitrogen atoms. Since indium is an element with an outermost shell electron deficiency, it is considered that NOx is easily adsorbed. Further, since zeolite has a cation exchange ability to adsorb ammonia and the like, it is considered that NO ₂ is adsorbed to the cation. In addition, those in which a cation is introduced into these compounds by a coordination bond or an ionic bond are preferable because they are excellent in NOx adsorption. That the NOx adsorbent has the property of adsorbing NOx is demonstrated by measuring the mass increase of the material in the presence of NOx. Also, it is NOx adsorption and not NOx absorption because the infrared absorption spectrum of the adsorbed material shows NOx characteristic group frequency but not nitrate ion or nitrite ion characteristic group frequency. Proven.

  The catalyst of the present invention is porous, and the micro-mesoporous NOx oxidation catalyst is a catalyst material in which most of the pores are supported on a support material having a pore distribution ranging from a micro region of 2 nm or less to a meso region of 50 nm. A catalyst. Since the pore distribution is usually expressed as an average pore diameter (diameter display), the preferred range of the average pore diameter of the carrier material is in the range of 0.4 to 10 nm, more preferably in the range of 1 to 5 nm. is there. The average pore diameter is preferably 0.4 nm or more from the viewpoint of supporting the catalyst, and preferably 10 nm or less from the viewpoint of selective permeation of NOx and oxygen. The mesoporous NOx adsorbent refers to an adsorbent supported on a carrier material having a pore distribution in which most of the pores span a meso region of 2 nm to 50 nm. The preferable range of the average pore diameter of the support material is in the range of 2 to 10 nm, more preferably in the range of 2 to 5 nm. The average pore diameter is preferably 2 nm or more from the viewpoint of supporting the adsorbent, and preferably 50 nm or less from the viewpoint of selective permeation of NOx and oxygen. The macro mesoporous NOx selective reduction catalyst refers to a catalyst in which the NOx selective reduction catalyst is supported on a support material in which most of the pores have a pore distribution of a 2 nm to 50 nm meso region and a macro region exceeding 50 nm. The preferable range of the average pore diameter of the support material is in the range of 2 nm to 10 μm, more preferably in the range of 10 nm to 1 μm. The average pore diameter is preferably 2 nm or more from the viewpoint of permeation of hydrocarbons, and preferably 10 μm or less from the viewpoint of uniform loading of the catalyst. The majority of the pores referred to here means that the pore volume occupied by the desired pores is 60% or more of the total pore volume.

Furthermore, the carrier material preferably has a high specific surface area. Excellent effects such as drastically increasing the specific surface area of the catalyst supported on the catalyst, and suppressing the reaggregation of the catalyst particles by supporting the catalyst in the pores to achieve uniform and high dispersion of the catalyst. Because there is. The specific surface area of the carrier material is preferably as high as possible unless there are special circumstances. The specific surface area of the support material which can be used in the present invention is 100~4000m ² / g, preferably 200~3000m ² / g, and more preferably from 400~2000m ² / g. The specific surface area is preferably 100 m ² / g or more in order to bring out the catalyst performance of the supported catalyst. Since the particle size of the catalyst supported in the pores of the support material is approximately the same as or smaller than the pore size, the above pore diameter range coincides with the particle size range of the catalyst exhibiting high activity. Yes. In general, catalyst particles atomized to nano size have a large number of high-order crystal planes such as edges, corners, and steps showing activity, so that not only catalytic activity is significantly improved but also catalytic activity in the bulk. It is known that even an inert metal such as this may exhibit unexpected catalytic activity. Therefore, the smaller the catalyst, the better from the viewpoint of catalytic ability. However, on the other hand, undesirable properties such as surface oxidation and side reaction due to atomization also appear, so there is an optimum range for the particle size of the fine particles. The average particle diameter of the catalyst exhibiting effective activity for the target NOx purification treatment in the present invention is in the range of 0.4 to 50 nm, preferably in the range of 1 to 20 nm, particularly preferably in the range of 1 to 10 nm.

  Examples of the carrier material having both the specific surface area and the pore distribution as described above include silica, alumina, titania, zirconia, magnesia, calcia, ceria, niobia, activated carbon, porous graphite, and the like. Titania, zirconia, magnesia, calcia, ceria, activated carbon, and a composite material thereof are preferable, and silica, alumina, magnesia, activated carbon, and a composite material thereof are more preferable. The specific surface area of the carrier material is a value measured by a BET nitrogen adsorption method using nitrogen as an adsorption / desorption gas, and the pore diameter is measured by a nitrogen adsorption method using nitrogen as an adsorption / desorption gas. This is a pore distribution (differential distribution display) in the range of 1 to 200 nm determined by the BJH method.

