JPH1085600A - Exhaust gas purifying catalyst and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the purifying performance of HC and SOF in a low temperature zone and control the oxidization of SO2 in a high temperature zone by zirconium oxide particles and a catalyst precious metal, with the total carrying amount of the catalyst precious metal in the highly oxidized state of specified amount or more. SOLUTION: 50% or more of the total carrying amount of a catalyst precious metal is in existence in the highly oxidized state in an exhaust gas catalyst. The adsorption of SO2 becomes hard and the oxidization of SO2 is controlled in the catalyst precious metal in the highly oxidized state. SO2 is adsorbed to the precious metal not in the highly oxidized state but in the metal state, and also SO2 is oxidized to form sulfate. On the other hand, in paraffins as a main component of SOF and the oxidization activities of HC, scarcely any difference can be recognized between the highly oxidized precious metal and the metal state precious metal. Sulfate formed by the oxidization of SO2 , therefore, can be controlled without lowering the purification activities of HC and SOF by the exhaust gas catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention can oxidize and purify at least hydrocarbons (HC) in exhaust gas discharged from an internal combustion engine, particularly a diesel engine from a low temperature range, and furthermore, can remove sulfur dioxide (SO ₂ ) even in a high temperature range. An exhaust gas purifying catalyst capable of suppressing oxidation to suppress sulfate emission;
It relates to the manufacturing method.

[0002]

2. Description of the Related Art Regarding gasoline engines, harmful substances in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of coping with it. However, due to the unique situation that harmful components are mainly emitted as particulates in diesel engines, regulations and development of technology are behind in comparison with gasoline engines, and the development of exhaust gas purification catalysts that can reliably purify diesel engines is desired. I have.

[0003] As a diesel engine exhaust gas purifying catalyst that has been developed to date, a method using a trap (without a catalyst and with a catalyst) and an open oxidation catalyst are known. Of these, the method using a trap captures diesel particulates and regulates their emission, and is particularly effective for exhaust gas with a high dry suit ratio. However, the method using a trap requires a reprocessing device to incinerate the captured diesel particulates, which causes many practical problems such as cracking of the catalyst structure during regeneration, blockage by ash, or complicated systems. Is leaving.

On the other hand, as an open type oxidation catalyst, a catalyst in which a catalytic metal such as a platinum group metal is supported on a supporting layer of activated alumina or the like is used as in a gasoline engine, as shown in Japanese Patent Application Laid-Open No. 1-171626, for example. , CO and HC
At the same time, it purifies by oxidatively decomposing a soluble organic component (SOF). This open type oxidation catalyst has the drawback of low drysuit removal rate, but the amount of drysuit can be reduced by improving the diesel engine and fuel itself, and the major advantage is that no reprocessing equipment is required. Therefore, further improvement of technology is expected in the future.

[0005] Although the open type oxidation catalyst can decompose SOF efficiently at high temperatures, it has a disadvantage that the catalytic action of the catalytic metal is low and the purification performance of SOF decreases at low temperatures. For this reason, when the engine is started or idling, the temperature of the exhaust gas is low and undecomposed SO
A phenomenon occurs in which F accumulates in the honeycomb passage. Then, there is a problem that the catalyst is clogged by the deposited SOF, and the catalyst performance is reduced.

On the other hand, when the oxidation performance of the catalyst is improved to improve the purification performance of HC and SOF in a low temperature range, even SO _{2 in} the exhaust gas is oxidized to form SO _{3, which} is converted into sulfate to conversely. There is a problem that the amount of particulates increases. This is because SO ₂ is not measured as particulates, but sulfate is measured as particulates. The increase in particulates due to sulfate formation is remarkable in a high temperature range. Particularly, in a diesel engine, a large amount of oxygen gas is present in exhaust gas, and S
Oxidation reaction of O ₂ is likely to occur.

