JP2018140367A - NOx OCCLUSION REDUCTION TYPE CATALYST AND METHOD FOR MANUFACTURING SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a NOx occlusion reduction type catalyst that can easily release sulfur even in the case of sulfur poisoning, and thereby can realize excellent NOx purification performance.SOLUTION: Provided is a NOx occlusion reduction type catalyst characterized by comprising a carrier containing a complex oxide of iron, magnesium and aluminum and a NOx occlusion material supported on the carrier and containing an active metal comprising a noble metal and at least Ba, represented by the following composition formula: MgFeΔAlO(In the formula, Δ represents a defect site of iron and magnesium, x is 0.02 to 0.70, y is 0.02 to 0.60, and x + y is 0.04 to 0.80.).SELECTED DRAWING: None

Description

  The present invention relates to a NOx occlusion reduction type catalyst and a method for producing the same, and more particularly to a NOx occlusion reduction type catalyst containing a composite oxide containing at least aluminum as a carrier and a method for producing the same.

  Conventionally, NOx has been catalyzed in a lean atmosphere in order to purify nitrogen oxides (NOx) contained in exhaust gas discharged from internal combustion engines such as diesel engines and lean burn engines with low fuel consumption. NOx occlusion reduction type that occludes in the upper adsorbent (Ba, K, etc.) and exposes the catalyst to a reducing atmosphere for a short time (intermittently rich atmosphere) to release the occluded NOx and reduce and purify it. Catalysts (NSR catalysts: NOx Storage Reduction catalysts) and the like have been developed.

As such a NOx occlusion reduction type catalyst, JP-A-2006-107260 (Patent Document 1) discloses a support containing a composite oxide containing at least zinc and aluminum, an active metal supported on the support, and A composite oxide comprising a composition formula: Zn _x Mg _y □ _(1-xy) Al ₂ O ₄ (wherein x represents a number of 0.02 to 0.70, y represents a number from 0 to 0.60, satisfies the condition that the sum of x and y is 0.02 or more and 0.70 or less, and □ represents a defect site of zinc and magnesium. Zn _x Mg _y □ _(1-xy) Al ₂ O ₄ of a diffraction peak obtained from an X-ray diffraction pattern using a CuKα ray obtained by X-ray diffraction measurement of the support. Main peak area (A) and Zn The value of the main peak area of to the total of (B) and the main peak area of MgO (C), the Zn _x □ _{1-x Al} 2 ratio of the main peak area of _{O 4 (A) (A /} [A + B + C]) Is a NOx occlusion reduction catalyst in which the active metal is a noble metal and the NOx occlusion material contains at least barium or potassium.

JP-A-2006-107260

  However, the present inventors have found that the NOx occlusion reduction type catalyst described in Patent Document 1 has a problem that, when sulfur poisoned, sufficient NOx purification performance is not exhibited even after sulfur is desorbed. It was.

  The present invention has been made in view of the above-described problems of the prior art, and even when sulfur poisoning (S poisoning) is performed, sulfur (S) can be easily detached, thereby An object of the present invention is to provide a NOx occlusion reduction type catalyst capable of exhibiting excellent NOx purification performance and a method for producing the same.

  As a result of intensive studies to achieve the above object, the present inventors have conducted a NOx occlusion reduction comprising a support containing a composite oxide containing at least aluminum, and an active metal and NOx occlusion material supported on the support. In the type catalyst, by containing iron and magnesium at a specific atomic ratio in the composite oxide, even when sulfur poisoning is performed, sulfur can be desorbed like this, The inventors have found that NOx purification performance is manifested, and have completed the present invention.

That is, the NOx storage reduction catalyst of the present invention has the following composition formula:
Mg _x Fe _y □ _(1-xy) Al ₂ O ₄
(In the formula, □ represents a defect site of iron and magnesium, x is 0.02 to 0.70, y is 0.02 to 0.60, and x + y is 0.04 to 0.80. .)
And a support containing a composite oxide of iron, magnesium and aluminum, and an NOx occlusion material which is supported on the support and contains an active metal composed of a noble metal and at least Ba. To do.

In such a NOx occlusion reduction type catalyst of the present invention, it is preferable that the specific surface area of the carrier is 60 m ² / g or more. Moreover, it is preferable that y in the said composition formula is 0.10-0.50.

