JP2007244934A - Catalyst for exhaust gas treatment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an innovative meso-porous catalyst capable of carrying out efficient treatment of diesel exhaust NO<SB>x</SB>, which is conventionally difficult, and a monolith catalyst bearing this catalyst. <P>SOLUTION: Platinum particles with an average particle diameter of 1 to 20 nm in an amount of 0.01 to 20% by mass are deposited as a main catalyst on amorphous meso-porous silica with fine pores with a fine pore diameter of 2 to 50 nm and a specific surface area of 100 to 1400 m<SP>2</SP>/g. This catalyst (meso-porous catalyst) is capable of efficiently carrying out treatment of NO<SB>x</SB>at 160 to 300°C in lean-burn atmosphere (generally, 5% or higher oxygen concentration atmosphere), which is conventionally difficult. A monolith catalyst obtained by applying the meso-porous catalyst to a formed body is provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a mesoporous catalyst having a high specific surface area and a monolith catalyst in which this catalyst is attached to the inner wall of a gas flow path of a monolith molded body. By using the monolith catalyst, NOx contained in lean burn automobile exhaust gas can be efficiently produced. It can be purified.

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 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 hundred nm to several μm 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 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 (patents). Reference 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.
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 that of gasoline (combustion of diesel fuel is called lean burn), the oxygen concentration in diesel exhaust gas is usually over 5% and contains almost no reducing substances. Because it is not. For the same reason, it is difficult to purify the exhaust gas from lean burn gasoline vehicles with a three-way catalyst.

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 small diesel vehicles (see Patent Document 3). In this method, lean burn and rich burn cycle combustion is performed, lean burn exhaust NOx is absorbed by the NOx storage agent, absorbed NOx is released in a rich burn atmosphere, and the released NOx is released from the carbon monoxide in the rich burn exhaust gas. It is based on the idea of reducing with a reducing substance such as hydrogen or hydrocarbon.
The purification method using the NOx occlusion reduction catalyst can purify NOx from about 250 ° C. to about 600 ° C. even in a high-concentration oxygen atmosphere in which a three-way catalyst used for exhaust gas treatment of gasoline passenger cars cannot be used. Although there is an advantage, there is a problem that NOx purification at about 200 ° C. or lower is very difficult and a problem that the NOx storage agent is remarkably deteriorated by moisture in the exhaust gas and a small amount of SOx.

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.
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. Therefore, the catalyst required for the exhaust NOx treatment of diesel passenger cars is desired to be a catalyst having high activity in the low temperature range of 120 ° C to 200 ° C.
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. It is estimated that. 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 smaller by one to two digits than the particle diameter of the conventional catalyst particles in the pores of the crystalline 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 4-9. However, it has not been possible 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 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

  In view of the above circumstances, an object of the present invention is to provide a novel catalyst for purifying NOx contained in lean burn exhaust gas. Specifically, in order to efficiently purify diesel exhaust NOx, which has been difficult in the past, a novel mesoporous catalyst that exhibits activity against exhaust NOx even in a relatively high concentration oxygen atmosphere of lean burn and this catalyst are attached. To provide a monolithic catalyst.

As a result of intensive studies to achieve the above object, the present inventors have found that a catalyst in which platinum is supported on an amorphous mesoporous silica support having a specific pore distribution and a high specific surface area is subjected to lean burn NOx treatment. As a result, the present invention has been completed based on this finding.
That is, the present invention
(1) 0.01 to 20% by mass of platinum particles having an average particle diameter of 1 to 20 nm on an amorphous mesoporous silica support having pores having a pore diameter of 2 to 50 nm and a specific surface area of 100 to 1400 m ² / g A mesoporous catalyst for purifying lean burn exhaust NOx, which is a supported catalyst.
(2) A lean burn NOx purifying monolith catalyst characterized in that the mesoporous catalyst of (1) is attached to the inner wall of the gas flow path of the monolith molded body.
(3) The amount of mesoporous catalyst attached to the monolith catalyst is 3 to 30% by mass of the monolith catalyst, the amount of platinum supported on the mesoporous catalyst is 0.1 to 10% by mass, and the amount of platinum supported per monolith catalyst The lean-burn exhaust NOx purification monolith catalyst according to the above (2), characterized in that is 0.03 to 3% by mass.
(4) An exhaust NOx purification catalyst for small diesel that performs rich burn and lean burn alternately, using the lean burn exhaust NOx purification monolith catalyst of (2) or (3) above.
(5) The present invention relates to an exhaust NOx purification catalyst for large diesel using the lean burn exhaust NOx purification monolith catalyst of the above (2) or (3).

