NITROGEN OXIDE SORPTIONS AND EXHAUST GAS CATALYST

Description

Technical Field

[0001] This invention relates to a nitrogen oxide storage material, a catalyst for exhaust gas purification, and a method for producing the nitrogen oxide storage material.Background Art

[0002] Techniques of using a nitrogen oxide (hereinafter, NO x ) storage material composed of a basic support, e.g., MgAl 2 O 4 or ZrO 2 , and a noble metal supported thereon in exhaust gas purification are known. With the recent demand for control of fuel consumption, not only diesel engines but lean-burn gasoline engines have been attracting attention. For these engines, a management system has been developed and put to practical use in which the air-fuel mixture is normally burnt under fuel-lean conditions (excess oxygen) and intermittently burnt under fuel-stoichiometric to rich conditions to make the exhaust gas reductive thereby to reduce NO x . As a catalyst suited to this system, NO x storage materials capable of storing NO x under lean conditions and releasing the stored NO x in fuel-stoichiometric to rich conditions and NO x storage-reduction (NSR) catalysts containing the NO x storage material have been developed. The NO x storage material usually oxidizes NO generated in the fuel lean atmosphere to NO 2 by the action of the noble metal, allows the NO 2 to be adsorbed onto the basic support, and desorbs NO or NO 2 in the fuel-stoichiometric to rich atmosphere. The NO and NO 2 released in the fuel-stoichiometric to rich atmosphere react with CO or HC (hydrogen carbide) in exhaust gas.

[0003] Various types of NO x storage materials have been reported. For instance, patent literature 1 listed below discloses a purification catalyst obtained by adding zirconia sol to an MgAl 2 O 4 spinel powder and further adding thereto Pt and potassium. Patent literature 2 below discloses a NO x storage material comprising MgAl 2 O 4 powder and an alkali metal and Pt supported thereon. Patent literature 2 describes the inconvenient of an alkaline earth metal as compared with alkali metals in improving NO x storage capacity in high temperatures, saying that "a NO x storage-reduction catalyst having an alkaline earth metal as a NO x storage material has the disadvantage of small storage capacity in high temperatures and, instead, a NO x storage-reduction catalyst composed of a basic support and an alkali metal and a noble metal supported thereon proved capable of storing large quantities of NO x at up to high temperatures."

[0004] Patent literature 3 relates to the technique of using an alkali metal to increase NO x storage capacity in high temperatures similarly to patent literature 2. Patent literature 3 recites in the section "Problems to be Solved by Invention" (para.

[0008] ) "a NO x storage-reduction catalyst composed of a basic support and a noble metal and an alkali metal supported thereon undergoes reduction in noble metal activity due to the interaction between the alkali metal and the basic support, thereby reducing in NO oxidative activity. As a result, the NO x storage capacity decreases totally. This inconvenience offsets both the improving effect on NO x storage performance and the nitrate stabilizing effect in high temperatures. Therefore, the catalyst cannot be said to have sufficient NO x storage performance in high temperatures". To solve this problem, patent literature 3 teaches supporting a noble metal on a porous support instead of a basic support.

[0005] Patent literatures 4 and 5 below describe a storage material having an increased NO x adsorption rate at low temperatures, which is obtained by supporting a basic support, such as zirconia, with a noble metal in the form of a colloid.

[0006] Patent literature 6 below describes in particular a NOx absorber catalyst comprising a substrate monolith coated with one or more washcoat layers and comprising a first component comprising a nitrogen oxide storage component, at least one precious metal and a dispersed rare earth oxide supported on a refractory support material, and a second component comprising a precious metal supported on a bulk reducible oxide that is substantially free of nitrogen oxide storage material, wherein the first component including a refractory support material and the second component which dopes the bulk reducible oxide or zirconia doping the bulk reducible oxide are merely contained / mixed in the washcoat.

[0007] Patent literature 7 below describes an exhaust system for an internal combustion engine, the exhaust system comprising, a lean NOx trap (LNT), a wall flow monolithic substrate having a NOx storage and reduction zone thereon, the wall flow monolithic substrate having a pre-coated porosity of 40% or greater, the NOx storage and reduction zone comprising a platinum group metal loaded on a first support, the first support comprising one or more alkaline earth metal compounds, a mixed magnesium / aluminum oxide, cerium oxide, and at least one base metal oxide selected from the group consisting of copper oxide, manganese oxide, iron oxide and zinc oxide.

