JP7310539B2 - Exhaust gas purification catalyst and exhaust gas purification device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying device, and more particularly to an exhaust gas purifying catalyst made of a platinum composite oxide and an exhaust gas purifying device using the same.

Exhaust gases emitted from internal combustion engines contain harmful substances such as CO, hydrocarbons (HC), nitrogen oxides (NO _x ) and particulate matter (PM). If such harmful substances are released into the atmosphere as they are, they cause air pollution and health hazards. Therefore, attempts have been made to purify harmful substances contained in the exhaust gas using various catalysts.

For example, in Non-Patent Document 1, although it is not a CO oxidation catalyst,
(a) preparing a calcined catalyst in which Pt particles are highly dispersedly supported on the surface of cobalt oxide (Co ₃ O ₄ ) by an impregnation method;
(b) A Pt/Co ₃ O ₄ -based catalyst obtained by reducing a calcined catalyst in hydrogen is disclosed.

In the same document,
(A) when the calcined catalyst is treated in hydrogen, an intermetallic compound Pt ₃ Co is formed;
(B) the reduced surface species are readily oxidized upon exposure to an oxidizing atmosphere; and
(C) It is described that strong CO adsorption on Pt can be suppressed by changing the electronic band structure of Pt.

Moreover, in Patent Document 1,
(a) zirconia powder, platinum nitrate, praseodymium nitrate, yttrium nitrate, and cobalt nitrate are dispersed in an aqueous solution and then dried at 150° C. for 2 hours;
(b) A diesel engine exhaust gas purifying catalyst obtained by calcining a dry powder at 500° C. for 1 hour is disclosed.

In the same document, with such a catalyst,
(A) capable of decomposing NO _x in exhaust gas containing a large amount of oxygen at low temperatures;
(B) can burn off untwisted hydrocarbons, carbon monoxide, and particulate matter in the exhaust gas, and
(C) Since the oxidation reaction of SO ₂ in the exhaust gas can be suppressed, the amount of sulfate produced by the oxidation of SO ₂ can be reduced.
is described.

The Pt/Co ₃ O ₄ -based catalyst described in Non-Patent Document 1 has the function of oxidizing CO weakly adsorbed on the Pt surface. However, the Pt/Co ₃ O ₄ group catalyst has a large CO oxidation overvoltage and low activity as a CO oxidation catalyst.

On the other hand, in the exhaust gas purifying catalyst described in Patent Document 1, only metal platinum is an active species, and this is responsible for the CO oxidation reaction and the oxygen reduction reaction, which is the accompanying reduction reaction. However, the surface of platinum, which is an active species, takes either a metal or an oxide state depending on the temperature and exhaust gas atmosphere. For CO oxidation, the activity in the oxide state is high and the activity in the metal state is low. The oxygen reduction reaction is the opposite, with high activity in the metal state and low activity in the oxide state. For this reason, it is impossible to achieve both high activity for the CO oxidation reaction and the accompanying oxygen reduction reaction.

Japanese Patent Application Laid-Open No. 09-308829

T. Matui, et al., "Electrochemical CO Oxidation and Microstructure in Pt/Co3O4-Based Catalysts," J. Electrochem. Soc., 156, K128(2009)

The problem to be solved by the present invention is to provide an exhaust gas purifying catalyst having high CO oxidation ability.
Another problem to be solved by the present invention is to provide an exhaust gas purifying apparatus using such an exhaust gas purifying catalyst.

In order to solve the above problems, an exhaust gas purifying catalyst according to the present invention has the following configuration.
(1) The exhaust gas purifying catalyst is
a carrier made of a metal oxide;
and active points formed on the surface of the carrier.
(2) the active point is
an active point (A) made of a platinum composite oxide containing platinum and metal M;
and an active point (B) made of metal platinum.
(3) The platinum composite oxide is a platinum bronze having a composition represented by the following formula (1) and/or a platinum-containing layered rock salt type composite oxide having a composition represented by the following formula (2) Including things.
_{CoxPt3 -yO4 -z} (1)
However, 0<x≤1, 0≤y≤1, 0≤z≤1.
M'PtO2 ( ₂ )
However, M' is Co, or Co and Mn.

The exhaust gas purifier according to the present invention includes:
Honeycomb and
and an exhaust gas purifying catalyst according to the present invention filled in the gas flow path of the honeycomb.

The active site (A) made of platinum composite oxide has high activity for CO oxidation reaction. On the other hand, the active site (B) made of metal platinum has high activity for the oxygen reduction reaction that accompanies the CO oxidation reaction. Therefore, when two types of active sites (A) and (B) having different functions are arranged close to the carrier surface, the CO oxidation reaction is promoted.

