JPH09108570A - Oxidation catalyst for cleaning exhaust gas and preparation thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively prevent the generation of sulfate and to reduce the discharge of particulates. SOLUTION: The quantity of noble metal supported on the surface layer of the secondary particles of a porous material on the incoming gas side located upstream in an exhaust gas flow is made greater than that on the outgoing gas side located downstream. Since HC is oxidized easily as compared with SO2 , HC is oxidized preferentially on the incoming gas side, while the oxidation of SO2 is controlled on the outgoing gas side because of poor Pt on the surface layer. Therefore, the oxidation of SO2 is prevented by the catalyst as a whole.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying oxidation catalyst, and more specifically, to carbon monoxide (CO) and hydrocarbon (which are harmful components contained in exhaust gas from a diesel engine (hereinafter referred to as DE)). The present invention relates to an oxidation catalyst for purifying exhaust gas, which purifies HC) and soluble organic components (SOF) and reduces the emission amount of sulfate (sulfate).

[0002]

2. Description of the Related Art With respect to gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of coping with the regulations. However, with respect to DE, due to the unique circumstances in which harmful components are discharged as particulates (hereinafter referred to as PM) such as carbon fine particles, sulfur fine particles such as sulfate, and high molecular weight hydrocarbon fine particles, both regulations and technological advances have been made in gasoline. It is behind the engine, and development of an exhaust gas purification device that can reliably purify the exhaust gas is desired.

Exhaust gas purifying apparatuses for DE that have been developed so far are roughly classified into a trap type exhaust gas purifying apparatus using a trap type exhaust gas purifying catalyst and an open type exhaust gas purifying apparatus using an open type exhaust gas purifying catalyst. The device is known. As a trap type exhaust gas purifying catalyst, a ceramic plugging type honeycomb body (diesel particulate filter (DPF)) and the like are known. In an exhaust gas purifying apparatus using this exhaust gas purifying catalyst, exhaust gas is filtered by a DPF or the like to collect PM, and if pressure loss increases, PM accumulated in a burner or the like is burned to regenerate the DPF or the like. ing. Also, PM
In order to oxidize and decompose CO, HC and SOF as well as capturing CO2, an exhaust gas purification catalyst in which a catalyst supporting layer is formed of alumina or the like on a carrier substrate such as DPF and platinum (Pt) or the like is supported on this catalyst supporting layer Are also being considered.

On the other hand, as an open type exhaust gas purifying catalyst, a carrier base material made of a ceramic straight flow type honeycomb body or the like, a catalyst support layer formed of alumina or the like on the carrier base material, and the catalyst support It is known that the layer is composed of Pt and the like supported in the same manner as a gasoline engine. According to this open type exhaust gas purifying apparatus, CO and the like can be oxidized and decomposed by the catalytic action of Pt and the like.

[0005]

However, in the trap type or open type exhaust gas purifying apparatus having the above platinum or the like, P
At time t, SO ₂ reacts with oxygen to generate sulfate, and the catalyst supporting layer adsorbs SO ₂ in the exhaust gas,
When the temperature of the catalyst becomes high, SO ₂ is oxidized by the catalytic action of Pt or the like and is discharged as SO ₃ . Particularly in DE, oxygen gas is sufficiently present in the exhaust gas, and SO ₂ is easily oxidized by this oxygen gas and is easily discharged as SO ₃ . SO ₃ easily reacts with a large amount of water vapor present in the exhaust gas to form a sulfuric acid mist and is discharged as a sulfate.

Comparing the non-oxidizing properties of HC and SO ₂ , HC is more easily oxidized than SO ₂ even at high temperatures. Therefore, on the gas entering side (front side) of the catalyst, since a large amount of HC is contained in the exhaust gas, HC is preferentially oxidized and SO ₂ is hardly oxidized. However, on the gas output side (rear side) of the catalyst, the HC in the exhaust gas is reduced because it is oxidized and purified on the gas input side, and conversely a large amount of SO ₂ is present. Therefore, SO ₂ is oxidized on the gas outlet side of the catalyst to generate sulfate.

