JP4688646B2 - Three-way catalyst and methane-containing gas purification method using the same - Google Patents

Description

  The present invention relates to a three-way catalyst for purifying methane-containing gas, such as exhaust gas from a gas engine using natural gas as fuel, and a method for purifying methane-containing gas using the same.

  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. Conventionally, the above three components are removed from the exhaust gas using a three-way catalyst method, and the exhaust gas is released. In the three-way catalyst method, the air-fuel ratio of exhaust gas is controlled to balance the oxidizing and reducing components in the exhaust gas, and then nitrogen oxides are passed through the exhaust gas through a catalyst containing platinum or rhodium (three-way catalyst). , Carbon monoxide, and hydrocarbons are simultaneously removed.

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 removed well when the air-fuel ratio is λ = 1.000 and the vicinity thereof. At an air-fuel ratio on the lean (fuel lean or oxygen-excess) side than λ = 1.000, the purification rate of carbon monoxide and hydrocarbons is maintained high, but the nitrogen oxide removal rate is reduced. On the other hand, at the air-fuel ratio on the rich side (excess 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 decreases. The reason why the purification performance decreases at the lean and rich air-fuel ratio is that 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” (for example, see Non-Patent Document 1).

Also, the application of a three-way catalyst has been proposed in the purification of exhaust gas whose hydrocarbon is mainly methane, such as exhaust gas of a gas engine (see, for example, Non-Patent Documents 1 and 2 and Patent Documents 1 and 2). ).
Yoshio Ono, Makoto Misono, Yoshihiko Morooka, “Encyclopedia of Catalysts”, Asakura Shoten, 2000 (page 260) Hanaki-Yasunari et al., “JSAE Review”, 17: 259-265, 1996 (FIGS. 4 and 5) Republished publication WO 96/25593 (background art, FIGS. 3 and 4) JP-A-5-23592 (paragraph number [0005])

  However, when the hydrocarbon in the exhaust gas is mainly methane like the exhaust gas of a gas engine, it is described in Non-Patent Document 2 and Patent Documents 1 and 2 (particularly FIGS. 4 and 5 of Non-Patent Document 2). Thus, it has been clarified that the window region is very narrow, and a high purification rate cannot be obtained with the same type of catalyst and use conditions as gasoline exhaust gas. Specifically, when the hydrocarbon is methane, the methane purification rate is low on the lean side, and the nitrogen oxide purification rate near λ = 1.000 is low. Furthermore, the air-fuel ratio for securing the nitrogen oxide purification rate moves to the rich side. These are considered to be caused by the fact that methane is the most stable hydrocarbon among the hydrocarbons and lacks reactivity. In addition, since the zirconium oxide oxygen sensor used for air-fuel ratio control has the highest sensitivity in the vicinity of λ = 1.000, the sensitivity decreases as the air-fuel ratio at which the optimum purification rate is obtained deviates from λ = 1.000, In such a case, there is a problem that air-fuel ratio control becomes more difficult.

  In recent years, the energy utilization efficiency of the engine has improved, and the temperature of the exhaust gas tends to decrease. Therefore, there is a concern that a conventional three-way catalyst requires a large amount of catalyst in order to obtain sufficient performance at a low temperature, and a highly active three-way catalyst that can be used even at a lower temperature is demanded.

  Furthermore, natural gas supplied as fuel may contain sulfur compounds as odorants. In such a case, the catalyst may be poisoned, leading to a decrease in activity and a shortened life. There was a problem.

  Therefore, in view of the above problems, an object of the present invention is to provide a three-way catalyst that is excellent in low-temperature performance, has high resistance to sulfur poisoning, and has excellent methane removal performance on the lean side.

To achieve this object, the three-way catalyst of the present invention is characterized in that, as described in claim 1, iridium is supported on an inorganic oxide mainly composed of monoclinic zirconium oxide. In the three-way catalytic reaction treatment of a methane-containing gas containing carbon monoxide, nitrogen oxides, and methane, combustion air for fuel is generated by combustion of a gas adjusted to the stoichiometric air-fuel ratio. The nitrogen oxides are reduced and removed . 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 characteristic configuration, as described in claim 2, it is preferable to carry platinum,
As described in claim 3, the supported amount of iridium is preferably 0.5 to 20% with respect to the mass of zirconium oxide,
Preferably, the monoclinic inorganic oxide contains more than 50% by mass of zirconium oxide.

