JP2012152744A - Selective reduction catalyst for cleaning exhaust gas and exhaust gas cleaning device using the catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for cleaning an exhaust gas which can effectively clean, especially starting from a low temperature time, nitrogen oxide contained in the exhaust gas emitted from a rarefied combustion engine by supplying atomized urea water as a reduction component to a selective reduction catalyst.SOLUTION: This exhaust gas cleaning method is to selectively reduce NOx contained in the exhaust gas emitted from a lean combustion engine with the help of the selective reduction catalyst and ammonia. That is, a urea aqueous solution is supplied by atomization to the selective reduction catalyst containing at least zeolite (A) and a hydrolysis promotion component (B) of urea. Then, the urea aqueous solution is allowed to come into contact with the selective reduction catalyst at 150 to 600°C, and thus, the ammonia is produced at a rate of [NH/NOx=0.5-1.5] for NOx in the exhaust gas in terms of ammonia, causing the decomposition of the nitrogen oxide into nitrogen and water. The zeolite (A) is zeolite containing an iron element and the hydrolysis promotion component (B) is a composite oxide containing at least one selected from the group consisting of titania or titanium, zirconium, tungsten. silicon and alumina.

Description

  The present invention relates to an exhaust gas purifying selective reduction catalyst and an exhaust gas purifying apparatus using the same, and more specifically, by supplying urea water as a reducing component by spraying to the selective reducing catalyst, a boiler, a gas turbine, and a lean burn. The present invention relates to a selective reduction catalyst for purifying exhaust gas that can effectively purify nitrogen oxides contained in exhaust gas from lean combustion engines such as gasoline engines and diesel engines, particularly from low temperatures, and an exhaust gas purifying apparatus using the same.

  The exhaust gas discharged from the lean combustion engine contains various harmful substances derived from fuel and combustion air. Such harmful substances include hydrocarbons (HC), soluble organic fraction (also called SOF), soot, carbon monoxide (CO), nitrogen oxides (NOx), and the like. Regulations on component emissions are getting stronger year by year. As a method for purifying these harmful components, a method for purifying exhaust gas by contacting it with a catalyst has been put into practical use.

  In such a lean combustion engine, it is also considered to control the generation amount of harmful substances by controlling the type, supply amount, supply timing, and air amount of fuel. However, conventional exhaust gases and control methods have not been satisfactory for exhaust gas purification. In particular, in lean combustion engines, nitrogen oxides are likely to be emitted, and in addition, the regulations are becoming increasingly strict. However, with existing NOx purification technologies, the operating conditions of diesel engines installed in automobiles are always the same. Due to the change, it was difficult to control the emission of harmful substances.

Among NOx purification (hereinafter, sometimes referred to as “De-NOx”) technologies, a catalyst is used to oxidize exhaust gas containing NOx in the presence of an ammonia (NH ₃ ) component. A technique for reducing denitration by contacting with a selective reduction catalyst mainly composed of titanium, vanadium oxide, zeolite or the like is known as a selective reduction method or a selective catalytic reduction (hereinafter, also referred to as SCR) method. It has been.

In the SCR using this NH ₃ component as a reducing agent, NOx is finally reduced to N ₂ mainly by the following reaction formulas (1) to (3).
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (1)
2NO ₂ + 4NH ₃ + O ₂ → 3N ₂ + 6H ₂ O (2)
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (3)
In such a denitration catalyst system, NH ₃ gas may be used as a reducing component, but NH ₃ itself has an irritating odor and harmfulness. Therefore, a method has been proposed in which urea water is added as an NH ₃ component from the upstream of the denitration catalyst, NH ₃ is generated by thermal decomposition or hydrolysis, and this is used as a reducing agent to exhibit denitration performance.

The reaction formula for obtaining NH ₃ by such decomposition of urea is as follows.
NH ₂ —CO—NH ₂ → NH ₃ + HCNO (urea pyrolysis)
HCNO + H ₂ O → NH ₃ + CO ₂ (isocyanic acid hydrolysis)
_{NH 2 -CO-NH 2 + H} 2 O → 2NH 3 + CO 2 ( hydrolysis of urea)

In denitration in exhaust gas, theoretically, in the denitration reactions (1) to (3), the NH ₃ / NOx molar ratio may be 1.0, but transient engine operation during operation of the diesel engine is acceptable. obtained when the conditions and, and the space velocity, temperature of the exhaust gas, when the temperature of the catalyst surface are not suitable, the choice but to increase the NH 3 _/ NOx ratio of the NH ₃ component supplied to obtain sufficient denitration performance In some cases, there was a risk that unreacted NH ₃ would leak out and cause secondary pollution such as new environmental pollution. Hereinafter, the leaked NH ₃ may be referred to as slip or NH ₃ slip.

For such NH ₃ slip, it is necessary to arrange an oxidation catalyst in order to oxidize and purify NH ₃ slipped downstream of the SCR catalyst. However, the arrangement of such a catalyst for purifying NH ₃ slip leads to an increase in cost, and it is difficult to secure a catalyst mounting location particularly in an automobile.
Further, when the amount of NH _{3 to be} slipped increases, the catalyst is required to have a high oxidizing ability, and it is necessary to use a large amount of expensive noble metal such as platinum which is an active species.

Further, in the purification of NOx by NH ₃ component, the reaction is promoted in an atmosphere containing approximately half of NO and NO ₂ as in the above formula (3) (Non-patent Document 1). However, most of the NOx component discharged from the lean combustion engine is nitric oxide (NO) (Patent Document 2). For this reason, in order to efficiently purify NOx, it has been proposed to dispose NO oxidation means in the exhaust gas flow path in order to increase the concentration of the NO ₂ component in the exhaust gas (Patent Document 2).
There has also been proposed a method of simultaneously purifying harmful particulate components and NOx with one catalyst system using such NO oxidation means. One of them is that an oxidation catalyst is disposed in the exhaust gas flow path, a filter is disposed in the subsequent stage, an ammonia component is sprayed in the subsequent stage, and an SCR catalyst is disposed in the subsequent stage (Patent Document 3).

Further, in order to promote a reaction between NH ₃ and NOx, plasma is generated in the denitration catalyst unit, a method of promoting a reaction between NH ₃ and NOx to improve the activity of the SCR catalyst has been proposed (Patent Documents 1). Such an exhaust gas purification technology using plasma is a so-called physical process, and the technical field is different from a chemical purification technology of exhaust gas by a general catalyst technology. Also, if plasma is used, the catalyst components may jump out into the gas phase (exhaust gas) and be released into the atmosphere, which may cause new pollution. The growth of the component particles may occur, the specific surface area value of the catalyst component particles may decrease, and the catalyst activity may decrease.
Such a phenomenon caused by plasma is effectively used in other fields as physical vapor deposition (PVD) or sputtering (sputtering), but is practically used in the field of exhaust gas purification catalysts. Was difficult.

  Further, in order to use plasma, a plasma generation device, a control device, and the like are necessary, and it is necessary to study from the viewpoint of cost and safety. Especially for automobiles, it must be able to be miniaturized due to the problem of installation space. Because of these various problems, the use of plasma in the exhaust gas purification catalyst technology is not easy and has not yet become widespread.

As the NH ₃ component used as the reducing component, urea is becoming the mainstream. This urea is spray-supplied as urea water from the upstream of the SCR catalyst. As described above, since NH ₃ mainly contributes to the reduction and purification of NOx, the reaction of NOx in the SCR catalyst is affected by the decomposition efficiency of urea. If the decomposition efficiency of urea is low, not only the efficiency of NOx purification is lowered, but also the amount of urea used increases, and there is a possibility that NH ₃ slip is induced by unreacted urea.

Therefore, in order to efficiently use urea as an NH ₃ component, instead of supplying urea itself to the SCR catalyst, a heating means or a means for decomposing urea into NH ₃ using a catalyst means is provided, resulting in decomposition. A method of supplying NH ₃ to the SCR catalyst has been proposed (Patent Documents 4 and 5). However, in this method, providing a separate decomposition part increases the number of parts, leading to an increase in cost, and the decomposition part may be clogged by the urea component. Patent Document 4 also describes means for improving clogging in the reducing component supply system. However, because of the complexity of the apparatus and the use of superheating means exceeding 450 ° C., there are concerns about cost increase and safety. There is. Further, even if such means is used, clogging in the reducing component supply system cannot be completely prevented, and NOx purification in the exhaust gas cannot be performed stably for a long period of time.

