JP2008212799A - Catalyst for performing catalytic reduction of nitrogen oxide in exhaust gas and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for performing catalytic reduction of nitrogen oxides with high selectivity without deteriorating the catalyst and without causing aggravation of fuel cost in a broad range of temperature by bringing an exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions into contact with sulfur oxides in the presence of carbon monoxide and hydrogen. <P>SOLUTION: The catalyst for performing catalytic reduction of the nitrogen oxide by bringing the exhaust gas containing the nitrogen oxides produced by burning the fuel under lean conditions into contact with the sulfur oxides in the presence of carbon monoxide and hydrogen consists of (A) 0.1-5.0 wt.% of rhodium, (B) 0.1-5.0 wt.% of at least one selected from copper, iron, vanadium, tungsten and molybdenum and (C) a remainder carrier. Preferably, the catalyst contains (D) 0.1-5.0 wt.% of at least one selected from an alkali metal, alkaline earth metal and rare earth element. <P>COPYRIGHT: (C)2008,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, according to the present invention, fuel is burned under lean conditions, and nitrogen oxides in exhaust gas generated by the combustion are composed of sulfur oxides (mainly SO ₂ and SO ₃ , hereinafter referred to as SOx). And a catalyst for catalytic reduction with high selectivity by contacting them in the presence of carbon monoxide and hydrogen. In addition, the present invention provides a high selectivity by combusting fuel under lean conditions and bringing the nitrogen oxides in the exhaust gas produced by the combustion into contact with the catalyst in the presence of SOx, carbon monoxide and hydrogen. It relates to a method for catalytic reduction. 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 containing sulfur oxides are, for example, a method of oxidizing them and then absorbing them in alkali, using ammonia or urea, hydrogen, carbon monoxide or hydrocarbons as a reducing agent, It is removed by a method such as reduction to nitrogen. 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 the 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, copper ion exchange ZSM-5 and H-type (hydrogen type or acid type) zeolite ZSM-5 (SiO ₂ / Al ₂ O ₃ molar ratio = 30 to 40) are considered to be optimal. However, 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 such circumstances, development of a more highly active catalyst 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. (See, for example, Patent Documents 1 and 2). However, since this catalyst has high oxidation activity and low selective reduction activity with respect to nitrogen oxides, it is known that the conversion efficiency of nitrogen oxides into 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 because of 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 support 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. It has been proposed (see Patent Document 5).

  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 mixtures 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).

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

  Therefore, the present inventors have heretofore been known as a result of earnest and research on an effective catalyst composition and reaction conditions for catalytic reduction of nitrogen oxides in exhaust gas generated by fuel combustion under lean conditions. When only the sulfur oxide and the reducing agent component carbon monoxide are present in the exhaust gas, the nitrogen oxide is hardly reduced under lean conditions, preferably a support, preferably a zeolite. A catalyst having rhodium supported thereon, more preferably, rhodium is supported on the zeolite in a highly dispersed manner by an ion exchange method or by synthesizing a zeolite using a rhodium precursor such as rhodium nitrate and a zeolite synthesis raw material. In addition to sulfur oxide, in addition to carbon monoxide, there is more hydrogen in addition to the sulfur oxide. The, at a wide range of temperatures, without the catalyst deterioration, without causing deterioration of fuel efficiency, have found that it is possible to catalytic reduction of nitrogen oxides in high selectivity.

  Furthermore, the present inventors have shown by reaction analysis by in situ FT-IR that, on the support, preferably on the zeolite when hydrogen is present in addition to carbon monoxide as a reducing agent component together with sulfur oxide. A new finding has been obtained that ammonia is produced on a catalyst in which rhodium is supported, and this ammonia is adsorbed on a carrier, and nitrogen oxide is selectively reduced by this ammonia in an oxygen-excess atmosphere.

  Accordingly, the inventors of the present invention have proposed a selective reduction catalyst for nitrogen oxide (hereinafter simply referred to as a selective reduction catalyst or SCR catalyst) using ammonia as a reducing agent on a catalyst in which rhodium is supported on a support, preferably zeolite. In other words, the present invention has been completed by finding that nitrogen oxides can be catalytically reduced more selectively by combining the above.

  Therefore, the present invention provides a catalyst for a wide range of temperatures by contacting exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions in the presence of sulfur oxides, carbon monoxide and hydrogen. An object of the present invention is to provide a catalyst for catalytic reduction of the nitrogen oxide with high selectivity without causing deterioration of fuel consumption without deterioration.

  Furthermore, the present invention provides a catalyst in the presence of sulfur oxides, carbon monoxide, and hydrogen with exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions, over a wide range of temperatures. It is an object of the present invention to provide a method for catalytic reduction of the nitrogen oxide with high selectivity without causing deterioration of the catalyst 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 sulfur oxides, carbon monoxide, and hydrogen, and the nitrogen oxides are catalytically reduced. A catalyst for,
(A) 0.1 to 5.0% by weight of rhodium,
(B) 0.1 to 5.0% by weight of at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum;
(C) A catalyst comprising the balance carrier is provided. Hereinafter, this catalyst is referred to as the first catalyst of the present invention.

