JP5264316B2 - Lean burn automobile exhaust gas purification catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst effective for purifying a diesel automobile exhaust gas which performs the purification treatment of the diesel automobile exhaust gas efficiently by overcoming the conventional difficulties in the purification. <P>SOLUTION: The catalyst for purifying the lean burn automobile exhaust gas purifies the lean burn exhaust gas using the composite catalyst comprised of a catalyst keeping a platinum group element carried by a mesoporous material and a cation exchange clinoptilolite and/or a cation exchange erionite. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a purification catalyst for purifying exhaust gas discharged from a lean burn automobile.

Conventionally, NOx, carbon monoxide, and hydrocarbons contained in exhaust gas from gasoline vehicles have been purified by a three-way catalyst made of a platinum group element. 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 nm to several hundreds of nm are supported. The purification method using a three-way catalyst controls the air-fuel ratio, which is the air: fuel weight mixing ratio, to be close to the theoretical air-fuel ratio (14.7: 1) (this combustion is called rich burn). Since the oxygen concentration contained in the exhaust gas 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, if the oxygen concentration in the exhaust gas exceeds several percent, There is a problem that significant oxidative degradation occurs.

In addition, a so-called urea SCR method using a transition metal compound and 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. This method has the advantage that NOx can be efficiently purified over a relatively high temperature region near 600 ° C. from a relatively low temperature region near 200 ° C., but it is necessary to mount 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, causing water quality environmental pollution.
As a method using a reducing agent other than urea water, a hydrocarbon SCR method using hydrocarbons in exhaust gas (lower olefins such as ethylene and propylene contained in a small amount have a reducing property) as a reducing agent has been studied for a long time. Yes. Although this method can obtain a high purification rate for rich burn exhaust NOx, it has not been put to practical use because it has a problem that it can be processed only in a very narrow temperature range around 200 ° C. for lean burn exhaust NOx. . Recently, hydrogen is generated from a carbon monoxide contained in exhaust gas by a reforming catalyst by a water gas shift reaction, and is reacted with NO ₂ on a platinum-based catalyst to generate ammonia. A method of purifying NOx by adsorbed and generated ammonia has been proposed (see Patent Document 1).

On the other hand, a three-way catalyst cannot be used for exhaust NOx treatment of small diesel vehicles such as diesel passenger cars. This is because the air-fuel ratio is more than several times that of gasoline (diesel fuel combustion is lean burn), and the oxygen concentration in diesel exhaust gas is usually 5% or more and contains almost no reducing substances. 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, The ability to fully oxidize reducing substances with oxygen is also high, so when a conventional platinum-based catalyst is brought into direct contact with lean burn exhaust gas with high oxygen concentration, hydrogen, carbon monoxide, This is because when a reducing substance such as hydrocarbon is supplied, the NOx treatment temperature range is limited to a very narrow region of approximately 200 ° C. to 250 ° C. On the other hand, transition metal oxides have not been put to practical use, but since their redox power is lower than that of platinum, it has been reported that the range of 250 ° C. to 400 ° C. is the NOx treatment temperature range. Conventionally, it has been studied to mix the platinum group catalyst and the transition metal oxide catalyst in order to expand the processing temperature range, but the object could not be achieved by simply mixing them. The reason is considered to be that most of the reducing substances contained in the exhaust gas components are consumed by the platinum catalyst because the oxidizing power of the platinum catalyst in the mixed catalyst is too strong.
In addition, a so-called NOx occlusion reduction catalyst in which a NOx occlusion agent is added to a platinum group catalyst as a catalyst is currently being studied for the exhaust NOx treatment of lean burn gasoline vehicles and small diesel vehicles (for example, Patent Document 2). In this method, since it is difficult to purify exhaust NOx only by lean burn, a combustion method in which lean burn and rich burn are alternately repeated is performed. In this method, NOx purification is performed by absorbing lean burn exhaust NOx with a NOx storage agent, releasing the absorbed NOx in a rich burn atmosphere, and supplying the released NOx into the rich burn exhaust gas or the rich burn exhaust gas. It is based on the idea that reduction treatment is carried out on a platinum group catalyst using a large amount of reducing substances such as carbon monoxide, hydrogen, and hydrocarbons. 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. However, there is an advantage that NOx can be purified over about 600 ° C., but NOx purification at about 200 ° C. or lower is very difficult. Further, the NOx storage agent is significantly deteriorated by moisture and SOx in the exhaust gas. Therefore, it is necessary to regenerate the catalyst by regular high-temperature treatment usually at 750 ° C. or higher, and there is a problem that the catalyst is significantly deteriorated by the regeneration treatment. .
In addition, the combustion method in which the lean burn and the rich burn are alternately repeated requires the installation of an advanced and complicated control system for precisely controlling the combustion and oxygen concentration in the internal combustion engine. Since the above control system can be simplified if NOx purification is possible, a NOx purification catalyst for such a lean burn vehicle is desired.