Under such circumstances, automobile manufacturers have been working on NOx purification technology using urea water as a reducing component, and the FLENDS (Friends) system of Nissan Diesel Co., Ltd. has been developed and spread. Accordingly, a 31.8 to 33.3% by weight urea aqueous solution has been standardized and has come to be distributed under the trade name Adblue. In the future, it is predicted that NH ₃ -SCR technology will be developed using this standardized urea water, and an effective NOx purification technology using urea water has been required.

Special Table 2002-538361 (Claim 7, Claim 27, 0008, 0016) JP 5-38420 (Claims 1, 0012, 0013) Special table 2002-502927 Patent No. 3869314 (Claims 1, 0009, 0013) JP 2002-1067 (FIG. 2) Catalysis Today 114 (2006) 3-12 (Left column, page 2)

An object of the present invention is to spray urea supply as a reducing component to a selective reduction catalyst and oxidize nitrogen contained in exhaust gas discharged from a boiler, gas turbine, lean burn gasoline engine, diesel engine, or other lean combustion engine. An object of the present invention is to provide an exhaust gas purifying selective reduction catalyst capable of effectively purifying an object from a low temperature and an exhaust gas purifying apparatus using the same.
The present invention also provides an exhaust gas purification device that purifies HC, CO, soot, and SOF together with NOx by the exhaust gas purification catalyst of the present invention. In the present invention, “soot” and “SOF” are sometimes collectively referred to as combustible particle components.

  As a result of intensive studies in view of the above-described conventional problems, the present inventors have used a SCR catalyst using a specific zeolite and a hydrolysis component of urea, and when the surface temperature is 150 ° C. or higher, nitrogen oxides are used. By supplying a specific concentration of aqueous urea solution together with exhaust gas discharged from a lean combustion engine containing ammonia, and generating ammonia with an SCR catalyst, the nitrogen oxide component in the exhaust gas can be purified with high efficiency. The present invention is completed by finding that it is possible to purify NOx by using a standardized and easily available urea water with a simple configuration without using urea or hydrolyzing urea outside the catalyst system. It came to.

That is, according to the first invention of the present invention, the following zeolite (A) and urea hydrolysis promoting component (B) for selectively reducing NOx in exhaust gas discharged from a lean combustion engine with ammonia are added. Including a selective reduction catalyst for purifying exhaust gas,
An aqueous urea solution is spray-supplied to the selective reduction catalyst and brought into contact at 150 to 600 ° C., and ammonia in a ratio of [NH ₃ /NOx=0.5 to 1.5] with respect to NOx in the exhaust gas in terms of ammonia. And a selective reduction catalyst for purifying exhaust gas, wherein nitrogen oxides are decomposed into nitrogen and water.
Zeolite (A): A zeolite containing an iron element essential to Fe-β type zeolite (A1), the content of which is 80 wt% or more of the total zeolite. Hydrolysis promoting component (B): titania or titanium , Zirconium, tungsten, silicon, or at least one selected from alumina

According to the second invention of the present invention, in the first invention, the zeolite (A) and urea hydrolysis promoting component (B) are the upper catalyst component layer and / or the lower catalyst of the ceramic honeycomb structure. The selective reduction catalyst for exhaust gas purification according to claim 1, which is contained in a component layer.
According to a third aspect of the present invention, there is provided the selective reduction catalyst for exhaust gas purification according to the first aspect, wherein the β-type zeolite (A1) contains 0.1 to 5 wt% of iron element. Provided.
According to the fourth aspect of the present invention, in the first aspect, the zeolite (A) is a β-type zeolite (A2) containing an iron element and a cerium element. A reduction catalyst is provided.
Further, according to the fifth aspect of the present invention, in the fourth aspect, the β-type zeolite (A2) contains 0.05 to 2.5 wt% of cerium element. A catalyst is provided.
According to a sixth aspect of the present invention, the exhaust gas purification method according to the first aspect, wherein the zeolite (A) is an MFI type zeolite (A3) containing an iron element and / or a cerium element. A selective reduction catalyst is provided.
Furthermore, according to the seventh aspect of the present invention, in the first aspect, the weight ratio [(B) / (A)] of the zeolite (A) and the hydrolysis promoting component (B) is 2/100 to 30. The selective reduction catalyst for purifying exhaust gas is provided.

On the other hand, according to the eighth invention of the present invention, in the first invention, the zeolite (A) and urea hydrolysis promoting component (B) are formed into a ceramic honeycomb structure having a cell density of 100 to 1500 cel / inch. The selective reduction catalyst for purifying exhaust gas is provided, which is coated with 55 to 330 g / L per unit volume.
Furthermore, according to the ninth invention of the present invention, in the second invention, the upper catalyst component layer of the ceramic honeycomb structure contains β-type zeolite (A1) containing iron element, and the lower catalyst component layer Contains β-type zeolite (A2) containing iron element and cerium element, and contains a hydrolysis promoting component (B) in at least one of the upper catalyst component layer and the lower catalyst component layer A selective reduction catalyst is provided.

Furthermore, according to the tenth aspect of the present invention, in the eighth or ninth aspect, the upper catalyst component layer of the ceramic honeycomb structure contains 25 to 150 g / L of zeolite (A1), and the lower catalyst component layer Contains 25 to 150 g / L of zeolite (A2), and the hydrolysis promoting component (B) contained in one layer is 2.5 to 15 g / L. A catalyst is provided.
On the other hand, according to the eleventh aspect of the present invention, the exhaust gas passage exhausted from the diesel engine is provided with an oxidizing means, a urea aqueous solution spraying means, and the exhaust gas purification selection according to any one of the first to tenth aspects of the invention. The reduction catalyst is arranged in this order, and includes a platinum component or a palladium component as a noble metal component as the oxidation means, and the amount of the noble metal component is 0.1 to 3 g / L in terms of metal, and in terms of metal in the noble metal component After oxidizing the hydrocarbon component, carbon monoxide, nitrogen monoxide, and nitrous oxide in the exhaust gas with the oxidation catalyst having an amount of 50 to 100% by weight, increasing the nitrogen dioxide concentration, the urea solution spraying means There is provided an exhaust gas purifying apparatus characterized in that an aqueous urea solution is spray-supplied to a selective reduction catalyst and brought into contact at 150 to 600 ° C., and nitrogen oxides are decomposed into nitrogen and water by the generated ammonia.
According to the twelfth aspect of the present invention, in the eleventh aspect, the oxidizing means, the urea aqueous solution spraying means, and the selective reduction catalyst are arranged in this order in the exhaust gas passage exhausted from the diesel engine. In addition, as an oxidation means, a platinum component or a palladium component is included as a noble metal component, the amount of this noble metal component is 0.1 to 3 g / L in terms of metal, and the amount of platinum in terms of metal in the noble metal component is 50 to 100 w% There is provided an exhaust gas purifying apparatus characterized by using a catalyst and a filter and collecting combustible particle components by the filter.

If the selective reduction catalyst for exhaust gas purification of the present invention is used, NOx purification by NH ₃ -SCR can be performed with a simple configuration without using a special urea decomposition mechanism or plasma assist. In particular, the surface of the SCR catalyst can purify NOx in the exhaust gas with high efficiency in a wide temperature range from low temperature to high temperature of 150 to 600 ° C., for example. Further, it is possible to suppress discharge of NH ₃ components that are not used for purification of NOx and slip from the SCR. Therefore, it can be effectively applied even when the installation space of a catalyst such as an automobile is limited.

  Hereinafter, the present invention will be described by taking a diesel engine mainly used for automobiles as an example, but it is not limited to diesel automobile applications, but is used for mobile vehicles such as gasoline automobiles and ships, and stationary generators and the like. Even in other applications such as applications, the present invention can be used for purifying NOx generated by lean combustion.