Therefore, in these exhaust gas purifying devices,
There is a problem that the amount of PM increases due to the formation of sulfate at high temperature. The present invention has been made in view of such circumstances, and by optimally controlling the oxidation reaction on the inlet gas side and the outlet gas side of the catalyst, it is possible to effectively prevent the formation of sulfate and reduce the PM emission amount. The purpose is to reduce

[0008]

Means for Solving the Problems An exhaust gas purifying oxidation catalyst of the present invention which solves the above-mentioned problems is characterized by an oxidation catalyst comprising a porous carrier and a noble metal supported on the porous carrier. The amount of the noble metal carried on the surface layer of the secondary particles of the porous body constituting the carrier is characterized in that the incoming gas side located upstream of the exhaust gas flow is more than the outgoing gas side located downstream. To do.

The method for producing an oxidation catalyst for exhaust gas purification according to the second aspect of the present invention, which is used for producing the above-mentioned oxidation catalyst, is characterized in that the carrier gas has a large specific surface area on the incoming gas side located upstream of the exhaust gas flow. The porous body powder is coated to form a porous coat layer by coating a porous body powder having a small specific surface area on the outlet gas side located on the downstream side of the exhaust gas flow of the carrier substrate.
The amount of noble metal supported on the surface layer of the secondary particles of the porous body that constitutes the porous coat layer is larger than that on the gas outlet side by supporting the precious metal on the porous coat layer. .

Further, the characteristic feature of the method for producing an oxidation catalyst for exhaust gas purification of the third invention used for producing the above-mentioned oxidation catalyst is that the porous carrier has high adsorbability on the incoming gas side located upstream of the exhaust gas flow. A solution of a noble metal salt is brought into contact, and a solution of a noble metal salt having low adsorptivity is brought into contact with the outlet gas side of the porous carrier, which is located on the downstream side of the exhaust gas flow, and the secondary particles of the porous body constituting the porous carrier. The amount of the noble metal supported on the surface layer of 1 was set to be larger on the gas side than on the gas side.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In a porous carrier, a porous body powder exists as secondary particles in which primary particles are aggregated. Then, in the oxidation catalyst of the present invention, the amount of noble metal carried on the surface layer of the secondary particles is different between the incoming gas side located upstream of the exhaust gas flow and the outgoing gas side located downstream, and the incoming gas The side has a larger amount of precious metal in the surface layer.

With the oxidation catalyst of the present invention configured as described above, the oxidation reaction on the inlet gas side and the outlet gas side of the catalyst can be optimally controlled as follows. (1) Entered gas side On the entered gas side, exhaust gas contains a large amount of HC.
Since SO ₂ is less likely to be oxidized than HC, HC tends to be preferentially oxidized over SO ₂ in an atmosphere containing a large amount of HC. Therefore, in the present invention, the amount of precious metal supported on the surface layer on the incoming gas side is increased. Therefore, since the oxidation reaction is accelerated on the incoming gas side, most of HC is oxidized on the incoming gas side, and SO ₂ is hardly oxidized and flows to the outgoing gas side.

(2) Outgas side On the outgas side, most of the HC has been oxidized on the incoming gas side, so the amount of HC in the exhaust gas is small and S
It contains a large amount of O ₂ . Therefore, in the present invention, the amount of noble metal supported on the surface layer of the secondary particles of the porous carrier on the gas outlet side is smaller than that on the gas inlet side. In exit gas side therefore since the reaction low surface oxidation activity is delayed, SO ₂
Is almost not oxidized and flows downstream and is discharged, and the discharge of sulfate is suppressed. On the other hand, since HC is more easily oxidized than SO ₂ , it is easily oxidized by the noble metal carried inside the porous carrier, and HC not oxidized on the inlet gas side is oxidized and purified on the outlet gas side. .

The volume ratio of the incoming gas side and the outgoing gas side referred to in the oxidation catalyst of the first invention varies depending on the amount of the noble metal carried, but is configured so that the incoming gas side volume is 50% or less of the whole. Good. If the volume of the incoming gas exceeds 50%, the amount of sulfate produced may increase rapidly. It should be noted that the input gas side and the output gas side may be continuous catalysts, or may be composed of two catalysts in which the input gas side and the output gas side are separated. The distribution of the amount of the noble metal carried may be a two-step distribution on the inlet gas side and the outlet gas side, or the amount of the secondary particle surface layer supported may gradually decrease from the inlet gas side to the outlet gas side. it can.

There are methods described in the second invention and the third invention as an effective method for making the amount of the noble metal supported on the surface layer of the secondary particles different between the incoming gas side and the outgoing gas side. In the manufacturing method of the second aspect of the invention, the inlet gas side is coated with the porous body powder having a large specific surface area, and the outlet gas side is coated with the porous body powder having a small specific surface area. Therefore, when a precious metal is supported on such a porous coat layer, a large amount of the precious metal is supported on the surface layer of secondary particles having a large specific surface area, and a small amount of the precious metal is supported on the surface layer of secondary particles having a small specific surface area. Thereby, the oxidation catalyst of the first invention can be easily manufactured.