The characteristic means of the purification method of methane-containing gas of the present invention for achieving this object, as described in claim 5, is generated by combustion of a gas adjusted to a stoichiometric air-fuel ratio, carbon monoxide, the methane-containing gas containing nitrogen oxides and methane, is come in contact to the three-way catalyst constituted by supported iridium inorganic oxide composed mainly of zirconium oxide monoclinic, the methane-containing gas Using methane as a reducing power , carbon monoxide, nitrogen oxides, and methane in the methane-containing gas are removed by a three-way catalytic reaction .

In the above characteristic means, as described in claim 6, an air-fuel ratio of the gas is adjusted to λ = 0.998 to 1.000, and the methane-containing gas generated by combustion is converted into the three-way catalyst. It is preferable to make it contact.
Further, as described in claim 7, it is preferable that the removal reaction of carbon monoxide, nitrogen oxide and methane in the methane-containing gas is performed at a reaction temperature of 400 ° C to 600 ° C,
Furthermore, as described in claim 8, the reaction temperature is preferably 400 to 550 ° C.

  As a result of diligent study by the inventors, as described in claim 1, a three-way catalyst constituted by supporting iridium on an inorganic oxide mainly composed of monoclinic zirconium oxide is theoretically Methane contained in methane-containing gas can be used as reducing power at an air-fuel ratio, and this catalyst has excellent low-temperature performance, high sulfur poisoning resistance, and excellent methane removal performance on the lean side, completing the invention It came to do.

  Furthermore, as described in claim 2, when the three-way catalyst is one that carries platinum in addition to the iridium, the low-temperature performance and the lean-side methane removal performance are very excellent.

  Here, if the supported amount of iridium is less than 0.5% with respect to the mass of zirconium oxide, the three-way catalytic activity is low, and if it is more than 20%, the particle size of iridium becomes large and the performance commensurate with the supported amount. It cannot be obtained and is not economical. Therefore, as described in claim 3, it is preferable that the amount of iridium supported be 0.5 to 20% with respect to the mass of zirconium oxide.

  Further, as described in claim 4, when the monoclinic inorganic oxide contains more than 50% by mass of zirconium oxide, a three-way catalyst having excellent heat resistance can be obtained.

  Further, as described in claim 5, iridium is supported on an inorganic oxide mainly composed of monoclinic zirconium oxide, and methane in a methane-containing gas is used as a reducing power at a stoichiometric air-fuel ratio. When an available three-way catalyst is brought into contact with a methane-containing gas at a stoichiometric air-fuel ratio, the three-way catalyst exhibits high low-temperature performance, as is clear from the examples. However, it exhibits carbon monoxide, nitrogen oxide and methane purification capabilities as a three-way catalyst. In addition, since the performance degradation due to sulfur poisoning is small, the life is long and the methane-containing gas can be purified over a long period of time.

  Here, as described in claim 6, when 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, it is clear from the examples. As described above, a very high three-way catalytic activity is exhibited, and carbon monoxide, nitrogen oxides and methane in the methane-containing gas can be almost removed. In addition, when adjusting the air-fuel ratio of the methane-containing gas brought into contact with the three-way catalyst, the air-fuel ratio is such that the sensitivity of the zirconium oxide oxygen sensor becomes sharp. Therefore, the air-fuel ratio of the methane-containing gas can be easily adjusted. Can be done accurately.

  In addition, as described in claim 7, when 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 600 ° C, the heat of the three-way catalyst It is possible to suppress deterioration due to (grain growth). Moreover, in the gas engine etc. which operate | move at low temperature, since the temperature of waste gas exists in this range, it can purify | clean, without adjusting temperature of waste gas. Such an effect is remarkable when the reaction temperature is 400 to 550 ° C. as described in claim 8.