[0007] Japanese Patent Application Laid-Open No. 59-36545 discloses that an activated alumina carrier is loaded with Pt.
A method is disclosed in which heat treatment is performed at a temperature of 1000 ° C. to cause grain growth in Pt and suppress oxidation of SO ₂ . However, in the method of growing grains on Pt, Pt
There is a problem in that the oxidation activity of the catalyst is suppressed, and the purification performance of HC, CO and SOF is also reduced.

[0008]

As described above, it has been difficult for conventional exhaust gas purifying catalysts to achieve both improvement in HC and SOF purifying performance and suppression of SO ₂ oxidation. The present invention has been made in view of such circumstances, and provides an exhaust gas purifying catalyst which has excellent purification performance of HC and SOF in a low temperature range and can suppress oxidation of SO ₂ even in a high temperature range, and a method for producing the same. With the goal.

[0009]

According to a first aspect of the present invention, there is provided an exhaust gas purifying catalyst for oxidizing and purifying at least hydrocarbons in exhaust gas, comprising a zirconium oxide. Containing particles and catalytic noble metal,
The reason is that 50% or more of the total supported amount of the catalytic noble metal is present in a highly oxidized state.

The method for producing an exhaust gas purifying catalyst according to claim 2, which is most suitable as a method for producing the exhaust gas purifying catalyst, is characterized in that the exhaust gas purifying catalyst oxidizes and purifies at least hydrocarbons in exhaust gas. A production method, comprising: a supporting step of supporting a catalytic noble metal on a solid hydroxide that becomes an oxide by firing; and a firing step of firing a solid hydroxide supporting the catalytic noble metal at a temperature of 700 ° C. or lower in an oxidizing atmosphere. And that it has.

[0011] A feature of the method for producing an exhaust gas purifying catalyst according to claim 3, which further characterizes the production method according to claim 2, is that the firing step is performed in an oxidizing atmosphere containing 3% by volume or more of water vapor. is there.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION SO_TwoOxidation of
According to the mechanism, first the catalyst noble metal was SO _TwoAdsorb
It has been found that an adsorption process is required. So book
In the exhaust gas purifying catalyst of the present invention, the total amount of the catalyst noble metal carried is
More than 50% are present in a highly oxidized state. This highly oxidized form
SO that has an electron-accepting type adsorption on the catalytic noble metal that exists in a state
_TwoAdsorption becomes difficult, and SO_TwoOxidation is suppressed. one
On the other hand, for precious metals that are not in a highly oxidized state,
SO_TwoIs adsorbed and easily SO_TwoIs oxidized to sulfur
Generated.

On the other hand, the main component of SOF is paraffins, and the oxidizing activity of paraffins and HC is almost the same between the catalytic noble metal existing in a highly oxidized state and the catalytic noble metal supported in a metal state. There is no. Therefore, the exhaust gas purifying catalyst of the present invention, which has 50% or more of a noble metal in a highly oxidized state, can suppress sulfate caused by oxidation of SO ₂ without lowering the activity of purifying HC and SOF.

Here, the oxidation state of the catalyst noble metal constituting the exhaust gas purifying catalyst of the present invention refers to the oxide of the catalyst noble metal or the state of δ + of the catalyst noble metal, and the high oxidation state is, for example, Pt. It refers to a state that exists divalent or tetravalent. According to the CO oxidation mechanism, a process of adsorbing CO to the catalytic noble metal is required. However, CO is a substance that is very easily oxidized, and sufficient oxidation activity can be ensured even when most of the catalytic noble metal exists in a highly oxidized state.

That is, according to the exhaust gas purifying catalyst of the present invention,
If the oxidation activity of CO, HC and SOF is kept high,
One, SO_TwoSuppresses the oxidation of sulfur and the production of sulfate
Is done. The catalyst noble metal present in the highly oxidized state
If less than 50%, the metal (zero valence) catalyst on the catalyst surface
Precious metal increases, SO_TwoIt is difficult to suppress the oxidation of the compound. SO
_TwoIn order to suppress the oxidation of metals, there are many catalytic noble metals in the high oxidation state.
Very preferred.