  The amount of catalyst or adsorbent supported when the NOx oxidation catalyst, adsorbent, or NOx selective reduction catalyst is supported on the support material is 0.01 to 20% by mass, preferably 0.1 to 10% by mass. However, if there is no problem with quantity, it is usually used at a loading amount of 1 to several mass%. The supported amount can be 20% by mass or more, but when the supported amount is excessive, the catalyst or adsorbent in the deep pores that hardly contribute to the reaction is increased, so 20% by mass or less is preferable. If less than 0.01% by mass, the activity is not sufficient, so 0.01% by mass or more is preferable.

  The functional catalyst of the present invention can be usually produced by applying a sol-gel method using a surfactant micelle as a template, which is a conventional method. In this method, a micro mesoporous NOx oxidation catalyst layer which is an inner core is first prepared, and then a mesoporous adsorbent layer and a macro mesoporous NOx selective reduction catalyst layer are sequentially laminated by a sol-gel method. . The micro-mesoporous NOx oxidation catalyst, which is the inner core, is prepared by supporting a catalytically active component on a micro-mesoporous support material produced by a sol-gel method using a precursor of a micro-mesoporous support material and a surfactant as a template. To do. Next, a mesoporous adsorbent layer is laminated by the sol-gel method in the same manner using the catalyst as a nucleus, and a macro mesoporous selective reduction catalyst layer is laminated by the sol-gel method in the same manner using the catalyst as a nucleus. . In the case of mesoporous silica, the precursor of the carrier material in each layer is usually tetramethoxysilane, tetraethoxysilane, tetra-i-propoxysilane, tetra-n-propoxysilane, tetra-i-butoxysilane, tetra-n- Alkoxides such as butoxysilane, tetra-sec-butoxysilane, and tetra-t-butoxysilane are used. Mesoporous alumina, titania, zirconia, magnesia, calcia, ceria and niobia can also be usually produced using an alkoxide. Examples of micelle-forming surfactants include long-chain alkylamines, long-chain quaternary ammonium salts, long-chain alkylamine N-oxides, long-chain sulfonates, polyethylene glycol alkyl ethers, polyethylene glycol fatty acid esters, etc. Any of these may be used. As the solvent, one or more of water, alcohols, and diols are usually used, and an aqueous solvent is preferable. When a small amount of a compound having a coordination ability to metal is added to the reaction system, the stability of the reaction system can be remarkably enhanced. As such a stabilizer, compounds having metal coordination ability such as acetylacetone, tetramethylenediamine, ethylenediaminetetraacetic acid, pyridine, and picoline are preferable. The composition of the reaction system comprising the precursor, surfactant, solvent and stabilizer is such that the molar ratio of the precursor is 0.01 to 0.60, preferably 0.02 to 0.50, and the precursor / surfactant The molar ratio is 1-30, preferably 1-10, the solvent / surfactant molar ratio is 1-1000, preferably 5-500, the stabilizer / precursor molar ratio is 0.01-1.0, Preferably it is 0.2-0.6. The reaction temperature is in the range of 20 to 180 ° C, preferably 20 to 100 ° C. The reaction time is 5 to 100 hours, preferably 10 to 50 hours. The reaction product is usually separated by filtration, sufficiently washed with water, dried, and then thermally decomposed and removed from the template by high-temperature baking at 500 to 1000 ° C. to obtain a mesoporous material. If necessary, the surfactant can be extracted with alcohol or the like before firing.

  The catalyst component can be supported on the carrier material by, for example, an ion exchange method or an impregnation method. These two methods are different in that the catalyst is deposited on the support, although the ion exchange method uses the ion exchange capacity of the support surface and the impregnation method uses the capillary action of the support. The general process is almost the same. That is, it can be produced by immersing a strongly basic mesoporous material in an aqueous solution of the catalyst raw material, followed by filtration, drying, washing with water as necessary, and reduction treatment with a reducing agent.

