JP2009166032A - Catalyst and method for conversion of nitrogen oxide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferric silicate catalyst being capable of converting nitrogen oxides efficiently over a wide temperature range from low to high temperatures and having high hydrothermal durability, and to provide a method of converting nitrogen oxides using the catalyst. <P>SOLUTION: The catalyst consists of a crystalline silicate or an aluminosilicate containing iron in the β structure. For the β ferric silicate of a SiO<SB>2</SB>/Fe<SB>2</SB>O<SB>3</SB>mol ratio of 20-300 and a log(SiO<SB>2</SB>/Al<SB>2</SB>O<SB>3</SB>) of ≥2, the catalyst uses at least one reducing agent selected from ammonia, urea and organic amines so as to exert high performance of reducing nitrogen oxides in exhaust gas selectively, especially a high activity after a durability treatment at low temperatures of ≤250°C. The iron in the β ferric silicate is preferably an isolated ferric ion having a symmetric tetrahedral structure confirmable by UV/visible absorption spectrometry and electron spin resonance measurement. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to the purification of nitrogen oxides discharged from an internal combustion engine, a nitrogen oxide purification catalyst comprising an iron silicate having a β structure, and at least one of ammonia, urea, and organic amines using the catalyst. The present invention provides a method for purifying nitrogen oxides to be reacted.

  Silicates substituted with different elements in the skeletal structure are expected to have different properties from ordinary aluminosilicate zeolites, and their use in catalytic reactions is being investigated.

  For example, iron as a xylene isomerization catalyst using an iron silicate carrying platinum (Patent Document 1), a selective methylation catalyst of a naphthalene compound using an iron silicate (Patent Document 2), and a ring-opening polymerization catalyst of a cyclic ether A method for producing polyalkylene glycol using silicate (Patent Document 3) and the like are disclosed.

  On the other hand, nitrogen oxide purification technology using iron silicate is also being studied.

  For example, a catalyst for purifying exhaust gas containing nitrogen oxide in which a coprecipitation complex oxide of copper and gallium is dispersed and supported on ZSM-5 type iron silicate (Patent Document 4), in an atmosphere where excess oxygen exists, Nitrogen oxide purification method (patent document 5) in which an alkali metal exchanger of ZSM-5 type iron silicate is contacted with exhaust gas containing nitrogen oxide in the presence of hydrocarbons or oxygen-containing compounds, nitrogen oxide, oxygen gas, and Nitrogen oxide removal method in which combustion exhaust gas containing sulfurous acid gas is contact-reacted in the presence of an iron silicate catalyst and a hydrocarbon reducing agent as required (Patent Document 6), platinum, palladium, rhodium and cobalt on iron silicate Among them, an exhaust gas purification catalyst that mainly removes nitrogen oxides supporting at least one of them (Patent Document 7) has been reported.

  In addition, since the iron silicate described in Patent Documents 6 and 7 uses a tetrapropylammonium salt at the time of synthesis, the skeleton structure of the obtained iron silicate is predicted to be a ZSM-5 structure.

  As for the purification catalyst for nitrous oxide, a method for producing a catalyst containing β-type iron silicate supporting copper, cobalt, etc., used for direct decomposition of nitrous oxide (Patent Document 8), an iron silicate having a β structure is used. In addition, a method of directly decomposing nitrous oxide, a method of non-selective catalytic reduction of nitrous oxide using carbon monoxide as a reducing agent (Non-patent Document 1), and the like are disclosed.

  On the other hand, for the purification catalyst of nitrogen oxides in exhaust gas, with respect to the purification of nitrogen oxides of oxygen excess exhaust gas represented by lean burn combustion exhaust gas and diesel combustion exhaust gas, an aluminosilicate zeolite catalyst supporting iron or copper is used, A method of selective catalytic reduction (usually called SCR) with ammonia (Patent Document 9) is known.

  However, in a method for reducing nitrogen oxide (NOx) using ammonia as a reducing agent, an iron silicate excellent in nitrogen oxide decomposition performance at low temperature and hydrothermal durability has not been known so far.

Japanese Patent No. 3269828 JP-T-2004-524142 Japanese Patent No. 3477799 JP-A-5-305240 Japanese Patent No. 2691643 JP-A-5-154349 Japanese Patent No. 2605956 US Patent Application Publication No. 2006-0088469 JP-A-2-2933021 Journal of Catalysis, 232 (2005) 318-334

  While efficient purification of nitrogen oxides in exhaust gas is desired, a nitrogen oxide purification catalyst having high nitrogen oxide purification activity at a low temperature of 250 ° C. or lower and high hydrothermal durability performance has been obtained. It was not done.

