Exhaust gas purification catalyst

Abstract

Provided is an exhaust gas purification catalyst that exhibits a high Nox conversion rate and excellent reduction catalyst activity not only in a low temperature region of 400°C or lower but also in a high temperature region of higher than 400°C. An exhaust gas purification catalyst (20) is used as a catalyst for a purification reaction for purifying an exhaust gas, and comprises: a noble metal (231); an oxygen absorption / release material (213) that has an oxygen absorption ability and an oxygen releasing ability; and an iron-containing compound (211) that is supported on the oxygen absorption / release material (213). The oxygen absorption / release material (213) contains yttrium, cerium, zirconium, and iron. When cerium exists as cerium oxide (CeO2) and iron exists as iron oxide (Fe2O3) in the oxygen absorption / release material (213), the following expression is satisfied: [content (mass%) of iron oxide in oxygen absorption / release material] > 0.38-0.02 × [content (mass%) of cerium oxide in oxygen absorption / release material].

Description

Exhaust gas purification catalyst

[0001] This invention relates to an exhaust gas purification catalyst.

[0002] Automobiles are equipped with exhaust gas purification catalysts, which are catalysts for the purification reaction that cleans exhaust gases. These exhaust gas purification catalysts have catalytic activity (oxidation catalytic activity) for the oxidation reaction that oxidizes hydrocarbons (HC) and carbon monoxide (CO) contained in exhaust gases and converts them into water (H2O) and carbon dioxide (CO2), and nitrogen oxides (NO2). X It has catalytic activity (reducing catalytic activity) for a reduction reaction that reduces ) to nitrogen gas (N2). Patent Document 1 discloses an exhaust gas purification catalyst having the above two catalytic activities, comprising a ceria-zirconia composite oxide containing yttrium, a noble metal, and . The exhaust gas purification catalyst disclosed in Patent Document 1 has excellent reducing catalytic activity in the low temperature range of 400°C or below, and therefore the ratio of nitrogen oxides converted to nitrogen gas (hereinafter sometimes referred to as "NOx conversion rate") in the low temperature range of 400°C or below was high.

[0003] Japanese Patent Publication No. 2016-185531

[0004] In actual use of exhaust gas purification catalysts in automobiles, the temperature conditions can range from low temperatures below 400°C to high temperatures exceeding 400°C (e.g., 500°C). However, the exhaust gas purification catalyst disclosed in Patent Document 1 did not exhibit sufficient catalytic reduction activity at high temperatures around 500°C. As a result, the NOx conversion rate was insufficient at high temperatures around 500°C. The object of the present invention is to provide an exhaust gas purification catalyst that exhibits excellent catalytic reduction activity and a high NOx conversion rate not only in the low-temperature range below 400°C but also in the high-temperature range exceeding 400°C.

[0005] An exhaust gas purification catalyst according to one aspect of the present invention is an exhaust gas purification catalyst used as a catalyst for a purification reaction that purifies exhaust gas, wherein the exhaust gas purification catalyst contains a noble metal, an oxygen absorption and release material having the ability to absorb and release oxygen, and an iron-containing compound supported on the oxygen absorption and release material, wherein the oxygen absorption and release material contains yttrium, cerium, zirconium, and iron, and assuming that cerium exists as cerium oxide (CeO2) and iron exists as iron oxide (Fe2O3) in the oxygen absorption and release material, the following equation is satisfied: [Content of iron oxide in the oxygen absorption and release material (mass%)] > 0.38 - 0.02 × [Content of cerium oxide in the oxygen absorption and release material (mass%)].

[0006] According to the present invention, it is possible to provide an exhaust gas purification catalyst that exhibits excellent reduction catalyst activity and a high NOx conversion rate not only in the low temperature range of 400°C or below, but also in the high temperature range of 400°C or above.

[0007] This is a schematic conceptual diagram illustrating an example of the configuration of an exhaust gas purification catalyst according to one embodiment of the present invention. This graph shows the results of evaluating the reduction performance of the exhaust gas purification catalysts of Example 1 and Comparative Example 1 by the hydrogen-temperature reduction method. This graph shows the results of measuring the BET specific surface area of ​​the exhaust gas purification catalysts of Example 1 and Comparative Example 4. This graph shows the results of measuring the NOx conversion rate of the exhaust gas purification catalysts of Example 1 and Comparative Example 5. This figure shows the results of analyzing the iron-containing compound of the exhaust gas purification catalyst of Example 1 by electron diffraction.

