Catalyst for purifying exhaust gas, and method for manufacturing the same

Abstract

To provide a catalyst for purifying exhaust gas which satisfies both catalytic performance and OSC performance, and a method for manufacturing the catalyst.SOLUTION: Provided are a catalyst for purifying exhaust gas containing noble metal, OSC material, barium carbonate, a carrier for barium on which the barium carbonate is carried, where the carrier for barium is a mixed crystal of α-alumina and θ-alumina, the barium carbonate is dispersed into mesopores of the mixed crystal, and the content of the barium carbonate is 2 wt.% to 15 wt.% for the total weight of the barium carbonate and the carrier for barium while using metal barium as a reference, and a method for manufacturing the catalyst.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purifying catalyst and a method for producing the same. [Background technology]

[0002] Exhaust gas emitted from internal combustion engines for automobiles and the like, such as gasoline engines or diesel engines, contains harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx).

[0003] For this reason, internal combustion engines are generally equipped with exhaust gas purification devices to decompose and remove these harmful components, and these harmful components are almost entirely neutralized by exhaust gas purification catalysts, such as three-way catalysts, installed in these exhaust gas purification devices.

[0004] For example, Patent Document 1 discloses a method for producing an exhaust gas purification catalyst, which comprises the steps of: preparing a pre-crystallization composition containing elemental components constituting a perovskite-type composite oxide containing a precious metal; mixing the pre-crystallization composition with powder of θ-alumina and / or α-alumina to prepare a mixture; and heat-treating the mixture.

[0005] Exhaust gas purification catalysts contain precious metals, which play a role in simultaneously promoting the oxidation reaction of CO and HC and the reduction reaction of NOx. Precious metals are expensive, and there is also a need to reduce their amount in terms of resource risks. To reduce the amount of precious metal, it is necessary to prevent the catalytic activity of the precious metal from decreasing due to use of the exhaust gas purification catalyst. To prevent the catalytic activity of the precious metal from decreasing, for example, it is possible to suppress the poisoning of the precious metal by HC in exhaust gas (HC poisoning), which is one factor that decreases the catalytic activity of the precious metal.

[0006] Patent Document 2 discloses an exhaust gas purification catalyst comprising a substrate and a catalytic layer facing the substrate, the catalytic layer containing a precious metal, alumina, an oxygen storage material, and an alkaline earth metal sulfate having an average particle size of 0.01 μm to 0.7 μm as determined by scanning electron microscope observation. The alkaline earth metal in the catalytic layer has the effect of suppressing HC poisoning of the precious metal and improving the NOx purification rate, which is reduced by HC poisoning. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-243306 [Patent Document 2] International Publication No. 2010 / 137658 Summary of the Invention [Problem to be solved by the invention]

[0008] On the other hand, exhaust gas purification catalysts are required to have oxygen storage capacity (OSC), and the catalyst coating layer may contain a material with OSC (OSC material) in addition to the catalytic metal. An "OSC material" is a material that can absorb and release oxygen. The OSC material can keep the oxygen concentration constant and maintain the purification performance (catalytic performance) of the exhaust gas purification catalyst even when the air-fuel ratio fluctuates.

[0009] In other words, it is desirable for the exhaust gas purification catalyst to have both catalytic performance, that is, the ability to suppress HC poisoning of precious metals, and OSC performance.

[0010] However, alkaline earth metals, such as barium, which are introduced to suppress HC poisoning of precious metals, can react with the OSC material under certain high-temperature conditions, resulting in a decrease in OSC performance. This is thought to be because barium, which is supported on the surface of the carrier, reacts with nearby OSC material at high temperatures, reducing the crystallinity of the OSC material.

[0011] Therefore, an object of the present invention is to provide an exhaust gas purification catalyst that achieves both catalytic performance and OSC performance, and a method for producing the same. [Means for solving the problem]

[0012] Therefore, the present inventors have investigated various means for solving the above-mentioned problems, and as a result, have found that in an exhaust gas purification catalyst containing a precious metal, an OSC material, barium carbonate, and a carrier on which the barium carbonate is supported (hereinafter also referred to as a "barium carrier"), by using a mixed crystal of α-alumina and θ-alumina as the barium carrier and dispersing and supporting a certain amount of barium carbonate in the mesopores of the mixed crystal, it is possible to suppress the reaction between the barium carbonate and the OSC material at high temperatures in the obtained exhaust gas purification catalyst, and as a result, the NOx purification rate is improved while maintaining high OSC performance, and have completed the present invention.

