Exhaust gas purification catalyst

Abstract

The present invention addresses the problem of providing an exhaust gas purification catalyst capable of further improving purification performance. This exhaust gas purification catalyst is characterized by comprising: carrier particles; zirconium dioxide particles which contain zirconium dioxide and in which a first cation having a higher oxidation number than zirconium and a second cation having a larger ion radius than zirconium are added together to the zirconium dioxide; and rhodium particles supported on the zirconium dioxide particles, (i) the carrier particles are porous carrier particles, and the zirconium dioxide particles and the rhodium particles are supported in pores of the porous carrier particles, or (ii) the carrier particles are composed of a plurality of primary particles, and the zirconium dioxide particles and the rhodium particles are dispersed inside the carrier particles.

Description

Technical Field

[0001] This invention relates to a catalyst for purifying waste gas. Background Technology

[0002] The exhaust gas from internal combustion engines contains harmful components such as CO (carbon monoxide), HC (hydrocarbons), and NOx (nitrogen oxides). To remove these harmful components from the exhaust gas, catalysts for exhaust gas purification are being used, in which precious metals such as Rh (rhodium) are supported on carriers such as Al2O3 (alumina).

[0003] As a catalyst for such waste gas purification, one known catalyst contains: a noble metal; Al2O3 support particles; and ZrO2 (zirconia) semiconductor particles supported on the surface of the support particles (Patent Document 1). Furthermore, one known catalyst contains: at least one noble metal selected from the group consisting of Pt (platinum); and a composite compound, which is a compound of at least one metal element selected from the group consisting of Al (aluminum) substantially uniformly dispersed in at least one oxide selected from the group consisting of Al2O3, wherein the noble metal is supported on the composite compound in such a state that a portion of its surface area is covered by the composite compound (Patent Document 2).

[0004] Patent Document 1: International Publication No. 2020 / 050464

[0005] Patent Document 2: Japanese Patent Application Publication No. 2006-198594 Summary of the Invention

[0006] The restrictions on the discharge of harmful components contained in exhaust gas are being strengthened year by year, requiring further improvement in the purification performance of catalysts used for exhaust gas purification.

[0007] The present invention has been made in view of this, and its object is to provide a catalyst for exhaust gas purification that can further improve purification performance.

[0008] To solve the above-mentioned problems, the catalyst for exhaust gas purification of the present invention is characterized by having: a carrier particle; a zirconium dioxide particle, wherein a first cation with an oxidation number higher than zirconium and a second cation with an ionic radius greater than zirconium are added together to the zirconium dioxide; and a rhodium particle loaded on the zirconium dioxide particle, (i) the carrier particle is a porous carrier particle, and the zirconium dioxide particle and the rhodium particle are loaded in the pores of the porous carrier particle, or (ii) the carrier particle is composed of a plurality of primary particles, and the zirconium dioxide particle and the rhodium particle are dispersed inside the carrier particle.

[0009] Invention Effects

[0010] According to the present invention, purification performance can be further improved. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the catalyst for purifying exhaust gas according to the first embodiment.

[0012] Figure 2 This is a schematic diagram of the catalyst for purifying waste gas according to the second embodiment.

[0013] Figure 3 The XRD patterns are, in particular, the range of 27° to 32° of the samples after durability tests of Examples 4 to 6 and Comparative Examples 2 and 3.

[0014] Figure 4 These are the samples after durability test 1 of Comparative Examples 1-3 and the samples after durability test 1 of Examples 1-3 (Y in the first powder).

[0015] Figure 5 These are the samples after durability test 2 of Comparative Examples 1-3, and the samples after durability test 2 of Examples 1-3 (Y in the first powder). Detailed Implementation

[0016] The following describes embodiments of the catalyst for exhaust gas purification according to the present invention. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the spirit of the present invention.

[0017] 1. Catalysts for waste gas purification

[0018] First, regarding the catalyst for exhaust gas purification involved in the embodiments, the first and second embodiments are exemplified. Figure 1 This is a schematic diagram of the catalyst for purifying waste gas according to the first embodiment. Figure 2 This is a schematic diagram of the catalyst for purifying waste gas according to the second embodiment.

[0019] like Figure 1 As shown, the catalyst 1 for purifying exhaust gas according to the first embodiment includes powdered porous support particles 30, powdered zirconium dioxide particles 20, and powdered rhodium particles 10 supported on the zirconium dioxide particles 20. The zirconium dioxide particles 20 contain ZrO2 (zirconia), and a first cation with an oxidation number higher than Zr (zirconia) and a second cation with an ionic radius greater than Zr are added (doped) into the ZrO2. The zirconium dioxide particles 20 and the rhodium particles 10 are supported in the pores of the porous support particles 30 and dispersed among the porous support particles 30.

[0020] like Figure 2As shown, the catalyst 2 for purifying exhaust gas according to the second embodiment includes powdered carrier particles 40, powdered zirconium dioxide particles 20, and powdered rhodium particles 10 supported on the zirconium dioxide particles 20. The carrier particles 40 are secondary particles formed by the aggregation of multiple primary particles. The zirconium dioxide particles 20 contain ZrO2 (zirconia), and a first cation with an oxidation number higher than Zr (zirconia) and a second cation with an ionic radius greater than Zr are added (doped) into the ZrO2. The zirconium dioxide particles 20 and the rhodium particles 10 are dispersed inside the carrier particles 40 and between the carrier particles 40.