The catalyst noble metals include Pt, Pd, R
h, etc., but P which is particularly excellent in HC oxidation activity
It is preferable to use t. In this case, the amount of Pt is desirably 0.005 to 10 g with respect to 120 g of zirconium oxide. If the amount is less than 0.005 g, the oxidizing activity of HC is lost. When the amount of Pt is 0.005 g or more, the effect of Pt in a highly oxidized state can be sufficiently exhibited. However, if the amount exceeds 10 g, the effect is saturated and the cost increases.

In order to suppress sulfate formation,
It is desirable that Pt be in a divalent or tetravalent high oxidation state.
Value is desirable. In order for the catalytic noble metal to be present in a highly oxidized state, the production method described below is useful.

That is, in the method for producing an exhaust gas purifying catalyst of the present invention, a catalytic noble metal is first supported on a solid hydroxide which becomes an oxide by firing in a supporting step, and then a solid hydroxide supporting the catalytic noble metal is supported in a firing step. The material is fired at a temperature of 700 ° C. or less in an oxidizing atmosphere. This turns the hydroxide into an oxide, at which time at least 50% of the catalytic noble metal is present in a highly oxidized state.

In the firing step, the solid hydroxide supporting the catalytic noble metal is fired in an oxidizing atmosphere at a temperature of 700 ° C. or less. This firing is performed in an oxygen-excess oxidizing atmosphere.
Any atmosphere may be used. When the firing temperature is 70
When the temperature exceeds 0 ° C., the catalytic noble metal in a highly oxidized state is less than 50%, and a large amount of the metallized catalytic noble metal is exposed on the oxide surface, which makes it difficult to suppress the oxidation of SO ₂ . Therefore, the upper limit of the firing temperature is set to 700 ° C.

Since the hydroxide easily becomes an oxide when calcined at about 300 ° C., if the atmosphere during use is oxidizing and the temperature during use is 300 ° C. or higher, the hydroxide for purification of exhaust gas of the present invention is used. It can be sufficiently used as a catalyst. For example, in the case of an exhaust gas purifying catalyst arranged in an exhaust system from a diesel engine, sufficient oxygen is present in the exhaust gas, and the exhaust gas temperature becomes 300 ° C. or higher.
Even if less than 50% or more of the catalytic noble metal is present in a highly oxidized state at the time of use, it can be in the same state as previously baked at 300 ° C. or more. Therefore, the lower limit of the firing temperature is not limited.

In order to ensure that the hydroxide can be converted into an oxide and that at least 50% of the catalytic noble metal is present in a highly oxidized state, the calcination temperature is preferably 500 to 600 ° C. desirable. Further, as described in claim 3, it is desirable that the firing step be performed in an oxidizing atmosphere containing 3% by volume or more of water vapor. This makes it possible to further increase the ratio of the catalytic noble metal present in a highly oxidized state in all the catalytic noble metals, and to further suppress the oxidation of SO ₂ . If the amount of water vapor is less than 3% by volume, the effect of including water vapor is not exhibited, and it is not expected that the amount of catalytic noble metal present in a highly oxidized state will be further increased as compared with the case where firing is performed in an oxidizing atmosphere containing no water vapor. . The amount of water vapor is 3% by volume
In the above description, the amount of catalytic noble metal present in a high oxidation state increases as the amount of water vapor increases. However, when the amount is too large, the effect is saturated. Therefore, the amount of water vapor of about 10% by volume is appropriate.