1. Selective reduction catalyst for exhaust gas purification The selective reduction catalyst for exhaust gas purification of the present invention (hereinafter sometimes referred to as the present catalyst) includes β-type zeolite containing iron element and titania as an essential component for hydrolysis of urea components. And a composition containing an oxide containing at least one of zirconium, tungsten, and silicon as required.

1.1. Zeolite The zeolite used in the present invention is an SCR component, and β-type and MFI-type zeolites having a three-dimensional pore structure are preferable. In particular, β-type zeolite has a relatively complicated three-dimensional pore structure composed of unidirectionally aligned linear pores having relatively large diameters and curvilinear pores intersecting with the pores. And diffusion of gas molecules such as NH ₃ is facilitated.

In addition, zeolite has acid sites that can adsorb basic compounds such as NH ₃ , and the number of acid sites varies depending on the Si / Al ratio. In general, zeolite with a low Si / Al ratio has a large number of acid sites, but the degree of deterioration is large in durability in the presence of water vapor, and on the contrary, zeolite with a high Si / Al ratio is excellent in heat resistance. In this catalyst, NH ₃ is adsorbed on the acid sites of the zeolite, and these serve as active sites to reduce and remove nitrogen oxides such as NO _2, so the one with more acid sites (the one with a lower Si / Al ratio). This is advantageous for the denitration reaction. Thus, regarding the Si / Al ratio, durability and activity are in a trade-off relationship, but considering these, the Si / Al ratio of zeolite is preferably 5 to 500, more preferably 10 to 100, and 15 to 50. Is more preferable. Such characteristics are also possessed by β-type zeolite and MFI-type zeolite.
Zeolite containing an iron element is used as a main component in the zeolite (A) of the present catalyst. Moreover, it is preferable to use the zeolite which the iron element ion-exchanged to the cation site of the zeolite as the zeolite containing such an iron element. Moreover, the iron in which the iron element is ion-exchanged may contain iron oxide as an iron component. The zeolite (A) as the main component is preferably a zeolite containing 80% by weight or more of the total amount of zeolite used in the present catalyst, more preferably 90% by weight or more. Zeolite containing no iron element has a slow NH ₃ adsorption / desorption rate and low activity as an SCR, so it is not desirable to increase the amount of such zeolite.
Further, when the present catalyst is used in multiple layers, the zeolite containing iron element contained in one layer is preferably 50 wt% or more, and 80 wt% or more of the total amount of zeolite used in the entire catalyst is 80 wt% or more. It is preferably a zeolite containing an iron element, and more preferably 90 wt% or more. When using multiple layers, if the amount of zeolite containing no iron element contained in one layer is at most, if the amount of zeolite containing iron element contained in one layer contained in another layer is sufficient, The activity of the SCR as a whole is supplemented.

1.2. β-type zeolite The β-type zeolite preferred as the zeolite of the present catalyst has the three-dimensional pore structure as described above, and can easily diffuse cations during ion exchange and gas molecules such as NH ₃ . In addition, such a structure has only a linear hole in which mordenite, faujasite, etc. are aligned in one direction, whereas it is a unique structure, and because it is such a complicated hole structure, β-zeolite is highly effective because it is difficult to cause structural breakdown due to heat and has high stability.

1.3 β-type zeolite to which iron element is added (A1)
In the zeolite, cations exist as counter ions as solid acid sites. As the cation, ammonium ions and protons are generally used, but an iron element is added as a cation species to the β-type zeolite used in the present catalyst (hereinafter sometimes referred to as “Fe-β”). Although the reason why the effect of the present invention is improved by the β-type zeolite ion-exchanged with iron element is not clear, NO is oxidized to NO ₂ on the zeolite surface to increase the reaction activity with NH _3, and the framework structure of the zeolite is It is thought that it is stabilized and contributes to the improvement of heat resistance. The addition amount of the ion-exchange species to zeolites, 0.01~5wt% _(Fe 2 _{O 3} equivalent) are preferred as the iron, 0.2~2.0wt% is more preferable. If the amount of iron element exceeds 5 wt%, the number of active solid acid sites cannot be ensured and not only the activity decreases, but also the heat resistance decreases, and if it is less than 0.01 wt%, sufficient NOx purification performance is obtained. Therefore, the exhaust gas purification performance deteriorates, which is not preferable. In addition, although all of the iron element added as an ion exchange seed may be ion-exchanged, a part thereof may exist in the state of iron oxide.

The β-type zeolite used in the present catalyst has a unit cell composition represented by the following average composition formula and is classified as a tetragonal synthetic zeolite.
M _{m / x} [Al _m Si _(64-m) O ₁₂₈ ] .pH ₂ O
(Wherein M is a cationic species, x is the valence of M, m is a number greater than 0 and less than 64, and p is a number greater than or equal to 0)

The ratio of β-type zeolite of the present catalyst is ion-exchanged with iron element, and the iron element (ion) 1, the monovalent ion exchange sites in the zeolite [AlO _4/2] ^- 2 pieces of unit Are preferably represented by the following formula (4) based on the formation of ion pairs.
[Number of moles of iron ions contained in unit weight of zeolite by ion exchange / {(number of moles of Al ₂ O ₃ present in unit weight of zeolite) × (1/2)}] × 100・ (4)
Here, the ion exchange rate is preferably 10 to 100%, more preferably 12 to 92%, and still more preferably 30 to 70%. When the ion exchange rate is 92% or less, the framework structure of the zeolite is further stabilized, the heat resistance of the catalyst and thus the life of the catalyst is improved, and more stable catalytic activity can be obtained. However, if the ion exchange rate is too low and less than 10%, sufficient denitration performance may not be obtained. In addition, when the said ion exchange rate is 100%, it means that all the cation seed | species in a zeolite is ion-exchanged with an iron ion. Thus, the ion-exchanged zeolite exhibits excellent purification ability.

1.4 β-type zeolite added with iron and cerium elements (A2)
As the zeolite used in the present catalyst, it is preferable to use β-type zeolite (hereinafter also referred to as “Fe, Ce-β”) to which cerium element is added together with iron element in addition to Fe-β. The reason why the action of the present invention is improved by adding cerium element in this way is not clear, but since Ce has adsorption performance with oxygen element, adsorption of oxygen element in NOx also with respect to NOx. In addition, since the reaction between the adsorbed NOx and NH ₃ is accelerated, the HC poisoning of the catalyst is suppressed by the oxygen storage / release function of Ce. In addition, the addition of both iron and cerium elements to the β-type zeolite will allow the effects of the iron element and the action of the cerium element to be exhibited at the same time, thereby improving the action of the catalyst. It is done.

  For this catalyst, not only Fe-β, Fe, Ce-β but also β-type zeolite (“Ce-β”) to which cerium element is added may be used. As described above, since Ce is expected to have a NOx purification action, it also exhibits a NOx purification action when added to the present catalyst.

The amount of Ce ions and Fe ions of β zeolite in “Fe, Ce-β” is 0.1 to 5 wt% (converted to Fe ₂ O ₃ ) as iron and 0.05 to 2.5 wt% (converted to CeO ₂ ) as cerium. More preferably, it is 0.5 to 2.5 wt% as iron and 0.1 to 1.5 wt% as cerium. If the amount of iron element exceeds 5 wt%, the number of active solid acid sites cannot be secured and the activity is lowered, and the heat resistance is also reduced. If the amount is less than 0.1 wt%, sufficient NOx purification performance is obtained. Therefore, the exhaust gas purification performance deteriorates, which is not preferable. On the other hand, if the amount of the cerium element exceeds 2.5 wt%, not only the number of active solid acid sites can be secured, but the activity is lowered, and the heat resistance is also reduced. The catalyst activity may be reduced. The iron element and cerium element may all be ion-exchanged, but some of them may be present in an oxide state.

  The zeolite used in the present invention preferably contains 20 to 80 wt%, more preferably 30 to 70 wt% of Fe-β with respect to the total amount of zeolite. Moreover, when mixing and using Fe-β and Fe, Ce-β, the ratio of Fe-β and Fe, Ce-β is preferably 20 to 50 wt% with respect to the total amount of zeolite, and 20 to 40 wt%. Is more preferable.