[0021] Next, the details of the catalyst for purifying waste gas involved in the implementation method will be explained.

[0022] Generally, Rh (rhodium) as a catalyst metal exhibits higher catalytic activity than Pd (palladium) or Pt (platinum), especially in NOx reduction. Improving the catalytic activity of Rh can potentially reduce the amount of Rh required.

[0023] Regarding this, the rate-controlling step in the reduction of NOx in exhaust gas is the CO2 (carbon dioxide) generation pathway. That is, the reaction between O (oxygen) atoms adsorbed on rhodium particles and CO (carbon monoxide) is rate-controlled. Therefore, if the adsorption energy of O atoms adsorbed on rhodium particles can be reduced, the NOx reduction performance of the catalyst for exhaust gas purification can be improved. In the catalyst for exhaust gas purification, rhodium particles are loaded onto zirconium dioxide particles. Moreover, the ZrO2 (zirconia) contained in the zirconium dioxide particles becomes electron-rich by the addition of a first cation with an oxidation state higher than Zr (zirconia) (e.g., Nb). This electron flows into the rhodium particles, reducing the adsorption energy of O atoms. Therefore, the NOx reduction performance of the catalyst for exhaust gas purification can be improved. Thus, the purification performance of the catalyst for exhaust gas purification can be improved. Furthermore, ZrO2 has a particularly high adsorption energy in metal oxides and readily adsorbs with Rh, such as RhO2 (rhodium oxide). Among them, ZrO2 with the addition of the first cation, especially Nb (niobium), exhibits particularly high adsorption energy compared to ZrO2 without the addition of the first cation. This suppresses the evaporation of rhodium particles and achieves high heat resistance.

[0024] On the other hand, in catalysts for waste gas purification, if a first cation (e.g., Nb) is added to the ZrO2 of zirconium dioxide particles, the crystal structure of the zirconium dioxide particles becomes unstable at high operating temperatures due to the stability of the monoclinic crystal structure of the oxide of the first cation (e.g., Nb2O5). This instability, rather than the stable tetragonal structure, may lead to a decrease in purification performance. However, in catalysts for waste gas purification, a second cation (e.g., Y) with an ionic radius greater than Zr is added to the ZrO2 of the zirconium dioxide particles along with the first cation. Therefore, for example, by using the same principle as partially stabilized zirconium (e.g., yttrium-stabilized zirconium) described in papers such as Garvie, RC, Hannink RH and Pascoe, RT (1975) Ceramic steel. Nature, 258(5537), 703-704. and Kobayashi, K., Kuwajima H. ​​and Masaki, T., Solid State Ionics., Vol. 3 / 4, pp. 489-493 (1981), the crystal structure of zirconium dioxide particles can remain tetragonal and stable even at high temperatures during use. Therefore, the decrease in the purification performance of the catalyst for exhaust gas purification caused by the addition of the first cation can be suppressed. Thus, the purification performance of the catalyst for exhaust gas purification can be further improved.

[0025] Furthermore, in the catalyst for exhaust gas purification according to the first embodiment, the zirconium dioxide particles, by being loaded within the pores of the porous support particles or dispersed and loaded between the porous support particles, can suppress the aggregation of zirconium dioxide particles, thus further improving heat resistance. Rhodium particles are also loaded onto the zirconium dioxide particles and are loaded within the pores of the porous support particles. Furthermore, in the catalyst for exhaust gas purification according to the second embodiment, the zirconium dioxide particles, by being dispersed and loaded within the support particles or between the support particles, can suppress the aggregation of zirconium dioxide particles, thus further improving heat resistance. Rhodium particles are also loaded onto the zirconium dioxide particles and dispersed within the support particles.

[0026] That is, the catalyst for exhaust gas purification possesses both high NOx reduction performance and high heat resistance. Furthermore, there are no particular limitations on the catalyst used for exhaust gas purification; for example, it can be a three-way catalyst.

[0027] The specific surface area of ​​the catalyst used for waste gas purification after durability testing can reach 90 m². 2 / g or less, 89.9m 2 / g or less, 89.8m 2 / g or below or 89.7m 2Below / g, it can also be 88.0m 2 / g or more, 88.1m 2 / g or more, 88.2m 2 / g or above or 88.3m 2 / g or more. In addition, durability tests are conducted, for example, by placing the catalyst for exhaust gas purification in a flow-through durability furnace and alternately and repeatedly introducing reducing gas (CO) and oxidizing gas (O2) at 1000°C for 5 hours.

[0028] The NOx 50% purification temperature after the durability test of the catalyst for waste gas purification can be, for example, below 250°C, below 249°C, below 248°C, below 247°C, or below 246°C, and can be above 245°C, above 246°C, above 247°C, or above 248°C. Furthermore, the NOx 50% purification temperature is determined as follows: First, the catalyst for waste gas purification is placed in a flow-through reactor and heated to 500°C in an evaluation model gas at a heating rate of 50°C / min. After maintaining this temperature for 10 minutes, it is cooled to 100°C. Next, it is heated at a heating rate of 20°C / min, and the gas temperature at the point where the NOx purification rate reaches 50% is determined as the NOx 50% purification temperature. The composition of the evaluation model gas can be, for example, the composition used in the examples described later.

