JP5416497B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

Description

  The present invention relates to an exhaust gas purification catalyst used for fuel-rich exhaust gas having an air-fuel ratio (A / F) of less than 14.5 discharged from an internal combustion engine such as a motorcycle and a method for manufacturing the same.

  Exhaust gas discharged from an internal combustion engine mounted on an automobile or a motorcycle includes harmful components such as carbon monoxide (CO), hydrocarbon (THC), and nitrogen oxide (NOx). In order to decompose and purify these harmful components before exhaust gas is discharged, an exhaust gas purifying catalyst containing a platinum group element, a refractory inorganic oxide, or the like may be mounted on the exhaust port of the internal combustion engine. (For example, refer to Patent Document 1).

  As for motorcycles, in recent years, there has been an increase in regulations on the concentration of harmful components in exhaust gas. In addition to this, in the future, even after driving several tens of thousands of kilometers, the regulations will be met. In addition, durability requirements are expected to be imposed on purification performance.

  However, since the mounting space of a motorcycle is limited compared to that of an automobile, the exhaust gas purification catalyst to be mounted on the motorcycle is required to exhibit a high degree of purification ability while having a small capacity.

  Since motorcycles tend to focus on output, a large amount of fuel is used, and the oxygen concentration in the exhaust gas decreases accordingly, so that the air-fuel ratio (A / F) in the exhaust gas is 10.5 in the theoretical air-fuel ratio. There are many scenes that become less than.

  Since the air-fuel ratio of exhaust gas from automobiles is near the stoichiometric air-fuel ratio (14.5 to 14.7), conventionally used exhaust gas purification catalysts are basically purified at such an air-fuel ratio. In the concentration range, harmful components can be efficiently removed by oxidizing carbon monoxide and hydrocarbons and reducing nitrogen oxides.

  However, the exhaust gas with less than the stoichiometric air-fuel ratio discharged from the internal combustion engine of the two-wheeled vehicle has a low oxygen concentration. Therefore, the conventional exhaust gas purifying catalyst has a significantly reduced ability to oxidize carbon monoxide and hydrocarbons. For this reason, it has been extremely difficult to efficiently purify carbon monoxide and hydrocarbons. From the above, it has become necessary to prepare a purifying catalyst specialized for purifying exhaust gas discharged from a motorcycle.

Japanese Patent No. 3235640

  In view of the above situation, the present invention is not limited to nitrogen oxides, but also carbon monoxide and hydrocarbons, even in fuel-rich exhaust gas with an air-fuel ratio (A / F) of less than 14.5, which is emitted by motorcycles and the like. An object of the present invention is to provide an exhaust gas purifying catalyst that can be efficiently purified and also has durability, and a method for producing the same.

  As a result of various studies to solve the above problems, the present inventors have found that according to an exhaust gas purification catalyst containing cerium oxide, zirconium oxide, aluminum oxide, a specific concentration of yttrium oxide and / or magnesium oxide, and a noble metal. The present inventors have found that in fuel-rich exhaust gas having an air-fuel ratio of less than 14.5, the purification efficiency of carbon monoxide and hydrocarbons is improved while maintaining the purification efficiency of nitrogen oxides.

That is, the present invention is an exhaust gas purification catalyst used for fuel-rich exhaust gas having an air-fuel ratio (A / F) of less than 14.5,
(1) cerium oxide,
(2) zirconium oxide,
(3) Aluminum oxide,
(4) yttrium oxide and / or magnesium oxide, and
(5) contains noble metals,
The present invention relates to an exhaust gas purifying catalyst in which the total concentration of yttrium and / or magnesium is 2.0 wt% to 5.0 wt% with respect to the total amount of the catalyst.

  The exhaust gas purifying catalyst of the present invention is preferably produced by impregnating (5) with a complex oxide in which (1) to (4) are uniformly mixed.

  The noble metal is preferably at least one selected from the group consisting of platinum, rhodium and palladium.

The present invention also includes a metal carrier,
The exhaust gas purifying catalyst coated on the surface of the metal carrier,
The present invention relates to an exhaust gas purification material used for fuel-rich exhaust gas having an air-fuel ratio (A / F) of less than 14.5.