Examples of the raw material of the platinum catalyst include H ₂ PtCl ₄ , (NH ₄ ) ₂ PtCl ₄ , H ₂ PtCl ₆ , (NH ₄ ) ₂ PtCl ₆ , Pt (NH ₃ ) ₄ (NO ₃ ) ₂ , Pt (NH ₃ ) ₄ (OH) ₂ , PtCl ₄ , platinum acetylacetonate, and the like can be used. A catalyst to which a co-catalytic component is added as necessary is manufactured in the same manner by mixing water-soluble salts such as nitrate, sulfate, carbonate, and acetate of the co-catalytic component with the platinum catalyst raw material. Can do. As the reducing agent for the catalytically active component, hydrogen, an aqueous hydrazine solution, formalin, or the like can be used. The reduction may be performed under normal conditions known for each reducing agent. For example, hydrogen reduction can be performed by placing a sample in a hydrogen gas stream diluted with an inert gas such as helium and treating the sample at 300 to 500 ° C. for several hours. After reduction, if necessary, heat treatment may be performed at 500 to 1000 ° C. for several hours under an inert gas stream.

  The functional catalyst of the present invention is usually used by adhering to a monolith molded article that is used as a support for automobile catalysts. A monolith molded body is a molded body in which the molded body has a mesh-like cross section and is provided with gas flow paths partitioned by thin walls parallel to each other in the axial direction. A catalyst is attached to the monolith molded body. Such a catalyst is hereinafter referred to as a monolith catalyst. Although the external shape of a molded object is not specifically limited, Usually, it is a cylindrical shape. The adhesion amount of the catalyst when the functional catalyst is adhered to the gas flow path inner wall of the monolith molded body is preferably 3 to 30% by mass. It is preferably 30% or less from the viewpoint of gas diffusion to the catalyst existing inside the carrier. Moreover, 3% or more is preferable in order to draw out sufficient catalyst performance. The adhesion amount corresponding to the coating amount of the catalyst on the monolith molded body is preferably 0.03 to 3% by mass of the molded body.

  Said monolith catalyst can be manufactured according to the manufacturing method of the monolith molded object which adhered the three-way catalyst for motor vehicles. For example, the functional catalyst of the present invention and colloidal silica as a binder are usually mixed in a mass ratio of 1: (0.01 to 0.2), and the mixture is dispersed in water to usually 10 to 50 mass. % Slurry, the monolith molded body is immersed in the slurry to adhere the slurry to the inner wall of the gas flow path of the monolith molded body, and after drying, under an inert atmosphere such as nitrogen, helium, argon, etc. It can be produced by heat treatment at a temperature of several hours. As a binder other than colloidal silica, methyl cellulose, acrylic resin, polyethylene glycol, or the like can be used as appropriate. Alternatively, it can also be produced by a method in which a porous carrier material is applied to a monolith molded body, and then a catalyst raw material is impregnated into the carrier material, followed by reduction treatment and heat treatment. The thickness of the functional catalyst layer attached to the monolith molded body is usually preferably 1 μm to 100 μm and particularly preferably in the range of 10 μm to 50 μm in the method of attaching the slurry. 100 μm or less is preferable from the viewpoint of diffusion of the reaction gas. From the viewpoint of suppressing deterioration of the catalyst performance, 1 μm or more is preferable.

In the NOx purification treatment using the functional catalyst of the present invention, as a NOx reducing agent, carbon monoxide, hydrogen, hydrocarbon, steam, hydrocarbon-derived reformed gas, alcohol, alcohol-derived reformed gas, etc. Reducing substances can be used, and the method of supplying the reducing agent is to increase the hydrogen, carbon monoxide, hydrocarbons, and steam contained in the exhaust gas of the rich burn, rich burn after reforming the fuel through the reforming catalyst For example, a method of supplying alcohol to the exhaust gas, a method of supplying alcohol directly to the exhaust gas, a method of supplying alcohol to the exhaust gas after reforming through a reforming catalyst, and the like can be used. Alcohol has recently attracted attention as an alternative fuel for gasoline. However, since it is a substance that is also used as a raw material for reducing substances such as aldehyde, carbon monoxide, and hydrogen, it can also be used as a reducing agent for NOx. According to the study by the present inventors, when an appropriate reforming catalyst for alcohol is used, it exhibits high activity against exhaust NOx in an oxidizing atmosphere, unlike a lower olefin that functions as a reducing agent in the hydrocarbon SCR method. It has been found. It has also been found that the reduction proceeds to the nitrogen stage even in the low to medium temperature range from about 200 ° C. or lower to about 300 ° C. As the alcohol, lower alcohols such as methanol, ethanol, propanol, and butanol and alcohols having 5 or more carbon atoms are effective. Among these, methanol, ethanol, propanol, and butanol are preferable because of their excellent reducibility, Of these, ethanol is more preferred because of its low flammability and low oral toxicity. The above alcohol is also effective as a hydrous alcohol. Hydrous alcohol can cause a steam reforming reaction of alcohol. The water content is preferably such that it does not affect the reforming temperature of the alcohol, and is usually within about 10%. When alcohol-derived reformed gas is used as the reducing agent, the amount of alcohol supplied through the reforming catalyst is somewhat affected by the oxygen concentration in the exhaust gas, but the oxygen concentration is within a range of 20% or less. The number of moles is not particularly limited as long as it is from the same number of moles as NOx to several tens of moles. Moreover, you may mix a small amount of air with alcohol as an oxidizing agent. The amount of air is not particularly limited as long as it is an amount that causes partial oxidation of alcohol, but is usually about several moles or less. The reforming temperature of the alcohol is set to an appropriate temperature depending on the type of alcohol and the type of reforming catalyst. In the present invention, it is usually carried out in the range of 100 ° C to 500 ° C. The reforming catalyst can be heated by a method such as exhaust heat recovery or direct heating.

