INCORPORATION BY REFERENCE

[0001] The disclosure of Japanese Patent Application No. 2015-027441 filed on

February 16, 2015 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0002] The present invention relates to an exhaust gas purifying catalyst, a method of manufacturing the same, and a method of purifying exhaust gas using the same. 2. Description of Related Art

[0003] In order to remove harmful components (for example, carbon monoxide (CO), hydrocarbons (HC), and the like) contained in gas discharged from an internal combustion engine such as a diesel engine or a lean burn engine having a low fuel consumption ratio, various types of exhaust gas purifying catalyst have been studied. In addition, as such an exhaust gas purifying catalyst, exhaust gas purifying catalysts which use various types of metal oxide as a carrier have been suggested.

[0004] As an exhaust gas purifying catalyst, for example, Japanese Patent Application Publication No. 2009-285623 (JP 2009-285623 A) discloses an exhaust gas purifying catalyst which is an exhaust gas purifying catalyst for removing at least HC and CO in exhaust gas discharged from a lean burn engine and includes, on a carrier, a catalyst layer having alumina particles that support Pt, Ce-containing oxide particles having an oxygen storage and release ability, and zeolite particles, in which a large amount of iron oxide particles are dispersed and contained in the catalyst layer, at least a portion of the iron oxide particles are fine iron oxide particles having a particle diameter of 300 nm or smaller, the fine iron oxide particles come into contact with the alumina particles, the Ce-containing oxide particles, and the zeolite particles, and a ratio of an area of the fine iron oxide particles to the total area of the iron oxide particles, which is observed by an electron microscope is 30% or higher. However, the exhaust gas purifying catalyst in the related art as described in JP 2009-285623 A has oxidative activity that is not necessarily sufficient for CO or HC at a low temperature.

[0005] Japanese Patent Application Publication No. 9-308829 (JP 9-308829 A) discloses a diesel engine exhaust gas purifying catalyst which includes catalyst components containing at least one type of element selected from the group consisting of platinum, ruthenium, rhodium, and palladium, in addition to praseodymium and yttrium, in which the catalyst components are supported on a fire-resistant carrier such as zirconia or alumina. However, the diesel engine exhaust gas purifying catalyst in the related art as described in JP 9-308829 A also has oxidative activity that is not necessarily sufficient for CO or HC at a low temperature.

[0006] Furthermore, Japanese Patent Application Publication No. 9-155193 (JP

9-155193 A) discloses an exhaust gas purifying catalyst which includes catalyst components containing copper, praseodymium, and yttrium, in which at least one type of element selected from the group consisting of cobalt, iron, nickel, lanthanum, cerium, and neodymium is added to the catalyst components as necessary. However, the exhaust gas purifying catalyst in the related art as described in JP 9-155193 A also has oxidative activity that is not necessarily sufficient for CO or HC at a low temperature.

[0007] Japanese Patent Application Publication No. 8-266865 (JP.8-266865 A) discloses a diesel engine exhaust gas purifying catalyst which includes a catalyst supporting layer that is formed of a fire-resistant inorganic oxide such as alumina, silica, titania, zeolite, silica-alumina, or titania-alumina, and a platinum-group element that is supported on the catalyst supporting layer, in which the catalyst supporting layer further includes a composite oxide of at least one of vanadium, lanthanum, cerium, yttrium, and tungsten supported thereon. In the description of JP 8-266865 A, it is possible to provide a diesel engine exhaust gas purifying catalyst which simultaneously realizes the maintenance and improvement of the ability to oxidize and decompose CO and the like and more adequate reduction in the generation of SO3.

[0008] However, in recent years, better characteristics have been required for the exhaust gas purifying catalyst, there is a demand for an exhaust gas purifying catalyst having sufficient oxidative activity toward CO or HC at a low temperature, and furthermore, there is a demand for an exhaust gas purifying catalyst which maintains sufficiently high oxidative activity toward CO and HC even when being exposed to a high temperature. SUMMARY OF THE INVENTION

[0009] The invention provides an exhaust gas purifying catalyst which has sufficiently high oxidative activity toward CO and HC at a low temperature and maintains sufficiently high oxidative activity toward CO and HC even when being exposed to a high temperature, a method of manufacturing the same, and a method of purifying exhaust gas using the same.

[0010] The inventors performed repeated and extensive research, and as a result, found that by using a carrier made of alumina, a specific amount of yttria, and a specific amount of iron oxide and obtaining an exhaust gas purifying catalyst which includes a noble metal supported on the carrier, oxidative activity toward CO and HC at a low temperature is enhanced, and high oxidative activity toward CO and HC even during exposure to a high temperature is maintained.

[0011] A first aspect of the invention relates to an exhaust gas purifying catalyst. The exhaust gas purifying catalyst includes: a carrier made of alumina, yttria, and iron oxide; and a noble metal supported on the carrier, wherein an amount of the yttria contained in the carrier is 0.3 mass% to 31 mass%, an amount of the iron oxide contained in the carrier is 0.5 mass% to 27 mass%, and an atomic ratio of yttrium in the yttria to iron in the iron oxide in the carrier is in a range of 80:20 to 10:90.

[0012] In the aspect, the noble metal may be at least one type selected from the group consisting of platinum and palladium. [0013] The noble metal in the aspect may include platinum and palladium, and at least a portion of the platinum and palladium may be in a solid solution state.

[0014] A second aspect of the invention relates to a method of manufacturing an exhaust gas purifying catalyst. The method of manufacturing an exhaust gas purifying catalyst includes: obtaining a carrier made of alumina, yttria, and iron oxide by bringing alumina particles into contact with a solution including a first compound containing yttrium and a second compound containing iron; allowing a noble metal to be supported on the carrier by using a solution of a noble metal salt; and obtaining the exhaust gas purifying catalyst by baking the carrier having the noble metal supported thereon.

[0015] A third aspect of the invention relates to a method of purifying exhaust gas.

The method of purifying exhaust gas includes purifying exhaust gas by bringing the exhaust gas from an internal combustion engine into contact with the exhaust gas purifying catalyst.

[0016] In addition, the reason that it is possible for the exhaust gas purifying catalyst of the invention to exhibit sufficiently high oxidative activity toward CO and HC at a low temperature and maintain sufficiently high oxidative activity toward CO and HC even during exposure to a high temperature is assumed to be as follows. That is, in an exhaust gas purifying catalyst in the related art, CO or HC is strongly adsorbed onto an active site of noble metals, the adsorption of oxygen from a gas state is impeded, and thus an oxidation reaction of CO or HC is impeded due to this (self-poisoning phenomenon). In the invention, since the carrier is made of alumina, yttria, and iron oxide and the carrier in which the amount of iron oxide contained in the carrier is in a specific range is used, the iron oxide in the carrier is present with high dispersibility and forms a large interface with the noble metals and oxygen is delivered to the noble metals as a portion of the iron oxide is reduced. Therefore, it is thought that the self-poisoning phenomenon described above is alleviated, and an oxidation reaction of CO and HC even at a low temperature sufficiently proceeds. In addition, iron oxide has an excellent oxygen storage and release ability and the presence of the iron oxide may contribute to the enhancement of the oxidative activity of the exhaust gas purifying catalyst toward CO and HC. Therefore, it is assumed that it is possible to exhibit sufficiently high oxidative activity toward CO and HC even at a low temperature.