Under such circumstances, recently, as a method of replacing the combustion method in which lean burn and rich burn are alternately repeated, a combustion method in which lean burn with a small amount of reducing agent and lean burn with a large amount of reducing agent are alternately started to be studied. This is a so-called post-injection method, which is a combustion method in which a small amount of fuel is injected in a very short time from the normal lean burn to the next lean burn. This system is a system in which a small amount of fuel is injected into a combustion chamber still in a high temperature state immediately after lean burn to thermally decompose the injected fuel, thereby generating lower hydrocarbons. If this method is performed, lean burn exhaust gas rich in lower hydrocarbons can be generated. However, this lower hydrocarbon is mainly composed of C ₆ -C ₁₂ saturated and unsaturated hydrocarbons having low reducing performance unlike unsaturated lower hydrocarbons having high reducing performance such as carbon monoxide, ethylene and propylene. . With known exhaust gas purification catalysts such as conventional three-way catalysts and NOx occlusion reduction catalysts, it is difficult to treat NOx of exhaust gas exhausted by the post-injection method. Therefore, even such hydrocarbons can reduce exhaust NOx. There is a need for the development of an innovative and highly active NOx purification catalyst that is not possible in the past.

  Zeolite has long been known for its ability to adsorb small molecules such as carbon monoxide, hydrocarbons, ammonia, oxygen, and catalytic activity, and many proposals have been made to use these properties for exhaust gas treatment. Has been. For example, a method of adsorbing using a zeolite-based molecular sieve to trap hydrocarbons contained in exhaust gas discharged during a cold start of a gasoline vehicle (see Non-Patent Document 1), a cation exchange mordenite which is a kind of zeolite NO reduction treatment using propylene with NO (see Non-Patent Document 2), Lean NOx treatment with a honeycomb catalyst in which ZSM-5, which is a synthetic zeolite, is double washed on the platinum-supported catalyst layer (see Non-Patent Document 3) In order to simultaneously decompose and remove hydrocarbons and NOx, a catalyst in which a noble metal catalyst such as platinum is supported on a molecular sieve such as zeolite, and a method for producing the same (see Patent Document 3), Oxidative decomposition removal (refer to Patent Document 4), NOx discharged into the tunnel is converted into zeolite. A method of adsorbing and removing (see Patent Document 5), a method of trapping hydrocarbons in exhaust gas using a zeolite material (see Patent Document 6), a zeolite layer provided in the lower layer of the honeycomb-coated catalyst layer, and a platinum-based material in the upper layer A method in which a catalyst layer is provided to simultaneously decompose and remove hydrocarbons and NOx (see Patent Document 7), hydrocarbons in exhaust gas are temporarily trapped by zeolite provided upstream, and desorbed by a three-way catalyst provided downstream A method for purifying hydrogen (see, for example, Patent Documents 8 to 12) has been proposed. However, until now, there have been few reports that lean burn exhaust NOx in a low temperature region at 200 ° C. or less can be efficiently purified using a zeolite-based hydrocarbon adsorbent.

By the way, most of the diesel cars that run in Japan are large cars such as trucks and buses, and there are almost no passenger cars. Therefore, a diesel car is essentially a large car. The temperature of the exhaust gas is about 100 ° C. to 200 ° C. during transient running, and is about 200 ° C. to 400 ° C. during stable running, and about 80% of the NOx discharged is discharged during transient running.
From the above, the catalyst required for exhaust gas treatment of diesel automobiles is higher than the exhaust gas discharged by the lean burn only combustion method with respect to lean burn exhaust NOx in the low temperature range of 100 ° C. to 200 ° C. Although it is desired to be active and highly active against lean burn NOx in the middle temperature range of 200 ° C. to 400 ° C., it is currently effective against lean burn NOx below 250 ° C. No catalyst for NOx purification has been found.

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 broad 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 13 to 14, and are named crystalline mesoporous molecular sieves because the pore arrangement of pores is similar to the atomic arrangement of a crystalline substance.

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 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. It is guessed. 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 15-19. However, it has been difficult to effectively remove low-temperature exhaust NOx around 100 ° C. to 200 ° C. discharged by diesel vehicles and the like.

WO2005 / 103461 JP 2001-170487 A JP 2004-269276 A JP 05-031359 A Japanese Patent Laid-Open No. 01-155934 JP 2004-019439 A JP 2001-300319 A Japanese Patent Laid-Open No. 06-074019 Japanese Patent Application Laid-Open No. 07-144119 Japanese Patent Laid-Open No. 06-142457 JP 05-059942 A Japanese Patent Application Laid-Open No. 07-102957 Japanese Patent Laid-Open No. 05-254827 Japanese National Patent Publication No. 05-503499 US Pat. No. 5,143,707 JP 2001-009275 A JP 2002-210369 A JP 2002-320850 A JP 2003-135963 A SAE, 2000-01-0892 (2000). The 99th Catalysis Conference, Discussion Session A Proceedings, 1P47 (March 2007). Applied Catalysis B: Environmental 19 (1998) 127-135.