The method for producing a NOx occlusion reduction catalyst of the present invention includes adding an alkaline compound to a solution containing aluminum ions, magnesium ions, and iron ions, generating a precipitate by a coprecipitation method, and firing the precipitate. According to the following composition formula:
Mg _x Fe _y □ _(1-xy) Al ₂ O ₄
(In the formula, □ represents a defect site of iron and magnesium, x is 0.02 to 0.70, y is 0.02 to 0.60, and x + y is 0.04 to 0.80. .)
A step of preparing a carrier containing a composite oxide of iron, magnesium and aluminum represented by:
The NOx occlusion reduction according to any one of claims 1 to 3, wherein the support is loaded with an active metal composed of a noble metal and a NOx occlusion material containing at least Ba, and then calcined. Obtaining a type catalyst; and
It is the method characterized by including.

  In such a method for producing a NOx storage reduction catalyst of the present invention, the alkaline compound is preferably ammonia water.

  In addition, even if it is a case where sulfur poisoning is carried out by the NOx occlusion reduction type catalyst of the present invention, it is not necessarily certain why sulfur can be easily desorbed and thereby exhibit excellent NOx purification performance. However, the present inventors infer as follows. That is, in the NOx occlusion reduction type catalyst described in Patent Document 1, in order to improve the NOx purification performance, by introducing magnesium into the composite oxide containing zinc and aluminum, the support made of the composite oxide is provided. The amount of base points is increased. However, when the amount of the base point of the carrier increases, it is presumed that the NOx purification performance deteriorates because sulfur poisoning easily occurs and sulfur does not easily desorb from the carrier.

  On the other hand, in the NOx occlusion reduction type catalyst of the present invention, in the composite oxide of zinc, magnesium and aluminum, iron is introduced instead of zinc to reduce the base point amount of the carrier, thereby eliminating sulfur. As a result, it is assumed that even when sulfur poisoning is performed, sulfur can be easily desorbed, thereby improving the NOx purification performance.

  According to the present invention, even when sulfur poisoning (S poisoning) is performed, sulfur (S) can be easily desorbed, and thereby excellent NOx purification performance can be exhibited. It becomes possible to obtain a NOx occlusion reduction type catalyst.

It is a graph which shows the sulfur desorption amount of the NOx occlusion reduction type catalyst obtained in Examples 1-2 and Comparative Examples 1-4. It is a graph which shows the NOx purification rate after sulfur desorption of the NOx occlusion reduction type catalyst obtained in Examples 1-2 and Comparative Examples 1-4.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

[NOx storage reduction catalyst]
First, the NOx storage reduction catalyst of the present invention will be described. The NOx occlusion reduction type catalyst of the present invention has the following composition formula:
Mg _x Fe _y □ _(1-xy) Al ₂ O ₄
(In the formula, □ represents a defect site of iron and magnesium, x is 0.02 to 0.70, y is 0.02 to 0.60, and x + y is 0.04 to 0.80. .)
And a support containing a composite oxide of iron, magnesium and aluminum, and an NOx occlusion material which is supported on the support and contains an active metal composed of a noble metal and at least Ba. Such a NOx occlusion reduction type catalyst can easily desorb sulfur even when sulfur poisoning is performed, thereby exhibiting excellent NOx purification performance.

(Carrier)
The carrier in the NOx storage reduction catalyst of the present invention has the following composition formula:
Mg _x Fe _y □ _(1-xy) Al ₂ O ₄
(In the formula, □ represents a defect site of iron and magnesium, x is 0.02 to 0.70, y is 0.02 to 0.60, and x + y is 0.04 to 0.80. .)
Containing a composite oxide of iron, magnesium and aluminum.

  In such a composite oxide, the value of x in the composition formula, that is, the atomic ratio of magnesium (Mg) is 0.02 to 0.70. When the value of x is less than the lower limit, it becomes difficult to maintain the crystal structure in a lean atmosphere. On the other hand, when the value exceeds the upper limit, sulfur is not easily desorbed when sulfur poisoning occurs, and NOx purification is performed. Performance decreases. As such a value of x, it is possible to maintain a crystal structure in a lean atmosphere, and when sulfur poisoning is performed, sulfur can be easily desorbed, and excellent NOx purification performance can be obtained. 0.10 to 0.65 is preferable, 0.15 to 0.60 is more preferable, and 0.20 to 0.60 is particularly preferable.

  In the composite oxide, the value of y in the composition formula, that is, the atomic ratio of iron (Fe) is 0.02 to 0.60. When the value of y is less than the lower limit, sulfur is not easily desorbed when sulfur poisoning occurs, and the NOx purification performance decreases. On the other hand, when the upper limit is exceeded, the crystal structure is maintained in a lean atmosphere. It becomes difficult. As such a value of y, when sulfur poisoning is performed, sulfur can be easily desorbed, excellent NOx purification performance is obtained, and from the viewpoint of maintaining a crystal structure in a lean atmosphere, 0.10 to 0.50 is preferable, 0.15 to 0.40 is more preferable, and 0.20 to 0.30 is particularly preferable.