Further, in the production method of the third invention, a solution of a highly adsorptive noble metal salt is brought into contact with the inlet gas side of the porous carrier and the outlet gas side of the porous carrier located downstream of the exhaust gas flow is adsorbed. The noble metal is supported by contacting with a solution of a noble metal salt having a low content. This noble metal salt also includes a noble metal complex salt. The second invention and the third invention can produce an oxidation catalyst in which the production of sulfate is prevented even if they are independently performed, but the production of sulfate is further prevented by performing both in combination. Different oxidation catalysts can be produced.

On the incoming gas side using a solution of a highly adsorbable noble metal salt, many noble metal ions or noble metal complex ions are adsorbed and supported on the secondary particle surface layer of the porous carrier, but on the outgas side, noble metal ions or noble metal ions are adsorbed. Due to the low adsorptivity of complex ions, the amount of noble metal supported on the surface layer of the secondary particles is small. According to the impregnation-supporting method, a necessary amount of noble metal can be supported inside the secondary particles on the gas outlet side. Thereby, the oxidation catalyst of the first invention can be easily manufactured.

The porous carrier referred to in the first invention is alumina,
Silica, titania, zeolite, silica-alumina and
Titania-Formed by refractory inorganic oxide such as alumina
Can be The refractory inorganic oxide has an average particle size of 20.
μm or less, specific surface area 10 m ^Two/ G or more
Is preferred. Flat refractory inorganic oxides exceeding 20 μm
Uniform particle size and 10m^Two/ G specific surface area
Therefore, there is a possibility that sufficient SOF decomposition performance may not be obtained.

Typical noble metals include Pt, palladium (Pd), rhodium (Rh), and ruthenium (R).
At least one of u), osmium (Os), and iridium (Ir) can be adopted. For example, the supported amount of Pt is 0.01 to 10 per unit volume of the oxidation catalyst.
It is preferably 0 g / L. Pt loading is 0.0
If it is less than 1 g / L, there is a fear that sufficient oxidation purification performance may not be obtained. On the contrary, even if Pt is loaded in excess of 10.0 g / L, the oxidation purification performance is slightly improved and the exhaust gas purification catalyst becomes expensive. In particular, the supported amount of Pt is 0.1 to 3.0
When it is g / L, it is preferable because both oxidation purification performance and cost can be balanced.

The amount of Pd supported is preferably 0.01 to 20.0 g / L per unit volume of the exhaust gas purifying catalyst. If the amount of Pd carried is less than 0.01 g / L, there is a possibility that sufficient oxidation purification performance may not be obtained. Conversely, 20.0 g
Even if Pd is supported in excess of / L, the improvement in oxidation purification performance is slight and the exhaust gas purification catalyst becomes expensive. In particular, Pd
It is preferable that the supported amount is 0.5 to 3.0 g / L because the oxidation purification performance and the cost can be balanced.

The supported amount of Rh is preferably 0.01 to 1.0 g / L per unit volume of the exhaust gas purifying catalyst. If the supported amount of Rh is less than 0.01 g / L, there is a fear that sufficient oxidation purification performance may not be obtained. Conversely, 1.0 g /
Even if Rh is supported in excess of L, the improvement in oxidation purification performance is slight and the exhaust gas purification catalyst becomes expensive. In particular, when the amount of Rh supported is 0.05 to 0.5 g / L, it is preferable because both the oxidation purification performance and the cost can be balanced.

The porous powder having a large specific surface area used on the incoming gas side in the manufacturing method of the second invention is preferably one having a specific surface area of 70 to 400 m ² / g. If the specific surface area of the porous carrier on the incoming gas side is less than 70 m ² / g, the HC purification activity will decrease, which is not preferable. Further, if the specific surface area exceeds 400 m ² / g, there is no heat resistance and the specific surface area during use is greatly reduced, which is not preferable.