[0017] In addition, in the exhaust gas purifying catalyst which uses an alumina carrier containing iron oxide in the related art, it is thought that the iron oxide easily undergoes a solid-phase reaction with alumina during exposure to a high temperature, the solid-phase reaction reduces the specific surface area of the carrier, and grain growth of noble metals is further accelerated. In the invention, since the carrier is made of alumina, yttria, and iron oxide and the carrier in which the amount of yttria contained in the carrier is in a specific range is used, the yttria in the carrier is present with high dispersibility and increases the basicity of the carrier. Therefore, the interaction between the noble metal and the carrier is strengthened, grain growth of the noble metal is suppressed even during exposure to a high temperature, and oxidative activity toward CO and HC is maintained. It is assumed that since the yttria in the carrier reacts with alumina and suppresses the solid-phase reaction between iron oxide and alumina, oxidative activity toward CO and HC is maintained, it is possible to exhibit sufficiently high oxidative activity toward CO and HC even at a low temperature due to the presence of the yttria, and it is possible to maintain sufficiently high oxidative activity toward CO and HC even during exposure to a high temperature.

[0018] Furthermore, in the invention, as the carrier, the carrier in which the ratio between the yttria and the iron oxide contained in the carrier is in a specific range in terms of the atomic ratio of metal elements therein is used. It is assumed by the inventors that since this carrier is used, the alleviation of the self-poisoning phenomenon due to the iron oxide, suppression of the grain growth of the noble metal particles due to an increase in the basicity of the yttria, and suppression of the solid-phase reaction between the iron oxide and alumina due to the yttria can be compatible with each other, and due to the synergy effect thereof, the object is accomplished.

[0019] According to the aspects, it is possible to provide an exhaust gas purifying catalyst which has sufficiently high oxidative activity toward CO and HC at a low temperature and maintains sufficiently high oxidative activity toward CO and HC even when being exposed to a high temperature, a method of manufacturing the same, and a method of purifying exhaust gas using the same.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a graph representing the 50% CO oxidation temperatures of exhaust gas purifying catalysts obtained in Examples 1 to 3 and Comparative Examples 1 to 4;

FIG. 2 is a graph representing the 50% HC oxidation temperatures of the exhaust gas purifying catalysts obtained in Examples 1 to 3 and Comparative Examples 1 to 4;

FIG. 3 is a graph representing the relationship between the atomic ratio of Fe of the catalysts obtained in Examples 1 to 3 and Comparative Examples 3 and 4 and the 50% CO oxidation temperature of the catalysts after a durability test;

FIG. 4 is a graph representing the relationship between the atomic ratio of Fe of the catalysts obtained in Examples 1 to 3 and Comparative Examples 3 and 4 and the 50% HC oxidation temperature of the catalysts after the durability test;

FIG. 5 is a graph representing the particle diameters of noble metals of the catalysts obtained in Example 1 and Comparative Examples 1 to 4; and

FIG. 6 is a graph representing the oxygen storage and release amounts of the catalysts after the durability test, which are obtained in Examples 1 to 3 and Comparative Examples l to 4.

DETAILED DESCRIPTION OF EMBODIMENTS

[0021] Hereinafter, the invention will be described according to exemplary embodiments thereof.

<Exhaust Gas Purifying Catalyst>

[0022] An exhaust gas purifying catalyst of the invention includes: a carrier made of alumina, yttria, and iron oxide; and a noble metal supported on the carrier, in which the amount of the yttria contained in the carrier is 0.3 mass% to 31 mass%, the amount of the iron oxide contained in the carrier is 0.5 mass% to 27 mass%, and the ratio between the yttria and the iron oxide contained in the carrier is in a range of 80:20 to 10:90 in terms of the atomic ratio of metal elements therein ([yttrium]: [iron]).

[0023] (Carrier)

The carrier in the exhaust gas purifying catalyst of the invention needs to be made of alumina (AI2O3), yttria (Y20 ₃ ), and ircn oxide (FeO _x ), allow the amount of the yttria contained in the carrier to be 0.3 mass%. to 31 mass%, allow the amount of the iron oxide contained in the carrier to be 0.5 mass% to 27 mass%, and allow the ratio between the yttria and the iron oxide contained in the carrier to be in a range of 80:20 to 10:90 in terms of the atomic ratio of metal elements therein ([yttrium] : [iron]).

[0024] The amount of the yttria (Y ₂ 0 ₃ ) contained in the carrier needs to be 0.3 mass% to 31 mass% with respect to 100 mass% of the total mass of the carrier. When the amount of the yttria is lower than the lower limit, a solid-state reaction between the iron oxide and the alumina cannot be sufficiently suppressed, oxidative activity of the exhaust gas purifying catalyst toward CO and HC cannot be sufficiently obtained, grain growth of the noble metal cannot be sufficiently suppressed when the exhaust gas purifying catalyst is exposed to a high temperature, and thus the noble metal such as platinum, palladium, and/or alloy particles thereof cannot be in a high dispersed state and the oxidative activity of the exhaust gas purifying catalyst toward the CO and HC cannot be sufficiently obtained. On the other hand, when the amount thereof is higher than the upper limit, it becomes difficult to allow the noble metal to be in a metal state, the specific surface area of the entirety thereof is reduced, and thus it becomes difficult to support the noble metal with high dispersibility. From the viewpoint of the compatibility between high dispersibility and metalation of the noble metal and compatibility between initial activity and durable performance, the amount of the yttria is more preferably 1 mass% to 20 mass%, and particularly preferably 5 mass% to 10 mass%.

[0025] In addition, the amount of the iron oxide (FeO _x ) in the carrier needs to be 0.5 mass% to 27 mass% with respect to 100 mass% of the total mass of the carrier. When the amount of the iron oxide is lower than the lower limit, an oxygen storage and release ability of the exhaust gas purifying catalyst cannot be sufficiently obtained, and sufficiently high oxidative activity toward CO and HC at a low temperature cannot be obtained. On the other hand, when the amount thereof is higher than the upper limit, a solid-state reaction with alumina in a case of being exposed to a high temperature and grain growth of the noble metal resulting therefrom are incurred. From the viewpoint of the compatibility between sufficient suppression of the grain growth of the noble metal particles, an exhibition of a high oxygen storage and release ability, and the alleviation of a self-poisoning phenomenon, the amount of the iron oxide is more preferably 1 mass% to 20 mass%, and particularly preferably 3 mass% to 9 mass%.

[0026] Furthermore, the ratio between the yttria and the iron oxide contained in the carrier needs to be in a range of 80:20 to 10:90 in terms of the atomic ratio of metal elements therein ([yttrium] .-[iron]). W en the ratio of the yttria is lower than the lower limit (that is, when the amount of the iron oxide is higher than the upper limit), an effect of suppressing the grain growth of the noble metal in a case of being exposed to a high temperature cannot be sufficiently obtained. On the other hand, when the ratio of the yttria is higher than the upper limit (that is, when the amount of the iron oxide is lower than the lower limit), it becomes difficult to obtain oxidative activity to CO and HC at a low temperature due to the alleviation of self-poisoning of CO or HC. From the viewpoint of the compatibility between high dispersibility of the noble metal and the alleviation of self-poisoning, the ratio of the yttria and the iron oxide contained in the carrier is more preferably 70:30 to 20:80, and particularly preferably 60:40 to 40:60 in terms of the atomic ratio of metal elements therein ([yttrium]: [iron]).