[0029] 1-1. Carrier particles

[0030] The support particles in a catalyst for waste gas purification can be porous support particles commonly used in waste gas purification catalysts, or they can be secondary particles composed of multiple primary particles. Support particles can be, for example, metal oxides, specifically metal oxides containing Al (aluminum), and more specifically, composite oxides containing Al and Zr, such as Al2O3 (alumina) or Al2O3-ZrO2 composite oxides.

[0031] The average particle size of porous carrier particles and carrier particles that are secondary particles composed of multiple primary particles is not particularly limited. For example, it can be 1 μm or more, 10 μm or more, 50 μm or more, or 100 μm or more, and it can be less than 1000 μm, less than 500 μm, less than 200 μm, or less than 100 μm. In addition, the average particle size refers to the numerical average of the equivalent diameters of at least 200 particles observed using a scanning electron microscope (SEM) and calculated based on the equivalent diameters of true circles of equal area.

[0032] The pore diameter of the porous carrier particles and the pore diameter of the internal pores of the carrier particles are not particularly limited as long as they are large enough to load zirconium dioxide particles and rhodium particles into the pores. For example, they can be above 10 nm, above 50 nm or above 100 nm, and below 1000 nm, below 500 nm or below 200 nm.

[0033] 1-2. Zirconia particles

[0034] The catalyst for exhaust gas purification contains zirconium dioxide particles (ZrO2), with a first cation having an oxidation number higher than Zr (zirconia) and a second cation having an ionic radius greater than Zr added (doped) into the ZrO2. After durability testing, the crystal structure of the zirconium dioxide particles in the catalyst for exhaust gas purification can, for example, be a tetragonal crystal structure. Furthermore, the crystal structure of the zirconium dioxide particles refers to the crystal structure of the compound in which the first and second cations are added together in the ZrO2 contained within the zirconium dioxide particles.

[0035] The content of zirconium dioxide particles relative to the total of zirconium dioxide particles and carrier particles can be from 1.0% to 70.0% by mass. Specifically, this percentage can be 1.0% or more by mass, 5.0% or more by mass, 10.0% or more by mass, or 15.0% or more by mass, and can be less than 70.0% by mass, less than 50.0% by mass, less than 30.0% by mass, or less than 10.0% by mass. If the percentage is above these lower limits, the amount of zirconium dioxide particles that can act on rhodium particles can be significantly increased. On the other hand, if the percentage is below these upper limits, the dispersion of zirconium dioxide particles within the pores of the porous carrier particles and within the interior of the carrier particles is particularly good.

[0036] There are no particular limitations on the first cation; for example, Nb (niobium) can be included. The content of the Nb (niobium) oxide portion in the zirconium dioxide particles can be from 0.1% to 12.0% by mass. Specifically, this content can be 0.1% or more by mass, 0.5% or more by mass, 1.0% or more by mass, 1.5% or more by mass, 2.0% or more by mass, 5.0% or more by mass, or 11.0% or more by mass, and can be less than 12.0% by mass, less than 11.0% by mass, less than 10.0% by mass, less than 9.0% by mass, less than 8.0% by mass, less than 6.0% by mass, or less than 3.0% by mass. Furthermore, the content of the Nb oxide portion in the zirconium dioxide particles refers to the ratio of the mass of the Nb oxide portion to the total mass of the zirconium dioxide particles [mass%]. Specifically, the Nb oxide portion in the zirconium dioxide particles is, for example, Nb₂O₅ (niobium pentoxide).

[0037] If the content is above these lower limits, the adsorption energy of zirconium dioxide particles becomes particularly large, thus easily suppressing the evaporation of rhodium particles. Furthermore, the electron-gathering effect resulting from the addition of Nb to ZrO2 is readily achieved. On the other hand, if the content is below these upper limits, the electron-donating / accepting of rhodium particles is particularly good, which can significantly reduce the NOx 50% purification temperature. Moreover, it is easy to prevent the crystal structure of zirconium dioxide particles from becoming unstable at high temperatures during use, such as turning into a monoclinic structure instead of a tetragonal stable structure.

[0038] There are no particular limitations on the second cation; for example, Y (yttrium) can be included. The content of the oxide portion of Y (yttrium) in the zirconium dioxide particles can be from 0.1% to 12.0% by mass. Specifically, this content can be 0.1% or more by mass, 0.5% or more by mass, 1.0% or more by mass, 1.5% or more by mass, 2.0% or more by mass, 5.0% or more by mass, or 11.0% or more by mass, and can be less than 12.0% by mass, less than 11.0% by mass, less than 10.0% by mass, less than 9.0% by mass, less than 8.0% by mass, less than 6.0% by mass, or less than 3.0% by mass. Furthermore, the content of the oxide portion of Y in the zirconium dioxide particles refers to the ratio of the mass of the oxide portion of Y to the total mass of the zirconium dioxide particles [mass%]. Specifically, the oxide portion of Y in the zirconium dioxide particles is, for example, Y₂O₃ (yttrium oxide).

[0039] If the content is above these lower limits, then although Nb is added to ZrO2, it can more effectively achieve the effect of making the zirconium dioxide particles form a tetragonal stable crystal structure at high temperatures during use. On the other hand, if the content is below these upper limits, it can more effectively suppress the decrease in electron gain effect caused by the addition of Y due to the addition of Nb to ZrO2.