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

As a result of intensive studies to achieve the above object, the present inventors have found that a specific zeolite selected from a catalyst in which a platinum group element is supported on a mesoporous material (hereinafter referred to as a mesoporous catalyst) and a zeolite. In order to complete the present invention based on this finding, it was found that a catalyst comprising a zeolite that had been subjected to cation exchange with respect to the catalyst was extremely effective against lean burn exhaust gas, particularly in a low temperature region to a medium temperature region. It came.
That is, the present invention
1. A lean-burn automobile exhaust gas purification catalyst comprising a catalyst having a platinum group element supported on a mesoporous material and a cation exchange clinoptilolite.
2. A lean burn automobile exhaust gas purification catalyst comprising a catalyst having a platinum group element supported on a mesoporous material and a cation exchange erionite.
3. The above-mentioned 1., wherein the cation is at least one cation selected from transition metal ions, alkali metal ions, and alkaline earth metal ions. Or 2. The lean burn automobile exhaust gas purification catalyst as described in 1.

The automobile exhaust gas purification catalyst of the present invention can be effectively used even with extremely low concentrations of hydrocarbons contained in the exhaust gas, so that lean burn NOx treatment that could not be achieved in the past is extremely efficient from the low temperature region to the intermediate temperature region. Can be done well. For example, with a three-way catalyst, NOx can hardly be purified under an atmosphere with an oxygen concentration of 10% even if a large amount of hydrocarbon is present in the exhaust gas. If there is a hydrocarbon of several hundred ppm to 500 ppm in the exhaust gas under the condition of superficial velocity (SV: abbreviation of s space- v elocity), 75 to 85% of lean burn exhaust NOx with an oxygen concentration of 10% is 180%. It can be purified at a temperature of from 200 ° C to 200 ° C, and 30 to 40% can be purified from 200 ° C to 300 ° C. At the same time, most hydrocarbons can be purified.

Hereinafter, the present invention will be described in detail.
The conventional three-way catalyst can purify NOx almost 100% over 250 ° C. to 600 ° C. when the reducing substance is about 10 times the molar concentration of NOx in an atmosphere having an oxygen concentration lower than 1%. However, in an atmosphere having an oxygen concentration of several percent or more, the NOx purification rate decreases drastically as the temperature increases, unless the reducing substance is abundant and the amount is not excessively large enough to meet the amount of oxygen consumed by oxidation. This cannot be fully explained because the three-way catalyst is poisoned by oxygen. This is probably because the complete combustion reaction of the reducing substance on the catalyst is significantly faster than the NOx reduction reaction when the temperature exceeds 250 ° C. or more.
The exhaust gas purifying catalyst in the present invention is a catalyst that improves such an undesirable catalytic reaction, and thereby efficiently purifies lean burn exhaust NOx from a low temperature region to a medium temperature region.

That is, the present invention is an automobile exhaust gas purification catalyst for purifying lean burn exhaust gas, specifically comprising a mesoporous catalyst, a cation exchange clinoptilolite and / or a cation exchange erionite. This is a lean burn automobile exhaust gas purification catalyst. The lean burn in the present invention means that the oxygen ion concentration in the exhaust gas is 1% or more, and usually means that the oxygen concentration is 5% or more. The role of the NOx oxidation catalyst is to oxidize NO to NO ₂ in a lean burn atmosphere in a low temperature region. The role of the specific zeolite subjected to the cation exchange is mainly to store hydrocarbons in the exhaust gas by adsorbing hydrocarbons in the exhaust gas and supply hydrocarbons consumed by the NOx oxidation catalyst. However, as described below, the effect of the present invention cannot be sufficiently explained only by the adsorptivity of hydrocarbons. In the present invention, it is composed of a specific zeolite subjected to cation exchange with a mesoporous catalyst. Normally, both materials are used as a mixture in which the materials are micro-dispersed. However, the respective materials can be used by being laminated in a thin-film multilayer structure. When used as a mixture, the blending ratio is usually in the range of the same amount to 10 times that of the NOx oxidation catalyst, but the amount is smaller than that of the NOx oxidation catalyst depending on the hydrocarbon content. You can also Further, the cation in the specific zeolite subjected to the cation exchange of the present invention is at least one cation selected from transition metal ions, alkali metal ions, and alkaline earth metal ions. As the transition metal ions, group 4 to group 11 metal ions are preferred, and iron ions and copper ions are more preferred. As the alkali metal ions, all of the alkali metal ions of group 1 can be used, among which sodium, potassium and cesium ions are more preferable. As the alkaline earth metal ions, all metal ions of group 2 alkaline earth metal ions can be used, among which calcium, magnesium, barium, and strontium ions are more preferable.