  An object of the present invention is an iron silicate having catalytic performance for efficiently purifying nitrogen oxides in a wide temperature range, particularly in a relatively low temperature range of 250 ° C. or less, and having hydrothermal durability superior to conventional ones. It is to provide a catalyst.

  Still another object is to provide a method for purifying nitrogen oxides using the above catalyst.

As a result of intensive studies on a selective reduction catalyst for nitrogen oxides using ammonia or the like, the present inventors have found that the SiO ₂ / Fe ₂ O ₃ molar ratio is 20 to 300, and the SiO ₂ / Al ₂ O ₃ molar ratio is In crystalline silicate having iron in β skeleton structure which is 2 or more as log (SiO ₂ / Al ₂ O ₃ ), excellent nitrogen oxide purification in selective reduction of nitrogen oxide using ammonia or the like as a reducing agent The present invention has been found to have performance, and the present invention has been completed.

  Hereinafter, the nitrogen oxide purification catalyst of the present invention will be described.

The nitrogen oxide purification catalyst of the present invention has iron in a β skeleton structure having a SiO ₂ / Fe ₂ O ₃ molar ratio of 20 to 300 and a log (SiO ₂ / Al ₂ O ₃ ) of 2 or more (molar ratio). Crystalline silicate (hereinafter referred to as “β-type iron silicate”).

The composition of β-type iron silicate is
(X + y) M _{(2 / n)} O.xFe ₂ O ₃ .yAl ₂ O ₃ .zSiO ₂ .wH ₂ O
(Where n is the valence of the cation M, x, y, and z are mole fractions of Fe ₂ O ₃ , Al ₂ O ₃ , and SiO ₂ , respectively, and x + y + z = 1. W is 0 or more. And z / y may be 100 or more, and y may be 0).

  The crystal structure of the β-type iron silicate of the present invention is β-type as confirmed by X-ray diffraction. β-type iron silicate is a metallosilicate having three-dimensional pores in which 0.76 × 0.64 nm and 0.55 × 0.55 nm pores composed of oxygen 12-membered rings intersect. The X-ray diffraction pattern of β-type iron silicate is characterized by the lattice spacing d (angstrom) shown in Table 1 below and its diffraction intensity.

本発明のβ型鉄シリケートは、四配位構造の鉄が骨格原子として酸素原子と連結した構造を有し、アルミノシリケートゼオライトと同様にシリケート骨格の電荷不足に由来する固体酸性質を有するものである。本発明の鉄シリケートは、通常のアルミノシリケートゼオライトに鉄を担持した触媒に比べ、触媒の活性金属としての鉄が高度に分散した孤立鉄イオン（Ｆｅ^３＋）として存在しており、アンモニア等を用いた選択還元反応において、鉄の凝集が抑制され、特に高い性能が発揮される。

The β-type iron silicate of the present invention has a structure in which four-coordinate iron is connected to an oxygen atom as a skeleton atom, and has a solid acid property derived from a lack of charge in the silicate skeleton as in the case of an aluminosilicate zeolite. is there. The iron silicate of the present invention exists as isolated iron ions (Fe ³⁺ ) in which iron as a catalyst active metal is highly dispersed compared to a catalyst in which iron is supported on a normal aluminosilicate zeolite. In the selective reduction reaction, iron aggregation is suppressed, and particularly high performance is exhibited.

The β-type iron silicate of the present invention is a crystalline silicate having iron in the β skeleton structure, and the SiO ₂ / Fe ₂ O ₃ molar ratio is in the range of 20 to 300, preferably 25 to 300.

If the SiO ₂ / Fe ₂ O ₃ molar ratio is less than 20, the total iron content increases, but the crystallinity is likely to be lowered by the hot water treatment, and it is difficult to sufficiently retain isolated Fe ³⁺ that contributes to activity. From the viewpoint of crystallinity, the SiO ₂ / Fe ₂ O ₃ molar ratio is particularly preferably 25 or more.

On the other hand, when the SiO ₂ / Fe ₂ O ₃ molar ratio exceeds 300, the absolute iron ion amount is small and sufficient catalytic activity cannot be obtained.

The SiO ₂ / Fe ₂ O ₃ molar ratio is preferably in the range of 20 to 150, more preferably 25 to 100.

The iron that contributes most to the reduction of nitrogen oxides in the β-type iron silicate of the present invention is dispersed as isolated iron ions (Fe ³⁺ ) in the silicate skeleton described later, and Fe ₂ O ₃ Not agglomerated. The SiO ₂ / Fe ₂ O ₃ molar ratio used in the definition of the composition of the β-type iron silicate of the present invention is a notation used for convenience to define the total iron content including isolated iron ions.