[0040] The grain diameter of zirconium dioxide particles can be 6.0 nm or more, 6.2 nm or more, 6.4 nm or more, or 6.8 nm or more, and can be 8.0 nm or less, 7.8 nm or less, 7.6 nm or less, or 7.4 nm or less. If the grain diameter is above these lower limits, the effect of suppressing sintering of catalyst metal particles is particularly good. On the other hand, if the grain diameter is below these upper limits, the heat resistance of the zirconium dioxide particles is particularly good, especially suppressing the aggregation of zirconium dioxide particles caused by heating. The secondary particle size (D50) of zirconium dioxide particles can exceed 0 nm, 5 nm or more, 10 nm or more, or 15 nm or more, and can be 40 nm or less, 35 nm or less, 30 nm or less, or 25 nm or less. If the secondary particle size (D50) is of this size, the dispersibility of the zirconium dioxide particles can be further improved. In addition, the D50 (median particle size) can be measured, for example, using a laser diffraction particle size distribution measuring device (SALD-2300) manufactured by Shimadzu Corporation, and is measured as the particle size at 50% cumulative frequency.

[0041] 1-3: Rhodium particles

[0042] The catalyst for exhaust gas purification contains rhodium particles (Rh) that are supported on zirconium dioxide particles.

[0043] The size and shape of the rhodium particles can be any size and shape of the catalyst metal used as a catalyst for waste gas purification. Specifically, the particle size (D50) of the rhodium particles can be 0.1 nm or more, 1.0 nm or more, 2.0 nm or more, or 2.5 nm or more, and can be less than 10.0 nm, 5.0 nm or less, 3.0 nm or less, or 2.5 nm or less. Furthermore, the method for determining the D50 of the rhodium particles is the same as the method for determining the D50 of the zirconium dioxide particles described above.

[0044] Regarding the loading amount of rhodium particles onto zirconium dioxide particles, for example, the ratio of the mass of rhodium particles to the total mass of the catalyst for exhaust gas purification can be 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, or 0.4% by mass or more, and the ratio of the mass of rhodium particles to the total mass of the catalyst for exhaust gas purification can be 5.0% by mass or less, 2.5% by mass or less, 1.0% by mass or less, or 0.5% by mass or less. If the ratio of the mass of rhodium particles to the total mass of the catalyst for exhaust gas purification is above these lower limits, more rhodium particles are available to aid in NOx purification, thus achieving higher NOx purification performance. On the other hand, if the ratio of the mass of rhodium particles to the total mass of the catalyst for exhaust gas purification is below these upper limits, the amount of Rh, an expensive precious metal, can be reduced, resulting in excellent cost performance.

[0045] 2. Method for manufacturing catalysts for waste gas purification

[0046] There are no particular limitations on the manufacturing method of catalysts for exhaust gas purification. For example, the manufacturing methods listed below (numbers 1 to 3) can be used.

[0047] 2-1. Manufacturing Method 1

[0048] The first manufacturing method comprises the following steps: obtaining a dispersion by dispersing porous support particles, a zirconium dioxide source, a first cation source, and a second cation source in an acidic dispersion medium; obtaining porous support particles containing zirconium dioxide particles with ZrO2 and the first and second cations added (doped) together in the ZrO2 by drying the dispersion and further calcining it; and loading rhodium particles onto the zirconium dioxide particles within the pores of the porous support particles. The rhodium particles are loaded within the pores of the porous support particles and onto the zirconium dioxide particles.

[0049] Citric acid can be used as an acidic dispersion medium, for example. Al₂O₃ can be used as a porous support particle, for example, specifically, a composite of La₂O₃ and Al₂O₃ (La₂O₃(lanthanum(III)):1 wt%). Zr nitrate can be used as a zirconium dioxide source, for example. Nb oxalate can be used as a first cation source, for example. Y nitrate can be used as a second cation source, for example.

[0050] The drying of the dispersion can be carried out under the following conditions: after evaporating the dispersion to obtain the precipitate, it is dried in a constant temperature furnace at 100°C to 200°C, preferably 100°C to 150°C, more preferably 100°C to 120°C, for 1 hour to 36 hours, preferably 10 to 24 hours. Calcination can be carried out, for example, under the following conditions: in the atmosphere, at 600°C to 1000°C, preferably 700°C to 900°C, more preferably 700°C to 800°C, for 1 to 10 hours, preferably 2 to 5 hours, more preferably 3 to 4 hours. In the process of drying and calcining the dispersion, a reduction treatment can be performed after calcination. The reduction treatment can be carried out, for example, under the following conditions: reduction at 800°C for 2 hours in a 3% hydrogen atmosphere. The pH of the dispersion medium can be, for example, above 1.0, above 1.5, or above 2.0, and can be below 5.0, below 4.5, or below 4.0, especially preferably 2.5 to 3.5.

[0051] There are no particular limitations to the method of loading rhodium particles onto zirconium dioxide particles within the pores of a porous carrier particle. Examples include dispersing the porous carrier particles with zirconium dioxide particles loaded within the pores in a dispersion medium (e.g., water such as distilled water), adding a rhodium source (e.g., nitric acid Rh), stirring, drying, and then calcining. The drying and calcination in this method can be carried out under the same conditions as the drying and calcination of the dispersion in the process of obtaining the porous carrier particles.

