JP2015180490A - Three-way catalyst, and clarification method of methane-containing gas using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a three-way catalyst excellent in low-temperature performance, having high sulfur poisoning resistivity, and also excellent in methane removal performance even on the lean side.SOLUTION: There is provided a three-way catalyst constituted by carrying ruthenium and platinum on an inorganic oxide carrier mainly composed of zirconium oxide of a monoclinic crystal, for removing reductively a nitrogen oxide by using methane as reducing power, in a three-way catalyst reaction treatment of methane-containing gas containing carbon monoxide, nitrogen oxide and methane which is generated by combustion of gas in which combustion air to a fuel is adjusted at a theoretical air-fuel ratio.

Description

  The present invention is, for example, a technique for performing a three-way catalytic reaction treatment of a methane-containing gas containing carbon monoxide, nitrogen oxides, and methane, which is generated by combustion of a gas whose combustion air for fuel is adjusted to a stoichiometric air-fuel ratio. About.

  The exhaust gas of an internal combustion engine such as an engine contains nitrogen oxides, carbon monoxide, and hydrocarbons. If these components are released into the atmosphere as they are, there is a problem from the viewpoint of the environment and the like, and therefore, an exhaust gas purification method (three-way catalyst method) that removes the three components from the exhaust gas using a three-way catalyst is widely used.

  The three-way catalyst method controls the air-fuel ratio of exhaust gas and balances the oxidizing component and reducing component in the exhaust gas, and then oxidizes nitrogen through the exhaust gas to a catalyst containing platinum or rhodium (three-way catalyst). It is intended to remove substances, carbon monoxide, and hydrocarbons simultaneously.

  The exhaust gas purification method using a three-way catalyst has been applied to exhaust gas purification of gasoline automobiles, and has had a great effect on reducing nitrogen oxides from automobile exhaust gas. When the three-way catalyst method is applied to gasoline automobile exhaust gas, any component of nitrogen oxides, carbon monoxide, and hydrocarbons can be satisfactorily removed at and near the air-fuel ratio (excess air ratio) λ = 1.000. .

  However, in the exhaust gas purification method using a three-way catalyst, the purification rate of carbon monoxide and hydrocarbons is maintained high at an air-fuel ratio on the lean (fuel lean or oxygen excess) side of λ = 1.000, but nitrogen There is a known tendency for the oxide removal rate to decrease. On the other hand, at an air-fuel ratio on the rich side (excessive fuel, that is, oxygen shortage) than λ = 1.000, the purification rate of nitrogen oxides is high, but the purification rate of carbon monoxide and hydrocarbons tends to decrease. Yes. The reason why the purification performance decreases at the lean and rich air-fuel ratio is considered to be because the balance between the oxidizing component and the reducing component is lost.

The range of the air-fuel ratio that can remove the three kinds of gas components in a well-balanced manner, which is a guideline for using the three-way catalyst, is generally called “window”.
It is also known that when the hydrocarbon in the exhaust gas is mainly methane, such as the exhaust gas of a gas engine, this window region is very narrow, and a high purification rate cannot be obtained with the same catalyst and operating conditions as gasoline exhaust gas. In particular, the lean side is characterized by a low methane purification rate. This is thought to be due to the fact that methane is the most stable hydrocarbon among the hydrocarbons and lacks reactivity. Therefore, a three-way catalyst for combustion exhaust gas of main component gas of methane has been developed that can obtain a high purification rate even for exhaust gas having a high proportion of methane in hydrocarbons (Patent Documents 1 and 2).

However, in recent years, as a result of improving the energy conversion efficiency of the engine, the temperature of the exhaust gas tends to be lowered, and there is a concern that a conventional three-way catalyst requires a large amount of catalyst to obtain sufficient performance at a low temperature. Therefore, there is a need for a highly active three-way catalyst that can be used at lower temperatures.
In view of such a situation, iridium and platinum are supported on an inorganic oxide mainly composed of monoclinic zirconium oxide, and methane in a methane-containing gas can be used as a reducing power at a stoichiometric air-fuel ratio. A three-way catalyst has also been proposed, and it has been shown that high methane oxidation activity can be obtained even on the lean side with improved performance in a low temperature range (Patent Document 3). However, since iridium is the rarest metal among the platinum group metals and expensive, there is a strong demand for a catalyst that can provide sufficient performance with a less expensive component and a larger amount of resources.

  It is known that a catalyst in which ruthenium and platinum are supported on an inorganic oxide mainly composed of monoclinic zirconium oxide exhibits high methane oxidation activity in an oxygen-excess (lean) atmosphere (Patent Document 4). However, this catalyst is a combustion exhaust gas containing methane and containing a large excess of oxygen (about 2% or more on a volume basis and having oxygen more than about 5 times the oxidation equivalent of reducing components such as hydrocarbons) The methane oxidation activity and nitrogen oxide removal performance in the vicinity of the theoretical air-fuel ratio (that is, excess air ratio λ = 1.000) was completely unknown. In addition, in an oxidizing atmosphere containing a large excess of oxygen, the supported ruthenium and platinum are presumed to be in an oxidized state, whereas under conditions near the stoichiometric air-fuel ratio that hardly contains oxygen, ruthenium and platinum are assumed. Is presumed to be close to a metal, and the chemical state of the active metal is completely different. Therefore, it is not easy to infer the reactivity in one condition from the reactivity in one condition.