  Furthermore, in the composite oxide, the sum (x + y) of x and y in the composition formula is 0.04 to 0.80. When the value of x + y is less than the lower limit, the amount of magnesium and iron introduced into the defect site of aluminum oxide is reduced, the effect of suppressing solid solution deterioration of the NOx storage material is reduced, and the properties of the support as aluminum oxide are reduced. Appears remarkably, and the NOx purification performance decreases. On the other hand, when the value of x + y exceeds the upper limit, the specific surface area becomes small, so that the NOx purification performance decreases. The value of x + y is preferably 0.10 to 0.80, more preferably 0.30 to 0.80, and particularly preferably 0.40 to 0.75, from the viewpoint of improving the NOx purification performance. .

  Note that the composition of such a complex oxide includes an XRF (X-ray Fluorescence Analysis), a post-inductively coupled plasma (ICP) emission analyzer, an EDX (energy dispersive X-ray detection). Apparatus), XPS (photoelectron spectrometer), SIMS (secondary ion mass spectrometer), HR-TEM (high resolution transmission electron microscope), FE-STEM (field emission-scanning transmission electron microscope), etc. It can confirm by the composition analysis combined suitably. Specifically, for example, in X-ray fluorescence analysis, composition analysis is performed by measuring X-ray intensity using a scanning X-ray fluorescence analyzer (for example, “ZSX Primus II” manufactured by Rigaku Corporation), Composition analysis of the composite oxide can be performed.

In the NOx occlusion reduction type catalyst of the present invention, the composite oxide represented by the composition formula is preferably a fine particle powder from the viewpoint of a large surface area per unit mass and an excellent NOx purification performance. . In addition, the average crystallite diameter of the composite oxide represented by the composition formula is preferably 2 nm or more, and more preferably 5 nm or more from the viewpoint that it is difficult to sinter even when used in a high temperature range. Further, the upper limit of the average crystallite diameter of the composite oxide represented by the composition formula is preferably 20 nm or less, and more preferably 10 nm or less, from the viewpoint of securing a large specific surface area. Note that the average crystallite size of such a complex oxide can be obtained by analysis by a powder X-ray diffraction method, observation by a transmission electron microscope (TEM), a scanning electron microscope (SEM), or the like. For example, in the analysis by the powder X-ray diffraction method, first, the powdered composite oxide represented by the composition formula is analyzed by the powder X-ray diffraction method, and a predetermined crystal plane (hkl) is obtained from the obtained X-ray diffraction pattern. ) The half-value width B _hkl (radian) of the diffraction line is obtained. Then, an average value D _hkl (nm) of crystallite diameters in a direction perpendicular to the (hkl) crystal plane of the composite oxide particles is calculated by Scherrer's formula: D _hkl = Kλ / B _hkl cos θ _hkl . In the Scherrer equation, the constant K is 0.89, λ is the X-ray wavelength (nm), and θ _hkl is the diffraction angle (°). The “average crystallite diameter” is a value obtained by the powder X-ray diffraction method and means an average value D ₃₁₁ (nm) of crystallite diameters in a direction perpendicular to the (311) plane.

In the NOx storage reduction catalyst of the present invention, the specific surface area of the support containing the composite oxide represented by the composition formula is preferably 60 m ² / g or more, and 70 m ² / g or more. Is more preferable. The upper limit of the specific surface area of the carrier is preferably 200 m ² / g or less. If the specific surface area of the support is less than the lower limit, the dispersibility of the active metal tends to be reduced, and sufficient NOx purification performance tends to be difficult to develop. On the other hand, if the upper limit is exceeded, grain growth of the support tends to occur. There is a tendency. Such a specific surface area can be calculated as a BET specific surface area from an adsorption isotherm using a BET isotherm adsorption formula. In addition, such a BET specific surface area can be calculated | required using a commercially available apparatus.

  Furthermore, in the NOx occlusion reduction type catalyst of the present invention, there is provided a carrier containing the composite oxide represented by the above composition formula from the viewpoint that the amount of occlusion of NOx is increased and more excellent NOx purification performance is expressed. It is preferably in powder form. The average particle size of the powdery carrier is preferably 0.5 to 100 μm, and more preferably 1 to 10 μm. When the average particle diameter of the carrier in powder form is less than the lower limit, grain growth of the carrier tends to occur. On the other hand, when the upper limit is exceeded, the specific surface area becomes small and sufficient NOx purification performance is exhibited. It tends to be difficult. In addition, such an average particle diameter is observed with a scanning electron microscope (SEM), a transmission electron microscope (TEM), an electron beam microanalyzer (EPMA), etc., for example, 20 or more (preferably, preferably) , 100 or more) carrier particles are extracted and the particle size is measured, and the average value thereof is calculated or the particle size distribution of the carrier particles is created. It can also be determined by analysis by a powder X-ray diffraction method.