[0008] One embodiment of the present invention is described below. This embodiment is merely an example of the present invention, and the present invention is not limited to this embodiment. Furthermore, various modifications or improvements can be made to this embodiment, and such modified or improved forms may also be included in the present invention.

[0009] The exhaust gas purification catalyst according to this embodiment is an exhaust gas purification catalyst used as a catalyst for a purification reaction that purifies exhaust gas, and contains a noble metal, an oxygen absorption and release material having the ability to absorb and release oxygen, and an iron-containing compound supported on the oxygen absorption and release material. The oxygen absorption and release material contains yttrium (Y), cerium (Ce), zirconium (Zr), and iron (Fe). Furthermore, if cerium exists as cerium oxide (CeO2) and iron exists as iron oxide (Fe2O3) in the oxygen absorption and release material, the following equation is satisfied: [Content of iron oxide in the oxygen absorption and release material (mass%)] > 0.38 - 0.02 × [Content of cerium oxide in the oxygen absorption and release material (mass%)].

[0010] As a result of having the above configuration, the exhaust gas purification catalyst according to this embodiment exhibits excellent catalytic reduction activity and a high NOx conversion rate not only in the low temperature range of 400°C or below, but also in the high temperature range of 400°C or above. More specifically, since the oxygen absorption and release material of the exhaust gas purification catalyst according to this embodiment contains iron, the interionic distance between cerium and zirconium in the oxygen absorption and release material is larger than in the case where iron is not present. As a result, the oxygen release capacity of the oxygen absorption and release material is increased. Furthermore, since iron has a high affinity for oxygen, when iron ions are present near cerium and zirconium ions, oxygen is more easily extracted from the lattice of cerium and zirconium ions. For these reasons, oxygen is more easily released from the oxygen absorption and release material, and therefore the exhaust gas purification catalyst according to this embodiment exhibits excellent catalytic reduction activity and a high NOx conversion rate not only in the low temperature range of 400°C or below, but also in the high temperature range of 400°C or above.

[0011] Here, the oxygen absorption and release material has the ability to absorb and release oxygen, and this ability supports the purification reaction of exhaust gases by precious metals. The oxygen absorption and release material may also function as a carrier or co-catalyst. Furthermore, when calculating the content in the above formula, it is assumed that cerium exists as cerium oxide and iron exists as iron oxide in the oxygen absorption and release material, and the respective content is calculated accordingly. That is, cerium and iron may exist as composite oxides or as oxides in the oxygen absorption and release material, but even if they exist as composite oxides, the content is calculated assuming they exist as oxides. For example, if there is 1 mole of cerium in the composite oxide in the oxygen absorption and release material, the content is calculated assuming that there is 1 mole of cerium oxide (CeO2).

[0012] In the exhaust gas purification catalyst according to this embodiment, if cerium exists as cerium oxide (CeO2) and iron exists as iron oxide (Fe2O3) in the oxygen absorption / release material, then the above-mentioned formula [Content of iron oxide in the oxygen absorption / release material (mass%)] > 0.38 - 0.02 × [Content of cerium oxide in the oxygen absorption / release material (mass%)] is satisfied, and it is even more preferable that the formula [Content of iron oxide in the oxygen absorption / release material (mass%)] < 1.0 - 0.02 × [Content of cerium oxide in the oxygen absorption / release material (mass%)] is satisfied, and it is even more preferable that the formula [Content of iron oxide in the oxygen absorption / release material (mass%)] < 0.8 - 0.02 × [Content of cerium oxide in the oxygen absorption / release material (mass%)] is satisfied. With this configuration, the balance between oxygen absorption and release from the oxygen absorption / release material is improved, resulting in superior catalytic reduction activity not only in the low-temperature range below 400°C but also in the high-temperature range above 400°C, leading to a higher NOx conversion rate.