[0013] That is, the gist of the present invention is as follows. (1) A catalyst for purifying exhaust gases, comprising a precious metal, an OSC material, barium carbonate, and a barium carrier on which the barium carbonate is supported, wherein the barium carrier is a mixed crystal of α-alumina and θ-alumina, the barium carbonate is dispersed in mesopores of the mixed crystal, and the content of the barium carbonate is 2% by weight to 15% by weight, based on the total weight of the barium carbonate and the barium carrier, on the basis of metallic barium. (2) The exhaust gas purifying catalyst according to (1), wherein the barium carbonate has a crystallite size of 20 nm or less when calculated from the half-width of the peak on the (111) plane in an XRD spectrum, and the mixed crystal has an average pore size of 20 nm or more when calculated by the BJH method based on pore analysis by nitrogen gas adsorption. (3) The exhaust gas purifying catalyst according to (1) or (2), wherein the content of the barium carbonate is 5% by weight to 13% by weight, based on the total weight of the barium carbonate and the barium carrier, on the basis of metallic barium. (4) The catalyst for purifying exhaust gas according to any one of (1) to (3), wherein the catalyst for purifying exhaust gas has a substrate and a catalyst coating layer coated on the substrate, and the catalyst coating layer contains the noble metal, the OSC material, the barium carbonate, and the barium carrier. (5) A method for producing an exhaust gas purification catalyst, comprising: (i) a step of mixing aluminum hydroxide with an aqueous solution of a barium compound, and firing the mixture at 1000°C to 1400°C to prepare a mixture of barium carbonate and a mixed crystal of α-alumina and θ-alumina carrying the barium carbonate, wherein the content of the barium carbonate is adjusted to 2% by weight to 15% by weight based on the total weight of the mixture, based on metallic barium; and (ii) a step of mixing a noble metal, an OSC material, and the mixture prepared in (i), and firing the mixture again at 400°C to 800°C to produce a catalyst for exhaust gas purification. [Effects of the Invention]

[0014] The present invention provides an exhaust gas purification catalyst that achieves both catalytic performance and OSC performance, and a method for producing the same. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram comparing a conventional exhaust gas purifying catalyst (A) with an embodiment of the exhaust gas purifying catalyst of the present invention (B). [Figure 2]1 is a graph showing XRD spectra of barium carbonate in Example 1 and a mixture of mixed crystals of α-alumina and θ-alumina on which the barium carbonate is supported. [Figure 3] 1 is a graph showing the OSC of the exhaust gas purifying catalysts of Example 1 and Comparative Examples 1 and 2. [Figure 4] 1 is a graph showing NOx-T50 of the exhaust gas purification catalysts of Example 1 and Comparative Examples 1 and 2. [Figure 5] 1 is a graph showing the relationship between the Ba content and NOx-T50 of the catalysts of Examples 2 and 3 and Comparative Examples 3 and 4. DETAILED DESCRIPTION OF THE INVENTION

[0016] Preferred embodiments of the present invention will now be described in detail. In this specification, the features of the present invention will be described with reference to the drawings as appropriate. In the drawings, the dimensions and shapes of each part are exaggerated for clarity, and the actual dimensions and shapes are not accurately depicted. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. The exhaust gas purification catalyst and its manufacturing method of the present invention are not limited to the following embodiments, and can be embodied in various forms including modifications and improvements that can be made by those skilled in the art, without departing from the gist of the present invention.

[0017] The exhaust gas purifying catalyst of the present invention contains a precious metal, an OSC material, barium carbonate, and a carrier (barium carrier) on which the barium carbonate is supported.

[0018] The noble metal is not limited as long as it functions as a catalytic metal. Examples of noble metals include gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os), iridium (Ir), and mixtures of two or more of these. The noble metal may also be an alloy of two or more elements including the elements listed above. In one embodiment, the noble metal is Pt, Pd, and / or Rh. In one embodiment, the noble metal is Pd.