The catalyst may have a single-layer structure, but preferably has a two-layer structure. In the case of a two-layer structure, the upper and lower layers may contain both Fe-β and Fe, Ce-β, but the upper layer should use Fe-β as the main zeolite component. It is preferable to use Fe, Ce-β for the lower layer. The reason why the purification performance of NOx in the exhaust gas is improved by such a configuration is not certain, but one of the reasons may be that Ce of Fe, Ce-β has the ability to occlude oxygen atoms. Seem. That is, it is considered that NOx stays in the catalyst due to the action of Ce adsorbing oxygen atoms of NOx, and the reaction with NH ₃ is promoted. However, the Ce component may be derived from its oxygen storage performance and promote the oxidation of NH ₃ that is a reducing agent. When NH ₃ is oxidized, SCR activity is decreased, and new NOx may be generated due to oxidation of NH ₃ . Therefore, if there is a Ce component in the upper layer, most of NH ₃ introduced into the catalyst is oxidized in the upper layer of the inlet, and the NOx purification activity in the lower layer may not be obtained. Therefore, in the present invention, it is preferable that Fe, Ce-β is mainly used in the lower layer.

On the other hand, Fe-β cannot be expected to have the same NOx adsorption ability as Fe and Ce-β, and Fe-β has lower NH ₃ oxidation characteristics than Fe and Ce-β. Therefore, the oxidation of NH ₃ is suppressed even under conditions of high reactivity at high temperatures. Further, the upper layer has a high contact opportunity with components in the exhaust gas, and is expected to exhibit higher reactivity than the lower layer. Therefore, it is preferable to use Fe-β for the upper layer.
In this way, configuring the upper layer and the lower layer can be said to purify NOx by using the concentration gradient of the reaction component in the catalyst. Since the concentration of NH ₃ is high on the catalyst surface side, NOx is easily adsorbed on the catalyst lower layer side, and the concentration is high, and NOx staying in the lower layer is exhausted through the surface layer. Therefore, it is considered that reduction and purification is performed by NH ₃ stored on the surface layer.
Note that both the upper layer and the lower layer may contain both Fe-β and Fe, Ce-β. With such a configuration, the effects of the two types of β-type zeolite may be exerted in a well-balanced manner, but the amount of Ce component in the upper layer is preferably smaller than the amount of Ce component in the lower layer.

Zeolite added with iron element and cerium element used in this catalyst can be purchased in various grades from major zeolite manufacturers, and can be produced as described in JP-A-2005-502451.
That is, the iron element and the cerium element (hereinafter also referred to as a metal catalyst component) may be supported in the pores of the β-type zeolite or in the vicinity of the pore entrance. It doesn't matter how. In the present invention, it is desirable that at least a part of the zeolite is promoted by ion exchange with the metal catalyst component. By appropriate ion exchange, the framework structure of the zeolite is stabilized and the heat resistance of the zeolite itself is improved. Note that the metal catalyst component may not be completely ion-exchanged, and a part thereof may exist as an oxide.
The method for obtaining the ion exchange zeolite is not particularly limited, and the zeolite may be subjected to an ion exchange treatment with an aqueous solution of a metal catalyst component compound (for example, ferric nitrate) and calcined after drying by a conventional method. The metal catalyst component compound is usually used in the form of nitrate, sulfate, carbonate, acetate or the like. The calcining conditions are not particularly limited as long as they are sufficient to obtain a zeolite in which the metal catalyst component is stably supported. The firing temperature is preferably 300 to 1200 ° C, more preferably 400 to 800 ° C. About a heating means, it can carry out by well-known heating means, such as an electric furnace and a gas furnace.

1.5 MFI type zeolite MFI type zeolite can be used for this catalyst together with various β type zeolites. MFI-type zeolite is also known as an SCR component, and has a three-dimensional pore structure like β-type zeolite. Here, the Si / Al ratio of the MFI type zeolite is the same as that of the β type zeolite. The MFI type zeolite used in the present catalyst preferably contains an iron element and / or a cerium element like the β type zeolite. Among these, the MFI-type zeolite containing an iron element may be referred to as “Fe-MFI” hereinafter.

Comparing the characteristics of MFI-type zeolite and β-type zeolite with NH ₃ -TPD, Fe-MFI type zeolite has excellent adsorption capacity, and β-type zeolite tends to have excellent ability to hold NH ₃ at higher temperatures. (See FIG. 1). FIG. 1 shows data comparing the performance of β-type zeolite containing iron element and Fe-MFI type zeolite used in the present catalyst. According to FIG. 1, in a low temperature region faster NH ₃ desorption rate of the Fe-MFI-type, it is understood that faster NH ₃ desorption rate of the β-type zeolite at high temperatures. This suggests that Fe-MFI-type zeolite has excellent reactivity of NH ₃ and NOx at low temperatures, and β-type zeolite has excellent reactivity of NH ₃ and NOx at high temperatures. is doing. FIG. 1 also shows the case of the H-MFI type. It is also noted that the NH ₃ desorption rate is much faster in the H-MFI type in spite of the low NH ₃ desorption rate in the low temperature range.
In an actual usage environment of a diesel engine, β-type and MFI-type zeolites are mixed and used as an NH ₃ selective reduction catalyst, so that a transient temperature change during engine operation can be dealt with in a wide range. In this case, the weight component ratio represented by “β-type zeolite / MFI-type zeolite” is preferably 0.1 to 10, and more preferably 0.2 to 5.

In addition to the above-mentioned zeolite, the zeolite species may be used in combination with one or more of various types of zeolite such as A, X, Y, MOR.
When this catalyst is used in combination with other types of zeolite, the total ratio of the various β-type zeolites or MFI-type zeolites in the total zeolite is preferably 50 to 100%.
In addition to the iron element and cerium element, the zeolite may contain other transition metals, rare earth metals, or noble metals. Specific examples include transition metals such as nickel, cobalt, zirconium and copper, rare earth metals such as lanthanum, praseodymium and neodymium, and noble metals such as gold, silver, platinum, palladium, rhodium, iridium and ruthenium.
In addition, niobium, tungsten, tantalum, ceria, cerium-zirconium composite oxide, lanthanum oxide, alumina, silica, zirconia, vanadia, tin, gallium, alkaline elements such as alkaline earth elements, etc. It can add suitably in the range which does not inhibit the objective of this invention.

2. Hydrolyzed component of urea component (B)
In addition to the above zeolite (A), a urea component hydrolysis component (hereinafter sometimes simply referred to as a hydrolysis component) is used for the catalyst. As such a hydrolysis component, titanium oxide is an essential component. In addition to titanium, if necessary, an oxide containing at least one of zirconium, tungsten, silicon, and aluminum (titania, zirconia, tungsten oxide, silica, alumina, or a composite oxide thereof) can be used. These hydrolysis components may be used as individual oxides of each element, but may be used as a composite oxide or as a cluster with one or more kinds of particles selected from the above oxides. Alternatively, a rare earth metal component, a transition metal component, or the like may be added.

Titanium oxide, a NH ₃ -SCR catalyst material for plant facility such as thermal power plants, vanadia, widely known with tungsten oxide. However, since Vanadia is a concern for human health and may volatilize in the exhaust gas, exhaust gas purification technology in recent years, especially automobile exhaust gas used in an environment close to human living environment. The development of purification technology tends to avoid its use.

Moreover, it is preferable that the hydrolysis component used for this catalyst is a complex oxide of titanium oxide and silica or alumina. In particular, a composite oxide of titanium oxide and silica is preferable. Such a complex oxide is desirable in terms of heat resistance.
The composition of the composite oxide of titanium oxide and silica or alumina is preferably, as a weight ratio, titanium oxide / silica or alumina = 98/2 to 50/50, more preferably 95/5 to 80/20. If the amount of titanium oxide is too large, the heat resistance may be inferior, and if the amount of titanium oxide is too small, the decomposition performance of urea may decrease, and the low-temperature activity of the catalyst may decrease. In purifying the exhaust gas, if a DPF as described later is disposed in front of the catalyst, the exhaust gas temperature may exceed 600 ° C. due to combustion of soot. If only titanium oxide is used, the activity may decrease at such times.
Titanium oxide uses tungsten and zirconia oxides in addition to silica and alumina. Tungsten and zirconia oxides have strong acidity and are expected to have a large adsorption effect on urea and ammonia, which are alkali components. it can.