In order to make the oxidizing atmosphere contain 3% by volume or more of water vapor, a method of bubbling an oxidizing gas stream in the water is used.
A method of injecting a predetermined amount of water into an oxidizing air stream by a micro feeder is exemplified. Further, even if the solid hydroxide supporting the catalytic noble metal is fired in a wet state, the vapor can be included in the gas phase. In the case of an exhaust gas purifying catalyst disposed in an exhaust system of a diesel engine, since the exhaust gas contains 3% by volume or more of sufficient water vapor and the exhaust gas temperature becomes 300 ° C. or more, it is calcined in the exhaust gas. A step may be performed. Since 3% by volume or more of steam is present in the exhaust gas, the same result as the above-described treatment method can be obtained. However, SOF and soot contained in the exhaust gas of diesel engines adhere to the catalyst and may not provide a sufficient effect. Therefore, it is necessary to perform the firing step beforehand in an oxidizing atmosphere containing water vapor before use. desirable.

Examples of the solid hydroxide which becomes an oxide upon firing include zirconium hydroxide, aluminum hydroxide, cerium hydroxide, magnesium hydroxide and the like. In order to support the catalyst noble metal on the solid hydroxide, for example, there is a method of bringing an aqueous solution of a noble metal salt into contact with the hydroxide.
As the hydroxide, a commercially available one may be used,
A hydroxide formed by precipitating a hydrochloride, a sulfate, or the like in an alkaline aqueous solution can also be used. When a noble metal alkali aqueous solution is used as the alkali aqueous solution, the noble metal can be supported simultaneously with the generation of the hydroxide.

For example, when Pt is supported on zirconium hydroxide, commercially available zirconium hydroxide (Zr (O
H) ₄ ) may be used, or zirconium hydroxide obtained by precipitating a zirconium salt such as zirconyl chloride, zirconyl nitrate, or zirconyl sulfate in an alkaline aqueous solution may be used. To obtain the same effect as Zr (OH) ₄ , amorphous zirconia may be used. Further, as a Pt source, platinum chloride, dinitrodiammine platinum, platinum sulfate or the like is used.

Although the obtained exhaust gas purifying catalyst is in a powder form, in order to use it as an automotive exhaust gas purifying catalyst, it may be pelletized by a conventional method into a pellet catalyst, or may be slurried to be made of cordierite or metal. A honeycomb catalyst coated on the surface of the honeycomb substrate can also be used.

[0026]

The present invention will be specifically described below with reference to examples and comparative examples. (Example 1) <Supporting step> Powdered zirconium hydroxide (Z
A predetermined concentration of dinitrodiammineplatinum nitrate aqueous solution was added to a suspension in which r (OH) ₄ ) was dispersed in ion-exchanged water, and the mixture was stirred with a stirrer for 2 hours or more. Thereafter, water was evaporated to dryness under stirring by a heating stirrer, and Pt was supported on zirconium hydroxide. The amount of Pt supported is determined by converting zirconium hydroxide to zirconium oxide (Zr
The amount was adjusted to 1 g with respect to 120 g in terms of O ₂ ).

<Firing Step> Next, the Pt-supported zirconium hydroxide obtained above was subjected to 50 hours in air using an electric furnace.
The mixture was heated to 0 ° C. and held at 500 ° C. for 3 hours for firing. After firing, the powder was pulverized in a mortar to prepare a powdery catalyst having a size of 100 mesh or less. (Example 2) A catalyst of Example 2 was prepared in the same manner as in Example 1 except that the calcination temperature was 300 ° C.

Example 3 A catalyst of Example 3 was prepared in the same manner as in Example 1 except that the calcination temperature was 700 ° C. Example 4 A catalyst of Example 4 was prepared in the same manner as in Example 1 except that the amounts of the aqueous dinitrodiammineplatinum solution and zirconium hydroxide were changed and that 2.0 g of Pt was supported on 120 g of zirconium oxide. did.

Example 5 The procedure of Example 5 was repeated in the same manner as in Example 1 except that the amounts of the aqueous solution of dinitrodiammine platinum and zirconium hydroxide were changed and that 0.01 g of Pt was supported on 120 g of zirconium oxide. A catalyst was prepared. (Example 6) The catalyst of Example 6 was prepared in the same manner as in Example 1 except that the amounts of the aqueous dinitrodiammine platinum solution and zirconium hydroxide were changed, and 4.0 g of Pt was supported on 120 g of zirconium oxide. did.