[0019] The average particle size of the noble metal is not limited and is usually 0.1 nm to 100 nm, and in one embodiment 1 nm to 10 nm, in terms of the diameter equivalent to a circle having a projected area as measured by a transmission electron microscope (TEM) (Heywood diameter).

[0020] The content of the precious metal is not limited. The content of the precious metal is typically 0.1% by weight to 10% by weight, and in one embodiment, 0.5% by weight to 2% by weight, calculated as the metal, relative to the total weight of the precious metal, OSC material, barium carbonate, and barium carrier. The contents of components such as the precious metal, OSC material, barium carbonate, and barium carrier contained in the exhaust gas purification catalyst typically depend on the amounts of precursors of each component added as materials during production of the exhaust gas purification catalyst.

[0021] By setting the type, average particle size and content of the noble metal as described above, it is possible to ensure sufficient catalytic performance of the exhaust gas purifying catalyst.

[0022] The OSC material is not limited as long as it has OSC properties, and examples of the OSC material include ceria (CeO), alumina (AlO)-ceria-zirconia (ZrO) composite oxides (ACZ), ceria-zirconia composite oxides (CZ), such as ceria-zirconia composite oxides having a fluorite structure (CeZrO) or ceria-zirconia composite oxides having a pyrochlore structure (CeZrO). Furthermore, the component ratios of the elements that make up the OSC material are not limited, and the OSC material may further contain additive elements such as praseodymium (Pr), scandium (Sc), yttrium (Y), lanthanum (La), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium (Yb), lutetium (Lu), titanium (Ti), and mixtures of two or more of these elements.

[0023] The average particle size of the OSC material is not limited, and is typically 1 nm to 500 nm, and in one embodiment 10 nm to 50 nm, in terms of the diameter (Heywood diameter) of a circle equivalent to the projected area of a transmission electron microscope (TEM) or a scanning electron microscope (SEM).

[0024] The content of the OSC material is not limited, but is usually 5% to 80% by weight, and in one embodiment 30% to 70% by weight, based on the total weight of the precious metal, OSC material, barium carbonate, and barium carrier.

[0025] By setting the type, average particle size, and content of the OSC material as described above, it is possible to ensure sufficient OSC performance of the exhaust gas purification catalyst.

[0026] Barium carbonate is a compound with the chemical formula BaCO3.

[0027] The crystallite size of barium carbonate is not limited, and is usually 20 nm or less, 18 nm or less in one embodiment, and 10 to 15 nm in one embodiment, as calculated from the half-width of the (111) plane peak in an X-ray diffraction (XRD) spectrum.

[0028] The content of barium carbonate is not limited, but is usually 2% by weight to 15% by weight, and in one embodiment 5% by weight to 13% by weight, based on metallic barium (Ba) relative to the total weight of barium carbonate and the carrier for barium.

[0029] By setting the crystallite size and content of barium carbonate as described above, it is possible to suppress HC poisoning of the precious metal, improve the NOx purification rate that can be reduced by HC poisoning, and further suppress an increase in the specific surface area of the barium carbonate and the barium carrier.

[0030] The barium carrier is a mixed crystal of α-alumina and θ-alumina. The crystalline form of alumina can be confirmed by XRD using CuKα. Therefore, in the XRD spectrum of the mixed crystal, peaks corresponding to α-alumina and θ-alumina are simultaneously observed.

[0031] The weight ratio of α-alumina to θ-alumina (α-alumina / θ-alumina) in the mixed crystal is not limited. The weight ratio is usually 1 to 6, and in one embodiment, 3 to 5, for example, 4.

[0032] The mixed crystal has mesopores, which refer to pores with a diameter of 2 nm to 50 nm that exist on the particle surface.

[0033] The average pore diameter of the mixed crystal is usually 20 nm or more, in one embodiment 25 nm or more, and in one embodiment 25 to 40 nm, when calculated by the BJH method based on pore analysis by nitrogen gas adsorption.