  [(B) / (A)] is 2/100 to 30/100 in the weight ratio of the zeolite (A) and the hydrolysis promoting component (B) used in the present catalyst. Zeolite (A) / Hydrolysis promoting component (B) is preferably 98/2 to 70/30, more preferably 95/5 to 80/20. If the amount of zeolite (A) is too large, the decomposition performance of urea as a reducing component may be inferior, and if the amount of zeolite (A) is too small, the NOx purification performance may be inferior.

In the present invention, when the catalyst has a laminated structure, titania as a hydrolysis promoting component may be used in either the upper layer, the lower layer, or both, but is usually used in the upper layer. Is preferred. Urea supplied in the exhaust gas diffuses from the surface of the SCR catalyst into the inside of the catalyst. However, when titania is contained in the upper layer, urea is quickly decomposed into NH ₃ , and NOx and Is supplied as highly reactive NH _{3 and} the purification of exhaust gas is promoted. However, when the catalyst is in contact with high-temperature exhaust gas exceeding 300 ° C., it is preferable to use titania as the lower layer.
Such a hydrolysis promoting component may be contained in a high concentration on the exhaust gas flow side in the upper layer of the SCR catalyst, or may be biased upstream in the exhaust gas flow.

3. Urea aqueous solution In the present invention, an aqueous urea solution is used as a reducing component. As this urea aqueous solution, a 31.8 to 33.3% by weight aqueous solution is distributed as an automobile standard “JASO E502: 2004” issued by the Japan Society of Automotive Engineers. In the present invention, this urea aqueous solution is supplied to the catalyst at a rate of 0.5 to 40 cc / min to purify NOx in the exhaust gas.
In the case of diesel engines, the combustion engines to which the present catalyst is applied range from small cars with a displacement of about 1 L to heavy-duty diesel engines with a displacement of more than 50 L, and exhausted from these diesel engines. NOx in the exhaust gas varies greatly depending on its operating state, combustion control method, and the like. And the SCR catalyst used in order to purify NOx in the exhaust gas discharged from these diesel engines can also be selected according to the variety of diesel engine exhaust amount exceeding about 1L to more than 50L.

4). Catalyst temperature Diesel engines have a relatively low exhaust gas temperature compared to gasoline engines due to their structural characteristics, and the temperature is approximately 150 to 600 ° C. The exhaust gas temperature is particularly low during start-up and low loads. However, when the temperature of the exhaust gas is low, the temperature of the catalyst does not rise sufficiently, the purification performance is not sufficiently exhibited, and NOx in the exhaust gas is easily exhausted without being sufficiently purified.
In the present invention, at such low temperatures, specifically, even when the surface temperature of the present catalyst is 150 to 180 ° C., excellent NOx purification performance is exhibited by using a urea aqueous solution distributed in the market.
Further, in the present invention, it is not necessary to convert the urea into NH ₃ having high reactivity without acting on plasma, and even with a simple catalyst layout that has been practically used in the past, NOx in the exhaust gas can be reduced. Purification is possible.

5. Honeycomb structure type catalyst This catalyst is preferably used as a honeycomb structure type catalyst by coating the honeycomb structure type carrier surface with a composition containing zeolite (A) and urea hydrolysis promoting component (B).

Here, the honeycomb structure type carrier is not particularly limited, and can be selected from known honeycomb structure type carriers. As such a honeycomb structure type carrier, there are a flow-through type carrier and a wall flow type carrier used for CSF. Both of them can be used in the present invention, but a flow-through type carrier is preferable.
In addition, such a honeycomb structure has an arbitrary overall shape, and can be appropriately selected according to the structure of the exhaust system to be applied, such as a cylindrical shape, a quadrangular prism shape, or a hexagonal prism shape. In addition, the number of holes in the opening can be determined in consideration of the type of exhaust gas to be processed, gas flow rate, pressure loss or removal efficiency, etc. About 100-1500 per square inch is preferable, and 100-900 is more preferable. If the cell density per square inch is 10 or more, the contact area between the exhaust gas and the catalyst can be secured, and a sufficient exhaust gas purification function can be obtained, and the cell density per square inch is 1500. If it is below, there will be no significant exhaust gas pressure loss and the performance of the internal combustion engine will not be impaired.

Further, the thickness of the cell wall of such a honeycomb structure type carrier is preferably 2 to 12 mil (milliinch), and more preferably 4 to 8 mil. The material of the honeycomb structure type carrier includes metals such as stainless steel and ceramics such as cordierite.
In addition to the honeycomb structure type carrier, the monolithic structure type carrier used in the present catalyst includes a sheet-like structure knitted from a thin fibrous material, and a felt-like noncombustible structure composed of a relatively thick fibrous material. The body can be used. Although these integrated structure type carriers may increase the back pressure, they have a large amount of supported catalyst components and a large contact area with the exhaust gas, so that they may have a higher processing capacity than other structural type carriers. .

When this catalyst component is used on the above-mentioned flow-through type honeycomb carrier, the amount of coating should be a carrier having 100 to 1500 holes per square inch and a cell wall thickness of 4 to 8 mil. For example, the total amount of the catalyst is preferably 55 to 330 g / L, more preferably 100 to 250 g / L.
The amount of zeolite (A) is preferably 50 to 300 g / L, more preferably 90 to 225 g / L. And it is preferable that the quantity of a hydrolysis promoting component (B) is 5-30 g / L, and 10-25 g / L is more preferable. If the amount of the honeycomb type SCR catalyst used is too small, the effects of the present invention may not be obtained sufficiently. If the amount of catalyst is too large, the honeycomb holes may be clogged, and the exhaust gas back pressure will increase significantly. In addition, the engine performance may be reduced.

The catalyst component is preferably laminated in two layers, the honeycomb structure side (lower layer) and the upper layer (upper layer). In that case, the upper layer contains Fe-β as an essential component, the lower layer contains Fe, Ce-β as an essential component, and the hydrolysis promoting component (B) is added to at least one of the upper catalyst component layer or the lower catalyst component layer. It is preferable to include.
When the internal combustion engine operates at a high speed, the exhaust gas becomes a high temperature and a high space velocity, and the reaction in the upper catalyst layer is promoted. In addition, the denitration reaction is likely to be accelerated at high temperatures without the aid of hydrolysis components.
The main catalyst component in the denitration reaction of this catalyst is zeolite. When attempting to improve the denitration performance at high rotation, it is effective to increase the zeolite concentration in the upper catalyst layer. Therefore, in the present invention, the catalyst can be designed so that the upper catalyst layer does not contain a hydrolysis component. Thereby, NOx in the exhaust gas can be effectively purified by the upper catalyst layer having a high zeolite concentration when the engine is running at a low speed and using the entire catalyst containing the hydrolysis component when the engine is running at a high speed.
Here, it is desirable that the catalyst amount in the upper layer and the catalyst amount in the lower layer are approximately the same amount. The lower catalyst component layer of the ceramic honeycomb structure contains 25 to 150 g / L of zeolite (A2) and 2.5 to 15 g / L of the hydrolysis promoting component (B), and the upper catalyst component layer is zeolite. It is preferable to contain 25 to 150 g / L of (A1) and 2.5 to 15 g / L of the hydrolysis promoting component (B).

6). Manufacture of monolithic catalyst The catalyst is mixed with a catalyst component and, if necessary, additives such as binders and surfactants with an aqueous medium to form a slurry mixture, and then applied to the monolithic carrier. It is produced as a monolithic catalyst by drying and firing.

  That is, the catalyst material and the aqueous medium are mixed at a predetermined ratio to obtain a slurry mixture. What is necessary is just to use the quantity which can disperse | distribute each catalyst component uniformly in a slurry for an aqueous medium. At this time, various additives can be added as necessary. As such an additive, in addition to the surfactant used for adjusting the viscosity and improving the dispersibility of the slurry, an acid and an alkali for adjusting the pH are blended, and the surfactant, the dispersing resin, etc. Can be blended. As a mixing method of the slurry, pulverization and mixing by a ball mill or the like can be applied, but other pulverization or mixing methods may be applied.