According to the present invention, the exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions is contacted in the presence of sulfur oxides, carbon monoxide and hydrogen, and the nitrogen oxides are contacted. A catalyst for reduction,
(A) 0.1 to 5.0% by weight of rhodium,
(B) 0.1 to 5.0% by weight of at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum;
(C) 0.1 to 5.0% by weight of at least one selected from alkali metals, alkaline earth metals and rare earth elements;
(D) A catalyst comprising the balance carrier is provided. Hereinafter, this catalyst is referred to as a second catalyst of the present invention.

  In the catalyst according to the present invention, the rhodium carrier is preferably a zeolite, and the selective catalytic reduction catalyst component carrier is preferably a solid acid.

Furthermore, according to the present invention, an exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions is contacted with a catalyst in the presence of sulfur oxides, carbon monoxide and hydrogen, and the nitrogen oxides In the method for catalytic reduction of the catalyst, the catalyst is (A) 0.1 to 5.0% by weight of rhodium,
(B) 0.1 to 5.0% by weight of at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum;
(C) A method comprising a balance carrier is provided. Hereinafter, the method using the first catalyst as described above is referred to as the first method of the present invention.

Further, according to the present invention, exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions is contacted with a catalyst in the presence of sulfur oxides, carbon monoxide and hydrogen, and the nitrogen oxides In the method for catalytic reduction of the catalyst, the catalyst is (A) 0.1 to 5.0% by weight of rhodium,
(B) 0.1 to 5.0% by weight of at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum;
(C) 0.1 to 5.0% by weight of at least one selected from alkali metals, alkaline earth metals and rare earth elements;
(D) A method comprising a balance carrier is provided. Hereinafter, such a method using the second catalyst is referred to as a second method of the present invention.

  In the method according to the present invention, the rhodium support is preferably a zeolite, and the selective catalytic reduction catalyst component support is preferably a solid acid.

  Further, according to the present invention, the fuel is burned under lean conditions, and the exhaust gas containing nitrogen oxides generated is brought into contact with the catalyst in the presence of sulfur oxides, carbon monoxide and hydrogen, and hydrogen is oxidized. There is provided a method for catalytic reduction of nitrogen oxides in exhaust gas, characterized by reacting with nitrogen to convert to ammonia and reducing nitrogen oxides with ammonia in an oxygen-excess atmosphere.

  In particular, according to the present invention, in the above method, the fuel is burned under lean conditions, and the generated exhaust gas containing nitrogen oxide is brought into contact with the catalyst in the presence of sulfur oxide, carbon monoxide and hydrogen, Hydrogen is reacted with nitric oxide on a rhodium catalyst supported on zeolite to convert it to ammonia, and an oxygen-excess atmosphere on a selective catalytic reduction catalyst consisting of at least one selected from copper, iron, vanadium, tungsten and molybdenum A method for selectively reducing nitrogen oxides with ammonia is provided below.

  In such a method, it is preferable that the rhodium catalyst supported on zeolite further supports at least one selected from alkali metals, alkaline earth metals, and rare earth elements.

In the method of the present invention described above, an exhaust gas containing 0.1 to 1.5% carbon monoxide, 0.1 to 0.5% hydrogen, and 0.1 to 10 ppm sulfur oxide is heated to 200 to 500 ° C. It is preferable to contact the catalyst at a space velocity of 5,000 to 150,000 h ⁻¹ .

  According to the present invention, exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions is brought into contact with the above-described catalyst in the presence of sulfur oxides, carbon monoxide, and hydrogen. At temperature, the nitrogen oxides in the exhaust gas can be catalytically reduced with low fuel consumption loss and high selectivity without deterioration of the catalyst, thus purifying the exhaust gas.

A first catalyst according to the present invention comprises contacting an exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions in the presence of sulfur oxides, carbon monoxide and hydrogen, and A catalyst for catalytic reduction,
(A) Rhodium 0.1 to 5.0 wt%, preferably 0.1 to 2.5 wt%,
(B) 0.1 to 5.0% by weight of at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum;
(C) The balance carrier.

A second catalyst according to the present invention comprises contacting an exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions in the presence of sulfur oxides, carbon monoxide and hydrogen, and A catalyst for catalytic reduction,
(A) 0.1 to 5.0% by weight of rhodium, preferably 0.1 to 1.25% by weight;
(B) 0.1 to 5.0% by weight of at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum;
(C) at least one selected from alkali metals, alkaline earth metals and rare earth elements, 0.1 to 5.0% by weight, preferably 0.25 to 2.5% by weight;
(D) The balance carrier.

  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.

  As described above, when catalytically reducing nitrogen oxides in exhaust gas using a reducing agent in the presence of a conventionally known rhodium catalyst, only sulfur monoxide and carbon monoxide as a reducing agent component are contained in the exhaust gas. When present, nitrogen oxides are hardly reduced and purified under lean conditions.

  However, according to the present invention, the zeolite is preferably supported on a support, preferably a catalyst with rhodium supported on the zeolite, more preferably by ion exchange or using a rhodium precursor such as rhodium nitrate and a zeolite synthesis raw material. By synthesis, the SCR catalyst was supported as an ion and / or oxide in a solid acid such as acid-type zeolite, titanium oxide, titanium oxide-silica, zirconia, etc. together with a catalyst in which rhodium was supported on zeolite in a highly dispersed manner. When the first catalyst comprising the catalyst component is used, there is no deterioration of the catalyst in a wide temperature range when hydrogen is present as a reducing agent component in addition to carbon monoxide as well as sulfur oxide in the exhaust gas. In addition, nitrogen oxides can be reduced at a high selectivity without deteriorating fuel consumption.

