JPH06327942A - Purifying method of waste gas - Google Patents

Abstract

PURPOSE:To efficiently remove nitrogen oxide by using the waste gas purifying material composed by combining the catalyst carrying a silver component on a porous inorg. oxide with the catalyst carrying Pt, etc., on the porous inorg. oxide, adding mixed hydrocarbons or a hydrocarbon compd. in the waste gas and allowing the waste gas to contact with the purifying material at a specified temp. CONSTITUTION:The waste gas purifying material having a first catalyst carrying 0.2-15wt.% silver or silver oxide being an activated seed (expressed in terms of element) on the porous inorg. oxide at the inflow side of the waste gas and a second catalyst carrying 0.01-5wt.% at least one kind element selected from the group consisting of Pt, etc., being the activaated seed on the porous inorg. oxide at the outflow side of the waste gas is provided in the midst of the waste gas conduit. Less than 5 times mixed hydrocarbons consisting of either, liquefied petroleum gas or city gas or liquefied natural gas, or the hydrocarbon such as methane per the weight of nitrogen oxide in the waste gas, is added at the upstream side of the catalyst, and the waste gas is allowed to contact with the catalyst at 200-650 deg.C and the nitrogen oxide is removed efficiently.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification method capable of effectively removing nitrogen oxides from combustion exhaust gas containing nitrogen oxides and excess oxygen.

[0002]

2. Description of the Related Art Excessive amounts of combustion exhaust gas emitted from internal combustion engines such as automobile engines, combustion equipment installed in factories, household fan heaters, etc. Nitrogen oxides such as nitric oxide and nitrogen dioxide are contained together with oxygen. Here, "containing excess oxygen" means containing more oxygen than the theoretical oxygen amount necessary to burn unburned components such as carbon monoxide, hydrogen, and hydrocarbons contained in the exhaust gas. To do. Moreover, the nitrogen oxide in the following refers to nitric oxide and / or nitrogen dioxide.

This nitrogen oxide is considered to be one of the causes of acid rain and is a serious environmental problem. Therefore, various methods for removing nitrogen oxides in exhaust gas discharged from various combustion devices have been studied.

As a method for removing nitrogen oxides from combustion exhaust gas containing excess oxygen, a selective catalytic reduction method using ammonia is used, particularly for large-scale fixed combustion devices (large combustors such as factories). It has been put to practical use.

However, in this method, ammonia used as a reducing agent for nitrogen oxides is expensive, and ammonia is toxic, so that unreacted ammonia is discharged so that nitrogen oxides in exhaust gas are not discharged. There are problems that the amount of ammonia injection must be controlled while measuring the concentration and that the apparatus is generally large.

[0006] As another method, there is a non-selective catalytic reduction method for reducing nitrogen oxides by using a gas such as hydrogen, carbon monoxide or hydrocarbon as a reducing agent, but this method is effective. In order to reduce and remove nitrogen oxides, it is necessary to add a reducing agent in an amount equal to or larger than a theoretical reaction amount with oxygen in exhaust gas, and there is a drawback that a large amount of reducing agent is consumed. Therefore, the non-selective catalytic reduction method is practically effective only for the exhaust gas having a low residual oxygen concentration that is burned in the vicinity of the theoretical air-fuel ratio, and is not versatile and impractical.

Therefore, there has been proposed a method for removing nitrogen oxides by adding a reducing agent in an amount equal to or less than a theoretical reaction amount with oxygen in exhaust gas by using zeolite or a catalyst supporting a transition metal thereon (for example, a special method). Kaisho 63-100919, 63-28372
No. 7, JP-A-1-130735, and 59th Annual Meeting of the Chemical Society of Japan
(1990) 2A526, 60th Autumn Meeting (1990) 3L420, 3L42
2, 3L423, "Catalyst" vol.33 No.2, page 59, 1991 etc.).

However, it has been found that these methods have a narrow temperature range for removing nitrogen oxides, and that the exhaust gas containing water and having a large temperature change depending on operating conditions has a significantly low nitrogen oxide removal rate. .

Therefore, an object of the present invention is to remove nitrogen oxides, carbon monoxide, hydrogen, hydrocarbons, etc., such as combustion exhaust gas from a fixed combustion device and a gasoline engine, a diesel engine, etc. that burn under an excess oxygen condition. From the combustion exhaust gas that contains more than the theoretical reaction amount of oxygen for unburned components,
An object of the present invention is to provide an exhaust gas purification method capable of efficiently removing nitrogen oxides.