  Disclosed is a method for producing an exhaust gas purification catalyst in which at least one of platinum and rhodium and at least one of iridium and ruthenium are supported on an inorganic support such as activated alumina by a specific method using citric acid. (See Patent Document 5). According to this publication, iridium and / or ruthenium forms a solid solution having a high melting point with platinum and / or rhodium, so that the heat resistance of the obtained catalyst is improved. However, this publication only shows that the NOx conversion rate of the obtained catalyst has been improved, and does not teach any oxidative decomposition of refractory methane, especially among hydrocarbons contained in exhaust gas. .

JP-A-5-23592 Japanese Patent Laid-Open No. 7-313878 JP 2006-181569 A JP 2007-90331 A Japanese Patent Laid-Open No. 3-98644

  That is, in any of the techniques, there is still room for improvement in order to maintain the reaction activity of the three-way catalyst easily high and exhibit sufficiently high activity even at low temperatures.

  In view of the above situation, an object of the present invention is to provide a highly active methane main component gas three-way catalyst for combustion exhaust gas capable of maintaining high performance even at low temperatures.

  The inventors of the present invention found out as a result of earnest research to solve the above-mentioned problems, and the characteristic configuration of the three-way catalyst of the present invention for achieving this purpose is an inorganic material mainly composed of monoclinic zirconium oxide. Composed of ruthenium and platinum supported on an oxide carrier, combustion air for fuel is generated by combustion of a gas adjusted to the theoretical air-fuel ratio, and a methane-containing gas containing carbon monoxide, nitrogen oxides and methane In the three-way catalytic reaction treatment, the nitrogen oxide is reduced and removed using the methane as a reducing power. In this specification, “theoretical air-fuel ratio” refers to a range in which the air amount is about 0.990 times to 1.005 times (λ = 0.990 to 1.005) the theoretical air amount.

In the above-described characteristic configuration, the amount of ruthenium supported is preferably 0.1 to 0.5% with respect to the mass of the inorganic oxide support. Moreover, it is preferable that the supported amount of platinum is 0.5 to 5% on the mass basis with respect to the inorganic oxide support.
The inorganic oxide support preferably contains more than 50% by mass of monoclinic zirconium oxide.

  In the present invention, “main component” means that the material X as a constituent material is one of the main active ingredients, and an additive may be added as necessary. As long as the characteristics possessed by the material appear, the blending ratio is not particularly limited, and the material X alone does not necessarily need to be the most abundant material in the mixture, and preferably 50% or more is composed of the material X. Although it is preferable, it may be less.

  In order to achieve this object, the characteristic means of the method for purifying a methane-containing gas of the present invention is a methane-containing gas generated by combustion of a gas adjusted to a stoichiometric air-fuel ratio and containing carbon monoxide, nitrogen oxides, and methane. Is contacted with a three-way catalyst constituted by supporting ruthenium and platinum on an inorganic oxide support mainly composed of monoclinic zirconium oxide, and using methane in the methane-containing gas as a reducing power, The point is that carbon monoxide, nitrogen oxides, and methane in the methane-containing gas are removed by a three-way catalytic reaction.

In the above characteristic means, it is preferable that the air-fuel ratio of the methane-containing gas is adjusted to λ = 0.998 to 1.000 and brought into contact with the three-way catalyst.
Moreover, it is preferable to perform the removal reaction of carbon monoxide, nitrogen oxides, and methane in the methane-containing gas at 400 ° C to 550 ° C.

  The catalyst of the present invention is composed of a low-resource component with a large amount of resources that is formed by supporting ruthenium and platinum on an inorganic oxide carrier mainly composed of monoclinic zirconium oxide, and the combustion air for the fuel is theoretically empty. In a three-way catalytic reaction treatment of a methane-containing gas containing carbon monoxide, nitrogen oxides and methane generated by combustion of a gas adjusted to the fuel ratio, the nitrogen oxides are reduced and removed using the methane as a reducing power. . As a result, it exhibits high performance at low temperatures even for exhaust gases such as natural gas combustion exhaust gas, in which methane, which has a low reactivity, occupies the majority of hydrocarbons, and improves the purification rate of nitrogen oxides, carbon monoxide and hydrocarbons. It can be kept high, and performance degradation is small even in the presence of sulfur oxides.

It is a figure which shows the purification performance of the conventional three-way catalyst. It is a figure which shows the purification performance of the three-way catalyst of this invention.

Embodiments of the present invention will be described below.
The catalyst of the present invention is constituted by supporting ruthenium and platinum on an inorganic oxide carrier mainly composed of monoclinic zirconium oxide, and is generated by combustion of gas in which combustion air for fuel is adjusted to a theoretical air-fuel ratio. In the three-way catalytic reaction treatment of a methane-containing gas containing carbon monoxide, nitrogen oxides, and methane, the three-way catalyst performs reduction removal of the nitrogen oxides using the methane as a reducing power.