[0034] When the barium carrier is a mixed crystal of α-alumina and θ-alumina and the mixed crystal has the above-mentioned pore size on the particle surface, barium carbonate having a crystallite size smaller than the pore size is dispersed and supported within the pores, reducing the probability of contact between barium carbonate and the OSC material in the exhaust gas purification catalyst. As a result, at high temperatures at which the exhaust gas purification catalyst may be used, the solid solution reaction between barium carbonate and the OSC material, such as ceria-zirconia composite oxide, is suppressed, which leads to the suppression of deterioration in OSC performance and ensures high OSC performance.

[0035] The content of the carrier for barium is not limited, but is usually 10% by weight to 90% by weight, and in one embodiment 30% by weight to 70% by weight, based on the total weight of the noble metal, OSC material, barium carbonate, and carrier for barium.

[0036] The exhaust gas purifying catalyst of the present invention may further contain other components within the range that does not impair the effects of the present invention.

[0037] Other components include metal oxides and additives used in exhaust gas purification catalysts for this type of application, such as metal oxides such as silica, magnesium oxide (MgO), zirconia, alumina other than a barium carrier, titania (TiO), yttria (YO), neodymium oxide (NdO), lanthanum oxide (LaO), composite oxides and solid solutions thereof, such as alumina-zirconia composite oxide (AZ), and alkali metals such as potassium (K), sodium (Na), lithium (Li), cesium (Cs), alkaline earth metals other than Ba, such as calcium (Ca) and strontium (Sr), rare earth elements such as La, Y, Ce, etc., transition metals such as iron (Fe), etc., and combinations of two or more thereof. The ratio of each oxide in the composite oxide, such as AZ, is not limited and may be any ratio commonly used in the technical field of exhaust gas purification catalysts.

[0038] The content of the other components is not limited, but is usually 1% by weight to 40% by weight, and in one embodiment 10% by weight to 30% by weight, based on the total weight of the precious metal, OSC material, barium carbonate, and barium carrier.

[0039] Acidic supports, such as silica, are compatible with catalytic metals that reduce NOx. Basic supports, such as magnesium oxide, are compatible with potassium and barium, which occlude NOx. Zirconia suppresses sintering of other support particles at high temperatures that would otherwise cause sintering. When combined with the precious metal Rh, it generates H2 through a steam reforming reaction, enabling efficient reduction of NOx. Acid-base amphoteric supports, such as alumina, have a high specific surface area and can be used to efficiently occlude and reduce NOx. Titania can suppress sulfur poisoning of catalytic metals. The addition of alumina, zirconia, and other metal oxides can also enhance the durability of the support.

[0040] The OSC material, the barium carrier, and any other optional components contained in the exhaust gas purification catalyst of the present invention may be a carrier for a precious metal. By supporting the precious metal on the OSC material, the barium carrier, and any optional carrier particles, a high specific surface area of the precious metal can be ensured, and high catalytic performance can be ensured.

[0041] Figure 1 is a schematic diagram comparing a conventional exhaust gas purification catalyst (A) with one embodiment of the exhaust gas purification catalyst of the present invention (B). Figure 1(A) shows that barium sulfate is present in a position where it can come into contact with the ceria-zirconia composite oxide serving as an OSC material, causing a decrease in the crystallinity of the ceria-zirconia composite oxide and resulting in a decrease in OSC performance. On the other hand, Figure 1(B) shows that barium carbonate is dispersed and supported within the mesopores of the mixed crystal of α-alumina and θ-alumina, preventing it from coming into contact with the ceria-zirconia composite oxide and maintaining the OSC performance.

[0042] The catalyst for purifying exhaust gases of the present invention can be used in the form of pellets, which are packed into a reaction tube or the like.

[0043] Furthermore, in consideration of practicality, the exhaust gas purification catalyst of the present invention may be in a form having a substrate and a catalyst coating layer coated on the substrate, and the above-mentioned noble metal, OSC material, barium carbonate, barium carrier, and any other components may be introduced into the catalyst coating layer for use.