[0010]

As a result of earnest research in view of the above problems, the present inventors have found that the first catalyst comprising a specific amount of a silver component of a specific shape supported on a porous inorganic oxide, Pt, etc. The second catalyst formed by carrying the component of is separately prepared, and the exhaust gas purifying material formed by the combination is used, and the mixed hydrocarbon or the hydrocarbon compound is added to the exhaust gas, and The inventors have found that if the exhaust gas is brought into contact with the purification material, the nitrogen oxides can be effectively removed in a wide temperature range even with the exhaust gas containing 10% of water, and the present invention has been completed.

That is, in the first exhaust gas purification method of the present invention, 0.2 to 15% by weight (elemental conversion value) of silver or silver oxide, which is an active species, is added to the porous inorganic oxide on the exhaust gas inflow side. At least one element selected from the group consisting of Pt, Pd, Ru, Rh, and Ir, which has a supported first catalyst and is an active species in a porous inorganic oxide, on the exhaust gas outflow side. .01-5
An exhaust gas purifying material having a second catalyst supporting wt% is installed in the middle of the exhaust gas conduit, and on the upstream side of the purifying material,
A mixed hydrocarbon of liquefied petroleum gas, city gas, or liquefied natural gas, which is 5 times or less the weight of nitrogen oxides in the exhaust gas, or a hydrocarbon such as methane or ethane is added, 20
Contacting the exhaust gas with the purification material at 0 to 650 ° C.,
Therefore, the nitrogen oxide and the hydrocarbon are reacted to remove the nitrogen oxide.

The second exhaust gas purification method of the present invention is
Silver or silver oxide which is an active species for the porous inorganic oxide.
A group consisting of a first catalyst supporting 2 to 15% by weight (elemental conversion value), and Pt, Pd, Ru, Rh and Ir in which the second catalyst is a porous inorganic oxide and active species. An exhaust gas purifying material obtained by mixing a second catalyst supporting more than 2 wt% of at least one element selected from the above and 5 wt% or less is installed in the middle of the exhaust gas conduit, On the upstream side, a mixed hydrocarbon of liquefied petroleum gas, city gas, or liquefied natural gas, which is 5 times or less the weight of nitrogen oxides in the exhaust gas, or a hydrocarbon such as methane or ethane is added, and the amount is adjusted to 200 to 6
The exhaust gas is brought into contact with the purification material at 50 ° C., and the nitrogen oxide and the hydrocarbon are reacted with each other to remove the nitrogen oxide.

The present invention will be described in detail below. In the purifying material used in the first method of the present invention, on the exhaust gas inflow side, silver that is an active species of porous inorganic oxide or silver oxide 0.2 to
It forms 15% by weight (element conversion value) of the first catalyst, and the porous inorganic oxide P on the outflow side is the active species.
An exhaust gas purifying material formed by forming a second catalyst containing 0.01 to 5% by weight of at least one element selected from the group consisting of t, Pd, Ru, Rh, and Ir in an exhaust gas conduit. Installed and added mixed hydrocarbon or hydrocarbon compound to the exhaust gas on the upstream side of the installation position of the purification material, and contact the exhaust gas with this purification material, and use the mixed hydrocarbon or hydrocarbon compound as a reducing agent to reduce nitrogen in the exhaust gas. The oxide is reduced and removed.

In the purifying material used in the second method of the present invention,
Silver or silver oxide which is an active species for the porous inorganic oxide.
2 to 15% by weight (elemental conversion value) supporting the first catalyst and Pt, Pd, Ru, which are active species in the porous inorganic oxide,
An exhaust gas purifying material obtained by mixing with a second catalyst supporting 0.01 to 5% by weight of at least one element selected from the group consisting of Rh and Ir is installed in an exhaust gas conduit to obtain a purifying material. A mixed hydrocarbon or hydrocarbon compound is added to the exhaust gas upstream of the installation position of the exhaust gas to bring the exhaust gas into contact with this purification material, and the mixed hydrocarbon or hydrocarbon compound is used as a reducing agent to reduce and remove nitrogen oxides in the exhaust gas. To do.

