JP2005224705A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce a catalyst in which the specific surface area of an oxide carrier can be restrained from being reduced and the grain growth of a noble metal to be deposited on the oxide carrier can also be restrained. <P>SOLUTION: This catalyst is composed of the oxide carrier the specific surface area of which, when heat-treated in an oxidizing atmosphere, is substantially equal to that when heat-treated in a reducing atmosphere and the noble metal deposited on the oxide carrier. Since the extent of reduction of the specific surface area in the reducing atmosphere is substantially equal to that of reduction of the specific surface area in the oxidizing atmosphere, the reduction of the specific surface area can be restrained in comparison with the conventional oxide carrier. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purifying catalyst in which a noble metal is supported on an oxide carrier, and more particularly to an exhaust gas purifying catalyst capable of suppressing noble metal grain growth at high temperatures.

Exhaust gas from automobiles contains harmful components such as CO, HC, and NO _x , and various exhaust gas purification catalysts such as oxidation catalysts, three-way catalysts, and NO _x storage reduction catalysts are used to reduce these emissions. It is installed in the exhaust system. These catalysts are mainly used by fixing a noble metal such as Pt as an active species and supporting the noble metal on an oxide carrier such as alumina.

  By the way, in order to extract the activity of the noble metal more, it is necessary to support the noble metal in a fine state to increase the surface area and to increase the number of active sites. Therefore, an aqueous solution of a soluble noble metal salt is used to support the noble metal on the oxide carrier. And, for example, an impregnation supporting method in which a predetermined amount of the noble metal salt aqueous solution is impregnated into the oxide carrier powder and then evaporated to dryness, or a coat layer made of the oxide carrier is formed on the honeycomb substrate, and this is added to the noble metal salt aqueous solution. There is known a supporting method such as a water absorbing supporting method in which the substrate is dipped in and then fired and fired.

However, since the exhaust gas-purifying catalyst is exposed to a high temperature, grain growth may occur on the supported noble metal. For example, Pt becomes PtO ₂ in a high-temperature oxygen-excess atmosphere and easily moves on the support, and Pt existing in the vicinity tends to aggregate and become coarse particles. When the noble metal becomes such coarse particles, there is a problem that the purification activity is lowered due to the decrease in the active points. In particular, when the specific surface area of the carrier supporting the noble metal is reduced due to use in a high temperature range, grain growth of the noble metal is promoted.

  Therefore, in order to suppress the grain growth of the noble metal, it is effective to suppress the decrease in the specific surface area of the carrier. For example, because ceria has oxygen absorption / release capacity (OSC), it is possible to mitigate fluctuations in the atmosphere of exhaust gas by using ceria as a carrier, and exhaust gas that can achieve maximum activity in an atmosphere near stoichiometric, such as a three-way catalyst. The activity of the purification catalyst can be further improved. However, the catalyst using ceria as a carrier has a problem that the specific surface area decreases due to use in a high temperature region, OSC decreases, and the supported noble metal grows, resulting in a decrease in activity.

In view of this, it has been conventionally proposed that a rare earth element oxide other than zirconia or cerium is dissolved in ceria. For example, Japanese Laid-Open Patent Publication No. 04-055315 discloses an oxidation process in which a ceria precursor and a zirconia precursor are co-precipitated from a mixed aqueous solution of a water-soluble salt of cerium (Ce) and a water-soluble salt of zirconium (Zr) and heat-treated. A method for producing a cerium fine powder is disclosed. According to this manufacturing method, when the coprecipitate is heat-treated, CeO ₂ and ZrO ₂ become a composite oxide, and an oxide solid solution in which they are in solid solution is generated.

  JP-A 09-221304 discloses that the average diameter of the crystallite is small and high by adding a surfactant when the oxide precursor is precipitated from an aqueous solution in which the metal salt of Ce and Zr is dissolved. A method for producing a ceria-zirconia solid solution having a solid solubility and high OSC is disclosed.