  In WO 2002/040152, it is disclosed that a catalyst in which iridium or platinum is supported on zirconium oxide removes hydrocarbons (particularly methane). 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) ) To purify the hydrocarbons in it. Here, in the technical common sense related to exhaust gas purification, in the state where the oxygen concentration of the purification target gas is greatly different, that is, in the state where the air-fuel ratio is greatly different, the oxidation-reduction state of the metal element that controls the catalytic function changes. It is usually thought that it cannot be expected. For example, as described in Claims 3 and 4 of Patent Document 1 and FIGS. 3 and 4, the “window” of the platinum / rhodium three-way catalyst substantially has an air-fuel ratio of 0.99 to 1. In the case of 00, palladium-based three-way catalyst, the air-fuel ratio is 0.85 to 0.95. Thus, the range of the air-fuel ratio that functions effectively as a three-way catalyst is quite narrow, and the range varies depending on the catalyst. Therefore, it is reasonable to consider that a catalyst having a similar composition does not maintain high activity on the other side when it exhibits high activity in either the stoichiometric air-fuel ratio region or the oxygen-excess air-fuel ratio region. In view of the above, the present invention cannot be easily conceived based on the disclosure of WO 2002/040152.

Embodiments of the present invention will be described below.
The three-way catalyst of the present invention is configured by supporting iridium on an inorganic oxide mainly composed of monoclinic zirconium oxide, and can use methane in a methane-containing gas as a reducing power at a stoichiometric air-fuel ratio. is there.

This three-way catalyst is obtained by impregnating zirconium oxide (ZrO ₂ ) with a solution containing iridium ions, followed by drying and firing. As a solution containing metal ions used for impregnation with iridium, when an aqueous solution is used, a water-soluble compound such as chloroiridium acid may be dissolved in pure water. Alternatively, an organic solvent system in which an organometallic compound such as tris (acetylacetonato) iridium is dissolved in acetone or the like may be used. Moreover, it is good also as a mixed solvent which added the water-soluble organic solvent to water as needed. When platinum is supported in addition to iridium, it can also be supported at once using an aqueous solution that dissolves both iridium salt and platinum salt. In this case, use a mixed solution of chloroiridate and chloroplatinic acid. Can do. In addition, iridium and platinum may be separately loaded and supported sequentially, and at this time, a process such as drying or calcining may be appropriately performed before the next loading.

If the specific surface area of zirconium oxide is too low, iridium cannot be kept in a highly dispersed state. On the other hand, if the surface area is too high, the carrier 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 such zirconium oxide, commercially available zirconium oxide for catalyst support may be used, or commercially available zirconium hydroxide may be used after calcining at 600 to 1000 ° C. The zirconium oxide support preferably has a specific surface area of ² to 90 m ² / g, and more preferably 5 to 60 m ² / g. 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. Further, a content of more than 65% is more preferable because activity inhibition by sulfur oxide is small and durability is further enhanced.

  If the supported amount of iridium is too small, the three-way catalyst activity is low, and if it is too large, the particle size of iridium becomes large and performance corresponding to the supported amount cannot be obtained, resulting in poor economic efficiency. Therefore, Preferably it is 0.5 to 20% with respect to the mass of a zirconium oxide, More preferably, you may be 1 to 5%. In the case of using platinum, if it is too small, there is no effect, and if it is too much, the activity of iridium is inhibited. Therefore, it is preferably 5 to 100%, more preferably 10 to 50% with respect to the mass of iridium.

  After loading the active metal, it is calcined to complete the catalyst. The gas that circulates at the time of firing may be ordinary air, but a gas obtained by appropriately mixing air or oxygen and an inert gas such as nitrogen may be used, or water vapor or carbon dioxide may be added. . If the calcination temperature is too high, grain growth of the supported noble metal proceeds and high activity cannot be obtained. On the other hand, if it is too low, there is no effect of calcination, and there is a possibility that the growth of noble metal grains progresses during use of the three-way catalyst, and stable activity cannot be obtained. Therefore, in order to obtain high activity stably, the firing temperature is preferably in the range of 450 ° C. to 650 ° C., more preferably in the range of 500 ° C. to 600 ° C.