1 is a schematic cross-sectional view of an exhaust gas purifying catalyst according to the present invention; FIG. CO oxidation voltammograms of CoPtO ₂ , Co—Pt bronze, and Pt/Vulcan®. 2 is a CO oxidation voltammogram at a high potential of Co—Pt bronze without surface metal platinization treatment. FIG. 2 is a CO oxidation voltammogram at high potential of CoPtO ₂ without surface metal platinization treatment. FIG. It is a temperature profile during an exhaust gas purification test. CO: 1000 ppm-O ₂ : 10%-H ₂ O: 0% gas atmosphere conditions (CO oxidation (a)) are the CO conversion rates of various catalysts.

CO: 1000 ppm-O ₂ : 10%-H ₂ O: 1% gas atmosphere conditions (CO oxidation (b)) are the CO conversion rates of various catalysts. CO: 1000 ppm-O ₂ : 10%-H ₂ O: 3% gas atmosphere conditions (CO oxidation (c)) are the CO conversion rates of various catalysts. CO: 1000 ppm-O ₂ : 10%-H ₂ O: 5% gas atmosphere conditions (CO oxidation (d)) are the CO conversion rates of various catalysts. CO: 1000 ppm-O ₂ : 1%-H ₂ O: 5% gas atmosphere conditions (CO oxidation (e)) are the CO conversion rates of various catalysts.

An embodiment of the present invention will be described in detail below.
[1. Exhaust gas purification catalyst]
The exhaust gas purifying catalyst according to the present invention is
a carrier made of a metal oxide;
and active points formed on the surface of the carrier.

[1.1. carrier]
[1.1.1. material]
The carrier consists of a metal oxide. Any metal oxide may be used as long as it has heat resistance enough to withstand the environment in which the exhaust gas purifying catalyst is used. Metal oxides may be electronic conductors or may be insulators. Since the CO oxidation reaction and the oxygen reduction reaction are carried out via active sites that are electronic conductors, the carrier does not necessarily have to be an electronic conductor. Also, the carrier may be a solid material or a porous material.
Examples of carrier materials include alumina, ceria, zirconia, and titania.

[1.1.2. Average particle size]
"Average particle size" refers to the median value ( _d50 ) of particle sizes measured using a laser diffraction scattering method.
The average particle size of the carrier is not particularly limited, and an optimum value can be selected depending on the purpose. In general, when the average particle diameter of the carrier is too small, the carrier grains tend to grow easily. Therefore, the average particle size of the carrier is preferably 0.01 μm or more. The average particle size is preferably 0.1 μm or more, more preferably 0.5 μm or more.

On the other hand, if the average particle size of the carrier is too large, the specific surface area tends to become small, making it impossible to obtain sufficient catalytic activity. Therefore, the average particle size of the carrier is preferably 100 μm or less. The average particle size is preferably 50 μm or less, more preferably 10 μm or less.

[1.2. active point]
The active point is
an active point (A) made of a platinum composite oxide containing platinum and metal M;
and an active point (B) made of metal platinum.

[1.2.1. Active point (A)]
In the present invention, the term "active site (A)" refers to a platinum composite oxide containing platinum and metal M, which is active with respect to CO oxidation reaction. Such platinum composite oxides include platinum bronze, platinum-containing layered rock salt type composite oxides, and the like. The active site (A) may be one of these, or may be two or more.

[A. Formation of active sites (A) on carrier surface]
The phrase "active sites (A) are formed on the surface of the carrier" means that particles of the platinum composite oxide are supported on the surface of the carrier. In this case, the platinum composite oxide may be supported on the outer surface of the solid carrier, or may be supported on the inner surfaces of the pores of the porous carrier.

[B. platinum bronze]
"Platinum bronze" refers to a composite metal oxide containing platinum, oxygen, and ions of metal element M as a third component. The compositional formula of platinum bronze is generally represented by M _x Pt _3-y O _4-z . The space group of platinum bronze is Pm3n, where the platinum atom is located at the 6c site, the oxygen atom at the 8e site, and the metal element M at the 2a site. The amount x of the metal element M varies depending on synthesis conditions (amount of metal charged, synthesis temperature, etc.). x is usually 0<x≦1.
Although y and z are zero in the basic composition, as long as the crystal structure can be maintained and the electrical neutrality can be maintained as a composition, they can be changed by adjusting the charging amount and processing conditions. can. y is usually 0≤y≤1. Also, z is usually 0≦z≦1.