  Furthermore, the present invention provides (5) a noble metal to a composite oxide containing (1) cerium oxide, (2) zirconium oxide, (3) aluminum oxide, and (4) yttrium oxide and / or magnesium oxide. The present invention also relates to a method for producing a catalyst for purifying exhaust gas, wherein the total concentration of yttrium and / or magnesium including the impregnation step is 2.0% by weight to 5.0% by weight with respect to the total amount of the catalyst.

  In the production method, each of the components (1) to (4) and water are mixed to prepare a suspension, and then a coprecipitate is generated from the suspension by a coprecipitation method. Preferably, the method further includes a step of obtaining a composite oxide containing the above (1)-(4) by firing the above.

  According to the present invention, in a fuel-rich exhaust gas having an air-fuel ratio of less than 14.5, which is mounted on a motorcycle or the like and exhausted by an internal combustion engine having a large air-fuel ratio fluctuation range, carbon monoxide and hydrocarbons that are harmful components, and Nitrogen oxide can be efficiently removed at the same time.

  Moreover, since the catalyst according to the present invention exhibits sufficient removal performance by itself, it is not necessary to have a two-layer structure in which different catalyst layers are laminated as in Patent Document 1, for example, and can be easily manufactured.

  Furthermore, since the catalyst according to the present invention has a simple configuration, it is resistant to vibrations that are likely to occur in a two-wheeled vehicle and is excellent in durability.

The graph which shows the purification rate of the harmful | toxic component by the catalyst with various yttrium density | concentrations measured in Experimental example 1 The graph which shows the purification rate of the harmful | toxic component by the catalyst with various yttrium density | concentrations measured in Experimental example 1 The graph which shows the purification rate of the harmful | toxic component by the catalyst with various yttrium density | concentrations measured in Experimental example 1 The graph which shows the purification rate of the harmful | toxic component by the catalyst with various magnesium concentration measured in Experimental example 2 Of harmful components by catalysts having various magnesium concentrations measured in Example 2 Of harmful components by catalysts having various magnesium concentrations measured in Example 2 The graph which shows the purification rate of the harmful | toxic component by the catalyst containing the yttrium or other element measured in Experimental example 3 The graph which shows the purification rate of the harmful | toxic component by the catalyst containing the yttrium or other element measured in Experimental example 3 The graph which shows the purification rate of the harmful | toxic component by the catalyst containing the yttrium or other element measured in Experimental example 3 The graph which shows the purification rate of the harmful | toxic component by the catalyst prepared by the coprecipitation method measured in Experimental Example 5, or the catalyst prepared by the simultaneous impregnation method The graph which shows the purification rate of the harmful | toxic component by the catalyst prepared by the coprecipitation method measured in Experimental Example 5, or the catalyst prepared by the simultaneous impregnation method The graph which shows the purification rate of the harmful | toxic component by the catalyst prepared by the coprecipitation method measured in Experimental Example 5, or the catalyst prepared by the simultaneous impregnation method Transmission electron micrograph of a catalyst containing no yttrium Transmission electron micrograph of catalyst containing yttrium

  The exhaust gas purifying catalyst of the present invention has the following five components: (1) cerium oxide, (2) zirconium oxide, (3) aluminum oxide, (4) yttrium oxide and / or magnesium oxide, and (5) Consists of precious metals. Regarding component (4), it may contain only yttrium oxide, may contain only magnesium oxide, or may contain both yttrium oxide and magnesium oxide. From the viewpoint of purification efficiency, yttrium oxide is preferable to magnesium oxide. With oxides other than yttrium oxide and magnesium oxide (for example, iron oxide, manganese oxide, cobalt oxide, nickel oxide, etc.), the purification performance for fuel-rich exhaust gas cannot be improved.

  In the exhaust gas catalyst of the present invention, the total concentration of yttrium and / or magnesium is 2.0 wt% or more and 5.0 wt% or less with respect to the total amount of the catalyst. Within this range, it is possible to exhibit excellent purification performance for fuel-rich exhaust gas having an air-fuel ratio of less than 14.5. Outside this range, the purification performance of carbon monoxide, hydrocarbons, and nitrogen oxides is significantly reduced. Since the purification performance of carbon monoxide and hydrocarbons is more excellent, the lower limit is preferably 3.0% by weight or more. Moreover, since the purification performance of nitrogen oxide is more excellent, the upper limit is preferably 4.0% by weight or less.