In addition, in the NOx occlusion reduction type catalyst of the present invention, the carrier containing the composite oxide represented by the composition formula is not limited to the composite oxide represented by the composition formula within a range not impairing the effects of the present invention. Other oxides may be included. Examples of such other oxides include iron (III) oxide (Fe ₂ O ₃ ), magnesium oxide (MgO), and aluminum oxide (Al ₂ O ₃ ).

(Active metal)
The active metal in the NOx occlusion reduction catalyst of the present invention is composed of a noble metal and is supported on the carrier. Examples of such noble metals include platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir), and ruthenium (Ru). These noble metals may be used alone or in combination of two or more. Among these noble metals, Pt, Pd, and Rh are preferable, Pt and Pd are more preferable, and Pt and Pd are particularly preferably used in combination from the viewpoint of improving the NOx purification performance.

  The supported amount of the active metal composed of such a noble metal is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 5 parts by mass in terms of metal with respect to 100 parts by mass of the carrier. If the loading amount of the active metal is less than the lower limit, sufficient NOx purification performance tends to be not obtained. On the other hand, even if the active metal is loaded exceeding the upper limit, the NOx purification performance is saturated and the cost increases. Tend to.

  Moreover, the average particle diameter of such an active metal composed of a noble metal is preferably 0.5 to 100 nm, and more preferably 1 to 500 nm. When the average particle diameter of the active metal is less than the lower limit, grain growth tends to occur. On the other hand, when the average particle diameter exceeds the upper limit, the NOx purification performance tends to decrease. Such an average particle diameter of the noble metal can be determined by a conventionally known CO chemical adsorption method, observation of an STEM image using Cs-STEM, or the like.

(NOx storage material)
The NOx occlusion material in the NOx occlusion reduction catalyst of the present invention contains at least Ba and is supported on the carrier. Such a NOx occlusion material may or may not contain other metals as long as it contains at least Ba. Examples of the other metals include alkali metals such as lithium (Li), sodium (Na), potassium (K), and cesium (Cs); alkaline earth metals such as magnesium (Mg), calcium (Ca), and strontium (Sr). A metal is mentioned. The alkali metal has a high NOx storage capability in a high temperature range, and the alkaline earth metal has a high NOx storage capability in a low temperature range.

  The amount of the NOx occlusion material supported is preferably 0.01 to 0.4 mol, more preferably 0.02 to 0.2 mol, per 100 g of the carrier. When the loading amount of the NOx storage material is less than the lower limit, sufficient NOx storage performance tends to be not obtained. On the other hand, when the amount exceeds the upper limit, the NOx storage material may block pores or coat active metal. It is generated and the catalytic activity of the active metal and the NOx purification performance tend to decrease. Further, when an alkali metal and an alkaline earth metal are used in combination as the NOx storage material, the molar ratio of the supported alkali metal to the alkaline earth metal (alkali metal: alkaline earth metal) is 5: 1 to 1: 5. Preferably there is. When the molar ratio of the alkali metal to the alkaline earth metal is less than the lower limit, the NOx storage capacity in the high temperature range tends to decrease. On the other hand, when the molar ratio exceeds the upper limit, the reaction of NOx and oxygen in the low temperature range. Tend to decrease.

(NOx storage reduction catalyst)
The NOx occlusion reduction type catalyst of the present invention comprises a support containing a composite oxide of iron, magnesium and aluminum represented by the above composition formula, an active metal composed of a noble metal and at least Ba supported on the support. The NOx occlusion material is included. Although there is no restriction | limiting in particular as a form of the NOx occlusion reduction type | mold catalyst of this invention, It is a powdery thing (catalyst powder), shape | molded into a pellet form (pellet catalyst), what coated the base material (monolith catalyst) Is mentioned. Examples of the substrate include a particulate filter substrate (DPF substrate), a honeycomb substrate, a pellet substrate, and a plate substrate. Moreover, there is no restriction | limiting in particular also about the material of such a base material, For example, metals, such as ceramics, such as cordierite, silicon carbide, and mullite, and stainless steel containing chromium and aluminum are mentioned.