[0027] Here, "being made of alumina, yttria, and iron oxide" means that the carrier contains only "alumina, yttria, and iron oxide", or the carrier is primarily made of "alumina, yttria, and iron oxide" and contains other components in a range without damage to the effects of the invention. As other components, other metal oxides, additives, and the like used as carriers in this type of application may be used. In the latter case, the amount of "alumina, yttria, and iron oxide" contained in the carrier is preferably 60 mass% or more, and more preferably 80 mass% or higher with respect to 100 mass% of the total mass of the carrier. When the amount of "alumina, yttria, and iron oxide" contained in the carrier is lower than the lower limit, there is a tendency toward the insufficient achievement of the effects of the invention.

[0028] As the alumina (A1 ₂ 0 ₃ ) in the carrier, alumina of at least one type selected from the group consisting of boehmite type, pseudo-boehmite type, χ type, κ type, p type, η type, γ type, pseudo-γ type, δ type, Θ type, and a type may be employed. From the viewpoint of heat resistance, a-alumina, γ-alumina, and θ-alumina are preferably used, and γ-alumina or θ-alumina having high activity are particularly preferably used.

[0029] The yttria (Y ₂ 0 ₃ ) in the carrier is not particularly limited, yttrium(III) oxide is also well-known, and a substance having the chemical formula Y2O3 may be used. For example, commercially available substances which are generally used as the raw material of catalysts (carriers) or ceramic may be used.

[0030] The iron oxide (FeO _x ) in the carrier is not particularly limited, and for example, a mixture including two or more types of iron(II) oxide (FeO), iron(II,III) oxide (Fe ₃ 0 ₄ ), and iron(III) oxide (Fe ₂ 03) may be employed. Among these, from the viewpoint of thermal stability and a high oxygen storage and release ability, iron(III) oxide is preferable.

[0031] Metal oxides used as other components that can be contained in the carrier in a range without damage to the effects of the invention are not particularly limited as long as the metal oxides can be used in the carrier of the exhaust gas purifying catalyst. For example, from the viewpoint of the thermal stability and the catalyst activity of the carrier, for example, oxides of metals including rare-earth elements, alkali metals, alkaline earth metals, and transition metals such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), magnesium ( g), calcium (Ca), strontium (Sr), barium (Ba), scandium (Sc), vanadium (V), mixtures of the oxides of the metals, solid solutions of the oxides of the metals, and composite oxides of the metals may be appropriately used. [0032] Furthermore, the shape of the carrier of the exhaust gas purifying catalyst of the invention is not particularly limited, and well-known shapes such as a ring shape, a spherical shape, a columnar shape, a particle form, and a pellet form may be used. From the viewpoint of a high amount of Pt and Pd contained in a highly dispersed state, the particle form is preferably used. In the case where the carrier is in the particle form, the average secondary particle diameter of the carrier is preferably 0.5 μιη to 10 μηι.

[0033] In addition, the specific surface area of the carrier is not particularly limited, and is preferably 5 m ² /g to 30C m ² /g, and more preferably 10 m ² /g to 200 m ² /g. When the specific surface area thereof is lower than the lower limit, there is a tendency toward a reduction in the dispersibility of the noble metal such as Pt or Pd and a reduction in the catalyst performance (oxidative activity toward CO and HC at a low temperature). On the other hand, when the specific surface area thereof is higher than the upper limit, the carrier easily undergoes grain growth even at a low temperature of 700°C or lower, the grain growth of the noble metal supported on the carrier is accelerated, and thus there is a tendency toward a reduction in the catalyst performance. The specific surface area may be calculated as a BET specific surface area using a BET isotherm adsorption equation from an adsorption isotherm. The BET specific surface area may be obtained by using a commercially available apparatus.

[0034] A method of manufacturing the carrier is not particularly limited, and well-known methods may be appropriately employed. As the carrier, a commercially available carrier may also be used.

(Noble Metal)

[0035] Next, in the exhaust gas purifying catalyst of the invention, the noble metal is supported on the carrier. The noble metal is not particularly limited, and at least one type selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir), and ruthenium (Ru) is preferably used. Among the metals, from the viewpoint of catalyst performance, at least one type selected from the group consisting of Pt, Rh, Pd, Ir, and Ru is more preferable, and at least one type selected from the group consisting of Pt and Pd is even more preferable. [0036] The amount of the supported noble metal is not particularly limited, and a necessary amount of the noble metal may be appropriately supported depending on the desired design or the like. The amount of the supported noble metal is preferably 0.1 parts by mass to 15 parts by mass with respect to 100 parts by mass of the carrier in terms of metal. When the amount of the supported noble metal is lower than the lower limit, sufficiently high oxidative activity toward CO and HC at a low temperature tends not to be obtained. On the other hand, when the amount thereof is higher than the upper limit, sintering of the noble metal is likely to occur, and the degree of dispersion of the noble metal is reduced, which results in a tendency toward disadvantageous in terms of effective use of the noble metal and cost. From the viewpoint of catalyst performance and cost, the amount of the supported noble metal is more preferably 0.5 parts by mass to 10 parts by mass. The particle diameter (average particle diameter) of the noble metal supported in the carrier as described above is preferably 1 nm to 100 nm (more preferably 2 nm to 50 nm). When the particle diameter of the noble metal is lower than the lower limit, there is a tendency toward a difficulty in achieving a metal state. On the other hand, when the particle diameter thereof is higher than the upper limit, there is a tendency toward a significant reduction in the amount of active sites.

[0037] In the exhaust gas purifying catalyst of the invention, it is particularly preferable that the noble metal includes platinum and palladium, and at least a portion of the platinum and palladium is solutionized. Since the platinum and palladium are in such a solid solution state, the characteristics of active sites during reactions with CO, HC, and the like (activity per the number of active sites) tend to be further enhanced. The solid solution can be generated, for example, by performing a heat treatment on a catalyst having platinum and palladium supported thereon at 700°C or higher. The presence of the solid solution can be identified by obtaining a lattice constant by measuring peaks derived from the (311) face of crystals of the platinum, palladium, and/or solid, solution between 81.2° and 82.1° in an X-ray diffraction method using CuKa radiation caused by the platinum, palladium, and/or solid solution. Furthermore, in a case where the X-ray diffraction measurement is performed on the solid solution, it is possible to obtain the amounts of the platinum and palladium solutionized on the basis of Vegard's law from a change in the lattice constant. The amount of the solid solution of the platinum and palladium obtained as described above is preferably 10 mass% to 90 mass% with respect to the total amount of the platinum and palladium, from the viewpoint of sufficiently enhancing the characteristics of active sites during reactions with CO, HC, and the like (activity per the number of active sites). For the same reason, the particle diameter (average particle diameter) of the solid solution is preferably 1 nm to 100 nm (more preferably 2 nm to 50 nm). In addition, in the exhaust gas purifying catalyst of the invention, diffraction line peaks derived from the (311 ) face between 81.2° and 82.1 ° in the X-ray diffraction method using CuK radiation caused by the platinum, palladium, and/or solid solution are 81.5° or higher. Since the diffraction line peaks are 81.5° or higher, it is seen that the solid solution of the platinum and palladium is sufficiently formed. In a case where the solid solution of the platinum and palladium is not sufficiently formed, the diffraction line peaks become lower than 81.5°.