[0008] Patent literature 8 below describes a catalyst for lowering the amount of nitrogen oxides in the exhaust gas from lean burn engines. The catalyst comprises aluminum oxide, magnesium oxide and at least one nitrogen oxide storage material as well as at least one noble metal from the platinum group of the periodic table of the elements (see paragraph 001). This document also discloses a method of preparation in which a suspension is formed and comprises at least the metal oxide, a barium compound (BaO / Ce / Zr oxide) mixed with a Mg / Al mixed oxide (see examples 1, 2). The suspension is then used for coating a honeycomb carrier.Citation List - Patent Literature

[0009] Patent literature 1: US 6306360B1 Patent literature 2: JP 2001-252563A Patent literature 3: JP 2001-259422A Patent literature 4: JP 2001-259416A Patent literature 5: JP 2001-300302A Patent literature 6: US 2013 / 336865 Patent literature 7: US 2017 / 314438 Patent literature 8: JP 2003-210988 Summary of Invention

[0010] During relatively low load operation of a lean-burn gasoline engine or in relatively low temperatures (e.g., not higher than 450°C, particularly 250° to 450°C) of emissions from a diesel engine, a NO x storage capacity is secured relatively easily by applying a sufficient amount of a NO x storage material to a catalyst substrate. However, with the recent increasing stringency of CO 2 emissions control, further improvement on NO x conversion performance has been sought. In order to further improve the NO x conversion performance in low temperatures, it is necessary to trap and store the NO x gases flowing through an NSR catalyst, allowing as little escape as possible. With this view, a technique for allowing increase of NO x storage rate is needed. As used herein, the term "NO x storage rate" refers to the amount of NO x stored by a NO x storage material in a certain period of time, e.g., per second or minute. While as stated above an NSR catalyst is an exhaust aftertreatment technique used in both diesel engines and lean-burn gasoline engines, diesel engines and lean-burn gasoline engines are characterized by their exhaust gas temperatures. The exhaust gas from diesel engines has relatively low temperatures (e.g., below 450°C), whilst that from lean-burn gasoline engines is higher in average (e.g., above 450°C) than that from diesel engines. Hence, there has been a demand for an NSR catalyst having a high NO x storage capacity in high temperatures and suited for use in not only diesel engines but lean-burn gasoline engines.

[0011] The techniques according to patent literatures 1 and 2 are designed with consideration for high-temperature NO x storage capacity to meet the demand but these techniques do not pay attention to obtaining a catalyst having a high NO x storage rate in low temperatures. Patent literature 3, as stated above, aims at avoidance of reduction in noble metal activity by supporting a noble metal on a porous support other than basic supports. However, because porous supports other than basic supports generally have lower NO x storage performance than basic supports, the technique of supporting a noble metal on a support other than basic supports as proposed in patent literature 3 still leaves room for improvement on low-temperature NO x storage performance and high-temperature NO x storage capacity.

[0012] In patent literatures 4 to 8, no considerations are given to increasing high-temperature NO x storage capacity of NO x storage materials.

[0013] Accordingly, an object of the invention is to improve a catalyst material for exhaust gas purification and more particularly to provide a NO x storage material having both low-temperature NO x storage rate and high-temperature NO x storage capacity in good balance.

[0014] The inventors have extensively investigated the structure of a NO x storage material that secures low-temperature NO x storage rate and high-temperature NO x storage capacity in good balance. They have found as a result that a NO x storage material that surprisingly achieves high levels of NO x storage rate in low temperatures and NO x storage capacity in high temperatures can be provided by selecting a NO x storage material composed of Mg 1-y Al 2 O 4-y (wherein y is a number satisfying 0 ≤ y ≤ 0.2) as a support and a barium compound and a noble metal supported thereon, and further supporting at least one specific metal oxide, such as ZrO 2 , thereon; as set out in the appended set of claims.

[0015] The invention thus provides a NO x storage material comprising: Mg 1-y Al 2 O 4-y , wherein y is a number satisfying 0 ≤ y ≤ 0.2, a noble metal, at least one metal oxide selected from zirconium oxide, praseodymium oxide, and niobium oxide, and a barium compound, the noble metal, the at least one metal oxide, and the barium compound being supported on Mg 1-y Al 2 O 4-y , wherein the metal oxide is present in an amount of 5 to 20 mass% based on the nitrogen oxide storage material, and the amount of Mg 1-y Al 2 O 4-y in the nitrogen oxide_storage material is 50 to 93.9 mass%.

[0016] The invention also provides an exhaust gas purification catalyst comprising the NO x storage material.