  Furthermore, Japanese Patent Publication No. 2001-524918 discloses a zirconia that optimally adjusts the particle size distribution, pore volume, etc. by adding a surfactant when the oxide precursor is precipitated from an aqueous solution of a metal salt. Alternatively, a method for producing ceria-zirconia solid solution or the like is disclosed.

  However, even if it is a ceria-zirconia-based composite oxide, a decrease in specific surface area in a high temperature region is inevitable. As a result of further research, it is clear that there is a difference in the degree of decrease in the specific surface area between the oxidizing atmosphere and the reducing atmosphere, and it is known that the specific surface area is more likely to decrease in the reducing atmosphere.

  The reason for this is not clear, but in consideration of the contents described in Japanese Patent Laid-Open No. 10-218668, it is considered as follows. Japanese Patent Application Laid-Open No. 10-218668 describes that sintering is promoted when a metal oxide having oxygen nonstoichiometry including ceria is repeatedly heated in an atmosphere in which the oxygen partial pressure varies. The reason is described that oxygen atoms move from the metal oxide due to fluctuations in the oxygen partial pressure, and the movement of the metal elements is also activated accordingly.

Taking this into consideration, the metal oxide having oxygen non-stoichiometry containing ceria has the largest valence in the atmosphere rich in oxygen, so it releases oxygen by heating in an oxygen-deficient reducing atmosphere, It is considered that the movement of the metal element becomes active and the sintering is promoted, thereby reducing the specific surface area. Furthermore, since the exhaust gas of automobiles is in a state in which an oxidizing atmosphere and a reducing atmosphere are repeated, a catalyst in which a noble metal is supported on a metal oxide having an oxygen non-stoichiometry containing ceria has a reduced specific surface area under actual use conditions. It is easy to cause noble metal grain growth.
JP 04-055315 JP 09-221304 A Special table 2001-524918 JP-A-10-218668

  The present invention has been made in view of the above-described circumstances, and an object to be solved is to provide a catalyst that can suppress a decrease in the specific surface area of an oxide carrier and can also suppress grain growth of a supported noble metal.

  The exhaust gas purifying catalyst of the present invention that solves the above problems is characterized in that the specific surface area when heat-treated in an oxidizing atmosphere and the specific surface area when heat-treated in a reducing atmosphere are substantially the same, and the oxidation support thereof And a noble metal supported on a material carrier.

  The oxide support may or may not contain a transition metal oxide having oxygen nonstoichiometry. When the oxide support includes a transition metal oxide having oxygen nonstoichiometry, the transition metal oxide is preferably ceria.

  According to the exhaust gas purifying catalyst of the present invention, even when used in an exhaust gas of an automobile in which an oxidizing atmosphere and a reducing atmosphere are repeated, the degree of decrease in the specific surface area of the oxide carrier is small and is supported on the carrier. Grain growth of precious metal is suppressed. Therefore, the initial high activity can be maintained for a long time, and the durability is excellent.

  The exhaust gas-purifying catalyst of the present invention uses an oxide carrier in which the specific surface area when heat-treated in an oxidizing atmosphere and the specific surface area when heat-treated in a reducing atmosphere are substantially the same. Conventional oxide carriers have a difference in the degree of reduction of the specific surface area between the oxidizing atmosphere and the reducing atmosphere, and the specific surface area tends to decrease when heated in a reducing atmosphere. Therefore, when used in the exhaust gas of an automobile in which an oxidizing atmosphere and a reducing atmosphere are repeated, the specific surface area is reduced to some extent in the oxidizing atmosphere, and the specific surface area is further reduced in the reducing atmosphere. Resulting in.

  However, in the exhaust gas purifying catalyst of the present invention, since the degree of reduction of the specific surface area in the reducing atmosphere is substantially the same as that in the oxidizing atmosphere, the reduction of the specific surface area can be suppressed as compared with the conventional case.