[0052] 2-2. Second manufacturing method

[0053] The second manufacturing method comprises the following steps in sequence: obtaining a first dispersion by dispersing porous carrier particles in an organic solvent; mixing a zirconium dioxide source, a first cation source, and a second cation source into the first dispersion; boiling and stirring the first dispersion; removing the organic solvent to obtain a powder; drying the powder and further calcining it to obtain porous carrier particles containing zirconium dioxide particles with ZrO2 and the first and second cations added together in the ZrO2 within their pores; and loading rhodium particles onto the zirconium dioxide particles within the pores of the porous carrier particles. The rhodium particles are loaded within the pores of the porous carrier particles and onto the zirconium dioxide particles.

[0054] Hexane can be used as an organic solvent, for example. Alumina (Al₂O₃) can be used as a porous support particle, for example; specifically, La₂O₃-composite Al₂O₃ (La₂O₃: 1 wt%) can be used. Zirconium butoxide can be used as a zirconium dioxide source, for example. Niobium butoxide can be used as a first cation source, for example. Yttrium butoxide can be used as a second cation source, for example.

[0055] The method of loading rhodium particles onto zirconium dioxide particles within the pores of porous carrier particles, as well as the drying and calcination, in the second manufacturing method can be the same as in the first manufacturing method.

[0056] 2-3. Third manufacturing method

[0057] The third manufacturing method comprises the following steps in sequence: obtaining a first dispersion by dispersing a zirconium dioxide source (e.g., oxygen Zr nitrate), a first cation source (e.g., Nb oxalate), and a second cation source (e.g., Y nitrate) in a solvent (e.g., water); generating a precipitate, for example, by adjusting the pH of the first dispersion to alkaline using ammonia; adding raw materials constituting the primary particles of the carrier particles (e.g., Al nitrate and La nitrate) to the first dispersion and mixing them; removing the solvent from the first dispersion by using a centrifuge and drying it, for example, and further calcining it to obtain a first powder (a powder in which zirconium dioxide particles are loaded onto carrier particles); and dispersing the first powder and a rhodium source (e.g., Rh nitrate) in a solvent (e.g., water) and then drying and further calcining it to obtain a second powder (Rh-loaded powder). The third manufacturing method may further include the following steps: dispersing the second powder and the raw materials constituting the primary particles of the carrier particles (e.g., Al2O3-CeO2-ZrO2 composite oxide, CZ composite oxide and any binder (e.g., Al2O3-based binder)) in a liquid (e.g., water) to obtain these slurries suspended in the liquid; and pulverizing the obtained slurry after pressing it, for example by cold isostatic pressing (CIP).

[0058] 3. Waste gas purification methods

[0059] The waste gas purification method described in the embodiments includes a step of contacting the waste gas with a catalyst for waste gas purification. The method of contacting the waste gas with the catalyst is not particularly limited. The waste gas may, for example, be a gas containing NOx (nitrogen oxides), CO (carbon monoxide), and HC (hydrocarbons).

[0060] The following examples and comparative examples provide a more detailed description of the catalyst for waste gas purification involved in the implementation.

[0061] 1. Preparation of samples of catalysts for waste gas purification

[0062] 1-1. Raw materials for the catalyst sample used for waste gas purification

[0063] Material 1 (Al2O3 (alumina)):

[0064] Powder containing La2O3 composite Al2O3 (La2O3 (lanthanum oxide (III)): 1 wt%) particles

[0065] Material 2 (ACZ):

[0066] Al2O3-CeO2-ZrO2 composite oxide (Al2O3: 30 wt% / CeO2 (cerium dioxide): 20 wt% / ZrO2 (zirconia): 44 wt% / Nd2O3 (neodymium(III)): 2 wt% / La2O3: 2 wt% / Y2O3 (yttrium(III)): 2 wt%)

[0067] Material 3 (CZ):

[0068] Pyrochlore-type CZ composite oxides (CeO2: 51.5 wt% / ZrO2: 45.5 wt% / Pr6O) 11 (3% by mass)

[0069] Material 4 (Al(aluminum) nitrate):

[0070] Reagents manufactured and commercially available by FUJIFILM Wako Pure Chemical Corporation

[0071] Material 5 (Zr(zirconium) nitrate):

[0072] Reagents manufactured and commercially available by FUJIFILM Wako Pure Chemical Corporation

[0073] Material 6 (Rhodium Nitrate):

[0074] Nitric acid Rh aqueous solution (Rh concentration: 2wt%)

[0075] Material 7 (La (lanthanum nitrate)):

[0076] Reagents manufactured and commercially available by FUJIFILM Wako Pure Chemical Corporation

[0077] Material 8 (Nb(niobium) oxalate):

[0078] Taniobis manufactured, commercially available reagents

[0079] Material 9 (Nitric Acid Y (Yttrium)):

[0080] Reagents manufactured and commercially available by FUJIFILM Wako Pure Chemical Corporation

[0081] Material 10 (ammonia):

[0082] Reagents manufactured and commercially available by FUJIFILM Wako Pure Chemical Corporation

[0083] 1-2. Preparation of samples of catalysts for waste gas purification

[0084] [Example 1]

[0085] Test samples of the catalyst for exhaust gas purification involved in the second embodiment were prepared.