This three-way catalyst is obtained by impregnating an inorganic oxide carrier mainly composed of monoclinic zirconium oxide with a solution containing ruthenium and platinum ions, followed by drying and firing.
If the specific surface area of the support is too low, the supported ruthenium and platinum cannot be kept highly dispersed. On the other hand, if the specific surface area is too high, the support may become unstable and sintering of the support may proceed during use. Therefore, it is good to set it as the range of 2-90 m < ² > / g, More preferably, it is the range of 5-60 m < ² > / g.

As monoclinic zirconium oxide satisfying such conditions, commercially available zirconium oxide for catalyst supports may be used, or commercially available zirconium hydroxide may be calcined at 600 to 1000 ° C. Zirconium oxide is known to be monoclinic, tetragonal, or cubic depending on the preparation conditions, the presence or absence of additives, and the amount thereof.
The support used in the three-way catalyst of the present invention may contain tetragonal or cubic zirconium oxide, but it must contain monoclinic zirconium oxide as a main component. In the case of the present invention, if the proportion of monoclinic zirconium oxide as a main component in the support including components other than zirconium oxide is more than 50% on a mass basis, nitrogen oxidation in the vicinity of the theoretical air-fuel ratio is performed. When the ratio of the monoclinic zirconium oxide is more than 60% on a mass basis, the nitrogen oxide purification ratio near the stoichiometric air-fuel ratio is further improved.

The carrier may contain cerium oxide, aluminum oxide (alumina) or the like as a component other than zirconium oxide. In particular, since cerium oxide has an oxygen storage capacity, it is highly effective in exhibiting stable purification performance under conditions where the air-fuel ratio varies. However, in order to obtain high purification performance at a low temperature, the content is preferably 20% or less on a mass basis.
The impregnation of ruthenium and platinum may be performed using an aqueous solution prepared by dissolving a water-soluble compound of these metals in pure water, or an organometallic compound such as an acetylacetonato complex in an organic solvent such as acetone. You may carry out using the melt | dissolved organic solvent solution.
Examples of water-soluble compounds include ruthenium chloride, ruthenium nitrate, hexaammineruthenium nitrate, chloroplatinic acid (hexachloroplatinic acid), and tetraammineplatinum nitrate. If a solution having a desired concentration cannot be obtained with water alone, a solution may be prepared by adding hydrochloric acid, nitric acid or aqueous ammonia as necessary.
Examples of organometallic compounds include tris (acetylacetonato) ruthenium and bis (acetylacetonato) platinum.
In the impregnation operation, depending on the type of the metal compound, precipitation may occur due to mixing. In such a case, different metals may be sequentially supported on the carrier. For example, the first active ingredient can be supported on a carrier, and if necessary, the second active ingredient can be supported after drying or after drying and calcination.

If the supported amount of platinum is too small, the three-way catalytic activity is low, and if it is too large, the particle size of platinum becomes large and performance corresponding to the supported amount cannot be obtained, resulting in poor economic efficiency. Therefore, it is preferably 0.5 to 5% with respect to the mass of the carrier.
If the supported amount of ruthenium is too small, the three-way catalyst activity is low, and if it is too large, the particle size of ruthenium becomes large and performance corresponding to the supported amount cannot be obtained, resulting in poor economic efficiency. Therefore, it is preferably 0.1 to 0.5% with respect to the mass of the carrier.
The mass ratio of supported platinum and ruthenium is usually about 20: 1 to 2: 1, preferably about 15: 1 to 5: 1.
The impregnation time is not particularly limited as long as a predetermined loading amount is ensured, but is usually about 1 to 50 hours, preferably about 3 to 20 hours.
Next, the carrier carrying the predetermined metal component is evaporated to dryness or dried as necessary, and then fired.
Firing may be performed under air circulation. Or you may carry out in oxidizing gas circulation, such as air or the gas which mixed oxygen and inert gas, such as nitrogen, suitably.
If the firing temperature is too high, grain growth of the supported metal proceeds and high activity cannot be obtained. In particular, ruthenium may vaporize by forming a gaseous oxide having a high oxidation number at a high temperature. On the other hand, if it is too low, the calcination is not performed sufficiently, so that the metal particles supported during the use of the catalyst may become coarse and stable activity may not be obtained. Therefore, in order to stably obtain a high catalytic activity, the calcination temperature is preferably about 450 to 650 ° C., more preferably about 500 to 600 ° C.
The firing time is not particularly limited, but is usually about 1 to 50 hours, preferably about 3 to 10 hours.

The three-way catalyst of the present invention may be used by molding its shape into an arbitrary shape such as a pellet shape or a honeycomb shape. For example, a wash-resistant honeycomb such as cordierite may be used as a wash coat, and pressure loss can be reduced by doing so.
When wash-coating on a fire-resistant honeycomb, the carrier can be preliminarily treated in the same manner by adding a zirconium oxide sol or the like to the three-way catalyst prepared by the above method to make a slurry. Ruthenium and platinum may be supported according to the above method after wash-coating on the fire-resistant honeycomb.