[0013] Furthermore, in the exhaust gas purification catalyst according to this embodiment, the type of precious metal is not particularly limited. Examples of precious metals include gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os). Among these precious metals, rhodium is preferred, as it results in a higher NOx conversion rate.

[0014] In the exhaust gas purification catalyst according to this embodiment, the precious metal may or may not be supported on a carrier. When the precious metal is supported on a carrier, the carrier may be the same type of carrier as the oxygen absorption / release material that supports the iron-containing compound, or it may be a different type of carrier. The carrier on which the precious metal is supported is preferably one that contains zirconium and yttrium, but may not contain cerium and iron. The method for supporting the precious metal on the carrier is not particularly limited, but for example, the precious metal can be supported on the carrier by mixing the precious metal and the carrier and sintering them.

[0015] Furthermore, in the exhaust gas purification catalyst according to this embodiment, the content of yttrium oxide in the oxygen absorption / release material is not particularly limited, but if yttrium exists as yttrium oxide (Y2O3) in the oxygen absorption / release material, the content of yttrium oxide in the oxygen absorption / release material may be 5% by mass or more.

[0016] Yttrium has the effect of improving the catalytic reduction activity in the low-temperature range of 400°C or below. Therefore, with the above configuration, the catalytic reduction activity of the exhaust gas purification catalyst according to this embodiment will be superior in the low-temperature range of 400°C or below. When calculating the yttrium content in the oxygen absorption and release material, the content is calculated assuming that yttrium exists as yttrium oxide. The specific calculation method is the same as that for cerium and iron described above, so a detailed explanation is omitted.

[0017] Furthermore, in the exhaust gas purification catalyst according to this embodiment, the oxygen absorption and release material may comprise a composite oxide having yttrium, cerium, zirconium, and iron. That is, the oxygen absorption and release material comprises yttrium, cerium, zirconium, and iron, but may also comprise a composite oxide having yttrium, cerium, zirconium, and iron. The bonding of yttrium and zirconium increases the heat resistance of the oxygen absorption and release material, resulting in improved catalytic reduction activity and a higher NOx conversion rate not only in the low-temperature range of 400°C or below, but also in the high-temperature range of 400°C or above.

[0018] Examples of compounds that make up oxygen absorption and release materials include zirconium cerium yttrium iron oxide (ZrCeYFeO x ), zirconium cerium neodymium yttrium iron oxide (ZrCeNdYFeO x ), Zirconium cerium lanthanum yttrium iron oxide (ZrCeLaFeYO x ) are some examples.

[0019] Furthermore, the yttrium, cerium, zirconium, and iron contained in the oxygen absorption / release material do not necessarily have to be in the form of a complex oxide. For example, it may be a mixture of yttrium oxide, cerium oxide, zirconium oxide, and iron oxide. The valency of iron in the oxygen absorption / release material is not particularly limited, but whether yttrium, cerium, zirconium, and iron form a complex oxide or not, the valency of iron may be divalent or trivalent.

[0020] Furthermore, in the exhaust gas purification catalyst according to this embodiment, the type of iron-containing compound is not particularly limited, and examples include iron oxide (FeO, Fe2O3), barium iron oxide, calcium iron oxide, neodymium strontium iron oxide, and lanthanum strontium iron oxide. A specific example of lanthanum strontium iron oxide is La 0.8 Sr 0.2 FeO3 is one example.

[0021] When manufacturing the exhaust gas purification catalyst according to this embodiment, the method of supporting the iron-containing compound on the oxygen absorption and release material is not particularly limited, and a general supporting method can be used. For example, the iron-containing compound can be supported on the oxygen absorption and release material by mixing and sintering the iron-containing compound and the oxygen absorption and release material.

[0022] Furthermore, the iron composite oxide (the composite oxide having yttrium, cerium, zirconium, and iron) contained in the oxygen absorption and release material can also be used as the iron-containing compound. That is, particles of the composite oxide having yttrium, cerium, zirconium, and iron may be supported on a carrier made of the composite oxide having yttrium, cerium, zirconium, and iron.