[0017] The invention also provides a method for producing the NO x storage material, said method comprising the following steps (1), (2), and (3) in this order: (1) mixing an aqueous solution containing a water-soluble salt of a specific metal which is selected from zirconium, praseodymium, and niobium and a Mg 1-y Al 2 O 4-y powder, evaporating the mixture to dryness, and firing the resulting mixture to give a M1 / MAO which is a powder of Mg 1-y Al 2 O 4-y on which a specific metal oxide M1 is supported; (2) mixing the M1 / MAO with an aqueous solution containing a water-soluble barium salt, evaporating the mixture to dryness, and firing the resulting mixture to give a M1 / Ba / MAO which is a powder of Mg 1-y Al 2 O 4-y on which the specific metal oxide and a barium compound are supported; and (3) mixing the M1 / Ba / MAO with an aqueous solution containing a water-soluble salt of a noble metal M2, evaporating the mixture to dryness, and firing the resulting mixture to yield a M2 / M1 / Ba / MAO which is a powder of Mg 1-y Al 2 O 4-y on which the specific metal oxide, the barium compound, and the noble metal M2 are supported. Description of Embodiments

[0018] The invention will be described on the basis of its preferred embodiment. The NO x storage material of the invention comprises Mg 1-y Al 2 O 4-y , wherein y is a number satisfying 0 ≤ y ≤ 0.2, a noble metal, at least one metal oxide selected from zirconium oxide, praseodymium oxide, and niobium oxide, and a barium compound, the noble metal, the at least one metal oxide, and the barium compound being supported on Mg 1-y Al 2 O 4-y , wherein the metal oxide is present in an amount of 5 to 20 mass% based on the nitrogen oxide storage material, and the amount of Mg 1-y Al 2 O 4-y in the nitrogen oxide_storage material is 50 to 93.9 mass%. The at least one metal oxide will hereinafter be also called a "specific metal oxide" or simply a "metal oxide".

[0019] The support of the NO x storage material of the invention on which a noble metal, a metal oxide, and a barium compound are supported is not a weakly basic or acidic support but Mg 1-y Al 2 O 4-y , where y is a number satisfying 0 ≤ y ≤ 0.2 (this support will hereinafter be also called MAO). The NO x storage material of the invention achieves high NO x storage performance because of the use of MAO. A barium compound generally easily reacts with a support, and such a reaction reduces the specific surface area of the NO x storage material, thereby causing reduction in NO x storage performance. MAO, on the other hand, is less likely to react with a barium compound. Thus, supporting a barium compound on MAO allows high-temperature NO x storage action of the barium compound to be maintained on a high level.

[0020] For MAO, y in the above formula is preferably equal to or smaller than 0.1 in view of NO x storage performance. MAO is more preferably spinel type MgAl 2 O 4 . MAO is prepared by, for example, co-precipitation using an aluminum material and a magnesium material.

[0021] With the view of improving both low-temperature NO x storage rate and high-temperature NO x storage capacity, the amount of MAO in the NO x storage material of the invention is 50 to 93.9 mass%, preferably 70 to 90 mass%. The amount of MAO may be obtained by, for example, determining by ICP-AES the amounts of Mg and Al in a NO x storage material sample solution prepared by, e.g., alkali fusion.

[0022] The NO x storage material of the embodiment may contain a support other than MAO for supporting a noble metal, a metal oxide, or a barium compound in addition to the MAO, noble metal, metal oxide, and barium compound. Examples of useful supports other than MAO include Al 2 O 3 , TiO 2 , SiO 2 , zeolite, MgO, CeO 2 , ZrO 2 , CeO 2 -ZrO 2 double oxide.

[0023] As used herein, the expression "A supported on B" does not mean that "A and B are in a merely mixed state" but that "particulate A is present on the surface of particulate B, the particulate B being much larger in size than the particulate A". The state of B having A supported thereon can be confirmed by, for example, measuring the particle sizes, for example in SEM (scanning electron microscope) microscopy. The average particle size of A present on the surface of B is preferably 10% or less, more preferably 3% or less, even more preferably 1% or less, of that of B. The term "average particle size" as used herein refers to an average of the maximum Feret's diameters of at least 30 particles in SEM microscopy. The "maximum Feret's diameter" is defined as the furthest distance between any two points on the perimeter of a particle.

[0024] In the case where A on B is too small to be observed for size measurement by SEM, the presence of small particles of A supported on the surface of B may be confirmed by (1) and (2): (1) distribution of the element constituting A in an area where large particles of B are observed is confirmed by the use of an energy-dispersive or wavelength-dispersive X-ray analyzer (EDX or WDX) attached to SEM; and (2) the absence of a complex compound composed of the element constituting A and the element constituting B is confirmed in powder X-ray diffractometry.

[0025] In the case where a noble metal as A on B is too small to be observed for size measurement by SEM, the average particle size of the noble metal particles supported on the support may be estimated by the method in which an adsorbate gas, such as CO, is adsorbed selectively on the surface of noble metal particles and analyzed to determine the ratio of the amount of exposed noble metal to the total amount of the supported noble metal. This method is called "pulse chemisorption analysis" or "measurement of metal dispersion ratio".