  The mesoporous catalyst of the present invention can perform lean burn exhaust NOx treatment, which could not be achieved in the past, extremely efficiently even in a low temperature region. For example, although a three-way catalyst can hardly purify nitric oxide in an atmosphere with an oxygen concentration of 14%, the platinum catalyst supported on the amorphous mesoporous silica of the present invention has a monoxide that exists in an atmosphere with an oxygen concentration of 14%. 80% or more of nitrogen can be purified at 160 ° C to 300 ° C.

Hereinafter, the present invention will be described in detail.
One of the features of the present invention is that amorphous mesoporous silica is used as a carrier for a NOx purification catalyst. The reason is that the catalyst can be supported in the pores of the amorphous mesoporous silica, the effect of gas diffusion through the pore channel can be expected, and the preferable particle size range of the catalytically active species can be maintained by controlling the pore distribution. This is because, by supporting the catalyst in the pores, there are excellent effects such as suppressing reaggregation of the catalyst particles and achieving uniform and high dispersion of the catalyst.
The amorphous mesoporous silica in the present invention is mesoporous silica having one small-angle X-ray diffraction peak due to pores (= vacancies) and no ordering of pore arrangement observed by TEM observation. .

  In general, vacancies present in a porous material can be observed by a small-angle X-ray diffraction method, or directly by a TEM. Voids with a meso-region size (2 nm to 50 nm) generally have a broad diffraction peak when the horizontal axis represents the diffraction angle 2θ and the vertical axis represents the X-ray diffraction intensity. Indicates. When the vacancies are pores of a porous material and are regularly arranged in a certain long distance range, in the region of the diffraction angle, a plurality of cavities generally observed in a crystalline substance are used. A diffraction peak is observed, and the pore array can be assigned from the pattern. Further, when this material is observed by TEM (= transmission electron microscope), an image in which pores are arranged in an orderly manner can be observed. A mesoporous material having such regularly arranged pores was invented by Mobil Oil, and named as a crystalline mesoporous molecular sieve (Japanese Patent No. 3444328).

When only one diffraction peak is observed, it cannot be determined that it is crystalline because the pattern cannot be assigned. In such a case, TEM observation is effective for determining whether it is crystalline or amorphous. The mesoporous silica of the present invention has only one small-angle X-ray diffraction peak due to pores. Furthermore, since the long-range order of the pore arrangement was not observed at all by TEM observation, it was confirmed that it was not crystalline but so-called amorphous. Crystalline mesoporous silica and amorphous mesoporous silica of the present invention can be clearly distinguished by small-angle X-ray diffraction measurement and TEM observation.
Next, when considering a mesoporous material as a catalyst support, parameters of pore characteristics such as pore morphology, pore distribution, specific surface area and the like are important. This is because the morphology of the pores affects the trapping state of the catalyst particles in the pores, the pore distribution affects the particle size of the catalyst particles, and the specific surface area affects the dispersion state of the catalyst. Since the crystalline mesoporous molecular sieve according to the invention of Mobil Oil described above belongs to the hexagonal system in the pore arrangement, the surfaces having pores are laminated in layers, and a fibrous elongated cylinder in the plane of each layer It has a honeycomb-like shape in which the fine pores are arranged in parallel. On the other hand, the amorphous mesoporous silica of the present invention has a structure in which amorphous shallow pores are connected in some places and dispersed randomly by small-angle X-ray diffraction measurement and TEM observation.