[0023] Furthermore, in the exhaust gas purification catalyst according to this embodiment, at least a part of the iron-containing compound may have a perovskite structure. The crystal structure of the iron-containing compound is not particularly limited, but the perovskite structure represented by the general formula ABO3 can be an inlet for oxygen absorption and also has an effect of improving the durability of the exhaust gas purification catalyst. Therefore, it is preferable that at least a part of the iron-containing compound has a perovskite structure.

[0024] Examples of the iron-containing compound having a perovskite structure represented by the general formula ABO3 include, for example, the chemical formula La p M 1-p FeO 3-q The compounds represented by are mentioned. In the chemical formula, La is lanthanum, M is at least one selected from barium (Ba), strontium (Sr), and calcium (Ca), Fe is iron, O is oxygen, p is a number satisfying 0 < p ≤ 1, and q is a number satisfying 0 ≤ q ≤ 1. Among the iron-containing compounds represented by the above chemical formula, compounds in which M is strontium are preferable.

[0025] Furthermore, the exhaust gas purification catalyst according to this embodiment may further include a partition material that separates the precious metal from the oxygen absorption / release material. The heat resistance of the exhaust gas purification catalyst is improved by the partition material, so the NOx conversion rate is higher. The partition material is interposed between the precious metal and the oxygen absorption / release material to suppress contact between the precious metal and the oxygen absorption / release material. By using a partition material, the decrease in catalytic activity due to contact between the precious metal and the iron-containing compound can be suppressed. The material of the partition material is not particularly limited, but examples include boehmite, alumina (Al2O3), zirconia oxide, and ceria-zirconia composite oxide.

[0026] The applications of the exhaust gas purification catalyst according to this embodiment are not particularly limited, but for example, it can be used to purify exhaust gases emitted from automobiles, factories, etc. Automobiles are equipped with exhaust gas purification devices, and the exhaust gas purification catalyst according to this embodiment can be used in these purification devices. Furthermore, the exhaust gas purification catalyst according to this embodiment can be used not only for automobile exhaust gases but also as a catalyst for purifying exhaust gases from internal combustion engines.

[0027] Next, an example of an exhaust gas purification catalyst according to this embodiment will be described with reference to the schematic conceptual diagram in Figure 1. The exhaust gas purification catalyst 20 comprises a first catalyst 21, a second catalyst 23, and a partition material 25, with the first catalyst 21 and the second catalyst 23 separated by the partition material 25. The first catalyst 21 contains an iron-containing compound 211 and an oxygen absorption / release material 213, with the oxygen absorption / release material 213 supporting the iron-containing compound 211. The second catalyst 23 contains a noble metal 231 and a carrier 233, with the carrier 233 supporting the noble metal 231.

[0028] The present invention will be described in more detail below with reference to examples and comparative examples. [Example 1] <Preparation of the first catalyst support> A hydroxide gel, which is a precursor of zirconium cerium neodymium yttrium iron oxide (52% by mass zirconium oxide (ZrO2) - 19% by mass cerium oxide (CeO2) - 9.9% by mass neodymium oxide (Nd2O3) - 19% by mass yttrium oxide (Y2O3) - 0.1% by mass iron oxide (Fe2O3)), was prepared.

[0029] Specifically, zirconium compounds (zirconium oxynitrate), cerium compounds (cerium nitrate), neodymium compounds (neodymium nitrate), yttrium compounds (yttrium nitrate), and iron compounds (iron nitrate) were weighed, and respective aqueous solutions were prepared. Next, the obtained zirconium compound aqueous solution, cerium compound aqueous solution, neodymium compound aqueous solution, yttrium compound aqueous solution, and iron compound aqueous solution were mixed so that they would be in a predetermined ratio in terms of oxide conversion to obtain a mixed aqueous solution.

[0030] Thereafter, while sufficiently stirring the obtained mixed aqueous solution, an aqueous sodium hydroxide solution as an alkaline aqueous solution was added to obtain a dispersion of a mixed hydroxide gel composed of zirconium hydroxide gel, cerium hydroxide gel, neodymium hydroxide gel, yttrium hydroxide gel, and iron hydroxide gel.