  More specifically, since the exhaust gas usually contains a large amount of oxygen, when there is no sulfur oxide in the exhaust gas, the surface of the rhodium catalyst is covered with oxygen and is a reducing agent component in the exhaust gas. Reducing agent components such as carbon monoxide and hydrogen are oxidized with little reaction with nitrogen oxide, and as a result, the reaction selectivity between nitrogen oxide and carbon monoxide is low. However, when sulfur oxide is present in the exhaust gas, even if oxygen is excessively present in the exhaust gas, the sulfur oxide preferentially covers the surface of the rhodium catalyst as described above. There is an improvement in the reaction selectivity between the oxide and carbon monoxide.

  Thus, even when sulfur oxide is present in the exhaust gas, when the reducing agent component in the exhaust gas is only carbon monoxide, nitrogen oxide still cannot react with carbon monoxide with high efficiency. . Here, according to the present invention, when hydrogen coexists with sulfur oxide and carbon monoxide in the exhaust gas, the presence of sulfur oxide and carbon monoxide suppresses oxidation of hydrogen by oxygen. In addition, this hydrogen reacts with nitric oxide on rhodium and is converted to ammonia, and the ammonia thus generated selectively converts nitrogen oxides onto the selective catalytic reduction catalyst component in an oxygen-excess atmosphere. Thus, according to the present invention, the nitrogen oxide removal efficiency is remarkably improved in the exhaust gas atmosphere in which oxygen is excessively present.

  Furthermore, according to the present invention, a reducing agent in exhaust gas is obtained by using a second catalyst that is a combination of at least one selected from alkali metals, alkaline earth metals, and rare earth elements in addition to the first catalyst described above. Oxidation of the reducing agent component such as carbon monoxide and hydrogen, which are components, by gas phase oxygen is suppressed, and the reaction selectivity between nitrogen oxide and carbon monoxide can be further improved.

  That is, according to the present invention, fuel is combusted under lean conditions, and the generated exhaust gas containing nitrogen oxides is brought into contact with the catalyst in the presence of sulfur oxides, carbon monoxide, and hydrogen, and hydrogen is oxidized. It can be converted to ammonia by reacting with nitrogen, and nitrogen oxides can be reduced with the ammonia in an oxygen-excess atmosphere, thus purifying nitrogen oxides in the exhaust gas.

  More specifically, a rhodium catalyst is produced by burning a fuel under lean conditions and bringing the generated exhaust gas containing nitrogen oxide into contact with the catalyst in the presence of sulfur oxide, carbon monoxide and hydrogen, and supporting the catalyst on zeolite. The hydrogen is reacted with nitrogen monoxide to convert it into ammonia, and then oxidized with the ammonia in an oxygen-excess atmosphere on a selective catalytic reduction catalyst made of at least one selected from copper, iron, vanadium, tungsten and molybdenum. The substances can be selectively reduced, and thus nitrogen oxides in the exhaust gas can be purified.

  According to the present invention, ammonia produced by the reaction of hydrogen and nitric oxide on a rhodium catalyst supported on zeolite is adsorbed on the zeolite.

  According to the present invention, as described above, it is preferable to further support at least one selected from alkali metals, alkaline earth metals, and rare earth elements on the rhodium catalyst supported on zeolite.

  Examples of the alkali metal include sodium and potassium, and examples of the alkaline earth metal include magnesium, calcium, strontium, barium and the like. Examples of rare earth elements include lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, ytterbium, and lutetium. However, alkali metals, alkaline earth metals, and rare earth elements are not limited to these examples.

  According to the present invention, as described above, preferably, rhodium is supported on the zeolite, and in such a case, the alkali metal, alkaline earth metal and rare earth element are preferably supported on rhodium. Is contained in the zeolite. That is, according to the present invention, as a preferred embodiment, when an alkali metal, an alkaline earth metal, and a rare earth element are contained in a zeolite crystal structure so that they can be relatively easily ion-exchanged as constituent elements, , They exist as metal ions, while they are present in the form of oxides when supported on zeolites. However, when the catalyst according to the present invention is used for the catalytic reduction of nitrogen oxides in exhaust gas, alkali metals, alkaline earth metals and rare earth elements are catalyzed in the form of sulfite or sulfate by sulfur dioxide in the exhaust gas. It seems to exist inside.

  In the second catalyst according to the present invention, the catalyst component comprising at least one selected from the alkali metals, alkaline earth metals, and rare earth elements is oxidized by reducing the amount of surface oxygen on rhodium under lean conditions. It is a co-catalyst component that has the effect of increasing the selectivity of the reduction reaction of nitrogen oxides by carbon and hydrogen. Therefore, when the amount of the catalyst component is less than 0.5% by weight, the promoter effect by the component is not effectively expressed, so that the nitrogen oxide purification rate is not improved, while 5.0% by weight is reduced. When exceeding, it has a bad influence on the reducing ability of rhodium, and as a result, the nitrogen oxide purification rate falls.