The iron silicate of the present invention has a log (SiO ₂ / Al ₂ O ₃ ) of 2 or more (molar ratio). When log (SiO ₂ / Al ₂ O ₃ ) is less than 2, the nitrogen oxide purification performance at a low temperature of 250 ° C. or lower is drastically reduced by the hydrothermal durability treatment.

The upper limit of log (SiO ₂ / Al ₂ O ₃ ) is not particularly limited, but may be larger than 2 (100 as SiO ₂ / Al ₂ O ₃ ), particularly 2.5 (SiO ₂ / Al ₂ O ₃ is preferably 310) or more, more preferably 3 (1000 as SiO ₂ / Al ₂ O ₃ ) or more. On the other hand, even if log (SiO ₂ / Al ₂ O ₃ ) is 4 (10000 as SiO ₂ / Al ₂ O ₃ ) or more, the effect tends to be saturated.

In nitrogen oxide purification by iron silicate, the direct reaction active site is an isolated iron ion, but it is known that aluminum in the skeleton also contributes to the activity improvement. Conventionally, in aluminosilicate, the higher the SiO ₂ / Al ₂ O ₃ ratio, the higher the heat resistance tends to be. However, the amount of Al ₂ O ₃ decreases and the solid acid point decreases, so the catalytic activity is not necessarily high. There wasn't. However, the β-type iron silicate of the present invention has remarkably high low-temperature activity (heat resistance) before and after hydrothermal durability treatment when the log (SiO ₂ / Al ₂ O ₃ ) is 2 or more.

The iron silicate of the present invention contains 70% of isolated iron ions in a state (hereinafter referred to as fresh) that does not include an organic structure directing agent (hereinafter referred to as SDA) in the micropores. The above preferably has a symmetric tetrahedral structure. The iron that exists as isolated iron ion here is not agglomerated like iron oxide (Fe ₂ O ₃ ) or the like, but is an iron ion that is isolated and dispersed in the silicate skeleton or at the ion exchange site. Specifically, it refers to an iron ion detected in electron spin resonance measurement.

  The structural symmetry of the iron component can be measured by electron spin resonance measurement (measurement temperature 77K).

Paramagnetic iron ion (Fe ³⁺ ) exhibits resonance absorption in electron spin resonance measurement, and has absorption peaks having at least three absorption peaks of g≈2.0, g≈4.3, and g> 4.3. It is known to belong to (See Journal of Catalysis, 249 (2007) 67, etc.) Those having an absorption peak of g≈2.0 are isolated iron ions having a symmetric tetrahedral structure (or highly symmetric multi-coordination structure), g≈4.3. And an iron ion having an absorption of g> 4.3 is attributed to an isolated iron ion having a distorted tetrahedral structure and a distorted multi-coordination structure.

  That is, the fact that the number of isolated iron ions having a symmetric tetrahedral structure is 70% or more of the isolated iron ions means that the electron spin resonance spectrum intensity of g≈2.0 in the electron spin resonance measurement is g≈2. It means 70% or more with respect to the sum of all spectrum intensities of 0, g≈4.3 and g> 4.3.

  Since the electron spin resonance spectrum is generally evaluated by a spectrum expressed in a differential form, the intensity of the electron spin resonance spectrum is also obtained in the present invention using the amplitude length of the differential curve. That is, the ratio of the isolated iron ions having a symmetric tetrahedral structure is the amplitude length of the differential curve of the electron spin resonance spectrum of g≈2.0, g≈2.0, g≈4.3, and g> 4.3. It is the value divided by the sum of each amplitude length.

  The electron spin resonance spectrum can be measured by a general method.

  For example, using an electron spin resonance apparatus (JES-TE200 manufactured by JEOL Ltd.), the measurement conditions are a measurement temperature of 77K, a microwave output of 1.0 mW, an observation range of 0 to 1000 mT, a modulation width of 0.32 mT, The constant can be 0.3 sec. About 10 mg of the sample is weighed into a quartz sample tube, inserted into a liquid nitrogen temperature measurement dewar, and then measured.

  In the present invention, the electron spin resonance spectrum is evaluated by using a value measured at a measurement temperature of 77K after the sample is baked and fresh at about 600 ° C. in an atmosphere of dry air and / or nitrogen.

  The β-type iron silicate of the present invention has a differential absorption curve in which the peak intensity of the differential absorption curve of the electron spin resonance spectrum (measurement temperature 77 K) attributed to g≈2.0 is g≈4.3 and g> 4.3. It is particularly preferred that it is greater than any peak intensity of the curve.

  The ratio of isolated iron ions having a symmetric tetrahedral structure is preferably 70% or more, particularly 80% or more of the entire isolated iron ions. The ratio does not theoretically exceed 100%.

  In the β-type iron silicate of the present invention, the number of isolated iron ions having a symmetrical tetrahedral structure is preferably 60% or more of the total iron content.