  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, zirconium oxide sol or the like may be added to the three-way catalyst prepared by the above method to form a slurry and wash-coated as required. Then, after wash-coating on the refractory honeycomb, iridium and, if necessary, platinum may be supported according to the above method. When the cordierite is wash-coated, the coating amount of the three-way catalyst is preferably 50 to 300 g as zirconium oxide and 0.5 to 30 g as iridium per liter of cordierite, more preferably 100 as zirconium oxide. ˜250 g, iridium as 5-20 g. When carrying | supporting platinum, it is 5 to 100% by mass ratio with respect to iridium, More preferably, you may be 10 to 50%.

The three-way catalyst of the present invention may be used in combination with a known catalyst as necessary. In the method of mixing with the three-way catalyst of the present invention, both powders may be mixed and pulverized, and when a washcoat catalyst is used, the mixed powder obtained by mixing and pulverizing both powders is washcoated. It may be divided into two or more layers and coated separately or with different mixing ratios.
If the amount of the catalyst used is too small, an effective oxidation performance cannot be obtained. Therefore, it is desirable to use the catalyst under conditions where the space velocity per gas hour (GHSV) is 200,000 h ⁻¹ or less. As the space velocity per gas time (GHSV) decreases, the amount of catalyst increases, so the catalyst performance improves. For example, when used at 1000 h ^-1 or less, in addition to the economical problem, There is a risk that the pressure loss will increase.

  In the methane-containing gas purification method using the catalyst as described above, the methane-containing gas at the stoichiometric air-fuel ratio is brought into contact with the above three-way catalyst, and the carbon monoxide, nitrogen oxide, and methane in the methane-containing gas are contacted. It is characterized by removing.

  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. Of 0.990 times to 1.005 times (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.

For simplicity, the composition of the methane-containing gas brought into contact with the three-way catalyst is adjusted so as to be in a range satisfying both of the following two conditions, thereby satisfying the air-fuel ratio.
Condition 1:
([H ₂ ] × 0.5 + [CO] × 0.5 + [CH ₄ ]) <([O ₂ ] + [NO] × 0.5 + [NO ₂ ])
Condition 2:
([H ₂ ] × 0.5 + [CO] × 0.5 + [CH ₄ ] × 2)> ([O ₂ ] + [NO] × 0.25 + [NO ₂ ] × 0.5)
However, the inside of [] represents gas concentration in ppm (volume basis).

  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. Further, 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.

  Here, the methane-containing gas may contain sulfur components 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.

[Example 1]
Zirconium oxide (Nippon Denko Corporation, N-PC, specific surface area 28 m ² / g) was calcined in air at 800 ° C. for 6 hours to obtain calcined zirconium oxide having a BET specific surface area of 16 m ² / g. In X-ray diffraction measurement using Cu-Kα rays, only monoclinic diffraction lines near 2θ = 28 ° and 31.5 ° were observed. Tetragonal diffraction lines around 2θ = 30 ° were not observed. From this, it is considered that 99% or more of the calcined zirconium oxide is monoclinic. After impregnating 40 g of this calcined zirconium oxide with an aqueous solution of chloroiridic acid (H ₂ IrCl ₆ ) and further evaporating to dryness with an evaporator, it was calcined in air at 550 ° C. for 6 hours to give 2 mass of Ir with respect to zirconium oxide. % Ir / zirconium oxide three-way catalyst was obtained. The specific surface area of this three-way catalyst was 16 m ² / g.

A three-way catalyst was formed by tableting and sized to a particle size of 1 to 2 mm, and 2 ml of this was filled into a stainless steel reaction tube. The temperature of the three-way catalyst layer is kept at a predetermined temperature of 400 to 550 ° C., and the composition shown in Table 1 (simulating a methane-containing gas having a ratio of air amount to theoretical air amount (λ) of 0.985 to 1.005) The initial reaction gas was circulated at a flow rate of 1.5 liters per minute (volume in the standard state; the same applies hereinafter), and the purification rate of NO _x , CH ₄ , and CO was measured.