  When the functional catalyst of the present invention is mounted on automobiles, particularly diesel automobiles and lean burn gasoline automobiles, the lean burn exhaust NOx discharged by automobiles can be removed extremely effectively in a low temperature range of 160 ° C to 400 ° C. In addition, since it is effective even when the oxygen concentration is low, when it is used for exhaust gas purification treatment of a small diesel engine that can alternately perform rich burn with high oxygen concentration and lean burn with low oxygen concentration in exhaust gas, Exhaust NOx can be purified efficiently over a wide temperature range of ˜600 ° C. It can also be used as a method for purifying exhaust NOx for large vehicles such as trucks.

The present invention will be specifically described below with reference to examples.
The specific surface area and pore distribution were measured with a Sorpmatic 1800 type apparatus manufactured by Carlo Elba using nitrogen as a desorption gas. The specific surface area was determined by the BET method. The pore distribution was measured in the range of 1 to 200 nm and indicated by a differential distribution obtained by the BJH method. Many of the produced porous support materials showed a peak at a specific pore diameter position in an exponentially increasing distribution. The pore diameter that gives this peak is the pore diameter.

As a model gas for automobile exhaust NOx, a mixed gas of helium diluted nitric oxide and oxygen was used. The reduced pressure chemiluminescence NOx analyzer (Nippon Thermo Co. preparation: Model 42i-HL and 46C-H) (total of NO and NO ₂₎ NOx contained in the gas before and after treatment by the _N 2 O (NOx The NOx purification rate and the nitrogen selectivity were calculated by Equation (1) and Equation (2), respectively.

“Production Example 1” Synthesis of a catalyst similar to a three-way catalyst as an exhaust gas purification catalyst 0.215 g of PtCl ₄ .5H ₂ O, 0.106 g of PdCl ₂ .2H ₂ O, and 0.162 g of Rh (NO ₃ ) ₃ · 2H ₂ O were placed an aqueous solution prepared by dissolving in distilled water of 20ml evaporation dish, to which 10g of active alumina (Nikki chemical Co., Ltd. producing specific surface area: 250 meters ² / g, average pore diameter 6.2 nm, particle size 2 to 3 μm fine particles) were added and evaporated to dryness in a steam bath, and then placed in a vacuum dryer and vacuum dried at 100 ° C. for 3 hours. This sample was put in a quartz tube and reduced at 500 ° C. for 3 hours under a helium-diluted hydrogen gas (10% v / v) stream to synthesize a catalyst having a noble metal content of about 2% by weight. This was used in a comparative experiment as a noble metal catalyst simulating a three-way catalyst.