Example 7 The procedure of Example 7 was repeated in the same manner as in Example 1 except that the amounts of the aqueous solution of dinitrodiammine platinum and zirconium hydroxide were changed, and 0.008 g of Pt was supported per 120 g of zirconium oxide. A catalyst was prepared. (Example 8) A catalyst of Example 8 was prepared in the same manner as in Example 1 except that in the calcination step, the atmosphere was bubbled in water to calcinate in an air stream containing 1% by volume of water vapor.

Example 9 The catalyst of Example 9 was prepared in the same manner as in Example 1 except that in the firing step, the air was bubbled into water to fire in an air stream containing 3% by volume of water vapor. Prepared. (Example 10) A catalyst of Example 10 was prepared in the same manner as in Example 1 except that in the firing step, the atmosphere was bubbled in water to fire in an airflow containing 10% by volume of water vapor.

(Example 11) In a firing step, the air was bubbled into water to reduce the volume of water vapor to 15% by volume.
A catalyst of Example 11 was prepared in the same manner as in Example 1 except that the catalyst was calcined in the contained air current. (Comparative Example 1) In the same manner as in Example 1 except that zirconium oxide powder (monoclinic powder having high crystallinity, manufactured by Mitsuka Chemical Co., Ltd.) was used instead of zirconium hydroxide,
A catalyst of Comparative Example 1 was prepared.

(Comparative Example 2) A catalyst of Comparative Example 2 was prepared in the same manner as in Example 1 except that the calcination temperature was 800 ° C. <Evaluation Test 1> The catalysts of Example 1 and Comparative Example 1 were placed in a CO gas for a predetermined time, and then the infrared absorption spectra of CO adsorbed on the catalyst were measured. The results are shown in FIG.

In the catalyst of Comparative Example 1, a strong peak attributable to CO stretching vibration was observed around 2070 cm ⁻¹ ,
Such a strong peak is not observed in the catalyst of Example 1. That is, it is shown that the catalyst of Example 1 hardly adsorbs CO to the noble metal. The following two points can be considered as causes that the adsorption of CO did not occur in the catalyst of Example 1. (1) Reduction of CO adsorption sites due to sintering of Pt. (2) Since most of Pt exists in a highly oxidized (divalent or tetravalent) state, CO cannot be adsorbed.

In order to verify the above hypothesis, an investigation was performed by TEM (electron microscope) and XPS (X-ray photoelectron spectroscopy). First, TEM examination confirmed that the catalyst of Comparative Example 1 had Pt particles on the surface of zirconium oxide, but the catalyst of Example 1 had Pt particles on the surface of zirconium oxide.
No particles were observed. From these results, Example 1
It is considered that the decrease in the amount of adsorbed CO in the catalyst is not due to sintering of Pt, but to the fact that most of Pt exists in a highly oxidized (divalent or tetravalent) state.

On the other hand, the ratio of Pt in a highly oxidized state to the total Pt amount was determined by using XPS. The measuring method is as follows. First, in a 0.1 atm oxygen stream,
A pretreatment was performed in which the catalyst was heated at 700 ° C. for 10 minutes. Thereafter, the spectrum of Pt for 4f was measured by an XPS apparatus. At this time, the X-ray source uses Mg-Kα radiation,
The electron extraction angle was 45 degrees. The obtained spectrum was subjected to peak fitting, and the proportions of divalent and tetravalent highly oxidized Pt were determined. The results are shown in Table 2.