  The oxide carrier is not particularly limited as long as the specific surface area when heat-treated in an oxidizing atmosphere and the specific surface area when heat-treated in a reducing atmosphere are substantially the same, and alumina, titania, ceria, zirconia, silica, magnesia A simple substance or a mixture selected from the above or a composite oxide composed of a plurality of kinds selected from these can be used. The method for producing the oxide carrier is not particularly limited, such as a precipitation method, a coprecipitation method, a sol-gel method, and a thermal decomposition method.

Here, “the specific surface area when heat-treated in an oxidizing atmosphere and the specific surface area when heat-treated in a reducing atmosphere” is substantially the same as the specific surface area when heat-treating in an oxidizing atmosphere and the heat-treated temperature in a reducing atmosphere. Although the absolute value of the difference from the surface area is small, the difference is relative. For example, even if the difference between the specific surface area when heat-treated in an oxidizing atmosphere and the specific surface area when heat-treated in a reducing atmosphere is 10 m ² / g, the specific surface area before heat treatment is 100 m ² / g and 20 m ² / g. As for the thing, the influence with respect to the particle size of a noble metal differs, and it is thought that the latter thing has a larger influence. Therefore, the ratio of the absolute value of the difference between the specific surface area when heat-treated in an oxidizing atmosphere and the specific surface area when heat-treated in a reducing atmosphere to the specific surface area when heat-treated in an oxidizing atmosphere is preferably used as an index. Is 10% or less, more preferably 5% or less.

The oxide support may or may not contain a transition metal oxide having oxygen non-stoichiometry, but when it is contained, the effect of the present invention is maximized. NiO, Fe ₂ O ₃ , CuO, Mn ₂ O ₅ , CeO _{2 and the} like are exemplified as transition metal oxides having oxygen non-stoichiometry, but CeO ₂ having a high OSC is used as a carrier for exhaust gas purification catalysts. since particularly preferred, CeO _2, CeO ₂ -ZrO ₂ composite oxide as the oxide support, it is desirable to use those containing CeO _2, etc. CeO ₂ -ZrO ₂ -Y ₂ O ₃ composite oxide.

  Further, as the noble metal supported on the oxide carrier, at least one selected from Pt, Rh, Pd, Ir and the like can be used, but it is preferable to contain at least Pt which has a particularly high oxidation activity and easily grows grains. The amount of the precious metal supported is generally about 0.1 to 20 g per liter of catalyst volume.

  In order to produce the oxide carrier referred to in the present invention, for example, a first step of obtaining a precipitate of an oxide precursor by adding an alkaline substance to an aqueous solution in which a compound containing a metal element to be an oxide is dissolved; A production method for sequentially performing a second step of washing the precipitate, a third step of stirring the washed precipitate in water together with a surfactant, and a fourth step of firing the precipitate after the third step. Can be used. Even if this production method is used, depending on the composition ratio of the raw materials or production conditions, oxide carriers having substantially different specific surface areas when heat-treated in an oxidizing atmosphere and specific surface areas when heat-treated in a reducing atmosphere are produced. There is a case.

  In this manufacturing method, first, in the first step, an oxide precursor precipitate is formed by adding an alkaline substance to an aqueous solution in which a compound containing a metal element to be an oxide is dissolved.

  As an aqueous solution in which a compound containing a metal element that becomes an oxide is dissolved, an aqueous solution of a water-soluble compound containing a metal such as Zr, Ce, or Y can be used. For example, when the metal is Ce or Zr, cerium nitrate is used. An aqueous solution of (III), cerium (IV) ammonium nitrate, cerium (III) chloride, cerium (III) sulfate, cerium (IV) sulfate, zirconium oxynitrate, zirconium oxychloride, or the like can be used.

  Moreover, as an alkaline substance, what shows alkalinity as aqueous solution can be used. Ammonia, which can be easily separated during heating, is particularly desirable. However, other alkaline substances such as alkali metal hydroxides can be used because they can be easily removed by washing with water. The alkaline substance is preferably added as an aqueous solution, which may be gradually added dropwise or mixed all at once. In order to prevent segregation of precipitates, it is desirable to add an aqueous solution in which a metal element is dissolved with stirring.