[0038] The form of the exhaust gas purifying catalyst of the invention is not particularly limited, and for example, may be a form of a honeycomb-shaped monolith catalyst, a pellet-like pellet catalyst, or the like, or a form in which a powder form is disposed at predetermined points as it is. A method of manufacturing the exhaust gas purifying catalyst in such a form is not particularly limited, and well-known methods may be appropriately employed. For example, a method of obtaining a pellet-like exhaust gas purifying catalyst by molding a catalyst in a pellet form, a method of obtaining a form of an exhaust gas purifying catalyst in which a catalyst is applied (fixed) to a catalyst substrate by coating the catalyst substrate with the catalyst, or the like may be appropriately employed. The catalyst substrate is not particularly limited and is appropriately selected, for example, depending on the purpose of the obtained exhaust gas purifying catalyst, and a honeycomb monolith substrate, a pellet-like substrate, a plate-like substrate, or the like is appropriately employed. In addition, the material of the catalyst substrate is not particularly limited, and for example, a substrate made of ceramic such as cordierite, silicon carbide, or mullite, and a material made of metal such as stainless steel containing chromium and aluminum are appropriately employed. Furthermore, the exhaust gas purifying catalyst of the invention may also be used in combination with other catalysts. Other catalysts are not particularly limited, and well-known catalysts (for example, oxidation catalysts, NOx reduction catalysts, NOx storage and reduction catalysts (NSR catalysts), and NOx selective reduction catalysts (SCR catalysts)) may be appropriately used.

<Method of Manufacturing Exhaust Gas Purifying Catalyst>

[0039] Next, a method of manufacturing a gas purifying catalyst of the invention will be described. A method of manufacturing an exhaust gas purifying catalyst of the invention includes: a process of obtaining a carrier made of alumina, yttria, and iron oxide by bringing alumina particles into contact with a solution including a first compound containing yttrium (Y) and a second compound containing iron (Fe) (carrier preparing process); a process of allowing a noble metal to be supported on the carrier by using a solution of a noble metal salt (noble metal supporting process); and a process of obtaining the exhaust gas purifying catalyst of the invention by baking the carrier having the noble metal supported thereon (baking process). According to this method, the exhaust gas purifying catalyst of the invention which has sufficiently high oxidative activity toward CO and HC at a low temperature and maintains sufficiently high oxidative activity toward CO and HC even when being exposed to a high temperature can be manufactured.

(Carrier Preparing Process)

[0040] In the method of manufacturing an exhaust gas purifying catalyst of the invention, first, a carrier made of alumina, yttria, and iron oxide is obtained by bringing alumina particles into contact with a solution including a first compound containing yttrium (Y) and a second compound containing iron (Fe) (carrier preparing process).

[0041] The alumina particles used in the carrier preparing process according to the manufacturing method of the invention is not particularly limited, and for example, alumina which can be obtained by appropriately employing a well-known method of manufacturing alumina, or commercially available alumina may be used. As the method of manufacturing alumina, for example, a method of obtaining alumina by baking precipitate, which is obtained by neutralization through the addition ammonia water to a solution of aluminum nitrate, at about 500°C to 1200°C for about 0.5 hour to 10 hours and thereafter dry-grinding the resultant may be employed.

[0042] Regarding the particle diameter of the alumina particles, the average secondary particle diameter thereof is preferably 0.5 μπι to 100 μιη, and more preferably 1 μπι to 10 μηι. When the average particle diameter of the alumina particles is lower than the lower limit, grain growth of the carrier tends to ^' easily occur. On the other hand, when the average particle diameter thereof is higher than the upper limit, the noble metal tends not to be supported with high dispersibility.

[0043] The specific surface area of the alumina particles is preferably 5 m ² /g to

300 m ² /g, and more preferably 10 m ² /g to 200 m ² /g. When the specific surface area thereof is lower than the lower limit, the dispersibility of the noble metal is reduced, and sufficient activity tends not to be obtained. On the other hand, when the specific surface area thereof is higher than the upper limit, grain growth of the carrier tends to easily occur.

[0044] Next, the first compound containing yttrium (Y) used in the carrier preparing process according to the manufacturing method of the invention is not particularly limited, and for example, yttrium salts such as nitrates, sulfates, halides (fluorides, chlorides, or the like), acetates, carbonates, citrates, and the like of yttrium (Y), and complexes thereof may be employed. These types may be used singly or in combination of two or more types. Among the types, from the viewpoint of uniform support on the carrier, at least one type selected from the group consisting of nitrates, acetates, and citric acid complex salts is preferable. Specifically, yttrium acetate tetrahydrate, yttrium nitrate hexahydrate, an yttrium citric acid complex that is obtained by mixing an aqueous solution of citric acid with an aqueous solution of yttrium acetate, or the like may be employed.

[0045] In addition, the second compound containing iron (Fe) used in the carrier preparing process according to the manufacturing method of the invention is not particularly limited, and for example, salts of iron and oxides, hydroxides, chlorides, acetates, nitrates, sulfates, ammonium salts, and organic acid salts of iron may be employed. These types may be used singly or in combination of two or more types. Among the types, from the viewpoint of uniform support on the carrier, at least one type selected from the group consisting of nitrates and citric acid complexes is preferable. Specifically, ammonium iron(III) citrate, iron(III) nitrate nonahydrate, or the like may be employed.

[0046] The solvent of the solution including the first compound and the second compound is not particularly limited, and for example, a solvent such as water (preferably pure water such as ion-exchange water or distilled water) may be employed. The concentration of yttrium and iron in the solution is not particularly limited, and it is preferable that yttrium (Y) ions have a concentration of 0.01 mol L to 1.0 mol L and iron (Fe) ions have a concentration of 0.01 mol/L to 1.0 mol/L.

[0047] Next, a method of manufacturing the solution including the first compound and the second compound is not particularly limited as long as the first compound and the second compound can be dissolved in the solvent in the method. For example, a method of first preparing each of a solution including the first compound (yttrium solution) and a solution including the second compound (iron solution) and mixing and stirring the yttrium solution (solution 1) and the iron solution (solution 2) may be employed. Otherwise, a solution including the first compound and the second compound may also be prepared.

[0048] In addition, a method of brining the alumina particles into contact with the solution including the first compound and the second compound is not particularly limited, and for example, well-known methods such as a method of allowing the alumina particles to be impregnated with the solution including the first compound and the second compound, a method of allowing the solution including the first compound and the second compound to be adsorbed and supported on the alumina particles, or the like may be appropriately employed. When the alumina particles are brought into contact with the solution, an aqueous solution including the first compound and the second compound is brought into contact with the alumina particles so that the amount of yttria contained in the carrier after baking is 0.3 mass% to 31 mass%, the amount of iron oxide contained in the carrier is 0.5 mass% to 27 mass%, and the ratio between the yttria and the iron oxide contained in the carrier after the baking is in a range of 80:20 to 10:90 in terms of the atomic ratio of metal elements therein ([yttrium]: [iron]). In addition, the amount of an yttrium element supported in the yttrium solution (the solution including the first compound) with respect to the alumina particles is preferably 0.00003 mol/g to 0.005 mol/g in terms of metal ([the number of moles of the yttrium element in the aqueous solution]/[the mass of the alumina particles]), and more preferably 0.0001 mol/g to 0.002 mol/g. When the amount of the supported yttrium element is lower than the lower limit, there is a tendency toward a reduction in the dispersibility of the noble metal in a case of exposure to a high temperature. On the other hand, when the amount thereof is higher than the upper limit, there is a tendency toward a difficulty in metalation of the noble metal. The amount of an iron element supported in the iron solution (the solution including the second compound) with respect to the alumina particles is preferably 0.00006 mol/g to 0.005 mol/g in terms of metal ([the number of moles of the iron element in the aqueous solution]/[the mass of the alumina partbles]), and more preferably 0.0002 mol/g to 0.002 mol/g. When the amount of the supported iron element is lower than the lower limit, high oxidative activity toward CO and HC at a low temperature tends not to be obtained. On the other hand, when the amount thereof is higher than the upper limit, there is a tendency toward grain growth of the noble metal in a case of exposure to a high temperature.