[0026] The noble metal supported on MAO in the NO x storage material of the embodiment is used to oxidize NO adsorbed onto the NO x storage material to NO 2 . The noble metal is exemplified by a platinum group element, such as Pt, Rh, Pd, Ir, and Ru. In particular, Pt and Pd are preferred for their excellent oxidative activity. With a view to enhancing the oxidative activity of the NO x storage material thereby to increase the total NO x storage capacity of the NO x storage material, the amount of the noble metal supported is preferably 0.1 mass% or more based on the total mass of the NO x storage material. With a view to reducing the amount of the noble metal to increase economy, the amount of the noble metal is preferably 10 mass% or less based on the total mass of the NO x storage material. From these considerations, the amount of the noble metal supported is more preferably 0.2 to 8 mass%, even more preferably 0.5 to 5 mass%, based on the total mass of the NO x storage material. It is preferred that the noble metal be present in the form of metal atoms on MAO, but it may be present in the form of an oxide. The amount of the supported noble metal may be obtained as the noble metal content in the NO x storage material. It is determined by, for example, ICP-AES on a NO x storage material sample solution prepared by, e.g., alkali fusion.

[0027] One of the characteristics of the invention lies in that the MAO support supports both a specific metal oxide and a barium compound in addition to the noble metal. The metal oxide on the MAO support in the co-presence of the barium compound and the noble metal adsorbs NO x , a product of oxidation by the noble metal, at a high rate and hands off the NO x to the barium compound efficiently, whereby the NO x storage rate by the barium compound increases. Simultaneously, an improving action on the thermal stability of the stored NO x is exhibited, bringing about an increase in high-temperature storage capacity. In other words, the effects of the invention are produced not by the mixed state of the specific metal oxide with MAO as in Example 5 of patent literature 1 but by the coexistence of, and proximity between, the barium compound and the specific metal oxide on the same support.

[0028] As will be understood by comparing Comparative Examples 2 and 3 with Examples hereinafter described, the low-temperature NO x storage rate rises when MAO supports the specific metal oxide as well as the barium compound and the noble metal. As will also be seen from the comparison between Comparative Example 1 and each Example, the high-temperature NO x storage capacity increases extremely by the existence of the barium compound in addition to the metal oxide and the noble metal on MAO.

[0029] The specific metal oxide includes at least one metal oxide selected from zirconium oxide, niobium oxide, and praseodymium oxide in terms of improvement on low-temperature NO x storage rate. Zirconium oxide is especially preferred in terms of further improvement on not only low-temperature NO x storage rate but high-temperature NO x storage capacity. It is preferred that zirconium oxide, praseodymium oxide, and niobium oxide, be those represented by ZrO 2 , Pr 6 O 11 , Nb 2 O 5 , respectively.

[0030] With a view to improving both low-temperature NO x storage rate and high-temperature NO x storage capacity, the metal oxide must be supported in a specific amount. From this viewpoint, the amount of the metal oxide supported is 5 to 20 mass%, preferably 5 to 15 mass%, relative to the total mass of the NO x storage material. The above range of supporting is particularly favorable when the metal oxide is zirconium oxide in the light of the improvement on the low-temperature storage rate and high-temperature storage capacity. The amount of the supported metal oxide may be obtained as the metal oxide content in the NO x storage material. The content of the metal oxide can be obtained by, for example, determining by ICP-AES the amounts of Zr, Pr, Nb, and Fe in a NO x storage material sample solution prepared by, e.g., alkali fusion.

[0031] Examples of the barium compound include barium carbonate, barium aluminate, and barium zirconate, with barium carbonate being preferred in terms of improvement on low-temperature storage rate and high-temperature NO x storage capacity.

[0032] The amount of the barium compound supported is preferably 5 mass% or more based on the total mass of the NO x storage material in terms of improvement in the total NO x storage capacity of the NO x storage material. In view of high-temperature storage performance, the amount is preferably 30 mass% or less relative to the total mass of the NO x storage material. From these viewpoints, the amount of the barium compound supported is more preferably 6 to 25 mass%, even more preferably 8 to 20 mass%, relative to the total mass of the NO x storage material. The amount of the supported barium compound may be obtained as the barium compound content in the NO x storage material. The barium compound content can be obtained by, for example, determining by ICP-AES the amount of Ba in a NO x storage material sample solution prepared by, e.g., alkali fusion.

[0033] The NO x storage material of the embodiment is in the form of, e.g., powder, and the BET specific surface area thereof is preferably 10 to 200 m 2< / g, more preferably 30 to 150 m 2< / g, in terms of high NO x storage performance. The BET specific surface area can be determined by the method described in Examples given later.