When producing a CeO ₂ —ZrO ₂ composite oxide, it is necessary to pay attention to the valence of cerium. In the case of tetravalent cerium, there is no problem because CeO ₂ dissolves relatively easily with ZrO ₂ , but in the case of trivalent cerium, Ce ₂ O ₃ is difficult to dissolve in ZrO ₂ . For example, it is desirable to coexist hydrogen peroxide in an aqueous solution. In this way, cerium (III) forms a complex with hydrogen peroxide and is oxidized to cerium (IV), so that CeO ₂ can be easily dissolved in ZrO ₂ .

The amount of hydrogen peroxide added is desirably ¼ or more of cerium ions. If the added amount of hydrogen peroxide is less than 1/4 of the cerium ion, the solid solution of CeO ₂ and ZrO ₂ becomes insufficient. The excessive addition of hydrogen peroxide has no particular adverse effect, but it is disadvantageous in terms of economy, has no merit, and is preferably in the range of 1/2 to 2 times that of cerium ions. In addition, the addition timing in particular of hydrogen peroxide is not restrict | limited, It may be before the addition of an alkaline substance, and can also be added simultaneously or after it. Hydrogen peroxide is a particularly desirable oxidizing agent because it does not require post-treatment, but in some cases, other oxidizing agents such as oxygen gas, peroxides such as ozone, perchloric acid and permanganic acid can be used. .

Further, it is also preferable to add an alkaline substance while stirring an aqueous solution in which a compound containing a metal element as an oxide is dissolved at a high shear rate of 10 ³ sec ⁻¹ or more, desirably 10 ⁴ sec ⁻¹ or more. Since the components in the precipitated fine particles, which are neutralized products, cannot avoid a certain degree of segregation, the segregation can be made uniform and the dispersibility can be improved by vigorous stirring. For example, when co-precipitated from an aqueous solution of cerium salt and zirconium salt, the same kind of precipitated particles are likely to be a group because the pH at which they precipitate is different. Therefore, high-speed stirring at a high shear rate destroys a group of precipitated fine particles of the same type and improves the contact degree between the CeO ₂ precursor and the ZrO ₂ precursor, so that the precipitated particles are mixed well. Therefore, the solid solubility of the CeO ₂ —ZrO ₂ solid solution can be improved and the average diameter of the crystallite can be reduced. When the shear rate is less than 10 ³ sec ⁻¹ , the solid solution promoting effect is not sufficient. The shear rate V is expressed by V = v / D. Here, v is a speed difference (m / sec) between the rotor and the stator of the stirrer, and D is a gap (m) between the rotor and the stator.

  In the conventional coprecipitation method, a compound containing a metal element that becomes an oxide, an alkaline substance, and a surfactant coexist in an aqueous solution. For this reason, there has been a problem that the property of the surfactant is changed by the pH change when the alkaline substance is added, and the effect of adding the surfactant is impaired.

  Therefore, in this production method, the precipitate of the oxide precursor is washed in the second step, and the washed precipitate is stirred in water together with the surfactant in the third step. Since the alkaline substance and free acid are washed away from the oxide precursor by washing in the second step, the surfactant is not exposed to pH change in the third step. Therefore, the effect of adding the surfactant is maximized, and a metal oxide having a pore volume of 0.1 cc / g or more and a pore volume of 0.2 cc / g or more can be produced by firing in the fourth step.

  The action of the surfactant is not clear, but is presumed as follows. That is, in the state just neutralized with the alkaline substance, the metal element that becomes an oxide precipitates in a very fine hydroxide or oxide state having a particle size of several nm or less. Then, the aggregation of the primary particles proceeds to some extent during the cleaning in the second step and becomes secondary particles.

  By adding the surfactant in the third step, the secondary particles are taken into the micelles of the surfactant, and the particles grow in the concentrated small space by aggregating and aging in the micelles. Progresses. Furthermore, the dispersibility of the secondary particles is improved by the dispersing effect of the surfactant. By these actions, it is considered that a metal oxide powder having a pore diameter of 0.1 袖 m or less and a pore volume of 0.2 cc / g or more can be produced.