[0044] When adopting such a form of exhaust gas purification catalyst, a substrate having a known honeycomb shape can be used as the substrate. Specific examples of the substrate that can be used include honeycomb-shaped monolith substrates (honeycomb filters, high-density honeycombs, etc.). The material of such a substrate is not particularly limited. Examples of the substrate include substrates made of ceramics such as cordierite, silicon carbide (SiC), silica (SiO), alumina, mullite, or mixtures thereof, and substrates made of metals such as stainless steel containing chromium and aluminum. In one embodiment, the substrate is cordierite from the viewpoint of cost.

[0045] The amount of the catalyst coating layer is not limited, but is usually 100 g to 300 g, and in one embodiment 150 g to 250 g, per 1 L of the volume of the portion of the substrate on which the catalyst coating layer is applied.

[0046] The thickness of the catalyst coating layer is not limited, but is usually 50 μm to 200 μm, and in one embodiment, 80 μm to 150 μm on average. The thickness of the first catalyst coating layer can be measured, for example, with a scanning electron microscope (SEM).

[0047] By ensuring that the coating amount and thickness of the catalyst coating layer are within the above ranges, it is possible to maintain a good balance between pressure loss, catalytic performance, and durability in the exhaust gas purifying catalyst.

[0048] The present invention further relates to a method for producing the exhaust gas purifying catalyst of the present invention.

[0049] The exhaust gas purification catalyst of the present invention can be produced by a method including the steps of: (i) mixing aluminum hydroxide with an aqueous solution of a barium compound, and firing the mixture at a specific temperature to prepare a mixture of barium carbonate and a mixed crystal of α-alumina and θ-alumina carrying the barium carbonate, wherein the barium carbonate content is adjusted to a specific amount; and (ii) mixing a precious metal, an OSC material, and the mixture prepared in (i), and firing again to produce the exhaust gas purification catalyst.

[0050] In step (i), the aluminum hydroxide is not limited. For example, aluminum hydroxide C31 manufactured by Sumitomo Chemical can be used. Before mixing with the barium compound, the aluminum hydroxide may be calcined, for example, at 400 to 600°C for 1 to 3 hours.

[0051] In step (i), the barium compound is not limited as long as it dissolves in water, and examples of the barium compound include barium acetate, barium chloride, and barium nitrate.

[0052] In step (i), the method for mixing aluminum hydroxide with the aqueous solution of the barium compound is not limited, and examples of the mixing method include impregnating aluminum hydroxide with the aqueous solution of the barium compound, or adding aluminum hydroxide to the aqueous solution of the barium compound and mixing them.

[0053] The mixture of aluminum hydroxide and the aqueous solution of a barium compound may optionally be dried, for example, typically at 100°C to 120°C for typically 1 to 3 hours, and then calcined, for example, typically at 400°C to 600°C for typically 1 to 3 hours.

[0054] The mixture of aluminum hydroxide and the aqueous solution of a barium compound is then fired at 1000 to 1400°C, and in one embodiment, 1100 to 1300°C, typically for 1 to 10 hours, and in one embodiment, 2 to 8 hours.

[0055] By calcining a mixture of aluminum hydroxide and an aqueous solution of a barium compound at a high temperature of 1000°C or higher, the aluminum hydroxide becomes a mixed crystal of α-alumina and θ-alumina, and relatively large mesopores with an average pore size of 20 nm or more are formed at the interface between the two crystals. Furthermore, the barium compound precipitates as fine barium carbonate particles within the mesopores of the mixed crystal and is fixed in place. As a result, a mixture of barium carbonate and a mixed crystal of α-alumina and θ-alumina in which the barium carbonate is dispersed and supported is prepared.

[0056] The content of barium carbonate is 2 to 15% by weight, and in one embodiment, 5 to 13% by weight, based on metallic barium (Ba) relative to the total weight of the mixture of barium carbonate and the mixed crystal of α-alumina and θ-alumina in which the barium carbonate is dispersed and supported. By keeping the barium carbonate content within this range, it is possible to suppress HC poisoning of the precious metal, improve the NOx purification rate that can be reduced by HC poisoning, and suppress an increase in the specific surface area of the barium carbonate and the barium support.

[0057] In step (ii), the noble metal, the OSC material, and the mixture prepared in step (i) are mixed together.