In the present invention, as the active site (A), at least one of platinum bronze (that is, Co—Pt bronze) having a composition represented by the following formula (1), or a platinum-containing layered rock salt type composite oxide described later including. Co—Pt bronze is suitable as the active site (A) because it has a high activity for CO oxidation reaction.
_{CoxPt3 -yO4 -z} (1)
However, 0<x≤1, 0≤y≤1, 0≤z≤1.

[C. Platinum-Containing Layered Rock Salt Type Composite Oxide]
"Platinum-containing layered rock salt type composite oxide" refers to a composite metal oxide containing platinum, oxygen, and ions of a metal element M' as a third component.
The composition formula of the platinum-containing layered rock salt type composite oxide is generally represented by M'PtO ₂ . The space group of the platinum-containing layered rocksalt-type composite oxide is R3m, where the platinum atom is located at the 3a site, the oxygen atom is located at the 8e site, and the metal element M' is located at the 3b site.

In the present invention, the active site (A) includes at least one of a platinum-containing layered rock salt-type composite oxide having a composition represented by the following formula (2), or the platinum bronze described above. A platinum-containing layered rocksalt-type composite oxide containing only Co as M′ or a platinum-containing layered rocksalt-type composite oxide containing both Co and Mn as M′ are both highly active against the oxidation reaction of CO. is suitable as the active point (A).
M'PtO2 ( ₂ )
However, M' is Co, or Co and Mn.

[D. Specific surface area]
The specific surface area of the active site (A) affects the activity of CO for the oxidation reaction. In general, the larger the specific surface area of the active site (A), the higher the effect obtained with a small amount of addition. In order to obtain high activity for CO oxidation reaction, the specific surface area of the active site (A) is preferably 15 m ² /g or more. The higher the specific surface area, the better.

[E. Amount of active sites (A)]
The amount of active sites (A) (that is, the amount of platinum composite oxide supported) is not particularly limited, and an optimum value can be selected according to the purpose.
In general, if the amount of the platinum composite oxide supported is too small, the CO oxidation reaction activity is lowered. Therefore, the supported amount of the platinum composite oxide is preferably 0.01 mass % or more. The supported amount is preferably 0.05 mass % or more, more preferably 0.1 mass % or more.

On the other hand, if the amount of platinum composite oxide carried becomes excessive, the catalytic activity tends to be saturated and the cost tends to rise. Therefore, the supported amount of the platinum composite oxide is preferably 10 mass % or less. The supported amount is preferably 5 mass % or less, more preferably 3 mass % or less.

[1.2.2. Active point (B)]
In the present invention, the active site (B) refers to metal platinum. Metallic platinum has high activity for the oxygen reduction reaction that accompanies the CO oxidation reaction.

[A. Formation of active sites (B) on carrier surface]
"Active sites (B) are formed on the surface of the carrier"
(a) Particles made of a platinum composite oxide are supported on the surface of the carrier, and a metal platinum layer is formed on a part of the surface of the particles made of the platinum composite oxide, or
(b) It means that metal platinum particles are carried on the surface of the carrier in addition to the platinum composite oxide particles.

As will be described later, in order to allow the CO oxidation reaction to proceed efficiently, it is preferred that the accompanying oxygen reduction reaction also proceed efficiently. For this purpose, it is preferable that the active point (A) where the CO oxidation reaction proceeds and the active point (B) where the oxygen reduction reaction proceeds are close to each other. However, when both platinum composite oxide particles and metal platinum particles are supported on the carrier surface, not all the platinum composite oxide particles are close to the metal platinum particles.

On the other hand, the active point (A) is made of platinum composite oxide particles supported on the surface of the carrier, and the active point (B) is a metal platinum layer formed on a part of the surface of the particle made of platinum composite oxide. , the distance between the active point (A) and the active point (B) is close. Therefore, the CO oxidation reaction and the accompanying oxygen reduction reaction can proceed efficiently.

[B. Specific surface area]
When the active sites (B) are metal platinum particles, the specific surface area of the active sites (B) affects the activity for the oxygen reduction reaction. In general, the larger the specific surface area of the active site (B), the higher the effect obtained with a small amount of addition. In order to obtain high activity for the oxygen reduction reaction, the higher the specific surface area of the active site (B), the better.

[C. Amount of active sites (B)]
The amount of active sites (B) is not particularly limited, and an optimum value can be selected according to the purpose. In general, if the amount of active sites (B) is too small relative to the amount of active sites (A), the oxygen reduction reaction becomes rate-determining, and the oxidation reaction of CO does not proceed efficiently.
On the other hand, even if the amount of active sites (B) is increased more than necessary with respect to the amount of active sites (A), there is no difference in the effect and there is no practical benefit.