[0031] Next, the dispersion of the obtained mixed hydroxide gel was filtered, and the collected mixed hydroxide gel was washed with ion-exchanged water. Further, the mixed hydroxide gel was decanted until sodium was no longer detected in the supernatant. Thereafter, the mixed hydroxide gel was dried at 100 °C for 16 hours and further heat-treated at 600 °C for 5 hours to obtain a powder of the carrier for the first catalyst used in this example. This is designated as powder 1-1.

[0032] <Preparation of the First Catalyst> La 0.8 Sr 0.2 Lanthanum carbonate, strontium carbonate, and iron carbonate were put into a mixed aqueous solution of citric acid and malic acid (concentration of citric acid: 25% by mass, concentration of malic acid: 25% by mass) so as to form a 10% by mass solution of La 0.8 Sr 0.2 FeO3, and they were stirred and dissolved to obtain a 10% by mass La

[0033] Next, the obtained 10% by mass La 0.8 Sr 0.2 FeO3 solution was added to powder 1-1 at 6% by mass La 0.8 Sr 0.2Impregnate to obtain FeO3, and calcine this in order at 200 °C for 1 hour, 400 °C for 1 hour, 580 °C for 1 hour, and 700 °C for 1 hour to obtain the powder of the first catalyst used in this example. Designate this as powder 1-2.

[0034] <Preparation of the support for the second catalyst> Prepare a hydroxide gel which is a precursor of zirconium lanthanum neodymium yttrium oxide (80 mass% zirconium oxide (ZrO2) - 3 mass% lanthanum oxide (La2O2) - 5 mass% neodymium oxide (Nd2O3) - 12 mass% yttrium oxide (Y2O3)). Specifically, weigh zirconium compounds (zirconium oxynitrate), lanthanum compounds (lanthanum nitrate), neodymium compounds (neodymium nitrate), and yttrium compounds (yttrium nitrate), and prepare each aqueous solution.

[0035] Next, mix the obtained zirconium compound aqueous solution, lanthanum compound aqueous solution, neodymium compound aqueous solution, and yttrium compound aqueous solution so that they are in a predetermined ratio in terms of oxide conversion to obtain a mixed aqueous solution. Thereafter, while sufficiently stirring the obtained mixed aqueous solution, add an aqueous sodium hydroxide solution as an alkaline aqueous solution to obtain a dispersion of a mixed hydroxide gel composed of zirconium hydroxide gel, lanthanum hydroxide gel, neodymium hydroxide gel, and yttrium hydroxide gel.

[0036] Next, filter the dispersion of the obtained mixed hydroxide gel, and wash the collected mixed hydroxide gel with ion-exchanged water. Further, decant the mixed hydroxide gel until no sodium is detected from the supernatant. Thereafter, dry the mixed hydroxide gel at 100 °C for 16 hours, and further heat-treat it at 600 °C for 5 hours to obtain the powder of the support for the second catalyst used in this example. Designate this as powder 1-3.

[0037] <Preparation of the second catalyst> Support rhodium on powder 1-3 by the incipient wetness method using an aqueous rhodium nitrate solution (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) to obtain the powder of the second catalyst used in this example. Designate this as powder 1-4. Drying is performed at 150 °C overnight, and calcination is performed at 400 °C for 1 hour in a muffler furnace.

[0038] <Preparation of Powder A> The powder of the first catalyst (powder 1-2) was pulverized in a bead mill until the average particle size (D50) was approximately 150 nm. The powder of the second catalyst (powder 1-4) was also pulverized in a bead mill until the average particle size (D50) was approximately 150 nm. A laser diffraction / scattering particle size distribution analyzer (LA-920, manufactured by Horiba, Ltd.) was used to measure the average particle size (D50).

[0039] Next, boehmite alumina and water were mixed, and the pulverized liquids of the first catalyst powder (powder 1-2) and the pulverized liquids of the second catalyst powder (powder 1-4) were added to this mixture and mixed to obtain a mixed solution. The mass ratio of the first catalyst powder (powder 1-2), the second catalyst powder (powder 1-4), and boehmite was 50:30:20. Next, the obtained mixed solution was filled into a spray-drying apparatus and dried. Then, this dried material was calcined in a muffle furnace at 550°C for 3 hours to obtain powder A used in this example.