The above exhaust gas purifying materials can take the following two forms. A first preferred form is a purification material obtained by coating a purification material substrate with a catalyst in which a powdery porous inorganic oxide carries a catalytically active species. As the ceramic material forming the substrate of the purification material, γ-alumina and its oxides (γ-alumina-titania, γ-alumina-silica, γ-alumina-zirconia, etc.), zirconia, titania-zirconia, and other porous materials are used. A heat-resistant material having a large surface area can be used. When high heat resistance is required, it is preferable to use cordierite, mullite, alumina and their composites. Also, a known metal material can be used for the substrate of the exhaust gas purifying material.

The shape and size of the substrate of the exhaust gas purifying material are
Various changes can be made according to the purpose. Practically, it is preferable to have two or more parts including an inlet part and an outlet part. Examples of the structure include a honeycomb structure type, a foam type, a plate type, a three-dimensional network structure type such as a fibrous refractory, a granular type, a pellet type and the like.

The second preferred form is a purification material obtained by filling a pellet-shaped, granular-shaped, or powdered porous inorganic oxide with a catalyst having a catalytically active species described below.

The purifying material used in the present invention is formed with the following two catalysts. (1) First catalyst The first catalyst comprises a porous inorganic oxide carrying a silver component, and is formed on the exhaust gas inflow side. As the porous inorganic oxide, porous alumina, silica, titania, zirconia, and their composite oxides can be used, but γ-alumina or alumina-based composite oxide is preferably used. By using γ-alumina or an alumina-based composite oxide, the reaction between the added mixed hydrocarbon or hydrocarbon compound and / or the residual hydrocarbon in the exhaust gas and the nitrogen oxide in the exhaust gas occurs efficiently.

The specific surface area of the porous inorganic oxide is 10 m ²
/ G or more is preferable. Specific surface area of 10 m ² / g
If it is less than the above range, the contact area between the exhaust gas and the inorganic oxide (and the silver component carried on the exhaust gas) becomes small, and the nitrogen oxide cannot be removed well.

The amount of the silver component supported on the above-mentioned inorganic oxide such as γ-alumina as an active species is 10
0.2 to 15% by weight (elemental conversion value) relative to 0% by weight. If it is less than 0.2% by weight, the removal rate of nitrogen oxides is lowered. Further, if the amount of silver supported exceeds 15% by weight, combustion of the mixed hydrocarbon or the hydrocarbon compound itself easily occurs,
The removal rate of nitrogen oxides rather decreases. The amount of the silver component supported is preferably 0.5 to 12% by weight. The silver component is in the state of metal or oxide in the temperature range of exhaust gas,
They can easily be converted to each other.

As a method for supporting the silver component on the inorganic oxide such as γ-alumina, a known impregnation method, a kneading method or the like can be used. Adjustment of the purifying material after loading is 50 to 15
After drying at about 0 ° C., it is preferable to raise the temperature stepwise at 100 to 600 ° C. and bake. The firing is preferably performed in air or under nitrogen flow, or under hydrogen gas flow, or while evacuating. It is preferable that the purification material fired under the flow of nitrogen gas or hydrogen gas is finally subjected to an oxidation treatment at 500 ° C.

In the first preferred embodiment of the purification material, the thickness of the first catalyst provided on the purification material substrate may be limited due to the difference in thermal expansion characteristics between the substrate material and this catalyst. Many. The thickness of the catalyst provided on the purification material base is 200
It is preferable that the thickness is less than or equal to μm. With such a thickness, it is possible to prevent the purification material from being damaged by thermal shock during use. The method of forming the catalyst on the surface of the purifying material substrate is a known washcoat method, sol-gel method, powder method or the like.

Further, the amount of the first catalyst provided on the surface of the purification material substrate is preferably 5 to 70% by weight of the purification material substrate. If the amount of the catalyst is less than 5% by weight, good NOx cannot be removed. On the other hand, when the amount of the catalyst exceeds 70% by weight, the removal characteristics do not improve so much and the pressure loss increases. More preferably, the first catalyst provided on the surface of the purification material substrate is 10 to 70% by weight of the purification material substrate.