  The present invention can find that clinoptilolite and erionite are most effective for the purposes of the present invention, among more than 30 kinds of natural zeolites and more than 80 kinds of synthetic zeolites. It was. Zeolite is a material that has hydrocarbon adsorption, ammonia adsorption, and various catalytic functions, so the hydrocarbon adsorption amount of clinoptilolite and erionite was compared with other zeolites. It wasn't expensive. For example, mordenite and ZSM-5 were far superior in terms of hydrocarbon adsorption, and zeolite X was superior in terms of ammonia adsorption. In addition, since zeolites having a pore size and crystal structure similar to clinoptilolite and erionite did not have a significant catalytic effect, it was a unique effect seen in clinoptilolite and erionite. Conceivable. Note that clinoptilolite and erionite are quite different in pore size and crystal structure, so there is not much structural similarity between the two materials. Since the cation exchange zeolite of the present invention is not a basic substance, there is no problem caused by SOx as seen in conventional NOx storage reduction catalysts.

The mesoporous catalyst in the present invention is a catalyst in which a platinum group element is supported on a mesoporous material. (However, the three-way catalyst is excluded). The platinum group element is a generic name of six elements of ruthenium, rhodium, palladium, osmium, iridium, and platinum, and among these, one or two selected from rhodium, palladium, iridium, and platinum. Species are preferred, with platinum being most preferred. Usually, 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 exhausted NOx, to NO ₂ , and N ₂ O depending on the reducing agent. This is because it has a high ability to reduce to N ₂ and is chemically stable even in a high-concentration oxygen atmosphere, and also because platinum is relatively active at a relatively low temperature among platinum group elements. By adding a co-catalytic component having a different function to the platinum-based catalyst, the catalyst performance can be improved by the synergy effect. As such a component, for example, a transition metal and its oxide can be used. Examples of the transition metal element include elements of Group 3 to Group 11 in the periodic table. Among these, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, molybdenum, tungsten, and rhenium are preferable. Of these, iron, cobalt, and tungsten are most preferable. 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.

The mesoporous material used as the catalyst support has a high specific surface area and a pore size of nano-size, so that the specific surface area of the catalyst supported on the mesoporous material can be dramatically increased and the catalyst is supported in the pores. Thus, there are excellent effects such as suppressing reagglomeration of the catalyst particles, suppressing reagglomeration of the catalyst particles, and achieving uniform and high dispersion of the catalyst. The higher the specific surface area of the mesoporous material, the better, unless there are special circumstances. The specific surface area of mesoporous materials that may be used in the present invention is 100~4000m ² / g, preferably 200~3000m ² / g, and more preferably from 400~2000m ² / g. When the specific surface area is less than 100 m ² / g, the supported amount of the catalyst is reduced, so that it is preferably 100 m ² / g or more in order to bring out the catalyst performance of the supported catalyst. On the other hand, in terms of material strength, the specific surface area is preferably 4000 m ² / g or less. Further, most of the pores of the mesoporous material in the present invention have a pore diameter (diameter display) in the range of 1 to 50 nm, preferably in the range of 2 to 20 nm, more preferably in the range of 2 to 10 nm. . The majority of the pores referred to here means that the pore volume occupied by pores of 1 to 50 nm is 60% or more of the total pore volume. The catalyst can be supported even if the pore diameter is less than 1 nm, but it is preferably 1 nm or more in consideration of the flow of the reducing agent and contamination due to impurities. If it exceeds 50 nm, the dispersed and supported catalyst tends to grow into giant particles by sintering (= sintering) under hydrothermal high temperature conditions and the like. Since the particle size of the catalyst supported in the pores of the mesoporous material is approximately the same as or smaller than the pore size, the above-mentioned pore size range coincides with the particle size range of the catalyst exhibiting high activity. Yes. The average particle size of the mesoporous material is 0.1 to 10 μm, preferably 0.1 to 1 μm, particularly preferably 0.1 to 0.5 μm. Generally, catalyst particles atomized to nanosize have a large number of high-order crystal planes such as edges, corners, and steps showing activity, so that not only catalytic activity is remarkably improved but also catalytic activity is shown in 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 reactions 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 1 to 50 nm, preferably in the range of 1 to 20 nm, and particularly preferably in the range of 1 to 10 nm.

Examples of the mesoporous material having both the specific surface area, the pore distribution, and the average particle size as described above include silica, alumina, titania, zirconia, magnesia, calcia, ceria, niobia, activated carbon, and porous graphite. Silica, alumina, titania, zirconia, magnesia, calcia, ceria, niobia, activated carbon, and a composite material thereof are preferable, and silica, alumina, magnesia, activated carbon, and a composite material thereof are more preferable. Mesoporous silica is roughly classified into crystalline mesoporous silica in which the pores are regularly arranged like a crystal lattice, and amorphous mesoporous silica in which the regularity such as crystals is hardly observed in the pore arrangement. For the purpose of the invention, amorphous mesoporous silica excellent in hydrothermal durability, heat resistance, gas diffusibility and the like is superior.
The specific surface area for adsorption / desorption is a value measured by a BET nitrogen adsorption method using nitrogen as an adsorption / desorption gas, and the pore size is measured by a nitrogen adsorption method using nitrogen as an adsorption / desorption gas. It is shown by a pore distribution (differential distribution display) in the range of 1 to 200 nm obtained by the BJH method.