[0058] The mixing method is not limited, and examples of the mixing method include mixing the OSC material with the mixture prepared in (i), for example, dry mixing or wet mixing, and then impregnating the mixture with an aqueous solution of a precious metal, or adding the mixture of the OSC material and the mixture prepared in (i) to an aqueous solution of a precious metal and further mixing.

[0059] The mixture of the noble metal mixed in step (ii), the OSC material, and the mixture prepared in step (i) is optionally dried, for example, typically at 100°C to 120°C, typically for 1 hour to 3 hours, and then calcined, specifically at 400°C to 800°C, and in one embodiment at 400°C to 600°C, typically for 1 hour to 3 hours, to produce a catalyst for purifying exhaust gases.

[0060] In step (ii), a catalyst for purifying exhaust gases may be produced by mixing the precious metal, the OSC material, and the mixture prepared in step (i) in a solvent, for example, water, to prepare a catalyst coating layer slurry, applying the catalyst coating layer slurry to the wall surface of the substrate as described above, and then drying the slurry, for example, typically at 100°C to 120°C, typically for 1 hour to 3 hours, and then calcining the slurry, specifically at 400°C to 800°C, in one embodiment at 400°C to 600°C, typically for 1 hour to 3 hours.

[0061] In steps (i) and (ii), the conditions described above, particularly the conditions other than the firing temperature in step (i), are not limited. For example, the firing atmosphere may be air or an inert gas, such as nitrogen gas and / or argon gas.

[0062] (Use of catalyst for purifying exhaust gas) The exhaust gas purification catalyst of the present invention can exhibit a significant effect in exhaust gas purification performance in a rich atmosphere, and can be used as an exhaust gas purification catalyst that exhibits a high HC poisoning suppression effect, which can be used even in an environment in which excess HC, etc. are adsorbed to the exhaust gas purification catalyst in a rich atmosphere and can poison the exhaust gas purification catalyst. [Example]

[0063] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to those shown in these examples.

[0064] I-1. Preparation of catalyst for exhaust gas purification Example 1: Aluminum hydroxide powder was calcined in air at 500°C, and the resulting powder was impregnated with a barium acetate aqueous solution, dried, and calcined at 500°C for 2 hours, and then calcined at 1200°C for 5 hours to produce a barium carbonate-dispersed α-alumina and θ-alumina mixed crystal. The weight ratio of α-alumina to θ-alumina in this mixed crystal (α-alumina / θ-alumina) was 4, as measured by the peak intensity ratio in XRD. Furthermore, the barium carbonate content in this mixed crystal was 10 wt% (based on Ba) of the total weight of the barium carbonate-dispersed α-alumina and θ-alumina mixed crystal. Next, the obtained powder was mixed with ceria-zirconia composite oxide in an amount of 50 wt % based on the total weight of the ceria-zirconia composite oxide and the barium carbonate-dispersed α-alumina and θ-alumina mixed crystal, and the mixture was further impregnated with an aqueous palladium nitrate solution, dried, and calcined for 2 hours at 500° C. This yielded a powder of a catalyst for exhaust gas purification in which Pd was supported in an amount of 1 wt % based on the total weight of the catalyst for exhaust gas purification.

[0065] Comparative Example 1: θ-alumina powder was mixed with ceria-zirconia composite oxide in an amount of 50 wt % based on the total weight of the ceria-zirconia composite oxide and θ-alumina, and the mixture was impregnated with an aqueous palladium nitrate solution, dried, and calcined for 2 hours at 500° C. This yielded a powder of a catalyst for purifying exhaust gases in which Pd was supported in an amount of 1 wt % based on the total weight of the catalyst for purifying exhaust gases.

[0066] Comparative Example 2: Barium carbonate-supported α-alumina was prepared by impregnating θ-alumina powder with a barium acetate aqueous solution, drying, and calcining at 500°C. The barium carbonate content of the α-alumina was 10 wt% based on Ba, relative to the total weight of the barium carbonate-supported α-alumina. Next, ceria-zirconia composite oxide was mixed with the obtained powder in an amount of 50 wt% relative to the total weight of the ceria-zirconia composite oxide and the barium carbonate-supported α-alumina, and the mixture was further impregnated with a palladium nitrate aqueous solution, dried, and calcined at 500°C for 2 hours. This resulted in a powder of a catalyst for exhaust gas purification in which 1 wt% of Pd was supported, relative to the total weight of the catalyst for exhaust gas purification.

