JP2009183925A - Catalyst and method for performing catalytic reduction of nitrogen compound in tail gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for performing a catalytic reduction on the nitrogen compound with high selectivity neither causing the aggravation of fuel consumption nor the deterioration of the catalyst at a wider range of temperatures allowing a tail gas containing nitrogen the compound produced when a fuel is burnt under a lean condition to contact the catalyst in the presence of sulfur oxide, carbon monooxide, and hydrogen. <P>SOLUTION: The catalyst that catalytically reduces the nitrogen compound allowing the tail gas containing the nitrogen compound produced when the fuel is burnt under the lean condition to contact the catalyst in the presence of carbon monooxide and hydrogen or in the presence of carbon monooxide, hydrogen, and sulfur oxide including an alkaline metal (and an alkaline earth metal)-ZSM-5 where a 0.5 to 5.0 wt.% rhodium is supported obtained by cleaning, drying, and sintering a solution after performing hydrothermal treatment on an aqueous mixture containing at least one of mineralizer selected from water, source of silicon, source of aluminium, source of rhodium, an alkaline metal hydroxide, and an alkaline earth metal hydroxide, a salting-out effect agent consisting of an alkaline metal salt, a hydrogen ion concentration regulator consisting of acid, and at least one of mold agent selected from source of tetraethylammonium and source of tratrapropylammonium is provided. In addition, a catalyst containing H-ZSM-5 where a 0.5 to 5.0 wt.% rhodium is supported is also provided. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a catalyst and method for catalytic reduction of nitrogen oxides (mainly composed of NO and NO ₂ , hereinafter sometimes referred to as NOx). Specifically, the present invention provides a catalyst for catalytically reducing nitrogen oxides in exhaust gas by supplying fuel to a combustion chamber of a diesel engine or a gasoline engine and burning it, and contacting the generated exhaust gas, and the exhaust gas. The present invention relates to a process for catalytic reduction of nitrogen oxides therein using such a catalyst. Such catalysts and methods are suitable for use, for example, to reduce and remove harmful nitrogen oxides contained in exhaust gas from mobile sources such as automobiles.

In particular, the present invention burns fuel under lean conditions and converts nitrogen oxides in exhaust gas produced by the combustion in the presence of carbon monoxide and hydrogen or carbon monoxide, hydrogen and sulfur oxides (mainly , SO ₂ and SO _3, hereinafter referred to as SOx), and a catalyst for catalytic reduction at a high selectivity by contacting with each other. Further, the present invention is to burn fuel under lean conditions, and to convert nitrogen oxides in exhaust gas generated by the combustion in the presence of carbon monoxide and hydrogen or between carbon monoxide, hydrogen and sulfur oxide. The present invention relates to a method for catalytic reduction with high selectivity by contacting with a catalyst in the presence. According to such a catalyst or method of the present invention, nitrogen oxides in the exhaust gas can be catalytically reduced at a high selectivity without causing catalyst deterioration and fuel consumption deterioration in a wide range of temperatures. .

  Conventionally, nitrogen oxides in exhaust gas are reduced to nitrogen by, for example, a method in which the oxides are oxidized and then absorbed by alkali, or ammonia or urea, hydrogen, carbon monoxide or hydrocarbons are used as a reducing agent. Has been removed by the method. However, each of these conventional methods has drawbacks.

  That is, according to the former method, in order to prevent environmental problems, a means for treating the generated alkaline waste water is necessary. According to the latter method, for example, when ammonia or urea is used as a reducing agent, ammonia produced by hydrolysis of ammonia or urea reacts with sulfur oxides in exhaust gas to form salts, As a result, the catalytic activity deteriorates at low temperatures. In addition, when nitrogen oxides from mobile sources such as automobiles are treated with ammonia, the safety becomes a problem. Furthermore, when urea is used, it is necessary to mount urea water as an ammonia source. There is also a need for infrastructure to supply urea water to automobiles. Further, the catalyst requires three types of catalysts, ie, a urea hydrolysis catalyst, a selective reduction catalyst of nitrogen oxide using ammonia as a reducing agent, and a leaked ammonia oxidation catalyst, and there is a problem that the catalyst volume increases.

  On the other hand, when using hydrogen, carbon monoxide, or hydrocarbon as the reducing agent, when using a known three-way catalyst, the reducing agent has a higher concentration of oxygen than nitrogen oxide. Therefore, if the nitrogen oxide is to be substantially reduced, a large amount of reducing agent is required, and the fuel consumption is greatly reduced.

Therefore, it has been proposed to catalytically decompose nitrogen oxides without using a reducing agent. However, conventionally known catalysts for directly decomposing nitrogen oxides have not been put into practical use because of their low decomposition activity. On the other hand, various zeolites have been proposed as nitrogen oxide catalytic reduction catalysts using hydrocarbons or oxygen-containing organic compounds as reducing agents. In particular, Cu-ZSM-5 (copper ion exchange ZSM-5) and H-ZSM-5 (hydrogen type or acid type ZSM-5, SiO ₂ / Al ₂ O ₃ molar ratio = 30 to 40) are optimal. Has been. However, the reaction selectivity between nitrogen oxides and hydrocarbons on these catalysts is low, and when water is contained in the exhaust gas, the active site (solid acid) of the zeolite has a zeolite structure under hydrothermal conditions. It is known that the catalyst performance deteriorates rapidly due to deterioration due to dealumination.

  Under these circumstances, development of a catalyst having higher activity for catalytic reduction of nitrogen oxides has been demanded. Recently, a catalyst in which silver or silver oxide is supported on an inorganic oxide support material has been proposed. (For example, see Patent Documents 1 and 2). However, since this catalyst has high oxidation activity and low selective reduction activity with respect to nitrogen oxide, it is known that the conversion efficiency of nitrogen oxide to nitrogen is low. In addition, this catalyst has a problem that its activity is quickly reduced in the presence of sulfur oxides. These catalysts have a catalytic action to selectively reduce nitrogen oxides to some extent using hydrocarbons under perfectly lean conditions, but have a lower nitrogen oxide removal rate than three-way catalysts, Since the operating temperature window (temperature range) is narrow, it is difficult to put such a catalyst into practical use. Thus, there is an urgent need for more active and useful catalysts for the catalytic reduction of nitrogen oxides.

  On the other hand, an automobile equipped with a lean combustion engine such as a diesel engine can be driven at a very large air / fuel ratio, and therefore an automobile equipped with an Otto engine that burns fuel stoichiometrically with air. As a result, the fuel consumption rate can be lowered. Therefore, the importance of automobiles equipped with a lean combustion engine such as a diesel engine that can reduce the amount of carbon dioxide gas, which is a greenhouse gas, and suppress global warming is increasing. Therefore, in order to overcome the above-mentioned problems, the NOx storage-reduction system that has recently supplied the combustion chamber with a quantity exceeding the stoichiometric amount in a short period of time in the lean combustion engine is the most. It has been proposed as a promising method (see, for example, Patent Documents 3 and 4). Such a lean-burn engine NOx storage-reduction system reduces nitrogen oxides by two periodic steps spaced 1-2 minutes apart.