[Method for producing NOx storage reduction catalyst]
Next, a method for producing the NOx storage reduction catalyst of the present invention will be described. The method for producing a NOx occlusion reduction catalyst of the present invention includes adding an alkaline compound to a solution containing aluminum ions, magnesium ions, and iron ions, generating a precipitate by a coprecipitation method, and firing the precipitate. According to the following composition formula:
Mg _x Fe _y □ _(1-xy) Al ₂ O ₄
(In the formula, □ represents a defect site of iron and magnesium, x is 0.02 to 0.70, y is 0.02 to 0.60, and x + y is 0.04 to 0.80. .)
A step of preparing a carrier containing a composite oxide of iron, magnesium and aluminum represented by:
A step of obtaining the NOx occlusion reduction type catalyst of the present invention by supporting an active metal composed of a noble metal and a NOx occlusion material containing at least Ba on the carrier;
Contains. As a result, even when sulfur poisoning is performed, sulfur can be easily desorbed, whereby a NOx occlusion reduction type catalyst capable of exhibiting excellent NOx purification performance can be obtained.

(Carrier preparation process)
In the method for producing a NOx storage reduction catalyst of the present invention, first, an alkaline compound is added to a solution containing aluminum ions, magnesium ions, and iron ions to form a precipitate by a coprecipitation method. By firing, the following composition formula:
Mg _x Fe _y □ _(1-xy) Al ₂ O ₄
(In the formula, □ represents a defect site of iron and magnesium, x is 0.02 to 0.70, y is 0.02 to 0.60, and x + y is 0.04 to 0.80. .)
A carrier containing a composite oxide of iron, magnesium and aluminum represented by

  The solution containing aluminum ions, magnesium ions and iron ions used in the carrier preparation step can be prepared by dissolving an aluminum compound, a magnesium compound and an iron compound in a solvent. The aluminum compound, magnesium compound and iron compound are not particularly limited as long as they are dissolved in a solvent to form ions. For example, salts (sulfates, nitrates, chlorides) of these metals (aluminum, magnesium, iron) , Acetate, etc.). Examples of the solvent include water and alcohol. Each concentration of the aluminum compound, the magnesium compound, and the iron compound is appropriately set according to the atomic ratio of iron, magnesium, and aluminum in the obtained composite oxide.

  In such a carrier preparation step, an alkaline compound is added to such a solution containing aluminum ions, magnesium ions, and iron ions, and a precipitate containing iron, magnesium, and aluminum is generated by a coprecipitation method. . The alkaline compound is not particularly limited as long as it can generate a precipitate containing iron, magnesium and aluminum by a coprecipitation method. For example, ammonia water, ammonium carbonate, sodium hydroxide, water Examples include potassium oxide and sodium carbonate. Among these alkaline compounds, aqueous ammonia and ammonium carbonate are preferable from the viewpoint that they are removed by volatilization when the precipitate is fired or the composite oxide is fired.

Next, the precipitate produced in this manner is fired to prepare a carrier containing a composite oxide of iron, magnesium and aluminum represented by the above composition formula. As a calcination temperature, 200-900 degreeC is preferable and 300-800 degreeC is more preferable. The firing time is preferably 0.5 to 20 hours, and more preferably 1 to 10 hours. When the firing temperature or firing time is less than the above lower limit, the precipitate is not sufficiently fired, and a carrier containing a desired composite oxide tends to be not obtained. Since the grains grow, there is a tendency that sufficient NOx purification performance cannot be obtained. The atmosphere in the baking treatment is not particularly limited, but an oxidizing atmosphere (for example, in the air) or an inert gas (for example, N ₂ ) atmosphere is preferable.

(Supporting process)
Next, the support containing the composite oxide of iron, magnesium and aluminum obtained in the support preparation step is loaded with an active metal composed of a noble metal and a NOx occlusion material containing at least Ba. The NOx occlusion material supported by the carrier containing a composite oxide of iron, magnesium, and aluminum represented by the composition formula is easily maintained in a high valence state, and solid solution deterioration is suppressed, Excellent NOx purification performance appears.

  In this loading step, the order of loading the active metal and the NOx occlusion material on the carrier is not particularly limited, and the method of carrying the NOx occlusion material after loading the active metal on the carrier; Any method such as a method of supporting the active metal after supporting the NOx storage material on a support; a method of supporting the active metal and the NOx storage material simultaneously on the support may be adopted. From the viewpoint of preventing elution of the occlusion material into water, a method of carrying the NOx occlusion material after the active metal is carried on the carrier is preferable.

  In such a supporting step, the method for supporting the active metal on the carrier is not particularly limited. For example, a salt of the active metal (for example, acetate, carbonate, nitrate, ammonium salt, citrate, Examples of the method include a method in which a solution containing a dinitrodiammine salt) or a complex thereof (for example, a tetraammine complex) is contacted with the carrier and then dried. The method for bringing the solution containing the active metal salt or complex thereof into contact with the carrier is not particularly limited. For example, the method of impregnating the carrier with the solution containing the active metal salt or complex thereof, Examples thereof include a method in which a solution containing an active metal salt or a complex thereof is adsorbed and supported on the carrier.