According to the present invention, the alkali metal and alkaline earth metal component in the second catalyst further contains nitrogen oxides in the exhaust gas containing sulfur dioxide so that the reduction performance in the low temperature range does not decrease. In order to improve the catalytic reduction property, it is preferable that neutralization with sulfur oxide is performed in advance. Neutralization treatment with sulfur oxide can be performed, for example, by bringing the catalyst to be used into contact with a gas containing about 10 ppm of SO ₂ that the exhaust gas contains for several hours.

  In the present invention, in order to enhance the selectivity of reduction and removal of nitrogen oxides by a reducing agent, selection of a support for each catalyst component and particularly high dispersion of rhodium on such a support is extremely important. . Therefore, in the present invention, the carrier for rhodium has an ion exchange ability, and the carrier which does not oxidize on the carrier by oxygen of carbon monoxide as a reducing agent component, that is, the consumption of the reducing agent. It is preferred to select a support that does not have surface oxygen involved, so according to the invention, zeolite is preferably used as a support for rhodium.

As such a zeolite, for example, alkali metal zeolite such as K-SUZ-4, Na-Y zeolite, Na-mordenite, Na-β zeolite, Na-ZSM-5 is preferably used. In addition, when rhodium is supported on zeolite to prepare a rhodium catalyst component, NH ₄ -zeolite is also used, but since the preparation of this rhodium catalyst component includes a calcination step, Specifically, it is obtained as an acid-type zeolite carrying rhodium. In particular, in the present invention, β zeolite is preferably used among various zeolite species.

  According to the present invention, the catalyst component in which rhodium is supported on the zeolite is supported by rhodium on the zeolite by an ion exchange method, or the zeolite is hydrothermally synthesized by using a rhodium precursor such as rhodium and a zeolite synthesis raw material. By doing so, it is preferable that rhodium is supported on zeolite in a highly dispersed manner. Thus, when rhodium is supported in a highly dispersed manner on the zeolite, rhodium can be supported in the form of fine particles having a particle size of 2 nm or less. At this time, the amount of rhodium supported on the zeolite is preferably in the range of 0.1 to 2.5% by weight.

  The particle size of such rhodium fine particles on the carrier can be measured, for example, by observation with a transmission electron microscope (Nakajo et al., Applied Catalyst B: Environmental 30 (2001) 209-223). Further, if necessary, rhodium may be supported on a zeolite together with a normal carrier component such as alumina.

  A solid acid such as acid-type zeolite, titanium oxide, titanium oxide-silica, zirconia or the like is used as the carrier for the SCR catalyst component. That is, the SCR catalyst component is supported on the solid acid carrier as ions or oxides. However, preferably, of the SCR catalyst components, copper or iron is supported on the acid-type zeolite. Such copper or iron catalyst components are usually prepared by ion exchange methods. On the other hand, among the SCR catalyst components, vanadium, tungsten or molybdenum is preferably supported on a solid acid such as titanium oxide, titanium oxide-silica, zirconia or the like. Such a catalyst component is usually prepared by an impregnation method or an evaporation to dryness method.

  In the first and second catalysts of the present invention, the amount of rhodium supported on the carrier is in the range of 0.1 to 5.0% by weight of the total of the weight of rhodium and the carrier, preferably as described above. In the range of 0.1 to 2.5% by weight. When the amount of rhodium supported on the carrier is less than 0.1% by weight, even when such a catalyst is used, the selective reduction reaction of nitrogen oxides is insufficient, while the amount of rhodium supported on the carrier When the amount exceeds 5.0% by weight, the oxidation of carbon monoxide and hydrogen occurs, and the nitrogen oxide purification rate decreases.

  When catalytically reducing nitrogen oxides in exhaust gas using the catalyst according to the present invention, the nitrogen oxides in the exhaust gas are composed of the rhodium catalyst as described above and carbon monoxide as a reducing agent component in the presence of sulfur oxides. The concentration of carbon monoxide as a reducing agent component in the exhaust gas is in the range of 0.1 to 1.5%, preferably 0.5 to 1.0% so that it is reduced with high selectivity by hydrogen. The fuel is supplied to the combustion chamber of the engine and burned so that the hydrogen concentration is in the range of 0.1 to 0.5%, preferably in the range of 0.25 to 0.5%. Is desirable. Furthermore, according to the present invention, the sulfur oxide concentration in the exhaust gas is in the range of 0.1 to 10 ppm, preferably in the range of 1 to 5 ppm.

According to such a method of the present invention, the temperature at which the exhaust gas is brought into contact with the catalyst, that is, the reaction temperature depends on the composition of the exhaust gas. In order to have activity, it is usually in the range of 200 to 500 ° C, and preferably in the range of 250 to 400 ° C. 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 .

  As the catalyst used in the present invention, both the first catalyst and the second catalyst can be obtained in various forms such as powder and granular materials. 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 and production example 1 of catalyst structure
To 20 mL of ion-exchanged water, 5 g of Nad-β zeolite (SiO ₂ / Al ₂ O ₃ molar ratio = 25) made by Sudden Chemie Catalyst was added to form a slurry, which was stirred with a magnetic stirrer, and an aqueous rhodium nitrate solution (Rh as 1). 5% by weight) was added. After maintaining at 60 ° C. for 1 hour to exchange sodium ions of Na-β-zeolite with rhodium ions, drying in air at 100 ° C., followed by firing at 500 ° C. for 1 hour, rhodium 1 A powder in which wt% was supported on Na-type-β zeolite was obtained. The particle diameter of the rhodium fine particles on the zeolite thus obtained was 2 nm or less as observed with a transmission electron microscope. The same was true for the catalysts obtained in the following examples.