[0034] The NO x storage material of the embodiment has each of the metal oxide, barium compound, and noble metal dispersed on the surface of the MAO support. The metal oxide, barium compound, and noble metal may be present at the same or different positions in the radial or height direction of the MAO support. When present at different positions, any one of the metal oxide, barium compound, and noble metal may be on the outermost side of the NO x storage material (the farthest from the support), and any other one of them may be on the support (closest to the center of the NO x storage material). For example, the barium compound may be located closer to the surface of the NO x storage material (farther from the support) than the metal oxide, and the noble metal may be located closer to the surface of the NO x storage material (farther from the support) than the barium compound. The positional relation of the metal oxide, barium compound, and noble metal in the direction of the height of the MAO support can be found out using an Scanning Electron Microscope (SEM) or Transmission Electron Microscope (TEM) equipped with an Energy Dispersive X-ray analyzer (EDX analyzer) or a Wavelength Dispersive X-ray analyzer (WDX analyzer).

[0035] The NO x storage material of the embodiment can be produced suitably by, for example, a method including the following steps.Metal oxide supporting step:

[0036] An aqueous solution containing a water soluble salt of a specific metal and MAO powder are mixed, and the mixture is evaporated to dryness on an evaporator and fired to give powder of MAO supporting the specific metal oxide M1 (M1 / MAO).Barium compound supporting step:

[0037] The M1 / MAO is mixed with an aqueous solution containing a water soluble barium salt, and the mixture is evaporated to dryness on an evaporator and fired to give powder of MAO supporting the specific metal oxide and the barium compound (M1 / Ba / MAO).Noble metal supporting step:

[0038] The M1 / Ba / MAO is mixed with an aqueous solution containing a water soluble salt of a noble metal M2, and the mixture is evaporated to dryness and fired to yield powder of MAO supported the specific metal oxide, barium compound, and noble metal M2 (M2 / M1 / Ba / MAO).

[0039] The water soluble salt of the specific metal used in the metal oxide supporting step is exemplified by a specific metal nitrate. The specific metal concentration of the aqueous solution of the water soluble specific metal salt is preferably 0.01 to 1 mol / L in terms of efficient supporting of the metal oxide and NO x storage performance. The firing may be carried out in the atmosphere under conditions, for example at 500° to 800°C for 1 to 10 hours.

[0040] The water soluble barium salt used in the barium compound supporting step is exemplified by barium acetate and barium nitrate. The barium concentration of the water soluble barium salt aqueous solution is preferably 0.01 to 1 mol / L in terms of efficient supporting of the barium compound and NO x storage performance. The firing may be carried out in the atmosphere under conditions, for example at 500° to 800°C for 1 to 10 hours.

[0041] The water soluble noble metal salt used in the noble metal supporting step is exemplified by a nitrate, an ammine complex salt, and a chloride. The noble metal concentration of the water soluble noble metal salt aqueous solution is preferably 0.001 to 0.1 mol / L in terms of efficient supporting of the noble metal and NO x storage performance. The firing may be carried out in the atmosphere under conditions, for example at 450° to 700°C for 1 to 10 hours.

[0042] The method for producing the NO x storage material of the invention is not limited to the above method. For instance, the metal oxide supporting step may be preceded by the step of supporting MAO with the barium compound, in which case the resulting Ba / MO then supports the metal oxide and the noble metal. The metal oxide, barium compound, and noble metal may be supported on MAO at the same time.

[0043] The NO x storage material of the invention may have any form, such as powder, paste, or granule. The NO x storage material of the embodiment is useful to make a catalyst for exhaust gas purification. For example, the NO x storage material of the embodiment may be used to form a catalyst active layer on a catalyst substrate to produce an exhaust gas purification catalyst. The catalyst substrate may be made, e.g., of ceramics or metallic materials. While the shape of the catalyst substrate is not particularly limited, the substrate usually has the form of a honeycomb, a plate, pellets, a DPF, a GPF, and so on. A honeycomb, DPF, or GPF is preferably used. These catalyst substrates may be made of ceramics, such as alumina (Al 2 O 3 ), mullite (3Al 2 O 3 -2SiO 2 ), cordierite (2MgO-2Al 2 O 3 -5SiO 2 ), aluminum titanate (Al 2 TiO 5 ), and silicon carbide (SiC); or metallic materials, such as stainless steel.