[Production Example 2] Production of a functional catalyst having a two-layer structure of the present invention In a 1 liter beaker, 300 g of distilled water, 240 g of ethanol and 30 g of dodecylamine N-oxide were added and dissolved. While stirring, 125 g of tetraethoxysilane was added and stirred at room temperature for 22 hours. The precipitate was filtered, washed with water, dried in warm air at 110 ° C. for 5 hours, and then baked in air at 750 ° C. for 5 hours to decompose and remove the contained surfactant to obtain a micro mesoporous silica material. As a result of small-angle X-ray diffraction measurement of the micro mesoporous silica material, one broad diffraction peak was shown. Further, as a result of observation with a transmission electric microscope, a regular arrangement was not observed in the arrangement of the pores, and a disordered state was observed. From these results, it was confirmed that the produced micro mesoporous silica material was amorphous. Moreover, as a result of pore distribution and specific surface area measurement, there is a pore peak at a position of about 1 nm, the specific surface area is 820 m ² / g, the pore volume is 1.30 cm ³ / g, and the pores are 1 to 50 nm. The volume was 1.29 cm ³ / g.
An aqueous solution of chloroplatinic acid H ₂ PtCl ₆ · 6H ₂ O and 0.267g and rhodium nitrate _{Rh (NO 3) 3 · 2H} 2 O were dissolved 0.0016g in distilled water 20g were placed in an evaporating dish, to which said micro -After adding 5 g of mesoporous silica material and evaporating to dryness with a steam bath, it was put in a vacuum dryer and vacuum-dried at 100 ° C for 3 hours. This sample was put into a quartz tube and reduced at 500 ° C. for 3 hours under a helium-diluted hydrogen gas (10 v / v%) stream, and the supported amount of platinum-rhodium was about 2 mass% [Pt-Rh / micro mesoporous silica] A catalyst was synthesized. The average particle size of the platinum-rhodium particles supported on the mesoporous catalyst was about 1.0 nm.

Next, 5 g of the [Pt-Rh / micro / mesoporous silica] catalyst and 100 g of distilled water were placed in a 1 liter beaker and dispersed uniformly, and then 80 g of ethanol, 5 g of dodecylamine, and 5 g of polyethylene glycol having a molecular weight of 100,000. added. Under stirring, 10 g of tetraethoxysilane was added and stirred at room temperature for 22 hours. The precipitate was filtered, washed with water, dried in warm air at 110 ° C. for 5 hours, and then calcined in air at 550 ° C. for 5 hours to decompose and remove the contained surfactant, and the catalyst coated with the macro mesoporous silica material Got. The macro mesoporous silica material had 3 nm pores and 100 nm pores, a specific surface area of 600 m ² / g, and a pore volume of 1.20 cm ³ / g. 5 g of the catalyst coated with the macro mesoporous silica material is placed in an evaporating dish, 10 g of an aqueous solution in which 0.5 g of cobalt nitrate is dissolved is added thereto, evaporated to dryness in a steam bath, and then placed in a vacuum dryer at 100 ° C. − Vacuum drying was performed for 3 hours. This sample was put in a quartz tube and calcined in air at 300 ° C. for 2 hours to obtain a catalyst in which a [Co ₃ O ₄ / macro-mesoporous silica] catalyst was laminated on a [Pt—Rh / mesoporous silica] catalyst. The supported rate of Co ₃ O ₄ in this [Co ₃ O ₄ / macro mesoporous silica] catalyst was about 4% by mass.

[Production Example 3] Production of functional catalyst having a three-layer structure of the present invention [Pt-Rh / micro mesoporous silica] catalyst produced in Production Example 2 and 100 g of distilled water were uniformly dispersed in a 1 liter beaker. Thereafter, 80 g of ethanol and 10 g of dodecylamine were added thereto. Under stirring, 10 g of tetraethoxysilane was added and stirred at room temperature for 22 hours. The precipitate was filtered, washed with water, dried in warm air at 110 ° C. for 5 hours, and then baked in air at 550 ° C. for 5 hours to decompose and remove the contained surfactant, and coated with a mesoporous silica material [Pt— Rh / micro mesoporous silica] catalyst was obtained. The mesoporous silica material had a specific surface area of 930 m ² / g, a pore volume of 1.35 cm ³ / g, and a volume occupied by 1-50 nm pores of 1.34 cm ³ / g. 5 g of the [Pt-Rh / micro mesoporous silica] catalyst coated with the mesoporous silica material was placed in an evaporating dish, and 10 g of an aqueous solution in which 0.3 g of indium nitrate and 0.2 g of stannic chloride were dissolved was added thereto. After evaporating to dryness in a bath, it was put in a vacuum dryer and vacuum dried at 100 ° C. for 3 hours. This sample was put in a quartz tube and reduced at 500 ° C. for 3 hours under a helium-diluted hydrogen gas (10 v / v%) stream, and an [In-Sn alloy / mesoporous silica] adsorbent on a [Pt—Rh / mesoporous silica] catalyst. The catalyst which laminated | stacked was obtained. The loading ratio of the In—Sn alloy in this [In—Sn alloy / mesoporous silica] adsorbent was about 4 mass%.