  Examples of the noble metal of component (5) include gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os). However, at least one selected from the group consisting of platinum, rhodium and palladium is preferable from the viewpoint of purification performance against exhaust gas. In particular, platinum and / or rhodium are suitable. The total concentration of noble metals in the exhaust gas catalyst of the present invention is suitably about 0.1 to 10% by weight based on the total amount of the catalyst.

  Of these five components, (4) yttrium oxide and / or magnesium oxide, and (5) noble metals are minor components, (1) cerium oxide, (2) zirconium oxide, and (3) aluminum oxide. Is the main component and occupies most of the entire catalyst. The weight ratio of these three components is appropriately about 1: 0.1 to 2.0: 0.1 to 2.0 from the viewpoint of purification performance.

  In the exhaust gas purification catalyst of the present invention, precious metal particles are more finely dispersed by blending yttrium oxide or magnesium oxide at a specific concentration. As a result, it is considered that the purification performance for exhaust gas is improved. .

  The exhaust gas purifying catalyst of the present invention is preferably one produced by impregnating (5) a noble metal into the composite oxide containing the above (1)-(4). As a result, it is possible to arrange a precious metal having a high purification capacity in the vicinity of the surface of the catalyst, and thus it is possible to improve the purification performance for fuel-rich exhaust gas having an air-fuel ratio of less than 14.5.

  The composite oxide containing (1)-(4) is a mixture of each component of (1)-(4) in the oxide state or the precursor of (1)-(4) and water. After preparing a suspension, a coprecipitate is produced from the suspension by a coprecipitation method, and the coprecipitate can be fired.

  Examples of the precursors (1) to (4) include organic salts and inorganic salts of each metal. Examples of inorganic salts include nitrates, sulfates, carbonates, chlorides, hydroxides, and the like, but are not particularly limited. After mixing these with water and stirring sufficiently to prepare a suspension, each component of the above (1)-(4) is prepared by a coprecipitation method in which an alkali such as aqueous ammonia is added to this suspension. Or the coprecipitate which a precursor mixes uniformly is produced | generated.

  The separated coprecipitate is then fired to obtain a fired product made of the composite oxide containing (1)-(4). By this firing, the precursors (1) to (4) are also converted into respective oxides. The firing temperature can be adjusted as appropriate depending on the type of the precursor, but may be, for example, about 400 to 800 ° C.

  Since the obtained fired product is coated on the carrier described later, it can be mixed with water again to form a slurry. At this time, component (3) or a precursor thereof may be further replenished. This slurry facilitates coating on the carrier. It is preferable to fire again after coating.

  With respect to the fired product (including the coating layer formed by coating the fired product on the support) made of the composite oxide in which the above (1) to (4) are uniformly mixed, obtained as described above, (5 ) The exhaust gas purifying catalyst of the present invention is produced by impregnating with a noble metal.

  In order to impregnate the noble metal, a fired product made of the complex oxide containing the above (1) to (4) may be coated with a noble metal or a precursor thereof dissolved in water. Examples of the noble metal precursor include organic salts and inorganic salts of each metal. Examples of inorganic salts include, but are not limited to, nitrates, sulfates, carbonates, chlorides, oxides, hydroxides, and the like.

  Dry after coating. The drying temperature may be appropriately adjusted according to the type of the noble metal precursor, and may be, for example, about 100 to 200 ° C.

  In the production method described above, first, a coprecipitate formed by uniformly mixing the components or precursors (1) to (4) is generated. Alternatively, yttrium (or magnesium) is not included in the coprecipitate, and yttrium is impregnated at the same time when impregnating the noble metal in a subsequent step (simultaneous impregnation method). In this simultaneous impregnation method, a system in which three components: cerium oxide, zirconium oxide and aluminum oxide are uniformly mixed is impregnated with yttrium oxide (or magnesium oxide) and a noble metal. The purification performance for fuel-rich exhaust gas with a fuel ratio of less than 14.5 did not reach a sufficient level. Therefore, a structure obtained by impregnating a noble metal into a system in which four components obtained by coprecipitation method: cerium oxide, zirconium oxide, aluminum oxide and yttrium oxide (or magnesium oxide) are uniformly mixed is sufficient. It is considered to be important for exerting a good purification performance.