On the other hand, 5 g of NH ₄ type-β zeolite (SiO ₂ / Al ₂ O ₃ molar ratio = 25, sodium content 1.57 wt%) made by Sued Chemie Catalyst was added to 20 mL of ion-exchanged water to form a slurry, which was then magnetically stirred. While stirring, 5 g of an iron nitrate aqueous solution (1 wt% as Fe) was added. After holding at 60 ° C. for 1 hour to exchange ammonium ions of NH ₄ -β zeolite with iron ions, drying in air at 100 ° C., followed by firing at 500 ° C. for 1 hour, iron A powder having 1% by weight supported on acid-β zeolite was obtained.

  2.4 g of the above-mentioned rhodium-supported Na type-β zeolite powder 2.4 g of iron 1 wt% of supported acid type-β zeolite powder 0.6 g of silica sol (Nissan Chemical Industry Co., Ltd.) Snowtex N, 20 as silica 0.8 g (% by weight, the same applies hereinafter) 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 coating was applied to a honeycomb substrate made of cordierite having 400 cells per square inch and dried, followed by firing in air at 500 ° C. for 1 hour, and 95 g of catalyst per liter of honeycomb substrate ( In addition, 5 g of a binder containing the binder, the same applies hereinafter) was obtained.

Production Example 2
0.26 g of potassium bromide, 0.14 g of sodium hydroxide and 1.16 g of sodium aluminate were dissolved in 14.4 g of ion-exchanged water, and 22.1 g of tetraethylammonium hydroxide (40% by weight) was stirred into the resulting solution. Added while. Next, 9.9 g of silica powder (white carbon VN-3, manufactured by Tosoh Corporation) was added to this solution little by little while gradually stirring to obtain a slurry. While stirring this slurry, 4.76 g of an aqueous rhodium nitrate solution (rhodium concentration 4.198%) was gradually added thereto. The obtained slurry was put into a Teflon beaker having an internal volume of 200 mL, and charged into an autoclave (model MMJ-200) manufactured by OM Labotech.

The hydrothermal synthesis reaction was carried out at 150 ° C. for 72 hours at a stirring speed of 60 rpm. The autoclave was opened and the product was filtered and washed. At this time, filtration and washing were repeated until the pH of the filtrate was 9.5. The product obtained by filtration was dried at 100 ° C. overnight. The obtained dried product was calcined at 550 ° C. for 5 hours, and 2 wt% rhodium-supported Na, K-type β zeolite powder (SiO ₂ / Al ₂ O ₃ molar ratio = 24, sodium content 0.80 wt%, 9.5 g of potassium content 1.06% by weight) was obtained.

On the other hand, 5 g of NH ₄ type-β zeolite (SiO ₂ / Al ₂ O ₃ molar ratio = 25) made by Sued Chemie Catalyst was added to 20 mL of ion-exchanged water to prepare a slurry, and this was stirred with a magnetic stirrer, and an aqueous copper nitrate solution ( 5 g of 1% by weight as Cu) was added. Holding at 60 ° C. for 1 hour, ammonium ions of NH ₄ -β zeolite were ion-exchanged with copper ions. After drying in air at 100 ° C., firing was performed at 500 ° C. for 1 hour to obtain a powder in which 1% by weight of copper was supported on acid-β zeolite.

  The rhodium 2 wt% supported Na-type zeolite powder 2.4 g, the copper 1 wt% supported acid type-β zeolite powder 0.6 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 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 supported honeycomb catalyst structure was obtained.

Production Example 3
5 g of NH ₄ type-β zeolite (SiO ₂ / Al ₂ O ₃ molar ratio = 25) made by Sued Chemie Catalyst was added to 20 mL of ion-exchanged water to form a slurry, and this was stirred with a magnetic stirrer, and an aqueous rhodium nitrate solution (as Rh) 1%) 5 g was added. After holding at 60 ° C. for 1 hour, the ammonium ions of NH ₄ -β zeolite are ion-exchanged with rhodium ions, dried in air at 100 ° C., and further calcined in air at 500 ° C. for 1 hour. Then, a powder in which 1% by weight of rhodium was supported on acid-β zeolite was obtained.

On the other hand, 5 g of titanium oxide (specific surface area 135 m ² / g) manufactured by Chemilla Co., Ltd. was added to 20 mL of ion-exchanged water to form a slurry. 5 g of ammonium molybdate (1 wt% as Mo) and 5 g of an aqueous solution of ammonium metatungstate (40 wt% as W) were added and evaporated to dryness with stirring. Then, after drying at 100 ° C. in the air, it was fired at 500 ° C. for 1 hour to obtain a titanium oxide powder carrying vanadium 1 wt%, molybdenum 1 wt% and tungsten 8 wt%.