The porous powder having a small specific surface area used for the porous carrier on the gas outlet side preferably has a specific surface area of 5 to 60 m ² / g. Specific surface area is 60m
If it exceeds ² / g, sulfate is likely to be generated.
If it is less than 5 m ² / g, it becomes difficult to support the precious metal. Examples of the noble metal salt referred to in the production method of the third invention include, for example, dinitrodiammine platinum, chloroplatinic acid, dinitrodichloroplatinum, tetraamminehydroxidoplatinum, rhodium chloride, rhodium nitrate, palladium nitrate, tetrachloropalladium, tetraamminepalladium, dinitrodiamminepalladium. , Hexaammine rhodium and the like. High and low adsorptivity are relative and change depending on the type of noble metal salt selected. For example, the adsorptivity of dinitrodiammine platinum is higher than that of dichlorodiammine platinum, and the adsorptivity of dichlorodiammine platinum is higher than that of chloroplatinic acid.

[0024]

The present invention will be described more specifically with reference to examples and comparative examples. (Example 1) The half of the cordierite honeycomb carrier having a volume of 1.3 liter on the gas inlet side has a specific surface area of 180 m ² /
g of alumina powder as a main component, and then pull up to blow off excess slurry and dry it.
A heat treatment of heating at 0 ° C. for 1 hour was performed to form a front coat layer on the half of the entire incoming gas side. Then, the honeycomb carrier having the front coat layer was immersed in a dinitrodiammine platinum nitric acid aqueous solution, pulled up, and the excess aqueous solution was blown off to dry it, so that Pt was supported on the front coat layer. The supported amount of Pt is 1.5 g per liter of the honeycomb carrier.

Then, a rear coat layer containing alumina powder having a specific surface area of 5.5 m ² / g as a main component was formed on the remaining rear half on the gas outlet side in the same manner as above. Then, the rear coat layer was made to absorb water with a predetermined concentration of an aqueous solution of tetraamminehydroxidoplatinum enough to absorb half of the carrier, and dried to support Pt so that the rear coat layer had 1.5 g per liter of the honeycomb carrier. Then, the whole was treated at 250 ° C. for 1 hour to prepare the oxidation catalyst of Example 1.

In this embodiment, Pt is used as the front coat layer.
Although the rear coat layer is formed after supporting the same, the same catalyst can be produced by forming the front coat layer and the rear coat layer on the same honeycomb carrier and then supporting Pt on both coat layers. it can. It is also possible to prepare two honeycomb bodies each having a half length, form a front coat layer and a rear coat layer on each of them, carry Pt, and then bond both catalysts.

A schematic diagram of the obtained oxidation catalyst is shown in FIG.
This oxidation catalyst has a different structure on the inlet gas side of the front half and the outlet gas side of the rear half, and the size of the secondary particles of alumina is small on the inlet gas side and large on the outlet gas side. Further, since Pt is carried by the adsorption method on the incoming gas side, most of Pt is carried on the surface layer of the secondary particles, whereas Pt is carried by the water absorbing method on the outgoing gas side. Pt is supported almost uniformly from the surface layer to the inside of the secondary particles. Therefore, the total amount of Pt carried on the front coat layer and the rear coat layer is the same, but the amount of Pt carried on the secondary particle surface layer is larger in the front coat layer.

The obtained oxidation catalyst was assembled in an exhaust system of 2.6 liter DE so that the incoming gas side (front coat layer) was the upstream side of the exhaust gas flow, and the evaluation was carried out on an engine bench. As the evaluation conditions, after operating for 1 hour at an engine speed of 2000 rpm and an inlet gas temperature of 500 ° C., the inlet gas temperature was lowered by 50 ° C. and the HC amount and PM amount before and after the catalyst were measured. And 50% reduction temperature of HC and zero% of PM
The reduced temperature was determined, and the results are shown in FIGS. Note that P
M zero% reduction temperature means that the production amount of sulfate that increases as the temperature increases becomes higher than P such as SOF excluding sulfate.
It is the temperature at which the amount of reduction of the M component is the same.

(Example 2) The specific surface area of alumina powder was 2
0 m ² / g, 50 m ² / g, 60 m ² / g and 100 m
Four levels of ² / g were taken, and four kinds of oxidation catalysts were prepared in the same manner as in Example 1 except that the back coat layer was formed from each. The specific surface area of alumina of the front coat layer is 180 m ² / g, which is the same as in Example 1. Then, the HC50% reduction temperature and the PM zero% reduction temperature were obtained in the same manner as in Example 1, and the results are shown in FIG.