That is, in the first step, NO is oxidized to NO ₂ on a platinum catalyst under (normal) lean conditions, and this NO ₂ is added to an absorbent composed of an alkaline compound such as potassium carbonate or barium carbonate. Absorbed as nitrate. Then, a rich condition for the second step is formed and this rich condition is maintained for a few seconds. Under this rich condition, the absorbed (stored) NO ₂ is released from the absorbent and is efficiently reduced to nitrogen by hydrocarbons, carbon monoxide or hydrogen on the platinum catalyst.

This NOx storage-reduction system works well for a long time in the absence of sulfur oxides, but in the presence of sulfur oxides, NOx on alkaline compounds under both lean and rich conditions. _{2 The} absorption site irreversibly absorbs sulfur oxide, the system deteriorates rapidly, and the deterioration catalyst has to be regenerated in a high temperature reducing atmosphere. Furthermore, since nitrogen oxides are absorbed as nitrates in this method, it is necessary to strengthen the rich conditions in order to decompose and reduce nitrates when rich, and there is a problem in terms of deterioration of fuel consumption. Further, since the NOx storage-reduction system contains a strong alkaline material in the catalyst, the reducibility at a low temperature is impaired. Therefore, it is considered that there is a problem in applying the NOx storage-reduction system to the aftertreatment technology of a lean combustion engine such as a diesel engine in which the engine exhaust gas temperature decreases due to high fuel efficiency.

Therefore, in order to reduce the disadvantage of the NOx storage-reduction system that the performance deteriorates and the fuel consumption loss is large in the presence of sulfur oxide, or to solve the problem,
(A) (a) ceria or (b) praseodymium oxide or (c) a mixture of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, gadolinium and lanthanum and a composite oxide of at least these two elements And (B) (d) at least one selected from platinum, rhodium, palladium and oxides thereof, and a surface catalyst layer having at least one selected from
(E) A catalyst composed of an internal catalyst layer having a carrier as an internal catalyst component exhibits high performance at low temperatures, and also exhibits nitrogen oxide purification ability close to NOx storage-reduction system and high SOx durability at intermediate temperatures. (See Patent Document 5).

  Further, a surface catalyst layer comprising a first catalyst component selected from rhodium, palladium and oxides thereof, and a second catalyst component selected from zirconia, cerium oxide, praseodymium oxide, neodymium oxide and a mixture thereof, A catalyst comprising an internal catalyst layer containing a third catalyst component selected from rhodium, palladium, platinum and a mixture thereof has been proposed as exhibiting high SOx durability (see Patent Document 6).

  However, in the method using these catalysts, in order to remove nitrogen oxides, as in the NOx storage-reduction system, a rich / lean stroke is required, so that deterioration of fuel consumption cannot be avoided. There is a problem.

  On the other hand, as a catalyst for reducing and removing exhaust gas containing nitrogen oxides generated by burning fuel under lean conditions using carbon monoxide as a reducing agent, alkali metal and / or alkaline earth metal, oxidation There are known metal oxides such as cobalt and noble metals such as rhodium (see Patent Document 7). Similarly, in a method for reducing and removing exhaust gas containing nitrogen oxides generated by burning fuel under lean conditions using carbon monoxide as a reducing agent, the reaction between nitrogen oxides and carbon monoxide In order to improve the selectivity, it is also proposed to coexist with sulfur dioxide (see Patent Document 8).

  Furthermore, as a reducing agent, at least one selected from hydrogen, carbon monoxide, hydrocarbons, and oxygen-containing compounds is used as a catalyst having improved selectivity in the presence of sulfur dioxide, together with iridium or rhodium. A catalyst in which a Group Ia, IIa or IIb metal is supported on silica has been proposed (see Patent Document 9). As a similar catalyst, a catalyst in which rhodium is supported on silica has also been proposed (see Patent Document 10).

  In addition, when a conventionally known rhodium catalyst is used, when only sulfur oxide and carbon monoxide as a reducing agent component are present in exhaust gas, nitrogen oxide is hardly reduced under lean conditions. In the case of using a catalyst having rhodium supported on a support, preferably zeolite, in addition to carbon monoxide as a reducing agent component together with sulfur oxide, in the presence of hydrogen, in a wide range of temperatures. It has been proposed that nitrogen oxides can be catalytically reduced at a high selectivity without deteriorating the catalyst and without causing deterioration of fuel consumption (see Patent Document 11).

However, even if these catalysts are used, nitrogen oxides cannot be sufficiently purified and removed under practical conditions depending on the properties of the exhaust gas and the mode of the catalyst structure.
European Patent Application No. 526099 European Patent Application No. 6794427 International Publication No. 93/7363 Specification International Publication No. 93/83833 Specification International Publication No. 02/89977 WO02 / 22255 specification Japanese Patent Laid-Open No. 2-122831 JP-A-2-187131 Japanese Patent Laid-Open No. 2004-73921 JP 2003-33654 A JP 2007-136329 A

  The present invention has been made in order to solve the above-mentioned problems relating to a catalyst and method for catalytically reducing and purifying NOx in exhaust gas, and is produced by burning fuel under lean conditions. The exhaust gas containing NO is brought into contact in the presence of carbon monoxide and hydrogen, or in the presence of carbon monoxide, hydrogen and sulfur oxide, and the fuel consumption is deteriorated without deterioration of the catalyst in a wide range of temperatures. An object of the present invention is to provide a catalyst for catalytic reduction of the nitrogen oxide with high selectivity.

  Furthermore, the present invention provides an exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions as a catalyst in the presence of carbon monoxide and hydrogen, or in the presence of carbon monoxide, hydrogen and sulfur oxides. An object of the present invention is to provide a method for catalytically reducing the nitrogen oxide at a high selectivity at a high temperature without causing deterioration of the fuel consumption at a wide range of temperatures without causing deterioration of the catalyst.

  According to the present invention, exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions is contacted in the presence of carbon monoxide and hydrogen, or in the presence of carbon monoxide, hydrogen and sulfur oxides. A catalyst for catalytic reduction of the nitrogen oxide, at least one mineralizer selected from water, a silicon source, an aluminum source, a rhodium source, an alkali metal hydroxide and an alkaline earth metal hydroxide, Hydrothermal treatment of an aqueous mixture containing a salting-out effect agent comprising an alkali metal salt, a hydrogen ion concentration regulator comprising an acid, and at least one type selected from a tetraethylammonium source and a tetrapropylammonium source, followed by washing, Contains alkali metal (and alkaline earth metal) -ZSM-5 loaded with 0.5-5.0 wt% rhodium obtained by drying and firing Medium is provided.