  The addition amount of the surfactant in the third step is in a range of 2 to 40% by weight with respect to the obtained metal oxide powder, that is, metal oxide powder: surfactant = 98 to 60: 2 to 40 in a weight ratio. The range of is preferable. If the added amount of the surfactant is less than 2% by weight, the effect of the addition is small. If the added amount exceeds 40% by weight, the dispersibility of the oxide precursor is reduced due to the aggregation of the surfactants, and firing in the fourth step Occasionally, the amount of heat generated by the combustion of the surfactant increases, so that the metal oxide aggregates and the specific surface area decreases.

The amount of water in the third step is preferably about the same as the amount of water in the first step, but is not particularly limited as long as it can be stirred. Further, the stirring speed in the third step is preferably 1000 sec ⁻¹ or more, and it is preferable to stir at a temperature of 10 to 30 ° C. for 5 minutes or more. If the shearing force by stirring is too large, heat will be generated or the apparatus will be exhausted excessively. If the shearing force is too small, the surfactant may not be sufficiently dispersed. Further, if the temperature during stirring is lower than this range, the stirring time becomes longer, and if the temperature is higher than this range, heat generation or exhaustion of the apparatus occurs.

  As the surfactant, any of anionic, cationic, and nonionic surfactants can be used. Among them, the micelle to be formed can easily form a narrow space inside, for example, a spherical micelle. A surfactant is desirable. A surfactant having a critical micelle concentration (cmc) of 0.1 mol / liter or less is desirable, and a surfactant having a concentration of 0.01 mol / liter or less is desirable. The critical micelle concentration (cmc) is the lowest concentration at which a surfactant forms micelles.

Examples of these surfactants include anionic surfactants such as alkylbenzene sulfonic acid and its salt, α-olefin sulfonic acid and its salt, alkyl sulfate ester salt, alkyl ether sulfate ester salt, phenyl ether sulfate ester salt, Methyl taurate, sulfosuccinate, ether sulfate, alkyl sulfate, ether sulfonate, saturated fatty acid and its salt, unsaturated fatty acid such as oleic acid and its salt, other carboxylic acid, sulfonic acid, sulfuric acid, phosphorus Acids, phenol derivatives, etc .;
Nonionic surfactants such as polyoxyethylene polypropylene alkyl ether, polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene polystyryl phenyl ether, polyoxyethylene polyoxypolypropylene alkyl ether, polyoxyethylene poly Polyhydric alcohols such as oxypropylene glycol, glycol, glycerin, sorbitol, mannitol, pentaerythritol, sucrose, fatty acid partial ester of polyhydric alcohol, polyoxyethylene fatty acid partial ester of polyhydric alcohol, polyoxyethylene fatty acid of polyhydric alcohol Ester, polyoxyethylenated castor oil, polyglycene fatty acid ester, fatty acid diethanolamide, polyoxyethylene alkyl Min, triethanolamine fatty acid partial esters, trialkylamine oxide and the like;
Cationic surfactants, fatty acid primary amine salt, fatty acid secondary amine salt, fatty acid tertiary amine salt, tetraalkylammonium salt, trialkylbenzylammonium salt, alkylpyrodinium salt, 2-alkyl-1- Quaternary ammonium salts such as alkyl-1-hydroxyethylimidazolinium salts, N, N-dialkylmorpholinium salts, polyethylene polyamines, fatty acid amide salts, etc .;
At least one selected from betaine compounds and the like that are amphoteric surfactants can be used.

  In a 4th process, the metal element in a precipitate is made into an oxide by baking the precipitate of the oxide precursor after a 3rd process. The firing atmosphere may be any of an oxidizing atmosphere, a reducing atmosphere, and a neutral atmosphere. The reason why the metal element in the precipitate becomes an oxide is that oxygen contained in water or the like of the aqueous solution used as a raw material is involved, and the metal element in the precipitate is oxidized during firing. Therefore, a metal oxide can be obtained even when fired in a reducing atmosphere. The firing temperature is preferably 150 to 800 ° C. When the firing temperature is lower than 150 ° C, it takes a long time for firing. When the firing temperature is higher than 800 ° C, the specific surface area decreases.