[0049] In the carrier preparing process according to the manufacturing method of the invention, it is preferable to obtain the carrier made of alumina, yttria, and iron oxide by bringing the alumina particles into contact with the solution including the first compound containing yttrium (Y) and the second compound containing iron (Fe) and thereafter baking the resultant. Baking conditions are not particularly limited, and it is preferable to perform heating in the air in a temperature range of 500°C to 900°C, and it is more preferable to perform heating in a temperature range of 750°C to 850°C. When the heating temperature is lower than the lower limit, the amount of the specific surface area reduced due to thermal deterioration in the carrier tends to be increased. On the other hand, when the heating temperature is higher than the upper limit, thermal deterioration proceeds, the dispersibility of the yttria and/or the iron oxide is degraded, and furthermore, grain growth of the alumina also tends to be incurred. The heating time varies depending on the heating temperature and is thus cannot be said sweepingly. However, the heating time is preferably 3 hours to 20 hours, and more preferably 4 hours to 15 hours.

[0050] In the carrier preparing process according to the manufacturing method of the invention, as another embodiment, the carrier made of alumina, yttria, and iron oxide can be obtained by first bringing a solution including a first compound containing yttrium (Y) into contact with alumina particles, then baking the resultant, thereafter bringing a solution including a second compound containing iron (Fe) into contact with the alumina particles after the baking, and baking the resultant. In this case, in the order in which the solutions including the compounds are brought into contact, the solution including the second compound may also be applied first. Furthermore, as still another embodiment, the carrier made of alumina, yttria, and iron oxide can be obtained by first bringing a solution including a first compound containing yttrium (Y) into contact with alumina particles, thereafter bringing a solution including a second compound containing iron (Fe) into contact with the alumina particles, and then baking the resultant. In this case, in the order in which the solutions including the compounds are brought into contact, the solution including the second compound may also be applied first.

[0051] In the carrier preparing process according to the manufacturing method of the invention, after the solution is brought into contact with the alumina particles and the solution is supported, a drying process may be appropriately performed before the baking. The drying process is not particularly limited, and well-known methods may be appropriately employed. For example, in addition to natural drying and evaporation to dryness, methods such as drying using a rotary evaporator, a blow dryer, or the like may be employed. The drying time is not particularly limited and is appropriately selected depending on the desired design or the like. For example, a . process of performing drying through heating at 80°C to 200°C for 5 hours to 20 hours may be employed.

(Noble Metal Supporting Process)

[0052] In the method of manufacturing an exhaust gas purifying catalyst of the invention, the carrier obtained in the carrier preparing process is allowed to support a noble metal by using a solution of a noble metal salt (noble metal supporting process).

[0053] The solution of the noble metal salt used in the noble metal supporting process according to the manufacturing method of the invention is not particularly limited. For example, a platinum salt is used as he noble metal salt, acetates, carbonates, nitrates, ammonium salts, citrates, dinitrodiammine salts of platinum (Pt), or complexes thereof may be employed. Among these, from the viewpoint of ease of supporting and high dispersibility, nitrates and dinitrodiammine salts are preferable. In a case where a palladium salt is used as the noble metal salt, for example, a solution of acetates, carbonates, nitrates, ammonium salts, citrates, and dinitrodiarrrmine salts of palladium (Pd), or complexes thereof may be employed. Among these, from the viewpoint of ease of supporting and high dispersibility, nitrates and dinitrodiarnmine salts are preferable. In addition, the solvent is not particularly limited, and for example, a solvent which dissolves a salt into ions, such as water (preferably pure water such as ion-exchange water or distilled water) may be employed. The concentration of the solution of the noble metal salt is not particularly limited, and is preferably 0.0002 mol/L to 0.1 mol/L in terms of noble metal salt ions.

[0054] In addition, a method of allowing the carrier to support the noble metal using the solution of the noble metal salt is not particularly limited, and for example, well-known methods such as a method of allowing the carrier to be impregnated with the solution of the noble metal salt, a method of allowing the solution of the noble metal salt to be adsorbed and supported on the carrier, or the like may be appropriately employed. When the carrier is allowed to support the solution of the noble metal salt, the amount of a noble metal element contained in the solution of the noble metal salt is preferably 0.1 parts by mass to 15 parts by mass with respect to 100 parts by mass of the carrier in terms of metal, and more preferably 0.5 parts by mass to 10 parts by mass. The amount of the supported noble metal element is lower than the lower limit, high oxidative activity toward CO and HC at a low temperature tends not to be sufficiently obtained. On the other hand, when the amount thereof is higher than the upper limit, there is a tendency toward disadvantageous in terms of effective use of the noble metal and cost. From the viewpoint of catalyst performance and cost, the amount of the supported noble metal element is preferably 0.1 parts by mass to 15 parts by mass, and more preferably 0.5 parts by mass to 10 parts by mass with respect to 100 parts by mass of the carrier in terms of metal.

<Baking Process>

[0055] Next, in the method of manufacturing an exhaust gas purifying catalyst of the invention, the exhaust gas purifying catalyst of the invention is obtained by baking the carrier (the carrier supporting the noble metal) that supports the noble metal obtained in the noble metal supporting process (baking process).

[0056] In the baking process according to the method of manufacturing an exhaust gas purifying catalyst of the invention, baking conditions are not particularly limited, and it is preferable to bake the carrier (the carrier supporting the noble metal) having the noble metal supported thereon at a temperature in a range of 300°C to 900°C. When the baking temperature is lower than the lower limit, the noble metal salt is not sufficiently decomposed, and noble metal tends not to be activated, and thus sufficiently high oxidative activity toward CO and HC at a low temperature tends not to be exhibited. On the other hand, when the baking temperature is higher than the upper limit, it becomes difficult to support the noble metal with high dispersibility, and there is a tendency toward a reduction in the oxidative activity toward CO and HC. In addition, thermal deterioration proceeds, grain growth of iron oxide particles and the like occurs, and there is a tendency toward a reduction in the dispersibility on the surface of the carrier. From the viewpoint of the compatibility of activation of the noble metal and high dispersibility, the baking temperature is preferably a temperature in a range of 300°C to 900°C, and more preferably a temperature in a range of 400°C to 700°C. The baking (heating) time varies depending on the baking temperature and is thus cannot be said sweepingly. However, the baking time is preferably 0.5 hour to 10 hours, and more preferably 1 hour to 5 hours. Furthermore, the atmosphere in the baking process is not particularly limited, and the air or an inert gas such as nitrogen (N ₂ ) is preferable. [Method of Purifying Exhaust Gas]

[0057] Next, a method of purifying exhaust gas of the invention will be described. The method of purifying exhaust gas of the invention is a method of purifying exhaust gas by bringing the exhaust gas from an internal combustion engine into . contact with the exhaust gas purifying catalyst of the invention.