  When a catalyst was supported on these two types of mesoporous silica and the catalytic activity of the NOx purification reaction was compared, it was found that the mesoporous catalyst of the present invention has a far superior low temperature activity. This unexpected difference in catalytic activity can be explained as follows. That is, the catalyst supported on the mesoporous silica MCM-41, which is a hexagonal mesoporous silica, is captured deep inside the elongated pores by the surface tension of the pores, and the catalyst supported on the amorphous mesoporous silica of the present invention is The amorphous mesoporous silica of the present invention, which is trapped in a shallow place near the entrance of the pores and can achieve mass transfer more quickly, is effective for the exhaust NOx purification reaction that is a gas phase reaction on the catalyst surface. This is because it is superior to crystalline mesoporous silica as a catalyst carrier.

The pore diameter of the support of the present invention and the particle diameter of the catalyst supported on the pores are closely related. As will be described below, since the particle size of the catalyst particles exhibiting high activity with respect to NOx is nano-sized, the pore size of the mesoporous silica that is the carrier must be about the same as that of the catalyst particles. Usually, the particle size of the catalyst supported in the pores of the mesoporous silica is approximately the same as the pore size. Therefore, by controlling the pore size of the mesoporous silica, a catalyst having a preferable particle size can be dispersed uniformly. Can have. Therefore, the pore size of mesoporous silica is an important design factor.
Most of the pores of the amorphous mesoporous silica of the present invention have a pore diameter (diameter display) in the range of 2 to 50 nm, preferably in the range of 2 to 20 nm, more preferably in the range of 2 to 10 nm. . As used herein, the majority of pores means that the pore volume occupied by pores of 2 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 2 nm, but 2 nm or more is preferable in consideration of the influence of contamination by impurities and the like. If it exceeds 50 nm, the dispersion-supported catalyst tends to grow into giant particles by sintering (= sintering) under hydrothermal high temperature conditions and the like. In addition, the pore diameter in the present invention is a value measured by a nitrogen adsorption method using nitrogen as an adsorption / desorption gas, and is represented by a pore distribution (differential distribution display) in a range of 1 to 200 nm obtained by the BJH method. .

Next, the specific surface area of the support is an important design factor for achieving uniform dispersibility of the catalyst. The specific surface area should be as high as possible unless there are special circumstances. The specific surface area of the mesoporous silica can be used in the present invention is 100~1400m ² / g, preferably 100~1200m ² / g, and more preferably from 400~1200m ² / g. When the specific surface area is less than 100 m ² / g, the supported amount of the catalyst is reduced. Therefore, 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 1400 m ² / g or less. The specific surface area in the present invention is a value measured by a BET nitrogen adsorption method using nitrogen as an adsorption / desorption gas.

The mesoporous silica used in the present invention is selected from the group consisting of a 3A group element, a 3B group element, a 4A group element, a 5A group element, and a 6A group element including a lanthanoid group as a part of silicon constituting the mesoporous silica. Furthermore, it is preferable to use a mesoporous silica composite in which at least one element is introduced because it has been surprisingly found that the catalyst is imparted with low temperature activity.
Of the group 3A elements, scandium, yttrium, lanthanum, cerium, samarium, and gadolinium are preferred, boron is preferred for group 3B elements, titanium and zirconium are preferred for group 4A elements, niobium and tantalum are preferred for group 5A, and group 6A In this case, chromium, molybdenum, and tungsten are preferable.