Subsequently, the gas having the composition shown in Table 2 was circulated for 20 hours at a flow rate of 1.5 liters per minute, and after the deterioration treatment, the gas having the composition shown in Table 3 was circulated at a flow rate of 1.5 liters per minute. Then, the purification rate of NO _x , CH ₄ , and CO was measured again.

The purification rate is defined as 100 × (1− (exit concentration) / (inlet concentration)) (%), and for NO _x , the total concentration of nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ ). Is used.

The results are shown in Table 4. The NO _x purification rate at an air-fuel ratio of λ = 1.000 was 100% at 450 ° C. or higher, and high purification rates were shown for all of NO _x , CH ₄ , and CO. Further, the methane purification rate on the lean side (λ = 1.005) reaches 98% at 450 ° C., and the methane removal performance can be maintained high even when the air-fuel ratio deviates to the lean side. Moreover, even after deterioration treatment with sulfur dioxide, a high methane removal rate of 87% at 450 ° C. is maintained, and a high purification rate for all of NO _x , CH ₄ , and CO at λ = 0.998 and 1.000 above 475 ° C. Was maintained.

[Example 2]
40 g of the same calcined zirconium oxide as in Example 1 was impregnated with a mixed aqueous solution of chlorinated iridium acid (H ₂ IrCl ₆ ) and chloroplatinic acid (H ₂ PtCl ₆ ), further evaporated to dryness with an evaporator, and then 550 in air. Firing at 6 ° C. for 6 hours gave an Ir—Pt / zirconium oxide three-way catalyst containing 2% by mass of Ir and 0.5% by mass of Pt based on zirconium oxide. The specific surface area of this three-way catalyst was 16 m ² / g.

The performance of this three-way catalyst was evaluated under the same conditions as in Example 1. The results are shown in Table 5. The NO _x purification rate when the air-fuel ratio is λ = 1.000 was 100% at 450 ° C. or higher. Further, the methane purification rate on the lean side (λ = 1.005) was almost 100% at 450 ° C. or higher. In the three-way catalyst, the air-fuel ratio is controlled so that any component can be removed. For example, when 90% of NO _x , CH ₄ , and CO are removed, the initial temperature is 400 ° C. Even after the deterioration treatment, if it was 450 ° C. or higher, the necessary purification performance was obtained. Moreover, even after the deterioration treatment with sulfur dioxide, very high three-way catalyst activity could be maintained particularly at λ = 0.998 to 1.000.

[Comparative Example 1]
Alumina (Gobain - Norton Co., SA6276) in 12.8g those sized crushing to a particle size 1~2mm a cerium nitrate _{(Ce (NO 3) 3 ·} 6H 2 O) and 3.2g of 18g pure An aqueous solution dissolved in water was impregnated. This was evaporated to dryness and then calcined at 800 ° C. for 6 hours to obtain a CeO ₂ —Al ₂ O ₃ support carrying 10% cerium oxide (CeO ₂ ) with respect to alumina. 6 g of this was impregnated with an aqueous solution dissolving tetraammineplatinum nitrate (Pt (NH ₃ ) ₄ (NO ₃ ) ₂ ) and pentaammine aquadium nitrate (Rh (NH ₃ ) ₅ (H ₂ O) (NO ₃ ) ₃ ). Pt—Rh / CeO ₂ —Al ₂ O ₃ ternary carrying 2% by mass of Pt and 0.5% by mass of rhodium with respect to the support by evaporating to dryness and further firing in air at 550 ° C. A catalyst was obtained.

The performance of this three-way catalyst was evaluated under the same conditions as in Example 1. The results are shown in Table 6. Compared to the three-way catalyst according to Examples 1 and 2, the three-way catalyst according to Comparative Example 1 has a lower purification rate at the lean side air-fuel ratio (λ = 1.000 to 1.005). After the deterioration treatment due to sulfur poisoning, the purification rate is further reduced, and when NO _x , CH ₄ , and CO are all removed by 90%, it is possible at 400 ° C at the beginning, but after the deterioration treatment, it is 500 ° C or more. Necessary purification performance could not be obtained unless the temperature was lower.