Next, the slurry-like mixture is applied to the monolithic structure type carrier. The coating method is not particularly limited, but a wash coat method is preferable. After the coating, drying and firing are performed to obtain a monolithic structure type catalyst on which the composition of the present catalyst is supported. In addition, 100-300 degreeC is preferable and, as for drying temperature, 100-200 degreeC is more preferable. Moreover, 300-700 degreeC is preferable and, as for baking temperature, 400-600 degreeC is especially preferable. The drying time is preferably 0.5 to 2 hours, and the firing time is preferably 1 to 3 hours. About a heating means, it can carry out by well-known heating means, such as an electric furnace and a gas furnace.
Further, in order to form a plurality of layers of the catalyst component on the monolithic support, a plurality of slurry-like mixtures may be prepared and the above operation may be repeated twice. At that time, it may be dried and baked after being coated twice by the wash coat method, or may be dried after being coated by the wash coat method, and dried and baked after coating the second and subsequent layers thereon. Good.

7). Catalyst layout (exhaust gas purifier)
The exhaust gas purification apparatus of the present invention is an apparatus in which an oxidation means, a urea aqueous solution spraying means, and a selective reduction catalyst (SCR) are arranged in this order in an exhaust gas flow path discharged from a diesel engine, or the oxidation means. As an apparatus, an oxidation catalyst, a filter, urea aqueous solution spraying means, and a selective reduction catalyst (SCR) are arranged in this order, and this is used to process an exhaust gas flow discharged from a diesel engine.

In purifying NOx in the exhaust gas, it is desirable to contact the SCR catalyst with the NO ₂ concentration in the exhaust gas increased. Since NO ₂ is excellent in reactivity with NH ₃ , NOx can be efficiently purified by increasing the concentration of the NO ₂ component in NOx. Therefore, it is preferable to arrange the NO oxidation function upstream of the present catalyst with respect to the exhaust gas flow. As such oxidizing means, an oxidation catalyst (hereinafter sometimes referred to as DOC) arranged to oxidize HC and CO in exhaust gas, and combustible particle components contained in exhaust gas are collected. There is a filter (hereinafter sometimes referred to as DPF).
As the oxidation catalyst, a known platinum or a catalyst mainly composed of activated alumina on which at least one of palladium is supported can be used, and the activated alumina preferably contains La. In addition to these components, a catalyst containing β-type zeolite ion-exchanged with cerium may be used. It is preferable to arrange this catalyst downstream of these DOC and DPF. The combustible particle components collected by the DPF are then burned and removed, and the DPF function is regenerated. NO ₂ is used for burning soot in the DPF. Combustion of soot by NO ₂ is milder than oxygen and hardly induces damage to the DPF due to combustion heat. Some DPFs are coated with an oxidation catalyst for the purpose of promoting this combustion regeneration, which is sometimes called CSF. In the present invention, DPF is described as a superordinate concept of CSF unless otherwise specified.

Here, the concentration of NO ₂ component in NOx (NO ₂ / NOx molar ratio) exhibits the highest purification characteristics when it is close to 0.5 (Non-patent Document 1). In the present invention, when the NO ₂ / NOx ratio exceeds 0.8, by-products such as ammonium nitrate are generated particularly at low temperatures, it is not preferable. Further, when the surface temperature of the present catalyst is a low temperature of 150 to 300 ° C., the reaction proceeds satisfactorily with a NO ₂ / NOx ratio of 0.4 to 0.75. And since reaction rate becomes quick at the high temperature of 300-600 degreeC, reaction advances favorably by 0.25-0.75. Since the catalyst exhibits high activity at a high temperature of 300 ° C. or higher, sufficient purification performance can be exhibited even when the NO ₂ / NOx ratio is not relatively high. Since the reaction rate on the catalyst may not be fast at a relatively low temperature of 300 ° C. or lower, it is preferable to supply exhaust gas to the catalyst at a more reactive NO ₂ / NOx ratio.

Thus, the DOC for adjusting the NO ₂ / NOx ratio preferably contains a platinum component or a palladium component as a noble metal component, and the amount of the noble metal component is preferably 0.1 to 3 g / L in terms of metal, More preferably, it contains 0.5 to 3 g / L. If the precious metal component is too much, the NO ₂ / NOx ratio may be too large, and if it is too small, the suitable NO ₂ / NOx ratio may not be obtained.
Moreover, it is preferable that this noble metal component contains 50-100 w% platinum in metal conversion, and it is more preferable that 80-100 w% platinum is included. Many diesel oils used as fuel for diesel vehicles contain sulfur components, and exhaust gases exhausted using fuels containing such sulfur components poison noble metals in the catalyst components. However, it is known that palladium tends to be poisoned with sulfur, whereas platinum has a tendency to be hardly poisoned with sulfur. Therefore, it is preferable to use platinum as a main component in the DOC used in the present invention as a noble metal component.

Further, in a normal diesel vehicle, the NO ₂ / NOx ratio in the exhaust gas is usually large when the exhaust gas is low and small when the temperature is high. In order to optimize the NO ₂ / NOx ratio in the exhaust gas as described above, it is preferable to arrange a catalyst that exhibits an oxidizing function, such as DOC and DPF, in front of this catalyst. The catalyst that exhibits such an oxidation function preferably has a function of increasing the value of the NO ₂ / NOx ratio more than twice when the exhaust gas temperature is 200 ° C. or higher, particularly 150 ° C. or higher.
If the purpose is not to remove all the main harmful components in the exhaust gas, but only to increase the NO ₂ concentration in the NOx, a form in which only the oxidation catalyst is laid out upstream of the catalyst may be used. .

In the present invention, urea water is used as a reducing agent. The urea water is decomposed by the present catalyst to generate NH ₃ and is used for purification of NOx. It is desirable to generate ammonia at a ratio of [NH ₃ /NOx=0.5 to 1.5] with respect to NOx in the exhaust gas in terms of ammonia. However, depending on the situation, not all NH ₃ may be used for NOx purification. At this time, NH _{3 that} has not been consumed in the purification of NOx leaks (slips) the catalyst and is discharged. Therefore, in order to purify this slipping NH ₃ , an oxidation catalyst may be laid out after the present catalyst. Even in such a case, since NH ₃ slips less than the conventional urea SCR, a small NH ₃ oxidation catalyst or a low noble metal NH ₃ oxidation catalyst can be used.

  Further, as a means for purifying NOx in the exhaust gas, a NOx occlusion catalyst may be used separately from the SCR as in the present invention, which is called LNT (Lean NOx Trap). The NOx occluded in the LNT purifies NOx using HC and CO, which are reducing components in the exhaust gas, as a reducing agent, but this catalyst may be combined with such an LNT.

8). Other Catalyst Layouts When the exhaust gas purification apparatus of the present invention is used for automobiles, it can be used in combination with a catalyst that exhibits an action different from that of the present catalyst. The arrangement will be exemplified below including those described above. In the following example, the present catalyst is represented by the present SCR, and “(Urea)” represents the spray supply position of the urea aqueous solution as the reducing agent.