When the platinum-based catalyst is supported on the mesoporous material, the supported amount of platinum is 0.01 to 20% by mass, preferably 0.1 to 10% by mass. It is used at a loading of 1 to several percent. The supported amount can be 20% by mass or more, but if the supported amount is excessive, the catalyst in the deep pores that hardly contributes to the reaction increases, so 20% by mass or less is preferable. Since activity is not enough if it is less than 0.01 mass%, 0.01 mass% or more is preferable.
The mesoporous material used in the present invention can be produced by applying a sol-gel method using a surfactant micelle as a template, which is a conventional method. As the precursor of the mesoporous material, in the case of mesoporous silica, usually, tetramethoxysilane, tetraethoxysilane, tetra-i-propoxysilane, tetra-n-propoxysilane, tetra-i-butoxysilane, tetra-n-butoxysilane An alkoxide such as tetra-sec-butoxysilane or tetra-t-butoxysilane is used. Mesoporous silica, alumina, titania, zirconia, magnesia, calcia, ceria and niobia can also be usually produced using alkoxides. 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, and polyethylene glycol fatty acid esters. Either may be sufficient. 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 improved. 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 of the mesoporous material, the surfactant, the solvent and the stabilizer is such that the molar ratio of the precursor of the mesoporous material is 0.01 to 0.6, preferably 0.02 to 0.5, The precursor / surfactant molar ratio is 1-30, preferably 1-10, and the solvent / surfactant molar ratio is 1-1000, preferably 5-500, stabilizer / main agent (reaction system excluding solvent). The molar ratio of the composition is 0.01 to 1, preferably 0.2 to 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 NOx oxidation catalyst whose main component is a catalyst having a platinum group element supported on a mesoporous material can be produced 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 the 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 platinum catalyst material 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. If necessary, water-soluble salts such as nitrates, sulfates, carbonates, and acetates of the promoter component can be mixed with the platinum catalyst raw material and produced in the same manner. 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 cation exchange zeolite can be produced, for example, by an ion exchange method. That is, it can be produced by immersing the zeolite in an aqueous solution containing a cation to be exchanged, followed by filtration, drying, and washing with water as necessary.
The catalyst of the present invention is usually used in a shape applied to the inner wall of the gas flow path of a ceramic or metal monolith molded body generally used as a carrier for automobile catalysts, but is not limited to these. Absent. The above-mentioned monolith molded body is a molded body having a mesh-shaped cross section and 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. Hereinafter, this catalyst is 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 amount of the catalyst deposited when the catalyst of the present invention is deposited on the gas flow path inner wall of the monolith molded body is preferably 3 to 30% by mass. The amount is preferably less than 30% by mass from the viewpoint of gas diffusion into the catalyst inside the support. Further, 3% by mass or more is preferable in order to bring out sufficient catalyst performance.

  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 mixture of the present invention catalyst and colloidal silica (also referred to as silica sol) as a binder is usually mixed at a mass ratio of 1: (0.01 to 0.2), and this is usually dispersed in water. After adjusting the slurry of 10 to 50% by mass, 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, an inert atmosphere such as nitrogen, helium or argon It can manufacture by processing at 500-1000 degreeC for several hours below. Alternatively, it is also possible to produce a monolith molded body by applying a mixture of a mesoporous material and a cation exchange zeolite in advance and then immersing the monolith molded body in an aqueous solution of a catalyst raw material, taking out, drying, reducing treatment, and heat treatment. .

As a binder other than colloidal silica, methyl cellulose, acrylic resin, polyethylene glycol, or the like can be used as appropriate. The thickness of the catalyst of the present invention to be attached to the monolith molded body is usually preferably 1 μm to 100 μm, particularly preferably in the range of 10 μm to 50 μm, in the method for attaching the slurry. If it exceeds 100 μm, the diffusion of the reaction gas becomes slow, so that it is preferably 100 μm or less. In order to suppress deterioration of catalyst performance, 1 μm or more is preferable.
The catalyst of the present invention is extremely effective in mounting NOx and hydrocarbon harmful substances in lean burn exhaust gas emitted by automobiles in the range of 160 ° C to 400 ° C when mounted on automobiles, particularly diesel automobiles and lean burn gasoline automobiles. Can be removed. In particular, it can be effectively used as an exhaust gas purification method 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 mesoporous materials peaked at specific pore diameter positions in an exponentially increasing distribution. The pore diameter system giving this peak is the pore diameter.
As a model gas for automobile exhaust NOx, a mixed gas of nitric oxide, oxygen and propylene diluted with helium was used. The reduced pressure chemiluminescent NOx analyzer: measuring the concentration of (Nippon Thermo Co. production model 42i-HL and 46C-H) by NOx contained in the gas before and after treatment (total of NO and NO ₂₎₎ The NOx purification rate was calculated by equation (1).