Among these, boron, tungsten, niobium, and cerium are more preferable, and tungsten and cerium are particularly preferable from the viewpoint of sustainability.
The amount of these elements introduced is preferably 1-20 mol%, more preferably 1-10 mol%, relative to the main metal constituting the mesoporous silica.
The mesoporous silica composite introduced with the group 3A element or the like refers to a composite structure in which a part of silicon constituting the mesoporous silica is substituted with an element such as the group 3A element.
Next, the catalyst used in the present invention is a platinum catalyst. Conventionally, a three-way catalyst is known as an automobile exhaust gas treatment catalyst containing platinum, but this catalyst is known to have little effect on diesel exhaust NOx purification treatment. The reason is that palladium and rhodium, which are constituent elements other than platinum, undergo surface oxidation by a low concentration of oxygen. Since the three-way catalyst is composed of platinum-palladium-rhodium, it is easily deactivated when subjected to surface oxidation.

The reason why the platinum catalyst is used in the present invention is that platinum has a high catalytic ability to oxidize nitrogen monoxide, which is the main component of exhaust NOx, into nitrogen dioxide by coexisting oxygen, and is chemically stable even in a high-temperature oxygen atmosphere. . Also, platinum was found to be low temperature active. Nitrogen dioxide produced by the catalytic reaction is easily converted into nitrogen and water by reducing substances such as lower olefins having 1 to 6 carbon atoms and lower paraffin (which is contained in a small amount in fuel) or ammonia urea (which can be mounted on trucks). Disassembled. Since the surface area of the catalyst particles is inversely proportional to the square of the particle diameter, the smaller the catalyst particles, the higher the catalytic activity. For example, the surface area of the catalyst particles of 1nm is 10 ⁴ times greater than that of 0.1 [mu] m.

  In addition, catalyst particles atomized into nano-sizes have a large number of high-order crystal planes such as edges, corners, and steps that exhibit activity, so that not only catalytic activity is significantly improved but also catalytic activity is exhibited in bulk. It is known that even an inert metal such as this may exhibit unexpected catalytic activity. Therefore, finer catalyst particles are preferable from the viewpoint of catalytic ability, but on the other hand, there are also undesirable properties such as surface oxidation and side reactions due to atomization, so there is an optimum range for the particle size of the fine particles. . It has been found that the average particle diameter of platinum particles exhibiting an effective activity for the target NOx decomposition and purification treatment in the present invention is in the range of 1 to 20 nm, particularly in the range of 1 to 10 nm.

The catalyst of the present invention is a supported catalyst supported on pores of mesoporous silica. The supported amount of platinum as the main catalyst is 0.01 to 20% by mass, preferably 0.1 to 10% by mass. If there is no quantitative problem, the supported amount is usually several percent. . The supported amount of catalyst of mesoporous silica can be 20% by mass or more. However, when 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. Moreover, 0.01 mass% or more is preferable in order to obtain sufficient catalyst activity.
By adding a co-catalytic component having a different function to the platinum catalyst which is the main catalyst of the present invention, the catalyst performance can be improved by the synergy effect. Examples of such components include chromium, manganese, iron, cobalt, nickel, copper, zinc, barium, scandium, yttrium, titanium, zirconium, hafnium, niobium, tantalum, molybdenum, tungsten, lanthanum, cerium, barium, and these. Can be mentioned. Among these, chromium, iron, cobalt, nickel, which is a passivating film, copper with a relatively high adsorptive power of reducing agents, cerium oxide and manganese dioxide with moderate oxidizing power, effective in preventing SOx poisoning Copper-zinc, iron-chromium, molybdenum oxide, and the like are preferable. The addition amount of this component is usually about 0.01 to 100 times to about 100 times the weight of platinum, but may be 100 times or more as necessary.