  When the cordierite is wash-coated, the coating amount of the three-way catalyst is 50 to 300 g as an inorganic oxide support, 0.05 to 1.5 g as ruthenium, and 0.25 to 15 g as platinum per liter of cordierite. More preferably, the inorganic oxide carrier is in the range of 100 to 250 g, ruthenium in the range of 0.1 to 1.25 g, and platinum in the range of 0.5 to 12.5 g.

  The methane-containing gas purification method using a catalyst as described above is generated by combustion of a gas adjusted to a theoretical air-fuel ratio, and methane-containing gas containing carbon monoxide, nitrogen oxides and methane is used as the three-way catalyst. The carbon monoxide, nitrogen oxide and methane in the methane-containing gas are removed by a three-way catalytic reaction using methane in the methane-containing gas as a reducing power.

When the air-fuel ratio of the methane-containing gas to be purified is at the stoichiometric air-fuel ratio, it is not necessary to adjust the air-fuel ratio. When a methane-containing gas that does not have such an air-fuel ratio is the target of purification, for example, a method of directly controlling the air-fuel ratio of the combustor, which is normally performed, or the oxygen excess of the combustion methane-containing gas is measured. Accordingly, the air-fuel ratio is adjusted by a method of adding an oxidizing gas such as air or a reducing gas such as fuel.
Here, the stoichiometric air-fuel ratio means that the amount of combustion air with respect to the fuel input to the combustor is usually the minimum value (theoretical air amount) necessary for complete combustion. For example, the air amount is the theoretical air amount. Is in the range of about 0.990 times to 1.005 times (that is, the excess air ratio λ = 0.990 to 1.005). In practice, when the air-fuel ratio is vibrated in units of several tens of milliseconds to several seconds, the time-averaged air-fuel ratio may be within the above range.
As a special case, when air, fuel, or the like is added at the subsequent stage of the combustor, the sum of these and the amount of fuel or air introduced into the combustor may be within the above range.
Even when a gas having a different oxygen concentration, such as oxygen-enriched air, is used as the combustion air, the theoretical gas amount can be calculated according to the oxygen content. It may be about 990 times to 1.005 times.
Preferably, a methane-containing gas having an air-fuel ratio of λ = 0.998 to 1.000 is brought into contact with the three-way catalyst according to the present invention. If it does in this way, very high purification ability can be exhibited.

Although the three-way catalyst of the present invention has a high activity, the activity is lowered at a too low temperature, and the desired oxidation performance may not be obtained. Therefore, the catalyst layer temperature should be maintained at 400 ° C. or higher. Is preferred. In addition, when used at a temperature exceeding 600 ° C., the durability of the three-way catalyst may be deteriorated. In particular, when air is circulated for a long time at a temperature of 600 ° C. or more, active metal aggregation (granular growth) is promoted, and there is a concern about deterioration of the three-way catalyst.
More preferably, the gas discharged from the engine operating at a low temperature of 400 to 550 ° C. is directly brought into contact with the catalyst.
Although the usage-amount of a catalyst can be suitably selected according to the purification | cleaning rate requested | required, it is the range of 1000-200,000h < ^-1 > as a space velocity per gas time (GHSV) normally. As the GHSV is lowered, the amount of the catalyst is increased, so that the catalyst performance is improved. For example, when used at 1000 h ⁻¹ or less, in addition to the economical problem, there is a problem that the pressure loss in the catalyst layer increases. May occur. On the other hand, there is a concern that sufficient performance cannot be secured under conditions where GHSV exceeds 200,000 h ⁻¹ . Preferably, it is in the range of 10,000 to 100,000 h ⁻¹ as GHSV.

The methane-containing gas may contain a sulfur component such as sulfur dioxide derived from the sulfur content in the fuel. However, as is clear from the examples, the three-way catalyst of the present invention has high resistance to sulfur poisoning, and thus high purification performance is maintained even in such a case.
In addition, the methane-containing gas may contain hydrocarbons other than methane and other organic components. Even in such a case, since the three-way catalyst of the present invention has a high oxidation activity that can also use inert methane, hydrocarbons other than methane and other organic components can be effectively removed, and purification performance is improved. There is no inhibition.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated in detail, this invention is not limited to these Examples.

(Comparative Example 1)
Aluminum oxide for catalyst (active type, 4-6 mm) manufactured by Kishida Chemical Co., Ltd. was crushed and sieved, and the particle size was adjusted to 1-2 mm. 100 g of this was impregnated with a solution of 31 g of cerium nitrate (III) hexahydrate (Ce (NO ₃ ) ₃ .6H ₂ O) in 180 g of pure water for 12 hours. This was evaporated to dryness, dried at 120 ° C. for 1 hour, and further fired in air at 700 ° C. for 6 hours to obtain cerium oxide-aluminum oxide (BET specific surface area 109 m ² / g).
A mixture of 3.16 g of a dinitrodiammine platinum nitrate solution (containing 20% by mass of nitric acid) containing 10.1% by mass as Pt and 0.64 g of an aqueous rhodium nitrate solution containing 5.0% by mass as Rh, Dilute with water. This was impregnated in 16 g of the above cerium oxide-aluminum oxide support, evaporated to dryness, dried at 120 ° C. for 1 hour, and further calcined in air at 550 ° C. for 6 hours to give 2% Pt−0.2% Rh / Cerium oxide-aluminum oxide (hereinafter referred to as catalyst A) was obtained. The BET specific surface area of the catalyst A was 104 m ² / g.