[0086] First, while stirring, Zr nitrate (Materials 5), Nb oxalic acid (Materials 8), and Y nitrate (Materials 9) were added to distilled water as a solvent. Then, the pH was adjusted to alkaline using ammonia (Materials 10), resulting in a precipitate. A solution premixed with Al nitrate (Materials 4) and La nitrate (Materials 7) was further added to this solution and mixed. The solvent was then removed using a centrifuge, and the mixture was dried at 120°C for one day. Finally, it was calcined in air at 800°C for 3 hours, thereby obtaining the first powder (a powder of zirconium dioxide particles loaded onto carrier particles). The first powder is a powder containing carrier particles composed of multiple primary particles and zirconium dioxide particles, with Nb and Y added together, dispersed within the carrier particles in ZrO2.

[0087] Next, while stirring, the first powder and nitric acid Rh (material 6) were added to distilled water, dried, and then calcined to obtain the second powder (Rh-loaded powder). In the second powder, rhodium particles are loaded onto zirconium dioxide particles and dispersed within the carrier particles. The second powder corresponds to the catalyst for exhaust gas purification according to the second embodiment.

[0088] Then, while stirring, the second powder, along with ACZ (material 2), CZ (material 3), and Al2O3-based binder, were added to distilled water to obtain a suspended slurry.

[0089] The suspended slurry was then dried and pressed into shape by cold isostatic pressing (CIP) at a pressure of 1 ton. It was then crushed and sieved simultaneously to obtain the test sample of the catalyst for exhaust gas purification in Example 1.

[0090] The content (mass ratio) of Nb₂O₅, Y₂O₃, ZrO₂, and the carrier particles (Al₂O₃) in the first powder was determined using XRF (X-ray fluorescence spectrometry). The content (mass ratio) of Nb, Y, Zr, and Al in the first powder was also determined using XRF. Then, assuming that Nb, Y, Zr, and Al exist as Nb₂O₅, Y₂O₃, ZrO₂, and the carrier particles (Al₂O₃) in the first powder, respectively, the content of Nb₂O₅, Y₂O₃, ZrO₂, and the carrier particles (Al₂O₃) in the first powder was calculated based on the content of Nb, Y, Zr, and Al. The content of Nb₂O₅, Y₂O₃, ZrO₂, and the carrier particles (Al₂O₃) in the first powder of Example 1 is shown in Table 1 below. Furthermore, assuming that Nb₂O₅, Y₂O₃, and ZrO₂ in the first powder are contained within the zirconium dioxide particles, the mass ratios of Nb₂O₅ (niobium oxide portion) and Y₂O₃ (yttrium oxide portion) in the zirconium dioxide particles were calculated based on the content of Nb₂O₅, Y₂O₃, and ZrO₂ in the first powder. These calculation results are also shown in Table 1 below.

[0091] (Examples 2-9)

[0092] By adjusting the amounts of nitric acid oxygen Zr (material 5), oxalic acid Nb (material 8), and nitric acid Y (material 9) added to the solvent, the contents of Nb2O5, Y2O3, and ZrO2 in the first powder and the contents of Nb2O5 and Y2O3 in the zirconium dioxide particles were set as shown in Table 1 below. Otherwise, test samples of the catalysts for exhaust gas purification in Examples 2 to 9 were obtained in the same manner as in Example 1.

[0093] <Comparative Example 1>

[0094] Without adding Nb oxalic acid (material 8), the amounts of nitric acid oxygen Zr (material 5) and nitric acid Y (material 9) added to the solvent were adjusted. Thus, the contents of Nb2O5, Y2O3 and ZrO2 in the first powder and the contents of Nb2O5 and Y2O3 in the zirconium dioxide particles were set as shown in Table 1 below. Otherwise, in the same manner as in Example 1, a test sample of the catalyst for exhaust gas purification of Comparative Example 1 was obtained.

[0095] [Compare Examples 2 and 3]

[0096] Nitric acid Y (material 9) was not added to the solvent, and the amounts of nitric acid oxygen Zr (material 5) and oxalic acid Nb (material 8) added to the solvent were adjusted. Thus, the contents of Nb2O5, Y2O3 and ZrO2 in the first powder and the contents of Nb2O5 and Y2O3 in the zirconium dioxide particles were set as shown in Table 1 below. Otherwise, test samples of the exhaust gas purification catalysts of Comparative Examples 2 and 3 were obtained in the same manner as in Example 1.

[0097] 2. Experiment

[0098] 2-1. X-ray diffraction test

[0099] The crystal structure of the zirconium dioxide particles in the samples of Examples 1-9 and Comparative Examples 1-3 after durability testing was determined. Specifically, for each example, after obtaining the samples after durability testing by calcining them at 1100°C for 5 hours under atmospheric conditions (a test different from durability tests 1 and 2 in "2-2. Durability Tests" described later), XRD patterns were determined by performing X-ray diffraction tests on the samples after durability testing using an X-ray diffraction apparatus. Then, the crystal structure of the zirconium dioxide particles in each example after durability testing was determined based on the XRD patterns. In the X-ray diffraction test, CuKα rays were excited, and measurements were performed in the range of 0° to 90° at a scan rate of 0.05° / min.

[0100] The results of identifying the crystal structure of zirconium dioxide particles in the samples after durability tests of Examples 1-9 and Comparative Examples 1-3 are shown in Table 1 below. Figure 3 The XRD patterns are, in particular, the range of 27° to 32° of the samples after durability tests of Examples 4 to 6 and Comparative Examples 2 and 3.