  The active metal salt is preferably a dinitrodiammine salt when a platinum salt is used, and a nitrate or dinitrodiammine salt is preferred when a palladium salt is used, from the viewpoint of easy loading and high dispersibility. The solvent is not particularly limited, but a solvent (for example, water) that can form ions by dissolving the salt of the active metal is preferable.

  In such a loading step, the method for loading the NOx occlusion material on the carrier is not particularly limited. For example, a salt of the NOx occlusion material containing at least barium (Ba) (for example, carbonate, nitrate, (Citrate, carboxylate, dicarboxylate, sulfate) or a solution containing the complex is brought into contact with the carrier, followed by a drying treatment. The method for bringing the solution containing the NOx storage material salt or its complex into contact with the carrier is not particularly limited. For example, the method for impregnating the carrier with the solution containing the NOx storage material salt or its complex is available. And a method of adsorbing and supporting a solution containing a salt of the NOx storage material or a complex thereof on the carrier.

  As the salt of the NOx storage material, a carboxylate (for example, acetic acid, for example) can be efficiently prevented from forming an oxide coating layer on the surface of the NOx storage material, and more excellent NOx purification performance is exhibited. Salt, butyrate, stearate) and dicarboxylate (for example, oxalate, malonate), and acetate is more preferable from the viewpoint of production cost.

In the method for producing the NOx occlusion reduction catalyst of the present invention, the active metal and the NOx occlusion material are supported on the carrier in this way, and then subjected to a firing treatment, whereby the NOx occlusion reduction type of the present invention is performed. A catalyst can be obtained. As a calcination temperature, 200-800 degreeC is preferable, 300-700 degreeC is more preferable, 400-600 degreeC is especially preferable. Moreover, as baking time, 1 to 20 hours are preferable and 2 to 10 hours are more preferable. When the firing temperature or firing time is less than the lower limit, firing is not sufficiently achieved and sufficient NOx purification performance tends to be not obtained. On the other hand, when the upper limit is exceeded, the active metal and the NOx storage material are not obtained. It is not dispersedly supported, and there is a tendency that sufficient NOx purification performance cannot be obtained. The atmosphere in the baking treatment is not particularly limited, but an oxidizing atmosphere (for example, in the air) or an inert gas (for example, N ₂ ) atmosphere is preferable. In addition, such a baking process may be performed for every carrying | support process, and may be performed after performing a carrying process in multiple times.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
First, 378.88 g of aluminum nitrate nonahydrate was dissolved in 800 g of ion-exchanged water. Moreover, 26.80 g of magnesium acetate tetrahydrate and 50.50 g of iron nitrate nonahydrate were dissolved in 800 g of ion-exchanged water. After mixing these solutions and stirring at room temperature for 5 minutes, 650 g of 25 mass% ammonia aqueous solution was added, and also stirred for 30 minutes, and the deposit was produced | generated. The resulting precipitate was calcined at 400 ° C. for 5 hours in the air, and further calcined at 800 ° C. for 5 hours in the air to prepare a catalyst carrier comprising a composite oxide powder E1 of iron, magnesium and aluminum. did. This composite oxide powder E1 was subjected to composition analysis by X-ray intensity measurement using a scanning X-ray fluorescence analyzer (“ZSX Primus II” manufactured by Rigaku Corporation). As a result, composition formula: Mg _0.25 Fe _0.25 □ _0.5 Al ₂ O ₄ (in the formula, □ represents a defect site of iron and magnesium). Further, the nitrogen adsorption isotherm of the catalyst carrier made of the composite oxide powder E1 was measured, and the specific surface area was determined by the BET method. As a result, it was 159 m ² / g.

  Next, 0.075 g of platinum and 0.025 g of palladium were supported on 10 g of the obtained catalyst support made of the composite oxide powder E1 using a dinitrodiammine platinum nitric acid solution and a palladium nitrate solution, and then in the atmosphere. The PtPd supported catalyst was obtained by calcination at 2 ° C. for 2 hours.

Next, 6.6 g of the obtained PtPd-supported catalyst was drawn out onto a cordierite honeycomb substrate (diameter: 30 mm, length: 50 mm, volume: 35 ml, number of cells: 400 cells / inch ² , wall thickness: 4 mil). Coated by the method. Thereafter, a PtPd-supported catalyst supported on the honeycomb substrate was impregnated with an aqueous barium acetate solution to support 0.007 mol of barium, and then fired in the atmosphere at 500 ° C. for 5 hours to be supported on the honeycomb substrate. A NOx occlusion reduction catalyst E1 was obtained.