Next, 5 g of a catalyst in which an adsorbent of [In-Sn alloy / mesoporous silica] was adsorbed on the [Pt—Rh / micro / mesoporous silica] catalyst and 100 g of distilled water were placed in a 1 liter beaker and uniformly dispersed. To this, 80 g of ethanol, 5 g of dodecylamine and 5 g of polyethylene glycol having a molecular weight of 100,000 were added. Under stirring, 10 g of tetraethoxysilane was added and stirred at room temperature for 22 hours. The precipitate was filtered, washed with water, dried in warm air at 110 ° C. for 5 hours, and then calcined in air at 550 ° C. for 5 hours to decompose and remove the contained surfactant, and the catalyst coated with the macro mesoporous silica material Got. The macro mesoporous silica material had 3 nm pores and 100 nm pores, a specific surface area of 600 m ² / g, and a pore volume of 1.20 cm ³ / g. 5 g of the catalyst coated with the macro mesoporous silica material is placed in an evaporating dish, 10 g of an aqueous solution in which 0.5 g of cobalt nitrate is dissolved is added thereto, evaporated to dryness in a steam bath, and then placed in a vacuum dryer at 100 ° C. − Vacuum drying was performed for 3 hours. This sample was put into a quartz tube, calcined in air at 300 ° C. for 2 hours, and sequentially adsorbed with an [In—Sn alloy / mesoporous silica] adsorbent and [Co ₃ O ₄ / macro • on a [Pt—Rh / mesoporous silica] catalyst. A catalyst in which a mesoporous silica] catalyst was laminated was obtained. The supported rate of Co ₃ O ₄ in this [Co ₃ O ₄ / macro mesoporous silica] catalyst was about 4% by mass.

“Production Example 4” Production of the Functional Catalyst of the Present Invention 1 g of the functional catalyst powder of the three-layer structure of Production Example 3 was placed in a vapor deposition device and about 0.01 g of metal cesium was deposited, and then stored in an ampoule in a nitrogen stream. .
"Production Example 5" Production of the functional catalyst of the present invention supported on a monolith 1 g of the functional catalyst of the present invention of Production Example 3 and 0.1 g of colloidal silica were added to 10 ml of distilled water and stirred to prepare a slurry. Five mini-molded bodies (21 cells, diameter 8 mm × length 9 mm, weight 0.15 g) cut from a commercially available cordierite monolith molded body (400 cells / in ² , diameter 118 mm × length 50 mm, weight 243 g) After immersion, the sample was taken out and air-dried, and then heat-treated at 500 ° C. for 3 hours under a nitrogen stream. The adhesion amount of the functional catalyst of the present invention was about 10% by mass of the mini-molded product.

"Examples 1-3", "Comparative Example 1"
Lean burn NOx treatment using propylene as a reducing agent Each of the catalysts of Production Examples 1 to 3 was charged into a quartz flow reaction tube. Also, five catalyst-supported mini-molded bodies, which are monolith catalysts of Production Example 5, were filled in a quartz continuous flow reaction tube. Each reaction tube was adjusted to an arbitrary temperature of 150 ° C. to 500 ° C. by an external heater. Next, a simulated lean burn gas was circulated through each reaction tube to perform NOx treatment. The lean burn simulated gas was a gas containing 250 ppm of nitrogen monoxide, 10% of oxygen, 10% of water vapor, and 400 ppm of propylene adjusted in concentration with helium, and the flow rate was 100 mL per minute. The exhaust gas was sampled, and the NOx purification rate and nitrogen selectivity were measured. The results are shown in Table 1.

“Examples 4 to 6”, “Comparative Example 2”
Lean Burn / Rich Burn NOx Treatment Using Propylene as Reducing Agent Example 1 except that lean burn and rich burn simulated gas were alternately circulated at intervals of 1 minute instead of the lean burn simulated gas of Example 1. Similarly, NOx treatment was performed. The lean burn simulated gas was a gas containing 250 ppm of nitrogen monoxide, 10% of oxygen, 10% of water vapor, and 400 ppm of propylene adjusted in concentration with helium, and the flow rate was 100 mL per minute. The simulated gas of rich burn was a gas containing 250 ppm of nitrogen monoxide, 1% of oxygen, 10% of water vapor, and 4000 ppm of propylene adjusted in concentration with helium, and the flow rate was 100 mL per minute. The exhaust gas was sampled, and the NOx purification rate and nitrogen selectivity were measured. The results are shown in Table 2.