[0040] <Preparation of Powder B> A hydroxide gel, which is a precursor of zirconium cerium neodymium oxide (70% by mass zirconium oxide (ZrO2) - 20% by mass cerium oxide (CeO2) - 10% by mass neodymium oxide (Nd2O3)), was prepared. Specifically, zirconium compounds (zirconium oxynitrate), cerium compounds (cerium nitrate), and neodymium compounds (neodymium nitrate) were weighed to obtain aqueous solutions of each. Next, the obtained aqueous solutions of the zirconium compound, cerium compound, and neodymium compound were mixed in a predetermined ratio in terms of oxides to obtain a mixed aqueous solution.

[0041] Subsequently, while thoroughly stirring the resulting mixed aqueous solution, an aqueous sodium hydroxide solution was added as an alkaline aqueous solution to obtain a dispersion of mixed hydroxide gels consisting of zirconium hydroxide gel, cerium hydroxide gel, and neodymium hydroxide gel. Next, the dispersion of mixed hydroxide gels obtained was filtered, and the filtered mixed hydroxide gels were washed with deionized water. Furthermore, the mixed hydroxide gels were decanted until sodium could no longer be detected in the supernatant. After that, the mixed hydroxide gels were dried at 100°C for 16 hours and then heat-treated at 600°C for 5 hours to obtain powder B used in this example. This powder B acts as a co-catalyst in the exhaust gas purification reaction.

[0042] <Coating onto the carrier> The obtained powders A and B, γ-alumina, and boehmite alumina as a binder were placed in a magnetic pot in a mass ratio of 53:17:15:15. Nitric acid, pure water, and alumina balls were further added to the magnetic pot and shaken to grind the mixture and obtain a slurry.

[0043] Furthermore, the obtained slurry was transferred to another container, and this slurry was suction-coated onto a ceramic honeycomb support (capacity: 0.119 L), and excess slurry was removed by airflow. Thereafter, the slurry-coated honeycomb support was dried at 120°C and fired at 400°C for 30 minutes under airflow to obtain the exhaust gas purification catalyst of this example.

[0044]

[0045] [Example 2] <Preparation of the first catalyst support> The powder of the first catalyst support used in this example was obtained in the same manner as in Example 1, except that the composition ratio of the first catalyst support (see Table 1) was different. This will be designated as powder 2-1. <Preparation of the first catalyst> The powder of the first catalyst used in this example was obtained in the same manner as in Example 1, except that powder 2-1 was used instead of powder 1-1. This will be designated as powder 2-2.

[0046] <Preparation of the second catalyst support> A hydroxide gel, which is a precursor of zirconium cerium neodymium yttrium oxide (80% by mass zirconium oxide (ZrO2) - 3% by mass cerium oxide (CeO2) - 5% by mass neodymium oxide (Nd2O3) - 12% by mass yttrium oxide (Y2O3)), was prepared. Specifically, zirconium compounds (zirconium oxynitrate), cerium compounds (cerium nitrate), neodymium compounds (neodymium nitrate), and yttrium compounds (yttrium nitrate) were weighed, and aqueous solutions of each were prepared.

[0047] Next, the obtained zirconium compound aqueous solution, cerium compound aqueous solution, neodymium compound aqueous solution, and yttrium compound aqueous solution were mixed in a predetermined ratio in terms of oxides to obtain a mixed aqueous solution. From this point onward, the same procedure as in Example 1 was performed to obtain the powder of the second catalyst support used in this example. This is referred to as powder 2-3.

[0048] <Preparation of the second catalyst> In the same manner as in Example 1, rhodium was supported on powder 2-3 to obtain the powder of the second catalyst used in this example. This will be designated as powder 2-4. <Preparation of powder A> Powder A used in this example was obtained in the same manner as in Example 1, except that powder 2-2 was used instead of powder 1-2, and powder 2-4 was used instead of powder 1-4.

[0049] <Preparation of Powder B> Powder B obtained in Example 1 was used as Powder B in this example. <Coating to the Carrier> The exhaust gas purification catalyst of this example was obtained in the same manner as in Example 1, except that Powder A and Powder B obtained in Example 2 were used.