The method for producing the amorphous mesoporous silica of the present invention is not particularly limited, and a desired material can be produced by applying a conventional sol-gel method using a surfactant micelle as a template. it can. The precursors of mesoporous silica are usually tetramethoxysilane, tetraethoxysilane, tetra-i-propoxysilane, tetra-n-propoxysilane, tetra-i-butoxysilane, tetra-n-butoxysilane, tetra-sec- Alkoxides such as butoxysilane and tetra-t-butoxysilane are used.
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, and the stabilizer / main agent molar ratio is 0.01-1.0, preferably Is 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 and dried, and then the template is thermally decomposed and removed by high-temperature baking at 500 to 1000 ° C. to obtain mesoporous silica. If necessary, the surfactant can be extracted with alcohol or the like before firing.

The mesoporous catalyst of the present invention can be produced, for example, by 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 mesoporous silica in an aqueous solution of a 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. As a raw material of the co-catalytic component added to the main catalyst as necessary, for example, water-soluble salts such as chloride, nitrate, sulfate, carbonate and acetate can be used. A catalyst obtained by adding a promoter component to platinum can also be produced in the same manner by mixing the raw material with the main catalyst raw material. As the reducing agent, 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.

Note that a mesoporous silica composite in which a group 3A element, a group 3B element, a group 4A element, a group 5A element, and / or a group 6A element is introduced instead of silicon of mesoporous silica is used as a precursor of a mesoporous material. An appropriate amount of alkoxide, acetylacetonate or the like can be added to produce the mesoporous material by the same method as described above.
The monolith molded body of the present invention is a molded body in which a cross section of the molded body is mesh-like and provided with gas flow paths partitioned by thin walls in parallel to the axial direction. Although the external shape of a molded object is not specifically limited, Usually, it is a cylindrical shape. The monolith catalyst of the present invention means a catalyst in which a mesoporous catalyst is adhered to the inner wall of the gas flow path of the monolith molded body. The adhesion amount of the mesoporous catalyst is preferably 3 to 30% by mass. It is preferably less than 30% from the viewpoint of gas diffusion to the catalyst existing inside the support. 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.

The monolith catalyst of this invention can be manufactured according to the manufacturing method of the monolith molded object which adhered the three-way catalyst for motor vehicles. For example, a mixture in which a mesoporous catalyst and colloidal silica as a binder are usually mixed at a mass ratio of 1: (0.01 to 0.2) is prepared, and this is usually dispersed in water to form a slurry of 10 to 50% by mass. After the monolith molded body is immersed in the slurry, the slurry is attached to the inner wall of the gas flow path of the monolith molded body, dried, and then dried at a temperature of 500 to 1000 ° C. under an inert atmosphere such as nitrogen, helium or argon. It can be manufactured by heat treatment for a long time.
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 amorphous mesoporous silica is applied to a monolith molded article, and then the mesoporous silica is impregnated with the catalyst raw material, followed by reduction treatment and heat treatment.

Further, as another production method, instead of attaching the slurry, a chemical vapor deposition method in which a thin film mesoporous silica is directly attached using a gaseous mesoporous silica precursor. The platinum catalyst supported on the monolith molded body to which thin film mesoporous silica is adhered by vapor deposition can be usually performed by a method of impregnating an aqueous solution of a catalyst raw material and drying, followed by reduction treatment and heat treatment.
The thickness of the mesoporous catalyst layer deposited on the monolith molded body varies depending on the deposition method, and in the above-described method of depositing the slurry, it is usually preferably 1 μm to 100 μm, and particularly preferably 10 μm to 50 μm. 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. Moreover, in the said chemical vapor deposition method, the range of 10 nm-10 micrometers is preferable normally, More preferably, it is 100 nm-1 micrometer. Although it is possible to make the thickness 10 μm or more, it is preferably 10 μm or less because there is little diffusion of exhaust gas into the inside.