[0101] 2-2. Durability Test

[0102] (Durability Test 1)

[0103] The samples from Examples 1-6 and Comparative Examples 1-3 were placed in a flow-through durability furnace and subjected to a durability test 1 by alternately introducing reducing gas (CO) and oxidizing gas (O2) at 1000°C for 5 hours. The flow rate and duration of the reducing and oxidizing gases were 20 L / min for 10 minutes each time.

[0104] (Durability Test 2)

[0105] The samples from Examples 1-9 and Comparative Examples 1-3 were placed in a flow-through durability furnace and subjected to a durability test 2 by alternately introducing reducing gas (CO) and oxidizing gas (O2) at 1050°C for 5 hours. The flow rate and duration of the reducing and oxidizing gases were 20 L / min for 10 minutes each time.

[0106] 2-3. Exhaust gas purification test

[0107] (Exhaust gas purification test 1)

[0108] First, 2g of the samples from each of Examples 1-6 and Comparative Examples 1-3 after durability test 1 were placed in a flow-through reactor and heated to 500°C in an evaluation model gas at a heating rate of 50°C / min. This temperature was maintained for 10 minutes, then cooled to 100°C. Next, heating was performed at a heating rate of 20°C / min, and the gas temperature at which the NOx purification rate reached 50% was determined as the NOx 50% purification temperature. The evaluation model gas consisted of CO (0.65 vol%), CO2 (10.00 vol%), C3H6 (0.10 vol%), NO (0.15 vol%), O2 (0.70 vol%), H2 (3.00 vol%), and N2 (remaining).

[0109] The NOx 50% purification temperature [°C] of the samples after durability test 1 of Examples 1 to 6 and Comparative Examples 1 to 3 is shown in Table 1 below. Figure 4 This is a graph showing the change in NOx 50% purification temperature [°C] relative to the Nb2O5 content [mass%] in the first powder for samples after durability test 1 of Comparative Examples 1-3, samples after durability test 1 of Examples 1-3 (Y2O3 content in the first powder: 1% by mass), and samples after durability test 1 of Examples 4-6 (Y2O3 content in the first powder: 4% by mass).

[0110] (Exhaust gas purification test 2)

[0111] First, the NOx 50% purification temperature was determined for the samples after durability test 2 of Examples 1-9 and Comparative Examples 1-3 using the same method as in exhaust gas purification test 1.

[0112] The NOx 50% purification temperature [°C] of the samples after durability test 2 of Examples 1-9 and Comparative Examples 1-3 is shown in Table 1 below. Figure 5This is a graph showing the change in NOx 50% purification temperature relative to the Nb2O5 content [mass%] in the first powder for samples after durability test 2 of Comparative Examples 1-3, samples after durability test 2 of Examples 1-3 (Y2O3 content in the first powder: 1% by mass), samples after durability test 2 of Examples 4-6 (Y2O3 content in the first powder: 4% by mass), and samples after durability test 2 of Examples 7-9 (Y2O3 content in the first powder: 8% by mass).

[0113] 3. Test Results

[0114] The manufacturing conditions of the samples in Examples 1 to 9 and Comparative Examples 1 to 3, as well as the results of X-ray diffraction tests and exhaust gas purification tests, are summarized in Table 1 below.

[0115] [Table 1]

[0116]

[0117] 4. Evaluation

[0118] 4-1. X-ray diffraction test

[0119] As shown in Table 1 above and Figure 3 As shown, based on the peaks in the XRD patterns of the samples after durability tests for Examples 1-9 and Comparative Example 1, the zirconia particles are considered to have a tetragonal crystal structure. On the other hand, based on the peaks in the XRD patterns of the sample after durability tests for Comparative Example 2, the zirconia particles are considered to be in the process of transitioning from tetragonal to monoclinic crystal structure. Furthermore, based on the peaks in the XRD patterns of the sample after durability tests for Comparative Example 3, the zirconia particles are considered to have a monoclinic crystal structure.

[0120] In these results, as with the samples of Comparative Examples 2 and 3, the addition of Nb to ZrO2 resulted in a monoclinic crystal structure instead of a tetragonal crystal structure in the zirconium dioxide particles after the durability test. It is believed that since the crystal structure of Nb₂O₅ is monoclinic, the addition of Nb to ZrO2 resulted in a monoclinic crystal structure in the zirconium dioxide particles at the high temperatures during the durability test. In contrast, as with the samples of Examples 1-9, the addition of Y along with Nb to ZrO2 resulted in a tetragonal crystal structure in the zirconium dioxide particles after the durability test. It is believed that by further adding Y to ZrO2, based on the same principle as with partially stabilized zirconium, the zirconium dioxide particles would achieve a tetragonal crystal structure at the high temperatures during the durability test.