(Comparative Example 2)
Dinitrodiammine platinum nitrate solution containing 10.1% by mass as Pt (containing 20% by mass nitric acid) 3.17g and ruthenium nitrate aqueous solution 0.82g containing 3.9% by mass as Ru were mixed, and 15g of pure Dilute with water. This was impregnated in 16 g of the same cerium oxide-aluminum oxide support as in Comparative Example 1, evaporated to dryness, dried at 120 ° C. for 1 hour, and further calcined in air at 550 ° C. for 6 hours to give 2% Pt-0.2. % Ru / cerium oxide-aluminum oxide (hereinafter referred to as catalyst B) was obtained.

(Comparative Example 3)
Zirconium hydroxide (manufactured by Hayashi Pure Chemical Industries, Ltd., containing 85% as ZrO ₂ ) 122.5 g and cerium nitrate hexahydrate (Ce (NO ₃ ) ₃ .6H ₂ O) 50 g were dissolved in 100 g of pure water. It was immersed in an aqueous solution for 15 hours and evaporated to dryness. Furthermore, after drying at 120 ° C. for 1 hour and baking at 700 ° C. for 6 hours, a cerium oxide-zirconium oxide having a BET specific surface area of 33 m ² / g (composite oxide having a mass ratio of cerium oxide: zirconium oxide = 16: 84) )
In the X-ray diffraction measurement using Cu-Kα ray, a tetragonal diffraction line near 2θ = 30 ° was strongly observed, and monoclinic diffraction lines near 2θ = 28 ° and 31.5 ° were weak. Observed. From these strength ratios, it was calculated that 79% of this cerium oxide-zirconium oxide was tetragonal or cubic, and the proportion of monoclinic crystal was 21%. If all monoclinic crystals are composed of zirconium oxide, the proportion of monoclinic zirconium oxide in the cerium oxide-zirconium oxide (that is, the inorganic oxide carrier) is about 20%.
3.17 g of dinitrodiammine platinum nitric acid solution containing 10.1% by mass as Pt (containing 20% by mass of nitric acid) and trinitratonitrosylruthenium (Ru (NO) (NO ₃ ) ₃ ) containing 12.5% as Ru 0.256 g of the aqueous solution was mixed and diluted with 10 g of pure water. This was impregnated in 16 g of the cerium oxide-zirconium oxide support, evaporated to dryness, dried at 120 ° C. for 1 hour, and further calcined in air at 550 ° C. for 6 hours to give 2% Pt−0.2% Ru / Cerium oxide-zirconium oxide (hereinafter referred to as catalyst C) was obtained.

Example 1
Zirconium oxide (Nippon Denko, N-PC, specific surface area 28 m ² / g) was calcined in air at 700 ° C. for 6 hours to obtain calcined zirconium oxide having a BET specific surface area of 17 m ² / g. In the X-ray diffraction measurement using Cu-Kα rays, only monoclinic diffraction lines near 2θ = 28 ° and 31.5 ° were observed. It is thought that it has become.
Instead of 16 g of the cerium oxide-zirconium oxide support 16 of Comparative Example 3, 10 g of the above-mentioned calcined zirconium oxide and 6 g of cerium oxide-zirconium oxide of Comparative Example 3 were used as a support in the same manner as Comparative Example 3. A 2% Pt-0.2% Ru / cerium oxide-zirconium oxide mixed carrier (hereinafter referred to as catalyst D) was obtained. The BET specific surface area of the catalyst D was 22 m ² / g. Further, the ratio (mass basis) of monoclinic zirconium oxide in the inorganic oxide support of catalyst D is calculated to be 70%. Further, the proportion (mass basis) of cerium oxide in the inorganic oxide carrier of the catalyst D is 6.0%.

(Examples 2 and 3)
2% Pt-0.1% Ru / cerium oxide-zirconium oxide mixed carrier (similar to Example 1 except that the Ru loading was changed to 0.1% and 0.5% by mass ratio to the carrier) Hereinafter, catalyst E) and 2% Pt-0.5% Ru / cerium oxide-zirconium oxide mixed carrier (hereinafter referred to as catalyst F) were obtained.

(Comparative Examples 4 and 5)
A mixture of 10 g of calcined zirconium oxide similar to Example 1 and 6 g of cerium oxide-zirconium oxide of Comparative Example 3 was used as a carrier, and 2% Pt-0.2% in the same manner as Example 3 of Patent Document 3. An Ir / cerium oxide-zirconium oxide mixed carrier (hereinafter referred to as catalyst G) and a 2% Pt-0.5% Ir / cerium oxide-zirconium oxide mixed carrier (hereinafter referred to as catalyst H) were obtained.