(2) Second catalyst The second catalyst comprises a porous inorganic oxide carrying a catalytically active species and is formed on the exhaust gas outflow side. As the porous inorganic oxide, γ-alumina and its oxide (γ-alumina-titania, γ-alumina-silica, γ-alumina-
Examples thereof include heat-resistant ceramics having a large surface area such as zirconia), zirconia, and titania-zirconia. Preferably, γ-alumina, titania, zirconia and a composite oxide containing them are used. Like the first catalyst, the porous inorganic oxide has a specific surface area of 10 m ² /
It is preferably at least g.

As the active species of the above-mentioned second catalyst, Pt, P
At least one element selected from the group consisting of d, Ru, Rh, and Ir is used. In particular, Pt and Rh; Pd and Rh; P
The combination of t, Pd and Rh is effective. By providing a platinum catalyst in addition to the silver catalyst, it is possible to effectively oxidize and remove harmful substances such as unburned hydrocarbons, residual hydrocarbons and carbon monoxide. The total amount of active species supported on the inorganic oxide by the second catalyst is 0.01 to 5% by weight based on the above-mentioned porous inorganic oxide (100% by weight). If the amount of the catalytically active species exceeds 5% by weight with respect to the above-mentioned substrate, the amount of the catalyst supported will exceed the oxidative combustion of the mixed hydrocarbon or the hydrocarbon compound, and the nitrogen oxide reducing property will deteriorate. Further, if it is less than 0.01% by weight, the effect is not sufficiently exhibited. The preferable loading amount is 0.05 to 5% by weight.

As the active species of the second catalyst, 10% by weight or less of a rare earth element such as La or Ce or at least one element selected from Ca, Mg, Ba and the like may be further supported. preferable. By supporting the rare earth element, the heat resistance of the platinum-based catalyst can be improved.

For supporting the active species on the second catalyst, a known impregnation method, precipitation method, sol-gel method or the like can be used. When using the impregnation method, the porous inorganic oxide is immersed in an aqueous solution of a carbonate, a hydrochloride, a nitrate, an acetate, or a hydroxide of a catalytically active element, dried at 70 ° C., and then staged at 100 to 700 ° C. The temperature is increased and the firing is performed. Although the supported component is shown as a metal element, the supported component exists in the state of a metal and an oxide under normal use temperature conditions of the purification material.

In the first preferred embodiment of the purification material, the thickness of the second catalyst provided on the purification material substrate is 100 μm.
It is preferably m or less. Further, the amount of the second catalyst provided on the surface of the purification material substrate is preferably 5 to 70% by weight of the purification material substrate. The method of coating the purifying material substrate with the second catalyst is the same as the method of the first catalyst described above.

In the present invention, the weight ratio of the first catalyst to the second catalyst is preferably 5: 1 to 1: 5. The ratio is less than 1: 5 (less first catalyst)
Then, the purification rate of nitrogen oxides is generally lowered in a wide temperature range of 250 to 600 ° C. On the other hand, when the ratio exceeds 5: 1 (the amount of the first catalyst is large), the purifying ability of nitrogen oxides at 400 ° C or lower does not increase. That is, the reaction between the reducing agent and the nitrogen oxide does not proceed sufficiently at a relatively low temperature.
More preferable weight ratio of the first catalyst to the second catalyst is from 4: 1 to
It is 1: 4.

If the purifying material having the above-mentioned structure is used,
In a wide temperature range of up to 650 ° C., good nitrogen oxides can be removed even with exhaust gas containing about 10% of water.

Next, the method of the present invention will be described. In the first exhaust gas purification method of the present invention, the first catalyst is installed in the middle of the exhaust gas pipe so that the first catalyst faces the exhaust gas inlet and the second catalyst faces the exhaust gas outlet. In the second exhaust gas purification method of the present invention, a purification material obtained by mixing the first catalyst and the second catalyst is installed in the middle of the exhaust gas conduit.

The present invention is a method for treating combustion exhaust gas from an engine which uses a mixed hydrocarbon composed of liquefied petroleum gas, city gas or liquefied natural gas as a fuel.
Such exhaust gas contains some residual hydrocarbons. If the residual hydrocarbons are not sufficient to reduce the nitrogen oxides in the exhaust gas, it is necessary to add organic compounds from the outside. At this time, the above-described purification material is installed in the middle of the exhaust gas conduit, and the mixed hydrocarbon or hydrocarbon compound that is the fuel of the engine is added to the exhaust gas on the upstream side of the installation site of the purification material.