[0121] 4-2. Exhaust Gas Purification Test

[0122] (Exhaust gas purification test 1)

[0123] As shown in Table 1 above and Figure 4 As shown, in the samples after durability test 1 of Comparative Examples 1-3, it was confirmed that the NOx 50% purification temperature tended to increase with the increase of Nb2O5 content. It is believed that although adding Nb to ZrO2 can increase electron capacity, the increased amount of Nb causes the zirconium dioxide particles to become monoclinic rather than tetragonal, thus becoming unstable and reducing the catalyst's purification performance. In contrast, although in the samples after durability test 1 of Examples 1-3 (Y2O3 content in the first powder: 1% by mass), it was confirmed that, similar to Comparative Examples 1-3, the NOx 50% purification temperature tended to increase with the increase of Nb2O5 content, the NOx 50% purification temperature was lower than that of the samples after durability test 1 of Comparative Examples 1-3. It is believed that by further adding Y to ZrO2, the crystal structure of zirconium dioxide particles becomes a tetragonal stable structure, thereby suppressing the decrease in catalyst purification performance caused by the increase in Nb addition. Furthermore, in the samples after durability tests 1 of Examples 4-6 (Y2O3 content in the first powder: 4% by mass), the NOx 50% purification temperature at a Nb2O5 content of 2% by mass was higher than that of the samples after durability tests 1 of Comparative Examples 1-3 and Examples 1-3. It is believed that the increase in electrons generated by Nb addition is reduced due to the excessive amount of Y relative to Nb addition. On the other hand, in this sample, the NOx 50% purification temperature at a Nb2O5 content of 8% by mass was significantly lower than that of the samples after durability tests 1 of Comparative Examples 1-3 and Examples 1-3. It is believed that the effect of increasing the amount of Y relative to Nb addition on the formation of a tetragonal stable crystal structure in the zirconium dioxide particles is greater.

[0124] (Exhaust gas purification test 2)

[0125] As shown in Table 1 above and Figure 5As shown, in the samples after durability test 2 of Comparative Examples 1-3, it was confirmed that the NOx 50% purification temperature tended to increase with the increase of Nb2O5 content. It is believed that although adding Nb to ZrO2 can increase electron capacity, the increased amount of Nb causes the zirconium dioxide particles to become monoclinic rather than tetragonal, thus becoming unstable and reducing the catalyst's purification performance. In contrast, although in the samples after durability test 2 of Examples 1-3 (Y2O3 content in the first powder: 1% by mass), it was confirmed that, similar to Comparative Examples 1-3, the NOx 50% purification temperature tended to increase with the increase of Nb2O5 content, the NOx 50% purification temperature was lower than that of the samples after durability test 2 of Comparative Examples 1-3. It is believed that by further adding Y to ZrO2, the crystal structure of zirconium dioxide particles becomes a tetragonal stable structure, thereby suppressing the decrease in catalyst purification performance caused by the increase in Nb addition. Furthermore, in the samples after durability tests 2 of Examples 4-6 (Y2O3 content in the first powder: 4% by mass) and the samples after durability tests 2 of Examples 7-9 (Y2O3 content in the first powder: 8% by mass), the NOx 50% purification temperature at an Nb2O5 content of 2% was lower than that of the samples after durability tests 2 of Comparative Examples 1-3, and higher than that of the samples after durability tests 2 of Examples 1-3. It is believed that the addition of Y achieves the effect of making the crystal structure of zirconium dioxide particles tetragonal stable, but the electron-increasing effect of Nb addition decreases when the amount of Y added is too high relative to the amount of Nb added. On the other hand, in this sample, the NOx 50% purification temperature at a Nb₂O₅ content of 8% by mass was significantly lower than that of the samples after durability test 2 of Comparative Examples 1-3 and Examples 1-3. It is believed that the increase in the amount of Y relative to the amount of Nb increases the effect of making the zirconium dioxide particles into a tetragonal stable crystal structure.

[0126] The embodiments of the catalyst for waste gas purification involved in the present invention have been described in detail above. However, the present invention is not limited to the embodiments described above, and various design changes can be made within the scope of the spirit of the present invention as described in the patent application.

[0127] Symbol Explanation

[0128] 1-Catalyst for waste gas purification, 2-Catalyst for waste gas purification, 10-Rhodium particles, 20-Zirconium dioxide particles, 30-Porous support particles, 40-Support particles.

Claims

1. A catalyst for purifying waste gas, characterized in that, have: Carrier particles; Zirconia particles, comprising zirconium dioxide, wherein a first cation with an oxidation number higher than zirconium and a second cation with an ionic radius greater than zirconium are added together to the zirconium dioxide; and Rhodium particles, which are loaded onto the zirconium dioxide particles. (i) The carrier particles are porous carrier particles, and the zirconium dioxide particles and the rhodium particles are loaded in the pores of the porous carrier particles, or (ii) The carrier particles are composed of multiple primary particles, and the zirconium dioxide particles and the rhodium particles are dispersed inside the carrier particles.

2. The catalyst for purifying waste gas according to claim 1, characterized in that, The zirconium dioxide particles contain 1.0% to 70.0% by mass relative to the total number of zirconium dioxide particles and carrier particles.

3. The catalyst for purifying waste gas according to claim 1 or 2, characterized in that, The first cation is niobium.

4. The catalyst for purifying waste gas according to claim 3, characterized in that, The content of the niobium oxide portion in the zirconium dioxide particles is 0.1% to 12.0% by mass.

5. The catalyst for purifying waste gas according to claim 1 or 2, characterized in that, The second cation is yttrium.

6. The catalyst for purifying waste gas according to claim 5, characterized in that, The content of the yttrium oxide portion in the zirconium dioxide particles is 0.1% to 12.0% by mass.

7. The catalyst for purifying waste gas according to claim 1 or 2, characterized in that, The carrier particles are aluminum oxide.

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