  Moreover, it is also preferable to perform a precipitate washing step between the third step and the fourth step. As a result, the surfactant can be removed, and heat generation due to combustion of the surfactant during firing can be avoided.

  In many cases, the oxide produced by the above production method has substantially the same specific surface area when heat-treated in an oxidizing atmosphere and specific surface area when heat-treated in a reducing atmosphere.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

(Example 1)
In a 3 liter beaker, stirring a solution of 900.0 g of ion exchange water and 358.0 g of 25% aqueous ammonia with a propeller stirrer, 589.7 g of cerium nitrate aqueous solution, 437.3 g of zirconium oxynitrate aqueous solution, and yttrium nitrate powder A solution in which 33.5 g and 199.5 g of 30% hydrogen peroxide solution were mixed was added to obtain a precipitate of an oxide precursor. The composition ratio of each metal was set to be Ce: Zr: Y = 60: 40: 5 in molar ratio.

  The resulting precipitate was centrifuged to discard the supernatant, and the same amount of ion-exchanged water as the discarded supernatant was added thereto, stirred, and again centrifuged. This operation was further performed twice to wash the precipitate.

  Finally, the precipitate after discarding the supernatant was again transferred to a 3 liter beaker, 900 g of ion exchange water was added, and the mixture was stirred using a propeller stirrer and a homogenizer. Thereto were added 21.9 g of a cationic surfactant (“Armac” Lion) and 21.9 g of an anionic surfactant (“Armoflow” Lion), and the mixture was further stirred for 5 minutes. This dispersion was centrifuged and the precipitate was washed in the same manner as described above.

Finally, the precipitate after discarding the supernatant is calcined at 400 ° C. for 5 hours in the atmosphere using a degreasing furnace, and further calcined at 700 ° C. for 5 hours in the atmosphere to obtain CeO ₂ —ZrO ₂ —Y ₂ O _3. A composite oxide powder was prepared.

Using this CeO ₂ —ZrO ₂ —Y ₂ O ₃ composite oxide powder, heat treatment was carried out in an oxidizing atmosphere heated at 1000 ° C. for 5 hours while circulating air humidified by bubbling. ”(Manufactured by Micro Data Co., Ltd.). In addition, CeO ₂ —ZrO ₂ solid solution powder that has not been heat-treated is used, and heat treatment is performed in a reducing atmosphere heated at 1000 ° C. for 5 hours while circulating 1% H ₂ / N ₂ gas humidified by bubbling. The specific surface area of was measured in the same manner. The results are shown in Table 1.

Next, CeO ₂ —ZrO ₂ —Y ₂ O ₃ composite oxide powder not subjected to the above heat treatment is impregnated with a predetermined amount of a dinitrodiaminoplatinum nitric acid solution having a predetermined concentration, and at 110 ° C. in air for 12 hours. After drying, it was calcined at 250 ° C. for 1 hour to carry 1% by weight of Pt.

Using the obtained catalyst powder, heat treatment was performed in a reducing atmosphere in which 1% H ₂ / N ₂ gas humidified by bubbling was circulated and heated at 1000 ° C. for 5 hours. Thereafter, the particle diameter of the supported Pt was measured using the measuring apparatus shown in FIG. The results are shown in Table 2.

  This measuring apparatus includes a reaction tube 1, a gas supply unit 2, a cooling unit 3, and a detection unit 4. The reaction tube 1 has a U-shaped cylindrical shape made of quartz glass and is packed with catalyst powder 10 inside. The reaction tube 1 is provided with a temperature sensor 11 so that the temperature of the catalyst powder 10 can be detected.

The gas supply means 2 includes four types of gas cylinders 21 to 24, and is connected to one end opening of the reaction tube 1 through a switching cock 20. The gas cylinder 21 is filled with He gas containing 5% O ₂ , the gas cylinder 22 is filled with He gas containing 10% H ₂ , the gas cylinder 23 is filled with He gas, and the gas cylinder 24 is filled with CO gas. Is filled. The gas cylinder 24 is connected to the gas flow path of the gas cylinder 23 via the switching cock 25.