  By mounting the monolith catalyst of the present invention on automobiles, particularly diesel automobiles and lean burn gasoline automobiles, lean burn exhaust NOx discharged by automobiles can be extremely effectively purified in a low temperature range of 160 to 300 ° C. Although a reducing agent is required for the treatment of exhaust NOx, in the case of small cars such as passenger cars, lower olefins and lower paraffins having 1 to 6 carbon atoms contained in a small amount of light oil as fuel become reducing agents. The fuel may be supplied directly or through the reformer onto the catalyst. Since the oxygen concentration is high at the time of rich burn and the oxygen concentration is low at the time of lean burn, if the monolith catalyst of the present invention is used for exhaust gas purification treatment of a small diesel capable of alternately performing rich burn and lean burn, 160 to Exhaust NOx can be efficiently purified over a wide temperature range of 600 ° C. Also, in the case of large vehicles such as trucks, it is usually possible to use a system that thermally decomposes urea water to generate ammonia as a reducing agent and supplies it onto the catalyst. It can also be used as an exhaust NOx purification catalyst.

The present invention will be specifically described below with reference to examples.
Small-angle X-ray diffraction patterns and powder X-ray diffraction patterns in the examples were measured with a RINT2000 X-ray diffraction apparatus manufactured by Rigaku Corporation.
The pores of the support and the catalyst were directly observed using an H-9000UHR transmission electron microscope manufactured by Hitachi, Ltd. It was confirmed that the particle diameter of the catalyst observed with a transmission electron microscope coincided with the value calculated by substituting the half width of the main peak of the powder X-ray diffraction pattern into the Scherrer equation.
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 silicas showed a peak at a specific pore diameter in an exponentially increasing distribution. The pore diameter that gives this peak is the pore diameter.
Helium-diluted nitric oxide, oxygen, and reducing gas (ethylene or ammonia) were used as model gases for automobile exhaust NOx. The treatment rate of nitric oxide was measured by measuring nitrogen monoxide contained in the treated gas with a reduced pressure chemiluminescence NOx analyzer (manufactured by Nippon Thermo Co., Ltd .: models 42i-HL and 46C-H). Calculated according to (1).

「製造例１」三元触媒類似の貴金属触媒の合成
０．２１５ｇのＰｔＣｌ_４・５Ｈ_２Ｏ、０．１０６ｇのＰｄＣｌ_２・２Ｈ_２Ｏ、及び０．１６２ｇのＲｈ（ＮＯ_３）_３・２Ｈ_２Ｏを２０ｍｌの蒸留水に溶解した水溶液を蒸発皿に入れ、これに１０ｇのγ−アルミナ（粒径２〜３μｍの微粒子）を加え、スチームバスで蒸発乾固した後、真空乾燥機に入れ１００℃で３時間真空乾燥を行った。この試料を石英管に入れヘリウム希釈水素ガス（１０％ｖ／ｖ）気流下５００℃で３時間還元し、貴金属の含有量が約２重量％の触媒を合成した。これを、三元触媒を模した貴金属触媒として比較実験に用いた。

“Production Example 1” Synthesis of a three-way catalyst-like noble metal 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 An aqueous solution in which O is dissolved in 20 ml of distilled water is placed in an evaporating dish, 10 g of γ-alumina (fine particles having a particle diameter of 2 to 3 μm) is added thereto, evaporated to dryness in a steam bath, and then placed in a vacuum dryer. Vacuum drying was performed at 0 ° 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 Synthesis of Pt / Crystalline Mesoporous Silica Catalyst MCM-41 type mesoporous silica was synthesized according to the method of Example 21 in US Pat. No. 5,143,707. That is, an aqueous solution of cetyltrimethylammonium hydroxide was obtained through a column packed with 300 g of an aqueous solution of 29% by mass of dodecyltrimethylammonium bromide and 10 g of hydroxide for halide exchange resin. This was mixed with 306 g of a 29% strength by weight aqueous solution of dodecyltrimethylammonium bromide, 30 g of tetraethylorthosilicate was added thereto, and the mixture was stirred at 40 ° C. for 1 hour. This solution was transferred to a polypropylene container, placed in a humidifier and allowed to stand at 100 ° C. for 48 hours.