  The exhaust gas purification catalyst of the present invention is particularly excellent in purification performance for fuel-rich exhaust gas having an air-fuel ratio of less than 14.5. In the region where the air-fuel ratio is less than 14.5, since the proportion of air is small and the atmosphere is in a reducing atmosphere, carbon monoxide and hydrocarbons are not easily oxidized and their purification rate tends to decrease. However, the exhaust gas catalyst of the present invention improves the purification rate of carbon monoxide and hydrocarbons in the region where the air-fuel ratio is less than 14.5, as compared with the conventional exhaust gas catalyst. Further, the purification rate of nitrogen oxides in the same region is equal to or improved as compared with the conventional exhaust gas catalyst. That is, the exhaust gas catalyst of the present invention is excellent in all three types of harmful component purification rates when the air-fuel ratio is less than 14.5. Therefore, the exhaust gas catalyst of the present invention can be suitably mounted on a two-wheeled vehicle having a large fluctuation range of the air-fuel ratio as compared with an automobile and a large number of operations in a region where the theoretical air-fuel ratio is less than 14.5. .

  Furthermore, durability requirements are imposed on motorcycles as regulations are tightened in the future, and a higher level of durability than the current level is required for the exhaust gas catalyst to be mounted. Unlike the two-layered catalyst formed by laminating two kinds of catalyst compositions as described in Patent Document 1, the exhaust gas purifying catalyst of the present invention is composed of a single layer, so that vibrations that are likely to occur in two-wheeled vehicles are avoided. On the other hand, it has excellent durability.

  The exhaust gas purifying catalyst of the present invention is supported on a carrier and used as a purifying material, so that it can be easily mounted on a motorcycle or the like. The carrier is preferably a three-dimensional structure, and among them, a carrier having a honeycomb shape is preferred from the viewpoint of purification efficiency. As the carrier to be used, a ceramic carrier and a metal carrier are known, but it is preferable to use a metal carrier from the viewpoint of durability. Although it does not specifically limit as a metal which comprises a metal support | carrier, Aluminum, aluminum alloy, stainless steel, etc. can be used.

  When the exhaust gas purifying catalyst of the present invention is supported on a carrier, particularly a metal carrier, first, a composite oxide containing the components (1) to (4) is formed into a slurry, and this is coated on the surface of the carrier. . After drying, the surface of the composite oxide is further coated with an aqueous solution of a noble metal or a precursor thereof to impregnate the noble metal. After that, by firing, an exhaust gas purification material in which the exhaust gas purification catalyst of the present invention is coated on the surface of the carrier can be produced. According to this, it is possible to configure an exhaust gas purifying material having a simple structure and an excellent purifying ability for fuel-rich exhaust gas having an air-fuel ratio of less than 14.5 with a simple structure.

  The coating amount of the exhaust gas purification catalyst of the present invention on the carrier can be appropriately determined according to the required purification performance. For example, about 50 to 300 g / L is appropriate.