“Production Example 1” Synthesis of a catalyst similar to a three-way catalyst as an exhaust gas purification catalyst 0.538 g of PtCl ₄ .5H ₂ O, 0.2065 g of PdCl ₂ .2H ₂ O, and 0.405 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 activated alumina (JGC Corporation manufacturing trade name: N-613N ,: specific surface area of 250 meters ² / g, average pore diameter 6.2 nm, fine particles having a particle diameter of 2 to 3 μm) 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 into 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 precious metal content of about 5 mass%. This was used in a comparative experiment as a noble metal catalyst simulating a three-way catalyst.

[Production Example 2] Production of platinum catalyst supported on mesoporous silica as NOx oxidation catalyst In a 1 liter beaker, 300 g of distilled water, 240 g of ethanol, and 30 g of dodecylamine were added and dissolved. While stirring, 125 g of tetraethoxysilane was added and stirred at room temperature for 22 hours. The product is 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 contained dodecylamine, and mesoporous silica material having an average particle size of 0.5 μm Got. As a result of small angle X-ray diffraction measurement of the mesoporous silica material, one broad diffraction peak was shown. Further, as a result of observation with a transmission electron microscope, a regular arrangement was not observed in the pore arrangement, and a disordered state was observed. From these results, it was confirmed that the produced 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 3.2 nm, a specific surface area of 933 m ^{2 /} g, a pore volume of 1.35 cm ³ / g, and a pore of 1 to 50 nm. The volume occupied by was 1.34 cm ³ / g. An aqueous solution of chloroplatinic acid _{H 2} PtCl ₄ · 6H 2 O and 0.668g and rhodium nitrate _{Rh (NO 3) 3 · 2H} 2 O were dissolved 0.004g of distilled water 20g were placed in an evaporating dish, to which the above mesoporous After adding 5 g of a 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 flow of helium-diluted hydrogen gas (10 v / v%), and a [Pt-Rh / mesoporous silica] catalyst having a platinum-rhodium loading of about 5% by mass was obtained. Synthesized. The average particle diameter of platinum-rhodium particles supported on mesoporous silica was about 3.0 nm.

[Production Example 3] Production of Mg ion-exchanged mordenite 100 g of commercially available mordenite (trade name: HSZ-640, manufactured by Tosoh Corporation) was placed in 2 liters of distilled water, and 50 g of a 20 mass% magnesium nitrate aqueous solution was added thereto, and the room temperature was increased. And stirred for a whole day. Magnesium ion exchanged zeolite was produced by vacuum filtration, washing with 100 ml of distilled water and vacuum drying at 100 ° C. The magnesium ion content was about 5 g.

“Production Example 4” Production of Mg ion-exchanged clinoptilolite 100 g of commercially available natural clinoptilolite (natural clinoptilolite is a potassium ion exchanger) was placed in 2 liters of distilled water, and 20 50 g of a mass% magnesium nitrate aqueous solution was added, and the mixture was stirred overnight at room temperature. Magnesium ion exchanged zeolite was produced by vacuum filtration, washing with 100 ml of distilled water and vacuum drying at 100 ° C. The magnesium ion content was about 5 g.

"Production Example 5" Production of Mg ion-exchanged erionite Commercially available natural erionite (natural erionite is a potassium ion exchanger) (product name: TSZ-450 manufactured by Tosoh Corporation) 2 g of distilled water Into this, 50 g of a 20 mass% magnesium nitrate aqueous solution was added, and the mixture was stirred at room temperature for a whole day and night. Magnesium ion exchanged zeolite was produced by vacuum filtration, washing with 100 ml of distilled water and vacuum drying at 100 ° C. The magnesium ion content was about 6 g.

[Production Example 6] Production of Fe ion exchange clinoptilolite 100 g of commercially available natural clinoptilolite (trade name: Zeolite 3S, manufactured by Sun Zeolite Co., Ltd.) was placed in 2 liters of distilled water, and 20% by mass of nitric acid was added thereto. 50 g of ferric aqueous solution was added and stirred at room temperature for a whole day and night. Fe ion-exchanged zeolite was produced by vacuum filtration, washing with 100 ml of distilled water, and vacuum drying at 100 ° C. The iron ion content was about 6 g.

“Production Example 7” Production of a monolithic catalyst coated with a conventional three-way catalyst 160 g of noble metal catalyst imitating the three-way catalyst of Production Example 1 and 16 g of colloidal silica are added to 2 liters of distilled water and stirred to prepare a slurry. (Conditioning liquid 1). A commercially available cordierite monolith molded body (4.5 mil / 400 cpsi, diameter 143.8 mm × length 118 mm) (trade name: HANICERAM, manufactured by NGK KK) (hereinafter the same) is immersed in the adjustment liquid 1, and then air-dried. The monolith catalyst which apply | coated the noble metal catalyst which imitated the three way catalyst was manufactured by heat-processing for 500 degreeC-3 hours under nitrogen stream. The amount of the precious metal catalyst imitating the three-way catalyst in the monolith catalyst was about 80 g. A mini-molded body (diameter 20 mm × length 12.7 mm) was cut out from the monolith catalyst.