[0044] When the NO x storage material of the embodiment is used to form a catalyst layer on a catalyst substrate to provide an exhaust gas purification catalyst as described, another catalyst layer made of a hitherto known catalyst material may be provided on the layer of the NO x storage material of the embodiment. Conversely, a catalyst layer containing the NO x storage material of the embodiment may be formed on a catalyst layer of a known catalyst material formed on a substrate. Furthermore, a catalyst layer of a mixture containing the NO x storage material of the embodiment and a known catalyst material may be formed on a catalyst substrate.

[0045] The NO x storage material of the invention and the exhaust gas purification catalyst containing the same achieve higher NO x storage rates in low temperatures as compared with conventional NO x storage materials and also retain high levels of NO x storage capacity in high temperatures. Such a NO x storage material exhibits high NO x storage performance in a broad range of temperature, from engine start-up to high-temperature operation. Accordingly, the exhaust gas purification catalyst containing the NO x storage material of the invention is capable of efficiently converting harmful components such as NO x in fossil fuel-powered internal-combustion engines, such as lean-burn gasoline engines and diesel engines, and provides high exhaust gas purification performance.EXAMPLES

[0046] The invention will now be illustrated in greater detail with reference to Examples, but it should be understood that the invention is not deemed to be limited thereto.Example 1

[0047] (1) A zirconium oxynitrate (ZrO(NO 3 ) 2 ·2H 2 O) aqueous solution having a Zr concentration of 0.2 mol / L was prepared. MgAl 2 O 4 powder was dispersed in the aqueous solution, and the dispersion was evaporated to dryness on a rotary evaporator. The resulting solid was fired in the atmosphere at 600°C for 5 hours to give MAO powder supporting zirconium oxide (ZrO 2 ) (hereinafter called Zr / MAO powder). (2) The Zr / MAO powder was dispersed in a barium acetate (Ba(CH 3 COO) 2 ) aqueous solution having a Ba concentration of 0.2 mol / L. The resulting dispersion was evaporated to dryness on a rotary evaporator, and the resulting solid was fired in the atmosphere at 600°C for 5 hours to give MAO powder supporting ZrO 2 and barium carbonate (BaCO 3 ) (hereinafter called Ba / Zr / MAO powder). (3) The Ba / Zr / MAO powder was dispersed in distilled water. To the dispersion was added an aqueous solution of Pt(NO 2 ) 2 (NH 3 ) 2 to prepare a dispersion having a Pt concentration of 0.01 mol / L. The resulting dispersion was evaporated to dryness on a rotary evaporator, and the resulting solid was fired in the atmosphere at 600°C for 5 hours to provide a NO x storage material powder having ZrO 2 , BaCO 3 , and Pt supported on the MAO powder (hereinafter called Pt / Ba / Zr / MAO powder).

[0048] The NO x storage material powder was found to contain 10 mass% of ZrO 2 , 10 mass% of barium carbonate, and 1 mass% of platinum relative to the total mass of the NO x storage material powder.Example 2

[0049] A NO x storage material powder having Pr 6 O 11 , BaCO 3 , and Pt supported on MAO powder (Pt / Ba / Pr / MAO) was obtained in the same manner as in Example 1, except for replacing the zirconium oxynitrate (ZrO(NO 3 ) 2 ·2H 2 O) with praseodymium nitrate (Pr(NO 3 ) 3 ·6H 2 O).Comparative Example 3

[0050] A NO x storage material powder having Fe 2 O 3 , BaCO 3 , and Pt supported on MAO powder (Pt / Ba / Fe / MAO) was obtained in the same manner as in Example 1, except for replacing the zirconium oxynitrate (ZrO(NO 3 ) 2 ·2H 2 O) with iron nitrate (Fe(NO 3 ) 3 ·9H 2 O).Example 4

[0051] A NO x storage material powder having Nb 2 O 5 , BaCO 3 , and Pt supported on MAO powder (Pt / Ba / Nb / MAO) was obtained in the same manner as in Example 1, except for replacing the zirconium oxynitrate (ZrO(NO 3 ) 2 ·2H 2 O) with niobic acid and a 0.01 mol / L aqueous solution of oxalic acid.Examples 5 to 7, Comparative Example 8

[0052] A NO x storage material powder having ZrO 2 , BaCO 3 , and Pt supported on MAO powder was obtained in the same manner as in Example 1, except for changing the mixing ratio of the zirconium oxynitrate (ZrO(NO 3 ) 2 ·2H 2 O) aqueous solution and MAO so that the content of ZrO 2 in the Pt / Ba / Zr / MAO might be as shown in Table 1 below. The resulting powders of Examples 5 to 8 are designated Pt / Ba / 5Zr / MAO, Pt / Ba / 15Zr / MAO, Pt / Ba / 20Zr / MAO, and Pt / Ba / 30Zr / MAO, respectively.Comparative Example 1