For example, when the active point (B) consists of a metal platinum layer formed on a part of the surface of a particle made of platinum composite oxide, the ratio of the area of the metal platinum layer to the surface area of the particle made of platinum composite oxide is Define as "coverage". In this case, the coverage is preferably 2% to 50% in order to allow the CO oxidation reaction to proceed efficiently.

[2. Exhaust gas purification device]
The exhaust gas purifier according to the present invention includes:
Honeycomb and
and an exhaust gas purifying catalyst according to the present invention filled in the gas flow path of the honeycomb.

[2.1. Honeycomb]
A honeycomb has a large number of gas passages for exhaust gas to flow. In the present invention, the honeycomb structure is not particularly limited, and an optimum structure can be selected according to the purpose.

[2.2. Exhaust gas purification catalyst]
The gas passages of the honeycomb are filled with the exhaust gas purifying catalyst according to the present invention. The details of the exhaust gas purifying catalyst are as described above, so the description is omitted.
The filling amount of the exhaust gas purifying catalyst is not particularly limited, and an optimum amount can be selected according to the purpose.

[3. Production method of platinum composite oxide (1) -solid phase method-]
Platinum bronzes or platinum-containing layered rock salt-type composite oxides can be produced by various methods. The first method is
(a) a blending step of adding a predetermined amount of raw material of the metal element M or M' (hereinafter collectively referred to as "(M, M') source") to the Pt source;
(b) a heat treatment step of heat-treating the raw material mixture under predetermined conditions to cause a solid phase reaction;
(c) if necessary, a purification step of removing metal platinum by-produced from the reaction product;
(d) a reduction step of subjecting the surface of the platinum composite oxide to reduction treatment, if necessary.

[3.1. Compounding process]
First, a predetermined amount of (M, M') source is added to the Pt source (mixing step).
The Pt source is not particularly limited as long as it is capable of synthesizing a platinum composite oxide. Examples of platinum sources include
(a) platinum oxide ( _PtO2 ),
(b) platinum nitrates, chloro complexes, ammine salts, hydroxy complexes, bromo complexes, iodine complexes, cyano complexes, and the like;

The (M, M') source is not particularly limited as long as it can synthesize a platinum composite oxide. (M, M') sources include, for example,
(a) salts with inorganic anions such as nitrates, fluorides, chlorides, bromides, iodides, carbonates, perchlorates, phosphates, sulfates, borates;
(b) salts with organic anions such as acetates, oxalates, citrates;
and so on.
The amount of the (M, M') source to be added is selected so as to obtain the desired platinum composite oxide.

[3.2. Heat treatment process]
Next, the raw material mixture is heat-treated under predetermined conditions to cause a solid phase reaction (reaction step). As for the heat treatment conditions, optimal conditions are selected according to the composition of the platinum composite oxide and the type of raw material.
For example, in the case of Pt--Co bronze, the heat treatment temperature is preferably 400-750.degree. In the case of a platinum-containing layered rock salt type composite oxide, the heat treatment temperature is preferably 600 to 700.degree.
The heat treatment time is usually several minutes to several hours. When the raw material mixture is heat-treated under predetermined conditions, a platinum composite oxide is produced and at the same time metal platinum is by-produced.

[3.3. Purification process]
Next, if necessary, metal platinum by-product is removed from the reaction product (purification step). Metal platinum is preferably removed by aqua regia treatment. By performing aqua regia treatment under predetermined conditions, the platinum composite oxide can be isolated from the reaction product.

[3.4. Reduction process]
Next, if necessary, the surface of the platinum composite oxide is reduced (reduction step). As a result, part of the surface of the platinum composite oxide is metal-platinized.
As a method of metal platinizing a part of the surface of the platinum composite oxide, for example,
(a) a method of applying a potential cycle to a platinum composite oxide;
(b) a method of treating a platinum composite oxide with a reducing agent (e.g., ethanol);
and so on.
A part of the surface of the platinum composite oxide may be subjected to metal platinization before or after it is supported on the carrier.

[4. Method for producing platinum composite oxide (2) -Template method-]
A second method for producing platinum bronze or platinum-containing layered rock salt type composite oxide is
(a) a filling step of filling the mesopores of mesoporous silica with a Pt source and a (M, M') source;
(b) a heat treatment step of heat-treating the mesoporous silica filled with the Pt source and the (M, M') source to prepare a composite in which the mesopores are filled with the platinum composite oxide;
(c) a recovery step of removing the mesoporous silica from the composite and recovering the reaction product;
(d) optionally, a purification step to remove metal platinum particles from the reaction product;
(e) a reduction step of reducing the surface of the platinum composite oxide, if necessary.