(Evaluation) From FIG. 2, the influence of the specific surface area of the alumina of the rear coating layer on the HC50% purification temperature is small, but the PM zero% reduction temperature greatly decreases as the specific surface area of the alumina of the rear coating layer increases. doing. P
The M zero% reduction temperature is a temperature at which the reduction of SOF and the like is offset by the sudden increase of the sulfate and the apparent PM reduction rate becomes 0%. Therefore, the reduction of the PM zero% reduction temperature facilitates the formation of sulfate. Means Since the PM zero% reduction temperature is preferably about 400 ° C. or higher, it can be seen from FIG. 2 that the specific surface area of alumina in the rear coat layer (outlet gas side) is preferably less than 60 m ² / g.

(Example 3) The specific surface area of alumina powder was 2
0 m ² / g, 50 m ² / g, 70 m ² / g and 250 m
Four kinds of oxidation catalysts were prepared in the same manner as in Example 1 except that the front coat layer was formed from each of 4 levels of ² / g. The specific surface area of alumina in the back coat layer is 5.5 m ² / g, which is the same as in Example 1. Then, the HC50% reduction temperature and the PM zero% reduction temperature were obtained in the same manner as in Example 1, and the results are shown in FIG.

(Evaluation) From FIG. 3, although the influence of the specific surface area of the alumina of the front coat layer on the PM zero% reduction temperature is small, the HC50% reduction temperature increases as the specific surface area of the alumina of the front coat layer decreases. ing. HC50
Since the% reduction temperature is preferably less than 250 ° C., it can be seen from FIG. 3 that the specific surface area of alumina in the front coat layer is preferably 70 m ² / g or more.

Example 4 and Comparative Examples 1 and 2 By changing the ratio of the front coat layer and the rear coat layer, the volume ratio of the front coat layer was 10%, 20% and 60% of the whole honeycomb carrier.
Three kinds of oxidation catalysts were prepared in the same manner as in Example 1 except that the above three levels were used. The volume ratio of the front coat layer in Example 1 was 50%. Further, for comparison, each of the front coat layer and the rear coat layer alone (volume ratio 10
The oxidation catalysts of Comparative Example 1 and Comparative Example 2 having a coating layer of 0% and 0%) were also prepared. Then, the HC 50% reduction temperature and the PM zero% reduction temperature were obtained in the same manner as in Example 1, and the results are shown in FIG.

(Evaluation) From FIG. 4, the HC50% purification temperature gradually decreases as the volume ratio of the front coat layer increases. However, it can be seen that when the volume ratio of the front coat layer exceeds 50%, the PM zero% reduction temperature sharply decreases, that is, the production amount of sulfate sharply increases. Therefore, it is understood that the volume ratio of the front coat layer is preferably 50% or less.

However, if the volume ratio of the front coat layer is 0%,
That is, the whole was formed from alumina with a small specific surface area.
In Comparative Example 2, the HC50% purification temperature is as high as about 270 ° C.
Therefore, it is necessary to form the front coat layer even if only slightly.
It is. (Example 5) Made of cordierite with a volume of 1.3 liters
A honeycomb carrier with a specific surface area of 120 m ^Two/ G of alumina powder
Dip it in a slurry whose main component is powder and pull it up to remove excess
After the slurry has been blown off, it is dried and heated at 500 ° C for 1 hour.
Heated heat treatment was performed. Next is the incoming gas side, the front half
Dip dinitrodiammine platinum nitric acid in water and pull up
The excess coating solution is blown off to dry it, and the front coat layer
Supported Pt. The loading amount of Pt is 1 liter of honeycomb carrier.
It is 1.5 g per torr.

Next, the remaining rear half on the gas outlet side was made to absorb an aqueous chloroplatinic acid solution, which was just absorbed by the carrier half, and dried to support Pt on the rear coat layer. Pt
The supported amount of was adjusted to 1.5 g per liter of the honeycomb carrier by repeating water absorption and drying. Then the whole 2
By treating at 50 ° C. for 1 hour, the oxidation catalyst of Example 5 was prepared. Then, the HC 50% reduction temperature and the PM zero% reduction temperature were obtained in the same manner as in Example 1, and the results are shown in FIG.

Further, when the coating layer was impregnated with a chloroplatinic acid aqueous solution and supported by adsorption, the ratio of Pt adsorbed to the coating layer in the solution was measured, and the loading efficiency is shown in FIG. (Example 6) An oxidation catalyst of Example 6 was prepared in the same manner as in Example 5 except that a dichlorodiammine platinum aqueous solution was used in place of the chloroplatinic acid aqueous solution in the rear half on the gas output side. For this catalyst, as in Example 1, H
The C50% reduction temperature and the PM zero% reduction temperature were determined, and the results are shown in FIG.