  Further, according to the present invention, the alkali metal (and alkaline earth metal) -ZSM-5 supporting rhodium is ion-exchanged with ammonium ions, and then washed and dried. A catalyst comprising H-ZSM-5 loaded with 0.5-5.0 wt% rhodium obtained by calcination is provided.

  According to the present invention, in the preparation of ZSM-5 carrying rhodium, a salting-out effect agent comprising an alkali metal salt and an aqueous solution containing water contain a hydrogen ion concentration regulator, a mold agent and an aluminum source. It is preferable to prepare an aqueous mixture by adding an aqueous solution and an aqueous solution containing a silicon source and a mineralizer to obtain a mixture, and then adding a rhodium source to the mixture.

  Furthermore, according to the present invention, an exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions is placed in the presence of carbon monoxide and hydrogen, or in the presence of carbon monoxide, hydrogen and sulfur oxides. In the method for catalytically reducing a nitrogen oxide by contacting with a catalyst, the catalyst is at least one selected from water, a silicon source, an aluminum source, a rhodium source, an alkali metal hydroxide and an alkaline earth metal hydroxide. Hydrothermal treatment of an aqueous mixture containing at least one type of agent selected from a mineralizing agent, a salting out effect agent comprising an alkali metal salt, a hydrogen ion concentration regulator comprising an acid, and a tetraethylammonium source and a tetrapropylammonium source Thereafter, it is obtained by washing, drying and firing, and an alkali metal (and alkaline earth metal) carrying 0.5 to 5.0% by weight of rhodium. ) -ZSM-5 is intended to include a method is provided.

  Further, according to the present invention, the exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions is placed in the presence of carbon monoxide and hydrogen, or in the presence of carbon monoxide, hydrogen and sulfur oxides. In a method for catalytically reducing a nitrogen oxide by contacting with a catalyst, the catalyst is an alkali metal (and alkaline earth metal) -ZSM-5 with the rhodium supported thereon (and alkaline earth metal). A method is provided comprising H-ZSM-5 loaded with 0.5 to 5.0 wt% rhodium obtained by ion exchange with ammonium ions followed by washing, drying and calcination.

  In the method of catalytic reduction of nitrogen oxides in exhaust gas using such a catalyst according to the present invention, the exhaust gas is 0.1 to 1.5% carbon monoxide and 0.1 to 0.5% hydrogen. Or 0.1 to 1.5% carbon monoxide, 0.1 to 0.5% hydrogen and 1 ppm or less of sulfur oxide.

Furthermore, in the method for catalytically reducing nitrogen oxides in the exhaust gas using such a catalyst according to the present invention, the exhaust gas is contacted with the catalyst at a temperature of 200 to 500 ° C. and a space velocity of 5000 to 150,000 h ⁻¹ . preferable.

  According to the present invention, an exhaust gas containing nitrogen oxides produced by burning a fuel under lean conditions in the presence of carbon monoxide and hydrogen or in the presence of carbon monoxide, hydrogen and sulfur oxide is described above. By contacting with the catalyst, the nitrogen oxides in the exhaust gas are contact-reduced with low fuel consumption loss and high selectivity without deterioration of the catalyst in a wide range of temperatures, thus purifying the exhaust gas. be able to.

  The catalyst according to the present invention contacts an exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions in the presence of carbon monoxide and hydrogen or in the presence of carbon monoxide, hydrogen and sulfur oxides. A catalyst for catalytic reduction of the nitrogen oxide, at least one mineralizer selected from water, a silicon source, an aluminum source, a rhodium source, an alkali metal hydroxide and an alkaline earth metal hydroxide, It is obtained by hydrothermally treating an aqueous mixture containing a salting-out effect agent comprising an alkali metal salt, a hydrogen ion concentration regulator comprising an acid, and at least one type selected from a tetraethylammonium source and a tetrapropylammonium source. It contains ZSM-5 carrying 0.5 to 5.0 wt% rhodium.

  In the present invention, the lean condition means a state in which the air / fuel ratio (air-fuel ratio) of the fuel in question is larger than the stoichiometric air / fuel ratio, and conversely, the rich condition is a problem. A state in which the air / fuel ratio of the fuel is smaller than the stoichiometric air / fuel ratio. In a normal gasoline engine vehicle, the stoichiometric air / fuel ratio is about 14.5.

  When catalytically reducing nitrogen oxides in exhaust gas using a reducing agent in the presence of a conventionally known rhodium catalyst, when only carbon monoxide is present as a reducing agent component in the exhaust gas, Nitrogen oxides are hardly reduced and purified.

  However, according to the present invention, by using a catalyst containing rhodium-supported ZSM-5 obtained using a synthetic raw material for ZSM-5 together with a rhodium salt as a rhodium source, as a reducing agent component in the exhaust gas. In addition to carbon monoxide, when hydrogen is present, nitrogen oxides can be reduced with high selectivity in a wide temperature range without deterioration of the catalyst and without deterioration of fuel consumption. The reason why the reduction reaction of nitric oxide on the catalyst according to the present invention is greatly improved when hydrogen is present in addition to carbon monoxide as a reducing agent component is that CO adsorbed on rhodium is absorbed by hydrogen. This is probably due to the accelerated dissociative adsorption of nitric oxide on the metal rhodium.

According to the present inventors, a rhodium salt and a raw material for synthesizing ZSM-5 are subjected to hydrothermal reaction to synthesize ZSM-5, and this is mixed with rhodium, which is calcined in air at 500 ° C. for 5 hours. Then, rhodium-supported ZSM-5 (hereinafter referred to as rhodium-supported ZSM-5) was prepared, and the chemical state analysis of the rhodium was performed by X-ray photoelectron spectroscopy (XPS). As a result, in the rhodium-supported ZSM-5 obtained by firing in air, the chemical state of rhodium is Rh ³⁺ , but after being used for catalytic reduction of exhaust gas containing nitrogen oxides, Rh ⁰ was confirmed. That is, rhodium supported on ZSM-5 is in a metal state under lean conditions, and nitric oxide adsorbed on the metal rhodium is dissociatively adsorbed to generate nitrogen and oxygen, thus It has been found that oxygen accumulated in the rhodium-supported ZSM-5 reacts with a reducing agent such as CO and hydrogen to produce carbon dioxide and water, thus forming a catalytic cycle.