・ DOC + (Urea) + this SCR
・ (Urea) + SCR + DOC
・ DOC + DPF + (Urea) + SCR
・ DOC + (Urea) + Book SCR + DPF
・ DOC + DPF + (Urea) + Book SCR + DOC
・ DOC + (Urea) + main SCR + DOC + DPF

・ LNT + DOC + (Urea) + this SCR
・ DOC + LNT + (Urea) + this SCR
・ DOC + (Urea) + Book SCR + LNT

・ LNT + DOC + DPF + (Urea) + book SCR
・ DOC + LNT + DPF + (Urea) + book SCR
・ DOC + DPF + LNT + (Urea) + book SCR
・ DOC + DPF + (Urea) + Book SCR + LNT

・ LNT + DOC + DPF + (Urea) + main SCR + DOC
・ DOC + LNT + DPF + (Urea) + main SCR + DOC
・ DOC + DPF + LNT + (Urea) + main SCR + DOC
・ DOC + DPF + (Urea) + Book SCR + LNT + DOC
・ DOC + DPF + (Urea) + main SCR + DOC + LNT

・ LNT + DOC + (Urea) + main SCR + DPF
・ DOC + LNT + (Urea) + main SCR + DPF
・ DOC + (Urea) + main SCR + LNT + DPF
・ DOC + (Urea) + main SCR + DPF + LNT

・ LNT + DOC + (Urea) + main SCR + DOC + DPF
・ DOC + LNT + (Urea) + main SCR + DOC + DPF
・ DOC + (Urea) + main SCR + LNT + DOC + DPF
・ DOC + (Urea) + main SCR + DOC + LNT + DPF
・ DOC + (Urea) + main SCR + DOC + DPF + LNT

In addition, such a catalyst may be a combination of catalysts having a plurality of functions in addition to a single catalyst specialized for one function. Specifically, as described below, the present catalyst and DPF are combined, and the present catalyst is coated on one of the front side or the rear side of the DPF, or on the surface of the DPF on the engine side or on the back surface thereof.
・ DOC + (Urea) + DPF / main SCR

  Examples and Comparative Examples are shown below to further clarify the features of the present invention, but the present invention is not limited to the embodiments. In addition, the catalyst used for a present Example and a comparative example was prepared by the method shown next.

[Production of the present SCR catalyst (1)]
Titanium-silicon composite oxide (SiO ₂ equivalent silicon content: 10 wt%, BET value: 100 m ² / g), water, and β-type zeolite ion exchanged with iron element (iron element equivalent: concentration 2 wt% ion exchange amount) 70% SAR = 35), MFI zeolite ion-exchanged with iron element (iron element equivalent: concentration 2 wt% ion exchange amount 70% SAR = 40), and β-type zeolite ion-exchanged with iron element and cerium element (iron) Element conversion: Concentration 2 wt% Ion exchange amount 70%, Cerium element conversion: Concentration 0.1 wt% Ion exchange amount 5%, SAR = 35) and silica as a binder are added, the concentration is adjusted with water, and a ball mill is used. Milling gave a slurry. This slurry was coated on a honeycomb flow-through cordierite carrier (cell density: 300 cel / inch, wall thickness: 5 mil, length: 6 inch, diameter: 7.5 inch) by the wash coat method, and dried at 150 ° C. for 1 hour. Then, it was calcined at 500 ° C. for 2 hours in an air atmosphere.
Table 1 shows the catalyst amount per unit volume and the composition of the obtained SCR catalyst (1). In Table 1, “TiO ₂ ” is a titanium-silicon composite oxide, and the numerical value is the amount [g / L] supported per unit volume of the honeycomb flow-through cordierite carrier.

[Production of the present SCR catalyst (2)]
= Lower layer =
Titanium-silicon composite oxide (SiO ₂ equivalent silicon content 10 wt% BET value: 100 m ² / g) and β-type zeolite ion-exchanged with iron element and cerium element (iron element equivalent: concentration 2 wt%, ion exchange amount 70% ) (Cerium element conversion: concentration 0.1 wt% ion exchange amount 5%) (SAR = 35) and β-type zeolite ion-exchanged with iron element (iron element conversion: concentration 2 wt% ion exchange amount 70% SAR = 35) Then, silica and water as a binder were added to adjust the concentration, and milling was performed using a ball mill to obtain a slurry. This slurry was coated on a honeycomb flow-through cordierite carrier (cell density: 300 cel / inch, wall thickness: 5 mil, length: 6 inch, diameter: 7.5 inch) by a wash coat method.
= Upper layer =
Thereafter, β-type zeolite ion-exchanged with iron element (iron element conversion: concentration 2 wt%, ion exchange amount 70%) (SAR = 35) and silica as a binder are added with water and milled in a ball mill to obtain a slurry. It was. This slurry was coated on a honeycomb flow-through cordierite carrier coated with a lower layer portion by a wash coat method, dried at 150 ° C. for 1 hour, and then fired at 500 C for 2 hours in an air atmosphere.
The amount of catalyst per unit volume and the composition of the obtained SCR catalyst (2) are shown in Table 1 as in the case of the SCR (1).

[Production of the present SCR catalyst (3)]
= Lower layer =
A β-type zeolite ion-exchanged with iron element (iron element conversion: concentration 2 wt%, ion exchange amount 70%) (SAR = 35) was milled with water and silica as a binder in a ball mill to obtain a slurry. This slurry was coated on a honeycomb flow-through cordierite carrier (cell density: 300 cel / inch, wall thickness: 5 mil, length: 6 inch, diameter: 7.5 inch) coated with a lower layer portion by a wash coat method.
= Upper layer =
Thereafter, titanium-silicon composite oxide (SiO ₂ equivalent silicon content 10 wt% BET value: 100 m ² / g) and β-type zeolite ion-exchanged with iron element and cerium element (iron element equivalent: concentration 2 wt% ion exchange amount) (70%) (cerium element conversion: concentration 0.1 wt% ion exchange amount 5%) (SAR = 35) and β-type zeolite ion-exchanged with iron element (iron element conversion: concentration 2 wt% ion exchange amount 70%) ( SAR = 35), silica as a binder and water were added to adjust the concentration, and milling was performed using a ball mill to obtain a slurry. This slurry was coated on a honeycomb flow-through cordierite support by a wash coat method, dried at 150 ° C. for 1 hour, and then fired at 500 ° C. for 2 hours in an air atmosphere.
The amount of catalyst per unit volume and the composition of the obtained SCR catalyst (3) are shown in Table 1 as in the case of the SCR catalyst (1).

[Production of the present SCR catalyst (4)]
= Lower layer =
Titanium-silicon composite oxide (SiO ₂ equivalent silicon content 10 wt% BET value: 100 m ² / g) and β-type zeolite ion-exchanged with iron element (iron element equivalent: concentration 2 wt% ion exchange amount 70%) (SAR = 35) and β-type zeolite ion-exchanged with iron element and cerium element (iron element conversion: concentration 2 wt% ion exchange amount 70%) (cerium element conversion: concentration 0.1 wt% ion exchange amount 5%) (SAR = 35), silica as a binder and water were added to adjust the concentration, and milling was performed with a ball mill to obtain a slurry. This slurry was coated on a honeycomb flow-through cordierite carrier (cell density: 300 cel / inch, wall thickness: 5 mil, length: 6 inch, diameter: 7.5 inch) coated with a lower layer portion by a wash coat method.
= Upper layer =
After that, β-type zeolite ion-exchanged with iron element (iron element conversion: concentration 2 wt% ion exchange amount 70%) (SAR = 35) and MFI-type zeolite ion-exchanged with iron element (iron element conversion: concentration 2 wt% ion) The amount of exchange was 70% SAR = 40), silica as a binder and water were added to adjust the concentration, and milling was performed using a ball mill to obtain a slurry. This slurry was coated on a honeycomb flow-through cordierite support by a wash coat method, dried at 150 ° C. for 1 hour, and then fired at 500 ° C. for 2 hours in an air atmosphere.
The amount of catalyst per unit volume and the composition of the obtained SCR catalyst (4) are shown in Table 1 as in the case of the SCR catalyst (1).

[Production of comparative SCR catalyst]
Comparative SCR catalysts (1) to (3) were obtained in the same manner except that the titanium-silicon composite oxide was removed from the present SCR catalysts (1) to (3) and silicon oxide was supplemented instead. Moreover, the MFI type | mold zeolite ion-exchanged with the iron element of this SCR catalyst (4) was changed to the proton type | mold MFI type | mold zeolite (SAR = 40), and the comparative SCR catalyst (4) was obtained.
About each obtained comparative SCR catalyst, the catalyst amount [g / L] per unit volume and a composition are described in Table 1 similarly to this SCR catalyst (1).