[4.1. Filling process]
First, the mesopores of mesoporous silica are filled with a Pt source and a (M, M') source (first step).

[A. mesoporous silica]
Mesoporous silica is a template for synthesizing fine platinum composite oxide particles. Various mesoporous silica materials with different pore sizes and pore structures are known. In the present invention, the type of mesoporous silica is not particularly limited, and an optimum material can be selected according to the purpose. In general, the smaller the pore size of mesoporous silica, the larger the specific surface area of platinum composite oxide particles obtained.

[B. Pt source, (M, M') source]
The filling of the Pt source and the (M.M') source into the mesopores is performed by filling the liquid containing Pt and (M, M') into the mesopores. Therefore, the Pt source and (M, M') source must be compounds that are liquid at room temperature or compounds that are soluble in a solvent at room temperature.
Other points regarding the Pt source and the (M, M') source are the same as those of the first method, so the description is omitted.

[4.2. Heat treatment process]
Next, the mesoporous silica filled with the Pt source and the (M, M') source is heat treated (heat treatment step). As a result, a composite is obtained in which the mesopores of the mesoporous silica are filled with the platinum composite oxide.

As the heat treatment conditions, an optimum temperature is selected according to the composition of the platinum composite oxide. In general, if the heat treatment temperature is too low, no platinum composite oxide is formed. On the other hand, if the heat treatment temperature is too high, the platinum composite oxide produced at low temperature may be decomposed to produce metallic platinum. Alternatively, platinum composite oxide may react with silica.
The rest of the heat treatment conditions are the same as in the first method, so the description is omitted.

[4.3. Recovery process]
Next, the template mesoporous silica is removed from the composite (recovery step). Thereby, a reaction product containing platinum composite oxide can be recovered.
Specifically, as a method for removing mesoporous silica,
(1) a method of heating the complex in an alkaline aqueous solution such as sodium hydroxide;
(2) a method of etching the composite with an aqueous hydrofluoric acid solution;
and so on.

[4.4. Purification process]
Next, if necessary, metal platinum is removed from the reaction product (purification step). Thereby, the platinum composite oxide can be isolated.
As a method for removing metal platinum from the reaction product,
(a) a method of treating the reaction product with aqua regia;
(b) There is a method of separating coarse metal platinum particles using a centrifugal sedimentation method.

[4.5. Reduction process]
Next, if necessary, part of the surface of the platinum composite oxide is metal-platinized (fourth step). The details of the reduction step are the same as those of the first method, so the explanation is omitted.

[5. Manufacturing method of exhaust gas purifying catalyst]
When a part of the surface of the platinum composite oxide particles is metal-platinized, the exhaust gas purifying catalyst according to the present invention can be obtained by supporting the surface of the carrier as it is.
On the other hand, when part of the surface of the platinum composite oxide particles is not metal-platinized, the exhaust gas purifying catalyst according to the present invention can be obtained by supporting the platinum composite oxide particles and the metal platinum particles on the carrier surface.
A method for supporting fine particles on the carrier surface is not particularly limited. Carrying is usually carried out by dispersing the carrier and fine particles in a dispersion medium and volatilizing the dispersion medium.

[6. action]
FIG. 1 shows a schematic cross-sectional view of an exhaust gas purifying catalyst according to the present invention. The active site (A) made of platinum composite oxide has high activity for CO oxidation reaction. The CO oxidation reaction (anode reaction) is represented by the following formula (1).
CO+H ₂ O→CO ₂ +2H ⁺ +2e ⁻ −0.12V (1)
From the formula (1), in order to efficiently proceed the CO oxidation reaction, H ₂ O is efficiently supplied to the active site (A), and at the same time, the reaction products, protons, electrons, and It can be seen that it is necessary to remove CO ₂ efficiently.

On the other hand, the active site (B) made of metal platinum has high activity for the oxygen reduction reaction that accompanies the CO oxidation reaction. The oxygen reduction reaction (cathode reaction) is represented by the following formula (2).
O ₂ +4H ⁺ +4e ⁻ →H ₂ O 1.23 V (2)
When the active point (B) made of metal platinum and the active point (A) made of platinum composite oxide are arranged in close proximity, the protons and electrons generated at the active point (A) are transported to the active point (B), Consumed for oxygen reduction reaction. On the other hand, the water produced at the active point (B) is transported to the active point (A) and consumed in the CO oxidation reaction. Therefore, when two types of active sites (A) and (B) having different functions are arranged close to the carrier surface, the CO oxidation reaction is promoted.