[Production Example 8] Production of monolith catalyst coated with [Pt-Rh / mesoporous silica] catalyst 160 g of [Pt-Rh / mesoporous silica] catalyst of Production Example 2 and 16 g of colloidal silica were added to 2 liters of distilled water and stirred. The slurry was adjusted (conditioning liquid 1). After immersing a commercially available cordierite monolith molded body (4.5 mil / 400 cpsi, diameter 143.8 mm × length 118 mm) in the adjustment liquid 1, the monolith was heat-treated at 500 ° C. for 3 hours under an air stream of nitrogen. A catalyst was prepared. The amount of [Pt-Rh / mesoporous silica] catalyst deposited on the monolith catalyst was about 80 g. A mini-molded body (diameter 20 mm × length 12.7 mm) was cut out from the monolith catalyst.

Production Example 9 Production of Monolith Catalyst Coated with Mixture of [Pt—Rh / Mesoporous Silica] Catalyst and Mg Ion Exchange Mordenite 160 g of Production Example 2 [Pt—Rh / Mesoporous Silica] Catalyst and Mg Ion Exchange of Production Example 3 160 g of mordenite and 32 g of colloidal silica were added to 4 liters of distilled water and stirred to prepare a slurry (conditioning liquid 1). After immersing a commercially available cordierite monolith molded body (4.5 mil / 400 cpsi, diameter 143.8 mm × length 118 mm) in the adjustment liquid 1, the monolith was heat-treated at 500 ° C. for 3 hours under an air stream of nitrogen. A catalyst was prepared. The adhesion amount of the mixture catalyst on the monolith catalyst was about 160 g. A mini-molded body (diameter 20 mm × length 12.7 mm) was cut out from the monolith catalyst.

[Production Example 10] Production of monolith catalyst coated with the catalyst of the present invention 160 g of [Pt-Rh / mesoporous silica] catalyst of Production Example 2, 160 g of Mg ion-exchanged clinoptilolite of Production Example 4 and 32 g of colloidal silica were distilled water. In addition to 4 liters, stirring was performed to prepare a slurry (conditioning liquid 1). After immersing a commercially available cordierite monolith molded body (4.5 mil / 400 cpsi, diameter 143.8 mm × length 118 mm) in the adjustment liquid 1, the monolith was heat-treated at 500 ° C. for 3 hours under an air stream of nitrogen. A catalyst was prepared. The adhesion amount of the catalyst of the present invention on the monolith catalyst was about 160 g. A mini-molded body (diameter 20 mm × length 12.7 mm) was cut out from the monolith catalyst.

[Production Example 11] Production of monolith catalyst coated with the catalyst of the present invention 160 g of [Pt-Rh / mesoporous silica] catalyst of Production Example 2, 160 g of Mg ion-exchanged erionite of Production Example 5 and 32 g of colloidal silica were added to 4 liters of distilled water. In addition to the above, stirring was performed to prepare a slurry (conditioning liquid 1). After immersing a commercially available cordierite monolith molded body (4.5 mil / 400 cpsi, diameter 143.8 mm × length 118 mm) in the adjustment liquid 1, the monolith was heat-treated at 500 ° C. for 3 hours under an air stream of nitrogen. A catalyst was prepared. The adhesion amount of the catalyst of the present invention on the monolith catalyst was about 160 g. A mini-molded body (diameter 20 mm × length 12.7 mm) was cut out from the monolith catalyst.

[Production Example 12] Production of monolith catalyst coated with catalyst of the present invention 160 g of [Pt-Rh / mesoporous silica] catalyst of Production Example 2, 160 g of Fe ion exchange clinoptilolite of Production Example 6 and 32 g of colloidal silica were distilled water. In addition to 4 liters, stirring was performed to prepare a slurry (conditioning liquid 1). After immersing a commercially available cordierite monolith molded body (4.5 mil / 400 cpsi, diameter 143.8 mm × length 118 mm) in the adjustment liquid 1, the monolith was heat-treated at 500 ° C. for 3 hours under an air stream of nitrogen. A catalyst was prepared. The adhesion amount of the catalyst of the present invention on the monolith catalyst was about 160 g. A mini-molded body (diameter 20 mm × length 12.7 mm) was cut out from the monolith catalyst.

[Production Example 13] Production of monolith catalyst coated with catalyst of the present invention [Pt-Rh / mesoporous silica] catalyst of Production Example 2 and commercially available natural clinoptilolite (potassium ion exchanger) (Sun Zeolite Co., Ltd.) Product name: 160 g of zeolite 3S) and 32 g of colloidal silica were added to 4 liters of distilled water and stirred to prepare a slurry (conditioning liquid 1). After immersing a commercially available cordierite monolith molded body (4.5 mil / 400 cpsi, diameter 143.8 mm × length 118 mm) in the adjustment liquid 1, the monolith was heat-treated at 500 ° C. for 3 hours under an air stream of nitrogen. A catalyst was prepared. The adhesion amount of the catalyst of the present invention on the monolith catalyst was about 160 g. A mini-molded body (diameter 20 mm × length 12.7 mm) was cut out from the monolith catalyst.