Example 4
Zirconium hydroxide (produced by Hayashi Junyaku Kogyo Co., Ltd., containing 79% as ZrO ₂ ) 39.4 g and cerium nitrate hexahydrate (Ce (NO ₃ ) ₃ .6H ₂ O) 36.6 g in 60 g of pure water It was immersed in the dissolved aqueous solution for 15 hours and evaporated to dryness. Furthermore, after drying at 120 ° C. for 1 hour and firing at 700 ° C. for 6 hours, cerium oxide-zirconium oxide B having a BET specific surface area of 35 m ² / g (cerium oxide: zirconium oxide = 32: 68 by mass ratio) was obtained. .
In the X-ray diffraction measurement using Cu-Kα ray, diffraction lines of monoclinic zirconium oxide around 2θ = 28 ° and 31.5 ° were not observed. Therefore, this cerium oxide-zirconium oxide B is substantially free of monoclinic crystals.
3.09 g of a dinitrodiammine platinum nitrate solution (containing 20% by mass of nitric acid) containing 10.1% by mass as Pt, and 0.80 g of a ruthenium nitrate (Ru (NO ₃ ) ₃ ) aqueous solution containing 3.9% as Ru, Were mixed and diluted with 11 g of pure water. This was impregnated in a carrier obtained by mixing 3.6 g of the above cerium oxide-zirconium oxide carrier B and 12.0 g of the calcined zirconium oxide obtained in Example 1, evaporated to dryness, and then dried at 120 ° C. for 1 hour. Further, it was calcined in air at 550 ° C. for 6 hours to obtain a 2% Pt-0.2% Ru / cerium oxide-zirconium oxide mixed carrier (hereinafter referred to as catalyst I). The BET specific surface area of the catalyst I was 21 m ² / g. Further, the ratio (mass basis) of monoclinic zirconium oxide in the inorganic oxide support of catalyst I is calculated to be 77%. Further, the ratio (mass basis) of cerium oxide in the inorganic oxide carrier of catalyst I is 7.38%.

(Example 5)
3.06 g of a dinitrodiammine platinum nitrate solution (containing 20% by mass of nitric acid) containing 10.1% by mass as Pt, and 0.79 g of an aqueous ruthenium nitrate (Ru (NO ₃ ) ₃ ) solution containing 3.9% as Ru, Were mixed and diluted with 11 g of pure water. This was impregnated in a carrier obtained by mixing 1.4 g of the above cerium oxide-zirconium oxide carrier B and 14.0 g of calcined zirconium oxide obtained in Example 1, evaporated to dryness, and then dried at 120 ° C. for 1 hour. Further, it was calcined in air at 550 ° C. for 6 hours to obtain a 2% Pt-0.2% Ru / cerium oxide-zirconium oxide mixed carrier (hereinafter referred to as catalyst J). The BET specific surface area of the catalyst J was 19 m ² / g. Further, the ratio (mass basis) of monoclinic zirconium oxide in the inorganic oxide support of catalyst J is calculated to be 91%. Further, the ratio (mass basis) of cerium oxide in the inorganic oxide carrier of the catalyst J is 2.91%.

(Example 6)
2% Pt-0.2% Ru / oxidized in the same manner as in Example 5 except that the mixing ratio of the carrier was changed to 7.7 g of cerium oxide-zirconium oxide carrier B and 7.7 g of calcined zirconium oxide obtained in Example 1. A cerium-zirconium oxide mixed carrier (hereinafter referred to as catalyst K) was obtained. The ratio (mass basis) of monoclinic zirconium oxide in the inorganic oxide support of catalyst K is calculated to be 50%. Further, the ratio (mass basis) of cerium oxide in the inorganic oxide carrier of the catalyst K is 16.0%.

(Example 7)
6.18 g of a dinitrodiammine platinum nitrate solution (containing 20% by mass of nitric acid) containing 10.1% by mass as Pt and 0.80 g of an aqueous ruthenium nitrate (Ru (NO ₃ ) ₃ ) containing 3.9% as Ru, Were mixed and diluted with 11 g of pure water. This was impregnated in a carrier obtained by mixing 3.6 g of the above cerium oxide-zirconium oxide carrier B and 12.0 g of the calcined zirconium oxide obtained in Example 1, evaporated to dryness, and then dried at 120 ° C. for 1 hour. Further, it was calcined in air at 550 ° C. for 6 hours to obtain a 4% Pt-0.2% Ru / cerium oxide-zirconium oxide mixed carrier (hereinafter referred to as catalyst L). Further, the ratio (mass basis) of monoclinic zirconium oxide in the inorganic oxide support of the catalyst L is calculated as 77%. Further, the proportion (mass basis) of cerium oxide in the inorganic oxide carrier of the catalyst L is 7.38%.