As the hydrocarbon compound, alkane, alkene, alkyne and the like are used. Preferably, methane, ethane, propane, butane, etc. are used. As the mixed hydrocarbon, liquefied petroleum gas, city gas, or liquefied natural gas is used. Also, other mixed hydrocarbons may be used. By using these mixed hydrocarbons, 600
Nitrogen oxide removal performance in a high temperature range of ℃ or higher is improved. When a mixed hydrocarbon is used, if a saturated hydrocarbon having a small number of carbon atoms is the main component, the nitrogen oxide removal characteristics at low temperatures deteriorate, so it is preferable to add a hydrocarbon having a large number of carbon atoms before use. .

The amount of the mixed hydrocarbon or hydrocarbon compound introduced from the outside is preferably such that the weight ratio (weight of reducing agent added / weight of nitrogen oxide in exhaust gas) is 5 or less. If this weight ratio exceeds 5, it leads to deterioration of fuel efficiency.

Further, in the present invention, the temperature of the exhaust gas at the purification material installation site where the mixed hydrocarbon or hydrocarbon compound reacts with the nitrogen oxides is maintained at 200 to 650 ° C. If the temperature of the exhaust gas is less than 200 ° C., the reaction between the reducing agent and the nitrogen oxide does not proceed, and the nitrogen oxide cannot be removed satisfactorily. On the other hand, if the temperature exceeds 650 ° C., the combustion of the mixed hydrocarbon or the hydrocarbon compound itself starts, and the nitrogen oxide cannot be reduced and removed. A preferable exhaust gas temperature is 300 to 600 ° C.

[0036]

The present invention will be described in more detail by the following specific examples. Example 1 10 g of commercially available pelletized γ-alumina (diameter 1.5 mm, length about 6 mm, specific surface area 200 m ² / g) was loaded with 5% by weight of silver using an aqueous solution of silver nitrate, and dried at 70 ° C., 15
Catalyst 1 was prepared by calcining stepwise to 0 ° C, 200 ° C, 300 ° C, 400 ° C, 500 ° C, and 600 ° C. Further, 5 g of the same pellet-shaped alumina was loaded with 3% by weight of Pt by using an aqueous solution of chloroplatinic acid, dried and calcined to 700 ° C. to prepare catalyst 2.

A purifying material in which 4 g of the silver-based catalyst and 2 g of the Pt-based catalyst were mixed was set in the reaction tube. Next, a gas having the composition shown in Table 1 (nitrogen monoxide, oxygen, liquefied petroleum gas, nitrogen, and water) was flowed at a flow rate of 2.2 liters per minute (standard state) (total apparent space velocity of about 10 1,000 h ⁻¹ , the contact time between the silver-based catalyst and the Pt-based catalyst is 0.1 and 0.05 seconds, respectively.
g / ml), the temperature of the exhaust gas in the reaction tube was kept in the range of 200 to 700 ° C., and the liquefied petroleum gas and nitrogen oxide were reacted.

The nitrogen oxide concentration of the gas after passing through the reaction tube was measured by a chemiluminescence type nitrogen oxide analyzer to determine the nitrogen oxide removal rate. The results are shown in Fig. 1.

Table 1 Concentration of components Nitrogen monoxide 800 ppm Oxygen 10% by volume Liquefied petroleum gas 1700 ppm Nitrogen balance Moisture 10% by volume based on the amount of gas composed of the above components

Example 2 Powdery γ-alumina (specific surface area 2
A catalyst having 5% by weight of silver supported on 00 m ² / g) was prepared, and about 1.0 g of this catalyst was commercially available and made of a cordierite honeycomb-shaped compact (diameter 30 mm, length 12.6 mm, 400 cells / square inch). ) Was coated, dried and then fired stepwise to 600 ° C. to prepare a purification material. In addition, a chloroplatinic acid solution was used for the same honeycomb-shaped compact (length 6 mm) to coat 0.4 g of a catalyst containing 3 wt% of Pt on powdery γ-alumina, and the temperature was reduced to 700 ° C after drying. Firing was performed to prepare a purification material. It was set in the reaction tube so that the inflow side of the exhaust gas was a silver-based purification material and the outflow side was a platinum-based purification material. This purification material was evaluated under the same conditions as in Example 1 using the gases having the components shown in Table 1 (total apparent space velocity of 10,000).
0h ^-1 ). The experimental results are shown in FIG.