[Production Example 14] Production of monolith catalyst coated with the catalyst of the present invention [Pt-Rh / mesoporous silica] catalyst of Production Example 2 and commercially available natural erionite (a potassium ion exchanger) (manufactured by Tosoh Corporation) : TSZ-450) 160 g and colloidal silica 32 g were added to 4 liters of distilled water and stirred to prepare a slurry (conditioning solution 1). After immersing a commercially available cordierite monolith molded body (4.5 mil / 400 cpsi, diameter 143.8 mm × length 118 mm) in the adjustment liquid 1, the monolith was heat-treated at 500 ° C. for 3 hours under an air stream of nitrogen. A catalyst was prepared. The adhesion amount of the catalyst of the present invention on the monolith catalyst was about 160 g. A mini-molded body (diameter 20 mm × length 12.7 mm) was cut out from the monolith catalyst.

<Purification of lean burn simulated gas>
“Example 1”, “Example 2”, “Example 3”, “Example 4”, “Example 5”, “Comparative Example 1”, “Comparative Example 2”, “Comparative Example 3”
A monolithic catalyst mini-molded body of Production Example 7 was filled in a quartz flow reaction tube. The reaction tube was adjusted to an arbitrary temperature of 160 ° C. to 400 ° C. by an external heater. Next, a lean burn simulated gas with a low propylene concentration and a lean burn simulated gas with a high propylene concentration were alternately circulated through the reaction tube to perform NOx treatment. The lean burn simulated gas having a low propylene concentration is a gas containing 250 ppm of nitric oxide, 10% of oxygen, and 400 ppm of propylene, the concentration of which is adjusted with helium, and the flow rate is 500 ml per minute. The lean burn simulated gas with a high propylene concentration was a gas containing 250 ppm of nitric oxide, 10% oxygen, and 2% propylene, the concentration of which was adjusted with helium, and the flow rate was 500 ml per minute. The supply cycle of the lean burn simulated gas having a low propylene concentration and the lean burn simulated gas having a high propylene concentration was 10 minutes: 10 minutes. The NOx purification rate when sampling exhaust gas and circulating lean burn simulated gas with low propylene concentration (5 minutes after the start of distribution) and Nox purification rate when circulating lean lean simulated gas with high propylene concentration It was measured.
In the same manner as described above, the NOx purification rates of the monolithic catalyst mini-molded bodies of Production Examples 8, 9, 10, 11, and 12 were also measured. These results are shown in Tables 1 and 2.

なお、比較例１では処理後のガスに未反応のプロピレンが検出されたが、比較例２〜３、及び実施例１〜５では、処理後のガスには未反応のプロピレンは全く検出されなかった。

In Comparative Example 1, unreacted propylene was detected in the treated gas, but in Comparative Examples 2-3 and Examples 1-5, no unreacted propylene was detected in the treated gas. It was.

なお、比較例１では処理後のガスに未反応のプロピレンが検出されたが、比較例２〜３、及び実施例１〜５では、処理後のガスには未反応のプロピレンは全く検出されなかった。

In Comparative Example 1, unreacted propylene was detected in the treated gas, but in Comparative Examples 2-3 and Examples 1-5, no unreacted propylene was detected in the treated gas. It was.

As shown in Table 1, for lean burn exhaust gas with a low reducing agent content, the catalyst of the present invention gives a higher NOx purification rate at 160 ° C. to 400 ° C. than the conventional catalyst. In Examples 1-2, a purification rate of 80% or more is obtained from 180 ° C. to 200 ° C., and a purification rate of 40% or more is obtained at 200 ° C. to 300 ° C. Also, as shown in Table 2, for lean burn exhaust gas with a high content of reducing agent, the catalyst of the present invention has a purification rate of 80% or more even at 160 ° C. where the conventional catalyst does not exhibit much activity. Gave.
From the above, it can be seen that the purification catalyst of the present invention can efficiently purify lean burn exhaust gas from a low temperature region to a medium temperature region. Further, it can be seen that the exhaust gas in which a large amount of reducing agent is instantaneously supplied to the lean burn exhaust gas as in the post-injection method can be efficiently purified from a low temperature to an intermediate temperature range.

  The purification catalyst of the present invention is useful as a diesel automobile exhaust gas purification catalyst.

Claims (3)

  A lean-burn automobile exhaust gas purification catalyst comprising a catalyst having a platinum group element supported on a mesoporous material and a cation exchange clinoptilolite.   A lean burn automobile exhaust gas purification catalyst comprising a catalyst having a platinum group element supported on a mesoporous material and a cation exchange erionite. The lean-burn automobile exhaust gas purification according to claim 1 or 2, wherein the cation is at least one kind of cation selected from transition metal ions, alkali metal ions, and alkaline earth metal ions. Catalyst.