[0053] A comparative NO x storage material powder having ZrO 2 and Pt supported on MAO powder (Pt / Zr / MAO) was obtained in the same manner as in Example 1, except the following: the barium compound was not supported on MAO and, in step (3), the Zr / MAO powder was dispersed in distilled water, followed by adding the Pt(NO 2 ) 2 (NH 3 ) 2 aqueous solution to the dispersion.Comparative Example 2

[0054] A comparative NO x storage material powder having BaCO 3 and Pt supported on MAO powder (Pt / Ba / MAO) was obtained in the same manner as in Example 1, except the following: ZrO 2 was not supported on MAO and, in step (2), the MAO powder was dispersed in the barium acetate (Ba(CH 3 COO) 2 ) aqueous solution.Comparative Example 4

[0055] A comparative NO x storage material powder having CeO 2 , BaCO 3 , and Pt supported on MAO powder (Pt / Ba / Ce / MAO) was obtained in the same manner as in Example 1, except for replacing the zirconium oxynitrate (ZrO(NO 3 ) 2 ·2H 2 O) with cerium nitrate (Ce(NO 3 ) 3 ·6H 2 O). Table 1SupportMetal OxideBarium CompoundNobel MetalMetal Oxide / NO x Storage Material (mass%)Example 1MAOZrO 2 BaCO 3 Pt10Example 2MAOPr 6 O 11 BaCO 3 Pt10Comp. Example 3MAOFe 2 O 3 BaCO 3 Pt10Example 4MAONb 2 O 5 BaCO 3 Pt10Example 5MAOZrO 2 BaCO 3 Pt5Example 6MAOZrO 2 BaCO 3 Pt15Example 7MAOZrO 2 BaCO 3 Pt20Comp. Example 8MAOZrO 2 BaCO 3 Pt30Comp. Example 1MAOZrO 2 -Pt10Comp. Example 2MAO-BaCO 3 Pt-Comp. Example 4MAOCeO 2 BaCO 3 Pt10 Evaluation:

[0056] The BET specific surface area of the NO x storage material powders prepared in Examples 1 and 2 and Comparative Examples 2 and 4 was determined by the following method. As a result, the BET specific surface area of the NO x storage material powders of Examples 1 and 2 and Comparative Examples 2 and 4 was found to be 75.6 m 2< / g, 49 m 2< / g, 81.9 m 2< / g, and 68.2 m 2< / g, respectively.Method for measurement of BET specific surface area:

[0057] The specific surface area was measured by the BET three-point method using a surface area analyzer QUADRASORB SI from Quantachrome Corp. As an adsorbate gas for the measurement, nitrogen gas was used.

[0058] The NO x storage material powders prepared in Examples 1 to 7 and Comparative example 8 and Comparative Examples 1, 2 and 4 were evaluated for NO x storage rate and storage capacity in accordance with the following methods. The results obtained are shown in Table 2.Method of evaluation of NO x storage rate and storage capacity:

[0059] A fixed-bed flow reactor was used for the evaluation. The NO x storage material powder weighing 100 mg was heated to 700°C and kept at that temperature for 30 minutes in a helium stream (pre-treatment). The sample powder was allowed to cool to 200°C. The powder at 200°C after the pre-treatment was contacted with a 100 ml / min of NO x gas (composition: 0.1 vol% of NO, 10 vol% of O 2 , and the balance of He) for 30 minutes. The storage capacity, i.e., the amount of NO x stored (mol / g) at 200°C in 200 seconds from the start of the contact with the NO x gas was calculated as an NO x storage rate from the NO x concentration in the flowing gas. The rate of NO x storage per second (mol / (g·sec)) is obtained by dividing the calculated storage capacity by the gas contact time (200 seconds).

[0060] The NO x storage material powder was then maintained at 200°C for 40 minutes and heated up to 700°C at a rate of 10°C / min in a helium (100 vol%) stream. The amount of NO x released during the temperature elevation from 450° to 700°C was calculated from the NO x concentration in the flowing gas. The NO x storage material desorbs the stored NO x with increase in temperature in a helium atmosphere. The amount of NO x released in a specific temperature range indicates the NO x storage capacity, i.e., the amount of NO x stored, in that temperature range.