[0058] In the method of purifying exhaust gas of the invention described above, a method of bringing the exhaust gas into contact with the exhaust gas purifying catalyst is not particularly limited, and well-known methods may be appropriately employed. For example, a method of bringing the exhaust gas from the internal combustion engine into contact with the exhaust gas purifying catalyst by disposing the exhaust gas purifying catalyst of the invention in an exhaust gas pipe through which gas discharged from the internal combustion engine passes may be employed.

[0059] The exhaust gas purifying catalyst of the invention used in the method of purifying exhaust gas of the invention has sufficiently high oxidative activity toward CO and HC at a low temperature and maintains sufficiently high oxidative activity toward CO and HC even when being exposed to a high temperature. Therefore, it is possible to exhibit sufficiently high oxidative activity toward CO and HC at a low temperature, it is possible to exhibit sufficiently high oxidative activity toward CO and HC even during exposure to a high temperature, and it is possible to sufficiently purify CO and HC in the exhaust gas by bringing the exhaust gas from the internal combustion engine such as a diesel engine into contact with the exhaust gas purifying catalyst of the invention. From this viewpoint, the method of purifying exhaust gas of the invention can be appropriately employed as a method of purifying CO and HC in exhaust gas discharged from an internal combustion engine such as a diesel engine.

<Examples>

[0060] Hereinafter, the invention will be more specifically described on the basis of Examples and Comparative Examples, and the invention is not limited to the following Examples.

(Example 1) [0061] First, as a yttrium solution, an aqueous solution of a yttrium citric acid complex was prepared by dissolving 34.6 g (0.18 mol) of citric acid (manufactured by Wako Pure Chemical Industries, Ltd., Special Grade) in 34 g of ion-exchange water, thereafter adding 20.3 g (0.06 mol) of yttrium acetate tetrahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) thereto, and stirring the resultant at room temperature (25°C) for about 6 hours. As an iron solution, an aqueous solution of iron, citrate was prepared by dissolving 29.3 g (0.06 mol) of ammonium iron(III) citrate (manufactured by Wako Pure Chemical Industries, Ltd., brown, 1st Grade) in 33 g of ion-exchange water and thereafter stirring the resultant at room temperature (25°C) for about 6 hours.

[0062] Next, an yttria-iron oxide-alumina carrier (carrier A) was obtained by mixing the obtained yttrium solution (corresponding to 0.10 mol of yttrium) with the iron solution (corresponding to 0.10 mol of iron), diluting the mixture with ion-exchange water into about 500 mL of an aqueous solution, thereafter allowing an amount corresponding to 0.10 mol of yttrium and an amount corresponding to 0.10 mol of iron to be supported on 150 g of alumina powder (manufactured by W.R. Grace and Company, MI307) using the obtained aqueous solution, drying the resultant with a rotary evaporator, and baking the resultant in the air at a temperature of 800°C for 5 hours. The amount of yttria contained in the carrier A was 6.7 mass%, the amount of iron oxide contained was 4.7 mass%, and the atomic ratio of yrtrium:iron was 50:50.

[0063] Next, catalyst powder was obtained by allowing the entirety of the obtained yttria-iron oxide-alumina carrier (carrier A) to be impregnated with an aqueous solution of dimtrodiammine platinum nitrate (0.014 mol/L) and an aqueous solution of palladium nitrate (0.0063 mol/L) to support platinum and palladium so as to achieve an amount of supported platinum of 5.4 g and an amount of supported palladium of 1.35 g with respect to 150 g of the alumina powder, and thereafter baking the resultant in the air at 550°C for 2 hours.

[0064] Next, a coating slurry was obtained by adding 66.6 g of ALUMINASOL (manufactured by Nissan Chemical Industries, Ltd., trade name "A520", solid content concentration 20 mass%) to the obtained catalyst powder, and grinding the resultant using an agitated media mill (ATTRITO ) for 30 minutes. An exhaust gas purifying catalyst made of a monolith catalyst was obtained by coating a cordierite monolith substrate having a test piece size (having a diameter of 30 mm, a length of 50 mm, a volume of 35 ml (35 cc), and a cell density of 400 cells/inch ² ) with the slurry and thereafter baking the resultant at a temperature of 500°C for 3 hours. In addition, the coating amount of the slurry was adjusted to achieve an amount of platinum of 5.4 g/L and an amount of palladium of 1.35 g/L per 1 L of the monolith substrate.

(Example 2)

[0065] An yttria-iron oxide-alumina carrier (carrier B) was obtained in the same manner as in Example 1 except that the amount of added yttrium acetate tetrahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was 28.4 g, the amount of added ammonium iron(III) citrate (manufactured by Wako Pure Chemical Industries, Ltd.) was

17.6 g, and an yttrium solution corresponding to 0.14 mol of yttrium and an iron solution corresponding to 0.06 mol of iron were mixed with each other. The amount of yttria contained in the carrier B was 9.3 mass%, the amount of iron oxide contained was 2.8 mass%, and the atomic ratio of yttrium:iron was 70:30.

[0066] Next, an exhaust gas purifying catalyst made of a monolith catalyst was obtained in the same manner as in Example 1 by using the obtained yttria-iron oxide-alumina carrier (carrier B). The coating amount of the slurry was adjusted to achieve an amount of platinum of 5.4 g/L and an amount of palladium of 1.35 g/L per 1 L of the monolith substrate.

(Example 3)

[0067] An yttria-iron oxide-alumina carrier (carrier C) was obtained in the same manner as in Example 1 except that the amount of added yttrium acetate tetrahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was 8.1 g, the amount of added ammonium iron(III) citrate (manufactured by Wako Pure Chemical Industries, Ltd.) was

46.7 g, and an yttrium solution corresponding to 0.04 mol of yttrium and an iron solution corresponding to 0.16 mol of iron were mixed with each other. The amount of yttria contained in the carrier C was 2.7 mass%, the amount of iron oxide contained was 7.6 mass%, and the atomic ratio of yttrium:iron was 20:80.

[0068] Next, an exhaust gas purifying catalyst made of a monolith catalyst was obtained in the same manner as i Example 1 by using the obtained yttria-iron oxide-alumina carrier (carrier C). The coating amount of the slurry was adjusted to achieve an amount of platinum of 5.4 g/L and an amount of palladium of 1.35 g/L per 1 L of the monolith substrate.

(Comparative Example 1)

[0069] Catalyst powder was obtained by allowing 150 g of alumina powder (manufactured by W.R. Grace and Company, MI307) to be impregnated with a dinitrodiammine platinum nitrate solution and a palladium nitrate solution to support platinum and palladium so as to achieve an amount of supported platinum of 5.4 g and an amount of supported palladium of 1.35 g with respect to 150 g of the alumina powder, and thereafter baking the resultant in the air at 550°C for 2 hours.

[0070] Next, a coating slurry was obtained by adding 66.6 g of ALUMINASOL to the obtained catalyst powder, and grinding the resultant using an agitated media mill (ATTRITOR) for 30 minutes. A comparative catalyst was obtained by coating a cordierite monolith substrate having a test piece size with the slurry and thereafter baking the resultant at a temperature of 500°C for 3 hours in the same manner as in Example 1. In addition, the coating amount of the slurry was adjusted to achieve an amount of platinum of 5.4 g/L and an amount of palladium of 1.35 g/L per 1 L of the monolith substrate.