In particular, as shown in FIG. 1, when the active point (B) is a metal platinum layer formed on the surface of the platinum composite oxide, the metal platinum surface exhibits higher oxygen reduction activity. Therefore, the CO oxidation reaction is accelerated compared to the case where the platinum composite oxide particles are present alone or the platinum composite oxide particles and metal platinum particles are supported on the surface of the carrier.

(Examples 1 and 2, Comparative Example 1)
[1. Sample preparation (solid-phase method)]
[1.1. Synthesis of Co—Pt bronze (Example 1)]
Platinum oxide (PtO ₂ ) and cobalt nitrate hexahydrate (Co(NO ₃ ) ₂ .6H ₂ O) were weighed out in a molar ratio of 3:1 and mixed in a mortar. The resulting mixture was heat-treated at 650° C. for 5 hours under air ventilation (1 L/min). A powder X-ray diffraction measurement revealed that the powder after the heat treatment was a mixture of the desired platinum bronze and metal platinum as a by-product.

After the obtained mixed powder was immersed in hot aqua regia for 30 minutes, the supernatant was removed. It was confirmed by powder X-ray diffraction measurement that the metal platinum pattern disappeared from the diffraction pattern after the treatment with aqua regia. That is, it was confirmed that by the aqua regia treatment, by-produced metal platinum was removed and platinum bronze was isolated.

[1.2. Synthesis of platinum-containing layered rock salt type composite oxide (CoPtO ₂ ) (Example 2)]
Platinum nitrate (Pt(NO ₃ ) ₂ ) and cobalt nitrate hexahydrate (Co(NO ₃ ) ₂ .6H ₂ O) were weighed out in a molar ratio of 1:1 and mixed in a mortar. The resulting mixture was heat-treated at 650° C. for 5 hours under air ventilation (1 L/min). A powder X-ray diffraction measurement revealed that the powder after heat treatment was a mixture of the desired CoPtO ₂ and by-product metal platinum.

After the obtained mixed powder was immersed in hot aqua regia for 30 minutes, the supernatant was removed. It was confirmed by powder X-ray diffraction measurement that the metal platinum pattern disappeared from the diffraction pattern after the treatment with aqua regia. That is, it was confirmed that the by-product metal platinum was removed by the aqua regia treatment and CoPtO ₂ was isolated.

[1.3. Platinum-supported carbon (Comparative Example 1)]
Commercially available platinum-supported carbon (Pt/Vulcan (registered trademark)) was directly used in the test.

[2. Test method]
[2.1. Examples 1 and 2]
Catalyst powder was dispersed in acetone. This dispersion was applied to the surface of a glassy carbon (GC) electrode and dried. Using this as a working electrode, an electrochemical measurement was performed to evaluate the CO oxidation activity in the liquid phase. A reversible hydrogen electrode (RHE) was used as a reference electrode, a gold mesh was used as a counter electrode, 0.1M perchloric acid was used as an electrolyte, and the liquid temperature was 30°C.

The surface layer of the catalyst particles was metal-platinized by repeating potential sweeping in a potential range of 0.05 to 1.2 V in an electrolytic solution saturated with Ar. The number of potential sweep repetitions was 100 cycles for Co--Pt bronze and 200 cycles for CoPtO ₂ .
Thereafter, the CO oxidation performance was evaluated by sweeping the potential in a potential range of 0.05 to 1.2 V while rotating the working electrode at 400 rpm in an electrolytic solution saturated with CO.

[2.2. Comparative Example 1]
The CO oxidation performance was evaluated in the same manner as in Examples 1 and 2, except that the potential sweep was not performed.

[3. result]
FIG. 2 shows the CO oxidation voltammograms of CoPtO ₂ , Co—Pt bronze, and Pt/Vulcan®. Both Co—Pt bronze and CoPtO ₂ show CO oxidation currents at potentials lower than those of Pt/Vulcan (registered trademark), indicating that they have high CO oxidation activity. As mentioned above, the standard redox potential for the CO oxidation reaction (equation (1)) is -0.12V. Both Co--Pt bronze and CoPtO ₂ have low overpotentials for CO oxidation, indicating that they have high CO oxidation activity.

On the other hand, for Pt/Vulcan®, CO oxidation currents are seen at potentials higher than 0.8V. This indicates that the platinum surface becomes oxidized at high potential and CO oxidation proceeds on the surface. That is, it indicates that the CO oxidation reaction does not proceed on the metallic platinum surface.