[0061] The NO x concentrations were measured using a chemoluminescence NO x meter NOA-7000 from Shimadzu Corp. Table 2NO x Storage MaterialNO x Storage Rate (initial 0 to 200 secs) (mol / g)High-temp. (450°-700°C) NO x Storage Capacity (mol / g)Example 1Pt / Ba / Zr / MAO9.58 × 10 -5< 2.61 × 10 -4< Example 2Pt / Ba / Pr / MAO9.10 × 10 -5< 1.38 × 10 -4< Comp. Example 3Pt / Ba / Fe / MAO8.21 × 10 -5< 1.98 × 10 -4< Example 4Pt / Ba / Nb / MAO9.24 × 10 -5< 1.51 × 10 -4< Example 5Pt / Ba / 5Zr / MAO8.51 × 10 -5< 2.70 × 10 -4< Example 6Pt / Ba / 15Zr / MAO9.23 × 10 -5< 2.35 × 10 -4< Example 7Pt / Ba / 20Zr / MAO8.65 × 10 -5< 2.20 × 10 -4< Comp. Example 8Pt / Ba / 30Zr / MAO8.37 × 10 -5< 1.68 × 10 -4< Comp. Example 1Pt / Zr / MAO1.05 × 10 -4< 0.37 × 10 -4< Comp. Example 2Pt / Ba / MAO8.01 × 10 -5< 1.38 × 10 -4< Comp. Example 4Pt / Ba / Ce / MAO7.54 × 10 -5< 1.86 × 10 -4<

[0062] As shown in Table 2, the NO x storage materials of Examples 1 to 7 and Comparative Example 8, which are MAO having a specific metal oxide and a barium compound as well as a noble metal supported thereon, achieve low-temperature (200°C) NO x storage rates of 8.21 × 10 -5< mol / g or higher and high-temperature (450° to 700°C) NO x storage capacity of 1.38 × 10 -4< mol / g or higher, proving capable of accomplishing high levels of both of low-temperature NO x storage rate and high-temperature NO x storage capacity.

[0063] In contrast, the NO x storage material of Comparative Example 1, which does not have a barium compound supported, is equivalent in low-temperature NO x storage rate performance to those of Examples but greatly inferior in high-temperature NO x storage capacity. The NO x storage material of Comparative Example 2, which has no specific metal oxide supported, and that of Comparative Example 4, which uses ceria in place of the specific metal oxide, are equivalent to those of Examples in high-temperature NO x storage capacity but inferior in low-temperature NO x storage rate.Industrial Applicability

[0064] The invention provides a NO x storage material exhibiting high NO x storage rate in low temperatures and high NO x storage capacity in high temperatures. The invention also provides an exhaust gas purification catalyst having the NO x storage material and exhibiting excellent NO x conversion performance.

Claims

1. A nitrogen oxide storage material comprising: Mg1-yAl2O4-y, wherein y is a number satisfying 0 ≤ y ≤ 0.2, a noble metal, at least one metal oxide selected from zirconium oxide, praseodymium oxide, and niobium oxide, and a barium compound, the noble metal, the at least one metal oxide, and the barium compound being supported on Mg1-yAl2O4-y, wherein the metal oxide is present in an amount of 5 to 20 mass% based on the nitrogen oxide storage material, and the amount of Mg1-yAl2O4-y in the nitrogen oxide_storage material is 50 to 93.9 mass%.

2. The nitrogen oxide storage material according to claim 1, wherein the metal oxide is zirconium oxide.

3. The nitrogen oxide storage material according to any one of claims 1 to 2, wherein the barium compound is barium carbonate.

4. The nitrogen oxide storage material according to any one of claims 1 to 3, wherein the noble metal is platinum.

5. The nitrogen oxide storage material according to any one of claims 1 to 4, wherein the Mg1-yAl2O4-y is MgAl2O4.

6. The nitrogen oxide storage material according to any one of claims 1 to 5, wherein the nitrogen oxide storage material is in a powder form and has a BET specific surface area of 10 to 200 m2 / g.

7. An exhaust gas purification catalyst comprising the nitrogen storage material according to any one of claims 1 to 6.

8. A method for producing the nitrogen oxide storage material according to claim 1, the method comprising the following steps (1), (2), and (3) in this order: (1) mixing an aqueous solution containing a water-soluble salt of a specific metal which is selected from zirconium, praseodymium, and niobium and a Mg1-yAl2O4-y powder, evaporating the mixture to dryness, and firing the resulting mixture to give a M1 / MAO which is a powder of Mg1-yAl2O4-y on which a specific metal oxide M1 is supported; (2) mixing the M1 / MAO with an aqueous solution containing a water-soluble barium salt, evaporating the mixture to dryness, and firing the resulting mixture to give a M1 / Ba / MAO which is a powder of Mg1-yAl2O4-y on which the specific metal oxide and a barium compound are supported; and (3) mixing the M1 / Ba / MAO with an aqueous solution containing a water-soluble salt of a noble metal M2, evaporating the mixture to dryness, and firing the resulting mixture to yield a M2 / M1 / Ba / MAO which is a powder of Mg1-yAl2O4-y on which the specific metal oxide, the barium compound, and the noble metal M2 are supported.