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

1. Catalyst preparation method (coprecipitation method)
(1) The following raw materials were mixed so that the oxide conversion, that is, 30 g in total of cerium oxide and zirconium oxide, and the molar ratio of cerium oxide: zirconium oxide was 1: 1. In this composition, the concentration of yttrium with respect to the total amount of catalyst finally obtained is 2.0% by weight.
Cerium nitrate hexahydrate (Ce (NO3) 3 × 6H2O) 37.8g
Zirconium oxynitrate dihydrate (ZrO (NO3) 2 × 2H2O) 32.6g
Yttrium nitrate hexahydrate (Y (NO3) 3 × 6H2O) 6.4g
γ-Alumina (TM-300D) 45.0g
The resulting mixture was added to 300 mL of water and stirred to prepare a suspension.
(2) A coprecipitate was formed by dropping 5.0 wt% aqueous ammonia into the suspension to raise the pH of the suspension to 9.
(3) The coprecipitate was left to mature for 15 hours, then separated by filtration and washed with water. The separated coprecipitate was dried in an oven at 120 ° C. for 4 hours, and further calcined in air at 600 ° C. for 2 hours to obtain 77.3 g of a calcined powder.
(4) The obtained fired powder was mixed with the following raw materials.
77.3 g of the calcined powder
Alumina sol (20 wt%) 51.6g
6.4g of aluminum nitrate (Al (NO3) 3) aqueous solution
6.4g of 10% nitric acid (HNO3) solution
Ion exchange water 96.7g
Each raw material was mixed and pulverized and mixed with a ball mill for 15 hours to obtain a slurry.
(5) The obtained slurry was transferred to an evaporating dish and baked in the air at 500 ° C. for 1 hour to obtain 90.7 g of a baked product. The obtained fired product was composed of cerium oxide, zirconium oxide, aluminum oxide, and yttrium oxide, and the weight ratio of each oxide in the fired product was 16.5% by weight, 16.6% by weight, and 64.4%, respectively. % By weight and 2.5% by weight.
(6) The obtained fired product was put into a mortar and ground to make it uniform, and then 5 g was weighed.
(7) Separately, 0.0361 g of tetraamminedichloroplatinum monohydrate (Pt (NH3) 4Cl2 × H2O) and 0.0140 g of rhodium nitrate (Rh (NO3) 3) were weighed and dissolved in 100 mL of pure water to prepare an aqueous solution. Got. The weight ratio of platinum and rhodium to the fired product (5 g) is 0.4 wt% (0.02 g) and 0.1 wt% (0.005 g), respectively.
(8) 5 g of the calcined product weighed above was mixed with the aqueous noble metal solution obtained in (7), and thoroughly mixed for 2 to 3 hours.
(9) The mixed solution was subjected to a rotary evaporator to remove water.
(10) The catalyst powder obtained after the distillation was sufficiently dried at 110 ° C. in a dryer.
(11) About 2 g of the obtained catalyst powder was weighed.
(12) The weighed catalyst powder was subjected to aging calcination in the air at 1000 ° C. for 5 hours. This calcination operation is an operation performed for forced deterioration of the catalyst for the purpose of evaluating the durability of the catalyst.
(13) The calcined catalyst was reduced by leaving it in a hydrogen stream at 400 ° C. for 30 minutes.

  As a result, an exhaust gas purification catalyst having an yttrium concentration of 2.0% by weight was obtained. The purification rate of harmful components by this catalyst was evaluated by the following test method.

2. Test Method (1) 1 g of exhaust gas purification catalyst was weighed.
(2) The weighed catalyst was pelletized and then pulverized to adjust the particle size in the range of about 1.0 to 1.5 mm.
(3) The granular catalyst was diluted with 20 g of quartz beads and charged into an atmospheric pressure fixed bed reactor.
(4) A gas mixture of concentration components simulating the exhaust gas of an actual vehicle was prepared by drawing and mixing each gas from a gas cylinder of each gas, and circulated through the reaction tube of the reactor.
(5) The reactor is heated in an electric furnace, and after the catalyst layer inlet temperature reaches 400 ° C., the carbon monoxide (CO) concentration, hydrocarbon (THC) concentration, and nitrogen of the catalyst layer inlet and outlet The oxide (NOx) concentration was measured.
(6) The purification rate of carbon monoxide (CO), hydrocarbon (THC), and nitrogen oxide (NOx) was calculated based on the following calculation formula.
Purification rate = (concentration at catalyst layer inlet−concentration at catalyst layer outlet) / concentration at catalyst layer inlet

(Experimental example 1)
Using the catalyst (yttrium concentration: 2.0% by weight) obtained by the above catalyst preparation method, the purification rate of each gas was measured by the above test method. However, as a mixed gas to be tested, an air / fuel usage ratio was adjusted so that the A / F value was in the range of 14.0 to 15.6.

  Furthermore, except that the amount of yttrium nitrate hexahydrate was changed so that the concentration of yttrium was 0% by weight, 6.0% by weight, or 10% by weight, the same procedure as in the above catalyst preparation method was performed. Different types of catalysts were prepared. The purification rate of each gas was measured in the same manner.

  The above results are shown in FIG. From FIGS. 1 (a) and (b), in the range where A / F is less than 14.5, the catalyst having an yttrium concentration of 2.0% by weight is compared with the catalyst containing no yttrium and carbon monoxide and Although the purification rate of hydrocarbons is improved, it can be seen that the purification rate of the catalyst having a high concentration of yttrium concentration of 6.0% by weight or 10% by weight is lowered. It can be seen that, in the range of A / F of 14.0 or more and 14.3 or less, the catalyst having an yttrium concentration of 2.0% by weight has a remarkable improvement in the purification rate of carbon monoxide and hydrocarbons. Further, FIG. 1 (c) shows that the catalyst having an yttrium concentration of 2.0% by weight has a nitrogen oxide purification rate equivalent to that of the other catalyst in the range where A / F is less than 14.5.