  Whether or not iron ions are present in the form of isolated iron ions without aggregating in the iron silicate crystal (including in the skeleton and including the ion exchange sites) can be measured by UV-visible absorption measurement. Therefore, the ratio of the isolated iron ion having a symmetric tetrahedral structure to the total iron contained is the ratio of the isolated iron ion obtained by ultraviolet-visible absorption measurement and the isolated iron ion having a symmetric tetrahedral structure obtained by electron spin resonance measurement. It can be determined as a value multiplied by the ratio.

  The ratio of isolated iron ions in the UV-visible absorption measurement is as follows: the integrated absorption intensity (C) and the peak wavelength of 211 ± 10 nm with respect to the total absorption integrated intensity (B) and the peak in the wavelength range of 220 to 700 nm of the UV-visible absorption spectrum. It is calculated | required by ratio (A = (C + D) / B) of the integral absorption intensity | strength (D) of wavelength 272 +/- 10nm.

Iron ions or iron oxides absorb in the ultraviolet and visible wavelength regions. In the UV-Vis absorption measurement, the absorption wavelength region of iron varies depending on the state of existence, absorption less than 300 nm is in isolated iron ions (Fe ³⁺ ), absorption in the range of 300 to 400 nm is in Fe ₂ O ₃ clusters, and exceeds 400 nm. Absorption is attributed to Fe ₂ O ₃ aggregated particles. That is, the decomposition waveforms C and D consisting of Gaussian curves with peak wavelengths of about 211 ± 10 nm and about 272 ± 10 nm in the UV-visible absorption spectrum are attributed to absorption based on isolated iron ions. The isolated iron ion is mainly composed of Fe ³⁺ located at the skeleton of the β-type iron silicate or the ion exchange site, and is attributed to iron existing in a highly dispersed state.

  In the β-type iron silicate of the present invention, the ratio of isolated iron ions to the total iron content is preferably 90% or more, but it is not sufficient that the iron ions are isolated and has a symmetrical tetrahedral structure. This greatly contributes to the nitrogen oxide purification performance.

  The ratio of isolated iron ions having a symmetric tetrahedral structure is preferably 60% or more, more preferably 70% or more, based on the total contained iron. The upper limit is not particularly limited, but theoretically does not exceed 100%.

  The following general methods are used for UV-visible absorption measurement.

  That is, the measurement of ultraviolet-visible light absorption was performed using a self-recording spectrophotometer (UV-3100, manufactured by Shimadzu Corporation) equipped with an integrating sphere attachment device (for example, ISR-3100 manufactured by Shimadzu Corporation) attached to the sample chamber. Done. The scan speed is 200 nm / min, the slit width is 5.0 nm, and barium sulfate powder is used for baseline correction. The powdered sample may be filled in a sample folder and the reflectance in the wavelength range of 220 to 700 nm may be measured.

  In addition, since β-type iron silicate immediately after hydrothermal synthesis contains SDA such as tetraethylammonium cation in its micropores, the measurement of the UV-visible absorption spectrum is carried out in an atmosphere such as dry air or nitrogen in the form of β-type iron. This is performed after the SDA removal operation (hereinafter referred to as “fresh baking”), which is usually performed by baking the silicate at about 600 ° C.

Next, in the β-type iron silicate of the present invention, the content of isolated iron ions having a symmetric tetrahedral structure is preferably 2 mol% or more (in a molar ratio of Fe ³⁺ / (Si + Al + Fe)).

  Even if the ratio of isolated iron ions having a symmetric tetrahedral structure is large, the purification performance of nitrogen oxides is low if the absolute amount is too small.

  The absolute amount of isolated iron ions having a symmetric tetrahedral structure is obtained from the ratio of isolated iron ions obtained by ultraviolet-visible absorption spectrum to the number of moles of iron Fe / (Si + Al + Fe) in β-type iron silicate and the electron spin resonance spectrum. It is a numerical value obtained by multiplying the ratio of isolated iron ions having a symmetric tetrahedral structure.

The content of isolated iron ions having a symmetric tetrahedral structure in the β-type iron silicate of the present invention is preferably 2 mol% or more, particularly 2.2 mol% or more. On the other hand, in the present invention, since the lower limit of the SiO ₂ / Fe ₂ O ₃ molar ratio is defined as 20 (Si / Fe molar ratio 10), the upper limit of the absolute amount of isolated iron ions having a symmetric tetrahedral structure is about 9.1 mol% (1 / (10 + 0 + 1)).

  The β-type iron silicate of the present invention has high nitrogen oxide reduction performance even after endurance treatment, but the ratio of isolated iron ions having a symmetric tetrahedral structure present in the β-type iron silicate even after endurance treatment Is expensive.