(Example 2)
A NOx occlusion reduction catalyst E2 was obtained in the same manner as in Example 1 except that the amount of iron nitrate nonahydrate was changed to 75.80 g. The composite oxide powder E2 of iron, magnesium, and aluminum in the NOx storage reduction catalyst E2 has a composition formula: Mg _0.25 Fe _0.5 □ _0.25 Al ₂ O ₄ (where □ is iron. And a defect site of magnesium.). Further, the specific surface area of the catalyst carrier made of the composite oxide powder E2 was 124 m ² / g.

(Comparative Example 1)
Except for using 10 g of alumina-zirconia-titania composite oxide powder (alumina / zirconia / titania (mass ratio) = 50/35/15, specific surface area: 113 m ² / g) prepared by a coprecipitation method as the catalyst carrier. In the same manner as in Example 1, a NOx storage reduction catalyst C1 was obtained.

(Comparative Example 2)
NOx occlusion reduction type catalyst C2 was obtained in the same manner as in Example 1 except that the amount of magnesium acetate tetrahydrate was changed to 53.5 g and iron nitrate nonahydrate was not used. In addition, the composite oxide powder C2 of magnesium and aluminum in the NOx occlusion reduction type catalyst C2 has a composition formula: Mg _0.5 □ _0.5 Al ₂ O ₄ (where □ represents a defect site of magnesium. ). Further, the specific surface area of the catalyst carrier made of the composite oxide powder C2 was 131 m ² / g.

(Comparative Example 3)
A NOx occlusion reduction type catalyst C3 was obtained in the same manner as in Comparative Example 2 except that the amount of magnesium acetate tetrahydrate was changed to 80.3 g. The composite oxide powder C3 of magnesium and aluminum in the NOx storage reduction catalyst C3 has a composition formula: Mg _0.75 □ _0.25 Al ₂ O ₄ (where □ represents a defect site of magnesium. ). Further, the specific surface area of the catalyst carrier made of the composite oxide powder C3 was 100 m ² / g.

(Comparative Example 4)
First, 2 g of citric acid was dissolved in 25 ml of ion-exchanged water, and 4.3 g of magnesium acetate tetrahydrate and 6.5 g of zinc acetate dihydrate were dissolved in the resulting citric acid solution to obtain a zinc citrate complex. And an aqueous solution containing a magnesium citrate complex were prepared. 10 g of Al ₂ O ₃ powder (trade name “MI307”, specific surface area 200 m ² / g) manufactured by Rhodia Co., Ltd. was added to this aqueous solution, and water was evaporated using a hot stirrer to obtain a powder. This powder was dried in air at 120 ° C. for 12 hours, and then calcined in air at 500 ° C. for 5 hours, and further in air at 800 ° C. for 5 hours, and then composite oxide C4 of zinc, magnesium and aluminum was obtained. A catalyst carrier consisting of This composite oxide powder C4 is represented by a composition formula: Mg _0.2 Zn _0.3 □ _0.5 Al ₂ O ₄ (where □ represents a defect site of zinc and magnesium). It was. Further, the specific surface area of the catalyst carrier made of the composite oxide powder C4 was 126 m ² / g.

  A NOx occlusion reduction type catalyst C4 was obtained in the same manner as in Example 1 except that 10 g of the catalyst support made of the composite oxide powder C4 was used instead of the catalyst support made of the composite oxide powder E1.

  In the NOx occlusion reduction type catalysts obtained in Examples 1 and 2 and Comparative Examples 1 to 4, the values of x and y in the composition formula of the composite oxide, the specific surface area of the catalyst carrier, the supported noble metal and the NOx occlusion material Are summarized in Table 1.

<Catalyst performance evaluation test>
(An endurance test)
After the obtained NOx occlusion reduction type catalyst (honeycomb-like catalyst) was installed in a normal pressure fixed bed flow type reactor (“CATA-8000” manufactured by Best Sokki Co., Ltd.), the temperature was 750 ° C. and the following composition was obtained. An endurance test was conducted by alternately flowing lean gas (110 seconds) and rich gas (10 seconds) at a flow rate of 10 L / min for a total of 5 hours.
Lean gas: O ₂ (7% by mass) + C ₃ H ₆ (0.06% by mass) + CO (0.01% by mass)
+ NO (0.04 mass%) + CO ₂ (11 mass%) + H ₂ O (3 mass%)
+ N ₂ (balance).
Rich gas: C ₃ H ₆ (0.32 mass%) + CO (6 mass%) + H ₂ (2 mass%)
+ NO (0.04 mass%) + CO ₂ (11 mass%) + H ₂ O (3 mass%)
+ N ₂ (balance).