Further, with respect to dichlorodiammineplatinum and dinitrodiammineplatinum, the loading efficiency of Pt on the coating layer was measured, and the results are shown in FIG. (Evaluation) From FIG. 5, the loading efficiency of Pt varies depending on the type of complex ion, and is 100 in the case of dinitrodiammine platinum.
%, 50% in the case of chloroplatinic acid, and the PM zero% reduction temperature decreases as the loading efficiency in the rear half increases. Further, since the PM zero% reduction temperature is preferably 400 ° C. or higher, it is understood that it is desirable to use complex ions having a loading efficiency of 50% or less for supporting Pt on the rear half coat layer.

(Example 7 / Comparative Example 3) Dinitrodiammineplatinum and chloroplatinic acid were changed in volume ratio,
Three types of oxidation catalysts were prepared in the same manner as in Example 5 except that the volume ratio supported by the dinitrodiammine platinum nitric acid aqueous solution was set to three levels of 10%, 60% and 80%. Also, for comparison, an oxidation catalyst of Comparative Example 3 in which Pt was supported entirely using a chloroplatinic acid aqueous solution was prepared. Then, the HC 50% reduction temperature and the PM zero% reduction temperature were obtained in the same manner as in Example 1, and the results are shown in FIG. 6 together with Comparative Example 1.

(Evaluation) As shown in FIG. 6, when the volume of the incoming gas side supported by dinitrodiammine platinum exceeds 50%, P
The M zero% reduction temperature decreases to 400 ° C. or lower. Therefore, when Pt is supported on the incoming gas side using dinitrodiammine platinum, the volume thereof is preferably 50% or less of the total volume.

[0041]

[Effects of the Invention] That is, according to the oxidation catalyst for purifying exhaust gas of the present invention, it is possible to have a high oxidation purification performance of HC and to effectively suppress the production and emission of sulfate. According to the method for producing an oxidation catalyst of the present invention, it is possible to easily and reliably produce an oxidation catalyst capable of preventing the formation of sulfate.

[Brief description of the drawings]

FIG. 1 is a schematic illustration of an oxidation catalyst according to one embodiment of the present invention.

FIG. 2 Specific surface area of alumina and HC50 of the rear coating layer
It is a graph which shows the relationship with% purification temperature and PM zero% reduction temperature.

FIG. 3 shows the specific surface area of alumina and HC50 of the front coat layer.
It is a graph which shows the relationship with% purification temperature and PM zero% reduction temperature.

FIG. 4 Volume ratio of high specific surface area alumina and HC 50%
It is a graph which shows the relationship between purification temperature and PM zero% reduction temperature.

FIG. 5 is a graph showing the relationship between the Pt loading efficiency of various Pt complex salts and the HC50% purification temperature and PM zero% reduction temperature.

FIG. 6 is a ratio of the carrying volume on the incoming gas side supported with dinitrodiammine platinum and the HC50% purification temperature and P
It is a graph which shows the relationship with M zero% reduction temperature.

Claims (3)

[Claims] 1. An oxidation catalyst comprising a porous carrier and a noble metal supported on the porous carrier, wherein the noble metal supported on the surface layer of secondary particles of a porous body constituting the porous carrier. The amount of is larger on the inlet gas side located upstream of the exhaust gas flow than on the outlet gas side located downstream thereof. 2. An inlet gas side of the carrier substrate, which is located upstream of the exhaust gas flow, is coated with a porous body powder having a large specific surface area, and a porous material powder is coated on the inlet gas side of the carrier substrate, which is located downstream of the exhaust gas flow. A porous body powder having a small surface area is coated to form a porous coat layer, the porous coat layer is loaded with a noble metal, and the porous coat layer is formed by loading the surface layer of secondary particles of the porous body. A method for producing an oxidation catalyst for exhaust gas purification, characterized in that the amount of said noble metal is increased on the side of said incoming gas than on the side of said outgoing gas. 3. A solution of a highly adsorbable precious metal salt is brought into contact with the incoming gas side of the porous carrier, which is located upstream of the exhaust gas flow, and the porous carrier is placed on the outgoing gas side of the porous carrier, which is located downstream of the exhaust gas flow. The amount of the noble metal supported on the surface layer of the secondary particles of the porous body constituting the porous carrier is brought into contact with a solution of a noble metal salt having low adsorptivity from the outgas side. A method for producing an oxidation catalyst for purifying exhaust gas, which is characterized by increasing the number.