In the fifth embodiment and the seventh embodiment,
Since the supported amount of Pt was 0.01 g or less, the peak intensity of the XPS spectrum was weak, and accurate measurement was difficult.
Therefore, Table 2, Example 1, Example 8, and Example 1
The values inferred from the measured value of 0 are shown. <Evaluation test 2>

[0038]

[Table 1] Each catalyst was pressurized with a cold isostatic press and then pulverized to form granules of 6 to 10 mesh. 3.6 g of this
After being weighed and installed in a fixed bed flow type apparatus, the model gas shown in Table 1 was charged at a gas temperature of 600 ° C. to 150 ° C.
Space temperature SV = 100,000 hr while lowering temperature at ° C / min
^Under the condition of ^-1 , the conversion of C ₆ H ₁₄ and SO ₂ was measured by the following equation.

Conversion rate (%) = [(incoming gas concentration−outgoing gas)
Concentration) / incoming gas concentration] × 100 Then, a temperature window ΔT is obtained by the following equation,
Table 2 shows the results. ΔT (° C.) = (SO_Two30% conversion temperature)-(C₆H₁₄5
0% conversion temperature) where SO_Two30% conversion temperature means 30% conversion of SO2
Temperature, and C₆H ₁₄50% conversion temperature is C₆H₁₄To
The temperature at which 50% conversion can be achieved. Of which C₆H ₁₄50%
Low conversion temperature, SO_Two30% higher conversion temperature
The value of ΔT is large and SO_TwoHardly oxidizes HC and oxidizes HC
Shows that the catalyst is easy (excellent in HC selectivity)
Things.

[0040]

[Table 2]

As shown in Table 2, the catalysts of Examples 1 to 11 had Pt in a highly oxidized state of 50% or more, and the catalysts of Comparative Examples 1 and 2 had less than 50%. Further, from Table 2, the T50 (HC) of the catalysts of Examples 1, 4 and 6 is almost the same as the T50 (HC) of Comparative Example 1. On the other hand, T30 (SO ₂ ) of the catalysts of Examples 1, 4 and 6 was
Is higher than T30 (SO ₂ ) of the catalyst. Therefore, it can be seen that the catalysts of Examples 1, 4 and 6 have a wider temperature window (ΔT) than the catalyst of Comparative Example 1 and are excellent in HC selectivity.

This is the effect of using zirconium hydroxide as a starting material in Examples 1, 4 and 6 to allow 90% of the supported Pt to be present in a highly oxidized state. That is, with respect to T50 (HC), there is almost no difference between the type of the oxidation state of Pt, that is, the type of the metal (0 valence) or the high oxidation state (2, 4 valence). on the other hand,
As for T30 (SO ₂ ), Pt in a highly oxidized state is 40
%, And the remaining 60% is in a metal state. In Comparative Example 1 where the Pt of the metal is large, sulfate is easily formed and T30 (SO ₂ ) is low, whereas 90%
In Examples 1, 4 and 6 in which Pt was supported in a highly oxidized state, the production of sulfate was suppressed, so that T30 (S
O ₂ ) is higher.

Also, comparing T50 (HC) and T30 (SO ₂ ) of the catalysts of Example 1 and Examples 8 to 11, the catalyst of Example 8 has almost the same performance as the catalyst of Example 1. The catalysts of Examples 9 to 11 have T30 (SO ₂ ) higher than that of Example 1, and T50 (HC) is almost equivalent to the catalyst of Example 1. That is, the catalysts of Examples 9 to 11 have a wider temperature window (ΔT) than the catalyst of Example 1,
It is understood that the selectivity of HC is excellent.

Next, T50 of the catalysts of Examples 2 and 3 was used.
(HC) is almost the same as T50 (HC) of the catalyst of Comparative Example 2.
And so on. Further, T30 of the catalysts of Example 2 and Example 3
(SO _Two) Is T30 (SO 2) of the catalyst of Comparative Example 2._Two)
Everything is expensive. Therefore, the catalysts of Example 2 and Example 3
The temperature window (ΔT) is wider than that of the catalyst of Comparative Example 2.
This shows that the catalyst is excellent in HC selectivity.