(Experimental example 2)
In the above catalyst preparation method, magnesium nitrate hexahydrate (Mg (NO3) 2 · 6H2O) is used instead of yttrium nitrate hexahydrate, magnesium is not included in yttrium and is 2.0% by weight based on the total amount of catalyst. , 6.0 wt%, or 10.0 wt% catalyst was prepared. About these, the purification rate of each gas was measured like Experimental example 1. FIG.

  The above results are shown in FIG. In FIG. 2, the result regarding the catalyst containing neither yttrium nor magnesium prepared in Experimental Example 1 is also shown.

  2 (a) and 2 (c), in the range where A / F is less than 14.5, the purification rate of carbon monoxide and nitrogen oxides by the catalyst having a magnesium concentration of 2.0% by weight includes magnesium. The purification rate of hydrocarbons by the 2.0 wt% concentration catalyst is higher than that of the catalyst not containing magnesium, and is 6.0 wt% or 10 wt% concentration. It can be seen that the hydrocarbon purification rate by this catalyst is decreasing.

(Experimental example 3)
In the above catalyst preparation method, instead of yttrium nitrate hexahydrate, iron (III) nitrate nonahydrate ([Fe (H2O) 6] (NO3) 3 · 3H2O), manganese (II) acetate tetrahydrate ( Mn (CH3COO) 2, 4H2O) (manganese oxide raw material), cobalt nitrate (II) hexahydrate (Co (NO3) 2, 6H2O) or nickel nitrate (II) hexahydrate (Ni (NO3) 2, 6H2O) ) Was used to prepare a catalyst containing 2.0% by weight of iron, manganese, cobalt or nickel without yttrium. About these, the purification rate of each gas was measured like Experimental example 1. FIG.

  The results are shown in FIG. FIG. 3 also shows the results regarding the catalyst containing 2.0% by weight of yttrium prepared by the above catalyst preparation method and the catalyst containing neither yttrium nor iron or the like prepared in Experimental Example 1.

  Further, from FIG. 3, the purification rate of each gas when the A / F value by each catalyst is 14.1 is extracted and shown in Table 1.

  From FIG. 3 and Table 1, the catalyst containing yttrium has significantly improved the purification rate of carbon monoxide and hydrocarbons as compared with the additive-free catalyst, but the catalysts containing other elements all decreased. I understand that. It can also be seen that the catalyst containing yttrium has reached a high level with respect to the purification rate of nitrogen oxides.

(Experimental example 4)
A catalyst was prepared by changing the yttrium concentration from 0 to 6.0% by weight in steps of 1.0% by weight according to the above catalyst preparation method. Using these catalysts, the purification rate of each gas was measured by the above test method. As the mixed gas to be tested, an air / fuel ratio was adjusted so that the A / F value was 14.1.

  The results are shown in Table 2.

  From Table 2, when the concentration of yttrium is in the range of 2.0 wt% to 5.0 wt%, the purification rate of carbon monoxide and hydrocarbons is remarkably improved as compared with the catalyst not containing yttrium, It can also be seen that the purification rate of nitrogen oxides is at least equivalent or improved.

(Experimental example 5)
An aqueous solution containing yttrium nitrate hexahydrate together with platinum salt and rhodium salt was prepared in step (7) without adding yttrium nitrate hexahydrate in step (1) of the catalyst preparation method (simultaneous impregnation method). Except for this, a catalyst having an yttrium concentration of 2.0% by weight was prepared in the same manner as in the catalyst preparation method described above.

  About this, the purification rate of each gas was measured similarly to Experimental example 1. FIG. The results are shown in FIG. FIG. 4 also shows the purification rate of the catalyst having the same concentration prepared by the above-described catalyst preparation method by the coprecipitation method.

  From FIG. 4, it was found that the catalyst prepared by the simultaneous impregnation method cannot exhibit a sufficient purification rate in the range where A / F is less than 14.5, compared with the catalyst prepared by the coprecipitation method.