Even in the case where a large amount of oxygen is contained in the exhaust gas, the chemical state of rhodium supported on ZSM-5 is mainly Rh ^0. When nitric oxide can be adsorbed, nitric oxide is dissociated and adsorbed, and nitric oxide is converted into nitrogen and oxygen. And the produced | generated oxygen reacts with a reducing agent component, and produces | generates a carbon dioxide and water. However, when the reducing agent is only CO, since rhodium is covered with CO, the progress of this nitric oxide reduction reaction is suppressed. However, when hydrogen coexists, a clean rhodium surface is formed by the cleaning action of hydrogen, and nitrogen monoxide is adsorbed thereto, so that the nitric oxide reduction reaction proceeds more smoothly.

  According to the present invention, the sulfur oxide preferentially coats the surface of the rhodium-supported ZSM-5 as described above, and suppresses oxidation of the reducing agents CO and hydrogen by gas phase oxygen, so that the reaction temperature is low. Even when the temperature is high, a high NOx purification rate can be obtained, that is, the temperature of the reduction reaction of nitrogen oxides by CO and hydrogen shifts to the high temperature side. Instead, the NOx purification rate in the low temperature range decreases. In particular, in the reaction in which nitrogen oxides in exhaust gas are catalytically reduced using rhodium-supported ZSM-5 according to the present invention as a catalyst, when the exhaust gas contains sulfur oxide, the amount of sulfur oxide in the exhaust gas is: A range of 1 ppm or less is preferable.

  That is, in the present invention, when the amount of sulfur oxide in the exhaust gas is in the range of 1 ppm or less, the NOx purification rate is not lowered, but rather the oxidation of CO and hydrogen as reducing agents is suppressed. There is an advantage that the selective reactivity between NOx and these reducing agents is improved. From this viewpoint, the amount of sulfur oxide in the exhaust gas is preferably in the range of 0.01 to 1 ppm. When the amount of sulfur oxide in the exhaust gas exceeds 1 ppm, the NOx purification rate decreases.

  The rhodium-supported ZSM-5 as described above is obtained by hydrothermal synthesis of ZSM-5 using a rhodium salt as a rhodium source and a synthesis raw material for ZSM-5, and rhodium is formed in the formed ZSM. Supported on clusters on -5. Thus, when rhodium is supported on ZSM-5 in a cluster shape, rhodium is supported in the form of fine particles having a particle size of about 5 to 10 nm. The particle size of the rhodium fine particles on such a carrier can be measured, for example, by observation with a transmission electron microscope.

  In the present invention, the amount of rhodium supported in the rhodium-supported ZSM-5 is usually in the range of 0.1 to 5.0% by weight, but preferably in the range of 0.1 to 2.5% by weight. In the rhodium-supported ZSM-5, when the supported amount of rhodium is less than 0.1% by weight, when the obtained rhodium-supported ZSM-5 is used as a catalyst, the catalytic reduction ability of nitrogen oxides is not sufficient, When the amount of rhodium supported is more than 5.0% by weight, the resulting rhodium-supported ZSM-5 has a strong oxidizing ability, and when used as a catalyst, the selective reducing ability of nitrogen oxides is lowered, which is not preferable.

  As described above, rhodium-supported ZSM-5 is obtained using a synthetic raw material for ZSM-5 together with a rhodium salt as a rhodium source. The production method will be described in detail later. As is apparent from the production method, rhodium is first supported on the rhodium salt by subjecting the synthetic raw material for the ZSM-5 and ZSM-5 to a hydrothermal reaction. In addition, an alkali metal (and alkaline earth metal) -type ZSM-5 containing an alkali metal ion and optionally an alkaline earth metal ion is obtained. That is, the catalyst according to the present invention obtained using a rhodium salt as a rhodium source and a synthetic raw material for ZSM-5 is rhodium-supported alkali metal (and alkaline earth metal) -ZSM-5. In the present invention, alkali metal (and alkaline earth metal) -ZSM-5 means alkali metal-ZSM-5 or alkali metal and alkaline earth metal-ZSM-5. Similarly, alkali metal (and alkaline earth metal) means alkali metal or alkali metal and alkaline earth metal.

  Thus, the alkali metal ion (and alkaline earth metal ion) in the alkali metal (and alkaline earth metal) -ZSM-5 having an alkali metal ion and optionally an alkaline earth metal ion together with rhodium. The amount is usually in the range of 0.1 to 7.5% by weight, most often in the range of 1 to 5% by weight.

  However, the catalyst according to the invention is preferably rhodium-supported H-ZSM-5. Such rhodium-supported H-ZSM-5 can be obtained by exchanging ammonium ions of those metal ions of the rhodium-supported alkali metal (and alkaline earth metal) -ZSM-5 and then firing in air. Such a rhodium-supported H-ZSM-5 has a wider window (reaction temperature range) and is excellent in reaction selectivity, that is, NOx purification performance.

  The production of rhodium-supported ZSM-5, which is a catalyst according to the present invention, will be described below. According to the present invention, rhodium-supported ZSM-5 is composed of at least one mineralizer selected from water, a silicon source, an aluminum source, a rhodium source, an alkali metal hydroxide and an alkaline earth metal hydroxide, an alkali metal salt. It is obtained by hydrothermally treating an aqueous mixture containing a salting-out effect agent, a hydrogen ion concentration regulator comprising an acid, and at least one type selected from a tetraethylammonium source and a tetrapropylammonium source.

In the present invention, as the silicon source, water glass which is an aqueous solution containing a silica component is preferably used. Specific examples of such water glass include water glass No. 3 (SiO ₂ content 29.1%, Na ₂ O content 9.3%). As the aluminum source, water-soluble aluminum sulfate or sodium aluminate is preferably used. As the rhodium source, a water-soluble rhodium salt is used, and among them, rhodium nitrate is preferably used.

  The mineralizer is an alkali for dissolving the silica and forming cation sites in the obtained ZSM-5, and in the present invention, an alkali metal hydroxide is preferably used. An alkaline earth metal hydroxide is used in combination with an alkali metal. However, when water glass is used as the silica source, it is not necessary to newly add a mineralizer because the water glass contains an alkali metal hydroxide. Also. As the alkali metal hydroxide, at least one selected from potassium hydroxide and sodium hydroxide is preferably used. When the alkaline earth metal is present in the ZSM-5 crystal, an alkaline earth metal hydroxide such as barium hydroxide or barium nitrate is preferably used.