Appropriate amount of titanium oxide powder 0.6 g and silica sol 0.8 g supporting 1 wt% rhodium supported NH ₄ type-β zeolite powder, 1 wt% vanadium, 1 wt% molybdenum and 8 wt% tungsten Of water. 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 liter of honeycomb substrate. A supported honeycomb catalyst structure was obtained.

Production Example 4
0.675 g of caustic barium and 1.16 g of sodium aluminate were dissolved in 14.4 g of ion-exchanged water. To this solution, 22.1 g of tetraethylammonium hydroxide (40 wt%) was added with stirring. Furthermore, 9.9 g of silica powder (white carbon VN-3, manufactured by Tosoh Corporation) was added to this solution little by little while gradually stirring to obtain a slurry. Further, 2.38 g of an aqueous rhodium nitrate solution (4.198% as Rh) was gradually added to the slurry while stirring.

The obtained slurry was put into a Teflon beaker having an internal volume of 200 mL, and charged into an autoclave (model MMJ-200) manufactured by OM Labotech. The hydrothermal synthesis reaction was performed at 150 ° C. for 72 hours at a stirring speed of 60 rpm. The autoclave was opened and the product was filtered and washed. At this time, filtration and washing were repeated until the pH of the filtrate was 9.5. The product obtained by filtration was dried overnight at 100 ° C., and the obtained dried product was calcined at 550 ° C. for 5 hours to obtain a Na, Ba type-β zeolite powder supporting 1% by weight of rhodium ( 9.1 g of SiO ₂ / Al ₂ O ₃ molar ratio = 24, sodium content 0.34 wt% and barium content 4.75 wt%).

On the other hand, 5 g of NH ₄ type ZSM-5 (SiO ₂ / Al ₂ O ₃ molar ratio = 32) manufactured by Zeolis Co., Ltd. was added to 20 mL of ion-exchanged water to form a slurry. 5 g of 1% by weight as Fe) was added. Holding at 60 ° C. for 1 hour, ammonium ions of NH ₄ type ZSM-5 were ion-exchanged with iron ions. After drying at 100 ° C. in air, the powder was fired at 500 ° C. for 1 hour to obtain a powder in which 1% by weight of iron was supported on the acid type ZSM-5.

Rhodium 1 wt% supported Na, Ba type-β zeolite powder 2.4 g, Iron 1 wt% supported NH ₄ type ZSM-5 powder 0.6 g, and Silex Sol (Nissan Chemical Industry Co., Ltd.) Snowtex N, 0.8 g of silica (20% by weight, the same applies hereinafter) 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 liter of honeycomb substrate. A supported honeycomb catalyst structure was obtained.

Production Example 5
In the same manner as in Production Example 3, a powder in which 1 wt% rhodium was supported on NH ₄ -β zeolite was obtained. 3 g of this powder, 20 mL of ion-exchanged water, and 3 g of praseodymium nitrate aqueous solution (1 wt% as Pr) were mixed to form a slurry, which was kept at about 80 ° C. and evaporated to dryness while stirring with a magnetic stirrer. The powder was calcined in air at 500 ° C. for 1 hour to obtain acid-β zeolite powder supporting 1% by weight of rhodium and 1% by weight of praseodymium.

2.4 g of this powder and 0.6 g of 1 wt% iron-supported NH ₄ type ZSM-5 powder used in Production Example 4 were mixed with 0.8 g of silica sol and an appropriate amount of water. 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 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 supported honeycomb catalyst structure was obtained.

  Table 1 shows the components of the respective catalysts obtained in Production Examples 1 to 5.

(2) Examples 1-5
Each of the catalyst structures produced in Production Examples 1 to 8 above was aged by contacting with a gas having the following composition containing 10 ppm of SO ₂ for 5 hours, and after the performance was stabilized, nitrogen oxidation was performed using each catalyst. The mixed gas containing the product was reduced under the 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 mixed gas used in each example are shown below and in Table 1. 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.

Mixed gas composition NO: 100ppm
SO ₂ : Table 2
O ₂ : 9%
CO: Table 2
H ₂ : Table 2
H ₂ O: 6%

Comparative Examples 1 and 2
The mixed gas shown in Table 2 and the above was treated in the same manner as in the above example using the catalyst structure obtained in Production Example 1 or 3. The results are shown in Table 2.

Comparative Example 3
Add 5 g of mesoporous silica (SILFAM manufactured by Nippon Kagaku Kogyo 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 liter of honeycomb substrate was obtained in the same manner as in Example 1. Using this catalyst structure, a mixed gas having the composition shown in Table 2 and above was treated in the same manner as in the example. The results are shown in Table 2.

  As can be seen from Table 2, according to the method of the present invention, nitrogen oxides are removed at a high removal rate. However, according to the methods of Comparative Examples 1 and 2, the removal rate of nitrogen oxides is generally high. Low. In Comparative Example 3, when sulfur oxide coexists in the mixed gas, the nitrogen oxide removal rate at 350 ° C. and 400 ° C. is somewhat improved, but the nitrogen oxide removal rate in the low temperature region is conversely reduced. did. That is, in Comparative Example 3, even when sulfur oxide coexists with carbon monoxide and hydrogen in the mixed gas to be treated, the nitrogen oxide removal rate was not improved.