  The cooling means 3 is composed of a container in which a refrigerant cooled to about -80 ° C. by putting dry ice into ethanol is put, and a portion packed with the catalyst powder 10 of the reaction tube 1 is immersed in the refrigerant. Yes. The other end of the reaction tube 1 is connected to a detection means 4 comprising a gas chromatograph and a mass spectrometer so that the CO concentration in the output gas can be quantified.

First, a heater was disposed in place of the cooling means 3. Next, 0.5 g of the above-mentioned heat-treated catalyst powder is packed in the reaction tube 1 and He gas is circulated from the gas cylinder 23 at a flow rate of 30 ml / min so that the temperature of the catalyst powder 10 becomes 400 ° C. with a heater. Heated. While maintaining the temperature of the catalyst powder 10 at 400 ° C., He gas containing 5% O ₂ is supplied from the gas cylinder 21 for 15 minutes, and then He gas containing 10% H ₂ is supplied from the gas cylinder 22 for 15 minutes. Thereafter, a pretreatment for supplying He gas from the gas cylinder 23 for 15 minutes was performed. In both cases, the gas flow rate is 30 ml / min.

  After the pretreatment, the heater was removed, and the catalyst powder 10 was cooled while flowing He gas from the gas cylinder 23 at a flow rate of 30 ml / min. When the temperature of the catalyst powder 10 was close to room temperature, the cooling means 3 was arranged as shown in FIG. 1 and the reaction tube 1 was immersed in the refrigerant to further cool the catalyst powder 10.

  In a state where the temperature of the catalyst powder 10 is −78 ° C., a predetermined amount of CO gas is supplied in pulses from the gas cylinder 24 into the He carrier, and the CO concentration in the output gas at that time is measured by the detection means 4. The amount of CO in the outgas was calculated. Then, the amount of CO adsorbed by the catalyst powder 10 was calculated from the difference between the supplied CO amount and the CO amount in the output gas. Further, from the calculated CO adsorption amount, the Pt particle size was calculated as follows, and the results are shown in Table 2.

When the surface area S of the noble metal supported is defined as a being the lattice constant of the noble metal, a single adsorbed gas molecule is adsorbed to a ² .
S = number of gas molecules adsorbed on 1 g of sample × a ²
= K × 6.02 × 10 ²² × a ²
(K is the number of moles of adsorbed gas)
Assuming that the particle is a cube (hexahedron) having a side length of D (m), the effective surface area (s) of one particle has five faces that do not contact the carrier.
s = 5D ²
If the volume of one noble metal particle is v,
v = D ³
If the number of noble metal particles in 1 g of the sample is n, the volume Vc of the noble metal in the sample of 1 g of sample is
Vc = nv = nD ³
Therefore, the surface area S of the noble metal per gram of sample is
S = ns = n5D ² By transforming this equation, nD ² = S / 5
From Vc = nD ³ and this equation, D = Vc / (nD ² ) = 5 Vc / S.

On the other hand, when the content (%) of the noble metal is C, it is C / 100, and the volume Vc of the noble metal in 1 g of the sample is d, where the specific gravity of the noble metal is d.
Since Vc = (C / 100) / d,
D = 5Vc / S = (5C / 100d) / S,
By measuring S, D can be calculated.

(Example 2)
ZrO ₂ —Y ₂ O was used in the same manner as in Example 1 except that 534.0 g of zirconium oxynitrate aqueous solution, 15.7 g of yttrium nitrate powder, and 137.3 g of 25% aqueous ammonia were used without using the cerium nitrate aqueous solution. _Three composite oxide powders were prepared.