  Thus, according to the present invention, according to the required characteristics of the obtained rhodium-supported ZSM-5, for example, by using a combination of potassium hydroxide and sodium hydroxide, the proportion of the cation sites of ZSM-5 is appropriately set. Rhodium-supported Na and K-ZSM-5 prepared by adjusting to the above can be obtained. Further, according to the required characteristics of the obtained rhodium-supported ZSM-5, for example, by using barium hydroxide in combination with potassium hydroxide and / or sodium hydroxide, the cation site of ZSM-5 can be used. A part can be occupied by barium ions. That is, rhodium-supported alkali metal and barium-ZSM-5 can be obtained.

  In addition, the mold has an effect of regulating the structure of the obtained ZSM-5 during the hydrothermal treatment of the aqueous mixture, and is also called a templating agent or a structured regulating agent. In the invention, as the mold agent, at least one tetraalkylammonium source selected from a tetraethylammonium source and a tetrapropylammonium source is used. The tetraalkylammonium source may be a tetraalkylammonium hydroxide or a tetraalkylammonium halide. The halogen is preferably chlorine or bromine. In the present invention, usually, tetrapropylammonium bromide or tetraethylammonium hydroxide is preferably used as the mold agent.

  Furthermore, in the method of the present invention, a salting-out effect agent comprising an alkali metal salt such as sodium chloride, potassium chloride, lithium sulfate, rubidium nitrate or cesium acetate is obtained in order to promote crystallization during the hydrothermal reaction. In order to adjust the pH of the resulting aqueous mixture to 9 to 11, a hydrogen ion concentration regulator comprising an acid such as sulfuric acid, hydrochloric acid, nitric acid, acetic acid, citric acid or the like is used. However, the salting-out effect agent and the hydrogen ion concentration regulator are not limited to the above examples.

  According to the present invention, by hydrothermally treating an aqueous mixture containing the above-described silicon source, aluminum source, rhodium source, salting-out effect agent, hydrogen ion concentration regulator, mineralizer and mold, Thus, rhodium-supported ZSM-5 can be obtained. Here, the method for preparing the aqueous mixture is not particularly limited, but contains a salting-out effect agent composed of an alkali metal salt and an aqueous solution containing water containing a hydrogen ion concentration regulator, a mold agent and an aluminum source. It is preferable to prepare an aqueous mixture by adding an aqueous solution containing a silicon source and a mineralizing agent to obtain a mixture, and then adding a rhodium source to the mixture.

  Furthermore, according to the present invention, the water, silicon source, aluminum source, rhodium source, mineralizer, mold agent, salting-out effect agent, and hydrogen ion concentration modifier that form the aqueous mixture have the following molar ratio to silica:

SiO ₂ = 1
Al ₂ O ₃ = 0.01 to 0.2
Rhodium = 0.001 to 0.03
Water = 1-50
Mold = 0.01-1
Mineralizer = 0.005-0.05
Salting out effect agent = 0.1-3
Hydrogen ion concentration regulator: 0.05 to 0.15

ZSM-5 having high crystallinity carrying rhodium can be obtained in a high yield.

When the molar ratio of Al ₂ O ₃ in the water-soluble mixture is smaller than 0.01, since the number of cation sites of ZSM-5 obtained is small, the dispersibility of rhodium supported on ZSM-5 is poor. However, when the molar ratio of Al ₂ O ₃ is larger than 0.2, ZSM-5 skeleton formation does not proceed in hydrothermal treatment, and rhodium-supported ZSM-5 with high crystallinity cannot be obtained.

  When the rhodium molar ratio is less than 0.001, rhodium is highly dispersed in the obtained ZSM-5, but the supported amount of rhodium is small, the formation of cluster rhodium is insufficient, and the catalyst function is high. Rhodium-supported ZSM-5 cannot be obtained. On the other hand, when the molar ratio of rhodium is larger than 0.03, the dispersibility of clustered rhodium in the obtained rhodium-supported ZSM-5 decreases, and as a result, the obtained rhodium-supported ZSM-5 has a high catalytic function. No. Furthermore, the presence of a high concentration of rhodium inhibits the formation of a crystal structure under hydrothermal treatment of ZSM-5, and as a result, rhodium-supported ZSM-5 with high crystallinity cannot be obtained.

  When the molar ratio of water is less than 1, the dissolution rate of silica is low and ZSM-5 crystals are not sufficiently formed under hydrothermal treatment. As a result, rhodium-supported ZSM-5 having excellent crystallinity cannot be obtained. However, when the molar ratio of water is greater than 50, the hydrolytic heat treatment has a high silica dissolution rate, and the ZSM-5 crystal formation formed under the equilibrium reaction of silica dissolution / precipitation does not proceed well. As a result, rhodium-supported ZSM-5 with high crystallinity cannot be obtained.

  When the molar ratio of the mold is less than 0.1, the structure regulating effect by the mold during hydrothermal treatment is insufficient, and the skeletal formation of ZSM-5 does not proceed well. High rhodium-supported ZSM-5 cannot be obtained. However, even if the molar ratio of the mold is larger than 1, the formation of the skeleton of the rhodium-supported ZSM-5 at the time of hydrothermal treatment does not change greatly, which is not preferable because it increases the manufacturing cost.

  When the molar ratio of the mineralizer is smaller than 0.005, the function as a mineralizer, that is, the function of controlling the equilibrium of dissolution / precipitation of silica during hydrothermal treatment does not work sufficiently. Sometimes rhodium-supported ZSM-5 does not progress well. On the other hand, when the molar ratio in the aqueous mixture is larger than 0.05, the dissolution / precipitation equilibrium of silica is biased toward the dissolution side under hydrothermal treatment, so that rhodium-supported ZSM-5 with high crystallinity cannot be obtained. .

  When the molar ratio of the salting-out effect agent is smaller than 0.1, the balance between precipitation and dissolution of silica and alumina is biased toward the dissolution side, and when the molar ratio is 3 or more, the balance is biased toward the production side. In any case, the function of controlling the equilibrium of dissolution / precipitation does not work sufficiently, and as a result, the skeleton formation of rhodium-supported ZSM-5 does not proceed well during hydrothermal treatment.

  When the molar ratio of the hydrogen ion concentration regulator is 0.05 or less, the balance between precipitation and dissolution of silica and alumina is biased toward the dissolution side, and when the molar ratio is 0.15 or more, the balance is biased toward generation, As a result, in any case, the function of controlling the equilibrium of dissolution / precipitation does not work sufficiently, and as a result, the skeleton formation of rhodium-supported ZSM-5 does not proceed well during hydrothermal treatment.

  According to the method of the present invention, ZSM-5 in which the target rhodium is highly dispersed and supported can be obtained by hydrothermally treating the aqueous mixture as described above. The temperature of the hydrothermal treatment is in the range of 140 to 180 ° C. When the hydrothermal treatment temperature is lower than 140 ° C., rhodium-supported ZSM-5 with high crystallinity cannot be obtained. However, when the temperature of the hydrothermal treatment is higher than 180 ° C., the mold is thermally decomposed, and the amount of the mold in the aqueous mixture is reduced. Therefore, the ZSM-5 structure is well formed during the hydrothermal treatment. As a result, rhodium-supported ZSM-5 with high crystallinity cannot be obtained.