The resulting precipitate was filtered under reduced pressure, washed with water, calcined at 540 ° C. for 1 hour in a nitrogen stream, and then further calcined at 540 ° C. in air for 6 hours. The specific surface area of the obtained material was 1450 m ² / g, and the pore diameter was 2.5 nm. The small-angle X-ray diffraction pattern shows four diffraction peaks, and the interplanar spacing (d value) is 3.2 nm (strong), 1.8 nm (weak), 1.6 nm (weak), and 1.2 nm ( very weak).
The above small-angle X-ray diffraction pattern agrees with the data of MCM-41 described in Example 21 in US Pat. No. 5,143,707, and thus it is confirmed that it is MCM-41 type mesoporous silica. It was done.
Next, put the aqueous solution 0.267g dissolved _{H 2} PtCl ₆ · 6H 2 O in distilled water 20g evaporation dish, to which the above MCM-41 type mesoporous silica material 5g added, evaporated to dryness in a steam bath Then, it put into the vacuum dryer and vacuum-dried at 100 degreeC for 3 hours. This sample was put in a quartz tube and reduced at 500 ° C. for 3 hours under a stream of helium diluted hydrogen gas (10 v / v%) to synthesize a mesoporous catalyst having a platinum content of about 2% by mass. The particle size of the supported platinum catalyst was about 2.5 nm.

Production Example 3 Synthesis of Platinum / Amorphous Mesoporous Silica Catalyst 300 g of distilled water, 240 g of ethanol, and 30 g of dodecylamine were placed in a 1 liter beaker and dissolved. While stirring, 125 g of tetraethoxysilane was added and stirred at room temperature for 22 hours. The product 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 contained dodecylamine to obtain mesoporous silica.
As a result of small-angle X-ray diffraction measurement of the mesoporous silica, one broad diffraction peak was observed at a 2θ angle of 2.72 degrees (d = 3.25 nm) as shown in FIG.
Further, as a result of observation with a transmission electric microscope, as shown in FIG. 2, a regular arrangement was not observed in the arrangement of the pores, and a state where the pores were randomly distributed was observed.

From these results, it was confirmed that the produced mesoporous silica was amorphous. Moreover, as a result of pore distribution and specific surface area measurement, there was 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 2 to 50 nm. The volume occupied by was 1.34 cm ³ / g.
Put an aqueous solution prepared by 0.267g dissolved _{H 2} PtCl ₆ · 6H 2 O in distilled water 20g evaporation dish, to which the mesoporous silica material 5g was added and after evaporation to dryness on a steam bath, placed in a vacuum dryer Vacuum drying was performed 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 mesoporous catalyst having a platinum content of about 2 mass%. The average particle size of the platinum particles supported on the mesoporous catalyst was about 3.0 nm.

“Production Example 4” Synthesis of Pt / Amorphous Mesoporous Silica / Monolith Catalyst 1 g of the catalyst 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. 5 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) Each piece was immersed, taken out and air-dried, and then heat-treated in a nitrogen stream at 500 ° C. for 3 hours. The adhesion amount of the mesoporous catalyst was about 10% by mass of the mini-molded product, and the supported amount of platinum per mini-molded product was about 0.2% by mass.

[Example 1] Lean burn NOx treatment using ethylene as a reducing agent Each of the catalyst samples of Production Examples 1 and 2 was filled in 0.3 g in a continuous flow reaction tube made of quartz, and the monolith catalyst of Production Example 3 was loaded with a catalyst. One mini-molded body was filled, and nitrogen monoxide whose concentration was adjusted with helium was circulated.
The component molar concentrations of the gas to be treated were 0.1% nitric oxide, 14% oxygen, 10% water vapor, and 0.3% ethylene. The flow rate of the mixed gas introduced into the reaction tube was 100 ml per minute, and the treatment temperature was 160 to 300 ° C. The exhaust gas was sampled and the treatment rate of nitric oxide was determined. The results are shown in Table 1.
From Table 1, it can be seen that the mesoporous catalyst of the present invention can efficiently purify NOx in the coexistence of high-concentration oxygen using a hydrocarbon such as ethylene as a reducing agent. Further, the platinum catalyst supported on the crystalline mesoporous silica MCM-41 has a temperature that gives the maximum reaction rate of nitric oxide at 200 ° C., whereas the mesoporous catalyst of the present invention has a nitrogen monoxide concentration at 170 ° C. Gives maximum response rate. Therefore, it can be seen that the mesoporous catalyst of the present invention is remarkably excellent in low temperature activity.