  In the β-type iron silicate of the present invention, the ratio of isolated iron ions having a symmetric tetrahedral structure to isolated iron ions detectable by electron spin resonance measurement is more than 40% even after endurance treatment, particularly 80% or more. Furthermore, it is preferable that 90% or more is maintained.

The “endurance treatment” referred to here is a treatment commonly used for evaluating the hydrothermal durability of the silicate catalyst, and refers to a heat treatment performed in a high-temperature atmosphere containing H ₂ O. The durability treatment is performed, for example, by filling a reaction tube with a silicate catalyst and circulating air containing H ₂ O at 700 ° C. for 20 hours. Specific processing conditions are described in the examples. The β-type iron silicate catalyst of the present invention has high hydrothermal durability and maintains high nitrogen oxide purification activity even after durability treatment.

  The ratio of the isolated iron ion having a symmetric tetrahedral structure to the total iron content in the β-type iron silicate of the present invention exceeds 20% even after the endurance treatment, particularly 40% or more, and more preferably 50% or more. Are preferred.

  The absolute amount of isolated iron ions having a symmetric tetrahedral structure in the β-type iron silicate of the present invention is more than 0.6 mol% even after endurance treatment, in particular 1 mol% or more, and further 1.5 mol. % Or more is preferred.

  The ratio of isolated iron ions in the β-type iron silicate of the present invention is preferably more than 50% even after endurance treatment, but it is not only isolated iron ions but also has the above-mentioned symmetrical tetrahedral structure. Is particularly preferred.

  Next, a method for producing the β-type iron silicate of the present invention will be described.

In the β-type iron silicate of the present invention, the composition of the obtained β-type iron silicate is such that the SiO ₂ / Fe ₂ O ₃ molar ratio is 20 to 300 and the log (SiO ₂ / Al ₂ O ₃ ) is 2 or more. It can manufacture by crystallizing with a preparation composition.

  In crystallization under such conditions, it is possible to introduce isolated iron ions that are highly dispersed as isolated iron ions in the skeleton structure of β-type iron silicate and that have a highly symmetric tetrahedral structure.

  If the amount of iron is too large, aggregation is likely to proceed due to fresh baking or durability treatment, and isolated iron ions having an isolated symmetric tetrahedral structure are not sufficiently introduced, and the crystallinity of β-type iron silicate is likely to deteriorate.

  Al and Fe are expected to be a competitive reaction when introduced into the skeleton of β-type silicate, but in the present invention, the content of competing Al is low, so that isolated iron ions having a symmetric tetrahedral structure. It is thought that it contributes to increasing the ratio.

  The raw material for synthesis is composed of a silica source, an iron source, SDA and water, and an aluminum source and a fluorine source are used as necessary.

  As the silica source, colloidal silica, amorphous silica, sodium silicate, tetraethylorthosilicate, aluminosilicate gel and the like can be used. As the iron source, iron nitrate, iron chloride, iron sulfate, metallic iron and the like can be used. These raw materials are preferably those that can be sufficiently uniformly mixed with other components.

  As SDA raw materials, tetraethylammonium hydroxide having tetraethylammonium cation, tetraethylammonium bromide, tetraethylammonium fluoride, octamethylenebiskinucridium, α, α′-diquinuclidium-p-xylene, α, α′-diquinuclidium- m-xylene, α, α′-diquinuclidium-o-xylene, 1,4-diazabicyclo [2,2,2] octane, 1,3,3, N, N-pentamethyl-6-azonium bicyclo [3,2 , 1] octane or at least one of a group of compounds containing N, N-diethyl-1,3,3-trimethyl-6-azonium bicyclo [3,2,1] octane cation can be used.

  As the aluminum source, aluminum sulfate, sodium aluminate, aluminum hydroxide, aluminum nitrate, aluminosilicate gel, metal aluminum, etc. can be used. As the fluorine source, hydrofluoric acid, sodium fluoride, potassium fluoride, ammonium fluoride, Tetraethylammonium fluoride or the like can be used.

  These raw materials are preferably easy to mix uniformly with other components.

  The following composition range is illustrated as a preparation composition of a raw material mixture. However, these composition ranges are not limited, and can be arbitrarily set so that the final product composition falls within the composition range of the β-type iron silicate of the present invention. In addition, a component having a crystallization promoting action such as a seed crystal may be added, and conditions under which a large crystal grain size can be obtained are preferable.