FIG. 3 shows the CO oxidation voltammogram at a high potential of Co—Pt bronze which is not subjected to metal platinization treatment on the surface. Regarding the Co—Pt bronze, when the oxidation performance of CO was evaluated in a potential range higher than 0.6 V without subjecting the surface layer to metal platinization treatment by potential cycling, as shown in FIG. was taken. From this, it was confirmed that CO oxidation proceeds on the surface of Co—Pt bronze, that is, on the surface of platinum composite oxide.

FIG. 4 shows the CO oxidation voltammogram at high potential of CoPtO ₂ with no surface metal platinization treatment. As for CoPtO ₂ , when the oxidation performance of CO was evaluated in a potential range higher than 0.6 V without subjecting the surface to metal platinization by potential cycling, as shown in FIG. was taken. From this, it was confirmed that CO oxidation proceeds on the surface of CoPtO ₂ , that is, on the surface of platinum composite oxide.

(Example 3, Comparative Example 2)
[1. Sample preparation (silica template method)]
[1.1. Synthesis of Fine Co—Pt Bronze (Example 3)]
Chloroplatinic acid (H ₂ PtCl ₆ ) and cobalt nitrate hexahydrate (Co(NO ₃ ) ₂ .6H ₂ O) were weighed out in a molar ratio of 3:1 and dissolved in water. A mesoporous silica (FSM22) having a mesopore diameter of 3.8 nm was weighed in a closed container, and the above-mentioned metal salt solution for the pore volume was added and mixed well while the container was closed.

The raw material powder after mixing was heat-treated at 650° C. for 1 hour under oxygen gas flow (200 mL/min). A powder X-ray diffraction measurement revealed that the powder after the heat treatment was a mixture of the target platinum bronze and metallic platinum as a by-product as crystal phases.
The resulting powder sample was treated with 10M sodium hydroxide to dissolve silica. After centrifuging the solids, the supernatant was removed. By repeating the above steps three times, the silica was sufficiently removed to obtain a mixture of Co—Pt bronze and platinum metal.

The resulting mixture was dispersed in ultrapure water, irradiated with ultrasonic waves, solids were sedimented by centrifugation, and the supernatant was removed. This operation was repeated until the supernatant became neutral, thereby sufficiently washing. The solid content was again dispersed in ultrapure water, irradiated with ultrasonic waves, and then centrifugally sedimented at such a centrifugal acceleration that only coarse particles of metal platinum were sedimented, and fine Co—Pt bronzes were recovered from the supernatant. It was confirmed by powder X-ray diffraction measurement that fine Co—Pt bronze was obtained by the above method, since the platinum bronze phase peak has a wide half width. From electron microscopic observation, it was confirmed that fine needle-like Co—Pt bronze reflecting the shape of the pores of the mesoporous silica used as the template was obtained.

[1.2. Platinum black (Comparative Example 2)]
Commercially available platinum black was directly used for the test.

[2. Test method]
[2.1. Measurement of specific surface area]
The nitrogen adsorption amount of each powder was measured using a gas adsorption amount measuring device (Autosorb-1, Quantachrome). The specific surface area was obtained by analyzing the nitrogen adsorption amounts at relative pressures of 0.1, 0.2 and 0.3 by the BET method.
The specific surface area was measured not only for Co-Pt bronze (Co-Pt bronze-T, Example 3) and platinum black (Comparative Example 2) synthesized by the silica template method, but also for Co- Pt bronze (Co--Pt bronze-S, Example 1) and CoPtO ₂ (Example 2) were also tested.

[2.2. Metal platinization of surface layer and measurement of coverage]
The surface layer was metal platinized by immersing the synthesized platinum composite oxide powder in an 80 vol % ethanol aqueous solution. The metal platinization treatment of the surface layer was performed not only on Co--Pt bronze-T (Example 3), but also on Co--Pt bronze-S (Example 1) and CoPtO ₂ (Example 2).

After the metal platinization treatment, the powder was washed with ultrapure water and dispersed in acetone. This dispersion was applied to the surface of a glassy carbon (GC) electrode and dried. Using this as a working electrode, an electrochemical measurement was performed in the liquid phase. A reversible hydrogen electrode (RHE) was used as a reference electrode, a gold mesh was used as a counter electrode, 0.1M perchloric acid was used as an electrolyte, and the liquid temperature was 30°C.
While maintaining the potential of the working electrode at 1.0V, it was immersed in an electrolytic solution saturated with Ar, and the potential was swept in the potential range of 0.05 to 1.0V. The surface area of platinum was obtained from the current value of the desorption wave of hydrogen. By dividing this by the BET surface area of each active species supported on the electrode, the metal platinum coverage of the surface layer of the active species was obtained.