  In the present invention, the hydrothermal treatment time is usually about 40 to 70 hours, although it depends on the temperature of the hydrothermal treatment and the composition of the aqueous mixture to be hydrothermally treated. When the hydrothermal treatment time is too short, formation of the ZSM-5 skeleton formed according to the molecular structure of the mold during the hydrothermal treatment is insufficient, and rhodium-supported ZSM-5 having a high degree of crystallinity cannot be obtained. However, when the hydrothermal treatment time is too long, the formed ZSM-5 crystal structure is once dissolved again, and the yield of rhodium-supported ZSM-5 is lowered.

  In this way, when the crystallization process by hydrothermal treatment is completed, in order to prevent re-dissolution of the obtained rhodium-supported ZSM-5, preferably, the obtained reaction mixture is quenched and then rhodium-supported ZSM-5 is used. -5 The crystals are separated from the mother liquor and washed thoroughly to remove free alkali metal ions and alkaline earth metal ions present in the crystals. Next, after drying the rhodium-supported ZSM-5 crystal, it is fired in air at 500 to 700 ° C. for an appropriate time, for example, about 5 to 10 hours, and the mold is burned and removed, whereby the used mineralizer is used. Rhodium-supported alkali metal (and alkaline earth metal) -ZSM-5 having a cation site corresponding to the above can be obtained. For example, when sodium hydroxide and potassium hydroxide are used in combination as mineralizers, rhodium-supported Na and K-ZSM-5 can be obtained.

  According to the present invention, the rhodium-supported alkali metal (and alkaline earth metal) -ZSM-5 thus obtained is immersed in, for example, an aqueous ammonium nitrate solution, and the above alkali metal ions (and alkaline earth metal ions) are obtained. After ion exchange with ammonium ions, rhodium-supported H-ZSM-5 can be obtained by baking in air at a temperature of 500 to 700 ° C. for 5 to 10 hours.

  According to the present invention, when the above-mentioned rhodium-supported ZSM-5 is used as a catalyst to reduce the nitrogen oxides in the exhaust gas by contact, the nitrogen oxides in the exhaust gas are composed of the rhodium catalyst as described above and a reducing agent component. The concentration of carbon monoxide as a reducing agent component in the exhaust gas is 0 so that it is reduced with high selectivity in the presence of certain carbon monoxide and hydrogen or in the presence of carbon monoxide, hydrogen and sulfur oxide. In the range of 0.1 to 1.5%, preferably in the range of 0.5 to 1.0%, and the hydrogen concentration is in the range of 0.1 to 0.5%, preferably 0.25 to 0. It is desirable to supply the fuel to the combustion chamber of the engine and burn it so that it is in the range of 5%. Furthermore, according to this invention, as above-mentioned, when exhaust gas contains a sulfur oxide, the density | concentration is the range of 1 ppm or less, Preferably, it is the range of 0.01-1 ppm.

The temperature at which the exhaust gas is brought into contact with the catalyst according to the present invention, i.e., the reaction temperature, depends on the composition of the exhaust gas. It is the range of -500 degreeC, Preferably, it is the range of 250-400 degreeC. At such a reaction temperature, the exhaust gas is preferably treated at a space velocity in the range of 5000 to 150,000 h ⁻¹ .

  The rhodium-supported ZSM-5 used as a catalyst in the present invention can be obtained in various forms such as powders and granules. Therefore, it is possible to form catalyst structures having various shapes such as honeycombs, annular materials, spherical materials, and the like, using such a catalyst by any method that has been well known. Further, in the production of such a catalyst structure, appropriate additives such as molding aids, reinforcing materials, inorganic fibers, organic binders and the like can be used as necessary.

  In particular, according to the present invention, the catalyst described above is made into a slurry together with a binder component, and this is applied to the surface of an inert base material for support having an arbitrary shape by, for example, a wash coat method. It is advantageous to use it as a catalyst structure. The inert base material may be made of, for example, a clay mineral such as cordierite or a metal such as stainless steel, preferably a heat-resistant metal such as Fe-Cr-Al. The shape may be a honeycomb, an annular shape, a spherical structure, or the like.

All of such catalyst structures are not only excellent in resistance to heat but also excellent in resistance to sulfur oxides. Therefore, the method of the present invention using such a catalyst structure is applied to a diesel engine. It is suitable for reducing and purifying NOx in the exhaust gas of automobiles and lean gasoline engines.

Hereinafter, the present invention will be described in detail by way of examples of production of catalyst structures and reduction of nitrogen oxides using the same, but the present invention is not limited to these examples.
(1) Production of catalyst structure

Example 1
Aluminum sulfate 16 hydrate (Al ₂ (SO ₄ ) ₃ .16H ₂ O) 11.3 g, sulfuric acid (95% or more) 11 g, tetrapropylammonium bromide ((n-C ₃ H ₇ ) ₄ NBr) 21 g and ions A solution A was prepared by mixing 153 g of exchange water. A liquid B was prepared by mixing 185 g of water glass No. 3 (manufactured by Nippon Chemical Industry Co., Ltd., SiO ₂ amount 29.1%, Na ₂ O amount 9.3%) and 118 g of water. Further, 70 g of sodium chloride was dissolved in 270 g of water to prepare solution C. Using these A liquid, B liquid and C liquid, 1 wt% rhodium-supported Na-ZSM-5 (SiO ₂ / Al ₂ O ₃ molar ratio = 50, sodium content 4.80 wt%, 47.4 g) (surface area 413 m ^{2 /} g) was prepared.

(A) The A liquid and the B liquid were gradually dropped into the C liquid, and during this time, the dropping speed of the A liquid and the B liquid was set to about A: B = 1: 2 so that the mixed solution did not solidify. The pH of the obtained mixed solution was 10.2.
(B) To the obtained mixed solution, 13.45 g of an aqueous rhodium nitrate solution (4.138 wt% as rhodium) was added with stirring, followed by stirring for 30 minutes to obtain a slurry.
(C) The obtained slurry was charged into an autoclave, and hydrothermal synthesis was performed at a temperature of 160 ° C. and a stirring speed of 60 rpm for 50 hours.
(D) The obtained reaction product was taken out from the autoclave as a slurry, washed with 5 L of hot water at 100 ° C., and then filtered to obtain a solid.
(E) This solid was washed and filtered until no chlorine ions were detected in the filtrate. The finally obtained solid was dried at 110 ° C. overnight.
(F) The dried material thus obtained was calcined in air at 500 ° C. for 5 hours to obtain a 1 wt% rhodium-supported Na-ZSM-5 powder.