SiO ₂ / Al ₂ O ₃ molar ratio is 15 or more, and Al ₂ O ₃ may not be added. SiO ₂ / Fe ₂ O ₃ molar ratio 20 to 300, preferably 100 or less H ₂ O / SiO ₂ molar ratio 5 50, preferably 5-10
SDA / SiO ₂ molar ratio of 0.1 to 5, preferably 0.1 to 1
F / SiO ₂ molar ratio 0-5, preferably 0-1
Β-type iron according to the present invention by crystallizing a raw material mixture of water, silica source, iron source, SDA, and optionally an aluminum source and a fluorine source in a sealed pressure vessel at a temperature of 100 to 180 ° C. A silicate can be obtained.

  At the time of crystallization, the raw material mixture may be mixed and stirred, but in order to obtain crystals having high heat resistance, a stationary state is preferable. After completion of crystallization, the mixture is allowed to cool sufficiently, separated into solid and liquid, washed with a sufficient amount of pure water, and dried at a temperature of 110 to 150 ° C. to obtain the β-type iron silicate according to the present invention.

  The β-type iron silicate of the present invention can be used as it is as a purification catalyst for nitrogen oxides. Since the β-type iron silicate immediately after synthesis contains SDA in the pores, it is preferably used as a purification catalyst for nitrogen oxides after removing these if necessary.

As the SDA removal treatment, a liquid phase treatment using a chemical solution containing an acidic solution or an SDA decomposition component, an exchange treatment using a resin or the like, or a thermal decomposition treatment may be employed, and these treatments may be combined. Furthermore, it can be used by converting into β-type or NH ₄ type by utilizing the ion exchange ability of β-type iron silicate.

  The crystal grain size (SEM particle size) of the β-type iron silicate of the present invention is preferably larger. The SEM diameter is more than 5 μm, preferably 7 μm or more, particularly preferably 10 μm or more. A large crystal grain size is easily obtained by performing crystallization without stirring.

  Since the β-type iron silicate of the present invention contains active isolated iron ions, it can be used as it is as a purification catalyst for nitrogen oxides, but it may also be used by supporting a catalytically active metal species. .

  The metal species to be supported is not particularly limited, but is, for example, at least one selected from the group consisting of elements of groups 8, 9, 10, and 11, particularly iron, cobalt, palladium, iridium, platinum, copper, silver, and gold. In particular, at least one of iron, palladium, platinum, copper, and silver is preferable.

  In addition, a promoter component such as rare earth metal, titanium or zirconia can be additionally added. There are no particular limitations on the loading method when loading a catalytically active metal species. As the loading method, methods such as an ion exchange method, an impregnation loading method, an evaporation to dryness method, a precipitation loading method, and a physical mixing method can be employed. Nitrate, sulfate, acetate, chloride, complex salt, oxide, composite oxide and the like can be used as raw materials for metal loading.

  The amount of metal supported is not limited, but a range of 0.1 to 10% by weight is particularly preferable.

  The catalyst of the present invention can be used after being mixed with a binder such as silica, alumina and clay mineral. Examples of clay minerals used for molding include kaolin, attapulgite, montmorillonite, bentonite, allophane, and sepiolite. Moreover, it can also be used by wash-coating on a cordierite or metal honeycomb substrate.

  By using β-type iron silicate of the present invention as a nitrogen oxide purification catalyst and reacting at least one of ammonia, urea, and organic amines as a reducing agent, nitrogen oxide in exhaust gas can be selectively reduced. it can.

  The nitrogen oxides purified by the present invention are nitric oxide, nitrogen dioxide and mixtures thereof. Here, the nitrogen oxide concentration in the exhaust gas treated according to the present invention is not limited.

  The method of adding the reducing agent is not particularly limited, and a method of directly adding the reducing component in a gaseous state, a method of spraying and vaporizing a liquid such as an aqueous solution, a method of spraying pyrolysis, and the like can be employed. The addition amount of these reducing agents can be arbitrarily set so that nitrogen oxides can be sufficiently purified.

  The exhaust gas may contain components other than nitrogen oxides, and may contain, for example, hydrocarbons, carbon monoxide, carbon dioxide, hydrogen, nitrogen, oxygen, sulfur oxides, and water. Specifically, the method of the present invention can purify nitrogen oxides from a wide variety of exhaust gases such as diesel vehicles, gasoline vehicles, boilers, gas turbines and the like.

In the method for purifying nitrogen oxides of the present invention, the space velocity when contacting the exhaust gas with the catalyst comprising the β-type iron silicate of the present invention is not particularly limited, but the preferred space velocity is 500 to 500,000 hr ^{−1 on a} volume basis. More preferably, it is 2,000 to 300,000 hr ⁻¹ .

  The β-type iron silicate of the present invention has high nitrogen oxide purification performance and can efficiently purify nitrogen oxide in a wide temperature range, particularly at a low temperature of 250 ° C. or lower. Moreover, it is excellent in durability and has high catalytic activity even after durability treatment.