(Comparative Example 2)

[0071] An yttria-alumina carrier (carrier D) was obtained by dissolving 38.3 g (0.10 mol) of yttrium(III) nitrate hexahydrate (manufactured by Mitsuwa Chemicals Co., Ltd.) in about 500 mL of ion-exchange water, thereafter adding 150 g of alumina powder (manufactured by W.R. Grace and Company, MI307) thereto, drying the resultant using a rotary evaporator, and baking the resultant in the air at a temperature of 800°C for 5 hours. The amount of yttria contained in the carrier D was 7.0 mass%.

[0072] Next, a comparative catalyst was obtained in the same manner as in Example 1 except that the obtained yttria-alumina carrier (carrier D) was used instead of the carrier A. In addition, the coating amount of the slurry was adjusted to achieve an amount of platinum of 5.4 g/L and an amount of palladium of 1.35 g/L per 1 L of the monolith substrate.

(Comparative Example 3)

[0073] An yttria-alumina carrier (carrier E) was obtained by dissolving 76.6 g (0.20 mol) of yttrium(III) nitrate hexahydrate (manufactured by Mitsuwa Chemicals Co., Ltd.) in about 500 mL of ion-exchange water, thereafter adding 150 g of alumina powder (manufactured by W.R. Grace and Company, MI307) thereto, drying the resultant using a rotary evaporator, and baking the resultant in the air at a temperature of 800°C for 5 hours. The amount of yttria contained in the carrier E was 13.1 mass%.

[0074] Next, a comparative catalyst was obtained in the same manner as in Example 1 except that the obtained yttria-alumina carrier (carrier E) was used instead of the carrier A. In addition, the coating amount of the slurry was adjusted to achieve an amount of platinum of 5.4 g/L and an amount of palladium of 1.35 g/L per 1 L of the monolith substrate.

(Comparative Example 4)

[0075] An iron oxide-alumina carrier (carrier F) was obtained by dissolving 80.8 g (0.20 mol) of iron(III) nitrate nonahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) in about 500 mL of ion-exchange water, thereafter adding 150 g of alumina powder (manufactured by W.R. Grace and Company, MI307) thereto, drying the resultant using a rotary evaporator, and baking the resultant in the air at a temperature of 800°C for 5 hours. The amount of iron contained in the carrier F was 9.6 mass%.

[0076] Next, a comparative catalyst was obtained in the same manner as in Example 1 except that the obtained iron-alumina carrier (carrier F) was used instead of the carrier A. In addition, the coating amount of the slurry was adjusted to achieve an amount of platinum of 5.4 g/L and an amount of palladium of 1.35 g/L per 1 L of the monolith substrate.

<Thermal Durability and Activity Evaluation Tests> [0077] First, a durability test was conducted on each of the exhaust gas purifying catalysts obtained in Examples 1 to 3 and the comparative catalysts obtained in Comparative Examples 1 to 4. That is, a thermal durability test (durability test) in which each catalyst was put in an electric furnace (muffle furnace) and was maintained under the condition of 750°C for 37 hours was conducted.

[0078] Next, by using each of the exhaust gas purifying catalysts obtained in Examples 1 to 3 and the comparative catalysts obtained in Comparative Examples 1 to 4 after the durability test, the oxidation performance of each of the catalysts was measured.

[0079] In a test for measuring the oxidation performance, first, the Catalyst after the durability test was loaded in a quartz reaction tube having an inner diameter of 30 mm using a fixed bed flow type reactor, and while supplying a model gas containing C0 ₂ (10 vol%), 0 ₂ (10 vol%), CO (800 ppm), C ₃ H ₆ (400 ppmC), NO (100 ppm), H ₂ 0 (5 vol%), and N ₂ (remainder), at a flow rate of 15 L/min, the temperature of the gas supported to the catalyst was increased from 100°C to 400°C at a temperature increase rate of 10 °C/min. In addition, the CO concentration in the gas emitted from the catalyst (gas discharged from the quartz reaction tube after coming into contact with the catalyst) during the temperature increase was measured using a continuous gas analyzer, a CO conversion (oxidation) ratio was calculated from the CO concentration in the model gas and the CO concentration in the emitted gas, and a temperature at which the CO conversion (oxidation) ratio had reached 50% was obtained as a 50% CO oxidation temperature (°C). In the same manner, a temperature at which an HC(C ₃ H ₆ ) conversion (oxidation) ratio had reached 50% was obtained as a 50% HC oxidation temperature (°C). A graph representing the 50% CO oxidation temperatures after the durability test of the catalysts obtained in Examples 1 to 3 and Comparative Examples 1 to 4 is shown in FIG. 1. In addition, a graph representing the 50% HC oxidation temperatures after the durability test of the catalysts obtained in Examples 1 to 3 and Comparative Examples 1 to 4 is shown in FIG. 2.

[0080] As is apparent from the results shown in FIGS. 1 and 2, it was confirmed that the exhaust gas purifying catalysts after the durability test in Examples 1 to 3 exhibited higher CO oxidative activity and HC oxidative activity at the 50% CO oxidation temperatures and the 50% HC oxidation temperatures.

[0081] Next, the relationship between the ratio of iron atoms in the catalyst and the oxidative activity of _. the catalyst after the durability test is represented. First, as a graph representing the results of the activity evaluation test (oxidation performance evaluation test) of each of the exhaust gas purifying catalysts obtained in Examples 1 to 3 and the comparative catalysts obtained in Comparative Examples 3 and 4 after the durability test, the relationship between the atomic ratio of Fe and the 50% CO oxidation temperature of the catalyst after the durability test in a case where the sum of the numbers of yttrium (Y) atoms and iron (Fe) atoms in the carrier is referred to as 100, which represents the ratios of yttria and iron oxide contained in the carrier, is shown in FIG. 3.

[0082] In addition, as a graph representing the results of the activity evaluation test (oxidation performance evaluation test) of each of the exhaust gas purifying catalysts obtained in Examples 1 to 3 and the comparative catalysts obtained in Comparative Examples 3 and 4 after the durability test, the relationship between the atomic ratio of Fe and the 50% HC oxidation temperature of the catalyst after the durability test in the case where the sum of the numbers of yttrium (Y) atoms and iron (Fe) atoms in the carrier is referred to as 100, which represents the ratios of yttria and iron oxide contained in the carrier, is shown in FIG. 4.

[0083] As is apparent from the comparison between the results of Examples 1 to 3 and the results of Comparative Examples 3 and 4 shown in FIGS. 3 and 4, it was confirmed that the exhaust gas purifying catalysts of Examples 1 to 3 exhibited higher CO oxidative activity and HC oxidative activity in a case where the ratio between yttria and iron oxide contained in the carrier as the atomic ratio between yttrium (Y) and iron (Fe) is in a range of 80:20 to 10:90 in terms of the atomic ratio of metal elements therein ([yttrium (Y)]:[iron (Fe)]). That is, it was confirmed that the exhaust gas purifying catalysts of Examples 1 to 3 exhibited sufficiently high oxidative activity toward CO and HC at a low temperature and maintained sufficiently high oxidative activity toward CO and HC even when being exposed to a high temperature.