[0050] [Examples 3-7] Powders of the first catalyst support used in Examples 3-7 were obtained in the same manner as in Example 1, except that the composition ratio of the first catalyst support (see Table 1) was different. Then, the exhaust gas purification catalysts of Examples 3-7 were obtained in the same manner as in Example 2, except that the obtained powders of the first catalyst support were used.

[0051] [Comparative Examples 1-4] Powders of the first catalyst support used in Comparative Examples 1-4 were obtained in the same manner as in Example 1, except that the composition ratio of the first catalyst support (see Table 1) was different. In other words, Comparative Examples 1-4 are examples in which the first catalyst support (oxygen absorption / release material) does not contain iron. Then, exhaust gas purification catalysts for Comparative Examples 1-4 were obtained in the same manner as in Example 2, except that the obtained powder of the first catalyst support was used.

[0052] [Comparative Example 5] La to the first catalyst support 0.8 Sr 0.2 A comparative example of the exhaust gas purification catalyst for Comparative Example 5 was obtained in the same manner as in Example 2, except that the FeO3 solution was not impregnated into the catalyst. In other words, Comparative Example 5 is an example in which the oxygen absorption / release material does not support an iron-containing compound.

[0053] <Evaluation of Exhaust Gas Purification Catalysts> The catalytic reduction activity of the exhaust gas purification catalysts in Examples 1-7 and Comparative Examples 1-5 was evaluated. First, the exhaust gas purification catalysts were subjected to the following durability conditions. Then, exhaust gas purification was performed using the durable exhaust gas purification catalysts under the following evaluation conditions. Then, the exhaust gas before and after purification was analyzed using an exhaust gas analyzer (MEXA-7500D manufactured by Horiba, Ltd.) to determine the nitrogen oxides (NOx) in the exhaust gas before and after purification. X The concentration of NOx was measured, and the NOx conversion rate at 500°C was calculated. The results are shown in Table 1. The NOx conversion rate was calculated using the following formula: NOx conversion rate (%) = (NOxin - NOxout) / NOxin × 100 NOxin is the concentration of nitrogen oxides in the exhaust gas before purification, and NOxout is the concentration of nitrogen oxides in the exhaust gas after purification.

[0054] (Durability Conditions) The exhaust gas purification catalysts of Examples 1 to 7 and Comparative Examples 1 to 5 were installed in the exhaust passage of a 3.5L V6 engine manufactured by Nissan Motor Co., Ltd. The engine was operated, and the exhaust gas purification catalysts were exposed to exhaust gas for 300 hours at an inlet temperature of 920°C.

[0055] (Evaluation conditions) In the exhaust passage of a Nissan Motor Co., Ltd. V6 3.5L engine, 119 cm³ 3An exhaust gas purification catalyst was installed, and the engine was operated. Exhaust gas was flowed through the exhaust passage at a catalyst inlet temperature of 500°C. The exhaust gas flow rate was 60 m³. 3 The value is / h. In addition, the air-fuel ratio of the gas supplied to the engine was adjusted to one of the values ​​between 13.4 and 15.4, and evaluations were performed at multiple air-fuel ratios (i.e., A / F = 14.4 at an amplitude of 1.0). The NOx conversion rate for each air-fuel ratio was then calculated, and the average value was calculated.

[0056] The exhaust gas composition (stoichiometric air-fuel ratio) is approximately 2000 ppm C1 of hydrocarbons (HC), 0.54% by volume of carbon monoxide, 1500 ppm of nitric oxide (NO), 0.56% by volume of oxygen (O2), and 14.6% by volume of carbon dioxide. Note that "ppm C1" above refers to the carbon equivalent; for example, 1 ppm of benzene is equivalent to 6 ppm C1.

[0057] The exhaust gas purification catalysts of Examples 1 to 7, because their first catalyst support (oxygen absorption / release material) contains iron, exhibited a higher NOx conversion rate at 500°C even after durability testing, and superior reduction catalyst activity even in high-temperature ranges exceeding 400°C, compared to the exhaust gas purification catalysts of Comparative Examples 1 to 4. The exhaust gas purification catalyst of Comparative Example 5, because its oxygen absorption / release material did not support an iron-containing compound, had a low NOx conversion rate (see Table 1 and the graph in Figure 4).