[Example 2] Rich burn NOx treatment using ethylene as a reducing agent Nitric oxide was treated using 0.3 g of each of the catalysts of Production Examples 3 and 4. The component molar concentration ratio of the gas to be treated was 0.1% nitric oxide, 1% oxygen, and 1% ethylene. The flow rate of the adjustment gas was 100 ml / min, and the processing temperature was 160 to 600 ° C. The exhaust gas was sampled and the treatment rate of nitric oxide was determined. The results are shown in Table 2.
From Table 2, it can be seen that the mesoporous catalyst of the present invention can efficiently purify NOx under rich burn conditions from a medium temperature region to a high temperature region using hydrocarbon as a reducing agent. Therefore, for example, if the lean burn and the rich burn are alternately performed, the mesoporous catalyst of the present invention can remove NOx in a wide temperature range. Therefore, the exhaust NOx of a small diesel vehicle capable of performing the lean burn and the rich burn alternately. It turns out that it is suitable for processing.

[Example 3] NOx treatment using ammonia as a reducing agent Nitric oxide was treated using 0.3 g of each of the catalysts of Production Examples 3 and 4. The component molar concentration ratio of the gas to be treated was 0.1% nitric oxide, 14% oxygen, 10% water vapor, and 0.3% ammonia. The flow rate of the adjustment gas was 100 ml / min, and the processing temperature was 150 to 600 ° C. The exhaust gas was sampled and the treatment rate of nitric oxide was determined. The results are shown in Table 3.
Table 3 shows that the mesoporous catalyst of the present invention can efficiently purify NOx in the presence of high-concentration oxygen even when ammonia is used as a reducing agent. Therefore, it turns out that it is suitable for the exhaust-NOx purification process of the large-sized diesel vehicle carrying the urea supply system as an ammonia source.

The mesoporous catalyst and monolith catalyst of the present invention are useful as a diesel exhaust NOx purification catalyst.

Small-angle X-ray diffraction pattern of mesoporous silica produced in Production Example 3 Transmission electron microscope observation of the mesoporous silica produced in Production Example 3

Claims (5)

An amorphous mesoporous silica support having pores with a pore diameter of 2 to 50 nm and a specific surface area of 100 to 1400 m ² / g supported 0.01 to 20% by mass of platinum particles with an average particle diameter of 1 to 20 nm. A mesoporous catalyst for purifying lean burn exhaust NOx, which is a catalyst. 2. A monolith catalyst for purifying lean burn exhaust NOx, wherein the mesoporous catalyst according to claim 1 is adhered to an inner wall of a gas flow path of a monolith molded body. The amount of mesoporous catalyst attached to the monolith catalyst is 3 to 30% by mass of the monolith catalyst, the amount of platinum supported on the mesoporous catalyst is 0.1 to 10% by mass, and the amount of platinum supported per monolith catalyst is 0. The monolith catalyst for purifying lean burn exhaust NOx according to claim 2, characterized in that it is in a range of 03 to 3 mass%. An exhaust NOx purification catalyst for small diesel, which performs rich burn and lean burn alternately, using the lean burn exhaust NOx purification monolith catalyst according to claim 2 or 3. An exhaust NOx purification catalyst for large diesel using the lean burn exhaust NOx purification monolith catalyst according to claim 2 or 3.

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