[3. result]
Table 1 shows the results. Table 1 shows the following.
(1) Co-Pt bronze-S, Co-Pt bronze-T, and CoPtO ₂ all had a specific surface area of 15 m ² /g or more.
(2) Co—Pt bronze has a higher coverage than CoPtO ₂ . This is probably because Co--Pt bronze is more easily reduced than CoPtO ₂ .
(3) Co--Pt bronze-S has a higher coverage than Co--Pt bronze-T. It is considered that this is because the heat treatment time is different.

(Examples 4.1 to 4.3, Comparative Examples 3.1 to 3.4)
[1. Preparation of sample]
Al ₂ O ₃ was used as a carrier. After mixing various active sites (catalyst powder) and a carrier in water, the mixture was dried and solidified to prepare a catalyst for an exhaust gas purification test.

The active point has
No. 1: Platinum black (Comparative Example 3.1)
No. 2: a mixture of Pt and CoO (Comparative Example 3.2),
No. 3: Co—Pt bronze-T whose surface layer is metal platinized (Example 4.1),
No. 4: Co-Pt-bronze-S whose surface layer is metal platinized (Example 4.2),
No. 5: Co-Pt bronze-S whose surface layer is not metal platinized (Comparative Example 3.3),
No. 6: CoPtO ₂ whose surface layer is not metal platinized (Comparative Example 3.4), or
No. 7: CoPtO ₂ with a metal platinized surface layer (Example 4.3)
was used.
The content of active sites in the catalyst was adjusted to 1 mass % in terms of platinum mass. Table 2 shows the specifications of each catalyst.

[2. Test method]
Each obtained catalyst was tested with a predetermined temperature profile under predetermined gas atmosphere conditions to determine the CO conversion rate at each temperature. Table 3 shows the gas atmosphere during the exhaust gas purification test. FIG. 5 shows the temperature profile during the exhaust gas purification test.

[3. result]
6 to 10 show the CO conversion of various catalysts under various gas atmosphere conditions. From FIGS. 6 to 10, the following can be understood.
(1) Co--Pt bronze-S, Co--Pt bronze-T, and CoPtO ₂ whose surface layer is metal platinized show rapid CO conversion at a temperature lower than that of platinum black under any gas atmosphere conditions. It was found to be highly active for CO oxidation.
(2) No. By comparing the catalytic activities of 4-7, Co-Pt-bronze-S (No. 4) and CoPtO ₂ (No. 7) are superior to Co-Pt bronze-S (No. 5) without metal platinization. ) and CoPtO ₂ (No. 6) without metal platinization treatment, and it was found to be highly active for CO oxidation. These exhibit special effects when H ₂ O coexists and under low oxygen concentration conditions (CO oxidation (b) to (e)).
(3) No. 1 and No. 2, the addition of Co to the Pt active site does not significantly improve the CO conversion rate, so metal platinization of the surface of the active site (A) made of platinum composite oxide is effective for improving activity. It turned out to be

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The exhaust gas purifying catalyst according to the present invention can be used in exhaust gas purifying devices for diesel engines, post-treatment devices for stationary furnaces, and the like.

Claims (3)

An exhaust gas purifying catalyst having the following configuration.
(1) The exhaust gas purifying catalyst is
a carrier made of a metal oxide;
and active points formed on the surface of the carrier.
(2) the active point is
an active point (A) made of a platinum composite oxide containing platinum and metal M;
and an active point (B) made of metal platinum.
(3) The platinum composite oxide is a platinum bronze having a composition represented by the following formula (1) and/or a platinum-containing layered rock salt type composite oxide having a composition represented by the following formula (2) Including things.
_{CoxPt3 -yO4 -z} (1)
However, 0<x≤1, 0≤y≤1, 0≤z≤1.
M'PtO2 ( ₂ )
However, M' is Co, or Co and Mn.
(4) the active sites (A) are composed of particles of the platinum composite oxide supported on the surface of the carrier;
The active point (B) includes a metal platinum layer formed on a part of the surface of the particles made of the platinum composite oxide. 2. The exhaust gas purifying catalyst according to claim 1, wherein the active sites (A) have a specific surface area of 15 m ² /g or more. Honeycomb and
3. An exhaust gas purifying apparatus comprising the exhaust gas purifying catalyst according to claim 1 or 2 filled in the gas flow path of the honeycomb.

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