This is the same as Example 2 and Example 3 except that P
This is an effect of baking the zirconium hydroxide supporting t at 700 ° C. or lower so that 50% or more of the supported Pt is present in a highly oxidized state. On the other hand, the catalyst of Comparative Example 2 was calcined at a temperature exceeding 700 ° C., so that the proportion of Pt in a highly oxidized state decreased and the Pt of metal increased,
It is easier to produce sulfate, and the temperature window is narrower than those of the catalysts of Examples 2 and 3.

The T50 of the catalysts of Examples 5 and 7
(HC) is higher than T50 (HC) of the other examples, and T30 (SO ₂ ) is extremely high. Therefore, it can be seen that the catalysts of Examples 5 and 7 have a wide temperature window (ΔT) in a high temperature range and are excellent in the selectivity of HC. That is, compared to the catalysts of the other examples, P
The catalysts of Example 5 and Example 7 in which the amount of t was reduced are effective when the operating temperature is high.

In the catalysts of Examples 9 to 11, the ratio of Pt in the highly oxidized state was 93% or more, and Examples 1 and 8
It is clear that the oxidation of SO ₂ was further suppressed. In other words, it is clear that by performing the firing step in an atmosphere containing 3% by volume or more of water vapor, the proportion of Pt existing in a highly oxidized state is further increased. In the catalysts of Example 10 and Example 11,
It can be seen that the effect is saturated when the proportion of Pt in the highly oxidized state is the same as 97%, and when the amount of water vapor is 10% by volume or more.

From the above results, the catalysts of Examples 1 to 8
Compared to the catalyst of the comparative example, it is possible to suppress the oxidation of SO ₂ in a high temperature range while securing the purification performance of oxidizing HC at a low temperature,
It is clear that the catalyst has a wide temperature window capable of suppressing the formation of sulfate, and this is clearly the effect of allowing 50% or more of the total amount of supported Pt to be present in a highly oxidized state.

Further, in the catalysts of Examples 9 to 11, the ratio of Pt in a highly oxidized state is 93% or more, which is higher than that of the other examples, whereby the oxidation of SO ₂ is further suppressed. That is, by performing the firing step in an atmosphere containing 3% by volume or more of water vapor, the proportion of Pt present in a highly oxidized state is further increased. It is clear that

[0050]

According to the exhaust gas purifying catalyst of the present invention, the purification performance of HC and SOF in a low temperature range is excellent.
Further, even in a high temperature range, oxidation of SO ₂ can be suppressed, and generation of sulfate can be suppressed. According to the method for producing an exhaust gas purifying catalyst of the present invention, 50% or more of the catalyst noble metal can be easily present in a highly oxidized state, and the catalyst can be easily and stably produced.

In the method for producing a catalyst for purifying exhaust gas of the present invention, the proportion of the catalytic noble metal present in a highly oxidized state can be further increased by including 3% by volume or more of water vapor in the oxidizing atmosphere. Thus, it is possible to produce a catalyst that can further suppress the production of sulfate.

[Brief description of the drawings]

FIG. 1 is an infrared absorption spectrum of CO adsorbed on catalysts of Example 1 and Comparative Example 1.

Claims (3)

[Claims] 1. An exhaust gas purifying catalyst for oxidizing and purifying at least hydrocarbons in exhaust gas, comprising zirconium oxide particles and a catalytic noble metal, wherein at least 50% of the total amount of the catalytic noble metal carried is present in a highly oxidized state. An exhaust gas purifying catalyst characterized in that: 2. A method for manufacturing an exhaust gas purifying catalyst for oxidizing and purifying at least hydrocarbons in exhaust gas, comprising: a supporting step of supporting a catalytic noble metal on a solid hydroxide which becomes an oxide by firing; And baking a solid hydroxide carrying a noble metal at a temperature of 700 ° C. or lower in an oxidizing atmosphere at a temperature of 700 ° C. or less. 3. The method for producing an exhaust gas purifying catalyst according to claim 2, wherein the calcination step is performed in an oxidizing atmosphere containing 3% by volume or more of water vapor.