  Thus, a rhodium salt and a raw material for synthesis of ZSM-5 are subjected to a hydrothermal reaction, and a rhodium-supported Na-ZSM-5 transmission electron micrograph of rhodium supported in a cluster form on ZSM-5. Is shown in FIG.

  The rhodium 1 wt% supported Na-ZSM-5 powder 3 g, silica sol (Nissan Chemical Industry Co., Ltd. Snowtex N, 20 wt% as silica, hereinafter the same) 0.8 g and an appropriate amount of water were mixed. Using a few grams of zirconia balls as a grinding medium, the mixture was shaken by hand to loosen the agglomerated powder to obtain a slurry for wash coating. The wash coat slurry was applied to a honeycomb substrate made of cordierite having 400 cells per square inch, dried, and then fired in air at 500 ° C. for 1 hour to obtain 95 g of catalyst per liter of honeycomb substrate ( A honeycomb catalyst structure including 5 g of silica as a binder (hereinafter the same applies) was obtained.

Example 2
2 wt% rhodium-supported Na-ZSM-5 powder in the same manner as in Production Example 1, except that 26.90 g of an aqueous rhodium nitrate solution (4.138 wt% as rhodium) was used in step (b) of Example 1 (SiO ₂ / Al ₂ O ₃ molar ratio = 50, sodium content 4.94% by weight, surface area 407 m ^{2 /} g) 47.9 g was obtained.

  The rhodium 2 wt% supported Na-ZSM-5 powder 3 g, silica sol 0.8 g and an appropriate amount of water were mixed. Using a few grams of zirconia balls as a grinding medium, the mixture was shaken by hand to loosen the agglomerated powder to obtain a wash coat slurry. The above-mentioned slurry for wash coat was applied to a honeycomb substrate made of cordierite having 400 cells per square inch, dried, and then fired in air at 500 ° C. for 1 hour to obtain 95 g of catalyst per 1 L of honeycomb substrate. A supported honeycomb catalyst structure was obtained.

Example 3
The 2 wt% rhodium-supported Na-ZSM-5 powder obtained in Example 2 was put into 1000 mL of 0.1 N aqueous ammonium nitrate solution, and ammonium ion exchange was performed at 80 ° C. for 3 hours, followed by filtration and washing of ZSM-5. Was repeated 5 times to remove sodium ions eluted in the aqueous ammonium nitrate solution. The ZSM-5 thus obtained was dried and then calcined in air at a temperature of 500 ° C. for 3 hours to obtain 47.0 g of 2 wt% rhodium-supported H-ZSM-5 powder.

  The rhodium 2 wt% supported H-ZSM-5 powder 3 g, silica sol 0.8 g and an appropriate amount of water were mixed. Using a few grams of zirconia balls as a grinding medium, the mixture was shaken by hand to loosen the agglomerated powder to obtain a wash coat slurry. The above-mentioned slurry for wash coat was applied to a honeycomb substrate made of cordierite having 400 cells per square inch, dried, and then fired in air at 500 ° C. for 1 hour to obtain 95 g of catalyst per 1 L of honeycomb substrate. A supported honeycomb catalyst structure was obtained.

Comparative Example 1
To 20 mL of ion-exchanged water, 5 g of NH ₄ —ZSM-5 (Zeolist Catalysts CBV552407, SiO ₂ / Al ₂ O ₃ molar ratio = 50) was added to form a slurry, and this was stirred with a magnetic stirrer. 5 g of 1% by weight as rhodium) was added. After holding at 60 ° C. for 1 hour, NH ₄ —ZSM-5 ammonium ions are ion-exchanged with rhodium ions, dried in air at 100 ° C., and further calcined in air at 500 ° C. for 1 hour. Then, 5 g of powder in which 1% by weight of rhodium was supported on H-ZSM-5 was obtained.

The rhodium 1 wt% supported NH ₄ -ZSM-5 powder 3 g, silica sol 0.8 g and an appropriate amount of water were mixed. Using a few grams of zirconia balls as a grinding medium, the mixture was shaken by hand to loosen the agglomerated powder to obtain a wash coat slurry. The above-mentioned slurry for wash coat was applied to a honeycomb substrate made of cordierite having 400 cells per square inch, dried, and then fired in air at 500 ° C. for 1 hour to obtain 95 g of catalyst per 1 L of honeycomb substrate. A supported honeycomb catalyst structure was obtained.

Comparative Example 2
To 20 mL of ion-exchanged water, 5 g of Na-β zeolite (SiO ₂ / Al ₂ O ₃ molar ratio = 25, specific surface area of 680 m ² / g made by Sued Chemie Catalyst) was added to form a slurry, and this was mixed with nitric acid while stirring with a magnetic stir bar. An aqueous rhodium solution (1% by weight as rhodium) (5.0 g) was added. By holding at 60 ° C. for 1 hour, sodium ions of Na-β zeolite were ion exchanged with rhodium ions. Then, it was dried in air at 100 ° C. and further calcined at 500 ° C. for 1 hour to obtain a powder in which rhodium was supported by 1% by weight on Na-β zeolite.

  3 g of the powder, 0.8 g of silica sol, and an appropriate amount of water were mixed. Using a few grams of zirconia balls as a grinding medium, the mixture was shaken by hand to loosen the agglomerated powder to obtain a slurry for wash coating. The above-mentioned slurry for wash coating is applied to a honeycomb substrate made of cordierite having 400 cells per square inch, dried, and then fired in air at 500 ° C. for 1 hour to give 100 g of catalyst per liter of honeycomb substrate. A supported honeycomb catalyst structure was obtained.

Comparative Example 3
Add 5 g of mesoporous silica (SILFAM manufactured by Nippon Chemical Industry Co., Ltd.) to 20 mL of ion-exchanged water to form a slurry, and add 5 g of an aqueous rhodium nitrate solution (1 wt% as rhodium) while stirring with a magnetic stirrer. Heated at a temperature of 80 ° C. to evaporate to dryness. The obtained dried product was calcined in air at 500 ° C. for 1 hour to obtain a catalyst powder in which 1% by weight of rhodium was supported on mesoporous silica. Using this catalyst powder, a honeycomb catalyst structure carrying 100 g of catalyst per 1 L of honeycomb substrate was obtained in the same manner as in Example 1.