(Experimental example 6)
Observation with a transmission electron microscope for each of the catalyst having the yttrium concentration of 2.0% by weight prepared by the above catalyst preparation method and the catalyst not containing yttrium nitrate prepared by adding the yttrium nitrate hexahydrate by the same method. (TEM) was performed.

  FIG. 5 shows a BF-STEP image for a catalyst containing no yttrium, and FIG. 6 shows a BF-STEP image for a catalyst having an yttrium concentration of 2.0 wt%. The BF-STEM image indicates a bright field of the STEM image. By using energy dispersive X-ray analysis (EDX) together, the following was found.

  In the field of view observed in the BF-STEM image of FIG. 5, the black particles having a size of about 30 to 300 nm indicated by the analysis positions 1, 2, 3, 4, 5, 6 are made of platinum. It was. Zirconium and cerium were detected from the black fine particle aggregate shown at the analysis position 7. Oxygen and aluminum were detected from the gray fine particle aggregate shown at the analysis position 8.

  In the visual field observed with the BF-STEM image in FIG. 6, the black particles having a size of about 50 to 200 nm shown at the analysis positions 1 and 2 were made of platinum. Zirconium and cerium were detected from the black fine particle aggregates shown at analysis positions 3 and 4. Oxygen and aluminum were detected from the gray fine particle aggregate shown at the analysis position 5.

  From the above TEM observation results, it can be seen that the size of the particles composed of the noble metal is remarkably reduced in the inside of the yttrium-containing catalyst of the present invention as compared with the catalyst not containing yttrium. From this, it is considered that the addition of yttrium tends to suppress the aggregation of the noble metal, and thus the purification rate of harmful components in the fuel-rich exhaust gas is improved.

(Experimental example 7)
X-ray photoelectron analysis (XPS) was applied to the catalyst having the yttrium oxide concentration of 2.0% by weight prepared by the above catalyst preparation method.

According to this, a large peak due to yttrium oxide (Y ₂ O ₃ ) was observed in the vicinity of 156 to 159 eV, whereas a peak due to yttrium in the vicinity of 155.8 eV was hardly observed. From this, in the catalyst of this invention, it was estimated that most of yttrium exists in the form of an oxide. Moreover, since the peak observed in the vicinity of 156 to 159 eV may indicate (Y ₂ O ₃ ) ₁₀ (ZrO ₂ ) ₉₀ , in that case, yttrium is more in the state of yttrium oxide than in cerium oxide. Is also present in zirconium oxide and is presumed to be complexed with zirconium oxide.

  The exhaust gas purifying catalyst and the exhaust gas purifying material according to the present invention are used to remove harmful components from fuel-rich exhaust gas having an air-fuel ratio of less than 14.5 discharged from an internal combustion engine such as a motorcycle. Can be used by attaching to the outlet of

Claims (6)

An exhaust gas purifying catalyst used for fuel-rich exhaust gas having an air-fuel ratio (A / F) of less than 14.5,
(1) cerium oxide,
(2) zirconium oxide,
(3) Aluminum oxide,
(4) yttrium oxide and / or magnesium oxide, and
(5) contains noble metals,
An exhaust gas purifying catalyst, wherein the total concentration of yttrium and / or magnesium is 3.0 wt% to 5.0 wt% with respect to the total amount of the catalyst.   The exhaust gas purifying catalyst according to claim 1, which is produced by impregnating (5) with a composite oxide in which (1)-(4) is uniformly mixed.   The exhaust gas purifying catalyst according to claim 1, wherein the noble metal is at least one selected from the group consisting of platinum, rhodium and palladium. A metal carrier;
The exhaust gas purifying catalyst according to any one of claims 1 to 3 coated on the surface of the metal carrier.
An exhaust gas purification material used for fuel-rich exhaust gas having an air-fuel ratio (A / F) of less than 14.5. (5) a step of impregnating a noble metal with a composite oxide containing (1) cerium oxide, (2) zirconium oxide, (3) aluminum oxide, and (4) yttrium oxide and / or magnesium oxide. The manufacturing method of the catalyst for exhaust gas purification in any one of Claims 1-4 .   After preparing each suspension of (1)-(4) and water to prepare a suspension, a coprecipitate is produced from the suspension by a coprecipitation method, and the coprecipitate is baked to produce the suspension. The method for producing an exhaust gas purifying catalyst according to claim 5, further comprising a step of obtaining a composite oxide containing (1)-(4).