[0067] I-2. Analysis of exhaust gas purification catalysts I-2-1. Crystal structure analysis of alumina XRD measurements were performed on the barium carbonate-dispersed α-alumina and θ-alumina mixed crystal of Example 1, the θ-alumina of Comparative Example 1, and the barium carbonate-supported α-alumina of Comparative Example 2, which were used in the production of each exhaust gas purification catalyst powder, and the crystal structures were identified from the XRD patterns.

[0068] I-2-2. Measurement of barium carbonate crystallite size XRD measurements were performed on the barium carbonate-dispersed α-alumina and θ-alumina mixed crystal of Example 1 and the barium carbonate-supported α-alumina of Comparative Example 2, which were used in the production of each exhaust gas purification catalyst powder, and the barium carbonate crystallite diameter (Ba crystallite diameter) was calculated from the half-width of the (111) plane of the XRD peak of barium carbonate.

[0069] I-2-3.Measurement of average pore diameter The pore size distribution of the barium carbonate-dispersed α-alumina and θ-alumina mixed crystal of Example 1, the θ-alumina of Comparative Example 1, and the barium carbonate-supported α-alumina of Comparative Example 2, which were used in producing each exhaust gas purification catalyst powder, was measured using a pore size measurement method based on nitrogen gas adsorption, and the average pore size was calculated using the BJH method.

[0070] I-2-4. Heat resistance evaluation (Durability treatment) To examine the heat resistance of each exhaust gas purification catalyst, the powder of each exhaust gas purification catalyst was compacted and then crushed to produce pellet catalyst with a diameter of 0.5 mm to 1.7 mm. The pellet catalyst was then exposed to 1000°C for 5 hours in an electric furnace.

[0071] (Performance test) After durability testing, the OSC amount of the pellet catalyst was measured at a gas temperature of 400°C. A 1% O2 / N2 gas was passed through 1g of pellet catalyst at a gas flow rate of 15 L / min for 2 minutes, followed by a 2% CO / N2 gas. The oxygen absorption and release amount was calculated from the amount of CO2 generated in the first 15 seconds. Next, the temperature was increased while a gas with the composition shown in Table 1 was passed through, and the temperature at which the NOx purification rate reached 50% was measured.

[0072] [Table 1]

[0073] I-3. Evaluation Results Figure 2 shows the XRD spectra of the barium carbonate-dispersed α-alumina and θ-alumina mixed crystals used in producing the exhaust gas purification catalyst powder of Example 1. Table 2 shows the alumina crystal structure, barium carbonate crystallite size (Ba crystallite size), and average pore size. Figure 3 shows the OSC of the exhaust gas purification catalysts of Example 1 and Comparative Examples 1 and 2. Figure 4 shows the NOx-T50 of the exhaust gas purification catalysts of Example 1 and Comparative Examples 1 and 2.

[0074] [Table 2]

[0075] In Example 1 of the present invention, fine barium carbonate particles with a crystallite size of 14 nm were dispersed in a mixed crystal of α-alumina and θ-alumina with an average pore size of 27 nm. Furthermore, it was found that Example 1 had higher OSC performance and NOx purification performance than Comparative Examples 1 and 2. Although the details of the mechanism by which such a structure is formed are unknown, it is presumed that by producing an exhaust gas purification catalyst under the conditions of the present invention, the mixed barium adequately suppresses the crystal type transition of alumina and is incorporated into the pores at the interface of the formed crystal, where it exists as fine barium particles.

[0076] II. Check the barium carbonate content II-1. Preparation of catalyst Example 2: Aluminum hydroxide powder was calcined in air at 500°C. The resulting powder was impregnated with a barium acetate aqueous solution, dried, and calcined at 500°C for 2 hours, followed by another calcination at 1200°C for 5 hours to produce a barium carbonate-dispersed α-alumina and θ-alumina mixed crystal. The weight ratio of α-alumina to θ-alumina (α-alumina / θ-alumina) in this mixed crystal was 4, as measured by the peak intensity ratio in XRD. Furthermore, the barium carbonate content in this mixed crystal was 5 wt% (Ba based) based on the total weight of the barium carbonate-dispersed α-alumina and θ-alumina mixed crystal. The resulting powder was then impregnated with a palladium nitrate aqueous solution, dried, and calcined at 500°C for 2 hours. This resulted in a catalyst powder with a Pd loading of 1 wt% based on the total weight of the catalyst powder.