Example 6
The powder catalyst obtained in Production Example 2 was filled in a diffuse reflection chamber manufactured by Thermo Spectro-Tech, and mixed gases (a) to (g) having the following composition were sequentially added at a rate of 200 mL (STP) / min. Supply to the diffuse reflection chamber and perform infrared spectrophotometric analysis in diffuse reflected light using an FT-IR infrared spectrophotometer (Nicolet AVATAR), thus treating each mixed gas with a catalyst The adsorbed species on the catalyst was analyzed. At this time, the infrared absorption spectra (a) to (g) when 20 minutes have elapsed since the respective mixed gases were supplied to the diffuse reflection chamber so that the infrared absorption in the diffuse reflected light is stabilized are shown in FIG. Shown in

Mixed gas composition (a) NO: 5000 ppm, balance: helium (b) CO: 0.5%, balance: helium (c) CO: 0.5%, O ₂ : 2.5%, balance: helium (d) NO: 5000 ppm, CO: 0.5%, balance: helium (e) NO: 5000 ppm, CO: 0.5%, O ₂ : 2.5%, balance: helium (f) NO: 5000 ppm, CO: 0. 5%, O ₂ : 2.5%, H ₂ : 0.5%, balance: helium (g) NO: 5000 ppm, CO: 0.5%, O ₂ : 2.5%, H ₂ : 0.5 %, SO ₂ : 20 ppm, balance: helium

The adsorbed species on the catalyst by infrared absorption spectrum were identified based on the following literature.
CO ₂ : Journal of Molecular Catalysis A: Chemical, Volume 220, Issue 2, 11 October 2004, pp. 229-237
CO and SO ₃ ^-1 : Applied Catalysis A: General, Volume 268, Issues 1-2, 10 August 2004, pp. 189-197
^{Rh-NO -:. Journal of} Catalysis, Volume 244, Issue 2, 10 December 2006, pp 169-182
NCO: Journal of Catalysis, Volume 239, Issue 1, 1 April 2006, pp. 51-64
(NH ₄ ) ₂ SO ₄ : NIST Standard Reference Database Number 69, June 2005
Went to see.

  As described above, Table 3 shows changes in the infrared absorption spectrum in the diffuse reflected light when the mixed gases (a) to (g) are sequentially supplied in this order to the diffuse reflective chamber filled with the catalyst. In Table 3, (i) after the wave number indicates that the infrared absorption at the wave number is increased compared to the infrared absorption at the same wave number when the mixed gas is treated with the catalyst before the wave number. d) shows that the infrared absorption of the wave number is reduced compared with the infrared absorption of the same wave number when the previous mixed gas is treated with the catalyst.

The following is shown from such a change in the infrared absorption spectrum. That is, corresponding to the composition (a) to (g) of each mixed gas,
(A) Under the NO-He atmosphere, almost no absorption peak derived from NO is observed.
(B) When CO is present instead of NO in the atmosphere, NCO and CO absorption peaks are observed.
(C) When oxygen is further present in the atmosphere, an NCO absorption peak is reduced and a CO ₂ (g) absorption peak appears.
Under (d) He-NO-CO atmosphere, NCO, CO and Rh-NO ^- absorption peak appears.
(E) When oxygen is further present in the He—NO—CO atmosphere, the absorption peak of CO disappears and the absorption peak of NCO decreases.
(F) When hydrogen is further present in the He—NO—CO—O ₂ atmosphere, the NCO yield peak is further reduced and Rh—NO 2 ^— is produced. At this time, in the infrared absorption spectrum, the spectrum of ammonia is not observed because the catalyst used has basicity, and therefore there is no ammonia adsorption site on the catalyst. It is for separating.
(G) When sulfur dioxide is further present in the He—NO—CO—O ₂ atmosphere, an absorption peak of SO ₂ or SO ₃ ⁻ (ammonia absorption peak overlaps) appears together with an absorption peak of ammonium sulfate, Rh—NO 2 ^— and NCO absorption peaks are reduced.

In relation to the above (f), even when SO ₂ is not present, when hydrogen is present in an oxygen-excess atmosphere, that is, in a NO—CO—H ₂ —N ₂ atmosphere, ammonia is present. It was confirmed qualitatively separately as follows.

Using the honeycomb catalyst structure on which the catalyst obtained in Production Example 2 was supported, a fixed bed flow type reaction was performed under the same conditions as described above, and the ammonia concentration in the outlet gas was measured using an FT-IR gas analyzer. As a result, it was confirmed that it was about 200 ppm. That is, in an atmosphere in which SO ₂ does not exist, the catalyst exhibits alkalinity, so ammonia is not adsorbed on the zeolite and is discharged to the gas phase side. However, in an atmosphere in which SO ₂ exists, Since SO ₃ is adsorbed on the cation sites of the zeolite and acidic sites are generated, as a result, ammonia is adsorbed on the catalyst, and the ammonia reacts with oxygen and NO to reduce NO.

  From the above, when the catalyst according to the present invention is used, nitrogen oxides in exhaust gas are catalytically reduced by the following reaction.