[Comparative Example 2]
15 g of the same calcined zirconium oxide as in Example 1 was impregnated with an aqueous solution of palladium nitrate (Pd (NO ₃ ) ₂ ), further evaporated to dryness with an evaporator, and then calcined in air at 550 ° C. for 6 hours. Thus, a Pd / zirconium oxide three-way catalyst supporting 2% by mass of Pd was obtained.

The performance of this three-way catalyst was evaluated under the same conditions as in Example 1. The results are shown in Table 7. The initial purification rate is high, and the NO _x purification rate when the air-fuel ratio is λ = 1.000 reaches 78% at 400 ° C. and 100% at 450 ° C., and methane purification on the lean side (λ = 1.005) The rate reached 89% even at 400 ° C. However, after the deterioration treatment with sulfur dioxide, the purification rate is greatly reduced, and the NO _x purification rate when the air-fuel ratio is λ = 1.000 is only 63% at 550 ° C., and in the region of 400 to 550 ° C., NO _x , CH _4, air-fuel ratio can be 90% removal of any CO was present. Therefore, it can be seen that the catalyst according to Comparative Example 2 is very vulnerable to sulfur poisoning and does not practically function as a three-way catalyst.

  When the catalyst according to Example 1 and the catalyst according to Comparative Example 1 are compared, the catalyst according to Example 1 has a leaner air-fuel ratio (λ = 1.000) and most of nitrogen oxides, methane, and carbon monoxide. It can be seen that it can be removed. Further, when the catalyst according to Example 1 and the catalyst according to Comparative Example 2 are compared, the catalyst according to Example 1 maintains high activity even after being deteriorated with a gas containing sulfur, whereas It can be seen that in the catalyst according to Example 2, the purification rate of nitrogen oxides and methane is greatly reduced. As described above, the three-way catalyst according to the present invention is excellent in that it has a sulfur poisoning resistance and exhibits a removal ability at an air-fuel ratio very close to λ = 1.000 on the lean side of the conventional catalyst. It can be said that.

  Further, as shown in Table 2, by loading zirconium oxide as a carrier and platinum together with iridium as shown in Example 2, the above effects are further improved as shown in Table 5.

[Comparative Example 3]
Zirconium hydroxide 60g (HayashiJunyaku Kogyo Co., 79% contained as ZrO _2), cerium nitrate hexahydrate _{(Ce (NO 3) 3 ·} 6H 2 O) 22g aqueous solution prepared by dissolving in pure water 60g It was immersed for 15 hours and evaporated to dryness. Thereafter, the mixture was calcined at 700 ° C. for 6 hours to obtain a ceria-zirconia carrier having a BET specific surface area of 39 m ² / g (cerium oxide: zirconium oxide = 16: 84 by mass ratio). In the X-ray diffraction measurement using Cu-Kα rays, a tetragonal diffraction line near 2θ = 30 ° was strongly observed. In addition, the monoclinic diffraction lines near 2θ = 28 ° and 31.5 ° were weakly observed. From these intensity ratios, it was calculated that 80% of this ceria-zirconia was tetragonal or cubic, and the proportion of monoclinic crystal was 20%. Even if the monoclinic crystal consists entirely of zirconium oxide, the proportion of monoclinic zirconium oxide in the ceria-zirconia support is about 20%.
This ceria-zirconia support was impregnated with a mixed aqueous solution of chloroiridic acid (H ₂ IrCl ₆ ) and chloroplatinic acid (H ₂ PtCl ₆ ), evaporated to dryness with an evaporator, and then fired at 550 ° C. in air for 6 hours. did. As a result, an Ir—Pt / ceria-zirconia three-way catalyst containing 2% by mass of Ir and 0.5% by mass of Pt with respect to the ceria-zirconia support was obtained. The specific surface area of this three-way catalyst was 38 m ² / g.

  The performance of this three-way catalyst was evaluated under the same conditions as in Example 1. Table 8 shows the purification rate after the deterioration treatment. The NOx purification rate at the theoretical air-fuel ratio (λ = 1.000) was 77% at 475 ° C., which was about the same as that of Comparative Example 1.