[0077] Example 3: A catalyst powder was obtained in the same manner as in Example 2, except that the content of barium carbonate was 10% by weight, based on Ba, relative to the total weight of the barium carbonate-dispersed α-alumina and θ-alumina mixed crystals.

[0078] Comparative Example 3: A catalyst powder was obtained in the same manner as in Example 2, except that the content of barium carbonate was 20% by weight, based on Ba, relative to the total weight of the barium carbonate-dispersed α-alumina and θ-alumina mixed crystals.

[0079] Comparative Example 4: Aluminum hydroxide powder was calcined in air at 500°C and then further calcined at 1200°C for 5 hours to produce α-alumina. The resulting powder was then impregnated with an aqueous palladium nitrate solution, dried, and calcined at 500°C for 2 hours. This yielded a catalyst powder carrying 1 wt% Pd relative to the total weight of the catalyst powder.

[0080] II-2. Catalyst Analysis: Heat Resistance Evaluation (Durability treatment) To examine the heat resistance of each catalyst, the powder of each catalyst was compacted and then crushed to prepare catalyst pellets with diameters of 0.5 mm to 1.7 mm, which were then exposed to 1000°C for 5 hours in an electric furnace.

[0081] (Performance Test) After the durability test, the pellet catalyst was heated while a gas having the composition shown in Table 1 was passed through it, and the temperature at which the NOx purification rate reached 50% was measured.

[0082] II-3. Evaluation Results Figure 5 shows the relationship between the NOx-T50 and the content of barium carbonate as metal barium (Ba) in the catalysts of Examples 2 and 3 and Comparative Examples 3 and 4. Figure 5 shows that performance changes depending on the Ba content of the barium carbonate. Catalysts with a Ba content of around 10% by weight in barium carbonate showed high performance, and a Ba content of 5% to 13% by weight in barium carbonate was particularly suitable.

Claims

1. A catalyst for purifying exhaust gases, comprising a precious metal, an OSC material, barium carbonate, and a barium carrier on which the barium carbonate is supported, the barium carrier is a mixed crystal of α-alumina and θ-alumina, the barium carbonate is dispersed in the mesopores of the mixed crystal; The content of the barium carbonate is 2% by weight to 15% by weight based on the total weight of the barium carbonate and the barium carrier, based on the barium metal. The exhaust gas purification catalyst.

2. 2. The exhaust gas purification catalyst according to claim 1, wherein the barium carbonate has a crystallite size of 20 nm or less when calculated from the half-width of the peak of the (111) plane in an XRD spectrum, and the mixed crystal has an average pore size of 20 nm or more when calculated by the BJH method based on pore analysis by nitrogen gas adsorption.

3. 2. The exhaust gas purifying catalyst according to claim 1, wherein the content of the barium carbonate is 5% by weight to 13% by weight, based on metallic barium, relative to the total weight of the barium carbonate and the barium carrier.

4. The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the exhaust gas purifying catalyst has a substrate and a catalyst coating layer coated on the substrate, and the catalyst coating layer contains the noble metal, the OSC material, the barium carbonate, and the barium carrier.

5. A method for producing an exhaust gas purification catalyst, (i) a step of mixing aluminum hydroxide with an aqueous solution of a barium compound and firing the mixture at 1000°C to 1400°C to prepare a mixture of barium carbonate and a mixed crystal of α-alumina and θ-alumina carrying the barium carbonate, wherein the content of the barium carbonate is adjusted to be 2% by weight to 15% by weight based on the weight of metallic barium relative to the total weight of the mixture; (ii) mixing a precious metal, an OSC material, and the mixture prepared in (i), and calcining the mixture again at 400°C to 800°C to produce a catalyst for purifying exhaust gas; A method comprising:

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