Example 3 The same purification material as in Example 2 was used, except that propane was used instead of liquefied petroleum gas among the gases having the compositions shown in Table 1.
Evaluation was performed under the same conditions as in Example 1. The experimental results are shown in FIG.

Comparative Example 1 The same γ-alumina pellet used in Example 1 was used as a purification material, and this purification material g was set in a reaction tube and evaluated with the gas having the composition shown in Table 1. The experimental results are shown in FIG.

As can be seen from the above, in Examples 1 to 3, good removal of nitrogen oxides was observed in a wide exhaust gas temperature range. On the other hand, in Comparative Example 1, the temperature range for removing nitrogen oxides was narrow.

[0044]

As described above in detail, the use of the exhaust gas purification method of the present invention makes it possible to efficiently remove nitrogen oxides in exhaust gas containing excess oxygen in a wide temperature range. INDUSTRIAL APPLICABILITY The exhaust gas purification method of the present invention can be widely used for exhaust gas purification of various combustors, automobiles and the like.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the exhaust gas temperature and the removal rate of nitrogen oxides in the exhaust gas in Examples 1 to 3 and Comparative Example 1.

Claims (5)

[Claims] 1. A combustion exhaust gas from an engine that uses a mixed hydrocarbon of any one of liquefied petroleum gas, city gas, and liquefied natural gas as a fuel, and has a larger amount of oxygen than the theoretical reaction amount for an unburned component coexisting with nitrogen oxides In the exhaust gas purification method for removing nitrogen oxides from combustion exhaust gas containing and, 0.2 to 15% by weight of elemental silver or silver oxide, which is an active species in a porous inorganic oxide, is provided on the exhaust gas inflow side. At least one element selected from the group consisting of Pt, Pd, Ru, Rh, and Ir, which are active species in the porous inorganic oxide, on the exhaust gas outflow side. An exhaust gas purifying material having a second catalyst supporting 0.01 to 5% by weight is installed in the middle of the exhaust gas conduit, and 5 times or less of the weight of nitrogen oxides in the exhaust gas is provided on the upstream side of the purifying material. Of liquefied petroleum gas, city gas, liquefied natural gas Mixed hydrocarbon or methane consisting or displacement, hydrocarbons ethane was added, 200-65
An exhaust gas purification method comprising contacting exhaust gas with the purification material at 0 ° C., thereby causing the nitrogen oxide and the hydrocarbon to react to remove the nitrogen oxide. 2. A combustion exhaust gas from an engine that uses a mixed hydrocarbon of any one of liquefied petroleum gas, city gas and liquefied natural gas as a fuel, and oxygen that is larger than a theoretical reaction amount with respect to an unburned component coexisting with nitrogen oxides. In the exhaust gas purification method for removing nitrogen oxides from combustion exhaust gas containing and, silver or silver oxide 0.2 to
15% by weight (elemental conversion value) is supported on the first catalyst, and the second catalyst is selected from the group consisting of Pt, Pd, Ru, Rh and Ir which are active species in the porous inorganic oxide. The exhaust gas purifying material obtained by mixing the second catalyst supporting 0.01 to 5% by weight of at least one element is installed in the middle of the exhaust gas conduit, and the exhaust gas is provided on the upstream side of the purifying material. The mixed gas consisting of liquefied petroleum gas, city gas, or liquefied natural gas, which is 5 times or less the weight of nitrogen oxides in the mixture, or hydrocarbon such as methane or ethane is added, and the exhaust gas is purified at 200 to 650 ° C. A method for purifying an exhaust gas, which comprises contacting a material with the nitrogen oxide and reacting the nitrogen oxide with the hydrocarbon to remove the nitrogen oxide. 3. The exhaust gas purifying method according to claim 1, wherein the purifying material is formed by coating the surface of a ceramic or metal three-dimensional structure with the first and second catalysts. A characteristic exhaust gas purification method. 4. The exhaust gas purification method according to claim 1, wherein the porous inorganic oxides of the first and second catalysts are in the form of pellets or granules, respectively. 5. The exhaust gas purification method according to claim 1, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide in the first catalyst, and alumina in the second catalyst. An exhaust gas purification method, which is a composite oxide comprising any one of titania and zirconia or two or more thereof.