Using this ZrO ₂ —Y ₂ O ₃ composite oxide powder, heat treatment was performed in an oxidizing atmosphere and a reducing atmosphere in the same manner as in Example 1, and the specific surface area thereafter was measured. The results are shown in Table 1. Further, Pt was supported on the ZrO ₂ —Y ₂ O ₃ composite oxide powder in the same manner as in Example 1. The supported amount of Pt was 0.63% by weight so that the supported amount per specific surface area after heat treatment in the reducing atmosphere was equivalent to that in Example 1. Then, using the obtained catalyst powder, heat treatment in a reducing atmosphere was performed in the same manner as in Example 1, and then the Pt particle size was measured in the same manner. The results are shown in Table 2.

(Comparative Example 1)
CeO ₂ —ZrO ₂ —Y As in Example 1, except that 884.6 g of cerium nitrate aqueous solution, 109.3 g of zirconium oxynitrate aqueous solution, 33.5 g of yttrium nitrate powder, and 396.5 g of 25% aqueous ammonia were used. ₂ O ₃ composite oxide powder was prepared. The composition ratio of each metal is Ce: Zr: Y = 90: 10: 5 in molar ratio.

Using this CeO ₂ —ZrO ₂ —Y ₂ O ₃ composite oxide powder, heat treatment was performed in an oxidizing atmosphere and a reducing atmosphere in the same manner as in Example 1, and the specific surface area thereafter was measured. The results are shown in Table 1. Further, Pt was supported on the CeO ₂ —ZrO ₂ —Y ₂ O ₃ composite oxide powder in the same manner as in Example 1. The amount of Pt supported is 1% by weight. Then, using the obtained catalyst powder, heat treatment in a reducing atmosphere was performed in the same manner as in Example 1, and then the Pt particle size was measured in the same manner. The results are shown in Table 2.

  <Evaluation>

  As is apparent from the table, the oxide support of Comparative Example 1 has a large difference between the specific surface area when heat-treated in an oxidizing atmosphere and the specific surface area when heat-treated in a reducing atmosphere, and the specific surface area is increased by heat treatment in a reducing atmosphere. Is significantly reduced. On the other hand, the specific surface area when heat-treated in an oxidizing atmosphere and the specific surface area when heat-treated in a reducing atmosphere of the oxide carriers of each Example are substantially the same.

  And in the catalyst of each Example, it is clear that Pt grain growth during heat treatment in a reducing atmosphere is suppressed, which is the specific surface area when heat-treated in an oxidizing atmosphere and when heat-treated in a reducing atmosphere. It is clear that this is an effect obtained by using an oxide support having substantially the same specific surface area.

Further, from Examples 1 and 2, the specific surface area when heat-treated in an oxidizing atmosphere and a reducing atmosphere, whether or not containing CeO ₂ , which is a transition metal oxide having oxygen nonstoichiometry, in the oxide carrier, It can also be seen that the specific surface area when heat-treated with can be made substantially the same.

In the exhaust gas purifying catalyst of the present invention, the increase in the specific surface area in the actual exhaust gas is suppressed, and the noble metal grain growth is suppressed. Therefore, the oxidation catalyst, the three-way catalyst, the NO _x selective reduction catalyst, the NO _x occlusion reduction It can be used for various catalysts such as a catalyst, a filter catalyst and a hydrogen generation catalyst.

It is a schematic explanatory drawing which shows the measuring apparatus used in one Example of this invention.

Explanation of symbols

1: Reaction tube 2: Gas supply means 3: Cooling means 4: Detection means 10: Catalyst powder

Claims (4)

  A specific surface area when heat-treated in an oxidizing atmosphere and an oxide carrier having substantially the same specific surface area when heat-treated in a reducing atmosphere, and a noble metal supported on the oxide carrier, Exhaust gas purification catalyst.   The exhaust gas-purifying catalyst according to claim 1, wherein the oxide carrier includes a transition metal oxide having oxygen nonstoichiometry.   The exhaust gas-purifying catalyst according to claim 1, wherein the oxide carrier does not contain a transition metal oxide having oxygen nonstoichiometry.   The exhaust gas-purifying catalyst according to claim 2, wherein the transition metal oxide is ceria.

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