(2) Nitrogen oxide catalytic reduction test The catalyst structures produced in Examples 1 to 3 and Comparative Examples 1 to 3 were 500 ppm NO, 1 ppm SO ₂ , 9% O ₂ and 1.5%, respectively. After aging by contacting with a mixed gas consisting of CO, 0.5% H ₂ , 6% H ₂ O and the balance nitrogen for 5 hours and stabilizing the performance, the following compositions were used with each catalyst. The test gas containing nitrogen oxide was treated under conditions of a temperature of 250 ° C., 300 ° C., 350 ° C. or 400 ° C., and a space velocity of 50000 h ⁻¹ . The concentrations of the reducing agent and sulfur oxide in the test gas used in each example are shown below and in Table 1. The balance is nitrogen. Further, the conversion rate (removal rate) from nitrogen oxides to nitrogen in each example is calculated based on the concentration analysis of NO, NO ₂ and N ₂ O by a FTTR gas analyzer manufactured by Gasmet. did.

(Test gas composition)
NO: 500ppm
SO ₂ : Table 1 O ₂ : 9%
CO: Table 1 H ₂ : Table 1 H ₂ O: 6%

  As is apparent from Table 1, according to the catalyst of the present invention, nitrogen oxides are removed at a high removal rate. However, according to the catalysts of Comparative Examples 1 to 3, the removal rate of nitrogen oxides is generally high. Is low.

It is an example of the rhodium carrying | support ZSM-5 which is a catalyst by this invention, Comprising: It is a transmission electron micrograph which shows the rhodium carry | supported by ZSM-5 in the cluster form.

Claims (13)

  Contacting an exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions in the presence of carbon monoxide and hydrogen, or in the presence of carbon monoxide, hydrogen and sulfur oxides, the nitrogen oxides A catalyst for catalytic reduction of at least one mineralizer selected from water, a silicon source, an aluminum source, a rhodium source, an alkali metal hydroxide and an alkaline earth metal hydroxide, a salt comprising an alkali metal salt Hydrothermal treatment of an aqueous mixture containing a deposition effect agent, a hydrogen ion concentration regulator comprising an acid, and at least one type selected from a tetraethylammonium source and a tetrapropylammonium source, followed by washing, drying and firing. A catalyst comprising alkali metal (and alkaline earth metal) -ZSM-5 loaded with 0.5-5.0% by weight of rhodium obtained by:   Contacting an exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions in the presence of carbon monoxide and hydrogen, or in the presence of carbon monoxide, hydrogen and sulfur oxides, the nitrogen oxides A catalyst for catalytic reduction of at least one mineralizer selected from water, a silicon source, an aluminum source, a rhodium source, an alkali metal hydroxide and an alkaline earth metal hydroxide, a salt comprising an alkali metal salt An aqueous mixture containing a deposition effect agent, a hydrogen ion concentration regulator comprising an acid, and at least one type selected from a tetraethylammonium source and a tetrapropylammonium source is hydrothermally treated, washed, dried, and calcined. , Rhodium-supported alkali metal (and alkaline earth metal) -ZSM-5, and then rhodium-supported alkali metal (and alkaline earth) Metal) -ZSM-5 obtained by ion-exchange of alkali metal (and alkaline earth metal) of alkali metal (and alkaline earth metal) with ammonium ion, followed by washing, drying, and firing, 0. Catalyst comprising H-ZSM-5 loaded with 5-5.0 wt% rhodium.   An aqueous solution containing a salting-out effect agent composed of an alkali metal salt and water and an aqueous solution containing a hydrogen ion concentration modifier, a mold and an aluminum source and an aqueous solution containing a silicon source and a mineralizer were added to obtain a mixture. The catalyst according to claim 1 or 2, wherein a rhodium source is added to the mixture to obtain an aqueous mixture.   The catalyst according to any one of claims 1 to 3, wherein water glass is used as a silicon source and sodium chloride is used as a salting-out effect agent.   The catalyst according to claim 1 or 2, wherein the hydrothermal treatment is carried out at a temperature in the range of 140 to 180 ° C.   The exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions is brought into contact with the catalyst in the presence of carbon monoxide and hydrogen, or in the presence of carbon monoxide, hydrogen and sulfur oxides, and the nitrogen In the method for catalytic reduction of an oxide, the catalyst is at least one mineralizer selected from water, a silicon source, an aluminum source, a rhodium source, an alkali metal hydroxide and an alkaline earth metal hydroxide, an alkali metal salt Hydrothermal treatment of an aqueous mixture containing a salting out effect agent consisting of, a hydrogen ion concentration regulator consisting of an acid and at least one type selected from a tetraethylammonium source and a tetrapropylammonium source, then washing and drying, Contains alkali metal (and alkaline earth metal) -ZSM-5 loaded with 0.5-5.0 wt% rhodium obtained by calcination The methods of.   The exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions is brought into contact with the catalyst in the presence of carbon monoxide and hydrogen, or in the presence of carbon monoxide, hydrogen and sulfur oxides, and the nitrogen In the method for catalytic reduction of an oxide, the catalyst is at least one mineralizer selected from water, a silicon source, an aluminum source, a rhodium source, an alkali metal hydroxide and an alkaline earth metal hydroxide, an alkali metal salt Hydrothermal treatment of an aqueous mixture containing a salting out effect agent consisting of, a hydrogen ion concentration regulator consisting of an acid and at least one type selected from a tetraethylammonium source and a tetrapropylammonium source, then washing and drying, Firing to obtain alkali metal (and alkaline earth metal) -ZSM-5 carrying rhodium, and then carrying the rhodium-supported alkali metal And 0.5 to 5.0% by weight obtained by ion-exchange of alkali metal (and alkaline earth metal) of ZSM-5 with ammonium ion, followed by washing, drying and firing. Of H-ZSM-5 carrying rhodium.   An aqueous solution containing a salting-out effect agent composed of an alkali metal salt and water and an aqueous solution containing a hydrogen ion concentration modifier, a mold and an aluminum source and an aqueous solution containing a silicon source and a mineralizer were added to obtain a mixture. 8. The method according to claim 6 or 7, wherein a rhodium source is added to the mixture to obtain an aqueous mixture.   The method according to any one of claims 6 to 8, wherein water glass is used as a silicon source and sodium chloride is used as a salting-out effect agent.   The method according to claim 6 or 7, wherein the hydrothermal treatment is performed at a temperature in the range of 140 to 180 ° C.   The method according to claim 6 or 7, wherein the exhaust gas contains 0.1 to 1.5% carbon monoxide and 0.1 to 0.5% hydrogen.   The method according to claim 6 or 7, wherein the exhaust gas contains 0.1 to 1.5% carbon monoxide, 0.1 to 0.5% hydrogen, and 1 ppm or less of sulfur oxide. The method according to any one of claims 6, 7, 11 and 12, wherein the exhaust gas is brought into contact with the catalyst at a temperature of 200 to 500 ° C and a space velocity of 5000 to 150,000 h- ¹ .