<Measurement of Noble Metal Particles and Particle Diameter> [0084] By using each of the exhaust gas purifying catalysts obtained in Examples

1 to 3 and the comparative catalysts obtained in Comparative Examples 1 to 4 after the durability test, the particle diameters of noble metal particles in each of the catalysts was measured.

[0085] In a test for measuring the particle diameters of the noble metal particles, first, test powder was obtained by scraping off catalyst components covering the cordierite monolith substrate in each of the catalysts after the durability test. An X-ray diffraction (XRD) pattern of the obtained test powder was measured using an X-ray diffractometer (manufactured by Rigaku Corporation, "RINT-TTR") under the conditions of a scan step of 0.01°, a divergence slit of 2/3°, a divergence slit of 8 mm, CuKa radiation, 40 kV, 40 mA, a scanning speed of 10 °/min. Next, the particle diameter (average particle diameter) of the noble metal (platinum, palladium, and/or the solid solution thereof) was measured by calculating the half of the width of diffraction line peaks (2Θ) between 81.2° and 82.1° derived from the (311) crystal face of the platinum, palladium, and/or the solid solution thereof using the Scherrer equation. FIG. 5 shows a graph representing the particle diameters of the noble metals of the catalysts obtained in Example 1 and Comparative Examples 1 to 4.

[0086] As is apparent from the comparison between the results of Example 1 and the results of Comparative Examples 1 to 4 shown in FIG. 5, it was confirmed that the particle diameter of the noble metal in the comparative catalyst of Comparative Example 4 in which alumina and iron oxide were contained in the carrier was significantly greater than that in Comparative Example 1 in which only alumina was contained in the carrier, and iron oxide accelerated grain growth of the noble metal. Contrary to this, the particle diameter of the noble metal in the exhaust gas purifying catalyst of Example 1 was smaller than that in Comparative Example 1 regardless of the iron oxide contained in the carrier and was substantially equal to that of the comparative catalysts of Comparative Examples

2 and 3 in which alumina and yttria were contained in the carrier. That is, it was confirmed that since the exhaust gas purifying catalyst of Example 1 contained yttria in the carrier, grain growth of the noble metal was suppressed, and an adverse effect of iron oxide was eliminated. It is thought that this is because yttria was present on the surface of alumina in the exhaust gas purifying catalyst and thus a solid-phase reaction between iron oxide and alumina and a reduction in the specific surface area of the carrier resulting therefrom were impeded.

<Oxygen Storage and Release Amount Measurement Test>

[0087] By using each of the exhaust gas purifying catalysts , obtained in Examples 1 to 3 and the comparative catalysts obtained in Comparative Examples 1 to 4 after the durability test, the oxygen storage and release amount of each of the catalysts was measured.

[0088] In a test for measuring the oxygen storage and release amount, first, a gas mixture of CO (1 mass%) and N ₂ (remainder) and a gas mixture of 0 ₂ (1 mass%) and N ₂ (remainder) and were alternately supplied to each catalyst sample after the durability test for 80 seconds and 40 seconds, respectively, under the condition of a temperature of 150°C and a gas flow rate of 15 L/min using a fixed bed flow reactor (manufactured by Best Sokki, Ltd.), and the amount of C0 ₂ generated during the supply of CO and N ₂ (atmosphere) was measured as the oxygen storage and release amount (mmol/L) by using a motor exhaust gas analyzer (manufactured by Best Sokki, Ltd., trade name "BEX-5000"). In addition, a graph representing the oxygen storage and release amounts of the catalysts after the durability test, which were obtained in Examples 1 to 3 and Comparative Examples 1 to 4 is shown in FIG. 6.

[0089] As is apparent from the comparison between the results of Examples 1 to 3 and the results of Comparative Examples 1 to 4 shown in FIG. 6, it was confirmed that the oxygen storage and release amount of the comparative catalyst of Comparative Example 4 in which alumina and iron oxide were contained in the carrier was significantly higher than that in Comparative Example 1 in which only alumina was contained in the carrier, and iron oxide contributed to an exhibition of oxygen storage and release performance. In addition, it was confirmed that the comparative catalysts of Comparative Examples 2 and 3 in which alumina and yttria were contained in the carrier had the same degree of oxygen storage and release amount as that of Comparative Example 1 and yttria did not contribute to an exhibition of oxygen storage and release performance. Contrary to this, it was confirmed that the exhaust gas purifying catalysts of Examples 1 to 3 in which alumina, yttria, and iron oxide were contained the carrier had similar or better oxygen storage and release performance than Comparative Example 4. It is thought that the oxygen storage and release amount at a relatively low temperature (150°C) is a factor of the enhancement of oxidative activity toward CO and HC at a low temperature.

<Test for Checking Solid Solution State of Platinum and Palladium in Noble Metals>

[0090] During the X-ray diffraction measurement, regarding the catalysts after the durability test among the exhaust gas purifying catalysts obtained in Examples 1 to 3, the values of the positions (2Θ) of the diffraction line peaks between 81.2° and 82.1° derived from the (311) crystal face of platinum, palladium, and/or the solid solution thereof were further checked. The obtained results are shown in Table 1. For comparison, X-ray diffraction measurement was performed on Pt/alumina and Pd/alumina in which only platinum or only palladium was supported on alumina powder (MB 07, manufactured by W.R. Grace and Company). The obtained results are shown in Table 1.

[Table 1]

[0091] As is apparent from the results shown in Table 1 and FIGS. 1 and 2, the diffraction peak derived from the (311) face in the Pt/alumina was observed at 81.26°, and the diffraction peak in the Pb/alumina was observed at 82.11°. On the other hand, in the exhaust gas purifying catalysts of Examples 1 to 3, a single diffraction peak between 81.2° and 82.1° was observed, and the position thereof was between 81.64° and 81.65°. From the results, it was confirmed that in the exhaust gas purifying catalysts of Examples 1 to 3, a considerable amount of platinum and palladium in the catalyst forms a solid solution. It is thought that due to the formation of the solid solution, sufficiently high oxidative activity toward CO and HC is exhibited.

[0092] As is apparent from the comparison between the results of Examples 1 to 3 and the results of Comparative Examples 1 to 4 shown in Table 1 and FIGS. 1 to 6 described above, it was confirmed that the exhaust gas purifying catalysts of Examples 1 to 3 had sufficiently high oxidative activity toward CO and HC at a low temperature and maintained sufficiently high oxidative activity toward CO and HC even when being exposed to a high temperature.

[0093] As described above, according to the invention, it is possible to provide an exhaust gas purifying catalyst which has sufficiently high oxidative activity toward CO and HC at a low temperature and maintains sufficiently high oxidative activity toward CO and HC even when being exposed to a high temperature. As described above, since the exhaust gas purifying catalyst of the invention can exhibit sufficiently high oxidative activity toward CO and HC at a low tsmperature and exhibit sufficiently high oxidative activity toward CO and HC even when being exposed to a high temperature, it is possible to sufficiently purify CO and HC in exhaust gas by bringing the exhaust gas from an internal combustion engine such as a diesel engine into contact with the exhaust gas purifying catalyst of the invention. From this point of view, the method of purifying exhaust gas of the invention can be appropriately applied as a method of purifying CO and HC in exhaust gas discharged from an internal combustion engine such as a diesel engine. Therefore, the invention is particularly useful for an exhaust gas purifying catalyst for purifying CO and HC contained in exhaust gas from an internal combustion engine such as a diesel engine, a method of manufacturing the same, and a method of purifying exhaust gas using the same.