(Sulfur desorption test)
Sulfur dioxide was passed through each NOx storage reduction catalyst (honeycomb catalyst) after the durability test at a temperature of 400 ° C. to poison 3 g of sulfur per liter of the catalyst. Next, a rich gas having the following composition was allowed to flow at a flow rate of 30 L / min for 0.5 hours while raising the temperature from 400 ° C. to 650 ° C. at 10 ° C./min. The SOx concentration in the outgas was measured to determine the amount of sulfur desorption. The result is shown in FIG.
Rich gas: CO (0.13 _wt%) + H 2 (0.09 wt%)
+ NO (0.04 mass%) + CO ₂ (10 mass%) + H ₂ O (5 mass%)
+ N ₂ (balance).

(NOx purification test)
Each NOx occlusion reduction catalyst (honeycomb-like catalyst) after the sulfur desorption test was made to flow at a flow rate of 15 L / min alternately with a lean gas (60 seconds) and a rich gas (3 seconds) having the following composition at a temperature of 450 ° C. The NOx purification rate was determined by measuring the NOx concentration in the catalyst exhaust gas in the steady state. The result is shown in FIG.
Lean gas: O ₂ (10% by mass) + C ₃ H ₆ (0.01% by mass)
+ NO (0.04 wt%) + _CO 2 (10 wt%)
+ H ₂ O (5% by mass) + N ₂ (remainder).
Rich gas: CO (6 wt%) _{+ H} 2 (2 wt%) + NO (0.04% by weight)
+ H ₂ O (5% by mass) + N ₂ (remainder).

  As is apparent from the results shown in FIG. 1 and FIG. 2, the NOx occlusion reduction type catalyst (Examples 1 and 2) of the present invention including the catalyst carrier made of the composite oxide represented by the composition formula is alumina- NOx occlusion reduction type catalyst (Comparative Example 1) having a catalyst carrier made of zirconia-titania composite oxide, NOx occlusion reduction type catalyst (Comparative Examples 2 to 3) having a catalyst carrier made of a composite oxide of magnesium and aluminum, And the NOx occlusion reduction type catalyst (Comparative Example 4) provided with a catalyst carrier made of a composite oxide of zinc, magnesium and aluminum, the desorption of sulfur is faster when sulfur is poisoned. It was confirmed that the NOx purification performance was excellent.

  As described above, according to the present invention, even when sulfur poisoning is performed, sulfur can be easily desorbed, and thereby NOx occlusion capable of exhibiting excellent NOx purification performance. A reduced catalyst can be obtained.

  Therefore, the NOx occlusion reduction type catalyst of the present invention is nitrogen oxide (NOx) and sulfur oxide (SOx) discharged from an internal combustion engine such as a diesel engine or a lean burn engine having a low fuel consumption rate. It is particularly useful as a NOx occlusion reduction type catalyst for purifying nitrogen oxides (NOx) in exhaust gas containing nitrogen.

Claims (5)

The following composition formula:
Mg _x Fe _y □ _(1-xy) Al ₂ O ₄
(In the formula, □ represents a defect site of iron and magnesium, x is 0.02 to 0.70, y is 0.02 to 0.60, and x + y is 0.04 to 0.80. .)
And a support containing a composite oxide of iron, magnesium and aluminum, and an NOx occlusion material which is supported on the support and contains an active metal composed of a noble metal and at least Ba. NOx storage reduction catalyst. ^2. The NOx occlusion reduction type catalyst according to claim 1, wherein the support has a specific surface area of 60 m ² / g or more.   3. The NOx occlusion reduction catalyst according to claim 1, wherein y in the composition formula is 0.10 to 0.50. An alkaline compound is added to a solution containing aluminum ions, magnesium ions, and iron ions to form a precipitate by a coprecipitation method, and the precipitate is baked to obtain the following composition formula:
Mg _x Fe _y □ _(1-xy) Al ₂ O ₄
(In the formula, □ represents a defect site of iron and magnesium, x is 0.02 to 0.70, y is 0.02 to 0.60, and x + y is 0.04 to 0.80. .)
A step of preparing a carrier containing a composite oxide of iron, magnesium and aluminum represented by:
The NOx occlusion reduction according to any one of claims 1 to 3, wherein the support is loaded with an active metal composed of a noble metal and a NOx occlusion material containing at least Ba, and then calcined. Obtaining a type catalyst; and
A process for producing a NOx occlusion reduction type catalyst, comprising:   The method for producing a NOx occlusion reduction catalyst according to claim 4, wherein the alkaline compound is aqueous ammonia.