[0058] Figure 2 is a graph showing the results of evaluating the reduction performance of the exhaust gas purification catalysts of Example 1 and Comparative Example 1 using the hydrogen-temperature reduction method. From the graph in Figure 2, it can be seen that the exhaust gas purification catalyst of Example 1 consumes more hydrogen in the high-temperature range around 500°C compared to the exhaust gas purification catalyst of Comparative Example 1. In other words, if the oxygen absorption / release material contains iron, it becomes an exhaust gas purification catalyst that has the characteristic of easily releasing oxygen in the high-temperature range.

[0059] When an oxygen absorber contains yttrium, there is a trade-off relationship where the amount of oxygen released increases in the low-temperature range below 400°C, but decreases in the high-temperature range above 400°C. However, when an oxygen absorber contains iron, the amount of oxygen released increases in the high-temperature range above 400°C, so even if the oxygen absorber contains yttrium, the catalytic reduction activity in the high-temperature range above 400°C is superior.

[0060] Figure 3 is a graph showing the results of measuring the BET specific surface area of ​​the exhaust gas purification catalysts of Example 1 and Comparative Example 4. From the graph in Figure 3, the exhaust gas purification catalyst of Example 1, in which the first catalyst support (oxygen absorption / release material) contains iron, had a larger BET specific surface area than the exhaust gas purification catalyst of Comparative Example 4, in which the first catalyst support (oxygen absorption / release material) does not contain iron.

[0061] Figure 5 shows the results of electron diffraction analysis of the iron-containing compound of the exhaust gas purification catalyst in Example 1. From the figure shown in Figure 5, the iron-containing compound La used in Example 1 0.8 Sr 0.2 It was found that the crystal structure of FeO3 is a perovskite structure represented by the general formula ABO3.

[0062] 20... Exhaust gas purification catalyst 21... First catalyst 23... Second catalyst 25... Partition material 211... Iron-containing compound 213... Oxygen absorption / release material 231... Precious metal 233... Carrier

Claims

1. An exhaust gas purification catalyst used as a catalyst for a purification reaction to purify exhaust gas, wherein the exhaust gas purification catalyst contains a noble metal, an oxygen absorption and release material having the ability to absorb and release oxygen, and an iron-containing compound supported on the oxygen absorption and release material, wherein the oxygen absorption and release material contains yttrium, cerium, zirconium, and iron, and assuming that the cerium exists as cerium oxide (CeO2) and the iron exists as iron oxide (Fe2O3) in the oxygen absorption and release material, the exhaust gas purification catalyst satisfies the following equation: [Content of iron oxide in the oxygen absorption and release material (mass%)] > 0.38 - 0.02 × [Content of cerium oxide in the oxygen absorption and release material (mass%)].

2. The exhaust gas purification catalyst according to claim 1, wherein, assuming that the cerium exists as cerium oxide (CeO2) and the iron exists as iron oxide (Fe2O3) in the oxygen absorption / release material, the following equation is further satisfied: [Content of iron oxide in the oxygen absorption / release material (mass%)] < 1.0 - 0.02 × [Content of cerium oxide in the oxygen absorption / release material (mass%)].

3. The exhaust gas purification catalyst according to claim 1, wherein, assuming that the cerium exists as cerium oxide (CeO2) and the iron exists as iron oxide (Fe2O3) in the oxygen absorption / release material, the following equation is further satisfied: [Content of iron oxide in the oxygen absorption / release material (mass%)] < 0.8 - 0.02 × [Content of cerium oxide in the oxygen absorption / release material (mass%)].

4. The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein the precious metal is rhodium.

5. The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein, assuming that the yttrium exists as yttrium oxide (Y2O3) in the oxygen absorption / release material, the content of yttrium oxide in the oxygen absorption / release material is 5% by mass or more.

6. The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein the oxygen absorption and release material comprises a composite oxide having yttrium, cerium, zirconium, and iron.

7. The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein at least a portion of the iron-containing compound has a perovskite structure.

8. The exhaust gas purification catalyst according to any one of claims 1 to 3, further comprising a partition material separating the noble metal and the oxygen absorption / release material.

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