  EXAMPLES Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples.

(UV-visible absorption measurement)
The ultraviolet-visible absorption measurement was performed under the following conditions.

Integrating sphere attachment device :: ISR-3100 manufactured by Shimadzu Corporation
Self-recording spectrophotometer: UV-3100 manufactured by Shimadzu Corporation
Scan speed: 200 nm / min
Slit width: 5.0nm
Baseline correction: Barium sulfate powder used Wavelength range: 220-700 nm reflectance measurement Sample pretreatment: 600 ° C. firing in dry air (fresh firing)
(Electron spin resonance measurement)
Electron spin resonance measurement was performed under the following conditions.

Measurement temperature: 77K
Microwave output: 1.0 mW
Observation range: 0 to 1000 mT
Modulation width: 0.32 mT
Time constant: 0.3 sec
Sample amount: about 10mg
(Nitrogen oxide purification test)
The iron silicate powders synthesized in Examples and Comparative Examples were pressed and then crushed and sized to 12-20 mesh. 1.5 cc of the sized powder was filled into a normal pressure fixed bed flow type reaction tube. While the gas having the composition shown in Table 2 below was passed through the catalyst layer at 1500 cc / min, the nitrogen oxide removal rate at a constant temperature of 100 to 500 ° C. was measured.

窒素酸化物の除去活性は下式で表される。

The nitrogen oxide removal activity is represented by the following formula.

ここで、Ｘ_ＮＯｘは窒素酸化物の浄化率（％）、［ＮＯｘ］_ｉｎは入りガスの窒素酸化物濃度、［ＮＯｘ］_ｏｕｔは出ガスの窒素酸化物濃度を示す。

Here, X _NOx represents the nitrogen oxide purification rate (%), [NOx] _in represents the nitrogen oxide concentration of the incoming gas, and [NOx] _out represents the nitrogen oxide concentration of the outgoing gas.

The performance after the endurance treatment was as follows: 3 cc of each catalyst was filled in a normal pressure fixed bed flow type reaction tube, and the air containing H ₂ O = 10 vol% was circulated at 300 cc / min for 20 hours at 700 ° C. The nitrogen oxide removal rate was measured under the same conditions as above.

Example 1
9.43 g of iron nitrate nonahydrate is dissolved in 264 g of a 35% aqueous solution of tetraethylammonium hydroxide (hereinafter referred to as “TEAOH”), and 214 g of tetraethylorthosilicate (TEOS) is added thereto, followed by thorough mixing at room temperature. Hydrolysis was performed and the ethanol produced was evaporated. Subsequently, the required amount of water was evaporated. After adding 21.45 g of 48% hydrofluoric acid to this and mixing in a mortar, the reaction mixture was filled in a stainless steel autoclave and crystallized by heating at 150 ° C. for 240 hours. The composition of the reaction mixture was 62SiO ₂ : Fe ₂ O ₃ : 31HF: 37.8TEAOH: 465H ₂ O. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C.

The dried powder was calcined at 600 ° C. for 2 hours under air flow. As a result of X-ray diffraction measurement, β-type iron silicate has the X-ray diffraction pattern of Table 1. As a result of ICP emission analysis, SiO ₂ / Al ₂ O ₃ molar ratio is 4743, SiO ₂ / Fe ₂ O ₃ molar ratio Was 62. The crystal grain size was about 7.5 μm.

Example 2
No. 3 sodium silicate (SiO ₂ ; 402 g / l, Na ₂ O; 129 g / l, Al ₂ O ₃ ; 0.22 g / l), 98% sulfuric acid, water and iron nitrate nonahydrate The produced gel was subjected to solid-liquid separation with Nutsche, and then a sufficient amount of washing was performed with pure water. A predetermined amount of water, TEAOH, and NaOH were added to the washed gel and mixed thoroughly with stirring. The composition ratio of the reaction mixture was 1965SiO ₂ : Al ₂ O ₃ : 30.05Fe ₂ O ₃ : 98.27Na ₂ O: 786.2TEAOH: 19654H ₂ O. The reaction mixture was sealed in a stainless steel autoclave and crystallized by heating at 150 ° C. for 90 hours under rotating conditions. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was calcined at 600 ° C. for 2 hours under air flow.

Example 3
The composition ratio of the reaction mixture to be crystallized was changed to 500SiO ₂ : Al ₂ O ₃ : 7.69Fe ₂ O ₃ : 250HF: 305TEAOH: 3750H ₂ O, and Example 1 was used except that aluminum nitrate nonahydrate was used. A reaction mixture was prepared in the same manner. This reaction mixture was charged in a stainless steel autoclave and crystallized by heating at 150 ° C. for 240 hours. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was calcined at 600 ° C. for 2 hours under air flow.