NO → Rh-NO ⁻ (1) (NO adsorption on Rh)
NO ⁻ → N (adsorption) + O (adsorption) → N (adsorption) + CO → NCO (adsorption) (2) (NO ⁻ cleavage on Rh and generation of NCO)
1 / 3NCO + 1 / 6O ₂ → 1 / 6N ₂ + 1 / 3CO ₂ (3) (oxidation of NCO)
H ₂ + 2O (adsorption) → H ₂ O (4) (formation of water on Rh)
(In the presence of SO ₂ ) 2/3 NCO (adsorption) + H ₂ O → 2/3 NH ₃ adsorption) +2/3 CO ₂ + 1 / 6O ₂ (5) (production of ammonia and adsorption onto solid acid sites)
2 / 3NO + 2 / 3NH ₃ adsorption) + 1 / 6O ₂ → 2 / 3N ₂ + H ₂ O (6) (selective catalytic reduction of ammonia)

 Thus, according to the present invention, the fuel is burned under lean conditions, and the generated exhaust gas containing nitrogen oxide is brought into contact with the catalyst in the presence of sulfur oxide, carbon monoxide and hydrogen, Ammonia is produced by reacting with nitrogen monoxide, and nitrogen oxides can be selectively and efficiently reduced with the ammonia in an oxygen-excess atmosphere.

  On the other hand, in the case of a conventional catalyst known so far, the following competitive reaction proceeds simultaneously on the catalyst, so the selectivity of the NO reduction reaction by CO is low, and nitrogen oxides are efficiently used. Cannot reduce well.

CO + 1 / 2O ₂ → CO ₂ (7)
NO + CO → 1 / 2N ₂ + CO ₂ (8)

It is an infrared absorption spectrum of the adsorbed species on the catalyst when the mixed gases (a) to (g) having various compositions are treated with the catalyst according to the present invention.

Claims (14)

A catalyst for catalytically reducing nitrogen oxides by contacting exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions in the presence of sulfur oxides, carbon monoxide and hydrogen. ,
(A) 0.1 to 5.0% by weight of rhodium,
(B) 0.1 to 5.0% by weight of at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum;
(C) A catalyst comprising the balance carrier. A catalyst for catalytically reducing nitrogen oxides by contacting exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions in the presence of sulfur oxides, carbon monoxide and hydrogen. ,
(A) 0.1 to 5.0% by weight of rhodium,
(B) at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum in an amount of 0.1 to 5.0% by weight;
(C) 0.1 to 5.0% by weight of at least one selected from alkali metals, alkaline earth metals and rare earth elements;
(D) A catalyst comprising the balance carrier.   The catalyst according to claim 1 or 2, wherein the rhodium support is zeolite.   The catalyst according to claim 1 or 2, wherein the support for the selective catalytic reduction catalyst component is a solid acid. In a method for catalytically reducing nitrogen oxides by contacting exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions with a catalyst in the presence of sulfur oxides, carbon monoxide and hydrogen The catalyst is (A) 0.1 to 5.0% by weight of rhodium,
(B) 0.1 to 5.0% by weight of at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum;
(C) A method comprising the balance carrier. In a method for catalytically reducing nitrogen oxides by contacting exhaust gas containing nitrogen oxides produced by burning fuel under lean conditions with a catalyst in the presence of sulfur oxides, carbon monoxide and hydrogen The catalyst is (A) 0.1 to 5.0% by weight of rhodium,
(B) 0.1 to 5.0% by weight of at least one selective catalytic reduction catalyst component selected from copper, iron, vanadium, tungsten and molybdenum;
(C) 0.1 to 5.0% by weight of at least one selected from alkali metals, alkaline earth metals and rare earth elements;
(D) A method comprising a balance carrier.   The method according to claim 5 or 6, wherein the rhodium support is zeolite.   7. The process according to claim 6, wherein the support for the selective catalytic reduction catalyst component is a solid acid.   The method according to any one of claims 5 to 8, wherein the exhaust gas contains 0.1 to 1.5% carbon monoxide, 0.1 to 0.5% hydrogen, and 0.1 to 10 ppm sulfur oxide. . The method according to any one of claims 5 to 9, wherein 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 .   The fuel is burned under lean conditions, and the generated exhaust gas containing nitrogen oxides is brought into contact with the catalyst in the presence of sulfur oxides, carbon monoxide and hydrogen, and hydrogen is reacted with nitrogen monoxide to convert it to ammonia. A method of catalytically reducing nitrogen oxides in exhaust gas, characterized by reducing nitrogen oxides with ammonia in an oxygen-excess atmosphere.   The fuel is burned under lean conditions, and the generated exhaust gas containing nitrogen oxides is brought into contact with the catalyst in the presence of sulfur oxides, carbon monoxide and hydrogen, and hydrogen is removed on the rhodium catalyst supported on the zeolite. Nitrogen oxide is reacted with nitrogen oxide to convert it to ammonia, and the nitrogen oxide is selectively converted by the ammonia under an oxygen-excess atmosphere on a selective catalytic reduction catalyst consisting of at least one selected from copper, iron, vanadium, tungsten and molybdenum. The method according to claim 1, wherein the reduction is performed.   The method according to claim 12, wherein ammonia is produced on a rhodium catalyst supported on zeolite and adsorbed on the zeolite. The method according to claim 12 or 13, wherein the rhodium catalyst supported on zeolite further supports at least one selected from alkali metals, alkaline earth metals and rare earth elements.

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