(Activity evaluation)
Catalysts A and B were used as they were, and Catalysts C to H were tablet-molded to have a particle size of 1 to 2 mm, and 1.45 g (Catalysts A and B were about 1.9 mL, Catalysts C to H were about 1.5 mL) filled in a quartz reaction tube, the catalyst layer temperature was changed to 525,500,475,450,425,400 ° C., and the gas having the composition shown in Table 1 at each temperature was 1.675 liters per minute ( The volume converted into the standard state at 0 ° C. and 1 atm, the same applies hereinafter, and the conversion rates of nitrogen oxide (NOx), carbon monoxide (CO) and hydrocarbon (CH ₄ ) were measured (initial performance) ). The conversion rate is defined as 100 × (1− (outlet concentration) / (inlet concentration)) (%). For NOx, the total concentration of nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ ) is used. Used.

  Subsequently, a simulated deterioration process was performed in which the catalyst layer temperature was kept at 400 ° C. and the gas shown in Table 2 was circulated at a flow rate of 1.675 liters per minute for 18 hours. This attaches a sulfur compound that reduces the activity by poisoning the catalyst and simulates deterioration in a state where the oxygen concentration generated at the time of starting and stopping is high.

  Subsequent to the simulated deterioration treatment, the catalyst layer temperature is changed to 400, 425, 450, 475, 500, and 525 ° C., and the gas having the composition shown in Table 3 is circulated at a flow rate of 1.675 liters per minute at each temperature. The conversion was measured (performance after deterioration treatment).

  The conversion rates of nitrogen oxides, carbon monoxide and hydrocarbons at 400 ° C. and 450 ° C. of catalyst A and catalyst D before and after the simulated deterioration treatment are shown in FIGS. Table 4 shows the conversion rate of each component at 400, 450, and 500 ° C. after the simulated deterioration treatment.

  Catalyst A has been conventionally used as a three-way catalyst for combustion exhaust gas of methane main component gas (FIG. 1). In this catalyst, nitrogen oxide (conversion rate 100%), carbon monoxide (conversion rate 100%), and near the theoretical air-fuel ratio (λ = 0.9995) at 450 ° C. in the initial stage (before deterioration treatment) A conversion rate of 90% or more can be obtained for all three components of hydrocarbon (conversion rate of 98%). Further, the conversion rate at 400 ° C. slightly decreases with nitrogen oxide (conversion rate of 93%), carbon monoxide (conversion rate of 100%) and hydrocarbon (conversion rate of 90%), but is slightly increased on the fuel excess side (λ = 0). 0.9975), high conversion rates can be secured for both nitrogen oxide (conversion rate 100%), carbon monoxide (conversion rate 99%) and hydrocarbons (conversion rate 96%). However, after simulated deterioration (after deterioration), the performance is greatly reduced, and even at 450 ° C., sufficient purification performance cannot be secured near the theoretical air-fuel ratio (λ = 0.9995). In particular, regarding the conversion rate of hydrocarbons, it can be seen that after the simulated deterioration treatment, even if the air-fuel ratio is optimized, the conversion rate does not reach 90%, so that sufficient purification performance is not exhibited. That is, the conventional catalyst can obtain a sufficient purification performance at a temperature exceeding 500 ° C., but the performance at a lower temperature is not sufficient.

  On the other hand, the catalyst D which is the catalyst of the present invention shows high performance even after simulated deterioration (after deterioration) (FIG. 2). For example, at 450 ° C., nitrogen oxide (conversion 95%), carbon monoxide (conversion 100%) and hydrocarbon (conversion 92%) 3 in the vicinity of the theoretical air-fuel ratio (λ = 0.9995). Conversion of 90% or more was obtained for both components, and even at 400 ° C., a slight excess of fuel (λ = 0.9975) was obtained under conditions of nitrogen oxide (conversion 98%), carbon monoxide (conversion 100). %) And hydrocarbons (conversion rate 82%) and a high conversion rate can be secured. With the catalyst of the present invention, sufficient purification performance can be ensured even in a temperature range of about 400 to 450 ° C.

  That is, it is a three-way catalytic reaction process catalyzed by a three-way catalyst, and the combustion air for the fuel contains carbon monoxide, nitrogen oxides, and methane generated by combustion of the gas adjusted to the stoichiometric air-fuel ratio. In a three-way catalytic reaction treatment of a methane-containing gas, a three-way catalyst constituted by supporting ruthenium and platinum on an inorganic oxide support containing 50 mass% or more of monoclinic zirconium oxide, and reducing the methane It can be seen that it is preferable to use a three-way catalyst for reducing and removing the nitrogen oxides as force.

  Catalysts G and H, which are catalysts formed by supporting iridium and platinum on an inorganic oxide mainly composed of monoclinic zirconium oxide, described in Patent Document 3, are also more preferable than catalyst A which is a conventional catalyst. Purification performance at low temperatures is high. However, this catalyst requires iridium, which is a rare metal, and requires a relatively high amount of iridium to exhibit sufficient performance (the amount of iridium supported by Catalyst H is 0.5%). . On the other hand, the catalyst of the present invention uses a less expensive ruthenium, and a sufficient performance can be obtained with a relatively small loading (the ruthenium loading on the catalyst D is 0.2%).