“Example 7”, “Comparative Example 3”
NOx treatment using ethanol as a reducing agent Each of the catalysts of Production Examples 1 and 4 was charged in a continuous flow reaction tube made of quartz in an amount of 0.3 g. Each reaction tube was adjusted to an arbitrary temperature of 150 ° C. to 500 ° C. by an external heater. Next, a simulated lean burn gas was circulated through each reaction tube to perform NOx treatment. The lean burn simulated gas was a gas containing 250 ppm of nitric oxide, 10% oxygen, 10% water vapor, and 4000 ppm ethanol adjusted in concentration with helium, and the flow rate was 100 mL per minute. The exhaust gas was sampled, and the NOx purification rate and nitrogen selectivity were measured. The results are shown in Table 3.

表１に示すように１８０℃から５００℃に渡って、本発明機能性触媒はリーンバーンＮＯｘに対して６０％以上の高いＮＯｘ浄化率が得られた。また、窒素の選択率は、１８０〜２００℃で約１０％、２５０〜５００℃で約９０％であった。

As shown in Table 1, over the range of 180 ° C. to 500 ° C., the functional catalyst of the present invention obtained a high NOx purification rate of 60% or more with respect to lean burn NOx. Moreover, the selectivity of nitrogen was about 10% at 180 to 200 ° C and about 90% at 250 to 500 ° C.

表２に示すように１８０℃から５００℃に渡って、本発明機能性触媒はリーン・リッチバーンＮＯｘに対して６０％以上の高いＮＯｘ浄化率が得られた。また、窒素の選択率は、１８０〜２００℃で約１０％、２５０〜５００℃で約９０％であった。

As shown in Table 2, over the range of 180 ° C. to 500 ° C., the functional catalyst of the present invention obtained a high NOx purification rate of 60% or more with respect to lean rich burn NOx. Moreover, the selectivity of nitrogen was about 10% at 180 to 200 ° C and about 90% at 250 to 500 ° C.

表３に示すように１８０℃から５００℃に渡って、本発明機能性触媒はリーンバーンＮＯｘに対して６０％以上の高いＮＯｘ浄化率が得られた。また、窒素の選択率は、１８０℃〜５００℃で約９０％であった。
以上のことから、本発明機能性触媒は、リーンバーン内燃機関の排ガス及びリーンバーンとリッチバーンを交互に繰り返す内燃機関の排ガスを低温領域から高温領域に渡って効率よく浄化できることがわかる。

As shown in Table 3, over a temperature range of 180 ° C. to 500 ° C., the functional catalyst of the present invention obtained a high NOx purification rate of 60% or more with respect to lean burn NOx. Moreover, the selectivity of nitrogen was about 90% at 180 ° C. to 500 ° C.
From the above, it can be seen that the functional catalyst of the present invention can efficiently purify the exhaust gas of a lean burn internal combustion engine and the exhaust gas of an internal combustion engine that alternately repeats lean burn and rich burn from a low temperature region to a high temperature region.

  The exhaust gas purifying catalyst of the present invention is useful as a diesel exhaust NOx purifying catalyst.

Claims (4)

  A catalyst for exhaust gas purification, which is a functional catalyst having a multilayer structure comprising a porous NOx oxidation catalyst layer provided in an inner core and a porous NOx selective reduction catalyst layer provided thereon. In the exhaust gas purifying catalyst according to claim 1, further exhaust gas purifying characterized the intermediate setting only kite porous NOx adsorbent layer the porous NOx oxidation catalyst layer and the porous NOx selective reduction catalyst layer catalyst.   The catalyst in the porous NOx oxidation catalyst layer is a micro mesoporous NOx oxidation catalyst in which a platinum group element is supported on a micro mesoporous material, and the catalyst in the porous NOx selective reduction catalyst layer carries a transition metal oxide on a macro mesoporous material. The exhaust gas purification catalyst according to claim 1, wherein the catalyst is a macro-mesoporous NOx selective reduction catalyst.   The exhaust gas purifying catalyst according to claim 2 or 3, wherein the adsorbent in the porous NOx adsorbent layer is a mesoporous adsorbent in which an indium alloy is supported on a mesoporous material.