[Comparative Example 4]
The same ceria-zirconia support as in Comparative Example 3 was prepared from hexaammineiridium nitrate ([Ir (NH ₃ ) ₆ ] (NO ₃ ) ₃ ) and tetraammine platinum nitrate ([Pt (NH ₃ ) ₄ ] (NO ₃ ) ₂ ). Was immersed in a mixed aqueous solution dissolved in 1% aqueous ammonia for 15 hours to evaporate to dryness. Thereafter, it was calcined in air at 550 ° C. for 6 hours to obtain an Ir—Pt / ceria-zirconia three-way catalyst (2) containing 2% by mass of Ir and 0.5% by mass of Pt based on the ceria-zirconia support. Obtained. The specific surface area of this three-way catalyst was 39 m ² / g.

  The performance of this three-way catalyst was evaluated under the same conditions as in Example 1. Table 9 shows the purification rate after the deterioration treatment. The NOx purification rate at the stoichiometric air-fuel ratio (λ = 1.000) was 84% at 475 ° C., which was slightly higher than that of Comparative Example 3, but is almost the same as that of Comparative Example 1.

Example 3
The calcined zirconium oxide 10 g used in Example 1 and the ceria-zirconia carrier 5 g used in Comparative Example 3 were mixed in a mortar. The resulting ceria-zirconia mixed support contains at least 66% by weight of monoclinic zirconium oxide. In this ceria-zirconia mixed carrier, platinum and iridium are supported in the same manner as in Comparative Example 4, and Ir—Pt / ceria-zirconia tris containing 2% by mass of Ir and 0.5% by mass of Pt with respect to the carrier. The original catalyst (3) was obtained.

  The performance of this three-way catalyst was evaluated under the same conditions as in Example 1. Table 10 shows the purification rate after the deterioration treatment. The NOx purification rate at the theoretical air fuel ratio (λ = 1.000) was 93% at 475 ° C. This result was similar to that of Example 1 and was much better than Comparative Example 1. Although not shown in Table 10, the methane purification rate at λ = 1.005 was 92% at 450 ° C. and 96% at 475 ° C., which was much better than Comparative Example 1.

  Summarizing the above results, the nitrogen oxide purification rate of the three-way catalyst is higher when the ratio of monoclinic zirconium oxide contained in the support is higher at least in the condition of low temperature or increased air-fuel ratio. In addition to the above, it has been clarified that it is increased by supporting platinum.

  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. And since it has high tolerance with respect to sulfur poisoning, the exhaust gas (methane containing gas) derived from the generally distributed natural gas containing the sulfur compound as an odorant can be utilized as it is. Therefore, it is possible to reduce the cost for treating methane-containing gas in various facilities using a natural gas engine, and to greatly improve the environment.

Claims (8)

Composed of iridium supported on inorganic oxide 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, nitrogen oxide And a three-way catalyst for reducing and removing the nitrogen oxides using the methane as a reducing power in a three-way catalytic reaction treatment of a methane-containing gas containing methane.   The three-way catalyst according to claim 1, wherein platinum is supported.   The three-way catalyst according to claim 1 or 2, wherein the amount of iridium supported is 0.5 to 20% with respect to the mass of zirconium oxide.   The three-way catalyst according to any one of claims 1 to 3, wherein the inorganic oxide contains more than 50 mass% of monoclinic zirconium oxide. A methane-containing gas containing carbon monoxide, nitrogen oxides, and methane is generated by combustion of a gas adjusted to a theoretical air-fuel ratio, and iridium is supported on an inorganic oxide mainly composed of monoclinic zirconium oxide. is come in contact to the three-way catalyst comprised Te, by using methane of the methane-containing gas as the reducing power, carbon monoxide of the methane-containing gas is removed by the nitrogen oxide and the three-way catalyst reaction of methane Purification method for methane-containing gas. The method for purifying a methane-containing gas according to claim 5, wherein an air-fuel ratio of the gas is adjusted to λ = 0.998 to 1.000, and the methane-containing gas generated by combustion is 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 600 ° C.   The method for purifying a methane-containing gas according to claim 7, wherein the reaction temperature is 400 to 550 ° C.