  Further, from the results of the catalysts E and F shown in Table 4, the supported amount of ruthenium used in the catalyst of the present invention has sufficient performance in the range of 0.1 to 0.5% on the mass basis with respect to the inorganic oxide alone. Is maintained. That is, it can be seen that the supported amount of ruthenium used in the catalyst of the present invention is preferably 0.1 to 0.5% on a mass basis with respect to the inorganic oxide support.

  Further, even in the case of the same combination of ruthenium and platinum as in the present invention, when the support is the same cerium oxide-aluminum oxide support as in the prior art (catalyst B), When the main component is not zirconium oxide (catalyst C), the performance is low.

The catalyst of the present invention contains monoclinic zirconium oxide as a main component of the inorganic oxide support. In the catalysts C, D, I, J, and K, the proportion of monoclinic zirconium oxide in the inorganic oxide support is compared. When the conversion rate of nitrogen oxides in the vicinity of the theoretical air-fuel ratio at 400 ° C. (λ = 0.9995) is compared, the catalysts D, I, J, and K, which are the catalysts of the present invention, each have 84% as shown in Table 4. 85%, 89%, and 81%. On the other hand, it was 41% for catalyst C. The proportion (mass basis) of monoclinic zirconium oxide in these inorganic oxide supports is 70%, 77%, 91%, and 50% for the catalysts D, I, J, and K, which are the catalysts of the present invention, respectively. It is. On the other hand, the catalyst C is 20%. That is, the higher the proportion of monoclinic zirconium oxide, the higher the purification performance of nitrogen oxides.
As these show, the higher the ratio of monoclinic zirconium oxide, the better the nitrogen oxide purification performance near the low-temperature stoichiometric air-fuel ratio.

  However, cerium is an essential component for securing the oxygen storage capacity in the three-way catalyst, and attention must be paid to the balance with the oxygen storage capacity when increasing the proportion of monoclinic zirconium oxide. In consideration of the result of the catalyst J having the smallest cerium oxide content in the evaluation of the catalyst of the present invention, the oxidation in the inorganic oxide support is necessary in order to exert an appropriate nitrogen oxide purification performance in the catalyst of the present invention. It can be said that the ratio (based on mass) of cerium is preferably at least 2.9% or more.

That is, it is preferable to contain cerium oxide as the inorganic oxide carrier.
The cerium oxide, once prepared as a composite oxide of cerium oxide and zirconium oxide, is separately mixed with monoclinic zirconium oxide and used as the inorganic oxide carrier. It is preferred that

  Further, the composite oxide of cerium oxide and zirconium oxide has at least a crystal system other than a monoclinic crystal as a main component, and is preferably tetragonal or cubic.

  The mixing ratio of cerium oxide and zirconium oxide when adjusting the composite oxide is preferably 1: 2 to 1: 6.

  The catalyst L shows the effect of the amount of Pt supported. Compared with catalyst I, it can be seen that when the amount of Pt supported is increased to 4%, as shown in Table 4, the purification performance near the low-temperature stoichiometric air-fuel ratio is greatly improved. In general, the supported amount of platinum is preferably 0.5 to 5% on a mass basis with respect to the inorganic oxide support.

  The three-way catalyst of the present invention is excellent in low-temperature performance and excellent in methane removal performance on the lean side, so that a methane-containing gas purification device is used to construct a methane-containing gas even under low exhaust temperature conditions. Purification performance can be obtained, and high-level methane-containing gas purification can be performed under economically advantageous conditions.

Claims (7)

  Composed of ruthenium and platinum supported on an inorganic oxide carrier mainly composed of monoclinic zirconium oxide, combustion air for fuel is generated by combustion of gas adjusted to the theoretical air-fuel ratio, carbon monoxide, A three-way catalyst for reducing and removing the nitrogen oxide by using the methane as a reducing power in a three-way catalytic reaction treatment of a methane-containing gas containing nitrogen oxides and methane.   The three-way catalyst according to claim 1, wherein the supported amount of ruthenium is 0.1 to 0.5% on a mass basis with respect to the inorganic oxide support.   The three-way catalyst according to claim 1 or 2, wherein a supported amount of platinum is 0.5 to 5% on a mass basis with respect to the inorganic oxide support.   The three-way catalyst according to any one of claims 1 to 3, wherein the inorganic oxide support contains more than 50 mass% of monoclinic zirconium oxide.   A methane-containing gas containing carbon monoxide, nitrogen oxides, and methane generated by combustion of a gas adjusted to a theoretical air-fuel ratio is converted to ruthenium and platinum on an inorganic oxide carrier mainly composed of monoclinic zirconium oxide. The carbon monoxide, nitrogen oxide, and methane in the methane-containing gas are brought into contact by a three-way catalytic reaction using methane in the methane-containing gas as a reducing power. A method for purifying methane-containing gas to be removed.   The method for purifying a methane-containing gas according to claim 5, wherein an air-fuel ratio of the methane-containing gas is adjusted to λ = 0.998 to 1.000 and brought into contact with the three-way catalyst.   The method for purifying a methane-containing gas according to claim 5 or 6, wherein the removal reaction of carbon monoxide, nitrogen oxides, and methane in the methane-containing gas is performed at a reaction temperature of 400 ° C to 550 ° C.

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