<DOC>
To 500 g of commercially available lanthanum-added γ-alumina (specific surface area: 220 m ² / g, Al ₂ O ₃ / La ₂ O ₃ (weight ratio) = 98.4 / 1.6), chloroplatinic acid aqueous solution was 2 wt in terms of Pt. % Impregnated, dried at 100 ° C. for 1 hour, calcined in an electric furnace at 500 ° C. for 1 hour in the air atmosphere, cooled and pulverized to obtain alumina carrying Pt.
To 500 g of commercially available lanthanum-added γ-alumina (specific surface area: 220 m ² / g, Al ₂ O ₃ / La ₂ O ₃ (weight ratio) = 98.4 / 1.6), a mixed solution of an aqueous palladium nitrate solution with Pd It was impregnated to 1 wt%, dried at 100 ° C. for 1 hour, calcined in an electric furnace at 500 ° C. for 1 hour in the air atmosphere, cooled and pulverized to obtain alumina carrying Pd.
Water was added to these two types of alumina and milled using an alumina ball to form a slurry. The slurry is impregnated into a cordierite flow-through carrier (400 cel / inch ² , cell wall thickness: 6/1000 [inch], diameter: 5.66 [inch], length: 6 [inch]), and unnecessary slurry The portion was blown off with an air gun, dried at 100 ° C. for 1 hour, and calcined at 500 ° C. to obtain a catalyst. The resulting catalyst was aged in an electric furnace at 800 ° C. for 20 hours in an air atmosphere. The catalyst composition in the obtained monolithic catalyst is shown in Table 2.

(Examples 1-4) (Comparative Examples 1-4)
For each SCR obtained as described above, an oxidation catalyst is disposed in front of the present SCR catalyst and the comparative SCR catalyst, and “DOC + SCR” is used as the NOx purification performance and slipping NH ₃ under the following measurement conditions. The concentrations of are measured and listed in Table 2. The NOx purification performance is “[NOx concentration at the catalyst inlet−NOx concentration at the catalyst outlet] / [NOx concentration at the catalyst inlet]”, expressed as “NOx conversion rate” in Table 3, and the concentration of NH ₃ that slips is “ The “NH ₃ concentration at the catalyst outlet” is shown in Table 3 as “NH ₃ slip concentration”.
Note that DOC and catalyzed DPF have an oxidation function and also a function of reducing the NO ratio in NOx. In this embodiment, the layout of “DOC + SCR” will be described. However, even when “DOC + DPF + SCR” is selected, since the oxidation reaction is performed in DPF, “DOC + DPF + SCR” exhibits the same effect as “DOC + SCR”. Nor.

<Measurement conditions>
-Engine: Diesel 5L engine-Surface temperature of this catalyst: 200 ° C (1200rpm), 400 ° C (2000rpm)
・ Space velocity: 72,000 / h (200 ° C), 120,000 / h (400 ° C)
Ammonia component: 32.5 wt% urea aqueous solution Urea water spray amount: NH ₃ / NOx ratio in exhaust gas controlled to 0.9 DOC front: NO ₂ / NO ratio in exhaust gas: 0.3 (200 ° C), 0.1 (400 ° C)
-DOC rear: NO ₂ / NO ratio in front of SCR: 0.7 (200 ° C), 0.3 (400 ° C)

[Evaluation]
The following can be understood by comparing the examples using the SCR catalysts (1) to (4) with the comparative examples using the comparative SCR catalysts (1) to (4).
That is, as shown in Table 2, the present SCR catalyst (1), which is a selective reduction catalyst according to the present invention, is superior in NOx purification performance and NH ₃ slip performance as compared to the conventional type SCR (1). Are better. This is because when the present SCR catalyst (2) is compared with the comparative SCR (2), when the present SCR catalyst (3) is compared with the comparative SCR (3), the present SCR catalyst (4) is further compared with the comparative SCR (4). It is the same when compared with.
In addition, in this SCR catalyst (2), since there is much content of the upper layer Fe- (beta) zeolite, the performance superior to others is exhibited. Moreover, although the result of the comparative SCR catalyst (4) is extremely bad, it is because the denitration function of H-MFI is much smaller than that of Fe-zeolite, the ammonia consumption rate is slow, and unreacted ammonia does not react with the catalyst and flows out. It is thought that it has done.

For β-type zeolite and Fe-MFI-type zeolite containing iron element used in the present invention, it is a graph showing the temperature dependence of the NH ₃ desorption rate.

Claims (12)

A selective reduction catalyst for purifying exhaust gas comprising the following zeolite (A) that selectively reduces NOx in exhaust gas discharged from a lean combustion engine with ammonia and a urea hydrolysis promoting component (B):
An aqueous urea solution is spray-supplied to the selective reduction catalyst and brought into contact at 150 to 600 ° C., and ammonia in a ratio of [NH ₃ /NOx=0.5 to 1.5] with respect to NOx in the exhaust gas in terms of ammonia. A selective reduction catalyst for purifying exhaust gas, wherein nitrogen oxides are decomposed into nitrogen and water.
Zeolite (A): A zeolite containing an iron element essential to Fe-β type zeolite (A1), the content of which is 80 wt% or more of the total zeolite. Hydrolysis promoting component (B): titania or titanium , Zirconium, tungsten, silicon, or at least one selected from alumina   2. The exhaust gas purification component according to claim 1, wherein the zeolite (A) and urea hydrolysis promoting component (B) are contained in the upper catalyst component layer and / or the lower catalyst component layer of the ceramic honeycomb structure. Selective reduction catalyst.   The selective reduction catalyst for exhaust gas purification according to claim 1, wherein the β-type zeolite (A1) contains 0.1 to 5 wt% of iron element.   The selective reduction catalyst for exhaust gas purification according to claim 1, wherein the zeolite (A) is β-type zeolite (A2) containing an iron element and a cerium element.   The selective reduction catalyst for exhaust gas purification according to claim 4, wherein the β-type zeolite (A2) contains 0.05 to 2.5 wt% of cerium element.   The selective reduction catalyst for exhaust gas purification according to claim 1, wherein the zeolite (A) is MFI type zeolite (A3) containing an iron element and / or a cerium element.   The selection for exhaust gas purification according to claim 1, wherein the weight ratio [(B) / (A)] of the zeolite (A) and the hydrolysis promoting component (B) is 2/100 to 30/100. Reduction catalyst.   A ceramic honeycomb structure having a cell density of 100 to 1500 cel / inch is coated with 55 to 330 g / L of the zeolite (A) and urea hydrolysis promoting component (B) per unit volume. The selective reduction catalyst for exhaust gas purification according to claim 2.   The upper catalyst component layer of the ceramic honeycomb structure contains β-type zeolite (A1) containing iron element, and the lower catalyst component layer contains β-type zeolite (A2) containing iron element and cerium element. The selective reduction catalyst for exhaust gas purification according to claim 8, wherein the hydrolysis promoting component (B) is contained in at least one of the catalyst component layer and the lower catalyst component layer.   The upper catalyst component layer of the ceramic honeycomb structure contains 25 to 150 g / L of zeolite (A1), and the lower catalyst component layer contains 25 to 150 g / L of zeolite (A2) and is included in one layer. The selective reduction catalyst for exhaust gas purification according to claim 8 or 9, wherein the hydrolysis promoting component (B) is 2.5 to 15 g / L.   An oxidation means, urea aqueous solution spray means, and the exhaust gas purifying selective reduction catalyst according to any one of claims 1 to 10 are arranged in this order in an exhaust gas flow path discharged from a diesel engine, and the oxidation means An oxidation catalyst containing a platinum component or a palladium component as a noble metal component, wherein the amount of the noble metal component is 0.1 to 3 g / L in terms of metal, and the amount of platinum in terms of metal in the noble metal component is 50 to 100 w%. , After oxidizing the hydrocarbon components, carbon monoxide, nitric oxide and nitrous oxide in the exhaust gas to increase the concentration of nitrogen dioxide, the urea aqueous solution spraying means sprays and supplies the urea aqueous solution to the selective reduction catalyst; 150 An exhaust gas purifying apparatus characterized by contacting at ˜600 ° C. and decomposing nitrogen oxides into nitrogen and water by the generated ammonia.   An oxidation means, a urea aqueous solution spraying means, and a selective reduction catalyst are arranged in this order in the exhaust gas flow path discharged from the diesel engine. The oxidation means contains a platinum component or a palladium component as a noble metal component. Using an oxidation catalyst having an amount of 0.1 to 3 g / L in terms of metal and 50 to 100 w% of platinum in terms of metal in the noble metal component, and a filter, the combustible particle component is collected by the filter. The exhaust gas purification and purification apparatus according to claim 11.

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