Comparative Example 1
A reaction mixture was prepared in the same manner as in Example 1, except that the composition ratio of the reaction mixture to be crystallized was changed to 70SiO ₂ : Al ₂ O ₃ : Fe ₂ O ₃ : 35HF: 42TEAOH: 490H ₂ O. This reaction mixture was charged in a stainless steel autoclave and crystallized by heating at 150 ° C. for 240 hours. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was calcined at 600 ° C. for 2 hours under air flow.

Comparative Example 2
Tosoh β-type zeolite (trade name: HSZ-940NHA) having a SiO ₂ / Al ₂ O ₃ molar ratio of 40 was calcined at 600 ° C. in a dry air stream. From the X-ray diffraction, the β-type zeolite had the X-ray diffraction pattern shown in Table 1, and the SiO ₂ / Al ₂ O ₃ molar ratio in the ICP emission analysis was 40. The β-type zeolite supporting iron amount is accurately weighed so as to be 3 _{wt% Fe (NO 3) 3 ·} 9 with an aqueous solution of hydrate was impregnated supporting iron. This was air calcined at 500 ° C.

Comparative Example 3
To 235 g of TEAOH, 1.26 g of aluminum hydroxide, 8.36 g of iron nitrate, 62.8 g of amorphous silica powder (trade name: Nipsil VN-3) manufactured by Tosoh Silica, and 172 g of water were added and mixed thoroughly. The composition of the reaction mixture was 90SiO ₂ : Al ₂ O ₃ : Fe ₂ O ₃ : 54TEAOH: 1800H ₂ O. The reaction mixture was sealed in a stainless steel autoclave and heated at 150 ° C. for 96 hours for crystallization. The slurry mixture after crystallization was white. This was separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. The dried powder was fired at 600 ° C. under air flow.

Table 3 (before endurance treatment; fresh) and Table 4 (after endurance treatment) show the results of evaluating the nitrogen oxide purification performance before and after the endurance treatment of the β-type iron silicate obtained in Examples and Comparative Examples. For the catalyst before and after the endurance treatment, the ratio of isolated iron ions, the ratio of isolated iron ions having a symmetric tetrahedral structure and the low temperature (200 ° C., 250 ° C.) determined by UV-visible absorption measurement and electron spin resonance measurement. Table 5 (before endurance treatment; fresh) and Table 6 (after endurance treatment) show the relationship between nitrogen oxide purification rates (after endurance), and log (SiO ₂ / Al ₂ O ₃ ) and nitrogen at low temperature (200 ° C.). Table 7 shows the relationship between the oxide purification rates.

ｌｏｇ（ＳｉＯ_２／Ａｌ_２Ｏ_３）が２以上において、耐久後における低温（２００℃）
の浄化率の急激な立ち上がりが認められ、ｌｏｇ（ＳｉＯ_２／Ａｌ_２Ｏ_３）が２において
臨界的な変曲点が認められた。

log (SiO ₂ / Al ₂ O ₃ ) of 2 or more, low temperature after durability (200 ° C.)
A sharp rise in the purification rate was observed, and a critical inflection point was observed when log (SiO ₂ / Al ₂ O ₃ ) was 2.

An example (Example 1) of the electron spin resonance spectrum of the β-type iron silicate before and after the durability treatment. An example of the electron spin resonance spectrum of the iron-supported β-type zeolite before and after the durability treatment (Comparative Example 2). The ultraviolet-visible absorption spectrum of the β-type iron silicate of the present invention (Example 1). Relationship between nitrogen oxide purification rate (200 ° C.) before and after endurance treatment and log (SiO ₂ / Al ₂ O ₃ ).

Claims (5)

A nitrogen oxide purification catalyst which is a crystalline silicate having iron in a β skeleton structure, wherein the SiO ₂ / Fe ₂ O ₃ molar ratio is 20 to 300 and the log (SiO ₂ / Al ₂ O ₃ ) is 2 or more. The nitrogen oxide purification catalyst according to claim 1, wherein the number of isolated iron ions having a symmetric tetrahedral structure is 70% or more of the isolated iron ions. 2. The nitrogen oxide purification catalyst according to claim 1, wherein the number of isolated iron ions having a symmetric tetrahedral structure is 60% or more of the total content of iron. 4. The nitrogen oxide purification catalyst according to claim 1, wherein the content of isolated iron ions having a symmetric tetrahedral structure (Fe ³⁺ / (Si + Al + Fe)) is 2 mol% or more. 5. The nitrogen oxide purification catalyst according to claim 1, wherein nitrogen oxide is selectively reduced by reacting with at least one of ammonia, urea, and organic amines as a reducing agent. Nitrogen oxide purification method.

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